Electrical conduction and photovoltaic effects of TPA-derivative solar cells

Thin Solid Films 519 (2011) 5219–5222

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Electrical conduction and photovoltaic effects of TPA-derivative solar cells Khaulah Sulaiman ⁎, Muhamad Saipul Fakir Low Dimensional Material Research Center, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

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Available online 20 January 2011 Keywords: Organic semiconductors Organic solar cells TDCV-TPA NPD

a b s t r a c t We report the charge conduction behaviours and photovoltaic properties of two different diodes containing low-molecular-weight organic semiconductors: N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′diamine (NPD) and tris[4-(5-dicyanomethylidenemethyl-2-thienyl)phenyl]amine (TDCV-TPA), respectively. Both low-molecular-weight materials were triphenyl-amine TPA derivatives, and each thin film was sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes. In our work, the NPD and TDCVTPA thin films were prepared via a solution-processed spin-coating technique instead of the conventional method of thermal evaporation. Higher electrical conduction and photocurrents were produced by utilizing a TDCV-TPA layer in the device instead of an NPD film. We attribute such phenomena to the greater carrier mobility and to the increase in charge generation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Triphenylamine (TPA) derivatives are well known for their starshape dendrimer structure and can be viewed as 3D conjugated systems of organic semiconductors with hole-injecting/transporting behaviour [1,2]. In this study, we considered two TPA derivatives: N,N′-Di-[(1naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPD) and tris [4-(5-dicyanomethylidenemethyl-2-thienyl)phenyl]amine (TDCVTPA). The triphenylamine (TPA) moiety has a central amine nitrogen atom connected to three phenyl rings; in NPD and TDCV-TPA, these phenyl rings consist of two molecules and one molecule of TPA, respectively (see Fig. 1). Many research studies have been carried out on the electrical properties of NPD [3–7], thin films of which are typically utilized as holetransporting layers in organic light emitting diodes (OLEDs). On the other hand, there are a very limited number of reports about TDCV-TPA [8,9]. Recently, Roncali and co-workers reported that TDCV-TPA possesses an internal charge transfer system that is used in light emitting diodes and solar cells [9]. As understood from theoretical modelling of the behaviour of organic solar cells, the carrier transport towards the electrodes occurs via a hopping process. The mobility of charge carriers can be reduced by traps in the organic layer [10]. In general, the production of thin films based on low-molecularweight organic semiconductors including NPD [3–7] and TDCV-TPA [8,9] has been performed by thermal evaporation. However, we used a spin-coating technique to deposit thin films of NPD and TDCV-TPA. There are several advantages of using spin-coating over thermal evaporation, including the simplicity of spin-coating, higher cost-

⁎ Corresponding author. E-mail address: [email protected] (K. Sulaiman). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.163

effectiveness and lower material consumption. Additionally, we compared the effects of the material structures of NPD and TDCV-TPA on the absorption spectra, current–voltage characteristics and capacitance– frequency behaviours. 2. Experimental details The organic NPD and TDCV-TPA semiconductors were purchased from Sigma-Aldrich and were used without further purification. The chemical structures of NPD and TDCV-TPA are shown in Fig. 1 (on the left and in the middle, respectively). For solar cell fabrication, commercial indium tin oxide (ITO) glass substrates with a sheet resistance of 7 Ω/square were used. The ITO glass substrates were cleaned sequentially with a detergent solution, acetone, isopropyl alcohol and de-ionized water. After blow-drying the substrates with nitrogen gas, they were spin-coated with a water-based solution of poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). This spin-coated PEDOT:PSS layer was annealed at 110 °C for 10 min to remove any water content. The photoactive layer was then deposited onto the PEDOT:PSS film by spin-coating the organic semiconductor material solutions in chloroform, to a concentration of 25 mg/ml. A surface profiler meter was used to measure the thickness of the PEDOT: PSS layer and the organic films. The thickness of PEDOT:PSS layer was determined to be approximately 40 nm, whereas the NPD and TDCVTPA films were found to be approximately 70 and 80 nm, respectively. Finally, the top aluminium electrode was deposited onto the organic film through a shadow mask using a thermal evaporator under a low vacuum pressure of 10−5 mbar. The schematic diagram of the device construction of the ITO/PEDOT: PSS/organic layer/Al is illustrated in Fig. 1 (right). The absorption spectra in a range of 200 nm to 800 nm were measured at room temperature using a Jasco V-570 UV–Vis–NIR spectrophotometer. The current–

