Reducing Carrier Density in Formamidinium Tin Perovskites and Its

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Reducing Carrier Density in Formamidinium Tin Perovskites and Its Beneficial Effects on Stability and Efficiency of Perovskite Solar Cells Seon Joo Lee, Seong Sik Shin, Jino Im, Tae Kyu Ahn, Jun Hong Noh, Nam Joong Jeon, Sang Il Seok, and Jangwon Seo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00976 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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ACS Energy Letters

Reducing Carrier Density in Formamidinium Tin Perovskites and Its Beneficial Effects on Stability and Efficiency of Perovskite Solar Cells Seon Joo Lee†, Seong Sik Shin†, Jino Im†, Tae Kyu Ahn‡, Jun Hong Noh†,§, Nam Joong Jeon†, Sang Il Seok†,║,*, and Jangwon Seo†,* †

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141

Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea ‡

Department of Energy Science, Sungkyunkwan University, 2066 Seobu-ro, Jangsan-gu, Suwon

440-746, Republic of Korea

§School

of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-713,

Republic of Korea ║

School of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea Corresponding Author *E-mail: [email protected] (S. I. S.), *E-mail: [email protected] (J. S.)

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ABSTRACT

In Sn-based halide perovskite solar cells (PSCs), the oxidation of Sn2+ to Sn4+ under ambient air leads to unwanted p-type doping in the perovskite film, which is main reason for increased background carrier density and low efficiency. Here, we find that the introduction of bromide into formamidinium tin iodide (CH(NH2)2SnI3, FASnI3) lattice significantly lowers carrier density of perovskite absorber, which is thought to be a result of reduction of Sn vacancies. It reduces the leakage current of devices and increases recombination lifetime, and finally improves Voc and FF of the resulting devices employing mesoporous TiO2 as an electron transport layer. Consequently, a high power conversion efficiency (PCE) of 5.5% is achieved with an average PCE of 5% and after encapsulation the devices are highly stable over 1000 hours under continuous one sun illumination including UV region. This study suggests a simple approach to improve stability and efficiency in FASnI3-based PSCs.

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With increasing attention to lead halide perovskite materials, the development of lead halide perovskite solar cells (PSCs) has been accelerated in a short period, finally achieving power conversion efficiency (PCE) of 22.1%.1 However, the toxicity issue for lead remains unresolved and limits further practical commercialization of lead-based PSCs.2,3 In this regard, alternative metals with less toxicity than Pb, including Sn,4-11 Bi,12 Ge,13 and Sb,14 have been suggested by many researchers. Despite extensive efforts to develop efficient Pb-free PSCs over the past three years, only Sn can be considered a promising candidate, on the basis of its device performance as well as its optical and electrical properties.4-11,15 However, the instability of Sn2+ upon exposure to air, which causes unwanted p-type doping in the perovskite film, is still a point of concern. In early photovoltaic applications, the use of methylammonium tin iodide (MASnI3) was consequently delicate and challenging work even in a nitrogen-filled glove box, resulting in low reproducibility of the device performance.4 Recently, formamidinium tin iodide (CH(NH2)2SnI3, FASnI3) perovskites have been intensively explored in Pb-free PSCs due to their better stability and higher formation energy of Sn vacancies than MASnI3,16,17 achieving a power conversion efficiency of 7.14%9 or 9%10 using a normal or inverted device structure, respectively. Moreover, the long-term stability (for over 100 days) of the FASnI3-based device against air through simple encapsulation is encouraging.7 This improved performance and stability is provided by the beneficial additive SnF2 that has been adopted as a reducing agent. It is known that SnF2 in a perovskite precursor solution increases the formation energy of Sn vacancies and thus decreases defect concentration and the metallic conductivity.18 Mathews et al. reported that the use of SnF2 reduces the charge carrier density by two orders of magnitude, which was revealed by Hall effect measurements.18 As a similar strategy, excess SnCl2 was introduced into tin-based perovskite films as an effective


