Enhanced Charge Carrier Transport and Device Performance

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Enhanced Charge Carriers Transport and Device Performance Through Dual-cesium Doping in Mixed-cation Perovskite Solar Cells with Near Unity Free Carrier Ratio Tao Ye, Miloš Petrovi#, Shengjie Peng, Jeremy Lee Kong Yoong, Chellappan Vijila, and Seeram Ramakrishna ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12845 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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ACS Applied Materials & Interfaces

Enhanced Charge Carriers Transport and Device Performance Through Dual-cesium Doping in Mixed-cation Perovskite Solar Cells with Near Unity Free Carrier Ratio

Tao Ye†,‡, Miloš Petrović†,‡, Shengjie Peng†*, Jeremy Lee Kong Yoong†, Chellappan Vijila†,‡*, Seeram Ramakrishna†,‡*

†

Department

of

Mechanical

Engineering

and

Centre

of

Nanofibers

and

Nanotechnology (NUSCNN), National University of Singapore, Singapore 117576, Singapore ‡

Institute of Materials Research and Engineering (IMRE), Agency for Science,

Technology and Research (A*STAR), #08-03, 2 Fusionopolis Way, Innovis, 138634, Singapore. *Corresponding authors:

Emails: [email protected], [email protected], [email protected]

Keywords: perovskite solar cells; interface engineering; CsI doping; exciton binding energy; carrier mobility; small perturbation method.

1

ACS Paragon Plus Environment


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Abstract PbI2-enriched mixed perovskite film [FA0.81MA0.15Pb(I0.836Br0.15)3] has been widely studied due to its great potential in perovskite solar cell (PSC) applications. Herein, a FA0.81MA0.15Pb(I0.836Br0.15)3 film has been fabricated, with the temperature dependent optical absorption spectra utilized to determine its exciton binding energy. A ~13 meV exciton binding energy is estimated and a near unity fraction of free carriers out of the total photoexcitons has been obtained in the solar cell operating regime at equilibrium state. PSCs are fabricated with this mixed perovskite film, but a significant electron transport barrier at the TiO2/perovskite interface limited their performance. Cs2CO3 and CsI are then utilized as functional enhancers to substantially balance the electron and hole transport and increase the carriers (both electrons and holes) mobilities in PSCs, resulting in much improved solar cell performance. The modified PSCs exhibit reproducible power conversion efficiency (PCE) values with little hysteresis effect in the J-V curves, achieving PCEs up to 19.5% for the Cs2CO3 modified PSC, and 20.6% when subsequently further doped with CsI.

Introduction Lead halide perovskite APbB3 [A = Cs (cesium), MA (CH3NH3, methylammonium), or FA (NH=CHNH3, formamidinium); B = Cl, Br or I] solar cells have achieved a certified efficiency as high as 22.1% in recent years.1-10 The widespread interest in this type of solar cell originates from their novel optoelectronic properties, which include tunable optical properties, high absorption coefficients, and millimeter-scale 2


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charge carrier diffusion lengths. The highest published PCE on record was from a PbI2-riched mixed perovskite film [FA0.81MA0.15Pb(I0.836Br0.15)3] based PSC.9-12 Perovskites with mixed cations and halides are an emerging class of materials with the potential to address many of the limitations plaguing pure perovskites, such as MAPbI3, FAPbI3, and CsPbI3 in photovoltaic (PV) applications. The PSCs of MAPbI3 is not particularly competitive, and has not been reported to achieve efficiencies greater than 20%.2,5 Additionally, the low temperature structural phase transition at 55 °C,13 degradation upon contact with moisture, and the formation of light-induced trapped-states further decrease their potential in commercial applications.14 FAPbI3 has therefore been proposed as a replacement of MAPbI3 due to its broader optical absorption and potential to obtain higher PCEs.15 However, the FAPbI3 can crystallize into two different phases at room temperature: 1) hexagonal δ-phase, a photoinactive and non-perovskite structure; 2) α-phase, a photoactive,13, 16,17

but moisture sensitive8 perovskite. Inorganic cesium lead trihalide perovskites

have exhibited good thermal stability,18 however, the bandgap of CsPbBr3 may be unsuitable for single junction solar cell applications, while CsPbI3 crystallizes steadily into a photoactive α-phase only at temperatures above 300 °C.19,20 The structural and thermal instabilities of pure perovskites have so far been the most crucial obstacle toward achieving improved perovskite PV performance. In this respect, the mixing of cations and halides could address these shortcomings, and become a key route to improving the performance and unlock the potential of PSCs, as discussed in some recent results.9,10 3



