Additive Enhanced Crystallization of Solution-Processed Perovskite for Highly Efficient...

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Additive Enhanced Crystallization of Solution-ProcessedPerovskite for Highly Efficient Planar-HeterojunctionSolar Cells

Po-Wei Liang, Chien-Yi Liao, Chu-Chen Chueh, Fan Zuo, Spencer T. Williams,

Xu-Kai Xin, Jiangjen Lin, and Alex K.-Y. Jen*

P.-W. Liang, C.-Y. Liao, Dr. C.-C. Chueh, Dr. F. Zuo,S. T. Williams, Dr. X.-K. Xin, Prof. A. K.-Y. Jen

Department of Materials Science and EngineeringUniversity of WashingtonSeattle, WA 98195, USAE-mail: [email protected]

Prof. A. K.-Y. JenDepartment of ChemistryUniversity of WashingtonSeattle, WA 98195, USA

C.-Y. Liao, Prof. J. J. LinInstitute of Polymer Science and EngineeringNational Taiwan UniversityTaipei 106, Taiwan

DOI: 10.1002/adma.201400231

the potentiality of the planar p-i-n heterojunction comprisedof thin-film intrinsic perovskite between solution processablep- and n-type charge transporting interlayers like PEDOT:PSSor Spiro-MeOTAD, and TiO2 or PCBM, respectively. Severalplanar-heterojunction configurations with no mesoporous TiO2 layer have been reported with increasing frequency.[11–13,19–24] Device architectures can be divided according to p-i-n hetero-

junction sequence as conventional (PEDOT:PSS (p)/Perovs-kite (i)/PCBM (n))[13,20,21,23] and inverted (compact TiO2 (n)/Perovskite (i)/organic semiconductors (p))[12,19,22,24] structure.Recently, a high efficiency perovskite planar-heterojunctioninverted solar cell with power conversion efficiency (PCE) over12% was demonstrated.[11,19]

At present, one of the main challenges encountered in per-ovskite thin film fabrication is the control of the crystallizationprocess and its impact on film quality. Poor perovskite mor-phology has been cited as very detrimental to device perfor-mance because it not only causes electrical shorting but alsodeleteriously impacts charge dissociation/transport/recom-bination.[21–24] Because of the sensitive dependence of growthkinetics on interfacial energy, solution concentration, precursorcomposition, solvent choice, and deposition temperature,improving perovskite morphology and coverage through con-trolling crystallization during film deposition and annealingis an attractive route to device optimization. It is possible toachieve optimal perovskite film morphology by finding effectiveways to manipulate its nucleation and growth.[21–24]

Burschka et al. have recently showed that enhanced perovs-kite crystallinity can be achieved in DSSCs by pre-deposition ofPbI2 from solution onto meso-porous TiO2 .

[7] The crystalliza-tion in the two-step process improved as a result of enhancedperovskite nucleation at the meso-porous TiO2 surface com-pared to that through the direct one-step deposition of the com-posite precursor. This example demonstrates that the surface

properties of the substrate have a strong influence on the nucle-ation and growth of a deposited film. As another example ofthe impact of phase transformation control, Snaith et al. haverecently demonstrated that perovskite crystallization rate canbe controlled by changing precursor composition.[2,9,10,19,22,23] By partially substituting I− with Cl− in CH3 NH3 PbI3 to formCH3 NH3 PbI3-x Clx , crystallization is prolonged as a result of thelattice distortion caused by Cl− doping. This is evident by theincreased time necessary to fully anneal deposited films, spe-cifically less than 1 h for pure iodine perovskite and between2 to 3 h for the mixed halide perovskite. More significantly,Cl− also increases the conductivity and charge diffusion length

Despite occurring only less than a year ago, the breakthroughof over 15% power conversion efficiency (PCE) in organo-metal halide perovskite solar cells has attracted significantattention and this hybrid system has been considered a viablemember of next generation photovoltaics that can addressthe scalability changes with a low-cost solution process.[1–13] Organometal halide perovskite absorbers possess several

appealing features such as intense light absorption, decentambipolar charge mobility, and small exciton binding energy.The band-gap of organometal perovskites can be easily tailoredthrough the choice of metal cation,[14] inorganic anion,[1] andorganic ligand.[15,16] Both p- and n-type conductivity of thisclass of perovskites are measured to be on the order 10−3 to10−2 S/cm.[3,16,17] The small exciton binding energy (∼20 meV)of these perovskites enable long exciton diffusion lengths(100-1000 nm) and lifetimes (∼100 ns) as compared with thepoor exciton diffusion lengths (∼10 nm) and lifetimes (∼10 ns)of organic semiconductors caused by tightly bounded elec-tron-hole pairs (>100 meV).[18–20] Complementary to this andcentral to their commercial viability is the low-temperature(∼100 °C) solution processability of organometal halide perovs-kites. All these advantages reveal their great potential to rivalsilicon-based solar cells for solar energy.

