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Efficient detection of chemical analytes using fluorescence-based sensors necessitates an in-depth understanding of the physical interaction between the analyte molecules and the sensor films. This study explores the interplay between the thermal properties of a series of triphenylamine-centered fluorescent dendrimers with different glass transition temperatures (Tg) for detecting nitroaromatic explosives. When exposed to 4-nitrotoluene (pNT) vapors, biphasic diffusion kinetics were observed for all the dendrimers, corresponding to Super Case II kinetics, suggesting rapid film swelling during initial analyte uptake. The diffusion kinetics were further analyzed using a diffusion–relaxation model, where a strong Tgdependence was observed for both the initial concentration-driven diffusion phase and the slower film relaxation phase. Additionally, a difference in kinetics between analyte uptake and release was observed. The photoluminescence (PL) kinetics also showed a Tgdependence, with more efficient PL recovery observed for films composed of dendrimers that had a lower Tg. Rapid quenching of over 40% with little PL recovery was seen in the dendrimer with the highest Tg(107 °C), while a smaller quench with efficient PL recovery was observed in the dendrimer that had a Tgclose to room temperature. The results highlight the critical role of the thermal properties of sensor films in achieving rapid and sensitive detection.

Luminescence-based sensing has been demonstrated to be a powerful method for rapid trace detection of chemical vapors (analytes). Analyte diffusion has been shown to be the critical factor for real-time luminescence-based detection of explosive analytes via photoinduced electron transfer in amorphous films of conjugated polymers and dendrimers. However, similar studies to determine the critical factors for sensing have not been performed on materials that employ photoinduced hole transfer (PHT) to detect low electron affinity analytes such as illicit drugs. Nor have such studies been performed on semicrystalline sensing films. We have developed a family of perylene diimide-based sensing materials capable of undergoing PHT with amine-group containing analytes. It was found that the choice of branched alkyl chain [1-hexylheptyl (PHH), 2-hexyloctyl (PHO), or 2,2-dihexyloctyl (PDHO)] attached to the nitrogen atoms of the imide moiety strongly affected the solution-processed film morphology. PHHand PHOwere found to contain crystalline phases, whereas PDHOwas essentially amorphous. The degree of crystallinity strongly influenced exciton diffusion, with PHHand PHOexhibiting exciton diffusion coefficients that were 20× and 10× greater than the value of the amorphous PDHO. The degree of film crystallinity was also found to be critical when the films were applied to detect N-methylphenethylamine (MPEA), a simulant of methamphetamine. While PHHhad the largest exciton diffusion coefficient [(1.0 ± 0.2) × 10–2cm2s–1] and analyte uptake (12.3 ± 1.8 ng) it showed the smallest quenching efficiency (2.6% ng–1). In contrast, PHO, which sorbed the least analyte (6.1 ± 0.4 ng) of the three compounds, had the largest quenching efficiency (7.1% ng–1) due to its molecular packing and hence exciton diffusion coefficient [(4.5 ± 1.4) × 10–3cm2s–1] not being affected by sorption of the analyte. These results show that when applying fluorescent films in practical detection scenarios there is a potential trade-off between a high exciton diffusion constant and analyte diffusion for semicrystalline sensing materials and that a high exciton diffusion coefficient in an as-cast film does not necessarily translate into a more efficient fluorescent quenching. The results also show that sensing materials that form semicrystalline films, whose packing is not disrupted by analyte diffusion, provide a route for overcoming these effects and achieving high sensitivity.

We study the effect of (2,3,4,5,6-pentafluorophenyl)alkylamine additives with differing alkyl chain lengths (methyl, ethyl, and n-propyl) on the performance of methylammonium lead triiodide (MAPbI3) perovskite solar cells. The results show that the length of the alkyl chain between the 2,3,4,5,6-pentafluorophenyl group and ammonium moiety has a critical effect on the perovskite film structure and subsequent device performance. The 2,3,4,5,6-pentafluorophenyl ammonium additive with the shortest linking group (a methylene unit), namely (2,3,4,5,6-pentafluorophenyl)methylammonium iodide, was found to be distributed throughout the bulk of the perovskite film with a 2D phase only being observable at high concentrations (>30 mol%). In contrast, the additives with ethyl and n-propyl linking groups phase-separate during solution processing and are found to concentrate at the surface of the perovskite film. Photoluminescence measurements showed that the fluorinated additives passivated the surface defects on the perovskite grains. Of the three additives, inverted devices containing 0.32 mol% of the 2,3,4,5,6-pentafluorophenyl ammonium additive with the methylene linking group achieved a maximum power conversion efficiency of 22.0%, with the device efficiency decreasing with increasing additive concentration. In contrast, the devices composed of the additive with the longest alkyl linker, 3-(2,3,4,5,6-pentafluorophenyl)propylammonium iodide, had the poorest performance, with PCEs less than that of the neat MAPbI3control and decreasing with increasing additive concentration.

