All back contact solar cells

The highest silicon wafer-based solar cell power conversion efficiencies reported to date have been achieved with the interdigitated back contact (IBC) architecture. IBC solar cells require interdigitated (or striped) doping on the rear surface and only have contacts on the rear. This doping can be achieved by masked diffusion, masked ion-implantation or laser doping. The solar cells are then metallised by forming metal fingers along each diffused region.

 

IBC_front_back.png

Figure 1: Photograph of the front (left) and back (right) of an industrial IBC solar cell.

 

With the exception of the Tunnel Oxide Passivated Contact (TOPCon) [1] and PERL solar cell [2] architectures, this type of silicon wafer-based solar cell structure is the only architecture to achieve or exceed 25 %. In addition, the IBC architecture (see Figure 1) is the dominant architecture for all silicon wafer-based solar cells with efficiencies over 25 % [3]. The highest efficiency IBC solar cell has been fabricated with a heterojunction structure with a energy conversion efficiency of > 26.63 % [4], whilst the highest known diffused homojunction IBC structures has achieved a conversion efficiency of 24.4 % [5], the polysilicon on oxide (POLO) cell which recently achieved 25.0 % with doped-polysilicon on SiO2 passivated contacts [6] and the most recent SunPower X-series cell, which reached 25.2% [7] . There are several advantages of the IBC architecture over the more commonly fabricated front and rear contact design;

  • the elimination of front grid shading, allowing for potentially higher short circuit currents,
  • the elimination of front surface doping allows for a wider range of front surface texturing and light trapping schemes to be deployed on the front surface [8, 9] and
  • the IBC solar cell is ideal for mechanically stacked tandem cells with higher bandgap technologies such as perovskites.

However, a key drawback of this technology is the significantly more complicated fabrication procedure required to isolate and fabricate the interdigitated carrier selective collector regions—although there have been numerous attempts to simplify this fabrication through selective etching [10] and laser processing [5, 11]. The second issue with this structure is the requirement for high minority carrier lifetime wafer material to ensure that the photogenerated electron-hole pairs can diffuse to the rear contacts, which mainly affects carriers generated near the front side of the cell. A third issue is the complications with rear surface passivation design and optimisation, due to the presence of dual polarities of dopants on the rear side of the solar cell, which is typically passivated by a single dielectric material. This final issue creates a significant challenge when optimising the rear passivation for enhanced carrier selectivity within the device since the optimisation of the effects of interface charge Q on the both the heavily doped p-type and n-type regions is required. The current state-of-the-art rear surface passivation designs used on IBC solar cells to optimise rear surface carrier selectivity are shown schematically in Figure 2 and listed as follows:

  • Using passivating dielectrics with a very low interface defect density such as thermally grown SiO2 [5, 9, 12] or hydrogenated amorphous silicon (a-Si) [13] on the rear side of the device. This has the effect of reducing Sn0 and Sp0, and hence the peak J0s (Figure 1a).
  • Eliminating the un-diffused surface area at the rear of the device, such that the heavily diffused n-type and p-type regions are adjacent or overlapping (Figure 1b).[14].
  • The fabrication of a heterojunction at the rear of the solar cell (Figure 1c) [15-18].

The third approach avoids the formation of a dual polarity diffused surface and moves the hole and electron selective regions out of the base material, and onto the rear surface where they are isolated. Note that the second approach has been adopted in the heterojunction architectures, whereby the p‑a-Si layer is adjacent to the n‑a-Si layer [18]. In this case, due to the low lateral mobility within the thin doped a-Si layers, the contact between the n‑type and p‑type a-Si is not device limiting.

IBC_schematics.png

Figure 2: Schematic of an IBC solar cell showing the various approaches to rear diffused region layout and passivation strategies. 

Whilst IBC solar cells have no optical shading losses, they can have electrical shading losses due to the emitter not covering the entire rear surface.

 

  1. Glunz, S.W., et al., The irresistible charm of a simple current flow pattern—25% with a solar cell featuring a full-area back contact. , in European Photovoltaic Solar Energy Conference. 2015: Hamburg.
  2. Zhao, J., A. Wang, and M.A. Green, 24· 5% Efficiency silicon PERT cells on MCZ substrates and 24· 7% efficiency PERL cells on FZ substrates. Progress in Photovoltaics: Research and Applications, 1999. 7(6): p. 471-474.
  3. Green, M.A., et al., Solar cell efficiency tables (version 49). Progress in Photovoltaics: Research and Applications, 2017. 25(1): p. 3-13.
  4. Yoshikawa, K. Record-breaking Efficiency Back-contact Heterojunction Crystalline Si Solar Cell and Module. in 44th IEEE Photovoltaics Specialists Conference (PVSC44). 2017. Washington DC.
  5. Franklin, E., et al., Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell. Progress in Photovoltaics: Research and Applications, 2016. 24(4): p. 411-427.
  6. Felix, H., et al., Interdigitated back contact solar cells with polycrystalline silicon on oxide passivating contacts for both polarities. Japanese Journal of Applied Physics, 2017. 56(8S2): p. 08MB15.
  7. Smith, D.D., et al. Silicon solar cells with total area efficiency above 25%. in Photovoltaic Specialists Conference (PVSC), 2016 IEEE 43rd. 2016. IEEE.
  8. Rahman, T., et al., Passivation of all-angle black surfaces for silicon solar cells. Solar Energy Materials and Solar Cells, 2017. 160: p. 444-453.
  9. Savin, H., et al., Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency. Nat Nano, 2015. 10(7): p. 624-628.
  10. Rienäcker, M., et al., Recombination Behavior of Photolithography-free Back Junction Back Contact Solar Cells with Carrier-selective Polysilicon on Oxide Junctions for Both Polarities. Energy Procedia, 2016. 92: p. 412-418.
  11. Chan, C.E., B.J. Hallam, and S.R. Wenham, Simplified Interdigitated Back Contact Solar Cells. Energy Procedia, 2012. 27: p. 543-548.
  12. Jeong, S., M.D. McGehee, and Y. Cui, All-back-contact ultra-thin silicon nanocone solar cells with 13.7% power conversion efficiency. Nature communications, 2013. 4: p. 2950.
  13. Lu, M., et al., Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero-interface band structures while maintaining surface passivation. Progress in Photovoltaics: Research and Applications, 2011. 19(3): p. 326-338.
  14. Zhang, X., et al. Development of high efficiency interdigitated back contact silicon solar cells and modules with industrial processing technologies. in 6th World Conference on Photovoltaic Energy Conversion. 2014.
  15. Yoshikawa, K., et al., Exceeding conversion efficiency of 26% by heterojunction interdigitated back contact solar cell with thin film Si technology. Solar Energy Materials and Solar Cells.
  16. Table I : Glass frit content in the four pastes Paste Glass frit by weight percentage Base. 2007(September): p. 20-23.
  17. Masuko, K., et al., Achievement of More Than 25% Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell. IEEE Journal of Photovoltaics, 2014. 4(6): p. 1433-1435.
  18. Nakamura, J., et al., Development of Heterojunction Back Contact Si Solar Cells. IEEE Journal of Photovoltaics, 2014. 4(6): p. 1491-1495.

 

Portions of the text in this section has been adapted and reproduced with the author’s permission from To, A., Improved carrier selectivity of diffused silicon wafer solar cells. 2017, (PhD Thesis)  University of New South Wales, Sydney.