NANO EXPRESS Open Access

Sb2S3 Thickness-Related Photocurrent andOptoelectronic Processes in TiO2/Sb2S3/P3HT Planar Hybrid Solar CellsFan Wu1,2* , Rajesh Pathak2, Lan Jiang1, Weimin Chen1, Chong Chen3, Yanhua Tong4, Tiansheng Zhang1,Ronghua Jian1 and Qiquan Qiao2*

Abstract

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 planarstructure for TiO2/Sb2S3/P3HT heterojunction hybrid solar cells was conducted, and the photon-to-electronprocesses 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 interfacialcharge recombination lifetime were studied to reveal the key factors that governed the device photocurrent. DarkJ–V curves, Kelvin probe force microscope, and intensity-modulated photocurrent/photovoltage dynamics indicatethat internal electric field is the main factors that affect the photocurrent when the Sb2S3 thickness is less than thehole diffusion length. However, when the Sb2S3 thickness is larger than the hole diffusion length, the inferior areain 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 ofholes in P3HT, when the Sb2S3 thickness is larger than the hole diffusion length, would increase the differencebetween the quasi-Fermi levels of electrons and holes for a lower Voc.

Keywords: Solar cells, Sb2S3, Photocurrent, Optoelectronic processes

IntroductionSb2S3 has been increasingly utilized for solid thin-film solarcells because of its moderate bandgap of 1.7 eV and an ab-sorption coefficient of 1.8 × 105 cm−1 [1, 2]. Sb2S3 thin filmscan be prepared by various methods, including spray pyroly-sis [3], electrodeposition [4], chemically deposition [5], andthermal vacuum evaporation technique [6]. In Sb2S3-basedphotovoltaic device, photoelectric conversion efficiency(PCE) has reached to 5.7–7.5% by improved technology anddevice design [1, 2, 7–10]. However, current efficiencies ofsolid-state devices still remain low compared to otherphotovoltaic 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 betteroptoelectronic performance in solid-state devices [7–10, 13–15]. In this regards, it is imperative to study the photo-electronic processes in Sb2S3-based solar cells for guidingthe device design and optimization. This includes a compre-hensive understanding of the balance among absorption, in-ternal electrical field, and transport path, and relativekinetics on Sb2S3 photovoltaic performance, which is im-portant to guide the optimization of the Sb2S3-based hybridsolar cells. In this work, the conventional TiO2/Sb2S3/poly(3-hexylthiophene-2,5-diyl(P3HT) n-i-p device structurewas used to study the charge carrier generation and dissoci-ation 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) theamount of photon harvesting, which influences thephoton-generated electron/hole concentration; (ii) themagnitude of internal electrical field across the Sb2S3layer, which influences the photon-generated electron/

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* Correspondence: wufanjay@126.com; qiquan.qiao@sdstate.edu1School of Sciences and Key Lab of Optoelectronic Materials and Devices,Huzhou University, Huzhou 313000, China2Center for Advanced Photovoltaics, Department of Electrical Engineeringand Computer Sciences, South Dakota State University, Brookings, SD 57007,USAFull list of author information is available at the end of the article

Wu et al. Nanoscale Research Letters (2019) 14:325 https://doi.org/10.1186/s11671-019-3157-x

hole drift; (iii) electron/hole transport distance to the re-spective electrode; and (iv) electron/hole recombination[16, 17]. However, the reason for the Sb2S3 thickness-dependent performance in n-i-p structure is still am-biguous, which has been simply attributed to the issueswith bulk resistance, photon absorption, generation/re-combination of charge carriers, and internal electric field[16–21], but the detailed and quantified analysis for thethickness-dependent photovoltaic parameters is not clearyet. To gain insight into the change of Jsc and Voc uponthe Sb2S3 thickness, TiO2/Sb2S3/P3HT n-i-p solar cellswere fabricated (Fig. 1), and the thickness of Sb2S3-re-lated photon-generated electron and hole transport pro-cesses which result in the different photocurrents wasstudied in this work. Moreover, we introduced dynamicintensity-modulated photocurrent/photovoltage spectra(IMPS/IMVS) and Kelvin probe force microscope(KPFM) characterization to study photon-to-electronprocesses and investigate the key factors that governedthe device performance in different thicknesses of Sb2S3solar cells.

