Temperature-Induced Lattice Relaxation of Perovskite Crystal Enhances Optoelectronic Properties and Solar Cell Performance

Impact of Residual Strains on the Carrier Mobility and Sability of Perovskite Films

Solution-based inorganic-organic halide perovskites are of great interest to researchers because of their unique optoelectronic properties and easy processing. However, polycrystalline perovskite films often show inhomogeneity due to residual strain induced during the film's post-processing phase. In turn, these strains can impact both their stability and performance. An exhaustive study of residual strains can provide a better understanding and control of how they affect the performance and stability of perovskite films. In this work, we explore this complex interrelationship between residual strains and electrical properties for methylammonium CH3NH3PbI3-xClx films using grazing incidence X-ray diffraction (GIXRD). We correlate their resistivity and carrier mobility using the Hall effect. The sin2(ψ) technique is used to optimize the annealing parameters for the perovskite films. We also establish that temperature-induced relaxation can yield a significant enhancement of the charge carrier transports in perovskite films. Finally, we also use Raman micro-spectroscopy to assess the degradation of perovskite films as a function of their residual strains.

Complete Suppression of Phase Segregation in Mixed-Halide Perovskite Nanocrystals under Periodic Heating

Under continuous light illumination, it is known that localized domains with segregated halide compositions form in semiconducting mixed-halide perovskites, thus severely limiting their optoelectronic applications due to the negative changes in bandgap energies and charge-carrier characteristics. Here mixed-halide perovskite CsPbBr1.2 I1.8 nanocrystals are deposited onto an indium tin oxide substrate, whose temperature can be rapidly changed by ≈10 °C in a few seconds by applying or removing an external voltage. Such a sudden temperature change induces a temporary transition of CsPbBr1.2 I1.8 nanocrystals from the segregated phase to the mixed phase, the latter of which can be permanently maintained when the light illumination is coupled with periodic heating cycles. These findings mark the emergence of a practical solution to the detrimental phase-segregation problem, given that a small temperature modulation is readily available in various fundamental studies and practical devices of mixed-halide perovskites.

Enhanced Phase Stability of Compressive Strain-Induced Perovskite Crystals

Control of strain in perovskite crystals has been considered as an effective strategy to ensure the phase stability of perovskite films where a compressive strain is particularly preferred over a tensile strain due to a lowered Gibbs free energy by the unit cell contraction effect. Here we adapt the strategy of strain control into perovskite solar cells in which the compressive strain is applied by utilizing a thermal expansion difference between the perovskite film and an adjacent layer. Poly(4-butylphenyldiphenylamine), with a higher thermal expansion coefficient compared to that of perovskite, is employed as a substrate for perovskite crystal growth at 100 °C, followed by cooling to room temperature. The applied compressive strain at the interface, as a result of a greater contraction of the polymer compared to the perovskite film, is confirmed by grazing incidence X-ray diffraction showing a red peak shift with increasing secondary angle. The compressive strain-induced perovskite film shows relatively constant absorbance spectra as a function of time. In the meantime, the absorbance spectra of a film without strain control exhibit a gradual decay with developing an Urbach tail. Importantly, the effect of strain engineering is remarkably prominent in the long-term photovoltaic performance. The photocurrent drops by 41% over 911 h without controlling strain, which is significantly improved by employing compressive strain, showing only a 6% drop in photocurrent from a shelf-stability test without encapsulation. It is also noted that an S-shaped kink appears in the current-voltage curves since 579-h-long storage for the device without strain control, leading to unreliable and overestimated fill factor and conversion efficiency. On the other hand, a 16% increase in fill factor with a stable performance is derived over 911 h from the compressive strain-induced device.

Tailored Lattice "Tape" to Confine Tensile Interface for 11.08%-Efficiency All-Inorganic CsPbBr3 Perovskite Solar Cell with an Ultrahigh Voltage of 1.702 V

The crystal distortion such as lattice strain and defect located at the surfaces and grain boundaries induced by soft perovskite lattice highly determines the charge extraction-transfer dynamics and recombination to cause an inferior efficiency of perovskite solar cells (PSCs). Herein, the authors propose a strategy to significantly reduce the superficial lattice tensile strain by means of incorporating an inorganic 2D Cl-terminated Ti3 C2 (Ti3 C2 Clx ) MXene into the bulk and surface of CsPbBr3 film. Arising from the strong interaction between Cl atoms in Ti3 C2 Clx and the under-coordinated Pb2+ in CsPbBr3 lattice, the expanded perovskite lattice is compressed and confined to act as a lattice "tape", in which the PbCl bond plays a role of "glue" and the 2D Ti3 C2 immobilizes the lattice. Finally, the defective surface is healed and a champion efficiency as high as 11.08% with an ultrahigh open-circuit voltage up to 1.702 V is achieved on the best all-inorganic CsPbBr3 PSC, which is so far the highest efficiency record for this kind of PSCs. Furthermore, the unencapsulated device demonstrates nearly unchanged performance under 80% relative humidity over 100 days and 85 °C over 30 days.

