Volume 7, Issue 9, September 2014
Index of content:
- ORGANIC ELECTRONICS AND PHOTONICS
105(2014); http://dx.doi.org/10.1063/1.4894637View Description Hide Description
Fullerenes and their derivatives have been widely used as n-type materials in organic transistor and photovoltaic devices. Though it is believed that they shall be ambipolar in nature, there have been few direct experimental proofs for that. In this work, fullerene C70, known as an efficient acceptor, has been employed as a p-type electron donor in conjunction with 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile as an electron acceptor in planar-heterojunction (PHJ) organic photovoltaic (OPV) cells. High fill factors (FFs) of more than 0.70 were reliably achieved with the C70 layer even up to 100 nm thick in PHJ cells, suggesting the superior potential of fullerene C70 as the p-type donor in comparison to other conventional donor materials. The optimal efficiency of these unconventional PHJ cells was 2.83% with a short-circuit current of 5.33 mA/cm2, an open circuit voltage of 0.72 V, and a FF of 0.74. The results in this work unveil the potential of fullerene materials as donors in OPV devices, and provide alternative approaches towards future OPV applications.
High performance unipolar inverters by utilizing organic field-effect transistors with ultraviolet/ozone treated polystyrene dielectric105(2014); http://dx.doi.org/10.1063/1.4895121View Description Hide Description
High performance unipolar inverters based on a significant variation of threshold voltage (Vth) of organic field-effect transistors (OFETs), which was realized by introducing UV/ozone (UVO) treatment to polystyrene (PS) dielectric, were fabricated. A controllable Vth shift of more than 10 V was obtained in the OFETs by adjusting the UVO treating time, and the unipolar inverters exhibited inverting voltage near 1/2 driving voltage and a noise margin of more than 70% of ideal value. From the analysis of scanning electron microscopy, atom force microscopy, and X-ray photoelectron spectroscopy, the dramatic controllable Vth of OFETs, which played a key role in high performance unipolar inverters, was attributed to the newly generated oxygen functional groups in the PS dielectric induced by UVO treatment.
Electrical switching and memory behaviors in organic diodes based on polymer blend films treated by ultraviolet ozone105(2014); http://dx.doi.org/10.1063/1.4895122View Description Hide Description
Resistive memory devices with resistive switching characteristics were fabricated based on poly (3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) doping with polyvinyl alcohol. It has been demonstrated that the resistive switching characteristics in the memory device was strongly dependent on the treatment of the polymer blend film by ultraviolet ozone (UV-ozone). The UV-ozone treated device exhibited improved performance with the ON/OFF current ratio of more than 102, and its ON and OFF states can be maintained over 96 h without deterioration. The resistive switching behavior in the UV-ozone treated device was attributed to the formation and rupture of the PEDOT:PSS filaments as well as the narrow conducting paths through the native oxide of aluminum.
Roles of solvent additive in organic photovoltaic cells through intensity dependence of current-voltage characteristics and charge recombination105(2014); http://dx.doi.org/10.1063/1.4895531View Description Hide Description
In this research, the enhanced power conversion efficiency of PBnDT-FTAZ:PC70BM bulk-heterojunction (BHJ) solar cells (over 7%), fabricated with a solvent additive (1,8-diiodooctane, 3%), was demonstrated. The appropriate ratio of solvent additive leads to a device with increased JSC, fill factor, and favorably controlled nanomorphology phase separation. Furthermore, the device with BHJ film fabricated with the solvent additive exhibited improved charge collection and simultaneously reduced charge recombination from a systematic light intensity analysis. The lack of Shockley-Read-Hall recombination at open circuit, the dominance of bimolecular recombination only when approaching open circuit, and an improved charge collection probability (95.5%) at short circuit contribute to high performance organic photovoltaic cells.
Contrary interfacial exciton dissociation at metal/organic interface in regular and reverse configuration organic solar cells105(2014); http://dx.doi.org/10.1063/1.4895675View Description Hide Description
An opposite interfacial exciton dissociation behavior at the metal (Al)/organic cathode interface in regular and inverted organic solar cells (OSCs) was analyzed using transient photocurrent measurements. It is found that Al/organic contact in regular OSCs, made with the blend layer of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-[3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7):3′H-Cyclopropa [8,25][5,6] fullerene-C70-D5h(6)-3′-butanoicacid,3′-phenyl-,methyl ester (PC 70BM), always hampers the electron collection. However, this is not observed in their reverse geometry OSCs fabricated using the same PTB7:PC70BM blend system. The detrimental interfacial exciton dissociation in regular OSCs originates the compensation of field drifted photo-generated electrons at Al/organic interface. The unfavorable interfacial exciton dissociation can be eliminated, e.g., by interposing a ZnO-based interlayer between Al and organic layer, attaining an efficient electron collection, thereby power conversion efficiency.