Reduced Interfacial barrier height at ITO/a-Si:H(p) by using oxygen reactive sputtering on silicon heterojunction solar cells S. H. Ahn 1* , J. H. Lee 1 , S. B. Kim 1 , V. A. Dao 1 , S. Q. Hussain 2 , Y. S. Lee 2 , J. H. Cho 1 , S. H. Kim 2 , D. Y. Kim 3 and J. S. Yi 1,2 1 College of Information and Communication Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, Gyeonggi-do 400-746, Republic of Korea 2 Department of Energy Science, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, Gyeonggi-do 400-746, Republic of Korea 3 School of Electrical and Electronic Engineering, Ulsan College West Campus, Ulsan, 680-749, Republic of Korea *Corresponding author: Email yi@skku.ac.kr Abstract ITO films with low carrier concentra- tion (n), and high mobility (μ) and work function were prepared by RF magnetron sputtering with addition of oxygen flow and used as anti-reflection layer in heterojunction silicon solar cells. The ITO work function (φ ITO ) was increased from 4.89 to 5.04 eV by varying the O 2 /(Ar + O 2 ) flow rate from 0 to 1.4%. This in- crease in the φ ITO leads to band bending, which in turns improved the built-in volt- age, and hence increases the V oc of the device up to 710 mV. With addition of ox- ygen, 1 cm 2 heterojunction silicon solar cells with industrial-compatible process were fabricated. The solar cell character- istics resulted in short-circuits current density, open-circuit voltage and efficien- cy of 34.79 mA/cm 2 , 710 mV and 17.79%, respectively. Introduction A change in work function of ITO films affect the efficiency of carrier extrac- tion from ITO/a-Si:H(p) interface which plays a critical role in key device parame- ters such as operating voltage. Therefore, the ITO film surface with higher work function is required to reduce the energy barrier for hole transport in the heterojunction solar cells. However, very few reports are available related to the modification of Ф ITO and interfacial barrier height of ITO/a-Si:H(p) of HIT solar cell by the oxygen reactive sputtering tech- nique. In this article, the effects of oxygen to argon gas ratio on the structural and electrical properties of sputtered ITO films were investigated. In addition, the HIT so- lar cells with high Ф ITO thin films as a front electrode were fabricated and the perfor- mance of the solar cell was characterized as a function of the O 2 /(Ar + O 2 ) gas ratio. Experiment The X-ray diffraction patterns of ITO films deposited at different oxygen to argon gas ratio is shown in Fig. 1. The ITO films deposited without oxygen showed complete polycrystalline nature with strong preferred orientations of (222) and (440) plane direction. The incorpora- tion of oxygen suppresses the crystalliza- tion of ITO films, as shown in Fig 1. As the oxygen gas ratio was increased, the preferential orientation of (222) plane di- rection became more dominant. By add- ing oxygen gas during sputtering process, the ITO film surface may absorbed more oxygen atoms. This absorbed process leads to metal adatoms trapping by the oxygen atoms, causing reduction of adatom mobility and also diffusion length. As a result, the nucleation step is more kinetically limited, and the crystallization is restrained [1]. Fig. 2. depicts the performance of an a-Si/c-Si heterojunction solar cell as a function of O 2 /(Ar + O 2 ) gas ratio. This figure indicates an apparent enhance- ment in the cell parameters, such as open-circuit voltage (V oc ), short-circuits current density (J sc ), and fill factor (FF). Therefore, the conversion efficiency of the solar cells is improved from 14.39 % to 17.80% with increasing O 2 /(Ar + O 2 ) gas ratio from 0 to 1.2%. Further increase in oxygen gas ratio resulted in deteriorat- ing slightly the cell efficiency. In order to