Ligang Yuan

A novel wide‐bandgap nonfullerene acceptor TfIF‐4FIC is synthesized. PBDB‐T‐2F:TfIF‐4FIC‐based organic solar cell acquires a power conversion efficiency (PCE) of 13.1%, a high open‐circuit voltage of 0.98 V, which is the best performed device with bandgap larger than 1.60 eV. When using PBDB‐T‐2F:TfIF‐4FIC as front cell and PTB7‐Th:PCDTBT:IEICO‐4F as back cell to construct tandem device, PCE of 15% is achieved.

Abstract

A tandem organic solar cell (OSC) is a valid structure to widen the photon response range and suppress the transmission loss and thermalization loss. In the past few years, the development of low‐bandgap materials with broad absorption in long‐wavelength region for back subcells has attracted considerable attention. However, wide‐bandgap materials for front cells that have both high short‐circuit current density (J _SC) and open‐circuit voltage (V _OC) are scarce. In this work, a new fluorine‐substituted wide‐bandgap small molecule nonfullerene acceptor TfIF‐4FIC is reported, which has an optical bandgap of 1.61 eV. When PBDB‐T‐2F is selected as the donor, the device offers an extremely high V _OC of 0.98 V, a high J _SC of 17.6 mA cm⁻², and a power conversion efficiency of 13.1%. This is the best performing acceptor with such a wide bandgap. More importantly, the energy loss in this combination is 0.63 eV. These properties ensure that PBDB‐T‐2F:TfIF‐4FIC is an ideal candidate for the fabrication of tandem OSCs. When PBDB‐T‐2F:TfIF‐4FIC and PTB7‐Th:PCDTBT:IEICO‐4F are used as the front cell and the back cell to construct tandem solar cells, a PCE of 15% is obtained, which is one of best results reported to date in the field of organic solar cells.

Fluorinated perylenediimide (F‐PDI) is first introduced to optimize photovoltaic performance and stability of perovskite solar cells. Conductive F‐PDI effectively passivates defects and promotes charge transfer. The hydrophobicity of F‐PDI preventing moisture penetration as well as the strong hydrogen bonding immobilizing methylamine ions, thereby, endow excellent moisture and thermal stability with nearly 70% efficiency retention after thermal treatment at 100 °C.

Abstract

The notoriously poor stability of perovskite solar cells is a crucial issue restricting commercial applications. Here, a fluorinated perylenediimide (F‐PDI) is first introduced into perovskite film to enhance the device's photovoltaic performance, as well as thermal and moisture stability simultaneously. The conductive F‐PDI molecules filling at grain boundaries (GBs) and surface of perovskite film can passivate defects and promote charge transport through GBs due to the chelation between carbonyl of F‐PDI and noncoordinating lead. Furthermore, an effective multiple hydrophobic structure is formed to protect perovskite film from moisture erosion. As a result, the F‐PDI‐incorporated devices based on MAPbI₃ and Cs_0.05 (FA_0.83MA_0.17)_0.95 Pb (Br_0.17I_0.83)₃ absorber achieve champion efficiencies of 18.28% and 19.26%, respectively. Over 80% of the initial efficiency is maintained after exposure in air for 30 days with a relative humidity (RH) of 50%. In addition, the strong hydrogen bonding of F···H‐N can immobilize methylamine ion (MA⁺) and thus enhances the thermal stability of device, remaining nearly 70% of the initial value after thermal treatment (100 °C) for 24 h at 50% RH condition.

A small molecule donor with appropriate energy levels and good compatibility is designed and synthesized as the third component for the construction of two types (fullerene/non‐fullerene) of all‐small‐molecule ternary solar cells with higher PCEs of over 10%. The results demo‐nstrate that introducing a homologous donor for the host donor is a promising way toward developing highly efficient all‐small‐molecule solar cells.

