Amorphous hydrogenated silicon (a-Si:H) is a well-explored medium for solar cells and photodetectors due to its simple fabrication process and abundance of raw materials [1,2]. The effect of the a-Si:H deposition power on the light sensing has been studied in the literature [3]. However, the influence of the n⁺ layer deposition power was rarely studied. In this paper, the authors investigated the effect of the n⁺ layer deposition power on the performance of pin a-Si:H diodes under solar, red, green, and blue light exposure conditions.

The a-Si:H pin tri-layer was fabricated using plasma-enhanced chemical vapor deposition (PECVD) process in the same chamber under one pump-down at 260°C with a 50 kHz RF generator. The Corning 1737 glass was used as the substrate. The deposition conditions are listed in Table I. The n⁺ layer deposition power was varied. It was the top layer where the device was first exposed to the incident light. Both the top and bottom electrodes were made of the sputter deposited 8 nm thick indium tin oxide (ITO) film. No metal back reflector was included in the diode structure. While this arrangement decreases light-sensing efficiency due to the lack of light-trapping [2], it has the versatility of allowing incident light through either side of the diode for testing purposes. The final annealing was done at 200°C in air for 30 min. Each cell was tested under the red (625 nm), green (530 nm), and blue (470 nm) LED exposure, respectively. The power density on the device was fixed at 27.6 W/m². For the solar light test, the power density was 1000 W/m².

Figure 1 shows the current density (J) – voltage (V) curve of the device. For each curve, the corresponding open circuit voltage (V_OC), short circuit current (J_SC), fill factor (FF), efficiency (η), and external quantum efficiency (EQE) were calculated. Under solar light exposure, all devices exhibited the same V_OC of 0.85 V independent of the n⁺ deposition condition. The J_SC and η increased with the increase of the deposition power. The FF increased non-linearly with the increase of power.  

Optical characteristics of the device changed with the wavelength of the exposure light and the n⁺ layer deposition power. In general, the best characteristics were obtained at the intermediate n⁺ layer deposition power of 300 W. Figure 2 shows the relationship between η and the deposition power of the n⁺ layer with the type of exposure light as the parameter. In spite of the difference in the n⁺ layer deposition power, the green light gave the highest η and the blue light gave the lowest η.

In general, the RF power in the PECVD process affects the characteristics of doped a-Si:H thin films. With n⁺ layers deposited at very low and at very high power, the light sensitivity declined under the single wavelength light exposure condition. However, under the solar light exposure, the maximum efficiency was observed at the highest power deposition condition. Therefore, in addition to the n⁺ layer deposition power, the spectrum of the exposure light affected the sensing efficiency.

Separately, influences of the n⁺ layer deposition power on the film deposition rate and sheet resistivity have been investigated. According to Learn et al. [4], the dopant concentration in the n⁺ a-Si:H thin films is inversely proportional to the deposition rate. Also, the sheet resistivity is inversely proportional to the dopant concentration, but this relationship breaks down at higher dopant concentrations due to the high defect density in the film [5]. In this study, it was observed that the light sensing of the a-Si:H pin diode improved with the n⁺ layer deposition power from 200 W to 300 W. However, the efficiency decreased with the further increase of the power probably due to the high defect density in the film or at the n⁺/ a-Si:H interface.  

[1] A.V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, and J. Bailat. Prog. Photovolt: Res. Appl. 12, 113 (2004).

[2] B. Rech and H. Wagner. Appl. Phys. A 69, 155 (1999).

[3] H. Miki, S. Kawamoto, T. Maejima, H. Sakamoto, M. Hayama, and Y. Onishi. Mat. Res. Soc. Symp. Proc. 95, 431 (1987).

[4] A. Learn and D. Foster. Appl. Phys. 61, 1898 (1987).

[5] S. Martín de Nicolás, D. Muñoz, A.S. Ozanne, N. Nguyen, and P.J. Ribeyron. Energy Procedia 8, 226 (2011).