Perovskite-based Optoelectronic Devices Research Articles

Polarity effects of ZnO on charge recombination at CsPbBr3 nanoparticles/ZnO interfaces

We report on the effects of the polarity of ZnO on the transfer of photogenerated electrons at CsPbBr3 nanoparticle/ZnO heterojunctions. Closely packed CsPbBr3 nanoparticles were coated onto the atomically flat Zn-polar (0001) and O-polar (0001¯) surfaces of ZnO via spin-coating. The photoluminescence intensity of the CsPbBr3 nanoparticles on the Zn-polar surface was only half of that of the O-polar surface. The lifetime of radiative emission from the CsPbBr3 nanoparticles on the Zn-polar surface also decreased. These results indicate that the polar surface of ZnO clearly affects the recombination of photogenerated electrons from CsPbBr3. This suggests that controlling the polarity of ZnO could be a significant approach for achieving high-efficiency perovskite-based optoelectronic devices, such as light-emitting diodes and photovoltaic devices.

Photodriven Dipole Reordering: Key to Carrier Separation in Metalorganic Halide Perovskites.

Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron-hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials' properties.

Understanding structures and properties of phosphorene/perovskite heterojunction toward perovskite solar cell applications

Two-dimensional black phosphorus (phosphorene) has drawn much attention in recent years due to its excellent electronic and optical properties. In this manuscript, we employ ab initio calculations to investigate the structural origin of the phosphorene/perovskite heterostructure. The calculations suggest that the chemical stability and the mechanical stability depend on the surface terminations, and the mechanical stability of the phosphorene/perovskite heterojunction should be further improved. The weak interactions between the P atoms in the phosphorene and the under-coordinated Pb atoms at the perovskite surfaces, as well as the weak interfacial charge transfer characters, are proposed to be mainly responsible for the moderate heterostructure stability. Suggestions to improve the stability of the heterojunction are provided. This study helps the fundamental understanding of the interaction between the phosphorene and the halide perovskite materials, and could provide a foundation for the better understanding of the low-dimensional materials in perovskite-based optoelectronic devices.

Light-Emitting Electrochemical Cells of Single Crystal Hybrid Halide Perovskite with Vertically Aligned Carbon Nanotubes Contacts

Based on the reported ion migration under electric field in hybrid lead halide perovskites we have developed a bright, light emitting electrochemical cell with CH3NH3PbBr3 single crystals directly grown on vertically aligned carbon nanotube (VACNT) forests as contact electrodes. Under the applied electric field, charged ions in the crystal drift and accumulate in the vicinity of the electrodes, resulting in an in operando formed p-i-n heterojunction. The decreased interface energy barrier and the strong charge injection due to the CNT tip enhanced electric field, result in a bright green light emission up to 1800 cd/m2 at room temperature (average = 60 cd/m2). Beyond the light emission, this original device architecture points to the possibility of implementing vertically aligned CNTs as electrodes in operationally-stable perovskite-based optoelectronic devices.

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects.

The photovoltaics of organic-inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regarding compositional engineering of the hybrid perovskite structure are discussed, including potentials for developing all-inorganic and lead-free perovskite materials. Looking at the latest cutting-edge research, the prospects for perovskite-based photovoltaic and optoelectronic devices, including non-photovoltaic applications such as X-ray detectors and image sensing devices in industrialization, are described. In addition to the aforementioned major topics, we also review, as a background, our encounter with perovskite materials for the first solar cell application, which should inspire young researchers in chemistry and physics to identify and work on challenging interdisciplinary research problems through exchanges between academia and industry.

Dry-pressed anodized titania nanotube/CH3NH3PbI3 single crystal heterojunctions: The beneficial role of N doping

Highly ordered, anodically grown TiO2 nanotubes on titanium supports were annealed in ammonia atmosphere in order to incorporate nitrogen doping (≤2 at.%) in the titanium oxide lattice. FESEM micrographs revealed nanotubes with an average outer diameter of 101.5 ± 1.5 nm and an average wall thickness of about 13 nm. Anatase crystals were formed inside the tubes after annealing in ammonia atmosphere for 30 min. With further annealing, rutile peaks appeared due to the thermal oxidation of the foil and rise as the duration of heat treatment was increased. The concentration and chemical nature of nitrogen in the nanotube arrays can be correlated to the optical response of dry-pressed heterojunctions of doped TiO2/CH3NH3PbI3 single crystals. The N-TiO2/perovskite heterojunction with the highest amount of interstitial nitrogen exhibited an improved photocurrent, indicating the importance of the semiconductor doping-based heterojunction optimization strategies to deliver competitive levels of halide perovskite-based optoelectronic devices to be envisioned for urban infrastructures.