Abstract

Two types of all‐small‐molecule ternary solar cells consisting of two small‐molecule donors and one acceptor (fullerene/non‐fullerene) are developed. Interestingly, both these devices have a common component: a carefully designed medium bandgap small molecule, which possesses appropriate energy levels and displays good compatibility with the host donor. In the fullerene system, the charge‐relaying role of the additive donor is confirmed by the improved charge transportation and suppressed charge recombination. While in the non‐fullerene system, the mixed face‐on and edge‐on orientation of the ternary film induced by the additive donor dominates the promotion of charge transportation. Accordingly, both ternary devices deliver higher short‐circuit current density, fill factor, and power conversion efficiencies of over 10% compared to binary ones. This work offers a promising guideline on the construction of high‐performance all‐small‐molecule ternary solar cells by incorporating a miscible small‐molecule donor.

The photophysics and nature of localized traps and their role on the optical properties of lead bromide perovskite films are investigated using optical spectroscopy and theoretical modeling. A clear understanding of the origin and nature of localized traps has important ramifications for perovskite light harvesting and emitting applications, as well as the design of new perovskites.

Abstract

Traps exert an omnipotent influence over the performance of halide perovskite optoelectronic devices. A clear understanding of the origin and nature of the traps in halide perovskites is the key to controlling them and realizing optimal devices. Herein, the role of localized traps on the optical properties of lead bromide perovskite films is investigated. In the low‐temperature orthorhombic phase of CH₃NH₃PbBr₃ perovskite, band‐edge carrier dynamics exhibit a power‐law decay due to the presence of structural‐disorder‐induced localized traps, which has a depth of ≈40 meV. The continuous distribution of these localized traps gives rise to a broad sub‐band‐gap emission that becomes more prominent in thicker films with a larger trap density. The presence of this emission only from the hybrid organic–inorganic perovskites points to the vital role of organic dipoles in localized trap states formation. This study explicates the nature of these localized traps as well as their nontrivial role in carrier recombination kinetics, which is of fundamental importance in perovskites optoelectronics.

The device with binary additive of octance‐1,8‐dithiol:1,8‐diiodooctane (ODT:DIO) (0.375%:0.125%) based on FTAZ:ITIC‐Th blends exhibits a higher power conversion efficiency of 10.93% than the devices processed with only 0.5% ODT, 0.5% DIO, or excess binary additive of ODT:DIO (0.5%:0.5%). The reason is that additives with different boiling point work in different stages during the whole filming process as in situ grazing incidence wide‐angle X‐ray scattering characterization indicates.

Abstract

The power conversion efficiency of polymer solar cells (PSCs) is strongly affected by active layer morphology. Here, two solvent additives (ODT: octance‐1,8‐dithiol; DIO: 1,8‐diiodooctane) are used to optimize the bulk heterojunction morphology of FTAZ:ITIC‐Th based PSCs and ≈11% efficiency is obtained, which is 10% higher than the untreated device. Based on the morphological characterizations, the influence of binary solvent additives on manipulating molecular packing and phase separation of blend films is successfully revealed. More importantly, in situ grazing incidence wide‐angle X‐ray scattering characterization is adopted to explore the crucial role played by these two solvent additives at different stages of the film‐forming process, that is, ODT influences the initial stage of the film‐forming process, while DIO later establishes the ultimate photoactive film formation. Due to the impacts of two additives at different film processing stages, an optimal ratio of ODT:DIO (0.375%:0.125%) is obtained, which helps in realizing the optimized morphology.

Methoxy‐functionalized triphenylamine‐imidazole derivatives, simultaneously working as hole transport materials and bifacial interface‐modifiers passivating defects in the perovskite and NiO _x layers, are developed for high‐performance and stable perovskite solar cell. They are advantageous to improve charge‐extraction kinetics of devices and significantly enhance the stability of devices under constant UV illumination in air.

