Microstructural study of the CdS/CuGaSe2 interfacial region in CuGaSe2 thin film solar cells

Abstract

The microstructure of the CdS/CuGaSe2 interface region in Cu-rich CuGaSe2-based polycrystalline thin film solar cells with KCN-treated absorber layers are characterized. Two recipes for the chemical bath deposition (CBD) of CdS with different bath temperatures (60 and 80 °C) are compared. Coherent Cu–Se precipitates are observed in both cases in the grains of the absorber layer. This precipitation cannot be avoided and seems to be a principal limitation for the performance of Cu-rich CuGaSe2-based thin film solar cells. There is a significant difference between both recipes concerning the interaction with the absorber layer surface. For bath temperatures of 80 °C the interaction is much stronger and Cu–S inclusions are found in the buffer layer. These may be responsible for shunts across the pn junction. Owing to the reduced interaction of the CdS deposited at 60 °C there are no Cu–S inclusions. For the 80 °C recipe the CdS/CuGaSe2 interface region consists of a continuous transition zone with low defect density, whereas for the 60 °C recipe the interface is sharper, but the CdS layer contains a high density of stacking faults. The structure of the CdS layer depends also on the bath temperature and the growth orientation of the CuGaSe2 grains. CdS(80 °C) crystallizes predominantly in the zincblende structure and contains less linear and planar defects than CdS(60 °C) which tends to incorporate hexagonal regions in the cubic matrix. Strains due to lattice mismatch as well as mixture between wurtzite and zincblende structures were revealed in high resolution transmission electron microscopy (HRTEM) images of the CdS(60 °C) layer. For CdS(80 °C) the strain is relaxed by twinning and small-angle grain boundaries which were imaged by HRTEM. A suitable CdS buffer layer for Cu-rich absorber layers could not be obtained by CBD because of either the low crystal quality [CdS(60 °C)] or the formation of Cu–S inclusions [CdS(80 °C)]. The enhanced interaction with the Ga-rich absorber layer and improved quality of CdS(80 °C) results in an improved device performance of Ga-rich CuGaSe2-based solar cells.

Intermixing and Formation of Cu-Rich Secondary Phases at Sputtered CdS/CuInGaSe2Heterojunctions

“…Cu was also found in the CBD-CdS, consistent with previous reports [11,21], but in a more patchy manner rather than being organized into uniform Cu-rich layers [13]. In contrast to the PVD samples, a much lower concentration of Cd (4 vs. at least 10, close to the PVD-CdS/CIGS interface) was found to be present in the CIGS.…”

Studies in CuInS2 based solar cells, including ZnS and In2S3 buffer layers

“…The diffusion of Cu towards the buffer layer of CuInS 2 based solar cells has been controlled using a double layer structure of CuInS 2 film, having a Curich first layer and In-rich second layer deposited using Chemical spray pyrolysis technique [2]. In addition, films of CuInS 2 tend to build phase segregation like Cu x S at the film's surface and in the bulk of the crystallites [3][4][5]. Since the formation of secondary phases affects the performance of the CIS based solar cells, is convenient to prevent it prior to the buffer deposition; this is usually performed by KCN etching or by optimized 3-stage growth process in samples prepared using vacuum based multistage processes like the one used in this work [6,7].…”

Development of thin‐film Cu(In,Ga)Se2and CdTe solar cells

“…[50][51][52] For the preparation of solar cells only slightly Cu-deficient compositions of p-type conductivity are suited. 53,54 Depending on the [Ga]/[In þ Ga] ratio, the bandgap of CIGS can be varied continuously between 1Á04 and 1Á68 eV, (Figure 3). The current high-efficiency devices are prepared with bandgaps in the range 1Á20-1Á25 eV, this corresponds to a [Ga]/[In þ Ga] ratio between 25 and 30%.…”