Supplementary Information

Harvesting Near- and Far-Field Plasmonic Enhancements from

Large Size Gold Nanoparticles for Improved Performance in

Organic Bulk Heterojunction Solar Cells

Jongmoon Shin1,a, Myungkwan Song1,a,*, Hassan Hafeez2,a, P. Justin Jeusraj2,a, Dong Hyun Kim2, Jong

Chan Lee2, Won Ho Lee2, Dae Keun Choi2, Chul Hoon Kim3,*, Tae-Sung Bae4, Seung Min Yu4, Kyoung-

Ho Kim5, Hong-Gyu Park6, Kwun-Bum Chung7, Aeran Song7, Yong-Cheol Kang8, Juuyn Park8, Chang Su

Kim1, Seung Yoon Ryu2,*

1Advanced Functional Thin Films Department, Korea Institute of Materials Science (KIMS),797 Changwondaero, 641-831, Republic of Korea

2School of Display and Semiconductor Physics, College of Science and Technology, Korea University Sejong Campus 2511 Sejong-ro, Sejong City, 339-770, Republic of Korea

3Department of Advanced Materials Chemistry, College of Science and Technology, Korea University Sejong Campus 2511 Sejong-ro, Sejong City, 339-770, Republic of Korea

4Jeonju Center, Korea Basic Science Institute (KBSI), Analysis & Researcher Division, 634-18 Geumam-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea

5Department of Physics, Chungbuk National University, Cheongju, 28644, Republic of Korea

6Department of Physics, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 136-701, Republic of Korea

7Division of Physics and Semiconductor Science, Dongguk University, Seoul, 04620, Republic of Korea

8Department of Chemistry, Pukyong National University 45 Yongso-Ro, Nam-gu, Busan 48513, Republic of Korea

Keywords: Organic solar cells; Gold nanoparticles; Bulk heterojunction; near-field plasmon

resonance; far-field scattering; Time resolved photoluminescence.

aThese authors equally contributed to this paper.

*Corresponding author List

*Prof. Dr. Seung Yoon Ryu,2School of Display and Semiconductor Physics, College of Science and Technology, Korea

University Sejong Campus, 2511 Sejong-ro, Sejong City, 30019 (Republic of Korea) Republic of

Korea, Tel: +82-44-860-1375, justie74@korea.ac.kr ,

*Dr. Myungkwan Song

1Advanced Functional Thin Films Department, Korea Institute of Materials Science (KIMS),

Changwon, 641-831 (Republic of Korea), Tel) +82-55-280-3696, smk1017@kims.re.kr

*Prof. Dr. Chul Hoon Kim,

3New Materials Chemistry Department, College of Science and Technology, Korea University

Sejong Campus 2511 Sejong-ro, Sejong City, 339-770, Republic of Korea

, Tel) +82-44-860-1337, chulhoon@korea.ac.kr

Figure S1: UHR FE-SEM images of 71, 80, 87, and 103 nm Au-NPs in PEDOT:PSS (Single particle model). The Au-NPs were partially covered by the 40 nm-thick PEDOT:PSS. (a) 71 nm (b) 80 nm (c) 87 nm and (d) 103 nm-sized Au-NPs.

The UHR FE-SEM images depicted below clearly show the size of various Au-NPs in PEDOT:

PSS. We have recorded the micrograph in various portions of the PEDOT:PSS+Au-NPs films.

The Au-NPs generally dispersed uniformly all over the substrate, however at edge areas of the

substrate aggregations were noted due to centrifugal force in spin coating process. These areas

have been excluded carefully while depositing electrodes. The coverage density graphs also

depicted that almost similar values, however in the aggregated areas it increased tremendously.

Figure S2. UHR FE-SEM images of Au-NPs partially covered by the 40 nm-thick PEDOT:PSS (Aggregated model). (a) 71 nm (b) 80 nm (c) 87 nm and (d) 103 nm-sized Au-NPs.

The UHR FE-SEM images depicted above clearly show the size of various aggregated Au-NPs

in PEDOT:PSS. In particular, the line or shadow around the NPs indicate the penetration of

particles from the PEDOT:PSS surface. This kind of aggregation is not good for device

performances and is found at the edges of the substrate due to centrifugal force in spin coating

process, which carefully avoided during cathode deposition.

Figure S3. AFM images of BHJ deposited on ITO/PEDOT:PSS+Au-NPs. (a) reference (w/o Au-NPs), (b) 71 nm, (c) 80 nm, (d) 87 nm and (e) 103 nm Au-NPs added PEDOT:PSS with BHJ. (f) represents the regression curve of the RMS values calculated with various sized Au-NPs.

