Photocarrier Recombination and Injection Dynamics in Long-Term

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Photocarrier Recombination and Injection Dynamics in Long-Term Stable Lead-Free CH3NH3SnI3 Perovskite Thin Films and Solar Cells Taketo Handa,+ Takumi Yamada,+ Hirofumi Kubota,‡ Shogo Ise,‡ Yoshihiro Miyamoto,‡ and Yoshihiko Kanemitsu*,+ +

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Chemical Materials Evaluation and Research Base (CEREBA), Tsukuba, Ibaraki 305-8565, Japan

‡

S Supporting Information *

ABSTRACT: We investigated the near-band-edge optical responses and photocarrier dynamics of encapsulated long-term stable CH3NH3SnI3 (MASnI3) thin films and solar-cell devices. The MASnI3 thin film prepared with SnF2 exhibited a bandgap of 1.25 eV, while the film without SnF2 had a significantly blueshifted absorption edge. On the contrary, the PL peak energies were not influenced by the addition of SnF2. These observations indicate that the blueshift of the absorption edge in the SnF2-free MASnI3 sample is due to the Burstein−Moss shift induced by a significant unintentional hole doping. Furthermore, timeresolved photoluminescence measurements revealed that by adding SnF2 the photocarrier lifetime of the film increased by one order of magnitude, which enables improved device performance of solar cells. We clarified that in the MASnI3 solar cells the short-circuit current stays significantly below the ideal value due to a large nonradiative recombination rate in the perovskite layer, resulting in a small photocarrier-injection efficiency into the charge-transport layers.

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performance of lead-free tin halide perovskite devices.22−27 Interestingly, many publications have demonstrated that the performance of tin halide perovskite solar cells improves significantly by adding 10−20 mol % SnF2 during the fabrication.24−27,29 Recent works revealed that the use of SnF2 as an additive enhances the device performance by improving the film morphology,25,27 as well as reducing Sn vacancies and suppressing the oxidation of Sn2+ to Sn4+.24−27 Such structural information is useful for developing advanced growth techniques. Furthermore, the optical properties of the semiconductor absorber layer play a crucial role for the solarcell performance. However, detailed studies about the optical properties of MASnI3 films with and without SnF2 additive are rare and the impact of the SnF2 additive on the near-band-edge optical responses of MASnI3 still remains unclear. Thus, to further improve the conversion efficiencies of MASnI3 solar cells, it is required to deeply understand the photocarrier behaviors in thin films and devices, and to clarify the limiting physical mechanism of the conversion efficiency. Since the luminescence efficiency is an indicator of the material quality of a semiconductor and is directly related to the solar-cell conversion efficiency, photoluminescence (PL) spectroscopy is one of the most powerful methods for understanding photocarrier dynamics of metal halide thin films and solar-cell devices.30

INTRODUCTION Recently, metal halide perovskites, which are known for their excellent optical and electrical properties, have attracted considerable attention as novel optoelectronic materials.1−8 Since the first report of perovskite solar cells in 2009, lead halide perovskites (APbX3, A = CH3NH3, HC(NH2)2, Cs; and X = I, Br) have attracted much attention for photovoltaic applications.1−6 Their important material properties are manifold: large absorption coefficients enabled by direct optical transitions,9−11 small exciton binding energies,12,13 existence of free carriers,14 and consequently long carrier diffusion lengths,15−17 high luminescence quantum yields,18,19 and photon recycling16,19 have been revealed. These unique characteristics are the reasons why conversion efficiencies exceeding 20% can be achieved with lead halide perovskite solar cells.6 Although lead halide perovskites provide many superior characteristics, the toxicity of lead is problematic for the practical implementation. Thus, the fabrication of lead-free perovskite materials and devices has been conducted very actively.20−28 In particular, the tin halide MASnI3 (MA = CH3NH3) solar cells have been intensively investigated since the beginning of the search for lead-free alternatives.20−23 It has been reported that MASnI3 has a bandgap around 1.2 to 1.3 eV,20−23 which is smaller than that of MAPbI3 (1.6 eV),9,11 and therefore, a large short-circuit current (JSC) is expected for MASnI3 solar cells. However, the so far reported conversion efficiencies of MASnI3 solar cells are limited to around 5−6%, and consequently tremendous efforts have recently been made to improve the © 2017 American Chemical Society

