Jisoo Shin

Highly crystalline low-bandgap polymer nanowires towards high-performance thick-film organic solar cells exceeding 10% power conversion efficiency

Preparing polymer nanowire (PNW) structures using donor–acceptor (D–A) conjugated polymers is one promising strategy to improve the power conversion efficiencies (PCEs) of bulk-heterojunction (BHJ) polymer solar cells (PSCs). Here, we report that a high PCE of 10.62% was obtained with a single-junction inverted cell with a 350 nm thick active layer containing highly crystalline PNWs based on a D–A conjugated polymer (P4TNTz-2F), which possesses a deep-lying HOMO level (−5.46 eV) and a low-bandgap (1.59 eV) as well as a planar/rigid backbone. The thick active layer in the P4TNTz-2F:PC71BM-based PSC absorbs incident light almost completely, which in turn contributes to a high short-circuit current density of 19.45 mA cm−2. This high PCE is attributed to the continuous and evenly distributed polymer network with narrow PNWs (≈6 nm in width and several hundred nanometers in length) in the thick film blended with PC71BM, which facilitates charge separation (QPL ≈ 98%) and transport (μh = 8.31 × 10−3 cm2 V−1 s−1). Moreover, this PNW structure in the BHJ active layer can be prepared using a facile film-forming process at a mild blending temperature (≈70 °C), which means that high efficiency BHJ PSCs can be fabricated with good reproducibility. These results demonstrate the great promise of polymer nanowire solar cells and provide important scientific insights that facilitate further improvements in the morphologies and performances of organic solar cells through material design and development.

Donor–Acceptor Alternating Copolymer Nanowires for Highly Efficient Organic Solar Cells

A donor-acceptor conjugated copolymer enables the formation of nanowire systems that can be successfully introduced into bulk-heterojunction organic solar cells. A simple binary solvent mixture that makes polarity control possible allows kinetic control over the self-assembly of the crystalline polymer into a nanowire structure during the film-forming process. The enhanced photoconductivity of the nanowire-embedded photoactive layer efficiently facilitates photon harvesting in the solar cells. The resultant maximum power conversion efficiency is 8.2% in a conventional single-cell structure, revealing a 60% higher performance than in devices without nanowires.

Synergistic effects of an alkylthieno[3,2-b]thiophene π-bridging backbone extension on the photovoltaic performances of donor–acceptor copolymers

The synergistic effects of a thiophene-based π-bridging backbone extension on the intrinsic and photovoltaic properties of electron donor–acceptor (D–A) copolymers were systematically investigated. A series of alternating D–A copolymers (PBTs) based on 4,8-bis(5-ethylhexylselenophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (EHSeBDT) and 5-(2-butyloctyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (BOTPD), which featured thiophene-based π-bridges, were synthesized: PBT without a π-bridge, PBT with a 3-octylthiophene (OT) π-bridge (PBT-OT), and PBT with a 3-octylthieno[3,2-b]thiophene (OTT) π-bridge (PBT-OTT). The light absorption and charge transport properties were significantly enhanced upon incorporation of the OTT π-bridge. The enhancements resulted from the strong π–π intermolecular interactions using the OTT π-bridging backbone extension between neighboring polymer chains. PBT-OTT was most miscible in PC71BM. As a result, the photoactive layers prepared using PBT-OTT and PC71BM formed a well-mixed bulk-heterojunction morphology and yielded organic solar cells (OSCs) with a high power conversion efficiency of 7.21%. Transient absorption analysis suggested that the π-electrons were further delocalized along the copolymer after incorporation of the OTT π-bridge, and the charge separation efficiency increased. These results suggested that incorporating OTT π-bridges into D–A copolymers provides a useful strategy for developing highly efficient OSCs.

Effects of conformational symmetry in conjugated side chains on intermolecular packing of conjugated polymers and photovoltaic properties

Introducing conjugated side chains onto the backbone of two-dimensionally (2D) conjugated polymers has been utilized for tuning the optoelectronic characteristics of the polymer and the morphological properties of organic photovoltaics. To investigate the effects of conformational symmetry of conjugated side chains, two benzo[1,2-b:3,4-b′]dithiophene (BDT)-based derivatives, one with the asymmetric alkoxythienyl (Th) side chain and the other with the symmetric alkoxyphenyl (Ph) side chain, were synthesized as donor units and copolymerized with fluorinated benzothiadiazole (2FBT). These two side chains were selected for the distinct differences between their structures, and were found to affect the intrinsic characteristics of these BDT polymers. The introduction of the symmetric conjugated side chain to the conjugated backbone of the polymer was observed to improve both light harvesting and the charge carrier mobility, apparently by increasing the extent of packing between the polymer chains. Power conversion efficiency (PCE) values of photovoltaic devices fabricated using these conjugated polymers were strongly related to the light absorbance and crystallinity in a film of the blend of polymer and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). PBDT2FBT-Ph showed effective light absorption, an optimum morphology that we argue is due to the symmetry of the conjugated Ph side chain, and a maximum PCE of 6.23%, with an open-circuit voltage of 0.83 V, a short-circuit current density of 11.33 mA cm−2, and a fill factor of 66.3%. These results demonstrate that symmetric conjugated side chains are promising groups to produce 2D-conjugated polymers for high-performance photovoltaics. This systematic study of side chain engineering provides a valuable strategy to synthesize 2D conjugated polymers and to achieve high PCE values in organic photovoltaics.

Impact of side-chain fluorination on photovoltaic properties: fine tuning of the microstructure and energy levels of 2D-conjugated copolymers

The control of the molecular energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is crucial to the design of highly efficient polymer solar cells (PSCs). In particular, the fine control of the HOMO energy level is vital because it can ensure that polymer solar cell devices have a high open circuit voltage (Voc) and a high short circuit current density (Jsc). We systematically synthesized a series of polymers substituted with different numbers of fluorine (F) atoms in conjugated side chains to fine-tune the HOMO energy levels of the donor polymer, and then optimized the associated solar cell devices by varying the length of the alkyl chain in the conjugated side chains. A series of conjugated side chains with non-fluorinated (0F), mono-fluorinated (1F), and di-fluorinated (2F) alkoxyphenyl groups at the meta position were synthesized and introduced into the donor polymer. The strong electron-withdrawing properties of fluorine were found to reduce the HOMO energy level and yield high Voc values up to 1.01 V. The substitution of the conjugated side chain with fluorine affects not only the energy level of the polymer but also its intermolecular packing and crystallinity due to the resulting intermolecular interactions, and was found to be effective in the control of Jsc and FF. A maximum efficiency of 7.64% was achieved for the polymer with the mono-fluorinated conjugated side chain.