G L Schulz

F S U Fischer

D Trefz

A Melnyk

A Hamidi-Sakr

M Brinkmann

D Andrienko

S Ludwigs

The PCPDTBT family: correlations between chemical structure, polymorphism, and device performance

We highlight the influence of processing conditions on polymorphism and structure formation on the mesoscale for the family of PCPDTBT polymers with branched alkyl side chains. Direct correlations of morphology to the chemical structure and to transistor device performance are established. We found that up to four different packing motifs could be realized depending on the polymer derivative and the processing conditions: amorphous, -stacked, cross-hatched and dimer-containing polymorphs. While C-and F-PCPDTBT display similar packing behavior organizing in -stacked and dimer-like structures, Si-PCPDTBT gives rise to cross-hatched structures upon simple deposition from solution. The observed differences in chain packing for C-/F-PCPDTBT versus Si-PCPDTBT are related to differences in backbone conformations and aggregation behavior in solution. The effect of polymorphism on charge transport is probed using field-effect transistors, in which both -stacked and cross-hatched polymer chain arrangements yield the highest hole mobilities. Structural and mobility simulations rationalize our experimental findings by relating mobility to distributions of electronic coupling elements between the chains.

Introduction

The study of the morphology of conjugated polymers has been an active area of research for several decades due to its well-established role in influencing their many opto-electronic properties such as light absorption and charge transport. Regioregular poly(3-hexylthiophene) (P3HT), for example, is a semicrystalline polymer, which has been shown to demonstrate various packing motifs, depending on the deposition conditions, from amorphous to highly ordered stacked arrangements. P3HT polymer chains crystallize in various orientations with respect to the substrate, from face-on to edge-on to standing chains. 1 Pioneering work in this field was done by Sirringhaus and coworkers, 2 who investigated the effect of different P3HT chain orientations on the transistor properties. In edge-on orientation, the field effect mobility is more than 100 times higher than for chains in face-on orientation, which is attributed to excellent charge transport along the chain axis and in the -stacking direction. 3,4 In addition, it was found that solvent vapor annealing could be used to precisely control the crystallization of P3HT. 5 Charge transport in anisotropic thin films was preferred along the polymer chains over transport in the stacking direction. [START_REF] Crossland | Anisotropic Charge Transport in Spherulitic poly(3-Hexylthiophene) Films[END_REF] Another conjugated polymer that has also been given much attention is poly(9,9-dioctylfluorene) (PFO). 7 PFO has two reported polymer conformations: and . The -type conformation prevails in PFO thin films exposed to vapors of good solvents, e.g., toluene.

Whereas the chain conformation is reported to crystallize in a dimer-containing orthorhombic structure of high symmetry (Pnb21) and to have a less planar backbone than the conformation. 8,9

Over the years a new class of donor-acceptor (D-A) conjugated copolymers has been developed.

In comparison to P3HT and PFO, fewer studies have been performed on the precise packing motifs of these, more advanced, materials. D-A copolymers are structurally more complex than homopolymers like P3HT and PFO, since they consist of electron rich and electron poor -conjugated building blocks. In what follows, we describe the processing techniques used to obtain various polymorphs as well as their characteristic traits, starting with C-PCPDTBT, followed by F-PCPDTBT and finally Si-PCPDTBT. This includes structural characterization and measurements of charge carrier mobilities of all polymers in different polymorphs under identical conditions. The experimental results are supported by theoretical calculations performed for the predicted crystalline packing motifs. To this end, we use molecular dynamics simulations to simulate atomistic morphologies of large supercells and evaluate charge transfer rates using polarizable force-fields and first principles methods. Kinetic Monte Carlo (KMC) simulations are then used to solve the Master equation for charge dynamics. Finally, we evaluate charge mobility from the KMC trajectories.

Simulation results on structures and mobilities are compared to experimental data allowing us to rationalize the molecular packing and the effect of local and mesoscale ordering on charge carrier mobility.

Chemical and physical properties

Morphology and charge transport simulations

We start by rationalizing the differences in hole mobilities exhibited by the three PCPDTBT derivatives:Si-and F-PCPDTBT have hole mobilities up to 10 -2 cm 2 /Vs, whereas the mobility of C-PCPDTBT is approximately one order of magnitude lower at 10 -3 cm 2 /Vs (see Table S1). [START_REF] Zhang | Significant Improved Performance of Photovoltaic Cells Made from a Partially Fluorinated Cyclopentadithiophene/Benzothiadiazole Conjugated Polymer[END_REF][START_REF] Hou | Characterization, and Photovoltaic Properties of a Low Band Gap Polymer Based on Silole-Containing Polythiophenes and 2,1,3-Benzothiadiazole[END_REF][START_REF] Morana | Nanomorphology and Charge Generation in Bulk Heterojunctions Based on Low-Bandgap Dithiophene Polymers with Different Bridging Atoms[END_REF][START_REF] Chen | Silicon Atom Substitution Enhances Interchain Packing in a Thiophene-Based Polymer System[END_REF][START_REF] Scharber | Influence of the Bridging Atom on the Performance of a Low-Bandgap Bulk Heterojunction Solar Cell[END_REF][START_REF] Horie | Cyclopentadithiophene-Benzothiadiazole Oligomers and Polymers; Synthesis, Characterisation, Field-Effect Transistor and Photovoltaic Characteristics[END_REF][START_REF] Kettle | Optimisation of PCPDTBT Solar Cells Using Polymer Synthesis with Suzuki Coupling[END_REF] These transistors were obtained from classical deposition conditions reported to yield mainly π-stacked structures. At this stage it remains unclear whether the change in the electronic structure, local molecular packing, or mesoscale ordering of chains in the film is responsible for the change in the hole mobility.

To rule out contributions due to the change in the electronic structure, we first analyze electronic coupling elements (transfer integrals) between two tetramers (four donor-acceptor pairs) in a -stacked arrangement. These couplings are plotted as a function of the relative shift between two backbones in Figure 1. The separation between two chains is fixed to , corresponds to ageometry of a dimer in a stack morphology with a minimal side-chain overlap, i.e. when the acceptor (BT) units are on top of each other, while the donor (CPDT) units are spatially separated due to backbone bending, as shown in Figure 7 (a,b). In this model system, branched ethyl-hexyl side-chains are replaced with hydrogens in order to reduce computational costs. Evidently, all three polymers have a similar dependence of the electronic coupling on the relative shift . Si-PCPDTBT has the lowest coupling for this given dimer geometry (see SI, Section C for details). This observation is in a clear contradiction to the experimental observations, with Si-PCPDTBT having the highest OFET mobility. We can therefore conclude that the change in the electronic structure alone cannot be responsible for the change in charge mobility. Hence, different polymers havedifferent mesoscale ordering of backbones.

To further assess the influence of chain packing on charge transport, we have performed molecular dynamics simulations of large supercells of 128 tetramers with ethyl-hexyl side-chains.

The simulation box of 4 lamellae with 32 chains per lamella has been equilibrated at 300 K and . In case of Si-PCPDTBT the corresponding values are and . We then used 10 snapshots of the 1 ns production run in order to perform averages of the distribution of electronic coupling elements, site energies, and mobilities.

Electronic coupling elements were evaluated using the projection method 36 with diabatic states constructed from HOMO energy levels of hydrogen-saturated tetramers. With the electronic couplings at hand we have evaluated intermolecular charge transfer rates using the high-temperature limit of non-adiabatic charge transfer theory 38 :

Here =0.1 eV is the reorganization energy, J AB the electronic coupling, and the driving force. All these parameters were evaluated using a combination of quantum-chemical methods and classical polarizable force-fields. 39,40

The rates and centers of mass of molecules were used to parameterize the master equation which was solved using the kinetic Monte Carlo (KMC) algorithm. [START_REF] Jansen | An Introduction To Monte Carlo Simulations Of Surface Reactions[END_REF] The intermolecular charge-carrier mobility along the direction of the external field was obtained from the charge trajectory as where ∆R is the displacement of charge over a simulation time ∆t. The electric field was along the -stacking axis. The results of simulations are summarized in Table 1. Since we do not take into account energetic disorder and defects, such as grain boundaries and amorphous regions, the calculated mobilities are higher than the experimental ones. The trend is, however, clear: the carbon-bridged polymer has the lowest and the silicon-based PCPDTBT the highest mobility.

Hence, we can conclude that the local chain packing favors larger hole mobility in Si-PCPDTBT.

This can indeed be confirmed by looking at the distributions of electronic couplings in the simulated morphologies (see Supporting Information, Figure S4): Si-PCPDTBT has the largest average coupling. Better registry of backbones in Si-PCPDTBT could be attributed to longer Si-C bonds and softer attachment of alkyl side chains due to a larger, as compared to carbon, Si atom. 42 Indeed, the side chains of Si-PCPDTBT are on average further away from the backbone, leading to larger interlamellar separations and, correspondingly, smaller distances. Energetic disorder in the lamellar arrangements of all three polymers is rather large, in part due to conformational disorder of side chains. If accounted for, it leads to a three order of magnitude decrease of mobility (at low charge carrier densities) for all three polymers.

Even though the local ordering already favors transport in Si-PCPDTBT as compared to other derivatives, the difference in simulated mobilities is still smaller than measured in OFETs. This indicates that large-scale molecular arrangement further favours transport in Si-PCPDTBT.

Aggregation in solution

As start of our experimental investigations, we first compare the aggregation behavior of all PCPDTBT derivatives using temperature dependent UV-vis absorption spectroscopy (Figure 2).

