Application of a compositionally graded heterojunction for organic photovoltaics

Thin film photovoltaic devices containing a compositional gradient of donor-acceptor (CuPc-PTCDA) organic molecules were fabricated to investigate the improvements in efficiency resulting from this photovoltaic design. Bound electron-hole pairs (excitons) arising from photon absorption require a donor-acceptor interface to become free charge carriers. A compositional gradient increases the interface area, facilitating the splitting of excitons. Donor (or acceptor) fraction increases approaching the hole (electron) collecting contact improving electron and hole drift away from the interface and subsequently towards the contacts.; The presence of impurities was found to affect the photovoltaic response. Mobile charged impurities within an organic layer and their collection in a specific region affect the internal energy levels of a device. This movement of charged impurities can be detrimental or beneficial to device efficiency depending on device structure. Monochromatic efficiency experiments found the charged impurity to be of positive charge and originating within the PTCDA.