Directing Hybrid Structures by Combining Self-Assembly of Functional Block Copolymers and Atomic Layer Deposition: A Demonstration on Hybrid Photovoltaics

The simplicity and versatility of block copolymer self-assembly offers their use as templates for nano- and meso-structured materials. However, in most cases, the material processing requires multiple steps, and the block copolymer is a sacrificial building block. Here, we combine a self-assembled block copolymer template and atomic layer deposition (ALD) of a metal oxide to generate functional hybrid films in a simple process with no etching or burning steps. This approach is demonstrated by using the crystallization-induced self-assembly of a rod–coil block copolymer, P3HT-<i>b</i>-PEO, and the ALD of ZnO. The block copolymer self-assembles into fibrils, ∼ 20 nm in diameter and microns long, with crystalline P3HT cores and amorphous PEO corona. The affinity of the ALD precursors to the PEO corona directs the exclusive deposition of crystalline ZnO within the PEO domains. The obtained hybrid structure possesses the properties desired for photovoltaic films: donor–acceptor continuous nanoscale interpenetrated networks. Therefore, we integrated the films into single-layer hybrid photovoltaics devices, thus demonstrating that combining self-assembly of functional block copolymers and ALD is a simple approach to direct desired complex hybrid morphologies

Chemical Composition of Additives That Spontaneously Form Cathode Interlayers in OPVs

Interlayers between the active layer and the electrodes in organic devices are known to modify the electrode work function and enhance carrier extraction/injection, consequently improving device performance. It was recently demonstrated that chemical interactions between the evaporated electrode and interlayer additive can induce additive migration toward the metal/organic interface to spontaneously form the interlayer. In this work we used P3HT:PEG blends as a research platform to investigate the driving force for additive migration to the organic/metal interface and the source of the work function modification in OPVs. For this purpose PEG derivatives with different end groups were blended with P3HT or deposited on top of P3HT layer, topped with Al or Au evaporated electrodes. The correlation between the additive chemical structure, the <i>V</i>_oc of corresponding devices, and the metal/organic interface composition determined by XPS revealed that the driving force for additive migration toward the blend/metal interface is the chemical interaction between the additives’ end group and the deposited metal atoms. Replacing the PEG additives with alkyl additives bearing the same end groups has shown that the Al work function is actually modulated by the PEG backbone. Hence, in this work we have identified and separated between structural features controlling the migration of the interlayer additive to the organic/metal interface and those responsible for the modification of the metal work function

Coexisting Glassy Phases with Different Compositions in NFA-Based Bulk Heterojunctions

Organic solar cell (OSC) bulk heterojunctions (BHJ) typically feature a rich phase morphology with the phase composition and distribution significantly affecting processes such as charge generation, recombination and extraction, and in turn, device performance. While fullerene-based BHJs are relatively well understood structurally, especially when blends with a flexible-chain donor are employed, donor: non-fullerene acceptor (NFA) blends are more challenging to elucidate. The reason is that NFAs often display different polymorphs; moreover, their glassy states can be complex. Focusing on blends of the widely investigated donor polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT), and the prototype NFA, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC), we reveal here the coexistence of two glassy phases: a molecularly intermixed and an ITIC-rich one. In P3HT-rich blends, both glassy phases are present as nanosized domains, evenly distributed in the BHJ, as visualized via vapor phase infiltration (VPI) “staining”. In contrast, the 1:1 (by weight) and NFA-rich blends show clear, lateral phase separation between large (>500 nm) domains of the glassy phases and thinner polymer-rich domains that are unaffected by annealing. Our observations help to explain earlier P3HT: ITIC device studies; and also highlight the complexity of NFA-based BHJs, emphasizing the need for a deeper understanding of the phase behavior of such systems

Mechanism of Metal Oxide Deposition from Atomic Layer Deposition inside Nonreactive Polymer Matrices: Effects of Polymer Crystallinity and Temperature

Atomic layer deposition (ALD) is conventionally used to deposit smooth and conformal coatings from the gas phase onto surfaces. ALD onto organic films, however, may lead to precursor infiltration into the sample and subsurface deposition. Hence, ALD into polymer films could be used for the preparation of inorganic-in-organic nanocomposite materials. However, harnessing this approach requires deep understanding of the mechanisms that govern the infiltration, nucleation, and <i>in situ</i> growth with respect to the processing and properties of the organic matrix. Here we investigate the effect of matrix crystallinity and growth temperature on the deposition into nonreactive polymer matrices (i.e., polymers that do not bear functional groups which interact with the ALD precursors). This is done by exposing films of a nonreactive polymer, poly­(3-hexylthiophene-2,5-diyl) (P3HT), with different extents of crystallinity, to ALD cycles of ZnO precursors at different deposition temperatures. In the case of polymer matrices that chemically react with the precursors, the amount of inorganic phase uptake is a result of the interplay between precursor diffusion and matrix reactivity. However, using absorption measurements and high-resolution scanning electron microscopy, we show that, in the case of nonreactive polymer matrices, the inorganic uptake is significantly affected by the rate of nucleation which is determined by the retention of the precursors in the matrix. Furthermore, we find that the retention in the film is facilitated by the presence of crystalline domains, probably due to physisorption of the precursor molecules. This retention-dependence mechanism is further supported by temperature dependence and deposition in amorphous/semicrystalline bilayers. We find that the precursors diffuse through the top amorphous layer but ZnO is deposited strictly in the bottom semicrystalline layer due to the preferred retention. Revealing the general growth mechanism in nonreactive polymer matrices offers new approaches for nanoscale engineering of hybrid materials with an eye toward creating inorganic–organic heterostructures for organic electronic device applications

Understanding and Promoting Molecular Interactions and Charge Transfer in Dye-Mediated Hybrid Photovoltaic Materials

The performances of hybrid organic–inorganic photovoltaics composed of conjugated polymers and metal oxides are generally limited by poor electronic coupling at hybrid interfaces. In this study, physicochemical interactions and bonding at the organic–inorganic interfaces are promoted by incorporating organoruthenium dye molecules into self-assembled mesostructured conjugated polymer–titania composites. These materials are synthesized from solution in the presence of surfactant structure-directing agents (SDA) that solubilize and direct the nanoscale compositions and structures of the conjugated polymer, dye, and inorganic precursor species. Judicious selection of the SDA and dye species, in particular, exploits interactions that direct the dye species to the inorganic–organic interfaces, leading to significantly enhanced electronic coupling, as well as increased photoabsorption efficiency. This is demonstrated for the hydrophilic organoruthenium dye N3, used in conjunction with alkyleneoxide triblock copolymer SDA, polythiophene conjugated polymer, and titania species, in which the N3 dye species are localized in molecular proximity to and interact strongly with the titania framework, as established by solid-state NMR spectroscopy. In contrast, a closely related but more hydrophobic organoruthenium dye, Z907, is shown to interact more weakly with the titania framework, yielding significantly lower photocurrent generation. The strong SDA-directed N3-TiO_<i>x</i> interactions result in a significant reduction of the lifetime of the photoexcited state and enhanced macroscopic photocurrent generation in photovoltaic devices. This study demonstrates that multicomponent self-assembly can be harnessed for the fabrication of hierarchical materials and devices with nanoscale control of chemical compositions and surface interactions to improve photovoltaic properties