The effects of high-temperature (500 °C) post-deposition annealing (PDA) on the properties of cesium lead bromide (CsPbBr3) films deposited by vacuum evaporation were studied. The PDA effectively improved the grain size of the CsPbBr3 films. This improvement of the grain size leads to the improvement of carrier diffusion length from 0.1 µm to 0.5 μm. A CsPbBr3 solar cell fabricated using a CsPbBr3 layer with PDA at 500 °C for 60 min showed a conversion efficiency of 6.62% (VOC = 1.465 V, JSC = 6.57 mA/cm2, and FF = 0.688). Our CsPbB3 solar cell also showed a conversion efficiency of 22.5% (VOC = 1.502 V, JSC = 53.7 mA/cm2, and FF = 0.574) for blue LED light (peak wavelength of 453 nm) with an intensity of 206 mW/cm2.

Optical wireless power transmission (OWPT) is one of the wireless power transmission techniques that uses a light source and an optical power converter.1 Semiconductor lasers or light emitting diodes (LEDs) are used for the light source, and solar cells are used for the optical power converter.2 In this application, the efficiency of the OWPT system is mainly determined by the product of the conversion efficiency of the light source and the conversion efficiency of the optical power converter. Therefore, the development of a high efficiency solar cell for monochromatic light is important. When a solar cell is illuminated with monochromatic light with a photon energy of Eph, the conversion efficiency η(Eph) can be expressed as follows:

ηEph=VOCEphJSCeFFF,

where F is the incident photon flux, VOC is the open circuit voltage of the solar cell, JSC is the short-circuit current density of the solar cell, FF is the fill factor of the solar cell, and e is the elementary charge. The first term on the right-hand side is referred to as voltage efficiency and the second term on the right-hand side is referred to as current efficiency that is equal to the external quantum efficiency (EQE) of the solar cell. According to the above equation, the upper limit of the voltage efficiency is obtained when Eph is equal to the bandgap (Eg) of the light absorption layer of the solar cell. The detailed balance theory reported by Shockley and Queisser indicates that VOC of the solar cell is an almost linear function of the Eg and VOC deficit (EgVOC) for an ideal solar cell that is about 0.25–0.3 V under the standard sunlight.3,4 Therefore, the maximum voltage efficiency, VOC/Eg, increases with an increase in Eg. This means that a solar cell using a wide-gap light absorbing material has a high potential for a high efficiency optical power converter. For wide-gap optical power converters, it is important to use short-wavelength monochromatic light. It is well known that blue light emitting diodes (LEDs) show a high wall-plug efficiency of above 80%.5 The wall-plug efficiency of blue lasers is limited to be about 40% at present, but there is room for improvement of the efficiency.6 This suggests that blue LEDs and lasers are some of the good candidates for the light source of OWPT. Therefore, it is important to develop an optical power converter using a wide bandgap material for a high efficiency OWPT system.

Cesium lead bromide (CsPbBr3) is a promising candidate for a light absorbing material for an optical power converter for OWPT. This material has been investigated as the top cell material of a four-terminal tandem solar cell since its large bandgap of approximately 2.3 eV.7 A further increase in the bandgap is possible by replacing some Br with Cl to obtain CsPbBrxCl3−x.8 CsPbBr3 is known as a higher stability material against moisture compared to other lead halide perovskite materials.9–12 Kulbak et al. demonstrated that a CsPbBr3 perovskite based solar cell has long-term stability under the relative humidity (RH) of 60%–70%.13 Solution-based processes by using lead bromide (PbBr2) and cesium bromide (CsBr) are widely used for CsPbBr3 film deposition14 Zhao et al. developed a CsPbBr3 solar cell with a conversion efficiency of 10.6%.15 Compared with the solution-based process, thermal evaporation is advantageous to obtain good film morphology.16 Post-deposition annealing is one of the key factors to obtain good CsPbBr3 films by using thermal evaporation of PbBr2 and CsBr.17 Zhang et al. obtained high quality CsPbBr3 films with grain size exceeding 2 μm for post-deposition annealing in RH = 30%.18 In previous reports, the temperature of the post-deposition annealing was less than 250 °C.14,19 According to Ref. 13, the sublimation temperatures of PbBr2, CsBr, and CsPbBr3 can be estimated to be approximately 500 °C, 700 °C, and 600 °C, respectively. Therefore, higher temperature annealing at about 500 °C is possible to apply for CsPbBr3. Higher annealing temperature (500 °C) has potential to improve the solar cell performance in terms of improvement of crystallinity.17 However, the effect of higher temperature annealing on the optoelectronic properties of CsPbBr3 films has not been investigated in detail. In this study, we investigated the effect of high temperature post-deposition annealing (PDA) on the properties of CsPbBr3 films deposited by the thermal evaporation method. We also discuss the potential of the CsPbBr3 solar cell as an optical power converter for OWPT.