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With the research deepening down to the fundamental level, the nature of elementary photoexcitons of the PV mechanisms has become a crucial issue. Determining the exciton binding energy in the perovskite under room temperature, and the free carrier fraction under typical solar cell operation provides mechanistic understanding into the nature of elementary photoexcitons.21,22 In particular, whether bound excitons are generated and transported to the heterojunctions (TiO2/perovskite and perovskite/spiro-OMeTAD) or whether photoinduced free carriers are generated at the same time within the bulk perovskite. A strong hysteresis effect was found in MAPbI3 PSCs, due to the strong free electron transport/injection barrier at the TiO2/MAPbI3 interface,23,24 in mixed PSCs with mesoporous TiO2 layer.25 To reduce the electron transport barrier, TiO2/graphene nanocomposite,26 CsBr27 and metal elements (aluminum28 and strontium29) doping were demonstrated to effectively improve the electron mobility and overall performance of MAPbI3 PSCs; Cs2CO3 is a frequently used interface modifier in MAPbI3 PSCs. For example, an exceeding 15% PCE was reported by using Cs2CO3 as an ITO modifier in a planar PSC;30 Cs2CO3 modified mesoporous TiO2 was reported and a nearly 20% PCE enhancement was obtained.31 As for the mixed perovskite PSCs, only one recent result shows the enhanced electronic properties of TiO2 electron transport layer (ETL) by lithium doping.25 Thus, refinement of these TiO2 ETL modification techniques could be an avenue to achieve yet more performance improvements in the mixed perovskite PSCs. Cs has attracted attention in mixing cations and halides perovskites recently and 4


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it possesses an 1.81 Å ionic radius, which is smaller than MA (2.70 Å) and FA (2.79 Å) cations.32 Cs can efficiently ‘‘pushing’’ FA crystallizes into the desirable photoactive phase due to the size difference of Cs and FA, resulting in defect-free and more pure perovskite films with improved structural and humidity stabilities.33-35 Apart from recent discussion of the perovskite structural and humidity stabilities improvement, from a view of the whole device, we try to find the evidence of the free carrier transport/injection enhancement within the PSCs after Cs doping. The two approaches discussed above, Cs2CO3 modified TiO2 layers and Cs doped mixed perovskites, have been shown to suppress the electron transport barrier and increase overall performance of PSCs. In this study, it is demonstrated that a small amount of Cs2CO3 and CsI is sufficient to balance electron and hole transport, increase charge carrier mobilities and further improve the PCEs. Employing a temperature dependent optical absorption spectra method to calculate the exciton binding energy of the FA0.81MA0.15Pb(I0.836Br0.15)3 film, the equilibrium fraction between free charges and total photoexcitons can then be derived from the Saha equation.36,37 A near unity fraction of free carriers out of total photoexcitons is estimated in the solar cell operating regime. Subsequently, PSCs with this FA0.81MA0.15Pb(I0.836Br0.15)3 film were fabricated and their carrier behaviors investigated with small perturbation transient photovoltage (TPV) and time-of-flight (TOF) photoconductivity measurements. Cs2CO3 and CsI have been utilized as a modification material to suppress the electron transport/injection barrier generation and enhance the carriers (both electrons and holes) mobilities. The modified PSCs 5



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with dual-cesium doping exhibit reproducible efficiency values with little hysteresis effect in the J-V curves and the highest overall PCE of 20.6% have been reached.

Experimental Section Device fabrication. Fluorine doped tin oxide (FTO) glass (NSG) was cleaned with detergent, DI water, acetone (Sigma), isopropanol (Sigma) and treated with UV ozone treatment at 100 °C for 10 min. A ~30 nm TiO2 layer was deposited on top of the FTO by spin-coating TiO2 precursor solution at 6000 rpm for 30 s. Then, it was heated at 450 °C for 30 min in air. To prepare the TiO2 precursor solution, titanium isopropoxide (1 mL, Sigma) and 12 M HCl solution (10 µL, Sigma) were diluted in ethanol (10 mL). Then a ~ 180 nm mesoporous TiO2 layer was deposited by spin-coating a 30-nm TiO2 nanoparticle paste (Dyesol) in ethanol (1:5 in weight ration) at a speed of 6000 rpm for 30 s. The substrate was then annealing at 500°C for 20 min. For the device with Cs2CO3 treatment, a 0.5% Cs2CO3 (Sigma) aqueous solution was spin coated on the prepared TiO2 layers at 3000 rpm for 30 s and dried at 100 °C for 5 min. The procedure can be repeated several times for a substrate. Next, perovskite film was deposited on the substrate by a spin-coating process in glovebox, with a precursor solution composition same as the one published in a recent study.9 The precursor solution was prepared by dissolving 0.265 g PbI2 (TCI), 0.037 g PbBr2 (TCI), 0.011 g MABr (TCI) and 0.094 g FAI (Dyesol) in 0.4 mL anhydrous N,N-dimethylformamide (Sigma) and 0.1 mL anhydrous dimethylsulfoxide (Sigma). First, 2000 rpm for 10 s; second, 6000 rpm for 30 s, then chlorobenzene (110 mL, 6