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additive in preventing oxidation of Sn2+.6 F or Cl with a high electronegativity may have some interaction with the neighboring Sn2+ of the perovskite lattice and prevent the Sn2+ from forming Sn4+ by oxidation at the same time as excess tin halide compensates for deficient Sn2+ stemming from the oxidation of Sn2+.19 For lead halide perovskite materials, a partial change of the halide composition in the lattice has brought synergetic effects on the entire material. For example, MA(or FA)PbI3-xClx accommodated only a small amount (~4 mol%) of Cl in the lattice due to the size mismatch of the two halides, but led to unexpected electrical and crystal structural changes of a long carrier diffusion length and a highly preferred crystal orientation.20,21 In the case of MA(or FA)PbI3xBrx,

chemical management with Br ions not only changed the crystal structure from tetragonal

to cubic by directly replacing I with Br in the lattice, but also controlled their bandgaps according to the substitution amount of Br.22,23 In addition, a specific composition of the mixed halide perovskites resulted in improved stability against humidity as an unexpected effect.22 Herein, we have extended this approach to FASnI3-based PSCs for higher efficiency and better stability. In this work, we introduced Br into the lattice of FASnI3 to investigate the role of bromide in mixed halide formamidinium tin PSCs. High quality mixed halide perovskite films with various bandgaps were prepared using our solvent engineering method through SnF2-pyrazine complex and anti-solvent dripping. As expected, Br doping into the lattice of FASnI3 raised the conduction band edge of the perovskite absorber. More importantly, we found that the carrier density in the Br-doped FASnI3 film was much lower compared to FASnI3. This indicated that the formation of Sn vacancy defects can be effectively suppressed in the presence of Br in the lattice. As a consequence, we successfully fabricated efficient Br-doped FASnI3 based PSCs,


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achieving a PCE of 5.5% for 25 mol%-doped FASnI3. The Br-doped FASnI3 device showed much better reproducibility than a bare FASnI3 device. Surprisingly, the encapsulated cells showed remarkable light-stability over 1000 hours under continuous one sun illumination (humidity = ~25%, temperature = ~30 °C). For preparation of the perovskite film, we adopted our previously reported solvent engineering method24 using a SnF2-pyrazine7 complex. Formamidinium tin iodide (FASnI3) perovskite layer was formed by using a solution of formamidinium iodide (FAI, 1 mmol), tin iodide (SnI2, 1 mmol), tin fluoride (SnF2), and pyrazine (C4H4N2) dissolved in a mixed solvent of dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) (4:1 volume ratio). To incorporate Br into the FASnI3 lattice, we chose formamidinium bromide (FABr) as a bromide source. The doping amount of Br was adjusted from 0 mol% to 33 mol% by changing the molar ratio of FAI:FABr from 1:0 to 0:1 in the precursor solutions. Table S1 shows that the quantified atomic ratios of Br and I from the Energy-dispersive X-ray spectroscopy (EDS) analysis are very close to the theoretical ratios given by stoichiometry of the precursor solutions. The colors of the perovskite films were changed from black to dark brown with increasing Br content from 0 mol% to 33 mol%. As shown in the surface scanning electron microscopy (SEM) images (Figure S1), dense and flat films were obtained without any by-products on the surface, irrespective of the doping amount of Br into the FASnI3 film. Interestingly, it was found that Br incorporation tends to increase the average grain size of the resulting perovskite film, guiding different nucleation dynamics.25


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Figure 1. a) XRD patterns and b) absorption spectra of FASnI3 perovskite films with various Br content. c) Evolution of the absorption spectra of FASnI3 perovskite films without Br (left) and with Br (right) prepared in the absence of SnF2. d) The normalized absorbance at 500 nm when the films were exposed to ambient air for 100 hours. e) Device structure of the FASnI3-based perovskite solar cell (PSC). f) Photocurrent density-voltage (J-V) curves for the FASnI3-based PSCs without and with Br fabricated in the absence of SnF2.