To meet the commercial requirement of high throughputmanufacturing processes, researchers are interested in devel-oping thin-film perovskite solar cells through simple, scalable,and low-temperature processing techniques.[18,20–24] Snaith et al. first discovered the feasibility of the planar thin-film architec-ture of solution processed perovskite solar cells as an evolutionfrom dye-sensitized photovoltaic systems.[10] This demonstrates

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(∼1000 nm) of the perovskite domain without affecting itsoptical properties.[19,25]

In this study, we develop a simple and very effective methodto enhance the crystallization of solution-processed perovskite.We demonstrate that crystallization rate of perovskite can be

controlled by incorporating additives into its precursor solu-tion to modulate thin film formation. The capacity of bidentatehalogenated additives to temporarily chelate with Pb+2 duringcrystal growth is evidenced by the improved solubility of PbCl 2 in the presence of 1,8-diiodooctane (DIO). The influence of thischelation encourages homogenous nucleation and likely modi-fies interfacial energy favorably, ultimately altering the kineticsof crystal growth. As a result, the morphology of the perovs-kite thin films processed from the solution with 1 wt% DIOshow a much smoother and more continuous surface than thatobtained from the pristine solution, as is illustrated in Figure 1 .The crystal size and homogeneity formed under the influenceof this halogenated additive lead to very high PCE (∼12%) inplanar-heterojunction perovskite solar cells.

Motivated by its low-temperature solution processability,we adopted a conventional p-i-n heterojunction architectureof ITO or FTO/PEDOT:PSS (35–40 nm)/CH3 NH3 PbI3-x Clx (∼400 nm)/PC61 BM (∼55 nm)/Bis-C60 (10 nm)/Ag (150 nm)to study the influence of perovskite thin-film morphology ondevice performance (Figure 1). Currently, the processing ofcompact TiO2 in the inverted architecture involves high-tem-perature annealing (∼500 °C) which negates some of the ben-efits offered by solution-based manufacturing process.[12,19,22] The conventional planar-heterojunction also offers the ben-efit of efficient charge transfer between the interfaces ofPEDOT:PSS/PC61 BM interlayers and perovskite thin films,ensuring sufficient charge dissociation and extraction for high

performance provided high quality perovskite films can begrown.[19–21,23] Notably, the Bis-C60 surfactant is employed asan efficient electron-selective interfacial layer that aligns theenergy levels at the organic/cathode interface and enables theutilization of stable metals such as Ag as the top electrodes,providing respectable environmental stability.[26,27] The mixedhalide CH3 NH3 PbI3-x Clx perovskite was chosen for study byvirtue of the fore mentioned merits demonstrated by Snaithet al.[2,9,10,19,22,23] Detailed information regarding the prepara-tion of the perovskite precursor solution, deposition of the per-ovskite thin-film, and the fabrication of devices is described inthe experimental section.

Solvent additives have enabled significant efficiencyenhancements in bulk-heterojunction (BHJ) organic solar cells(OPVs) by modulating BHJ morphology.[28–30] Bazan et al. firstdiscovered that the morphology of BHJ layers could be effec-tively optimized by simply incorporating additives like alkane

dithiols or 1,8-di(R)octanes into the processing solution.[28,29]

Two important features of the processing additives were identi-fied for the further optimization of BHJ morphology: the selec-tive solubility of fullerenes and the higher boiling point withrespect to the host solvent.[28,29] Inspired by this finding, weare interested in exploring the additive's influence on both thecrystallization of perovskite thin films and device performance.The additive studied here is DIO as it bears iodide in commonwith CH3 NH3 PbI3-x Clx and as a soft Lewis base may interactwith Pb2+ , a soft Lewis acid. We hypothesize that this additivecan temporarily coordinate with Pb2+ during crystal growthand modulate crystallization kinetics as the transient captureof additive in the growing crystal lattice will increase both theinternal energy and entropy of the crystals.[12,19,25]

