Tsz-Ki Lau

A spirobifluorene and diketopyrrolopyrrole moieties based non-fullerene acceptor for efficient and thermally stable polymer solar cells with high open-circuit voltage

In this study, we design and synthesize a new non-fullerene electron acceptor, SF(DPPB)4, in which a spirobifluorene (SF) core is installed with four benzene endcapped diketopyrrolopyrrole (DPP) arms. SF(DPPB)4 exhibits energy levels matching perfectly with those of the commonly used poly(3-hexyl thiophene) (P3HT) donor in polymer solar cells (PSCs). Furthermore, a designed cross-shaped molecular geometry helps in suppressing strong intermolecular aggregation in the P3HT : SF(DPPB)4 blend, leading to efficient non-fullerene PSCs. The resultant devices give a maximum power conversion efficiency (PCE) of 5.16% with an extremely high open-circuit voltage (Voc) of 1.14 V. In contrast, the devices based on P3HT : PC61BM blends provide a PCE of 3.18% with a Voc of 0.62 V. Finally, we observe that the P3HT : SF(DPPB)4 devices exhibit significantly improved thermal stability from that of the P3HT : PC61BM devices; upon thermal treatment at 150 °C for 3 h, the PCEs of P3HT : SF(DPPB)4 devices remain unchanged, whereas those of the P3HT : PC61BM devices drop drastically to below 1%. The abovementioned results demonstrate that the new design strategy of employing a high-performance non-fullerene acceptor, SF(DPPB)4, is promising for the future practical application of PSCs.

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.

A non-fullerene acceptor with a fully fused backbone for efficient polymer solar cells with a high open-circuit voltage

Herein, we design and synthesize a perylene diimide derivative with a fully fused backbone, FITP, which possesses an elevated lowest unoccupied molecular orbital level and high electron mobility. Consequently, polymer solar cells with FITP as the acceptor can provide the best efficiency of 7.33% with a high voltage of 0.99 V.

Improved photon-to-electron response of ternary blend organic solar cells with a low band gap polymer sensitizer and interfacial modification

Incorporating two polymer donors with different bandgaps to compose a ternary blend bulk heterojunction (BHJ) is proved to be an effective approach to improve the device performance of BHJ polymer solar cells (PSCs). Here, we demonstrate an efficient ternary PSC consisting of a polythieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host blend sensitized by a low band gap (LBG) polymer poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:20,30-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothiadiazole)] (PDTP-DFBT). The addition of the PDTP-DFBT sensitizer remarkably extended the PSC photon to electron response from 750 to 900 nm, which increased the Jsc from 15.12 to 16.27 mA cm−2, and the device performance from 8.08% to 8.63%. A study on the morphology involving the atomic force microscopy mapping and grazing incident X-ray diffraction showed that the incorporation of PDTP-DFBT improved the crystallinity of the PTB7 film with most of the sensitizers associated with the PTB7 domains when blending with a PC71BM film. This observation, together with the unchanged Voc of the ternary PSC, implies a sensitizing mechanism with addition of PDTP-DFBT. After further interfacial modification with a capronic acid self-assembling monolayer (C3-SAM), a higher PCE of 9.06% was achieved, which is among the highest values of efficient ternary PSCs. Our work suggests that a sensitizing mechanism of ternary blends compensates for the light absorbing limitation of binary blend PSCs for high device performance.

Enhancing the Performance of Polymer Solar Cells via Core Engineering of NIR‐Absorbing Electron Acceptors

In order to utilize the near-infrared (NIR) solar photons like silicon-based solar cells, extensive research efforts have been devoted to the development of organic donor and acceptor materials with strong NIR absorption. However, single-junction organic solar cells (OSCs) with photoresponse extending into >1000 nm and power conversion efficiency (PCE) >11% have rarely been reported. Herein, three fused-ring electron acceptors with varying core size are reported. These three molecules exhibit strong absorption from 600 to 1000 nm and high electron mobility (>1 × 10-3 cm2 V-1 s-1 ). It is proposed that core engineering is a promising approach to elevate energy levels, enhance absorption and electron mobility, and finally achieve high device performance. This approach can maximize both short-circuit current density (  JSC ) and open-circuit voltage (VOC ) at the same time, differing from the commonly used end group engineering that is generally unable to realize simultaneous enhancement in both VOC and JSC . Finally, the single-junction OSCs based on these acceptors in combination with the widely polymer donor PTB7-Th yield JSC as high as 26.00 mA cm-2 and PCE as high as 12.3%.

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.

Low-temperature solution-processed NiOx films for air-stable perovskite solar cells

The use of organic hole transporting layers (HTLs) in organolead halide perovskite solar cells (PSCs) often limits the air and thermal stability of the devices. In this work, we developed a low-temperature solution process that enables the fabrication of nickel oxide (NiOx) based HTLs on top of perovskite active layers. The NiOx film exhibits a uniform and dense morphology, rendering the PSCs air-stable and thermally stable. We further found that by introducing an interfacial layer (e.g. CuSCN) between the NiOx film and the top metal electrode, the power conversion efficiency (PCE) of the PSCs can be largely improved from 10.4% to 17.2%, while the air stability continues to be excellent. The role of the interfacial layer was investigated through impedance analysis and ultraviolet photoelectron spectroscopy. Remarkably, the PSCs with the NiOx/CuSCN hybrid inorganic HTL exhibit no degradation in their PCE after being exposed to ambient air (humidity level: 50–60%) for 4 months without encapsulation.

A low-temperature formation path toward highly efficient Se-free Cu2ZnSnS4 solar cells fabricated through sputtering and sulfurization

A novel low-temperature Cu2ZnSnS4 (CZTS) formation path using co-sputtered SnS2–ZnS–Cu precursors was employed for CZTS solar cell fabrication, which gave rise to a cell with a power conversion efficiency of 8.58%, a big step forward from the previous record of 6.77% reported by Katagiri et al. for this kind of solar cell. This method consists of a low-temperature annealing stage for CZTS phase formation followed by a short high-temperature annealing stage for grain growth and secondary phase removal. The employment of SnS2 as a precursor causes the CZTS phase to readily form at low temperature when SnS2 has not dramatically decomposed into volatile SnS. The two-stage process wisely separates the phase formation and crystal coalescence, which makes the fabrication of CZTS films more controllable. Furthermore, the demonstration of the low-temperature formation path provides new opportunities to fabricate highly efficient, cost-effective and environmentally friendly CZTS solar cells on low-weight and flexible substrates such as polyimides.

Searching for a fabrication route of efficient Cu2ZnSnS4 solar cells by post-sulfuration of co-sputtered Sn-enriched precursors

We have systematically investigated the Sn loss in the Cu2ZnSnS4 (CZTS) thin film formation process using a sputtering/sulfuration approach with highly Sn-enriched precursors. It was found that the remaining Sn content in the CZTS absorber is strongly dependent on the peak sulfuration temperature, H2S concentration in the sulfuration atmosphere, the temperature ramping rate, and the Zn content in the precursors. Since the Sn content could be controlled by the Sn loss during the thermal treatment process, the final composition of the CZTS thin films could be manipulated simply by tailoring the Cu/Zn ratio in the precursors. With such a route, highly efficient solar cells could be achieved from Sn-rich precursors.