High-Efficiency Semitransparent Organic Solar ... - ACS Publications

Subscriber access provided by EASTERN KENTUCKY UNIVERSITY

Article

High efficiency semitransparent organic solar cells with non-fullerene acceptor for window application Mushfika Baishakhi Upama, Matthew Wright, Naveen Kumar Elumalai, Md Arafat Mahmud, Dian Wang, Cheng Xu, and Ashraf Uddin ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00618 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

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

ACS Photonics

High efficiency semitransparent organic solar cells with non-fullerene acceptor for window application Mushfika Baishakhi Upama*, Matthew Wright, Naveen Kumar Elumalai*, Md Arafat Mahmud, Dian Wang, Cheng Xu, Ashraf Uddin School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, 2052, Sydney, Australia

ABSTRACT- Organic solar cells possess multiple desirable traits, such as low-cost, flexibility and semi transparency, which opens up potential avenues unavailable to other solar technologies, a prime example of this being window applications. For this specific application, a delicate balance between the transmission of light through the device and power conversion efficiency (PCE), dependent on the amount of light absorbed, must be optimized. Here, we report a high efficiency semitransparent device based on a novel fullerene-free material system. Using an active layer based on the material system PBDB-T: ITIC, optimized devices exhibited PCEs exceeding 7%, whilst also achieving an average visible transmittance (AVT) of 25%. The concurrent demonstration of high efficiency with an AVT of 25% represents a notable step forward for semitransparent organic solar cells. Additionally, the influence of the active layer thickness on the color rendering properties of these cells was studied. Optimisation of the active layer thickness can lead to high efficiency cells, with high visible transmission as well as the ability to display an image accurately.

KEYWORDS: semitransparent, organic solar cells, non-fullerene acceptor, ITIC, MoO3/Ag/MoO3

Organic solar cells (OSCs) have showcased great potentials as a competitive renewable energy source due to the low-cost technique of fabrication, light weight, flexibility and semitransparency nature.1-5 Recently, the power conversion efficiency (PCE) of OSCs has exceeded 12% due to advanced photovoltaic materials.6-7 One notable characteristic of OSCs is their semi-transparency which leads to prospects in power-generating windows and curtains for buildings and automobiles, and architectural and fashion applications.5, 8-15 To fabricate a semitransparent OSC, the light absorption inside the active layer must be precisely tuned to allow sufficient light to transmit.16-17 In addition, both electrodes must be transparent. There is a variety of possible approaches to fabricate transparent electrodes.18 Carbon nanotubes are a suitable candidate, as they can be produced cheaply and exhibit good mechanical flexibility.19 Another carbon-based solution, graphene, has been explored as both a top and bottom electrode, and has been demonstrated in semitransparent organic solar cells.

ACS Paragon Plus Environment

ACS Photonics

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

Liu et al.20 incorporated a doped graphene layer as the top electrode into a P3HT:PCBM semitransparent solar cell, which achieved an efficiency of 2.7%. However, these electrodes have relatively low conductivity and require additional chemical doping.20-21 Single or multiple pairs of one-dimensional photonic crystals have been used as top electrode to achieve 5-6% efficiency at an average visible transmittance (AVT) of 25%, the construction of which involves deposition of multiple layers of metal oxide (MoO3 or WO3)/LiF pairs.15, 22-23 Another possible choice for the top electrode is a dielectric/metal/dielectric (D/M/D) structure. This is advantageous due to its relatively simple construction and fabrication technique, unlike alternative approaches such as photonic crystals or graphene based electrodes.15, 20, 22 This approach has been used in this study. One of the key metrics for a semitransparent solar cell used in window applications is the AVT.9-10, 16 A fundamental trade off exists between the desire to maximise both the transmittance and generated photocurrent. Multiple reports have indicated that a minimum AVT of 25% is required for window applications.10, 15-16 The transmission of the cell can also be used to calculate the color rendering index (CRI). This parameter describes the accuracy of the depiction of colors in the transmitted light, compared to a standard illuminant, as perceived by the human eye. This is considered important for solar cells in window applications, such that the image outside is accurately displayed inside. Multiple traditional photoactive material combinations, including P3HT:PCBM11 and PTB7:PC71BM,24 have been demonstrated in a semitransparent architecture, however, the key issue remains achieving a high efficiency in a device which also displays high transmission in the visible region. Recently, non-fullerene small molecule acceptors have emerged as attractive alternatives to conventional fullerene acceptors used in OSCs as electron collector.6, 25-30 The superior performance of fullerene-free OSCs originates from the enhanced optical and electronic properties of non-fullerene acceptors including easily tuneable molecular energy levels,6, 26, 3132 superior optical absorption properties,33 and lower synthetic costs compared to fullerene acceptors.6, 25-26 These advantageous properties have accelerated the improved performance exhibited by OSCs over the past few years. The non-fullerene acceptor 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl) dithieno [2,3d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) has shown particularly promising results. This novel small molecule material has a high lying LUMO level (-3.78 eV), which leads to high open circuit voltage (Voc).26 When coupled with the novel polymer poly[(2,6(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2thienyl-5’,7’-bis(2-ethylhexyl) benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))] (PBDB-T), efficiencies exceeding 11% have been recorded. When combined, these materials have complementary absorption profiles, and well aligned electronic structure, leading to high Voc and short circuit current density (Jsc).6, 26, 33-34 In this work, we demonstrated the use of a non-fullerene material system in an inverted semitransparent solar cell architecture. The PBDB-T:ITIC material system achieved an efficiency of more than 7%, one of the highest reported values in a semitransparent device. Importantly, this high efficiency was achieved in a device with an AVT of 25%, which is


