This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL OF PHOTOVOLTAICS 1 Investigation of Electrical Parameters of Amorphous–Crystalline Silicon Heterojunction Solar Cell: Correlations Between Carrier Dynamics and S-Shape of Current Density–Voltage Curve Sapna Mudgal , Sonpal Singh, and Vamsi Krishna Komarala Abstract—We have analyzed a-Si:H(p)/a-Si:H(i)/c-Si(n) hetero- junction silicon solar cell having the S-shaped current density– voltage characteristics with a low fill factor and open-circuit volt- age, using quantum efficiency (QE) characterization technique under forward/reverse voltage and different light (blue, infrared, and white) bias conditions. The curvature of S-shape is sensitive to excitation light intensities because of modification in junction barrier potential (variation in quasi-Fermi levels splitting). With forward-bias voltage alone near/above S-shaped region, cell’s QE is uniformly reduced because of reduction in junction field and dom- inance of barrier for collection of holes. However, with blue and white light at bias voltages close to S-shaped characteristics, a uni- form improvement of QE in broad wavelength region is observed because of defects saturation at the junction interface and photo- conductivity in the a-Si layers. With white light and voltage bias, cell’s QE is anomalously improved and it has even crossed the QE response at no voltage/light bias conditions in the blue region be- cause of defects saturation in a-Si:H layers, whereas under infrared light and voltage bias conditions defect saturation is not displayed in the QE because of carrier generation in a deeper region of the cell after crossing unabsorbed photons front region. Index Terms—Band offset, silicon heterojunction (SHJ) solar cells, S-shape, quantum efficiency (QE), voltage and light bias. I. INTRODUCTION S ILICON heterojunction (SHJ) opens the possibility of us- ing various semiconducting materials as charge selective contacts for solar cells application [1], [2]. With the thin hydro- genated amorphous silicon (a-Si:H) on the crystalline silicon (c-Si) wafer, one can have minimal discontinuities in the bulk properties than the totally different materials as charge selective contacts (emitters), which already demonstrated record conver- sion efficiency of 26.3% on large area by Kaneka Corporation Manuscript received February 2, 2018; accepted March 28, 2018. This work was supported in part by the Department of Science and Technology, India, under Clean Energy Research Initiative Grant RP03240 for establishing sili- con heterojunction solar cell test facilities and in part by the University Grant Commission (UGC), India. (Corresponding author: Vamsi Krishna Komarala.) The authors are with the Center for Energy Studies, Indian Institute of Tech- nology Delhi, New Delhi 110016, India (e-mail:, sapnamudgal89@gmail.com; spsinghin@gmail.com; vamsi@ces.iitd.ac.in). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2018.2821839 [3]. The SHJ cell’s (Ag/TCO/a-Si:H(p+)/a-Si:H(i)/c-Si(n)/a- Si:H(i)/a-Si:H(n+)/TCO/Ag) microscopic current transport is under intense investigation, [4]–[7] apart from the passivation studies of the c-Si surface for the better device performance [1], [8], [9]. Because of the complexity involved in device structure with different thin layers and their heterointerfaces, one can still consider for using various characterization techniques to understand the current transport mechanism better. The SHJ de- vices usually suffer from low voltage and poor fill factors (FFs) with S-shape in light current density–voltage (J–V) characteris- tics because of charge carrier transport issues, such as barrier for charge collection, tunneling through band spike/interface states, multistep tunneling in a-Si:H layers, and recombination via a- Si:H gap states/interface states [4], instead of simple charge carrier diffusion (in neutral bulk) and recombination (in space charge region) in case of homojunction-based silicon solar cells. From the literature, it can be foretold that the S-shape in the SHJ cells may arise by two types of hindrances in charge carrier collection: 1) Schottky contact (injection barrier) at the transpar- ent conducting oxide (TCO)/a-Si:H interface either in front or back side of the cell because of reverse junction formation [10], and 2) charge carrier accumulation (extraction barrier) at the a-Si:H/c-Si interface because of band offsets and discontinuous bulk/electronic properties [4]. In reality, the band offset at the p-n junction of the SHJ can behave like both as an injection and extraction barrier simultaneously depending on how it blocks the flow of charge carriers from either side of a junction. The J–V measurements alone are not enough to distinguish S-shape from either because of the Schottky contact at TCO/a-Si:H in- terface or because of band offsets at the a-Si:H/c-Si interface of the SHJ cell [4], [5], [11]. Recently, Das et al. [4] adopted the Suns-V oc and quantum efficiency (QE) measurements, and demonstrated the difference of QE behavior from the Schottky barrier and the band offset present within the SHJ cells. Their study was focused only on the QE change in response because of energy barriers existing at the interfaces for the charge carriers, but the responses from the constituent layers of the device are not included. QE of a solar cell in short-circuit conditions is frequently used to understand charge carrier collection efficiency at dif- ferent wavelengths of the incident light spectrum [12]. One 2156-3381 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.