Dual‐Source Precursor Approach for Highly Efficient Inverted Planar Heterojunction Perovskite Solar Cells

The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium and formamidinium mixed cations. Currently, high‐quality mixed‐cation perovskite thin films are normally made by use of antisolvent protocols. However, the widely used “antisolvent”‐assisted fabrication route suffers from challenges such as poor device reproducibility, toxic and hazardous organic solvent, and incompatibility with scalable fabrication process. Here, a simple dual‐source precursor approach is developed to fabricate high‐quality and mirror‐like mixed‐cation perovskite thin films without involving additional antisolvent process. By integrating the perovskite films into the planar heterojunction solar cells, a power conversion efficiency of 20.15% is achieved with negligible current density–voltage hysteresis. A stabilized power output approaching 20% is obtained at the maximum power point. These results shed light on fabricating highly efficient perovskite solar cells via a simple process, and pave the way for solar cell fabrication via scalable methods in the near future.     

Infrared Solution‐Processed Quantum Dot Solar Cells Reaching External Quantum Efficiency of 80% at 1.35 µm and Jsc in Excess of 34 mA cm−2

Developing low‐cost photovoltaic absorbers that can harvest the short‐wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si‐based and perovskite photovoltaic technologies, is a prerequisite for making high‐efficiency, low‐cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic–organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short‐circuit current density of 34 mA cm^?2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI₃ perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.     

Assembling Mesoscale‐Structured Organic Interfaces in Perovskite Photovoltaics

Mesoscale‐structured materials offer broad opportunities in extremely diverse applications owing to their high surface areas, tunable surface energy, and large pore volume. These benefits may improve the performance of materials in terms of carrier density, charge transport, and stability. Although metal oxides–based mesoscale‐structured materials, such as TiO₂, predominantly hold the record efficiency in perovskite solar cells, high temperatures (above 400 °C) and limited materials choices still challenge the community. A novel route to fabricate organic‐based mesoscale‐structured interfaces (OMI) for perovskite solar cells using a low‐temperature and green solvent–based process is presented here. The efficient infiltration of organic porous structures based on crystalline nanoparticles allows engineering efficient “n‐i‐p” and “p‐i‐n” perovskite solar cells with enhanced thermal stability, good performance, and excellent lateral homogeneity. The results show that this method is universal for multiple organic electronic materials, which opens the door to transform a wide variety of organic‐based semiconductors into scalable n‐ or p‐type porous interfaces for diverse advanced applications.     

Efficient Flexible Solar Cell based on Composition‐Tailored Hybrid Perovskite

Organic–inorganic hybrid perovskites (OIHPs) are new photoactive layer candidates for lightweight and flexible solar cells due to their low‐temperature process capability; however, the reported efficiency of flexible OIHP devices is far behind those achieved on rigid glass substrates. Here, it is revealed that the limiting factor is the different perovskite film deposition conditions required to form the same film morphology on flexible substrates. An optimized perovskite film composition needs a different precursor ratio, which is found to be essential for the formation of high‐quality perovskite films with longer radiative carrier recombination lifetime, smaller density of trap states, reduced precursor residue, and uniform and pin‐hole free films. A record efficiency of 18.1% is achieved for the flexible perovskite solar‐cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low temperature (≤100 °C) solution process.     

Slow Hot‐Carrier Cooling in Halide Perovskites: Prospects for Hot‐Carrier Solar Cells

Rapid hot‐carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot‐carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot‐carrier properties: slow hot‐carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, long‐range hot‐carrier transport (up to ≈600 nm), and highly efficient hot‐carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot‐carrier cooling properties in halide perovskites and prospects for perovskite hot‐carrier solar cells are also discussed.     