High-Performance Photodetectors Based on Lead-Free 2D Ruddlesden-Popper Perovskite/MoS2 Heterostructures.

Two-dimensional (2D) Ruddlesden-Popper perovskites have attracted great interest for their promising applications in high-performance optoelectronic devices owing to their greatly tunable band gaps, layered characteristics, and better environmental stability over three-dimensional (3D) perovskites. Here, we for the first time report on photodetectors based on few-layer MoS2 (n-type) and lead-free 2D perovskite (PEA)2SnI4 (p-type) heterostructures. The heterojunction device is capable of sensing light over the entire visible and near-infrared wavelength range with a tunable photoresponse peak. By using few-layer graphene flakes as the electrical contact, the performance of the heterostructures can be improved with a responsivity of 1100 A/W at 3 V bias, a fast response speed of ∼40 ms under zero bias, and an excellent rectification ratio of 500. Importantly, the quantum efficiency can achieve 38.2% at zero bias, which is comparable or even higher than that of 3D perovskite/2D material photodetectors. Importantly, the spectral response peak of heterojunctions gradually shifts in a wide spectral range from the band edge of MoS2 toward that of (PEA)2SnI4 with the external bias. We believe these 2D perovskite/2D material heterostructures with a great diversity represent an interesting system for investigating the fundamental optoelectronic properties and open up a new pathway toward 2D perovskite-based optoelectronic devices.

Constructing the Electronic Structure of CH3NH3PbI3 and CH3NH3PbBr3 Perovskite Thin Films from Single-Crystal Band Structure Measurements.

Photovoltaic cells based on halide perovskites, possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single-crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3), we reveal the band dispersion in two high-symmetry directions and identify the global valence band maxima. With these benchmark data, we construct "standard" photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature for determining the valence band onset with high reliability. Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.

Organometal halide perovskite-based optoelectronic devices

Abstract Organometal halide perovskites are potential optoelectronic materials due to high solar cell efficiencies of 22.1% and light-emitting diodes with wide color gamut emission and external quantum efficiencies of 8%. Although perovskite light-emitting devices are still in research and development phase, in only a few years perovskite-based light emitting diodes have already exceeded the performance of organic light-emitting diodes. Further progress in this new material will depend on material optimization and development of scalable deposition process. Here, the advancements in perovskite-based optoelectronic devices at National university of Tainan are presented.

Organic interfacial materials for perovskite-based optoelectronic devices

We summarize the development and utilization of organic interfacial materials in solar cells, photodetectors and light-emitting diodes based on organic–inorganic halide perovskites.

Highly Efficient and Ultrasensitive Large-Area Flexible Photodetector Based on Perovskite Nanowires.

Hybrid organic-inorganic perovskites have shown exceptional semiconducting properties and microstructural versatility for inexpensive, solution-processable photovoltaic and optoelectronic devices. In this work, an all-solution-based technique in ambient environment for highly sensitive and high-speed flexible photodetectors using high crystal quality perovskite nanowires grown on Kapton substrate is presented. At 10 V, the optimized photodetector exhibits a responsivity as high as 0.62 A W-1 , a maximum specific detectivity of 7.3 × 1012 cm Hz1/2 W-1 , and a rise time of 227.2 µs. It also shows remarkable photocurrent stability even beyond 5000 bending cycles. Moreover, a deposition of poly(methyl methacrylate) (PMMA) as a protective layer on the perovskite yields significantly better stability under ambient air operation: the PMMA-protected devices are stable for over 30 days. This work demonstrates a cost-effective fabrication technique for high-performance flexible photodetectors and opens opportunities for research advancements in broadband and large-scale flexible perovskite-based optoelectronic devices.

High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes

Perovskite-based optoelectronic devices have gained significant attention due to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes (LEDs), non-radiative charge carrier recombination has limited electroluminescence (EL) efficiency. Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL half-life of 46 hours under continuous operation. This performance is achieved with an emissive layer comprising quasi-2D and 3D perovskites and an insulating polymer. Transient optical spectroscopy reveals that photogenerated excitations at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps. The dominant component of the photoluminescence (PL) is primarily bimolecular and is characteristic of the 3D regions. From PL quantum efficiency and transient kinetics of the emissive layer with/without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated. Light outcoupling from planar LEDs, as used in OLED displays, generally limits EQE to 20-30%, and we model our reported EL efficiency of over 20% in the forward direction to indicate the internal quantum efficiency (IQE) to be close to 100%. Together with the low drive voltages needed to achieve useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that perovskite-based LEDs have significant potential for light-emission applications.

Piezo-phototronic Effect Enhanced Photodetector Based on CH3NH3PbI3 Single Crystals.

