No abstract

No abstract

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.

Thin‐film photovoltaic device efficiencies are limited by carrier recombination, thus understanding recombination mechanisms is critical for performance improvements. Bulk minority carrier lifetime (τ bulk) is a critical parameter for solar cells but is difficult to determine in P–N junction devices, especially for high doping. As doping ≥1016 cm−3 is required for efficient drift‐charge‐carrier‐collection devices, a method for τ bulk determination in doped P–N junction devices is necessary. This work utilizes time‐resolved photoluminescence (TRPL) simulations to quantify bulk and interface recombination properties in highly doped, graded absorber CdSeTe structures. The two methods developed here for τ bulk determination include utilization of an instantaneous lifetime representation to guide TRPL fitting and direct comparison between measured and simulated decays. Simulations verified that both methods are valid for state‐of‐the‐art device architectures which include graded bandgap absorbers, graded doping, and graded lifetimes. Shifts in the dominant recombination mechanism are identified for sufficiently long τ bulk, where front and back interface quality plays a more prominent role. Evaluation of surface recombination velocities and conduction band offset illustrate electro‐optical advantages of a positive conduction band offset and highlight the necessity of improved interfaces as bulk quality in photovoltaic devices improves.

No abstract

No abstract

The optimal fraction of Mg incorporation in sputter‐deposited MgXZn1−XO (MZO) emitters for thin‐film CdTe‐based solar cells is evaluated by varying it over a range of x from 0 to 0.35. This range allows a variation in the conduction band offset from −0.1 eV (cliff like) to +0.32 eV (spike like). A maximum efficiency of 18.5% for cells with the bilayer CdSeTe/CdTe absorber occurs at x = 0.15, which corresponds to a spike‐like band offset near 0.2 eV, as confirmed by X‐ray photoelectron spectroscopy. In addition, good cell performance is seen over a fairly broad range of x extending from 0.1 to 0.25. The MZO optical bandgap increases with the Mg fraction, consistent with an increasing conduction band offset. Temperature‐dependent current−voltage measurements and time‐resolved photoluminescence show improvement in the emitter/absorber interface with the incorporation of Mg. Capacitance−voltage measurements show that the depletion region extends further into the absorber with more Mg, and X‐ray diffraction confirms a change from a hexagonal‐dominant crystal structure toward zinc blende at x = 0.35.