Active-Material-Independent Color-Tunable Semitransparent Organic

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Active-material-independent Color-tunable Semitransparent Organic Solar Cells Shafidah Shafian, Jieun Son, Youngji Kim, Jerome Kartham Hyun, and Kyungkon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03254 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Materials & Interfaces

Active-Material-Independent Color-Tunable Semitransparent Organic Solar Cells Shafidah Shafian‡, Jieun Son‡, Youngji Kim‡, Jerome K.Hyun*‡, Kyungkon Kim*‡

‡Department of Chemistry and Nanoscience, Ewha Womans University, Seoul, Korea

KEYWORDS: semitransparent organic solar cells, colored OPV, color filter, solution-processed dielectric material, power-generating window

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ABSTRACT: Semitransparent colorful organic solar cells (OSC) provide exciting opportunities for harnessing sunlight as colored windows. Previously, color filter (CF) electrodes on (OSC) were demonstrated via vacuum deposition techniques, resulting in deposition-induced damage. Thus, we present CF integrated organic photovoltaics (CFOPVs) using solution processed TiO2-AcAc as the dielectric component. The noninvasive processing substantially expands the range of usable active materials, allows the device to display pure and vibrant colors that are independent of the inherent color of the active material and show superior optical and photovoltaic characteristics. These results provide practical pathways to realizing colored semitransparent solar cells.


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While solar cells offer a promising pathway to renewable power generation, finding installable areas is difficult especially in cities where land is scarce and costly. To this end, windows provide an excellent opportunity for contributing to the power grid because of their vast areal coverage in buildings and vehicles. Colorful windows, in particular, provide both the ability to harness sunlight and impart aesthetic and artistic value to modern buildings. Organic solar cells offer strong potential as power-generating colorful windows as they exhibit semitransparency, scalability, color, and significant power conversion efficiencies (PCEs).1-3 Previous efforts to impart color to organic solar cells centered on the incorporation of active materials with designed absorption bands.4-7 Nevertheless, such a strategy requires the complex synthesis and processing of organic molecules, challenging practical implementation due to high costs and varied performances among differently colored devices. To overcome these challenges, we previously demonstrated the integration of a Fabry-Perot color filtering (CF) electrode with the organic solar cell, endowing freely tunable colors to the device while maintaining uniform photovoltaic performance with a single active material.8 In this previous study, however, the filter fabrication involved electron-beam (e-beam) vacuum deposition


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techniques of the dielectric CF component, which degraded the open-circuit voltage (Voc) of the device due to UV-induced damage of the active layer (0.75V with PTB7-Th:PC70BM using e-beam processed CF compared to 0.81V without using e-beam processed CF).8 As some organic active molecules are more susceptible to UV damage than others, vacuum deposition techniques can restrict the range of usable active materials. Herein, we report on CF-OPVs optimized in both photovoltaic and optical performance by employing an inexpensive and scalable CF made by non-invasive processing. The device fabrication involves solution processing of the dielectric CF component, allowing the solar cell to achieve superior photovoltaic performance with a PCE of around 4.30 to 5.80%. Furthermore, solution processing of the dielectric component expands the range of usable active materials permitting any arbitrary active material that does not exhibit perfect absorption in the visible range to display a full palette of colors regardless of its inherent color. In this paper we used three different active materials: PTB7-Th:ITIC, PCDTBT:PC70BM and PTB7-Th:PC70BM to represent intrinsically blue, red and colorless active materials, respectively, while using solution processed TiO2-AcAc as the dielectric layer for the CF. The ability to generate pure colors with the CF from distinctly colored


