M Arzel

M Morsli

J C Addou

L Bernède

Cattin

Jean Christian Bernède

Linda Cattin

Keywords: Organic solar cells, energy materials, photovoltaic, semiconductor materials, interface, thin film

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

1

The effect of the band structure on the Voc value of ternary planar heterojunction organic solar cells based on pentacene, boron subphthalocyanine chloride and different electron acceptors.

Z. El Jouad, 1,5 E.M. El-Menyawy, 2,3 G. Louarn, 2 L. Arzel, 2 M. Morsli, 4 M. Addou, 5 J. C. Bernède 1* and L. Cattin 2*

Introduction

Nowadays, organic photovoltaic solar cells (OPVs) show great potential due to their high efficiency, flexibility, low weight and semi-transparency [START_REF] Hu | Towards a bright future: polymer solar cells with power conversion efficiencies over 10%[END_REF][START_REF] Ma | Ternary non-fullerene polymer solar cells with efficiency >13.7% by integrating the advantages of the materials and two binary cells[END_REF]. The progress in this area of research has been possible due to the exploration of new materials and configurations, which has led to an improvement in various organic photovoltaic solar cell (OPV) parameters; for instance, the use of new organic materials and optimized configurations, such as tandem OPVs [START_REF] Che | High fabrication yield organic tandem photovoltaics combining vacuum and solution processed subcells with 15% efficiency[END_REF]. Although it is interesting to develop high-performance products and/or configurations, it is important to keep in mind other important criteria such as the facile synthesis of the requisite organic materials and control over the fabrication process. For instance, ternary structures with a cascade energy alignment allow exciton dissociation and carrier collection without the need of recombination layers, which are necessary in tandem OPVs [START_REF] Yuen | A simple parallel tandem organic solar cell based on metallophthalocyanines[END_REF]. On the other hand, charge generation can take place not only at an electron donor/electron acceptor (ED/EA) interface, which corresponds to the classical four step process [START_REF] Peumans | Separation of geminate charge-pairs at donor-acceptor interfaces in disordered solids[END_REF], but also via a two-step exciton generation process at the ED/EA interface [START_REF] Coffey | Photovoltaic charge generation in organic semiconductors based on long-range energy transfer[END_REF], opening up the possibility for three-layer devices. This last process consists of two-steps involving exciton diffusion from the donor to the acceptor over long length scales and dissociation. Diffusion over a long length scale is possible through Förster resonance energy transfer (FRET) [START_REF] Stoltzfus | Charge generation pathways in organic solar cells: Assessing the contribution from the electron acceptor[END_REF][START_REF] An | Versatile ternary organic solar cells: A critical review[END_REF].

