Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystal

REPORTS

(MAPbX 3)perovskite solar cells (PSCs)have now achieved 20.1%certified power con-version efficiencies (1),following a rapid surge of development since perovskite-based devices were first reported in 2009(2).A key to the success of PSCs is the long diffusion length of charge carriers in the absorber perovskite layer (3).This parameter is expected to depend strong-ly on film crystallinity and morphology.Ther-mally evaporated MAPbI 3films fabricated using a Cl –-based metal salt precursor were reported to exhibit carrier diffusion lengths three times those of the best solution-processed materials,yet no measurable Cl –was incorporated in the final films,hinting at a major but unclear mecha-nism in the control of crystallinity and morphol-ogy (4,5).These observations suggest that there may be room to improve upon already remark-able PSC efficiencies via the optimization of three key parameters:charge carrier lifetime,mobility,and diffusion length.

The quest for further improvements in these three figures of merit motivated our exploration of experimental strategies for the synthesis of large single-crystal MAPbX 3perovskites that would ex-hibit phase purity and macroscopic (millimeter)

methods failed to produce single crystals with macroscopic dimensions large enough to enable electrode deposition and practical characteriza-tion of electrical properties (6).Past efforts based on cooling-induced crystallization were hindered by (i)the limited extent to which solubility could be influenced by controlling temperature,(ii)the complications arising from temperature-dependent phase transitions in MAPbX 3,and (iii)the impact of convective currents (arising from thermal gra-dients in the growth solution)that disturb the ordered growth of the crystals.

We hypothesized that a strategy using anti-solvent vapor-assisted crystallization (AVC),in which an appropriate antisolvent is slowly dif-fused into a solution containing the crystal pre-cursors,could lead to the growth of sizable MAPbX 3crystals of high quality (with crack-free,smooth surfaces,well-shaped borders,and clear bulk trans-parency).Prior attempts to grow hybrid perov-skite crystals with AVC have fallen short of these qualities —a fact we tentatively attributed to the use of alcohols as antisolvents (7).Alcohols act as good solvents for the organic salt MAX (8)due to solvent-solute hydrogen bond interactions;as a result,they can solvate MA +during the ionic assembly of the crystal,potentially disrupting long-range lattice order.

We instead implemented AVC (Fig.1A)using a solvent with high solubility and mod-erate coordination for MAX and PbX 2[N ,N -dimethylformamide (DMF)or g -butyrolactone (GBA)]and an antisolvent in which both perov-skite precursors are completely insoluble [dichlo-romethane (DCM)].We reasoned that DCM,unlike alcohols,is an extremely poor solvent for both

MAX and PbX 2and lacks the ability to form hydrogen bonds,thus minimizing asymmetric interactions with the ions during their assembly into crystal form.When combined with a slow and controlled diffusion rate into DMF or GBA,our approach established the conditions for all the ionic building blocks of the perovskite to be coprecipitated from solution https://www.wendangku.net/doc/bd6378817.html,ing this method,we grew high-quality,millimeter-sized MAPbBr 3and MAPbI 3single crystals (fig.S1)(9)whose shape conformed to the underlying symmetry of the crystal lattice.The phase purity of the as-grown crystals was confirmed by x-ray diffraction (XRD)performed on powder ground from a large batch of crystals (Fig.1B).

The synthesized crystals were of sufficient quality and macroscopic dimensions to enable a detailed investigation of the optical and charge transport properties.The absorbance of MAPbX 3(X =Br –or I –)(Fig.2)shows a clear band edge cutoff with no excitonic signature,which sug-gests a minimal number of in-gap defect states.For comparison,the absorption spectrum from the polycrystalline MAPbBr 3(fig.S2)(9)and MAPbI 3(5)thin films shows a peak near the band gap,which is often attributed to an ex-citonic transition.This observation is consistent with a substantial amount of disorder and lack of long-range structural coherence in nanostruc-tured thin films (10).By extrapolating the linear region of the absorption edge to the energy-axis intercept (fig.S3)(9),we determined the opti-cal band gaps of MAPbBr 3and MAPbI 3single

RESEARCH

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1

Solar and Photovoltaic Engineering Research Center (SPERC),King Abdullah University of Science and

Technology (KAUST),23955-6900Thuwal,Saudi Arabia.2

Department of Electrical and Computer Engineering,University of Toronto,Toronto,Ontario M5S 3G4,Canada.3

Department of Chemistry,Indiana University,Bloomington,IN 47405,USA.4Department of Physics and Astronomy,University of Nebraska,Lincoln,NE 68588,USA.

