- Email: [email protected]
Epoxy catalyzed sol–gel method for pinhole-free pyrite FeS2 thin films
Journal of Alloys and Compounds 607 (2014) 169–176
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Epoxy catalyzed sol–gel method for pinhole-free pyrite FeS2 thin films S. Kment a,b,⇑, H. Kmentova c,b, A. Sarkar b, R.J. Soukup b, N.J. Ianno b, D. Sekora b, J. Olejnicek a, P. Ksirova a, J. Krysa d, Z. Remes a, Z. Hubicka a a
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 14800 Prague, Czech Republic Department of Electrical Engineering, University of Nebraska-Lincoln, 209N Scott Engineering Center, Lincoln, NE 68588-0511, USA c Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojova 2/135, 165 02 Prague, Czech Republic d Department of Inorganic Technology, Institute of Chemical Technology Prague, Technicka 5, 16628 Prague, Czech Republic b
a r t i c l e
i n f o
Article history: Received 10 May 2013 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Pyrite FeS2 Thin films Sol–gel Photoelectrochemical solar cells Heterojunction
a b s t r a c t The uniform pinhole-free iron disulfide (FeS2) pyrite thin films were fabricated. In the first step the FeO(OH)x xerogel solution was synthetized by means of a novel epoxy catalyzed sol–gel method and next spin-coated onto various substrates to form the thin films. In the second step the xerogel coatings were annealed to yield hematite (a-Fe2O3) films, which were transformed into pyrite by chemical sulfurization. Among the main deposition parameters studied were the temperature of xerogel–hematite transformation and the optimal sufulrization temperature and duration. It has been observed that 15 min sulfurization at the temperature of 450 °C provided the pyrite films of sufficient quality. The estimated optical band gap of 0.98 eV is very close to the theoretical value of 0.95 eV. Auger electron spectroscopy showed almost ideal stoichiometry of FeS2. However a trace amount of oxygen (approximately 0.5 at.%) present in the film was still detected. The photoinduced functionality of the films was assessed based on photoelectrochemical experiments. In order to demonstrate the ability of pyrite to photosensitize a large band gap semiconductor, a bilayer system of TiO2 anatase/FeS2 pyrite was prepared. The highest photocurrent reached was 45 lA cm 2 at the light intensity of 10.5 mW cm 2. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The iron disulfide (FeS2) in pyrite crystalline structure has been listed among potentially suitable candidates for scalable thin film photovoltaics (PV) in an economic study of long term module cost effectiveness owing strictly to its band gap, 0.95 eV, and the abundance of iron and sulfur [1]. FeS2 has many desirable properties such as very high absorption coefficient a = 6 105 cm 1 for k = 633 nm [2], ease of n/p doping and the composition of abundant, cheap and non-toxic elements. On the other hand, its band gap (0.95 eV) does ultimately limit its efficiency (maximum efficiency is obtained with a band gap of 1.37 eV) [3,4]. In addition, detailed studies of pyrite’s electronic properties, such as minority carrier lifetime, grain boundary recombination rate and as grown carrier concentration indicate that limited solar cell efficiency may be the norm [5–8]. In fact, every FeS2 cell fabricated to date is less than 3% efficient even though theoretical analysis predicts ⇑ Corresponding author at: Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 14800 Prague, Czech Republic. Tel.: +420 266 052 419; fax: +420 286 581 448. E-mail address: [email protected] (S. Kment). http://dx.doi.org/10.1016/j.jallcom.2014.04.060 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
that greater than 18% should be attainable [3,4,9]. The main problem is that pyrite provides only a very low photovoltage. The open circuit voltages rarely exceeded about 200 mV at room temperature [10]. Many of these issues have been traced to the fact that FeS2 is inherently sulfur deficient [11,12]. However, it has also been shown that secondary phases (especially hexagonal troilite FeS, but also marcasite – the metastable orthorhombic modification of FeS2), present in the pyrite films, significantly affects the overall photoefficiency [13]. One of the strategies for PV applications is to develop a sensitization solar cell in which the pyrite FeS2 serves as the absorbing layer of the visible light and further injects photogenerated electrons into the conduction band of the large gap window layer via a photoelectrochemical junction [4] and/or p-i-n junctions [10] with pyrite as the light absorbing i layer. A photoelectrochemical solar cell using the iodide/iodine electrolyte has shown, for example, to yield 90% quantum efficiency with current densities of up to 42 mA/cm2 under 100 mW/cm2 illumination [9]. Though the photovoltages still remained relatively low (VOC = 200 mV). In the p-i-n design, most of the photogenerated carriers are created in the space charge region and further drain away towards the external contact by drift. Among the main advantages of thin film
170
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
pyrite solar cells would be a low material consumption and a fast and low cost production process. Pyrite thin films have been prepared by several methods such as sputtering [14], sulfurization of iron or iron oxide layers [15,16], metal–organic chemical vapor deposition [17], electrodeposition [18], spray pyrolysis [3], and thermal or flash evaporation [19,20]. In this work, we report on a promising epoxide catalyzed sol–gel chemical pathway providing phase-pure, uniform, surface defectless, and polycrystalline pyrite films. This technique has several advantages: simplicity, treatment of large areas, easy control of film thickness, the utilization of metallic and nonmetallic substrates, feasible doping and finally, low cost. It is based on chemical sulfurization of FeO(OH)x xerogel film prepared throughout the epoxide catalyzed sol–gel synthesis. Another attempt is to first thermally transform the xerogel film into crystalline a-Fe2O3 layer followed by its sulfurization. The second method is the main subject matter of this communication. The effects of number of coating cycles and the temperature of the chemical sulfurization on the properties of pyrite films are discussed. Furthermore, using this sol–gel method we recently succeeded in producing ultra-thin pyrite films (10–50 nm), which are, as mentioned above, good candidates for application in p-i-n junctions. The prepared pyrite films were also coupled with the wide band gap material TiO2 (3.2 eV) and tested photoelectrochemically in order to investigate the ability of pyrite to provide the photoelectrochemical junction. Other sol–gel chemical pathways (e.g. [21–23] to fabricate pyrite films have already been applied in the past. In none of these works, however, the produced pyrite films have been combined with a large band gap semiconductor partner to assemble the photoelectrochemical solar cell. 2. Experimental 2.1. Sol–gel method An epoxide catalyzed sol–gel method was applied to grow the films [24]. All syntheses were performed under ambient conditions. In this procedure iron(III) nitrate (2.6 g), serving as the molecular precursor, was dissolved in absolute alcohol (10 mL). Into this solution 2 mL of propylene oxide was slowly dropped. Soon after (within 2 min) the solution turned to intense dark red-brown color simultaneously releasing heat due to the self-propagating exothermic redox reaction. The prepared solution was used to spin coat previously cleaned substrates at 2200 rpm. By a single spin, thin (50 nm) layers were obtained; whereas thicker layers were obtained by multiple (up to 2–5) coatings. Contrary the thinner films were achieved by increasing the spinning speed (up to 3500 rpm). Some of the spin coated films were thermally treated in air by raising the temperature at approximately 100 °C/h and keeping the temperature at different level for 1 h to promote the crystallization of xerogel (as deposited) films. The purpose of the crystallization was to yield a-Fe2O3 (hematite) phase of the films. Soda lime glass slides were used as substrates. The substrates were first cleaned by a water–soup mixture, followed by 20 min sonication in the deionized water bath. The next steps involved very quick immersion of dried substrates into the 20% HF solution and again washing in deionized water. Then the samples were washed with 99.99% ethanol and finally sonicated for another 20 min in isopropyl alcohol and finally dried.
2.3. Thin film characterization Crystallographic structures were determined using Raman spectroscopy and grazing incidence X-ray diffraction spectroscopy (GI-XRD). Raman spectra were recorded by means of a back scattering Raman microscope (Renishaw) with the excitation laser wavelength of 514 nm focused to a spot of 2 lm diameter on the sample surface. X-ray diffraction patterns were taken on an XRD Rigaku D/ Max-B diffractometer (Cu Ka radiation 1.544 Å, 2h = 20–70°) and with the aid of X’Pert MRD powder diffractometer (parallel beam geometry realized with Goebel mirror in the primary beam and parallel plate collimator and graphite monochromator in the secondary beam) to examine the crystallographic phase of prepared samples and it was also used for the estimation of the samples’ crystallite size. Chemical compositions were analyzed by Auger Electron Spectroscopy – AES (Physical Electronics 560 AES/XPS). The optical absorbance was evaluated by Photothermal Deflection Spectroscopy (PDS) based on the deviation of the laser beam collinear with the sample surface [25]. Light absorbance could be measured down to 0.1% accuracy with this technique. The spectrally resolved optical absorption coefficient, the index of refraction, and the film thickness were evaluated from the specular reflectance and the optical absorbance spectra using the effective media approximation. The same method has been used previously to determine optical properties of another photoactive semiconductor material [26]. The surface images were captured by SEM (Hitachi S-520). Hematite and pyrite 3D surface pictures and topographical features of the layers were taken with the AFM microscope (Thericroscopes) and analyzed for their relative surface roughness (rms).