Fig. S3 (a)-(e) show the AFM images of BHJ deposited on ITO/PEDOT:PSS with different size

Au-NPs. Even though, the different Au-NPs various coverage from the PEDOT:PSS, the BHJ

layer smoothen over the surface and exhibited a uniform roughness. The calculated RMS value

has been varied with a factor of 0.1-0.3 nm, hence it does not increase the roughness of the BHJ

significantly.

Figure S4. (a) Schematic view of P3HT:PC61BM-based OSCs incorporating Au-NP in PEDOT: PSS, (b) Energy-level diagram of corresponding OSCs with different anode work functions calculated from UPS, (c) Normalized absorption spectrum of reference device and 50 nm Au-NPs plasmonic device, (d) Simulated absorption spectrum of BHJ with and without Au-NPs in PEDOT: PSS. Inset shows the electromagnetic field distribution around the 50 nm Au-NP in PEDOT: PSS.

To initially check the plasmonic enhancements, we inserted 50 nm AuNPs in PEDOT: PSS and

fabricated OSC devices with P3HT:PC61BM BHJs as shown in Fig. S4a. The corresponding

energy level alignment between adjacent layers of the OSC is illustrated in Fig. S4b. In the

PEDOT: PSS layer, the distribution of 50 nm-sized Au-NPs is found to be uniform as depicted

by UHR FE-SEM and AFM images (Fig. S5). Interestingly, the Au-NPs infused devices

exhibited improved absorption in experimental as well as in theoretical full wave simulation

(Fig. S6). Further, the performances of OSCs also found to be enhanced due to near-field

coupling (LSPR) for devices with 50 nm Au-NPs are depicted in Fig. S7 and Table S1.

Figure S5: (a) UHR-FE-SEM image of Au-NPs in PEDOT:PSS network, AFM images of PEDOT:PSS film (b) without and (c) with Au-NPs coated on ITO.

The distribution of the 50 nm-sized Au-NPs (white dot) is found to be uniform as depicted by

UHR FE-SEM image in Fig. S5a. Some Au-NPs were aggregated in the PEDOT: PSS layer with

a size of 100 nm because the Au-NPs’ surfactant solution was diluted by the PEDOT: PSS

solution. The AFM images and the surface roughness of the PEDOT: PSS with (RMS-1.31 nm)

and without Au-NPs (RMS 1.02 nm) are shown in Fig. S5b and S5c, respectively. This increase

in the RMS also indicates an improvement in the Jsc, because hole collection can be enhanced by

the morphological/chemical changes of the PEDOT:PSS+Au-NPs HTL.

Figure S6. (a) Electric-field distribution of generated localized surface plasmons (LSP) Au-NPs with the cross-sectional view of xz- and xy-planes. (b) Light absorption spectrum for P3HT:PC61BM active layer. (c) Schematic of the device with Au-NP and absorption calculation region of active layer.

Full-wave numerical simulation revealed strongly excited LSP indicated by high |E|2 regions

around the surface of the particle in the lateral direction. The generated LSP by Au-NPs

increased the light absorption by the active layer in the wavelength region of 550–800 nm for the

plasmonic device in comparison to the reference device. The light absorption by the active layer

is mainly dependent on the refractive index and dielectric constant of the layer; these layer

properties are shown in Fig. S6(c). With environmental media, red-/or blue-shifts in the

extinction peak for the plasmons of nearby particles in a cluster originated from near-field inter-

particle coupling (LSPR).

Figure S7: (a) The J-V curves, (b) The dark J-V plots, (c) IPCE spectra and (d) Reflectance spectrum of the P3HT:PC61BM OSCs with and without 50 nm Au-NPs in PEDOT:PSS.

The device performance of the OSCs with and without the 50 nm Au-NPs under simulated AM

of 1.5 G illumination (100 mW/cm2) are compared in Fig. S7a-d. The reference device without

the NPs exhibit a Voc of 0.62 V, a Jsc of 8.77 mA/cm2, a FF of 59.18 % and a final PCE of 3.21%.

In the case of the 50 nm Au-NPs-infused devices, the Voc remain unchanged, however, an

enhancement in the PCE to 3.91% was observed with an increment of 22% compared to the

reference. In addition, the FF also progressed from 59.18% to 63.81%, together with the

increment in Jsc from 8.77 mA/cm2 to 9.85 mA/cm2 for the plasmonic devices. The above

mentioned efficiency improvements are mainly attributed to the near-field coupling (LSPR).

