Plasmonic Forward Scattering Effect in Organic Solar Cells

[Supporting information]

Plasmonic Forward Scattering Effect in Organic Solar Cells:

A Powerful Optical Engineering Method

Se-Woong Baek1,3, Jonghyeon Noh1,3, Chun-Ho Lee2,3, BongSoo Kim4, Min-Kyo Seo2,3, and Jung-

Yong Lee1,3,*

1Graduate School of Energy, Environment, Water, and Sustainability (EEWS), 2Department of Physics, 3KI for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea, 4Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul, 136-791, Republic of Korea *All correspondence should be addressed to J.-Y.L (email: [email protected])

1. Scattering efficiency of silver and gold nanoparticles

Figure S1. The calculated absorption and scattering power of an AgNP 70 nm in size (red circles) and an AuNP 70 nm in size (blue rectangles) embedded in PEDOT:PSS. The filled and the open symbols respectively denote the absorption and the scattering power of both types of NPs.

Figure S1 shows the calculated absorption and scattering power of AgNPs 70 nm in size (red circles) and AuNPs 70 nm in size (blue rectangles) embedded in PEDOT:PSS. The filled and open symbols denote the absorption power and scattering power, respectively. Although the scattering

1 power of the AuNPs has broader wavelength range between 500 and 700 nm, the scattering efficiency of AuNPs is much lower than that of the AgNPs and the absorption power of the AuNPs is greater than that of the AgNPs. Thus, the AgNPs are far more effective than the AuNPs in terms of scattering. We also fabricated plasmonic OPVs using AuNPs, but the power conversion efficiency

(PCE) enhancement was not as great as that of devices with AgNPs partly due to the lower scattering efficiency.

2. Optical properties of AgNPs: experiments and simulations

Figure S2. The normalized extinction spectra of the various sizes of AgNPs. The experimental (a) and optical simulation (b) results of size-controlled AgNPs are compared. AgNPs are dispersed in ethanol.

Figure S2 show the normalized extinction spectra of mono-dispersed AgNPs of various sizes. The experimental (Fig. S2(a)) and optical simulation (Fig. S2(b)) results are compared in terms of the size of the AgNPs. The locations of the experimental extinction peaks are in good agreement with the theoretical predictions1. The experimental data (Fig. S2(a)) was attained by means of a UV-vis spectrophotometer while the AgNPs were dispersed in ethanol. Simulation spectra (Fig. S2(b)) were also acquired assuming a medium of ethanol. The experimental results showed slightly broader peaks than the simulation results due to the actual size distribution of the AgNPs. Especially for the AgNP-

94 samples, we could not find the two clear peaks because there were various sizes (60 ~ 120 nm) of

2 the AgNPs in the solvent. The extinction peaks of AgNPs became red-shifted as the size of the

AgNPs increased. Quadruple resonance peaks of the AgNPs near 350 nm were observed for AgNP-

67 and AgNP-94. There were no sub-peaks in the long-wavelength range, implying that the most of the nanoparticles were spherical and not aggregated2-4.

3. Angular plots of the scattering power of the AgNPs

Figure S3. Angular plots of the calculated scattering power of AgNPs of various sizes embedded in PEDOT:PSS at an incident wavelength of 430 nm: AgNP-13 (black), AgNP-40 (red), AgNP-67 (blue), and AgNP-94 (green).

Figure S3 shows angular scattering power plots of AgNPs of various sizes embedded in

PEDOT:PSS according to analytical simulations based on a previous report1. The incident light is propagating in the upward direction (i.e., from 180° to 0°), and the wavelength of the incident light is

430 nm, which is near the LSPR. With an increase in the size, the forward (centered at 0°) scattering power becomes stronger and the scattered angle wider up to AgNP-67. The total scattering power of

AgNP-94 is larger than that of AgNP-67, as shown in Fig. 1(a), but the backward (centered at 180°)

3 scattering power comprises a large fraction of the total scattering power, potentially reducing the

OPV’s PCE.

4. Statistics of various sizes of AgNPs

Figure S4. The SEM images and size distribution histogram of various sizes of AgNPs: (a) AgNP-13, (b) AgNP-40, (c) AgNP-67, and (d) AgNP-94

Figure S4 illustrates SEM images and size distribution histograms of the fabricated AgNPs. The dimensions were analyzed directly from SEM images. Table S1 summarizes detailed information

4 about the AgNPs. We estimated the average sizes and standard deviations by measuring at least 200

AgNPs.

5. Performances of PTB7-based OPVs

Figure S5. (a) EQE (black circles), absorption (red rectangles), and IQE (blue triangles) of a PTB7- based control OPV (filled symbols) and a plasmonic OPV (open symbols). The EQE and absorption were enhanced near the LSPR wavelength (400 ~ 500 nm), and IQE was not significantly affected. (b) EQE enhancement (red circles), absorption enhancement (blue rectangles) and IQE enhancement (green triangles) of a plasmonic OPV. The EQE and absorption were significantly enhanced by up to approx. 45%.

Figure S5 illustrates the performances of PTB7-based plasmonic OPVs. Figure S5(a) shows the

EQE, absorption efficiency and IQE of a control device (filled) and of the plasmonic device (open).

