Optimization of MOCVD grown MQW structures for triple junction solar cells

Triple junction solar cells (TJSC) are the main components of concentrator photovoltaic (CPV) systems. Metalorganic chemical vapor deposition (MOCVD) is the manufacturing process of choice used to produce state of the art solar cells. An improvement in cell efficiency through improvement in cell design lowers the overall cost of solar electricity and stimulates widespread adoption of solar systems. Recently, the introduction of multiquantum well (MQW) structures into TJSC devices has been shown to improve short circuit currents and improve cell efficiency. Optimization of the MOCVD growth process for MQW layers using In_.09Ga_.91As wells and GaAs_0.9P_0.1 barriers is important for best cell performance. Since these layers are strained, the need to strain balance the MQW stack is important to reduce defects causing SRH trap/defect assisted recombination. The use of in-situ wafer curvature measurements allow for evaluation of strain compensation at growth temperature. Growth temperature also plays a role in growth of MQW layers with higher temperatures causing interdiffusion of wells and barriers. Optimum growth of MQW layers show sharp x-ray rocking curves, PL spectra with narrow line width, and TEM images with abrupt well/barrier interfaces. In this work, we discuss MOCVD growth optimization leading to high material quality of the MQW structures with up to 50 periods. Material characterization of optimized MQW structures with x-ray, PL, SIMS and TEM are discussed. To demonstrate the optimized growth conditions, a solar cell was fabricated with a MQW and external quantum efficiency was measured showing the absorption to higher wavelength than bulk InGaAs. Triple junction solar cells with MQWs are thus a proven way to improve cell efficiency and cell currents.