Nanjing University of Posts and Telecommunications achieves 13% efficiency copper-zinc-tin-sulfur thin-film solar cells

On October 6, 2022, Beijing time, the international academic journal Nature Energy published the latest research results in the field of copper-zinc-tin-sulfur thin-film solar cells in the field of copper-zinc-tin-sulfur thin-film solar cells of Nanjing University of Posts and Telecommunications/Fudan University/Chinese Academy of Sciences, “Elemental de-mixing-induced epitaxial kesterite/CdS interface enabling 13%-efficiency.” kesterite solar cells”。

Professor Xin Hao of Nanjing University of Posts and Telecommunications, Professor Chen Shiyou of Fudan University and researcher Meng Qingbo of the Institute of Physics of the Chinese Academy of Sciences are the co-corresponding authors of the paper, and Gong Yuancai, doctoral student of Nanjing University of Posts and Telecommunications, and Zhu Qiang, a master’s student, are the co-first authors of the paper.

The copper-zinc-tin-sulfur selenium (Cu2ZnSn(S, Se)4, CZTSSe) semiconductor material of the zinc tin (Kesterite) structure can be seen as derived from the copper indium gallium selenium (Cu(In, Ga) Se2, CIGS) of the chalcopyrite structure by elemental substitution (Zn2++Sn4+ instead of In3+/G3+), so CZTSSe and CIGS have similar crystal structure, optical properties and more than 32% photoelectric conversion efficiency. Compared with CIGS, CZTSSe has low toxicity of the constituent elements and abundant earth reserves, so it is a new type of green low-cost photovoltaic material with great application potential. However, since 2013, the record conversion efficiency of CZTSSe solar cells has been stagnant at 12.6% for a long time, far lower than the 23.35% of CIGS solar cells, and the key factor restricting the efficiency of CZTSSe cells is the low open circuit voltage. CZTSSe’s cell structure is also derived from CIGS, p-type CZTSSe absorption layer and n-type CdS to build heterojunction, compared with CIGS, CZTSSe solar cells performance is seriously subject to defects caused by heterojunction interface recombination, however, for CZTSSe/CdS heterojunction interface defects formation mechanism has been unclear.

Early studies by Xin Hao’s team found that heterojunction heat treatment (JHT, 200°C) can significantly reduce the open circuit voltage of CZTSSe batteries prepared with DMSO solution with Sn4+ as a precursor compound (Sci. China Mater. 2021, 64, 1304; EES, 2021, 14, 2369; AFM, 2021, 2101927), and obtained copper-zinc-tin-sulfur battery devices with minimal open-circuit voltage losses. In this work, they found that the heat treatment of silver alloyed silver copper zzts, tin sulfur selenium ACZTSSe/CdS heterojunction (JHT) at low temperature (110 °C) conditions can significantly improve the open circuit voltage and filling factor of the cell (Figures 1b, d), one of the cells by the NREL certification photoelectric conversion efficiency reached 13.0%, which is the current world record for copper-zinc-tin-sulfur solar cells (Figure 1e, f). The paper did a comprehensive and systematic study to understand the root cause of heterojunction interface recombination of copper-zinc-tin-sulfur solar cells. It is found that during the chemical bath deposition of CdS, Zn2+ on the surface of the copper-zinc-tin-sulfur absorbing layer reacts with ammonia water to form a zinc-poor surface layer, resulting in Cd2+ entering the absorption layer and Zn2+ entering the CdS layer to form a defective heterojunction interface, and the low-temperature annealing treatment promotes the reverse mixing of elements, realizing the epitaxial heterojunction interface.

Figure 1: Effect of copper-zinc-tin-sulfur cell preparation process and low-temperature heterojunction heat treatment on photovoltaic performance of the cell.

Figure 2: Analysis of ACZTSSe and CISSe battery carrier composite characteristics.

(a) The effect of low-temperature annealing on heterojunction interface recombination was studied and compared with copper indium gallium selenium batteries

Transient photovoltage/transient photocurrent (M-TPV/M-TPC) and capacitance-voltage (CV, DLCP) analysis (Figure 2a-e) show that the improvement in device performance is mainly due to the reduction in the density of defects at ACZTSSe/CdS heterojunction interfaces. The voltage-temperature (VOC-T) results show that the composite activation energy (Ea) of the unheat-treated (Ref) device is only 0.95 eV, which is lower than the band gap of the material (1.11 eV), and after heat treatment Ea is raised to 1.10 eV, which is very close to the band gap, indicating that the low-temperature heat treatment almost eliminates the non-radiated recombination at the heterojunction interface (Figure 4f); The VOC-T results of the copper-indium sulfur selenium (CISSe) device prepared by the same method show that the unheat-treated battery has a perfect CISSe/CdS heterojunction interface (Ea=Eg), and the heat treatment significantly reduces Eaa (Figure 4h); Raman spectroscopy shows that CdS has low crystallinity in untreated ACZTSSe/CdS films and significantly enhanced crystallinity after heat treatment, while in CISSe/CdS films it has a high crystallinity without heat treatment, and heating treatment has no significant effect on its crystallinity (Figure 4g). These results show that copper-zinc-tin sulfur has completely different heterojunction interface properties from copper-indium-gallium selenium batteries.