DeviceJ sc

(mA/cm2)

Voc

(V)

FF

(%)

Rsh

( cm2)

Rs

( cm2)

PCE

(%)

Rectification ratio

Reference 8.77 0.62 59.18 6.95×103 4.86 3.21 2.24102

Plasmonic

(50 nm)9.85 0.62 63.81 1.92×104 3.50 3.91 6.63103

Table S1 Performance characteristics of typical P3HT:PC61BM active layer’s OSCs parameters for reference device (without Au-NPs) and plasmonic device (50 nm Au-NPs) in PEDOT: PSS.

Table S1 shows the performance enhancement of OSCs with 50 nm Au-NPs by increased RSH and the decreased RS. The RS is

reduced from 4.86 cm2 to 3.50 cm2, and the RSH increased from 6.95×103 to 1.92×104 cm2 for the plasmonic devices as

compared to the reference. This decrease in RS and increase in RSH are attributed to the lower recombination rate and the improved

hole extraction by the addition of Au-NPs.

Figure S8: UPS analysis to measure the work function of the PEDOT: PSS layer without Au-NPs by Ecutoff and Fermi level measurements.

The analysis reveals an increase in the binding energy by analyzing the shift in the Fermi level

for the plasmonic device in comparison to the reference device. This change in the binding

energy by the addition of the Au-NPs is due to an increase in the work function of the layer and

provides a better alignment for the device. Thus, the interfacial resistance for hole extraction is

reduced, which is also reflected by the low Rs of the solar cells.

Figure S9: (a) Comparison of photocurrent-effective voltage (Jph-Veff) curve for the reference device and the plasmonic device with the inset showing compensation voltage (V0) analysis. (b) exciton dissociation probability [P(E, T)] plotted as a function of Veff.

The results are shown in Fig. S9 (a) in terms of Jph as a function of Veff. Jph is analyzed using the

difference between the current densities under illumination (JL) and dark conditions (JD) shown

in the inset of Fig. S9 (a). The point where, Jph: JL – JD = 0, a shift in voltage axis can be observed

to account for the built-in potential which is the V0. The difference between the V0 and the

applied voltage (V) provides us with an analysis of Veff. At low Veff < 0.1, the Jph for both devices

increases linearly and starts saturating at higher Veff > 1. A higher photocurrent density was

achieved by the plasmonic device in comparison to the reference device, which is an indication

of improved exciton generation. The maximum exciton generation rate (Gmax) for the reference

device and the incorporated Au-NPs device are presented, since the value of Gmax represents the

light absorption. A high value of Gmax for the plasmonic device indicates an increase in light

absorption by the active layer. The value of Gmax is directly related to the saturated photocurrent

(Jsat) of the device by the relation Jsat = eGmaxL, where e is the electronic charge and L is the

thickness of the active layer [1, 2]. The normalized current density given by Jph / Jsat, is utilized to

analyze the values of P(E, T), which is dependent on the electric field [E] and the temperature

[T]. At the short-circuit condition point shown as a solid arrow facing downwards, when V = 0

and Veff is equal to V0, then [P(E, T)] represents the small portion of generated excitons which

dissociate into free charge carriers. A slightly higher value of [P(E, T)] for the plasmonic device

in comparison to the reference device was observed, indicating that slightly more excitons were

dissociated into free charge carriers. The increase in [P(E, T)] increases the hole mobility,

reduces the recombination rate, and consequently improves the device performance [1-3].

Figure S10: UPS measurements of large size Au-NPs added ITO/PEDOT:PSS surface. (a) comparison of work function with respect to 70, 80 and 87 nm Au-NPs added surfaces, (b)-(d) UPS spectra of 71, 80 and 87 nm Au-NPs added ITO/PEDOT:PSS surfaces.

Fig. S10 depicts the work function analysis of large size Au-NPs added PEDOT:PSS surfaces on

ITO. For example, 71 nm Au-NPs exhibited a work function of 5.04 eV against the 4.49 eV of

ITO/PEDOT:PSS. Further, it is increased to 5.09 and 5.12 eV by the insertion of 80 and 87 nm

Au-NPs respectively. However, we failed to get a proper work function for 103 nm Au-NPs

added sample. But we believe that as work function of samples already reached the work

function of bulk gold, 103 nm Au-NPs+PEDOT:PSS samples’ work function also around 5.1 eV.

