Mechanism of the Anomalous Dependence between Spin–Orbit Coupling and Dimensionality in Lead Halide Perovskites

The spin–orbit coupling (SOC) effect of lead (Pb) atoms is a consequential attribute of the unique optoelectronic and defect properties of lead halide perovskites (LHPs). It has been found that the SOC effect varies significantly as the structural dimensionality changes with an anomalous dependence; i.e., while the SOC strength monotonically decreases as structural dimensionality decreases from three-dimensional (3D) to two-dimensional (2D) and then to one-dimensional (1D), the zero-dimensional (0D) SOC strength is greater than the 1D SOC strength. The underlying mechanism of such a SOC dimensionality dependence anomaly remains elusive. In this work, we show that Pb 6p energy splitting increases from 3D to 2D and to 1D LHPs due to the increased degree of distortion, leading to a reduced SOC strength. However, the degree of distortion decreases for the 1D to 0D transformation, resulting in reverse SOC enhancement. The mechanism described in this work can be employed to regulate the SOC effect in the design of perovskite materials

Predictions for p‑Type CH₃NH₃PbI₃ Perovskites

Approaches for doping organic–inorganic CH₃NH₃PbI₃ halide perovskite solar cell materials are investigated by density-functional theory calculations of the extrinsic doping properties of CH₃NH₃PbI₃. Our results reveal that p-type CH₃NH₃PbI₃ halide perovskites can be realized by incorporation of some group IA, IB, or VIA elements such as Na, K, Rb, Cu, and O at I-rich growth conditions. We further show that n-type CH₃NH₃PbI₃ halide perovskites are more difficult to realize due to the formation of neutral defects or compensation from intrinsic point defects. Our results suggest that nonequilibrium growth conditions and/or processes may be required to produce n-type CH₃NH₃PbI₃ halide perovskites

Solution-Processed Nb-Substituted BaBiO₃ Double Perovskite Thin Films for Photoelectrochemical Water Reduction

Photoelectrochemical (PEC) water reduction is a long-term strategical technology for hydrogen production. In this work, we synthesize a series of compact and nano/mesoporous Nb-substituted BaBiO₃ [i.e., Ba₂Bi­(Bi_1–<i>x</i>Nb_<i>x</i>)­O₆, 0 ≤ <i>x</i> ≤ 0.93, BBNO] thin films using cost-effective chemical solution methods. The synthesized BBNO alloy based thin films demonstrate tunable bandgaps from 1.41 eV (<i>x</i> = 0) to 1.89 eV (<i>x</i> = 0.93) to efficiently absorb the solar spectrum and p-type conductivities suitable for hydrogen production. The photoelectrodes with a configuration fluorine-doped SnO₂/BBNO (0 ≤ <i>x</i> ≤ 0.93)/Pt produce cathodic photocurrents of 0.05–1 mA·cm^–2 at 0 V_RHE (volt versus reversible hydrogen electrode) measured in a neutral (pH = 7.2) phosphate buffer and under simulated AM 1.5G illumination (100 mW·cm^–2). The BaBiO₃ without Nb alloying based electrode delivers the best photocurrent of 1 mA·cm^–2 at 0 V_RHE but is subjected to severe corrosions during the PEC related tests. Alloying Nb has an obvious influence on enhancing the material stability against corrosion. With Nb alloying, the screen-printed nanoporous BBNO (<i>x</i> = 0.6, bandgap = 1.62 eV) based photoelectrode generates a better photocurrent of 0.2 mA·cm^–2 at 0 V_RHE with a highly positive onset at 1.5 V_RHE enabling unbiased water reduction

Stability, Electronic and Optical Properties of M₄M′X₄ (M = Ga or In, M′ = Si, Ge, or Sn, X = Chalcogen) Photovoltaic Absorbers