Received: May 17, 2017 Revised: July 4, 2017 Published: July 5, 2017 16158

DOI: 10.1021/acs.jpcc.7b06199 J. Phys. Chem. C 2017, 121, 16158−16165

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The Journal of Physical Chemistry C

sectional SEM image for a representative solar-cell device, and the thickness of each layer was determined as follows: c-TiO2 ≈ 20 nm, mp-TiO2 ≈ 300 nm, MASnI3 ≈ 350 nm, PTAA ≈ 25 nm, and Au ≈ 100 nm. The active area of the cell was 0.49 cm2 (Figure S2 in the Supporting Information). The scan rate for measuring the current−voltage curves was 0.05 V/s. For accurate comparison of the optical properties, we studied MASnI3 thin films and devices fabricated from the same batched samples. Steady-State PL and Absorption Measurements. For the excitation of the samples, we used a picosecond pulsed laser with wavelength of 650 ± 10 nm and a repetition rate of 5 MHz, obtained from a supercontinuum light source (Fianium) in combination with a wavelength-tunable bandpass filter (Fianium). The laser was incident from the transparent glass side, and the diameter of the excitation spot was determined to be about 0.1 mm for all PL measurements. Long pass filters were used in the detection path to filter the scattered excitation light, and only the PL emission from the MASnI3 layer was detected using a liquid-nitrogen-cooled CCD detector with a monochromator (Roper Scientific). The steady-state absorption spectra were measured using a UV−vis−NIR spectrophotometer and an integrating sphere system (JASCO). For the resonantly excited PL measurements, the excitation light was obtained from a monochromator (JASCO) and the white light generated by a supercontinuum light source (Fianium, repetition rate 40 MHz). The power of the excitation light was set to 4.8 μW. By using crossed polarizers in the excitation and detection path, the scattered excitation light was suppressed by several orders of magnitude. The PL emission was detected by a liquid-nitrogen-cooled InGaAs detector with a monochromator (Roper Scientific). Time-Resolved PL Measurements. The samples were excited by a femtosecond pulsed laser at a wavelength of 650 nm using a wavelength-tunable Yb:KGW-based femtosecond laser system with an optical parametric amplifier (Light Conversion; pulse duration, 300 fs; repetition rate, 200 kHz). The scattered excitation light was suppressed by using appropriate long pass filters, and only the PL emission from the samples was recorded using a monochromator and a streak camera (Hamamatsu); the time resolution of the system was about 50 ps. Before we record the PL decay curves, the samples were light soaked for 1 h with an excitation fluence of 100 nJ/ cm2. The PL decay curves were obtained by spectrally integrating from 1.24 to 1.32 eV.

In this work, we fabricated encapsulated long-term stable MASnI3 thin films and solar-cell devices and studied their PL spectra and dynamics in order to reveal the photocarrier recombination and injection dynamics in the device. While the MASnI3 thin film prepared with the SnF2 additive showed a steep absorption onset at 1.25 eV, a significant blueshift of the absorption edge appears in the thin film prepared without SnF2. Independent of the presence of the additive, the PL peak energy exhibited almost no change. These results indicate that the sample without the additive experiences a blueshift of the absorption edge due to the Burstein−Moss effect from unintentionally doped holes. Furthermore, the resonantly excited PL spectra evidenced that the PL emission width is independent of the excitation energy, suggesting that the broad PL spectrum from MASnI3 is determined solely by the intrinsic broadening due to strong electron−phonon interactions. By performing time-resolved PL measurements, it became clear that by adding SnF2 during fabrication, the photocarrier lifetime in the thin film increases by an order of magnitude. This longer carrier lifetime is the reason for the improved performance of devices prepared with SnF2. Additionally, through analyzing the carrier lifetimes in the thin film and the device, we clarified the origin of the strong deviation of JSC from the ideal value in the MASnI3 solar cell: the nonradiative recombination rates of photocarriers in the perovskite layer is very high, which impedes the photocarrier injection into the charge transport layer.

