Charge injection and transport studies of poly(2,7-carbazole) copolymer PCDTBT and their relationship to solar cell performance

The charge injection and transport properties of a high performance semiconducting polymer for organic photovoltaic (OPV) applications, poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), are investigated by time-of-flight (TOF) and dark-injection space-charge-limited current (DI-SCLC) techniques. OPV cells employing PCDTBT are known to possess power conversion efficiency (PCE) exceeding 6% [1,2]. While TOF probes only the hole mobilities of a thick film, DI-SCLC is shown to be useful down to a sample thickness of ∼200 nm, which is comparable to thicknesses used in OPV cells. We show that for pristine PCDTBT, the hole mobilities for both thick used in TOF and thin films for DI-SCLC are essentially the same, and they are in the range of 0.4-3.0 × 10^-4 cm²/Vs at room temperature. Both poly(3,4-ethylene dioxythioplene) doped with poly(strenesulfonate) (PEDOT:PSS) and molybdenum (VI) oxide (MoO₃) form quasi-Ohmic contacts to PCDTBT with better hole injection from MoO₃. Furthermore, the Gaussian Disorder Model (GDM) was employed to analyze the hopping transport of PCDTBT thin films. We show that PCDTBT possesses a relatively large energetic disorder (σ) of ∼129 meV, which is significantly higher than the σ of poly(3-hexylthiophene) (P3HT) processed under similar conditions. The correlation between σ and OPV device performance is addressed.