Determining Structures of Layer-by-Layer Spin-Coated Zinc Dicarboxylate-Based Metal-Organic Thin Films.

Thin films of crystalline solids with substantial free volume built from organic chromophores and metal secondary building units (SBUs) are promising for engineering new optoelectronic properties through control of interchromophore coupling. Zn-based SBUs are especially relevant in this case because they avoid quenching the chromophore's luminescence. We find that layer-by-layer spin-coating using Zn acetate dihydrate and benzene-1,4-dicarboxylic acid (H2BDC) and biphenyl-4,4'-dicarboxylic acid (H2BPDC) linkers readily produces crystalline thin films. However, analysis of the grazing-incidence wide-angle X-ray scattering (GIWAXS) data reveals the structures of these films vary significantly with the linker, and with the metal-to-linker molar ratio used for fabrication. Under equimolar conditions, H2BPDC creates a type of structure like that proposed for SURMOF-2, whereas H2BDC generates a different metal-hydroxide-organic framework. Large excess of Zn2+ ions causes the growth of layered zinc hydroxides, irrespective of the linker used. Density functional theory (DFT) calculations provide structural models with minimum total energy that are consistent with the experimentally observed diffractograms. In the broader sense, this work illustrates the importance in this field of careful structure determination, e. g., by utilizing GIWAXS and DFT simulations to determine the structure of the obtained crystalline metal-organic thin films, such that properties can be rationally engineered and explained.

Enhancement of Amplified Spontaneous Emission by Electric Field in CsPbBr3 Perovskites.

Perovskite gain materials can sustain continuous-wave lasing at room-temperature. A first step toward the unachieved goal of electrically excited lasing would be an improvement in gain when electrical stimulation is added to the optical. However, to date, electrical stimulation supplementing optical has reduced gain performance. We find that amplified spontaneous emission (ASE) in a CsPbBr3 perovskite light-emitting diode (LED) held under invariant subthreshold optical excitation can be turned on/off by the addition/removal of an electric field. A positive bias voltage leads to a factor of 3 reduction in the optical ASE threshold, the cause of which can be attributed to an enhancement of the radiative rate. The slow components (10 s time scale) of the modulation in the photoluminescence and ASE when the voltage is changed suggest that the relocation of mobile ions trigger the increased radiative rate and observed lowering of ASE thresholds.

Solar-pumped fiber laser using a solid-state luminescent solar collector.

We have developed a fully planar solar-pumped fiber laser using a solid-state luminescent solar collector (LSC). This laser does not use any focusing device, such as a lens or mirror; thus, it can lase without tracking the sun. Our developed device with an aperture of 30 cm emits 15 mW, corresponding to an optical-to-optical conversion efficiency of 0.023% and a collection efficiency of 0.21 W/m2. A 12-fold improvement over a previously developed liquid LSC is achieved by combining the total internal reflection of the solid-state LSC with dielectric multilayer mirrors. The observed laser power is in good agreement with that predicted via numerical simulation, demonstrating the effectiveness of our proposed method.

Photon Upconversion for Photovoltaics and Photocatalysis: A Critical Review.

Opportunities for enhancing solar energy harvesting using photon upconversion are reviewed. The increasing prominence of bifacial solar cells is an enabling factor for the implementation of upconversion, however, when the realistic constraints of current best-performing silicon devices are considered, many challenges remain before silicon photovoltaics operating under nonconcentrated sunlight can be enhanced via lanthanide-based upconversion. A photophysical model reveals that >1-2 orders of magnitude increase in the intermediate state lifetime, energy transfer rate, or generation rate would be needed before such solar upconversion could start to become efficient. Methods to increase the generation rate such as the use of cosensitizers to expand the absorption range and the use of plasmonics or photonic structures are reviewed. The opportunities and challenges for these approaches (or combinations thereof) to achieve efficient solar upconversion are discussed. The opportunity for enhancing the performance of technologies such as luminescent solar concentrators by combining upconversion together with micro-optics is also reviewed. Triplet-triplet annihilation-based upconversion is progressing steadily toward being relevant to lower-bandgap solar cells. Looking toward photocatalysis, photophysical modeling indicates that current blue-to-ultraviolet lanthanide upconversion systems are very inefficient. However, hope remains in this direction for organic upconversion enhancing the performance of visible-light-active photocatalysts.

