Gerardo Terán-Escobar,
Jonas Pampel,
José M. Caicedo and
Mónica Lira-Cantú*
Catalan Institute of Nanoscience and Nanotechnology (ICN2), Campus UAB, Building ICN2, Bellaterra, Barcelona, E-0193, Spain. Consejo Superior de Investigaciones Cientificas (CSIC), Campus UAB, Building ICN2, Bellaterra, Barcelona, E-0193, Spain. E-mail: monica.lira@cin2.es; Fax: +34 937373606; Tel: +34 937374615
First published on 4th September 2013
Layered V2O5 hydrate has been applied as the hole transport layer (HTL) in organic solar cells (OSCs). V2O5 is obtained from a sodium metavanadate solution in water under ambient conditions, resulting in a final thin film of formula V2O5·0.5H2O. The 0.5 water molecules are not removed from the V2O5 layered structure unless the sample is heated above 250 °C, which makes the thin film highly stable under real working conditions. The HTL was used in OSCs in the normal and the inverted configurations, applying metallic Ag as the back-metal electrode in both cases. Fabrication of both OSC configurations completely by solution-processing printing methods in air is possible, since the Al electrode needed for the normal-configuration OSC is not required. The work function (WF) and band gap energy (BG) of the V2O5 thin films were assessed by XPS, UPS and optical analyses. Different WF values were observed for V2O5 prepared from a fresh V2O5–isopropanol (IPA) solution (5.15 eV) and that prepared from a 24 h-old solution (5.5 eV). This difference is due to the gradual reduction of vanadium (from V5+ to V4+) in IPA. The OSCs made with the V2O5 thin film obtained from the 24 h-old V2O5–IPA solution required photo-activation, whereas those made with the freshly obtained V2O5 did not. Outdoor stability analyses of sealed OSCs containing a V2O5 HTL in either configuration revealed high stability for both devices: the photovoltaic response at T80 was retained for more than 1000 h.
Broader contextOrganic Solar Cells (OSCs) have achieved an impressive increase in power conversion efficiency in the past few years, with values above the 12% range. Yet, in order to be competitive with existing energy sources from fossil fuels and modern inorganic photovoltaic technologies, OSCs must reduce fabrication costs and improve its energy payback time (EPBT). To achieve the latter, the fabrication of OSCs by large scale, solution processing methods applying inexpensive, low temperature techniques is required. An important aim is the exclusion of toxic organic solvents, being water-based or alcohol-based solutions is highly desired. In this work, a layered V2O5 hydrate has been applied as the hole transport layer in stable OSCs. V2O5 is obtained from the dissolution of sodium metavanadate in water under ambient atmospheric conditions, resulting in a final thin film with the V2O5·0.5H2O formula. OSCs with normal and inverted configuration applying metallic Ag as the back metal electrode in both cases have been fabricated. The use of a Ag electrode eliminates the need for a highly reactive work function metal electrodes (Al, Ca) for the normal configuration OSC, and permits the fabrication of both OSC configurations completely by solution processing printing methods in air. Outdoor stability analyses of sealed devices showed high stability, maintaining the photovoltaic response at T80 for more than 1000 h. |
Transition metal oxides (TMOs) have been employed in organic solar cells, especially TiO2 and ZnO. Their most attractive feature is the possibility to be deposited by low temperature solution processing methods. Among them are also V2O5,12–21 NiO,22–30 MoO3,31–35 WO336–39 or Sb2O3.40 These TMOs exhibit a wide range of energy level alignments,41–44 good transparency as thin films, are easy to manipulate, and confer low-resistance ohmic contacts to the OSC.45 Moreover, TMOs can enhance the adhesion to the active layer8,16 and show higher stability to ambient atmosphere relative to PEDOT:PSS, which has been shown to be detrimental for OSCs due to its high hygroscopicity and its acidic pH.46–50 Additionally, the high power conversion efficiency requirement for future OSCs rely on small-molecule OSCs (SmOSCs)39,40,51,52 and multi-junction or tandem solar cell (TmOSC) structures, whose PCE values presently range from 10–12%.5,10,53–56 One interesting TMO is V2O5, which has been reported to be a good candidate for the HTL in OSCs. To date, V2O5 HTLs have been synthesised chiefly by multistep techniques. Examples include the suspension of V2O5 nanoparticulates obtained from the hydrolysis of vanadium(III) acetyl acetate19 or the fabrication of a bronze V2O5 HTL from a suspension of the metal oxide obtained after the reaction between the metal powder and H2O2.21 Among the most widespread fabrication methods is the application of sol–gels made from vanadium(V) oxytriisopropoxide (ViPr),5,14,15,57,58 which is a compound known for its high toxicity, reactivity and cost.