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Al Organic layer PEDOT:PSS ITO-glass substrate Fig. 1. Chemical structures of NPD (left) and TDCV-TPA (middle) and the schematic diagram of the organic solar cells (right).

voltage characteristics of the ITO/PEDOT:PSS/TDCV-TPA/Al and ITO/ PEDOT:PSS/NPD/Al devices were determined in air using a Keithley 2400 sourcemeter in the dark and under AM1.5G-filtered light at 100 mW/cm 2 from an Oriel solar simulator. In addition, the capacitance–frequency (C–F) characteristics were measured using an LCR meter. 3. Results and discussion Fig. 2(a) shows the absorption spectra of NPD and TDCV-TPA thin films. The absorption spectrum of the NPD film exhibited only two small absorption bands in the UV region, whereas the spectrum of the TDCVTPA film extended into part of the UV region (200 to 400 nm) and covered the whole range of the visible region (400 to 700 nm). The peaks of the spin-coated NPD film occurred at 290 nm and 352 nm due to the triphenylamine monomer and the triphenyl dimer, respectively; the positions of these peaks are in agreement with those of the thermally evaporated NPD film [11]. The redshift of the spectrum of the TDCV-TPA film compared to that of the NPD film could be attributed to the extended conjugation length of the dicyanovinylene in the TDCVTPA photoactive layer. Furthermore, there was a significant increase in the absorption coefficient in the TDCV-TPA film, and it was thought that such a characteristic corresponds to the existence of the three thienylene rings attached to the TPA molecule. Furthermore, there was a distinct peak between 400 nm and 650 nm in the visible region for the TDCV-TPA film, which can be assigned to the intramolecular charge transfer transition between the triphenylamine-thienylenevinylene electron-donor unit and the dicyanovinylene electron-acceptor unit [9]. The observed absorption spectrum of the spin-coated TDCV-TPA film was comparable to that reported earlier for a thermally deposited film [8]. To estimate the optical energy gap (from the onset of the photon energy at the absorption peak) of both TPA derivatives, the absorption

(a)

spectra are plotted against photon energy in Fig. 2(b). It can be clearly seen from this figure that the optical energy gaps, the estimated Eg values, were 1.9 eV and 2.9 eV for TDCV-TPA and NPD, respectively, and these values were in agreement with the values reported in the literature [8,12]. It should be also noted that, based on this result, the energy gap can be reduced by introducing dicyanovinylene and thienylenevinylene moieties into the TPA molecules. Fig. 3(a) shows the linear plot of the current density–voltage (J–V) characteristic of the ITO/PEDOT:PSS/TDCV-TPA/Al and ITO/PEDOT: PSS/NPD/Al devices, which was measured in the dark. The device containing the TDCV-TPA active layer exhibited a rectification ratio (RR) of 104 at ±3 Volts, and this RR value was two orders of magnitude lower for the device containing NPD (the negative applied voltages are not shown in Fig. 3(a)). One obvious J–V characteristic that can be observed in this linear plot is that at the same applied voltage (for example at 4 V), the electrical current is four times higher for the TDCV-TPA-based device than for the NPD-based device. This electrical conduction behaviour is known to be correlated with the mobility of charge carriers. We believe that the superior electrical conduction of the ITO/PEDOT:PSS/TDCV-TPA/Al device is due to the higher charge carrier mobility of TDCV-TPA than NPD, even though we cannot deduce the mobility value from this result. Not only was the current higher, but the turn-on voltage of the TDCV-TPA device was also reduced (see Fig. 3(a)), indicating that the barrier height at the aluminium/TDCV-TPA interface is decreased by inserting dicyanovinylene and thienylenevinylene moieties into the TPA molecules. The lower value of the turn-on voltage could originate from the superior charge injection through the metal contact, generating a better electrical conduction in the ITO/PEDOT:PSS/TDCV-TPA/Al device compared to that of ITO/PEDOT:PSS/NPD/Al devices. To explore the electrical conduction mechanisms of the devices, the current is shown as a function of the applied bias on a double logarithmic

(b) 3.0

Absorption Coefficient ( x 105) (cm-1)

Absorption Coefficient ( x 105) (cm-1)

3.0

TDCV-TPA

2.5 2.0 1.5 1.0 0.5 0.0 200

NPD

Eg (optical) = 1.9 eV for TDCV-TPA

2.5 Eg (optical) = 2.9 eV for NPD

2.0 1.5 1.0 0.5 0.0

400

600

Wavelength (nm)

800

1

2

3

4

5

Energy (eV)

Fig. 2. The absorption spectra of NPD (open circles) and TDCV-TPA (closed circles): (a) absorption-wavelength and (b) absorption-energy.