Figure S11: Four probe conductivity measurements of Au-NPs+PEDOT:PSS films.

The four-probe conductivity measurements of PEDOT:PSS and PEDOT:PSS+Au-NPs are

illustrated in Fig. S11. In general, the conductivity of PEDOT:PSS film is observed to be reduced

slightly with the addition of 71, 80, 87 and 103 nm Au-NPs. This reduction is attributed to the

fact that the citrate stabilizing agent present in Au-NP prevents the conduction between them to

the polymer chains [4]. However, to our knowledge it has not been examined before that the

conductivity increases with size of Au-NPs. The changes encountered in conductivity and most

importantly in work function causing the solar cells to realize higher short circuit current density.

Figure S12: TRPL exciton lifetime plots of BHJs deposited on ITO/PEDOT:PSS with and without the 50 nm Au-NPs, measured at (a) 660 nm and (b) 720 nm.

Figure S12 (a) and (b) show the TRPL lifetime measurement of P3HT:PC61BM BHJ deposited

on ITO/PEDOT: PSS substrate with or without Au-NPs. When measuring at 660 nm, BHJ with

Au-NPs exhibited slightly reduced average lifetime as compared to samples with no particles.

This indicated the increased exciton decay and reduced exciton lifetime by the near-field

coupling (LSPR) with Au-NPs. Even though, this behavior is superior at Au-NPs absorption

wavelength (530 nm), prolonged near-field coupling (LSPR) is effective until 660 nm. However,

at or after 720 nm, near-field coupling (LSPR) was deteriorated and far-field scattering behavior

was expected to occur. This will increase the exciton lifetime due to improved optical path way

of photons into BHJs, i.e. improved absorption of BHJs. In summary, if we compared this result

with IPCE values in Fig. S7 (c) showed higher increment around 550 nm. Hence, we concluded

that in 50 nm Au-NPs based performances near-field coupling (LSPR) contribution is relatively

higher as compared to far-field scattering.

Figure S13: Simulated absorption of BHJ with single and aggregated Au-NPs.

Figure 13(a) shows the comparison of simulated field distribution of different size single Au-NPs

(without aggregation) along with the spectral absorption graph for the BHJ. Even though the

boundary conditions were slightly modified (in comparison to the analysis in the main

manuscript) to adjust for the aggregated Au-NPs analysis. But, the single Au-NPs demonstrated

similar field distribution and average spectral absorption trend, where the electric field

distribution around the Au-NPs was maximum at around 87/90 nm size and then became

saturated. However, in the aggregated Au-NPs model (Fig. 13b), the field distribution seems to

be quenched due to the overlap of the enhancement region of the adjacent Au-NPs. As a result,

the average spectral absorption for the BHJ in the aggregated model was less than that the single

Au-NPs model, despite the fact that the absorption was slightly increasing with the size of the

nanoparticle. This kind of aggregation was present in our substrate edges which were neglected

while depositing cathodes or fabricating the device architecture therefore, the aggregation zone

was excluded and is not expected to adversely affect the resulting device performance.

Figure S14: Schematic of devices with (a) 71 nm (b) 80 nm (c) 87 nm and (d) 103 nm Au-NPs in the PEDOT:PSS layer and their vertical coverage in the active layer.

Active layer vertical coverage of the incorporated Au-NPs in PEDOT:PSS was found to be one

of the important parameters which defines the device efficiency. The vertical coverage of 71 nm

Au-NPs was about 42.3%, and the highest vertical coverage was achieved with 103 nm Au-NPs

(61.2%). Due to the reduced plasmon energy and high probability for exciton quenching (very

poor near-field coupling (LSPR)) with adjacent excitons in the BHJ, the 103 nm Au-NPs-infused

devices exhibited poor performances relative to the 87 nm device.

Figure S15: (a) Normalized absorption of Au-NP solution with larger-sized (>50 nm) NPs. (b) TRPL analysis of PEDOT:PSS with and without the incorporation of various sizes of Au-NPs (70, 80, 87, and 103 nm) at an emission wavelengths of 620 nm.