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METHODS Thin-Film Samples. We prepared MASnI3 thin films with and without SnF2 additive at 20 mol % in a nitrogen-filled glovebox. Methylammonium iodide (MAI) and SnI2 were dissolved in dimethyl sulfoxide (DMSO) with stoichiometric ratio. A 20 mol % sample of SnF2 was added when necessary. The concentration of MASnI3 was 39 wt % for the samples with and without SnF2. The precursor solution was spin-coated onto cleaned glass substrates at 3000 rpm for 60 s. During the spinning, toluene was dropped on the samples, which were subsequently annealed at 65 °C for 1 h. The prepared samples were encapsulated in the nitrogen-filled glovebox, and the thickness of the MASnI3 film was determined to be ≈450 nm using a cross-sectional scanning electron microscope (SEM) image. Solar Cell Devices. A 20 nm thick layer of compact TiO2 (c-TiO2) was coated onto a fluorine doped SnO2 (FTO; Asahi Glass) substrate by plasma enhanced atomic layer deposition (the source material was titanium(IV) isopropoxide). Then a mesoporous TiO2 (mp-TiO2) layer was deposited onto the cTiO2/FTO substrate by spin-coating a TiO2 paste (Nikki Syokubai Kasesi, PST-18NR) diluted with ethanol (1:3 by weight). The substrates were calcined at 500 °C for 1 h and afterward transferred to a glovebox filled with nitrogen. The perovskite layer was deposited on a mp-TiO2/c-TiO2/FTO substrate by the above-mentioned procedure. Then, a PTAA/ toluene (15 mg/1 mL) solution with 7.5 μL of Li− bis(trifluoromethanesulfonyl) imide (Li−TFSI)/acetonitrile (170 mg/1 mL) and 7.5 μL of tert-butylpyridine (t-BP) (1 mL t-BP/1 mL acetonitrile) was spin-coated on each MASnI3/ mp-TiO2/c-TiO2/FTO substrate at 3000 rpm for 60 s, and afterward, the samples were annealed at 65 °C for 5 min. Subsequently, gold was thermally evaporated, and the prepared devices were encapsulated in the nitrogen-filled glovebox. Figure S1 in the Supporting Information shows the cross-

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RESULTS AND DISCUSSION Figure 1a shows the current−voltage characteristics of the solar cell based on the MASnI3 absorber layer prepared with SnF2, and this device was used for optical measurements. The blue data was measured 20 days after preparation. The extracted photovoltaic parameters were JSC = 26.1 mA/cm2, VOC = 0.25 V, and FF = 0.30, and the conversion efficiency was 1.94%. Since the device was encapsulated and stored in the dark, there was almost no drop in the conversion efficiency during the following 146 days (Figure 1a, red data, showing JSC = 25.0 mA/cm2, VOC = 0.25 V, FF = 0.31, and conversion efficiency = 1.93%). This long-term stability is similar to the previously reported shelf stability of FASnI3 [FA = HC(NH2)2] perovskite solar cells.26,27 Furthermore, our device showed a long-term stability even under continuous 1-sun irradiation in air. We performed a stability test for another encapsulated MASnI3 solar cell. Between each current−voltage measurement, the 16159

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Figure 1. (a) Current−voltage curves of a MASnI3 solar cell (prepared with SnF2) recorded under AM 1.5G 1-sun irradiation before aging (blue curve, sample age 20 days) and after 5 months kept in dark (red curve, sample age 166 days). (b) Time evolution of the power conversion efficiency of an encapsulated MASnI3 solar cell (with SnF2). The stability test was performed under continuous 1-sun irradiation and open-circuit condition in ambient air.