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Sigma) was dropped in 8-10 s during the second spin-coating process. The substrate was then heated at 100 °C, 60 min. As for the Cs doped mixed perovskites, corresponding amount of CsI (Sigma) was added to achieve the desired triple cation composition.

2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene

(spiro-OMeTAD) (Merck) was deposited by spin-coating at 3000 rpm for 30 s. The spiro-OMeTAD solution was prepared by dissolving 74 mg spiro-OMeTAD, 28.5 µL 4-tert-butylpyridine (Sigma), 17.5 µL of a stock solution of 520 mg/mL lithium bis (trifluoromethylsulphonyl) imide (Sigma) in acetonitrile (Sigma) and 29 µL of a stock solution of 100 mg/mL tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl) imide) (Dyenamo) in acetonitrile in 1 mL anhydrous chlorobenzene. Finally, 80 nm of gold was deposited as an electrode by thermal evaporation. Perovskite sample for optical characterization. The FA0.81MA0.15Pb(I0.836Br0.15)3 samples on quartz substrates were prepared with the same methods as perovskite layer deposition in solar cell fabrication. The MAPbI3 sample was made with a spin-coating procedure (4000 rpm for 30 s and 100 mL chlorobenzene was dropped in 3-5 s during the process). The precursor solution was prepared by dissolving 0.100 g MAI (Dyesol) and 0.289 g PbI2 (TCI) in 0.5 mL N,N-dimethylformamide. Characterization. J-V characteristics were measured in the glovebox with a solar simulator (SAN-EI Electric XES-301S 300W Xe Lamp JIS Class AAA) and a Keithley 2400 sourcemeter. The solar cells were masked with metal apertures to define the active areas which were typically 0.09 or 0.08 cm2. IPCE was recorded 7



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with a Keithley 2400 sourcemeter combined with an Oriel 300-W Xe lamp, an Oriel Cornerstone 130 monochromator and a SRS 810 lock-in amplifier (Stanford Research Systems). A calibrated Si diode was used as the reference. The surface and cross section morphologies of the samples were investigated by a SEM (JEOL JSM-7001F) at 10 kV. XRD experiments were conducted by a Bruker AXS (D8 ADVANCE) X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). The temperature dependent absorption spectra were measured on a PerkinElmer Lambda 950UV/VIS/NIR spectrometer with a liquid nitrogen cooled device detector. PL spectra were obtained from a triple-grating micro-Raman spectrometer (Horiba-JY T64000) setup. A 532 nm Nd:YAG pulsed laser (NT341A–10–AW, pulse duration shorter than 4ns and 1 Hz repetition frequency) was used as excitation light source in the small perturbation TPV experiments. Neutral density filters were used to afford a small perturbation (below 20 mV) of the cell photovoltage. A white light halogen lamp was employed to vary the back illumination level. The TPVs were monitored by a digital oscilloscope (Agilent 54845A). The TOF photoconductivity measurements were operated in an integrated mode with the same laser used for the TPV experiments and an external resistance of 5,000 Ω has been used in the study. The background voltages of the system were generated by a Keithley 2400 sourcemeter. The photocurrents at different background voltages were documented by the same oscilloscope.