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active materials provides one the freedom to explore and balance photovoltaic performance and aesthetic figures of merit. Figure 1a-b describes the basic schematic of the CF-OPVs and a colorless semitransparent OPV without the CF for reference purposes. Both devices share an inverted structure that is identical except at the anode. The inverted structure consists of the following layers (thickness) arranged sequentially: an ITO cathode (140 nm) on a glass substrate, a ZnO electron transport layer (ETL, 30 nm), active material (75(±5) nm), and MoO3 hole transport layer (HTL, 10 nm for CF-OPVs and 2 nm for the reference semitransparent OPV). For the anode, the reference semitransparent OPV employs a 10 nm-thick Ag layer followed by a 40 nm-thick MoO3 layer, constituting an Oxide/Metal/Oxide (OMO) semitransparent electrode, whereas the CF-OPV employs a 35 nm-thick Ag layer, a dielectric layer comprising TiO2-AcAc of variable thickness followed by a 25 nm-thick Ag layer with a measured sheet resistance of 2.0-2.4 Ωcm-1. The CF operates by supporting a half-wavelength resonance between the two Ag layers, defined by the thickness of the TiO2-AcAc layer. This mode corresponds to the lowest energy mode, also known as the fundamental mode. Light undergoing the resonance


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leaks into free space, resulting in colored transmission, while light outside the resonance reflects back into the device, contributing to additional exciton generation. To demonstrate the expanded range of active materials compatible with the CF, we built OPVs using different sets of donors and acceptors. Figure 1c shows the chemical structure of two donors: low band gap polymer PTB7-Th and wide band gap polymer PCDTBT with -5.30 eV and -5.35 eV HOMO and -3.71 eV and -3.42 eV LUMO, respectively, and two acceptors: non-fullerene ITIC and fullerene PC70BM with -5.62 eV and -5.81 eV HOMO and -3.93 eV and -3.91 eV LUMO, respectively.9-12 PTB7-Th:ITIC, PCDTBT:PC70BM and PTB7-Th:PC70BM were chosen to represent three classes of bulk heterojunction (BHJ) active material that display different intrinsic colors. Figure 1d shows the normalized absorption for each active layer normalized by its maximum intensity. PTB7-Th:ITIC absorbs strongly in the red, rendering the active material blue, while PCDTBT:PC70BM absorbs in the blue, rendering the active material red. PTB7Th:PC70BM absorbs uniformly across the visible range, therefore produces no inherent color. Figure 1e shows the corresponding CIE colors based on transmission spectra of the active layer film as shown in Supplementary Figure S1.


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Figure 1. Device structure of (a) CF-OPV and (b) reference semitransparent OPV. (c) Chemical structure of donor and acceptor molecules used in the set of active materials:PTB7-Th:ITIC,

PCDTBT:PC70BM,

PTB7-Th:PC70BM.

(d)

Normalized

absorption and (e) CIE chromaticities of PTB7-Th:ITIC, PCDTBT:PC70BM, PTB7Th:PC70BM.

The CF was fabricated in a non-invasive manner using solution processed TiO2-AcAc as the dielectric layer. Monodisperse TiO2 nanoparticles (NPs) were first obtained through hydrolysis of titanium butoxide in the presence of acetylacetone (AcAc) and para-toluene


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sulfonic acid. The surface of the TiO2 NPs were stabilized with AcAc ligands, which allows the NPs to be dispersed without aggregation in alcoholic solutions at concentration higher than 1M, improving the processability of the NPs.13 We obtained the XRD spectrum of TiO2-AcAc NPs and compared it with that of commercial anatase TiO2 particles as shown in Figure 2a. Although the TiO2-AcAc is largely amorphous in nature, weak peaks corresponding to the reference TiO2 anatase crystalline peaks are apparent, confirming the presence of nanocrystalline TiO2-AcAc. HR-TEM images further confirm its presence, as displayed in Figure 2b. Crystallites with sizes of 4~5 nm are identified via lattice fringes encircled by white dotted lines in the inset of Figure 2b. To examine the structure of the OPV, we prepared cross-sections of three different colored devices using focused ion beam milling (FIB) and imaged them with the TEM. As seen in Figure 2c-e, all layers, including the TiO2-AcAc layer, exhibit uniform and homogeneous coverage. Although the internal structure is consistent for the three devices, the TiO2-AcAc layer thickness differs, giving rise to distinct colors due to the changed cavity length. The range of thicknesses from 75 to 115 nm is shown to produce resonances in the visible range, which is slightly larger than that produced by e-beam


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processed TiOx films.8 This suggests that the refractive index of solution-processed TiO2 films is smaller than that of e-beam processed films to compensate for the increased thickness. Refractive index measurements performed through ellipsometry indeed reveal that the index of the solution processed TiO2-AcAc film is on average 24% smaller than that of e-beam processed TiOx film, as shown in Supplementary Figure S2, due to the lower density of the TiO2-AcAc film.

