Towards all-inorganic antimony sulphide semi-transparent solar cells

doi:10.21203/rs.3.rs-4991889/v1

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Towards all-inorganic antimony sulphide semi-transparent solar cells

https://doi.org/10.21203/rs.3.rs-4991889/v1

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NiO, a wide band gap hole-transporting material (HTM), is gaining attention in photovoltaics due to its optical transparency, chemical stability, and favourable band alignment with absorber. This study uses NiO_x nanoparticle-based HTM in semi-transparent Sb₂S₃ solar cells via a simple chemical precipitation method. We optimized NiO_x layer by varying precursor solution concentration and studied its impact on optical and structural properties, composition of nanoparticles and subsequent effect on the performance of semi-transparent Sb₂S₃ solar cell. NiO_x nanoparticles, deposited from nickel(II)nitrate hexahydrate (precursor solution concentrations of 0.2 M to 1.2 M), were thermally treated by two steps at 90°C for 6 h and 270°C for 3 h. Nanoparticles with crystallite sizes of 6–9 nm had band gaps (Eg) of ca 3.65–3.70 eV. Using 1.2 M concentration yielded the largest crystallites (9 nm), lowest Eg (3.65 eV) while retaining the most organic residues. The highest power conversion efficiency (2.65%) was achieved with NiO_x from a 0.5 M precursor, a 60% improvement over HTM-free cells. The effect of precursor solution concentration on the solar cell parameters (efficiency, fill factor, open circuit voltage and short circuit current) are discussed. Present work paves a path toward stable, efficient, and cost-effective all-inorganic Sb₂S₃ solar cells using NiO_x HTM instead of organic counterparts.

Physical sciences/Materials science/Materials for devices

Physical sciences/Materials science/Materials for energy and catalysis

Hole transport layer

Sb2S3 solar cells

NiOx nanoparticles

Chemical precipitation

NiO thin film

Utilization of solar energy and its conversion to suitable form is presently envisioned as one of the best options to meet the global energy demand¹. Currently, majority of the solar photovoltaic (PV) market is dominated by crystalline silicon solar cells^2,3. However, research on thin film solar cells is progressing rapidly in recent years owing to advantages of low-cost solution-based fabrication processes, flexible and semi-transparent devices that extend the application of thin film PV to solar windows⁴. Numerous efforts are underway to further improve the performance of these solar cells by choosing a wide variety of materials as electron (ETM) and hole transport materials (HTM)^5,6. Mostly, high efficiency solar cells rely on organic HTMs like poly(3,4-ethylenedioxythiophene): poly(styrene-sulfonate) (PEDOT: PSS), poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA),2,2′,7,7′-tetrakis (N, N-dimethoxyphenyl amine)-9,9′-spirobifluorene (Spiro-OMeTAD) and poly(3-hexylthiophene) (P3HT)^7,8. However, organic HTMs suffer from hygroscopicity, high cost, tedious preparation methods and instability that inhibits their commercial application despite the high performance of solar cells^9,10. Inorganic p-type metal oxide materials including NiO_x, Fe₂O₃, CuSCN, CuOx, CoOx, MoO₃, V₂O₅, and WO₃ are recently being most explored HTMs in different thin film based solar cells due to their wide band gap, higher stability, high work function, and good hole-transport properties, which would be ideal characteristics for the replacement of organic HTMs^11–13.

