Dual-layer tandem technique develops highly efficient perovskite–CIGS tandem solar cell

Waited for three years, the perovskite–CIGS tandem solar cell developed by UCLA Engineering researchers reaches 22.43 % power conversion efficiency.

This research was led by Yang Yang, UCLA’s Carol and Lawrence E. Tannas Jr. Professor of Materials Science, also, it was conducted by Qifeng Han, a visiting research associate in Yang’s laboratory, and Yao-Tsung Hsieh and Lei Meng, who both recently earned their doctorates at UCLA. The study’s other authors are members of Yang’s research group and researchers from Solar Frontier Corp.’s Atsugi Research Center in Japan.
The combination of hybrid perovskite and Cu(In,Ga)Se2 (CIGS) has the potential for realizing high-efficiency thin-film tandem solar cells because of the complementary tunable bandgaps and excellent photovoltaic properties of these materials. In tandem solar device architectures, the interconnecting layer plays a critical role in determining the overall cell performance, requiring both an effective electrical connection and high optical transparency. 
The device is made by spraying a thin layer of perovskite onto a commercially available solar cell as the first layer of the device. The bottom layer of the device is made of a compound of copper, indium, gallium and selenide — CIGS.

Adding a 1 micron-thick perovskite layer, the efficiency is increased by 3.73%
The CIGS base layer is about 2 microns thick (or two thousandths of a millimeter). After absorbing sunlight, the conversion efficiency is about 18.7%. However, adding a 1 micron-thick perovskite layer can increase efficiency.And the entire assembly sits on a glass substrate that’s about 2 millimeters thick.
The team used nanoscale interface engineering of the CIGS surface and a heavily doped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport layer between the subcells that preserves open-circuit voltage and enhances both the fill factor and short-circuit current. A monolithic perovskite/CIGS tandem solar cell achieved a 22.43% efficiency, and unencapsulated devices under ambient conditions maintained 88% of their initial efficiency after 500 hours of aging under continuous 1-sun illumination.(The previous record of this type of tandem solar cells, reported in 2015 by a group at IBM’s Thomas J. Watson Research Center, was 10.9 percent.)

Effects of CMP on CIGS surface and resulting performance of CIGS solar cells

▲Fig. Effects of CMP on CIGS surface and resulting performance of CIGS solar cells. (A) Atomic force microscopy (AFM) image of the CIGS surface before CMP polishing. (B) AFM image of the CIGS surface after CMP polishing. (C) Cross-sectional SEM images of the CMP processing on the CIGS surface. (D) J-V curves of original CIGS solar cells and after CMP polishing with a step size of 0.02 V and a scan velocity of 0.1 V/s, measured under AM1.5G illumination. (E) EQE of original CIGS solar cells and after CMP polishing.

Performance of the perovskite/CIGS tandem cells

Fig. Performance of the perovskite/CIGS tandem cells. (A) Schematic and cross-sectional SEM image of the monolithic perovskite/CIGS tandem solar cell. (B) J-V curve (NREL-certified; see fig. S8) and efficiency at the maximum power point (inset) of the champion tandem device. (C) EQE spectra for the subcells of the monolithic perovskite/CIGS tandem solar cell. (D) Stability test of the monolithic perovskite/CIGS tandem solar cell. The unencapsulation device maintained 88% of their initial PCE after 500 hours of aging under continuous 1-sun illumination and maximum power point tracking at 30°C ambient environment. The inset shows that the device can recover 93% of its initial performance after a 12-hour resting period without load and illumination.

The team uses Enli Tec QE-R Perovskite Solar Cell Quantum Efficiency Measurement System to measure EQE spectra of perovskite and CIGS single junction solar cells.

Dual-layer tandem technique could eventually approach 30 % power conversion efficiency
Professor Yang said, “With our tandem solar cell design, we’re drawing energy from two distinct parts of the solar spectrum over the same device area. This increases the amount of energy generated from sunlight compared to the CIGS layer alone.” Professor Yang also said, “Our technology boosted the existing CIGS solar cell performance by nearly 20 percent from its original performance. That means a 20 percent reduction in energy costs." He added that devices using the two-layer design could eventually approach 30 percent power conversion efficiency. That will be the research group’s next goal.

The study was supported by the National Science Foundation and the Air Force Office of Scientific Research. Over the years, Professor Yang Yang and the team of Yang Yang Lab have been working on tandem solar cells, and have many internationally renowned research results, including transparent tandem solar cells that can be applied to windows.

Cited from: High-performance perovskite/Cu(In,Ga)Se2 monolithic tandem solar cells
Qifeng Han, Yao-Tsung Hsieh, Lei Meng, Jyh-Lih Wu, Pengyu Sun, En-Ping Yao, Sheng-Yung Chang, Sang-Hoon Bae, Takuya Kato, Veronica Bermudez, Yang Yang 
Science31 Aug 2018; Vol. 361, Issue 6405, pp. 904-908; DOI: 10.1126/science.aat5055
References:Matthew Chin | August 30, 2018. Dual-layer solar cell developed at UCLA sets record for efficiently generating power. (UCLA NEWROOM)