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.

No additional funding sources recorded.