Low-temperature carrier selective contacts for silicon solar cells

Most crystalline silicon-based carrier selective contacts technology include high-temperature diffusion processes in the range of 800-900 ˚C. On the other hand, “low-temperature” carrier selective contacts exploit the relative position of the conduction and valence band compared to silicon for thin films as and these semiconductor materials are typically grown at substrate temperatures below 300 ˚C. Hole selective contacts typically use materials with a very high work function which can induce band bending in the silicon and move the Fermi level close to the valence band facilitating the collection of holes and blocking electrons. Conceivably, low work function materials are suitable for electron selective contacts. Adopted from the research of perovskite and organic solar cells, transition metal oxides like molybdenum oxide (MoOx), tungsten oxide (WOx), vanadium oxide (VOx), nickel oxide (NiOx), copper oxide (CuOx) and chromium oxide (CrOx) are promising candidates for hole selective contacts, while titanium oxide (TiOx), niobium oxide (NbOx), magnesium oxide (MgOx) have shown properties for electron selective contacts. Other low work function materials like magnesium fluoride (MgFx), lithium fluoride (LiFx) are also proven to be good electron selective contacts materials. These materials enable the fabrication of dopant-free silicon solar cells using only low-temperature processes. Table I summarises different carrier selective materials that have been investigated to date.

Table I Summary of dopant-free selective contacts [1-4]

  Contact details Deposition details
  Wafer doping (cm-3) IV behaviour Extracted ρc (mΩcm2)  
Hole contacts  
c-Si(p)/MoOx/Pd/Ag 7×1015 Ohmic 1 TE
c-Si(p)/WOx/Pd/Al 9×1015 Ohmic 4 TE
c-Si(p)/PEDOT:PSS/Pd/Ag 9×1015 Ohmic 50 Spin coated
c-Si(p)/CuSCN/Al 2×1016 Ohmic 58 Spin coated
c-Si(p)/CuOx:N/Pd/Ag 1.5×1016 Ohmic 11 Reactive sputtered
c-Si(p)/CuPc/Au/Ag 2×1016 Ohmic 180 TE
Electron contacts  
c-Si(n)/TiOx/Al 5×1015 Ohmic 30 Thermal ALD
c-Si(n)/TiOx/LiFx/Al 5×1015 Ohmic 490 Thermal ALD
c-Si(n)/LiFx/Al 5×1015 Ohmic 1 TE
c-Si(n)/CsFx/Al 5×1015 Ohmic 1 TE
c-Si(n)/KFx/Al 5×1015 Ohmic 7.6 TE
c-Si(n)/CsOx/Al 5×1015 Ohmic 1.8 TE
c-Si(n)/MgFx/Al 1×1016 Ohmic 35 TE

Note: ALD = Atomic Layer Deposition, TE=Thermal evaporation


[1] – Bullock, J., Y.M. Wan, M. Hettick, J. Geissbuhler, A.J. Ong, D. Kiriya, D. Yan, T. Allen, J. Peng, X.Y. Zhang, C.M. Sutter-Fella, S. De Wolf, C. Ballif, A. Cuevas, A. Javey, and Ieee, Survey of Dopant-Free Carrier-Selective Contacts for Silicon Solar Cells, in 2016 Ieee 43rd Photovoltaic Specialists Conference. 2016. p. 210-213.

[2] – J. Bullock, M. Hettick, J. Geissbühler, A. J. Ong, T. Allen, Carolin M. Sutter-Fella, T. Chen, H. Ota, E. W. Schaler, S. De Wolf, C. Ballif, A. Cuevas, and A. Javey, “ Efficient silicon solar cells with dopant-free asymmetric heterocontacts,” Nat. Energy 1, 15031 (2016).

[3] – J. Bullock, A. Cuevas, T. Allen, and C. Battaglia, “ Molybdenum oxide MoOx: A versatile hole contact for silicon solar cells,” Appl. Phys. Lett. 105(23), 232109 (2014).

[4] – Wan, Y.M., C. Samundsett, D. Yan, T. Allen, J. Peng, J. Cui, X.Y. Zhang, J. Bullock, and A. Cuevas, “A magnesium/amorphous silicon passivating contact for n-type crystalline silicon solar cells,” Applied Physics Letters 109, 5 (2016).