2.4. Photoelectrochemistry The photoinduced activity of pyrite films was assessed in a complex photoelectrochemical study. The general experimental set-up was described elsewhere [27,28]. Briefly, the reactions were carried out using three electrodes in this system. Here the tested pyrite layer was incorporated in a multilayer structure consisting of the ITO (transparent conductive tin-doped indium oxide film) substrate/TiO2 (anatase)/FeS2 (pyrite) that served as the working electrode. Such assembly should demonstrate ability of pyrite to sensitize high band gap material (TiO2, Eg 3.2 eV – UV, the window layer) under visible light absorption. The remaining two electrodes were a platinum sheet working as the counter electrode and the Ag/AgCl (environment of 3 M KCl) electrode was employed as the reference one (potential of 207 mV against Standard hydrogen Electrode – SHE). The experimental arrangement included an optical bench (Melles Griot, Albuquerque, NM, USA) with geometrically centered parts of the system (electrochemical cell, lamp, filters, shade, etc.). As the source of radiation DC Arc polychromatic lamp (LOT LSH201/2 Hg, Xe) was used (LOT-Oriel). The monochromatic filter guaranteed exposure of the films by narrow flux of photons with the wavelength of 625 nm. The size of the working electrode was limited up to 1 cm2 (using PTFE tape) and 0.1 M solution of Na2SO4 was used as the electrolyte. The photocurrents were measured using Voltalab PGZ-100 potentiostat (Radiometer Analytical SAS, Lyon, France, and Voltalab software pack Volta Master 4, version 7) generating a well-defined applied potential (rate of potential increase 10 mV s 1). Two types of electrochemical experiments were performed [29]. These were amperometry and the open circuit voltage measurement (OCP). Amperometry provides determination of the current density in lA cm 2 (the photocurrent) as a function of time at a constant applied potential on the working electrode. The applied potential was in our case 700 mV. The photocurrents were recorded in a sequence of 30 s dark periods followed by 60 s of the sample illumination and repeated twice. The OCP gives the information of the electron/hole pair’s recombination kinetics. This measurement is again based on the combination of the dark/light periods. It is focused on monitoring changes of the potential in 0.2 s intervals over the total interval of 150 s. The procedure starts for 30 s in the dark. Then the shutter is opened and the working electrode is exposed to the photon flux for 60 s. The remaining interval of 60 s is devoted to monitoring the potential changes again in the dark.
2.2. Thermal sulfurization
3. Results and discussion The xerogel films and/or calcinated hematite layers prepared by the sol–gel method were sulfurized in a quartz tube using the Rapid Thermal Annealing (RTA) irradiation lamp as a heating source. via the RTA system a precise annealing ramp can be programmed. However, in many optimizing tests, as discussed later, it was revealed that a complicated thermal program was not needed. The most used annealing procedure was the following. The temperature was increased at the rate of 150 °C/min up to 450 °C. At this temperature level, the samples dwelled for 15 min and then the RTA was turned, off leaving the temperature decreasing slowly to room temperature. During sulfurization, the samples were kept in a graphite container together with sulfur powder. The sulfur amount of 1 g was the same for all the experiments. The container was placed in the quartz tube, which was connected via a thermocouple to a station operating the RTA lamp. The tube was first evacuated and then filled with Argon. The influence of two parameters, temperature and time of sulfurization, was studied. First, the sulfurization temperature was varied between 400 °C and 500 °C (heating rate 150 °C/min) and second sulfurization time was varied from 15 min to 5 h at the temperature of 450 °C.
The main target of the work was to fabricate photoactive pinhole-free FeS2 pyrite thin films with a non-vacuum approach. For this purpose we previously used a chemical bath deposition method [30]. However, this technique bears several drawbacks, such as very long deposition time (up to 20 h) and relatively high extent of films’ inhomogeneity. Another method we have previously used was a chemical sulfurization of magnetron sputtered iron thin films [31]. Although this approach yielded a sufficient homogeneity of the films and the time of the preparation was remarkably reduced, the produced layers suffered from a relatively large number of surface pinholes. Moreover the high vacuum processing would substantially increase the final costs of a solar cell.
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
To the contrary, the non-vacuum sol–gel method seems to be a suitable candidate to produce high-quality pyrite layers overcoming these problems. Basically, the method is a two steps process. In the first step Fe2O3 films of hematite structure (a-Fe2O3) are prepared according to the aforementioned protocol. Generally, the propylene oxide catalyzed gelation of Fe(H2O)3+ 6 yields amorphous iron oxide/hydroxide phase termed as ferrihydrite. In this reaction the epoxide (propylene oxide) serves as a base and an irreversible proton scavenger that induces hydrolysis of the Fe(III) species and a slow uniform condensation yielding an iron(III) oxide/oxyhydroxide sol. Such an inorganic Fe(III) hydrolysis consists of several stages (i) formation of low-molecular-weight species (homogenous solution) before a significant increase in pH; (ii) further condensation to form a red cationic polymer – formation of dimers and oligomers; (iii) aging of the polymer through olation and oxolation causing conversion to iron(III) oxide phases; (iv) as the pH further rises, the clusters eventually condense to make a gel [16]. The films were formed from the iron oxide/hydroxide xerogel by spin coating (15 drops) at 2200 rpm. The dip-coating method is also possible. To convert the iron amorphous xerogel films to their oxide form, the as deposited ferrihydrite films were thermally treated at different temperatures. The film thickness after the annealing was about 50 nm. In order to achieve thicker samples the whole process was repeated several times. In this case the thermal annealing was an inevitable step after each coating. The set of layers with three different thicknesses corresponding to one, two, and three coating cycles were prepared. The hematite films (before sulfurization) are coded in the text as HL1, HL2, and HL3, in which the L1, L2, and L3 mean the one, two, and three coating cycles and H stands for the hematite. After the subsequent sulfurization the coding is PL1, PL2, and PL3, in which P means the pyrite form. The basic physical parameters of the films are summarized in Table 1. 3.1. Crystal Structure Fig. 1a shows the Raman spectrum of the HL3 annealed at 600 °C. The Raman spectrum corresponds to the a-Fe2O3 hematite. Hematite belongs to the D63d crystal space group and there are seven bands expected in the Raman spectrum. There are two A1g modes (225 and 498 cm 1) and five Eg modes (247, 293, 299, 412 and 613 cm 1) [32]. All the peaks can be found in the spectrum but slightly shifted towards lower wavenumbers. Next, the hematite films were sulfurized to achieve the pyrite FeS2. Three different temperatures (400 °C, 450 °C and 500 °C) were chosen for this study. The sulfurization time was always 1 h. In Fig. 1b, the Raman spectra of the FeS2 forms obtained are seen. The FeS2 pyrite has a cubic structure and it belongs to the space group Pa3 with four formula units per a unit cell. With respect to the particular space group, six modes representing the vibrations of FeS2 pyrite can be identified: C = Ag + Eg + 3Tg + 2Au + 2Eu + 6Tu. The modes 5Tu are infrared active and the vibrations (Ag + Eg + 3Tg) are all active in the first-order Raman spectrum. The remaining (2Au + 2Eu) modes and the rigid lattice translations (Tu) are optically inactive Table 1 Basic parameters of the pyrite thin films (L1, L2, L3 mean one, two, three coatings of the xerogel; after each coating followed by calcination at 300 °C and then proceeded by 15 min sulfurization at 450 °C with the temperature increase of 150 °C/min).
*
The layer
Thickness (nm)
Grain size (nm)*
Indirect band gap (eV)
Surface roughness (nm)
PL1 PL2 PL3
90 160 240
30 50 55
1.08 1.05 0.98
19 27 32
The grain sizes are estimated within the standard deviation of 10%.
171
[33]. Using the temperature of 400 °C led to the chemical sulfurization of hematite film to pyrite FeS2. Three basic Raman bands of pyrite, corresponding to the peaks at 339 cm 1 Eg, 374 cm 1 Ag and 426 cm 1 Tg, are observed in the spectrum (see Fig. 1b). However, the chemical transformation is incomplete. This is demonstrated by the presence of the two peaks at around 1500 cm 1, which are apparently attributed to the residual organic carbon in the form of graphite coming from the sol–gel reagents. The low intensities of pyrite Raman peaks probably reflect the low extent of crystallinity of the sample. The peaks of the graphite Raman bands are not seen in case of the sample sulfurized at 500 °C. However, different crystallographic structure corresponding to marcasite FeS2 was revealed in the spectrum. The phase transformation is obviously due to the higher sulfurization temperature. The best result in terms of desired pyrite crystal phase was accomplished by performing the sulfurization at 450 °C. At this point it is worth mentioning that this temperature also significantly influences the creation of pinholes. It was discovered and verified in many tests that using the temperature of sulfurization of 450 °C remarkably suppressed the pinholes production to nearly zero. Nonetheless, from a technological point of view, the annealing temperature of 600 °C to obtain hematite crystallographic phase is rather high. For this reason attention was also paid to this topic. The experiments in which the samples were annealed at 100 °C, 200 °C, 400 °C and 600 °C were done in order to find out the lowest possible annealing temperature still convenient for pyrite production. Although the temperatures of 100 °C and 200 °C were still too low to yield the hematite structure, the subsequent sulfurization of these samples yielded pyrite films. Nevertheless these samples still contained an undesirable amount of organic carbon. At a temperature of about 300 °C the hematite structure started to develop and based on the Raman spectra, it seemed that it is a temperature sufficient enough to obtain highly crystalline pyrite films after sulfurization. This temperature was, therefore, used for annealing of the as deposited xerogels’ films. The findings regarding the crystallographic transformations were verified by means of X-ray diffraction spectroscopy. In Fig. 2 there is a GI-XRD spectrum of the three-layers coating, annealed after each deposition at 300 °C for 1 h, followed by chemical sulfurization at 450 °C for 1 h (the films prepared under these particular conditions are identified as the PL3 type of films). Besides the pyrite crystallographic phase, no other structures were detected. The spectrum included all the peaks related to polycrystalline pyrite: 2h = 28. 51 (1 1 1), 33.08 (2 0 0), 37.10 (2 1 0), 40.78 (2 1 1), 47.40 (2 2 0), 56.27 (3 1 1). The ratios of their intensities correspond well to those from Powder Diffraction File (PDF) database, which means that any significant prevailing orientation, often connected with the internal tension of the films, did not occur. This is in good agreement with the hypotheses that the presence of the pinholes is reduced due to the low tension stress throughout the films. The pyrite crystal lattice is also visualized in Fig. 2. The crystalline size was calculated from a full width at half maximum (FWHM) of XRD peaks according to the Scherrer equation [34]. The values of one, two, and three layers coatings are summarized in Table 1. Although it should not be regarded as precise due to the relatively high standard deviation, the observed trend of the grain sizes with the thicknesses remains reasonable. It has been established previously that the thicker the film, the more rapid crystallization under the same temperature and duration. Such observation is attributed to tensile stress [35,36]. Large particles are developed at the onset of the crystallization and then remain unchanged. The average crystalline size was assessed to be around 50 nm. Furthermore, the effect of the sulfurizarion duration on the films properties was studied. The sulfurizations were running according to the procedure described in the experimental section.