Abstract

Methoxy‐functionalized triphenylamine‐imidazole derivatives that can simultaneously work as hole transport materials (HTMs) and interface‐modifiers are designed for high‐performance and stable perovskite solar cells (PSCs). Satisfying the fundamental electrical and optical properties as HTMs of p‐i‐n planar PSCs, their energy levels can be further tuned by the number of methoxy units for better alignment with those of perovskite, leading to efficient hole extraction. Moreover, when they are introduced between perovskite photoabsorber and low‐temperature solution‐processed NiO _x interlayer, widely featured as an inorganic HTM but known to be vulnerable to interfacial defect generation and poor contact formation with perovskite, nitrogen and oxygen atoms in those organic molecules are found to work as Lewis bases that can passivate undercoordinated ion‐induced defects in the perovskite and NiO _x layers inducing carrier recombination, and the improved interfaces are also beneficial to enhance the crystallinity of perovskite. The formation of Lewis adducts is directly observed by IR, Raman, and X‐ray photoelectron spectroscopy, and improved charge extraction and reduced recombination kinetics are confirmed by time‐resolved photoluminescence and transient photovoltage experiments. Moreover, UV‐blocking ability of the organic HTMs, the ameliorated interfacial property, and the improved crystallinity of perovskite significantly enhance the stability of PSCs under constant UV illumination in air without encapsulation.

A novel wide‐bandgap nonfullerene acceptor TfIF‐4FIC is synthesized. PBDB‐T‐2F:TfIF‐4FIC‐based organic solar cell acquires a power conversion efficiency (PCE) of 13.1%, a high open‐circuit voltage of 0.98 V, which is the best performed device with bandgap larger than 1.60 eV. When using PBDB‐T‐2F:TfIF‐4FIC as front cell and PTB7‐Th:PCDTBT:IEICO‐4F as back cell to construct tandem device, PCE of 15% is achieved.

Abstract

A tandem organic solar cell (OSC) is a valid structure to widen the photon response range and suppress the transmission loss and thermalization loss. In the past few years, the development of low‐bandgap materials with broad absorption in long‐wavelength region for back subcells has attracted considerable attention. However, wide‐bandgap materials for front cells that have both high short‐circuit current density (J _SC) and open‐circuit voltage (V _OC) are scarce. In this work, a new fluorine‐substituted wide‐bandgap small molecule nonfullerene acceptor TfIF‐4FIC is reported, which has an optical bandgap of 1.61 eV. When PBDB‐T‐2F is selected as the donor, the device offers an extremely high V _OC of 0.98 V, a high J _SC of 17.6 mA cm⁻², and a power conversion efficiency of 13.1%. This is the best performing acceptor with such a wide bandgap. More importantly, the energy loss in this combination is 0.63 eV. These properties ensure that PBDB‐T‐2F:TfIF‐4FIC is an ideal candidate for the fabrication of tandem OSCs. When PBDB‐T‐2F:TfIF‐4FIC and PTB7‐Th:PCDTBT:IEICO‐4F are used as the front cell and the back cell to construct tandem solar cells, a PCE of 15% is obtained, which is one of best results reported to date in the field of organic solar cells.

Fluorinated perylenediimide (F‐PDI) is first introduced to optimize photovoltaic performance and stability of perovskite solar cells. Conductive F‐PDI effectively passivates defects and promotes charge transfer. The hydrophobicity of F‐PDI preventing moisture penetration as well as the strong hydrogen bonding immobilizing methylamine ions, thereby, endow excellent moisture and thermal stability with nearly 70% efficiency retention after thermal treatment at 100 °C.

Abstract

The notoriously poor stability of perovskite solar cells is a crucial issue restricting commercial applications. Here, a fluorinated perylenediimide (F‐PDI) is first introduced into perovskite film to enhance the device's photovoltaic performance, as well as thermal and moisture stability simultaneously. The conductive F‐PDI molecules filling at grain boundaries (GBs) and surface of perovskite film can passivate defects and promote charge transport through GBs due to the chelation between carbonyl of F‐PDI and noncoordinating lead. Furthermore, an effective multiple hydrophobic structure is formed to protect perovskite film from moisture erosion. As a result, the F‐PDI‐incorporated devices based on MAPbI₃ and Cs_0.05 (FA_0.83MA_0.17)_0.95 Pb (Br_0.17I_0.83)₃ absorber achieve champion efficiencies of 18.28% and 19.26%, respectively. Over 80% of the initial efficiency is maintained after exposure in air for 30 days with a relative humidity (RH) of 50%. In addition, the strong hydrogen bonding of F···H‐N can immobilize methylamine ion (MA⁺) and thus enhances the thermal stability of device, remaining nearly 70% of the initial value after thermal treatment (100 °C) for 24 h at 50% RH condition.

