skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Sputter-deposited WO x and MoO x for hole selective contacts

Abstract

Here, reactive sputter deposited tungsten and molybdenum oxide (WO x, MoO x) thin films are tested for their ability to form a hole selective contact for Si wafer based solar cells. A characterization approach based on analyzing the band bending induced in the c-Si absorber and the external and implied open-circuit voltage of test structures was used. It is shown that the oxygen partial pressure allows to tailor the selectivity to some extent and that a direct correlation between induced band bending and hole selectivity exists. Although the selectivity of the sputtered films is inferior to the reference films deposited by thermal evaporation, these results demonstrate a good starting point for further optimizations of sputtered WO x and MoO x towards higher work functions to improve the hole selectivity.

Authors:
 [1];  [1];  [2];  [1]
  1. Fraunhofer Institute for Solar Energy Systems ISE, Freiburg (Germany)
  2. Fraunhofer Institute for Solar Energy Systems ISE, Freiburg (Germany); National Renewable Energy Lab. (NREL), Golden, CO (United States)
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
German Federal Ministry for Economic Affairs and Energy; USDOE
OSTI Identifier:
1408996
Report Number(s):
NREL/JA-5J00-70502
Journal ID: ISSN 1876-6102
Grant/Contract Number:
AC36-08GO28308
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Energy Procedia
Additional Journal Information:
Journal Volume: 124; Journal Issue: C; Journal ID: ISSN 1876-6102
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; metal oxide; work function; induced junction; sputtering

