TiO2 bulk-heterojunction solar cell sensitized by a perylene derivative

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Solar Energy Materials & Solar Cells 91 (2007) 1782–1787 www.elsevier.com/locate/solmat

P3HT/TiO2 bulk-heterojunction solar cell sensitized by a perylene derivative Mingqing Wang, Xiaogong Wang Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084, PR China Received 26 April 2007; received in revised form 19 June 2007; accepted 20 June 2007 Available online 27 July 2007

Abstract In this work, a new type of dye-sensitized bulk-heterojunction hybrid solar cells has been developed. The heterojunction films were prepared to contain poly(3-hexylthiophene) (P3HT), N,N0 -diphenyl glyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide (PDI) and TiO2. In the architecture, TiO2 and P3HT were designed to act as the electron acceptor and donor. PDI was used as sensitizer to enhance the photon absorption. Results showed that by incorporation of PDI in the P3HT/TiO2 composite, the light absorption, exciton separation and photocurrent under white light were dramatically enhanced. Solar decay analyses showed that devices contained TiO2 required 12 h to obtain maximum current density and the addition of PDI did not affect the solar decay behavior and stability of device composed of P3HT/TiO2. The devices of P3HT, P3HT/TiO2, P3HT/TiO2/PDI could work for 5, 42, 45 h under continuous white light illumination (100 mW/m2) under the ambient condition. r 2007 Elsevier B.V. All rights reserved. Keywords: Perylene derivatives; Hybrid solar cells; TiO2; Polythiophene; Stability

1. Introduction Photovoltaics have received increasing attention over the past decades as a feasible way to replace the diminishing fossil fuels and reduce the environmental damaging. Inorganic semiconductors are ideal for fabricating highly efficient solar cells, as they can absorb a broad range of light and transport charge effectively, but the expensive processing processes of the classical inorganic photovoltaics limit their applications in many areas. Polymer-based photovoltaic materials and devices are attracting more and more attention for the advantages such as low-cost, light-weight, feasibility for large size [1]. Moreover, flexible solar panel can be developed based on polymeric systems. Compared with inorganic photovoltaics, performance of polymer-based photovoltaics is limited by weak absorption in the red, poor charge transport, and low stability [2], but improvements are available through optimizing materials Corresponding author. Tel.: +86 10 62784561; fax: +86 10 62770304.

E-mail address: [email protected] (X. Wang). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.06.006

and device structures [3]. To solve the problem of limited exciton diffusion length (4–20 nm) and low electrical conductivity of conjugated polymers (CPs), the strategy using interpenetrating networks of electron-accepting and hole-accepting components within the device has been proposed. This can be achieved by mixing p-type CPs with n-type CPs [4,5], dyes [6,7], inorganic semiconductor (e.g., TiO2 [8–14], ZnO [15–20], or CdSe [21–26]), and C60 derivatives [27–29]. Although the efficiency of CP solar cells can be improved upon the introduction of electron acceptor materials, in many cases, the active layers of the solar cells cannot absorb photons over all the solar emission spectrum and produce high photocurrents. The light absorption in the active layer can be increased by introducing dyes with large absorbance. Perylene derivatives possess high electron affinity, high absorption coefficient (about 10 times higher than P3HT). They have been used as photosensitizer in dyesensitized TiO2 solar cells with polythiophene as hole conductors [30,31]. It has been reported that perylene derivatives were used as electron donors in CP/dye composites [32]. However, due to the low solubility and

ARTICLE IN PRESS M. Wang, X. Wang / Solar Energy Materials & Solar Cells 91 (2007) 1782–1787

high crystalline of most perylene derivatives, the CPs/dye composites cannot show a significant increase in the photoelectric efficiency for many cases. In this article, a soluble dye, N,N0 -diphenyl glyoxaline3,4,9,10-perylene tetracarboxylic acid diacidamide (PDI), was used to improve the performance of poly(3-hexylthiophene) (P3HT)/TiO2 bulk-heterojunction films through enhancing the light absorption and charge generation efficiency. P3HT/PDI/TiO2 composite films were prepared by spin-coating and solar cells were prepared by sandwiching the composite films between two electrodes. Composite film preparation, photoelectric properties of composite films, and the solar cell performance and stabilities will be presented in the following parts.

