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Based on a “TADF–Linker–Host” strategy, a series of polyimide (PI)‐based thermally activated delayed fluorescence (TADF) polymers with excellent thermal stability (T _g>308.7 °C, T _d>519.5 °C) and refractive index (1.76–1.79) are reported. Through device optimization, a current efficiency of 60.2 cd A⁻¹ was realized.

Abstract

A series of rigid nonconjugated polyimide (PI)‐based thermally activated delayed fluorescence (TADF) polymers were reported for the first time, based on a “TADF‐Linker‐Host” strategy. Among of which, the TADF unit contains a typical TADF luminous core structure, the “Host” unit exhibits effective conjugation length that endows polyimide with high triplet energy, and the “Linker” unit has an aliphatic ring structure to improve solubility and inhibits intramolecular charge transfer effect. All the TADF polymers exhibit high thermal stability (T _g>308.7 °C) and refractive index (1.76–1.79). Remarkably, highly‐efficient polymer light‐emitting diodes (PLEDs) based on the polymers are successfully realized, leading to a maximal external quantum efficiency of 21.0 % along with low efficiency roll‐off. Such outstanding efficiency is amongst the state‐of‐the‐art performance of nonconjugated PLEDs, confirming the effectiveness of structural design strategy, providing helpful and valuable guidance on the development of highly‐efficient fluorescent polymer materials and PLEDs.

To enable highly efficient and stable CsFA perovskite solar cells, a joint experimental–computational study is conducted. It is shown that by treating the perovskite with an n‐type molecular dopant, the band bending is increased, shaping the electric field across the active layer to overcome limited diffusive transport. Using this strategy, CsFA solar cell devices are fabricated with stabilized power conversion efficiencies of 20.3%.

Abstract

Perovskites with the multi‐cation composition of cesium (Cs), methylammonium (MA), and formamidinium (FA) (CsMAFA) are pursued for their high power conversion efficiencies, but they are limited by their thermal stability. To withstand damp‐heat accelerated aging MA‐free compositions such as CsFA are of interest, but these exhibit lower carrier diffusion lengths and thus lesser performance in photovoltaic devices. A band engineering strategy that overcomes limited carrier diffusion within inverted perovskite solar cells based on CsFA is reported. A joint experimental‐computational study shows that treating the perovskite with an n‐type molecular dopant increases band bending, shaping the electric field across the active layer to overcome limited diffusive transport. Using this strategy, CsFA solar cell devices with stabilized power conversion efficiencies of 20.3%, a high value for devices using CsFA active layers, are fabricated.

In article number 2009131, Gang Liu and co‐workers conduct high‐pressure isotope research to discover a significantly suppressed lattice disorder realized by H/D substitution in hybrid halide perovskites, which reveals large emission enhancement and strong structural robustness in isotope‐functionalized perovskite materials. The CD₃ND₃PbI₃‐based device also exhibits slower degradation of photovoltaic performance, which is promising for better materials‐by‐design and more stable photovoltaic applications.

3D TiO₂ nanotube arrays decorated using ultrafine silver nanocrystals are demonstrated as a confined space host for lithium metal deposition. Li metal anode deposited on such architectures delivers a high Coulomb efficiency at around 99.4% even after 300 cycles, ultralow overpotential of 4 mV, and long‐term cycling life over 2500 h in Li symmetric cells.

Abstract

3D scaffolds and heterogeneous seeds are two effective ways to guide Li deposition and suppress Li dendrite growth. Herein, 3D TiO₂ nanotube (TNT) arrays decorated using ultrafine silver nanocrystals (7–10 nm) through cathodic reduction deposition are first demonstrated as a confined space host for lithium metal deposition. First, TiO₂ possesses intrinsic lithium affinity with large Li absorption energy, which facilitates Li capture. Then, ultrafine silver nanocrystals decoration allows the uniform and selective nucleation in nanoscale without a nucleation barrier, leading to the extraordinary formation of lithium metal importing into 3D nanotube arrays. As a result, Li metal anode deposited on such a binary architecture (TNT‐Ag‐Li) delivers a high Coulomb efficiency at around 99.4% even after 300 cycles with a capacity of 2 mA h cm⁻². Remarkably, TNT‐Ag‐Li exhibits ultralow overpotential of 4 mV and long‐term cycling life over 2500 h with a capacity of 2 mAh cm^–2 in Li symmetric cells. Moreover, the full battery with 3D spaced Li nanotubes anode and LiFeO₄ cathode exhibits a stable and high capacity of 115 mA h g^–1 at 5 C and an excellent Coulombic efficiency of ≈100% over 500 cycles.

