Structural Evolution of Crystalline Conjugated Polymer/Fullerene Domains from Solution to the Solid State in the Presence and Absence of an Additive

The power conversion efficiencies of polymer/fullerene solar cells are critically dependent on the nanometer-scale morphologies of their active layers, which are typically processed from solution. Using synchrotron wide- and small-angle X-ray scattering, we have elucidated the intricate mechanism of the structural transitions from solutions to solid films of the crystalline polymer poly­[bis­(dodecyl)­thiophene-thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione] (PBTTPD) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), including the effect of the solvent additive 1,6-diiodohexane (DIH). We found that the local assembly of rigid-rod PBTTPD segments that formed in solution instantly and then relaxed within several hundred seconds upon cooling to room temperature from 90 °C could re-emerge and develop into seeds for subsequent crystallization of the polymer in the solid films. At room temperature (25 °C), the presence of DIH in chlorobenzene slightly enhanced the formation of local assembly PBTTPD segments in the supersaturated PBTTPD in PBTTPD/PC₇₁BM blend solution. Two cases of films were subsequently developed from these blend solutions with drop-casted and spin-coated methods. For spin-coated thin films (90 nm thick), which evolve quickly, polymer’s crystallinity and the fullerene packing in the solid-state thin films were enhanced in the case of involving DIH. Regarding the effect of DIH for processing the drop-casted thick films (2.5 μm thick), which evolve slowly, DIH has no observable effect on PBTTPD/PC₇₁BM structure. Our results provide some understanding of the mechanism behind the structural development of polymer/fullerene blends upon their transitions from solution to the solid state, as well as the key functions of the additive

Quantitative Characterization and Mechanism of Formation of Multilength-scale Bulk Heterojunction Structures in Highly Efficient Solution-Processed Small-Molecule Organic Solar Cells

In this study we used simultaneous grazing-incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively) to probe the multilength-scale structures of thin active layers comprising the linear A–D–A-type π-conjugated donor molecule TBDTCNR and the fullerene acceptor molecule PC₆₁BM for use in solution-processed small-molecule-based organic solar cells (SMOSCs). We found that the pseudo-two-dimensional fractal-like networks in the bulk heterojunction (BHJ) structure were determined by mutual interactions between the small-molecule (SM) crystallites and the nanoscale PC₆₁BM clusters during their formation and phase separation, and deduced quantitatively, at multiple length scales, the BHJ structures comprising these SM crystallites and PC₆₁BM clusters. We also conducted in situ GIWAXS measurements to study the temporal behavior and kinetics of SM crystallization from solution to the solid film state. Our GISAXS/GIWAXS study revealed that the multilength-scale BHJ structures in the thin films could be tuned effectively by varying the amount of incorporated PC₆₁BM and the annealing temperature. This study provides fundamental information relating to the mechanism of formation of hierarchical BHJ structures through relatively rapid crystallization of a highly crystalline SM, as well as the relationships among the hierarchical structure, the photovoltaic performance, and the mechanism of formation, thereby allowing greater control over BHJ structures in SMOSCs with optimized fabrication and performance

Reaction Kinetics and Formation Mechanism of TiO₂ Nanorods in Solution: An Insight into Oriented Attachment

The reaction kinetics and formation mechanism of oriented attachment for shaped nanoparticles in solution are not well-understood. We present the reaction kinetics and formation mechanism of organic-capped anatase TiO₂ nanorods in solution as a case study for the oriented attachment process using small-angle X-ray scattering (SAXS) and transmission electronic microscopy. The SAXS analysis qualitatively and quantitatively provides in-depth understanding of the mechanism, including the structural evolution, interparticle interaction, and spatial orientation of nanoparticles developed from nanodots to nanorods during the nucleation, isotropic, and anisotropic growth steps. The present study demonstrates the growth details of oriented attachment of nanoparticles in solution. An ordered lamellar structure in the solution is constructed by the balance of interaction forces among surface ligands, functional groups, and solvent molecules serving as a natural template. The template allows the alignment of spherical nanoparticles into ordered chain arrays and facilitates simultaneous transformation from spherical to rod shape via proximity attachment. The proposed model reveals an insight into the oriented attachment mechanism. This multistep formation mechanism of TiO₂ nanorods in solution can provide the fundamental understanding of how to tune the shape of nanoparticles and further control the aggregation of spatial nanorod networks in solution

Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives

The aim of this study is to quantitatively investigate the effect of different fullerene type (PC₆₀BM and PC₇₀BM) on various morphological structures and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) P3HT/PC_<i>x</i>BM solar cells. The solar cells are fabricated by spin coating without thermal annealing. The quantitative investigations of three-dimensional self-organized nanostructures are performed by using combined grazing-incidence small- and wide-angle X-ray scattering technique (GISAXS/GIWAXS). Two types of nanostructures are observed due to the phase separation in the BHJ films during the processing. They include (1) intercalated PC_<i>x</i>BM molecules around boundary of P3HT crystalline domain and within amorphous domain and (2) aggregated PC_<i>x</i>BM clusters in PC_<i>x</i>BM domains. The lamellar spacing of P3HT crystalline domains in P3HT/PC₇₀BM is larger than that in P3HT/PC₆₀BM. This result indicates more interfacial areas are generated between PC₇₀BM and P3HT at the molecular scale for more efficient charge separation. On the other hand, the size, volume fraction, partial attachment, and spatial distribution of PC₆₀BM clusters are larger than that of PC₇₀BM clusters, which reveals more efficient electron transport in P3HT/PC₆₀BM. We deduce the correlation between nanostructures and PCE (3.25% and 2.64%, respectively, for P3HT/PC₇₀BM and P3HT/PC₆₀BM). The structure of fullerene intercalated with P3HT rather than the size of fullerene cluster plays a major role in the PCE performance of BHJ solar cell without thermal annealing