device was kept under continuous 1-sun irradiation in air at room temperature and open-circuit condition. Having the stability over 500 h as shown in Figure 1b, the encapsulated MASnI3 device demonstrates its potential for practical implementation. The long-term stability of our devices seen in Figure 1 is important for the reproducibility of the optical data. On the other hand, the device based on the MASnI3 absorber layer without SnF2 additive was not able to generate electrical power, since under illumination a rather metallic behavior was observed, instead of the required diode characteristic (Figure S3 in the Supporting Information). The MASnI3 solar-cell device (with SnF2) showed a JSC of 26.1 mA/cm2 (Figure 1a). This exceeds the ≈24 mA/cm2 of the MAPbI3 solar cell,31 and evidence that the MASnI3 has an absorption edge at a longer wavelength, resulting in a larger current generation. Note that this JSC value is higher than those reported for MASnI 3 solar cells.20−23 However, when considering the reported bandgap Eg of MASnI3,20−23 the theoretical limit would be about 40 mA/cm2 (for the case Eg = 1.2 eV),32 and that implies the existence of an extremely large current loss. To clarify the details of the current loss mechanism and the influence of the SnF2 addition on the optoelectronic properties, we studied optical properties of MASnI3 thin films prepared with and without SnF2. Figure 2a shows the steady-state PL spectra for the MASnI3 thin film with (red curve) and without SnF2 (blue curve), which were obtained after a laser illumination time of 120 s (see Figure S4 in the Supporting Information). The inset shows the normalized spectra. The excitation wavelength and power density were 650 nm and 10 nJ/cm2, respectively (the detailed experimental setup is explained in Methods). The PL spectrum of the MASnI3 thin film with (without) SnF2 additive exhibited a peak at 1.26 eV (1.27 eV), and had a full-width half-maximum of about 90 meV (100 meV). Note that the PL of the film without SnF2 additive exhibited a tail in the high-energy region, whose origin is explained later. The PL intensity of the thin film with SnF2 is about 5 times larger than that for the film without SnF2 (Figure 2a), which indicates a reduction of the nonradiative recombination centers in the perovskite layer due to the SnF2 additive. Figure 2b shows the absorption spectra of the MASnI3 thin films prepared with (red curve) and without SnF2 (blue curve). The absorption spectrum of the MASnI3 film with SnF2 shows a steep increase at 1.25 eV, which is in good agreement with the peak energy of the PL spectrum. On the other hand, the film without SnF2 shows an onset at around 1.3 eV and the slope is

Figure 2. (a) PL spectra of the MASnI3 thin films prepared with SnF2 (red) and without SnF2 (blue). The inset shows the PL spectra normalized at the peak intensity. (b) Absorption spectra for the thin films with SnF2 (red) and without SnF2 (blue). (c) PL spectra of the film with SnF2 obtained for different excitation energies. The excitation photon energy Eex is shown in the figure. The scattered excitation light is visible as sharp diverging peaks in the figure.

also less steep. This means that the optical absorption edge of the film without SnF2 is blueshifted. The steep absorption onset of the thin film with SnF2 (Figure 2b, red) indicates small densities of localized states within the bandgap,10,11 promising higher luminescence efficiency. The PL spectra under resonant excitation (i.e., excitation at energies within the PL band) of the thin film with SnF2 additive are shown in Figure 2c. The resonantly excited PL can provide us with important information about the luminescence origin.33,34 In Figure S5 in the Supporting Information, the normalized PL spectra are also shown, and we can clearly confirm that the PL width is independent of the excitation energy. This data suggests that the observed broad spectrum in the sample with SnF2 is not a result of extrinsic effects like impurities and defects, but most likely the intrinsic width determined by the homogeneous broadening due to strong electron−phonon coupling.35−37 Additionally, even for excitation energies below the peak energy, a strong anti-Stokes PL was observed (Figure 2c, red dots). The same characteristics have been also observed for MAPbI3,35,36 which indicates that the luminescence properties of MASnI3 are similar to those of MAPbI3. We found that the SnF2 addition causes (i) the redshift of the absorption edge, (ii) no significant change of the PL peak, and (iii) the reduction of the high-energy tail of the PL spectrum. These experimental results can be explained with the Burstein− Moss shift due to an abundant number of doped holes in the sample without SnF2 additive. It has been reported that the tin halide perovskites, MASnI3, FASnI3, and CsSnI3, are unintentionally hole-doped to a high degree (therefore they are p-type semiconductors).21,24−27,38,39 This is triggered for example due to the formation of Sn vacancies, and also the oxidation of Sn2+, forming Sn4+.24−26,38,39 The above-mentioned metallic behavior of the MASnI3 solar cell based on the thin film without the additive (Figure S3 in the Supporting Information) supports the existence of unintentionally high-doped carriers. Previous work demonstrated that the addition of SnF2 reduces the hole 16160

