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Title: Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance

Abstract

Amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers—inserted between substrate and (front or rear) contacts—since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. As a consequence, device implementation of such films as window layers—without degraded carrier collection—demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.

Authors:
; ;  [1]; ;  [2]; ;  [3]
  1. Photovoltaics and Thin-Film Electronics Laboratory, Institute of Microengineering (IMT), Ecole Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71b, CH-2002 Neuchâtel (Switzerland)
  2. Department of Physics, Yıldız Technical University, Davutpasa Campus, TR-34210 Esenler, Istanbul (Turkey)
  3. CSEM, PV-Center, Jaquet-Droz 1, CH-2002 Neuchâtel (Switzerland)
Publication Date:
OSTI Identifier:
22597738
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 120; Journal Issue: 5; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; ASYMMETRY; COMPARATIVE EVALUATIONS; ELECTRIC POTENTIAL; ELECTRONS; FIELD EMISSION; FILL FACTORS; FILMS; HETEROJUNCTIONS; HOLES; IMPLEMENTATION; INTERFACES; LAYERS; PASSIVATION; PERFORMANCE; SILICON OXIDES; SILICON SOLAR CELLS; SUBSTRATES; SURFACES; TOPOLOGY; DARK CURRENT

Citation Formats

Seif, Johannes Peter, E-mail: johannes.seif@alumni.epfl.ch, Ballif, Christophe, De Wolf, Stefaan, Menda, Deneb, Özdemir, Orhan, Descoeudres, Antoine, and Barraud, Loris. Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance. United States: N. p., 2016. Web. doi:10.1063/1.4959988.
Seif, Johannes Peter, E-mail: johannes.seif@alumni.epfl.ch, Ballif, Christophe, De Wolf, Stefaan, Menda, Deneb, Özdemir, Orhan, Descoeudres, Antoine, & Barraud, Loris. Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance. United States. doi:10.1063/1.4959988.
Seif, Johannes Peter, E-mail: johannes.seif@alumni.epfl.ch, Ballif, Christophe, De Wolf, Stefaan, Menda, Deneb, Özdemir, Orhan, Descoeudres, Antoine, and Barraud, Loris. Sun . "Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance". United States. doi:10.1063/1.4959988.
@article{osti_22597738,
title = {Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance},
author = {Seif, Johannes Peter, E-mail: johannes.seif@alumni.epfl.ch and Ballif, Christophe and De Wolf, Stefaan and Menda, Deneb and Özdemir, Orhan and Descoeudres, Antoine and Barraud, Loris},
abstractNote = {Amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers—inserted between substrate and (front or rear) contacts—since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. As a consequence, device implementation of such films as window layers—without degraded carrier collection—demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.},
doi = {10.1063/1.4959988},
journal = {Journal of Applied Physics},
number = 5,
volume = 120,
place = {United States},
year = {Sun Aug 07 00:00:00 EDT 2016},
month = {Sun Aug 07 00:00:00 EDT 2016}
}
  • Here, amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers -- inserted between substrate and (front or rear) contacts -- since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. Asmore » a consequence, device implementation of such films as window layers -- without degraded carrier collection -- demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.« less
  • Cited by 5
  • Amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers—inserted between substrate and (front or rear) contacts—since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap siliconoxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. As a consequence, device implementation of such films as window layers—without degraded carrier collection—demands electron collection at the front andmore » hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.« less
  • ZnSnN 2 (ZTN) has been proposed as a new earth abundant absorber material for PV applications. While carrier concentration has been reduced to values suitable for device implementation, other properties such as ionization potential, electron affinity and work function are not known. Here, we experimentally determine the value of ionization potential (5.6 eV), electron affinity (4.1 eV) and work function (4.4 eV) for ZTN thin film samples with Zn cation composition Zn/(Zn+Sn) = 0.56 and carrier concentration n = 2x10 19cm -3. Using both experimental and theoretical results, we build a model to simulate the device performance of a ZTN/Mg:CuCrOmore » 2 solar cell, showing a potential efficiency of 23% in the limit of no defects present. We also investigate the role of band tails and recombination centers on the cell performance. In particular device simulations show that band tails are highly detrimental to the cell efficiency, and recombination centers are a major limitation if present in concentration comparable to the net carrier density. The effect of the position of the band edges of the p-type junction partner was assessed too. Through this study, we determine the major bottlenecks for the development of ZTN-based solar cell and identify avenues to mitigate them.« less
  • We have studied by Raman spectroscopy and electro-optical characterization the properties of thin boron doped microcrystalline silicon layers deposited by plasma enhanced chemical vapor deposition (PECVD) on crystalline silicon wafers and on amorphous silicon buffer layers. Thin 20{endash}30 nm p{sup +} {mu}c-Si:H layers with a considerably large crystalline volume fraction ({approximately}22{percent}) and good window properties were deposited on crystalline silicon under moderate PECVD conditions. The performance of heterojunction solar cells incorporating such window layers were critically dependent on the interface quality and the type of buffer layer used. A large improvement of open circuit voltage is observed in these solarmore » cells when a thin 2{endash}3 nm wide band-gap buffer layer of intrinsic a-Si:H deposited at low temperature ({approximately}100{degree}C) is inserted between the microcrystalline and crystalline silicon [complete solar cell configuration: Al/(n)c-Si/buffer/p{sup +}{mu}c-Si:H/ITO/Ag]. Detailed modeling studies showed that the wide band-gap a-Si:H buffer layer is able to prevent electron backdiffusion into the p{sup +}{mu}c-Si:H layer due to the discontinuity in the conduction band at the amorphous-crystalline silicon interface, thereby reducing the high recombination losses in the microcrystalline layer. At the same time, the discontinuity in the valence band is not limiting the hole exit to the front contact and does not deteriorate the solar cell performance. The defect density inside the crystalline silicon close to the amorphous-crystalline interface has a strong effect on the operation of the cell. An extra atomic hydrogen passivation treatment prior to buffer layer deposition, in order to reduce the number of these defects, did further enhance the values of V{sub oc} and fill factor, resulting in an efficiency of 12.2{percent} for a cell without a back surface field and texturization. {copyright} {ital 1997 American Institute of Physics.}« less