Chen Weijie

Transient photovoltage (TPV) mostly probes the dynamics of charge carriers close to the transport‐layer interfaces. These dynamics are affected by other properties of the device such as the charge‐carrier mobility. A different optoelectronic technique that can reliably measure the lifetime of charge carriers is developed. By applying both techniques to a series of devices, the TPV lifetime is found to be misleading.

Transient photovoltage (TPV) measurements are frequently used to study recombination processes in thin‐film solar cells by probing the decay of a small optically induced voltage perturbation to infer the charge carrier dynamics of devices at open circuit. However, the validity of this method to probe organic semiconductors has recently come into doubt due to large discrepancies in the reported carrier lifetime values for the same systems and the reporting of unrealistic reaction order values. Herein, the validity of TPV to extract reliable charge carrier lifetimes in thin‐film solar cells is explored through the use of time‐dependent drift‐diffusion simulations and measurements. It is found that in low‐mobility materials, TPV serves primarily as a probe of charge carrier redistribution in the bulk rather than bulk recombination dynamics and that the extracted time constant is highly mobility dependent. To address this shortcoming, transient photocharge, a new technique to measure the charge carrier density during photovoltage decay, is introduced and applied to study the recombination dynamics in a series of (fullerene and nonfullerene) organic solar cell systems. It is shown that using this technique the charge carrier recombination lifetime in the active layer is more accurately determined.

Addition of the n‐type dopant benzyl viologen (BV) into several best‐in‐class organic bulk‐heterojunctions (BHJ) is shown to consistently improve the power conversion efficiency (PCE) of the resulting solar cells. The presence of BV inside the BHJs increases the absorption coefficient, balances charge transport, and enhances the charge‐carrier density. These synergistic effects result in organic photovoltaics with a maximum PCE of 17.1%.

Abstract

Molecular doping is often used in organic semiconductors to tune their (opto)electronic properties. Despite its versatility, however, its application in organic photovoltaics (OPVs) remains limited and restricted to p‐type dopants. In an effort to control the charge transport within the bulk‐heterojunction (BHJ) of OPVs, the n‐type dopant benzyl viologen (BV) is incorporated in a BHJ composed of the donor polymer PM6 and the small‐molecule acceptor IT‐4F. The power conversion efficiency (PCE) of the cells is found to increase from 13.2% to 14.4% upon addition of 0.004 wt% BV. Analysis of the photoactive materials and devices reveals that BV acts simultaneously as n‐type dopant and microstructure modifier for the BHJ. Under optimal BV concentrations, these synergistic effects result in balanced hole and electron mobilities, higher absorption coefficients and increased charge‐carrier density within the BHJ, while significantly extending the cells' shelf‐lifetime. The n‐type doping strategy is applied to five additional BHJ systems, for which similarly remarkable performance improvements are obtained. OPVs of particular interest are based on the ternary PM6:Y6:PC₇₁BM:BV(0.004 wt%) blend for which a maximum PCE of 17.1%, is obtained. The effectiveness of the n‐doping strategy highlights electron transport in NFA‐based OPVs as being a key issue.

Strong Faraday rotation is demonstrated for methylammonium lead bromide (MAPbBr₃) single crystals. Coupled with low defect density, they are poised to represent a solution‐processed alternative for Faraday rotator applications, such as optical isolators and optical switches. Additionally, their Faraday rotation fits well with classical theory, allowing prediction of other perovskites for this application.

Abstract

Lead halide perovskites (LHPs) have become a promising alternative for a wide range of optoelectronic devices, thanks to their solution‐processability and impressive optical and electrical properties. More recently, LHPs have been investigated in magneto‐optic studies and have exhibited spin‐polarized emission, photoinduced magnetization, and long spin lifetimes. Here, the viability of methylammonium lead bromide (MAPbBr₃) single crystals as solution‐processed Faraday rotators is demonstrated. Compared to terbium gallium garnet, the industry standard in the visible, it is found that MAPbBr₃ exhibits Verdet constants (i.e., strength of Faraday effect) of similar or greater magnitude (up to 2.5x higher), with lower temperature dependence. Due to its low trap absorption, it is calculated that an optical isolator made from MAPbBr₃, with appropriate antireflection coatings, should reach ≈95% transmission and achieve 40 dB isolation for incoming powers of over 2 W. It is also shown that the Verdet constant of MAPbBr₃ can be calculated accurately from its dispersion in refractive index, allowing the possibility to predict similar effects in other perovskite materials.

