Covalently Connecting Crystal Grains with Polyvinylammonium Carbochain Backbone To Suppress Grain Boundaries for Long-Term Stable Perovskite Solar Cells

Grain boundaries act as rapid pathways for nonradiative carrier recombination, anion migration, and water corrosion, leading to low efficiency and poor stability of organometal halide perovskite solar cells (PSCs). In this work, the strategy suppressing the crystal grain boundaries is applied to improve the photovoltaic performance, especially moisture-resistant stability, with polyvinylammonium carbochain backbone covalently connecting the perovskite crystal grains. This cationic polyelectrolyte additive serves as nucleation sites and template for crystal growth of MAPbI₃ and afterward the immobilized adjacent crystal grains grow into the continuous compact, pinhole-free perovskite layer. As a result, the unsealed PSC devices, which are fabricated under low-temperature fabrication protocol with a proper content of polymer additive PVAm·HI, currently exhibit the maximum efficiency of 16.3%. Remarkably, these unsealed devices follow an "outside-in" corrosion mechanism and respectively retain 92% and 80% of the initial PCE value after being exposed under ambient environment for 50 days and 100 days, indicating the superiority of carbochain polymer additives in solving the long-term stability problem of PSCs.

Quantification of re-absorption and re-emission processes to determine photon recycling efficiency in perovskite single crystals

Photon recycling, that is, iterative self-absorption and re-emission by the photoactive layer itself, has been speculated to contribute to the high open-circuit voltage in several types of high efficiency solar cells. For organic–inorganic halide perovskites that have yielded highly efficient photovoltaic devices, however, it remains unclear whether the photon recycling effect is significant enough to improve solar cell efficiency. Here we quantitatively evaluate the re-absorption and re-emission processes to determine photon recycling efficiency in hybrid perovskite with its single crystals by measuring the ratio of the re-emitted photons to the initially excited photons, which is realized by modulating their polarization to differentiate them. The photon recycling efficiencies are revealed to be less than 0.5% in CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ single crystals under excitation intensity close to one sun, highlighting the intrinsically long carrier recombination lifetime instead of the photon-recycling-induced photon propagation as the origin of their long carrier diffusion length.

Fang et al. develop a method to determine the photon recycling efficiency for organic-inorganic hybrid single crystal perovskites by differentiating between emitted and re-absorbed photons based on their polarization difference. For these systems efficiencies of less than 0.5% are reported.

Wide-angle polarization-free plasmon-enhanced light absorption in perovskite films using silver nanowires

Since the successful implementation of organic-inorganic hybrid perovskites as light-absorbing materials, stunning progresses have been made towards the efficiency boost of perovskite solar cells. To build upon these successes, further impetus may derive from revisits to the intrinsic properties of perovskites, such as their optical properties. Herein, we introduce periodic Ag nanowire (AgNW) structures into perovskite films to optimize their solar absorption efficiency through plasmonic interactions. Numerical simulations show a remarkable integrated solar absorption enhancement of 25.9% attained by incorporating properly tailored AgNW arrays into perovskite films. The AgNW crosses are further introduced to achieve polarization-independent light harvesting capability. The omnidirectional light absorption enhancement ability of the AgNW embedded perovskite films is also demonstrated.

Fabrication and Characterization of High-Quality Perovskite Films with Large Crystal Grains

Solution-processable organometal perovskite materials have been widely used in various kinds of devices. In these devices, the perovskite materials normally act as active layers. Grain boundaries and structural disorder in the perovskite layer would interfere the charge transport and increase recombination probability. Here we proposed a novel fabrication method to dramatically increase the crystal size by more than 20 times as compared with previously reported values. Exceptional structural order in the large crystals is illustrated by nanoscale surface morphology and a simple recrystallization method. Because of reduced grain boundaries and increased crystal order in perovskite layers, the lateral charge transport is significantly improved, as demonstrated by conductive atomic-force microscopy and performance of photodetectors. This deposition technology paves the way for future high-performance devices based on perovskite thin films.

