(390e) Optoelectronic Characterization of Silver-Doped Cu3AsS4 for Photovoltaic Applications

Enargite (Cu₃AsS₄) is an emerging photovoltaic material, consisting of earth abundant elements, with a bandgap in the vicinity of 1.41 eV. Its constituent atoms, Cu, As, and S, have differing ionic radii of 91, 60, and 170 pm respectively, reducing the likelihood of cation disorder. Theoretical studies conducted on this material have suggested that enargite is expected to have high carrier mobilities, high defect tolerance with shallow defects, and benign grain boundaries.¹ Subsequent experimental work on enargite, Cu₃AsS₄, from our group has demonstrated that enargite films have a Shockley-Read-Hall, or defect assisted, recombination lifetime on the timescale of nanoseconds, relatively shallow defect energies, and carrier concentrations on the order of 10¹⁵ cm^-3.^2,3 However, carbonaceous fine grain layers, secondary phases, and pinholes may be part of the reason that power conversion efficiencies for enargite-based photovoltaic devices remained low. In this work, silver doping of Cu₃AsS₄ films facilitated by a solution-processed route using amine-thiol based molecular precursor inks is shown to improve film morphology and the optoelectronic properties of enargite-based photovoltaic devices.

The amine-thiol solvent system is capable of dissolving a variety of metal chalcogenides for the fabrication of thin films and nanoparticles.^4,5 Arsenic sulfides and copper sulfides can undergo reactive dissolution in amine-thiol solvents to form complexes that can decompose at temperatures below 400°C. These molecular precursor solutions can be combined into inks that can directly be coated onto glass substrates to create tetragonal luzonite (Cu­₃AsS₄) films. These nanocrystalline films can then be coarsened in an arsenic-sulfide atmosphere at 425°C to produce an orthogonal enargite (Cu₃AsS₄) film with 0.5 µm sized grains. Unfortunately, devices made using these enargite films demonstrate low shunt resistances, poor film morphology, and low external quantum efficiency (EQE).

Photoluminescence (PL) characterization on the coarsened enargite film shows a large peak at 0.9 eV. As this peak is roughly 0.5 eV from the band edge, it would be considered a mid-gap defect and is likely a large contributor to the poor performance of enargite-based devices. Computational analysis done on the defect properties of enargite shows that S_Cu and S_As anti-site defects would have energies near 0.5 eV from the enargite valence band and could be the cause for the large PL peak seen at 0.9 eV.⁶ In hopes of increasing shunt resistances, improving film morphology, and improving optoelectronic properties, inspiration was taken from ongoing research on silver doping of Cu₂ZnSnSe₄ (CZTSe) and Cu(In,Ga)Se₂ (CIGSe) thin film solar cells.

In both CZTSe and CIGSe films, silver doping has been shown to decrease the impact of copper-based defects by modifying their formation energy and their location in the absorber band gap.^7,8 Furthermore, silver alloying has been shown to increase grain size and reduce structural defects, which may help improve the morphology of enargite films and increase shunt resistances. The amine-thiol solvent system allows for facile incorporation of varying concentrations of Ag₂S into coating inks for the creation of silver doped enargite films. Films with a silver content ranging from 0 to 0.1 Ag/(Cu+Ag) were created and analyzed through various optoelectronic characterization techniques.

Silver incorporation increases the average size of grains in coarsened enargite films from 560 nm to 1.11 µm. The overall morphology of the film also improves with no visible pinholes in SEM micrographs. Kelvin-probe force microscopy (KPFM) shows that the grain boundaries of enargite films at all levels of silver doping are negatively charged (having a positive contact potential difference) in comparison to the bulk grains, suggesting that grain boundaries will not act as shunt pathways for minority carriers in the device. Time-resolved photoluminescence shows a doubling in minority carrier lifetimes from 0.23 ns for 0 Ag/(Cu+Ag) to 0.46 ns for 0.1 Ag/(Cu+Ag). The maximum EQE of enargite increases with increasing silver content from 1% to over 8% at 0.5 Ag/(Cu+Ag). The bandgap of enargite was extracted from the EQE spectra, and similar to what occurs in silver doped CZTSe and CIGSe, silver doping increases the band gap of enargite from 1.43 eV for no silver to 1.50 eV for 0.1 Ag/(Cu+Ag). Photoluminescence measurements on silver-doped enargite films show that the relative intensity of the primary band-to-band recombination peak decreases with increasing silver content. This may be due to silver-based defects in the films leading to an increase in non-radiative recombination. The relative intensity of the photoluminescence peak at 0.9 eV does not change with silver doping, suggesting that this peak may not be related to a copper-based defect.

