当前位置：文档库 › Low-temperature_solution-processed_wavelength-tunable_perovskites_for_lasing.1

Low-temperature_solution-processed_wavelength-tunable_perovskites_for_lasing.1

Low-temperature solution-processed wavelength-tunable perovskites for lasing

Guichuan Xing1?,Nripan Mathews2,3,4*?,Swee Sien Lim1,3,Natalia Yantara2,3,Xinfeng Liu1, Dharani Sabba2,3,Michael Gr?tzel3,5,Subodh Mhaisalkar2,3and Tze Chien Sum1*

Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light https://www.wendangku.net/doc/7f4201861.html,anic semiconductors and colloidal quantum dots are considered the main candidates for this application. However,stumbling blocks in organic lasing1–4include intrinsic losses from bimolecular annihilation and the con?icting requirements of high charge carrier mobility and large stimulated emission;whereas challenges pertaining to Auger losses and charge transport in quantum dots5–7still remain. Herein,we reveal that solution-processed organic–inorganic halide perovskites(CH3NH3PbX3where X=Cl,Br,I),which demonstrated huge potential in photovoltaics8–11,also have promising optical gain.Their ultra-stable ampli?ed spontaneous emission at strikingly low thresholds stems from their large absorption coe cients,ultralow bulk defect densities and slow Auger recombination.Straightforward visible spectral tunability(390–790nm)is demonstrated. Importantly,in view of their balanced ambipolar charge transport characteristics8,these materials may show electrically driven lasing.

Organic–inorganic halide perovskites have recently emerged as a new class of photovoltaic materials with high e?ciencies driven by the large absorption coe?cients and long-range balanced electron and hole transport lengths8–11.Surprisingly,we found that they also exhibit excellent coherent light emission properties. Figure1a shows the transition from spontaneous emission(SE) to amplified spontaneous emission(ASE)with increasing pump fluence in a65-nm-thick CH3NH3PbI3film spin-coated on a quartz substrate.At low pump levels,the broad SE(with full width at half maximum(FWHM)～50nm,Fig.1b)from CH3NH3PbI3 increases linearly with increasing pump fluence.Correspondingly, the average transient photoluminescence(PL)lifetimes(τPL) progressively decrease(Fig.1b).Above the threshold fluence (12±2μJ cm?2;Fig.1c),the emission intensity increases superlinearly,withτPL dramatically shortened owing to the occurrence of a new short lifetime(<10ps)dynamical process (Supplementary Information).Concurrently,the emission band collapses to yield a sharp peak at788nm(Fig.1a).These are clear signatures of optical amplification of the SE from CH3NH3PbI3—that is,ASE behaviour.The balance between optical gain and self-absorption gives rise to a red-shifted ASE peak that is located near the tail of the absorption edge(Supplementary Information)12.In this work,we focus on the intrinsic gain properties of

perovskites by examining the ASE behaviour in a cavity-free

configuration.The ASE values provide a better benchmark for

comparing di?erent material sets on their intrinsic suitability

for gain applications.From the measured threshold fluence

(12±2μJ cm?2)and absorption coe?cient(α=5.7×104cm?1 at600nm;Supplementary Information),the ASE threshold carrier

density is calculated to be～1.7×1018cm?3.The threshold carrier

density corresponds to the ease with which a material can attain net

gain through optical or electrical generated https://www.wendangku.net/doc/7f4201861.html,paratively,

for highly crystalline high-temperature-grown ZnSe and CdS

nanowires(with similarα～105cm?1at the excitation wavelengths),

the typical threshold carrier densities are nearly one order larger

under similar measurement conditions13,14.Similarly,the typical

ASE threshold carrier density for solution-processed organic thin

films is approximately one order larger1–4.As a point of comparison,

state-of-the-art cavity-free solution-processed polymer films such

as poly[9,9-dioctylfluorene-co-9,9-di(4-methoxyphenyl)-fluorene]

(F8DP;ref.15)and Super Y ellow2exhibited an ASE threshold of

～6μJ cm?2(calculated from the reported threshold pump energy of

0.1μJ per pulse;excitation stripe400μm×4mm)and～36μJ cm?2

(calculated from the reported values of315nJ/pulse over a

rectangular spot of length2.5mm and width～350μm)respectively.