Photocurrent Spectroscopy of Perovskite Layers and Solar Cells: A Sensitive Probe of Material Degradation

Optical absorptance spectroscopy of polycrystalline CH₃NH₃PbI₃ films usually indicates the presence of a PbI₂ phase, either as a preparation residue or due to film degradation, but gives no insight on how this may affect electrical properties. Here, we apply photocurrent spectroscopy to both perovskite solar cells and coplanar-contacted layers at various stages of degradation. In both cases, we find that the presence of a PbI₂ phase restricts charge-carrier transport, suggesting that PbI₂ encapsulates CH₃NH₃PbI₃ grains. We also find that PbI₂ injects holes into the CH₃NH₃PbI₃ grains, increasing the apparent photosensitivity of PbI₂. This phenomenon, known as modulation doping, is absent in the photocurrent spectra of solar cells, where holes and electrons have to be collected in pairs. This interpretation provides insights into the photogeneration and carrier transport in dual-phase perovskites.

Electron-acoustic phonon coupling in single crystal CH3NH3PbI3 perovskites revealed by coherent acoustic phonons

Despite the great amount of attention CH₃NH₃PbI₃ has received for its solar cell application, intrinsic properties of this material are still largely unknown. Mobility of charges is a quintessential property in this aspect; however, there is still no clear understanding of electron transport, as reported values span over three orders of magnitude. Here we develop a method to measure the electron and hole deformation potentials using coherent acoustic phonons generated by femtosecond laser pulses. We apply this method to characterize a CH₃NH₃PbI₃ single crystal. We measure the acoustic phonon properties and characterize electron-acoustic phonon scattering. Then, using the deformation potential theory, we calculate the carrier intrinsic mobility and compare it to the reported experimental and theoretical values. Our results reveal high electron and hole mobilities of 2,800 and 9,400 cm² V⁻¹ s⁻¹, respectively. Comparison with literature values of mobility demonstrates the potential role played by polarons in charge transport in CH₃NH₃PbI₃.

Carrier mobility is a basic semiconductor property. Mante et al., use femtosecond lasers to investigate coherent acoustic phonons and relate their deformation potentials to estimate the intrinsic electron and hole mobilities of CH₃NH₃PbI₃ single crystals to be 2,800 and 9,400 cm² V⁻¹ s⁻¹, respectively.

Enhanced Optoelectronic Performance on the (110) Lattice Plane of an MAPbBr3 Single Crystal

Hybrid organic-inorganic lead halide perovskites have attracted significant attention due to their impressive optoelectronic properties. MAPbX₃ (MA= CH₃NH₃⁺, X= Cl, Br or I), the most popular member of this family, has been recognized as an important next-generation optoelectronic materials contender, and remarkable progress has been achieved in both thin films and single crystals. However, the lack of optimizations in energy harvest, transportation, carrier extraction, and process compatibility is hindering their future development. In this study, a triangle prism MAPbBr₃ single crystal exposing (100) and (110) crystallographic planes was successfully synthesized, and the optoelectronic performances of these two lattice planes were systematically explored by employing a planar metal-semiconductor-metal (MSM) device. Compared to the device fabricated on the (100) plane, a 153.33% enhancement of responsivity was achieved under 10 μW irradiation and 10 V bias on the (110) plane. Finally, possible mechanism for such an enhancement was discussed based on the different defect migration behaviors of (100) and (110) planes.

Photoresponse of CsPbBr3 and Cs4PbBr6 Perovskite Single Crystals

High-quality and millimeter-sized perovskite single crystals of CsPbBr3 and Cs4PbBr6 were prepared in organic solvents and studied for correlation between photocurrent generation and photoluminescence (PL) emission. The CsPbBr3 crystals, which have a 3D perovskite structure, showed a highly sensitive photoresponse and poor PL signal. In contrast, Cs4PbBr6 crystals, which have a 0D perovskite structure, exhibited more than 1 order of magnitude higher PL intensity than CsPbBr3, which generated an ultralow photoresponse under illumination. Their contrasting optoelectrical characteristics were attributed to different exciton binding energies, induced by coordination geometry of the [PbBr6]4- octahedron sublattices. This work correlated the local structures of lead in the primitive perovskite and its derivatives to PL spectra as well as photoconductivity.