Results and discussion Figure 1a shows the surface morphology of prepared FA0.81MA0.15Pb(I0.836Br0.15)3 film 8


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on clean quartz substrate obtained from scanning electron microscopy (SEM). The X-ray diffraction (XRD) spectrum of the thin film (Figure 1b) reveals it’s the crystal structure, with the small peak at 12.7o corresponding to the cubic PbI2 derived from the incomplete transformation of the FA compound into the photoactive phase.9,33,35,38,39 The steady state photoluminescence (PL) spectrum of this material has been documented (Figure 1c) was then obtained. It showed the perovskite film had a PL peak position at ~777 nm. To study the fraction of free charges out of the total number of photoexcitons generated, the temperature dependent optical absorption spectra40-41 at the bandedge was recorded from at intervals between 330 K to 77 K (Figure 2a). Between 720 nm and 800 nm, the spectra show the different gradients of the absorption bandedge, where the slope at the 150~77 K range is larger than that at the 330~150 K range, within the cooling period (the full absorption spectra are shown in Figure S1). This behavior can be attributed to a space group transition from a P3m1 trigonal space group to a P3 trigonal space group of the perovskite crystal, notably occurring at ~150 K in the absorption edge due to a broad array of distortions generated from the significant shifting of Pb off-centering. This space group transition temperature is congruent with other results reported in the literature.13,17 At room temperature, the perovskite film exhibited a bandgap of ~1.5 eV, with blue shifting of the absorption band edge as the temperature increases, corresponding to a positive thermal expansion coefficient of the bandgap (‘Varshni Trend’).40-42 Some recent studies show that the exciton binding energy of MAPbI3 was found 9



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within the 2~62 meV range through different measurements.43-45 Although the exciton binding energy of the same perovskite material obtained with different experimental methods is not identical, the exciton binding energies of different perovskites, which are obtained with same setup under identical conditions, can be utilized as an evaluation criterion of the free carrier generation capacity among these different perovskites.40 Exciton binding energies of 50 meV and 23 meV were estimated for the MAPbI3-xClx film and CH3NH3PbI3/carbon nanocomposite,40,41 respectively, by using the temperature dependent optical absorption of the excitonic transition. Generally, the elastic scattering events (with rate γ) and natural population decay ( t1 ) of the excited state would affect the coherence lifetime ( t 2 ) of the perovskite excited state transition and contribute to the broadening of the excitonic transition at different temperatures. The population decay rate 1/t1 can be described as the sum of the thermal dissociation rate ( kT ) and intrinsic decay rate ( k 0 ),46,47 1 = k 0 + kT t1

(1)

The exciton thermal dissociation rate can be defined by: −

kT = vT e

EB k BT

(2)

Where EB is the exciton binding energy, T is the absolute temperature (in Kelvin), k B is the Boltzmann constant and vT is the attempt frequency. Also, t1 and t2 can be defined as:46,47 1 1 = +γ t2 2t1

(3)

10


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Equation (3) can be expressed as (for the line width ∆v =

1 ) π t2

E

∆v = ∆v0 +

− B 1 vT e kBT 2π

(4)

is the temperature-independent broadening of line width. The binding energy value is a typical parameter reflecting the free carrier generation capacity of a system.36,46,47 By fitting the experimental data with equation (4), and using the first derivation of the optical absorption spectra, a ~13 meV exciton binding energy is obtained for FA0.81MA0.15Pb(I0.836Br0.15)3 (Figure 2c, detailed calculation is shown in Supplementary Methods). Similarly, the temperature dependent optical absorption spectra and their corresponding first derivatives at the bandedge for the MAPbI3 film are shown in Figure 2b and Figure 2d, giving an estimated exciton binding energy of ~43 meV, almost four times larger than that of FA0.81MA0.15Pb(I0.836Br0.15)3.

Thus,

the

free

carrier

generation

capacity

of

FA0.81MA0.15Pb(I0.836Br0.15)3 is much higher than that in MAPbI3 and MAPbI3-xClx. The binding energy of the FA0.81MA0.15Pb(I0.836Br0.15)3 and MAPbI3 films are then utilized in the Saha-Langmuir equation to determine the free carrier fractions over the total photoexcitons in them. The exciton binding energy is independent of total photoinduced excitations density in perovskites.48,49 The mass action law could be applied for Wannier-Mott excitons, which are described by Saha-Langmuir equation.36,37,46 The ratio of free charges over the total density of photoexcitons x, in bulk semiconductor can be expressed by the following equation:36,46 11



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‫ݔ‬2 1 2ߨ݉݇‫ ܶ ܤ‬3/2 − ‫ܤܧ‬ = ൬ ൰ ݁ ݇‫ܶ ܤ‬ 1−‫ܰ ݔ‬ ℎ2

(5)

