Introduction

Sb2S3 has been increasingly utilized for solid thin-film solar cells because of its moderate bandgap of 1.7 eV and an absorption coefficient of 1.8 × 105 cm−1 [1, 2]. Sb2S3 thin films can be prepared by various methods, including spray pyrolysis [3], electrodeposition [4], chemically deposition [5], and thermal vacuum evaporation technique [6]. In Sb2S3-based photovoltaic device, photoelectric conversion efficiency (PCE) has reached to 5.7–7.5% by improved technology and device design [1, 2, 7,8,9,10]. However, current efficiencies of solid-state devices still remain low compared to other photovoltaic devices, such as dye-sensitized solar cells [11] and perovskite solar cells [12]. At present, most of the works usually focus on finding the best technology to get better optoelectronic performance in solid-state devices [7,8,9,10, 13,14,15]. In this regards, it is imperative to study the photo-electronic processes in Sb2S3-based solar cells for guiding the device design and optimization. This includes a comprehensive understanding of the balance among absorption, internal electrical field, and transport path, and relative kinetics on Sb2S3 photovoltaic performance, which is important to guide the optimization of the Sb2S3-based hybrid solar cells. In this work, the conventional TiO2/Sb2S3/poly(3-hexylthiophene-2,5-diyl(P3HT) n-i-p device structure was used to study the charge carrier generation and dissociation dynamic processes for different thicknesses of Sb2S3.

It is obvious that the different thickness of Sb2S3 in TiO2/Sb2S3/P3HT n-i-p solar cells can change (i) the amount of photon harvesting, which influences the photon-generated electron/hole concentration; (ii) the magnitude of internal electrical field across the Sb2S3 layer, which influences the photon-generated electron/hole drift; (iii) electron/hole transport distance to the respective electrode; and (iv) electron/hole recombination [16, 17]. However, the reason for the Sb2S3 thickness-dependent performance in n-i-p structure is still ambiguous, which has been simply attributed to the issues with bulk resistance, photon absorption, generation/recombination of charge carriers, and internal electric field [16,17,18,19,20,21], but the detailed and quantified analysis for the thickness-dependent photovoltaic parameters is not clear yet. To gain insight into the change of Jsc and Voc upon the Sb2S3 thickness, TiO2/Sb2S3/P3HT n-i-p solar cells were fabricated (Fig. 1), and the thickness of Sb2S3-related photon-generated electron and hole transport processes which result in the different photocurrents was studied in this work. Moreover, we introduced dynamic intensity-modulated photocurrent/photovoltage spectra (IMPS/IMVS) and Kelvin probe force microscope (KPFM) characterization to study photon-to-electron processes and investigate the key factors that governed the device performance in different thicknesses of Sb2S3 solar cells.

Fig. 1
figure 1

Illustration of TiO2/Sb2S3/P3HT n-i-p solar cell architecture.h+ denotes the hole and e− denotes the electron

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Methods

Reagents

Etched FTO-coated glass substrates were purchased from Huanan Xiangcheng Co., Ltd., China. SbCl3 (99%), Na2S2O3 (99%), and titanium diisopropoxide (75% in isopropyl alcohol) were purchased from Adamas-beta. P3HT was ordered from Xi’an Polymer Company, China, and Ag (99.999%) was ordered from Alfa.

