Room Temperature Deposition of Crystalline Nanoporous ZnO Nanostructures for Direct Use as Flexible DSSC Photoanode
© Han et al. 2016
Received: 10 January 2016
Accepted: 14 April 2016
Published: 26 April 2016
A facile approach to fabricate dye-sensitized solar cells (DSSCs) is demonstrated by depositing (001) oriented zinc oxide (ZnO) nanostructures on both glass and flexible substrates at room temperature using pulsed laser deposition. Unique crystallographic characteristics of ZnO combined with highly non-equilibrium state of pulsed laser-induced ablated species enabled highly crystalline ZnO nanostructures without aid of any chemically induced additives or organic/inorganic impurities at room temperature. Film morphology as well as internal surface area is tailored by varying ambient oxygen pressure and deposition time. It is revealed that the optimization of these two experimental factors was essential for achieving structure providing large surface area as well as efficient charge collection. The DSSCs with optimized ZnO photoanodes showed overall efficiencies of 3.89 and 3.4 % on glass and polyethylene naphthalate substrates, respectively, under AM 1.5G light illumination. The high conversion efficiencies are attributed to elongated electron lifetime and enhanced electrolyte diffusion in the high crystalline ZnO nanostructures, verified by intensity-modulated voltage spectroscopy and electrochemical impedance measurements.
KeywordsZinc oxide (ZnO) Room temperature Dye-sensitized solar cell Pulsed laser deposition (PLD) Electron recombination
Flexible, light weight dye-sensitized solar cells (DSSCs) based on plastic substrates are one of the most attractive topics in the field of renewable energy . Incorporating such flexible substrates would allow light weight, shock-resistant power conversion devices . Typical DSSC is consisted of nanoporous film of interconnected TiO2 nanoparticles that provide large internal surface area for dye adsorption, interpenetration for liquid redox electrolyte, and pathway for charge transport. However, the fabrication of such nanoparticle-based film requires high temperature annealing of the mesoporous photoanodes films at 400–500 °C in order to improve its crystallinity, interconnection between particles and to eliminate residual organic substances. The annealing step has prevented the utilization of plastic substrates to DSSCs where elevated temperature above 200 °C becomes problematic . Zinc oxide (ZnO)-based photoanodes have been studied in this context as a candidate to replace TiO2 nanoparticle films which are dominantly used in DSSCs. While the band gap of ZnO is similar to that of TiO2, ZnO exhibits higher electron mobility and lower crystallization temperature than that of TiO2. Intrinsically anisotropic crystal structure of ZnO hexagonal wurzite (P63mc) are suitable for low temperature synthesis of the mesoporous films which rapidly collect photogenerated electrons from surrounding dye molecules and minimize the carrier recombination [4–8].
Recently, significant progresses were made in tailoring the morphology and surface properties of nanostructured ZnO. With different surface energies between crystallographic planes, a wide range of high crystalline ZnO nanostructures has been fabricated rather easily at mild conditions compared to other wideband gap metal oxide materials. However, with increased surface-to-volume ratio, ZnO nanostructures are very sensitive to the growth condition and chemical environment, often lacking reproducibility and controllability [9–12]. It has been challenging to prepare efficient ZnO photoanode with enlarged surface area at low temperature because the synthesis of high crystalline nanoparticles and the formation of mesoporous structure with good electronic connectivity between particles were rather challenging. Various alternative methods such as cathodic electrodeposition , chemically activated solution process , direct metal transfer , compression method , water vapor treatment , and hydrothermal growth  have been developed as viable methods at low temperature. Although the highest overall conversion efficiencies among flexible ZnO devices were in the range of 3.1–3.8 %, further treatments were required in order to remove aggregation with structural guiding agents (cathodic electrodeposition), to dissolve surface chemisorbed species (chemical activation), or to achieve crystallinity before being transferred to flexible substrate. To the best of our knowledge, there has been no report on demonstrating one-step room temperature synthesis method of self-assembled nanostructured photoanodes with enlarged surface area so far. By using pulsed laser deposition (PLD), a versatile deposition method using highly energetic-ablated species, fabrication process can be free from organic residues and stoichiometry of the material can be reliably controlled.
In this study, we demonstrate synthesis route of c-axis-oriented ZnO nanostructures at room temperature obtaining wurzite structure directly on transparent conducting oxide (TCO) substrate. With the aid of instantaneous, non-equilibrium nature of excimer laser-ablated species combined with anisotropic nature of wurzite phase, high-crystalline naonoporous ZnO films were realized on to glass and flexible substrates alike. Morphology and thickness of nanoporous ZnO films were tailored by controlling deposition parameters of pulsed laser deposition under ambient oxygen environment. Morphology of ZnO photoanodes are found to be strongly correlated to photoelectrochemical properties of the fabricated DSSCs as dye adsorption and electrolyte diffusion in the nanostructures are enhanced. The DSSC with the optimized ZnO photoanode on polyethylene naphthalate substrate show an overall efficiency of 3.4 % under AM 1.5G light illumination which is among the best photon to electron conversion efficiency for devices fabricated at room temperature.
