Post-annealing Effect on Optical and Electronic Properties of Thermally Evaporated MoOX Thin Films as Hole-Selective Contacts for p-Si Solar Cells

Exceptional Hole-Selective Properties of Ta2O5 Films via Sn4+ Doping for High Performance Silicon Heterojunction Solar Cells

Carrier-selective passivating contacts using transition metal oxides (TMOs) have attracted great attention for crystalline silicon (c-Si) heterojunction solar cells recently. Among them, tantalum oxide (Ta2O5) exhibits outstanding advantages, such as a wide bandgap, good surface passivation, and a small conduction band offset with c-Si, which is typically used as an electron-selective contact layer. Interestingly, it is first demonstrated that solution-processed Ta2O5 films exhibit a high hole selectivity, which blocks electrons and promotes hole transport simultaneously. Through the ozone pre-treatment of Ta2O5/p-Si interface and optimization of the film thickness (≈9 nm), the interfacial recombination is suppressed and the contact resistivity is reduced from 178.0 to 29.3 mΩ cm2. Moreover, the Sn4+ doping increases both the work function and oxygen vacancies of the film, contributing to the improved hole-selective contact performance. As a result, the photoelectric conversion efficiencies of Ta2O5/p-Si heterojunction solar cells are significantly improved from 14.84% to 18.47%, with a high thermal stability up to 300 °C. The work has provided a feasible strategy to explore new features of TMOs for carrier-selective contact applications, that is, bipolar carrier transport properties.

Greatly enhanced hole collection of MoO x with top sub-10 nm thick silver films for gridless and flexible crystalline silicon heterojunction solar cells

Greatly enhanced hole collection of MoO_x is demonstrated experimentally with a top sub-10 nm thick Ag film, allowing for an efficient dopant-free contacted crystalline silicon (c-Si) heterojunction solar cell without a front grid electrode. With the removal of shadows induced by the front grid electrode, the gridless solar cell with the MoO_x/Ag hole-selective contact (HSC) shows an increment of ∼8% in its power conversion efficiency (PCE) due to the greatly improved short-circuit current density (J_sc) as well as the almost undiminished fill factor (FF) and open-circuit voltage (V_oc), while the gridless solar cells with the conventional MoO_x/ITO and pure MoO_x HSCs exhibit ∼20% and ∼43% degradations in PCE due to the overwhelming decrease in their FF and J_sc, respectively. Through systematic characterizations and analyses, it is found that the ultrathin Ag film (more conductive than ITO) provides an additional channel for photogenerated holes to transport on MoO_x, contributing to the great enhancement in the hole collection and the great suppression of the shunt loss in the gridless solar cells. A 50 μm thick gridless c-Si heterojunction solar cell with the MoO_x/Ag HSC is 75% thinner but is 86% efficient compared to its 200 μm thick counterpart (while the 50 μm thick gridless solar cell with the MoO_x/ITO HSC is much less efficient). It is over 82% efficient after being bent to a curvature radius as small as 4 mm, also showing superior mechanical flexibility to its counterpart with the MoO_x/ITO HSC. Our MoO_x/Ag double-layer HSC can be easily fabricated through thermal evaporation without breaking the vacuum, saving both the time and cost of the fabrication of the whole device. Therefore, this work provides a guide for the design of efficient HSCs for high-efficiency, low-cost, and flexible solar cells.

Greatly enhanced hole collection of MoO_x is demonstrated experimentally with a top sub-10 nm thick Ag film. With this hole-selective contact, a 50 μm thick gridless c-Si solar cell, showing excellent flexibility, is 75% thinner but 86% efficient.