Our present results on CH3NH3PbI3also compare favourably

to a recent breakthrough in CdSe/ZnCdS core/shell colloidal

quantum dot(QD)films,where an ASE threshold of90μJ cm?2

was reported5.Photoluminescence quantum yield(PLQY)values

approaching20%at pump fluence above the ASE thresholds

were also measured using an integrating sphere(Supplementary

Information).The relatively low yield may be a consequence of the

low exciton binding energy(19±3meV;ref.16)as well as high

electron and hole mobilities17.Nonetheless,variable stripe length

(VSL)measurements on CH3NH3PbI3revealed a gain of～250cm?1

(fitted with Chan’s method7,typically used for colloidal QDs)or

～40cm?1(fitted with Shaklee and Leheny’s method18,largely used

for films of conjugated polymers)at a pump fluence of14μJ cm?2

(Supplementary Information).These gain values obtained from the

respective methods compare favourably with those for colloidal

QDs(refs5,19)and conjugated polymer thin films20at comparable

excitation intensities.

The noteworthy performances of CH3NH3PbI3with respect to

other solution-processed systems,present a compelling case to

understand the origins driving gain amplification in this system.

1Division of Physics and Applied Physics,School of Physical and Mathematical Sciences,Nanyang Technological University,21Nanyang Link,Singapore 637371,Singapore,2School of Materials Science and Engineering,Nanyang Technological University,Nanyang Avenue,Singapore639798,Singapore, 3Energy Research Institute@NTU(ERI@N),Research Techno Plaza,X-Frontier Block,Level5,50Nanyang Drive,Singapore637553,Singapore,

4Singapore-Berkeley Research Initiative for Sustainable Energy,1Create Way,Singapore138602,Singapore,5Laboratory of Photonics and Interfaces, Department of Chemistry and Chemical Engineering,Swiss Federal Institute of Technology,Station6,CH-1015Lausanne,Switzerland.?These authors contributed equally to this work.*e-mail:Nripan@https://www.wendangku.net/doc/7f4201861.html,.sg;Tzechien@https://www.wendangku.net/doc/7f4201861.html,.sg

15 μJ cm ?2

13 μJ cm ?2

11 μJ cm ?2

10 μJ cm

1.0

PL (ns)

0.1

8121.2 μJ cm ?2

900

800700

Wavelength (nm)

Pump ?uence (μ

J cm ?2)

Pump ?uence (μJ cm ?2)

P L i n t e n s i t y (a .u .)

F W H M (n m )

τFigure 1|Coherent light emission from solution-processed perovskite ?lm.a ,Steady-state PL emission spectra from a 65-nm-thick CH 3NH 3PbI 3?lm photoexcited using 600nm,150fs and 1kHz pump pulses with increasing pump ?uence (per pulse)—illustrating the transition from SE to ASE.b ,FWHM of the emission peak and average transient PL lifetime (τPL )as a function of the pump ?uence.τPL is the time taken for the intensity to decrease to 1/e of its

initial value.c ,PL intensity as a function of pump ?uence.The arrows indicate the trap state saturation threshold ?uence (P trap

th )and the ASE threshold ?uence (P ASE th ).The blue and red lines represent the linear ?ts to experimental data in the two linear regimes of SE and ASE,respectively.The dashed vertical black lines in b and c indicate the onset of ASE.

In particular,we seek to examine why the typical competing non-radiative pathways that can rapidly deplete the carrier population and make ASE unfavourable in other solution-processed semiconductors,are not dominant here.These non-radiative pathways include:bulk defects (such as vacancies,interstitials,antisites and so on)with fast trapping in the fs to ps timescales 21,22;surface traps which typically require >100ps for carrier di?usion through a few tens of nanometres of the material 8;and multi-particle loss mechanisms (such as bimolecular recombination in organic thin films or Auger recombination in QDs;refs 1–7).

