Organic LEDs promise highly efficient lighting and display technologies. We introduce a new class of linear donor-bridge-acceptor light-emitting molecules, which enable solution-processed LEDs with near-100% internal quantum efficiency at high brightness. Key to this performance is their rapid and efficient utilization of triplet states. Using time-resolved spectroscopy, we establish that luminescence via triplets occurs within 350 ns at ambient temperature, after reverse intersystem crossing to singlets. We find that molecular geometries exist at which the singlet-triplet energy gap (exchange energy) is close to zero, such that rapid interconversion is possible. Calculations indicate that exchange energy is tuned by relative rotation of donor and acceptor moieties about the bridge. Unlike other low exchange energy systems, substantial oscillator strength is sustained at the singlet-triplet degeneracy point

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Cs 2 AgBiBr 6 thin film preparation for characterization. The double-perovskite thin films studied in this work were all prepared through sequential vapour deposition. In a vacuumsealed chamber, AgBr (99% Fluka), BiBr 3 (≥98% Sigma Aldrich) and CsBr (99.9% Sigma Aldrich) were placed in separate crucibles and sequentially thermally evaporated onto the substrates. In particular, the standard procedure we optimized evaporated 90 nm of AgBr, 120 nm of BiBr 3 and 150 nm of CsBr to obtain 300 nm of Cs 2 AgBiBr 6 . This basic stack was repeated the necessary number of times to achieve the desired total film thickness. To achieve thicknesses that are not multiples of 300 nm (like the 750 nm reported in the text), we ran the last evaporation cycle depositing half of each precursor thickness, keeping always the same precursors ratio (1:1.3:1.6 AgBr:BiBr 3 :CsBr). After the deposition of the desired thickness, we annealed the samples on a hotplate in air at 250 ºC for 30 minutes. The post-deposition annealing temperature and time were optimised to deliver maximum solar cell performance.Solar cell preparation. FTO or ITO coated glasses were cleaned by sequential sonication in soap, water, acetone and isopropanol. After being dried with a N 2 gun, the substrates were further cleaned by O 2 plasma for 10 minutes. Titanium isopropoxide (140 µl in 1 ml of EtOH) was added to 1 ml of acidic EtOH (10 µl of HCl 2M in 1 ml EtOH), and deposited on the FTO substrates by spincoating at 2000 rpm for 45 sec with 2000 rpm/sec acceleration. Following this, the films were annealed at 150°C for 15 min and 500°C for 30 min. SnO 2 layers were prepared by spincoating at 3000 rpm (200 rpm/sec) for 30 sec of a solution of SnCl 4 ⋅5H 2 O in isopropanol (17.5 mg/ml) on top of the FTO or ITO coated glasses. The so-prepared films were annealed at 100°C for 10 min followed by an annealing at 180°C for 30 min. The SnO 2 and TiO 2 films were placed in the vacuum chamber, and the Cs 2 AgBiBr 6 film was deposited as previously presented. The hole transport material (Spiro-OMeTAD, Lumtec) was dissolved in chlorobenzene (85 mg/ml) and doped with 20 µl of LiTFSI (500 mg/ml in BuOH) and 30 µl of tert-butylpyridine. The solution was then deposited on the active layer by spincoating in air at 2000 rpm (2000 rpm/sec) for 45 sec. The devices were then left overnight in a desiccator in air atmosphere, and then completed by the evaporation of 100 nm silver contacts. All the