Among them, NiO_x appears to be the most promising candidate for desirable HTM in emerging thin film technologies owing to its high optical transmittance, easy processability, a wide band gap, suitable energy level alignment with most of the chalcogenide and perovskite solar cells, and excellent electron blocking ability^14,15. The stoichiometric NiO is naturally an insulator but with the presence of Ni vacancies the material exhibits p-type conductivity^14,15. It has been shown that NiO has suitable band offset with most of chalcogenides solar cells¹⁶ and is a suitable electron blocking layer in Sb₂S₃ or perovskite solar cells¹⁶. Numerous methods to synthesize NiO_x HTMs are employed like sol-gel spin casting¹⁷, solution processed synthesis and thermal decomposition of Ni(OH)₂¹⁸, sputtering¹⁶, pulse laser deposition¹⁹, and atomic layer deposition²⁰. Utilization of physical methods has yielded power conversion efficiencies (PCE) up to 12 % and even 17.3 % in peovskite solar clls using pulse laser deposition but these methods require complex equipment involving vacuum leading to high cost^19,21. Therefore, extensive efforts are dedicated on chemical methods like spin coating, spray pyrolysis, electrodeposition, doctor blade, spun cast, screen-printing and combustion. The resulting devices with solution processed NiO_x as HTM have shown excellent performance with PCEs ranging from 7.6 to 16.47 % for perovskite solar cells^21–24 Recently, spin coating is most frequently used chemical method to deposit NiO_x due to its low cost, easy and facile processing, and ability to reproduce effectively at lab scale²¹. Spin coating of NiO_x HTM is carried out using two kinds of precursors: nickel containing organic sol or crystalline NiO_x nanoparticles^25,26. Synthesis of NiO_x HTM with nanoparticles as a sol is achieving a lot of attention as it requires low temperature annealing to evaporate solvent during the device fabrication stage²⁶.

Some of the best performance of perovskite and up to date existing Sb₂S₃ solar cells using NiO as HTM layer are summarized in Table 1. According to the literature, NiO as HTM has often been incorporated in various perovskite solar cell architectures. At the same time, antimony sulphide (Sb₂S₃) exhibits great potential for PV power generation owing to its band gap (1.70 eV), chemical stability, high absorption coefficient (~ 10⁵ cm^− 1 at 450 nm), and rich elemental content which are environmentally friendly, earth abundant, low cost and nontoxic. Owing to these distinct characteristics, intensive efforts are ongoing to develop Sb₂S₃ as absorber material in single junction solar cells, light harvester in tandem cells and semi-transparent solar window applications. Till now, mainly organic HTMs like poly(3-hexylthiophene) (P3HT)^27,28 and 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD)^29,30 are employed in Sb₂S₃ solar cells which impact the long-term stability of devices. Although NiO_x HTMs can be employed in Sb₂S₃ solar cells due to favourable band alignment³¹, only few groups have studied solution processed NiO and P3HT-NiO composite HTMs in antimony sulphide solar cells and reported 2.45 % and 2.65 PCE^32,33.

Table 1

A selection of published reports showing performance of NiO_x based solar cells synthesized using different methods.
Device configuration	Synthesis method of NiO	V_OC (V)	J_SC (mA·cm^− 2)	FF (%)	PCE (%)	Ref
Perovskite solar cells
ITO/NiO_x/CH₃NH₃PbI₃/ PCBM/Ag	Spun cast	0.92	12.43	68.0	7.80	³⁴
FTO/NiO_x/CH₃NH₃PbI₃/ PCBM/BCP/Au	Magnetron Sputtering	1.10	15.17	59.0	9.84	³⁵
FTO/NiO_x/CuSCN/ MAPBI_3 − xCl_x/PCBM/Ag	Electro deposition	0.94	19.4	75.0	7.26	³⁶
ITO/NiO_x/CH₃NH₃PbI₃ /PCBM/LiF/Al	Pulse Laser Deposition	1.06	20.2	81.3	17.3	¹⁹
ITO /NiO_x/CH₃NH₃PbI₃/ PCBM/Al	Atomic Layer Deposition	1.04	21.9	72.0	16.4	²⁰
ITO/NiO_x/MAPbI₃ /PCBM/ZrAcac/Ag	Spin coating	1.08	22.2	78.1	18.7	³⁷
ITO/NiO_x/(CsFA)PbI₃/ C60/BCP/Ag	Spin coating (NP)	1.14	23.6	82.0	20.7	³⁸
ITO/NiO/(CsFAMA)Pb(ICl)/ BCBM/ZrAcac/Ag	Spin coating (NP)	1.15	22.9	79.5	21.0	³⁹
FTO/NiO_x/(CsFAMA)Pb(BrI) /PCBM + PEI/Ag	Spin coating	1.03	20.8	81.0	17.4	⁴⁰
Antimony sulphide solar cells
FTO/c-TiO₂/Sb₂S₃/NiO_x/Au	Spin coating (NP)	0.49	12.7	39.7	2.45	³³
FTO/c-TiO₂/Sb₂S₃/P3HT-NiO_x/Au	Spin coating (NP)	0.51	11.9	47.8	2.90	³²

