Shirong Lu

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

Highly Efficient Cu(OAc)2‐Catalyzed Dimerization of Monofunctionalized Hydrofullerenes Leading to Single‐Bonded [60]Fullerene Dimers

The single-bonded fullerene dimer RC60 C60R, which has a direct covalent bond between two C60 cages, is an interesting and unusual structure that is expected to display interesting optical and electronic properties through the interaction of two adjacent fullerene cages. Since the pioneering work by Krusic and co-workers, studies on ESR and X-ray crystalstructure analysis revealed that singly bonded fullerene dimers consist of racemic and meso isomers, which are in equilibrium with the monomer radical (RC60C) in solution. [3]

Cobalt-catalyzed hydroalkylation of [60]fullerene with active alkyl bromides: selective synthesis of monoalkylated fullerenes.

The Co-catalyzed hydroalkylation of C(60) with reactive alkyl bromides 1 (RBr) in the presence of Mn reductant and H(2)O at ambient temperature gave the monoalkylated C(60) (2) in good to high yields. The use of CoLn/Mn/H(2)O under Ar atmosphere is crucial for the success of the present transformation. The reaction most probably proceeds through the Co(0 or I) complex-promoted generation of a radical (R(•)) followed by addition to C(60). This hydroalkylation method was applied to the synthesis of zinc porphyrin attached C(60) (2l), dendrimer attached C(60) (2m), and fullerene dimer (2n), which were not easily available through the previously known methods.

Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer

Organic–inorganic hybrid perovskites have recently attracted considerable interest for application in solar cells due to their low cost, high absorption coefficient and high power conversion efficiency (PCE). Herein, we utilize a CdSe quantum dot/PCBM composite as an electron transport layer (ETL) to investigate the structure, stability and PCE of CH3NH3PbI3−xClx perovskite solar cells. It is found that adsorption of the CdSe/PCBM composite reduces the roughness of the perovskite, leading to a high-quality film with a compact morphology. Density functional theory (DFT) based first-principles calculations show that CdSe enhances the chemical stability of CH3NH3PbI3−xClx involving strong atomic orbital hybridization. Interestingly, an inorganic-terminated perovskite surface has much stronger interaction with CdSe compared to the surface with organic CH3NH3 termination, with noticeable interfacial charge redistribution. Experiments on solar cells incorporating the CdSe/PCBM composite as the ETL show enhanced photocurrent and fill factor, which is related to the in-built electric field between CH3NH3PbI3−xClx and CdSe that greatly facilitates the separation of electron and hole pairs. We show an improved PCE of 13.7% with enhanced device stability in a highly humid atmosphere. These joint theoretical–experimental results may provide a new aspect for improving the structural stability and operating performance of optoelectronic devices based on perovskite structures.

Effect of molecular weight on the properties and organic solar cell device performance of a donor–acceptor conjugated polymer

Donor–acceptor conjugated polymers with 2-(2-ethylhexyl)-3-hexyl thienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor building block and 5,6-difluorobenzo[c][1,2,5]thiadiazole as acceptor building block have been synthesized by Stille coupling polymerization. The polymerization conditions were optimized to achieve high molecular weight polymers (number-average molecular weight, Mn, up to 139 kg mol−1). The molecular weight dependent polymer properties were studied and compared. Photovoltaic applications of the polymers in bulk heterojunction (BHJ) solar cells revealed that the power conversion efficiency increased significantly (from 0.9% to 4.1%) as the Mn increased from 10 kg mol−1 to 73 kg mol−1 while further increase of the molecular weight had less influence on the solar cell performance.

Co-catalyzed radical cycloaddition of [60]fullerene with active dibromides: selective synthesis of carbocycle-fused fullerene monoadducts.

An efficient and highly selective Co-catalyzed radical cycloaddition of [60]fullerene with active dibromides for the synthesis of three-, five-, six-, and seven-membered carbocycle-fused fullerene monoadducts has been reported. The controlled experiments unambiguously disclosed that the reaction proceeds through the formation of a fullerene monoradical as a key intermediate.

2D/3D perovskite hybrids as moisture-tolerant and efficient light absorbers for solar cells

The lifetime and power conversion efficiency are the key issues for the commercialization of perovskite solar cells (PSCs). In this paper, the development of 2D/3D perovskite hybrids (CA2PbI4/MAPbIxCl3-x) was firstly demonstrated to be a reliable method to combine their advantages, and provided a new concept for achieving both stable and efficient PSCs through the hybridization of perovskites. 2D/3D perovskite hybrids afforded significantly-improved moisture stability of films and devices without encapsulation in a high humidity of 63 ± 5%, as compared with the 3D perovskite (MAPbIxCl3-x). The 2D/3D perovskite-hybrid film did not undergo any degradation after 40 days, while the 3D perovskite decomposed completely under the same conditions after 8 days. The 2D/3D perovskite-hybrid device maintained 54% of the original efficiency after 220 hours, whereas the 3D perovskite device lost all the efficiency within only 50 hours. Moreover, the 2D/3D perovskite hybrid achieved comparable device performances (PCE: 13.86%) to the 3D perovskite (PCE: 13.12%) after the optimization of device fabrication conditions.

Cu-catalyzed C-H amination of hydrofullerenes leading to 1,4-difunctionalized fullerenes.

A novel and highly efficient Cu-catalyzed C-H amination of the monofunctionalized hydrofullerenes for the synthesis of 1,4-difunctional fullerenes has been reported. A new series of 1,4-fullerene derivatives having various monoamine addends were synthesized in good to high yields under mild reaction conditions. The controlled experiments revealed that the reaction proceeds through the formation of a fullerene monoradical as a key intermediate followed by coupling with an amine radical.

NaOH-catalyzed dimerization of monofunctionalized hydrofullerenes: transition-metal-free, general, and efficient synthesis of single-bonded [60]fullerene dimers.

Unprecedented, transition-metal-free NaOH-catalyzed homo- and cross-dimerizations of monofunctionalized hydrofullerenes are reported. Various single-bonded fullerene dimers were synthesized under mild reaction conditions with remarkably high yields. The use of a NaOH catalyst combined with tetrahydrofuran as a cosolvent under an air atmosphere is critical in achieving highly efficient catalytic dimerization.

Deuterium Isotope Effect on Bulk Heterojunction Solar Cells. Enhancement of Organic Photovoltaic Performances Using Monobenzyl Substituted Deuteriofullerene Acceptors

A series of novel monobenzyl-substituted deuteriofullerenes (BnDCs) were synthesized efficiently through Co-catalyzed selective monofunctionalization of C60. Bulk heterojunction solar cells, based on poly(3-hexylthiophene) as the donor and BnDCs as the acceptors, exhibited higher photovoltaic performances as compared to the corresponding protonated BnHCs devices.