Chen Weijie

A methylammonium chloride (MACl) additive is used to synthesize FA_1–x MA _x PbI₃ films. The best molar fraction of this additive is determined. The MA content in thin films actually used in solar cells is x = 0.06. This amount is thermodynamically the best for the stabilization of this highly efficient perovskite. The perovskite solar cell achieves a stabilized power conversion efficiency as high as 22.06%.

Nowadays, complex chemistry and precursor solution compositions are developed to stabilize hybrid perovskite films and boost the efficiency of perovskite solar cells (PSCs). In this context, determining the actual composition of these layers, especially in organic cations, and understanding the chemistry behind is challenging. Herein, the introduction of methylammonium (MA⁺) in formamidinium lead iodide (FAPbI₃) 3D perovskite is considered to stabilize the α‐phase, whose quantity must be minimized to reduce the material hydrophilicity and its possible destabilization by degassing. The key effects of methylammonium chloride (MACl) additive on the growth of FA_1–x MA _x PbI₃ perovskite layers are studied. Liquid nuclear magnetic resonance (NMR) is used to analyze the photovoltaic layers. NMR peaks and their origin are identified. The MA and FA content in films actually used in PSCs is reliably measured and prepared over a large additive molar concentration ratio. x is quantified at 0.06 ± 0.01 for pure films, which corresponds to the best entropic compound stabilization. It results in PSCs with a stabilized power conversion efficiency as high as 22.06%. These PSCs are shown to be highly stable under solar irradiation and high moisture.

The drastic open‐circuit voltage drop at low temperatures in bulk heterojunction and bilayer organic solar cells is found to be dominated by the competition between the photocurrent and the parasitic leakage current. The leakage current should thus be carefully optimized in temperature dependent analysis as well as in practical applications.

While the efficiency of organic solar cells (OSCs) has increased considerably in recent years, there remains a significant gap between the experimental open‐circuit voltage (V _OC) and the theoretical limit. Understanding the origin of this energy loss is important for the future development of OSCs, with temperature‐dependent measurement of V _OC an important approach to help unlock the underlying physics. Interestingly, previous studies have observed a reduction in V _OC at low temperature that has been attributed by different studies to different phenomena. To resolve this issue, herein the temperature dependence of V _OC of various polymer‐based OSC systems covering a range of acceptor types (fullerene, polymer, and non‐fullerene small molecule) as well as device architectures (conventional, inverted, blend and bilayer) is studied. Across all systems studied, V _OC reduction at low temperatures is associated with high parasitic leakage current, providing a universal explanation for this phenomenon in OSCs. Moreover, it is shown that leakage current, which causes complexity in the analysis and raises reliability concerns in potential applications, can be suppressed by varying device architecture, providing an effective approach for analyzing the true temperature dependence of V _OC.

MACl additive is used to suppress the excessive solvate phase during the crystal nucleation and growth of Dion–Jacobson (DJ) perovskites and crystals with vertical orientation and increased crystallinity are obtained. A high power conversion efficiency of 15.6% for PXD(MA)₂Pb₃I₁₀ perovskite solar cells is achieved. The DJ perovskites exhibit suppressed ion migration and enhanced humidity and illumination stability.

Dion–Jacobson (DJ)‐type quasi‐two‐dimensional perovskites exhibit improved stabilities than their 3D counterparts but meanwhile limited charge transport properties. Knowledge to manipulate the crystal orientation and crystallinity is the primary issue for DJ perovskite with high power conversion efficiencies (PCEs). Herein, the nucleation of DJ perovskite films is divided into three stages and the formation of PbI₂–N,N‐dimethylformamide (DMF)‐based solvated phase (PDS) is highlighted as the initial stage. For the first time, it is demonstrated that regulating the amount of PDS precipitation in stage I by MACl additive is the key to ensure the downward growth of DJ perovskites with out‐of‐plane orientation and high crystallinity in stage III, which is valid for DJ perovskites with different bukly organic cations including p‐phenylenediamine (PPD), p‐xylylenediamine (PXD), and propane‐1,3‐diammonium (PDA). For (PXD)(MA)₂Pb₃I₁₀‐based perovskite solar cells, the PDS engineering lead to a dramtically improved PCE from 1.2% to 15.6%. Moreover, based on temperature‐dependent ionic conductivity measurement, it is confirmed that the ion migration in DJ perovskite films is efficiently suppressed, despite the possible coexisting 3D perovskite phase. The unencapsulated PXD‐based DJ perovskite devices retain over 90% efficiencies after 700 h of continuous illumination or 1500 h of storage in glove box.

