Radiative Efficiency Limit with Band Tailing Exceeds 30% for Quantum Dot Solar Cells

Thin films of colloidal quantum dots (QDs) are promising solar photovoltaic (PV) absorbers in spite of their disordered nature. Disordered PV materials face a power conversion efficiency limit lower than the ideal Shockley-Queisser bound because of increased radiative recombination through band tail states. However, investigations of band tailing in QD solar cells have been largely restricted to indirect measurements, leaving their ultimate efficiency in question. Here we use photothermal deflection spectroscopy (PDS) to robustly characterize the absorption edge of lead sulfide (PbS) QD films for different bandgaps, ligands, and processing conditions used in leading devices. We also present a comprehensive overview of band tailing in many commercial and emerging PV technologies including c-Si, GaAs, a-Si:H, CdTe, CIGS, and perovskites then calculate detailed-balance efficiency limits incorporating Urbach band tailing for each technology. Our PDS measurements on PbS QDs show sharp exponential band tails, with Urbach energies of 22 +/- 1 meV for iodide-treated films and 24 +/- 1 meV for ethanedithiol-treated films, comparable to those of polycrystalline CdTe and CIGS films. From these results, we calculate a maximum efficiency of 31%, close to the ideal limit without band tailing. This finding suggests that disorder does not constrain the long-term potential of QD solar cells.