Scott Crossen and Dr. John Colton, Department of Physics and Astronomy
Thin film photovoltaics are a promising candidate in the search for cheap, efficient solar cells. In particular, cadmium telluride is one of the most widely-used materials for this application because of its leading dollar-per-watt ratio and high efficiency of around 22.3%. However, for materials with bulk band-gaps close to 1.48 eV such as CdTe, the theoretical efficiency limit is 30%. To fully realize the maximum efficiency, this project was designed to study the optical properties that lead to superior thin-film photovoltaic design so that we can better understand the limiting factor in solar cell absorption efficiency. In this experiment, a special specimen of Arsenic-doped CdTe was used to analyze what properties affect overall photovoltaic performance.
The goal of this project was to investigate optical properties for a p-type variety of CdTe that contains vacancy defects in the crystal-lattice formed through a type of doping that’s predicted to yield superior results over current techniques commonly used. This characterization consisted of two parts: A general photoluminescence study and a time-resolved photoluminescence study. The first is used to gather information about material-specific information such as the band gap and emission range, and the second uses this information to gather the photoluminescence lifetime of the material which is extracted from the exponential decay form of the periodic emission. This later value is essential in comparing the performance of this material with others. Ideally this information will be gathered across a series of temperatures from 17.5 K to room temperature. We have not yet finished the final part of the project, but we are well on our way to more fully characterizing the essential properties of CdTe and which properties lead to enhanced solar cell efficiency.
This work is ongoing and is done in collaboration with Dr. Mike Scarpulla of the University of Utah. He has provided us with the CdTe sample needed for our experiments. General photoluminescence was studied using a 400 nm laser at various intervals in temperature from 17.5 K to room temperature. A general spectrometer was used to select frequencies to a resolution of .1 nm, and the intensity was read using a calibrated silicon sensor attached to the output of the spectrometer. The time-resolved photoluminescence study is incomplete. This part of the project used a tunable pulsed laser centered at 800 nm (with the necessary lenses, mirrors, and other optical elements), an APD to start a single photon counting timer on each laser emission, and another APD to end the timer when a resulting photon was recovered from the stimulated CdTe. A cryostat and a specially designed single photon counting computer card were also included in this experiment. All equipment was controlled using a collection of LabVIEW software that is written and maintained by the members of our research group.
As previously mentioned, this project consists of two parts: A general photoluminescence study and a time-resolved photoluminescence study. The first study produced valuable information for this specimen of CdTe such as the location of the primary spectral emission. For the case of 17.5 K this was between 850 nm and 1050 nm and is shown in Figure 1. Note that the values discussed are for a preliminary sample of CdTe and are not the p-type material for which we ultimately expect to publish on. The second part of the study — the time-resolved photoluminescence is incomplete and so no concrete results can be displayed at this time.
Discussion & Conclusion
The primary conclusion of this project cannot yet be formulated until the final portion of this project is completed. The intermediate conclusion is related to the band-gap and spectral emission range of CdTe and is included in figure 1. As shown in the photoluminescence spectrum above, at 17.5 K, CdTe shows a very smooth spectral peak at 925 nm. At higher temperatures this peak shifts and becomes broader.