We report the first example of a thermally activated delayed fluorescence (TADF) poly(dendrimer), composed of a norbornenyl-derived polymer backbone and dendritic side-chain chromophores comprising 2,3-dicyanopyrazino as the electron acceptor and a first-generation fluorenylcarbazole derivative as the electron donor. The TADF poly(dendrimer) homopolymer, with one dendritic side chain attached to each monomer unit, emitted deep-red light. The emission of the poly(dendrimer) was found to be red-shifted relative to the nonpolymeric doubly dendronized emitter composed of the same components. The simple dendrimer was found to have a solution photoluminescence quantum yield (PLQY) of around 70%. In contrast, the poly(dendrimer) had a PLQY of 9%, which was attributed to intramolecular interchromophore interactions. An interesting feature of the poly(dendrimer) was that oxygen did not quench the TADF emission. We found that the PLQY of the simple dendrimer decreased markedly in neat films, whereas that of the poly(dendrimer) did not, with both having a solid-state PLQY of around 10%. The results suggest that intrapolymer chromophore–chromophore interactions observed in solution for the poly(dendrimer) were similar to the intermolecular chromophore–chromophore interactions of the dendrimer in the solid state. Simple two-layer organic light-emitting diodes comprising nondoped films of the materials and an electron transport layer showed red emission with CIE coordinates of (x> 0.66, y< 0.34). The dendrimer-based device had a maximum external quantum efficiency of 2.4%, which is among the best for solution-processed deep-red emissive TADF-based OLEDs but in a simpler device architecture.

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor–acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor–acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where Drepresents 9,9-di-n-propylfluorene, A1represents benzothiadiazole, and A2represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-Dor D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1* and S1on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-Dand D-A1-A2in solution and film, and for D-A1-A1-Din solution. In the film, the singlet excited state of D-A1-A1-Dpredominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOCloss commonly associated with NFA-based OPV devices.

Multication metal-halide perovskites exhibit desirable performance and stability, compared to their monocation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanisms in solution-processed MAPbI3(MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95(triple cation) perovskite solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi-Fermi level splitting than for the triple cation perovskite absorbers, which defines the open-circuit voltage of the MAPI cells. PL measurements at low temperatures (∼20 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the nonradiative recombination loss at room temperature. We conclude that while triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states within the MAPI films is the primary consideration for device optimization.

Poly(dendrimers) are an emerging class of fluorescent organic semiconductors that have been investigated for their ability to detect nitro-based explosives and taggants. In this work, the sensing chromophores were dendritic side chains attached to a norbornenyl-derived polymer backbone. The sensing chromophore of each poly(dendrimer) was based on fluorene and carbazole moieties, both of which have been shown to be sensitive toward nitro-containing explosives. The monomers, and corresponding polymers, were screened against 2,4-dinitrotoluene (DNT) and 2,3-dimethyl-2,3-dinitrobutane (DMNB) vapors and a range of household interferents such as mouthwash and ethanol. An important result of this work was the rapid and sensitive detection of DMNB, a taggant added to commercial plastic explosives, which is usually difficult to detect via fluorescence quenching due to its relatively low electron affinity and spontaneous diffusion out of the sensing film. Testing of the poly(dendrimer)s was also performed with a hand-held prototype detector for explosives. Selectivity was demonstrated with a range of interferents and explosive or taggant vapors, with the interferents giving an increase in fluorescence intensity and the explosive related materials a quenching of the luminescence. The results show that organic semiconductor poly(dendrimer)s are an interesting class of material for the detection of nitro-based explosives and taggants, especially DMNB.

