Efficient perovskite/fullerene planar heterojunction solar cells with enhanced charge extraction and suppressed charge recombination

Enhancement of Perovskite Solar Cells by TiO2-Carbon Dot Electron Transport Film Layers

The high performance of perovskite solar cells was produced with the help of an electron transport layer (ETL) and hole transport layer. The film ETL (mesoporous (meso)-TiO₂/carbon dot) boosted the efficiency of the perovskite solar cells. A perovskite cell was fabricated by a coating of carbon dot on a meso-TiO₂ ETL. The fabricated meso-TiO₂/carbon dot-based device has decreased the pin-holes of the perovskite film layer compared to the meso-TiO₂-based device, which boosted 3% of the averaged PCE value of the devices. The UV-visible spectroscopy confirmed that the meso-TiO₂/carbon dot ETL showed better absorbance, that is, absorbed more incident light than meso-TiO₂ ETL to generate higher power conversion efficiency. Coating of carbon dot on meso-TiO₂ reduced carrier recombination, and fadeaway of the perovskite film cracks. The X-ray diffraction spectra displayed the removal of the perovskite component after spin-coating of carbon dot to the meso-TiO₂ ETL, indicating that the suppression of non-radiative recombination improves the device performance compared to meso-TiO₂ ETL. The stability after four weeks on the performance of the device was improved to be 92% by depositing carbon dot on meso-TiO₂ ETL compared to the meso-TiO₂ ETL-based device (82%). Thus, the high-quality perovskite cell was fabricated by coating carbon dot on a meso-TiO₂ ETL, because the electron transport between ETL and perovskite film layer was improved by the injection of electrons from carbon dot.

Electrode Engineering in Halide Perovskite Electronics: Plenty of Room at the Interfaces

Contact engineering is a prerequisite for achieving desirable functionality and performance of semiconductor electronics, which is particularly critical for organic-inorganic hybrid halide perovskites due to their ionic nature and highly reactive interfaces. Although the interfaces between perovskites and charge-transporting layers have attracted lots of attention due to the photovoltaic and light-emitting diode applications, achieving reliable perovskite/electrode contacts for electronic devices, such as transistors and memories, remains as a bottleneck. Herein, a critical review on the elusive nature of perovskite/electrode interfaces with a focus on the interfacial electrochemistry effects is presented. The basic guidelines of electrode selection are given for establishing non-polarized interfaces and optimal energy level alignment for perovskite materials. Furthermore, state-of-the-art strategies on interface-related electrode engineering are reviewed and discussed, which aim at achieving ohmic transport and eliminating hysteresis in perovskite devices. The role and multiple functionalities of self-assembled monolayers that offer a unique approach toward improving perovskite/electrode contacts are also discussed. The insights on electrode engineering pave the way to advancing stable and reliable perovskite devices in diverse electronic applications.

Effects of CsSnxPb1-xI3 Quantum Dots as Interfacial Layer on Photovoltaic Performance of Carbon-Based Perovskite Solar Cells

In this work, inorganic tin-doped perovskite quantum dots (PQDs) are incorporated into carbon-based perovskite solar cells (PSCs) to improve their photovoltaic performance. On the one hand, by controlling the content of Sn²⁺ doping, the energy level of the tin-doped PQDs can be adjusted, to realize optimized band alignment and enhanced separation of photogenerated electron-hole pairs. On the other hand, the incorporation of tin-doped PQDs provided with a relatively high acceptor concentration due to the self-p-type doping effect is able to reduce the width of the depletion region near the back surface of the perovskite, thereby enhancing the hole extraction. Particularly, after the addition of CsSn_0.2Pb_0.8I₃ quantum dots (QDs), improvement of the power conversion efficiency (PCE) from 12.80 to 14.22% can be obtained, in comparison with the pristine device. Moreover, the experimental results are analyzed through the simulation of the one-dimensional perovskite/tin-doped PQDs heterojunction.

Interfacial and structural modifications in perovskite solar cells

The rapid and continuous progress made in perovskite solar cell (PSC) technology has drawn considerable attention from the photovoltaic research community, and the application of perovskites in other electronic devices (such as photodetectors, light-emitting diodes, and batteries) has become imminent. Because of the diversity in device configurations, optimization of film deposition, and exploration of material systems, the power conversion efficiency (PCE) of PSCs has been certified to be as high as 25.2%, making this type of solar cells the fastest advancing technology until now. As demonstrated by researchers worldwide, controlling the morphology and defects in perovskite films is essential for attaining high-performance PSCs. In this regard, interface engineering has proven to be a very efficient way to address these issues, obtaining better charge collection efficiency, and reducing recombination losses. In this review, the interfacial modification between perovskite films and charge-transport layers (CTLs) as well as CTLs and electrodes of PSCs has been widely summarized. Grain boundary (GB) engineering and stress engineering are also included since they are closely related to the improvement in device performance and stability.

