Piperazine Suppresses Self-doping in CsSnI3 Perovskite Solar Cells

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Piperazine Suppresses Self-doping in CsSnI3 Perovskite Solar Cells Tze-Bin Song, Takamichi Yokoyama, Jenna Leigh Logsdon, Michael R. Wasielewski, Shinji Aramaki, and Mercouri G. Kanatzidis ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00866 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Energy Materials

Piperazine Suppresses Self-doping in CsSnI3 Perovskite Solar Cells Tze-Bin Song,† Takamichi Yokoyama,†‡ Jenna Logsdon,† Michael R. Wasielewski,† Shinji Aramaki,‡ and Mercouri G. Kanatzidis†* †

Department of Chemistry, Northwestern University, 2145, Sheridan Road, Evanston, Illinois

60208, United States ‡

Mitsubishi Chemical Group Science & Technology Research Center, Inc., 1000 Kamoshida-

cho, Aoba-ku, Yokohama 227-8502, Japan *Corresponding Author: Email: [email protected]

Keywords: Perovskite, Lead-free, Solar Cells, Piperazine, Diamine

1 Environment ACS Paragon Plus

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Tin-based halide perovskite materials are promising candidates for lead-free halide perovskite solar cells. However, they suffer from poor device reproducibility and limited overall power conversion efficiencies due to their tendency to become semi-metallic from p-type defect states. Herein, we demonstrate an effective approach to address this issue via the addition of piperazine to the precursor solution of tin-based halide perovskite films, to suppress the undesirable pdoping of CsSnI3 films. Piperazine is found to significantly reduce the conductivity of CsSnI3 films, improve the film coverage and at the same time suppress the crystallization of excess SnI2. Consequently, short circuit behaviors are eliminated, with significantly improved CsSnI3 solarcell performance. Moreover, the effects of incorporating SnCl2 and SnF2 into the CsSnI3 devices were investigated in conjunction with addition of piperazine to achieve CsSnI3 devices with a maximum power conversion efficiency of 3.83%.


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Organic-inorganic metal halide perovskite compounds are promising candidates for high performance and low-cost optoelectronic devices. In particular, lead (Pb)-based halide perovskites (with a chemical structure of ABX3, where A is a monovalent caion, B is a divalent metal caion and X is a halide) have demonstrated impressive photovoltaic performance with power conversion efficiencies (PCEs) over 22% within less than a decade of development.1-4 Due to their superior optical and electrical properties, Pb-based halide perovskites are also being explored for their application in other applications, such as light emitting diode (LED) and lasing.5-7 However, the use of the Pb element could pose a serious barrier toward mass production and commercialization because of its toxicity. In addition, Pb-based halide perovskites have band gap (Eg) values between 1.5 and 2.4 eV, which are higher than the ideal Eg (1.34 eV) derived from the Shockley–Queisser limit for single-junction solar-cell applications.8 Therefore, the development of Pb-free halide perovskite materials with smaller Eg is becoming increasingly important. Recently, many Pb-free halide perovskite derivatives have been explored as alternatives, including divalent Sn-, Mn- and

Ge-based halide perovskites [A(Sn, Mn,

Ge)X3],9-11 layered Sb-based halide perovskites,12 molecular halide perovskites,13-15 and ordered double perovskites (A2(I)(III)X6, where I = monovalent metal caion; III = trivalent metal caion).16-19 Among these, Sn-based halide perovskitesis a particularly promising alternative not only because they are direct analogs of Pb-based halide perovskites with three dimensional crystal structure, but also possess less toxicity and narrower Eg values ranging from 1.2 to 2.2 eV.9,20,21 Moreover, Sn-based halide perovskite solar cells have achieved a PCE of over 9%, the highest among all the Pb-free halide perovskite materials.22 Although Sn-based halide perovskites have seen increased device performances, a strong “self-doping” effect due to the easy oxidation of divalent Sn (Sn2+) to quadrivalent Sn (Sn4+) as