Two acceptors of PCl and PCl‐Si are synthesized, and both acceptors have a similar unit that exists on the backbone of the donor PM6. The similar unit acts as a bridge to improve the interfacial interaction and miscibility between the donor and acceptor. The PM6:PCl‐based all‐polymer solar cell achieves a high fill factor of 70.25% and a high device stability.

Poor miscibility between the polymer donor and acceptor in the active layer leads to low fill factors (FF). PCl and PCl‐Si are synthesized by polymerization of the accessible and inexpensive IDIC‐C16 with BDT‐Cl and BDT‐Cl‐Si, respectively. PCl and PCl‐Si involve a BDT skeleton that is definitely used in most highly efficient polymer donors, such as PM6. Guided by the law of similarity and intermiscibility, the similar building block acts as a bridge to improve the interfacial interaction and miscibility between the donor and acceptor, leading to a favorable morphology of the active layer. It is found that the miscibility of the active layer is sensitive to the structural similarity degree of the similar unit of the donor and acceptor. The PCl‐Si‐based device delivers a power conversion efficiency (PCE) of 9.25% with a moderate FF of 67.86%, whereas the PM6:PCl‐based device achieves a PCE of 10.02% with a higher FF of 70.25%, which is the highest FF of the device with an IDIC‐C16‐based polymer acceptor. In addition, the improved interaction between the donor and acceptor improves the device stability. These results demonstrate that regulating the structural similarity between donor and acceptor is a promising strategy to optimize and stabilize morphology for high‐performance all‐polymer solar cells.

The effect of carbon–oxygen (CO)‐bridge isomerization is found to depend on the side‐chain and end‐group modifications. Their combined role determines the end‐group π–π stacking and aggregation behaviors of the CO‐bridge acceptor–donor–acceptor (A–D–A) acceptors.

For bulk heterojunction organic solar cells (OSCs), controlling molecular self‐aggregation during solution processing is crucial to obtain ideally phase‐separated morphology and high device performance. Recently, fused‐ring regiochemistry, for example, carbon–oxygen (CO)‐bridge isomerization, has been found to effectively modulate the aggregation structures and photovoltaic properties of acceptor–donor–acceptor (A–D–A) small‐molecule acceptors (SMAs). Strikingly, the relative aggregation ability for the CO‐bridge isomers turns out to be reverse after simultaneous replacement of the linear alkyl side chains with branched ones and fluorination of the end groups. Herein, to understand the molecular origin of such an observation, the aggregation behaviors of three pairs of CO‐bridge isomeric SMAs in solutions are systematically investigated by atomistic molecular dynamics simulations. Because of the large side‐chain steric hindrance around the fused‐ring core, the molecular self‐aggregation for all of these SMAs is dominated by end‐group π–π stacking. Moreover, the end‐group π–π interaction is controlled by the synergistic effect of CO‐bridge isomerization, side‐chain branching, and end‐group fluorination, which are responsible for the reversal of the aggregation ability of the isomeric SMAs. This work provides the rationalization of experimental observations and is helpful for modulating the blending morphologies for high‐efficiency OSCs based on CO‐bridge SMAs.

This work presents effective grain boundary defect passivation in a 1.78 eV quadruple cation wide‐bandgap perovskite using a combination of two approaches for passivation: four cations (RbCsFAMA) and secondary growth (by guanidinium iodide) to achieve high efficiency.

Development of high‐performance wide‐bandgap perovskites is a key component to enable tandem solar cells with either a silicon or low‐bandgap perovskites. However, the presence of defects in the Br‐rich wide‐bandgap perovskites, especially in the grain boundaries (GBs) has been particularly challenging and limits its performance. Herein, to accomplish the passivation of these defects, a combination of cation management with rubidium (Rb) introduction into the triple cation combination of cesium/formamidinium/methylammonium (CsFAMA) is exercised. Passivation is further enhanced by secondary growth (SG) using guanidinium iodide. In‐depth assessments of GB defect passivation are performed using Kelvin probe force microscopy (KPFM) and nanoscale charge‐carrier dynamics mappings provide insightful details on the presence of GBs defects and their suppression by the cation management and SG techniques. Reduction of unreacted PbX₂ to realize a highly crystalline perovskite surface is achieved after incorporating Rb and SG treatment. As a result, a champion cell for 1.78 eV (FA_0.79MA_0.16Cs_0.05)_0.95Rb_0.05Pb(I_0.6Br_0.4)₃ wide‐bandgap perovskite with an efficiency of 17.71% along with enhancement in all photovoltaic parameters is achieved. This study introduces a new way to analyze GB defects and reveals the consequence of defect passivation on charge‐carrier dynamics for realizing efficient perovskites.