In the present study, we chose to mainly work with commercial molecules in planar heterojunctions (PHJ). This configuration exhibits low performance when compared to bulk heterojunctions (BHJ), but they are easier to implement. If BHJs achieve high efficiency, good performances can also be expected to be obtained using PHJs, as demonstrated in the literature [START_REF] Sista | Enhancement in open circuit voltage through a cascade-type energy band structure[END_REF][START_REF] Heidel | Reducing recombination losses in planar organic photovoltaic cells using multiple step charge separation[END_REF][START_REF] Schlenker | Cascade organic solar cells[END_REF][START_REF] Cnops | Enhanced photocurrent and open-circuit voltage in a 3-layer cascade organic solar cell[END_REF][START_REF] Vereet | Decreased recombination through the use of a non-fullerene acceptor in a 6.4% efficient organic planar heterojunction solar cell[END_REF][START_REF] Cnops | 8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer[END_REF][START_REF] Barito | Universal design principles for cascade heterojunction solar cells with high fill factors and internal quantum efficiencies approaching 100%[END_REF][START_REF] Stevens | Open-Circuit voltage exceeding the outmost HOMO-LUMO offset in cascade organic solar cells[END_REF][START_REF] Lin | Enhanced sub-bandgap efficiency of a solid-state organic intermediate band solar cell using triplet-triplet annihilation[END_REF]. Small molecules present some advantages, such as facile purification and more reproducible synthesis, leading to a unique final product without any batch-to-batch variation [START_REF] Leliège | Small D-p-A systems with s-phenylene-bridged accepting units as active materials for organic photovoltaics[END_REF]. The use of thermal evaporation under vacuum permits facile control over the structural architecture and management of the film morphology using the deposition rate [START_REF] Jouad | Ca/Alq3 hybrid cathode buffer layer for the optimization of organic solar cells based on a planar heterojunction[END_REF] and nature of the substrate [START_REF] Makha | MoO3/Ag/MoO3 anode in organic photovoltaic cells: Influence of the presence of a CuI buffer layer between the anode and the electron donor[END_REF]. Moreover, deposition via sublimation results in an improvement in the layer purity, leading to highly reproducible device fabrication [START_REF] Scholes | Intra phase microstructure-understanding the impact on organic solar cell performance[END_REF][START_REF] Galindo | Influence of the density of states on the open-circuit voltage in small-molecule solar cells[END_REF][START_REF] Chang | Carbazole based small molecules for vacuum deposited organic photovoltaic devices with open-circuit voltage exceeding1 V[END_REF]. Therefore, in the present work we have studied OPVs based on small molecules deposited using a vacuum process. In the ternary structure, the central organic layer must be ambipolar (EDA) with sufficient carrier mobility, holes and electrons. Moreover, the use of a ternary PHJ induces an increase in the series resistance of the OPV device, therefore, it is recommended to use organic molecules with good carrier mobility. Boron subphthalocyanine chloride (SubPc), which exhibits a very high absorption coefficient and ambipolar conductivity [START_REF] Ebenhoch | Solution-processed boron subphthalocyanine derivatives as acceptors for organic bulk-heterojunction solar cells[END_REF], was chosen as the central layer in our ternary structure. We chose fullerene (C60) as the electron acceptor due to its high electron mobility and very high efficiency toward collecting excited electrons [START_REF] Sariciftci | Photoinduced electron transfer from a conducting polymer to buckminsterfullerene[END_REF]. The third layer, the electron donor, was pentacene due to its high hole mobility [START_REF] Gundlach | Pentacene Organic thin-film transistors-molecular ordering and mobility[END_REF][START_REF] Kang | Influence of post-annealing on polycrystalline pentacene thin film transistor[END_REF]. Schematic representations of typical PHJ-OPVs are shown in the supporting information (Fig. S1). In regard the pentacene layer, it has been shown that the substrate properties significantly influence its properties, such as grain size, and therefore, the carrier diffusion length [START_REF] Puigdollers | Pentacene thin-film transistors on polymeric gate dielectric: Device fabrication and electrical characterization[END_REF]. In addition, it has been shown that when the grain size is increased, the scattering probability of the charge carriers is decreased due to the reduced grain boundary density, which results in an increase in the maximum drain current observed in thin film transistors [START_REF] Kang | Influence of post-annealing on polycrystalline pentacene thin film transistor[END_REF]. For instance, the carrier mobility increases by one order of magnitude after annealing due to an increase in the grain size [START_REF] Kim | Influence of post-deposition annealing of PVP/pentacene films on characteristics of organic thin film transistors[END_REF]. On the other hand, as shown by Hsiao Wen Zan et al. [START_REF] Zan | Surface energy and adhesion energy of solutionbased patterning in organic thin film transistors[END_REF], the grain size in pentacene thin films increases with the surface energy of the substrate. In the case of OPVs, the under layer of the ED, in this case pentacene, is a thin layer called the hole extracting layer (HEL), which increases the band matching between the ITO anode and the ED. Actually, in the case of some phthalocyanine dyes, we have shown that if both MoO3 and CuI are used as the HEL, the resulting hybrid MoO3/CuI layer can allow the best OPV efficiency to be achieved [START_REF] Cattin | Improved performance of organic solar cells by growth optimization of MoO3/CuI double-anode buffer[END_REF]. Nevertheless, we have also shown that the surface energy of MoO3 is significantly higher than that of CuI. Therefore, in the present work, we used MoO3 as the HEL to optimize the properties of the pentacene layer. After a systematic characterization of the PHJ-OPVs based on Pentacene/SubPc/C60 we widened our study to two others EAs in order to increase the open circuit voltage, which is quite small in the case of the Pentacene/C60 couple. These EAs were PCBM (phenyl-C61butyric acid methyl ester) and EH-IDTBR (C72H88N6O2S8 ) (Figure 1), and the reason for their selection will be discussed below. The chemical structures of the small molecules used in this study are shown in Figure 1. We named the OPVs based on the three layer structure comprised of Pentacene/SubPc/EA as "ternary OPVs", while those using a simple couple, i.e. Pentacene/SubPc, Pentacene/EA or SubPc/EA, were called binary structures (Supporting information S1a and S1b).

Experimental

Organic molecules

The majority of the chemical products used in this study, including pentacene, SubPc, C60, BCP, MoO3 and Al, were obtained from CODEX-International (France), while PCBM and EH-IDTBR were purchased from Aldrich. Their chemical structures are shown in Figure 1.

