Improvement of Open-Circuit Voltage in Organic Photovoltaic Cells with Chemically Modified Indium-Tin Oxide

Khayankhyarvaa Sarangerel, Byambasuren Delgertsetseg, Namsrai Javkhlantugs, Masaru Sakomura, Chimed Ganzorig

Center for Nanoscience and Nanotechnology, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, Ulaanbaatar, Mongolia.
Center for Nanoscience and Nanotechnology, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, Ulaanbaatar, Mongolia;Faculty of Engineering, New Mongol Institute of Technology, Ulaanbaatar, Mongolia.
Department of Chemistry, Chemical Engineering and Life Science, Yokohama National University, Yokohama, Japan.
Department of Electronics and Computer, School of Power Engineering, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia.

DOI: 10.4236/wjnse.2013.34016                

Abstract

The possibility of the increase in open-circuit voltage of organic photovoltaic cells based primarily indium-tin oxide (ITO)/rubrene/fullerene/Al structure by changing the work function of ITO anodes and Al cathodes was described in this work. To change built-in potential preferably in order to increase the open-circuit voltage, the work function of ITO should be increased and work function of Al should be decreased. The correlation between the change in work functions of electrodes and performance of the organic photovoltaic cells before and after surface modifications was examined in detail. The enhancement of open-circuit voltage depends on a function of work function change of both ITO and Al electrode. We could show that the built-in potential in the cells played an important role in open-circuit voltage.

Keywords

Open-Circuit Voltage; Chemical Modification; Indium-Tin Oxide

1. Introduction

Organic photovoltaic (PV) cells have been attracted much attention in recent decades due to their potentials as fabrication, low-cost production, and technological advantages of semiconductor materials [1-5]. Since the first report of donor-acceptor heterojunction with a power conversion efficiency (h_p) of about 1% by Tang [6], new materials and device structures have been developed in PV cells [7-15]. After the first report of organic PV cells, the performances of this type of cells have been significantly improved to reach h_p in a range of 3% - 8% [8,9,16,17]. However, such efficiency is not sufficient for practical use, and further improvement is required.

To obtain large open-circuit voltage (V_oc), Taima et al. introduced a p-type semiconductor 5, 6, 11, 12-tetraphenylnaphthacene (rubrene), which has the HOMO level of 5.4 eV. They obtained the V_oc of 0.91 V [18]. Forrest et al. introduced an excellent p-type semiconductor boron subphthalocyanine chloride (SubPc) with a low HOMO level of 5.6 eV [19].

Indium-tin-oxide (ITO) is the most widely used as a transparent anode in organic PV cells due to its high conductivity, work function, and transparency in the visible spectral range [6]. Thus, various surface treatments of ITO have been attempted to change the work function of ITO in order to improve the properties of ITO substrates and control the charge injection barrier height reviewed in previous reports [20,21]. Although a number of groups have shown that chemical modification of ITO can be used to optimize the performance of organic lightemitting diodes (OLEDs) [20,21], there have been limited attempts to use chemical modification or chemically selfassembled monolayers (SAMs) in organic PV cells [22,23].

To investigate the possibility of increase in V_oc by controlling the work functions of the electrodes, we report here the use of chemically modified ITO with different terminal groups (Hand Cl-) of p-benzenesulfonyl chlorides and p-chlorophenyldichlorophospate (-P) forming effective monolayers. We examine the correlation between the change in the work function of ITO and the performance of the PV cells by the chemical modification and find that the large increase in V_oc. In this work, we selected tris(8-hydroxyquinoline)aluminum (Alq₃) as an electron transport layer (ETL) to substitute for bathocuproine (BCP) in cells based on rubrene (Rub)/buckminsterfullerene (C₆₀) heterojunction. Moreover, to examine the further improvement of V_oc, we used a lithium carboxylate (C₆H₅COOLi) [24] as a cathode interface material with low-work function which was inserted between ETL and Al.

2. Experimental

ITO coated glass substrates with a sheet resistance of ca. 15 W/square (Sanyo Vacuum Industries) were cleaned by sonication successively in two detergents (Extran MA 03, pH 6.8, MERCK and Kontaminon O, pH 10, WAKO), rinsed with deionized water, and stored in isopropanol until being required. After cleaning with acetone and isopropanol (this cleaned ITO will be called hereafter “as-cleaned ITO” with notation of “ac”) the ITO substrates were immersed for 5 min in dichloromethane solutions containing 1 mM of (Hand Cl-) of p-benzenesulfonyl chlorides (Tokyo Chemical Industry) and p-chlorophenyldichlorophospate (Tokyo chemical industry). The modified ITO anodes were rinsed in pure dichloromethane and then vacuum dried for ~1 h.

C₆₀ (purity > 99%) (Tokyo Chemical Industry), the sublimed grade rubrene (Aldrich Co.) and Alq₃ (Dojindo Labs), the reagent grade BCP (Kanto Chemical), and lithium benzoate (purity~99%) (Aldrich Co.) were used without further purification. All the materials were deposited using vacuum evaporation under a pressure of 5 - 7 ´ 10⁻⁶ Torr at deposition rates of 1 - 1.5 Ǻ/s for organic layers and 3 - 4 Ǻ/s for Al cathode. The active area for all the cells was defined to be 5 ´ 5 mm² by using a shadow mask. The current density-voltage (J-V) curves were measured under illumination of a simulated solar light with 100 mW ´ cm⁻² (AM1.5G) by a solar simulator (Yamashita Denso, YSS-50). Electric data were taken using an Advantest R6145 DC voltage current source unit at room temperature in ambient atmosphere.