Device Fabrication

The substrates were cleaned via ultrasonication in soap water, acetone, and isopropanol for 60 min each, followed by treatment with UV-ozone for 30 min. A thin layer of compact TiO2 (0.15 M titanium diisopropoxide in ethanol) was spin coated at 4500 rpm for 60 s, followed by annealing at 125 °C for 5 min and 450 °C for 30 min. Deposition of Sb2S3 on the top of the TiO2 thin layer was performed by a chemical bath deposition (CBD) method [5, 10, 22]. An acetone solution containing SbCl3 (0.3 M) was added dropwise into Na2S2O3 (0.28 M) with stirring in an ice bath (~ 5 °C). The FTO substrate was covered with a thin layer of TiO2 and then suspended upside-down in the aqueous solution when the color of the solution changed to orange. After 1 h, 1.5 h, 2 h, and 3 h of the CBD process, a smooth and uniform amorphous Sb2S3 layer was deposited onto the TiO2-coated FTO substrates, and the sample was thoroughly rinsed with de-ionized water and dried under N2 flow. The substrate was further annealed for 30 min in a glovebox (O2: 0.1 ppm, H2O: 0.1 ppm) under an N2 atmosphere. The fabrication of n-i-p heterojunction was completed by spin casting (1500 rpm for 60 s) of P3HT (15 mg/mL) film on top of Sb2S3 inside a glovebox (O2: 0.1 ppm, H2O: 0.1 ppm) under an N2 atmosphere. Finally, the MoO3 (10 nm) and Ag (100 nm) electrode was deposited by evaporation through a shadow mask.

Instruments and Characterization

X-ray diffraction (XRD) patterns of the film were recorded by an MXP18AHF X-ray diffractometer with Cu Kα irradiation (λ = 1.54056 Å). Scanning electron microscopy (SEM) measurements were performed on a field-emission scanning electron microscope (ZEISS, GeminiSEM 300). The absorption spectra were recorded with a Shimadzu UV-2600 spectrophotometer. Current density–voltage (J–V) characteristics were measured under AM 1.5 illumination with an intensity of 100 mW/cm2 using a 94023A Oriel Sol3A solar simulator (Newport Stratford, Inc.). The light intensity from a 450 W xenon lamp was calibrated with a standard crystalline silicon solar cell. The J–V curves were collected using an Oriel I–V test station (Keithley 2400 Source Meter, Newport). External quantum efficiency (EQE) spectra of the solar cells were measured by using a QE/IPCE measurement kit (Zolix Instruments Co., Ltd.) in the spectral range of 300–900 nm. Intensity-modulated photocurrent spectra (IMPS) and intensity-modulated photovoltage spectra (IMVS) were measured using an electrochemistry workstation (IviumStat.h, Netherlands) under ambient conditions with a background intensity of 28.8 mW/cm2 from a white light-emitting diode, with a small sinusoidal perturbation depth of 10%. Kelvin probe force microscope (KPFM) was performed by an Agilent SPM 5500 atomic force microscope equipped with a MAC III controller (comprising three lock-in amplifiers) to map the surface potential (SP).

Results and Discussion

Deposition and Characterization of Sb2S3/TiO2 Film

FE-SEM images (Fig. 2a) clearly show that different thicknesses of Sb2S3 film are deposited on TiO2 layer-coated glass substrates with the different CBD time t (1.0 h, 1.5 h, 2.0 h, 3.0 h). It can be seen that the uniform Sb2S3 layers were successfully obtained by CBD techniques. The average thickness of the Sb2S3 film estimated from the cross-sectional FE-SEM images is plotted in Fig. 2b as a function of the CBD time. The average thickness d of Sb2S3 film increases linearly with t (Fig. 2b). The average thickness increased almost linearly from 96 to 373 nm by changing CBD time from 1 to 3 h. The XRD patterns of Sb2S3 film with different thickness of Sb2S3 film on FTO glass are shown in Fig. 3. The measured XRD spectrum is indexed to orthorhombic Sb2S3 (JCPDS PCPDFWIN #42-1393) [23].