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

Device FabricationThe substrates were cleaned via ultrasonication in soapwater, acetone, and isopropanol for 60 min each,followed by treatment with UV-ozone for 30 min. A thinlayer of compact TiO2 (0.15M titanium diisopropoxidein ethanol) was spin coated at 4500 rpm for 60 s,followed by annealing at 125 °C for 5 min and 450 °C for30 min. Deposition of Sb2S3 on the top of the TiO2 thinlayer was performed by a chemical bath deposition(CBD) method [5, 10, 22]. An acetone solution contain-ing SbCl3 (0.3 M) was added dropwise into Na2S2O3

(0.28M) with stirring in an ice bath (~ 5 °C). The FTOsubstrate was covered with a thin layer of TiO2 and thensuspended upside-down in the aqueous solution whenthe color of the solution changed to orange. After 1 h,1.5 h, 2 h, and 3 h of the CBD process, a smooth anduniform amorphous Sb2S3 layer was deposited onto theTiO2-coated FTO substrates, and the sample was thor-oughly rinsed with de-ionized water and dried under N2

flow. The substrate was further annealed for 30 min in aglovebox (O2: 0.1 ppm, H2O: 0.1 ppm) under an N2 at-mosphere. The fabrication of n-i-p heterojunction wascompleted 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. Fi-nally, the MoO3 (10 nm) and Ag (100 nm) electrode wasdeposited by evaporation through a shadow mask.

Instruments and CharacterizationX-ray diffraction (XRD) patterns of the film were recordedby an MXP18AHF X-ray diffractometer with Cu Kα irradi-ation (λ = 1.54056 Å). Scanning electron microscopy (SEM)measurements were performed on a field-emission scan-ning electron microscope (ZEISS, GeminiSEM 300). Theabsorption spectra were recorded with a Shimadzu UV-2600 spectrophotometer. Current density–voltage (J–V)characteristics were measured under AM 1.5 illuminationwith an intensity of 100mW/cm2 using a 94023A OrielSol3A solar simulator (Newport Stratford, Inc.). The lightintensity from a 450W xenon lamp was calibrated with astandard crystalline silicon solar cell. The J–V curves werecollected using an Oriel I–V test station (Keithley 2400Source Meter, Newport). External quantum efficiency(EQE) spectra of the solar cells were measured by using aQE/IPCE measurement kit (Zolix Instruments Co., Ltd.) inthe spectral range of 300–900 nm. Intensity-modulatedphotocurrent spectra (IMPS) and intensity-modulatedphotovoltage spectra (IMVS) were measured using an elec-trochemistry workstation (IviumStat.h, Netherlands) underambient conditions with a background intensity of 28.8mW/cm2 from a white light-emitting diode, with a small si-nusoidal perturbation depth of 10%. Kelvin probe forcemicroscope (KPFM) was performed by an Agilent SPM5500 atomic force microscope equipped with a MAC IIIcontroller (comprising three lock-in amplifiers) to map thesurface potential (SP).

Results and DiscussionDeposition and Characterization of Sb2S3/TiO2 FilmFE-SEM images (Fig. 2a) clearly show that different thick-nesses of Sb2S3 film are deposited on TiO2 layer-coatedglass 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 layerswere successfully obtained by CBD techniques. The averagethickness of the Sb2S3 film estimated from the cross-

Fig. 1 Illustration of TiO2/Sb2S3/P3HT n-i-p solar cell architecture.h+

denotes the hole and e− denotes the electron

Wu et al. Nanoscale Research Letters (2019) 14:325 Page 2 of 10

sectional FE-SEM images is plotted in Fig. 2b as a functionof the CBD time. The average thickness d of Sb2S3 film in-creases linearly with t (Fig. 2b). The average thickness in-creased almost linearly from 96 to 373 nm by changingCBD time from 1 to 3 h. The XRD patterns of Sb2S3 filmwith different thickness of Sb2S3 film on FTO glass areshown in Fig. 3. The measured XRD spectrum is indexed toorthorhombic Sb2S3 (JCPDS PCPDFWIN #42-1393) [23].As shown in Fig. 4, the TiO2 samples exhibit the ab-

sorption onset at 386 nm (3.21 eV) corresponding to thebandgap absorption of TiO2 [24]. All the as-depositedTiO2/Sb2S3 layers with different t of CBD exhibit an ab-sorption edge at ca. 750 nm [25]. The absorption inten-sity of Sb2S3 on the TiO2 surfaces is clearly in the order3 h > 2 h > 1.5 h > 1 h. This result also indicates that theSb2S3 film gradually becomes thicker with a longer CBDt, which also agrees with the SEM results.