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The Journal of Physical Chemistry C density.24,27 In a same manner, it is considered that the doped hole density in the sample prepared with SnF2 (Figure 2a,b, red data) is lower than that prepared without SnF2 (Figure 2a,b, blue data). In heavily doped semiconductors, a large shift of the absorption edge to higher energies is observed with an increase of the doping density,40−44 which is known as the Burstein− Moss shift.40,41 In our MASnI3 thin film without SnF2, the states in the vicinity of the valence band maximum are occupied with doped holes, and thus only the higher optical transitions are observed in the absorption spectrum (Figure 2b, blue data). Meanwhile, since PL emission occurs due to carrier recombination at the band edges, a change in the PL peak energy is not observed, and the excess holes result in a tail of the PL toward higher energies (Figure 2a). Furthermore, it can be considered that the variations in the absorption edges of MASnI3 reported in the literature20−23 are the result of the Burstein−Moss shift of thin films with different doping densities. Next, we discuss the photocurrent loss in the MASnI3 solar cell. From the absorption spectrum of the MASnI3 thin film with SnF2 additive (Figure 2b, red curve), we determined the absorption edge to be at 1.25 eV. When we consider the AM 1.5G illumination (NREL, ASTM G173−03), and assume that all photons with energy larger than 1.25 eV are absorbed, the upper limit of JSC is 39.0 mA/cm2. The JSC with 26.1 mA/cm2 of our device corresponds to 67% of the ideal value, meaning that there is a large loss in the current extraction process. We note that, solar cells based on MAPbI3 or other inorganic semiconductors usually have a small current loss; i.e., their JSC values are close to the ideal theoretical limit determined by the bandgap of the absorber.45 For example, the highest reported JSC in MAPbI3 solar cells is about 24 mA/cm2.31 This value is very close to the theoretical limit of 26.5 mA/cm2, which is determined only from the bandgap (1.61 eV)9,11 and all other optical or electrical losses are neglected. In addition, the VOC of the MASnI3 solar cell is also small. The small JSC and VOC indicate that many energy loss mechanisms exist in MASnI3 devices. We utilize time-resolved PL measurements to discuss the details of the underlying current extraction loss mechanism. Parts a−c of Figures 3 show the excitation−fluence dependence of the PL decay curves for the thin film without SnF2 additive, with additive, and the solar cell (with additive), respectively. The PL of the solar cell was measured under opencircuit condition, i.e., without use of contacts. The excitation wavelength for all three samples was 650 nm. Note that for each sample in Figure 3, the scale of the x-axis is different. When we consider an absorption coefficient of ≈3.3 × 104 cm−1 for MASnI3 at 650 nm (the layer thickness is about 400 nm),21 we can estimate that the excitation fluence of 1000 nJ/ cm2 corresponds to an initial photocarrier density of 1.1 × 1017 cm−3. The thin film with SnF2 additive (Figure 3b) exhibited a slow and nearly single-exponential PL decay, while the other two samples showed fast nonexponential decays. The nonexponential profile of the film without SnF2 (Figure 3a) can be explained with a complicated recombination process as a result of the large number of doped holes and/or nonradiative recombination centers.46,47 With regard to the nonexponential decay in the device (Figure 3c), qualitatively the same trend has been also reported for the MAPbI3 solar cell, and this has been assigned due to the internal electric field or the injection into the transport layer.48,49

Figure 3. Excitation−fluence dependence of PL decay curves for the MASnI3 thin film (a) without and (b) with SnF2 and (c) the solar-cell device. The broken curves represent the fitting results. Excitationfluence dependence of the initial PL intensity for the thin film (d) without and (e) with SnF2 and (f) the solar-cell device. The broken lines represent the linear dependence of the excitation fluence.