Direct-Writing Multifunctional Perovskite Single Crystal Arrays by Inkjet Printing

Perovskite single-crystalline microplate arrays are directly achieved in large scale by inkjet printing, which present high performance lasing property with quality factors up to 863 and RGB (red-green-blue) emission. This facile, nonlithographic method makes its promising applications on multi-integrated coherent light sources and other high-performance integrated optoelectronic applications.

π-Conjugated Lewis Base: Efficient Trap-Passivation and Charge-Extraction for Hybrid Perovskite Solar Cells

A π-conjugated Lewis base is introduced into perovskite solar cells, namely, indacenodithiophene end-capped with 1.1-dicyanomethylene-3-indanone (IDIC), as a multifunctional interlayer, which combines efficient trap-passivation and electron-extraction. Perovskite solar cells with IDIC layers yield higher photovoltages and photocurrents, and 45% enhanced efficiency compared with control devices without IDIC.

Ultrafast carrier dynamics in bimetallic nanostructure-enhanced methylammonium lead bromide perovskites

In this work, we examine the impact of hybrid bimetallic Au/Ag core/shell nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr₃) mesoporous perovskite solar cells (PSCs). Plasmon-enhanced PSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in the photobleach formation time, which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr₃ perovskites. TAS also showed a decrease in carrier decay lifetimes, indicating that nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO₂ prior to bulk recombination. Furthermore, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.

Gate-Induced Insulator to Band-Like Transport Transition in Organolead Halide Perovskite

Understanding the intrinsic charge transport in organolead halide perovskites is essential for the development of high-efficiency photovoltaics and other optoelectronic devices. Despite the rapid advancement of the organolead halide perovskite in photovoltaic and optoelectronic applications, the intrinsic charge-carrier transport in these materials remains elusive partly due to the difficulty of fabricating electrical devices and obtaining good electrical contact. Here we report the fabrication of organolead halide perovskite microplates with mono- or bilayer graphene as low barrier electrical contact. Systematic charge-transport studies reveal an insulator to band-like transport transition. Our studies indicate that the insulator to band-like transport transition depends on the orthorhombic-to-tetragonal phase-transition temperature and defect densities of the organolead halide perovskite microplates. Our findings not only are important for the fundamental understanding of charge-transport behavior but also offer valuable practical implications for photovoltaics and optoelectronic applications based on the organolead halide perovskite.

Substrate-dependent electronic structure and film formation of MAPbI3 perovskites

We present investigations on the interface formation between the hybrid perovskite MAPbI₃ and various substrates, covering a wide range of work functions. The perovskite films are incrementally evaporated in situ while the electronic structure is evaluated using photoelectron spectroscopy. Our results show that there is an induction period in the growth of the perovskite during which volatile compounds are formed, catalyzed by the substrate. The duration of the induction period depends strongly on the nature of the substrate material, and it can take up to 20–30 nm of formal precursor deposition before the surface is passivated and the perovskite film starts forming. The stoichiometry of the 2–3 nm thin passivation layer deviates from the expected perovskite stoichiometry, being rich in decomposition products of the organic cation. During the regular growth of the perovskite, our measurements show a deviation from the commonly assumed flat band condition, i.e., dipole formation and band bending dominate the interface. Overall, the nature of the substrate not only changes the energetic alignment of the perovskite, it can introduce gap states and influence the film formation and morphology. The possible impact on device performance is discussed.

Long-Distance Charge Carrier Funneling in Perovskite Nanowires Enabled by Built-in Halide Gradient

The excellent charge carrier transportation in organolead halide perovskites is one major contributor to the high performance of many perovskite-based devices. There still exists a possibility for further enhancement of carrier transportation through nanoscale engineering, owing to the versatile wet-chemistry synthesis and processing of perovskites. Here we report the successful synthesis of bromide-gradient CH₃NH₃PbBr_xI_3-x single-crystalline nanowires (NWs) by a solid-to-solid ion exchange reaction starting from one end of pure CH₃NH₃PbI₃ NWs, which was confirmed by local photoluminescence (PL) and energy dispersive X-ray spectroscopy (EDS) measurements. Due to the built-in halide gradient, the long-distance carrier transportation was driven by the energy funnel, rather than the spontaneous carrier diffusion. Indeed, local PL kinetics demonstrated effective charge carrier transportation only from the high-bandgap bromide-rich region to the low-bandgap iodine-rich region over a few micrometers. Therefore, these halide gradient NWs might find applications in various optoelectronic devices requiring long-distance and directional delivery of excitation energy.

