Sb2S3 Thickness-Related Photocurrent and Optoelectronic Processes in TiO2/Sb2S3/P3HT Planar Hybrid Solar Cells

In this work, a comprehensive understanding of the relationship of photon absorption, internal electrical field, transport path, and relative kinetics on Sb2S3 photovoltaic performance has been investigated. The n-i-p planar structure for TiO2/Sb2S3/P3HT heterojunction hybrid solar cells was conducted, and the photon-to-electron processes including illumination depth, internal electric field, drift velocity and kinetic energy of charges, photo-generated electrons and hole concentration-related surface potential in Sb2S3, charge transport time, and interfacial charge recombination lifetime were studied to reveal the key factors that governed the device photocurrent. Dark J–V curves, Kelvin probe force microscope, and intensity-modulated photocurrent/photovoltage dynamics indicate that internal electric field is the main factors that affect the photocurrent when the Sb2S3 thickness is less than the hole diffusion length. However, when the Sb2S3 thickness is larger than the hole diffusion length, the inferior area in Sb2S3 for holes that cannot be diffused to P3HT would become a dominant factor affecting the photocurrent. The inferior area in Sb2S3 layer for hole collection could also affect the Voc of the device. The reduced collection of holes in P3HT, when the Sb2S3 thickness is larger than the hole diffusion length, would increase the difference between the quasi-Fermi levels of electrons and holes for a lower Voc.


Introduction
Sb 2 S 3 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 × 10 5 cm −1 [1,2]. Sb 2 S 3 thin films can be prepared by various methods, including spray pyrolysis [3], electrodeposition [4], chemically deposition [5], and thermal vacuum evaporation technique [6]. In Sb 2 S 3 -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 photoelectronic processes in Sb 2 S 3 -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 Sb 2 S 3 photovoltaic performance, which is important to guide the optimization of the Sb 2 S 3 -based hybrid solar cells. In this work, the conventional TiO 2 /Sb 2 S 3 / 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 Sb 2 S 3 .
It is obvious that the different thickness of Sb 2 S 3 in TiO 2 /Sb 2 S 3 /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 Sb 2 S 3 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 Sb 2 S 3 thicknessdependent 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 J sc and V oc upon the Sb 2 S 3 thickness, TiO 2 /Sb 2 S 3 /P3HT n-i-p solar cells were fabricated (Fig. 1), and the thickness of Sb 2 S 3 -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 Sb 2 S 3 solar cells.

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 TiO 2 (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 Sb 2 S 3 on the top of the TiO 2 thin layer was performed by a chemical bath deposition (CBD) method [5,10,22]. An acetone solution containing SbCl 3 (0.3 M) was added dropwise into Na 2 S 2 O 3 (0.28 M) with stirring in an ice bath (~5°C). The FTO substrate was covered with a thin layer of TiO 2 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 Sb 2 S 3 layer was deposited onto the TiO 2 -coated FTO substrates, and the sample was thoroughly rinsed with de-ionized water and dried under N 2 flow. The substrate was further annealed for 30

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/cm 2 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/cm 2 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 Sb 2 S 3 /TiO 2 Film   Fig. 2b as a function of the CBD time. The average thickness d of Sb 2 S 3 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 Sb 2 S 3 film with different thickness of Sb 2 S 3 film on FTO glass are shown in Fig. 3. The measured XRD spectrum is indexed to orthorhombic Sb 2 S 3 (JCPDS PCPDFWIN #42-1393) [23].
As shown in Fig. 4, the TiO 2 samples exhibit the absorption onset at 386 nm (3.21 eV) corresponding to the bandgap absorption of TiO 2 [24]. All the as-deposited TiO 2 /Sb 2 S 3 layers with different t of CBD exhibit an absorption edge at ca. 750 nm [25]. The absorption intensity of Sb 2 S 3 on the TiO 2 surfaces is clearly in the order 3 h > 2 h > 1.5 h > 1 h. This result also indicates that the Sb 2 S 3 film gradually becomes thicker with a longer CBD t, which also agrees with the SEM results.

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 Sb 2 S 3 thickness of 175 nm can be determined by comparison of device efficiencies, at which point a maximum PCE of 1.65%, J sc of 6.64 mA cm −2 , V oc 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/ Sb 2 S 3 /P3HT n-i-p cells with Sb 2 S 3 layers of three different thickness (50, 100, and 350 nm) by thermal evaporation achieving the highest PCE (~2%) with the Fig. 2 a Cross-sectional FE-SEM images of Sb 2 S 3 films on TiO 2 dense layer-coated glass substrates. b Average Sb 2 S 3 thickness d plotted as a function of the CBD reaction time t for the Sb 2 S 3 film deposition. The values were estimated by the FE-SEM cross-section images 100-nm-thick Sb 2 S 3 [12]. Kamruzzaman et al. studied TiO 2 /Sb 2 S 3 /P3HT n-i-p cells with Sb 2 S 3 thicknesses of 45-120 nm by a thermal evaporation method, and the absorber Sb 2 S 3 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 Sb 2 S 3 indeed strongly affects the device performance, even by different deposition strategies of Sb 2 S 3 film or annealing condition.