Figure 2. (a) XRD of TiO2-AcAc and commercial TiO2 anatase powder. (b) HR-TEM of


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TiO2-AcAc film. Inset: magnified lattice fringes indicating the presence of TiO2 nanoparticles. (c-e) Cross-sectional TEM images of a (c) blue, (d) green and (e) orange colored CF-OPV.

The solution processed TiO2-AcAc CF renders excellent color saturation, as shown in Figure 3. A semitransparent OMO electrode is also shown for reference. Figure 3a shows photographs of the fabricated OMO and CFs with size of 2.5 x 2.5 cm2. Each CF shows distinct colors throughout its area. To quantify the optical characteristics, we measured the transmission spectra of the OMO and CFs as shown in Figure 3b. The OMO shows spectrally broad transmission of around 50~60% while the CFs show spectrally selective transmission characterized by a single peak with FWHM of 61, 59, 57 and 57 nm, for the blue, green, orange and red CFs, respectively. These results were compared to transfer matrix calculations14-16 using experimental refractive indices of the TiO2-AcAc layer extracted through ellipsometry and tabulated refractive indices of Ag from Johnston and Christy.17 The calculated FWHMs for the blue, green, orange and red colors were 25, 42, 45 and 47 nm, respectively, which are smaller than those from measurement due to the


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preclusion of surface roughness and spatial inhomogeneities in the model. Even so, mapping the measured chromaticities onto the CIE color diagram, as shown in Figure 3c, one can see that all colors reside on the borders of the sRGB triangle, indicating excellent color saturation. The OMO anode from the reference device exhibits CIE chromaticities corresponding to a white color. Further comparisons between measurement and calculation show excellent agreement both qualitatively and quantitatively in spectral position and intensity of the peaks. The transmission dependence on TiO2-AcAc thickness was also calculated as shown in Figure 3d. We considered a TiO2-AcAc thickness range from 40 to 150 nm with fixed Ag thickness of 35 nm and 25 nm for the bottom and top layer, respectively. For a TiO2AcAc thickness range from 40 to 130 nm, spectrally pure colors spanning from violet to red with peak transmission values of 65-75% are achievable. Above a thickness of 130 nm, a second order resonance emerges into the blue end of the spectrum compromising the spectral purity. To confirm the calculated trend, we plotted the measured resonant wavelength against its respective TiO2-AcAc layer thickness, extracted from AFM measurements (Table 1), as shown by the white square markers in Figure 3d. The


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measurement was found to exhibit a linear relationship that agreed well with the calculated results. Color stability of the solution processed CF electrodes stored under a nitrogen glove box was excellent after 4 months and 26 days as shown in Supplementary Figure S3.

Figure 3. Optical characteristic of CFs. (a) Photos of CFs illuminated through a white sheet. (b) Experimental (top) and calculated (bottom) transmission spectra of OMO


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reference and blue, green, orange and red CFs. (c) CIE chromaticities of the OMO reference and CFs. (d) Calculated transmission of the CFs as a function of wavelength and TiO2-AcAc thickness. The white square marker denotes the measured resonance of each CF and corresponding TiO2-AcAc thickness.

Table 1. Measured thickness of CF components and optical characteristics of CFs.

