Fundamental electronic processes such as charge-carrier transport and recombination play a critical role in determining the efficiency of hybrid perovskite solar cells. The presence of mobile ions complicates the development of a clear understanding of these processes as the ions may introduce exceptional phenomena such as hysteresis or giant dielectric constants. As a result, the electronic landscape, including its interaction with mobile ions, is difficult to access both experimentally and analytically. To address this challenge, we applied a series of small perturbation techniques including impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) to planar $\mathrm{MAPbI_3}$ perovskite solar cells. Our measurements indicate that both electronic as well as ionic responses can be observed in all three methods and assigned by literature comparison. The results reveal that the dominant charge-carrier loss mechanism is surface recombination by limitation of the quasi-Fermi level splitting. The interaction between mobile ions and the electronic charge carriers leads to a shift of the apparent diode ideality factor from 0.74 to 1.64 for increasing illumination intensity, despite the recombination mechanism remaining unchanged.
One of the most significant features of lead-halide perovskites are their ability to have comparably slow recombination despite the fact that these materials are mostly processed from solution at room temperature. The slow recombination allows achieving high open-circuit voltages when the lead-halide perovskite layers are used in solar cells. This perspective discusses the state of the art of our understanding and of experimental data with regard to recombination and open-circuit voltages in lead-halide perovskites. A special focus is put onto open questions that the community has to tackle to…
Phase segregation is a critical phenomenon that influences the stability and performance of mixed halide perovskite based opto-electronic devices. In addition to the underlying physical mechanisms, the spatial pattern and randomness associated with the nanoscale morphology of phase segregation significantly influence performance degradation a topic which, along with the multitude of parameter combinations, has remained too complex to address so far. Given this, with MAPbI1.5Br1.5 as a model system, here we address the influence of critical factors like the spatial randomness of phase segregati…
We demonstrate the critical role of surface recombination in mixed-cation, mixed-halide perovskite, FA0.83Cs0.17Pb(I0.85Br0.15)3. By passivating non-radiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS) we transform these thin films. We demonstrate average minority carrier lifetimes > 4 μs, nearly single exponential monomolecular PL decays, and concomitantly high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We confirm both the composition and …
Halide perovskites exhibit slow rates of non-radiative electron-hole recombination upon illumination. Chu et al. [Sci. Adv. 6 7, eaaw7453 (2020)] use the results of first-principles simulations to argue that this arises from the nature of the crystal vibrations and leads to a breakdown of Shockley-Read-Hall theory. We highlight flaws in their methodology and analysis of carrier capture by point defects in crystalline semiconductors.
We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination, over multiple orders of photo-excitation density. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our resul…
The ideality factor determined by measuring the open circuit voltage (VOC) as function of light intensity is often used as a means to identify the dominant recombination mechanism in solar cells. However, applying this Suns-VOC technique to perovskite cells is problematic since the VOC evolves with time in a way which depends on the previously applied bias (Vpre), the light intensity, and the device architecture/processing. Here we show that the dominant recombination mechanism in two structurally similar CH3NH3PbI3 devices containing either mesoporous Al2O3 or TiO2 layers can be identified fr…
Charge carrier recombination in the perovskite solar cells (PSCs) has a deep influence on the electrical performance, such as open circuit voltage, short circuit current, fill factor and ultimately power conversion efficiency. The impacts of injection barrier, recombination channels, doping properties of carrier transport layers and light intensity on the performance of PSCs are theoretically investigated by drift-diffusion model in this work. The results indicate that due to the injection barrier at the interfaces of perovskite and carrier transport layer, the accumulated carriers modify the …
Distinguishing Carrier Transport and Interfacial Recombination at Perovskite-Transport Layer Interfaces Using Ultrafast Spectroscopy and Numerical Simulation