Absolute quantum yield for understanding upconversion and downshift luminescence in PbF2:Er3+,Yb3+ crystals.

The search for new materials capable of efficient upconversion continues to attract attention. In this work, a comprehensive study of the upconversion luminescence in PbF2:Er3+,Yb3+ crystals with different concentrations of Yb3+ ions in the range of 2 to 7.5 mol% (Er3+ concentration was fixed at 2 mol%) was carried out. The highest value of upconversion quantum yield (ϕUC) 5.9% (at 350 W cm-2) was found in the PbF2 crystal doped with 2 mol% Er3+ and 3 mol% Yb3+. Since it is not always easy to directly measure ϕUC and estimate the related key figure of merit parameter, saturated photoluminescence quantum yield (ϕUCsat), a method to reliably predict ϕUCsat can be useful. Judd-Ofelt theory provides a convenient way to determine the radiative lifetimes of the excited states of rare-earth ions based on absorption measurements. When the luminescence decay times after direct excitation of a level are also measured, ϕUCsat for that level can be calculated. This approach is tested on a series of PbF2:Er3+,Yb3+ crystals. Good agreement between the estimates obtained as above and the directly experimentally measured ϕUCsat values is demonstrated. In addition, three methods of Judd-Ofelt calculations on powder samples were tested and the results were compared with Judd-Ofelt calculations on single crystals, which served as the source of the powder samples. Taken together, the results presented in our work for PbF2:Er3+,Yb3+ crystals contribute to a better understanding of the UC phenomena and provide a reference data set for the use of UC materials in practical applications.

Tunable J-type aggregation of silicon phthalocyanines in a surface-anchored metal-organic framework thin film.

Organic chromophores and semiconductors, like anthracene, pentacene, perylene, and porphyrin, are prone to aggregation, and their packing in the solid state is often hard to predict and difficult to control. As the condensed phase structures of these chromophores and semiconductors are of crucial importance for their optoelectronic functionality, strategies to control their assembly and provide new structural motifs are important. One such approach uses metal-organic frameworks (MOFs); the organic chromophore is converted into a linker and connected by metal ions or nodes. The spatial arrangement of the organic linkers can be well-defined in a MOF, and hence optoelectronic functions can be adjusted accordingly. We have used such a strategy to assemble a phthalocyanine chromophore and illustrated that the electronic inter-phthalocyanine coupling can be rationally tuned by introducing bulky side grounds to increase steric hindrance. We have designed new phthalocyanine linkers and using a layer-by-layer liquid-phase epitaxy strategy thin films of phthalocyanine-based MOFs have been fabricated and their photophysical properties explored. It was found that increasing the steric hindrance around the phthalocyanine reduced the effect of J-aggregation in the thin film structures.

BODIPY-pyrene donor-acceptor sensitizers for triplet-triplet annihilation upconversion: the impact of the BODIPY-core on upconversion efficiency.

Triplet-triplet annihilation upconversion (TTA-UC) is an important type of optical process with applications in biophotonics, solar energy harvesting and photochemistry. In most of the TTA-UC systems, the formation of triplet excited states takes place via spin-orbital interactions promoted by heavy atoms. Given the crucial role of heavy atoms (especially noble metals, such as Pd and Pt) in promoting intersystem crossing (ISC) and, therefore, in production of UC luminescence, the feasibility of using more readily available and inexpensive sensitizers without heavy atoms remains a challenge. Here, we investigated sensitization of TTA-UC using BODIPY-pyrene heavy-atom-free donor-acceptor dyads with different numbers of alkyl groups in the BODIPY scaffold. The molecules with four and six alkyl groups are unable to sensitize TTA-UC in the investigated solvents (tetrahydrofuran (THF) and dichloromethane (DCM)) due to negligible ISC. In contrast, the dyad with two methyl groups in the BODIPY scaffold and the dyad with unsubstituted BODIPY demonstrate efficient intersystem crossing (ISC) of 49-58%, resulting in TTA-UC with quantum yields of 4.7% and 6.9%, respectively. The analysis of the elementary steps of the TTA-UC process indicates that heavy-atom-free donor-acceptor dyads are less effective than their noble metal counterparts, but may equal them in the future if the right combination of solvent, donor-acceptor sensitizer structure, and new luminescent molecules as TTA-UC emitters can be found.