Most of the research based on NiO_x HTM employs chemical precipitation method⁴¹; however, to the best of authors’ knowledge, there is no systematic study on type and concentration of precursor used for the synthesis of NiO_x nanoparticles and the application of resulting NiO_x HTM in Sb₂S₃ semi-transparent solar cells. The NiO_x HTM used in highly efficient perovskite solar cells generally employs nickel nitrate as precursor with concentration 0.5 M^26,42–44 while for Sb₂S₃ solar cells limited reports are available wherein the concentration of nickel precursor is around 0.05 M^32,33. Therefore, it is important to understand the effect of precursor concentration on the properties and the subsequent performance of NiO_x HTM in solar cells.

In this research, we study the synthesis of NiO_x nanoparticles using simple and cost-effective chemical precipitation method. The physical and optical properties of nanoparticles are studied systematically by varying the concentration of precursor solution during synthesis of nanoparticles. These nanoparticles are further employed for the formation of NiO_x HTM on Sb₂S₃ solar cells using spin coating method. The focus is put on studying the effect of precursor concentration on the properties of NiO_x nanoparticles and the subsequent impact of NiO_x HTM layer on the performance of Sb₂S₃ solar cells.

2.1. Materials

The following materials were used: Nickel(II)nitrate hexahydrate – 99.99 wt%, Sodium hydroxide pellets ≥ 98%, titanium(IV)tetraisopropoxide (TTIP) – 99 wt% (Acros Organics), acetylacetone – 99 wt% (Acros Organics), ethanol – 96.6 vol % (Estonian Spirit), methanol – 99.9 vol % (Sigma-Aldrich), antimony trichloride – 99.99 wt% (Sigma-Aldrich) and thiourea – 99 wt% (Sigma-Aldrich). The materials were used as received.

2.2. Synthesis of NiO_x nanoparticles

In this work chemical precipitation method has been used for the synthesis of NiO_x nanoparticles as it is simple process, cost effective, rapid, low temperature and easy scaling up for industrialization⁴⁵. This method allows good control of the size of nanoparticles by determining the relative rates of nucleation and growth⁴⁶. The main reactions involved during the chemical precipitation as reported by Mahaleh et al. are mentioned in Equations (1)–(5)⁴⁷:

$$\:2\:\text{N}\text{a}\text{O}\text{H}\left(\text{s}\right)\to\:2{\text{N}\text{a}}^{+}\left(\text{a}\text{q}\right)+2{\text{O}\text{H}}^{-}\left(\text{a}\text{q}\right)$$

$$\:\text{N}\text{i}{\left({\text{N}\text{O}}_{3}\right)}_{2}.6{\text{H}}_{2}\text{O}\left(\text{s}\right)\to\:{\text{N}\text{i}}^{2+}\left(\text{a}\text{q}\right)+2{\text{N}\text{O}}_{3}^{-}\left(\text{a}\text{q}\right)+6{\text{H}}_{2}\text{O}\left(\text{a}\text{q}\right)$$

$$\:{\text{N}\text{i}}^{2+}\left(\text{a}\text{q}\right)+2{\text{O}\text{H}}^{-}\left(\text{a}\text{q}\right)+\text{x}{\text{H}}_{2}\text{O}\left(\text{a}\text{q}\right)\to\:\text{N}\text{i}{\left(\text{O}\text{H}\right)}_{2}.\text{x}{\text{H}}_{2}\text{O}\left(\text{s}\right)\downarrow\:$$

