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.
Highlights
C harge-carrier multiplication processes where an initial hot exciton is converted into multiple electron−hole pairs are considered promising mechanisms in photovoltaic devices to bypass efficiency losses due to thermalization of carriers generated by above-bandgap photons.[1−3] While carrier multiplication in inorganic bulk semiconductors is inefficient and only significant at very high photon energies,[4] the same process is significantly amplified in semiconducting quantum dots (QDs) where the process is termed multiple exciton generation (MEG).[5]
It is believed that this phenomenon is due to the quantum confinement in QDs that provides a strong coulomb matrix element for the generation of multiple excitons from a hot single exciton and valence band electrons.[6−8] the restricted spatial extent of the crystal weakens the requirement for crystal momentum conservation, thereby reducing the photon energy that is necessary to drive MEG.[9,10]
The early time yield of multiple-exciton states is determined by a competition between the MEG process and rapid cooling of the photogenerated state.[11−14] The multiple-exciton state may decay by an Auger process, which in lead chalcogenide QDs is found to occur on 20−200 ps time scales,[15,16] and it is this process that is typically used as the signature of multiple excitons in spectroscopic measurements of MEG yields.[11,16,17]
Summary
Ultrafast transient absorption and two-pulse ultrafast photocurrent spectroscopy we confirm delayed intraband relaxation of hot carriers at early times and estimate the threshold energy necessary to extract charge carriers generated by MEG. The additional photocurrent response for high pump energies decays with increasing delay between pump and push pulses on a time scale of ∼900 ± 200 fs (see Supporting Information S12). We attribute this push-induced photocurrent response to additional carriers generated via MEG. We identify a high-energy ground-state bleach signal that indicates a delayed intraband cooling process of the initial hot single exciton As this mechanism is believed to directly compete with MEG at early times we argue that the time for converting single excitons into multiple charge carrier pairs is extended. Together with a calculated IQE of more than 150% at around 3.3Eg, this demonstrates the potential of QD solar cells to recover significant above-bandgap energy that would normally be lost to thermalization
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