Copper indium gallium selenide (Cu(In, Ga)Se2, or CIGS) nanoparticles as the absorber layer of high-efficiency thin-film solar cells have become one of the global photovoltaic research hotspots because of their low production cost, low pollution, and good low-light performance, etc. The photoelectric conversion efficiency of the CIGS solar cells is currently the highest among all kinds of thin-film solar cells, and it is regarded as the next generation of new and very promising thin-film solar cells. Thin film solar cells. Although the current maximum efficiency has reached 23.35%, there are still many challenges to further improve the efficiency.
Solar cell researchers at Uppsala University recently demonstrated a series of strategies to improve the efficiency of CIGS solar cells, leading to a certified efficiency of 23.64 per cent. The team introduced a relatively high amount of silver ([Ag]/([Ag] + [Cu]) = 0.19) into the absorber and achieved a 'hockey-stick'-like distribution of gallium: a high concentration of Ga near the contacting Mo back electrode, and a lower, constant concentration in the region closer to the CdS buffer layer. This elemental distribution minimises lateral and longitudinal bandgap fluctuations, yielding a bandgap close to 1.15 eV, which reduces the open-circuit voltage loss.
Fig. 1 (a) Current-voltage characteristic curve and (b) external quantum efficiency (EQE) spectrum. Solar cell parameters: (c) FF, (d) JSC, (e) VOC and (f) η.
In addition, the researchers employed a post-deposition treatment of RbF, leading to the formation of an Rb-In-Se phase, possibly RbInSe2, which passivates the absorber surface. The cell efficiency was measured in the laboratory to be 23.75% and certified by the Fraunhofer Institute for Solar Energy Systems (ISE) to be 23.64%. This efficiency is higher than previously reported values for high efficiency solar cells (η= 22.9% and η= 22.6%).
Composite Rate and Bandgap Optimization
Through analysis, it was found that the total composite rate in the space charge region (SCR) is reduced, mainly due to lower activation energy or lower defect density in the SCR. Compounding is mainly achieved through intermediate bandgap defects, while interfacial compounding is not an issue for existing devices. Scanning electron microscopy (SEM) images show the formation of large (Ag, Cu)(In, Ga)Se2 (ACIGS) particles, most of which are larger than 1 µm in all dimensions. This large particle structure contributes to the reduction of composite losses at the interface and improves the photoelectric conversion efficiency.
Role of Rb Elements
In order to study the Rb-rich regions in more detail and provide a quantification of the elemental depth profiles, the researchers analyzed specific locations of the films by STEM-EDS. The results show that Rb is mainly concentrated in the region under the CdS layer (size<100 nm), but small amounts of Rb can be found everywhere at the ACIGS/CdS interface. The Rb signal peaks under the buffer layer and the glow discharge emission spectroscopy (GDOES) also reveals Rb aggregation at the back-bottom contact (i.e., between MoSe2 and ACIGS).
Fig. 2 (a) Elemental distribution at the 'Pos2.2' position. (b) STEM-BF image of the Rb-rich region found in a. (c) The same image with the superimposed Rb concentration map from a. (d) Quantified elemental concentrations extracted from the circular regions 1 and 2 shown in b. (e) STEM-BF and (f) STEM-DF images of the Rb-rich region at high magnification.
Interestingly, very thin (Ag, Rb)-In-Se compounds (possibly (Ag, Rb)InSe2) formed on the absorber surface, which is consistent with the Cu and Ga signals dropping by about 5 nm on the absorber surface. Summarising all the findings, the researchers speculate that a very thin (<5 nm) RbInSe2 phase forms at the CdS/ACIGS interface.
Conclusion
In summary, this is a high-performance copper indium gallium selenide (CIGS) solar cell with a certified efficiency of 23.64%. The improved performance is attributed to the high concentration of silver ([Ag]/([Ag] + [Cu] = 0.19) alloyed in the absorber layer and the minimization of compositional fluctuations on the surface of the absorber layer as well as in the space charge region (SCR), both laterally and longitudinally.
Future studies should further avoid parasitic absorption in the window and buffer layers while maintaining the same open-circuit voltage (VOC) and fill factor (FF) levels, potentially the most direct way to increase efficiency up to 25%. These findings not only provide new ideas for further optimization of CIGS solar cells but also provide an important reference for the development of other types of thin film solar cells.
Reference
- High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency. Nature Energy (2024).