Organic photovoltaic cell deposition process and characterization

The substrate preparation and OPV deposition processes have already been described in the literature [START_REF] Cattin | Improvement in the lifetime of planar organic photovoltaic cells through the introduction of MoO3 into their cathode buffer layers[END_REF] and are briefly described in supporting information S2, except for pentacene. In regard pentacene, it has been shown previously that depositing pentacene at a deposition rate of 0.05 nm/s allows the formation of a larger grain size [START_REF] Yang | Effects of thin film processing on pentacene/C60 bilayer solar cell performance[END_REF][START_REF] Karak | Organic photovoltaic devices based on pentacene/N, N′-dioctyl-3,4,9,10-perylenedicarboximide heterojunctions[END_REF]. Schematic representations of the OPVs used in this study are shown in supporting information S1a and S1b, while the characterization techniques are presented in supporting information S3. In order to obtain a high extraction efficiency for the electrical carriers, hole and electron extracting layers are desirable to improve the power conversion efficiency (η) in the OPVs. Therefore, an exciton blocking layer (EBL) was inserted between the EA and Al cathode. The EBL was bathocuproine [START_REF] Peumans | Very-high-efficiency double-heterostructure copper phthalocyanine/C60 photovoltaic cells[END_REF]. MoO3 was used as the HEL inserted between the ITO anode and ED, as justified above. Some attempts were carried out using a MoO3/CuI HEL to check the validity of this choice. Therefore, the organic molecules used in the active layers were pentacene, SubPc, C60 and PCBM or EH-IDTBR. Among them, C60, PCBM and EH-IDTBR were the EA in the ternary OPVs, while due to the fact that it is ambipolar, SubPc was used as the central layer. Finally, aluminium film was used as the cathode. Regarding the choice of SubPc as the central layer, it was used not only due to the fact that it is ambipolar, but also because it has intermediate HOMO and LUMO values between those of the donor (pentacene) and acceptors. It must be noted that SubPc, which has an electron mobility of 8×10 -3 cm 2 V -1 s -1 , has already been successfully inserted between an ED and EA [START_REF] Ebenhoch | Solution-processed boron subphthalocyanine derivatives as acceptors for organic bulk-heterojunction solar cells[END_REF][START_REF] Beaumont | Boron subphthalocyanine chloride as an electron acceptor for high-voltage fullerene-free organic Photovoltaics[END_REF]. Therefore, the structures used were as follows: Glass/ITO (100 nm)/MoO3 (3 nm)/organic active layers/BCP (9 nm)/Al (120 nm).

Results

After an optical and electrical study of the different OPVs, we proceeded to perform some morphological characterization using SEM and AFM in order to help toward understanding the different behaviour observed for the OPVs.

Optical and electrical (J-V characteristics and external quantum efficiency (EQE)) studies on the OPVs using C60 as an electron acceptor.

