Effect of oxide thin films in back contact on the optical absorption efficiency of thin crystalline Si solar cells Seok Yong Byun a, c , Seok-Joo Byun a , Taek Sung Lee b , Dongwoo Sheen c , Jeung-hyun Jeong d , Won Mok Kim b, * a INSIDEOPTICS Co. LTD, Seoul 139-743, Republic of Korea b Electronic Materials Research Center, Thin Film Materials Design Lab, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea c Interdisciplinary Program in Computational Science & Technology, Seoul National University, Seoul 151-747, Republic of Korea d Solar Cell Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea article info Article history: Received 6 November 2012 Received in revised form 7 January 2013 Accepted 8 January 2013 Available online 9 February 2013 Keywords: Thin crystalline Si solar cells Back contact Optical absorption efficiency Oxide thin films Ray tracing abstract The optical absorption efficiency (OAE) of thin crystalline Si (c-Si) solar cells was examined by using a direct absorption calculation algorithm based on the three-dimensional modeling and ray-tracing technique. The back contact was assumed to be made of ZnO based oxide layers with different optical constants and metallic electrode, and the front surface was assumed to be Lambertian. Simulation results showed that the insertion of non-absorptive oxide layer between c-Si and Al improved OAE, and that the relative amount of enhancement increased with decreasing refractive index, manifesting the reduction in absorption loss in Al electrode due to the increased total internal reflection. In the case of absorptive oxide layer, although increase in OAE was still attainable when compared with the back contact without oxide, the OAE was subdued significantly due to large absorption loss in oxide layer. The optimal oxide layer thickness was around 200 nm for non-absorptive oxide, and that of absorptive oxide decreased with increasing extinction coefficient. In the case of Ag metal contact, the enhancement of OAE due to the use of oxide layer was much less than the case of Al because of inherent high reflectivity and low ab- sorption loss in Ag layer. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Reduction of production cost is ever-demanding in photovoltaic industry. In crystalline Si (c-Si) solar cell, much effort has been given to develop crystalline Si solar cells with reduced thickness in order to achieve higher cost-effectiveness [1e5]. For thin crystalline Si solar cells, maximizing the incorporation of the incoming light into the active layer is of great importance to maintain or even improve the conversion efficiency. One of the main activities for maximizing the light absorption is texturing and applying anti- flection coating for the front surface, and the other is designing the back contact [6e8]. Proper design of back contact, in which utilized double layered structure composed of thin oxide layer and metal contact, has been shown to be effective in improving the optical absorption efficiency (OAE) in Si by lowering the absorption losses at the metal contact and redirecting the reflected light into Si by total internal reflection (TIR) [9e12]. In order to increase TIR at the c-Si/oxide interface, oxide films with lower refractive index were studied and tested [9]. ZnO based transparent conducting oxide (TCO) thin films have been studied extensively as an alter- native for Sn-doped In2O3 [13e15], and frequently adopted as an oxide layer in back contact. Such transparent oxide films with reasonable amount free carriers inevitably possess non-zero extinction coefficient in long wavelength region of visible and near infra-red region due to free electron absorption [16,17]. However, very little attention has been given to the effect of ab- sorption characteristics of the oxide layer on the OAE in Si based solar cells. Furthermore, analytical studies on the effect of the thickness of thin oxide layer on OAE in Si solar cells are rare. In this study, we examined the thickness dependent OAE in thin c-Si solar cells for various ZnO thin films with different dielectric properties by performing simulations using ray tracing technique. The optical absorption in c-Si and the absorption loss in metal and oxide layer together with the reflection loss were calculated and analyzed for the double-layered back contacts with undoped ZnO films having different refractive indices and doped ZnO films hav- ing different extinction coefficients. * Corresponding author. Tel.: þ82 2 958 5384. E-mail address: wmkim@kist.re.kr (W.M. Kim). Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.01.045 Current Applied Physics 13 (2013) S140eS143