Session T49: Focus Session: Organic Electronics and Photonics: Structure - Property Correlations

Correlating the efficiency and nanomorphology of polymer blend solar cells utilizing resonant soft x-ray scattering

Enhanced scattering contrast afforded by resonant soft x-ray scattering (R-SoXS) is used to probe the nanomorphology of all-polymer solar cells based on blends of the donor poly(3-hexylthiophene) (P3HT) with the acceptor either poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3- benzothiadiazole]- 2',2"-diyl) (F8TBT) or poly([N,N'- bis(2-octyldodecyl)-11 naphthalene-1,4,5,8- bis(dicarboximide)- 2,6-diyl]- alt-5,5'- (2,2'-12 bithiophene)) (P(NDI2OD-T2)). A bimodal distribution of domain sizes is observed for P3HT:P(NDI2OD-T2) blends with small domains of $\sim $ 5 - 10 nm that evolve with annealing and larger domains of $\sim $ 100 nm insensitive to annealing. P3HT:F8TBT blends in contrast show a broader distribution but with the majority structured on 10 nm length scale. For both blends an evolution in device performance is observed, correlated with a coarsening and purification of domains on the 5 - 10 nm length scale. Grazing-Incidence Wide Angle X-ray Scattering (GI-WAXS) reveals 25 - 40 nm thick P(NDI2OD-T2) crystallites embedded in the larger domains observed by R-SoXS. A higher degree of P3HT crystallinity is observed in blends with P(NDI2OD-T2) compared to F8TBT. The propensity of the polymers to crystallize in P3HT:P(NDI2OD-T2) blends is also observed to hinder the morphological development on the sub-10 nm length scale. More broadly, this work also highlights the complementarity of R-SoXS and GI-WAXS.   

Hierarchical Nanomorphologies Promote Exciton Dissociation in Polymer: Fullerene Bulk Heterojunction Solar Cells

In the last fifteen years, research efforts have led to organic photovoltaic (OPV) devices with power conversion efficiencies (PCEs) up to $\sim $8{\%}, but these values are still insufficient for the devices to become widely marketable. To further improve solar cell performance a thorough understanding of the complex structure-property relationships in the OPV devices is required. In this work, we demonstrated that the OPV active layer of PTB7:fullerene bulk heterojunction (BHJ) solar cells, which set a historic record of PCE (7.4{\%}), involves hierarchical nanomorphologies ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains, themselves hundreds of nanometers in size. These hierarchical nanomorphologies with optimum crystallinity and intermixing of PTB7 with fullerenes are coupled to significantly enhanced exciton dissociation, which consequently contribute to photocurrent, leading to the superior performance of PTB7:fullerene BHJ solar cells. New insights of performance-related structures afforded by the current study should aid in the rational design of even higher performance polymeric solar cells.   

Unexpected Morphological Traits of Bulk Heterojunction Organic Solar Cells with Exceptional Power Conversion Efficiencies

The synthesis of new polymers for polymer/fullerene bulk heterojunction solar cells has boosted the power conversion efficiency (PCE) of these devices to levels now exceeding 5{\%}. Even with these advancements in efficiency, relatively little is known of the morphological characteristics of the active layer including domain size and purity, material crystallization and orientation, and miscibility of the bulk heterojunction components. Herein, we employ a suite of soft and hard x-ray scattering and microscopy techniques to probe defining traits of the morphology for the high-performing polymers, poly[4,8-(3-butylnonyl)benzo[1,2-b:4,5-b']dithiophene-\textit{alt}-2-(2-butyloctyl)-5,6-difluoro-2H-benzo[d][1,2,3]triazole] (BnDT-FTAZ) and thieno[3,4-b]thiophene-\textit{alt}-benzodithiophene (PTB7) blended with phenyl-C61-butyric acid methyl ester (PC$_{61}$BM) and PC$_{71}$BM, respectively. PCEs of 7.4{\%} for BnDT-FTAZ and 5.3{\%} for PTB7 based solar cells are achieved when processing methods result in smaller, more mixed polymer/fullerene phases where non-zero miscibility is measured for each system. Furthermore, the polymers do not strongly crystallize in the active layer and average domain sizes larger than 50 nm are noted for both systems.   

