Optical Profile and Nanostructure Effects in the Charge Carrier Transport and Performance of Photovoltaic Devices.

摘要

Charge carrier plays a significant role in energy harvesting in photovoltaic devices. Due to recombination, the inadequate charge carrier transport length prevents the devices from achieving efficient absorption by increasing active layer thickness. Fundamental research on the charge transport is important as it is a critical factor to determine the optimal device structures. In this thesis, the charge carrier transport process is studied in photovoltaic devices by linking local characteristic light absorption profiles to photocurrent measurements. Local light absorption profile can be approximated as the free charge generation profile, which determines the average charge transport distance. Together with incident light wavelength, illumination direction effectively controls the generation profile and hence the charge transport distance. And this charge transport distance is demonstrated to relate to recombination that can be measured from photocurrent. Therefore, the charge carrier transport length can be estimated.;On the other hand, the potential of the nanostructured solar cells as a key to solve the problem lies between adequate light absorption and efficient charge carrier collection. In this thesis, the discussion focuses on the nanostructured bulk heterojunction (BHJ) organic photovoltaics (OPVs). As photonic crystal nanostructures have been proposed to increase the light trapping effects without increasing the volumes of the active materials, intuitively, it is believed that the nanostructure will affect only the optical absorption. However, in this thesis it is demonstrated that there is a tradeoff between light trapping enhancement and charge carrier collection deterioration due to the nanopatterning effects. Furthermore, the nanopatterning process is shown to affect the material composition in BHJ OPVs as well. Improvement of BHJ OPVs' performance by nanostructures is not a simple task of increasing light absorption. Comprehensive considerations are demonstrated necessary for design of optimal device structures.