Through the appropriate choice of solvent and a wide temperature window, one can in an ideal case, observe a transition from the fully dissolved (high temperature) to the highly aggregated state (low temperature). Before going into detail about the absorption spectra of each polymer, it should also be noted that for all PCPDTBT polymers, the high energy band seen at around 400 nm and the low energy band at about 700 nm are assigned to a and charge transfer transition, respectively. 30 Figure 2a displays the measured absorption profiles for C-PCPDTBT in CB from -15 (blue) to 100°C (red). From high temperature down to room temperature, the maximum absorption wavelength was found to be at 710 nm and the spectral shape can be described as rather broad with a slight shoulder at high energy (~ 660 nm). Cooling towards -15°C (spectrum shown in blue), a 15 nm red-shift of the absorption maximum up to 725 nm is observed. In a recent publication we have described the aggregation behavior of C-PCPDTBT in 2-methyltetrahydrofuran (m-THF) between -93°C and 67°C using absorption and photoluminescence spectroscopy. These experiments in combination with an evaluation via Franck Condon analysis revealed an isosbestic point and thus two polymer conformations: a disordered random coil and an aggregated conformer with planarized chains. 30 At low temperature it was deviated that the aggregated phase of C-PCPDTBT consisted of long-range stacked polymer chains. Both in m-THF and in THF PCPDTBT is slightly more aggregated at room temperature, Figure S1.

For the F-PCPDTBT polymer, the temperature dependent absorption spectra are displayed in Figure 2b. The spectrum measured at 100°C shows a maximum absorption at 690 nm, with a hint of a shoulder at 660 nm. At room temperature, the maximum is shifted to 710 nm. Upon further cooling to -15°C, is at 720 nm and a shoulder at 780 nm becomes clearly visible.

Si-PCPDTBT displays analogous trends in its absorption behavior as the other two polymers, but with much stronger aggregation tendency. At 100°C (Figure 2c, red spectrum), the maximum absorption is at 655 nm, whereas at 20°C and lower temperatures two distinct maxima at 655 and 750 nm become visible. In Figure 2d, the decay of the aggregated species in solution is depicted by plotting the absorption intensity evolution of the low energy peak (750-800 nm) as a function of temperature for all three polymers. For C-PCPDTBT, no clear shoulder can be observed indicating that very little aggregation occurs in chlorobenzene solution (refer to Figure 2a). For F-PCPDTBT, the shoulder at 780 nm is less intense than for Si-PCPDTBT and decreases steadily in intensity from -15°C to 40°C. The silicon containing polymer however displays a shoulder at 750 nm over the entire temperature range. The results clearly indicate the tendency to undergo aggregation increases in the series: C-PCPDTBT, F-PCPDTBT, Si-PCPDTBT. Depending on the solvent in which the polymer is dissolved, the aggregates can be present in solution at room temperature, for example like the case of Si-PCPDTBT in CB or C-PCPDTBT in THF (refer to Figure S1) but not for C-PCPDTBT in CB (see Figure 2). Additionally, we further observed a significant decrease in polymer solubility from C-PCPDTBT to F-PCPDTBT to Si-PCPDTBT. We note that materials with low solubility can in extreme cases, limit the processing and characterization techniques that can be applied to study such conjugated polymer systems. Later during the spincoating process aggregation in solution becomes important because it influences structure formation if pre-aggregated species are deposited or forming during the drying process. In addition, both F-and C-PCPDTBT form a semicrystalline arrangement whose crystal structure has been assigned to the Pccn space group which is very similar to that which has been reported for α-PFO (Pnb21). [START_REF] Fischer | Structural Models of Poly(cyclopentadithiophene-Alt -Benzothiadiazole) with Branched Side Chains: Impact of a Single Fluorine Atom on the Crystal Structure and Polymorphism of a Conjugated Polymer[END_REF] The orthorhombic unit cell contains 4 polymer chains grouped into pairs (dimers, Figure 3c and3d). CPDT and BT units are stacked in a segregated manner in the dimers, forming stacked PCPDTBT polymer chains. The alkyl side chains are surrounding the two planar backbones, and insulate the individual dimer units from one another. An important finding that has emerged during the investigations of the dimer crystal structure is that it is thermodynamically more stable than the -stacked polymorph. 33 The proposed dimer-like polymer arrangement for C-PCPDTBT and F-PCPDTBT is examined in more detail in References 32,33 . Figure 3c exemplarily shows the projection of the crystal structure of F-PCPDTBT in the plane. It has to be noted that the stacking distance between two chains within a dimer was estimated to be between , no long range stacking is possible in these structures. In order to validate the experimentally predicted unit cell of a dimer structure, we have additionally performed molecular dynamics simulations with this molecular arrangement.

Supercells ( unit cells, with four tetramers in each unit cell) were constructed from the proposed unit cells and equilibrated at 300 K and ambient pressure for 1 ns. All supercells were stable during the entire equilibration run. As an example, the final unit cell and super cell for F-PCPDTBT are shown in Figure 3d, 3e, and 3f. The unit cell dimensions, summarized in Table 2, are in good agreement with experiment. Note that even though our simulations support experimental findings only indirectly (since free energy calculations of large supercells are computationally prohibitive), a combination of molecular dynamics simulations and TEM/ED allows the identification of detailed molecular arrangements in unit cells, accounting for the excluded volume of branched side chains.

Preparation Conditions -Structural Analysis

Next we present the preparation techniques which can be used to obtain the various polymer morphologies as well as their characteristic traits. The thin film morphology of C-PCPDTBT spincoated from two different solvents: CHCl 3 and CB/DIO are shown using atomic force microscopy (AFM) height images in Figure 4a and 4b, respectively. The sample deposited using chloroform displays no characteristic features in the AFM images, 4a, and the corresponding absorption profile is broad and featureless, see Figure 4d. 31 These results as well as the rather fuzzy and weak -stacking reflection in the ED pattern (Fig. S5e) led us to the conclusion that the sample spincoated from chloroform is rather amorphous in nature. In contrast, films deposited from high boiling point mixture of CB and DIO yielded excellent solar cell performance 15 and polymer chains have extended stacking interactions as suggested from a rather sharp and intense Scherrer ring at 3.75Å in the ED pattern (see Figure S6).

Such -stacked aggregates have also been observed for P3HT, and for C-PCPDTBT consist of an edge-on orientation with respect to the substrate. 20,28 The AFM height image shown below reveals round aggregates on the film surface, approximately 40 nm in diameter, 4b. A distinct shoulder at 800 nm can be seen in the absorption spectrum of the thin film spincoated from CB/DIO (Figure 4d). In analogy to the peak assignment performed in temperature dependent absorption spectroscopy experiments, 30 in the thin films spincoated from CB/DIO, we also assign the presence of the shoulder at long wavelength, to ordered, -stacked, C-PCPDTBT polymer chains.

DIO as well as other high boiling point solvents (e.g. 1-CN) are needed for C-PCPDTBT to force the polymers in the long-range pi-stacked structure; pre-aggregation in solution seems to play a major role in this context. -7.

To access the dimer crystal structure we found that melt crystallization on PTFE films and solvent vapor annealing of spincoated films can be used. 32,33 (compare Figure S7). Using the latter approach with carbon disulfide as solvent vapor homogeneous films are accessible, Figure 4c.

Fiber-like structures with a diameter of 40 nm can be seen on the surface in the AFM height image.

The absorption spectra of the solvent annealed film shows a blue-shift of the maximum of about 40 nm compared to the film spincoated from CB/DIO and a pronounced shoulder at 650 nm. Using TEM/ED (Figure 7e + h) and GIWAXS measurements (Figure S7f), the film exposed to solvent vapor was confirmed to adopt the dimer structure (Figure 3c-f). Figure S7h shows the characteristic traits in the ED pattern for C-PCPDTBT in the dimer arrangement with 1 1 0 and 0 0 4 reflections, displayed at 10.5 and 5.9 Å, respectively. 33,49

Overall, three different polymer morphologies were obtained for the C-PCPDTBT derivatives:

amorphous, -stacked and dimer crystal structure.

A similar approach to that used for C-PCPDTBT was applied to samples consisting of F-PCPDTBT. Interestingly no amorphous films were accessible by solution-deposition.

Spincoating from a solvent with a high boiling point such as chlorobenzene led to rod-like structures (diameter = 30 nm) in the AFM height image shown in Figure 5a. TEM also revealed rod-like structures whereas the ED pattern shows an intense pi-stacking ring at 3.7 Å and a weak ring with a 11.0Å interlayer spacing (see Figure S8). These reflections point indeed at a -stacked structure with a dominant population of edge-on oriented crystals.

The corresponding absorption profile of the same thin film shows a band centered around 750 nm with an intense shoulder at 790 nm (Figure 5e). The shoulder observed at 790 nm is assigned to aggregates in the thin film that are of the same nature as those seen in the CB solution at -15°C (Figure 2b) which consist of long-range -stacked polymer chains. For the carbon bridged analogue of PCPDTBT, additives with high boiling points such as diiodooctane (b.p. 365°C) were needed to induce the stacking, however for F-PCPDTBT, chlorobenzene (b.p. 132°C) was sufficient since it has a higher tendency to aggregate in solution than C-PCPDTBT.

Again solvent vapor annealing had been shown to lead to the dimer crystal structure, with the only problem that no continuous films could be obtained. 33 High temperature mechanical rubbing proved to be a good alternative in this polymer, Figure 5. The existence of the dimer crystal structure is highlighted by the characteristic reflections at 10.5 Å and 5.8 Å in the ED pattern in Figure 5b. 33 Bright field TEM images as well as AFM images of such films were recorded and can be seen in Figure 5c and 5d, respectively. In both the TEM and AFM images, lamellae perpendicular to the rubbing direction can be seen, consistent with polymer chain alignment. The semicrystalline nature of F-PCPDTBT is evidenced in the TEM image which shows alternating light and dark regions, corresponding to amorphous and crystalline domains within the polymer film. Polarized optical absorption spectroscopy was employed to assess the degree of alignment after HT rubbing. Through comparison of the ratio of intensities at the wavelength of maximum absorption, a moderate dichroic ratio of 2.3 was obtained, Figure S2a. Not surprisingly, it was also found that the absorption spectrum of the rubbed film is blue shifted compared to the thin film spincoated from chlorobenzene (see Figure 5e), and a new absorption maximum at 695 nm is observed. Accordingly for F-PCPDTBT, only two polymorphs were obtained: -stacked and dimer.