Nontoxic tin‐based perovskite solar cells (PSCs) have attracted attention, but are easily oxidized, which causes their performance and stability to be far behind lead‐based PSCs. Here, strategies to improve the stability of tin‐based PSCs (additive engineering, deoxidizer, partial substitution, and reduced dimensions) are reviewed. Outlooks are also proposed to avoid the shortcoming for fabricating highly efficient and stable PSCs.

Abstract

Although lead‐based perovskite solar cells (PSCs) are highly efficient, the toxicity of lead (Pb) limits its large‐scale commercialization. As such, there is an urgent need to find alternatives. Many studies have examined tin‐based PSCs. However, pure tin‐based perovskites are easily oxidized in the air or just in glovebox with an ultrasmall amount of oxygen. Such a characteristic makes their performance and stability less ideal compared with those of lead‐based perovskites. Herein, how to address the instability of tin‐based perovskites is introduced in detail. First, the crystalline structure, optical properties, and sources of instability of tin‐based perovskites are summarized. Next, the preparation methods of tin‐based perovskite are discussed. Then, various measures for solving the instability problem are explained using four strategies: additive engineering, deoxidizer, partial substitution, and reduced dimensions. Finally, the challenges and prospects are laid out to help researchers develop highly efficient and stable tin‐based perovskites in the future.

A stable intermediate‐state film is obtained by using teramethylene sulfoxide (TMSO), originating from the formation of stronger coordination bond between TMSO and all perovskite precursors, which extends the annealing window and promotes the formation of a high‐quality film with larger grains and textured surface. 21.14% efficiency is achieved attributable to the improvement of the long‐wavelength response and fill factor.

Abstract

As the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is increased to as high over 25%, it becomes pre‐eminent to study a scalable process with wide processing window to fabricate large‐area uniform perovskite films with good light‐trapping performance. A stable and uniform intermediate‐state complex film is obtained by using tetramethylene sulfoxide (TMSO), which extends the annealing window to as long as 20 min, promotes the formation of a high‐quality perovskite film with larger grains (over 400 nm) and spontaneously forms the surface texture to result in an improved fill factor and open‐circuit voltage (V _oc). Moreover, the superior surface texture significantly increases the long‐wavelength response, leading to an improved short‐circuit current density (J _sc). As a result, the maximum PCE of 21.14% is achieved based on a simple planar cell structure without any interface passivation. Moreover, a large area module with active area of 6.75 cm² is assembled using the optimized TMSO process, showing efficiency as high as 16.57%. The study paves the way to the rational design of highly efficient PSCs for potential scaled‐up production.

Fill factor losses in nonfullerene‐acceptor‐based organic solar cells under illumination are caused by morphological traps due to diffusion limited aggregation of nonfullerene acceptors in the mixed matrix. To achieve stable and high‐performance organic solar cells under illumination, it is essential to engineer the mixed regions from both thin film formation kinetics and materials intrinsic properties, e.g., materials compatibility and diffusion constant.

Abstract

As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long‐term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state‐of‐the‐art polymer donors with a commonly used nonfullerene acceptor (NFA), IT‐4F, are investigated. The active‐layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion‐limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene‐based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA‐based solar cells, which may be the key tactics to develop next‐generation OSCs with high efficiency as well as excellent stability.

A new S‐atom‐containing small molecule (TPE‐S) is introduced as a dopant‐free hole‐transporting layer in all‐inorganic and organic/inorganic hybrid perovskite solar cells (PVSCs) with a p–i–n inverted structure, leading to improved power conversion efficiencies of 15.4% and 21%, respectively. In addition, these devices also show enhanced photostability, with performance comparable to state‐of‐the‐art PVSCs based on the conventional n–i–p structure.