Page 2 of 19

Page 3 of 19

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

ACS Photonics

crucial for window applications. At the time of writing, this is the highest reported efficiency at an AVT of 25% for single junction OSCs. MoO3/Ag/MoO3 was used as transparent anode. Additionally, the CRI was calculated as a function of the direction of illumination. This provides insights into the color rendering properties from the perspective of both viewers inside and outside of a building. The analysis provides compelling insight into future architectural applications.

RESULTS AND DISCUSSION The inverted device structure used for this study was ITO/ ZnO/ PBDB-T: ITIC/ MoO3/ Ag/ MoO3, as shown in Figure 1a. MoO3/ Ag/ MoO3 (6/10/40 nm) acts as a transparent top electrode, the inner MoO3 layer also acts as the hole transport layer. Sol-gel processed ZnO layer was used as the electron transport layer and ITO is the transparent bottom electrode. Novel polymer, PBDB-T and non-fullerene small molecule, ITIC, as shown in Figure 1b, were selected as the donor and acceptor, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the materials used are displayed in Figure 2a. The combination of PBDB-T and ITIC in opaque device structure has previously shown excellent photovoltaic performance with PCE exceeding 11%.26. When it was employed in the semitransparent (ST) structure (Figure 1a), PCE>7% was achieved at 25% AVT. Figure 2b exhibits the transmission of the MoO3/ Ag/ MoO3 anode. The anode has sufficiently high transmission with a maximum transmission of 63.7% at λ= 485 nm. The sheet resistance is 37.3 Ω/□, which is in the same order of that of commercial ITO thin films (36 Ω/□).35 This indicates that the electrode is conductive enough to extract charge from the device and yield high efficiency ST devices. In order to optimize the device structure, a range of ST devices were fabricated with various active layer thicknesses, ranging from 53 nm to 143 nm. Different thicknesses were achieved by changing the spin rate from 1200-6500 rpm. The thickness reduces with increasing spin speed; this relationship is shown graphically in Figure S1. The J-V characteristics curves are illustrated in Figure 3a. The figure clearly indicates that all the ST devices have a similar Voc, which is close to 0.9 V. The value is ~0.2 V higher than that of devices containing traditional fullerene acceptors, due to the high lying LUMO of non-fullerene acceptors.16, 26 This is a major factor contributing to the enhanced photovoltaic performance of this material system. As the active layer thickness was increased, the Jsc also increased. As the active layer thickness was increased from 53 nm to 143 nm, the Jsc almost doubled, it increased from 6.85 mA/cm2 to 13.82 mA/cm2. These values are displayed in Table 1. The absorption of the active layer film, for various film thicknesses, is shown in Figure 3b. This displays a clear trend, as the film thickness is increased, the absorption increases over the entire wavelength range. To link the absorption to the Jsc, we have also measured the EQE for all semitransparent devices, in a bottom illumination configuration. These EQE spectra are displayed in Figure 3c. The trend at long wavelengths (> 500 nm) directly matches the trend


ACS Photonics

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

Page 4 of 19

displayed in the absorption curves. Increasing the thickness causes an increase in the EQE, which is conclusively shown to be related to increased photon absorption. For short wavelengths (< 500 nm), the trend is less clear. For small thicknesses (< 100 nm), the trend matches that seen in long wavelengths; the EQE increases as the thickness increases. As the thicknesses increases beyond 100 nm, the EQE sharply reduces. A peak at ~ 380 nm is seen in all spectra. According to the absorption of the individual components, ITIC contributes very little photocurrent generation in this region.26 The reduced EQE in this region is largely related to some reduction in photocurrent generated in the PBDB-T phase. The exact mechanism causing this is not clear; we believe this may be due to increased recombination in the thick film, which leads to reduced carrier collection.34 In contrast, the fill factor (FF) declined with the increment in active layer thickness. The peak FF (>66%) occurred in devices with 59 and 55 nm active layer. Although the Jsc is highest for 143 nm thick active layer, at this thickness, the FF is at a minimum value of 49.6%. We believe the reduced FF for larger thicknesses, as well as the reduced EQE at short wavelengths, is related to increased recombination. The drop in FF is linked to an increase in series resistance (Rs) and a decrease in shunt resistance (Rsh) values. As a result, an optimum thickness is determined by the increased Jsc for thicker films and increased FF for thinner films. The optimum PCE is obtained at 100 nm active layer thickness (2000 rpm) with a maximum PCE value of 7.4%. To the best of our knowledge, this PCE is one of the few reported PCEs exceeding 7% for a single junction semitransparent OSC. The Jsc, Voc and FF of the best device are 13.7 mA/cm², 0.883 V and 60.9%, respectively. A comparison of the ST champion cell, with an opaque device of the same active layer thickness, is shown in Figure S2. The Voc and FF are almost the same (Table S1) which suggests that the transparent D/M/D top electrode can make good electrical contact. The efficiency of the ST cell (7.4%) is 67% of the efficiency of the opaque cell (11%). The main difference in the efficiency is the reduced Jsc for the ST cell, caused by the transparent top electrode, which reduces the path length of incident light in the active layer. The ST device is capable of retaining 71% Jsc of its opaque counterpart.

