Photoelectrochemically deposited Sb₂Se₃ thin films: deposition mechanism and characterization

Abstract

Sb₂Se₃ thin films were photoelectrochemically deposited (PED) and displayed a compelling photoelectrochemical (PEC) performance. The main influence of the illumination mechanism on Sb₂Se₃ deposition is that the photoconductive effect accelerates the deposition rate and the photogenerated electrons (in the conduction band of the deposited Sb₂Se₃ thin film) promote the electroreduction of SbO⁺. Electrochemical impedance spectroscopy (EIS) and potentiostatic polarization show evidence that illumination can promote the rate of cathodic reduction. Linear sweep photovoltammetry (LSPV) and X-ray fluorescence spectrometry (XRF) indicate that illumination facilitates the reduction of SbO⁺. The PED process can improve the homogeneity and compactness of the films, facilitate the growth of stoichiometric Sb₂Se₃ and further enhance the photocurrent response of films, compared to the conventional electrochemical deposition (CED) process.

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Sb₂Se₃ is widely considered as a new non-toxic and earth-abundant light absorber with an excellent light absorption coefficient (>10⁵ cm⁻¹ at short wavelengths) and suitable band gap (approximately 1.1–1.3 eV).^1,2 It is beneficial for the low-cost and large-scale production of thin film solar cells. In addition, sensitized cells,^3,4 solid-state cells⁵ and photoelectrochemical cells⁶ based on Sb₂Se₃ (or Sb₂S₃) thin films have been fabricated and displayed a good performance, which draws worldwide attention to the photoresponse property of Sb₂Se₃.

Several methods have been employed to fabricate Sb₂Se₃ thin films such as vacuum thermal evaporation,⁵ chemical bath deposition,⁷ successive ionic layer adsorption and reaction methods,⁸ spray pyrolysis,⁹ pulsed laser deposition¹⁰ and electrochemical deposition.^3,11–13 Among these methods, electrochemical deposition (or electrodeposition) is reported to be a very simple¹⁴ and versatile³ process for semiconductor film fabrication due to economic and convenient considerations. However, because there are different deposition potentials for various ions, it is difficult to control composition¹⁵ for compound semiconductor electrodeposition. Low conductive semiconductors grown on cathodes also lead to some problems¹⁶ such as low growth rate and increased ohmic potential drop at the electrode during deposition.

Photoelectrochemical deposition (PED) is developing as a general preparative strategy, which opens up new avenues for the synthesis of compound semiconductors. Due to the photosensitivity of deposited semiconductor films,^15,17–20 illumination has a great influence on the enhancement of deposition current and the inhibition of ohmic potential drop at the electrode. Therefore, the PED process has attracted researchers' attention and has been explored by many groups.^{16,19,21–26} Moreover, we conducted a research¹⁶ that demonstrates the PED process benefits high quality and well performing CuInSe₂ semiconductor thin film fabrication. This study is mainly focused on revealing the deposition mechanism and characterizing the effects of illumination on electrodeposited Sb₂Se₃ thin films.

According to the transmittance of the electrolyte solution (ESI, Fig. S1†), light in the wavelength range of 900–2000 nm with a low transmittance value is absorbed by electrolyte solutions for calefaction (photothermal effect),²⁷ but light in the wavelength range of 300–900 nm with a very high transmittance value can be absorbed by as-deposited films for exciting electrons and holes^16,19 (photoelectric effect). For xenon lamp spectra, light in the wavelength range of 300–900 nm covers almost the total emitted light energy.²⁸ Consequently, the influence of illumination on electrochemical behavior is dominated by the photoelectric effect rather than the photothermal effect.

The potential–energy diagram drawn in Fig. 1 illustrates the photoelectric effects of illumination on Sb₂Se₃ electrodeposition mechanisms. The relative position of the conduction and valence band edges in the Sb₂Se₃ layer is adopted from the literature.²⁹ The beginning reduction potential of H₂SeO₃ and SbO⁺ are about 0.05 V and −0.43 V, respectively, and thereby the deposition of Sb is more difficult¹⁴ than that of Se. Once Sb₂Se₃ is potentiostatic cathodic deposited (e.g. at −0.55 V) on the surface of the cathode, the Fermi level of Sb₂Se₃ move along the positive direction to a location equal to this cathodic potential. Thus, band alignment in deposited Sb₂Se₃ favors electron flow from the conduction band to the electrolyte and hole injection into the valence band from the electrolyte. The driving force for the electroreduction of SbO⁺ and H₂SeO₃ is due to the difference between the Fermi level of deposited Sb₂Se₃ and the relevant redox potential in the electrolyte.

