Plasmonic organic solar cells with gold nanoparticles
Date of Issue2015
School of Electrical and Electronic Engineering
Bulk heterojunction (BHJ) organic solar cells (OSCs) have many promising characteristics such as lower material cost, simple processing at ambient conditions and the possibility of roll-to-roll printing. The present major limitation of OSCs is relatively low power conversion efficiency (PCE) which is mainly caused by insufficient light absorption of thin active layers. In this thesis, the PCE of both polymer and small molecule (SM) OSCs are enhanced by increasing light trapping using the localized surface plasmon resonance (LSPR) effect of gold (Au) nanoparticles (NPs) embedded in the organic layers. Two types of sub-wavelength Au NPs were studied. The first is large-sized core shell Au-silica nanorods with a length of about 100 nm. The nanorods have dual LSPR peaks for which the major peak wavelength can be tuned by the aspect ratio (length:diameter). The second type of NPs is Au nanospheres with a diameter of about 10 nm and one LSPR peak at 520 nm when dispersed in chlorobenzene. Au-silica nanorods with major absorption peak at 720 nm and a silica shell thickness of 17 nm were directly blended into the regioregular poly (3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC60BM) BHJ layer of a conventional OSC device structure. At optimum nanorod concentration, the average PCE of the plasmonic device was enhanced to 3.5 ± 0.06% with an increase of 13% relative to the reference device due to increased optical absorption from 480 nm to 600 nm. When these nanorods were embedded within the BHJ layer comprising poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM), the PCE of 4.4 ± 0.11% was obtained with a relative enhancement of 25% with respect to the reference device. The significantly greater enhancement than that observed in P3HT: PC60BM device is due to the broader absorption and external quantum efficiency improvement from 500 to 800 nm, especially near the LSPR peak of nanorod. By using diffuse scattering measurements, the plasmonic enhancement in the PCPDTBT:PC70BM device is mainly ascribed to increased light scattering in thick shell large-sized nanorods. Since solution-processed SM donors have the advantage of monodispersity relative to polymer donors, Au-silica nanords with shell thickness of 5 nm and aspect ratio of 2.6 were introduced into OSCs based on 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6- ﬂuoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole):PC70BM (p-DTS(FBTTh2)2:PC70BM). A high PCE of 8.2 ± 0.12% with 27% relative enhancement in the p-DTS(FBTTh2)2:PC70BM device was achieved. Both current density and fill factor were higher than the P3HT:PC60BM and PCPDTBT:PC70BM devices because the major LSPR peak at 680 nm is well-matched with the absorption peak of p-DTS(FBTTh2)2. In addition, its well-balanced and high carrier mobility enhanced the carrier transport and collection as confirmed by electrical characterization. Finite-difference time-domain simulation showed that the localized electric field intensity outside the silica shell decreased with the increase of the silica shell thickness. As a result, both increased localized electric field intensity and increased light scattering were involved in PCE enhancement for a shell thickness of 5 nm. Furthermore, silica shell was proved that it can avoid the carrier recombination at the surface of Au nanorods by conducting light intensity dependence of short-circuit current density measurements. The optimum nanorod concentration was determined by decrease of carrier mobility and increase of BHJ layer surface roughness at high nanorod concentration. In order to realize further PCE improvement in the SM device, an alternative device architecture consisting of bare Au nanospheres incorporated into the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) anode buffer layer and Au-silica nanorods into the p-DTS(FBTTh2)2:PC70BM layer simultaneously was adopted. A high PCE of 8.5 ± 0.22% with a relative increase of 31% over the reference device was achieved. The combination of two types of Au NPs resulted in superior broadband absorption enhancement. Moreover, the fill factor was improved by Au nanospheres which can facilitate hole collection due to reduced series resistance.