Optimization of bulk heterojunction organic photovoltaic devices

Abstract

Stabilizing the global climate and energy security are the biggest challenges of humanity in this century and thus created substantial political, academic, and industrial interest in the renewable energy resources. Among renewable energy sources, solar photovoltaic has the most promise for becoming amajor energy source. In the field of photovoltaic technologies, the organic solar cells represent a transformative technology with great potential for extremely high-throughput manufacturing at very low cost, low environmental impact, mechanical flexibility, molecular tailorability and made from nontoxic, earth-abundant materials with short energy payback times. The introduction of new light absorbing materials, device architectures, and light management structures has resulted in enhancement of the power conversion efficiencies from 2.5% in 2001, to 5% in 2006, to greater than 10% in 2016 for small solar cells, predicting a bright future for organic solar cells. However, before large-scale commercialization and entering a direct competition with state of the art inorganic PV technologies, further improvements especially in the power conversion efficiency are required. It is strange that despite of rapid progress in organic solar cells, there is no standard validation tool for device optimization. The experimental optimization is expensive and time-consuming as reduced feature size needs more complicated and costly manufacturing processes. Thus, simulation and modeling becomes indispensable tool for cost-effective and accurate optimization of such nanoscale devices. Optical modeling enables a quantitative comparison of optical performance of alternative materials, the optimization of the physical structure of the device, finding the dependency of devices efficiency on structure parameters and material properties, the analysis of loss mechanisms, and the calculation of the generation profile for electronic modeling. From the optical point of view, thin-film organic solar cells are multilayer structures, thus interference effects between forward- and backward- going (reflected) light have to be considered in the analysis. The transfer matrix method, where transmission and reflection are calculated for each interface in the stack as well as attenuation in each layer is employed. This chapter summarizes the various optical modeling techniques employed for the optical optimization of bulk heterojunction (BHJ) structure and other OPV solar cells and their corresponding development in recent years based on device physics and its working principle. Optical optimization of PBDTTPD:PCBM BHJ OPV has been carried out with respect to various parameters.