Fig. 1 Illumination influence on the potential–energy diagram during Sb₂Se₃ electrodeposition.

In the conventional electrochemical deposition (CED) process, majority carriers (holes) and very few minority carriers (electrons at the conduction band) exist in p-type Sb₂Se₃, which means that only holes act as charge carriers and electroreduction is inhibited by deposited Sb₂Se₃. The only reduction route is that positive charges inject into the Sb₂Se₃ film from H₂SeO₃ and SbO⁺. Moreover, the energy level difference between the surface valance band and redox is the energy barrier for the electroreduction of H₂SeO₃ and SbO⁺. It can be noted that the reduction of SbO⁺ is much more difficult than that of H₂SeO₃, because the SbO⁺/Sb redox has a higher energy barrier and lower driving force relative to the H₂SeO₃/Se redox.

In the PED process, both the majority carriers (holes and photogenerated holes) and minority carriers (photogenerated electrons) act as charge carriers, and the photogenerated holes and photogenerated electrons improve the conductivity²⁰ (photoconductive effect) of the cathode and inhibit the actual potential drop (for potentiostatic electrodeposition) of the film/electrolyte interface. Therefore, compared to the CED process, the electroreduction of species in the electrolyte is enhanced and the Sb₂Se₃ growth rate is promoted accordingly. Furthermore, SbO⁺ can receive photogenerated electrons from the conduction band directly, which is driven by the bending band without any energy barrier, and be easily reduced to Sb. The newly formed Sb may further react with electroreduced Se, according to eqn (1), due to the large Gibbs energy release (−135 kJ mol⁻¹),³⁰ which further accelerates the deposition rate.

2Sb + 3Se = Sb₂Se₃ (1)

Consequently, from the potential–energy diagram, we may draw the conclusion that there are two effects of illumination on the electrodeposition of Sb₂Se₃: the first is accelerating the deposition rate; the second is promoting the electroreduction of SbO⁺ by changing the electroreduction route.

To understand the underlying mechanism, Fig. 2 shows the Nyquist plots of the CED process (a) and PED process (b) after different potentiostatic polarization times (at −0.55 V). The main EIS difference between the CED process and PED process is that there exists obvious low-frequency Warburg diffusion resistance in the Nyquist plot of the latter. The difference in EIS indicates that the CED process is electroreduction controlled, whereas the PED process is simultaneously controlled by electroreduction and diffusion.^31–33 Combined with the smaller diameter of the high-frequency semicircle, these phenomena give the conclusion that the electroreduction rate in the PED process is faster than that in the CED process. An equivalent circuit (Fig. 2(c)) was also designed to simulate the deposition³³ occurring at the cathode/electrolyte interface. R_ct is charge transfer resistance of the cathode/electrolyte interface, C is the capacitance of the electrical double layer at the electrode, R_s is the ohmic resistance of the electrolyte and W_s is the Warburg diffusion resistance.

Fig. 2 Nyquist plots of (a) CED process and (b) PED process after different potentiostatic polarization time; (c) equivalent circuit designed to simulate the deposition; (d) linear sweep photovoltammogram on an SnO₂ substrate under chopped illumination at a scan rate of 2 mV s⁻¹ (black solid line is measured value; blue/red dashed line is fitted value under dark/illumination, respectively).

The values of R_ct, C, R_s and W_s for both systems are illustrated in Table 1.

