Fabrication of Hybrid Bulk Heterojunction Solar Cells based on inorganic Aerogels as acceptors and organic Porphyrin dyes as donors

Hybrid bulk heterojunctions provide a technically and economically credible alternative to conventional silicon based photovoltaic technologies. The best efficiency of organic based solar cells to date is limited to 5 to 6 % which is about three times less than that of silicon based solar cells. Therefore there is still a strong need for novel materials which can improve the carrier transport and stability of bulk heterojunction solar cells. The main goal of this work was to fabricate hybrid bulk heterojunction solar cells using inorganic aerogels as electron acceptor materials and organic porphyrin dyes as electron donor materials. Porphyrin dyes are a sunlight absorber, electron donor, and hole-transporter, while aerogels are an electron acceptor and electron-transporter. The transport of carriers in this type of solar cell depends mainly on the morphology of the material, the processing temperature and the solvents used. Six types of hybrid bulk heterojunction solar cells were fabricated. The active layer was one of three inorganic oxides (TiO2, SnO2, and (Sn+Ti)O2) as electron acceptors and one of four novel organic porphyrin compounds (SBPorH2, SBPorZn, H2TPP(OH)4 and H2TPP(OMe)4) as electron donors. The fabrication process included synthesis of aerogels by sol-gel and spin coating methods. The metal electrodes were deposited using thermal evaporation. Highly transparent nanostructured aerogel films with transmission in excess of 90 % and nanocrystals of about 15 nm in diameter were developed. The photoactive layers were characterized by atomic force microscopy, UV-visible spectroscopy, X-ray Diffraction and photocurrent measurements. The conversion efficiency of the devices was quite low as 0.4 % and was highly dependent on the dye attachment to aerogel films which was controlled by the hydrophilic nature of the porphyrins. The low efficiency was attributed to a low shunt resistance resulting from the reaction between Al back contact and porphyrin dye. Future work should include depositing a very thin (< 1 nm) LiF insulating layer between the photo-active layer and Al back contact which may increase the shunt resistance since LiF will prevent Al reaction with the dye. A polyaniline layer should also be deposited between the Al counter electrode and photo-active layer to improve hole transport due high hole mobility of polyaniline.