Spin-dependent nonlinear processes in organic materials such as singlet-fission and triplet-triplet annihilation could increase the performance for photovoltaics, detectors, and light emitting diodes. Rubrene/C60 light emitting diodes exhibit a distinct low voltage (half-bandgap) threshold for emission. Two origins for the low voltage turn-on have been proposed: (i) Auger assisted energy up-conversion, and (ii) triplet-triplet annihilation. We test these proposals by systematically altering the rubrene/C60 interface kinetics by introducing thin interlayers. Quantitative analysis of the unmodified rubrene/C60 device suggests that higher order processes can be ruled out as the origin of the sub-bandgap turn-on. Rather, band-to-band recombination is the most likely radiative recombination process. However, insertion of a bathocuproine layer yields a 3-fold increase in luminance compared to the unmodified device. This indicates that suppression of parasitic interface processes by judicious modification of the interface allows a triplet-triplet annihilation channel to be observed.

Cascade heterojunction (CHJ) organic solar cells have recently emerged as an alternative to conventional bulk heterojunctions and series-connected tandems due to their signifi cant promise for high internal quantum effi ciency (IQE) and broad spectral coverage. However, CHJ devices thus far have also exhibited poor fi ll factor (FF), resulting in minimal enhancements (or even decreases) in power conversion effi ciency (PCE) when compared with single heterojunction (SHJ) cells. In this study, the major variables controlling the CHJ maximum power point and FF are determined using a combinatorial approach. By matching the maximum power point voltage (V MPP) of the constituent parallel-connected heterojunctions (subjunctions) and minimizing the injection barriers intrinsic to CHJs, high FF and PCE can be achieved. Optimized CHJ devices are demonstrated with >99% IQE in the interlayer and a 46% increase in PCE compared to a SHJ reference (4.1% versus 2.8%). Devices with a transparent exciton dissociation layer (EDL)/interlayer/acceptor structure are employed, such that each CHJ has absorption effi ciency identical to its interlayer/acceptor SHJ counterpart. Using these results, a clear map of performance as a function of material parameters is developed, providing straightforward, universal design rules to guide future engineering of molecules and layer architectures for CHJ organic photovoltaic devices.

In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that “reflect” excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.

A substantial broadband increase in the external quantum effi ciency (EQE) of thin-fi lm organic photovoltaic (OPV) devices using near-fi eld coupling to surface plasmons is reported, signifi cantly enhancing absorption at surface plasmon resonance (SPR). The devices tested consist of an archetypal boron subpthalocyanine chloride/fullerene (SubPc/C 60 ) donor/acceptor heterojunction embedded within a planar semitransparent metallic nanocavity. The absorption and EQE are modeled in detail and probed by attenuated total internal refl ection spectroscopy with excellent agreement. At SPR, the EQE can be enhanced fourfold relative to normal incidence, due to simulated ninefold enhancement in active layer absorption effi ciency. The response at SPR is thickness-independent, down to a few monolayers, suggesting the ability to excite monolayer-scale junctions with an EQE of ≈6% and a 16-fold absorption enhancement over normal incidence. These results potentially impact the future design of plasmonically enhanced thin-fi lm photovoltaics and photodetectors and enable the direct analysis of the dynamics of photocurrent production at OPV heterojunctions.

To realize the full promise of solution deposited photovoltaic devices requires processes compatible with high-speed manufacturing. We report the performance and morphology of blade-coated bulk heterojunction devices based on the small molecule donor p-DTS(FBTTh2)2 when treated with a post-deposition solvent vapor annealing (SVA) process. SVA with tetrahydrofuran improves the device performance of blade-coated films more than solvent additive processing (SA) with 1,8-diiodooctane. In spin-coating, SA and SVA achieve similar device performance. Our optimized, blade coated, SVA devices achieve power conversion efficiencies over 8 % and maintain high efficiencies in films up to ≈ 250 nm thickness, providing valuable resilience to small process variations in high-speed manufacturing. Using impedance spectroscopy, we show that this advantageous behavior originates from highly suppressed bimolecular recombination in the SVA-treated films. Electron microscopy and grazing-incidence X-ray scattering experiments show that SA and SVA both produce highly crystalline donor domains, but SVA films have a radically smaller domain size compared to SA films. We attribute the different behavior to variations in initial nucleation density and relative ability of SVA and SA to control subsequent crystal growth.