Bulk Heterojunction Perovskite Hybrid Solar Cells With Large Fill Factor

Energy &EnvironmentalScience

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Bulk heterojunct

aDepartment of Polymer Engineering, The Un

E-mail: [email protected]; Fax: +1 330 97bDepartment of Chemical & Biomolecular En

OH 44325, USA

Cite this: Energy Environ. Sci., 2015, 8,1245

Received 21st January 2015Accepted 5th February 2015

DOI: 10.1039/c5ee00222b

www.rsc.org/ees

This journal is © The Royal Society of C

ion perovskite hybrid solar cellswith large fill factor

Kai Wang,a Chang Liu,a Pengcheng Du,a Jie Zhengb and Xiong Gong*a

Broader context

Efficiently and economically harnessing solar energy via photovoltaic (PV)devices is one of the most promising solutions to the global energy crisis.However, over 80% of the available PVs are silicon-based and/or inorganicthin-lm solar cells, which are expensive and require critical processingtechniques. Organic PV cells have been developed to lower the costs, buttheir efficiency still lags behind their inorganic counterparts. Hybridorganic–inorganic halide perovskites have shown great potential as novel,highly efficient and low-cost PV devices. However, the fundamental limi-tation of unbalanced charge carrier diffusion lengths in perovskitematerials requires a mesoporous structure and/or a highly conductiveelectron transfer layer to balance the charge extraction efficiencies, whichcomplicates device processing. We report here a simple method of fabri-cating bulk heterojunction (BHJ) perovskite hybrid solar cells (pero-HSCs)from a BHJ composite consisting of perovskite materials blended withwater-/alcohol-soluble fullerene derivatives to balance the charge extrac-tion efficiencies. A greater than 22% enhanced efficiency was observedfrom these BHJ pero-HSCs compared with planar heterojunction pero-HSCs. This fabrication method for BHJ pero-HSCs offers an efficient andsimple way to increase the efficiency of pero-HSCs.

A major limitation to increasing the efficiency of perovskite hybrid

solar cells (pero-HSCs) is the fact that the diffusion length of the

electrons is shorter than that of the holes. To facilitate the electron

extraction efficiency in pero-HSCs and to make this efficiency

comparable with that of the holes, we fabricated bulk heterojunction

(BHJ) pero-HSCs by mixing perovskite materials with water-/alcohol-

soluble fullerene derivatives. The observed enhanced short-circuit

current densities (JSC) and enlarged fill factors (FF) were a result of the

balance in the charge carrier extraction efficiency and the enlarged

interfacial area between the perovskite materials and the fullerene

derivatives. Significantly improved power conversion efficiencies were

obtained for these BHJ pero-HSCs. A greater than 22% increase in

power conversion efficiency was observed for the BHJ pero-HSCs

compared with planar heterojunction pero-HSCs. A remarkable 86.7%

FF, the highest reported value for pero-HSCs, was observed for the

BHJ pero-HSCs. Our strategy of using a BHJ structure in pero-HSCs

offers an efficient and simple way to further increase the performance

of these devices.

Introduction

Cost-effective and high-performance photovoltaic cells arecurrently being investigated in both the academic and indus-trial sectors.1–7 Methylammonium lead halide (MAPbX3) perov-skites have attracted much attention as novel photovoltaicmaterials as a result of their excellent optical and electricalproperties.8–10 Power conversion efficiencies (PCEs) of >17%have been reported for perovskite hybrid solar cells (pero-HSCs).11 However, as a result of the unbalanced electron–holediffusion lengths (the diffusion length for the electrons isshorter than that for the holes; Leff, e�/Leff, h+ < 1),12 there is afundamental limitation to increasing the PCEs of pero-HSCs.13

iversity of Akron, Akron, OH 44325, USA.

2 3406

gineering, The University of Akron, Akron,

hemistry 2015

To circumvent these problems, meso-superstructured pero-HSCs (MS pero-HSCs) and planar heterojunction pero-HSCs(PHJ pero-HSCs) have been developed. In MS pero-HSCs, meso-structured titanium dioxide (TiO2) is used as the electrontransport layer (ETL) and aluminum oxide (Al2O3) is used as theinsulating scaffold to ensure efficient charge carrier separationby enlarging the interfacial area.14–17 However, the low electricalconductivity of TiO2 contributes an additional charge transportresistance within TiO2 itself,18 resulting in low PCEs for MSpero-HSCs. In addition, TiO2 needs to be sintered at hightemperatures, which means that MS pero-HSCs are incompat-ible with large-scale, high-throughput manufacturing andfabrication on exible plastic substrates. In PHJ pero-HSCs,solution-processed n-type materials, such as fullerene deriva-tives with high electrical conductivity, are required for theETL.19–22 However, the electron extraction efficiencies of these n-type materials are still not high enough because the interfacebetween the perovskite materials and fullerene derivatives is

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insufficient, which prevents any further increase in the PCE ofPHJ pero-HSCs. In addition, in both MS pero-HSCs and PHJpero-HSCs, the coarse surface of the solution-processed perov-skite layer not only creates a poor contact between the fullerenederivative and the perovskite lm, which decreases the electronextraction efficiency, but also introduces a large shunt resis-tance, resulting in a large leakage current and a low ll factor(FF). As a result, low PCEs are observed for these pero-HSCs.23–26

We report here bulk heterojunction (BHJ) pero-HSCs fabri-cated by blending perovskite materials with fullerene deriva-tives to improve the electron extraction efficiency. An increasedshort-circuit current density (JSC) and an increased FF withcorrespondingly enhanced PCEs were observed for these BHJpero-HSCs. A remarkable FF of 86.7%, the highest reported forpero-HSCs, was obtained for the BHJ pero-HSCs.