We introduce a catalyst-free, highly efficient, ambient temperature Diels–Alder reaction employing o-methylbenzaldehyde derivatives as photocaged dienes as an ideal approach for forming three-dimensional insoluble networks for inkjet printing of OLED emissive layer. Herein, poly(methyl methacrylate) based polymers containing 4-(9H-carbazol-9-yl)-2-(3′-hydroxy-[1,1′-biphenyl]-3-yl)isoindoline-1,3-dione as a blue-green (λmax= 495–500 nm) thermally activated delayed fluorescence (TADF) emitter and a photochemically active maleimide/o-methylbenzaldehyde cross-linker couple were synthesized and their photo-cross-linking behavior was studied. Time resolved fluorescence measurements confirm that the TADF properties are maintained upon integration in a polymer network and HOMO/LUMO levels of the emitter species remain unchanged by the photo-cross-linking at 365 nm of the polymer chains. The network formation of the fluorescent films is evidenced by solvent resistance tests and monitored by Fourier transform infrared (FT-IR) spectroscopy as well as time of flight secondary ion mass spectroscopy (ToF-SIMS), showing the consumption of maleimide and o-methylbenzaldehyde groups with increasing irradiation time. The surface roughness is investigated via atomic force microscopy (AFM) and found to be unchanged by a solvent wash after the cross-linking. Furthermore, confirmation that the polymer solution can be printed on an inkjet-printer and subsequently photo-cross-linked for multilayer OLED device fabrication is obtained.

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6blended with high performance NFA materials, Y6or ITIC-4F. We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6and ITIC-4F, respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

Low-weight-percent donor content fullerene-based organic solar cells have been reported to have good efficiencies although the exact reason for their performance is still a matter of discussion. We use a solution processable hole-transporting poly(dendrimer) with minimal absorption in the visible region to study the blend-ratio dependency of energy-loss mechanisms in fullerene low-donor content organic solar cells in which [6,6]-phenyl-C71-butyric acid methyl ester is used as the primary absorber. The photocurrent losses for each of the steps that govern the device performance have been assessed, with the optimized device performance achieved when the donor was at a concentration of 6 wt %. The 6 wt % donor device had balanced charge transport, and transient absorption spectra revealed that although the low-donor content devices suffered from a low-exciton-quenching rate, the dissociation efficiency of the resulting charge-transfer states was almost unity. That is, essentially all of the formed charge-transfer states led to charge-separated states. A gradual increase in the open-circuit voltage was observed as the donor ratio decreased from 50 to 6 wt %. Internal quantum-efficiency measurements indicated efficient formation of charge-separated states from the intermediate charge-transfer states and high charge-collection efficiency at low-donor content resulting in an overall higher external quantum efficiency despite the lower interfacial area. Overall, these measurements highlight the competing effects of exciton dissociation (favored by the high surface area associated with large donor content) and charge-transfer state dissociation, which we observe to be more efficient when the donor content is low.

PNNThas been prepared as a polymeric electron acceptor for organic solar cells. The polymer has an A–A′–A acceptor motif linked alternatively with thiophene and vinyl moieties. The A′-unit is a naphthalene diimide, while the A groups are thiazoles. PNNTfilms were found to have an estimated electron affinity of ≈4.3 eV and an electron mobility of the order of 10–4cm2V–1s–1. Its relatively low solubility in common chlorinated solvents at ambient temperature allowed the manufacture of sequentially deposited (SD) devices, which were found to have significantly higher efficiency than that of bulk heterojunction (BHJ) solar cells containing the same materials. Grazing-incidence wide-angle X-ray scattering measurements indicated that the SD films retained the ordering of the individual polymers to a greater extent compared to the BHJ films. The best SD devices were found to have a power conversion efficiency of up to 4.5%, with stable performance under thermal stress.

The power conversion efficiencies of solar cells incorporating non-fullerene donor–acceptor blends now approach those of fullerene-based devices in the best performing examples. However, the lack of clear structure–property relationships means that the poor device performance of many novel non-fullerene systems cannot be readily explained. We report a series of non-fullerene acceptors, comprising essentially the same chromophore, that differ in terms of their shape and number of chromophores. To understand the impact of these structural differences on charge generation, we have employed transient absorption spectroscopy to investigate the photophysical properties of the acceptors in blends with the conjugated polymer PTB7. To minimize the impact of morphology, we employed a broad range of acceptor concentrations and compared the results with those of blends containing the fullerene derivative PC70BM. In terms of singlet exciton harvesting, the non-fullerene acceptors exhibited similar performance to PC70BM in blends with PTB7. The rate of singlet exciton quenching as a function of acceptor concentration was consistent with exciton diffusion mediated quenching with a diffusion length of 4–5 nm for the PTB7 singlet exciton. The polaron generation efficiency of the non-fullerene acceptors was comparable to that of PC70BM although the fraction of polarons that subsequently underwent geminate recombination was much greater in the non-fullerene blends. Furthermore, the photophysical properties of the non-fullerene acceptors were not influenced by the shape of the acceptor, the chromophore number, or the donor–acceptor ratio, which indicates that the higher geminate recombination is related to the structure of the acceptor chromophore and the interface formed with the donor. The implication of these results is that despite appropriate energetics and optical absorption, the performance of some non-fullerene donor–acceptor blends will be intrinsically limited by the choice of acceptor chromophore and the nature of the interface it forms with the donor material.