The solvent additive was incorporated into theCH3 NH3 PbI3-x Clx precursor solution prior to the thin film depo-sition. The optimized blending ratio of DIO in the precursor solu-tion was found to be around 1 wt% with respect to the weight ofperovskite (denoted as 1% DIO in the following discussion). Atfirst, we constructed the planar-heterojunction on an ITO sub-strate. Encouragingly, the device processed from the precursorsolution containing 1% DIO showed a ∼30% PCE enhancementcompared to the control as shown in Figure 2 a. Open-circuitvoltage (V oc ), short-circuit current density ( J sc ), fill factor (FF),and PCE of all devices are summarized in Table 1 . The controldevice fabricated from the pristine composite solution showeda PCE of 7.8% with a V oc of 0.90 V, a J sc of 15.0 mA/cm2 ,

and a FF of 0.58 while that derived from the solution containing1% DIO exhibited a significantly enhanced PCE of 10.3% with aV oc of 0.92 V, a J sc of 15.6 mA/cm2 , and a FF of 0.71. All param-eters improved as a result of the incorporation of DIO in theprecursor solution, which suggests the generation of improvedperovskite morphology due to the influence of DIO.

It is well known that the crystallinity of perovskite absorberdomains determines ultimate performance of the fabricateddevices since defects in the crystals will create severe shortingand trapping sites for charge recombination. Crystallinity willalso greatly affect the efficiency of charge dissociation, trans-port, and diffusion length.[19,22] As shown in Figure 1, the film

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Figure 1. Device configuration of the planar-heterojunction solar cell, the cross-section SEM of the planar-heterojunction, and the surface SEM imagesof the CH3 NH3 PbI3− x Clx layer.



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prepared from the 1% DIO solution showed better coverage,less surface roughness, and more regular crystallites withmore ordered growth directions than the pristine thin film. Itis worthwhile to point out that the high V oc over 0.90 V for bothdevices implies the small potential loss for photoexciton dis-sociation (band-gap of CH3 NH3 PbI3-x Clx is ∼1.5 eV, estimatedfrom Figure 2c). This highlights prominent benefits of perovs-kites: small exciton binding energy (∼20 meV) and respectableambipolar charge conduction.[11–13,16–20] The high FF of over0.70 further demonstrates the efficient charge transfer andextraction of such a planar p-i-n heterojunction. [13,20]

Recently, Snaith et al. revealed that the perovskite perfor-mance is strongly dependent on the surface roughness of thesubstrate due to the sensitivity of crystallization on interfacialstructure.[23] They found that on FTO/PEDOT:PSS the perovs-kite film was more homogeneous (with ∼90% surface coverage)than the one on ITO/PEDOT:PSS (with only ∼80% surface

coverage), benefiting from the rough surface of FTO (100 nm)relative to ITO (∼5 nm, herein).[23] This resulted in the superiorperformance of the device constructed on FTO which motivatedus to explore our additive’s influence on this favored system.Very impressively, an increased PCE with similar enhancementfactor (∼31%) to that of the ITO case was achieved on FTO,suggesting that DIO’s beneficial influence on crystal growthkinetics is not limited by interfacial structure.

The device derived from the 1% DIO solution on FTO pos-sessed a promising PCE of 11.8% with a V oc of 0.92 V, a J sc of17.5 mA/cm2 , and a FF of 0.73, compared to the 9.0% PCE ofthe control device with a V oc of 0.90 V, a J sc of 16.0 mA/cm2 , anda FF of 0.62. Similar to the case on ITO substrates, the improvedquality and surface coverage of the crystalline perovskite thinfilms caused by the presence of DIO significantly contributesto the enhancement of J sc (16.0 to 17.5 mA/cm2 ) and FF (0.62to 0.73) (Surface images were shown in Figure S1). The results

of our top-performing devices are among

the best reported for state-of-the-art low-temperature solution-processed photovol-taics. The external quantum efficiency (EQE)plotted in Figure 2b confirms the increased J sc of these devices, in which the integrated J sc for the devices derived from 1% DIO con-taining solution on FTO and ITO is 17.3 and15.4 mA/cm2 , respectively (the integrated J sc of control device on ITO is 14.5 mA/cm2 ). Ascan be seen, the maximum EQE peaks of thetop-performing devices can reach over 70%(ITO-substrate) and 80% (FTO-substrate)

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Figure 2. (a) Current−voltage characteristics and (b) external quantum efficiency (EQE) spectra of the studied solar cells. (c) UV-vis absorption spectraand (d) XRD spectra of the solution-processed perovskite with and without additive.

