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Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Ziqi Liang, Miaomiao Li, Qi Wang, Yunpeng Qin, Sam J. Stuard, Zhongxiang Peng, Yunfeng Deng, Harald Ade, Long Ye, Yanhou Geng

Two isomeric crossconjugated polymer hole‐transporting materials (HTMs) are developed to demonstrate significantly distinct device power conversion efficiencies (PCEs) under the same device fabrication conditions, 11.1% PPE1 and 19.3% for PPE2 , which is found to be due to the improved quality of perovskite films made on top of PPE2 . More excitingly, the PPE2 ‐based perovskite solar cells (PVSCs) can further achieve a more impressive PCE of 21.3% through suitable surface passivation.

Abstract

Currently, there are only very few dopant‐free polymer hole‐transporting materials (HTMs) that can enable perovskite solar cells (PVSCs) to demonstrate a high power conversion efficiency (PCE) of greater than 20%. To address this need, a simple and efficient way is developed to synthesize novel crossconjugated polymers as high performance dopant‐free HTMs to endow PVSCs with a high PCE of 21.3%, which is among the highest values reported for single‐junction inverted PVSCs. More importantly, rational understanding of the reasons why two isomeric polymer HTMs (PPE1 and PPE2 ) with almost identical photophysical properties, hole‐transporting ability, and surface wettability deliver so distinctly different device performance under similar device fabrication conditions is manifested. PPE2 is found to improve the quality of perovskite films cast on top with larger grain sizes and more oriented crystallization. These results help unveil the new HTM design rules to influence the perovskite growth/crystallization for improving the performance of inverted PVSCs.

Organic solar cells with unusual inverted structure (sequentially processed heterojunction) are fabricated by sequentially spin coating the acceptor layer FOIC1 after the donor layer PTB7‐Th, which yields better‐controlled vertical phase separation and improved efficiency compared with traditional bulk heterojunction devices.

A new fused‐ring electron acceptor FOIC1 is designed and synthesized. FOIC1 exhibits intense absorption in the range of 600–1000 nm, the highest occupied molecular orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO) energy levels of −5.39/−3.99 eV, and electron mobility of 1.8 × 10⁻³ cm² V⁻¹ s⁻¹. Organic solar cells based on sequentially processed heterojunction (SHJ) with an unusual inverted structure are fabricated. Through sequentially spin‐coating polymer donor PTB7‐Th as the bottom layer and acceptor FOIC1 as the top layer, a better vertical phase distribution is formed in this SHJ compared with that in traditional bulk heterojunction (BHJ). In the upper‐half part, a more balanced donor/acceptor distribution is beneficial for exciton dissociation. At the bottom interface, more FOIC1 accumulation is beneficial for exciton generation and charge transport. Overall, the SHJ cells exhibit power conversion efficiency as high as 12.0%, higher than that of the BHJ counterpart (11.0%).

The incorporation of potassium can remarkably stabilize wide‐bandgap perovskites with a high Br content by the synergistic effect of the formation of 2D K₂PbI₄ at the grain boundaries and the interstitial occupancy in the perovskite lattices, which can effectively reduce the trap density and inhibit ion migration, thus suppressing the nonradiative recombination and photoinduced phase segregation.

Wide‐bandgap perovskites have great potential to enable high‐efficiency tandem photovoltaics by combining with the well‐established low‐bandgap absorbers. However, such wide‐bandgap perovskites are often necessarily constructed with a high Br content, and thus faced with issues of phase segregation–induced photoinstability and high defect density, severely hindering their photovoltaic performance. Herein, a remarkable boost of the stability and efficiency of wide‐bandgap perovskite solar cells (PSCs) is demonstrated by simply incorporating potassium ions. Experiments have shown the interstitial occupancy of potassium ions in the perovskite lattice and the formation of 2D K₂PbI₄ at the grain boundaries, both can reduce the trap density and inhibit ion migration, and thus suppress nonradiative recombination and photoinduced phase segregation. The average power conversion efficiency (PCE) of photovoltaic devices based on the perovskite with 40% Br is improved from 15.28% to 17.94%, among which the champion efficiency is 18.38% with an optimal 15% KI incorporation. Importantly, the champion open‐circuit voltage (V _oc) remains unchanged (≈1.25 V) even when the bandgap reduces from 1.80 to 1.75 eV due to KI doping, effectively reducing the V _oc deficit. In addition, the unencapsulated cells can sustain 94% of the initial PCE after 2000 h of storage in ambient atmosphere, affirming their outstanding stability.