Interfacial Engineering and Photon Downshifting of CsPbBr3 Nanocrystals for Efficient, Stable, and Colorful Vapor Phase Perovskite Solar Cells

Photovoltaic devices employing lead halide perovskites as the photoactive layer have attracted enormous attention due to their commercialization potential. Yet, there exists challenges on the way to the practical use of perovskite solar cells (PSCs), such as light stability and current-voltage (J-V ) hysteresis. Inorganic perovskite nanocrystals (IPNCs) are promising candidates for high-performance photovoltaic devices due to their simple synthesis methods, tunable bandgap, and efficient photon downshifting effect for ultraviolet (UV) light blocking and conversion. In this work, CsPbBr3 IPNCs modification could give rise to the vapor phase and solution-processed PSCs with a power conversion efficiency (PCE) of 16.4% and 20.8%, respectively, increased by 11.6% and 5.6% compared to the control devices for more efficient UV utilization and carrier recombination suppression. As far as is known, 11.6% is the most effective enhanced factor for PSCs based on photon downshifting effect inside of devices. The CsPbBr3 layer could also significantly retard light-induced degradation, leading to the lifetime over 100 h under UV illumination for PSCs. Additionally, the modified PSCs exhibit weak hysteresis and multiple colors of fluorescence. These results shed light on the future design of combining a photon downshifting layer and carrier interfacial modification layer in the applications of perovskite optoelectronic devices.

Passivation of the grain boundaries of CH3NH3PbI3 using carbon quantum dots for highly efficient perovskite solar cells with excellent environmental stability

Organic-inorganic hybrid perovskites are prone to defect formation due to iodine and methylamine ion/defect migration, leading to the formation of lots of defects at the perovskite surface and grain boundaries. Passivation of the defects is an effective method to improve the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). To achieve stable passivation, the interaction between the perovskite and additive materials should be taken into consideration. In this work, we for the first time introduced carbon quantum dots (CQDs) as an additive for the stabilization of MAPbI3via passivation of the grain boundaries of the perovskite. Because the carboxylic groups, hydroxyl groups and amino-groups on the edge of CQDs can bond with the uncoordinated Pb in MAPbI3, strong and stable interactions between the perovskite and CQDs can be generated, inducing a lower trap-state density and better optoelectronic properties. The typical PCE of the PSCs based on CQD modified MAPbI3 films increases from 17.59% to 18.81% and the PCE of the optimized champion PSCs reaches 19.38%. Furthermore, the hydrophobic CQD molecules can block the contact between water and MAPbI3, and even if the CQD modified perovskite is kept under ambient atmosphere without controlling the humidity for 4 months, the MAPbI3 film still retained its original black color.

Recent Advance in Solution-Processed Organic Interlayers for High-Performance Planar Perovskite Solar Cells

Planar heterojunction perovskite solar cells (PSCs) provide great potential for fabricating high-efficiency, low-cost, large-area, and flexible photovoltaic devices. In planar PSCs, a perovskite absorber is sandwiched between hole and electron transport materials. The charge-transporting interlayers play an important role in enhancing charge extraction, transport, and collection. Organic interlayers including small molecules and polymers offer great advantages for their tunable chemical/electronic structures and low-temperature solution processibility. Here, recent progress of organic interlayers in planar heterojunction PSCs is discussed, and the effect of chemical structures on device performance is also illuminated. Finally, the main challenges in developing planar heterojunction PSCs based on organic interlayers are identified, and strategies for enhancing the device performance are also proposed.

Oxygen- and Water-Induced Energetics Degradation in Organometal Halide Perovskites

Organometal halide perovskites are under rapid development, and significant focus has been placed on their stability that currently presents a major obstacle for practical application. Energetics plays a vital role in charge injection/extraction and transport properties in devices. Here, we in situ investigate oxygen- and water-induced energetics degradation in organometal halide perovskite films. Oxygen gas induces an upward shift of the vacuum level of the perovskite films because of the formation of an oxygen-induced surface dipole, water vapor causes a significant vacuum-level downshift, and the valence band binding energy referenced to the Fermi level simultaneously increases so as to keep the ionization potential of the perovskite films unchanged. Moreover, the chemical compositions, crystalline structures, surface morphologies, and dynamical properties also are monitored and analyzed in detail. These results are indispensable to understand the degradation mechanisms and to perform the optimizations of stable materials and devices in the future.