ExperimentalMaterials

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)were purchased from Clevious and 1-Material Inc., respectively,and were used as received without further purication. Lead(II)iodide (PbI2), anhydrous N,N-dimethylformamide, ethanol(99.5%), hydroiodic acid (99.99%) and methylamine werepurchased from Sigma-Aldrich and were used as receivedwithout further purication. Methylammonium iodide (MAI)was synthesized following a previously reported method.27

A10C60 fullerene derivative

A10C60 is a fullerene derivative, where A is a carboxylic acidgroup regiospecically functionalized on the C60 head, whichimparts hydrophilicity on the surface of C60. A10C60 wassynthesized according to a previously reported method.28 An HP8453 UV-visible spectrophotometer was used to measure theabsorption spectrum of a thin lm of A10C60 spin-cast onto aquartz substrate from an ethanol solution. Cyclic voltammetry(CV) measurements were carried out on a Gamry Reference 3000instrument. We used a saturated silver chloride (AgCl) referenceelectrode, a glassy carbon working electrode and a platinumelectrode counter. The CV curves were measured under anitrogen atmosphere in an ethanol solution containing 0.1 Mtetrabutylammoniumhexauorophosphate as the electrolyte.The concentrations of the analytes were between 0.50 and1.0 mmol L�1. The potentials were calibrated with respect to thepotential of the Ag/AgCl electrode.

Preparation of the perovskite precursor

A PbI2 solution with a concentration of 400 mg mL�1 wasprepared by dissolving PbI2 in N,N-dimethylformamide. ThePbI2 solution was stored overnight to give a clear PbI2 super-natant aer stirring at 70 �C for 12 h. A pure MAI solution with aconcentration of 70 mgmL�1 was prepared by dissolving MAI inethanol for further attenuation. A10C60 solutions with concen-trations from 0 to 40 mg mL�1 were prepared by dissolvingA10C60 in ethanol. A mixed MAI:A10C60 solution was prepared by

1246 | Energy Environ. Sci., 2015, 8, 1245–1255

simultaneously mixing the MAI and A10C60 solutions together.The concentration of MAI in both the pristine MAI ethanolsolution and the MAI:A10C60 ethanol solution was 35 mg mL�1;the concentration of A10C60 in the MAI:A10C60 ethanol solutionranged from 2.5 to 20 mg mL�1.

Preparation of perovskite lm

The CH3NH3PbI3 lms were prepared by a two-step solutiondeposition method. PbI2 was rst spin-cast on the top of thepre-heated (70 �C) substrate from the warm (70 �C) PbI2supernatant, followed by thermal annealing at 70 �C for 10 minand then cooling to room temperature. Either a pure MAI layeror a MAI:A10C60 BHJ composite layer was then spin-coated ontop of the PbI2 layer from the corresponding ethanol solutions,followed by thermal annealing at 100 �C for 2 h to covert PbI2and MAI to CH3NH3PbI3.

Characterization of perovskite lm

Top-view and cross-sectional scanning electron microscopy(SEM) images were obtained using a JEOL-7401 eld-emissionscanning electron microscope. Atomic force microscopy (AFM)images were obtained using a Digital Instrument NanoScopeNS3A system to observe the surface morphologies and thick-nesses of various thin lms. The X-ray diffraction (XRD)patterns of the thin lms coated on polyethylene terephthalatesubstrates were obtained using a Bruker AXS Dimension D8X-ray system. The photoluminescence (PL) spectra of the CH3-NH3PbI3, CH3NH3PbI3:A10C60, CH3NH3PbI3/PC61BM and CH3-NH3PbI3:A10C60/PC61BM thin lms were obtained on a Picoharp300 instrument aer pre-amplication by PAM 102.

Fabrication of pero-HSCs

Both PHJ and BHJ pero-HSCs were fabricated on pre-cleanedindium tin oxide (ITO) coated glass. Aer the pre-cleaned ITOsubstrates had been treated with UV-ozone for 20 min under anambient atmosphere, an approximately 40 nm thick lm ofPEDOT:PSS was spin-cast on top of the ITO substrates, followedby thermal annealing at 150 �C for 10 min under an ambientatmosphere. Either a CH3NH3PbI3 or CH3NH3PbI3:A10C60 layerwas then spin-coated on top of the PEDOT:PSS layer from thecorresponding solution by the described two-step depositionmethod. The thickness of the CH3NH3PbI3 and CH3NH3PbI3:-A10C60 thin lms were measured to be about 300 nm. Anapproximately 200 nm thick PC61BM ETL was then spin-castfrom a chlorobenzene solution on to the top of either theCH3NH3PbI3 or CH3NH3PbI3:A10C60 layer. Finally, an approxi-mately 100 nm thick layer of Al was deposited thermally on topof the PC61BM ETL through a shadow mask under a vacuumwith a base pressure of 6 � 10�6 mbar. The area of the devicewas measured to be 0.045 cm2.