Abstract

Designing new hole‐transporting materials (HTMs) with desired chemical, electrical, and electronic properties is critical to realize efficient and stable inverted perovskite solar cells (PVSCs) with a p–i–n structure. Herein, the synthesis of a novel 3D small molecule named TPE‐S and its application as an HTM in PVSCs are shown. The all‐inorganic inverted PVSCs made using TPE‐S, processed without any dopant or post‐treatment, are highly efficient and stable. Compared to control devices based on the commonly used HTM, PEDOT:PSS, devices based on TPE‐S exhibit improved optoelectronic properties, more favorable interfacial energetics, and reduced recombination due to an improved trap passivation effect. As a result, the all‐inorganic CsPbI₂Br PVSCs based on TPE‐S demonstrate a remarkable efficiency of 15.4% along with excellent stability, which is the one of the highest reported values for inverted all‐inorganic PVSCs. Meanwhile, the TPE‐S layer can also be generally used to improve the performance of organic/inorganic hybrid inverted PVSCs, which show an outstanding power conversation efficiency of 21.0%, approaching the highest reported efficiency for inverted PVSCs. This work highlights the great potential of TPE‐S as a simple and general dopant‐free HTM for different types of high‐performance PVSCs.

A low‐dimensional DJ: A Dion–Jacobson‐phase lead‐free perovskite, (BEA)FA _n−1Sn _n I_3n+1 (BEA=1,4‐butanediamine, FA=formamidinium), is used to fabricated perovskite solar cells. The representative (BEA)FA₂Sn₃I₁₀ perovskite exhibits excellent optical absorption and carrier transport, resulting in a record power conversion efficiency of 6.43 %.

Abstract

1,4‐butanediamine (BEA) is incorporated into FASnI₃ (FA=formamidinium) to develop a series of lead‐free low‐dimensional Dion–Jacobson‐phase perovskites, (BEA)FA _n−1Sn _n I_3n+1. The broadness of the (BEA)FA₂Sn₃I₁₀ band gap appears to be influenced by the structural distortion owing to high symmetry. The introduction of BEA ligand stabilizes the low‐dimensional perovskite structure (formation energy ca. 10⁶ j mol⁻¹), which inhibits the oxidation of Sn²⁺. The compact (BEA)FA₂Sn₃I₁₀ dominated film enables a weakened carrier localization mechanism with a charge transfer time of only 0.36 ps among the quantum wells, resulting in a carrier diffusion length over 450 nm for electrons and 340 nm for holes, respectively. Solar cell fabrication with (BEA)FA₂Sn₃I₁₀ delivers a power conversion efficiency (PCE) of 6.43 % with negligible hysteresis. The devices can retain over 90 % of their initial PCE after 1000 h without encapsulation under N₂ environment.

An effective non‐preheating film‐casting method to realize highly oriented quasi‐2D perovskite films is proposed by replacing the BA⁺ spacer partially with MA⁺ cation as (BA)_{2− x} (MA)_{3+ x} Pb₄I₁₃ (x = 0, 0.2, 0.4, and 0.6). At the optimal x‐value of 0.4, the resultant quasi‐2D perovskite solar cell yields a best efficiency of 15.44% with an impressive fill factor of 80.20%.

Abstract

Although the hot‐casting (HC) technique is prevalent in developing preferred crystal orientation of quasi‐2D perovskite films, the difficulty of accurately controlling the thermal homogeneity of substrate is unfavorable for the reproducibility of device fabrication. Herein, a facile and effective non‐preheating (NP) film‐casting method is proposed to realize highly oriented quasi‐2D perovskite films by replacing the butylammonium (BA⁺) spacer partially with methylammonium (MA⁺) cation as (BA)_{2− x} (MA)_{3+ x} Pb₄I₁₃ (x = 0, 0.2, 0.4, and 0.6). At the optimal x‐value of 0.4, the resultant quasi‐2D perovskite film possesses highly orientated crystals, associated with a dense morphology and uniform grain‐size distribution. Consequently, the (BA)_1.6(MA)_3.4Pb₄I₁₃‐based solar cells yield champion efficiencies of 15.44% with NP processing and 16.29% with HC processing, respectively. As expected, the HC‐processed device shows a poor performance reproducibility compared with that of the NP film‐casting method. Moreover, the unsealed device (x = 0.4) displays a better moisture stability with respect to the x = 0 stored in a 65% ± 5% relative humility chamber.