The EQE (circles) and absorption (rectangles) were enhanced near the LSPR wavelength (400 ~ 500 nm). The EQE and absorption were significantly enhanced by as much as 45 %, while the IQE was only slightly affected, as depicted in Fig. S5(b). The tendencies of the EQE and absorption enhancement of the PTB7-based plasmonic OPVs were similar to those of PCDTBT-based plasmonic

5 6. Scattering vs. absorption

Figure S6. The optical simulations of the absorption (open) and scattering (filled) power with various sizes of AgNPs: (black circles) AgNP-13, (red rectangles) AgNP-40, (blue triangles (up)) AgNP-67, and (green triangles (down)) AgNP-94. As the size of AgNPs increased, both the absorption and scattering increased. A simulation was performed analytically for a single AgNP in PEDOT:PSS at wavelengths ranging from 300 nm to 800 nm.

Figure S6 shows the optical simulation results of the absorption (open symbols) and the scattering

(filled symbols) power at various sizes of AgNPs embedded in PEDOT:PSS. Figure 1(a) in the manuscript demonstrates the integrations of each curve from 300 nm to 800 nm. With an increase in the size of the AgNPs, both the absorption and the scattering power increased. However, the scattering power grows much faster than the absorption power such that the ratio of the scattering to absorption power increases as the size of AgNPs increases, as illustrated in Fig. 1(a).

6 7. NSOM topography of AgNP-67-embedded PEDOT:PSS

Figure S7. An NSOM topography image and line profiles of an AgNP-67-embedded PEDOT:PSS layer. The scan size was 15 μm × 15 μm. (a) The line profile across the AgNPs. (b) The line profile

7 across an area where no AgNPs existed. (c) The line profiles across an AgNP embedded in PEDOT:PSS film.

Figure S7 shows an NSOM topography image and line profiles of the PEDOT:PSS with AgNP-67 embedded. Figure S7(b) shows that the root mean square (RMS) roughness of the bare PEDOT:PSS layer was only ca. 2 nm while the peak height of AgNP-67 ranged from approximately 10 to 30 nm, as illustrated in Figs. S7(a) and (c), demonstrating that AgNPs having an average diameter of 67 nm were well embedded in the 30 nm-thick PEDOT:PSS layer. Indeed, as the size of the AgNPs was increased from AgNP-28 to AgNP-94, the peak height increased from 4 nm to 45 nm.

8. Concentration effect of AgNPs

Figure S8. SEM images of AgNP-67 embedded in PEDOT:PSS at various concentrations: (a) optimum density (4.5 × 109/cm2), (b) high density (9.0 × 109/cm2), (c) low density (1.2 × 108/cm2), and (d) medium density (1.3 × 109/cm2). The samples were prepared on clean Si wafers. The inset of (b) shows large silver clusters in the cross-section of an OPV with AgNPs embedded at a high concentration.

8 Figure S8 show SEM images of AgNP-67 embedded in PEDOT:PSS layers at various concentrations. The samples were prepared on Si wafers for better imaging. In the manuscript, we showed that the doping concentration significantly influenced the device performance. Specifically,

Fig. S8(b) shows that a high density of AgNPs yielded large clusters consisting of smaller AgNPs.

These large clusters of AgNPs (> 200 nm) are thicker than the active materials of the OPVs, as shown in the inset of Fig. S8(b).

9. Sheet resistance variation upon the insertion of AgNPs

Figure S9. The sheet resistance data of ITO/AgNP-67-doped PEDOT:PSS at various concentrations. The sheet resistance of the reference film was 9 Ω/sq. The measurement was done immediately after the samples were prepared.

Figure S9 shows the sheet resistance data of ITO/AgNP-67 doped PEDOT:PSS at various concentrations. A four-point probe was used to measure the sheet resistance. The sheet resistance of the films was scarcely affected by the addition of the AgNPs, and it was very similar to that of

ITO/PEDOT:PSS film at approx. 9 Ω/sq. Interestingly, we found that the sheet resistance even increased slightly at a high concentration. Consequently, we can rule out the possibility that the Jsc

9 could be overestimated due to the enhanced conductivity when the MNPs are doped in the anodic buffer layer of PEDOT:PSS.

Table S1. The statistics with various sizes of AgNPs. The average sizes, standard deviations, minimum sizes and maximum sizes are summarized. The results were obtained from at least 200

AgNPs in each SEM image.

Average size Standard Minimum Maximum (nm) deviation size (nm) size (nm) AgNP-13 13.7 6.7 7.14 28.6 AgNP-40 40.9 12.6 14.9 58.6 AgNP-67 67.7 14.8 28.0 100.0 AgNP-94 94.7 19.7 46.2 128.2

Table S2. The standard deviations of device performance at various sizes of AgNPs. The statistical data were acquired from at least 50 devices at each condition.

Polymer AgNP Voc Jsc FF PCE

Control device 0.001 0.023 0.012 0.119

AgNP-13 0.001 0.060 0.005 0.056

PCDTBT AgNP-40 0.001 0.069 0.003 0.079

AgNP-67 (optimum) 0.001 0.084 0.004 0.082

AgNP-94 0.002 0.012 0.005 0.077

Control device 0.001 0.031 0.007 0.078 PTB7 AgNP-67 (optimum) 0.001 0.088 0.005 0.069

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