A family of first-generation dendrimers containing 3,5-bis(carbazolyl)phenyl dendrons attached to a green emissive fac-tris(2-phenylpyridyl)iridium(III) core were prepared. The solubility of the dendrimers was imparted by the attachment of tert-butyl surface groups to the carbazole moieties. The dendrimers differed in the number of dendrons attached to each ligand (one or two dendrons) as well as the degree of rotational restriction within the dendrons. The densities of the films containing the doubly dendronized materials were higher than those of their mono-dendronized counterparts, with the dendrimer containing two rotationally constrained dendrons per ligand having the highest density at 1.12 ± 0.04 g cm–3. The dendrimers were found to have high photoluminescence quantum yields (PLQYs) in solution of between 80 and 90%, with the doubly dendronized materials having the lower values and a red-shifted emission. The neat film PLQY values of the dendrimers were less than those measured in solution although the relative decrease was smaller for the doubly dendronized materials. The dendrimers were incorporated into solution-processed bilayer organic light-emitting diodes (OLEDs) composed of neat or blend emissive layers and an electron transport layer. The best-performing devices had the dendrimers blended with a host material and external quantum efficiencies as high as 14.0%, which is higher than previously reported results for carbazole-incorporating emissive dendrimers. A feature of the devices containing blends of the doubly dendronized materials was that the maximum efficiency was relatively insensitive to the concentration in the host between 1 and 7 mol %.

Three-dimensional (3D) perovskite solar cells (PSCs) containing additives capable of forming two-dimensional (2D) structures in neat films have attracted attention due to their ability to enhance power conversion efficiency (PCE) in combination with improved operational stability. Herein, a newly designed fluorinated ammonium salt, 2-(perfluorophenyl)ethanaminium bromide:chloride50:50(FEABr:Cl50:50), is introduced into CsMAFAPbI3-based PSCs with a standard n–i–p architecture. FEABr:Cl50:50was used as an additive in the tin(IV) oxide (SnO2) electron transporting layer (ETL) as well as a surface treatment for the perovskite film. Used in this dual way, the additive was found to passivate charge-trapping defects within the SnO2ETL and regulate the crystal growth of the perovskite layer. When FEABr:Cl50:50was deposited onto the surface of the 3D perovskite film, it formed a thin hydrophobic 2D capping layer. Adopting this dual strategy led to the perovskite film having larger grain sizes, improved quality, and overall better device performance. As a result, the best-performing device exhibited a PCE of over 23% with negligible hysteresis in an n–i–p device architecture with an area of 0.2 cm2. Furthermore, unencapsulated devices with the hydrophobic 2D capping layer showed improved stability compared to the control device when measured under continuous light irradiation at a maximum power point (MPP) at 80 ± 5 °C in a humid (≈50%) environment.

To date, nonfullerene acceptors for organic solar cells have yet to reach the overall device performance achieved with fullerene derivatives. Power conversion efficiencies are low, even when combined with narrow optical gap polymers, with the processing conditions and blend microstructure difficult to optimize. To understand the potential origins of the decreased performance, we performed a photophysical study on a model system of blends of poly(3-n-hexylthiophene) (P3HT) with the planar small molecule electron acceptor 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12), focusing, in particular, on the effects of the crystallinity of the K12 phase on exciton dissociation and charge generation. Our results show that the microstructure of the blends can be manipulated by the processing conditions to give amorphous through to (semi)crystalline films. The amorphous blends show strong quenching of the photoluminescence, which indicates that there is a fine mixture of P3HT and K12. After annealing, the blends all showed increased photoluminescence and signs of phase separation with the formation of large-scale crystalline K12 domains. Photoinduced absorption spectroscopy confirmed the presence of positive polarons in the P3HT and revealed triplet excitons in blends containing crystalline K12 domains, indicating that exciton harvesting from the K12 phase was inefficient. However, charge generation in the 1:2 blend (the best for devices) was enhanced in the (semi)crystalline films, which suggests that aggregation of the P3HT and K12 aids charge separation. Hence, the complex phase behavior of K12 results in a trade-off between charge generation and collection against exciton harvesting from the K12 phase.

