Enhanced performance of planar perovskite solar cells using low

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Enhanced performance of planar perovskite solar cells using low- temperature processed Ga-doped TiO ₂ compact ﬁ lm as ef ﬁ cient electron-transport layer

Hui Liu

^a

, Zongbao Zhang

^a

, Xiang Zhang

^a

, Yangyang Cai

^a

, Yang Zhou

^a

, Qiqi Qin

^a

, Xubing Lu

^a

, Xingsen Gao

^a

, Lingling Shui

^a

, Sujuan Wu

^a^,^*

, Jun-Ming Liu

^b

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

bLaboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China

a r t i c l e i n f o

Article history:

Received 27 December 2017 Received in revised form 23 March 2018 Accepted 25 March 2018 Available online 27 March 2018

Keywords:

Planar perovskite solar cell Ga doping

Low-temperature TiO2compact layer electric properties

Stability

a b s t r a c t

Planar perovskite solar cells (PSCs) based on low-temperature processed gallium (Ga)-doped TiO2(Ga- TiO2)ﬁlms and CH3NH3PbI3-xClxactive layer have been fabricated by one-step method. The annealing temperature of Ga-TiO2electron-transport layer (ETL) is only 200C. The effect of Ga doping concentrations on the electric properties of Ga-TiO2ETL and PSCs has been systematically investigated. At the optimum Ga concentration, the PSCs based on Ga-TiO2 and TiO2ETL achieve the champion power conversion efﬁciency (PCE) of 17.09% and 14.63%, and an average PCE of 16.51% and 14.15%, respectively.

The higher PCE of PSCs based on Ga-TiO2ETL (Ga-PSCs) can be attributed to the reduced trap state density, enhanced conductivity, increased electron mobility and suppressed charge recombination, which will lead to higherVoc,Jscand FF, thus the improved PCE. Moreover, the Ga-PSC shows negligible hysteresis and improved stability compared to the reference TiO2-PSC. After being stored in air for 28 days, the PCE of unsealed Ga-PSCs can remain to be 86% of its initial value. This work provides an excellent strategy to fabricate efﬁcient and stable PSCs by low-temperature process.

1. Introduction

Due to high absorption coefficient and long charge carrier diffusion length of perovskite materials, organic-inorganic perovskite solar cells (PSCs) have attracted much attention in the photovoltaic application[1-4]. The power conversion efficiency (PCE) of PSCs has rapidly improved to over 22.1% from 3.8% in a few years[5-8]. Nowadays, most high-efficiency PSCs that have been certified are composed of mesoscopic configurations[6-8]. The mesoscopic layer acts as the scaffold which offers the larger interface contact area and improves the charge transfer[9,10]. But the mesoscopic electron-transport layer (ETL) needs high-temperature annealing, which hampers the application onflexible plastic sub- strate. In fact, planar PSCs can be fabricated by low-temperature process and have been widely studied due to their dramatically increased efficiency, low fabrication cost and simple structure[11-

14]. In planar PSCs, the ETLs play an important role on extracting photo-generated free electrons and controlling charge recombination. Owing to the chemical stability and superior electron transfer capability, compact TiO2have been widely used as ETL in PSCs[15,16]. But TiO₂ETLs simultaneously suffer from more trap states which have a negative impact on the stability and efﬁciency of PSCs[17,18]. Moreover, the conductivity of perovskite and hole transport layer (HTL) is much higher than that of TiO₂ETLs, which can result in unbalanced charge transport in PSCs. Furthermore, obvious hysteresis in planar PSCs based on TiO2ETLs can be easily detected from theJ-Vmeasurements[19,20].

To solve these issues in PSCs based on TiO₂ ETLs, PCBM or fullerene derivatives have been successfully used to modify the surface of TiO2 ETLs[21,22]. In addition, ion doping as another important method has been widely applied to fabricate doped TiO₂ ETLs for PSCs. It was found that the incorporation of a certain amount of lithium[23], indium[9], niobium[24], lanthanum[25]

and magnesium[26] could effectively decrease trap density, tailor the energy band structure and modify surface morphology of TiO2

*Corresponding author.

E-mail address:[email protected](S. Wu).