In situ diagnostics (X‐ray scattering and optical absorbance) reveal nearly identical crystallization behaviors of all MAPbBr_{3− x} Cl _x perovskite alloys and pure halide systems from a dimethyl sulfoxide (DMSO) solution, which is vastly different from methylammonium lead iodide (MAPbI₃). The similarities in the structure and the phase transformation pathway promote halide homogeneity in the mixed‐halide perovskite alloys.

The current understanding of the crystallization, morphology evolution, and phase stability of wide‐bandgap hybrid perovskite thin films is very limited, as much of the community's focus is on lower bandgap systems. Herein, the crystallization behavior and film formation of a wide and tunable bandgap MAPbBr_{3− x} Cl _x system are investigated, and its formation and phase stability are contrasted to the classical MAPbI_{3− x} Br _x case. A multiprobe in situ characterization approach consisting of synchrotron‐based grazing incidence wide‐angle X‐ray scattering and laboratory‐based time‐resolved UV–Vis absorbance measurements is utilized to show that all wide‐bandgap perovskite compositions of MAPbBr_{3− x} Cl _x studied (0 < x < 3) crystallize the same way: the perovskite phase forms directly from the colloidal sol state and forms a solid film in the cubic structure. This results in significantly improved alloying and phase stability of these compounds compared with MAPbI_{3− x} Br _x systems. The phase transformation pathway is direct and excludes solvated phases, in contrast to methylammonium lead iodide (MAPbI₃). The films benefit from antisolvent dripping to overcome the formation of discontinuous layers and enable device integration. Pin‐hole‐free MAPbBr_{3− x} Cl _x hybrid perovskite thin films with a tunable bandgap are, thus, integrated into working single‐junction solar cell devices and achieve a tunable open‐circuit voltage as high as 1.6 V.

Herein, the two‐step deposition of perovskite in mesoporous‐carbon‐based perovskite solar cells with repeatable power conversion efficiency over 12% is analyzed. Stability characterizations show degradation with time, however, a complete recovery of the devices in the dark was revealed. Analyzing the mechanism for this shows that the perovskite's unit cell shrinks during the recovery process due to internal stress relief.

Lead halide perovskites attract much attention in recent years as a realistic solution for efficient and low‐cost solar cells. One of the interesting solar cell structures is the fully mesoporous‐carbon‐based perovskite solar cells. The mesoporous layers can be fabricated entirely by screen printing with the potential for upscaling. Herein, the two‐step deposition of perovskite in mesoporous‐carbon‐based perovskite solar cells is studied. The influence of the dipping time on the photovoltaic parameters is investigated using charge extraction and intensity‐modulated photovoltage spectroscopy (IMVS) measurements. A power conversion efficiency of 15% is observed for cells fabricated using two‐step deposition which is one of the highest reported for this solar cell structure. Stability characterizations at maximum power point (MPP) tracking show degradation with time, however a complete recovery of the devices in the dark is revealed. Analyzing the mechanism for this shows that the perovskite's unit cell shrinks during the recovery process due to internal stress relief. This interesting phenomenon opens the possibility to optimize the stability of these solar cells for commercial applications.

Antimony chalcogenides‐based triple‐junction tandem solar cells can achieve a theoretical efficiency of 32.98%. Herein, Sb₂S₃/Sb₂(S_0.7Se_0.3)₃/Sb₂Se₃ tandem solar cells have been theoretically simulated by tuning their thickness and compound stoichiometry. Besides, Sb₂(S_1−x Se _x )₃ single‐junction solar cells with an increasing Se content profile could promote the incident light absorption and the hole transport, which leads to a higher efficiency of 15.50%.

