Effects on Organic Photovoltaics Using Femtosecond-Laser-Treated

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Letter

The effects on organic photovoltaics using femtosecond-laser treated indium tin oxides Mei-Hsin Chen, Ya-Hsin Tseng, Yi-Ping Chao, Sheng-Yang Tseng, Zong-Rong Lin, Hui-Hsin Chu, Jan-Kai Chang, and Chih Wei Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06263 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Applied Materials & Interfaces

The effects on organic photovoltaics using femtosecond-laser treated indium tin oxides Mei-Hsin Chen,*,a Ya-Hsin Tseng,b Yi-Ping Chao,a Sheng-Yang Tseng,b Zong-Rong Lin,a Hui-Hsin Chu,b Jan-Kai Chang,c and Chih-Wei Luo*,b

a

Department of Optoelectronic Engineering, National Dong Hwa University, Hualien 974, Taiwan

b

Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan

c

Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan

[*] E-mail: [email protected] ; [email protected]

Abstract The effects of femtosecond-laser-induced periodic surface structures (LIPSS) on an indium tin oxide (ITO) surface applied to an organic photovoltaic (OPV) system were investigated. The modifications of ITO induced by LIPPS in OPV devices result in more than 14 % increase in power conversion efficiency (PCE) and short-circuit current density relative to those of the standard device. The basic mechanisms for the enhanced short-circuit current density are attributed to better light harvesting, increased scattering effects, and more efficient charge collection between the ITO and photoactive layers. Results show that higher PCEs would be achieved by laser pulses treated electrodes.

Keywords: organic photovoltaics, P3HT:PCBM, laser-treated ITO, femtosecond lasers pulses, ultraviolet ACS Paragon Plus Environment


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photoemission spectroscopy, scattering effect

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Bulk-heterojunction (BHJ) solar cells based on blends of organic donors and fullerene derivative acceptors have been developed rapidly during the past decade.1-4 The standard BHJ solar cell was initially implemented in a conventional structure in which the BHJ active layer is sandwiched between a low-work-function metal cathode and an indium tin oxide (ITO) anode coated with poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS). In order to optimize the efficiency of these composite materials, increasing numbers of techniques have been utilized to study the basic operational mechanisms in BHJ solar cells, such as ultrafast time-resolved spectroscopy,5 atomic force microscopy,6,7 photoemission spectroscopy,8,9 and grazing-incidence wide-angle X-ray scattering.10,11 Furthermore, increasing the short-circuit current density (JSC) is one of the key steps in improving the performance of organic photovoltaic (OPV) devices. Therefore, studies have focused on interfacial modification techniques to increase the JSC in an OPV system, such as the use of copolymers with poly(3-hexylthiophene) (P3HT) molecules12 and self-assembled monolayers13,14 . In this study, we applied laser-induced periodic surface structures (LIPSS) to a basic OPV structure through a relatively easy process. The formation of LIPSS has been investigated since five decades ago.15 Compared with other physical and chemical methods for the preparation of nanostructures, laser-assisted methods are simple, efficient, and environmentally friendly. Because of their high peak power and minimal thermal effects, femtosecond lasers have been proven to be a powerful tool for material processing16,17 and for nano- and microstructural fabrication18-20. For instance, they have been used for creating surface texture to enhance frictional behavior21,22 and to increase light absorption by black metals23-25. Additionally, this technique has been applied to prepare pure wurtzite-phase ZnSe particles from zinc-blende-phase ZnSe single crystals26 to produce microstructures on the surface of high-Tc superconductor oxide thin films27 and ACS Paragon Plus Environment