Silver-doped enargite devices show a maximum champion efficiency of 0.6% at 0.02 Ag/(Cu+Ag). Improved film morphology and increased grain size leads to an initial increase in shunt resistance and J_sc. However, likely due to increases in bandgap energies with increasing silver content, J_sc decreases after 0.05 Ag/(Cu+Ag) from a maximum of 1.9 mA/cm². Increasing bandgap energies also contributes to the initial increase in V_oc. However, increases in non-radiative recombination at higher levels of silver doping causes V_oc to decrease from a maximum of 170 mV at 0.02 Ag/(Cu+Ag) to 80 mV at 0.1 Ag/(Cu+Ag).

This work demonstrates that silver-doping can be employed on emerging photovoltaic materials and may produce similar effects as is seen in AZTSe and CIGSe, including increased grain size, increased bandgap, and improved minority carrier lifetimes. In enargite films, the ideal amount of silver doping is around 0.02 Ag/(Cu+Ag) as it maximizes improvements in film morphology and minority carrier lifetimes while minimizing the drawbacks involved with enlarged bandgap energies and increased silver-based defects.

¹Wallace, S. K., Svane, K. L., Huhn, W. P., Zhu, T., Mitzi, D. B., Blum, V., Walsh, A., “Candidate photoferroic absorber materials for thin-film solar cells from naturally occurring minerals: enargite, stephanite, and bournonite,” Sustainable Energy and Fuels, 1, 1339, (2017).

²McClary, S. A., Andler, J., Handwerker, C. A., Agrawal, R., “Solution-processed copper arsenic sulfide thin films for photovoltaic applications,” Journal of Materials Chemistry C, 5, 28, pp. 6913-6916 (2017).

³McClary, S. A., Taheri, M. M., Blach, D. D., Pradhan, A. A., Li, S., Huang, L., Baxter, J. B., Agrawal, R., ”Nanosecond carrier lifetimes in solution-processed enargite (Cu₃AsS₄) thin films,” Applied Physics Letters, 117, 16, pp. 162102 (2020).

⁴McCarthy, C. L, Brutchey, C. L., “Solution processing of chalcogenide materials using thiol-amine “alkahest” solvent systems,” Chemical Communications, 53, pp. 4888-4902 (2017).

⁵Zhao, X., Deshmukh, S. D., Rokke, D. J., Zhang, G., Wu, Z., Miller, J. T., Agrawal, R., “Investigating Chemistry of Metal Dissolution in Amine-Thiol Mixtures and Exploiting It Toward Benign Ink Formulation for Metal Chalcogenide Thin Films,” Chemistry of Materials, 31, 15, pp. 5674-5682 (2019).

⁶McClary, S. A., Yao, C., Rokke, D. J., Pradhan, A. A., Andler, J., Yan, Y., Agrawal, R., “A combined Experimental and Theoretical Study of the Defect Properties of Enargite Cu₃AsS₄,” In Prep.

⁷Hages, C. J., Koeper, M. J., Agrawal, R., “Optoelectronic and material properties of nanocrystal-based CZTSe absorbers with Ag-alloying,” Solar Energy Materials and Solar Cells, 145, 3, pp. 342-348 (2016).

⁸Zhao, Y., Yuan, S., Kou, D., Zhou, Z., Wang, X., Xiao, H., Deng, Y., Cui, C., Chang, Q., Wu, S., “High Efficiency CIGS Solar Cells by Bulk Defect Passivation through Ag Substitution Strategy,” ACS Applied Materials & Interfaces, 12, 11, pp. 12717-12726 (2020).