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 Cu 2 ZnSn(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, NHCHNH) in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations reveal the FA agitates about an equilibrium position, with NH groups pointing at opposite faces of the inorganic lattice cube, and undergoes 90° flips on picosecond time scales. Time-resolved infrared measurements revealed a prominent vibrational transient feature arising from a vibrational Stark shift: photogenerated charge carriers increase the internal electric field of perovskite thin films, perturbing the FA antisymmetric stretching vibrational potential, resulting in an observed 5 cm 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 the observed low charge recombination rates have been attributed to.
A single-precursor solution approach is developed for depositing stoichiometric BiSI thin films featuring pure paraelectric orthorhombic (Pnam) phase. The compact and homogenous films are composed of flake-shaped grains oriented anti-planar to the substrate and display a sharp optical transition corresponding to a bandgap of 1.57 eV. Optical and Raman signatures of the thin films are rationalized using the quasiparticle G0W0@PBE0 and Density Functional Perturbation Theory calculations. Electrochemical impedance spectroscopy revealed n-type doping with valence and conduction band edges located at 4.6 and 6.2 eV below vacuum level, respectively. Planar BiSI solar cells are fabricated with the architecture: glass/FTO/SnO2/BiSI/F8/Au, where F8 is poly(9,9-din -octylfluorenyl-2,7-diyl), showing record conversion efficiency of 1.32% under AM 1.5 illumination.
The present work delivers the first assessment of BiFeO 3 (BFO) thin films as an absorber for sustainable all-oxide photovoltaic devices. Films are deposited from a metal−organic precursor complex solution followed by annealing in air at 673 K for 2 h. Xray diffraction, complemented by quantitative analysis, indicated formation of pure BFO with rhombohedral structure (R 3 C). Atomic force microscopy suggests deposition of compact and smooth films with spherical particles of sizes ∼150 nm. A direct band gap of 2.2 eV is ascertained from UV−vis−NIR spectroscopy. Mechanistic aspects of the BFO formation are discussed based on thermograveminetric analysis, differential scanning calorimetry, and infrared spectroscopy of the precursor complex. A proof-of-concept BFO/ ZnO heterojunction based solar cell fabricated by solution processing delivered a photoconversion efficiency of 3.98% with open-circuit voltage (V oc ), short-circuit current density, and fill factor of 642 mV, 12.47 mA/cm 2 , and 50.4%, respectively. The device exhibits a maximum external quantum efficiency of nearly 70%. These parameters are among the highest values reported for all oxide PV. Analysis of the V oc , series resistance, and conversion efficiency as a function of temperature revealed valuable information about recombination processes.
Chalcogenide perovskite materials are anticipated to have favourable structural, optical and electronic characteristics for solar energy conversion, yet experimental verification of the numerous computational studies is still lacking. In this perspective we summarise and critically review the computational and synthetic achievements, whilst suggesting new pathways for achieving the goal of developing this exiting class of materials. Greater knowledge of phase chemistry would allow the realisation of bandgap engineering through mixed cation and anion compositions. Combining this with fabrication and characterisation of thin films could yield promising new tailored materials for photovoltaic absorbers in the near future.
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