Following photo-excitation across the CH 3NH 3PbI 3bandgap (at low pump fluence where Auger recombination is not dominant),the excited charge carriers could either relax through bandedge emission or trap-mediated non-radiative pathways.The former gives rise to SE with a lifetime (τ0)of 4.5±0.3ns (Fig.1b).An estimate of the bulk and surface trap densities can be made under these conditions where trap state recombination is much slower than bandedge radiative recombination.The photo-generated charge carrier density (n C (t ))after photoexcitation can be described with a set of di?erential equations (Supplementary Information).The model reveals the presence of two types of trap in these CH 3NH 3PbI 3thin films,with the bulk (surface)traps exhibiting fast (slow)trapping times 8,21,22.The bulk trap

density is n F TP ～5×1016cm ?3

whereas the surface trap density is n S TP ～1.6×1017cm ?3

.The trap densities measured in CH 3NH 3PbI 3are comparable to defect densities in highly ordered organic crystals (1015–1018cm ?3)23and superior to those of solution-processed organic thin films (1019cm ?3)24.Solution-deposited,high-temperature annealed Cu–In–Ga–S/Se (CIGS)chalcogenide layers 25also exhibit comparable defect densities to that reported here (1016cm ?3).These low bulk defect densities in perovskite are also consistent with the high solar cell e?ciencies in this material 8–11.To examine the e?ects of the more prevalent surface traps on the carrier dynamics and ASE,PL measurements on bare CH 3NH 3PbI 3were compared against CH 3NH 3PbI 3/([6,6]-phenyl-C61-butyric acid methyl ester (PCBM),C 60)bilayers to mimic the presence of infinite interfacial electron trap states.Selective excitation of the CH 3NH 3PbI 3layer (～65nm thick for both cases)was performed with 600nm laser pulses.The presence of the PCBM

layer (～45nm)is expected to severely quench the SE from the CH 3NH 3PbI 3layer;see Fig.2a for the PL spectra and Fig.2b for the PL decay transients.Such e?cient PL quenching originates from the long-range electron di?usion in the CH 3NH 3PbI 3film,where the di?usion-limited electron trapping time by the surface states can be estimated to be ～0.40ns (ref.8).Surprisingly,under high pump fluence excitation,the ASE is impervious to the presence of the PCBM layer,which acts as a perfect electron quencher.Figure 2b clearly shows that the carrier avalanche proceeds at a much faster timescale than the carrier trapping at the surface states.Thus the surface states will not a?ect the ASE processes,only the fast bulk traps.Indeed,the ASE threshold fluence for the CH 3NH 3PbI 3/PCBM film is measured to be 10±2μJ cm ?2(Fig.2c).This value is slightly smaller than that of the bare CH 3NH 3PbI 3film (12±2μJ cm ?2)because of the better light confinement and propagation due to the presence of the PCBM cladding layer which improves gain buildup (Supplementary Information).Remarkably,ASE can also be observed in functional photovoltaic devices (η=11.4%;Device structure:FTO/TiO 2compact layer/TiO 2mesoporous layer/CH 3NH 3PbI 3/Spiro-OMeTAD/Au)with optical excitation;see Supplementary Information.The presence of the Spiro-OMeTAD layer,which acts as a perfect hole quencher,has no e?ect on the ASE from CH 3NH 3PbI 3—further exemplifying its exceptional gain properties.

Although a low bulk defect density is favourable for obtaining reduced ASE thresholds,a critical criterion for achieving ASE is suppressed multi-particle non-radiative recombination rates (for example,bimolecular recombination noted in organics or Auger recombination in inorganic semiconductors).Bimolecular recombination (which is a limiting process in organic lasing)has been reported to be extremely low in CH 3NH 3PbI 3—defying the Langevin recombination limit by at least four orders of magnitude 17.These low bi-molecular charge recombination constants are consistent with our findings of low bulk defect densities as discussed earlier.The Auger recombination process in perovskite,which manifests under high pump fluence (nonlinear regime),typically yields Auger lifetimes (τAuger )from a few ps to ns,depending on the photo-generated charge carrier density 6.The Auger recombination in CH 3NH 3PbI 3is e?cient (τAuger ～300ps )compared with SE

750

Wavelength (nm)

Time (ns)700

P L i n t e n s i t y (a .u .)