$$\:\text{N}\text{i}{\left(\text{O}\text{H}\right)}_{2}.\text{x}{\text{H}}_{2}\text{O}\left(\text{s}\right)\underrightarrow{{90}^{0}\text{C}}{\text{N}\text{i}\left(\text{O}\text{H}\right)}_{2}\left(\text{s}\right)+\text{x}{\text{H}}_{2}\text{O}\left(\text{g}\right)\uparrow\:$$

$$\:{\text{N}\text{i}\left(\text{O}\text{H}\right)}_{2}\left(\text{s}\right)\underrightarrow{{270}^{0}\text{C}}\:\text{N}\text{i}\text{O}+{\text{H}}_{2}\text{O}\left(\text{g}\right)\uparrow\:$$

The steps of NiO_x nanoparticles synthesis carried out in this study can be seen on Fig. 1. In a typical procedure, different amount of Ni(NO₃)₂·6H₂O was dispersed in 10 mL of deionized (DI) water to obtain a dark green solution with varying concentration of 0.2 M, 0.5 M, 0.8 M, and 1.2 M. The pH of the solution was adjusted to 10 by adding 10 M NaOH solution with continuous stirring according to the literature reports^26,41,48. After the completion of reaction, as obtained green coloured colloidal precipitate of Ni(OH)₂ was collected and thoroughly washed with DI water twice, and then with ethanol. The washed and filtered precipitates were dried at 90°C for 6 h and grinded to form a powder. The obtained green powder was then annealed in air at 270°C for 3 h to obtain a dark-black powder⁴³. After annealing, the black NiO_x powder was collected and dispersed in DI water with concentration of 20 mg·ml^− 1 and ultrasonicated for 6 h yielding a very stable colloidal ink⁴¹. The colloidal ink was then filtered using polytetrafluoroethylene filter (0.45 µm) and used for spin coating.

2.3. Device Fabrication

Semi-transparent Sb₂S₃ solar cells in this work were constructed in superstate configuration (glass/FTO/TiO₂/Sb₂S₃/HTM/Au) as reported earlier by our group^28,49. The substrates (glass/FTO (7 Ω·sq^–1)) were first thoroughly cleaned with DI water, acetone, and iso-propyl alcohol in an ultrasonic bath for 10 min each followed by boiling the substrates in DI water and purging with N₂ gas. Then TiO₂ as electron transport layer (ETL) and Sb₂S₃ as absorber layer were deposited by an indigenously made ultrasonic spray deposition system as shown in Fig. 1(b). For the deposition of TiO₂, a solution mixture of 0.2 M TTIP and 0.2 M acetylacetone in ethanol was used as precursor. The precursor was then sprayed on glass/FTO substrate maintained at around 340°C on a hot plate with spray rate of ≈ 2.5 mL·min^− 1 for 30 min. The as deposited TiO₂ samples were annealed at 450°C for 30 min in air. Sb₂S₃ thin films were deposited using precursor containing a mixture of 60 mM SbCl₃ and 180 mM thiourea in methanol. The films were deposited at a spray rate of 2 mL·min^− 1 while maintaining a substrate temperature of ≈ 200°C. The as-deposited Sb₂S₃ films were annealed at around 250–260°C for 5 min under nitrogen. The NiO_x hole transporting layer was fabricated on Sb₂S₃ absorber layer by spin-coating NiO_x colloidal ink at 2500 rpm for 30 s with a 3 s ramp. The NiO_x coated samples were then heated at 130°C for 20 min in air. Here after in this study for the sake of clarity, nanoparticles synthesized using different precursor concentration of 0.2 M, 0.5 M, 0.8 M and 1.2 M during synthesis of NiO_x nanoparticles will be mentioned as NP-1, NP-2, NP-3, and NP-4 and corresponding NiO_x HTM/Sb₂S₃ solar cells will be referred as NS-1, NS-2, NS-3, and NS-4, respectively (NS-0 corresponds to device without HTM).

