Xaysatith Souliyavongsa
email: xaysatith1@hotmail.com

Alain Pierret

Vidhaya Trelo-Ges

Supat Isarangkool

Na Ayutthaya

Saysongkham Sayavong

Christian Hartmann

Does forest conversion to tree plantations affect properties of subsoil horizons? Findings from mainland Southeast Asia (Lao PDR, Yunnan-China)

Keywords: Acrisols, soil compaction, soil organic carbon, land use change, rubber tree plantation, teak tree plantation

While forest conversion to tree plantations in northern Laos is ongoing since the 1990s, its impact on soil properties is yet to be assessed. Our objectives were (i) to assess changes in soil properties, including subsoil horizons down to 160 cm, in <14-year-old teak and 21-yearold rubber tree plantations compared to paired forest soils, in Luang Namtha province, Laos;

(ii) review the recently published studies on the impact of tree plantations in neighboring Xishuangbanna, China; and (iii) to provide, based on (i) and (ii) broad recommendations for sustainable soil management in areas of northern Laos converted to tree plantations.

We analyzed selected chemical and physical properties of soil collected at 6 depths, from surface to a depth of 160 cm, along hillslopes, at the locations of 4 paired sites. All layers (0-Xishuangbanna reported comparable degradations, also limited to the upper layers, following conversion to rubber tree plantations. Some isolated studies mentioned some apparent improvements of soil properties, related to textural differences.

Overall, the only significant subsoil degradation consisted in N depletion (0-160 cm) under rubber plantations. Even though the impact of forest conversion on soil properties, up to two decades following tree plantation, appeared limited to the uppermost soil layers, the observed alteration of soil porosity may have an impact on runoff and erosion, inducing irreversible degradation of soil functions and productive potential. The subtle nature of such changes hampers adequate perception of their actual importance by farmers and stakeholders who likely have the false impression that soils are unaffected by forest conversion to tree plantations. Consequently, an enforcement of soil conservation practices is recommended when establishing commercial tree plantations on the naturally nutrient-depleted and low porosity Acrisols of this region.

160 cm) were very acidic pH (≤ 4.3) and depleted in exchangeable cations. Compared to their levels in the surface layer (0-10 cm), organic carbon content (21 g kg -1 ) and specific pore volume (0.68 cm 3 g -1 ) decreased by 62% and 44%, respectively at 40 cm, and by 81% and 53%, respectively at 110 cm. Forest conversion to tree plantations induced a decrease in pore volume and carbon content in the topsoil, but the subsoil remained unaffected. In rubber tree plantations, the nitrogen content dropped in all layers, including subsoil. In teak tree plantations, pH and phosphorus content increased in the deepest layers. Studies from

Introduction

The forests of Northern Laos are hotspots of biodiversity of both global and local importance.

In this landlocked country, a conjunction of factors including (i) a sparse population of shifting cultivation farmers, (ii) a rugged terrain, hampering the development of transport infrastructures and trade with the neighboring countries, and (iii) biodiversity legislation ((Ministry of Natural Resources and the Environment (MoNRE), 2016), contributed to the conservation of such natural environment [START_REF] Lestrelin | Territorialising Sustainable Development: The Politics of Land-use Planning in Laos[END_REF]). Nevertheless, since the 1980s deforestation, land conversion, extractive activities, hydropower, and forest encroachment have impacted this environment; the pace of changes increased in the 1990s with the introduction of commercial plantations of teak (Tectona grandis) and rubber tree (Hevea brasiliensis). The characteristics of rubber tree expansion have been largely documented [START_REF] Lu | Tapping into rubber: China's opium replacement program and rubber production in Laos[END_REF][START_REF] Sayavong | Recent co-occurring expansion of rubber tree plantations and land use/land cover change in Luangnamtha district, Northern Laos[END_REF][START_REF] Shi | Rubber boom in Luang Namtha: a transnational perspective, Rural Development in Mountainous Areas of Northern Lao PDR Component 1: Natural Resource Management and Local and Regional Economic Development[END_REF][START_REF] Vongvisouk | Falling Rubber Prices in Northern Laos: Local Responses and Policy Options[END_REF][START_REF] Xiao | Sentinel-2 red-edge spectral indices (RESI) suitability for mapping rubber boom in Luang Namtha Province, northern Lao PDR[END_REF]. Tree plantations of Northern Laos were mostly established on sloping forested land, prone to soil erosion, due to shortage of lowlands already used for rice and vegetable cultivation [START_REF] Brady | The nature and properties of soils[END_REF][START_REF] Singer | Soils: an introduction[END_REF]. While such sloping land often bears conspicuous signs of erosion (gullies, landslides, etc.), more concealed forms of soil degradation (soil compaction, acidification, carbon loss, nutrient depletion, etc.) may also occur. The extent and rate of soil degradation (whether transient of irreversible) resulting from forest conversion to tree plantation appears highly context-dependent, influenced by baseline soil properties (i.e. as observed under forests), slope, climate, management practices, etc. [START_REF] Vongsiharath | Forest cover and land-use changes in Lao PDR according to the National Forest Reconnaissance Survey (Country report)[END_REF]. A comparative study by [START_REF] Sang | Carbon sequestration and soil fertility of tropical tree plantations and secondary forest established on degraded land[END_REF] found that soil fertility of 17-year Acacia mangium plantations in Vietnam was similar to that of secondary forest and pasture. [START_REF] Ramesh | Effect of multipurpose tree species on soil fertility and CO2 efflux under hilly ecosystems of Northeast India[END_REF] reported that, in uplands of Northeast India, 26-year-old A. nepalensis and M. oblonga plantations improved soil fertility. On the other hand, in the uplands of Luang Prabang province, Laos, severe soil erosion was observed in teak tree plantations without understory [START_REF] Lacombe | Linking crop structure, throughfall, soil surface conditions, runoff and soil detachment: 10 land uses analyzed in Northern Laos[END_REF]. Soil fertility loss was also observed in Hainan (China), following 41 years of continuous rubber tree plantation [START_REF] Cheng | Variation of soil fertility and carbon sequestration by planting Hevea brasiliensis in Hainan Island, China[END_REF].

While forest conversion to tree plantations in Luang Namtha may have lifted local populations out from poverty and improved both local and national economies, its environmental impact, particularly on soil resources, was not yet assessed when we undertook this survey. Such appraisal is increasingly needed as the rapid economic development of Laos will likely come at the cost of further deforestation, following a pattern similar to what previously happened a few decades ago in neighboring Xishuangbanna, southern China.

Xishuangbanna and northern Laos share common physiographic features including comparable geomorphology, climate, flora and fauna. The forested area of this Chinese province dwindled with promotion of extensive rubber tree plantations by the Government of China in the late 1950s [START_REF] Li | Demand for rubber is causing the loss of high diversity rain forest in SW China[END_REF]. With increasing pressure on the soil resources of Xishuangbanna, research focusing on soil properties in rubber tree plantations gained momentum: in the past 6 years, 16 papers focusing on different aspects of soils in forest and tree plantations of this region appeared in international peer reviewed journals, including research on soil physical degradation (Liu et al., 2018;Nespoulos et al., 2019;[START_REF] Zhu | Effects of land-use changes on runoff and sediment yield: Implications for soil conservation and forest management in Xishuangbanna, Southwest China[END_REF], soil organic carbon [START_REF] Li | Estimating changes in soil organic carbon storage due to land use changes using a modified calculation method[END_REF][START_REF] Liu | Carbon Stocks across a Fifty Year Chronosequence of Rubber Plantations in Tropical China[END_REF][START_REF] Sun | Temporal Changes of Ecosystem Carbon Stocks in Rubber Plantations in Xishuangbanna, Southwest China[END_REF][START_REF] Yang | Land-use change impact on time-averaged carbon balances: Rubber expansion and reforestation in a biosphere reserve, South-West China[END_REF], soil respiration [START_REF] Goldberg | Soil respiration in sloping rubber plantations and tropical natural forests in Xishuangbanna, China. Agriculture[END_REF][START_REF] Wu | Soil respiration under three different land use types in a tropical mountain region of China[END_REF][START_REF] Zhao | Spatial and seasonal variation in soil respiration along a slope in a rubber plantation and a natural forest in Xishuangbanna, Southwest China[END_REF], spatial variability of soil characteristics [START_REF] De Blécourt | Spatial variability in soil organic carbon in a tropical montane landscape: associations between soil organic carbon and land use, soil properties, vegetation, and topography vary across plot to landscape scales[END_REF]Xia et al., 2016), soil improvement using intercrops [START_REF] Chen | Can intercropping with the cash crop help improve the soil physico-chemical properties of rubber plantations?[END_REF][START_REF] Li | Enhancement of soil carbon and nitrogen stocks by abiotic and microbial pathways in three rubber-based agroforestry systems in Southwest China[END_REF]Liu C.-A. et al., 2018;Liu C. et al., 2018).

It seemed interesting to take stock of all these results in order to verify whether there are general rules of evolution and whether they correspond to what can be observed in Laos.

In Northern Laos, given the potentially devastating environmental effects of wide-scale, poorly planned and managed conversion to tree plantations, both in terms of biodiversity [START_REF] Bremer | Does plantation forestry restore biodiversity or create green deserts? A synthesis of the effects of land-use transitions on plant species richness[END_REF][START_REF] Li | Demand for rubber is causing the loss of high diversity rain forest in SW China[END_REF]), soil (Liao et al., 2012;[START_REF] Yuen | Uncertainty in below-ground carbon biomass for major land covers in Southeast Asia[END_REF] and water resources (Ziegler 2009), it appears essential to support an informed decision-making process through the provision of environmental data. In this perspective, our study aimed to: (i) measure selected soil and soil characteristics in forested areas (F), 12-to 14-year-old teak (T) and 21-year rubber plantations (R) of plots located in Luang Namtha province with a special attention to the subsoil layers; (ii) review recent literature on the impact of tree plantations in neighboring Xishuangbanna, China, in an attempt to identify regional trends and finally, (iii) provide broad outlines of recommendations for future development of tree plantations and agricultural practices in Northern Laos.

