Bandgap Perovskite Research Articles

Narrowband Monolithic Perovskite–Perovskite Tandem Photodetectors

AbstractNarrowband photodetectors (PDs) are sought after for many applications requiring selective spectral response. The most common systems combine optical bandpass filters with broadband photodiodes. This work reports a method to obtain a narrowband response in a perovskite PD by the monolithic integration of a perovskite photoconductor and a perovskite photodiode. The spectral response of the tandem PD is determined by the bandgap energy difference of the two perovskites, and exhibits a full width at half maximum below 85 nm, an external quantum efficiency up to 68% and a high specific detectivity of ≈1012 Jones in reverse bias, enabling the device to detect weak light signals. The absorption profile of the narrowband PD can be tuned by changing the thickness and bandgap of the wide bandgap perovskite absorber.

Fast HCl-free Synthesis of Lead-free Rb2ZrCl6:xSb3+ Perovskites.

Due to the toxicity and instability issues of lead halide perovskites, lead-free perovskites have recently emerged as a viable alternative. However, significant optical band gaps of lead-free perovskites exert influence on their luminescent properties. Fortunately, the addition of dopants becomes an efficacious solution. The current widely utilized methods for synthesizing perovskites almost require high temperatures, a long period, and atmosphere protection, which cost more energy and resources. In this paper, we report that Rb2ZrCl6:xSb3+ perovskite phosphors can be easily prepared by a wet grinding approach at room temperature, which is a more efficient and facile process. Due to the self-trapped excitons of the host structure and Sb3+ ions, the produced samples display blue-white and orange fluorescence under UV lamp irradiation at 254 and 365 nm, respectively. In the photoluminescence spectrum, the doped perovskite exhibits an emission peak at 630 nm under excitation at 365 nm. Importantly, the prepared phosphors have tunable emissions related to the excitation wavelength. In addition, our produced powders show remarkable stability at room temperature, laying the foundations for this approach to be widely used in perovskite production.

Low temperature preparation of W-doped In2O3 transparent electrodes for p-i-n structured perovskite solar cells

W-doped In2O3 (IWO) films prepared by reactive plasma deposition (RPD) technique at 150 °C have been studied to replace Sn-doped In2O3 (ITO) serving as transparent electrodes for p-i-n structured perovskite solar cells. The optical and electrical properties of IWO films were found to depend on the oxygen partial pressure of deposition process. Compared with the reference ITO films, the IWO films obtained at an oxygen partial pressure of 0.12 Pa exhibited prominent performance, including higher near-infrared transmittance and carrier mobility. Furthermore, the IWO films showed higher work function than that of the ITO films, resulting in the enhancement of open-circuit voltage (0.982 V vs 0.901 V) and conversion efficiency (12.9 % vs 11.9 %). It can be attributed to that high work function of the IWO films reduced the carrier transport barrier at the interface between transparent electrode and PEDOT:PSS and enhanced the carrier collection near the edge of the perovskite bandgap. The excellent properties of the RPD-prepared IWO films indicate that it is a promising transparent electrode material for p-i-n structured perovskite solar cells, especially for applications requiring low-temperature fabrication.

Unraveling the Effect of Halogen Ion Substitution on the Noise of Perovskite Single-Crystal Photodetectors.

Halide mixing in perovskites has been an efficient way to engineer the bandgap, stability, and charge carrier mobility of lead halide perovskites, while its effect on the noise of lead halide perovskite photodetectors is still unknown. We present the preparation of mixed halide methylammonium lead perovskite single crystals by the solution temperature-decreasing method. Planar-structured photodetectors were constructed on the basis of these mixed halide perovskite single crystals. The effect of halogen ion substitution on the noise of devices was investigated by analyzing their dark current spectra. It is shown that the single-crystal photodetectors with higher levels of chloride suffer from larger noise thus have lower detectivity. Density functional theory calculations have also been proposed to reveal the effect of halogens on band structure. These results provide a comprehensive understanding of mixed halide perovskites and may help in the design and preparation of higher-performance devices.