Contents lists available atScienceDirect

Electrochimica Acta

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / e le c t a c t a

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ETLs, resulting in the reduced hysteresis and improved efficiency of PSCs. However, all of these doped TiO₂films need to be annealed at 500C and the stability of PSCs has not been investigated. Recently, TiO2compactfilms can be fabricated at low temperature by simple chemical bath method (CBD) which has been widely used in planar PSCs[27-30]. It is noted that Ga-doped SnO2and Ga-doped TiO2

(Ga-TiO2) mesoporousfilms had been used as photoanodes in dye- sensitized solar cells to improve the performance [31,32]. However, there is no report about low-temperature processed Ga-TiO2film used as ETL to fabricate efficient and stable planar PSCs. Moreover, the effect of Ga doping on the electric properties at nanoscale level, mobility and electron trap state density of TiO2ETL, and charge extraction, carrier transfer and recombination process in the PSCs have not been systematically investigated.

Based on these considerations, the Ga-TiO2ETL was fabricated by a low-temperature solution process. The annealing temperature of Ga-TiO2 ETL is only 200C. Planar PSCs based on the low- temperature processed TiO₂ ETL, CH₃NH₃PbI_3-xCl_x perovskite layer and 2,2⁰,7,7⁰-tetrakis(N,N-di-4-methoxy-phenylamino)-9,9⁰- spirobifluorene (Spiro-OMeTAD) have been fabricated. The planar PSCs consist of the structure of FTO/TiO2(Ga-TiO2)/CH3NH3PbI3- xCl_x/Spiro-OMeTAD/Ag. The effect of Ga concentrations on the electric properties of Ga-TiO2films and PSCs has been systematically investigated. For convenience of presentation, two types of PSCs are discussed in this work: TiO₂-PSC as a reference where the TiO2 film is used as ETL, and Ga-PSC where the Ga-TiO2 film is selected as ETL. Upon optimization, the Ga-PSCs demonstrate the champion PCE of 17.09% and average PCE of 16.51%. The TiO₂-PSCs prepared with the same process demonstrate the best PCE of 14.63% and average PCE of 14.15%, respectively. Obviously, the Ga- PSC show higher PCE than that of reference PSC prepared in the same process. To study the effect of Ga doping on the electric properties of TiO2ETLs at nanoscale level, the local electric properties such as photo-current and contact potential difference (CPD) were characterized by conductive force microscopy (CFM) and Kelvin probe force microscopy (KPFM), respectively. The electron trap state density (Dtrap), charge extraction, carrier transfer and recombination process in the PSCs were investigated by current- voltage (I-V) characteristic curves, steady-state photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). The results indicate that the Ga doping at the optimum concentration can promote the charge transfer and suppress carrier recombination at Ga-TiO2/perovskite interface, resulting in the improved performance of PSCs. Moreover, Ga doping reduces trap state density and increase the mobility of electron in TiO₂films, contributing to diminished hysteresis and enhanced stability of Ga- PSCs. This work provides a simple method to fabricate efficient and stable planar PSCs by low-temperature process.

2. Experimental section

2.1. Preparation of TiO₂and Ga-TiO₂ﬁlms

Fluorine-doped SnO2 (FTO) coated glass substrates were suc- cessively cleaned with deionized water, acetone, isopropanol and ethanol by sonication for 20 min, then dried with nitrogen (N₂) ﬂow. Oxygen UV treatments were subsequently applied to the FTO substrates for 15 min, then the cleaned FTO substrates were immersed in a 0.2 M aqueous solution of titanium chloride (TiCl₄, 99.6%, Alfa Aesar) or gallium nitrate hydrate (Ga(NO3)3$xH2O, 99.99%, Aladdin)/titanium chloride (molar ratios: 3%, 5%, 7%, 9%) in a closed vessel at 70C for 55min. The deposited substrates were washed with deionized water several times to remove loosely bound materials, dried with N2ﬂow and annealed at 200C for

30 min in air.