N is the total density of excitations and N= n fc (density of free charges) + nexc (density of excitons), m is the reduced exciton mass (~0.15 me)40,41,46 and EB is the exciton binding energy. Figure 2e shows the fraction model of free photoinduced carriers in the FA0.81MA0.15Pb(I0.836Br0.15)3 over a wide range (1013-1022 cm-3) of total photoexciton densities. The free charge fraction is near unity throughout the temperature range (77~340 K) with reasonable excitation densities (1013~1015 cm-3) for solar cell operations. The fraction model of free photoinduced carriers in the pure MAPbI3 film with the same photoexciton densities range can be seen in Figure 2f. Under

same

photoexciton

density,

the

free

charge

fraction

of

FA0.81MA0.15Pb(I0.836Br0.15)3 is much larger than that of MAPbI3 at the same temperature especially in the low temperature range (77-170 K), showing that more free carriers are generated in the FA0.81MA0.15Pb(I0.836Br0.15)3. Carefully tuning the total photoexciton density of FA0.81MA0.15Pb(I0.836Br0.15)3 can be effective method to adjust the nature of the primary photoexciton, resulting in wide optoelectronic applications for this perovskite material. PSCs

were

fabricated

with

FA0.81MA0.15Pb(I0.836Br0.15)3

by

a

process

schematically illustrated in Figure S2. The front view and back views of the prepared devices are shown in Figure S3a and Figure S3b depicting the smooth and highly reflective

gold

electrodes

along

with

the

dense,

optically

opaque

FA0.81MA0.15Pb(I0.836Br0.15)3. As shown in Figure 3a, a standard device exhibits a short 12


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circuit current density (Jsc) of 22.2 mA/cm2, open-circuit voltage (Voc) of 1.13 V, fill factor (FF) of 0.63 and overall PCE of 16.1% during forward scan, whereas a Jsc of 22.2 mA/cm2, Voc of 1.13 V, FF of 0.68 and overall PCE of 17.1% is recorded during the backward scan. The slight hysteresis in the plotted J-V curves suggests that the electron transport/injection within the device may be suppressed. To study the free carrier behavior in the PSC, small perturbation TPV was utilized to observe the faster decay component occurrence within the system.6,50-52 The TPV spectra and carrier lifetimes (insert) are shown in Figure 3b. The TPVs show that for FA0.81MA0.15Pb(I0.836Br0.15)3 PSCs, carrier lifetimes ranged from 4 to 237 µs in a wide back illumination level from 1 V to 0.5 V, much longer than the MAPbI3 counterpart (3 to 92 µs with the back illumination level 0.7 V to 0.1 V),50,52 indicating that the TPV lifetime may benefit from substantially trap densities reducing in the mixed perovskite.6 Thus, the free carriers can be transported longer distances in the trap densities reduced FA0.81MA0.15Pb(I0.836Br0.15)3 film and result in enhanced PCEs in PSCs. Charge mobility in the whole device was further studied with TOF photoconductivity measurement at room temperature,53,54 obtaining the electron and hole transit times separately by changing the applied background voltages of the device. Specifically, the pulsed laser excitation generates excitons near the transparent electrode and the applied voltage across the device separates the excitons into free charge carriers. Depending on the applied voltage on the device, one of the charge carriers is removed at the transparent electrode and another type charge carrier drift 13



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across the photoactive layer and recombined at the opposite electrode. The drift mobility of the charge travelling across the film is directly measured by the oscilloscope. By changing the bias condition, the drift mobility of the opposite charge can be measured. Figure 3c shows the TOF transient times and mobilities within the device for holes obtained at different background voltages (the full TOF is given in Figure S4a). Transit times can be obtained from the peak position of the TOF transient spectra, with and the corresponding charge mobility (µ) calculated from the relation µ=d2/VtT,54 where d is the carrier transport distance (perovskite/HTM thickness 700

nm), V is the applied voltage, and tT is the transit time. The calculated hole mobility within the device is 2.2 × 10-3 cm2 V-1 s-1, an order magnitude higher than that in MAPbI3 PSCs (~10-4 cm2 V-1 s-1),50,56 at an external applied electric field of 7.70 × 103 V cm-1. The TOF transient times and mobilities for free electrons within the device at different background voltages are shown in Figure 3d (the full TOF spectra are given in Figure S4b). In this work, the electron mobility is calculated to be 1.4 × 10-3 cm2 V-1 s-1 at an applied electric field of 1.1 × 104 V cm-1 for a 700 nm thick TiO2/FA0.81MA0.15Pb(I0.836Br0.15)3 heterojunction in the device. The electron and hole mobilities of this PSC are both one order magnitude higher than PSCs fabricated from MAPbI3,56 however, the electron mobility in the device is smaller than the hole mobility, implying that an electron transport/injection barrier is generated at the TiO2/perovskite interface, which could negatively affect the performance of the device. This imbalance between electron and hole mobilities can be ameliorated by 14