*These authors contributed equally to this work.?Corresponding author.E-mail:

osman.bakr@https://www.wendangku.net/doc/bd6378817.html,.sa

o n J a n u a r y 29, 2015

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crystals to be 2.21and 1.51eV (Fig.2),respec-tively.Both materials in their single-crystalline than the corresponding films,which could en-hance photon harvesting and hence improve As also shown in Fig.2,both MAPbBr 3and MAPbI 3exhibit a narrow photoluminescence 520

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300200100

Time decay (ns)Time trace @ 580nm 3P L i n t e n s i t y

Photoluminescence

Transient absorption (590nm)MAP bBr 3

Slow component Fast component

A )Time-of-flight time t =0in a corner in each trace time versus inverse crystals,evaluated at with a slower decay (t ≈978T 22ns).(D cence (PL)color map,markers).(E )PL time decay exponential fits showing (F )PL time decay trace fast (t ≈22T 6ns)and RESEARCH |REPORTS

shoulder at ~790nm in the PL of MAPbI 3single crystals is in agreement with the PL from thin films (5),with the main PL peaking at 820nm attributed to the intrinsic PL from the MAPbI 3crystal lattice.A more structured PL spectrum was observed for polycrystalline MAPbBr 3thin films (fig.S2)(9).

We investigated the key quantities that direct-ly affect a material ’s potential for application in PSCs:carrier lifetime t ,carrier mobility m ,and carrier diffusion length L D .In addition,we estimated the in-gap trap density n traps in order to correlate the trap density with the observed diffusion length.For MAPbBr 3single crystals,we first measured carrier mobility using the time-of-flight technique (11).The transient current was measured for various driving voltages (V ),and the corresponding traces are shown in Fig.3A on a bilogarithmic scale.The transit time t t ,defined as the position of the kink in the time traces,is marked by the blue squares,and the correspond-ing values are plotted in Fig.3B as a function of V –1.The mobility m [m =m p ≈m n ,where m p and m n are the hole and electron mobility,respectively (12,13)]can be directly estimated from the transit time t t ,sample thickness d ,and applied voltage V as m =d 2/V t t (Fig.3B)(9).Estimating mobility via a linear fit of t t versus V –1led to an estimate of 115cm 2V –1s –https://www.wendangku.net/doc/bd6378817.html,plementary Hall effect measurements at room temperature con-firmed a carrier (holes)concentration of between 5×109and 5×1010cm –3,and provided a mobility estimate in the range from 20to 60cm 2V –1s –1.Slightly lower mobilities obtained via the Hall effect may be ascribed to surface effects that are negligible for time-of-flight,which constitutes a bulk probe.

For MAPbI 3single crystals,we estimated the carrier mobility using the space-charge-limited current (SCLC)technique.We measured the current-voltage (I-V )trace for the crystals and observed a region showing a clear quadratic de-pendency of the current on the applied voltage at 300K (see fig.S8for details).From this region,we could conservatively estimate the carrier mo-bility,

obtaining the value m =2.5cm 2V –1s –1.From the linear ohmic region,we also identified the conductivity of the crystal to be s =1×10?8(ohm·cm)–https://www.wendangku.net/doc/bd6378817.html,bining the information on mo-bility and conductivity,we estimated a carrier concentration of n c =s /e m ≈2×1010cm ?3(where e is the electronic charge).

We estimated the carrier lifetime t from tran-sient absorption (TA)and PL spectra.Nano-second pump-probe TA spectroscopy was carried out over a window covering the nanosecond-to-microsecond time scales in order to evaluate the fast (t ≈74ns)as well as the slow (t ≈978ns)carrier dynamics,as determined from biexponen-tial fits.Time (t )–and wavelength (l )–resolved PL maps IPP (t ,l )(Fig.3D)of single-crystalline MAPbBr 3were acquired in the wavelength re-gion around the main band-to-band recombina-tion peak at 580nm (l =500to 680nm).The time-dependent PL signals in single-crystalline samples of MAPbBr 3and MAPbI 3are shown in Fig.3,E and F,respectively;the data were mea-sured at the wavelength of the main PL peak —i.e.,l =580nm and l =820nm for MAPbBr 3and MAPbI 3,respectively (see insets).

