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. 2018 Aug 11;6:272–279. doi: 10.1016/j.isci.2018.08.005

Room Temperature Processing of Inorganic Perovskite Films to Enable Flexible Solar Cells

Dianyi Liu 1, Chenchen Yang 1, Matthew Bates 1, Richard R Lunt 1,2,3,∗
PMCID: PMC6137715  PMID: 30240617

Summary

Inorganic lead halide perovskite materials have attracted great attention recently due to their potential for greater thermal stability compared with hybrid organic perovskites. However, the high processing temperature to convert from the non-perovskite phase to the cubic perovskite phase in many of these systems has limited their application in flexible optoelectronic devices. Here, we report a room temperature processed inorganic perovskite solar cell (PSC) based on CsPbI2Br as the light harvesting layer. By combining this composition with key precursor solvents, we show that inorganic perovskite films can be prepared by the vacuum-assist method under room temperature conditions in air. Unencapsulated devices achieved power conversion efficiency up to 8.67% when measured under 1-sun irradiation. Exploiting this room temperature process, flexible inorganic PSCs based on an inorganic metal halide perovskite material are demonstrated.

Subject Areas: Inorganic Materials, Materials Chemistry, Energy Materials

Graphical Abstract

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Highlights

  • •

    Developed room temperature processing of inorganic PSCs based on CsPbI2Br

  • •

    Flexible inorganic PSCs based on an inorganic metal halide perovskite are demonstrated

  • •

    Films show improved humidity stability compared with films annealed at high temperature

  • •

    Power conversion efficiencies up to 8.67% and 6.50% for rigid and flexible devices

Inorganic Materials; Materials Chemistry; Energy Materials

Introduction

Halide perovskite materials have emerged as excellent candidates for photovoltaic applications in recent years (Burschka et al., 2013, Kojima et al., 2009, Liu et al., 2013, Zhou et al., 2014). High device efficiency and low materials costs have given perovskite solar cells (PSCs) strong potential as a competitor for silicon solar cells (Liu and Kelly, 2014, Mei et al., 2014, Yang et al., 2017). To date, the highest reported power conversion efficiency (PCE) of hybrid organic-inorganic PSCs is up to 22.7% (Yang et al., 2017), which is higher than the efficiencies of polycrystalline silicon solar cells, cadmium telluride solar cells, and copper-indium-gallium selenide solar cells. However, the stability issue of organic-inorganic halide perovskite materials is still a key challenge for the commercial application of PSCs due to the high volatility of organic components in hybrid perovskite compounds (Eperon et al., 2014, Gratzel, 2014, Leijtens et al., 2015).

In contrast, inorganic perovskite materials could have better intrinsic thermal stability (Sharma et al., 1992). Previous research suggests that the pure CsPbI3 perovskite can maintain a stable cubic phase over 400°C (Sharma et al., 1992, Sutton et al., 2016). Thus, significant effort has been focused on developing PSCs with the inorganic cesium lead halide light absorbers (Beal et al., 2016, Chen et al., 2017, Frolova et al., 2017, Li et al., 2017, Liang et al., 2016, Ma et al., 2017, Nam et al., 2017a, Nam et al., 2017b, Niezgoda et al., 2017, Sutton et al., 2016, Swarnkar et al., 2016, Zeng et al., 2018, Zhang et al., 2018a). Sutton et al. systematically investigated the stability behavior of the cesium lead halide compounds and reported inorganic PSCs with a PCE of 9.84% (Sutton et al., 2016). Chen et al. used the vacuum-deposition method to prepare inorganic cesium halide PSCs and achieved a device efficiency over 11% (Chen et al., 2017). Zeng et al. reported a polymer-passivated cesium lead halide PSC based on inorganic perovskite nanocrystals with a PCE of over 12% and an open-circuit voltage (VOC) of over 1.3 V (Zeng et al., 2018). Wang et al. very recently reported a certified efficiency of 14.67%, which is also the highest efficiency of inorganic PSCs to date (Wang et al., 2018). Despite the rapid research progress in improving efficiencies with inorganic PSCs, the processing of inorganic perovskite film is still challenging. Because the conversion temperature of CsPbI3 from the non-perovskite phase to the cubic perovskite phase occurs at over 300°C (Sharma et al., 1992, Sutton et al., 2016), the fabrication process of CsPbI3-xBrx-based inorganic perovskite films generally requires a thermal annealing treatment at temperatures up to 350°C (Chen et al., 2017, Duan et al., 2018, Frolova et al., 2017, Liang et al., 2016, Nam et al., 2017a, Nam et al., 2017b, Niezgoda et al., 2017, Sutton et al., 2016, Zeng et al., 2018, Zhang et al., 2018a). The high temperature thermal annealing treatment not only increases the cost of inorganic PSCs but also can prevent the application of inorganic perovskite materials on polymer-based flexible substrates.

