Device Physics and Characterization of Silicon Point-Contact Solar Cells.

The silicon point-contact solar cell is a candidate solar cell for use in highly concentrated sunlight. Recent progress is described for a point-contact solar cell optimized for incident power densities of 36 Watts/cm^ {2}. The design and fabrication for several device runs is discussed in detail. Extensive data from these solar cells is presented in order to display the major trends in device design and performance. The data culminates in a solar cell which is 28% efficient for incident power densities between 4 and 15 watts/cm^ {2} at 25^circC. Utilizing measurements from several runs of solar cells, a three-dimensional model is compared to the data in order to establish a strong baseline case from which future design enhancements can be evaluated. A modeling study of the optimum geometries for point contact solar cells provides the context for a discussion of the device physics of the solar cell operation. These studies indicate that efficiencies exceeding 30% may soon be attained. The ultimate efficiency of silicon solar cells will be limited by parasitic Auger recombination. With the model, a device is optimized to measure the carrier lifetimes in highly-injected silicon in the carrier density range of interest for solar cell operation, 1 times 10^{15} to 2 times 10^{17} carriers/cm ^{2}. With this test device, a simple unambiguous determination of the recombination rates is possible using both steady-state and time dependent measurements. The recombination rates determined by both methods are in excellent agreement and are best fit by an ambipolar Auger coefficient of 1.66 times 10^{-30}{rm cm}^ {6}/sec. This coefficient is four times greater than the value most commonly used for modeling. The lifetime data from doped and undoped silicon are brought into much closer agreement by these new results. This raises the possibility that the lifetime limiting mechanisms for both undoped and doped silicon are the same and depend only upon the carrier density, independent of whether this carrier density results from dopants or excitation.