Owing to the low dielectric constant of organic materials, organic photovoltaic (OPV) is regarded as an excitonic solar cell that excitons are generated upon photo-excitation. Such intrinsic small dielectric constant (ε) in organic materials results in large exciton binding energy (Eb). That becomes a key detrimental factor limiting the further improvement in organic photovoltaic cells. Increasing the material dielectric constant seems to be a straight-forward strategy to reduce the strong coulombic attraction of the photo-generated electron-hole pairs. Despite the matter of importance, there are limited reports in measuring the Eb and ε in organic photovoltaic materials and the correlation between the dielectric constant and the exciton binding energy is unclear. Here, we extend our demonstration by using quantum efficiency measurement  and electro-absorption to access the transporting gap and exciton binding energy in pristine organic photovoltaic materials for polymeric donor, fullerene and non-fullerene small molecular acceptors. It is found that Eb varies from 0.3 eV to 1.2 eV in those prototypical materials and it apparently follows a second power law with the inverse of the dielectric constant of the materials, i.e. Eb ∝ 1 / ε2. Instead of widely assumed first-order dependence, this second order dependent relationship is firstly reported. Interestingly, we have also found that the binding energy is more dependent on the molecular-molecular interaction rather than the intrinsic properties of single molecule. In this presentation, we will also demonstrate how the higher dielectric material benefits the exciton dissociation at donor/acceptor interface.
: Ho-Wa Li, Zhiqiang Guan, Yuanhang Cheng, Taili Liu, Qingdan Yang, Chun-Sing Lee, Song Chen, Sai-Wing Tsang, On the Study of Exciton Binding Energy with Direct Charge Generation in Photovoltaic Polymers, Adv. Electron. Mater., 2016, 2 (11), 1600200.
We demonstrate that poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) can act as an
excellent hole injection material for small organic charge transporters. With PEDOT:PSS as a conducting anode, it is
possible to achieve nearly Ohmic hole injection contacts to phenlyamine-based materials with HOMO values of up to
5.5 eV. In current-voltage experiment, the PEDOT:PSS anode can achieve nearly Ohmic hole injection to NPB (N,N'-
diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine), and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl) (1,1'-
biphenyl)-4,4'diamine). Meanwhile, dark-injection space charge limited current (DI-SCLC) transients are clearly
observed and are used to evaluate the charge-carrier mobility of these phenylamine compounds. The carrier mobilities
extracted by DI-SCLC are in excellent agreement with independent time-of-flight (TOF) technique. It is conceivable that
PEDOT:PSS can be used as a general conducting anode for the electrical characterizations of organic materials that
require Ohmic hole contacts.
We show that admittance spectroscopy (AS) can be used to determine charge carrier mobilities and transport parameters in materials relevant to organic light-emitting diodes (OLEDs). Via computer simulation, we found that a plot of the negative differential susceptance vs frequency yields a maximum at a frequency τr-1. The position of the maximum τr-1 is related to the average carrier transit time τdc by τdc = 0.56 τr. Thus knowledge of τr can be used to determine the carrier mobility in the material. Devices with the structure anode/phenylamines/Ag have been designed to evaluate their mobilities. The extracted hole mobility data from AS in pristine and doped material systems are in excellent agreement with those independently extracted from time-of-flight (TOF) technique. In addition, materials with different energy levels of highest occupied molecular orbital (HOMO), are further examined in order to study the effects of injection barrier on the extracted mobility by AS. In the case of an Ohmic hole contact (e.g. ITO or Au /m-MTDATA), the mobility data is good agreement with TOF results. However, for a non-Ohmic contact, the extracted mobility appears to be smaller. Thus AS can be used a means of evaluating the quality of electric contact between the injection electrode and the organic material.