Citation Formats

Bivour, Martin, Zähringer, Florian, Ndione, Paul F., and Hermle, Martin. Sputter-deposited WOx and MoOx for hole selective contacts. United States: N. p., 2017. Web. doi:10.1016/j.egypro.2017.09.259.
Bivour, Martin, Zähringer, Florian, Ndione, Paul F., & Hermle, Martin. Sputter-deposited WOx and MoOx for hole selective contacts. United States. doi:10.1016/j.egypro.2017.09.259.
Bivour, Martin, Zähringer, Florian, Ndione, Paul F., and Hermle, Martin. 2017. "Sputter-deposited WOx and MoOx for hole selective contacts". United States. doi:10.1016/j.egypro.2017.09.259. https://www.osti.gov/servlets/purl/1408996.
@article{osti_1408996,
title = {Sputter-deposited WOx and MoOx for hole selective contacts},
author = {Bivour, Martin and Zähringer, Florian and Ndione, Paul F. and Hermle, Martin},
abstractNote = {Here, reactive sputter deposited tungsten and molybdenum oxide (WOx, MoOx) thin films are tested for their ability to form a hole selective contact for Si wafer based solar cells. A characterization approach based on analyzing the band bending induced in the c-Si absorber and the external and implied open-circuit voltage of test structures was used. It is shown that the oxygen partial pressure allows to tailor the selectivity to some extent and that a direct correlation between induced band bending and hole selectivity exists. Although the selectivity of the sputtered films is inferior to the reference films deposited by thermal evaporation, these results demonstrate a good starting point for further optimizations of sputtered WOx and MoOx towards higher work functions to improve the hole selectivity.},
doi = {10.1016/j.egypro.2017.09.259},
journal = {Energy Procedia},
number = C,
volume = 124,
place = {United States},
year = 2017,
month = 9
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Save / Share:
  • Here, the applicability of atomic-layer-deposited titanium oxide (TiO x) thin films for the formation of carrier selective contacts to crystalline silicon (c-Si) is investigated. While relatively good electron selectivity was presented recently by other groups, we show that carrier selectivity can be engineered from electron to hole selective depending on the deposition conditions, post deposition annealing and the contact material covering the TiOx layer. For both the electron and hole contacts, an open-circuit voltage (Voc) of ~ >650 mV is obtained. The fact that the Voc is correlated with the (asymmetric) induced c-Si band bending suggests that carrier selectivity ismore » mainly governed by the effective work function and/or the fixed charge rather than by the asymmetric band offsets at the Si/TiOx interface, which provides important insight into the basic function of metal-oxide-based contact systems.« less
  • Spectrally selective Cr{sub x}O{sub y}/Cr/Cr{sub 2}O{sub 3} multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O{sub 2} and Ar plasmas to deposit a Cr{sub x}O{sub y} (bottom layer)/Cr/Cr{sub 2}O{sub 3} (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899-0.912) and low emittance (0.05-0.06). The x-ray diffraction data in thin film mode showed that the Cr{sub x}O{sub y}/Cr/Cr{sub 2}O{sub 3} coating consists of an amorphous phase; the Raman data of the coating, however,more » showed the presence of A{sub 1g} and E{sub g} modes, characteristic of Cr{sub 2}O{sub 3}. The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr{sup 3+} and no phases of CrO{sub 2} and CrO{sub 3} were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical constants (n and k). The optical constants of the three layers indicate that the bottom two layers are the main absorber layers and the top Cr{sub 2}O{sub 3} layer, which has higher oxygen content, acts as an antireflection coating. In order to study the thermal stability of the Cr{sub x}O{sub y}/Cr/Cr{sub 2}O{sub 3} coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The coating deposited on Cu substrates exhibited high solar selectivity ({alpha}/{epsilon}) of 0.895/0.06 even after heat treatment in air up to 300 deg. C for 2 h. At higher temperatures, the solar selectivity decreased significantly (e.g., {alpha}/{epsilon}=0.855/0.24 at 350 deg. C in air), which is attributed to oxidation of Cr crystallites, increased surface roughness, and formation of CuO. The formation of CuO and the increase in Cr{sup 3+} vacancies due to the outward diffusion of Cr at higher annealing temperatures were confirmed by XPS. In the case of vacuum annealing, for temperatures greater than 500 deg. C the outward diffusion of Cu was the dominating degradation mechanism. The microstructural stability of the absorber coatings heat treated in air (up to 325 deg. C) and vacuum (up to 600 deg. C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests indicated that the absorber coatings on Cu were stable in air up to 250 h at 250 deg. C with a solar selectivity of 0.898/0.11.« less
  • We present a novel method for electrodeposition of ultra-thin films of poly-3-hexylthiophene (e-P3HT) on chemically modified indium-tin oxide (ITO) electrodes, to produce a hole-selective contact with an easily tuned work function (Φ), as demonstrated by a combination of spectroelectrochemistry and ultraviolet photoemission spectroscopy (UPS). Selective contacts for optimized charge injection have become essential components for both thin film organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Electrochemically doped e-P3HT thin films, using counter ions such as PF 6 - do not suffer from stability issues associated with more “acidic” polymer layers (e.g. PEDOT:PSS). By controlling the oxidation state ofmore » the e-P3HT film via electrochemical doping we control the charge density within the film, resulting in an increase in work function with an increase in degree of oxidation. The method of electrochemical formation and doping of the e-P3HT film, using either constant potential step (CA) versus pulsed-potential step (PPS) protocols, has a significant secondary impact on the work function, as a result of the interface dipole effects from entrapment of these counter ions in the near-surface region of the polymer film. These results have significance for the performance of both OLEDs and OPVs built on these doped e-P3HT layers.« less
  • Sputter-deposited tantalum (Ta) and reactively sputter-deposited Ta-nitride films were studied with respect to the passivation capability against copper (Cu) oxidation in thermal O{sub 2} ambient. A 200 {angstrom} Ta or Ta-nitride film was sputter-deposited on a 2,000 {angstrom} Cu film using a Ta target in an Ar/N{sub 2} gas mixture. With Ta passivation, Cu was not oxidized at temperatures up to 400 C, which can be further improved by using passivation of an amorphous Ta-nitride film deposited in an appropriate condition. The absence of long-range defects in the Ta-nitride film was presumably responsible for this improvement. However, sputtering-induced surface damagemore » by excess N{sub 2} in the sputter gas mixture may reduce the passivation capability of Ta-nitride films. When the passivated Cu was oxidized, the Cu oxides always resided in the top surface region. That is, in the oxidation process, Cu diffused through the defects of the passivation layers to the outer surface.« less