CsPbBr3 films were deposited on a Corning 7059 glass substrate by sequential evaporation of PbBr2 and CsBr. The distance between the crucibles for the source materials and the substrate was approximately 35 cm. The back pressure was about 3 × 10−3 Pa. The Cs/Pb ratio of the films was controlled by changing the weight ratio of PbBr2 and CsBr sources that were placed in each crucible. The PbBr2 and CsBr at a stoichiometric molar ratio of 1:1 were used in this study. After film deposition, thermal annealing was carried out at 500 °C for 1–120 min in nitrogen (N2) ambient by using a hot plate. A clean glass sheet was placed on the hot plate, and the samples were placed with the film side down on the glass sheet to prevent sublimation of the deposited films. This annealing process is referred to as PDA. The total thickness of the PbBr2/CsBr stack layer was about 320 nm. The crystal structure and grain size were evaluated by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. Steady-State Photo-Conductivity (SSPC) measurements were carried out by using the films with coplanar Au electrodes. The width and gap of the electrodes were 3 mm and 0.2 mm, respectively. The carrier diffusion length of the CsPbBr3 films was estimated from SSPC measurements under monochromatic light with a wavelength of 400 nm. The chopped monochromatic light with a power density of 0.52 mW/cm2 was irradiated between the gap of the coplanar Au electrodes. Photoconductivity (σph) was measured by using a lock-in measurement system. The measured photoconductivity can be described by

σph=eμτFA/d,
(1)

where e is the elementary charge (1.60 × 10−19 C), μ is the mobility, τ is the excited carrier lifetime, F is photon flux (1.0 × 1015 cm−2 s−1), A is the absorbance of the films, and d is the thickness of the films. The value of A was estimated by transmittance and reflectance measurements. By applying Einstein’s relationship, Eq. (1) can be rearranged as follows:

L=(Dτ)1/2={(σphkBTd)/(e2FA)}1/2,
(2)

where L is the carrier diffusion length, kB is the Boltzmann constant, and T is the temperature (298 K).

We also fabricated CsPbBr3 solar cells by using a compact titanium oxide (c-TiO2) electron transport layer and a Poly(3-hexylthiophene-2,5-diyl) (P3HT) hole transport layer. The structure of the solar cell was glass/fluorine-doped tin oxide (FTO)/c-TiO2/CsPbBr3 (330 nm, 680 nm, 1020 nm, 1360 nm)/P3HT/Au. The sheet resistance of the glass/FTO substrate was 10 Ω−2. The c-TiO2 layer was spin-coated (4000 rpm for 20 s) by using a mixture of titanium butoxide (150 μl)/ethanol (3 ml)/HNO3 (0.1M, 150 μl). After spin-coating, the sample was annealed at 500 °C for 1 h in air. Subsequently, the substrate was immersed in an aqueous solution of 50 mM titanium chloride at 70 °C for 30 min and then annealed at 500 °C for 30 min in air. After the deposition and PDA of the CsPbBr3 layer, a P3HT layer was prepared by spin coating of a mixture of P3HT (5 mg)/chloroform (1 ml) and subsequently annealed at 180 °C for 10 min in N2 ambient. Finally, Au electrodes with an area of 0.09 cm2 were thermally evaporated. The fabricated solar cells were characterized by the illuminated current–voltage (IV) measurements under AM1.5 spectrum and external quantum efficiency (EQE) measurements.