172
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
Fig. 1. Raman spectra of (a) sol–gel three layers (after each coating annealed at 600 °C in air for 1 h) a-Fe203 hematite coating – HL3 type, and (b) FeS2 films obtained from hematite layers sulfurized at different temperatures for 1 h using 1 g of sulfur. In the graph: P – pyrite, M – marcasite and C – carbon.
Fig. 2. Grazing incident X-ray diffractogram of FeS2 pyrite thin film, three-layers sol–gel coating with annealing at 300 °C for 1 h after each deposition, sulfurization with one gram of sulfur at 450 °C for 1 h (PL3).
Fig. 3. Raman spectra of pyrite samples (PL3) prepared by sulfurizing the hematite films at different time durations.
Briefly, the temperature increase was 150 °C per minute with the target point at 450 °C. The dwelling time was varied between 15 min and 6 h. The somewhat surprising results were observed in the Raman spectra (seen in Fig. 3) of the tested samples (PL3). It is clearly seen from the graph that overall time of the sulfurization did not influence the crystallographic structure. The three main Raman bands are present in every spectrum. The complete chemical transformation from the hematite to pyrite was achieved after 15 min. It should be pointed out that conventional sulfurization approaches usually utilize the evacuated quartz capsules in which the sample is placed together with an appropriate amount of sulfur and sealed. Obviously, such a procedure is much more difficult compared to that introduced in this work. Fig. 3 also contains the spectrum of natural mineral pyrite supplied from Alfa Aesar. Its Raman spectrum is identical to those of all the laboratory prepared samples. These films were also a subject of XRD analysis. The results correlate with the Raman spectroscopy data. The intensity of XRD peaks (all from only pyrite phase) are almost equal indicating the same extent of crystallization.
is seen in Fig. 4. Before this analysis, the AES analysis of natural pyrite mineral was run in order to evaluate the sensitivity correction factor for the Auger peaks of Fe and S. The depth sputtering of the film started at the third cycle. The measurement revealed that the stoichiometry of the film was slightly below the ideal ratio of 2 (S):1 (Fe) and it was relatively constant through the entire film. From the measured data, and taking into account the range of precision of the AES system, the concentration of oxygen was estimated to be 0.5%. An average stoichiometry of FeO0.5S1.5 has been established according to similar data by Smestad et al. in [16]. The detected sulfur deficiency resulted in n-type semiconductor behavior. Any other contaminants were not detected within the limits of the Auger system.
3.2. Elemental depth profile Very important diagnostics of the exact chemical composition of the pyrite films were performed with help of Auger electron spectroscopy (AES). A typical depth profile Auger scan of a PL3 film
3.3. Optical properties The optical properties of the pyrite films are of the utmost interest considering their uses in photoinduced applications. Of major interest is the energy band gap value. Although several manuscripts have claimed that the material contains a direct band gap [37,13], it is believed that the band gap is indirect [38], in some cases even both [4,39]. The absorption spectra based on photothermal deflection spectroscopy are shown in the inset of Fig. 5. It is seen the exceptionally large optical absorption in the visible region (a > 105 at hm > 1.5 eV). The abrupt absorption region also indicates
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
Fig. 4. Auger electron spectroscopy elemental depth profile of the pyrite sample (PL3).
that the films are of typical semiconductor characteristic. The graph also presents the plot of (ahm)n vs. energy, with n = 1/2 for the indirect transition giving the band gap energy of 0.98 eV (PL3), which is very close to the theoretical value of 0.95 eV. A slight increase of the band gap of 1.05 and 1.08 for PL2 and PL1 respectively is certainly not due to the sulfur deficiency and instead is a consequence of the presence of oxygen. The effect of the sulfides’ band gap broadening due to the oxygen has previously been reported [40,41]. 3.4. Surface properties In Fig. 6 the surface properties of hematite films and after their thermal sulfurization are visualized as 3D topography images captured via AFM. Despite the number of hematite coatings the hematite films exhibit a smooth surface lacking significant defects. For the hematite precursive HL1, HL2, and HL3 films, the RMS roughness of 3.4 nm, 3.1 nm, and 2.8 nm was estimated, respectively. It is also clearly seen that the films consist of small grains. Contrarily, after the thermal sulfurization of these films the grain sizes increased considerably. It was reflected in a noteworthy increase of the RMS roughness, which was determined to be
Fig. 5. Band-edge absorption plot of the PL3 type of the films; the intersection of the blue line with the x-axis denotes the band gap. The inner graph – the absorption coefficient data pyrite films with different thicknesses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
173
19 nm, 27 nm, and 32 nm for PL1, PL2, and PL3 films, respectively. Moreover, besides the increase of the grain sizes and the surface roughness the film volume (thickness) expansion was also observed. As stated previously the most suitable temperature for the thermal sulfurization was in our case was 450 °C. At this temperature level the majority of elemental sulfur is in the form of quite large cyclic octaatomic S8 molecules [42]. The diameter of S8 ranges from 7.6 to 8.4 Å depending on whether the molecule exists as a ring or a chain. Thus, the volume expansion can be attributed to both the diffusion of large S8 molecules throughout the hematite precursive films and the following phase transformation. The enlargement of the grains may have also resulted from sub-grain agglomeration, since the volume expansion and the increase of the surface roughness are more distinct for the thicker films. Such transformation often produces stresses in the film and at the interface of the film and substrate. The phase transformation stress usually results in defects such as micro cracks, delaminations, and pinholes. However, none of these problems were observed in these films. This was even more evident when the samples were analyzed by SEM. Fig. 7 displays surface SEM images with a broad range of the spatial resolutions. The surface images clearly demonstrate the pinhole-free surface of the fabricated pyrite films. Moreover this result conclusively correlates with all the aforementioned findings and statements regarding the stress of the films and their morphology. 3.5. Photoelectrochemical functionality The photoinduced functionality of pyrite films was tested on the basis of photolectrochemical experiments. The methodology of the photosensitization of a wide band gap semiconductor (in this case, TiO2 with the anatase crystal phase) acting as a window layer for the low band gap absorbing pyrite in a photoelectrochemical solar cell (PEC) has been proposed previously [2,4]. When pyrite absorbs visible light the photons create an electron–hole pair. The electron is further injected to the valance band of the large gap TiO2 and thus generates a photovoltage. In this study the anatase TiO2 thin film was deposited onto an ITO substrate by means of magnetron sputtering. On the top of TiO2 the pyrite FeS2 was deposited using the spin-coating with elevated speed of rotation (3500 rpm) in order to prepare a very thin film (30 nm). This assembly was then annealed at 300 °C for 1 h to obtain the hematite Fe2O3 followed by the thermal sulfurization at 450 °C for 15 min. The TiO2 did not react with the sulfur and, moreover, it protected the ITO from sulfurization. In Fig. 7 the UV–Vis transmission spectra are shown on bare TiO2 film on the ITO substrate and with very thin film of pyrite deposited onto this structure. It can be seen that the absorption onset occurs at around 370 nm for anatase TiO2 and a shift of the onset to 850 nm occurs in case of the anatase/pyrite system. The anatase structure is verified by the Raman spectrum (inner graph of Fig. 8). One of the basic photoelectrochemical characteristics is the dependency of current density on time at a constant applied bias potential to the working electrode, photoamperometry. The applied potential provides for an enhanced separation of the electrons and holes and thus increases the overall photoefficiency. The photoamperometry data of the system TiO2/FeS2 are depicted in Fig. 9a. A cutoff filter with the transmission peak maximum at 625 nm was used to illuminate the system. The same filter was used to illuminate a single TiO2 film (without FeS2) for the purpose of excluding the effect of visible light photoactivation of the TiO2 itself. Since no response was recorded under these conditions it is possible to conclude that the photoactivity shown in Fig. 9a is due to the photosensitization of TiO2 by the pyrite. The light was chopped in order to demonstrate how fast the photoresponse is and to show both the photocurrent and the dark current. Initially,
174
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
Fig. 6. AFM images of hematite films (HL1, HL2, and HL3) and after their chemical sulfurization (pyrite films – PL1, PL2, and PL3).
Fig. 7. SEM images verifying the pinhole-free surface of the fabricated pyrite films; (a–e) SEM surface images of the PL3 type with the resolution of 100 lm, 20 lm, 5 lm, 2 lm, and 1 lm, respectively, and (f) the cross section SEM image with the resolution of 1 lm.
the photocurrent reaches the maximum of about 50 lA cm 2 which subsequently decreases to a steady state photocurrent at 45 lA cm 2 at a light intensity of 10.5 mW cm 2. It is also possible to observe a dark current, probably coming from a large density of
defect states positioned within the pyrite band gap. The result is a lowering of the band gap at the surface [43,44]. Fig. 9b shows the dependence of photovoltage on time. This graph illustrates a different kinetics of the reactions of holes and electrons. Moreover, it
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
175
is determined by the reaction of the electrons with the redox electrolyte closing the photocurrent cycle. This complex system of reactions is predominantly influenced by the junction between the interfaces such as electrolyte/pyrite, pyrite/TiO2, TiO2/ITO and thus, the surface chemistry of anatase and pyrite. The relatively low photocurrent and photovoltage values reached indicate that the films contain a higher degree of defect states both in the bulk and on the surface serving as recombination centers. Moreover, a problematic junction at the interface of TiO2/FeS2 probably also contributes to the weaker photoactivity. 4. Conclusion
Fig. 8. UV–Vis transmission spectra of bare magnetron sputtered TiO2 thin film and the bilayer system of TiO2/FeS2. The inner graph represents Raman spectrum of the magnetron sputtered TiO2 film denoting the anatase crystallographic structure.