Interfacial Embedding for High‐Efficiency and Stable Methylammonium‐Free Perovskite Solar Cells with Fluoroarene Hydrazine

AbstractPerovskite solar cells (PSCs) with state‐of‐the‐art efficiencies contain thermally unstable methylammonium (MA). Here, interfacial passivation with pentafluorophenylhydrazine (5F‐PHZ) to fabricate efficient and stable MA/Br‐free PSCs is introduced. The 5F‐PHZ surface treatment quenches the PbI2 and δ‐perovskite phase formed in the pristine film. The surface passivation ameliorates the film chemistries at the surface with modulation of interface band alignment as a consequence of halogen bonding with fluoroarene moieties or NH–NH2 terminals. This results in a much longer carrier lifetime with the passivation at the surface and grain boundaries trap centers. As a result, it boosts the power conversion efficiency (PCE) (area ≈ 1 cm2) from 18.10% to 22.29% (VOC ≈ 1.096–1.178 V) with superior operational thermal stability. A certified PCE of 21.01% with a large area of ≈1.026 cm2 is also achieved. It is found that the surface passivation forms an interfacial embedded layer subsequent to attenuation of defect densities and suppression of ion migration, which is supported by density‐function‐theory calculation. Importantly, this approach is effective in enhancing the PCE of narrow and wide bandgap perovskite systems. Thus, this work opens up a new technique for interface modulation with fluoroarene functional derivatives to achieve superior device performance and stability.

Strain-induced enhancement of carrier transport and optical absorption in Cs3Bi2Br9 perovskite

Based on the density functional theory, we investigated the electronic structure, the density of states, the effective mass, and optical properties of the Cs3Bi2Br9 perovskite under strains. It was found that the tuning ranges of the bandgap of the Cs3Bi2Br9 perovskite are 0.08, 0.07, and 0.29 eV under uniaxial strain along the a-, b-, c-axes, and 0.42 eV under triaxial strain for variations from −4% to 4%, respectively. We integrated the optical absorption over a range from 1 to 7 eV and established that the optical absorption can be improved by up to 138% under −4% uniaxial strain along the b-axis. In addition, the optical absorption also was enhanced by up to 129% under a 4% uniaxial tensile strain along the a-axis. Furthermore, we found that triaxial compressive strain is a highly effective method to promote election diffusion by decreasing the effective mass of the electrons in the material.

Carbonyl functional group assisted crystallization of mixed tin–lead narrow bandgap perovskite absorber in ambient conditions

Tin–lead (Sn–Pb) perovskite solar cells are receiving growing interest due to their applications in tandems and lead mitigation. Nonetheless, fast crystallization and facile Sn2+ oxidation restrict their ambient fabrication, which increases fabrication costs. This Letter presents an experimental study on additive assisted growth of FA0.2MA0.8Sn0.5Pb0.5I2.4Br0.6 narrow bandgap perovskite films employing a Lewis-base molecule, caffeine (1,3,7-trimethylpurine-2,6-dione), having two carbonyl functional groups (C = O) in ambient conditions (relative humidity < ∼10%). The C = O interacts with metallic ions (Sn2+ and Pb2+) via chelation to form an acid–base adduct, slowing down the fast crystallization of FA0.2MA0.8Sn0.5Pb0.5I2.4Br0.6 perovskite films. As a result, the grain size improves resulting in better structural and optical properties. In contrast, Urbach energy values showed higher electronic disorder near the band edges even upon caffeine doping implying Sn4+ doping in an ambient environment. This work accentuates the potential of the acid–base adduction to regulate uncontrolled crystallization of Sn–Pb perovskites in the ambient environment.

