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Efficient silicon solar cells with dopant-free asymmetric heterocontacts

Nature Energy volume 1, Article number: 15031 (2016) Cite this article

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Abstract

A salient characteristic of solar cells is their ability to subject photo-generated electrons and holes to pathways of asymmetrical conductivity—‘assisting’ them towards their respective contacts. All commercially available crystalline silicon (c-Si) solar cells achieve this by making use of doping in either near-surface regions or overlying silicon-based films. Despite being commonplace, this approach is hindered by several optoelectronic losses and technological limitations specific to doped silicon. A progressive approach to circumvent these issues involves the replacement of doped-silicon contacts with alternative materials which can also form ‘carrier-selective’ interfaces on c-Si. Here we successfully develop and implement dopant-free electron and hole carrier-selective heterocontacts using alkali metal fluorides and metal oxides, respectively, in combination with passivating intrinsic amorphous silicon interlayers, resulting in power conversion efficiencies approaching 20%. Furthermore, the simplified architectures inherent to this approach allow cell fabrication in only seven low-temperature (≤200 ∘C), lithography-free steps. This is a marked improvement on conventional doped-silicon high-efficiency processes, and highlights potential improvements on both sides of the cost-to-performance ratio for c-Si photovoltaics.

The majority of c-Si solar cells within both industry and research laboratories make use of doped homojunctions to separate photo-generated electrons and holes. Researchers tasked with optimizing these doped homojunctions are faced with a myriad of interrelated optical, carrier transport and recombination based losses, most notably parasitic absorption1, Auger recombination and other heavy doping effects2,3 (for details see Supplementary Table 1). In addition, technological complexities associated with doping, such as high processing temperatures (>800 ∘C, with a concomitant necessity for cleanliness), small contact fractions (<0.5%), dopant glass removal and junction isolation must be considered4,5. These issues can be partially alleviated by switching to architectures which instead use a set of asymmetric carrier-selective heterocontacts—a strategy that has long been considered a crucial technological step to attaining the intrinsic efficiency limit of c-Si (ref. 6). Carrier-selective heterocontacts provide a negligible resistance to the collected carrier (synonymous with a low contact resistivity) whilst simultaneously ‘blocking’ the other carrier (equivalent to low contact recombination). This can be achieved through a number of possible mechanisms at the heterocontact—for example using surface passivating layers or stacks which provide conductivity asymmetry through band offsets, tunnelling probabilities or band bending when applied to c-Si (ref. 7).

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Figure 1: Conceptual structure of the DASH solar cell.
Figure 2: Optoelectronic properties of carrier-selective layers.
Figure 3: Contact-level analysis of electron-selective contacts.
Figure 4: DASH cell level results.

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Acknowledgements

We would like to thank P. Frischmann for his assistance with I–V measurements and A. Fell for his suggestions regarding the simulations. Device design, fabrication and characterization were funded by the Bay Area Photovoltaics Consortium (BAPVC). Materials characterization was supported by the Electronic Materials Programs, funded by the Director, Office of Science, Office of Basic Energy Sciences, Material Sciences and Engineering Division of the US Department of Energy under Contract No. DE-AC02- 05CH11231. XPS characterization was performed at the Joint Center for Artificial Photosynthesis, supported through the Office of Science of the US Department of Energy under Award Number DE-SC0004993. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy (Contract No. DE-AC02-05CH11231). Work at EPFL was supported by the Office fedéral de l’ énergie (OFEN). Work at the ANU was supported by the Australian Renewable Energy Agency (ARENA). The authors would like to thank the CSEM PV-center for wafer preparation and device metallization.

Author information

Authors and Affiliations

  1. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA

    James Bullock, Mark Hettick, Alison J. Ong, Carolin M. Sutter-Fella, Hiroki Ota, Ethan W. Schaler & Ali Javey

  2. Berkeley Sensor and Actuator Center, University of California, Berkeley, California 94720, USA

    James Bullock, Mark Hettick, Alison J. Ong, Carolin M. Sutter-Fella, Hiroki Ota & Ali Javey

  3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    James Bullock, Mark Hettick, Alison J. Ong, Carolin M. Sutter-Fella, Hiroki Ota & Ali Javey

  4. Research School of Engineering, The Australian National University (ANU), Canberra, Australian Capital Territory 0200, Australia

    James Bullock, Thomas Allen & Andrés Cuevas

  5. École Polytechnique Fédérale de Lausanne (EPFL), Institute of Micro Engineering (IMT), Photovoltaics and Thin Film Electronic Laboratory (PVLab), Maladière 71b, CH-200 Neuchatel, Switzerland

    Jonas Geissbühler, Stefaan De Wolf & Christophe Ballif

  6. The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    Teresa Chen

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Contributions

J.B. and A.J. conceived the idea. J.B. and J.G. carried out the device fabrication, electrical characterization and analysis. A.J.O., T.A. and T.C. assisted with device fabrication. M.H. and C.M.S.-F., assisted with materials characterization. H.O. and E.W.S. assisted with mask fabrication. A.C., S.D.W. and C.B. discussed the results. J.B. wrote the paper and all other authors provided feedback.

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Correspondence to Ali Javey.

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The authors declare no competing financial interests.

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Supplementary Information

Supplementary Notes 1-4, Supplementary Tables 1-3, Supplementary Figures 1-4, Supplementary References. (PDF 911 kb)

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Bullock, J., Hettick, M., Geissbühler, J. et al. Efficient silicon solar cells with dopant-free asymmetric heterocontacts. Nat Energy 1, 15031 (2016). https://doi.org/10.1038/nenergy.2015.31

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