To reduce the operation temperature of cesium lead halide perovskite films, several approaches have been examined in the past 2 years (Akkerman et al., 2016, Beal et al., 2016, Eperon et al., 2015, Hu et al., 2017, Luo et al., 2016, Wang et al., 2017c, Wang et al., 2017d, Zhang et al., 2017). For example, it was shown that doping a small amount of bromide (Br) can dramatically decrease the formation temperature of CsPbI3 films (Yin et al., 2014). Following this work, Beal et al. reported the low-temperature processing of CsPbBrI2 films as the light absorber (Beal et al., 2016). The device was fabricated at 135°C and achieved a PCE of up to 6.5%. Other efforts have introduced various additives to decrease the fabrication temperature, which include hydroiodic acid, bismuth iodide, sulfobetaine zwitterions, and ethylammonium iodide (Eperon et al., 2015, Hu et al., 2017, Luo et al., 2016, Wang et al., 2017c, Wang et al., 2017d, Zhang et al., 2017). With these additives, the cubic phase CsPbI3-xBrx film could be formed at 90°C–150°C. However, the thermal annealing treatment still remained an essential step for preparation of the cesium lead halide perovskite films.

Room temperature processing is not only important to simplify the fabrication procedure but also enables fabrication on flexible substrates (Liu and Kelly, 2014). To date, only a couple of studies have reported inorganic lead halide films fabricated under room temperature that then required high-temperature annealing of TiO2 (450°C–500°C) and pre-synthesized perovskite quantum dots (Akkerman et al., 2016, Swarnkar et al., 2016). In addition, despite many reports of flexible solar cells based on the organic-inorganic hybrid perovskite materials, flexible inorganic PSCs have not yet been reported (Bi et al., 2017, Docampo et al., 2013, He et al., 2017, Kaltenbrunner et al., 2015, Li et al., 2018, Ling et al., 2017, Liu and Kelly, 2014, Remeika et al., 2018, Roldan-Carmona et al., 2014, Wang et al., 2017a, Zhang et al., 2016, Zhang et al., 2018b). Here, we develop a room temperature processed inorganic PSC with CsPbI2Br as the light harvesting layer. By choosing a suitable precursor solvent, combined with the vacuum-assist method, we show that inorganic perovskite films can be prepared at room temperature in air with a PCE up to 8.67% when measured at 1-sun irradiation. We subsequently show that this low-temperature processing enables fabrication of highly flexible inorganic halide perovskite photovoltaics.

Results and Discussion

Due to the limited solubility of lead halide compounds, the precursor solvents generally chosen are N,N-dimethylformamide (DMF), DMSO, and DMF/DMSO mixtures (Burschka et al., 2013, Chen et al., 2016, Jeon et al., 2014, Liu et al., 2018a, Liu et al., 2018b, Zhou et al., 2014). The solubility of mixed halide cesium lead precursors are particularly limited in DMF (Figure 1A) (Sutton et al., 2016), leading some researchers to utilize pure DMSO (Beal et al., 2016, Hu et al., 2017, Li et al., 2017, Wang et al., 2017c, Zhang et al., 2018a). However, DMSO is a Lewis base with strong coordination capability, which can result in colorless coordination complexes with lead halide compounds (Ahn et al., 2015, Jeon et al., 2014, Jo et al., 2016, Lee and Baik, 2018, Wu et al., 2014) and can lead to difficulties in converting the lead halide perovskite precursors to the perovskite phase under room temperature (Figure 1A). Another polar aprotic solvent used to fabricate organic-inorganic hybrid PSCs is 1-methyl-2-pyrrolidone (NMP) (Hao et al., 2015, Jo et al., 2016, Nie et al., 2015, Tsai et al., 2017, Zhou et al., 2015). Compared with DMF and DMSO, NMP has good solubility for cesium lead halide precursors and weak coordination affinity for lead compounds. NMP has other advantages as well, including better crystallization of perovskite films and miscibility with other solvents, and has been reported as the solvent to fabricate hybrid organic-inorganic PSCs under room temperature (Tsai et al., 2017, Zhou et al., 2015). Hence, we focus on NMP as the solvent for the preparation of inorganic lead halide perovskite films with room temperature processing.

