awn

Two compatible non‐fullerene acceptors with similar lowest unoccupied molecular orbital levels are finely selected to prepare efficient ternary organic solar cells (OSCs). The optimized ternary OSCs exhibit a power conversion efficiency of 15.74% and fill factor of 75.64%.

Abstract

Efficient organic solar cells (OSCs) are fabricated using polymer PM6 as donor, and IPTBO‐4Cl and MF1 as acceptors. The power conversion efficiency (PCE) of IPTBO‐4Cl based and MF1 based binary OSCs individually arrive to 14.94% and 12.07%, exhibiting markedly different short circuit current density (J _SC) of 23.18 mA cm⁻² versus 17.01 mA cm⁻², fill factor (FF) of 72.17% versus 78.18% and similar open circuit voltage (V _OC) of 0.893 V versus 0.908 V. The two acceptors, IPTBO‐4Cl and MF1, have similar lowest unoccupied molecular orbital levels, which is beneficial for efficient electron transport in the ternary active layer. The PCE of optimized ternary OSCs arrives to 15.74% by incorporating 30 wt% MF1 in acceptors, resulting from the simultaneously increased J _SC of 23.20 mA cm⁻², V _OC of 0.897 V, and FF of 75.64% in comparison with IPTBO‐4Cl based binary OSCs. The gradually increased FFs of ternary OSCs indicate the well‐optimized phase separation and molecular arrangement with MF1 as morphology regulator. This work may provide a new viewpoint for selecting an appropriate third component to achieve efficient ternary OSCs from materials and photovoltaic parameters of two binary OSCs.

A strategy is proposed to precisely control CsPbIBr₂ crystallization behaviors by incorporating sulfamic acid sodium salt (SAS), thus resulting in a high‐quality film. More importantly, SAS in perovskite possibly introduces an additional internal electric field effect that favors the electron transport and injection. Encouragingly, a higher efficiency of 10.57% is achieved with this strategy.

Abstract

Excellent power conversion efficiency (PCE) and stability are the primary forces that propel the all‐inorganic cesium‐based halide perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high density of trap state and internal nonradiative recombination of CsPbIBr₂ perovskite film are the barriers that limit its development. In the present study, a facile additive strategy is introduced to fabricate highly efficient CsPbIBr₂ PSCs by incorporating sulfamic acid sodium salt (SAS) into the perovskite layer. The additive can control the crystallization behaviors and optimize morphology, as well as effectively passivate defects in the bulk perovskite film, thereby resulting in a high‐quality perovskite. In addition, SAS in perovskite has possibly introduced an additional internal electric field effect that favors electron transport and injection due to inhomogeneous ion distribution. A champion PCE of 10.57% (steady‐output efficiency is 9.99%) is achieved under 1 Sun illumination, which surpasses that of the contrast sample by 16.84%. The modified perovskite film also exhibits improved moisture stability. The unencapsulated device maintains over 80% initial PCE after aging for 198 h in air. The results provide a suitable additive for inorganic perovskite and introduce a new conjecture to explain the function of additives in PSCs more rationally.

Metal oxides (MO) with unique optoelectronic properties and outstanding stability are increasingly developed as effective electron transport layers (ETLs) for perovskite solar cells (PSCs). This Review focuses on the recent advances of MO ETLs from systematical synthesis to strategical optimization and provides feasible directions for future development of MO ETLs in higher‐performing PSCs.

Abstract

Organometallic mixed halide perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology with increasingly improved device efficiency exceeding 24%. Charge transport layers, especially electron transport layers (ETLs), are verified to play a vital role in device performance and stability. Recently, metal oxides (MOs) have been widely studied as ETLs for high‐performance PSCs due to their excellent electronic properties, superb versatility, and great stability. This Review briefly discusses the development of PSCs' architecture and outlines the requirements for MO ETLs. Additionally, recent progress of MO ETLs from preparation to optimization for efficient PSCs is systematically summarized and highlighted to associate the versatility of MO ETLs with the performance of devices. Finally, a summary and prospectives for the future development of MO ETLs toward practical application of high‐performance PSCs are drawn.

