PCBM organic solar cells with Al-doped ZnO electrode

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 1020–1023

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Effects of intrinsic ZnO buffer layer based on P3HT/PCBM organic solar cells with Al-doped ZnO electrode Sungeun Park, Sung Ju Tark, Joon Sung Lee, Heejin Lim, Donghwan Kim  Department of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-701, Republic of Korea

a r t i c l e in f o

a b s t r a c t

Article history: Received 14 January 2008 Accepted 21 November 2008 Available online 17 January 2009

Organic solar cell devices were fabricated using poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), which play the role of an electron donor and acceptor, respectively. The transparent electrode of organic solar cells, indium tin oxide (ITO), was replaced by Al-doped ZnO (AZO). ZnO has been studied extensively in recent years on account of its high optical transmittance, electrical conduction and low material cost. This paper reports organic solar cells based on Al-doped ZnO as an alternative to ITO. Organic solar cells with intrinsic ZnO inserted between the P3HT/PCBM layer and AZO were also fabricated. The intrinsic ZnO layer prevented the shunt path in the device. The performance of the cells with a layer of intrinsic ZnO was superior to that without the intrinsic ZnO layer. & 2008 Elsevier B.V. All rights reserved.

Keywords: Organic solar cells ZnO Intrinsic ZnO Shunt resistance

1. Introduction Organic solar cells have considerable technological potential as an alternative, renewable source of electrical energy. The demand for inexpensive renewable sources of energy has encouraged new approaches in the development of low-cost solar cells [1]. Recently the efficiency of a tandem structure organic solar cell reached more than 6% at illumination [2]. The materials used in organic solar cells have many practical advantages over conventional solar cell materials such as silicon due to the use of solution processing techniques to fabricate the cells. Blend heterojunctions, which consist a bulk mixing of poly(3-hexylthiophene) (P3HT) as a donor and 6,6-phenyl C61-butyric acid methyl ester (PCBM) as an acceptor, are quite promising [3]. The transparent electrode of organic solar cells, indium tin oxide (ITO), has been replaced by Al-doped ZnO (AZO) [4–6]. ZnO has attracted considerable attention in recent years because it exhibits high optical transmittance and electrical conduction and has a low material cost [7]. Therefore, there is a need to make organic solar cell with different transparent electrodes, such as ZnO. One of the main reasons for the reduced organic solar cell efficiency is the shunt path in the organic solar cell device [8]. Fig. 1 shows a schematic diagram of a variety of shunt paths in organic solar cells. Since the intrinsic ZnO layer is highly resistive, it can be expected to protect the solar cell from shunts [9]. Furthermore, the protection against shunts improves with increasing intrinsic ZnO layer thickness because a thicker layer  Corresponding author. Tel.:+82 2 3290 3275.

E-mail address: [email protected] (D. Kim). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.033

can be assumed to increase the resistance in the shunt path and hence, a decrease the leakage current. In this study, a structure was designed using intrinsic ZnO on an Al-doped ZnO transparent electrode as a buffer layer.

2. Experimental Al-doped ZnO films on Corning glass (Eagle 2000) were prepared by radio frequency (rf) magnetron sputtering of ZnO targets containing Al2O3 at a substrate temperature of 150 1C. ZnO targets with a 2 wt% Al2O3 content were used. Before approximately 250–300-nm-thick AZO deposition, the glass substrates were sequentially cleaned ultrasonically in acetone, alcohol and de-ionized water, and finally dried in nitrogen gas. The distance between the target and the substrate was approximately 50 mm. The rf magnetron working power and the deposition time were 50 W and 60 min, respectively. The base pressure was 1 10 6 Torr. Intrinsic ZnO films were deposited on AZO thin films using an ALD system. Zn (C5H5)2 and H2O were placed in bubblers kept at 20 1C in temperature-controlled baths. The chamber was evacuated using a rotary pump and maintained at 2.5 Torr. For ALD growth, diethylzinc (DEZn) and H2O were injected alternatively into the chamber with argon as the carrier gas. The open and close sequences of the air valves were controlled automatically using a computer. The pulse time was 1 s for the reactants and 8 s for the purging line and chamber. The flow rates of the carrier gas for both H2O and DEZn were 20 sccm. The substrate temperature was 100 1C.

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metal Au PCBM 11

22

33

44

55

Active layer

P3HT/PCBM i-ZnO AZO

ZnO:Al

glass

Band alignment

e

metal Ef PCBM h

P3HT/PCBM P3HT/PCBM Intrinsic ZnO AZO glass

ZnO:Al

Fig. 1. Schematic diagram of the shunt path in organic solar cells without (a) and with (b) the intrinsic ZnO buffer layer.

i-ZnO

P3HT:PCBM

Au

Fig. 2. Schematic diagram of the device structure (a) and band diagram (b) with the intrinsic ZnO buffer layer.

Table 1 Organic solar cell performance with and without the intrinsic ZnO layer.. Types

VOC (mV)

Rshunt (O)

FF (%)

Efficiency (%)

AZO Intrinsic ZnO/AZO

126 177

1180 1590

25.6 29.9

0.07 0.20

Through a preliminary experiment, AZO thin films were deposited by radio frequency magnetron sputtering using a ceramic target containing 2 wt% of Al at a substrate temperature 150 1C. The resulting film had a low resistivity of 3.21 10 4 O cm and a transmittance in the visible light range of 85% or higher. Bulk heterojunction solar cells were prepared from an active layer of poly(3-hexylthiophene)/[6,6]-phenyl-C61-butric acid methyl ester at 1:0.8 weight ratio in a chloroform solvent [9]. An active layer with a thickness 100–120 nm was cast on top of the intrinsic ZnO layer on the AZO film by spin coating. After a drying period of approximately 1 h, a top electrode, 100 nm Au, was deposited by thermal evaporation in vacuum better than 10 5 mbar. The devices were annealed at 150 1C on a hot plate for 30 min [10].

