Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120%

Abstract

Multiple-exciton generation—a process in which multiple charge-carrier pairs are generated from a single optical excitation—is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley–Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation.

Multiple exciton generation in quantum dot-based solar cells

“…There is general consensus that such an increased number of under-coordinated surface atoms and the relatively small (B) Transmission electron microscopy images of as-synthesised lead selenide nanorods and their different shaped by-products. Part A has been reproduced from data provided in [46] and Part B has been taken from [27].…”

“…PVs made using these films, however, do not show clear MEG enhancement to the photocurrent. Instead, supplementing these shorter ligands with longer, thiol-based surface molecules such as mercaptopropionic acid [90] or a mixture of EDT and hydrazine [100,101] shows a clear contribution of MEG, but reduces carrier mobility and overall device efficiency [26][27][28].…”

“…Tuning QD properties to promote MEG in PVs through different organic ligands has been explored extensively [26,27,100]. There is general consensus that improved QD trap state passivation played a vital role in this improvement particularly in improving the typically poor V oc of QD devices [97,104].…”

“…nanorods) have shown improved MEG efficiencies over spherical QDs not only on the single particle level [45,46], but also in devices [27]. This has mainly been attributed to two effects: first, the stronger Coulomb matrix element in nanorods, which has been shown to increase MEG without altering the reverse process (i.e.…”

“…Singlet exciton fission materials in combination with suitable charge acceptors and QD-based solar cells have both been shown to produce external quantum efficiencies (EQEs) beyond 100% in a PV device [23][24][25][26][27][28]. While these proofof-concept studies show that CM effects can increase the photocurrent of a solar cell, PCEs in both types of device remain low.…”