800850

Figure 2|E ects of surface/interfacial traps on the coherent light emission.a ,Time-integrated PL spectra of CH 3NH 3PbI 3(black)and

CH 3NH 3PbI 3/PCBM (red).b ,Time-resolved PL (TRPL)decay transients for quartz/CH 3NH 3PbI 3(65nm)(black,～1.3μJ cm ?2)and

quartz/CH 3NH 3PbI 3(65nm)/PCBM (red,～1.3μJ cm ?2;blue,

～17μJ cm ?2)?lms in vacuum following excitation at 600nm (1kHz,150fs).Through modifying the surface/interfacial trap density,these measurements reveal that,whereas SE is strongly quenched by the surface/interfacial traps,ASE—which occurs on a much faster timescale—could e ectively compete with these carrier trapping

processes.The solid lines in b are the single-exponential ?ts of the PL decay transients.c ,The pump ?uence-dependent PL spectra and PL intensity (inset)of quartz/CH 3NH 3PbI 3(65nm)/PCBM ?lm.In the inset,the black and red lines represent the linear ?ts to experimental data in the two linear regimes of SE and ASE,respectively.

(4.5±0.3ns)because of the long-range electron-hole di?usion lengths within them 8.However,the timescale for the occurrence of ASE (<10ps—limited by the instrument response)signifies that the carrier build-up time for population inversion and the subsequent avalanche,out-competes the Auger processes in these CH 3NH 3PbI 3thin films (Supplementary Information).In contrast to solution-processed colloidal QDs (typical biexciton τAuger ～50ps for 5nm diameter CdSe QDs;ref.6),such an Auger loss mechanism is less dominant in this ‘bulk-like’CH 3NH 3PbI 3film.

The photostability of the CH 3NH 3PbI 3thin films was assessed by monitoring the ASE intensity as a function of time under laser irradiation at a 1kHz repetition rate at room temperature.

Figure 3a shows the variation in ASE intensity,with a standard deviation of 0.2%about the mean intensity for ～26h of continuous irradiation (that is ～108laser shots in all).The near invariance of the output intensity bears testimony to the excellent optical stability of these perovskite gain media.This performance compares favourably against the state-of-the-art organic semiconducting thin films (50%drop in output power after ～107laser shots;ref.4)and colloidal QDs (50%drop in output power after ～106laser shots;ref.7).The impressive ASE stability of the perovskite layers is also evident from tests of perovskite solar cells irradiated for ～8h under ambient conditions (Supplementary Information).

In the absence of any significant defect concentrations,the SE originates from the bandedge emission.Because the SE provides the seed photons for the photon cascade in ASE,the ASE wavelengths are in turn dependent on the bandgap of the semiconducting film.This is clearly evident from our temperature-dependent studies,where an increase in the bandgap due to a tetragonal to orthorhombic phase transition results in a blue-shifted SE and a corresponding shift in the ASE (Fig.3b;ref.26).The orthorhombic phase gives rise to three emission peaks,attributed to two bound exciton emissions (815nm and 782nm)and a free exciton emission (746nm),which yields the low-temperature ASE peak (at a threshold fluence of 10±2μJ cm ?2).Such an intrinsic dependence of the ASE on the bandgap allows wavelength tunability through halide substitution.By using either mixtures of bromides and iodides or chlorides and iodides,the bandgap is continuously tunable over the entire visible spectral range (from ～390to 790nm).We realize this through a simple physical mixing of the precusor solutions before spin-coating.Figure 3c shows the ASE from CH 3NH 3PbCl 3,CH 3NH 3PbCl 1.5Br 1.5,CH 3NH 3PbBr 3,CH 3NH 3PbBrI 2and CH 3NH 3PbI 3,thin films,demonstrating its wide wavelength-tunability.The ability of the perovskites to encompass the full visible spectrum allows them to address the ‘green gap’seen in III-nitrides and III-phosphides https://www.wendangku.net/doc/7f4201861.html,sing in perovskites can be achieved with a suitably designed cavity resonator (for example,with microspheres as whispering galley mode lasing or with gratings as distributed feedback lasing).Towards this,lasing has also been observed from CH 3NH 3PbI 3single crystals from dropcast thin films (Supplementary Information).This shows that despite the relatively lower PLQY measured,the impressive gain,the large absorption cross-section,low defect densities,low bimolecular recombination and slow Auger recombination in CH 3NH 3PbI 3enables lasing.

