The potential applications of organic materials in solar cells have been widely explored for the creation of inexpensive and flexible modules compared with their inorganic counterparts. However, the power conversion efficiency (PCE) of organic solar cells (OSCs) have been severely restricted on account of the insufficient light absorption in the organic active layers. A simple method to achieve higher absorption efficiency is to increase the thickness of the active layer, but considerable electrical loss can occur during charge transport to the electrodes. Therefore, it is necessary to seek some effective ways to enhance light absorption in active layer without increasing its thickness. OSCs with inverted configuration usually present higher PCE and longer lifetime than corresponding devices with regular configuration. In this study, we demonstrate an improvement in photovoltaic properties in inverted OSCs by introducing the patterned structures in the active layer (PTB7:PC70BM) using a nano-imprinting technique with a PDMS stamp. By adjusting pressure of imprinting the active layer, the imprinted OSCs were optimized, showing the optimal optoelectronic performances. The results indicated when the imprinted cell was pressed by a 200 g weight, the absorption of the nanoimprinted cell dramatically increased compared with the control cell. Meanwhile, the fill factor (FF) also increased from 68.0% for the control to 70.0% for the optimal imprinted cell. In addition, the open voltage (Voc) was maintained in 0.73 V. Overall, the PCE of 6.95% with a 6.0% enhancement compared to the control cell (6.54%) was achieved.
Organic solar cells (OSCs) are very attractive as a clean and renewable energy technology owing to their advantages of low cost, abundant material sources, good flexibility, etc. Nevertheless, OSCs are faced with the contradictions between the optical and electrical properties. Their low absorption efficiency requires a thick active layer for efficient light harvesting, while the short carrier transport distance implies a thin active layer is necessary for efficient charge extraction. One way to solve this contradiction is to effectively enhance light absorption in the active layer without increasing the thickness. In this work, a core-shell structured plasmonic nanoparticles in the form of Au nanorod core coated with a SiO2 shell (in short of Au NR@SiO2) were introduced at the interface between the active and the cathode buffer layer of an inverted OSC based on PTB7:PC70BM active layer. By adjusting the concentration of the plasmonic nanoparticles of Au NRs@SiO2, we optimized the optoelectronic performances of OSCs. The results indicated when we spin-coated 1 pM Au NRs@SiO2 on top of the buffer layer, the device performances were optimized with the short circuit current increasing significantly while the open circuit voltage bearing negligible change. Overall, the power conversion efficiency of the OSC increases from 6.52% to 7.03%, corresponding to an enhancement of 8% as compared to that of the structurally identical control cell without Au NRs@SiO2. The performance improvement in inverted OSCs is mainly resulted from efficient light trapping effect of the core-shell plasmonic nanoparticles.
It has been demonstrated experimentally that the presence of metallic nanoparticles (MNPs) in the active layer assists in improving the power conversion efficiency of organic solar cells (OSCs), due to the combination of favorable optical as well as electrical effects. In this work, the optical effects of two different spherical MNPs (Ag and Au nanospheres) on absorption enhancement in the active layer with the optimal thickness are analyzed in detail using finite-difference time-domain simulation. The results reveal clearly that the absorption enhancement in the OSCs is dependent on both the properties of MNPs and the types of the donor/acceptor blend systems. We conclude that Au nanospheres are less effective as compared to Ag nanospheres on absorption enhancement in OSCs, and large sized MNPs are favorable for light trapping in the organic active layer due to the prominent plasmonic excitations. For a low bandgap polymer PSBTBT:PC71BM blend system incorporating Ag nanospheres, a 11.2% increase in the integrated absorption is obtained due to the excitation of magnetic and electric resonances of surface plasmons. This work could contribute to the development of high efficiency plasmonic OSCs.
A high performance proximity sensor that integrates a front semitransparent organic photodiode (OPD) and an organic light-emitting diode (OLED) is demonstrated. A 0.3-nm-thick plasma-polymerized fluorocarbon film (CFX)-modified thin silver interlayer, serving simultaneously as a semitransparent cathode for the OPD and an anode for OLED, is used to vertically connect the functional organic electronic components. A microcavity OLED is formed between a semitransparent Ag/CFX interlayer and the rear Al cathode enhancing the forward electroluminescence emission in the integrated device. The semitransparent-OPD/OLED stack is designed using an optical admittance analysis method. In the integrated sensor, the front semitransparent OPD component enables a high transmission of light emitted by the integrated OLED unit and a high absorption when light is reflected from objects, thereby to increase the signal/noise ratio. The design and fabrication flexibility of an integrated semitransparent-OPD/OLED device also has cost benefit, making it possible for application in organic proximity sensors.
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