Lithography-Free Broadband Ultrathin Film. Photovoltaics

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1 Supporting Information Lithography-Free Broadband Ultrathin Film Absorbers with Gap Plasmon Resonance for Organic Photovoltaics Minjung Choi 1, Gumin Kang 1, Dongheok Shin 1, Nilesh Barange 2, Chang-Won Lee 3, Doo-Hyun Ko 4*, and Kyoungsik Kim 1* 1 School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea. 2 Korea Institute of Science and Technology, Hwarangno 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. 3 Samsung Advanced Institute of Technology, Suwon-si, Gyeonggi-do 16678, Republic of Korea. 4 Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 17104, Republic of Korea. * kks@yonsei.ac.kr, dhko@khu.ac.kr. S-1

2 Figure S1. Diagrams (left) and corresponding optical photographs of 2 cm 2 cm (right) throughout the transfer process. a. BOE etching of the silicon native oxide layer process to separate metal NPs grasped by PMMA mediator from Si substrate. b. Rinsing with plenty of DI water after separating PMMA-Au NPs film. c. Transferring PMMA-Au NPs film to spacerreflector substrate. d. Selectively lifting off PMMA layer by gently soaking in acetone. S-2

3 Figure S2. Result of the transfer process. Top-view SEM images of NPs 600 thermally dewetted after fast deposition before (a) and after (b) transferring process. Figure S3. Angle of orientation according to the major axis of the NP with respect to horizontal line of top-view SEM image. S-3

4 Figure S4. Major and minor axes diameters of S-700 (a) in C 0.8 and of F-400 (b), F-600 (c), and F-700 (d) in C >0.8. We plotted distribution histograms of major axis and minor axis diameters simultaneously as shown in Figure S4a d. We evaluated NPs of S-700 sample in C 0.8, as shown in Figure S4a, and the average major- and minor-axes diameters were 47.7 ± 17.7 nm and 28.0 ± 10.0 nm, respectively. The analysis of particles with circularity of >0.8 shows that the average major (minor) axes diameters are 77.6 ± 30.6 nm (56.8 ± 23.0 nm), 72.4 ± 20.1 nm (52.3 ± 13.4 nm), and ± 40.1 nm (80.6 ± 30.7 nm) for the samples F-400, F-600, and F-700, respectively. As a result, the aspect ratios of NPs are experimentally observed as 1.4, 1.4, and 1.3 for F-400, F- 600, and F-700. For example, F-600 have 33.9% NPs of hemiellipsoidal shape with a = 2b (aspect ratio = 2.0) and the other 66.1% NPs of hemispherical shape with a = 1.4b (aspect ratio = 1.4). S-4

5 Figure S5. a. SEM image employed for the particle analysis process by ImageJ. Figure shows fast deposited then annealed Au NPs at 500 (F-500) and the inset shows SEM image with higher magnification. b. Circularity distribution histograms of above depicted SEM image. c, d. Histograms of particle counts with respect to the major axis diameter (red columns) and minor axis diameter (blue columns) of each NP. The histograms display the statistic results of (a) for in C 0.8 (c) and in C >0.8 (d). Figure S5a is the SEM image of Ostwald ripened NPs after thermal dewetting at 500 (F-500) using a 4-nm thick nominal Au film with fast deposition rate of ca. 2 A /sec. The inset shows SEM image with higher magnification. For the fast deposited then annealed sample F-500, the circularity distribution histograms are presented in Figure S5b. The percentage of particles with circularity of 0.8 is 42.8%. We evaluate particles of F-500 sample, as shown in Figure S5 c, d. The average major and minor axes diameters are ± 54.4 nm and 75.5 ± 23.4 nm in C 0.8 (c) and 95.6 ± 38.8 nm and 71.6 ± 29.2 nm in C >0.8 (d), respectively. S-5

6 Figure S6. Normalized absorption spectra of S-700 and F-600 samples. Experimental normalized absorption of absorbers for spacer thicknesses of d = 10 nm (a) and d = 40 nm (b). Figure S7. Error bar included FWHM of measured absorption spectra of absorber samples with HSNPs (S-700) and with HENPs (F-600) with respect to the Al 2 O 3 spacer thicknesses. S-6