The time-resolved traces are representative of the transient evolution of the electron-hole population after impulsive (D t ≈0.7ns)photo-excitation.Biexponential fits were performed to quantify the carrier dynamics (fig.S4,blue traces)(9).Both the bromide-and iodide-based perovskite crystals exhibited a superposition of fast and slow dynamics:t ≈41and 357ns for MAPbBr 3,and t ≈22and 1032ns for MAPbI 3.We assign these two very different time scales to the presence of a surface component (fast)together with a bulk component (slow),which reveals the lifetime of carriers propagating deeper in the material.The relative contribution of these two terms to the static PL can be readily eval-uated by integrating the respective exponential traces (the integral is equal to the product of the amplitude A and the decay time t ),which shows that the fast (tentatively surface)component amounts to only 3.6%of the total TA signal in MAPbBr 3,and to 12%and 7%of the total PL signal in MAPbBr 3and MAPbI 3,respectively.

Ultimately,by combining the longer (bulk)car-rier lifetimes with the higher measured bulk mo-bility,we obtained a best-case carrier diffusion length L D =(k B T /e ·m ·t )1/2(where k B is Boltzmann ’s constant and T is the sample temperature)of ~17m m in MAPbBr 3;use of the shorter lifetime and lower mobility led to an estimate of ~3m m.The same considerations were applied for the MAPbI 3crystals to obtain a best-case diffusion length of ~8m m and a worst-case length of ~2m m.For comparison,we also investigated the PL decay of solution-processed thin films of MAPbBr 3(fig.S5).We again found two dynamics:a fast decay (t ≈13ns)and a longer-lived component (t ≈168ns),in both cases faster than the single crystals.This result suggests a larger trap-induced recombination rate in the thin films,which are expected to possess a much higher trap density than the single crystals.Previous studies on non –Cl-doped MAPbI 3nanostructured thin films also corroborate this trend,revealing a PL life-time of ~10ns and a carrier diffusion length of ~100nm (3,5).

Crystalline MAPbX 3is characterized by a charge transport efficiency that outperforms thin film –based materials in mobility,lifetime,and diffu-sion length.To unveil the physical origins of this difference,we investigated the concentration of in-gap deep electronic trap states.We measured the I -V response of the crystals in the SCLC re-gime (Fig.4).Three regions were evident in the experimental data.At low voltages,the I -V re-sponse was ohmic (i.e.,linear),as confirmed by the fit to an I ≈V functional dependence (blue line).At intermediate voltages,the current exhib-ited a rapid nonlinear rise (set in at V TFL =4.6V for MAPbBr 3and 24.2V for MAPbI 3)and sig-naled the transition onto the trap-filled limit (TFL)—a regime in which all the available trap states were filled by the injected carriers (14).The onset voltage V TFL is linearly proportional to the density of trap states n traps (Fig.4A).Cor-respondingly,we found for MAPbBr 3single crystals a remarkably low trap density n traps =5.8×109cm –3,which,together with the extremely clean absorption and PL profiles (see again Fig.2A),points to a nearly defect-free electronic structure.At high fields,the current showed a quadratic voltage dependence in the Child ’s regime.In this region,we extracted the value for the trap-free mobility m .We found m =38cm 2V –1s –1(Fig.4A),a value in good agreement with the mobility extracted using time-of-flight and Hall effect mea-surements (fig.S7)(9).We determined a com-parable low trap density n traps =3.3×1010cm –3for MAPbI 3single crystals using the same meth-od (Fig.4B).

The defect density measured for the room temperature –grown MAPbX 3crystals was supe-rior to a wide array of established and emerging optoelectronic inorganic semiconductors includ-ing polycrystalline Si (n traps ≈1013to 1014cm –3)(15,16),CdTe/CdS (n traps ≈1011to 1013cm –3)(17),and copper indium gallium selenide (CIGS)(n traps ≈1013cm –3)thin films (18),as well as organic materials such as single-crystal rubrene (n traps ≈1016cm –3)(19)and pentacene (n traps ≈1014to 1015cm –3)(20).Only

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Fig.4.Current-voltage traces and trap density.Characteristic I -V trace (purple markers)showing three different regimes for (A )MAPbBr 3(at 300K)and (B )MAPbI 3(at 225K).A linear ohmic regime (I V V ,blue line)is followed by the trap-filled regime,marked by a steep increase in current (I V V n >3,green line).The MAPbBr 3trace shows a trap-free Child ’s regime (I V V 2,green line)at high voltages.

RESEARCH |REPORTS

ultrahigh-quality crystalline silicon,grown at high temperatures,offers comparable or better deep trap densities (108

REFERENCES AND NOTES

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7.Y.Tidhar et al .,J.Am.Chem.Soc.136,13249–13256(2014).8.M.Xiao et al .,Angew.Chem.Int.Ed.53,9898–9903(2014).9.See supplementary materials on Science Online.

