Sequential "click" functionalization of mesoporous titania for energy-relay dye enhanced dye-sensitized solar cells.

Energy relay dyes (ERDs) have been investigated previously as a mean to achieve panchromatic spectral response in dye-sensitized solar cells via energy transfer. To reduced the distance between the ERDs and energy-accepting injection dyes (IDs) on the surface of a mesoporous titanium dioxide electrode, the ERDs were immobilized adjacent to the IDs via a sequential functionalization approach. In the first step, azidobenzoic acid molecules were co-adsorbed on the mesoporous titanium dioxide surface with the ID. In the second step, the highly selective copper(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition "click" reaction was employed to couple an alkyne-functionalized ERD to the azidobenzoic acid monolayer. The cycloaddition step in the mesoporous electrode was slowed dramatically due to reactants and catalysts forming agglomerates. In solar cell devices, the close proximity between the surface-immobilized ERD and energy-accepting squaraine sensitizer dyes results in energy transfer efficiencies of up to 91%. The relative improvement in device performance due to the additional ERD spectral response was 124%, which is among the highest reported. The sequential functionalization approach described herein is transferrable to other applications requiring the functionalization of electrodes with complex molecules.

Enhancing the hole-conductivity of spiro-OMeTAD without oxygen or lithium salts by using spiro(TFSI)2 in perovskite and dye-sensitized solar cells.

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), the prevalent organic hole transport material used in solid-state dye-sensitized solar cells and perovskite-absorber solar cells, relies on an uncontrolled oxidative process to reach appreciable conductivity. This work presents the use of a dicationic salt of spiro-OMeTAD, named spiro(TFSI)2, as a facile means of controllably increasing the conductivity of spiro-OMeTAD up to 10(-3) S cm(-1) without relying on oxidation in air. Spiro(TFSI)2 enables the first demonstration of solid-state dye-sensitized solar cells fabricated and operated with the complete exclusion of oxygen after deposition of the sensitizer with higher and more reproducible device performance. Perovskite-absorber solar cells fabricated with spiro(TFSI)2 show improved operating stability in an inert atmosphere. Gaining control of the conductivity of the HTM in both dye-sensitized and perovskite-absorber solar cells in an inert atmosphere using spiro(TFSI)2 is an important step toward the commercialization of these technologies.

Melt-infiltration of spiro-OMeTAD and thermal instability of solid-state dye-sensitized solar cells.

A method for achieving complete pore-filling in solid-state dye-sensitized solar cells termed melt-infiltration is presented: after the customary solution-processed deposition of spiro-OMeTAD, the device is heated above the glass transition temperature of spiro-OMeTAD to soften the material and allow capillary action to pull additional spiro-OMeTAD from the overlayer reservoir into the pores. The pore-filling fraction increases from 60-65% to 90-100% as a result of melt-infiltration. The organic D-π-A dye used in this study is found to withstand the thermal treatment without performance loss, unlike ruthenium-based dyes. Through our experiments, we find that the 4-tert-butylpyridine (tBP) additive, commonly used in dye-sensitized solar cells, evaporates from the device during heat treatment at temperatures as low as 85 °C. This significantly impacts device performance, potentially excluding its use in commercial applications, and demonstrates the need for a more thermally stable tBP alternative. Melt-infiltration is expected to be a viable method for achieving complete pore-filling in systems where volatile additives are not required for operation.

Bicolour, large area, inkjet-printed metal halide perovskite light emitting diodes.

We demonstrate a bicoloured metal halide perovskite (MHP) light emitting diode (LED) fabricated in two sequential inkjet printing steps. By adjusting the printing parameters, we selectively and deliberately redissolve and recrystallize the first printed emissive layer to add a pattern emitting in a different color. The red light emitting features (on a green light emitting background) have a minimum size of 100 µm and originate from iodide-rich domains in a phase-segregated, mixed MHP. This phase forms between the first layer, a bromide-based MHP, which is partially dissolved by printing, and the second layer, an iodide-containing MHP. With an optimised printing process we can retain the active layer integrity and fabricate bicolour, large area MHP-based LEDs with up to 1600 mm2 active area. The two emission peaks at 535 nm and 710 nm are well separated and produce a strong visual contrast.

Highly soluble energy relay dyes for dye-sensitized solar cells.

