Steady state and transient absorption spectroscopy in metal halide perovskites

Metal halide perovskites (MHPs) have emerged as the most promising materials due to superior optoelectronic properties and great applications spanning from photovoltaics to photonics. Absorption spectroscopy provides a broad and deep insight into the carrier dynamics of MHPs, and is a critical complement to fluorescence and scattering spectroscopy. However, absorption spectroscopy is often misunderstood or underestimated, being seen as UV-vis spectroscopy only, which can lead to various misinterpretations. In fact, absorption spectroscopy is one of the most important branches of spectroscopic techniques (others including fluorescence and scattering), which plays a critical role in understanding the electronic structure and optoelectrical dynamics of MHPs. In this tutorial, the basic principles of various types of absorption spectroscopy as well as their recent developments and applications in MHP materials and devices are summarized, covering comprehensive advances in steady state and transient absorption spectroscopy. Given the significance of absorption spectroscopy in directing the design of different optoelectronic applications of MHPs, this tutorial will comprehensively discuss absorption spectroscopy, covering wavelengths from optical to terahertz (THz) and microwave, and timescales from femtoseconds to hours, and it specifically focuses on time-dependent steady-state and transient absorption spectroscopy under light illumination bias to study MHP materials and devices, allowing researchers to select suitable characterization techniques.

Eliminating performance loss from perovskite films to solar cells

Preoptimizing perovskite films may generally improve the performance of the final perovskite solar cells (PSCs). However, the research on whether the film optimization fully contributes to the enhancement of the final PSCs has been long neglected. We demonstrated that the preparation of metal electrodes by high-vacuum thermal evaporation, an unavoidable step in almost all device fabrication processes, will damage the surface of perovskite films, resulting in component escape, defect density rebound, carrier extraction barrier, and film stability deterioration. Therefore, the prepared perovskite film and the final film actually working in devices are not exactly the same, and the contribution of film optimization to the device improvement was weakened. We designed a bilayer structure composed of graphene oxide and graphite flakes to eliminate the unwanted film inconsistencies and thus save the film optimization loss. Therefore, the efficient PSCs with power conversion efficiency of 25.55% were obtained, which demonstrated negligible photovoltaic performance loss after operating for 2000 hours.

Hole Localization in Bulk and 2D Lead-Halide Perovskites Studied by Time-Resolved Infrared Spectroscopy

Scattering and localization dynamics of charge carriers in the soft lattice of lead-halide perovskites impact polaron formation and recombination, which are key mechanisms of material function in optoelectronic devices. In this study, we probe the photoinduced lattice and carrier dynamics in perovskite thin films (CsFAPbX3, X = I, Br) using time-resolved infrared spectroscopy. We examine the CN stretching mode of formamidinium (FA) cations located within the lead-halide octahedra of the perovskite structure. Our investigation reveals the formation of an infrared mode due to spatial symmetry breaking within a hundred picoseconds in 3D perovskites. Experiments at cryogenic temperatures show much-reduced carrier localization, in agreement with a localization mechanism that is driven by the dynamic disorder. We extend our analysis to 2D perovskites, where the precise nature of charge carriers is uncertain. Remarkably, the signatures of charge localization we found in bulk perovskites are not observed for 2D Ruddlesden-Popper perovskites ((HexA)2FAPb2I7). This observation implies that the previously reported stabilization of free charge carriers in these materials follows different mechanisms than polaron formation in bulk perovskites. Through the exploration of heterostructures with electron/hole excess, we provide evidence that holes drive the formation of the emerging infrared mode.

Charge Transfer Kinetics in Halide Perovskites: On the Constraints of Time-Resolved Spectroscopy Measurements

Understanding photophysical processes in lead halide perovskites is an important aspect of optimizing the performance of optoelectronic devices. The determination of exact charge carrier extraction rate constants remains elusive, as there is a large and persistent discrepancy in the reported absolute values. In this review, we concentrate on experimental procedures adopted in the literature to obtain kinetic estimates of charge transfer processes and limitations imposed by the spectroscopy technique employed. Time-resolved techniques (e.g., transient absorption-reflection and time-resolved photoluminescence spectroscopy) are commonly employed to probe charge transfer at perovskite/transport layer interfaces. The variation in sample preparation and measurement conditions can produce a wide dispersion of the measured kinetic parameters. The selected time window and the kinetic fitting model employed introduce additional uncertainty. We discuss here evaluation strategies that rely on multiexponential fitting protocols (regular or stretched) and show how the dispersion in the reported values for carrier transfer rate constants can be resolved.

Dye-induced photoluminescence quenching of quantum dots: role of excited state lifetime and confinement of charge carriers

We investigate the role of quantum confinement and photoluminescence (PL) lifetime of photoexcited charge carriers in semiconductor core/shell quantum dots (QDs) via PL quenching due to surface modification. Surface modification is controlled by varying the number of dye molecules adsorbed onto the QD shell surface forming QD-dye nanoassemblies. We selected CuInS₂/ZnS (CIS) and InP/ZnS (InP) core/shell QDs exhibiting relatively weak (664 meV) and strong (1194 meV) confinement potentials for the conduction band electron. Moreover, the difference in the emission mechanism gives rise to a long and short excited state lifetime of CIS (ca. 290 ns) and InP (ca. 37 ns) QDs. Dye molecules of different ionic characters (rhodamine 575: zwitterionic and rhodamine 560: cationic) are used as quenchers. A detailed analysis of Stern-Volmer data shows that (i) quenching is generally more pronounced in CIS-dye assemblies as compared to InP-dye assemblies, (ii) dynamic quenching is dominating in all QD-dye assemblies with only a minor contribution from static quenching and (iii) the cationic dye shows a stronger interaction with the QD shell surface than the zwitterionic dye. Observations (i) and (ii) can be explained by the differences in the amplitude of the electronic component of the exciton wavefunction near the dye binding sites in both QDs, which results in the breaking up of the electron-hole pair and favors charge trapping. Observation (iii) can be attributed to the variations in electrostatic interactions between the negatively charged QD shell surface and the cationic and zwitterionic dyes, with the former exhibiting a stronger interaction. Moreover, the long lifetime of CIS QDs facilitates us to easily probe different time scales of the trapping processes and thus differentiate the origins of static and dynamic quenching components that appear in the Stern-Volmer analysis.

