The physical properties of polymer films of nanoscale thickness have been shown to exhibit film thickness-dependent behavior, largely associated with intermolecular interactions between the film and its external interfaces. These effects are typically manifested in the thickness dependencies of the glass transition temperatures (T(g)s), and segmental dynamics, over length scales ranging from a few nanometers to tens of nanometers in many polymer/substrate systems. Moreover, different experimental techniques, including incoherent neutron scattering, spectroscopic ellipsometry, and broadband dielectric spectroscopy, have yielded thickness-dependent T-g trends of the same systems that appear to be at odds. To this end, the first direct quantitative measurements of the length scales of interfacial interactions of strategically selected polymer/substrate systems, obtained using Kelvin force probe microscopy, are reported. These findings reveal pathways to understand and reconcile seemingly the contradictory experimental observations about T-g vs. film thickness trends, and dynamics, at the nanoscale. Our findings have broader implications for understanding the physical properties of polymer films in the nanoscale thickness regime.

Charge-carrier mobilities in poly(3-hexylthiophene) (P3HT) organic thin-film transistors (OTFTs) increase 5-fold when OTFTs composed of P3HT films on trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (FTS) monolayers supported on SiO2 dielectric substrates (P3HT/FTS/SiO2/Si) are subjected to supercritical carbon dioxide (scCO2) processing. In contrast, carrier mobilities in P3HT/octadecyltrichlorosilane (OTS)/SiO2 OTFTs processed using scCO2 are comparable to mobilities measured in as-cast P3HT/OTS/SiO2/Si devices. Topographical images of the free and buried interfaces of P3HT films reveal that scCO2 selectively alters the P3HT morphology near the buried P3HT/FTS-SiO2 interface; identical processing has negligible effects at the P3HT/OTS-SiO2 interface. A combination of spectroscopic ellipsometry and grazing-incidence X-ray diffraction experiments indicate insignificant change in the orientation distribution of the intermolecular π-π stacking direction of P3HT/FTS with scCO2 processing. The improved mobilities are instead correlated with enhanced in-plane orientation of the conjugated chain backbone of P3HT after scCO2 annealing. These findings suggest a strong dependence of polymer processing on the nature of polymer/substrate interface and the important role of backbone orientation toward dictating charge transport of OTFTs.

ABSTRACTAn epitaxy‐directing solvent additive 1,3,5‐trichlorobenzene is combined with an off‐center spin‐casting technique to produce poly(3‐hexylthiophene) (P3HT) fibers with uniaxial in‐plane alignment on the centimeter scale. Photoconductive atomic force microscopy (pc‐AFM) is used to characterize planar heterojunction devices assembled from phenyl‐C61 butyric acid methyl ester (PCBM) acceptor and both aligned and unaligned P3HT donor. By varying the relative positions of the laser spot (site of carrier generation) and probe (site of hole extraction), it is found that devices with aligned P3HT exhibit anisotropic and greatly enhanced long‐range photocarrier transport, with nearly 10% of original photocurrent measured 400 µm from the laser spot along the direction parallel to the alignment. Complementary thin film transistor (TFT) measurements reveal a factor of ∼3.5 difference in the hole mobilities parallel and perpendicular to the direction of alignment. Together, these findings highlight the importance of macroscopic alignment as a strategy to overcome the low mobilities of disordered polymer semiconductors.1 © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 180–188

A larger interfacial area between the copolymer and fullerene is obtained with the gradient copolymer relative to the block architecture. This is correlated with two orders of magnitude higher initial carrier density.

We investigate dipolar donor materials mixed with a C-70 acceptor in an organic photovoltaic (OPV) cell. Dipolar donors that have donor-acceptor-acceptor (d-a-a') structure result in high conductivity pathways due to close coupling between neighboring molecules in the mixed films. We analyze the charge transfer properties of the dipolar donor:C-70 mixtures and corresponding neat donors using a combination of time-resolved electroluminescence from intermolecular polaron pair states and conductive tip atomic force microscopy, from which we infer that dimers of the d-a-a' donors tend to form a continuous network of nanocrystalline clusters within the blends. Additional insights are provided by quantum-mechanical calculations of hole transfer coupling and hopping rates between donor molecules using nearest-neighbor donor packing motifs taken from crystal structural data. The approximation using only nearest-neighbor interactions leads to good agreement between donor hole hopping rates and the conductive properties of the donor:C-70 blends. This represents a significant simplification from requiring details of the nano- and mesoscale morphologies of thin films to estimate their electronic characteristics. Using these dipolar donors, we obtain a maximum power conversion efficiency of 9.6 +/- 0.5% under 1 sun, AM1.5G simulated illumination for an OPV comprised of an active layer containing a dipolar donor mixed with C-70.