Interface Pattern Engineering in Core-Shell Upconverting Nanocrystals: Shedding Light on Critical Parameters and Consequences for the Photoluminescence Properties.

Advances in controlling energy migration pathways in core-shell lanthanide (Ln)-based hetero-nanocrystals (HNCs) have relied heavily on assumptions about how optically active centers are distributed within individual HNCs. In this article, it is demonstrated that different types of interface patterns can be formed depending on shell growth conditions. Such interface patterns are not only identified but also characterized with spatial resolution ranging from the nanometer- to the atomic-scale. In the most favorable cases, atomic-scale resolved maps of individual particles are obtained. It is also demonstrated that, for the same type of core-shell architecture, the interface pattern can be engineered with thicknesses of just 1 nm up to several tens of nanometers. Total alloying between the core and shell domains is also possible when using ultra-small particles as seeds. Finally, with different types of interface patterns (same architecture and chemical composition of the core and shell domains) it is possible to modify the output color (yellow, red, and green-yellow) or change (improvement or degradation) the absolute upconversion quantum yield. The results presented in this article introduce an important paradigm shift and pave the way toward the emergence of a new generation of core-shell Ln-based HNCs with better control over their atomic-scale organization.

Tuning Optical Properties by Controlled Aggregation: Electroluminescence Assisted by Thermally-Activated Delayed Fluorescence from Thin Films of Crystalline Chromophores.

Several photophysical properties of chromophores depend crucially on intermolecular interactions. Thermally-activated delayed fluorescence (TADF) is often influenced by close packing of the chromophore assembly. In this context, the metal-organic framework (MOF) approach has several advantages: it can be used to steer aggregation such that the orientation within aggregated structures can be predicted using rational approaches. We demonstrate this design concept for a DPA-TPE (diphenylamine-tetraphenylethylene) chromophore, which is non-emissive in its solvated state due to vibrational quenching. Turning this DPA-TPE into a ditopic linker makes it possible to grow oriented MOF thin films exhibiting pronounced green electroluminescence with low onset voltages. Measurements at different temperatures clearly demonstrate the presence of TADF. Finally, this work reports that the layer-by-layer process used for MOF thin film deposition allows the integration of the TADF-DPA-TPE in a functioning LED device.

Critical Power Density: A Metric To Compare the Excitation Power Density Dependence of Photon Upconversion in Different Inorganic Host Materials.

In photon upconversion (UC) based on triplet-triplet annihilation, the upconversion photoluminescent quantum yield (UC-PLQY) depends on the excitation power density in a way that can be described by a single figure of merit. This figure of merit, the threshold value, allows the excitation power density required for efficient UC-PLQY to be compared between different triplet-triplet annihilation systems. Here, we investigate the excitation power density dependence of two-photon UC processes in a series of four lanthanide-doped inorganic host materials (oxides, fluorides, and chlorides) all doped with 18 mol % Yb3+ sensitizer ions and 2 mol % Er3+ activator ions. We demonstrate that an analogous figure of merit, which we call the critical power density (CPD), accurately describes the UC power dependence of these samples. Better CPD values are obtained when the lifetime of the intermediate states is long. The UC-PLQY at the CPD is linked to the saturation UC-PLQY. Thus, a measurement of the UC-PLQY at this low power density can be used to estimate the theoretical saturation UC-PLQY in the absence of deleterious effects such as laser-induced heating. This is compared to another method to estimate the saturation based on the CPD model, namely, taking half of the level's PLQY under direct excitation. Our careful analysis of the upconversion spectra as a function of excitation power density gives several insights into the differing upconversion pathways in the hosts and proves to be a useful tool for their comparison.

Synthesis of dipolar molecular rotors as linkers for metal-organic frameworks.

We report the synthesis of five dicarboxylic acid-substituted dipolar molecular rotors for the use as linker molecules in metal-organic frameworks (MOFs). The rotor molecules exhibit very low rotational barriers and decent to very high permanent, charge free dipole moments, as shown by density functional theory calculations on the isolated molecules. Four rotors are fluorescent in the visible region. The linker designs are based on push-pull-substituted phenylene cores with ethynyl spacers as rotational axes, functionalized with carboxylic acid groups for implementation in MOFs. The substituents at the phenylene core are chosen to be small to leave rotational freedom in solids with confined free volumes. The dipole moments are generated by electron-donating substituents (benzo-1,3-dioxole, benzo-1,4-dioxane, or benzo-2,1,3-thiadiazole annelation) and withdrawing substituents (difluoro, or dicyano substitution) at the opposite positions of the central phenylene core. A combination of 1,4-dioxane annelation and dicyano substitution generates a theoretically predicted, very high dipole moment of 10.1 Debye. Moreover, the molecules are sufficiently small to fit into cavities of 10 Å3. Hence, the dipolar rotors should be ideally suited as linkers in MOFs with potential applications as ferroelectric materials and for optical signal processing.

