Improved Hole Collection in Polymer Heterojunction Solar Cells with DNA/Pt-DNA Layers
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Improved Hole Collection in Polymer Heterojunction Solar Cells with DNA/Pt-DNA Layers Ali Bilge Guvenc1, Shirui Guo2, Cengiz S. Ozkan3, 4, Mihrimah Ozkan1 1 Department of Electrical Engineering, University of California-Riverside, Riverside, CA 92521, U.S.A. 2 Department of Chemistry, University of California-Riverside, Riverside, CA 92521, U.S.A. 3 Department of Mechanical Engineering, University of California-Riverside, Riverside, CA, U.S.A. 4 Materials Science and Engineering Program, University of California-Riverside, Riverside, CA, U.S.A. ABSTRACT In this study, we investigated the effects of DNA/Pt-DNA strands as hole collecting layers in polymer heterojunction solar cells based on ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al structure. We demonstrated that by introducing DNA or Pt-DNA layers between the polymer electrode (PEDOT:PSS) and the active layer (P3HT:PCBM), lead to an improvement in the hole collection efficiency and power conversion efficiency and the absorbance spectra of the devices indicate that Pt particles work as surface plasmons and increase the absorbance of the devices. INTRODUCTION Bulk heterojunction (BHJ) organic solar cells have attracted great attention due to the advantages of light weight, flexibility, tunable optical properties and low cost large area fabrication.1-3 The active layer of BHJ solar cells is fabricated by cascading solutions of conjugated polymers and fullerene derivatives or nanoparticles. Despite of their promising benefits, the power conversion of BHJ solar cells are limited due to their short exciton diffusion lengths of the materials in the active layer4, short drift length of the charge carriers5, non-ordered phase separation6, low dissociation probability of excitons and inefficient hopping carrier transport. Improving the performance of the BHJ solar cells has been resolved by adding nanostructures including carbon nanotubes7, nanorods8, nanoparticles9 and nanowires10. One other approach for improving the performance of the devices is enhancing the efficiency of the hooping carrier transport. This can be achieved by arranging the band energy diagram of the BHJ solar cell devices by introducing hole collecting and electron blocking layers into the device structure. The hole collection and electron blocking effect of deoxyribonucleic acid (DNA) complexes has already been shown.11 In this study, we presented metalized DNA complexes as hole collecting and electron blocking layer on the anode side of the devices. The metallization of the DNA strands with platinum (Pt) arranges the highest occupied molecular orbital (HOMO) level of the Pt-DNA complex to a level between 5.6 eV (work function of DNA12) and 5.9 eV (work function of Pt13 )14 and may work as a better hole collection. Additionally, the lowest unoccupied molecular orbital (LUMO) level of the mixture is assumed to be almost equal to the LUMO of the DNA12 and this energy difference may work as an electron blocking layer on the anode side of the device which definitely will improve the charge transport of the device.
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