Organic solar cells (OSCs) based on polymers and small molecules have seen a tremendous increase in interest during the past few years. Signifi cant progress in this fi eld seeded the prospect for a cost-effective and easy-to-fabricate photovoltaic technology-typical advantages claimed for organic (opto-)electronic devices. Very recently, certifi ed cell effi ciencies in excess of 7% have been reported for polymer based cells. [ 1 ] For large-scale and high-throughput production of OSCs, liquid processing of the functional layers is desirable. Aside from the active organic layers, inter-layers are typically required to facilitate the extraction of the photo-generated charges. Specifi cally, on the anode side, polyethylene dioxythiophene:polystyrenesulfonate (PEDOT:PSS) is regularly used. [ 2 ] However, PEDOT:PSS is burdened with structural and electrical inhomogeneity [ 3,4 ] and has been demonstrated to be an origin of limited device lifetime. [ 5 ] Particularly, the aqueous PEDOT:PSS dispersion and the acidic nature can cause substantial degradation. [6,7 ] Very recently, transition metal-oxides (TMOs) such as molybdenum-, vanadium-, or tungsten-oxide (MoO 3 , V 2 O 5 , and WO 3 ) with high work functions (WFs) of up to 6.9 eV have been shown to be promising alternatives to PEDOT:PSS. [8][9][10][11] TMOs have also been used as constituents of the connecting architecture in stacked organic light-emitting diodes and organic tandem solar cells. [12][13][14][15] The unique energetics of these TMOs has so far been predominantly accessible for fi lms thermally evaporated in high-vacuum.The fi rst results for TMO layers obtained by solution processing from nano-particle (NP) dispersions have been reported only very recently. [ 16,17 ] Meyer et al. prepared MoO 3 layers by dispersing MoO 3 NPs using a polymer as dispersing agent. After deposition, the layers had to be treated by an oxygen plasma to remove the polymer. A high WF of the resulting layers of 5.7-6 eV was obtained. A substantial drawback of the approach, however, is the observation of larger NP aggregates with a size of 100 nm and an overall high roughness of 25 nm (rms). Owing to their roughness these NP-layers are critical sources of shorts, especially over a large device area.In contrast, TMO layers (WO 3 , V 2 O 5 and MoO 3 ) have been prepared by sol-gel deposition, predominantly for electrochromic, catalytic and sensing applications. [18][19][20][21] Post processing of the sol-gel TMO layers at high temperatures (300 ° C-600 ° C) is routinely applied in order to achieve specifi c microstructures or crystalline phases in the materials, as required by the particular application. These high processing temperatures are not compatible with the temperature-sensitive substrates (e.g. poly mer foils) envisaged for low-cost, high-throughput fabrication of organic solar cells. In spite of this limitation, Steirer et al. have very recently used NiO prepared via a sol-gel route as a replacement for PEDOT:PSS in an organic solar cell. [ 22 ] The requirement of pos...

For large‐scale and high‐throughput production of organic solar cells (OSCs), liquid processing of the functional layers is desired. We demonstrate inverted bulk‐heterojunction organic solar cells (OSCs) with a sol–gel derived V2O5 hole‐extraction‐layer on top of the active organic layer. The V2O5 layers are prepared in ambient air using Vanadium(V)‐oxitriisopropoxide as precursor. Without any post‐annealing or plasma treatment, a high work function of the V2O5 layers is confirmed by both Kelvin probe analysis and ultraviolet photoelectron spectroscopy (UPS). Using UPS and inverse photoelectron spectroscopy (IPES), we show that the electronic structure of the solution processed V2O5 layers is similar to that of thermally evaporated V2O5 layers which have been exposed to ambient air. Optimization of the sol gel process leads to inverted OSCs with solution based V2O5 layers that show power conversion efficiencies similar to that of control devices with V2O5 layers prepared in high‐vacuum.

A common phenomenon of organic solar cells (OSCs) incorporating metal‐oxide electron extraction layers is the requirement to expose the devices to UV light in order to improve device characteristics – known as the so‐called “light‐soaking” issue. This behaviour appears to be of general validity for various metal‐oxide layers, various organic donor/acceptor systems, and regardless if single junction devices or multi stacked cells are considered. The requirement of UV exposure of OSCs may impose severe problems if substrates with limited UV transmission, UV blocking filters or UV to VIS down‐conversion concepts are applied. In this paper, we will demonstrate that this issue can be overcome by the use of Al doped ZnO (AZO) as electron extraction interlayer. In contrast to devices based on TiOx and ZnO, the AZO devices show well‐behaved solar cell characteristics with a high fill factor (FF) and power conversion efficiency (PCE) even without the UV spectral components of the AM1.5 solar spectrum. As opposed to previous claims, our results indicate that the origin of s‐shaped characteristics of the OSCs is the metal‐oxide/organic interface. The electronic structures of the TiOx/fullerene and AZO/fullerene interfaces are studied by photoelectron spectroscopy, revealing an electron extraction barrier for the TiOx/fullerene case and facilitated electron extraction for AZO/fullerene. These results are of general relevance for organic solar cells based on various donor acceptor active systems.

