Surface Trap States Research Articles

Synthetic Control of the Photoluminescence Stability of Organolead Halide Perovskites

An optimized synthetic procedure for preparing photostable nanocrystalline methylammonium lead halide materials is reported. The procedure was developed by adjusting the lead halide to methylammonium/octylammonium halide precursor ratio. At a high precursor ratio (1:3), a blue-shifted photoinduced luminescence peak is measured at 642 nm for CH3NH3PbI3 with 0.01 to 12 mJ pulsed-laser irradiation. The appearance of this peak is reversible over 300 min upon blocking the irradiation. In order to determine if the peak is the result of a phase change, in situ x-ray diffraction measurements were performed. No phase change was measured with an irradiance that causes the appearance of the photoinduced luminescence peak. Luminescence microscpectroscopy measurements showed that the use of a lower precursor ratio (1:1.5) produces CH3NH3PbI3 and CH3NH3PbBr3 perovskites that are stable over 4 min of illumination. Given the lack of a measured phase change, and the dependence on the precursor ratio, the photoinduced luminesce peak may derive from surface trap states. The enhanced photostability of the resulting perovskite nanocrystals produced with the optimized synthetic procedure supports their use in stable optoelectronic devices.

New strategies for colloidal-quantum-dot-based intermediate-band solar cells.

The intermediate-band solar cell (IBSC) concept promises to increase the efficiency limit in a single-junction solar cell through the absorption of below-bandgap-energy photons. Despite their operating principle having been proposed over 20 years ago, IBSCs have not delivered on this promise yet, and the devices fabricated so far, mainly based on embedded epitaxial quantum dots, have instead operated with lower efficiency than conventional solar cells. A new paradigm, based on the exploitation as the intermediate band of the intragap states naturally occurring in the density functional theory description of colloidal (i.e., chemically synthesized) quantum dots, was suggested recently. Here, we revisit this intriguing concept unveiling its shortcomings and propose two alternative schemes: in the first, the localized electron surface trap states, ubiquitously found in commonly synthesized colloidal quantum dots, are used as intermediate bands in strongly coupled films made of small InAs nanocrystals and, in the second scheme, the intermediate band is provided by the conduction-band-minimum-derived miniband in films of larger InAs nanocrystals. Both schemes yield estimated limiting IBSC efficiencies exceeding Shockley-Queisser's limit for a single absorber.

Enhanced thermal stability of electron transport layer-free perovskite solar cells via interface strain releasing

The thermal decomposition of perovskite films on ZnO surfaces is generally believed to originate from specific surface states of ZnO and the impact from the lattice mismatch between ZnO and perovskite films on this process has long been ignored. In this research, the role of lattice mismatch in the thermal degradation process of cesium-containing perovskite films on Al doped ZnO (AZO) is studied. A Ba(OH)2 buffer layer on the surface of AZO is employed to release the lattice mismatch and suppress the thermal degradation of perovskite films resulted from ZnO. Consequently, perovskite films with enhanced thermal stability and crystalline properties are obtained. Meanwhile, the Ba(OH)2 films efficiently passivate the surface trap states and reduce the vacuum level of the AZO surfaces. On this basis, electron transport layer-free perovskite solar cells yield the best efficiency of 18.18% and the thermal stability is obviously improved.

Red-emitting CsPbBrI2/PbSe heterojunction nanocrystals with high luminescent efficiency and stability for bright light-emitting diodes

