Quantitative Bimolecular Recombination in Organic Photovoltaics through Triplet Exciton Formation

The nanoscale morphology and high charge densities in organic photovoltaics (OPVs) lead to a high rate of bimolecular encounters between spin-uncorrelated electrons and holes. This process can lead to the formation of low-energy triplet excitons on the donor polymer that decay nonradiatively and limit the device performance. We use time-resolved optical spectroscopy to characterize the effect of morphology through the use of solvent additives such as 1,8-octane­dithiol (ODT) on triplet dynamics and charge recombination in blends of poly­[2,6-(4,4-bis­(2-ethyl­hexyl)-4<i>H</i>-cyclo­penta­[2,1-<i>b</i>;­3,4-<i>b</i>′]-dithio­phene)-<i>alt</i>-4,7-(2,­1,3-benzo­thia­diazole)] and [6,6]-phenyl-C₇₁-butyric acid methyl ester. This is an attractive OPV system since the extended absorption of the polymer into the near-infrared gives good coverage of the solar spectrum, but nevertheless, the internal quantum efficiency (IQE) has not been reported to be higher than ∼65% under short circuit conditions. We find that, without ODT, the IQE is 48% and 16% of excitations decay via bimolecular triplet formation. With ODT treatment, which improves crystallinity and carrier mobility, the IQE increases to 65%, but bimolecular triplet formation significantly increases and now accounts for all of the recombination (35% of charges)

Compatibilization of All-Conjugated Polymer Blends for Organic Photovoltaics

Compatibilization of an immiscible binary blend comprising a conjugated electron donor and a conjugated electron acceptor polymer with suitable electronic properties upon addition of a block copolymer (BCP) composed of the same building blocks is demonstrated. Efficient compatibilization during melt-annealing is feasible when the two polymers are immiscible in the melt, i.e. above the melting point of ∼250 °C of the semicrystalline donor polymer P3HT. To generate immiscibility at these high temperatures, the acceptor polymer PCDTBT is equipped with fluorinated side chains leading to an increased Flory–Huggins interaction parameter. Compatibilization in bulk and thin films is demonstrated, showing that the photovoltaic performance of pristine microphase separated and nanostructured BCPs can also be obtained for compatibilized blend films containing low contents of 10–20 wt % BCP. Thermodynamically stable domain sizes range between several tens of microns for pure blends and ∼10 nm for pure block copolymers. In addition to controlling domain size, the amount of block copolymer added dictates the ratio of edge-on and face-on P3HT crystals, with compatibilized films showing an increasing amount of face-on P3HT crystals with increasing amount of compatibilizer. This study demonstrates the prerequisites and benefits of compatibilizing all-conjugated semicrystalline polymer blends for organic photovoltaics

Aqueous Self-Assembly of an Electroluminescent Double-Helical Metallopolymer

A new type of water-soluble copper-containing polymer has been synthesized using the technique of subcomponent self-assembly. Copper­(I)-directed imine bond formation between triethylene glycol functionalized 1,2-phenylenediamine and 2,9-diformylphenanthroline subcomponents resulted in the formation of a chain in which two conjugated helical ligand strands wrap around a linear array of metal ions. Characterization data from a variety of analytical methods are consistent with our formulation of this material. After purification by dialysis, the polymer was shown to possess several properties of conceptual and practical interest. (1) Individual double-helical strands appear to further aggregate through entanglement of their side chains to form well-defined superstructures such as nanoscale bow ties and macrocycles, which can be imaged on a surface. (2) The material’s copper­(I) ions underwent reversible electrochemical oxidation in solution, whereas analogous model compounds were observed to decompose upon oxidation: the polymer’s greater length appeared to stabilize oxidized states through delocalization or entrapment. (3) Photophysical measurements reveal this material to be photo- and electroluminescent. It has been successfully used for the fabrication of electroluminescent devices and shows a weak emission of white-blue light with CIE coordinates of (0.337, 0.359). This study further demonstrates the utility of the technique of subcomponent self-assembly for the straightforward generation of materials with useful properties

Enhancing Phase Separation and Photovoltaic Performance of All-Conjugated Donor–Acceptor Block Copolymers with Semifluorinated Alkyl Side Chains