Our findings show that these organic–inorganic halide semiconductors are a new class of robust solution-processed gain media with highly desirable characteristics.The low ASE threshold and the long-range balanced charge carrier di?usion length stems from the low bulk defect density in CH 3NH 3PbI 3films.The highly crystalline PbX 6three-dimensional network lends crystalline inorganic character to CH 3NH 3PbX 3while maintaining its solution processability.Broad wavelength tunability is possible with both cation and anion replacement 28.Their low-temperature solution processing is highly compatible with unconventional substrates (Supplementary Information),printing technologies and monolithic integration with silicon-based electronics.Together with the long-range balanced electron and hole di?usion 8,high charge carrier mobilities and low bimolecular charge recombination rates 17,as well as large wavelength range continuously tunable coherent emission,our findings indicate that the simple solution-processed CH 3NH 3PbX 3may hold the key to realizing electrically driven solution-processed on-chip coherent light sources.

Methods

Materials preparation.The CH 3NH 3PbI 3films on quartz substrates were

prepared by spin-coating 10vol%solutions in DMF.[6,6]-phenyl-C 61-butyric acid methyl ester (PCBM)layers were spin-coated from a solvent mixture

Wavelength (nm)

PL intensity (a.u.)

Temperature (K)

W a v e l e n g t h (n m )

0.10.01

Laser shots (×107)

A S E i n t e n s i t y (a .u .)

Time (h)

Figure 3|ASE photostability,temperature-dependent PL and facile wide emission wavelength tunability.a ,Shot-dependent ASE intensity of the solution-processed CH 3NH 3PbI 3?lm with over 9×107laser excitation shots at 600nm (1kHz,50fs,～18μJ cm ?2)performed at room temperature.b ,PL spectrum at 10K—where the dotted lines are the deconvolved Gaussian peaks.The dashed lines in the false colour temperature-dependent PL map show the evolution of the emission peaks with temperature.c ,Wide wavelength tunability of ASE wavelengths from low-temperature solution-processed organic–inorganic halide perovskite ?lms fabricated by mixing the precursor solutions.(10mgml ?1)of anhydrous chlorobenzene and anhydrous chloroform (1:1v/v).The samples were put in vacuum for more than three days to get rid of any

residual solvent before the optical measurements.Mixed halides were prepared by blending appropriate molar ratios of CH 3NH 3PbI 3,CH 3NH 3PbBr 3and

CH 3NH 3PbCl 3solutions.The solar cells were fabricated using the sequential deposition procedure,as previously reported 9,29,and characterized under simulated air mass 1.5global (AM1.5G)solar irradiation in the dark.

Optical spectroscopy.For femtosecond optical spectroscopy,the laser sources were a Coherent Legend regenerative amplifier (150fs,1kHz,800nm)seeded by a Coherent Vitesse oscillator (100fs,80MHz)and a Coherent Libra regenerative amplifier (50fs,1kHz,800nm)seeded by a Coherent Vitesse oscillator (50fs,80MHz).800nm wavelength laser pulses were from the regenerative amplifier’s output whereas 400nm wavelength laser pulses were obtained with a BBO doubling crystal.600-nm laser pulses were generated from the Coherent TOPAS-C and Coherent OPerA-Solo optical parametric amplifiers.The laser pulses (circular spot,diameter 1.5mm)were directed to the films under vacuum in a cryostat.The emission from the samples was collected at a backscattering angle of 150?by a pair of lenses into an optical fibre that was coupled to a

spectrometer (Acton,Spectra Pro 2500i)and detected by a charge coupled device (Princeton Instruments,Pixis 400B).Time-resolved PL (TRPL)was collected using an Optronis Optoscope streak camera system which has an ultimate

temporal resolution of ～10ps.All optical measurements were performed at room temperature,except for ASE from CH 3NH 3PbCl 3(at 150K).Room-temperature photoluminescence quantum yield (PLQY)of the perovskite thin films was measured using an integrating sphere.The samples were excited with 600nm pulses generated from the Coherent OPerA-Solo.The emission was corrected for CCD and grating responsivity.Room-temperature gain measurements were carried out using standard VSL methods.The excitation stripe was focused by a cylindrical lens (with focal length f =20cm)to a stripe and the emission

collection configuration was the same as described above.The excitation stripe length was varied through an adjustable slit actuated by a micrometer which was placed at the focal line of the cylindrical lens.