The novel approach for the production of the pinhole-free thin films of FeS2 pyrite was described. The method is based on the sol–gel preparation of a-Fe2O3 (hematite) thin layers and their subsequent chemical sulfurization. The effects of thermal annealing of xerogel films and temperature of the sulfurization, on the properties of the pyrite films were discussed. It was shown that the temperature of sulfur annealing of 450 °C guarantees a complete transformation of hematite precursive films into the pyrite regardless of the thickness of the films after only 15 min of sulfurization. None of the usually accompanied FeS2 phases such as marcasite and trotille were revealed either by GI-XRD or Raman spectroscopy. According to the AES measurements sulfur was slightly deficient in the films owing to the presence of oxygen. The oxygen was apparently responsible for a certain broadening of the band gap, which was more evident for thicker films. Furthermore, the higher the number of coatings, the higher the thickness expansion caused by penetration of sulfur molecules during the deposition. The defect-less, pinhole-free surface of the pyrite films is apparent from SEM morphology scrutiny. Photoelectrochemical investigation of the films exhibited certain features that might be connected to a higher extent of defects acting as recombination centers resulting in relatively low photofunctionality. Although iron pyrite has many properties suitable for inexpensive, efficient solar cells, much more research is necessary in order to fulfill its promise. The impurity and stoichiometry must be improved considerably and a suitable heterojunction partner must be found. Alternatively, a p-i-n structure or a pn junction may be used if the purity problem can be solved. Acknowledgements The authors acknowledge the Academy of Sciences of the Czech Republic – Project M100101215 and the Ministry of Education, Youth and Sports of the Czech Republic – Project LH12043. References
Fig. 9. (a) Photoamperometry characteristic of the ITO/TiO2/FeS2/electrolyte photoelectrochemical system at applied potential 700 mV vs. Ag/AgCl electrode, and the light intensity of 10.5 mW cm 2 and (b) the open circuit potential measurement.
reflects the extent and kinetics of backward electron–hole pair recombination. When the light is on, the photovoltage is governed by the reaction of photogenerated holes at the pyrite/electrolyte interface that, however, compete with electron injection into the TiO2 layer [45]. During the dark sequence the photovoltage decay
[1] C. Wadia, A. Paulalvisatos, Daniel M. Kammen, Environ. Sci. Technol. 43 (2009) 2072. [2] A. Ennaoui, H. Tributsch, Sol. Cells 13 (1984) 197. [3] A. Yamamoto, M. Nakamura, A. Seki, E.L. Li, A. Hashimoto, S. Nakamura, Sol. Energy Mater. Sol. Cells 75 (2003) 451. [4] A. Ennaoui, S. Fiechter, Ch. Pettenkofer, N. Alonso-Vante, K. Buker, M. Bronold, Ch. Hopfner, H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (1993) 289. [5] A. Gomes, J.R. Ares, I.J. Ferrer, M.I. da Silva, Pereira, C. Sanchez, Mater. Res. Bull. 38 (2003) 1123. [6] I.J. Ferrer, J.R. Ares, C. Sanchez, Sol. Energy Mater. Sol. Cells 76 (2003) 183. [7] S. Guo, D.P. Young, R.T. Macaluso, D.A. Browne, N.L. Henderson, J.Y. Chan, L.L. Henry, J.F. Ditusa, Phys. Rev. B 81 (2010) 144424. [8] J.R. Ares, A. Pascual, I.J. Ferrer, C.R. Sanchez, Thin Solid Films 451–452 (2004) 233. [9] P.P. Altermatt, T. Kiesewetter, K. Ellmer, H. Tribtsch, Sol. Energy Mater. Sol. Cells 71 (2002) 181. [10] K. Büker, N. Alonso-Vante, H. Tributsch, J. Appl. Phys. 72 (1992) 5721. [11] R. Schieck, A. Hartmann, S. Fiechter, R. Konenkamp, H. Wetzel, J. Mater. Res. 5 (1990) 1567.
176
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
[12] S. Fiechter, M. Birkholz, A. Hartmann, P. Dulski, H. Tributsch, R.J.D. Tikkey, J. Mater. Res. 7 (1992) 1829. [13] C. Wadia, Y. Wu, S. Gul, S.K. Volkman, J. Guo, A.P. Alivisatos, Chem. Mater. 21 (2009) 2568. [14] D. Lichtenberger, K. Ellmer, R. Schieck, S. Fiechter, Appl. Surf. Sci. 70–71 (1993) 583. [15] C. de las Heras, J.L.M. de Vidales, I.J. Ferrer, C. Sanchez, J. Mater. Res. 11 (1996) 211. [16] G. Smestad, A. Ennaoui, S. Fiechter, H. Tributsch, W.K. Hofmann, M. Birkholz, W. Kautek, Sol. Energy Mater. 20 (1990) 149. [17] B. Meester, L. Reijnen, A. Goossens, J. Schoonman, Chem. Vapor Dep. 6 (2000) 121. [18] S. Nakamura, A. Yamamoto, Sol. Energy Mater. Sol. Cells 65 (2001) 79. [19] A.M. Karguppikar, V.G. Vedeshawar, Phys. Status Solidi A 95 (1986) 717. [20] C. de las Heras, C. Sanchez, Thin Solid Films 199 (1991) 259. [21] L.Y. Huang, F. Wang, Z.J. Luan, L. Meng, Mater. Lett. 64 (2010) 2612. [22] B. Kilic, J. Roehling, O.T. Ozmen, J. Nanoelectron. Opt. 8 (2013) 260. [23] F. Wang, L.Y. Huang, Z.J. Luan, J.S. Huang, L. Meng, Mater. Chem. Phys. 132 (2012) 505. [24] Ch. Park, J. Walker, R. Tannenbaum, A.E. Stiegman, J. Frydrych, L. Machala, Appl. Mater. Interfaces 1 (2009) 1843. [25] W.B. Jackson, N.M. Amer, A.C. Boccara, D. Fournier, Appl. Opt. 20 (1981) 1333. [26] S. Kment, P. Kluson, Z. Hubicka, J. Krysa, M. Cada, I. Gregora, A. Deyneka, Z. Remes, H. Zabova, L. Jastrabik, Electrochim. Acta 55 (2010) 1548–1556. [27] S. Kment, P. Kluson, V. Stranak, P. Virostko, J. Krysa, M. Cada, J. Pracharova, M. Kohout, M. Morozova, P. Adamek, Z. Hubicka, Electrochim. Acta 54 (2009) 3352–3359.
