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SUPPLEMENTARY

Surface plasmon excitation in semitransparent

inverted polymer photovoltaic devices and their

applications as label-free optical sensors

Byoungchoo Park*1, Soo Hong Yun1, Chan Youn Cho1, Young Chan Kim1, Jung Chul Shin1, Hong Goo Jeon1, Yoon Ho Huh1, Inchan Hwang2, Ku Youn Baik1, Young In Lee1, Han Sup Uhm1, Guang Sup Cho1 and Eun Ha Choi1 1Department of Electrical and Biological Physics, Kwangwoon University,

2Department of Electronic Materials Engineering, Kwangwoon University,

Wolgye-Dong, Nowon-gu, Seoul 139-701, Republic of Korea

*e-mail: bcpark @ kw.ac.kr (Supplementary Information)

SUPPLEMENTARY FIGURES

1 Fig. S1 Energy-level diagram of the semitransparent IPSCs studied and optical properties of the functional multilayers used in the IPSCs. (a) Schematic illustration of the energy-level diagram of the semitransparent IPSCs. The inset shows the molecular structures of P3HT and PCBM. (b) The optical absorption spectra of the P3HT:PCBM layer. The inset shows a photograph of a P3HT:PCBM layer on a glass substrate. The

P3HT:PCBM PV layer used for the IPSCs exhibited optical absorption with a peak at around 495-510 nm, which is mainly attributed to the π-π* transition of the P3HT (band edge: ~650 nm). (c) Upper panel: The transmission spectra (T) of the IPSCs investigated

(dashed curves), together with T for the single thin (~45 nm) Ag and Au metal films

(solid curves). The inset shows a photograph of a pair of semitransparent IPSCs. Lower panel: The reflectance spectra (R) of IPSCs investigated (dashed curves), together with

R of the single thin (~45 nm) Au and Ag metal films (solid curves). The lowest

2 transmission of the IPSC with a thin Au anode (Au-IPSC) occurred at around 500 nm due to the strong absorption of the P3HT:PCBM layer, in contrast with the thin Au layer, which reaches its maximum transmission at this wavelength. The low transmission of the Au-IPSC at wavelengths longer than the absorption edge (~650 nm) of the P3HT:PCBM layer is mainly attributed to the strong optical reflection of the Au-

IPSC. Although the transmission and reflection spectra differ between the Au and the

Ag films, the optical characteristics of semitransparent IPSCs with a thin Ag anode (Ag-

IPSC) were found to be similar to those of the Au-IPSC.

3 Fig. S2 Complex refractive indices of the functional multilayers used in the semitransparent IPSCs. (a) Left: A typical example of spectroscopic ellipsometry measurement results of a P3HT:PCBM layer on a Si wafer coated with SiO2 (300 nm) for a range of different incidence angles (60° ~ 80°). Ellipsometric alpha (α) and beta

(β) parameters were analyzed using the Fresnel model and were found to be in good agreement with the experimental data, as shown in the figure (solid curves). Right:

Estimated real (n) and imaginary () refractive indices of the P3HT:PCBM layer, which are comparable with the n values of Reference 25 in the manuscript. (b) Complex refractive indices of the ITO, PEO:Cs2CO3 and MoO3 functional layers obtained from ellipsometry measurements. Ellipsometric parameters were acquired for the polarization change () of the incident light wave as reflected at the different interfaces in the films, where the ellipsometric angles Delta (∆) and Psi () are related to α and β: and . Here,

P is the polarizer azimuth angle (45o) in the rotating analyser ellipsometer (RAE), and ‘’ and ‘’ are the complex Fresnel reflection coefficients of the sample film for TM

(transverse-magnetic or p-) and TE (transverse-electric or s-) polarized lights, respectively.1 The optical constants obtained show good agreement with those published previously.2,3 (c) Complex refractive indices of the thin Au and Ag metal layers (ca. 45 nm, sputter-deposited), obtained from variable wavelength attenuated-total-reflection

(ATR) measurements (symbols). The curves show the results fitted using an analytical model of the frequency ()-dependent Drude-critical points model for the electric permittivity () of gold and silver4:

4 where the first and second terms are the standard contribution of a Drude model5 with a high-frequency limit dielectric constant , a plasma frequency and a damping term , while the last two terms are the critical point transition contributions6 from the interband transitions (gap) with critical point amplitude , interband transition frequency , phase and broadening 4. Here, we used = 1.1431, = 1.320×1016 rad s-1, = 1.7306×1014 rad s-1,

= 0.26698, = 3.8711×1015 rad s-1, = -1.2371, = 3.7693×1014 rad s-1, = 3.0834, =

4.1684×1015 rad s-1, = -1.0968, = 2.5419×1015 rad s-1 for the thin Au film, and =

15.833, = 1.3861×1016 rad s-1, = 19.8847×1013 rad s-1, = 1.0171, = 6.6327×1015 rad s-1,

= -0.93935, = 6.685×1014 rad s-1, = 15.797, = 9.2726×1017 rad s-1, = 1.8087, =

1.2258×1017 rad s-1 for the thin Ag film.