Herein we present the synthesis, optimisation and application of water-based, solution processable V2O5 as the HTL in OSCs. The HTL was fabricated at low temperature in air without the need for any high temperature post-deposition treatments or multistep reactions. The water-based layered V2O5 hydrate is highly compatible with the fabrication of OSCs by large-area, low-cost, fast processing and high-throughput printing,5,15 and also enables the preparation of ZnO/V2O5 recombination layers required for TmOSCs.59 We demonstrate here that the application of the low-temperature water-based V2O5 solution can be tuned in order to fabricate OSCs in either the inverted or the normal configurations, on either glass or flexible substrates. Moreover, we have fabricated OSCs with both configurations applying only Ag as the back metal electrode. Thus, our OSCs can be made completely by solution-processing methods, as they do not require an Al electrode for the normal-configuration OSCs10 and the Ag metal electrode can be deposited by established solution-processing printing techniques.11 A careful optimisation of the V2O5 hydrate solution permitted to obviate the requirement for photo-activation of the solar cell in air.19 Finally, the OSC devices also show good outdoor stability maintaining T80 for more than 1000 h.
The V2O5 film used in this work was prepared by spin coating and the substrate was then treated at 120 °C for several minutes before being applied to the OSCs. The final formula of the thin film is V2O5·0.5H2O (the number of water molecules was calculated from the TGA analyses performed in air). Once the films have been prepared, the water molecules can only be removed if the film is heated above 250 °C. However, under real working conditions the OSCs will never reach those temperatures and therefore, the water in the V2O5 interlayer can be considered stable.
Fig. 1 shows the photovoltaic response of inverted organic solar cells with the glass/FTO/TiO2/PCM:P3HT/V2O5/Ag configuration, depending on the V2O5 concentration. For the fabrication of inverted devices, the deposition of V2O5 on top of the active P3HT:PCBM layer requires mixing of V2O5 hydrate with isopropanol (IPA) to improve adherence. The optimum ratio of V2O5 to IPA was found to be 1:1 (as determined by contact angle measurements; see ESI Fig. S1†). The solution obtained was then spin-coated at 1000 rpm in an ambient atmosphere, and finally, heated at 120 °C for 5 min. There is a clear improvement on the photovoltaic response of the device (ca. in FF, Jsc and PCE) when the concentration of the oxide is increased from 2 mg mL−1 to 9 mg mL−1, as observed in Fig. 1. The PCE and FF increase and ultimately stabilise at 3% and 50%, respectively. As the FF increased, the Jsc also stabilised at a V2O5 concentration of 9 mg mL−1. Increasing the photovoltaic response by increasing the concentration of V2O5 above these values was not achievable due its limited solubility. Thus, the optimal value chosen for fabrication of the inverted OSCs was 9 mg mL−1. An increase in the thin film layer thickness was also observed when raising the concentration of the oxide from 2 mg mL−1 to 9 mg mL−1, with values ranging from 60 nm to 125 nm (as measured by SEM and profilometry, Fig. 1e). The possibility to fabricate thick film layers without compromising the photovoltaic performance of the device (i.e. by increasing series resistance) has also been observed by Brabec et al.19 In our case, we attributed this response to the mixed ionic-electronic conductivity that characterises the V2O5·0.5H2O thin film. Finally, Fig. 1f shows the atomic force microscopy (AFM) image, at a 2 mm × 2 mm scan size, of the thin film V2O5 made with a solution concentration of 9 mg mL−1. A nanostructured surface with high surface roughness is observed for the thin film.
