Colloidal Quantum Dot Solar Cell Band Alignment using Two-Step Ionic Doping

Colloidal quantum dot (CQD) solar cells composed of ionic halide passivated active layers benefit from improved passivation and high carrier mobility because of short interparticle distance. However, non-ideal band alignment of the active layer limits the potential open-circuit voltage (VOC) produced by the solar cell. We initiated a suite of simulation-based studies of CQD solar cells and found a route to improved performance by increasing the degree of p-type behavior. Fluoride, while it is a p-type ionic ligand as desired, is incompatible with traditional ligand exchange processes. In prior studies, it has shown to etch the lead sulfide surface uncontrollably. Instead, we develop a multistep halide exchange method, in which the CQD active layer is doped with fluoride ions after ligand exchange. This new method prevents CQD surface etching without impeding charge transport, resulting in a statistically significant improvement in VOC, fill factor, and power conversion efficiency.