Fig. S3 Optimum thickness of the Au anode in IPSCs. (a) To determine the optimum

thickness of the Au anode in the semitransparent IPSC for resonant excitation of SPs,

the optical absorptions of the Au anodes were calculated numerically by FDTD as a

function of the thickness of the Au electrode for an incident light of  = 632.8 nm at

o three incident angles (θs) of the SPR angle of 43.8 (θ = θR), the lower off-resonance

o o angle of 42.0 (θ < θR) and the higher off-resonance angle of 48.0 (θ > θR). The figure

5 clearly shows that the most appropriate thickness of the Au anode to cause resonance

of SPs is about 45 nm. (b) In order to confirm the optimum thickness (ca. 45 nm) of

the Au anode in the semitransparent IPSC studied, the optical absorptions of all the

functional multilayers used in the Au-IPSC were also calculated as a function of the

incidence angle  for an incident light of  = 632.8 nm at the given Au thickness of 45

nm. The figure clearly shows that at the SPR angle (θ = θR), the optical absorption of

the Au anode is strongest among the functional layers and the total absorption of the all

multilayers almost reaches 1.0. This result indicates that the main origin of the sharp

dip in the R() spectra (e.g., Figure 2b) is the strong absorption of the Au anode due to

SPR excitation, and is not attributed to the effects of any destructive interference or

transmission of incident light.

Fig. S4 Optical dispersion relationships between the frequency f and the in-plane

wavevector kX for the TM-polarized modes in the IPSCs studied. Optical dispersion

relationships of an Au-IPSC (left panel) and an Ag-IPSC (right panel) for the TM-

polarized modes as determined by means of the finite difference time domain (FDTD)

6 method. In the FDTD calculations, as input parameters we used the refractive indices

of the multilayers in the IPSCs, as determined by optical measurements

(Supplementary Fig. S2). In the figures, the dotted lines labelled “air-mode” and

“glass-mode” represent the frequencies and wavevectors accessible by light

propagating in air and in the glass substrate, respectively. The solid curves labelled

“ITO-mode” and “P3HT:PCBM & MoO3-mode” represent the waveguide modes in the

ITO and P3HT:PCBM/MoO3 layers, respectively. The dispersion relationships clearly

show that the semitransparent IPSC structure under investigation supports two TM-

polarized resonant modes, arising from the strong coupling of the incident photons

with the SPs at the two metal/dielectric interfaces, i.e., the metal anode-air (SP1 mode)

and metal anode-(MoO3)-PV (SP2 mode) interfaces.

Fig. S5 PV performance of the Ag-IPSC. (a) Semilogarithmic plot of the dark J-V curve of an Ag-IPSC. (b) J-V curve of the Ag-IPSC under bottom illumination (open symbols), together with the curve under top illumination (closed symbols). (c) IPCE spectral curve of the Ag-IPSC under bottom illumination (open symbols), together with the curve under top illumination (closed symbols).

7 Fig. S6 Optical power absorbed by the functional layers in an Au-IPSC. Plots of the calculated fraction of the optical power absorbed by the functional multilayers in an Au-

IPSC, as a function of the angle of incidence for TM- (upper) and TE-polarized (lower) incident light with wavelengths of 633 nm (a), 500 nm (b), and 750 nm (c). The data were obtained from FDTD simulations.