High solubility is a requirement for energy relay dyes (ERDs) to absorb a large portion of incident light and significantly improve the efficiency of dye-sensitized solar cells (DSSCs). Two benzonitrile-soluble ERDs, BL302 and BL315, were synthesized, characterized, and resulted in a 65% increase in the efficiency of TT1-sensitized DSSCs. The high solubility (180 mM) of these ERDs allows for absorption of over 95% of incident light at their peak wavelength. The overall power conversion efficiency of DSSCs with BL302 and BL315 was found to be limited by their energy transfer efficiency of approximately 70%. Losses due to large pore size, dynamic collisional quenching of the ERD, energy transfer to desorbed sensitizing dyes and static quenching by complex formation were investigated and it was found that a majority of the losses are caused by the formation of statically quenched ERDs in solution.

Large area inkjet-printed metal halide perovskite LEDs enabled by gas flow assisted drying and crystallization.

We demonstrate the upscaling of inkjet-printed metal halide perovskite light-emitting diodes. To achieve this, the drying process, critical for controlling the crystallization of the perovskite layer, was optimized with an airblade-like slit nozzle in a gas flow assisted vacuum drying step. This yields large, continuous perovskite layers in light-emitting diodes with an active area up to 1600 mm2.

Comparing spiro-OMeTAD and P3HT hole conductors in efficient solid state dye-sensitized solar cells.

Two hole conductor materials, spiro-OMeTAD and P3HT, were compared in solid-state dye-sensitized solar cells. Two organic dyes containing one anchor unit (D35) or two anchor units (M3) were used in the comparison. Absorbed photon to current conversion efficiency close to unity was obtained for the devices with spiro-OMeTAD. Energy conversion efficiencies of 4.7% and 4.9% were measured for the devices with spiro-OMeTAD and the dyes D35 and M3, respectively. For the devices using the P3HT hole conductor the results were rather different comparing the two dye molecules, with energy conversion efficiencies of 3.2% and 0.5% for D35 and M3, respectively. Photo-induced absorption measurements suggest that the regeneration of the dyes, and the polymer infiltration, is not complete using P3HT, while spiro-OMeTAD regenerates the dyes efficiently. However, the TiO(2)/D35/P3HT system shows rather high energy conversion efficiency and electrochemical oxidation of the dyes on TiO(2) indicates that D35 have a more efficient dye to dye hole conduction than M3, which thereby might explain the higher performance. The dye hole conduction may therefore be of significant importance for optimizing the energy conversion in such hybrid TiO(2)/dye/polymer systems.

Contribution from a hole-conducting dye to the photocurrent in solid-state dye-sensitized solar cells.

The hole transporting medium in solid-state dye-sensitized solar cells can be utilized to harvest sunlight. Herein we demonstrate that a triphenylamine-based dye, used as hole-transporting medium, contributes to the photocurrent in a squaraine-sensitized solid-state dye-sensitized solar cell. Steady-state photoluminescence measurements have been used to distinguish between electron transfer and energy transfer processes leading to energy conversion upon light absorption in the hole-transporting dye.

Origin of Ionic Inhomogeneity in MAPb(IxBr1-x)3 Perovskite Thin Films Revealed by In-Situ Spectroscopy during Spin Coating and Annealing.

Irradiation-induced phase segregation in mixed methylammonium halide perovskite samples such as methylammonium lead bromide-iodide, MAPb(IxBr1-x)3, is being studied intensively because it limits the efficiency of wide band gap perovskite solar cells. It has been postulated that this phenomenon depends on the intrinsic ionic (in)homogeneity in samples already induced during film formation. A deeper understanding of the MAPb(IxBr1-x)3 formation processes and the influence of the halide ratio, solvents, and the perovskite precursor composition as well as the influence of processing parameters during deposition, e.g., spin coating and annealing parameters, is still lacking. Here, we use a fiber optic-based optical in-situ setup to study the formation processes of the MAPb(IxBr1-x)3 series on a subsecond time scale during spin coating and thermal annealing. In-situ UV-vis measurements during spin coating reveal the influence of different halide ratios, x, in the precursor solution on the preferential crystallization of the phase. Pure bromide samples directly form a perovskite phase, samples with high iodide content form a solvate intermediate phase, and samples with a mixed stoichiometry between 0.1 ≤ x ≤ 0.6 form both. This leads to a heterogeneous formation process via two competing reaction pathways, that leads to a heterogeneous mixture of phases, during spin coating and rationalizes the compositional heterogeneity of mixed bromide-iodide samples even after annealing.

Relating Defect Luminescence and Nonradiative Charge Recombination in MAPbI3 Perovskite Films.