Direct Tracking of Charge Carrier Drift and Extraction from Perovskite Solar Cells by Means of Transient Electroabsorption Spectroscopy

The best perovskite solar cells currently demonstrate more than 25% efficiencies, yet many fundamental processes that determine the operation of these devices are still not fully understood. In particular, even though the device performance strongly depends on charge carrier transport across the perovskite layer to selective electrodes, information about this process is still very controversial. Here, we investigate charge carrier motion and extraction from an archetypical CH3NH3PbI3 (MAPI) perovskite solar cell. We use the ultrafast electric-field-modulated transient absorption technique, which allows us to evaluate the electric field dynamics from the time-resolved electroabsorption spectra and to visualize the motion of charge carriers with subpicosecond time resolution. We demonstrate that photogenerated holes drift across the mesoporous TiO2/perovskite layer during hundreds of picoseconds. On the other hand, their extraction into the spiro-OMeTAD hole transporting layer lasts for more than 1 nanosecond, suggesting that the hole extraction is limited by the perovskite/spiro-OMeTAD interface rather than by the hole transport through the perovskite layer. Additionally, we use the ultrafast time-resolved fluorescence technique that reveals fluorescence decay during tens of picoseconds, which we attribute to the spatial separation of electrons and holes.

Deep Blue and Highly Emissive ZnS-Passivated InP QDs: Facile Synthesis, Characterization, and Deciphering of Their Ultrafast-to-Slow Photodynamics

InP-based quantum dots (QDs) are an environment-friendly alternative to their heavy metal-ion-based counterparts. Herein we report a simple procedure for synthesizing blue emissive InP QDs using oleic acid and oleylamine as surface ligands, yielding ultrasmall QDs with average sizes of 1.74 and 1.81 nm, respectively. Consecutive thin coating with ZnS increased the size of these QDs to 4.11 and 4.15 nm, respectively, alongside a significant enhancement of their emission intensities centered at ∼410 nm and ∼430 nm, respectively. Pure phase synthesis of these deep-blue emissive QDs is confirmed by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Armed with femtosecond to millisecond time-resolved spectroscopic techniques, we decipher the energy pathways, reflecting the effect of successive ZnS passivation on the charge carrier (electrons and holes) dynamics in the deep-blue emissive InP, InP/ZnS, and InP/ZnS/ZnS QDs. Successive coating of the InP QDs increases the intraband relaxation times from 200 to 700 fs and the lifetime of the hot electrons from 2 to 8 ps. The lifetime of the cold holes also increase from 1 to 4 ps, and remarkably, the Auger recombination escalates from 15 to 165 ps. The coating also drastically decreases the quenching by the molecular oxygen of the trapped charge carriers at the surfaces of the QDs. Our results provide clues to push further the emission of InP QDs into more energetically spectral regions and to increase the fluorescence quantum yield, targeting the construction of efficient UV-emissive light-emitting devices (LEDs).

Quantifying Charge Carrier Recombination Losses in MAPbI3/C60 and MAPbI3/Spiro-OMeTAD with and without Bias Illumination

To increase the open-circuit voltage in perovskite-based solar cells, recombination processes at the interface with transport layers (TLs) should be identified and reduced. We investigated the charge carrier dynamics in bilayers of methylammonium lead iodide (MAPbI3) with C60 or Spiro-OMeTAD using time-resolved microwave conductance (TRMC) measurements with and without bias illumination (BI). By modeling the results, we quantified recombination losses in bare MAPbI3 and extraction into the TLs. Only under BI did we find that the density of deep traps increases in bare MAPbI3, substantially enhancing trap-mediated losses. This reversible process is prevented in a bilayer with C60 but not with Spiro-OMeTAD. While under BI extraction rates reduce significantly in both bilayers, only in MAPbI3/Spiro-OMeTAD does interfacial recombination also increases, substantially reducing the quasi Fermi level splitting. This work demonstrates the impact of BI on charge dynamics and shows that adjusting the Fermi level of TLs is imperative to reduce interfacial recombination losses.

There is plenty of room at the top: generation of hot charge carriers and their applications in perovskite and other semiconductor-based optoelectronic devices

Hot charge carriers (HC) are photoexcited electrons and holes that exist in nonequilibrium high-energy states of photoactive materials. Prolonged cooling time and rapid extraction are the current challenges for the development of future innovative HC-based optoelectronic devices, such as HC solar cells (HCSCs), hot energy transistors (HETs), HC photocatalytic reactors, and lasing devices. Based on a thorough analysis of the basic mechanisms of HC generation, thermalization, and cooling dynamics, this review outlines the various possible strategies to delay the HC cooling as well as to speed up their extraction. Various materials with slow cooling behavior, including perovskites and other semiconductors, are thoroughly presented. In addition, the opportunities for the generation of plasmon-induced HC through surface plasmon resonance and their technological applications in hybrid nanostructures are discussed in detail. By judiciously designing the plasmonic nanostructures, the light coupling into the photoactive layer and its optical absorption can be greatly enhanced as well as the successful conversion of incident photons to HC with tunable energies can also be realized. Finally, the future outlook of HC in optoelectronics is highlighted which will provide great insight to the research community.