well as a low formation energy of Sn vacancies (VSn) results in a near metallic electrical behavior, which in turn leads to poor device reproducibility and stability.23-25 Hence, near short-circuit diode behaviors were generally observed in Sn-based halide perovskite solar cells.26-29 CsSnI3 is particularly prone to this behavior.30,31 Recently, it has been reported that the incorporation of various organic cations including butylammonium (BA), phenylethylammonium (PEA), and ethane-1,2-diammonium (EDA) into Sn-based halide perovskites can improve the device performances and stabilities.32-35 Additionally, we have shown that incorporating excess Sn2+ compounds can work as Sn vacancy suppressors and the p-type background carrier density can be significantly reduced. A reducing vapor atmosphere process using hydrazine was developed to suppress the formation of Sn4+, and improved carrier lifetimes, reduced defects/traps-induced carrier recombinations in Sn-based halide perovskite films were observed.31,36 Despite these progress, the incorporation of larger cations will lead to a larger Eg beyond 1.4 eV, and the use of hydrazine vapor could raise potential safety concerns. Thus, finding alternative molecules based on previous findings without changing the Eg could pave the way to achieving a more controllable and safe process for high performance Sn-based halide perovskite solar cells. Here, piperazine was chosen because of its diamine character, which is similar to hydrazine while at the same time, also it has a chemical structure that is too bulky to fit into the crystal structure of ASnX3-based perovskite materials, thus avoiding disruption of the crystal structure and the increase in Eg. In addition, piperazine is solid at room temperature and has relatively high vaporization temperature which can facilitate a well-controlled process. We add piperazine to CsSnI3 perovskite films with excess SnI2 (optimal CsI to SnI2 ratio ~0.4) based on our previous report.31 Significantly reduced conductivity from electrical measurements is observed with the addition of piperazine. We are able to eliminate the shorting of devices and


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achieve improved solar cell performances. It is also confirmed that the diamine group plays a major role in suppressing self-doping effects in Sn-based halide perovskites as well as formation of crystalline SnI2. Moreover, based on these findings, in the presence of piperazine we investigated the effects of adding SnX2 (X = Cl and F) on the behavior of CsSnI3 perovskite solar cells. We find that SnF2 can improve device performance, mainly the open circuit voltage, while SnCl2 can improve device stability. CsSnI3 devices with a PCE of 3.83% are achieved with 10 mol% SnF2 using piperazine. The CsSnI3 perovskite films were prepared using the one-step spin-coating process on mesoporous TiO2 (meso-TiO2)/compact TiO2 (c-TiO2)/FTO substrates with a CsI/SnI2 ratio of 0.4 (referred to as 0.4-CsSnI3) dissolved in a mixed solvent as described in details in the supporting information (SI). Various amounts of 2M piperazine solution were added into the 0.4CsSnI3 solution to tune the molar ratio of piperazine to Sn from 0 to 25%. The deposited films were post-annealed at 90°C for 25 min. In the x-ray diffraction (XRD) pattern shown in Figure 1a, a strong SnI2 peak at 2θ=12.8° was observed coexisting with CsSnI3 Bragg peaks. It is interesting to note that the SnI2 peak diminished significantly with increasing amount of piperazine and completely disappeared when more than 10 mol% of piperazine was added. Moreover, we do not observe significant peak shifting, which is a common issue related with the addition of BA, FEA or EDA cation.32-34 This indicates that piperazine does not incorporate into the CsSnI3 perovskite crystal structure. This also suggests that the excess SnI2 is transformed into an amorphous state. To confirm the amorphous SnI2 formation, we directly characterize the pure SnI2 film with and without piperazine. In Figure S1, it is observed that pure SnI2 film shows extremely strong XRD pattern which is three orders of magnitude higher than the background signals from the meso-TiO2/c-TiO2/FTO substrate. When 20 mol% piperazine is added into SnI2,



there are no observable crystalline SnI2 Bragg peaks. Meanwhile, the SnI2 film and SnI2 with piperazine both show yellowish color (as shown in Figure S1b, c). These results prove that piperazine will suppress the crystalline SnI2 formation, which can lead to poor film coverage.37,38 Furthermore, we performed Fourier-transform infrared spectroscopy (FTIR) characterization as shown in Figure S2, which confirms that the piperazine molecules are still present in the film. These suggest that the piperazine interacts strongly with SnI2 to make an amorphous complex suppressing SnI2 crystallization. In Figure 1b, Tauc plots of the CsSnI3 films prepared from 0, 10 and 20 mol% of piperazine containing solutions show similar Eg at ~1.3 eV, in agreement with previous reported values.30,39,40 Thus the piperazine treated CsSnI3 film consists of crystalline CsSnI3 diluted by amorphous SnI2, with the Eg determined by the CsSnI3. From the optical images of the films with various amounts of piperazine in Figure 1c, the colors of the films gradually change from yellowish to brownish with increasing amount of piperazine. The yellowish color could be due to the presence of SnI2, while the brownish color is a signature of the better coverage of the CsSnI3 layer, which will be discussed in detail below. To confirm that these effects are due to the diamine functionality of piperazine, we examined the aromatic pyrazine molecule which has similar molecular structure with piperazine but has no diamine character, Figure S3a. As expected, we do not observe the film color change and the disappearance of the crystalline SnI2 phase when 20 mol% pyrazine is added as shown in the optical image and XRD spectrum (Figure S3b, c). By comparing these two molecules, we confirm that the diamine group plays an important role in the film formation and Sn halide crystallinity properties.


