Hybrid solar cells using solution-processed TiO

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Hybrid solar cells using solution-processed TiO

₂

/Sb

₂

S

₃

bilayer as electron transport layer

Xiaoxiao Ma

^a

, Jian Zhong

^a

, Mingqiu Li

^a

, Jianpo Chen

^a

, Yingxia Zhang

^a

, Sujuan Wu

^a,^⇑

, Xingsen Gao

^a

, Xubing Lu

^a

, Jun-Ming Liu

^b,^⇑

, Haiying Liu

^c

aInstitute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China

bLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

cSchool of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China Received 23 September 2015; received in revised form 30 January 2016; accepted 25 March 2016

Communicated by: Associate Editor Sam-Shajin Sun

Abstract

Efficient inverted hybrid solar cells using poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction as active layers and TiO₂/Sb₂S₃ bilayers as electron transport layers (ETL) have been prepared in ambient air. The microstructural, photoelectronic, and electrochemical characteristics on these cells have been investigated. Photoluminescence measure- ment indicates that the exciton dissociation in TiO₂/Sb₂S₃is more effective than that in ITO/Sb₂S₃structure. The Nyquist plots obtained from electrochemical impedance spectroscopy implies that charge recombination has been suppressed in TiO2/Sb2S3 with respect to Sb2S3structure. In addition, the TiO2/Sb2S3bilayer structure exhibits significantly enhanced external quantum efficiency in the visible region. The cell structure consisting of ITO/TiO₂/Sb₂S₃/P3HT:PCBM/PEDOT:PSS/Ag with the TiO₂/Sb₂S₃bilayer synthesized under the optimized conditions gains an average efficiency of 3.52%, which is enhanced for28.5% and209% referring to the cells only with the TiO₂layer and Sb₂S₃layer as the ETL, respectively.

Keywords: TiO2/Sb2S3bilayer; Electron transport layer; Air-processed P3HT:PCBM inverted cells

1. Introduction

In recent years, hybrid bulk-heterojunction solar cells (HSCs) have attracted much attention due to the possibility of low cost, mechanical ﬂexibility, and easy manufacturing by solution process (Dennler et al., 2009; Gao et al., 2015;

Cha et al., 2014; Kang et al., 2014). Although encouraging progress has been made, low power conversion eﬃciency

(PCE) and performance stability as compared to inorganic solar cells seem to be one of the major drawbacks beyond commercialization. ThePCEimprovement requires imple- mentation of new materials and exploration of new device architectures. In regular structure HSCs, a blend of electron-donor conjugated polymer and electron acceptor fullerene derivatives as active layer is sandwiched between a transparent indium tin oxide (ITO) anode and a low-work-function metal cathode (e.g., Ca, Al). The device performance in regular structure often degrades due to the instability of the interface between ITO and poly(3,4-ethyle nedioxythiophene):poly-(styrene sulfonate) (PEDOT:PSS),

⇑ Corresponding authors.

E-mail addresses: [email protected] (S. Wu), [email protected] (J.-M. Liu).

www.elsevier.com/locate/solener

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Solar Energy 133 (2016) 103–110

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the oxidation of the reactive metal cathode, and the morphology evolution in the photoactive layer (J.J. Zhu et al., 2011). In order to overcome these problems in regular structure HSCs, inverted structure devices have been introduced, where the degradation at the ITO/PEODT:

PSS interface induced by the strong acidic nature of PEDOT:PSS can be avoided. Moreover, the vertical spon- taneous phase separation can be eﬀectively utilized (Dennler et al., 2009; Gao et al., 2015). In the inverted HSCs, an electron transport layer (ETL) between the ITO and the active layer can be introduced to allow the bottom ITO cathode to eﬀectively collect electrons.

