Chapter 41 Temperature Dependence of Carrier Mobility in SubPc and C 60 Organic Semiconductors for Photovoltaic Applications Nesrine Mendil, Mebarka Daoudi, Zakarya Berkai and Abderrahmane Belghachi Abstract Carrier mobility is an important parameter in determining device performance in optoelectronics. Here, we study the charge carrier mobility of C 60 and SubPc neat layers for different temperatures calculated in darkness. The results show that temperature affects the disorder energy ( σ) of organic semiconductors; when the temperature is < 289 K, this parameter is around 0.03 ev and it is double when the temperature is > 289 K. In these conditions, the mobility of electrons and holes are around 0.14 and 10 7 Cm 2 /Vs, respectively. Our results are in good agreement with experimental data reported by Pandey et al., Adv Funct Mater 22:617 624. 2012) Keywords Organic semi conductors (C 60, SubPc) Effect of temperature Charge carrier mobility Characteristic current density voltage 41.1 Introduction One new type of photovoltaic (PV) technology, organic solar cell technology, is based on fullerene (C 60 ), boron subphthalocyanine chloride (SubPc) and molecules. In organic solar cells, it is essential to deduce and understand the laws that determine the charge transport and the parameters which affect the charge transport as carrier mobility, charge distribution and temperature. These parameters play a critical role in determining the fluency and efficiency of the organic solar cells. In order N. Mendil ( ) M. Daoudi Z. Berkai A. Belghachi Laboratory of semiconductor devices physics, (LPDS) University of Bechar, PB n 417, 08000 Bechar, Algeria e-mail: mendil_nesrine@yahoo.com M. Daoudi e-mail: Mebarkadaoudi@yahoo.fr Z. Berkai e-mail: Berakai_zakarya@yahoo.com A. Belghachi e-mail: abelghachi@yahoo.fr Springer International Publishing Switzerland 2016 A. Sayigh (ed.), Renewable Energy in the Service of Mankind Vol II, DOI 10.1007/978-3-319-18215-5_41 469
470 N. Mendil et al. Fig. 41.1 Organic (C 60 or SubPc) Schottky diode structure Aluminium (Al) Ac ve layer (C60 or SubPc) Indium n oxide (ITO) Glass to do this, the use of sophisticated modeling to simulate the charge transport is necessary. In contrast to the situation with crystalline and amorphous organic materials, there has been little research on charge mobility in liquid crystals. Here, we initiated a study on the mobility and current density of charge carrier results obtained for different temperatures of C 60 or SubPc layers calculated without illumination. The results present nonlinear behavior. The structure of a solar cell is shown in Fig. 41.1. Starting from the bottom, there is a glass substrate (the indium tin oxide [ITO] layer) which is a transparent layer which serves as the hole contact. In the middle is the active layer which is based on C 60 or SubPc.C 60 organic semiconductors, and is used as the organic electronacceptor due to its high stability and high carrier mobility [2]. SubPc organic semiconductors are used as organic hole-donor semiconductors. Lastly, the aluminum layer is the electron contact. The aim of our work is to investigate the effect of temperature on mobility and current density voltage characteristics for both C 60 and SubPc Schottky solar structures. 41.2 Theoretical Model The key factor is to decide the efficiency of the carrier transport and collection process by the electrodes. Therefore, carrier mobility is a critical factor in determining the efficiency. Some theoretical models have been established for organic solar cells. In the absence of traps, the space charge limited current density (J) can be written as a function of the applied voltage (V) as [3]: V J = 9 εµ 8 d 2 3 (41.1) where ε is the relative dielectric constant of the organic thin film and is approximated as ε = 4 [1], V is the applied voltage, μ is the charge carrier mobility of material, and d is the organic film thickness. The mobility dependence of temperature as determined from the various electron-only or hole-only devices is given by the Gaussian disorder model. It is governed by the width of a Gaussian density-of-states as follows [4]:
