Zhi-Kuan Chen

Additive to regulate the perovskite crystal film growth in planar heterojunction solar cells

We reported a planar heterojunction perovskite solar cell fabricated from MAPbI3−xClx perovskite precursor solution containing 1-chloronaphthalene (CN) additive. The MAPbI3−xClx perovskite films have been characterized by UV-vis, SEM, XRD, and steady-state photoluminescence (PL). UV-vis absorption spectra measurement shows that the absorbance of the film with CN additive is significantly higher than the pristine film and the absorption peak is red shift by 30u2009nm, indicating the perovskite film with additive possessing better crystal structures. In-situ XRD study of the perovskite films with additive demonstrated intense diffraction peaks from MAPbI3−xClx perovskite crystal planes of (110), (220), and (330). SEM images of the films with additive indicated the films were more smooth and homogenous with fewer pin-holes and voids and better surface coverage than the pristine films. These results implied that the additive CN is beneficial to regulate the crystallization transformation kinetics of perovskite to...

PDI Derivative through Fine-Tuning the Molecular Structure for Fullerene-Free Organic Solar Cells

A perylenediimide-based (PDI-based) small molecular (SM) acceptor with both an extended π-conjugation and a three-dimensional structure concurrently is critical for achieving high-performance PDI-based fullerene-free organic solar cells (OSCs). Herein, a novel PDI-based SM acceptor has been successfully synthesized through fusing PDI units with a spiro core 4,4-spirobi[cyclopenta[2,1-b;3,4-b]dithiophene (SCPDT) together via β-position coupling with thiophene bridges. An enhanced absorption from 350 to 520 nm has been observed. Moreover, compared with previously reported acceptor SCPDT-PDI4, in which the PDI units and SCPDT are not fused together, the LUMO energy level of FSP (the new SCPDT-based molecule) increases. OSCs containing PTB7-Th as a donor and FSP as an acceptor have been demonstrated to show an excellent performance with a power conversion efficiency as high as 8.89%. This result might be attributed to the efficient and complementary photoabsorption, balanced carrier mobilities, and favorable phase separation in the blend film. This research could offer an effective strategy to design novel high-performance PDI-based acceptors.

A random copolymer approach to develop nonfullerene acceptors for all-polymer solar cells

We report a donor–acceptor type of random copolymer PNDI-TT-TVT as a replacement for fullerene-based acceptors in organic solar cells (OSCs). PNDI-TT-TVT is composed of naphthalene tetracarboxylic diimide (NDI) acceptor units, randomly distributed thieno[3,2-b]thiophene (TT) and thienylene-vinylene-thienylene (TVT) donor units. OSCs fabricated from a blend of PTB7-Th and PNDI-TT-TVT exhibit a power conversion efficiency of 4.86%, which is significantly higher than the PCEs of the devices based on alternating copolymers of PNDI-TT and PNDI-TVT. The results indicate that random copolymerization is a promising approach to develop novel polymer acceptors for organic photovoltaics.

Dopant-free hole transport materials based on alkyl-substituted indacenodithiophene for planar perovskite solar cells

Two dopant-free hole transporting materials (HTMs) comprising a planar indacenodithiophene (IDT) core with different alkyl chains (either C4 or C6) and electron-rich methoxytriphenylamine (TPA) side arms were synthesized (namely IDTC4-TPA and IDTC6-TPA, respectively) and successfully employed in CH3NH3PbI3 perovskite solar cells. These HTMs can be obtained from relatively cheap starting materials by adopting a facile preparation procedure that does not use expensive and complicated purification techniques. In the crystal lattice, each of these molecules interacted with either the CH/π or CH/O hydrogen bonds. At the same time, the IDTC6 backbone enables a tight molecular arrangement based on π–π stacking interactions (3.399 A); this causes the new material to have a higher hole mobility than the standard IDTC4-based HTM. Moreover, the photoluminescence quenching in a perovskite/HTM film structure was shown to be more effective at the perovskite/IDTC6-TPA interface than at the perovskite/IDTC4-TPA interface. Consequently, devices fabricated using IDTC6-TPA show superior photovoltaic properties (exhibiting an optimal performance of 15.43%) compared with IDTC4-TPA-containing devices.

Effect of fluorination on n-type conjugated polymers for all-polymer solar cells

Four naphthalene diimide (NDI) based donor–acceptor conjugated polymers for using in all-polymer organic solar cells were synthesized and characterized. The effect of inclusion of a different number of fluorine atoms on the donor portion of the polymer was thoroughly investigated via a range of techniques. Fluorination of the polymer backbone lowered both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and simultaneously broadened the energy bandgap of the polymer. Incorporation of a different number of fluorine atoms on the donor portion of the polymer significantly affected the solar cells power conversion efficiency from 0.67% to 2.50%. The “fluorine” effect suggests fluorine substitution should be considered for achieving high performance PSCs in the future design of new materials.

