Photoelectrochemical solar cells fabricated from porous CdSe and CdS layers

Electrochimica Acta 55 (2010) 5665–5669

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Photoelectrochemical solar cells fabricated from porous CdSe and CdS layers Sang Hyuk Im, Yong Hui Lee, Sang Il Seok ∗ KRICT-EPFL Global Research Laboratory, Advanced Materials Division, Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong, Daejeon 305-600, Republic of Korea

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Article history: Received 15 January 2010 Received in revised form 29 April 2010 Accepted 29 April 2010 Available online 6 May 2010 Keywords: Porous CdSe and CdS layers Photoelectrochemical solar cells Spray pyrolysis deposition

a b s t r a c t Porous CdSe layers were prepared by spray pyrolysis deposition using sodium selenosulfate as a selenium source and its surface area and porosity were increased by the dissolution of sodium sulfate formed as by-product. The porous CdSe as both photoanode and absorber could efficiently transport electrons to fluorine-doped tin oxide electrode and extract holes to the electrolyte. The cells were optimized by controlling the number of spray pyrolysis deposition cycles and then etching with sodium sulfate. An efficient solar cell having a power conversion efficiency of 2.6% at 1 sun illumination (100 mW cm−1 ) was fabricated. Further, we extend this approach to fabricate an efficient porous CdS-sensitized solar cell with power conversion efficiency greater than 1.0% at 1 sun illumination. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSSCs) are one of the promising candidates for replacing conventional p–n junction solar cells because of their low cost and high energy conversion efficiency of up to 11% [1]. In DSSCs, effective light harvesting is achieved by using a combination of porous TiO2 (p-TiO2 ) with a large surface area as a passive photoanode and a dye as a sensitizer. However, the electron drift mobility of mesoporous TiO2 is ∼10−6 cm2 /V s, which is 5–6 orders of magnitude lower than the electron Hall mobility of single-crystal TiO2 [2–4]. Hence, many researchers have attempted to increase the electron transport rate while maintaining the porosity of the TiO2 electrode. For instance, TiO2 nanorods and nanotubes have been synthesized to reduce the number of surface traps between particles, and various surface treatments have been developed to improve the interfacial connectivity between particles [5–7]. Basically, TiO2 is not a good electron conductor as compared with metal chalcogenides such as CdS, CdSe, and CdTe, whose electron mobility is ∼103 cm2 /V s [8]. Photoelectrochemical cells based on single-crystal CdS and CdSe have been studied extensively, and their potential as solar cells has been demonstrated [9]. To date, CdS and CdSe quantum dotsensitized solar cells (QDSSCs) incorporated with p-TiO2 have been shown to be a promising alternative to conventional DSSCs [10,11]. Although CdSe and CdS act as photoanodes for electron transport and sensitizers at the same time, no attempts have been made to exploit their advantages. Therefore, we carry out the fabrication of porous CdSe (p-CdSe)-sensitized solar cells that can serve as both

∗ Corresponding author. Tel.: +82 42 860 7314; fax: +82 42 861 4151. E-mail address: [email protected] (S.I. Seok). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.04.106

electron conductors and sensitizers. We adopted the spray pyrolysis deposition (SPD) method to form a porous structure and used sodium selenosulfate as a selenium source [12]. By this method, the porosity of the p-CdSe layers preformed by SPD could be increased through the dissolution of sodium sulfate formed as a by-product. In addition, we extended the approach to fabricate porous CdS (p-CdS) layers to demonstrate the versatility of this novel technique. 2. Experimental details 2.1. Preparation of porous CdSe and CdS by SPD First, a 50-nm-thick TiO2 blocking layer (bl-TiO2 ) is deposited on fluorine-doped tin oxide (FTO) glass (TEC15, USA) by SPD using 0.02 M solution of titanium diisopropoxide bis(acetylacetonate) to prevent direct contact between FTO and the electrolyte. For the preparation of sodium selenosulfite, 7.9 g (0.2 M) of selenium powder was mixed in excess 0.5 M of sodium sulfite aqueous solution (500 mL) to completely dissolve selenium powder and was heated at 70 ◦ C for 10 h with a magnetic stirring. The reactant was then filtered to remove tiny amount of unreacted selenium powder. The 0.2 M of cadmium chloride solution (500 mL) was then mixed to 0.2 M of sodium selenosulfate solution (500 mL) and finally the pH of this solution was titrated to 12.6 by ammonium hydroxide solution to get stable clear solution. To obtain the p-CdSe, the pHcontrolled (pH of 12.6 controlled by aqueous ammonia) aqueous solution was deposited by SPD on the bl-TiO2 /FTO substrate that was preheated to 450 ◦ C. 0.8 mL of a precursor solution was sprayed for 2 s and left for 5 s to form crystalline CdSe particles in one cycle. To increase the amount of deposited sensitizer, we repeated the process cycles 5, 10, 15, and 20 times denoted by SPD5, SPD10, SPD15, and SPD20, respectively. To further increase the porosity