For organic electronics, the organic/electrode interface plays a critical role in terms of device performance. This interface can be modiﬁed by inserting a functional interlayer to improve the device performance. Inorganic ETL such as titanium oxide (TiOx) (Xiong et al., 2014; Kim et al., 2006, 2011; Hadipour et al., 2013; Yusoﬀ et al., 2013), zinc oxide (ZnO), (Cha et al., 2014; Wilken et al., 2012; Cho et al., 2014; Kundu et al., 2013; Liao et al., 2013; Chang and Leu, 2013) cesium carbonate (Cs2CO3), (Xiao et al., 2013; Park et al., 2013a,b, 2009; R. Zhu et al., 2011) chromium oxide (CrOx), (Wang et al., 2010) and Cadmium selenide (CdSe) quantum dot (Lee et al., 2012; Jeon et al., 2012) have been widely used as the interlayers in the inverted HSCs, due to their suitable energy level alignment and good electron-transporting property.

These inorganic interlayers cannot absorb the sunlight in the visible range. New n-type inorganic ETLs which can act as junction with the active layer materials for charge separation and increase the sunlight absorption are partic- ularly popular in inverted HSCs. For example, inserted cadmium sulﬁde (CdS) layer does enhance the visible range absorption and improves the performance (J.J. Zhu et al., 2011; Yavuz and Yu, 2013). It provides a promising path- way for the fabrication of eﬃcient inverted HSCs.

In fact, in order to simultaneously enhance the light harvesting and charge transfer of the HSCs, one may anchor inorganic semiconductor materials on the ETL surface or select n-type inorganic sensitizer as the ETL in inverted HSCs. Inorganic semiconductors offer advantages such as high extinction coefficient and capability of optical absorption tailoring over a wider wavelength range (Cardoso et al., 2012; Chang et al., 2010; Im et al., 2011). Sb2S3as one of metal chalcogenides is an attractive candidate. High performance hybrid solar cells with Sb2S3as absorber have been fabricated (Chang et al., 2010; Im et al., 2011). How- ever, the HSCs with Sb₂S₃or TiO₂/Sb₂S₃as ETL has not been reported. Chemical bath deposition has been widely employed to fabricate the Sb2S3layer, which is easy to con- trol and cost-effective with regard to equipment cost and energy needs. Thus, the TiO2/Sb2S3ETL by chemical bath deposition are selected with high preference in this work.

Another problem associated with P3HT:PCBM cells is the charge recombination in the P3HT:PCBM active layer induced by oxygen, which deteriorates the cell perfor- mances (Krebs and Norrman, 2007; Seemann et al., 2009;

Chellappan et al., 2009; Schaﬀerhans et al., 2010). There- fore, almost all HSCs based on P3HT:PCBM are fabricated in inert atmosphere or vacuum, which adds technique complexity and cost. A method that can solve this oxygen-relevant problem and allow device fabrication in air is highly desirable, and will be both cost and technology-competitive.

Taking care of these considerations, in this work we address fabrication and characterization of inverted HSCs using TiO2/Sb2S3bilayer as the ETL in ambient air. In this structure, the P3HT:PCBM is used as the active layer, and PEDOT:PSS as the hole-selective layer. The duration for the Sb2S3layer deposition is the key issue and will be investigated in details. Subsequently, we characterize the photo- electric properties and cell performance of the inverted HSCs based on the P3HT:PCBM active layer with the TiO2/Sb2S3 bilayer as the ETL (denoted as TiO2/Sb2S3

devices), the TiO₂ layer alone as the ETL (denoted as TiO2 devices), and the Sb2S3 layer alone as the ETL (denoted as ITO/Sb2S3devices). Our data shows the significantly enhanced external quantum eﬃciency (EQE) in the visible region in the TiO2/Sb2S3devices with respect to the other two types of devices and improved charge transfer compared to Sb2S3devices. The average PCEat the optimized conditions is improved to 3.52%, contrasted by the 2.74% for the TiO2devices and 1.14% for the Sb2S3devices.

2. Experimental details

All materials for the fabrication of HSCs were used as received. P3HT (4002-EE, regioregularity is 90–93%) and PCBM were purchased from Rieke Metals and Solenne BV, respectively. The PEDOT:PSS (Clevios Al4083) was bought from Heraeus. The sheet resistance of the ITO coated glass substrate was 8X/square. Blend solutions with a 1:0.8 weight ratio of P3HT to PCBM were prepared in chlorobenzene.

