INTERFACIAL CHARGE TRANSFER PROCESSES IN QUANTUM DOT SOLAR CELLS

Abstract

Quantum dots (QDs) are extensively used in photovoltaic devices due to their unique properties: bandgap tunability, capability of multiple exciton generation, up/down wavelength conversion. The improvement in device performance is attributed to the enhancement in optical absorption, quantum efficiency, and reduction in thermalization losses. Good optical, as well as electrical properties of the QDs, are essential for efficient device operation. Charge transport and carrier recombination in the QDs are the key processes that affect the device performance and these processes can be easily tuned during the synthesis of QDs and fabrication of the device. In this work, current-voltage characteristics in bilayer heterojunction diodes are studied (effects of energy barriers, layer thicknesses, etc.) and separated into three working regimes based on the energy band diagram of the device. Subsequently, a model for multilayer quantum dot organic solar cells has been developed that explores the impact of electronic processes (carrier recombination, tunneling, injection, etc.) in QDs on the current-voltage (J-V) characteristic of the solar cells. Solar cell characteristics can be controlled by the quantum dot layers. The bimolecular recombination coefficient of QDs is a prime factor that controls the open-circuit voltage without any significant reduction in short circuit current. To verify our proposed model, various core-shell QDs have been fabricated and its interlayer is inserted between the donor and acceptor layer in the device. The addition of QDs has improved the optical absorption in the device resulting in an increase in photo-current/short circuit current density and open-circuit voltage of the solar cell but the current-voltage characteristics show an s-shaped curve in the fourth quadrant which results in drastically reduced fill factor. The reason behind the appearance of s-kink in experimentally obtained J-V characteristic of QD solar cells has been analyzed with the model. According to the model, the capture/emission time and tunneling rate coefficient in QDs are individually responsible for degradation in device performance via an undesirable s-shaped J-V characteristic of hybrid organic/inorganic quantum dot solar cells. Thus, injection/extraction rate, tunneling among QDs and recombination in QDs are essential factors that are required to be optimized for efficient QD solar cells. The structural and energetic disorders at various interfaces, surface properties of QDs, fabrication process, etc. must be taken into consideration to achieve an efficient device.