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Scheme 1. Schematic of p-CdSe and p-CdS layers formed by SPD.

of the p-CdSe layers, sodium sulfate formed as a by-product of SPD was etched away using double distilled water at 60 ◦ C for 1 h which are denoted to SPD5R, SPD10R, SPD15R and SPD20R. The thickness of each p-CdSe layer was approximately 0.6, 1.0, 1.5 and 2.3 ␮m, respectively. For the fabrication of porous CdS (p-CdS), saturated aqueous solution of 0.15 M CdCl2 , 0.1 M thiourea, and excess sodium sulfate was deposited by SPD onto the bl-TiO2 /FTO substrate preheated to 450 ◦ C. The saturated aqueous solution was prepared by filtering the above mixture. The thickness of rinsed p-CdS layer (SPD15R) was 0.5 ␮m. 2.2. Fabrication of p-CdSe and p-CdS photoelectrochemical solar cells A Pt-coated counter electrode was prepared by dropping 5 mM of H2 PtCl6 in i-propanol onto FTO glass and heating it up to 400 ◦ C for 20 min. The cells were assembled by sandwiching pCdSe deposited on the bl-TiO2 /FTO substrate and Pt-coated counter electrode using a thermal adhesive film (Surlyn, 60 ␮m, DuPont). Methanol/water (7:3, v/v) solution containing 0.5 M Na2 S, 2 M S, and 0.2 M KCl was used as the redox electrolyte [13]. The electrolyte was injected by vacuum backfilling, and then the hole was sealed with a Surlyn film and cover glass. To improve electrical contact, lead contact pads were deposited on both sides of electrodes using an ultrasonic soldering iron. The active area of the photoelectrode was found to be 0.18 cm2 . 2.3. Measurements Photocurrent–voltage (I–V) characteristics of the cells were measured under 1 sun illumination (air mass 1.5 global, 100 mW/cm2 ) from a solar simulator (Newport, Class A, 91195A) using a Keithley 2420 sourcemeter and calibrated Si reference cell (certified by NREL). The external quantum efficiency (EQE) was measured using a fully computerized homemade system comprising a light source (300-W Xe lamp, Newport, 66902), monochromator (Newport Cornerstone 260), and multimeter (Keithley 2002). All measurements were carried out at least five times. 3. Results and discussion 3.1. Structure and morphology in SPD cycles CdSe was immediately formed by reaction of Se2− ions and cadmium–ammonia complex ions during SPD at high temperature. Possible chemical route for CdSe formation from a precursor solution was given in Supporting information. Scheme 1 is a schematic illustration of the control of the thickness of the p-CdSe layer by SPD. In general, droplet size is dependent on many parameters such as flow rate, nozzle structure, and spray distance. Therefore, we var-