We report three generations of dendrimers incorporating either a fluorene or spirobifluorene core with carbazole dendrons and fluorene surface groups that are effective sensing materials for the detection of nitrated explosives by fluorescence quenching. The photophysical properties of the dendrimers were investigated with a combination of steady-state absorption and photoluminescence and time-resolved photoluminescence. We show that the first-generation dendrimers behave as single chromophores while the higher-generation dendrimers contain multiple chromophores that interact to give excimer-like emissive states. Stern–Volmer measurements with nitrated analytes show that the quenching efficiency decreases with generation for the planar fluorene-cored dendrimers and increases with generation for the more three-dimensional spirobifluorene-cored dendrimers. These contrasting trends are shown to be caused primarily by changes in the quenching efficiency of static interactions with the nitrated analytes, which is a consequence of the choice of core. Our results highlight the potential for exploiting such excimer-like states for chemical sensing, particularly in the case of nitrated explosives.

We compare two dendrimers, which contain the same luminescent chromophores but differ in dimensionality, for the detection of an explosive analyte via PL quenching. Each dendrimer has first generation biphenyl dendrons with 2-ethylhexyloxy surface groups but differ in the core units. One dendrimer has a bifluorene core and hence has a “planar” structure, whereas the second has four bifluorene units tetrahedrally arranged around an adamantyl center and hence has a “three-dimensional” structure. Solution Stern–Volmer measurements have previously been reported to show that the three-dimensional dendrimer has a higher binding constant than that of the more planar compound. Films of the dendrimers rapidly detect 2,4-dinitrotoluene (DNT) with thinner films (∼25 nm) being more responsive than thicker films (∼85 nm). Neutron reflectometry measurements show that the analyte can diffuse completely through the films with the three-dimensional dendrimer absorbing more of the analyte. The rate of recovery of the PL was faster for the planar dendrimer than the three-dimensional material showing that large binding constants are not necessary for reversible detection of analytes.

Stable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)3] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)3in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)3:CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo­luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.

Fluorescence quenching dynamics were studied in poly(9,9-dioctylfluorene) films with a wide range of β-phase concentrations. Measurements of time-resolved fluorescence and fluorescence quantum yield showed that the radiative decay rate was independent of the β phase content, whereas the nonradiative decay rate was found to be higher in β-phase rich films. The trend was analyzed in terms of diffusion-mediated excitation energy transfer to quenchers. The concentration of fluorescence quenchers is found to be less than 0.01% of the number of fluorene repeat units and independent of the fraction of the film in the β phase. The results suggest that fluorescence quenching is predominantly by chemical defects rather than by excimer formation.

Singlet–singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of β‐phase chains using time‐resolved fluorescence. On a timescale of >15 ps after excitation, the results are fitted well by a time‐independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time‐dependent annihilation rate is observed during the first 15 ps in the glassy phase and in the β‐phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low‐energy sites. The annihilation rate in the mixed‐phase films increases with increasing fraction of β‐phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersedβ‐phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line‐dipole approximation for an exciton wavefunction extending over 2.5 nm. The results indicate that glassy andβ‐phase chromophores are intimately mixed rather than clustered or phase‐separated.

p-Type inorganic nickel oxide (NiOx) exhibits high transparency, tunable-optoelectronic properties, and a work function (WF) that is potentially suitable for hole extraction in inverted perovskite solar cells (PSCs). However, NiOx films possess surface defects that lead to high interfacial recombination and an energy offset with the ionization potential of the perovskite. Herein, we show that fluorinated 3-(2,3,4,5,6-pentafluorophenyl)propan-1-aminium iodide (FPAI) can be used to modify the electronic properties of the NiOx anode interlayer. The FPAI modification led to good perovskite crystal growth and films with reduced surface defects. The FPAI modification also increased the WFof NiOx and improved charge extraction. These improvements led to an increased Vocvalue compared with control devices without FPAI modification, 1.05 V versus 1.00 V, and a higher short-circuit current and larger fill factor. As a result, the best PSCs with FPAI-modified NiOx had a power conversion efficiency of 19.3%. Finally, the PSCs with the FPAI-modified NiOx layer were found to have improved stability.

Stable film morphology is critical for durable, high‐performance organic light‐emitting diodes. On page 2225, Ian R. Gentle, Paul L. Burn, and co‐workers use neutron reflectometry to study the out‐of‐plane structure of blended thin films and multilayer structures. Uniformly blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)3] in 4,4'‐bis(N‐carbazolyl)biphenyl (CBP) can be formed by evaporation, while 6 wt% films of Ir(ppy)3in CBP, typically used in OLEDs, were found to phase‐separate with moderate heating. Luminescence microscopy shows that phase separation leads to fiber‐like structures of CBP (blue) and Ir(ppy)3(green).