Figure 3: Analysis of ACZTSSe/CdS heterojunction interface properties before and after low-temperature heat treatment.

(b) An in-depth analysis of the chemical composition, elemental distribution and microstructure of the heterojunction interface before and after annealing was conducted

The EDX line scan map of the interface region (Figure 2c, d) shows that the two samples have different element mixing region widths at the interface, and the JHT treatment reduces the interface element mixing region from 16 nm (Ref) to 10 nm (JHT), indicating that elemental inverse mixing behavior occurs between the absorption layer and the buffer layer, which is different from the heating-induced cross-diffusion behavior of heterojunction interface elements commonly observed in the literature. HRTEM analysis showed that JHT treatment significantly enhanced the order of the lattice at the interface, the crystallinity of CdS increased, and the interface defects decreased. Combined with FFT image analysis of HRTEM images, JHT processing converts the disordered ACZTSSe/CdS interface into an ordered epitaxial interface with (1-11) CdS ‖ (112)ACZTSSe. The formation of epitaxial interface is the internal reason for the reduction of device interface defects, the reduction of interface composites, and the improvement of device performance.

Figure 4: Migration and rearrangement of copper-zinc-tin sulfur surfaces and heterojunction interfaces

(c) Further in-depth study was carried out on the construction process and defect formation mechanism of CZTSSe/CdS heterostructure

The results show that the Cd2+ occupies the absorption layer during the deposition of the CdS buffer layer of CIGS solar cells, indicating that the Cu vacancy forms a “shallow buried pn junction” and naturally epitaxial growth on the CIGS surface, and in the process of depositing CdS on the surface of ACZTSSe, the surface Zn2+ is dissolved by NH4OH in solution to form a poor-Zn surface (Figure 4a), resulting in Cd2+ occupying the Zn vacancy (not the Cu vacancy), Cd2+ and Zn2+ Differences in ion radius size result in lattice distortion and a large number of unoccupied Zn vacancies, while Zn2+ dissolved into solution is re-deposited into the CdS buffer layer (Figure 4b), resulting in low crystallinity of the CdS layer at the interface, forming a severely defective ACZTSSe/CdS interface. Heterojunction low-temperature heat treatment induces the migration and rearrangement of elements near the interface, including the reverse mixing of Cd2+ and Zn2+ ions at the interface and the diffusion of Zn2+ from the bulk phase to the absorption layer interface, and the element migration and rearrangement realize the gradient distribution of Zn and Cd near the interface, which promotes the formation of the epitaxial ACZTSSe/CdS interface (Figure 4c), effectively reduces the concentration of defects near the interface, inhibits the non-radiative recombination of the heterojunction interface, and greatly improves the open circuit voltage and filling factor of the battery.

Figure 5: Large Area Devices and Device Stability.

(d) Large area device and stability research

Based on the low-temperature heterojunction heat treatment process, the research group prepared a large number of ACZTSSe cells with an area of 1.1 cm2 with a maximum efficiency of 12.7% (Figure 5a). One of the devices has been certified by the National Photovoltaic Metrology and Testing Center of Fujian Provincial Institute of Metrology with an efficiency of 11.7% (Figure 5c), which is the highest value of copper-zinc-tin-sulfur 1-cm2 area batteries. In addition, the work also reported on the stability of the ACZTSSe battery. An unencapsulated battery with an efficiency of 12.7% does not degrade performance for 194 days in a normal atmospheric environment (Figure 5d). The copper-zinc-tin-sulfur battery reported in this work not only has high photoelectric conversion efficiency, but also has quite excellent stability, showing the great application prospects of this type of battery.

In short, the paper for the first time reveals the construction process and the inherent mechanism of defect formation of the heterojunction interface of copper-zinc-tin-sulfur thin film solar cells, reveals the chemical roots of copper-zinc-tin-sulfur and copper-indium gallium selenium interface properties with completely different heterojunction interface properties, and truly realizes the epitaxial heterojunction interface by inducing element reverse mixing through low-temperature heat treatment, which not only creates a new world record efficiency, but also breaks through the bottleneck of heterojunction interface recombination that limits the performance of copper-zinc-tin sulfur cells. Moreover, it provides new ideas and strategies for further improving the efficiency of such batteries.

This work has been supported by the National Key R&D Program of China (2019YFE0118100), the National Natural Science Foundation of China (22075150, U2002216, 51972332, 12174060), Shanghai Excellent Academic Leader (19XD1421300) and other projects. (Source: Science Network)

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