Morphological control over the bulk heterojunction (BHJ) microstructure of a high-efficiency small molecule photovoltaic system composed of a quinquethiophene based molecule (DRCN5T) as electron donor and [6, 6]-phenyl-C71-butyric acid methyl ester (PC 70 BM) as electron acceptor is demonstrated using three different post-processing strategies, including thermal annealing (TA), solvent vapor annealing (SVA), and two-step annealing (TA-SVA) treatments. We systematically analyze the processing condition-microstructure-device property relationships, explore the corresponding morphology evolution and their effects on
Ternary blends with broad spectral absorption have the potential to increase charge generation in organic solar cells but feature additional complexity due to limited intermixing and electronic mismatch. Here, a model system comprising the polymers poly[5,5‐bis(2‐butyloctyl)‐(2,2‐bithiophene)‐4,4‐dicarboxylate‐alt‐5,5‐2,2‐bithiophene] (PDCBT) and PTB7‐Th and PC70BM as an electron accepting unit is presented. The power conversion efficiency (PCE) of the ternary system clearly surpasses the performance of either of the binary systems. The photophysics is governed by a fast energy transfer process from PDCBT to PTB7‐Th, followed by electron transfer at the PTB7‐Th:fullerene interface. The morphological motif in the ternary blend is characterized by polymer fibers. Based on a combination of photophysical analysis, GIWAXS measurements and calculation of the intermolecular parameter, the latter indicating a very favorable molecular affinity between PDCBT and PTB7‐Th, it is proposed that an efficient charge generation mechanism is possible because PTB7‐Th predominantly orients around PDCBT filaments, allowing energy to be effectively relayed from PDCBT to PTB7‐Th. Fullerene can be replaced by a nonfullerene acceptor without sacrifices in charge generation, achieving a PCE above 11%. These results support the idea that thermodynamic mixing and energetics of the polymer–polymer interface are critical design parameter for realizing highly efficient ternary solar cells with variable electron acceptors.
Organic solar cells based on multinary
components are promising
to further boost the device performance. The complex interplay of
the morphology and functionality needs further investigations. Here,
we report on a systematic study on the morphology evolution of prototype
ternary systems upon adding sensitizers featuring similar chemical
structures but dramatically different crystallinity, namely poly(3-hexylthiophene)
(P3HT) and indene-C60-bis-adduct (ICBA) blends with poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadi-azole)-5,5′-diyl]
(Si-PCPDTBT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT), employing
energy-filtered transmission electron microscopy (EFTEM) and resonant
soft X-ray scattering (RSoXS). In addition, a combined density functional
theory (DFT) and artificial neuronal network (ANN) computational approach
has been utilized to calculate the solubility parameters and Flory–Huggins
intermolecular parameters to evaluate the influence of miscibility
on the final morphology. Our experiments reveal that the domain spacing
and purity of ICBA-rich domains are retained in Si-PCPDTBT-based systems
but are strongly reduced in C-PCPDTBT-based ternary systems. The P3HT
fiber structure are retained at low sensitizer content but dramatically
reduced at high sensitizer content. The theoretical calculations reveal
very similar miscibility/compatibility between the two sensitizers
and ICBA as well as P3HT. Thus, we conclude that mainly the crystallization
of Si-PCPDTBT drives the nanostructure evolution in the ternary systems,
while this driving force is absent in C-PCPDTBT-based ternary blends.
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