[13] K. A. Nagamatsu, S. Avasthi, G. Sahasrabudhe, G. Man, J. Jhaveri, A. H. Berg, J. Schwartz, A. Kahn, S. Wagner, and J. C. Sturm, âTitanium dioxide/silicon ...
Survey of Dopant-Free Carrier-Selective Contacts for Silicon Solar Cells James Bullock1,2, Yimao Wan2, Mark Hettick1, Jonas Geissbühler3, Alison J. Ong1, Daisuke Kiriya1, Di Yan2, Thomas Allen2, Jun Peng2, Xinyu, Zhang2, Carolin M. Sutter-Fella1, Stefaan De Wolf3, Christophe Ballif3, Andrés Cuevas2 and Ali Javey1 1
Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA.2 Research School of Engineering, The Australian National University (ANU), Canberra, ACT 0200, Australia.3 École Polytechnique Fédérale de Lausanne (EPFL), Photovoltaics and Thin Film Electronic Laboratory (PVLab), Maladière 71b, CH-200 Neuchatel, Switzerland. Abstract — In recent years a significant amount of effort has been devoted towards the development of dopant-free carrier selective contacts for crystalline silicon (c-Si) solar cells. This short manuscript surveys a range of materials which have the potential to induce carrier-selectivity when applied to c-Si, including metals, metal oxides, alkali / alkaline earth metal salts, and organic conductors. Simple Ohmic test structures are used to assess the selectivity of these materials, that is, hole contacts are tested on ptype c-Si and electron contacts on n-type c-Si. Among these alternatives, a number of systems with exceptional potential are identified.
I. INTRODUCTION To function effectively, all crystalline silicon (c-Si) solar cells must separate the photo-excited electrons and holes and collect them at opposite contacts. The majority of c-Si solar cells attain this by diffusing dopants of opposite polarity into the two surfaces of the wafer which create regions of very high conductivity for only one of the two carriers. Even with its wellproven efficacy, this approach can limit device performance via optoelectronic losses and technological issues [1]. These issues have motivated research into alternative approaches whereby ‘carrier-selective’ materials or stacks of materials are deposited on the surface of a c-Si wafer (also known as ‘heterocontacts’, ‘passivating contacts’ or ‘carrier-selective contacts’). As it currently stands, the most successful heterocontacts are achieved via deposited doped-silicon films, either amorphous or polycrystalline [2]–[6]. Such contacts are now utilized in high performance cells with power conversion efficiencies at and above 25% [6]–[8]. However such films still introduce optical losses and complex processing [9], [10]. An alternative to doped-silicon, discussed here, is to use other materials, for example metal oxides [11]–[19], alkali and alkaline earth metal salts [1], [20]–[22], organic materials [23], [24] and metals [25] which do not incur the same limitations and practical difficulties. Many of these contact systems have already been successfully implemented on other (predominately organic) semiconductor absorbers. The use of dopant-free, carrierselective materials, rather than doped-silicon layers, opens up a wider material space, commonly allowing simpler deposition
methods to be used in the fabrication of c-Si solar cells – those presented here are only a rather small subset of obvious candidates. The promise of the dopant-fee approach has recently been demonstrated in a solar cell featuring a set of dopant free asymmetric heterocontacts (DASH) achieving an efficiency of 19.4% [1] – signifying the beginning of its competitiveness with conventional approaches. II. EXPERIMENTAL Candidate selective materials were deposited using a wide range of low temperature techniques, as detailed in Table 1. To analyze the suitability of these materials, Ohmic contact test structures were fabricated and the contact resistivity was extracted via either the Cox and Strack or the transfer length method. Electron and hole contact test structures were fabricated using lightly doped (≤ 2×1016 cm-3) n-type and p-type c-Si wafers, respectively. These simple Ohmic test structures are used as a proxy to estimate the relative performance of the different materials. Metal over layers > 250 nm are deposited via thermal evaporation. The IV behavior of the different contact structures is measured at room temperature. The extracted contact resistivity comprises the interfacial resistances as well as the bulk resistance of the selective material. The approximate thicknesses of the films are estimated from either crystal monitors in the deposition chambers or ellipsometry measurements taken of the deposited films on polished silicon wafers.
Fig. 1. Ohmic electron (left) and hole (right) test structures used to assess potential selective materials in this study.
TABLE I SUMMARY OF TRIALED DOPANT-FREE SELECTIVE HETEROCONTACTS Contact Details Wafer doping IV behavior Extracted ρc (cm-3) (mΩcm2) Electron contacts c-Si(n)/TiOx/Al c-Si(n)/TiOx/LiFx/Al c-Si(n)/LiFx/Al c-Si(n)/CsFx/Al c-Si(n)/KFx/Al c-Si(n)/C60/Al c-Si(n)/CsOx/Al c-Si(n)/CsOx/Ag c-Si(n)/MgFx/Al c-Si(n)/Ca/Al Hole contacts c-Si(p)/MoOx/Pd/Ag c-Si(p)/WOx/Pd/Al c-Si(p)/PEDOT:PSS/Pd/Ag c-Si(p)/CuPc /Au/Ag c-Si(p)/CuSCN/Al c-Si(p)/CuOx:N/Pd/Ag
Deposition details Extraction method
Refs.
5×1015 5×1015 5×1015 5×1015 5×1015 5×1015 5×1015 5×1015 1×1016 5×1015
Ohmic Ohmic Ohmic Ohmic Ohmic Ohmic Ohmic Ohmic Ohmic Ohmic
30 490 1 1 7.6 200 1.8 7.5 35 2
C&S C&S C&S C&S C&S C&S C&S TLM C&S C&S
Thermal ALD, 230oC, titanium tetraisopropoxide / water, 2.8nm Thermal ALD, 230oC, titanium tetraisopropoxide / water, 6 nm TE, LiF powder source, ~1Å/sec, ~10-6 mbar, 1.5 nm TE, CsF powder source, ~1Å/sec, ~10-6 mbar, 1.5 nm TE, KF powder source, ~1Å/sec, ~10-6 mbar, 1.5 nm TE, C60 powder source, ~1Å/sec, ~10-5 mbar, 5 nm TE, CsCO3 powder source, ~1Å/sec, ~10-5 mbar, 1 nm TE, CsCO3 powder source, ~1Å/sec, ~10-5 mbar, 1 nm TE, MgF2 powder source, ~1Å/sec, ~10-6 mbar, 1 nm TE, Ca pellet source, ~1Å/sec,