Spectroscopic investigations of electron and hole dynamics in MAPbBr3 perovskite film and carrier extraction to PEDOT hole transport layer

Organometallic halide perovskite (MAPPbBr3), Rust-based Vapor Phase Polymerization (RVPP)-PEDOT hole transporting layers and (RVPP-PEDOT)/MAPPbBr3 dual-layer, deposited on fluorine doped tin oxide glass were studied at room temperature using steady-state absorption, time-resolved photoluminescence imaging and femtosecond time-resolved absorption spectroscopy. Application of these techniques in conjunction with diverse excitation intensities allowed determination of various optoelectronic properties of the perovskite film and the time constant of the hole extraction process. Spectral reconstruction of the bandedge absorption spectrum using Elliot's formula enabled separation of the exciton band. The binding energy of the exciton was determined to be 19 meV and the bandgap energy of the perovskite film was 2.37 eV. Subsequent time-resolved photoluminescence studies of the perovskite film performed using a very weak excitation intensity followed by a global analysis of the data revealed monomolecular recombination dynamics of charge carriers occurring with an amplitude weighted lifetime of 3.2 ns. Femtosecond time-resolved transient absorption of the film performed after excitation intensity spanning a range of over two orders of magnitude enabled determining the rate constant of bimolecular recombination and was found to be 2.6 × 10-10 cm3 s-1. Application of numerous high intensity excitations enabled observation of band filling effect and application of the Burstein-Moss model allowed to determine the reduced effective mass of photoexcited electron-hole pair in MAPPbBr3 film to be 0.19 rest mass of the electron. Finally, application of transient absorption on RVPP-PEDOT/MAPPbBr3 enabled determination of a 0.4 ps time constant for the MAPPbBr3-to-PEDOT hole extraction process.

Impact of improvements in mesoporous titania layers on ultrafast electron transfer dynamics in perovskite and dye-sensitized solar cells

Improvement in the performance of perovskite solar cells (PSC) and dye-sensitized solar cells (DSSC) upon modifications of mesoporous titania layers has been studied. For PSC with triple cation perovskite (FA0.76 MA0.19 Cs0.05 Pb (I0.81 Br0.19)3) about 40% higher photocurrent (up to ∼24 mA cm-2) was found for more homogenous, made of larger particles (30 nm) and thinner (150-200 nm) titania layer. For DSSC (both with liquid cobalt-based electrolyte as well as with solid state hole transporter - spiro-OMeTAD), a greater dye loading, rise in photovoltage, and the enhancement in relative photocurrent were observed for the cells prepared from the diluted titania paste (2 : 1 w/w ratio) with respect to those prepared from undiluted one. The impact of these improvements in titania layers on charge transfer dynamics in the complete solar cells as well as in pristine TiO2 layers was investigated by femtosecond transient absorption. Shorter photocarriers lifetime in perovskite material observed in better PSC, indicated that faster electron transfer at the titania interface was responsible for the higher photocurrent. Moreover, the photoinduced changes close to TiO2 interface were revealed in better PSC, which may indicate that in the efficient devices halide segregation takes place in perovskite material. In liquid DSSC, the fast component of unwanted recombination was slower in the samples with the diluted titania paste than in those made with undiluted ones. In solid state DSSC, hole injection from MK2 dye to spiro-OMeTAD takes place on the very fast ps time scale (comparable to that of electron injection) and the evidence of better penetration of spiro-OMeTAD into thinner and more porous titania layers was provided.

Hot Carrier Cooling and Recombination Dynamics of Chlorine-Doped Hybrid Perovskite Single Crystals

Controlling the photoexcited properties and behavior of hybrid perovskites by halide doping has the potential to impact a wide range of emerging technologies, including solar cells and radiation detectors. Crystalline samples of methylammonium lead bromide substituted with chlorine (MAPbBr_3-xCl_x) were examined by transient reflectivity spectroscopy and nonadiabatic molecular dynamics simulations. At picosecond time scales, the addition of chlorine to the perovskite crystal increased the observed rate of hot carrier cooling and the calculated electron-phonon coupling constants. Chlorine-doped samples also exhibit a slower surface recombination velocity and a smaller ambipolar mobility.

Photoinduced Dynamics of Charge Carriers in Metal Halide Perovskites from an Atomistic Perspective

Perovskite solar cells have attracted intense attention over the past decade because of their low cost, abundant raw materials, and rapidly growing power conversion efficiency (PCE). However, nonradiative charge carrier losses still constitute a major factor limiting the PCE to well below the Shockley-Queisser limit. This Perspective summarizes recent atomistic quantum dynamics studies on the photoinduced excited-state processes in metal halide perovskites (MHPs), including both hybrid organic-inorganic and all-inorganic MHPs and three- and two-dimensional MHPs. The simulations, performed using a combination of time-domain ab initio density functional theory and nonadiabatic molecular dynamics, allow emphasis on various intrinsic and extrinsic features, such as components, structure, dimensionality and interface engineering, control and exposure to various environmental factors, defects, surfaces, and their passivation. The detailed atomistic simulations advance our understanding of electron-vibrational dynamics in MHPs and provide valuable guidelines for enhancing the performance of perovskite solar cells.

Reducing Amplified Spontaneous Emission Threshold in CsPbBr3 Quantum Dot Films by Controlling TiO2 Compact Layer

Amplified spontaneous emission (ASE) threshold in CsPbBr3 quantum dot films is systematically reduced by introducing high quality TiO2 compact layer grown by atomic-layer deposition. Uniform and pinhole-free TiO2 films of thickness 10, 20 and 50 nm are used as a substrates for CsPbBr3 quantum dot films to enhance amplified spontaneous emission performance. The reduction is attributed indirectly to the improved morphology of TiO2 compact layer and subsequently CsPbBr3 active layer as grown on better quality substrates. This is quantified by the reduced roughness of the obtained films to less than 5 nm with 50 nm TiO2 substrate. Considering the used growth method for the quantum dot film, the improved substrate morphology maintains better the structure of the used quantum dots in the precursor solution. This results in better absorption and hence lower threshold of ASE. Besides that, the improved film quality results further in reducing light scattering and hence additional slight optical enhancement. The work demonstrates a potential venue to reduce the amplified spontaneous emission threshold of quantum dot films and therefore enhanced their optical performance.