Interplays between charge and electric field, which play a critical role in determining the charge transport, recombination, storage and hysteresis in the perovskite solar cell, have been systematically investigated by both electrical transient experiments and theoretical calculations. It is found that the light illumination can increase the carrier concentration in the perovskite absorber, thus enhancing charge recombination and causing the co-existence of high electric field and free carriers. Meanwhile, the cell shows a similar charge storage and junction mechanism to that of the multicryst…
The hybrid perovskite CH3NH3PbI3 (MAPI) exhibits long minority-carrier lifetimes and diffusion lengths. We show that slow recombination originates from a spin-split indirect-gap. Large internal electric fields act on spin-orbit-coupled band extrema, shifting band-edges to inequivalent wavevectors, making the fundamental gap indirect. From a description of photoluminescence within the quasiparticle self-consistent GW (QSGW) approximation for MAPI, CdTe and GaAs, we predict carrier lifetime as a function of light intensity and temperature. At operating conditions we find radiative recombination …
Recently we proposed that defect tolerance in the hybrid perovskites is due to their characteristic low-frequency lattice phonon modes that decrease the non-adiabatic coupling and weaken the overlap between the free carrier and defect states [Sci. Adv. 6 7, eaaw7453 (2020)]. Kim and Walsh disagree with the interpretation and argue that there are flaws in our employed methodology. Herein we address their concerns and show that their conclusions are not valid due to misunderstandings of nonadiabatic transition.
Wavelength-Dependent Photoluminescence Mapping of Depth-Dependent Degradation Under Thermal Stress in Phenethylammonium Tetrafluoroborate Treated Perovskite Solar Cells
In this perspective, we explore the insights into the device physics of perovskite solar cells gained from modeling and simulation of these devices. We discuss a range of factors that influence the modeling of perovskite solar cells, including the role of ions, dielectric constant, density of states, and spatial distribution of recombination losses. By focusing on the effect of non-ideal energetic alignment in perovskite photovoltaic devices, we demonstrate a unique feature in low recombination perovskite materials - the formation of an interfacial, primarily electronic, self-induced dipole th…
The ability to reduce energy loss at semiconductor surfaces through passivation or surface field engineering has become an essential step in the manufacturing of efficient photovoltaic (PV) and optoelectronic devices. Similarly, surface modification of emerging halide perovskites with quasi-2D heterostructures is now ubiquitous to achieve PV power conversion efficiencies (PCEs) > 22% and has enabled single-junction PV devices to reach 25.7%, yet a fundamental understanding to how these treatments function is still generally lacking. This has established a bottleneck for maximizing beneficial i…
Using cross-correlation current noise spectroscopy, we have investigated carrier dynamics in methylammonium lead triiodide solar cells. This method provides a space selectivity for devices with planar multi-layered structure, effectively amplifying current noise contributions coming from the most resistive element of the stack. In the studied solar cells, we observe near full-scale shot noise, indicating the dominance of noise generation by a single source, likely the interface between the perovskite and the spiro-OMeTAD hole-transport layer. We argue that the strong 1/f noise term has contrib…
Due to their exceptional photovoltaic properties, metal halide perovskites (MHPs) are extensively studied for their potential applications in solar cells. In recent years, the power conversion efficiencies of MHPs-based solar cells rapidly increased from the initial few \% towards more than 25\,\% for single-junction devices. Therefore, also taking into account their low costs and ease of manufacturing, MHPs-based solar cells have become the fastest-advancing photovoltaic technology. In this regard, much of the recent work has been dominated by absorber materials based on methylammonium MHPs, …
Over the last two decades, intensive research efforts have been devoted to the suppressions of photoluminescence (PL) blinking and Auger recombination in metal-chalcogenide nanocrystals (NCs), with significant progresses being made only very recently in several specific heterostructures. Here we show that nonblinking PL is readily available in the newly-synthesized perovskite CsPbI3 (cesium lead iodide) NCs, and their Auger recombination of charged excitons is greatly slowed down, as signified by a PL lifetime about twice shorter than that of neutral excitons. Moreover, spectral diffusion is c…