Fig. 1(a) shows a schematic diagram of TiO2/Sb2S3

device to be fabricated in this work. For synthesizing the TiO2/Sb2S3 bilayers, cleaned ITO substrates were first coated with a 40 nm compact TiO2 films by spin-coating the sol–gel solution (0.25 M titanium n-butoxide in ethanol) at 6000 rpm. These samples were annealed under O2at 500° C for 15 min by rapid thermal processing (RTP) to allow sufficient crystallization of the TiO2 layers. Subsequently, several of the annealed samples were chosen as the TiO2

reference samples and the other samples were submitted to next processing. Each of the TiO₂ﬁlms except the reference samples was covered with a Sb2S3layer using a chemical bath deposition method in an aqueous solution of SbCl3

and Na2S2O3at 4°C in ambient air (Cardoso et al., 2012;

Boix et al., 2012a,b; Lim et al., 2012; Zhong et al., 2013).

The growth rate of Sb2S3 is about 1.0 nm/min (as shown inFig. S1, see supporting information). For the Sb2S3refer- ence samples, cleaned ITO substrates were used to deposit the Sb2S3layers. The thickness of the Sb2S3layers was con- trolled by the deposition time. The as-prepared TiO2/Sb2S3

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bilayers, the TiO2reference layers, and the Sb2S3reference layers on ITO substrates were rinsed thoroughly with deion- ized water and dried. Using the reported method (Zhong et al., 2013), these samples were annealed in nitrogen (N2) ambient at 350°C for 20 min, followed by cooling in air.

To fabricate the inverted HSCs, the P3HT:PCBM blend solution in chlorobenzene (20 mg/mL and 16 mg/mL for P3HT and PCBM, respectively) was spin-coated on these samples in air. The thickness of the coated P3HT:PCBM active layers was 150 nm. On each of the P3HT:PCBM active layers was spin-coated the PEDOT:PSS solution at 3000 rpm for 30 s in air, followed by thermal annealing at 140°C for 10 min in a glove box ﬁlled with high purity N2. Then these samples were taken out of the glovebox and kept in air until the deposition of the top Ag electrodes. The Ag electrodes with a thickness of80 nm were deposited on the active layers by thermal evaporation through a shadow mask. The active area of each device is 0.2 cm². These devices were then taken out to air and trans- ferred to the glovebox for postannealing at 120°C for 20 min. The purpose of the postannealing is to remove oxygen in the active layer and optimize the contact between the active layers and Ag cathodes. After the post-annealing, the devices were encapsulated by glass caps and epoxy in the glovebox. It should be mentioned that except the Sb2S3 layer, synthesis conditions for the other layers in the devices have been optimized and no details will be given here (Zhong et al., 2013; Wu et al., 2010).

The films’ thicknesses were measured with a TalyForm P-6 profilometer (Tencor). The structure of Sb2S3 is characterized using X-ray diffraction pattern (XRD, X⁰Pert PRO, Cu Ka radiation). The photoluminescence (PL) spectra of the TiO2/Sb2S3/P3HT:PCBM, TiO2/P3HT:PCBM and Sb2S3/P3HT:PCBM films were measured by a fluores- cence spectrophotometer (HITACHI F-5000) exited at 500 nm. The PL spectra have been normalized to absorbance and are measured at the same conditions. The surface morphology and roughness of the ETL were characterized with atomic force microscopy (AFM, Asy- lum Research, Cypher) in tapping mode. The morphology of Sb2S3 layer coated ITO is characterized by scanning

electron microscopy (SEM, JEOL 5700, Japan). The photovoltaic performance of the as-prepared HSCs was characterized using a Keithley 2400 source meter under an illumination of 100 mW cm² (Newport 91160, 150 W solar simulator equipped with an AM 1.5G ﬁlter). The radiation intensity was calibrated by a standard silicon solar cell (certiﬁed by NREL) as the reference. The EQE and the UV–vis absorption spectra were measured using a standard EQE system (Newport 66902). The electrochemical impedance spectroscopy (EIS) measurements were performed on the Zahner Zennium electrochemical workstation in the dark at 0.60 V applied bias. A 10 mV AC sinusoidal signal source was employed over the con- stant bias with the frequency ranging from 1 Hz to 1 MHz.