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Figure 1. (a) XRD patterns of the 0.4-CsSnI3 with piperazine from 0-25 mol%. (b) Absorption spectra of the 0.4-CsSnI3 with 0, 10 and 20 mol% piperazine. (c) Pictures of the 0.4-CsSnI3 with piperazine from 0-25 mol% (Left to Right) under white light in glovebox. All the films are characterized on meso-TiO2/c-TiO2/FTO. The morphologies of the 0.4-CsSnI3 ﬁlms, prepared from 0, 10 and 20 mol% of piperazine containing solutions, are characterized by scanning electron microscopy (SEM), as shown in Figure 2a-c. Without piperazine, the 0.4-CsSnI3 films formed crystalline islands which could be the result of the formation of crystalline SnI2 which impedes the growth of CsSnI3. The crystallization of SnI2 results in poor coverage over the mesoporous substrates that could lead to a decrease in the optical absorption, making the film appear more transparent as shown in Figure 1c. When piperazine is incorporated, the film coverage improves leaving no meso-TiO2 layer exposed even when observed in high magnification, as shown in Figure 2b, c and Figure S4. When 20 mol% piperazine was added, there are observable glass-like filling in between the grains in Figure 2c. We believe that this glass-like formation is the amorphous SnI2 complex 7 Environment ACS Paragon Plus


formed due to the interaction between the excess SnI2 and piperazine as discussed previously. We speculate that the incorporation of piperazine suppresses the nucleation of SnI2 crystallites, leading to enhanced CsSnI3 grain growth in the early stages of the annealing process while the amorphous SnI2 complex develops at a slower rate and becomes the filling around the perovskite grains covering the surface of the perovskite layer. The color change from yellowish to brownish observed in Figure 1c could be due to the improved CsSnI3 film coverage and better filling into the meso-TiO2 layer. Apart from the film morphologies, electrical characterization of the Snbased halide perovskite films with and without piperazine was performed as shown in Figure 2d. It is well known that Sn-based halide perovskite is easily doped to show the electrical properties of a conductor rather than a semiconductor as mentioned before. The self-doping effect of Snbased halide perovskite could cause a shorting behavior in photovoltaic devices and poor device reproducibility. We performed four probe measurements to the 0.4-CsSnI3 films with and without piperazine on meso-TiO2/c-TiO2/glass substrates. The pure 0.4-CsSnI3 film shows a lower resistance than those of piperazine-contained films, despite the discontinuous nature of the pure 0.4-CsSnI3 film. Interestingly, the film resistance increased over four orders of magnitude when 20 mol% piperazine is incorporated. Thus the 0.4-CsSnI3 film with piperazine treatment exhibits significantly reduced conductivity revealing an effective suppression of the hole doping despite the more continuous morphologies. These results suggest that piperazine plays the dual role of affecting the film formation and coverage as well as suppressing self-doping in Sn-based halide perovskite materials.


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Figure 2. SEM images of the 0.4-CsSnI3 perovskite films prepared with various piperazine molar ratios over meso-TiO2 substrates: (a) 0 mol%; (b) 10 mol% and (c) 20 mol%. Scale bars are 2 µm. (d) Semi-log plot of the current-voltage (I-V) characteristics for each condition. The measurements were performed using the four point probe technique.

Solar cell devices were fabricated by depositing hole transporting layer (HTL) and Au top electrode on the perovskite films, as described in detail in the SI. Representative current density-voltage (J-V) curves as well as key device parameters’ dependence on the molar ratio of piperazine are shown in Figure 3a. The 0.4-CsSnI3 devices prepared from solutions without piperazine exhibit short-circuit diode behavior, and the device performance improves significantly when the piperazine is used. The improvement is consistent with the electrical measurements which showed that the presence of piperazine suppresses the electrical