In parallel to the study of ternary OPVs, we studied binary OPVs constructed from all possible combinations in order to verify if the multiple interfaces within the ternary structure are all active or not. Therefore, we fabricated a set of planar heterojunction OPVs based on the Pentacene/C60, SubPc/C60 and Pentacene/SubPc couples as control devices. The absorption spectra of the different layers used and those of their stacks are represented in Figure 2. The total absorption of the stacked layers corresponds to the sum of the absorption of the three stacked layers. The typical J-V characteristics observed for these devices are presented in Figure 3, while the results are summarized in Table 1. The results given in Table 1 were obtained after optimisation of the thickness of the different layers constituting the active OPV. These optimisations are reported in supporting information S4. Firstly, it can be noted that the efficiency of the Pentacene/C60 binary OPV was in good agreement with values reported in the literature [START_REF] Yang | Effects of thin film processing on pentacene/C60 bilayer solar cell performance[END_REF][START_REF] Potscavage | Encapsulation of pentacene/C60 organic solar cells with Al2O3 deposited by atomic layer deposition[END_REF]. Similarly, the SubPc/C60couple gives results in accordance with those previously reported [38,[START_REF] Zheng | Photocurrent enhancement via structural templating of phthalocyanine based planar heterojunction photovoltaic by a thin layer of dinaphthothienothiophene (DNTT) or 3,4,9,19-perylenetetracarboxylic-dianhydride (PTCDA)[END_REF]. On the other hand, the performance of the Pentacene/subPc OPV was very poor. However, the ternary OPV allowed a significant increase in the performance since, when compared with the best result obtained for the binary structures (i.e., the SubPc/C60 OPV), there was a 10% increase in the efficiency. It must be noted that the ternary structures using MoO3/CuI as the HEL exhibit lower efficiencies to those obtained using MoO3. This confirms that our choice of HEL was judicious. In regard the EQE, if we compare the shape of absorption and EQE spectra (Figure 2b and4) we can see that the EQE spectra reproduce the shape of their corresponding absorption curves. Therefore, the contribution of the three organic molecules to the current generation is clearly visible. Nevertheless, at first glance, when looking at Figure 3, it is immediately noticeable that the presence of pentacene strongly limits the Voc value. More precisely, as shown in Table 1, the ternary OPV comprised of Pentacene/SubPc/C60 achieved the highest Jsc value. Upon comparison with the binary OPVs constructed using pentacene as the ED, the ternary structure showed a significant increase in the OPV performance. Moreover, due to its high Jsc value, the efficiency of the ternary Pentacene/AlPcCl/C60 OPV was higher than that of the binary OPV constructed from SubPc/C60, and was also better than the performance observed for the ternary OPV prepared using MoO3/CuI as the HEL. In order to check the influence of MoO3 and CuI on the morphology of the pentacene layers, we carried out SEM and AFM analyses (Figure 5). The SEM images of the surface of the ITO/MoO3/pentacene (Figure 5c) and ITO/MoO3/CuI/pentacene (Figure 5d) structures show that the grain size of the pentacene layers deposited onto MoO3 was higher than that of the thin films deposited onto CuI. This result was confirmed by AFM analysis. To determine the surface roughness and grain size of the samples, several representative parameters were selected. The surface roughness was investigated using the mean roughness (Ra) and mean square roughness (Rq), while a specific module of Gwyddion software was used to evaluate the particle statistics (grain number, mean grain size and total projected boundary length). It should be stressed here that to mark and quantify particles from the AFM images, the thresholding method was used (Table 2). The AFM images obtained for ITO/MoO3/pentacene (Figure 5a) and ITO/MoO3/CuI/pentacene (Figure 5b) confirm the difference in the grain sizes observed during SEM analysis. More precisely, the particle statistics (grains number, mean grain size and total projected boundary length) reported in Table 2 clearly show that the grain size of the pentacene films deposited onto MoO3 was significantly higher than that of the films deposited onto CuI, and it follows that the grain boundary length decreases. The larger grain size and smaller grain boundary length induces a lower carrier trap density, which results in the higher Jsc value. Moreover, it can be seen in Table 2 that the pentacene films deposited onto MoO3 are rougher than those deposited onto CuI, which makes the interface area with the top layer larger, which induces a higher charge separation efficiency. All these morphological differences justify that the Jsc value is higher in the OPV constructed using MoO3 as the HEL. The EQE measurements are reported in Figure 4 and as-mentioned above, the shape of the curve shows that all the organic layers in the ternary OPVs contribute to the charge carrier generation process. This means that while the very low efficiency observed for the binary Pentacene/SubPc OPV shows that the interface between these two layers is not efficient in terms of charge carrier separation, both layers are active in the ternary structures. It is known that SubPc/C60 is an efficient couple in OPVs, which justifies the contribution of SubPc in the EQE curves. In the case of pentacene, the Pentacene/SubPc couple is inefficient as a charge separator and the contribution of pentacene to the EQE spectrum must be attributed to the FRET process. This energy transfer, to be efficient, needs significant overlapping of the ED emission spectrum with the EDA absorption spectrum [START_REF] Coffey | Photovoltaic charge generation in organic semiconductors based on long-range energy transfer[END_REF]. The photoluminescence spectrum of pentacene has already been studied and six photoluminescent bands (A1-A6) were identified [START_REF] Anger | Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films[END_REF]. If the main band is situated at λ = 677 nm, the A5 and A6 bands are situated between 520 and 620 nm. Since SubPc absorbs between 500 nm and 650 nm, there is a significant overlap between the photoluminescence of pentacene and the absorbance of SubPc.

OPVs constructed using electron acceptors others than C60.

