A Low Resistance Calcium/Reduced Titania Passivated Contact for High Efficiency Crystalline Silicon Solar Cells

Advances in the efficiency of crystalline silicon (c-Si) photovoltaic (PV) devices above the long-held record efficiency value of 25% have all come from solar cell architectures with passivated contacts fabricated on n-type silicon.[1] The most successful devices to date have a silicon heterojunction (SHJ) cell structure, featuring a thin intrinsic amorphous silicon (a-Si) film that passivates c-Si surface defects, effectively separating the solar cell absorber (c-Si) from the remaining contact materials (doped a-Si, transparent conductive oxides, and metals). This separation of absorber and contact materials avoids extraneous Auger recombination and free carrier absorption losses associated with heavily doped regions, a detrimental feature of diffused junction solar cells. However, the amorphous silicon heterocontact structure necessitates the implementation of large contact fractions due to a relatively high contact resistivity. The trade-off between contact resistivity (ρc), contact recombination (J0c), and contact fraction (fc) is optimized for these devices by minimizing J0c and combating the increase in ρc by applying the contacts over a large area. However, optical losses arising from parasitic absorption in the doped and intrinsic a-Si layers, as well as the transparent conductive oxide, are a limiting factor in this cell design.[2] Interdigitated back contact structures minimize these losses and have achieved the highest performance for c-Si solar cells,[3] but they still pose challenges for mass production. Another strategy to achieve high efficiencies is to develop transparent, dopant-free, passivating heterocontacts that minimize ρc, thereby allowing the application of the contact structure to devices with low contact fractions, as in the partial rear contact (PRC) architectures commonly known as PERC (passivated emitter and rear cell) and PERL (passivated emitter with rear locally diffused) cells.[4] By minimizing ρc the constraints on recombination at the contact can be relaxed and the contact can be applied to small areas (fc < 10%), leaving the remaining surfaces to be passivated with materials that have been utilized by the c-Si PV industry for decades, like silicon nitride (SiNx) and aluminum oxide (Al2O3). These materials are known to effectively eliminate Shockley–Read–Hall, or defect-assisted, recombination at the silicon surface. Optically this approach Recent advances in the efficiency of crystalline silicon (c-Si) solar cells have come through the implementation of passivated contacts that simultaneously reduce recombination and resistive losses within the contact structure. In this contribution, low resistivity passivated contacts are demonstrated based on reduced titania (TiOx) contacted with the low work function metal, calcium (Ca). By using Ca as the overlying metal in the contact structure we are able to achieve a reduction in the contact resistivity of TiOx passivated contacts of up to two orders of magnitude compared to previously reported data on Al/ TiOx contacts, allowing for the application of the Ca/TiOx contact to n-type c-Si solar cells with partial rear contacts. Implementing this contact structure on the cell level results in a power conversion efficiency of 21.8% where the Ca/TiOx contact comprises only ≈6% of the rear surface of the solar cell, an increase of 1.5% absolute compared to a similar device fabricated without the TiOx interlayer.