A solution-processed pillar[5]arene-based small molecule cathode buffer layer for efficient planar perovskite solar cells

A room-temperature solution-processed pillar[5]arene-based small molecule material, termed C3, has been designed, synthesized, and incorporated between a conventional PCBM electron transport layer (ETL) and a metal electrode to function as a single-layer cathode buffer layer (CBL) for efficient planar p-i-n perovskite solar cells (PVSCs). It has been found that C3 has a work function tunneling effect, which can decrease the work function of the Ag electrode; therefore, introduction of C3 successfully enhances the interface contact and reduces the interface barriers, which usually exist between fullerene derivatives and metal electrodes. It was also found that the C3 capping layer could improve the surface quality of PCBM, forming a smooth, dense and pinhole-free morphology with fewer surface defects. Thus, C3 can modify the interface between PCBM and Ag, enhance the diode properties of devices and facilitate electron transport through the devices; therefore, it is a very promising CBL material for PVSCs. A device with a hybrid PCBM ETL and a single cathode buffer layer of C3 exhibited a high power conversion efficiency (PCE) of 17.42% with negligible hysteresis, which was dramatically higher than that of a device based on a pure PCBM ETL. With the major advantages of a low-temperature solution process and interface modification, the excellent PCE of PVSCs on flexible substrates can exceed 13%. These results demonstrate that solution-processed pillar[5]arene-based small molecule materials can serve as high performance CBLs in PVSCs.

Stable and Efficient Organo-Metal Halide Hybrid Perovskite Solar Cells via π-Conjugated Lewis Base Polymer Induced Trap Passivation and Charge Extraction

High-quality pinhole-free perovskite film with optimal crystalline morphology is critical for achieving high-efficiency and high-stability perovskite solar cells (PSCs). In this study, a p-type π-conjugated polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)-benzo[1,2-b:4,5-b'] dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl) benzo[1',2'-c:4',5'-c'] dithiophene-4,8-dione))] (PBDB-T) is introduced into chlorobenzene to form a facile and effective template-agent during the anti-solvent process of perovskite film formation. The π-conjugated polymer PBDB-T is found to trigger a heterogeneous nucleation over the perovskite precursor film and passivate the trap states of the mixed perovskite film through the formation of Lewis adducts between lead and oxygen atom in PBDB-T. The p-type semiconducting and hydrophobic PBDB-T polymer fills in the perovskite grain boundaries to improve charge transfer for better conductivity and prevent moisture invasion into the perovskite active layers. Consequently, the PSCs with PBDB-T modified anti-solvent processing leads to a high-efficiency close to 20%, and the devices show excellent stability, retaining about 90% of the initial power conversion efficiency after 150 d storage in dry air.

Low-temperature solution-processed vanadium oxide as hole transport layer for efficient and stable perovskite solar cells

Planar perovskite solar cells employing low-temperature-processed vanadium oxide as the hole transport layer with enhanced stability.

Facile fabrication of perovskite layers with large grains through a solvent exchange approach

A solvent exchange strategy (SES) was used to obtain perovskite films without a dripping anti-solvent.

High performance planar p-i-n perovskite solar cells based on a thin Alq3 cathode buffer layer

As a thin cathode buffer layer (CBL) tris-(8-hydroxyquinoline), aluminum (Alq₃) is successfully introduced into the planar p-i-n perovskite solar cells (PSC) between the PCBM layer and cathode with a device structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃(Cl)/PCBM/Alq₃/Ag. Due to the as-introduced thin Alq₃ CBL, a high performance planar PSC has been achieved with a fill factor (FF) of 72% and maximum power conversion efficiency (PCE) of 14.22%. The PCE value is approximately 29% higher than that of the reference device without Alq₃ CBL. Concerning the results of AC impedance spectra and transient photocurrent measurements, such a remarkable improvement of PCE is mainly attributed to the Alq₃-caused better charge-extraction at the cathode, which is induced by reducing charge accumulation between PCBM and Ag.

Alq₃ can shift the band bending to the cathode contact in the PCBM layer which is beneficial to electron transportation.