Fig. 1 Optimisation of the concentration of the V2O5 hydrate solution used to create the hole transport layer in an inverted organic solar cell (glass/FTO/TiO2/P3HT:PCBM/V2O5/Ag). (a) PCE (%) and (b) Jsc (mA cm−2). Measurements made at 100 W cm−2 AM1.5G. (c) Voc and (d) FF (%), (e) layer thickness vs. V2O5 concentration and (f) AFM analyses of the V2O5 thin film made with a concentration of 9 mg mL−1. |
To compare the performance of our V2O5 HTLs with that of the most widely used HTL, PEDOT:PSS, we fabricated and assessed OSCs of both configurations. The devices were prepared on glass/FTO substrates (the fabrication of flexible OSCs applying the V2O5 HTL is also possible, see ESI Fig. S2†). Fig. 2 shows the IV curves and the IPCE spectra obtained for the devices. In all cases the ETL was ZnO. Table 1 shows the photovoltaic parameters obtained for the different OSCs; for comparison purposes, we have also included solar cells containing a TiO2 ETL. The reported values are mean values from six samples. The best photovoltaic performance (PCE: 3%) was generally observed for the OSC fabricated on glass/FTO substrates, with TiO2 as the ETL and V2O5 hydrate as the HTL. In this case, the OSCs employing the V2O5 hydrate resulted in better performance when compared to PEDOT:PSS. The OSCs with ZnO and V2O5 showed a very similar response with photovoltaic PCEs of ca. 2.5 to 2.6%. Our results indicate that a similar response can be achieved for OSCs with layered V2O5 hydrate when compared to the PEDOT:PPS HTL.
Fig. 2 IV curves (a) and the corresponding IPCE spectra (b) for the inverted configuration organic solar cell in glass/FTO/ZnO/P3HT:PCBM/V2O5/Ag and, for comparison purposes, similar cells containing PEDOT:PSS instead of V2O5. Measurements were taken at 100 mW cm−2 AM1.5G. |
Inverted configuration | Voc (V) | Jsc (mA cm−2) | FF (%) | PCE (%) |
---|---|---|---|---|
Glass/FTO/TiO2/P3HT:PCBM/PEDOT:PSS/Ag | 0.557 ± 0.015 | 9.84 ± 0.43 | 45.43 ± 2.74 | 2.53 ± 0.17 |
Glass/FTO/TiO2/P3HT:PCBM/V2O5/Ag | 0.563 ± 0.015 | 10.69 ± 0.38 | 50.49 ± 1.90 | 3.09 ± 0.18 |
Glass/FTO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag | 0.543 ± 0.013 | 10.07 ± 0.37 | 45.06 ± 1.16 | 2.64 ± 0.12 |
Glass/FTO/ZnO/P3HT:PCBM/V2O5/Ag | 0.540 ± 0.016 | 9.54 ± 1.10 | 47.20 ± 1.90 | 2.58 ± 0.22 |
Fig. 3 Organic solar cells withV2O5 hydrate as the hole transport layer in the inverted configuration (glass/FTO/TiO2/P3HT:PCBM/V2O5/Ag). Photovoltaic response of the cells fabricated from a freshly prepared (1) or a 24 h-old (2) V2O5–IPA solution. Using the fresh solution obviates the need for photo-activation of the device in air, as shown in the IV-curves and IPCE spectra from (2) to (3). Measurements were taken at 100 mW cm−2 AM1.5G. |
Since the only difference in the fabrication of the two OSCs was the V2O5 HTL, we attributed the need for photo-annealing to the V2O5 thin film properties that are probably affected by the interaction between V2O5 with IPA.62,66,67 Layered vanadium(V) oxides in their hydrated state tend to accommodate foreign molecules in their interlayer region,64,66–68 including organic compounds such as alcohols.69 Alcohols intercalate, via their –OH group, at the polar site of V2O5. In this partially reversible reaction the H2O molecules of V2O5 hydrate are exchanged with alcohol molecules, leading to the reduction of V2O5 from V+5to V+4. V2O5 reduces relatively quickly when in solution with organic molecules, as indicated by a gradual change in the colour of the solution from red (indicative of V5+) to green (indicative of the reduction of V5+ to V4+). Thus, the thin film obtained from the 24 h-old V2O5–IPA solution could be partially reduced and therefore photo-annealing is required in order to eliminate the undesirable shunts and inflection points (S-shape IV curve) and to achieve maximum power conversion efficiency.4,70,71,80
To understand the changes that we observed in the OSCs as a function of the freshness of the V2O5–IPA solution, we characterised the V2O5 thin films by XPS, UPS and optical analyses.76–78 Fig. 4a shows the XPS spectra of the V2O5·0.5H2O thin films fabricated from the fresh (red) and 24 h-old (green) solutions. The binding energy (BE) values of the main peaks and their assignment are detailed in Table 2.