Table 1. Performance of the studied solar cells under AM 1.5G Illumination (100 mW cm−2 ).

Voc

(V)

FF Jsc

(mA/cm2 )

PCE

(%)

On ITO Substrate (Roughness: ∼5 nm; 15 ohm/sq)

Pristine Perovskite 0.90 0.58 15.0 7.9

Perovskite-1% DIO 0.92 0.71 15.6 10.3

On FTO Substrate (Roughness: ∼100 nm; 8 ohm/sq)

Pristine Perovskite 0.90 0.62 16.0 9.0

Perovskite-1% DIO 0.92 0.73 17.5 11.8



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EQE. This high photon-to-electron conversion together withthe panchromatic absorption over the visible range (Figure 2c)highlights the intense bandgap light harvesting and excellentambipolar transport properties of the perovskites.

The additive enhanced crystallization of CH3 NH3 PbI3-x Clx isevident in the absorption and X-ray diffraction (XRD) spectra inFigure 2c–d. As can be seen, the perovskite processed with 1%DIO solution exhibits clearly increased light absorption acrossthe visible range into the near-infrared wavelengths, consistentwith the increase in the EQE curves. The increased absorptionshould be the result of improved surface coverage and moreuniform crystal formation in perovskite thin films. The rising

band-edge absorption around 780–790 nm proves the increasedcrystallinity of perovskites processed with additive. This canalso be observed in the enhanced intensity of the reflections at14.2° and 28.5° in the XRD spectrum of the DIO processed per-ovskite in Figure 2d. Although intensity increase in XRD is pos-sibly due to several factors, in this case, identical instrumentalparameters, sample quantities, and compositions were usedfor analysis. After being annealed for 3 h, we are confident thatmost DIO has evaporated; which, taken with the above state-ment, points to increased crystallinity in the DIO sample as thecause for the markedly increased observed peak intensity ascompared to the pristine sample.

These strong reflections at 14.2° and 28.5° are assigned tothe (110) and (220) crystal planes of the orthorhombic lattice ofmixed halide perovskites.[8] In the XRD analysis of all sampleswe fabricate on PEDOT:PSS, we see a significant degree of tex-ture with the (110) plane preferentially oriented parallel to thefilm surface. A close analysis of the data in Figure 2d revealsthat in addition to the overall intensity increase in the DIOsample, there is a noticeable increase in the ratio between theintensity of both the (110) and (220) peaks and that of the (310),(224), and (314) peaks located at 31.9°, 40.7°, and 43.3° respec-tively, as inserted in Figure 2d.[21,31] This indicates that in addi-tion to improving the degree of crystallinity, DIO enhances the

generation of texture already encouraged by the PEDOT:PSS/perovskite interface. This can be explained either through amodification of interfacial energy caused by DIO or as a conse-quence of the role DIO chelation of Pb2+ plays in phase trans-formation, but currently the texture inherent to these systemsis so strong that most reflections do not extend far above thenoise threshold. A pole figure analysis is necessary to quantita-tively characterize this phenomenon.

To get more in-depth understanding of the function of DIOduring perovskite crystallization, time resolved morphologicalcharacterization was made and recorded with scanning elec-tron microscopy (SEM) and XRD, as presented in Figure 3 . The

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Figure 3. (a) Time resolved SEM images of the surface of the evolving CH3 NH3 PbI3− x Clx films. The scale bars are all 5 µm. (b) Time resolved XRDspectra of the evolving CH

3

NH3

PbI3− x

Clx

films.



5



preparation of these perovskite thin films was identical to theconditions used for the device fabrication. As cast, both filmscontain many pin-holes and voids, but the additive assistedfilm already demonstrates markedly improved surface coverage.Additionally, the unique contrast features in the SEM image ofthe as cast DIO film along with its dense XRD pattern suggest ageneration of order before annealing that is unique to the DIO

sample. The increased coverage and smoothness of the DIOassisted film suggest that DIO encourages homogenous nuclea-tion and reduces the kinetics of the transformation enough toallow the thermodynamic influence of the interface betweenPEDOT:PSS and CH3 NH3 PbI3-x Clx to play a more dominantrole. The presence of DIO may also modify this interfacialenergy, making it more favorable for the crystal to grow in con-tact with the surface.