Inkjet-printed perovskite distributed feedback lasers.

We report on digitally printed distributed feedback lasers on flexible polyethylene terephthalate substrates based on methylammonium lead iodide perovskite gain material. The perovskite lasers are printed with a digital drop-on-demand inkjet printer, providing full freedom in the shape and design of the gain layer. We show that adjusting the perovskite ink increases the potential processing window and decreases the surface roughness of the active layer to less than 7 nm, which is essential for low lasing thresholds. Prototype inkjet-printed perovskite lasers processed on top of nanopatterned rigid as well as flexible substrates are demonstrated. Optimized perovskite gain layers printed on PET substrates demonstrated lasing and showed a linewidth of 0.4 nm and a lasing threshold of 270 kW/cm2. In addition, printing of a distinct shape shows a high level of uniformity, demonstrated by a low spatial resolved full width half maximum variation over the whole printing area. These results reveal the possibilities of digital printed perovskite layers towards large-scale and low-cost laser applications of arbitrary shape.

Absolute upconversion quantum yields of blue-emitting LiYF4:Yb3+,Tm3+ upconverting nanoparticles.

The upconversion quantum yield (ΦUC) is an essential parameter for the characterization of the optical performance of lanthanoid-doped upconverting nanoparticles (UCNPs). Despite its nonlinear dependence on excitation power density (Pexc), it is typically reported only as a single number. Here, we present the first measurement of absolute upconversion quantum yields of the individual emission bands of blue light-emitting LiYF4:Yb3+,Tm3+ UCNPs in toluene. Reporting the quantum yields for the individual emission bands is required for assessing the usability of UCNPs in various applications that require upconverted light of different wavelengths, such as bioimaging, photocatalysis and phototherapy. Here, the reliability of the ΦUC measurements is demonstrated by studying the same batch of UCNPs in three different research groups. The results show that whereas the total upconversion quantum yield of these UCNPs is quite high-typically 0.02 at a power density of 5 W cm-2-most of the upconverted photon flux is emitted in the 794 nm upconversion band, while the blue emission band at 480 nm is very weak, with a much lower quantum yield of ∼6 × 10-5 at 5 W cm-2. Overall, although the total upconversion quantum yield of LiYF4:Yb3+,Tm3+ UCNPs seems satisfying, notably for NIR bioimaging, blue-light demanding phototherapy applications will require better-performing UCNPs with higher blue light upconversion quantum yields.

Reaction of porphyrin-based surface-anchored metal-organic frameworks caused by prolonged illumination.

Crystalline surface-anchored metal-organic framework (SURMOF) thin films made from porphyrin-based organic linkers have recently been used in both photon upconversion and photovoltaic applications. While these studies showed promising results, the question of photostability in this organic-inorganic hybrid material has to be investigated before applications can be considered. Here, we combine steady-state photoluminescence, transient absorption, and time-resolved electron paramagnetic resonance spectroscopy to examine the effects of prolonged illumination on a palladium-porphyrin based SURMOF thin film. We find that phototreatment leads to a change in the material's photoresponse caused by the creation of stable products of photodecomposition - likely chlorin - inside the SURMOF structure. When the mobile triplet excitons encounter such a defect site, a short-lived (80 ns) cation-anion radical pair can be formed by electron transfer, wherein the charges are localized at a porphyrin and the photoproduct site, respectively.

Excitonically Coupled States in Crystalline Coordination Networks.