2.2. Fabrication of PSCs

The 40 wt% perovskite precursor solution was prepared by dis- solving CH3NH3I (synthesized according to previous report[33]) and PbCl2(99.99%, Aladdin) of a 3:1 M ratio in anhydrous N, N- dimethylformamide (DMF, 99.8%, Sigma-Aldrich) at 60C. After TiO2/FTO or Ga-TiO2/FTO substrates being treated by Oxygen UV for 10 min, perovskite precursor solution was spin-coated on them at a speed of 3000 rpm for 30 s in the glove box. All the samples were then heated on a hot plate at 100C for 60 min, the dark perovskite ﬁlms can be formed. Subsequently, a thin layer of Spiro-OMeTAD was deposited on the perovskite layer by spin-coating a chloro- benzene solution containing 80 mM Spiro-OMeTAD, 64 mM 4-tert- butylpyridine (TBP, Aladdin) and 24 mM Li-bis (tri- ﬂuoromethylsulfonyl) imide lithium salt (Li-TFSI, Sigma-Aldrich) (520 mg/ml in acetonitrile) at 4000 rpm for 30 s. Finally, silver (Ag) electrode with the thickness of ~100 nm was evaporated as top electrode via thermal evaporation through a shadow mask. The active area of these devices was 0.045 cm².

2.3. Characterizations

The Cyclic voltammetry (C-V) measurement was carried out in an electrochemical workstation with aqueous solution of Fe(CN)63/

4- as electrolyte. X-ray photoelectron spectroscopy (XPS) was measured by using AL K-Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific, UK). The percentages of Ti and Ga in the film are checked by inductively coupled plasma mass spectrometry (ICP-MS, SPECTRO ARCOS MV). The current density-voltage (J-V) characteristics of PSCs, FTO/TiO2/Ag and FTO/Ga-TiO2/Ag were performed by Keithley 2400 source meter under an illumination of 100 mW/cm² (Newport 91160, 150 W solar simulator equipped with an AM 1.5 Gfilter). The thickness of ETLs was performed by a profilometer (Dektak XT). The surface morphology of the TiO2, Ga- TiO₂and perovskitefilms was characterized by scanning electron microscopy (SEM, ZEISS ULTRA 55). Atomic force microscopy (AFM, Asylum Research, Cypher) was employed to investigate the surface potentials and conductivity of TiO₂and Ga-TiO₂films. The external quantum efficiency (EQE) was measured by a standard EQE system (Newport 66902). EIS measurements were performed on a LED (l¼526 nm) with 300 W/m²as a light source with an AC amplitude of 20 mV from 1 Hz to 1 MHz by the Zahner Zennium electrochemical workstation. The stable photoluminescence spectra (PL) was recorded by a fluorescence spectrophotometer (HITACHI F- 5000), which had been normalized to the absorbance and measured in the same conditions. For PL measurement, the exci- tation wavelength is 515 nm.

3. Results and discussion

XPS measurement was carried to investigate the effect of Ga doping on chemical composition of TiO2and Ga-TiO2ﬁlms.Fig. 1(a) shows the XPS full spectrum of 7% Ga-doped TiO2 ﬁlms. The inserted spectrum inFig. 1(a) is the peak of Ga 3d, demonstrating the presence of Ga in ETLs. As shown inFig. 1(b), the peaks located at about 464.3 and 458.6 eV can be ascribed to the binding energy of Ti 2p3/2and Ti 2p1/2of TiO2, which results from theþ4 oxidation state[34]. It is noted that the binding energy of Ti 2p_1/2and Ti 2p_3/2 peaks for Ga-TiO2slightly shift toward the higher value. The electronegativity value of Ti and Ga is about 1.5 and 1.6, respectively[35, 36]. According to the Pauling electronegativity theory[37], the negative charge will prefer to transfer toward Ga, which results in the increased binding energy of Ti 2p[38,39]. Meanwhile, the peaks

H. Liu et al. / Electrochimica Acta 272 (2018) 68e76 69

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of O1sin Ga-TiO2ﬁlm shifts toward higher binding energy as shown inFig. 1(c, d), which suggests that the chemical environment of Ga- Ti composite has changed. These changes indicated the existence of Ga^3þin the TiO2crystal lattice[9]. In addition, the radius of Ga^3þ and Ti^4þis 0.062 and 0.0605 nm, respectively, which beneﬁt for Ga³^þdoping into the TiO₂lattice[34].