Contrasting Electron and Hole Transfer Dynamics from CH(NH2)2PbI3 Perovskite Quantum Dots to Charge Transport Layers

In this work, the ultrafast transient absorption spectroscopy (TAs) was utilized to first investigate the charge transfer from the emerging FAPbI3 (FA = CH(NH2)2) perovskite quantum dots (PQDs) to charge transport layers. Specifically, we compared the TAs in pure FAPbI3 PQDs, PQDs grown with both electron and hole transfer layers (ETL and HTL), and PQDs with only ETL or HTL. The TA signals induced by photoexcited electrons decay much faster in PQDs samples with the ETL (~20 ps) compared to the pure FAPbI3 PQDs (>1 ns). These results reveal that electrons can effectively transport between coupled PQDs and transfer to the ETL (TiO2) at a time scale of 20 ps, much faster than the bimolecular charge recombination inside the PQDs (>1 ns), and the electron transfer efficiency is estimated to be close to 100%. In contrast, the temporal evolution of the TA signals in the PQDs with and without HTL exhibit negligible change, and no substantive hole transfer to the HTL (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], PTAA) occurs within 1 ns. The much slower hole transfer implies the further potential of increasing the overall photo-carrier conversion efficiency through enhancing the hole diffusion length and fine-tuning the coupling between the HTL and PQDs.

Reducing Anomalous Hysteresis in Perovskite Solar Cells by Suppressing the Interfacial Ferroelectric Order

Despite the booming research in organometal halide perovskite solar cells (PSCs) of recent years, considerable roadblocks remain for their large-scale deployment, ranging from undesirable current-voltage hysteresis to inferior device stability. Among various plausible origins of hysteresis, interfacial ferroelectricity is particularly intriguing and warrants a close scrutiny. Here, we examine interfacial ferroelectricity in MAPbI₃ (FAPbI₃)/TiO₂ and MAPbI₃/phenyl-C61-butyric-acid-methyl-ester (PCBM) heterostructures and explore the correlations between the interfacial ferroelectricity and the hysteresis from the perspective of nonadiabatic electronic dynamics. It is found that the ferroelectric order develops at the MAPbI₃/TiO₂ interface owing to the interaction between the polar MA ions and TiO₂. The polarization switching of the MA ions under an applied gate field would drastically result in different rates in interfacial photoelectron injection and electron-hole recombination, contributing to the undesirable hysteresis. In sharp contrast, ferroelectricity is suppressed at the FAPbI₃/TiO₂ and MAPbI₃/PCBM interfaces, thanks to elimination of the interfacial electric field between perovskite and TiO₂ via substitution of strong polar MA (dipole moment: 2.29 debye) by weak polar FA ions (dipole moment: 0.29 debye) and interface passivation, leading to consistent interfacial electronic dynamics and the absence of hysteresis. The present work sheds light on the physical cause for hysteresis and points to the direction to which the hysteresis could be mitigated in PSCs.

Improved Carrier Collection and Hot Electron Extraction Across Perovskite, C60, and TiO2 Interfaces

The use of C₆₀ as an interfacial layer between TiO₂ and methylammonium lead iodide perovskite is probed to reduce the current-voltage hysteresis in perovskite solar cells (PSCs) and, in turn, to impact the interfacial carrier injection and recombination processes that limit solar cell efficiencies. Detailed kinetic analyses across different time scales, that is, from the femtoseconds to the seconds, reveal that the charge carrier lifetimes as well as the charge injection and charge recombination dynamics depend largely on the presence or absence of C₆₀. In addition, we corroborate that C₆₀ is applicable in hot carrier PSCs as it is capable of extracting hot carriers generated throughout the early time scales following photoexcitation.

Investigating ultrafast carrier dynamics in perovskite solar cells with an extended π-conjugated polymeric diketopyrrolopyrrole layer for hole transportation

Here, we show a new diketopyrrole based polymeric hole-transport material (PBDTP-DTDPP, (poly[[2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[[2,2′-(4,8-bis(4-ethylhexyl-1-phenyl)-benzo[1,2-b:4,5-b′]dithiophene)bis-thieno[3,2-b]thiophen]-5,5′-diyl]])) for application in perovskite solar cells. The material performance was tested in a solar cell with an optimized configuration, FTO/SnO₂/perovskite/PBDTP-DTDPP/Au, and the device showed a power conversion efficiency of 14.78%. The device charge carrier dynamics were investigated using transient absorption spectroscopy. The charge separation and recombination kinetics were determined in a device with PBDTP-DTDPP and the obtained results were compared to a reference device. We find that PBDTP-DTDPP enables similar charge separation time (<∼4.8 ps) to the spiro-OMeTAD but the amount of nongeminate recombination is different. Specifically, we find that the polymeric PBDTP-DTDPP hole-transport layer (HTL) slows-down the second-order recombination much less than spiro-OMeTAD. This effect is of particular importance in studying the charge transportation in optimized solar cell devices with diketopyrrole based HTL materials.

Diketopyrrole based hole-transport organic semiconductor was employed in perovskite solar cells and charge carrier dynamics was explained.

