Solvent Engineering Improves Efficiency of Lead-Free Tin-Based

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Solvent Engineering Improves Efficiency of Lead-free Tin-based Hybrid Perovskite Solar Cells beyond 9% Xinghua Liu, Kang Yan, Dawei Tan, Xiao Liang, Hongmei Zhang, and Wei Huang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01588 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Energy Letters

Solvent Engineering Improves Efficiency of Leadfree Tin-based Hybrid Perovskite Solar Cells beyond 9% Xinghua Liu, † Kang Yan, † Dawei Tan, † Xiao Liang, ‡ Hongmei Zhang,*, † Wei Huang*, † †

Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced

Materials (IAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing, 210023, China ‡

School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (H.Z.). *Email: [email protected] (W.H.).

ACS Paragon Plus Environment

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ABSTRACT. Here, we report on the effect of different anti-solvent dripping on film morphology and charge recombination of mixed formamidinium-methylammonium tin iodide (FA0.75MA0.25SnI3) as light absorber in perovskite solar cells. N, N-dimethyl methanamide (DMF) and dimethyl sulfoxide (DMSO) were used as the mixed solvent in the perovskite precursors together with tin fluoride (SnF2) as additives. Diethyl ether (DE), toluene (TL), and chlorobenzene (CB) were employed as anti-solvents for comparison. Our results show that CB as anti-solvent leads to a dense and uniform Sn-based perovskite film. The maximum power conversion efficiency of our Sn-based perovskite solar cell achieves 9.06% (9.02%) under forward (reverse) voltage scan under AM 1.5G 100 mW/cm2 illumination. The encapsulated cells show good long-term stability with ~75% of its initial efficiency retained over a period of 30 days’ storage. Our work suggests the promising potential to further improve the performance of Pb-free Sn-based perovskite solar cells.


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Organic-inorganic hybrid halide perovskite solar cells (PSCs) are a promising photovoltaic (PV) technology. Lead (Pb)-based hybrid perovskite solar cells (HPSCs) have achieved an impressive power conversion efficiency (PCE) of 23.2%1 since the first report in 2009.2 However, the use of toxic Pb severely limits their broad applications and commercialization. Theoretical studies have shown that superior photovoltaic properties of the Pb-based halide perovskites are partially attributed to the perovskite structure and the inactive Pb 6s orbitals.3-6 It has been proved that other cations that are environmentally friendly and contain inactive outer shells orbitals such as germanium (II) (Ge2+),7 tin (II) (Sn2+),8, 9 antimony (III) (Sb3+),10 bismuth (III) (Bi3+),11, 12 and copper (Cu2+)13 replace Pb to form Pb-free perovskites.14, 15 Recently, the development of Pb-free PSCs is highly desired due to the rising demand in the nontoxic elements in the absorbers. As an ideal alternative to Pb, Sn-based hybrid perovskites display excellent optical and electrical properties such as high absorption coefficients, small exciton binding energies, and high charge carrier mobility, leading to reasonable device performance.16-21 Pb-based perovskites typically have the bandgap of 1.5-1.8 eV, larger than 1.34 eV which is ideal for approaching the Shockley–Queisser efficiency limit (33%) for single-junction solar cells.22,

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Compared to their Pb-based counterparts,24,

25

Sn-based perovskites with slightly

narrower bandgaps (~1.3 eV) promise higher short-circuit current density (Jsc) exhibiting a theoretical PCE very close to 33%.26 The record PCE of Sn-based HPSCs still remains at about 6% in the past three years,8, 9 although a recent PCE of 9 % has been reported,27 indicating large room to further improve their performance. The primary origins for chemical instability of Sn-based perovskites include low formation energy of Sn vacancies, fast oxidation of divalent Sn2+ into Sn4+,28 and poor film morphology due to the fast crystallization and reaction between Sn halide and organic ammonium salts,


