Optimizing Light‐Harvesting Polymers via Side Chain Engineering

Abstract

wileyonlinelibrary.comdevices by matching energy levels and optimizing morphologies. [1][2][3][4] Semiconducting π-conjugated polymers made of donor-acceptor (D-A) hybrids are still leading the development, which show facile tuning of band gaps and energy levels by introducing new or different constituent units. [ 5,6 ] However, in comparison to the development of new conjugated building blocks, modifying side chains offers an alternative, perhaps more convenient and cost-effective route to optimize materials. [7][8][9][10][11][12][13][14][15][16][17][18] Modifying side chain constituents is usually done in a framework of an established high-performance backbone with the aim of fi ne tuning the morphology. The selection of side chain substituents, usually alkyl units, needs to balance polymer solubility with the ability for the main chains to self-assemble into ordered units. Polymers with bulky side chains have good solubility in organic solvents for processing, and less bulky side chains provide good interchain packing that ensures charge-carrier transport in solid fi lms. Using advanced structural characterization tools, a number of recent studies have shown the importance of side chain tuning. For example, Beaujuge et al. showed that side chains can infl uence the polymer self-assembly and preferential crystalline orientation; [ 19 ] Cho et al. demonstrated that the solubility, energy level, light absorption, surface tension, and intermolecular packing of the resulting polymers could be changed by altering the functional groups of side chains; [ 20 ] Osaka et al. found that the backbone orientation can be changed by tuning the length between adjacent side chains; [ 21,22 ] McGehee et al. studied the molecular interactions between polymer segments and fullerenes using linear and branched side chains. [ 23 ] Each of these parameters strongly affects the morphology and power conversion effi ciencies (PCEs) of photovoltaic devices. There is no universal guidance to select the chemical constitution of the ideal side chains. From the viewpoint of solubility, longer linear alkyl chains afford comparable solubility to shorter, branched alkyl substituents. [ 9 ] From the molecular ordering viewpoint, linear side chains will promote π-π stacking between the backbones, leading to better charge transport and, usually, higher short circuit currents ( J sc ). [ 10 ] However, strong intermolecular interactions can lead to greater dark currents ( J so ) that reduce the open circuit voltage ( V oc ). [ 14 ] Consequently, there is a delicate balance that needs to be struck to generate conjugated polymers with a suitable mixture of linear and branched side Optimizing Light-Harvesting Polymers via Side Chain EngineeringPeng Liu , Sheng Dong , Feng Liu , * Xiaowen Hu , Liqian Liu , Yaocheng Jin , Shengjian Liu , Xiong Gong , Thomas P. Russell , * Fei Huang , * and Yong Cao A series of conjugated polymers using naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole and benzodithiophene alternating backbone is synthesized to investigate the...

A Facile Method to Fine‐Tune Polymer Aggregation Properties and Blend Morphology of Polymer Solar Cells Using Donor Polymers with Randomly Distributed Alkyl Chains

“…[1][2][3][4][5][6][7] Recent advancements have resulted in a range of material systems with over 10% power conversion efficiencies (PCEs) in a single junction cell. [21][22][23][24][25][26][27][28][29][30] In particular, the device performance is very sensitive to the size of alkyl side chains in donor polymers. One of the most successful strategies for improving device performance is by engineering the side chains of the donor polymer, which strongly affects the solubility, aggregation properties, domain crystallinity/purity, and thus the overall morphology of the polymeric blend.…”

Effects of alkyl side chain length of low bandgap naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole‐based copolymers on the optoelectronic properties of polymer solar cells

“…Owing to the characteristics of low cost, light weight, flexibility, and facile fabrication of roll‐to‐roll processing, bulk‐heterojunction (BHJ) polymer solar cells (PSCs) have been widely focused on and are considered as promising alternative for inorganic solar cells . Assisted by exploring and developing novel conjugated polymers (CPs), device structure innovation, film morphology and interfacial engineering, PSCs have gained great progress in a brief span of years. Recently, the power conversion efficiency (PCE) of single junction module has surpassed over 13% .…”

“…Among the various A units, naphtho[1,2‐ c :5,6‐ c ′]bis[1,2,5]thiadiazole (NT) bearing an enlarged planarity and a stronger electron‐withdrawing feature exhibited the interesting electronic properties and highly self‐assembling nature . And thus, these NT‐based CPs have attracted the increasing attention . Since Huang et al .…”

“…In comparison with exploring the new backbone CPs, adjusting side chain can offer an alternative, which is likely to afford more convenient and cost‐effective synthetic routes to optimize materials and improve the PV performance . Typically, CPs are composed of π‐conjugated backbones and peripheral flexible solubilizing side chains .…”

Diethynylbenzo[1,2‐b:4,5‐b′]dithiophene‐based small molecule and cross‐conjugated copolymers for organic solar cells

“…Typically, there are two categories of the electron‐donating materials for OSCs, which can be described as the π‐conjugated small molecules and polymers. In the past decade, much effort has been exerted to the rational molecular design of these materials, such as tuning conjugated backbones, changing the side chains and varying the substituents . As a result, a range of conjugated polymers and small molecules have been developed for single junction BHJ organic solar cells that presented remarkable power conversion efficiencies (PCEs) over 9–10% .…”