Optical, Electrical, and Magnetic Studies of Organic Solar Cells Based on Low Bandgap Copolymer with Spin ½ Radical Additives

The charge photogeneration and recombination processes in organic photovoltaic solar cells based on blend of the low bandgap copolymer, PTB7 (fluorinated poly-thienothiophene-benzodithiophene) with C60-PCBM using optical, electrical, and magnetic measurements in thin films and devices is studied. A variety of steady state optical and magneto-optical techniques were employed, such as photoinduced absorption (PA), magneto-PA, doping-induced absorption, and PA-detected magnetic resonance (PADMR); as well as picosecond time-resolved PA. The charge polarons and triplet exciton dynamics in films of pristine PTB7, PTB7/fullerene donor-acceptor (D-A) blend is followed. It is found that a major loss mechanism that limits the power conversion efficiency (PCE) of PTB7-based solar cell devices is the back reaction that leads to triplet exciton formation in the polymer donor from the photogenerated charge-transfer excitons at the D-A interfaces. A method of suppressing this back reaction by adding spin1/2 radicals Galvinoxyl to the D-A blend is presented; this enhances the cell PCE by approximate to 30%. The same method is not effective for cells based on PTB7/C70-PCBM blend, where high PCE is reached even without Galvinoxyl radical additives.