41 Temperature Dependence of Carrier Mobility in SubPc 471. δ δ µ ( E) = µ exp 2 1 5 3. ( ) KBT + 078 2 5 KBT (41.2) where μ is the mobility as the temperature is going to infinity, δ is the Gaussian disorder, a is the inter-site spacing, E is the electric field, and K B is the Boltzmann constant. eae δ 41.3 Results and Discussion In this work we investigated carrier mobility and the effect of applied voltage on the current densities of the neat electron layer (C 60 ) and neat hole layer (SubPc), each one sandwiched between a cathode and an anode as illustrated in Fig. 41.1. Figure 41.2 shows the electron-only (a) and hole-only (b) current density voltage characteristics using Eq. 41.2, with the corresponding parameters for C 60 and SubPc organic semiconductors in darkness shown in Table 41.1. It is clear that the J V characteristics exhibit two regions. When the applied bias is < 0.2 V, the electron and hole current densities are almost zero. Above this region, the current densities continue to increase as the voltage increases. The overall behavior of these characteristics is nonlinear. Our results are in a good agreement with the experiment data. We can see that our results in Fig. 41.3a exhibit two regions for range of voltage (0.3, 0.4, 0.6, 0.8 and 1.2 V) with various temperature values. The first one is from 120 K to T c = 185 K (the critical temperature); the electron mobility increases with increasing temperature and voltage. In contrast, when the temperature is above T c (the second region) we observe that the electron mobility increases with increasing temperature and decreasing voltage. In Fig. 41.3b the electron mobility increases with increasing temperature from 300 K to T c = 371.98 K and decreasing voltage; however, after T c, the electron mobility increases with increasing temperature and voltage. Figure 41.4a shows two regions for range of voltage (0.3, 0.4, 0.6, 0.8 and 1.2 V) with various values of temperature. In Fig. 41.4a, the first region is from 120 K to T c = 187 K (the critical temperature). We can see when the bias is between 0.3 and 0.4 V the hole mobility increases with increasing of temperature. In contrast, when the bias is > 0.4 V mobility decreases. In the second region we observe that the hole mobility decreases with increasing temperature and voltage. As a result, in graph (b) when voltage is > 0.08 V, the hole mobility increases with increasing temperature except for 1.2 V. However, in the second region, after T c the hole mobility decreases with increasing temperature and decreasing voltage. The effect of voltage has a strong influence on the carrier mobility of both SubPc and C 60.
472 N. Mendil et al. Fig. 41.2 Dependence of the current density voltage characteristic versus bias voltage; for electron transport in neat C 60 (a) and hole transport in neat SubPc (b) at room temperature 298 K. The solid lines represent our results and black squares represent experimental data taken from ref [1]
41 Temperature Dependence of Carrier Mobility in SubPc 473 Table 41.1 Material properties of C 60 and SubPc a (10 8 cm) μ (Cm 2 /Vs) Δ (ev) T 289 K T >300 K C 60 14.17 [5] 0.3 0.03 [6] 0.06 [7] SubPc 12 [8] 1.5 10 7 0.03 0.06 Fig. 41.3 Electron mobility dependence of temperature in neat C 60 at various voltages. Two cases of Gaussian disorder a 0.03 ev and b 0.06 ev
474 N. Mendil et al. Fig. 41.4 Hole mobility as a function of temperature in neat SubPc for various voltage values. Two cases of Gaussian disorder a 0.03 ev and b 0.06 ev
41 Temperature Dependence of Carrier Mobility in SubPc 475 41.4 Conclusion In this study, we investigated electron mobility in neat C 60 n-type Schottky diodes and neat hole mobility in SubPc p-type Schottky diodes using a recent analytical model assuming a Gaussian density-of-states distribution. Our results show good agreement with experimental results. Furthermore, the temperature has a dramatic influence on the ability of carrier transportation on device performance. For each characteristic there are two distinct critical temperature regions. References 1. Pandey R, Gunawan AA, Mkhoyan KA, Holmes RJ (2012) Efficient organic photovoltaic cells based on nanocrystalline mixtures of Boron subphthalocyanine chloride and C 60. Adv Funct Mater 22:617 624. (WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 2. Wallace Jason U (2009) Thesis PHD carrier mobility in organic charge transport materials: methods of measurement, analysis and modulation. Rochester, New York 3. Martinelli N (2011) Charge transport in organic conjugated materials: impact of lattice dynamics and Coulombic interaction effects. Dissertation, 26 Sept 2011 4. Lenes M, Shelton SW, Sieval AB, Kronholm DF, (Kees) Hummelen JC, Blom PWM (2009) Electron trapping in higher adduct fullerene-based solar cells. Adv Funct Mater 19:3002 3007. (WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 5. Hu JY, Niu NN, Piao GZ, Yang Y, Zhao Q, Yao Y, Gu CZ, Jin CQ, Yu RC (2012) Carbon 50:5458 5462. (Elsevier Ltd. All rights reserved) 6. Copyright 2008 by Yang, Fan Fan Yang Santa Clara, CA May, 2008 7. Deibel C, Strobel T arxiv:0906.2486v2 [cond-mat.mtrl-sci] 16 Jul 2009 8. Trelka M, Medina A, Ecija D, Urban C, Groning O, Fasel R, Gallego JM, Claessens CG, Otero R, Torres T, Miranda R (2011) Subphthalocyanine-based nanocrystals. Chem Commun 47:9986 9988. (This journal is The Royal Society of Chemistry 2011)