When chromophores are brought into close proximity, noncovalent interactions (π-π/CH-π) can lead to the formation of excitonically coupled states, which bestow new photophysical properties upon the aggregates. Because the properties of the new states not only depend on the strength of intermolecular interactions, but also on the relative orientation, supramolecular assemblies, where these parameters can be varied in a deliberate fashion, provide novel possibilities for the control of photophysical properties. This work reports that core-substituted naphthalene diimides (cNDIs) can be incorporated into surface-mounted metal- organic structures/frameworks (SURMOFs) to yield optical properties strikingly different from conventional aggregates of such molecules, for example, formed in solution or by crystallization. Organic linkers are used, based on cNDIs, well-known organic chromophores with numerous applications in different optoelectronic devices, to fabricate MOF thin films on transparent substrates. A thorough characterization of the properties of these highly ordered chromophoric assemblies reveals the presence of non-emissive excited states in the crystalline material. Structural modulations provide further insights into the nature of the coupling that gives rise to an excited-state energy level in the periodic structure.

The impact of donor-acceptor phase separation on the charge carrier dynamics in pBTTT:PCBM photovoltaic blends.

The effect of donor-acceptor phase separation, controlled by the donor-acceptor mixing ratio, on the charge generation and recombination dynamics in pBTTT-C14:PC70 BM bulk heterojunction photovoltaic blends is presented. Transient absorption (TA) spectroscopy spanning the dynamic range from pico- to microseconds in the visible and near-infrared spectral regions reveals that in a 1:1 blend exciton dissociation is ultrafast; however, charges cannot entirely escape their mutual Coulomb attraction and thus predominantly recombine geminately on a sub-ns timescale. In contrast, a polymer:fullerene mixing ratio of 1:4 facilitates the formation of spatially separated, that is free, charges and reduces substantially the fraction of geminate charge recombination, in turn leading to much more efficient photovoltaic devices. This illustrates that spatially extended donor or acceptor domains are required for the separation of charges on an ultrafast timescale (<100 fs), indicating that they are not only important for efficient charge transport and extraction, but also critically influence the initial stages of free charge carrier formation.

Control of charge generation and recombination in ternary polymer/polymer:fullerene photovoltaic blends using amorphous and semi-crystalline copolymers as donors.

Charge generation and recombination processes occurring in ternary photoactive copolymer:copolymer:fullerene blends consisting of different mixing ratios between entirely amorphous and semi-crystalline PPE-PPV copolymers are investigated by transient absorption pump-probe and pump-push photocurrent spectroscopy. The experiments reveal that an excess of semi-crystalline polymer facilitates exciton dissociation into free charge carriers, slows down geminate recombination, and suppresses non-geminate recombination leading to increased short-circuit currents and high fill factors. In contrast, blends utilizing solely the amorphous polymer for their donor phase suffer from a large fraction of sub-nanosecond geminate recombination of interfacially bound charge-transfer states and also from fast non-geminate recombination of free charges, resulting in a significantly reduced photovoltaic performance. However, small fractions of the amorphous polymer blended into the semi-crystalline polymer increase the open-circuit voltage and the fill factor, while keeping the charge generation and recombination parameters largely unaltered in turn leading to an optimized device performance for the ternary PPE-PPV copolymer:copolymer:fullerene blends.

High Quantum Yield Shortwave Infrared Luminescent Tracers for Improved Sorting of Plastic Waste.

More complete recycling of plastic waste is possible only if new technologies that go beyond state-of-the-art near-infrared (NIR) sorting are developed. For example, tracer-based sorting is a new technology that explores the upconversion or down-shift luminescence of special tracers based on inorganic materials codoped with lanthanide ions. Specifically, down-shift tracers emit in the shortwave infrared (SWIR) spectral range and can be detected using a SWIR camera preinstalled in a state-of-the-art sorting machine for NIR sorting. In this study, we synthesized a very efficient SWIR tracer by codoping Li3Ba2Gd3 (MoO4)8 with Yb3+ and Er3+, where Yb3+ is a synthesizer ion (excited near 976 nm) and Er3+ emits near 1550 nm. Fine-tuning of the doping concentration resulted in a tracer (Li3Ba2Gd(3-x-y)(MoO4)8:xYb3+, yEr3+, where x = 0.2 and y = 0.4) with a high photoluminescence quantum yield for 1550 nm emission of 70% (using 976 nm excitation). This tracer was used to mark plastic objects. When the object was illuminated by a halogen lamp and a 976 nm laser, the three parts could be easily distinguished based on reflectance and luminescence spectra in the SWIR range: a plastic bottle made of polyethylene terephthalate, a bottle cap made of high-density polyethylene, and a label made of the tracer Li3Ba2Gd3(MoO4)8:Yb3+, Er3+. Importantly, the use of the tracer in sorting may require only the installation of a 976 nm laser in a state-of-the-art NIR sorting system.

