Trapping Recombination Research Articles

Influence of Fermi Level Alignment with Tin Oxide on the Hysteresis of Perovskite Solar Cells.

We tune the Fermi level alignment between the SnO x electron transport layer (ETL) and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 and highlight that this parameter is interlinked with current-voltage hysteresis in perovskite solar cells (PSCs). Furthermore, thermally stimulated current measurements reveal that the depth of trap states in the ETL or at the ETL-perovskite interface correlates with Fermi level positions, ultimately linking it to the energy difference between the Fermi level and conduction band minimum. In the presence of deep trap states, charge accumulation and recombination at the interface are promoted, affecting the charge collection efficiency adversely, which increases the hysteresis of PSCs.

Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States, Charge Transport, and Recombination

AbstractAdding cesium (Cs) and rubidium (Rb) cations to FA0.83MA0.17Pb(I0.83Br0.17)3 hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.

Enhancing the efficiency of low-temperature planar perovskite solar cells by modifying the interface between perovskite and hole transport layer with polymers

In this work, planar perovskite solar cells (PSCs) based on CH3NH3PbI3 perovskite layer and low-temperature processed TiO2 have been fabricated. Polymers including poly(methylmethacrylate) (PMMA), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-pheny- lenevinylene] (MEH-PPV) and polyethylene glycol (PEG) in chlorobenzene solution have been selected to modify the interface between perovskite and hole transport layer (HTL), respectively. The concentrations of the three polymer solutions have been optimized. The effect of interfacial modification by different polymer solutions on the photoelectric properties of perovskite layer and the performance of PSCs has been systematically investigated. The microstructure and photoelectric properties of the modified perovskite films has been systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), conducting force microscopy (CFM) and Kelvin probe force microscopy (KPFM). The results reveal that the modified perovskite films with tetrahedral perovskite structure have lager grain size, lower roughness and better photoelectric properties compared with the reference sample. The electron trap state density (Dtrap), charge extraction, carrier transfer and recombination process in the PSCs have been investigated by current-voltage (I-V) characteristic curves, steady-state photoluminescence (PL), photo-voltage decay and electrochemical impedance spectroscopy (EIS). The results indicate that the polymeric interface modification at the optimum concentration can reduce the Dtrap, promote the charge transfer and suppress carrier recombination, resulting in the improved performance of PSCs. All of the modified PSCs at an optimum concentration exhibit the improved fill factor (FF) and open circuit voltage (Voc), thus the power conversion efficiency (PCE) is enhanced to over 17% from 15.49%.

Charge transfer dynamics between MPA capped CdTe quantum dots and methyl viologen

Abstract The understanding of charge carrier dynamics in hybrid materials based on colloidal semiconductor nanocrystals (or quantum dots) and organic moieties is fundamental for the design of efficient photonic and photovoltaic devices. In the present work, we investigate the interactions occurring between CdTe quantum dots, capped with a strong capping agent such as 3-mercaptopropionic acid, and a well known electron acceptor such as methylviologen molecule. The nature of the interactions and of exciton dynamics is investigated by stationary and time-resolved spectroscopies. Luminescence data recorded in presence of increasing methylviologen concentrations, indicate that the organic molecule is able to statically interact with the surface sites of CdTe quantum dots; a biphasic interaction behavior is evidenced by determining the apparent association constants. These latter are obtained through the analysis of luminescence data, and values in the range 103–104 are determined. The nature of the interactions is characterized by nanosecond and femtosecond transient absorption spectroscopies, to clarify the dynamics and the conditions able to foster charge mobility. Nanosecond flash photolysis measurements, carried out upon quantum dots excitation, shows the absorption of methylviologen radical cation specie at 605 nm, suggesting the occurrence of electron transfer from CdTe nanocrystals to the organic acceptor; the relatively long decay time of the transient signal (10.3 μs) indicates that back electron transfer processes are negligible. Ultrafast transient absorption measurements confirm the occurrence of an ultrafast electron transfer process; spectral and kinetic analysis of the transient data show that methylviologen radical cation is formed almost instantaneously on the ps-time scale but mainly when the samples are pumped in the energy continuum at 400 nm. This finding suggests that electron mobility from the nanocrystals to the organic units is achieved mainly when the excitonic states possess an excess of energy. The comparison of the kinetic behaviour of the signals at increasing methylviologen concentrations indicates that the electron transfer process competes with the radiative exciton recombination. In addition, the kinetic data suggests that surface trapping and Auger recombination processes might slow down the charge mobility.

