JIANG ZHI XUAN

2D perovskites are of great importance to increase both the efficiency and stability of perovskite interfaces. Motivated by the stronger halogen bond interaction, (5FBzAI)₂PbI₄ used as a capping layer in 3D/2D systems self‐organizes with an in‐plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V _oc, and remarkable operational stability.

Abstract

Despite organic/inorganic lead halide perovskite solar cells becoming one of the most promising next‐generation photovoltaic materials, instability under heat and light soaking remains unsolved. In this work, a highly hydrophobic cation, perfluorobenzylammonium iodide (5FBzAI), is designed and a 2D perovskite with reinforced intermolecular interactions is engineered, providing improved passivation at the interface that reduces charge recombination and enhances cell stability compared with benchmark 2D systems. Motivated by the strong halogen bond interaction, (5FBzAI)₂PbI₄ used as a capping layer aligns in in‐plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V _oc, a twofold improvement compared with its analogous monofluorinated phenylethylammonium iodide (PEAI) recently reported. This endows the system with high power conversion efficiency of 21.65% and extended operational stability after 1100 h of continuous illumination, outlining directions for future work.

A dual‐ion‐migration phenomenon and its underlying possible mechanism are reported for the lead‐free double perovskite Cs₂AgBiBr₆, where the diffusive behavior of both Ag and Br contribute significantly to the degradation of the perovskite thin‐film and long‐term operational stability of the Cs₂AgBiBr₆ solar cells.

Abstract

Lead‐free double perovskite Cs₂AgBiBr₆ has attracted increasing research interest in addressing the toxicity and stability challenges confronted by lead halide perovskites. While most of the studies on this Cs₂AgBiBr₆ material have been focusing on photovoltaic performance and potential applications, its long‐term stability and degradation mechanism are well under‐explored. Herein, high‐quality Cs₂AgBiBr₆ thin‐films are developed for lead‐free double perovskite solar cells with a decent efficiency of 1.91%. By exploring the ambient stability of these photovoltaic devices, it is found that the Cs₂AgBiBr₆ exhibits a unique dual‐ion‐migration phenomenon, where Ag and Br ions gradually diffuse through the hole‐transporting layer in the long‐term operation. This phenomenon leads to the degradation of the Cs₂AgBiBr₆ perovskite and subsequent device failure. Theoretical calculations indicate that low formation energies of the Ag and Br vacancies, and low diffusive energy barriers contribute to the dual‐ion‐migration effect. A possible mechanism involving a vacancy‐mediated ion‐migration is proposed to explain this phenomenon. These key findings are essential for halide double perovskites not only in providing a new knowledge base for further addressing the challenge of double perovskite stability, but also in extending their optoelectronic/electronic applications where mixed electronic, ionic and photonic properties may be desired.

The properties of trap states that limit the performance of hybrid perovskite solar cells and light‐emitting devices are still under much debate. Herein, a unified model is presented, that accurately describes trap‐related and higher‐order charge‐carrier recombination. This work reveals the importance of explicit accounting for charge‐carrier trapping, detrapping and accumulation, and disentangles radiative and nonradiative recombination channels.

Abstract

Trap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA_0.95Cs_0.05PbI_3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

By identifying the bulk polarization in the magnetic field under photoexcitation conditions, the existence of photoinduced bulk polarization is verified by removing the interferences of photoinduced mobile ions. In addition, the linear/circular polarization modulated photocurrent (ΔJ _sc) measurements indicate that the orbit–orbit interaction between excitons in hybrid perovskites can be tuned by bulk polarization through the dipole moment of organic cations.

Abstract

Photoinduced polarization and orbit–orbit interaction are important issues in hybrid perovskites toward developing optoelectronic functionalities. This paper identifies that photoinduced polarization occurs in hybrid perovskites with mixed‐cation methylammonium (MA)/formamidinium (FA) (MA _x FA_{(1− x )}PbI₃) by measuring bulk polarization at 1 MHz in a magnetic field. Interestingly, when the internal dipole moment is increased upon increasing the MA:FA ratio, the photoinduced dipolar polarization can be substantially enhanced, clarifying the controversial issue of whether photoexcitation can induce a dielectric polarization within dipolar polarization regime in hybrid perovskites. Furthermore, upon increasing photoinduced dipolar polarization, it is found that the intrinsic orbit–orbit interaction between excitons can be increased, revealed by monitoring photocurrent change (ΔJ _sc) upon switching the photoexcitation between linear and circular polarizations. This presents that organic cations are directly involved in the orbit–orbit interaction within band structures. Clearly, the studies provide an insightful understanding of the dipole moment effects on photoinduced dipolar polarization and orbit–orbit interaction between excitons in hybrid perovskites toward controlling the optoelectronic properties.