The effect of Ga doping concentrations on the performance of Ga-PSCs has been investigated.Fig. 2(a) shows theJ-Vcurves of reference TiO₂-PSC and Ga-PSCs with different Ga concentrations.

Fig 2(b, c) shows the detailed photovoltaic parameters of PSCs including short-circuit current density (Jsc), open circuit voltage (V_oc),ﬁll factor (FF), and PCE as functions of Ga concentrations. The reference TiO2-PSC shows a PCE of 14.63%, with theVoc,Jscand FF of 0.92 V, 21.80 mA/cm²and 0.730, respectively. For Ga-PSCs, the PCE increases with the increase of Ga concentrations at the beginning.

When the Ga concentration increases to 7%, Ga-PSC shows the highest PCE, yielding aJscof 22.45 mA/cm², aVocof 0.98 V, a FF of 0.774 and a PCE of 17.09%. Then the PCE decrease with the increase of Ga concentration when it is over 7%. Therefore, the optimum Ga concentration is 7%, which is the condition for Ga-TiO2ﬁlm and Ga- PSCs in all of our other experiments fromFigs. 4e8andFigs. S1eS8 (Supporting Information, SI).Fig. 2(d) shows the efﬁciency distribution histograms of TiO2 and Ga-TiO2 PSCs evaluated from 40 devices every batch. The average PCE for reference TiO2-PSCs and Ga-PSCs is 14.15% and 16.51%, respectively. It is clear that Ga-PSCs demonstrate the higher PCE than that of TiO2-PSCs.

The obtained positive effects of Ga doping simulate us to investigate the Ga distribution and morphology of Ga-TiO₂ﬁlms.

Fig. S1 and S2show SEM images and elemental distribution mapping images of the TiO2and Ga-TiO2films, respectively. It is found that Ga distribution in Ga-TiO2films is homogenous on the surface of thefilms.Fig. S3 and S4show the cross-sectional SEM images and elemental distribution mapping images of the TiO2and Ga-TiO2

ﬁlms, respectively. As shown inFig. S3 and S4, there is no apparent change in Ga concentration from top to bottom in the Ga-TiO₂ﬁlm.

Fig. S5shows the SEM images of perovskitefilm deposited on TiO₂ and Ga-TiO2 film, respectively. It could be seen that the micro- graphs of perovskitefilms and TiO2with and without Ga doping do not show significant difference.

To explore the reason of the improved electric performance in Ga-PSCs, the electric properties of TiO2and Ga-TiO2ﬁlms such as electron conductivity (s0) and trap state density (D_trap) have been systematically investigated. TheI-Vcurves of the device with the structure of FTO/TiO2(Ga-TiO2)/Ag. As shown in Fig. S6, the red ﬁtting line at low voltage region indicates an ohmic response of device. In this low bias low voltage region, the conductivity (s0) can be estimated from the slope of I-V plot by equation(1)[40,41].

I¼

s

0AL¹V (1)

Where A is the area of sample (0.045 cm²),Lis the thickness of TiO₂or Ga-TiO₂ﬁlm (z50 nm, as shown inFig. S7). The calculated s0values are summarized inFig.3(a). It is noted that the value ofs0

in TiO2film increase with Ga concentration at the beginning, and the bests0of TiO₂is obtained with 7% Ga concentration. When Ga concentration is over 9%, thes0becomes worse. The results suggest that Ga doping does improve the conductivity of TiO2. With the increase of current, the bias voltage exceeds the kink point, indicating the electron trap states are completelyfilled. The current increases significantly with the increase of voltage. The density of the electron traps (D_trap) can be calculated by equation(2)[42].

V_TFL¼eDtrapL²

2εε0 (2)

In the equation, e is the elementary charge of the electron Fig. 1.XPS spectrum of Ga-TiO2film with 7% Ga doping (a), Insert: Ga 3d peak; Ti 2p peak (b), the O1s peak of the TiO2film (c); the O1s peak of 7% Ga-TiO2film (d).