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to increase the conductivity of ITO thin films28. After application of LIPPS to the ITO substrates, the power conversion efficiency (PCE) of the device increased to 4.06% from the standard value of 3.56%. These results suggest that modifying the ITO surface can enhance the device performance, thus enabling production of high-performance BHJ solar cells through electrode engineering. As shown in Figure 1, 20 mm × 3 mm × 300 nm ITO thin films on 20 mm × 20 mm glass substrates were irradiated by a femtosecond laser with normal incidence and a pulse energy of 0.12 mJ with different numbers of femtosecond pulses (N). N was varied by changing the exposure time (t). The laser fluence of 0.15 mJ/cm2 was delivered to the center of ITO films with the spot size of 1 cm in diameter. In this study, the laser source is a coherent, regenerative amplified Ti:sapphire laser system (Legend USP) with a central wavelength of 800 nm, pulse duration of 30 fs, pulse energy of 0.12 mJ, and a repetition rate of 5 kHz. From the repetition rate and exposure time (t = 10, 20, 40, 60, 90, and 120 minutes), we calculated N (3 × 106, 6 ×

106, 1.2 × 107, 1.8 × 107, 2.7 × 107 and 3.6 × 107, respectively). After femtosecond laser annealing,

the morphology of the laser-treated ITO thin films was examined under a scanning electron microscope, and the resistivity along the long axis of laser-treated ITO thin films was measured by four-point probe resistivity measurements. The orientation of the ripple structure on the laser-treated ITO thin films was found to be perpendicular to the laser polarization. In our experiments, P3HT/PCBM based BHJ solar cells were used as the test devices because they are well-established systems. Except for the control device, the ITO substrates were treated with femtosecond laser pulses, for which the factors could be changed according to exposure time. Figure 2(a) shows the J–V characteristics at exposure times of 0, 10, 60, and 90 minutes. The open-circuit voltages (VOC) of all the devices remained in the standard voltage region of 0.62 V to 0.63 V, indicating that the enhancement of the ACS Paragon Plus Environment

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power conversion efficiency (PCE) of the cells is largely due to an increase in JSC. The maximum PCE and JSC values of the device for which t = 60 minutes markedly improved, increasing to 4.06% and 9.62 mA/cm2 respectively relative to those of the control device (3.5% and 8.27 mA/cm2, respectively). In addition, leakage was suppressed in the dark-current measurement, as shown in Figure 2(b). From this observation, we can conclude that all the devices with laser treatment (t = 10, 60, and 90 minutes) have dark-current densities more than one order of magnitude lower than that of the control sample (without laser treatment) under the reversed bias and that LIPPS can suppress the leakage current in the devices. To understand further the mechanisms of the effects of LIPPS on BHJ solar cells, P3HT:PCBM devices both with and without LIPPS were compared with EQE measurements. The integrated area below the curve represents the number of photons that are absorbed and then converted into photocurrents as a function of wavelength. As shown in Figure 2(c), the EQE spectrum of the LIPPS-treated OPV device (t = 60 minutes) is enhanced in the region of 300 to 800 nm in comparison with that of the control device. This result indicates that LIPPS can help increasing the light harvest in the device, thereby increasing the current density via the better efficiency of dissociated and transferred excitons. It also suggests that the efficiency improvement induced by the LIPPS arises from the scattering properties in the entire absorption region of P3HT:PCBM devices, rather than from those at a specific absorption wavelength. P3HT:PCBM devices with laser-treated-ITO were produced at incremental exposure times and then tested to determine the influence of exposure time on the photovoltaic properties. The photovoltaic parameters are summarized in Table 1. For the ITO-treatment process, the exposure time was varied from 0 to 120 minutes. The table shows that, across the various exposed ITO substrates under different conditions, the VOC and fill factor remain within the standard region of 0.62 to 0.63 V and 68% to 70%, respectively. It ACS Paragon Plus Environment


