An effective bilayer cathode buffer for highly efficient small molecule organic solar cells

Organic Electronics 13 (2012) 1925–1929

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An effective bilayer cathode buffer for highly efficient small molecule organic solar cells Hao-Wu Lin a,⇑, Hao-Wei Kang a, Zheng-Yu Huang a, Chang-Wen Chen a, Yi-Hong Chen a, Li-Yen Lin b, Francis Lin b, Ken-Tsung Wong b a b

Department of Materials Science and Engineering, National Tsing Hua University, Hsin Chu 30013, Taiwan Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

a r t i c l e

i n f o

Article history: Received 3 April 2012 Received in revised form 16 May 2012 Accepted 24 May 2012 Available online 18 June 2012 Keywords: Organic photovoltaics Cathode buffer Electron transporting layer Small molecule solar cells

a b s t r a c t Novel organic/ultrathin low work function metal bilayer cathode buffers for small molecule organic solar cells are proposed. Ultrathin low work function metal layers possess a high built-in electric field for effective carrier extraction and a high cathode reflectivity for maximum absorption in the photoactive layers. This leads to a significant increase of short circuit current density and fill factor of cells. By integrating this bilayer cathode buffer with DTDCTB:C60 small molecular heterojunction, the device exhibits a high power conversion efficiency of up to 5.28%, which is an improvement of 22% compared to a device with a traditional single organic layer buffer. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, organic solar cells (OSCs) have attracted much attention for their potential as low cost, flexible and renewable energy conversion devices. Extensive research has been devoted to improving the power conversion efficiencies (PCEs) of OSCs, which are determined by various factors, including the optical properties of organic active materials, device structures, process conditions and the effects of electrode buffer layers. One of the most effective ways to improve cell efficiencies is the integration of a cathode or anode buffer layer [1–5]. The most commonly used cathode buffer layer materials in small molecule OSCs are large-bandgap 2,9-dimethyl4,7-diphenyl-1,10-phenanthroline (BCP) and 4,7-diphenyl-1,10-phenanthroline (Bphen), which serve as electron extraction layers that contribute to efficient charge transport and also act as an optical spacer for modulating the optical fields inside the devices [3,6]. Also, such buffer layers often behave as an exciton blocking layer to prevent ⇑ Corresponding author. E-mail address: [email protected] (H.-W. Lin). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.05.049

excitons generated in the active layers from quenching at the organic/metal interface [7]. It has been reported that cathode buffer materials such as BCP and Bphen transport carriers via metal deposition induced trapping states. Accordingly, the depths of the damage-induced trapping states restrict the thickness of the buffer layer to within 10 nm. In polymer OSCs, especially with highly crystalline organic active layers, thick Ca (20–40 nm) is usually adopted as the cathode buffer layer due to significant morphology modification [8,9]. However, in small molecule OSCs, of which much thinner and smoother active organic layers are usually utilized, the low reflectivity of thick Ca may result in lower short circuit current density (Jsc) [10]. Therefore, Ca is rarely reported as an effective cathode buffer layer in small molecule OSCs. Recently, planar mixed heterojunction (PMHJ) small molecule OSCs based on new molecule DTDCTB (2-{[7(5-N,N-ditolylaminothiophen-2-yl)-2,1,3-benzothiadiazol4-yl]methylene}malononitrile) as the donor and C60 as the acceptor have been reported. The optimized devices with BCP buffer layer exhibited PCEs as high as 4.4% [11]. In this study, an effective organic/ultrathin low work function metal bilayer cathode buffer structure is proposed. The

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H.-W. Lin et al. / Organic Electronics 13 (2012) 1925–1929

Current Density (mA/cm2)

organic layer acts as an electron extraction layer and optical spacer while the ultrathin low work function metal layer establishes a high built-in electric field for effective carrier extraction without diminishing cathode reflectivity. By integrating this bilayer buffer into DTDCTB:C60 based PMHJ solar cells, increase of Jsc from 10.8 to 11.9 mA/cm2, and fill factor (FF) from 50.1% to 54.6% are observed. This results in a high PCE of 5.28% and a large PCE improvement of >20% compared to reference devices with traditional single organic layer as cathode buffer.