The optical power absorbed by the multilayers in the Au-IPSC from the incident light was investigated by means of FDTD simulations with the optical parameters of the layers (see Supplementary Fig. S2). Supplementary Fig. S6 shows that for incident light of = 633 nm, which corresponds to the wavelength just above the bandgap of the PV layer, the amount of power absorption in the IPSC can be split into five regions; Region 1 (Transmission) and Region 2 (Reflection) represent the light transmitted directly through the IPSC and the light reflected from the IPSC, respectively. The other regions represent the amount of light-absorption of the PV layer, the Au anode (‘Surface plasmons’), and other functional layers including the MoO3, ITO, and PEO:Cs2CO3 layers. At normal incidence, for both TM- and TE-polarized incident light, 45.7% of the incident light is absorbed into the PV layer, 9.6% is absorbed into the Au layer, and 7.1% is absorbed into the other stacks. As  increases, for TM-polarized incident light, SP coupling in the Au absorption begins to increase at angles just above the critical

8 angle (θC) of 41.3°, with a more rapid rise at an SPR angle (θR) of 43.7°, showing the highest fraction of SP-coupled light (about 60.4%). While the absorption of the other layers decreases at this SPR angle, above this SPR angle the SP coupling decreases and the absorption levels of the other layers recover to their high values. For TE- polarization, however, as increases the absorption of each layer changes continuously without any contribution from SP excitation. We therefore note that at θ = θR for TM- polarized excitation light, the power absorption of the Au-IPSC probably has two main contributors: PV absorption and SP excitation. For comparative purposes, the wavelength-dependent absorption of light by the Au-IPSC was also investigated (Supplementary Figs. S6b and S6c). For incident light of = 500 nm, which corresponds to the wavelength of maximum absorbance of the PV layer, most of the incident light is absorbed directly into the PV layer, implying a very small contribution of SP, even for TM-polarization. In contrast, for incident light of  = 750 nm, which corresponds to the wavelength just below the bandgap of the PV layer, most of the incident light is reflected at θ < θR due to the high n and small  of the PV layer on the highly reflective Au anode. Alternatively, it is absorbed by the Au anode layer when θ ≈

θR (only for TM-polarized light), with a small amount of absorption by the PV layer. It is also noted that the strong reflection at θ ~ 0° for incident light of λ = 750 nm is the main reason for the low transmission of the Au-IPSC at wavelengths longer than the absorption edge of the PV layer (~650 nm).

9 Fig. S7 Excitation of SPs on Ag-IPSC. Excitation characteristics of SPs on the Ag-IPSC and their effects on the PV characteristics were investigated using the ATR prism- coupling technique as a function of angle of incidence (θ). (a) Angular TM-polarized reflectivity spectra (R) of the Ag-IPSC assembled with an ATR coupling prism (Ag-

IPSC/prism) for an incident light of  = 632.8 nm. The symbols represent the experimental data, and the solid curve represents the theoretical simulation. The SPs are

o excited at a resonant angle of θR = 43.4 . (b) Simulations of depth profiles of electric field intensity (|E|2) in the Ag-IPSC structure as functions of incident angle  and depth z. The colours show the strength of the electric field intensity. (c) Measured J-V curves

o of the Ag-IPSC/prism at the resonance angle (θ = θR = 43.4 ), together with the curves at

o o the lower off-resonance ( = 41.7 < θR) and the higher off-resonance ( = 48.6 > θR) regions for a monochromatic incident light (λ = 632.8 nm, 0.35 mW cm-2). In the J-V

10 curves, it is clear that the observed JSC values depend strongly on the excitation of SPs.

(d) Dependence of normalized JSC of the Ag-IPSC/prism on the angle of incidence  for the 632.8 nm excitation. The circles represent the experimental data, and the solid curve represents the normalized optical absorption of the PV layer, calculated by the FDTD simulation. It is noted that the SPR angle of the JSC(θ) spectra (APG) is nearly identical to that of the R(θ) spectra (ATR, Fig. S2a), confirming that the observed results of the

Ag-IPSC exhibit similar behaviour to those of the Au-IPSC, as described in the main text.

Fig. S8 Comparison of stabilities and lifetimes for the semitransparent IPSCs studied. In order to investigate the stabilities of the fabricated semitransparent IPSCs, we measured the relative PCEs as a function of storage time. After fabrication, the IPSCs were not encapsulated. Because the devices were exposed to ambient air, they degraded, mainly as a result of the presence of oxygen and water in the air.7 The operational lifetimes of

11 the devices were measured under intermittent illumination (solar simulator, 100 mW cm-2, AM 1.5G) at a temperature of 26.1 ± 2.0 °C and a relative humidity of 31 ± 9.7%.

Between measurements, the devices were kept in the dark under open-circuit conditions.

The symbols represent experimental data, and the dotted lines show curves fitted using bi-exponential decay functions.7 As shown in the figure, the Ag-IPSC (closed circles) clearly exhibits a pronounced bi-exponential decay with , where the fast lifetime (t1) is about 3 hrs and the slow lifetime (t2) is about 950 hrs. In contrast to the relatively rapid degradation of the Ag-IPSC, it is clear that the Au-IPSC (open circles) exhibits a nearly single-exponential decay of with t1 ~ t2 ~ 2800 hrs, indicating that the Au anode has the advantage of long term stability. This long term stability of the Au-IPSC is mainly due to the higher ionization energy (or higher work function) of Au than Ag (See

Supplementary Fig. S1a). Note that the PCE values obtained are averages of more than four individual devices on different substrates.