Graphdiyne (GDY) serves as the hole accumulator and transporter in the perovskite layer for stable and efficient planar perovskite solar cells. Benefiting from the higher Fermi level of GDY than perovskite, the unidirectional hole transport from perovskite to GDY is ensured, and the charge separation ability and environmental stability of perovskite are significantly improved.

Abstract

Traditional carbon materials have demonstrated immense potential in perovskite solar cells (PSCs) owing to their superior electrical properties and environmental stability. Graphdiyne (GDY), as an emerging carbon allotrope, features uniformly distributed pores, endless design flexibility, and unique electronic character compared with traditional carbon materials. Herein, graphdiyne is introduced into the upper part of the perovskite (CH₃NH₃PbI₃) layer by utilizing a GDY‐containing antisolvent during the one‐step synthesis of perovskite. Intriguingly, GDY plays an essential role in hole accumulation and transportation because of its higher Fermi level than perovskite. As a result, the automatic separation of photogenerated carriers inside the perovskite film is achieved. Furthermore, the Schottky barrier formed on the interface between perovskite and GDY guarantees the unidirectional hole transport from perovskite to GDY, thereby benefiting further extraction to the hole transport layer. Consequently, GDY‐modified perovskite‐based planar PSCs exhibit a boosted J _sc of 24.21 mA cm⁻² and up to 19.6% power conversion efficiency owing to the increased efficient light utilization and charge extraction. The device with GDY modification exhibits less than 10% shrinkage after a month in ambience. Overall, this work demonstrates an easy method for the utilization of GDY to boost the charge extraction and environmental stability in PSCs.

A series of novel Ruddlesden–Popper perovskite films of (PBA_{1− x} BA _x )₂MA₃Pb₄I₁₃ are successfully designed and fabricated to reveal the interplay of binary organic spacers on the precursor chemistry, film morphology, crystal orientation, trap states, and charge transport to obtain highly efficient solar cells, providing an effective design strategy to further develop stable and efficient perovskite materials and devices.

Abstract

Ruddlesden–Popper perovskite (RPP) materials have attracted great attention due to their superior stability, where the organic spacer dominantly determines the stability and efficiency of RPP solar cells, but research still lacks the systematical understanding of the interplay of binary spacer in the overall mixture range of 0–100% in RPPs on the precursor chemistry, film quality, and carrier behavior. Herein, a series of novel binary spacer RPP films of (PBA_{1− x} BA _x )₂MA₃Pb₄I₁₃ (BA = n‐butylammonium, PBA = 4‐phenylbutan‐1‐aminium, and MA = methylammonium) is successfully fabricated to reveal the interplay of binary spacers. The incorporation of 50% BA into the (PBA)₂MA₃Pb₄I₁₃ precursor solution increases the colloidal size and reduces nucleation sites, and therefore forms a very smooth film with much larger crystal grains and a higher degree of crystal preferential orientation, resulting in a significant reduction of trap states. The resulting combination of fast electron transfer and efficient electron extraction facilitates to effectively suppress the trap‐assisted charge recombination and remarkably decrease charge recombination losses. Consequently, the (PBA_0.5BA_0.5)₂MA₃Pb₄I₁₃ device achieves a champion efficiency of 16.0%, among the highest reported efficiencies for RPP devices. Furthermore, this device demonstrates good ambient, illumination, and thermal stabilities, retaining 60–93% of its initial efficiency after 30 days of various ageing.

Highly efficient all‐inorganic perovskite solar cells based on CsPbI _x Br_{3‐ x} are fabricated through the introduction of a spontaneous interfacial manipulation method. A spontaneously formed ultrathin 2D perovskite top interface can not only eliminate interfacial defects but also effectively prevent moisture penetration. As a result, the device exhibits a power conversion efficiency of 18% with extended device stability.

