Chargeand Light Management in Organic Solar Cells

Sammanfattning: Solar cells based on organic materials are extensively investigated both in academia and industry, owing to their potential of being inexpensive, lightweight, flexible and suitable for roll-to-roll production. Performance of organic solar cells has been rapidly improving, and the state of the art organic solar cells have an efficiency over 10%.This thesis aims to provide a comprehensive study of device engineering and device physics of organic solar cells. The ambition has been to investigate and optimize the conditions for light in-coupling and charge extraction in organic solar cells with alternative geometries. The ultimate goal is to propose methods that could potentially get the power conversion efficiency of organic solar cells with industrially compatible geometries over the current limit of 10%.To achieve such a goal, we employed a conjugated polymer as an interface modifier to improve the surface and electrical properties of substrate electrodes for solar cells with active layers based on blends containing conjugated polymers and fullerenes. A possibility of improving photovoltaic performance of easily fabricated reverse geometry solar cells (bottom metal cathode/active layer/top  transparent anode) has been demonstrated via this simple interface modification. Systematic investigations revealed that the enhanced device performance was not only related to the modified active layer/electrode interface but also changes in the bulk of the active layer. Consequently, the device performance of a reversed solar cell was found to be as good as that of a standard solar cell. The understanding of the effects of the interlayer modification on the performance of reversed solar cells, and the methodology provided in this thesis is a guideline for further studying and developing interface materials for highly efficient organic solar cells.An ITO electrode was further converted into a cathode by the polymer interlayer to construct semi-transparent solar cells with great potentials for building integrated photovoltaic applications. By stacking semi-transparent solar cells on top of each other, we showed that light can be more efficiently harvested compared to a conventional architecture solar cell. It has also been demonstrated that semitransparent solar cells combined with macroscopic light trapping elements can also be highly efficient.However, this thesis is not only about papers that I have published as a graduate student. The first five chapters have been included to provide background in the field of organic photovoltaics.