To improve power conversion efficiencies (PCEs) of organic solar cells (OCSs), it is critical to reduce the relatively large photon energy loss (E_loss) which is defined by E_ex−eV_OC, where E_ex and V_oc are the optical bandgap (the exciton energy) and the open-circuit voltage (V_OC), respectively. The E_loss is typically 0.7–1.0 eV in high-efficiency OSCs, while those of ca. 0.6 eV have been recently reported by several groups [1]. However, the mechanism of the energy loss has not yet been fully understood.

In the operation of OSC, the exciton generated by the photo-absorption in an organic layer is separated through the charge-transfer (CT) state at the donor/acceptor interface to the free carriers of the hole in the donor and the electron in the acceptor, called the charge-separation (CS) state. The free carriers are collected by the electrodes where the difference in the quasi Fermi levels at the electrode is V_oc. Thus, the energy of each step is necessary to understand the mechanism of the energy loss. Compared with the energies of the excition and CT states as well as V_oc, the energy of the CS state has been only roughly evaluated. Basically, the CS energy is the sum of the ionization energy of donor I_D and the electron affinity of acceptor A_A. These values are usually estimated from the oxidation potential of the donor and the reduction potential of the acceptor measured using cyclic voltammetry, respectively. Recently, it has been pointed out that the energy levels around the donor and acceptor interface can depend on the molecular orientation (or the orientation of polymer backbone in polymers) and roughness at the bulk-heterojunction [2]. Such variation of energy levels is explained by the electrostatic interaction between the molecular ion and the permanent charge of the surrounding molecules which depends on the molecular orientation and the macroscopic shape of the film. Thus the CS energy should be determined by I_D and A_A of the film with the bulk-heterojunction structure instead of I_D and A_A of the individual materials measured in the solution.

In the previous work, we have demonstrated that the energy levels of hole and electron states can be precisely determined by use of the ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS) [3,4], respectively. Further, the electrostatic energy has been evaluated from the UPS and LEIPS data [5,6]. In this work, extending this idea, we demonstrate a procedure to determine the energy of the CS level in the bulk-heterojunction. We apply this method to the low-energy loss OSC, the bulk-heterojunction of PNOz4T:PCBM [1] and the result was compared with that of the high-efficiency OSC with the ordinal E_loss, PNTz4T:PCBM. The determined CS energy is found to be higher by 0.1-0.2 eV than the CT energy in each D-A combination indicating that difference in the energy loss between the ordinary and the low energy loss OCSs mainly occurs at the process from the exciton to the CT states. The resultant energy increase between the CT and CS states will also be discussed in terms of a role of the entropy [7].