Lead Telluride Quantum Dot Solar Cells Displaying External Quantum Efficiencies Exceeding 120%

Abstract

Multiple exciton generation (MEG) in semiconducting quantum dots is a process that produces multiple charge-carrier pairs from a single excitation. MEG is a possible route to bypass the Shockley-Queisser limit in single-junction solar cells but it remains challenging to harvest charge-carrier pairs generated by MEG in working photovoltaic devices. Initial yields of additional carrier pairs may be reduced due to ultrafast intraband relaxation processes that compete with MEG at early times. Quantum dots of materials that display reduced carrier cooling rates (e.g., PbTe) are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we demonstrate PbTe quantum dot-based solar cells, which produce extractable charge carrier pairs with an external quantum efficiency above 120%, and we estimate an internal quantum efficiency exceeding 150%. Resolving the charge carrier kinetics on the ultrafast time scale with pump–probe transient absorption and pump–push–photocurrent measurements, we identify a delayed cooling effect above the threshold energy for MEG.

Multiple exciton generation in quantum dot-based solar cells

“…This transiently stable, hot carrier population close to or above the material's MEG th E extends the time window for MEG. Such a phonon scattering bottleneck is thus consistent with the high MEG yields observed in dispersed PbTe QDs [31,50,59] and is also in agreement with the high EQEs (>120%) reported for devices based on the same QD material [28]. Figure 3A) [46,68].…”

“…In PbTe QDs, the energy of this effective phonon scattering bottleneck is approximately at the same position as MEG th , E which has been determined independently under operational solar cell conditions [28]. This transiently stable, hot carrier population close to or above the material's MEG th E extends the time window for MEG.…”

“…However, recent spectroscopic measurements by Geiregat et al indicate that quantum-confined lead chalcogenide materials show transiently stable carrier populations at distinct energies high-up in the condensed energy manifold [28,67]. To explain this transient accumulation of charge carriers far from the band gap, the authors consider a high-energy point in the material's band structure, which requires the relaxing charge carriers to change the direction of the scattering wave vector of the emitted phonons.…”

“…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].…”

“…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.…”