Wide Bandgap Perovskite Photovoltaic Cells for Stray Light Recycling in a System Emitting Broadband Polarized Light

AbstractPerovskite based photovoltaic (PV) cells are unique in combining low open‐circuit voltage losses with a broad bandgap tunability. This makes them an ideal PV cell to recycle photons back into electrical power in a variety of illumination systems or light emitting devices. Here, advantage of these features is taken and wide bandgap (WBG) perovskite PV cells are incorporated in devices suitable for display illumination and demonstrate a high yield in stray light recycling back into electricity with up to a 37.5% power conversion efficiency. The specific device considered is a modified half‐cylinder photonic plate designed to emit diffused broadband polarized light using a nonabsorbing reflective polarizer based on a random dielectric layer distribution. It is experimentally demonstrated that light recycling using appropriately tuned WBG perovskite PV cells becomes very efficient when implemented in systems where the light is emitted from narrowband sources, even if the emission spans a broad wavelength range.

Cation‐Diffusion‐Based Simultaneous Bulk and Surface Passivations for High Bandgap Inverted Perovskite Solar Cell Producing Record Fill Factor and Efficiency

AbstractHigh bandgap perovskite solar cells are integral to perovskite‐based multi‐junction tandem solar cells with efficiency potentials over 40%. However, at present, high bandgap perovskite devices underperform compared to their mid bandgap counterparts in terms of voltage outputs and fill factors resulting in lower than ideal efficiencies. Here, the low fill factor aspect of high bandgap perovskite is addressed by developing a cation‐diffusion‐based double‐sided interface passivation scheme that simultaneously provides bulk passivation for a 1.75 eV perovskite cell that is also compatible with a p‐i‐n cell architecture. The champion cell achieves a record fill factor of 86.5% and a power conversion efficiency of 20.2%. Results of ionic distribution profiling, Fourier transform infrared spectroscopy, and X‐ray diffraction crystallography reveal evidence of cation diffusion from the surface perovskite passivation layer into bulk. The diffused cations reduce Shockley–Read–Hall recombination in the perovskite bulk and at the surfaces with the latter being more dominant as confirmed by light‐intensity dependent and temperature‐dependent open‐circuit voltage measurements as well as thermal admittance spectroscopy. This concurrent bulk and surface passivation scheme renders record fill factor and efficiency in the double‐side passivated cells. This provides new insights for future passivation strategies based on ionic diffusion of functionalized materials.

Narrowband Near-Infrared Perovskite/Organic Photodetector: TCAD Numerical Simulation

Narrowband photodetectors (PD) established in the near-infrared (NIR) wavelength range are highly required in a variety of applications including high-quality bioimaging. In this simulation study, we propose a filter-less narrowband PD based on the architecture of perovskite/organic heterojunction. The most decisive part of the photodetector is the hierarchical configuration of a larger bandgap perovskite material with a thicker film followed by a lower bandgap organic material with a narrower layer. The design of the structure is carried out by TCAD numerical simulations. Our structure is based on an experimentally validated wideband organic PD, which is modified by invoking an additional perovskite layer having a tunable bandgap. The main detector device comprises of ITO/perovskite (CsyFA1−yPb(IxBr1−x)3)/organic blend (PBDTTT-c:C60-PCBM)/PEDOT:PSS/Al. The simulation results show that the proposed heterojunction PD achieves satisfactory performance when the thickness of perovskite and organic layers are 2.5 µm and 500 nm, respectively. The designed photodetector achieves a narrow spectral response at 730 nm with a full width at half-maximum (FWHM) of 33 nm in the detector, while having a responsivity of about 0.12 A/W at zero bias. The presented heterojunction perovskite/organic PD can efficiently detect light in the wavelength range of 700 to 900 nm. These simulation results can be employed to drive the development of filter-less narrowband NIR heterojunction PD.