2.4. Characterization methods

The crystal structure and phase constitution of the samples were examined using a Rigaku Ultima IV X-ray diffractometer with a Cu Kα source (λ = 1.5406 Å). Fourier Transform Infrared (FTIR) spectroscopy was carried out to investigate the functional groups present in NiO_x nanoparticles using a Bruker Alpha FTIR spectrometer with diamond crystal at room temperature in the range of 400–4000 cm^− 1. A Zeiss HR FESEM was used to analyse the cross-sectional and surface morphologies of the layers. The optical absorption and transmittance measurements were made using a Jasco V-670 ultraviolet–visible spectrophotometer (UV–VIS) in the range of 300–1000 nm. The current–voltage (I–V) characteristics of the fabricated solar cells were measured using Wave labs LS-2 LED solar simulator with an AM1.5G (100 mW·cm^− 2) light source. A Newport 69911 system with a 300 W Xenon lamp was used to measure the external quantum efficiency (EQE) of the fabricated solar cells.

Characterization of NiO _x nanoparticles

X-ray diffraction was used to study the phase composition of as synthesized nanoparticles with different precursor concentrations (NP-1, NP-2, NP-3, and NP-4) and the patterns recorded are shown in Fig. 2(a). According to XRD patterns, all the NiO_x nanoparticles demonstrated five major diffraction peaks at 2θ values of 37.2°, 43.2°, 62.8°, 75.4°, and 79.4° corresponding to the (111), (200), (220), (311), and (222) crystallographic planes of cubic NiO [JCPDS, No. 04-0835], respectively. No other peaks were found on the diffractogram.

The average crystallite size (D) of NiO_x nanoparticles was estimated using Debye Scherrer formula along the (200) diffraction plane⁵⁰ using Eq. (6):

$$\:D=\:\frac{0.9\:\lambda\:}{\beta\:\text{cos}\theta\:}$$

where λ is the X-ray wavelength, β is the full width half maximum and θ is the diffraction angle of the concerned peak. The nanoparticles synthesized at concentrations of 0.2 M, 0.5 M and 0.8 M (NP-1, NP-2, and NP-3) show similar crystallite size of 6 nm whereas nanoparticles grown at higher concentration of 1.2 M (NP-4) depict increased crystallite size of 9 nm. This phenomenon can be easily understood by the fact that with the increase in the concentration of solution number of precursor ions per unit of time and volume increases that leads to increase in growth rate yielding larger particles. The similar behaviour was reported by some other groups for different material systems synthesized by chemical precipitation method^51,52.

Figure 2(b) shows the FTIR spectra of NiO_x nanoparticles (NP-1, NP-2, NP-3, and NP-4) synthesized at different precursor concentration. For all the samples, a wide band at around 3400–3600 cm^− 1 has been detected which could be assigned to O-H stretching vibrations of free H₂O or OH group⁵³. The peak at around 1630 cm^− 1 probably arises from the O-H bending mode of H₂O that is adsorbed on the material surface. Sharp peak at ca 420 cm^− 1 is characteristic of Ni-O vibration^54,55. A strong absorption band in the FTIR spectra of all the samples appears in the region of 1200–1515 cm^− 1 with the peak positioned at around 1350 cm^− 1, whereat this band is the strongest in the sample NP-4 prepared from the most concentrated solution. Strong IR bands in this spectral region were also recorded for Ni(OH)₂ phase synthesized from NiNO₃⁵³. Namely, vibrations at ~ 1390 cm^− 1 and ~ 1490 cm^− 1 were assigned to O-H bend and lattice OH in Ni(OH)₂, respectively, and vibrations at around 1280 cm^− 1, 1310 cm^− 1 and 1340 cm^− 1 were assigned NO₃⁻ ions⁵³. IR bands positioned at ~ 1350 cm^− 1 and ~ 830 cm^− 1 were recorded in spectra of layered nickel hydroxynitrate phases and assigned to the nitrate stretch and bend modes, respectively⁵⁵.

