Effect of Thermal Annealing on Charge Transfer States and Charge Trapping in PCDTBT:PC₇₀BM Solar Cells

Abstract

an increase in dielectric constant, thermal annealing also has a positive impact on the effectiveness of the CT states. More effective CT states were shown to facilitate exciton dissociation and charge carrier generation. Even though the resulting photogenerated current in annealed devices is higher, a signifi cant decrease in the device fi ll factor (FF) was observed, resulting in an overall decrease in device power conversion effi ciency. We found that the decrease in FF is resulted from an increase in monomolecular recombination due to the formation of mid-gap states determined from photothermal defl ection spectroscopy (PDS).The current density-voltage ( J-V ) characteristics for PCDTBT:PC 70 BM devices with and without annealing are presented in Figure 1 a. As shown in the fi gure, annealing at 150 °C results in a decrease in the overall device performance as has previously been reported. [ 8 ] The decrease in device performance after annealing is caused primarily by a decrease in FF from 56% to 46%, whereas the short circuit current ( J sc ) and open circuit voltage ( V oc ) remain mostly unchanged. A summary of the device parameters is given in Table S1 (Supporting Information). Interestingly, under reverse bias the current output for the annealed PCDTBT:PC 70 BM device is slightly higher than for the as-prepared device.In order to determine whether the current output for the annealed PCDTBT:PC 70 BM device at higher reverse bias is due to an increase in free carrier generation, the external quantum effi ciencies (EQEs) were measured at 540 nm as a function of reverse bias. The wavelength chosen for these measurements corresponds to a regime where the EQEs are the same for both devices under short circuit conditions (Figure 1 b inset). Figure 1 b shows the EQE under reverse bias for both devices in the fi eldassisted dissociation voltage regime. It is expected that the EQEs for the two devices should saturate at high reverse bias voltages where the IQE is approaching100%. [ 9 ] It is apparent from Figure 1 b that the carrier generation rate is higher for the annealed PCDTBT:PC 70 BM device and there are overall more available carriers to be collected compared to the device that was not annealed. The higher EQE at high reverse bias for the annealed device is in agreement with our J-V data confi rming that the charge generation rate is higher for the annealed devices. Even though the difference in performance observed under reverse bias is relatively small, it was consistently observed during the measurement of over 50 devices and has previously been observed in the literature. [ 10 ] We believe that this increase in charge generation after annealing is an indication of the formation of more effective CT states after thermal annealing.In order to confi rm that the increase in charge generation after annealing is not due to a dramatic change in the morphology of the blend, we examined the effects of thermal annealing on the morphological properties of the photoactive Thermal annealing has been commonly used i...

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“…As shown in Figure S1 (Supporting Information), both the ITIC and PC 71 BM devices show a higher photoresponse at energies below 1.55 eV compared to the SF‐PDI4 device. These results suggest that the CT states in PC 71 BM and ITIC‐based devices result in more efficient charge transfer and exciton dissociation, compared to that in the SF‐PDI4‐based device …”

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