Fig. 2
figure 2

a Cross-sectional FE-SEM images of Sb2S3 films on TiO2 dense layer-coated glass substrates. b Average Sb2S3 thickness d plotted as a function of the CBD reaction time t for the Sb2S3 film deposition. The values were estimated by the FE-SEM cross-section images

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Fig. 3
figure 3

XRD patterns of the as-synthesized Sb2S3 film on FTO with different deposition time. Sample 1 is the pure FTO glass substrate, and samples 2–5 are Sb2S3 films with t of 1 h, 1.5 h, 2 h, and 3 h, respectively

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As shown in Fig. 4, the TiO2 samples exhibit the absorption onset at 386 nm (3.21 eV) corresponding to the bandgap absorption of TiO2 [24]. All the as-deposited TiO2/Sb2S3 layers with different t of CBD exhibit an absorption edge at ca. 750 nm [25]. The absorption intensity of Sb2S3 on the TiO2 surfaces is clearly in the order 3 h > 2 h > 1.5 h > 1 h. This result also indicates that the Sb2S3 film gradually becomes thicker with a longer CBD t, which also agrees with the SEM results.

Fig. 4
figure 4

The UV-vis absorption of TiO2 and TiO2/Sb2S3 films with t of 1–3 h, respectively

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Solar Cells

J-V characteristics of solar cells with different thickness d (i.e., CBD t) are compared in Fig. 5a. Table 1 presents the overall photovoltaic performance of these devices. Increasing thickness d (i.e., CBD time t) significantly affects device performance. The PCE increases as d increases from 96 to 175 nm (i.e., t increased from 1.0 to 1.5 h) and decreases thereafter, especially decreases largely after d > 280 nm (i.e., t > 2 h). Optimum Sb2S3 thickness of 175 nm can be determined by comparison of device efficiencies, at which point a maximum PCE of 1.65%, Jsc of 6.64 mA cm−2, Voc of 0.61 V, and FF of 40.81% can be achieved. This result is comparable to the other’s reports [16, 26]. Liu et al. studied hybrid ZnO/Sb2S3/P3HT n-i-p cells with Sb2S3 layers of three different thickness (50, 100, and 350 nm) by thermal evaporation achieving the highest PCE (~ 2%) with the 100-nm-thick Sb2S3 [12]. Kamruzzaman et al. studied TiO2/Sb2S3/P3HT n-i-p cells with Sb2S3 thicknesses of 45–120 nm by a thermal evaporation method, and the absorber Sb2S3 and hole transporting layer P3HT were annealed under atmospheric conditions. In their studies, the thickness of 100–120 nm showed a better power conversion efficiency of 1.8–1.94% [26]. Obviously, the thickness of Sb2S3 indeed strongly affects the device performance, even by different deposition strategies of Sb2S3 film or annealing condition.

Fig. 5
figure 5

a J–V curves and b EQE spectra of the solar cells with different CBD t for Sb2S3 film

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Table 1 Device performance of the TiO2/Sb2S3/P3HT n-i-p planar solar cells under AM 1.5 illumination of 100 mW/cm2
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Charge Transport

The device Jsc increases remarkably with increasing Sb2S3 thickness d from 96 to 175 nm and then decreases as the d increases (Fig. 5 and Table 1). The device Jsc is significantly dependent on the Sb2S3 thickness d. The charge carrier generation and dissociation are key processes for photocurrent generation. Firstly, visible light will pass through the TiO2 layer due to its visible light window property (Fig. 4) and begin to be absorbed from TiO2/Sb2S3 interface. Sb2S3 has been proved to be a high absorption coefficient α around 105 cm−1 in the visible region [27]. Here, we take α = 105 cm−1 for Sb2S3. The thickness-dependent illumination depth is depicted in Fig. 6 according to the Beer-Lambert law I(x) = I0 e-ax, in which the I0 is the incident photon flux and the I(x) is the photon flux in Sb2S3. Obviously, the incident photons cannot be absorbed fully when the Sb2S3 has a thickness of 100 nm or 200 nm (Fig. 6b). The d-related ratio of absorbed photon (Na)/incident photons (Ni) can be calculated by integration of the area of the shaded area in coordinate. As shown in Fig. 6b (also refer Fig. 7b), the Na/Ni is 61% when the d = 96 nm and Na/Ni is enhanced to 82% when the d = 175 nm. It can be believed that the further 21% photons absorbed might cause the increase in Jsc from 5.50 to 6.64 mA/cm2. When the d increases to 280 nm, the extra 11% photons are absorbed and the Na/Ni is further enhanced to 93%, which shows that more photons could be further absorbed and then might generate more electrons. However, the device Jsc decreased to 5.06 mA/cm2 which is lower than the case of d = 96 nm. When the d increases to 373 nm, the Na/Ni is close to 100%, and the device Jsc is sharply decreased to 2.64 mA/cm2. Therefore, absorption is not the sole factor that affects Jsc.