These improvements may also be attributed to the increasedsolubility of PbCl2 in the mixed solvent DMF/DIO, as shown inFigure 4 . We speculate that this improvement results from thetemporary coplanar chelation of Pb2+ with DIO as Cl− ligandsreside in axial octahedral positions on Pb2+ .[8,25] As soft Lewisbases, iodocarbons can coordinate with soft metal ions based onthe hard-soft acid-base principle.[32] Thus it can be envisionedthat the transient metal-solvent coordination can improve the sol-ubility of PbCl2 , as proposed in Figure 4a (and Scheme S1).[32,33] On the other hand, bidentate chelation is also more favorablethan monodentate chelation from a thermodynamic perspectiveas observed in some metal complex systems.[34,35] The formationof the chelated ring structures of the former allow less configu-rational entropy loss during coordination, resulting in a muchsmaller Gibbs free energy. Therefore, the temporary chelation ofPb2+ with DIO will participate in the evolving dynamic equilib-rium of the drying and annealing film, co-existing with the coor-dination of Pb2+ with methyl ammonium iodide (MAI) duringcrystal growth until DIO fully evaporates.

As a result, transformation kinetics are retarded enough toallow and encourage more defect-free crystal growth. More-over, it can be seen in the SEM images that the crystallites ofthe additive-assisted film display more regular faceting andimproved interconnectivity during crystal growth, which canalso be attributed to the prolonged growth caused by temporarychelation of Pb2+ by DIO. The distorted crystal lattice inducedby captured DIO will increase the internal energy and con-figurational entropy of growing crystals thus modulating theirgrowth rate and shape. The impact of these modifications ongrowth kinetics is apparent in the device data discussed above,correlating well with the increase of all relevant parameters.

Time resolved photoluminescence (PL) behavior was charac-terized to probe the influence of the enhanced crystallizationof perovskite thin films on charge dissociation. Detailed infor-mation regarding the preparation, measurement, and fittingmethodology can be found in the experimental section. The PLlifetime of the samples was fitted with a bi-exponential decayfunction containing a fast decay and a slow decay process. We

consider the fast decay process to be the result of the quenchingof free carriers in the perovskite domain through transport toPEDOT:PSS or PC61 BM, and the slow decay process to be theresult of radiative decay. Figure 5 displays the PL decay and therelated parameters are summarized in Table 2 . For the pris-tine thin-film perovskite, the fast decay lifetime is 12.9 ± 0.8 nsand the slow decay lifetime is 104 ± 3 ns while their weightfractions are 81% and 19% respectively, indicating that chargetransfer is the dominating decay mechanism. To mimic thereal charge behavior in our p-i-n planar heterojunction device,the bilayer CH3 NH3 PbI3-x Clx /PC61 BM systems were examined.The existence of the electron quenching PC61 BM layer atop theperovskite significantly decreases the fast decay lifetime from12.9 ± 0.8 ns to 3.4 ± 0.4 ns and increases the weight fraction offast decay from 81% to 94%. This suggests that most of the freecarriers generated by illumination are efficiently transferred toPEDOT:PSS and PC61 BM, which confirms the potential of theconventional p-i-n heterojunction.

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Figure 4. (a) Schematic diagram for the transient chelation of Pb2+ with DIO. (b) Solubility of the PbCl2 and PbI2 in DMF with or without additives.From left to right, PbCl2 in DMF, PbCl2 in DMF/DIO, PbI2 in DMF, and PbI2 in DMF/DIO. The blending ratio of DIO to DMF for PbCl2 and PbI2 is1:1.6 and 1:2.8 by mol, respectively.

Figure 5. Time resolved photoluminescence characterization of thesolution-processed perovskite with and without additive.



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As for the thin-film perovskite prepared from the 1% DIOsolution, the fast decay lifetime decreased from 12.9 ± 0.8 nsto 8.5 ± 0.5 ns and the weight fraction increased from81% to 88%. The same trend was observed in the bilayerCH3 NH3 PbI3-x Clx /PC61 BM film with lifetime decreasing from3.4 ± 0.4 ns to 1.2 ± 0.1 ns and weight fraction increasingfrom 94% to 99%. As evidenced in Figure 3, the addition ofDIO improved the coverage and crystallinity of the perovskitethin films on ITO/PEDOT:PSS, thus facilitating the diffusionof free carriers and increasing the efficiency of charge transfer.