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the introduction of a Cs2CO3 treated TiO2 ETL in order to further increase the efficiency of the PSCs.56 A diluted Cs2CO3 aqueous solution (0.5%) was used to modify the TiO2 ETL (compact and mesoporous layers). Figure S2 schematically illustrates the detailed fabrication process of a PSC with a Cs2CO3 treated TiO2. Four different TiO2 samples were prepared, with each undergoing a different number of Cs2CO3 treatment cycles denoted as Cs2CO3-0 (primary TiO2 without treatment); Cs2CO3-1(one cycle); Cs2CO3-2 (2 cycles) and Cs2CO3-3 (three cycles). The number of treatment cycles do not result in the formation of discrete layers as observed in Figure 4a which shows the cross-section SEM of the Cs2CO3-2 PSC and also in Figure S5 which shows no additional layers are visible in the samples. Optical absorption spectra of TiO2 samples with different times of Cs2CO3 deposition are shown in Figure 4. The modulation of the TiO2 layers by Cs2CO3 deposition results in a more efficient electron transport at the TiO2/perovskite interface by passivating the TiO2 surface traps (reducing of the charge recombination), as corroborated in a recent study.31 We also observed enhanced electron transport within the Cs2CO3 modified devices through TOF photoconductivity measurements (as will be discussed later). As depicted in Figure 4c, the champion Cs2CO3-2 devices exhibited a Jsc of 23.2 mA/cm2, Voc of 1.15 V, FF of 0.73 and overall PCE of 19.5%. This compared with the best performing control Cs2CO3-0 PSC which gave performance metrics Jsc of 22.3 mA/cm2, Voc of 1.13 V, FF of 0.68 and an overall PCE of 17.2%, represent a ~12% enhancement. To confirm this finding, incident photon to electron conversion 15



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efficiency (IPCE) spectra for the best performing devices were recorded under 1 sun (Figure 4d). There was an appreciable IPCE response enhancement at almost the wavelength range for Cs2CO3-2 PSC over the control. Furthermore, according to the PCE’s histogram in Figure S6, the devices show good reproducibility and unremarkable hysteresis. Comparing the performances of the PSCs with different number of Cs2CO3 treatment cycles, the main J-V parameters for the different devices are plotted with errors in Figure 5. It is apparent that the VOC, FF and PCE of Cs2CO3-2 PSCs were superior compared to the other devices. The TPV spectra and carrier lifetimes (insert) of the Cs2CO3-2 PSC are shown in Figure 6a. Carrier lifetimes of the device span from 102 to 338 µs, which is longer than the control group, in the 0.8 V to 0.5 V back voltage range, indicating that the free carrier lifetime may benefit from substantially reduced trap densities at the TiO2(Cs2CO3-2)/perovskite interface by Cs2CO3 doping of the TiO2 ETL.6 Thus, the performance of the device has been remarkably enhanced. Charge mobility in this Cs2CO3-2 device was also studied with TOF photoconductivity measurement. The TOF hole transient times and mobilities within the device at different background voltages are shown in Figure 6b (the TOF spectra are shown in Figure S7a). The hole mobility within the device was measured at 1.3 × 10-3 with an external electric field of 1.43 × 104 V cm-1, very close to the measurements from of the control group. Further, as shown in Figure 6c, the electron mobility within the device is 1.8 × 10-3 cm2 V-1 s-1 at 7.1 × 103 V cm-1 for the modified system, much higher than the control group (the TOF electron spectra are shown in Figure S7b). The free electron transport at the 16