Facet-controlled preparation of hybrid perovskite microcrystals in the gas phase and the remarkable effect on optoelectronic properties

Particle shape of hybrid perovskite microcrystals influences PL properties via differences in the abundant facets and associated surface trap states.

Unveiling the irreversible performance degradation of organo-inorganic halide perovskite films and solar cells during heating and cooling processes

During a heating process, degradation of perovskite films occurred at 70 °C, resulting in a deeper trap depth leading to irreversible performance degradation of perovskite solar cells.

Understanding charge transport in lead iodide perovskite thin-film field-effect transistors

Fundamental understanding of the charge transport physics of hybrid lead halide perovskite semiconductors is important for advancing their use in high-performance optoelectronics. We use field-effect transistors (FETs) to probe the charge transport mechanism in thin films of methylammonium lead iodide (MAPbI₃). We show that through optimization of thin-film microstructure and source-drain contact modifications, it is possible to significantly minimize instability and hysteresis in FET characteristics and demonstrate an electron field-effect mobility (μ_FET) of 0.5 cm²/Vs at room temperature. Temperature-dependent transport studies revealed a negative coefficient of mobility with three different temperature regimes. On the basis of electrical and spectroscopic studies, we attribute the three different regimes to transport limited by ion migration due to point defects associated with grain boundaries, polarization disorder of the MA⁺ cations, and thermal vibrations of the lead halide inorganic cages.

Efficiency enhancement in inverted planar perovskite solar cells by synergetic effect of sulfated graphene oxide (sGO) and PEDOT:PSS as hole transporting layer

We report a simple solution route for preparing a sGO-PEDOT composite HTL by combining solution-processable sGO with commercialized PEDOT:PSS solution. The PSCs based on these sGO-PEDOT composite HTLs were systematically investigated.

Combining theory and experiment in the design of a lead-free ((CH3NH3)2AgBiI6) double perovskite

A lead-free double perovskite, (CH₃NH₃)₂AgBiI₆, which is rather stable, was investigated using a combination of experiment and density functional theory.

Ligand-assisted thickness tailoring of highly luminescent colloidal CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets

We present quantum size-confined colloidal CH₃NH₃PbX₃ (X = Br and I) perovskite nanoplatelets with remarkably high photoluminescence quantum yield up to 90%.

Growth of centimeter-sized [(CH3)2NH2][Mn(HCOO)3] hybrid formate perovskite single crystals and Raman evidence of pressure-induced phase transitions

The fewer the number of the nucleation sites formed in the vessel, the larger the size of the obtained crystals.

Local temperature reduction induced crystallization of MASnI3 and achieving a direct wafer production

A local temperature reduction induced crystallization (LTRIC) method has been developed to prepare CH₃NH₃SnI₃ wafer with 110 μm-thick, which shows great orientation along [001] direction, absorption onset at 1015 nm and a narrow band gap of 1.21 eV.

Blended additive manipulated morphology and crystallinity transformation toward high performance perovskite solar cells

The morphology and crystallinity of MAPbI₃ thin films were regulated using blended-additive engineering for high performance perovskite solar cells.

A micron-scale laminar MAPbBr3 single crystal for an efficient and stable perovskite solar cell

A novel strategy is used to prepare a MAPbBr₃ single-crystal with a controllable thickness of 16 μm and a size of 6 × 8 mm.

Organic Cations Might Not Be Essential to the Remarkable Properties of Band Edge Carriers in Lead Halide Perovskites

A charge carrier in a lead halide perovskite lattice is protected as a large polaron responsible for the remarkable photophysical properties, irrespective of the cation type. All-inorganic-based APbX₃ perovskites may mitigate the stability problem for their applications in solar cells and other optoelectronics.

Insights into charge carrier dynamics in organo-metal halide perovskites: from neat films to solar cells

Various transport measurements for perovskites are reviewed with profound insights into charge dynamics from neat films to solar cells.