Thickness (nm)

Optical parameter of CFs

Bottom Ag layer

TiO2-AcAc layer

Top Ag layer

Peak transmission (%)

Center of resonance (nm)

FWHM (nm)

Ref

-

-

-

~60

400-700

-

Blue

35

60

25

62

426

61

Green

35

95

25

68

534

59

Orange

35

115

25

66

592

57

Red

35

130

25

59

647

57

Name

One key advantage of the solution processed TiO2-AcAc CFs is the ability to produce distinct and highly saturated colors regardless of the inherent color of the active material. Figure 4 shows the transmission spectra (Figure 4a), CIE color diagram (Figure 4b) and optical images (Figure 4c) of OPVs made with different active materials displaying different intrinsic colors: PTB7-Th:ITIC (Figure 4(i)), PCDTBT:PC70BM (Figure 4(ii)) and PTB7-Th:PC70BM(Figure 4(iii)). Despite the differences in intrinsic colors, integration of


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the solution processed TiO2-AcAc CF yields completely distinct and vibrant colors in the solar cells as shown by CF-OPVs illuminated through a white sheet in Figure 4c. Comparison of the transmission spectra between reference semitransparent OPV and CF-OPVs shows that the intrinsic absorption modulates the transmission intensity of all colors (Table 2). Colors from blue to red show a variation from 14 to 43% for PTB7Th:ITIC; 31 to 43% for PCDTBT:PC70BM; and 38 to 46% for PTB7-Th:PC70BM. In particular, PTB7-Th:PC70BM OPVs show consistent peak transmission efficiencies in the considered wavelength range. We note that the peak transmission efficiencies from the CF-OPVs coincide with that of the reference at the corresponding wavelength, suggesting that the CF is efficient in its transmission since it does not experience additional optical losses. Furthermore, the choice of active material does not affect the color saturation because the color generating mechanism occurs outside the absorbing layer.


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Figure 4. Optical characteristics of CF-OPVs comprising PTB7-Th:ITIC (top row), PCDTBT:PC70BM (middle row) and PTB7-Th:PC70BM (bottom row). (a) Measured transmission spectra, (b) corresponding CIE chromaticities and (c) photo of semitransparent reference, blue, green, orange and red CF-OPVs.

Table 2. Measured thickness of CF-component and optical characteristics of CF-OPVs comprising PTB7-Th:ITIC, PCDTBT:PC70BM and PTB7-Th:PC70BM.

Thickness (nm) Active Material

PTB7-Th:ITIC

PCDTBT:PC70BM

Optical parameters of CF-OPV

Ref

-

TiO2AcAc layer -

-

~12-48

420-780

Blue

35

65

25

43

444

Device

Bottom Ag layer

Top Ag layer

Peak transmission (%)

Center of resonance (nm)

Green

35

95

25

36

538

Orange

35

115

25

20

595

Red

35

125

25

14

620

Ref

-

-

-

~29-49

420-780


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PTB7-Th:PC70BM

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Blue

35

75

25

31

474

Green

35

100

25

33

556

Orange

35

115

25

36

586

Red

35

125

25

43

623

Ref

-

-

-

~38-58

420-780

Blue

35

65

25

46

432

Green

35

100

25

42

543

Orange

35

120

25

44

617

Red

35

135

25

38

667

We further examined the photovoltaic performance of the solution processed TiO2-AcAc CF-OPVs. Figure 5 compares the photovoltaic characteristics of CF-OPVs made from three distinct active materials to that of the reference semitransparent OPVs made from the respective active materials. For all cases, the CF-OPVs generate higher short-circuit current (Jsc) values compared to that of reference semitransparent OPVs as the CFs reflect non-resonant light back into the active material for additional exciton generation. Jsc enhancement of each CF-OPV relative to the reference semitransparent OPV is shown in Table 3. The CF-OPVs show a Jsc enhancements between 41 to 47% for PTB7Th:ITIC, 63 to 70% for PCDTBT:PC70BM and 52 to 59% for PTB7-Th:PC70BM. Despite the drastically different spectral absorption characteristics, each type of CF-OPV is found to experience a consistent enhancement in its Jsc. In addition, Figure 5a and Table 3 show that distinctly colored CF-OPVs of the same active material give rise to consistent


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photovoltaic characteristics, including the Voc and Fill Factor (FF). These results indicate that regardless of the active material and spectral absorption, distinctly colored CF-OPVs of the same active material maintain uniform performance.


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Figure 5. (a) J-V characteristics and (b) EQE spectra of semitransparent reference OPV and blue, green, orange and red CF-OPVs using PTB7-Th:ITIC (top row), PCDTBT:PC70BM (middle row) and PTB7-Th:PC70BM (bottom row).