Received 6November 2013;accepted 11February 2014;published online 16March 2014

References

1.Kozlov,V .G.,Bulovic,V .,Burrows,P .E.&Forrest,https://www.wendangku.net/doc/7f4201861.html,ser action in organic

semiconductor waveguide and double-heterostructure devices.Nature 389,362–364(1997).

2.Namdas,E.B.et al.Low thresholds in polymer lasers on conductive substrates

by distributed feedback nanoimprinting:progress toward electrically pumped plastic lasers.Adv.Mater.21,799–802(2009).

3.Clark,J.&Lanzani,https://www.wendangku.net/doc/7f4201861.html,anic photonics for communications.Nature Photon.

4,438–446(2010).

4.Grivas,C.&Pollnau,https://www.wendangku.net/doc/7f4201861.html,anic solid-state integrated amplifiers and lasers.

Laser Photon.Rev.6,419–462(2012).

5.Dang,C.et al.Red,green and blue lasing enabled by single-exciton gain in

colloidal quantum dot films.Nature Nanotech.7,335–339(2012).6.Klimov,V .I.Spectral and dynamical properties of multiexcitons in

semiconductor nanocrystals.Annu.Rev.Phys.Chem.58,635–673(2007).7.Chan,Y.et al.Blue semiconductor nanocrystal laser.Appl.Phys.Lett.86,

073102(2005).

8.Xing,G.et al.Long-range balanced electron-and hole-transport lengths in

organic–inorganic CH 3NH 3PbI 3.Science 342,344–347(2013).

9.Burschka,J.et al.Sequential deposition as a route to high-performance

perovskite-sensitized solar cells.Nature 499,316–319(2013).

10.Kim,H.S.et al.Lead iodide perovskite sensitized all-solid-state submicron thin

film mesoscopic solar cell with e?ciency exceeding 9%.Sci.Rep.2,591(2012).11.Heo,J.H.et al.E?cient inorganic-organic hybrid heterojunction solar cells

containing perovskite compound and polymeric hole conductors.Nature Photon.7,486–491(2013).

12.Ding,J.,Hagerott,M.,Ishihara,T.,Jeon,H.&Nurmikko,A.V .(Zn,Cd)Se/ZnSe

quantum-well lasers:Excitonic gain in an inhomogeneously broadened quasi-two-dimensional system.Phys.Rev.B 47,10528–10542(1993).

13.Xing,G.et al.Ultrafast exciton dynamics and two-photon pumped lasing from

ZnSe nanowires.Adv.Opt.Mater.1,319–326(2013).

14.Liu,X.,Zhang,Q.,Xiong,Q.&Sum,T.C.Tailoring the lasing modes in

semiconductor nanowire cavities using intrinsic self-absorption.Nano Lett.13,1080–1085(2013).

15.Xia,R.,Heliotis,G.&Bradley,D.D.Fluorene-based polymer gain media for

solid-state laser emission across the full visible spectrum.Appl.Phys.Lett.82,3599–3601(2003).

16.Sun,S.et al.The origin of high e?ciency in low-temperature

solution-processable bilayer organometal halide hybrid solar cells.Energy

Environ.Sci.7,399–407(2014).

17.Wehrenfennig,C.,Eperon,G.E.,Johnston,M.B.,Snaith,H.J.&Herz,L.M.

High charge carrier mobilities and lifetimes in organolead trihalide perovskites.

Adv.Mater.https://www.wendangku.net/doc/7f4201861.html,/10.1002/adma.201305172(2013).

