Development of Heterojunction Solar Cells Comprising of Boron-Doped Silicon-Carbon Based Semiconductor Thin Films and n-Si

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1. Introduction Multi-layered structure is one of the methods to enhance the photoelectric conversion efficiency of solar cells. The optical gap of amorphous silicon carbide (a-SixC1-x) semiconductor thin films can be controlled by changing the ratio of silicon and carbon atoms. Moreover, a-SixC1-x thin films can easily form the stacked cell compared to single-crystalline and polycrystalline materials because clean lattice constant cannot be defined. Therefore, a-SixC1-x thin film is a promising material that enables us to realize the multi-junction solar cells with high-efficiency. Our research group has reported the fabrication of a-SixC1-x semiconductor thin films with controllable optical gap in the range from 1.25 to 2.76 eV [1]. However, the conversion efficiency of heterojunction solar cell comprised of p-type a-SixC1-x thin film and n-Si substrate was approximately 2.7×10-4 %. The reason of this small value might be the formation of defects such as dangling bond at interface between p-type a-SixC1-x film and n-Si substrate because high power is needed for the decomposition of source gases in the deposition [3]. In this study, the damage of junction interface was decreased by decreasing power of applied radio-frequency input. Furthermore, the decomposition rate of source gases was tried to be enhanced by increasing frequency to improve the performance of solar cells. 2. Experimental The heterojunction solar cells comprising boron-doped a-SixC1-x thin film and n-Si substrate were fabricated by plasma-enhanced chemical vapor deposition (SAMCO, BPD-1) equipped with variable-frequency radio frequency generator (THAMWAY, T162-5767C) using trimethylsilane and trimethylborate as source materials. B/(Si + C) ratio in the liquid source was 7.50 at.%. The boron-doped a-SixC1-x thin films were deposited by changing discharge frequency and applied power. The substrate temperature and deposition time were adjusted to 200˚C and 20 min, respectively. The performance of heterojunction solar cells was investigated by current-voltage (I-V) measurement (KEYSIGHT, B2901A) with simulated AM 1.5 sunlight (ASAHI SPECTRA, HAL-320). In addition, in order to evaluate the quality of pn junction, the junction capacitance was calculated from AC impedance measurement (Solartron Analytical, SI 1260 and 1287). 3. Results and discussion As described in the previous paper, the conversion efficiency of heterojunction solar cell prepared at frequency of 40.68 MHz and power of 100 W was 2.7×10-4 % [2]. The conversion efficiency was increased to 1.83×10-2 % by decreasing power from 100 to 10 W. Moreover, the efficiency was increased to 3.03×10-1 % with increasing frequency from 40.68 to 60.00 MHz. Figure 1 shows I-V characteristics of solar cells fabricated at 40.68 and 60.00 MHz under simulated sunlight irradiation. As a result of increase frequency, short-circuit current density and open-circuit voltage, which are contributed to the conversion efficiency, were drastically increased. Figure 2 shows capacitance-voltage (C-V) characteristics of heterojunction solar cells fabricated at 40.68 and 60.00 MHz. The capacitance of solar cells prepared at 40.68 and 60.00 MHz were 2.61 and 3.12 nF/cm2 (at zero bias), respectively. It has been reported that the bombardment energy of ion in plasma is decreased by increasing frequency [4]. Therefore, we consider that the capacitance is increased, since the defect at interface might be decreased with increasing frequency. From 1/C2-V plot, the slope of solar cell prepared at 60.00 MHz was smaller than that of solar cell prepared at 40.68 MHz. This result implies that the carrier density of boron-doped a-SixC1-x thin film deposited at 60.00 MHz is larger than that of the thin film deposited at 40.68 MHz. This might be because the decomposition rate of source gases might be enhanced by increasing frequency. From these results, the conversion efficiency of boron-doped a-SixC1-x/n-Si heterojunction solar cell was successfully enhanced by the improvement of junction quality and increasing carrier density.