AVS 58th Annual International Symposium and Exhibition
    Energy Frontiers Focus Topic Wednesday Sessions
       Session EN+NS-WeM

Paper EN+NS-WeM12
Correlation of Interfacial Electronic Structures and Open Circuit Voltages in Organic Solar Cells

Wednesday, November 2, 2011, 11:40 am, Room 209

Session: Organic Photovoltaics
Presenter: Mei-Hsin Chen, National Dong Hwa University, Taiwan, Republic of China
Authors: M.H. Chen, National Dong Hwa University, Taiwan, Republic of China
W.H. Tseng, National Taiwan University
J.Y. Wang, National Taiwan University
C.T. Tseng, National Taiwan University
C.I. Wu, National Taiwan University
Correspondent: Click to Email
In this study, the interfacial electronic structures and energy band diagrams of polymer/fullerene bulk heterojunction (BHJ) solar cells are investigated. To study the device performance influenced by the interlayer, poly(3-hexylthiophene) (P3HT) mixed with 6,6-phenyl C61-butyric acid methylester (PCBM) are used as an active layer and bathocuproine (BCP) and calcium (Ca) are incorporated respectively with the cathodes. Since the mechanisms leading to the improvement of power conversion efficiency are more chemically and electronically complicated and have not been understood, the ultraviolet and x-ray photoemission spectroscopy (UPS and XPS) are used to investigated the properties of energy band, electronic structures and the interface chemistry at the interface. For the devices incorporated with BCP, highest occupied molecular orbital (HOMO) level of PCBM is pulled down about 0.3 eV with respect to the Fermi level after the deposition of BCP. It indicates that the BCP layer could modify the energy band by shifting the energy level of acceptors, causing the increase of built-in potential (Vbi). Thus, the open circuit voltage (Voc) attributed to the increase of Vbi resulted from BCP could be enhanced. Moreover, AFM image indicates that BCP molecules cannot form a uniform layer on the active layer surface, pointing out the band modulation is the key reason to the Voc improvement. For the devices with Ca as cathodes, the UPS results with deposition of Ca illustrate a 0.8 eV-downward shift in energy levels of P3HT, but not in those of PCBM. Therefore, the energy difference between the HOMO of P3HT and the LUMO of PCBM is widened, resulting in the increase of Voc and the enhancement of device efficiency. Moreover, from the XPS spectra, there is an extra new peak appear at the lower binding energy about 162 eV, which suggests that the addition of electrons around the reactive sulfur (S) atoms in P3HT molecules after Ca deposition. The combinations of S and Ca spectra imply that the formation of Ca-S bonding at the Ca/P3HT interface, which is in good agreement with the findings of UPS spectra.