Correlating Free-Carrier production in P3HT with Solid-State Microstructure using Time-Resolved Microwave Conductivity

The nature of the primary photoexcitation in conjugated polymers has been a subject of interest for a number of years, and two models have emerged: neutral excitons and free charge carriers. While excitons are recognized as the dominant of the two species, there are a small fraction of carriers that appear directly upon photoexcitation that have been detected experimentally either spectroscopically or through conductivity measurements. The fraction of near-instantaneous free charge carriers produced depends both on the chemical structure of the polymer and on the time-scale on which the study is performed. For example, poly(3-hexylthiophene) (P3HT) thin films have been reported to have free carrier yields as high as 15\%, when measured on a fast time scale, but as low as 3\% when measured on a slow time scale. It is unclear why these numbers are so different, and from where these carriers originate. This presentation will report studies using flash photolysis, time-resolved microwave conductivity (fp-TRMC) to probe carriers produced in a number of thin films of P3HT of differing molecular weights. By correlating the free carrier yield with the solid-state microstructure of the polymer, and the corresponding electronic absorption spectra, we will propose a model that explains the origins of these carriers.   

Controlling Active Layer Morphology in Polymer/Fullerene Solar Cells

The active layer in most polymer solar cells is based on the bulk heterojunction (BHJ) design. BHJs are prepared by arresting the phase separation of a polymer/fullerene blend to produce a nanoscale, interpenetrating network. Such non-equilibrium structures are very difficult to control and reproduce, posing a significant challenge for fundamental structure-property investigations. We demonstrate a new approach to control the active layer morphology with a simple two-step process: First, a thin film of poly(3-hexylthiophene) (P3HT) is cross-linked into stable nanostructures or microstructures with electron-beam lithography [1]. Second, a soluble fullerene is spun-cast on top of the patterned polymer to complete the heterojunction. Significantly, irradiated P3HT films retain good optoelectronic properties and bilayer P3HT/fullerene heterojunctions yield power-conversion efficiencies near 0.5{\%}. We have performed preliminary studies with model nanostructured devices and we find that efficiency increases with interfacial area [2]. These model devices are very valuable for fundamental studies because the interfacial area is accurately measured with small-angle X-ray scattering, and the active layer can be ``deconstructed'' for imaging with atomic force microscopy. \\[4pt] [1] S. Holdcroft, Adv. Mater. 2001, 13, 1753-1765.\\[0pt] [2] He et al., Adv Funct. Mater. 2011, 21, 139-146.   

The Miscibility of PCBM in Low Band-Gap Conjugated Polymers in Organic Photovoltaics

Understanding the morphology of the photoactive layer in organic photovoltaics (OPVs) is essential to optimizing conjugated polymer-based solar cells to meet the targeted efficiency of 10{\%}. The miscibility and interdiffusion of components are among the key elements that impact the development of morphology and structure in OPV active layers. This study uses neutron reflectivity to correlate the structure of low band gap polymers to their miscibility with PCBM. Several low band gap polymers that exhibit power conversion efficiencies exceeding 7{\%}, including PBnDT-DTffBT were examined. The intermixing of low band-gap polymer and PCBM bilayers was monitored by neutron reflectivity before and after thermal annealing, providing quantification of the miscibility and interdiffusion of PCBM within the low band gap polymer layer. These results indicate that the miscibility of PCBM ranges from 3{\%} to 26{\%} with the low band-gap polymers studied. The correlation between low band gap polymer structure and miscibility of PCBM will also be discussed.   

Self-assembly Columnar Structure in Active Layer of Bulk Heterojunction Solar Cell

Bulk Heterojunction (BHJ) polymer solar cells are an area of intense interest due to their flexibility and relatively low cost. However, due to the disordered inner structure in active layer, the power conversion efficiency of BHJ solar cell is relatively low. Our research provides the method to produce ordered self-assembly columnar structure within active layer of bulk heterojunction (BHJ) solar cell by introducing polystyrene (PS) into the active layer. The blend thin film of polystyrene, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C$_{61}$ butyric acid methyl ester (PCBM) at different ratio are spin coated on substrate and annealed in vacuum oven for certain time. Atomic force microscopy (AFM) images show uniform phase segregation on the surface of polymer blend thin film and highly ordered columnar structure is then proven by etching the film with ion sputtering. TEM cross-section technology is also used to investigate the column structure. Neutron reflectometry was taken to establish the confinement of PCBM at the interface of PS and P3HT. The different morphological structures formed via phase segregation will be correlated with the performance of the PEV cells to be fabricated at the BNL-CFN.   

Dissipative Particle Dynamics Studies of Rod-Coil Polymer Nanocomposites

Organic solar cell efficiency depends strongly on the morphology within the active layer consisting of donor (e.g. conjugated polymer) and acceptor (e.g. fullerene derivative) materials. Higher device efficiency can be obtained if the donor-acceptor morphology has high interfacial area, small domains, and continuous pathways. One way to control donor-acceptor morphology is via copolymerization of the conjugated ``rod'' polymer and an acceptor ``coil'' block. Past theory and experimental studies have characterized the phase behavior of pure rod-coil block copolymers. In this talk we will present dissipative particle dynamics simulation studies of composites of symmetric rod-coil block copolymers and nanoscale additives of varying selectivity (rod-, coil- and non-selective). With increasing volume fraction of non- and rod- selective nanoadditives we see a shift in the liquid crystalline and microphase transitions to lower temperatures, and new morphologies (e.g. helical twists in rod block domain and zig-zag lamellae) not seen in pure symmetric rod-coil polymers. These shifts in phase transition are explained by where the nanoadditives reside, which in turn is dictated by where the system can maximize enthalpic gain and minimize loss of nanoadditive translational entropy.   