Parts d−f of Figure 3 show the excitation−fluence dependence of the PL intensities just after the excitation (t = 0 ns). For the thin film samples (Figure 3d,e), the initial PL intensity has a linear dependence upon the excitation fluence. This result can be understood with the following consideration. When we denote the density of the unintentionally doped holes with Ndope, and the photogenerated electrons and holes with n and p, respectively, the internal luminescence intensity of the sample (IPL) can be written as14 IPL = Bn(p + Ndope) ≈ BnNdope

(1)

Here, B stands for the bimolecular recombination rate for electrons and holes, which we consider being the radiative process. Since the number of photogenerated electrons and holes at t = 0 nm is equal (n0 = p0), the initial PL intensity can be written as I0 = Bn0(n0 + Ndope). As indicated with the approximate symbol in eq 1, in case that the density of doped holes Ndope is much larger than the photogenerated electron and hole densities, the initial PL intensity shows a linear dependence upon the excitation fluence. The observed linear dependence can be explained with the radiative recombination of photogenerated electrons with doped holes. Previous works have reported that in MAPbI3 the radiative recombination process changes from monomolecular (linear dependence) to bimolecular (quadratic dependence) for increasing excitation power.14,50 From the crossing point of the mono- and bimolecular regions, the doped carrier density Ndope can be obtained.14 By performing the same analysis for our MASnI3 thin film prepared with 20 mol % SnF2 (Figure 3e), we can estimate the density of unintentionally doped holes. In the sample with SnF2, the doped hole density should be reduced, but we find that within the experimental excitationfluence range the bimolecular recombination of photogenerated electrons with photogenerated holes is minor. This means that the density of doped holes is at least 5 × 1017 cm−3 for the film prepared with SnF2. It has also been reported that for FASnI3 and CsSnI3 thin films prepared with SnF2 at 20 mol %, the doped hole density is on the order of 1017−1018cm−3.24,27,51 When we consider that the SnF2 addition reduces the doped hole density by about 1−2 orders,24,27 the doped hole density 16161

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The Journal of Physical Chemistry C in the MASnI3 thin film without SnF2 additive (Figure 3d) is at least 1019 cm−3. This large hole density in the thin film without SnF2 might cause a large Burstein−Moss shift of the absorption edge. Figure 3f shows the excitation-fluence dependence of the initial PL intensity for the solar-cell device prepared using the SnF2 additive. In contrary to the thin film sample with SnF2, above an excitation power of 1000 nJ/cm2 (corresponding to an initially photogenerated carrier density of 1.1 × 1017 cm−3) a trend deviating from the linear law toward higher exponents is observed, suggesting a dominant bimolecular recombination at higher excitation powers. This power dependence may have several reasons, which we explain in the following, but for a deep understanding further studies are required. One possibility we consider is that, due to the contact between the hole transport and perovskite layers, the unintentionally doped holes diffuse into the transport layer. As a consequence, the density of unintentionally doped holes in the MASnI3 is reduced. Figure 4 summarizes the excitation−fluence dependence of the average PL lifetimes for each sample, extracted from the PL