[28] S. Kment, H. Kmentova, P. Kluson, J. Krysa, Z. Hubicka, V. Cirkva, I. Gregora, O. Solcova, L. Jastrabik, J. Colloid Interface Sci. 348 (2010) 198–205. [29] M. Morozova, P. Kluson, J. Krysa, M. Zlamal, O. Solcova, S. Kment, T. Steck, J. Sol–gel Sci. Technol. 52 (2009) 398. [30] P. Prabukanthan, R.J. Soukup, N.J. Ianno, A. Sarkar, S. Kment, et al., IEEE Photovoltaic Spec. Conf. (2010) 2965. [31] R.J. Soukup, P. Prabukanthan, N.J. Ianno, A. Sarkar, C.A. Kamler, D.G. Sekora, J. Vac. Sci. Technol. A 29 (2011). Article Number: 011001. [32] C. Barrato, P.P. Lottici, D. Bersani, G. Antonioli, J. Sol–gel Sci. Technol. 13 (1998) 667. [33] P. Terrence, P. Mernagh, A.G. Trudu, Chem. Geol. 103 (1993) 113. [34] H.G. Jiang, M. Ruhle, E.J. Lavernia, J. Mater. Res. 14 (1999) 549. [35] R. Kuzel, L. Nichtova, Z. Matej, D. Heiman, J. Musil, Z. Kristallogr. 26 (2007) 247. [36] R. Kuzel, L. Nichtova, Z. Matej, J. Musil, Thin Solid Films 519 (2010) 1649. [37] Y.Z. Dong, Y.F. Zheng, H. Duan, Y.F. Sun, Y.H. Chen, Mater. Lett. 59 (2005) 2398. [38] F. Wang, L. Huang, Z. Luan, J. Huang, L. Meng, Mater. Chem. Phys. 132 (2012) 505. [39] H.A. Macpherson, C.R. Stoldt, ACS Nano 6 (2012) 8940. [40] R. Robles, A. Vega, A. Mokrani, Opt. Mater. 17 (2001) 497. [41] N. Barreau, J.C. Berne’de, S. Marsillac, A. Mokrani, J. Cryst. Growth 235 (2002) 430. [42] B. Meyer, Chem. Rev. 76 (1976) 367. [43] R. Murphy, D.R. Strongin, Surf. Sci. Rep. 64 (2009) 1. [44] Y. Bi, Y. Yuan, C.I. Exstrom, S.A. Darveau, J. Huang, Nano Lett. 11 (2011) 4953. [45] E. Ennaoui, S. Fiechter, H. Tributsch, M. Giersig, R. Vogel, H. Weller, J. Electrochem. Soc. 139 (1992) 2514.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Epoxy catalyzed sol–gel method for pinhole-free pyrite FeS2 thin films S. Kment a,b,⇑, H. Kmentova c,b, A. Sarkar b, R.J. Soukup b, N.J. Ianno b, D. Sekora b, J. Olejnicek a, P. Ksirova a, J. Krysa d, Z. Remes a, Z. Hubicka a a
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 14800 Prague, Czech Republic Department of Electrical Engineering, University of Nebraska-Lincoln, 209N Scott Engineering Center, Lincoln, NE 68588-0511, USA c Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojova 2/135, 165 02 Prague, Czech Republic d Department of Inorganic Technology, Institute of Chemical Technology Prague, Technicka 5, 16628 Prague, Czech Republic b
a r t i c l e
i n f o
Article history: Received 10 May 2013 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Pyrite FeS2 Thin films Sol–gel Photoelectrochemical solar cells Heterojunction
a b s t r a c t The uniform pinhole-free iron disulfide (FeS2) pyrite thin films were fabricated. In the first step the FeO(OH)x xerogel solution was synthetized by means of a novel epoxy catalyzed sol–gel method and next spin-coated onto various substrates to form the thin films. In the second step the xerogel coatings were annealed to yield hematite (a-Fe2O3) films, which were transformed into pyrite by chemical sulfurization. Among the main deposition parameters studied were the temperature of xerogel–hematite transformation and the optimal sufulrization temperature and duration. It has been observed that 15 min sulfurization at the temperature of 450 °C provided the pyrite films of sufficient quality. The estimated optical band gap of 0.98 eV is very close to the theoretical value of 0.95 eV. Auger electron spectroscopy showed almost ideal stoichiometry of FeS2. However a trace amount of oxygen (approximately 0.5 at.%) present in the film was still detected. The photoinduced functionality of the films was assessed based on photoelectrochemical experiments. In order to demonstrate the ability of pyrite to photosensitize a large band gap semiconductor, a bilayer system of TiO2 anatase/FeS2 pyrite was prepared. The highest photocurrent reached was 45 lA cm 2 at the light intensity of 10.5 mW cm 2. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The iron disulfide (FeS2) in pyrite crystalline structure has been listed among potentially suitable candidates for scalable thin film photovoltaics (PV) in an economic study of long term module cost effectiveness owing strictly to its band gap, 0.95 eV, and the abundance of iron and sulfur [1]. FeS2 has many desirable properties such as very high absorption coefficient a = 6 105 cm 1 for k = 633 nm [2], ease of n/p doping and the composition of abundant, cheap and non-toxic elements. On the other hand, its band gap (0.95 eV) does ultimately limit its efficiency (maximum efficiency is obtained with a band gap of 1.37 eV) [3,4]. In addition, detailed studies of pyrite’s electronic properties, such as minority carrier lifetime, grain boundary recombination rate and as grown carrier concentration indicate that limited solar cell efficiency may be the norm [5–8]. In fact, every FeS2 cell fabricated to date is less than 3% efficient even though theoretical analysis predicts ⇑ Corresponding author at: Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 14800 Prague, Czech Republic. Tel.: +420 266 052 419; fax: +420 286 581 448. E-mail address: [email protected] (S. Kment). http://dx.doi.org/10.1016/j.jallcom.2014.04.060 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
that greater than 18% should be attainable [3,4,9]. The main problem is that pyrite provides only a very low photovoltage. The open circuit voltages rarely exceeded about 200 mV at room temperature [10]. Many of these issues have been traced to the fact that FeS2 is inherently sulfur deficient [11,12]. However, it has also been shown that secondary phases (especially hexagonal troilite FeS, but also marcasite – the metastable orthorhombic modification of FeS2), present in the pyrite films, significantly affects the overall photoefficiency [13]. One of the strategies for PV applications is to develop a sensitization solar cell in which the pyrite FeS2 serves as the absorbing layer of the visible light and further injects photogenerated electrons into the conduction band of the large gap window layer via a photoelectrochemical junction [4] and/or p-i-n junctions [10] with pyrite as the light absorbing i layer. A photoelectrochemical solar cell using the iodide/iodine electrolyte has shown, for example, to yield 90% quantum efficiency with current densities of up to 42 mA/cm2 under 100 mW/cm2 illumination [9]. Though the photovoltages still remained relatively low (VOC = 200 mV). In the p-i-n design, most of the photogenerated carriers are created in the space charge region and further drain away towards the external contact by drift. Among the main advantages of thin film
170
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
pyrite solar cells would be a low material consumption and a fast and low cost production process. Pyrite thin films have been prepared by several methods such as sputtering [14], sulfurization of iron or iron oxide layers [15,16], metal–organic chemical vapor deposition [17], electrodeposition [18], spray pyrolysis [3], and thermal or flash evaporation [19,20]. In this work, we report on a promising epoxide catalyzed sol–gel chemical pathway providing phase-pure, uniform, surface defectless, and polycrystalline pyrite films. This technique has several advantages: simplicity, treatment of large areas, easy control of film thickness, the utilization of metallic and nonmetallic substrates, feasible doping and finally, low cost. It is based on chemical sulfurization of FeO(OH)x xerogel film prepared throughout the epoxide catalyzed sol–gel synthesis. Another attempt is to first thermally transform the xerogel film into crystalline a-Fe2O3 layer followed by its sulfurization. The second method is the main subject matter of this communication. The effects of number of coating cycles and the temperature of the chemical sulfurization on the properties of pyrite films are discussed. Furthermore, using this sol–gel method we recently succeeded in producing ultra-thin pyrite films (10–50 nm), which are, as mentioned above, good candidates for application in p-i-n junctions. The prepared pyrite films were also coupled with the wide band gap material TiO2 (3.2 eV) and tested photoelectrochemically in order to investigate the ability of pyrite to provide the photoelectrochemical junction. Other sol–gel chemical pathways (e.g. [21–23] to fabricate pyrite films have already been applied in the past. In none of these works, however, the produced pyrite films have been combined with a large band gap semiconductor partner to assemble the photoelectrochemical solar cell. 2. Experimental 2.1. Sol–gel method An epoxide catalyzed sol–gel method was applied to grow the films [24]. All syntheses were performed under ambient conditions. In this procedure iron(III) nitrate (2.6 g), serving as the molecular precursor, was dissolved in absolute alcohol (10 mL). Into this solution 2 mL of propylene oxide was slowly dropped. Soon after (within 2 min) the solution turned to intense dark red-brown color simultaneously releasing heat due to the self-propagating exothermic redox reaction. The prepared solution was used to spin coat previously cleaned substrates at 2200 rpm. By a single spin, thin (50 nm) layers were obtained; whereas thicker layers were obtained by multiple (up to 2–5) coatings. Contrary the thinner films were achieved by increasing the spinning speed (up to 3500 rpm). Some of the spin coated films were thermally treated in air by raising the temperature at approximately 100 °C/h and keeping the temperature at different level for 1 h to promote the crystallization of xerogel (as deposited) films. The purpose of the crystallization was to yield a-Fe2O3 (hematite) phase of the films. Soda lime glass slides were used as substrates. The substrates were first cleaned by a water–soup mixture, followed by 20 min sonication in the deionized water bath. The next steps involved very quick immersion of dried substrates into the 20% HF solution and again washing in deionized water. Then the samples were washed with 99.99% ethanol and finally sonicated for another 20 min in isopropyl alcohol and finally dried.
2.3. Thin film characterization Crystallographic structures were determined using Raman spectroscopy and grazing incidence X-ray diffraction spectroscopy (GI-XRD). Raman spectra were recorded by means of a back scattering Raman microscope (Renishaw) with the excitation laser wavelength of 514 nm focused to a spot of 2 lm diameter on the sample surface. X-ray diffraction patterns were taken on an XRD Rigaku D/ Max-B diffractometer (Cu Ka radiation 1.544 Å, 2h = 20–70°) and with the aid of X’Pert MRD powder diffractometer (parallel beam geometry realized with Goebel mirror in the primary beam and parallel plate collimator and graphite monochromator in the secondary beam) to examine the crystallographic phase of prepared samples and it was also used for the estimation of the samples’ crystallite size. Chemical compositions were analyzed by Auger Electron Spectroscopy – AES (Physical Electronics 560 AES/XPS). The optical absorbance was evaluated by Photothermal Deflection Spectroscopy (PDS) based on the deviation of the laser beam collinear with the sample surface [25]. Light absorbance could be measured down to 0.1% accuracy with this technique. The spectrally resolved optical absorption coefficient, the index of refraction, and the film thickness were evaluated from the specular reflectance and the optical absorbance spectra using the effective media approximation. The same method has been used previously to determine optical properties of another photoactive semiconductor material [26]. The surface images were captured by SEM (Hitachi S-520). Hematite and pyrite 3D surface pictures and topographical features of the layers were taken with the AFM microscope (Thericroscopes) and analyzed for their relative surface roughness (rms).
