Baobing Fan

Optimisation of processing solvent and molecular weight for the production of green-solvent-processed all-polymer solar cells with a power conversion efficiency over 9%

Increasing interest has been devoted to developing high-performance all-polymer solar cells (all-PSCs) owing to their specific advantages in light absorption and long-term stability. In this work, we systematically investigated the synergistic effects of processing solvents and molecular weight on the photovoltaic performance of all-PSCs, which consist of an n-type polymer N2200 and a p-type wide bandgap polymer PTzBI that are made up of benzodithiophene and imide-functionalized benzotriazole units. It is noted that increasing the molecular weight of N2200 can simultaneously enhance exciton generation and dissociation, reduce bimolecular recombination, and facilitate charge extraction. The films processed with the environmentally-friendly solvent 2-methyl-tetrahydrofuran (MeTHF) exhibit a more favourable film morphology than those processed with commonly used halogenated solvents. The all-PSC consisting of the high molecular weight N2200 and PTzBI processed with the environmentally friendly solvent MeTHF presents a remarkable power conversion efficiency of 9.16%, which is the highest value so far observed for all-PSCs. Of particular interest is that the PCE remains 6.37% with the active layer thickness of 230 nm. These observations imply the great promise of the developed all-PSCs for practical applications toward high-throughput roll-to-roll technology.

All‐Polymer Solar Cells Based on a Conjugated Polymer Containing Siloxane‐Functionalized Side Chains with Efficiency over 10%

A novel wide-bandgap conjugated copolymer based on an imide-functionalized benzotriazole building block containing a siloxane-terminated side-chain is developed. This copolymer is successfully used to fabricate highly efficient all-polymer solar cells (all-PSCs) processed at room temperature with the green-solvent 2-methyl-tetrahydrofuran. When paired with a naphthalene diimide-based polymer electron-acceptor, the all-PSC exhibits a maximum power conversion efficiency (PCE) of 10.1%, which is the highest value so far reported for an all-PSC. Of particular interest is that the PCE remains 9.4% after thermal annealing at 80 °C for 24 h. The resulting high efficiency is attributed to a combination of high and balanced bulky charge carrier mobility, favorable face-on orientation, and high crystallinity. These observations indicate that the resulting copolymer can be a promising candidate toward high-performance all-PSCs for practical applications.

A high dielectric constant non-fullerene acceptor for efficient bulk-heterojunction organic solar cells

The majority of organic semiconductors have a low relative dielectric constant (er 6) has attracted a very limited attention. Moreover, high performance OSCs based on high dielectric constant photovoltaic materials are still in their infancy. Herein, we report an oligoethylene oxide side chain-containing non-fullerene acceptor (ITIC-OE) with a high relative dielectric constant of er ≈ 9.4, which is two times larger than that of its alkyl chain-containing counterpart ITIC. Encouragingly, the OSCs based on ITIC-OE show a high power conversion efficiency of 8.5%, which is the highest value for OSCs that employ high dielectric constant materials. Nevertheless, this value is lower than those of ITIC-based control devices. The less phase-separated morphology in blend films due to the reduced crystallinity of ITIC-OE and the too good miscibility between PBDB-T and ITIC-OE are responsible for the lower device performance. This work suggests additional prerequisites to make high dielectric constants play a significant role in OSCs.

Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1,2,5]thiadiazole Unit for Polymer Solar Cells

We designed and synthesized two isomeric nonfullerene acceptors, IFBR-p and IFBR-d. These molecular semiconductors contain indacenodithiophene (IDT) as the central unit, adjacent asymmetric 5-fluorobenzo[c][1,2,5]thiadiazole units, and are flanked with rhodanine as the peripheral units. The orientation of the two fluorine atoms (proximal, p, or distal, d), relative to IDT impacts most severely the film morphologies when blended with the electron-donating polymer PTzBI. Polymer solar cells based on PTzBI:IFBR-p give rise to a power conversion efficiency (7.3 ± 0.2%) that is higher than what is achieved with PTzBI:IFBR-d (5.2 ± 0.1%). This difference is attributed to the lower tendency for (over)crystallization by IFBR-p and the resulting more favorable morphology of the photoactive layer. These results highlight the subtle impact of substitution regiochemistry on the properties of nonfullerene acceptors through modulation of their self-assembly tendencies.