Table 1. Photovoltaic performance parameters of the semitransparent OSCs with different active layer thicknesses under AM1.5G 100 mW cm-2 illumination and calculated color rendering indices (CRI). All data was acquired under bottom illumination. rpm

Active layer thickness (nm)

Jsc (mA/cm2)

Voc (V)

FF (%)

Avg. PCE (%)

Max PCE (%)

Rs (Ω.cm2)

Rsh (Ω.cm2)

AVT (%)

1200

143

13.82+0.30

0.897+0.01

49.6+2.43

6.1+0.20

6.3

19.5

582.4

20.2

1600

114

15.08+0.26

0.875+0.01

51.5+1.65

6.7+0.10

6.8

15.4

542.4

23.2

2000

100

13.8+0.11

0.88+0.01

59.8+1.10

7.3+0.10

7.4

11.9

977.4

25.2

2500

91

12.62+0.44

0.875+0.01

59.4+1.16

6.6+0.34

6.8

12.9

1176.3

26.6

3500

72

8.78+0.08

0.897+0.01

63.2+0.19

5+0.15

5.1

14

1446

31.1

5000

59

8.41+0.21

0.890+0.01

66.9+0.98

5+0.20

5.2

10.4

1578

33

6500

53

6.85+0.14

0.884+0.01

66.4+0.71

4+0.21

4.2

13.6

1868

35.3


Page 5 of 19

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

ACS Photonics

The AVT, which is the average of the transmittance of the solar cells in the visible region (370–740 nm), of the ST devices are presented in Table 1. As the thickness of the active layer varies from 53 nm to 143 nm, the AVT can be tuned accordingly from 35.3% to 20.2%. The result indicates that the transparency of the devices is significantly controlled by the active layer thickness. An AVT of 25% is the benchmark for window applications of building integrated photovoltaic devices,10 which was achieved at the optimum active layer thickness (100 nm) with 7.3% average device efficiency. Previous reports of ST cells with efficiencies higher than 7% have had significantly lower AVT values.10, 22 This result represents a notable advancement in the field of ST organic solar cells. The relationship between the PCE and AVT, as a function of active layer thickness, is illustrated in Figure 4. The PCE increased with increasing thickness until it reached the peak value at 100 nm. For larger thicknesses, PCE reduced due to reduced FF. Unlike PCE, the AVT reduced linearly with increased active layer thickness. The total device transmittance spectra in the visible region (370-740 nm), under bottom illumination (light incident from ITO side), for all the fabricated devices are displayed in Figure 5a. The device transmittance decreases with the increase in active layer thickness, which explains the steady reduction in AVT of thicker devices. The transmission is highest in the short wavelength region (λ< 500 nm). The highest transmission (47% at 400 nm) is achieved for the thinnest layer (53 nm at 6500 rpm), on the other hand, for the thickest layer (143 nm at 1200 rpm), the highest transmittance value is 34% at 460 nm. When λ> 500 nm, the transmission for all the devices reduces, except for devices with 53 nm active layer which displays a second peak 525 nm and yields the highest AVT (35.3%). The transmission for all devices increases again when the wavelength exceeds 640 nm. In building integration, it is important to find the device transparency from both top and bottom sides. Sometimes it is ideal for the window to be transparent enough from both inside and outside of a building such that people standing on both sides have similar visibility. Hence, we measured the total device transmittance under top illumination (light incident from top MoO3/Ag/MoO3 electrode side) as well. Figure 5b shows the device transmission for both top and bottom illumination, for 2000 rpm devices. It is interesting to find that the transmittance spectra do not vary when the direction of illumination is changed; this trend is applicable to all the fabricated ST devices. The total device transmittance spectra of all the ST devices under top illumination are shown in Figure S3. As a result, the AVT values under top and bottom illumination, listed in Table 2, are essentially the same. Figure 5c shows photographs of the UNSW logo with and without the ST device. As can be seen from the figure, the photographs, with the same ST device in front of the logo, look almost similar regardless of the direction of illumination. Despite similar transmission from top and bottom illumination, the photovoltaic performance, including the device Jsc, is vastly different when the direction of illumination is changed. The photovoltaic performance of the ST devices under top illumination is summarized in Table S2 in order to compare with the device performance under bottom illumination. Although Voc is independent of both active layer thickness and direction of illumination, Jsc and FF reduced under top illumination. The main reason behind lower Jsc under top illumination is expected to be the lower exciton generation rate under this circumstance. It is well-known that organic thin film solar cells act as multilayer optical cavities where the optical field distribution is governed by the optical


