CdTe solar cells by low temperature processes

ABSTRACT:CdTe polycryst. thin film solar cells have a strong potential in scalability. They have shown long-term stable performance and high efficiency up to 16.5% under AM1.5 illumination. Amongst several attractive features, high chem. stability of CdTe and a simple compd. formation are the most important ones for large area prodn. of solar modules. A further simplification has been done by substituting the CdCl2 step by treating CdTe films in an atm. contg. a non toxic gas that is inert at room temp., like HCF2Cl, that belongs to the Freon family. The treatment temp. is typically 400°C, for a few minutes and in an atm. contg. Cl, typically 100 mbar of Ar contg. 15% of HCF2Cl. The change in the morphol. of CdTe films after treatment is very similar to that obtained with CdCl2 treatment and an increase in the size of small grains is always obsd. This process has been applied by N. Romeo et al. on CdTe deposited by close-spaced sublimation (CSS) with very interesting results (15.8% efficiency). The application of the regular CdCl2 treatment and of this novel "activation process" on low and high temp. processed solar cells will be described Moreover, there are new promising device configurations like bifacial solar cells, ultra-thin solar cells and flexible devices. The highest efficiencies in CdTe solar cells have been obtained using CSS deposition method, which requires a high substrate temp. (500÷550 °C). Instead, conventional phys. vapor deposition (PVD) process where CdTe is evapd. in a high vacuum evapn. (HVE) system at lower substrate temps. (typically 300°C) has provided solar cells with efficiencies of more than 12%. For these reasons HVE process is attractive for a very simple in-line deposition of large area CdTe solar modules on soda-lime glass substrates, as well as on polymer foils thereby facilitating the roll-to-roll manufg. of flexible solar modules. Flexible CdTe/CdS solar cells of 11% efficiency in superstate and 7% efficiency in substrate configurations have been developed with a "lift-off" approach. However, roll-to-roll manufg. is desired in future. Therefore, flexible superstate solar cells have also been directly grown on com. available polyimide foils. Solar cells with AM1.5 efficiency of 12.4% on Upilex foils (highest efficiency recorded for flexible CdTe cell) have been developed. A different possibility in making flexible CdTe solar cells is to use a metal foil as substrate, provided that the stacks would be deposited in the opposite order to have light coming from the top and not through the substrate (the so-called "substrate configuration" process). However making a CdTe photovoltaic device in inverted structure implies a variety of different scientifical issues, that will be addressed. The latest development is the application of a transparent conducting oxide (TCO) ITO as a back elec. contact on CdTe leading to first bifacial CdTe solar cells, which can be illuminated from either or both sides. Accelerated long term stability tests show that light soaking improves the efficiency of CdTe solar cells with ITO back contacts and performance does not degrade. Such solar cells are attractive for tandem solar cells though larger band gap absorbers based on CdTe would be desired. Application of light tapping concepts can be employed to reduce the CdTe layer thickness to below 1 μ.