Charge Formation in Pentacene Layers During Solar‐Cell Fabrication: Direct Observation by Electron Spin Resonance

Abstract

Microscopic characterization of charge carriers in solar cells is useful for high-performance cell fabrication because the formation and accumulation of charges in cells greatly affect the device performance. Electron spin resonance (ESR) is suitable for such characterization because it can directly observe charge carriers with spins in these cells. In this work, the ESR method is applied to organic thin-fi lm solar cells to investigate charge formation in such devices. Heterojunction cells of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/pentacene/C 60 /bathocuproine (BCP)/Al are investigated. Clear ESR signals are observed by inserting a typical PEDOT:PSS hole buffer layer. From analysis of the dependence of the ESR characteristics on the external magnetic fi eld direction, the bias voltage, and the duration of solar-simulated irradiation, the charges (mobile holes) in pentacene layers are successfully identifi ed and it can be deduced that these holes are formed at the PEDOT:PSS/pentacene interface during device fabrication. This ESR analysis provides useful knowledge for understanding device operation and improving device performance at the microscopic level. 592 www.MaterialsViews.com www.advenergymat.de

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].…”

“…Anion formation decreases J sc and concomitantly decreases V oc [11]. Anion formation should also decrease V oc because of the shift in energy levels at the interfaces [19,20]. The expected decrease in V oc was, indeed, observed (see Fig.…”

“…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].…”

Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiOx layer: Microscopic elucidation of the light-soaking phenomenon

“…[9][10][11][12][13][14]16 To fully elucidate the light-soaking phenomenon, electron spin resonance (ESR) spectroscopy is the most suitable technique because ESR is a highly sensitive and nondestructive technique for directly observing the charges in organic semiconductors and their devices at the molecular level. [17][18][19][20][21][22][23][24] ESR studies have demonstrated a clear correlation between performance deterioration and charge accumulation (or trappings) in a normal-type OSC, 18 and have clarified degradation mechanism due to charge formation during device fabrication. 19 As noted above, the light-soaking phenomenon has been considered to be related to charge trappings.…”