Light-induced electron spin resonance study of galvinoxyl-doped P3HT/PCBM bulk heterojunctions

We report the effects of doping of P3HT/PCBM layers with spin 1/2 radicals of galvinoxyl (Gx) based on light-induced electron spin resonance (LESR), photoluminescence-detected magnetic resonance (PLDMR), and post-annealing experiments. LESR showed both a P3HT + and PCBM − signal for undoped P3HT/PCBM; however, as Gx doping increased (above ∼1 wt%), only the P3HT + signal was evident in the LESR spectra, with no PCBM − signal. The PLDMR exhibited a strong narrow signal at g = 2.002 that originates from nongeminate polaron pairs; no triplet PLDMR signal has been observed throughout the whole range of Gx concentrations (x = 0, 0.1, 1, 2, 4, 12 wt%). Adding Gx to ∼3 wt% led to a decrease of the PL-enhancement. There was big difference between the slow-dried P3HT/PCBM samples and the post-annealed samples. For the slow-dried samples, efficiency monotonously decreased with Gx additives. When post-annealed, however, an enhancement in η was observed at ∼2 wt% for P3HT/PCBM(1:2) samples. The LESR spectra for post-annealed samples revealed disappearance of Gx spin signals, and thus no spin interactions with PCBM − spins. It is unlikely that the increase of efficiency after Gx doping of P3HT/PCBM solar cell is due to an increase of triplet states.