Eﬃcient room temperature aqueous Sb 2 S 3 synthesis for inorganic–organic sensitized solar cells with 5.1% efficiencies †

Sb 2 S 3 sensitized solar cells are a promising alternative to devices employing organic dyes. The manufacture of Sb 2 S 3 absorber layers is however slow and cumbersome. Here, we report the modified aqueous chemical bath synthesis of Sb 2 S 3 absorber layers for sensitized solar cells. Our method is based on the hydrolysis of SbCl 3 to complex antimony ions decelerating the reaction at ambient conditions, in contrast to the usual low temperature deposition protocol. This simplified deposition route allows the manufacture of sensitized mesoporous-TiO 2 solar cells with power conversion eﬃciencies up to g = 5.1%. Photothermal deflection spectroscopy shows that the sub-bandgap trap-state density is lower in Sb 2 S 3 films deposited with this method, compared to standard deposition protocols.

Sb 2 S 3 sensitized solar cells are a promising alternative to devices employing organic dyes.The manufacture of Sb 2 S 3 absorber layers is however slow and cumbersome.Here, we report the modified aqueous chemical bath synthesis of Sb 2 S 3 absorber layers for sensitized solar cells.Our method is based on the hydrolysis of SbCl 3 to complex antimony ions decelerating the reaction at ambient conditions, in contrast to the usual low temperature deposition protocol.This simplified deposition route allows the manufacture of sensitized mesoporous-TiO 2 solar cells with power conversion efficiencies up to g = 5.1%.Photothermal deflection spectroscopy shows that the sub-bandgap trap-state density is lower in Sb 2 S 3 films deposited with this method, compared to standard deposition protocols.
Antimony sulfide (Sb 2 S 3 ) is a promising material for several optoelectronic applications.Due to its high absorption coefficient (a E 1.8 Â 10 5 cm À1 at l = 450 nm) and a suitable direct bandgap of E g E 1.7 eV, crystalline Sb 2 S 3 (stibnite) is interesting as light absorber for solid-state sensitized solar cells (Fig. 1). 1,2In particular, Sb 2 S 3 -based solar cells excel in their stability of operation when compared to other organic-inorganic hybrid devices.Recently, power-conversion efficiencies of Z = 6.2% (ref.3) and Z = 7.5% (ref.4) were achieved using Sb 2 S 3 as the absorber material obtained from chemical bath deposition.Further, the material has been used to improve the stability of methyl-ammonium lead iodide perovskite solar cells. 57][8][9] The standard method is the aqueous chemical bath deposition (CBD) using antimony chloride and sodium thiosulfate.This technique is however problematic since it requires a precise temperature control of the solution when cooling below 10 1C and maintaining the sample at low temperatures.For large-scale applications such a cooling protocol is cumbersome, costly and energy-intensive.
Here, we present an aqueous room temperature (RT) deposition route of Sb 2 S 3 using the same precursor materials as the standard CBD method.We have fabricated Sb 2 S 3 -sensitized solar cells using this RT deposition method and demonstrate excellent device performance with efficiencies of up to Z = 5.1%.
The low-temperature synthesis of Sb 2 S 3 for photovoltaic applications involves the chemical reaction equations 7 These reactions have to be slowed down by cooling below 10 1C to avoid immediate precipitation 7 and to enable strong adhesion of Sb 2 S 3 to the substrate.Thus, the standard CBD method, termed low-T deposition, requires cooling of the reaction solution, whereas our modified method, RT deposition, can be performed at room temperature.By changing the order of reactant addition at RT, Sb 2 S 3 formation is slowed down and well-adhering films are obtained.
For the RT deposition, a 1.4 M SbCl 3 solution in acetone was prepared.Note that the optimal concentration of SbCl 3 for the RT method is slightly higher compared to low-T deposition (Fig. S7, ESI †).SbCl 3 can be used without dissolution in acetone, changing the reaction behaviour very little (Fig. S8, ESI †).The addition of acetone facilitates however the handling of the highly hygroscopic SbCl 3 .Deionised water is added under vigorous stirring to reduce the total concentration of SbCl 3 to 46 mM.The addition of water hydrolyses SbCl 3 , which leads to a solid white precipitate.The product of the hydrolysis reaction of SbCl 3 is not very well defined.It depends on many parameters such as the dilution of the reaction medium, the pH value of the solution and solvent composition. 11,12The aqueous solution containing the hydrolysed SbCl 3 has a pH of 1.4.