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X-ray diffraction (XRD) patterns for the prepared FASnI3 perovskite films with various Br content are shown in Figure 1a. The XRD pattern of FASnI3 film without Br closely matches with that of the orthorhombic FASnI3 crystal with an Amm2 space group (JCPDS No. 01-0842959). As the larger I atoms are substituted with the smaller Br atoms in the FASnI3 lattice, the diffraction peaks are gradually shifted toward higher degrees, which is attributed to the reduction of the lattice spacing.22 Unit cell parameters of FASnI3 perovskite with different Br amount calculated from XRD reflections were summarized in Table S2. It is worth to note that lattice parameter b and c of FASn(BrxI1-x)3 become equal around x=0.17. It means that orthorhombic structure of FASnI3 is maintained until x=0.08 and converts to more symmetric tetragonal structure around x=0.17, and finally cubic structure when x=1.26 Figure 1b shows that the bandgap of the perovskite film is widened with increasing amounts of Br, as expected. The calculated bandgaps of the Br-doped FASnI3 perovskite films from absorption spectra correspond to 1.4 eV, 1.47 eV, 1.53 eV, 1.63 eV, and 1.7 eV when the doping amount of Br is 0 mol%, 8 mol%, 17 mol%, 25 mol%, and 33 mol%, respectively (Figure S2). From the above XRD and UV data, we can confirm that Br is successfully incorporated into the FASnI3 perovskite lattice. To explore the air-stability of FASnI3 perovskite films without and with Br, we monitored the absorption spectra for 100 hours upon exposure to air (Figure 1c). In order to exclude any additional effect of SnF2 on the film stability, FASnI3 films without and with Br were prepared in the absence of SnF2. Figure 1d shows that the FASnI3 film with Br maintains ~40% of its initial absorbance at 500 nm even after 100 hours’ exposure to ambient air without any encapsulation. In contrast, the absorbance of the FASnI3 film without Br nearly vanishes, implying that the introduction of Br to the FASnI3 perovskite significantly improves the film stability. This was


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indirectly supported by the photovoltaic performance of the full solar cells employing a FTO/blTiO2/mp-TiO2/FASnI3-xBrx (no addition of SnF2)/spiro-OMeTAD/Au structure (See Figure 1e); the FASnI3 PSC showed a negligible open circuit voltage (Voc) (of 0.02 V) and almost shortcircuit diode behavior, likely due to the metallic behavior of FASnI3 induced by a strong “selfdoping” effect upon easy oxidation of Sn2+ to Sn4+. In contrast, the Br-doped FASnI3 PSC exhibited good diode behavior with a typical J-V curve for a solar cell despite low efficiency in the absence of SnF2 (Figure 1f and Table S3). It is noteworthy that incorporation of Br into the FASnI3 perovskite lattice not only alters the bandgap but also suppresses the oxidation of Sn2+ to some degree, thereby enhancing the air-stability of the film. The result of the first-principle electronic structure calculations within the density functional theory (DFT) formalism supports our experimental observation. According to Figure S3, the heat of formation (∆ ) of FASnI3(1x)Br3x

decreases as x increases, which implies that a higher Br content is expected to produce a

more stable material with respect to the dissociation into reference phases of chemical elements. It can be due to the more symmetric and compact structure of perovskite materials with a higher Br content.22 The increased average grain size is also related to the enhanced air-stability of the Br-doped FASnI3 (Figure S4).


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Figure 2. The distribution of a) short circuit current density (Jsc), b) open circuit voltage (Voc), c) fill factor (FF), and d) power conversion efficiency (PCE) for FASnI3 PSCs with various Br content in the presence of SnF2.

To investigate the effect of the Br content in the FASnI3 on each photovoltaic parameter, typical n-i-p structured FASnI3 PSCs with various Br content were fabricated in the presence of SnF2. As the amount of Br was increased, we found that the Voc of the device gradually increased and the short circuit current density (Jsc) gradually decreased. That is, there is trade-off relation between the Voc and Jsc, which is attributed to widened bandgap and blue-shifted absorption band


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edge of the perovskite material. Interestingly, the fill factor (FF) was enhanced rapidly at higher doping amounts of Br than 17 mol% and was optimized in the 25 mol% Br-doped perovskite device (Figure 2). As a result of the overall effects of Br doping, the FASnI3-xBrx device showed the best performance with the 25 mol% Br-doped perovskite film and also showed much better reproducibility than the device without Br (Figure 2d).

