Concomitant development of [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) aggregation and poly(3-hexylthiophene) (P3HT) crystallization in bulk heterojunction (BHJ) thin-film (ca. 85 nm) solar cells has been revealed using simultaneous grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS). With enhanced time and spatial resolutions (5 s/frame; minimum q ≈ 0.004 Å(-1)), synchrotron GISAXS has captured in detail the fast growth in size of PCBM aggregates from 7 to 18 nm within 100 s of annealing at 150 °C. Simultaneously observed is the enhanced crystallization of P3HT into lamellae oriented mainly perpendicular but also parallel to the substrate. An Avrami analysis of the observed structural evolution indicates that the faster PCBM aggregation follows a diffusion-controlled growth process (confined by P3HT segmental motion), whereas the slower development of crystalline P3HT nanograins is characterized by constant nucleation rate (determined by the degree of supercooling and PCBM demixing). These two competing kinetics result in local phase separation with space-filling PCBM and P3HT nanodomains less than 20 nm in size when annealing temperature is kept below 180 °C. Accompanying the morphological development is the synchronized increase in electron and hole mobilities of the BHJ thin-film solar cells, revealing the sensitivity of the carrier transport of the device on the structural features of PCBM and P3HT nanodomains. Optimized structural parameters, including the aggregate size and mean spacing of the PCBM aggregates, are quantitatively correlated to the device performance; a comprehensive network structure of the optimized BHJ thin film is presented.
A novel approach to modulate the nucleation and growth of perovskite crystals by intermixing precursor-capped nanoparticles has been reported.
We simultaneously employed grazing incidence small-angle and wide-angle X-ray scattering (GISAXS and GIWAXS) techniques to quantitatively study the structural evolution and kinetic behavior of poly(3-hexylthiophene) (P3HT) crystallization, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) aggregation and amorphous P3HT/PCBM domains from a bulk heterojunction (BHJ) to a thermally unstable structure. The independent phase separation regimes on the nanoscale (∼10 nm), mesoscale (∼100 nm) and macroscale (∼μm) are revealed for the first time. Bis-PCBM molecules as inhibitors incorporated into the P3HT/PCBM blend films were adopted as a case study of a control strategy for improving the thermal stability of P3HT/PCBM solar cell. The detailed information on the formation, growth, transformation and mutual interaction between different phases during the hierarchical structural evolution of P3HT/PCBM:xbis-PCBM (x = 8-100%) blend films are presented herein. This systematic study proposes the mechanisms of thermal instability for a polymer/fullerene-based solar cell. We demonstrate a new fundamental concept that the structural evolution and thermal stability of mesoscale amorphous P3HT/PCBM domains during heating are the origin of controlling thermal instability rather than those of nanoscale thermally-stable BHJ structures. It leads to a low-cost and easy-fabrication control strategy for effectively tailoring the hierarchical morphology against thermal instability from molecular to macro scales. The optimum treatment achieving high thermal stability, control of mesoscale domains, can be effectively designed. It is independent of the original BHJ nanostructure design of a polymer/fullerene-based solar cell with high performance. It advances the general knowledge on the thermal instability directly arising from the nanoscale structure.
Polymer solar cells (PSCs) based on fullerene derivatives often require additives to optimize active layer morphology. Here, the novel additive 1‐naphthalenethiol (SH‐na) is proposed for processing the PSC active layer of PTB7:PC71BM. Spin‐casting with SH‐na as additive achieves a power conversion efficiency (PCE) of 7.3%, compared to 6.7% for preparations containing the conventional 1,8‐diiodooctane additive. Dipping of the active layer into a methanol solution of critical SH‐na concentration increases the PCE further to 8.75%. This is mainly due to an improved open‐circuit voltage (from 0.72 to 0.79 V) together with a high achieved fill factor of 0.70. The improved PCE is correlated to the morphology optimization according to measurements of grazing incidence small/wide‐angle X‐ray scattering, neutron reflectivity, atomic force microscopy, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. The integrated results suggest that the halogen‐free additive SH‐na can form hydrogen bonds with both PTB7 and PC71BM, resulting in substantially improved PTB7 crystallization and multi‐length‐scale PC71BM dispersion for appropriate aggregation and networks. The subsequent dipping treatment with SH‐na further modifies the active layer morphology for a more PC71BM‐enriched surface and better PC71BM networks in the bulk film for an optimized electron‐to‐hole mobility ratio of 2.04, hence resulting in improved device performance.
Using simultaneously scanning small-angle X-ray scattering (SAXS) and UV-vis absorption with integrated online size exclusion chromatography, supplemental with molecular dynamics simulations, we unveil the long-postulated global structure evolution of a model multidomain protein bovine serum albumin (BSA) during acid-induced unfolding. Our results differentiate three global packing structures of the three molten globule domains of BSA, forming three intermediates I, I, and E along the unfolding pathway. The I-I transition, overlooked in all previous studies, involves mainly coordinated reorientations across interconnected molten globule subdomains, and the transition activates a critical pivot domain opening of the protein for entering into the E form, with an unexpectedly large unfolding free energy change of -9.5 kcal mol, extracted based on the observed packing structural changes. The revealed local packing flexibility and rigidity of the molten globule domains in the E form elucidate how collective motions of the molten globule domains profoundly influence the folding-unfolding pathway of a multidomain protein.
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