Bilayer and bulk heterojunction solar cells with functional poly(2,2′-bithiophene) films electrochemically deposited from aqueous emulsion

Ultra thin films of poly(2,2′-bithiophene) with thickness of 11–33 nm were electrochemically deposited on poly(3,4-ethylenedioxithiophene):poly(styrene sulfonic acid) modified indium tin oxide substrates using a new approach for electrochemical synthesis. Ultrasonic emulsification of aqueous perchloric acid and 2,2′-bithiophene monomer is an effective method to disperse this water insoluble monomer in micro droplets that improved the contact with the electrode. Rigorous control of the poly(2,2′-bithiophene) thickness was achieved by controlling the potential applied and the charge during the deposition. We studied thin films solar cells based on poly(2,2′-bithiophene) polymer as bilayer with C60, resulting a power conversion efficiency of 0.3%, and as buffer layer in bulk heterojunction solar cells. We show that poly(2,2′-bithiophene) films influenced positively the open circuit voltage and increased about 1.6 times the short circuit current, increasing the efficiency from 1.3% up to 2.9% from devices without buffer layer or with 11 nm of poly(2,2′-bithiophene), respectively.