Effect of In Situ Thermal Annealing Process on Structural, Optical and Electrical Properties of CdS\CdTe Thin-Film Solar Cells Fabricated by Pulsed Laser Deposition

Abstract

Cadmium Telluride has long been recognized as the second lowest- cost material after Si in the world photovoltaic market, specifically for thin-film solar cells. The two attractive properties of the CdTe are its nearly ideal band gap of ~1.5 eV for single p-n junction photovoltaic and its high optical absorption coefficient up to 〖10〗^5 cm-1. Therefore, a thickness of ~1 µm of CdTe can absorb up to 90% of the incident light. The key to high-performance thin film CdTe-based solar cells is controlling microstructure of the CdS/CdTe through obtaining high-quality crystalline CdTe thin films that have low density pinholes and other defects and form high-quality p-n heterojunction interfaces on the CdS or other window layers. Considering these, the relative high temperatures used for CdTe thick film growth may not be suitable in the thin film case due to lack of control in CdTe microstructure evolution. Therefore, development of low-temperature processes for CdTe thin film solar cells is important to achieving a precise control of the CdS/CdTe microstructure and optoelectronic properties. In addition, low temperatures provide benefits in wider selection of substrates especially those for low-cost, flexible solar cells applications. However, the CdS/CdTe solar cells based on thin CdTe films fabricated at low temperature have generally poor performance as a result of increased density of grain boundaries and defects. In order to address this issue, we have developed an in situ thermal annealing process (iTAP) immediately after the CdS/CdTe deposition using Pulsed laser deposition (PLD) at 200 °C and before the common ex situ CdCl2 annealing typically employed for optimization of the CdTe-based solar cells. A systematic study on the microstructure, optical and optoelectronic properties of CdS/CdTe solar cells processed under different iTAP conditions has been carried out. It has been found that these physical properties depend sensitively on the iTAP processing conditions and appropriate iTAP in the optimal window enhances grain growth, improves grain boundary connectivity, and reduces crystal defects. This leads to considerably improved CdTe crystallinity and as a result, improved optoelectronic properties of the CdS/CdTe solar cells. Our result suggests that the iTAP is important for optimizing the chemical composition and microstructure of CdTe thin films and its heterojunction with CdS, both of which are critical to the performance of the CdS/CdTe thin film solar cells. In addition, it was found that smaller CdTe thickness provides advantages in reduced charge recombination.