CuInSe₂ nanocrystal photovoltaics : device physics, defects, and ligand chemistry

Nanocrystal based photovoltaics (PVs) have the potential to be printed at room temperature on a variety of substrates with the performance and stability required to enable low-cost, lightweight portable power applications. CuInSe₂ nanocrystal (PVs), fabricated using methods like those developed for polycrystalline CuIn [subscript x] Ga [subscript 1-x] Se₂ (CIGS) PVs, have exhibited some success in this direction, but the efficiency of the devices has been limited. Simulations of CuInSe₂ nanocrystal photovoltaic devices using Solar Cell Capacitance Simulator (SCAPS) software indicate that higher performance is not likely to be achieved though ligand exchange procedures that influence the nanocrystals’ electron affinity, as reported for PbS nanocrystal photovoltaics. Higher PV performance requires a combination of increased carrier mobility and longer carrier lifetimes. Both are influenced by the capping ligand chemistry and the presence of defects in the nanocrystals. The nanocrystal synthesis conditions are critical for controlling the defects and phase of the nanocrystal products. X-ray diffraction (XRD) shows that CuInSe₂ tends to form the tetragonal chalcopyrite structure while under fast nucleation conditions CuIn [subscript x] Ga [subscript 1-x] Se₂ nanocrystals form the hexagonal wurtzite phase. Raman spectroscopy has shown that the CuInSe₂ nanocrystals contain a significant concentration of InC [subscript u]+2V [subscript Cu] pair defects that depends on the synthesis conditions. These pair defects tend to order under light excitation and can be eliminated by high temperature annealing. Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy indicates that the oleylamine and diphenylphosphine capping ligands used in a typical CuInSe₂ nanocrystal synthesis bind strongly to the nanocrystal surface and exhibit very limited desorption. The addition of I₂ induces ligand desorption and facilitates ligand exchange on these nanocrystals. Furthermore, the tightly bound ligands do not evaporate when heated under inert conditions, like the free molecule would. The bound ligands are found to form disordered sp² carbon that vaporizes over tens of minutes. These studies provide insight for improving the performance of CuInSe₂ nanocrystal photovoltaics by showing that the ligand chemistry and defects in CuInSe₂ are relatively unique compared to other types of nanocrystals