Attempts to improve the efficiency of kesterite solar cells by changing the intrinsic stoichiometry have not helped to boost the device efficiency beyond the current record of 12.6%. In this light, the addition of extrinsic elements to the Cu₂ZnSn(S,Se)₄ matrix in various quantities has emerged as a popular topic aiming to ameliorate electronic properties of the solar cell absorbers. This article reviews extrinsic doping and alloying concepts for kesterite absorbers with the focus on those that do not alter the parent zinc-blende derived kesterite structure. The latest state-of-the-art of possible extrinsic elements is presented in the order of groups of the periodic table. The highest reported solar cell efficiencies for each extrinsic dopant are tabulated at the end. Several dopants like alkali elements and substitutional alloying with Ag, Cd or Ge have been shown to improve the device performance of kesterite solar cells as compared to the nominally undoped references, although it is often difficult to differentiate between pure electronic effects and other possible influences such as changes in the crystallization path, deviations in matrix composition and presence of alkali dopants coming from the substrates. The review is concluded with a suggestion to intensify efforts for identifying intrinsic defects that negatively affect electronic properties of the kesterite absorbers, and, if identified, to test extrinsic strategies that may compensate these defects. Characterization techniques must be developed and widely used to reliably access semiconductor absorber metrics such as the quasi-Fermi level splitting, defect concentration and their energetic position, and carrier lifetime in order to assist in search for effective doping/alloying strategies.

Organic cation rotation in hybrid organic-inorganic lead halide perovskites has previously been associated with low charge recombination rates and (anti-)ferroelectric domain formation. Two-dimensional infrared spectroscopy (2DIR) was used to directly measure 470 ± 50 fs and 2.8 ± 0.5 ps time constants associated with the reorientation of formamidinium cations (FA+, NH2CHNH2+) in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations reveal the FA+ agitates about an equilibrium position, with NH2 groups pointing at opposite faces of the inorganic lattice cube, and undergoes 90° flips on picosecond timescales. Time-resolved infrared measurements revealed a prominent vibrational transient feature arising from a vibrational Stark shift: photo-generated charge carriers increase the internal electric field of perovskite thin films, perturbing the FA+ anti-symmetric stretching vibrational potential, resulting in an observed 5 cm-1 shift. Our 2DIR results provide the first direct measurement of FA+ rotation inside thin perovskite films, and cast significant doubt on the presence of long-lived (anti-)ferroelectric domains, which have been attributed to the observed low charge recombination rates.

Phase-pure BiI3 films obtained by a versatile gas phase iodination of Bi2S3 are investigated as an absorber in photovoltaic devices. This preparation method leads to highly crystalline BiI3 films featuring rhombohedral phase and a high-degree of stacking order. The films are composed of micrometer-sized flat grains distributed homogenously across the F:doped SnO2 (FTO) substrate exhibiting an indirect band gap transition at 1.72 eV. High-level calculations based on G0W0 approximation are used to rationalize the electronic structure of BiI3, confirming the band gap value estimated experimentally. The films show p-type conductivity with an acceptor density of the order of 1015 cm-3. Solar cells with the architecture Glass/FTO/TiO2/BiI3/F8/Au, where F8 is Poly(9,9-di-n-octylfluorenyl-2,7-diyl), display record open-circuit voltage above 600 mV and overall power conversion efficiency of 1.2% under AM 1.5G illumination. The large open-circuit potential is rationalized in terms of carrier lifetimes longer than 1 ns as probed by time-resolved photoluminescence spectroscopy.

This review provides an overview of porous carbon materials and their hydrogen storage properties with insight on strategic improvement by structural and chemical modifications and current trends of using machine learning and lifecycle assessment.