Electrostatic Screening of Charged Defects in Monolayer MoS2.

Defects in monolayer transition-metal dichalcogenides (TMDCs) may lead to unintentional doping, charge-carrier trapping, and nonradiative recombination. These effects impair electronic and optoelectronic technologies. Here we show that charged defects in MoS2 monolayers can be effectively screened when they are in contact with an ionic liquid (IL), leading to an increase in photoluminescence (PL) yield by up to two orders of magnitude. The extent of this PL enhancement by the IL correlates with the brightness of each pretreated sample. We propose the existence of two classes of nonradiative recombination centers in monolayer MoS2: (i) charged defects that relate to unintentional doping and may be electrostatically screened by ILs and (ii) neutral defects that remain unaffected by the presence of ILs.

100-fold improvement in carrier drift mobilities in alkanephosphonate-passivated monocrystalline TiO2 nanowire arrays

Single crystal rutile titania nanowires grown by solvothermal synthesis are actively being researched for use as electron transporting scaffolds in perovskite solar cells, in low detection limit ultraviolet photodetectors, in photoelectrochemical water-splitting, and in chemiresistive and electrochemical sensing. The electron drift mobility (μn) in solution-grown TiO2 nanowires is very low due to a high density of deep traps, and reduces performance in optoelectronic devices. In this study, the effects of molecular passivation of the nanowire surface by octadecylphosphonic acid (ODPA), on carrier transport in TiO2 nanowire ensembles, were investigated using transient space charge limited current measurements. Infrared spectroscopy indicated the formation of a highly ordered phosphonate monolayer with a high likelihood of bidentate binding of ODPA to the rutile surface. We report the hole drift mobility (μp) for the first time in unpassivated solvothermal rutile nanowires to be 8.2 × 10−5 cm2 V−1 s−1 and the use of ODPA passivation resulted in μp improving by nearly two orders of magnitude to 7.1 × 10−3 cm2 V−1 s−1. Likewise, ODPA passivation produced between a 2 and 3 order of magnitude improvement in μn from ∼10−5–10−6 cm2 V−1 s−1 to ∼10−3 cm2 V−1 s−1. The bias dependence of the post-transit photocurrent decays in ODPA-passivated nanowires indicated that minority carriers were lost to trapping and/or monomolecular recombination for small values of bias (<5 V). Bimolecular recombination was indicated to be the dominant recombination mechanism at higher bias values.

Electron trapping energy states of TiO2–WO3 composites and their influence on photocatalytic degradation of bisphenol A

Impregnation method was used to synthesize TiO2–WO3 composites with two different TiO2 morphologies (nanorods (R-TiO2–WO3) and polyhedral nanoparticles (P-TiO2–WO3)). Their structural, morphological, surface properties and electron trapping states were analyzed and correlated to performance in photocatalytic bisphenol A oxidation. TiO2 nanorods were prepared with alkaline hydrothermal digestion of commercially available high surface TiO2 nanopowder (DT-51), which was used as a reference for TiO2 nanoparticles. TEM analysis showed that in R-TiO2–WO3 composites WO3 is dispersed over the surface of TiO2 nanorods in amorphous form and with increasing amount of WO3 the TiO2 crystal structure deteriorates. However, in the case of P-TiO2–WO3 composite, multiphase system with monoclinic WO3 intermixed with anatase TiO2 was observed. Moreover, P-TiO2–WO3 composite showed strong surface acidic sites, which were absent in R-TiO2–WO3 composites; this information is significant to understand the depth of the electron trapping states. UV light-induced electron–hole pair excitations and decay dynamics in both TiO2–WO3 composites were studied by infrared spectroscopy measurements and information about the conduction band (CB) electrons and surface trapping states of the composites were collected. In the case of composites with high density of shallow trapping states, enhanced photocatalytic activity was observed. On the contrary, lower photocatalytic activity of solids was observed in cases where deep trapping (TiO2 nanoparticles) or fast recombination (R-TiO2–WO3 composite) prevailed.

