Congcheng Fan

Low temperature solution processed planar heterojunction perovskite solar cells with a CdSe nanocrystal as an electron transport/extraction layer

A cadmium selenide (CdSe) nanocrystal was used as an electron transport/extraction layer for perovskite solar cells due to its high electron mobility and solution-processability at low temperatures. Power conversion efficiency (PCE) up to 11.7% was achieved under standard AM1.5G conditions in air.

Solution-grown organic single-crystalline p-n junctions with ambipolar charge transport.

Organic single-crystalline p-n junctions are grown from mixed solutions. First, C60 crystals (n-type) form and, subsequently, C8-BTBT crystals (p-type) nucleate heterogeneously on the C60 crystals. Both crystals continue to grow simultaneously into single-crystalline p-n junctions that exhibit ambipolar charge transport characteristics. This work provides a platform to study organic single-crystalline p-n junctions.

Pyrene and Diketopyrrolopyrrole-Based Oligomers Synthesized via Direct Arylation for OSC Applications

In this report, an atom efficient and facile synthetic strategy for accessing multi-diketopyrrolopyrrole (DPP)-based oligomers used in solution-processed organic field effect transistors (OFETs) and organic solar cells (OSCs) has been developed. The DPP units were successfully installed onto benzene and pyrene cores via palladium-catalyzed dehydrohalogenative coupling of mono-capped DPPs with multi-bromo-benzene or -pyrene (direct arylation), affording four oligomer small molecules (SMs 1-4) containing bis-, tri-, tri-, and tetra-DPP, respectively, in high yields of 78-96%. All the designed linear or branched DPP-based oligomers exhibit broad light absorptions, narrow band-gaps (1.60-1.73 eV), deep highest occupied molecular orbital (HOMO) levels (-5.26∼-5.18 eV), and good thermal stability (Td=390-401 °C). OFETs based on SMs 1-4 showed hole mobilities of 0.0033, 0.0056, 0.0005, and 0.0026 cm2 V(-1) s(-1), respectively. OSCs based on SMs 1-4 under one sun achieved power conversion efficiencies of 3.00%, 3.71%, 2.47%, and 1.86% accordingly, along with high open-circuit voltages of 0.86-0.94 V. For OSC devices of SM 1, SM 3, and SM 4, the solvent CHCl3 was solely employed to the formation of active layers; neither high boiling point additives nor annealing post-treatment was needed. Such a simple process benefits the large-scale production of OSCs via roll to roll technology.

Solution-grown organic single-crystalline donor-acceptor heterojunctions for photovoltaics.

Organic single crystals are ideal candidates for high-performance photovoltaics due to their high charge mobility and long exciton diffusion length; however, they have not been largely considered for photovoltaics due to the practical difficulty in making a heterojunction between donor and acceptor single crystals. Here, we demonstrate that extended single-crystalline heterojunctions with a consistent donor-top and acceptor-bottom structure throughout the substrate can be simply obtained from a mixed solution of C60 (acceptor) and 3,6-bis(5-(4-n-butylphenyl)thiophene-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (donor). 46 photovoltaic devices were studied with the power conversion efficiency of (0.255±0.095)% under 1 sun, which is significantly higher than the previously reported value for a vapor-grown organic single-crystalline donor-acceptor heterojunction (0.007%). As such, this work opens a practical avenue for the study of organic photovoltaics based on single crystals.

Boosting the electron mobility of solution-grown organic single crystals via reducing the amount of polar solvent residues

Enhancing electron transport to match with the development in hole transport is critical for organic electronics in the future. As electron motion is susceptible to extrinsic factors, seeking these factors and avoiding their negative effects have become the central challenge. Here, the existence of polar solvent residues in solution-grown single-crystals of 6,13-bis(triisopropylsilylethynyl)-5,7,12,14-tetraazapentacene is identified as a factor detrimental to electron motion. Field-effect transistors of the crystals exhibit electron mobility boosted by about 60% after the residues are removed. The average electron mobility reaches up to 8.0 ± 2.2 cm2 V−1 s−1 with a highest value of 13.3 cm2 V−1 s−1; these results are significantly higher than those obtained previously for the same molecule (1.0–5.0 cm2 V−1 s−1). Furthermore, the achieved mobility is also higher than the maximum reported electron mobility for organic materials (11 cm2 V−1 s−1). This work should greatly accelerate the advancement of organic electron-transporting materials.

