Photocrosslinking of low band-gap conjugated polymers using alkyl chloride sidechains: Toward high-efficiency, thermally
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We report the synthesis and photovoltaic characterization of four novel polymers based on poly ({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b9]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) incorporating various numbers of photocrosslinkable n-octyl chloride sidechains (PTB-Cl). These polymers showed similar optoelectronic properties to PTB7 and readily cross-linked in the form of thin films after deep-UV exposure. Photolithography with micrometer-scale patterning is demonstrated. PTB-Cls exhibit similar PV performances to PTB7 and lightly cross-linked PTB-Cls showed stable high photoconversion efficiencies after prolonged thermal treatment. However, it is found that a high-degree crosslinking is needed to prevent the formation of PCBM crystallites at high annealing temperatures even though the PV performance is stabilized with a much lower degree of cross-linking. This implies that the complete prevention of PCBM crystallite formation is not necessary to affect the stabilization of PV devices against excessive heat.
I. INTRODUCTION
Solution-processed organic solar cells based on conjugated polymers and fullerenes are potentially costeffective alternatives to conventional inorganic solar cells.1–3 The active layer of a polymer solar cell (PSC) is typically composed of a bulk heterojunction of polymer and a fullerene.4 Bulk heterojunctions ideally contain finely separated, bicontinuous phases of polymer and a fullerene, which provide interfaces for exciton dissociation and percolating pathways for charge transportation.5–7 The formation of bulk heterojunctions during casting is kinetically driven rather than thermodynamically favored. The as-cast morphologies are thus subject to further phase segregation, which are typically caused by fullerene’s tendency to diffuse and crystalize.8,9 The change in morphology is exacerbated at high temperature and leads to increased charge recombination of the photoexcited state.10–13 As a consequence, thermally stable bulk heterojunctions are desired because excessive thermal energy is generated during device operation.14,15 Strategies explored to achieve stable bulk heterojunctions include seeking a reduction in the rate of fullerene diffusion.9 One strategy consists of synthesizing block copolymers where one of the blocks is capable of bonding fullerenes.16,17 These block copolymers are usually mixed in the bulk heterojunction to act as a compatibilizer and increase thermal stability without compromising the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.78
photoconversion efficiency (PCE).18–20 In another strategy, thermally cleavable sidechains are introduced to the backbones of a conjugated polymer. The sidechains can be subsequently thermally removed prior to device operation.21 The cleavage of sidechains significantly increases the glass transition temperature of the polymer (Tg) and impedes the diffusion of the fullerenes.22–25 Cross-linking is another strategy used to stabilize the morph
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