ACS Photonics

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

interference effect, due to reflections of the incident light at the layer interfaces.36-37 When light enters through the D/M/D electrode under top illumination, the maximum optical field intensity is located close to the anode/active layer interface, followed by a steady decrease along the optical path of incident light.17, 37 The behavior is representative of the Beer−Lambert law, causing an exponential reduction in exciton generation rate which explains the drastic reduction in device Jsc, compared to that under bottom illumination. For devices with thicker active layer (>100 nm), the FF was lower due to increased series resistance which could be related to the uneven distribution of exciton generation inside the active layer (most exciton being generated near the anode/active layer interface) and subsequent carrier transport towards respective electrodes. We have also investigated the color rendering properties of the ST solar cells fabricated in this study. We calculated the tristimulus value (X, Y, Z) and the color coordinates (x, y) from the transmission spectra of the semitransparent OSCs using the following formulae:16

× , = ×

× , = ×

= × × , =

,

(4)

= ,

(5)

(1) (2) (3)

,

and are the color matching Here, S(λ) is the standard D65 illuminant spectrum, functions as defined by the CIE protocol and T(λ) is the experimental transmittance spectrum. The tristimulus values can evaluate the cell colors according to human eye perception through an objective description of color sensations registered in the eye. (x, y) are the color coordinates in the CIE 1931 color space. The color coordinates shows how the light from an object changes when it transmits though the ST devices as perceived by the viewer’s eye. The calculated CIE color coordinates under both top and bottom illumination are listed in Table 2. The CIE color space, including the coordinates of ST devices consisting of different active layer thicknesses, is shown in Figure 6a. The color coordinates of the semitransparent OPV devices with thin active layer thicknesses (tactivelayer< 100 nm) are located close to the achromatic or so-called ‘white point’ on the CIE chromaticity diagram. The close proximity indicates good achromatic or neutral color sensations when looking through the devices under AM1.5G illumination. The transmitted lights through the devices with 72 nm (3500 rpm) and 59 nm (5000 rpm) active layer thicknesses have their color coordinates quite close to the standard daylight illuminant D65


Page 6 of 19

Page 7 of 19

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

ACS Photonics

[(xD65, yD65) = (0.31, 0.33)], at (0.27, 0.30) and (0.28, 0.31), respectively. Hence, these devices can produce high quality light transmission with near white sensation to the human eye without altering the original color of an object. However, as the active layer thickness increases (tactivelayer > 100 nm), the color coordinates move in the direction of the intersection of cyan and blue color of the CIE chromaticity diagram. For the device with the best PCE and AVT, the active layer thickness is 100 nm and its color coordinates are slightly distant from the achromatic point, nevertheless, the device does not alter the transmitted light by a large extent. From Figure 5c, it can be seen that, in the transmitted image, the original colors of the objects (logo background, font color, building, sky) are almost correctly represented, with a slight hint of the color blue. This slight bluish tone in the photograph correlated with the position of this device in the CIE color space. The images are accurately represented; however, the intensity of the image is slightly reduced when viewed through the ST device. The photographs of UNSW logo taken for all the fabricated ST devices are presented in Figure S4. The device with the thickest active layer (143 nm at 1200 rpm) is situated at the farthest location (x= 0.23, y= 0.26) in the CIE diagram from the achromatic point. As a result, a strong blue tone can be seen in the photograph taken in front of this device in Figure S4. The light intensity is the lowest in this case; however, the image is still recognizable. The color rendering index (CRI) provides an objective analysis of the color rendering properties of a device. Essentially, the CRI, which is expressed as a percentage, provides a description of the ability for a device to accurately present an image, as compared to a standard illuminant, such as daylight. The CRI was calculated according to the CIE 13.3 1995 protocol.5, 38 To calculate the CRI, the (x, y) coordinates from the CIE 1931 color space must first be converted to (u, v) coordinates in the CIE 1960 color space. These coordinates must then be modified according to the 8 test color samples, as specified in the CIE standard. Each test color sample provides a special CRI. The average of these 8 special CRI values gives the general CRI. Full details of the methodology used are shown in ref 1b.16 This value, referred to as CRI, is shown in Table 2, for all active layer thicknesses. The CRI value for the 1200 rpm device is very low (14%). As the thickness reduced, a clear trend appears. The CRI increases, up to a maximum value of 76% for bottom illumination. The CRI value is dependent on the transmission. As the transmittance value is very similar for both bottom and top illumination, the CRI values for top illumination, which are also shown in Table 2, are almost identical. In the CIE protocol, 8 test color samples are specified, which cover the visible spectrum. The reflection from each sample, when illuminated by the D65 illumination source, accounts for the different colors of the solar spectrum. The CRI depends on the interaction between the light and that sample in all regions of the solar spectrum. To analyze the light passing through the sample in specific regions, these test color samples are used. In Figure 6b, both a ‘ref’ and ‘test’ image are shown for the 8 test color samples. The ref sample represents the specific test colors when illuminated by the D65 illuminant. The test sample represents light reflected from the test color samples, and then transmitted through the ST solar cell. In the figure, three different ST solar cell thickness are compared with the test color samples. For the 1200 rpm case, where the active layer is thick, in each portion of the spectrum, the light is modified such that it appears bluer than the original color. This provides a physical depiction of the