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(a)

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(b) 0.30

1.E-05

0.25

1.E-06

0.20

1.E-07

Current (A)

Current density (mA/cm2)

I ~ V7.3

0.15

I ~ V2.4 1.E-08

I ~ V2.5

0.10

1.E-09

0.05

1.E-10

0.00 0

1

2

3

4

I ~ V6.8

I ~ V1.6

I ~ V1.8

1.E-11 0.01

0.1

Voltage (V)

1

10

Voltage (V)

Fig. 3. The current density–voltage (J–V) characteristics in the dark of ITO/PEDOT:PSS/TDCV-TPA/Al (open squares) under ambient conditions in forward bias and of ITO/PEDOT:PSS/ NPD/Al (open circles): (a) the linear plots and (b) log–log plots.

plot. In Fig. 3(b), the log–log plot reveals that the current follows a power law in the form of I ~ Vm, and three different regimes can be identified. In the first lower voltage regime (V b 0.4 V), the currents show an ohmic behaviour with I ∝Vmwhere the values of m are 1.6 and 1.8 for TDCV-TPA and NPD devices, respectively. The values of m in both cases were greater than unity, which can be attributed to the formation of an aluminium oxide layer between the Al/TDCV-TPA and Al/NPD interfaces. When the applied voltage was increased (0.5 Vb V b 1.0 V), the I–V behaviour became bulk controlled, which can be treated as a space-charge-limited current (SCLC) with a single discrete shallow trap of I ∝V2.4 for the TDCVTPA device and I ∝V2.5 for the NPD device. Further increasing the applied voltage (V N 2 V) can be explained by the trap-filling limited current with m= 7.3 and 6.8 for the TDCV-TPA and NPD devices, respectively. Unfortunately, we were not able to measure the current at higher applied voltages because device breakdown occurred at approximately 5 to 6 V due to the thin layers of the organic films (approximately 70 to 80 nm). A power-law dependence of the current on the applied voltage has also been observed for other low-molecular-weight organic materials [13,14]. Fig. 4 shows the photovoltaic effect characteristics of the ITO/PEDOT: PSS/TDCV-TPA/Al and ITO/PEDOT:PSS/NPD/Al devices under white light

1

illumination through ITO contact under ambient conditions. The performance of both solar cells revealed that the open-circuit currentdensity (Jsc) of the TDCV-TPA device was three times higher than that of the NPD-based device. This behaviour can be explained by the superior electrical conductivity of the TDCV-TPA device, as previously discussed. Additionally, the generation of charge carriers in TDCV-TPA was greater than in NPD due to the larger portion of light that can be absorbed by the TDCV-TPA photoactive layer, as shown in the absorption spectrum. In addition, this TDCV-TPA single-layer organic solar cell device had a high open circuit voltage, Voc, of about 0.8 V. Nevertheless, overall performance of the investigated solar cells requires further improvement (very low efficiency approximately 3 × 10−3) in future work, such as through the introduction of an electron acceptor material to form a bulk heterojunction device. This bulk heterojunction layer can also be fabricated via a solution-processed spin-coating method such as the kind used in this study. The heterojunction structure can reinforce the dissociation of the exciton, thus generating more efficient charge transfer [10]. Fig. 5 shows the results of the capacitance–frequency (C–F) measurements of the ITO/PEDOT:PSS/TDCV-TPA/Al and ITO/PEDOT: PSS/NPD/Al devices under short-circuit conditions at room temperature for a frequency range of 100 Hz to 6 MHz. The C–F relationships for both TPA derivative-based devices were in agreement with the

NPD 20

0 0.0

0.2

0.4

0.6

0.8

Voltage (V)

-1

-2

-3

NPD

1.0

TDCV-TPA

TDCV-TPA

15

Capacitance (nF)

Current density (µA/cm2)

-0.2

10

5

-4 0 1.E+02 -5 Fig. 4. The current density–voltage (J–V) characteristics under light irradiation of ITO/ PEDOT:PSS/TDCV-TPA/Al (closed circles) and ITO/PEDOT:PSS/NPD/Al (open circles).