Three-dimensional cubic M₄M′X₄ (M = Ga or In, M′ = Si, Ge, or Sn, and X = S, Se, or Te) have been proposed as photovoltaic absorber materials. Herein, we present density functional theory investigation of the stability, electronic and optical properties of M₄M′X₄. We find that M₄M′X₄ exhibit unique electronic properties. M elements lose partially both the outmost s and p electrons, whereas M′ elements only lose a small fraction of the valence electrons. As a result, the conduction band edges of M₄M′X₄ consist of a large contribution from the M s orbitals, leading to rather small electron effective masses. The valence bands are derived from M, M′, and X p orbitals. The band gap of this family can be tuned by selecting the combination of M and X elements. Among these semiconductors, In₄GeS₄, In₄GeSe₄, In₄SnS₄, and In₄SnSe₄ are suitable for photovoltaic applications due to their stability and suitable band gaps. However, the inclusion of scarce In may hinder their large-scale applications

Distant-Atom Mutation for Better Earth-Abundant Light Absorbers: A Case Study of Cu₂BaSnSe₄

Thin-film Cu­(In,Ga)­Se₂ and CdTe solar cells have demonstrated high power conversion efficiencies, but they cannot provide a sustainable clean energy pathway because of the scarcity of Te and In. Here, we propose a distant-atom concept to mutate In by a group II element (Ba) and a group IV element (Sn) that are at rather different locations on the periodic table. Because of the very different electronic properties between the cations, the resultant earth-abundant orthorhombic Cu₂BaSnSe₄ absorber does not have the detrimental cation–cation disorder issue seen in the earth-abundant kesterite Cu₂ZnSnSe₄ absorber. We anticipate that Cu₂BaSnSe₄ solar cells should not have large open-circuit voltage deficits as seen in CuZnSnSe₄ solar cells. Density functional theory calculation of the electronic and defect properties of Cu₂BaSnSe₄ confirms these expectations

The Interfacial Reaction at ITO Back Contact in Kesterite CZTSSe Bifacial Solar Cells

The synthesis route based on co-electroplating of copper, zinc, tin, and chalcogen precursor plus post-chalcogenization demonstrates the tremendous potential to realize industrial manufacture of earth-abundant kesterite materials for sustainable photovoltaics. Exploration of appropriate annealing temperature is significant to gain insight into the crystallization of kesterite solar materials on the back contacts based on transparent conducting oxides in bifacial device. The Cu₂ZnSn­(S_<i>x</i>, Se_1–<i>x</i>)₄ (CZTSSe) absorber films have been fabricated by post-selenizing co-electroplated metal–sulfide precursors on ITO substrate at 500, 525, and 550 °C. Experimental proof, including electron microscopies, X-ray diffraction, optical transmission/reflection spectra, polarized Raman, and IR techniques, is presented for the interfacial reaction between the ITO back contact and CZTSSe absorber. This reaction contributes to substitutional diffusion of In into CZTSSe (CZTISSe) to a considerable extent and formation of a SnO₂ interfacial layer when the temperature is higher than 500 °C. In incorporation does not much change the optical absorption, band gap, and phonon spectra of CZTSSe; whereas, it leads to lattice expansion more or less. The bifacial kesterite solar devices are successfully fabricated, and the device performance is analyzed and discussed

Characteristics of In-Substituted CZTS Thin Film and Bifacial Solar Cell

Implementing bifacial photovoltaic devices based on transparent conducting oxides (TCO) as the front and back contacts is highly appealing to improve the efficiency of kesterite solar cells. The p-type In substituted Cu₂ZnSnS₄ (CZTIS) thin-film solar cell absorber has been fabricated on ITO glass by sulfurizing coelectroplated Cu–Zn–Sn–S precursors in H₂S (5 vol %) atmosphere at 520 °C for 30 min. Experimental proof, including X-ray diffraction, Raman spectroscopy, UV–vis–NIR transmission/reflection spectra, PL spectra, and electron microscopies, is presented for the interfacial reaction between the ITO back contact and CZTS absorber. This aggressive reaction due to thermal processing contributes to substitutional diffusion of In into CZTS, formation of secondary phases and electrically conductive degradation of ITO back contact. The structural, lattice vibrational, optical absorption, and defective properties of the CZTIS alloy absorber layer have been analyzed and discussed. The new dopant In is desirably capable of improving the open circuit voltage deficit of kesterite device. However, the nonohmic back contact in the bifacial device negatively limits the open circuit voltage and fill factor, evidencing by illumination-/temperature-dependent <i>J</i>–<i>V</i> and frequency-dependent capacitance–voltage (<i>C</i>–<i>V</i>–<i>f</i>) measurements. A 3.4% efficient solar cell is demonstrated under simultaneously bifacial illumination from both sides of TCO front and back contacts