2. Experimental 2.1. Device manufacture Tetrabutyl titanate was purchased from Acros. P3HT and PDI were synthesized and purified according to Refs. [33,34]. Clean ITO glasses were used for device preparation. Photovoltaic devices were fabricated by preparing the photo-active films on substrates coated with thin PEDOT:PSS films. For P3HT/PDI devices, a chloroform solution of P3HT (10 mg/mL) and PDI (1 mg/mL) was spin-coated on the substrates. The content of PDI was 10 wt% in the P3HT/PDI composite. To prepare the P3HT/TiO2 bulk-heterojunction, a chloroform solution containing P3HT (10 mg/mL) and Ti(OC4H9)4 (100 mg/mL) was spin-coated on the substrates. The content of TiO2 in

the composite was 60 wt%. Subsequent conversion of the Ti(OC4H9)4 precursor, via hydrolysis in dark under an ambient condition, resulted in the formation of a TiO2 phase in the polymer films. To improve the crystalline degree of TiO2 and the composite film quality, the composite films were annealed at 160 1C vacuum oven for 1 h. To prepare the P3HT/TiO2/PDI bulk-heterojunction films, PDI was added into the chloroform solution of P3HT/Ti(OC4H9)4 and the content of PDI in the composite was 5 wt%. At last, the back electrodes, 100 nm thick Al films, were evaporated in high vacuum. 2.2. Characterization and testing UV–vis spectra of the spin-coated films were recorded on a Perking–Elmer Lambda Bio-40 spectrometer. Photoluminescence (PL) measurements were made using Hitachi F-4500 florescence spectrophotometer. Current–voltage (I–V) measurements were taken in air at room temperature using a Keithley 236 high current source power meter under 100 mW/cm2 white light illumination from a halogen lamp. To measure the solar cell stabilities, the short circuit current (ISC) as a function of time during continuous illumination was recorded. 3. Results and discussion The energy levels of the materials used in this work are shown in Scheme 1. The energy gap of PDI lies between the LUMO of the P3HT and the CB of the TiO2. After absorbing photons from sunlight, PDI can transfer

C6H13

*

S

n *

Vaccum Level

HOMO e

-3.0eV

S+/S*

hv

e-

-4.0eV

CB

e-

hv -5.0eV

Al PEDOT

-6.0eV

-7.0eV

h+

LUMO P3HT

1783

h+ S+/S PDI

VB

TiO2

Scheme 1. Chemical structures and energy level diagram: (a) P3HT, (b) PDI, and (c) energy levels relative to vacuum.

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electrons to the TiO2 and holes to P3HT in the P3HT/PDI/ TiO2 films. P3HT can also absorb photons and transfer the excited electrons to adjacent PDI or TiO2. TiO2 and dye both can act as electron donors for P3HT. Nanodispersed TiO2 phase can provide a large interfacial area. As PDI can absorb light more efficiently, the two electron donors can work synergistically to improve the cell efficiency. Fig. 1 shows the UV–vis absorption spectra of P3HT, P3HT/TiO2, P3HT/PDI, and P3HT/TiO2/PDI thin films. It can be seen that P3HT shows a strong absorption in the visible wavelength range from 400 to 600 nm. TiO2 absorbs light in the wavelength range from 200 to 300 nm. The absorption peaks of the P3HT/TiO2 composite show the overlapping absorption bands of P3HT and TiO2. Fig. 2 shows UV–vis absorption spectrum of the PDI film. PDI shows two absorption bands, one in ranges from 200 to 400 nm and the other from 450 to 750 nm. As PDI has broad and strong absorption nearly throughout all of the visible wavelength range, the absorption range and efficiency are obviously increased. PL quenching can evidence the photo-induced charge transfer in P3HT/PDI/TiO2 composite films. Fig. 3 shows

P3HT

1.6

P3HT/TiO2 P3HT/PDI P3HT/PDI/TiO2

Absorbance (a.u.)

1.2

0.8

0.4

0.0 200

400 600 Wavelength (nm)

800

Fig. 1. UV–vis absorption spectra of P3HT, P3HT/TiO2, P3HT/PDI, and P3HT/PDI/TiO2.