Effect of InGaN thickness on assisted trap recombination and behaviour of InGaN/AlGaN double heterostructure LED

This work is dedicated to the study of InGaN based LED on the thickness variation effect. The operating voltage, total emission rate, efficiency droop and spontaneous recombination rate was improved by increasing the thickness of InGaN layer of InGaN/AlGaN electron blocking layer (EBL). Based on optical and electrical results, the thicker InGaN layer could have lower operating voltage and higher total emission rate. LED with 5nm thick InGaN layer also could prevent electron leakage into p-region and improve hole injection efficiency. As a result it decreases 60% of efficiency droops and reduces the trap assisted recombination in InGaN/AlGaN active light emitting region. This indicates that InGaN layer may decrease the non-radiative recombination

The Role of Graphene Nanostructures on Colloidal Quantum Dot Photovoltaic Devices

PbS quantum dots (QDs) have received considerable attention in the past decade due to their low-cast solution processing, bandgap tenability by control the size, and multiexciton generation make them particularly attractive as an active absorbing layer in optoelectronic devices such as solar cells. Recently, combining semiconductor quantum dots and graphene are a new strategy to improve the optoelectronic properties of photovoltaics devices. Herein, we have reviewed our recent works which are expolred the role of graphene as a passivation agent on the surface of PbS QDs and an interface layer between the electron transfer layer and absorber layer. First, we have demonstrated that the graphene can wrap the surface of PbS QDs and passivate the deep trap states by using a novel solution proces. Simulation by density functional theory of a prototypical model of a non-stoichiometric Pb-rich core (400 atoms) coated by graphene (20 atoms for each graphene sheet) indicates the possibility of surface passivation of (111) planes of PbS with graphene resulting in a decrease in trap states and recombination sites. The graphene coating of the PbS quantum dots decrease the exciton lifetime up to 0.78 µs as compared to 1.2 µs for the oleic acid passivated PbS quantum dots due to the fast extraction of carriers. We have employed PbS/graphene as well as Cd-doped PbS/graphene quantum dots as active layers of bulk heterojunction solar cells, and achieved solar power conversion efficiencies of 3.6% and 4.1%, respectively. In addition to the surface passivation role, a thin layer of graphene as an interface layer can increase the performance of PbS QDs device due to the improvement of carriers transfer and enhancement of current density. In our work, a simple and novel method is employed to synthesize a new structure of graphene, i.e. hollow graphene. In this regards., ZnO-Graphene QDs nanocomposite synthesized by a solution method and then ZnO QDs were dissolved from the structure using an acidic solution to obtain hollow graphene. Afterward, this structure was utilized in PbS QDs solar cell in order to improve the carriers transport. It is shown that holow graphene electrodes enhance electron extraction, yielding 30% improvement in performance compared with conventional device. Figure 1

Discovery of Fe-Ce Oxide/BiVO4 Photoanodes through Combinatorial Exploration of Ni-Fe-Co-Ce Oxide Coatings.

