We investigate the effect of high work function contacts in halide perovskite absorber-based photovoltaic devices. Photoemission spectroscopy measurements reveal that band bending is induced in the absorber by the deposition of the high work function molybdenum trioxide (MoO). We find that direct contact between MoO and the perovskite leads to a chemical reaction, which diminishes device functionality. Introducing an ultrathin spiro-MeOTAD buffer layer prevents the reaction, yet the altered evolution of the energy levels in the methylammonium lead iodide (MAPbI) layer at the interface still negatively impacts device performance.

The recombination of injected electrons with oxidized redox species and regeneration behavior of copper redox mediators are investigated for four copper complexes, [Cu(dmby) 2 ] 2+/1+ (dmby = 6,6′-dimethyl-2,2′bipyridine), [Cu(tmby) 2 ] 2+/1+ (tmby = 4,4′,6,6′-tetramethyl-2,2′-bipyridine), [Cu(eto) 2 ] 2+/1+ (eto = 4-ethoxy-6,6′-dimethyl-2,2′-bipyridine), and [Cu-(dmp) 2 ] 2+/1+ (dmp = bis(2,9-dimethyl-1,10-phenantroline). These complexes were examined in conjunction with the D5, D35, and D45 sensitizers, having various degrees of blocking moieties. The experimental results were further supported by density functional theory calculations, showing that the low reorganization energies, λ, of tetra-coordinated Cu(I) species (λ = 0.31−0.34 eV) allow efficient regeneration of the oxidized dye at driving forces down to approximately 0.1 eV. The regeneration electron transfer reaction is in the Marcus normal regime. However, for Cu(II) species, the presence of 4-tertbutylpyridine (TBP) in electrolyte medium results in penta-coordinated complexes with altered charge recombination kinetics (λ = 1.23−1.40 eV). These higher reorganization energies lead to charge recombination in the Marcus normal regime instead of the Marcus inverted regime that could have been expected from the large driving force for electrons in the conduction band of TiO 2 to react with Cu(II). Nevertheless, the recombination resistance and electron lifetime values were higher for the copper redox species compared to the reference cobalt redox mediator. The DSC devices employing D35 dye with [Cu(dmp) 2 ] 2+/1+ reached a record value for the open circuit voltage of 1.14 V without compromising the short circuit current density value. Even with the D5 dye, which lacks recombination preventing steric units, we reached 7.5% efficiency by employing [Cu(dmp) 2 ] 2+/1+ and [Cu(dmby) 2 ] 2+/1+ at AM 1.5G full sun illumination with open circuit voltage values as high as 1.13 V.

Infrared-to-visible photon upconversion could benefit applications such as photovoltaics, infrared sensing, and bioimaging. Solid-state upconversion based on triplet exciton annihilation sensitized by nanocrystals is one of the most promising approaches, albeit limited by relatively weak optical absorption. Here, we integrate the upconverting layers into a Fabry−Peŕot microcavity with quality factor Q = 75. At the resonant wavelength λ = 980 nm, absorption increases 74-fold and we observe a 227-fold increase in the intensity of upconverted emission. The threshold excitation intensity is reduced by 2 orders of magnitude to a subsolar flux of 13 mW/ cm 2 . We measure an external quantum efficiency of 0.06 ± 0.01% and a 2.2-fold increase in the generation yield of upconverted photons. Our work highlights the potential of triplet−triplet annihilation-based upconversion in low-intensity sensing applications and demonstrates the importance of photonic designs in addition to materials engineering to improve the efficiency of solid-state upconversion.