Table 1 Parameters used for fitting the EIS data in Fig. 2(a) and (b)

	CED			PED
	4 min	8 min	12 min	4 min	8 min	12 min
Rct	145.2	221.3	531.3	117.7	158.4	171.7
C	1.1 × 10^−4	1.0 × 10^−4	1.4 × 10^−4	7.4 × 10^−5	7.8 × 10^−5	6.6 × 10^−5
Rs	59.2	58.6	55.4	57.5	67.7	84.1
Ws	113.4	102.3	129.3	665.2	270.6	197.3

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For the CED process, with the increase of polarization time, R_s and W_s remained relatively steady (below 60 Ω and about 110 Ω, respectively), which can be interpreted by the relatively steady electrolyte status. However, the diameters of the capacitive semicircles obviously increase and R_ct increased accordingly (145.2 Ω for 4 min, 221.3 Ω for 8 min and 531.3 Ω for 12 min) due to the decrease in conductivity of the cathode (low electrical conductive Sb₂Se₃ ³⁴ covering about 10⁻⁶ to 10⁻² Ω⁻¹ m⁻¹).

For the PED process, photogenerated carriers promote the cathodic reduction, which further results in a lower R_ct³² (117.7 Ω for 4 min, 158.4 Ω for 8 min and 171.7 Ω for 12 min) than that for the CED process. C for PED (70 μF cm⁻²) is lower than that for CED (100 μF cm⁻²) probably due to the smoother surface (ESI, Fig. S3†) of the PED thin film. However, with the increase of polarization time, the slight increase of R_ct (from 117.7 Ω to 171.7 Ω) still relates to the deposited film with low electrical conductivity (even under illumination). R_s increase³⁵ (from 57.5 Ω to 84.1 Ω) can be attributed to ion consumption in the electrolyte and W_s decrease (from 665.2 Ω to 197.3 Ω) mainly results from the low concentration gradient.

Fig. 2(d) shows the linear sweep photovoltammogram under chopped illumination from 0.40 V to −0.80 V (negative scanning) with an interval of 5 s. The curve displays an initial reduction peak at about −0.09 V, which can be assigned to the four-electron predeposition of selenium.^14,24 The reduction peak at around −0.65 V (blue dash line, off) mainly corresponds to the reduction of SbO⁺ and the subsequent formation of Sb₂Se₃,^14,24 which positively shifts to −0.48 V under illumination (red dash line, on). The 0.17 V positive shift of the reductive peak indicates that the illumination facilitates the reduction of SbO⁺ (also shown in Fig. 1). In addition, a higher current density is obtained, which indicates that the reduction of SbO⁺ and the according formation of Sb₂Se₃ can be significantly promoted under illumination.

The largest photocurrent response is observed at the potential between −0.45 V and −0.55 V, which reveals that the significant influence of illumination on Sb₂Se₃ film deposition can be observed at this potential region. Hence, Sb₂Se₃ was deposited at −0.45 V and −0.55 V for further research.

The composition analysis by XRF shows that the Sb content is higher in PED Sb₂Se₃ films (41.09 at%, deposited at −0.45 V and 44.10 at%, deposited at −0.55 V) than that in CED Sb₂Se₃ films (4.87 at%, deposited at −0.45 V and 22.04 at%, deposited at −0.55 V). This finding furnishes additional evidence in support of the illumination promoting the electroreduction of SbO⁺ with a more negative reduction potential compared to H₂SeO₃. Moreover, films with a similar stoichiometry of Sb : Se = 2 : 3 can be easily obtained by the PED process.

The enhanced deposition rate of Sb₂Se₃ can be also verified by the higher deposition current (ESI, Fig. S2†) and the thicker film obtained under illumination. The thicknesses of the films are 0.05 μm/0.60 μm deposited at −0.45 V and 0.1 μm/0.70 μm deposited at −0.55 V for CED/PED, respectively. From the SEM micrographs (ESI, shown in Fig. S3†), it can be observed that the PED Sb₂Se₃ films have a more homogeneous and smooth morphology. According to the analysis of the visual images (ESI, shown in Fig. S3† insert), −0.55 V is beneficial to the film deposition because the films electrodeposited at −0.45 V are much more transparent than the films electrodeposited at −0.55 V.

Fig. 3(a) shows the Raman spectra of the Sb₂Se₃ films deposited by PED and CED at −0.55 V. The characteristic peaks from Sb₂Se₃ can be observed at^36–41 83 cm⁻¹, 118 cm⁻¹, 189 cm⁻¹, 253 cm⁻¹, 372 cm⁻¹ and 450 cm⁻¹. All the peaks of the PED film exhibit a narrower peak width (more acute shape) and greater peak intensity than those of the CED film, which generally indicates the enhancement of crystallinity.^41,42 The main peaks at about 189 cm⁻¹ and 253 cm⁻¹ correspond to the vibrations^36,41 related to Sb bonds. Therefore, the obvious enhancement of main peak intensity should be related to the increase of Sb content (from 22.1 at% to 44.1 at%).