Enhancing the performance and stability of carbon-based perovskite solar cells by the cold isostatic pressing method

The cold isostatic pressing method was used as a post-treatment process for enhancing the power conversion efficiency and stability of carbon-based perovskite solar cells without hole transport materials.

Valence band dispersion measurements of perovskite single crystals using angle-resolved photoemission spectroscopy

The ARPES study of perovskite single crystals revealed the band structure along theΓXandΓMdirections.

Ultrathin TiO2nanosheets synthesized using a high pressure solvothermal method and the enhanced photoresponse performance of CH3NH3PbI3–TiO2composite films

Highly crystalline ultrathin (2–3 nm) TiO₂nanosheets are synthesized using a high pressure solvothermal method. The perovskite–TiO₂films exhibit strikingly enhanced photoresponse performance.

Interfacial electronic structures revealed at the rubrene/CH3NH3PbI3 interface

The promising rubrene-based PSC device performance demonstrates the potential of rubrene as a suitable hole transport material in PSCs due to an optimal energy level alignment at the rubrene/CH₃NH₃PbI₃ interface.

Size-dependent one-photon- and two-photon-pumped amplified spontaneous emission from organometal halide CH3NH3PbBr3 perovskite cubic microcrystals

In the past few years, organometal halide light-emitting perovskite thin films and colloidal nanocrystals (NCs) have attracted significant research interest in the field of highly purified illuminating applications.

Current progress and scientific challenges in the advancement of organic–inorganic lead halide perovskite solar cells

The solution-processed organic–inorganic lead halide perovskite solar cells have recently emerged as promising candidates for the conversion of solar power into electricity.

Effects of TiCl4treatment on the structural and electrochemical properties of a porous TiO2layer in CH3NH3PbI3perovskite solar cells

TiCl₄treatment with an appropriate concentration decreases adsorbed water and increases interconnectivity in apTiO₂layer to improve electron transfer ability.

Role of Intrinsic Ion Accumulation in the Photocurrent and Photocapacitive Responses of MAPbBr3 Photodetectors

We studied steady state and transient photocurrents in thin film and single-crystal devices of MAPbBr₃, a prototype organic-inorganic hybrid perovskite. We found that the devices' capacitance is abnormally large, which originates from accumulation of large densities of Pb²⁺ and Br^- in the active perovskite layer. Under applied bias, these ions are driven toward the opposite electrodes leading to space-charge fields close to the metal/perovskite interfaces. The ion accumulation, in turn, causes photocurrent reversal polarity that depends on the history of the applied bias and excitation photon energy with respect to the optical gap. Furthermore, the large capacitive response dominates the transient photocurrent and, therefore, obscures the weaker contribution from the photocarriers' drift. We show that these properties depend on the ambient conditions in which the measurements are performed. Understanding these phenomena may lead to better control over the stability of perovskite photodetectors for visible light.

Ti/Au Cathode for Electronic transport material-free organic-inorganic hybrid perovskite solar cells

We have fabricated organic-inorganic hybrid perovskite solar cell that uses a Ti/Au multilayer as cathode and does not use electron transport materials, and achieved the highest power conversion efficiency close to 13% with high reproducibility and hysteresis-free photocurrent curves. Our cell has a Schottky planar heterojunction structure (ITO/PEDOT:PSS/perovskite/Ti/Au), in which the Ti insertion layer isolate the perovskite and Au layers, thus proving good contact between the Au and perovskite and increasing the cells’ shunt resistance greatly. Moreover, the Ti/Au cathode in direct contact with hybrid perovskite showed no reaction for a long-term exposure to the air, and can provide sufficient protection and avoid the perovskite and PEDOT:PSS layers contact with moisture. Hence, the Ti/Au based devices retain about 70% of their original efficiency after 300 h storage in the ambient environment.