Piezoelectric organic-inorganic lead halide perovskites have recently attracted much attention in the field of optoelectronic devices. However, their piezoelectric properties as a possible way to modulate device performances have rarely been reported. Here, we study experimentally a photodetector based on CH3NH3PbI3(MAPbI3) single crystals, whose performance is effectively modulated via an emerging effect-the piezo-phototronic effect, which is to use the piezoelectric polarization charges to tune the optoelectronic processes at the interface. A piezoelectric coefficient of 10.81 pm/V of the CH3NH3PbI3 single crystal is obtained. Under 680 nm laser illumination with a power density of 3.641 mW/cm2 and at an external bias of 2 V, compared to the case without straining, the light current of the photodetector is enhanced by ∼120% when a 43.48 kPa compressive pressure is applied. The response speed of the photocurrent is 3 and 2 times faster than the cases without applying pressure for the light-on and light-off states, respectively. This work proves that the performance of the photodetector based on MAPbI3 single crystals can be effectively enhanced by the piezo-phototronic effect, providing a good method for optimizing the performance of future perovskite-based optoelectronic devices.

Effects of Oxygen Plasma on the Chemical, Light-Emitting, and Electrical-Transport Properties of Inorganic and Hybrid Lead Bromide Perovskite Nanocrystal Films

We show that oxygen plasma affects in different ways the structural, chemical, optical, and electrical properties of methylammonium and cesium lead bromide nanocrystals. Hybrid organic–inorganic nanocrystals were severely and quickly degraded by oxygen plasma at 50 W. Their photoluminescence was quenched with almost 100% loss of the initial quantum yield, which is linked to decomposition of the nanocrystals. Inorganic nanocrystals were more resistant to oxygen plasma in the same conditions. Despite a moderate loss of photoluminescence and electrical conductivity, oxygen plasma had a positive impact, removing unbound ligands and resulting in more ohmic behavior of the film. This paves the way for the application of oxygen plasma in the development of perovskite-based optoelectronic devices.

Double-edged sword effects of cation rotation and additive passivation on perovskite solar cell performance: an ab initio investigation

Perovskite solar cells, mainly based on the prototypical CH3NH3PbI3, have recently emerged as a disruptive technology in the solar cell industry. Despite the great successes in the exceptional power conversion efficiency, many underlying mechanisms involving the structures and performance of the perovskite solar cells remain elusive, such as the role of the methylammonium cation and the optimization of the passivation technique. These hinder the formulation of a universal design principle that can guide us to rationally improve the perovskite solar cell performance. In addition, the notorious stability issue in perovskite solar cells prevents the solar cell from wide industrial application. In this manuscript, we closely examine the cation rotation and the passivating layer at the halide perovskite surface in the context of the stability issue. An experimentally-proved passivating agent iodopentafluorobenzene (IPFB) is used as an example. The ab initio investigation is performed via constraining/relaxing the cation rotation at the perovskite surface and manipulating the coverage of IPFB. Many existing reports tend to agree that the methylammonium cation rotation could be beneficial to the outstanding performance of the perovskite solar cell due to the ferroelectric effect. On the other hand, the passivating agents such as molecular additives tend to universally improve the device stability. In this study, our calculation suggests that: 1) the methylammonium cation rotation could undesirably accelerate the water infiltration process at the perovskite surface; 2) the passivating layer can counter-intuitively facilitate the water interaction with the perovskite surface; both of these could severely degrade the solar cell stability. Therefore, we emphasize on the double-edged sword effects of cation rotation and additive passivation on the stability of the perovskite solar cells. This study facilitates the convergence to the fundamental understanding of the organic-inorganic perovskite solar cells and is beneficial for the rational design of the halide perovskite-based optoelectronic devices.

Understanding interactions between halide perovskite surfaces and atmospheric/VOC gas molecules: an ab initio investigation

Halide perovskite materials have scored great successes in a number of optoelectronic devices such as solar cells. The industrial sector requires an inevitable possibility of contacts between the halide perovskite materials and a number of atmospheric gases and volatile organic compounds (VOCs), which could strongly influence the stability and charge transfer properties of perovskite-based optoelectronic devices. Understanding the interactions between various gas molecules and the perovskite layer could thus be of utmost importance to further optimize perovskite solar cells from an industrial perspective. Surprisingly, a systematic study of the interactions between gas molecules highlighting atmospheric and VOCs and the prototypical CH3NH3PbI3 surfaces is unavailable at the moment. Aiming to bridge the gap, we perform first principles and molecular dynamic calculations to investigate the interactions between CH3NH3PbI3 surfaces and gas molecules, employing CO, CO2, H2, H2S, NH3, NO, NO2, O2, and SO2 gas molecules as the adsorbates. Our study suggests that the NH3 gas molecules exhibit detrimental effects on the structural and optical properties of the perovskite material; consequently, halide perovskite solar cells should be strongly protected against these gases. However, the influences of gas molecules on the perovskite layer is not always undesirable, since NH3 molecules can strongly adsorb onto the perovskite surface, increasing the conductivity and initiating optical bleaching in the perovskite underlayer, leading to the viability of the halide perovskite materials as an effective gas sensor. The structure–property relationships of the molecule/perovskite systems investigated in this study could initiate the design protocol of halide perovskite materials toward different applications. This study provides a foundation for the convergence to a fundamental understanding of designing different halide perovskite-based optoelectronic devices.