The A‐D‐A′‐D‐A strategy is applied to develop two new Y6‐type unfused‐ring acceptors. The resulting fluorinated unfused acceptors lock more planar conformation, thus exhibiting red‐shifted absorption and better aggregation properties, leading to high device efficiencies of over 12%.

Unfused‐ring acceptors (UFAs) have gained considerable research attention as they offer simple chemical structures through simplified synthesis methods, which would boost the commercialization of organic solar cells (OSCs). Recently, a new small molecule acceptor (SMA) named Y6 was reported, yielding high‐performance OSCs. Herein, the Y6‐like A‐DA′D‐A framework is developed to A‐D‐A′‐D‐A‐type backbone adopted in constructing UFAs. Two new Y6‐like UFAs are synthesized within four steps and the effect of noncovalent atoms at the central electron‐deficient core on material properties and device performances is studied. It is found that the introduction of fluorine atoms can bring larger red‐shift in the absorption spectra and better aggregation of the resulting UFA film states compared with those of oxygen atoms. Interestingly, the variations in the noncovalent interaction atoms induce different intermolecular charge transfer between donors and UFAs. When blended with another economical donor, PTQ10, F substitution at the benzothiadiazole ring is more effective than O substitution, leading to the increased short‐circuit current density (J _SC) and higher efficiency of over 12%, among the best performances of UFA‐based OSCs. This contribution demonstrates the appropriate introduction of noncovalent interaction is a promising method for tuning energy levels, absorption, and aggregation of UFAs for high‐performance OSCs.

The optimized ratio of chlorinated organic salt benzyltriethylammonium chloride ([BZTAm]Cl) is helpful for the interfacial defect passivation at the perovskite/PC₆₁BM interface. The corresponding perovskite film treatment produces high‐quality film, suppresses nonradiative recombination, and promotes the energy levels matching, which results in remarkably improved device performance and environmental stability.

In perovskite solar cells (PSCs), the interfaces between perovskite film and charge transport layers have an enormous influence on the device performance and stability. Recently, it has been proven that the surface defect passivation of perovskite layer is an effective strategy to improve the device efficiency. Herein, an organic ammonium salt benzyltriethylammonium chloride ([BZTAm]Cl) is used as an ultra‐thin modification layer in perovskite films in MAPbI₃ PSCs for passivating the surface defects. The obtained results demonstrate that the [BZTAm]Cl modifier improves the crystallization/morphology of perovskite film and effectively aligns the energy levels with the corresponding charge‐transporting layers, suppressing the nonradiative recombination and reducing the trap state density. As a result, a champion device efficiency of 20.45% is achieved for optimized concentration of [BZTAm]Cl in comparison with 17.87% for the control device. Moreover, the unencapsulated device presents a good long‐term stability after aging in an ambient environment with 40–50% relative humidity conditions for 30 days.

Efficient triple‐junction tandem PSCs are achieved by optimizing the middle subcell thickness (t _M) in the presence of controlled hybrid interconnection layers between each subcell. The high FF (74.3%) and V _OC (2.14 V), leading to 13% PCE, can be realized at t _M = 60 nm due to the harmony between the physical leakage and light‐harvesting characteristics.

