Shinuk Cho

Effect of processing additive on the nanomorphology of a bulk heterojunction material.

The bulk heterojunction (BHJ) material Si-PDTBT:PC(70)BM is sensitive to the use of a small amount of 1-chloronaphthalene (CN) as a processing additive; CN as a cosolvent (e.g., 4% in chlorobenzene) causes in a factor of 2 increase in the power conversion efficiency of BHJ solar cells. The morphology of the BHJ material, prepared with and without the CN additive is studied with top-down transmission electron microscopy, cross-sectional transmission electron microscopy, and atomic force microscopy. The improved performance is the result of changes in the nanoscale morphology. Field-effect transistor measurements are consistent with the observed changes in morphology.

Functionalized Methanofullerenes Used as n-Type Materials in Bulk-Heterojunction Polymer Solar Cells and in Field-Effect Transistors

The synthesis of two well-solubilized [60]methanofullerene derivatives ( p- EHO-PCBM and p- EHO-PCBA) is presented for usage in organic solar cells and in field-effect transistors. The para position of the PCBMs phenyl ring was substituted with a branched alkoxy side chain, which contributes to higher solubility, facilitating synthesis, purification, and processing. We find a small change of the open-circuit voltage ( V oc) as a slight improvement in performance upon application in P3HT/[60]methanofullerene bulk-heterojunction-photovoltaic cells, when compared to PCBM, because of the electron donation of the alkoxy group. In the case of the devices with a TiO x layer, the best power conversion efficiencies (PCE, eta e) is observed in a layered structure of P3HT/ p- EHO-PCBA/TiO x (eta e = 2.6%), which slightly exceeds that of P3HT/PCBM/TiO x (eta e = 2.3%) under conditions reported here. This can be attributed, in part, to the carboxylic acid group in p- EHO-PCBA that leads to an effective interface interaction between the active layer and TiO x phase. In addition, n-channel organic field-effect transistor (OFET) devices were fabricated with thin films of p- EHO-PCBM and p- EHO-PCBA, respectively cast from solution on SiO 2/Si substrates. The values of field-effect mobility (mu) for p- EHO-PCBM and p- EHO-PCBA are 1 x 10 (-2) and 1.6 x 10 (-3) cm (2)/V.s, respectively. The results in this paper demonstrate the effects of a carboxylic acid group and an electron-donating substituent in [60]methanofullerenes as n-type materials with respect to organic solar cells and OFET applications.

Thermal annealing-induced enhancement of the field-effect mobility of regioregular poly(3-hexylthiophene) films

Polymer field-effect transistors with a field-effect mobility of μ≈0.3cm2s−1V−1 have been demonstrated using regioregular poly(3-hexylthiophene) (rr-P3HT). Devices were fabricated by dip coating the semiconducting polymer followed by annealing at 150°C for 10min. The heat annealed devices exhibit an increased field-effect mobility compared with the as-prepared devices. Morphology studies and analysis of the channel resistance demonstrate that the annealing process increases the crystallinity of rr-P3HT and improves the contact between the electrodes and the P3HT films, thereby increasing the field-effect mobility of the films.Polymer field-effect transistors with a field-effect mobility of μ≈0.3cm2s−1V−1 have been demonstrated using regioregular poly(3-hexylthiophene) (rr-P3HT). Devices were fabricated by dip coating the semiconducting polymer followed by annealing at 150°C for 10min. The heat annealed devices exhibit an increased field-effect mobility compared with the as-prepared devices. Morphology studies and analysis of the channel resistance demonstrate that the annealing process increases the crystallinity of rr-P3HT and improves the contact between the electrodes and the P3HT films, thereby increasing the field-effect mobility of the films.

Columnlike structure of the cross-sectional morphology of bulk heterojunction materials.

The cross-sectional morphology of the bulk heterojunction (BHJ) films comprising regio-regular poly(3-hexylthiophene) (rrP3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) was observed with transmission electron microscopy (TEM). The cross-sectional TEM images of the BHJ film provide information on the pathways for charge transport through the film thickness. The length scale of the phase separation was obtained from spatial Fourier transform analysis of the TEM images and from calculations of the autocorrelation function.