Antimony chalcogenides have become a family of promising photoelectric materials for high‐efficiency solar cells. To date, single‐junction solar cells based on individual antimony selenide or sulfide are dominant and show limited photoelectric conversion efficiency. Therefore, great gaps remain for the multiple junction solar cells. Herein, triple‐junction antimony chalcogenides‐based solar cells are designed and optimized with a theoretical efficiency of 32.98% through band engineering strategies with Sb₂S₃/Sb₂(S_0.7Se_0.3)₃/Sb₂Se₃ stacking. The optimum Se content of the mid‐cell should be maintained low, i.e., 30% for achieving a low defect density in an absorber layer. Therefore, Sb₂(S_0.7Se_0.3)₃‐based mid solar cells have contributed to elevate the external quantum efficiency in triple‐junction devices by the full utilization of the solar spectrum. In a single‐junction solar cell, the bandgap gradient is regulated through the Se content gradient along the depth profile of Sb₂(S_1−x Se _x )₃. Besides, an increasing Se content profile provides an additional built‐in electric field for boosting hole charge carrier collection. Thus, the high charge carrier generation rate leads to a 17.96% improvement in the conversion efficiency compared with a conventional cell. This work may pave the way to boost the conversion efficiency of antimony chalcogenides‐based solar cells to their theoretical limits.

A new pyridine‐based polymer semiconductor (PPY2) was introduced as the dopant‐free hole‐transporting material in inverted perovskite solar cells. It exhibits suitable energy levels, high hole mobility, effective passivation effects, and the capability of promoting the formation of a high‐quality polycrystalline perovskite film. The devices based on PPY2 delivered an encouraging power‐conversion efficiency up to 22.41 % with a high V _OC of 1.16 V.

Abstract

Currently, the performance improvement for inverted perovskite solar cells (PVSCs) is mainly limited by the high open circuit voltage (V _OC) loss caused by detrimental non‐radiative recombination (NRR) processes. Herein, we report a simple and efficient way to simultaneously reduce the NRR processes inside perovskites and at the interface by rationally designing a new pyridine‐based polymer hole‐transporting material (HTM), PPY2, which exhibits suitable energy levels with perovskites, high hole mobility, effective passivation of the uncoordinated Pb²⁺ and iodide defects, as well as the capability of promoting the formation of high‐quality polycrystalline perovskite films. In absence of any dopants, the inverted PVSCs using PPY2 as the HTM deliver an encouraging PCE up to 22.41 % with a small V _OC loss (0.40 V), among the best device performances for inverted PVSCs reported so far. Furthermore, PPY2‐based unencapsulated devices show an excellent long‐term photostability, and over 97 % of its initial PCE can be maintained after one sun constant illumination for 500 h.

(NiCo)_1−y Fe _y O _x decorated GO is used as a hole booster in all‐inorganic CsPbIBr₂ PSC. A champion efficiency of 10.95 % is achieved arising from the charge transfer doping effect between (NiCo)_1−y Fe _y O _x and GO.

Abstract

The precise regulation of interfacial charge distribution highly determines the power conversion efficiency of perovskite solar cells (PSCs). Herein, inorganic (NiCo)_1−y Fe _y O _x nanoparticle decorated graphene oxide (GO) is successfully demonstrated as a hole booster for all‐inorganic CsPbIBr₂ PSC free of precious metal electrode. Arising from the spontaneous electron transfer induced p‐type doping of GO from edged oxygen‐containing functional groups to (NiCo)_1−y Fe _y O _x , the best all‐inorganic CsPbIBr₂ PSC achieves an efficiency of 10.95 % under one standard sun owing to the eliminated paradox between charge extraction and charge localization in GO surface. Furthermore, the champion device exhibits an excellent long‐term stability at 10 % relative humidity without encapsulation over 70 days because of the suppressed ions migration.

Lead halide perovskites are well known for their facile crystallization due to their ionic nature. Herein, the synthesis and characterization of amorphous lead halide thin films are reported. The amorphous lead halide perovskite has a large and tunable optical bandgap and improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite.

Abstract

Lead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high‐quality crystals with tunable bandgaps in the visible and near‐infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin‐film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells.

Flexible organic solar cells (OSCs) come to the forefront of organic electronics. It is critical to develop high‐merit flexible transparent electrodes (FTEs). The work covers the frontier progress of PEDOT:PSS, graphene, metallic nanostructures, metal oxide/metal/metal oxide, Mxene, and hybrid electrodes. It raises the awareness for the importance of developing the FTEs and reveals their critical role in flexible OSCs.