Since, as discussed below, the Voc value depends on the energy difference between the energy levels of the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, in order to try to increase the Voc value, we have substituted the C60 EA with smaller absolute LUMO values (Table 3). One of them is well-known, PCBM, which is used in BHJs in place of C60 due to its far higher solubility; the other is EH-IDTBR (Figure 1), which has been recently used with success in a C60 free BHJ [START_REF] Sharapov | High performance ternary organic solar cells due to favoured interfacial connection by a nonfullerene electron acceptor with cross-like molecular geometry[END_REF]. The results obtained with these EA's are summarized in Table 4. As-mentioned above, in order to avoid the Voc limitation induced by the small ∆(HOMOPentacene -LUMOC60) value, we chose other acceptors which, theoretically, may lead to a higher Voc value (Table 3). If there is an increase in the Voc value, in return, the Jsc value will be reduced, which results in limited efficiency. In the case of EH-IDTBR, if the efficiency of the ternary Pentacene/SubPc/EH-IDTBR OPV was higher than that of the binary OPVs, Pentacene/EH-IDTBR (Table 4) and Pentacene/SubPc (Table 1), it is smaller than that observed for the SubPc/EH-IDTBR OPV. Moreover, the highest efficiency obtained with OPVs prepared using EH-IDTBR was due to the ternary OPV comprised of Pentacene/EH-IDTBR/C60, which shows that, when deposited under vacuum, EH-IDTBR is not as efficient acceptor as C60. The same tendency was observed with PCBM. The efficiency of the ternary structure constructed from Pentacene/SubPc/PCBM was η = 0.50%, while that of the binary structure (Pentacene/PCBM) was η = 0.20%. If there was a small increase in the Voc value when compared with the OPVs prepared using C60 as the EA, which was expected, and if the FF was acceptable, the Jsc value is very small. Although the deposition of these materials via sublimation under vacuum is not usual, we have attempted to check whether some degradation took place during the deposition process. In order to check if the deposition of EH-IDTBR under vacuum induced some degradation, we measured its optical density. Its spectrum is shown in Figure 6. By comparison with the absorption of the thin film deposited via spin coating, there was no difference observed in its spectrum [START_REF] Holliday | High-efficiency and air-stable P3HT-based polymer solar cells with new non-fullerene acceptor[END_REF]. The preservation of EH-IDTBR was also confirmed using XPS analysis. In accordance with the optical study, no difference was observed between the powder and thin film XPS spectra (Figure 7). The small extra peaks visible in the thin film spectrum were attributed to In and Si, due to the EH-IDTBR being deposited on ITO coated glass. It can be concluded that there was no significant degradation of EH-IDTBR during the sublimation step. In regard the PCBM layer, some studies have already been performed. Hergen Eilers showed using infrared spectroscopy that the main contributions to the infrared spectra of PCBM were preserved after deposition under vacuum with the bands corresponding to the side chains of PCBM being observed [START_REF] Eilers | Multispectral visible/infrared sensors based on polymer-metal nanocomposites[END_REF]. Therefore, EH-IDTBR and PCBM were not significantly damaged during their deposition. All these results are discussed below.

Discussion

In OPVs, the energy levels of the HOMO and LUMO of the organic materials are primordial. Not only does the energy difference between the LUMO and HOMO (i.e. the band gap) of the organic molecules determine the light absorption, but also the relative energy levels of the LUMO and HOMO of the ED and EA are very important. Actually, in order to obtain highly efficient exciton charge separation, the offset of the LUMOs (HOMOs) values must be greater than the energy of the exciton (∆Eexc). On the other hand, it is now generally accepted that the open circuit voltage of a binary OPVs is as follows (HOMO and LUMO in absolute values) [START_REF] Pode | On the problem of open circuit voltage in metal phthalocyanine/C60 organic solar cells[END_REF][START_REF] Benduhn | Intrinsic nonradiative voltage losses in fullerene-based organic solar cells[END_REF]: qVoc = HOMOED-LUMOEA-X ( 0.3 eV < X< 0.6 eV in the case of a PHJ) [START_REF] Hu | Towards a bright future: polymer solar cells with power conversion efficiencies over 10%[END_REF] It is therefore necessary to find the best compromise between the gap value of the ED, the energy difference between the LUMOs (HOMOs) of ED and EA, and the Voc value (Figure 8).