(a) 0 -3

-9 -12 0.0 70

2. Experimental

Reference Ca-1 nm Ca-3 nm Ca-30 nm Mg-1 nm Mg-3 nm Mg-30 nm

-6

0.2

0.4

0.6

0.8

1.0

Voltage(V)

(b)

60 50

EQE (%)

Devices were fabricated with the following structure: glass/ITO/5 nm MoO3/7 nm DTDCTB/40 nm DTDCTB:C60 (1:1)/20 nm C60/6 nm Bphen/0, 1, 3, 30 nm Ca or Mg/ 150 nm Ag. A device utilizing single 6 nm Bphen cathode buffer without ultrathin Ca or Mg layers was taken as reference cell. Organic materials such as Bphen and C60 were purified by vacuum thermal gradient sublimation. All devices were prepared with a transparent ITO as anode with sheet resistance 15 X/sq. ITO substrates were cleaned in ultrasonic bath with de-ionized water, acetone and methanol for 15 min, and then treated by ultraviolet–ozone for 30 min before being loaded into a high vacuum chamber (base pressure <1  106 torr). Organic layers were thermally evaporated at the rate of 0.1–0.2 nm/s. Ca and Mg were deposited at the rate of 0.1 nm/s and Ag was deposited at the rate of 0.4 nm/s through a shadow mask to define a cell area of 5 mm2. Finally the fabricated cells were encapsulated in a N2 glove box. The accurate device areas were measured device-by-device using calibrated optical microscope. The current density versus voltage (J–V) characteristics were measured with a Keithley SourceMeter 2636A in the dark and under AM 1.5G simulated solar illumination with an intensity of 100 mW/cm2 (1 sun, calibrated by NREL-traceable KG5 filtered Silicon reference cell). Light intensities of <1 sun were achieved

40 30 20 10 0

400

500

600

N

S

CN

N

NC DTDCTB

900

by placing metallic neutral density filters in the light path. The external quantum efficiency (EQE) spectra were taken by illuminating a chopped monochromatic light with a continuous-wave bias white light (from halogen lamp) on the solar cells. The monochromatic light intensities were measured with NIST-traceable power meter (Ophir). The photocurrent signals were then extracted with lock-in technique using a current preamplifier (Stanford Research System) followed by a lock-in amplifier (AMETEK). The simulation program was coded with Matlab software (The MathWorks, Inc.) and performed with dual-core Intel-CPU

N

S

800

Fig. 2. (a) Dark (open symbol) and AM 1.5G, 1 sun illuminated (solid symbol) J–V characteristics of devices with traditional single organic layer (reference cell) and new bilayer buffer layers. (b) Corresponding EQE spectra of the devices.

(a) N

700

Wavelength(nm)

C60

N

Bphen

(b)

Fig. 1. (a) Molecular structures and (b) energy level diagram of devices in this study.

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H.-W. Lin et al. / Organic Electronics 13 (2012) 1925–1929 Table 1 Performance of solar cells in this work.

Reference cell Ca 1 nm Ca 3 nm Ca 30 nm Mg 1 nm Mg 3 nm Mg 30 nm

JSC (mA/cm2)

FF (%)

PCE (%)

Rs (X/cm2)a

Rsh (X/cm2)a

0.80 0.81 0.81 0.80 0.80 0.81 0.81

10.8 11.9 10.7 9.4 10.0 10.3 10.4

50.1 54.6 55.5 51.7 51.2 52.1 51.9

4.33 5.28 4.82 3.90 4.11 4.33 4.35

10.9 8.2 7.9 10.0 10.2 10.2 10.2

347.8 303.3 300.4 329.8 308.8 297.7 336.3

Under AM 1.5G 1 sun illumination.

