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Organic‐free (all‐inorganic) and lead‐free halide perovskites in combination with dopant‐free hole transport materials (HTMs) are summarized in terms of potential photovoltaic performance, and progress in compositional and morphological design of solution‐processed perovskite absorbers. Strategies to enhance device efficiency are focused on preparation of high quality perovskite and HTM interface.

Abstract

Having demonstrated incredibly fast progress in power conversion efficiency, rising to a level comparable with that of crystalline silicon cells, lead‐based organic–inorganic hybrid perovskite solar cells are now facing the stability tests needed for industrialization. Poor thermal stability (<150 °C) owing to organic constituents and interlayer diffusion of materials (dopants), and environmental incompatibility due to Pb has surged the development of organic‐free, Pb‐free perovskites and dopant‐free hole transport materials (HTMs). The recent rapid increase in efficiency of cells based on inorganic perovskites, crossing 18%, demonstrates the great potential of inorganic perovskites as thermally stable and high‐efficiency cells. Although all kinds of Pb‐free perovskites lag in efficiency in comparison to the hybrid and inorganic perovskites, they also demonstrate better structural and environmental stability. The performance of dopant‐free HTMs matching/surpassing dopant‐containing HTMs makes the former a better choice for stability. Even though the efforts to enhance the stability of Pb‐based hybrid perovskites should continue by different techniques, organic‐free and lead‐free perovskites, and dopant‐free HTMs must be pursued with greater interest for the future. This review describes the present issues and possible strategies to address them, and thus will help to improve the overall performance of robust organic‐free, Pb‐free, and dopant‐free perovskite solar cells.

A water‐based TiO₂ nanocrystal solution is developed to use as an electron transport layer for perovskite solar cells that show substantially reduced organic molecules and a high Cl content on the TiO₂ nanocrystal surface, which effectively passivate the interface between TiO₂ and perovskite layer with significantly reduced defects. Corresponding solar cells demonstrate a 20.5% power conversion efficiency and 500 h of operational stability.

Halide perovskite solar cells (PSCs) provide a new opportunity for next‐generation photovoltaic applications. However, traditional low‐temperature solution‐processed TiO₂ that acts as an electron transport layer for PSCs shows an inferior stability compared with solar cells based on high‐temperature (typically 500 °C) TiO₂; however, the high‐temperature process is energy consuming and is not compatible with flexible device processing. Traditional TiO₂ nanoparticles made from titanium tetrachloride dispersed in an organic solvent usually have many organic molecules attached on their surface that lead to the formation of deep‐level defect states during long‐term operations. Herein, environmentally friendly, water‐based Cl‐passivated TiO₂ nanoparticles (W‐TiO₂) are invented, and surface organic molecules are removed by a vacuum rotary evaporation process. W‐TiO₂‐based PSCs can reach up to a 20.5% power conversion efficiency with reduced hysteresis and can maintain 80% of their initial performance after 500 h of continuous operation under 1 sun illumination at the maximum power point. This improved performance is ascribed to the organic‐molecule‐free and Cl‐passivated surfaces. The water‐based TiO₂ nanoparticle dispersion also offers a convenient and universal way to introduce other passivation agents to further improve the photovoltaic performance of PSCs.

The technological potential of emerging perovskite solar cells is determined by the stability of the power generated. Alkali salts used as interface modifiers positively affect device performance and stability and mitigate current–voltage hysteresis. Devices modified with potassium nitrate deliver a power conversion efficiency of 19.2% attributed to an improved charge carrier extraction and suppression of transient capacitive effects in device operation.

After demonstration of a 23% power conversion efficiency, a high operational stability is the next most important scientific and technological challenge in perovskite solar cells (PSCs). A potential failure mechanism is tied to a bias‐induced ion migration, which causes current–voltage hysteresis and a decay in the device performance over time. Herein, alkali salts are shown to mitigate hysteresis and stabilize device performance in n‐i‐p hybrid planar PSCs. Different alkali salts of potassium chloride, iodide, and nitrate as well as sodium chloride and iodide are deposited from aqueous solution onto the n‐type contact, based on SnO₂, prior to deposition of the perovskite absorber Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. Introduction of potassium‐based alkali salts suppresses the current–voltage hysteresis and stabilizes the operational device stability at the maximum power point. This is attributed to the suppression of hole trapping at the n‐type selective transport layer (SnO₂)/perovskite interface observed by surface photovoltage spectroscopy, which is interpreted to reduce interfacial recombination and improve charge carrier extraction. The best and most stable performance of 19% is achieved using potassium nitrate as the interface modifier. Devices with higher and more stable performance exhibit substantially lower current transients, analyzed during maximum power point tracking.