Materials and methods

Site description

The study sites were located in Luang Namtha province, Northern Lao PDR, which borders Yunnan province (China) (Fig. S1). Luang Namtha is a predominately mountainous area with an average elevation > 500 m a.s.l., and dominant sloping land. Average annual rainfall and temperature of the past 20 years were 1,450 mm and 23°C, respectively (Fig. S2). The main land uses on the sloping areas were firstly mixed secondary deciduous forest (F) and secondly tree plantations that mainly consist of rubber trees (R) and teak trees (T); in the lowlands the main crop was paddy rice (Fig. S1).

We initially sought paired plots (forest v.s. tree plantations) within micro-watersheds in order to minimize differences in the underlying pedo-climatic conditions of each pair. We succeeded to find only three sites (A, B, C) for which the three land uses (F, R, T) were located within the same watershed, at distances <5 km from each other, although with different slopes. We failed to find a fourth site that presented similar conditions, a difficulty frequently encountered in ecological/environmental studies [START_REF] Davies | Don't let spurious accusations of pseudo replication limits our ability to learn from natural experiments (and other messy kinds of ecological monitoring)[END_REF]. For the fourth site (D), each land use was located in a separate micro-watersheds, different from that where sites A, B, C are located, more than 5 km apart and on different slopes. The location of sites A, B, C and D are presented in Fig. S1 and their environmental characteristics are summarized in Table 1.

On the Lao soil map, soils at the sampled locations were described as Ferric Acrisols (sites A & C) and Ferric Alisols (sites B & D) (Table 1). The similarity of sites B and D in terms of soils offered some perspective to test the relative importance of pedological factors compared to climate and slope. The fact that three studied land uses were much more scattered at site D than at the three other site also provided a means to assess the importance of the paired plots approach in this context.

We selected teak and rubber tree plantations owned by smallholder farmers. Teak tree plantations, 21 years of age at the time of sampling, were found almost exclusively on gentle slopes (ranging 16-27%; Table 1), in places where forests had been cleared in 1995. Trees were randomly planted in 1996, with an average -but quite variable -spacing of about 2.5 m corresponding to an average tree density of about 2,000 trees per ha. Plantations had an approximate surface area of 2,000, 5,000, 5,000, 3,000 m 2 at locations A, B, C and D, respectively. During the first 3 years following tree planting, farmers intercropped young teak trees with upland rice, controlling weeds using hand tools. Following this early phase, weeds were simply slashed to minimize competition with growing teak trees.

The rubber tree plantations, 12 to 14 years of age at the time of sampling, were established on plots that had been cleared from 2003 to 2005. Unlike teak tree plantations, rubber trees were established on steep slopes (slope gradient ranging 50-77%; Table 1). The reason for this specific positioning of rubber tree plantations within the landscape is that all land with gentle slope nearby villages had already been used and only steeper and higher land, further away from villages, remained available [START_REF] Sayavong | Recent co-occurring expansion of rubber tree plantations and land use/land cover change in Luangnamtha district, Northern Laos[END_REF]. Lacking access to machinery, farmers laid out very narrow terraces that consisted of small flattenings (~0.3-0.5 m wide) cut by hand at regular intervals along the slope, following contour lines, resulting in minimal soil disturbance. Tree spacing was 2.5 m along tree rows with an inter-row distance of 4 m. Rubber tree plantations selected were 12,000, 7,000, 9,000 and 5,000 m 2 at plots A, B, C, D, respectively. During the first 3 years of the plantations, trees were intercropped with rice, without fertilization; weeds were controlled using hand tools. After the first 3 years, weeds and coppices were controlled by means of herbicide applications until complete rubber tree canopy closure. Farmers started to harvest latex when trees reached 7 years of age and at the time of this survey, the trees were still harvested.

Forest study sites were all located on steep slopes (ranging 51-74%; Table 1). Even if information about previous deforestation has to be taken with caution [START_REF] Shi | Rubber boom in Luang Namtha: a transnational perspective, Rural Development in Mountainous Areas of Northern Lao PDR Component 1: Natural Resource Management and Local and Regional Economic Development[END_REF], the selected plots were described by local farmers as not having been deforested during at least

Soil sampling

Soil samples were collected on two successive occasions: first, in rubber tree plantations and forests in January 2017 (i.e. middle of the dry season, Fig. S2), then in teak tree plantations in June 2017 (i.e. early rainy season, Fig. S2). Composite soil samples were collected at distance >= 2 m from a randomly chosen tree, at up-, mid-and low-slope positions, at least 3 collection points were thus pooled, depending on the length of the slope. At each sampling point, disturbed soil samples were taken using a standard auger at 6 soil depth increments: 0-10, 10-20, 30-40, 60-70, 100-110 and 150-160 cm, hereafter referred to as: 10, 20, 40, 70, 110 & 160 cm. Bulk density samples were collected using a soil core sampler with a core volume of 100 cm 3 (5 cm in diameter and height) at all but the deepest soil depth increment (i.e. at 0-10, 10-20, 30-40, 60-70, 100-110 cm). Following collection, soil samples were transferred to air-tight plastic bags to prevent evaporation of soil water. The number of soil samples could vary depending on study sites due to time and constraints needed to reach the sites, collect samples and bring them back to our vehicle (Table 1).

Soil analysis

Analyses were performed at the soil laboratory of the Department of Agricultural Land Management (DALaM, Lao ministry of Agriculture). Soil samples were oven dried at 105 °C for 48 h to determine soil dry bulk density (BD) and water content (WC). From bulk density we calculated the specific pore volume (cm 3 g -1 ): specific pore volume = (1/BD) -1/PD = (1/BD) -0.377 BD is the bulk density (g cm -3 ), PD is the particle density i.e. the volumetric mass of the solid soil particles that was assumed to be 2.65 g cm -3 in the absence of high organic matter or oxide/hydroxides content. Subsequently, dried soil samples were sieved through a 2 mm mesh and stored in sealed plastic bags for laboratory analysis according to standard methods detailed in [START_REF] Pansu | Handbook of soil analysis: mineralogical, organic and inorganic methods[END_REF]. Soil pH was measured in 1:2.5 soil:water suspensions with a glass electrode pH meter (Jenway 3520). Organic carbon content (OC) was determined using the Walkley and Black method. Total nitrogen (N) was determined by Kjeldalh's method, while available phosphorus (P) was extracted following the Bray II method. Exchangeable potassium (K)

was determined by extraction with neutral 1N NH4OAc. Particle size analysis was carried out according to the pipette method after organic matter destruction using H2O2.

The soil organic carbon (SOC) stock was estimated using the equation: SOC (Mg ha -1 ) = 100 x BD x OC x thickness with 'BD' the soil bulk density (g cm -3 or Mg m -3 ), 'thickness' is the thickness of the soil layer (m), OC in mg g -1 .

Statistical analysis

After having checked normality and homogeneity of variance, we applied ANOVA (Analysis of Variance), to compare the mean values of the soil properties (clay content, BD, WC, pH, OC, N, P, K, Ca), for each soil depth increment, considering the 4 sites A, B, C and D as replicates. This was done using R software 3.6.1 (R Core Team, 2020) and Agricolae package [START_REF] Mendiburu | Agricolae: statistical procedures for agricultural research[END_REF].

Results

Soil organic carbon content

In forested areas, topsoil (0-10 cm) organic carbon (SOC) was 21 g kg -1 and decreased sharply with depth to 13 and 8 g kg -1 at 20 and 40 cm, respectively; in deeper layers, the SOC decreased at a lower rate, from 5 to 3 g kg -1 at 70 and 160 cm, respectively (Table 3).

Compared to forested areas, in rubber tree plantations, only the topsoil (0-10 cm) displayed significantly lower soil carbon content (reduction of 5 g kg -1 compared to forest; p<0.05). In teak tree plantations, two layers were significantly affected: at 0-10 cm and 30-40 cm depth, the soil carbon content reduction compared to forest being 7 g kg -1 and 3 g kg -1 , respectively.

Soil bulk density

In forest plots, soil bulk density was low at the surface (average: 0.99 g cm -3 ) and increased with depth (average: 1.44 g.cm -3 at depth ≥ 70 cm; table 2). Such an increase in bulk density equates to a loss of more than half of the total pore volume (i.e. from 0.68 cm 3 .g -1 to 0.32 cm 3 .g -1 ). Compared to the forested areas, rubber tree plantations did not induce any significant change in soil density. In contrast, teak tree plantations significantly (p<0.05) increased soil bulk density of the uppermost soil layers: the porosity was reduced by 40% at 10 cm, 33% at 20 cm and 16% at 40 cm depth. Tree plantations also homogenized the soil at the surface compared to the forest: the coefficient of variation of porosity in the top layer was 32% and 16% under the forested areas and the tree plantations, respectively.

Soil acidity

All forest soils studied were strongly acidic, with pH ranging from 4.1 to 4.3 at all depth.

Forest conversion to tree plantations did not have any significant impact on soil surface pH.

At depth, significant (p<0.05) differences were found only in teak tree plantations where pH increased (up to 4.5-4.8) at depths of 40 to 160 cm.

Plant nutrients

In the forested areas, N, P and K contents presented depth patterns similar to that observed for organic carbon, i.e. higher contents close to the soil surface with a steep decrease at 20 to 40 cm, and then a more gradual decrease down to 160 cm depth (Table 3). The C/N ratio decreased more regularly from 13 in the top layer to 6 at 160 cm. In contrast, exchangeable Ca as well as Mg and Na were low throughout the profiles, consistently with the observed acidic pH values, which under tropical climate, concur to induce nutrient leaching.

We only observed a significant (p<0.05) reduction in N content, along the whole profile, in places where forest had been converted to rubber tree plantations.

Conversion of forests to tree plantations did not appear to have impacted phosphorus profiles, as no significant differences were observed depth-wise between the phosphorus contents of the three land covers. In contrast, a significant decrease in potassium content was observed in the teak tree plantations, but only in the topsoil (-36% at 0-10 cm depth). Under rubber tree plantations the situation was different as a significant increase (p<0.05) was observed in two deep layers (+75% and +87% at 70 and 110 cm depth, respectively).

Discussion

Content and stocks of soil organic carbon

Soil organic carbon (SOC) profiles were similar to that reported for other tropical forests worldwide, i.e. with the highest contents observed in the top layer, a fast decrease in the upper layers (0-40 or 50 cm) and a slow decrease down to ≈150 cm, followed by very low but stable carbon contents over several meters [START_REF] Ngo | Carbon stocks in primary and secondary tropical forests in Singapore[END_REF].