Characterization of pero-HSCs

The current density versus voltage (J–V) characteristics of thepero-HSCs were obtained using a Keithley Model 2400 sourcemeasuring unit. A Newport Air Mass 1.5 Global (AM1.5G) full-

This journal is © The Royal Society of Chemistry 2015


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spectrum solar simulator was used as the light source. The lightintensity was 100 mW cm�2 and was calibrated using a mono-silicon detector with a KG-5 visible color lter from the NationalRenewable Energy Laboratory to reduce the spectral mismatch.The incident photon-to-electron conversion efficiency (IPCE)spectra were measured with the IPCE measurement set-up usedat European Solar Test Installation (ESTI) for cells and mini-modules: a 300 W steady-state xenon lamp provides the sourcelight; up to 64 lters (8–20 nm width, range 300–1200 nm) areavailable on four lter wheels to produce the monochromaticinput, which is chopped at 75 Hz, superimposed on the bias lightand measured via the usual lock-in technique (the bias light isnecessary to examine the device close to the operating irradianceconditions). Aer collecting the IPCE data, the soware inte-grated the data with the AM1.5G spectrum and gave the calcu-lated JSC value, which was helpful for checking the accuracy of themeasurement. To extract the Nyquist plots, impedance spectrawere obtained using an HP 4194A impedance/gain-phaseanalyzer, in the dark or under 1 sun illumination, with anoscillating voltage of 10 mV and a frequency of 1 Hz to 1 MHz.The pero-HSCs were held at their respective open-circuit poten-tials obtained from the (J–V) measurements while the impedancespectra were recorded.

Results and discussion

Fig. 1a shows the UV-visible absorption spectrum of the A10C60

thin lm. The inset shows the molecular structure of A10C60,where the C60 head is regiospecically functionalized by10 carboxylic acid groups. Similar to PC61BM,29 A10C60 has astrong absorption at wavelengths < 500 nm, which implies thatA10C60 contributes the photocurrent in the pero-HSCs with adevice structure of ITO/PEDOT:PSS/CH3NH3PbI3:A10C60/PC61BM/Al. Fig. 1b shows the CV curve of A10C60. The ELUMO andEHOMO values of A10C60 are estimated to be �4.1 and �6.1 eV,respectively, according to the empirical equations ELUMO ¼ �e[Eonsetred + 4.4] and EHOMO ¼ �e[Eonsetox + 4.4],30 where ELUMO is thelowest unoccupied molecular orbital energy level, Eonsetred is the

Fig. 1 (a) UV-visible absorption spectrum of A10C60 thin film; inset, molechead is regiospecifically functionalized with 10 carboxylic acid groups. (b


reduction potential, EHOMO is the highest occupied molecularorbital energy level and Eonsetox is the oxidization potential. Boththe LUMO and HOMO energy levels of A10C60 are nearly thesame as those of PC61BM.29 These results indicate that the 10carboxylic acid groups within C60 do not change the optical andelectrical properties of C60, but signicantly tune A10C60 topossess hydrophobicity.

Fig. 2a shows the device structures of the BHJ pero-HSCs:ITO/PEDOT:PSS/CH3NH3PbI3:A10C60/PC61BM/Al. For compar-ison, Fig. 2b shows the device structure of the PHJ pero-HSCs:ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al. Fig. 2c shows theLUMO and HOMO energy levels of CH3NH3PbI3, A10C60 andPC61BM and the work functions of the ITO, PEDOT:PSS and Alelectrodes. In both devices, PC61BM acts as the ETL. Based onthe band alignment, there is no energy barrier to the electronstransported from A10C60 to PC61BM ETL, thus there is a higherelectron extraction efficiency and, consequently, a higher JSC isexpected from the BHJ pero-HSCs.

Fig. 3a shows the J–V characteristics of the PHJ pero-HSCsand the BHJ pero-HSCs. The PHJ pero-HSCs have a JSC of17.31 mA cm�2, an open-circuit voltage (VOC) of 0.86 V, a FF of77.2% and a corresponding PCE of 11.45%. The device perfor-mance parameters of PHJ pero-HSCs are comparable withpreviously reported values.31 However, the BHJ pero-HSCs havea JSC of 19.41 mA cm�2, a VOC of 0.88 V, a FF of 81.6% and acorresponding PCE of 13.97%. The PCE of the BHJ pero-HSCs is>22% higher than that of the PHJ pero-HSCs. The VOC valuesfrom both the PHJ pero-HSCs and the BHJ pero-HSCs are thesame, which is in good agreement with the band alignmentshown in Fig. 2c. However, enhanced values of the JSC and FFare seen for the BHJ pero-HSCs. The enhanced JSC value indi-cates that more electrons are extracted from the CH3NH3PbI3:-A10C60 BHJ composite through the PC61BM ETL to thecorresponding Al electrode. The signicantly enlarged FF(81.6%) promisingly indicates that more balanced chargecarriers are obtained from the BHJ pero-HSCs.32

The series resistance (RS) and shunt resistance (RSH) areestimated from the slopes of the J–V curves under open-circuit

ular structure of A10C60, a [5:1]-hexakisadducts of C60, in which the C60

) Cyclic voltammetry curve of A10C60.



Fig. 2 Device structures of (a) PHJ and (b) BHJ pero-HSCs. (c) LUMO andHOMOenergy levels of CH3NH3PbI3, A10C60 and PC61BM and theworkfunctions of the ITO, PEDOT:PSS and Al electrodes.

Fig. 3 (a) J–V characteristics of the PHJ and BHJ pero-HSCs under AM 1.5 simulated illumination. (b) IPCE spectra of PHJ and BHJ pero-HSCs.


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and short-circuit conditions, respectively. Table 1 summarizesthe RS and RSH values from both the PHJ and BHJ pero-HSCs.Compared with the PHJ pero-HSCs, the smaller RS value fromthe BHJ pero-HSCs originates from both the smaller contactresistance and the bulk resistance of the CH3NH3PbI3:A10C60

BHJ composite, which indicates that high photocurrents areanticipated to be generated from the BHJ pero-HSCs at lowapplied voltages; the larger RSH for the BHJ pero-HSCs indicatesthat short circuits or current leakages are minimized. As aresult, high JSC values, a large FF and consequently high PCE

Table 1 Photovoltaic performance of PHJ pero-HSCs and BHJ pero-HSCs

DeviceVOC(V)

JSC(mA cm�2) FF (%)

RS(kU cm2)

RSH

(kU cm2)PCE(%)

PHJ pero-HSC 0.86 17.31 77.2 0.92 246.7 11.45BHJ pero-HSC 0.88 19.41 81.6 0.75 252.9 13.97


values are observed for the BHJ pero-HSCs. These results areconsistent with those observed from the J–V characteristics ofthe PHJ pero-HSCs and the BHJ pero-HSCs (Fig. 3a).