Phase separation of all-conjugated donor–acceptor block copolymers is more difficult to achieve compared to classical coil–coil systems owing the intrinsic similarity of the two blocks having both rigid conjugated backbones and alkyl side chains and their generally low degrees of polymerization. Here we demonstrate that side chain fluorination of a poly­(carbazole-<i>alt</i>-dithienylbenzothiadiazole) segment (SF-PCDTBT), to be used as electron acceptor block in combination with poly­(3-hexylthiophene) P3HT as donor block in all-conjugated donor–acceptor block copolymers of type SF-PCDTBT-<i>b</i>-P3HT, strongly increases dissimilarity between P3HT and SF-PCDTBT leading to phase separation for already moderate molar masses. Key to the successful synthesis of a new TBT-monomer with semifluorinated side chains is a direct arylation step that elegantly bypasses classical cross-coupling reactions in which the semifluorinated side chain causes low yields. Suzuki polycondensation of the semifluorinated TBT monomer with a suitable carbazole comonomer and in situ termination by P3HT-Br is optimized extensively with respect to the yield of the end-capping efficiency and molar mass control of the PCDTBT segment. When the fluorinated side chains are replaced by hydrogen (H-PCDTBT) or by <i>n</i>-hexyl chains (hex-PCDTBT), the tendency for phase separation with covalently connected P3HT is much reduced as shown by differential scanning calorimetry and grazing incidence small-angle scattering measurements on thin films. Favorably, of all the block copolymers made only SF-PCDTBT-<i>b</i>-P3HT is microphase separated, exhibits face-on orientation of P3HT domains, and additionally displays surface segregation of the SF-PCDTBT segment at the polymer/air interface. All of these properties are beneficial for single layer single component solar cells. SF-PCDTBT-<i>b</i>-P3HT exhibits the best solar cells performance with a high open-circuit voltage of 1.1 V and a power conversion efficiency of ∼1% which largely outperforms devices based on the analogous H-PCDTBT-<i>b</i>-P3HT and hex-PCDTBT-<i>b</i>-P3HT

In Situ Optical Measurement of Charge Transport Dynamics in Organic Photovoltaics

We present a novel experimental approach which allows extraction of both spatial and temporal information on charge dynamics in organic solar cells. Using the wavelength dependence of the photonic structure in these devices, we monitor the change in spatial overlap between the photogenerated hole distribution and the optical probe profile as a function of time. In a model system we find evidence for a buildup of the photogenerated hole population close to the hole-extracting electrode on a nanosecond time scale and show that this can limit charge transport through space-charge effects under operating conditions

In Situ Optical Measurement of Charge Transport Dynamics in Organic Photovoltaics

We present a novel experimental approach which allows extraction of both spatial and temporal information on charge dynamics in organic solar cells. Using the wavelength dependence of the photonic structure in these devices, we monitor the change in spatial overlap between the photogenerated hole distribution and the optical probe profile as a function of time. In a model system we find evidence for a buildup of the photogenerated hole population close to the hole-extracting electrode on a nanosecond time scale and show that this can limit charge transport through space-charge effects under operating conditions

Improved Performance of ZnO/Polymer Hybrid Photovoltaic Devices by Combining Metal Oxide Doping and Interfacial Modification

Photoinduced charge separation at hybrid organic–inorganic interfaces is poorly understood and challenging to control. We investigate charge separation at a model system of ZnO/poly­(3-hexylthiophene) (P3HT) and employ Sr doping of ZnO and phenyl-C61-butyric acid (PCBA) self-assembled modification to study and enhance the charge separation efficiency. We find that doping alone lowers the efficiency of charge separation due to the introduction of defect states at the oxide surface. However, with the combination of doping and molecular modification, charge separation efficiency is significantly enhanced due to the passivation of interfacial traps and improved modifier coverage. This demonstrates a complex noncumulative effect of doping and surface modification and shows that with the correct choice of metal oxide dopant and organic modifier, a poorly performing hybrid interface can be turned into an efficient one