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Table 3. Device performance parameters of PTB7-Th:ITIC, PCDTBT:PC70BM and PTB7Th:PC70BM

PCDTBT:PC70BM

PTB7-Th:ITIC

Active Material

PTB7-Th:PC70BM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a,b,d,e)

Jsc enhancement (%) c)

FFd)

PCE (%) e)

PCE enhancement (%)f)

Jsc EQE (mA/cm2)

7.85 / 8.11 ± 0.22

-

0.55 / 0.54 ± 0.03

3.52 / 3.50 ± 0.15

-

8.04

0.82 / 0.82 ± 0.00

11.54 / 11.60 ± 0.16

+47.00

0.60 / 0.57 ± 0.02

5.68 / 5.40 ± 0.22

+61.36

11.22

Green

0.82 / 0.82 ± 0.00

11.34 / 11.26 ± 0.11

+44.46

0.60 / 0.58 ± 0.03

5.58 / 5.37 ± 0.30

+58.52

11.03

Orange

0.82 / 0.81 ± 0.01

11.53 / 11.42 ± 0.17

+46.88

0.61 / 0.56 ± 0.07

5.78 / 5.24 ± 0.75

+64.20

10.92

Red

0.82 / 0.82 ± 0.00

11.07 / 10.83 ± 0.23

+41.02

0.60 / 0.61 ± 0.01

5.48 / 5.42 ± 0.05

+55.68

10.98

Ref

0.90 / 0.89 ± 0.00

5.24 / 5.28 ± 0.04

-

0.51 / 0.51 ± 0.01

2.40 / 2.39 ± 0.06

-

5.84

Blue

0.91 / 0.91 ± 0.00

8.52 / 8.41 ± 0.13

+62.60

0.57 / 0.57 ± 0.00

4.42 / 4.34 ± 0.08

+84.17

8.38

Green

0.91 / 0.91 ± 0.01

8.59 / 8.35 ± 0.16

+63.93

0.55 / 0.55 ± 0.01

4.30 / 4.19 ± 0.13

+79.17

8.33

Orange

0.91 / 0.91 ± 0.00

8.56 / 8.57 ± 0.03

+63.36

0.58 / 0.58 ± 0.00

4.53 / 4.52 ± 0.01

+88.75

8.67

Red

0.91 / 0.91 ± 0.00

8.91 / 8.85 ± 0.09

+70.04

0.59 / 0.58 ± 0.02

4.78 / 4.69 ± 0.20

+99.17

9.12

Ref

0.80 / 0.81 ± 0.00

7.35 / 7.20 ± 0.12

-

0.58 / 0.57 ± 0.01

3.43 / 3.31 ± 0.08

-

7.66

Blue

0.81 / 0.81 ± 0.00

11.50 / 11.62 ± 0.16

+56.46

0.52 / 0.51 ± 0.01

4.85 / 4.80 ± 0.05

+41.40

11.78

Green

0.81 / 0.81 ± 0.00

11.71 / 11.36 ± 0.30

+59.32

0.52 / 0.52 ± 0.00

4.94 / 4.74 ± 0.15

+44.02

11.5

Orange

0.81 / 0.81 ± 0.00

11.17 / 11.04 ± 0.25

+51.97

0.53 / 0.50 ± 0.02

4.77 / 4.49 ± 0.27

+39.07

11.27

Red

0.81 / 0.81 ± 0.00

11.65 / 11.00 ± 0.22

+58.50

0.51 / 0.50 ± 0.00

4.78 / 4.44 ± 0.13

+39.36

11.72

Device

Voc (V)a)

Ref

0.81 / 0.80 ± 0.01

Blue

Jsc (mA/cm2) b)

Value from best PCE/Average with standard deviation; c) Best Jsc enhancement; f)

