Accurate control and characterization of Cu depletion layer for highly efficient Cu(In,Ga)Se₂ solar cells

Abstract

The unintentional formation of a Cu depletion layer (CDL) on the surface of Cu(In,Ga)Se2 was observed in 1993, and CDLs have been expected to improve the performance of Cu(In,Ga)Se2 solar cells. However, methods of controlling CDLs have hitherto been unavailable. For the first time, we succeeded in controlling a uniform CDL on a Cu(In,Ga)Se2 surface by introducing irradiation with Se into the typical three‐stage growth process of Cu(In,Ga)Se2. We discuss the characterization and effects on device performance of CDLs. A uniform, smooth CDL with a thickness of 200 nm was formed at a Se irradiation time of 5 minutes, whereas the CDL formed at an irradiation time of 10 minutes was rough and non‐uniform. A maximum efficiency of 19.8% was achieved at an irradiation time of 5 minutes via the formation of a complete CDL. This simple, unique method may help to maximize efficiency of Cu(In,Ga)Se2 solar cells.

Controllable Formation of Ordered Vacancy Compound for High Efficiency Solution Processed Cu(In,Ga)Se2 Solar Cells

“…[ 11,12 ] The benefits of copper‐poor OVC are as follows: it could widen the CIGS band gap through decreasing the valence band maximum (VBM), which forms a hole barrier; the formation of a buried homo‐junction owing to the similar crystal structure between OVC and CIGS; it is helpful to quench the deep level defects in CIGS. [ 11–15 ] Thus, the OVC is believed to improve the device performance by minimizing the carrier recombination at the CIGS/CdS interface. [ 13–15 ] In vacuum deposition process, every element deposition rate can be adjusted in real time.…”

“…[ 11–15 ] Thus, the OVC is believed to improve the device performance by minimizing the carrier recombination at the CIGS/CdS interface. [ 13–15 ] In vacuum deposition process, every element deposition rate can be adjusted in real time. Hence, the OVC formation can be accurately controlled.…”

“…Tremendous research efforts have been implemented to study the formation of OVC phase in vacuum based process. [ 15–17 ] Nevertheless, limited studies are investigating the OVC formation through solution process. Current studies have reported the existence of OVC phase in high efficiencies solution processed CIGS solar cells, confirming the indispensability of OVC phase.…”

Electron-Beam-Induced Current Measurements of Thin-Film Solar Cells

“…Consequently, the net doping of this semiconductor layer needs not to be homogeneously distributed across the thin film. Evidence for this inhomogeneous distribution of the net-doping density has already been provided by several authors 44,45,46,47 via analyses conducted via scanning spreading-resistance microscopy. Spatial changes in the net doping affect the transport properties and also introduce band bending via Poisson's equation, both having impact on the EBIC signal.…”

“…The different EBIC levels in the two adjacent grains (normalized about 1 and 1.1) can be attributed to slightly different net-doping densities (see Refs. 44,45,46,47 , and also Sec. 5.4 below for further details on this matter).…”