Rational Design of High Performance Conjugated Polymers for Organic Solar Cells

Evaluation of heterocycle-modified pentathiophene-based molecular donor materials for solar cells

Two novel solution-processable acceptor-donor-acceptor (A-D-A)-structured organic small molecules with diketopyrrolopyrrole (DPP) as terminal acceptor units and pentathiophene (PTA) or pyrrole-modified pentathiophene (NPTA) as the central donor unit, namely, DPP2(PTA) and DPP2(NPTA), were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [DPP2(PTA)] to nitrogen [DPP2(NPTA)] in the small-molecule donor material. Replacement of the bridging atom with a different electronic structure has a visible effect on both the optical and electrical properties: DPP2(NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP2(PTA) with the less electron-rich thiophene in the same position. More importantly, substitution of the bridging atoms results in a change of the substituting alkyl chains due to the nature of the heteroatoms, which significantly tailored the crystallization behavior and the ability to form an interpenetrating network in thin-film blends with an electron acceptor. Compared to DPP2(PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP2(NPTA) exhibits improved crystallinity and better miscibility with PC71BM probably because of a dodecyl chain on the central nitrogen atom of the NPTA unit. These features endow the DPP2(NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP2(NPTA)/PC71BM (1.5:1, w/w) has resulted in an average power conversion efficiency (PCE) as high as 3.69% (the maximum PCE was 3.83%). This study demonstrates that subtle changes and tailoring of the molecular structure, such as simply changing the bridging heteroatom in the thienoacene unit in D/A-type small molecules, can strongly affect the physical properties that govern their photovoltaic performances.

Low band-gap conjugated polymers with strong interchain aggregation and very high hole mobility towards highly efficient thick-film polymer solar cells

Absorption spectra of polymer FBT-Th4 (1,4) (M n = 46.4 Kg/mol, E g = 1.62 eV, and HOMO = -5.36 eV) indicate strong interchain aggregation ability. High hole mobilities up to 1.92 cm(2) (V s)(-1) are demonstrated in OFETs fabricated under mild conditions. Inverted solar cells with active layer thicknesses ranging from 100 to 440 nm display PCEs exceeding 6.5%, with the highest efficiency of 7.64% achieved with a 230 nm thick active layer.

Donor-Acceptor Block Copolymers: Synthesis and Solar Cell Applications

Fullerene derivatives have been widely used for conventional acceptor materials in organic photovoltaics (OPVs) because of their high electron mobility. However, there are also considerable drawbacks for use in OPVs, such as negligible light absorption in the visible-near-IR regions, less compatibility with donor polymeric materials and high cost for synthesis and purification. Therefore, the investigation of non-fullerene acceptor materials that can potentially replace fullerene derivatives in OPVs is increasingly necessary, which gives rise to the possibility of fabricating all-polymer (polymer/polymer) solar cells that can deliver higher performance and that are potentially cheaper than fullerene-based OPVs. Recently, considerable attention has been paid to donor-acceptor (D-A) block copolymers, because of their promising applications as fullerene alternative materials in all-polymer solar cells. However, the synthesis of D-A block copolymers is still a challenge, and therefore, the establishment of an efficient synthetic method is now essential. This review highlights the recent advances in D-A block copolymers synthesis and their applications in all-polymer solar cells.

25th anniversary article: high-mobility hole and electron transport conjugated polymers: how structure defines function

The structural organization of three different families of semicrystalline π-conjugated polymers is reported (poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-alkyl-4H-cyclopenta-[2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](cyclopentadithiophene-benzothiadiazole) (CDT-BTZ) and poly(N,N"-bis-2-octyldodecylnaphtalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene (P(NDI2OD-T2))). These have triggered significant interest for their remarkable charge-transport properties. By performing molecular mechanics/dynamics simulations with carefully re-parameterized force fields, it is illustrated in particular how the supramolecular organization of these conjugated polymers is driven by an interplay between the length and nature of the conjugated monomer unit and the packing of their alkyl side chains, and to what extent it impacts the charge-carrier mobility, as monitored by quantum-chemical calculations of the intermolecular hopping transfer integrals. This Progress Report is concluded by providing generic guidelines for the design of materials with enhanced degrees of supramolecular organization.

