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.
We demonstrate an inverted low bandgap semitransparent organic solar cell with improved absorption as well as reasonable transparency perception based on a nanopatterned MoO3/Ag/MoO3 (MAM) multilayer film as the transparent anode under illumination from the MAM side. The integrated absorption efficiency of the active layer at normal hybrid-polarized incidence considering an AM 1.5G solar spectrum is up to 51.69%, increased by 18.53% as compared to that of the equivalent planar device (43.61%) and reaching 77.3% of that of the corresponding opaque nanopatterned device (66.90%). Detailed investigations reveal that the excitation of plasmonic waveguide modes (at transverse magnetic polarization) and photonic modes (at transverse electric polarization) are responsible for the observed enhancement in absorption. Importantly, the proposed device exhibits an average transmittance of up to 28.4% and an average transparency perception of 26.3% for the human eyes under hybrid-polarized light illumination along with a good color rendering property. Additionally, our proposal works very well over a fairly wide angular range.
The study of the transmission properties of subwavelength apertures has become a very active area of research in
electromagnetism. It is generally accepted that structuring the input surface of the metal film by periodic corrugations is
very effective in the process of transmission enhancement through single apertures. Here instead of periodic
corrugations, we propose to use periodic nano-strips placed before the input surface of the metal film to enhance the
transmission of light through a nano-slit milled in the film. Influences of the structural parameters of periodic nano-strips
on the transmission enhancement are investigated. The transmission efficiency through a 25nm-width silver nano-slit can
be boosted to be η = 164 when six pairs of nano-strips are placed 50nm distant away from the incident surface of the
silver film at λ0 = 1μm, which is originally η = 7.8 without any strips. This indicates that a large part of the incident light
can be transformed into the localized guided wave with strong intensity, and then more light can flow through the
nano-slit. We emphasize that periodic nano-strips can serve as an efficient receiving antenna to harvest light into the
nano-slit.
We propose to use a nearby metallic nano-particle to greatly enhance the transmission efficiency of a nano-slit aperture
in a metallic film. The metallic nano-particle helps to form a T shape cavity which could effectively transform the
propagating incident light into localized near-filed light. Harvest efficiency is enhanced by 20 times for a non-resonant
25 nm-width nano-slit.
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