a The average values shown in parentheses were obtained from 15 devices.
To further understand the changes in above-mentioned photovoltaic properties, charge dynamics were systematically investigated. The photocurrent density (J ph) versus effective voltage (V eff) curves were firstly measured to explore the charge generation and extraction mechanism. Generally,J ph is defined as J LJ D, where J L andJ D are the photocurrent densities under illumination and in the dark conditions, respectively.V eff can be defined asV eff = V 0V bias, where V 0 is the voltage at which J ph = 0 andV bias is the applied voltage.[43] Thus, the exciton dissociation probability can be calculated as the ratio ofJ ph/J sat, whereJ sat is the saturation photocurrent density. As shown in Figure 3c, the P diss values ofTz-H - and Tz-Cl -based devices were calculated to be 91.7% and 96.1%, respectively. The results demonstrated that theTz-Cl -based device exhibited a higher exciton dissociation efficiency, which was well consistent with the higherJ sc value of 20.71 mA cm−2.
To investigate the charge transport properties in the active layers, hole and electron mobilities were measured by the space charge limited current (SCLC) method. As shown in Figure S2, the hole (μ h) and electron (μ e) mobilities of Tz-H -based blend film were estimated to be 2.35 × 10−5 and 4.06 × 10-5cm2 V−1 s−1 with aμ h/μ e ratio of 0.58. In contrast, Tz-Cl -based blend film exhibited not only enhanced (5.54 × 10−5 and 5.47 × 10-5cm2 V−1 s−1) but also balanced (μ h/μ e = 1.01) charge mobilities, which contributed to the observed higherJ sc and FF in the PM6:Tz-Cl -based photovoltaic device. In addition, the charge recombination process was investigated by measuring the light intensity (P light) dependent J sc. The relationship between P light andJ sc follows the formula ofJ scP lightα . The exponentα value approaches to unit, when free charge carriers are almost swept out and collected at the electrodes. As shown in Figure 3d, theα value was estimated to be 0.952 for the Tz-Cl -based device under the short-circuit condition, which is a little higher than that of PM6:Tz-H (0.927). The higher α for theTz-Cl -based device indicated its weaker bimolecular recombination that may contribute to the higherJ sc and FF relative to the Tz-H -based device.