The precipitate was filtered and dried and X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and energy-dispersive X-ray (EDX) spectroscopy were performed on the white powder (Fig. S10 13,14 They also reporting the formation of a white precipitate, which they identify as antimony oxychloride SbOCl.The XRD pattern of Fig. S10 (ESI †) however shows no evidence of crystalline SbOCl formation.According to Chen et al. hydrolysis at pH 1-2 leads to the formation of Sb 4 O 5 Cl 2 for mixed solvents such as water and ethanol or water and ethylene glycol. 12We also do not observe an immediate colour change of the precipitate to orange upon the addition of the sulphur source, as reported by Li et al. 13 and Yu et al. 14 Subsequently, a 1 M aqueous Na 2 S 2 O 3 solution was added at a final concentration of 0.25 M in the chemical bath.This causes the solution to turn clear as most of the precipitate dissolves, suggesting the formation of a water-soluble complex.After 5-10 min at 20 1C, the solution starts to turn orange, indicating the formation of amorphous Sb 2 S 3 .This is accompanied by a pH change of the solution from pH = 3.3 upon sodium thiosulfate (pH = 7.3) addition to pH = 4.3 when antimony sulfide deposition is complete after two hours.The annealed antimony sulfide was characterised by powder X-ray diffraction (XRD).Fig. 3a compares the XRD pattern of the low-T and RT deposition methods.Both pattern are very similar and match the stibnite reference pattern. 15Sb 2 S 3 films deposited by the RT method onto mesoporous TiO 2 substrates also show the characteristic XRD peaks of crystalline Sb 2 S 3 (Fig. S3, ESI †).A Rietveld analysis of the two patterns using the powder diffraction software ReX 16    This journal is © The Royal Society of Chemistry 2015 layer can also be harvested due to the formation of PCBM electron conducting channels.The PCBM-PCPDTBT blend thus contributes to the current and causes the near-infrared shoulder in the EQE spectrum. 17Earlier cells using P3HT as hole conductor showed lower efficiencies compared to the PCPDTBT:PCBM blend.We therefore concentrated on the donor-acceptor PCPDTBT-PCBM blend.A comparison of a RT solar cell and a solar cell with the same device architecture using the low-temperature deposition method is shown in the ESI † (Fig. S4).The reference cells were prepared in the same laboratory using the identical procedures as the better performing earlier published devices. 4The lower performance may reflect the variation in this system that possibly arises from minor variations in device fabrication.It is however important to point out that the reference devices and devices made with our new methodology were created in parallel and therefore are much less likely to be subject to this type of variation.
X-ray photoelectron spectroscopy (XPS) measurements were carried out on Sb 2 S 3 films formed by both deposition techniques, shown in Fig. 3b.
To compare the oxide content of the samples, the antimony Sb3d 3/2 peak was examined, because the oxygen O1s peak directly overlaps with the antimony Sb3d 5/2 peak.The Sb3d 3/2 peak can be modelled using a superposition of two Gaussians, one at E538.5 eV representing Sb 2 S 3 and one at E539.5 eV for SbO x , most likely Sb 2 O 3 . 18The RT sample has a marginally higher oxide content compared to the low-T material.This probably causes the lower conductivity seen in these films (Fig. S6, ESI †).
One of the biggest challenges of using antimony sulfide as absorber in sensitized solar cells is the high density of electronic traps in this material, i.e. the number of energy states which lie in the band-gap of Sb 2 S 3 . 4,19These trap-states lead to a significant loss in potential and to charge carrier recombination in the solar cell.To explore this, we employed photo-thermal deflection spectroscopy (PDS) to determine the trap-state density and the energetic disorder of Sb 2 S 3 .PDS is a highly sensitive absorption measurement technique, which can detect absorbance values down to 10 À5 AU.Thus, PDS is able to accurately measure weak absorption in the bandgap.Fig. 3c shows the PDS spectra of Sb 2 S 3 samples on mesoporous TiO 2 for both deposition methods.The corresponding Urbach energies are given in the inset.
The absorption in the RT-deposited sample was significantly lower at energies below the band-gap of Sb 2 S 3 compared to the low-T sample, by nearly one order of magnitude at energies below 1.5 eV.This indicates a clear reduction in the density of deep-trap states for the RT deposited Sb 2 S 3 .The difference in the opencircuit voltage for low-T and RT deposited Sb 2 S 3 in optimized solar cells are however similar (Fig. S4, ESI †).As the band-gap of antimony oxide is higher than that of Sb 2 S 3 , 20 the PDS spectrum cannot show a potential increase of deep traps caused by the higher content of antimony oxide in the RT-deposited sample.
We have demonstrated an aqueous deposition technique of antimony sulfide for sensitized solar cells, which can be carried out at room temperature.The chemical bath deposition method is based on the same precursor materials but uses the hydrolysis of SbCl 3 to complex antimony ions.The resulting Sb 2 S 3 films were investigated using UV-vis spectroscopy, XRD, PDS and XPS.PDS shows a reduction in sub-band gap trap states in RT-deposited Sb 2 S 3 .Manufactured devices achieved a maximum power conversion efficiency Z = 5.1% for Sb 2 S 3 sensitized solar cells using the RT deposition method.A more detailed optimization of the deposition step, interfacial surface treatments 21,22 or doping of the Sb 2 S 3 23 could lead to a further

Fig. 1
Fig. 1 (a) Schematic of the solar cell cross section.(b) Simplified banddiagram of the Sb 2 S 3 sensitised photovoltaic cells.The band edge values were taken from ref. 10.

