Perovskite
quantum dots (PQDs) have many properties that make them
attractive for optoelectronic applications, including expanded compositional
tunability and crystallographic stabilization. While they have not
achieved the same photovoltaic (PV) efficiencies of top-performing
perovskite thin films, they do reproducibly show high open circuit
voltage (V
OC) in comparison. Further understanding
of the V
OC attainable in PQDs as a function
of surface passivation, contact layers, and PQD composition will further
progress the field and may lend useful lessons for non-QD perovskite
solar cells. Here, we use photoluminescence-based spectroscopic techniques
to understand and identify the governing physics of the V
OC in CsPbI3 PQDs. In particular, we probe
the effect of the ligand exchange and contact interfaces on the V
OC and free charge carrier concentration. The
free charge carrier concentration is orders of magnitude higher than
in typical perovskite thin films and could be tunable through ligand
chemistry. Tuning the PQD A-site cation composition via replacement of Cs+ with FA+ maintains the
background carrier concentration but reduces the trap density by up
to a factor of 40, reducing the V
OC deficit.
These results dictate how to improve PQD optoelectronic properties
and PV device performance and explain the reduced interfacial recombination
observed by coupling PQDs with thin-film perovskites for a hybrid
absorber layer.
The 1.24 eV bandgap, 18.8% power conversion efficiency Ag‐alloyed chalcopyrite (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells are characterized to relate voltage and efficiency improvements to electro‐optical (EO) characteristics. Shockley–Read–Hall recombination center defect density, identified and characterized through deep level transient spectroscopy and time‐resolved photoluminescence (TRPL), is reduced through potassium and copper treatment optimization. Concomitantly, longer minority carrier lifetimes are achieved, which increases open‐circuit voltage (VOC). Near‐conduction band defects associated in earlier studies with light‐induced current instability are also mitigated. Analysis of charge‐carrier dynamics after single‐ and two‐photon excitation is used to separate recombination at the front interface and in the absorber bulk. From TRPL decay simulations, the authors estimate ranges of key solar cell material characteristics: bulk carrier lifetime τbulk = 110–210 ns, charge‐carrier mobility μ = 110–160 cm2 V−1 s−1, and front interface recombination velocity Sfront = 700–1050 cm s−1. This lowest‐reported Sfront for ACIGS absorbers originates from the notched conduction band grading, which also makes the impact of the back interface recombination negligible. It is suggested in the results that solar cell performance enhancements can be made most readily with two distinct strategies: improving device architecture and reducing semiconductor defect densities. Using these approaches, power conversion efficiency in large‐area solar cells is improved by 1.1% absolute.
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