The complete electronic structure inside a practical organic photovoltaic (OPV) device consisting of a trilayer structure of copper‐phthalocyanine (CuPc), fullerene (C60), and bathocuproine (BCP) is demonstrated using low‐energy ultraviolet photoelectron spectroscopy (LE‐UPS) and photoelectron yield spectroscopy (PYS). The molecular orbital energy alignment and electrostatic potential distribution throughout the entire device is illustrated based on the LE‐UPS results. A favorable potential gradient to carry the photogenerated holes and electrons is manifested to be built spontaneously in the CuPc and BCP layers, respectively. Furthermore, the ultrahigh sensitivity measurements of LE‐UPS clearly unveil the distributions of faint density‐of‐states in the energy‐gap region in the organic films. Substantially barrierless contacts to both electrodes are fulfilled by the existence of these gap states. The electronic structure under simulated sunlight illumination is examined for the purpose of elucidating the electronic structures inside the working devices in the open‐circuit condition. These results indicate experimentally the electronic functionalities of each organic material, in particular of the BCP buffer layer, on the cell efficiency.

Low-energy photoelectron spectroscopy combined with photoelectron yield spectroscopy was developed to investigate the buried organic interfaces in the practical device thickness. Ultralow background signal and charging durability were achieved by utilizing monochromatic low-energy photons. C60/rubrene/Au interfaces were studied as a prototypical system of organic solar cells. The low density of gap states was detected in the rubrene film and a small feature due to the C60-induced morphological change was observed in the C60/rubrene interface.

The carrier extraction property of a prototypical small molecule organic solar cell (OSC) composed of copper phthalocyanine (CuPc), C 60 , and bathocuproine (BCP) was studied on the basis of the internal potential distribution and carrier dynamics in the device. The internal potential distribution in the OSC structure at the interfaces and in the bulk region was determined by the Kelvin probe method. Significant potential gradients were found in the CuPc film on indium tin oxide and in the C 60 film on CuPc, consistent with charge transfer through the contacts. Moreover, surface potential of the BCP layer grew linearly with increasing film thickness with a slope of ca. 35 mV/nm (giant surface potential: GSP), which indicated spontaneous orientation polarization in the film. The potential gradient and GSP significantly changed the built-in potential of the device. Current-voltage and modified time-of-flight measurements revealed that the BCP layer worked as an electron injection and extraction layer despite the wide energy gap. These results were discussed based on the contributions of GSP and the gap states in the BCP layer. V

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.

Carrier injection, which is a key factor in controlling and improving organic device properties, has been predominantly studied using the injection barrier height derived from HOMO and LUMO positions. The weak density of states (DOS) within the HOMO-LUMO energy gap is also important to understand the practical injection properties. In this study, the DOS of the α-NPD/electrode model interfaces are investigated using high-sensitivity UV photoemission spectroscopy. The nature of hole injection is discussed based on the observed DOS and a simple simulation. The results indicate that the weak DOS close to the Fermi level is critical for carrier injection