An efficient photoanode is a prerequisite for a viable solar fuels technology. The challenges to realizing an efficient photoanode include the integration of a semiconductor light absorber and a metal oxide electrocatalyst to optimize corrosion protection, light trapping, hole transport, and photocarrier recombination sites. To efficiently explore metal oxide coatings, we employ a high-throughput methodology wherein a uniform BiVO4 film is coated with 858 unique metal oxide coatings covering a range of metal oxide loadings and the full (Ni-Fe-Co-Ce)Ox pseudoquaternary composition space. Photoelectrochemical characterization of the photoanodes reveals that specific combinations of metal oxide composition and loading provide up to a 13-fold increase in the maximum photoelectrochemical power generation for oxygen evolution in pH 13 electrolyte. Through mining of the high-throughput data we identify composition regions that form improved interfaces with BiVO4. Of particular note, integrated photoanodes with catalyst compositions in the range Fe(0.4-0.6)Ce(0.6-0.4)Ox exhibit high interface quality and excellent photoelectrochemical power conversion. Scaled-up inkjet-printed electrodes and photoanodic electrodeposition of this composition on BiVO4 confirms the discovery and the synthesis-independent interface improvement of (Fe-Ce)Ox coatings on BiVO4.

Severe signal loss in diamond beam loss monitors in high particle rate environments by charge trapping in radiation‐induced defects

The beam condition monitoring leakage (BCML) system is a beam monitoring device in the compact muon solenoid (CMS) experiment at the large hadron collider (LHC). As detectors 32 poly‐crystalline (pCVD) diamond sensors are positioned in rings around the beam pipe. Here, high particle rates occur from the colliding beams scattering particles outside the beam pipe. These particles cause defects, which act as traps for the ionization, thus reducing the charge collection efficiency (CCE). However, the loss in CCE was much more severe than expected from low rate laboratory measurements and simulations, especially in single‐crystalline (sCVD) diamonds, which have a low initial concentration of defects. After an integrated luminosity of a few corresponding to a few weeks of LHC operation, the CCE of the sCVD diamonds dropped by a factor of five or more and quickly approached the poor CCE of pCVD diamonds. The reason why in real experiments the CCE is much worse than in laboratory experiments is related to the ionization rate. At high particle rates the trapping rate of the ionization is so high compared with the detrapping rate, that space charge builds up. This space charge reduces locally the internal electric field, which in turn increases the trapping rate and recombination and hence reduces the CCE in a strongly non‐linear way. A diamond irradiation campaign was started to investigate the rate‐dependent electrical field deformation with respect to the radiation damage. Besides the electrical field measurements via the transient current technique (TCT), the CCE was measured. The experimental results were used to create an effective deep trap model that takes the radiation damage into account. Using this trap model, the rate‐dependent electrical field deformation and the CCE were simulated with the software SILVACO TCAD. The simulation, tuned to rate‐dependent measurements from a strong radioactive source, was able to predict the non‐linear decrease of the CCE in the harsh environment of the LHC, where the particle rate was a factor 30 higher.

Proton-Induced Trap States, Injection and Recombination Dynamics in Water-Splitting Dye-Sensitized Photoelectrochemical Cells.

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize a sensitized metal oxide and a water oxidation catalyst in order to generate hydrogen and oxygen from water. Although the Faradaic efficiency of water splitting is close to unity, the recombination of photogenerated electrons with oxidized dye molecules causes the quantum efficiency of these devices to be low. It is therefore important to understand recombination mechanisms in order to develop strategies to minimize them. In this paper, we discuss the role of proton intercalation in the formation of recombination centers. Proton intercalation forms nonmobile surface trap states that persist on time scales that are orders of magnitude longer than the electron lifetime in TiO2. As a result of electron trapping, recombination with surface-bound oxidized dye molecules occurs. We report a method for effectively removing the surface trap states by mildly heating the electrodes under vacuum, which appears to primarily improve the injection kinetics without affecting bulk trapping dynamics, further stressing the importance of proton control in WS-DSPECs.