Nature Energy, Published online: 13 April 2020; doi:10.1038/s41560-020-0598-5

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Yuanmin Zhu, Zhigang Gui, Qi Wang, Fanxu Meng, Shihui Feng, Bing Han, Peiyi Wang, Li Huang, Hsing-Lin Wang, Meng Gu

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Wei Hui, Ying Xu, Fei Xia, Hui Lu, Bixin Li, Lingfeng Chao, Tingting Niu, Bin Du, Haiyan Du, Xueqin Ran, Yingguo Yang, Yingdong Xia, Xingyu Gao, Yonghua Chen, Wei Huang

Publication date: 15 April 2020

Source: Joule, Volume 4, Issue 4

Author(s): Xiangyue Meng, Yanbo Wang, Jianbo Lin, Xiao Liu, Xin He, Julien Barbaud, Tianhao Wu, Takeshi Noda, Xudong Yang, Liyuan Han

Publication date: 20 May 2020

Source: Joule, Volume 4, Issue 5

Author(s): Wei Wang, Qiang Wu, Rui Sun, Jing Guo, Yao Wu, Mumin Shi, Wenyan Yang, Hongneng Li, Jie Min

The popular poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole transporter layer in perovskite light‐emitting diodes will cause some loss of photons and result in limited device performance. Herein, to overcome this problem, an ultrathin PEDOT:PSS is prepared, and performance is successfully improved in 3D, quasi‐3D, and quasi‐2D perovskites.

Abstract

Recently, metal halide perovskite light‐emitting diodes (Pero‐LEDs) have achieved significant improvement in device performance, especially for external quantum efficiency (EQE). And EQE is mostly determined by internal quantum efficiency of the emitting material, charge injection balancing factor (η_c), and light extraction efficiency (LEE) of the device. Herein, an ultrathin poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (UT‐PEDOT:PSS) hole transporter layer is prepared by a water stripping method, and the UT‐PEDOT:PSS can enhance η_c and LEE simultaneously in Pero‐LEDs, mostly due to the improved carrier mobility, more matched energy level alignment, and reduced photon loss. More importantly, the performance enhancement from UT‐PEDOT:PSS is quite universal and applicable in different kinds of Pero‐LEDs. As a result, the EQEs of Pero‐LEDs based on 3D, quasi‐3D, and quasi‐2D perovskites obtain enhancements of 42%, 87%, and 111%, and the corresponding maximum EQE reaches 17.6%, 15.0%, and 6.8%, respectively.

CsPbX₃ perovskite nanoplate (PNPL) synthesis was driven by SnX₄ (X=Cl, Br, I) salts. The role played by these hard Lewis acids in directing the formation of PNPLs is addressed. Sn⁴⁺ disturbs the acid–base equilibrium of the system, increasing the protonation rate of oleylamine and inducing an anisotropic growth of the nanocrystals.

Abstract

CsPbX₃ perovskite nanoplates (PNPLs) were formed in a synthesis driven by SnX₄ (X=Cl, Br, I) salts. The role played by these hard Lewis acids in directing PNPL formation is addressed. Sn⁴⁺ disturbs the acid–base equilibrium of the system, increasing the protonation rate of oleylamine and inducing anisotropic growth of nanocrystals. Sn⁴⁺ cations influence the reaction dynamics owing to complexation with oleylamine molecules. By monitoring the photoluminescence excitation and photoluminescence (PL) spectra of the PNPLs grown at different temperatures, the influence of the thickness on their optical properties is mapped. Time‐resolved and spectrally resolved PL for colloidal dispersions with different optical densities reveals that the dependence of the overall PL lifetime on the emission wavelength do not originate from energy transfer between PNPLs but from the contribution of PNPLs with distinct thickness, indicating that thicker PNPLs exhibit longer PL lifetimes.

An oxidized phenothiazine‐based (OPTZ) hole transporting material (HTM) synthesized from its neutral form (NPTZ) is used to accurately tune the concentration of radical cations in HTMs via its stoichiometric ratio. Using the optimized ratio of OPTZ as the dopant in the HTM, the hole transporting mobility is effectively enhanced, due to the intra‐and intermolecular charge transfer process, thus increasing the fill‐factor of perovskite solar cells.