3. Results and discussion

The energy level diagram of the TiO2/Sb2S3 devices is shown in Fig. 1(b), where the energy levels across the TiO2, Sb2S3, P3HT, and PCBM are aligned so that the excitation dissociation and charge transfer at the interfaces are energetically favorable (Liao et al., 2013; Boix et al., 2012b; Khan et al., 2011). Both P3HT and Sb2S3 absorb sunlight and generate excitons. Electron aﬃnities of P3HT, Sb2S3 and PCBM are 3.37 eV, 3.75 eV and 4.2 eV (Boix et al., 2012b; Khan et al., 2011), respectively. It is energetically favorable for electron transfer from P3HT to Sb2S3or PCBM and Sb2S3to PCBM or hole injection from Sb2S3 to P3HT as indicated by arrows in Fig. 1(b) (Khan et al., 2011; Freitas et al., 2010). Moreover, success- ful sensitization of TiO2by Sb2S3in the TiO2/P3HT hybrid solar cells was once demonstrated, indicating that the energy levels of Sb2S3 and TiO2 match with each other (Chang et al., 2010; Im et al., 2011; Zhong et al., 2013;

Khan et al., 2011).

The surface morphology and roughness of the TiO2

layer and the TiO2/Sb2S3 bilayers with diﬀerent Sb2S3

deposition time (t) were investigated by AFM.Fig. 2shows the AFM micrographs of the compact TiO2layer only (a), and the TiO2/Sb2S3bilayers with the Sb2S3deposition time t= 20 min (b), 40 min (c), and 60 min (d), respectively.

Fig. 1. (a) A schematic diagram of the TiO2/Sb2S3device. (b) The corresponding energy level diagram of the corresponding materials used in our devices.

The energy levels of P3HT, Sb2S3and PCBM indicate that it facilitates the charge transfer at the P3HT/Sb2S3and PCBM interface. The arrows show the expected charge transfer in TiO2/Sb2S3structure.

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There is a significant change in the surface feature of the TiO2/Sb2S3bilayers with increasing t. Astincreases from 0 to 60 min, the root mean square (RMS) roughness increases from 2.24 nm at t= 0, 9.35 nm at t= 20 min, 19.96 nm att= 40 min, and 6.18 nm att= 60 min, respectively. The RMS reaches the maximum of 19.96 nm at t40 min. It is believed that the rough surface will be greatly beneficial to the interfacial bonding of the TiO2/ Sb2S3/polymer blend, favoring the charge transfer. At t reaches to 60 min, the RMS decreases, and this film demonstrates dendrite-like microstructure (see Fig. S2 in supporting information). Fig. S2 and S3 (see supporting information) show the SEM images of TiO2/Sb2S3 and ITO/Sb2S3 films with differentt, respectively. It is clearly seen that the dendrite-like microstructure evolves by the coalescence processing of these dendrites and grain growth.

Eventually, the large grains and clear grain boundaries can be obtained(see Fig. S2 and S3). Therefore, now one can establish the microstructural evolution of the Sb2S3layer with the growth time on TiO2 layer, as shown in Fig. 2 andFig. S2. In the initial stage, the Sb2S3takes the small island-like pattern. Then these islands grow and merge to form large grains which develop dendrite-like pattern in early state. Eventually, the large grains and clear grain boundaries can be observed. Fig. S4(a) (see supporting information) shows the energy dispersive X-ray spectrum of a TiO₂/Sb₂S₃layer, indicating that the ﬁber contains ele- ments Ti, O, Sb, and S.Fig. S4(b) (see supporting information) shows the XRD patterns of Sb2S3 layer on a slide glass substrate and the prepared condition is identical to that for the Sb2S3 layer in the TiO2/Sb2S3 devices. We can see that the peaks can be well indexed by the stibnite crystal structure (JCPDS 042-1393).