Tree plantations had an impact in the upper (0-10 cm) layer: SOC decreased in both rubber and teak tree plantations by 25 and 37% respectively, compared to forest, which probably related to differences in above ground biomass or litter production. In the 10-20 cm layer no significant difference was observed between forest, rubber and teak, while in the 30-40 cm layer, SOC under teak trees was significantly lower compared to forest and rubber; no difference was observed in the deeper layers. According to [START_REF] Cusack | Soil carbon stocks across tropical forests of Panama regulated by base cation effects on fine roots[END_REF], fine root biomass is an endogenous factor controlling SOC, equally important as above ground or litter biomass. Lower fine root development under planted teak trees compared to rubber trees and forest could consequently explain such a localized difference.

In forested areas, the total soil carbon stock was 130.9 Mg ha -1 (table 4) approximately half of it (69.5 Mg ha -1 ) being located in the 0-40 cm layer and 90 % in the 0-110 cm layer. Forest conversion to tree plantations decreased the carbon stock of the top layer (0-10 cm) but increased that of the 10-20 cm layer, a counter-intuitive contrast explained by the dramatic increase in bulk density in the 10-20 cm layer observed in tree plantations. Over the whole soil profile (0-160 cm), forest conversion to rubber tree and teak tree plantations decreased carbon stocks by 4% (~5 Mg) and 15% (~20 Mg), respectively. This is a much more limited reduction than that reported by [START_REF] Van Straaten | Conversion of lowland tropical forests to tree cash crop plantations loses up to one-half of stored soil organic carbon[END_REF] who found that conversion of lowland tropical forest to cash crop plantations (including rubber trees) induced a loss up to one-half of the SOC, and that layers deeper than 1 m were affected. We also found that forest conversion to tree plantation did not alter the partitioning of the carbon stock: half being located in the 0-40 cm and half in the 40-160 cm volume. Our measurements of the impact of rubber tree plantations may be underestimated as the impact of slope cuts made to plant trees, which likely locally decreased SOC [START_REF] Bruun | Long rotation swidden systems maintain higher carbon stocks than rubber plantations[END_REF], was not taken into account.

Additional investigations should be conducted to assess whether future surveys should analyze separately cuts and undisturbed slopes.

Soil pH

pH is more sensitive to land use changes than clay for example, but in the absence of large chemical inputs, it varies slowly, especially in the deepest soil layers. A strongly acidic pH (4.1 to 4.3) was measured at all depths of the Luang Namtha forested areas which is in agreement with the characteristics of deeply weathered tropical soils. Such pH values less than 5.0 are known to induce aluminum toxicity [START_REF] Kochian | Cellular Mechanisms of Aluminum Toxicity and Resistance in Plants[END_REF], which in turn can limit microbial activity and plant development [START_REF] Fageria | Fertility management of tropical acid soils for ustainable crop production[END_REF]. Compared to forest, no significant change was observed in rubber tree plantations, when in teak tree plantations a significant (p<0.05) pH increase was measured at 40 cm and deeper layers. This increase was approximately 0.5 pH unit and could result from soil sampling made at a different period compared to the forest and rubber tree plantations. The absence of acidification indicates that tree plantations did not accelerate leaching processes nor nutrient exports [START_REF] Veldkamp | Deforestation and reforestation impacts on soils in the tropics[END_REF].

Soil bulk density

In Luang Namtha, soils under forest or planted trees were characterized by an increase in soil bulk density with depth: from soil surface to 160 cm, the pore volume decreased by ≈ 50%. Such a reduced porosity could be surprising as, at the same time, the clay content increased by ≈ 30% and, all other parameters being equal, clay content and porosity are positively correlated as clay is structurally associated with high microporosity (Chaudhari et al. 2013).

Thus, the decrease in the total porosity with depth is necessarily related to a decrease in macroporosity. This suggests a high and intense biological activity in the upper layers (macrofaunal and fine root density) and a dramatic decrease of this activity with depth [START_REF] Lal | Effects of macrofauna on soil properties in tropical ecosystems[END_REF]. These two characteristics were indeed observed during our field observations and are consistent with observations made elsewhere (e.g. [START_REF] Nanganoa | Impact of Different Land-Use Systems on Soil Physicochemical Properties and Macrofauna Abundance in the Humid Tropics of Cameroon[END_REF]. Following forest conversion to rubber tree plantations, soil bulk density was unchanged; but in the case of conversion to teak tree plantations, soil bulk density increased in the upper layers (0-40 cm). This lower impact of rubber compared to teak trees may be related to age differences: in the plots that we studied, rubber trees being approximately 10 years younger than teak trees, a slow soil collapse due to decreased biological activity may only become detectable after sufficient time passed by. But it could also result in differences in soil macrofauna activity and fine roots development, the later being in agreement with reduced SOC content under teak tree plantations. More compact subsoil layers are generally considered as a hampering root development but in tropical forest those layers contribute to deep water storage and can play an important role in maintaining moist conditions in the root zone of the forest trees [START_REF] Toriyama | Soil physicochemical properties and moisture dynamics of a large soil profile in a tropical monsoon forest[END_REF]. Throughout southeast Asia, planted trees, have been observed to develop deep root systems in compact subsoils [START_REF] Clément | Linking tree-rooting profiles to leaf phenology: a first attempt on Tectona Grandis Linn F[END_REF][START_REF] Legendre | Understanding deep roots and their functions in ecosystems: an advocacy for more unconventional research[END_REF].

However, the dramatic decrease in soil macroporosity that we measured in the top layer has potential to drastically limit water infiltration and groundwater recharge, while increasing surface runoff and erosion, as observed in Luang Prabang.

Plant nutrients

Nutrient profiles were similar to SOC profiles, with the highest contents found in the top layer and a fast decrease with depth. Even in the top layer, nutrient contents were low, in agreement with very acidic pH values. The most striking impact of tree plantations was the decrease in N content observed from soil surface to 160 cm depth under rubber tree plantations and that seems strongly related to nitrogen uptake corresponding to latex harvesting. At the same time, a significant increase in K was observed under rubber plantations at 70 and 110 cm, which remains unexplained. The decrease in the K content at soil surface in the teak plantations is most probably related to a decrease in litter biomass.

Comparison between Luang Natham and Xishuangbanna

Our literature review indicated that soil types similar to those found in Luang Namtha (ALUPC, 2008) were also present in Xishuangbanna with a higher diversity as Alisols [START_REF] Wu | Soil respiration under three different land use types in a tropical mountain region of China[END_REF]), Ferralsols (De Blécourt et al., 2014;[START_REF] Chen | Can intercropping with the cash crop help improve the soil physico-chemical properties of rubber plantations?[END_REF]Liu et al., 2018;Nespoulos et al., 2019;[START_REF] Sun | Temporal Changes of Ecosystem Carbon Stocks in Rubber Plantations in Xishuangbanna, Southwest China[END_REF] and Cambisols [START_REF] Zhu | Effects of land-use changes on runoff and sediment yield: Implications for soil conservation and forest management in Xishuangbanna, Southwest China[END_REF][START_REF] Zhu | Effects of forest transition on litterfall, standing litter and related nutrient returns: Implications for forest management in tropical China[END_REF]. This similarity suggests that comparing the two regions is valid, at least in the perspective of a rough, preliminary assessment. Deeper knowledge of clay mineralogy (in particular the presence of high and low activity clays) should however be gained for a more detailed comparison, as this is a parameter known to influence many soil properties and functions [START_REF] Veldkamp | Deforestation and reforestation impacts on soils in the tropics[END_REF].

Content and stocks of soil organic carbon

In the forested areas of Xishuangbanna (Fig. 1 bottom left, Table S2), the average SOC value was >30 g kg -1 and decreased with depth; at 75 cm, values close to 0 were reported. The average SOC reported in the surface layers was higher than that we measured in Luang Namtha. It is noteworthy that the range of SOC values reported in Xishuangbanna was considerable: in the topsoil (0-15 cm), the lowest and highest reported SOC were <30 g kg -1 (de [START_REF] De Blécourt | Soil Carbon Stocks Decrease following Conversion of Secondary Forests to Rubber (Hevea brasiliensis) Plantations[END_REF][START_REF] Li | Soil restoration potential with corridor replanting engineering in the monoculture rubber plantations of Southwest China[END_REF][START_REF] Liu | The conversion of tropical forests to rubber plantations accelerates soil acidification and changes the distribution of soil metal ions in topsoil layers[END_REF] and >60 g kg -1 , respectively [START_REF] De Blécourt | Spatial variability in soil organic carbon in a tropical montane landscape: associations between soil organic carbon and land use, soil properties, vegetation, and topography vary across plot to landscape scales[END_REF][START_REF] Wu | Soil respiration under three different land use types in a tropical mountain region of China[END_REF]. In some cases, such discrepancies may arise from inconsistent reporting of analytical results, most probably expressed as 'organic 'matter' (SOM), i.e. SOC multiplied by a constant value of approximately 1.7; an hypothesis also confirmed by the excessively high C/N ratios reported in some papers [START_REF] Wu | Soil respiration under three different land use types in a tropical mountain region of China[END_REF].

However, in other cases, SOC values remain surprisingly high, even if this potential reporting issue is accounted for: SOC of 35 and 38.6 g kg -1 were reported by [START_REF] De Blécourt | Spatial variability in soil organic carbon in a tropical montane landscape: associations between soil organic carbon and land use, soil properties, vegetation, and topography vary across plot to landscape scales[END_REF] and [START_REF] Wu | Soil respiration under three different land use types in a tropical mountain region of China[END_REF] respectively, i.e. 50 % higher than values reported by de Blécourt et al.

(2013), [START_REF] Chen | Can intercropping with the cash crop help improve the soil physico-chemical properties of rubber plantations?[END_REF], [START_REF] Li | Soil restoration potential with corridor replanting engineering in the monoculture rubber plantations of Southwest China[END_REF] and [START_REF] Li | Estimating changes in soil organic carbon storage due to land use changes using a modified calculation method[END_REF] and nearly 100% higher than that of [START_REF] Liu | The conversion of tropical forests to rubber plantations accelerates soil acidification and changes the distribution of soil metal ions in topsoil layers[END_REF]. Such a large dispersion within a small region is quite puzzling as all soils have been similarly classified, and the composition and biomass of the forest trees and litter should not be significantly different. One possible explanation could be insufficient replication.

