A Novel Conductive Mesoporous Layer with a Dynamic Two-Step Deposition Strategy Boosts Efficiency of Perovskite Solar Cells to 20

Interface Passivation of a Pyridine-Based Bifunctional Molecule for Inverted Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have recently been demonstrated to be promising renewable harvesters because of their prominent photovoltaic power conversion efficiency (PCE), although their stability and efficiency still have not reached commercial criteria. Trouble-oriented analyses showcase that defect reduction among the grain boundaries and interfaces in the prepared perovskite polycrystalline films is a practical strategy, which has prompted researchers to develop functional molecules for interface passivation. Herein, the pyridine-based bifunctional molecule dimethylpyridine-3,5-dicarboxylate (DPDC) was employed as the interface between the electron-transport layer and perovskite layer, which achieved a champion PCE of 21.37% for an inverted MAPbI3-based PSC, which was greater than 18.64% for the control device. The mechanistic studies indicated that the significantly improved performance was mainly attributed to the remarkably enhanced fill factor with a value greater than 83%, which was primarily due to the nonradiative recombination suppression offered by the passivation effect of DPDC. Moreover, the promoted carrier mobility together with the enlarged crystal size contributed to a higher short-circuit current density. In addition, an increase in the open-circuit voltage was also observed in the DPDC-treated PSC, which benefited from the improved work function for reducing the energy loss during carrier transport. Furthermore, the DPDC-treated PSC showed substantially enhanced stability, with an over 80% retention rate of its initial PCE value over 300 h even at a 60% relative humidity level, which was attributed to the hydrophobic nature of the DPDC molecule and effective defect passivation. This work is expected not only to serve as an effective strategy for using a pyridine-based bifunctional molecule to passivate perovskite interfaces to enhance photovoltaic performance but also to shed light on the interface passivation mechanism.

Advances in the Application of Perovskite Materials

Nowadays, the soar of photovoltaic performance of perovskite solar cells has set off a fever in the study of metal halide perovskite materials. The excellent optoelectronic properties and defect tolerance feature allow metal halide perovskite to be employed in a wide variety of applications. This article provides a holistic review over the current progress and future prospects of metal halide perovskite materials in representative promising applications, including traditional optoelectronic devices (solar cells, light-emitting diodes, photodetectors, lasers), and cutting-edge technologies in terms of neuromorphic devices (artificial synapses and memristors) and pressure-induced emission. This review highlights the fundamentals, the current progress and the remaining challenges for each application, aiming to provide a comprehensive overview of the development status and a navigation of future research for metal halide perovskite materials and devices.

Water-Regulated Lead Halide Perovskites Precursor Solution: Perovskite Structure Making and Breaking

Identifying the impact of water on iodoplumbate complexes in various solutions is essential for linking the coordination environment of the perovskite precursor to its final perovskite solar cell (PSC) properties. In this study, we propose a digital twin approach based on X-ray absorption fine structure and molecular dynamic simulation to investigate the structure evolution of iodoplumbate complexes in precursor solutions as a function of storage time under a constant humidity environment. A full picture about what water does in the perovskite formation process is brought out, and the "making and breaking" role of water molecules is uncovered to link the structure of iodoplumbate complexes to its final properties. This study sheds light on a full picture about what water does in the perovskite formation process and the role of water, which will lead to developing water-involved strategies for consistent PSC fabrication under ambient conditions.

Compositional gradient engineering and applications in halide perovskites

Organic-inorganic halide perovskites (HPs) have attracted respectable interests as active layers in solar cells, light-emitting diodes, photodetectors, etc. Besides the promising optoelectronic properties and solution-processed preparation, the soft lattice in HPs leads to flexible and versatile compositions and structures, providing an effective platform to regulate the bandgaps and optoelectronic properties. However, conventional solution-processed HPs are homogeneous in composition. Therefore, it often requires the cooperation of multiple devices in order to achieve multi-band detection or emission, which increases the complexity of the detection/emission system. In light of this, the construction of a multi-component compositional gradient in a single active layer has promising prospects. In this review, we summarize the gradient engineering methods for different forms of HPs. The advantages and limitations of these methods are compared. Moreover, the entropy-driven ion diffusion favors compositional homogeneity, thus the stability issue of the gradient is also discussed for long-term applications. Furthermore, applications based on these compositional gradient HPs will also be presented, where the gradient bandgap introduced therein can facilitate carrier extraction, and the multi-components on one device facilitate functional integration. It is expected that this review can provide guidance for the further development of gradient HPs and their applications.

Green-solvent-processed formamidinium-based perovskite solar cells with uniform grain growth and strengthened interfacial contact via a nanostructured tin oxide layer

Green-solvent-processed perovskite solar cells (PSCs) have reached an efficiency of 20%, showing great promise in safe industrial production. However, the nucleation process in green-solvent-based deposition is rarely optimized, resulting in randomized crystallization and much lowered reported efficiencies. Herein, a nanostructured tin oxide nanorods (SnO₂-NRs) substrate is utilized to prepare a high-quality formamidinium (FA)-based perovskite film processed from a green solvent of triethyl phosphate (TEP) with a low toxic antisolvent of dibutyl ether (DEE). Compared with SnO₂ nanoparticles, the oriented SnO₂-NRs can accelerate the formation of heterogeneous nucleation sites and retard the crystal growth process of the perovskite film, resulting in a high-quality perovskite film with uniform grain growth. Furthermore, a chlorine-terminated bifunctional supramolecule (Cl-BSM) is introduced to passivate the increasing interfacial defects due to the vast contact area in SnO₂-NRs. Correspondingly, the substrate design of SnO₂-NRs with Cl-BSM increases the power conversion efficiency (PCE) of green-solvent-processed PSCs to 22.42% with an open-circuit voltage improvement from 1.02 to 1.12 V, which can be attributed to the uniform grain growth and reduced carrier recombination at the SnO₂-NRs/perovskite interface. More importantly, the photo and humidity stabilities of the unencapsulated device for up to 500 and 1000 hours are also achieved with negligible interfacial delamination after aging. This work provides a new perspective on the future industrial scale production of PSCs using environment-friendly solvents with compatible substrate design.

A Morphological Study of Solvothermally Grown SnO2 Nanostructures for Application in Perovskite Solar Cells

Tin(IV) oxide (SnO₂) nanostructures, which possess larger surface areas for transporting electron carriers, have been used as an electron transport layer (ETL) in perovskite solar cells (PSCs). However, the reported power conversion efficiencies (PCEs) of this type of PSCs show a large variation. One of the possible reasons for this phenomenon is the low reproducibility of SnO₂ nanostructures if they are prepared by different research groups using various growth methods. This work focuses on the morphological study of SnO₂ nanostructures grown by a solvothermal method. The growth parameters including growth pressure, substrate orientation, DI water-to-ethanol ratios, types of seed layer, amount of acetic acid, and growth time have been systematically varied. The SnO₂ nanomorphology exhibits a different degree of sensitivity and trends towards each growth factor. A surface treatment is also required for solvothermally grown SnO₂ nanomaterials for improving photovoltaic performance of PSCs. The obtained results in this work provide the research community with an insight into the general trend of morphological changes in SnO₂ nanostructures influenced by different solvothermal growth parameters. This information can guide the researchers to prepare more reproducible solvothermally grown SnO₂ nanomaterials for future application in devices.

Black Phosphorus Quantum Dot-Engineered Tin Oxide Electron Transport Layer for Highly Stable Perovskite Solar Cells with Negligible Hysteresis

An effective combination of smart materials plays an important role in charge transfer and separation for high photoelectric conversion efficiency (PCE) and stable solar cells. Black phosphorus quantum dots (BPQDs) have been revealed as a direct band gap semiconductor with ultrahigh conductivity, which have been explored in the present work as an additive component to a precursor solution of SnO₂ nanoparticles that can effectively improve the performance of SnO₂ electron transport layer (ETL)-based perovskite solar cells. Such a device can yield a high PCE of 21% with the SnO₂/BPQDs mixed ETL, which is higher than those of perovskite solar cells based on SnO₂ single layer (18.2%), BPQDs/SnO₂ bilayer (19.5%), and SnO₂/BPQDs bilayer (20.5%) samples. The mixed samples still possess good stability of more than 90% efficiency after 1000 h under AM 1.5G lamp irradiation and negligible hysteresis. It is found that the strong interaction of BPQDs with SnO₂ can not only modify the defects inherent to the SnO₂ layer but also inhibit the oxidation of BPQDs. This work provides a promising functional material for SnO₂ ETL-based perovskite solar cells and proves that the BPQD-based modification strategy is useful for designing other solar cells with high performance.

Methods of Stability Control of Perovskite Solar Cells for High Efficiency

The increasing demand for renewable energy devices over the past decade has motivated researchers to develop new and improve the existing fabrication techniques. One of the promising candidates for renewable energy technology is metal halide perovskite, owning to its high power conversion efficiency and low processing cost. This work analyzes the relationship between the structure of metal halide perovskites and their properties along with the effect of alloying and other factors on device stability, as well as causes and mechanisms of material degradation. The present work discusses the existing approaches for enhancing the stability of PSC devices through modifying functional layers. The advantages and disadvantages of different methods in boosting device efficiency and reducing fabrication cost are highlighted. In addition, the paper presents recommendations for the enhancement of interfaces in PSC structures.

Diluted-CdS Quantum Dot-Assisted SnO2 Electron Transport Layer with Excellent Conductivity and Suitable Band Alignment for High-Performance Planar Perovskite Solar Cells

An electron transport layer (ETL) with excellent conductivity and suitable band alignment plays a key role in accelerating charge extraction and transfer for achieving highly efficient planar perovskite solar cells (PSCs). Herein, a novel diluted-cadmium sulfide quantum dot (CdS QD)-assisted SnO₂ ETL has been developed with a low-temperature fabrication process. The slight addition of CdS QDs first enhances the crystallinity and flatness of SnO₂ ETLs so that it provides a promising workstation to obtain high-quality perovskite absorption layers. It also amazingly increases the conductivity of the SnO₂ ETL by an order of magnitude and regulates the energy level matching between the SnO₂ ETL and perovskite. These outstanding properties greatly accelerate the charge extraction and transfer. Thus, the MAPbI₃-based PSCs with such a diluted-CdSQD-assisted SnO₂ ETL achieve a maximum power conversion efficiency of 20.78% and obtain a better stability of devices in air. These findings testify the importance and potential of semiconductor QD modification on ETLs, which may pave the way for developing such composite ETLs for further enhancing photovoltaic performance of planar PSCs.

Intermediate-Adduct-Assisted Growth of Stable CsPbI2 Br Inorganic Perovskite Films for High-Efficiency Semitransparent Solar Cells

Thanks to the tunable bandgap and excellent photoelectric characteristics, perovskites have been widely used in semitransparent solar cells (ST-SCs). Most works present unsatisfactory power conversion efficiencies (PCEs) through reducing the thickness of the perovskite films because there is a trade-off between PCE and average visible transmittance (AVT). As a consequence, most PCEs are less than 12% when the AVT is higher than 20% due to the limited voltage (V_oc ) and short-circuit current (J_sc ). Herein, a strategy of intermediate adduct (IMAT) engineering is developed to improve the film quality of the inorganic perovskite CsPbI₂ Br, which is a challenging issue to limit its performance of efficiency and stability. A normal n-i-p-structured PSC based on the optimal CsPbI₂ Br film delivers a PCE of 16.02% with excellent stability. Furthermore, through optimizing the electrode type and interface, the ST-PSC shows a high V_oc larger than 1.2 V and the PCE reaches 14.01% and 10.36% under an AVT of 31.7% and 40.9%, respectively. This is the first demonstration of a CsPbI₂ Br ST-PSC, and it outperforms most of other types of perovskites.

Inorganic Materials by Atomic Layer Deposition for Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have received much attention with their rapid progress during the past decade, coming close to the point of commercialization. Various approaches in the process of PSC development have been explored with the motivation to enhance the solar cell power conversion efficiency—while maintaining good device stability from light, temperature, and moisture—and simultaneously optimizing for scalability. Atomic layer deposition (ALD) is a powerful tool in depositing pinhole-free conformal thin-films with excellent reproducibility and accurate and simple control of thickness and material properties over a large area at low temperatures, making it a highly desirable tool to fabricate components of highly efficient, stable, and scalable PSCs. This review article summarizes ALD’s recent contributions to PSC development through charge transport layers, passivation layers, and buffer and recombination layers for tandem applications and encapsulation techniques. The future research directions of ALD in PSC progress and the remaining challenges will also be discussed.

In Situ-Formed and Low-Temperature-Deposited Nb:TiO2 Compact-Mesoporous Layer for Hysteresis-Less Perovskite Solar Cells with High Performance

Recently, reported perovskite solar cells (PSCs) with high power conversion efficiency (PCE) are mostly based on mesoporous structures containing mesoporous titanium oxide (TiO₂) which is the main factor to reduce the overall hysteresis. However, existing fabrication approaches for mesoporous TiO₂ generally require a high-temperature annealing process. Moreover, there is still a long way to go for improvement in terms of increasing the electron conductivity and reducing the carrier recombination. Herein, a facile one-step, in situ, and low-temperature method was developed to prepare an Nb:TiO₂ compact-mesoporous layer which served as both scaffold and electron transport layer (ETL) for PSCs. The Nb:TiO₂ compact-mesoporous ETL-based PSCs exhibit suppressed hysteresis, which is attributed to the synergistic effect of the increased interface surface area caused by nano-pin morphology and the improved carrier transportation caused by Nb doping. Such a high-quality compact-mesoporous layer allows the PSCs assembled using optimized 2% Nb-doped TiO₂ to achieve a remarkable PCE of 19.74%. This work promises an effective approach for creating hysteresis-less and high-efficiency PSCs based on compact-mesoporous structures with lower energy consumption and cost.

High-Efficiency Perovskite Solar Cells Enabled by Anatase TiO2 Nanopyramid Arrays with an Oriented Electric Field

One-dimensional (1D) nanostructured oxides are proposed as excellent electron transport materials (ETMs) for perovskite solar cells (PSCs); however, experimental evidence is lacking. A facile hydrothermal approach was employed to grow highly oriented anatase TiO₂ nanopyramid arrays and demonstrate their application in PSCs. The oriented TiO₂ nanopyramid arrays afford sufficient contact area for electron extraction and increase light transmission. Moreover, the nanopyramid array/perovskite system exhibits an oriented electric field that can increase charge separation and accelerate charge transport, thereby suppressing charge recombination. The anatase TiO₂ nanopyramid array-based PSCs deliver a champion power conversion efficiency of approximately 22.5 %, which is the highest power conversion efficiency reported to date for PSCs consisting of 1D ETMs. This work demonstrates that the rational design of 1D ETMs can achieve PSCs that perform as well as typical mesoscopic and planar PSCs.

In Situ Formed Gradient Bandgap-Tunable Perovskite for Ultrahigh-Speed Color/Spectrum-Sensitive Photodetectors via Electron-Donor Control

Integration of various photodetectors with different light-sensitive materials and detection capacity is an inevitable way to achieve entire color/spectrum detection. However, the uneven capacity of each photodetector would drag the overall performance behind, especially the response speed. A response time down to nanosecond level has not previously been reported for a filter-free color/spectrum-sensitive photodetector, as far as is known. Here, a self-powered filterless color-sensitive photodetection array based on an in situ formed gradient perovskite absorber film with continuously tunable bandgap is demonstrated. Ultrahigh-speed response at nanosecond level is achieved in all the ingredient photodetectors. The junction capacitance being influenced by carrier concentration in the absorber is identified to be responsible for the detection speed. Without any optic or mechanical supporting system, the designed color detector exhibits an external quantum efficiency (EQE) up to 94% and a high spectral resolution of around 80 nm for the whole visible spectrum. This work offers a guidance to achieve fast response of perovskite-based photodetectors from the point of view of carrier-donor control and demonstrates a new avenue to establish color-sensitive photodetectors/spectrometers.

Realizing Stable Artificial Photon Energy Harvesting Based on Perovskite Solar Cells for Diverse Applications

As the fastest developing photovoltaic device, perovskite solar cells have achieved an extraordinary power conversion efficiency (PCE) of 25.3% under AM 1.5 illumination. However, few studies have been devoted to perovskite solar cells harvesting artificial light, owing to the great challenge in the simultaneous manipulation of bandgap-adjustable perovskite materials, corresponding matched energy band structure of carrier transport materials, and interfacial defects. Herein, through systematic morphology, composition, and energy band engineering, high-quality Cs_0.05 MA_0.95 PbBr_x I_{3- x} perovskite as the light absorber and Nb_y Ti_{1- y} O₂ (Nb:TiO₂ ) as the electron transport material with an ideal energy band alignment are obtained simultaneously. The theoretical-limit-approaching record PCEs of 36.3% (average: 34.0 ± 1.2%) under light-emitting diode (LED, warm white) and 33.2% under fluorescent lamp (cold white) are achieved simultaneously, as well as a PCE of 19.5% (average: 18.9 ± 0.3%) under solar illumination. An integrated energy conversion and storage system based on an artificial light response solar cell and sodium-ion battery is established for diverse practical applications, including a portable calculator, quartz clock, and even environmental monitoring equipment. Over a week of stable operation shows its great practical potential and provides a new avenue to promote the commercialization of perovskite photovoltaic devices via integration with ingenious electronic devices.

Novel NiO Nanoforest Architecture for Efficient Inverted Mesoporous Perovskite Solar Cells

Inverted perovskite solar cells (PSCs) demonstrate attractive features in developing an air-stable photovoltaic device, by employing inorganic hole transport layers (HTLs). However, their power conversion efficiencies are still inferior to that of mesoporous n-i-p devices, mainly attributed to the undesirable hole extraction and interfacial recombination loss. Here, we design a novel one-dimensional NiO nanotube (NT) nanoforest as efficient mesoporous HTLs. Such a NiO NT mesoporous structure provides a highly conductive pathway for rapid hole extraction and depresses interfacial recombination loss. Furthermore, excellent light capturing could be achieved by optimizing the length and branch growth of the NiO NT nanoforest, which mimics the evolution of the natural forest. Therefore, this inverted mesoporous PSCs yield an optimal efficiency of 18.77%, which is still prominent in state-of-the-art NiO-based devices. Alternatively, the mesoporous device exhibits greatly improved long-term stability. This work provides a new design perspective for developing high-performance inverted PSCs.

Unravelling the effects of oxidation state of interstitial iodine and oxygen passivation on charge trapping and recombination in CH3NH3PbI3 perovskite: a time-domain ab initio study

Understanding nonradiative charge recombination mechanisms is a prerequisite for advancing perovskite solar cells. By performing time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations, we show that electron-hole recombination in perovskites strongly depends on the oxidation state of interstitial iodine and oxygen passivation. The simulations demonstrate that electron-hole recombination in CH3NH3PbI3 occurs within several nanoseconds, agreeing well with experiment. The negative interstitial iodine delays charge recombination by a factor of 1.3. The deceleration is attributed to the fact that interstitial iodine anion forms a chemical bond with its nearest lead atoms, eliminates the trap state, and decreases the NA electron-phonon coupling. The positive interstitial iodine attracts its neighbouring lattice iodine anions, resulting in the formation of an I-trimer and producing an electron trap. Electron trapping proceeds on a very fast timescale, tens of picoseconds, and captures the majority of free electrons available to directly recombine with free holes while inhibiting the recombination of free electrons and holes, delaying the recombination by a factor of 1.5. However, the positive interstitial iodine easily converts to a neutral iodine defect by capturing an electron, giving rise to a singly occupied state above the valence band maximum and acting as a hole trap. The photoexcitation valence band hole becomes trapped by the hole trap state very rapidly, followed by acceleration of recombination with the conduction band free electron by a factor of 1.6. Surprisingly, molecular oxygen interacting with interstitial iodine anion forms a stable IO3 -1 species, which inhibits ion migration, stabilizes perovskites, and suppresses the electron-hole recombination by a factor of 2.7. Our simulations reveal the microscopic effects of the oxidation state of interstitial iodine defects and oxygen passivation in perovskites, suggesting an effective way to improve perovskite photovoltaic and optoelectronic devices.

High-Performance All-Optical Terahertz Modulator Based on Graphene/TiO2/Si Trilayer Heterojunctions

In this paper, we demonstrate a trilayer hybrid terahertz (THz) modulator made by combining a p-type silicon (p-Si) substrate, TiO₂ interlayer, and single-layer graphene. The interface between Si and TiO₂ introduced a built-in electric field, which drove the photoelectrons from Si to TiO₂, and then the electrons injected into the graphene layer, causing the Fermi level of graphene to shift into a higher conduction band. The conductivity of graphene would increase, resulting in the decrease of transmitted terahertz wave. And the terahertz transmission modulation was realized. We observed a broadband modulation of the terahertz transmission in the frequency range from 0.3 to 1.7 THz and a large modulation depth of 88% with proper optical excitation. The results show that the graphene/TiO₂/p-Si hybrid nanostructures exhibit great potential for terahertz broadband applications, such as terahertz imaging and communication.

In Situ Back-Contact Passivation Improves Photovoltage and Fill Factor in Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge-transport layers. Here, in situ back-contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits-in contrast with previously studied insulator-based passivants-the use of a relatively thick passivating layer. It is shown that a flat-band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.

Gradient Energy Band Driven High-Performance Self-Powered Perovskite/CdS Photodetector

Self-powered photodetectors are highly desired to meet the great demand in applications of sensing, communication, and imaging. Manipulating the carrier separation and recombination is critical to achieve high performance. In this paper, a self-powered photodetector based on the integrated gradient O-doped CdS nanorod array and perovskite is presented. Through optimizing the degree of continuous built-in band bending in the gradient-O CdS, the photodetector demonstrates a remarkable detectivity of 2.1 × 10¹³ Jones. Under the self-powered voltage mode, the responsivity can be as high as 0.48 A W^-1 , and the rise and decay time are 0.54/2.21 ms. The comprehensive performance is comparable and even better than reported perovskite and other types of self-powered photodetectors. The improved mechanism reveals that the gradient band bending promotes the photogenerated carrier transfer and hinders the recombination at the interface.

N-Methyl-2-pyrrolidone as an excellent coordinative additive with a wide operating range for fabricating high-quality perovskite films

N-Methyl-2-pyrrolidone (NMP), forming only one PbI₂·NMP complex, is demonstrated as an excellent coordinative solvent for the fabrication of high-quality perovskite thin films.

Furrowed hole-transport layer using argon plasma in an inverted perovskite solar cell

In this study, a novel process was found to be effective using the argon-plasma treatment, in which the ion cluster was used to scour the PEDOT:PSS surface instead of the traditional bombardment method. The photoelectric conversion efficiency of the device reaches 14.8%.