Model Hamiltonian for predicting the bandgap of conjugated systems

We calculate the bandgaps for conjugated systems using the adapted Su-Schrieffer-Heeger Hamiltonian and find good agreement with 130 independent experimental points. The 2D version of the model correctly demonstrates the decrease in bandgap from the addition of vinylene bridges to both poly(\emph{p}-phenylene) and polythiophene indicating that planarization is not a significant effect. Expanding the model to 3D shows that interchain interactions systematically reduces the bandgap. In fused rings sharing dissimilar bonds, such as in isothianaphthene, the bond length dimerization along the carbon backbone decreases leading to a decrease in the bandgap. In contrast, when fusing two of the same rings along equivalent bonds, for example thienoacene, the bandgap change is less than 10\% at best when normalized by the number of carbon atoms in the conjugation path. From porphyrin and pyrrole-benzothiadiazole we learn that tautomerization significantly affects the bandgap, as the $\varepsilon$ value for NH had to be used for both NH and N, indicating that H is being shared by both. In modeling donor-acceptor co-polymers we accurately calculate the reduction in the bandgaps when compared to their parent homopolymers.   

The Effect of Side-Chain Length on the Solid-State Structure and Optical Properties of F8BT: A DFT Study

Using the long-range corrected hybrid density functional theory (DFT/B97D) approach, we have performed bulk solid state calculations to investigate the influence of side-chain length on the molecular packing and optical properties of poly (9,9-di-n-octylfluorene-alt-benzothiadiazole) or F8BT. Two different packing structures, the lamellar and nearly hexagonal, were obtained corresponding to longer and shorter side-chains respectively. This behavior can be attributed to the micro-phase separations between the flexible side-chains and the rigid backbones and is in agreement with previous investigations for other hairy-rod polymers. In addition, as a result of the efficient inter-chain interactions for the lamellar structure, the dihedral angle between the F8 and BT units is reduced providing a more planar configuration for the backbone which leads to the decreased band gap (by 0.2-0.3 eV) in comparison to the hexagonal phase and the gas phase with no side-chain. Time-dependent DFT (TDDFT/B3LYP) was also used to study the excited states of the monomer of F8BT optimized in solid-state structures with different side-chain lengths. It is found that the absorption spectrum is red shifted for the polymers with lamellar structure relative to the polymers in hexagonal and gas phases.   

Polymer blend photovoltaics with conjugated block copolymers as surfactants

Conjugated polymer blend photovoltaics are a class of devices where the active layer is composed of a polymer acceptor and a polymer donor. These devices suffer from macrophase separation in the active layer, where it is challenging to kinetically trap domains with characteristic sizes below micron length scales. Thus, for mixtures of poly(3-hexylthiophene) (P3HT) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (PFOTBT), we have synthesized a conjugated block copolymer to act as an A/B surfactant and stabilize a microstructure. The performance of devices where the active layer is composed of P3HT, PFOTBT, and P3HT-PFOTBT block copolymer blends is found to depend on the composition of the mixture and processing conditions. In addition, we have utilized energy-filtered transmission electron microscopy to characterize the morphology of the blends and correlated the microstructure with device performance.   

Self-assembly and characterization of two-component films of semiconducting nanoparticles

Polymer-based semiconducting materials are promising candidates for large-scale, low-cost photovoltaic devices. To date, the efficiency of these devices has been low in part because of the challenge of optimizing molecular packing while also obtaining a bicontinuous structure with a length scale of approximately 10nm. Here we demonstrate an alternative approach to solving this problem by packing nanoparticles of electron- and hole-transporting semiconductors into a two-component film. We first make nanoparticles of semiconducting materials (P3HT, PCBM, CdSe, etc) suspended in liquid. Then a binary suspension of nanoparticles is dried onto a non-volatile liquid surface to form a solid, two-component film with uniform thickness. The absorbance, photoluminescence, structure, and charge mobility of the films are measured. For a range of stoichiometries, we obtain bicontinuous structures and significant luminescence quenching, indicating charge transfer. This study shows that two-component nanoparticulate films may be an effective route toward bulk heterojunctions with controlled morphology. We acknowledge support from the US Department of Energy, Office of Basic Energy Sciences, through grant DE-SC0001087.