Fig. 3 (a) Raman patterns of Sb₂Se₃ thin films prepared by PED and CED at −0.55 V; (b) X-ray diffraction patterns of CED and PED Sb₂Se₃ samples after RTA; (c) the optical characteristics of as-deposited Sb₂Se₃ films prepared by PED and CED and inset shows the estimated optical band gap; (d) photocurrent–potential response curve of the Sb₂Se₃ films prepared by PED (red line) and CED (black line) in 0.5 M H₂SO₄.

The XRD patterns of the CED and PED Sb₂Se₃ thin films (on SnO₂/glass) after RTA (rapid thermal annealing treatment) are shown in Fig. 3(b). It can be seen that polycrystalline films of antimonselite phase⁴ (Sb₂Se₃ (JCPDS no. 65-2433)) are achieved for both the CED and PED thin films. The intensity of the SnO₂ (JCPDS no. 77-0452) diffraction peaks for the PED thin film is weaker than that for the CED thin film, due to the difference thicknesses of the two types of films. It can also be noticed that the Sb₂Se₃ diffraction intensity of the PED thin film is stronger than that of the CED thin film, which indicates an improvement in crystalline quality by photoelectrochemical deposition. Similar phenomena have also been reported.⁴³ The improvement in crystalline quality can partly be related to the difference of antimony content in these two types of films.⁴⁴

The absorption coefficient, α, and optical band gap (in insert, estimated from the interception of the linear fitting) of the CED and PED Sb₂Se₃ films are both shown in Fig. 3(c). Because the component of the CED Sb₂Se₃ film deviates from stoichiometry, this film shows a low absorption coefficient, α, (about 8 × 10⁴ cm⁻¹ in visible light) and narrow band gap value (0.93 eV). The PED Sb₂Se₃ film shows excellent optical property (absorption coefficient α = 1.4 × 10⁵ cm⁻¹ in visible light, band gap value E_g = 1.37 eV), which is close to the value of single crystal Sb₂Se₃.^45,46 This excellent optical property can meet the needs for thin film solar cell materials.⁴⁷

The obvious difference in the PEC performance of the PED and that of the CED Sb₂Se₃ films is shown in Fig. 3(d). Both films are identified as p-type semiconductors due to their photocurrent density increase with a negative shift of the cathodic potential. At negative bias, the dark current (off) of the PED film is lower than that of the CED film, which implies that the PED film shows better rectification.⁴⁸ Since the rectification behaviour is considered to be limited by the deficiency, we may conclude that the deficiency (exposed substrate, impurities (undetected excess Se phase), cracks or boundary between the granules) in the PED film is much less than that in the CED film. This conclusion is in accordance with the compact morphology and near stoichiometry of the PED sample.

To confirm the conductivity of the deposited Sb₂Se₃ thin films again, photovoltage measurement^49,50 was employed by determining the difference between the illuminated and dark voltages of the Sb₂Se₃ electrode with respect to the counter electrode. The p- and n-type silicon wafers were used for comparison and calibration of the system. It is shown from Table 2 that the positive photovoltage demonstrates the p-type bulk conductivity of the deposited Sb₂Se₃ thin films. In agreement with the difference in photocurrent, the PED Sb₂Se₃ thin film has a higher photovoltage than that of CED Sb₂Se₃ thin film.

Table 2 Photovoltage produced at the Sb₂Se₃ electrode/electrolyte junction during light illumination

Sample	Measurement		Analysis
	Vdark/mV	Vlight/mV	Vphoto/mV	Conductivity
p-Si	−615	−573	42	p-type
n-Si	−330	−507	−177	n-type
PED	−352	−265	87	p-type
CED	−371	−359	12	p-type

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Based on the abovementioned analysis, the PED and CED Sb₂Se₃ films are both identified as p-type semiconductors. Its stronger and more sensitive photoresponse suggests that the PED Sb₂Se₃ film has better photon-to-electron conversion ability.

