Electro-Optics of Conventional and Inverted Thick Junction Organic Solar Cells

Bulk heterojunctions continue to be the dominant architecture for solution processed organic solar cells. In general, photoactive films on the order of 100 nm thickness have delivered the highest power conversion efficiencies. However, it is becoming increasingly apparent that thicker junctions are needed for high yield, high throughput, low cost manufacturing of commercial organic solar cells. Very few organic semiconductors are suitable for maintaining optimal efficiencies in cells with thicker junctions. This paradigm is beginning to shift with the recent high mobility donor polymers, where electrically inverted thick heterojunction structures deliver impressive efficiencies. The inverted architecture seems to be an essential feature of these solar cells. The reason for this has yet to be explained, and in this work, we address this question. We present analytical simulations and experimental evidence showing how the charge generation and extraction physics is significantly different in thin and thick heterojunctions, inverted and conventional. In particular, our predictive model shows how the inverted architecture compensates for strongly imbalanced carrier mobilities, which would otherwise cause debilitating recombination. Thick bulk heterojunctions can be designed to deliver high efficiencies, but for high mobility donors, this is only in an inverted architecture. These findings have profound implications for manufacturing of commercial organic solar cells.