Morphology-Dependent Charge Photogeneration in Donor–Acceptor Block Copolymer Films Based on Poly(3-hexylthiophene)-<i>bloc<i>k</i></i>-Poly(perylene bisimide acrylate)

We have examined how the nanomorphology and crystallinity of semiconducting double-crystalline block copolymers determine their photophysical and photovoltaic responses. The block copolymers consist of a poly­(3-hexylthiophene) (P3HT) donor block coupled to a polymerized perylene bisimide acrylate (PPerAcr) acceptor. Different molecular weights and processing solvents allow the modification of the donor–acceptor interface with regard to their morphology and crystallinity. Transient absorption spectroscopy was used to resolve photoinduced charge transfer seen on the ∼1 ps time scale, consistent with substantial photoluminescence quenching caused by finely dispersed, disordered donor–acceptor interfaces. For high molecular weight block copolymers, microphase separation is enhanced by slow film formation, leading to slower charge photogeneration. The crystallinity of the P3HT component is of particular importance, which has been monitored spectroscopically. Crystalline P3HT/PPerAcr interfaces lead to high levels of long-lived charge pairs that are more easily extracted in an applied electric field. While external quantum efficiencies of over 25% were obtained, the overall power conversion efficiency of the best block copolymer device is still limited. This is due to the unsuitable orientation of the block copolymer nanomorphology, and the performance lies below that achieved for a blend of equivalent homopolymers. This suggests that increasing the molecular weight of the block copolymers to tune the microphase separation could further improve the photovoltaic efficiency. Our photophysical results give guidelines for future development of promising block copolymer-derived devices, highlighting the importance of interfacial crystallinity and sufficient phase separation

Red-shifted delayed fluorescence at the expense of photoluminescence quantum efficiency ‒ an intramolecular charge-transfer molecule based on a benzodithiophene-4,8-dione acceptor

Employing the thiophene based quinone, benzo[1,2-b:4,5-b′]dithiophene-4,8-dione, as the electron-accepting moiety alongside N-phenylcarbazole donors to produce a donor–π–acceptor–π–donor (D–π–A–π–D) molecule has yielded a new red emitter displaying delayed fluorescence. This new molecule shows strongly (over 100 nm) red-shifted emission when compared to an anthraquinone based analogue. Cyclic voltammetry complemented by computational insights prove that this red-shift is due to the significantly stronger electron-accepting ability of the thiophene quinone compared to anthraquinone. Photophysical and computational studies of this molecule have revealed that while the presence of the thiophene containing acceptor facilitates rapid intersystem crossing which is comparable to anthraquinone analogues, the reverse intersystem crossing rate is slow and non-radiative decay is rapid which we can attribute to low-lying locally excited states. This limits the total photoluminescence quantum efficiency to less than 10% in both solution and the solid state. These results provide a useful example of how very minor structural variations can have a defining impact on the photophysical properties of new molecular materials

Interface-Dependent Radiative and Nonradiative Recombination in Perovskite Solar Cells

Interfacial engineering has shown to play an essential role to optimize recombination losses in perovskite solar cells; however, an in-depth understanding of the various loss mechanisms is still underway. Herein, we study the charge transfer process and reveal the primary recombination mechanism at inorganic electron-transporting contacts such as TiO₂ and its modified organic rivals. The modifiers are chemically ([6,6]-phenyl C₆₁ butyric acid, PC₆₀BA) or physically ([6,6]-phenyl C₆₁ butyric acid methyl ester, PC₆₀BM and C₆₀) attached fullerene to the TiO₂ surface to passivate the density of surface states. We do not observe any change in morphology, crystallinity, and bulk defect density of halide perovskite (CH₃NH₃PbI₃ in this case) upon interface modification. However, we observe compelling results via photoluminescence and electroluminescence studies that the recombination dynamics at both time scales (slow and fast) are largely influenced by the choice of the selective contact. We note a strong correlation between the hysteresis and the so-called slow charge dynamics, both significantly influenced by the characteristics of the selective contact, for example, the presence of surface traps at the selective contact not only shows a larger hysteresis but also leads to higher charge accumulation at the interface and distinguishable slow dynamics (a slower stabilization of recombination dynamics at a time scale of several minutes)