Diammonium Cations in the FASnI3 Perovskite Structure Lead to Lower Dark Currents and More Efficient Solar Cells Weijun Ke, Constantinos C. Stoumpos, Ioannis Spanopoulos, Michelle Chen, Michael R. Wasielewski, and Mercouri G. Kanatzidis* ACS Energy Lett. 2018.3:1470-1476. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/12/18. For personal use only.
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: Hybrid halide perovskite solar cells with mixed cations demonstrate superior optical and electrical properties, especially for lead-based perovskite devices. Here, we report leadfree tin-based perovskite solar cells with diammonium cations, which can significantly improve the device performance. Formamidinium tin iodide (FASnI3) perovskite can incorporate propylenediammonium (PN) and trimethylenediammonium (TN) and retain its three-dimensional structure while at the same time providing better film morphology and optoelectronic properties. As a result, solar cell devices using FASnI3 absorbers mixed with 10% PN and 10% TN achieve higher power conversion efficiencies of 5.85% and 5.53%, respectively, compared to 2.53% of the pristine FASnI3 solar cell. This difference in device performance can be mainly attributed to the reduced leakage current, lower trap-state density, and reduced recombination, as evidenced by our dark current−voltage, space-charge-limited current, and impedance measurements. The results suggest that perovskite absorbers with mixed diammonium cations are beneficial in achieving high-performance perovskite solar cells.
T
an additive, Seok et al. reported that formamidinium tin iodide (FASnI3) perovskite solar cells with a regular mesoporous structure can achieve a PCE of 4.8%.27 More recently, several groups achieved significant breakthroughs for the tin-based perovskite solar cells with inverted planar structures.28−32 For example, Yan et al. reported the inverted planar FASnI3 solar cells with a solvent-engineering technology for the film fabrication can achieve an efficiency of 6.22%.28 On the other hand, some researchers reported tin-based perovskite with larger mixed cations forming low-dimension perovskites to achieve higher efficiencies.29−32 For example, Liao and coworkers demonstrated FASnI3-based solar cells with phenylethylammonium as an additive and an inverted planar structure can achieve a high PCE of 5.94%.29 Previously, we showed that ethylenediammonium (EN) can serve as a pseudo A-site cation in the three-dimensional (3D) perovskite structure giving rise to a “hollow” perovskite 3D structure, with improved device efficiency of over 7% and significantly enhanced stability.33−36 For the mixed cations, researchers mainly focus on single
ernary tin and lead halide perovskites are efficient light absorbers in solar cells and could impact nextgeneration photovoltaic technology.1−6 Solar cells using lead-based halide perovskites have achieved a high record power conversion efficiency (PCE) of 22.7% within a few years, which is comparable with that of the commercialized silicon and cadmium telluride solar cells.7−14 The high-performance lead-based perovskite solar cells are attractive for the solar electricity market, but the inherent toxicity of lead remains a concern and bottleneck for further commercialization.15,16 Many researchers are seeking less-toxic and lead-free light absorbers,17 such as bismuth,18,19 tin,20,21 germanium,22 antimony,23 and copper compounds,24 to replace the leadbased perovskites. In these lead-free candidates, only tin-based perovskite solar cells have shown very promising performance.20,21,25,26 Compared with the lead-based perovskites, the tin-based perovskites have low toxicity and exhibit similar optical and electrical properties but with a much worse airstability arising from easy oxidation of Sn2+.3 Since the first report on the tin-based perovskite solar cells with an encouraging PCE of ∼6%,20,21 much attention has focused on increasing their efficiency and stability.15 During the past couple of years, the PCE of the tin-based perovskite solar cells has increased gradually. For example, by employing pyrazine as © 2018 American Chemical Society
Received: April 29, 2018 Accepted: May 24, 2018 Published: May 24, 2018 1470
DOI: 10.1021/acsenergylett.8b00687 ACS Energy Lett. 2018, 3, 1470−1476
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Cite This: ACS Energy Lett. 2018, 3, 1470−1476
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ACS Energy Letters ammonium cations such as butylammonium37,38 and phenylethylammonium,29,39 among others.40 The single-ammonium mixed cations can potentially form 2D perovskites and have attracted much attention because of the hydrophobic behavior of the long organic chains which lead to the materials’ moisture resistance.37,38 Many reports have demonstrated that the leadbased perovskite solar cells with mixed organic cations can achieve high performance and remarkably improved stability.41 However, there are no reports on tin-based perovskites with mixed diammonium cations. In this work, we demonstrate that the diammonium cations such as propylenediammonium (PN) and trimethylenediammonium (TN) can provide two new