Abstract

Cesium‐based all‐inorganic halide perovskites solar cells (PSCs) have recently attracted increasing attention. Currently, due to the existence of high defects density and unoptimized interfacial morphology, “state‐of‐the‐art” performances of all‐inorganic PSCs are still far away from their theoretical limits. Although commonly used two‐step passivation methods can effectively passivate the perovskite surface, they will inevitably detriment the original perovskite morphology due to the use of weak‐polarity solvents. This will potentially result in the unintentional doping, uncontrollable interfacial band alignment, and the additional defects formation. Hence, a spontaneous interfacial manipulation (SIM) method is developed to self‐organize a 2D/3D multidimensional perovskite top interface. It is demonstrated that the spontaneously formed ultrathin 2D perovskite can not only eliminate the interfacial defects, but also effectively prevent moisture penetration. As a result, a significant power conversion efficiency enhancement from 13.64% to over 18% is obtained along with greatly extended device lifetime, for CsPbI _x Br_{3‐ x} ‐based all‐inorganic PSC.

The built‐in electric field of a perovskite solar cell is reinforced by introducing electric dipole molecules, and the oriented charge transfer and collection are significantly improved. An efficiency of 21.5% is demonstrated and the average stability of the NMFL device retains 95% of the power conversion efficiency after storing over 2000 h under ambient conditions.

Abstract

Perovskite solar cells (PSCs) have received great attention due to their outstanding performance and their low processing costs. To boost their performance, one approach is to reinforce the built‐in electric field (BEF) to promote oriented carrier transport. The BEF is maximized by reinforcing the work function difference between cathode and anode (Δμ₁) and increasing the work function difference between lower and upper surfaces of perovskite film (Δμ₂) via introduction of electric dipole molecules, denoted as PTFCN and CF3BACl. The synergistic reinforcement of BEF improves charge transport and collection, and realizes markedly high photovoltaic performances with the best power conversion efficiency (PCE) up to 21.5%, a growth of 15.6% as compared to the control device, which is higher than the superposition of improvements achieved by either raising Δμ₁ or Δμ₂. Importantly, dual‐functional CF3BACl not only supplies dipole effect for tuning the surface potential of perovskite but offers hydrophobic trifluoride group toward the long‐term stable unencapsulated PSCs retaining more than 95% PCE after storing 2000 h under ambient conditions. This work demonstrates the synergistic effect of Δμ₁ and Δμ₂, providing an effective strategy for the further development of PSC in terms of photovoltaic conversion and stability.

A new nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, is designed and synthesized. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance with a power conversion efficiency (PCE) of 11.28%. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing treatment at 150 °C.

Abstract

A nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, based on 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene as an electron‐donating core and 2‐(6‐fluoro‐2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)‐propanedinitrile as electron‐withdrawing end groups, is designed and synthesized. i‐IEICO‐2F has a twist structure in the main conjugated chain, which causes blueshifted absorption and leads to harmonious absorption with a high bandgap donor. The bandgap of i‐IEICO‐2F compliments the bandgap of suitable wide bandgap donor polymers such as J52, leading to complete light absorption throughout the visible spectrum. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance including an open‐circuit voltage of 0.93 V, a short‐circuit current density of 16.61 mA cm⁻², and a fill factor of 73%, and result in a power conversion efficiency (PCE) of 11.28%. The i‐IEICO‐2F‐based devices reach PCEs of >11% without using any additives or post‐treatments. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing (TA) treatment at 150 °C. Based on UV, atomic force microscopy (AFM), and grazing incidence wide angle X‐ray scattering (GIWAXS) results, i‐IEICO‐2F devices show almost identical morphology and molecular orientation throughout the TA treatment and excellent stability compared to other IEICO derivatives.

A generic guideline for accurately controlling phase purity and arrangement in 2D perovskite films is provided by utilizing coordination engineering of a single‐crystal precursor solution. The resulting films with narrow distribution and preferentially perpendicular crystal orientation result in a significant improvement in device performance and stability, which is not typically found in conventional stoichiometric precursors.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) have recently drawn significant attention because of their structural variability that can be used to tailor optoelectronic properties and improve the stability of derived photovoltaic devices. However, charge separation and transport in 2D perovskite solar cells (PSCs) suffer from quantum well barriers formed during the processing of perovskites. It is extremely difficult to manage phase distributions in 2D perovskites made from the stoichiometric mixtures of precursor solutions. Herein, a generally applicable guideline is demonstrated for precisely controlling phase purity and arrangement in RPP films. By visually presenting the critical colloidal formation of the single‐crystal precursor solution, coordination engineering is conducted with a rationally selected cosolvent to tune the colloidal properties. In nonpolar cosolvent media, the derived colloidal template enables RPP crystals to preferentially grow along the vertically ordered alignment with a narrow phase variation around a target value, resulting in efficient charge transport and extraction. As a result, a record‐high power conversion efficiency (PCE) of 14.68% is demonstrated for a (TEA)₂(MA)₂Pb₃I₁₀ (n = 3) photovoltaic device with negligible hysteresis. Remarkably, superior stability is achieved with 93% retainment of the initial efficiency after 500 h of unencapsulated operation in ambient air conditions.

