Violet electroluminescence is rare in both inorganic and organic light-emitting diodes (LEDs). Low-cost and room-temperature solution-processed lead halide perovskites with high-efficiency and color-tunable photoluminescence are promising for LEDs. Here, we report room-temperature color-pure violet LEDs based on a two-dimensional lead halide perovskite material, namely, 2-phenylethylammonium (C6H5CH2CH2NH3(+), PEA) lead bromide [(PEA)2PbBr4]. The natural quantum confinement of two-dimensional layered perovskite (PEA)2PbBr4 allows for photoluminescence of shorter wavelength (410 nm) than its three-dimensional counterpart. By converting as-deposited polycrystalline thin films to micrometer-sized (PEA)2PbBr4 nanoplates using solvent vapor annealing, we successfully integrated this layered perovskite material into LEDs and achieved efficient room-temperature violet electroluminescence at 410 nm with a narrow bandwidth. This conversion to nanoplates significantly enhanced the crystallinity and photophysical properties of the (PEA)2PbBr4 samples and the external quantum efficiency of the violet LED. The solvent vapor annealing method reported herein can be generally applied to other perovskite materials to increase their grain size and, ultimately, improve the performance of optoelectronic devices based on perovskite materials.

We demonstrate a high open circuit voltage (Voc), high fill factor, single-junction organic photovoltaic (OPV) cell consisting of tetraphenyldibenzoperiflanthene (DBP) as the donor and pyrrolo[3,4-c]pyrrole- 1,4-dione, 3,6-bis(4-chlorophenyl)-2, 5-dihydro (DPP) as the acceptor in a vapor-deposited, fullerene free organic solar cell. Under one sun illumination, the DBP/DPP based planar heterojunction solar cell exhibits a Voc of 1.19 V with a fill factor of 0.67, leading to a final power conversion efficiency (PCE) of 2.2%. Our achieved Voc is the highest value reported thus far for a single-junction, high fill factor organic solar cell. A planar-mixed device architecture allows for high exciton generation while maintaining high exciton dissociation, leading to a PCE of 3.2%. The DBP and DPP interface creates a high interface energy gap (ΔEDA) and relatively low saturation current (JS) due to poor coupling across the donor-acceptor interface. These results support earlier theories that high Voc values are dependent on large ΔΕDA and low JS for a given donor-acceptor combination in OPVs.

We report an unassisted solar water splitting system powered by a diketopyrrolopyrrole (DPP)-containing semitransparent organic solar cell. Two major merits of this fullerene-free solar cell enable its integration with a BiVO photoanode. First is the high open circuit voltage and high fill factor displayed by this single junction solar cell, which yields sufficient power to effect water splitting when serially connected to an appropriate electrode/catalyst. Second, the wavelength-resolved photoaction spectrum of the DPP-based solar cell has minimal overlap with that of the BiVO photoanode, thus ensuring that light collection across these two components can be optimized. The latter feature enables a new water splitting device configuration wherein the solar cell is placed first in the path of incident light, before the BiVO photoanode, although BiVO has a wider bandgap. This configuration is accessed by replacing the reflective top electrode of the standard DPP-based solar cell with a thin metal film and an antireflection layer, thus rendering the solar cell semitransparent. In this configuration, incident light does not travel through the aqueous electrolyte to reach the solar cell or photoanode, and therefore, photon losses due to the scattering of water are reduced. Moreover, this new configuration allows the BiVO photoanode to be back-illuminated, i.e., through the BiVO/back contact interface, which leads to higher photocurrents compared to front illumination. The combination of a semitransparent single-junction solar cell and a BiVO photoanode coated with oxygen evolution catalysts in a new device configuration yielded an unassisted solar water splitting system with a solar-to-hydrogen conversion efficiency of 2.2% in water.

We demonstrated an indium tin oxide (ITO)-free, highly transparent organic solar cell with the potential to be integrated into window panes for energy harvesting purposes. A transparent, conductive ZnO/Ag/ZnO multilayer electrode and a Ag:Ca thin film electrode were used in this transparent device as the bottom and top electrode, respectively. To further improve the transmittance of the solar cell, the thickness of the top ZnO layer was investigated both experimentally and with simulations. An average visible transmittance of >60% was reached, with a maximum transmittance of 73% at 556 nm. Both top and bottom illumination of the solar cell generated comparable power conversion efficiencies, which indicates the wide application of this solar cell structure. In addition, we fabricated distributed Bragg reflector mirrors with sputtered SiO 2 and TiO 2 , which efficiently increased the power conversion efficiency over 20% for the solar cells on glass and poly(ethylene terephthalate) (PET) substrates.

Vapor-deposited, planar heterojunction organic solar cells containing a periflanthene donor and either a fullerene or non-fullerene acceptor are investigated. A high V OC of 1.16 V is observed in devices containing the non-fullerene, pyrrolo[3,4-c]pyrrole-1,4-dione, 3,6-bis(4-chlorophenyl)-2,5dihydro acceptor, whereas analogous devices containing C 60 only result in a V OC of 0.8 V. The measured band energy levels of the two different acceptors do not readily explain the observed difference. Small-perturbation transient photovoltage and transient photocurrent measurements reveal that interfacial charge recombination is comparatively slower for the non-fullerene acceptor, resulting in relatively higher V oc values. Published by AIP Publishing.