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.

The desired α‐FAPbI₃ perovskite phase is stabilized by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium) overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇‐based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. It showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress.

Abstract

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α‐FAPbI₃ more suitable for solar‐cell applications than methylammonium lead iodide (MAPbI₃). However, its spontaneous conversion into the yellow non‐perovskite phase (δ‐FAPbI₃) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α‐FAPbI₃ perovskite phase by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.

Ion migration plays critical roles in the functionalities of metal halide perovskites. Herein, using newly developed time‐resolved time‐of‐flight secondary ion mass spectrometry, hysteretic CH₃NH₃ ⁺ and I⁻ migrations in CH₃NH₃PbI₃ are observed, where CH₃NH₃ ⁺ migration hysteresis is illumination‐dependent. The ion redistribution in CH₃NH₃PbI₃ can lead to a permanent spontaneous current after the application of electric bias.

Abstract

The gap in understanding how underlying chemical dynamics impact the functionality of metal halide perovskites (MHPs) leads to the controversy about the origin of many phenomena associated with ion migration in MHPs. In particular, the debate regarding the impact of ion migration on current–voltage (I–V) hysteresis of MHPs devices has lasted for many years, where the difficulty lies in directly uncovering the chemical dynamics, as well as identifying and separating the impact of specific ions. In this work, using a newly developed time‐resolved time‐of‐flight secondary ion mass spectrometry CH₃NH₃ ⁺ and I⁻ migrations in CH₃NH₃PbI₃ are directly observed, revealing hysteretic CH₃NH₃ ⁺ and I⁻ migrations. Additionally, hysteretic CH₃NH₃ ⁺ migration is illumination‐dependent. Correlating these results with the I–V characterization, this work uncovers that CH₃NH₃ ⁺ redistribution can induce a remanent field leading to a spontaneous current in the device. It unveils that the CH₃NH₃ ⁺ migration is responsible for the illumination‐associated I–V hysteresis in MHPs. Hysteretic ion migration has not been uncovered and the contribution of any ions (e.g., CH₃NH₃ ⁺) has not been specified before. Such insightful and detailed information has up to now been missing, which is critical to improving MHPs photovoltaic performance and developing MHPs‐based memristors and synaptic devices.

The influence of solvent additives on operation‐induced changes of the morphology in the active layer of organic solar cells is studied in operando. High boiling point solvent additives introduce an interpenetrating network of donor and acceptor in the films and change the device degradation mechanism from a short‐circuit current dominated one to a open‐circuit voltage dominated degradation.

Abstract

Solvent additives are known to modify the morphology of bulk heterojunction active layers to achieve high efficiency organic solar cells. However, the knowledge about the influence of solvent additives on the morphology degradation is limited. Hence, in operando grazing‐incidence small and wide angle X‐ray scattering (GISAXS and GIWAXS) measurements are applied on a series of PffBT4T‐2OD:PC₇₁BM‐based solar cells prepared without and with solvent additives. The solar cells fabricated without a solvent additive, with 1,8‐diiodoctane (DIO), and with o‐chlorobenzaldehyde (CBA) additive show differences in the device degradation and changes in the morphology and crystallinity of the active layers. The mesoscale morphology changes are correlated with the decay of the short‐circuit current J _sc and the evolution of crystalline grain sizes is codependent with the decay of open‐circuit voltage V _oc. Without additive, the loss in J _sc dominates the degradation, whereas with solvent additive (DIO and CBA) the loss in V _oc rules the degradation. CBA addition increases the overall device stability as compared to DIO or absence of additive.

