Two dimensional simulation studies on amorphous silicon stack as front surface field for interdigitated back contact solar cells
Introduction
The interdigitated back contact (IBC) solar cells have attracted much attention due to its inherent advantages [1]. Placing all the electrodes on the rear surface eliminates shadow loss, leading to a high Jsc and independent optimization of the front surface [2]. Series resistance can get reduced by utilizing a high proportion of metallization area on the rear side. The avoidance of lateral current in doped layer also contributes to the reduction of internal series resistance. No series resistance degradation is observed under high intensity light, which makes it suitable for application in concentrated photovoltaics. In addition, in terms of module fabrication, IBC solar cells exhibit simplicity for interconnection and improved packing factor.
Recently the Australian National University (ANU) has reported an efficiency of 24.4% on a small area (2 × 2 cm2) n type IBC solar cell [3]. An efficiency as high as 25% on large area (121 cm2) has been achieved by SunPower Corporation [4]. Besides diffusion, ion-implantation was also proposed for junction-formation in IBC solar cells and the conversion efficiency of 24.1% was achieved [5]. In early 2014, Panasonic Corporation achieved the world-record conversion efficiency of 25.6% by applying the interdigitated back contact design on its heterojunction solar cells [6].
IBC structure requires a high minority carrier lifetime and exceptionally low surface recombination velocity because most electron–hole pairs are generated near the front surface and have to diffuse through the bulk of substrate to reach the rear surface in order to be collected by electrodes. Therefore, the effective front surface passivation is deemed to be critical to achieve high conversion efficiency. Traditionally, thermal silicon oxidation has been used as effective passivation layer [7], [8], [9]. Another commonly used passivation layer is SiNx whose refractive index is easily adjustable, which makes it extremely suitable for antireflective coating as well as passivation layer [10], [11], [12]. Owing to the low density of the interface defects and the high density of the negative fixed charges, the Al2O3 layers can afford excellent chemical and field-effect passivation even for nano-textured front surface of IBC solar cells [13], [14], [15].
Compared with SiO2 and SiNx, amorphous silicon (a-Si) deposited by plasma enhanced chemical vapor deposition (PECVD) has the advantage of low temperature (∼200 °C) deposition. The excellent passivating ability of intrinsic amorphous silicon (i-a-Si) has been shown through the great success of HIT solar cells [16].
On the other hand, front surface field (FSF) and front floating emitter (FFE) have been proved to be capable of improving front surface passivation and reducing lateral resistance [17], [18]. Many previous studies have been done concerning the optimization of the diffused FSF of IBC solar cells, such as diffusion profile, sheet resistance and UV stability [19], [20], [21], [22], [23]. Ion-implanted FSF has also been investigated to passivate IBC solar cells lately [17], [24].
In the present work, we propose the application of stack of doped and intrinsic amorphous silicon layers as FSF which integrates heterojunction structure to IBC solar cells with the assistance of TCAD simulation tools. Our preliminary experiments on such stack have shown promising results in the passivation of crystalline silicon wafer. Minority carrier lifetime of more than 2 ms indicated the excellent surface passivation capability of this FSF structure. In our simulation, the layer of n-type doped amorphous silicon (n-a-Si) is optimized in terms of thickness and doping level. The intrinsic amorphous silicon (i-a-Si) layer is also optimized with respect to thickness as well as energy band gap (Eg). In addition, the influence of interface defect density between crystalline Si (c-Si) substrate and i-a-Si layer is also investigated.
Section snippets
Simulation method
The simulations were performed using TCAD tools, the commercial device simulation package produced by Silvaco International with the version number of 5.19.20.R, which allows simulating 2D device required by the design of IBC solar cells [25]. A schematic cross-section of the IBC solar cell structure analyzed in this work is shown in Fig. 1.
A layer of n-a-Si is deposited following the deposition of i-a-Si on the front surface of 5 Ω•cm (Nbase = 1015 cm−3), and 300 μm thick crystalline silicon
The optimization of n-a-Si layer
Firstly, a series of doping level, varying from 1 × 1015 cm−3 to 1 × 1021 cm−3, of n-a-Si layer with different thickness (5 nm, 10 nm, 15 nm, 20 nm) are simulated systematically.
Fig. 2 demonstrates the influence of the doping level of n-a-Si layers with different thicknesses on the efficiency of IBC solar cells. It can be clearly seen that the efficiency sharply increases when doping level is close to 1 × 1018 cm−3 for all investigated n-a-Si layers' thickness. The efficiency then decreases
Conclusion
The FSF formed by depositing i-a-Si layer and n-a-Si layer on the front surface of c-Si substrate is competent for repelling minority carriers and reducing surface recombination, which ensures a high conversion efficiency. Simulations have been done to optimize the doped and intrinsic a-Si layer, during which the electric field and current density distribution are analyzed. The results show that, on the one hand, the doping level of n-a-Si should be high enough to form a strong electric field
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grant Nos. 11104319, 11274346, 51202285, 61234005, 51172268 and 51402347), the Chinese Academy of Sciences Solar Energy Action Plan (Grant Nos. Y1YT064001, Y1YF034001 and Y2YF014001), and the Opening Project of Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences.
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