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Publication date: 19 May 2021

Source: Joule, Volume 5, Issue 5

Author(s): Fujin Bai, Jianquan Zhang, Anping Zeng, Heng Zhao, Ke Duan, Han Yu, Kui Cheng, Gaoda Chai, Yuzhong Chen, Jiaen Liang, Wei Ma, He Yan

A templated growth approach, in which the crystallization of a 3D perovskite is guided by a dynamically dominant 2D structure, was established for the fabrication of a low-dimensional perovskite thin film with an out-of-plane orientation and a large grain size. The template growth is enabled by the reduced crystallization barrier of the 2D PEA₂SnI_4−x SCN _x intermediate.

Abstract

The manipulation of the dimensionality and nanostructures based on the precise control of the crystal growth kinetics boosts the flourishing development of perovskite optoelectronic materials and devices. Herein, a low-dimensional inorganic tin halide perovskite, CsSnBrI_2−x (SCN) _x , with a mixed 2D and 3D structure is fabricated. A kinetic study indicates that Sn(SCN)₂ and phenylethylamine hydroiodate can form a 2D perovskite structure that acts as a template for the growth of the 3D perovskite CsSnBrI_2−x (SCN) _x . The film shows an out-of-plane orientation and a large grain size, giving rise to reduced defect density, superior thermostability, and oxidation resistance. A solar cell based on this low-dimensional film reaches a power conversion efficiency of 5.01 %, which is the highest value for CsSnBr _x I_3−x perovskite solar cells. Furthermore, the device shows enhanced stability in ambient air.

A ligand molecule containing carbonyls (carboxyl and amide) and a long hydrophobic alkyl chain is incorporated into a perovskite precursor to achieving improved crystallinity, reduced trap state density, and inhibited ion migration. This strategy enables an impressive power conversion efficiency exceeding 23% with inhibited hysteresis.

Abstract

The nonradiative recombination losses resulting from the trap states at the surface and grain boundaries directly hinder the further enhancement of power conversion efficiency (PCE) and stability of perovskite solar cells. Consequently, it is highly desirable to suppress nonradiative recombination through modulating perovskite crystallization and passivating the defects of perovskite films. Here, a simple and effective multifunctional additive engineering strategy is reported where 11 Maleimidoundecanoic acid (11MA) units with carbonyls (carboxyl and amide) and long hydrophobic alkyl chain are incorporated into a perovskite precursor solution. It is revealed that improved crystallinity, reduced trap state density, and inhibited ion migration are achieved, which is ascribed to the strong coordination interaction between the carbonyl groups at both sides of 11MA molecules and Pb²⁺. As a result, improved efficiency and stability are achieved simultaneously after introducing 11MA additive. The device with 11MA additive delivers a champion PCE of 23.34% with negligible hysteresis, which is significantly higher than the 18.24% of the control device. The modified device maintains around 91% of its initial PCE after aging under ambient conditions for 3000 h. This work provides a guide for developing multifunctional additive molecules for the purpose of simultaneous improvement of efficiency and stability.

A mesoporous charge-transporting layer is embedded into quasi-interdigitated back-contact perovskite devices. The increased interfacial contact area significantly enhances the charge extraction behavior leading to a record high current density of 21.3 mA cm⁻² on a back-contact perovskite device.

Abstract

As the performance of organic–inorganic halide perovskite solar cells approaches their practical limits, the use of back-contact architectures, which eliminate parasitic light absorption, provides an effective route toward higher device efficiencies. However, a poor understanding of the underlying device physics has limited further performance improvements. Here a mesoporous charge-transporting layer is introduced into quasi-interdigitated back-contact perovskite devices and the charge extraction behavior with an increased interfacial contact area is studied. The results show that the incorporation of a thin mesoporous titanium dioxide layer significantly shortens the charge-transfer lifetime and results in more efficient and balanced charge extraction dynamics. A high short-circuit current density of 21.3 mA cm^–2 is achieved using a polycrystalline perovskite layer on a mesoscopic quasi-interdigitated back-contact electrode, a record for this type of device architecture.

The Li/Na-ion exchange strategy can be regarded as an effective way to address significant obstacles within Li-excess layered cathodes by tuning oxygen stacking arrangements and the coordination environment of alkali metals, which motivates various advanced experimental techniques and theory calculations for designing next-generation Li-excess oxides with high energy density.