Figure 1.

Figure 1

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Characterization of CsPbI2Br Precursor Solutions and Films

(A) Photograph of CsPbI2Br precursor solutions prepared by various solvents. The inset shows the films prepared by the corresponding precursor solutions with the room temperature process.

(B) Scanning electron micrograph of CsPbI2Br films with various annealing temperatures. The inset shows the photograph of the films. Scale bar, 1 μm.

It can be seen from Figure 1A that the CsPbI2Br perovskite films can be successfully prepared by the vacuum-assist deposition process under room temperature (Li et al., 2016, Liu et al., 2018a, Liu et al., 2018b). After the NMP solvent was extracted from the film under vacuum, the light brown CsPbI2Br perovskite film was formed. Scanning electron micrographs (Figure 1B) show that the CsPbI2Br film is smooth and homogeneous. Due to the rapid solvent extraction the CsPbI2Br film was formed quickly with a low level of crystallization, and led to a grain size of ∼50 nm. To investigate the thermal stability of the room temperature processed CsPbI2Br film, we annealed the films under various temperatures in an inert atmosphere. The photograph of these films clearly shows that the films maintain a brown color from room temperature to 280°C. The color of the 100°C film and the 150°C film are different from the previous report by Sutton (Sutton et al., 2016), but agree with the report by Beal (Beal et al., 2016). We infer that the film preparation method can dramatically influence the phase conversion temperature and the thermal stability of CsPbI2Br films. Accordingly, the scanning electron micrographs clearly indicate that the grain size of CsPbI2Br films gradually increased from ∼50 nm at room temperature to ∼1 μm at 280°C (Figure S1).

The as-prepared CsPbI2Br films with various annealing temperatures are also investigated by absorption spectroscopy and powder X-ray diffraction (XRD) (Figure S2). The absorption spectra show that all the films have strong absorption from 300 to 650 nm wavelength range (Figure S2A). After being annealed, the absorption of films presents a peak at 640 nm.

XRD patterns (Figure S2C) show that at room temperature the film has cubic perovskite peaks at 2θ = 14.7, 20.9, 29.6, and 42.6°, which are indexed to the (100), (110), (200), and (220) planes of CsPbI2Br, respectively. After being annealed, these diffraction peaks become stronger and sharper, which confirms the improved crystallization of the perovskite films. It is important to note that when the film is annealed to 150°C–250°C, new peaks at 2θ = 12.7 and 38.8° are observed. Since the new peaks are in agreement with the diffraction peaks of PbI2 (Liu et al., 2014), we infer that a small amount of CsPbI2Br subsequently decomposes. However, the film can maintain a metastable cubic phase up to 150°C (Figures S3 and S4). We suspect that this is due to structural tolerance factors that are close to the borderline of stable perovskite phases. This is reflected in the slight variation in the lattice constants found for the phases formed at room temperature (a = 6.11 Å) and 280°C (a = 6.02 Å). Because the lattice constant of the room temperature phase is larger, this implies that the ionic radius of Pb(II) or Br is slightly larger, leading to a reduced tolerance factor compared with the cubic phase obtained from high-temperature processing.

The humidity stability of CsPbI2Br films prepared at room temperature and 280°C was also investigated. Figure 2A is the photograph of CsPbI2Br films before and after a short period (10 min) of storage under relative humidity (RH) of 30% ± 4% in open air, respectively. It is clear that the room temperature processed film can maintain the brown color. In contrast, the 280°C film changes color in only 5–10 min, which suggests that the film undergoes a phase change or decomposition. The scanning electron micrograph for the room temperature processed CsPbI2Br film after storage has no obvious change (Figure S5). In comparison, the 280°C film shows a clear change with the formation of pinholes on the film surface after storage.

