Depleted-Heterojunction Colloidal Quantum Dot Solar Cells

Abstract

Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processability with quantum size-effect tunability to match absorption with the solar spectrum. Rapid recent advances in CQD photovoltaics have led to impressive 3.6% AM1.5 solar power conversion efficiencies. Two distinct device architectures and operating mechanisms have been advanced. The first-the Schottky device-was optimized and explained in terms of a depletion region driving electron-hole pair separation on the semiconductor side of a junction between an opaque low-work-function metal and a p-type CQD film. The second-the excitonic device-employed a CQD layer atop a transparent conductive oxide (TCO) and was explained in terms of diffusive exciton transport via energy transfer followed by exciton separation at the type-II heterointerface between the CQD film and the TCO. Here we fabricate CQD photovoltaic devices on TCOs and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation, and that they also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. We report the highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety.

Multiple exciton generation in quantum dot-based solar cells

“…These strategies use for such surface passivation either a ligand sphere of organic [55,82,83] and/or inorganic molecules [49,80,84] [82,83,89]. While QDs passivated with dithiol ligands showed high charge-carrier mobilities [90,91] and competitive device performances in solar cells, no evidence for MEG could be found in devices [26,60].…”

“…Furthermore, the reducing character of molecules such as hydrazine has been shown to increase the n-type doping density of IV-VI QD films significantly [96]. Most efficient QD-based PV devices employ a heterojunction analogous to a p-n junction at an interface between an n-type MO and a depleted, p-type QD layer [82,103,104]. It follows that the significant, ligand-associated changes in QD energetics and doping level to promote MEG in a device environment (e.g.…”

“…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].…”

“…In a conventional QD-based solar cell, the voltage is mainly defined by the quasi-Fermi level splitting between the (mostly p-type) QD donor and the (n-type) MO acceptor at the p-n junction [82,108]. The increased n-type doping density of the QD film introduced by the hydrazine ligands reduces this splitting and consequently lowers the extractable photovoltage.…”

Solid‐State Ligand‐Exchange Fabrication of CH3NH3PbI3 Capped PbS Quantum Dot Solar Cells

“…The surface ligands of CQDs play a crucial role in determining the optoelectronic properties of CQD solar cells 5. Generally, CQDs that bear long‐chain organic ligands offer the solubility, yet those ligands unavoidably form insulating barriers of charge transport between CQDs.…”