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(¼1.610¹⁹C),Lis the thickness of TiO2or Ga-TiO2film,εis the relative dielectric constant of TiO₂ (z50), ε0 is the vacuum permittivity (¼8.85410¹²F/m). The calculated values are summarized inFig. 3(b). The results show that the value of Dtrapde- creases with Ga concentration increasing to 7%. When Ga concentration is over 7%, the D_trapincreases. It is reported that Ga doping reduces the trap states that acts as recombination centres in TiO2film, which will lead to the decreased recombination rate and the increased V_oc[43]. The change of D_trap in Ga-TiO₂films with different Ga concentrations is consistent with Vocof Ga-PSCs shown inFig. 2(b). To understand the effect of Ga doping on Dtrapexplicitly,

the I-V curves of reference TiO2and 7% Ga doped TiO2are shown in Fig. 4(a). It is found that theV_TFLof device based on TiO₂(0.56 V) is higher than that of the device based on Ga-TiO2(0.31 V), and corresponding value of Dtrap is 1.9910¹⁶ and 1.1010¹⁶cm³ for TiO2and Ga-TiO2film, respectively. Ga-TiO2film shows the lower D_trapthan that of TiO₂film, which contributes to better efficiency and stability of PSCs[35]. These results demonstrate that 7% Ga-TiO2

shows better electric properties with higher s0and lower Dtrap, which is consistent with the results inFig.2.

In order to investigate the effect of Ga doping on electron transport property of TiO2ﬁlms, cyclic voltammetry (C-V) curves of Fig. 2.(a)J-Vcurves of reference PSC and Ga-PSC, (b) and (c)Voc,Jsc, FF and PCE of Ga-PSCs as a function of Ga concentrations, (d) Efﬁciency statistics histograms of reference TiO2- PSCs and Ga-PSCs.

Fig. 3.The values0of (a) and Dtrap(b) of TiO2(Ga-TiO2)ﬁlms as a function of Ga concentrations.

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Fig. 4.(a) I-V curves of the FTO/TiO2(Ga-TiO2)/Ag; (b) C-V curves of TiO2and Ga-TiO2ﬁlms in an aqueous solution containing the Fe(CN)6 redox system. Insert: corresponding DEpcurve parameter; (c) Log (J)-log (V) curves of the FTO/TiO2(Ga-TiO2)/PCBM/Ag, (b) EQE and integrated products of the EQE curves of the reference PSC and Ga-PSC.

Fig. 5.CFM images of the TiO2film (a) and Ga-TiO2film (b); KPFM images of the TiO2film (c) and Ga-TiO2film (d). The scan size is 10mm10mm.

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TiO2and Ga-TiO2film were characterized in the Fe(CN)63/4-redox system.Fig. 4(b) shows the C-V curves. It can be observed that there are anodic and cathodic peaks. The potential difference between the two peaks is labeled asDÊp.The histograms ofDÊpfor TiO2and Ga-TiO₂films are inserted inFig. 4(b). The largerDÊpindicates that the transport capability of the layer is inferior[44,45]. As shown in Fig. 4(b), the calculatedDÊpfor Ga-TiO2film is smaller than that of TiO₂film, implying that Ga doping can promote electron transfer from perovskite layer to TiO2layer[44]. Moreover, the mobility of ETL is also a straightforward signature that can illustrate the charge transfer at the ETL/perovskite interface. A high mobility reveals less trap states in ETL which will benefit to promote charge transport, resulting in the improved performance of Ga-TiO2 PSC[46]. This result is consistent with the J-V measurement discussed inFig. 2.

Moreover, the mobility of TiO2 and Ga-TiO2 films had been measured by the space-charge-limited current (SCLC) model, the fitting Log(J)-Log(V) curves of the TiO2and Ga-TiO2films with the FTO/TiO₂ (Ga-TiO₂)/PCBM/Ag structure are presented inFig. 4(c).

The mobility of TiO2and Ga-TiO2 ﬁlms can be calculated by the Mott-Gurney equation(3)[46].

J¼9εε₀

m

^V²

8L³ (3)

In equation(2),Jis the current density,εis the relative dielectric constant of TiO2 (¼50), ε0 is the vacuum permittivity(¼8.85410¹²F/m).mis the electron mobility,Vis the applied voltage of the device, andL(z50 nm) is the thickness of TiO2or Ga-TiO2ﬁlm. The calculated mobility for TiO2and Ga-TiO2

ﬁlms are 1.3510⁵and 2.5710⁵cm²V¹S¹, respectively. The higher mobility of Ga-TiO₂indicates that Ga doping can help to promote the electron transport at the ETL/perovskite interface, which will beneﬁt to improve theJsc. The EQE spectra of reference PSC and Ga-PSC were shown inFig. 4(d). It can be seen that Ga-PSC demonstrates the higher EQE than that of the reference TiO2-PSC.