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also shows that the PCE values are dependent on the changes in JSC. To determine the reproducibility of the sequential vacuum deposition technique, at least 12 devices from three different batches were fabricated. Measurements were taken by using the same optimized fabrication conditions for ITO substrates with different exposure time. The details of the dependence of the photovoltaic parameters JSC and PCE on the exposure times for the P3HT:PCBM devices are shown in Figures 3(a) and (b), respectively. The parameters were highly reproducible. Figure 3 presents averages and standard deviations of the devices for each process. It suggests that the devices fabricated by using the laser-treated ITO showed a remarkable enhancement (especially the devices with t = 40, 60, and 90 minutes) over the devices made without laser-treated-ITO. The dependence of the photovoltaic parameters on the exposure time are clear; PCE curves at differing laser-exposure times are dependent on the changes in JSC. The enhancement of JSC may be due to an increased ability for photoelectron conversion, transport, and collection between ITO and the photoactive layer. These results show the advantage of this fabrication method in solar-module production. To verify the functions of LIPPS on the ITO surface in OPV devices, experiments on the electronic structure, as well as electrical and optical properties of the devices, were evaluated, as shown in Figure S1 and Figure 4. Figure S1 shows that the resistivity of laser-treated ITO thin films decreases significantly with increasing exposure time, thus facilitating carrier transport in the photovoltaic devices. A similar phenomenon observed by Wang et al.28 is due to the formation of indium metal-like clusters on the surface of ITO films, which are caused by the breaking of In–O bonds and the appearance of an In–In bonding state after femtosecond laser treatment. Because of an effective increase in volume resulting from In–O bond breakage to form In–In bonds, indium metal-like clusters on the surface of ITO films appeared as bulges of ACS Paragon Plus Environment

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around 5 nm in height. The resistivity and surface morphology of laser-treated ITO thin films could therefore be controlled by varying the number (N) of fs laser pulses. The lower resistivity helps JSC value, which increases with the exposure time initially. However, the surface roughness of laser-treated ITO gets larger with number (N) of fs laser pulses increases28, which will increase the carrier scattering probability at the interface and lower the ability of carrier collection.29-30 Thus, this phenomena results in the decrease of device performance after 90 minutes. UPS was performed to detect the changes in energy level of the devices both with and without laser-treated ITO thin films, as shown in Figure 4(a). The right hand side of the figure represents the valence band structure of the ITO thin films. The position of the valence band cut-off between ITO and laser-treated-ITO (t = 60 minutes) is the same, indicating that the binding energy of the ITO valence band does not change after the laser-treatment process. The left hand side of the figure represents the onset position of the samples and indicates the shift in vacuum level for each device. Figure 4(a) shows an obvious shift in valence band of the laser-treated-ITO toward lower binding energy (0.2 eV), suggesting that the work function of the laser-treated-ITO is 0.2 eV larger than that of untreated-ITO. The work function is mostly determined by the surface condition, especially the surface dipoles. For the laser-treated ITO thin films, the surface topographic images and their corresponding surface current images were investigated by atomic force microscopy (AFM) and current-sensing AFM (CSAFM) measurements in previous report28. Combined with the UPS data in this study, it can be proposed that the localized high-current region caused by the localized surface dipoles, and hence the higher local work function, in the area of laser-treated ITO. Therefore, the laser-treated ITO is more suitable as an anode than is untreated-ITO and that it can enhance carrier extraction (or carrier collection) in an OPV system. ACS Paragon Plus Environment


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Figure 4(b) shows the optical effects resulting from the LIPPS. From the light-absorption behavior, the blue curve which represents the laser-treated-ITO (t = 60 minutes) has higher absorbance value in all measurement regions than untreated-ITO has. It is clear that the LIPPS structure can significantly enhance the absorbance of P3HT:PCBM thin films (after annealing) in whole visible range, which implies that the indium metal-like clusters provided a wavelength-independent scatter center for incident photons. This result is in a good agreement with the EQE shown in Figure 2(c). In summary, this study shows that P3HT:PCBM devices with femtosecond laser pulse treatment to induce LIPPS formation on an ITO surface can perform better than that of the untreated devices. The modifications of ITO induced by LIPPS in OPV devices result in more than 14 % increase in power PCE and short-circuit current density (4.06 % and 9.62 mA/cm2, respectively) for LIPPS devices based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester, relative to those of the standard device (3.56 % and 8.27 mA/cm2, respectively). The basic mechanisms for the enhanced short-circuit current density are attributed to better light harvesting, increased scattering effects, and more efficient charge collection between the ITO and photoactive layers. Such mechanisms are confirmed by analysis of the current density–voltage characteristics, external quantum efficiency, ultraviolet photoemission spectroscopy, resistivity, and absorption measurements. Results of this study show that higher PCEs would be achieved in OPV systems by engineering of electrode structures using femtosecond laser pulses.

