Defect Characterization in Organic Semiconductors by Forward Bias Capacitance–Voltage (FB-CV) Analysis

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.