Polythiophenes Comprising Conjugated Pendants for Polymer Solar Cells: A Review

Polythiophene (PT) is one of the widely used donor materials for solution-processable polymer solar cells (PSCs). Much progress in PT-based PSCs can be attributed to the design of novel PTs exhibiting intense and broad visible absorption with high charge carrier mobility to increase short-circuit current density (J_sc), along with low-lying highest occupied molecular orbital (HOMO) levels to achieve large open circuit voltage (V_oc) values. A promising strategy to tailor the photophysical properties and energy levels via covalently attaching electron donor and acceptor pendants on PTs backbone has attracted much attention recently. The geometry, electron-donating capacity, and composition of conjugated pendants are supposed to be the crucial factors in adjusting the conformation, energy levels, and photovoltaic performance of PTs. This review will go over the most recent approaches that enable researchers to obtain in-depth information in the development of PTs comprising conjugated pendants for PSCs.

Additive-assisted control over phase-separated nanostructures by manipulating alkylthienyl position at donor backbone for solution-processed, non-fullerene, all-small-molecule solar cells

A non-fullerene, all-small-molecule solar cell (NF-SMSC) device uses the blend of a small molecule donor and a small molecule acceptor as the active layer. Aggregation ability is a key factor for this type of solar cell. Herein, we used the alkylthienyl unit to tune the aggregation ability of the diketopyrrolopyrrole (DPP)-based small molecule donors. Replacing two alkoxyl units in BDT-O-DPP with two alkylthienyl units yields BDT-T-DPP, and further introducing another two alkylthienyl units into the backbone produces BDT-T-2T-DPP. With the introduction of alkylthienyl, the backbone becomes twisted. As a result, the ππ-stacking strength, aggregation ability, and crystallite size all obey the sequence of BDT-O-DPP > BDT-T-DPP > BDT-T-2T-DPP. When selected a reported perylene diimide dimer of bis-PDI-T-EG as acceptor, the best NF-SMSC device exhibits a power conversion efficiency of 1.34, 2.01, and 1.62%, respectively, for the BDT-O-DPP, BDT-T-DPP, and BDT-T-2T-DPP based system. The BDT-T-DPP/bis-PDI-T-EG system yields the best efficiency of 2.01% among the three combinations. This is due to the moderate aggregation ability of BDT-T-DPP yields moderate phase size of 30-50 nm, whereas the strong aggregation ability of BDT-O-DPP gives a bigger size of 50-80 nm, and the weak aggregation ability of BDT-T-2T-DPP produces a smaller size of 10-30 nm. The BDT-T-DPP/bis-PDI-T-EG combination exhibits balanced hole/electron mobility of 0.022/0.016 cm(2)/(V s), whereas the BDT-O-DPP/bis-PDI-T-EG and the BDT-T-2T-DPP/bis-PDI-T-EG blend show a hole/electron mobility of 0.0011/0.0057 cm(2)/(V s) and 0.0016/0.11 cm(2)/(V s), respectively.

Small molecule BODIPY dyes as non-fullerene acceptors in bulk heterojunction organic photovoltaics

A series of acceptor-donor-acceptor molecules containing terminal BODIPY moieties conjugated through the meso position were synthesized. Deep LUMO energy levels and good visible absorption led to their use as acceptors in bulk heterojunction solar cells. Inverted devices were fabricated, reaching efficiencies as high as 1.51%.

Dithienocarbazole and isoindigo based amorphous low bandgap conjugated polymers for efficient polymer solar cells

Three highly rigid and planar low-bandgap conjugated polymers comprising alternate isoindigo and dithienocarbazole groups are synthesized for the fabrication of high performance polymer solar cells. Power conversion efficiencies of up to 7.2% for conventional devices and 8.2% for inverted devices are demonstrated.