We attribute the absence of the amorphous phase to the higher tendency of F-PCPDTBT to aggregate versus C-PCPDTBT as demonstrated in Figure 2.

Charge Transport Measurements

The hole mobilities of the different polymorphs of C-and F-PCPDTBT were evaluated using field-effect transistors in a top-gate bottom-contact geometry. This device architecture was employed since the same polymer interface investigated in AFM is probed during transistor operation. The resulting output and transfer curves can be found in the supporting information order of magnitude lower than the samples with long-range stacking. This result indicates that these structures are not well suited for effective transport in FETs. In the case of the dimer structure this is likely due to the large distance between dimer units as a result of the insulating alkyl chains. Although the dimer structure of the PCPDTBT chains is highly crystalline, transport can really only occur along the polymer chains.

(Figures S12 + S13
In the case of F-PCPDTBT we compared -stacked (prepared from CB, Figure 5a) vs. dimer (prepared via HT-rubbing, Figure 5b-d) and found very similar charge carrier mobilities in the order of 2x10 -4 cm 2 /Vs. In the literature charge carrier mobilities of F-PCPDTBT have been reported to be between 1x10 -3 and 1.4x10 -2 cm 2 /Vs, 21,22 which is one to two orders of magnitude higher than the values we measured. We attribute this discrepancy to the low molecular weight of our polymer batch, as molecular weight has frequently been shown to be an important factor in determining charge carrier mobilities of conjugated polymers. 50,51

The transistor characteristics of the HT rubbed film were obtained by rubbing in the direction parallel to the transport direction between source and drain contacts. We also performed the same rubbing experiments perpendicular to the transport direction (see Table S2 in the supporting information), which gives a mobility of 6x10 -5 cm 2 /Vs. The observation of anisotropic transport in these dimer structures indicates that charges are transported more efficiently along the conjugated backbones. Intermolecular transport occurs either via a superexchange mechanism, via tunneling over side chains, or through numerous structural defects, since these structures do not provide a direct electronic overlap between molecules. This charge transport anisotropy is in accordance to findings on P3HT and other D-A copolymers. 6,12,48

Overall we conclude that stacking of the PCPDTBT chains best supports charge transport in OFETs for C-and F-PCPDTBT.

Si-PCPDTBT -Structural Analysis & Charge Transport

The last polymer in the series is the silicon-bridged cyclopentadithiophene analogue of PCPDTBT.

It is also the least soluble material in the series and due to its very high tendency to aggregate in solution, it was not possible to make homogeneous thin films using standard spincoating conditions. We found that films of sufficient quality could be obtained by spincoating from hot solutions of chlorobenzene (80°C). The AFM phase image of the resulting film is displayed in For the silicon containing analogue, the hole mobility of the predominately cross-hatched oriented morphology (spincoated sample) is compared to that of the mostly face-on oriented pi-stacked chains (HT-rubbing). In both cases, all transistor characteristics were very similar and mobility values between 1 and 2 x10 -2 cm 2 /Vs were equal within the error of the measurement. Although additional thermal annealing at 280°C of the mechanically rubbed film led to a clear enhancement of the intensity of the ED reflections, no measurable effect could be seen on the hole mobilities.

From the standpoint of charge transport, OFET measurements show that the highest charge mobilities are observed for Si-PCPDTBT films with a cross-hatched morphology as compared to the p-stacked and dimer-structures observed for C-PCPDTBT and F-PCPDTBT. Simulations of charge mobilities showed that local ordering tends to favor transport in Si-PCPDTBT as compared to other derivatives. However, the difference in simulated mobilities is smaller than that measured in OFETs, suggesting that large-scale structural and morphological aspects must be considered. In particular, the difference in polymorphism between the analogues must have some impact on

Conclusion

PCPDTBT-based low bandgap polymers showed the formation of aggregates both in solution and when deposited as thin films. Amorphous, -stacked, cross-hatched, or dimer polymorphs can be obtained by adjusting the processing conditions (nature of solvent, temperature, rubbing, etc).

Both experimentally measured and computer simulated hole mobilities confirmed that the stacked Si-PCPDTBT derivative has the highest mobilities in the series. Simulations showed that going from the carbon to the fluorine to the silicon derivative, the local charge mobility of the stacked systems increased. This increase is due to a better registry of the polymer chain backbones and more optimized packing of side chains, which leads to larger effective electronic coupling elements. The dimer-based and the amorphous thin films demonstrated lower mobility values than the -stacked and cross-hatched structures. For the dimer containing morphology, it seems likely that the transport is limited to one direction (along the polymer chain) as the bulky alkyl side chains create a large gap between dimer units. On the whole, small changes in the polymer structure drastically modified aggregation and solubility behavior, thereby strongly influencing the extent of morphology tunability. C-PCPDTBT was the most versatile in the series, next came F-PCPDTBT and finally Si-PCPDTBT. The observed differences in chain packing i.e.

dimer-like versus cross-hatched are related, at least in part, to the differences in backbone conformations.

  A typical example of a D-A copolymer is poly[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalene-dicarboximide-2,6-diyl]-alt-5,5'-(2,2'-bithiophe ne) (P(NDI2OD-T2)). This low bandgap copolymer has demonstrated impressive optoelectronic properties and has found application in organic field-effect transistors (OFETs) 10 and organic solar cells 11 as an n-type semiconductor. Due to the alternating nature of the chemical structure, new packing motifs in comparison to homopolymers, such as segregated and mixed stacking of the donor and acceptor blocks are possible. Two polymorphs of the semicrystalline P(NDI2OD-T2) (known as form I and form II) have been induced in thin films by applying different temperature annealing protocols. 12,13 Another example of a D-A polymer is the p-type semiconductor poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadi azole)] (C-PCPDTBT), which consists of alternating cyclopentadithiophene (CPDT, donor) and benzothiadiazole (BT, acceptor) units (for chemical structure see Scheme 1). C-PCPDTBT was introduced in 2006 by Brabec and coworkers, in an attempt to reduce the bandgap and thus to increase efficiency of a bulk heterojunction solar cell. Indeed, promising power conversion efficiencies (PCEs) up to 3.16% were demonstrated. 14 In the following year, Peet and Bazan reported that solvent additives instead of thermal annealing, can lead to PCEs up to 5.5% for C-PCPDTBT:PC 71 BM blends. 15 Further studies indicated that the presence of solvent additive 1,8-diiodooctane (DIO) could improve blend morphology and interchain coupling as a result of enhanced stacking. 16,17 Later, an edge-on orientation of the C-PCPDTBT polymer chains could be identified using grazing incident X-Ray diffraction techniques (GIXRD) by Nelson, 17 Bazan, 18,19 and Russell and coworkers. 20 Considering the promising nature of C-PCPDTBT, structural modifications on the polymer backbone have also been investigated over the last decade. In 2012, Neher 21 and Jen 22 both reported on a fluorinated PCPDTBT derivative by replacing one of the hydrogen atoms on the benzothiadiazole ring with a fluorine atom, yielding a regiorandom copolymer: poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(5-fluoro-2,1,3-be nzothiadiazole)] (F-PCPDTBT). PCEs up to 6.6% 23 were reported for F-PCPDTBT:PC 71 BM blends as well as hole mobilities of 10 -2 cm 2 /Vs in organic field-effect transistors (OFETs). 24 Several years earlier, in 2008, Yang had described another low band gap polymer in which the bridging carbon atom in the cyclopentadithiophene building block was replaced with a silicon atom: poly[(4,4'-bis(2-ethylhexyl)dithiene-[3,2-b:2',3'-d]silole)-alt-4,7-(2,1,3-benzothiadiazole)] (Si-PCPDTBT). 25 For the Si containing derivative, PCEs of 5.1% were obtained as well as hole mobilities up to 10 -2 cm 2 V -1 s -1 were measured in OFETs. 26 Further investigations of Si-PCPDTBT were carried out by Neher and coworkers who implemented multiple fullerene derivatives with Si-PCPDTBT to study the charge transfer state splitting in solar cells. 27 The differences in charge transport and photovoltaic properties of these three structurally similar low band gap polymers are often rationalized by the strength of their pi-pi stacking interactions. The arguments and packing motif used to support these claims are analogous to those applied to the P3HT semicrystalline morphology. For Si-and C-PCPDTBT, an edge-on morphology is proposed and the relative crystallinity of the two are compared using GIXRD techniques. The silicon containing derivative is more crystalline than C-PCPDTBT and has displayed stacking distances as low as 3.48 Å versus 3.6-3.8 Å for C-PCPDTBT. 20,28,29 Such close packing of the polymer backbone is attributed to the longer Si-C bond versus the C-C bond attaching the solubilizing branched alkyl chain to the aromatic units for Si-and C-PCPDTBT, respectively. In the case of F-PCPDTBT versus C-PCPDTBT, -1488172410 the stronger interactions of the fluorinated derivative were attributed to intra or intermolecular F-F or H-F interactions.-1488172410 22 -1488172410 As a part of our own investigations on the family of PCPDTBT polymers, we have examined the behavior of C-PCPDTBT in solution 30 and the thin film morphology 31,32 as well as compared the structure and polymorphism of C-versus F-PCPDTBT. 33 In this report we complete the series with inclusion of the silicon bridged analogue of PCPDTBT which shows yet another polymorph.