Degradation of Two-Dimensional CH3NH3PbI3 Perovskite and CH3NH3PbI3/Graphene Heterostructure.

Hybrid organic-inorganic metal halide perovskites have been considered as promising materials for boosting the performance of photovoltaics and optoelectronics. Reduced-dimensional condiments and tunable properties render two-dimensional (2D) perovskite as novel building blocks for constructing micro-/nanoscale devices in high-performance optoelectronic applications. However, the stability is still one major obstacle for long-term practical use. Herein, we provide microscale insights into the degradation kinetics of 2D CH3NH3PbI3 (MAPbI3) perovskite and CH3NH3PbI3/graphene heterostructures. It is found that the degradation is mainly caused by cation evaporation, which consequently affects the microstructure, light-matter interaction, and the photoluminescence quantum yield efficiency of the 2D perovskite. Interestingly, the encapsulation of perovskite by monolayer graphene can largely preserve the structure of the perovskite nanosheet and maintain reasonable optical properties upon exposure to high temperature and humidity. The heterostructure consisting of perovskite and another 2D impermeable material affords new possibilities to construct high-performance and stable perovskite-based optoelectronic devices.

A general printing approach for scalable growth of perovskite single-crystal films.

Perovskite single-crystal films, which exhibit exceptionally low trap density and nearly perfect translational symmetry, are believed to achieve the highest performance of perovskite-based optoelectronic devices. However, fabrication of these perovskite single-crystal films is quite difficult because of the uncontrollable nucleation caused by the rapid reaction of two perovskite precursors. We report a facile seed printing approach to selectively create millimeter-sized perovskite single-crystal films with controlled thickness and high yield. We show that perovskite single-crystal films can be perfectly transferred to almost arbitrary substrates through the printing process. The as-grown perovskite single-crystal films have excellent crystalline quality and morphology. We further demonstrate that perovskite single-crystal films can be directly printed for scalable fabrication of photodetectors and effective image sensors. This strategy allows high-yield fabrication of large perovskite single-crystal films for functional devices and may extend to other solution-processed materials for wide applications.

Design principles for electronic charge transport in solution-processed vertically stacked 2D perovskite quantum wells

State-of-the-art quantum-well-based devices such as photovoltaics, photodetectors, and light-emission devices are enabled by understanding the nature and the exact mechanism of electronic charge transport. Ruddlesden–Popper phase halide perovskites are two-dimensional solution-processed quantum wells and have recently emerged as highly efficient semiconductors for solar cell approaching 14% in power conversion efficiency. However, further improvements will require an understanding of the charge transport mechanisms, which are currently unknown and further complicated by the presence of strongly bound excitons. Here, we unambiguously determine that dominant photocurrent collection is through electric field-assisted electron–hole pair separation and transport across the potential barriers. This is revealed by in-depth device characterization, coupled with comprehensive device modeling, which can self-consistently reproduce our experimental findings. These findings establish the fundamental guidelines for the molecular and device design for layered 2D perovskite-based photovoltaics and optoelectronic devices, and are relevant for other similar quantum-confined systems.

Direct Observation and Quantitative Analysis of Mobile Frenkel Defects in Metal Halide Perovskites Using Scanning Kelvin Probe Microscopy

Ion migration is seen as a primary stability concern of halide perovskite-based photovoltaic and optoelectronic devices. Here, we provide experimental studies of long-distance, reversible ion migration in methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3) films. We use time-resolved scanning Kelvin probe microscopy on insulator-coated lateral electrodes to probe the dynamic redistribution of charged Frenkel defects over micrometer distances after application of an electric field. We combine these dynamic measurements with drift–diffusion simulations that yield self-consistent pictures of the sign, distribution, mobility, and activation energy of the associated, mobile Frenkel defects. This comprehensive approach is applied to study the impact of an organic cation on ionic mobility in metal halide perovskites, which we find to be significantly reduced in the case of FAPbI3 films compared to MAPbI3 films.