Nonhalogenated Solvent Processable and High-Density Photopatternable Polymer Semiconductors Enabled by Incorporating Hydroxyl Groups in the Side Chains.

Polymer semiconductors hold tremendous potential for applications in flexible devices, which is however hindered by the fact that they are usually processed by halogenated solvents rather than environmentally more friendly solvents. An effective strategy to boost the solubility of high-performance polymer semiconductors in nonhalogenated solvents such as tetrahydrofuran (THF) by appending hydroxyl groups in the side chains is herein presented. The results show that hydroxyl groups, which can be easily incorporated into the side chains, can significantly improve the solubility of typical p- and n-types as well as ambipolar polymer semiconductors in THF. Meanwhile, the thin films of these polymer semiconductors from the respective THF solutions show high charge mobilities. With THF as the processing and developing solvents these polymer semiconductors with hydroxyl groups in the side chains can be well photopatterned in the presence of the photo-crosslinker, and the charge mobilities of the patterned thin films are mostly maintained by comparing with those of the respective pristine thin films. Notably, THF is successfully utilized as the processing and developing solvent to achieve high-density photopatterning with ≈82 000 device arrays cm-2 for polymer semiconductors in which hydroxyl groups are appended in the side chains.

Surpassing 13% Efficiency for Polythiophene Organic Solar Cells Processed from Nonhalogenated Solvent.

Benefiting from low cost and simple synthesis, polythiophene (PT) derivatives are one of the most popular donor materials for organic solar cells (OSCs). However, polythiophene-based OSCs still suffer from inferior power conversion efficiency (PCE) than those based on donor-acceptor (D-A)-type conjugated polymers. Herein, a fluorinated polythiophene derivative, namely P4T2F-HD, is introduced to modulate the miscibility and morphology of the bulk heterojunction (BHJ)-active layer, leading to a significant improvement of the OSC performance. The Flory-Huggins interaction parameters calculated from the surface energy and differential scanning calorimetry results suggest that P4T2F-HD shows moderate miscibility with the popular nonfullerene acceptor Y6-BO (2,2'-((2Z,2'Z)-((12,13-bis(2-butyloctyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2',3':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile), while poly(3-hexylthiophene) (P3HT) is very miscible with Y6-BO. As a result, the P4T2F-HD case forms desired nanoscale phase separation in the BHJ film while the P3HT case forms a completely mixed BHJ film, as revealed by transmission electron microscopy (TEM) and grazing-incidence wide-angle X-ray scattering (GIWAXS). By optimizing the cathode interface and the morphology of the P4T2F-HD:Y6-BO films processed from nonhalogenated solvents, a new record PCE of 13.65% for polythiophene-based OSCs is demonstrated. This work highlights the importance of controlling D/A interactions for achieving desired morphology and also demonstrates a promising OSC system for potential cost-effective organic photovoltaics.

Highly Efficient All-Polymer Solar Cells Processed from Nonhalogenated Solvents.

The remarkable advance of all-polymer solar cells (all-PSCs) achieved in the past decades is primarily powered by the innovation of polymer acceptors. However, most of high-performance all-PSCs are dominantly fabricated with halogenated solvents, which are detrimental to human bodies and the environment. Herein, eco-friendly solvent-processed all-PSCs were developed, based on wide-bandgap polymer poly[4,8-bis(5-(2-ethylhexylthio)thiophen-2-yl)-benzo-[1,2-b;4,5-b']dithiophene-alt-2,5-di(butyloctylthiophen-2-yl) -thiazolo[5,4-d]thiazole] (PSTZ) as donor and newly synthesized narrow-bandgap polymer 5,6-dicyano-2,1,3-benzothiadiazole indacenodithiophene (DCNBT-IDT) as acceptor. When processed with o-xylene and THF, PSTZ : DCNBT-IDT-based all-PSCs yielded remarkable power conversion efficiencies of 7.23 and 8.77 % with high short-circuit currents of 12.94 and 14.12 mA cm-2 , respectively. The results indicated that the utilization of an all-polymer blend based on narrow polymer acceptor and compatible polymer donor is an effective strategy for advancing eco-friendly solvent-processed all-PSCs.

Highly Efficient, Stable, and Ductile Ternary Nonfullerene Organic Solar Cells from a Two-Donor Polymer Blend.

Organic solar cells (OSCs) are one of the most promising cost-effective options for utilizing solar energy, and, while the field of OSCs has progressed rapidly in device performance in the past few years, the stability of nonfullerene OSCs has received less attention. Developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable mechanical durability. Building upon the ink (toluene:FTAZ:IT-M) that broke the 10% benchmark when blade-coated in air, a second donor material (PBDB-T) is introduced to stabilize and enhance performance with power conversion efficiency over 13% while keeping toluene as the solvent. More importantly, the ternary OSCs exhibit excellent thermal stability and storage stability while retaining high ductility. The excellent performance and stability are mainly attributed to the inhibition of the crystallization of nonfullerene small-molecular acceptors (SMAs) by introducing a stiff donor that also shows low miscibility with the nonfullerene SMA and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer. The study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of nonfullerene OSCs.

A New Wide Bandgap Donor Polymer for Efficient Nonfullerene Organic Solar Cells with a Large Open-Circuit Voltage.

Significant progress has been made in nonfullerene small molecule acceptors (NF-SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF-OSCs). To achieve better compatibility with high-performance NF-SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5-dibromo-4-fluorothiophene-3-carboxylate (FE-T), is synthesized and copolymerized with benzo[1,2-b:4,5-b']dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1-P3 with a distinct side chain on FE-T. The incorporation of FE-T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of -5.60-5.70 eV, a complementary absorption to NF-SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC-Th, the solar cell based on P1 with the shortest methyl chain on FE-T achieves a PCE of 11.39% with a large V oc of 1.01 V and a J sc of 17.89 mA cm-2. Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7-Th, and acceptor IEICO-4F. These results demonstrate that the new FE-T is a highly promising acceptor unit to construct WBG polymers for efficient NF-OSCs.

Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent.

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale-up. Despite the rapid increase in power conversion efficiency (PCE) of spin-coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high-performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a single halogen-free solvent. More importantly, chlorine-free, blade coating of FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X-ray scattering results reveal that large π-π coherence length, high degree of face-on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost, low-toxicity, and high-efficiency OSCs by high-throughput production.

Morphology Evolution in High-Performance Polymer Solar Cells Processed from Nonhalogenated Solvent.

A new processing protocol based on non-halogenated solvent and additive is developed to produce polymer solar cells with power conversion efficiencies better than those processed from commonly used halogenated solvent-additive pair. Morphology studies show that good performance correlates with a finely distributed nanomorphology with a well-defined polymer fibril network structure, which leads to balanced charge transport in device operation.