Figure 1 shows the XRD patterns of the CsPbBr3 films on glass without and with PDA at 500 °C. The typical diffraction peaks of CsPbBr3 located at 2θ = 15.2° and 30.7° were observed. The peak intensity of the CsPbBr3 film with PDA at 500 °C is markedly larger than that of the CsPbBr3 film without PDA. The peak intensity reaches its maximum at 90 min and slightly decreased at 120 min. This suggests that longer time annealing enhances the crystallization, but simultaneously sublimation of CsPbBr3 occurred. The effect of PDA on the surface morphology of the CsPbBr3 film on glass was investigated by AFM, as shown in Fig. 2. The as-deposited CsPbBr3 film has a small grain size of approximately 200 nm. The PDA for 5 min increased the grain size from sub-micrometer order to micrometer order. Longer time PDA up to over 90 min further increases the grain size. In general, a large grain size is preferable for high-performance optoelectronic devices due to the reduction of the defective grain boundaries.

FIG. 1.

XRD patterns of the CsPbBr3 films on glass with and without post-annealing at 500 °C.

FIG. 1.

XRD patterns of the CsPbBr3 films on glass with and without post-annealing at 500 °C.

Close modal
FIG. 2.

AFM images of the (a) as-deposited film and 500 °C-annealed for (b) 1 min, (c) 5 min, (d) 10 min, (e) 30 min, (f) 60 min, (g) 90 min, and (h) 120 min CsPbBr3 films on glass.

FIG. 2.

AFM images of the (a) as-deposited film and 500 °C-annealed for (b) 1 min, (c) 5 min, (d) 10 min, (e) 30 min, (f) 60 min, (g) 90 min, and (h) 120 min CsPbBr3 films on glass.

Close modal

Figure 3 shows the carrier diffusion length of the CsPbBr3 films. The carrier diffusion length increased with an increase in the PDA time up to 90 min. The PDA for 90 min leads to the highest carrier diffusion length of 0.5 μm. According to the surface morphology shown in Fig. 2, this increase in the carrier diffusion length is attributed to the carrier mobility and/or suppression of the carrier recombination at the grain boundaries.

FIG. 3.

Carrier diffusion length of the as-deposited and 500 °C-annealed CsPbBr3 films estimated by photo-conductivity measurements.

FIG. 3.

Carrier diffusion length of the as-deposited and 500 °C-annealed CsPbBr3 films estimated by photo-conductivity measurements.

Close modal

Figure 4 shows the illuminated IV characteristics of the CsPbBr3 solar cells. To emphasize the effect of the PDA, we only show reverse scan curves of the champion cells for each annealing condition. Forward scan curves showed slightly worse photovoltaic performance than the corresponding reserve scan curves. The solar cell using CsPbBr3 without PDA showed very poor conversion efficiency less than 0.01%. On the other hand, the solar cells using CsPbBr3 with PDA clearly showed a substantial improvement of all solar cell parameters. These results demonstrate that the PDA can enhance the solar cell performance by increasing the carrier diffusion length. The solar cell with PDA for 60 min has a VOC of 1.388 V, a JSC of 5.52 mA/cm2, and a FF of 0.706, yielding a conversion efficiency of 5.41%. We also fabricated the solar cell with 90-min-PDA, but no photovoltaic performance was observed due to the many pinholes on the solar cell. When we characterized the electrical properties of CsPbBr3 films on a glass substrate, no pinholes were observed from the 90-min-PDA films. This suggests that the formation of pinholes was influenced by the substrate. In the case of PDA for 60 min, the use of the thicker CsPbBr3 layer (680 nm) improved the solar cell performance and we obtained a conversion efficiency of 6.62% (VOC = 1.465 V, JSC = 6.57 mA/cm2, and FF = 0.688). We also fabricated CsPbBr3 solar cells with thicker CsPbBr3 layers (1020 nm and 1360 nm). The conversion efficiency for the solar cell with a 1020-nm-thick CsPbBr3 layer showed a similar conversion efficiency of 6.10% (VOC = 1.428 V, JSC = 6.40 mA/cm2, and FF = 0.667); however, the conversion efficiency of the solar cell with a 1360-nm-thick CsPbBr3 layer deteriorated to 5.48% (VOC = 1.425 V, JSC = 5.73 mA/cm2, and FF = 0.671). The deterioration of the JSC for the solar cell with a thicker CsPbBr3 layer is attributed to the smaller carrier diffusion length. The diffusion length of the CsPbBr3 layer in the solar cell structure is discussed below.