Thus, according to the IR spectra, the synthesized NiO_x nanoparticles still contain an intermediate phase of synthesis, Ni(OH)_2, as well as the traces of the Ni(NO₃)₂ precursor, and their concentration is higher in nanoparticles synthesized from more concentrated solutions although only NiO phase was detected by XRD.

UV–Visible absorption spectra of semiconductor materials have been regarded as a significant tool to analyse fundamental information related to its optical properties. The band gap (Eg) of different nanomaterials has been calculated using absorption spectra and following Tauc plots as shown in Eq. (7):

$$\:{\left(\text{A}\text{h}{\nu\:}\right)}^{\text{n}}=\text{B}(\text{h}{\nu\:}-\text{E}\text{g})$$

where hυ is the energy of photon; A is absorbance, B is the constant related to the material; and n indicates either 2 or 1/2 for direct transition and indirect transition, respectively⁵⁶. Therefore, the optical band gap of the material can be obtained by drawing tangent to the linear portion of the (Ahυ)ⁿ – hυ curve and calculating intercept on hυ axis.

Figure 2(c) depicts the estimation of band gap of different nanoparticles synthesized by varying concentration using Tauc plots. Band gap values for NP-1, NP-2 and NP-3 are around 3.70 eV, but NP-4 shows a little smaller band gap of 3.65 eV. Band gap values of NiO_x nanoparticles are close to that measured for the ultrasonically sprayed NiO_x thin films^57,58 The smaller band gap of NP-4 could be explained by slightly larger crystallite size of 9 nm compared to 6 nm for NP-1 – NP-3. In addition, the higher amount of Ni(OH)₂ in NP-4, as shown by FTIR study, may influence the NiO_x band gap as the Eg of Ni(OH)₂ is 3–3.5 eV⁵⁹.

Characterization of Sb ₂ S ₃ solar cell

After examining the physical and structural properties of nanoparticles, it is equally significant to study the properties of NiO_x HTM deposited on Sb₂S₃ absorber layer before introducing gold contact. The structural properties of the glass/FTO/TiO₂/Sb₂S₃/NiO_x structure were studied by XRD, corresponding XRD patterns are shown in Fig. 3(a). The XRD pattern shows the peaks corresponding to Sb₂S₃, FTO and anatase TiO₂ layers. The peaks at 2θ values of 15.9°, 17.8°, 29.2° and 30.5° observed in diffractogram, are characteristic of the orthorhombic stibnite structure of Sb₂S₃ (ICDD PDF 01-075-4013)⁶⁰.There are no peaks observed related to NiO_x suggesting formation of an extremely thin coating over glass/FTO/TiO₂/Sb₂S₃ stack. The transmission spectra of glass/FTO/TiO₂/Sb₂S₃ with and without NiO_x in the stacked structure were recorded by UV–VIS spectrometry as shown on Fig. 3(b). It can be observed from the figure that the transmission spectrum of glass/FTO/TiO₂/Sb₂S₃/NiO_x is similar to that of glass/FTO/TiO₂/Sb₂S₃ owing to transparent nature of thin NiO_x film. The average visible transmittance (AVT %) was calculated and found to be around 22 % The transmission spectra clearly demonstrate semi-transparent behaviour of Sb₂S₃ solar cells in the visible range of the spectrum.

Figure 4(a), (b) and (c) illustrate surface morphology of NS-1, NS-2, NS-4 devices, respectively and demonstrate the formation of NiO_x porous layer over Sb₂S₃ absorber layer. The morphological studies reveal uniform coverage of NiO_x nanoparticles over Sb₂S₃ absorber layer in all samples. The cross-sectional image of NS-2 device (Fig. 4(d)) shows the thicknesses of TiO₂ and Sb₂S₃ layers of approximately 40 nm and 90 nm, respectively, while NiO_x layer is a noticeably thinner, ca 20 nm.