A diketopyrrolopyrrole molecule end-capped with a furan-2-carboxylate moiety: the planarity of molecular geometry and photovoltaic properties

We designed and synthesized a diketopyrrolopyrrole (DPP) molecule with a fully-planar molecular geometry, 3,6-bis{5-[(ethylfuran-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CF)2), for solution-processable organic solar cells (OSCs). It is theoretically calculated that the dihedral angles between the two furan-2-carboxylate end-groups and the DPP core are both only 0.56°. Due to this negligible steric distortion, the molecular conformation of DPP(CF)2 can be considered fully coplanar, leading to a higher crystallinity for the DPP(CF)2 film. As a result, the hole mobility of DPP(CF)2 is one order of magnitude higher than that of the DPP derivative with thiophene-2-carboxylate as the end-group (DPP(CT)2). DPP(CF)2 exhibits both a low optical band gap (Eg) of 1.60 eV and a low-lying highest occupied molecular orbital (HOMO) energy level of −5.33 eV, implying that DPP(CF)2 is a promising electron donor for OSCs. OSCs with DPP(CF)2 or DPP(CT)2 as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor were fabricated. It is found that the DPP(CF)2-based devices exhibit much better photovoltaic performance than the DPP(CT)2-based devices, with the highest power conversion efficiency of 5.37% and a short-circuit current density of 11.4 mA cm−2. This phenomenon can be ascribed to the superior charge-transporting ability of DPP(CF)2 due to its fully-planar molecular geometry.

Ambipolar charge transport of TIPS-pentacene single-crystals grown from non-polar solvents

Charge transport of solution-grown TIPS-pentacene single-crystals depends on the polarity of the used solvents. Crystals grown from non-polar solvents exhibit ambipolar transport (μe 0.020 cm2 V−1 s−1; μh 5.0 cm2 V−1 s−1), while polar solvents suppress electron transport. This work indicates that appropriate selection of solvents should further harvest the n-type behaviors of organic semiconductors.

A direct arylation-derived DPP-based small molecule for solution-processed organic solar cells

A diketo-pyrrolo-pyrrole (DPP) oligomer containing three DPP cores (Ph4Th4(DPP)3) was synthesized via direct arylation of C-H bonds (DACH). Ph4Th4(DPP)3 has good solubility in many organic solvents, and shows a broad absorption band from the visible to near-infrared region as well as a field-effect hole mobility as high as 0.006 cm(2) V(-1) s(-1). Solution-processed bulk heterojunction organic solar cells based on blends of Ph4Th4(DPP)3 as electron donor and fullerene derivative as electron acceptor were fabricated. An optimized power conversion efficiency of 3.76% with a high open-circuit voltage of 0.85 V was achieved after finely tuning the morphology by changing the blend ratio and by adding additives. These results indicate that DACH is an effective way to produce π-conjugated oligomers for organic solar cells.

Interfacing Solution-Grown C60 and (3-Pyrrolinium)(CdCl3 ) Single Crystals for High-Mobility Transistor-Based Memory Devices.

Aligned ferroelectric single crystals of (3-pyrrolinium)(CdCl3 ) can be prepared from solution on top of aligned semiconducting C60 single crystals using an orthogonal solvent. Memory devices based on these ferroelectric/semiconductor bilayered heterojunctions exhibit much larger hysteresis compared with that of only C60 single crystals. More importantly, the introduction of the ferroelectric layer induces the memory window without dramatically reducing the charge mobility.

The orbital symmetry of carbon monoxide on a Cs-precovered Ru(1010) surface

The molecular orbitals of CO are rearranged under the strong influence of coadsorbed Cs atoms. The peak for CO on the clean surface, at 7.5 eV BE attributed to the and orbits, is split into two peaks; one is at 6.3 eV BE and the other 7.8 eV BE. The peak at 6.3 eV BE shows an anti-symmetry character about the plane parallel to the crystallographic direction in the surface.