Figure 2.

Figure 2

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Stability Characterization of CsPbI2Br Films

(A) Photograph of CsPbI2Br films stored under ambient air with RH = 30% ± 4%. The left row is the room temperature film, and the right row is the film with 280°C annealing treatment.

(B) XRD patterns of the film with 280°C annealing treatment measured continuously in ambient air with RH = 22% ± 4%. Perovskite peaks are denoted with a “#” and decomposition products are denoted with a “∗”.

(C) XRD patterns of the room temperature film before and after storage in ambient air with RH ≤ 22% ± 4% for a week.

To further investigate the phase change of the 280°C film under humidity, we continuously measured the XRD spectra of the film under room temperature and RH = 22% ± 4% in air (Figure 2B). The initially prepared 280°C film shows the characteristic cubic perovskite peaks. During exposure to humidity, the cubic peaks begin to fade, whereas new diffraction peaks at 2θ = 10.0, 13.3, 26.8, 28.1, and 38.4° emerge. After 200 min the peaks of cubic phase completely disappear. In contrast, no change is observed in the XRD spectra for the room temperature processed CsPbI2Br film for over 1 week (Figure 2C). This indicates that the room temperature processed film has improved humidity stability and will therefore likely lead to improved operational lifetime as well.

Films with larger grain size are generally more compact than those with small grain size, and the more compact film should have better stability because of the better resistance to degradation from moisture and oxygen (Liu et al., 2018a). However, recent studies suggest that cesium lead halide perovskite films with small grain size have significantly improved stability than films with larger grain size (Wang et al., 2017c, Zhang et al., 2017). The reduction in the number of pinholes and defect passivation on the surface are believed to be the main contributions to the improved stability. Our observation is consistent with these studies. Previous studies also have found that grain boundaries play an important role in the perovskite film degradation process (Wang et al., 2017b, Yun et al., 2018). Chemical residues at the grain boundaries are suggested to be one possible reason for accelerated degradation of perovskite films. Overall, the room temperature processed technique provides an effective approach to improve the humidity stability of CsPbI2Br films.

Subsequently, the room temperature processed CsPbI2Br solar cells were prepared with the architecture shown in Figure 3A. An ultrathin poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT) layer was first deposited on pre-cleaned indium tin oxide (ITO) substrates. The CsPbI2Br layer was then prepared on top of the PEDOT layer by the vacuum-assist method in ambient atmosphere (Liu et al., 2018a, Liu et al., 2018b). A 20-nm C60 and a 7.5-nm 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline layer were then thermally evaporated onto the CsPbI2Br layer followed by the silver electrode.

Figure 3.

Figure 3

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Device Architecture and Performance

(A) PSC device architecture.

(B) Current-voltage (J-V) curves of perovskite devices with room temperature CsPbI2Br film and 280°C annealing treatment film measured under 1-sun illumination, respectively.

(C) The steady current density and power output under 1-sun illumination at a bias of 0.78 V.

(D) The J-V curves of room temperature processed CsPbI2Br device (different devices within B and C) measured with reverse and forward bias.