This work examines differences in structure and optoelectronic properties of two‐dimensional metal halide perovskites formed with two different diammonium ligands. Although of similar length and bonding motifs, the ligands differ by their strength of hydrogen‐bonding to halide anions, resulting in different lead‐iodine octahedra twisting, film structure, degree of carrier localization and energy gap in these materials.

Abstract

Reduced dimensionality forms of perovskites with alternating layers of organic ligands are a promising class of materials for achieving stable perovskite solar cells. Most work until now has focused on phases utilizing two ammonium terminated ligands per formula unit. However, phases utilizing a single diammonium ligand per formula unit are advantageous in that they can potentially have a thinner insulating organic layer between Pb‐halide layers, yet the structural effects on their optoelectronic properties are not yet well understood. In this study two organic ligands, butane 1,4‐diammonium (BDA) and N,N‐dimethylpropane diammonium (DMPD), are investigated as spacers in n = 1, 2D perovskites. Using ultraviolet and inverse photoelectron spectroscopies, BDAPbI₄ is shown to have a larger transport gap by 350 meV and a larger exciton binding energy by 140 meV than DMPDPbI₄. Through density functional theory calculations, the cause of this difference is traced to the out‐of‐plane tilting of the Pb‐halide octahedra provoked by the asymmetric ligand in DMPDPbI₄. Parallel channels of nearly straight PbIPb bonds are formed in one direction, leading to enhanced electronic coupling and higher band dispersion in that direction. In BDAPbI₄, no such channels exist, resulting in greater electronic confinement and a larger bandgap and exciton binding energy.

A unique ammonium salt, 1‐naphthylmethylamine iodide (NMAI) is shown to passivate the surface defects of perovskite, induce upward energy level bending and block electrons at the interface between the perovskite and hole transport layer in perovskite solar cells. These combined effects result in reduced non‐radiative recombination. Hence, more intensified electroluminescence and a champion open‐circuit voltage of 1.20 V are achieved in NMAI‐based devices.

Abstract

The presence of non‐radiative recombination at the perovskite surface/interface limits the overall efficiency of perovskite solar cells (PSCs). Surface passivation has been demonstrated as an efficient strategy to suppress such recombination in Si cells. Here, 1‐naphthylmethylamine iodide (NMAI) is judiciously selected to passivate the surface of the perovskite film. In contrast to the popular phenylethylammonium iodide, NMAI post‐treatment primarily leaves NMAI salt on the surface of the perovskite film. The formed NMAI layer not only efficiently decreases the defect‐assisted recombination for chemical passivation, but also retards the charge accumulation of energy level mis‐alignment for vacuum level bending and prevents minority carrier recombination due to the charge‐blocking effect. Consequently, planar PSCs with high efficiency of 21.04% and improved long‐term stability (98.9% of the initial efficiency after 3240 h) are obtained. Moreover, open‐circuit voltage as high as 1.20 V is achieved at the absorption threshold of 1.61 eV, which is among the highest reported values in planar PSCs. This work provides new insights into the passivation mechanisms of organic ammonium salts and suggests future guidelines for developing improved passivation layers.

Time of flight secondary ion mass spectrometry (TOF‐SIMS) is a versatile characterization technique which can provide key insights into the spatial location of all components of perovskite solar cell materials, and how those distributions change with performance/degradation. The technique is summarized here, past uses from the literature are covered, and example data and mitigation of known measurement artifacts are described.