Publication date: 16 September 2020

Source: Joule, Volume 4, Issue 9

Author(s): Sheng Dong, Tao Jia, Kai Zhang, Jianhua Jing, Fei Huang

The properties of trap states that limit the performance of hybrid perovskite solar cells and light‐emitting devices are still under much debate. Herein, a unified model is presented, that accurately describes trap‐related and higher‐order charge‐carrier recombination. This work reveals the importance of explicit accounting for charge‐carrier trapping, detrapping and accumulation, and disentangles radiative and nonradiative recombination channels.

Abstract

Trap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA_0.95Cs_0.05PbI_3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

All‐inorganic hole‐transport layer (HTL)‐free CsPbBr₃‐based perovskite solar cells doped with Eu²⁺ are studied. The decrement in trap‐state density and suppression of nonradiative recombination after doping is achieved with a higher power conversion efficiency (PCE) of 7.28% and V _OC of 1.45 V.

All‐inorganic perovskite of CsPbBr₃ thin‐films solar cells has attracted increasing interest in recent years due to its potential long‐term stability over the generally used hybrid perovskites. Herein, all‐inorganic CsPbBr₃ perovskites are doped with Eu²⁺ to enhance the efficiency of perovskite solar cells (PVSCs). The perovskite films exhibit a better crystallinity with smooth morphology after the introduction of rare‐earth elements. Hence, the hole‐transport layer‐free device with presence of Eu²⁺ and low‐cost carbon electrode achieves both enhanced efficiency and stability. In particular, the power conversion efficiency (PCE) enhances from 5.66% to 7.28% with high V _OC of 1.45 V by optimizing the doping concentration of Eu²⁺. In addition, the storage stability measurements reveal excellent performances of PCE without encapsulation in air with relative humidity of 70–80%. These results can pave changes in future inorganic PVSCs.

Ultralong charge‐carrier lifetimes >6 μs are achieved in polycrystalline halide perovskites by decorating the grain boundaries with a trace amount of electron‐rich anchors, which benefits from weak excitonic effects and the weakening of electron–phonon couplings in passivated films, fulfilling reduced voltage deficits and enhanced efficiencies in perovskite photovoltaics. This finding provides a new insight into realizing superior carrier properties of polycrystalline perovskite films and high‐performance perovskite optoelectronics.

Abstract

Lead halide perovskite films have witnessed rapid progress in optoelectronic devices, whereas polycrystalline heterogeneities and serious native defects in films are still responsible for undesired recombination pathways, causing insufficient utilization of photon‐generated charge carriers. Here, radiation‐enhanced polycrystalline perovskite films with ultralong carrier lifetimes exceeding 6 μs and single‐crystal‐like electron–hole diffusion lengths of more than 5 μm are achieved. Prolongation of charge‐carrier activities is attributed to the electronic structure regulation and the defect elimination at crystal boundaries in the perovskite with the introduction of phenylmethylammonium iodide. The introduced electron‐rich anchor molecules around the host crystals prefer to fill the halide/organic vacancies at the boundaries, rather than form low‐dimensional phases or be inserted into the original lattice. The weakening of the electron‐phonon coupling and the excitonic features of the photogenerated carriers in the optimized films, which together contribute to the enhancement of carrier separation and transportation, are further confirmed. Finally the resultant perovskite films in fully operating solar cells with champion efficiency of 23.32% are validated and a minimum voltage deficit of 0.39 V is realized.

Nature Materials, Published online: 24 August 2020; doi:10.1038/s41563-020-0774-9

The non‐fullerene acceptor small molecule (5Z,5′Z)‐5,5′‐((7,7′‐(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(benzo[c]1,2,5]thiadiazole‐7,4‐diyl))‐bis(methanylyl‐idene))bis(3‐ethyl‐2‐thioxothiazolidin‐4‐one) (FBR) is blended with the poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene)‐co‐(1,3‐di(5‐thiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PBDB‐T): 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) binary system to form the ITIC:FBR alloys, which simultaneously improves the performance and stability of solar cells.