In the case of three stacked layers, in terms of the Voc value, there is some controversy in regard the requirement concerning the band structure of the materials used. In the cascade configuration of the band structure (Figure 9) it is generally thought that it is necessary to use an ED with a HOMO (or EA with LUMO) aligned with that of the central layer EDA in order to avoid any reduction in the Voc value. This hypothesis has been supported in the literature, for instance, Cnops et al. confirmed that the Voc value is ultimately limited by the energy levels of the outer layers in the cascade structure [START_REF] Cnops | 8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer[END_REF]. However, this assertion was weighted by other works dedicated to BHJs, where both ED's were blended. In this case, these OPVs have a tuneable Voc value, which is dependent on the composition of the mixed layer, instead of being restricted to the lowest Voc value dictated by the lower HOMO level. The Voc value observed for the ternary OPVs is an intermediate value between the low and high Voc (Figure 9) [START_REF] Williams | Vacuum deposited ternary mixture organic solar cells[END_REF]. More recently, it has been shown that planar OPVs prepared with a four layer cascade architecture exhibit a Voc value that is greater than the offset in energy between the HOMO of the outmost ED and the LUMO of the outmost EA (Low Voc in Figure 3) [START_REF] Stevens | Open-circuit voltage exceeding the outmost HOMO-LUMO offset in cascade organic solar cells[END_REF]. In fact, as shown in Table 1, the Voc value does not surpass 0.46 eV, which confirms that the Voc value is limited by the offset in energy between the HOMO of the outmost ED and the LUMO of the outmost EA (Low Voc in Figure 9). SubPc has already been used as an ambipolar layer in ternary PHJ-OPVs [START_REF] Ebenhoch | Solution-processed boron subphthalocyanine derivatives as acceptors for organic bulk-heterojunction solar cells[END_REF][START_REF] Cattin | Comparison of performances of three active layers cascade OPVCs with those obtained from corresponding bi-layers[END_REF], however, in this work, the properties of the ED were different from those of pentacene. For instance, in Ref. [START_REF] Cattin | Comparison of performances of three active layers cascade OPVCs with those obtained from corresponding bi-layers[END_REF], the hole mobility of the ED was small, while in the present work, the hole mobility of pentacene is high. More importantly, the electron donor and the central ambipolar layers have the same HOMO value, therefore there is no need to discuss the Voc value (high Voc-low Voc) [START_REF] Cattin | Comparison of performances of three active layers cascade OPVCs with those obtained from corresponding bi-layers[END_REF], while it is a major contribution in the present study. Consequently, the objective in the present work is not achieving very high efficiency, but to contribute to the current debate surrounding the Voc value. The different LUMOs and HOMOs absolute values of the organic molecules used during this work are given in Table 1. It can be deduced from formula (1) that the maximum theoretical Voc value of the ternary PHJ Pentacene/SubPc/C60 OPV was only 0.6 eV with the HOMO value of pentacene being 4.9 eV. Therefore, beyond an exhaustive study of ternary Pentacene/SubPc/C60 OPVs, we performed an extensive study of the ternary OPVs using an EA with LUMO values expected to give high Voc values (Table 1). Actually, the maximum theoretical Voc values associated with PCBM are: "low" Voc = 1.2 V and "high" Voc = 1.9 V, and "low" Voc = 1 V and "high" Voc = 1.7 V with EH-IDTBR (Figure 3 and Table 1). A comparison of the results presented in Tables 2 and3 shows that there is indeed an increase in the Voc value. Nevertheless, the Voc increase is far from its maximum theoretical value and corresponds clearly to the "low" Voc value, which confirms that the Voc value is limited by the offset in energy between HOMO of the outmost ED and the LUMO of the outmost EA (Low Voc in Figure 9). Nevertheless, given the small Voc value, interesting OPV efficiencies were achieved when using MoO3 as the HTL due to the improved morphology of pentacene when deposited onto MoO3. The influence of the ITO/MoO3 anode on the OPV properties using pentacene as the ED has already been discussed upon comparison with bare ITO [START_REF] Hancox | The effect of a MoOx hole-extracting layer on the performance of organic photovoltaic cells based on small molecule planar heterojunctions[END_REF]. In the present work we have shown that the as-obtained performance was not as good as those obtained with MoO3, even when a CuI HTL was deposited onto ITO (Table1). As evocated in the previous paragraph, the high Jsc value was not only due to MoO3, but also to the occurrence of FRET. As a matter of fact, our experimental EQE spectra demonstrate that the very low efficiency of the binary Pentacene/SubPc OPV shows that the interface between these two layers is not efficient in terms of the charge carrier separation, but both these layers are active in the ternary structures. Firstly, the different behaviours of the interfaces observed in the Pentacene/SubPc (inefficient, i.e. unfavourable configuration) and Pentacene/C60 (efficient, i.e. favourable configuration) couples are a good illustration of the theoretical work of Ryno et al. [START_REF] Ryno | Polarization energies at organic-organic interfaces: Impact on the charge separation barrier at donor-acceptor interfaces in organic solar cells[END_REF]. Actually, these authors have shown that if the charge separation in the planar configuration is not easy due to the presence of a barrier to charge separation, there are several effects that will act to reduce or negate this large barrier. They have shown that such positive effects are present at the Pentacene/C60 interface. In the present work, if we confirm the high efficiency of charge separation at this interface, we can show that this is not the case for the Pentacene/SubPc interface, even if they both have a favourable cascade energy alignment. In regard the FRET process, it is well documented in review papers such as those of Stoltzfus et al. [START_REF] Stoltzfus | Charge generation pathways in organic solar cells: Assessing the contribution from the electron acceptor[END_REF] and An et al. [START_REF] An | Versatile ternary organic solar cells: A critical review[END_REF] that it is possible between pentacene and SubPc, which justifies the pentacene contribution to the EQE spectrum. This contribution via FRET to the charge collection results in the enhanced Jsc and η values. This alternative migration route for excitons created in the ED is decisive toward obtaining improved ternary OPVs when compared to their corresponding binaries. As-mentioned above, the sensitization of pentacene to SubPc was asserted from Figure 4. An EQE contribution between 625 and 700 nm corresponding to the main absorption peak of pentacene is clearly visible. This behaviour suggests that the pentacene excitons produced via optical absorption are transferred to the SubPc layer, since, as shown by the poor behaviour of the Pentacene/SubPc OPV, the Pentacene/SubPc interface is inefficient in separating the exciton charges. Such FRET has already been confirmed in other ternary PHJ-OPVs, such as a-6T/SubNc/SubPc [START_REF] Cnops | 8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer[END_REF]. This example is not isolated, for instance, it has been shown that the planar multi-heterojunction comprised of (thiophene/phenylene)co-oligomers/metalphthalocyanine/fullerene results in an improved short circuit current via efficient FRET [START_REF] Ichikawa | Sensitization of organic photovoltaic cells based on interlayer excitation energy transfer[END_REF]. Moreover, not only has the efficient energy transfer from pentacene to PbSe nanocrystals been reported [START_REF] Tabachnyk | Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals[END_REF], but also that observed between pentacene and ZnPc, a phthalocyanine dye similar to SubPc [START_REF] Hong | Antenna effects and improved efficiency in multiple heterojunction photovoltaic cells based on pentacene, zinc phthalocyanine and C60[END_REF]. All these findings confirm the presence of the FRET process.

