Zuo Xiao

A carbon–oxygen-bridged hexacyclic ladder-type building block for low-bandgap nonfullerene acceptors

A hexacyclic carbon–oxygen-bridged ladder-type unit, COi6, was developed. Three nonfullerene acceptors (COi6IC, COi6FIC and COi6DFIC) based on COi6 were prepared. They present low optical bandgaps of 1.31–1.37 eV and strong absorbance in the near-infrared region. A 9.12% power conversion efficiency was achieved from the solar cells based on COi6FIC and a wide-bandgap copolymer donor (FTAZ).

A D–A copolymer donor containing an alkylthio-substituted thieno[3,2-b]thiophene unit

A D–A copolymer donor, PSTTF2T, based on an alkylthio-substituted thieno[3,2-b]thiophene unit (STT), was developed for polymer solar cells. PSTTF2T is compatible with fullerene and non-fullerene acceptors. PSTTF2T:PC71BM and PSTTF2T:ITIC solar cells gave power conversion efficiencies of 4.73% and 5.22%, respectively.

Understanding the side-chain effects on A–D–A acceptors: in-plane and out-of-plane

Systematic side-chain engineering on nonacyclic acceptor–donor–acceptor (A–D–A) nonfullerene acceptors, NNFA[n, m], was carried out. “n” and “m” stand for the number of alkyl carbon atoms in the in-plane and out-of-plane side chains, respectively. Five acceptors, NNFA[0, 6], NNFA[6, 3], NNFA[6, 6], NNFA[12, 3] and NNFA[12, 6], were prepared and applied in organic solar cells by blending with a wide-bandgap copolymer donor (FTAZ). The alkyl chains substantially affect the NNFAs’ solubility and photovoltaic performance. The solubility varies from 23 mg mL−1 to 226 mg mL−1 in chloroform when changing the total alkyl carbons (2n + 4m). If the total alkyl carbons are equal, the NNFA with longer out-of-plane alkyl chains (higher “m”) shows higher solubility than that with longer in-plane alkyl chains (higher “n”). NNFA[6, 6] and NNFA[12, 3] with medium solubility (∼100 mg mL−1) present suitable miscibility with FTAZ, and afford more favorable morphology and higher device performance than other NNFAs. FTAZ:NNFA[12, 3] solar cells gave the highest power conversion efficiency of 10.81%.

A heptacyclic carbon–oxygen-bridged ladder-type building block for A–D–A acceptors

A heptacyclic carbon–oxygen-bridged ladder-type unit, COi7, was designed. Two corresponding nonfullerene acceptors, COi7IC and COi7DFIC, were developed. The single crystal structure of COi7IC indicates an S-shaped backbone and a packing via π–π interaction of the end groups. PTB7-Th:COi7DFIC solar cells gave a power conversion efficiency of 8.32%, with a decent fill factor of 71.9%.

Simultaneously improved efficiency and average visible transmittance of semitransparent polymer solar cells with two ultra-narrow bandgap nonfullerene acceptors

One narrow bandgap donor (PTB7-Th) and two ultra-narrow bandgap acceptors (COi8DFIC and IEICO-4F) were elaborately selected to fabricate semitransparent ternary polymer solar cells (PSCs). To find out the optimal content of IEICO-4F in the acceptors, a series of opaque PSCs were first fabricated with 100 nm of Ag as the electrode. The power conversion efficiency (PCE) of the optimized opaque ternary PSCs arrived at 11.94% by incorporating 15 wt% IEICO-4F in the acceptors. Then, the thickness of Ag was adjusted to balance its electrical conductivity and transmittance, and the 100 nm of Ag was replaced by 15 nm of Ag as a semitransparent electrode. The semitransparent electrode possesses high transmittance in the visible light range and low transmittance in the long wavelength range, which is propitious for photon harvesting of ternary active layers with two ultra-narrow bandgap acceptors. The optimized semitransparent ternary PSCs show a synchronously improved PCE of 8.23% and an average visible transmittance (from 370 nm to 740 nm) of 20.78% compared with COi8DFIC based semitransparent binary PSCs. This work suggests the potential of a ternary strategy for fabricating highly efficient semitransparent PSCs with narrow or ultra-narrow bandgap materials as active layers.

A facilely synthesized lactam acceptor unit for high-performance polymer donors

An aromatic lactam acceptor unit, [2,2′-bidithieno[3,2-b:2′,3′-d]pyridine]-5,5′(4H,4′H)-dione (BDTPi), was developed for making D–A copolymer donors. Two D–A copolymers, PThBDTPi and PSeBDTPi, gave power conversion efficiencies (PCEs) of 8.11% and 6.50%, respectively, when using PC71BM as the acceptor.