Efficient Planar Perovskite Solar Cells with Reduced Hysteresis and Enhanced Open Circuit Voltage by Using PW12-TiO2 as Electron Transport Layer

Hydrogen-Bonded Organic Framework-Polyoxometalate-Based System for Electrochromic Devices

A porous hydrogen-bonded organic framework (HOF) structure was explored for the first time in the design of high-performance electrochromic devices (ECDs) using polyoxometalate (POM)-based charge-balancing layers as counter electrodes (CEs). The novelty of this work lies in the facile construction of films using small molecule-based EC materials to form a porous HOF structure. A full-cell model of an ECD was constructed by utilizing a POM-based CE to optimize the voltage distribution on the HOF-coated working electrode (WE). The addition of PW12O403- (PW12) on CE significantly enhanced the voltage distribution on EC electrodes and decreased the overvoltage on the WE, further preventing the formation of non-EC species and resulting in a 3.3-fold increase in the lifetime of the ECD. The optical contrast was enhanced from 47% (TiO2 only) to 68%, and the coloration efficiency was enhanced from 185 (TiO2 only) to 373 cm2 C-1. The optimized voltage distribution on the WE, leading to the fast response time and high optical EC contrast, could be explained by the charge-balancing effect. Overall, this new finding provides a robust framework for designing high-performance ECDs, taking advantage of the porous morphology and potential matching of the HOF and PW12.

Metal oxide charge transporting layers for stable high-performance perovskite solar cells

This review summarizes the recent progress in metal oxide charge transporting layers to achieve stable high-performance perovskite solar cells.

Low-Temperature-Processed Brookite-Based TiO2 Heterophase Junction Enhances Performance of Planar Perovskite Solar Cells

In the design of electron-transport layers (ETLs) to enhance the efficiency of planar perovskite solar cells (PSCs), facile electron extraction and transport are important features. Here, we consider the effects of different titanium oxide (TiO₂) polymorphs, anatase and brookite. We design and fabricate high-phase-purity, single-crystalline, highly conductive, and low-temperature (<180 °C)-processed brookite-based TiO₂ heterophase junctions on fluorine-doped tin oxide (FTO) as the substrate. We test and compare single-phase anatase (A) and brookite (B) and heterophase anatase-brookite (AB) and brookite-anatase (BA) as ETLs in PSCs. The power-conversion efficiencies (PCEs) of PSCs with low-temperature-processed single-layer FTO-B as the ETL were as high as 14.92%, which is the highest reported efficiency of FTO-B-based single-layer PSC. This implies that FTO-B serves as an active phase and can be a potential candidate as an n-type ETL scaffold in planar PSCs. Moreover, the surface of highly crystalline brookite TiO₂ exhibits a tendency toward interparticle necking, leading to the formation of compact scaffolds. Furthermore, PSCs with heterophase junction FTO-AB ETLs exhibited PCEs as high as 16.82%, which is superior to those of PSCs with single-phase anatase (FTO-A) and brookite (FTO-B) as the ETLs (13.86% and 14.92%, respectively). In addition, the PSCs with FTO-AB exhibited improved efficiency and decreased hysteresis compared with those with FTO-BA (13.45%) due to the suitable band alignment with the perovskite layer, which resulted in superior photogenerated charge-carrier extraction and reduced charge accumulation at the interface between the heterophase junction and perovskite. Thus, the present work presents an effective strategy by which to develop heterophase junction ETLs and manipulate the interfacial energy band to further improve the performance of planar PSCs and enable the clean and eco-friendly fabrication of low-cost mass production.

The Role of Excitation Energy in Photobrightening and Photodegradation of Halide Perovskite Thin Films

We study the impact of excitation energy on the photostability of methylammonium lead triiodide (CH₃NH₃PbI₃ or MAPI) perovskite thin films. Light soaking leads to a transient increase of the photoluminescence efficiency at excitation wavelengths longer than 520 nm, whereas light-induced degradation occurs when exciting the films with wavelengths shorter than 520 nm. X-ray diffraction and extinction measurements reveal the light-induced decomposition of CH₃NH₃PbI₃ to lead iodide (PbI₂) for the high-energy excitation regime. We propose a model explaining the energy dependence of the photostability that involves the photoexcitation of residual PbI₂ species in the perovskite triggering the decomposition of CH₃NH₃PbI₃.