Fig.
Fig. S2 (ESI †) shows the UV-vis spectra of RT-deposited Sb 2 S 3 films on mesoporous-TiO 2 , annealed at 300 1C for 5 min.The annealed antimony sulfide was characterised by powder X-ray diffraction (XRD).Fig.3acompares the XRD pattern of the low-T and RT deposition methods.Both pattern are very similar and match the stibnite reference pattern.15Sb 2 S 3 films deposited by the RT method onto mesoporous TiO 2 substrates also show the characteristic XRD peaks of crystalline Sb 2 S 3 (Fig.S3, ESI †).A Rietveld analysis of the two patterns using the powder diffraction software ReX16 yields an average Sb 2 S 3 crystallite size of 40 nm and 35 nm for RT and low-T deposition, respectively.The structure of the solar cell studied in this work and a schematic band diagram are shown in Fig. 1.Fig. 2 shows (a) the current-voltage-characteristic and (b) the external quantum efficiency EQE of the best performing solar cell employing RT deposited Sb 2 S 3 .Its photovoltaic parameters are summarised in Fig. 2. The shunt and series resistances were obtained by a least square fit of the diode function.The RT method enables the fabrication of solar cells with high reproducibility.The deviations from batch to batch and from device to device were small (Fig. S13, ESI †).The hole transport layer consists of a PCPDTBT-PCBM blend.Charge carriers which are generated by photon absorption in the hole transport Fig. S2 (ESI †) shows the UV-vis spectra of RT-deposited Sb 2 S 3 films on mesoporous-TiO 2 , annealed at 300 1C for 5 min.The annealed antimony sulfide was characterised by powder X-ray diffraction (XRD).Fig.3acompares the XRD pattern of the low-T and RT deposition methods.Both pattern are very similar and match the stibnite reference pattern.15Sb 2 S 3 films deposited by the RT method onto mesoporous TiO 2 substrates also show the characteristic XRD peaks of crystalline Sb 2 S 3 (Fig.S3, ESI †).A Rietveld analysis of the two patterns using the powder diffraction software ReX16 yields an average Sb 2 S 3 crystallite size of 40 nm and 35 nm for RT and low-T deposition, respectively.The structure of the solar cell studied in this work and a schematic band diagram are shown in Fig. 1.Fig. 2 shows (a) the current-voltage-characteristic and (b) the external quantum efficiency EQE of the best performing solar cell employing RT deposited Sb 2 S 3 .Its photovoltaic parameters are summarised in Fig. 2. The shunt and series resistances were obtained by a least square fit of the diode function.The RT method enables the fabrication of solar cells with high reproducibility.The deviations from batch to batch and from device to device were small (Fig. S13, ESI †).The hole transport layer consists of a PCPDTBT-PCBM blend.Charge carriers which are generated by photon absorption in the hole transport

Fig. 2
Fig. S2 (ESI †) shows the UV-vis spectra of RT-deposited Sb 2 S 3 films on mesoporous-TiO 2 , annealed at 300 1C for 5 min.The annealed antimony sulfide was characterised by powder X-ray diffraction (XRD).Fig.3acompares the XRD pattern of the low-T and RT deposition methods.Both pattern are very similar and match the stibnite reference pattern.15Sb 2 S 3 films deposited by the RT method onto mesoporous TiO 2 substrates also show the characteristic XRD peaks of crystalline Sb 2 S 3 (Fig.S3, ESI †).A Rietveld analysis of the two patterns using the powder diffraction software ReX16 yields an average Sb 2 S 3 crystallite size of 40 nm and 35 nm for RT and low-T deposition, respectively.The structure of the solar cell studied in this work and a schematic band diagram are shown in Fig. 1.Fig. 2 shows (a) the current-voltage-characteristic and (b) the external quantum efficiency EQE of the best performing solar cell employing RT deposited Sb 2 S 3 .Its photovoltaic parameters are summarised in Fig. 2. The shunt and series resistances were obtained by a least square fit of the diode function.The RT method enables the fabrication of solar cells with high reproducibility.The deviations from batch to batch and from device to device were small (Fig. S13, ESI †).The hole transport layer consists of a PCPDTBT-PCBM blend.Charge carriers which are generated by photon absorption in the hole transport

Fig. 2
Fig. 2 (a) Current-voltage curve of the best performing solar cell device employing Sb 2 S 3 synthesised using the RT method.The inset table shows the photovoltaic parameters of the device.(b) External quantum efficiency of the device.