18.Shaklee,K.L.&Leheny,R.F.Direct determination of optical gain in

semiconductor crystals.Appl.Phys.Lett.18,475–477(1971).

19.Liao,Y.,Xing,G.,Mishra,N.,Sum,T.C.&Chan,Y.Low threshold,

amplified spontaneous emission from core-seeded semiconductor

nanotetrapods incorporated into a sol–gel matrix.Adv.Mater.24,

159–164(2012).

https://www.wendangku.net/doc/7f4201861.html,mpert,Z.E.,Reynolds,C.L.Jr.,Papanikolas,J.M.&Aboelfotoh,M.O.

Controlling morphology and chain aggregation in semiconducting conjugated polymers:the role of solvent on optical gain in MEH-PPV.J.Phys.Chem.B.

116,12835–12841(2012).

21.Xing,G.et al.Charge transfer dynamics in Cu-doped ZnO nanowires.Appl.

Phys.Lett.98,102105(2011).

22.Uhd Jepsen,P.et al.Ultrafast carrier trapping in microcrystalline silicon

observed in optical pump-terahertz probe measurements.Appl.Phys.Lett.79, 1291–1293(2001).

23.Mehraeen,S.,Coropceanu,V.&Bredas,J.L.Role of band states and trap states

in the electrical properties of organic semiconductors:Hopping versus mobility edge model.Phys.Rev.B87,195209(2013).

24.Willa,K.et al.From organic single crystals to solution processed thin-films:

charge transport and trapping with varying degree of order.J.App.Phys.113, 133707(2013).

25.Graetzel,M.,Janssen,R.A.J.,Mitzi,D.B.&Sargent,E.H.Materials

interface engineering for solution-processed photovoltaics.Nature488,

304–312(2012).

26.Baikie,T.et al.Synthesis and crystal chemistry of the hybrid perovskite

(CH3NH3)PbI3for solid-state sensitised solar cell applications.J.Mater.Chem.

A1,5628–5641(2013)https://www.wendangku.net/doc/7f4201861.html,ubsch,A.,Sabathil,M.,Baur,J.,Peter,M.&Hahn,B.High-power and

high-e?ciency InGaN-based light emitters.IEEE T.Electr.Dev.57,

79–87(2010).

28.Zhang,S.et al.Preparations and characterizations of luminescent two

dimensional organic–inorganic perovskite semiconductors.Materials3,

3385–3406(2010).

29.Mulmudi,H.K.et al.Flexible,low-temperature,solution processed ZnO-based

perovskite solid state solar https://www.wendangku.net/doc/7f4201861.html,mun.49,11089–11091(2013). Acknowledgements

We would like to acknowledge D.Giovanni for the data fitting and C.W.Kiang for the electroluminescence measurement,as well as S.Shuangyong,J.Chua,T.Krishnamoorthy and S.Kulkarni for sample and precursor preparation.Financial support from NTU start-up grants M4080514and M4081293,SPMS collaborative Research Award

M4080536,Ministry of Education AcRF Tier2grant MOE2013-T2-1-081and from the Singapore NRF through the Competitive Research Program(NRF-CRP4-2008-03)and the Singapore-Berkeley Research Initiative for Sustainable Energy(SinBeRISE)CREATE Programme is gratefully acknowledged.M.G.thanks the European Research Council for financial support under the Advanced Research Grant(ARG247404)‘Mesolight’. Author contributions

G.X.,N.M.and T.C.S.conceived the idea for the manuscript and designed the experiments.G.X.developed the basic concepts,conducted the spectroscopic characterization and coordinated the experiments.N.M.and D.S.fabricated and characterized the samples.N.Y.,X.L.,M.G.and S.M.contributed to the data analysis. T.C.S.,N.M.,G.X.,S.M.and S.S.L.analysed the data and wrote the paper.T.C.S.and N.M. led the project.

Additional information

Supplementary information is available in the online version of the paper.Reprints and permissions information is available online at https://www.wendangku.net/doc/7f4201861.html,/reprints. Correspondence and requests for materials should be addressed to N.M.or T.C.S. Competing?nancial interests

The authors declare no competing financial interests.