Herein, a facile strategy that can carry out double passivation to improve the performance of perovskite solar cells (PSCs) is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film; double passivation improves the performance of PSCs significantly.

Material passivation is essential to enhance the quality of perovskite materials and boost the performance of perovskite solar cells (PSCs). However, most of the previous reports only paid attention to improving the quality of perovskite films by adopting single passivation. Here, a facile strategy that can carry out double passivation to improve the performance of PSCs is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film, and by combining PEABr and PbI₂, the double passivation improves the performance of PSCs significantly. Very high short‐circuit current density of 24.30 mA cm⁻², open‐circuit voltage of 1.10 V, and fill factor of 79.75% are achieved which lead to a surprising efficiency of 21.32% for the passivated device. The improved efficiency is mainly according to the available surface passivation of the perovskite material, leading to repressed nonradiative recombination and unhindered charge collection. In addition, the passivated device exhibits better power conversion efficiency stability relative to the control device.

The studies from the steady‐state and time‐dependent measurements indicate that the extended absorption range, short charge carrier extraction time, and high charge carrier mobility by the non‐fullerene electron acceptors in the photoactive layer are responsible for enhanced photocurrent in ternary organic solar cells.

Ternary organic solar cells, a single active layer comprising three different components, are demonstrated to be one of the most efficient ways to approach high‐performance organic solar cells. But nevertheless, most of the ternary organic solar cells are characterized by steady‐state measurements, which are helpful but inadequate to fully understand the underlying charge carrier behavior at a short time scale. Herein, a comparison of the steady‐state and time‐dependent measurements is used to investigate the functionality of non‐fullerene electron acceptors in ternary organic solar cells. The steady‐state measurements indicate that non‐fullerene electron acceptors enlarge the absorption range of the photoactive layer, suppress charge carrier recombination, reduce charge carrier transfer resistance, and thereby increase photocurrent in ternary organic solar cells. The time‐dependent measurements demonstrate that a short charge carrier extraction time and a high charge carrier mobility are responsible for enhanced photocurrent in ternary organic solar cells. A comprehensive method understanding the underlying of enhanced efficiency of ternary organic solar cells is provided herein.

Homojunction bilayer electron transport layers (ETLs) are developed by stacking Sb‐doped SnO₂ (Sb‐SnO₂) and SnO₂ ETLs via a low‐temperature process. The perovskite solar cells with the Sb‐SnO₂/SnO₂ bilayer ETLs achieve the best power conversion efficiency of 20.73%. Due to Sb‐SnO₂/SnO₂, the bilayer ETL with a cascade energy arrangement enhances charge separation and reduces carrier recombination.

Energy alignment between electron transport layers (ETLs) and perovskite has a strong influence on the device performance of perovskite solar cells (PSCs). Two approaches are deployed to tune the energy level of ETLs: 1) doping ETLs with aliovalent metal cations and 2) constructing heterojunction bilayers with different materials. However, the abrupt interfaces in the heterojunction bilayers introduce undesirable carrier recombination. Herein, a homojunction bilayer ETL is developed by stacking Sb‐doped SnO₂ (Sb‐SnO₂) and SnO₂ ETLs via low‐temperature spin‐coating processes. The energy levels of ETLs are tuned by the incorporation of Sb and altering stacking orders. Bilayer ETL of Sb‐SnO₂/SnO₂ with cascade energy alignment promotes the best power conversion efficiency of 20.73%, surpassing single‐layer ETLs of SnO₂ (18.23%) and Sb‐SnO₂ (19.15%), whereas the SnO₂/Sb‐SnO₂ bilayer with barricade energy alignment receives the poorest device performance. The cascade bilayer ETL facilitates charge separation and suppresses carrier recombination in PSCs, which is verified by photoluminescence, conductivity, and impedance characterizations. The homojunction bilayer ETLs with adjustable energy levels open a new direction for interface engineering toward efficient PSCs.

