Enhancing the photovoltaic performance of inverted type organic solar cells by co-introducing carbon quantum dots and isopropanol additive in photoactive layer

Abstract

The introduction of carbon quantum dots (CQDs) and isopropanol (IPA) solvent additive in photoactive layer has been found to enhance the outdoor photovoltaic performance of inverted type organic solar cell (OSC). However, the combined effects of IPA solvent additive and CQDs on the outdoor and indoor photovoltaic performance of inverted type OSCs have not been explored. In this study, the photoactive layer consisted of poly(3-hexylthiophene) (P3HT) as donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor was used. The CQDs were first dispersed in IPA before being incorporated into the photoactive layer solution. The structure of the OSC was fluorine-doped tin oxide (FTO)/ZnO/P3HT:PCBM:CQDs:IPA/Ag. The FTO, ZnO and Ag acted as cathode, electron transport layer, and anode, respectively. In this study, spin coating method was utilized to prepare OSC devices. The I-V measurement under 1-sun illumination and white LED illumination, and incident photon to current conversion efficiency (IPCE) measurements were carried out to the device. While UV-Vis spectrometer, photoluminescence (PL) spectrophotometer, field emission scanning electron microscopy (FESEM) and atomic force microscope (AFM) characterizations were conducted to thin film samples. The first objective of this study focused on the effect of CQDs and IPA under 1-sun and white LED illumination. The power conversion efficiency (PCE) of pristine P3HT:PCBM device, device with P3HT:PCBM:40 vol% IPA, and device with P3HT:PCBM:2 wt% CQDs:40 vol% IPA were 0.50%, 0.76% and 1.00%, respectively under 1-sun illumination. Interestingly, under white LED, the PCE of above devices were 3.42%, 1.84% and 2.97%, respectively. It is believed that the introduction of IPA additive mainly improves the phase separation and P3HT crystallinity, whereas the CQDs contributes to the enhancement of optical absorption, exciton dissociation and charge transport of the OSC. However, under indoor white LED illumination, it is worth noting that the device with pristine photoactive layer exhibited the best photovoltaic performance compared to the devices with IPA and CQDs, possibly due to the less dominant effect of charge recombination under weak illumination intensity. The second objective focused on the optimization of CQDs doping concentration (1 wt%, 2 wt%, 3 wt% and 4 wt% with respect to P3HT:PCBM weight). Under 1-sun illumination, the device with 2 wt% CQDs/40 vol% IPA exhibited the highest PCE, an increment of almost 100% relative to that of pristine device. Finally, the effect of P3HT:PCBM:CQDs:IPA photoactive layer solution concentration was investigated using concentrations of 24, 36, 48 and 60 mg/mL. The result showed that the optimum solution concentration was different under outdoor and indoor light illumination. Under 1-sun illumination, device with solution concentration of 36 mg/mL exhibited the highest PCE 0.95% due to low recombination loss. Under white LED, device with solution concentration of 48 mg/mL showed the best performance with PCE of 3.59% due to efficient light absorption and exciton generation.