Abstract
The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.
Highlights
The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications
Previous research conducted on inorganic perovskite NCs (IP-NCs) NCs, failed to reveal carrier multiplication (CM): comparison of PL decay dynamics upon excitation with high and low energy photons did not show any signatures of CM43
Very little is known on the material parameters governing the efficiency and the threshold of CM—the most important features determining the possible impact of CM for practical applications
Summary
The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications. If the excess energy reaches a certain threshold, an interaction between a hot electron (hole) and other valence electrons (holes) can take place instead, such that a second e–h pair is generated In bulk semiconductors, this phenomenon is known as impact ionization[1,2] and was first observed in crystalline bulk semiconductors Si and Ge3. By comparing carrier transients at different pump photon energies, we demonstrate the fingerprint of CM in the form of a fast component induced by AR of multiple e–h pairs appearing in the same NC. We investigate in detail the dynamics for above threshold pumping, on a picosecond time scale This reveals that the CM process coincides with a longer build-up of the free carrier concentration and provides new and important insights into the CM phenomenon. We provide an unambiguous evidence of CM in allinorganic CsPbI3 NCs
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