The effect of anion exchange on the electronic and optical properties of vacancy ordered double perovskites K2PdX6 (X = Cl, Br, I) using first principle calculations

ABSTRACT Recent studies are confirmed that lead-free perovskite is a possible alternative solution for conventional hybrid lead-based perovskites to achieve better photovoltaic efficiency. This work investigates the structural, mechanical, electronic, and optical properties of vacancy ordered double perovskite of K2PdX6 (X = Cl, Br, I) for photovoltaic applications through the density functional theory at the HSE06 level of theory. The bond length of Pd-X is linearly increased while exchanging the anions from Cl to I. The mechanical study reveals that the vacancy-ordered double perovskites K2PdCl6, K2PdBr6, and K2PdI6 are elastically classified as ductile materials. The K2PdBr6 hold a favourable bandgap of 1.62 eV for single junctions solar cell applications. The studied perovskite bandgaps depend on the Pd-4d, s and p orbitals of anions. The optical study uncover that the studied perovskites are higher optical absorption in the visible regions. Based on calculated results, we conclude that these K2PdBr6 vacancy ordered double perovskites might be suitable material for photovoltaic applications.

Tuning the Band Gap in the Halide Perovskite CsPbBr3 through Sr Substitution.

The ability to continuously tune the band gap of a semiconductor allows its optical properties to be precisely tailored for specific applications. We demonstrate that the band gap of the halide perovskite CsPbBr3 can be continuously widened through homovalent substitution of Sr2+ for Pb2+ using solid-state synthesis, creating a material with the formula CsPb1-xSrxBr3 (0 ≤ x ≤ 1). Sr2+ and Pb2+ form a solid solution in CsPb1-xSrxBr3. Pure CsPbBr3 has a band gap of 2.29(2) eV, which increases to 2.64(3) eV for CsPb0.25Sr0.75Br3. The increase in band gap is clearly visible in the color change of the materials and is also confirmed by a shift in the photoluminescence. Density-functional theory calculations support the hypothesis that Sr incorporation widens the band gap without introducing mid-gap defect states. These results demonstrate that homovalent B-site alloying can be a viable method to tune the band gap of simple halide perovskites for absorptive and emissive applications such as color-tunable light-emitting diodes, tandem solar cells, and photodetectors.

Improving Symbolic Regression for Predicting Materials Properties with Iterative Variable Selection.

Symbolic regression offers a promising avenue for describing the structure-property relationships of materials with explicit mathematical expressions, yet it meets challenges when the key variables are unclear because of the high complexity of the problems. In this work, we propose to solve the difficulty by automatically searching for important variables from a large pool of input features. A new algorithm that integrates symbolic regression with iterative variable selection (VS) was designed for optimization of the model with a large amount of input features. Using the recent method SISSO for symbolic regression and random search for variable selection, we show that the VS-assisted SISSO (VS-SISSO) can effectively manage even hundreds of input features that the SISSO alone was computationally hindered, and it fastly converges to (near) optimal solutions when the model complexity is not high. The efficiency of this approach for improving the accuracy of symbolic regression in materials science was demonstrated in the two showcase applications of learning approximate equations for the band gap of inorganic halide perovskites and the stability of single-atom alloy catalysts.

Tuning bandgap and energy stability of Organic-Inorganic halide perovskites through surface engineering

Organohalide perovskite with a variety of surface structures and morphologies have shown promising potential owing to the choice of the type of heterostructure dependent stability. We systematically investigate and discuss the impact of 2-dimensional molybdenum-disulphide (MoS2), molybdenum-diselenide (MoSe2), tungsten-disulphide (WS2), tungsten-diselenide (WSe2), boron-nitiride (BN) and graphene monolayers on bandgap and energy stability of organic–inorganic halide perovskites. We found that MAPbI3 deposited on BN-ML shows room temperature stability (-25 meV ∼ 300 K) with an optimal bandgap of ∼ 1.68 eV. The calculated absorption coefficient also lies in the visible-light range with a maximum of 4.9 × 104 cm−1 achieved at 2.8 eV photon energy. On the basis of our calculations, we suggest that the encapsulation of an organic–inorganic halide perovskite monolayers by semiconducting monolayers potentially provides greater flexibility for tuning the energy stability and the bandgap.

