Most efforts at device optimization and analysis of transport in polymer-fullerene solar cells assume implicitly that the active layer is an intrinsic semiconductor. We instead focus on the effect that different amounts of inadvertent $p$-type doping has on two thickness series of different batches of high-performance DPP-TT-T:PC71BM devices. Understanding how device performance, and, in particular, charge carrier transport and recombination dynamics, responds to changes in the electric-field distribution resulting from inadvertent photoactive layer doping is a key consideration. We investigate device performance both with regard to the synthetic and purification strategies employed to control such doping levels and with regard to the choice of device architecture for optimum solar-cell performance.

Chemical doping in the active layer of solar cells affects device functionality. By combining charge extraction, transient photovoltage, study of the linearity of the short-circuit current with light intensity, and drift-diffusion modeling, we use experiments and simulations to demonstrate how commonly used methods to study recombination dynamics can be misleading if doping is not taken into account. We highlight, in particular, experimental evidence of doping as one potential cause of high reaction orders. We focus on two levels of doping: $1.5\times {10}^{16}$ and $7\times {10}^{16}{\mathrm{cm}}^{-3}$. Our work emphasizes how, even without the use of any particular molecular dopant, the unintentional doping levels in some efficient donor materials can be sufficient to totally impede carrier transport in thick active layers, although the thin devices exhibit good transport and absorption properties. The doping thereby prevents the use of devices thicker than 100 nm. As a result, we suggest ways to control unintentional doping of the photoactive layer.

We expect that our work will highlight the importance of doping, which can characteristically vary by over an order of magnitude in the polymer making up solar cells. Since power conversion efficiency is a measure of a solar cell’s effectiveness, it is critical to ensure that device performance does not suffer unnecessarily because of ignorance of doping levels.