Absorbance (a.u.)

2.0

1.5

1.0

0.5 200

400

600

Wavelength (nm) Fig. 2. UV–vis absorption spectrum of PDI.

800

P3HT

Photoluminescence (a.u.)

1784

P3HT/TiO2

60

P3HT/PDI P3HT/PDI/TiO2

40

20

0 500

600

700

800

Wavelength (nm)

Fig. 3. Fluorescence spectra of P3HT, P3HT/TiO2, P3HT/PDI, and P3HT/PDI/TiO2.

PL spectra of P3HT, P3HT/TiO2, P3HT/PDI, and P3HT/ PDI/TiO2 thin films, respectively. For P3HT, there is an emission peak at the wavelength of 617 nm. PDI and TiO2 both can act as electron acceptor when they are blended with P3HT. PL emission intensity is decreased from 60 to 29 for P3HT/PDI and 24 for P3HT/TiO2 composite films. When both PDI and TiO2 are blended into P3HT, 80% of PL is quenched. In the P3HT/PDI/TiO2 composite film, PDI can absorb more photons from light, but the interface area between the components is also increased. As a result, the excitons can be more efficiently separated to electrons and holes, which causes the decrease of the PL. The solar cells containing P3HT, P3HT/PDI, P3HT/ TiO2, and P3HT/TiO2/PDI photo-active layers were prepared to study the photoelectric performance of the materials. The layout of P3HT/TiO2/PDI solar cell is given in Fig. 4(a). All other solar cells for the comparison have a similar layout. The phase morphology of the P3HT/TiO2/ PDI bulk-heterojunction film is illustrated in Fig. 4(b). PDI is easy to crystallize and form fine crystals with high aspect ratio in the system. These crystals, overlapped with each other between the TiO2 nanoparticles, form continuous channels for effective transfer of the electrons toward the electrodes. I–V curves of the solar cells were studied upon exposure to white light. Fig. 5 shows the I–V curves of P3HT, P3HT/ PDI, P3HT/TiO2, and P3HT/TiO2/PDI solar cells. Doping with either PDI or TiO2 can improve the ISC of the cells, which increase from 27.14 to 62.65 mA/cm2 for P3HT/PDI and to 130.56 mA/cm2 for P3HT/TiO2. The open circuit voltages (VOC) of the cells are also increased, which increase from 0.32 to 0.35 V for P3HT/PDI and to 0.52 V for P3HT/TiO2. In the case that the active composite film of the solar cell is composed of P3HT/TiO2/PDI, the highest ISC of 256.7 mA/cm2 is obtained, which is near ten times higher than that of the pure P3HT device. Its VOC is 0.43 V, which is 0.07 V higher compared with P3HT/PDI and 0.09 V less compared with P3HT/TiO2.

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Fig. 4. (a) Layout of the P3HT/PDI/TiO2 photovoltaic devices, active layer is sandwiched between a PEDOT/PSS layer and aluminum top electrode. (b) Schematic morphology of the bulk-heterojunction layer.

400

30

P3HT/PDI

Isc (µA/cm2)

Current Density (µA/cm2)

P3HT

P3HT/TiO2

200

P3HT/PDI/TiO2

24 18 12 6

0

0 0.0

0.5

1.0 Time (h)

1.5

2.0

-200

0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

Fig. 5. Current density–voltage characterization of the P3HT/PDI, P3HT/TiO2, and P3HT/PDI/TiO2 photovoltaic devices.

Isc (µA/cm2)

140

120

100

80 0

3

6 9 Time (h)

12

15

0

3

6

12

15

300 Isc (µA/cm2)

The results given above can be understood by considering the properties of the components. As the easy crystallization of PDI, the content of PDI in the composite film cannot be too high. In this study, the content of PDI in P3HT/PDI film is 10 wt%. The low content of PDI leads to insufficient PL quench and the limited improvement of the photovoltaic properties. The content of TiO2 in P3HT/TiO2 film can reach 60 wt%. TiO2 with such high concentration can efficiently quench the PL, but it can only absorb the light in the wavelength ranges from 200 to 300 nm. When 5 wt% of PDI is added in the P3HT/PDI/ TiO2 solar cells, the absorption range of the active layer is significantly extended (Fig. 1). Fig. 3 shows that PL is also significantly quenched in the P3HT/PDI/TiO2 composite films, which indicates a high efficiency of charge separation. Therefore, the increase in the photocurrent can be attributed to the increased light absorption and efficient charge separation. However, VOC of the bulk-heterojunction solar cells is not significantly improved. VOC of P3HT/ PDI/TiO2 composite solar cells might not only be related with the different energy levels of electron acceptor PDI