Highly Efficient Perovskite Solar Cells Based on Zn2 Ti3 O8 Nanoparticles as Electron Transport Material

Developing ternary metal oxides as electron transport layers (ETLs) for perovskite solar cells is a great challenge in the field of third-generation photovoltaics. In this study, a highly mesoporous Zn₂ Ti₃ O₈ (m-ZTO) scaffold is synthesized by ion-exchange method and used as ETL for the fabrication of methyl ammonium lead halide (CH₃ NH₃ PbI₃ ) perovskite solar cells. The optimized devices exhibit 17.21 % power conversion efficiency (PCE) with an open circuit voltage (V_oc ) of 1.02 V, short-circuit current density (J_sc ) of 21.97 mA cm^-2 and fill factor (FF) of 0.77 under AM 1.5G sunlight (100 mW cm^-2 ). The PCE is significantly higher than that based on mesoporous ST01 (m-ST01; 10 nm TiO₂ powder) layer (η=14.93 %), which is ascribed to the deeper conductive band of ZTO nanoparticles, better light absorption and smaller charge recombination. The devices stored for 100 days at ambient temperature with humidity of 10 % showed excellent stability with only 12 % reduction of the PCE. The charge transmission kinetic and long-term stability parameters of the ZTO-based perovskite film growth are discussed as well.

Plasmonic Effects of Metallic Nanoparticles on Enhancing Performance of Perovskite Solar Cells

We report systematic design and formation of plasmonic perovskite solar cells (PSCs) by integrating Au@TiO₂ core-shell nanoparticles (NPs) into porous TiO₂ and/or perovskite semiconductor capping layers. The plasmonic effects in the formed PSCs are examined. The most efficient configuration is obtained by incorporating Au@TiO₂ NPs into both the porous TiO₂ and the perovskite capping layers, which increases the power conversion efficiency (PCE) from 12.59% to 18.24%, demonstrating over 44% enhancement, compared with the reference device without the metal NPs. The PCE enhancement is mainly attributed to short-circuit current improvement. The plasmonic enhancement effects of Au@TiO₂ core-shell nanosphere photovoltaic composites are explored based on the combination of UV-vis absorption spectroscopy, external quantum efficiency (EQE), photocurrent properties, and photoluminescence (PL). The addition of Au@TiO₂ nanospheres increased the rate of exciton generation and the probability of exciton dissociation, enhancing charge separation/transfer, reducing the recombination rate, and facilitating carrier transport in the device. This study contributes to understanding of plasmonic effects in perovskite solar cells and also provides a promising approach for simultaneous photon energy and electron management.

It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis

Solar cells made from inorganic-organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 °C is presented. The inorganic metal oxides TiO₂ and SnO₂, the organic fullerene derivatives C₆₀, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO₂, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current-voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO₂/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.

SiW12 -TiO2 Mesoporous Layer for Enhanced Electron-Extraction Efficiency and Conductivity in Perovskite Solar Cells

High quality electron-transport layer (ETL) with superior optical and electrical properties is an essential part in high efficient perovskite solar cells (PSCs). In this work, SiW₁₂ -TiO₂ mesoporous film is prepared by a facile one-step spin-coating deposition method and successfully applied as ETL in PSCs. Compared with pristine TiO₂ -based PSC, the SiW₁₂ -TiO₂ -based one shows a remarkable enhanced power conversion efficiency (PCE) from 12.00 to 14.66 %, which is owed to the higher conductivity, electron-extraction efficiency, and well-matched energy level alignment of SiW₁₂ -TiO₂ film. Moreover, the SiW₁₂ -TiO₂ -based device also shows a good long-time stability in under ambient conditions. This work demonstrates that using polyoxometalates (POMs) to modify the metal-oxide semiconductors is an effective approach for further enhancing the performance of PSCs.

Efficient perovskite solar cells by combination use of Au nanoparticles and insulating metal oxide

Achieving high open-circuit voltage and high short-circuit current density simultaneously is a big challenge in the development of highly efficient perovskite solar cells, due to the complex excitonic nature of hybrid organic-inorganic semiconductors. Herein, we developed a facile and effective method to fabricate efficient plasmonic PSC devices. The solar cells were prepared by incorporating Au nanoparticles (NPs) into mesoporous TiO₂ films and depositing a MgO passivation film on the Au NP-modified mesoporous titania via wet spinning and pyrolysis of magnesium salt. The PSCs obtained by combining Au NPs and MgO demonstrated a high power conversion efficiency of 16.1%, with both a high open-circuit voltage of 1.09 V and a high short-circuit current density of 21.76 mA cm^-2. The device achieved a 34.2% improvement in the power conversion efficiency compared with a device based on pure TiO₂. Moreover, a significant improvement of the UV stability in the perovskite solar cell was achieved due to the combined use of Au NPs and insulating MgO. The fundamental optics and physics behind the regulation of energy flow in the perovskite solar cell and the concept of using Au NPs and MgO to improve the device performance were explored. The results indicate that the combined use of Au NPs and a MgO passivation film is an effective way to design high performance and high stability organic-inorganic perovskite photovoltaic materials.

Keggin-Type PMo11V as a P-type Dopant for Enhancing the Efficiency and Reproducibility of Perovskite Solar Cells

The conventional perovskite solar cells (PSCs) with 2,2',7,7'-tetrakis(N,N-dimethoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) as a hole transporting material commonly suffer from poor stability and reproducibility mainly due to the process of placing the devices in air and illumination for oxidizing the spiro-OMeTAD. Herein, Keggin-type polyoxometalates (POMs)-phosphovanadomolybdate (H₄PMo₁₁V·nH₂O, denoted as PMo₁₁V) is for the first time employed as a p-type dopant for promoting the oxidation of spiro-OMeTAD. Thereby, without illumination and air, the conductivity and hole extraction efficiency of the PMo₁₁V doped spiro-OMeTAD with assistance of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (TBP) can be dramatically enhanced. On the basis of this strategy, the corresponding PSCs exhibit substantially improved photovoltaic performance and good reproducibility. The best performing device yields a power conversion efficiency (PCE) of 14.05%. This work indicates a great potential of polyoxometalates for further applications in solar cells and other optoelectronics devices.

Amorphous TiO2 Compact Layers via ALD for Planar Halide Perovskite Photovoltaics

A low-temperature (<120 °C) route to pinhole-free amorphous TiO2 compact layers may pave the way to more efficient, flexible, and stable inverted perovskite halide device designs. Toward this end, we utilize low-temperature thermal atomic layer deposition (ALD) to synthesize ultrathin (12 nm) compact TiO2 underlayers for planar halide perovskite PV. Although device performance with as-deposited TiO2 films is poor, we identify room-temperature UV-O3 treatment as a route to device efficiency comparable to crystalline TiO2 thin films synthesized by higher temperature methods. We further explore the chemical, physical, and interfacial properties that might explain the improved performance through X-ray diffraction, spectroscopic ellipsometry, Raman spectroscopy, and X-ray photoelectron spectroscopy. These findings challenge our intuition about effective electron selective layers as well as point the way to a greater selection of flexible substrates and more stable inverted device designs.

Recent Advances in Interface Engineering for Planar Heterojunction Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells are considered as one of the most promising next-generation solar cells due to their advantages of low-cost precursors, high power conversion efficiency (PCE) and easy of processing. In the past few years, the PCEs have climbed from a few to over 20% for perovskite solar cells. Recent developments demonstrate that perovskite exhibits ambipolar semiconducting characteristics, which allows for the construction of planar heterojunction (PHJ) perovskite solar cells. PHJ perovskite solar cells can avoid the use of high-temperature sintered mesoporous metal oxides, enabling simple processing and the fabrication of flexible and tandem perovskite solar cells. In planar heterojunction materials, hole/electron transport layers are introduced between a perovskite film and the anode/cathode. The hole and electron transporting layers are expected to enhance exciton separation, charge transportation and collection. Further, the supporting layer for the perovskite film not only plays an important role in energy-level alignment, but also affects perovskite film morphology, which have a great effect on device performance. In addition, interfacial layers also affect device stability. In this review, recent progress in interfacial engineering for PHJ perovskite solar cells will be reviewed, especially with the molecular interfacial materials. The supporting interfacial layers for the optimization of perovskite films will be systematically reviewed. Finally, the challenges remaining in perovskite solar cells research will be discussed.