The literature about forest conversion to rubber tree plantations in Xishuangbanna does not indicate any general trend regarding its impact on soil carbon content [START_REF] Chen | Can intercropping with the cash crop help improve the soil physico-chemical properties of rubber plantations?[END_REF][START_REF] Liu | The conversion of tropical forests to rubber plantations accelerates soil acidification and changes the distribution of soil metal ions in topsoil layers[END_REF][START_REF] Goldberg | Soil respiration in sloping rubber plantations and tropical natural forests in Xishuangbanna, China. Agriculture[END_REF][START_REF] Li | Soil restoration potential with corridor replanting engineering in the monoculture rubber plantations of Southwest China[END_REF][START_REF] Li | Estimating changes in soil organic carbon storage due to land use changes using a modified calculation method[END_REF], highlighting the need to document soil and environmental conditions as comprehensively as possible to enable interpretation of conflicting reports.

In the forested areas of Xhishuangbanna, [START_REF] Li | Soil restoration potential with corridor replanting engineering in the monoculture rubber plantations of Southwest China[END_REF] reported a stock of 33 Mg ha -1 in the 0-20 cm layer, i.e. a lower value than the 42 Mg ha -1 we found in Luang Namtha for the same soil depth range. In Luang Namtha, for the 0-40 cm layer, we found a value of ~70 Mg ha -1 , comparable to stocks of 61-63 Mg ha -1 in the 0-30 cm layer reported by [START_REF] Li | Estimating changes in soil organic carbon storage due to land use changes using a modified calculation method[END_REF] and [START_REF] Chen | Can intercropping with the cash crop help improve the soil physico-chemical properties of rubber plantations?[END_REF], but lower than values up to 117 Mg ha -1 reported by other authors [START_REF] Yang | Land-use change impact on time-averaged carbon balances: Rubber expansion and reforestation in a biosphere reserve, South-West China[END_REF][START_REF] De Blécourt | Soil Carbon Stocks Decrease following Conversion of Secondary Forests to Rubber (Hevea brasiliensis) Plantations[END_REF][START_REF] De Blécourt | Spatial variability in soil organic carbon in a tropical montane landscape: associations between soil organic carbon and land use, soil properties, vegetation, and topography vary across plot to landscape scales[END_REF]. The wide range of carbon stocks values reported in Xishuangbanna (61 to 117 Mg ha -1 ), while possibly reflecting extreme field variability, raises the issue of standardization and homogeneity of field and laboratory procedures, already pointed out nearly 10 years ago [START_REF] Yuen | Uncertainty in below-ground carbon biomass for major land covers in Southeast Asia[END_REF] and still apparently unresolved.

A wide range of carbon stock values were also reported for rubber plantations of Xishuangbanna; the lowest stock being 47 Mg ha -1 (Liu et al, 2018), intermediate stock of [START_REF] De Blécourt | Soil Carbon Stocks Decrease following Conversion of Secondary Forests to Rubber (Hevea brasiliensis) Plantations[END_REF], Li et al, 2020;[START_REF] Sun | Temporal Changes of Ecosystem Carbon Stocks in Rubber Plantations in Xishuangbanna, Southwest China[END_REF][START_REF] Yang | Land-use change impact on time-averaged carbon balances: Rubber expansion and reforestation in a biosphere reserve, South-West China[END_REF]) and the highest stock being of 113.9 Mg ha -1 [START_REF] Li | Estimating changes in soil organic carbon storage due to land use changes using a modified calculation method[END_REF], i.e. over twice the lowest stock.

≈60 Mg ha -1 (
Only a few papers compared the carbon stocks under forest and tree plantations: some reports indicate a decrease associated with forest conversion to tree plantation [START_REF] De Blécourt | Soil Carbon Stocks Decrease following Conversion of Secondary Forests to Rubber (Hevea brasiliensis) Plantations[END_REF][START_REF] Yang | Land-use change impact on time-averaged carbon balances: Rubber expansion and reforestation in a biosphere reserve, South-West China[END_REF], while others did not find any change [START_REF] Chen | Can intercropping with the cash crop help improve the soil physico-chemical properties of rubber plantations?[END_REF], but also moderate to dramatic stock increases [START_REF] Li | Soil restoration potential with corridor replanting engineering in the monoculture rubber plantations of Southwest China[END_REF][START_REF] Li | Estimating changes in soil organic carbon storage due to land use changes using a modified calculation method[END_REF]. This review on carbon stocks conducted in a small region, confirms that consistent conclusions and predictions of SOC and C stocks require a sufficient number of replicates and should be based on standard procedures of sampling as well as standard procedures of soil analysis.

Soil pH

In Xishuangbanna, all soil layers under forested areas were strongly acidic with slightly higher mean values compared to Luang Namtha (pH=4.7 to 4.8). In tree plantations, average pH slightly increased in deeper layers as was observed in teak tree plantations of Luang Namtha. It is noteworthy that several surveys reported a pH decrease in the top layer of tree plantations compared to that in forested areas [START_REF] Liu | The conversion of tropical forests to rubber plantations accelerates soil acidification and changes the distribution of soil metal ions in topsoil layers[END_REF][START_REF] Goldberg | Soil respiration in sloping rubber plantations and tropical natural forests in Xishuangbanna, China. Agriculture[END_REF][START_REF] De Blécourt | Soil redistribution by terracing alleviates soil organic carbon losses caused by forest conversion to rubber plantation[END_REF][START_REF] De Blécourt | Spatial variability in soil organic carbon in a tropical montane landscape: associations between soil organic carbon and land use, soil properties, vegetation, and topography vary across plot to landscape scales[END_REF][START_REF] Wu | Soil respiration under three different land use types in a tropical mountain region of China[END_REF]Nespoulos et al., 2019;[START_REF] Li | Soil restoration potential with corridor replanting engineering in the monoculture rubber plantations of Southwest China[END_REF], even if they also mentioned a marginal pH increase at depth. Cation lixiviation from soil surface and accumulation in deeper layers could explain such characteristics. In both Luang Namtha and Xishuangbanna, pH changes induced by forest conversion to tree plantation appear limited and, anyway, insufficient to impact soil biological or geochemical functioning.

Bulk density

Bulk density values from Xishuangbanna forests are presented in Fig. 3 (top left) and table S1. It is noteworthy that, in most papers (i) data concerned the topsoil layers (0-20 cm) and there is a paucity of data below 50 cm, (ii) one or two depth increments only were investigated and (iii) different numbers of replicates were used to characterize different layers (Table S2). The linear regression of bulk density vs soil depth indicates that the lowest mean values are observed at the surface (1.05 g cm -3 ) and increase with depth (1.3 g cm -3 at 75 cm).

Even if the general trend is the same as in Lung Namtha, a wider range of values is observed in each layer, corresponding to three main patterns: (i) loose soil throughout the profile (M and B), (ii) compact soil throughout the profile (A, E, G and H), and (iii) intermediate bulk density throughout the profile (C and L). These differences could result from textural differences between different areas of Xishuangbanna but they could equally result from different analytical procedures. These large differences observed within a small region highlight the need for inter-site comparisons to ensure that standard procedures have been used.

In Xishuangbanna, conversion of forested areas to tree plantations increased soil bulk density in the upper layers (0 to ≈30 cm) when the subsoil remained unchanged (Fig. 3, top right), i.e. a trend similar to that observed in Luang Namtha, but with a larger diversity of situations. As in Luang Namtha, no heavy machinery was used, the increased bulk density following conversion to tree plantations probably resulted from a significant soil biological activity reduction. A few papers reported lower bulk densities following conversion to tree plantations (C, E, L down to 20, 15 and 50 cm, respectively). This apparent soil improvement could result from textural differences, i.e. more clayey horizons in the tree plantation areas (Nespoulos et al, 2019).

Nutrients

In the forested areas of Xishuangbanna, the few reported N contents were either similar [START_REF] Zhu | Effects of forest transition on litterfall, standing litter and related nutrient returns: Implications for forest management in tropical China[END_REF][START_REF] Li | Soil restoration potential with corridor replanting engineering in the monoculture rubber plantations of Southwest China[END_REF] or much higher [START_REF] Wu | Soil respiration under three different land use types in a tropical mountain region of China[END_REF] than values we measured in Luang Namtha. For other elements, comparison between the results in Xishuangbanna and Luang Namtha were not possible because of the use of variable analytical methods in different surveys.

Conclusion

Overall, the results of our soil survey in Luang Namtha province, Laos, showed that (i) forest conversion to tree plantations mostly affected the porosity and carbon content of the topsoil and (ii) neither rubber nor teak tree plantations resulted in soil acidification. When considering the full 160 cm depth of the soil profiles, the decrease in carbon stocks compared to forest was lower in the rubber compared to the teak tree plantations: 4% (≈ 5 Mg) and 15% (≈20 Mg), respectively. Additionally, under rubber plantations, N was depleted throughout the whole soil profile (0-160 cm).

Our review of recent soil surveys in Xishuangbanna did not allow us to generalize our results: while general trends for some properties were similar in the two regions, published observations proved very diverse, difficult to compare and often contradictory; subsoil properties were rarely reported. Further studies on soil degradation in this area should therefore (i) be based on more standardized procedures (survey design, sampling depth, analytical methods, statistical analysis, etc..) , (ii) take into account pedo-geomorphological parameters that can have substantial impact on conclusions (clay content and type).

The limited level of soil degradation that we observed might be related to the fact that soils of the studied area are naturally very acidic, nutrient-depleted with a compact subsoil (i.e. with very few, if any, macropores). Our work is also indicative that time-scales to be considered to be able to detect changes in subsoil properties induced by land use change may be more than one or two decades. Nevertheless, the fact that we found that the most consistently predictable change in soil properties associated with forest conversion to tree plantations is, in the short term, topsoil compaction, is in itself valuable with regards to soil management practices. Indeed, such topsoil compaction is known to have major impacts on water balance and soil erosion, through decreased infiltration and increased runoff. Therefore, when converting forest to tree plantations, implementing practices that maintain topsoil porosity, such as, e.g. conserving a densely rooted and diverse understory and a healthy soil macrofauna, by means of agroforestry / intercropping designs, appears essential and were also strongly supported by several experiments recently conducted in Xishuangbanna.