1.E+04

1.E+06

1.E+08

Frequency (Hz) Fig. 5. The variation in the C–F characteristics of ITO/PEDOT:PSS/TDCV-TPA/Al (closed circles) and ITO/PEDOT:PSS/NPD/Al (open circles).

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results presented in the literature for the organic materials [15,16]. The capacitance gradually decreased at lower frequencies for both the NPD and TDCV-TPA devices. However, at a certain critical frequency of about 1 × 105 Hz, an instantaneous drop in the capacitance was observed. Such a frequency dispersion of capacitance suggests the presence of a distribution of localized states in the band gap of these organic semiconductors [17]. Nevertheless, we did not see any significant effect of the different types of TPA derivatives on the C–F characteristics within this high frequency range; to test for this, measurements at lower frequencies (lower than 100 Hz) will be performed in our future studies. 4. Conclusion We investigated the electrical conduction of two diodes containing low-molecular-weight materials: NPD and TDCV-TPA, respectively. The absorption spectrum of the TDCV-TPA film covers a part of the UV region as well as shows a distinct peak between 400 nm and 650 nm in the visible range, whereas the NPD film covers only a small portion of the UV range. We observed an enhancement in electrical conductivity when utilizing TDCV-TPA instead of NPD, which could be attributed to higher carrier mobility and lower barrier height at the Al/TDCV-TPA interface (yet to be confirmed in future works). The efficiency of the fabricated photovoltaic devices is still low. Further improvement in the photocurrent is essential for enhancing the efficiency of these devices.

Acknowledgement The authors acknowledge the financial support of the UMRG provided by the University of Malaya under project number RG053/ 09AFR. References [1] M.M. Stylianakis, J.A. Mikroyannidis, Q.F. Dong, J.N. Pei, Z.Y. Liu, W.J. Tian, Sol. Energy Mater. Sol. Cells 93 (2009) 1952. [2] Y. Shirota, J. Mater. Chem. 15 (2005) 75. [3] T. Sugiyama, Y. Furukawa, H. Fujimura, Chem. Phys. Lett. 405 (2005) 330. [4] Y. Shao, Y. Yang, Appl. Phys. Lett. 86 (2005) 073510. [5] A. Heppa, G. Ulrichb, R. Schmechela, H.V. Seggerna, R. Ziessel, Synth. Met. 146 (2004) 11. [6] A. Wan, J. Hwang, F. Amy, A. Kahn, Org. Electron. 6 (2005) 47. [7] V. Bulovic´, V.B. Khalfin, G. Gu, P.E. Burrows, D.Z. Garbuzov, S.R. Forrest, Phys. Rev. B 58 (1998) 3730. [8] S. Roquet, A. Cravino, P. Leriche, O. Aleveque, P. Frere, J. Roncali, J. Am. Chem. Soc. 128 (2006) 3459. [9] A. Cravino, P. Leriche, O. Aleveque, S. Roquet, J. Roncali, Adv. Mater. 18 (2006) 3033. [10] A. Militon, J.-M. Nunzi, Polym. Int. 55 (2006) 583. [11] T. Kato, T. Mori, T. Mizutani, Thin Solid Films 393 (2001) 109. [12] A. Kahn, N. Koch, W. Gao, J. Polym. Sci., B: Polym. Phys. 41 (2003) 2529. [13] W. Brütting, S. Berleb, A.G. Mückl, Synth. Met. 122 (2001) 99. [14] M. Shah, M.H. Sayyad, Kh.S. Karimov, M. Maroof-Tahir, Phys. B 405 (2010) 1188. [15] G.D. Sharma, V.S. Choudury, Y. Janu, M.S. Roy, Mater. Sci. Pol. 25 (2007) 1173. [16] U. Manna, H.M. Kim, M. Gowtham, J. Yi, S. Sohn, D. Jung, Thin Solids Films 495 (2006) 380. [17] P. Viktorovitch, J. Appl. Phys. 51 (1980) 4847.