ACS Photonics

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

Page 8 of 19

reason causing the low CRI. For the 2000 rpm case, which provided the highest efficiency, the color rendering properties are improved. It can be clearly seen the view of the test colors is more accurate than for the 1200 rpm case, however, a bluish tinge is still observed across all colors. This correlates with the position on the CIE 1931 color space, shown in Figure 6a. For the 5000 rpm case, corresponding to a film thickness of 59 nm, the depiction of the reflected light from the test color samples, after it has transmitted through the ST device, is very accurate. Across the entire wavelength spectrum, the bluish tinge is significantly reduced. For this case, where the active layer is thin, the generated photocurrent was limited, leading to an efficiency of 5%. The result presented here represents a noticeable enhancement; however, future work should focus on achieving high efficiencies with excellent color rendering properties. Table 2. Calculated color rendering indices (CRIs) under both bottom and top illumination. Bottom illumination rpm

Active layer thickness (nm)

1200

Top illumination

AVT (%)

General CRI

Color coordinates (x, y)

AVT (%)

General CRI

Color coordinates (x, y)

143

20.2

14

(0.23, 0.26)

20.1

14

(0.23, 0.26)

1600

114

23.2

27

(0.24, 0.27)

23.1

28

(0.24, 0.26)

2000

100

25.2

44

(0.25, 0.27)

25.2

44

(0.25, 0.27)

2500

91

26.6

43

(0.25, 0.28)

26.4

42

(0.25, 0.28)

3500

72

31.1

63

(0.27, 0.30)

30.8

63

(0.27, 0.29)

5000

59

33

76

(0.28, 0.31)

32.8

77

(0.28, 0.31)

6500

53

35.3

68

(0.28, 0.32)

35.1

69

(0.28, 0.32)

CONCLUSIONS In summary, we have demonstrated high efficiency semitransparent fullerene-free organic solar cells with a D/M/D top electrode. The novel polymer: non-fullerene system (PBDB-T: ITIC) was used as the photo-absorbing layer which contributed to high Voc and Jsc, resulting in enhanced device performance. The top electrode included a simple D/M/D layer construction that does not require sophisticated nano-structures or dielectric mirrors. By varying the active layer thicknesses, the device efficiency and overall device transmittance were controlled. An optimum thickness was achieved which yielded an average PCE of 7.3% with a remarkably high AVT of 25%. The optimized ST OSC is one of the few reported ST OSCs with a PCE value greater than 7%. The combination of high PCE with an AVT of 25% represents an appreciable step forward for this technology. In addition, the color rendering properties of the fabricated ST devices were analyzed by their position in CIE 1931 color space and CRI values. As with PCE and AVT, the color coordinates and CRI values of the devices varied significantly with the active layer thickness. The optimum ST device had a CRI value of 44 with a slight bluish tone on the transmitted image, whereas devices with 59


Page 9 of 19

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

ACS Photonics

nm thick active layer exhibited the highest CRI of 76 with the most accurate interpretation of the transmitted colors. We also analyzed the color rendering property as a function of the direction of illumination (both top and bottom illumination). The fabricated ST devices showed similar CRI values under both top and bottom illumination implying that these devices are capable of rendering the true color of an object irrespective of the viewer’s position from the device. All the advantages suggest that inverted semitransparent OSC based on the high efficiency material system PBDB-T: ITIC can provide sufficient transparency and color rendering property that is suitable for window applications.

METHODS Device fabrication: PBDB-T (donor polymer) and ITIC (non-fullerene acceptor) were purchased from Solarmer Energy, Inc. and used as received. Patterned ITO-coated glass substrates from Lumtec were used for all films and devices. These substrates were first cleaned by ultrasonication in soapy deionized (DI) water, DI water, acetone, and isopropanol. Sol-gel processed ZnO ETL was fabricated on top of ITO/glass substrate in a process reported in our earlier works.39-41 In brief, the ZnO sol-gel solution (0.48 M) was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2.2H2O, Sigma-Aldrich, >99.0%, 0.109 g) and ethanolamine (NH2CH2CH2OH, Sigma-Aldrich, >99.5%, 32 mL) in 2-methoxyethanol (CH3OCH2CH2OH, Sigma-Aldrich, 99.8%, anhydrous, 1 mL) under vigorous stirring for at least 24 h. The ZnO solution was spin-cast on top of the pre-cleaned ITO glass substrates at 4000 rpm for 60 s. The samples were then annealed at 170 °C for 30 min. Active layer solution of PBDB-T:ITIC (1:1 weight ratio) was prepared with the concentration of 20 mg/mL by blending in chlorobenzene: DIO (99.5:0.5 volume ratio) from Sigma Aldrich. Subsequently, the active layer was spin coated on top of the ZnO layer at different spin rates for 60 s to achieve various thicknesses within 53-143 nm range. Finally, the active layer coated substrates were loaded in a vacuum chamber (106 mBar) where MoO3/Ag/MoO3 layers were deposited by thermal evaporation through a shadow mask. The device area was 0.12 cm2. Device characterization: The film thickness was measured by an Alpha-Step D600 profiler. The resistivity of the MoO3/Ag/MoO3 electrode was measured by using a four point probe system (Jandel model RM3). The current density–voltage (J–V) measurements were performed using an IV5 solar cell I–V testing system from PV measurements, Inc. (using a Keithley 2400 source meter) under illumination power of 100 mW/cm2 by an AM 1.5G solar simulator (Oriel model 94023A; 100 mW/cm2). For optical characterization of the electrodes, a UV–VIS-NIR spectrometer (Perkin Elmer – Lambda 950) was used. External quantum efficiency (EQE) measurements were performed using a QEX10 spectral response system from PV measurements, Inc. During the J-V measurements of the semitransparent cells, the illumination was always from the ITO side (bottom illumination). For CRI calculation, device transmission from both top and bottom side was measured.