enables a higher probability for the successful photocarrier injection into the transport layers before recombination within the absorber layer occurs, and thus contributes to an improved device performance. Figure 4 also shows that under high excitation fluences the PL lifetime of the thin film with SnF2 tends to becomes slightly shorter. This suggests that the contribution of bimolecular recombination starts to become visible, but the decrease in lifetime is not as significant as that observed for MAPbI3.14,50 The PL lifetime obtained from the MASnI3 layer in the solar cell prepared with SnF2 (Figure 4, blue data), is about 1.4 ns at low excitation fluences, and thus faster than that of the corresponding thin film sample with SnF2 (Figure 4, red data). The faster time constant indicates a carrier injection from the perovskite layer into the charge transport layers.15,48 Note that the PL lifetime of the MASnI3 thin film is much shorter than that of MAPbI3 thin films under similar excitation conditions,14,48,50 which means that the bulk recombination rate of MASnI3 is much higher than that of MAPbI3. Consequently, the bulk recombination rate in MASnI3 can easily exceed the interfacial recombination rate (which is different from the case for MAPbI3 solar cells53) and therefore, we consider that the dominant loss process for photogenerated carriers is the bulk recombination. If we assume that the contribution of interfacial recombination is negligibly small, we can calculate the effective time for carrier injection (τinj) as follows: 1/τinj = 1/τdevice − 1/ τfilm, where τdevice and τfilm represent PL lifetimes of the device and film, respectively. Using this relation, we obtain τinj∼ 1.8 ns for low excitation fluences from Figure 4. Unfortunately, the separation into electron and hole injection time is difficult, but still very important conclusions can be drawn from this value. In the low excitation regime, the bimolecular recombination is minor, and thus the carrier-injection efficiency can be calculated using the injection and recombination time constants as the following:48

Figure 4. Excitation−fluence dependence of the PL lifetimes for the MASnI3 thin films (with and without SnF2) and the device extracted from Figure 3a−c. The dotted lines are guides for the eye.

decay curves in Figure 3. The average lifetime τPL was obtained from fitting with a stretched exponential function to take into account for the nonexponential nature of the PL decays:52 ⎡ ⎛ t ⎞β⎤ IPL(t ) = I0 exp⎢ −⎜ ⎟ ⎥ ⎣ ⎝τ⎠ ⎦ τPL =

τ ⎛1⎞ Γ⎜ ⎟ β ⎝β⎠

ηinj =

1/τinj 1/τfilm + 1/τinj

(3)

By using τfilm = 6.9 ns and τinj = 1.8 ns, we can determine the injection efficiency of ηinj = 79% for our MASnI3 solar cell. When we assume that the optical losses (reflection at the glass surface and scattering in the substrate) are 10%, and consider a bandgap of Eg = 1.25 eV, the carrier-injection efficiency permits to estimate a short-circuit current of JSC = 39.0 mA/cm2 × 0.9 × 0.79 = 27.7 mA/cm2. Our sample provided a JSC of 26.1 mA/cm2 (Figure 1a), which means that the value obtained from the PL measurement is in good agreement with the electrical measurement. This indicates that the main reason for the fast PL decay observed in the solar-cell device is the photocarrier injection into the transport layer, while traps or other recombination paths at the heterointerface plays a minor role. Furthermore, this result clarified that the large current loss in the present MASnI3 solar cell can be assigned due to the carrier recombination within the perovskite layer, which reduces the injection efficiency. We note that, in contrast, the difference between the PL lifetimes of the lead halide MAPbI3 thin film and solar cell under similar low excitation fluences is significantly larger.48 The corresponding calculated carrier-injection efficiency for the MAPbI3 is nearly 100%, which explains the experimentally observed high conversion efficiency of the MAPbI3 solar cells.48 To further enhance the present injection efficiency of 79% for the MASnI3

(2a)

(2b)

Here, Γ(x) is the Gamma function. The variable I0 is the initial PL intensity and β is a fitting parameter which expresses the deviation from the monoexponential decay. The fitting results are shown in Figure 3a−c with the broken curves. In Figure 4, it can be seen that under low excitation fluences the MASnI3 thin film without SnF2 (green circles) exhibits an extremely fast decay with a PL lifetime of 0.4 ns. This fast decay is considered to be a result of nonradiative recombination centers formed by Sn vacancies or also the tail states due to the existence of Sn 4+ .24,25 In addition, the high radiative recombination probability between photogenerated electrons and the large number of doped holes would also contribute to the fast PL decay.21 By adding SnF2, the formation of the vacancies or Sn4+ and the doping concentration are suppressed, and thus the film with the additive (Figure 4, red circles) has a PL lifetime of 6.9 ns at low excitation fluences, i.e., an improvement of more than one order. The longer lifetime 16162