Charge Transport
The device J sc increases remarkably with increasing Sb 2 S 3 thickness d from 96 to 175 nm and then decreases as the d increases ( Fig. 5 and Table 1). The device J sc is significantly dependent on the Sb 2 S 3 thickness d. The charge carrier generation and dissociation are key processes for photocurrent generation. Firstly, visible light will pass through the TiO 2 layer due to its visible light window property (Fig. 4) and begin to be absorbed from TiO 2 /Sb 2 S 3 interface. Sb 2 S 3 has been proved to be a high absorption coefficient α around 10 5 cm −1 in the visible region [27]. Here, we take α = 10 5 cm −1 for Sb 2 S 3 . The thickness-dependent illumination depth is depicted in Fig. 6 according to the Beer-Lambert law I(x) = I 0 e -ax , in which the I 0 is the incident photon flux and the I(x) is the photon flux in Sb 2 S 3 . Obviously, the incident photons cannot be absorbed fully when the Sb 2 S 3 has a thickness of 100 nm or 200 nm (Fig. 6b). The d-related ratio of absorbed photon (N a )/incident photons (N i ) can be calculated by integration of the area of the shaded area in coordinate. As shown in Fig. 6b (also refer Fig. 7b), the N a /N i is 61% when the d = 96 nm and N a /N i is enhanced to 82% when the d = 175 nm. It can be believed that the further 21% photons absorbed might cause the increase in J sc from 5.50 to 6.64 mA/cm 2 . When the d increases to 280 nm, the extra 11% photons are absorbed and the N a /N i is further enhanced to 93%, which shows that more photons could be further absorbed and then might generate more electrons. However, the device J sc decreased to 5.06 mA/cm 2 which is lower than the case of d = 96 nm. When the d increases to 373 nm, the N a /N i is close to 100%, and the device J sc is sharply decreased to 2.64 mA/cm 2 . Therefore, absorption is not the sole factor that affects J sc .
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 (V in ) normally can be estimated at the turning point where the dark curve begins to follow a quadratic behavior (Fig. 7a). Dependences of V in , J sc , N a /N i , E ke , and E kh on CBD t are shown in Fig. 7b. When d increased from 96 to 175 nm, the N a /N i enhanced 34.44%; however, the J sc only increased by 20.72%, which means that there is another factor limiting the J sc increment. It has been inferred that this was might due to the decreased internal electrical field across the Sb 2 S 3 layer, which weakened the photon-generated electron/hole drift [16]. Therefore, we calculated the internal electrical field E in cross the Sb 2 S 3 based on the relation of E in = V in /d (Table 2). Moreover, the drift velocity of electron v e and hole v e , kinetic energy of electron E ke , and hole E kh under internal electric field E in were also calculated ( Table 2 and Fig. 7b). When the d is 96 nm, the E ke is 296.56 meV, and E kh is 53.25 meV. When the d increased to 175 nm, the E ke largely  decreases to 95.29 meV and E kh decreased to 17.12 meV, which is lower than the thermal energy at ambient temperature (E kt , 26 meV). This result indicates that the internal electric field has little effects on hole drift when Sb 2 S 3 thickness is or larger than 175 nm. Obviously, the reduced E ke and E kh with the thicker Sb 2 S 3 should be the reason that limits the increment of J sc . Further increasing d from 175 to 280 nm, the N a /N i enhanced to 13.84%; however, the J sc get decreased. This might be due to the decrease in E ke which is close to the E kt (d = 280 nm) but much lower than the E kh (d = 373 nm), which means the E in gradually has little effects on electron drift when d > 280 nm as observed in this work. Therefore, E in decrement-related electron drift might be responsible for the J sc reduction when d increased from 175 to 280 nm. However, when the d increased to 373 nm, the E in has little effects on electron and hole drift, but J sc still largely decreased, which indicates that E in is also not the sole factor that affects the J sc .
We used KPFM to characterize the photo-generated electrons and hole concentration-related surface potential (SP) in Sb 2 S 3 /P3HT. The sample for the KPFM measurement was prepared by drop casting the P3HT precursor solution onto part of the FTO/TiO 2 /Sb 2 S 3 film surface (Fig. 8). As the Sb 2 S 3 thickness increases from 96 to 373 nm, the SP on the top of Sb 2 S 3 gradually becomes smaller, which means the Fermi level on the Sb 2 S 3 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 Sb 2 S 3 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 Sb 2 S 3 . P3HT might be excited by light to generate excitons and then separate into electrons and holes [29,30], when Sb 2 S 3 is very thin (< 200 nm). When Sb 2 S 3 becomes thicker, P3HT only acts as the hole transport layer, because most of the photons are absorbed by Sb 2 S 3 (Fig. 3). Therefore, when the thickness of Sb 2 S 3 is less 280 nm, P3HT could be photo-excited, resulting in the Fermi level of P3HT gradually decreases as Sb 2 S 3 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 Sb 2 S 3 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 Sb 2 S 3 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 J sc .  Furthermore, IMPS and IMVS, as the powerful dynamic photoelectrochemical methods in dyesensitized 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/opencircuit 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 τ IMPS for the electrons to reach the collection electrode under short-circuit condition or the electron lifetime τ IMVS related to interfacial charge recombination under open-circuit condition. According to the relation τ = (2πf) −1 [31][32][33][34][35], τ IMPS and τ IMVS in the devices were calculated ( Table 1). The increased τ IMPS suggests a longer transport path of charges to collection electrode, whereas the unchanged τ IMVS infers the same interfacial charge recombination [33]. The interfacial charge collection efficiency η c is typical considered as η c = 1-τ IMPS /τ IMVS [31][32][33][34][35]. Obviously, the longer transport time of the τ IMPS and the short interfacial charge recombination lifetime of the τ IMVS would cause a worse charge collection and vice versa. In this study, the τ IMPS increases with the thicker Sb 2 S 3 while the τ IMVS is unchanged. Therefore, interfacial charge collection efficiency η c decreases with the thicker Sb 2 S 3 , and the changes of J sc in different Fig. 6 The illustration of the Sb 2 S 3 thickness d-dependent illumination depth x and E in Fig. 7 a Semilogarithmic plots of J-V characteristic in the dark of the solar cells with different CBD t for Sb 2 S 3 film. b Dependences of V in , J sc , N a / N i , E ke , and E kh on Sb 2 S 3 thickness d thickness of Sb 2 S 3 solar cells should be caused by the transport path and charge collection efficiency, not by charge recombination.
The increase in Sb 2 S 3 thickness could absorb more photons which could enhance the photocurrent. However, in thicker Sb 2 S 3 layer, most of electrons and holes are generated near the TiO 2 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 Sb 2 S 3 layer, which is demonstrated by longer τ IMPS in Fig. 10d. When the thickness exceeds the hole diffusion length, the inferior area in Sb 2 S 3 for an inefficient hole generation and transport would decrease the photocurrent and weaken the J sc and EQE. The hole diffusion length in Sb 2 S 3 is around 180 nm [18]. When the thickness of Sb 2 S 3 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 Sb 2 S 3 than that from short band (photogenerated holes from short band could close to TiO 2 side), resulting in a more efficient collection of the hole from long band. Therefore, the EQE in longwavelength part did not get a large decreased as much as short-wave part with Sb 2 S 3 thickness of 373 nm (Fig. 5b).
As shown in Fig. 10d, it is easily understood that a smaller τ IMPS is accompanied by a thinner Sb 2 S 3 (i.e., a shorter charge transport path); however, τ IMVS mainly remains the same when Sb 2 S 3 thickness increased from 96 to 373 nm in this experiment, which means that there is no direct dependence of J sc and V oc on τ IMVS (i.e., interfacial recombination) when Sb 2 S 3 thickness changes. It is well known that the V oc of the TiO 2 / Sb 2 S 3 /P3HT solar cells is normally determined by the difference between the quasi-Fermi levels of the electrons in the TiO 2 and the holes in the P3HT [36]. As the collection of holes is reduced in P3HT when the thickness of Sb 2 S 3 is larger than the hole diffusion length, it would increase the difference between the quasi-Fermi levels of electrons and holes for a lower V oc . In addition, a thicker Sb 2 S 3 would increase the higher series resistance and worse charge collection efficiency; these unfavorable factors may cause a lower FF in thicker Sb 2 S 3 device.
Although, the efficiency of planar TiO 2 /Sb 2 S 3 /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). The mean value of d for a certain Sb 2 S 3 deposition time t is used for calculation of the above parameters E in ,v e, v h, E ke , and E kh Fig. 8 Illustration of SP measurement of Sb 2 S 3 /P3HT interface by KPFM

