Processing of pristine single and multiwalled carbon nanotubes as different stacking layers in bulk heterojunction solar
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Processing of pristine single and multiwalled carbon nanotubes as different stacking layers in bulk heterojunction solar cells M. Alam Khan,1 Michio Matsumura2 and Omar Manasreh1 1
Optoelectronic Laboratory, Department of Electrical Engineering, University of Arkansas, Fayetteville, 72701, AR, U.S.A. 2 Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka Campus, Osaka 560-8531, Japan ABSTRACT A study on the individualization of commercially purchased SWCNT and MWCNT were made in an N-N dimethyl tetraformamide solvent by a combination of ultrasonication and centrifugation, and processing of these individualized CNTs are applied as a stacking layer in bulk-heterojunction (BHJ) solar cells. The wt% (mg) of pristine CNTs loading was optimized in respect to volume of solvent (ml). Then as prepared BHJ devices were modified by spin casting at 4000 rpm/30s of these pristine individualized CNTs by incorporating a stacking layer (~30 nm) for efficiency evaluation. Comparisons of the devices made with known functionalized CNTs (acid treated) and found that stacking of pristine individualized CNTs between the PEDOT:PSS and active (P3HT:PCBM) layer demonstrate a significantly enhanced efficiency of 2.1% (JSC of 9 mA/cm2, VOC of 0.6, FF of 39) from the normal BHJ of 1.2% (JSC of 5.3 mA/cm2, VOC of 0.62, FF of 33). However, SWCNT with acid treated when applied in BHJ shows degrading efficiency (0.51%) which can be attributed to the degradation of corrugated tubular side walls leading to potential loss of opt-oelectric properties. The enhanced efficiency of devices with pristine individualize CNTs can be conjectured due to better opto-electrical properties and undamaged tubes. The microstructures of the heterojunction active layer were examined by using UV-Vis spectra, IV curve and EQE techniques. INTRODUCTION Plastic photovoltaics based on bulk-heterojunction concept consists of light-harvesting conjugated polymer and electron accepting fullerenes [1] are perceived as a promising and effective potential alternative to silicon solar cells owing to their light-weight, availability for large area fabrication, cost-effectiveness and flexible photovoltaics [2]. Significant approaches are devised to increase the efficiencies of these solar cells [3,4] and many effective processing has been developed to optimize the interpenetrating network such as mixing of solvents, additives [5,6] and optical spacers into the devices framework [7]. Unlike highly ordered crystalline inorganic solar cells, where photoexcitation yields a pair of free carriers, photons incident on the active layer of a polymer BHJ solar cell create excitons, that is, electron-hole pairs bound by their Coulomb attraction. In most cases of BHJ semiconducting polymers spin casted from the solution phase and the generated excitons can diffuse less than 10-20 nm in the cell before decaying to the ground state [8]. Since open circuit voltage (VOC) in polymer solar cells are primarily limited due to the limitation of subtraction of
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