The present study investigates the electrical properties of transition metal oxide (TMO) emitters in dopant‐free n‐Si back contact solar cells by comparing the properties of solar cells employing three TMOs (WOx, MoOx and V2Ox) with varying electrical properties acting as p‐type contacts. The TMOs are found to induce large band bending in n‐Si, which reduces the injection level dependent interfacial recombination speed Seff and contact resistivity ρc. Among the TMO/n‐Si contacts considered, the V2Ox/n‐Si contact achieves the lowest Seff of 138 cm/s and ρc of 0.034 Ω cm2, providing the significant advantages over heavily doped a‐Si:H(p)/n‐Si contacts. The best device performance was achieved by the V2Ox/n‐Si solar cell, demonstrating an efficiency of 16.59% and an open‐circuit voltage of 610 mV relative to solar cells based on MoOx/n‐Si (15.09%, 594 mV) and WOx/n‐Si (12.44%, 539 mV). Furthermore, the present work is the first to employ WOx, V2Ox and Cs2CO3 in back contact solar cells. The fabrication process employed offers great potential for the mass production of back contact solar cells owing to simple, metal mask patterning with high alignment quality and dopant‐free steps conducted at a lower temperature.

A high recombination rate and high thermal budget for aluminum (Al) back surface field are found in the industrial p-type silicon solar cells. Direct metallization on lightly doped p-type silicon, however, exhibits a large Schottky barrier for the holes on the silicon surface because of Fermi-level pinning effect. As a result, low-temperature-deposited, dopant-free chromium trioxide (CrO , x< 3) with high stability and high performance is first applied in a p-type silicon solar cell as a hole-selective contact at the rear surface. By using 4 nm CrO between the p-type silicon and Ag, we achieve a reduction of the contact resistivity for the contact of Ag directly on p-type silicon. For further improvement, we utilize a CrO (2 nm)/Ag (30 nm)/CrO (2 nm) multilayer film on the contact between Ag and p-type crystalline silicon (c-Si) to achieve a lower contact resistance (40 mΩ·cm). The low-resistivity Ohmic contact is attributed to the high work function of the uniform CrO film and the depinning of the Fermi level of the SiO layer at the silicon interface. Implementing the advanced hole-selective contacts with CrO /Ag/CrO on the p-type silicon solar cell results in a power conversion efficiency of 20.3%, which is 0.1% higher than that of the cell utilizing 4 nm CrO . Compared with the commercialized p-type solar cell, the novel CrO-based hole-selective transport material opens up a new possibility for c-Si solar cells using high-efficiency, low-temperature, and dopant-free deposition techniques.

Dopant-free carrier-selective contacts are becoming increasingly attractive for application in silicon solar cells because of the depositions for their fabrication being simpler and occurring at lower temperatures. However, these contacts are limited by poor thermal and environmental stability. In this contribution, the use of the conductive high work function of cuprous iodide, with its characteristic thermal and ambient stability, has enabled a hole-selective contact for p-type silicon solar cells because of the large conduction band offset and small valence offset at the CuI/p-Si interface. The contact resistivity (≈30 mΩ•cm 2 ) of the Ag/CuI (20 nm)/p-Si contact after annealing to 200 °C represents the CuI-based hole-selective contact with low resistance and high thermal stability. Microscopic images and elemental mapping of the Ag/CuI/p-Si contact interface revealed that a nonuniform, continuous CuI layer separates the Ag electrode and p-type Si. Thermal treatment at 200 °C results in the intermixing of the Ag and CuI layers. As a result, the 200 °C thermal process improves the efficiency (20.7%) and stability of the p-Si solar cells featuring partial CuI hole-selective contact. Furthermore, the devices employing the CuI/Ag contact are thermally stable upon annealing to temperatures up to 350 °C. These results not only demonstrate the use of metal iodide instead of metal oxides as hole-selective contacts for efficient silicon solar cells but also have important implications regarding industrial feasibility and longevity for deployment in the field.

Novel multilayer back contact (MLBC) solar cells employing V2Ox (8 nm)/metal/V2Ox (8 nm) multilayers achieve an efficiency of 19.02%.

Dopant-free passivating contacts for photovoltaics have the potential to be deposited at low costs while providing excellent surface passivation and low contact resistance. However, one pressing issue of dopant-free carrier selective contacts is their lower environmental stability compared to conventional silicon-based contacts. In this contribution, we study the degradation in the ZnO/LiF x /Al electron selective nanocontact with experiments and simulations and suggest design modifications for higher performance and stability. Using a thicker metallization and optimal ZnO deposition temperature (130 °C), we improved open-circuit voltage and fill factor, together with improved stability with retention of over 93 and 88% of the initial open-circuit voltage and fill factor after storage in air for 380 h. The champion device has reached an efficiency of 21.3% with V OC of 727 mV, J SC of 37.6 mA/cm2, and FF of 78.0%. Furthermore, the enhanced stability in vacuum, scanning transmission electron microscopy (STEM) images, and the current-exchange simulation suggests that the degradation of the a-Si:H­(i)/ZnO/LiF x /Al contact is caused by a drop of the LiF x /Al work function, due to interaction with air. This work has developed a deep understanding of the degradation mechanism and the methodology of stability analysis for dopant-free silicon solar cells.