Capacitance measurements to directly investigate exciton behaviors in organic photovoltaic materials

Tristep Mechanism To Explain the Illuminated C–V Characteristic of an Organic Device

“…The 5 nm Al layer on the ITO electrode was used to efficiently modify the ITO work function in order to form symmetric electrodes. 16 The normalized capacitances (ratio of measured capacitance to geometric value) of device A (without a 5 nm Al layer) and device D (with a 5 nm Al layer) are shown in Figure 2. It can been seen that the capacitance of the device with symmetric electrodes (device D, red circle) remained unchanged during the whole sweeping range from −15 to 15 V. The different structure of devices and device D shows obviously that the capacitance asymmetry is caused by the electrode materials.…”

“…C−V measurements of the parallel mode were conducted with a Keithley 4200 semiconductor charaterization system with a 4210-CVU, and the ac signal was set at 1 kHz, 50 mV. 16 Forward bias is defined as the potential of the ITO anode being positive. In the illuminated mode, C−V measurements were conducted on samples under illumination generated by a solar simulator (AM 1.5G).…”

“…13−15 Therefore, the capacitance−voltage (C−V) method can reveal rich information on carriers and excitons in organic semiconductors. 16−20 Through C−V measurements in the dark, the concentration of free carriers in a pristine C 60 layer 16 and the electric-field-assisted generation of mobile holes from CT complexes formed at the CuPc/MoO 3 interface have been studied, both qualitatively and quantitatively. 19,20 The illuminated C−V method has been introduced to investigate the influence of trapped and interfacial charge accumulation in organic optoelectronic devices, 21−23 but a mechanism involving multiple physical processes to explain the C−V characteristic of organic devices is not yet available.…”

“…In recent years, ultrafast broadband pump–probe spectroscopy was used to monitor the exciton dissociation process. − However, these transient technologies (TPV and ultrafast pump–probe spectroscopy) could not be used to effectively detect exciton dissociation in steady working devices. In the current-injection-prevented device, the measured capacitance is sensitive to space charges trapped or accumulated at the inerface and their spatial distribution. − Therefore, the capacitance–voltage ( C – V ) method can reveal rich information on carriers and excitons in organic semiconductors. − Through C – V measurements in the dark, the concentration of free carriers in a pristine C 60 layer and the electric-field-assisted generation of mobile holes from CT complexes formed at the CuPc/MoO 3 interface have been studied, both qualitatively and quantitatively. , The illuminated C – V method has been introduced to investigate the influence of trapped and interfacial charge accumulation in organic optoelectronic devices, − but a mechanism involving multiple physical processes to explain the C – V characteristic of organic devices is not yet available. However, an in-depth study of the steady illuminated C – V characteristic is essential to understanding the underlying physical process in organic optoelectronic devices and further improving the device performance.…”

“…Therefore, the increase of measured capacitance is closely related to the existence of an interface favorable to exciton dissociation.To study the effect of different electrode materials on the asymmetric capacitance characteristic, device D with a structure of ITO/Al (5 nm)/Bphen (100 nm)/NPB (50 nm)/Bphen (100 nm)/Al (100 nm) was prepared. The 5 nm Al layer on the ITO electrode was used to efficiently modify the ITO work function in order to form symmetric electrodes 16. The normalized capacitances (ratio of measured capacitance to geometric value) of device A (without a 5 nm Al layer) and device D (with a 5 nm Al layer) are shown in Figure2.…”