Solar CellsJ-V characteristics of solar cells with different thicknessd (i.e., CBD t) are compared in Fig. 5a. Table 1 presentsthe overall photovoltaic performance of these devices.Increasing thickness d (i.e., CBD time t) significantly af-fects device performance. The PCE increases as d in-creases from 96 to 175 nm (i.e., t increased from 1.0 to1.5 h) and decreases thereafter, especially decreaseslargely after d > 280 nm (i.e., t > 2 h). Optimum Sb2S3thickness of 175 nm can be determined by comparisonof device efficiencies, at which point a maximum PCE of1.65%, Jsc of 6.64 mA cm−2, Voc of 0.61 V, and FF of40.81% can be achieved. This result is comparable to theother’s reports [16, 26]. Liu et al. studied hybrid ZnO/Sb2S3/P3HT n-i-p cells with Sb2S3 layers of threedifferent thickness (50, 100, and 350 nm) by thermalevaporation achieving the highest PCE (~ 2%) with the

Fig. 2 a Cross-sectional FE-SEM images of Sb2S3 films on TiO2 dense layer-coated glass substrates. b Average Sb2S3 thickness d plotted as afunction 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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100-nm-thick Sb2S3 [12]. Kamruzzaman et al. studiedTiO2/Sb2S3/P3HT n-i-p cells with Sb2S3 thicknessesof 45–120 nm by a thermal evaporation method, andthe absorber Sb2S3 and hole transporting layerP3HT were annealed under atmospheric conditions.In their studies, the thickness of 100–120 nmshowed a better power conversion efficiency of 1.8–1.94% [26]. Obviously, the thickness of Sb2S3 indeedstrongly affects the device performance, even bydifferent deposition strategies of Sb2S3 film or an-nealing condition.

Charge TransportThe device Jsc increases remarkably with increasingSb2S3 thickness d from 96 to 175 nm and then decreases

as the d increases (Fig. 5 and Table 1). The device Jsc issignificantly dependent on the Sb2S3 thickness d. Thecharge carrier generation and dissociation are key pro-cesses for photocurrent generation. Firstly, visible lightwill pass through the TiO2 layer due to its visible lightwindow property (Fig. 4) and begin to be absorbed fromTiO2/Sb2S3 interface. Sb2S3 has been proved to be a highabsorption coefficient α around 105 cm−1 in the visibleregion [27]. Here, we take α = 105 cm−1 for Sb2S3. Thethickness-dependent illumination depth is depicted inFig. 6 according to the Beer-Lambert law I(x) = I0 e

-ax, inwhich the I0 is the incident photon flux and the I(x) isthe photon flux in Sb2S3. Obviously, the incident pho-tons cannot be absorbed fully when the Sb2S3 has athickness of 100 nm or 200 nm (Fig. 6b). The d-relatedratio of absorbed photon (Na)/incident photons (Ni) canbe calculated by integration of the area of the shadedarea in coordinate. As shown in Fig. 6b (also referFig. 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 be-lieved that the further 21% photons absorbed mightcause the increase in Jsc from 5.50 to 6.64 mA/cm2.When the d increases to 280 nm, the extra 11% photonsare absorbed and the Na/Ni is further enhanced to 93%,which shows that more photons could be furtherabsorbed and then might generate more electrons. How-ever, the device Jsc decreased to 5.06 mA/cm2 which islower than the case of d = 96 nm. When the d increasesto 373 nm, the Na/Ni is close to 100%, and the device Jscis sharply decreased to 2.64 mA/cm2. Therefore, absorp-tion is not the sole factor that affects Jsc.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) ex-ponential increase for diffusion dominated current, and(iii) quadratic increase for space-charge-limited current.The built-in voltage (Vin) normally can be estimated atthe turning point where the dark curve begins to followa 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 en-hanced 34.44%; however, the Jsc only increased by20.72%, which means that there is another factor limit-ing the Jsc increment. It has been inferred that this wasmight due to the decreased internal electrical field acrossthe Sb2S3 layer, which weakened the photon-generatedelectron/hole drift [16]. Therefore, we calculated the in-ternal electrical field Ein cross the Sb2S3 based on the re-lation of Ein =Vin/d (Table 2). Moreover, the driftvelocity of electron ve and hole ve, kinetic energy of elec-tron Eke, and hole Ekh under internal electric field Einwere also calculated (Table 2 and Fig. 7b). When the dis 96 nm, the Eke is 296.56 meV, and Ekh is 53.25 meV.When the d increased to 175 nm, the Eke largely

