S-shape current-voltage characteristics in quantum dot integrated bilayer organic solar cells
Upendra Kumar Verma, Aditya Nath Bhatt, Medha Joshi, Brijesh Kumar
Department of Electronics and Communication Engineering, Indian Institute of Technology Roorkee, Roorkee, India

In the numerical analysis of our previously proposed model on charge transport in quantum dots (QD), it was observed that on inserting a QD interlayer between the donor and acceptor layer, the device performance can be improved by optimizing the bimolecular recombination coefficient and capture/emission time constant. In order to experimentally verify the model, the CdSe-ZnS core-shell QDs have been fabricated and its interlayer is inserted between the donor and acceptor layer in the device. Addition of QDs has improved the optical absorption in the device; results in increase in photo-current/short circuit current density (JSC) and open circuit voltage (VOC) of the solar cell but, fill factor is drastically reduced with an s-shaped current-voltage (I-V) characteristic. The enhancement in JSC and VOC and reduction in series resistance can be attributed to improvement in charge transport and minimization in carrier recombination in the device. Further numerical analysis with the proposed model has revealed that the large capture/emission time constant between donor and QDs is the key factor for the degradation in device performance by affecting the accumulated photo-carrier concentration at the donor/QD interface. The measured I-V characteristics with increasing illumination intensity show the reduction in s-kink due to improvement in fill factor as expected form the analysis of the model. Thus, the concentration of accumulation of photo-carriers should be minimum for efficient photovoltaic devices. It can be achieved by minimizing the structural and energetic disorders in the vicinity of the donor-QD interface which provides better charge extraction from the active region of the device by reducing the capture/emission time constant.

Area: Sub-Area 1.2: Quantum-well, Wire, and Dot-Architectured Devices