Figure 3. a) Capacitance-voltage (Mott-Schottky) plots and b) recombination time constant as a function of the open-circuit voltage of the bare FASnI3 and Br-doped FASnI3 PSCs. c) Timeresolved photoluminescence (TRPL) spectra of the bare FASnI3 and Br-doped FASnI3 perovskite films on the Al2O3 and TiO2 layer. d) Schematic energy diagram of TiO2, FASnI3, and Br-doped FASnI3 films.


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Considering that there is a trade-off between the increased Voc and the decreased Jsc induced by broadening the bandgap, the enhanced FF is the main reason for the improved PCE in FASnI3 PSCs with 25 mol% Br. To reveal the origin of the increased FF, capacitance voltage (C-V) measurements were conducted for perovskite films with and without Br. The charge carrier densities of the perovskite films were derived from the slope of the capacitance-voltage (MottSchottky) plot (Figure 3a):18 slope =

(1)

where ε is the dielectric constant of the perovskite, the permittivity of free space, A the area, e the electronic charge, and is the charge carrier density. According to our calculation based on C-V measurements, the FASnI3 film including SnF2 has a carrier density of 6.76 × 1017 cm-3, which is acceptable when considering the previously reported value.8 For the 25 mol% Br-doped FASnI3 film (in the presence of SnF2), the carrier density is reduced by three orders of magnitude (7.80 × 1014 cm-3). A similar result was reported in the case of CsSnI3. For CsSnI2Br + SnF2 (33 mol% of Br), the carrier density was reduced by one order of magnitude (1.42 × 1017 cm-3) compared to CsSnI3 + SnF2 (5.28 × 1018 cm-3).27 For the 25 mol% Br-doped FASnI3 film, the much larger reduction of the carrier density reflects a significant reduction of unwanted p-doping caused by the oxidation of Sn2+ to Sn4+. The reduced background carrier density also lowers the leakage current of the device. (Figure S5). Therefore, it is considered that the introduction of Br into the halide position considerably restricts the formation of Sn vacancies in the film during the fabrication process and it decreases the carrier density of the perovskite film and, in turn, the dark current of the PSC, which is closely related to the improved FF of the PCSs. We further calculated the defect formation energy (∆ ) of FASnI3(1-x)Br3x. Here, we considered Sn vacancy only, which has been expected to be one of the most dominant defect configurations and a main


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source of hole carrier.28 As a result, the formation energy of Sn vacancy monotonically increases, and thus it leads to lower hole carrier density as Br content increases (Figure S6). This computational result is consistent with our experimental observation. As for the charge recombination of the device, we performed transient photovoltage (TPV) decay measurements (See Figure 3b); the Br-doped FASnI3 perovskite device still showed longer recombination lifetime (τn) as compared with the FASnI3 device, which is associated with the higher FF for the Br-doped FASnI3 device. For further investigation of the effect of Br doping on the charge transfer at the interface of the perovskite-TiO2, we carried out a time-resolved photoluminescence (TRPL) study using a bilayer film employing a perovskite upper-layer and a perovskites embedded mesoporous (mp)TiO2 or Al2O3 layer (see Figure 3c). For the case of the Al2O3 layer as an inactive scaffold, the PL lifetime of the Br-doped FASnI3 perovskite (τavg=2.84 ns) is twofold longer than that of the bare FASnI3 perovskite (τavg=1.49 ns). This indicates that the bare FASnI3 perovskite film has a much greater amount of non-radiative decay pathways, which originate from defect sites. On the mpTiO2 layer as an n-type scaffold, the Br-doped FASnI3 perovskite film has a shorter carrier lifetime (τavg=0.72 ns) than the bare FASnI3 perovskite film (τavg=1.02 ns). More specifically, the Br-doped FASnI3 perovskite film on the TiO2 layer exhibited an apparent reduction in PL lifetime compared with that on the Al2O3 layer, whereas the bare FASnI3 perovskite film showed similar PL lifetimes on both TiO2 and Al2O3 layers. As previously reported, the conduction band minimum of FASnI3 lies below that of TiO2.16,29 The introduction of Br into the perovskites not only widened bandgap but lifted the conduction band edge of the perovskites to a higher level than that of TiO2 (Figure 3d), which is supported by the ultraviolet photoelectron spectroscopy (UPS) results (see Figure S7). Taking all the results together, the Br-doped FASnI3 perovskite