Figure 1 .

 1 Figure 1. Electronic coupling element between two backbones (tetramers with side chains substituted by hydrogens) versus the backbone-backbone shift . Backbones are parallel to each other and constrained to a fixed separation of . Electronic couplings are evaluated using the projection method 36 , B3LYP density functional, and 6-31g basis set 37 . The monomer length is similar for all three derivatives and set to 11.8 Å.

  ambient pressure using Berendsen thermostat and barostat[36(??)]. Typical simulated morphologies are shown in Figures3a (unit cell) and 3b (super cell). The corresponding unit cell parameters are similar for C-and F-PCPDTBT: -stacking distance is and inter-lamellar spacing is

Figure 2 .

 2 Figure 2. Temperature dependent absorption spectra of polymer solutions in CB (1 mg/mL) a) C-PCPDTBT, b) F-PCPDTBT, and c) Si-PCPDTBT, measured in the temperature range between 100°C (red) and -15°C (blue) with 10°C temperature steps shown in grey. Spectra measured at RT shown as dashed line. d) Change in intensity of the shoulder at long wavelengths (750-800 nm) as a function of temperature for the three PCPDTBT polymers.

2. 4 C

 4 -and F-PCPDTBTThe film morphology of a conjugated polymer is influenced by a variety of factors such as chain rigidity, molecular weight, length and density of alkyl side chains, solubility, aggregation and crystallinity, etc. It has been found that subtle changes in chemical structure, e.g. the inclusion of a fluorine atom on a benzothiadiazole unit (as is the case for F-PCPDTBT versus C-PCPDTBT) or variations of the solubilizing side chains can result in significant changes in polymer properties.[START_REF] Albrecht | Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells[END_REF][START_REF] Zhang | Significant Improved Performance of Photovoltaic Cells Made from a Partially Fluorinated Cyclopentadithiophene/Benzothiadiazole Conjugated Polymer[END_REF][START_REF] Horie | Cyclopentadithiophene-Benzothiadiazole Oligomers and Polymers; Synthesis, Characterisation, Field-Effect Transistor and Photovoltaic Characteristics[END_REF] The protocol and the solvent used to fabricate the thin films plays an equally important role in determining the final morphology, as shown for example in depth for P3HT as model system. 1,48 Finally, after film formation, further modification of the morphology can be achieved by thermal or solvent annealing. In the upcoming section it will be shown how chemical structure, deposition technique, and post-deposition treatment influence polymer film morphology for C-and F-PCPDTBT.To date, the PCPDTBT polymers have been shown to demonstrate up to three different morphologies: amorphous, -stacked and the more recently reported 'dimer' structure. 32,33 The latter two packing motifs are depicted in Figure3as a point of reference for the reader. Figures 3a and 3b depict the polymer chain arrangement of the -stacked polymorph after equilibration using molecular dynamic simulations. The long range -stacked structure of C-PCPDTBT (and its Si-analogue) has been proposed based on the results of GIXRD experiments. It is noteworthy to mention that such -stacked structures typically display an edge-on orientation. 20,28

Figure 3 .

 3 Figure 3. Unit cells illustrating polymorphs of PCPDTBT polymers: (a) -stacked unit cell and (b) supercell, equilibrated using molecular dynamics. Dimer-containing unit cell of F-PCPDTBT: (c) proposed crystal structure from experimental studies, 33 (d), (e), and (f) after equilibration using molecular dynamics simulations. The final unit cell dimensions are given in Table 2. The -stacked polymorph has been observed for all C-, Si-, and F-PCPDTBTs, whereas the dimer-like structure for F-PCPDTBT 33 and C-PCPDTBT. 32,33

Figure 4 .

 4 Figure 4. AFM height images (1x1 μm) of C-PCPDTBT: a) spincoated from CHCl 3 yielding a mainly amorphous film, b) spincoated from CB/DIO yielding the pi-stacked structure, and c) spincoated from CHCl 3 , and exposed to CS 2 vapor afterwards yielding the dimer crystal structure. (d) Absorption spectra of thin films: spincoated from CHCl 3 : blue, spincoated from CB/DIO: black, and after exposure to CS 2 vapor: red. For further morphological data on C-PCPDTBT we refer to Figures S5-7.

Figure 5 .

 5 Figure 5. AFM height image of F-PCPDTBT spincoated from CB (a) exhibiting a -stacked morphology. ED pattern (b), HR-TEM image (c), and AFM phase image (d) of a thin film of F-PCPDTBT after HT rubbing at 240°C. White arrow indicates the rubbing direction. e) Corresponding absorption spectra of the -stacked and dimer-containing structures, black and red lines, respectively. For further morphological data on F-PCPDTBT we refer to Figures S8 + 9.

  ). For C-PCPDTBT we found the following values: the -stacked morphology (prepared from CB/DIO, Figure4a) gave values around …. with a highest mobility of …. . Films containing the dimer crystal structure (prepared via SVA-CS 2 , Figure4b) gave average values of around …. The amorphous films (prepared from CHCl 3 , Figure4c), gave average values of …. The dimer morphology and the amorphous film of C-PCPDTBT yield hole mobility values almost one

Figure 6a and reveals

  Figure6aand reveals round aggregates approximately 30 nm in diameter. Surprisingly, the HR-TEM image in Figure6bevidences the formation of cross-hatched structures. Cross-hatched structures have been demonstrated for polypropylenes and poly(paraphenylene benzobisoxazole), 52 but were only recently reported for Si-PCPDTBT by Takacs and coworkers in very thin films.[START_REF] Takacs | Quadrites and Crossed-Chain Crystal Structures in Polymer Semiconductors[END_REF] The cross-hatched structures consist of alternating layers of polymer chains in which the angle between two chains from adjacent layers was found to be 53.6 1.3°. This angle of 53.6° can be seen in the FFT pattern shown in the upper right hand corner in Figure6bwhich reveals two clear sets of reflections 53.6° from one another. Incidentally, the same cross-hatched domains were also observed when aggregates of Si-PCPDTBT formed in solution were cast on a carbon-coated TEM grid, indicating that such aggregates are formed in solution and not upon drying during spincoating (see supplementary information FigureS11g+h). The periodicity between successive chains in a layer was 17.4 Å, in agreement with earlier findings.[START_REF] Takacs | Quadrites and Crossed-Chain Crystal Structures in Polymer Semiconductors[END_REF] The absorption spectrum of the spincoated thin film displays a maximum intensity at 765 nm (Figure6d), this low energy peak is attributed to arise from absorption from the aggregated species in the polymer film. We refer to the aggregation in chlorobenzene solution in Figure2c.

Figure 6 .

 6 Figure 6. a) AFM phase image of Si-PCPDTBT spincoated from CB at 80°C. b) HR-TEM image and c) corresponding ED pattern of spincoated sample. d) Absorption profile of spin coated (orange) and rubbed film (black), respectively. e) HR-TEM image after HT rubbing at 230°C. f) ED pattern of Si-PCPDTBT after HT rubbing at 230°C. For further morphological data on Si-PCPDTBT we refer to Figures S10 + 11.

  Figure x. It has been proposed that such cross-hatched domains help interconnect nanocrystals with different in-plane orientations and thus help create quasi-three dimensional transport pathways (2 directions in-plane plus -stacking perpendicular to the substrate plane). (refs) The situation is different for both the classical pi-stacked and the dimer polymorphs observed for C-PCPDTBT and F-PCPDTBT. For these systems, charge transport should be essentially 2D in the -stacked crystals and even 1D in the dimer-structure. Macroscopic charge transport in C-PCPDTBT and F-PCPDTBT will imply charges to transit between ordered domains but without the highly interconnected domains found for Si-PCPDTBT. Accordingly, the comparison of charge transport in the analogous serie of PCPDTBTs highlights the essential role of polymorphism that is controlled by a molecular parameter i.e. the Phi angle in the conjugated backbone conformation.

Figure 7 .

 7 Figure 7. Optimized geometries of one repeat unit for C-PCPDTBT (a) and Si-PCPDTBT (b) highlighting the effect of substituting the bridged carbon atom in the cyclopentadithiophene with silicon. Supercells of the three different packing motifs: dimer (c), -stacked (d) and cross-hatched (e). The cross-hatched morphology was obtained by equilibrating (molecular dynamics, 300 K, NPT ensemble) a supercell 53 of decamers.

Table 1 . Summary of transistor characteristics of the -stacked polymers. Simulated mobility (μ sim ) values are given without taking into account energetic disorder (i.e. assuming high charge carrier densities in OFETs). Averaging is based on 1280 points for each system, the largest error is less than 10 -3 cm 2 /V.s. With energetic disorder, the simulated zero-field mobility is than 10 -6 cm

 1

	2 /V . s .

Table 2 . Unit cell sizes for dimer-containing polymer morphologies obtained from molecular dynamics simulations.

 2

		(along the backbone, nm)	(nm)	(nm)
	C-PCPDTBT (simulated)	2.46	2.08	1.25
	C-PCPDTBT (experiment 33 ) 2.36	1.93	1.24
	F-PCPDTBT (simulated)	2.40	2.20	1.17
	F-PCPDTBT (experiment 33 ) 2.32	2.06	1.10

Acknowledgements

This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG) under the Emmy Noether program, the Priority Program "Elementary Processes of Organic Photovoltaics" (SPP 1355), BMBF grant MESOMERIE (FKZ 13N10723) and MEDOS (FKZ 03EK3503B), and DFG program IRTG 1404 and TRR146. The project has received funding from the NMP-20-2014 -"Widening materials models" program under grant agreement No. 646259 (MOSTOPHOS). We are very grateful to S. Janietz who provided us with F-PCPDTBT and H. Klauk, U. Zschieschang, R. di Pietro, and S. Tannert for support with the transistor measurements.