Herein, amorphous‐Zn₂SnO₄ (am‐ZTO) is used to provide a large free energy difference (ΔG) to improve electron injection from perovskite to electron transport layers. In addition, the introduction of the am‐ZTO also leads to a dense physical contact between the am‐ZTO and the FTO substrate, leading to decreased leakage current. The optimized device exhibits a power conversion efficiency of 20.04%.

Charge extraction by electron transport layers (ETLs) plays a vital role in improving the performance of perovskite solar cells (PSCs). Here, PSCs with four different types of ETLs, such as SnO₂, amorphous‐Zn₂SnO₄ (am‐ZTO), am‐ZTO/SnO₂, and SnO₂/am‐ZTO, are successfully synthesized. The interface recombination behavior and the charge transport properties of the devices affected by four types of ETLs are systematically investigated. For dual am‐ZTO/SnO₂ ETLs, compact am‐ZTO ETL prepared by the pulsed laser deposition method provides a dense physical contact with FTO than the spin coating films, decreasing leakage current and improving charge collection at the interface of ETL/FTO. Moreover, dual am‐ZTO/SnO₂ ETLs lead to large free energy difference (ΔG), improving electron injection from perovskite to ETLs. One additional electron pathway from perovskite to am‐ZTO is formed, which can also improve electron injection efficiency. A power conversion efficiency of 20.04% and a stabilized efficiency of 19.17% are achieved for the device based on dual am‐ZTO/SnO₂ ETLs. Most importantly, the devices are fabricated at a low temperature of 150 °C, which offers a potential method for large‐scale production of PSCs, and paves the way for the development of flexible PSCs. It is believed that this work provides a strategy to design ETLs via controlling ΔG and interface contact to improve the performance of PSCs.

Using the oxygen‐containing functional group (—COOH and/or —C—OH)‐decorated multiwalled carbon nanotubes as the electrode, the power conversion efficiency of perovskite solar cells shows an improvement after long‐term storage. The reason is confirmed to be the self‐reconstruction ability of perovskite material and the interface reconstruction for its morphology and charge transfer resistance showing significant improvement.

Perovskite solar cells (PSCs) have attracted a lot of interest because of their high efficiency and low cost. However, in commercial applications, standard PSCs suffer from low stability of the cell components, including the hole transportation material (HTM). Owing to their characteristics of high chemical stability, hydrophobicity, and high conductivity, carbon nanotubes (CNTs) can be an alternative electrode to use to form HTM‐free PSCs. Enhancing the interaction with perovskite is vital not only for photovoltaic performance but also for the stability of CNT‐based PSCs. Herein, oxygen‐containing functional groups are introduced into CNTs via acid treatment to enhance the chemical interactions with perovskite. The self‐recrystallization ability of the perovskite material is discovered; its morphology shows significant improvement after long‐term storage. Results show that acid oxidization of CNTs enable the self‐recrystallization characteristics of MAPbI₃‐induced interfacial improvement, such that even with a dispersed initial photovoltaic performance, through storage in an ambient medium with relative humidity of 20–50%, the PSCs possess better interface contact, which results in lower charge transfer resistance, higher photovoltaic performance, and stability. As a result, PSCs with an initial power conversion efficiency range of 3.21–7.89% finally converge to within the range of 9.54–12.14% after long‐term storage.

This review provides a systematic overview of self‐powered integrated systems based on perovskite solar cells, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices are discussed to further the understanding of fundamental device physics. The current challenges and future perspective are provided.

Integrated smart portable devices (e.g., self‐powered devices) that utilize the environment‐friendly energy (e.g., solar energy) by means of photovoltaic technology (e.g., solar cell) are a popular concept in the current technological development trend. As a key component of integrated devices, photovoltaic devices acting as a bridge between solar energy and working devices play an important role in the whole system performance. The emergence of perovskite solar cells (PSCs) with high power conversion efficiencies (over 25%) allows for the possibility and appearance of many multifunctional self‐powered integrated devices. In this review, a systematic overview of self‐powered integrated devices based on PSCs that are reported so far is provided, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices and performance are also discussed to further the understanding of fundamental device physics. Finally, the current challenging issues and future perspective are provided to promote the development of self‐powered integrated devices based on PSCs in the near future.