ACS Photonics

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

ASSOCIATED CONTENT Supporting Information Relationship between spin speed and active layer thickness; J–V curves of opaque and optimum semitransparent devices; list of photovoltaic performance parameters of the opaque and optimum semitransparent devices; transmittance spectra of the semitransparent OSCs under top illumination; list of photovoltaic performance parameters of the semitransparent OSCs under top illumination; photographs of devices with different active layer thicknesses.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected], [email protected]

ACKNOWLEDGEMENTS The authors would like to thank the Australian Centre for Advanced Photovoltaics, UNSW staff and technicians for their support. We also acknowledge Future Solar Technologies for providing funding. The authors thank Professor Martin Green for useful scientific discussions.

REFERENCES 1. Guo, W.; Xu, Z.; Zhang, F.; Xie, S.; Xu, H.; Liu, X. Y., Recent Development of Transparent Conducting Oxide-Free Flexible Thin-Film Solar Cells. Adv. Funct. Mater. 2016, 26, 8855-8884. 2. Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K. Y., Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371-1388. 3. Forrest, S. R., The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 2004, 428, 911-918. 4. Kaltenbrunner, M.; White, M. S.; Głowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S., Ultrathin and lightweight organic solar cells with high flexibility. Nat. Commun. 2012, 3, 770. 5. Colsmann, A.; Puetz, A.; Bauer, A.; Hanisch, J.; Ahlswede, E.; Lemmer, U., Efficient SemiTransparent Organic Solar Cells with Good Transparency Color Perception and Rendering Properties. Adv. Energy Mater. 2011, 1, 599-603. 6. Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J., Energy-Level Modulation of Small-Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Adv. Mater. 2016, 28, 9423-9429. 7. Zhao, F.; Dai, S.; Wu, Y.; Zhang, Q.; Wang, J.; Jiang, L.; Ling, Q.; Wei, Z.; Ma, W.; You, W.; Wang, C.; Zhan, X., Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency. Adv. Mater. 2017, 1700144.


Page 10 of 19

Page 11 of 19

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

ACS Photonics

8. Betancur, R.; Romero-Gomez, P.; Martinez-Otero, A.; Elias, X.; Maymo, M.; Martorell, J., Transparent polymer solar cells employing a layered light-trapping architecture. Nat Photon 2013, 7, 995-1000. 9. Ameri, T.; Dennler, G.; Waldauf, C.; Azimi, H.; Seemann, A.; Forberich, K.; Hauch, J.; Scharber, M.; Hingerl, K.; Brabec, C. J., Fabrication, Optical Modeling, and Color Characterization of Semitransparent Bulk-Heterojunction Organic Solar Cells in an Inverted Structure. Adv. Funct. Mater. 2010, 20, 1592-1598. 10. Chen, K.-S.; Salinas, J.-F.; Yip, H.-L.; Huo, L.; Hou, J.; Jen, A. K. Y., Semi-transparent polymer solar cells with 6% PCE, 25% average visible transmittance and a color rendering index close to 100 for power generating window applications. Energy Environ. Sci. 2012, 5, 9551-9557. 11. Cho, J. M.; Lee, S. K.; Moon, S.-J.; Jo, J.; Shin, W. S., MoO3/Ag/MoO3 top anode structure for semitransparent inverted organic solar cells. Current Applied Physics 2014, 14, 1144-1148. 12. Liu, F.; Zhou, Z.; Zhang, C.; Zhang, J.; Hu, Q.; Vergote, T.; Liu, F.; Russell, T. P.; Zhu, X., Efficient Semitransparent Solar Cells with High NIR Responsiveness Enabled by a Small-Bandgap Electron Acceptor. Adv. Mater. 2017, 1606574. 13. Liu, Z.; You, P.; Liu, S.; Yan, F., Neutral-Color Semitransparent Organic Solar Cells with AllGraphene Electrodes. ACS Nano 2015, 9, 12026-12034. 14. Tian, X.; Zhang, Y.; Hao, Y.; Cui, Y.; Wang, W.; Shi, F.; Wang, H.; Wei, B.; Huang, W., Semitransparent inverted organic solar cell with improved absorption and reasonable transparency perception based on the nanopatterned MoO3/Ag/MoO3 anode. J. Nanophotonics 2015, 9, 093043093043. 15. Yu, W.; Jia, X.; Long, Y.; Shen, L.; Liu, Y.; Guo, W.; Ruan, S., Highly Efficient Semitransparent Polymer Solar Cells with Color Rendering Index Approaching 100 Using One-Dimensional Photonic Crystal. ACS Appl. Mater. Interfaces 2015, 7, 9920-9928. 16. Upama, M. B.; Wright, M.; Elumalai, N. K.; Mahmud, M. A.; Wang, D.; Chan, K. H.; Xu, C.; Haque, F.; Uddin, A., High performance semitransparent organic solar cells with 5% PCE using nonpatterned MoO3/Ag/MoO3 anode. Current Applied Physics 2017, 17, 298-305. 17. Upama, M. B.; Wright, M.; Elumalai, N. K.; Mahmud, M. A.; Wang, D.; Chan, K. H.; Xu, C.; Haque, F.; Uddin, A., Optical modelling of P3HT:PC71BM semi-transparent organic solar cell. Opt. Quantum Electron. 2017, 49, 28. 18. Tai, Q.; Yan, F., Emerging Semitransparent Solar Cells: Materials and Device Design. Adv. Mater. 2017, 1700192. 19. Jeon, I.; Delacou, C.; Kaskela, A.; Kauppinen, E. I.; Maruyama, S.; Matsuo, Y., Metalelectrode-free Window-like Organic Solar Cells with p-Doped Carbon Nanotube Thin-film Electrodes. Sci. Rep. 2016, 6, 31348. 20. Liu, Z.; Li, J.; Sun, Z.-H.; Tai, G.; Lau, S.-P.; Yan, F., The Application of Highly Doped SingleLayer Graphene as the Top Electrodes of Semitransparent Organic Solar Cells. ACS Nano 2012, 6, 810-818. 21. Shi, Y.; Kim, K. K.; Reina, A.; Hofmann, M.; Li, L.-J.; Kong, J., Work Function Engineering of Graphene Electrode via Chemical Doping. ACS Nano 2010, 4, 2689-2694. 22. Xu, G.; Shen, L.; Cui, C.; Wen, S.; Xue, R.; Chen, W.; Chen, H.; Zhang, J.; Li, H.; Li, Y.; Li, Y., High-Performance Colorful Semitransparent Polymer Solar Cells with Ultrathin Hybrid-Metal Electrodes and Fine-Tuned Dielectric Mirrors. Adv. Funct. Mater. 2017, 1605908. 23. Yu, W.; Jia, X.; Yao, M.; Zhu, L.; Long, Y.; Shen, L., Semitransparent polymer solar cells with simultaneously improved efficiency and color rendering index. PCCP 2015, 17, 23732-23740. 24. Romero-Gomez, P.; Betancur, R.; Martinez-Otero, A.; Elias, X.; Mariano, M.; Romero, B.; Arredondo, B.; Vergaz, R.; Martorell, J., Enhanced stability in semi-transparent PTB7/PC71BM photovoltaic cells. Sol. Energy Mater. Sol. Cells 2015, 137, 44-49. 25. Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C. B.; Tan, C.-H.; Dimitrov, S. D.; Shang, Z.; Gasparini, N.; Alamoudi, M.; Laquai, F.; Brabec, C. J.; Salleo, A.; Durrant, J.


