All inorganic lead halide perovskite nanocrystals (PNCs) typically suffer from poor stability against moisture and UV radiation as well as degradation during thermal treatment. The stability of PNCs can be significantly enhanced through polymer encapsulation, often accompanied by a decrease of photoluminescence quantum yield (PLQY) due to the loss of highly dynamic oleylamine/oleic acid (OLA/OA) ligands. Herein, we propose a solution for this problem by utilizing partially hydrolyzed poly(methyl methacrylate) (h-PMMA) and highly branched poly(ethylenimine) (b-PEI) as double ligands stabilizing the PNCs already during the mechanochemical synthesis (grinding). The hydrophobic polymer of h-PMMA imparts excellent film-forming properties and water stability to the resulting NC−polymer composite. In its own turn, the b-PEI forms an amino-rich, strongly binding ligand layer on the surface of the PNCs being responsible for the significant improvement of the PLQY and the stability of the resulting material. Moreover, the introduction of b-PEI promotes a partial phase conversion from CsPbBr 3 to CsPb 2 Br 5 to obtain CsPbBr 3 /CsPb 2 Br 5 nanocrystals with a core− shell-like structure. As-prepared PNCs solutions are directly processable as inks, while their PLQY drops only slightly from 75% in colloidal solution to 65% in films. Moreover, the final PNC−polymer film exhibits excellent stability against water, heat, and ultraviolet light irradiation. These superior properties allowed us to fabricate a proof of concept thin film OLED with h-PMMA/b-PEI-stabilized PNCs as an easily processable, narrowly emitting color conversion composite material. KEYWORDS: CsPbBr 3 −CsPb 2 Br 5 , partially hydrolyzed PMMA, highly branched PEI, high photoluminescence quantum yield, stability B ecause of their excellent photophysical properties, such as adjustable band gaps, high molar extinction coefficients, and excellent charge−transfer performance, all-inorganic cesium lead halide perovskite nanomaterials CsPbX 3 (X = Cl, Br, or I) have been perfect candidates for many optoelectronic applications, such as solar cells, 1 LEDs, 2,3 lasers, 4 photodetectors, 5 field effect transistors (FETs), 6 and Xray scintillators. 7 However, moisture, heat, and oxygen make perovskite nanomaterials suffering from poor stability. 8,9 For example, they dissolve in polar solvents, such as water, due to the ionic nature of the material itself. Additionally, perovskite nanomaterials easily undergo phase transitions and decompose

After 60 years of research, silicon solar cell efficiency saturated close to the theoretical limit, and radically new approaches are needed to further improve the efficiency. The use of tandem systems raises this theoretical power conversion efficiency limit from 34% to 45%. We present the advantageous spectral stability of using voltage-matched tandem solar cells with respect to their traditional series-connected counterparts and experimentally demonstrate how singlet fission can be used to produce simple voltage-matched tandems. Our singlet fission silicon–pentacene tandem solar cell shows efficient photocurrent addition. This allows the tandem system to benefit from carrier multiplication and to produce an external quantum efficiency exceeding 100% at the main absorption peak of pentacene.

For almost 70 years, Förster resonance energy transfer (FRET) has been investigated, implemented into nowadays experimental nanoscience techniques, and considered in a manifold of optics, photonics, and optoelectronics applications. Here, we demonstrate for the first time simultaneous and efficient energy transfer from both donating singlet and triplet states of a single photoluminescent molecular species. Using a biluminescent donor that can emit with high yield from both excited states at room temperature allows application of the FRET framework to such a bimodal system. It serves as an exclusive model system where the spatial origin of energy transfer is exactly the same for both donating spin states involved. Of paramount significance are the facts that both transfers can easily be observed by eye and that Förster theory is successfully applied to state lifetimes spanning over 8 orders of magnitude.

efficiency (PCE). [4,5] The main reason is their low open-circuit voltage (V OC ) as compared to the optical gap (E opt ) of the main absorbing materials. [6] All photovoltaic (PV) technologies suffer from voltage losses, arising from fundamental radiative recombination and parasitic nonradiative recombination. Radiative recombination is inevitable, and is the only recombination process taking place in an ideal solar cell. [7][8][9][10] This process determines the upper limit of the V OC , denoted as the radiative open-circuit voltage V r . In reality, the measured V OC is lower than V r due to the presence of nonradiative decay channels, lowering V r by ΔV nrRau has shown that ΔV nr is proportional to the natural logarithm of the quantum efficiency of emission (EQE EL ). [7] The validity of Equation (1) for OSCs has been shown previously, [11,12] where ΔV nr typically accounts for 0.25-0.40 V of the total voltage losses (ΔV OC = E CT − V OC ). [8,[12][13][14] This is a much higher value than in inorganic and Perovskite solar cells, where ΔV nr ≤0.15 V. [15][16][17] In addition to voltage losses due to radiative and nonradiative recombination, OSCs suffer voltage losses because the photogenerated excitons on the donor (D) or acceptor (A) undergo a charge transfer to form an interfacial charge-transfer (CT) state with energy E CT . However, it has been recently shown that the energy difference between the optical gap of the donor or acceptor and the CT state (E opt − E CT ) can be minimized to less than 0.05 eV [13,18] and even down to 0.01 eV, [6] without sacrificing efficient free charge carrier generation. Therefore, in the OSCs with the currently lowest voltage losses, nonradiative recombination is the main reason for the low V OC as compared to other PV technologies employing absorber with similar optical gaps.In a previous study, we have shown for a whole range of solution and vacuum processed OSCs that ΔV nr correlates with E CT . This led us to the conclusion that nonradiative decay is mediated by CT state decay via electron-phonon coupling. [12] However, in the related OLED technology, the major nonradiative decay channel is mediated by the triplet excited states. [19] In OSCs, triplet states are present on both the D and A materials, and for high voltage OSCs the energy of the lowest energy The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn-and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C 60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T 1 ) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanc...

Organic light-emitting diodes (OLEDs) have become a major pixel technology in the display sector, with products spanning the entire range of current panel sizes. The ability to freely scale the active area to large and random surfaces paired with flexible substrates provides additional application scenarios for OLEDs in the general lighting, automotive, and signage sectors. These applications require higher brightness and, thus, current density operation compared to the specifications needed for general displays. As extended transparent electrodes pose a significant ohmic resistance, OLEDs suffering from Joule self-heating exhibit spatial inhomogeneities in electrical potential, current density, and hence luminance. In this article, we provide experimental proof of the theoretical prediction that OLEDs will display regions of decreasing luminance with increasing driving current. With a two-dimensional OLED model, we can conclude that these regions are switched back locally in voltage as well as current due to insufficient lateral thermal coupling. Experimentally, we demonstrate this effect in lab-scale devices and derive that it becomes more severe with increasing pixel size, which implies its significance for large-area, high-brightness use cases of OLEDs. Equally, these non-linear switching effects cannot be ignored with respect to the long-term operation and stability of OLEDs; in particular, they might be important for the understanding of sudden-death scenarios.