ied only the number of SPD cycles and fixed the other conditions to increase the thickness of the p-CdSe layers. In a typical SPD cycle, we sprayed 0.8 mL of 0.1 M CdSe precursor aqueous solution for 2 s on the bl-TiO2 /FTO substrate preheated at 450 ◦ C and waited an additional 5 s (one SPD cycle) to form crystalline CdSe nanoparticles. Of course, the final size of CdSe nanoparticles produced from sprayed droplets also depends on the concentration of the CdSe precursor solution, which is related to the porosity of the p-CdSe layer. However, here, we focused not on device optimization but on fabrication of a p-CdSe layer by SPD and its application to solar cells. To study the development in the structure and morphology in SPD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffractometry were performed, as shown in Fig. 1. When sodium selenosulfite was used as a nontoxic selenium source to produce CdSe via SPD, water-soluble sodium sulfate was inevitably formed as a by-product, which could contribute to the increase in porosity of the preformed p-CdSe layer. Morphological differences were determined by rinsing out sodium sulfate with water, and typical SEM surface images are shown in Fig. 1(a) and (b). Fig. 1(a) is a representative SEM image of the as-deposited CdSe layer formed after 15 SPD cycles (SPD15). The morphology appeared to be porous but the pores have not been developed very well and submicron-sized particles have aggregated. Fig. 1(b) shows an SEM image of the SPD15R sample (shown in Fig. 1(a)) that was rinsed with water at 60 ◦ C for 1 h; the image clearly confirms the increase in porosity of the preformed p-CdSe layer. The cross-sectional SEM image (see Fig. S1(a)) of the SPD15R sample confirms also the formation of pCdSe film (thickness = ∼1.5 ␮m) through whole cross-section. This implies that the as-deposited p-CdSe layer was a mixture of CdSe and sodium sulfate because an equivalent amount of sodium sulfate was formed with the production of CdSe. The formation of water-soluble sodium sulfate as a by-product is very positive from the viewpoint of the surface area of the sensitizer because the well-developed pores and large surface areas will enhance hole extraction from the CdSe sensitizer to polysulfide electrolyte, which will further decrease recombination among the aggregated CdSe particles. Unlike as-deposited p-CdSe, rinsed p-CdSe in Fig. 1(b) appeared to be composed of aggregated particles with a size of ∼100 nm. In addition, the image was clearer, implying that only crystalline CdSe particles remained after rinsing sodium sulfate. The TEM image of rinsed p-CdSe of Fig. 1(b), as shown in Fig. 1(c), was examined to estimate the crystal size of primary CdSe particles in p-CdSe. The image showed that the size of the primary CdSe particles was between 2 and 10 nm and they formed secondary CdSe particles with a size of ∼100 nm. The broad size distribution might be helpful in exploiting a wide spectrum of light. To confirm the composition and structure of p-CdSe after rinsing, XRD patterns were measured as shown in Fig. 1(d). As expected from Fig. 1(a) and (b), the XRD peaks of the as-deposited CdSe and p-CdSe films rinsed with water indicate a mixture of crystalline sodium sulfate

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Fig. 1. Structure and morphology characterization. SEM surface images of as-deposited p-CdSe (SPD15) (a) and p-CdSe rinsed with water at 60 ◦ C for 1 h (SPD15R) (b), transmission electron microscopy image of rinsed p-CdSe (c), and XRD patterns (d).

Fig. 2. Characterization of device performance with SPD followed by rinsing with water in terms of J–V curves (a) and (b), transmittance spectra (c), and EQE (d).

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and CdSe and pure CdSe. This result clearly showed that watersoluble sodium sulfate formed as a by-product could be effectively removed by simple rinsing, and thus, the porosity of the p-CdSe layer could be increased easily. 3.2. Device performance To demonstrate the effectiveness of p-CdSe structures in exploiting solar energy, device performances were measured as shown in Fig. 2. First, we studied the effect of rinsing with water to remove sodium sulfate in p-CdSe, as shown in Fig. 2(a). Samples were prepared after five SPD cycles (SPD5) were rinsed (SPD5R). The rinsing out of sodium sulfate resulted in improved efficiency; the open circuit voltage (Voc ) increased from 0.45 to 0.48 V, current density (Jsc ) increased from 7.4 to 8.9 mA/cm2 , fill factor increased from 25% to 33%, and efficiency increased from 0.8% to 1.4%. The improvement in device efficiency was attributed to the increased porosity owing to the removal of sodium sulfate covering the surface of p-CdSe, because the highly exposed surface of p-CdSe resulted in a simultaneous increase in Jsc and Voc . Further, the expanded pores enhanced hole transportation toward the Pt counter electrode and improved the fill factor. In addition, p-CdSe not only transported electrons to the FTO electrode but also produced electron–hole pairs through the absorption of external light. This implies that n-type CdSe served as a good photoanode, i.e., the holes generated in p-CdSe could be effectively extracted to the electrolyte in the case of suitable surface areas and pore sizes even though the CdSe particles were aggregated. Because device performance is strongly dependent on the amount of sensitizer, we