dications for FASnI3 solar cells that enable the formation of 3D hollow perovskites. Compared with the FA cation, PN and TN diammonium cations have slightly larger sizes, but we find they can still be incorporated to a small degree into the perovskite structure without lowering the dimensionality. The FASnI3 materials that contain these diammonium cations exhibit band gaps similar to those of FASnI3 itself but a slightly larger unit cell volume, confirmed by ultraviolet−visible (UV−vis) absorption, photoluminescence (PL), and X-ray diffraction (XRD) measurements for both the films and crystal powders. In addition to getting incorporated to a certain degree in the perovskite structure, the PN and TN diammonium cations can lead to better film morphology and reduce trap-state density, dark current, and recombination of the devices. Therefore, the average efficiencies of the FASnI3 solar cells with additional 10% PN and 10% TN (FA:PN/TN = 1:0.1) diammonium cations are almost two times higher than that of the pristine FASnI3 solar cells. Our results suggest that tin-based perovskites with diammonium cations could be a promising approach for achieving high-performance lead-free perovskite solar cells. Figure 1a shows the 3D crystal structure of FASnI3, which is a typical organic−inorganic halide perovskite crystallizing in the
making them larger and more challenging to be incorporated into the FASnI3 perovskite. We first investigated the effect of the diammonium ions on the perovskite film morphology. Figure 2a shows the scanning
Figure 2. Top view SEM images of (a) pristine FASnI3 film and the FASnI3 films (b) with 10% PN and (c)10% TN deposited on mesoporous TiO2 films.
electron microscopy (SEM) image of a pristine FASnI3 film on a mesoporous TiO2 film, which was prepared by a one-step method. It is clear that in this case the mesoporous TiO2 film is only partly coated with star-shaped FASnI3 crystallite forming large grains. Therefore, most of the TiO2 remains exposed, leading to direct contact of the TiO2 electron-transporting layer and the hole-transporting layer. The poor coverage causes the solar cells to short circuit, but adding EN into the precursor solution can significantly improve the morphology of the perovskite films.33,34 In this work, we found the same improvement for the films with PN and TN diammoniums. As shown in Figure 2b,c, the FASnI3 films have much smoother surface, much better coverage, and fewer pinholes after including additional 10% PN and 10% TN. The grain size in the films decreases as FASnI3 > {PN}FASnI3 > {TN}FASnI3. Typically, perovskite films with large grains can reduce the recombination at grain boundaries but need to be pinhole free.42 Adding too much PN and TN into the perovskite precursor solutions causes the grain sizes to grow even smaller but the FASnI3 films look similar. As shown in Figure S1, the FASnI3 film with 40% PN is inferior because it has too many pinholes. By contrast, the FASnI3 films with 10% PN and 10%
Figure 1. Schematic illustration of the structure of (a) the 3D pristine FASnI3 perovskite and (b) 3D hollow FASnI3 with PN or TN. (c) Molecular structures of FA, PN, and TN.
Amm2 space group.3 Figure 1b shows the structure of 3D hollow FASnI3 with SnI2 vacancies, which can be filled with some PN and TN diammonium cations. The molecular structures of the different diammonium cations are shown in Figure 1c. Similar to EN, the PN and TN molecules have two NH2 amine groups, but they are expanded by one carbon atom, 1471
DOI: 10.1021/acsenergylett.8b00687 ACS Energy Lett. 2018, 3, 1470−1476
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ACS Energy Letters TN are compact and have a grain size of 2−3 μm, which is adequate for transporting charge carriers and reducing the electron−hole recombination in the perovskite absorber. Apart from the film morphology, the diammonium cations also influence the optical properties of materials. As shown in Figure 3a, the pristine FASnI3 film shows an absorption onset at
a small increase in unit cell size arising possibly from the incorporation of these diammonium cations in the structure. To further prove that PN and TN diammonium were successfully incorporated into the FASnI3 films, we measured the proton nuclear magnetic resonance (1H NMR) spectra of powders obtained from scratching away the FASnI3 films with 10% PN and TN and dissolved them in DMSO-d6. The peaks for PN and TN are visible in the spectra. The molar ratio of FA and PN can be determined from the comparison between the integration of the −NH2− protons (signal at 9.0 ppm) of FA and the −CH3 proton (signal at 1.24 ppm) of PN. Similarly, the molar ratio of FA and TN can be extracted from the comparison between the −NH2− protons (signal at 9.0 ppm) of FA and the −CH2− protons (signal at 1.87 ppm) of TN. The molar ratios of FA/PN and FA/TN are ∼1.0:0.10, in agreement with the amounts of PN and TN added in the film precursors. We also synthesized the corresponding bulk FASnI3 crystals with and without diammoniums. Figure S3 shows that all the FASnI3 crystals with and without 10% PN and 10% TN have the strong diffraction peaks of the (100), (102), (200), (122), (222), and (300) planes, characteristic of the 3D FASnI3 perovskite structure and in agreement with the film XRD results. The unit cell parameters of FASnI3/{PN}FASnI3/ {TN}FASnI3 crystals are a = 6.305/6.315/6.323, b = 8.926/ 8.932/8.944, and c = 8.930/8.948/8.952, suggesting a small increase in unit cell size after the incorporation of PN and TN diammonium cations. The NMR results of the crystals indicate that not all PN or TN used in the reaction can be incorporated in the final FASnI3 crystals (Figures S4−S6). The final molar ratios of FA/PN and FA/TN are ∼1:0.007 and 1:0.030 for the crystals with nominal solutions of 10% PN and 10% TN, respectively (Table S1). The larger degree of incorporation of TN than PN can be mainly attributed to the different molecule shapes and sizes of these diammonium cations. This is much lower than the degree of incorporation found in EN.33 TN has a larger size than PN, which itself is larger than EN. The fact that TN and EN consist of a linear chain, whereas PN is branched, suggests that shape is a more critical factor in being incorporated in the inorganic [SnI3]− framework. This in return compensates for the larger molecule incorporation in the structure by creating Sn vacancies (i.e., creating a hollow structure). This hypothesis has been recently confirmed in the case of HOCH2CH3NH3+ containing deficient perovskites with ordered vacancies.43 The larger incorporation of TN explains why the band gap of FASnI3 with 10% TN has a blue-shift that is larger than that of the 10% PN sample. We then studied the effect of the PN and TN diammonium cations on device performance. Figure 4a shows the complete solar cell device architecture. Here, a thick TiO2 mesoporous layer deposited on a fluorine-doped tin oxide (FTO) substrate coated with a thin compact TiO2 film was employed as the scaffold and electron transport layers. The perovskite FASnI3 films with and without PN and TN were used as the light absorber, and a thin poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) film was used as the hole-transporting/ electron-blocking layers; finally, a top gold film was used as the metal electrode. All the perovskite films coated on mesoporous TiO2 (∼1 μm) with a total thickness of ∼1.2 μm were prepared by a one-step method and added with 15% SnF2. The photocurrent density−voltage (J−V) results of the solar cells show that the performance is significantly enhanced when TN and PN are present (see Figure 4b). The pristine
Figure 3. (a) UV−vis absorption spectra, (b) band gaps, (c) PL spectra, and (d, e) XRD patterns of the FASnI3 films with and without 10% PN and 10% TN. (f) NMR spectra of dissolved powders obtained from scratching away the FASnI3 films with 10% PN and 10% TN in DMSO-d6.
∼910 nm, consistent with a band gap of ∼1.4 eV (Figure 3b). The optical absorption onset of the 10% PN film is the same as that of the pristine FASnI3 film, but that of the 10% TN film is slightly blue-shifted by ∼20 nm. To better prove this trend, we also fabricated the FASnI3 films with 40% PN and 40% TN added in the solution. As shown in Figure S2, the optical absorption spectrum of the 40% TN derived film is even more blue-shifted by ∼30 nm, while for the 40% PN film, the absorption edge does not change. Previously, we demonstrated that EN addition in the precursor solutions can clearly open the band bap of FASnI3 because these molecules enter the 3D perovskite structure and create massive Sn vacancies that cause discontinuities in the 3D perovskite network.33,34 We also showed that the greater the EN incorporation in the structure the greater the number of vacancies and consequently the band gap blue-shift. The similar band gap widening observed with TN addition suggests that Sn vacancies may be occurring in this case as well. The small degree of blue-shift in TN suggests that the incorporation of TN and subsequently the level of Sn vacancies are likely much lower than the EN case. PL measurements on the FASnI3 films without and with 10% TN and 10% PN confirm the band gap trend. As shown in Figure 3c, the PL emission peaks of the neat FASnI3 film and that with 10% PN occur at around 890 nm. Consistent with optical absorption results, the PL emission peak of the film with 10% TN shows a 20 nm blue-shift due to the changed band gap. The role of the PN and TN diammonium cations in the 3D FASnI3 structure was investigated with XRD. Figure 3d shows that all FASnI3 films with and without the diammonium cations have good crystalline quality, adopting a 3D FASnI3 perovskite structure. No low-angle Bragg peaks are observed, suggesting absence of a 2D layered perovskite structure. The zoomed-in XRD patterns show that the films with and without diammonium cations are slightly different (Figure 3e). Specifically, the diffraction patterns of the films with 10% PN and 10% TN slightly shift toward smaller 2θ angles, suggesting 1472
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Figure 5. (a) Plots of dV/dJ versus (Jsc − J)−1 of FASnI3 solar cells with and without 10% PN and 10% TN. (b) Dark I−V plots of the electron-only devices. (c) Dark J−V curves and (d) Nyquist plots of the FASnI3 solar cells with and without 10% PN and 10% TN.