Hole Trapping in Halide Perovskites Induces Phase Segregation

ConspectusMetal halide perovskites have garnered a great deal of attention for their applications in photovoltaics, LEDs, and radiation detection. The ease of solution processing high-quality perovskite semiconductors with large absorption coefficients and tolerance to native defects is decidedly attractive. Additionally, the ability to precisely tune the band gap of halide perovskites through compositional alloying of the halide ion is of particular interest for a range of applications, especially for tandem solar cells. However, under steady state light irradiation, an initially homogeneous mixed halide perovskite (MHP) will form local domains that are rich in one halide ion (e.g., Br or I). This light-induced phase segregation in MHPs forms iodide-rich domains that act as charge carrier traps and lowers the efficiency of perovskite-based devices. Thus, phase segregation poses a serious challenge to the implementation of MHPs in real-world device settings. Interestingly, when a phase segregated MHP film is placed in the dark, entropic driving forces become dominant and the segregated perovskite remixes and returns to its initially homogeneous state. Several key mechanistic details of phase segregation have been elucidated over the years. However, there are still aspects of halide segregation that are not clear, and there is ongoing debate in the literature as to what are the key factors that contribute to the mechanism.This Account discusses recent results that point to the specific role of hole trapping in phase segregation. Interestingly, generation of holes through above-band-gap excitation or through electrochemical injection increases ion migration and leads to phase segregation. The thermodynamic and redox properties of halide perovskites provide a strong driving force for hole trapping and oxidation of iodide species in MHPs. However, mobile halide species within the perovskite lattice take time to migrate and generate halide-rich domains. When in contact with a nonpolar solvent, the migration of iodine species is further extended to expulsion of iodine from the perovskite film. Thus, the mobility of halides and their susceptibility to hole-induced oxidation play a crucial role in determining the long-term stability of metal halide perovskites. Strategies to gain kinetic control over ion migration to slow phase segregation are needed to overcome these hurdles and achieve stable mixed halide perovskites. Modification of the perovskite composition through introduction of different cations or halide ions, or introduction of low-dimensional perovskite phases may suppress phase segregation. Thus, in achieving stability and improving the efficiency of perovskite solar cells and light emitting devices with minimal impacts, suppression of segregation remains the key factor.

Screening of Excitons by Organic Cations in Quasi-Two-Dimensional Organic-Inorganic Lead-Halide Perovskites.

Interlayer organic cations in quasi-two-dimensional halide perovskites are a versatile tuning vehicle for the optoelectronic properties of these complex systems, but chemical intuition for this design route is yet to be established. Here, we use density functional theory, the GW approximation, and the Bethe-Salpeter equation approach to understand the contribution of the organic cation to the quasiparticle band gap and exciton binding energy of layered perovskites. We show that organic cations in quasi-two-dimensional perovskites contribute significantly to the dielectric screening in these systems, countering quantum confinement effects on the quasiparticle band gap and the exciton binding energy. Using a simple electrostatics model inspired by parallel-plate capacitors, we decouple the organic cation and inorganic layer contributions to the effective dielectric constants and show that dielectric properties of layered perovskites are broadly tunable via the interlayer cation, providing a direct means of tuning photophysical properties for a variety of applications.

Methylammonium and Bromide‐Free Tin‐Based Low Bandgap Perovskite Solar Cells

AbstractLead halide‐based perovskite solar cells (PSCs) are intriguing candidates for photovoltaic technology because of their high efficiency, low cost, and simple process advantages. Owing to lead toxicity, PSCs based on partially/fully substituted Pb with tin have attracted tremendous attention, which would enable the ideal bandgap to approach the Shockley‐Queisser (S‐Q) limit. Especially, methylammonium (MA), bromide‐free, tin‐based perovskites are striking, because of the intrinsic poor stability of MA and blue shift caused by the incorporation of Br−. The first section of this review emphasizes the motivation for studying single‐junction MA, Br‐free, and Sn‐based perovskites. The film quality improvement strategies of Sn‐based perovskites, including additive, composition, dimensional, and interface engineering toward high‐efficiency devices are comprehensively overviewed. Moreover, strategies to improve stability, where shelf, thermal and operational stabilities of the devices are summarized. Finally, this review concludes with a discussion of actual limitations and future prospects for Sn‐based PSCs.

