Figure 2. a) Accumulated number of related articles as a function of time for the search strings given in the legend taken from ISI Web of Science. Proportion of all-inorganic perovskite solar cells are also shown. b) Relationship between champion efficiencies and the accumulated number of related articles. c) The champion power conversion efficiencies of all-inorganic perovskite solar cells along with years. The devices are classified into 5 groups by the iodine to bromine ratio quantified via the value of x. The efficiencies of hybrid perovskite solar cells are also shown for comparison. The devices with Sn doping process are not shown in the figures, because the bandgap deviates significantly from the intrinsic values. d) The champion efficiency of different all-inorganic perovskite solar cells compared with the SQ limit. The record efficiency of c-Si, GaAs and hybrid perovskite solar cells are shown for comparison.

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.

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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.