Fig. 4 XPS (a), band gap (b) and UPS (c) spectra of the V2O5·0.5H2O thin film obtained from freshly prepared (red) or 24 h-old (green) V2O5–IPA solution. |
Peak | Fresh (red) | 24 h-old (green) | Assignment |
---|---|---|---|
V2p3/2 | 516.20 | 516.20 | V4+ |
V2p3/2 | 517.20 | 517.20 | V5+ |
V2p1/2 | 524.75 | 524.70 | V5+ |
O 1s | 529.95 | 529.90 | O2− |
O 1s | 533.20 | — | H2O |
XPS analyses revealed only slight differences in the intensity of the spectra between the two films (see ESI Fig. S3†). Despite these small differences, the two thin films gave very similar XPS results: the main peaks of V2p3/2 and V2p1/2 were almost identical. The characteristic peaks of V2O5 are observed at 517 eV and 524 eV (corresponding to V5+), and at 529.9 eV (the O 1s from the O2− ions). The XPS plot was subject to a Lorentzian–Gaussian fitting: the region of the V5+ peak at 517 eV reveals a shoulder at ca. 516 eV. This peak is attributed to the presence of V4+, which is commonly observed in the hydrated form of V2O572 as well as in reduced films.73 But, it is not present in a crystalline V2O5 film that has been subjected to thermal evaporation or annealed at high temperatures, as these procedures eliminate all water.72 This shoulder at 516 eV has also been observed by Ziberberg et al., in the XPS analyses of a V2O5 thin film (10 nm) obtained from vanadium(V)-oxytriisopropoxide (ViPr). However, they attributed the presence of the V4+ peak to air exposure and not to any possible organic residues from the ViPr (despite having observed residual carbon by XPS).
The calculated composition analyses of the films show that V4+ accounts for a very small amount (less than 10% of total V), indicating that both thin films are partially reduced if compared to the stoichiometric V2O5. Taking into account the atomic ratio of V and O (expected V:O ratio of 1:2.46 for V2O5), we can be aware of the content of oxygen vacancies in the films. A deviation from the stoichiometric V:O ratio, of 1:2.46, was observed for both films, an indication of the presence of oxygen vacancies that arise from the reduction of V2O5, as expected.72 Moreover, the peak at 533.2 eV of the O 1s is slightly higher in intensity for the film made from the freshly prepared solution, and almost disappears in the thin film made from the aged V2O5–IAP solution (as can be seen in Fig. 4a and S3†). This is in good agreement with the replacement of the water molecules intercalated in V2O5 by the IAP molecules in solution. Once prepared as a thin film, the IAP evaporates from the V2O5 layer leaving behind a thin film without (or at least less amount) of water molecules.
We can infer from these results that both thin films are partially reduced: the film prepared with fresh solution, by water, and the film prepared with the 24 h-old solution, by IPA.
The optical band gap (BG), calculated from Tauc's formula, plot of α2E2 against photo energy,75 is shown in Fig. 4b. It reveals a slight difference in BGs between the thin films fabricated from either fresh (red) or 24 h-old (green) V2O5–IPA solution, with values of 2.7 eV and 2.8 eV, respectively. The full He I scan of the ultraviolet photoelectron spectroscopy (UPS) analyses of the films is shown in Fig. 4c. The work function (WF) values obtained were 5.15 eV (fresh) and 5.5 eV (24 h-old), respectively, as reflected in the photoemission offset around 16 eV. These values are in good agreement with WF values of thin films of V2O5 fabricated in air.15 Finally, the values for the ionisation potential (IP), defined as the energy difference between the valence band (VB) edge and the vacuum level (Ev), are 7.66 eV (fresh) and 8.0 eV (24 h-old).
