Prashant V. Kamat

Size quantization in semiconductor quantum dots (QDs) offers a convenient way to tune their bandgap and energy levels of conduction and valence bands. We have now employed a room-temperature synthesis of CsPbBr3 and CsPbI3 quantum dots, with sizes from 5 to 13.5 nm, to probe hole transfer to p-phenylenediamine and m-phenylenediamine. The apparent hole-transfer rate constants, as estimated from transient absorption spectroscopy measurements, indicate a strong dependence on QD size. The shift in the valence-band energy with decreasing QD size increases the driving force (−ΔG) for hole transfer. The apparent hole-transfer rate constants, which vary in the range of (1.5 ± 0.2) – (17.3 ± 0.6) × 1010 s–1 exhibit a dependence on the energy difference between the QD valence band energy and the oxidation potential of the hole acceptor. This dependence of the rate constants further establishes the normal and inverted regions, as expected from Marcus electron-transfer theory. The ability to modulate charge-transfer rate constants through size control paves the way for tailoring the performance of quantized photocatalytic systems.

Ternary semiconductor quantum dots (QDs) such as silver indium sulfide (AgInS2) represent an emerging class of heavy-metal-free nanomaterials with strong potential for light-harvesting applications. However, their photophysical properties are strongly influenced by intrinsic and surface defects, which act as donor–acceptor pair (DAP) states. Incorporating gallium into the crystal lattice is an effective way to reduce the number of defect states. We have alloyed the AgInS2 lattice with gallium to make AgInxGa1–xS2 QDs, subsequently growing a GaSy shell around the gallium-alloyed core to make AgInxGa1–xS2–GaSy core/shell QDs. These QDs are stable for several weeks when stored in the dark. The photovoltaic performance of these QDs in liquid-junction quantum dot-sensitized solar cells (QDSSCs) is evaluated to investigate the effect of gallium insertion in these ternary semiconductors. The electron injection from excited AgInS2 and its two gallium derivatives into mesoporous TiO2, probed using femtosecond transient absorption spectroscopy, indicated rate constants on the order of 109–1010 s–1. Insertion of gallium into the AgInS2 QDs enhances the photovoltaic performance, as evidenced by the increase in open-circuit voltage (Voc), short-circuit current density (JSC), and overall power conversion efficiency (PCE). The gallium insertion strategy presented in this study demonstrates the feasibility of improving the photoconversion efficiency of group I–III–VI ternary QD semiconductors by mitigating interstitial defect states.

Cooperative hole scavenging via a localized EDTA–Na ₂ SO ₃ catalytic cycle outcompetes trap-mediated recombination, fundamentally overcoming kinetic bottlenecks to maximize photoelectrochemical performance.

Photoinduced phase segregation remains a major roadblock to the long-term stability of photovoltaic devices employing mixed-halide perovskite absorbers. To address this issue, we use carbazole derivatives, often referred to as “self-assembled monolayers” (SAMs) with phosphonic acid as anchoring groups, typically used as hole transport materials at the buried interface, as surface treatments. Beyond their affinity for metal oxides, the phosphonic acid anchoring groups in these SAMs bind strongly to undercoordinated Pb2+ ions. 2PACz and MeO-2PACz exhibit distinct functionalities due to the energy alignment between the perovskite valence band maximum and the carbazole HOMO levels. While 2PACz passivates surface defects as evidenced by the increase in PL emission, only MeO-2PACz suppresses phase segregation and I2 expulsion via combined effects of defect passivation and hole extraction. By incorporating “SAMs” between the mixed-halide perovskite film and the spiro-OMeTAD hole transport layer, we fabricated photovoltaic devices with improved open-circuit voltage and stability under continuous illumination in open-circuit conditions.

Two-dimensional (2D) and quasi-2D layered perovskites, composed of alternating spacer cations and inorganic layers, have emerged as promising alternatives to conventional three-dimensional (3D) perovskites due to their suppressed halide ion mobility and improved ambient stability. Nevertheless, both halide anion and spacer cation migration can still occur in these reduced-dimensional perovskites, and ion migration remains a critical challenge for perovskite optoelectronic applications. Under photoirradiation and electrochemical bias, intrinsic iodine electrochemistry drives defect-mediated halide ion migration, further promoting halide segregation in mixed halide systems. Several effective strategies have been proposed to mitigate such dynamic ion migration, including controlling the inorganic layer number (n), crystallographic phase (Ruddlesden-Popper or Dion-Jacobson), crystal orientation, and, most importantly, the molecular structure of the intercalated spacer cations. However, a mechanistic understanding that links these structural parameters (spacer, A-, B-, and X-site composition) to lattice stability and ion migration remains limited. The ion migration processes discussed in this review provide insights into the thermodynamic and kinetic factors governing ion migration and offer design principles for improving the long-term operational stability of perovskite-based devices.