Abstract

Li-rich and Li-excess oxides have been regarded as a promising category of cathode materials for next-generation Li-ion batteries due to their high energy density on basis of anionic/cationic redox chemistry. However, the application of Li-excess oxides suffers from significant problems, such as irreversible lattice oxygen loss and structural distortion. A Li/Na-ion exchange strategy can be regarded as an effective way to address these obstacles by tuning oxygen stacking arrangements and the coordination environment of alkali metal within emerging structures. Herein, the state-of-the-art progress made within conventional and Li-excess layered cathodes generated by ion exchange strategy at first is reviewed. Moreover, the mechanisms of oxygen redox, the migrations of Li/transition metals, and structural evolutions within Li-excess oxides have been further clarified by advanced characterizations, which play an important role in understanding lattice oxygen release, irreversible transition metal migration, phase transition, and the root of capacity/voltage decay. In addition, the theory calculations based on density of states, oxygen release energy, and migration energy barriers of Li/transition metals are systematically summarized, providing an essential guidance to harvest stable oxygen-related redox reactions within Li-excess materials. Altogether, this review offers fundamental understanding and future perspectives toward the rational design of high-energy-density Li-excess oxides with reversible anionic/cationic redox chemistry.

Rapid degradation of ion migration, measurement source‐induced damage, phase transition, and separation of perovskite materials hinder accurate evaluation by conventional characterization tools. Recent advanced characterization tools, such as cryogenic temperature assisted measurement, in situ observation, and multidimensional imaging/mapping are presented here that enable the correct diagnose perovskite properties.

Abstract

In the last 10 years, organic–inorganic hybrid perovskite solar cells have achieved unprecedented advances, to the point where they now exhibit extremely high efficiency. However, long‐term stability and areal scalability limitations impede the commercial application of perovskite materials, and appropriate diagnosistic tools have become necessary to evaluate perovskite materials. Characterization of perovskite materials is regularly misinterpretated, due to unique intrinsic and extrinsic factors: degradation from the measurement source, ion migration, phase transition, and separation. Herein, studies on perovskites are reviewed that have used advanced characterization tools to overcome characterization challenges. Cryogenic temperature assisted measurements mitigate degradation or phase transitions induced by the measurement source. In situ measurements can track the variation of perovskite materials depending on external stimuli. Spatial material properties are able to be evaluated by the use of multidimensional mapping techniques. An overview of these advanced characterization tools that can overcome the challenges associated with established tools provides the opportunity for further understanding perovskite materials and solving the remaining challenges on the road to commercialization.

The power conversion efficiencies (PCEs) of single‐junction organic solar cells have now reached over 18%. Recent progress that has been made in understanding the morphology and the device photophysics of high performing polymer:non‐fullerene acceptor blends and some of the major challenges that must be overcome to attain PCEs of over 20% are highlighted.

Abstract

The power conversion efficiencies (PCEs) of single‐junction organic solar cells (OSC) have now reached over 18%. This rapid recent progress can be attributed to the development of new nonfullerene electron acceptors (NFAs) that are paired with suitable high performing polymer electron donors. Substantial improvements in the PCEs and long‐term stability enabled by NFA OSCs have allowed the development and integration of these systems into many niche and novel applications. Here, the recent progress that has been made in understanding the device photophysics of high performing polymer:NFA blends is highlighted. As the bulk heterojunction morphology is intrinsically linked to the device photophysics, this review focuses on studies that have provided noteworthy morphological insights using advanced techniques such as solid‐state NMR and resonant soft X‐ray scattering. Through this, some of the major challenges that must be overcome to attain PCEs of over 20% in NFA OSCs are addressed.

The effects of post treatments of thermal annealing (TA) and solvent annealing (SVA) on morphology evolution and efficiency are systematically investigated. The results show that CS₂ annealing induces better molecular interconnection and lower non-radiative recombination than that of TA treatment, which enables the best voltage and fill factor improvements and gives a record efficiency of 15.39%.

Abstract

Post-treatment is of great importance to form nanoscale phase-separated morphology for all-small-molecule organic solar cells (ASM-OSCs), while the reasons for the difference between thermal annealing (TA) and solvent annealing (SVA) remain unclear. In this work, the influences of TA and SVA (with three common solvents of THF, CS₂, and CF) are systematically investigated based on BT-2F:N3 through characterization of photovoltaic performance, molecular stacking, charge transfer, etc. The results indicate that: i) solvents with good solubility induce stronger molecular interaction than that of TA treatment, and thus endowing molecules with better mobility to migrate for crystallization and phase separation, which leads to better J-aggregation and molecular interconnection. ii) Donor-selectively dissolved CS₂ is better for optimizing the donor domain for its suitable domain size, improved molecular interaction and interconnection, and reduced trap states. iii) CS₂ imposes a small impact on N3 acceptors and thus alleviates the increment of non-radiative recombination. As a result, CS₂ SVA with unique multifunctions enables a PCE of 15.39% with simultaneously improved voltage (0.845 V) and fill factor (75.02%), which is much higher than 14.66% of TA treatment. Moreover, 15.39% efficiency is also the highest value in binary ASM-OSCs.

