On the possible role of the interfacial inversion layer in the improvement of the performance of hydrogenated amorphous silicon/crystalline silicon heterojunction solar cells [HIT] Moustafa Ghannam n , Ghadah Shehadah, Yaser Abdulraheem, Jef Poortmans 1 Kuwait University, EE Department, College of Engineering & Petroleum, P.O. Box 5969, 13060 Safat, Kuwait article info Article history: Received 31 March 2014 Received in revised form 10 July 2014 Accepted 9 September 2014 Keywords: Heterojunction silicon solar cells HIT solar cell Inversion layer solar cell AFORS-HET Band-gap narrowing in inversion layer abstract The energy band diagram of the hetero-interface between p-type hydrogenated amorphous silicon (a-Si: H(p þ )) and n-type crystalline silicon (c-Si(n)) obtained using AFORS-HET one dimensional device simulations reveals that a p þ inversion layer is induced at the hetero-interface, in the c-Si side, with or without the presence of a buffer intrinsic a-Si:H(i) spacer. Such an inversion layer controls the performance of the a-Si:H(p þ )/a-Si:H(i)/c-Si(n) HIT cell, because it pushes the p/n junction 20 nm in the c-Si and acts as the cell effective emitter. The formation of the inversion layer is controlled by the valence band offset and by the positioning of the Fermi energy in the energy band-gap. The latter is influenced by the active doping level in the doped a-Si:H(p þ ) and by the dangling bond defect density in the a-Si:H bulk and at the a-Si:H/c-Si interface. By inserting an intrinsic a-Si:H spacer, the defect density at the interface is strongly reduced, which not only decreases the interface recombination, but also ensures the proper formation of the inversion layer. The study also suggests that significantly reduced band-gap narrowing in the inversion layer emitter contributes to the higher open circuit voltage achieved in the HIT cell compared to c-Si cell with excellent front surface passivation. & 2014 Elsevier B.V. All rights reserved. 1. Introduction One sided current injection through the hydrogenated amor- phous silicon/crystalline silicon a-Si:H/c-Si interface has been con- firmed by the improved emitter efficiency in npn silicon bipolar transistors with a wide band-gap a-Si:H(n þ ) emitter deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) on p-type c-Si base [1], as well as by the high open circuit voltage reported for the heterojunction silicon solar cell or HIT cell [2]. Recently a record efficiency of 24.7% and open circuit voltage of 745–750 mV have been announced for the latest HIT cell fabricated by Sanyo [3]. All relevant reports on high performance a-Si:H/c-Si HIT cells attribute its high performance solely to the excellent front surface/contact passivation of the a-Si:H/c-Si interface, especially with the presence of an ultrathin intrinsic amorphous silicon a-Si:H(i) buffer layer (spacer) [2,4,5]. Although excellent surface passivation of p-type c-Si surface by ultrathin Al 2 O 3 deposited by ALD (atomic layer deposition) has been demonstrated [6], the open circuit voltage of advanced c-Si solar cells is still significantly smaller than that achieved in the HIT cell. This seems to indicate that another factor contributes to the emitter current reduction in the HIT cell. Such a factor has not been investigated nor evoked so far. Band line-up between the large energy band-gap a-Si:H (1.6– 1.8 eV) and the smaller energy band-gap c-Si (1.1 eV) results in conduction and valence band discontinuities (ΔE C and ΔE V offsets) at the interface. These discontinuities lead to abrupt changes in the carrier concentrations at the interface, which may cause inversion of the interface in the c-Si side. The presence of such an inversion layer has been confirmed experimentally by means of conductance and capacitance measurements and conductive probe atomic force microscopy (AFM) [7–14]. This inversion layer was even tested as a low resistive path for lateral transport of photogenerated carriers to replace the conductive top ITO layer in the HIT cell, but was found to be too resistive [7]. To our knowledge, apart from our recent suggestion [15], there have been no reports to date investigating a possible role of the interfacial inversion layer in controlling the operation of the HIT cell or in improving its open circuit voltage. Such a role is investigated in the present work using AFORS-HET one dimensional device simulations Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.09.008 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail addresses: mghannam@aucegypt.edu, moustafa.ghannam@ku.edu.kw (M. Ghannam). 1 IMEC, Kapeldreef 75, 3001, Leuven, Belgium. Solar Energy Materials & Solar Cells 132 (2015) 320–328