The current-voltage (J-V) characteristics of the devices are shown in Figure 3B. Under standard AM1.5G illumination, the room temperature processed device shows a PCE of 8.67%, with a short circuit current (JSC) of 12.4 mA cm−2, a VOC of 1.16 V, and a fill factor (FF) of 60.1%. The device shows an external quantum efficiency (EQE) spectrum (Figure S6) above 60% from 390 to 620 nm. The integrated photocurrent from the EQE gives a JSC of 12.1 mA cm−2, which is in good agreement with the measured value from the J-V data. For the device annealed at 280°C, a PCE of 8.02% was obtained. Although the annealed-film device shows a higher JSC, which is attributed to better carrier mobility and transport properties from the larger grain size (Senanayak et al., 2017), the loss of voltage leads to a slightly lower overall device performance. The VOC of the 280°C film device is only 1.01 V, which is significantly lower than that of the room temperature device. We measured the photoluminescence (PL) spectra for the two films and found that the room temperature film shows a strong PL peak at 645 nm, and that the peaks shift to 655 nm (with a decrease in intensity) after being annealed to 280°C. This PL spectral shift could stem from an increase in defect state emission or development of charge transfer states and actually suggests that there is a lower defect density or a reduction in charge transfer states in the room temperature-processed film. The high VOC of the room temperature device can be attributed to fewer defects (or possibly charge transfer states) and shunting pathways, which is confirmed by the stronger (and blueshifted) PL spectrum (Figure S7). It is also possible that the blueshift in the PL stems from very small grain size formation, which leads to a degree of quantum confinement and a potential increase in the bandgap (Figure S2B). In addition, the smooth film surface results in good contact between the perovskite film and the evaporated fullerene electron transfer layer, which also contributes to the improved VOC over the annealed device. Figure 3C shows that the steady photocurrent and power output of room temperature device is 10.6 mA cm−2 and 8.30 mW cm−2 under a bias of 0.78 V, respectively. Moreover, the room temperature processed device only shows a small photocurrent hysteresis when measured under forward and reversed scan modes (Figure 3D).

Based on the room temperature processing technique, inorganic cesium lead halide perovskite device were prepared on flexible substrates (Figure 4A). An ITO/polyethylene terephthalate (PET) flexible substrate was used to replace the rigid ITO/glass substrate to prepare the flexible device. Figure 4B shows the J-V curve of the flexible cesium lead halide PSC. The flexible device shows a PCE of 6.50% under 1-sun illumination, with a JSC of 12.0 mA cm−2, a VOC of 1.05 V, and an FF of 51.4%. The unencapsulated flexible device maintained an efficiency of 6.05% after 2 months storage in inert atmosphere, which is 93% of the original efficiency.

Figure 4.

Figure 4

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Photograph and Performance of Flexible PSCs

(A) Photograph of a flexible CsPbI2Br device.

(B) J-V curves of the champion flexible CsPbI2Br device measured before and after 2 months storage in a glove box without any encapsulation.

(C) Normalized parameters of flexible CsPbI2Br devices under various bending cycleswith a bending radius of 4.05 mm.

Bending tests were carried out to check the performance of flexible device after repeated bend cycles. After 100 bending cycles around a radius of 4.05 mm, the device only showed slight fluctuations in the efficiency (Figure 4C). However, the efficiency of the flexible device drops to ∼80% after 200 bending circles, largely due to drops in the FF, which likely stem from cracking of the ITO/PET flexible substrate. Although the performance of flexible inorganic PSC can still be improved, these first demonstrations are encouraging for the development of low-cost, flexible, and stable inorganic PSCs.

Conclusion

In this work, we demonstrate an approach to prepare inorganic lead perovskite films with a room temperature process. The room temperature film shows improved humidity stability over films prepared by high-temperature annealing treatment. Utilizing the room temperature approach, the inorganic lead PSCs are successfully prepared on rigid and flexible substrates. This work demonstrates the integration of inorganic halide perovskites into flexible solar cells and highlights the great potential of inorganic perovskite materials for a range of flexible optoelectronic devices.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.

Acknowledgments

The authors acknowledge financial support from the Michigan State University Strategic Partnership Grant (SPG) (D.L.) and from the U.S. Department of Energy (DOE) Office of Science, under Award # DE-SC0010472.

Author Contributions

D.L. and R.R.L. conceived the project and designed the experiments. D.L. fabricated and tested the films and devices. C.Y. assisted with measuring the PL data. M. B. assisted with device measurements. All authors contributed to the discussion of the data. D.L. and R.R.L wrote the manuscript.

Declaration of Interests

D.L. and R.R.L. have filed a patent application based on the work in this manuscript. All other authors declare no competing financial interest.

Published: August 31, 2018

Footnotes

Supplemental Information includes Transparent Methods, seven figures, one table and can be found with this article online at https://doi.org/10.1016/j.isci.2018.08.005.

Supplemental Information

Document S1. Transparent Methods, Figures S1–S7, and Table S1
mmc1.pdf (1.1MB, pdf)

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Supplementary Materials

Document S1. Transparent Methods, Figures S1–S7, and Table S1
mmc1.pdf (1.1MB, pdf)

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