In traditional n‐i‐p‐type perovskite solar cells (PSCs), most hole transporting materials (HTMs) rely on an uncontrolled oxidative process using Li salt and Co (III) complex to achieve sufficient hole mobilities. Herein, a stabilized oxidized phenothiazine‐based HTM (OPTZ) synthesized from its neutral form (NPTZ) through a photoredox reaction is demonstrated. This controllable and stable oxidation state is mainly derived from the planar structure and π conjugation of phenothiazine core in OPTZ. The energy gap between the singly occupied molecular orbital (SOMO) of OPTZ and highest occupied molecular orbital (HOMO) of NPTZ suitably promotes hole hopping in hole transporting layers. Using an optimized ratio of OPTZ as the dopant in NPTZ, the hole transporting mobility is effectively enhanced due to an intra‐ and intermolecular charge transfer process, resulting in an enhancement in the fill factor of the PSCs. Herein, a new strategy to obtain stabilized oxidized HTMs, which deliver significantly enhanced hole mobilities of HTMs in PSCs, is provided.

Combined data of transient optical and electro‐optical experiments reveals the efficiency determining processes in all‐polymer solar cells and precisely quantifies their yields. For the test system presented here, field‐dependent charge separation limits the fill factor and thus the performance evident by comparing the experimentally measured current–voltage characteristics to those reproduced by drift‐diffusion simulations using the spectroscopically determined kinetic parameters.

All‐polymer solar cells lag behind the state‐of‐the‐art in small molecule nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells (OSCs) for reasons still unclear. Herein, the efficiency‐limiting processes in all‐polymer solar cells are investigated using blends of the common donor polymer PBDT‐TS1 with different acceptor polymers, namely P2TPD[2F]T and P2TPDBT[2F]T. Combining data from steady‐state optical spectroscopy and time‐resolved photoluminescence, transient absorption, and time‐delayed collection field experiments, provides not only a concise but also quantitative assessment of the losses due to limited photon absorption, geminate and nongeminate charge carrier recombination, field‐dependent charge generation, and inefficient carrier extraction. Although both systems exhibit a similar charge separation efficiency in the absence of external bias, charge separation is significantly enhanced in P2TPDBT[2F]T‐based blends when biased. Kinetic parameters obtained via pulsed laser spectroscopy are used to reproduce the experimentally measured device current–voltage (J –V ) characteristics and indicate that low fill factors originate either from nongeminate recombination competing with charge extraction, or from a pronounced field dependence of charge generation, depending on the acceptor polymer. The methodology presented here is generic and can be used to quantify the loss processes in BHJ OSCs including both all‐polymer and small molecule NFA systems.

A narrow‐bandgap polymer acceptor PF3‐DTCO is developed by increasing the conjugation of the acceptor unit from the five‐ring‐fused IDIC16 to the seven‐ring‐fused ITIC16 and enhancing the electron‐donating ability of the donor unit from carbon‐bridged DTC to carbon–oxygen‐bridged DTCO, and a high power conversion efficiency of 10.13% with a high J _sc of 15.75 mA cm⁻² in all‐polymer solar cells is achieved.

The limited light absorption capacity for most polymer acceptors hinders the improvement of the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs). Herein, by simultaneously increasing the conjugation of the acceptor unit and enhancing the electron‐donating ability of the donor unit, a novel narrow‐bandgap polymer acceptor PF3‐DTCO based on an A–D–A‐structured acceptor unit ITIC16 and a carbon–oxygen (C–O)‐bridged donor unit DTCO is developed. The extended conjugation of the acceptor units from IDIC16 to ITIC16 results in a red‐shifted absorption spectrum and improved absorption coefficient without significant reduction of the lowest unoccupied molecular orbital energy level. Moreover, in addition to further broadening the absorption spectrum by the enhanced intramolecular charge transfer effect, the introduction of C–O bridges into the donor unit improves the absorption coefficient and electron mobility, as well as optimizes the morphology and molecular order of active layers. As a result, the PF3‐DTCO achieves a higher PCE of 10.13% with a higher short‐circuit current density (J _sc) of 15.75 mA cm⁻² in all‐PSCs compared with its original polymer acceptor PF2‐DTC (PCE = 8.95% and J _sc = 13.82 mA cm⁻²). Herein, a promising method is provided to construct high‐performance polymer acceptors with excellent optical absorption for efficient all‐PSCs.