3. Results and discussion Fig. 1(a) shows the molecular structures of the organic materials used in this study and Fig. 1(b) shows the energy level diagram. Fig. 2 illustrates the J–V characteristics as well as EQE spectra of devices with traditional single organic layer and new bilayer buffers. All devices exhibited open circuit voltage Voc = 0.80 ± 0.01 V, independent of buffer layer compositions. Studies have shown that Voc is determined by the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor (HOMODonor  LUMOAcceptor) [12]. The cell performances are summarized in Table 1. The results show that by using ultrathin Ca film (1–3 nm) as electron extraction layer, the efficiencies could be improved significantly compared to the reference device with only Bphen buffer layer. Best performance was obtained in a cell with Bphen/1 nm Ca buffer, which shows very high Jsc of 11.9 mA/cm2 and FF of 54.6%. It is believed that the increase in Jsc and FF is attributed to the higher magnitude of built-in electric field induced by low work function metal, resulting in higher exciton dissociation efficiency at the heterojunction and faster carrier extraction. Similar trends in improvements of Jsc and FF in polymer solar cells using poly[(9,9-bis(30 (N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) cathode buffer have been reported [13]. As shown in Fig. 2(b), the shapes of EQE spectra are almost identical in the reference cell and the device with bilayer buffer. This indicates that enhancement of device performances are wavelength-independent, thus occurring throughout the active layers. Upon increase of Ca thickness, the efficiency decreases due to the reduction of Jsc. Nevertheless, Jsc remains constant in cells with various Mg thicknesses. This originates from the different reflectivities of Ca and Mg on Ag cathode with 1–30 nm thickness. To support this argument, reflections of different thicknesses of Ca or Mg films on 150 nm Ag were measured and shown in Fig. 3. The reflectivities of 1–3 nm Ca on 150 nm Ag are almost the same as bare Ag electrode. However, as Ca thickness increases to 30 nm, reflectivity decreases remarkably. On the other hand, the reflectivity of Mg film on 150 nm Ag is of a high value 0.9 regardless of the thickness of Mg. To gain more

insight into the optical effect of the bilayer buffer proposed in this study, total harvested photons under standard 1 sun AM 1.5G solar illumination were calculated using panchromatic optical field and exciton generation simulation program [14]. As shown in Fig. 4, unlike Bphen/Mg buffer layer, the Jsc of Bphen/Ca decreases with the increase of the thickness of Ca layer. According to the simulation, a higher Jsc of Bphen/Ca(1 nm) cell (11.9 mA/cm2) compared to Bphen reference cell (10.8 mA/cm2) is not due to the optical effect but is attributed to the higher carrier collection of Ca. The harvested photons in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester

1.0

(a)

0.8

Reflectivity

personal computer. Reflection spectra were measured in N2 atmosphere by a miniature fiber optics spectrometer equipped with a reflection probe (Ocean Optics).

Ag Ca (1 nm)/Ag Ca (3 nm)/Ag Ca (30 nm)/Ag

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1.0

(b)

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Ag Mg (1 nm)/Ag Mg (3 nm)/Ag Mg (30 nm)/Ag

0.6 400

500

600

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Wavelength(nm) Fig. 3. Reflectivity spectra of different thicknesses of (a) Ca and (b) Mg on 150 nm Ag.

Harvested Photons/sec

a

VOC (V)

7x1019

6x1019

5x1019

DTDCTB:C cell with Bphen/Ca buffer 60

DTDCTB:C cell with Bphen/Mg buffer 60

P3HT:PCBM cell with Ca buffer

19

4x10

0

5

10

15

20

25

30

35

40

Ca,MgThickness(nm) Fig. 4. Simulated total harvested photons of DTDCTB:C60 (67 nm active layer) device with Bphen/0–40 nm Ca or Mg bilayer buffer and P3HT:PCBM (200 nm active layer) device with 0–40 nm Ca buffer layer.

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8

Reference Ca-1nm Ca-3nm Ca-30nm

0.8

Voc (V)

Jsc (mA/cm2)

12

4 0

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Intensity(sun)

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FF

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(d) 0.2

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Fig. 5. Light intensity dependence of cell performances: (a) Jsc (b) Voc (c) FF and (d) PCE.