Capacitance measurements as a function of voltage, frequency and temperature are useful tools to identify fundamental parameters that affect solar cell operation. Techniques such as capacitance-voltage (CV), Mott-Schottky analysis and thermal admittance spectroscopy (TAS) measurements are therefore frequently employed to obtain relevant parameters of the perovskite absorber layer in perovskite solar cells. However, state-of-the-art perovskite solar cells employ thin electron and hole transport layers that improve contact selectivity. These selective contacts are often quite resistive in nature…
Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution processed semiconductors since they possess high photoluminescence quantum yield (PLQY), and colour tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed halide (CsPbBrI2) NCs obtained via anion exchange reactions in ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I-, thus inducing a PL red-shift up to 676 nm, and obtaining a high PLQY in film (65%). Solar cell devices operating in t…
The same optical and electronic properties that make perovskite thin films ideal absorber materials in photovoltaic applications are also beneficial in photon upconversion devices. In this contribution, we investigate the rubrene-triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA). To elucidate the role of trap states which affect the free carrier lifetimes, we fabricate MAFA perovskite thin films with three different thicknesses. By measuring the change in the photoluminescence properties under different excitation fluences, we find that …
In halide perovskites, electrons (holes) exist as electronic (hole) polarons, excitons, free and trapped electrons (holes). Six kinds of collisions could lead to annihilation of electron and hole, three of them involve polaron(s). In the annihilation channel of each collision process, there is a certain probability to form a dying pair which the average distance between electron and hole is smaller than a critical distance. The annihilation probability per unit time of a collision process is a product of the formation probability of the dying pair in the annihilation channel and the annihilati…
Nanoscale investigations by scanning probe microscopy have provided major contributions to the rapid development of organic-inorganic halide perovskites (OIHP) as optoelectronic devices. Further improvement of device level properties requires a deeper understanding of the performance-limiting mechanisms such as ion migration, phase segregation and their effects on charge extraction both at the nano- and macroscale. Here, we have studied the dynamic electrical response of Cs0.05(FA0.83-MA0.17)0.95PbI3-xBrx perovskite structures by employing conventional and microsecond time-resolved Kelvin prob…
Two-dimensional (2D) perovskite materials have recently re-attracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. In this report, the band-edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)2PbI4 are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2x10^3cm/s and an intrinsic lifetime of 185…
Unraveling the Roles of Shallow, Deep and Auger Trapping in Charge Carrier Recombination in Triple-Cation Perovskites
Lund University（隆德大学）；Leibniz Institute for Solid State and Materials Research Dresden（德累斯顿莱布尼兹固体与材料研究所）；Technical University of Dresden（德累斯顿工业大学）；Indian Institute of Technology Roorkee（鲁尔基印度理工学院）；Forschungszentrum Jülich（于利希研究中心）；University of Duisburg-Essen（杜伊斯堡-埃森大学）
The extraction of photogenerated charge carriers and the generation of a photovoltage belong to the fundamental functionalities of any solar cell. These processes happen not instantaneously but rather come with finite time constants, e.g., a time constant related to the rise of the externally measured open circuit voltage following a short light pulse. The present paper provides a new method to analyze transient photovoltage measurements at different bias light intensities combining rise and decay times of the photovoltage. The approach uses a linearized version of a system of two coupled diff…
The enhancement of the fill factor in the current generation of perovskite solar cells is the key for further efficiency improvement. Thus, methods to quantify the fill factor losses are urgently needed. A classical method to quantify Ohmic and non-Ohmic resistive losses in solar cells is based on the comparison between the voltage in the dark and under illumination analysed at equal recombination current density. Applied to perovskite solar cells, we observe a combination of an Ohmic series resistance with a voltage-dependent resistance that is most prominent at short circuit and low forward …