Abstract

Time‐of‐flight secondary‐ion mass spectrometry (TOF‐SIMS), a powerful analytical technique sensitive to all components of perovskite solar cell (PSC) materials, can differentiate between the various organic species within a PSC absorber or a complete device stack. The ability to probe chemical gradients through the depth of a device (both organic and inorganic), with down to 100 nm lateral resolution, can lead to unique insights into the relationships between chemistry in the absorber bulk, at grain boundaries, and at interfaces as well as how they relate to changes in performance and/or stability. In this review, the technique is described; then, from the literature, several examples of how TOF‐SIMS have been used to provide unique insight into PSC absorbers and devices are covered. Finally, the common artifacts that can be introduced if the data are improperly collected, as well as methods to mitigate these artifacts are discussed.

The effects of the donating ability and bulkiness of the fluorenyl‐based donor in donor–π–acceptor structured organic dyes are investigated to establish the structure–property relationship in terms of molecular properties and photovoltaic performance. Through a simple cock‐tailed co‐sensitization strategy with porphyrin dye, the state‐of‐the‐art efficiencies of 14.2% and 11.6% with cobalt and iodine electrolytes, respectively, are achieved.

Abstract

A new series of 4‐hexyl‐4H‐thieno[3,2‐b]indole (HxTI) based organic chromophores is developed by structural engineering of the electron donor (D) group in the D–HxTI–benzothiadiazole‐phenyl‐acceptor platform with different fluorenyl moieties, such as unsubstituted fluorenyl (SGT‐146) and hexyloxy (SGT‐147), decyloxy (SGT‐148) and hexyloxy‐phenyl substituted (SGT‐149) fluorenyl moieties. In comparison to a reference dye SGT‐137 with a biphenyl‐based donor, the effects of the donating ability and bulkiness of the fluorenyl based donor in this D–π–A‐structured platform on molecular properties and photovoltaic performance are investigated to establish the structure–property relationship. The photovoltaic performance of dye‐sensitized solar cells (DSSCs) is improved according to the bulkiness of the donor groups. As a result, the DSSCs based on SGT‐149 show high power conversion efficiencies (PCEs) of 11.7% and 10.0% with a [Co(bpy)₃]^2+/3+ (bpy = 2,2′‐bipyridine) and an I⁻/I₃ ⁻ redox electrolyte, respectively. Notably, the co‐sensitization of SGT‐149 with a SGT‐021 porphyrin dye by utilizing a simple “cocktail” method, exhibit state‐of‐the‐art PCEs of 14.2% and 11.6% with a [Co(bpy)₃]^2+/3+ and an I⁻/I₃ ⁻ redox electrolyte, respectively.

This article aims to present an in‐depth review of the current understanding of metal halide perovskite devices and module stability by outlining how basic material intrinsic and extrinsic degradation mechanisms as well as additional complications from the presence of other layers and nonequilibrium conditions impact device and module performance over time.

Abstract

Metal halide perovskite solar cells (PSCs) have risen in efficiency from just 3.81% in 2009 to over 25.2% today. While metal halide perovskites have excelled in efficiency, advances in stability are significantly more complex and have progressed more slowly. The advance of efficiency, which is readily measured, over stability, which can require literally thousands of hours to demonstrate, is to be expected given the rapid rate of innovation in the field. In the face of changing absorber composition, synthetic approaches, and device stack components it is necessary to understand basic material properties to rationalize how to enable stability in devices. In this article the aim is to present an in‐depth review of the current understanding of metal halide perovskite devices and module stability by focusing on what is known retarding intrinsic and extrinsic degradation mechanisms at the material, device, and module level. Once these considerations are presented the discussion then moves to connecting different degradation mechanisms to stresses anticipated in operation and how they can impact efficiency of cells and ultimately modules over time.

Folding‐flexible semitransparent organic solar cells with over 10% efficiency and 21% average visible light transmission are realized by using xylitol microdoping and acid treatment on poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate transparent electrodes for supplying power and promoting plant growth in future multifunctional self‐powered greenhouses.