Organic solar cells based on a terpolymer donor and ITIC nonfullerene acceptor exhibit negligible energy level offsets that still lead to efficient hole and electron transfer. A small amount of PC₇₁BM is then added to optimize the blend film morphology, which yields significantly suppressed charge recombination and affords a maximum power conversion efficiency of 11.4% without any additional treatment.

Abstract

Nonfullerene acceptors (NFAs)‐based organic solar cells (OSCs) have recently drawn considerable research interests; however, their excitonic dynamics seems quite different than that of fullerene acceptors‐based devices and remains to be largely explored. A random terpolymer of PBBF11 to pair with a paradigm NFA of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITIC) such that both complementary optical absorption and very small offsets of both highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are acquired is designed and synthesized. Despite the small energy offsets, efficient electron/hole transfer between PBBF11 and ITIC is both clearly observed from steady‐state photoluminescence and transient absorption spectra and also supported by the measured low exciton binding energy in ITIC. Consequently, the PBBF11:ITIC‐based OSCs afford an encouraging power conversion efficiency (PCE) of 10.02%. Although the good miscibility of PBBF11 and ITIC induces a homogenous blend film morphology, it causes severe charge recombination. The fullerene acceptor of PC₇₁BM with varying loading ratios is therefore added to modulate film morphology to effectively reduce the charge recombination. As a result, the optimal OSCs based on PBBF11:ITIC:PC₇₁BM yield a better PCE of 11.4% without any additive or annealing treatment.

The effectiveness of using perovskite/Zr‐metal–organic framework (MOF) heterojunction in realizing efficient and stable inverted p–i–n perovskite solar cells (PVSCs) is demonstrated in article number 1801715 by Kevin C.‐W. Wu, Chu‐Chen Chueh, and co‐workers. Using perovskite/Zr‐MOF hybrid heterojunction to fabricate PVSCs is demonstrated to possess improved performance and ambient stability, benefiting from the grain‐locking effects introduced by the hybrid MOFs. It even reveals better merits than the perovskite/MOF bilayer heterojunction.

The effectiveness of using perovskite/Zr‐metal–organic frameworks (MOFs) heterojunctions in realizing efficient and stable inverted p–i–n perovskite solar cells (PVSCs) is demonstrated. Using a perovskite/Zr‐MOF hybrid heterojunction to fabricate PVSCs is demonstrated to possess improved performance and ambient stability, benefitting from the grain‐locking effects introduced by the hybrid MOFs. It even reveals better merits than the perovskite/MOF bilayer heterojunction.

Abstract

In this study, the effectiveness of using a perovskite/Zr‐metal–organic frameworks (MOFs) heterojunction in realizing efficient and stable inverted p–i–n perovskite solar cells (PVSCs) is demonstrated. Two types of Zr‐MOFs, UiO‐66 and MOF‐808, are investigated owing to their respectable moisture and chemical stabilities. The MOFs while serving as an interlayer in conjunction with the perovskite film are shown to possess the advantages of UV‐filtering capability and enhancing perovskite crystallinity. Consequently, the UiO‐66/MOF‐808‐modified PVSCs yield enhanced power conversion efficiencies (PCEs) of 17.01% and 16.55%, outperforming the control device (15.79%). While further utilizing a perovskite/Zr‐MOF hybrid heterojunction to fabricate the devices, the hybrid MOFs are found to possibly distribute over the perovskite grain boundary providing a grain‐locking effect to simultaneously passivate the defects and to reinforce the film's robustness against moisture invasion. As a result, the PCEs of the UiO‐66/MOF‐808‐hybrid PVSCs are further enhanced to 18.01% and 17.81%, respectively. Besides, over 70% of the initial PCE is retained after being stored in air (25 °C and relative humidity of 60 ± 5%) for over 2 weeks, in contrast to the quick degradation observed for the control device. This study demonstrates the promising potential of using perovskite/MOF heterojunctions to fabricate efficient and stable PVSCs.