Nonradiative losses in semiconductors are related to defects. At cryogenic temperatures, defect-related photoluminescence (PL) at energies lower than the band-edge PL is observed in methylammonium lead triiodide perovskite. We applied multispectral PL imaging to samples prepared by two different procedures and exhibiting 1 order of magnitude different PL quantum yield (PLQY). The high-PLQY sample showed concentration of the emitting defect sites around 1012-1013 cm-3. No correlation between PLQY and the relative intensity of the defect emission was found when micrometer-sized local regions of the same sample were compared. However, a clear positive correlation between the lower PLQY and higher defect emission was observed when two preparation methods were contrasted. Therefore, although the emissive defects are not connected directly with the nonradiative centers and may be spatially separated at the nano scale, chemical processes during the perovskite synthesis promote/prevent formation of both types of defects at the same time.

In Situ TEM Monitoring of Phase-Segregation in Inorganic Mixed Halide Perovskite.

Photoinduced phase separation, which limits the available band gap energies for photovoltaic applications, was reported for a range of mixed-halide perovskites. A microscopic understanding of the phase separation mechanism is still lacking but may be beneficial to rationalize limitations as well as enable the design of phase-stable perovskite semiconductors. In this letter, electron-beam-induced phase separations and transformations were investigated in a small crystallite of CsPb(Br0.8I0.2)3 by means of in situ high-resolution imaging in a transmission electron microscope. The acquired time series was evaluated using principal and independent component analysis to classify the structural change during the illumination by the electron beam. A more iodine-rich phase with the approximate composition of CsPb(Br0.6I0.4)3 was found to form at the edges of the particle, while a ternary pure bromide phase of CsPbBr3 remained at its center. These results provide an atomistic picture of in-grain phase segregation into iodide-rich phases at grain boundaries and bromide-rich phases in the interior of the grain.

Large lattice distortions and size-dependent bandgap modulation in epitaxial halide perovskite nanowires.

Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials. However, despite ongoing research from multiple perspectives, some fundamental questions regarding their optoelectronic properties remain controversial. One reason is the high-variance of data collected from, often unstable, polycrystalline thin films. Here we use ordered arrays of stable, single-crystal cesium lead bromide (CsPbBr3) nanowires grown by surface-guided chemical vapor deposition to study fundamental properties of these semiconductors in a one-dimensional model system. Specifically, we uncover the origin of an unusually large size-dependent luminescence emission spectral blue-shift. Using multiple spatially resolved spectroscopy techniques, we establish that bandgap modulation causes the emission shift, and by correlation with state-of-the-art electron microscopy methods, we reveal its origin in substantial and uniform lattice rotations due to heteroepitaxial strain and lattice relaxation. Understanding strain and its effect on the optoelectronic properties of these dynamic materials, from the atomic scale up, is essential to evaluate their performance limits and fundamentals of charge carrier dynamics.

Perfluorinated Self-Assembled Monolayers Enhance the Stability and Efficiency of Inverted Perovskite Solar Cells.

Perovskite solar cells are among the most exciting photovoltaic systems as they combine low recombination losses, ease of fabrication, and high spectral tunability. The Achilles heel of this technology is the device stability due to the ionic nature of the perovskite crystal, rendering it highly hygroscopic, and the extensive diffusion of ions especially at increased temperatures. Herein, we demonstrate the application of a simple solution-processed perfluorinated self-assembled monolayer (p-SAM) that not only enhances the solar cell efficiency, but also improves the stability of the perovskite absorber and, in turn, the solar cell under increased temperature or humid conditions. The p-i-n-type perovskite devices employing these SAMs exhibited power conversion efficiencies surpassing 21%. Notably, the best performing devices are stable under standardized maximum power point operation at 85 °C in inert atmosphere (ISOS-L-2) for more than 250 h and exhibit superior humidity resilience, maintaining ∼95% device performance even if stored in humid air in ambient conditions over months (∼3000 h, ISOS-D-1). Our work, therefore, demonstrates a strategy towards efficient and stable perovskite solar cells with easily deposited functional interlayers.

Luminescent Intermediates and Humidity-Dependent Room-Temperature Conversion of the MAPbI3 Perovskite Precursor.