Ab initio nonadiabatic molecular dynamics of charge carriers in metal halide perovskites

Photoinduced nonequilibrium processes in nanoscale materials play key roles in photovoltaic and photocatalytic applications. This review summarizes recent theoretical investigations of excited state dynamics in metal halide perovskites (MHPs), carried out using a state-of-the-art methodology combining nonadiabatic molecular dynamics with real-time time-dependent density functional theory. The simulations allow one to study evolution of charge carriers at the ab initio level and in the time-domain, in direct connection with time-resolved spectroscopy experiments. Eliminating the need for the common approximations, such as harmonic phonons, a choice of the reaction coordinate, weak electron-phonon coupling, a particular kinetic mechanism, and perturbative calculation of rate constants, we model full-dimensional quantum dynamics of electrons coupled to semiclassical vibrations. We study realistic aspects of material composition and structure and their influence on various nonequilibrium processes, including nonradiative trapping and relaxation of charge carriers, hot carrier cooling and luminescence, Auger-type charge-charge scattering, multiple excitons generation and recombination, charge and energy transfer between donor and acceptor materials, and charge recombination inside individual materials and across donor/acceptor interfaces. These phenomena are illustrated with representative materials and interfaces. Focus is placed on response to external perturbations, formation of point defects and their passivation, mixed stoichiometries, dopants, grain boundaries, and interfaces of MHPs with charge transport layers, and quantum confinement. In addition to bulk materials, perovskite quantum dots and 2D perovskites with different layer and spacer cation structures, edge passivation, and dielectric screening are discussed. The atomistic insights into excited state dynamics under realistic conditions provide the fundamental understanding needed for design of advanced solar energy and optoelectronic devices.

Photovoltaic Effect Related to Methylammonium Cation Orientation and Carrier Transport Properties in High-Performance Perovskite Solar Cells

Solar cells based on organic-inorganic hybrid halide perovskites (OIHHPs) have been widely studied because of their increasing power conversion efficiency. Extensive research has been conducted in electrical and optical properties and device fabrication. However, in terms of material science, the photovoltaic effects of OIHHP are still not well understood. Here, we theoretically investigate the photovoltaic phenomena of MAPbI₃ (MA = CH₃NH₃⁺) under standard AM 1.5G sunlight illumination, considering the MA cation orientation, light incident angle, polarization, and photon energy, using Keldysh nonequilibrium Green's function formalism combined with density functional theory calculations. It is found that the short-circuit current density J_sc has a maximum value of 383.149 A/m² when the MA orientation is parallel to the transport direction, whereas it is negligible when the MA orientation is orthogonal to the transport direction. In addition, full consideration is also given to the direction of incidence of sunlight and its polarization state. Nevertheless, of all factors considered, MA orientation plays the decisive role, for J_sc still has a respectable value of 364.112 A/m² even for a 90° sunlight incident angle relative to the transport direction, so long as the MAs are aligned in the transport direction. The increase in the photocurrent is attributed to an increase in the transmission coefficient of low-energy holes, as well as improvement of the velocities and mobilities of electrons and holes in the MAPbI₃-based device with [001] MA orientation. The results suggest that during the designing of high-performance OIHHP-based solar cells and photoelectronic devices, the crystal orientation and MA cation orientation relative to the transport direction should be carefully considered as they directly affect carrier transport properties.

Interfacial Residual Stress Relaxation in Perovskite Solar Cells with Improved Stability

To improve the photovoltaic performance (both efficiency and stability) in hybrid organic-inorganic halide perovskite solar cells, perovskite lattice distortion is investigated with regards to residual stress (and strain) in the polycrystalline thin films. It is revealed that residual stress is concentrated at the surface of the as-prepared film, and an efficient method is further developed to release this interfacial stress by A site cation alloying. This results in lattice reconstruction at the surface of polycrystalline thin films, which in turn results in low elastic modulus. Thus, a "bone-joint" configuration is constructed within the interface between the absorber and the carrier transport layer, which improves device performance substantially. The resultant photovoltaic devices exhibit an efficiency of 21.48% with good humidity stability and improved resistance against thermal cycling.

Strain engineering in perovskite solar cells and its impacts on carrier dynamics

The mixed halide perovskites have emerged as outstanding light absorbers for efficient solar cells. Unfortunately, it reveals inhomogeneity in these polycrystalline films due to composition separation, which leads to local lattice mismatches and emergent residual strains consequently. Thus far, the understanding of these residual strains and their effects on photovoltaic device performance is absent. Herein we study the evolution of residual strain over the films by depth-dependent grazing incident X-ray diffraction measurements. We identify the gradient distribution of in-plane strain component perpendicular to the substrate. Moreover, we reveal its impacts on the carrier dynamics over corresponding solar cells, which is stemmed from the strain induced energy bands bending of the perovskite absorber as indicated by first-principles calculations. Eventually, we modulate the status of residual strains in a controllable manner, which leads to enhanced PCEs up to 20.7% (certified) in devices via rational strain engineering.