Fig. 3b presents the IPCE spectra of both the PHJ pero-HSCsand the BHJ pero-HSCs. The IPCE spectra specify the ratio of theextracted electrons to the incident photons at a given wavelength.The PHJ pero-HSCs show a wide response from the visible to thenear-infrared region, with more than about 60% of the IPCE inthe range 500–800 nm, which indicates that the charge-transfercomplexes are formed in the CH3NH3PbI3 thin lms.33 The BHJpero-HSCs exhibit a strong photo-response over the entire spec-tral region, with over 85% of the IPCE in the range 375–800 nm.In contrast with the PHJ pero-HSCs, an obvious enhanced IPCEfrom the BHJ pero-HSCs is seen in the range 350–500 nm, whichprobably originates from the contribution of A10C60 in the CH3-NH3PbI3:A10C60 BHJ active layer resulting from the strongabsorption of A10C60 between 350 and 500 nm (Fig. 1a). Thephotocurrent densities estimated from the IPCE spectra are 16.21and 18.83 mA cm�2 for the PHJ pero-HSCs and BHJ pero-HSCs,




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respectively. These values are consistent with those obtainedfrom the J–V characteristics (Fig. 3a).

Fig. 4 shows a histogram of the photovoltaic parameters forthe BHJ pero-HSCs. Over 200 BHJ pero-HSCs devices werefabricated and characterized. Fig. 4 shows that the reproduc-ibility of the JSC, FF, VOC and PCE were fairly high. The relativestandard deviation was 1.3%, which is extremely low comparedwith previous reported data.34

A major issue in the pero-HSCs was the hysteresis of thephotocurrent.35 It has been reported36,37 that the origin of this

Fig. 4 Histograms of photovoltaic parameters for BHJ pero-HSCs: (a) V

Fig. 5 J–V characteristics of BHJ pero-PSCs with two different scan dirscan (from positive bias to negative bias) and at different scan rates: (a)


photocurrent hysteresis can be ascribed to traps, the ferroelectricproperties of the perovskite materials and/or the electro-migra-tion of ions in the pero-HSCs. To investigate the photocurrenthysteresis in the BHJ pero-HSCs, we measured the J–V charac-teristics using different scan directions [reverse scans (from apositive to a negative bias) and forward scans (from a negative toa positive bias)] and different scan rates (a high scan rate of 0.6 Vs�1 and a low scan rate of 0.03 V s�1).37 Fig. 5 shows the photo-current hysteresis results for the BHJ pero-HSCs. Table 2summarizes the device performance parameters of the BHJ pero-

OC; (b) JSC; (c) FF; and (d) PCE (n ¼ 200).

ections: forward scan (from negative bias to positive bias) and reverse0.6; and (b) 0.03 V s�1.



Table 2 Device performance parameters of BHJ pero-HSCsmeasured at different scan rates and in different scan directions

Scan rate(V s�1)

Scandirection

VOC(V)

JSC(mA cm�2) FF (%)

PCE(%)

0.6 Forward 0.88 19.40 80.80 13.85Reverse 0.88 19.54 83.10 14.34

0.03 Forward 0.86 19.58 75.10 12.65Reverse 0.88 19.72 78.30 13.59

Fig. 6 (a) J–V characteristics and (b) IPCE spectra of BHJ pero-HSCs coJSC, VOC, FF and PCE values versus different percentages by weight of A



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HSCs using different scan directions and rates. Larger PCEs wereseen for the BHJ pero-HSCs under the reverse scans comparedwith those under the forward scans. At a high scan rate of 0.6 Vs�1, the average PCE was 14.10%, higher than the 13.00%obtained at a low scan rate of 0.03 V s�1. These are similar topreviously reported values.37However, the differences in the PCEsof the BHJ pero-HSCs using either different scan directions ordifferent scan rates were smaller than those reported from PHJpero-HSCs,37,38 indicating that the photocurrent hysteresis in theBHJ pero-HSCs is weaker than that in the PHJ pero-HSCs.

ntaining different percentages by weight of A10C60 to CH3NH3PbI3. (c)

10C60 to CH3NH3PbI3.



Table 3 Photovoltaic performancea of BHJ pero-HSCs containing different ratios of A10C60 to CH3NH3PbI3

DeviceRatio of A10C60 toCH3NH3PbI3 (% w/w) VOC (V) JSC

b (mA cm�2) JSCc (mA cm�2) FF (%) PCE (%)

BHJ pero-HSC 1.6 0.87 � 0.02 17.28 � 0.43 15.99 � 1.05 79.7 � 1.5 11.75 � 0.213.2 0.89 � 0.02 17.52 � 0.29 16.47 � 0.89 82.6 � 1.6 12.88 � 0.386.4 0.86 � 0.03 18.08 � 0.32 17.23 � 0.95 86.7 � 1.8 13.48 � 0.279.6 0.88 � 0.04 19.41 � 0.35 18.53 � 1.13 81.6 � 1.6 13.97 � 0.31

12.8 0.88 � 0.03 20.17 � 0.42 19.20 � 1.53 72.3 � 2.1 12.83 � 0.16

a Average and standard deviation values were obtained based on over 30 cells from 6 different batches for each ratio. b JSC is observed from J–Vcharacteristics. c JSC is estimated from the IPCE spectrum.