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TiO2(Cs2CO3-2)/perovskite interface is significantly enhanced by the modification with Cs2CO3-2, enabling higher PCEs to be achieved, which is in good agreement with our J-V results. Subsequently, a further modification using CsI to partially replace MA/FA in the mixed perovskite was performed on the devices with the Cs2CO3-2 ETL. The detailed fabrication process of the PSCs can be found in the Method Section. For convenience, Cs-x represents the different types of mixed perovskites with a formula Csx(FA0.81MA0.15)100-xPb(I0.836Br0.15)3, where x is the percentage of CsI. The surface SEM images in Figure S8 show high crystallization and relatively larger crystal sizes for Cs-x (compared to perovskite in Figure 1a). Figure 7a plots the XRD patterns for the mixed perovskite and Cs-x with x =5, 10, 20%. A typical ~14° perovskite peak can be observed from all compositions, however, there was a disappearance of the 12.7° peak of cubic PbI2 upon CsI doping. The effective cation radius of the new three cations mixed perovskite was lowered as a result of the smaller Cs ionic radius. The tolerance factor of the material therefore favors the development of a photoactive phase that is stable at room temperature.32,35,38,39 However, it is noteworthy that at high Cs doping (where x approaches 20%), phase separation32 is observed to occur due to the large size difference between FA and Cs. Figure 7b shows the J-V curves and device statistics (insert table) for the four types of devices. The PV parameters were found to have improved from the Cs2CO3-2 group (abbreviated as Cs-2 but without CsI doping for the sample) to Cs-5 devices: Voc improved from 1.15 to 1.16 V, Jsc improved from 23.2 to 23.8 mA/cm2, FF 17



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improved from 0.73 to 0.75 and PCE improved from 19.5 to 20.6%. From the efficiencies histogram in Figure S9 and J-V parameters in table S1, the performance was found to have good reproducibility and insignificant J-V hysteresis. The long-term device stability of the Cs-5 device was further subject to investigation (Table S2), which found only a 1% decrease in the PCE over a 90-day period. Since Jsc and Voc did not decrease significantly, the PCE deterioration was largely be

attributed to the FF, which can be suppressed by using more suitable HTMs or device sealing techniques.58,59 The TPV spectra and free carrier lifetimes of the best performing device that have undergone both the Cs2CO3-2 and the subsequent CsI (Cs-5) modifications are shown in Figure 8a. Lifetimes spanning from 81 to 542 µs at a background voltage between 0.9 V and 0.6 V for this device, longer than that of the control and the Cs2CO3-2 PSCs, indicating that the trap densities may be significantly reduced in the tri-cations mixed perovskite material and the its interface by dual-cesium doping. Charge mobility in this Cs-5 device was also studied with TOF measurement. The TOF hole transient times and mobilities within the device are shown in Figure 8b (the TOF hole spectra are shown in Figure S8a). The hole mobilities within the device increases to 4.6 × 10-3 cm2 V-1 s-1 at an external electric field of 1.43 × 104 V cm-1, higher than both the control and Cs2CO3-2 PSCs. The corresponding TOF electrons transient times and mobilities within the device are shown in Figure 8c (the TOF electron spectra are shown in Figure S8b). The electron mobility is 4.2 × 10-3 cm2 V-1 s-1 at 1.43 × 104 V cm-1, 2~3 times that of the control and single-cesium doped 18


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(Cs2CO3-2) devices. The mobility of the free electrons is very close to the hole mobility in the dual-cesium doped PSCs; also the free carriers mobilities of the system outclass the control and Cs2CO3-2 PSCs. These results reveal that the dual-cesium doping is benefit for electron transport barrier suppression as well as carriers (both electrons and holes) mobilities enhancement so it is a possible method to push the PSCs to approach their theoretical limits for efficiency.

Conclusion In conclusion, the FA0.81MA0.15Pb(I0.836Br0.15)3 films have been synthesized via a typical

spin-coating

method.

The

exciton

binding

energy

of

FA0.81MA0.15Pb(I0.836Br0.15)3 has been calculated as ~13 meV. Free charges fraction is nearly unity through the whole temperature range with reasonable excitation densities for solar cell operations. Efficient solar cells based on this perovskite have been fabricated utilizing standard techniques. With the help of TPV and TOF methods, the detailed free carrier behaviors in the system are investigated and a strong electron transport barrier is observed at the TiO2/perovskite interface. Cs2CO3 and CsI doping have been introduced for the purpose of balancing the electron and hole transport as well as increasing the free carriers (both electrons and holes) mobilities of the PSCs. The fabricated devices with dual-cesium doping exhibit the highest overall PCE of 20.6% with little hysteresis effect in the J-V curve. Our work provides compelling evidence that optimizations of the total photoexciton densities at the thermodynamic equilibrium stage and device configurations by dual-cesium doping could lead to 19



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Page 20 of 38

solution-processed PSCs approaching their theoretical limits for efficiency.