An all‐layer‐inorganic perovskite solar cell (PSC) based on inorganic CsPbI₂Br perovskite absorber layer and tailored NiO hole‐transporting layer (HTL) is fabricated. The tailored NiO nanocrystalline films exhibit uniform, pinhole‐free morphologies, efficient charge‐extraction capabilities, and intrinsic chemical stability, which gives the whole photovoltaic device a high efficiency and much improved stability compared with PSCs based on the organic HTLs.

Cesium‐based inorganic perovskite solar cells (PSCs) have attracted great attention due to the superior thermal stability of the light absorbers. However, the reported devices normally contain organic charge‐transporting layers (CTLs), such as spiro‐OMeTAD, which is expensive and highly sensitive to ambient atmosphere and temperature. It is of great significance to develop inorganic CTLs with low cost and robust stability. To date, it is still a big challenge to achieve high‐quality inorganic CTL films via the solution process, especially for the hole‐transporting layer (HTL) in conventional n‐i‐p structures. Herein, tailored NiO nanocrystalline films as HTLs in an all‐layer‐inorganic CsPbI₂Br‐based PSCs are developed, which exhibit uniform, pinhole‐free morphologies and efficient charge‐extraction capabilities. Consequently, the as‐constructed all‐layer‐inorganic PSCs, with an optimal power conversion efficiency (PCE) of 15.14% and a stabilized power output of 14.82%, present robust long‐term thermal stability: retained 85% of their initial PCEs after a thermal treatment at 85 °C in the dark in a nitrogen atmosphere with encapsulation for 1000 h, greatly surpassing the performance of the PSCs based on the organic HTLs.

Changing the alkyl chain position of a small‐molecule donor provides optimized conformation, improved phase aggregation, and enhanced photovoltaic properties. The strategy affords 12.3% efficiency single‐junction all‐small‐molecule organic solar cells (ASM OSCs) with reduced recombination and enhanced carrier lifetimes. The power conversion efficiency of 12.3% is higher than all reported single‐junction ASM OSCs.

Molecular stacking plays an important role in defining the active layer morphology in all‐small‐molecule organic solar cells (ASM OSCs). However, the precise control of donor/acceptor stacking to afford optimal phase separation remains challenging. Herein, the molecular stacking of a small‐molecule donor is tuned by changing the alky chain position to match a high‐performance small‐molecule nonfullerene acceptor (NFA), Y6. The alky chain engineering not only affects the planarity of the small‐molecule donor, but also the molecular aggregation and the active layer morphology, and thus the photovoltaic performance. Notably, single‐junction ASM OSCs with 12.3% power conversion efficiency (PCE) are achieved. The PCE of 12.3% is among the top efficiencies of single‐junction ASM OSCs reported in the literature to date. The results highlight the importance of fine‐tuning the molecular structure to achieve high‐performance ASM OSCs.

Atomic layer deposition (ALD) is key to improving the efficiency and stability of perovskite solar cells (PSCs). Moreover, ALD unlocks novel options regarding device architecture and processing, not achievable otherwise. Herein, the state of the art in ALD‐grown functional charge transport layers for PSCs is highlighted and the most urgent scientific issues and opportunities for further research are outlined.

Within less than a decade of development, perovskite solar cells (PSCs) have reached efficiency levels that trigger the question about commercialization of the technology. However, the steadily increasing efficiency of PSCs is still accompanied by concerns of long‐term stability and open questions about upscaled manufacturing. Atomic layer deposition (ALD) is a technique that can provide unique contributions to both issues. On top of that, ALD is an enabling technology to unlock novel options regarding device architecture and processing that are not achievable by other coating techniques. As such, ALD has enjoyed some notable recent interest from the community of perovskite photovoltaics. Herein, the current state of the art in ALD‐grown functional charge transport layers for PSCs is highlighted. The most urgent scientific issues that have to be tackled are emphasized and opportunities for further research are outlined.