A Thermally Stable Semiconducting Polymer

A bs or pt io n RT 120 170 200 250 300 350 400 Early research on polymer electronic devices successfully demonstrated function and performance adequate for specific applications. As a result, the performance of devices fabricated from semiconducting polymers has improved to the point where ‘‘plastic’’ electronics are now expected to develop into a significant industry with a large market opportunity. However, the limited stability of polymer-based devices continues to hinder the path toward commercialization. Because stability in air is critical to the commercialization of polymer electronic devices, discussions concerning the stability of semiconducting polymers have focused on degradation caused by reaction with oxygen and water vapor. Conjugated polymers are, however, generally believed to be incapable of withstanding high temperatures (i.e., temperatures well above the glasstransition temperature, Tg), [6,7] thus, stability at high temperatures has received less attention. The availability of semiconducting polymers that can survive exposure to elevated temperatures would open a variety of new possibilities. For example, since inorganic electronic devices typically require process steps that must be carried out at high temperature (often over 300 8C), semiconducting polymers capable of withstanding high temperatures will enable the fabrication of novel organic–inorganic hybrid devices. Here, we report the remarkable stability of the poly(2,7carbazole) derivative, poly[N-900-hepta-decanyl-2,7-carbazole-alt5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)], (PCDTBT; see the inset of Fig. 1a). Prior to this report, there was no known example of a semiconducting polymer that is both stable in air at (and above) room temperature and capable of withstanding high temperatures for extended periods of time. PCDTBT is one of a relatively large class of ‘‘donor–acceptor’’ polycarbazole co-polymers. Recently, polymer bulkheterojuction solar cells fabricated with phase-separated blends of PCDTBT and PC71BM were demonstrated with internal quantum efficiency approaching 100%, power conversion efficiency of 17% in response to monochromatic radiation within the absorption band, and power conversion efficiency of 6.1% in response to solar radiation. To investigate the stability of PCDTBT, we have carried out spectroscopic studies on PCDTBT thin films and transport studies using the field-effect transistor (FET) architecture with PCDTBTas the semiconductor material in the channel. Figure 1 shows UV–visable (UV–vis) absorption spectra of PCDTBT thin films annealed for 15 minutes at various temperatures in air (Fig. 1a) and under N2 atmosphere (Fig. 1b). In air, the p–p* absorption spectrum is not affected after exposure to temperatures up to 150 8C. Under N2 atmosphere (Fig 1b), the electronic band structure of PCDTBT is stable after exposure to temperatures as high as 350 8C.

Titanium suboxide as an optical spacer in polymer solar cells

The use of a layer of titanium suboxide (TiOx) between the bulk heterojunction (BHJ) thin film and the metal contact in “plastic” solar cell has been shown to increase both the short-circuit current and the fill factor. Using a combination of optical modeling and device experiments, we clarify the role of the TiOx layer as an optical spacer. For an appropriate choice of the BHJ thickness and composition, the TiOx layer can enhance the absorption within the active layer, thereby increasing the photocurrent and the power conversion efficiency.

High‐Performance Planar Perovskite Optoelectronic Devices: A Morphological and Interfacial Control by Polar Solvent Treatment

Highly efficient planar perovskite optoelectronic devices are realized by amine-based solvent treatment on compact TiO2 and by optimizing the morphology of the perovskite layers. Amine-based solvent treatment between the TiO2 and the perovskite layers enhances electron injection and extraction and reduces the recombination of photogenerated charges at the interface.

Amine-Based Polar Solvent Treatment for Highly Efficient Inverted Polymer Solar Cells

The interfacial dipolar polarization in inverted structure polymer solar cells, which arises spontaneously from the absorption of ethanolamine end groups, such as amine and hydroxyl groups on ripple-structure zinc oxide (ZnO-R), lowers the contact barrier for electron transport and extraction and leads to enhanced electron mobility, suppression of bimolecular recombination, reduction of the contact resistance and series resistance, and remarkable enhancement of the power conversion efficiency.

Enhanced diode characteristics of organic solar cells using titanium suboxide electron transport layer

The (dark) diode characteristics of the organic bulk heterojunction solar cell based on the phase separated blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thenyl-2′,1′,3′-benzothiadiazole)] with [6,6]-phenyl C70-butyric acid methyl ester have been analyzed with a focus on the effect of the titanium suboxide (TiOx) electron transport layer. The addition of the TiOx layer into the device structure causes the saturation current density to decrease by a factor of 26 and the shunt resistance to increase by a factor of 12. The diode ideality factor and series resistance are, respectively, almost the same for diodes made with and without the TiOx layer. The results indicate that the TiOx layer increases the energy barrier for hole transport and reduces the minority carrier density.

Improved Injection in n-Type Organic Transistors with Conjugated Polyelectrolytes

To improve injection in n-type organic thin film transistors (OTFTs), a thin conjugated polyelectrolyte (CPE) layer was interposed between electrodes and the semiconductor layer. OTFTs were fabricated with [6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) and Au source and drain contacts. We demonstrate that the insertion of CPEs beneath top-contact Au source/drain electrodes can be a very effective strategy for improving the carrier injection and reducing turn-on threshold voltages of n-channel OTFTs. Ultraviolet photoemission spectroscopy (UPS) indicates that the decrease of the electron injection barrier is consistent with organized dipoles at the metal/organic interface.