First quadrant phototransistor behavior in CuInSe2 photovoltaics

• A detailed analysis of the temperature-dependence of a Cu(In,Ga)Se2 solar cell is presented
• The analysis separates components of the device from each other (series, shunt, main diode, back diode, etc.).
• The nature and reverse breakdown behavior of the back contact Schottky diode and its temperature dependence is described.
• The device is shown to exhibit operation as a phototransistor with photocurrent collection at the back contact.

Temperature-dependent current voltage measurements on a CuInSe2 (CIS) solar cell are described and analyzed in detail. At relatively low temperatures, the device is shown to exhibit operation as a phototransistor, evidenced by light flux dependent current in the first quadrant of the current/voltage curve prior to the onset of a high differential resistance region of the curve. The phototransistor behavior is not restricted to light with short wavelengths (such as would stimulate the front contact materials—CdS and ZnO), and has a similar sensitivity to light over a wide range of intensities. Other aspects of the device operation are analyzed including observation of a photoconductive shunt and series differential resistance component, neither of which requires short wavelength light to be observable, a non-light-sensitive diode reverse-saturation current, and characterization of the back diode in the device. It is argued that the presence of an effective back diode also requires a bias voltage to develop across that diode under a variety of operating conditions that could reduce the open circuit voltage of the device. The potential for characterization of minority carrier properties in the device absorber layer through further controlled study of phototransistor behavior in the devices is noted.