Nature Communications, Published online: 03 October 2019; doi:10.1038/s41467-019-12513-x

A UV‐inert ZnTiO₃ is demonstrated to be an electron selective layer in perovskite solar cells. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Planar perovskite solar cells based on ZnTiO₃ exhibit power conversion efficiency of 20.1% with improved photostability. The best device holds 90% of its initial efficiency after 100 h of ultraviolet soaking.

Abstract

Although planar‐structured perovskite solar cells (PSCs) have power conversion efficiencies exceeding 24%, the poor photostability, especially with ultraviolet irradiance (UV) severely limits commercial application. The most commonly‐used TiO₂ electron selective layer has a strong photocatalytic effect on perovskite/TiO₂ interface when TiO₂ is excited by UV light. Here a UV‐inert ZnTiO₃ is reported as the electron selective layer in planar PSCs. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Solar cells are fabricated with a structure of indium doped tin oxide (ITO)/ZnTiO₃/Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45/Sprio‐MeOTAD/Au. The champion device exhibits a stabilized power conversion efficiency of 19.8% with improved photostability. The device holds 90% of its initial efficiency after 100 h of UV soaking (365 nm, 8 mW cm⁻²), compared with 55% for TiO₂‐based devices. This work provides a new class of electron selective materials with excellent UV stability in perovskite solar cell applications.

Recent progress on additive engineering during perovskite film formation is reported according to the following common categories: Lewis acid, Lewis base, ammonium salts, low‐dimensional perovskites, and ionic liquid. Then, various additive‐assisted strategies for interface optimization are compared. Finally, an outlook on the research trends with respect to additive engineering in perovskite solar cell development is provided.

Abstract

Perovskite solar cells (PSCs) have reached a certified 25.2% efficiency in 2019 due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap. However, due to the nature of solution processing and rapid crystal growth of perovskite thin films, a variety of defects can form as a result of the precursor compositions and processing conditions. The use of additives can affect perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, as well as influence the interface tuning of structure and energetics. Here, recent progress in additive engineering during perovskite film formation is discussed according to the following common categories: Lewis acid (e.g., metal cations, fullerene derivatives), Lewis base based on the donor type (e.g., O‐donor, S‐donor, and N‐donor), ammonium salts, low‐dimensional perovskites, and ionic liquid. Various additive‐assisted strategies for interface optimization are then summarized; additives include modifiers to improve electron‐ and hole‐transport layers as well as those to modify perovskite surface properties. Finally, an outlook is provided on research trends with respect to additive engineering in PSC development.

The electron extracting properties of the widely used electron transporting layer (ETL) material Phen‐NaDPO are remarkably enhanced via simple addition of the wide‐bandgap inorganic material tin (II) thiocyanate (Sn(SCN)₂). Use of this hybrid ETL system in organic and perovskite solar cells results in consistent efficiency improvements due to the reduced trap‐assisted recombination and efficient electron extraction.

Abstract

A simple approach that enables a consistent enhancement of the electron extracting properties of the widely used small‐molecule Phen‐NaDPO and its application in organic solar cells (OSCs) is reported. It is shown that addition of minute amounts of the inorganic molecule Sn(SCN)₂ into Phen‐NaDPO improves both the electron transport and its film‐forming properties. Use of Phen‐NaDPO:Sn(SCN)₂ blend as the electron transport layer (ETL) in binary PM6:IT‐4F OSCs leads to a remarkable increase in the cells' power conversion efficiency (PCE) from 12.6% (Phen‐NaDPO) to 13.5% (Phen‐NaDPO:Sn(SCN)₂). Combining the hybrid ETL with the best‐in‐class organic ternary PM6:Y6:PC₇₀BM systems results to a similarly remarkable PCE increase from 14.2% (Phen‐NaDPO) to 15.6% (Phen‐NaDPO:Sn(SCN)₂). The consistent PCE enhancement is attributed to reduced trap‐assisted carrier recombination at the bulk‐heterojunction/ETL interface due to the presence of new energy states formed upon chemical interaction of Phen‐NaDPO with Sn(SCN)₂. The versatility of this hybrid ETL is further demonstrated with its application in perovskite solar cells for which an increase in the PCE from 16.6% to 18.2% is also demonstrated.