Hot electron injection into semiconducting polymers in polymer based-perovskite solar cells and their fate

Metal halide perovskites are known to possess upon photoexcitation long-lived hot carriers. By using femtosecond laser transient absorption spectroscopy, we probed in the current work interfacial charge transfer, that is, hot electrons and holes in methylammonium lead iodide perovskite. The focus was, on the one hand, on titanium dioxide as an electron transporting material and, on the other hand, on several organic semiconducting materials as hole transporting materials in perovskite solar cells. An unexpected carrier loss pathway for hot electrons was found in the form of injection into the low lying LUMOs of several organic semiconducting materials. Of great importance is the fact that the final photocurrents of perovskite solar cells scale with the suppression of this newly discovered loss pathway.

Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their V_OC to values well below the Shockley-Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi-Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the V_OC of operational devices. These measurements prove that in state-of-the-art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump-probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome-paving the way to the thermodynamic efficiency limit.

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

Metal halide perovskites (MHPs) have recently attracted great attention from the scientific community due to their excellent photovoltaic performance as well as their tremendous potential for other optoelectronic applications such as light-emitting diodes, lasers, and photodetectors. Despite the rapid progress in device applications, a solid understanding of the photophysical properties behind the device performance is highly desirable for MHPs. Here, the properties of excitons and photogenerated charge carriers in MHPs are explored. The unique dielectric constant properties, crystal-liquid duality, and fundamental optical processes of MHPs are first discussed. The properties of excitons and related phenomena in MHPs are then detailed, including the exciton binding energy determined by various methods and their influence factors, exciton dynamics, exciton-photon coupling and related applications, and exciton-phonon coupling in MHPs. The properties of photogenerated free charge carriers in MHPs such as the carrier diffusion length, mobility, and recombination are described. Recent progress in various applications is also demonstrated. Finally, a conclusion and perspectives of future studies for MHPs are presented.

Origin of Extended UV Stability of 2D Atomic Layer Titania-Based Perovskite Solar Cells Unveiled by Ultrafast Spectroscopy

The inherent instability of UV-induced degradation in TiO₂-based perovskite solar cells was largely improved by replacing the anatase-phase compact TiO₂ layer with an atomic sheet transport layer (ASTL) of two-dimensional (2D) Ti_1-δO₂. The vital role of microscopic carrier dynamics that govern the UV stability of perovskite solar cells was comprehensively examined in this work by performing time-resolved pump-probe spectroscopy. In conventional perovskite solar cells, the presence of a UV-active oxygen vacancy in compact TiO₂ prohibits current generation by heavily trapping electrons after UV degradation. Conversely, the dominant vacancy type in the 2D Ti_1-δO₂ ASTL is a titanium vacancy, which is a shallow acceptor and is not UV-sensitive. Therefore, it significantly suppresses carrier recombination and extends UV stability in perovskite solar cells with a 2D Ti_1-δO₂ ASTL. Other carrier dynamics, such as electron diffusion, electron injection, and hot hole transfer processes, were found to be less affected by UV irradiation. Quantitative pump-probe data clearly show a correlation between the carrier dynamics and UV aging of perovskite solar cells, thus providing a profound insight into the factors driving UV-induced degradation in perovskite solar cells and the origin of its performance.

The Applications of Polymers in Solar Cells: A Review

The emerging dye-sensitized solar cells, perovskite solar cells, and organic solar cells have been regarded as promising photovoltaic technologies. The device structures and components of these solar cells are imperative to the device’s efficiency and stability. Polymers can be used to adjust the device components and structures of these solar cells purposefully, due to their diversified properties. In dye-sensitized solar cells, polymers can be used as flexible substrates, pore- and film-forming agents of photoanode films, platinum-free counter electrodes, and the frameworks of quasi-solid-state electrolytes. In perovskite solar cells, polymers can be used as the additives to adjust the nucleation and crystallization processes in perovskite films. The polymers can also be used as hole transfer materials, electron transfer materials, and interface layer to enhance the carrier separation efficiency and reduce the recombination. In organic solar cells, polymers are often used as donor layers, buffer layers, and other polymer-based micro/nanostructures in binary or ternary devices to influence device performances. The current achievements about the applications of polymers in solar cells are reviewed and analyzed. In addition, the benefits of polymers for solar cells, the challenges for practical application, and possible solutions are also assessed.

Refractive index change dominates the transient absorption response of metal halide perovskite thin films in the near infrared

Perovskite films have an uncommon, previously unreported transient absorption response in the NIR, which is caused by a change in the refractive index.

Perovskites for Light Emission

Next-generation displays require efficient light sources that combine high brightness, color purity, stability, compatibility with flexible substrates, and transparency. Metal halide perovskites are a promising platform for these applications, especially in light of their excellent charge transport and bandgap tunability. Low-dimensional perovskites, which possess perovskite domains spatially confined at the nanoscale, have further extended the degree of tunability and functionality of this materials platform. Herein, the advances in perovskite materials for light-emission applications are reviewed. Connections among materials properties, photophysical and electrooptic spectroscopic properties, and device performance are established. It is discussed how incompletely solved problems in these materials can be tackled, including the need for increased stability, efficient blue emission, and efficient infrared emission. In conclusion, an outlook on the technologies that can be realized using this material platform is presented.

Highly efficient MoOx-free semitransparent perovskite cell for 4 T tandem application improving the efficiency of commercially-available Al-BSF silicon

In this work, the fabrication of MoOx-free semitransparent perovskite solar cells (PSC) with Power Conversion Efficiencies (PCE) up to 15.7% is reported. Firstly, opaque PSCs up to 19.7% were fabricated. Then, the rear metal contact was replaced by a highly transparent and conductive indium tin oxide (ITO) film, directly sputtered onto the hole selective layer, without any protective layer between Spiro-OMeTAD and rear ITO. To the best of our knowledge, this corresponds to the most efficient buffer layer-free semitransparent PSC ever reported. Using time-resolved photoluminescence (TRPL) technique on both sides of the semitransparent PSC, Spiro-OMeTAD/perovskite and perovskite/TiO2 interfaces were compared, confirming the great quality of Spiro-OMeTAD/perovskite interface, even after damage-less ITO sputtering, where degradation phenomena result less important than for perovskite/TiO2 one. Finally, a 4-terminal tandem was built combining semitransparent PSC with a commercially-available Aluminium Back Surface Field (Al-BSF) silicon wafer. That silicon wafer presents PCE = 19.52% (18.53% after being reduced to cell size), and 5.75% once filtered, to generate an overall 4 T tandem efficiency of 21.18% in combination with our champion large semitransparent PSC of 15.43%. It means an absolute increase of 1.66% over the original silicon wafer efficiency and a 2.65% over the cut Si cell.

Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites

The rapid relaxation of above-band-gap "hot" carriers (HCs) imposes the key efficiency limit in lead-halide perovskite (LHP) solar cells. Recent studies have indicated that HC cooling in these systems may be sensitive to materials composition, as well as the energy and density of excited states. However, the key parameters underpinning the cooling mechanism are currently under debate. Here we use a sequence of ultrafast optical pulses (visible pump-infrared push-infrared probe) to directly compare the intraband cooling dynamics in five common LHPs: FAPbI₃, FAPbBr₃, MAPbI₃, MAPbBr₃, and CsPbBr₃. We observe ∼100-900 fs cooling times, with slower cooling at higher HC densities. This effect is strongest in the all-inorganic Cs-based system, compared to the hybrid analogues with organic cations. These observations, together with band structure calculations, allow us to quantify the origin of the "hot-phonon bottleneck" in LHPs and assert the thermodynamic contribution of a symmetry-breaking organic cation toward rapid HC cooling.

Picosecond Capture of Photoexcited Electrons Improves Photovoltaic Conversion in MAPbI3 :C70 -Doped Planar and Mesoporous Solar Cells

In this work, solar cells based on methylammonium lead iodide (MAPbI3 ) doped in solution with C70 fullerene in a mesoporous as well as planar electron-transporting layer (ETL)-free architecture are realized, showcasing in the latter case a record efficiency of 15.7% and an improved open-circuit voltage (VOC ). Contrary to the bulk heterojunction previously reported, the C70 molecules do not phase segregate and they are rather finely dispersed in the perovskite film, possibly infiltrating at the grain boundaries, while assisting the growth of a highly uniform perovskite layer. By means of time-resolved femtosecond-to-nanosecond optical spectroscopy, with an extended spectral coverage, it is observed that electrons photogenerated in the perovskite are transferred to C70 with a time constant of 20 ps. Despite being captured by C70 , electrons are not deeply trapped and can potentially bounce back into the perovskite, as suggested by the high fill factor and enhanced VOC of the MAPbI3 :C70 solar cells, especially in the case of the ETL-free device configuration.

Reflectivity Effects on Pump-Probe Spectra of Lead Halide Perovskites: Comparing Thin Films versus Nanocrystals

Due to the sizable refractive index of lead halide perovskites, reflectivity off their interface with air exceeds 15%. This has prompted a number of investigations into the prominence of photoreflective contributions to pump-probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI₃) perovskite. By analyzing how complex refractive index changes impact the two experiments, results demonstrate that changes in absorption and not reflection dominate transient transmission measurements in thin films of these materials. None of the characteristic spectral signatures reported in such experiments are exclusively due to or even strongly affected by changes in sample reflectivity. This finding is upheld by another experiment where a methyl ammonium lead iodide (MAPbI₃) perovskite film was formed on high-index flint glass and probed after pump irradiation from either face of the sample. We conclude that interpretations of ultrafast pump-probe experiments on thin perovskite films in terms of photoinduced changes in absorption alone are qualitatively sound, requiring relatively minor adjustments to factor in photoreflective effects.

Dual nanocomposite carrier transport layers enhance the efficiency of planar perovskite photovoltaics

In photovoltaic devices, more effective transfer of dissociated electrons and holes from the active layer to the respective electrodes will result in higher fill factors and short-circuit current densities and, thus, enhanced power conversion efficiencies (PCEs). Planar perovskite photovoltaics feature an active layer that can provide a large exciton diffusion length, reaching several micrometers, but require efficient carrier transport layers for charge extraction. In this study, we employed two nanocomposite carrier transfer layers-an electron transport layer (ETL) comprising [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) doped with the small molecule 4,7-diphenyl-1,10-phenanthroline (Bphen), to enhance the electron mobility, and a hole transfer layer (HTL) comprising poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) doped with molybdenum disulfide (MoS2) nanosheets, to enhance the hole mobility. We used ultraviolet photoelectron spectroscopy to determine the energy levels of these composite ETLs and HTLs; atomic force microscopy and scanning electron microscopy to probe their surface structures; and transmission electron microscopy and synchrotron grazing-incidence small-angle X-ray scattering to decipher the structures of the ETLs. Adding a small amount (less than 1%) of Bphen allowed us to tune the energy levels of the ETL and decrease the size of the PC61BM clusters and, therefore, generate more PC61BM aggregation domains to provide more pathways for electron transport, leading to enhanced PCEs of the resulting perovskite devices. We used quantitative pump-probe data to resolve the carrier dynamics from the perovskite to the ETL and HTL, and observed a smaller possibility of carrier recombination and a shorter injection lifetime in the perovskite solar cell doubly modified with carrier transport layers, resulting in an enhancement of the PCE. The PCE reached 16% for a planar inverted perovskite device featuring an ETL incorporating 0.5 wt% Bphen within PC61BM and 0.1 wt% MoS2 within PEDOT:PSS; this PCE is more than 50% higher than the value of 10.2% for the corresponding control device.

