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Li, Cheng,Credgington, Dan,Ko, Doo-Hyun,Rong, Zhuxia,Wang, Jianpu,Greenham, Neil C. The Royal Society of Chemistry 2014 Physical chemistry chemical physics Vol.16 No.24
<P>The performance of organic solar cells incorporating solution-processed titanium suboxide (TiO<SUB><I>x</I></SUB>) as electron-collecting layers can be improved by UV illumination. We study the mechanism of this improvement using electrical measurements and electroabsorption spectroscopy. We propose a model in which UV illumination modifies the effective work function of the oxide layer through a significant increase in its free electron density. This leads to a dramatic improvement in device power conversion efficiency through several mechanisms – increasing the built-in potential by 0.3 V, increasing the conductivity of the TiO<SUB><I>x</I></SUB> layer and narrowing the interfacial Schottky barrier between the suboxide and the underlying transparent electrode. This work highlights the importance of considering Fermi-level equilibration when designing multi-layer transparent electrodes.</P> <P>Graphic Abstract</P><P>The mechanism by which UV exposure enhances the performance of organic solar cells incorporating TiO<SUB><I>x</I></SUB> layers is investigated by current–voltage and electroabsorption measurements. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4cp01251h'> </P>
Saturation, Relaxation, and Dissociation of Excited Triplet Excitons in Conjugated Polymers
Yang, Xudong,Lee, Chang-Lyoul,Westenhoff, Sebastian,Zhang, Xinping,Greenham, Neil C. WILEY-VCH Verlag 2009 ADVANCED MATERIALS Vol.21 No.8
<B>Graphic Abstract</B> <P>Femtosecond pump-probe spectroscopy is used to measure the excited state absorption dynamics in triplet excitons generated by continuous wave excitation of a conjugated polymer. Saturation of the T<SUB>1</SUB>–T<SUB>n</SUB> transition is observed, allowing a cross-section of 2.0 × 10<SUP>−16</SUP> cm<SUP>2</SUP> to be established. Relaxation to T<SUB>1</SUB> occurs with a 300 fs timescale, but 20% of T<SUB>n</SUB> states dissociate into charges. <img src='wiley_img/09359648-2009-21-8-ADMA200802597-content.gif' alt='wiley_img/09359648-2009-21-8-ADMA200802597-content'> </P>
Triplet Exciton and Polaron Dynamics in Phosphorescent Dye Blended Polymer Photovoltaic Devices
Lee, Chang‐,Lyoul,Hwang, In‐,Wook,Byeon, Clare Chisu,Kim, Bok Hyeon,Greenham, Neil C. WILEY‐VCH Verlag 2010 Advanced Functional Materials Vol.20 No.17
<P><B>Abstract</B></P><P>The triplet exciton and polaron dynamics in phosphorescent dye (PtOEP) blended polymer (MEH‐PPV) photovoltaic devices are investigated by quasi‐steady‐state photo‐induced absorption (PIA) spectroscopy. According to the low‐temperature PIA and photoluminescence (PL) results, the increase in strength of the triplet‐triplet (<I>T</I><SUB>1</SUB>‐<I>T<SUB>n</SUB></I>) absorption of MEH‐PPV in the blend system originates from the triplet‐triplet energy transfer from PtOEP to MEH‐PPV. The PtOEP blended MEH‐PPV/C<SUB>60</SUB> bilayer photovoltaic device shows a roughly 30%–40% enhancement in photocurrent and power‐conversion efficiency compared to the device without PtOEP. However, in contrast to the bilayer device results, the bulk heterojunction photovoltaic devices do not show a noticeable change in photocurrent and power‐conversion efficiency in the presence of PtOEP. The PIA intensity, originating from the polaron state, is only slightly higher (within the experimental error), indicating that carrier generation in the bulk heterojunction is not enhanced in the presence of PtOEP. The rate and probability of the exciton dissociation between PtOEP and PCBM is much faster and higher than that of the triplet‐triplet energy transfer between PtOEP and MEH‐PPV.</P>