DNA Based Thin Film As Hole Transport Layer In Bulk Heterojunction Polymer Solar Cells

In the field of organic solar cell, the conjugated polymer-fullerene bulk heterojunction concept is one of the ongoing research interests. In this work, the use of de-oxyribose nucleic acid/ hexadecyl trimethyl ammonium chloride (DNA/CTMA) thin films as hole transport layer in poly(3-hexylthiophene)-methanofullerene (P3HT-PCBM) bulk heterojunction polymer solar cell is demonstrated. P3HT-PCBM bulk heterojunction polymer solar cell was fabricated with a DNA complex layer replacing commonly used PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS). While PEDOT-PSS, as a hole transport and electron blocking layer, often improves the solar cell performance by improving fill factor, it requires careful engineering of spin-coating for the aqueous solution of PEDOT-PSS because of the hydrophobic nature of indium tin oxide on which PEDOT-PSS is generally deposited. Contrarily butanol solution of the DNA complex easily wets any oxide surface effectively forming DNA complex thin film. Subsequent spin-coating of P3HT-PCBM solution does not dissolve the DNA layer either. The band-gap of DNA complex layer is about 4.1 eV and it is transparent to broad spectrum of UV and visible light. The DNA complex layer is well known hole transport layer and its lowest unoccupied molecular orbital is quite high (1.1 eV) blocking electron quite efficiently.
In this work, we report the use of DNA complex layer in polymer solar cell structure for the first time and provide the indirect evidence of hole conduction in DNA complex layer. Optical absorption for individual layers was carried out and the band gap of DNA complex layer was extracted from the transmittance data. Cyclic voltammetry for DNA complex layer was carried out for the first time and the signal of oxidation was observed which relates to the oxidation of the guanine bases. From this oxidation peak, highest occupied molecular orbital (HOMO) level of DNA complex layer was found and used for the band alignment of the solar cell.