Improving the performance of Hybrid TiO 2 /polymer Solar Cells

1. e- LUMO e- CB h+ HOMO n contact VB (e.g. ITO) p contact e (e.g. Au/ Ag) e e e acceptor donor (TiO2) (polymer) 2 e 3 1 poly(3-hexylthiophene) (P3HT) poly (styrenesulfonate) doped Poly (ethylenedioxythiophene) (PEDOT: PSS) e 4 TiO2 dense layer Improving the performance of Hybrid TiO2/polymer Solar Cells using functionalized Single Wall Carbon Nanotube P.A.Amalraj, K.Balashangar, and P.Ravirajan# Department of Physics, University of Jaffna, Sri Lanka #p_ravirajan@jfn.ac.lk, pravirajan@gmail.com Abstract The nanocrystallineTiO2in hybrid titanium dioxide/polymer solar cells serves as electron acceptor have several merits compared to the all-organic solar cells. Although the chemical and mechanical stability of the hybrid titanium dioxide/polymer solar cells is better than those of polymer solar cells, the power conversion efficiency is still lower than organic counterparts. Several strategies together with the introduction of novel device structure and materials were reported in this context in order to enhance the performance of the hybrid TiO2/polymer solar cells. This study focuses on enhancing the performance of hybrid TiO2/polymer solar cells through incorporating carboxyl functionalized Single Wall carbon Nanotube (SWNT) in the active layer. Here the carboxyl group functionalized SWNT is introduced as an interfacial modifier within the active layer. , the overall power conversion efficiency was improved by 75% due to the introduction of the SWNT –COOH, compared to control device. Why metal oxide/ Polymer Solar Cells ? Favorable aspects of SWNT Materials and Device Design • SWNT posses favorable electrical and mechanical properties • Has been used in photovoltaic researches • SWNT played wide range of roles, for instance serving as • Transparent flexible electrode • Used to modify electrode • Buffer layer • Active component blended with polymer. • Chemical functionalization of carbon nanotubes provides • Improved solubility and processibility • Better alignment • It has also been shown that the COOH functionalised SWNT used as an interfacial layer provides improved photoinduced charge separation and transport of carriers to the collecting electrode. • Organic solar cells typically made from electron and hole transporting polymers. • But compared to inorganic materials, • electron mobility of photoactive polymers is extremely low • stability of photoactive polymers is poor Carboxyl group functionalized SWNT Diameter range=0.8-1.4 nm Length=>5 μm (micron). The SWNTs come in bundles of 10-30 Hybrid metal oxide/ polymer solar cells offer • Reasonable electron mobility • Stability in air • Ease of fabrication and low cost • Control of morphology due to the metal oxides  Compatible to the TiO2nanoporous of diameter ~ 20 nm SEM image of bundles of SWNT COOH functionalized SWNT P3HT TiO2 Nanoporous layer Device performance mainly limited by • Photogeneration rate • Interfaces Optical Characterization Current density - Voltage Characterization Possible roles of SWNT carboxylic acid functionalized single-walled Carbon nanotube Optical Characterization • Increased performance of the device with SWNT may be attributed to • Increased interfacial area • Organic P3HT – organic SWNT –COOH miscibility may be advantageous than organic P3HT – inorganic TiO2 • Faster electron path ways facilitated by SWNT –COOH after the exciton dissociation occurred at TiO2/Polymer interface • Note that electron mobility in SWNT –COOH is ~109 times that in TiO2 denoted by path (3) • Faster electron path ways by • scaffolding (1) • freeing from dead zones, supported by SWNT–COOH (2) • the electron transport through SWNT –COOH , if there is a possibility for exciton dissociating at SWNT –COOH /Polymer interface (4) • SWNT-COOH dipped in TiO2negligibly contribute to the visible range optical absorbance so as that of bare TiO2 - confirming the transparent nature of the SWNT-COOH in TiO2. Overall power conversion efficiency of the device with functionalized SWNT is increased by 75 % in comparison with the respective control device. • SWNT-COOH in this structure not affected polymer intake. Conclusion Overall power conversion efficiency of the device with SWNT is increased by 75% in comparison with its control. This may be due to the introduction of the SWNT –COOH in the hybrid TiO2 / polymer solar cells, attributed to the enhanced charge transport associated with reduction in the series resistance, reduced shunt pathways, reduced recombination and back direction electron transport, reduced exciton quenching increased charge separation efficiencies due to increased interfacial area, SWNT-COOH scaffolding, reduction in dead ends and incomplete pathways. Acknowledgment P.R, P.A.A and K.B thank National Science Foundation, Sri Lanka for the financial support