Carrier Decay Properties of Mixed Cation Formamidinium-Methylammonium Lead Iodide Perovskite [HC(NH2)2]1-x[CH3NH3]xPbI3 Nanorods

Organic-inorganic lead iodide perovskites are efficient materials for photovoltaics and light-emitting diodes. We report carrier decay dynamics of nanorods of mixed cation formamidinium and methylammonium lead iodide perovskites [HC(NH₂)₂]_1-x[CH₃NH₃]_xPbI₃ (FA_1-xMA_xPbI₃) synthesized through a simple solution method. The structure and FA/MA composition ratio of the single-crystal FA_1-xMA_xPbI₃ nanorods are fully characterized, which shows that the mixed cation FA_1-xMA_xPbI₃ nanorods are stabilized in the perovskite structure. The photoluminescence (PL) emission from FA_1-xMA_xPbI₃ nanorods continuously shifts from 821 to 782 nm as the MA ratio (x) increases from 0 to 1 and is shown to be inhomogeneously broadened. Time-resolved PL from individual FA_1-xMA_xPbI₃ nanorods demonstrates that lifetimes of mixed cation FA_1-xMA_xPbI₃ nanorods are longer than those of the pure FAPbI₃ or MAPbI₃ nanorods, and the FA_0.4MA_0.6PbI₃ displays the longest average PL lifetime of about 2 μs. These results suggest that mixed cation FA_1-xMA_xPbI₃ perovskites are promising for high-efficiency photovoltaics and other optoelectronic applications.

Decoupling Interfacial Charge Transfer from Bulk Diffusion Unravels Its Intrinsic Role for Efficient Charge Extraction in Perovskite Solar Cells

In a perovskite solar cell, the overall photoinduced charge-transfer (CT) process comprises both charge diffusion through the bulk to perovskite/electrode interfaces and interfacial electron and hole transfer to electrodes. In this study, we decoupled these two entangled processes by investigating the film thickness-dependent CT dynamics from CH₃NH₃PbI₃ perovskites to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (electron acceptor) and spiro-OMeTAD (hole acceptor). By fitting ultrafast transient absorption kinetics to an explicit "diffusion-coupled charge-transfer" model, we found that the charge diffusion from the film interior to perovskite/electrode interfaces took ∼200 ps to a few nanoseconds, depending on the thickness of perovskite film; the subsequent interfacial charge transfer was ultrafast, ∼6 ps for electron transfer to PCBM and ∼8 ps for hole transfer to spiro-OMeTAD, and led to efficient charge extraction (>90%) to electrodes in a 400 nm thick film. Our results indicate that the picosecond interfacial charge transfer is a key to high-performance perovskite solar cells.

Synthesis and Optical Properties of Lead-Free Cesium Tin Halide Perovskite Quantum Rods with High-Performance Solar Cell Application

Herein, the fabrication of a lead-free cesium tin halide perovskite produced via a simple solvothermal process is reported for the first time. The resulting CsSnX₃ (X = Cl, Br, and I) quantum rods show composition-tunable photoluminescence (PL) emissions over the entire visible spectral window (from 625 to 709 nm), as well as significant tunability of the optical properties. In this study, we demonstrate that through hybrid materials (CsSnX₃) with different halides, the system can be tunable in terms of PL. By replacing the halide of the CsSnX₃ quantum rods, a power conversion efficiency of 12.96% under AM 1.5 G has been achieved. This lead-free quantum rod replacement has demonstrated to be an effective method to create an absorber layer that increases light harvesting and charge collection for photovoltaic applications in its perovskite phase.

Imaging the Long Transport Lengths of Photo-generated Carriers in Oriented Perovskite Films

Organometal halide perovskite has emerged as a promising material for solar cells and optoelectronics. Although the long diffusion length of photogenerated carriers is believed to be a critical factor responsible for the material's high efficiency in solar cells, a direct study of carrier transport over long distances in organometal halide perovskites is still lacking. We fabricated highly oriented crystalline CH₃NH₃PbI₃ (MAPbI₃) thin-film lateral transport devices with long channel length (∼120 μm). By performing spatially scanned photocurrent imaging measurements with local illumination, we directly show that the perovskite films prepared here have very long transport lengths for photogenerated carriers, with a minority carrier (electron) diffusion length on the order of 10 μm. Our approach of applying scanning photocurrent microscopy to organometal halide perovskites may be further used to elucidate the carrier transport processes and the vastly different carrier diffusion lengths (∼100 nm to 100 μm) in different types of organometal halide perovskites.