Slow‐Photon‐Effect‐Induced Photoelectrical‐Conversion Efficiency Enhancement for Carbon‐Quantum‐Dot‐Sensitized Inorganic CsPbBr3 Inverse Opal Perovskite Solar Cells

All‐inorganic cesium lead halide perovskite is suggested as a promising candidate for perovskite solar cells due to its prominent thermal stability and comparable light absorption ability. Designing textured perovskite films rather than using planar‐architectural perovskites can indeed optimize the optical and photoelectrical conversion performance of perovskite photovoltaics. Herein, for the first time, this study demonstrates a rational strategy for fabricating carbon quantum dot (CQD‐) sensitized all‐inorganic CsPbBr₃ perovskite inverse opal (IO) films via a template‐assisted, spin‐coating method. CsPbBr₃ IO introduces slow‐photon effect from tunable photonic band gaps, displaying novel optical response property visible to naked eyes, while CQD inlaid among the IO frameworks not only broadens the light absorption range but also improves the charge transfer process. Applied in the perovskite solar cells, compared with planar CsPbBr₃, slow‐photon effect of CsPbBr₃ IO greatly enhances the light utilization, while CQD effectively facilitates the electron–hole extraction and injection process, prolongs the carrier lifetime, jointly contributing to a double‐boosted power conversion efficiency (PCE) of 8.29% and an increased incident photon‐to‐electron conversion efficiency of up to 76.9%. The present strategy on CsPbBr₃ IO to enhance perovskite PCE can be extended to rationally design other novel optoelectronic devices.     

Multichannel Interdiffusion Driven FASnI3 Film Formation Using Aqueous Hybrid Salt/Polymer Solutions toward Flexible Lead‐Free Perovskite Solar Cells

Tin (Sn)‐based perovskites are increasingly attractive because they offer lead‐free alternatives in perovskite solar cells. However, depositing high‐quality Sn‐based perovskite films is still a challenge, particularly for low‐temperature planar heterojunction (PHJ) devices. Here, a “multichannel interdiffusion” protocol is demonstrated by annealing stacked layers of aqueous solution deposited formamidinium iodide (FAI)/polymer layer followed with an evaporated SnI₂ layer to create uniform FASnI₃ films. In this protocol, tiny FAI crystals, significantly inhibited by the introduced polymer, can offer multiple interdiffusion pathways for complete reaction with SnI₂. What is more, water, rather than traditional aprotic organic solvents, is used to dissolve the precursors. The best‐performing FASnI₃ PHJ solar cell assembled by this protocol exhibits a power conversion efficiency (PCE) of 3.98%. In addition, a flexible FASnI₃‐based flexible solar cell assembled on a polyethylene naphthalate–indium tin oxide flexible substrate with a PCE of 3.12% is demonstrated. This novel interdiffusion process can help to further boost the performance of lead‐free Sn‐based perovskites.     

Ultraflexible and Lightweight Bamboo‐Derived Transparent Electrodes for Perovskite Solar Cells

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light‐weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo‐based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass‐based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.     

Hybrid PbS Quantum‐Dot‐in‐Perovskite for High‐Efficiency Perovskite Solar Cell

In this study, a facile and effective approach to synthesize high‐quality perovskite‐quantum dots (QDs) hybrid film is demonstrated, which dramatically improves the photovoltaic performance of a perovskite solar cell (PSC). Adding PbS QDs into CH₃NH₃PbI₃ (MAPbI₃) precursor to form a QD‐in‐perovskite structure is found to be beneficial for the crystallization of perovskite, revealed by enlarged grain size, reduced fragmentized grains, enhanced characteristic peak intensity, and large percentage of (220) plane in X‐ray diffraction patterns. The hybrid film also shows higher carrier mobility, as evidenced by Hall Effect measurement. By taking all these advantages, the PSC based on MAPbI₃‐PbS hybrid film leads to an improvement in power conversion efficiency by 14% compared to that based on pure perovskite, primarily ascribed to higher current density and fill factor (FF). Ultimately, an efficiency reaching up to 18.6% and a FF of over ≈0.77 are achieved based on the PSC with hybrid film. Such a simple hybridizing technique opens up a promising method to improve the performance of PSCs, and has strong potential to be applied to prepare other hybrid composite materials.     