The thickness of the Sb2S3layer is crucial for the device performance and should be optimized to keep a balance between optical absorbance and charge transfer. Fig. 3(a)

shows the current density–voltage (J–V) characteristics of a series of TiO2/Sb2S3devices with different Sb2S3deposi- tion time t, while all the other structure components in these devices are identical.Fig. 3(b) and (c) shows the photovoltaic parameters including the short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) andPCEas functions oft, respectively. In the initial stage, both theJscandPCEincrease witht, while theVocandFF decrease. When t increases to 40 min, the device demonstrates the best performance. It exhibits the J_sc11.1 mA/cm²,V_oc0.601 V andFF0.527, yielding an averagePCEof 3.52%. This will be confirmed addition- ally by the EQE spectra below. Increasingtto 60 min, the PCEdecreases. Therefore the optimized t for TiO2/Sb2S3

device is 40 min, which is the deposition time of Sb2S3layer for TiO2/Sb2S3device in all of the other experiments, such as J–V, EIS and PL samples shown in Fig. 6–8, respectively. It is known that the thicker-than-optimal Sb2S3

layer yields a lowerPCE, which is attributed to insuﬃcient charge transport and thus decreased performance

Fig. 2. AFM images of (a) TiO2ﬁlm, TiO2/Sb2S3ﬁlm at (b)t= 20 min, (c)t= 40 min, and (d)t= 60 min.

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(b)

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Fig. 3. (a)J–Vcurves of TiO2/Sb2S3devices with diﬀerent growth time for Sb2S3layer. MeasuredVocandJsc(b), and evaluatedFFandPCE(c) as a function of the growth time for Sb2S3layer in the TiO2/Sb2S3devices.

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parameters and PCE (Chang et al., 2010; Zhong et al., 2013). The optimized performance of the HSCs is essen- tially attributed to the increased J_sc. In the TiO₂/Sb₂S₃ devices, P3HT and Sb2S3both absorb sunlight compared to that of P3HT/PCBM device where PCBM contribution is very small. Thus light harvesting is more effective in TiO2/Sb2S3device. The hybrid solar cells based on TiO2/ Sb2S3/P3HT structure are denoted as TiO2/Sb2S3/P3HT devices. Fig. S5(a) (see supporting information) shows theJ–Vcurves of TiO2/Sb2S3/P3HT devices with different Sb2S3deposition timet.Fig. S5(b) and (c)shows the photovoltaic parameters including Jsc, Voc, FF and PCE as functions of t, respectively. It indicates that inorganic TiO2/Sb2S3and organic P3HT interface can act as the junction for charge carrier separation and the junction contribute to the Jsc. This is consistent with the reported results (Chang et al., 2010). To further clarify the origin of the J_sc variation, the measured EQE data on the TiO2/Sb2S3 devices with different t are presented in Fig. 4, indicating dramatic enhancement of the EQE in the visible range during the first 40 min. It implies that the Jsc increment results from the improved absorption (Fig. S6, see supporting information) and good electron collection by the TiO2/Sb2S3bilayers (Chang et al., 2010;

Zhong et al., 2013). The highest EQE in the best device (t= 40 min) suggests a good light absorption and balanced charge transport, thus indicating the best performance in this structure.

EIS is found to be an effective tool for investigating charge collection in HSCs (Xu et al., 2014). The charge collection can be correlated to device performance. Thus we investigate the competition between transport and recombination in different Sb₂S₃layer thickness devices by impedance. The measured EIS results on the TiO2/Sb2S3devices with different Sb2S3 layer growth time are presented in Fig. S7 (see supporting information). Fig. S7(a) shows the Nyquist plots of TiO2/Sb2S3 device with different growth time for Sb2S3layer. The solid lines in Fig. S7(a) are the fits using the equivalent circuit model given in Fig. S7(b) (Boix et al., 2012a; Xu et al., 2014; Belmonte et al., 2008; Leever et al., 2012; Bisquert, 2002). Taking into

account the surface roughness and heterojunction, the con- stant phase element (CPE) is used instead of the ideal capacitances for characterization. It can be seen that the measured Nyquist plots can be ﬁtted well, where theRsrep- resents the contact resistance. The parallel combination of the transport resistance R1 and device capacitance C1 is attributed to the high-frequency region of the impedance response, while the parallel association of recombination resistance R2 and CPE contributes to the low-frequency response which is associated with the internal charge transfer (Xu et al., 2014; Belmonte et al., 2008; Leever et al., 2012). It can be found that the recombination resistance reduce with the increase of growth time for Sb2S3 layer.