Abstract

Substantial effort has been devoted to both chemical doping and design of flexible transparent electrodes (FTEs) for flexible organic solar cells (OSCs) in the past decade. Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate), graphene, metal nanostructures, metal oxide/ultrathin metal/metal oxide, Mxene, and their hybrid electrodes emerge to be the most promising flexible conducting materials over indium tin oxide. The FTE fabrications play a critical role in flexible OSCs. This feature review article summarizes the current status on the researches of the FTEs including various approaches and strategies to boost the conductivity, work function, mechanical flexibility, wettability, etc, which directly affect the performances of the flexible OSCs. The most cutting edge progresses on both FTEs and flexible OSCs are highlighted along the line. Advantages and plausible issues are pointed out. Perspectives are provided that can advance the developments of the flexible OSCs. This review raises the awareness for the importance of developing plenty of FTEs and reveals their critical role in flexible OSCs.

The strong interaction between F and Sn changes the electron cloud density around Sn atoms by introducing RbF into SnO₂ colloidal dispersion, contributing to the improved electron mobility of SnO₂. While spin‐coating RbF onto the SnO₂ surface, the Rb⁺ cations escape into the bulk perovskite, which inhibits ion migration and decreases the trap density.

Abstract

Regulating the electron transport layer (ETL) has been an effective way to promote the power conversion efficiency (PCE) of perovskite solar cells (PSCs) as well as suppress their hysteresis. Herein, the SnO₂ ETL using a cost‐effective modification material rubidium fluoride (RbF) is modified in two methods: 1) adding RbF into SnO₂ colloidal dispersion, F and Sn have a strong interaction, confirmed via X‐ray photoelectron spectra and density functional theory results, contributing to the improved electron mobility of SnO₂; 2) depositing RbF at the SnO₂/perovskite interface, Rb⁺ cations actively escape into the interstitial sites of the perovskite lattice to inhibit ions migration and reduce non‐radiative recombination, which dedicates to the improved open‐circuit voltage (V _oc) for the PSCs with suppressed hysteresis. In addition, double‐sided passivated PSCs, RbF on the SnO₂ surface, and p‐methoxyphenethylammonium iodide on the perovskite surface, produces an outstanding PCE of 23.38% with a V _oc of 1.213 V, corresponding to an extremely small V _oc deficit of 0.347 V.

Temperature‐dependent transient absorption and time‐resolved photoluminescence investigations provide insights into electronic processes in heterogeneous organic‐inorganic halide perovskites. By taking 3D and quasi‐2D perovskite model systems, evidence is provided that charge carrier transfer (a double charge transfer) rather than energy migration dominates in heterogeneous quasi‐2D perovskite films.

Abstract

Heterogeneous organic‐inorganic halide perovskites possess inherent non‐uniformities in bandgap that are sometimes engineered and exploited on purpose, like in quasi‐2D perovskites. In these systems, charge carrier and excitation energy migration to lower‐bandgap sites are key processes governing luminescence. The question, which of them dominates in particular materials and under specific experimental conditions, still remains unanswered, especially when charge carriers comprise excitons. In this study transient absorption (TA) and transient photoluminescence (PL) techniques are combined to address the excited state dynamics in quasi‐2D and other heterogeneous perovskite structures in broad temperature range, from room temperature down to 15 K. The data provide clear evidence that charge carrier transfer rather than energy migration dominates in heterogeneous quasi‐2D perovskite films.

Long-term thermal stability is critical for perovskite solar cells (PSCs) operation. Un-encapsulated co-evaporated MAPbI₃ PSCs, contrary to similar spin-coated PSCs, exhibit remarkable structural robustness and retain 80% of their initial efficiency after 3600 h aging at 85 °C. This excellent intrinsic stability is driven by the strain-stress-free MAPbI₃ grown by co-evaporation, where post-annealing is not required to fully form the perovskite.

Abstract

Thermal stability is a critical criterion for assessing the long-term stability of perovskite solar cells (PSCs). Here, it is shown that un-encapsulated co-evaporated MAPbI₃ (TE_MAPbI₃) PSCs demonstrate remarkable thermal stability even in an n-i-p structure that employs Spiro-OMeTAD as hole transport material (HTM). TE_MAPbI₃ PSCs maintain over ≈95% and ≈80% of their initial power conversion efficiency (PCE) after 1000 and 3600 h respectively under continuous thermal aging at 85 °C. TE_MAPbI₃ PSCs demonstrate remarkable structural robustness, absence of pinholes, or significant variation in grain sizes, and intact interfaces with the HTM, upon prolonged thermal aging. Here, the main factors driving TE_MAPbI₃ stability are assessed. It is demonstrated that the excellent TE_MAPbI₃ thermal stability is related to the perovskite growth process leading to a compact and almost strain-stress-free film. On the other hand, un-encapsulated PSCs with the same architecture, but incorporating solution-processed MAPbI₃ or Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ as active layers, show a complete PCE degradation after 500 h under the same thermal aging condition. These results highlight that the control of the perovskite growth process can substantially enhance the PSCs thermal stability, besides the chemical composition. The TE_MAPbI₃ impressive long-term thermal stability features the potential for field-operating conditions.

