Incorporation of Fluorine onto Different Positions of Phenyl Substituted Benzo[1,2-b:4,5-b′]dithiophene Unit: Influence on Photovoltaic Properties

Abstract

We have designed and synthesized two low bandgap conjugated copolymers containing alternating meta-fluoro-p-alkoxyphenyl-(m-FPO-) or p-fluoro-m-alkoxyphenyl-(p-FPO-) substituted benzodithiophenes-co-benzooxadiazole (BO), named PBO-m-FPO and PBO-p-FPO. The properties, including UV−vis absorption, charge mobility and photovoltaic performance of the two polymers have been intensively investigated. The results indicated that the introduction of fluorine atom at m, p positions of phenyl substituted benzodithiophene unit hardly affected their absorption spectra and highest occupied molecular orbital (HOMO) level. However, the two polymers showed different photovoltaic properties. Power conversion efficiencies (PCEs) based on the device structure of ITO/PEDOT:PSS/polymer:PC 71 BM/ Ca/Al demonstrated a large distinction (5.9% for PBO-m-FPO vs 2.8% for PBO-p-FPO) at optimal weight ratio. When replacing the Ca layer with zirconium acetylacetonate (ZrAcac), using 3% 1,8-diiodooctane (DIO) as the active layer additive, the PCEs of PBO-m-FPO and PBO-p-FPO increased by 36% (8.0% vs 5.9%) and 85% (5.1% vs 2.8%), respectively. The active layer's mobilities, morphology and molecular packing resulted in a significant difference in short-circuit current density (J sc ) and fill factor (FF). ■ INTRODUCTIONPolymer solar cells (PSCs) based on bulk heterojunction (BHJ) have made tremendous advances toward commercialization. 1 Recently, the power conversion efficiencies (PCEs) of several polymers with single junction device have reached 11%. 2−5 As impressive and important renewable energy sources, PSCs have particular advantages such as simple device structure, lightweight, flexibility and low fabrication cost using simple ambient-condition solution or the roll-to-roll coating process. 6−8 Compared to inorganic-based solar cells, however, they still do not achieve the targeted 15% efficiency and satisfying lifetime, which is required for widespread commercialization. 9 Although optimizing the device fabrication 10,11 will improve photovoltaic performance, a promising candidate polymer with simple synthesis process is extremely in demand for broad applications of PSCs. 12 For high-performance PSCs, the ideal strategy is to design donor−acceptor (D−A) alternating molecular structure for BHJ solar cell, which can offer the unique feature of tuning the energy levels and the bandgap. 13 As well-known, PCE is proportional to the open-circuit voltage (V oc ), short-circuit current density (J sc ), and fill factor (FF). So we can independently modulate D−A copolymers to obtain a lowlying the highest occupied molecular orbital (HOMO) energy

A comparative study of the effect of fluorine substitution on the photovoltaic performance of benzothiadiazole-based copolymers

“…6 It is well-known that low band gap polymers with high V oc are crucial to attaining the objective of high performance PSCs. [8][9][10][11][12][13][14][15][16][17][18][19][20] Due to the strong electronwithdrawing characteristic of the uorine atom, the introduction of F as a substituent to the conjugated polymer would lower both energetic positions of the HOMO and LUMO levels of the synthesized polymer, as demonstrated by Brédas et al in a theoretical study. However, adding electron-withdrawing groups to the polymer can generally lower the energetic position of the LUMO level.…”

Small Changes With Big Consequences: Swapping Two Atoms In Side Chains Changes Phenylene‐Ethynylene Packing And Fluorescence

“…[16,17] Our previous work has shown that cofacialfluoroarene-arene (ArF-ArH) interactions of perfluorinated side chains can dictate the conformations and assemblies of phenylene-ethynylenes( PEs), and therefore determine their solid-state opticalp roperties. [18][19][20] Although examples exist for which regiochemistry of arene fluorination alters crystalp acking with modest effects on material properties, [21][22][23][24] slightd ifferences in fluorine regiochemistry of non-conjugated side chains would not generally be expected to impact the opticalp roperties of conjugated materials. Here, we describe surprisingly different opticala nd mechanofluorochromic (MFC) properties of solid PEs that depend on slight positional differences of Ha nd Fa toms (2,4,6v s. 2,3,6-trifluoro) on pendant arenes not conjugated to the PE moiety.…”

High‐Efficiency Polymer Solar Cells Over 13.9% With a High VOC Beyond 1.0 V by Synergistic Effect of Fluorine and Sulfur

“…To date, benzo(1,2‐b:4,5‐b')dithiophene (BDT), as a backbone building block to construct donor polymers, has received considerable attention because of its large planar structure and excellent photovoltaic performance in PSCs . The side chains flanking on the two‐dimensional (2D) conjugated unit BDT, such as phenyl substituted BDT (BDTP) and thienyl substituted (BDTT), play an critical role not only in the solubility for polymer but also in their optical and electrical properties . Compared with the two building blocks, the weaker electron‐donating nature of BDTP has relatively lower HOMO energy levels, which has been used to effectively lower the polymer energy levels than thienyl substituted BDTT unit .…”