Synthesis and characterization of organic conjugated polymer/ ZnO nano-particle films for solar cell application

<br>Hybrid thin films show great promise as a novel material for solar energy conversion devices. Using present knowledge from polymer bulk hetero-junction solar cells, the design of a simple, and potentially low cost, hybrid solar cell based on metal oxides and conjugated polymers is presented. The hybrid material properties are often superior to the sum of the intrinsic properties of the components and often have a functionality that is not present in either of the individual materials [1]. In this research, hybridization of organic conducting polymer and metal oxide semiconductor on nanometer scale in fabricating of a bulk hetero-junction solar cell will be investigated. ZnO nanoparticles were chosen as nanoparticle semiconductors and Poly(3-hexylthiophene-2,5-diyl) (P3HT) is used as a conductive polymer. ZnO and P3HT are individually very important functional materials.<br><br>Additionally, ZnO nanoparticles have transparency over the whole visible region and have good electron mobility compared to other wide band gap materials such as TiO2 [61,62]. The P3HT semiconductor polymer is important because of its high absorption coefficient in the visible region. In addition to the aforementioned intrinsic properties of these functional materials, in the hybrid of ZnO/P3HT energy levels of ZnO nanoparticles and P3HT polymer are matching and the hybrid layer can act as an active layer to absorb the visible solar light and release the electric charges. Uniform ZnO nanoparticles with small particle size distribution were synthesized. High resolution TEM imaging was performed to study the size and shape of the ZnO nanoparticles. The synthesized particles mainly range between 5-7 nm. To prepare the ZnO/P3HT hybrid solution we optimized the dispersion of the ZnO nanoparticles in the methanol/chloroform mixture solution (avoiding ZnO agglomeration). We obtained good dispersion of ZnO nanoparticles in a mixed solution of 4.5% methanol and 95.5% chloroform. Glass coated with ITO was used as a substrate followed by a poly (3,4ethylenedioxythiophene) poly (styrene sulfonate) (PEDOT:PSS) layer, spin coated on the ITO substrate. <br><br>Above the PEDOT:PSS layer we spin coated a hybrid (ZnO/P3HT) layer followed by depositing aluminium electrodes using the metal evaporation technique. We studied the quality of the film formation and the aluminium electrodes using optical and scanning electron microscopies. Also we studied the absorption and emission hybrid films using UV-Visible and florescent spectroscopies. By changing the blend (ZnO/P3HT) concentration and the ratio of ZnO and P3HT as the initial parameters we could optimise the optical properties. 5 mg/mL ZnO nanoparticles and 10 mg/mL P3HT concentrations with 50% ZnO and 50% P3HT ratio resulted in the best optical characteristics in the hybrid films. The concentration of the PEDOT:PSS was also optimised. We found that 0.65% PEDOT:PSS concentration results the highest electrical measurements of the prepared Bulk heterojunction solar cell. Using the optimised parameters we prepared the bulk hetero-junction solar cells and conducted the electrical tests including incident photon to electron conversion efficiency IPCE and J-V characterization. We found that optimized parameters resulted in 19% IPCE and 1.13 mA.Cm-2 ISC and 0.23 VOC. We chose three prepared solar cells; 1. best performing with 50% -50% ZnO/P3HT ratio , 2.worst performing with 50% -50% ZnO/P3HT ratio 3. solar cell with 30%-70% ZnO/P3HT ratio) to study the morphology of their hybrid layer using AFM and surface profiler. Up to some extend we could understand and relate the hybrid film morphologies to the obtained electrical performance of these three bulk hetero-junction solar cells. In compar to other lab-prepared bulk hetero-junction solar cells the best performing bulk hetero-junction solar cell has a better hybrid film uniformity. Also in the best performing bulk hetero-junction solar cell the roughnesses and mean granular sizes of the film over the surface were very similar. We used AFM to correlate the quality of the film surface with the electrical performance and found that uniform and continuous films are very important in the performance of the solar cell device.<br><br><br><br>