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ACS Energy Letters

which hinder the development of Sn-based PSCs.29 Many efforts have been made in optimizing the Sn-perovskite film morphology, tuning the film composition, using additives to suppress the oxidation like tin fluoride (SnF2), and engineering the device structure and interfaces.30-37 For example, adding SnF2 in cesium tin triiodide (CsSnI3) and formamidinium tin iodide (FASnI3) with an n–i–p device structure effectively prevents the oxidation of Sn2+ and increases the formation energy of Sn vacancies, leading to the PCE of 2.02% and 2.1%, respectively.30, 38 Seok and co-workers have improved the film morphology of FASnI3 by using SnF2-pyrazine complex as additives that slows down the perovskite crystallization, achieving a high PCE of 4.8%.31 It has been demonstrated that using dimethyl sulfoxide (DMSO) as the precursor solvent can form an intermediate phase (SnI2-3DMSO) that retards the rapid crystallization of Sn-based perovskites.28 Recently, Zhu and coworkers have demonstrated that SnF2 also facilitates the formation of more tin-based perovskite nucleuses and enables more homogeneous crystal growth with full coverage.39 Except for SnF2 as a reducing agent additive, solvent engineering40 has been proven to be an effective method to obtain high-quality perovskite film and enhance the device performance. Zhao, Yan and coworkers have reported a PCE of 6.22% for FASnI3 PSCs with an inverted p–i–n planar structure, in which diethyl ether (DE) is applied as anti-solvent to form FASnI3,32 which created the precedent for anti-solvent application in Sn-based perovskites. Zhao et al. have used mixed-cation tin triiodide (FA0.75MA0.25SnI3, where MAI is methylammonium iodide) as light absorbers and DMSO as the precursor solvent, where chlorobenzene (CB) is applied as anti-solvent, leading to a PCE of 8.12%.26 Shao et al. have incorporated a small amount of 2-phenylethylammonium iodide (PEAI) into FAI to form a 2D/3D-based PSCs and achieve a PCE of 9.0% with DE as anti-solvent.27 Since pure DMSO has relatively low


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evaporation rate and high viscosity, it is challenging to form a thin film by spin coating.41 Thus, DMF is commonly used to dilute DMSO in order to deposit a smooth film. Here, we report on efficient Pb-free FA0.75MA0.25SnI3 PSCs with 9.06% (9.02%) PCE under forward (reverse) voltage scan under AM 1.5G 100 mW/cm2 illumination. DE, toluene (TL), and CB have been employed as anti-solvent for comparison. The results indicate CB could lead to a dense and uniform perovskite film, producing an average PCE of 8.26±0.38%, suggesting a good reproducibility. Our encapsulated Sn-based cell retains 75% of its initial efficiency stored in a glovebox over a period of 30 days, exhibiting good long-term stability. The perovskite precursors consist of FAI, MAI, and tin iodide (SnI2) with a molar ratio of 0.75:0.25:1 dissolved in a mixture of DMSO and DMF. In order to prevent the easy formation of Sn vacancies, a small amount of SnF2 (10 mol%) was used as additives into Sn-based perovskite films to fill Sn vacancies and suppress oxidation of Sn2+.30-34 The FA0.75MA0.25SnI3 perovskite films were deposited via a one-step method, where, for comparison, DE, TL, and CB was dripped as the anti-solvent.

(b)

(a)

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FA0.75MA0.25SnI3

FA0.75MA0.25SnI3

3.0

Absorbance

CB

Normalized Intensity

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TL DE

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Figure 1. (a) XRD patterns of FA0.75MA0.25SnI3 films deposited via single-step method with/ without antisolvent on ITO/PEDOT:PSS substrates. The peak intensity is normalized for comparison. The asterisks (*) mark the diffraction peaks of ITO substrates. (b) Absorption spectra of the different perovskite films on quartz substrates. To investigate the effect of DE, TL, and CB on the crystallinity of Sn-based perovskite films, we measured the X-ray diffraction (XRD) patterns of these perovskite films (Figure 1a). The main diffraction peaks at 14°, 24°, 28°, 32°, and 40°, which are assigned to the crystallographic planes (100), (120), (200), (122), and (222), respectively, confirming that the perovskite possesses the orthorhombic crystal structure and preferred orientation.26,