Efficient Planar Structured Perovskite Solar Cells with Enhanced Open-Circuit Voltage and Suppressed Charge Recombination Based on a Slow Grown Perovskite Layer from Lead Acetate Precursor

For planar structured organic-inorganic hybrid perovskite solar cells (PerSCs) with the poly(3,4-ethylenedioxythiophene:polystyrene sulfonate) (PEDOT:PSS) hole transport layer, the open-circuit voltage (V_oc) of the device is limited to be about 1.0 V, resulting in inferior performance in comparison with TiO₂-based planar counterparts. Therefore, increasing V_oc of the PEDOT:PSS-based planar device is an important way to enhance the efficiency of the PerSCs. Herein, we demonstrate a novel approach for perovskite film formation and the film is formed by slow growth from lead acetate precursor via a one-step spin-coating process without the thermal annealing (TA) process. Because the perovskite layer grows slowly and naturally, high-quality perovskite film can be achieved with larger crystalline particles, less defects, and smoother surface morphology. Ultraviolet absorption, X-ray diffraction, scanning electron microscopy, steady-state fluorescence spectroscopy (photoluminescence), and time-resolved fluorescence spectroscopy are used to clarify the crystallinity, morphology, and internal defects of perovskite thin films. The power conversion efficiency of p-i-n PerSCs based on slow-grown film (16.33%) shows greatly enhanced performance compared to that of the control device based on traditional thermally annealed perovskite film (14.33%). Furthermore, the V_oc of the slow-growing device reaches 1.12 V, which is 0.1 V higher than that of the TA device. These findings indicate that slow growth of the perovskite layer from lead acetate precursor is a promising approach to achieve high-quality perovskite film for high-performance PerSCs.

Comprehensive Study of Sol-Gel versus Hydrolysis-Condensation Methods To Prepare ZnO Films: Electron Transport Layers in Perovskite Solar Cells

Owing to the high charge mobility and low processing temperature, ZnO is regarded as an ideal candidate for electron transport layer (ETL) material in thin-film solar cells. For the film preparation, the presently dominated sol-gel (SG) and hydrolysis-condensation (HC) methods show great potential; however, the effect of these two methods on the performance of the resulting devices has not been investigated in the same frame. In this study, the ZnO films made through SG and HC methods were applied in perovskite solar cells (Pero-SCs), and the performances of corresponding devices were compared under parallel conditions. We found that the surface morphologies and the conductivities of the films prepared by SG and HC methods showed great differences. The HC-ZnO films with higher conductivity led to relatively higher device performance, and the best power conversion efficiencie (PCE) of 12.9% was obtained; meanwhile, for Pero-SCs based on SG-ZnO, the best PCE achieved was 10.9%. The better device performance of Pero-SCs based on HC-ZnO should be attributed to the better charge extraction and transportation ability of HC-ZnO film. Moreover, to further enhance the performance of Pero-SCs, a thin layer of pristine C₆₀ was introduced between HC-ZnO and perovskite layers. By doing so, the quality of perovskite films was improved, and the PCE was elevated to 14.1%. The preparation of HC-ZnO film involves relatively lower-temperature (maximum 100 °C) processing; the films showed better charge extraction and transportation properties and can be a more promising ETL material in Pero-SCs.

Optimization of the Ag/PCBM interface by a rhodamine interlayer to enhance the efficiency and stability of perovskite solar cells

Effective control of the interface between the metal cathode and the electron transport layer (ETL) is critical for achieving high performance p-i-n planar heterojunction perovskite solar cells (PSCs). Several organic molecules have been explored as interlayers between the silver (Ag) electrode and the ETL for the improvement in the photovoltaic conversion efficiency (PCE) of p-i-n planar PSCs. However, the role of these organic molecules in the charge transfer at the metal/ETL interface and the chemical degradation processes of PSCs has not yet been fully understood. In this work, we systematically explore the effects of the interfacial modification of the Ag/ETL interface on PSCs using rhodamine 101 as a model molecule. By the insertion of rhodamine 101 as an interlayer between Ag and fullerene derivatives (PC60BM and PC70BM) ETLs improve the PCE as well as the stability of p-i-n planar PSCs. Atomic force microscopy (AFM) characterization reveals that rhodamine passivates the defects at the PCBM layer and reduces the band bending at the PCBM surface. In consequence, charge transfer from the PCBM towards the Ag electrode is enhanced leading to an increased fill factor (FF) resulting in a PCE up to 16.6%. Moreover, rhodamine acts as a permeation barrier hindering the penetration of moisture towards the perovskite layer as well as preventing the chemical interaction of perovskite with the Ag electrode. Interestingly, the work function of the metal cathode remains more stable due to the rhodamine incorporation. Consequently, a better alignment between the quasi-Fermi level of PCBM and the Ag work function is achieved minimizing the energy barrier for charge extraction. This work contributes to reveal the relevance of proper interfacial engineering at the metal-cathode/organic-semiconductor interface.