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has much lower carrier density due to efficient suppression of Sn vacancies compared to the bare FASnI3 perovskite and facilitates electron transfer into TiO2 due to more suitable energy level matching with TiO2, resulting in improved performance in the photovoltaic device.

Figure 4. a) Histogram of power conversion efficiency (PCE) for 40 devices and b) external quantum efficiency (EQE) of the optimized bare FASnI3 and Br-doped FASnI3 PSCs. c) Photocurrent density-voltage (J-V) curve of the device with the best performance under reverse voltage scanning. d) Stabilized PCE and photocurrent density of the best-performing device measured at a maximum power voltage of 0.293 V for 100 seconds.


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To improve the device performance, further optimization was carried out for the bare FASnI3 and Br-doped FASnI3 PSCs. Figure 4a and Figure S8 display the results comparing the distribution of the photovoltaic parameters for the optimized devices of FASnI3 and Br-doped FASnI3. The Br-doped FASnI3 PSCs shows better reproducibility as well as better performance with an average PCE of 5% (5.01±0.14%) as compared with the bare FASnI3 PSCs (3.46±0.41%). From the results based on the distribution curve of each photovoltaic parameter in Figure S8, we conclude that the enhanced PCE of the Br-doped FASnI3 PSCs originates from the respective increase of the FF and Voc. External quantum efficiency (EQE) spectra (Figure 4b) suggest that the difference in the Jsc values between the bare FASnI3 and Br-doped FASnI3 PSCs is mainly due to the different absorption onset of each material related to light-harvesting capability. A small hysteresis was observed for the Br-doped FASnI3 PSCs when measured by forward (a PCE of 4.91% with a Voc of 0.371 V, a Jsc of 20.1 mA/cm2, and a FF of 65.9%) and reverse (a PCE of 5.15% with a Voc of 0.377 V, a Jsc of 20.2 mA/cm2, and a FF of 67.6%) scans (Figure S9). Among Br (25 mol%)-doped FASnI3 PSCs, the best cell had a PCE of 5.5% (a Jsc of 19.8 mA/cm2, a Voc of 0.414 V, and a FF of 66.9%) under one sun illumination at 100 mW/cm2 when measured by reverse scans (See Figure 4c). Steady-state photocurrent for the device with the best performance measured at a maximum power voltage of 0.293 V for 100 seconds under one sun illumination at 100 mW/cm2 is 17.0 mA/cm2, which corresponds to a stabilized PCE of 5% (Figure 4d).


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Figure 5. a) Normalized PCE of the encapsulated Sn-based PSC and Pb-based PSC with the same structure of FTO/bl-TiO2/mp-TiO2/perovskite/spiro-OMeTAD/Au under continuous light exposure for 1000 h. b) XPS spectra of the Sn (3d) bands on the Sn-based perovskite surface with different etching time.

In order to evaluate the long-term stability of the Sn-based PSC, the encapsulated cell was kept in the ambient air atmosphere under continuous one sun full illumination for 1000 hours (without UV filter). During light exposure, the temperature of the device was maintained at the ambient temperature within 30 oC. It is noted that the devices were thermally encapsulated with a cover glass using a low-cost epoxy resin under a nitrogen atmosphere. Surprisingly, the devices showed remarkable photostability. Nearly 100% of the original device performance was retained after 600 hours’ light exposure even though a simple encapsulation strategy was applied, and the