All inorganic perovskite CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have been endowed great promise for optoelectronic device applications. However, further practical applications of these NCs are blocked because of their poor stability. In the present work, we propose a novel strategy to synthesize highly luminescent and stable red-emitting CsPbBrI2/PbSe heterojunction nanocrystals (h-NCs) via an epitaxial solution growth method, in which lattice-matching condition between CsPbBrI2 and PbSe was satisfied, and each CsPbBrI2 NC was partially covered by PbSe in the CsPbBrI2/PbSe heterodimers. The ultrafast transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy revealed that incorporation of PbSe can modify surface and hence passivate the surface trap states of the CsPbBrI2 NCs, helping to enhance the photoluminescence quantum yields (PLQY) (up to 83.4%) of these CsPbBrI2/PbSe h-NCs. First-principle calculations based on DFT confirmed that the significantly improved stability of these CsPbBrI2/PbSe h-NCs was attributed to the strong chemical bonding of selenium atoms of PbSe and lead atoms of PbX2-terminated surface from CsPbBrI2. Thin films of these CsPbBrI2/PbSe h-NCs can maintain bright red PL brightness and cubic phase even after 15-day storage under a high humidity condition. Benefiting from the performances of high stability and luminescent efficiency, these red-emitting CsPbBrI2/PbSe h-NCs have a positive implication for bright light-emitting diodes (LEDs).

Alkali acetate-assisted enhanced electronic coupling in CsPbI3 perovskite quantum dot solids for improved photovoltaics

Fully inorganic CsPbI3 perovskite quantum dots (CsPbI3-PQDs) are known as the best-performing photovoltaic absorber in colloidal quantum dot solar cells. This is achieved by improving the cubic-phase-stabilization and electronic-coupling in CsPbI3-PQD solids. In conventional approaches, the hydrolysis of methyl acetate (MeOAc) resulting in acetic acid and methanol as intermediate substances plays a key role in replacing long-chain hydrocarbons with short-chain ligands, which improves charge transport in the CsPbI3-PQD solids. However, CsPbI3-PQDs suffer from lattice distortion and instability under acidic conditions including protons and polar media, leading to CsPbI3-PQD fusion and poor photovoltaic performance. Herein, we report that electronic coupling and photovoltaic performance of CsPbI3-PQD solids are improved by efficient removal of long-chain oleate ligands using a solution of sodium acetate (NaOAc) in MeOAc, which results in the direct generation of OAc ions without forming protons and methanol. NaOAc-based ligand exchange of CsPbI3-PQDs enables preservation of their nanocrystal size without fusion and minimization of surface trap states originating from metal hydroxide formation on their surfaces. Consequently, the best solar cell comprising NaOAc-treated CsPbI3-PQDs shows an improved device performance with a power conversion efficiency (PCE) of 13.3%, as compared with a lead nitrate-treated control device (12.4% PCE).

Electrochemical Modulation of the Photophysics of Surface-Localized Trap States in Core/Shell/(Shell) Quantum Dot Films.

In this work, we systematically study the spectroelectrochemical response of CdSe quantum dots (QDs), CdSe/CdS core/shell QDs with varying CdS shell thicknesses, and CdSe/CdS/ZnS core/shell/shell QDs in order to elucidate the influence of localized surface trap states on the optoelectronic properties. By correlating the differential absorbance and the photoluminescence upon electrochemically raising the Fermi level, we reveal that trap states near the conduction band (CB) edge give rise to nonradiative recombination pathways regardless of the CdS shell thickness, evidenced by quenching of the photoluminescence before the CB edge is populated with electrons. This points in the direction of shallow trap states localized on the CdS shell surface that give rise to nonradiative recombination pathways. We suggest that these shallow trap states reduce the quantum yield because of enhanced hole trapping when the Fermi level is raised electrochemically. We show that these shallow trap states are removed when additional wide band gap ZnS shells are grown around the CdSe/CdS core/shell QDs.

Phosphate-Passivated SnO2 Electron Transport Layer for High-Performance Perovskite Solar Cells.