Fig. 6
figure 6

The illustration of the Sb2S3 thickness d-dependent illumination depth x and Ein

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Fig. 7
figure 7

a Semilogarithmic plots of J–V characteristic in the dark of the solar cells with different CBD t for Sb2S3 film. b Dependences of Vin, Jsc, Na/Ni, Eke, and Ekh on Sb2S3 thickness d

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The semilogarithmic plots of the J–V curve of solar cells in the dark normally exhibit three distinct regimes: (i) linear increase for leakage dominated current, (ii) exponential increase for diffusion dominated current, and (iii) quadratic increase for space-charge-limited current. The built-in voltage (Vin) normally can be estimated at the turning point where the dark curve begins to follow a quadratic behavior (Fig. 7a). Dependences of Vin, Jsc, Na/Ni, Eke, and Ekh on CBD t are shown in Fig. 7b. When d increased from 96 to 175 nm, the Na/Ni enhanced 34.44%; however, the Jsc only increased by 20.72%, which means that there is another factor limiting the Jsc increment. It has been inferred that this was might due to the decreased internal electrical field across the Sb2S3 layer, which weakened the photon-generated electron/hole drift [16]. Therefore, we calculated the internal electrical field Ein cross the Sb2S3 based on the relation of Ein = Vin/d (Table 2). Moreover, the drift velocity of electron ve and hole ve, kinetic energy of electron Eke, and hole Ekh under internal electric field Ein were also calculated (Table 2 and Fig. 7b). When the d is 96 nm, the Eke is 296.56 meV, and Ekh is 53.25 meV. When the d increased to 175 nm, the Eke largely decreases to 95.29 meV and Ekh decreased to 17.12 meV, which is lower than the thermal energy at ambient temperature (Ekt, 26 meV). This result indicates that the internal electric field has little effects on hole drift when Sb2S3 thickness is or larger than 175 nm. Obviously, the reduced Eke and Ekh with the thicker Sb2S3 should be the reason that limits the increment of Jsc. Further increasing d from 175 to 280 nm, the Na/Ni enhanced to 13.84%; however, the Jsc get decreased. This might be due to the decrease in Eke which is close to the Ekt (d = 280 nm) but much lower than the Ekh (d = 373 nm), which means the Ein gradually has little effects on electron drift when d > 280 nm as observed in this work. Therefore, Ein decrement-related electron drift might be responsible for the Jsc reduction when d increased from 175 to 280 nm. However, when the d increased to 373 nm, the Ein has little effects on electron and hole drift, but Jsc still largely decreased, which indicates that Ein is also not the sole factor that affects the Jsc.

