Direct Observation of Hole Accumulation in Polymer Solar Cells During Device Operation using Light‐Induced Electron Spin Resonance

Abstract

A clear correlation between the number of accumulated holes (Nspin) in poly(3‐hexylthiophene) (P3HT) and the deterioration of the performance is observed in polymer solar cells under simulated solar irradiation. The sites of hole accumulation with deep trapping levels are formed at the interfaces between P3HT:[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) and poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) layers.

ESR Study of Degradation Mechanism Due to Charge Formation in Polymer Solar Cells

“…The g factor of 2.002 is consistent with that of radical cations (positive polarons) on P3HT in organic field-effect devices [23,24]. The ESR signal of the PEDOT:PSS and the P3HT overlapped with each other because of their similar g factors [19,20].…”

“…No change in N spin was observed, which indicates that the PCBM radical anions are stable, most likely due to the formation of PCBM -Li + complexes rather than due to electron trappings in the PCBM. Notably, the ESR signal for radical anions on PCBM cannot usually be observed at room temperature because of the rapid spin relaxation of the fullerene electrons at room temperature [20,23,27]. …”

“…The solar cells with active areas of 2 mm × 2 mm were fabricated on a 20 mm × 20 mm glass substrate for the J-V characteristics and those of 2 mm × 10 mm were fabricated on a 3 mm × 20 mm quartz substrate for the ESR measurements [19,20].…”

“…Electron spin resonance (ESR) is a promising method for microscopic characterization because it is highly sensitive and can be utilized to investigate organic materials and devices at the molecular level [17][18][19][20]. We previously studied the effects of thermal annealing on device performance using ESR [21].…”

Dependence of the Device Performance of Polymer Solar Cells on the Insertion of Metal Nanoparticle Layers at the Electron-collecting Electrodes

“…[1][2][3][4][5][6][7][8][9][10][11] In particular, the devices containing blend films of regioregular poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C 61 -butyric acid methyl ester (PCBM) have been widely studied, and have exhibited a high power conversion efficiency (PCE), almost 7.0%. 8 During the preparation of the P3HT:PCBM active layer, the mixed phase spontaneously segregates into interpenetrated domains, creating bulk heterojunction (BHJ) devices.…”