Mechanism-Based Approach of CdS/Cu(In,Ga)Se2 (CIGS) Interfaces for CIGS Solar Cells through Deposition in Different Stages of Continuous Chemical Bath Deposition Reaction: Key to Achieving High Photovoltaic Performance

To accurately obtain an adjustable CdS layer, a quartz crystal microbalance (QCM) system is used in the chemical bath deposition (CBD) method by measuring the frequency change. However, although the thickness of the CdS layers deposited using the QCM system can be accurately adjusted to the same thickness, CdS layers of the same thickness formed in each section of the continuous reaction are expected to have different properties due to the reaction rate and mechanism, ultimately affecting the performance of solar cells. Therefore, 30 nm-thick CdS layers are formed in the various reaction sections (initial, middle, and late ranges) and their characteristics and performances are examined and compared with the conventional 60 nm-thick CdS buffer layer. Most Cu(In,Ga)Se-2 (CIGS) solar cells with the 30 nm-thick CdS buffer exhibit a higher efficiency than those with the 60 nm-thick CdS buffer due to improved J(sc). However, the CIGS with the 30 nm CdS formed in the middle range exhibits the lowest photovoltaic performance. To investigate this unexpected result, all the deposited CdS layers are chemically, structurally, and optically examined and the results are explained by a deposition mechanism study. Furthermore, the effect of heat treatment is demonstrated by band alignment.

Automated summary

Using a QCM system to accurately obtain an adjustable CdS layer, researchers examined the characteristics and performances of 30 nm-thick CdS layers compared to conventional 60 nm-thick CdS buffer. Most CIGS solar cells with the 30 nm-thick CdS buffer showed higher efficiency than those with the 60 nm-thick CdS buffer due to improved J(sc), despite CIGS with the 30 nm CdS formed in the middle range exhibiting the lowest photovoltaic performance. Further analysis on all deposited CdS layers and the effect of heat treatment was conducted to explain these results.