Photo-induced Traps in Organic Semiconductor blends

Thin film photovoltaic cells based on solution processable organic semiconductors have attracted remarkable interest as a possible alternative to conventional, inorganic thin film solar cells. The attraction of the organic photovoltaic (OPV) approach lies in the potential for very low-cost, light weight and flexible photovoltaic material and the potentially disruptive effect of the new technology on the PV market. During the last years substantial progress has been made in increasing the power conversion efficiency (PCE) of solution processed OPV. In 2001 record efficiencies were in the range of 2 %. About 10 years later, several research groups have reported efficiencies around 10 %. In addition, roll-to-roll processing of organic solar cells has been demonstrated and first fully printed prototypes are available today. In contrast to the efficiency and manufacturing successes, the durability of organic solar cells is still moderate and processes causing degradation are not well understood. In the project we would like to shed some light on light-induced degradation processes in organic solar cells. We will develop a method to characterize light-induced charge carrier traps which are believed to be one of the main sources of degradation. The method is based on electron spin resonance (ESR) spectroscopy which is known for its high sensitivity. We will correlate the light induced trap states to changes in the optical and charge transport properties and the performance of organic photovoltaic devices. The overall goal of the project is to establish ESR as a method to investigate the photo-stability of organic semiconductor blends. The explored experimental technique may allow an estimation of the ultimate lifetime of organic solar cells and could turn into one of the standard charac-terization methods of organic semiconductors.

For several years organic solar cells have been explored as alternative to conventional silicon- based photovoltaic devices for the conversion of solar radiation into electricity. Organic solar cells offer several advantages. They are easy to make, can be flexible and low weight and they are aesthetically pleasing. At the same time state-of-the-art organic solar cells exhibit severe disadvantages. On the one hand, power conversion efficiencies are lower (5-10 % compared to 15-20 % for silicon solar cells) and lifetimes are moderate. Both disadvantages limit the applications of organic solar cells to niche markets where flexibility, light weight and aesthetics are mandatory and high efficiencies and long lifetimes are not required. Goal of the project was to explore the intrinsic photo-stability of organic semiconductors applied in solar cells. The investigations should lead to a better understanding of the degradation processes and should identify the semiconductors with the best photo-stability. In the framework of the project we applied spectroscopic techniques to follow the degradation of different semiconductor materials. Electron Spin Resonance played a central role in our experiments. This method is extremely sensitive and can detect smallest changes in the properties of semiconductor materials. In addition, we built and degraded solar cells to correlate the spectroscopic and device data. The intrinsic photo-stability of organic semiconductors was found to be excellent. Even after 1000 hours of exposure to intense solar radiation, no changes in the Electron Spin Resonance spectra could be detected. This suggests that photo-induced charge generation and recombination of these charges is highly reversible. The situation changes completely in the presence of oxygen. Light and O2 induce a degradation of organic semiconductors leading to a loss of the electronic properties of the materials. I.e. for the photo-stability, the exclusion of oxygen is of utmost importance. The device degradation studies and the spectroscopic investigations show a poor correlation. Even in the absence of oxygen, most organic solar cells show distinct degradation after a few hundred hours of continuous illumination. Our investigations suggest that in addition to the stability of the organic semiconductors, the solar cells design and applied interlayer and electrode materials play an important role for the device stability. Our findings support the idea that there are not fundamental and intrinsic mechanisms limiting the stability of organic solar cells. However, for long-term stability an optimized set of materials, interlayer and electrodes are required. For many high performance semiconductors, these components have not been found by now and more research is required to develop stable and high performance organic solar cells.