Declaration of Competing Interest

The authors declare they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Table 1. Location of the 4 sites and description of their position, main environmental characteristics and number of replicates for each of the 3 land cover (forest, rubber and teak tree plantations).

Tables captions

Table 2. Soil physical properties at 6 various depths and under forest, rubber tree and teak tree plantations in Luang Namtha district (Lao PDR).

Table 3. Soil chemical and physico-chemical properties at 6 various depths and under forest, rubber tree and teak tree plantations in Luang Namtha district (Lao PDR).

Table 4. Profiles of soil carbon stocks under forest, rubber and teak tree plantations. av is the mean value, sd the standard deviation, cv(%) is the coefficient of variation in percentage, n the number of replicates was presented in Table 1. Different letters along the same column and at the same depth, indicate significant differences (P<0.05).

Figure captions

Table 3. Soil chemical and physico-chemical properties at 6 various depths and under forest, rubber tree and teak tree plantations in Luang Namtha district (Lao PDR).

Depth pH (1:2.5) SOC (g kg -1 ) N (g kg -1 ) C/N P (mg kg -1 ) K (mg kg -1 ) Ca (mg kg SOC is soil organic carbon. av is the mean value, sd the standard deviation, cv(%) is the coefficient of variation in percentage, n the number of replicates was presented in Table 1. Different letters along the same column and at the same depth, indicate significant differences (P<0.05). 'per layer' indicates the stock for the layer of the thickness indicated in the second column; 'cumul.' is for the cumulated stock from the soil surface to the depth indicated in the first column; 'ratio' is for the ration between the cumulated value and the total carbon stock from soil surface to 160 cm depth.

  30 years. At plot A, forest (F) was a protected area of the village surrounded by rubber plantations. At plot B, F was part of the protected Luang Namtha catchment (set up for the provision of clean water to the town). Plots C and D were located within the boundaries of the Nam Ha National Protected Area, in relation with their protected status and recognized outstanding biodiversity (national protected area (NPA) since 1995; ASEAN Heritage Park since 2003; UNESCO-Lao 'Nam Ha Ecotourism Project' since 2006, awarded the 'Equator Prize' from United Nations Development Programme (UNDP).

Fig. 1 .

 1 Fig. 1. Xishuangbanna region soil properties: bulk density (top), pH (middle) and organic carbon

Fig. 1 .

 1 Fig. 1. Xishuangbanna region soil properties: bulk density (top), pH (middle) and organic carbon content (bottom) at different depths, under forest (left) and tree plantations (right) in 16 recently published papers. The letter indicates the publication from which the data are coming from; the straight line is an estimate of the average trend (linear model) and the grey area represents the limit of confidence interval.

Table 1 .

 1 Location of the 4 sites and description of their position, main environmental characteristics and number of replicates for each of the 3 land cover (forest, rubber and teak tree plantations).

	Sites	Village	Dist.* (km)	Dir.*	Soil type**	Slope (%) Forest Rubber Teak	Number of replicates Forest Rubber Teak
	A	Phinhor	7	NW Acrisols	74	77	20	6	9	4
	B	Namdee	3	N	Alisols	64	65	16	3	3	4
	C	Chalernsouk	12	W	Alisols	67	68	25	3	3	4
	D	Sopsim	15	E	Acrisols	51	50	27	4	3	4

Table 2 .

 2 Soil physical properties at 6 various depths and under forest, rubber tree and teak tree plantations in Luang Namtha district (Lao PDR).

		Depth									BD				Pore			WC
		(cm)	Clay (%)			Silt (%)			Sand (%)		(g cm -3 )				(cm 3 g -1 )			(g g -1 )
			av	sd cv(%)	av	sd cv(%)	av	sd	av	sd	cv(%)		av	sd	cv(%)	av	sd
	Forest	10	28	6	21	24	4	19	49	9 19	0.99 0.22	22	b	0.68	0.22	32 a	0.35	0.08
		20	33	8	24	22	4	20	45	11 23	1.11 0.17	15	b	0.54	0.13	24 a	0.30	0.06
		40	37	7	19	20	4	17	42	9 22	1.34 0.15	11	b	0.38	0.08	21 a	0.26	0.05
		70	40	8	19	18	4	21	42	11 25	1.44 0.13	9		0.32	0.06	19	0.25	0.06
		110	40	10	25	16	4	25	43	11 26	1.43 0.09	6		0.32	0.04	13	0.24	0.06
		160	36	12	35	16	4	26	48	14 30	-	-			*	*		0.20	0.06
	Rubber	10	31	8	25	19	4	21	50	8 15	1.09 0.11	10	b	0.55	0.09	16 a	0.42	0.06
		20	36	9	24	18	4	25	46	7 16	1.21 0.11	9	b	0.45	0.07	16 a	0.34	0.05
		40	42	9	21	17	5	28	41	9 23	1.35 0.09	7	b	0.37	0.05	14 a	0.31	0.03
		70	45	10	22	16	4	25	40	9 23	1.41 0.07	5		0.33	0.03	9	0.30	0.03
		110	45	10	23	15	5	35	40	11 27	1.42 0.04	3		0.33	0.02	6	0.29	0.03
		160	41	13	32	15	3	24	44	13 29	-	-			-	-		0.26	0.05
	Teack	10	34	15	45	28	5	17	38	17 44	1.29 0.12	9	a	0.41	0.07	17 b	0.32	0.06
		20	38	16	43	26	3	12	36	17 48	1.35 0.12	9	a	0.36	0.06	17 b	0.27	0.04
		40	40	19	46	22	3	13	35	18 52	1.43 0.04	3	a	0.32	0.02	6 b	0.26	0.05
		70	44	19	44	22	3	12	32	19 59	1.45 0.15	10		0.32	0.07	22	0.26	0.08
		110	43	19	44	23	6	24	34	19 55	1.47 0.19	13		0.31	0.09	29	0.26	0.07
		160	40	17	43	22	6	29	38	18 48	-	-			-	-		0.23	0.05

Table 4 .

 4 Profiles of soil carbon stocks under forest, rubber and teak tree plantations.

	Depth	Thickness		Forest			Rubber			Teak
			per			per
	(cm)	(cm)	layer	cumul.	ratio	layer	cumul.	ratio	per layer cumul.	ratio
	10	0.10	21.2	21.2	0.16	17.9	17.9	0.14	17.4	17.4	0.16
	20	0.15	20.8	42.0	0.32	21.4	39.3	0.31	21.7	39.1	0.35
	40	0.25	27.5	69.5	0.53	27.0	66.3	0.53	19.3	58.4	0.52
	70	0.35	26.2	95.7	0.73	25.2	91.5	0.73	19.3	77.7	0.70
	110	0.45	23.2	118.8	0.91	23.6	115.1	0.92	21.8	99.5	0.89
	160	0.30	12.0	130.9	1.00	10.2	125.3	1.00	11.9	111.4	1.00

Acknowledgements

Funding

This study was supported by the EcoRubber-JAEI project, a cooperation project between the Institute Research for Development (IRD), France, Khon Kaen University (KKU, Thailand), the National University of Laos (NUOL, Lao PDR) and the Department of Agricultural Land Management (DALaM, Lao PDR)

analysis data; XS carried out the data collection and sample analyses; SS provided information about location of the different land cover; AP, CH and XS analyzed interpreted the results.

It would not have been possible to complete this study without cooperation of different institutions. Therefore, all co-authors would like to express their deep gratitude to the Institute Research for Development (IRD, France), and the JEAI-EcoRubber, the National University of Laos (NUoL), and Khon Kaen University (KKU, Thailand) for financial and technical support. We would also like to thank local Lao public officers in Luang Namtha who provided invaluable support during our field work, as well as farmers and villagers who provided information and gave the permission to work in their field. Finally, we would like to express our gratitude to Ms. Phonethip Banouvong who was in charge of all administrative and logistical aspects of this work, both in the office and in the field. XS sincerely thanks the Department of Agricultural Land Management (DALaM, Lao PDR) for providing access to equipment and analytical facilities needed to complet his PhD work. All co-authors would like to express their gratitude to native English-speaking colleagues who checked the quality of the English language and to the anonymous reviewers who provided many relevant suggestions that helped making this paper more concise, informative and readable.


Amélie Schultheiss
email: amelie.schultheiss@gmail.com

Viet Abderrahime Sekkat

Huong Nguyen

Alexandre Carella

Anass Benayad

Amélie Revaux

Renaud Demadrille

David Muñoz-Rojas
email: david.munoz-rojas@grenoble-inp.fr

Jean-Pierre Simonato
email: pierre.simonato@cea.fr

┴ Viet

High performance encapsulation of transparent conductive polymers by spatial atomic layer deposition

Keywords: poly(3, 4-ethylenedioxythiophene), PEDOT, SALD, aging, encapsulation, degradation

