Design of Semiconducting Indacenodithiophene Polymers for High Performance Transistors and Solar Cells

Abstract

The prospect of using low cost, high throughput material deposition processes to fabricate organic circuitry and solar cells continues to drive research towards improving the performance of the semiconducting materials utilized in these devices. Conjugated aromatic polymers have emerged as a leading candidate semiconductor material class, due to their combination of their amenability to processing and reasonable electrical and optical performance. Challenges remain, however, to further improve the charge carrier mobility of the polymers for transistor applications and the power conversion efficiency for solar cells. This optimization requires a clear understanding of the relationship between molecular structure and both electronic properties and thin film morphology. In this Account, we describe an optimization process for a series of semiconducting polymers based on an electron rich indacenodithiophene aromatic backbone skeleton. We demonstrate the effect of bridging atoms, alkyl chain functionalization, and co-repeating units on the morphology, molecular orbital energy levels, charge carrier mobility, and solar cell efficiencies. This conjugated unit is extremely versatile with a coplanar aromatic ring structure, and the electron density can be manipulated by the choice of bridging group between the rings. The functionality of the bridging group also plays an important role in the polymer solubility, and out of plane aliphatic chains present in both the carbon and silicon bridge promote solubility. This particular polymer conformation, however, typically suppresses long range organization and crystallinity, which had been shown to strongly influence charge transport. In many cases, polymers exhibited both high solubility and excellent charge transport properties, even where there was no observable evidence of polymer crystallinity. The optical bandgap of the polymers can be tuned by the combination of the donating power of the bridging unit and the electron withdrawing nature of co-repeat units, alternating along the polymer backbone. Using strong donors and acceptors, we could shift the absorption into the near infrared.

Fused Dithienogermolodithiophene Low Band Gap Polymers for High-Performance Organic Solar Cells without Processing Additives

“…3 The enforced co-planarity reduces rotational disorder thereby lowering reorganization energy and potentially increasing charge carrier mobility. 4 The bridging atoms also serve as a point of attachment for the necessary solubilizing groups needed to ensure processable materials.…”

“…3 The enforced co-planarity reduces rotational disorder thereby lowering reorganization energy and potentially increasing charge carrier mobility. 4 The bridging atoms also serve as a point of attachment for the necessary solubilizing groups needed to ensure processable materials.Within the class of donor-acceptor ladder polymers, bridged bithiophenes have proven to be a particularly useful building block. For example donor-acceptor type copolymers of cyclopentadithiophene (a C bridge) with 2,1,3-benzothiadiazole have exhibited FET mobilities up to 3.3 cm 2 V -1 s -1 when substituted with long hexadecyl sidechains.…”

Post-Polymerization Ketalization for Improved Organic Photovoltaic Materials

“…This is a commonly observed feature of donor-acceptor type copolymers. 5 Photoelectron spectroscopy in air was used to determine the highest occupied molecular orbital (HOMO) levels of the polymer thin films ( Table 1 Having established the fundamental properties of these three new BTT-polymers, attention was turned to BHJ OPV devices with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as the acceptor material using a conventional device configuration and a 1:2 polymer:PC 71 BM blend ratio as this was previously established to be the best blend ratio for these systems. 11 The limited solubility of BTT-DTBT-CO meant that no working OPV devices could be obtained with this material.…”

“…4 Controlling the blend morphology through chemical modifications of the donor polymer is typically pursued by means of alkyl side chain variations or subtle modifications to the conjugated backbone. [5][6][7] While these approaches often have proven successful, very laborious synthetic efforts -typically involving a repeat of most of the synthetic protocol -are required since the solubilising side chains generally are introduced early in the synthetic route for practical purposes.…”

New Fused Bis-Thienobenzothienothiophene Copolymers and Their Use in Organic Solar Cells and Transistors

“…[1][2][3][4][5] Most of the highest performing polymers show short pi-stacking distances between polymer backbones with wave functions often widely delocalized over the polymer conjugated aromatic system. 6 These properties, leading to high charge carrier mobilities, have often been obtained with ring fusion of heteroarenes. [7][8][9][10] Herein we have designed and synthesised an extended and soluble tetradodecylthieno [3',2':6,7][1]benzothieno [3,2-b]thieno [3,2-g] [1]benzothiophene core unit (DTBTBT, Scheme 1) which was incorporated into light absorbing electron donor copolymers for OPV and hole and electron transport OFET polymer devices.…”