Sol-gel processed MoO(x) (sMoO(x)) hole-extraction layers for organic solar cells are reported. A Bis(2,4-pentanedionato)molybdenum(VI)dioxide/isopropanol solution is used and only a moderate thermal post deposition treatment at 150 °C in N(2) ambient is required to achieve sMoO(x) layers with a high work-function of 5.3 eV. We demonstrate that in P3HT:PC(60)BM organic solar cells (OSCs) our sMoO(x) layers lead to a high filling factor of about 65% and an efficiency of 3.3% comparable to that of reference devices with thermally evaporated MoO(3) layers (eMoO(3)). At the same time, a substantially improved stability of the OSCs compared to devices using a PEDOT:PSS hole extraction layer is evidenced.

A hybrid approach for the realization of In-free transparent conductive layers based on a composite of a mesh of silver nanowires (NWs) and a conductive metal-oxide is demonstrated. As metal-oxide room-temperature-processed sol-gel SnO x or Al:ZnO prepared by low-temperature (100 ° C) atomic layer deposition is used, respectively. In this concept, the metal-oxide is intended to fuse the wires together and also to "glue" them to the substrate. As a result, a low sheet resistance down to 5.2 Ω sq −1 is achieved with a concomitant average transmission of 87%. The adhesion of the NWs to the substrate is signifi cantly improved and the resulting composites withstand adhesion tests without loss in conductivity. Owing to the low processing temperatures, this concept allows highly robust, highly conductive, and transparent coatings even on top of temperature sensitive objects, for example, polymer foils, organic devices. These Indium-and PEDOT:PSS-free hybrid layers are successfully implemented as transparent top-electrodes in effi cient allsolution-processed semitransparent organic solar cells. It is obvious that this approach is not limited to organic solar cells but will generally be applicable in devices which require transparent electrodes.

Transparent and electrically conductive gas diffusion barriers are reported. Tin oxide (SnOx ) thin films grown by atomic layer deposition afford extremely low water vapor transmission rates (WVTR) on the order of 10(-6) g (m(2) day)(-1) , six orders of magnitude better than that established with ITO layers. The electrical conductivity of SnOx remains high under damp heat conditions (85 °C/85% relative humidity (RH)), while that of ZnO quickly degrades by more than five orders of magnitude.

In organic solar cells (OSCs), the necessity of UV activation that comes with the use of ZnO‐ and TiOx‐based electron extraction layers (EELs) can be avoided by using tin oxide (SnOx), which can be prepared at temperatures as low as 80 °C. In contrast to devices based on TiOx and ZnO, OSCs comprising SnOx as the EEL show well‐behaved solar cell characteristics with a high fill factor (FF) and high efficiency, even without the UV spectral range of the AM1.5 solar spectrum.

low fi ll-factors (FFs) and overall low power conversion efficiency are found. This phenomenon is frequently referred to as "light-soaking" issue. [ 31,32 ] Development of charge extraction materials that do not rely on UV activation has been identifi ed to be of paramount importance to achieve highly effi cient and long-term stable devices. [ 33,34 ] In this sense, doped metaloxide EELs, e.g., Al:ZnO, [ 31,35,36 ] have been shown to mitigate the need for UV activation. While there are several reports of OSCs incorporating ZnO-based EELs in organic solar cells, which show a promising "shelf-life," [ 37 ] photoinduced shunts have been found to occur in the devices upon illumination "in actual operation." [38][39][40] Analogous to the case of the light activation discussed above, these photoinduced shunts are associated with the illumination by UV light (i.e., hν > E g ). As a result, a signifi cantly lowered shunt resistance along with a substantial decay of the FF and V oc is typically found to occur within minutes of illumination. The origin of this photoinduced shunt has been related to the UV-induced desorption of chemisorbed oxygen at the ZnO surface. [ 38 ] Approaches to modify and thereby to stabilize the ZnO surface range from the use of passivating mole cules [ 41 ] to the evaporation of thin aluminum layers onto the ZnO EEL. [ 39 ] Here, we will show that the photoinduced shunting behavior is a general phenomenon in OSCs comprising "neat or electrically doped" ZnO-based electron extraction layers, i.e., Al:ZnO (AZO) or Ga:ZnO (GZO), and it is found regardless if the EEL is prepared from nanoparticle dispersions or by vacuumbased techniques ( Figure 1 ). The photoinduced shunting of ZnO-based OSCs occurs for devices operated in air or under inert atmosphere, and it can therefore not be avoided by using a proper encapsulation. Moreover, we will show that while the photoinduced shunting is reversible in air, it is irreversible under the exclusion of oxygen. Opposed to ZnO-based EELs, we will demonstrate that the photoinduced shunting and the con-