Random vs regularized OPV: Limits of performance gain of organic bulk heterojunction solar cells by morphology engineering

Abstract

Inexpensive solution processing of bulk heterojunction (BHJ) type organic photovoltaic (OPV) cells offers an attractive option for the low cost solar energy conversion. Solution processing creates a disordered morphology consisting of two organic semiconductors, intermixed randomly within the light-absorbing layer of the cell. In this paper, we use a detailed three-dimensional process-device co-modeling framework to show that in spite of the inherent structural randomness of the morphology, the efficiency of solution-processed BHJ cells is nearly optimal – close to those of the perfectly ordered structures. In addition, we show that the morphological randomness by itself does not increase the performance variability of large-area cells. Both the results indicate that the inexpensive solution processing of BHJ cells imposes no inherent limitation on the performance/variability and the ultimate efficiency of such solution-processed films should compare favorably to the other ordered OPV cells fabricated by more expensive techniques. Finally, we explore the theoretical optimum morphology for BHJ cells and find that fill factor is the only parameter through which efficiency can be enhanced by morphology engineering. We conclude by exploring the performance gains/limits of organic solar cells with the improvement in transport parameters.