Table 2 Parameters of Vin, internal electric field Ein, drift velocity of the electron (ve) and hole (vh), and kinetic energy of the electron (Eke) and hole (Ekh) in the dark of the solar cells with different CBD t
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We used KPFM to characterize the photo-generated electrons and hole concentration-related surface potential (SP) in Sb2S3/P3HT. The sample for the KPFM measurement was prepared by drop casting the P3HT precursor solution onto part of the FTO/TiO2/Sb2S3 film surface (Fig. 8). As the Sb2S3 thickness increases from 96 to 373 nm, the SP on the top of Sb2S3 gradually becomes smaller, which means the Fermi level on the Sb2S3 surface becomes lower [28]. This demonstrates that electrons which could diffuse to the top surface are being gradually reduced, indicating that there is an inferior region for photo-generated electrons in thicker Sb2S3 film as shown in Fig. 6. We also examined the SP of P3HT part. The changes of SP of the P3HT are different from that of Sb2S3. P3HT might be excited by light to generate excitons and then separate into electrons and holes [29, 30], when Sb2S3 is very thin (< 200 nm). When Sb2S3 becomes thicker, P3HT only acts as the hole transport layer, because most of the photons are absorbed by Sb2S3 (Fig. 3). Therefore, when the thickness of Sb2S3 is less 280 nm, P3HT could be photo-excited, resulting in the Fermi level of P3HT gradually decreases as Sb2S3 thickness gradually increases (decreased photo-exciton). In the case of 280 nm, the SP of P3HT drops rapidly, because there is no photo-exciton and the P3HT works just as a hole transport layer to collect holes. As the Sb2S3 thickness increases to 373 nm which is much larger than the hole transport length, the hole collection also drops rapidly, causing the Fermi level in P3HT to rise again. Moreover, the changes of SP in P3HT is much larger than that in the Sb2S3 in the case of d = 373 nm, which means that hole collection is worse than electron collection and therefore would probably lead to a much decreased Jsc.

Fig. 8
figure 8

Illustration of SP measurement of Sb2S3/P3HT interface by KPFM

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Furthermore, IMPS and IMVS, as the powerful dynamic photoelectrochemical methods in dye-sensitized solar cells [31] and perovskite solar cells [32], have been applied to study the charge transport dynamics in this work. IMPS/IMVS measures the photocurrent/photovoltage response to a small sinusoidal light perturbation superimposed on the background light intensity under short-circuit/open-circuit condition [31,32,33]. The measured IMPS or IMVS responses appear in the fourth quadrant of the complex plane with a shape of the distorted semicircle (Fig. 10a, b). The time constant τ defined by the frequency (fmin) of the lowest imaginary component of IMPS or IMVS response is an evaluation of the transit time τIMPS for the electrons to reach the collection electrode under short-circuit condition or the electron lifetime τIMVS related to interfacial charge recombination under open-circuit condition. According to the relation τ = (2πf)−1 [31,32,33,34,35], τIMPS and τIMVS in the devices were calculated (Table 1). The increased τIMPS suggests a longer transport path of charges to collection electrode, whereas the unchanged τIMVS infers the same interfacial charge recombination [33]. The interfacial charge collection efficiency ηc is typical considered as ηc = 1-τIMPS/τIMVS [31,32,33,34,35]. Obviously, the longer transport time of the τIMPS and the short interfacial charge recombination lifetime of the τIMVS would cause a worse charge collection and vice versa. In this study, the τIMPS increases with the thicker Sb2S3 while the τIMVS is unchanged. Therefore, interfacial charge collection efficiency ηc decreases with the thicker Sb2S3, and the changes of Jsc in different thickness of Sb2S3 solar cells should be caused by the transport path and charge collection efficiency, not by charge recombination.

The increase in Sb2S3 thickness could absorb more photons which could enhance the photocurrent. However, in thicker Sb2S3 layer, most of electrons and holes are generated near the TiO2 side due to exponential photon absorption (Fig. 10c); therefore, the transport path of most of the electrons are almost the same. However, most of the holes need to be diffused in a longer path than electrons in the thicker Sb2S3 layer, which is demonstrated by longer τIMPS in Fig. 10d. When the thickness exceeds the hole diffusion length, the inferior area in Sb2S3 for an inefficient hole generation and transport would decrease the photocurrent and weaken the Jsc and EQE. The hole diffusion length in Sb2S3 is around 180 nm [18]. When the thickness of Sb2S3 exceeds hole diffusion length, the collection performance of holes is reduced which is also responded by EQE spectra (Fig. 5b) since the absorption coefficient of the long wave is much lower than the short wave, resulting in a longer illumination depth for long wave (Fig. 9) [35]. Photo-generated holes from long band could distribute more uniform in Sb2S3 than that from short band (photo-generated holes from short band could close to TiO2 side), resulting in a more efficient collection of the hole from long band. Therefore, the EQE in long-wavelength part did not get a large decreased as much as short-wave part with Sb2S3 thickness of 373 nm (Fig. 5b).

