To realize efficient photoconversion in organic semiconductors, photogenerated excitons must be dissociated into their constituent electronic charges. In an organic photovoltaic (OPV) cell, this is most often accomplished using an electron donor–acceptor (D–A) interface. Interestingly, recent work on MoO_x/C₆₀ Schottky OPVs has demonstrated that excitons in C₆₀ may also undergo efficient bulk-ionization and generate photocurrent as a result of the large built-in field created by the MoO_x/C₆₀ interface. Here, it is demonstrated that bulk ionization processes also contribute to the short-circuit current density (J_SC) and open-circuit voltage (V_OC) in bulk heterojunction (BHJ) OPVs with fullerene-rich compositions. Temperature-dependent measurements of device performance are used to distinguish dissociation by bulk-ionization from charge transfer at the D–A interface. In optimized fullerene-rich BHJs based on the D–A pairing of boron subphthalocyanine chloride (SubPc)–C₆₀, bulk-ionization is found to be responsible for ≈16% of the total photocurrent, and >30% of the photocurrent originating from C₆₀. The presence of bulk-ionization in C₆₀ also impacts the temperature dependence of V_OC, with fullerene-rich SubPc:C₆₀ BHJ OPVs showing a larger V_OC than evenly mixed BHJs. The prevalence of bulk-ionization processes in efficient, fullerene-rich BHJs underscores the need to include these effects when engineering device design and morphology in OPVs.

Previous studies have identified triplet-triplet annihilation and triplet-polaron quenching as the exciton density-dependent mechanisms which give rise to the efficiency roll-off observed in phosphorescent organic light-emitting devices (OLEDs). In this work, these quenching processes are independently probed, and the impact of the exciton recombination zone width on the severity of quenching in various OLED architectures is examined directly. It is found that in devices employing a graded-emissive layer (G-EML) architecture the efficiency roll-off is due to both triplet-triplet annihilation and triplet-polaron quenching, while in devices which employ a conventional double-emissive layer (D-EML) architecture, the roll-off is dominated by triplet-triplet annihilation. Overall, the efficiency roll-off in G-EML devices is found to be much less severe than in the D-EML device. This result is well accounted for by the larger exciton recombination zone measured in G-EML devices, which serves to reduce exciton density-driven loss pathways at high excitation levels. Indeed, a predictive model of the device efficiency based on the quantitatively measured quenching parameters shows the role a large exciton recombination zone plays in mitigating the roll-off.

An experimental approach to determine the spatial extent and location of the exciton recombination zone in an organic light-emitting device (OLED) is demonstrated. This technique is applicable to a wide variety of OLED structures and is used to examine OLEDs which have a double- (D-EML), mixed- (M-EML), or graded-emissive layer (G-EML) architecture. The location of exciton recombination in an OLED is an important design parameter, as the local optical field sensed by the exciton greatly determines the efficiency and angular distribution of far-field light extraction. The spatial extent of exciton recombination is an important parameter that can strongly impact exciton quenching and OLED efficiency, particularly under high excitation. A direct measurement of the exciton density profile is achieved through the inclusion of a thin, exciton sensitizing strip in the OLED emissive layer which locally quenches guest excitons and whose position in the emissive layer can be translated across the device to probe exciton formation. In the case of the G-EML device architecture, an electronic model is developed to predict the location and extent of the exciton density profile by considering the drift, diffusion, and recombination of charge carriers within the device.

The electrical and structural behavior of uniformly mixed films of boron subphthalocyanine chloride (SubPc) and C₆₀ and their performance in organic photovoltaic cells is explored. Device performance shows a strong dependence on active-layer donor–acceptor composition, and peak efficiency is realized at 80 wt.% C₆₀. The origin of this C₆₀-rich optimum composition is elucidated in terms of morphological changes in the active layer upon diluting SubPc with C₆₀. While neat SubPc is found to be amorphous, mixed films containing 80 wt.% C₆₀ show clear nanocrystalline domains of SubPc. Supporting electrical characterization indicates that this change in morphology coincides with an increase in the hole mobility of the SubPc:C₆₀ mixture, with peak mobility observed at a composition of 80 wt.% C₆₀. Organic photovoltaic cells constructed using this optimum SubPc:C₆₀ ratio realize a power conversion efficiency of (3.7 ± 0.1)% under 100 mW cm⁻² simulated AM1.5G solar illumination.

This work demonstrates an approach for measuring the Förster radius of energy transfer between electron donating and accepting materials commonly used in organic photovoltaic cells (OPVs). While energy transfer processes are surprisingly common in OPVs, they are often incorrectly ignored in measurements of the exciton diffusion length and in models of device performance. Here, the efficiency of energy transfer between an emissive donor and an absorptive acceptor is investigated through complementary experimental and theoretical techniques. This is accomplished by spatially separating the donor and acceptor materials using a wide-energy-gap spacer layer to suppress direct charge transfer and tracking donor photoluminescence as a function of spacer layer thickness. Fitting experimental data obtained for a variety of donor materials allows for the extraction of Förster radii that are in good agreement with predicted values. The impact of donor–acceptor excitonic energy transfer on device performance and on measurements of the exciton diffusion length is also investigated using the archetypical small molecule donor material boron subphthalocyanine chloride (SubPc). An average exciton diffusion length of 7.7 nm is extracted from photoluminescence quenching experiments using SubPc. This value is independent of the quenching material when the role of energy transfer is properly modeled.