2.4. Photoelectrochemistry The photoinduced activity of pyrite films was assessed in a complex photoelectrochemical study. The general experimental set-up was described elsewhere [27,28]. Briefly, the reactions were carried out using three electrodes in this system. Here the tested pyrite layer was incorporated in a multilayer structure consisting of the ITO (transparent conductive tin-doped indium oxide film) substrate/TiO2 (anatase)/FeS2 (pyrite) that served as the working electrode. Such assembly should demonstrate ability of pyrite to sensitize high band gap material (TiO2, Eg 3.2 eV – UV, the window layer) under visible light absorption. The remaining two electrodes were a platinum sheet working as the counter electrode and the Ag/AgCl (environment of 3 M KCl) electrode was employed as the reference one (potential of 207 mV against Standard hydrogen Electrode – SHE). The experimental arrangement included an optical bench (Melles Griot, Albuquerque, NM, USA) with geometrically centered parts of the system (electrochemical cell, lamp, filters, shade, etc.). As the source of radiation DC Arc polychromatic lamp (LOT LSH201/2 Hg, Xe) was used (LOT-Oriel). The monochromatic filter guaranteed exposure of the films by narrow flux of photons with the wavelength of 625 nm. The size of the working electrode was limited up to 1 cm2 (using PTFE tape) and 0.1 M solution of Na2SO4 was used as the electrolyte. The photocurrents were measured using Voltalab PGZ-100 potentiostat (Radiometer Analytical SAS, Lyon, France, and Voltalab software pack Volta Master 4, version 7) generating a well-defined applied potential (rate of potential increase 10 mV s 1). Two types of electrochemical experiments were performed [29]. These were amperometry and the open circuit voltage measurement (OCP). Amperometry provides determination of the current density in lA cm 2 (the photocurrent) as a function of time at a constant applied potential on the working electrode. The applied potential was in our case 700 mV. The photocurrents were recorded in a sequence of 30 s dark periods followed by 60 s of the sample illumination and repeated twice. The OCP gives the information of the electron/hole pair’s recombination kinetics. This measurement is again based on the combination of the dark/light periods. It is focused on monitoring changes of the potential in 0.2 s intervals over the total interval of 150 s. The procedure starts for 30 s in the dark. Then the shutter is opened and the working electrode is exposed to the photon flux for 60 s. The remaining interval of 60 s is devoted to monitoring the potential changes again in the dark.
2.2. Thermal sulfurization
3. Results and discussion The xerogel films and/or calcinated hematite layers prepared by the sol–gel method were sulfurized in a quartz tube using the Rapid Thermal Annealing (RTA) irradiation lamp as a heating source. via the RTA system a precise annealing ramp can be programmed. However, in many optimizing tests, as discussed later, it was revealed that a complicated thermal program was not needed. The most used annealing procedure was the following. The temperature was increased at the rate of 150 °C/min up to 450 °C. At this temperature level, the samples dwelled for 15 min and then the RTA was turned, off leaving the temperature decreasing slowly to room temperature. During sulfurization, the samples were kept in a graphite container together with sulfur powder. The sulfur amount of 1 g was the same for all the experiments. The container was placed in the quartz tube, which was connected via a thermocouple to a station operating the RTA lamp. The tube was first evacuated and then filled with Argon. The influence of two parameters, temperature and time of sulfurization, was studied. First, the sulfurization temperature was varied between 400 °C and 500 °C (heating rate 150 °C/min) and second sulfurization time was varied from 15 min to 5 h at the temperature of 450 °C.
The main target of the work was to fabricate photoactive pinhole-free FeS2 pyrite thin films with a non-vacuum approach. For this purpose we previously used a chemical bath deposition method [30]. However, this technique bears several drawbacks, such as very long deposition time (up to 20 h) and relatively high extent of films’ inhomogeneity. Another method we have previously used was a chemical sulfurization of magnetron sputtered iron thin films [31]. Although this approach yielded a sufficient homogeneity of the films and the time of the preparation was remarkably reduced, the produced layers suffered from a relatively large number of surface pinholes. Moreover the high vacuum processing would substantially increase the final costs of a solar cell.
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
To the contrary, the non-vacuum sol–gel method seems to be a suitable candidate to produce high-quality pyrite layers overcoming these problems. Basically, the method is a two steps process. In the first step Fe2O3 films of hematite structure (a-Fe2O3) are prepared according to the aforementioned protocol. Generally, the propylene oxide catalyzed gelation of Fe(H2O)3+ 6 yields amorphous iron oxide/hydroxide phase termed as ferrihydrite. In this reaction the epoxide (propylene oxide) serves as a base and an irreversible proton scavenger that induces hydrolysis of the Fe(III) species and a slow uniform condensation yielding an iron(III) oxide/oxyhydroxide sol. Such an inorganic Fe(III) hydrolysis consists of several stages (i) formation of low-molecular-weight species (homogenous solution) before a significant increase in pH; (ii) further condensation to form a red cationic polymer – formation of dimers and oligomers; (iii) aging of the polymer through olation and oxolation causing conversion to iron(III) oxide phases; (iv) as the pH further rises, the clusters eventually condense to make a gel [16]. The films were formed from the iron oxide/hydroxide xerogel by spin coating (15 drops) at 2200 rpm. The dip-coating method is also possible. To convert the iron amorphous xerogel films to their oxide form, the as deposited ferrihydrite films were thermally treated at different temperatures. The film thickness after the annealing was about 50 nm. In order to achieve thicker samples the whole process was repeated several times. In this case the thermal annealing was an inevitable step after each coating. The set of layers with three different thicknesses corresponding to one, two, and three coating cycles were prepared. The hematite films (before sulfurization) are coded in the text as HL1, HL2, and HL3, in which the L1, L2, and L3 mean the one, two, and three coating cycles and H stands for the hematite. After the subsequent sulfurization the coding is PL1, PL2, and PL3, in which P means the pyrite form. The basic physical parameters of the films are summarized in Table 1. 3.1. Crystal Structure Fig. 1a shows the Raman spectrum of the HL3 annealed at 600 °C. The Raman spectrum corresponds to the a-Fe2O3 hematite. Hematite belongs to the D63d crystal space group and there are seven bands expected in the Raman spectrum. There are two A1g modes (225 and 498 cm 1) and five Eg modes (247, 293, 299, 412 and 613 cm 1) [32]. All the peaks can be found in the spectrum but slightly shifted towards lower wavenumbers. Next, the hematite films were sulfurized to achieve the pyrite FeS2. Three different temperatures (400 °C, 450 °C and 500 °C) were chosen for this study. The sulfurization time was always 1 h. In Fig. 1b, the Raman spectra of the FeS2 forms obtained are seen. The FeS2 pyrite has a cubic structure and it belongs to the space group Pa3 with four formula units per a unit cell. With respect to the particular space group, six modes representing the vibrations of FeS2 pyrite can be identified: C = Ag + Eg + 3Tg + 2Au + 2Eu + 6Tu. The modes 5Tu are infrared active and the vibrations (Ag + Eg + 3Tg) are all active in the first-order Raman spectrum. The remaining (2Au + 2Eu) modes and the rigid lattice translations (Tu) are optically inactive Table 1 Basic parameters of the pyrite thin films (L1, L2, L3 mean one, two, three coatings of the xerogel; after each coating followed by calcination at 300 °C and then proceeded by 15 min sulfurization at 450 °C with the temperature increase of 150 °C/min).
*
The layer
Thickness (nm)
Grain size (nm)*
Indirect band gap (eV)
Surface roughness (nm)
PL1 PL2 PL3
90 160 240
30 50 55
1.08 1.05 0.98
19 27 32
The grain sizes are estimated within the standard deviation of 10%.
171
[33]. Using the temperature of 400 °C led to the chemical sulfurization of hematite film to pyrite FeS2. Three basic Raman bands of pyrite, corresponding to the peaks at 339 cm 1 Eg, 374 cm 1 Ag and 426 cm 1 Tg, are observed in the spectrum (see Fig. 1b). However, the chemical transformation is incomplete. This is demonstrated by the presence of the two peaks at around 1500 cm 1, which are apparently attributed to the residual organic carbon in the form of graphite coming from the sol–gel reagents. The low intensities of pyrite Raman peaks probably reflect the low extent of crystallinity of the sample. The peaks of the graphite Raman bands are not seen in case of the sample sulfurized at 500 °C. However, different crystallographic structure corresponding to marcasite FeS2 was revealed in the spectrum. The phase transformation is obviously due to the higher sulfurization temperature. The best result in terms of desired pyrite crystal phase was accomplished by performing the sulfurization at 450 °C. At this point it is worth mentioning that this temperature also significantly influences the creation of pinholes. It was discovered and verified in many tests that using the temperature of sulfurization of 450 °C remarkably suppressed the pinholes production to nearly zero. Nonetheless, from a technological point of view, the annealing temperature of 600 °C to obtain hematite crystallographic phase is rather high. For this reason attention was also paid to this topic. The experiments in which the samples were annealed at 100 °C, 200 °C, 400 °C and 600 °C were done in order to find out the lowest possible annealing temperature still convenient for pyrite production. Although the temperatures of 100 °C and 200 °C were still too low to yield the hematite structure, the subsequent sulfurization of these samples yielded pyrite films. Nevertheless these samples still contained an undesirable amount of organic carbon. At a temperature of about 300 °C the hematite structure started to develop and based on the Raman spectra, it seemed that it is a temperature sufficient enough to obtain highly crystalline pyrite films after sulfurization. This temperature was, therefore, used for annealing of the as deposited xerogels’ films. The findings regarding the crystallographic transformations were verified by means of X-ray diffraction spectroscopy. In Fig. 2 there is a GI-XRD spectrum of the three-layers coating, annealed after each deposition at 300 °C for 1 h, followed by chemical sulfurization at 450 °C for 1 h (the films prepared under these particular conditions are identified as the PL3 type of films). Besides the pyrite crystallographic phase, no other structures were detected. The spectrum included all the peaks related to polycrystalline pyrite: 2h = 28. 51 (1 1 1), 33.08 (2 0 0), 37.10 (2 1 0), 40.78 (2 1 1), 47.40 (2 2 0), 56.27 (3 1 1). The ratios of their intensities correspond well to those from Powder Diffraction File (PDF) database, which means that any significant prevailing orientation, often connected with the internal tension of the films, did not occur. This is in good agreement with the hypotheses that the presence of the pinholes is reduced due to the low tension stress throughout the films. The pyrite crystal lattice is also visualized in Fig. 2. The crystalline size was calculated from a full width at half maximum (FWHM) of XRD peaks according to the Scherrer equation [34]. The values of one, two, and three layers coatings are summarized in Table 1. Although it should not be regarded as precise due to the relatively high standard deviation, the observed trend of the grain sizes with the thicknesses remains reasonable. It has been established previously that the thicker the film, the more rapid crystallization under the same temperature and duration. Such observation is attributed to tensile stress [35,36]. Large particles are developed at the onset of the crystallization and then remain unchanged. The average crystalline size was assessed to be around 50 nm. Furthermore, the effect of the sulfurizarion duration on the films properties was studied. The sulfurizations were running according to the procedure described in the experimental section.