Investigation of Electrode Electrochemical Reactions in CH3NH3PbBr3 Perovskite Single‐Crystal Field‐Effect Transistors

Optoelectronic devices based on metal halide perovskites, including solar cells and light‐emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic–inorganic hybrid perovskite materials can enable high‐performance, solution‐processed field‐effect transistors (FETs) for next‐generation, low‐cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single‐crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source–drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such “ideal” interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom‐contact, bottom‐gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single‐crystal FETs with high mobility of up to ≈15 cm² V^?1 s^?1 at 80 K. This work addresses one of the key challenges toward the realization of high‐performance solution‐processed perovskite FETs.     

Facile fabrication of Si nanowire arrays for solar cell application

Large-area Si nanowire arrays have been fabricated on phosphorus doped Si surface by a facile silver-catalyzed chemical etching process. The solar cell incorporated with Si nanowire arrays shows a power conversion efficiency of 6.69% with an open circuit voltage of 558 mV and a short circuit current density of 25.13 mA/cm2 under AM 1.5 G illumination without using any extra antireflection layer and surface passivation technique. The high power conversion efficiency of Si nanowires based-solar cell is attributed to the low reflectance loss of Si nanowire arrays for incident sunlight. Optimization of electrical contact and phosphorus diffusion process will be critical to improve the performance of Si nanowires-based solar cell in the future.     

Site‐Selective Catalysis of a Multifunctional Linear Molecule: The Steric Hindrance of Metal–Organic Framework Channels

The site‐selective reaction of a multifunctional linear molecule requires a suitable catalyst possessing both uniform narrow channel to limit the molecule rotation and a designed active site in the channel. Recently, nanoparticles (NPs) were incorporated in metal–organic frameworks (MOFs) with the tailorable porosity and ordered nanochannel, which makes these materials (NPs/MOFs) highly promising candidates as catalytic nanoreactors in the field of heterogeneous catalysis. Inspired by a “Gondola” sailing in narrow “Venetian Canal” without sufficient space for a U‐turn, a simple heterogeneous catalyst based on NPs/MOFs is developed that exhibits site‐selectivity for the oxidation of diols by restricting the random rotation of the molecule (the “Gondola”) in the limited space of the MOF channel (the narrow “Venetian Canal”), thereby protecting the middle functional group via steric hindrance. This strategy is not limited to the oxidation of diols, but can be extended to the site‐selective reaction of many similar multifunctional linear molecules, such as the reduction of alkadienes.     

A Layer‐by‐Layer ZnO Nanoparticle–PbS Quantum Dot Self‐Assembly Platform for Ultrafast Interfacial Electron Injection

Absorbent layers of semiconductor quantum dots (QDs) are now used as material platforms for low‐cost, high‐performance solar cells. The semiconductor metal oxide nanoparticles as an acceptor layer have become an integral part of the next generation solar cell. To achieve sufficient electron transfer and subsequently high conversion efficiency in these solar cells, however, energy‐level alignment and interfacial contact between the donor and the acceptor units are needed. Here, the layer‐by‐layer (LbL) technique is used to assemble ZnO nanoparticles (NPs), providing adequate PbS QD uptake to achieve greater interfacial contact compared with traditional sputtering methods. Electron injection at the PbS QD and ZnO NP interface is investigated using broadband transient absorption spectroscopy with 120 femtosecond temporal resolution. The results indicate that electron injection from photoexcited PbS QDs to ZnO NPs occurs on a time scale of a few hundred femtoseconds. This observation is supported by the interfacial electronic‐energy alignment between the donor and acceptor moieties. Finally, due to the combination of large interfacial contact and ultrafast electron injection, this proposed platform of assembled thin films holds promise for a variety of solar cell architectures and other settings that principally rely on interfacial contact, such as photocatalysis.     

