Effect of Substrate Temperature on CdTe Thin Film Property and Solar Cell Performance

doi:10.15541/jim20150310

Abstract

Abstract:

Vapor transport deposition is an excellent method for preparing large area CdTe thin films with high quality and uniformity. Polycrystalline CdTe thin films were deposited by home-made vapor transport deposition system (VTD). The effects of substrate temperature on the property of CdTe film and the performance of CdTe solar cell were investigated. CdTe thin films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Vis spectrometer, and Hall Effect system. The results show that the CdTe thin films deposited by vapor transport deposition are cubic phase with a preferred orientation in (111) direction. The average grain size increases from 2 μm to 6 μm and the carrier concentration increases from 1.93×10¹⁰ cm^-3to 2.36×10¹³cm^-3when the substrate temperature increases from 520 ℃ to 620 ℃. This suggests that high substrate temperature can increase the carrier density significantly due to the suppressed defect recombination. The performance of CdTe thin film solar cells deposited at different substrate temperatures demonstrates that high substrate temperature (610℃) can greatly improve the efficiency, open circuit voltage and fill factor of the solar cells. But the substrate temperature higher than 610℃ will reduce the spectral response of the cells in long wavelength region, which results in the degradation of solar cell performance. The small-area CdTe thin film solar cell without back contact layer deposited at substrate temperature of 610℃ obtains the best conversion efficiency of 11.2%.

Key words: vapor transport deposition, CdTe thin film, substrate temperature, conversion efficiency

CLC Number:

TQ174

CAO Sheng, WU Li-Li, FENG Liang-Huan, WANG Wen-Wu, ZHANG Jing-Quan, YU Xiao-Qi, LI Xin-Xin, LI Wei, LI Bing. Effect of Substrate Temperature on CdTe Thin Film Property and Solar Cell Performance[J]. Journal of Inorganic Materials, 2016, 31(2): 141-147.

Figures/Tables 10

Fig. 1 Schematic diagram of VTD system

Fig. 2 XRD patterns of CdTe films deposited at different substrate temperatures

Table 1 Texture coefficient Ci and standard deviation σ of CdTe films for different substrate temperatures

Substrate temperature/℃	C_i					σ
Substrate temperature/℃	(111)	(220)	(311)	(440)	(331)	σ
520	2.98	0.10	0.96	0.27	0.67	1.04
550	4.79	0.01	0.05	0.01	0.13	1.89
580	3.44	0.14	0.43	0.31	0.66	1.23
620	4.45	0.03	0.15	0.06	0.36	1.72
640	3.96	0.06	0.22	0.11	0.65	1.49

Fig. 3 SEM images of CdTe films deposited at different substrate temperatures (a) 520℃; (b) 550℃; (c) 580℃; (d) 620℃; (e) 640℃

Fig. 4 Distribution of average grain size of CdTe films deposited at different substrate temperatures

Fig. 5 Optical transmittance spectra (a) and band-gap of CdTe films (b) deposited at different substrate temperatures

Table 2 Electrical parameters of CdTe films deposited at different substrate temperatures

Substrate temperature	Electrical parameters (300 K)			Activation energy
Substrate temperature	ρ/ (Ω·cm)	R_sh/ (Ω·□^-1)	P /cm^-3	μ/ (cm²·V^-1·s^-1)	E_a/eV	E_ap/eV	E_aμ/eV
520℃	1.76×10⁶	3.84×10⁹	1.93×10¹⁰	-	0.576	0.608	-
580℃	2.94×10⁶	6.75×10⁹	1.96×10¹²	1.171	0.554	0.369	0.178
620℃	5.84×10⁵	1.34×10⁹	2.36×10¹³	0.756	0.509	0.277	0.192

Fig. 6 Temperature-dependency of electronic properties of CdTe films deposited at different substrate temperatures

Fig. 7 Performance parameters of CdTe cells as a function of substrate temperature

Fig. 8 Measured EQE spectra of CdTe cells deposited at different substrate temperatures

References 13

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