Correlation among the ionization potential, built-in potential, and the open-circuit voltage of multi-layered organic ph
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Correlation among the ionization potential, built-in potential, and the open-circuit voltage of multi-layered organic photovoltaic devices Eiji Itoh and Toshiki Shirotori Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano, 380-8553, ABSTRACT We have investigated the current-voltage characteristics of the multi-layered photovoltaic devices consisting of ITO/oxide /p-type (donor)/fullerene/ bathocuproine (BCP)/ Al structures. We chose various p-type (donors) small molecules and polymers in order to tune the values of ionization potential (IP) of donor molecules. The open-circuit voltage (Voc) increases with the increment of IP of donor materials. However, VOC was limited at ~0.6-0.7V for the devices without oxide layer. On the other hand, the VOC increases up to 0.9V for the devices with NiO and to ~ 1.1V for the devices with MoOX as a hole extraction buffer layer, respectively. We also estimated the work-function differences between Al and the oxide as 0.7, 0.9-1.0, and 1.2-1.3 eV for the device without oxide, with NiO, and with MoOX, respectively. We therefore concluded the value of VOC is limited by the lower part of VOC and energy difference between the LUMO of fullerene and the HOMO of donor ΔE. INTRODUCTION Organic semiconductors are considered as a potential candidate for fabricating low-cost, flexible, and efficient photovoltaic devices (OPVs). The power conversion efficiency (ηP) of solar cells depends on the open-circuit voltage (VOC), the short-circuit current density (JSC), and the fill factor (FF), and all of which still need improving for practical applications [1-7]. One of the strategies to improve the efficiency of organic solar cells with a given set of organic semiconductor materials is to insert an interfacial buffer layer between active layers and electrodes in order to improve the charge transport and extraction. Although PEDOT-PSS allows good adjustment of the work function of an indium tin oxide (ITO)/p-type semiconductor (donor) interface and smoothing of the ITO surfaces, PEDOT-PSS is problematic since it degrades under UV illumination and behaves as an acidic material in the presence of water molecules[8-11]. Recently, transition-metal oxides such as MoO3 [11-13], V2O5[11], WO3 [14], and NiOX [15] have been demonstrated as alternative effective buffer layers of PEDOT:PSS for organic photovoltaic device applications because of their optical and electronic properties. Moreover, the origin of the VOC is not clarified yet because the energy level difference in donoracceptor molecule interface, the built-in potential (Vbi) defined as the work-function difference of the electrode materials, the charge recombination, and the contact at electrode/organic interface are responsible. We considered that the insertion of oxide layers in multi-layered device with various donor materials is an effective way to control the Vbi and therefore straight way to investigate the origin of VOC and FF in OPVs. In this study, we have investigated the relationship between the ionization potential (IP)
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