A PbI2-xClx seed layer for obtaining efficient planar-heterojunction perovskite solar cells via an interdiffusion process

Despite the previous reports on the fabrication of CH₃NH₃PbI_3-xCl_x films via sequential deposition, the positioning and formation of PbI₂ in MAPbI_3-xCl_x perovskite films made from the seed layer containing PbI₂ and PbCl₂ in different ratios have not yet been addressed. In this study, the PbI₂ content in a perovskite absorber layer is controlled by changing the PbCl₂ ratio in a PbI_2-xCl_x seed layer. The addition of PbCl₂ in the seed layer facilitates PbI₂ generation and affects the morphology of the perovskite film. By integrating a perovskite absorber via the PbI_2-xCl_x seed-layer into a solar cell, we investigated the effects of the correlation between the chlorine and PbI₂ contents on the device performance through intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy. Elemental depth profiling analyses confirm that not only was the formed PbI₂ preferentially located near the metal-oxide layer, but residual chlorine was adsorbed at the TiO₂ layer. Our findings demonstrate that the geometric features of the formed PbI₂ affected the perovskite solar cells according to the chlorine content, likely because of the elemental gradient induced by annealing. The PbI_2-xCl_x-derived planar-heterojunction perovskite solar cells exhibited maximum power-conversion efficiencies of 17.56% at reverse scan and 17.21% at forward scan, suppressed current density-voltage hysteresis, and good performance distributions.

Exploring the Photovoltaic Performance of All-Inorganic Ag2PbI4/PbI2 Blends

We present an all-inorganic photoactive material composed of Ag₂PbI₄ and PbI₂, which shows unexpectedly good photovoltaic performance in planar junction solar cells delivering external quantum efficiencies of ∼60% and light power conversion efficiencies of ∼3.9%. The revealed characteristics are among the best reported to date for metal halides with nonperovskite crystal structure. Most importantly, the obtained results suggest a possibility of reaching high photovoltaic efficiencies for binary and, probably, also ternary blends of different inorganic semiconductor materials. This approach, resembling the bulk heterojunction concept guiding the development of organic photovoltaics for two decades, opens wide opportunities for rational design of novel inorganic and hybrid materials for efficient and sustainable photovoltaic technologies.

Lead-Free Organic-Inorganic Hybrid Perovskites for Photovoltaic Applications: Recent Advances and Perspectives

Organic-inorganic hybrid halide perovskites (e.g., MAPbI₃ ) have recently emerged as novel active materials for photovoltaic applications with power conversion efficiency over 22%. Conventional perovskite solar cells (PSCs); however, suffer the issue that lead is toxic to the environment and organisms for a long time and is hard to excrete from the body. Therefore, it is imperative to find environmentally-friendly metal ions to replace lead for the further development of PSCs. Previous work has demonstrated that Sn, Ge, Cu, Bi, and Sb ions could be used as alternative ions in perovskite configurations to form a new environmentally-friendly lead-free perovskite structure. Here, we review recent progress on lead-free PSCs in terms of the theoretical insight and experimental explorations of the crystal structure of lead-free perovskite, thin film deposition, and device performance. We also discuss the importance of obtaining further understanding of the fundamental properties of lead-free hybrid perovskites, especially those related to photophysics.

Effect of Energy Alignment, Electron Mobility, and Film Morphology of Perylene Diimide Based Polymers as Electron Transport Layer on the Performance of Perovskite Solar Cells

For organic-inorganic perovskite solar cells (PerSCs), the electron transport layer (ETL) plays a crucial role in efficient electron extraction and transport for high performance PerSCs. Fullerene and its derivatives are commonly used as ETL for p-i-n structured PerSCs. However, these spherical small molecules are easy to aggregate with high annealing temperature and thus induce morphology stability problems. N-type conjugated polymers are promising candidates to overcome these problems due to the tunable energy levels, controllable aggregation behaviors, and good film formation abilities. Herein, a series of perylene diimide (PDI) based polymers (PX-PDIs), which contain different copolymeried units (X), including vinylene (V), thiophene (T), selenophene (Se), dibenzosilole (DBS), and cyclopentadithiophene (CPDT), are introduced as ETL for p-i-n structured PerSCs. The effect of energy alignment, electron mobility, and film morphology of these ETLs on the photovoltaic performance of the PerSCs are fully investigated. Among the PX-PDIs, PV-PDI demonstrates the deeper LUMO energy level, the highly delocalized LUMO electron density, and a better planar structure, making it the best electron transport material for PerSCs. The planar heterojunction PerSC with PV-PDI as ETL achieves a power conversion efficiency (PCE) of 10.14%, among the best values for non-fullerene based PerSCs.