Design and synthesis of molecular donors for solution-processed high-efficiency organic solar cells

Organic semiconductors incorporated into solar cells using a bulk heterojunction (BHJ) construction show promise as a cleaner answer to increasing energy needs throughout the world. Organic solar cells based on the BHJ architecture have steadily increased in their device performance over the past two decades, with power conversion efficiencies reaching 10%. Much of this success has come with conjugated polymer/fullerene combinations, where optimized polymer design strategies, synthetic protocols, device fabrication procedures, and characterization methods have provided significant advancements in the technology. More recently, chemists have been paying particular attention to well-defined molecular donor systems due to their ease of functionalization, amenability to standard organic purification and characterization methods, and reduced batch-to-batch variability compared to polymer counterparts. There are several critical properties for efficient small molecule donors. First, broad optical absorption needs to extend towards the near-IR region to achieve spectral overlap with the solar spectrum. Second, the low lying highest occupied molecular orbital (HOMO) energy levels need to be between -5.2 and -5.5 eV to ensure acceptable device open circuit voltages. Third, the structures need to be relatively planar to ensure close intermolecular contacts and high charge carrier mobilities. And last, the small molecule donors need to be sufficiently soluble in organic solvents (≥10 mg/mL) to facilitate solution deposition of thin films of appropriate uniformity and thickness. Ideally, these molecules should be constructed from cost-effective, sustainable building blocks using established, high yielding reactions in as few steps as possible. The structures should also be easy to functionalize to maximize tunability for desired properties. In this Account, we present a chronological description of our thought process and design strategies used in the development of highly efficient molecular donors that achieve power conversion efficiencies greater than 7%. The molecules are based on a modular D(1)-A-D(2)-A-D(1) architecture, where A is an asymmetric electron deficient heterocycle, which allowed us to quickly access a library of compounds and develop structure-property-performance relationships. Modifications to the D1 and D2 units enable spectral coverage throughout the entire visible region and control of HOMO energy levels, while adjustments to the pendant alkyl substituents dictate molecular solubility, thermal transition temperatures, and solid-state organizational tendencies. Additionally, we discuss regiochemical considerations that highlight how individual atom placements can significantly influence molecular and subsequently device characteristics. Our results demonstrate the utility of this architecture for generating promising materials to be integrated into organic photovoltaic devices, call attention to areas for improvement, and provide guiding principles to sustain the steady increases necessary to move this technology forward.

Acceptor-acceptor type isoindigo-based copolymers for high-performance n-channel field-effect transistors

Two acceptor-acceptor (A-A) type copolymers (PIIG-BT and PIIG-TPD) with backbones composed exclusively of electron-deficient units are designed and synthesized. Both copolymers show unipolar n-type operations. In particular, PIIG-BT shows electron mobility of up to 0.22 cm(2) V(-1) s(-1). This is a record value for n-type copolymers based on lactam cores.

Diindenocarbazole-based large bandgap copolymers for high-performance organic solar cells with large open circuit voltages

Diindenocarbazole-based large bandgap copolymers exhibit a power conversion efficiency of 7.26% with a high open-circuit voltage of 0.93 V.

Third-generation solar cells: a review and comparison of polymer:fullerene, hybrid polymer and perovskite solar cells

Third-generation solar cells have excellent potential for delivering large scale, low-cost solar electricity. We review and compare the current understanding of the operation principles, performance improvements and future prospects for polymer:fullerene, hybrid polymer and perovskite solar cells.