The light absorption of Au-NPs greater than 71 nm in size, shifted slightly towards longer

wavelength (red shift) in comparison to 50 nm Au-NPs. This red-shift is due to a decrease of the

free-electron oscillation of the Au-NPs in the layer that reduce the overall electric potentials on

its surface. In other words, we can claim that the plasmon energy associated with larger-size

nanoparticles have been reduced. Fig. S15(b) depicts the TRPL spectra (at 620 nm) of PBDTTT-

C:PC61BM deposited on ITO/PEDOT:PSS with different-size Au-NPs. The near-field coupling

(LSPR) dominates up to a size of 80 nm; in the 87 nm and 103 nm Au-NPs, far-field plasmonic

enhancements are prevalent. This lifetime variation of the BHJ is based on the combinations of

the near-field coupling (LSPR) and far-field plasmonic interactions of Au-NP with various

vertical coverage, as explained in the main manuscript. In comparison to Fig. 4, the lifetime

variation of the BHJ with different Au-NPs remains the same. However, a sudden drop in the

lifetime occurred for 71 and 80 nm Au-NPs, which is attributed to the reduced or lack of near-

field coupling (LSPR), because the measured energy (i.e. 620 nm) is much smaller than that of

the Au-NP plasmonic absorption. TRPL measurements were made on two different wavelengths

(470, 620 nm) in order to understand near- and far-field enhancements by Au-NPs. For example,

low wavelength region (420 nm) is very near to the absorption maximum of Au-NPs, which is

beneficial to better observe the near-field plasmonic interaction. Whereas, the longer wavelength

(620 nm) measurement is far from the absorption maximum of Au-NPs and was not expected to

demonstrate difference in life-times. However, we still could observe some difference in life-

times for various sized Au-NPs, which was not due to the near-field plasmons (LSPR) but

indicated the presence of far-field scattering. So, this difference in life-time for various sized Au-

NPs was not attributed to the near-field coupling (LSPR) effect. However, it was crucial

indication that far-field scattering occurred in larger-sized Au-NPs demonstrating an increase in

exciton life-time.

Figure S16: Steady-state PL analysis for the reference device and plasmonic device with 71, 80, 87, and 103 nm Au-NPs.

Steady-state PL analysis show an improvement in the normalized intensity of plasmonic device

in comparison to the reference device, for the wavelength region of 670-760 nm. This additional

intensity is due to the generated near-field coupling (LSPR) by Au-NPs in this region. Moreover,

a slight red-shift in the intensity profile can be observed in the case of the plasmonic device

because of the enhanced fluorescence by near-field coupling (LSPR). However, with another

more efficient active layer PBDTTT-C:PC61BM, a red-shift with 71 – 80 nm Au-NPs in the

steady-state PL was shown in case of plasmonic device. We could observe a red-shift in PL for

Au-NPs up to 80 nm size, and blue-shift in PL for Au-NPs larger than 80 nm. This trend could

be correlated with the life-time of the excitons observed in the TRPL analysis of the respective

Au-NPs. That means, with Au-NPs size below 80 nm, we could detect a decrease in exciton life-

time (TRPL) and red-shift in PL in comparison to the reference indicating near-field plasmon

(LSPR) effect due to the energy transfer process (energy loss). And for above 80 nm Au-NPs, an

increase in exciton life-time (TRPL) and blue-shift in PL was observed, which indicates a far-

field effect due to the far-field scattering (improved light trapping). The variation occurred on the

vertical coverage of Au-NP in the photoactive layer, with different sizes contributing to near-

field coupling (LSPR) and far-field scattering, which might be a reason for the observed slight

shift in the PL as described above.

Table S2: Calculation of p-values for JSC and PCE:

Type of BHJ

Size of

AuNPs

Short-circuit current (JSC) Power conversion efficiency

(PCE)

Z value P-value Z value P-value

PBDTTT-

C:PC61BM

71 nm 9.83 <0.00001 15.09 <0.00001

80 nm 18.13 <0.00001 20.12 <0.00001

87 nm 33.54 <0.00001 42.48 <0.00001

103 nm 19.87 <0.00001 26.08 <0.00001

PTB7:

PC71BM

71 nm 7.57 <0.00001 5.45 <0.00001

80 nm 10 <0.00001 12.07 <0.00001

87 nm 27.95 <0.00001 32.54 <0.00001

103 nm 29.90 <0.00001 19.67 <0.00001

The above table represents the calculated results of Z-score and p-values of JSC and PCE

extracted for different cells. The Z-score calculated from,

;

where η1, η0 are JSC (or PCE) of Au-NPs infused and reference devices respectively.

σ1- standard deviation of (JSC or PCE) Au-NPs device.

N- number of cells (5 in our case).

The increased Z-score will give low p-value, p-value less than 0.05 is always considered as

significance result. In our case, all the data showing very low p-values as because of very small

standard deviations. Meanwhile, Z values also very high for Au-NPs incorporated devices

indicating the high reproducibility of the results.

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