Consequently, the weight fraction of fast decay, which relatesto the charge transfer process, increased and the lifetime offast decay decreased. For perovskite processed from the 1%DIO precursor with PC61 BM, the weight fraction related toquenching was almost 100%, which indicated a very efficientcharge dissociation in the perovskite domain with PEDOT:PSSand PC61 BM. The observed high J sc (15.6 – 17.5 mA/cm2 ) andFF (0.71 – 0.73) of the devices derived from the 1% DIO solu-tion can also be attributed to this enhanced charge separationand collection.

Based on these findings, the enhanced crystallization of thesolution processed perovskite films should be mainly due tothe additive chelating effect and is less sensitive to the inter-facial structure and the congruent improvement on both ITOand FTO substrates. The chelation efficiency may be furthermodulated by changing the additive's properties: alkane chainlength and the nature of the end groups. Given the efficientcoordination between Pb2+ and halides (Cl− , Br− , and I− ) in per-ovskite structures,[1] the effect of alkane chain length will affectbidentate chelation as it will influence the thermodynamics andkinetics of the formation of the proposed chelated ring struc-tures. More in-depth works regarding the understanding of theoptimization of processing additives are ongoing.

In summary, we have described a significantly enhancedPCE (∼31%) of the planar-heterojunction perovskite solar cellsfrom 9.0% up to 11.8% by incorporating small amount ofrationally chosen additives into the perovskite precursor solu-

tions to improve the crystallization of perovskite thin films.We showed that incorporated additives facilitate homogenousnucleation and modulate the kinetics of growth during crystal-lization, as evidenced from the surface SEM images and XRDspectra. The enhanced crystallization facilitates the chargetransfer efficiency between the charge transporting interlayersand the perovskite absorber. As a result, very promising PCEs(∼12%) were achieved in planar-heterojunction solar cells, fab-ricated through the low-temperature solution processes (under150 °C). These results are among the best reported for the state-of-the-art solution-processed photovoltaics. We expect that evengreater performance enhancement can be achieved through

further rational design of processing additives as revealed byour work.

Experimental Section

Materials and Sample Preparation : Methylammonium iodide (MAI)was synthesized by reacting 24 mL of 0.20 mol methylamine (33 wt% inabsolute ethanol, Aldrich), 10 mL of 0.04 mol hydroiodic acid (57 wt%in water with 1.5% hypophosphorous acid, Alfa Aesar), and 100 mLethanol in a 250 mL round bottom flask under nitrogen at 0 °C for 2 h

with stirring. After reaction, the white precipitate of MAI was recoveredby rotary evaporation at 40 °C and then dissolved in ethanol followed bysedimentation in diethyl ether by stirring the solution for 30 min. Thisstep was repeated three times and the MAI powder was finally collectedand dried at 50 °C in a vacuum oven for 24 h. To prepare the perovskiteprecursor solution, MAI and lead chloride (PbCl2 , Aldrich) powder weremixed in anhydrous dimethylformamide (DMF, Aldrich) with a molarratio of 3:1. The perovskite/1,8-diiodooctane (DIO, Aldrich) solution wasprepared via adding 1 wt% of DIO with respect to perovskite weight intothe perovskite precursor solution. The solutions (40 wt%) were stirredovernight at 80 °C and filtered with 0.45 µm PVDF filters before devicefabrication.

Fabrication of thin-film perovskite solar cells : The devices were fabricatedin the configuration of indium tin oxide (ITO) or fluorine-doped tinoxide (FTO)/PEDOT:PSS/ CH3 NH3 PbI3-x Clx /[6,6]-phenyl-C61 -butyric acidmethyl ester (PC61 BM)/fullerene surfactant (C60 -bis)/Ag. ITO (15 ohm/

sq) and FTO (8 ohm/sq) glass substrates were cleaned sequentiallywith detergent and deionized water, acetone, and isopropanol undersonication for 10 min. After drying under a N 2 stream, substrates werefurther cleaned by a plasma treatment for 30 s. PEDOT:PSS (Baytron PVP Al 4083, filtered through a 0.45 µm nylon filter) was first spin-coatedonto the substrates at 5k rpm for 30 s and annealed at 150 °C for 10 minin air. To avoid oxygen and moisture, the substrates were transferredinto a N2 -filled glovebox, where the thin-film perovskite layers were spin-coated from a homogeneous 40 wt% CH3 NH3 PbI3-X ClX and CH3 NH3 PbI3-

