Drastic Change of Molecular Orientation in a Thiazolothiazole Copolymer by Molecular‐Weight Control and Blending with PC₆₁BM Leads to High Efficiencies in Solar Cells

Abstract

A thiazolothiazole-thiophene copolymer is examined as the active material in bulk heterojunction (BHJ) solar cells. By optimizing the molecular weight, the polymer-based cells exhibit power conversion efficiencies as high as 5.7%. The increase in molecular weight improves the orientational order, and blending with phenyl-C61-butyric acid methyl ester (PC61BM) changes the orientational motif from edge-on to face-on, which accounts for the trend in photovoltaic performances. These results might give new insight into the structure-property relationships in BHJ solar cells.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Firstly, extending the conjugation system and strengthening intra-chain and inter-chain interaction is a direct way to improve electronic properties. Based on the classic conjugated polymers such as P3ATs (P1), a straight strategy is inserting more conjugated moieties [3,82] such as carbon double bond (-HC=CH-, P2) [83][84][85], unsubstituted multiple or fused aromatic rings (i.e., thiophenes and thieno[3,2-b]thiophene, etc., P3-P7) [13,14,16,37,82,[86][87][88][89][90][91], other heterocyclic rings containing nitrogen(and sulfur) (P8 and P9) or selenium atoms (P10) [13,15,81,[92][93][94][95][96][97] and their combination (P11) [98]. Thus, the intra-chain conjugation is extended and strengthened by coupling with the more conjugated moieties, and the conjugation length is elongated due to the improved planarity.…”

“…Besides, the π-π interactions between conjugated units is enhanced due to more segments, leading to stronger intra-chain and inter-chain conjugation indicated by red-shifted spectra and increased 0-0 absorptions in films. The trade-off between the size of crystalline regions with high carrier mobilities and their connections by tie-chains dominates the optimal MW for the highest carrier mobility [96,133]. Besides electronic properties, the chains with larger MWs have problems like larger units hard to diffuse, lower solubility and severe entanglements, which limit the ordered aggregation of them and need solutions being discussed in Section 4.…”

Can computational approaches aid in untangling the inherent complexity of practical organic photovoltaic systems?

“…[22][23][24][25] Also critical is the ability to synthetically control molecular weight (recent work on thiophene-based polymers has demonstrated how molecular weight has drastic consequences on molecular orientation and subsequently PCE; see ref. 26) and to tailor the PT backbone with alkyl substitutents that induce aggregation into crystalline p-stacked domains on the size scale of tens of nanometers. 27 Additionally, some of the desirable photophysical properties likely originate from the existence of low-lying triplet states that are accessible from singlet fission, provided there is proper electronic alignment.…”

Synthesis and Characterization of an Alkoxythiazole-thiazolothiazole Semiconducting Polymer for Organic Solar Cells

“…15,[20][21][22][23][24][25][26][27][28][29][30] We have reported on a series of TzTz-based polymers with alkylthiophenes as the donor unit. 15,[20][21][22][23]31,32 One of the highest performing polymers was PTzBT-BOHD (Fig. 1a), which demonstrated quite high PCEs of ³7.5% in conventional single-junction cells.…”

“…crystallinity and backbone orientation. 4,[14][15][16][17][18][19][20] Thus, the ordering structure must also be carefully controlled.…”