TiO2 nanoparticles (NPs) with a size of 240 nm (T240), used as a light-scattering layer, were applied on 25-nm-sized TiO2 NPs (T25) that were used as a dye-absorbing layer in the photoelectrodes of dye-sensitized solar cells (DSSCs). In addition, the incident light was concentrated via a condenser lens, and the effect of light concentration on the capacity of the light-scattering layer was systematically investigated. At the optimized focal length of the condenser lens, T25/T240 double layer (DL)-based DSSCs with the photoactive area of 0.36 cm2 were found to have the short circuit current (Isc) of 11.92 mA, the open circuit voltage (Voc) of 0.74 V, and power conversion efficiency (PCE) of approximately 4.11%, which is significantly improved when they were compared to the T25 single layer (SL)-based DSSCs without using a solar concentrator (the corresponding values were the Isc of 2.53 mA, the Voc of 0.69, and the PCE of 3.57%). Thus, the use of the optimized light harvesting structure in the photoelectrodes of DSSCs in conjunction with light concentration was found to significantly enhance the power output of DSSCs.
Dye-sensitized solar cellSolar concentratorPlano-convex lensFocal lengthLight-scattering layer
Dye-sensitized solar cells (DSSCs) have been developed extensively because of the relatively low cost involved in their manufacturing processes . Numerous research groups have reported the enhancement of the light-to-electricity power output of DSSCs by employing newly developed materials and modifying the intrinsic solar cell structures [2–10]. An alternative approach for enhancing the light-to-electricity power output of DSSCs is to use a solar concentrator, which generally employs optical lenses or mirrors [11, 12]. The optical lens is incorporated to improve the power output of photovoltaic cells (PVs) by concentrating a large amount of sunlight onto a small area of photoactive layers in various PVs. In general, the power output of DSSCs decreases with an increase in the cell area of the photoactive layer. However, this problem can be solved by employing a solar concentrator that provides the advantages of increased power output. The application of an optical lens-based solar concentrator system mounted on top of DSSCs still poses several challenges in terms of efficiency, cost-effectiveness of optical design, and the provision of uniform and concentrated illumination on a DSSC regardless of surrounding environmental changes [13–15]. Furthermore, various complex phenomena, including light scattering, recombination of electron-hole pairs, and dye degradation, in the photoactive layers of DSSCs can occur when the intensity of incident light is changed by varying the beam focus of solar concentrator . The question arises as to how we can optimize the effects of the intrinsic cell structure and solar concentrator when concentrated light is incident on the photoactive layer structures in DSSCs.
In this work, we systematically investigated the effects of using a light-scattering layer in the photoelectrodes of DSSCs along with studying the effects of using a condenser lens-based solar concentrator on the photovoltaic performance of DSSCs. Briefly, three different photoelectrode structures fabricated with a T25/T25-accumulated double layer (T25/T25 DL), a T25/T240-accumulated double layer (T25/T240 DL), and a T240/T240-accumulated double layer (T240/T240 DL) were examined for verifying the effects of using a light-scattering layer under intensified light irradiation conditions tuned by a condenser lens-based solar concentrator. Here, T25 and T240 indicate commercialized TiO2 nanoparticles (NPs) with an average diameter of approximately 25 and 240 nm, respectively. With the optimized design of the condenser lens-based solar concentrator developed in this approach, we report a novel T25/T240 DL-based DSSC system with condenser lens-based solar concentrator that exhibits a photocurrent output of approximately 11.92 mA, an open circuit voltage of 0.74 V, and power conversion efficiency (PCE) of approximately 4.11%, which exhibits a much better photovoltaic performance compared to T25/T25 DL- and T240/T240 DL-based DSSCs with condenser lens-based solar concentrator.