Influence of thermally induced structural and morphological changes, and UV irradiation on photoluminescence and optical absorption behavior of CdS nanoparticles

Polycrystalline cubic CdS nanoparticles (NPs) with a crystallite size () ~3 nm were synthesized by chemical precipitation method at room temperature. Thermal induced structural and morphological changes have been investigated using x-ray diffraction, high-resolution transmission electron microscope, x-ray fluorescence, Fourier transform infrared and Raman spectroscopy. The influence of these changes on optical absorption and photoluminescence (PL) characteristics have been studied. It was found that increasing annealing temperature (Ta), results in structural phase transitions at 300 and 700 °C, increasing and red shift of the optical band gap () due to the improvement in crystallinity. The photoluminescence emission spectrum of nonstoichiometric CdS (Cd-rich) nanopowder reveals emission bands at 365, 397, and 434 nm. Furthermore, PL spectrum of colloidal solution exhibits additional green and red emission bands at 535, 570 and 622 nm. To explain the mechanism of PL emission in CdS NPs, trapping and radiative recombination levels have been identified and the corresponding energy band diagrams are suggested. Annealing process results in an overall enhancement in PL intensity due to the improvement in crystallinity associated with the reduction of nonradiative surface state defects. Irradiation of CdS NPs colloidal solution at UV irradiation dose <13 J cm−2 leads to the enhancement of PL quantum efficiency and blue shift of (i.e. photo-brightening) due to the decrease in the particle size deduced from Brus equation , This behavior is due to UV irradiation effects such as photopolymerization, the formation of CdSO4 passivation layers due to photooxidation and the reduction in by photocorrosion process. At UV irradiation dose <13 J cm−2, PL emission intensity continuously enhances without any change in both and . This behavior is discussed in terms of electron filling model. Boltzmann curve fitting successfully describes the dependence of both and on UV irradiation dose.

Reliability investigation of high-k/metal gate in nMOSFETs by three-dimensional kinetic Monte-Carlo simulation with multiple trap interactions

Degradation behaviors in the high-k/metal gate stacks of nMOSFETs are investigated by three-dimensional (3D) kinetic Monte-Carlo (KMC) simulation with multiple trap coupling. Novel microscopic mechanisms are simultaneously considered in a compound system: (1) trapping/detrapping from/to substrate/gate; (2) trapping/detrapping to other traps; (3) trap generation and recombination. Interacting traps can contribute to random telegraph noise (RTN), bias temperature instability (BTI), and trap-assisted tunneling (TAT). Simulation results show that trap interaction induces higher probability and greater complexity in trapping/detrapping processes and greatly affects the characteristics of RTN and BTI. Different types of trap distribution cause largely different behaviors of RTN, BTI, and TAT. TAT currents caused by multiple trap coupling are sensitive to the gate voltage. Moreover, trap generation and recombination have great effects on the degradation of HfO2-based nMOSFETs under a large stress.

Quasi Core/Shell Lead Sulfide/Graphene Quantum Dots for Bulk Heterojunction Solar Cells

Hybrid nanostructures combining semiconductor quantum dots and graphene are attracting increasing attention because of their optoelectronic properties promising for photovoltaic applications. We present a hot-injection synthesis of a colloidal nanostructure which we define as quasi core/shell PbS/graphene quantum dots due to the incomplete passivation of PbS surfaces with an ultrathin layer of graphene. Simulation by density functional theory of a prototypical model of a nonstoichiometric Pb-rich core (400 atoms) coated by graphene (20 atoms for each graphene sheet) indicates the possibility of surface passivation of (111) planes of PbS with graphene resulting in a decrease in trap states and recombination sites. The graphene coating of the PbS quantum dots decreases the exciton lifetime up to 0.78 μs as compared to 1.2 μs for the oleic acid passivated PbS quantum dots due to the fast extraction of carriers. We have employed PbS/graphene as well as Cd-doped PbS/graphene quantum dots as active layers of bu...

Towards the efficiency limits of silicon solar cells: How thin is too thin?

It is currently possible to fabricate crystalline silicon solar cells with the absorber thickness ranging from a few hundreds of micrometres (conventional wafer-based cells) to devices as thin as 1μm. In this work, we use a model single-junction solar cell to calculate the limits of energy conversion efficiency and estimate the optimal absorber thickness. We have found that the limiting efficiency for cells in the thickness range between 40 and 500μm is very similar and close to 29%. In this regard, we argue that decreasing the thickness below around 40μm is counter-productive, as it significantly reduces the maximum achievable efficiency, even when an optimal light trapping is implemented. We analyse the roles of incomplete light trapping and extrinsic (bulk and surface) recombination mechanisms. For a reasonably high material quality, consistent with present-day fabrication techniques, the optimal thickness is always higher than a few tens of micrometres. We identify incomplete light trapping and parasitic losses as a major roadblock for improving the efficiency upon the current record of 25.6% for silicon solar cells. Finally, considering the main parameters that impact the solar cell performance, we quantify the requirements for achieving a given efficiency, which helps us to establish a proper design strategy for high efficiency silicon solar cells.