Fig. 7 XRD patterns of thin films of the CH3NH3PbI3:A10C60 BHJcomposite, pristine CH3NH3PbI3 and pristine A10C60.

Fig. 8 Tapping-mode AFM height images and corresponding AFMphase images of (a and c) the pristine CH3NH3PbI3 layer and (b and d)the CH3NH3PbI3:A10C60 BHJ composite layer.


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To investigate the contribution of A10C60 in the BHJ pero-HSCs fabricated from the CH3NH3PbI3:A10C60 BHJ composite,the change in device performance of the BHJ pero-HSCs withthe concentration (by weight) of A10C60 in the CH3NH3PbI3:-A10C60 BHJ composite was studied. Fig. 6a and b show the J–Vcharacteristics and the IPCE spectra of the BHJ pero-HSCs.Fig. 6c shows the JSC, VOC, FF and PCE values plotted against theratios of A10C60 to CH3NH3PbI3. Table 3 summarizes thephotovoltaic performance parameters. As the ratio of A10C60 toCH3NH3PbI3 was increased from 1.6 to 12.8%, the VOC had asimilar value of 0.88 � 0.04 V, but the JSC value increased from17.28 to 20.17 mA cm�2, an increment of about 17%, and the FFinitially increased from 79.7 to 86.7% and then decreased to72.3%. Consequently, the PCE initially increased from 11.75 to13.97% and then decreased to 12.83%. The FF of 86.7% is thehighest reported value for pero-HSCs.

The increased JSC values with increasing ratios of A10C60 toCH3NH3PbI3 probably originate from the strong absorption ofA10C60, which contributes the photocurrent at wavelengths <500 nm andmore balanced electron–hole transport by blendingmore A10C60 with CH3NH3PbI3. In addition, the lmmorphology of CH3NH3PbI3A10C60 is modied by A10C60. Alarger amount of A10C60 probably forms a larger domain, whichis favorable for electron transport from the CH3NH3PbI3:A10C60

BHJ composite to the PC61BM ETL, resulting in an increased JSCvalue from the BHJ pero-HSCs containing large amounts ofA10C60.

Based on the IPCE spectra, the JSC values of the pero-HSCscontaining different concentrations of A10C60 were estimated byintegrating the IPCE spectra from 375 to 800 nm. Table 3summarizes the estimated JSC values. These estimated JSC valuesare consistent with those observed from the J–V characteristics(Fig. 6a).

XRD was used to study the differences between the pristineCH3NH3PbI3 and CH3NH3PbI3:A10C60 BHJ composite thin lmsto help understand the different photovoltaic performances ofthe PHJ pero-HSCs and the BHJ pero-HSCs. Fig. 7 shows theXRD patterns of pristine CH3NH3PbI3 and the CH3NH3PbI3:-A10C60 BHJ composite thin lms. The pristine A10C60 thin lmwas also investigated for comparison. Diffraction peaks wereobserved at 13.9, 28.5, 32.0, 40.6 and 43.6� for both the pristineCH3NH3PbI3 and the CH3NH3PbI3:A10C60 BHJ composite thinlms. These peaks correspond to the (110), (220), (310), (224)and (314) planes,39,40 respectively, which indicates that

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CH3NH3PbI3 was formed with an orthorhombic crystal struc-ture in both pristine CH3NH3PbI3 and the CH3NH3PbI3:A10C60

BHJ composite thin lms. No representative peak for PbI2 wasdetected in either the pristine CH3NH3PbI3 or the CH3NH3-PbI3:A10C60 BHJ composite thin lms, indicating that PbI2 andMAI react completely to generate CH3NH3PbI3 polycrystals.However, there were small peaks at 17 and 25� in the spectra ofpristine A10C60 and the CH3NH3PbI3:A10C60 BHJ composite thinlms, although the intensities were relatively weak in theCH3NH3PbI3:A10C60 BHJ composite thin lms. These resultsindicate that a small-scale A10C60 crystallinity is formed in theCH3NH3PbI3:A10C60 BHJ composite thin lms. In addition, theCH3NH3PbI3:A10C60 BHJ composite thin lm had a moreintense diffraction peak at 32.0� than the pristine CH3NH3PbI3thin lm, indicating that the high crystallinity of the CH3NH3-PbI3 thin lm was formed by the introduction of A10C60 into theCH3NH3PbI3:A10C60 BHJ composite. In summary, the A10C60 inthe CH3NH3PbI3:A10C60 BHJ composite did not change thecrystal structure of the CH3NH3PbI3, but inuenced the crys-tallinity of CH3NH3PbI3 on a small scale, indicating that thesetwo materials were not miscible.

Both AFM images and top-view SEM images were obtained tostudy the morphological differences between pristine CH3NH3-PbI3 and the CH3NH3PbI3:A10C60 BHJ composite thin lms. Fig. 8shows the AFM topographic images of pristine CH3NH3PbI3 andthe CH3NH3PbI3:A10C60 BHJ composite thin lms. Both thin

Fig. 9 Top-view SEM images of (a) the pristine CH3NH3PbI3 layer and (b) tresolution images. Cross-sectional SEM images of (e) ITO/PEDOT:PSPC61BM (some of the A10C60 domains inside CH3NH3PbI3:A10C60 can be


lms show a relatively smooth surface morphology with a root-mean-square roughness (Rq) of about 10 nm. Such a at surfacesuggests that either the CH3NH3PbI3 layer or the CH3NH3PbI3:-A10C60 BHJ composite layer was completely covered by a PC61BMlayer with a thickness of about 200 nm, which resulted in indirectcontact between the active perovskite layer and the top cathode. Asmall Rq value indicates a smoother surface with less sharp trapsor pinholes, which results in a high JSC value and a large FF.41