We find that mixtures of C60 with the wide energy gap, small molecular weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron conductivity and efficient exciton blocking when employed as buffer layers in organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C60 mixed layer has an exciton blocking efficiency of 84 ± 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency is accompanied by a 100-fold increase in electron conductivity compared with neat BPhen. Transient photocurrent measurements show that charge transport through a neat BPhen buffer is dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Thus, we infer that C60 undergoes nanoscale (<10 nm domain size) phase segregation even at very high (>80%) BPhen fractions.

The power conversion efficiency (PCE) of a bulk heterojunction (BHJ) organic solar cell is influenced by the morphology (domain size and connectivity, phase purity, and interfacial structure) of the donor:acceptor blend active layer. The design of experiments to understand interrelationships between structure, transport properties, and device performance remains an important challenge. To this end, we created different types of morphologies in the poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC61BM) active layer by exploiting different processing strategies: conventional solvent casting, supercritical carbon dioxide (scCO(2)) processing, and thermal annealing. We investigated the device characteristics and transport behavior (carrier densities, mobilities and recombination) of samples possessing comparable domain sizes, which exhibited comparable initial carrier densities upon illumination. Notably, however, one morphology exhibited PCE, short circuit current (J(SC)), and carrier mobility that were each approximately a factor of 3 larger than the other morphologies. We also investigated another case where, in spite of significant differences between the domain dimensions, the PCE and J(SC) values were quite similar. These observations are rationalized on the basis of interrelations between aspects of the active material morphology, the transport properties, and the device efficiencies. This work provides insight into morphological design of active layers for optimum device performance.

Bulk heterojunction (BHJ) solar cells are fabricated using active material blends of poly(3‐hexylthiophene) (P3HT) donor, indene‐C60 bisadduct (ICBA) acceptor, and an all‐conjugated random copolymer (RCP) additive. By optimizing RCP loading, power conversion efficiencies (PCEs) up to 20% higher than those of a binary P3HT:ICBA mixture are achieved. The improved device characteristics are rationalized in terms of the differences between the photoactive thin film morphologies. Energy‐filtered transmission electron micro­scopy reveals that incorporation of the RCP improves the degree of structural order of the BHJ fibrillar network and increases the extent of microphase separation between P3HT and ICBA. Additionally, a combination of atomic force microscopy and X‐ray photoelectron spectroscopy analysis indicates segregation of the RCP at the free interface, leading to a shift in the surface potentials measured by Kelvin probe force microscopy. These changes, both in the bulk morphology and in the interfacial composition/energetics, are correlated to improved carrier collection efficiency due to a reduction of non‐geminate recombination, which is measured by charge extraction of photo­generated carriers by linearly increasing voltage.

A substantial broadband increase in the external quantum efficiency (EQE) of thin‐film organic photovoltaic (OPV) devices using near‐field coupling to surface plasmons is reported, significantly 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 reflection 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 efficiency. 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‐film photovoltaics and photodetectors and enable the direct analysis of the dynamics of photocurrent production at OPV heterojunctions.

Loss to the open circuit voltage (Voc) in organic photovoltaic cells is a critical bottleneck to achieving high power conversion efficiency. We demonstrate that the insertion of multilayers of a poly(phenylene ethynylene) spacer into the planar heterojunction between poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester incrementally escalates the Voc of a polymer solar cell from 0.43 V to 0.9 V. Through a combination of light intensity and temperature dependent measurements, we show that this control over the molecular structure local to the interface increases Voc by raising the polaron pair energy and by suppressing the dark-diode current.

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.

We propose a sustainable low temperature alternative, using supercritical carbon dioxide (scCO(2)), to the conventional high temperature thermal annealing protocol for processing poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC61BM) organic photovoltaic devices. This new strategy enabled the fabrication of devices that exhibited comparable, and often better, short circuit currents, J(SC)'s, and efficiencies than those prepared using the conventional heat treatment protocol. While the fill factors (FF) of devices processed using either of the prorocols were comparable, the best performing scCO(2) processed devices provided the largest J(SC)'s. With the use of energy-filtered transmission electron microscopy (EFTEM), and electron energy loss spectroscopy (EELS), we show that the supercritical solvent protocol enabled the formation of similar macro- and nanoscale morphology as the heat treatment protocol. The active materials in the devices with the best efficiencies possessed purer P3HT and PC61BM phases, as shown by EFTEM/EELS. UV-vis measurements, moreover, corroborated this observation, revealing a higher average degree of packing and order of P3HT chains throughout the films. Conductive- and photoconductive-atomic force microscopy (cAFM and pcAFM) revealed that the higher efficiency devices possessed larger fractions of photoactive regions throughout the active material. The variations in photoconductivity are associated with changes in the local composition throughout the active material.