Stable Pure Iodide MA0.95Cs0.05PbI3 Perovskite toward Efficient 1.6 eV Bandgap Photovoltaics.

Perovskite photovoltaics with the advantages of facile fabrication and high efficiency have been the rising star in the field for a decade. Methylammonium lead triiodide (MAPbI3) was the first widely studied perovskite to initiate the boom of perovskite photovoltaics, but it was later considered thermodynamically instable for commercialization. Here, we demonstrate that simple cesium (Cs) doping without any complicated process can form a stable MA-based perovskite with a widened bandgap, which may broaden the application of MA-based perovskites in tandem solar cells. A record-high efficiency of ≤22% is thus achieved for a 1.6 eV bandgap perovskite solar cell. This work not only provides a new stable and efficient pure iodide candidate as a 1.6 eV bandgap perovskite but also reveals that Cs incorporation can help improve the efficiency and stability of MA-based perovskites.

Flexible Perovskite and Organic Semiconductor Heterojunction Devices for Tunable Band-Selective Photodetection

Filter-free band-selective photodetectors with tunable band edges possess extensive applications in smart sensors, artificial intelligence, the internet of everything, and so forth. However, such photodetectors operated with the charge collection narrowing effect require a thick active layer over millimeters, leading to rather a large device size, high material cost, and limited mechanical flexibility. In this work, we report a flexible thin-film perovskite and organic semiconductor (OSC) heterojunction device, which can achieve adjustable band-selective photodetection by utilizing the easy tunability of the perovskite band gap. First, heterojunction photodiodes with only two active layers of perovskites and OSCs are fabricated, in which the perovskite contributes to light absorption and rectification characteristics simultaneously. The fundamental device operation mechanism is investigated by combining the band alignments of the heterojunctions and their potential distributions observed with Kelvin probe force microscopy. It uncovers that the presence of built-in electric fields in the heterojunctions caused by moderate band alignments and appropriate difference in hole and electron concentrations between the perovskites and the OSCs is significant for the properties of the photodiodes. The resultant photodiodes exhibit a high rectification ratio of up to 103, decent photodetection performance, and mechanical flexibility. Furthermore, photodetectors based on perovskite/OSC/perovskite stack heterojunctions can realize the functions of filterless band-selective photodetection in a single device, and the band edges in the photodetection can be modulated by tuning the perovskite band gap. Therefore, this work provides flexible thin-film photodiodes with the potential of miniaturization and low-cost fabrication and demonstrates tunable band-selective photodetection for advanced applications.

Photovoltaic performance of bifacial perovskite/c-Si tandem solar cells

Many researchers have pointed out the advantages of bifacial perovskite/c-Si tandem solar cells, but the theoretical treatment has not been well resolved because of the existence of current matching loss (CML). Here we start from the photovoltaic performance of bifacial sub-cells through graphically analyzing the current characteristics of the sub-cells under bifacial illumination and propose a feasible method to calculate the CML of the bifacial tandem cells. The key success to determine the CML lies in establishing the numerical relationship between the bifacial sub-cells and the bifacial tandem cells according to the strict principle of energy balance. We yield the photovoltaic performance of bifacial perovskite/c-Si tandem solar cells under different perovskite bandgaps and albedos. The reliability of our approach has been demonstrated by comparison with the existing experimental data. We further predict the energy yield and loss of the bifacial tandem solar cell application in different latitudes in the world, and put forward possible solutions for increasing energy yield and reducing energy costs. Our present work establishes the calculation and prediction base of photovoltaic performance of the bifacial perovskite/c-Si tandem solar cells.