Preventing Interfacial Recombination in Colloidal Quantum Dot Solar Cells by Doping the Metal Oxide

Abstract

Recent research has pushed the efficiency of colloidal quantum dot solar cells toward a level that spurs commercial interest. Quantum dot/metal oxide bilayers form the most efficient colloidal quantum dot solar cells, and most studies have advanced the understanding of the quantum dot component. We study the interfacial recombination process in depleted heterojunction colloidal quantum dot (QD) solar cells formed with ZnO as the oxide by varying (i) the carrier concentration of the ZnO layer and (ii) the density of intragap recombination sites in the QD layer. We find that the open-circuit voltage and efficiency of PbS QD/ZnO devices can be improved by 50% upon doping of the ZnO with nitrogen to reduce its carrier concentration. In contrast, doping the ZnO did not change the performance of PbSe QD/ZnO solar cells. We use X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, transient photovoltage decay measurements, transient absorption spectroscopy, and intensity-dependent photocurrent measurements to investigate the origin of this effect. We find a significant density of intragap states within the band gap of the PbS quantum dots. These states facilitate recombination at the PbS/ZnO interface, which can be suppressed by reducing the density of occupied states in the ZnO. For the PbSe QD/ZnO solar cells, where fewer intragap states are observed in the quantum dots, the interfacial recombination channel does not limit device performance. Our study sheds light on the mechanisms of interfacial recombination in colloidal quantum dot solar cells and emphasizes the influence of quantum dot intragap states and metal oxide properties on this loss pathway.

Ethanedithiol Treatment of Solution‐Processed ZnO Thin Films: Controlling the Intragap States of Electron Transporting Interlayers for Efficient and Stable Inverted Organic Photovoltaics

“…This fact suggests that the Fermi level is close to the conducting band edge, implying a high electron concentration induced by the defects, such as oxygen vacancies. [40] After EDT treatment, the position of the valence band edge relative to the Fermi level decreases to 2.4 eV. Therefore the Fermi level is moving towards the center of the band gap, suggesting a significant reduction of electron concentration.…”

Multiple exciton generation in quantum dot-based solar cells

“…On a more general level, surface trapped carriers are highly localised and are therefore challenging to extract during solar cell operation [79,80]. Additionally, charge transport properties are intimately linked to the interparticle coupling constant within the QD film [81].…”

“…The second effect of using hydrazine as a ligand is the very short interparticle spacing; this reduces the width of the tunnelling barrier between QDs thereby increasing carrier mobility of the film as a whole. Both effects are particularly beneficial for holes that are produced via MEG as they are commonly located far away from the holeextracting electrode and are thus prone to recombination during charge extraction [79,82,83].…”

“…The increased n-type doping density of the QD film introduced by the hydrazine ligands reduces this splitting and consequently lowers the extractable photovoltage. This is particularly problematic as the V oc in QD-based solar cells is already relatively poor due to other effects such as sub-band gap QD trap states [113] and interfacial trap-states (see Section 4) [79]. Furthermore, while ultra-short ligand molecules increase charge extraction, they also weaken the quantum confinement within the particles, which ultimately interferes with the MEG mechanism itself.…”

“…There are, however, challenges which still need to be overcome (i.e. many shunting pathways due to long and non-uniform nanowires [139] and enhanced trap-mediated recombination caused by the significantly increased interfacial area) [79]. It can be concluded at this stage that adopting QD solar cell architectures according to a tandem-like principle or a nanostructured device environment may enable MEG to contribute to the photocurrent of a PV device more efficiently.…”