Fig. 3 XRD patterns of the as-synthesized Sb2S3 film on FTO withdifferent 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, and3 h, respectively

Fig. 4 The UV-vis absorption of TiO2 and TiO2/Sb2S3 films with t of1–3 h, respectively

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decreases to 95.29 meV and Ekh decreased to 17.12 meV,which is lower than the thermal energy at ambienttemperature (Ekt, 26 meV). This result indicates that theinternal electric field has little effects on hole drift whenSb2S3 thickness is or larger than 175 nm. Obviously, thereduced Eke and Ekh with the thicker Sb2S3 should bethe reason that limits the increment of Jsc. Further in-creasing d from 175 to 280 nm, the Na/Ni enhanced to13.84%; however, the Jsc get decreased. This might bedue 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 elec-tron drift when d > 280 nm as observed in this work.Therefore, Ein decrement-related electron drift might beresponsible for the Jsc reduction when d increased from175 to 280 nm. However, when the d increased to 373nm, the Ein has little effects on electron and hole drift,but Jsc still largely decreased, which indicates that Ein isalso not the sole factor that affects the Jsc.We used KPFM to characterize the photo-generated

electrons and hole concentration-related surface poten-tial (SP) in Sb2S3/P3HT. The sample for the KPFMmeasurement was prepared by drop casting the P3HTprecursor solution onto part of the FTO/TiO2/Sb2S3film surface (Fig. 8). As the Sb2S3 thickness increasesfrom 96 to 373 nm, the SP on the top of Sb2S3 graduallybecomes smaller, which means the Fermi level on the

Sb2S3 surface becomes lower [28]. This demonstratesthat electrons which could diffuse to the top surface arebeing gradually reduced, indicating that there is an in-ferior region for photo-generated electrons in thickerSb2S3 film as shown in Fig. 6. We also examined the SPof P3HT part. The changes of SP of the P3HT are dif-ferent from that of Sb2S3. P3HT might be excited bylight to generate excitons and then separate into elec-trons and holes [29, 30], when Sb2S3 is very thin (< 200nm). When Sb2S3 becomes thicker, P3HT only acts asthe hole transport layer, because most of the photonsare absorbed by Sb2S3 (Fig. 3). Therefore, when thethickness of Sb2S3 is less 280 nm, P3HT could bephoto-excited, resulting in the Fermi level of P3HTgradually decreases as Sb2S3 thickness gradually in-creases (decreased photo-exciton). In the case of 280nm, the SP of P3HT drops rapidly, because there is nophoto-exciton and the P3HT works just as a hole trans-port layer to collect holes. As the Sb2S3 thickness in-creases to 373 nm which is much larger than the holetransport length, the hole collection also drops rapidly,causing the Fermi level in P3HT to rise again. More-over, the changes of SP in P3HT is much larger thanthat in the Sb2S3 in the case of d = 373 nm, whichmeans that hole collection is worse than electron col-lection and therefore would probably lead to a muchdecreased Jsc.

Fig. 5 a J–V curves and b EQE spectra of the solar cells with different CBD t for Sb2S3 film

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

CBD time (h) Thickness d (nm) Voc (V) Jsc (mA cm−2) FF (%) PCE (%) τIMPS (μs) τIMVS (ms)

1.0 96 0.59 5.50 43.93 1.44 5.04 1.01

1.5 175 0.61 6.64 40.81 1.65 5.52 1.01

2.0 280 0.60 5.06 40.87 1.25 6.34 1.01

3.0 373 0.46 2.64 37.56 0.45 6.64 1.01

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Furthermore, IMPS and IMVS, as the powerfuldynamic photoelectrochemical methods in dye-sensitized solar cells [31] and perovskite solar cells[32], have been applied to study the charge transportdynamics in this work. IMPS/IMVS measures thephotocurrent/photovoltage response to a small sinus-oidal light perturbation superimposed on thebackground light intensity under short-circuit/open-circuit condition [31–33]. The measured IMPS orIMVS responses appear in the fourth quadrant of thecomplex plane with a shape of the distorted semicircle(Fig. 10a, b). The time constant τ defined by the fre-quency (fmin) of the lowest imaginary component ofIMPS or IMVS response is an evaluation of the transittime τIMPS for the electrons to reach the collectionelectrode under short-circuit condition or the electron