Fig. 8c and d show the phase images of CH3NH3PbI3 and theCH3NH3PbI3:A10C60 BHJ composite thin lms. A binary phasepattern was seen in the CH3NH3PbI3:A10C60 BHJ composite thinlms (Fig. 8d), similar to that of the bi-continuous phasemorphology observed for the BHJ composite in polymer solarcells.42 The BHJ interpenetrating network was essential for theefficient extraction of electrons from the BHJ composite becausethe enlarged interface areas between A10C60 and CH3NH3PbI3facilitate more dissociation of the excitons. As a result, more freeelectrons were extracted from CH3NH3PbI3 into the n-type A10C60

as a result of the decreased travel length.43 Thus the electronextraction efficiency was improved, resulting in a balancedcharge carrier extraction efficiency and an enlarged FF wasobserved for the BHJ pero-HSCs.

Fig. 9 compares the top-view and cross-sectional SEMimages of pristine CH3NH3PbI3 and the CH3NH3PbI3:A10C60

BHJ composite thin lms. Many large voids were seen in thepristine CH3NH3PbI3 thin lms, whereas fewer large voids

he CH3NH3PbI3:A10C60 BHJ composite layer; insets (c and d) are higherS/CH3NH3PbI3/PC61BM and (f) ITO/PEDOT:PSS/CH3NH3PbI3:A10C60/observed in the region marked by broken lines).




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were seen in the CH3NH3PbI3:A10C60 BHJ composites thinlms. This indicates that the thin lms formed by the CH3-NH3PbI3:A10C60 BHJ composites were more homogeneousthan the pristine CH3NH3PbI3 thin lms. Thus it was specu-lated that these large voids were lled by A10C60 and a newphase of A10C60, making the CH3NH3PbI3:A10C60 BHJcomposite thin lms more compact and denser. The insets inFig. 9c and d show high-resolution images of pristine CH3-NH3PbI3 and the CH3NH3PbI3:A10C60 BHJ composite thinlms at a magnication of �50 000. It is clear that the crystalsizes in the pristine CH3NH3PbI3 thin lm (Fig. 9a) are almostidentical and homogeneous, whereas the crystal sizes in theCH3NH3PbI3:A10C60 BHJ composite thin lms (Fig. 9b) areinhomogeneous. The inhomogeneous crystal domainsobserved in the CH3NH3PbI3:A10C60 BHJ composite thin lmsare of two different sizes: the domain size identical to thatshown in Fig. 9a is probably formed of CH3NH3PbI3 poly-crystals, whereas the other type with different domain sizes isprobably formed from A10C60 crystals.

Fig. 9e and f show the cross-sectional SEM images of the ITO/PEDOT:PSS/CH3NH3PbI3:A10C60/PC61BM and ITO/PEDOT:PSS/

Fig. 10 (a) PL spectra of CH3NH3PbI3, CH3NH3PbI3:A10C60, CH3NH3PbI3/spectra of CH3NH3PbI3/PC61BM and CH3NH3PbI3:A10C60/PC61BM thin filand at an applied voltage close to the open-circuit voltage of the perotrometry measurements fitted for different applied biases at 1 sun illumi


CH3NH3PbI3/PC61BM devices, respectively. The thickness ofeach layer was consistent with that in the nal devices. Fig. 9eshows the PHJ structure of CH3NH3PbI3/PC61BM, where thepristine CH3NH3PbI3 layer is sandwiched between thePEDOT:PSS layer and the PC61BM ETL. In contrast, Fig. 9f showsthe CH3NH3PbI3:A10C60 composite layer sandwiched betweenthe PEDOT:PSS layer and the PC61BM ETL. The A10C60 domainscould be clearly seen in the CH3NH3PbI3:A10C60 compositelayer. This observation conrms that A10C60 was mixed withCH3NH3PbI3 to form the CH3NH3PbI3:A10C60 BHJ compositelayer.

To further understand the underlying device performance ofboth the PHJ and BHJ pero-HSCs, the PL of the CH3NH3PbI3,CH3NH3PbI3/PC61BM, CH3NH3PbI3:A10C60 and (CH3NH3PbI3:-A10C60)/PC61BM thin lms was investigated. Fig. 10a shows thePL spectra of these thin lms. A more striking PL quenchingeffect was seen with the CH3NH3PbI3:A10C60 BHJ composite thinlms than with the pristine CH3NH3PbI3 thin lms, whichindicates that efficient electron extraction took place in theH3NH3PbI3:A10C60 BHJ composite thin lms rather in the pris-tine CH3NH3PbI3 thin lms. The enlarged PL spectra of

PC61BM and CH3NH3PbI3:A10C60/PC61BM thin films; insets, enlarged PLms. (b) Nyquist plots of PHJ and BHJ pero-HSCs measured in the dark-HSCs. (c) Recombination resistance resulting from impedance spec-nation.




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CH3NH3PbI3/PC61BM and CH3NH3PbI3:A10C60/PC61BM thinlms in Fig. 10a (inset) clearly demonstrate that more PLquenching took place in the (CH3NH3PbI3:A10C60)/PC61BM thinlms than in the CH3NH3PbI3/PC61BM thin lms. These resultsindicate that more efficient electron extraction occurred in the(CH3NH3PbI3:A10C60)/PC61BM thin lms than in the CH3NH3-PbI3/PC61BM thin lms. Therefore the PL spectra furtherconrm that the high electron extraction efficiency originatedfrom the enlarged interface between the CH3NH3PbI3 andA10C60 in the CH3NH3PbI3:A10C60 BHJ composite.

