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Cho, Shinuk,Lee, Kwanghee,Heeger, Alan J. WILEY-VCH Verlag 2009 Advanced Materials Vol.21 No.19
<B>Graphic Abstract</B> <P>A thin capping layer of titanium sub-oxide (TiOx) prepared by sol–gel synthesis from titanium alkoxides extends the lifetime of organic FETs. The TiO<SUB>x</SUB> layer functions as an ‘active’ passivation/barrier layer that actually removes oxygen and water vapor from the organic semiconductor. The results demonstrate a significant improvement in the lifetime of organic field-effect transistors when exposed to air. <img src='wiley_img/09359648-2009-21-19-ADMA200803013-content.gif' alt='wiley_img/09359648-2009-21-19-ADMA200803013-content'> </P>
Cho, Shinuk,Seo, Jung Hwa,Lee, Kwanghee,Heeger, Alan J. WILEY-VCH Verlag 2009 Advanced Functional Materials Vol.19 No.9
<P>Enhanced performance of n-channel organic field-effect transistors (OFETs) is demonstrated by introducing a titanium sub-oxide (TiO<SUB>x</SUB>) injection layer. The n-channel OFETs utilize [6,6]-phenyl-C<SUB>61</SUB> butyric acid methyl ester (PC<SUB>61</SUB>BM) or [6,6]-phenyl-C<SUB>71</SUB> butyric acid methyl ester (PC<SUB>71</SUB>BM) as the semiconductor in the channel. With the TiO<SUB>x</SUB> injection layer, the electron mobilities of PC<SUB>61</SUB>BM and PC<SUB>71</SUB>BM FET using Al as source/drain electrodes are comparable to those obtained from OFETs using Ca as the source/drain electrodes. Direct measurement of contact resistance (R<SUB>c</SUB>) shows significantly decreased R<SUB>c</SUB> values for FETs with the TiO<SUB>x</SUB> layer. Ultraviolet photoelectron spectroscopy (UPS) studies demonstrate that the TiO<SUB>x</SUB> layer reduces the electron injection barrier because of the relatively strong interfacial dipole of TiO<SUB>x</SUB>. In addition to functioning as an electron injection layer that eliminates the contact resistance, the TiO<SUB>x</SUB> layer acts as a passivation layer that prevents penetration of O<SUB>2</SUB> and H<SUB>2</SUB>O; devices with the TiO<SUB>x</SUB> injection layer exhibit a significant improvement in lifetime when exposed to air.</P> <B>Graphic Abstract</B> <P>Enhanced performance of fullerene organic field-effect transistors (OFETs) is demonstrated by introducing a TiO<SUB>x</SUB> injection layer that essentially eliminates the contact resistance between the electrodes and the active layer. The TiO<SUB>x</SUB> layer also serves to protect the device against penetration of O<SUB>2</SUB> and H<SUB>2</SUB>O, and thereby leads to a significant improvement in the lifetime of OFETs when exposed to air. <img src='wiley_img/1616301X-2009-19-9-ADFM200900189-content.gif' alt='wiley_img/1616301X-2009-19-9-ADFM200900189-content'> </P>
End‐Capping Effect of a Narrow Bandgap Conjugated Polymer on Bulk Heterojunction Solar Cells
Park, Jin Kuen,Jo, Jang,Seo, Jung Hwa,Moon, Ji Sun,Park, Yeong Don,Lee, Kwanghee,Heeger, Alan J.,Bazan, Guillermo C. WILEY‐VCH Verlag 2011 Advanced Materials Vol.23 No.21
<P><B>Device performances of BHJ solar cells</B> based on poly[(4,4‐didodecyldithieno[3,2‐b:2’,3’‐d]silole)‐2,6‐diyl‐<I>alt</I>‐(2,1,3‐benzoxadiazole)‐4,7‐diyl]and PC<SUB>71</SUB>BM improve by capping the chain ends with thiophene fragments. This structural modification yields materials that are more thermally robust and that can be used in devices with thicker films – important considerations for enabling the mass production of plastic solar cells. </P>
Sequential Processing: Control of Nanomorphology in Bulk Heterojunction Solar Cells
Wang, Dong Hwan,Moon, Ji Sun,Seifter, Jason,Jo, Jang,Park, Jong Hyeok,Park, O Ok,Heeger, Alan J. American Chemical Society 2011 Nano letters Vol.11 No.8