Device performance

The current density–voltage (J–V) characteristics of all the champion devices with varied NiO_x concentrations in HTM layers measured under one sun illumination (AM 1.5G) are depicted in Fig. 5(a), while solar cell output parameters along with shunt and series resistance are summarized in Table 2. It could be seen from Fig. 5(a) that the NiO_x precursor concentration of 0.5 M (NS-2) results in the most efficient solar cell compared to solar cells with other NiO_x concentrations. The external quantum efficiency (EQE) curve and the integrated current density curve of the best performing cell, NS-2, is presented in Fig. 5(b). To understand the performance of these devices and obtain information about reproducibility, statistical box plots of Voc, Jsc, FF and efficiency are shown in Fig. 6. Figure 6(a) shows the effect of NiO_x HTM precursor concentration on the Voc of Sb₂S₃ solar cells. The figure clearly shows that with increase in the precursor concentration, the Voc value spirals up from ca 360 mV, characteristic of the cell without an HTM, to ca 560 mV, and stabilizes around this value. Thus, application of NiO_x HTM layer significantly reduces the recombination of charge carriers at the back contact by transferring holes and blocking electrons at the interface, and hence resulting in higher value of Voc.

The impact of NiO_x type on the device current density (Jsc), is shown in Fig. 6(b). Current density first increases from ca 10 mA·cm^− 2 (NS-0) to ca 12 mA·cm^− 2 (NS-2, NiO_x precursor solution 0.5 M) and decreases thereupon to ca 7 mA·cm^− 2 (NS-4). The external quantum efficiency (EQE) curve and the integrated current density curve of the best performing cell, NS-2, are presented in Fig. 5(b). The integrated photocurrent Jsc of 12.35 mA·cm^− 2, as measured for the NS-2 device, corresponds well to that obtained from the J-V curve, 12.4 mA·cm^− 2 (Table 2). It should be noted that EQE curves do not exhibit a dip in spectral region of 500–650 nm, characteristic of solar cells that use P3HT as an HTM^49,61. Consequently, NiO_x HTM exhibits good perspective to be used in semi-transparent devices.

Fill factor (FF) values depending on the NiO_x precursor concentration are presented in Fig. 6(c) and show downtrend, from 40% (NS-0, without HTM) to 30% (NS-4). A possible explanation for the downtrend trend in the FF value in Fig. 6(c) may be related to a large series resistance, changing the boundary condition for the predominant recombination path in the device, under illumination condition. A higher NiO_x precursor concentration induces a high series resistance at the back interface (Table 2) and this effect is further enhanced under illumination conditions where changes from low to high injection generation occurs. This effect generates enhanced recombination at the absorber/NiO_x interface due to a light modulated potential barrier. An enhanced back surface recombination mechanism predominant around the maximum power point results in a low FF. In Fig. 5(a), the shapes of the light J–V characteristics of solar cells with high NiO_x precursor concentration indicate the presence of kink anomaly phenomenon. This effect seems to be more pronounced in the fourth quadrant, implying that a large series resistance (Table 2) together with a low doping in the Sb₂S₃ absorber promotes accumulation of charge carrier at the Sb₂S₃/NiO_x back interface. The later effect changes the boundary conditions for the charge recombination under light conditions – from interface and bulk recombination to intense recombination at the Sb₂S₃/NiO_x back interface, resulting in the degradation of all PV parameters and final PCE of the solar cells.

So far, the highest efficiency of 2.65% is recorded for NS-2 device with NiO_x from 0.5 M solution, which is 60% higher than the Sb₂S₃ solar cells without HTM while efficiencies of devices using NiO_x from more concentrated solutions demonstrate lower performance (Fig. 6(d) and Table 2) mainly due to drop in Jsc and FF. Thus, more concentrated solutions are not recommended for the production of NiO_x nanoparticles, as these may contain higher quantity of residues from the precursor and intermediates, as confirmed by FTIR study.