lifetime τIMVS related to interfacial charge recombin-ation under open-circuit condition. According to therelation τ = (2πf)−1 [31–35], τIMPS and τIMVS in the de-vices were calculated (Table 1). The increased τIMPS

suggests a longer transport path of charges to collec-tion electrode, whereas the unchanged τIMVS infers thesame interfacial charge recombination [33]. The inter-facial charge collection efficiency ηc is typical consid-ered as ηc = 1-τIMPS/τIMVS [31–35]. Obviously, thelonger transport time of the τIMPS and the short inter-facial charge recombination lifetime of the τIMVS

would cause a worse charge collection and vice versa.In this study, the τIMPS increases with the thickerSb2S3 while the τIMVS is unchanged. Therefore, inter-facial charge collection efficiency ηc decreases with thethicker Sb2S3, and the changes of Jsc in different

Fig. 6 The illustration of the Sb2S3 thickness d-dependent illumination depth x and Ein

Fig. 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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thickness of Sb2S3 solar cells should be caused by thetransport path and charge collection efficiency, not bycharge recombination.The increase in Sb2S3 thickness could absorb more

photons which could enhance the photocurrent. How-ever, in thicker Sb2S3 layer, most of electrons andholes are generated near the TiO2 side due to expo-nential photon absorption (Fig. 10c); therefore, thetransport path of most of the electrons are almost thesame. However, most of the holes need to be diffusedin a longer path than electrons in the thicker Sb2S3layer, which is demonstrated by longer τIMPS inFig. 10d. When the thickness exceeds the hole diffu-sion length, the inferior area in Sb2S3 for an ineffi-cient hole generation and transport would decreasethe photocurrent and weaken the Jsc and EQE. Thehole diffusion length in Sb2S3 is around 180 nm [18].When the thickness of Sb2S3 exceeds hole diffusionlength, the collection performance of holes is reducedwhich is also responded by EQE spectra (Fig. 5b)since the absorption coefficient of the long wave ismuch lower than the short wave, resulting in a longerillumination depth for long wave (Fig. 9) [35]. Photo-

generated holes from long band could distribute moreuniform 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 thehole from long band. Therefore, the EQE in long-wavelength part did not get a large decreased asmuch as short-wave part with Sb2S3 thickness of 373nm (Fig. 5b).As shown in Fig. 10d, it is easily understood that a

smaller τIMPS is accompanied by a thinner Sb2S3 (i.e., ashorter charge transport path); however, τIMVS mainlyremains the same when Sb2S3 thickness increased from96 to 373 nm in this experiment, which means that thereis no direct dependence of Jsc and Voc on τIMVS (i.e.,interfacial recombination) when Sb2S3 thicknesschanges. It is well known that the Voc of the TiO2/Sb2S3/P3HT solar cells is normally determined by thedifference between the quasi-Fermi levels of the elec-trons in the TiO2 and the holes in the P3HT [36]. Asthe collection of holes is reduced in P3HT when thethickness of Sb2S3 is larger than the hole diffusionlength, it would increase the difference between thequasi-Fermi levels of electrons and holes for a lower Voc.In addition, a thicker Sb2S3 would increase the higherseries resistance and worse charge collection efficiency;these unfavorable factors may cause a lower FF inthicker Sb2S3 device.Although, the efficiency of planar TiO2/Sb2S3/P3HT

n-i-p solar cells is very low, and how to further im-prove the device efficiency is a challenge. However,our results still demonstrated that some further im-provements could be carried out. For example, enhan-cing the built-in electric field by employing somedifferent electron transport layer or hole transportlayer could enhance charge transport and collection.Moreover, how to improve the hole diffusion abilityshould be considered; maybe some conductive addi-tives is helpful. In addition, interfacial engineer is alsoimportant for improving charge transfer and dissoci-ation. Last but not least, the method that expressedin this paper might be offering some helpful referencefor other relative high-efficiency solar cells (e.g., or-ganic solar cells, perovskite solar cells).

