Probing the charge generation and recombination in thin-film, optoelectronic devices

Sustainably and environment-friendly manufactured semiconductors are at-tractive candidates for next generation electronic and optoelectronic appli-cations ranging from memory storage and computation, to power manage-ment and energy generation. In this regard, organic semiconductors, i.e., semiconductors based on conjugated carbon-based molecules and polymers derived from earth abundant elements, are the subject of intense basic re-search and technological development efforts. Understanding the funda-mental processes governing these low-mobility and disordered semiconduct-ing materials is therefore key to establish next generation applications based upon flexible and solution-processible organic semiconductors as global com-mercial technologies.The work presented in this thesis focuses on the investigation of charge generation and recombination processes on thin film optoelectronic devices based upon organic semiconductors. A suite of experimental techniques, im-proved measurement setups, and expanded approaches are presented, and form the basis of comprehensive studies on state-of-the-art, high-efficiency organic photovoltaic systems. Specifically, an external quantum efficiency measurement technique with unprecedented dynamic range will be detailed. Using this enhanced apparatus, an approach allowing one to accurately de-termine charge generation quantum yields is introduced. After this, an extended technique to probe photogenerated charge carrier densities is out-lined and applied to thin-film solar cells. Having emphasized the importance of studying charge generation, a combined theoretical and experimental ex-ploration of the light intensity dependence of photocurrent and charge col-lection efficiency under the influence of various loss mechanisms is described. These insights provide the basis of a comprehensive study on organic so-lar cells, where recombination caused by localized trap states is found to be universally present under operational conditions limiting photocurrent and power-conversion efficiency. Overall, the work presented in this thesis expands on existing techniques and approaches, and yields important new understanding as to the device physics of thin-film, optoelectronic applica-tions