Making multijunctions in organic solar cells with solution‐processed polymeric bulk heterojunction (BHJ) layers, i.e., tandem polymer solar cells (PSCs), has been one of the state‐of‐the‐art approaches for the last two decades. Tandem PSCs can overcome the single‐junction Shockley–Queisser limit by improving light absorption as they can exploit the polymeric BHJ layers with poor charge carrier mobility as subcells with limited thickness. Herein, 13% efficient triple‐junction tandem PSCs with a nanocrated hybrid interconnection layer (hICL) can be achieved by controlling the BHJ thickness (60 nm) of middle subcells is demonstrated. The open‐circuit voltage and fill factor (FF) of the optimized triple‐junction tandem PSCs reach 2.14 V and 74.3%, respectively. The present approach of middle subcell thickness control, using the reproducible hICL‐based multilayer stacking technology, delivers a promising way to further extend the number of junctions leading to high efficiency tandem PSCs with enhanced open‐circuit voltages and FFs.

Different concentrations of ester‐substituted thiophene are introduced into the conjugated backbone of the polymer acceptor to rationally adjust the aggregation behaviors, absorption properties, and energy levels, and finally improve the photovoltaic performance of the PYEx‐based all‐polymer solar cells. Among them, blends of PYE2 with polymer donor PBDB‐T are achieved with a maximum power conversion efficiency (PCE) of 13.57%.

Finding effective molecular design strategies and fine tuning the molar ratios of donor/acceptor (D/A) random copolymers to optimize the blend microstructure of the photoactive layer is one of the main long‐standing challenges in developing and fabricating highly efficient all‐polymer solar cells (all‐PSCs). Herein, a random ternary copolymerization strategy to develop four random copolymer acceptors PYEx (x = 10, 20, 30, 40) is used by polymerizing a fused‐ring A–D–A‐type acceptor unit modified from Y5 with a thiophene‐connecting unit and a controlled amount of an ester‐substituted thiophene (EST) unit. Compared with PYT (PYE0) of only Y5‐like units and thiophene units, the ternary copolymers PYEx show slightly down‐shifted lowest unoccupied molecular orbital (LUMO) energy levels, reduced absorption coefficients, and decreased electron mobilities. However, it is also demonstrated that this design approach rationally modifies the molecular aggregations of polymer acceptors, effectively fine tuning the blend morphology and physical mechanisms, and enhances the device performance of the PYEx‐based all‐PSCs. Among them, blends of PYE20 with donor polymer PBDB‐T combine 13.6% power conversion efficiency (PCE). Of particular note is that all of the PYEx‐based devices exhibit the best PCEs of over 13%, indicating the high tolerance on molar ratios.

The chemical composition modulation and electric field‐induced ion migration of organic‐inorganic hybrid perovskites are utilized to fabricate performance‐enhanced triboelectric nanogenerators (TENGs). The chemical composition modulation induced conductive type conversion and electric field‐induced self‐doping on the surfaces enable controlled performance of the TENGs.

Abstract

In this paper, new strategies are proposed to design high‐performance organic–inorganic hybrid perovskite (PVK)‐based triboelectric nanogenerators (TENGs) via both chemical composition modulation and electric field‐induced ion migration in the films. Both composition variation and ion migration under electric field are found to change the type of conductivity of the perovskite films, then modify their surface potentials and electron affinities. These are utilized to fabricate PVK‐based TENGs in pairs with poly‐tetrafluoroethylene (PTFE) or nylon films, respectively. Results show that PVK films are able to work as either a positive or a negative tribo‐material depending on the tribo‐material pair used; the optimal performances are obtained for PTFE/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 2 and forward polarization, and for nylon/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 0.4 and reverse polarization, respectively. The maximum output voltage and peak power density of PTFE/PVK TENGs are about 979 V and 24 W m⁻², 2.5 and 6.5 times higher than those of TENGs with nonoptimal composition ratio or that are poorly polarized. This work provides a new material design method for high‐performance TENGs and a novel polarization strategy for TENG performance enhancement.