Figure 4. (a) Device structure of the solar cells. (b) J−V curves, (c) EQE curves, (d) and PCE statistics of the FASnI3 solar cells with and without 10% PN and 10% TN.
FASnI3-based solar cell achieved only a modest PCE of 2.53% with a short-circuit current density (Jsc) of 22.52 mA cm−2 and a fill factor (FF) of 48.85%, showing a very low open-circuit voltage (Voc) of 230.02 mV. The solar cell using the FASnI3 absorber with 10% PN achieved a PCE of 5.85% with a Voc of 435.34 mV, a Jsc of 22.15 mA cm−2, and a FF of 60.67%. A similarly enhanced PCE of 5.53% with a Voc of 398.59 mV, a Jsc of 22.72 mA cm−2, and a FF of 61.04% were achieved for the solar cell using the FASnI3 absorber with 10% TN. The J−V curves of the solar cells with different amounts of PN and TN are shown in Figures S7 and S8, respectively. The photovoltaic parameters are summarized in Tables S2 and S3. Both the devices with 10% PN and 10% TN show the optimum performance. Figure 4c shows the external quantum efficiency (EQE) spectra of the solar cells using the FASnI3 absorbers with different diammoniums. The Jsc integrated from the EQE curves of the solar cells using the neat FASnI3 absorber and the FASnI3 absorbers with 10% PN and 10% TN are 19.5, 19.3, and 20.4 mA cm−2, respectively, in agreement with the trend of the Jsc obtained from the J−V measurements. We also measured the average performance of the solar cells using the FASnI3 absorbers without and with PN and TN. The statistical values of PCEs are shown in Figure 4d. The 12 solar cells using neat FASnI3 achieved an average efficiency of 1.90 ± 0.46%. The 12 solar cells using FASnI3 with 10% PN and 10% TN achieved average efficiencies of 4.46 ± 0.75% and 4.11 ± 0.96%, respectively. To understand the improved performance of the {TN}FASnI3 and {PN}FASnI3 devices, we measured the series resistances (Rs), shunt resistance (Rsh), trap-state densities, dark currents, and impedances of the devices. As shown in Figure 5a, the devices using different absorbers have different series resistances. The J−V behavior can be defined by the following diode equation:44,45 −dV /dJ = AKBT (Jsc − J )−1e−1 + R s
The solar cell made with the neat FASnI3 has the lowest Rs and Rsh, which is attributed to the higher conductivity of the film arising from high dark current density due to the easy formation of Sn4+ resulting in p-type doping.46 Despite the use of SnF2, the neat FASnI3 perovskite absorber still has high conductivity and serious electron−hole recombination. Therefore, the reduced dark conductivity of the FASnI3 films with PN and TN suggests suppression of the p-type doping leading to enhanced device performance. We also used the space charge limited current method to evaluate the trap state density in the perovskite films. The dark I−V curves of the electron-only devices using different absorbers are shown in Figure 5b. The electron-only device consisted of a TiO2 layer, the perovskite absorbers with and without TN and PN, a phenyl-C61-butyric acid methyl ester (PCBM) layer, and a gold electrode. The plot at low bias voltage shows a linear relationship, indicating an ohmic-contact response. At higher bias voltages, the current increases sharply, indicating the trap states are fully filled. The voltage at which all the traps are filled can be marked as trap-filled limit voltage (VTFL), which can be determined by eq 2:47−49 VTFL = entL2 /2εε0
(2)
where ε is the relative dielectric constant of perovskite, ε0 the vacuum permittivity, L the thickness of the perovskite film (∼1.2 μm), e the elementary charge of the electron, and nt the trap-state density. The VTFL values of the devices without and with 10% PN and 10% TN are around 0.224, 0.151, and 0.157 V, respectively. The corresponding trap-state densities nt of the neat FASnI3 film and the FASnI3 films with 10% PN and 10% TN are estimated to be 6.0, 4.1, and 4.2 × 1014 cm−3, respectively. Therefore, the FASnI3 films with diammonium cations have ∼30% lower trap-state densities, resulting in the improved FF and Voc of the devices. The charge transport and recombination processes were evaluated by the dark current measurements.50 Figure 5c shows the dark J−V curves of the solar cells using the FASnI3 absorbers with and without the PN and TN cations. Both devices with 10% PN and 10% TN have lower dark currents at negative and positive bias voltages. The lower leakage dark current suggests improved charge transport and decreased recombination loss in the devices. This is also supported by the electrochemical impedance spectroscopy (EIS) measurements.