In this work, tandem solar cells using wide bandgap hydrogenated amorphous silicon and narrow bandgap organic bulk heterojunction photovoltaics are explored. By chemically texturing transparent conductive oxide layers, the current matching between two subcells can be optimized to give a power-conversion efficiency of 15% while greatly improving operational stability compared to single junction organic photovoltaics.

Abstract

Monolithically stacked tandem solar cells present opportunities to absorb more of the sun's radiation while reducing the degree of energetic loss through thermalization. In these applications, the bandgap of the tandem's constituent subcells must be carefully adjusted so as to avoid competition for photons. Organic photovoltaics based on nonfullerene acceptors (NFAs) have recently exploded in popularity owing to the ease with which their electrical and optical properties can be tuned through chemistry. Here, highly complementary and efficient 2-terminal tandem solar cells are reported based on a wide bandgap amorphous silicon absorber, and a narrow bandgap NFA bulk-heterojunction with power conversion efficiencies (PCEs) exceeding 15%. Interface engineering of this tandem device allows for high PCEs across a wide range of light intensities both above and below “1 sun.” Furthermore, the addition of an inorganic silicon subcell enhances the operational stability of the tandem by reducing the light-stress experienced by the bulk heterojunction, resolving a long-standing stumbling block in organic photovoltaic research.

This review aims to discuss challenges and recent advances in all-inorganic perovskites for advanced photovoltaics. After discussing the structural and electronic properties of the materials, the focus of this review moves towards all-inorganic perovskite solar cells, reporting the most effective approaches to improve device performance. Finally, efforts and challenges toward the fabrication of all-inorganic perovskite solar modules are discussed.

Abstract

In the last ten years, organic–inorganic hybrid perovskites have been skyrocketing the field of innovative photovoltaics (PVs) and now represent one of the most promising solution for next-generation PVs. Within the family of halide perovskites, increasing attention has been focused on the so-called all-inorganic group, where the organic cation is replaced by cesium, as in the case of CsPbI₃. This subclass of halide perovskites features desirable optoelectronic properties such as easily tunable bandgap, strong defect tolerance, and improved thermal stability compared to the hybrid systems. When integrated in PV cells, they exhibit high power conversion efficiency (PCE) with record values of 19.03%. However, all-inorganic perovskite solar cells (PCSs) face several challenges such as i) instability of the CsPbI₃ photoactive phase in ambient conditions, ii) inhomogeneous film morphology, and iii) high surface defect density. This work focuses on the mentioned challenges with a special attention on discussing the Cs–Pb–X system (X = I, Br). Then, the most recent and effective approaches for increasing both the PCE and the stability of devices are reviewed, which include material doping, interface engineering, and device optimization. Finally, the first efforts toward the upscaling of Cs-based PSCs, and predicted methods for enabling large-scale production, are discussed.

The dominant deep defect states in freshly prepared CsPbI_{3− x} Br _x films are mainly antisite defect pairs (Pb_I and I_Pb) and interstitial defects (Pb_i). All these defects are reduced because of self-regulation process after resting the films overnight in the dark. Based on this strategy, the reduced-defect high quality CsPbI_{3− x} Br _x films can be obtained and thus higher photovoltaic performance.

Abstract

Deep defects often act as Shockley–Read–Hall recombination centers in semiconductor materials, degrading the photoelectric performance and long-term stability of assembled photovoltaic devices. In this report, deep level transient spectroscopy is probed to determine defect concentrations and defect energy levels in all-inorganic CsPbI_{3− x} Br _x perovskite solar cells. Combining that data with the density functional theory calculation, the dominant deep defect states are assigned to antisite defect pairs (Pb_I and I_Pb) and interstitial defects (Pb_i) in freshly prepared CsPbI_{3− x} Br _x films. Astonishingly, all these defects are reduced by approximately one or two orders of magnitude after resting the films overnight, in excellent agreement with the defect-reduced trends from the fluorescence spectra, transient photovoltage, and space-charge-limited current measurements. The reduced defect concentrations are proposed to be connected with their self-regulation during the storage. To assess the thermodynamics possibilities, two reaction procedures are designed to calculate their formation enthalpies and negative Gibbs energy change revealed their spontaneous processes. Then, strain relief is the direct driving force for ion migration, thus defect-regulation by tracing the X-ray diffraction patterns. Furthermore, the power conversion efficiency is improved and the J–V hysteresis is suppressed due to reduced ion migration via relaxed strain.