These results permitted the construction of the band energy diagram for both thin films as shown in Fig. 5. To calculate the voltage of the OSC, we used the LUMO level of ZnO at 4.4 eV57 and the HOMO level obtained experimentally for V2O5 at 5.0 to 5.16 eV. The latter yields a Voc value of 0.56 V to 0.6 V, which is in good agreement with the experimental Voc values obtained for the OSCs shown in Fig. 3 (and very similar to the Voc values between 0.56 and 0.58 V observed in Fig. 1). However, we were unable to arrive at a clear conclusion regarding the Voc value of the solar cell that contained the V2O5 thin film made from the 24 h-old (green) V2O5–IPA solution (experimentally 0.38 V) since there is a wide range of possible reduction stages for V2O5 that can be detected in IPA over time. Thus, based on the experimental and calculated values of Voc, we reasoned that the fabrication steps followed to obtain the V2O5 thin film affect the final photovoltaic response of the OSC. Moreover, the final Voc of the device is probably chiefly dictated by the semiconductor oxide layers and by the HOMO/LUMO levels of the donor and acceptor materials of the active P3HT:PCBM layer.
Fig. 5 Band diagrams for V2O5 thin films obtained from freshly prepared (a) or 24 h-old and (b) V2O5–IPA solutions. Ev: vacuum level; CB: conduction band; Ef: Fermi level; and VB: valence band. |
Fig. 6 shows the solar cells' energy band diagrams (a and b), the solar cell architectures in both configurations (c and d), and the corresponding IV curves and IPCE analyses for both types of devices (e and f). The photovoltaic parameters obtained are detailed in Table 3. The band energy diagrams in Fig. 6a and b are represented in relation to the relative energy levels of the acceptor (PCBM) and the donor (P3HT). The experimental values observed for the V2O5 thin film (5.1 eV) are very close to the energy level of the P3HT, and in good agreement with the work function of the Ag and the FTO electrodes responsible for the hole and electron collection respectively. Comparison of the photovoltaic response indicates a very similar behaviour, with Voc ranging between 0.54 V and 0.56 V and the FF between 47 and 48%. The main difference is observed on the Jsc, which is lower for the OSC in the normal configuration in comparison with the inverted configuration. The difference in Jsc also limits the PCEs, which is observed between 2.6% and 3% for the inverted configuration, and at around 2% for the normal configuration (see Table 3). This difference in PCE is further validated by the corresponding IPCE responses: with 70% and 40% for inverted and the normal configuration, respectively. The dissimilarity in the performance between the two types of OSCs can be attributed to the greater light reflection and the UV-filter effect imposed by the V2O5 layer on the device. In the case where the device is illuminated from the FTO/V2O5 side (see Fig. 6d and e), the V2O5 layer could be acting as a UV-filter, limiting the amount of light reaching the cell. Adsorption spectra of the ZnO and the V2O5 layers are shown in Fig. 6f. V2O5 adsorbs at wavelengths up to 450 nm while in the inverted configuration (Fig. 6c), light enters the device from the FTO/ZnO side, where the ZnO layer blocks only the UV wavelength region below 380 nm. An interesting aspect observed is the value of Voc that is almost the same for both types of devices. This is an indication that the LUMO level of ZnO at 4.4 eV and the HOMO level of V2O5 at 5.16 eV can be used to calculate the Voc of the normal configuration OSC.57 In the same way it was described before for the inverted OSC in Section 3.2. OSCs applying V2O5 as the HTL5,10,13,14,16–20,81–83 have been usually reported with an Ag metal electrode in the inverted configuration,5,10,15,57 and an Al or Ca electrode in the normal configuration.14,19,21,58 In our work, the photovoltaic response of both types of OSCs seems to be independent of the Ag back metal electrode employed. This makes the OSCs amenable to fabrication by printing methods as the Ag metal electrode can simply be printed from solution.5,10,44,79 This also could be a step forward to the fabrication of more compatible recombination layers for TmOSCs.10
Fig. 6 Schematic representation of the band energy diagram for the inverted (a) and normal (b) configuration of organic solar cells containing ZnO as the electron transport layer and water-based, solution-processed V2O5 as the hole transport layer. The architecture of the inverted (e) and the normal (d) configuration OSCs. IV curves (c) and IPCE spectra (f) of the OSCs in each configuration. In both cases, an Ag back metal electrode was used. Measurements were taken at 100 mW cm−2 AM1.5G. |