<P>Bulk heterojunction organic photovoltaic devices based on poly[<I>N</I>-9′′-hepta-decanyl-2,7-carbazole-<I>alt</I>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT)/[6,6]-phenyl C<SUB>70</SUB> butyric acid methyl ester (PC<SUB>70</SUB>BM) can be successfully fabricated by a sequential solution deposition process. When the top layer is deposited from an appropriate cosolvent, the PC<SUB>70</SUB>BM penetrates a predeposited bottom layer of PCDTBT during the spin-casting process, resulting in an interdiffused structure with a layer-evolved bulk heterojunction (LE-BHJ) nanomorphology. The PCDTBT:PC<SUB>70</SUB>BM LE-BHJ solar cells prepared with an optimized cosolvent ratio have comparable power conversion efficiency to the conventional BHJ solar cells. The nanomorphology of the optimized PCDTBT:PC<SUB>70</SUB>BM LE-BHJ mixture was found to have better vertical connectivity than the conventional BHJ material.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-8/nl202320r/production/images/medium/nl-2011-02320r_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl202320r'>ACS Electronic Supporting Info</A></P>
A low-bandgap alternating copolymer containing the dimethylbenzimidazole moiety
Song, Suhee,Jin, Youngeup,Park, Sung Heum,Cho, Shinuk,Kim, Il,Lee, Kwanghee,Heeger, Alan J.,Suh, Hongsuk Royal Society of Chemistry 2010 Journal of materials chemistry Vol.20 No.31
<P>A new acceptor unit containing dimethyl-2<I>H</I>-benzimidazole was synthesized and utilized for the synthesis of a conjugated polymer containing electron donor–acceptor pairs for OPV devices. The dimethyl-2<I>H</I>-benzimidazole unit was designed to act as a substitute for the BT unit of <B>PCDTBT</B>. This novel dimethyl-2<I>H</I>-benzimidazole unit has two methyl groups which can supply higher solubility than those of the BT series. A thin film of <B>PCDTMBI</B>, containing the dimethyl-2<I>H</I>-benzimidazole unit, shows two broad absorption bands with maxima at 400 and 636 nm and an absorption onset of 756 nm, corresponding to a band gap of 1.64 eV. The oxidation onset of <B>PCDTMBI</B> was estimated to be 0.67 V, which corresponds to a HOMO energy level of −5.47 eV. The LUMO energy level of the polymer was thus determined to be −3.82 eV. A device with a <B>PCDTMBI</B>:PC<SUB>71</SUB>BM blend had a <I>V</I><SUB>OC</SUB> value of 0.65 V, a <I>J</I><SUB>SC</SUB> value of 10.0 mA cm<SUP>−2</SUP>, and a <I>FF</I> of 0.48, leading to an efficiency of 3.12%. The enhanced efficiency of <B>PCDTMBI</B> was caused by the higher IPCE value between 400 and 700 nm and high mobility (2.2 × 10<SUP>−3</SUP> cm<SUP>2</SUP> V<SUP>−1</SUP> s<SUP>−1</SUP>).</P> <P>Graphic Abstract</P><P>A new acceptor unit containing dimethyl-2<I>H</I>-benzimidazole was synthesized and utilized for the synthesis of a conjugated polymer containing electron donor–acceptor pairs for OPV devices. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c0jm00772b'> </P>
Isomeric iminofullerenes as acceptors in bulk heterojunction organic solar cells
Park, Sung Heum,Yang, Changduk,Cowan, Sarah,Lee, Jae Kwan,Wudl, Fred,Lee, Kwanghee,Heeger, Alan J. Royal Society of Chemistry 2009 Journal of materials chemistry Vol.19 No.31
<P>Two stable iminofullerene isomers, [5,6]-open azafulleroid (open APCBM) and [6,6]-closed aziridinofullerene (closed APCBM) enable us to scrutinize the use of these new acceptors in polymer bulk heterojunction (BHJ) solar cells and compare the effects of open trans- and closed trans-annlar subunits. When we compared the performance of both isomer devices, the poly(3-hexylthiophene)(P3HT):open APCBM device demonstrates an enhancement in photocurrent in comparison with the P3HT:closed APCBM device. From the comparative study, we attribute the enhanced current to the lower degree of symmetry of open APCBM. The alteration of fullerene structure from closed to open breaks its high degree of symmetry and consequently leads to an improved bulk heterojunction with the electron donating conjugated polymer.</P> <P>Graphic Abstract</P><P>The comparative study of bulk heterojunction solar cells based on two isomeric iminofullerenes, [5,6]-open azafulleroid and [6,6]-closed aziridino-fullerene as electron acceptors in combination with P3HT material are carried out. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b904535j'> </P>