ASSOCIATED CONTENT Supporting Information available: Device fabrication process and resistivity of laser-treated ITO thin films with different exposure times. ACS Paragon Plus Environment

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Author information

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.



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Acknowledgment

This study is supported by the Ministry of Science and Technology, Republic of China, under contract no. MOST 103-2112-M-259-007-MY3.


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Reference

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Table caption

Table 1 The optimal parameter set for OPV devices utilizing ITO that had been laser-treated for varying exposure times.



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Figure caption

Figure 1 Schematics of the femtosecond laser annealing system and a typical SEM image of the surface of a laser-treated ITO film (N = 1.8 × 107; t = 90 minutes; pulse energy = 0.12 mJ). The spot size of the femtosecond laser beam on the sample surface is 1 cm in diameter. The cross-sectional view of the ITO thin film shows the ITO crystal structure after laser annealing.

Figure 2 (a) J–V characteristics of the solar cells based on P3HT:PCBM with ITO thin films that had been laser-treated for exposure times (t) of 0 (control), 10, 60 and 90 minutes under AM1.5G illumination with an intensity of 100 mW/cm2. (b) The dark current of the solar cells based on P3HT:PCBM with laser-treated ITO thin films (t =0, 10, 60, and 90 minutes). (c) EQE spectra of the solar cells based on P3HT:PCBM with ITO thin films that had been laser-treated for exposure times (t) of 0 (control) and 60 minutes.

Figure 3 Evolution of average and standard deviation of the photovoltaic parameters (a) JSC and (b) PCE with exposure time (t; t = 0, 10, 20, 40, 60, 90 and 120 minutes) for P3HT:PCBM BHJ solar cells with laser-treated ITO thin films .

Figure 4 (a) UPS spectra of laser-treated ITO thin films with exposure times (t) of 0 (control) and 60 minutes. (b) Absorption spectra of P3HT:PCBM with laser-treated ITO thin films and exposure times (t) of 0 (control) and 60 minutes.


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Table of Contents



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Table1

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Rsh (Ω-cm2)

Rs (Ω-cm2)

0 minute(control)

0.63

8.27

69

3.56

2860

9

10 minutes

0.62

8.31

70

3.64

1433

8

20 minutes

0.62

8.77

69

3.75

1571

9

40 minutes

0.62

9.76

68

3.86

2597

8

60 minutes

0.62

9.62

68

4.06

1049

7

90 minutes

0.62

8.99

70

3.86

1495

7

120 minutes

0.62

8.29

70

3.59

2238

8

Factors Time


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Current density (mA/cm2)

(a)

2 0

Control 10 minutes 60 minutes 90 minutes

-2 -4 -6 -8 -10 -0.2

0.0

0.2

0.4

0.6

Voltage (V)

(b)

3

10

2

Dark Current(mA/cm2)

10

1

10

0

10

-1

10

-2

10

-3

10

Control 10 minutes 60 minutes 90 minutes

-4

10

-5

10

-6

10

-7

10

-2

-1

0

1

2

Voltage(V)

(c) External quantum efficiency (%)

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 20 of 22

80 70

Control 60 minutes

60 50 40 30 20 10 0 300

400

500

600

700

Wavelength (nm)


800

Page 21 of 22

Current density (mA/cm2)

(a) 9.8 9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8

0

20

40

60

80

100

120

Exposure time (minutes)

(b) Power conversion efficiency (%)

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


4.0 3.9 3.8 3.7 3.6 3.5 3.4 0

20

40

60

80

100

Exposure time (minutes)


120


(a) Intensity (arb. unit)

He I 21.2 eV 0.2 eV 60 minutes

Control

17.5 17.0 16.5 16.0 9 8 7 6 5 4 3 2 1 0

Binding Energy (eV) w.r.t. EF

(b)

1.0 0.8

Absorbance

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 22 of 22

0.6 0.4 0.2 0.0 300

Control 60 minutes 400

500

600

700

Wavelength (nm)


800

Effects on Organic Photovoltaics Using Femtosecond-Laser-Treated

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