Eu‐porphyrin complex is introduced into perovskite film to perfectly fabricate 2D perovskite inlaying the grain boundary of 3D polycrystalline. Such a modified device significantly increases humid, heating and UV light stability.

Abstract

The formation of defects at surfaces and grain boundaries (GBs) during the fabrication of solution‐processed perovskite film are thought to be responsible for its instability. Herein, Eu‐porphyrin complex (Eu‐pyP) is directly doped into methylammonium lead triiodide (MAPbI₃) precursor, perfectly fabricating 2D (Eu‐pyP)_0.5MA _{n ‐1}Pb _n I_{3 n +1} platelets inlaying the GBs of 3D polycrystalline interstices in this protocol. The device based on Eu‐pyP doped perovskite film possesses a champion efficiency of 18.2%. More importantly, the doped perovskite solar cells device shows beyond 85% retention of its pristine efficiency value, whereas the pure MAPbI₃ device has a rapid drop in efficiency down to 10% within 100 h under 45% humidity at 85 °C in AM 1.5 G. The above acquired perovskite films reveal an unpredictable thermodynamic self‐healing ability. Consequently, the findings provide an avenue for defect passivation to synchronously improve resistibility to moisture, heat, and solar light including UV.

A one‐pot solution method to synthesize lead‐free 2D perovskite (BA)₂SnI_{4− x} Cl _x crystals with various Cl incorporation concentrations is reported. The Cl element is successfully incorporated into the crystal lattice. The Cl incorporation greatly alters the morphology, optical properties, phase transition temperature, and charge transport behavior of the as‐synthesized crystals.

Abstract

The incorporation of chloride (Cl) into methylammonium lead iodide (MAPbI₃) perovskites has attracted much attention because of the significantly improved performance of the MAPbI₃‐based optoelectronic devices with a negligible small amount of Cl incorporation. It is expected that the Cl incorporation in 2D perovskites with layered nature would be much more efficient and thus can greatly alter the morphology, optical properties, phase transition, and charge transport; however, studies on those aspects in 2D perovskites remain elusive up to date. Here, a one‐pot solution method to synthesize the Cl‐doped lead‐free 2D perovskite (BA)₂SnI₄ with various Cl incorporation concentrations is reported and how the Cl incorporation affects the morphology change, photoluminescence, phase transition, and charge transport is investigated. The Cl element is successfully incorporated into the crystal lattice in the solution‐processed perovskite materials, confirmed by X‐ray photoelectron spectroscopy and energy dispersive X‐ray spectroscopy measurements. The temperature‐dependent photoluminescence studies indicate that the emission properties and phase transition behavior in (BA)₂SnI_{4− x} Cl _x can be tuned by varying the Cl incorporation concentration. Electrical measurement suggests that the charge transport behavior can also be greatly altered by the Cl doping concentration and the electrical conductivity can be significantly improved under a higher Cl incorporation concentration.

The efficiency limits in non‐fullerene organic solar cells are examined using a numerical simulator. Power conversion efficiency (PCE) of over 18% using recently reported carrier mobility values and voltage losses, are predicted. Increasing the mobility to >10⁻³ cm² V⁻¹ s⁻¹ and decreasing the recombination constant to <10⁻¹² cm³ s⁻¹ is shown to yield a single‐junction and 2T‐tandem cell with PCEs of >20% and >25%, respectively.

Abstract

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10⁻³ cm² V⁻¹ s⁻¹ in combination with low nongeminate recombination rate constants of 10⁻¹² cm³ s⁻¹ could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics.

Through the rational molecular design of fluorination, the work function of the conjugated polymer (CP) is enhanced from 4.83 to 5.00 eV. Consequently, the CP can be used to modify efficient active layers consisting of polymer donors with a deep HOMO level, such as PBDB‐T‐2F:IT‐4F, and an outstanding power conversion efficiency of 12.7% is achieved in the corresponding device without V _oc loss.