conductivity. In addition to mitigating the self-doping effect of the perovskite layer, the improvement of device parameters including open circuit voltage (VOC), short-circuit photocurrent density (JSC) and fill factor (FF) can also be attributed to the better film coverage. A highest average PCE of 2.42% is achieved in the 0.4-CsSnI3 devices with 15 mol% of piperazine. Further increasing the piperazine fraction leads to increasing amount of residual piperazine in the perovskite film which hinders the charge transport causing lower JSC and FF. Time-resolved photoluminescence (TrPL) decay spectra of 0.4-CsSnI3 with and without 15 mol% piperazine are shown in Figure 3b, c. The TrPL data shows enhancement in the PL lifetime (τ1, from ~134 ps to ~298 ps) when using piperazine in the 0.4-CsSnI3 films. Note that the τs obtained from a bi-exponential decay fit, see Figure 3c, is attributed to the scattering signals. The enhancement of the PL lifetime was likely a result of a reduced defect density that suppressed nonradiative recombination channels by piperazine. Using this piperazine process as a baseline, we then examined the effects of incorporating SnCl2 and SnF2 on the solar cell device performance. It has been reported that too much SnF2 leads to agglomeration (>20 mol%) and results in poor coverage.39,41,42 Thus, we choose 10 mol% SnCl2 and SnF2 in place of SnI2 in order to keep the same concentration of SnX2 and the same CsI/SnX2 ratio (referred to as 0.4-CsSnI3-10Cl and 0.4-CsSnI3-10F). We compared the PL lifetime of 0.4-CsSnI3, 0.4-CsSnI3-10Cl and 0.4-CsSnI3-10F films with 15 mol% of piperazine as shown in Figure 4a, b. It is clear that 0.4-CsSnI3 and 0.4-CsSnI3-10Cl show very similar PL lifetimes, and the 0.4-CsSnI3-10F shows the longest lifetime among the three. The longer PL lifetime suggests that the SnF2 could be further reducing the defect density and improving the device performance over that achieved with piperazine incorporation. The representative J-V curves and device performance of solar cells corresponding to 0.4-CsSnI3-10Cl and 0.4-CsSnI3-


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10F with various molar ratios of piperazine are shown in Figure 4c and Figure S5. Interestingly, in terms of device performance, the optimal amount of piperazine is the same across all three conditions, Figure S5. Average PCEs of 2.22% and 2.87% were achieved in 0.4-CsSnI3-10Cl and 0.4-CsSnI3-10F with optimized amount of (15 mol%) piperazine, respectively. The increase in the solar cell performance with adding SnF2 is consistent with the longer lifetime observed in the TrPL, which is a direct consequence of the reduction of the carrier concentration in Sn-based halide perovskite by modulating its vacancies.39 This could also be the reason that short circuit behavior did not occur in 0.4-CsSnI3-10F devices regardless of the molar ratio of the piperazine, Figure S5b, d. On the other hand, adding SnCl2 does not improve device efficiency, but has a significant effect on device stability, we will discuss in the next section.



0 mol% Piperazine 5 mol% Piperazine 10 mol% Piperazine 15 mol% Piperazine 20 mol% Piperazine 25 mol% Piperazine

PCE (%) 2

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500 400 300 200 100 0 100 80 60 40 20 0

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Figure 3. (a) Representative J–V characteristics of the 0.4-CsSnI3 devices prepared from various piperazine amount of solutions. Summarised device parameters are plotted as a function of the piperazine amount. The results shown are from four devices per condition prepared in the same batch. Note: The fill factor is shown as 0 when VOC value was lower than 0.02 V. The solid lines show the trend of average values of the device parameters. Time resolved photoluminescence (TrPL) decay spectra of 0.4-CsSnI3 films with (b) 0 mol% piperazine and (c) 15 mol% piperazine on glass (measured with encapsulation).

Device stability is another important concern in halide perovskite devices, especially for Sn-based halide perovskite. It has been reported that the Sn2+ in ASnX3 can be oxidized to Sn4+ when exposed to air resulting in a heavily doped semiconductor behavior which in turn leads to device failure.32,43 As shown in Figure 4d, the performances of un-encapsulated 0.4-CsSnI3 and 0.4-CsSnI3-10F devices degrade completely within 30 min when operated under 1 sun


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illumination in ambient conditions. The 0.4-CsSnI3-10Cl on the other hand showed relatively stable device performance and SnCl2 is considered to be acting as desiccant, slowing down the oxidation of the underlying CsSnI3.44 Although using SnCl2 did not improve the device efficiency, the significant improvement in device stability is noteworthy. These results show that alternative SnX2 sources together with piperazine can offer unique benefits towards achieving high performance and stable Sn-based halide perovskite devices.