Tin oxide (SnO2) is widely used in perovskite solar cells (PSCs) as an electron transport layer (ETL) material. However, its high surface trap density has already become a strong factor limiting PSC development. In this work, phosphoric acid is adopted to eliminate the SnO2 surface dangling bonds to increase electron collection efficiency. The phosphorus mainly exists at the boundaries in the form of chained phosphate groups, bonding with which more than 47.9% of Sn dangling bonds are eliminated. The reduction of surface trap states depresses the electron transport barriers, thus the electron mobility increases about 3 times when the concentration of phosphoric acid is optimized with 7.4 atom % in the SnO2 precursor. Furthermore, the stability of the perovskite layer deposited on the phosphate-passivated SnO2 (P-SnO2) ETL is gradually improved with an increase of the concentration. Due to the higher electron collection efficiency, the P-SnO2 ETLs can dramatically promote the power conversion efficiency (PCE) of the PSCs. As a result, the champion PSC has a PCE of 21.02%. Therefore, it has been proved that this simple method is efficient to improve the quality of ETL for high-performance PSCs.

Mechanism of surface passivation of methylammonium lead tribromide single crystals by benzylamine

Hybrid organic-inorganic perovskites are semiconductors that have great potential for optoelectronic applications such as light-emitting diodes, photodetectors, and solar cells. In such devices, the surface plays a crucial role in the performance and stability, as it strongly influences the recombination rate of excited charge carriers. It is reported that molecular ligands such as benzylamine are capable of reducing the surface trap state density in thin films. In this work, we aim to clarify the mechanisms that govern the surface passivation of hybrid perovskites by benzylamine. We developed a versatile approach to investigate the influence of benzylamine passivation on the well-defined surface of freshly cleaved hybrid perovskite crystals. We show that benzylamine permanently passivates surface trap states in these single crystals, resulting in enhanced photoluminescence and charge carrier lifetimes. Additionally, we show that exposure to benzylamine leads to the replacement of the methylammonium cations by benzylammonium, thereby creating a thermodynamically more stable two-dimensional (2D) perovskite (BA)2PbBr4 on the surface of the three-dimensional crystal. This conversion to a 2D perovskite drives an anisotropic etching of the crystal surface, with the {100} planes being most prone to etching. Initially, square etching pits appear spread over the surface. As time elapses, these etching pits broaden and merge to yield large flat terraces that are oriented normally to the cleaving plane when they form. A thorough understanding of the mechanisms governing the surface passivation is crucially important to optimize and design novel passivation schemes, with the ultimate goal of further advancing the efficiency of optoelectronic devices based on hybrid perovskites.

A facile method to fabricate high-quality perovskite nanocrystals based on single crystal powder

Although great advancements have been successfully achieved in ligand-assisted reprecipitation strategy (LARP) towards lead halide perovskite nanocrystals (NCs) synthesis, it still remains challenging to develop bright and stable iodide-based perovskite NC via facile LARP. Herein, strikingly bright MAPbI3 NCs with photoluminescence quantum yield (PLQY) as high as 79% are synthesized via replacing the raw material (MAI and PbI2) with MAPbI3 crystal powder during LARP procedure. It has been found that crystal powder derived MAPbI3 NCs are more iodide-rich compared with that based on raw material, which is favorable to passivate the surface trap state. Accordingly, femtosecond transient absorption spectroscopies and space charge limited current measurements have corroborated that the trap density is much less in crystal powder resulted MAPbI3 NCs. Further analyses indicate stronger solvation with reduced precursor colloid size has been observed in crystal powder derived precursor solution probably due to the formation of MAPbI3·DMF. This work has provided a facile but valid method to enhance the photoluminescence of perovskite NC via modulating the beginning precursor solution.

Energetics of Nonradiative Surface Trap States in Nanoparticles Monitored by Time-of-Flight Photoconduction Measurements on Nanoparticle-Polymer Blends.