One multifunctional molecule, 5‐amino‐2,4,6‐triiodoisophthalic acid (ATPA), is used as an interfacial layer between CsPbI₃ and TiO₂. The ATPA not only results in cascade energy‐level alignment, but also interacts strongly with the CsPbI₃ layer and effectively passivates the defects. Optimized devices based on the ATPA‐modified CsPbI₃ deliver a high efficiency of 18.12% with excellent stability.

A highly effective interface engineering approach uses a multifunctional molecule, 5‐amino‐2,4,6‐triiodoisophthalic acid (ATPA), to anchor on TiO₂ and CsPbI₃ simultaneously by reacting with dangling hydroxyl groups on TiO₂ surfaces and passivating the defects of CsPbI₃ films. In addition, the introduction of ATPA results in cascade energy‐level alignment between the perovskite and TiO₂ electron‐transporting layer (ETL) to improve the electron extraction property. Based on the ATPA‐modified TiO₂ substrates, optimized CsPbI₃ perovskite solar cells (PVSCs) deliver the highest power conversion efficiency (PCE) of over 18% with suppressed hysteresis. Moreover, the unencapsulated TiO₂/ATPA‐based devices exhibit much better long‐term stability and photostability than the only TiO₂‐based devices.

Morphology of an efficient ternary polymer blend is manipulated and the impacts on photovoltaic properties are explored. The morphology of ternary all‐polymer blending is determined by the interplay of the heterogeneous components from solution to solid state. Morphology with nanosized crystallite fibers in a mixing matrix assists in enhancing the solar cell performances.

Morphology control in multiblend all‐polymer solar cells is crucial for improving charge generation processes. Herein, it is demonstrated that the film morphology of the light‐harvesting layer of all‐polymer solar cells can be manipulated by incorporating a copolymer as the compatibilizer. Through in situ grazing‐incidence wide‐angle X‐ray scattering characterization, the insights of the crystallization kinetics of the polymer blends from solution to thin‐film state are provided. Of particular importance is that by kinetic and thermodynamic control of the film‐processing conditions, an optimal morphology with appropriate nanoscale fibrillar structure in a well‐mixed matrix is achieved. These findings indicate that the interplay between the crystalline regions and weakly/noncrystalline regions that are induced by the compatibilizer polymer plays a critical role in determining the morphology in multicomponent blend all‐polymer solar cells.

An environmentally stable acetamidinium (Aa⁺)‐incorporated MAPbI₃ film is successfully fabricated via hot casting in air. The large Aa⁺ immobilizes ions and improves crystal structure of MAPbI₃ through strong coordination bonds. The corresponding Aa–MAPbI₃ device shows 20.68% power conversion efficiency. Its 80% is maintained after 1300 h testing at 85 °C and 85 relative humidity (RH)%.

Ion migration in organometal halide perovskite solar cell (OHPSC) and crystal structure evolution of organometal halide perovskites (OHPVSKs) in air are considered as one of the critical factors for unstable performance and of the urgent issues for the reliability of OHPSCs. Herein, a novel cation of acetamidinium (Aa⁺) with stronger coordinated bond with I⁻ than methylammonium is induced into OHPVSK to stabilize its crystal structure. By incorporating Aa⁺ ions into OHPVSKs, the power conversion efficiency (PCE) of OHPSC without an encapsulation can maintain higher than 75% of its initial PCE after a 200 h humidity (60–80% relative humidity (RH) in air) or a 24 h thermal stress test (85 °C in dry N₂). The Aa–MAPbI₃ device exhibits an outstanding efficiency of 20.68%, and over 80% of initial PCE is maintained after a 1300 h damp heat as encapsulated. This novel cation can be easily incorporated into OHPVSK via a hot casting process in air with a high environmental tolerance as compared with that from the conventional coating process, which suffers from sophisticated crystallization steps and a strict processing atmosphere. It extends processing windows for OHPVSK fabrication and provides a promising path toward mass production and further commercialization.

