Dahui Zhao

High Performance All-Polymer Solar Cell via Polymer Side-Chain Engineering

Acknowledge support from the Office of Naval Research (N00014-14-1-0142), KAUST Center for Advanced Molecular Photovoltaics at Stanford and the Stanford Global Climate and Energy Program, NSF DMR-1303742 and the National Natural Science Foundation of China (Projects 21174004 and 21222403). Soft X-ray characterization and analysis by NCSU supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under Contract DE-FG02-98ER45737. Soft X-ray data was acquired at beamlines 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Professor Michael D. McGehee, Dr. George F. Burkhard and Dr. Eric T. Hoke for their help in discussion of the recombination mechanism.

Nucleation–elongation: a mechanism for cooperative supramolecular polymerization

The kinetic and thermodynamic characteristics of polymerizations following a cooperative, nucleation-elongation mechanism are discussed in comparison to those of non-cooperative, isodesmic polymerizations. Nucleation-elongation polymerization is a relatively unexplored avenue of synthetic polymer chemistry and offers some unique and interesting thermodynamic and kinetic attributes not found in the more classical mechanisms of polymer chemistry.

Towards rational design of organic electron acceptors for photovoltaics: a study based on perylenediimide derivatives

A series of PDI dimers featuring various arylene linkers are developed as electron acceptors in organic solar cells. Using P3HT as the donor, power conversion efficiency of up to 2.3% is achieved with two PDI dimers having spirobifluorene linkers. The results indicate that such non-planar, three-dimensional structures effectively suppress self-aggregation and crystallization of the PDI units, which is favourable for their solar cell performance.

A Vinylene-Bridged Perylenediimide-Based Polymeric Acceptor Enabling Efficient All-Polymer Solar Cells Processed under Ambient Conditions

All-polymer solar cells with 7.57% power conversion efficiency are achieved via a new perylenediimide-based polymeric acceptor. Furthermore, the device processed in ambient air without encapsulation can still reach a high power conversion efficiency (PCE) of 7.49%, which is a significant economic advantage from an industrial processing perspective. These results represent the highest PCE achieved from perylenediimide-based polymers.

Flow-enhanced solution printing of all-polymer solar cells.

Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.

Improved Performance of All-Polymer Solar Cells Enabled by Naphthodiperylenetetraimide-Based Polymer Acceptor

A new polymer acceptor, naphthodiperylenetetraimide-vinylene (NDP-V), featuring a backbone of altenating naphthodiperylenetetraimide and vinylene units is designed and applied in all-polymer solar cells (all-PSCs). With this polymer acceptor, a new record power-conversion efficiencies (PCE) of 8.59% has been achieved for all-PSCs. The design principle of NDP-V is to reduce the conformational disorder in the backbone of a previously developed high-performance acceptor, PDI-V, a perylenediimide-vinylene polymer. The chemical modifications result in favorable changes to the molecular packing behaviors of the acceptor and improved morphology of the donor-acceptor (PTB7-Th:NDP-V) blend, which is evidenced by the enhanced hole and electron transport abilities of the active layer. Moreover, the stronger absorption of NDP-V in the shorter-wavelength range offers a better complement to the donor. All these factors contribute to a short-circuit current density (J sc ) of 17.07 mA cm-2 . With a fill factor (FF) of 0.67, an average PCE of 8.48% is obtained, representing the highest value thus far reported for all-PSCs.

Shape-persistent arylene ethynylene macrocycles: syntheses and supramolecular chemistry

This article describes recent developments in the synthesis of macrocycles having rigid, monocyclic skeletons composed of arylene and ethynylene units and the studies on their self-assembling behavior.

New polymer acceptors for organic solar cells: the effect of regio-regularity and device configuration

Two polymers, r-PDI-diTh and i-PDI-diTh, were synthesized as acceptors applicable for solution-processed BHJ OSCs. By introducing a bulky, dove tailed side chain and thereby suppressing the π–π interactions between perylenediimide units in the backbones of acceptor polymers, more effective phase segregation of these acceptors with a donor polymer (P3HT) was realized. By employing the inverted device configuration to better match the vertical phase separation of donor–acceptor polymers produced by solution processing, undesirable polaron pair recombination was suppressed, and PCE up to 2.17% was achieved from the regio-regular acceptor r-PDI-diTh.

Conjugated dimeric and trimeric perylenediimide oligomers.

Dimeric and trimeric molecules comprising perylenediimide units conjugatively linked by phenylene, ethynylene, or a butadiynylene spacer via the bay positions were prepared. Electrochemical and photophysical characterizations showed that oligomers connected by C-C triple bond(s) exhibited effectively lowered LUMO compared to the monomer. Molecular modeling confirmed that the C-C triple bond realized efficient delocalization of frontier orbitals, while phenylene was less competent in extending the conjugation, partially due to steric interactions.

Oligo(p-phenyleneethynylene)s with hydrogen-bonded coplanar conformation.

A series of monodispersed oligo( p-phenyleneethynylene)s were synthesized bearing intramolecular hydrogen bonds between side chains of adjacent phenylene units in the backbone. Thus, all repeating units of the molecules are constrained in a coplanar orientation. Such planarized conformation is considered favorable for single-molecule conductance. Photophysical characterization results show narrowed bandgaps and extended conjugation lengths, consistent with a rigid, planar backbone framework as a result of intramolecular hydrogen bonding.