Preparation of metal-halide perovskites under room temperature attracts attention because of energy saving by removing thermal annealing. Room-temperature transformation of spin-cast wet films consisting of methylammonium (MA) iodide, PbI2, and dimethylformamide toward solid MAPbI3 perovskite proceeds via several intermediate crystalline states and is strongly dependent on ambient humidity. Light transmission and photoluminescence (PL) microscopy and spectroscopy were used to monitor the growth of crystals and transformation of their properties in time under nitrogen atmosphere at room temperature. Under low humidity, a highly luminescent intermediate phase with low absorption in the visible range appears, with the PL spectra composed of several bands in the range from 600 to 760 nm. We assign these bands to low-dimensional (nanocrystals and two-dimensional inclusions) MAPbI3 intermediates, where the exciton confinement shifts the spectrum to higher energies in comparison with the bulk MAPbI3. The intermediate levels of ambient humidity (10-50%) appear to catalyze the conversion of the intermediate phase to MAPbI3. At a high ambient humidity (>80%), the initially formed MAPbI3 is quickly transformed to the transparent hydrate phase of MAPbI3. The role of ambient water catalyzing the material transformation by competing for Pb coordination with the solvent molecules is discussed.

Transformation from crystalline precursor to perovskite in PbCl2-derived MAPbI3.

Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI3) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl2 + 3MAI) (MA = CH3NH3). The as-spun film consists of crystalline MA2PbI3Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.

Defect-induced local variation of crystal phase transition temperature in metal-halide perovskites.

Solution-processed organometal halide perovskites are hybrid crystalline semiconductors highly interesting for low-cost and efficient optoelectronics. Their properties are dependent on the crystal structure. Literature shows a variety of crystal phase transition temperatures and often a spread of the transition over tens of degrees Kelvin. We explain this inconsistency by demonstrating that the temperature of the tetragonal-to-orthorhombic phase transition in methylammonium lead triiodide depends on the concentration and nature of local defects. Phase transition in individual nanowires was studied by photoluminescence microspectroscopy and super-resolution imaging. We propose that upon cooling from 160 to 140 K, domains of the crystal containing fewer defects stay in the tetragonal phase longer than highly defected domains that readily transform to the high bandgap orthorhombic phase at higher temperatures. The existence of relatively pure tetragonal domains during the phase transition leads to drastic photoluminescence enhancement, which is inhomogeneously distributed across perovskite microcrystals.Understanding crystal phase transition in materials is of fundamental importance. Using luminescence spectroscopy and super-resolution imaging, Dobrovolsky et al. study the transition from the tetragonal to orthorhombic crystal phase in methylammonium lead triiodide nanowires at low temperature.

It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis.

Solar cells made from inorganic-organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 °C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current-voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.

"Supertrap" at Work: Extremely Efficient Nonradiative Recombination Channels in MAPbI3 Perovskites Revealed by Luminescence Super-Resolution Imaging and Spectroscopy.

Organo-metal halide perovskites are some of the most promising materials for the new generation of low-cost photovoltaic and light-emitting devices. Their solution processability is a beneficial trait, although it leads to a spatial inhomogeneity of perovskite films with a variation of the trap state density at the nanoscale. Comprehending their properties using traditional spectroscopy therefore becomes difficult, calling for a combination with microscopy in order to see beyond the ensemble-averaged response. We studied photoluminescence (PL) blinking of micrometer-sized individual methylammonium lead iodide (MAPbI3) perovskite polycrystals, as well as monocrystalline microrods up to 10 µm long. We correlated their PL dynamics with structure employing scanning electron and optical super-resolution microscopy. Combining super-resolution localization imaging and super-resolution optical fluctuation imaging (SOFI), we could detect and quantify preferential emitting regions in polycrystals exhibiting different types of blinking. We propose that blinking in MAPbI3 occurs by the activation/passivation of a "supertrap" which presumably is a donor-acceptor pair able to trap both electrons and holes. As such, nonradiative recombination via supertraps, in spite being present at a rather low concentrations (1012-1015 cm-3), is much more efficient than via all other defect states present in the material at higher concentrations (1016-1018 cm-3). We speculate that activation/deactivation of a supertrap occurs by its temporary dissociation into free donor and acceptor impurities. We found that supertraps are most efficient in structurally homogeneous and large MAPbI3 crystals where carrier diffusion is efficient, which may therefore pose limitations on the efficiency of perovskite-based devices.

Silicon-naphthalo/phthalocyanine-hybrid sensitizer for efficient red response in dye-sensitized solar cells.

Introduction of a naphthalocyanine moiety to phthalocyanine allows for a gradual red shift of the absorption spectrum in the resulting chromophore. Using silicon as a core atom allows for the introduction of additional siloxane side chains which mitigate dye aggregation. A dye-sensitized solar cell with this hybrid sensitizer exhibits a broad and flat IPCE of 80% between 600 and 750 nm and high photocurrent densities of 19.0 mA/cm(2).