Co-electroplated Kesterite Bifacial Thin-Film Solar Cells: A Study of Sulfurization Temperature

Earth-abundant material, kesterite Cu₂ZnSnS₄ (CZTS), demonstrates the tremendous potential to serve as the absorber layer for the bifacial thin-film solar cell. The exploration of appropriate sulfurization conditions including annealing temperature is significant to gain insight into the growth mechanism based on the substrates using transparent conductive oxides (TCO) and improve device performance. The kesterite solar absorbers were fabricated on ITO substrates by sulfurizing co-electroplated Cu–Zn–Sn–S precursors in argon diluted H₂S atmosphere at different temperatures (475–550 °C) for 30 min. Experimental proof, including cross-section scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, UV–vis–NIR transmission spectrum, and Raman and far-infrared spectroscopy, is presented for the crystallization of CZTS on an ITO substrate and the interfacial reaction between the ITO back contact and CZTS absorber. The complete conversion of precursor into CZTS requires at least 500 °C sulfurization temperature. The aggressive interfacial reaction leading to the out-diffusion of In into CZTS to a considerable extent, formation of tin sulfides, and electrically conductive degradation of ITO back contact occurs at the sulfurization temperatures higher than 500 °C. The bifacial devices obtained by 520 °C sulfurization exhibit the best conversion efficiencies and open circuit voltages. However, the presence of non-ohmic back contact (secondary diode), the short minority lifetime, and the high interfacial recombination rates negatively limit the open circuit voltage, fill factor, and efficiency, evidenced by illumination/temperature-dependent <i>J</i>–<i>V</i>, frequency-dependent capacitance–voltage (<i>C</i>–<i>V</i>–<i>f</i>), time-resolved PL (TRPL), and bias-dependent external quantum efficiency (EQE) measurements

Heterovalent B-Site Co-Alloying Approach for Halide Perovskite Bandgap Engineering

Compositional engineering, which can enrich the database of prospective materials and offer new or enhanced properties, represents one of the key focal points within halide perovskite research. Compositional engineering studies often focus on A⁺ and X^– site substitutions, within the ABX₃ perovskite structure, due to the relative ease of varying these sites. However, alloying on the B site can play a more important role in generating novel properties and decreasing Pb toxicity for Pb-based systems. To date, B site substitution has primarily been confined to single-element alloying. Herein, a heterovalent co-alloying strategy for the B site of halide perovskites is proposed. Ag^IBi^III and Ag^ISb^III are co-alloyed into a host crystal of APbBr₃ (A = Cs and methylammonium), leading to a larger range of prospective alloying elements on the perovskite B site. Density functional theory-based first-principles calculations provide a possible rational for the red shift of the bandgap and blue shift of the photoluminescence (PL) in the alloying experiments

Alloying and Defect Control within Chalcogenide Perovskites for Optimized Photovoltaic Application

Through density functional theory calculations, we show that the alloy perovskite system BaZr_1–<i>x</i>Ti_<i>x</i>S₃ (<i>x</i> < 0.25) is a promising candidate for producing high power conversion efficiency (PCE) solar cells with ultrathin absorber layers. To maximize the minority carrier lifetime, which is important for achieving high PCE, the defect calculations show that BaZr_1–<i>x</i>Ti_<i>x</i>S₃ films should be synthesized under moderate (i.e., near stoichiometric) growth conditions to minimize the formation of deep-level defects. The perovskite BaZrS₃ is also found to exhibit ambipolar self-doping properties, indicating the ability to form homo p–n junctions. However, our theoretical calculations and experimental solid-state reaction efforts indicate that the doped perovskite BaZr_1–<i>x</i>Ti_<i>x</i>S₃ (<i>x</i> > 0) may not be stable under thermal equilibrium growth conditions. Calculations of decomposition energies suggest that introducing compressive strain may be a plausible approach to stabilize BaZr_1–<i>x</i>Ti_<i>x</i>S₃ thin films