Conclusions

This study demonstrates the beneficial effects of illumination on Sb₂Se₃ electrodeposition. The influence mechanism has been illustrated by a schematic, which is supported by many experiments: the electroreduction of SbO⁺ with a more negative reduction potential is promoted and the deposition rate of Sb₂Se₃ is further increased. Moreover, PED improves the content of antimony, homogeneity, compactness in morphology and Sb₂Se₃ crystallinity. As a result, Sb₂Se₃ films with excellent optical properties and photon-to-electron conversion ability can be obtained by PED. Accordingly, PED is indeed a very competitive strategy and PED Sb₂Se₃ films should be suitable for high efficiency solar cells application.

Experimental section

Deposition and analysis

Electrochemical experiments were carried out in a stagnant three-electrode Pyrex electrolytic cell configuration at 25 °C with a SnO₂-coated glass substrate (SnO₂/glass, 20 Ω sq⁻¹) as the working electrode, a Pt gauze as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. All potentials are reported with respect to this reference electrode. SnO₂/glass substrates were ultrasonically cleaned in acetone, ammonia and alcohol, then rinsed with deionized water (18.2 MΩ cm⁻¹), and subsequently dried in a nitrogen flow. Linear sweep photovoltammetry (LSPV), PED and CED were performed using a Princeton Applied Research PARSTAT 4000 Potentiostat, and electrochemical impedance spectroscopy (EIS) was performed using a Princeton Applied Research PARSTAT 4000 EIS analyzer. An electrolyte solution containing 5.5 mM K(SbO)C₄H₄O₆·0.5H₂O (antimony potassium tartrate), 4.5 mM H₂SeO₃, and 100 mM NH₄Cl was used for LSPV, PED, CED and EIS. The pH of the electrolyte solution was adjusted to 2.3 using concentrated hydrochloric acid. Linear sweep photovoltammograms were measured at a scan rate of 2 mV s⁻¹ under chopped illumination. Sb₂Se₃ films were prepared by PED and CED and all the films were deposited for 30 min potentiostatically. EIS measurements were carried out under the amplitude of 10 mV at −0.55 V with a frequency range of 100 kHz to 0.1 Hz. When it was necessary, a Newport 300 W xenon lamp was used as the light source and the light intensity was at 100 mW cm⁻².

Characterization

The chemical composition, morphology and phase of the films were characterized using energy dispersive X-ray fluorescence spectrometry (XRF, Shimadzu, LAB CENTER XRF-1800, operated at 40 kV, 95 mA), environmental scanning electron microscopy (ESEM, FEI Quanta-200, at a 20 keV accelerating voltage) and Raman spectrometry (Jobin-Yvon LabRAM HR-800, Horiba), respectively. The thickness of the films was measured using a Veeco Dektak 150 surface profiler. The crystalline properties of the prepared films after RTA (rapid thermal annealing treatment in a flowing Ar atmosphere (20.00 sccm) at 300 °C for 3 min) were characterized using an X-ray diffractometer (XRD, Rigaku3014). The optical properties of the films and electrolytes were measured using a UV-VIS-NIR spectrophotometer (UV-VIS-NIR, Varian Cary-5000) in the wavelength range of 300 nm–2000 nm at room temperature.

The photoelectrochemical (PEC) property measurement was performed in a three-electrode configuration containing 0.5 M H₂SO₄ solution (also at a scan rate of 2 mV s⁻¹, under chopped illumination) to determine photon-to-electron conversion ability, using a Princeton Applied Research PARSTAT 4000 Potentiostat. The three-electrode configuration consisted of Sb₂Se₃ deposited on SnO₂/glass, a Pt gauze and a saturated calomel electrode (SCE), which acted as the working electrode, counter electrode and reference electrode, respectively. Photovoltage measurements were performed in the two-electrode configuration, which only consisted of a working electrode (Sb₂Se₃ deposited on SnO₂/glass) and a counter electrode (a Pt gauze).

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 51222403 and 51272292) and the Hunan Provincial Natural Science Foundation of China (13JJ1003).

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Footnote

† Electronic supplementary information (ESI) available: The transmittance of electrolytes, potentiostatic polarization curves for deposition process, SEM and visual photographs for deposited films. See DOI: 10.1039/c5ra16055c

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