A relatively high Jsc value was obtained, although the carrier mobility of the organic materials used is quite different. Actually, even without any annealing, the hole mobility (μh) in pentacene was 0.1 cm 2 V -1 s -1 [START_REF] Gundlach | Pentacene Organic thin-film transistors-molecular ordering and mobility[END_REF][START_REF] Kang | Influence of post-annealing on polycrystalline pentacene thin film transistor[END_REF], which is higher than the carriers mobility in SubPc (μh = 1.69×10 -5 cm 2 V -1 s -1 ), while the electron mobility (μe) was 8×10 -3 cm 2 V -1 s -1 [START_REF] Cattin | Comparison of performances of three active layers cascade OPVCs with those obtained from corresponding bi-layers[END_REF]. Nevertheless, the electron mobility of C60 was of the same order of magnitude as the hole mobility of pentacene. In C60, the µe value is 0.5-2×10 -1 cm 2 V -1 s -1 [START_REF] Anthopoulos | High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films[END_REF][START_REF] Rand | Solar cells utilizing small molecular weight organic semiconductors[END_REF]. Tress et al. [START_REF] Tress | Imbalanced mobilities causing S-shaped IV curves in planar heterojunction organic solar cells[END_REF] have shown that, in the case of a planar binary OPV, an imbalanced mobility ratio of nearly 100 is acceptable and that, in any case, regarding the OPV performance, imbalanced high/low mobilities are preferable when compared to balanced low mobilities. Moreover they have shown that a very high imbalanced mobility ratio induces S-shaped J-V characteristics. In the case of our ternary PHJ-OPVs, there was no S-shaped tendency, which confirmed that the difference in carrier mobility between pentacene and SubPc was not fatal, even if the decrease in the FF value of the pentacene/SubPc/C60 OPV relative to that obtained for the Pentacene/C60 OPV was probably due to this mobility difference. Nevertheless, if these ternary OPVs exhibit high Jsc values, their efficiency is limited by their small Voc values due to the small HOMOED-LUMOEA value. Therefore, we used ED/EA pairs with larger HOMOED-LUMOEA values to try to increase the Voc value of the OPVs, the new EAs chosen were PCBM and EH-HIDTBR (Figure 1). The two OPVs prepared using these EAs have a classical cascade band structure with significant LUMO and HOMO offsets (Table 3), however, the results shown in Table 4 were quite disappointing. In fact, the obtained efficiencies were smaller than those obtained with C60, even though the EH-IDTBR and PCBM molecules were not significantly damaged during their deposition. This is probably due to the fact that when used in the PHJ with a corresponding high EA film thickness, their conductivity is small and the morphology of these films was not optimized as in the case of the BHJ. Nevertheless, the increase in the Voc value obtained is promising and will encourage future efforts to improve the morphology of the multilayer device.

