Optical, Electrical, and Magnetic Studies of Organic Solar Cells Based on Low Bandgap Copolymer with Spin ½ Radical Additives. (18th February 2015)

• Record Type: Journal Article
• Authors: Basel, Tek; Huynh, Uyen; Zheng, Tianyue; Xu, Tao; Yu, Luping; Vardeny, Z. Valy
• Is Part Of: Advanced functional materials. Volume 25:Number 12(2015)
• Page Start: 1895
• Page End: 1902
• Publication Date: 2015-02-18
• Subjects: organic photovoltaics -- charge transfer processes -- charge loss mechanisms -- magneto‐photoinduced absorption; Materials -- Periodicals; Chemical vapor deposition -- Periodicals; 620.11
• DOI: 10.1002/adfm.201403191
• Languages: English
• ISSNs: 1616-301X

Abstract

Abstract : 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 spin½ radicals Galvinoxyl to the D–A blend is presented; this enhances the cell PCE by ≈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. Abstract : Charge transfer process in an organic photovoltaic (OPV) cell is studied in thin films and devices of a low bandgap polymer. Major loss in copolymer‐based OPV devices is the formation of triplet excitons in the polymer donor from 3 CT at the donor–acceptor interfaces. A method is presented to circumvent this process by incorporating spin ½ additives.