INTRODUCTION

Conductive polymers have a wide range of applications including in organic electronics, photovoltaics, energy storage, and smart biomaterial devices. [START_REF] Das | Review on Conducting Polymers and Their Applications[END_REF][START_REF] Balint | Conductive polymers: Towards a smart biomaterial for tissue engineering[END_REF][START_REF] Zhan | Conductive polymer nanocomposites: a critical review of modern advanced devices[END_REF][START_REF] Nezakati | Conductive Polymers: Opportunities and Challenges in Biomedical Applications[END_REF][START_REF] Fan | PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications[END_REF][START_REF] Ouyang | Recent Advances of Intrinsically Conductive Polymers[END_REF] In electronic and optoelectronic devices, poly (3,4-ethylenedioxythipohene) (PEDOT), one of the most promising conductive polymers, balances conductivity and transparency just as well as ITO (indium tin oxide), with the additional benefits of scalable processing, low cost, and flexibility. [START_REF] Eom | Polymer solar cells based on inkjet-printed PEDOT:PSS layer[END_REF][START_REF] Lu | Si/PEDOT:PSS core/shell nanowire arrays for efficient hybrid solar cells[END_REF][START_REF] Thomas | High-efficiency hybrid solar cells by nanostructural modification in PEDOT:PSS with co-solvent addition[END_REF][START_REF] Thomas | Defect-Minimized PEDOT:PSS/Planar-Si Solar Cell with Very High Efficiency[END_REF][START_REF] Jin | Efficient, large area ITO-and-PEDOT-free organic solar cell sub-modules[END_REF][START_REF] Sharma | Alternative transparent conducting electrode materials for flexible optoelectronic devices[END_REF][START_REF] Yvenou | Spray-coated PEDOT:OTf films: thermoelectric properties and integration into a printed thermoelectric generator[END_REF][START_REF] Schultheiss | Water content control during solution-based polymerization: a key to reach extremely high conductivity in PEDOT thin films[END_REF][START_REF] Schultheiss | Electrical and Mechanical Properties of Intrinsically Flexible and Stretchable PEDOT Polymers for Thermotherapy[END_REF] However, one of the main drawbacks of this material, and of organic materials in general, is their poor stability under environmental stress (high humidity, solar radiations). [START_REF] Schultheiss | Insight into the Degradation Mechanisms of Highly Conductive Poly(3,4ethylenedioxythiophene) Thin Films[END_REF] The development of techniques to improve the stability of polymeric conductive films is therefore crucial. For PEDOT, several encapsulation films and methods have been evaluated in this context. For example, Chen et al. extended the half-life of PEDOT more than 10-fold by capping it with poly(divinylbenzne-co-maleic anhydride, PDVB-MA), [START_REF] Chen | Conformal single-layer encapsulation of PEDOT at low substrate temperature[END_REF] although the latter did still degrade over time. Spatial atomic layer deposition (SALD), an alternative to conventional ALD, has shown promise in device integration since it was first published in 2008. [START_REF] Levy | Stable ZnO thin film transistors by fast open air atomic layer deposition[END_REF] In this straightforward and scalable method, reactive precursors are separated in space rather than in time, allowing metal oxide films to be deposited under ambient conditions at remarkably high rates. [START_REF] Poodt | Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition[END_REF][START_REF] Muñoz-Rojas | Spatial atmospheric atomic layer deposition: a new laboratory and industrial tool for low-cost photovoltaics[END_REF][START_REF] Muñoz-Rojas | Spatial Atomic Layer Deposition (SALD), an emerging tool for energy materials. Application to newgeneration photovoltaic devices and transparent conductive materials[END_REF][START_REF] De La Huerta | Gas-Phase 3D Printing of Functional Materials[END_REF][START_REF] Nguyen | Impact of precursor exposure on process efficiency and film properties in spatial atomic layer deposition[END_REF][START_REF] Sekkat | Open-air printing of Cu2O thin films with high hole mobility for semitransparent solar harvesters[END_REF] Metal oxide films are efficient encapsulating coatings since they protect the underlying layer from UV radiations and act as a barrier against water and oxygen. Aluminum oxide (Al2O3) has interesting gas diffusion barrier properties, [START_REF] Choi | Three-dimensionally stacked Al2O3/graphene oxide for gas barrier applications[END_REF] , [START_REF] Groner | Gas diffusion barriers on polymers using Al 2 O 3 atomic layer deposition[END_REF] while zinc oxide and titanium dioxide (ZnO and TiO2) are commonly used as UV filters. [START_REF] Schneider | A review of inorganic UV filters zinc oxide and titanium dioxide[END_REF] , [START_REF] Smijs | Titanium dioxide and zinc oxide nanoparticles in sunscreens: Focus on their safety and effectiveness[END_REF] Gas barrier oxides are useful for sunlight protection because materials are generally sensitive to the combination of radiation (mainly UV) and water vapor or oxygen. [START_REF] Schultheiss | Insight into the Degradation Mechanisms of Highly Conductive Poly(3,4ethylenedioxythiophene) Thin Films[END_REF] For example, metal oxide films have successfully been used to improve the stability of silver and copper nanowire networks under harsh conditions. [START_REF] Khan | Stability Enhancement of Silver Nanowire Networks with Conformal ZnO Coatings Deposited by Atmospheric Pressure Spatial Atomic Layer Deposition[END_REF] , [START_REF] Celle | Oxidation of copper nanowire based transparent electrodes in ambient conditions and their stabilization by encapsulation: Application to transparent film heaters[END_REF] The effects of metal oxide coatings on organic materials have not been so well studied because it is difficult to deposit a protective layer uniformly and conformally on ultrathin polymeric substrates without damaging them. [START_REF] Ali | High rate roll-to-roll atmospheric atomic layer deposition of Al2O3 thin films towards gas diffusion barriers on polymers[END_REF][START_REF] Maydannik | Roll-to-roll atomic layer deposition process for flexible electronics encapsulation applications[END_REF][START_REF] Ling | Ultra-thin ALD-AlOx/PEDOT:PSS hole selective passivated contacts: An attractive low cost approach to increase solar cell performance[END_REF] Further requirements for the deposition technique are that it should be cost-effective, high-throughput, low-temperature, vacuum-free and compatible with roll-to-roll processing while still yielding high-quality, pinhole-free coatings. SALD thus appears as an ideal technique to apply protective oxide coatings to organic devices. Indeed, Mutee Ur Rehman et al. enhanced the lifetime of PEDOT:PSS in an organic sensor thanks to an Al2O3 layer, applied by SALD. [START_REF] Mutee Ur Rehman | Significance of encapsulating organic temperature sensors through spatial atmospheric atomic layer deposition for protection against humidity[END_REF] Similarly, Choi et al. and Maydannik et al. used an Al2O3 layer as a gas barrier to improve the water vapor transmission rate (WVTR) of organic displays. [START_REF] Maydannik | Spatial atomic layer deposition: Performance of low temperature H 2 O and O 3 oxidant chemistry for flexible electronics encapsulation[END_REF][START_REF] Choi | Fast spatial atomic layer deposition of Al 2 O 3 at low temperature (<100 °C) as a gas permeation barrier for flexible organic light-emitting diode displays[END_REF] In this study, we evaluated the performance of several multilayer structures as encapsulation layers for PEDOT:triflate (PEDOT:OTf, see Figure S1), PEDOT:sulfate/hydrogen sulfate (PEDOT:Sulf, the sulfuric treated version of PEDOT:OTf, with higher conductivity but lower stability) and PEDOT:polystyrene sulfonate, post-treated with ethylene glycol (PEDOT:PSS-EG), as reference product. We focused on their stability under solar radiations because of the widespread use of PEDOT in photovoltaics, [START_REF] Mehmood | Review on recent advances in polythiophene based photovoltaic devices[END_REF][START_REF] Seo | Progress in photovoltaic performance of organic/inorganic hybrid solar cell based on optimal resistive Si and solvent modified poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) junction[END_REF][START_REF] Dauzon | Stretchable and Transparent Conductive PEDOT:PSS-Based Electrodes for Organic Photovoltaics and Strain Sensors Applications[END_REF][START_REF] Zhang | The Role of Mineral Acid Doping of PEDOT:PSS and Its Application in Organic Photovoltaics[END_REF] and because these materials are known to be particularly susceptible to radiation damage (mainly because of chain scission, degradation of counter-anions and crystallinity). [START_REF] Schultheiss | Insight into the Degradation Mechanisms of Highly Conductive Poly(3,4ethylenedioxythiophene) Thin Films[END_REF] Given their intended use in optoelectronic devices, the PEDOT films were prepared in thin layers of around 30 nm for at least 85% total transmittance (knowing that the haze factor of these materials is less than 1%), and a sheet resistance of around 150 Ω.sq -1 . The polymer films were protected with layers of Al2O3, ZnO, and/or TiO2. [START_REF] Resende | Planar and Transparent Memristive Devices Based on Titanium Oxide Coated Silver Nanowire Networks with Tunable Switching Voltage[END_REF] The novelty of this work lies in the deposition of metal oxides on organic layers under ambient conditions (temperatures below 200°C, without need of vacuum chamber), using SALD.

Moreover, we used the superposition of several (two or three) oxide layers to combine their advantages. [START_REF] Aghazadehchors | Versatility of bilayer metal oxide coatings on silver nanowire networks for enhanced stability with minimal transparency loss[END_REF] The effects of the precursors on the polymeric films, notably on the doping level, were studied by UV-Vis-NIR spectroscopy and soft and hard X-ray photoelectron spectroscopy (S-XPS and HAXPES).The oxide layers were then tested alone and in combination to encapsulate the different PEDOT films under sunlight radiations.

EXPERIMENTAL SECTION

2.1. Materials 3,4-ethylenedioxythiophene (EDOT), N-methyl-2-pyrrolidone (NMP), isopropanol (IPA), iron(III) trifluoromethanesulfonate (Fe(OTf)3), polyethylene glycol -polypropylene glycolpolyethylene glycol (PEG-PPG-PEG) Mw = 5800 g/mol and ethylene glycol (EG) were obtained from Sigma-Aldrich. Anhydrous ethanol was purchased from CARLO ERBA Reagents.

PEDOT:PSS CLEVIOS TM PH1000 was purchased from Heraeus. Glass substrates (7.5 × 2.5 cm², 0.96 to 1.06 mm thick) were purchased from Corning.

PEDOT deposition

PEDOT:OTf and PEDOT:Sulf films were prepared as follows: Fe(OTf)3 was dissolved at 0.126 mg/mL in a solution containing 72.25 wt % of ethanol, 20.00 wt % of PEG-PPG-PEG, and 7.75 wt % of NMP. The solution was sonicated for 2 h, then 0.25 mL of this solution was mixed with 5 μL of EDOT before being spin-coated onto the substrate (Glass or PEN), at 4500 rpm for 30 s. The resulting films were heated on a hot plate at 70°C before being rinsed with ethanol and dried again at 70 °C, yielding to PEDOT:OTf. To prepare PEDOT:Sulf, PEDOT:OTf film was dipped in 0.1 M sulfuric acid for 30 min before being dried on a hot plate at 120 °C for 30 min, to allow substitution of a large part of the triflate anions by sulfate or hydrogen sulfate groups.