Table 2

Photovoltaic parameters of NiO_x HTM based Sb₂S₃ champion solar cells with variation in NiO_x HTM precursor concentration during synthesis.
Solar cell	Concentration of precursor (M)	Voc (mV)	Jsc (mA·cm^− 2)	FF (%)	Efficiency (%)	Series resistance (Ωcm²)	Shunt resistance (Ωcm²)
NS-0	0	356	11.2	42	1.66	5	162
NS-1	0.2	500	6.5	30	0.96	37	96
NS-2	0.5	555	12.4	39	2.65	12	175
NS-3	0.8	563	9.9	37	2.08	26	169
NS-4	1.2	545	8.7	32	1.51	19	103

Results obtained in this study show that an inorganic HTM composed of NiO_x nanoparticles synthesized by a chemical precipitation method is an effective material to decrease the recombination at the solar cell back interface with no reducing the optical transparency of the device. We also showed that the nanoparticles preparation parameters viz, precursor solution concentration has strong impact on the properties of nanoparticles and solar cell output performance and should be carefully optimized for highly efficient devices.

The impact of precursor concentration on the quality of NiO_x nanoparticles synthesized using chemical precipitation method and the subsequent influence on NiO_x HTMs in semi-transparent Sb₂S₃ solar cells was studied. A comparative study of optical and structural properties of different nanoparticles like phase, crystallinity, size, band gap, and functional moieties present on surface using various characterization techniques was carried out. Increasing the precursor solution concentration from 0.2 M to 1.2 M increased the crystallite size of cubic NiO nanoparticles from 6 nm to 9 nm, respectively. The corresponding increase in precursor solution concentration however led to the decrease in band gap from 3.70 eV to 3.65 eV which could be also due to the presence of secondary phase originating from higher precursor amount. The NiO_x colloidal ink with precursor solution concentration of 0.5 M and deposited on Sb₂S₃ solar cells using spin coating method demonstrated an efficiency of 2.65% which is about 60% higher than Sb₂S₃ solar cells without HTM. All the NiO_x HTM devices have shown improved Voc in the range of 500–563 mV that is higher as compared to Sb₂S₃ solar cells without HTM (363 mV). The present study puts forward a cost effective, simple, and feasible way of solution-processed high quality inorganic HTMs in the form of crystalline nanoparticles for efficient Sb₂S₃ solar cells.

Additional information

The authors declare no competing interests.

Author Contribution

Study conception and design: A.P., I.O.A., M.K.. Acquisition of data: A.P., A.K. Analysis and interpretation of data: A.P., A.K., M.K., N.S., I.O.A, M.K.. Drafting of manuscript and preparing figures and tables: A.P., M.K. ,N.S., I.O.A., M.K.. Critical revision of manuscript: M.K., N.S., I.O.A, M.K.. All authors have reviewed and approved the final manuscript.

Acknowledgements

The study was funded by the Estonian Research Council projects SJD78 “Development of NiO_x Thin Films as Electrode Material for Semi-transparent Solar Cells”, PRG627 “Antimony chalcogenide thin films for next-generation semi-transparent solar cells applicable in electricity producing windows”, the Estonian Ministry of Education and Research project (TK210/TK210U8) „Centre of Excellence in Sustainable Green Hydrogen and Energy Technologies“, and the European Union’s Horizon 2020 programme under the ERA Chair project 5GSOLAR grant agreement No 952509. Olga Volobujeva is thanked for the SEM measurements.

Data Availability

All generated and/or analysed data is provided within the manuscript.

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No competing interests reported.

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Towards all-inorganic antimony sulphide semi-transparent solar cells

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Version 1

Abstract

Figures

1. Introduction

2. Methods

2.1. Materials

2.2. Synthesis of NiO_x nanoparticles

2.3. Device Fabrication

2.4. Characterization methods

3. Results and discussion

4. Conclusions

Declarations

Additional information

Author Contribution

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1

Towards all-inorganic antimony sulphide semi-transparent solar cells

Status:

Version 1

Abstract

Figures

1. Introduction

2. Methods

2.1. Materials

2.2. Synthesis of NiOx nanoparticles

2.3. Device Fabrication

2.4. Characterization methods

3. Results and discussion

4. Conclusions

Declarations

Additional information

Author Contribution

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1

2.2. Synthesis of NiO_x nanoparticles