Time resolved photoluminescence on Cu(In, Ga)Se-2 absorbers: Distinguishing degradation and trap states

Recent reports have suggested that the long decay times in time resolved photoluminescence (TRPL), often measured in Cu(In, Ga)Se-2 absorbers may be a result of detrapping from sub-bandgap defects. In this work, we show via temperature dependent measurements, that long lifetimes >50 ns can be observed that reflect the true minority carrier lifetime not related to deep trapping. Temperature dependent time resolved photoluminescence and steady state photoluminescence imaging measurements are used to analyze the effect of annealing in air and in a nitrogen atmosphere between 300K and 350K. We show that heating the Cu(In, Ga)Se-2 absorber in air can irreversibly decrease the TRPL decay time, likely due to a deterioration of the absorber surface. Annealing in an oxygen-free environment yields a temperature dependence of the TRPL decay times in accordance with Schockley Read Hall recombination kinetics and weakly varying capture cross sections according to T-0.6. Published by AIP Publishing

Chemistry and Dynamics of Ge in Kesterite : Toward Band-Gap- Graded Absorbers

peer reviewe

Development and characterization of nanoparticle-based kesterite solar cells

Kesterite solar cells based off of the Cu2ZnSn(S,Se) 4 (CZTSSe) material system are a promising technology for earth-abundant photovoltaic applications. Thin-film absorbers fabricated from kesterite materials demonstrate a tunable band gap ideal for photovoltaic applications, high absorption coefficient ideal for low material usage, and success with low-cost, scalable, solution-based processing techniques. However, CZTSSe solar cells achieve low power-conversion efficiencies relative to other successful solar absorber technologies with cubic/tetragonal crystal structure, such as c-Si, GaAs, CdTe, and Cu(In,Ga)Se2. In this work, the development of kesterite-based solar cells from nanocrystal inks is discussed in terms of understanding and improving device performance for this material system. Herein, an overview of CZTSSe solar technology, nanoparticle synthesis, thin film formation, and optoelectronic characterization is presented, illustrating the challenges associated with the formation of high-quality quaternary semiconductors. Detailed optoelectronic and material characterization is used to develop an understanding of the fundamental limitations to improved device performance, with general applicability to the CZTSSe material system; namely, detailed analysis of the origin of voltage limitations commonly reported for CZTSSe solar cells is considered. Fundamental device performance limitations are associated here with a high propensity for detrimental defect formation in CZTSSe absorbers. Ultimately, an improved understanding of these device performance limitations and their association with the absorber material properties is used to develop novel thermal processing techniques to improve the absorber transport properties. Optimized absorber processing has resulted in the fabrication of record performance nanocrystal-based CZTSSe solar cells. In addition to novel CZTSSe absorber processing, modification of the absorber properties is also presented in terms of alloyed-kesterite materials based off of the CZTSSe material system. Namely, the development of Ge-alloyed Cu2Zn(Sn,Ge)(S,Se)4 (CZTGeSSe) and Ag-alloyed (Ag,Cu)ZnSnSe4 (ACZTSe) absorbers is described. CZTGeSSe and ACZTSe are shown to demonstrate promise as alternative kesterite material absorbers due to improved optoelectronic properties and device performance relative to CZTSSe. Record device performance is reported here for CZTGeSSe and ACZTSe solar cells. Lastly, detailed optoelectronic characterization—including current-voltage, quantum efficiency, and capacitance spectroscopy—is used to develop novel analysis methodologies for non-ideal diodes. This work demonstrates how traditional optoelectronic characterization analysis techniques, commonly applied to CZTSSe solar cells, can result in misinterpretation of device limitations if non-ideal diode behavior is not accounted for in the diode analysis. Here, a consideration of the complex material properties in the developed optoelectronic characterization analysis methods has resulted in improved accuracy in understanding fundamental device performance limitations in kesterite solar cells

Controlled Grain Growth for High Performance Nanoparticle-Based Kesterite Solar Cells

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 Cu₂ZnSnSe₄ (CZTSe) absorbers from Cu₂ZnSnS₄ nanoparticles by proposing a liquid-assisted grain growth mechanism. Through the use of a multizone rapid-thermal-processing 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

Atomic Scale Structure of (Ag,Cu)2_{2}ZnSnSe4_{4} and Cu2_{2}Zn(Sn,Ge)Se4_{4} Kesterite Thin Films

Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both (Ag,Cu)$_{2}$ZnSnSe$_{4}$ and Cu$_{2}$Zn(Sn,Ge)Se$_{4}$ based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X-ray diffraction and low temperature X-ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu$_{2}$Zn(Sn,Ge)Se$_{4}$, the thin film absorbers show a very similar behavior as Cu$_{2}$Zn(Sn,Ge)Se$_{4}$ powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn-S bonds instead of Ge-S bonds. In (Ag,Cu)$_{2}$ZnSnSe$_{4}$, the dependence of the Ag-Se and Cu-Se bond lengths on Ag/(Ag+Cu) might indicate an energetic advantage in the formation of certain local configurations

Suppressed Deep Traps and Bandgap Fluctuations in Cu 2

The identification of performance-limiting factors is a crucial step in the development of solar cell technologies. Cu2ZnSn(S,Se)4-based solar cells have shown promising power conversion efficiencies in recent years, but their performance remains inferior compared to other thin-film solar cells. Moreover, the fundamental material characteristics that contribute to this inferior performance are unclear. In this paper, the performance-limiting role of deep-trap-level-inducing 2CuZn+SnZn defect clusters is revealed by comparing the defect formation energies and optoelectronic characteristics of Cu2ZnSnS4 and Cu2CdSnS4. It is shown that these deleterious defect clusters can be suppressed by substituting Zn with Cd in a Cu-poor compositional region. The substitution of Zn with Cd also significantly reduces the bandgap fluctuations, despite the similarity in the formation energy of the CuZn+ZnCu and CuCd+CdCu antisites. Detailed investigation of the Cu2CdSnS4 series with varying Cu/[Cd+Sn] ratios highlights the importance of Cu-poor composition, presumably via the presence of VCu, in improving the optoelectronic properties of the cation-substituted absorber. Finally, a 7.96% efficient Cu2CdSnS4 solar cell is demonstrated, which shows the highest efficiency among fully cation-substituted absorbers based on Cu2ZnSnS4.Ministry of Education (MOE)Accepted versionL.H.W. and S.H. acknowledge the funding support from the CREATE Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), which was supported by the National Research Foundation, Prime Minister’s Office, Singapore; and the Ministry of Education (MOE) Tier 2 Project (MOE2016-T2-1-030). E.A.C. acknowledges support from U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0002120, and from Princeton University for computing resources. V.I.-R. acknowledges support by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the Spanish Ministry of Science, Innovation and Universities under the IGNITE (ENE2017-87671-C3-1-R), and by the European Regional Development Funds (ERDF, Fons Europeu de Desenvolupament Regional (FEDER) Programa Competitivitat de Catalunya 2007–2013). V.I.-R. belongs to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the “Generalitat de Catalunya” (Ref. 2017 SGR 862). T.U. and S.L. acknowledge support by the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968