Fabrication of Bilayer ZnO Based Hybrid Photoanode for Enhanced Photovoltaic Performance in CdS
5.2 EXPERIMENTAL METHODS
5.2.1 SYNTHESIS OF HOLLOW MESOPOROUS ZnO MICROSPHERES (ZnO HMSP) 3D ZnO microspheres were synthesized by following a reported protocol and is presented schematically in scheme 5.2.1.32 In a typical procedure, zinc nitrate hexahydrate (10 mmol) was dissolved in 100 mL polyethylene glycol 200 (PEG 200). The solution was then transferred into a 250 mL round bottom flask, gradually heated to 160 C and refluxed for 6 h. The solution was
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then allowed to cool down to the room temperature. The precipitate was collected by centrifugation at 10,000 RPM, washed with ethanol and distilled water several times to remove the excess solvent and impurities. The precipitate was dried in a hot air oven at 90 C for 24 h. The obtained brown powder was annealed at 500 C for 4 h in a muffle box furnace and white powders of ZnO HMSP were finally obtained.
Scheme 5.2.1 Schematic presentation of step-by-step synthetic process of 3D ZnO microspheres (HMSPs).
5.2.2 SYNTHESIS OF CdS NANOPARTICLES
Synthesis of CdS nanoparticles (NPs) was carried out by a simple precipitation method in aqueous media. Briefly a 0.5 M Na2S solution (25 mL) in deionized (DI) water was added drop wise to an equal volume of 0.5 M Cd(NO3)2.4H2O solution under vigorous stirring at room temperature. The reaction was continued upto 12 h for completion and then centrifuged, washed with ethanol, DI water and dried in hot air oven at 60 C for 12 h.
5.2.3 GROWTH OF 1D ZnO NWs ON FTO SUBSTRATES
Growth of 1D ZnO NWs was carried out on precleaned FTO substrates coated with ZnO seed layer as reported by Gradečak et.al.33 First, the fluorine doped tin oxide (FTO) substrates were washed with soap solution and plenty of distilled water, followed by ultrasonic treatment in ethanol and isopropanol and dried under Argon. ZnO seed layer was deposited on FTO substrates by spin coating a solution of 300 mM of zinc acetate dihydrate and ethanolamine in 2- methoxyethanol at 4000 rpm for 60 s under inert atmosphere. The substrates were then subjected to anneal at 175 ºC for 10 min under ambient conditions. The process was repeated twice to form a uniform ZnO seed layer, which offers a platform for the well aligned, uniform growth of ZnO nanowire arrays over a large area. The role of ZnO seed layer in growing ZnO NW is to initialize the uniform growth of one dimensionally oriented NWs. The seed layer also acts as a thin ZnO
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blocking layer that resists the direct contact between the two electrodes through electrolyte solution in photovoltaic devices. The seeded substrates were then immersed in an aqueous solution containing 50 mM zinc nitrate hexahydrate and 50 mM hexamethylenetetramine for 4 h at 90 C to grow ZnO NWs. After cooling down to room temperature, the substrates were taken out from the solution, thoroughly rinsed with DI water and finally annealed at 200 C for 5 min. We have schematically presented step-by-step experimental processes for growth of 1D ZnO NWs on FTO substrates in scheme 5.2.2.
5.2.4 FABRICATION OF (ZnO NW ZnO HMSPCdS) HYBRID PHOTOANODE
ZnO NWZnO HMSPCdS hybrid photoanode was prepared as follows. As synthesized ZnO HMSPs (0.5 g) were added to a mixture of 1 mL of terpineol and 0.2 g of PEG-PPG-PEG and stirred well till a homogeneous slurry was obtained. The slurry was then coated over the ZnO NWs grown over FTO substrates by doctor blade technique. The films were then dried in air and calcined at 480 C for 30 min to remove the polymer. The measured thickness of ZnO NWZnO HMSP layer using surface profilometer was found to be in the range of 12−15 µm, which was further confirmed by the cross-sectional field emission scanning electron microscopy (FESEM) analysis. The ZnO NWZnO HMSP electrodes were in-situ sensitized with CdS QDs using successive ionic layer adsorption and reaction (SILAR) process. The schematic of the fabrication procedure is represented in scheme 5.2.2.
Scheme 5.2.2 Step-by-step device fabrication of photovoltaic cell based on all solution processable route.
Briefly, the electrodes were dipped into an ethanol solution containing Cd(NO3)2.4H2O (0.5 M) for 1 min, rinsed with ethanol, and dried on a hot plate at 70 C. Again they were dipped into a
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Na2S (0.5 M) solution in methanol for 1 min, rinsed with methanol and dried at 70 C. The amount of CdS grown onto the ZnO system was controlled by the number of repeated SILAR cycles, however, the exact concentration of CdS QDs might vary from batch to batch. This cycle was repeated for 5 times to adsorb the CdS QDs on the ZnO microspheres as well as ZnO NWs.
Hereafter, the fabricated photoanode is represented as ZnO NWZnO HMSPCdS. The FTO substrates having bare ZnO NWs (ZnO microsphere free) and ZnO HMSP (ZnO NW free) were sensitized by CdS QDs following the same procedure and named hereafter as ZnO NWCdS and ZnO HMSPCdS, respectively. In order to perform the powder X-ray diffraction (PXRD), UV–
Vis diffused reflectance absorption (DRS) and FESEM analysis, identical films of ZnO NWZnO HMSPCdS, ZnO NWCdS and ZnO HMSPCdS were made on glass substrates.
5.2.5 DEVICE FABRICATION AND CHARACTERIZATION
Devices were fabricated by sandwiching the as prepared photoanodes with platinized FTO counter electrodes. The Pt counter electrode preparation was performed by spin coating a solution of chloroplatinic acid (5 mM in isopropanol) on a pre-cleaned FTO substrate. The FTO substrates were then placed in a muffle furnace at 450 C (heating ramp of 5 C/min) for 15 min and cooled down to room temperature naturally. The photovoltaic cell construction was completed by injecting a drop of sulfide/polysulfide (S2−/Sn2−) electrolyte into the devices. Electrolyte solution was prepared by dissolving 2 M S and 2 M Na2S in methanol–water (7:3, v/v). The active areas of the QD sensitized ZnO films were found to be ~1.0 cm2. The cells were ready for photocurrent and incident photon-to-current conversion efficiency (IPCE) measurements.