250

200

9 Time (h)

Fig. 6. Solar decay analyses of devices composed of (a) P3HT, (b) P3HT/ TiO2, and (c) P3HT/PDI/TiO2 photovoltaic devices, measured in ambient atmosphere with 100 mW/cm2 white light.

and TiO2, which could also be affected by the morphology of the active layer. Based on the consideration, solar cell efficiency can be further improved by using the components and optimizing their morphology at the same time.

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Fabricating solar cells with long lifetime is a great challenge in the fields of hybrid cells [35,36]. Lifetime data of the devices composed of P3HT, P3HT/TiO2, and P3HT/ TiO2/PDI were studied to evaluate stability under illumination. Fig. 6 shows the cell decay analyses performed on the devices under continuous white light illumination with 100 mW/m2 in ambient atmosphere. For devices containing P3HT, ISC decreases from 25.12 to 2.42 mA/cm2 in 120 min (Fig. 6a). For the devices containing TiO2, more complicated ISC variation with time was observed. For devices containing P3HT/TiO2, solar decay shows an initial decrease in ISC then follows by a slow increase until it reaches a maximum and stabilizes at that level. The maximum ISC is reached at about 12 h approximately (Fig. 6b). This behavior is associated with the atmosphere to which the devices were exposed before testing and its influence on the semiconductor oxide in the thin films. It has been reported that oxygen is required for the devices containing metal oxides for its function to stabilize the electronic structure under illumination [37]. It has been observed that the photovoltaic response stops in vacuum or argon, but if air is admitted the device starts working again. In this work, after metal electrodes were prepared under vacuum evaporation, the devices were transferred again to the ambient atmosphere and a gradual diffusion of oxygen from the atmosphere into the photo-active layers could take place. In the process, Ti3+ present in the TiO2 structure was slowly re-oxidized to Ti4+ during the process of gradual diffusion of oxygen from the ambient atmosphere into the TiO2 structure [38]. Therefore, the apparent initial increase of ISC can be observed. Addition of PDI does not affect the light decay behavior of the P3HT/TiO2 devices (Fig. 6c). The lifetime of the solar cells was characterized by the decrease of ISC to almost zero, it was observed to be 5 h for devices containing P3HT, 42 h for P3HT/TiO2, 45 h for P3HT/TiO2/PDI. The final decay of the devices could be attributed to the degradation due to photo-oxidation in air. Above results clearly show that the deterioration of the device performance is closely related to the nature of the organic polymer. The addition of TiO2 can significantly improve the stability of devices composed of P3HT. Doping with PDI has no observable effect on the stability of the P3HT/TiO2 devices. 4. Summary A soluble perylene derivative (PDI), which can absorb a broad range of the sunlight, was used to enhance the photoelectric efficiency of the hybrid P3HT/TiO2 bulkheterojunction solar cells. Energy level diagram of the components shows that PDI can act as sensitizer and transfer the excited electrons and holes to P3HT and TiO2, respectively. Both PDI and TiO2 can act as electron acceptor when P3HT absorb light and act as electron donor. The incorporation of the PDI in the P3HT/TiO2 composite can dramatically increase the layer absorption and decrease the PL. When 5 wt% of PDI is added into

P3HT/TiO2 composite film, the photocurrent can be significantly increased in comparison with the corresponding P3HT/TiO2 and P3HT/PDI cells. Solar decay analyses show that devices containing TiO2 require 12 h to obtain the maximum current density and the addition of PDI does not affect the solar decay behavior and stability of the P3HT/TiO2 devices. The devices of P3HT, P3HT/TiO2, P3HT/TiO2/PDI can work for 5, 42, 45 h upon continuous white light illumination (100 mW/m2) under the ambient condition.

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