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initial value only declined by 17% after 1000 hours; Jsc and FF showed little change while Voc was reduced by only 16% after 1000 hours’ light irradiation (See Figure 5a and Figure S10). For the comparison, we fabricated Pb-based PSC with the same structure of FTO/bl-TiO2/mpTiO2/perovskite/spiro-OMeTAD/Au and photostability test was conducted under the same condition. In this case, a severe decrease in the PCE of 76% was observed in the encapsulated device of Pb-based PSC after 400 hours’ illumination (Figure 5a). It was reported that typical ni-p devices including a mp-TiO2 layer showed UV light-induced degradation because the photocatalytic effect of TiO2 works upon UV light exposure, producing deep traps at the surface.30-33 For a better understanding of the improved photostability of the Sn-based PSCs including a TiO2 layer, we carried out X-ray photoelectron spectroscopy (XPS) depth profile analysis of Snbased perovskite film deposited on mp-TiO2 with various Ar+ sputtering time. From an intersection of Sn and Ti content in Figure S11, we could estimate that it takes about 80 seconds to etch above perovskite layer. The Sn (3d) bands remain unchanged until the etching time is 75 sec (Figure S12). As the etching time is increased from 75 seconds to 285 seconds, the XPS spectra of the Sn (3d) bands shift gradually from 486.85 eV to 487.45 eV, corresponding to the 3d5/2 binding energy of Sn2+ and Sn4+, respectively (See Figure 5b), while the Ti (2p) bands of TiO2 at around 459.1 eV are unchanged (See Figure S13). In other words, the Sn (3d) bands start to shift to higher energy near the TiO2 surface. Prolonged Ar+ sputtering appears to yield a small amount of metallic Sn,34 which is supported by the emerging Sn (3d) band at around 485.65 eV after 180 seconds etching. From these results of the XPS study, we speculate that a thin layer incorporating Sn4+ can be spontaneously generated at the interface between the perovskite and TiO2 during the formation of the Sn-based perovskite layer on the mp-TiO2. Some researchers


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deliberately introduced a thin blocking layer such as Sb2S332 or CsBr33 between the perovskite and TiO2 to hinder degradation of the perovskite at the interface with TiO2, which is accelerated by the photocatalytic reaction with TiO2 under continuous light illumination. In the present case, it is thought that unexpected oxidation of Sn2+ from the Sn-based perovskite layer attached on the TiO2 surface may occur, thereby producing a self-generated blocking layer between the perovskite and mp-TiO2 and effectively improving the light stability of the devices. The Snbased PSC without Br also showed superior photostability to the Pb-based PSC after 100 hour’s light irradiation, probably due to the same reason as the Sn-based PSC with Br (Figure S14). In conclusion, we have theoretically and experimentally demonstrated that the introduction of Br halide into a FASnI3 perovskite lattice produces more stable phase, improving air stability. Moreover, Br doping into the FASnI3 perovskite plays a key role in lowering the defect concentration, and hence decreases the carrier density of the perovskite material. As a result, Br (25 mol%)-doped FASnI3 PSCs exhibited device efficiency of 5.5%. More interestingly, the encapsulated device showed remarkable photostability; 83% of the initial efficiency for 1000 hours was maintained under continuous one sun irradiation containing UV light, which is surprising value compared to lead-based PSCs with the same configuration. This is likely due to a self-generated blocking layer between the perovskite and mp-TiO2. The present work presents a novel and simple approach to achieve improved efficiency and better stability in FASnI3-based PSCs.


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ASSOCIATED CONTENT Supporting Information. Experimental details, computational details, additional supplementary figures and/or tables about material/device characterization and computational results, statistics of Sn-based PSCs photovoltaic parameters, and light stability of Sn-based PSCs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. I. S.), *E-mail: [email protected] (J. S.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a grant from the Korea Research Institute of Chemical Technology (KRICT), Republic of Korea (KK1702-A01), the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade Industry & Energy (MOTIE) of the Republic of Korea (No. 20163010012470), and Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (NRF-2016M3A6A7945503). This work was also supported by the KRICT-SKKU DRC program.


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Reducing Carrier Density in Formamidinium Tin Perovskites and Its

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