ACS Photonics

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

R.; McCulloch, I., High-efficiency and air-stable P3HT-based polymer solar cells with a new nonfullerene acceptor. Nat. Commun. 2016, 7, 11585. 26. Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganäs, O.; Gao, F.; Hou, J., Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734-4739. 27. Zheng, Z.; Awartani, O. M.; Gautam, B.; Liu, D.; Qin, Y.; Li, W.; Bataller, A.; Gundogdu, K.; Ade, H.; Hou, J., Efficient Charge Transfer and Fine-Tuned Energy Level Alignment in a THF-Processed Fullerene-Free Organic Solar Cell with 11.3% Efficiency. Adv. Mater. 2017, 29, 1604241. 28. Huang, J.; Wang, H.; Yan, K.; Zhang, X.; Chen, H.; Li, C.-Z.; Yu, J., Highly Efficient Organic Solar Cells Consisting of Double Bulk Heterojunction Layers. Adv. Mater. 2017, 1606729. 29. Yao, H.; Ye, L.; Hou, J.; Jang, B.; Han, G.; Cui, Y.; Su, G. M.; Wang, C.; Gao, B.; Yu, R.; Zhang, H.; Yi, Y.; Woo, H. Y.; Ade, H.; Hou, J., Achieving Highly Efficient Nonfullerene Organic Solar Cells with Improved Intermolecular Interaction and Open-Circuit Voltage. Adv. Mater. 2017, 1700254. 30. Chen, S.; Zhang, G.; Liu, J.; Yao, H.; Zhang, J.; Ma, T.; Li, Z.; Yan, H., An All-Solution Processed Recombination Layer with Mild Post-Treatment Enabling Efficient Homo-Tandem Non-fullerene Organic Solar Cells. Adv. Mater. 2017, 29, 1604231. 31. Zhang, Y.; Guo, X.; Guo, B.; Su, W.; Zhang, M.; Li, Y., Nonfullerene Polymer Solar Cells based on a Perylene Monoimide Acceptor with a High Open-Circuit Voltage of 1.3 V. Adv. Funct. Mater. 2017, 27, 1603892. 32. Suman; Gupta, V.; Bagui, A.; Singh, S. P., Molecular Engineering of Highly Efficient Small Molecule Nonfullerene Acceptor for Organic Solar Cells. Adv. Funct. Mater. 2017, 27, 1603820. 33. Qin, Y.; Uddin, M. A.; Chen, Y.; Jang, B.; Zhao, K.; Zheng, Z.; Yu, R.; Shin, T. J.; Woo, H. Y.; Hou, J., Highly Efficient Fullerene-Free Polymer Solar Cells Fabricated with Polythiophene Derivative. Adv. Mater. 2016, 28, 9416-9422. 34. Min Nam, Y.; Huh, J.; Ho Jo, W., Optimization of thickness and morphology of active layer for high performance of bulk-heterojunction organic solar cells. Solar Energy Materials and Solar Cells 2010, 94, 1118-1124. 35. Ram Narayan Chauhan, C. S., R. S. Anand, Jitendra Kumar, Effect of Sheet Resistance and Morphology of ITO Thin Films on Polymer Solar Cell Characteristics. International Journal of Photoenergy 2012, 2012. 36. Armin, A.; Velusamy, M.; Wolfer, P.; Zhang, Y.; Burn, P. L.; Meredith, P.; Pivrikas, A., Quantum Efficiency of Organic Solar Cells: Electro-Optical Cavity Considerations. ACS Photonics 2014, 1, 173-181. 37. Wilken, S.; Wilkens, V.; Scheunemann, D.; Nowak, R.-E.; von Maydell, K.; Parisi, J.; Borchert, H., Semitransparent Polymer-Based Solar Cells with Aluminum-Doped Zinc Oxide Electrodes. ACS Appl. Mater. Interfaces 2015, 7, 287-300. 38. C.I.d.l.É. International, ISBN 3 900 734 57 7, (1995). C.I.d.l.É. International, ISBN 3 900 734 57 7, (1995). 39. Upama, M. B.; Elumalai, N. K.; Mahmud, M. A.; Sun, H.; Wang, D.; Chan, K. H.; Wright, M.; Xu, C.; Uddin, A., Organic solar cells with near 100% efficiency retention after initial burn-in loss and photo-degradation. Thin Solid Films 2017, 636, 127-136. 40. Upama, M. B.; Elumalai, N. K.; Mahmud, M. A.; Wright, M.; Wang, D.; Xu, C.; Haque, F.; Chan, K. H.; Uddin, A., Interfacial engineering of electron transport layer using Caesium Iodide for efficient and stable organic solar cells. Appl. Surf. Sci. 2017, 416, 834-844. 41. Upama, M. B.; Wright, M.; Puthen-Veettil, B.; Elumalai, N. K.; Mahmud, M. A.; Wang, D.; Chan, K. H.; Xu, C.; Haque, F.; Uddin, A., Analysis of burn-in photo degradation in low bandgap polymer PTB7 using photothermal deflection spectroscopy. RSC Adv. 2016, 6, 103899-103904.