(P3HT:PCBM) model polymer solar cells were also calculated for comparison. Interestingly, because of commonly used thicker active layer (200 nm) in typical P3HT:PCBM solar cells, there is no influence of different Ca thickness on harvesting efficiency. This explains the high efficiencies obtained from P3HT:PCBM cells utilizing popular 20– 40 nm Ca buffer layers [15–19]. Note that by using either ultrathin or thick low work function metal as electron extraction layer, FF can be improved remarkably. High FF can be attributed to low series resistance (Rs) or high shunt resistance (Rsh) of devices under illumination. Rs and Rsh are summarized in Table 1. Devices with 1–3 nm Ca show largely decrease of Rs from 10.9 to 7.9 X/cm2 compared to devices with only Bphen buffer layer. Devices using Mg or 30 nm Ca show slight decrease in Rs. The influence of shunt resistance is revealed to be negligible in all devices. Two mechanisms can be assigned to the reduction of Rs. First, the higher magnitude of builtin electric field induced by Ca may enhance both carrier extraction efficiency and field dependent exciton dissociation. A better carrier transportation property will cause the enhancement of both lower series resistance and higher Jsc simultaneously [13]. Second, it has been observed that a rough surface and void tend to increase series resistance of organic solar cells [20]. By introducing a Ca thin layer on Bphen, void may be reduced between Bphen and Ag. This modification of the morphology by incorporation of Ca or Mg between organic film and Ag cathode, is evident in the dark currents shown in Fig. 2(a), which may also contribute to the raise of FF. Generally, better performance was observed in Bphen/ultrathin Ca than in Bphen/ultrathin Mg. This could be due to the much lower work function of Ca (2.9 eV) compared to Mg (3.68 eV). Fig. 5 shows the illumination intensity dependence of cell performances. Devices with Bphen/Ca bilayer buffer show very stable FF values down to 0.1 sun while FFs of reference cell decrease with the reduction in illumination intensity. The results indicate that there is less geminate

recombination in cells with bilayer buffer [21]. This could be due to decrease of traps between organic layer and cathode, thus reducing trap assisted recombination by effective surface morphology modification and/or internal electrical field assisted carrier transportation. Since upon increasing of illumination intensity, the dominant recombination mechanism changes from geminate recombination to non-geminate recombination [21,22], the effects are more obvious at lower light intensity (e.g. FF of 52% (Bphen/ 1 nm Ca buffer) vs. 39% (Bphen buffer) at 0.1 sun) than at higher light intensity (e.g. FF of 54% (Bphen/1 nm Ca buffer) vs. 49% (Bphen buffer) at 1 sun). Note that, as shown in Fig. 5(d), unlike the reference cell efficiency which reduces with decrease of illumination intensities, the optimized cell (Bphen/1 nm Ca buffer layer) shows almost constant >5% efficiency under various illuminated intensities (from 0.02 sun to 1 sun), which is a desired characteristic for wholeday, all-weather, outdoor and indoor light harvesting. 4. Conclusions To summarize, a novel organic/ultrathin low work function metal bilayer buffer is proposed for small molecule OSCs. Compared to cells with a traditional single organic layer buffer, significant increases of Jsc and FF in cells utilizing bilayer buffers are demonstrated. By integrating Bphen/1 nm Ca buffer layer with DTDCTB:C60 PMHJ active layers, cells attained Jsc = 11.9 mA/cm2, FF = 54.6%, and PCE = 5.28%. This is >20% efficiency enhancement compared to a device with a traditional Bphen buffer layer (PCE = 4.33%). To our knowledge, the PCE value is among the highest reported for OSCs using C60 as the acceptor [4,11,23–27]. Devices utilizing bilayer buffer also exhibit an excellent constant efficiency under very dim to very bright illumination. The bilayer cathode buffer that strikes a balance between a large internal electric field (high FF) and a high cathode reflection (high Jsc) can be regarded as an advancement of the single organic cathode buffer