172
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
Fig. 1. Raman spectra of (a) sol–gel three layers (after each coating annealed at 600 °C in air for 1 h) a-Fe203 hematite coating – HL3 type, and (b) FeS2 films obtained from hematite layers sulfurized at different temperatures for 1 h using 1 g of sulfur. In the graph: P – pyrite, M – marcasite and C – carbon.
Fig. 2. Grazing incident X-ray diffractogram of FeS2 pyrite thin film, three-layers sol–gel coating with annealing at 300 °C for 1 h after each deposition, sulfurization with one gram of sulfur at 450 °C for 1 h (PL3).
Fig. 3. Raman spectra of pyrite samples (PL3) prepared by sulfurizing the hematite films at different time durations.
Briefly, the temperature increase was 150 °C per minute with the target point at 450 °C. The dwelling time was varied between 15 min and 6 h. The somewhat surprising results were observed in the Raman spectra (seen in Fig. 3) of the tested samples (PL3). It is clearly seen from the graph that overall time of the sulfurization did not influence the crystallographic structure. The three main Raman bands are present in every spectrum. The complete chemical transformation from the hematite to pyrite was achieved after 15 min. It should be pointed out that conventional sulfurization approaches usually utilize the evacuated quartz capsules in which the sample is placed together with an appropriate amount of sulfur and sealed. Obviously, such a procedure is much more difficult compared to that introduced in this work. Fig. 3 also contains the spectrum of natural mineral pyrite supplied from Alfa Aesar. Its Raman spectrum is identical to those of all the laboratory prepared samples. These films were also a subject of XRD analysis. The results correlate with the Raman spectroscopy data. The intensity of XRD peaks (all from only pyrite phase) are almost equal indicating the same extent of crystallization.
is seen in Fig. 4. Before this analysis, the AES analysis of natural pyrite mineral was run in order to evaluate the sensitivity correction factor for the Auger peaks of Fe and S. The depth sputtering of the film started at the third cycle. The measurement revealed that the stoichiometry of the film was slightly below the ideal ratio of 2 (S):1 (Fe) and it was relatively constant through the entire film. From the measured data, and taking into account the range of precision of the AES system, the concentration of oxygen was estimated to be 0.5%. An average stoichiometry of FeO0.5S1.5 has been established according to similar data by Smestad et al. in [16]. The detected sulfur deficiency resulted in n-type semiconductor behavior. Any other contaminants were not detected within the limits of the Auger system.
3.2. Elemental depth profile Very important diagnostics of the exact chemical composition of the pyrite films were performed with help of Auger electron spectroscopy (AES). A typical depth profile Auger scan of a PL3 film
3.3. Optical properties The optical properties of the pyrite films are of the utmost interest considering their uses in photoinduced applications. Of major interest is the energy band gap value. Although several manuscripts have claimed that the material contains a direct band gap [37,13], it is believed that the band gap is indirect [38], in some cases even both [4,39]. The absorption spectra based on photothermal deflection spectroscopy are shown in the inset of Fig. 5. It is seen the exceptionally large optical absorption in the visible region (a > 105 at hm > 1.5 eV). The abrupt absorption region also indicates
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
Fig. 4. Auger electron spectroscopy elemental depth profile of the pyrite sample (PL3).
that the films are of typical semiconductor characteristic. The graph also presents the plot of (ahm)n vs. energy, with n = 1/2 for the indirect transition giving the band gap energy of 0.98 eV (PL3), which is very close to the theoretical value of 0.95 eV. A slight increase of the band gap of 1.05 and 1.08 for PL2 and PL1 respectively is certainly not due to the sulfur deficiency and instead is a consequence of the presence of oxygen. The effect of the sulfides’ band gap broadening due to the oxygen has previously been reported [40,41]. 3.4. Surface properties In Fig. 6 the surface properties of hematite films and after their thermal sulfurization are visualized as 3D topography images captured via AFM. Despite the number of hematite coatings the hematite films exhibit a smooth surface lacking significant defects. For the hematite precursive HL1, HL2, and HL3 films, the RMS roughness of 3.4 nm, 3.1 nm, and 2.8 nm was estimated, respectively. It is also clearly seen that the films consist of small grains. Contrarily, after the thermal sulfurization of these films the grain sizes increased considerably. It was reflected in a noteworthy increase of the RMS roughness, which was determined to be
Fig. 5. Band-edge absorption plot of the PL3 type of the films; the intersection of the blue line with the x-axis denotes the band gap. The inner graph – the absorption coefficient data pyrite films with different thicknesses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
173
19 nm, 27 nm, and 32 nm for PL1, PL2, and PL3 films, respectively. Moreover, besides the increase of the grain sizes and the surface roughness the film volume (thickness) expansion was also observed. As stated previously the most suitable temperature for the thermal sulfurization was in our case was 450 °C. At this temperature level the majority of elemental sulfur is in the form of quite large cyclic octaatomic S8 molecules [42]. The diameter of S8 ranges from 7.6 to 8.4 Å depending on whether the molecule exists as a ring or a chain. Thus, the volume expansion can be attributed to both the diffusion of large S8 molecules throughout the hematite precursive films and the following phase transformation. The enlargement of the grains may have also resulted from sub-grain agglomeration, since the volume expansion and the increase of the surface roughness are more distinct for the thicker films. Such transformation often produces stresses in the film and at the interface of the film and substrate. The phase transformation stress usually results in defects such as micro cracks, delaminations, and pinholes. However, none of these problems were observed in these films. This was even more evident when the samples were analyzed by SEM. Fig. 7 displays surface SEM images with a broad range of the spatial resolutions. The surface images clearly demonstrate the pinhole-free surface of the fabricated pyrite films. Moreover this result conclusively correlates with all the aforementioned findings and statements regarding the stress of the films and their morphology. 3.5. Photoelectrochemical functionality The photoinduced functionality of pyrite films was tested on the basis of photolectrochemical experiments. The methodology of the photosensitization of a wide band gap semiconductor (in this case, TiO2 with the anatase crystal phase) acting as a window layer for the low band gap absorbing pyrite in a photoelectrochemical solar cell (PEC) has been proposed previously [2,4]. When pyrite absorbs visible light the photons create an electron–hole pair. The electron is further injected to the valance band of the large gap TiO2 and thus generates a photovoltage. In this study the anatase TiO2 thin film was deposited onto an ITO substrate by means of magnetron sputtering. On the top of TiO2 the pyrite FeS2 was deposited using the spin-coating with elevated speed of rotation (3500 rpm) in order to prepare a very thin film (30 nm). This assembly was then annealed at 300 °C for 1 h to obtain the hematite Fe2O3 followed by the thermal sulfurization at 450 °C for 15 min. The TiO2 did not react with the sulfur and, moreover, it protected the ITO from sulfurization. In Fig. 7 the UV–Vis transmission spectra are shown on bare TiO2 film on the ITO substrate and with very thin film of pyrite deposited onto this structure. It can be seen that the absorption onset occurs at around 370 nm for anatase TiO2 and a shift of the onset to 850 nm occurs in case of the anatase/pyrite system. The anatase structure is verified by the Raman spectrum (inner graph of Fig. 8). One of the basic photoelectrochemical characteristics is the dependency of current density on time at a constant applied bias potential to the working electrode, photoamperometry. The applied potential provides for an enhanced separation of the electrons and holes and thus increases the overall photoefficiency. The photoamperometry data of the system TiO2/FeS2 are depicted in Fig. 9a. A cutoff filter with the transmission peak maximum at 625 nm was used to illuminate the system. The same filter was used to illuminate a single TiO2 film (without FeS2) for the purpose of excluding the effect of visible light photoactivation of the TiO2 itself. Since no response was recorded under these conditions it is possible to conclude that the photoactivity shown in Fig. 9a is due to the photosensitization of TiO2 by the pyrite. The light was chopped in order to demonstrate how fast the photoresponse is and to show both the photocurrent and the dark current. Initially,
174
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
Fig. 6. AFM images of hematite films (HL1, HL2, and HL3) and after their chemical sulfurization (pyrite films – PL1, PL2, and PL3).
Fig. 7. SEM images verifying the pinhole-free surface of the fabricated pyrite films; (a–e) SEM surface images of the PL3 type with the resolution of 100 lm, 20 lm, 5 lm, 2 lm, and 1 lm, respectively, and (f) the cross section SEM image with the resolution of 1 lm.
the photocurrent reaches the maximum of about 50 lA cm 2 which subsequently decreases to a steady state photocurrent at 45 lA cm 2 at a light intensity of 10.5 mW cm 2. It is also possible to observe a dark current, probably coming from a large density of
defect states positioned within the pyrite band gap. The result is a lowering of the band gap at the surface [43,44]. Fig. 9b shows the dependence of photovoltage on time. This graph illustrates a different kinetics of the reactions of holes and electrons. Moreover, it
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
175
is determined by the reaction of the electrons with the redox electrolyte closing the photocurrent cycle. This complex system of reactions is predominantly influenced by the junction between the interfaces such as electrolyte/pyrite, pyrite/TiO2, TiO2/ITO and thus, the surface chemistry of anatase and pyrite. The relatively low photocurrent and photovoltage values reached indicate that the films contain a higher degree of defect states both in the bulk and on the surface serving as recombination centers. Moreover, a problematic junction at the interface of TiO2/FeS2 probably also contributes to the weaker photoactivity. 4. Conclusion
Fig. 8. UV–Vis transmission spectra of bare magnetron sputtered TiO2 thin film and the bilayer system of TiO2/FeS2. The inner graph represents Raman spectrum of the magnetron sputtered TiO2 film denoting the anatase crystallographic structure.