To investigate the electrical properties of the interfaces,impedance spectrometry was used to study the internal seriesresistance (RS) of both the BHJ pero-HSCs and the PHJ pero-HSCs. The impedance spectrometry analysis enabled us tomonitor the detailed electrical properties of the interfaces thatcould not be determined by the direct current measurements.The RS consists of the sheet resistance (RSHEET) of the electrodes,the charge-transfer resistance (RCT) at the interfaces betweenthe electrode and the carrier selective layer and between thecarrier selective layer and the internal BHJ active layer.44 Themain difference between the PHJ and BHJ pero-HSCs is the RCTat the interface between CH3NH3PbI3 and A10C60. Fig. 10bshows the Nyquist plots of both the PHJ and BHJ pero-HSCs inthe dark at an applied voltage close to the VOC of the pero-HSCs.From the plots, RCT values of 0.85 and 0.66 kU were obtained forthe PHJ and BHJ pero-HSCs, respectively. In the PHJ pero-HSCs,a large RCT value indicated that the large charge-transfer resis-tance resulted from the inferior contact at the interface with thePC61BM/CH3NH3PbI3. For the BHJ pero-HSCs, a small RCT wasobtained as a result of the larger interfacial area between CH3-NH3PbI3 and A10C60. Therefore a higher PCE is expected fromthe BHJ pero-HSCs than from the PHJ pero-HSCs.

Fig. 10c shows the recombination resistance as a function ofthe applied bias in the relevant voltage range. The recombina-tion processes were associated with the feature observed in thelow-frequency region of the Nyquist plot of the pero-HSCs. Therecombination rate was inversely proportional to the recombi-nation resistance. In this study, the major difference in thedevice structure was the introduction of A10C60 into CH3NH3-PbI3 to form the CH3NH3PbI3:A10C60 BHJ composite. Therecombination resistance was signicantly enhanced at a lowapplied bias, which was in good agreement with the large FFobserved for the BHJ pero-PSCs.45 On the other hand, at a highapplied bias, close to the VOC, both the PHJ and BHJ pero-HSCshad a similar recombination resistance, indicating a similar VOCfor both cells.46 These observations were consistent with thoseof the J–V characteristics (Fig. 3a).

Conclusions

BHJ pero-HSCs were fabricated to address the fundamentallimitation of the unbalanced electron–hole diffusion length inpero-HSCs. High-efficiency BHJ pero-HSCs were fabricated bysolution-processing the BHJ composite from an ethanol solu-tion consisting of CH3NH3PbI3 and A10C60 rather than from apristine CH3NH3PbI3 ethanol solution. XRD, AFM, TEM, PL andimpedance spectrometry studies showed that the high


efficiency of the BHJ pero-HSCs originated from the balancedcharge carrier extraction efficiency and enlarged interfacial areabetween the CH3NH3PbI3 and A10C60. The fabrication of theseBHJ pero-HSCs offers an efficient and simple way to increase theefficiency of pero-HSCs.

Acknowledgements

The authors thank the NSF (1351785) for nancial support.

References

1 M. Graetzel, R. A. J. Janssen, D. B. Mitzi and E. H. Sargent,Nature, 2012, 488, 304–312.

2 Y. Chen, X. Wan and G. Long, Acc. Chem. Res., 2013, 46, 2645–2655.

3 G. Chen, H. Sasabe, Z. Wang, X. Wang, Z. Hong, J. Kido andY. Yang, Phys. Chem. Chem. Phys., 2012, 14, 14661–14666.

4 C. Zhang, H. You, Z. Lin and Y. Hao, Jpn. J. Appl. Phys., 2011,50, 082302.

5 F. Guo, X. Zhu, K. Forberich, J. Krantz, T. Stubhan,M. Salinas, M. Halik, S. Spallek, B. Butz, E. Spiecker,T. Ameri, N. Li, P. Kubis, D. M. Guldi, G. J. Matt andC. J. Brabec, Adv. Energy Mater., 2013, 3, 1062–1067.

6 Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazanand A. J. Heeger, Nat. Mater., 2012, 11, 44–48.

7 G. Hodes, Perovskite-Based Solar Cells, Science, 2013, 342,317–318.

8 S. Pang, H. Hu, J. Zhang, S. Lv, Y. Yu, F. Wei, T. Qin, H. Xu,Z. Liu and G. Cui, Chem. Mater., 2014, 26, 1485–1491.

9 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami andH. J. Snaith, Science, 2012, 338, 643–647.

10 C. C. Stoumpos, C. D. Malliakas and M. G. Kanatzidis, Inorg.Chem., 2013, 52, 9019–9038.

11 N. K. Noel, S. D. Stranks, A. Abate, C. Wehrenfennig,S. Guarnera, A.-A. Haghighirad, A. Sadhanala,G. E. Eperon, S. K. Pathak, M. B. Johnston, A. Petrozza,L. M. Herza and H. J. Snaith, Energy Environ. Sci., 2014, 7,3061–3068.

12 E. Edri, S. Kirmayer, S. Mukhopadhyay, K. Gartsman,G. Hodes and D. Cahen, Nat. Commun., 2014, 5, 3461.

13 E. Edri, S. Kirmayer, A. Henning, S. Mukhopadhyay,K. Gartsman, Y. Rosenwaks, G. Hodes and D. Cahen, NanoLett., 2014, 14, 1000–1004.

14 R. Lindblad, D. Bi, B.-w. Park, J. Oscarsson, M. Gorgoi,H. Siegbahn, M. Odelius, E. M. J. Johansson andH. Rensmo, J. Phys. Chem. Lett., 2014, 5, 648–653.