Abstract

Since the highest occupied molecular orbital (HOMO) level of donors in organic solar cells (OSCs) is being constantly downshifted for achieving high open‐circuit voltage (V _oc), a further enhancement of the anode work function (WF) is required. Herein, an effective approach of fluorination is demonstrated to simultaneously improve the WF and transparency for anode interlayer (AIL) material. By fluorination, in combination with the dialysis treatment in LiCl solution, the WF of PCP‐2F‐Li could be significantly enhanced from 4.86 to 5.0 eV, as compared to PCP‐Na. Meanwhile, the transparency of the polymer is also improved. As a result, PCP‐2F‐Li can be used to modify efficient active layers consisting of polymer donors with deep HOMO levels, such as PBDB‐T‐2F:IT‐4F, and an outstanding power conversion efficiency (PCE) of 12.7% is achieved in the corresponding device with a high V _oc of 0.84 V. This result represents the highest efficiency for the OSCs using a solution‐processed pH‐neutral AIL, which is beneficial to the low‐cost fabrication of high‐performance OSCs with improved stability. More importantly, PCP‐2F‐Li could be processed by blade coating for making large‐area device of 1 cm², and a PCE of 10.6% is achieved, bringing a promising prospect for the large‐area device fabrication.

The elemental sulfur (S₈) added to the perovskite precursor solution ((FAPbI₃)_0.95(MAPbBr₃)_0.05 in dimethylformamide/dimethyl sulfoxide) not only increases the stability of the solution owing to amine–sulfur coordination but also significantly improves the thermalstability and photostability due to the increase in chemical stability of the perovskite material itself without compromising the power conversion efficiency of the resulting perovskite solar cells.

Abstract

Efficient perovskite solar cells (PSCs) are mainly fabricated by a solution coating processes. However, the efficiency of such devices varies significantly with the aging time of the precursor solution used to fabricate them, which includes a mixture of perovskite components, especially methylammonium (MA), and formamidinium (FA) cations. Herein, how the inorganic–organic hybrid perovskite precursor solution of (FAPbI₃)_0.95(MAPbBr₃)_0.05 degrades over time and how such degradation can be effectively inhibited is reported on, and the associated mechanism of degradation is discussed. Such degradation of the precursor solution is closely related to the loss of MA cations dissolved in the FA solution through the deprotonation of MA to volatile methylamine (CH₃NH₂). Addition of elemental sulfur (S₈) drastically stabilizes the precursor solution owing to amine–sulfur coordination, without compromising the power conversion efficiency (PCE) of the derived PSCs. Furthermore, sulfur introduced to stabilize the precursor solution results in improved PSC stability.

A very simple rod‐shaped bithiophene‐based small molecule, T2‐ORH, is synthesized in only two steps to obtain a nonfullerene acceptor for use in efficient organic photovoltaic cells. The additive‐free inverted PTB7‐Th:T2‐ORH single‐junction device exhibits a power conversion efficiency of 9.33%, with a remarkably low E _loss of 0.51 eV due to a smooth homogeneous film morphology and vertical and parallel charge transport.

Abstract

The introduction of rigid and extended ladder‐type fused‐ring cores, such as indacenodithiophene, has enabled the synthesis of a variety of nonfullerene small molecules for use as electron acceptors in high‐performance organic photovoltaic cells. Contrasting with recent trends, a very simple‐structured nonfullerene acceptor (NFA), T2‐ORH, consisting of a bithiophene core and octyl‐substituted rhodanine ends, is synthesized in two steps from inexpensive commercially available raw materials. Its relatively short π‐conjugation results in a wide bandgap and a blue‐shifted UV–vis absorption profile complementary to those of poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th). Despite a sufficient offset between T2‐ORH and PTB7‐Th, the lowest unoccupied molecular orbital (LUMO) energy level of T2‐ORH is still higher than the LUMOs of other NFAs (e.g., ITIC). Therefore, the PTB7‐Th:T2‐ORH blend film exhibits an efficiency of 9.33% with a high open‐circuit voltage of 1.07 V and a short‐circuit current of 14.72 mA cm⁻² in an additive‐free single‐junction cell. Importantly, the optimized device displays a remarkably low energy loss of 0.51 eV, in which bimolecular and monomolecular charge recombination is effectively suppressed by solvent vapor annealing treatment. The blend film has a very smooth and homogeneous morphology, providing both vertical and parallel charge transport in the devices.