Combining the layer‐by‐layer processing method and a ternary strategy, 18.16% efficiency, which is among the highest values reported to date, is achieved in single‐junction organic photovoltaics (OPVs) based on the PM6:BO‐4Cl:BTP‐S2 blend, superior to that (18.03%) of bulk‐heterojunction OPVs, proving that layer‐by‐layer processed ternary OPVs could be a promising approach to high efficiencies.

Abstract

Obtaining a finely tuned morphology of the active layer to facilitate both charge generation and charge extraction has long been the goal in the field of organic photovoltaics (OPVs). Here, a solution to resolve the above challenge via synergistically combining the layer‐by‐layer (LbL) procedure and the ternary strategy is proposed and demonstrated. By adding an asymmetric electron acceptor, BTP‐S2, with lower miscibility to the binary donor:acceptor host of PM6:BO‐4Cl, vertical phase distribution can be formed with donor‐enrichment at the anode and acceptor‐enrichment at the cathode in OPV devices during the LbL processing. In contrast, LbL‐type binary OPVs based on PM6:BO‐4Cl still show bulk‐heterojunction like morphology. The formation of the vertical phase distribution can not only reduce charge recombination but also promote charge collection, thus enhancing the photocurrent and fill factor in LbL‐type ternary OPVs. Consequently, LbL‐type ternary OPVs exhibit the best efficiency of 18.16% (certified: 17.8%), which is among the highest values reported to date for OPVs. The work provides a facile and effective approach for achieving high‐efficiency OPVs with expected morphologies, and demonstrates the LbL‐type ternary strategy as being a promising procedure in fabricating OPV devices from the present laboratory study to future industrial production.

A thermally stable perovskite solar cell is developed by capturing mobile lithium ions using a new molecular hole transporter, 1,3‐bis(5‐(4‐(bis(4‐methoxyphenyl)amino)phenyl)thieno[3,2‐b]thiophen‐2‐yl)‐5‐octyl‐4H‐thieno[3,4‐c]pyrrole‐4,6(5H)‐dione (coded HL38), where a strong interaction of the lithium ions in lithium bis(trifluoromethanesulfonyl)imide with the 5‐octylthieno[3,4‐c]pyrrole‐4,6‐dione (octyl‐TPD) moiety in HL38 is responsible for maintaining ≈86% of the initial power conversion efficiency for over 1000 h at 85 °C.

Abstract

A thermally stable perovskite solar cell (PSC) based on a new molecular hole transporter (MHT) of 1,3‐bis(5‐(4‐(bis(4‐methoxyphenyl) amino)phenyl)thieno[3,2‐b]thiophen‐2‐yl)‐5‐octyl‐4H‐thieno[3,4‐c]pyrrole‐4,6(5H)‐dione (coded HL38) is reported. Hole mobility of 1.36 × 10⁻³ cm² V⁻¹ s⁻¹ and glass transition temperature of 92.2 °C are determined for the HL38 doped with lithium bis(trifluoromethanesulfonyl)imide and 4‐tert‐butylpyridine as additives. Interface engineering with 2‐(2‐aminoethyl)thiophene hydroiodide (2‐TEAI) between the perovskite and the HL38 improves the power conversion efficiency (PCE) from 19.60% (untreated) to 21.98%, and this champion PCE is even higher than that of the additive‐containing 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐MeOTAD)‐based device (21.15%). Thermal stability testing at 85 °C for over 1000 h shows that the HL38‐based PSC retains 85.9% of the initial PCE, while the spiro‐MeOTAD‐based PSC degrades unrecoverably from 21.1% to 5.8%. Time‐of‐flight secondary‐ion mass spectrometry studies combined with Fourier transform infrared spectroscopy reveal that HL38 shows lower lithium ion diffusivity than spiro‐MeOTAD due to a strong complexation of the Li⁺ with HL38, which is responsible for the higher degree of thermal stability. This work delivers an important message that capturing mobile Li⁺ in a hole‐transporting layer is critical in designing novel MHTs for improving the thermal stability of PSCs. In addition, it also highlights the impact of interface design on non‐conventional MHTs.