Commercially available TiO2 NPs (T25, Degussa; T240, Sigma Aldrich, St. Louis, MO, USA) were used without further treatment. In order to prepare TiO2 NP paste for the screen-printing process, 6 g of TiO2 NPs, 15 g of ethanol, 1 mL of acetic acid (CH3COOH), and 20 g of terpineol were mixed in a vial and sonicated for 1 h. A solution of 3 g of ethylcellulose dissolved in 27 g of ethanol was separately prepared and subsequently added to the TiO2 NP-dispersed solution, which was then sonicated for 30 min [5, 17]. As a photoelectrode layer, TiO2 NP-accumulated thin layer was applied via a screen-printing process on a fluorine-doped tin oxide (FTO) glass (SnO2:F, 7 Ω/sq, Pilkington, Boston, USA) with a photoactive area of 0.6 × 0.6 cm2, as shown in Figure 1. The T25 single layer (T25 SL), T25/T25 DL, T25/T240 DL, and T240/T240 DL were separately prepared for comparison purposes. The resulting TiO2 NP-accumulated layer formed on the FTO glass via the screen-printing process was then sintered in an electric furnace at 500°C for 30 min and subsequently immersed in anhydrous ethanol containing 0.3 mM of Ru dye (Bu4N)2[Ru(Hdcbpy)2-(NCS)2] (N719 dye, Solaronix, Zollikon, Switzerland) for 24 h at room temperature in order to allow the dye molecules to attach themselves to the entire surface of the TiO2 NPs. The dye-soaked TiO2-NP-based photoelectrode was then rinsed with ethanol and dried in a convection oven at 80°C for 10 min. As a counter electrode, we prepared Pt-coated FTO glass using an ion sputter (model no. E1010, Hitachi, Chiyoda-ku, Japan) operated at 2.5 kV. Both the dye-soaked TiO2 NP-based photoelectrode and the Pt-coated counter electrode were sealed together with a hot-melt polymer film (60-μm thick, Surlyn, DuPont, Wilmington, Delaware, USA) that was inserted between them, and an iodide-based liquid electrolyte (AN-50, Solaronix) was then injected into the interspace between the electrodes. The current-voltage (I–V) characteristics of the resulting DSSCs fabricated in this study were measured under AM 1.5 simulated illumination with an intensity of 100 mW/cm2 (PEC-L11, Peccell Technologies, Inc., Yokohama, Kanagawa, Japan). The intensity of sunlight illumination was calibrated using a standard Si photodiode detector with a KG-5 filter. The I–V curves were automatically recorded using a Keithley SMU 2400 source meter (Cleveland, OH, USA) by illuminating the DSSCs. The condenser lens-based solar concentrator employed in this study had a diameter of 15 mm, a center thickness of 3.35 mm, an edge thickness of 1.36 mm, and an effective focal length of 22.5 mm. The condenser lens was supported by a homemade vertical holder, and the focal length was changed by adjusting the rotating gauge.
Results and discussion
First, in order to examine the effects of the condenser lens-based solar concentrator on the photovoltaic performance of DSSCs, we varied the focal length of the light pathway in the condenser lens system such that a reference DSSC with an approximately 10-μm-thick T25 single layer (T25 SL) was exposed to various concentrated sunlight conditions, as shown in Figure 1. Here, by simulating the optical geometries in the given condenser lens system, we estimated that the circular area of the focused beam can fully cover a 0.6 × 0.6 cm2 photoactive layer as long as the optical length is less than 10 mm. Also, when condenser lens system was applied, the temperature measured by a thermocouple installed on top of DSSC was approximately 40°C or less, in which no additional cooling system was required.
From Table 1, we can clearly observe that the short circuit current (Isc) significantly increases with the focal length of the condenser lens system (i.e., higher light concentration). With the use of the condenser lens system, the PCE of the reference T25 SL-based DSSC was found to slightly decrease from approximately 3.57% (without the condenser lens) to approximately 3.38%, when the focal length was set to the maximum value of approximately 10 mm. This is owing to the increase of power input caused by higher light concentration with longer focal length. However, as the light concentration increased, both Isc and Voc were observed to make a significant increase. This is consistent with the general theoretical model given in Equation 1 for conventional inorganic solar cells that Isc increases linearly with increasing light intensity (X), and Voc increases logarithmically with increasing Isc and X:
where, n is the diode quality factor, k is the Boltzmann's constant, T is the absolute temperature, q is the electronic charge, and Io is the reverse saturation current.
Summary of photovoltaic characteristics of T25-accumulated single layer (T25 SL)-based DSSCs
Focal length (mm)
Light concentration (Suns)
Isc, photocurrent; Voc, open circuit voltage; FF, fill factor; PCE, power conversion efficiency.
In this study, we obtained the optimized intensity and focal area of incident light in a simple condenser lens-based solar concentrator by adjusting the focal length of light pathways for a reference DSSC with a T25 SL. Further, we verified the role of a T240-accumulated layer applied on top of the T25-accumulated dye-absorbing layer to serve as a strong light-scattering medium. Furthermore, the light-scattering capacity of the T240 layer in the photoelectrodes of DSSCs was found to be enhanced upon precisely concentrating the incident light with the assistance of the condenser lens-based solar concentrator. On comparison of the photovoltaic performance of the T25/T25-DL-, T25/T240-DL-, and T240/T240-DL-based DSSCs with and without the use of concentrated incident light, the optimized photoelectrode structures exhibited both enhanced dye-absorbing properties (T25) and light-scattering properties (T240) under highly concentrated light irradiation, resulting in increased values of Isc and PCE. The presence of the light-scattering layer in the photoelectrodes of DSSCs and the use of the condenser lens system to concentrate the irradiated light can synergistically enhance the inherent photovoltaic performance of DSSCs.
This study was supported by the National Research Foundation of Korea (NRF), funded by the Korean government (MEST) (2011–0013114).
Department of Samsung Advanced Integrated Circuit Engineering, Pusan National University
Department of Nanomechatronics Engineering, Pusan National University
Department of Nano Fusion Technology, Pusan National University
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