PEDOT:PSS-EG films were prepared as follows: commercial PEDOT:PSS (Clevios PH1000) was mixed with IPA (50:50 vol). The surface was covered by the solution and spincoated at 1000 rpm for 1 min, with an acceleration of 500 rpm/s. The film was heated at 120 °C for 10 min on a hot plate, then dipped in EG for 30 min before being rinsed in ethanol and dried once more on a hot plate at 120 °C for 30 min.

To measure the electrical resistance of the films, gold electrodes were deposited at the edges of square samples (2.5×2.5 cm²) so that the resistance values obtained were equivalent to the sheet resistance of the film in Ω.sq -1 .

SALD of metal oxide layers

The TiO2 thin films were deposited using a custom-built SALD system. [START_REF] Nguyen | Increasing the Electron Mobility of ZnO-Based Transparent Conductive Films Deposited by Open-Air Methods for Enhanced Sensing Performance[END_REF] Titanium tetrachloride (TiCl4) and water vapor were carried to the manifold head at flow rates of 10 sccm for TiCl4 and 100 sccm of H2O, diluted in 25 and 100 sccm of N2, respectively. Nitrogen was used as an inert gas barrier with a total flow rate of 450 sccm. The temperature of the substrate was maintained at 150 °C. The distance between the substrate and the SALD injection head was approximately 150 µm with a scanning speed of 10 cm/s. The precursor for the Al2O3 layers was trimethylaluminium (TMA), which was carried at 15 sccm with 150 sccm H2O, diluted in 235 and 225 sccm of N2, respectively. The N2 barrier total flow was 750 sccm. Different deposition temperatures (100, 150 and 200 °C) were tested and the number of deposition cycles was varied from 10 to 90, to adjust the thickness of the films from 9 to 81 nm. Diethylzinc precursor (C2H5)2Zn (DEZ) was used to deposit ZnO, using a flor of 30 sccm with 150 sccm of H2O, respectively diluted in 270 and 300 sccm of N2. The N2 barrier total flow was 900 sccm. The deposition temperature was varied between 50 °C and 200 °C and the number of deposition cycles was varied from 10 to 60 to obtain thicknesses between 5 and 55 nm.

Characterizations

Soft-X-ray photoelectron spectra were recorded using a Versaprobe II ULVAC-PHI spectrometer. A monochromatic beam (X-ray source, Al Kα, 1486.6 eV; diameter, 100 μm; power, 50 W) was focused on the surface of the samples. High-resolution spectral analyses were performed using a pass energy of 23 eV, corresponding to a resolution of 0.5 eV. All XPS measurements were carried out in an ultrahigh vacuum chamber (7x10 -10 mbar). The spectrometer was calibrated using the photoemission lines of gold (Au 4f7/2 = 84.0 eV, with reference to the Fermi level). Dual charge neutralization was used to avoid any eventual charging effect induced at the surface of the samples. All XPS spectra binding energies were corrected using the C1s line of carbon sp 3 at 285.0 eV. As the S-XPS is a surface analysis technique (depth analyses of 5 nm). To probe deeper (~20 nm) into the films, HAXPES data were recorded on a PHI-Quantes spectrometer. A monochromatic beam (X-ray source, Cr Kα, 5414.9 eV) of 100 μm of diameter and 50 W was used. High-resolution spectral analyses were performed using a pass energy of 69 eV, corresponding to a resolution of 0.8 eV. Depth profiling analyses were performed using an argon Gas Cluster Ion Beam (GCIB), at 8 eV/atom with a current density of 5 nA/mm 2 , which was used instead of a conventional Ar ion beam to avoid chemical destruction during depth profiling.

Resistances were measured at room temperature (T= 20-25°C, for relative humidity RH≈ 40%) with a two-probe ISO-TECH IDM19 multimeter. UV-vis-NIR spectra were recorded on a Varian Cary 5000 device. Sun aging experiments were performed using an ABET Sun 3000 solar simulator calibrated at 1000 W/m².

RESULTS AND DISCUSSION

We first studied the effects of SALD on the electrical performance of PEDOT:OTf. ZnO, Al2O3 and TiO2 where thus deposited at an initial temperature of 150 °C (due to the organic nature of PEDOT) and 10 to 90 deposition cycles (corresponding to layer thicknesses of 9-81 nm), to preserve high optical transparency. Figure 1a shows how the resistance of the polymeroxide systems varied over time under 1 sun (1000 W/m²) illumination, using the measurement setup shown in Figure 1b.

For Al2O3, the initial R/R0 ratio ranged from 1.81 after 10 SALD cycles to 2.46 after 90 (Figure S2a), possibly because of the reducing effect of the trimethylaluminum (TMA)

precursor. [START_REF] Juppo | Trimethylaluminum as a reducing agent in the atomic layer deposition of Ti (Al) N thin films[END_REF] Increasing the thickness of the alumina layer does not drastically increase the resistance of the composite, presumably because reduction only occurs at the interface and also acts as a protective gas barrier. The overall transmittance does not decreases linearly as the thickness increases because the change in density of the film affects its refractive index. Overall, the protective effect of alumina does not compensate for the initial deterioration in electrical properties.

With ZnO, while the increase in resistance for the thinnest deposited layer was similar to that observed for Al2O3, R/R0 varied substantially with the thickness of the deposited layer (sevenfold higher when 30 deposition cycles were used instead of 10, as displayed in Figure S2b). The diethylzinc precursor ((C2H5)2Zn, DEZ) is reactive, [START_REF] Hoye | Perspective: Maintaining surface-phase purity is key to efficient open air fabricated cuprous oxide solar cells[END_REF] as is TMA, but contrary to Al2O3, ZnO is less efficient as a gas barrier (being mostly a UV filter), and therefore does not afford the same protection that Al2O3 does as the number of deposition cycles is increased. To reduce the reactivity of DEZ on PEDOT, ZnO was also deposited at a lower temperature (50 °C), but the resulting coatings provided less protection than did the thinnest layer deposited at 150 °C.

At 50 °C indeed, the deposited layers are probably not dense or crystalline enough to provide effective UV protection. [START_REF] Bedia | Morphological and Optical Properties of ZnO Thin Films Prepared by Spray Pyrolysis on Glass Substrates at Various Temperatures for Integration in Solar Cell[END_REF][START_REF] Nguyen | Deposition of ZnO based thin films by atmospheric pressure spatial atomic layer deposition for application in solar cells[END_REF] Figure 1a shows that aside from the initial alteration of the PEDOT:OTf due to the deposition process, the samples protected by a thin layer of ZnO (10 SALD cycles at 150 °C) remained remarkably stable, with a R/R0 slope of just 0.0005 h -1 compared with 0.0096 h -1 for the unprotected sample, equating to a 19-fold decrease in the degradation rate.

The precursor for TiO2, TiCl4 leads to the formation of HCl as by-product during deposition. Acids have been shown to increase the electrical conductivity of PEDOT, [START_REF] Fan | Transfer-Printed PEDOT:PSS Electrodes Using Mild Acids for High Conductivity and Improved Stability with Application to Flexible Organic Solar Cells[END_REF][START_REF] Bießmann | Highly Conducting, Transparent PEDOT:PSS Polymer Electrodes from Post-Treatment with Weak and Strong Acids[END_REF] even in the gas phase. [START_REF] Fallahzadeh | Ultra-smooth poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) films for flexible indium tin oxide-free organic light-emitting diodes[END_REF] Here, TiO2 deposition was indeed observed to slightly increase the electrical conductivity of PEDOT:OTf, even for the thickest layers (63 nm, 90 SALD cycles, Figure S2c).

