Advanced Solar Cell Modelling

Available online at www.sciencedirect.com

Energy Procedia 33 (2013) 99 – 103

PV Asia Pacific Conference 2012

Advanced Solar Cell Modelling Marius Peters*, Ma Fajun, Ke Cangming, Guo Siyu, Nasim Sahraei, Bram Hoex, Armin G. Aberle Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Block E3A, Singapore 117574, Singapore

Abstract Solar cells today enter a regime where they become cost-competitive with conventional methods of electricity generation. However, there is still a need to further reduce costs. An efficient way to do this is to increase the conversion efficiency of the solar cells. For crystalline silicon wafer solar cells this has been demonstrated for various concepts and the task here is to find low-cost processes, suitable for mass-production for these solar cells. Silicon wafer solar cells have the largest market share, but also thin-film technologies have developed rapidly and show excellent performance. However, while silicon wafer solar cells operate already close to their theoretical potential, this gap has yet to be filled for thin-film solar cells and more fundamental work is required here. Computer-based modelling can assist in many aspects to pave the way for an era that is relying more and more on renewable energies. The tasks addressed by modelling reach from fundamental research to production levels and it is important to keep these different levels in mind. A wide range of commercial and open-source software is available for these tasks. This paper presents two examples of current activities at the Solar Energy Research Institute of Singapore (SERIS).

2013 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Solar Energy ©©2013 The Authors. Published by Elsevier Ltd. University Singapore (NUS). (SERIS) The PV –Asia Pacific Researchand Institute of Singapore (SERIS) ofNational Selection peer-review under responsibility Solar Energy ResearchofInstitute of Singapore National University of Singapore (NUS). Theorganised PV Asia Pacifi Conference jointly organised byIndustry SERIS and the Asian Conference 2012 was jointly by cSERIS and2012 the was Asian Photovoltaic Association Photovoltaic (APVIA). Industry Association (APVIA) Keywords: Solar cells; modeling; overview

* Corresponding author. Tel.: +65 66011988; fax: +65 775 1943 E-mail address: [email protected]

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of Solar Energy Research Institute of Singapore (SERIS) – National University of Singapore (NUS). The PV Asia Pacific Conference 2012 was jointly organised by SERIS and the Asian Photovoltaic Industry Association (APVIA) doi:10.1016/j.egypro.2013.05.045

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Marius Peters et al. / Energy Procedia 33 (2013) 99 – 103

1. Introduction Solar cells today enter a regime where they become cost-competitive with conventional methods of electricity generation. However, there is still a need to further reduce costs. An efficient way to do this is to increase the conversion efficiency of the solar cell. In the lab, 25% efficiency for a crystalline waferbased silicon solar cell has been demonstrated [1]. This is more than 85% of the theoretical potential for these solar cells taking into account radiative and Auger recombination. The task for silicon wafer solar cell developers now is to transfer these high efficiencies into low-cost processes that are suitable for massproduction. Currently, the concepts with the highest potential to achieve high-efficiencies in massproduction are all-back-contact and hetero-junction silicon wafer solar cells. Silicon wafer solar cells have the largest market share, but also thin-film technologies have developed rapidly and show excellent performance. However, thin-film solar cells are still further away from their theoretical efficiency limits and technology advances are required. The highest efficiency for CIGS solar cells, for example, is 19.6% [1] with a theoretical limit comparable to that of silicon. Computer-based modelling is an inherent part of solar cell R&D. Modelling assists cell development from fundamental base to production level. A wide range of commercial and open-source software is available for this task. This paper presents two examples of current activities at the Solar Energy Research Institute of Singapore (SERIS) in this field: fundamental studies on surface passivation for silicon wafer solar cells and studies on scattering for thin-film silicon solar cells. 2. Fundamental studies on surface passivation With silicon wafers improving in quality, surface passivation is of more and more importance. In the past years, significant progress has been made using field-effect passivation [2, 3]. For wafers of high quality, surface effects contribute a major part to the total recombination. In recent research, therefore, the mechanisms that result in an increase in surface recombination have been closely investigated. One such mechanism, that has been suggested, is the presence of a region with a higher degree of damages located directly at the solar cell surface. The characteristic of this region would be a reduced minority chargecarrier lifetime. In a theoretical study, the impact of such a locally reduced lifetime on the minority charge-carrier lifetime eff was investigated. For this purpose, a symmetric setup for lifetime testing was implemented in Sentaurus TCAD [4]. A schematic sketch of this setup is shown in Fig. 1a. The bulk lifetime was set to 50 ms in order to be only sensitive to surface effects. The damaged region has a width of 300 nm and a reduced lifetime for minority and majority charge carriers ( n0, p0) of 7 s. Furthermore, the presence of surface fixed charges with a density of 1.3×1013 was assumed [5]. Experimentally, this feature is realised by a thin layer of Al2O3, for example. In the investigation, the polarity of these charges was switched. Experimentally, this can be realised using a corona charger. The result of this investigation is shown in Fig. 1b