This work introduces an ultra‐thin MgO layer between SnO₂ electron transport layer and CsPbIBr₂ perovskite layer to reduce the interface and nonradiative recombination. Meanwhile, the MgO provides better substrate for pure α‐phase perovskite growth. As a result, it achieves power conversion efficiency of 11.04% and maintains ≈90% after 1250 hours, with an open‐circuit voltage up to 1.36 V.

Although the power conversion efficiency (PCE) of thermally stable inorganic CsPbIBr₂ perovskite solar cells (PSCs) is over 10%, the severe interfacial and nonradiative recombination deteriorates the open‐circuit voltage (V _oc). Herein, an ultrathin wideband MgO is mediated between the SnO₂ electron transport layer (ETL) and the CsPbIBr₂ photoabsorber to passivate the undesirable recombination, thereby enhancing the V _oc. Meanwhile, the δ‐phase perovskite located at the interface between SnO₂ ETL and CsPbIBr₂ film is reduced after MgO modification, because the MgO layer provides a substrate for perovskite growth and reduces vacancy. Moreover, the tunneling effect and better band alignment effectively block holes and accelerate electrons to the electrode. Consequently, for optimal MgO‐modified devices, a shining improvement of V _oc from 1.25 to 1.36 V without short‐circuit current losses boosts the champion CsPbIBr₂ PSCs to obtain a PCE of 11.04%, which is the highest value among the pure‐CsPbIBr₂ PSCs. However, the MgO layer significantly reduces severe hysteresis and increases device stability.

The thermomechanical properties of donor–acceptor polymers with systematically tuned side‐chain lengths are investigated. A predictive linear mass‐per‐flexible bond model is introduced to capture the side‐chain length effect on their glass transition temperature. This provides guidance toward the design of future application‐driven conjugated polymers with desired thermomechanical performances.

Abstract

Semiconducting donor–acceptor (D–A) polymers have attracted considerable attention toward the application of organic electronic and optoelectronic devices. However, a rational design rule for making semiconducting polymers with desired thermal and mechanical properties is currently lacking, which greatly limits the development of new polymers for advanced applications. Here, polydiketopyrrolopyrrole (PDPP)‐based D–A polymers with varied alkyl side‐chain lengths and backbone moieties are systematically designed, followed by investigating their thermal and thin film mechanical responses. The experimental results show a reduction in both elastic modulus and glass transition temperature (T _g) with increasing side‐chain length, which is further verified through coarse‐grained molecular dynamics simulations. Informed from experimental results, a mass‐per‐flexible bond model is developed to capture such observation through a linear correlation between T _g and polymer chain flexibility. Using this model, a wide range of backbone T _g over 80 °C and elastic modulus over 400 MPa can be predicted for PDPP‐based polymers. This study highlights the important role of side‐chain structure in influencing the thermomechanical performance of conjugated polymers, and provides an effective strategy to design and predict T _g and elastic modulus of future new D–A polymers.

Simplified perovskite solar cells without employing electron/hole‐transport layers are attractive owing to their reduced fabrication process and low cost. The developments of simplified perovskite solar cells are discussed systematically. Device structure design, perovskite‐film optimization, interface modification, underlying mechanism, and other research hotspots are considered to present the bright future of simplified perovskite solar cells.