Effect of Selective Contacts on the Thermal Stability of Perovskite Solar Cells

Thermal stability of CH₃NH₃PbI₃ (MAPbI₃)-based perovskite solar cells was investigated for normal structure including the mesoporous TiO₂ layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI₃ was found to be intrinsically stable from 85 °C to 120 °C in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 °C. Photoluminescence (PL) intensity and the time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI₃ into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 °C-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD, and MAPbI₃ is proved to be thermally stable. Spiro-MeOTAD with additives was crystallized at 85 °C due to a low glass transition temperature, and hole mobility was significantly deteriorated, which was responsible for the thermal instability. Thermal stability was significantly improved for the inverted structure with the NiO hole transporting layer, where the power conversion efficiency (PCE) was maintained at 74% of its initial PCE of 14.71% after the 80th thermal cycle (one cycle: heating at 85 °C for 2 h and cooling at 25 °C for 2 h). This work implies that the thermal stability of perovskite solar cells depends on selective contacts.

Current status of electron transport layers in perovskite solar cells: materials and properties

Methyl ammonium lead halide-based hybrid perovskite solar cells (PSCs) have been intensively studied in recent years because of their high efficiency and low processing costs.

Simultaneously enhanced Jsc and FF by employing two solution-processed interfacial layers for inverted planar perovskite solar cells

A high-performance perovskite solar cell has been achieved by embedding s-MoO_x and TOPD buffer layers between active layer and electrodes, showing a remarkable enhancement in FF from 69.2% to 80.7%, and in efficiency from 11.2% to 16.04%.

Improved Performance and Stability of Inverted Planar Perovskite Solar Cells Using Fulleropyrrolidine Layers

Inverted planar structure perovskite solar cells (PSCs), due to their low-temperature precessing and lack of hysteretic problems, are attracting increased attention by researchers around the world. Fullerene derivatives are the most widely used electron transport materials (ETMs) in inverted planar perovskite solar cells, especially [6,6]-phenyl-C₆₁-butyric acid methylester (PC₆₁BM), which exhibits very good performance. However, to the best of our knowledge, the influence of adducts on fullerene-based PSCs performance has not been fully explored to date. In this work, two fullerene derivatives, 2,5-(dimethyl ester) C₆₀ fulleropyrrolidine (DMEC₆₀) and the analogous C₇₀ derivative (DMEC₇₀), were synthesized in high yield via a 1,3-dipolar cycloaddition reaction at room temperature and incorporated into CH₃NH₃PbI₃ perovskite solar cells as electron transport materials. Possibly because the attached pyrrolidine ester groups are able to coordinate with the perovskite layer, the devices based on DMEC₆₀ and DMEC₇₀ achieved power conversion efficiencies (PCE) of 15.2% and 16.4%, respectively. Not only were both devices' efficiencies higher than those based on PC₆₁BM and PC₇₁BM, but their stabilities were also higher than those for PCBM-based devices. The results suggest that DMEC₆₀ and DMEC₇₀ are better alternatives than PC₆₁BM and PC₇₁BM for the ETMs in PSCs.

Metal-encapsulated organolead halide perovskite photocathode for solar-driven hydrogen evolution in water

Lead-halide perovskites have triggered the latest breakthrough in photovoltaic technology. Despite the great promise shown by these materials, their instability towards water even in the presence of low amounts of moisture makes them, a priori, unsuitable for their direct use as light harvesters in aqueous solution for the production of hydrogen through water splitting. Here, we present a simple method that enables their use in photoelectrocatalytic hydrogen evolution while immersed in an aqueous solution. Field's metal, a fusible InBiSn alloy, is used to efficiently protect the perovskite from water while simultaneously allowing the photogenerated electrons to reach a Pt hydrogen evolution catalyst. A record photocurrent density of −9.8 mA cm⁻² at 0 V versus RHE with an onset potential as positive as 0.95±0.03 V versus RHE is obtained. The photoelectrodes show remarkable stability retaining more than 80% of their initial photocurrent for ∼1 h under continuous illumination.