Nanoparticles (NPs) have had increasingly successful applications including in emissive or photovoltaic devices; however, trap states associated with the surface of NPs often drastically reduce the efficiency of devices and are difficult to detect spectroscopically. We show the applicability of photoconduction as the means of detecting and quantifying trap states in NPs. We performed time-of-flight (ToF) photoconduction measurements, using semiconducting poly[bis(4-phenyl)(4-butyphenyl)amine] (P-TPD) doped with either core/shell CdSeS/CdS quantum dots (QDs) or perovskite CsPbBr3 NPs, both of which are carefully designed to be energetically matched. In the case of the QDs, a drop in the hole mobility from ∼10-3 to ∼10-4 cm2 V-1 s-1 was observed when compared to a control sample, suggesting the presence of a hole trapping. These trap states were found to be around -5.0 to -4.9 eV from the vacuum level. The presence of the trap states was further supported by a coincident reduction in the photoluminescence (PL), quantum yield (QY), and lifetime of the core/shell QDs after purification. Using the measured reductions in the PL, QY, and lifetime, the surface trap state density was estimated to increase by between 20 and 40%, most likely due to a ligand detachment. In the case of the perovskite NP-doped samples, a mobility of ∼10-3 cm2 V-1 s-1 was measured. Thus, doping with perovskite NPs did not generate any obvious hole trapping from the P-TPD matrix. The absence of a trapping may be related to the reduced surface-to-volume ratio and NP number density of the perovskite NPs compared to the core/shell QDs, since the perovskite NPs are approximately 10 times larger in radius than that of the core/shell QDs. Our results suggest that to minimize the presence of trap states with a view to improving device performance, large-size perovskite NPs appear to be better than small-size QDs.

Thermodynamically stabilized β-CsPbI3-based perovskite solar cells with efficiencies >18.

Although β-CsPbI3 has a bandgap favorable for application in tandem solar cells, depositing and stabilizing β-CsPbI3 experimentally has remained a challenge. We obtained highly crystalline β-CsPbI3 films with an extended spectral response and enhanced phase stability. Synchrotron-based x-ray scattering revealed the presence of highly oriented β-CsPbI3 grains, and sensitive elemental analyses-including inductively coupled plasma mass spectrometry and time-of-flight secondary ion mass spectrometry-confirmed their all-inorganic composition. We further mitigated the effects of cracks and pinholes in the perovskite layer by surface treating with choline iodide, which increased the charge-carrier lifetime and improved the energy-level alignment between the β-CsPbI3 absorber layer and carrier-selective contacts. The perovskite solar cells made from the treated material have highly reproducible and stable efficiencies reaching 18.4% under 45 ± 5°C ambient conditions.

Tetragonal phases for perovskite solar cells

Solar Cells The power conversion efficiencies (PCEs) of all-inorganic perovskites are lower than those of materials with organic cations. This is in part because these materials have larger bandgaps. The cubic crystal phases of these materials also exhibit poor stability. Wang et al. synthesized the tetragonal β-phase of CsPbI3 from HPbI3 and CsI. The material exhibited higher stability and a more favorable bandgap, which allowed for PCEs of 15%. Passivation of the surface trap state with choline iodide boosted PCEs to 18%. Science , this issue p. [591][1] [1]: /lookup/doi/10.1126/science.aav8680

Facile precursor stoichiometry engineering for efficient inverted perovskite solar cells without any dopants

Full coverage of MAPbI3 layer could be easily obtained by two-step method, while the perovskite precursor stoichiometry ratio is very sensitive to the performance of the perovskite solar cells (PSCs). Here, we demonstrated the high quality MAPbI3 film can be formed on the hydrophobic poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) without any surface treatment and dopant. We systematically adjusted the stoichiometry ratio of the first PbI2:MAI precursor from 1.1:0.3 to 1.3:0.3 and the surface morphology as well as optoelectronic properties of perovskite films were also measured. Benefits from the more uniform surface, higher crystallinity and lower trap states, the high photoelectric conversion efficiency (PCE) of 18.38% and lower hysteresis were achieved based on the optimized ratio of PbI2:MAI. That is one of highest performance of the inverted PSCs based on free-dopants both in MAPbI3 active layer and PTAA hole transport layer. In addition, the devices also showed good stability, and the PCE could maintain 73.85% of its initial value when they were exposed in air and 95.63% in N2 for 216 h. This work suggested a facile precursor stoichiometry engineering to obtain efficient and stable MAPbI3 PSCs based on free-dopants.