Abstract

Perovskite solar cells (PSCs) are considered one of the most promising next‐generation examples of high‐tech photovoltaic energy converters, as they possess an unprecedented power conversion efficiency with low cost. A typical high‐performance PSC generally contains a perovskite active layer sandwiched between an electron‐transport layer (ETL) and a hole‐transport layer (HTL). The ETL and HTL contribute to the charge extraction in the PSC. However, these additional two layers complicate the manufacturing process and raise the cost. To extend this technology for commercialization, it is highly desired that the structure of PSCs is further simplified without sacrificing their photovoltaic performances. Thus, ETL‐free or/and HTL‐free PSCs are developed and attract more and more interest. Herein, the commonly used methods in reducing the defect density and optimizing the energy levels in conventional PSCs in order to simplify their structures are summarized. Then, the development of diverse ETL‐free or/and HTL‐free PSCs is discussed, with the PSCs classified, including their working principles, implemented technologies, remaining challenges, and future perspectives. The aim is to redirect the way toward low‐cost and high‐performance PSCs with the simplest possible architecture.

Purely vibrationally excited lead–iodide perovskites are prepared using off‐resonance, infrared optical excitation far below the bandgap. The transient optical response manifested as bandgap oscillations below and above the static bandgap is attributed to the A _g optical phonon mode at 25 cm⁻¹. This mode, arising from antiphase octahedral rotations, is observed in both 3D perovskite CH₃NH₃PbI₃ and layered 2D perovskite [CH₃(CH₂)₃NH₃]₂PbI₄.

Abstract

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the A _g vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales.

A novel material called phenylhydrazinium iodide (PHAI) is effective for defects minimization, surface passivation, and efficient charge transportation in hybrid perovskite solar cells. It plays multiple roles in controlled crystallization, stabilizing under‐coordinated ions, and as a self‐supported moisture barrier in perovskite films.

Abstract

In recent years, hybrid perovskite solar cells (HPSCs) have received considerable research attention due to their impressive photovoltaic performance and low‐temperature solution processing capability. However, there remain challenges related to defect passivation and enhancing the charge carrier dynamics of the perovskites, to further increase the power conversion efficiency of HPSCs. In this work, the use of a novel material, phenylhydrazinium iodide (PHAI), as an additive in MAPbI₃ perovskite for defect minimization and enhancement of the charge carrier dynamics of inverted HPSCs is reported. Incorporation of the PHAI in perovskite precursor solution facilitates controlled crystallization, higher carrier lifetime, as well as less recombination. In addition, PHAI additive treated HPSCs exhibit lower density of filled trap states (10¹⁰ cm⁻²) in perovskite grain boundaries, higher charge carrier mobility (≈11 × 10⁻⁴ cm² V⁻¹ s), and enhanced power conversion efficiency (≈18%) that corresponds to a ≈20% improvement in comparison to the pristine devices.

A fibril network strategy is demonstrated to fabricate semitransparent organic solar cells (ST‐OSCs). An effective hole transport pathway can be maintained even when a small amount of PBT1‐C‐2Cl donor is incorporated in the blends due to the well distributed fibril nanostructure formed by PBT1‐C‐2Cl. A high efficiency of 9.1% with an average visible transmittance of over 40% is achieved for ST‐OSCs.

Abstract

The development of semitransparent organic solar cells (ST‐OSCs) represents a significant step toward the commercialization of OSCs. However, the trade‐off between power conversion efficiency (PCE) and average visible transmittance (AVT) restricts further improvements of ST‐OSCs. Herein, it is demonstrated that a fibril network strategy can enable ST‐OSCs with a high PCE and AVT simultaneously. A wide‐bandgap polymer PBT1‐C‐2Cl that can self‐assemble into a fibril nanostructure is used as the donor and a near‐infrared small molecule Y6 is adopted as the acceptor. It is found that a tiny amount of PBT1‐C‐2Cl in the blend can form a high speed pathway for hole transport due to the well distributed fibril nanostructure, which increases the transmittance in the visible region. Meanwhile, the acceptor Y6 guarantees sufficient light absorption. Using this strategy, the optimized ST‐OSCs yield a high PCE of 9.1% with an AVT of over 40% and significant light utilization efficiency of 3.65% at donor/acceptor ratio of 0.25:1. This work demonstrates a simple and effective approach to realizing high PCE and AVT of ST‐OSCs simultaneously.