Lewis Base Passivation of Hybrid Halide Perovskites Slows Electron-Hole Recombination: Time-Domain Ab Initio Analysis

Nonradiative electron-hole recombination plays a key role in determining photon conversion efficiencies in solar cells. Experiments demonstrate significant reduction in the recombination rate upon passivation of methylammonium lead iodide perovskite with Lewis base molecules. Using nonadiabatic molecular dynamics combined with time-domain density functional theory, we find that the nonradiative charge recombination is decelerated by an order of magnitude upon adsorption of the molecules. Thiophene acts by the traditional passivation mechanism, forcing electron density away from the surface. In contrast, pyridine localizes the electron at the surface while leaving it energetically near the conduction band edge. This is because pyridine creates a stronger coordinative bond with a lead atom of the perovskite and has a lower energy unoccupied orbital compared with thiophene due to the more electronegative nitrogen atom relative to thiophene's sulfur. Both molecules reduce two-fold the nonadiabatic coupling and electronic coherence time. A broad range of vibrational modes couple to the electronic subsystem, arising from inorganic and organic components. The simulations reveal the atomistic mechanisms underlying the enhancement of the excited-state lifetime achieved by the perovskite passivation, rationalize the experimental results, and advance our understanding of charge-phonon dynamics in perovskite solar cells.

Tuning optical/electrical properties of 2D/3D perovskite by the inclusion of aromatic cation

Demonstration of the tuning of the dielectric confinement effect by the inclusion of a highly polarizable cation in 2D/3D perovskites.

Differences in photoinduced optical transients in perovskite absorbers for solar cells

Methylammonium lead iodide films and powdered crystals were studied by time-resolved absorption and emission spectroscopy on the time scales from femtoseconds to nanoseconds. Strikingly different transient absorption signals were observed, changing from strong long-wavelength band-edge bleach to weak signatures of band-shift, which depended on the absorber form (films or polycrystals) and preparation method (stoichiometric or non-stoichiometric). The observed differences were correlated with the variation in absorption and emission spectra, changes in photo-induced carrier lifetimes and solar cell efficiency. These differences also pointed out that similar perovskite absorbers can provide significantly different transient responses and emphasize that special care must be taken when interpolating the obtained findings to the processes occurring in the most efficient devices.

Striking differences in transient absorption signal are observed for perovskite absorbers prepared with different methods.

Multiple-Trapping Model for the Charge Recombination Dynamics in Mesoporous-Structured Perovskite Solar Cells

The photovoltaic performance of organic-inorganic hybrid perovskite solar cells has reached a bottleneck after rapid development in last few years. Further breakthrough in this field requires deeper understanding of the underlying mechanism of the photoelectric conversion process in the device, especially the dynamics of charge-carrier recombination. Originating from dye-sensitized solar cells (DSSCs), mesoporous-structured perovskite solar cells (MPSCs) have shown many similarities to DSSCs with respect to their photoelectric dynamics. Herein, by applying the multiple-trapping model of the charge-recombination dynamic process for DSSCs in MPSCs, with rational modification, a novel physical model is proposed to describe the dynamics of charge recombination in MPSCs that exhibits good agreement with experimental data. Accordingly, the perovskite- and TiO₂ -dominating charge-recombination processes are assigned and their relationships with the trap-state distribution are also discussed. An optimal balance between these two dynamic processes is required to improve the performance of mesoporous-structured perovskite devices.

Improved Carrier Transport in Perovskite Solar Cells Probed by Femtosecond Transient Absorption Spectroscopy

CH₃NH₃PbI₃ perovskite thin films have been deposited on glass/indium tin oxide/hole transport layer (HTL) substrates, utilizing two different materials as the HTLs. In the first configuration, the super hydrophilic polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), known as PEDOT:PSS, was employed as the HTL material, whereas in the second case, the nonwetting poly(triarylamine) semiconductor polymer, known as PTAA, was used. It was found that when PTAA is used as the HTL material, the averaged power conversion efficiency (PCE) of the perovskite solar cells (PSCs) remarkably increases from 12.60 to 15.67%. To explore the mechanism behind this enhancement, the aforementioned perovskite/HTL arrangements were investigated by time-resolved transient absorption spectroscopy (TAS) performed under inert conditions. By means of TAS, the charge transfer, carrier trapping, and hole injection dynamics from the photoexcited perovskite layers to the HTL can be directly monitored via the characteristic bleaching profile of the perovskite at ∼750 nm. TAS studies revealed faster relaxation times and decay dynamics when the PTAA polymer is employed, which potentially account for the enhanced PCE observed. The TAS results are correlated with the structure and crystalline quality of the corresponding perovskite films, investigated by scanning electron microscopy, X-ray diffraction, atomic force microscopy, micro-photoluminescence, and transmittance spectroscopy. It is concluded that TAS is a benchmark technique for the understanding of the carrier transport mechanisms in PSCs and constitutes a figure-of-merit tool toward their efficiency improvement.

First-Principles Study of Electron Injection and Defects at the TiO2/CH3NH3PbI3 Interface of Perovskite Solar Cells

We investigated electron injection rates and vacancy defect properties by performing first-principles calculations on the interface of an anatase-TiO₂(001) and a tetragonal CH₃NH₃PbI₃(110) (MAPbI₃(110)). We found that the coupling matrix element between the lowest unoccupied molecular orbital of MAPbI₃ and the TiO₂ conduction band (CB) minimum is negligibly small, the indication being that electron-injection times for low-energy excited states are quite long (more than several tens of picoseconds). We also found that higher-lying CB states coupled more strongly; injection was expected to take place on a femtosecond time scale. Furthermore, we found that vacancy defects in the TiO₂ layer produced undesired defect levels that caused hole traps and recombination centers. Whereas most of the vacancy defects in the MAPbI₃ layer produced no additional states in the MAPbI₃ gap, a Pb vacancy (V_Pb) at the interface created an energy level below the MAPbI₃ CB edge and had a lower energy of formation than the V_Pb defect in bulk because of the interaction with the TiO₂ surface.

Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy

In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation. In hybrid perovskites, the cooling of such thermal carrier distributions occurs on timescales of about 300 fs via carrier-phonon scattering. However, the initial build-up of the thermal distribution proved difficult to resolve with pump-probe techniques due to the requirement of high resolution, both in time and pump energy. Here, we use two-dimensional electronic spectroscopy with sub-10 fs resolution to directly observe the carrier interactions that lead to a thermal carrier distribution. We find that thermalization occurs dominantly via carrier-carrier scattering under the investigated fluences and report the dependence of carrier scattering rates on excess energy and carrier density. We extract characteristic carrier thermalization times from below 10 to 85 fs. These values allow for mobilities of 500 cm² V^-1 s^-1 at carrier densities lower than 2 × 10¹⁹ cm^-3 and limit the time for carrier extraction in hot carrier solar cells.Carrier-carrier scattering rates determine the fundamental limits of carrier transport and electronic coherence. Using two-dimensional electronic spectroscopy with sub-10 fs resolution, Richter and Branchi et al. extract carrier thermalization times of 10 to 85 fs in hybrid perovskites.

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films

In this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I_1-xBr_x)₃) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I_1-xBr_x)₃ from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, we observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I_1-xBr_x)₃ samples exhibit a lower Burstein-Moss shift of 0.07-0.08 eV compared to pure MAPbI₃ (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron-phonon coupling and phonon propagation in the lattice.

Direct Observation of Ultrafast Hole Injection from Lead Halide Perovskite by Differential Transient Transmission Spectroscopy

Efficient charge separation at the interfaces of the perovskite with the carrier transport layers is crucial for perovskite solar cells to achieve high power conversion efficiency. We present a systematic experimental study on the hole injection dynamics from MAPbI₃ perovskite to three typical hole transport materials (HTMs). We extract the carrier dynamics directly related to the hole injection by employing a pump light with short absorption depth and comparing the transient transmission signals excited on the two sides of the sample. The differential transmission signals reveal the hole injections to PTAA and PEDOT:PSS to be complete within 1 and 2 ps, respectively, and that to NiO_x to exhibit an additional slow process on a 40 ps time scale. The obtained injection dynamics are discussed in comparison with the device performance of the solar cells containing the same MAPbI₃/HTM interfaces.

Femtosecond Charge-Injection Dynamics at Hybrid Perovskite Interfaces

With a power conversion efficiency (PCE) exceeding 22 %, perovskite solar cells (PSCs) have thrilled photovoltaic research. However, the interface behavior is still not understood and is a hot topic of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, which makes perovskite interface physics intriguing. Herein, through femtosecond transient absorption spectroscopy with spectral coverage extending into the crucial IR region, the ultrafast interface-specific processes at standard and newly molecularly engineered perovskite interfaces in state-of-the-art PSCs are interrogated. Ultrafast interfacial charge injection occurs with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the timescale for the first step involved in the complex charge-transfer process. This is also true for 20 % efficient devices measured under real operation, for which the femtosecond injection is followed by a slower picosecond component. These findings provide compelling evidence for the femtosecond interfacial charge-injection step and demonstrate a robust method for the straightforward identification of interfacial non-equilibrium processes on the ultrafast timescale.

Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots

Methylammonium lead iodide perovskite (MAPbI₃) is a promising material for photovoltaic devices. A modification of MAPbI₃ into confined nanostructures is expected to further increase efficiency of solar energy conversion. Photoexcited dynamic processes in a MAPbI₃ quantum dot (QD) have been modeled by many-body perturbation theory and nonadiabatic dynamics. A photoexcitation is followed by either exciton cooling (EC), its radiative (RR) or nonradiative recombination (NRR), or multiexciton generation (MEG) processes. Computed times of these processes fall in the order of MEG < EC < RR < NRR, where MEG is on the order of a few femtoseconds, EC is in the picosecond range, while RR and NRR are on the order of nanoseconds. Computed time scales indicate which electronic transition pathways can contribute to increase in charge collection efficiency. Simulated mechanisms of relaxation and their rates show that quantum confinement promotes MEG in MAPbI₃ QDs.

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr₃ and CsPb(ClBr)₃ perovskite nanocrystals in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L₃-edge, and the Cs L₂-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L₂-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs⁺ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.

Ferroelectric Alignment of Organic Cations Inhibits Nonradiative Electron-Hole Recombination in Hybrid Perovskites: Ab Initio Nonadiabatic Molecular Dynamics

Hybrid organic-inorganic perovskites show impressive potential for photovoltaic applications and currently give rise to one of the most vibrant research areas in the field. Until recently, the electrostatic interactions between their organic and inorganic components were considered mostly for stabilization of the fragile perovskite structure. We study the effect of local interactions of polar C-N bonds in the organic layer on the nonradiative electron-hole recombination in the recently reported room-temperature ferroelectric hybrid perovskite, (benzylammonium)₂PbCl₄. Using nonadiabatic molecular dynamics and real-time time-dependent density functional theory, we show that ferroelectric alignment of the polar groups weakens the electron-phonon nonadiabatic coupling and inhibits the nonradiative charge recombination. The effect is attributed to suppression of contributions of higher frequency phonons to the electron-phonon coupling. The coupling is dominated in the ferroelectric phase by slower collective motions. We also demonstrate the importance of van der Waals interactions for the charge-phonon relaxation in the hybrid perovskite systems. Combined with the long-range charge separation achievable in the ferroelectric phase, the weakened electron-phonon coupling indicates that ferroelectric order in hybrid perovskites can lead to increased excited-state lifetimes and improved solar energy conversion performance.