Yunlong Ma

Solution-derived poly(ethylene glycol)-TiOx nanocomposite film as a universal cathode buffer layer for enhancing efficiency and stability of polymer solar cells

Highly efficient and stable polymer solar cells (PSCs) have been fabricated by adopting solution-derived hybrid poly(ethylene glycol)-titanium oxide (PEG-TiOx) nanocomposite films as a novel and universal cathode buffer layer (CBL), which can greatly improve device performance by reducing interface energy barriers and enhancing charge extraction/collection. The performance of inverted PSCs with varied bulk-heterojunctions (BHJs) based on this hybrid nanocomposite CBL was found to be much better than those of control devices with a pure TiOx CBL or without a CBL. An excellent power conversion efficiency up to 9.05% under AM 1.5G irradiation (100 mW·cm−2) was demonstrated, which represents a record high value for inverted PSCs with TiOx-based interface materials.

Recent advances in wide bandgap semiconducting polymers for polymer solar cells

As key components in the active layer of multi-junction, ternary blend, or non-fullerene polymer solar cells (PSCs), wide bandgap semiconducting polymers have the characteristic of exhibiting strong absorption bands in the short-wavelength region. In multi-junction, ternary blend, or non-fullerene PSCs, wide bandgap polymers can provide complementary absorption profiles with low bandgap counterparts, thereby further increasing power conversion efficiencies. In this review, we summarize the recent advances of wide bandgap semiconducting polymers and their applications in PSCs, and discuss the designs as well as the material structure–property-device performance correlations. Finally, we conclude by highlighting the challenges and future developments of wide bandgap semiconducting polymers.

Improving the photovoltaic performance of ladder-type dithienonaphthalene-containing copolymers through structural isomerization

A ladder-type angular-shaped dithienonaphthalene (aDTN), an isomer of ladder-type linear-shaped dithienonaphthalene (DTN), was designed and synthesized as an electron-rich unit to construct donor–acceptor copolymers with deep-lying highest occupied molecular orbital (HOMO) energy levels. Benzo[c][1,2,5]thiadiazole (BT) with various substituents were used as electron deficient units for synthesizing the target copolymers (PaDTNBTO, PaDTNBTH, and PaDTNBTF) via the Stille coupling reaction. Incorporating different substituents onto the BT moiety has significant effects on the photophysical and electrochemical properties of the copolymers, as well as on the roughness of the polymer/PC71BM blends. With four solubilizing alkyl chains on the aDTN unit, all its three copolymers have good solubility in common solvents. The synthesized copolymers exhibit deep-lying HOMO energy levels, leading to high open circuit voltages (Voc ≥ 0.90 V) of the resulting polymer solar cells. The bulk heterojunction solar cell based on the aDTN-containing copolymers (PaDTNBTO) shows an improved efficiency of 6.44% and an increased Voc of 0.92 V compared to that based on the linear-shaped DTN containing counterpart (efficiency = 4.78%, Voc = 0.86 V). Whereas, under the same device fabrication conditions, PaDTNBTH- and PaDTNBTF-based devices exhibit efficiencies of 5.22% and 1.73%, respectively. Our results demonstrate that aDTN is a better building block in constructing p-type copolymers for high open circuit voltage devices compared to the linear-shaped DTN.

Push–Pull Type Non-Fullerene Acceptors for Polymer Solar Cells: Effect of the Donor Core

There has been a growing interest in the design and synthesis of non-fullerene acceptors for organic solar cells that may overcome the drawbacks of the traditional fullerene-based acceptors. Herein, two novel push-pull (acceptor-donor-acceptor) type small-molecule acceptors, that is, ITDI and CDTDI, with indenothiophene and cyclopentadithiophene as the core units and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the end-capping units, are designed and synthesized for non-fullerene polymer solar cells (PSCs). After device optimization, PSCs based on ITDI exhibit good device performance with a power conversion efficiency (PCE) as high as 8.00%, outperforming the CDTDI-based counterparts fabricated under identical condition (2.75% PCE). We further discuss the performance of these non-fullerene PSCs by correlating the energy level and carrier mobility with the core of non-fullerene acceptors. These results demonstrate that indenothiophene is a promising electron-donating core for high-performance non-fullerene small-molecule acceptors.