In this study, efficient inverted organic photovoltaics using hexagonal boron nitride as an electron blocking layer is fabricated and the device stability, as compared to the reference devices, is improved.

Abstract

In this study, monolayer hexagonal boron nitride (h-BN) grown via chemical vapor deposition (CVD) as an effective electron blocking layer (EBL) for the organic photovoltaics (OPVs) is proposed. Unexpectedly, it is found that h-BN can replace the commonly used hole transport layers (HTLs), i.e., molybdenum trioxide (MoO₃) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) in an inverted device architecture. Here, a wet-transfer technique is employed and a single layer of h-BN on top of the PV2000:PC₆₀BM blend is successfully placed. Analysis of the bandgap diagram shows that the monolayer h-BN makes smaller barrier for holes but significantly larger barrier for electrons. This makes the h-BN effective in blocking electrons while creating a possible path for the holes through tunneling to the electrode, due to the low energy barrier at the PV2000/h-BN interface. Using h-BN as an EBL, efficient inverted OPVs are achieved with an average solar-to-power conversion efficiency of 6.13%, which is comparable with that of reference devices based on MoO₃ (7.3%) and PEDOT:PSS (7.6%) as HTLs. Interestingly, the devices with h-BN shows great light-soak stability. The study reveals that the monolayer h-BN grown by CVD could be an effective alternative EBL for the fabrication of efficient, lightweight, and stable OPVs.

A novel strategy is developed for preparing high-efficient perovskite solar cells (PSCs) with ferroelectricity by incorporating 1D ferroelectric perovskite with 3D organic–inorganic hybrid perovskite (OIHP). The 1D/3D mixed OIHP films exhibit evident ferroelectricity, and the 1D perovskite is randomly distributed. The poling of the 1D/3D mixed PSCs increase V _oc, and the ferroelectric-polarization is retained for a long time.

Abstract

With the capability to manipulate the built-in field in solar cells, ferroelectricity is found to be a promising attribute for harvesting solar energy in solar cell devices by influencing associated device parameters. Researchers have devoted themselves to the exploration of ferroelectric materials that simultaneously possess strong light absorption and good electric transport properties for a long time. Here, it is presented a novel and facile approach of combining state-of-art light absorption and electric transport properties with ferroelectricity by the incorporation of room temperature 1D ferroelectric perovskite with 3D organic–inorganic hybrid perovskite (OIHP). The 1D/3D mixed OIHP films are found to exhibit evident ferroelectric properties. It is notable that the poling of the 1D/3D mixed ferroelectric OIHP solar cell can increase the average V _oc can be increased from 1.13 to 1.16 V, the average PCE from 20.7% to 21.5%. A maximum power conversion efficiency of 22.7%, along with an enhanced fill factor of over 80% and open-circuit voltage of 1.19 V, can be achieved in the champion device. The enhancement is by virtue of reduced surface recombination by ferroelectricity-induced modification of the built-in field. The maximum power point tracking measurement substantiates the retention of ferroelectric-polarization during the continued operation.

Light soaking (LS) is found to activate halide ion migration and significantly passivate the defects. By adding excessive PbI₂ in the precursor, the LS effect can be controlled and suppressed. An efficiency of 18.14% is achieved in all-inorganic CsPb(I_0.8Br_0.2)₃ perovskite solar cells with reduced LS time.

Abstract

Light soaking (LS) has been reported to positively influence the device performance of perovskite solar cells (PSCs), which, however, could be potentially harmful to the loaded devices due to the unstable output. There are very few reports on controls over the LS effect, especially in all-inorganic PSCs. In this study, a remarkable LS induced performance enhancement of CsPb(I_{1− x} Br _x )₃ based PSCs is presented. In situ grazing-incidence wide-angle X-ray scattering measurements quantize the temperature increase under illumination and reveal a radiative heating-induced lattice expansion. The device curing time is shortened with the increased Br/I ratio, evidently correlated with their distinct mobility and activation energy. It is suggested that LS could promote the migration of halide ions, giving rise to notable defect passivation and thus device improvements. Based on these understandings, an effective means is proposed to suppress the LS effect, which is to incorporate slightly over-stochiometric PbI₂ in precursor, and a champion PCE of 18.14% in all-inorganic PSCs with significantly reduced device curing time is obtained.

Application of intercalated π-conjugated aromatic spacers is proposed as a novel strategy toward engineering quasi-2D Dion–Jacobson perovskites, which can significantly enhance out-of-plane charge transport, with improved stability and reduced bandgap for achieving a photovoltaic conversion efficiency around 17%.