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

d (nm) Vin (V) Ein (× 104 Vcm−1) ve (cm s−1) Eke (meV) vh (cm s−1) Ekh (meV)

96 0.61 6.35 42.55 296.56 16.51 53.25

175 0.63 3.60 24.12 95.29 9.36 17.12

280 0.61 2.18 14.61 34.96 5.67 6.28

373 0.49 1.31 8.78 12.63 3.67 2.26

The mean value of d for a certain Sb2S3 deposition time t is used for calculation of the above parameters Ein,ve,vh, Eke, and Ekh

Fig. 8 Illustration of SP measurement of Sb2S3/P3HT interfaceby KPFM

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ConclusionIn this paper, the mechanism of photocurrent changes inTiO2/Sb2S3/P3HT n-i-p solar cells with different thicknessof Sb2S3 was studied. When the thickness is less than thehole transport length, the absorption and internal electricfield are the main factors that affect the photocurrent;when the thickness is larger than the hole transportlength, the inferior area in Sb2S3 for an inefficient holegeneration and transport is the main reason for photocur-rent decrement. Results showed that device short-circuits’current density (Jsc) is increased with the enhanced pho-ton absorption when the Sb2S3 thickness is less than thehole transport length; however, when the Sb2S3 thicknessis larger than the hole transport length, device Jsc issharply decreased with further increased absorption. In-ternal electric field decrement-related electron drift could

lead to the reduction in the Jsc when the thickness ofSb2S3 is less than the hole transport length. However,when the thickness of Sb2S3 is larger than the holetransport length, the internal electric field has littleeffects on electron and hole drift, but Jsc still largelydecreased. KPFM and IMPS/IMVS characterizationdemonstrated that there is an inferior region forphoto-generated electrons in thicker Sb2S3 film. Theinferior area in Sb2S3 for a reduction of holes thatcan diffuse into the P3HT when the Sb2S3 thicknessis larger than the hole diffusion length, leading to theobviously decreased Jsc. Moreover, the reduced collec-tion of holes in P3HT with the increased thickness ofSb2S3 would increase the difference between thequasi-Fermi levels of electrons and holes for a lowerVoc.

Fig. 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 lightillumination from FTO glass, respectively. i, j The corresponding SP distributions of Sb2S3 and P3HT

Fig. 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 forshort and long wavelength illumination. d Dependence of τIMPS and τIMVS on CBD t

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AbbreviationsCBD: Chemical bath deposition; Ein: Internal electric field; Eke: Kinetic energyof the electron; Ekh: Kinetic energy of the hole; Ekt: Thermal energy atambient temperature; EQE: External quantum efficiency; FF: Fill factor;IMPS: Intensity-modulated photocurrent spectra; IMVS: Intensity-modulatedphotovoltage spectra; J–V: Current density–voltage; Jsc: Short-circuit current;KPFM: Kelvin probe force microscope; Na: Absorbed photon; Ni: Incidentphotons; P3HT: Poly (3-hexylthiophene-2,5-diyl; PCE: Photoelectric conversionefficiency; SEM: Scanning electron microscopy; SP: Surface potential; UV-vis: Ultraviolet-visible spectroscopy; ve: Drift velocity of the electron; vh: Driftvelocity of the hole; Vin: Built voltage; Voc: Open-circuit voltage; XRD: X-raydiffraction

AcknowledgementsThis work was supported by the National Natural Science Foundation ofChina (21607041, 11747312, 11647306), China Scholarship Council(201708330103), Science and Technology Planning Project of ZhejiangProvince, China (2017C33240), and Zhejiang Provincial Natural ScienceFoundation of China (LQ14F040003). The authors also acknowledge thesupport provided under the “1112 Talents Project” of Huzhou city.

Authors’ ContributionsFW carried out the experiments. FW and RP drafted the manuscript. LJ andWC participated in the device preparation. TH, TS, RJ, and CC participated inthe design of the study. QQ conceived of the study and helped to draft themanuscript. All authors read and approved the final manuscript.

Competing InterestsThe authors declare that they have no competing interests.

Author details1School of Sciences and Key Lab of Optoelectronic Materials and Devices,Huzhou University, Huzhou 313000, China. 2Center for AdvancedPhotovoltaics, Department of Electrical Engineering and Computer Sciences,South Dakota State University, Brookings, SD 57007, USA. 3Henan KeyLaboratory of Photovoltaic Materials and School of Physics and Electronics,Henan University, Kaifeng 475004, China. 4Department of MaterialsChemistry, Huzhou University, Huzhou 313000, China.

Received: 23 July 2019 Accepted: 23 September 2019

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