Fig. 1. (a) Schematic sketch of the structure implemented for the carrier lifetime simulation. (b) Injection-level dependent effective minority charge carrier lifetime for a solar cell with a region with surface damage. The bulk doping type was n-type with a dopant density of 2.5×1015 cm-3

Depending on the polarity of the surface charges, the curves for the injection-dependent effective lifetime as depicted in Fig. 1(b) show significantly different characteristics. While the lifetime for positive surface charges is steadily decreasing with an increasing injection level (purple dashed line), there is a local maximum for negative surface charges (blue line). To verify this result, an experiment has been designed with the goal to reproduce the simulated curves. It was found that there is a significant difference in the injection-dependent lifetime characteristics when different surface charges are used, however, simulated results indicate that also other effects play a role here and further studies on the subject are required. More details about this can be found in Ref. [6]. 3. Scattering studies for silicon thin-film solar cells Increasing the efficiency of thin-film silicon solar cells is one of the key challenges in this field and light trapping is one of the main features that can help to accomplish this task. Efficient light trapping is achieved by scattering and can be realised by using textured glass on which the solar cell is deposited. The scattering process increases the light path length and with it the absorption. An example for a scattering structure fabricated using the aluminium induced texturing (AIT) technique is shown in Fig. 2a. More details about this process can be found in Ref. [7]. To assess the light trapping potential of such a structure, the angular distribution function (ADF) of the scattered light inside the absorber needs to be calculated. The ADF tells how much light is scattered into which direction. The function is calculated based on a scalar scattering approach [8]. In a subsequent step this function is used to calculate. This calculation was done using the commercial ASA software. As an exemplary structure, we implemented an a a-Si:H solar cell with a thickness of 250 nm between two layers of TCO (1 m) m and on textured glass. The wavelength dependent ADF in the absorber for this setup is shown in Fig. 2. Based on this result, we calculated solar cell characteristics for the implemented setup. An increase in current of 1 mA/cm2 or 10% compared to a planar reference was calculated. More details about this procedure can be found in Ref. [9].

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Fig. 2. (a) Surface profile of a scattering texture used for light trapping in thin-film silicon solar cells; (b) angular scattering function (ADF) calculated from the profile shown on the left side.

4. Summary In this paper, we have presented two examples for simulation based studies currently performed at the solar energy research institute of Singapore (SERIS). In the first example, the effect of a damaged region with a reduced lifetime close to the surface on the effective lifetime was investigated. In this investigation, the injection level was varied and additionally the presence of surface charges was assumed. The surface charge polarity was switched and it was found that for different polarities different signatures in the effective lifetime are found. These signatures are fingerprints for the existence of a damaged region close to the surface. In the second example, the light scattering properties of textured glass were investigated theoretically. For this purpose, first the angular distribution function for the scattered light was calculated. Subsequently, the absorption and current generation were simulated for this scattering function in a 250 nm thick a-Si:H solar cell. It was found that the scattering increases the current generation by 1 mA/cm2. Acknowledgements ional Research Foundation (NRF) through the Singapore Economic Development Board (EDB). References [1]

Green MA, Emery K, Hishikawa Y, Warta W, Dunlop ED. Solar cell efficiency tables (version 39). Progress in Photovoltaics 2012;20:12-20.


[2] [3] [4] [5] [6] [7] [8] [9]

Hoex B, Heil SBS, Langereis E, van de Sanden MCM, Kessels WMM. Ultralow surface recombination of c-Si substrates passivated by plasma-assisted atomic layer deposited Al2O3. Appl. Phys. Lett. 2006;89:042112. Aberle AG, Lauinger T, Schmidt J, Hezel R. Injection level dependent surface recombination velocities at the silicon plasma silicon nitride interface. Appl. Phys. Lett. 1995;66:2828. Sentaurus, 2010.03 ed. (Synopsys Inc., Mountain View, CA, 2008). Hoex B, Gielis JJH, van de Sanden MCM, Kessels WMM. On the c-Si surface passivation mechanism by the negativecharge-dielectric Al2O3. J. Appl. Phys. 2008;104:113703 Ma F, Samudra GG, Peters M, Aberle AG, Werner F et al. Advanced modeling of the effective minority carrier lifetime of passivated crystalline silicon wafers. J. Appl. Phys. 2012; 112:054508. Wang J, Venkataraj S, Battaglia C, Vayalakkara P, Aberle AG. Analysis of optical and morphological properties of aluminium induced texture glass superstrates. Jap. J. Appl. Phys. 2012; 51:10NB08 Dominé D, Haug FJ, Battaglia C, Ballif C. Modeling of light scattering from micro- and nanotextured surfaces. J. Appl. Phys. 2010;107: 044504. Sahraei N, Venkataraj S, Aberle AG, Peters IM. Investigation of the optical absorption of a-Si:H solar cells on micro- and nano-textured surfaces. Proc. PV Asia Pacific Conf. 2012, Singapore (2013, in press).

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