Photocurrent Mapping in Single-Crystal Methylammonium Lead Iodide Perovskite Nanostructures

We investigate solution-grown single-crystal methylammonium lead iodide (MAPbI₃) nanowires and nanoplates with spatially resolved photocurrent mapping. Sensitive perovskite photodetectors with Schottky contacts are fabricated by directly transferring the nanostructures on top of prepatterned gold electrodes. Scanning photocurrent microscopy (SPCM) measurements on these single-crystal nanostructures reveal a minority charge carrier diffusion length up to 21 μm, which is significantly longer than the values observed in polycrystalline MAPbI₃ thin films. When the excitation energy is close to the bandgap, the photocurrent becomes substantially stronger at the edges of nanostructures, which can be understood by the enhancement of light coupling to the nanostructures. These perovskite nanostructures with long carrier diffusion lengths and strong photonic enhancement not only provide an excellent platform for studying their intrinsic properties but may also boost the performance of perovskite-based optoelectronic devices.

Improving the photovoltaic performance of perovskite solar cells with acetate

In an all-solid-state perovskite solar cell, methylammonium lead halide film is in charge of generating photo-excited electrons, thus its quality can directly influence the final photovoltaic performance of the solar cell. This paper accentuates a very simple chemical approach to improving the quality of a perovskite film with a suitable amount of acetic acid. With introduction of acetate ions, a homogeneous, continual and hole-free perovskite film comprised of high-crystallinity grains is obtained. UV-visible spectra, steady-state and time-resolved photoluminescence (PL) spectra reveal that the obtained perovskite film under the optimized conditions shows a higher light absorption, more efficient electron transport, and faster electron extraction to the adjoining electron transport layer. The features result in the optimized perovskite film can provide an improved short-circuit current. The corresponding solar cells with a planar configuration achieves an improved power conversion efficiency of 13.80%, and the highest power conversion efficiency in the photovoltaic measurements is up to 14.71%. The results not only provide a simple approach to optimizing perovskite films but also present a novel angle of view on fabricating high-performance perovskite solar cells.

Crystal Engineering for Low Defect Density and High Efficiency Hybrid Chemical Vapor Deposition Grown Perovskite Solar Cells

Synthesis of high quality perovskite absorber is a key factor in determining the performance of the solar cells. We demonstrate that hybrid chemical vapor deposition (HCVD) growth technique can provide high level of versatility and repeatability to ensure the optimal conditions for the growth of the perovskite films as well as potential for batch processing. It is found that the growth ambient and degree of crystallization of CH₃NH₃PbI₃ (MAPI) have strong impact on the defect density of MAPI. We demonstrate that HCVD process with slow postdeposition cooling rate can significantly reduce the density of shallow and deep traps in the MAPI due to enhanced material crystallization, while a mixed O₂/N₂ carrier gas is effective in passivating both shallow and deep traps. By careful control of the perovskite growth process, a champion device with power conversion efficiency of 17.6% is achieved. Our work complements the existing theoretical studies on different types of trap states in MAPI and fills the gap on the theoretical analysis of the interaction between deep levels and oxygen. The experimental results are consistent with the theoretical predictions.

Formation of Perovskite Heterostructures by Ion Exchange

Thin-film optoelectronic devices based on polycrystalline organolead-halide perovskites have recently become a topic of intense research. Single crystals of these materials have been grown from solution with electrical properties superior to those of polycrystalline films. In order to enable the development of more complex device architectures based on organolead-halide perovskite single crystals, we developed a process to form epitaxial layers of methylammonium lead iodide (MAPbI₃) on methylammonium lead bromide (MAPbBr₃) single crystals. The formation of the MAPbI₃ layer is found to be dominated by the diffusion of halide ions, leading to a shift in the photoluminescence and absorption spectra. X-ray diffraction measurements confirm the single-crystal nature of the MAPbI₃ layer, while carrier transport measurements show that the converted layer retains the high carrier mobility typical of single-crystal perovskite materials. Such heterostructures on perovskite single crystals open possibilities for new types of devices.