A novel method for defects passivation in perovskite solar cells via controlling the electron density distribution of D‐π‐A molecule is proposed. As the polarity of the passivated molecule increases, the passivation effect on the under‐coordinated Pb²⁺ defects will be more obvious, leading to an increase of 80 mV in the open circuit voltage of the devices.

Abstract

Organic–inorganic hybrid perovskite solar cells (PSCs) are a promising photovoltaic technology that has rapidly developed in recent years. Nevertheless, a large number of ionic defects within perovskite absorber can serve as non‐radiative recombination center to limit the performance of PSCs. Here, organic donor‐π‐acceptor (D‐π‐A) molecules with different electron density distributions are employed to efficiently passivate the defects in the perovskite films. The X‐ray photoelectron spectroscopy (XPS) analysis shows that the strong electron donating N,N‐dibutylaminophenyl unit in a molecule causes an increase in the electron density of the passivation site that is a carboxylate group, resulting in better binding with the defects of under‐coordinated Pb²⁺ cations. Carrier lifetime in the perovskite films measured by the time‐resolved photoluminescence spectrum is also prolonged by an increase in donation ability of the D‐π‐A molecules. As a consequence, these benefits contribute to an increase of 80 mV in the open circuit voltage of the devices, enabling a maximum power conversion efficiency (PCE) of 20.43%, in comparison with PCE of 18.52% for the control device. The authors' findings provide a novel strategy for efficient defect passivation in the perovskite solar cells based on controlling the electronic configuration of passivation molecules.

Zwitterions have emerged as a novel class of materials for organic/perovskite solar cells, light‐emitting devices, and lithium ion batteries. These materials have demonstrated tremendous performance in the enabling of highly efficient devices. The mechanism of action, structure–property relationships, and more device applications of zwitterionic materials must be explored to exploit the potential of these materials to the full.

Abstract

Zwitterions, a class of materials that contain covalently bonded cations and anions, have been extensively studied in the past decades owing to their special features, such as excellent solubility in polar solvents, for solution processing and dipole formation for the transfer of carriers and ions. Recently, zwitterions have been developed as electrode modifiers for organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light‐emitting devices (OLEDs), as well as electrolyte additives for lithium ion batteries (LIBs). With the rapid advances of zwitterionic materials, high‐performance devices have been constructed with enhanced efficiencies by introducing them as interface layers and electrolyte additives. In this review, recent progress in OSCs, PVSCs, OLEDs, and LIBs by using zwitterions is highlighted. The authors also elaborate the role of various zwitterionic materials as interfacial layers and additives for highly efficient OSCs, PVSCs, OLEDs, and LIBs. This article presents an overview of device performance of zwitterionic materials. The structure–property relationship is also discussed. Finally, the prospects of zwitterion materials are also addressed.

2D Dion–Jacobson perovskites have better carrier charge transport because of the closer interlayer distance. Solar cells based on Dion–Jacobson perovskites having mixed organic cations and using solvent‐engineering methods and hydriodic acid additive achieve higher efficiencies with high fill factors. Most importantly, the Dion–Jacobson perovskite solar cells exhibit better environmental stability compared with butylammonium‐based perovskites and 3D analogs.

Abstract

Hybrid halide 2D perovskites deserve special attention because they exhibit superior environmental stability compared with their 3D analogs. The closer interlayer distance discovered in 2D Dion–Jacobson (DJ) type of halide perovskites relative to 2D Ruddlesden–Popper (RP) perovskites implies better carrier charge transport and superior performance in solar cells. Here, the structure and properties of 2D DJ perovskites employing 3‐(aminomethyl)piperidinium (3AMP²⁺) as the spacing cation and a mixture of methylammonium (MA⁺) and formamidinium (FA⁺) cations in the perovskite cages are presented. Using single‐crystal X‐ray crystallography, it is found that the mixed‐cation (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ perovskite has a narrower bandgap, less distorted inorganic framework, and larger PbIPb angles than the single‐cation (3AMP)(MA)₃Pb₄I₁₃. Furthermore, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ films made by a solvent‐engineering method with a small amount of hydriodic acid have a much better film morphology and crystalline quality and more preferred perpendicular orientation. As a result, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃‐based solar cells exhibit a champion power conversion efficiency of 12.04% with a high fill factor of 81.04% and a 50% average efficiency improvement compared to the pristine (3AMP)(MA)₃Pb₄I₁₃ cells. Most importantly, the 2D DJ 3AMP‐based perovskite films and devices show better air and light stability than the 2D RP butylammonium‐based perovskites and their 3D analogs.