ZnO ceramic target for laser ablation was prepared from ZnO powders purchased from Sigma Aldrich (Puriss. p.a. ACS reagent grade) using solid state reaction. Target was pressed in pellet shape and sintered at 800 °C for 2 h. For ZnO photoanodes, tin-doped indium oxide (ITO) glass (Samsung Corning, 8.3 ohm sq−1, Korea) and PEN (Peccell, 13 ohm sq−1, Japan) were used as the substrates; 0.3 mM N719 dye anhydrous acetonitrile-tert butanol 1:1 solution was prepared for dye sensitization. N719 dye ((2,2ʹ bipyridyl-4,4ʹ dicarboxilate)2(NCS)2) was obtained from Solaronix. Electrolyte used for dye-sensitized solar cell fabrication is commercially available Merck (SI16L1535-01) iodide-based liquid electrolyte. Anhydrous acetonitrile and tert butanol was obtained from Sigma Aldrich. Pt-sputtered FTO (TEC8, 8 ohm sq−1, Pilkington) was used for counter electrode.
Target surface was exposed to concentrated laser pulses by an optic lense. ZnO structure was prepared on both rigid glass and flexible polymer substrates. ZnO target was ablated with KrF excimer laser (248 nm, 1.5 J cm−2, 5 Hz, Compex Pro 205F, Coherent, USA). At room temperature, ambient oxygen pressure of high purity O2 gas was varied from 100 to 400 mTorr. The thicknesses of nanostructured ZnO films were controlled by deposition time (number of laser pulses) and they were measured by a scanning electron microscope. As-deposited ZnO films were immersed in an N719 solution for 2 h at 50 °C (see Additional file 1: Figure S1, Table S1) [19–21]. Sandwich-type cells were sealed by pressing heat-melted Surlyn tape between sensitized ZnO photoanode and Pt counter electrode. After 3 h, the fabricated cells have shown saturated efficiencies (see Additional file 1: Figure S2, Table S3).
Photovoltaic properties were measured using a potentiostat (CHI 608C, CH instrument, USA) under AM 1.5G, 100 mW light illumination generated by solar simulator (PEC-L11, Peccel, Japan) as well as dark condition. The light intensity of the solar simulator was calibrated with a reference cell (PV Measurements, USA). An electrochemical workstation (Zennium, Zahner, USA) with an attached frequency response analyzer and a light-emitting diode (667 nm) was utilized for the intensity-modulated photovoltage spectroscopy study. A specially designed system (K3100, McScience) was used to obtain incident photon-to-current efficiency (IPCE) in the range 300–800 nm. Electrochemical impedance spectroscopy (EIS) of the cells was also measured using the potentiostat under illumination and applying open-circuit voltage as the bias. Amount of dye adsorbed was determined by desorbing sensitized film in 10 ml 0.1 M NaOH aqueous solution and optical absorption spectra was measured using UV-vis spectrophotometer (Cary 5000, Agilent technologies, USA). X-ray diffraction (XRD; D8-advance, Bruker Miller Co., USA) was used to determine crystalline structure, and film morphology was observed with field emission scanning microscope (FESEM; FSM-6330F, FEOL, Japan). High-resolution transmission electron microscopy (HRTEM) and selective area electron diffraction (SAED) with JEOL JEM-2100F microscope at an acceleration voltage of 200 kV were carried out to investigate the microstructure of the ZnO films.
Results and Discussion
Deposited films show hierarchical morphology constructed with 100-nm-sized ZnO nanoparticles, while porosity of the film changes as gas pressure inside the chamber is tuned. Morphology transition from dense, cone-like structure at 100 mTorr to porous open structure at higher ambient gas pressure (200–400 mTorr) enabled large internal surface area where the local maximum was obtained at 300 mTorr. All as-prepared samples showed P63mc wurzite phase of ZnO without further treatment.
Nanocluster generation during PLD has been investigated until recently . However, exact mechanism under ambient gas pressure has yet to be fully understood. By combining insights earned from our experimental results and reviewing theoretical and fundamental studies so far, we suggest the formation mechanism of oriented nanostructure deposition during PLD. Ablated species ejected from the target surface induced by concentrated excimer laser pulses initially forms highly concentrated, a non-equilibrium gas state called Knudsen layer, followed by rapid vapor plume expansion pushing ambient gas away. Expanded gas is then pushed back and compressed. Nanoparticles are reported to be concentrated in the region between plume and buffer gas where the highest supersaturation is reached [25, 26] due to the extremely fast quenching rate right after the initial ablation. High spatial localization of nanoparticles leads to three-dimensional fractal aggregations kinetically deposited onto the substrate forming columnar nanostructures suggesting diffusion limited aggregation  is the main factor when cluster formation takes place during PLD plume condensation and deposition.
Photovoltaic parameters of DSSCs with nanostructured ZnO photoanodes under simulated AM 1.5 G light illumination
J sc (mA cm−2)
V oc (V)
Flexible 10 μm
In conclusion, we demonstrate the direct synthesis of ZnO nanostructures on ITO/glass and ITO/polymer substrates with controllable surface area using PLD method. All films showed crystalline ZnO wurzite phase as deposited at room temperature and morphology of the nanostructured film were tuned with the function of ambient gas pressure. J sc which is strongly correlated with surface area for dye adsorption sites was tuned out to be the most important parameter determining overall conversion efficiency. The optimal conversion efficiency of 3.89 % was achieved under AM 1.5 G light illumination on ITO/glass substrate and 3.4 % on ITO/PEN flexible substrate, which is the highest among flexible, room temperature-fabricated ZnO DSSCs. Our results suggest the versatility of PLD combined with anisotropic characteristic of ZnO resulted in successful synthesis of crystalline nanostructures providing well-necked, oriented nanostructures with enlarged surface area at room temperature, showing promising potential uses for other electrochemical applications and materials as well.
This work was financially supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT and Future Planning as the Global Frontier Project, the Fusion Research Program for Green Technologies and the Outstanding Young Researcher Program through the National Research Foundation of Korea.
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