Side-chain modification of polyethylene glycol on conjugated polymers for ternary blend all-polymer solar cells with efficiency up to 9.27%

With the rapid progress achieved by all-polymer solar cells (all-PSCs), wide-bandgap copolymers have attracted intensive attention for their unique advantage of constructing complementary absorption profiles with conventional narrow-bandgap copolymers. In this work, we designed and synthesized a wide bandgap ternary copolymer PEG-2% which has the benzodithiophene-alt-difluorobenzotriazole as the backbone and the polyethylene glycol (PEG) modified side chain. The PBTA-PEG-2%:N2200 can be processed with a non-chlorinated solvent of 2-methyl-tetrahydrofuran (MeTHF) for the binary all-PSC, which exhibits a moderate photovoltaic performance. In particular, the ternary all-PSCs that consisting an additional narrow bandgap polymer donor PTB7-Th can also be processed with MeTHF, resulting in an unprecedented power conversion efficiency (PCE) of 9.27%, and a high PCE of 8.05% can be achieved with active layer thickness of 240 nm, both of which are the highest values so far reported from all-PSCs. Detailed investigations revealed that the dramatically improved device performances are attributable to the well-extended absorption band in the photoactive layer. Hence, developing novel copolymers with tailored side chains, and introducing additional polymeric components, can broaden the horizon for high-performance all-PSCs.

Processing a pyridyl-based polymeric additive for improved photovoltaic performance of a wide-bandgap π-conjugated polymer

In this manuscript we introduced a novel conjugated polymeric additive PBTA-Py with benzodithiophene-alt-benzo[1,2,3]triazole as the backbone and polar pyridyl as the terminal group of the side chains. Relative to its polymeric counterpart that consists of the same main chain but a non-polar alkyl side chain, the resulting PBTA-Py exhibits quite comparable absorption and electrochemical properties and significantly improved surface energy. Of particular interest is that, by incorporating a certain amount of PBTA-Py as a polymeric additive into the bulk-heterojunction film of pristine polymer PBTA-BO and PC71BM, the power conversion efficiency was obviously enhanced from 4.96% to 6.48% primarily by virtue of the improved current density. The improved current density can be attributed to the slightly increased hole mobility as revealed by the space charge limited current model. Furthermore X-ray photoelectron spectra confirm the formation of the supramolecular interaction of the nitrogen atom with the electron-deficient PC71BM, leading to increased charge transfer in the bulk-heterojunction layer. These observations, in combination with the formation of more favorable phase separation morphology, demonstrate the great promise of developing polymeric additives for improving the photovoltaic performance of polymer solar cell devices.

Asymmetric Alkyl Side-Chain Engineering of Naphthalene Diimide-Based n-Type Polymers for Efficient All-Polymer Solar Cells

The design and synthesis of three n-type conjugated polymers based on a naphthalene diimide-thiophene skeleton are presented. The control polymer, PNDI-2HD, has two identical 2-hexyldecyl side chains, and the other polymers have different alkyl side chains; PNDI-EHDT has a 2-ethylhexyl and a 2-decyltetradecyl side chain, and PNDI-BOOD has a 2-butyloctyl and a 2-octyldodecyl side chain. These copolymers with different alkyl side chains exhibit higher melting and crystallization temperatures, and stronger aggregation in solution, than the control copolymer PNDI-2HD that has the same side chain. Polymer solar cells based on the electron-donating copolymer PTB7-Th and these novel copolymers exhibit nearly the same open-circuit voltage of 0.77 V. Devices based on the copolymer PNDI-BOOD with different side chains have a power-conversion efficiency of up to 6.89%, which is much higher than the 4.30% obtained with the symmetric PNDI-2HD. This improvement can be attributed to the improved charge-carrier mobility and the formation of favorable film morphology. These observations suggest that the molecular design strategy of incorporating different side chains can provide a new and promising approach to developing n-type conjugated polymers.

11.2% All-Polymer Tandem Solar Cells with Simultaneously Improved Efficiency and Stability

All-polymer solar cells (all-PSCs) that contain both p-type and n-type polymeric materials blended together as light-absorption layers have attracted much attention, since the blend of a polymeric donor and acceptor should present superior photochemical, thermal, and mechanical stability to those of small molecular-based organic solar cells. In this work, the interfacial stability is studied by using highly stable all-polymer solar cell as a platform. It is found that the thermally deposited metal electrode atoms can diffuse into the active layer during device storage, which consequently greatly decreases the power conversion efficiency. Fortunately, the diffusion of metal atoms can be slowed down and even blocked by using thicker interlayer materials, high-glass-transition-temperature interlayer materials, or a tandem device structure. Learning from this, homojunction tandem all-PSCs are successfully developed that simultaneously exhibit a record power conversion efficiency over 11% and remarkable stability with efficiency retaining 93% of the initial value after thermally aging at 80 °C for 1000 h.