Overall, these findings indicate that there is little effect of the two different packing motifs on the charge transport in FETs for Si-PCPDTBT.

Discussion

Depending on the processing conditions we can access three different polymorphs in the PCPDTBT family. Figure 7 c, d ande show supercells of the three packing motifs for illustration.

In the case of C-PCPDTBT it is possible to access both an amorphous and stacked film morphology by varying the deposition solvent, whereas for the other two polymers, F-PCPDTBT and Si-PCPDTBT, we could not find the appropriate conditions to make amorphous films. We believe this is due to increased tendency for the polymers to aggregate within the series: C-PCPDTBT, F-PCPDTBT, and Si-PCPDTBT. In fact, the backbone stiffness, evaluated from the scans of the dihedral potentials between the donor and acceptor units of the backbone (Figure S3), was found to be similar for all three polymers, indicating that solvent-polymer interactions play a dominant role in determining polymer film properties.

For all 3 polymers we could access -stacked morphologies. In the case of C-PCPDTBT the addition of diiodooctane to the processing solvent was required, while F-PCPDTBT formed pi-stacked structures upon deposition from CB. Both polymers form edge-on textures on substrates. For Si-PCPDTBT we found -stacked structures upon high temperature rubbing.

While mostly face-on textures were observed directly after HT rubbing, additional annealing at 280°C lead to mostly edge-on domains. The -stacking distance decreases from 3.75 Å, to 3.7 Å to 3.5 Å for C-PCPDTBT, F-PCPDTBT and Si-PCPDTBT. The interlayer spacings are around 17 Å for Si-PCPDTBT and around 11 Å for C-and F-PCPDTBT.

The dimer-like packing motif was exclusively found for F-and C-PCPDTBT. It can be accessed by applying techniques such as high temperature mechanical rubbing, melt-crystallization on friction-transferred PTFE substrates or solvent vapor annealing. The data suggest that this crystal structure is the most thermodynamically stable polymorph for these two polymers.

The silicon containing polymer shows no evidence of dimer-like packing. Even temperature annealing after high temperature rubbing did not lead to the dimer-structures. On the other hand, cross-hatched structures are observed exclusively for Si-PCPDTBT. An interlayer spacing of 17 Å was found in our samples, which is similar to the pi-stacked structures.

Experimental Section

Materials.

Poly [2,6-(4,4- NIR 2,2 µm) in transmission mode using fiber optics.

Film processing. As substrates 1x1 cm² microscope slides (for UV-vis absorption), pure Si-wafers (for AFM), and Si-wafers with a 300 nm SiOx layer (for TEM) were employed and cleaned with a CO 2 snow jet followed by exposure to oxygen plasma (Diener Femto 100 W) for at least 600 s. Both, solution and film preparation was done under nitrogen atmosphere. The solvent-additive processed films were prepared from 3 mg/mL CB solutions in the presence of 2 wt% DIO, spincoating was performed at 1500 rpm for 300 seconds. The initial films for solvent vapor annealing were prepared from 3 mg/mL CHCl 3 solutions. The films were prepared by spincoating at 1500 rpm for 30 s within 24 h after preparation of the solutions. For solvent vapor annealing CHCl 3 precast films were exposed to controlled CS 2 vapor as previously described. [START_REF] Fischer | Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto-Electronic Properties[END_REF] The films were swollen to a solution like state at a vapor pressure of 92%; recrystallization was induced by decreasing the vapor pressure first to 88 % and then to 85 % within 10 minutes. The temperature of the vapor was set to 22°C and the sample temperature to 20°C. The F-PCPDTBT thin films were deposited from CB solution (3 mg/mL) and the Si-PCPDTBT films were also made from 3 mg/mL solutions deposited at 80°C. High-temperature rubbing of the F-PCPDTBT thin films was performed following a procedure described in the literature. [START_REF] Fischer | Structural Models of Poly(cyclopentadithiophene-Alt -Benzothiadiazole) with Branched Side Chains: Impact of a Single Fluorine Atom on the Crystal Structure and Polymorphism of a Conjugated Polymer[END_REF] The rubbing was performed at 240°C by applying a rotating cylinder equipped with a fiber cloth with a pressure of 2 bar on the sample holder.

Film characterization. Oriented areas were identified for transmission electron microscopy (TEM) analysis by optical microscopy (Leica DMR-X microscope). The polymer films were coated with a thin amorphous carbon film and removed from the glass substrates by floating on a diluted aqueous HF solution (5 wt %) and subsequently recovered on TEM copper grids. Aggregates present in solution were studied by drop-casting CB solutions on TEM copper grids. TEM was performed in bright field, high-resolution and diffraction modes using a CM12 Philips microscope equipped with a MVIII (Soft Imaging System) charge coupled device camera. Calibration of the reticular distances in the ED patterns was made with an oriented PTFE film. UV-vis-NIR spectroscopy was performed with a Zeiss spectrometer (light source: CLH600, detector: MCS621

VIS II and MCS611 NIR 2,2 µm) in transmission mode using fiber optics. AFM was performed on a Dimension Icon AFM from Bruker operating in tapping mode.

Organic field-effect transistors. Charge transport was probed in organic field-effect transistors in a top-gate bottom-contact geometry in a nitrogen atmosphere. Gold source and drain electrodes of 30 nm thickness with a 5 nm thick chromium adhesion layer were evaporated using a shadow mask on glass substrates defining transistor channels with a channel length (L) of 100-250 µm and channel width (W) of 1 mm. The transistor substrates were cleaned by successive ultrasonication in acetone and isopropanol each for 10 min, rinsing in isopropanol and drying in a nitrogen stream. The polymer solution was prepared in the glove box by stirring at elevated temperature for at least 4h to allow complete dissolution of the polymer.

Polymethylmethacrylate (PMMA) layers (500 nm) (EG-PMMA, Mn = 300 kDa, purchased from Polymer Source Inc.) served as dielectric and were spincoated from n-butyl acetate (60 mg/mL) followed by annealing at 80°C for 30 min. In a last step, aluminum (30 nm) was evaporated through a shadow mask and used as gate electrode. Transistor testing was performed with a SUSS Microtech setup connected to a Keithley 2636 Source meter under inert atmosphere. The charge carrier mobilities were extracted from the saturation regime at -60 V using , where I SD the source-drain current, C i is the capacitance per unit area and V G is the gate voltage. Transfer rve ere avera ed over all annel len t 0 00 0 00 μm or -16 devices.

Simulation methods. In this paper, MD simulations of C-/F-and Si-PCPDTBT morphologies were performed using the GROMACS simulation package 54 (v 4.6.7). Charge transfer integrals, site energies and mobilities were evaluated using a combination of the Gaussian G09 simulation package 55 and the VOTCA-CTP module 39 .

For Table of Contents Use

Subtle changes in molecular structure lead to new packing motifs for PCPDTBT polymers.

TOC graphic.


Catherine Lejealle

IS MOBILE TV A SMALL TV?

Keywords: Mobile TV, uses, sociability, digital television, viewing practices

Whereas most of the studies and papers present mobile television as a personal mobile TV with isolated niche usage, mostly in mobile situations, our findings show that half of the usage is non-mobile and about 20% is shared with a family member, a friend or a colleague. Moreover, it is incorporated in routine social activities such as work and family life. Indeed, the content viewed depends on the other activities and other content accessed previously, read or seen on home TV, radio, MP3 devices or newspapers. On his way to work, the user will watch the news if he hasn't already done so, otherwise he will select a musical mobile TV channel. We found that working class participants view different content on mobile TV and on their home TV, as they focus on music, sport and news on mobile TV, a content which can easily fragmented to suit short or long viewing duration. Thus mobile TV is complementary to other devices and media. Finally, each usage is a new challenge to the user's capabilities and competences, redefining his representation and sometimes preferring telephony, TV, Windows or surfing on the Web.

INTRODUCTION

Traditionally, not only in France but in other Western countries, studies on TV (nondigital as it appears in every home since the 60s') focus on the way viewers receive a program: does the program have limited or broad effects on the public, both in terms of alienation and of construction of ideas, beliefs, tastes, moral values and opinions ( [START_REF] Adorno | La production industrielle des biens culturels[END_REF], [START_REF] Berelson | The people's choice. How the voter makes up his mind in the presidential campaign[END_REF], [START_REF]Qu'est-ce qu'une sociologie de la télévision ?[END_REF], [START_REF] Guern | Consommation et réception de la culture. La démocratisation des publics[END_REF], [START_REF] Silverstone R | Television and everyday life[END_REF]). These studies define three different usages of TV ( [START_REF] Lull | The social uses of television[END_REF]). First, it helps build a link between people (at home, at work or between neighbours) as they give shared topics to speak of, without threatening a relation as more personal topics could. Secondly, it structures the schedule as a backbone, especially for retired people or for woman staying at home. Thirdly, it contributes to learn social codes, as it's the case for instance of series that teenagers enjoy because they get familiar with love code and practices ( [START_REF] Chalvon-Demersay S | Drôles de stars[END_REF], [START_REF] Pasquier | Vingt ans de recherches sur la télévision: une sociologie post-lazarsfeldienne ?[END_REF], [START_REF] Pasquier | Télévision et apprentissage sociaux: les séries pour adolescents[END_REF], [START_REF] Pasquier | La culture des sentiments. L'expérience télévisuelle des adolescents[END_REF], [START_REF] Pasquier | Une nouvelle amie: le héros télévisuel comme promesse d'amour[END_REF]). Researchers also stress the fact that a same program is received by viewers not as a unique message meant by a sender. There are multiple ways of interpreting and decoding a program, depending on your social or ethnical background and even on the country the program is seen: in 1990, Liebes & Katz compared the reception of the serial Dallas ( [START_REF] Liebes | The export of meaning: cross-cultural readings of Dallas[END_REF]); in 1980 Morley did the same for the BBC news ( [START_REF] Morley | The « Nationwide » audience: structure and decoding[END_REF]). Thus, those social, ethnical and national groups do not only watch different contends but also receive it differently. Those studies criticize mass consumption and culture for the mass, following the views of the Frankfurt School. They see TV as the leisure for the working class as opposed to the culture of the upper class. In that matter, TV is a tool to reproduce social differentiation, allowing the very literate to educate well their children and giving them access to elite culture, whereas the working class doesn't have access to it, due to cost or lacking the codes. TV is seen as less legitimate, responsible for alienation, violence and the development of individualism.