Nanoscale morphology of the active layer plays a crucial role in the power conversion efficiency (PCE) and stability of polymer solar cells (PSCs). Blending the photoactive layer with a third component to produce a ternary system is considered a reliable approach to tune the nanomorphology, thereby improving the device performance. Herein, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene)‐co‐(1,3‐di(5‐thiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PBDB‐T): 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) solar cells doped with a third small molecule are systematically investigated, namely, (5Z,5′Z)‐5,5′‐((7,7′‐(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(benzo[c]1,2,5]thiadiazole‐7,4‐diyl))‐bis(methanylyl‐idene))bis(3‐ethyl‐2‐thioxothiazolidin‐4‐one) (FBR). Owing to the wide optical bandgap of FBR, blending PBDB‐T:ITIC with FBR increases the device's light‐harvesting capability in the short wavelength range (400–550 nm), which improves the short circuit current. Differential scanning calorimetry and grazing incidence wide angle X‐ray scattering analyses reveal that the FBR exhibits impressive miscibility with ITIC, leading to the formation of ITIC:FBR alloys. Optimum performance is achieved with a PBDB‐T:ITIC:FBR (1:0.8:0.2) cell, which yields a PCE of 11.17%, demonstrating a 10% improvement relative to the PBDB‐T:ITIC binary cell. Crucially, the ternary solar cells also show improved device stability, which is attributed to the formation of ITIC:FBR alloys suppressing the crystallization of ITIC. This study provides deep insights into the performance‐ and stability‐related improvements available to PSCs devices that incorporate a third conjugated small molecule.

All‐inorganic hole‐transport layer (HTL)‐free CsPbBr₃‐based perovskite solar cells doped with Eu²⁺ are studied. The decrement in trap‐state density and suppression of nonradiative recombination after doping is achieved with a higher power conversion efficiency (PCE) of 7.28% and V _OC of 1.45 V.

All‐inorganic perovskite of CsPbBr₃ thin‐films solar cells has attracted increasing interest in recent years due to its potential long‐term stability over the generally used hybrid perovskites. Herein, all‐inorganic CsPbBr₃ perovskites are doped with Eu²⁺ to enhance the efficiency of perovskite solar cells (PVSCs). The perovskite films exhibit a better crystallinity with smooth morphology after the introduction of rare‐earth elements. Hence, the hole‐transport layer‐free device with presence of Eu²⁺ and low‐cost carbon electrode achieves both enhanced efficiency and stability. In particular, the power conversion efficiency (PCE) enhances from 5.66% to 7.28% with high V _OC of 1.45 V by optimizing the doping concentration of Eu²⁺. In addition, the storage stability measurements reveal excellent performances of PCE without encapsulation in air with relative humidity of 70–80%. These results can pave changes in future inorganic PVSCs.

Ion migration plays critical roles in the functionalities of metal halide perovskites. Herein, using newly developed time‐resolved time‐of‐flight secondary ion mass spectrometry, hysteretic CH₃NH₃ ⁺ and I⁻ migrations in CH₃NH₃PbI₃ are observed, where CH₃NH₃ ⁺ migration hysteresis is illumination‐dependent. The ion redistribution in CH₃NH₃PbI₃ can lead to a permanent spontaneous current after the application of electric bias.

Abstract

The gap in understanding how underlying chemical dynamics impact the functionality of metal halide perovskites (MHPs) leads to the controversy about the origin of many phenomena associated with ion migration in MHPs. In particular, the debate regarding the impact of ion migration on current–voltage (I–V) hysteresis of MHPs devices has lasted for many years, where the difficulty lies in directly uncovering the chemical dynamics, as well as identifying and separating the impact of specific ions. In this work, using a newly developed time‐resolved time‐of‐flight secondary ion mass spectrometry CH₃NH₃ ⁺ and I⁻ migrations in CH₃NH₃PbI₃ are directly observed, revealing hysteretic CH₃NH₃ ⁺ and I⁻ migrations. Additionally, hysteretic CH₃NH₃ ⁺ migration is illumination‐dependent. Correlating these results with the I–V characterization, this work uncovers that CH₃NH₃ ⁺ redistribution can induce a remanent field leading to a spontaneous current in the device. It unveils that the CH₃NH₃ ⁺ migration is responsible for the illumination‐associated I–V hysteresis in MHPs. Hysteretic ion migration has not been uncovered and the contribution of any ions (e.g., CH₃NH₃ ⁺) has not been specified before. Such insightful and detailed information has up to now been missing, which is critical to improving MHPs photovoltaic performance and developing MHPs‐based memristors and synaptic devices.