FAPbI₃‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap; however, FAPbI₃‐based materials suffer from notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This study reveals that 1‐hexyl‐3‐methylimidazolium iodide ionic liquid incorporation stabilizes the α‐FAPbI₃ phase and exhibits a promising efficiency exceeding 20% with excellent long‐term operational and shelf‐stability.

Abstract

Formamidinium lead triiodide (FAPbI₃)‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap, allowing absorption of photons over a broad solar spectrum. However, FAPbI₃‐based materials suffer from a notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This transition dramatically reduces light absorbtion, thus, degrading the photovoltaic performance and stability of ensuring solar cells. In this study, 1‐hexyl‐3‐methylimidazolium iodide (HMII) ionic liquid (IL) is employed as an additive for the first time in FAPbI₃ perovskite to overcome the above‐mentioned issues. HMII incorporation facilitates the grain coarsening of FAPbI₃ crystal owing to its high‐polarity and high‐boiling point, which yields liquid domains between neighboring grains to reduce the activation energy of the grain‐boundary migration. As a result, the FAPbI₃ active layer exhibits micron‐sized grains with substantially suppressed parasitic traps with an Urbach energy reduced by 2 meV. Hence, the resulting perovskite solar cell achieves an efficiency of 20.6% with notable increase in open circuit voltage (V _OC) of 80 mV compared with HMII‐free cells (17.1%). More importantly, the HMII‐doped FAPbI₃‐based cells show a striking enhancement in shelf‐stability under high humidity and thermal stress, retaining >80% of their initial efficiencies at 60 ± 10% relative humidity and ≈95% at 65 °C.

An effective strategy is demostrated to create a double barrier that not only blocks the invasion of the moisture but also takes advantage of the permeated moisture to increase the moisture durability of perovskite films, which results in an n–i–p perovskite solar cell with moisture stability over 115 days in a relative humidity of 70% and a champion efficiency up to 21.34%.

Abstract

The rapid growth in the device efficiency of perovskite solar cells (PSCs) has raised great demands for tackling their long‐term stability upon external environmental stimuli that restricts the commercialization of PSCs, in which the instability upon exposure to moisture has been one of the major obstacles. Herein, an effective way of building up double barriers for moisture degradation of the perovskite films is demonstrated by modifying them with rationally selected hydrolyzable hydrophobic molecules (1H,1H,2H,2H‐perfluorooctyl trichlorosilane, PFTS). The layer of oligomer derived from the hydrolyzed PFTS at the surface that increases the hydrophobicity of perovskite film could serve as an efficient wall preventing the moisture invasion. The long‐term exposure of the film upon moisture allows for the formation of a secondary wall that employs the hydrolyzation of PFTS at grain boundaries, favoring defects passivation to further improve the humidity stability. Such gradual hydrolyzation is encouragingly helpful for the enhancement of the open‐circuit voltage of the PSCs from the original 1.136 up to 1.205 V. The PSCs constructed with the double barriers demonstrate excellent stability upon moisture and improved thermal and light stabilities, as well as a champion power conversion efficiency up to 21.34%.

A ligand‐assisted matrix to regulate surface and packing states of perovskite quantum dots (QDs) is demonstrated, which involves a ligand exchange and a mild thermal annealing process that are triggered by guanidinium thiocyanate. Consequently, the CsPbI₃ QD solar cells (QDSCs) deliver a champion power conversion efficiency of 15.21%, which is the highest report among all CsPbI₃ QDSCs.