Abstract

Quasi-2D CsPbI₃ perovskites have emerged as excellent candidates for advanced photovoltaic technologies due to their fundamentally enhanced stability than conventional 3D counterparts. However, the applications of quasi-2D perovskites are plagued with their poor out-of-plane carrier mobility induced by the intercalated insulating organic layers. In this work, a new strategy is explored to significantly enhance the out-of-plane charge transport in quasi-2D Dion–Jacobson (DJ) CsPbI₃ perovskites via leveraging the intercalation of aromatic diamine cations (p-phenylenediamine, PPDA) with unique π-conjugated bond based on the first-principles calculations. The strong interactions between PPDA²⁺ cations and inorganic Pb-I framework (i.e., I–I interaction, p-π coupling, and H-bonds) provide three carrier pathways to facilitate the out-of-plane charge transport. Furthermore, the restricted in-plane and out-of-plane structural distortion induced by the π-conjugated bond could improve the electronic coupling and charge mobility along the out-of-plane direction with reduced bandgaps. As a proof of concept, the calculated average photovoltaic conversion efficiency of such engineered DJ CsPbI₃ perovskite solar cells is ≈17%, which is very close to the certificated champion efficiency of 3D α-CsPbI₃, underscoring their potential for solar cell applications.

Two D–A copolymers, PBQ5 and PBQ6, are designed and synthesized based on difluoroquinoxaline (DFQ) units with different side chains. The organic solar cell (OSC) based on PBQ6 as donor and Y6 as acceptor achieves a high power conversion efficiency of 17.62%, which is one of the highest efficiencies for binary OSCs with a polymer donor and Y6 acceptor.

Abstract

Side-chain engineering has been an effective strategy in tuning electronic energy levels, intermolecular interaction, and aggregation morphology of organic photovoltaic materials, which is very important for improving the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, two D–A copolymers, PBQ5 and PBQ6, are designed and synthesized based on bithienyl-benzodithiophene (BDTT) as the donor (D) unit, difluoroquinoxaline (DFQ) with different side chains as the acceptor (A) unit, and thiophene as the π-bridges. PBQ6 with two alkyl-substituted fluorothiophene side chains on the DFQ units possesses redshifted absorption, stronger intermolecular interaction, and higher hole mobility than PBQ5 with two alkyl side chains on the DFQ units. The blend film of the PBQ6 donor with the Y6 acceptor shows higher and balanced hole/electron mobilities, less charge carrier recombination, and more favorable aggregation morphology. Therefore, the OSC based on PBQ6:Y6 achieves a PCE as high as 17.62% with a high fill factor of 77.91%, which is significantly higher than the PCE (15.55%) of the PBQ5:Y6-based OSC. The PCE of 17.62% is by far one of the highest efficiencies for the binary OSCs with polymer donor and Y6 acceptor.

Publication date: 19 May 2021

Source: Joule, Volume 5, Issue 5

Author(s): Deborah L. McGott, Christopher P. Muzzillo, Craig L. Perkins, Joseph J. Berry, Kai Zhu, Joel N. Duenow, Eric Colegrove, Colin A. Wolden, Matthew O. Reese

A new wide-bandgap conjugated polymer, PBQx-TCl, exhibits 18% efficiency for outdoor application and 28.5% efficiency for indoor application. Simultaneously, the PBQx-TCl-based device also shows light-emitting function with broad emission ranges from 630 to 1000 nm and moderate external quantum efficiency approaching 0.2%.

Abstract

Exploring the intriguing bifunctional nature of organic semiconductors and investigating the feasibility of fabricating bifunctional devices are of great significance in realizing various applications with one device. Here, the design of a new wide-bandgap polymer named PBQx-TCl (optical bandgap of 2.05 eV) is reported, and its applications in photovoltaic and light-emitting devices are studied. By fabricating devices with nonfullerene acceptors BTA3 and BTP-eC9, it is shown that the devices exhibit a high power conversion efficiency (PCE) of 18.0% under air mass 1.5G illumination conditions and an outstanding PCE of 28.5% for a 1 cm² device and 26.0% for a 10 cm² device under illumination from a 1000 lux light-emitting diode. In addition, the PBQx-TCl:BTA3-based device also demonstrates a moderate organic light-emitting diode performance with an electroluminescence external quantum efficiency approaching 0.2% and a broad emission range of 630–1000 nm. These results suggest that the polymer PBQx-TCl-based devices exhibit outstanding photovoltaic performance and potential light-emitting functions.