Supramolecular control of organic p/n-heterojunctions by complementary hydrogen bonding

The supramolecular structure of organic semiconductors (OSCs) is the key parameter controlling their performance in organic electronic devices, and thus methods for controlling their self-assembly in the solid state are of the upmost importance. Recently, we have demonstrated the co-assembly of p- and n-type organic semiconductors through a three-point hydrogen-bonding interaction, utilizing an electron-rich dipyrrolopyridine (P2P) heterocycle which is complementary to naphthalenediimides (NDIs) both in its electronic structure and H bonding motif. The hydrogen-bonding-mediated co-assembly between P2P donor and NDI acceptor leads to ambipolar co-crystals and provides unique structural control over their solid-state packing characteristics. In this paper we expand our discussion on the crystal engineering aspects of H bonded donor–acceptor assemblies, reporting three new single co-crystal X-ray diffraction structures and analyzing the different packing characteristics that arise from the molecular structures employed. Particular attention is given toward understanding the formation of the two general motifs observed, segregated and mixed stacks. Co-assembly of the donor and acceptor components into a single, crystalline material, allows the creation of ambipolar semiconductors where the mutual arrangement of p- and n-conductive channels is engineered by supramolecular design based on complementary H bonding.

Synthesis characterization and bulk-heterojunction photovoltaic applications of new naphtho[1,2-b:5,6-b′]dithiophene–quinoxaline containing narrow band gap D–A conjugated polymers

New naphtho[1,2-b:5,6-b′]dithiophene–quinoxaline containing donor–acceptor conjugated copolymers with alkyl/alkoxy side chains positioned differently were developed and investigated for polymer solar cells.

Effects of cyano (CN)-groups on the planarity, film morphology and photovoltaic performance of benzodithiophene-based polymers

A class of polymers (POT-DH, POT-HCN and POT-DCN) were synthesized and they contain the same donor (BDT) and acceptor units but different numbers of cyano (CN)-groups, i.e. 0, 1 and 2.

Synthesis, characterization and photovoltaic properties of benzo[1,2-b:4,5-b′]dithiophene-bridged molecules

Four benzo[1,2-b:4,5-b′]dithiophene (BDT) centred small molecules with different acceptor and end-capping groups were synthesized as the donor materials for organic solar cells.

The effect of heteroatom conformation on optoelectronic properties of cyclopentadithiophene derivatives

Cyclopentadithiophene (CPDT) derivatives with different heteroatom conformations have been synthesized.

A cyclopenta[1,2-b:5,4-b′]dithiophene–porphyrin conjugate for mesoscopic solar cells: a D–π–D–A approach

In this article, cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) was introduced as a spacer between the porphyrin chromophore and cyanoacetic acid to obtain a porphyrin dye (coded as LW9).

A high-performance photovoltaic small molecule developed by modifying the chemical structure and optimizing the morphology of the active layer

High performance organic solar cell (8.26% PCE) developed through modifying the chemical structure of molecule and optimizing the morphology of active layer.

Low band-gap copolymers derived from fluorinated isoindigo and dithienosilole: synthesis, properties and photovoltaic applications

Fluorination of isoindigo affords a dithienosilole-based low band-gap copolymer with low-lying energy levels, strong and broad absorption, high carrier mobility as well as efficient power conversion efficiency.

The effect of different chalcogenophenes in isoindigo-based conjugated copolymers on photovoltaic properties

Three low bandgap conjugated copolymers based on isoindigo and three different chalcogenophenes (thiophene, selenophene and tellurophene) were synthesized to investigate the effect of different chalcogenophenes on their photovoltaic properties.

Recent developments on isoindigo-based conjugated polymers

A review of recent advances in isoindigo-based conjugated polymers for organic photovoltaic and field-effect transistor applications is presented.

Rational design on D–A conjugated P(BDT–DTBT) polymers for polymer solar cells

This review summarizes the various structural modifications and photovoltaic properties of the benzodithiophene–benzothiadiazole conjugated polymers [P(BDT–DTBT)] and their derivatives.

A new class of organic photovoltaic materials: poly(rod-coil) polymers having alternative conjugated and non-conjugated segments

A new class of organic photovoltaic donor materials, poly(rod-coil) polymers, has been proposed and studied.