(1)
where A is the ideality factor, KB Boltzmann’s constant, Rs series resistance, and T the absolute temperature. According to eq 1, the Rs values of the neat FASnI3, 10% PN, and 10% TN solar cells are estimated to be 0.62, 0.85, and 0.74 Ω cm2, respectively. We also estimated Rsh by the slopes in J−V curves. The Rsh values of the neat FASnI3, 10% PN, and 10% TN solar cells are 84.7, 264.8, and 421.9 Ω cm2, respectively. 1473
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ACS Energy Letters Figure 5d shows the Nyquist plots of the FASnI3 solar cells without and with 10% PN and 10% TN. The Nyquist plots show a main semicircle at low frequency, which can be attributed primarily to the recombination resistance (Rrec) and capacitance.51−53 The estimated Rrec values of the solar cells with the neat FASnI3 absorber and the FASnI3 absorbers with 10% PN and 10% TN are 135, 992, and 908 Ω, respectively. Because the recombination rate is inversely proportional to Rrec,51−53 the devices employing the FASnI3 absorbers with 10% PN and 10% TN have much lower recombination rates. On the basis of these results, the PN and TN diammonium cations seem to reduce the trap-state density of the FASnI3 perovskite and recombination rate of the FASnI3 films and devices. Despite their size being larger than that of FA, PN and TN diammonium cations can still be incorporated into the 3D FASnI3 perovskite, thereby favorably modifying the optoelectronic properties of FASnI3. The FASnI3 perovskites containing these diammonium cations therefore have a “hollow” structure similar to the EN-containing perovskites,31 showing a slightly blue-shifted band gap and an expanded unit-cell volume. Most importantly, the modified “hollow” FASnI3-based solar cells have twice the efficiency of the pristine FASnI3 solar cells. The enhanced device performance can be mainly attributed to the better film morphology, reduced trap-state density, reduced leakage current, and reduced recombination, resulting in the higher Voc and FF and therefore PCE. Our results suggest that developing tin-based perovskites with suitable diammonium cations such as TN, PN, and EN in their structure is a new promising approach for fabricating high-performance tin-based perovskite solar cells. We anticipate that incorporation of diammonium cations of proper size and shape has a universal tendency to improve the properties of tin-based perovskites and possibly also those of lead-based perovskites.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported in part by the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award DE-SC0001059. This work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center, and the Nanoscale Science and Engineering Center (EEC-0118025/003), both programs of the National Science Foundation, the State of Illinois, and Northwestern University.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00687. Experimental details, SEM images and UV−vis absorption spectra of the FASnI3 films with 40% PN and 40% TN, XRD patterns and NMR spectra of the FASnI3 crystals with and without 10% PN and 10% TN, J−V curves and photovoltaic parameters of the FASnI3 solar cells with various amounts of PN and TN loading (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Weijun Ke: 0000-0003-2600-5419 Constantinos C. Stoumpos: 0000-0001-8396-9578 Ioannis Spanopoulos: 0000-0003-0861-1407 Michelle Chen: 0000-0002-4336-2971 Michael R. Wasielewski: 0000-0003-2920-5440 Mercouri G. Kanatzidis: 0000-0003-2037-4168 Notes
The authors declare no competing financial interest. 1474
DOI: 10.1021/acsenergylett.8b00687 ACS Energy Lett. 2018, 3, 1470−1476
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