Sharp Exponential Band Tails in Highly Disordered Lead Sulfide Quantum Dot Arrays

We employ temperature-dependent, illumination intensity modulated photocurrent spectroscopy (IMPS) to investigate the intra-band-gap density of states in films of PbS quantum dots (QDs). Using both coplanar electrode and stacked photovoltaic device configurations, IMPS measurements of PbS QD arrays show evidence of carrier trapping in exponential band tails extending from the band edges into the gap. The band tails have characteristic energies near 14 meV, similar to those found in other larger grain, polycrystalline bulk semiconductors, rather than the large Urbach energies normally associated with nanocrystals and porous/polycrystalline films. This result helps explain recent success in using QD solids in device applications and indicates potential for QD materials to compete with bulk materials in semiconductor applications.