Optical Optimization of Thin-Film Polymer Solar Cells

Abstract

Photovoltaics are now slowly replacing fossil fuels, aiming at higher efficiencies and lower costs to bring PV to cost parity with grid electricity. Solar energy is a clean and renewable energy, which is generated from the natural source sun. Solar cells are devices that convert solar energy into electricity, either directly via the photovoltaic effect, or indirectly by first converting the solar energy to heat or chemical energy. Both inorganic and organic types of solar cells are available. Unfortunately, the solar cells dominating the market are all made of inorganic materials requiring expensive and complicated manufacturing processes and have limited applications basically to the rooftops. One of the promising alternatives to inorganic solar cells is the polymer ones. Polymer solar cells (PSCs) are attracting interest as potential sources of renewable and clean energy because of their attractive advantages of low-cost large-area fabrication on lightweight flexible substrates. Though the efficiency of polymer devices have not yet reached those of their inorganic counterparts (≈10–24%); the perspective of low cost, low temperature and energy processing, low material requirement, can be used on a flexible substrate, can be shaped to suit architectural applications are some advantages of polymer solar cell that drives the development of polymer photovoltaic devices further in a dynamic way. This chapter is devoted to the optimization of layer thickness in a polymer photovoltaic cell. It presents the applied calculation method which is based on the optical transfer matrix 2 × 2 formalism. Optical modelling results show that the distribution of light energy determined by optical interference and optimization of thickness of each layer in the OPV would help in the improvement of its performance. The influence of thickness of active layer, electron transport layer and hole transport layer on the normalized modulus squared of optical electric fields distribution inside devices and on the distributions of exciton generation rate and hence current density has been investigated. A mapping of total and useful absorbed energy and parasitic absorption have been done which helps in accurate measurement of IQE and EQE. The distribution of exciton generation rate has been predicted by modelling.