Since the recombination rate is inversely proportional to the recombination resistance (Bisquert, 2002), it is understood that the recombination rates increases with the increase of the thickness Sb2S3layer. This is consistent with the reported results (Boix et al., 2012a). Fig. S7(c)shows the curves ofFFand (R2/Rl)^1/2as a function of growth time for Sb2S3 layer in TiO2/Sb2S3devices. The R2 and Rl are extracted from the ﬁtting data in (a) to the equivalent circuit in (b). As seen in Fig. S7(c), the (R2/Rl)^1/2 values reduce with the increase of growth time for Sb2S3 layer.

It was reported that charge collection can be quantiﬁed by (R2/Rl)^1/2, which is proportional to device FF (Xu et al., 2014). This can explain the decrease ofFF with the growth time inFig. 3.

Fig. 5(a) demonstrates the J–V curves of ITO/Sb2S3

devices with diﬀerent t. Fig. 5(b) and (c) shows the cells’

parameters of Jsc, Voc, FF and PCE as functions of t, respectively. At the beginning, the PCEof the ITO/Sb2S3

devices increases with the increase of t. When t increases to 60 min, the device shows the best performance, yielding an averagePCEof 1.14% with aJscof 5.82 mA/cm², aVoc

of 0.533 V and a FFof 0.370. When tis over 60 min, the device performance becomes worse with the increase of t.

Therefore the optimized deposition time of Sb2S3 in the ITO/Sb2S3 device is 60 min, which is the condition for ITO/Sb2S3device in all of our other experiments, such as J–V, EIS, PL, AFM and SEM samples shown inFig. 6–8 and Fig. S8, respectively. Fig. 6compares theJ–Vcurves and their EQE spectra of the TiO2 reference device, the ITO/Sb2S3 reference device and the TiO2/Sb2S3 device.

The photovoltaic performance values are summarized in Table 1. As shown inFig. 6(a), the TiO2reference device gives an average PCE of 2.74% with Voc0.620 V, J_sc7.50 mA/cm², and FF0.590, while the ITO/Sb₂S₃ reference device yields an average PCE of 1.14%. When the Sb2S3 layer is deposited on the TiO2 layer to obtain the TiO2/Sb2S3bilayer (att= 40 min) as the ETL, the average PCE increases up to 3.52%, exhibiting Jsc11.1 mA/

cm², Voc0.601 V and FF0.527. As a supplementary to the above statement, Fig. 6(b) shows the EQE spectra for these devices. It can be seen that the TiO2/Sb2S3device indicates the maximum EQE in the visible range, while the minimal EQE is from the ITO/Sb2S3 reference device.

Clearly, the eﬃciency improvement of the TiO2/Sb2S3 Fig. 4. EQE curves of TiO2/Sb2S3devices with diﬀerent growth time for

Sb2S3layer.

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device results from the increased Jsc, consistent with the EQE data.

Now we look at the possible microscopic mechanism for the high performance of the TiO2/Sb2S3 device (at t= 40 min). We ﬁrst compare the photoluminescence (PL) spectra, which can be used to evaluate charge transfer (Khan et al., 2011; Lin et al., 2014; Gollu et al., 2014).

Fig. 7 shows the PL spectra of the TiO2/P3HT:PCBM reference film, the Sb2S3/P3HT:PCBM reference film and the TiO2/Sb2S3/P3HT:PCBM film. Reasonably, the PL intensity of the TiO2/Sb2S3/P3HT:PCBM film is between the two reference films, indicating that excitons effectively dissociate in TiO2/Sb2S3/P3HT:PCBM compared with that in Sb2S3/P3HT:PCBM film (Khan et al., 2011; Lin et al., 2014; Gollu et al., 2014; Ginger and Greenham, 1998; Fu et al., 2015). This implies that the TiO2/Sb2S3 bilayer as the ETL is better than the Sb2S3 layer alone for more effective photogenerated charge transfer and excitonic dissociation. The PL intensity of the TiO2/P3HT:PCBM reference film is the lowest, implying that the TiO2 ETL can contribute most to the charge transfer and thereby

recombination suppression, in consistence with the high VocandFFof the TiO2device. Together with the fact that the Sb2S3layer allows better light absorption in the visible range(Fig. S6, see supporting information)and the highest EQE shown in Fig. 6, the best performance of the TiO2/Sb2S3device (att= 40 min) is reasonable.

