Cenqi Yan

Realizing Small Energy Loss of 0.55 eV, High Open‐Circuit Voltage >1 V and High Efficiency >10% in Fullerene‐Free Polymer Solar Cells via Energy Driver

A new, easy, and efficient approach is reported to enhance the driving force for charge transfer, break tradeoff between open-circuit voltage and short-circuit current, and simultaneously achieve very small energy loss (0.55 eV), very high open-circuit voltage (>1 V), and very high efficiency (>10%) in fullerene-free organic solar cells via an energy driver.

Fused Hexacyclic Nonfullerene Acceptor with Strong Near-Infrared Absorption for Semitransparent Organic Solar Cells with 9.77% Efficiency

A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 105 m-1 cm-1 , a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10-3 cm2 V-1 s-1 . The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.

Alloy Acceptor: Superior Alternative to PCBM toward Efficient and Stable Organic Solar Cells.

The alloy acceptor (indene-C60 bis-adduct (ICBA)/[6,6]-phenyl-C71 -butyric acid-methyl-ester (PC71 BM)) is employed to replace the widely used fullerene acceptor (PC71 BM) in organic solar cells based on five different polymer donors, which exhibit a higher efficiency and much better device stability than the PC71 BM counterpart.

Fused Tris(thienothiophene)‐Based Electron Acceptor with Strong Near‐Infrared Absorption for High‐Performance As‐Cast Solar Cells

A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600-950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 105 m-1 cm-1 , low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10-3 cm2 V-1 s-1 . Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.

Diluting concentrated solution: a general, simple and effective approach to enhance efficiency of polymer solar cells

Diluting concentrated solution (DCS) is a new, simple, general and effective approach to improve power conversion efficiencies (PCEs) of polymer solar cells (PSCs). PCEs of binary blend PSCs, ternary blend PSCs and all-polymer solar cells fabricated using this method are enhanced by a factor as high as 37% relative to those using the general process.

Rhodanine flanked indacenodithiophene as non-fullerene acceptor for efficient polymer solar cells

A fused-ring electron acceptor IDT-2BR1 based on indacenodithiophene core with hexyl side-chains flanked by benzothiadiazole rhodanine was designed and synthesized. In comparison with its counterpart with hexylphenyl side-chains (IDT-2BR), IDT-2BR1 exhibits higher highest occupied molecular orbital (HOMO) energy but similar lowest unoccupied molecular orbital (LUMO) energy (IDT-2BR1: HOMO=−5.37 eV, LUMO=−3.67 eV; IDT-2BR: HOMO=−5.52 eV, LUMO=−3.69 eV), red-shifted absorption and narrower bandgap. IDT-2BR1 has higher electron mobility (2.2×10–3 cm2 V–1 s–1) than IDT-2BR (3.4×10–4 cm2 V–1 s–1) due to the reduced steric hindrance and ordered molecular packing. Fullerene-free organic solar cells based on PTB7-Th:IDT-2BR1 yield power conversion efficiencies up to 8.7%, higher than that of PTB7-Th:IDT-2BR (7.7%), with a high open circuit voltage of 0.95 V and good device stability.

Efficient and stable organic solar cells via a sequential process

Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted considerable attention in the last two decades. Sequentially solution processed BHJ (s-BHJ) have been developed in recent years. s-BHJ not only maintain some advantages of mixed BHJ (m-BHJ), but also exhibit other advantages over m-BHJ. However, to date, s-BHJ OSCs exhibit relatively lower efficiency and have received much less attention compared with m-BHJ OSCs. Moreover, there have been rare systematic comparisons between m-BHJ and s-BHJ OSCs. In this work, we systematically compare the m-BHJ and s-BHJ OSCs based on a classical system PTB7-TH/PC71BM in terms of film morphology, domain size and purity, molecular orientation and aggregation, vertical phase separation, charge transport, efficiency and stability. The s-BHJ OSCs without additives exhibit efficiencies as high as 8.6%, which is similar to that of m-BHJ OSCs with additives (8.5%) and is the highest reported for s-BHJ OSCs. More importantly, the s-BHJ OSCs show much better device stability than the m-BHJ OSCs. This study demonstrates that employing s-BHJ is a promising strategy towards efficient and stable OSCs.