Oxygen‐Vacancy‐Introduced BaSnO3−δ Photoanodes with Tunable Band Structures for Efficient Solar‐Driven Water Splitting

To achieve excellent photoelectrochemical water‐splitting activity, photoanode materials with high light absorption and good charge‐separation efficiency are essential. One effective strategy for the production of materials satisfying these requirements is to adjust their band structure and corresponding bandgap energy by introducing oxygen vacancies. A simple chemical reduction method that can systematically generate oxygen vacancies in barium stannate (BaSnO₃ (BSO)) crystal is introduced, which thus allows for precise control of the bandgap energy. A BSO photoanode with optimum oxygen‐vacancy concentration (8.7%) exhibits high light‐absorption and good charge‐separation capabilities. After deposition of FeOOH/NiOOH oxygen evolution cocatalysts on its surface, this photoanode shows a remarkable photocurrent density of 7.32 mA cm^?2 at a potential of 1.23 V versus a reversible hydrogen electrode under AM1.5G simulated sunlight. Moreover, a tandem device constructed with a perovskite solar cell exhibits an operating photocurrent density of 6.84 mA cm^?2 and stable gas production with an average solar‐to‐hydrogen conversion efficiency of 7.92% for 100 h, thus functioning as an outstanding unbiased water‐splitting system.     

Activated Electron‐Transport Layers for Infrared Quantum Dot Optoelectronics

Photovoltaic (PV) materials such as perovskites and silicon are generally unabsorptive at wavelengths longer than 1100 nm, leaving a significant portion of the IR solar spectrum unharvested. Small‐bandgap colloidal quantum dots (CQDs) are a promising platform to offer tandem complementary IR PV solutions. Today, the best performing CQD PVs use zinc oxide (ZnO) as an electron‐transport layer. However, these electrodes require ultraviolet (UV)‐light activation to overcome the low carrier density of ZnO, precluding the realization of CQD tandem photovoltaics. Here, a new sol–gel UV‐free electrode based on Al/Cl hybrid doping of ZnO (CAZO) is developed. Al heterovalent doping provides a strong n‐type character while Cl surface passivation leads to a more favorable band alignment for electron extraction. CAZO CQD IR solar cell devices exhibit, at wavelengths beyond the Si bandgap, an external quantum efficiency of 73%, leading to an additional 0.92% IR power conversion efficiency without UV activation. Conventional ZnO devices, on the other hand, add fewer than 0.01 power points at these operating conditions.     

300% Enhancement of Carrier Mobility in Uniaxial‐Oriented Perovskite Films Formed by Topotactic‐Oriented Attachment

Organic–inorganic perovskites with intriguing optical and electrical properties have attracted significant research interests due to their excellent performance in optoelectronic devices. Recent efforts on preparing uniform and large‐grain polycrystalline perovskite films have led to enhanced carrier lifetime up to several microseconds. However, the mobility and trap densities of polycrystalline perovskite films are still significantly behind their single‐crystal counterparts. Here, a facile topotactic‐oriented attachment (TOA) process to grow highly oriented perovskite films, featuring strong uniaxial‐crystallographic texture, micrometer‐grain morphology, high crystallinity, low trap density (≈4 × 10¹⁴ cm^?3), and unprecedented 9 GHz charge‐carrier mobility (71 cm² V^?1 s^?1), is demonstrated. TOA‐perovskite‐based n‐i‐p planar solar cells show minimal discrepancies between stabilized efficiency (19.0%) and reverse‐scan efficiency (19.7%). The TOA process is also applicable for growing other state‐of‐the‐art perovskite alloys, including triple‐cation and mixed‐halide perovskites.     