Page 12 of 19

Page 13 of 19

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

ACS Photonics

List of figures

Figure 1. a) Schematic diagram of the inverted semitransparent OSCs with MoO3/Ag/MoO3 as the transparent electrode; b) chemical structures of active layer materials (donor polymer: PBDB-T and non-fullerene acceptor: ITIC).


ACS Photonics

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

Figure 2. a) Energy level diagram of the materials used in the semitransparent OSCs; b) Transmittance spectra of transparent electrode.


Page 14 of 19

Page 15 of 19

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

ACS Photonics

Figure 3. a) J–V curves of the semitransparent devices under AM1.5G 100 mW cm−2 illumination; b) absorption spectra of the active layer with different thicknesses and transparent MoO3/Ag/MoO3 electrode reflectance; c) EQE spectra of the devices with different active layer thicknesses.


ACS Photonics

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

Figure 4. Device efficiency (%) and AVT (%) vs. active layer thicknesses (nm) of the semitransparent devices.


Page 16 of 19

Page 17 of 19

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

ACS Photonics

Figure 5. a) Total transmittance spectra of the semitransparent OSCs under bottom illumination; b) total device transmittance spectra (active layer spin rate= 2000 rpm) under both top and bottom illumination; c) comparison of images through ST device from both top and bottom side; photographs of the devices with 93 nm active layer thickness (2000 rpm).


ACS Photonics

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

Figure 6. a) Representation of the color coordinates (x, y) of the semitransparent solar cells with different active layer thicknesses under standard D65 illumination light source on the CIE 1931 color space; b) simulated color appearance of 8 CRI reflective samples for different semitransparent solar cells (test- 1200, 2000 and 5000 rpm) and when illuminated by D65 (Ref).


Page 18 of 19

Page 19 of 19

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

ACS Photonics

For Table of Contents Use Only

High efficiency semitransparent organic solar cells with non-fullerene acceptor for window application Mushfika Baishakhi Upama*, Matthew Wright, Naveen Kumar Elumalai*, Md Arafat Mahmud, Dian Wang, Cheng Xu, Ashraf Uddin

The graphic displays the device structure and photovoltaic performance of a high performance semitransparent organic solar cell, which includes conjugated polymer and nonfullerene acceptor as the photo active layer. Our reported device exhibited an average efficiency of 7.3% at 25% average visible transmittance. To the best of our knowledge, this efficiency is one of the few reported efficiencies exceeding 7% for a single junction semitransparent organic solar cell. The semitransparent nature of organic solar cells makes them an ideal candidate for applications in power-generating, aesthetic windows. The high efficiency achieved with our device structure will open new route to the design and fabrication of high performance photovoltaic modules with excellent potential to be incorporated into windows of buildings, especially in populated urban areas.


High-Efficiency Semitransparent Organic Solar ... - ACS Publications

Recommend Documents