Peculiarities of the capacitance-voltage characteristic of a photoelectric solar energy convertor based on a silicon p-n junction with a porous silicon antireflection coating

Abstract

The application of thin porous silicon (por Si) films as an antireflection coating of the frontal surface of traditional photoelectric convertors (PECs) of solar energy was considered by many authors. It has been shown convincingly that the application of por Si makes it possible to increase the efficiency of solar energy conversion. At the same time, the electrophys ical properties of such PECs have not been studied comprehensively.In this communication, we report on the results of analysis of a high frequency capacitance-voltage (C-V) characteristic of a silicon PEC based on the n + -p junction with a thin por Si film on the n + surface.The structure under investigation is based on the n + -p junction formed by diffusion of phosphorus. The substrate was a silicon single crystal of solar cell qual ity with the p type conductivity, with the (100) orien tation of the surface, and a resistivity of 1 Ω cm. The depth of the n + -p junction was 0.5 μm. The por Si film was grown on the surface of the n + layer by electro chemical anode etching. We investigated a PEC with a 0.4 μm thick por Si film. As shown in our previous pub lication [1], such a thickness of the por Si layer is optimal from the standpoint of the PEC efficiency. Indium con tacts were formed for measurements in the p region of the structure and on the por Si surface.The C-V characteristics were measured by an E7 20 immitance meter for a test signal frequency of 1 MHz at a temperature of 300 K. We tested a batch of ten identical samples with almost identical C-V charac teristics. A typical high frequency C-V curve for the tested batch of samples is shown in the figure. The polarity of a constant bias voltage U applied to the structure is determined by the sign of the voltage at a contact with por Si.The characteristic shown in the figure differs from that for an n + -p diode (initial structure) for which the capacitance minimum must be observed in the region of positive bias voltages.The resultant C-V curve (see figure) is typical of the metal-insulator-semiconductor (MIS) structure based on a semiconductor with n type conductivity. The regime of accumulation of the majority charge carriers is observed in the region of positive voltages. The depletion also begins for high positive voltages (see figure). For a negative bias voltage, deep depletion takes place. The local peak near U = 0 V is associated with the effect of surface states [3]. The voltage of pla nar regions is strongly shifted to the range of positive values, which can be explained by the effect of a nega tive charge in the insulator layer [3].Abstract-Experimental results on the high frequency capacitance-voltage characteristic of a photoelectric solar energy converter based on the n + -p junction with a thin porous silicon film on the frontal surface are considered. It is shown that the capacitance-voltage characteristic is determined by the surface metal-insu lator-semiconductor (MIS) structure formed as a result of growing of a porous silicon layer by electrochem ical anode etching. The ...