Best PCE enhancement


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More importantly, we observe that the Voc does not degrade significantly compared to the reference semitransparent OPV, in contrast to results obtained by e-beam processed CF- OPVs.8 The degraded performance in the latter case originates from damage to the active layer by UV light generated by the e-beam irradiated target. In the case of solution processed TiO2-AcAc CF-OPVs, such damage mechanism is absent, ensuring that the Voc stays consistent with that of the reference semitransparent OPV. Detailed comparison of the performance between e-beam processed TiOx and solution processed TiO2-AcAc CF-OPVs indeed shows that the solution processing does not impair the Voc as seen in Figure 6 and Table 4. Figure 6 shows the Jsc characteristics of solution

processed

and

e-beam

processed

CF-OPVs

PCDTBT:PC70BM, and PTB7-Th:PC70BM as the active layer.

with

PTB7-Th:ITIC,

One can see that

regardless of active material, OPVs with CFs fabricated through e-beam deposition result in lower Vocs and FFs compared to those with solution processed TiO2-AcAc CFs. As UV damage occurs mostly in fullerene materials, the e-beam processed CF-OPV with PTB7Th:PC70BM shows a larger drop in Voc (0.81V to 0.75V) compared to that with an active material of the same donor but of a non-fullerene acceptor, PTB7-Th:ITIC.18


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Figure 6. J-V characteristics of semitransparent reference OPV (black), OPV with 65nm solution processed TiO2-AcAc CF (red), and OPV with 65nm e-beam TiOx processed CF (blue) using (a) PTB7-Th:ITIC, (b) PCDTBT:PC70BM and (c) PTB7-Th:PC70BM as the active material.

Table 4. Device performance parameters of semitransparent reference OPV, OPV with solution processed TiO2-AcAc CF and OPV with e-beam processed TiOx CF using PTB7Th:ITIC, PCDTBT:PC70BM and PTB7-Th:PC70BM as the active material.

PTB7-Th:ITIC

Active Material

PCDTBT:PC70BM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Device

Voc (V) a)

Jsc (mA/cm2) b)

FF c)

PCE (%)d)

Ref

0.81 / 0.80 ± 0.01

7.85 / 8.11 ± 0.22

0.55 / 0.54 ± 0.03

3.52 / 3.50 ± 0.15

Solution processed TiO2–AcAc

0.82 / 0.82 ± 0.00

11.87 / 11.79 ± 0.12

0.61 / 0.59 ± 0.01

5.96 / 5.69 ± 0.19

E-beam processed TiOx

0.81 / 0.81 ± 0.01

12.86 / 12.52 ± 0.23

0.57 / 0.56 ± 0.02

5.94 / 5.62 ± 0.31

Ref

0.90 / 0.89 ± 0.00

5.24 / 5.28 ± 0.04

0.51 / 0.51 ± 0.01

2.40 / 2.39 ± 0.06


0.91 / 0.90 ± 0.02

8.18 / 8.28 ± 0.12

0.58 / 0.53 ± 0.05

4.31 / 3.93 ± 0.37


0.89 / 0.89 ± 0.01

8.17 / 8.11 ± 0.05

0.55 / 0.53 ± 0.02

4.02 / 3.83 ± 0.19


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PTB7-Th:PC70BM


a,b,c,d)

Page 22 of 32

Ref

0.80 / 0.81 ± 0.00

7.35 / 7.20 ± 0.12

0.58 / 0.57 ± 0.01

3.43 / 3.31 ± 0.08


0.81 / 0.81 ± 0.00

11.50 / 11.62 ± 0.16

0.52 / 0.51 ± 0.01

4.85 / 4.80 ± 0.05


0.75 / 0.75 ± 0.00

11.73 / 11.57 ± 0.22

0.51 / 0.51 ± 0.00

4.46 / 4.31 ± 0.21

Value from best PCE/Average with standard deviation;

To further study the loss in Voc with fullerene material by e-beam processing, the fullerene acceptor was paired with a different donor, PCDTBT:PC70BM. No significant drop in Voc was found by using PCDTBT:PC70BM with e-beam processed CFs. Solution processed and e-beam processed CF-OPVs showed a similar Voc of 0.90V and 0.89V, respectively, presumably due to the lower surface energy of PCDTBT than that of PCB7Th. Previous studies reported phase separation of PCDTBT from the acceptor toward the top of the active layer due to lower surface energy.19, 20 The polymer rich region of the active layer shares an interface with the hole transport layer, MoO3 and bottom Ag component of the CF, therefore providing additional protection against UV irradiation during the dielectric layer formation. It has also been reported that PCDTBT is more stable than PTB7-Th, inhibiting rearrangement of the active material morphology upon UV