Improved Power Conversion Efficiency of Inverted Organic Solar Cells by Incorporating Au Nanorods into Active Layer.

This Research Article describes a cooperative plasmonic effect on improving the performance of organic solar cells. When Au nanorods(NRs) are incorporated into the active layers, the designed project shows superior enhanced light absorption behavior comparing with control devices, which leads to the realization of organic solar cell with power conversion efficiency of 6.83%, accounting for 18.9% improvement. Further investigations unravel the influence of plasmonic nanostructures on light trapping, exciton generation, dissociation, and charge recombination and transport inside the thin films devices. Moreover, the introduction of high-conductivity Au NRs improves electrical conductivity of the whole device, which contributes to the enhanced fill factor.

Excitation correlation photoluminescence in the presence of Shockley-Read-Hall recombination

Excitation correlation photoluminescence (ECPL) measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non-radiative recombination path, but not for a non-linear recombination rate as in the general Shockley-Read-Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley-Read-Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non-linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non-radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs/InAlAs that were additionally cross-checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreement.

Phase transformation of ZrO2:Tb3+ nanophosphor: Color tunable photoluminescence and photocatalytic activities

The current study involves synthesis of a series of Tb3+ doped ZrO2 nanophosphors by solution combustion method using oxalyl dihydrazide as fuel. The as-formed ZrO2:Tb3+ nanophosphors having different concentrations of Tb3+ (1–11mol%) were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV–Visible spectroscopic techniques and the materials were subjected to photoluminescence and photocatalytic dye decolorization studies. The PXRD analysis indicates the formation of tetragonal symmetry up to 5mol% concentration of Tb3+. Further increase in Tb3+ concentration has lead to cubic phase formation and the same was confirmed by Rietveld refinement analysis. SEM images revealed that material was highly porous in nature comprising of large voids and cracks with irregular morphology. TEM and SAED images clearly confirm the formation of high quality tetragonal nanocrystals. The emissive properties of nanophosphors were found to be dependent on Tb3+ dopant concentration. The green emission of the material was turned to white emission with the increase of Tb3+ ion concentration. The photocatalytic activities of these nanophosphors were probed for the decolorization of Congo red under UV and Sunlight irradiation. All the photocatalysts showed enhanced activity under UV light compared to Sunlight. The photocatalyst with 7mol% Tb3+ showed enhanced activity attributed to effective separation of charge carriers due to phase transformation from tetragonal to cubic. The influence of crystallite size and PL on charge carrier trapping–recombination dynamics was investigated. The study successfully demonstrates synthesis of tetragonal and cubic ZrO2:Tb3+ green nanophosphors with superior photoluminescence and photocatalytic activities.

Photocarrier recombination dynamics in perovskite CH3NH3PbI3 for solar cell applications.

Using time-resolved photoluminescence and transient absorption measurements at room temperature, we report excitation-intensity-dependent photocarrier recombination processes in thin films made from the organo-metal halide perovskite semiconductor CH3NH3PbI3 for solar-cell applications. The photocarrier dynamics are well described by a simple rate equation including single-carrier trapping and electron-hole radiative recombination. This result provides clear evidence that the free-carrier model is better than the exciton model for interpreting the optical properties of CH3NH3PbI3. The observed large two-carrier recombination rate suggests the promising potential of perovskite semiconductors for optoelectronic device applications. Our findings provide the information about the dynamical behaviors of photoexcited carriers that is needed for developing high-efficiency perovskite solar cells.