Alkyl Chain Engineering of Solution‐Processable Star‐Shaped Molecules for High‐Performance Organic Solar Cells

Abstract

The impact of alkyl side‐chain substituents on conjugated polymers on the photovoltaic properties of bulk heterojunction (BHJ) solar cells has been studied extensively, but their impact on small molecules has not received adequate attention. To reveal the effect of side chains, a series of star‐shaped molecules based on a triphenylamine (TPA) core, bithiophene, and dicyanovinyl units derivatized with various alkyl end‐capping groups of methyl, ethyl, hexyl and dodecyl is synthesiyed and studied to comprehensively investigate structure‐properties relationships. UV‐vis absorption and cyclic voltammetry data show that variations of alkyl chain length have little influence on the absorption and highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) levels. However, these seemingly negligible changes have a pronounced impact on the morphology of BHJ thin films as well as their charge carrier separation and transportation, which in turn influences the photovoltaic properties of these small‐molecule‐based BHJ devices. Solution‐processed organic solar cells (OSCs) based on the small molecule with the shortest methyl end groups exhibit high short circuit current (Jsc) and fill factor (FF), with an efficiency as high as 4.76% without any post‐treatments; these are among the highest reported for solution‐processed OSCs based on star‐shaped molecules.

Star-shaped D–π–A oligothiophenes with a tris(2-methoxyphenyl)amine core and alkyldicyanovinyl groups: synthesis and physical and photovoltaic properties

“…The replacement of the active hydrogen by an alkyl group was found to be an efficient tool not only to increase the electrochemical stability of donor-acceptor small molecules, 15,16 but also to control their solubility and a number of important solid-state physical properties by simple adjustment of the alkyl chain length in the DCV substituent. 17,18 To fine-tune the energy levels in small molecules, some additional electron-donating blocks such as dithienosilole, [19][20][21][22] benzodithiophene, 23,24 and dithienopyrrole, 25 and etc., are often used. For the design of star-shaped molecules, a triphenylamine (TPA) core is one of the widely used electron-donating blocks due to its low cost, high electrochemical stability and hole transporting ability.…”

“…34 In this work we report on the synthesis and a comprehensive study of a series of four D-p-A oligomers having the tris-(2-methoxyphenyl)amine (m-TPA) core as a donor, either alkyl-DCV or DCV as acceptor units, and either bi-or terthiophene as p-bridges (Scheme 1). A comparison of properties of these four molecules with those of their full analogs with a TPA core [15][16][17]35 reveals the benefits and drawbacks of this chemical design strategy. Furthermore, variation of the p-bridge and the alkyl chain length as well as the type of substituent at the DCV group allows elucidating the structure-property relationships in this series of star-shaped molecules systematically.…”

Tuning the Ferroelectric Properties of Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

“…The issue of flexible chain length in defining material characteristics has been discussed multiple times for semiconducting and/or supramolecular materials. [22,[24][25][26] However, comparatively few works have addressed structureproperty relations of molecular ferroelectrics in a systematic manner. [8,19,27] One of the few studies focusing on proper organic ferroelectric materials is by Kishikawa et al, who probed ferroelectric columnar hexagonal liquid crystalline urea with C 4 H 9 to C 16 H 33 long alkyl chains.…”

Fabrication of highly reproducible polymer solar cells using ultrasonic substrate vibration posttreatment

“…3,[10][11][12] It is noted that although P3HT:PCBM is the standard and most studied D/A system, other polymer blends with lower bandgaps have also been used to obtain compelling photovoltaic performance. [13][14][15][16][17] In this fundamental study, however, the P3HT:PCBM system is used, while the findings may be extended to solar cells with similar architecture. Table 1 lists the power conversion efficiency (PCE) of several solar cells based on P3HT: PCBM system, reported in the literature.…”