Conclusion
In this paper, the mechanism of photocurrent changes in TiO 2 /Sb 2 S 3 /P3HT n-i-p solar cells with different thickness of Sb 2 S 3 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 Sb 2 S 3 for an inefficient hole generation and transport is the main reason for photocurrent decrement. Results showed that device short-circuits' current density (J sc ) is increased with the enhanced photon absorption when the Sb 2 S 3 thickness is less than the hole transport length; however, when the Sb 2 S 3 thickness is larger than the hole transport length, device J sc is sharply decreased with further increased absorption. Internal electric field decrement-related electron drift could lead to the reduction in the J sc when the thickness of Sb 2 S 3 is less than the hole transport length. However, when the thickness of Sb 2 S 3 is larger than the hole transport length, the internal electric field has little effects on electron and hole drift, but J sc still largely decreased. KPFM and IMPS/IMVS characterization demonstrated that there is an inferior region for photo-generated electrons in thicker Sb 2 S 3 film. The inferior area in Sb 2 S 3 for a reduction of holes that can diffuse into the P3HT when the Sb 2 S 3 thickness is larger than the hole diffusion length, leading to the obviously decreased J sc . Moreover, the reduced collection of holes in P3HT with the increased thickness of Sb 2 S 3 would increase the difference between the quasi-Fermi levels of electrons and holes for a lower V oc .