A new strategy is established to improve the performance of perovskite solar cells, which sheds more light on the currently proposed mechanism governing the action of moisture on the quality of perovskite film. Self‐passivated perovskite solar cells show an extraordinary V_OC of 1.17 V and the highest efficiency of 21.38%.

The performance of perovskite solar cells (PSCs) is known to be extremely sensitive to humidity in the preparation environment. However, the main mechanism by which the moisture influences the quality of the perovskite film and the device performance is not yet fully understood. Herein, a new strategy is established to obtain inverted PSCs with a remarkabll high V _OC by including a high‐humidity treatment and sufficient DMSO‐atmosphere annealing in the preparation process. It is found that the lattice distortion on the surface of perovskite grains caused by the high‐humidity treatment plays a key role in the self‐passivation of perovskite. Inverted (p‐i‐n) PSCs based on the self‐passivated perovskite films show effective suppression of nonradiative recombination, which increase the device V _OC to 1.17 V and achieve the highest efficiency of 21.38%. It is expected that the findings of this work shed more light on the currently proposed mechanism governing the action of moisture on the performance of the PSCs.

Novel fullerene dimers are designed and employed as interfacial materials in perovskite solar cells, and shown to be effective at passivating and stabilizing devices with a maximum efficiency of 22.3% without any hysteresis and with 98% retained efficiency after ambient storage for 1000 h.

Abstract

A major limit for planar perovskite solar cells is the trap‐mediated hysteresis and instability, due to the defective metal oxide interface with the perovskite layer. Passivation engineering with fullerenes has been identified as an effective approach to modify this interface. The rational design of fullerene molecules with exceptional electrical properties and versatile chemical moieties for targeted defect passivation is therefore highly demanded. In this work, novel fulleropyrrolidine (NMBF‐X, XH or Cl) monomers and dimers are synthesized and incorporated between metal oxides (i.e. TiO₂, SnO₂) and perovskites (i.e. MAPbI₃ and (FAPbI₃) _x (MAPbBr₃)_{1‐ x} ). The fullerene dimers provide superior stability and efficiency improvements compared to the corresponding monomers, with chlorinated fullerene dimers being most effective at coordinating with both metal oxides and perovskite via the chlorine terminals. The non‐encapsulated planar device delivers a maximum power conversion efficiency of 22.3% without any hysteresis, while maintaining over 98% of initial efficiency after ambient storage for 1000 h, and exhibiting an order of magnitude improvement of the T₈₀ lifetime.

Herein, 17% efficient and stable ternary organic solar cells are realized by introducing a delayed fluorescence material 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐TPDA) in a non‐fullerene system. Long‐lifetime singlet excitons on APDC‐TPDA can transfer to the polymer donor to prolong the excitons lifetime and suppress the reverse energy transfer from charge transfer state to triplet state, and then reduce the recombination energy loss of the device.

Abstract

Charge transfer state (CT) plays an important role in exciton diffusion, dissociation, and charge recombination mechanisms. Enhancing the utilization and suppressing the recombination process of CT excitons is a promising way to improve the performance of organic solar cells (OSCs). Here, an effective method is presented via introducing a delayed fluorescence (DF) emitter 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐TPDA) in OSCs. The long‐lifetime singlet excitons on APDC‐TPDA can transfer to polymer donors to prolong exciton lifetime, which ensures sufficient time for diffusion and dissociation. Concurrently, the high triplet energy level (T₁) of the DF material can also prevent the reverse energy transfer from CT to T₁. APDC‐TPDA‐containing ternary OSCs shows a high PCE of 16.96% with a reduced recombination energy loss of 0.46 eV. It is noteworthy that the ternary OSC also exhibits superior storage stability. After 55 days of storage, the PCE of the ternary OSC still retains about 96% of its primitive state. Furthermore, this ternary strategy is efficient and universally applicable to OSCs, and positive results can be obtained in different systems with different DF emitters. These results indicate that the ternary strategy provides a new design idea to realize high performance OSCs.