A modified detailed balance model is built to understand and quantify efficiency loss of perovskite solar cells. The modified model captures the light-absorption dependent short-circuit current, contact and transport-layer modified carrier transport, as well as recombination and photon-recycling influenced open-circuit voltage. Our theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, non-radiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors fo…
Engineering the energetics of perovskite photovoltaic devices through the deliberate introduction of dipoles to control the built-in potential of the devices offers the opportunity to enhance their performance without the need to modify the active layer itself. In this work, we demonstrate how the incorporation of molecular dipoles into the bathocuproine (BCP) hole-blocking layer of inverted perovskite solar cells improves the device open-circuit voltage (VOC) and consequently, its performance. We explore a series of four thiaazulenic derivatives that exhibit increasing dipole moments and demo…
Metal halide perovskites feature excellent absorption, emission and charge carrier transport properties. These materials are therefore very well suited for photovoltaics applications where there is a growing interest. Still, questions arise when looking at the unusual long carrier lifetime that, regarding the micrometric diffusion length, would imply a very low diffusion coefficient as compared to commonly used photovoltaic absorbers. In this paper, we provide an experimental insight into this long lifetime of charge carriers in slow-motion. Our approach relies on an improved model to analyze …
Open circuit voltage decay measurements are performed on methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells to investigate the charge carrier recombination dynamics. The measurements are compared to the two reference polymer-fullerene bulk heterojunction solar cells based on P3HT:PC60BM and PTB7:PC70BM blends. In the perovskite devices, two very different time domains of the voltage decay are found, with a first drop on a short time scale that is similar to the organic solar cells. However, two major differences are also observed. 65-70% of the maximum photovoltage persists on much…
Although methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) perovskite has attracted enormous scientific attention over the last decade or so, important information on the charge extraction dynamics and recombination processes in perovskite devices is still missing. Herein we present a novel approach to evaluate the quality of CH$_3$NH$_3$PbI$_3$ layers, via in-situ monitoring of the perovskite layer charge carrier dynamics during the thermal annealing crystallization process, by means of time-resolved femtosecond transient absorption spectroscopy (TAS). In particular, CH$_3$NH$_3$PbI$_3$ films …
Nanocrystals based on metal-halide perovskites offer a promising material platform for highly efficient lighting. Using transient optical spectroscopy, we study excitation recombination dynamics in manganese-doped CsPb(Cl,Br)3 perovskite nanocrystals. We find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. Supported by ab initio calculations, we can attribute the enhanced emission rates to increased exciton localization through lattice periodicity breaking from Mn dopants, which increases exciton eff…
Drift-diffusion model is an indispensable modeling tool to understand the carrier dynamics (transport, recombination, and collection) and simulate practical-efficiency of solar cells (SCs) through taking into account various carrier recombination losses existing in multilayered device structures. Exploring the way to predict and approach the SC efficiency limit by using the drift-diffusion model will enable us to gain more physical insights and design guidelines for emerging photovoltaics, particularly perovskite solar cells. Our work finds out that two procedures are the prerequisites for pre…
We report a proof-of-concept two-terminal perovskite/Cu(In, Ga)Se2 (CIGS) monolithic thin-film tandem solar cell grown on ultra-thin (30-microns thick), light-weight, and flexible polyimide foil with a steady-state power conversion efficiency of 13.2% and a high open-circuit voltage over 1.75 V under standard test condition.
Space-charge-limited current (SCLC) measurements are used to estimate charge carrier mobilities and electronic trap densities of semiconductors by analysing the current density-voltage (JV) relationship for unipolar devices predicted by the Mott-Gurney (MG) law. However, the interpretation of SCLC measurements for metal-halide perovskites is problematic due to mobile ionic defects which redistribute to screen electrostatic fields in devices during measurements. To overcome this, an SCLC measurement protocol was recently suggested that minimises ionic charge redistribution by probing the curren…
Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first order recombination at short circuit to bimolecular recombinat…