15 J.-H. Im, H.-S. Kim and N.-G. Park, APL Mater., 2014, 2,081510.

16 A. Marchioro, J. C. Brauer, J. Teuscher, M. Gratzel andJ.-E. Moser, Proc. SPIE, 2013, 8811, 881108–881111.

17 V. Roiati, E. Mosconi, A. Listorti, S. Colella, G. Gigli andF. D. Angelis, Nano Lett., 2014, 14, 2168–2174.

18 C. Deibel and V. Dyakonov, Rep. Prog. Phys., 2010, 73,096401.

19 J.-Y. n. Jeng, Y.-F. Chiang, M.-H. Lee, S.-R. Peng, T.-F. Guo,P. Chen and T.-C. Wen, Adv. Mater., 2013, 25, 3727–3732.




Publ

ishe

d on

06

Febr

uary

201

5. D

ownl

oade

d by

Sun

gkyu

nkw

an U

nive

rsity

on

30/0

5/20

15 0

7:16

:10.

View Article Online

20 J. Seo, S. Park, Y. C. Kim, N. J. Jeon, J. H. Noh, S. C. Yoon andS. I. Seok, Energy Environ. Sci., 2014, 7, 2642–2646.

21 O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas,M. Graetzel, M. K. Nazeeruddin and H. J. Bolink, Nat.Photonics, 2014, 8, 128–132.

22 J.-Y. Jeng, K.-C. Chen, T.-Y. Chiang, P.-Y. Lin, T.-D. Tsai,Y.-C. Chang, T.-F. Guo, P. Chen, T.-C. Wen and Y.-J. Hsu,Adv. Mater., 2014, 26, 4107–4113.

23 T. C. Sum and N. Mathews, Energy Environ. Sci., 2014, 7,2518–2534.

24 Q. Wang, Y. Shao, Q. Dong, Z. Xiao, Y. Yuan and J. Huang,Energy Environ. Sci., 2014, 7, 2359–2365.

25 J. M. Ball, M. M. Lee, A. Hey and H. J. Snaith, Energy Environ.Sci., 2013, 6, 1739–1743.

26 H. J. Snaith, N. C. Greenham and R. H. Friend, Adv. Mater.,2004, 16, 1641–1645.

27 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami andH. J. Snaith, Science, 2012, 338, 643–647.

28 X. Yu, W.-B. Zhang, K. Yue, X. Li, H. Liu, Y. Xin, C.-L. Wang,C. Wesdemiotis and S. Z. D. Cheng, J. Am. Chem. Soc., 2012,134, 7780–7787.

29 H. Wang, Y. He, Y. Li and H. Su, J. Phys. Chem. A, 2012, 116,255–262.

30 J. L. Bredas, R. Silbey, D. S. Boudreux and R. R. Chance, J. Am.Chem. Soc., 1983, 105, 6555–6559.

31 C.-H. Chiang, Z.-L. Tseng and C.-G. Wu, J. Mater. Chem. A,2014, 2, 15897–15903.

32 Y. Zhang, X.-D. Dang, C. Kim and T.-Q. Nguyen, Adv. EnergyMater., 2011, 1, 610–617.

33 K. G. Stamplecoskie, J. S. Mansera and P. V. Kamat, EnergyEnviron. Sci., 2015, 8, 208–215.


34 Z. Wu, S. Bai, J. Xiang, Z. Yuan, Y. Yang, W. Cui, X. Gao,Z. Liu, Y. Jin and B. Sun, Nanoscale, 2014, 6, 10505–10510.

35 E. L. Unger, E. T. Hoke, C. D. Bailie, W. H. Nguyen,A. R. Bowring, T. Heumuller, M. G. Christoforod andM. D. McGehee, Energy Environ. Sci., 2014, 7, 3690–3698.

36 Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang,Y. Gao and J. Huang, Energy Environ. Sci., 2014, 7, 2619–2623.

37 H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens,N. K. Noel, S. D. Stranks, J. T.-W. Wang, K. Wojciechowskiand W. Zhang, J. Phys. Chem. Lett., 2014, 5, 1511–1515.

38 M. D. McGehee, Nat. Mater., 2014, 13, 845–846.39 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501,

395–398.40 B.-w. Park, B. Philippe, T. Gustafsson, K. Sveinbjornsson,

A. Hagfeldt, E. M. J. Johansson and G. Boschloo, Chem.Mater., 2014, 26, 4466–4471.

41 T. Salim, S. Sun, Y. Abe, A. Krishna, A. C. Grimsdalea andY. M. Lam, J. Mater. Chem. A, 2015, DOI: 10.1039/C4TA05226A.

42 C. Liu, K. Wang, X. Hu, Y. Yang, C.-H. Hsu, W. Zhang,S. Xiao, X. Gong and Y. Cao, ACS Appl. Mater. Interfaces,2013, 5, 12163–12167.

43 T. C. Sum and N. Mathews, Energy Environ. Sci., 2014, 7,2518–2534.

44 J.-S. Kim, W.-S. Chung, K. Kim, D. Y. Kim, K.-J. Paeng,S. M. Jo and S.-Y. Jang, Adv. Funct. Mater., 2010, 20, 3538–3546.

45 E. J. J. Perez, M. Wubler, F. F. Santiago, K. L. Wollny,E. Mankel, T. Mayer, W. Jaegermann and I. Mora-Sero, J.Phys. Chem. Lett., 2014, 5, 680–685.

46 F. F. Santiago, G. G. Belmonte, I. M. Seroa and J. Bisquert,Phys. Chem. Chem. Phys., 2011, 13, 9083–9118.



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