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irradiation.21-24 Therefore, appropriate selection of the donor material is required to help mitigate UV-induced damage for e-beam processed CFs. One important application of CF-OPVs is in colorful solar windows. Solar windows are candidates for replacing conventional windows because of their ability to harvest sunlight into electricity. Meanwhile, colored windows can enhance the utility of sunlight by brightening and guiding colored light into the room to create a wide range of visual ambiences.25 Figure 7 and Supporting Movie S1-S3 show the colorful shadow of each CF-OPV illuminated by a white light source. The colored shadow appears on the white paper because the filtered light scatters off the microscopic sites of the paper. The CFOPV efficiently divides the utility of sunlight by allowing filtered light to create decorative and aesthetically pleasing environments and unfiltered light to generate power.


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Figure 7. (a) Illustration and (b) photo of setup showing the semitransparent reference OPV with CF-OPVs and projected shadow. (c) Photos of semitransparent reference OPV with CF-OPVs made from PTB7-Th:ITIC, PCDTBT:PC70BM and PTB7-Th:PC70BM, and their colorful shadows projected onto white paper.

In summary, we demonstrated solution processed TiO2-AcAc CFs for use in semitransparent, colored OPVs. Due to the soft processing, the fabrication preserves the electrical characteristics of the device while imparting optical functionality to the device. This enables any arbitrary active material that does exhibit perfect absorption in the visible range to produce a full palette of vibrant colors regardless of the intrinsic color of the active material. We further demonstrated consistent photovoltaic performances among


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differently colored CF-OPVs of the same active material regardless of its spectral absorption characteristics. In addition, superior Voc, FF and PCE values relative to those of e-beam processed CF-OPVs were observed. These results suggest that solution processed TiO2-AcAc CFs offer one the toolbox to explore and design colorful powergenerating windows with optimized color and photovoltaic performance. As solution processing represents an inexpensive and scalable fabrication approach, we believe that the results promote the practical use of CF-OPVs for colorful power-generating windows.


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ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the http://pubs.acs.org.

Experimental section, absorption and transmission of active materials, refractive index of dielectric materials and color stability of CF electrodes (PDF)

Semitransparent

reference

OPV

and

CF-OPVs

made

from

PTB7-Th:ITIC,

PCDTBT:PC70BM and PTB7-Th:PC70BM, and their colorful shadows projected onto white paper (MPEG)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (K.K.K.)

*Email: [email protected] (J.K.H.)

ORCID

Jerome K. Hyun: 0000-0002-2630-5051


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Kyungkon Kim: 0000-0002-0115-8112

Author Contributions ‡These authors contributed equally.

Funding Sources This research was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT and Future Planning (NRF-2015M1A2A2057506, NRF-2016M1A2A2940914, and NRF-2017R1A5A1015365).

Notes The authors declare no competing financial interests. ACKNOWLEDGEMENT This research was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT and Future Planning (NRF-2015M1A2A2057506, NRF2016M1A2A2940914, and NRF-2017R1A5A1015365). ABBREVIATIONS

OSC, organic solar cell; CF, color filter; CF-OPVs, color filter organic photovoltaics; AcAc; acetyl acetone; PCE, power conversion efficiency; Voc, open circuit voltage; Jsc,


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short circuit current; FF, fill factor; ETL, electron transport layer; HTL, hole transport layer; OMO, oxide/metal/oxide; BHJ, bulk heterojunction; NP, nanoparticle; FIB, focused ion beam; e-beam, electrom beam; XRD; x-ray powder diffraction; TEM, transmission electron microscopy; AFM, atomic force microscopy.


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“Graphic for manuscript”


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Active-Material-Independent Color-Tunable Semitransparent Organic

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