Effect of Isomerization on High-Performance Nonfullerene Electron Acceptors

We design and synthesize two isomeric fused-ring electron acceptors, FNIC1 and FNIC2, which have the same end-groups and side-chains, but isomeric fused-nine-ring cores. Subtle changes in the two isomers influence their electronic, optical, charge-transport, and morphological properties. As compared with FNIC1, FNIC2 film exhibits a red-shifted absorption peak at 794 nm (752 nm for FNIC1), larger electron affinity of 4.00 eV (3.92 eV for FNIC1), smaller ionization energy of 5.56 eV (5.61 eV for FNIC1), and higher electron mobility of 1.7 × 10-3 cm2 V-1 s-1 (1.2 × 10-3 cm2 V-1 s-1 for FNIC1). The as-cast organic solar cells based on PTB7-Th:FNIC2 blends exhibit a power conversion efficiency (PCE) of 13.0%, which is significantly higher than that of PTB7-Th:FNIC1-based devices (10.3%). Semitransparent devices based on PTB7-Th:FNIC2 blends exhibit PCEs varying from 9.51% to 11.6% at different average visible transmittance (AVT, 20.3- 13.6%), significantly higher than those of PTB7-Th:FNIC1-based devices (7.58-9.14% with AVT of 20.2- 14.7%).

Ladder-type nonacyclic indacenodithieno[3,2-b]indole for highly efficient organic field-effect transistors and organic photovoltaics

Developing electron-donating building blocks for organic semiconductors is still one big chemical challenge to achieve high performance active materials for organic photovoltaics (OPVs). In this work, we have successfully designed and synthesized a novel ladder-type nonacyclic indacenodithieno[3,2-b]indole (IDTI) unit via intramolecular annulation with rigid and coplanar features. Two donor–acceptor copolymers of PIDTI-BT and PIDTI-DTBT were synthesized by utilizing the Suzuki and Stille coupling polymerization method with IDTI units. Both copolymers displayed excellent solubility, high thermal stability, broad absorption and a low band gap. The FET hole mobility reaches 2.1 × 10−2 and 1.4 × 10−2 cm2 V−1 s−1 for PIDTI-BT and PIDTI-DTBT, respectively. The conventional bulk-heterojunction (BHJ) polymer solar cell (PSC) devices based on the PIDTI-BT : PC71BM (1 : 2 in wt%) blend exhibit a moderate PCE of 4.02% with a Voc of 0.82 V, a Jsc of 8.99 mA cm−2 and a FF of 54.6% under AM 1.5G, 100 mW cm−2 illumination, which is among the highest values for polymer donor materials based on multifused TI units. The improved performance may be associated with the extended conjugation length, which optimizes the interchain interactions and improves molecular organization for accelerating charge transport. Our results demonstrate that the multifused nonacyclic TIBDP as the donor unit is very promising for application in PSCs and FETs.

Small molecule donors based on benzodithiophene and diketopyrrolopyrrole compatible with both fullerene and non-fullerene acceptors

Herein, two small molecule donors, BDTTT-DPP and BDTTVT-DPP, compatible with both fullerene and non-fullerene acceptors were developed. Both molecules exhibited medium bandgaps with complementary absorption to and appropriate energy level offsets with typical acceptors like PC61BM and IDIC. The optimized devices showed the best power conversion efficiency (PCE) of 5.53% for fullerene organic solar cells and 5.48% for non-fullerene organic solar cells. Moreover, ternary devices containing blends of the small molecule donors, IDIC and PC61BM produced PCEs of 6.53% for BDTTT-DPP cells and 6.55% for BDTTVT-DPP cells, even higher than the binary ones.