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solar cell, the ratio between the nonradiative recombination and the carrier-injection time constants has to be increased (see eq 3). In other words, by either extending the carrier recombination time within the absorber layer, or shortening the carrier-injection time, the conversion efficiency should be improved. Finally, we would like to comment on the slower PL lifetime that is observed for higher excitation fluences in the MASnI3 solar-cell device (Figure 4, blue circles). Similar behavior has also been observed for the MAPbI3 solar cell and was explained with a suppressed carrier-injection rate in case of high photocarrier densities.48 However, the excitation-fluence dependence of the PL lifetimes in the MASnI3 device is much weaker compared to that in high-efficiency MAPbI3 devices and GaAs solar cells.48,54 This result also shows that the MASnI3 solar cell performance is determined by the nonradiative recombination in the MASnI3 layer.

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CONCLUSIONS

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ASSOCIATED CONTENT

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work at Kyoto was supported by JST-CREST (Grant No. JPMJCR16N3) and JSPS (17J09650). Part of this work at Tsukuba was supported by New Energy and Industrial Technology Development Organization (NEDO).

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In summary, we prepared encapsulated long-term stable MASnI3 thin films and solar-cell devices with and without 20 mol % SnF2, and studied their optical properties to clarify photocarrier dynamics in solar cells. The MASnI3 thin film with 20 mol % SnF2 additive showed a bandgap of 1.25 eV. On the contrary, the MASnI3 thin film without SnF2 exhibited a blueshifted absorption as a result of the Burstein−Moss shift due to a large number of unintentionally doped holes. Moreover, the resonantly excited PL spectra evidenced that the broad MASnI3 PL spectrum is not due to extrinsic effects such as impurities or defects, but due to the intrinsic homogeneous broadening by strong electron−phonon interactions. The time-resolved PL results clarified that the SnF2 addition improves the luminescence lifetime of the thin film by more than one order, and this longer lifetime is the reason for the improved performance in devices prepared with SnF2. Furthermore, by analyzing the carrier lifetimes of the thin film and the solar cell, we clarified that the large deviation of JSC from the ideal value in the MASnI3 solar cell is due to the large nonradiative recombination rate of photocarriers in the perovskite layer, which leads to a reduced injection efficiency into the transport layers. MASnI3 is a material with many defects, but through improving the injection rate into the transport layers, an improvement of the solar cell efficiency is expected.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06199. Scanning electron microscope image and film morphology, transient photoresponse of MASnI3 thin films and devices, resonantly excited PL spectroscopy, lightsoaking, and the power-dependence of PL decay (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*(Y.K.) E-mail: [email protected]. Telephone: +81 774 38 4510. ORCID

Yoshihiko Kanemitsu: 0000-0002-0788-131X 16163

DOI: 10.1021/acs.jpcc.7b06199 J. Phys. Chem. C 2017, 121, 16158−16165

Article

The Journal of Physical Chemistry C

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The Journal of Physical Chemistry C Amplified Spontaneous Emission in HC(NH2)2SnI3 Perovskite Films. J. Phys. Chem. Lett. 2016, 7, 4178−4284. (52) Lindsey, C. P.; Patterson, G. D. Detailed Comparison of the Williams−Watts and Cole−Davidson Functions. J. Chem. Phys. 1980, 73, 3348. (53) Yamada, Y.; Yamada, T.; Shimazaki, A.; Wakamiya, A.; Kanemitsu, Y. Interfacial Charge-Carrier Trapping in CH3NH3PbI3Based Heterolayered Structures Revealed by Time-Resolved Photoluminescence Spectroscopy. J. Phys. Chem. Lett. 2016, 7, 1972−1977. (54) Tex, D. M.; Imaizumi, M.; Kanemitsu, Y. Analyzing the Electrical Performance of a Solar Cell with Time-Resolved Photoluminescence: Methodology for Fast Optical Screening. Phys. Rev. Appl. 2017, 7, 014019.

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DOI: 10.1021/acs.jpcc.7b06199 J. Phys. Chem. C 2017, 121, 16158−16165