Abstract

Semitransparent organic solar cells (ST‐OSCs) have attracted extensive attention for their potential greenhouse applications. Conventional ST‐OSCs are typically based on indium tin oxide (ITO) electrodes which suffer from mechanical brittleness. Therefore, alternatives for ITO are required for realization of foldable‐flexible ST‐OSCs (FST‐OSCs). Herein, flexible poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes are prepared as ITO alternatives via polyhydroxy compound (xylitol) microdoping and acid treatment. As a result, flexible opaque OSCs based on PBDB‐T‐2F:Y6 photoactive system yield a high efficiency of 14.20%. The desirable optical properties of modified PEDOT:PSS electrodes in the visible light region and PBDB‐T‐2F:Y6 photoactive layer in the near‐infrared region facilitate the fabrication of FST‐OSCs with over 10% efficiency and 21% average visible light transmittance. Those FST‐OSCs also display excellent mechanical stability against bending and folding due to the xylitol doping, where over 80% of the initial efficiency can still be maintained even after 1000 folding cycles. Meanwhile, parallel comparisons between plants grown under direct sunlight with a FST‐OSCs roof and those under direct sunlight yield very similar results in terms of branch sturdiness and hypertrophic leaves. The results pave the way for realizing high‐performing FST‐OSCs based on PEDOT:PSS electrodes that could utilize visible light for plant growth and infrared light for power generation.

A new nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, is designed and synthesized. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance with a power conversion efficiency (PCE) of 11.28%. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing treatment at 150 °C.

Abstract

A nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, based on 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene as an electron‐donating core and 2‐(6‐fluoro‐2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)‐propanedinitrile as electron‐withdrawing end groups, is designed and synthesized. i‐IEICO‐2F has a twist structure in the main conjugated chain, which causes blueshifted absorption and leads to harmonious absorption with a high bandgap donor. The bandgap of i‐IEICO‐2F compliments the bandgap of suitable wide bandgap donor polymers such as J52, leading to complete light absorption throughout the visible spectrum. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance including an open‐circuit voltage of 0.93 V, a short‐circuit current density of 16.61 mA cm⁻², and a fill factor of 73%, and result in a power conversion efficiency (PCE) of 11.28%. The i‐IEICO‐2F‐based devices reach PCEs of >11% without using any additives or post‐treatments. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing (TA) treatment at 150 °C. Based on UV, atomic force microscopy (AFM), and grazing incidence wide angle X‐ray scattering (GIWAXS) results, i‐IEICO‐2F devices show almost identical morphology and molecular orientation throughout the TA treatment and excellent stability compared to other IEICO derivatives.

A novel one‐step multifunctional chemical approach is demonstrated to realize a silver nanonetwork‐based flexible transparent electrode through simultaneously controlling the Ag nanowires welding, eliminating the insulating surfactants, and improving the electrical contacts with adjacent layers. Highly performing flexible organic solar cells are achieved by the ultrasmooth surface and unprecedented stability simultaneously under electrical bias and mechanical bending.

Abstract

For ideal flexible transparent electrodes, the features of good electrical/optical properties, low surface roughness, efficient charge transportation, robust electrical stability under simultaneously continuous operation bias, and mechanical bending are critical. Herein, a flexible transparent electrode fulfilling all these features is demonstrated by silver (Ag) nanonetwork composites semi‐embedded in low‐temperature‐processed colorless polyimide (cPI), which shows a figure of merit over 1000 (5.4 Ω sq⁻¹ sheet resistance and >94% diffused transmission at 550 nm wavelength), extremely smooth topography (<1 nm root‐mean‐square roughness and <3 nm peak‐to‐valley roughness), remarkable bending stability under continuous operation bias, and increased work function favoring the band alignment with typical charge transport layers for efficient devices. These characteristics are attributed to one‐step multifunctional chemical treatment on the composite of Ag nanowires and an example polymer of poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The strategic one‐step process simultaneously offers selective welding at nanowires cross junctions to form an Ag nanonetwork, and removing polyvinylpyrrolidone surfactant from Ag nanowires and PSS from PEDOT:PSS. The flexible electrode also favors the residue‐free cPI transfer for applications. Flexible organic solar cells (OSCs) made from the electrode achieve an averaged power conversion efficiency of 14.46% (best, 15.12%), which is the best flexible OSCs reported so far.