Exploring Deep and Shallow Trap States in a Non-Fullerene Acceptor ITIC-Based Organic Bulk Heterojunction Photovoltaic System

Nonfullerene organic molecules that usually act as acceptors have exhibited prominent optoelectronic properties in organic bulk heterojunction (BHJ) solar cells. Although the highly efficient exciton dissociation can happen even with a relatively smaller driving force, the underlying energy loss including exciton dissociation and electron–hole recombination in the nonfullerene acceptor (NFA) system remains unclear. In principle, the energetic disorder characteristic is a common feature in organic semiconductors as well as their organic blends due to the molecular random distribution and the donor–acceptor interpenetration network. Owing to their completely different molecular structures in contrast to the bulky-ball shaped fullerene-derivative counterpart such as PCBM, a comprehensive understanding for the trap-associated energetic disorder characteristic and distribution are merits to further eliminate the open-circuit voltage (Voc) loss. In this work, a highly efficient planar organic BHJ solar cell comprising ITO(glass)/ZnO/PBDB-T:ITIC/MoO3/Al was fabricated. Both the polymeric donor PBDB-T and NFA ITIC form an interpenetrating network. With an assist from the nondestructive impedance spectroscopic technique, surface and bulk trap states can be fully revealed. More importantly, both of them respectively contain deep and shallow traps with different energetic locations in an energy gap. The results help to explain the appearance of double and triple capacitance–voltage (C–V) bands. Furthermore, with pump–probe laser spectroscopic measurements, the trap states may locate far below the photoabsorption band-edge. This work serves an insightful view on the trap states in the NFA-based organic BHJ system, and it is valuable for minimizing the energy loss.

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the band gap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states, providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of the QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.

Surface passivated aqueous mediated synthesis of CdTe QDs: potential nano thin films

Cadmium Telluride (CdTe) semiconductor quantum dots (QDs) have potential applications in optoelectronics, biomedical and photovoltaic solar cells. In spite of having tremendous potentiality in the applied fields, the literature reveals scanty reports about the low cost, environmentally sustainable and ambient temperature methodology for the synthesis of CdTe QDs. Currently, the aqueous synthesis route is more promising due to its more reproducibility, less toxicity and biocompatibility. However, the reported aqueous methods also find defect associated photoluminescence due to the presence of surface trap states, which diminish luminescence property drastically with red-shift emission. The present work reportssynthesis of surface passivated FCC cubic crystalline CdTe QDs (size ∼4–7 nm from x-ray diffraction and transmission electron microscopic techniques) in an atom economic greener way, where Polyvinylpyrollidone is acting as capping agent. Polyvinylpyrollidone effectively passivates the surface of QDs by quenching surface dangling bonds, which is proved by photoluminescence investigation with quality single band edge emission at 500 nm (λex = 335 nm, Eg = 2.48 eV). The characterized CdTe nanomaterials were subjected to the fabrication of thin films by simple and cost-effective spin coating method.

Enhanced Performance of Perovskite Light-Emitting Diodes via Diamine Interface Modification

Interfacial engineering between charge transport layers and perovskite light-emitting layers has been applied as an effective strategy to enhance performance of perovskite light-emitting diodes (PeLEDs). Herein, we introduce a Lewis base diamine molecule [2,2-(ethylenedioxy)bis(ethylammonium), EDBE] to modify the interface between the ZnMgO electron transport layer (ETL) and perovskite light-emitting layer in PeLEDs. With two amino groups in EDBE, one amine can interact with ZnMgO beneath to tune the growth of perovskite films, resulting in improved electron injection and suppressed current leakage. Meanwhile, the other amine can passivate the surface trap states of the polycrystalline perovskite films, which would eliminate trap-mediated nonradiative recombination. An enhanced performance for near-infrared PeLEDs is achieved with external quantum efficiency from 9.15 to 12.35% after incorporating the EDBE interfacial layer. This work demonstrated that the introduction of Lewis base diamine molecules as the ETL/perovskite interfacial agent is a promising way for developing high-performance PeLEDs.

