Understanding the Performance-Limiting Factors of Cs₂AgBiBr₆ Double-Perovskite Solar Cells

Double perovskites have recently emerged as possible alternatives to lead-based halide perovskites for photovoltaic applications. In particular, Understanding the Performance-Limiting Factors of Cs₂AgBiBr₆ Double-Perovskite Solar Cells has been the subject of several studies because of its environmental stability, low toxicity, and its promising optoelectronic features. Despite these encouraging features, the performances of solar cells based on this double perovskite are still low, suggesting severe limitations that need to be addressed. In this work we combine experimental and theoretical studies to show that the short electron diffusion length is one of the major causes for the limited performance of Cs₂AgBiBr₆ solar cells. Using EQE measurements on semitransparent Cs₂AgBiBr₆ solar cells we estimate the electron diffusion length to be only 30 nm and corroborated this value by terahertz spectroscopy. By using photothermal deflection spectroscopy and surface photovoltage measurements we correlate the limited electron diffusion length with a high density of electron traps. Our findings highlight important faults affecting this double perovskite, showing the challenges to overcome and hinting to a possible path to improve the efficiency of Cs₂AgBiBr₆ solar cells

Highly Absorbing Lead-Free Semiconductor Cu₂AgBiI₆ for Photovoltaic Applications from the Quaternary CuI-AgI-BiI₃ Phase Space

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu_{2}AgBiI_{6}: a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 10^{5} cm^{–1} near the absorption onset, several times that of CH_{3}NH_{3}PbI_{3}. Solution-processed Cu2AgBiI6 thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm^{2} V^{–1} s^{–1}), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI_{3}. The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs2AgBiI6 thin films have not been synthesized to date, Cu_{2}AgBiI_{6} is a valuable example of a stable Ag^{+}/Bi^{3+} octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space

Polarons and charge localization in metal-halide semiconductors for photovoltaic and light-emitting devices

Metal-halide semiconductors have shown excellent performance in optoelectronic applications such as solar cells, light-emitting diodes, and detectors. In this review the role of charge–lattice interactions and polaron formation in a wide range of these promising materials, including perovskites, double perovskites, Ruddlesden–Popper layered perovskites, nanocrystals, vacancy-ordered, and other novel structures, is summarized. The formation of Fröhlich-type “large” polarons in archetypal bulk metal-halide ABX3 perovskites and its dependence on A-cation, B-metal, and X-halide composition, which is now relatively well understood, are discussed. It is found that, for nanostructured and novel metal-halide materials, a larger variation in the strengths of polaronic effects is reported across the literature, potentially deriving from variations in potential barriers and the presence of interfaces at which lattice relaxation may be enhanced. Such findings are further discussed in the context of different experimental approaches used to explore polaronic effects, cautioning that firm conclusions are often hampered by the presence of alternate processes and interactions giving rise to similar experimental signatures. Overall, a complete understanding of polaronic effects will prove essential given their direct influence on optoelectronic properties such as charge-carrier mobilities and emission spectra, which are critical to the performance of energy and optoelectronic applications

Interplay of structure, charge-carrier localization and dynamics in copper-silver-bismuth-halide semiconductors

Silver-bismuth based semiconductors represent a promising new class of materials for optoelectronic applications because of their high stability, all-inorganic composition, and advantageous optoelectronic properties. In this study, charge-carrier dynamics and transport properties are investigated across five compositions along the AgBiI4–CuI solid solution line (stoichiometry Cu4x(AgBi)1−xI4). The presence of a close-packed iodide sublattice is found to provide a good backbone for general semiconducting properties across all of these materials, whose optoelectronic properties are found to improve markedly with increasing copper content, which enhances photoluminescence intensity and charge-carrier transport. Photoluminescence and photoexcitation-energy-dependent terahertz photoconductivity measurements reveal that this enhanced charge-carrier transport derives from reduced cation disorder and improved electronic connectivity owing to the presence of Cu+. Further, increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies, and higher mobilities. Finally, ultrafast charge-carrier localization is observed upon pulsed photoexcitation across all compositions investigated, lowering the charge-carrier mobility and leading to Langevin-like bimolecular recombination. This process is concluded to be intrinsically linked to the presence of silver and bismuth, and strategies to tailor or mitigate the effect are proposed and discussed

Charge-carrier dynamics, mobilities and diffusion lengths of 2D-3D hybrid butylammonium-caesium-formamidinium lead halide perovskites

Perovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature‐ and humidity‐induced instability. Materials that combine high charge‐carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D–3D hybrid systems, the hybrid perovskite BAx(FA0.83Cs0.17)1‐xPb(I0.6Br0.4)3 is investigated across the composition range 0 ≤ x ≤ 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap‐mediated charge‐carrier recombination and enhancing charge‐carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge‐carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge‐carrier diffusion lengths up to 7.7 µm for x = 0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D–3D hybrid materials

Charge-carrier mobility and localization in semiconducting CU2AGBiI6 for photovoltaic applications

Lead-free silver–bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu2AgBiI6 using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu2AgBiI6 are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm2 V–1 s–1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ∼29 meV and a direct band gap near 2.1 eV, these findings highlight Cu2AgBiI6 as an attractive lead-free material for photovoltaic applications

Charge‐carrier trapping and radiative recombination in metal halide perovskite semiconductors

Trap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA0.95Cs0.05PbI3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process

Charge-carrier trapping dynamics in bismuth-doped thin films of MAPbBr3 perovskite

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr3 host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast charge-carrier decay component, enhanced infrared emission, and a notable decrease of charge-carrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr3, which may enhance the presence of bromide interstitials

Chemical control of the dimensionality of the octahedral network of solar absorbers from the CuI-AgI-BiI3 phase space by synthesis of 3D CuAgBiI5

A newly reported compound, CuAgBiI5, is synthesized as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag+/Bi3+ network as in spinel, but occupancy of the tetrahedral interstitials by Cu+ differs from those in spinel. The 3D octahedral network of CuAgBiI5 allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI–AgI–BiI3 phase field, giving the ability to chemically control structural dimensionality. To investigate composition–structure–property relationships, we compare the basic optoelectronic properties of CuAgBiI5 with those of Cu2AgBiI6 (which has a 2D octahedral network) and reveal a surprisingly low sensitivity to the dimensionality of the octahedral network. The absorption onset of CuAgBiI5 (2.02 eV) barely changes compared with that of Cu2AgBiI6 (2.06 eV) indicating no obvious signs of an increase in charge confinement. Such behavior contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in photoluminescence spectra and lifetimes between the two compounds mostly derive from the difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI5 powder samples are found to be more sensitive to degradation under solar irradiation compared to Cu2AgBiI6