Photovoltaic Performance of Ultrasmall PbSe Quantum Dots

Abstract

We investigated the effect of PbSe quantum dot size on the performance of Schottky solar cells made in an ITO/PEDOT/PbSe/aluminum structure, varying the PbSe nanoparticle diameter from 1 to 3 nm. In this highly confined regime, we find that the larger particle bandgap can lead to higher open-circuit voltages (∼0.6 V), and thus an increase in overall efficiency compared to previously reported devices of this structure. To carry out this study, we modified existing synthesis methods to obtain ultrasmall PbSe nanocrystals with diameters as small as 1 nm, where the nanocrystal size is controlled by adjusting the growth temperature. As expected, we find that photocurrent decreases with size due to reduced absorption and increased recombination, but we also find that the open-circuit voltage begins to decrease for particles with diameters smaller than 2 nm, most likely due to reduced collection efficiency. Owing to this effect, we find peak performance for devices made with PbSe dots with a first exciton energy of ∼1.6 eV (2.3 nm diameter), with a typical efficiency of 3.5%, and a champion device efficiency of 4.57%. Comparing the external quantum efficiency of our devices to an optical model reveals that the photocurrent is also strongly affected by the coherent interference in the thin film due to Fabry-Pérot cavity modes within the PbSe layer. Our results demonstrate that even in this simple device architecture, fine-tuning of the nanoparticle size can lead to substantial improvements in efficiency.

Nonmonotonic Size Dependence in the Hole Mobility of Methoxide-Stabilized PbSe Quantum Dot Solids

“…Films of initially well-dispersed quantum dots with a mean interparticle spacing of roughly 15 Å undergo a contraction to ∼2À3 Å upon exchange with methoxide. We note a similar effect reported in PbSe nanoparticle films where hydrous methanol was used to replace Pb(oleate) 2 . 15 In that work, partial fusion of nanoparticles was reported upon ligand exchange, which we do not observe with the treatment presented here.…”

“…Previous studies using carboxylic acids and PbSe have found similar quick reversals of excitonic red shift upon air exposure. 13 We demonstrate the reduction of interparticle spacing upon ligand exchange from Pb(oleate) 2 to Pb(MeO) 2 by a comparative TEM study in Figure 4. Films of initially well-dispersed quantum dots with a mean interparticle spacing of roughly 15 Å undergo a contraction to ∼2À3 Å upon exchange with methoxide.…”

“…For the smallest particles (3 nm), the exciton was not measurable in thin films, possibly due to either extremely large exciton shifts or the dominance of FabryÀPérot cavity modes. 2 For more details on these arguments, the reader is referred to the SI. For comparison, we include two reference points for EDT-capped PbSe nanoparticles at a given size in the diagram.…”

“…Oxidation of PbSe in air is a multistep process leading to the formation of PbO, Pb(OH) 2 , and PbSeO x species. 4,32 O 2 is readily incorporated into the PbSe lattice, resulting in a smaller semiconductor nanoparticle diameter and larger interparticle spacing.…”

“…32,33 Water can have a similarly detrimental effect on the structure and chemical nature of PbSe by substitutional incorporation into the lattice and formation of H 2 Se and Pb(OH) 2 . For example, Zhang et al have studied the gradual formation of an amorphous Pb(OH) 2 shell onto the surface of PbTe nanoparticles upon exposure to aqueous bases. 34 Specifically in the case of MeO À -capped nanoparticles, exposure to moisture will lead to the formation of methanol in the ligand sphere.…”