Supporting Information: Optical photographs of prepared mixed perovskite film and PSC, the full absorption spectra of this perovskite, schematic of fabrication process of PSCs, linear plots of TOF results, SEM images of Cs2CO3 modified TiO2 surfaces, histogram of solar cell efficiencies, SEM images of CsI doped mixed perovskites, Photovoltaic parameters of Cs-5 devices, estimation of binding energy. This material is available free of charge via the website at http://pubs.acs.org.

Acknowledgements This

work

is

supported

by

the

Grantor

Lloyd's

Register

Foundation

(R-265-000-553-597) and T. Y. acknowledges the National University of Singapore for the research scholarship. The authors thank Dr. Xizu Wang, Dr. Changyun Jiang, Dr. Lim Siew Lay and Mr. Goh Wei Peng from IMRE and Mr. Xingzhi Wang from Nanyang Technological University for fruitful discussion and access to various equipment and resources.

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Figure 1. (a) SEM images of as prepared perovskite film, insert shows the image with high resolution. (b) XRD pattern of the prepared perovskite film. α and # denote the identified diffraction peaks corresponding to the photoactive perovskite and PbI2, 30


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respectively. (c) photoluminescence spectrum of the prepared perovskite film.

Figure 2. Optical absorption results. Temperature dependent UV-vis spectra of the (a) FA0.81MA0.15Pb(I0.836Br0.15)3 and (b) MAPbI3 samples. First derivative of the optical absorption spectra of the (c) FA0.81MA0.15Pb(I0.836Br0.15)3 and (d) MAPbI3 at bandedges. (e) Modeling of the fraction of free charges over the total photoexcitons 31



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with a ~13 meV exciton binding energy in FA0.81MA0.15Pb(I0.836Br0.15)3 film. (f) Modeling of the fraction of free charges over the total photoexcitons with a ~43 meV exciton binding energy in MAPbI3 film. The shaded area represents PV operating conditions. For better comparison, UV-vis spectra are offset: for (a), the spectrum of 330 K is fixed and other spectra are offset; for (b), the 77 K spectrum is fixed and other spectra are offset.

Figure 3. (a) Standard current–voltage characteristics of the PSC measured under dark and simulated 1 sun condition with forward (FW) and backward (BW) scans. (b) Normalized small perturbation TPVs of the PSC at different voltages. The insert is the carrier lifetimes extracted from the TPV spectra. (c) Plots of TOF hole transient time and mobilities measured for the same device at different voltages. (d) TOF transient 32


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time and mobilities of free electron at different voltages.

Figure 4. (a) Cross-section SEM image of the PSC based on Cs2CO3-2 (Cs-2). (b) Optical absorption spectra of the TiO2 samples with different number of Cs2CO3 treatment cycles, the experiment was operated with 12 samples (three samples for each condition). (c) Current–voltage characteristics of the champion PSCs based on Cs2CO3-0 (control) and Cs2CO3-2 measured under simulated 1 sun condition. (d) The corresponding IPCE spectrum of the PSCs.

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Figure 5. a) Jsc; b) Voc; c) FF and d) PCE for PSCs based on different times of Cs2CO3 treatment. 40 devices have been made for each group.

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Figure 6. TPV and TOF results of the PSC based on Cs2CO3-2. (a) Normalized small perturbation TPVs of the device at different voltages. The insert is the carrier lifetimes extracted from the TPV spectra. (b) Plots of TOF hole transient time and mobilities measured for the same device at different voltages. (c) TOF transient time and mobilities of free electron at different voltages.

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Figure 7. (a) XRD diffraction results of the perovskite layers with different amount of CsI doping. α, #, δ and ν denote the identified diffraction peaks corresponding to the photoactive perovskite, PbI2, photoinactive (non-perovskite) polymorphs and CsPbI3, respectively. (b) Current–voltage characteristics of the champion PSCs based on Cs2CO3-2 (Cs-2), Cs-5, Cs-10 and Cs-20 samples measured under simulated 1 sun condition. Insert table shows the main J-V parameters extracted from the J-V curves.

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Figure 8. TPV and TOF results of the PSC based on Cs-5 perovskites. (a) Normalized small perturbation TPVs of the device at different voltages. The insert is the carrier lifetimes extracted from the TPV spectra. (b) Plots of TOF hole transient time and mobilities measured for the same device at different voltages. (c) TOF transient time and mobilities of free electron at different voltages.

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Table of Contents Graphic

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Enhanced Charge Carrier Transport and Device Performance

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