32

In

addition, the (100) and (200) peaks become much higher and (120) peak becomes much weaker for FA0.75MA0.25SnI3 compared with those for pristine FASnI3 (Figure S1a), in agreement with those reported in literature.25 We attribute the peak shift to the partial replacement of the larger FA cations by the smaller MA cations, suggesting the lattice contraction. The film without anti-solvent dripping has different crystallinity and crystal orientation from those with anti-solvent dripping. The intensity of its main peak at 24° for the film without anti-solvent dripping is much stronger than that of other peaks, implying its preferred (120) orientation. When the anti-solvent is applied, the intensity of the (120) peak is largely decreased and the intensities of (100) and (200) peaks become dominant, indicating the more preferred orientation for the perovskites with anti-solvent dripping. Additionally, the intensity of the main peak for the films treated by CB or TL is much higher than that of film treated by DE. This indicates the significant influence of different anti-solvent process on crystallinity of perovskite films. CB or TL may be a good choice for anti-solvent in FA0.75MA0.25SnI3 perovskite films. More detailed explanations about the interaction between anti-solvent and film quality will be given below.


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The absorption spectra of the FA0.75MA0.25SnI3 samples are presented in Figure 1b. The spectra of films with anti-solvent treatment are consistent with the previous reports,32 which have significantly higher intensity than that without anti-solvent treatment. The absorption edge is around 950 nm, suggesting an extended light-harvesting ability beyond near infrared region. A series of photographs of different perovskite films deposited on ITO/PEDOT:PSS before and after thermal annealing are shown in Figure S2. It is obvious that the film without anti-solvent treatment is almost transparent before and after thermal annealing, consistent with its absorption spectrum. The perovskite films with anti-solvent treatment have the mirror-like appearance in black. However, the film with DE treatment looks hazy. The morphology of perovskite film is crucial for device performance and the film quality is correlated with anti-solvents used in solvent engineering process to a large degree. Then, we examined the film morphology of Sn-based perovskites processed by different anti-solvents. Figure 2a-d show the top-view scanning electron microscope (SEM) images of FA0.75MA0.25SnI3 films with and without anti-solvent treatment. A discontinuous film with several flower-like grains is formed when the anti-solvent is not applied (Figure 2a). A similar flower-like pattern of crystalline FASnI3 without antisolvent dripping has also been observed in literature.32 Perovskite growth is a process that combines nucleation and crystal growth. Deposition of a compact and uniform perovskite film requires more nucleation sites and low crystal growth rate.42 Antisolvent dripping is used to make the precursors supersaturated quickly so that the density of nucleation sites increases simultaneously.43 When the film is deposited without any anti-solvent dripping, the precursor is far from supersaturation until the solvents evaporate completely. As a result, the density of nucleation sites is quite low, leading to a flower-like film with low surface coverage. Meanwhile, some white grains aggregate on bare substrates, indicating the excess


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SnF2. Many pinholes are found in the FA0.75MA0.25SnI3 film dripped by DE, the size of which is smaller than that of pinholes found in the film dripped by TL, but the number of pinholes is higher (Figure 2b and c). The film with CB dripping has uniform surface with full coverage and clear features of grains. The boiling point of solvents determines the crystal growth time.44 CB has a high boiling point of 131 °C, which has a slower evaporation rate that extends the crystal growth time during the thermal annealing process. The presence of the solvent provides sufficient fluidity in the film for the neighboring nuclei to coalesce to larger grains. The role of DMSO in forming a uniform and compact perovskite film has been studied before.40, 45 It is obviously seen that dripping by DE leads to a formation of reddish film and the color turns black after the next 20 s. The color of the film is much blacker after annealing at 100 °C for 10 mins, indicating the formation of intermediate phase partially retards the reaction rate. However, the spin-coated films turn brown immediately when CB or TL is dripped onto the films, and then the color of the films also turns black after the next 10 s. The film gets darker after annealing at 100 °C for 10 mins, but no much difference is found. This implies that the crystallization of the perovskites dripped by CB or TL is much faster than that by DE. We speculate that the rapid reaction between SnI2 and FAI (MAI) in the film with CB or TL dripping should be attributed to the miscibility of CB or TL in DMSO, leading to a nonstoichiometric intermediate phase. Thus, they have similar surface features.


