Mechanism and control of the structural evolution of a polymer solar cell from a bulk heterojunction to a thermally unstable hierarchical structure

Abstract

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.

Effects of amorphous poly(3‐hexylthiophene) on active‐layer structure and solar cells performance

“…[17][18][19] The morphological transition has been suggested to involve a largescale aggregation of fullerene that reduces the interfacial area for charge generation, and thus leads to a decay of PCE. 20 Taking the most widely studied regioregular poly(3-hexylthiophene)/ [6,6]-phenyl C60-butyric acid methyl ester (P3HT/ PCBM) blending system as an example, the macrophase separation between polymer and fullerene results from a combination of two aspects. First, regioregular P3HT is a semicrystalline polymer and upon crystallization, the impurities, such as PCBM, are expelled by the growing P3HT crystals.…”

“…Such a scattering profile has previously been interpreted to originate either from PCBM clusters 20,[46][47][48][49] or from the spatial arrangement of P3HT crystallites. 25,50 Since the scattering shoulder is absent in B-0 sample that contains PCBM clusters [Supporting Information Fig.…”

Morphological Control Agent in Ternary Blend Bulk Heterojunction Solar Cells

“…Referring to the established insights into the P3HT/PC61BM BHJ system [64], the P3HT/PC61BM blend film is generally comprised of three kinds of structures: (1) P3HT crystallites; (2) PC61BM clusters with size of 10-20 nm and (3) meso-scaled domains of amorphous P3HT chains intercalated with PC61BM molecules (denoted as PC61BM/amorphous P3HT). The authors stated that the thermally degraded device performance was actually attributed to the growth and densification of PC61BM/amorphous P3HT domains (structure (3)) instead of the nano-scaled PC61BM clusters (structure (2)) [63]. In other words, the authors clarified the mistaken impression in the developing or coalescing of nano-scaled PC61BM clusters during long-time thermal annealing, which was discovered to be stably confined with unnoticeable change in both size and volume fraction (Figure 7c).…”

“…By adding 8.3 wt% of bis-PC61BM, the devices showed enhanced performance from 3.09% of reference cell to 3.62% with extremely high thermal stability. This work was subsequently investigated by employing powerful grazing incidence wide/small angle X-ray scattering (GIWAXS/GISXAS) technique (Figure 7c) to explore the morphological evolutions and mechanisms in which the findings beyond the knowledge at that time were pointed out [63]. The GIWAXS/GISAXS technique with modeling has been intensively utilized to quantitatively characterize the nanostructures of BHJ blend films [24,64,65].…”

Fullerene mixtures enhance the thermal stability of a non-crystalline polymer solar cell blend

“…In addition, micrometer-sized fullerene crystallites grow, 9,10 which are detrimental for the device performance. 5,8,11 One increasingly explored route to improve the thermal stability of polymer:fullerene blends is the use of fullerene mixtures, which either hinders crystallization of the fullerene acceptor, [12][13][14] or results in the controlled nucleation of submicrometer-sized fullerene crystals. 15,16 Here, mixtures of phenyl-C 61 -butyric acid methyl ester (PC 61 BM, Fig.…”