Fig. 9
figure 9

KPFM images of Sb2S3 of 1 h (a), 1.5 h (b), 2 h (c), and 3 h (d) and P3HT on Sb2S3 of 1 h (e), 1.5 h (f), 2 h (g), and 3 h (h) under white light illumination from FTO glass, respectively. i, j The corresponding SP distributions of Sb2S3 and P3HT

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As shown in Fig. 10d, it is easily understood that a smaller τIMPS is accompanied by a thinner Sb2S3 (i.e., a shorter charge transport path); however, τIMVS mainly remains the same when Sb2S3 thickness increased from 96 to 373 nm in this experiment, which means that there is no direct dependence of Jsc and Voc on τIMVS (i.e., interfacial recombination) when Sb2S3 thickness changes. It is well known that the Voc of the TiO2/Sb2S3/P3HT solar cells is normally determined by the difference between the quasi-Fermi levels of the electrons in the TiO2 and the holes in the P3HT [36]. As the collection of holes is reduced in P3HT when the thickness of Sb2S3 is larger than the hole diffusion length, it would increase the difference between the quasi-Fermi levels of electrons and holes for a lower Voc. In addition, a thicker Sb2S3 would increase the higher series resistance and worse charge collection efficiency; these unfavorable factors may cause a lower FF in thicker Sb2S3 device.

Fig. 10
figure 10

a IMPS and b IMVS characterizations of solar cells with different CBD t for Sb2S3 film. c Illustration of electron and hole diffusion area for short and long wavelength illumination. d Dependence of τIMPS and τIMVS on CBD t

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Although, the efficiency of planar TiO2/Sb2S3/P3HT n-i-p solar cells is very low, and how to further improve the device efficiency is a challenge. However, our results still demonstrated that some further improvements could be carried out. For example, enhancing the built-in electric field by employing some different electron transport layer or hole transport layer could enhance charge transport and collection. Moreover, how to improve the hole diffusion ability should be considered; maybe some conductive additives is helpful. In addition, interfacial engineer is also important for improving charge transfer and dissociation. Last but not least, the method that expressed in this paper might be offering some helpful reference for other relative high-efficiency solar cells (e.g., organic solar cells, perovskite solar cells).

Conclusion

In this paper, the mechanism of photocurrent changes in TiO2/Sb2S3/P3HT n-i-p solar cells with different thickness of Sb2S3 was studied. When the thickness is less than the hole transport length, the absorption and internal electric field are the main factors that affect the photocurrent; when the thickness is larger than the hole transport length, the inferior area in Sb2S3 for an inefficient hole generation and transport is the main reason for photocurrent decrement. Results showed that device short-circuits’ current density (Jsc) is increased with the enhanced photon absorption when the Sb2S3 thickness is less than the hole transport length; however, when the Sb2S3 thickness is larger than the hole transport length, device Jsc is sharply decreased with further increased absorption. Internal electric field decrement-related electron drift could lead to the reduction in the Jsc when the thickness of Sb2S3 is less than the hole transport length. However, when the thickness of Sb2S3 is larger than the hole transport length, the internal electric field has little effects on electron and hole drift, but Jsc still largely decreased. KPFM and IMPS/IMVS characterization demonstrated that there is an inferior region for photo-generated electrons in thicker Sb2S3 film. The inferior area in Sb2S3 for a reduction of holes that can diffuse into the P3HT when the Sb2S3 thickness is larger than the hole diffusion length, leading to the obviously decreased Jsc. Moreover, the reduced collection of holes in P3HT with the increased thickness of Sb2S3 would increase the difference between the quasi-Fermi levels of electrons and holes for a lower Voc.