The integratedJscfor TiO2-PSC and Ga-PSC is 20.01 and 20.61 mA/

cm², respectively. This agrees well with theJ_scvalues inFig. 2.

The CFM have been widely used to study local photocurrent characteristics at the surface, which can reach high sensitivity of picoampere (pA) in PSCs[47]. It is reported thatJ_scis related to the current distribution at the surface of ETLs or HTLs[47,48]. Here, CFM was employed to characterize the TiO2and Ga-TiO2films.Fig. 5 shows the CFM and KPFM images of TiO₂ and Ga-TiO₂ film, respectively. As shown inFig. 5(a,b), the Ga-TiO₂ surface shows significantly higher current, which is in accordance with higherJsc

in Ga-PSCs. Similarly, KPFM was used to detect the surface potential of the TiO₂and Ga-TiO₂films[48]. As shown inFig. 5(c, d), the Ga- TiO2 film demonstrates the lower average surface potential compared to the TiO2 films, which is reduced by approximately 0.15 V. The result confirms that Ga doping reduces the work function of TiO2surface[49]. The reduced work function suggests that the photo-induced electrons in the perovskite layer can be extracted more easily, which agrees well with the higherV_ocof Ga- PSC[49].

The steady-state PL measurement is a useful tool that can help to investigate the effect of Ga doping on electron extraction at the TiO₂/perovskite interface.Fig. 6(a) shows the PL spectra of perov- skiteﬁlms deposited on TiO2and Ga-TiO2ﬁlms, respectively. The peaks of both samples locate at the wavelength of about 775 nm, which is consistent with the previous reports[50]. The PL intensity of the sample deposited on Ga-TiO2is lower than that of the TiO2

sample, indicating more efﬁcient electron extraction at the Ga- TiO₂/perovskite interface[35]. It is noted that the enhanced electron extraction could inhibit excess charge accumulation in the perovskite layer or at the surface of perovskite[34]. The excess charge

accumulation in perovskite will lead to the reduced hysteresis[14], which indicate that the hysteresis in Ga-PSCs can be reduced. The effect of Ga doping on hysteresis behavior of PSCs is discussed below. Furthermore, EIS measurement was carried out to investigate the charge recombination at ETL/perovskite interface.Fig. 6(b) demonstrates the Nyquist plots measured under illumination and the inset is the equivalent circuit used toﬁt the data. The transfer resistance (R_tr) and recombination resistance (R_rec) at the ETL/

perovskite interface can be obtained byfitting the impedance data with the equivalent circuit.Fig. 6(c) shows the histograms of Rtrand R_recfor the reference TiO₂-PSC and Ga-PSC. It can be seen that the Fig. 6.(a) Steady state photoluminescence spectra of perovskitefilms respectively deposited on FTO/TiO2and Ga-TiO2, EIS Nyquist plots (b) and thefitted Rrec(c) of the reference PSC and Ga-PSC.

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value of R_trdecreases from 47 to 38U^{and R}recincreases from 340 to 435U after Ga doping. The smaller Rtr is referred to enhanced charge transport and larger Rrecis related to decreased recombination in PSCs[23]. Compared to the reference TiO₂-PSC, the lower Rtr and higher Rrec of Ga-PSC suggests the promoted electron extraction and reduced charge recombination at the Ga-TiO2/ perovskite interface, resulting in the improved performance of Ga- PSC[14,30].

The hysteresis behavior is a signiﬁcant parameter for planar PSCs, which can be caused by some factors such as ion migration [51], ferroelectricity[52], and unbalanced charge transportation or extraction[53]. Fig. 7shows the J-Vcurves of reference TiO2-PSC and Ga-PSC under reverse (RS) and forward (FS) voltage scan.

Hysteresis index can be used to describe the hysteresis behavior of PSCs[54]. It can be calculated by the following equation(4)[55].