Elucidation of Photoluminescence Blinking Mechanism and Multiexciton Dynamics in Hybrid Organic-Inorganic Perovskite Quantum Dots.

Halide perovskites (ABX3 ) have emerged as promising materials in the past decade owing to their superior photophysical properties, rendering them potential candidates as solar cells, light-emitting diode displays, and lasing materials. To optimize their utilization into optoelectronic devices, fundamental understanding of the optical behaviors is necessary. To reveal the comprehensive structure-property relationship, CH3 NH3 PbBr3 (MAPbBr3 ) perovskite quantum dots (PQDs) of three different sizes are prepared by controlling the precipitation temperature. Photoluminescence (PL) blinking, a key process that governs the emission efficiency of the PQD materials, is investigated in detail by the time-resolved spectroscopic measurements of individual dots. The nature of the generated species in the course of blinking events is identified, and the mechanism governing the PL blinking is studied as a function of PQD sizes. Further, the practical applicability of MAPbBr3 PQDs is assessed by studying the multiexciton dynamics under high photoexcitation intensity under which most of the display devices work. Ultrafast transient absorption spectroscopy helped in uncovering the volume-dependent Auger recombination rates, which are further explored by comparing the early-time transitions related to surface trap states and higher band states.

Efficient and Stable Perovskite Solar Cell Achieved with Bifunctional Interfacial Layers.

The elaborate control of the surface morphologies and trap states of solution-processed perovskite films significantly governs the photovoltaic performance and moisture resistance of perovskite solar cells (PSCs). Herein, a thin layer of poly(triaryl amine) (PTAA) was unprecedentedly devised on top of perovskite quasi-film by spin-coating PTAA/chlorobenzene solution before annealing the perovskite film. This treatment induced a smooth and compact perovskite layer with passivated surface defects and grain boundaries, which result in a significantly reduced charge recombination. Besides, the time-resolved photoluminescence spectra of the PTAA-treated perovskite films confirmed a faster charge transfer and a much longer lifetime compared to the control cells without the PTAA treatment. Moreover, such a hydrophobic polymer atop the perovskite layer could effectively protect the perovskite against humidity and retain 83% of its initial efficiency in contrast to 56% of control cells stored for 1 month in ambient conditions (25 °C, 35 RH%). As a result, the PTAA-treated PSCs displayed an average efficiency of 17.77% (with a peak efficiency of 18.75%), in contrast to 16.15% of the control cells, and enhanced stability. These results demonstrate that PTAA and the method thereof constitute a promising passivation strategy for constructing stable and efficient PSCs.

Bamboo shoots shaped FeVO4 passivated ZnO nanorods photoanode for improved charge separation/transfer process towards efficient solar water splitting

Among many novel and promising semiconductors, the triclinic iron vanadate (FeVO4) has been rarely applied in the field of solar water splitting due to its poor charge carrier separation ability. In this work, we prepared FeVO4-passivated ZnO heterojunction nanorods (FVO/ZnO NRs) photoanodes by simple spin-coating and calcination method for the first time. The optimal photoanode exibihits stable and effective PEC water oxidation performance, compared to pristine ZnO, affording 130 mV negative shift in onset potential and 2.4 folds augment in photocurrent density. Particularly, X-ray photoelectron spectroscopy and Mott-Schottky test reveal the establishment of heterojunction, which subdue photo-induced carriers bulk recombination and improve the charge separation efficiency. Open-circuit photovoltage and electrochemical impedance spectroscopy were recorded to further clarify that the FeVO4-passivated layer eliminates the surface trapping state, thus increasing the photovoltage and promoting the interface charge transfer kinetics. These demonstrations are expected to provide new thread for the rational application of new photoanode candidates and to optimize PEC performance.