Long lifetime stable and efficient semitransparent organic solar cells using a ZnMgO-modified cathode combined with a thin MoO3/Ag anode

Semitransparent organic solar cells (STOSCs) have shown great promise as advanced window integrated photovoltaics for architectural and automotive utilization. In the applications, STOSCs should possess both long-term stability and high power conversion efficiency (PCE). Here, long lifetime stable and efficient STOSCs with good transparency color perception are demonstrated by combining a transparent ZnMgO-modified cathode with thin MoO3/Ag anodes. The resulting devices exhibit high PCEs (6.83–8.15%) with tunable average visible transmittances (21.6–3.8%) and good color perception close to white light. The combination of the ZnMgO-modified cathode with the MoO3/Ag anode in the STOSCs leads to long-term device shelf lifetime due to their barrier effects for oxygen and water. The STOSCs can maintain over 90% of their original PCEs over two-month storage under ambient conditions. More importantly, record high PCEs of 7.08% and 7.02% were retained for the STOSCs after storage for 1 year and 2 years, respectively, demonstrating the long lifetime stability for high-efficiency STOSCs. These findings offer a promising path to develop STOSCs with high efficiencies, long lifetime and good color perception towards practical applications.

Side-chain engineering of diindenocarbazole-based large bandgap copolymers toward high performance polymer solar cells

Five wide bandgap conjugated polymers based on diindenocarbazole (DIC) and dithienylbenzothiadiazole (DTBT) alternating units have been designed and prepared to investigate the effects of side chains on the photovoltaic performance of the polymers. All the polymers are soluble in common organic solvents and they show similar optical bandgaps of around 2.0 eV as well as deep-lying highest occupied molecular orbital (HOMO) energy levels (below −5.48 eV). Bulk heterojunction (BHJ) polymer solar cells (PSCs) using phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor material were fabricated. The side chains on the polymer backbone have a strong impact on the film morphology of the polymer:PC71BM blends. A phase separation with a relatively larger domain size was found for the polymers with longer side chains, while those with relatively shorter alkyl chains could form uniform films featuring a phase separation with a smaller domain size. Finally, PSCs based on PC1BT6:PC71BM exhibited an outstanding power conversion efficiency of 7.34% with a high open-circuit voltage (Voc) of 0.95 V. Our results demonstrate that the judicious design of side-chains is effective in improving the photovoltaic performance of DIC-based polymers. With a larger bandgap than P3HT but an improved power conversion efficiency (PCE) with a large Voc, this type of DIC-based polymer should be a promising bottom layer material for tandem solar cells.

Ladder-type tetra-p-phenylene-based copolymers for efficient polymer solar cells with open-circuit voltages approaching 1.1 V

Side-chain engineering of polymer backbones can induce subtle variations in polymer properties, resulting in a significant impact on their photovoltaic performance. In this work, four ladder-type tetra-p-phenylene containing copolymers with different alkyl side chains (P3FTBT1, P3FTBT1F, P3FTBT8O6 and P3FTBT1O6) were designed and synthesized. These copolymers have large bandgaps (∼2.0 eV) and deep-lying highest occupied molecular orbital (HOMO) energy levels (from −5.44 eV to −5.53 eV). The substitution of two hexyl groups with two methyl groups on the ladder-type tetra-p-phenylene unit afforded polymer P3FTBT1 which exhibits an enhanced power conversion efficiency (PCE) of 5.39%. Incorporation of fluorine into the benzo[c][1,2,5]thiadiazole (BT) unit gave polymer P3FTBT1F which exhibits a PCE of 4.50% with an open circuit voltage (Voc) of 1.09 V. By introducing two alkoxy groups to the BT unit, P3FTBT1O6 was synthesized, and it exhibits a PCE of 5.73% with a Voc of 1.02 V. The results suggest that the ladder-type tetra-p-phenylene is an excellent building block to construct donor–acceptor copolymers with high PCEs and large Vocs.

Modulation of bulk heterojunction morphology through small π-bridge changes for polymer solar cells with enhanced performance

Investigations on the relationships among the chemical structures, morphology and photovoltaic properties of conjugated polymers are crucial in designing high-efficiency semiconducting polymers. Here, two novel copolymers, PIBTO-T and PIBTO-TT, were designed and synthesized to demonstrate the improvement in photovoltaic performance of conjugated polymers by a small change in their chemical structures. PIBTO-TT with thieno[3,2-b]thiophene π-bridges has a more linear backbone conformation, thereby resulting in enhanced intermolecular π–π interactions compared to PIBTO-T with thiophene π-bridges. Benefiting from the closer intermolecular π–π stacking, PIBTO-TT:PC71BM exhibits a higher hole mobility than PIBTO-T:PC71BM. Morphological studies reveal that the miscibility of PC71BM in PIBTO-TT is better than that in PIBTO-T. This enhanced miscibility could shorten the distances between adjacent fullerenes and improve electron transportation in the miscible region. Meanwhile, PIBTO-TT:PC71BM blends have larger donor/acceptor interfacial areas than PIBTO-T:PC71BM samples. All these factors contribute to the better photovoltaic performance of PIBTO-TT-based devices. This study clearly shows that the morphological characteristics and photovoltaic properties of conjugated polymers are closely related to their molecular structures, and the manipulation of backbone conformation through π-bridge modulation is a promising molecular engineering approach to improve the photovoltaic properties of conjugated polymers.