H.-W. Lin et al. / Organic Electronics 13 (2012) 1925–1929

layer which has been widely used in small molecule OSCs for the past decade. The buffer structure in this study provides a very simple but extremely effective way to further increase the efficiencies of the existing small molecule and even polymer OSCs with various kinds of thin (<100 nm) active layer configurations, e.g. bilayer, bulk heterojunction and PMHJ. Devices employing bilayer cathode buffer with different combinations of organic materials and low work function metals are currently under investigation. Acknowledgement We thank the National Science Council of Taiwan and the Low Carbon Energy Research Center, National TsingHua University, for financial support. References [1] M.Y. Chan, S.L. Lai, K.M. Lau, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 89 (2006) 163515. [2] H. Tsuji, K. Sato, Y. Sato, E. Nakamura, Chem. An Asian J. 5 (2010) 1294. [3] B.E. Lassiter, G.D. Wei, S.Y. Wang, J.D. Zimmerman, V.V. Diev, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 98 (2011) 243307. [4] M. Hirade, C. Adachi, Appl. Phys. Lett. 99 (2011) 153302. [5] C.C. Chang, C.F. Lin, J.M. Chiou, T.H. Ho, Y. Tai, J.H. Lee, Y.F. Chen, J.K. Wang, L.C. Chen, K.H. Chen, Appl. Phys. Lett. 96 (2010) 263506. [6] B.P. Rand, J. Li, J.G. Xue, R.J. Holmes, M.E. Thompson, S.R. Forrest, Adv. Mater. 17 (2005) 2714. [7] P. Peumans, V. Bulovic, S.R. Forrest, Appl. Phys. Lett. 76 (2000) 2650. [8] H. Jin, M. Tuomikoski, J. Hiltunen, P. Kopola, A. Maaninen, F. Pino, J. Phys. Chem. C 113 (2009) 16807.

1929

[9] Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang, Y. Chen, Adv. Energy Mater. 1 (2011) 771. [10] A.K. Pandey, P.E. Shaw, I.D.W. Samuel, J.-M. Nunzi, Appl. Phys. Lett. 94 (2009) 103303. [11] L.Y. Lin, Y.H. Chen, Z.Y. Huang, H.W. Lin, S.H. Chou, F. Lin, C.W. Chen, Y.H. Liu, K.T. Wong, J. Am. Chem. Soc. 133 (2011) 15822. [12] M.F. Lo, T.W. Ng, T.Z. Liu, V.A.L. Roy, S.L. Lai, M.K. Fung, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 96 (2010) 113303. [13] Z. He, C. Zhong, X. Huang, W.Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Adv. Mater. 23 (2011) 4636. [14] L.A.A. Pettersson, L.S. Roman, O. Inganas, J. Appl. Phys. 86 (1999) 487. [15] D. Gupta, M. Bag, K.S. Narayan, Appl. Phys. Lett. 92 (2008) 093301. [16] A. Kumar, G. Li, Z. Hong, Y. Yang, Nanotechnology 20 (2009) 165202. [17] G. Li, V. Shrotriya, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4 (2005) 864. [18] M.O. Reese, M.S. White, G. Rumbles, D.S. Ginley, S.E. Shaheen, Appl. Phys. Lett. 92 (2008) 053307. [19] V. Shrotriya, G. Li, Y. Yao, C.W. Chu, Y. Yang, Appl. Phys. Lett. 88 (2006) 073508. [20] D. Gupta, S. Mukhopadhyay, K.S. Narayan, Sol. Energy Mater. Sol. Cells 94 (2010) 1309. [21] W.L. Leong, S.R. Cowan, A.J. Heeger, Adv. Energy Mater. 1 (2011) 517. [22] S.R. Cowan, R.A. Street, S.N. Cho, A.J. Heeger, Phys. Rev. B 83 (2011) 035205. [23] J.G. Xue, B.P. Rand, S. Uchida, S.R. Forrest, J. Appl. Phys. 98 (2005) 124903. [24] R. Fitzner, E. Reinold, A. Mishra, E. Mena-Osteritz, H. Ziehlke, C. Korner, K. Leo, M. Riede, M. Weil, O. Tsaryova, A. Weiss, C. Uhrich, M. Pfeiffer, P. Bauerle, Adv. Funct. Mater. 21 (2011) 897. [25] N.M. Kronenberg, V. Steinmann, H. Burckstummer, J. Hwang, D. Hertel, F. Wurthner, K. Meerholz, Adv. Mater. 22 (2010) 4193. [26] J. Wagner, M. Gruber, A. Hinderhofer, A. Wilke, B. Broker, J. Frisch, P. Amsalem, A. Vollmer, A. Opitz, N. Koch, F. Schreiber, W. Brutting, Adv. Funct. Mater. 20 (2010) 4295. [27] D. Wynands, M. Levichkova, K. Leo, C. Uhrich, G. Schwartz, D. Hildebrandt, M. Pfeiffer, M. Riede, Appl. Phys. Lett. 97 (2010) 073503.