Characterization of Interface Between Accurately Controlled Cu‐Deficient Layer and Cu(In,Ga)Se₂ Absorber for Cu(In,Ga)Se₂ Solar Cells

Abstract

We recently succeeded in controlling a Cu-deficient Cu(In,Ga)Se 2 layer (CDL) on a Cu(In,Ga)Se 2 (CIGS) surface by introducing an Se irradiation after the completion of the second stage in a three-stage process during CIGS growth. The CDL on the surface causes the formation of a valence band offset (ΔE V ) between the CDL and CIGS because the Cu vacancies decrease the valence band maximum of the CDL. Therefore, we can expect the suppression of recombination at the CdS/CIGS interface in CIGS solar cells due to the repelling of holes by ΔE V . The amount of knowledge regarding the properties of CDL/CIGS interfaces is observed to be quite small because a control technique for CDL has not been developed so far. In this study, the compositional and structural properties of an accurately controlled CDL/CIGS interface are investigated in detail. The composition of the interface between the CDL and CIGS is observed using an energy dispersive X-ray spectroscopy with the help of a transmission electron microscope. Using nanobeam electron diffraction and Fourier transfer mapping analysis, it is confirmed that the 1 12Þ ð plane in the CDL continuously grows on the 11 2Þ ð plane in CIGS. Further, these results indicate that a high-quality interface is formed between the CDL and CIGS, which contains only a small amount of dangling bonds. Finally, a high conversion efficiency of 19.4% is achieved in the CIGS solar cell, which can be attributed to the formation of CDL and effect of ΔE V .Cu(In,Ga)Se 2 (CIGS) is considered to be one of the most promising materials that can be incorporated into the thin film solar cells because it is possible to achieve high performance along with low costs. Recent performance enhancements achieving efficiencies over 20% have been realized by applying an alkaline post-deposition treatment (PDT) process. Currently, a high efficiency of 22.6% has been achieved by introducing RbF-PDT. [1] Using KF-PDT, a conversion efficiency of 20.4% for flexible CIGS solar cell containing a polyimide substrate has been achieved. It has been remarkably reported that a Cu-deficient condition is formed on the CIGS surface using KF-PDF. [2] To date, Cu-deficient layers (CDLs) have been unintentionally formed on a CIGS surface during CIGS growth as a result of the general fabrication process. [3,4] A positive effect is expected when a CDL is formed on the CIGS surface. It has been reported that the valence band maximum (E V ) decreases in Cu-deficient condition, which results in the formation of a valence band offset (ΔE V ) between CIGS and CDL, [5][6][7] whereas ΔE V causes the suppression of interfacial recombination at the n-type CdS buffer layer/CIGS absorber interface. [4,8,9] However, an accurate control method for CDL formation has not yet been revealed, despite numerous studies on CIGS solar cells. Recently, our group reported a control technique for CDL formation on a CIGS surface [10] by modifying the three-stage process without using the PDT process. A secondary Cu 2 Se layer is temporally segregated on the ...

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“…We quantitatively evaluated the suppression of interfacial recombination by valence band offset at the CdS/CIGS interface and clarified the role of the CDL. We therefore reviewed the CIGS growth process, constructed a CDL formation model, and proposed methods for the control of CDLs during the growth of CIGS . To form a CDL on the surface of CIGS, it is important to control the non‐stoichiometric Cu 2 − x Se layer with Cu‐deficient conditions after the second stage in the three‐stage process .…”

“…CIGS films can be formed by roll-to-roll deposition in continuous high-speed manufacturing processes with low production and installation costs and high functionality. The world record conversion efficiency of 22.6% has been achieved in CIGS solar cells 5 via alkaline-post deposition treatment, such as NaF, KF, and RbF PDT. 1,5,6 This technique leads to improvements in quality at interfaces between n-type buffer layers and p-type CIGS absorbers, which results in increases in cell performance.…”

“…We therefore reviewed the CIGS growth process, constructed a CDL formation model, and proposed methods for the control of CDLs during the growth of CIGS. 21,22 To form a CDL on the surface of CIGS, it is important to control the non-stoichiometric Cu 2 − x Se layer with Cu-deficient conditions after the second stage in the three-stage process. 23,24 Cu-depleted Cu 2 − x Se and a Cu-Se liquid phase can coexist (Equation 1) after the addition of Se to Cu 2 Se, which temporarily forms on the CIGS surface under Cu-rich conditions at the end of the second stage.…”