assessed the performance of SPD5R, SPD10R, SPD15R, and SPD20R produced after SPD5, SPD10, SPD15, and SPD20 and rinsed with water at 60 ◦ C for 1 h, respectively, as shown in Fig. 2(b). As the amount of p-CdSe sensitizer increased with the number of SPD cycles, Jsc increased monotonically and a maximum efficiency of 2.6% was obtained after SPD15 and rinsing (SPD15R). After SPD20 and rinsing (SPD20R), Jsc still increased owing to the increased amount of p-CdSe sensitizer but the fill factor and Voc deteriorated, possibly because of increased recombination. To measure the amount of deposited p-CdSe, UV–vis transmittance spectra were measured, as shown in Fig. 2(c). The transmittance spectra showed that p-CdSe absorbed light up to a wavelength of 750 nm, confirming the broad size distribution of the CdSe particles. After SPD15, the p-CdSe layer absorbed light steadily up to a wavelength of 700 nm, but as the number of SPD cycles increased, the p-CdSe layer appeared to absorb only slightly more light and even appeared to be excessively deposited. The excess p-CdSe could not convert light to electricity and only acted as resistance, recombining the generated electron and hole pairs. This might explain why Jsc was slightly higher and Voc and fill factor were lower for the SPD20R sample as compared with the SPD15R sample. In other words, the slightly increased absorption generated more electron and hole pairs that resulted in an increase in Jsc and the excessively deposited p-CdSe intensified recombination, thus leading to a decrease in Voc and fill factor. Fig. 2(d) shows the changes in EQE with the number of SPD cycles and indicates the tendency of Jsc for a closed circuit. To demonstrate the versatility of this method and extend its use to other semiconductors, we prepared a p-CdS layer by SPD using saturated aqueous solution of 0.15 M CdCl2 , 0.1 M thiourea, and excess

Fig. 3. Representative morphology of p-CdS obtained by SEM image (a), transmittance spectra (b), J–V curves (c), and EQE (d).

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sodium sulfate. The excessive amounts of CdCl2 and sodium sulfate were added to maximize the porosity of p-CdS; they could be easily removed by rinsing with water after SPD. The representative morphology of p-CdS obtained by SEM is shown in Fig. 3(a), and it clearly indicates the formation of a porous structure by SPD followed by rinsing. The cross-sectional SEM image of p-CdS (see Fig. S1(b)) confirms also the formation of p-CdS film (thickness = ∼0.5 ␮m). Fig. 3(b) shows the transmittance of the p-CdS layer fabricated by SPD15 at 450 ◦ C followed by rinsing with water at 60 ◦ C for 1 h. The result indicates that the p-CdS layer (SPD15R) obtained after SPD15 absorbed light sufficiently well. In addition, the J–V and EQE measurements shown in Fig. 3(c) and (d) indicate that an efficient p-CdS-sensitized solar cell with a power conversion efficiency of more than 1.0% and high EQE up to ∼60% could be fabricated. From these series of experiments, we believe that porous semiconductors can serve as both efficient photoanodes and sensitizers if holes are extracted to an electrolyte through the expansion of surface areas and pores.

solar cell having a power conversion efficiency of 2.6%. In addition, we extended this approach to another semiconductor, CdS, and fabricated an efficient p-CdS-sensitized solar cell having a power conversion efficiency in excess of 1.0%. We believe that this porous sensitizer will be helpful in the design of solar cells, including QDSSCs incorporated with p-TiO2 .

4. Summary

References

We fabricated CdSe-sensitized solar cells by SPD using sodium selenosulfate as a selenium source and further increasing the porosity of the p-CdSe layers through the dissolution of sodium sulfate formed as a by-product. In addition, we showed that p-CdSe could act as both a photoanode and a sensitizer. The p-CdSe structure could transport electrons to the FTO electrode and extract them to the electrolyte because of its expanded surface area and pores even though the sensitizing CdSe particles were aggregated. This might imply that semiconductors such as CdSe and CdS can serve as both good photoanodes and sensitizers if the holes generated in the porous semiconductors can be efficiently transported to the electrolyte through the porous structure. We fabricated an efficient

Acknowledgement This study was supported by the Global Research Laboratory (GRL) Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, Republic of Korea. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2010.04.106.

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