To predict the open‐circuit voltage (V _oc) of polymer–fullerene solar cells, three independent methods, square‐wave voltammetry (SWV), ultraviolet photoelectron spectroscopy, and density functional theory, are compared. For 19 diketopyrrolopyrrole polymers, SWV gives the best correlation. Remarkably, the slope of V _oc with the blend's electrochemical gap is less than unity and possible reasons for this result are discussed.

Abstract

For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square‐wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open‐circuit voltage (V _oc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]‐phenyl‐C₆₁‐butyl acid methyl ester or [6,6]‐phenyl‐C₇₁‐butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with V _oc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the V _oc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.

The ITC‐2Cl‐based device yields an excellent power conversion efficiency of 13.6% with a low E _loss of 0.67 eV, which is superior to those of the devices based on ITCPTC, IT‐4F, and IT‐4Cl.

Abstract

Generally, highly efficient organic solar cells require both a high open‐circuit voltage (V _OC) and a high short‐circuit current density (J _SC). Reducing the energy loss (E _loss) is an effective way to achieve a high V _OC without compromising the photocurrent, which is ideal for enhancing the power conversion efficiencies (PCEs). Herein, a new chlorinated nonfullerene acceptor (ITC‐2Cl) with chlorinated thiophene‐fused end groups is developed. In comparison with the unchlorinated counterpart (ITCPTC), the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the lowest unoccupied molecular orbital energy levels, but also the molecular packing and thus thin‐film morphology. The PM6:ITC‐2Cl‐based device yields a significantly higher PCE (13.6%) with a lower E _loss (0.67 eV) than the ITCPTC‐based device (PCE of 12.3% with E _loss of 0.70 eV). More importantly, compared to the archetypal nonfullerene acceptors such as IT‐4F (PCE of 12.9% with E _loss of 0.73 eV) and IT‐4Cl (PCE of 12.7% with E _loss of 0.76 eV), the ITC‐2Cl‐based device shows a higher PCE and a lower E _loss. These results demonstrate that the chlorinated thiophene‐fused end group is a promising candidate for a high‐performance nonfullerene acceptors with low energy loss.

The ITC‐2Cl‐based device yields an excellent power conversion efficiency of 13.6% with a low E _loss of 0.67 eV, which is superior to those of the devices based on ITCPTC, IT‐4F, and IT‐4Cl.

Abstract

Generally, highly efficient organic solar cells require both a high open‐circuit voltage (V _OC) and a high short‐circuit current density (J _SC). Reducing the energy loss (E _loss) is an effective way to achieve a high V _OC without compromising the photocurrent, which is ideal for enhancing the power conversion efficiencies (PCEs). Herein, a new chlorinated nonfullerene acceptor (ITC‐2Cl) with chlorinated thiophene‐fused end groups is developed. In comparison with the unchlorinated counterpart (ITCPTC), the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the lowest unoccupied molecular orbital energy levels, but also the molecular packing and thus thin‐film morphology. The PM6:ITC‐2Cl‐based device yields a significantly higher PCE (13.6%) with a lower E _loss (0.67 eV) than the ITCPTC‐based device (PCE of 12.3% with E _loss of 0.70 eV). More importantly, compared to the archetypal nonfullerene acceptors such as IT‐4F (PCE of 12.9% with E _loss of 0.73 eV) and IT‐4Cl (PCE of 12.7% with E _loss of 0.76 eV), the ITC‐2Cl‐based device shows a higher PCE and a lower E _loss. These results demonstrate that the chlorinated thiophene‐fused end group is a promising candidate for a high‐performance nonfullerene acceptors with low energy loss.