Facile synthesis of a dopant-free hole transporting material with a phenothiazine core for planar perovskite solar cells

A novel electron-rich small-molecule, 4,4′-(10-(4-octylphenyl)-10H-phenothiazine-3,7-diyl)bis(N,N-(4-methoxyphenyl)anilene) (PTZ-TPA), containing phenothiazine as the core with triphenylamine side groups, was synthesized via a Suzuki–Miyaura cross-coupling reaction. When PTZ-TPA was incorporated into a CH3NH3PbI3 perovskite solar cell as a dopant-free hole transporting material (HTM), a short circuit photocurrent density of 21.5 mA cm−2, an open circuit voltage of 0.982 V, and a fill factor of 0.679 were obtained, giving rise to an overall power conversion efficiency of 14.3%, which is comparable to the power conversion efficiency obtained using the current state-of-the-art HTM 2,20,7,70-tetrakis(N,N′-di-p-methoxyphenylamine)-9,90-spirobifluorene with dopant (Spiro-MeOTAD, power conversion efficiency of 17.1%). PTZ-TPA is thus a promising HTM with the potential to replace the expensive Spiro-MeOTAD owing to its comparable performance and much simpler synthesis route; it also presented a better stability during a one week aging test compared with Spiro-MeOTAD.

Diketopyrrolopyrrole-based acceptors with multi-arms for organic solar cells

Three small molecules SBF-1DPPDCV, SBF-2DPPDCV and SBF-4DPPDCV consisting of a spirobifluorene (SBF) unit as the core and one, two, and four diketopyrrolopyrrole dicyanovinyl (DPPDCV) units as the arms have been designed and synthesized for solution-processed bulk-heterojunction (BHJ) solar cells. The UV-Vis absorption and cyclic voltammetry measurement of these compounds showed that all these compounds have an intense absorption band over 300–750 nm with a LUMO energy level at around −3.87 eV. When pairing with PTB7-Th as the donor, devices fabricated based on PTB7-Thu2006:u2006SBF-4DPPDCV blends showed a decent PCE of 3.85%, which is the highest power conversion efficiency (PCE) amongst the three DPP acceptor fabricated devices without extra treatment. Devices with SBF-1DPPDCV and SBF-2DPPDCV acceptors showed lower PCEs of 0.26% for SBF-1DPPDCV and 0.98% for SBF-2DPPDCV respectively. The three dimensional (3D) structure of SBF-4DPPDCV facilitates the formation of a 3D charge-transport network and thus enables a rational electron-transport ability (1.04 × 10−4 cm2 V−1 s−1), which further leads to a higher Jsc (10.71 mA cm−2). These findings suggest that multi-arm acceptors present better performance than one-arm or two-arm molecules for organic solar cells.

Phenylquinoline fused cyclic derivatives as electron acceptors of exciplex forming hosts for solution-processable red phosphorescent OLEDs

Skeletons of phenylquinoline fused cyclic derivatives for electron accepting materials are synthesized and their thermal, electrochemical, photophysical and device optoelectronic properties are fully characterized. Density functional theory simulations coincide with the experimental results, which reveal the core role of the phenylquinolinyl moiety in both intra- and inter-molecular electronic transitions. Due to the high electron accepting capability of the quinolinyl moiety, blends of these compounds with the electron donor material 4,4′,4′′-tris(N-3-methylphenyl-N-phenylamino)triphenylamine form efficient exciplex hosts. Through the utilization of the exciplex hosts, simplified phosphorescent emitting devices with bis(2-phenylquinolinato)(2,4-pentanedionato)iridium [Ir(PQ)2acac] are constructed by the solution process. All of the devices exhibit a high performance ascribed to an efficient energy transfer and balanced charge carriers in the emission zone. External quantum efficiencies of the devices of up to a maximum of 14.1% are achieved and maintain a high level of 9.6–12.2% at 1000 cd m−2 and 6.5–8.2% at 10u2006000 cd m−2, respectively. The excellent performance of the device with the newly designed acceptor materials is in the top level of Ir(PQ)2acac based phosphorescent devices fabricated by the solution process.

Low-Temperature Atomic Layer Deposition of Metal Oxide Layers for Perovskite Solar Cells with High Efficiency and Stability under Harsh Environmental Conditions.

Rapid progress achieved on perovskite solar cells raises the expectation for their further development toward practical applications. Moisture sensitivity of perovskite materials is one of the major obstacles which limits the long-term durability of the perovskite solar cells, especially in outdoor operation where rainfall and water accumulation on the solar panels often occur. Micro/nanopinholes within the functional layers of the devices usually lead to water vapor penetration, thus subsequent decomposition of perovskites, and finally poor device performance and shortened operational lifetime. In this work, low-temperature atomic layer deposition (ALD) technique was utilized to incorporate pinhole-free metal oxide layers (TiO2 and Al2O3) into an inverted perovskite solar cell consisting of indium tin oxide/NiO/perovskite/PC61BM/TiO2/Ag. The interface properties between the inserted TiO2 layer and the perovskite layer were investigated by X-ray photoelectron spectroscopy. The results showed that TiO2 ALD fabrication process had made negligible degradation to the perovskite layer. The TiO2 layer can significantly reduce interfacial charge recombination loss, improve interfacial contact, and enhance water resistance. A maximum power conversion efficiency (PCE) of 18.3% was achieved for devices with TiO2 interface layers. A stacked Al2O3 encapsulation layer was designed and deposited on top of the devices to further improve device stability under harsh environmental conditions. The encapsulated devices with the best performance retained 97% of the initial PCE after being stored in ambient condition for a thousand hours. They also showed great water resistance, and no significant degradation in terms of PCE and photocurrent of the devices was observed after they were immersed in deionized water for as long as 2 h. Our approach offers a promising way of developing highly efficient and stable perovskite solar cells under real-world operational conditions.