Field‐assisted charge generation upon illumination of neat SubNc sandwiched between charge selective electrodes is shown to be efficient, and results in low voltage losses with high quantum efficiencies. The described effects can play an important role in the current state‐of‐the‐art, high efficiency organic solar cells with low driving force for charge generation.

Abstract

Efficient charge generation in organic semiconductors usually requires an interface with an energetic gradient between an electron donor and an electron acceptor in order to dissociate the photogenerated excitons. However, single‐component organic solar cells based on chloroboron subnaphthalocyanine (SubNc) have been reported to provide considerable photocurrents despite the absence of an energy gradient at the interface with an acceptor. In this work, it is shown that this is not due to direct free carrier generation upon illumination of SubNc, but due to a field‐assisted exciton dissociation mechanism specific to the device configuration. Subsequently, the implications of this effect in bilayer organic solar cells with SubNc as the donor are demonstrated, showing that the external and internal quantum efficiencies in such cells are independent of the donor‐acceptor interface energetics. This previously unexplored mechanism results in efficient photocurrent generation even though the driving force is minimized and the open‐circuit voltage is maximized.

Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.

Abstract

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V _oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V _oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.

A straightforward polyTPD passivation is introduced to reduce the defect‐mediated recombination by elucidating the imperfections on the surface and grain boundaries of perovskite materials. Suppressed non‐radiative recombination and improved interfacial hole extraction result in perovskite solar cells with stabilized efficiency exceeding 21%. Moreover, ultra‐hydrophobic and thermally robust polyTPD passivated devices retain 94% of the initial efficiency after 800 h under operational conditions.

Abstract

The failure of perovskite solar cells (PSCs) to maintain their maximum efficiency over a prolonged time is due to the deterioration of the light harvesting material under environmental factors such as humidity, heat, and light. Systematically elucidating and eliminating such degradation pathways are critical to imminent commercial use of this technology. Here, a straightforward approach is introduced to reduce the level of defect‐states present at the perovskite and hole transporting layer interface by treating the various perovskite surfaces with poly(N,N′‐bis‐4‐butylphenyl‐N,N′‐bisphenyl)benzidine (polyTPD) molecules. This strategy significantly suppresses the defect‐mediated non‐radiative recombination in the ensuing devices and prevents the penetration of degrading agents into the inner layers by passivating the perovskite surface and grain boundaries. Suppressed non‐radiative recombination and improved interfacial hole extraction result in PSCs with stabilized efficiency exceeding 21% with negligible hysteresis (≈19.1% for control device). Moreover, ultra‐hydrophobic polyTPD passivant considerably alleviates moisture penetration, showing ≈91% retention of initial efficiencies after 300 h storage at high relative humidity of 80%. Similarly, passivated device retains 94% of its initial efficiency after 800 h under operational conditions (maximum power point tracking under continuous illumination at 60 °C). In addition to interfacial passivation function, hole‐selective role of dopant‐free polyTPD is also evaluated and discussed in this study.

Donor‐acceptor (D‐A) polymers are promising materials for organic electrochemical transistors (OECTs), as they minimize detrimental faradaic side‐reactions during OECT operation, yet their steady‐state OECT performance still lags far behind their all‐donor counterparts. Here, we report three D‐A polymers based on the diketopyrrolopyrrole unit that afford OECT performances similar to those of all‐donor polymers, hence representing a significant improvement to the previously developed D‐A copolymers. In addition to improved OECT performance, DFT simulations of the polymers and their respective hole polarons also revealed a positive correlation between hole polaron delocalization and steady‐state OECT performance, providing new insights into the design of OECT materials. More importantly, we demonstrate how polaron delocalization can be tuned directly at the molecular level by selection of the building blocks comprising the polymers’ conjugated backbone, thus paving the way for the development of even higher performing OECT polymers.