As mobile TV is concerned, most of the studies of this new service ( [START_REF] Goggin | Cell phone culture[END_REF], [START_REF] Harper R | Taking hold of TV: learning from the literature[END_REF]) present its usage as niche solitary isolated viewing, mostly in mobile situations ( [START_REF] Sodergard | Mobile television-technology and user experiences[END_REF]) and for new contents ( [START_REF] Slayden | Consuming video on Mobile devices[END_REF]). They don't consider the link with the other functionalities of the mobile phone (phone call, sending or receiving a SMS or a MMS, MP3 device embedded, radio embedded, games, camera…), with other devices (dedicated MP3, dedicated radio, other TV) nor with other activities at work or at home. Moreover, they tend to focus on the technical capabilities of the users and on the way they leverage them, thus focusing mainly on young people and on early adopters ( [START_REF] Harper R | Taking hold of TV: learning from the literature[END_REF]). It's also the same market segment that mobile operators stress in their ads: they show a junior executive, fully equipped with the newest technology, technically expert that uses his mobile TV mostly for stock exchange updates, news and sport, for very short durations, in mobile situations. Therefore, in this study, we have chosen to focus on the way users link mobile TV usage with other media through the day, how it competes with or complement other activities or access to several devices or contents (home TV, radio, MP3 devices, papers and magazines). We also choose to focus on working class users, not necessarily technical experts. All studies of cultural practices ( [START_REF] Donnat | Les pratiques culturelles des Français. Enquête 1997[END_REF], [START_REF] Donnat | Regards croisés sur les pratiques culturelles[END_REF]) show that retired people and non working adults are the most intensive TV viewers (over 4 hours per day) and that they favour series and entertainment programs. We wanted to see if they duplicate on this new service and artefact or if they are affected by the situation and the device. This study on urban mobility was funded by the Ile de France (Paris region) local authority in association with Orange Labs. It includes other mobile usage such as camera, visio-conference, MP3 device, which are not covered in this paper. Due to the subject, we chose a qualitative method based on a diary study for 2 weeks, augmented by two individual interviews, before and after. Participants were given a paper notebook to report and document each viewing. We interviewed 20 women and 20 men, living in Paris or in the suburb, aged from 21 to 45. They all have subscribed to the mobile TV option that costs about 7 to 10 euros monthly for unlimited usage.

The first interview is focused on general usage of the mobile phone, of mobile TV, of home TV and other cultural activities. To fully understand mobile TV, we follow the life cycle of the service: buying motivation and process, learning difficulties, content sharing…In the second interview, users comment the diary, which records each viewing: date, beginning and ending time, location, previous and following activity, potential interruptions and how they deal with them, people present and their participation to the viewing, body posture, choice of channel and of program. These 80 interviews have being recorded, transcribed and completed by ethnological study. Pictures and comments of users watching mobile TV have being captured, in order to trace back how they dynamically adapt to the situation.

IS MOBILE TV LIMITED TO MOBILE INDIVIDUAL USAGE ?

The decision of the French telecommunications regulatory authority to call mobile TV « Mobile Individual TV » emphasizes specific types of usage and has led us to question that choice. The dairy analysis shows that 60 % of the viewings are nonmobile compared to 40% in mobile context; 40% on private time and 60% at work, with minor differences between women and men. Moreover, women interviewed watch on their own 72% and men 85%. The 28% of women shared usage consists of 15% with somebody at work (colleague, customer) and 13% with a family member (partner, child, mother, brother, friend). The 15% of men shared usage consists of 4% with somebody at work and 11% with a family member. This huge part of shared or non-mobile usage shows that mobile TV isn't limited to single or mobile usage but is part of daily life, at home and at work, alone or not. Amongst non-mobile usage, mobile TV is sometimes the only access to TV, while in a hotel or in a holiday rental, especially during business trips abroad. If the rooms are TV-equipped, there is no access to French-speaking content and the users can watch their favourite shows on their mobile TV. "I was in Oslo and, thanks to my mobile phone; I did not miss the Karembeu 1 show on Saturday evening. I lie on the bed and watch the entire show (an hour and a half) there because the room's TV has only local channels or the usual international channels. For news or music, it is all right, but not for my favourite show on French TV." (a 36 year-old stewardess on medium-haul flights); « Last week, we went skiing and had TV in the room but were too exhausted to watch. We were isolated from the world. Fortunately, I checked my mobile on the ski-tow and this is how we learned live about Benazir Bhutto's death ». (a 21 year-old student).

¢¡ ¤£ ¦¥ ¨ § © © © ¥ ¨ ! " # $ § &% '© )( 0¥ ¨ "1 2 43 65 87 49 @5 ¦£ A B ¢C ED F¥ ¨© 0% GB H© D &5 ¦¥ I H% P "5 8D & RQ At home, at friends' or relatives' (parents, grand-parents), mobile TV is used when no other resource is free. « My husband uses the living room TV, my son the bedroom's, therefore I watch my mobile in the bedroom, next to my son. But as soon as my son releases the TV, I watch it. The screen is larger. » (a 39 year-old administrative assistant). « I was at my parents' in the country. It was the end of the afternoon and they were watching their show « Questions pour un champion ». I really dislike it, so I went to the kitchen and had a coffee. I watched what else was on. I zapped because it is not a time when I usually watch TV. I was watching casually, until they were done. I do not remember what it was. I preferred to be on my own and watch on my own screen » (Victor, 34-year old, sales representative).

Mobile TV allows watching one's own program, either quietly in another room, or close to relatives, or even to watch multiple channels at once, switching from the mobile phone's screen to the TV. "my lessons finish at 7PM and I get home late. When I get there, my parents are watching the news so I watch it with them, but when the weather forecast starts, I watch "Kameloot" on my mobile phone. I put on my headphones and watch my show, it's not long, just the time they watch the weather forecast and then we have dinner." (a 21-year old student living with his parents) "We have one TV in the living-room and one in the kitchen. So, when there are two of us at home, my girlfriend and I, it allows watching peacefully in the kitchen. I watch the news and she watches her Thursday soap opera. But, as we don' see each other during the day because we both work and I work shift, sometimes I have to leave at 4 am, so in the evening, we like being together on the couch. Then I watch the news on y mobile phone in order not to go to the kitchen. The screen is smaller but we are together and have each our own program. If I watch LCI 2 in the kitchen, I watch it half-heartedly. The mobile phone allows to have peace and watch whatever I want while being with her." (a 29 year-old men, living together with his girlfriend).

It is therefore possible to check the start of a show and avoid missing other shows that would start simultaneously or ignore news that colleagues would discuss the next day. "Yesterday evening, I was watching "Pop Star" on TV. I was on the couch with my girlfriend and at the same time, I was watching the show on TF1 3 on the mobile phone because I knew that today my colleague were going to discuss it and I wanted to be aware. There 300 channels non-stop and therefore I'm often on another one on the mobile phone in order to stay connected to the world. Last week, my colleagues spoke about a cat in a retirement home in Ireland. Every person this cat would stop in front of, dies in the following days. I was the only one unaware. Since then, I got the news on my mobile phone because it came up again and I could see it but I was upset to have missed the news. I turn the volume very low or I switch it off and turn it up only if it is really important" (Patrick, 29-year old, bus driver). The information has less value in itself than staying connected to the world. It is a relational usage (Chalvon-Demersay S., [START_REF] Pasquier | Télévision et apprentissage sociaux: les séries pour adolescents[END_REF].

¢¡ ¤£ ¦¥ % D F© "£ % ¨ § 5 ¦¥ ¢© ¡ © £ 2 A© F " ¥ § 5 ¦© ( 0¥ ¨ "1 2 " % '1 2 "© A " B "! $# &% (' 0) 1 "% 2! 43 5# 26 7) 8' 3 9! A@ 2B
C P3 P( ED GF 89 @5 A% GD 07 "5 87 A# B E© ¥ P( 0¥ ¨ "1 2 $1 2 © A " B C ED F¥ G© 0% ¨B H© D &5 ¦¥ I H% ¢ "5 8D Q On the bed or the couch, the comfort "like hospital TVs that are well oriented to watch in bed" justifies the use of mobile TV because one does not have to get up to switch it off to go to sleep. I was in the living room, lying on the couch, and I watched on my mobile phone and not on the living room's TV in order not to get up and grab the remote control. The mobile phone, I have it handy and therefore, being lazy, I watched the news with it." The small size of the screen and the handiness of the handset turn it into "one's own TV" and a bodily engagement different from a TV set. When coming home late after a party or tired from work, the interviewee appreciate to stretch out in order to watch on his own and relax.

At home, it also allows access to channels that are not available or that have an extra cost on ADSL (asynchronous digital subscriber line), cable or satellite, causing the cancellation since the channel is available at no extra cost on the mobile phone. To have both the comfort of a large screen and no cost, the mobile phone is liked to the LCD or plasma living room TV set, allowing a collective viewing with relatives. « I connect my mobile to the home TV screen and watch mangas with my nephews ». (a 21 year-old student living with his parents). In other cases, Mobile TV provides viewing continuity while changing places, such as starting in the tube or next to a child getting to sleep, and then watching the rest of the program on the living room TV. It can also provide temporary viewing while going to the toilets or to the kitchen to get a snack. " I watch the beginning of the shows before I get home. For instance, for Koh Lanta 4 , as tonight I finish work at 8 PM, even if I do not have time to watch because I'm driving, I switch on my mobile phone in order not to miss the beginning and, once at home, I proceed on the living room TV" (a 29-year-old employee). "Every evening, my one year-old son needs my presence to go to sleep. Therefore, I stretch on a carpet next to his bed. Before having this option, I waited sometimes over half an hour before I could join my husband on the sofa to watch TV. I often missed the beginning of a show and had to ask him for a summary and more of all, I got bored of waiting in the darkness. Now, I have my earphones and my mobile phone and I watch TV. He smells his mum's scent next to him and that's what he needs. I check what's on and then I watch the beginning of the evening show before joining my husband. » (a mother of a one-year old kid).

At work or in college, we see three different types of usage. The first one, which is shared and daily, happens when colleagues or school mates gather to watch together their favourite show, often a soap opera or the news. They access it directly and its timing structures the daily organization, even leading to have lunch earlier in order to be ready for the beginning of the program. Sharing create links and generates a routine with viewing optimization such as using a stand in order to orient the mobile phone for everybody to see. "At work, I watch TV. As we do not have a TV set in the office, my mobile phone is a way for me to watch my shows. And my colleagues benefit from it. Yesterday again, we watched "Les feux de l'amour" There were two colleagues with me in the office. My co-worker, which whom we usually watch the show together. So I was sitting, and so was she. We switch it on maybe five minutes early. As we know that it starts at 1:50 pm, we switch it on at 1:45 pm. Then, I remove the headset or else they would not hear anything" (a public servant).

P¡ 7 "5 87 A# "B © ¥ % G2 5 G 5 ¦ © )( 0¥ ¨ "1 2 3 ¢¡ 1 2 "© A " B 0C ED H¥ ¨© 0% GB F© D 5 ¦¥ I H% P "5 8D & Q "During the morning and noon breaks, we watch the France 2 news with my friends. We are in business prep scool and therefore supposed to read printed newspapers every day, like "Le Monde" and "Le Figaro" and "Les Echos" but we don't have the determination, so we watch the news flash on ITV during the break and the news while having lunch. During the break, I sit on a chair and two or three other classmates watch the news with me. It's like doing homework because we're supposed to read the press. It's instructive." (a 20-year old student in business prep school). « At the bar, the evening of the debate between Mr. Sarkozy and Mrs. Royal, I turned up the volume high so that everybody could hear, because customers who came to have dinner also wanted to hear the debate. It was hot because many of us were close by to listen, but in the restaurant there is no TV so we felt that we were cut from everything without my mobile phone." (a 27 year-old bartender).

Opposite to that, a second use is individual and allows in a noisy open space to get a quiet break, alone in one's bubble. A way to relive stress, the viewed content is of little important and zapping allows to ease tensions. The content is less important than the opportunity to bet quiet alone. "It was after lunch. I had quickly brought my sandwich to the office. I had time, at least 45 minutes left. Therefore, at this moment, I was not necessarily willing to chat with colleagues, because at that time, their discussions were getting on my nerves. You see, we have a generation and education gap, literally speaking, because I have four years of university education, but in my work environment I'm with people who attended only high school so therefore I have difficulties sharing thoughts with them. I need it, it's a question of survival to have a moment just by myself. It's a moment when I am cut from the world. We have a room where we can met between us, be quiet, a room somewhat isolated where archives are stored. I watch the news and travel or documentary channels. I surf a little. If I have lunch with them, we will either discuss kids and I have none, or policy. For a while, it's like a TV remote control. You see, I have the ability to do something else. I have this ability, while being physically there to have a break. It's some oxygen, to think about something else, to get out of the box, out of the work at hand. To get away, like others would make cross words or others call friends." (a 34-years old administrative clerk) A third usage, also individual, is for isolated workers (shop attendants, bus drivers, sales representatives) who are expecting Mobile TV to provide them with company and will leave it on for hours as a background noise without necessarily watching the screen continuously. "I run a photo shop on my own and it helps me to feel less lonely. Company policy prevents us from bringing a TV" (a 34 year-old college degree, franchise shop manager); "I watch all the time at the terminal before leaving. I have to stay in my bus. Before, I had a portable radio, but mobile TV is better because I can watch it" (a 28 year-old bus driver) « Before a meeting with a customer, I relax and get my brain washed from any ideas. I stay in my car, in the parking lot. I get there early, and it allows to think about something else than the stress of the negotiation" (a 34-year old, sales representative). With the three uses of TV, namely structured usage such as "background use" ( [START_REF] Lull | The social uses of television[END_REF]) pacing the day and providing company, relational usage in the conversations and cognitive ( [START_REF] Pasquier | Télévision et apprentissage sociaux: les séries pour adolescents[END_REF])usage as norm learning, there is a "physical relief"( [START_REF] Memmi | La dépendance. Esquisse pour un portrait du dépendant[END_REF]) and an individual bubble to isolate oneself from external noise and from others, to restore one's calm in open space offices. It's a special relational usage, because it makes the presence of others bearable, not by sharing by thy isolation.

Mobile TV is an additional resource to share time and space, but usage remains not very legitimate and sometimes hidden to avoid comments from relatives (mother, spouse) or colleagues, especially managers or subordinates who front of which one should not lose face. At home, the interviewee can watch secretly, leaving all the TV sets switched off, making believe that he's not watching and, in one instance, pretending to brood and leaving the large screen to the husband and in another, pretending to watch over the kids, while watching Mobile TV or else pretending to make homework or to sleep. For the adult at work or for the parent who need to take care of his kids, Mobile TV does have the same legitimacy than a book, so that the interviewee comes up with up self-regulation rules. "Mobile TV, I use it to brood. When I fought with my husband and I leave him the living room TV, I go to the bedroom to brood, I do not dare switch on the bedroom's TV because he would hear it and my pretence would be uncovered, so I watch it on [my Mobile TV] and he knows about nothing. He believes I'm upset and sleeping. As soon as he comes into the room, I switch it off and he's unaware" (a wedded interviewee); "I use it at my mother's, when she watches the living room TV and I go to the room to play with my kids (one and five year old). Sometimes playing gets boring and I want to switch on the TV. My mother has one in the bedroom but I do not date switch it on at that moment because I am supposed to play with my kids and not watch TV. So I take my mobile phone and my mother knows about nothing, else she would comment. She's in from of the TV, even while we're here. So, I let them play a little while and watch it." (a mother); « I do not have a TV in my bedroom and often I watch TV on my mobile phone instead of working. My parents believe that I do my homework, but really I watch TV" (a 20 year-old student); "I watched it every day during Christmas week. We are at my grand parents and they are old-fashioned. It's bedtime at 9:30 pm and I go to bed late. I watched the end of the evening movie and the following show." (a 21 year-old student). "We were in a meeting. It was a highly boring meeting because we were currently in a period of meeting frenzy. So I was both listening intently but also willing to get away so I decided to watch the "Planète" channel because they often have punchy shows. It's true that at that moment, instead of watching the news that I'm more interested but that I can't follow without the sound, I switched on "Planète" because I can watch it while muted. It's more for the images." (a 34-year-old administrative clerk). Specific observation on the workplace is complementary to work sociology studies or usage studies that study mobile phone especially its business purpose, notably the effect of home office on the hierarchical link and on the limits between private life and life, showing that it is a hindrance to professional usage between colleagues. Entertainment usage (Mobile TV and games) allow on the contrary to easy relationships or create bonding around routines and social customs. It maintains or soothes social links, around the ability to do or speak together, allowing to reconcile private and collective time. Being present or absent has a new meaning, allowing to escape on one's own or on the contrary to integrate and tell a story like a community sharing social rites.

Finaly, observation of the use of Mobile TV shows that there are also the 3 uses of standard TV at home: relational, structural and cognitive use, but with new context such as work, academic and mobility. These results tend to prove that Mobile TV is one additional device that interlaces with other devices, offering a wide variety of collective or individual usages, static or mobile, linked with previous and following activities. As soon as one resource is freed up (a larger screen, a partner back for dinner), the usage changes. "I watch it lying on the couch coming back from work and I stop it when he arrives to have dinner" (a 34-year-old administrative clerk). We find out that the taste is built jointly on the spot as a mediation of four elements that are "the tasted object, the community of users, devices and conditions enabling the tasting, the body that feels it…The taste is a reflexive, body-embedded, framed, collective, equipped activity, mixing in one gesture the user's expertise and the set of tools he values" ( [START_REF] Henion | Pour une pragmatique du goût[END_REF]). Usage is not isolated in niches but rather inserted seamlessly in the user's daily timetable, taking into account the environment (mobility, other people), the other activities and the access to several devices and contents.