which enable internal quantum efficiencies (IQEs) of up to 100%. [2,3] However, they rely on the use of expensive noble metals. [4] Moreover, blue phosphorescent emitters degrade rapidly, which result in low operational lifetimes. [5] As a consequence, blue colors are currently being produced by fluorescent emitters. Those emitters have long been limited to internal quantum efficiencies of 25% but can reach internal quantum efficiencies of up to 62.5% by employing triplet fusion. [6,7] Thermally activated delayed fluorescence (TADF) is a more efficient alternative to classical fluorescence and it can exhibit IQEs up to 100%. [8][9][10][11] In devices, three quarters of excitons are statistically formed in a triplet state, which are nonemissive in case of fluorescent emitters. [12] TADF molecules harness both singlet and triplet excitons by introducing appreciable reverse intersystem crossing (RISC) to convert nonemissive triplet excitons to emissive singlet excitons. [13,14] By spatially separating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the exchange energy is lowered and singlettriplet splitting (ΔE ST = E S1 − E T1 ) is reduced. [8] As a consequence, the singlet state becomes thermally accessible from the triplet state at room temperature. It has also been revealed that the resulting RISC proceeds via vibronic coupling of the triplet charge transfer state ( 3 CT) state with a triplet localized exciton state ( 3 LE), which in turn couples to the singlet manifold. [15] While many TADF molecules have been reported, [16][17][18][19][20][21][22] widely applicable molecular design principles beyond trial-anderror approaches remain scarce. [23][24][25] Recently, Adachi and coworkers [26] and Huang et al. [27] have shown that it is possible to enhance RISC in blue fluorescent molecules with carbazole donors by dihedral angle tuning (Figure 1a,b). The dihedral angle between donor and linker (θ 1 ) and the dihedral angle between linker and acceptor (θ 2 ) can be tuned by attaching methyl groups to the phenylene linker. Advantageously, this can be done without significantly changing emission wavelengths. As a result, deep blue emitters with external quantum efficiencies (EQEs) slightly above 10% were obtained. Herein, we report our latest efforts to test the general validity of the dihedral angle tuning strategy by applying it to a promising Efficient and stable blue emitters for organic light-emitting diodes are urgently needed for next-generation display and lighting applications. The discovery of thermally activated delayed fluorescence (TADF) has revealed a new class of promising candidates. After pairing the iminodibenzyl donor with the triazine acceptor via a phenylene linker, dihedral angle tuning is employed to regulate the difference between the energy levels of singlet and triplet excited states. Enhanced reverse intersystem crossing rates are observed in response to increased methylation at the phenylene linker. This behavior agrees with the density function...

Many efforts have been made to understand OLED degradation behavior. [3][4][5][6][7][8][9][10] While extrinsic degradation mechanisms have been identified and minimized, [11] routine identification of primary intrinsic degradation mechanisms has not been developed. Indeed, diagnostic spectr oscopy at the microscopic level is challenging given the exceedingly rare processes involved. Red phosphorescent OLEDs, for example, exhibit a decay time to 95% of initial luminance (LT95) of ≈20 000 h at an operating brightness of 1000 nits. [12] Given that the active phosphorescent dye is present at 3% loading in a 30 nm thick emissive layer, this yields more than 60 billion excitons per dye to LT90. One approach to the experimental challenge is to build models that consider an array of possible phenomena that are then fit to degradation data for a specific combination of materials in a specific device structure. [3,4,[13][14][15][16][17][18] Due to the multitude of fitting parameters, however, such models do not allow for an unequivocal, and more importantly, generalized quantification of physical parameters governing degradation.Given the challenges that confront the rational design of OLED materials, it is important to determine general characteristics of OLED failure processes by isolating individual key parameters in direct experimental probes. We focus on long-lived triplet excitons, which are common to all OLEDs, and are hypothesized as an important energy source for degradation processes. The total energy stored in triplet excitons is dependent on the triplet exciton density which, in turn, is determined by the triplet exciton generation rate, G, and the triplet exciton lifetime, τ. Using these two key parameters, we construct a simple model of OLED degradation based on three primary classes of known failure mechanisms.As shown in Table 1, the first class (i) of degradation mechanisms is unimolecular pathways. Examples include spontaneous degradation of a given molecule in its excited state, or an impurity-assisted process such as photo-oxidation. [19] Unimolecular processes are distinguished from the other classes because they scale linearly with the number of triplet excited states in the OLED. The second class (ii) of degradation mechanism is triplet-charge interactions. [14,20] Here, triplet excitons collide with a charged molecule, forming a high energy state which initiates permanent damage to one of the molecules. Assuming that the charge density is determined by nongeminate recombination, Organic light-emitting devices (OLEDs) are widely used for mobile displays, but the relatively short lifetime of blue OLEDs remains a challenge in many applications. Typically, instability is viewed as a material-specific chemical degradation problem. It is known to be alleviated by reducing the operating current or otherwise decreasing the exciton density. It is shown here that this view is incomplete. For archetypical phosphorescent materials, it is observed that the dependence of photostability on the triplet exciton life...