Abstract

Metal halide perovskite quantum dots (Pe‐QDs) are of great interest in new‐generation photovoltaics (PVs). However, it remains challenging in the construction of conductive and intact Pe‐QD films to maximize their functionality. Herein, a ligand‐assisted surface matrix strategy to engineer the surface and packing states of Pe‐QD solids is demonstrated by a mild thermal annealing treatment after ligand exchange processing (referred to as “LE‐TA”) triggered by guanidinium thiocyanate. The “LE‐TA” method induces the formation of surface matrix on CsPbI₃ QDs, which is dominated by the cationic guanidinium (GA⁺) rather than the SCN⁻, maintaining the intact cubic structure and facilitating interparticle electrical interaction of QD solids. Consequently, the GA‐matrix‐confined CsPbI₃ QDs exhibit remarkably enhanced charge mobility and carrier diffusion length compared to control ones, leading to a champion power conversion efficiency of 15.21% when assembled in PVs, which is one of the highest among all Pe‐QD solar cells. Additionally, the “LE‐TA” method shows similar effects when applied to other Pe‐QD PV systems like CsPbBr₃ and FAPbI₃ (FA = formamidinium), indicating its versatility in regulating the surfaces of various Pe‐QDs. This work may afford new guidelines to construct electrically conductive and structurally intact Pe‐QD solids for efficient optoelectronic devices.

An autonomously longitudinal scaffold constructed by the interspersion of in situ polymerized methyl methacrylate in PbI₂ is introduced to effectively eliminate the dependence of sequential deposition on mesoporous TiO₂, and is applied in planar perovskite solar cells, with excellent performance. Moreover, this scaffold's cross‐linking grains are capable of releasing mechanical stress, impeding ion migration, and water/oxygen permeation.

Abstract

Sequential deposition is certified as an effective technology to obtain high‐performance perovskite solar cells (PVSCs), which can be derivatized into large‐scale industrial production. However, dense lead iodide (PbI₂) causes incomplete reaction and unsatisfactory solution utilization of perovskite in planar PVSCs without mesoporous titanium dioxide as a support. Here, a novel autonomously longitudinal scaffold constructed by the interspersion of in situ self‐polymerized methyl methacrylate (sMMA) in PbI₂ is introduced to fabricate efficient PVSCs with excellent flexural endurance and environmental adaptability. By this strategy perovskite solution can be confined within an organic scaffold with vertical crystal growth promoted, effectively inhibiting exciton accumulation and recombination at grain boundaries. Additionally, sMMA cross‐linked perovskite network can release mechanical stress and occupy the main channels for ion migration and water/oxygen permeation to significantly improve operational stability, which opens up a new strategy for the commercial development of large‐area PVSCs in flexible electronics.

The efficiency and operational stability of MA‐free FA_0.95Cs_0.05PbI₃ perovskite solar cells can be simultaneously enhanced by the incorporation of the β‐guanidinopropionic acid (β‐GUA) molecule. The introduction of β‐GUA forms a 2D/3D hybrid perovskite phase, which effectively passivates the surface defects, resulting in an impressive power conversion efficiency of 22.2% with a substantial increase in V _oc (from 1.01 to 1.14 V).

Abstract

Almost all highly efficient perovskite solar cells (PVSCs) with power conversion efficiencies (PCEs) of greater than 22% currently contain the thermally unstable methylammonium (MA) molecule. MA‐free perovskites are an intrinsically more stable optoelectronic material for use in solar cells but compromise the performance of PVSCs with relatively large energy loss. Here, the open‐circuit voltage (V _oc) deficit is circumvented by the incorporation of β‐guanidinopropionic acid (β‐GUA) molecules into an MA‐free bulk perovskite, which facilitates the formation of quasi‐2D structure with face‐on orientation. The 2D/3D hybrid perovskites embed at the grain boundaries of the 3D bulk perovskites and are distributed through half the thickness of the film, which effectively passivates defects and minimizes energy loss of the PVSCs through reduced charge recombination rates and enhanced charge extraction efficiencies. A PCE of 22.2% (certified efficiency of 21.5%) is achieved and the operational stability of the MA‐free PVSCs is improved.