INTEGRATION IN THE WORKING CLASS'ECOSYSTEM

As previously mentioned, most of the 40 people interviewed belong to the working class, who are the most intensive TV viewers (over four hours per day) along with retired people as evidenced by Olivier Donnat's studies ( [START_REF] Donnat | Les pratiques culturelles des Français. Enquête 1997[END_REF], [START_REF] Donnat | Regards croisés sur les pratiques culturelles[END_REF]) for INSEE (French National Institute of statistics). As contents are concerned, they prefer series and entertainment programs. Studying an entire day shows that home TV viewing durations are not impacted by mobile TV, as the latter is used at times when the home TV wasn't switched on or in addition to it. Therefore these lower classes consume even more TV, in « an additional logic » that Olivier Donnat observes for higher social classes, cumulating visits to museum, opera attendance and reading. Mobile TV changes the content working classes watch on TV. But mobile TV isn't a small TV that reproduces identically TV usages, as contents are different. Indeed, Olivier Donnat defines four types of « TV program necessary to measure TV audience: entertainment and games, soap and series, cultural programs, sports ». On the mobile phone, sport counts for only 5% of the women's usage and 20% of men's. The three remaining categories account for less than 20% of the total duration. To measure mobile TV usage, two additional categories are needed: music (30% of women's usage and 20% of men's) and news (respectively 15% and 30%). Indeed, usage context suits better these contents than an entire feature movie, because the available time for viewing is often unpredictable (waiting for someone or something) or because the time slot differs from one day to another, preventing the user from watching his favourite show, so that he continues to watch it on his home TV, as usual. As they access content such as news or sport, they tend to get closer to what upper classes watch on TV at home. Mobile TV decreases the gap but doesn't suppress it as home routines remain unchanged.

Mobile TV is particularly suited to following live events (sport, elections). Formerly, users had no TV access while working and had to wait for the end of the day to get the update. Mobile TV enables them to access regularly, sometimes quickly to get an update at work, to stay tuned, often sharing with colleagues also eager to have the update. The latter will feed back into their conversation and fit in their activities.

As women are concerned, studies ( [START_REF] Lehingue | Les différenciations sexuelles dans les pratiques culturelles[END_REF]) on French TV uses show that they watch more TV than men. It is also the case on mobile TV: the analysis of the diaries gives 42 minutes per day of women and 20 minutes for men. They access the service 1.4 times a day, compared to 2.1 for men, because once on the service, they stay longer, sometimes leaving it as a TV company, listening and not watching. In situ observations underline that when waiting or queuing, men mechanically and automatically access to the service for very short duration, sometimes for relieve the stress as a cigarette could do. Women tend to wait for a non-mobile longer situation to connect to the service. « It takes a long time to get connected, so I wait until I have enough time available. Otherwise it takes longer to get connected than to enjoy it » (a woman); « I watch it regularly, as soon as I have a minute, often not thinking of it, simply mechanically on the tube platform, at the self service for lunch at work, at the coffee machine or while having a cigarette ». (a man).

As contents are concerned, Olivier Donnat shows that they prefer series and soap operas, to news and sport. We also observe that mobile TV enables them not to miss their favourite soap, making a daily ritual of it, sometimes inviting colleges to watch it, and that men tend to be uninterested by such contents. Series represent 30% of women contents on mobile and 14% of men. Musical channels represent 25% of women contents and 34% for men, taking the place of radio at home or in the car or only enabling to continue on a mobile situation. Regarding music, if they continue to hear it on their radio at home, mobile TV enables women to take music with them, adding the picture, as they favour music channel, sometimes watching, sometimes only hearing as a presence.

As mentioned, the mobile TV context invites women to open themselves more to sport and news than they do on home TV. Mobile TV tends to reduce the gender difference but doesn't suppress it: news represent 50% of men content and 12% of women. Sport represents 35% of their contents and only 5% of women. Again, we see how home TV and mobile TV are complementary, both in usage and content, and do not compete.

IS MOBILE TV CLOSER TO TELEPHONY OR TO TELEVISION?

This brings us to ask the users to qualify the service, which appears to them « not TV on the mobile » nor « a small scale TV but something different ». Each connection or channel choice reminds them that mobile TV is « not ready to use like home TV, in front of which I let myself passively go for hours without noticing. It's not like an easy button you push and it's ready for you to rest for hours. Not at all that kind of experience, as you have to stay alert, to manage it constantly and to adapt yourself to the changing situation ». Mobile TV appears as less outstanding and of unpredictable quality. « It takes forever [to access the service], I start it again, when it warms up, I stop it, then connection is interrupted for a while, it fluctuates, it bugs often ». The service is perceived as more complex than home TV and they master it poorly. « I don't control it, it works that way, I don't know how to use it, I've asked the operator for explanation but nobody knows how to do it, it depends, sometimes they change the first screen with the channel list ». While viewing, the user must constantly adapt the screen's orientation to the light so that he can watch comfortably. In spite of all those differences, some experiments brings mobile TV close to home TV: zapping while viewing, hearing without watching as a background noise for presence, uploading a TV remote control to mimic the home TV's, to leverage TV expertise.

Their representation of mobile TV is sometimes close to Windows organization on a computer with the logic « of unfolding menus and icons where I select the channel logo and I scroll into the menu ». Sometimes it's closer to Internet as « I connect to it, I surf, I navigate ». The channel list is « organized by pages like the result of a Google search. It's the same. You see the number of pages at the bottom of the screen. Now that I know that my favourite channel is on page 4 out of 5, I got there directly ». Some users customize the organization by adding like on the Web « a direct access by inserting TV as an icon on the front screen for my mobile so that I can access mobile TV directly, as soon as I switch my mobile on. I made that shortcut in order to avoid all the steps, from connecting to 3G, waiting for it and then going to the service and then selecting my favourite channel, which I know is on page 3 ». Thus, they leverage their Internet expertise, more useful according to them than the home TV expertise. « I spend my days surfing on the Web and that helps a lot. I've my habits »; « I've added mobile TV to my Favourite list, as I do on the computer for Web surfing »; « I programmed it to keep the last five channels I've seen in a log and to suggest them automatically, because I always watch the same ». Some trial usage reinforces this representation of mobile TV being close to Internet, such as interactivity. « Like on the Web, the operator attracts us with advertisement to inform us of new channels available or new programs. For example, I got to hear about Les têtes à claques that way. It's exactly like on the Web, you always have advertisement of new products in relation with what you like to give you new ideas. Home TV doesn't do that ». (Kaîs, 27, barman); «I've discovered that channel because they pushed it, by saying that it was a new one. Therefore I was tempted to go and see it and stayed tuned. It's exactly like on the Web, when you mail or google information. On the link you have your google findings and on the right side, they push information related to your topics. It gives you ideas. It's like on the Internet, sites are linked and not partitioned as competitors. The operator's portal federates and doesn't position channels as competitors, whereas once you watch one home TV channel, you stay on it without having information about the others. It's partitioned ». (Sam, 35, bookshop salesman). At last, both mobile TV and Internet have stream and stock programs, sometimes on a week or more, enabling the user to free himself from the TV schedule. Home TV isn't that flexible.

But other challenges while using mobile TV leads the interviewer to rethink his representation of the service, as closer to telephony. It's the case when they receive a call, a SMS or a MMS, technically impossible to avoid. It interrupts the viewing and disconnects. « I have to take the call » « I can't avoid it. It has a priority ». These recording of users show that the telephony universe breaks into the viewing with no choice left. Manual reconnection is compulsory to restart. Sometimes, the calling function is welcomed by the viewers because the content invites him to call to comment on the program. « Yesterday, I've watched the whole football match on my mobile but the paid channel Canal + broadcasts it and at home we only have the regular free channels. I lay in my bed all the time, watching the whole 2 hours, calling my friends during breaks to comment the game live ». (Simon, 20, student) Sometimes it's the mobile material limitations (battery, thermal effect, screen adaptation to the light) during daily usage out of home that reminds the user that « he doesn't watch TV at home but [needs] to adapt the viewing to the mobile phone's limitations. Very often, on my way back home in the evening, I stop watching in order to keep some battery in case I need to call home if the tube is delayed. »; « After a certain amount of time, it heats so that I stop viewing, switch it off and start again after a while ». Thus some functions of the mobile phone such as call, SMS or MMS reception, network disconnection or battery limitations remind the user in the middle of the viewing that he's watching on a mobile and not on his home TV.

CONCLUSION

This study of mobile TV usage shows that in spite of the name (mobile individual TV) given by the French regulatory authority, it is not limited to individual usage while being mobile but extends to shared or non-mobile usage. Dedicated or in parallel to other activities, for short or long duration, it allows the viewer to be quiet in an isolated bubble or to build a link with relatives or colleagues. Each session challenges the user's representation of the service, be it telephony, television, Windows presentation on the computer or Internet. Indeed, as call, SMS or MMS reception, network disconnection or battery breakdown in the middle of the viewing present the service close to telephony, while zapping and hearing without watching all day long as a presence when alone is closer to television. Interactivity, surf in the unfolding menus, presentation of the service with icons is closer to the computer and to Internet. Each viewing is a co-production, evolving dynamically, taking into account the availability of a resource, built as a mediation between four elements: "the tasted object, the community of users, devices and conditions enabling the tasting, the body that feels it…The taste is a reflexive, body-embedded, framed, collective, equipped activity, mixing in one gesture the user's expertise and the set of tools he values" ( [START_REF] Henion | Pour une pragmatique du goût[END_REF]. Usage is not isolated in niches but rather inserted in the user's daily timetable, taking into account the environment (mobility management, other people), the other activities and the access to several devices and contents. For example, if he hasn't seen the news before leaving home, the user will watch news in the tube on his way to work in the morning, otherwise he will chose musical channels. Mobile TV also enables him to decide to watch a program with his family on the sofa at night instead of watching alone another program, knowing he can keep it back alone the next morning on his mobile. The consecutive contents, devices and media available throughout the day define the usage. Lastly, a focus on lower classes shows that contents on mobile and home TV are different: more music, sport and news on the mobile, as these programs can be fragmented and are suited to short or long durations. Therefore both experiences are complementary, neither in competition nor niche usage.