Organic photovoltaic cells and graphene transparent conductors [electronic resource]
Keywords: Materials Science and Engineering

The Organic photovoltaic cell (OPV) is a promising technology because of its potential for low-cost high-throughput roll-to-roll manufacturing. Significant improvements have been achieved in power conversion efficiency (PCE) of OPV cells during last two decades. While recent progress in raising the PCE has been encouraging, the PCE of organic solar cells is still limited and needs to be improved to meet the requirement for commercial applications. Further improvements in both material properties and device architectures are necessary. Photocurrent generation in an OPV cell is fundamentally different from the process that takes place in their inorganic counterparts. A detailed understanding of the operation mechanisms of OPV cells and optimization of the fundamental electronic properties of the system (or material) are critical. In this work, I will first discuss major factors that limit the efficiency of bilayer OPV cells, such as exciton binding energy, exciton diffusion length, charge separation and open-circuit voltage. The exciton binding energy is one of the key parameters that govern the operation of OPV cells, and determines the required energy band offset between donor and acceptor, and thus the achievable open-circuit voltage of the donor-acceptor combination. Exciton diffusion is a main bottleneck limiting photocurrent of a bi-layer OPV cell, which depends on material properties and film morphology. The energy loss between optical excitation and extracted electrical power is mainly due to the energy band offset between donor and acceptor in OPV cells. The PCE limit for single junction OPV cell can be estimated based on the findings. In the second part of this work, I will focus on transparent conductors, which are essential components of thin-film optoelectronic devices. Sputtered Indium-Tin-Oxide (ITO) is currently the most commonly used transparent electrode material, but it has a number of shortcomings. There is a clear need for alternative transparent electrodes whose optical and electrical performance is similar to that of ITO but without its drawbacks. The next generation transparent conductor should also be lightweight, flexible, cheap, environmental attractive, and compatible with large-scale manufacturing methods. I will discuss the possibility of using graphene thin films as a replacement for ITO. Theoretical estimates indicate that graphene thin films are promising transparent electrodes for thin-film optoelectronic devices, with an unmatched combination of sheet resistance and transparency. For the first time, we demonstrated that solution-processed graphene thin films can serve as transparent conductive anodes for both OPV cells and organic light-emitting diodes (OLEDs). The graphene electrodes were deposited on quartz substrates by spin-coating of an aqueous dispersion of functionalized graphene, followed by a vacuum anneal step to reduce the sheet resistance. Small molecular weight organic materials and a metal cathode were directly deposited on the graphene anodes, resulting in devices with a performance comparable to control devices on ITO transparent anodes. Device modeling has been explored to compare the performance between graphene-based device and ITO-based control device. Transfer of graphene films to a foreign flexible substrate was also demonstrated which opens up new opportunities for low-cost flexible organic opto-electronics.