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Figure 2. Top-view SEM images of FA0.75MA0.25SnI3 films deposited on ITO/PEDOT:PSS substrates (a) without antisolvent, (b) with diethyl ether, (c) with toluene and (d) with chlorobenzene.


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Figure 3. (a) Schematic illustration of the device structure. (b) Band alignment diagram. (c) Cross-sectional SEM image of a completed device. The schematic device structure and the band alignment diagram are illustrated in Figure 3. The main photovoltaic parameters of these devices are summarized in Table 1. Figure 4a shows the photocurrent-voltage (J-V) curves of the devices with perovskites dripped by different antisolvents under AM 1.5G 100 mW cm-2 irradiation under reverse voltage scan. The performance of the device with DE dripping is relatively low, with an average PCE of 5.85±0.51%, compared with an average PCE of 6.36±0.64% or 8.26±0.38% of the device with TL or CB dripping, respectively. The Voc of the devices with TL and CB dripping increases from 0.47±0.01 V of the device with DE dripping to 0.49±0.02 V and 0.54±0.01 V, FF from 52.9±3.3% to 59.8±4.7% and 66.1±2.0%, respectively, leading to the notable enhancement in PCE. The improvement of


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device performance can be attributed to improved perovskite film morphology as discussed above. Typically, the loss of Voc is mainly due to the non-radiative recombination loss that originates from charge carrier trap states present in the perovskites.46 In theory, all of the recombination in perovskite active layer should be radiative for a solar cell in order to approach its efficiency limit. The higher Voc may be ascribed to lower non-radiative recombination losses, likely caused by the pinholes serving as non-radiative recombination center. In addition, pinholes create shunt paths and direct contact between hole transport layer and electron transport layer, leading to high leakage current and low FF (Figure 2). Table 1. Photovoltaic parameters of FA0.75MA0.25SnI3-based PSCs with different anti-solvent dripping. Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

Champion

0.49

23.7

55.3

6.39

Average

0.47±0.01

22.6±0.9

52.9±3.3

5.85±0.51

Champion

0.52

22.7

62.7

7.52

Average

0.49±0.02

21.5±1.1

59.8±4.7

6.36±0.64

Champion

0.55

24.3

67.3

9.06

Average

0.54±0.01

23.0±0.6

66.1±2.0

8.26±0.38

Devicea) DE TL CB

a) The statistical data in this table including average values and standard deviations are calculated from 30 separate devices for each category.

The best-performing device with CB dripping achieves a maximum PCE of 9.06% (9.02%) with a Voc of 0.55 (0.55) V, a Jsc of 24.3 (23.8) mA cm-2, an FF of 67.3% (68.6%) under forward and reverse voltage scan under AM 1.5G 100 mW/cm2 illumination, showing a negligible J-V hysteresis, as shown in Figure 4b. To confirm the accuracy of our photocurrent, we measured the incident photo-to-electron conversion efficiency (IPCE) and steady-state efficiency of the device with CB dripping (Figure 4c-d). The integrated Jsc from the IPCE spectrum over the AM 1.5G


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solar spectrum is 22.15 mA cm-2. Our best-performing device with CB dripping has a steadystate photocurrent of ~20.02 mA cm-2 and a steady-state PCE of 8.71% over a period of 100 s at a bias of 0.435 V under 100 mW cm-2 AM 1.5G irradiation, verifying the reliability of device performance. 30 devices were fabricated in several batches to testify the reproducibility. The PCE statistics of these devices are in Figure S4, producing an average PCE of 8.26±0.38%, an average Voc of 0.54±0.01 V, an average Jsc of 22.9±0.66 mA cm−2, and an average FF of 66.1±1.9% under forward voltage scan, indicating good reproducibility of our device performance. We also tested the stability of the encapsulated device with CB dripping, which was stored in a nitrogen-filled glovebox. Over a period of 30 days, 75% of its initial PCE is retained and the degradation of our encapsulated devices is likely due to the hydrophilic organic cation and charge transport layers. 31