3. Results and discussion The solar cells were based on poly(3-hexylthiophene) as the donor and 6,6-phenyl C61-butyric acid methyl ester (C60) as the acceptor molecule. The cells had the following layer sequence: AZO/intrinsic ZnO/active layer/Au. Fig. 2(a) shows the device structure in these studies. Fig. 2(b) shows the band alignment indicating a suitable arrangement for transporting the separated electrons and holes [11]. The electrons move to the AZO and the holes move to Au. Cells without an intrinsic ZnO buffer layer were examined for comparison. The intrinsic ZnO layer was 130 nm thick. This is because an intrinsic ZnO layer thickness o130 nm had no observable effect due to the morphology of the ZnO film. When electrons migrate through the intrinsic ZnO layer, they pass through a short way in the ZnO film so the electrons are not influenced by the intrinsic ZnO layer. However, at a thicker

Fig. 3. Transmittance (a) and optical bandgap (b) of ITO and AZO and intrinsic ZnO in AZO.

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µm

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5

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3

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Fig. 4. AFM images of the AZO and intrinsic ZnO on AZO surfaces.

Fig. 5. Current–voltage characteristics of the organic solar cells with and without the intrinsic ZnO layer under illumination.

intrinsic ZnO layer, the ZnO layer is covered and electrons are influenced by this layer. Fig. 3 shows the transmittance and optical band gap of ITO and AZO and intrinsic ZnO. Fig. 3(a) shows the a2 versus photon energy plot near the band edge. The optical band gap of 3.3 eV (ZnO/AZO), 3.4 eV (AZO) and 3.8 eV (ITO) was estimated by extrapolating the linear segments of the curves towards the x-axis. The average transmittance in the visible range was measured to be approximately 92%, which is sufficient for a transparent conductive electrode. AZO showed better transmittance value in the visible range compared with ITO. The fermi level of the intrinsic ZnO layer was located under the conduction band of the other side of the fermi level of AZO. For this reason, the band gaps of the two materials are different (shown band diagram Fig. 2(b)). The surface roughness of the AZO and intrinsic ZnO on AZO was measured by atomic force microscopy (AFM). Fig. 4. shows the AFM images of AZO and intrinsic ZnO on the AZO film surface. Both can be seen clearly as small islands. The root-mean-square (RMS) roughness values were 0.8 and 1.3 nm, respectively. The RMS roughness is a quantitative value for describing the surface roughness of AZO and intrinsic ZnO on the AZO film surface. However, the top to valley was 410 nm in the case of the intrinsic ZnO on AZO layer. This suggests that the carriers are not collected efficiently on the electrode (intrinsic ZnO on AZO) because the diffusion length of the carriers in organic solar cells are approximately 10 nm only. The device consisted of AZO/intrinsic ZnO/active layer/Au. The thickness of the active layer (P3HT and

PCBM mixed layer) is only 100 nm. Hence the device is weak for sharp shape and stress. These make a shunt path in organic solar cells. The front contact of this solar cell consists of a high resistive intrinsic ZnO layer on top of the high-conductivity ZnO:Al layer. Since the intrinsic ZnO layer is highly resistive, it can be expected to protect the solar cells from shunts. Furthermore, it can be expected that the level of protection against shunts improves with a thicker intrinsic ZnO layer because a thicker layer can be assumed to increase the resistance in the shunt path and decrease the leakage current. The performance of the cells was improved with a intrinsic ZnO layer compared with that without an intrinsic ZnO. Fig. 5 shows the performance of the organic solar cells with and without an intrinsic ZnO layer. In the case of non-intrinsic layer, the shunt resistance Voc and fill factor were less than that with the device with the intrinsic ZnO layer. The shunt resistance was improved from 1180 to 1590 O. In addition, the open-circuit voltage was increased from 126 to 177 mV. The fill factor was also improved from 25.6% to 29.9%, as shown in Fig. 5. These results suggest that the highly resistive and thick intrinsic ZnO layer prevented the abnormal electron path in the ZnO layer such as holes in the intrinsic ZnO layer, the electron path of a thinner spot and any other shunt paths. The performance of solar cell with and without intrinsic ZnO layer is shown in Table 1.

4. Conclusions In this study, ZnO thin films were used in organic solar cell applications. The interesting aspects of ZnO include the anisotropy in crystal structure, wide band gap, and optical transparency in the visible range. In addition, it has a fairly high refractive index, and large piezoelectric constant. Organic solar cells with P3HT/PCBM were fabricated and an organic solar cell was applied to an AZO anode. In the case of the ZnO/P3HT–PCBM/Au device, the power conversion results were poor. However, these AZO based solar cells have potential applications to solar cells. The intrinsic ZnO layer on AZO prevented the shunt paths in these organic solar cell devices. The experiments indicate that an intrinsic ZnO layer is needed to protect the organic solar cells from efficiency losses. Organic solar cells with an intrinsic ZnO layer appear to have higher efficiencies than those without the intrinsic ZnO layer.

Acknowledgement This work is outcome of the fostering project of the Best Lab supported financially by the Ministry of Commerce, Industry and Energy (MOCIE).

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