The HCl(g) by-product presumably leads to the release of chloride counter ions which have a doping effect on the PEDOT. Surprisingly however given the moderate gas barrier and UV filter properties of TiO2, these layers did not reduce the degradation rate of PEDOT:OTf under solar radiations (Figure 1a), indicating that TiO2 is not effective in this context when used alone. The optical and material properties of the encapsulated films were investigated to better understand their behavior. Figure 2a shows the visual aspect of the films (from light blue to light purple) after deposition of the three metal oxides. Changes in color generally indicate a de-doping effect, from oxidized to neutral chains. [START_REF] Massonnet | Improvement of the Seebeck coefficient of PEDOT:PSS by chemical reduction combined with a novel method for its transfer using free-standing thin films[END_REF] The UV-Vis-NIR spectroscopy results for the different systems, displayed in Figure 2b, show large variations in absorbance. For the ZnO and Al2O3 systems in particular, the broad feature that appears between 700 and 1200 nm indicates the presence of less oxidized PEDOT chains, [START_REF] Zozoulenko | And Absorption Spectroscopy of PEDOT[END_REF] which must be an effect of metal oxide deposition. Furthermore, all the coated samples have higher absorption than bare PEDOT:OTf in the blue/violet visible region (wavelengths below 500 nm), while in in the visible-infrared region, the absorbance of the coated films is more variable and generally lower than that of bare PEDOT:OTf. In order to further understand the individual effect of the oxide layers on the PEDOT:OTf, we performed soft-and hard-XPS (respectively S-XPS and HAXPES) measurements. The chemical impact of the Al2O3, ZnO and TiO2 deposited films was studied through the variation of sulfonate/thiophene ratio (extracted from S2p S-XPS spectra) within the PEDOT:OTf film, with respect to a reference bare sample (Figure 3a-d). The attribution and corresponding fits of the different sulfur doublet peaks in the S2p3/2-1/2 spectra (163.5 -164.7 eV for thiophene; 168.0 -169.2 eV for sulfonate) were taken from Gueye et al. [START_REF] Gueye | Structure and Dopant Engineering in PEDOT Thin Films: Practical Tools for a Dramatic Conductivity Enhancement[END_REF] In S1s spectra (from HAXPES measurements) the peak at 2470.9 eV was assigned to thiophene groups and the peak at 2477.5 eV to sulfonate groups. The main difference between S-XPS and HAXPES is that the penetration depth with HAXPES is much greater. The penetration depth in XPS varies with the photoelectron mean free path (λ) and take-off angle (θ, here 45°) as 3λ•sinθ. The mean free path can be estimated using the Tanuma, Powell and Penn TPP2M formula implemented in the QUASES-IMFP-TPP2M software. [START_REF] Tanuma | Calculations of electron inelastic mean free paths. V. Data for 14 organic compounds over the 50-2000 eV range[END_REF][START_REF] Tougaard | QUASES-IMFP-TPP2M[END_REF] For S-XPS the estimated penetration depths, based on the inelastic electron mean free path of S2p electron are ~6.6, 5.3 and 5.4 nm for respectively Al2O3, ZnO and TiO2 encapsulation layers. For HAXPES, the estimation of the penetration depth of S1s signal is 12.4, 9.9 and 10.3 nm depth, for Al2O3, ZnO and TiO2 respectively. The strong increases observed in the sulfonate/thiophene ratio (by S-XPS), from 0.28 without oxide to 0.55 with Al2O3 and 0.44 with ZnO (Figure 3a-c), for samples coated with a thin layer (1-2 nm thick, two SALD cycles) to be able to probe the PEDOT:OTf layer, show that Al2O3 and ZnO are highly damaging for PEDOT:OTf. For PEDOT:OTf/TiO2 (Figure 3d) the same effect was observed, but the sulfonate/thiophene ratio only increased to 0.36. Depth profiling of the PEDOT:OTf/TiO2 sample, using gas cluster ion beam (GCIB) etching, revealed the presence of chloride (from HCl) in the TiO2 layer (as reported previously [START_REF] Moitzheim | Chlorine Doping of Amorphous TiO2 for Increased Capacity and Faster Li+-Ion Storage[END_REF]) and at the interface with PEDOT:OTf (Figure 3.e). This is important because it confirms that TiCl4 promotes chloride doping at the interface, potentially increasing the conductivity of the PEDOT:OTf films. In agreement with the conductivity variations observed after oxide deposition, these data indicate that the precursors (DEZ, TMA and TiCl4) react with the PEDOT thin film during SALD. Moreover, the S-XPS measurements performed during abrasion show that the sulfonate/thiophene ratio (i.e. the doping level) of the PEDOT:OTf film decreases with increasing depth (from 0.28 to 0.14 after 8 min of GCIB etching; Figure 3f). Since the conductivity of PEDOT-based materials increases with the level of oxidation, [START_REF] Shi | Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review[END_REF] this means that the outer surface (5 nm) of the PEDOT:OTf film is more oxidized than the bulk. HAXPES measurements (with a greater penetration depth) were also performed on samples treated with 10 cycles of SALD and therefore more representative to the systems used for the aging tests. Figure 4a-d shows the S1s spectra obtained. As expected, the average doping level (sulfonate/thiophene ratio) of the PEDOT:OTf reference sample is lower when calculated from these HAXPES data (0.19 -Figure 4a, representative to the bulk doping) compared to the S-XPS experiment (0.28 -Figure 3a, representative to the outer surface doping). For the PEDOT:OTf/Al2O3 film, the increase in the sulfonate/thiophene ratio due to deposition of a thicker oxide layer is less significant (0.44, Figure 4b, versus 0.55, Figure 3b), because Al2O3 only damages a few nanometers of the PEDOT:OTf film and acts as a gas barrier thereafter. For the PEDOT:OTf/ZnO sample conversely, the increase in the sulfonate/thiophene ratio is greater after more SALD cycles (0.62, Figure 4c, versus 0.44, Figure 3c). In the case of PEDOT:OTf/TiO2, the sulfonate/thiophene ratio was measured at 0.28 (Figure 4d) versus 0.36 after just two SALD cycle (Figure 3d), indicating, that the effect of TiO2 deposition is limited to the extreme outer surface of the PEDOT:OTf sample. This is in agreement with the etching results, as shown in Figure 3e.

It is worth noting that the difference between the sulfonate/thiophene ratios measured in the reference PEDOT:OTf sample by S-XPS and HAXPES only reflects the intrinsically different doping levels at the surface and in the bulk of the film. For the encapsulated systems, the variation of sulfonate/thiophene ratio is mainly due to side reactions during oxide deposition that lead to the degradation of the thiophene ring in the PEDOT chains. These results are consistent with the data presented in Figure 1. Altogether, the results indicate that the only oxide layer that does not increase the resistance of the underlying PEDOT:OTf film during SALD is TiO2, while only Al2O3 and ZnO offer effective encapsulation. Since TiO2 acts as a gas barrier, we investigated whether it might protect the PEDOT films from the damaging effects of the other metal oxide layers. Bilayer PEDOT:OTf/TiO2/ZnO and PEDOT:OTf/TiO2/Al2O3 constructs were therefore manufactured and tested as a means to achieve effective protection against solar radiations, without altering the initial resistance of the underlying film during SALD. Based on previous results, the TiO2 layer was deposited with 30 cycles (≈20 nm thick) at 150 °C. Figure 5a shows that both bilayer architectures (TiO2/ZnO and TiO2/Al2O3, Figure 5b) provide effective and stable protection under 1 sun illumination, with little increase in resistance either initially or over time. The resistance of the PEDOT:OTf/TiO2/ZnO sample increased slightly at the very start of the experiments, then increased gradually with a R/R0 gradient of about 0.0013 h -1 and an overall increase of just 2.5-fold over 800 h of sun radiations (compared with a 9-fold increase for the reference film), using the best conditions for ZnO (20 SALD cycles at 200 °C, see Figure S3 for a list of all deposition conditions). For PEDOT:OTf/TiO2/Al2O3 in contrast, the resistance increased sharply in the first 24 h, before leveling off and remaining virtually constant (R/R0 gradient of about 0.0006 h -1 ) for the rest of the experiments. In summary, TiO2/ZnO encapsulation leads to practically no initial change in the resistance, but the increase over time is about twice as fast as measured for the TiO2/Al2O3 system. ).

This FoM has the dimension of a time (h) and corresponds to the delay required for the ratio R(t)/R0 to reach the value + , with a higher FoM corresponding to better protected system. When there is no initial degradation, the value + is equal to 2 (because R0 = Rstab) and the FOM corresponds then to the time to double the resistance R0. Different FoM curves are represented in Figure 6 as dotted lines. The FoM of the non-encapsulated sample is 100 (see Figure 6b), which means that the resistance doubles in just 100 h, compared with 500 h for the TiO2/Zn system. Looking directly at the scatter plot of FoM values for the different systems (Figure 6a), it is clear that several systems provide effective sun protection. The two bilayer encapsulations offer comparable levels of protection and are on the same FoM line, but not in the same part of FoM space. The pink rectangle in Figure 6a corresponds to samples with extremely low degradation rates (ΔR/R0*1h sun-aging) but potentially high initial degradation (Rstab/R0), while the horizontal part (blue rectangle) corresponds to a assemblies with moderate stability and almost no initial degradation. Trilayer protection with TiO2, Al2O3, and ZnO was tested to see whether the benefits of TiO2/Al2O3 and TiO2/ZnO -low initial degradation and extremely high stability -could be combined, but the resulting protective effect was inferior to that of the two component bilayers (Figure S4). The likely explanation is delamination, which was visible on the surface of the samples, probably due to interface problems, as is classically observed for multilayer systems. [START_REF] Hu | Defect structures at thin film-substrate interfaces[END_REF][START_REF] Dwarakanath | Vapor phase infiltration of aluminum oxide into benzocyclobutene-based polymer dielectrics to increase adhesion strength to thin film metal interconnects[END_REF][START_REF] Abedini | Mechanisms and control of edge interfacial delamination in a multilayer system containing a functionally graded interlayer[END_REF] Further optimization would be required for this system.

The bilayer TiO2/ZnO coating, with little initial degradation and good UV protection, was also tested on other PEDOT films having different counter-anions: PEDOT:PSS-EG and PEDOT:Sulf (Figure S5a,b). The results for PEDOT:PSS-EG are similar to those described above for PEDOT:OTf, with deposition leading to a small amount of degradation and the slope of the resistance profile under solar illumination divided by 6 compared with the unprotected film.

For PEDOT:Sulf in contrast, although the equilibrium rate of degradation under UV radiation was lower than measured for the bare sample, this did not compensate for the strong initial degradation (R/R0 < 4.5). This difference in behavior compared with the other PEDOT films might be due to the surface reactivity of the already highly doped PEDOT:Sulf with the TiCl4 precursor (used to deposit TiO2). These results show that to optimize their protective effect, metal oxide coatings have to be adapted specifically to the chemistry of the underlying polymer film.

CONCLUSION

This work demonstrates the potential of metal oxides deposited using Spatial Atomic Layer Deposition, an ambient atmosphere and low-cost technique, as UV protective layers for organic PEDOT-based films. Bilayer TiO2/ZnO and TiO2/Al2O3 protections are shown for the first time here to substantially decrease the degradation rate of the underlying film, while limiting deposition related damage. The combination of TiO2 and ZnO was found to be particularly effective for both PEDOT:OTf and PEDOT:PSS. XPS and UV-Vis-NIR measurements show that the precursors for ZnO and Al2O3 react strongly with PEDOT materials, with a negative effect on their electrical performance. The decrease in the sulfonate/thiophene ratio measured by XPS suggests these reactions probably lead to PEDOT degradation. In contrast, the TiO2 precursor has a doping effect on the PEDOT films (due to the HCl formed during deposition) and enhances their conductivity. Overall, this work proves that SALD of metal oxides is an efficient method to preserve the electrical performance of conductive polymers used in optoelectronic applications.

Novel bilayer structures (TiO2/ZnO and TiO2/Al2O3) were found to offer the most effective protection and may also be of interest for many other organic materials.

ACKNOWLEDGEMENT

Figure 1 .

 1 Figure 1. (a) Time evolution of the resistance of different PEDOT:OTf systems under 1 sun

Figure 2 .

 2 Figure 2. (a) Photographs of bare PEDOT:OTf and PEDOT:OTf encapsulated with Al2O3, ZnO

Figure 3 .

 3 Figure 3. S2p XPS spectra of (a) PEDOT:OTf, (b) PEDOT:OTf/Al2O3, (c) PEDOT:OTf/ZnO

Figure 4 .

 4 Figure 4. S1s HAXPES spectra of (a) PEDOT:OTf, (b) PEDOT:OTf/Al2O3, (c)

Figure 5 .

 5 Figure 5. (a) Time evolution of the resistance under 1 sun illumination (1000 W/m²) of

Figure

  Figure 6a compares all the systems in terms of Rstab/R0, plotted as a function of the slope

Figure 6 .

 6 Figure 6. Comparative protective performance under 1 sun illumination of bare PEDOT:OTf