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Nano Lett.14,127–133(2014).

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ACKNOWLEDGMENTS

We thank N.Kherani,B.Ramautarsingh,A.Flood,and P.O ’Brien for the use of the Hall setup.Supported by KAUST (O.M.B.)and by KAUST award KUS-11-009-21,the Ontario Research Fund Research Excellence Program,and the Natural Sciences and Engineering Research Council of Canada (E.H.S.).

SUPPLEMENTARY MATERIALS

https://www.wendangku.net/doc/bd6378817.html,/content/347/6221/519/suppl/DC1Materials and Methods Figs.S1to S12References (23–44)

12November 2014;accepted 19December 2014T

rovskites offers promising routes for the de-velopment of low-cost,solar-based clean global energy solutions for the future (1–4).Solution-processed organic-inorganic hybrid

333(X =Cl,Br,I),have achieved high average power conversion efficiency (PCE)values of ~16%using a titania-based planar structure (1–7)or ~10to 13%in the phenyl-C 61-butyric acid methyl ester (PCBM)–based architecture (8–10).Such high PCE values have been attributed to strong light absorption and weakly bound excitons that easily dissociate into free carriers with large diffusion length (11–13).The average efficiency is typically lower by 4to 10%relative to the reported most efficient device,indicating persistent challenges of stability and re-producibility.Moreover,hysteresis during device operation,possibly due to defect-assisted trapping,has been identified as a critical roadblock to the commercial viability of perovskite photovoltaic technology (14–17).Therefore,recent efforts in the field have focused on improving film surface cov-

erage (18)by increasing the crystal size and im-the crystalline quality of the grains (19),is expected to reduce the overall bulk defect and mitigate hysteresis by suppressing trapping during solar cell operation.Al-approaches such as thermal annealing 20,21),varying of precursor concentrations and solvents (22),and using mixed solvents 23)have been investigated,control over structure,size,and degree of crystallinity remain key challenges for the realization of high-devices.

Here,we report a solution-based hot-casting to achieve ~18%solar cell efficiency on millimeter-scale crystalline grains,with small variability (~2%)in the overall PCE one solar cell to another.Figure 1A schemat-describes our hot-casting process for depo-of organometallic perovskite thin films.Our involves casting a hot (~70°C)mixture of iodide (PbI 2)and methylamine hydrochloride solution onto a substrate maintained at a of up to 180°C and subsequently spin-(15s)to obtain a uniform film (Fig.1A).In conventional scheme,the mixture of PbI 2and is spin-coated at room temperature and then for 20min on a hot plate main-at 100°C.Figure 1,B to D,illustrates the crystal grain structures using this hot-casting technique for various substrate tem-peratures and processing solvents.We chose a PbI 2/MACl molar ratio of 1:1in all experiments described in this Report to achieve the basic pe-rovskite composition and the best morphology (see fig.S1for images).We observed large,mil-limeter-scale crystalline grains with a unique leaf-like pattern radiating from the center of the grain [see scanning electron microscopy (SEM)image of microstructure in fig.S2].The x-ray dif-fraction (XRD)pattern shows sharp and strong perovskite (110)and (220)peaks for the hot-casted film,indicating a highly oriented crystal structure (fig.S3).The grain size increases markedly (Fig.1,B and D)as the substrate temperature increases from room temperature up to 190°C or when using solvents with a higher boiling point,such

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1

Materials Physics and Application Division,Los Alamos National Laboratory,Los Alamos,NM 87545,USA.2Physical Chemistry and Applied Spectroscopy Division,Los Alamos National Laboratory,Los Alamos,NM 87545,USA.3School of Electrical and Computer Engineering,Purdue University,West Lafayette,IN 47907,USA.4Theoretical Chemistry and Molecular Physics Division,Los Alamos National Laboratory,Los Alamos,NM 87545,USA.5Center for Nonlinear Studies,Los Alamos National Laboratory,Los Alamos,NM 87545,USA.6Materials Science and Engineering,Rutgers University,Piscataway,NJ 08854,USA.*These authors contributed equally

to this work.?These authors contributed equally to this work.?Corresponding author.E-mail:amohite@https://www.wendangku.net/doc/bd6378817.html, (A.D.M.);hwang@https://www.wendangku.net/doc/bd6378817.html, (H.-L.W.)

RESEARCH |REPORTS

DOI: 10.1126/science.aaa2725

, 519 (2015);

347 Science et al.

Dong Shi trihalide perovskite single crystals Low trap-state density and long carrier diffusion in organolead

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