A series of two-dimensional conjugated polymers containing π-conjugated oligothienyl side chains, namely PBDT2FBT-T1, PBDT2FBT-T2, PBDT2FBT-T3, and PBDT2FBT-T4, was designed and synthesized to investigate the effect of two-dimensionally extended π-conjugation on the polymer solar cell (PSC) performance. The oligothienyl units introduced into the side chains significantly affect the optoelectronic properties of the parent polymers as well as the performances of the resulting solar cell devices by altering the molecular arrangement and packing, crystalline behavior, and microstructure of the polymer:PC 71 BM blend films. The crystallinity and blend morphology of the polymers can be systematically controlled by tuning the π-conjugation length of side chains; PBDT2FBT-T3 exhibited the most extended UV/vis light absorption band and the highest charge mobility, leading to a high short-circuit current density up to 12.5 mA cm −2 in the relevant PSCs. The PBDT2FBT-T3:PC 71 BM-based PSC exhibited the best power conversion efficiency of 6.48% among this series of polymers prepared without the use of processing additives or post-treatments. These results provide a new possibility and valuable insight into the development of efficient medium-bandgap polymers for use in organic solar cells.
We
designed, synthesized, and characterized a series of three medium-bandgap
conjugated polymers (PBDTfDTBO, PBDTfDTBT, and PBDTfDTBS)
consisting of fused dithienobenzochalcogenadiazole (fDTBX)-based
weak electron-deficient and planar building blocks, which possess
bandgaps of ∼2.01 eV. The fDTBX-based medium-bandgap polymers
exhibit deep-lying HOMO levels (∼5.51 eV), which is beneficial
for use in multijunction polymer solar cell applications. The resulting
polymers with chalcogen atomic substitutions revealed that the difference
in the electron negativity and atomic size of heavy atoms highly affects
an intrinsic property, morphological feature, and photovoltaic property
in polymer solar cells. The polymer solar cells based on sulfur-substituted
medium-bandgap polymer showed power conversion efficiencies above
6% when blended with [6,6]-phenyl-C71-butyric acid methyl
ester in a typical bulk-heterojunction single cell. These results
suggest that the fDTBX-based medium-bandgap polymer is a promising
alternative material for P3HT in tandem polymer solar cells for achieving
high efficiency.
A low-bandgap acceptor (ITIC) was added to a binary system composed of a wide-bandgap polymer (PBT-OTT) and an acceptor (PC
71
BM) to increase the light harvesting efficiency of the associated organic solar cells (OSCs). A ternary blend OSC with an acceptor ratio of PC
71
BM:ITIC = 8:2 was found to exhibit a power conversion efficiency of 8.18%, which is 18% higher than that of the binary OSC without ITIC. This improvement is mainly due to the enhanced light absorption and optimized film morphology that result from ITIC addition. Furthermore, an energy level cascade forms in the blend that ensures efficient charge transfer, and bimolecular and trap-assisted recombination is suppressed. Thus the use of ternary blend systems provides an effective strategy for the development of efficient single-junction OSCs.
Fine tuning the energy levels of donor polymers is a critically important step toward achieving high power conversion efficiencies in polymer solar cells (PSCs). We systematically controlled the energy levels of donor polymers by introducing cyano (CN) and alkoxy (OR) groups into the 4,4′-didodecyl-2,2′bithiophene (BT) unit in a step-by-step fashion, thereby varying the inductive and resonance effects. The three monomer units (BT, BTC, and BTCox) were polymerized with benzo[1,2-b:4,5-b′]dithiophene (BDT) as a counter unit to afford three polymers (PBDT-BT, PBDT-BTC, and PBDT-BTCox). The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels decreased significantly upon the introduction of CN groups, and these levels increased slightly upon attachment of the OR groups, in good agreement with the measured open-circuit voltages of the three polymer devices. The strong inductive and resonance effects present in PBDT-BTCox narrowed the polymer band gap to 1.74 eV to afford a power conversion efficiency of 5.06%, the highest value achieved among the three polymers.
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