Experimental investigation of the excess charge

Abstract

The open-circuit voltage of about 600 mV developed by 0.1 ohm-cm silicon solar cells under air mass zero illumination is about 100 mV less than voltages predicted from simple diffusion theory. The lower measured voltages appear to be controlled by junction current transport processes associated with the thin top diffused layer. Mechanisms such as low n⁺ layer minority carrier lifetime and bandgap narrowing due to heavy doping effects (HDE) have been suggested to explain these results. Experimental determinations of the properties of the diffused layer are required to assess which of these mechanisms predominate. While direct measurement is difficult, an indirect measurement methodology exists by which the lifetime or transit time in the diffused layer can be obtained. Nine p-type, 1×2 cm, 〈111〉 orientation silicon wafers were phosphorus diffused at 880°C for 45 minutes using P0Cl₃. Open-circuit voltages of 595-612 mV, typical of all 0.1 ohm-cm cell voltages, were obtained. From the open-circuit voltage and short-circuit current, the diffusion controlled I₀ was obtained. In addition to illuminated I-V characteristics, the time constants from the Open-Circuit Voltage Decay method, and the minority carrier diffusion lengths in the base region were measured. The base region charge was determined using the base region diffusion length measured by an X-ray method. The data from these experiments combined with simple theory can imply the minority carrier time constant and the excess charge in the diffused layer. From this, certain conclusions are drawn about the relative roles of bandgap shrinkage and recombination rates in the diffused layer.