Figure 4. (a) J-V curves of the champion devices from different anti-solvent dripping using reverse voltage scan at a scan rate of 80 mV s-1 under the simulation of AM 1.5G 100 mW cm-2 irradiation. (b) J-V curves of the champion device measured under forward and reverse voltage scan. (c) IPCE spectrum of the encapsulated FA0.75MA0.25SnI3 device. (d) Steady-state Jsc and


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PCE of the champion device at a bias of 0.435 V under the simulation of AM 1.5G 100 mW cm-2 irradiation. (e) J-V curves of the device from different anti-solvent dripping under dark condition. (f) Normalized PCE of a device with CB dripping stored in glovebox over a period of 30 days. In order to gain deep insight into the improvement in device performance with CB dripping, we performed steady-state photoluminescence (PL) and time-resolved PL (TRPL) on FA0.75MA0.25SnI3 films with different anti-solvent dripping. Figure 5a shows their corresponding PL spectra, where the PL peaks are located at around 920 nm, which is slightly smaller than the absorption onset wavelength,26 as found by different authors.47 Moreover, the film with CB dripping possesses the highest PL intensity, indicating the suppression of nonradiative recombination.48, 49 The TRPL spectra are shown in Figure 5b, which were measured by exciting the films with a 479-nm pulsed laser and the PL decay was fitted by single-exponential decay model. The acquired PL lifetimes of these films are estimated to be 3.42±0.03, 3.51±0.02, and 3.81±0.07 ns for the films with DE, TL, and CB dripping, respectively. The trend in slight increase in carrier lifetime and enhanced intensity of PL clearly reveals the suppressed charge recombination in the FA0.75MA0.25SnI3 films with CB dripping. Therefore, the film with CB dripping has improved quality, benefiting the better optoelectronic performance. In order to evaluate the extent of charge recombination in the devices, the electrochemical impedance spectroscopy (EIS) was employed under dark condition with bias from 0 V to Voc. The Nyquist plots of devices with different anti-solvent dripping are showed in Figure 5c and the corresponding equivalent circuit is schemed in the inset, which includes the series resistance (Rs), recombination resistance (Rrec), and constant phase element (CPE) of electrical double layer. For all PSCs, the spectra display a main arc at low frequencies (Figure 5c), which can be attributed to the Rrec and CPE. Figure 5c represents the corresponding Rrec fitted by ZSim software and the detailed values are listed in Table S2 in the Supporting Information. The


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FA0.75MA0.25SnI3 device with CB dripping has the largest Rrec at high applied voltages, which validates its outstanding Voc as a result of the reduction in recombination rate. (b)

(a) 5x104

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Figure 5. (a) Steady-state PL spectra and (b) TRPL spectra of the encapsulated FA0.75MA0.25SnI3 films with different anti-solvents dripping on quartz substrates. (c) Nyquist plots of FA0.75MA0.25SnI3-based PSCs with different anti-solvents dripping measured at 0.4 V under dark condition. Inset: Zoomed-in patterns and the equivalent circuit model for fitting the plots. (d) The fitted Rrec at different applied voltages obtained from the EIS analysis. We have demonstrated high-efficiency Pb-free Sn-based PSCs with PCEs up to 9%. The mixture of FA and MA cations and usage of CB as anti-solvent lead to improvement in the film morphology and reduction of charge carrier recombination in the devices. Our work suggests that fine solvent engineering is one of the important approaches to achieve higher Voc and PCE of Snbased PSCs.


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ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Experimental Section and additional characterization data AUTHOR INFORMATION Corresponding Author *Email: [email protected] (H.Z.). *Email: [email protected] (W.H.) ORCID Hongmei Zhang: 0000-0002-7759-5706 Wei Huang: 0000-0001-7004-6408 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the National Key Basic Research Program of China (973) (Grant No. 2015CB932203), the National Natural Science Foundation of China (Grant No. 91233117).


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