Time‐resolved photoluminescence (TRPL) is a powerful characterization technique to study carrier dynamics and quantify absorber quality in semiconductors. The minority carrier lifetime, which is critically important for high‐performance solar cells, is often derived from TRPL analysis. However, here it is shown that various nonideal absorber properties can dominate the TRPL signal making reliable extraction of the minority carrier lifetime not possible. Through high‐resolution intensity‐, temperature‐, voltage‐dependent, and spectrally resolved TRPL measurements on absorbers and devices it is shown that photoluminescence (PL) decay times for kesterite materials are dominated by minority carrier detrapping. Therefore, PL decay times do not correspond to the minority carrier lifetime for these materials. The lifetimes measured here are on the order of hundreds of picoseconds in contrast to the nanosecond lifetimes suggested by the decay curves. These results are supported with additional measurements, device simulation, and comparison with recombination limited PL decays measured on Cu(In,Ga)Se2. The kesterite material system is used as a case study to demonstrate the general analysis of TRPL data in the limit of various measurement conditions and nonideal absorber properties. The data indicate that the current bottleneck for kesterite solar cells is the minority carrier lifetime.

In this work, the benefits of Ag-alloying in kesterite solar cells are explored in terms of tunable band gap, improved grain growth, improved minority carrier lifetime, reduced defect formation, and reduced potential fluctuations for (Ag,Cu) 2 ZnSnSe 4 (ACZTSe) absorbers relative to Cu 2 ZnSnSe 4 (CZTSe). The enhanced optoelectronic properties are shown to scale here with the degree of Ag-alloying in ACZTSe. The impacts of these effects on device performance are discussed, with improvement in average device performance/open-circuit voltage reported for ACZTSe (5%-Ag) absorbers relative to CZTSe absorbers with similar band gap. These initial results are promising for the Ag-alloyed ACZTSe material system as V OC limitations are the primary cause of poor device performance in kesterite solar cells, and cation substitution presents a unique method to tune the defect properties of kesterite absorbers. Herein, nanoparticle synthesis and large-grain ACZTSe absorber formation is described followed by material and optoelectronic characterization. Additionally, RTP processing is presented to achieve fully selenized large-grain chalcogenide absorbers from sulfide nanocrystal inks.In addition to modification of the defect properties, Ag-alloying may also be beneficial for band gap tuning/grading of the absorber for improved performance. For CZTSSe, the absorber band gap (E G ) is determined mainly by Cu d orbital and S/Se p orbital anti-bonding (valance band maximum -VBM) and Sn s orbital and S/Se sp

Large-grain absorber formation through selenization techniques is a promising route for high performance chalcogenide solar cells. Understanding and subsequently controlling such grain growth is essential in improving absorber quality and developing absorbers with unique optoelectronic and morphological properties. We explain the essential role of liquid selenium in the grain growth of Cu 2 ZnSnSe 4 (CZTSe) absorbers from Cu 2 ZnSnS 4 nanoparticles by proposing a liquid-assisted grain growth mechanism. Through the use of a multizone rapid-thermalprocessing furnace, control of liquid Se delivery to the film and the Se (g) atmosphere during processing is shown to result in novel absorbers with tunable properties. Additionally, the processing parameters necessary for high quality CZTSe absorbers, the role of nanoparticle properties, and the role of alkali metal dopants in the liquid-assisted growth mechanism are shown. Ultimately, record nanoparticle-based device performance of 9.3% is achieved for selenized CZTSe absorbers.

A monoamine–dithiol mixture is used to prepare homogeneous Cu(In, Ga)Se2 (CIGSe) molecular precursor solution, which yields a highly sulfur depleted CIGSe thin-film solar cell with a power conversion efficiency of 12.2%.

Admittance spectroscopy is a useful tool used to study defects in semiconductor materials. However, metastable defect responses in non-ideal semiconductors can greatly impact the measurement and therefore the interpretation of results. Here, admittance spectroscopy was performed on Cu2ZnSn(S,Se)4 where metastable defect response is illustrated due to the trapping of injected carriers into a deep defect state. To investigate the metastable response, admittance measurements were performed under electrically and optically relaxed conditions in comparison to a device following a low level carrier-injection pretreatment. The relaxed measurement demonstrates a single capacitance signature while two capacitance signatures are observed for the device measured following carrier-injection. The deeper level signature, typically reported for kesterites, is activated by charge trapping following carrier injection. Both signatures are attributed to bulk level defects. The significant metastable response observed on kesterites due to charge trapping obscures accurate interpretation of defect levels from admittance spectroscopy and indicates that great care must be taken when performing and interpreting this measurement on non-ideal devices.

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