Impact of Doping on Absorbance Tails and Power Conversion Efficiency of Photovoltaics

Efficient and affordable photovoltaics are critical to widespread adoption of renewable energy. Photoelectric conversion efficiency (PCE) is limited by a few key losses—radiative, resistive, and recombinative. Doping is a common strategy for improving electronic properties in other devices. However, doping introduces positional and energetic disorder, particularly in organic materials, which has been shown to create Urbach tails in the absorbance spectrum. These absorbance tails increase radiative losses, significantly reducing PCE. Because of this, doping is often shown to be harmful to PCE, and the optimal doping concentration in photovoltaics remains an open question. We numerically compute the PCE from absorbance and show that doping is not always harmful to PCE and, in fact, can even improve it. While dopant-induced disorder always broadens the tails in the Density of States (DOS), its effect on absorbance tails depends on the position of the Fermi level. At high doping concentrations, the Fermi level can be positioned inside of one DOS tail, filling the trap states in the tail and "cutting off" its contribution to the absorbance. This mitigates the absorbance tail, creating a step-like absorbance spectrum that results in a high PCE. This is particularly exciting for intrinsically disordered materials such as organic photovoltaics, whose intrinsic absorbance tails are an obstacle to improving PCE. Additionally, heavy doping increases electrical conductivity, decreasing open-circuit voltage losses. In our work, we have identified conditions at which doping can increase the PCE.