The effect of solvent additives on morphology and excited-state dynamics in PCPDTBT:PCBM photovoltaic blends.

The dependence of the thin film morphology and excited-state dynamics for the low-bandgap donor-acceptor copolymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) in pristine films and in blends (1:2) with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) on the use of the solvent additive 1,8-octanedithiol (ODT) is studied by solid-state nuclear magnetic resonance (NMR) spectroscopy and broadband visible and near-infrared pump-probe transient absorption spectroscopy (TAS) covering a spectral range from 500-2000 nm. The latter allows monitoring of the dynamics of excitons, bound interfacial charge-transfer (CT) states, and free charge carriers over a time range from femto- to microseconds. The broadband pump-probe experiments reveal that excitons are not only generated in the polymer but also in PCBM-rich domains. Depending on the morphology controlled by the use of solvent additives, polymer excitons undergo mainly ultrafast dissociation (<100 fs) in blends prepared without ODT or diffusion-limited dissociation in samples prepared with ODT. Excitons generated in PCBM diffuse slowly to the interface in both samples and undergo dissociation on a time scale of several tens of picoseconds up to hundreds of picoseconds. In both samples a significant fraction of the excitons creates strongly bound interfacial CT states, which exhibit subnanosecond geminate recombination. The total internal quantum efficiency loss due to geminate recombination is estimated to be 50% in samples prepared without ODT and is found to be reduced to 30% with ODT, indicating that more free charges are generated in samples prepared with solvent additives. In samples prepared with ODT, the free charges exhibit clear intensity-dependent recombination dynamics, which can be modeled by Langevin-type recombination with a bimolecular recombination coefficient of 6.3 × 10(-11) cm(3) s(-1). In samples prepared without ODT, an additional nanosecond recombination of polaron pairs is observed in conjunction with an increased intensity-independent trap-assisted nongeminate recombination of charges. Furthermore, a comparison of the triplet-induced absorption spectra of PCPDTBT with the charge-induced absorption in PCPDTBT:PCBM blends reveals that triplets have a very similar excited-state absorption spectrum compared to the free charge carriers, however, in contrast have a distinct intensity-independent lifetime. Overall, our results suggest that whether free charges or strongly bound CT states are created upon dissociation of excitons at the PCPDTBT:PCBM interface is determined instantaneously upon exciton dissociation and that once formed strongly bound CT states rapidly recombine and thus are unlikely to dissociate into free charges. The observation of a significantly larger bimolecular recombination coefficient than previously determined for poly(3-hexylthiophen-2,5-diyl):PCBM (P3HT:PCBM) and PCDTBT:PCBM samples indicates that nongeminate recombination of free charges considerably competes with charge extraction in PCPDTBT:PCBM photovoltaic devices.

Synthesis and controlled self-assembly of covalently linked hexa-peri-hexabenzocoronene/perylene diimide dyads as models to study fundamental energy and electron transfer processes.

We report the synthesis and photophysical characterization of a series of hexa-peri-hexabenzocoronene (HBC)/perylenetetracarboxy diimide (PDI) dyads that are covalently linked with a rigid bridge. Both the ratio of the two components and the conjugation of the bridging element are systematically modified to study the influence on self-assembly and energy and electron transfer between electron donor HBC and acceptor PDI. STM and 2D-WAXS experiments reveal that both in solution and in bulk solid state the dyads assemble into well-ordered two-dimensional supramolecular structures with controllable mutual orientations and distances between donor and acceptor at a nanoscopic scale. Depending on the symmetry of the dyads, either columns with nanosegregated stacks of HBC and PDI or interdigitating networks with alternating HBC and PDI moieties are observed. UV-vis, photoluminescence, transient photoluminescence, and transient absorption spectroscopy confirm that after photoexcitation of the donor HBC a photoinduced electron transfer between HBC and PDI can only compete with the dominant Förster resonance energy transfer, if facilitated by an intimate stacking of HBC and PDI with sufficient orbital overlap. However, while the alternating stacks allow efficient electron transfer, only the nanosegregated stacks provide charge transport channels in bulk state that are a prerequisite for application as active components in thin film electronic devices. These results have important implications for the further design of functional donor-acceptor dyads, being promising materials for organic bulk heterojunction solar cells and field-effect transistors.