Two fully non‐fused acceptors are precisely designed and easily prepared. The side chain encapsulation can induce a planar molecular backbone conformation, endowing the acceptor with broad light absorption. Thermal annealing promotes molecular rearrangement to form J‐aggregates with even broader absorption and higher absorption coefficient. A PCE over 10 % is one of the highest PCE for fully non‐fused ring acceptors.

Abstract

Fused‐ring electron acceptors have made significant progress in recent years, while the development of fully non‐fused ring acceptors has been unsatisfactory. Here, two fully non‐fused ring acceptors, o‐4TBC‐2F and m‐4TBC‐2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o‐4TBC‐2F formed planar backbones, while m‐4TBC‐2F displayed a twisted backbone. Additionally, the o‐4TBC‐2F film showed a markedly red‐shifted absorption after thermal annealing, which indicated the formation of J‐aggregates. For fabrication of organic solar cells (OSCs), PBDB‐T was used as a donor and blended with the two acceptors. The o‐4TBC‐2F‐based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o‐4TBC‐2F gave a PCE of 10.26 %, which was much higher than those based on m‐4TBC‐2F at 2.63 %, and it is one of the highest reported PCE values for fully non‐fused ring electron acceptors.

Comprehensive high‐pressure experiments show that the structural and optoelectronic properties of methylammonium lead iodide perovskite (MAPbI₃) will be drastically improved when hydrogen is replaced with deuterium in organic cation. The improved lattice stability boosts the photoluminescence intensity of MAPbI₃ by threefold. The pressure‐treated CD₃ND₃PbI₃ exhibits a nearly reversible emission properties, demonstrating its superior mechanical robustness. CD₃ND₃PbI₃‐based device also exhibits slower degradation of photovoltaic performance.

Abstract

The soft nature of organic–inorganic halide perovskites renders their lattice particularly tunable to external stimuli such as pressure, undoubtedly offering an effective way to modify their structure for extraordinary optoelectronic properties. Here, using the methylammonium lead iodide as a representative exploratory platform, it is observed that the pressure‐driven lattice disorder can be significantly suppressed via hydrogen isotope effect, which is crucial for better optical and mechanical properties previously unattainable. By a comprehensive in situ neutron/synchrotron‐based analysis and optical characterizations, a remarkable photoluminescence (PL) enhancement by threefold is convinced in deuterated CD₃ND₃PbI₃, which also shows much greater structural robustness with retainable PL after high peak‐pressure compression–decompression cycle. With the first‐principles calculations, an atomic level understanding of the strong correlation among the organic sublattice and lead iodide octahedral framework and structural photonics is proposed, where the less dynamic CD₃ND₃ ⁺ cations are vital to maintain the long‐range crystalline order through steric and Coulombic interactions. These results also show that CD₃ND₃PbI₃‐based solar cell has comparable photovoltaic performance as CH₃NH₃PbI₃‐based device but exhibits considerably slower degradation behavior, thus representing a paradigm by suggesting isotope‐functionalized perovskite materials for better materials‐by‐design and more stable photovoltaic application.

Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.

Abstract

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V _oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V _oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.

Vacancy‐ordered double perovskites, A ₂RuCl₆ and A ₂RuBr₆, were studied. The synthesis, crystal structures, and optical and magnetic properties of the materials were determined. Differences in the halide and the A cation size influence the charge‐transfer spectra. On the basis of temperature‐dependent magnetic moments, accurate values were estimated for the spin‐orbit coupling constants of the Ru^IV d ⁴ ion.

Abstract

Vacancy‐ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of Ru^IV halides are presented; A ₂RuCl₆ and A ₂RuBr₆, where A is K, NH₄, Rb or Cs. We show that the optical properties and spin‐orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand‐to‐metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non‐magnetic ground state with J _eff=0 caused by SOC. Ru‐X covalency, and consequently, the delocalization of metal d‐electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion.