STOPV Application

II. Exploiting Ternary Blends to Accurately Control the Coloration of Semi-Transparent, Non-

2.3 Results and Discussion

2.3.2 STOPV Application

Figure 2. 2. J–V characteristics of (a) T2-ORH based devices with variable CN concentrations and (b) corresponding IPCE. J–V characteristics of (c) T2-OEHRH with variable donor to acceptor (D:A) ratio and (d) corresponding IPCE spectra.

To investigate the potential application of color-tunable, ternary, active-layer blends in STOPVs, photovoltaic devices were prepared and characterized. Active layer blends consisting of two NFAs were first optimized by varying the solution concentration, donor:acceptors (D:A) ratio, solvent additive content, and treatment by thermal annealing and solvent vapor annealing. A conventional structure of Glass/ITO/PEDOT:PSS/Active Layer/ZnO NPs/Al was used during the optimization procedure. As shown in Figure 2. 2 and Table 2. 1, it was observed that colors of binary blend films (PTB7-Th:T2- ORH or :T2-OEHRH) changed significantly from purple to reddish-purple upon increasing the volumetric ratio of CN additive or D:A ratio. Solvent additives are known to improve the packing order of films compared to films processed from pure solvents, which often leads to red- or blue-shifted absorption spectra in films processed with additives.33, 129, 130 In this study, the absorption of the blend films were observed to shift bathochromically compared to the as-cast blend films as the amount of CN additive increased, leading to a change in color from purple to reddish-purple. Each blend showed

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -12

-10 -8 -6 -4 -2 0 2

Current density (mA cm-2 )

Voltage (V)

w/o

w/ 0.1 vol% CN w/ 0.25 vol% CN w/ 0.5 vol% CN

300 400 500 600 700 800

0 10 20 30 40 50 60

IPCE (%)

Wavelength (nm)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 -12

-10 -8 -6 -4 -2 0 2

Current density (mA cm-2)

Voltage (V)

1:1 1:1.5 1:2

300 400 500 600 700 800

0 10 20 30 40 50 60 70

IPCE (%)

Wavelength (nm)

a b

c d

63

optimal performance when the active layer composition had a reddish color (1:1.5 ratio with 0.25 vol%

CN for PTB7-Th:T2-ORH and 1:2 ratio with 0.25 vol% CN for PTB7-Th:T2-OEHRH). Based on these results, we tried to demonstrate various film colors including red, green and blue by extending to a ternary system. Here, we selected PTB7-Th as the donor polymer and IEICO-4F as the NFA, which were blended with T2-ORH or T2-OEHRH ternary components. The PTB7-Th:IEICO-4F binary blend film, as shown in Figure 2. 3a, b, exhibited a cyan color. The color of the ternary blend films could be changed dramatically by increasing relative ratios of T2-ORH and T2-OEHRH. In this study, we prepared ternary blend films by physically mixing two solutions of binary donor:acceptor mixtures, where the same donor was used in both solutions. The ratios reported throughout the manuscript can be defined as volumetric ratios of each pair of binary solutions; the first and second number in the ratio correspond to the ratio of PTB7-Th:IEICO-4F blend and PTB7-Th:T2-ORH or :T2-EOHRH blend solutions, respectively. For example, 0.9:0.1 ternary blend film indicates that a solution consisting of 90% by volume of a PTB7-Th:IEICO-4F solution and 10% by volume of a PTB7-Th:T2-ORH was used. Thus, 0.9:0.1 and 0.8:0.2 ratios for PTB7-Th:IEICO-4F:T2-ORH and 0.95:0.05 and 0.85:0.15 for PTB7-Th:IEICO-4F:T2-OEHRH system, changed from aqua to purple, while the binary blend films (PTB7-Th:T2-ORH or :T2-OEHRH) yielded a reddish-purple colors. the ternary blend films at each volumetric ratio, denoted as 0.9:0.1 and 0.8:0.2 for PTB7-Th:IEICO-4F:T2-ORH and 0.95:0.05 and 0.85:0.15 for PTB7-Th:IEICO-4F:T2-OEHRH system, changed aqua to purple then the binary blend film (PTB7-Th:T2-ORH or :T2-OEHRH) yielded a reddish-purple color. Although a wider range of ratios were initially screened, these specific ratios were selected for further optimization due to their aesthetic appearance.

Table 2. 1. Summary of solar cell characteristics of T2-ORH and T2-OEHRH based devices results using variable amounts of CN or D:A ratios.

Donor

:Acceptor D:A Solvent JSC

(mA/cm²)

Cal. JSC

(mA/cm²) VOC

(V) FF PCE

(%)

Film Color

PTB7-Th

:T2-ORH 1:1.5

w/o 7.23 - 0.93 0.46 3.08

0.1 vol%

CN 8.95 - 0.73 0.43 2.80

0.25 vol%

CN 11.31 10.73 1.08 0.60 7.37

0.5 vol%

CN 4.67 - 0.85 0.56 2.21

PTB7-Th

:T2-OEHRH 1:1 0.25 vol%

CN 9.37 10.12 1.05 0.39 3.83

64

1:1.5 11.08 11.62 1.05 0.50 5.83

1:2 11.97 11.79 1.06 0.56 7.03

* Device Structure = Glass/ITO/PEDOT:PSS/PTB7-Th:T2-ORH or :T2-OEHRH/ZnO NPs/Al

Figure 2. 3. High resolution color photos for (a) T2-ORH and (b) T2-OEHRH based active layers at various blend ratios. Color tunable semi-transparent devices for (c) T2-ORH and (d) T2-OEHRH showing semi-transparent SAS electrodes.

In a previous report, ZnO NPs-Ti exhibited improved colloidal stability and possible processing at lower solution concentration.¹³¹ Therefore, the ZnO NPs were replaced with ZnO NPs-Ti under optimized conditions. In addition, to enhance the color tunability of the devices, dichroic electrodes consisting of thermally evaporated Sb2O3 and Ag films were employed. Semi-transparent electrodes consisting of SAS stacks have previously been shown to transmit specific colors of light while reflecting complimentary wavelengths,¹³² causing a dichroic effect and enhancing the perceived color of the devices. Because the same SAS architecture and layer thicknesses were used for all of the SAS devices in this study, the SAS electrodes did not contribute to changes in the spectra of transmitted or reflected light and the same dichroic effect due to the SAS stack occurred in all devices. The reflectance and

1:0 0.90:0.10 0.80:0.20 0:1 a

b

1:0 0.95:0.05 0.85:0.15 0:1 c

d

1:0 0.90:0.10 0.80:0.20 0:1

1:0 0.95:0.05 0.85:0.15 0:1

65

transmittance spectra of the devices are shown in Figure 2. 4. For these experiments, light is either reflected, absorbed or transmitted at each wavelength. Notably, light absorption by the active layer caused a decrease in both the transmittance and reflectance spectra at wavelengths where the active layer absorbed strongly. The reflectance of all devices was low at short wavelengths and generally increased with wavelength from 530−1050 nm in all blend films with minor relative decreases in reflectance that coincided with strong absorption bands in each active layer. The spectrum of light transmitted through the active layers was more clearly affected by the absorption bands of the active layers, and showed a general downward trend (the opposite of the reflectance spectra) at wavelengths beyond the absorption onsets of the active layers. For direct comparison, opaque OPVs were fabricated using optimized conditions, with a top electrode of Sb2O3/Ag then the STOPVs were replaced with a transparent electrode based on SAS electrodes.¹²⁵ In this study, the Sb2O3 based electrodes were deposited using an optimized combination of thicknesses for the Sb2O3 and Ag layers, as described in experimental section. In other words, using a device architecture consisting of Glass/ITO/PEDOT:PSS/Active Layer/ZnO NPs-Ti, Sb2O3/Ag and SAS were used as atop electrodes for the opaque OPVs and STOPVs, respectively.

Figure 2. 4. (a) Reflectance and (b) Transmittance spectra of color tunable devices with the structure of Glass/ITO/PEDOT:PSS/Active Layer/ZnO NPs-Ti/SAS.

In order to thoroughly optimize the STOPVs, the thickness of active layer was varied and optimized for all the blend systems; these photovoltaic characteristics are shown in Figure 2. 5. and summarized in Table 2. 2. It is evident that the semi-transparent devices for all formulations require thicker active layers compared to the opaque devices in order to achieve optimal PCE values, however, we found that the differences in PCE were not large for these thick devices compared to the conditions which were used for optimal semi-transparency and detailed optical characterization. J‒V characteristics of the devices are shown in Figure 2. 6a, c and the photovoltaic parameters are summarized in Table 2. 3.

With Sb2O3/Ag electrodes, PTB7-Th:IEICO-4F, 0.9:0.1, 0.8:0.2 and PTB7-Th:T2-ORH showed PCE

300 400 500 600 700 800 900 1000 0

10 20 30 40 50 60 70

Reflectance (%)

Wavelength (nm) IEICO-4F 0.95:0.05 0.90:0.10 0.85:0.15 0.80:0.20 T2-OEHRH T2-ORH

300 400 500 600 700 800 900 1000 0

10 20 30 40 50 60 70

IEICO-4F 0.95:0.05 0.90:0.10 0.85:0.15 0.80:0.20 T2-OEHRH T2-ORH

Transmittance (%)

Wavelength (nm)

a b

66

values of 10.53%, 10.40%, 10.77% and 6.01%, respectively. Likewise, the 0.95:0.05, 0.85:0.15 and

-0.2 0.0 0.2 0.4 0.6 0.8

-16 -14 -12 -10 -8 -6 -4 -2 0 2

Current density (mA cm-2)

Voltage (V)

0.95:0.05 160 nm 130 nm 110 nm 90 nm

-0.2 0.0 0.2 0.4 0.6 0.8

-16 -14 -12 -10 -8 -6 -4 -2 0 2

Current density (mA cm-2 )

Voltage (V)

0.85:0.15 160 nm 110 nm 100 nm 90 nm

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 -8

-6 -4 -2

0 T2-OEHRH

140 nm 110 nm 70 nm 60 nm

Current density (mA cm-2)

Voltage (V)

-0.2 0.0 0.2 0.4 0.6 0.8

-16 -14 -12 -10 -8 -6 -4 -2 0 2

Current density (mA cm-2)

Voltage (V)

IEICO-4F 160 nm 130 nm 90 nm 70 nm

-0.2 0.0 0.2 0.4 0.6 0.8

-16 -14 -12 -10 -8 -6 -4 -2 0 2

Current density (mA cm-2 )

Voltage (V)

0.9:0.1 170 nm 110 nm 100 nm 90 nm

-0.2 0.0 0.2 0.4 0.6 0.8

-16 -14 -12 -10 -8 -6 -4 -2 0 2

Current density (mA cm-2)

Voltage (V)

0.8:0.2 150 nm 140 nm 120 nm 90 nm

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 -8

-6 -4 -2 0

Current density (mA cm-2)

Voltage (V)

T2-ORH 140 nm 110 nm 90 nm 80 nm

a b

c d

e f

g

Figure 2. 5. J–V characteristics of STOPVs based (a) IEICO-4F, (b) 0.9:0.1, (c) 0.8:0.2, (d) T2-ORH, (e) 0.95:0.05, (f) 0.85:0.15 and (g) T2-OEHRH compositions with variable active layer thickness.

67

PTB7-Th:T2-OEHRH based opaque OPVs yielded similar PCEs of 10.58%, 10.64% and 6.40%, respectively. The addition of T2-ORH or T2-OEHRH to the IEICO-4F binary blends did not have a significant beneficial effect on the photovoltaic properties. Meanwhile, interesting phenomena were observed in the ternary blends in terms of VOC. The IEICO-4F based binary system exhibited a lower VOC of 0.70 V compared to that of T2-ORH (1.07 V) and T2-OEHRH (1.08 V). It is obvious that large difference VOCs between IEICO-4F and T2-ORH or T2-OEHRH binary system arise from the significantly lower LUMO of IEICO-4F (−4.19 eV) than that of T2-ORH (−3.59 eV) and T2-OEHRH (−3.58 eV), which can be seen in Figure 2. 1c. For both T2-ORH and T2-OEHRH ternary systems, the VOCs continuously increased with increasing T2-ORH or T2-OEHRH content, suggesting that IEICO- 4F and T2-ORH or T2-OEHRH preferentially form an alloy state in the ternary blends. The previously reported alloy-like model can be used to describe ternary OPVs which exhibit continuously variable VOCs due to mixing of one donor with two acceptors having different LUMO energy levels;^133-135 the same effect which was observed in the present system.

Table 2. 2. Summary of photovoltaic parameters for STOPVs based on T2-ORH and T2-OEHRH with variable active layer thickness. The results of opaque devices are also listed for comparison.

Active Layer

IEICO-4F :Acceptor

(v/v)

Electrode Thickness (nm)

JSC

(mA cm^-2) VOC

(V) FF PCE

(%)

Rsh

(Ω cm²) Rs

(Ω cm²)

PTB7-Th

:IEICO-4F 1:0 SAS

160 14.66 0.68 0.56 5.61 684.95 11.73 130 14.24 0.71 0.62 6.22 717.60 9.18

90 13.65 0.70 0.58 5.52 - - 70 13.33 0.70 0.54 5.04 - - Ag/Sb2O3 160 22.07 0.70 0.68 10.53 667.48 2.87

PTB7-Th :IEICO-4F

:T2-ORH

0.9:0.1 SAS

170 16.41 0.68 0.49 5.50 - - 110 15.17 0.71 0.63 6.75 788.60 8.68 100 14.59 0.73 0.58 6.15 649.99 12.13

90 14.45 0.70 0.56 5.60 - - Ag/Sb2O3 100 21.75 0.73 0.66 10.4 659.84 3.05

0.8:0.2 SAS

150 13.07 0.70 0.61 5.53 - - 140 15.40 0.70 0.64 6.93 712.28 7.51 120 14.23 0.73 0.60 6.21 633.28 10.60

90 13.20 0.72 0.56 5.32 - - Ag/Sb2O3 120 22.28 0.72 0.67 10.77 659.90 3.05

PTB7-Th

:T2-ORH 0:1 SAS

140 6.68 0.95 0.45 2.88 - - 110 6.96 1.06 0.55 4.04 829.45 27.20

90 6.66 1.06 0.56 3.97 - - 80 6.33 1.06 0.51 3.39 700.98 22.99 Ag/Sb2O3 80 10.38 1.07 0.54 6.01 526.87 8.76

PTB7-Th :IEICO-4F :T2-OEHRH

0.95:0.05 SAS

160 15.32 0.67 0.49 5.02 - - 130 15.01 0.71 0.63 6.73 736.24 7.73 110 14.12 0.71 0.59 5.95 704.79 10.99

90 14.69 0.70 0.45 4.65 - - Ag/Sb2O3 110 21.90 0.72 0.67 10.58 704.37 3.27

68

0.85:0.15 SAS

160 15.41 0.67 0.44 4.62 - - 110 15.13 0.72 0.61 6.68 758.36 9.00 100 14.03 0.73 0.58 5.96 789.28 10.99

90 14.85 0.71 0.47 4.90 - - Ag/Sb2O3 100 21.56 0.74 0.67 10.64 730.55 3.40

PTB7-Th

:T2-OEHRH 0:1 SAS

140 7.62 0.94 0.39 2.82 - - 110 7.44 1.06 0.46 3.60 454.70 28.12

70 6.80 1.07 0.47 3.39 679.59 26.21 60 6.33 1.07 0.47 3.16 - - Ag/Sb2O3 70 12.03 1.08 0.49 6.40 350.40 12.07

Figure 2. 6. J–V characteristics of color tunable opaque and semi-transparent devices with SAS electrodes based on (a) T2-ORH and (c) T2-OEHRH. Corresponding IPCE spectra for (b) T2-ORH and (d) T2-OEHRH devices.

After the thick Sb2O3/Ag electrodes were replaced with semi-transparent SAS electrodes, STOPVs were characterized upon illumination from the ITO side. All JSCs decreased compared to opaque devices.

This is mainly due to the reduced reflectivity of the thin SAS electrodes compared to the thick Ag electrodes, which increases light transmittance through the device and results in a lower intensity of light absorbed by the active layer. IEICO-4F or IEICO-4F dominant formulations exhibited no significant difference in Rsh and even increased Rshs were observed in T2-ORH or T2-OEHRH blend

b

300 400 500 600 700 800 900 1000 0

10 20 30 40 50 60 70

IPCE (%)

Wavelength (nm)

300 400 500 600 700 800 900 1000 0

10 20 30 40 50 60 70

IPCE (%)

Wavelength (nm) -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2

Current density (mA cm-2 )

Voltage (V) Ag SAS

IEICO-4F 0.95:0.05 0.85:0.15 T2-OEHRH -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -22

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2

Ag SAS IEICO-4F 0.90:0.10 0.80:0.20 T2-ORH Current density (mA cm-2 )

Voltage (V)

a

c d

69

systems when SAS electrodes were used. Therefore, we did not observe any influence of Rsh on the JSCs.

Instead, it can be also attributed to increased sheet resistance^{113, 125} of SAS electrode (0.4 Ω sq^-1and ~6 Ω sq^-1for Sb2O3/Ag and SAS electrodes, respectively. As shown in Figure 2. 5, Table 2. 2, Figure 2.

6a, c and Table 2. 3, most semi-transparent devices exhibited marginal differences in VOC (0.01−0.02 V) for each condition, compared to opaque devices, which is close to the experimental error for the measurement. A general enhancement in VOC was observed with increasing T2-ORH and T2-OEHRH content for ternary blends in both opaque OPVs and STOPVs. In contrast, a significant decrease in FF was observed for STOPVs compared with the opaque devices. This can be attributed to a significant increase in Rs for devices with SAS electrodes, where Rs values were consistently 2~4 times higher for SAS electrodes than in opaque devices with thick silver electrodes (Table 2. 2).

Table 2. 3. Summary of photovoltaic parameters for color tunable devices based on T2-ORH and T2- OEHRH.

Active Layer

IEICO-4F :Acceptor

(v/v)

Electrode JSC

(mA cm^-2)

Cal. JSC

(mA cm^-2)

VOC

(V) FF PCE

(%)

AVT (%) PTB7-Th

:IEICO-4F 1:0

Sb2O3/Ag 22.07

(21.73±0.99) 20.80 0.70 (0.70±0.02)

0.68 (0.66±0.03)

10.53

(10.04±0.49) -

SAS 14.24

(14.41±0.88) 15.36 0.71 (0.71±0.01)

0.62 (0.60±0.03)

6.22

(6.05±0.18) 34.65

PTB7-Th :IEICO-4F

:T2-ORH

0.90:0.10

Sb2O3/Ag 21.75

(21.75±0.57) 21.21 0.73 (0.71±0.02)

0.66 (0.65±0.02)

10.40

(10.10±0.31) -

SAS 15.17

(14.95±1.72) 14.10 0.71 (0.71±0.03)

0.63 (0.61±0.06)

6.75

(6.43±0.33) 34.77

0.80:0.20

Sb2O3/Ag 22.28

(21.46±0.72) 21.33 0.72 (0.70±0.02)

0.67 (0.68±0.03)

10.77

(10.18±0.59) -

SAS 15.40

(14.85±1.69) 16.48 0.70 (0.71±0.02)

0.64 (0.60±0.05)

6.93

(6.33±0.60) 34.03 PTB7-Th

:T2-ORH 0:1

Sb2O3/Ag 10.38

(10.27±0.47) 10.11 1.07 (1.07±0.01)

0.54 (0.52±0.03)

6.01

(5.73±0.31) -

SAS 6.96

(6.54±0.51) 7.53 1.06 (1.05±0.03)

0.55 (0.52±0.04)

4.04

(3.56±0.47) 23.03

PTB7-Th :IEICO-4F

:T2- OEHRH

0.95:0.05

Sb2O3/Ag 21.90

(21.45±0.46) 21.17 0.72 (0.71±0.01)

0.67 (0.66±0.01)

10.58

(10.11±0.47) -

SAS 15.01

(14.93±1.88) 15.47 0.71 (0.71±0.01)

0.63 (0.61±0.06)

6.73

(6.50±0.46) 32.20

0.85:0.15

Sb2O3/Ag 21.56

(21.38±0.43) 21.03 0.74 (0.74±0.02)

0.67 (0.67±0.00)

10.64

(10.46±0.20) -

SAS 15.13

(14.10±1.50) 14.71 0.72 (0.73±0.01)

0.61 (0.59±0.05)

6.68

(6.44±0.24) 30.32 PTB7-Th

:T2- OEHRH

0:1

Sb2O3/Ag 12.03

(11.36±0.67) 10.63 1.08 (1.06±0.05)

0.49 (0.48±0.05)

6.40

(5.77±0.87) -

SAS 7.44

(7.17±0.79) 8.77 1.06 (1.02±0.08)

0.46 (0.44±0.05)

3.60

(3.35±0.46) 26.66

70

In order to calculate AVT in the wavelength range of 300−900 nm, we followed a previously reported method which involves integration of the transmission spectrum and AM 1.5G photon flux weighted against the photopic response of the human eye.¹²⁸ The transmission spectra of the STOPVs are shown in Figure 2. 4b; all the devices retained 23−35% transparency. The minimum AVT required for practical applications is considered to be around 25%,100, 136, 137 thus, the transparencies achieved in our devices can be considered suitable for practical applications. It is also evident that the AVTs for devices remained high regardless of the active layer blend (binary or ternary) used. From the photographs in Figure 2. 3c, d, we can see that the colorful devices have very good transparency, and the background logos can be clearly seen through all of the STOPVs.

For the IEICO-4F binary system, a PCE of 6.22% and 34.65% AVT were achieved, giving devices with a cyan color. For the T2-ORH ternary device, a PCE of 6.75% and 34.77% AVT were achieved yielding aqua colored (0.90:0.10) devices. A PCE of 4.04% and 23.03% AVT were obtained for reddish-purple colored (T2-ORH binary) devices. A champion device yielded a PCE of 6.93% including a JSC of 15.40 mA cm^-2, a VOC of 0.70 V, a FF of 0.64 and 34.03% AVT, which appeared as an indigo colored (0.80:0.20) device. Likewise, for the T2-OEHRH based system, a PCE of 6.68% and 30.32%

AVT were achieved for purple (0.85:0.15) devices and a PCE of 3.60% and 26.66% AVT were achieved for reddish-purple (T2-OEHRH binary) devices. The champion device yielded a JSCof 15.01 mA cm^-2, a VOC of 0.71 V and a FF of 0.63 with an overall PCE of 6.73% while the device appeared to have an aqua color (0.95:0.05).

The IPCE spectra of the opaque OPVs are shown in Figure 2. 6b, d. All the devices containing IEICO-4F showed photo responses in the wavelength range of 300−1050 nm. The maximum IPCE values were larger than 66% for PTB7-Th:IEICO-4F binary and ternary blends and 53% for both PTB7- Th:T2-ORH and :T2-OEHRH binary blend devices. The IPCE curves of STOPVs are also shown in Figure 2. 6b, d. Transmittance of the SAS devices was higher than the opaque electrodes, hence the IPCE values were lower than the opaque OPVs. JSCs were calculated by integrating the IPCE data and compared to JSCs measured in J‒V curves; the integrated JSC values were in good agreement with the measured JSCs from J‒V measurements for all devices.

71