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Flexible CdTe Achieves Top Photovoltaic Efficiency



Empa, a research institute for material sciences and technology development based in Dübendorf, Switzerland, has demonstrated a photovoltaic conversion efficiency of 13.8 percent using Kapton, a colorless polyimide film from Dupont film. This leapfrogs the institute’s previous world record of 12.6 percent and nears that of glass. The company is currently developing Kapton for use as a flexible superstrate for cadmium telluride (CdTe) thin film photovoltaic (PV) modules. The film is over 100 times thinner and 200 times lighter than the weight of glass typically used for photovoltaics.

CdTe is a compound semiconductor consisting of the metal cadmium and the semiconductor tellurium, in a ratio of 1:1. The bandgap of CdTe is about 1.5 eV, which is close to the optimum for conversion of sunlight to electricity. Additionally, the material is very stable chemically and against heat. The absorption characteristics of this material make it an excellent candidate for the use in photovoltaic applications. The material deposition is very easy with varying technologies and thus allows high throughput in production.

Empa is involved in the research and development of high-efficiency thin film solar cells with emphasis on novel concepts for enhancing the performance of solar cells, simplifying the fabrication processes, and advancing device structures for next generation. They have been doing groundbreaking work in developing and optimizing a low deposition temperature (<450°C) process for high-efficiency CdTe solar cells on glass (15.6 percent efficiency) and polymer film (12.6 percent efficiency) and now 13.8 percent using Kapton. The Kapton material polyimide film enables lightweight, flexible CdTe solar modules.

There are inherent advantages in transitioning to flexible, film-based versus rigid glass CdTe systems. High-speed and low-cost roll-to-roll deposition technologies can be applied for high throughput manufacturing of flexible solar cells on polymer film as substrates. This new film may enable significantly thinner and lighter-weight flexible modules that are easier to handle and less expensive to install, making them ideal for applications including building integrated photovoltaics.

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