X ClX /DIO precursor solution at 6k rpm for 45 s (300-500 nm thickness)and then annealed at 90 °C for 2-3 h. Afterward, the PC61 BM (15 mg/mL in chloroform) and C60 -bis surfactant (2 mg/mL in isopropyl alcohol)were then sequentially deposited by spin coating at 1k rpm for 60 s and3k rpm for 60 s, respectively. Silver electrodes with a thickness of 150 nmwere finally evaporated under high vacuum (<2 × 10−6 Torr) through a

shadow mask. The device area is defined as 3.14 mm2

. All the J–V curvesin this study were recorded using a Keithley 2400 source meter unit. Thedevice photocurrent was measured under AM1.5 illumination conditionat an intensity of 100 mW cm−2 . The illumination intensity of the lightsource was accurately calibrated with a standard Si photodiode detectorequipped with a KG-5 filter, which can be traced back to the standard cellof the National Renewable Energy Laboratory (NREL). The EQE spectraperformed here were obtained from an IPCE setup consisting of a Xenonlamp (Oriel, 450 W) as the light source, a monochromator, a chopperwith a frequency of 100Hz, a lock-in amplifier (SR830, Stanford ResearchCorp), and a Si-based diode (J115711-1-Si detector) for calibration.

Time-resolved Photoluminescene Measurements : ITO-coated glasssubstrates were cleaned sequentially in detergent, acetone, iso-propanoland oxygen plasma. PEDOT:PSS solution was spin-coated at 5 k rpm

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Table 2. Time resolved photoluminescence characterization of the solution processed perovskite.

Subs. ETM Additive τ 1 (ns) Fraction 1 τ 2 (ns) Fraction 2 Average (ns)

ITO/PEDOT:PSS - - 12.9 ± 0.8 81% 104 ± 3 19% 30

DIO 8.5 ± 0.5 88% 88 ± 3 12% 18

PC61 BM - 3.4 ± 0.4 94% 139 ± 10 6% 12

DIO 1.2 ± 0.1 99% 18 ± 2 1% 1.4



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Received: January 15, 2014Revised: February 8, 2014

Published online:

Adv. Mater. 2014,

DOI: 10.1002/adma.201400231

for 30 s and annealed at 150 °C for 10 min in air. The preparation ofthe thin-film perovskite is identical to the method described in methodssection: MAI and PbCl2 were dissolved under heating in anhydrousDMF at a 3:1 molar ratio of MAI to PbCl2 , with a final weight percentof 40%. For samples with 1,8-diiodooctane (DIO), 1.0 wt% of DIOwith respect to the weight of perovskite was added in DMF solutionbefore adding MAI and PbCl2 . The precursors were then filtered andspin-coated onto the ITO/PEDOT:PSS substrate at 6k rpm inside the

glove box. After spin-coating, the CH3 NH3 PbI3-x Clx films were annealedat 90°C for 2 to 3 h. The top quenchers were then deposited via spin-coating chlorobenzene solutions of 10 mg/mL poly(methylmethacrylate)(PMMA) or chloroform solutions of 15 mg/mL PC61 BM at 1k rpm. Time-resolved PL measurements were measured by a time-correlated singlephoton counting (TCSPC) system (FluoTime 100, PicoQuant GmbH).Samples were photoexcited using a 467 nm laser beam (LDH-P-C-470,PicoQuant GmbH) pulsed at frequencies between 0.5-10MHz, with apulse duration of 60 ps and fluence of ∼10 nJ/cm2 , to avoid nonlineareffects such as exciton-charge annihilation. The lifetime was obtained byfitting the PL measured from perovskite films with a bi-exponential decayfunction of the form:

( ) exp exp1

12

2I t A

t A

tτ τ

= −

+ −

Supporting Information

Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements

The authors thank the support from the Air Force Office of ScientificResearch (No. FA9550-09-1-0426), the Asian Office of AerospaceR&D (No. FA2386-11-1-4072), and the Office of Naval Research (No.N00014-11-1-0300). A.K.Y.J. thanks the Boeing Foundation for support.S.T.W. thanks the financial support from National Science FoundationGraduate Research Fellowship Program (DGE-1256082). Part of thiswork was conducted at the University of Washington NanoTech User

Facility, a member of the NSF National Nanotechnology InfrastructureNetwork (NNIN). C.Y.L. and C.C.C. thank the financial support from theNational Taiwan University (101R4000).

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