Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy

Abstract

The carrier dynamics in organic photovoltaic (OPV) cells were investigated by impedance spectroscopy. We introduced a novel impedance spectrum representation called dynamic modulus plot (DMP), which allowed us to observe the layer-to-layer carrier injection behavior graphically. In this work, the impedance responses were characterized in the N,N'-diphenyl-N,N'-di-m-tolyl-4,4'-diaminobiphenyl (TPD)/C 60 p-n heterostructured OPV cells against applied voltages. The dependence of impedance responses on the layer thickness revealed a constant internal electric field that disturbed the carrier transport within the OPV cells. We applied this technique to new donor materials, in which thiophene units were inserted to the center of TPD. By increasing the number of thiophene units in TPD the fill-factor (FF) improved from 33% to 59%, which increased the power conversion efficiency (PCE). Based on the DMP analysis, we assigned the improvement in device performance to the reduction of the internal electric field.

Theory of carrier accumulation in organic heterojunctions

“…Next, we discuss the multi-layered devices. The carrier behavior in organic multilayered devices has been studied in detail by displacement current method and impedance spectroscopy [9,12,[18][19][20][21][22]. The organic materials used in these devices are insulating dielectrics with few carriers in the neutral state.…”

“…We have analyzed ultrathin organic multi-layered devices (UOMD) using impedance spectroscopy and reported that device physics in them is quite distinct from that of semiconductor devices [9][10][11][12]. One of the characteristic features of UOMD is the carrier accumulation.…”

Pressure-Induced Abnormal Electrical Transport Transition from Pure Electronic to Mixed Ionic–Electronic in Multiferroic BiFeO3 Ceramics

“…22,23 With regard to electrical transport properties, studies have seldom focused on the BFO under a high pressure, but this topic deserves attention because many practical applications are closely tied with electrical transport properties. 24 Based on the above consideration, we conducted an alternating current (AC) impedance spectroscopy investigation on BFO ceramics up to 40 GPa. Abnormal electrical transport behavior was found under the high pressure.…”

“…For instance, through neutron diffraction, Kozlenko et al. found that the pressure-induced structural phase transition to an antipolar phase of Pbam results in atomic displacements of antiferroelectric character and G-type antiferromagnetic order at 3 GPa. , In addition, Alexander et al discovered pressure-induced spin crossover and insulator–metal transition at 46 and 55 GPa, respectively. , With regard to electrical transport properties, studies have seldom focused on the BFO under a high pressure, but this topic deserves attention because many practical applications are closely tied with electrical transport properties …”

In-situ evaluation of the carrier mobility using Mott–Schottky-equivalent analysis in organic light emitting diodes

“…4,5) In order to apply the immittance spectroscopy to the analysis of OLED devices, we have devised a representation called dynamic modulus plot (DMP) to show the dependence on the bias voltage, which is a generalization of the Mott-Schottky (MS) plot used in the semiconductor physics. [6][7][8] We analyzed the step-by-step injection of a twolayered OLED and found that the accumulation should be considered as a special type of doping rather than the carrier injection from the electrode. 8) Then we developed a junction theory of the organic heterojunctions to describe it.…”