In addition, we have performed the EIS measurements on these structures so that the charge recombination at the interfaces can be evaluated (Gollu et al., 2014).Fig. 8 shows the Nyquist plots of the TiO2 reference device, TiO2/Sb2S3 device, and ITO/Sb2S3 reference device. The solid lines inFig. 8are the ﬁts using the equivalent circuit model given inFig. S7(b) (see supporting information). It can be seen that the measured Nyquist plots can be ﬁtted

Fig. 5. (a)J–Vcurves of ITO/Sb2S3devices with diﬀerent growth time for Sb2S3layer. (b) MeasuredVocandJsc, and (c) evaluatedFFandPCEas a function of the growth time for Sb2S3layer in the ITO/Sb2S3devices.

Fig. 6. (a) J–V curves and (b) EQE spectra of TiO2 reference device, ITO/Sb2S3reference device and TiO2/Sb2S3device.

Fig. 7. Photoluminescence spectra of TiO2/P3HT:PCBM reference film, TiO2/Sb2S3/P3HT:PCBM film and Sb2S3/P3HT:PCBM reference film, respectively.

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well. As shown in Fig. 8, the TiO2/Sb2S3 device and ITO/Sb2S3 device show reduced recombination resistance compared with the TiO2 device (Xu et al., 2014; Leever et al., 2012; Bisquert, 2002). It is understood that the Sb2S3 layer benefits to the recombination pathways, as found earlier in TiO2/Sb2S3/P3HT hybrid solar cells (Boix et al., 2012a; Zhong et al., 2013). Although the Sb2S3 layer may slightly enhance the recombination rate, it significantly improves the light absorption (Chang et al., 2010; Zhong et al., 2013), resulting in enhancedJsc. The best performance in the TiO2/Sb2S3 device at t= 40 min can be attributed to the intimate contact between the Sb₂S₃and P3HT, high and broad light absorption, and proper band alignment at the interfaces (Chang et al., 2010; Im et al., 2011).Fig. S3 and S8 (see supporting information)indicate that Sb2S3layer is not a compact film on ITO substrate. For the worse performance of the ITO/Sb2S3 device, the possible reason is that the Sb2S3

does not form a continuous layer. There is a direct contact between the ITO and P3HT even in the presence of the Sb2S3 layer, resulting in the reduced recombination resistance and consequently eﬃciencies.

4. Conclusions

We have developed an inverted solar cell using solution processed TiO2/Sb2S3bilayer as an ETL. The eﬀect of the deposition time for Sb2S3 layer on performance of TiO2/Sb2S3 device has been studied. It is found that the TiO2/Sb2S3 device with the Sb2S3 growth time of 40 min shows the best performance. The average PCE of TiO2/Sb2S3 structure enhances to 3.52% from 1.14% for ITO/Sb2S3 structure and 2.74% for TiO2 structure. The underlying mechanism is intrinsically related to the increased EQE and eﬀective charge transfer of the TiO2/Sb2S3ETL.

Acknowledgements

We acknowledge the ﬁnancial support of the National Natural Science Foundation of China (Grant No.

51003035, 51431006, 61271127, 51472093, 61201102, 21303060, 61574065), Program for Pearl River Star (Grant No. 2012J2200030), International Science & Technology Cooperation Platform Program of Guangzhou (No.

2014J4500016), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), Science and Technology Planning Project of Guangdong Province, China (2015B090927006), and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13064).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/

j.solener.2016.03.051.

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Fig. 8. Nyquist plots of the TiO2reference device, ITO/Sb2S3 reference device and TiO2/Sb2S3device measured at the bias voltage of 0.6 V (close toVoc). Solid line is the corresponding ﬁtting curve. Equivalent circuit employed to ﬁt the Nyquist plots inFig. S7 (see supporting information).

Table 1

Summary ofVoc,Jsc,FFandPCEobtained from the devices shown in Fig. 6(a).

Voc(V) Jsc(mA/cm²) FF PCE(%)

TiO2 0.620 7.50 0.590 2.74

TiO2/Sb2S3 0.601 11.1 0.527 3.52

ITO/Sb2S3 0.533 5.82 0.370 1.14

(8)

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