Predictive Modeling of CuInSe₂ Nanocrystal Photovoltaics: The Importance of Band Alignment and Carrier Diffusion

Nanocrystal inks have been used to make printed photovoltaic devices (PVs) with reasonably high efficiencies; however, little is actually known about the material properties that limit the performance of these devices. Here, we model the output characteristics of PVs with CuInSe2 nanocrystal absorber layers using the Solar Cell Capacitance Simulator (SCAPS) software package and obtain the thickness-limited response and typical current–voltage behavior, power conversion efficiency (PCE), and external quantum efficiency (EQE) of experimentally fabricated PVs. The device behavior is accurately modeled using a low carrier mobility of 5 × 10–4 cm2 V–1 s–1 and measured optical properties for the CuInSe2 nanocrystal films. The simulations reveal that a reduction in the energy barrier for hole transport at the back contact and increased mobility and lifetime of charge carriers in the CuInSe2 nanocrystal layer could improve device performance. Furthermore, this system is qualitatively different than the well-studied PVs of PbS nanocrystals, as manipulating the CuInSe2 nanocrystal electron affinity by ligand exchange actually leads to unfavorable band alignment across the device and does not improve device performance.