τs=< 50 ps

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Figure 4. Time resolved photoluminescence (TrPL) decay spectra of 0.4-CsSnI3 films fabricated using 15 mol% piperazine with substituting (a) 10 mol% SnCl2 (0.4-CsSnI3-10Cl) and (b) 10 mol% SnF2 (0.4-CsSnI3-10F) on glass (measured with encapsulation). (c) J–V characteristics of the 0.4-CsSnI3, 0.4-CsSnI3-10Cl and 0.4-CsSnI3-10F devices fabricated using 15 mol% piperazine. (d) Stability of the devices in an ambient environment (40–50% humidity, T=25 °C) without encapsulation under continuous one-sun illumination.



Based on the above results, we then prepared 0.4-CsSnI3-10F devices with optimal mesoTiO2 thickness (~1 µm) to explore the capability of our current approach using piperazine. Figure 5a shows the J–V characteristic of the best performing 0.4-CsSnI3-10F solar cell using 15 mol% of piperazine measured via reverse bias scan with a PCE of 3.83%. No significant hysteresis behavior (less than 5% difference) was observed between the reverse (from VOC to JSC) and forward (from JSC to VOC) scans (Figure S6). Moreover, to further examine the reproducibility of device performance, we fabricated batches of devices and the statistics of the PCEs are shown in Figure S7 with average PCE of 3.32%. The low VOC value observed in these devices is the next big challenge for high performance Sn-based halide perovskite devices. The significant voltage loss (Eg-VOC) is more than 750 mV for the current study which is twice as large as Pb-based perovskite solar cells.31,36,39,40,44-47 Beyond improving the material quality, adopting different device structures and transporting materials in order to achieve better energy alignment and interface between the adjacent transporting materials and Sn-based halide perovskite materials, would be an important step to reducing the voltage loss as discussed in previous studies.31,37 The representative incident photon-to-current conversion efficiency (IPCE) spectrum is shown in Figure 5b. IPCE spectrum gradually drops in the long wavelength region near Eg presumably because of the lower amount of absorber present in the 0.4-CsSnI3-10F layer. Increasing the absorber thickness and improving the material quality could further improve the photocurrent collection near band edge and improve the device performance.


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b) 100

20

80 EQE (%)

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25

15 10

VOC : 338.83 mV JSC

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: 20.63 mA/cm

5 FF

: 54.18 % PCE : 3.83 %

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Figure 5. (a) J–V characteristic of the best-performing 0.4-CsSnI3 with 10 mol% SnF2 device. (b) A representative IPCE spectrum and integrated JSC. In summary, we have demonstrated that piperazine enables the deposition of CsSnI3 films with reduced doping level and high resistivity. By comparing the aromatic pyrazine and the aliphatic piperazine, we also confirmed that the diamine group plays a key role in interacting with Sn-based halide perovskite. This is the result of the greater electron donor tendency of the aliphatic amines relative to the aromatic ones. Moreover, the film coverage is significantly improved by using piperazine. Our current approach can prevent short-circuit diode behavior and achieve well-behaving J-V characteristics in CsSnI3 solar cells. Based on the piperazine approach, we systematically showed the effects of adding SnCl2 and SnF2 in CsSnI3 solar cell devices. We found that incorporating SnCl2 can enhance device stability while incorporating SnF2 can improve device performance. A best-performing CsSnI3 solar cell with 3.83% PCE was further achieved to demonstrate the viability of our approach. These results shed light into the film formation process of CsSnI3 perovskite solar cells and can provide a general guideline for future work towards high performance and stable Sn-based perovskite solar cells.



ASSOCIATED CONTENT Supporting Information: Experimental section and Figures including XRD patterns and photographs of SnI2 films with and without piperazine; FTIR spectra of SnI2 film, piperazine powder and SnI2 film with piperazine; pyrazine molecule and the photograph and XRD patterns of 0.4-CsSnI3 films with and without pyrazine; SEM images of 0.4-CsSnI3 film with and without piperazine; J-V characteristics and device performances of 0.4-CsSnI3 devices using various molar ratio of piperazine additive and substituting SnI2 with 10 mol% SnCl2 or SnF2; Hysteresis behavior of a 0.4-CsSnI3 devices using 15 mol% of piperazine and substituting SnI2 with 10 mol% SnF2; Histogram of PCE of 0.4-CsSnI3 devices using 15 mol% of piperazine and substituting SnI2 with 10 mol% SnF2. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT 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, Office of Basic Energy Sciences under Award Number DE-SC0001059 (solar absorber material synthesis and solar cell characterization) and the Institute for Sustainable Energy at Northwestern University. T.-B.S.


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acknowledges financial support from Mitsubishi Chemical Group Science & Technology Research Center, Inc. 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 (EEC0118025/003), both programs of the National Science Foundation; the State of Illinois; and Northwestern University.

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