Development of Silicon and Carbon Based p-Type Amorphous Semiconductor Films with Optical Gap Variable for High-Efficiency Multi-Junction Solar Cells
Authors: Naragino Hiroshi , Nagata Yoshiya , Okafuji Keigo , Ohtomo Shinpei , Shimizu Yuta , Honda Kensuke 
Published: February 2020

1. Introduction

Multi-layered structure is one of the methods to enhance the conversion efficiency of solar cells. In order to realize the multi-junction solar cell with high conversion efficiency, the development of semiconductor materials with high semiconducting property and selective optical gap in wide range is required. The optical gap of amorphous silicon carbide (a-SixC1-x) is known to be controllable. a-SixC1-x, which is composed of silicon and carbon atoms, is expected to realize low-cost multi-junction solar cells. Moreover, a-SixC1-x can easily form the junction between solar cells with different optical wavelength compared to single-crystalline and polycrystalline materials because clean lattice constant cannot be defined. Our research group has reported the fabrication of the n-type a-SixC1-x semiconductor with controllable optical gap in the range from 1.8 to 2.8 eV [1]. If a p-type a-SixC1-x semiconductor with controllable optical gap in the same range can be synthesized, the multi-junction solar cell can be realized. Although previous studies reported that the optical gap of p-type a-SixC1-x films could be controlled, the controllable range was narrow (approximately 0.3 eV) [2]. It is assumed that the film cannot maintain the function as a p-type semiconductor when the chemical composition of the film is drastically changed in order to control the optical gap. The objective of this study is to control simultaneously the optical gap in a wide range and p-type semiconducting property of a-SixC1-xby changing power and frequency of applied RF input to change Si/C ratio drastically and enhancing activation ratio of boron atom incorporated as a dopant.

2. Experimental

The p-type a-SixC1-xfilms were deposited on glass and Si substrates by RF plasma-enhanced chemical vapor deposition (CVD) using trimethylsilane (TMS) and trimethylborate (TMOB) as source materials. The RF power and frequency were controlled from 100 to 400 W and from 13.56 to 40.68 MHz, respectively. The substrate temperature and deposition time were adjusted to 200˚C and 40 minutes, respectively.
The chemical composition and optical gap of a-SixC1-xfilms were investigated by X-ray photoemission spectroscopy (XPS) and UV-visible absorption spectroscopy, respectively. The photoelectrochemical measurement and current-voltage (I-V) measurement using Hg-Xe lamp with the wavelength of 365 nm were carried out to clarify the function to convert photons into electrons at the p-type semiconductor and the performance of solar cells.

3. Results and discussion

From XPS measurement, the fabricated films were mainly composed of Si and C atoms, and contained B atoms with concentration of 1.5-2.6 at.%. The Si/C ratio of a-SixC1-x film deposited at RF power of 400 W and frequency of 13.56 MHz was estimated to be 2.53. On the other hand, that of a-SixC1-x film deposited at low RF power (100 W) and high frequency (40.63 MHz) was estimated to be 1.03. These results indicate that the Si/C ratio of a-SixC1-x films can be controlled by RF power and frequency. The optical gap of a-SixC1-x films was increased from 1.8 to 2.5 eV with decreasing the Si/C ratio from 2.53 to 1.03. The carrier density was increased from 1012 to 1014 cm-3 with increasing the boron concentration in the film from 1.5 to 2.6 at.%. Moreover, in the electrochemical measurements, the cathodic currents of H2 evolution were not observed in aqueous solution in the dark, and the photocurrents were observed under UV illumination at both a-SixC1-x electrodes. These results indicate that B-doped a-SixC1-x film has the rectification property of a p-type semiconductor and the function of photoelectric conversion. The conversion efficiencies of a-SixC1-x films were calculated to be approximately 1.63%. It is confirmed that the boron atoms can act as an acceptor in the film by enhancing the decomposition of source materials at high RF power or frequency. These results indicate that the p-type a-SixC
1-x
semiconductor with wide-controllable optical gap can be realized.
Figure 1 shows the J-V curve of heterojunction solar cell comprising p-type a-SixC1-x film with optical gap of 2.5 eV and n-type Si substrate under UV illumination. The open-circuit voltage of heterojunction solar cell was approximately 200 mV. It implies that the generation of photocarriers at depletion region and the difference between Fermi level of p-type and n-type layers were emerged by UV illumination. These results suggest that the p-type a-SixC1-xfilms have the potential applicability to the multi-junction solar cells with operation range of approximately 500-700 nm in wavelength.

References

[1] K. Yoshinaga et al, J. Phys.: Conf. Series
441, 012140 (2012).

[2] T. Inoue et al., Appl. Phys. Let. 44, 871 (1984).

Figure 1