12 Fig. S9 Surface morphologies of the bi-adlayers on Au-IPSC. AFM morphologies (10 ×

10 μm2) for the formation (upper) and desorption (lower) of the Cytop and BSA bi- adlayers on the Au-IPSC were observed using an atomic force microscope set to scan in the static force mode (AFM, Nanosurf EasyScan2 AFM, Nanosurf AG, Switzerland

Inc.). (See also Figs. 3a-b). The root mean square (RMS) surface roughnesses of the single Cytop adlayer and the Cytop/ BSA bi-adlayers on the Au anodes decreased to

3.23 and 2.17 nm, respectively, from a relatively high surface roughness of 5.21 nm of the bare Au anode. After washing with water, the surface roughness (ca. 3.26 nm) of the washed surface was similar to that of the single Cytop adlayer, implying redissolution of the BSA adlayer in water. This result is consistent with the estimated layer thickness of ca. 20.4 nm for the bi-adlayers after washing with water, which is close to that (17.1 nm) of the single Cytop adlayer (See Figure 3b), confirming that the upper BSA was almost removed by the wash water. After successive exposure to the pure Ar plasma, the roughness (ca. 3.69 nm) of the treated surface is still similar to that before the exposure of the Ar plasma, implying little influence of the pure Ar plasma on the adlayers. In contrast, in the areas re-exposed to the Ar/O2 plasma, an area with a rough surface (ca.

5.55 nm) may clearly be seen in the AFM images, indicating that the adlayers were almost etched out by the plasma. These AFM results are also consistent with the SPR observation that the thickness of the adlayers exhibited a slow decrease (ca. 1.7 nm) for the pure Ar plasma, while showing a rapid decrease (ca. 17.1 nm) for the Ar/O2 plasma

(See Fig. 3b), due to the enhanced reaction of adlayers with ozone, hydroxyl radicals and oxygen atoms in the Ar/O2 AP plasma. The AFM results thus confirmed the results of the SPR investigations described above for the nanoscale formation and/or desorption of adlayers.

13 Fig. S10 In-situ monitoring ATR (R) and APG (JSC) signals. (a) Comparison of real time

in-situ monitoring ATR (R) and APG (JSC) signals at four different incidence angles (s) of 43.50°, 45.15°, 45.80° and 46.60° for Cytop/ BSA adlayers on Au-IPSC during an Ar plasma treatment process using an AP plasma jet.8 The adlayers were treated using an Ar plasma jet and the ATR (R) and APG (JSC) signals were recorded as a function of treatment time. As shown in the figure, time-dependent changes in the R curves (ATR) during the treatment are almost identical to those (APG) in the JSC curves. (b) A photograph of an operating Ar plasma jet treating the Cytop/ BSA bi-layers on the Au-

IPSC/prism. To generate the AP plasma jet, we used a needle-type plasma jet source derived from a dc–ac inverter of several tens of kHz in the voltage range 1–3 kV.8 We generated the plasma jet (plume length of about 10 mm), treating the bi-adlayers using a flow of Ar gas (2.5 ml min-1), and investigated the ATR and APG signals as a function of treatment time.

Table S1: Summary of the device performance of the Au-IPSC after each step in the formation and desorption processes for the bi-adlayers of Cytop and BSA on Au anodes.

14 VOC JSC FF PSC Step (V) (mA cm-2) (%) (%) Virgin Au-IPSC 0.59±0.01 9.68±0.42 49.55±1.30 2.82±0.10

9.21± 50.12 2.72± Cytop coating 0.59±0.01 0.38 ±2.99 0.05

0.61± 9.59± 49.46 2.87± BSA coating 0.01 0.63 ±2.72 0.16

0.60± 9.33± 50.45 2.84± Washing with water 0.01 0.88 ±2.04 0.35

0.61± 9.41± 47.97 2.76± Ar plasma treatment 0.01 0.98 ±4.14 0.33

Ar/O2 plasma 0.61± 8.87± 51.87 2.79± treatment 0.01 0.86 ±1.29 0.25

* The values shown are the averages and standard deviations obtained from several (more than five) individual PSCs on independent substrates for the device studied.

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