The absorption spectral data for all the thin film were taken using an UV-visible spectrophotometer (UV-265 FW, Shimadzu) at room temperature in ambient atmosphere.

3. Results and Discussions

3.1. Expected Energy Diagrams of PV Cells

For a cell based on exciton dissociation by charge transfer at a donor-acceptor (D/A) interface, h_p is the product of the efficiencies [1] of four sequential steps 1) photon absorption leading to the generation of an exciton, 2) diffusion of the exciton to the D/A interface, 3) exciton dissociation (or charge separation) by charge-transfer (CT) at the D/A interface, and 4) collection of the free charge carriers at electrodes, i.e., charge transport to the anode (holes) and cathode (electrons), to supply a direct current.

Figure 1 shows the interfacial energy diagrams with shifts of vacuum level (D) at the interfaces due to dipole layer formation in four types of cells studied in the present work. In general, the work function of metal is changed by covering the metal surface with different materials [25]. First, we will discuss the shift at a C₆₀/Al cathode interface. The photoemission study of the C₆₀/Al interface revealed an abrupt vacuum-level shift of D = ~ +0.9 eV [26]. Namely, the work function of the Al electrode (4.2 eV) was increased to 5.1 eV by depositing a C₆₀ film on an Al surface. This shift is schematically illustrated in Figures 1(a) and (b). The same energy level shift at the C₆₀/Al interface was also reported previously [27]. Another group reported the shift of +0.7 eV for the C₆₀/Al interface and that of +0.9 eV for the C₆₀/LiF(0.5 nm)/Al interface [28]. In the latter case, the work function was increased from 3.6 eV (LiF/Al) to 4.5 eV (C₆₀/LiF/Al). The increase in the work function for all cases described above is possibly interpreted by partial electron transfer from Al to C₆₀ [26-28]. The HOMO and LUMO levels of C₆₀ are reported to be 6.2 eV and 3.7 eV, respectively [10]. The increase in the work function of the Al electrode, however, is not preferable to create the built-in potential (V_bi) to separate the charge effectively in the PV cells.

In order to decrease the work function of the Al electrode, we have to put another layer of less electron affinity than C₆₀. As such materials, we examined Alq₃ and BCP [10,16,20,29-31] LUMO levels of which are higher (i.e., less electron affinity) than that of C₆₀. In fact, the organic side for these interfaces is charged positively, making this side more comfortable (low energy) for an electron, and making the sign of D negative. Taking into account the D at Alq₃/Al interface of ~−1.0 eV [25], the resulting work function of Alq₃/Al is decreased from the value of metallic Al (4.2 eV) [16] down to 3.2 eV as shown in Figures 1(c)-(d). The work function of the LiF/Al substrate was also gradually decreased upon Alq₃ deposition, from 3.6 eV to 3.1 eV for Alq₃ film deposition [28,32]. Toyoshima et al. reported the electronic structure at the interface between BCP and Al by UV photoemission spectroscopy [33]. Their results for BCP /Al interface were similar to the shift in the work function as observed at Alq₃/Al interface [25,32]. In this way, we constructed the energy diagrams of the Al cathode side as shown in Figure 1.

Figure 1. Interfacial energy diagrams with the shifts of vacuum level (D) at the interfaces due to dipole layer formation in the PV cells. These lead to buildup of built-in potential (V_bi) as shown in (d).

Next, we discuss the work function control of the anode side. The molecular approach allows for fine-tuning the work function using organic molecules on ITO depending upon magnitude and direction of the dipole moment [34]. The effective work functions formed by chemical modification of ITO shown in Figure 1 were estimated from the contact potential difference (CPD) values [34,35].

An interface dipole with its negative end pointing toward the organic layer and its positive end toward the electrode surface increases the ITO work function (i.e., the Fermi energy is down) and HOMO energy level in the organic layer is relatively up by adding an electrostatic energy [8] as shown in Figure 1. When the cells studied have the same cathode material, the changes in V_bi obtained for cells with variously modified ITO electrodes are equal to the changes in the ITO work function. This is illustrated on the left side of Figure 1, where we consider that the ITO work function is in the range 4.5 - 5.0 eV. The HOMO and LUMO values for rubrene are reported to be 5.4 eV and 3.2 eV, respectively [18]. The work function control at the anode as well the cathode leads to buildup of a large V_bi as shown in Figure 1(d). The dipole layers at interfaces may have a deep impact on the V_bi and consequently on the V_oc of organic PV cells.

3.2. Characteristics of PV Cells

Figure 2 shows the effect of ITO work function on the current density-voltage (J-V_bias) characteristics under 100 mW ´ cm⁻² illumination and in dark of four kinds of the PV cells with various surface treatments of ITO. Figure 2(a) shows the room temperature J-V_bias characteristics of ITO(variously treated)/C₆₀(60 nm)/Al single-layer cells with a focus on the dark conduction properties. A linear fitting of the log-log plot (not shown) for these cells shows that the current for forward bias (electrons injection from the top contact) increases much slower (a slope is ~1) than the space-charge limited conduction (SCLC) [36]. Conducting charge transfer complex formed on C₆₀/metal interface was studied in previous report [37]. The gap state, pinning the Fermi level close to the LUMO of C₆₀ molecules, is originating from the C₆₀-metal complex formation at the interface [37]. The unoccupied

Conflicts of Interest

The authors declare no conflicts of interest.

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