The residual strains in the mixed halide perovskite thin films and their effects on the solar cell devices are less understood. Here Zhu et al. study the impact of the gradient in-plane strain on the carrier dynamics of the strained perovskite films and optimize the device efficiency.

Single-crystalline perovskite wafers with a Cr blocking layer for broad and stable light detection in a harsh environment

Herein, ultrathin (∼35 μm) CH3NH3PbI3 (MAPbI3) single-crystalline wafers have been successfully prepared by using an appropriate geometry-regulated dynamic-flow reaction system. The measurement results proved that the obtained wafers have high crystallinity, and showed broad light absorption from ultraviolet to near infrared (850 nm) which can be attributed to the indirect bandgap. Straight after, such an MAPbI3 wafer was used to fabricate high-quality photodetectors (PDs). On account of its faster carrier transport and significantly reduced defect density, the device exhibits a high photoresponse (R) of 5 A/W and short on/off response (0.039 s/0.017 s). Interestingly, by introducing a Cr interlayer between the MAPbI3 wafer and the Au electrode to avoid the migration of Au, the PD shows nearly no degradation when it works at 200 °C. Furthermore, the device performance shows very little degradation over the course of 60 days of storage under ambient conditions owing to its lack of grain boundaries. We believe the strategy reported here is very promising for achieving broad photodetection in a harsh environment.

Concurrent Ultrafast Electron- and Hole-Transfer Dynamics in CsPbBr3 Perovskite and Quantum Dots

Ultrafast charge-transfer (i.e., electron and hole) dynamics has been investigated between the cesium lead bromide (CsPbBr₃, CPB) perovskite nanocrystals (NCs) and cadmium selenide (CdSe) quantum dots (QDs) as a new composite material for photocatalytic and photovoltaic applications. The CPB NCs have been synthesized and characterized by high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD) pattern. The redox levels (i.e., conduction band (CB) and valence band (VB)) of the CPB NCs and CdSe QDs suggest the feasibility of photoexcited electron transfer from CPB NCs to CdSe QDs and photoexcited hole transfer from CdSe QDs to CPB NCs, and it has been confirmed by both steady-state and time-resolved spectroscopy. To investigate the electron- and hole-transfer dynamics in ultrafast time scale, we have performed femtosecond up-conversion and femtosecond transient absorption studies. The measured electron-transfer time from CPB NCs to CdSe QDs and hole-transfer time from CdSe QDs to CPB NCs were found to be 550 and 750 fs, respectively. Interestingly, the charge-transfer process found to be restricted in CPB/CdSe@CdS core-shell system where electron transfer from CPB NCs to core shell takes place, but the hole transfer from core shell to CPB NCs found to be restricted due to CdS shell making the process thermodynamically nonviable. Our observation has suggested that after the photoexcitation of CPB NCs/CdSe QDs composite system, a charge-separated state is formed where the electrons are localized in CB of CdSe QDs and holes are localized in VB of CPB NCs. This makes the composite system a better material for efficient light harvesting and photocatalytic material as compared to the individual ones.

Continuous-wave optically pumped green perovskite vertical-cavity surface-emitter

We report an optically pumped green perovskite vertical-cavity surface-emitter operating in continuous-wave (CW) with a power density threshold of ∼89  kW/cm². The device has an active region of CH₃NH₃PbBr₃ embedded in a dielectric microcavity; this feat was achieved with a combination of optimal spectral alignment of the optical cavity modes with the perovskite optical gain, an adequate Q-factor of the microcavity, adequate thermal stability, and improved material quality with a smooth, passivated, and annealed thin active layer. Our results signify a way towards efficient CW perovskite emitter operation and electrical injection using low-cost fabrication methods for addressing monolithic optoelectronic integration and lasing in the green gap.

Modulated CH3NH3PbI3-xBrx film for efficient perovskite solar cells exceeding 18

The organic-inorganic lead halide perovskite layer is a crucial factor for the high performance perovskite solar cell (PSC). We introduce CH₃NH₃Br in the precursor solution to prepare CH₃NH₃PbI_3−xBr_x hybrid perovskite, and an uniform perovskite layer with improved crystallinity and apparent grain contour is obtained, resulting in the significant improvement of photovoltaic performance of PSCs. The effects of CH₃NH₃Br on the perovskite morphology, crystallinity, absorption property, charge carrier dynamics and device characteristics are discussed, and the improvement of open circuit voltage of the device depended on Br doping is confirmed. Based on above, the device based on CH₃NH₃PbI_2.86Br_0.14 exhibits a champion power conversion efficiency (PCE) of 18.02%. This study represents an efficient method for high-performance perovskite solar cell by modulating CH₃NH₃PbI_3−xBr_x film.

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

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.