Conclusions

In the present work, we have extensively studied the Pentacene/SubPc/C60 cascade PHJ for the first time, which showed systematically that the measured Voc values are smaller than the offset in energy between the HOMO of the outmost ED and the LUMO of the outmost EA. Therefore, the Voc value is limited by the "low Voc" (Figure 3). However, we have shown that ternary OPVs prepared using the Pentacene/SubPc/C60 active structure result in improved cell performance when compared with those obtained with their corresponding binary structures via an improvement in the Jsc. The increase in the Jsc value is attributed to the presence of FRET. Nevertheless, this improvement is limited by the Voc value. Our first attempts using a more favourable band structure, i.e. an EA with smaller LUMO value, to increase the Voc value failed to increase the OPV efficiency due to the limited Jsc values. However, recent studies have shown that many new molecules behave as effective acceptors, which may be successfully used in ternary OPVs with pentacene as the ED.

Figures

Figure 1: The chemical structures of the small molecules investigated in this research study. Table 1: A summary of the parameters of the ternary OPV (Pentacene/SubPc/C60) and its corresponding binary structures (* the HTL of this OPV is MoO3/CuI, while it is MoO3 for all the other OPVs presented in this table).

Tables

Table 2: Grain analysis and surface roughness parameters obtained for the different samples studied. Each analysis was performed from AFM images over a 5µm × 5µm area. The original data was plane-levelled to remove tilt by applying a numerical second-order correction. Particle analysis was carried out using the thresholding method (Gwyddion software).

Table 3: The LUMO and HOMO values of the different electron acceptors studied. * [START_REF] Anger | Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films[END_REF] and ** [START_REF] Huang | Enhanced performance in inverted polymer solar cells employing microwave-annealed sol-gel as electron transport layers[END_REF]. Table 4: A summary of the parameters of the OPV prepared using the new EA and its corresponding binary structures

Figure 2 :

 2 Figure 2: (a) The optical density of Pentacene (60 nm)/AlPcCl (15 nm)/C60 (40 nm) and the individual Pentacene, SubPc and C6O layers. (b) The OD and EQE spectra of the ternary structure.

Figure 3 :

 3 Figure 3: The typical J-V characteristics observed for the OPVs based on (♦) Pentacene/C60, (■) SubPc/C60, (•) Pentacene/SubPc and (▲) Pentacene/SubPc/C60 (the filled symbol denotes dark conditions; the empty symbol denotes light-AM1.5-conditions).

Figure 4 :

 4 Figure 4: The EQE spectra recorded for (■) SubPc/C60, (•) Pentacene/C60 and ( ▲ ) Pentacene/SubPc/C60.

Figure 5 :

 5 Figure 5: AFM images of the pentacene layer deposited onto (a) ITO/MoO3 and (b) ITO/MoO3/CuI.SEM images of the pentacene layer deposited onto (c) ITO/MoO3 and (d) ITO/MoO3/CuI.

Figure 6 :

 6 Figure 6: The optical density spectrum recorded for a thin film comprised of EH-IDTBR film deposited via sublimation under vacuum.

Figure 7 :

 7 Figure 7: The XPS spectra recorded for thin film and powdered EH-IDTBR.

Figure 8 :

 8 Figure 8: A schematic representation of the band structure of a contact ED/EA device.

Figure 9 :

 9 Figure 9: The band structure of the organic materials in aternary OPVs using Pentacene/SubPc/C60as anexample.

Figure 1 :

 1 Figure 1: The chemical structures of the small molecules investigated in this research study.

Figure 3 :

 3 Figure 3: The typical J-V characteristics observed for the OPVs based on (♦) Pentacene/C 60 , (■) SubPc/C 60 , (•) Pentacene/SubPc and (▲) Pentacene/SubPc/C60 (the filled symbol denotes dark conditions; the empty symbol denotes light-AM1.5-conditions).

Figure 4 :Figure 5 :

 45 Figure 4: The EQE spectra recorded for (■) SubPc/C 60 , (•) Pentacene/C 60 and (▲) Pentacene/SubPc/C 60 .

Figure 6 :

 6 Figure 6: The optical density spectrum recorded for a thin film comprised of EH-IDTBR film deposited via sublimation under vacuum.

Figure 7 :

 7 Figure 7: The XPS spectra recorded for thin film and powdered EH-IDTBR.

Figure 8 :Figure 9 :

 89 Figure 8: A schematic representation of the band structure of a contact E D /E A device.

Acknowledgments:

The authors acknowledge the funding received from the CNRST (PPR/2015/9) Ministère, Morocco).

Each analysis was performed from AFM images over a 5µm × 5µm area. The original data was plane-leveled to remove tilt by applying a numerical second-order correction. Particle analysis was carried out using the thresholding method (Gwyddion software).