Efficient Narrow Band Gap Cu(In,Ga)Se₂ Solar Cells with Flat Surface

Abstract

In this study, the influences of bromine-based etching (Br etching) of narrow band gap CuInSe 2 (CIS) absorbers and Cu(In,Ga)Se 2 absorbers with various single Ga gradings (CIS:Ga) on the properties of solar cells were investigated. Absorbers with narrow absorption edge energies (E abs ) of 1.0−1.02 eV, ideal for the application as a bottom cell in a tandem device, were fabricated using a modified three-stage process and subjected to Br etching. The evolution of surface flatness and their optical and electrical properties upon Br etching were investigated. Br etching typically reduced the root-mean-square deviation of the surface roughness height (R q ) for a CIS:Ga absorber from several hundreds to several tens of nanometers, whereas for some CIS absorbers, R q reduction was limited by the remaining voids. Moreover, Br etching reduced the leakage current across the pn junction. The high shunt resistances (R sh ) typically up to >10 kΩ•cm 2 were obtained by introduction of Br etching. However, etching sometimes adversely increased the V OC deficit. The investigation of the minority carrier lifetime and diode parameters revealed that backsurface recombination in CIS and low-Ga CIS:Ga solar cells increased as the absorber layer thickness decreased. A higher Ga grading significantly reduced back-surface recombination. Narrow band gap CIGS solar cells with improved surface flatness and high V OC were achieved by introducing Br etching and proper Ga grading.

III‐V//CuxIn1−yGaySe2 multijunction solar cells with 27.2% efficiency fabricated using modified smart stack technology with Pd nanoparticle array and adhesive material

“…The roughness was flattened to Rq = 28.3 nm. The etching process is an effective method to reduce the large and irregular roughness of the multicrystalline CIGSe layer, which also improves the quality of the p‐n junction 55 . Subsequently, by applying CMP treatment on the ZnO layer, the surface was sufficiently improved to Rq = 0.36 nm, as shown in Figure 3C.…”

“…In contrast, Figure 3B–D shows the image of the CIGSe cell in this study. Figure 3B shows an image of the CIGSe cell after a wet etching using a bromine‐based solution 55 . The roughness was flattened to Rq = 28.3 nm.…”

“…Although the narrow band‐gap CIGSe layer as a bottom cell is required, such CIGSe cells tend to cause low shunt resistance and recombination loss at the back‐surface. Recently, it has been reported that wet etching of the CIGSe surface and controlled Ga grading (Ga distribution in the CIGSe layer) are effective 55 . The shunt resistance can be improved by wet etching with a bromine‐based solution (Section 2.1), probably due to the reduced surface defects (voids) at the CIGSe surface.…”

“…The new design mainly focused on increasing the photocurrent generated in the CIGSe bottom cell and current‐matching level. The CIGSe bottom cell was optimized by a modified three‐stage co‐evaporation process using the molecular beam epitaxy (MBE) apparatus 55 . Due to the low Ga composition ( y ) of <0.1 of the Cu x In 1− y Ga y Se absorption layer, the band‐gap energy decreased to approximately 1.05 eV.…”

“…As shown in Figure 1, the rear‐passivation structure, which consists of a 30 nm‐thick‐Al 2 O 3 layer fabricated by atomic layer deposition and contact holes fabricated via photolithography, 56 was introduced between the Mo electrode and CIGSe absorption layer. In addition, the CIGSe absorption layer was subjected to etching with a bromine‐based solution to improve the surface flatness 55 . The final thickness of the CIGSe absorption layer was approximately 2300 nm.…”