A strong fluorine‐containing Lewis acid tris(pentafluorophenyl) phosphine (TPFP) is developed to passivate mixed perovskite solar cells, achieving a champion efficiency of 22.02% and a high stability under 85% relative humidity. The moisture degradation mechanism is phase segregation of I‐rich black phase and Cs/Br‐rich yellow phase resulting from water‐assisted synergistic Cs and halide ion migrations.

Abstract

Multiple‐cation lead mixed‐halide perovskites (MLMPs) have been recognized as ideal candidates in perovskite solar cells in terms of high efficiency and stability due to decreased open‐circuit voltage loss and suppressed yellow phase formation. However, they still suffer from an unsatisfactory long‐term moisture stability. In this study, phosphorus‐containing Lewis acid and base molecules are employed to improve device efficiency and stability based on their multifunction including recombination reduction, phase segregation suppression, and moisture resistance. The strong fluorine‐containing Lewis acid treatment can achieve a champion PCE of 22.02%. Unencapsulated and encapsulated devices retain 63% and 80% of the initial efficiency after 14 days of aging under 75% and 85% relative humidity, respectively. The better passivation of Lewis acid implies more halide defects than Pb defects at the MLMP surface. This unbalanced defect type results from phase segregation that is the synergistic effect of Cs and halide ion migrations. Identifying defect type based on different passivation effects is beneficial to not only choose suitable passivators to boost the efficiency and slow down the moisture degradation of MLMP solar cells, but also to understand the mechanism of defect‐assisted moisture degradation.

Graphitic carbon nitride (g‐C₃N₄) is doped into PEDOT:PSS to improve the conductivity by weakening the shield effect of PSS on conductive PEDOT. Employing g‐C₃N₄ doped PEDOT:PSS as a hole transport layer for PM6:Y6‐based organic solar cells, a device efficiency of up to 16.4% is achieved, partly as a result of improved charge transport and suppressed charge recombination at the interface.

Abstract

The power‐conversion efficiency (PCE) of single‐junction organic solar cells (OSCs) has exceeded 16% thanks to the development of non‐fullerene acceptor materials and morphological optimization of active layer. In addition, interfacial engineering always plays a crucial role in further improving the performance of OSCs based on a well‐established active‐layer system. Doping of graphitic carbon nitride (g‐C₃N₄) into poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer (HTL) for PM6:Y6‐based OSCs is reported, boosting the PCE to almost 16.4%. After being added into the PEDOT:PSS, the g‐C₃N₄ as a Bronsted base can be protonated, weakening the shield effect of insulating PSS on conductive PEDOT, which enables exposures of more PEDOT chains on the surface of PEDOT:PSS core‐shell structure, and thus increasing the conductivity. Therefore, at the interface between g‐C₃N₄ doped HTL and PM6:Y6 layer, the charge transport is improved and the charge recombination is suppressed, leading to the increases of fill factor and short‐circuit current density of devices. This work demonstrates that doping g‐C₃N₄ into PEDOT:PSS is an efficient strategy to increase the conductivity of HTL, resulting in higher OSC performance.

By employing HOOCCH₂NH₃ ⁺ (Gly⁺) with its outstanding additive effect, self‐additive low‐dimensional Ruddlesden–Popper perovskites are first designed. As a result, the Gly‐based self‐additive low‐dimensional RP perovskites with large grain sizes exhibit remarkable photoelectric properties, yielding the highest power conversion efficiency of 18.06% with negligible hysteresis. More importantly, Gly‐based devices exhibit markedly improved stability against humidity, heat, and UV light.