Light Confinement Effect Induced Highly Sensitive,Self‐Driven Near‐Infrared Photodetector and Image Sensor Based on Multilayer PdSe2/Pyramid Si Heterojunction

In this study, a highly sensitive and self‐driven near‐infrared (NIR) light photodetector based on PdSe₂/pyramid Si heterojunction arrays, which are fabricated through simple selenization of predeposited Pd nanofilm on black Si, is demonstrated. The as‐fabricated hybrid device exhibits excellent photoresponse performance in terms of a large on/off ratio of 1.6 × 10⁵, a responsivity of 456 mA W^?1, and a high specific detectivity of up to 9.97 × 10¹³ Jones under 980 nm illumination at zero bias. Such a relatively high sensitivity can be ascribed to the light trapping effect of the pyramid microstructure, which is confirmed by numerical modeling based on finite‐difference time domain. On the other hand, thanks to the broad optical absorption properties of PdSe₂, the as‐fabricated device also exhibits obvious sensitivity to other NIR illuminations with wavelengths of 1300, 1550, and 1650 nm, which is beyond the photoresponse range of Si‐based devices. It is also found that the PdSe₂/pyramid Si heterojunction device can also function as an NIR light sensor, which can readily record both “tree” and “house” images produced by 980 and 1300 nm illumination, respectively.     

Low‐Temperature Soft‐Cover Deposition of Uniform Large‐Scale Perovskite Films for High‐Performance Solar Cells

Large‐scale high‐quality perovskite thin films are crucial to produce high‐performance perovskite solar cells. However, for perovskite films fabricated by solvent‐rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low‐temperature soft‐cover deposition (LT‐SCD) method is presented, where the thermal convection‐induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection‐induced‐defects‐free perovskite films are obtained on an area of 12 cm², which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm². This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low‐temperature processing. Hence, the present LT‐SCD technology provides a new non‐spin‐coating route to the deposition of large‐area uniform perovskite films for both rigid and flexible perovskite devices.     

Quasi‐Heteroface Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted unprecedented attention due to their rapidly rising photoelectric conversion efficiency (PCE). In order to further improve the PCE of PSCs, new possible optimization path needs to be found. Here, quasi‐heteroface PSCs (QHF‐PSCs) is designed by a double‐layer perovskite film. Such brand new PSCs have good carrier separation capabilities, effectively suppress the nonradiative recombination of the PSCs, and thus greatly improve the open‐circuit voltage and PCE. The root cause of the performance improvement is the benefit from the additional built‐in electric field, which is confirmed by measuring the external quantum efficiency under applied electric field and Kelvin probe force microscope. Meanwhile, an intermediate band gap perovskite layer can be obtained simply by combining a wide band gap perovskite layer with a narrow band gap perovskite layer. Tunability of the band gap is obtained by varying the film thicknesses of the narrow and wide band gap layers. This phenomenon is quite different from traditional inorganic solar cells, whose band gap is determined only by the narrowest band gap layer. It is believed that these QHF‐PSCs will be an effective strategy to further enhance PCE in PSCs and provide basis to further understand and develop the perovskite materials platform.     

A Strategy for Architecture Design of Crystalline Perovskite Light‐Emitting Diodes with High Performance

All present designs of perovskite light‐emitting diodes (PeLEDs) stem from polymer light‐emitting diodes (PLEDs) or perovskite solar cells. The optimal structure of PeLEDs can be predicted to differ from PLEDs due to the different fluorescence dynamics and crystallization between perovskite and polymer. Herein, a new design strategy and conception is introduced, “insulator–perovskite–insulator” (IPI) architecture tailored to PeLEDs. As examples of FAPbBr₃ and MAPbBr₃, it is experimentally shown that the IPI structure effectively induces charge carriers into perovskite crystals, blocks leakage currents via pinholes in the perovskite film, and avoids exciton quenching simultaneously. Consequently, as for FAPbBr₃, a 30‐fold enhancement in the current efficiency of IPI‐structured PeLEDs compared to a control device with poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) as hole‐injection layer is achieved—from 0.64 to 20.3 cd A^?1—while the external quantum efficiency is increased from 0.174% to 5.53%. As the example of CsPbBr₃, compared with the control device, both current efficiency and lifetime of IPI‐structured PeLEDs are improved from 1.42 and 4 h to 9.86 cd A^?1 and 96 h. This IPI architecture represents a novel strategy for the design of light‐emitting didoes based on various perovskites with high efficiencies and stabilities.