Defect Characterization in Organic Semiconductors by Forward Bias Capacitance-Voltage (FB-CV) Analysis Biswajit Ray, † Aditya G. Baradwaj, ‡ Bryan W. Boudouris, ‡ and Muhammad A. Alam* ,† † School of Electrical and Computer Engineering and ‡ School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States * S Supporting Information ABSTRACT: Transport in organic semiconductors (OSCs) generally is poorer relative to their inorganic counterparts, mainly due to the high defect density that trap the free charge carriers. In this article, we demonstrate a new defect characterization method based on forward bias capacitance- voltage (FB-CV) measurements, which is appropriate for a broad range of low mobility OSCs with relatively large (>1.5 eV) band gaps. The characterization method, developed using numerical modeling and experimental data, relates the capacitance peaks in the FB-CV sweep to the deep level defect states; these states are inaccessible to classical reverse bias (RB) impedance spectroscopy. We validate the proposed technique by interpreting FB-CV data for organic photodiodes made of a commonly used semiconducting polymers, poly(3-hexylthiophene) (P3HT), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV), and copper(II) phthalocyanine (CuPc). We find that P3HT and MEH-PPV contain both shallow and deep level states, but deep traps in CuPc depend on process conditions, consistent with reports in the recent literature. We demonstrate that these deep traps corrupt the interpretation of the classical Mott-Schottky analysis (of RB-CV data), leading to an underestimation of the built-in voltage of a device. O rganic semiconductors (OSCs) are widely used in many applications such as light emitting diodes, 1 solar cells, 2-5 and transistors. 6 As van der Waals solids, 7 OSCs form disordered films with weak lattice forces that results in a large number of defect states 8-10 within the energy band gap, E g . These defects scatter and trap free carriers, so that the carrier mobility is reduced and recombination is enhanced. 11-13 Thus, it is important to understand the origin of these defects and characterize the energy levels and the density of these defect states. In this manner, the design of new OSCs with improved transport properties will be facilitated. The energy levels of defect states within the band gap of a material generally are characterized by impedance spectroscopy (IS) or, more specifically, capacitance-frequency (C-f) measurements at zero bias (the horizontal line separating RB and FB regions in Figure 1). 14-18 The IS analysis correlates the transition frequencies (ω i ) with the defect energy levels (E i ) according to the following relationship: E i = kTln(ω 0 /ω i ), where ω 0 is a material dependent constant, 14 T is the temperature, k is the Boltzmann constant, and E i is referred from the HOMO level of the OSC. For deep level states, ω i values are exponentially suppressed below the measurement window (W 1 in Figure 1a). The shallow states can be measured only if their ω i values are lower than dielectric relaxation frequency (ω dr ). 19 For OSCs, however, ω dr values character- istically are very low, which precludes the use of IS analysis for defect level characterization 20 (see W 4 in Figure 1a). As an illustrative example, in Figure 1b we show the measured IS data for a P3HT photodiode, which shows a single transition frequency corresponding to ω p ≈ 10 kHz. Therefore, classical IS analysis has limited applicability for measuring the trap levels of typical OSCs (e.g., poly(3-hexylthiophene) (P3HT), poly[2- methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH- PPV), copper (II) phthalocyanine (CuPc)). The validity of IS technique and the corresponding measurement window (ω min , ω dr ) are dependent on the material, device, and the measurement system. 10,15,21-23 This failure of the widely used IS method led the community to look for alternative techniques. For example, Nicolai et al. 12 have developed a space charge limited current-voltage (SCL- IV) measurement technique to determine the deep level defect states in OSCs. Unfortunately, their results can be consistently interpreted only for specialized test structures with relatively small built-in voltages (V bi ). It is not clear if the technique can be generalized to interpret defects in Schottky junction or PN junction diodes. In addition, the SCL-IV method cannot distinguish between deep and shallow level states and therefore is not suitable for devices with mixed and/or distributed defect levels. In this work, we develop a new defect characterization technique based on a FB-CV measurement, which is ideally Received: June 3, 2014 Revised: July 16, 2014 Article pubs.acs.org/JPCC © XXXX American Chemical Society A dx.doi.org/10.1021/jp505500r | J. Phys. Chem. C XXXX, XXX, XXX-XXX