Device structure | Voc (V) | Jsc (mA cm−2) | FF (%) | PCE (%) |
---|---|---|---|---|
a Average value from six samples. | ||||
Inverted | ||||
Glass/FTO/ZnO/P3HT:PCBM/V2O5/Ag | 0.540 ± 0.01 | 9.54 ± 1.1 | 47.20 ± 1.9 | 2.58 ± 0.2 |
Normal | ||||
Glass/FTO/V2O5/P3HT:PCBM/ZnO/Ag | 0.565 ± 0.01 | 7.65 ± 0.3 | 48.35 ± 2.3 | 2.10 ± 0.2 |
Reference cell | ||||
Glass/FTO/TiO2/P3HT:PCBM/V2O5/Ag | 0.563 ± 0.01 | 10.69 ± 0.3 | 50.49 ± 1.9 | 3.09 ± 0.1 |
The selection of the adequate back metal electrode in OSCs has been the subject of extensive research work. The OSCs that have been studied to date contain only one oxide semiconductor used as the ETL (usually TiO2, TiOx or ZnO), and PEDOT:PSS as the HTL.44,84 Since the use of TMOs as both ETL and HTL is relatively new, we have not found any other work in which a high work-function metal electrode (e.g. Ag) is used for the normal configuration OSC. Greiner et al. recently described the effect of metal electrodes on the work function and band structure of MoO3 at metal/metal oxide interfaces. The reduction of the oxide (from Mo6+ to Mo3+) in contact with the metal electrode results in a lower work function of the oxide, and the maximum value depends on the thickness of the oxide layer.85 Hadipour et al. have employed an Ag metal electrode in different OSCs in the normal configuration, including the ones in which MoO3 is the HTL. However, a thin layer of Ca between the active layer of P3HT:PCBM and the Ag metal electrode was employed for the normal configuration OSCs.86 Lidzey et al. have reported a study on different back metal electrodes in normal configuration OSCs in which MoO3 is also the HTL.87 The authors fabricated OSCs of the type ITO/MoO3/PCDTBT:PC70BM/metal electrode (note that no ETL was applied between the active layer and the metal electrode), using diverse, thermally evaporated metals (Ag, Al, Ca, Ca/Ag and Ca/Al). The final photovoltaic performance of the solar cells was very similar in all cases, showing only slight differences among the devices. The authors chose the Ca/Al back electrode as the best one, owing to its slightly better photovoltaic response. Although their work involved only one TMO as the HTL (MoO3) and did not entail the use of any ETL, it is the closest research work related to the one presented here by us (in terms of set-up and results). It also supports the idea that the photovoltaic performance of normal configuration OSCs containing metal oxides is probably independent of the back metal electrode used. Despite the advances made by Lidzey et al., our group and others, substantial studies are needed to clarify the role of the back electrode in these TMO-based OSCs.
Fig. 7 Outdoor stability analysis of sealed OSCs containing water-based, solution-processed V2O5 as the hole transport layer. Comparison of normalised PCE response: (a) normal configuration vs. inverted configuration (both without the UV filter); and (b) with the UV filter vs. without the UV filter (both in inverted configuration). The cells were analysed outdoors in Barcelona, Spain (41.30° N, 2.09° W). The PCE values were calculated using the maximum irradiance level per day. Average temperatures: 10 to 15 °C (day) and 5 to 7 °C (night). Average RH: 70%. Normal configuration: glass/FTO/V2O5/P3HT:PCBM/ZnO/Ag. Inverted configuration: glass/FTO/ZnO/P3HT:PCBM/V2O5/Ag. |
In order to demonstrate that the OSCs would be more stable in the absence of UV light, we applied a UV filter to the inverted-configuration OSC. Two samples, one with the UV filter and the other without, were analysed outdoors under the same conditions. The filter (an adhesive UV filter film that cuts UV light below 400 nm) was applied on top of the test cell. Fig. 7b shows the observed response for the first 1000 h of analysis. The control sample performed just like the inverted OSC analysed in Fig. 7a, reaching T60 at ∼500 h and T40 at ∼1000 h. However, the sample with the UV filter remained at T80 for many hours and was still stable after ∼1000 h of testing. Thus we can demonstrate that elimination of UV light can improve the lifetime of the inverted-configuration OSC by several orders of magnitude.
In this work, we have demonstrated the high stability of OSCs containing V2O5·0.5H2O as the HTL despite the presence of water molecules in the layer. The degradation of the OSC lacking the UV filter indicates that V2O5 is photoactive under UV light, and that the active P3HT:PCBM layer or the Ag electrode can interact with the V2O5 HTL. Nevertheless, a UV filter is beneficial and improves the OSC’s stability.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ee42204f |
This journal is © The Royal Society of Chemistry 2013 |