Lead-halide perovskites are sensitive to humidity, which limits their use in water splitting applications. Here, the authors protect the perovskite layer with Field's metal, driving photoelectrocatalytic hydrogen evolution in an aqueous solution for approximately one hour under constant illumination.

Morphology Engineering for High-Performance and Multicolored Perovskite Light-Emitting Diodes with Simple Device Structures

The film morphology is extremely significant for solution processed perovskite devices. Through fine morphology engineering without using any additives or further posttreatments, a full-coverage and high quantum yield perovskite film has been achieved based on one-step spin-coating method. The morphologies and film characteristics of MAPbBr3 with different MABr:PbBr2 starting material ratios are in-depth investigated by scanning electron microscopy, atomic force microscopy, X-ray diffraction, photoluminescence, and time resolved photoluminescence. High performance organometal halide perovskite light-emitting didoes (PeLEDs) based on simple device structure of indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (

PEDOT

PSS)/perovskite/TPBi/Ca/Al are demonstrated. The green PeLED based on MAPbBr3 shows a maximum luminance of 8794 cd m(-2) (at 7.3 V) and maximum current efficiency of 5.1 cd A(-1) (at 5.1 V). Furthermore, a class of hybrid PeLEDs by adjusting the halide ratios of methylammonium lead halide (MAPbX3 , where X is Cl, Br, or I) are also demonstrated at room temperature. These mix-halogenated PeLEDs show bright luminance (above 100 cd m(-) (2) ) with narrow and clean emission bands over the wide color gamut.

Power Conversion Efficiency and Device Stability Improvement of Inverted Perovskite Solar Cells by Using a ZnO:PFN Composite Cathode Buffer Layer

We have demonstrated in this article that both power conversion efficiency (PCE) and performance stability of inverted planar heterojunction perovskite solar cells can be improved by using a ZnO:PFN nanocomposite (PFN: poly[(9,9-bis(3'-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene]) as the cathode buffer layer (CBL). This nanocomposite could form a compact and defect-less CBL film on the perovskite/PC61BM surface (PC61BM: phenyl-C61-butyric acid methyl ester). In addition, the high conductivity of the nanocomposite layer makes it works well at a layer thickness of 150 nm. Both advantages of the composite layer are helpful in reducing interface charge recombination and improving device performance. The power conversion efficiency (PCE) of the best ZnO:PFN CBL based device was measured to be 12.76%, which is higher than that of device without CBL (9.00%), or device with ZnO (7.93%) or PFN (11.30%) as the cathode buffer layer. In addition, the long-term stability is improved by using ZnO:PFN composite cathode buffer layer when compare to that of the reference cells. Almost no degradation of open circuit voltage (VOC) and fill factor (FF) was found for the device having ZnO:PFN, suggesting that ZnO:PFN is able to stabilize the interface property and consequently improve the solar cell performance stability.

Acetate Salts as Nonhalogen Additives To Improve Perovskite Film Morphology for High-Efficiency Solar Cells

A two-step method has been popularly adopted to fabricate a perovskite film of planar heterojunction organo-lead halide perovskite solar cells (PSCs). However, this method often generates uncontrollable film morphology with poor coverage. Herein, we report a facile method to improve perovskite film morphology by incorporating a small amount of acetate (CH3COO(-), Ac(-)) salts (NH4Ac, NaAc) as nonhalogen additives in CH3NH3I solution used for immersing PbI2 film, resulting in improved CH3NH3PbI3 film morphology. Under the optimized NH4Ac additive concentration of 10 wt %, the best power conversion efficiency (PCE) reaches 17.02%, which is enhanced by ∼23.2% relative to that of the pristine device without additive, whereas the NaAc additive does not lead to an efficiency enhancement despite the improvement of the CH3NH3PbI3 film morphology. SEM study reveals that NH4Ac and NaAc additives can both effectively improve perovskite film morphology by increasing the surface coverage via diminishing pinholes. The improvement on CH3NH3PbI3 film morphology is beneficial for increasing the optical absorption of perovskite film and improving the interfacial contact at the perovskite/spiro-OMeTAD interface, leading to the increase of short-circuit current and consequently efficiency enhancement of the PSC device for NH4Ac additive only.

Vertically aligned nanostructured TiO2 photoelectrodes for high efficiency perovskite solar cells via a block copolymer template approach

We fabricated perovskite solar cells with enhanced device efficiency based on vertically oriented TiO2 nanostructures using a nanoporous template of block copolymers (BCPs). The dimension and shape controllability of the nanopores of the BCP template allowed for the construction of one-dimensional (1-D) TiO2 nanorods and two-dimensional (2-D) TiO2 nanowalls. The TiO2 nanorod-based perovskite solar cells showed a more efficient charge separation and a lower charge recombination, leading to better performance compared to TiO2 nanowall-based solar cells. The best solar cells employing 1-D TiO2 nanorods showed an efficiency of 15.5% with VOC = 1.02 V, JSC = 20.0 mA cm(-2) and fill factor = 76.1%. Thus, TiO2 nanostructures fabricated from BCP nanotemplates could be applied to the preparation of electron transport layers for improving the efficiency of perovskite solar cells.

Blending of n-type Semiconducting Polymer and PC61BM for an Efficient Electron-Selective Material to Boost the Performance of the Planar Perovskite Solar Cell

The highly efficient CH3NH3PbI3 perovskite solar cell (PeSC) is simply achieved by employing a blended electron-transport layer (ETL) consisting of PC61BM and P(NDI2OD-T2). The high molecular weight of P(NDI2OD-T2) allows for a thinned ETL with a uniform morphology that optimizes the PC61BM ETL more effectively. As a result of this enhancement, the power conversion efficiency of a PC61BM:P(NDI2OD-T2)-based PeSC is 25% greater than that of the conventional PC61BM based-PeSC; additionally, the incorporation of P(NDI2OD-T2) into PC61BM attenuates the dependence of the PeSC on the ETL-processing conditions regarding its performance. It is revealed that, in addition to the desirable n-type semiconducting characteristics of PC61BM:P(NDI2OD-T2)-including a higher electron-mobility and a more-effective electron selectivity of a blended ETL for an efficient electron extraction-the superior performance of a PC61BM:P(NDI2OD-T2) device is the result of a thinned and uniformly covered ETL on the perovskite layer.

Credible evidence for the passivation effect of remnant PbI₂ in CH₃NHCH₃PbICH₃ films in improving the performance of perovskite solar cells

The role of remnant PbI2 in CH3NH3PbI3 films is still controversial, some investigations have revealed that the remnant PbI2 plays a passivation role, reduces the charge recombination in perovskite solar cells (PSCs), and improves the performance of PSCs, but the opposing views state that remnant PbI2 has no passivation effect and it would deteriorate the stability of the devices. In our investigation, the CH3NH3PbI3 films have been prepared by a two-step spin-coating method and the content of the remnant PbI2 in CH3NH3PbI3 films has been tuned by varying the preparation temperature. It has been found that increasing the heating temperature could increase the coverage of spin-coated PbI2 films, which has led to high coverage CH3NH3PbI3 films and more remnant PbI2 in CH3NH3PbI3 films, and as a result, the performance of PSCs was enhanced obviously and the maximum power conversion efficiency of 14.32 ± 0.28% was achieved by the PSCs prepared at 130/120 °C (PbI2 films were heated at 130 °C and CH3NH3PbI3 films were heated at 120 °C). Furthermore, the dark current, electrochemical impedance spectroscopy and time-resolved fluorescence emission decay measurements revealed that the charge recombination in PSCs has been gradually suppressed and the fluorescence emission lifetime has gradually increased with the content of remnant PbI2 increasing. Thus, the passivation effects of the unreacted and decomposed PbI2 in improving the performance of PSCs have been confirmed unquestionably.

Null current hysteresis for acetylacetonate electron extraction layer in perovskite solar cells

Solution processed zirconium acetylacetonate (Zr(acac)) is successfully employed as an electron extraction layer, replacing conventional titanium oxide, in planar CH3NH3PbI3 perovskite solar cells. The as-prepared Zr(acac) film possesses high transparency, high conductivity, a smooth morphology, high wettability, compatibility with PbI2 DMF solution, and an energy level matching that of CH3NH3PbI3 perovskite material. An average power conversion efficiency of about 11.93%, along with a high fill factor of 74.36%, an open circuit voltage of 1.03 V, and a short-circuit current density of 15.58 mA cm(-2) is achieved. The overall performance of the devices is slight better than that of cells using ruthenium acetylacetonate (Ru(acac)). The differences between solar cells with different electron extraction layers in charge recombination, charge transport and transfer and lifetime are further explored and it is demonstrate that Zr(acac) is a more effective and promising electron extraction layer. This work provides a simple, and cost effective route for the preparation of an effective hole extraction layer.

Polyethylenimine as a dual functional additive for electron transporting layer in efficient solution processed planar heterojunction perovskite solar cells

The device performance is enhanced by doping a small percentage of polyethylenimine (PEI) into the PCBM.