7 Figure S8. External quantum efficiency (EQE) of reference and Au NPs OPV device for active layer thickness of ~80 nm. The external quantum efficiency (EQE) curves are presented in Figure S8. Table S1. Characteristics of both bare and with Au NP OPV devices Devices Photoactive layer Thickness (nm) V oc a (V) J sc b (ma/cm 2 ) FF c (%) Efficiency (%) Bare W/ Au NPs Open circuit voltage, a short circuit current, b and fill factor c of each device. The parasitic absorption, i.e., heat dissipation in metal NPs is a big issue for the enhanced energy harvest in OPV. Recently, it is reported that the heat dissipation is strong for metal NPs smaller than 50 nm but significantly decreases for metal NPs larger than 50 nm. 1,2 Our NPs are much larger than 50 nm thus heat dissipation is not significant. S-7

8 Figure S9. Calculated solar photon flux-weighted absorption for HENP absorber (red lines, a = c = 100 nm, b = 50 nm) and HSNP absorber (blue lines, D = 35 nm) with 30 nm-thick photoactive layer (PDTP-DFBT:PC 71 BM), against to the two-layered structure (black lines) of photoactive layer-back electrode with same thickness. The 100 nm-thick (a) Au and (b) Ag layers are used as electrode. In FDTD simulation, we used the PDTP-DFBT:PC 71 BM 3,4 as a spacer material. 5 The photoactive material has low bandgap (LBG) energy of 1.38 ev. Because our MDM absorber with Au HENPs has strong broadband absorption from VIS to NIR range, we choose this LBG polymer of organic solar cell (OSC) device as photoactive layer to confirm feasibility of our absorber for practical applications. We simulated total power absorbed within the photoactive layer, 6 then calculated solar photon flux-weighted absorption in the wavelength range of 300 nm to 950 nm (Figure S9). Optical constants of the polymer were imported from literature, 4 though other simulation details are same with our previous calculations in Figure 1 in the main text. With our HENP absorber structure, we achieve 80.4% (70.7%) of enhancement for the spectrally integrated photon flux absorbed within the only 30 nm-thick photoactive layer compared to the two-layered structure without Au NPs with Au (Ag) back electrodes. These enhancement S-8

9 percentages are significantly higher than those of HSNP absorbers, 41.1% and 35.2% with Au and Ag electrodes, respectively. By comparing the integrated photon flux directly between HSNP absorber and HENP absorber, we can describe that the efficiency of a device will be improved by 1.3 times if we employ HENPs instead of HSNPs with both Au and Ag electrodes. To investigate the absorption enhancements we additionally calculate the electric fields distribution of simulation structure. In Figure S10, we observed that the electric field is highly confined within the ultrathin active layer propagating in z-direction with HENPs, whereas comparatively weak field confinement is observed with HSNPs. S-9

10 Figure S10. Simulated E z -field (λ = 800 nm) For the HENP and HSNP absorbers with 30 nmthick photoactive layer as a spacer. E z -field profile (a) in xz-plane at y = 0 nm and (b) in xy-plane (b) at z = 30 nm for HENP absorber (a = c = 100 nm, b = 50 nm). E z field profile (a ) in xz-plane plane at y = 0 nm and (b ) in xy-plane at z = 30 nm for HSNP absorber (D = 35 nm). S-10

11 Figure S11. SEM image of the ZnO film on Au nanoparticles/ito-coated glass substrate. REFERENCES (1) Fleetham, T.; Choi, J.-Y.; Choi, H. W.; Alford, T.; Jeong, D. S.; Lee, T. S.; Lee, W. S.; Lee, K.-S.; Li, J.; Kim, I. Photocurrent Enhancements of Organic Solar Cells by Altering Dewetting of Plasmonic Ag Nanoparticles. Sci. Rep. 2015, 5, (2) Lakowicz, J. R. Radiative Decay Engineering 5: Metal-Enhanced Fluorescence and Plasmon Emission. Anal. Biochem. 2005, 337, (3) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.- C.; Gao, J.; Li, G.; Yang, Y. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, S-11

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