Microscopic Inhomogeneities in Organic Solar Cells Caused by PEDOT:PSS

Organic semiconductors play an important role in modern technology and society. Their comparatively easy processability at low-cost and flexible molecular design to meet required physical properties make them highly attractive. First realizations, e.g. in mobile phone displays, are already on the market. However, especially in photovoltaics, a couple of problems are to tackle before reliable, large-area application becomes likely. One problem is the considerable batch-to-batch and device-to-device variation of organic photovoltaic devices. It has been shown that devices prepared by the same experimentalist under identical conditions and materials show performance variations of up to 10% and so do published research results scatter for identical systems. From microscale investigations of organic solar cells it has been found that there are considerable spatial variations in device performance. As active layer systems are usually engineered to nanoscale perfection, it has been suspected that rather used charge selective interlayers, like PEDOT:PSS, could be responsible. However, there is still a gap in understanding of the microscale physical processes caused by PEDOT:PSS in these devices. The proposed project will investigate the very basic microscale diode physics in organic devices with a focus on the conditions of the PEDOT:PSS interlayer. In thin film diodes the smallest spatial variation has detrimental effects on the local device physics and thus on the entire device. The investigations will range from simple effects like height variations, to more delicate questions as effects of trapped PEDOT:PSS particles in the active layer or the influence of underlying indium-tin-oxide electrode due to PEDOT:PSS layer porosity. Experimentally this will be realized with a systematic study of diode characteristics with spatial resolution and their correlation to PEDOT:PSS related inhomogeneities. Thereby every measurement spot will be treated as an independent microdiode. These will be analysed especially regarding charge trapping effects and transport physics at the PEDOT:PSS/ITO and the PEDOT:PSS/active-layer interface. Finally, a set of characteristics associated to certain PEDOT:PSS layer inhomogeneities should be available. Re-assembling of these subunits with their specific characteristics to a 2D- array of parallel-connected microdiodes it should be possible to draw conclusions to the origin of the entire device response. Further, this will allow forecasts of performance variations of devices, depending on the density of according inhomogeneities associated with the PEDOT:PSS interlayer. The expected new insights gained from this project will not only help understanding of the device behaviour of solar cells comprising PEDOT:PSS interlayers, but shall also allow conclusions to other colloidal systems in use and in future, like graphene or nanowires.

Organic semiconductors, such as conjugated molecules and polymers, play an important role in modern technology, e.g. in OLED-based mobile phone and TV displays. Their easy processability and flexible molecular design make them also attractive for sensors and photovoltaics. However, organic solar cells still have to battle some drawbacks before they become competitive for large scale use, such as their batch-to-batch variations, spatial inhomogeneity of the photoactive area and lack of environmental stability. We dedicated this project to the investigation of reasons for performance inhomogeneities of organic solar cells, with special focus on one particular device component: the hole-transport layer (HTL). It is an inevitable part of the device architecture and located between the transparent electrode and the semiconductor layer. For quite some time, the material PEDOT:PSS was commonly used for that purpose, despite its reported negative effect on device stability, by inhomogeneity of particular films and corrosivity of the acid PSS. In our solar cells, we tested different particle-based and continuous HTLs from PEDOT:PSS or molybdenum oxide. The devices were evaluated by mapping of their generated photocurrents across the area and investigated regarding their local physical and chemical properties. This way we gained knowledge on type and distribution of defect sites and their impact on the performance of the entire solar cell. Different to our expectations, we found that neither particle size or distribution, nor the actual material or film thickness of the HTL had any impact on homogeneity or total performance, as long as the devices were characterized immediately after preparation. This changed with environmental exposure. When solely the HTLs were exposed before device assembly, still all devices were unaffected and remained uniform, except for humidity-exposed PEDOT:PSS. That device suffered from large areas of performance losses, which could be identified to originate from partial delamination of the HTL from the electrode by consecutive swelling and shrinking upon water uptake/loss. This rather mechanical effect is a very different observation to the commonly reported electrode or semiconductor degradation by reactions with PSS or/and humidity, which could not be detected here. Further, localized punctual defects on solar cells, which are often reported in connection with chemical reactions of PEDOT:PSS, were only created in our experiments with HTL-free solar cells upon humidity exposure of the whole device. In this case of clear absence of PEDOT:PSS, we could clearly assign the effect again to electrode contact loss, but in this case to isolating oxide islands formed between the reflective aluminum back-electrode and the semiconductor. This again was unexpected because they were not formed solely by oxygen exposure. Overall, our new findings indicate that the HTLs in organic solar cells are in many cases not the reason for spatial performance inhomogeneities, not even PEDOT:PSS. This also shows that organic solar cells are more stable that usually thought.