Critical Review of Cu-Based Hole Transport Materials for Perovskite Solar Cells: From Theoretical Insights to Experimental Validation.

Despite the remarkable efficiency of perovskite solar cells (PSCs), long-term stability remains the primary barrier to their commercialization. The prospect of enhancing stability by substituting organic transport layers with suitable inorganic compounds, particularly Cu-based inorganic hole-transport materials (HTMs), holds promise due to their high valence band maximum (VBM) aligning with perovskite characteristics. This review assesses the advantages and disadvantages of these five types of Cu-based HTMs. Although Cu-based binary oxides and chalcogenides face narrow bandgap issues, the "chemical modulation of the valence band" (CMVB) strategy has successfully broadened the bandgap for Cu-based ternary oxides and chalcogenides. However, Cu-based ternary oxides encounter challenges with low mobility, and Cu-based ternary chalcogenides face mismatches in VBM alignment with perovskites. Cu-based binary halides, especially CuI, exhibit excellent properties such as wider bandgap, high mobility, and defect tolerance, but their stability remains a concern. These limitations of single anion compounds are insightfully discussed, offering solutions from the perspective of practical application. Future research can focus on Cu-based composite anion compounds, which merge the advantages of single anion compounds. Additionally, mixed-cation chalcogenides such as CuxM1-xS enable the customization of HTM properties by selecting and adjusting the proportions of cation M.

Semitransparent Perovskite Solar Cells with > 13% Efficiency and 27% Transperancy Using Plasmonic Au Nanorods.

Semitransparent hybrid perovskites open up applications in windows and building-integrated photovoltaics. One way to achieve semitransparency is by thinning the perovskite film, which has several benefits such as cost efficiency and reduction of lead. However, this will result in a reduced light absorbance; therefore, to compromise this loss, it is possible to incorporate plasmonic metal nanostructures, which can trap incident light and locally amplify the electromagnetic field around the resonance peaks. Here, Au nanorods (NRs), which are not detrimental for the perovskite and whose resonance peak overlaps with the perovskite band gap, are deposited on top of a thin (∼200 nm) semitransparent perovskite film. These semitransparent perovskite solar cells with 27% average visible transparency show enhancement in the open-circuit voltage (Voc) and fill factor, demonstrating 13.7% efficiency (improved by ∼6% compared to reference cells). Space-charge limited current, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analyses shed more light on the trap density, nonradiative recombination, and defect density in these Au NR post-treated semitransparent perovskite solar cells. Furthermore, Au NR implementation enhances the stability of the solar cell under ambient conditions. These findings show the ability to compensate for the light harvesting of semitransparent perovskites using the plasmonic effect.

Strong self-trapping by deformation potential limits photovoltaic performance in bismuth double perovskite.

Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10-11 cm3 s-1) and low charge carrier mobility (around 0.05 cm2 s-1 V-1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

Improving Carrier-Transport Properties of CZTS by Mg Incorporation with Spray Pyrolysis.

High nonradiative recombination, low diffusion length and band tailing are often associated with a large open circuit voltage deficit, which results in low efficiency of Cu2ZnSnS4 (CZTS) solar cells. Recently, cation substitution in CZTS has gained interest as a plausible solution to suppress these issues. However, the common substitutes, Ag and Cd, are not ideal due to their scarcity and toxicity. Other transition-metal candidates (e.g., Mn, Fe, Co, or Ni) are multivalent, which may form harmful deep-level defects. Magnesium, as one of the viable substitutes, does not have these issues, as it is very stable in +2 oxidation state, abundant, and nontoxic. In this study, we investigate the effect of Mg incorporation in sulfur-based Cu2ZnSnS4 to form Cu2MgxZn1-xSnS4 by varying x from 0.0 to 1.0. These films were fabricated by chemical spray pyrolysis and the subsequent sulfurization process. At a high Mg content, it is found that Mg does not replace Zn to form a quaternary compound, which leads to the appearance of the secondary phases in the sample. However, a low Mg content (Cu2Mg0.05Zn0.95SnS4) improves the power conversion efficiency from 5.10% (CZTS) to 6.73%. The improvement is correlated to the better carrier-transport properties, as shown by a lesser amount of the ZnS secondary phase, higher carrier mobility, and shallower acceptor defects level. In addition, the Cu2Mg0.05Zn0.95SnS4 device also shows better charge-collection property based on the higher fill factor and quantum efficiency despite having lower depletion width. Therefore, we believe that the addition of a small amount of Mg is another viable route to improve the performance of the CZTS solar cell.

Doping and Switchable Photovoltaic Effect in Lead-Free Perovskites Enabled by Metal Cation Transmutation.

Creating defect tolerant lead-free halide perovskites is the major challenge for development of high-performance photovoltaics with nontoxic absorbers. Few compounds of Sn, Sb, or Bi possess ns2 electronic configuration similar to lead, but their poor photovoltaic performances inspire us to evaluate other factors influencing defect tolerance properties. The effect of heavy metal cation (Bi) transmutation and ionic migration on the defects and carrier properties in a 2D layered perovskite (NH4 )3 (Sb(1-x) Bix )2 I9 system is investigated. It is shown, for the first time, the possibility of engineering the carriers in halide perovskites via metal cation transmutation to successfully form intrinsic p- and n-type materials. It is also shown that this material possesses a direct-indirect bandgap enabling high absorption coefficient, extended carrier lifetimes >100 ns, and low trap densities similar to lead halide perovskites. This study also demonstrates the possibility of electrical poling to induce switchable photovoltaic effect without additional electron and hole transport layers.