Hysteresis index¼ ðPCERSPCE_FSÞ=PCERS (4) The PCE of Ga-PSC recorded from RS and FS scan is 16.89% and 15.61%, respectively. For the reference TiO2-PSC, the PCE under RS and FS scan is 15.41% and 11.33%, respectively. The hysteresis index for the reference TiO2-PSC and Ga-PSC is 26.48% and 7.58%, respectively. Compared to the reference TiO2-PSC, the hysteresis in Ga-PSC has been signiﬁcantly reduced. Obviously, Ga-TiO₂ ETL could alleviate the hysteresis of PSCs. The result further conﬁrms that Ga doping can promote electron transfer at the ETL/perovskite

and reduce theJ-Vhysteresis, which is consistent with the results of PL[41].

It is well-known that good long-term stability is very important for planar PSCs[56]. The stability of the unsealed TiO₂-PSC and Ga- PSC has been investigated under a 50±5% humidity at room temperature.Fig. 8(a) shows the normalized PCE of TiO2-PSC and Ga- PSC over time. The PCE of Ga-PSC remains more than 85% of the initial value after 28 days, while the PCE of reference TiO₂-PSC has decayed approximately 30% under the same circumstance. To further understand the origin of stability, the sample with the structure of FTO/TiO₂(Ga-TiO₂)/ MAPbI_3-xCl_x/PCBM/Au are fabricated to measure the value of Dtrapin the perovskite layer deposited on different ETLs after stored for 28 days in ambient air (50±5%

humidity). The I-V curves for devices based on TiO₂and Ga-TiO₂ETL are shown inFig. S8. Then the Dtrapcan be obtained from I-V curves by equation(2). The thickness of perovskite layer is about 350 nm, as shown inFig. S7(b). Theε(z18) of MAPbI_3-xCl_xis cited from the literature[57].Fig. 8(b) shows the variation of Dtrap with storing time. It can be seen that the Dtrapof reference TiO2-PSC and Ga-PSC increase with storing time. The increased Dtrapleads to the reduced PCE [35]. Compared to the reference TiO₂-PSC, the increase of D_trap in the Ga-PSC is much slower. The results indicate that more electron traps appear in perovskiteﬁlms deposited on reference TiO2

with storing time, which may lead to inferior stability of the reference TiO2-PSCs.

Fig. 7.J-V curves of the reference TiO2-PSC (a) and Ga-PSC (b) under reverse and forward voltage scan.

Fig. 8.Normalized PCE (a) and Dtrap(b) of the reference TiO2-PSC and Ga-PSC stored in air without any encapsulation.

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4. Conclusion

In summary, PSCs based on CH3NH3PbI3-xClxperovskite layer and Ga doped TiO2ETL have been fabricated by low-temperature solution process. It is found that the Ga doping at optimum concentration can improve the performance of PSCs. The Ga-PSCs demonstrate the champion PCE of 17.09%, being much higher than 14.63% of the reference TiO₂-PSC. Moreover, Ga-PSC exhibits reduced hysteresis and better stability compared to the reference TiO2-PSC. The enhanced performance of Ga-PSC can be attributed to the improved electric properties of Ga-TiO₂ film. The Ga-TiO₂ film demonstrates the higher mobility, lower trap state density and the reduced work function, resulting in the enhanced electron extraction/collection and suppressed charge recombination in Ga- PSC. This work provides an excellent method to obtain efficient and stable perovskite solar cells by low-temperature solution process.

Acknowledgements

We acknowledge theﬁnancial support of the National Key R&D Program of China (2016YFB0401502, 2016YFA0201002), the Natu- ral Science Foundation of Guangdong Province (No.

2016A030313421, 2016A030308019), the Characteristic Innovation Project of Guangdong Provincial Department of Education (Science 2016, 22), the National Natural Science Foundation of China (Grant Nos. 51431006, 51472093, 61574065), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016), the Science and Technology Planning Project of Guangdong Province (Grant No. 2016B090907001, 2016B090906004, 2015B090927006), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_17R70), the Guangdong Innovative Research Team Program (No. 2011D039) and the MOE International Laboratory for Optical Information Technologies.

Appendix A. Supplementary data

Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2018.03.158.

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Enhanced performance of planar perovskite solar cells using low