Thin film silicon alloys with enhanced stability made by PECVD and HWCVD for multibandgap solar cells
This paper presents the development of high deposition rate protocrystalline Si:H and /spl mu/c-Si:H as obtained by hot wire chemical vapor deposition (HWCVD, or catalytic CVD) and very high frequency plasma enhanced CVD (VHF PECVD) and protocrystalline SiGe:H as obtained by conventional PECVD. Using the latter two low bandgap materials as absorber layer, it is feasible to obtain stable devices showing virtually no light-induced changes in fill factor. We have obtained stable proto-SiGe:H cells using high H/sub 2/ dilution. A disadvantage, however, is the very low deposition rate for proto-SiGe:H (/spl sim/0.6 /spl Aring//s). By HWCVD, much higher deposition rates have been achieved for silicon-based thin films (both proto-Si:H and pc-Si:H). During the last few years, improved control of parameters is also obtained in HWCVD, which presently makes this method interesting for reducing the production cost of thin film solar cells. Recently, we have demonstrated that protocrystalline Si:H can be obtained from undiluted silane at 250 /spl deg/C in HWCVD at a high deposition rate of 1 nm/s. The light induced degradation in single junction n-i-p cells is less than 10%. Here, we also present for the first time p-i-n cells on commercial SnO/sub 2/:F with proto-Si intrinsic layers deposited at rates up to 3 nm/s, with again remarkable stability. Further, HWCVD microcrystalline materials with a crystalline fraction V/sub f/ of /spl sim/50% possess excellent electronic quality: the dangling bond density extracted from computer simulations, is only 2 /spl times/ 10/sub 15/ cm/sup -3/ and therefore we could obtain fill factors of 0.77 and 0.75 in dual-junction stacked /spl mu/c-Si:H//spl mu/c-Si:H cells, and proto-Si:H//spl mu/c-Si:H 'micromorph' n-i-p type solar cells, respectively. Both VHF PECVD /spl mu/c-Si:H made in the high pressure depletion regime and HWCVD /spl mu/c-Si:H have excellent potential for multibandgap solar cells.
Parameter estimation of three-diode solar photovoltaic model using an Improved-African Vultures optimization algorithm with Newton–Raphson method
Parameter identification and accurate photovoltaic (PV) modeling from basic I–V information are necessary for simulation, optimization, and control of the PV systems. Therefore, this paper proposes an Improved-African Vultures Optimization (I-AVO) algorithm, which combines the general Opposition-Based Learning (OBL) and Orthogonal Learning to extract the unknown parameters of the solar Photovoltaic (PV) modules accurately and effectually. The proposed I-AVO algorithm is developed from the basic version of the recently proposed African Vultures Optimization (AVO) algorithm. The solar PV parameters estimation problem is considered to be a complex optimization problem with the characteristics such as multi-dimensional, nonlinear, Transcendental, and multi-modal. Therefore, the basic variant of AVO struggles to produce the optimal and is stuck at local optima when it handles this complex optimization problem. Therefore, the I-AVO is formulated by combining the features of OL and OBL, along with the AVO, to generate the optimal solution. Out of various PV models, Three-Diode Model has been considered to determine the parameters. Furthermore, Newton–Raphson (NR) technique is discussed to solve the chaotic behavior of the I–V curve relation. The obtained results proved that the proposed I-AVO along with NR, called I-AVO-NR, can accurately obtain the optimal solution. The superiority of the proposed algorithm is proved to be better than other advanced algorithms based on the obtained results and their comparison. Based on the statistical test value obtained from Friedman’s test, the proposed algorithm stood first among eight algorithms with the ranking value of 1.542 for two case studies.
Defect characterization in amorphous silicon based solar cells by subband‐gap spectroscopy with constant photocurrent measurements
We used the constant photocurrent method to characterize a‐Si:H based pin cells in order to discuss the correlation between the device characterizing quantity, the fill factor (FF), and the number of defects which is related to the quantity αD (subbandgap optical absorption constant). We show that the changes in the i layer due to the creation of metastable defects after light soaking or current injection−characterized by αD−directly correlate with the changes in solar cell performance (FF). We are able to show that other reasons than an increased defect density can lead to a reduction of the fill factor. Further we try to quantify αD in terms of a defect density of states.