The novel approach for the production of the pinhole-free thin films of FeS2 pyrite was described. The method is based on the sol–gel preparation of a-Fe2O3 (hematite) thin layers and their subsequent chemical sulfurization. The effects of thermal annealing of xerogel films and temperature of the sulfurization, on the properties of the pyrite films were discussed. It was shown that the temperature of sulfur annealing of 450 °C guarantees a complete transformation of hematite precursive films into the pyrite regardless of the thickness of the films after only 15 min of sulfurization. None of the usually accompanied FeS2 phases such as marcasite and trotille were revealed either by GI-XRD or Raman spectroscopy. According to the AES measurements sulfur was slightly deficient in the films owing to the presence of oxygen. The oxygen was apparently responsible for a certain broadening of the band gap, which was more evident for thicker films. Furthermore, the higher the number of coatings, the higher the thickness expansion caused by penetration of sulfur molecules during the deposition. The defect-less, pinhole-free surface of the pyrite films is apparent from SEM morphology scrutiny. Photoelectrochemical investigation of the films exhibited certain features that might be connected to a higher extent of defects acting as recombination centers resulting in relatively low photofunctionality. Although iron pyrite has many properties suitable for inexpensive, efficient solar cells, much more research is necessary in order to fulfill its promise. The impurity and stoichiometry must be improved considerably and a suitable heterojunction partner must be found. Alternatively, a p-i-n structure or a pn junction may be used if the purity problem can be solved. Acknowledgements The authors acknowledge the Academy of Sciences of the Czech Republic – Project M100101215 and the Ministry of Education, Youth and Sports of the Czech Republic – Project LH12043. References
Fig. 9. (a) Photoamperometry characteristic of the ITO/TiO2/FeS2/electrolyte photoelectrochemical system at applied potential 700 mV vs. Ag/AgCl electrode, and the light intensity of 10.5 mW cm 2 and (b) the open circuit potential measurement.
reflects the extent and kinetics of backward electron–hole pair recombination. When the light is on, the photovoltage is governed by the reaction of photogenerated holes at the pyrite/electrolyte interface that, however, compete with electron injection into the TiO2 layer [45]. During the dark sequence the photovoltage decay
[1] C. Wadia, A. Paulalvisatos, Daniel M. Kammen, Environ. Sci. Technol. 43 (2009) 2072. [2] A. Ennaoui, H. Tributsch, Sol. Cells 13 (1984) 197. [3] A. Yamamoto, M. Nakamura, A. Seki, E.L. Li, A. Hashimoto, S. Nakamura, Sol. Energy Mater. Sol. Cells 75 (2003) 451. [4] A. Ennaoui, S. Fiechter, Ch. Pettenkofer, N. Alonso-Vante, K. Buker, M. Bronold, Ch. Hopfner, H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (1993) 289. [5] A. Gomes, J.R. Ares, I.J. Ferrer, M.I. da Silva, Pereira, C. Sanchez, Mater. Res. Bull. 38 (2003) 1123. [6] I.J. Ferrer, J.R. Ares, C. Sanchez, Sol. Energy Mater. Sol. Cells 76 (2003) 183. [7] S. Guo, D.P. Young, R.T. Macaluso, D.A. Browne, N.L. Henderson, J.Y. Chan, L.L. Henry, J.F. Ditusa, Phys. Rev. B 81 (2010) 144424. [8] J.R. Ares, A. Pascual, I.J. Ferrer, C.R. Sanchez, Thin Solid Films 451–452 (2004) 233. [9] P.P. Altermatt, T. Kiesewetter, K. Ellmer, H. Tribtsch, Sol. Energy Mater. Sol. Cells 71 (2002) 181. [10] K. Büker, N. Alonso-Vante, H. Tributsch, J. Appl. Phys. 72 (1992) 5721. [11] R. Schieck, A. Hartmann, S. Fiechter, R. Konenkamp, H. Wetzel, J. Mater. Res. 5 (1990) 1567.
176
S. Kment et al. / Journal of Alloys and Compounds 607 (2014) 169–176
[12] S. Fiechter, M. Birkholz, A. Hartmann, P. Dulski, H. Tributsch, R.J.D. Tikkey, J. Mater. Res. 7 (1992) 1829. [13] C. Wadia, Y. Wu, S. Gul, S.K. Volkman, J. Guo, A.P. Alivisatos, Chem. Mater. 21 (2009) 2568. [14] D. Lichtenberger, K. Ellmer, R. Schieck, S. Fiechter, Appl. Surf. Sci. 70–71 (1993) 583. [15] C. de las Heras, J.L.M. de Vidales, I.J. Ferrer, C. Sanchez, J. Mater. Res. 11 (1996) 211. [16] G. Smestad, A. Ennaoui, S. Fiechter, H. Tributsch, W.K. Hofmann, M. Birkholz, W. Kautek, Sol. Energy Mater. 20 (1990) 149. [17] B. Meester, L. Reijnen, A. Goossens, J. Schoonman, Chem. Vapor Dep. 6 (2000) 121. [18] S. Nakamura, A. Yamamoto, Sol. Energy Mater. Sol. Cells 65 (2001) 79. [19] A.M. Karguppikar, V.G. Vedeshawar, Phys. Status Solidi A 95 (1986) 717. [20] C. de las Heras, C. Sanchez, Thin Solid Films 199 (1991) 259. [21] L.Y. Huang, F. Wang, Z.J. Luan, L. Meng, Mater. Lett. 64 (2010) 2612. [22] B. Kilic, J. Roehling, O.T. Ozmen, J. Nanoelectron. Opt. 8 (2013) 260. [23] F. Wang, L.Y. Huang, Z.J. Luan, J.S. Huang, L. Meng, Mater. Chem. Phys. 132 (2012) 505. [24] Ch. Park, J. Walker, R. Tannenbaum, A.E. Stiegman, J. Frydrych, L. Machala, Appl. Mater. Interfaces 1 (2009) 1843. [25] W.B. Jackson, N.M. Amer, A.C. Boccara, D. Fournier, Appl. Opt. 20 (1981) 1333. [26] S. Kment, P. Kluson, Z. Hubicka, J. Krysa, M. Cada, I. Gregora, A. Deyneka, Z. Remes, H. Zabova, L. Jastrabik, Electrochim. Acta 55 (2010) 1548–1556. [27] S. Kment, P. Kluson, V. Stranak, P. Virostko, J. Krysa, M. Cada, J. Pracharova, M. Kohout, M. Morozova, P. Adamek, Z. Hubicka, Electrochim. Acta 54 (2009) 3352–3359.
[28] S. Kment, H. Kmentova, P. Kluson, J. Krysa, Z. Hubicka, V. Cirkva, I. Gregora, O. Solcova, L. Jastrabik, J. Colloid Interface Sci. 348 (2010) 198–205. [29] M. Morozova, P. Kluson, J. Krysa, M. Zlamal, O. Solcova, S. Kment, T. Steck, J. Sol–gel Sci. Technol. 52 (2009) 398. [30] P. Prabukanthan, R.J. Soukup, N.J. Ianno, A. Sarkar, S. Kment, et al., IEEE Photovoltaic Spec. Conf. (2010) 2965. [31] R.J. Soukup, P. Prabukanthan, N.J. Ianno, A. Sarkar, C.A. Kamler, D.G. Sekora, J. Vac. Sci. Technol. A 29 (2011). Article Number: 011001. [32] C. Barrato, P.P. Lottici, D. Bersani, G. Antonioli, J. Sol–gel Sci. Technol. 13 (1998) 667. [33] P. Terrence, P. Mernagh, A.G. Trudu, Chem. Geol. 103 (1993) 113. [34] H.G. Jiang, M. Ruhle, E.J. Lavernia, J. Mater. Res. 14 (1999) 549. [35] R. Kuzel, L. Nichtova, Z. Matej, D. Heiman, J. Musil, Z. Kristallogr. 26 (2007) 247. [36] R. Kuzel, L. Nichtova, Z. Matej, J. Musil, Thin Solid Films 519 (2010) 1649. [37] Y.Z. Dong, Y.F. Zheng, H. Duan, Y.F. Sun, Y.H. Chen, Mater. Lett. 59 (2005) 2398. [38] F. Wang, L. Huang, Z. Luan, J. Huang, L. Meng, Mater. Chem. Phys. 132 (2012) 505. [39] H.A. Macpherson, C.R. Stoldt, ACS Nano 6 (2012) 8940. [40] R. Robles, A. Vega, A. Mokrani, Opt. Mater. 17 (2001) 497. [41] N. Barreau, J.C. Berne’de, S. Marsillac, A. Mokrani, J. Cryst. Growth 235 (2002) 430. [42] B. Meyer, Chem. Rev. 76 (1976) 367. [43] R. Murphy, D.R. Strongin, Surf. Sci. Rep. 64 (2009) 1. [44] Y. Bi, Y. Yuan, C.I. Exstrom, S.A. Darveau, J. Huang, Nano Lett. 11 (2011) 4953. [45] E. Ennaoui, S. Fiechter, H. Tributsch, M. Giersig, R. Vogel, H. Weller, J. Electrochem. Soc. 139 (1992) 2514.