AbstractsChemistry

The replacement of the toxic chemical bath deposited CdS buffer layer in the heterostructure device ZnO/CdS/Cu(In,Ga)Se2 is a challenge for research and development of the thin film solar cell based on the Cu(In,Ga)Se2 (CIGS) material. Though a variety of alternative Cd-free buffers has been tested with considerable success, their solar cells in general are unstable and less efficient compared to the CdS buffer device. The aim of this work is to elucidate the superiority of the CdS buffer as well as the inferiority of an alternative In(OH,S) buffer by comparing the chemical and electronic properties at the heterointerfaces in the solar cells with those buffers. The first part of this thesis studies the formation of the CdS/CIGS interface, focusing on the interaction between the CIGS absorber layer and a standard chemical bath solution for the CdS deposition. The second part investigates the role of the heterointerfaces in the device with chemical bath deposited In(OH,S) buffer in comparison to that in the CdS buffer device. The study of the interface formation in the CdS buffer device first presents the growth characteristics of the CdS buffer from a standard chemical bath onto the CIGS substrate. X-ray photoelectron spectroscopy (XPS) reveals a complete coverage of the CIGS absorber layer after 4 min of CdS deposition. Taking into account that damage by ZnO sputtering can reach 5 nm depth in the substrate layer, a CdS layer deposited in 5 min is required to separate successfully the ZnO and CIGS layers. This result explains why the performance of the CdS buffer solar cells is best when the CdS layer is deposited in 5 min. A reduction of the CdS deposition duration, aiming at a decrease of the buffer thickness, results in a drastic decrease of the cell performance due to an incomplete coverage of the CIGS absorber surface by CdS, which is in turn leads to shunting between the ZnO and Cu(In,Ga)Se2 layers. The incomplete coverage also results in a deviation of the S/Cd ratio from unity during the CdS deposition. X-ray photoelectron spectroscopy investigations reveal a large excess of Cd compared to S (S/Cd ≈ 0.1) in the early stage of the CdS deposition. This strong deviation from stoichiometry is mainly due to an incorporation of Cd from the chemical bath into the Cu(In,Ga)Se2 surface layer. The S/Cd ratio increases as long as the absorber surface stays in contact with the chemical bath solution. As the absorber surface is completely covered by CdS, the S/Cd ratio saturates at a value of 0.75. Another factor that influences the S/Cd ratio is a co-deposition of an impurity containing Cd in the form of Cd(OH)2. During the chemical bath process, Cd(OH)2 can be converted into CdS due to a metathesis of thiourea on the surface of Cd(OH)2. However, this process plays a minor role in the trend of increasing the S/Cd ratio. The chemical mechanism of the incorporation of Cd into the absorber layer and the formation of Cd(OH)2 are next targeted by investigations of the treatment of the CIGS absorber in the Cd-NH3…