Abstract

The recent rise of low‐dimensional Ruddlesden–Popper (RP) perovskites is notable for superior humidity stability, however they suffer from low power conversion efficiency (PCE). Suitable organic spacer cations with special properties display a critical effect on the performance and stability of perovskite solar cells (PSCs). Herein, a new strategy of designing self‐additive low‐dimensional RP perovskites is first proposed by employing a glycine salt (Gly⁺) with outstanding additive effect to improve the photovoltaic performance. Due to the strong interaction between CO and Pb²⁺, the Gly⁺ can become a nucleation center and be beneficial to uniform and fast growth of the Gly‐based RP perovskites with larger grain sizes, leading to reduced grain boundary and increased carrier transport. As a result, the Gly‐based self‐additive low‐dimensional RP perovskites exhibit remarkable photoelectric properties, yielding the highest PCE of 18.06% for Gly (n = 8) devices and 15.61% for Gly (n = 4) devices with negligible hysteresis. Furthermore, the Gly‐based devices without encapsulation show excellent long‐term stability against humidity, heat, and UV light in comparison to BA‐based low‐dimensional PSCs. This approach provides a feasible design strategy of new‐type low‐dimensional RP perovskites to obtain highly efficient and stable devices for next‐generation photovoltaic applications.

The active layer morphology of all‐small‐molecule organic solar cells (SM‐OSCs) is tuned by side chain engineering of the donor molecules and thermal annealing (TA) of the devices. An SM‐OSC based on A–D–A‐structured SM1‐F with fluorine and alkyl substituents as the donor and Y6 as the acceptor, and with TA, demonstrates a high power conversion efficiency of 14.07%.

Abstract

It is very important to fine‐tune the nanoscale morphology of donor:acceptor blend active layers for improving the photovoltaic performance of all‐small‐molecule organic solar cells (SM‐OSCs). In this work, two new small molecule donor materials are synthesized with different substituents on their thiophene conjugated side chains, including SM1‐S with alkylthio and SM1‐F with fluorine and alkyl substituents, and the previously reported donor molecule SM1 with an alkyl substituent, for investigating the effect of different conjugated side chains on the molecular aggregation and the photophysical, and photovoltaic properties of the donor molecules. As a result, an SM1‐F‐based SM‐OSC with Y6 as the acceptor, and with thermal annealing (TA) at 120 °C for 10 min, demonstrates the highest power conversion efficiency value of 14.07%, which is one of the best values for SM‐OSCs reported so far. Besides, these results also reveal that different side chains of the small molecules can distinctly influence the crystallinity characteristics and aggregation features, and TA treatment can effectively fine‐tune the phase separation to form suitable donor–acceptor interpenetrating networks, which is beneficial for exciton dissociation and charge transportation, leading to highly efficient photovoltaic performance.

State‐of‐the‐art perovskite solar cells generally consist of an excess of lead iodide (PbI₂) as passivator. In this work, ligand‐modulation technology is demonstrated to fabricate vertically distributed PbI₂ nanosheets between the perovskite grain boundaries, which enhances the passivation effect of PbI₂ and improves the power conversion efficiency and stability of perovskite solar cells.

Abstract

Excess lead iodide (PbI₂), as a defect passivation material in perovskite films, contributes to the longer carrier lifetime and reduced halide vacancies for high‐efficiency perovskite solar cells. However, the random distribution of excess PbI₂ also leads to accelerated degradation of the perovskite layer. Inspired by nanocrystal synthesis, here, a universal ligand‐modulation technology is developed to modulate the shape and distribution of excess PbI₂ in perovskite films. By adding certain ligands, perovskite films with vertically distributed PbI₂ nanosheets between the grain boundaries are successfully achieved, which reduces the nonradiative recombination and trap density of the perovskite layer. Thus, the power conversion efficiency of the modulated device increases from 20% to 22% compared to the control device. In addition, benefiting from the vertical distribution of excess PbI₂ and the hydrophobic nature of the surface ligands, the modulated devices exhibit much longer stability, retaining 72% of their initial efficiency after 360 h constant illumination under maximum power point tracking measurement.