FIG. 4.

IV curves of the champion cells of the conversion efficiencies with and without post-annealing at 500 °C.

FIG. 4.

IV curves of the champion cells of the conversion efficiencies with and without post-annealing at 500 °C.

Close modal

The EQE spectra of the champion cells plotted in Fig. 5 also indicate that the PDA drastically enhanced the carrier diffusion length in the CsPbBr3 layer. The EQE spectra were analyzed using e-ARC software version 2.0.20,21 We calculated EQE spectra of the CsPbBr3 solar cell by varying the effective carrier collection length (Lc)21 in the CsPbBr3 layer from 0.3 μm to 1.0 μm and infinite. The structure of the calculated device is the same as that of the fabricated devices. The optical properties of glass, FTO, c-TiO2, and Au layers were taken from the database of e-ARC. For CsPbBr3, we used the optical properties estimated by spectroscopic ellipsometry measurements. The optical properties of P3HT were taken from Ref. 22. The calculated EQE spectra are also shown in Fig. 5 using solid lines and broken lines. As indicated in Fig. 5, EQE in the range of 350 nm to 550 nm increases with increasing PDA time in the case of the solar cell using a 340-nm-thick CsPbBr3 layer. The calculated EQE using Lc of 0.3 µm and 0.4 μm well reproduces the measured EQE for the 30-min-PDA device and the 60-min-PDA device, respectively. These carrier collection lengths are very close to the measured carrier diffusion length of CsPbBr3 films. The calculated EQE for the devices with a carrier collection length of 1.0 μm and infinite suggest that further improvement of EQE is possible and a carrier collection length of about 1.0 μm is enough to obtain good EQE. According to these analyses, the JSC of our device is smaller than other reports,17,23,24 even though the VOC and FF are almost comparable. This small JSC is caused by an insufficient carrier diffusion length and a relatively thin CsPbBr3 layer thickness. The 60-min-PDA device with a 680-nm-thick CsPbBr3 layer showed improvement of EQE, as shown in Fig. 5(b), and this measured EQE was well reproduced by the calculated EQE using a Lc of 0.45 μm. This analysis also indicates that further improvement of EQE is possible to use the CsPbBr3 layer with a Lc of 1.0 μm. Therefore, optimization of the annealing condition for a larger carrier diffusion length would enhance solar cell performance, especially JSC.

FIG. 5.

(a) EQE spectra of the CsPbBr3 solar cells with and without PDA at 500 °C. The thickness of the CsPbBr3 was 340 nm. (b) EQE spectra of the CsPbBr3 solar cell with PDA at 500 °C. The thickness of the CsPbBr3 was 680 nm. The solid lines and broken lines indicate the calculated EQE spectra for different carrier collection length (Lc).

FIG. 5.

(a) EQE spectra of the CsPbBr3 solar cells with and without PDA at 500 °C. The thickness of the CsPbBr3 was 340 nm. (b) EQE spectra of the CsPbBr3 solar cell with PDA at 500 °C. The thickness of the CsPbBr3 was 680 nm. The solid lines and broken lines indicate the calculated EQE spectra for different carrier collection length (Lc).

Close modal

We also measured the I–V characteristics of CsPbBr3 solar cells under monochromatic light with a peak wavelength of 453 nm by using blue LED illumination. Due to the shunt path formation during the IV measurements under AM1.5 illumination before the measurements under the blue LED, the best CsPbBr3 solar cell with a 680-nm-thick CsPbBr3 layer showed deterioration of the VOC, as shown in Fig. 6. This problem of shunt path formation was not observed in solar cells with thicker CsPbBr3 layers, as shown in Fig. 6. The CsPbBr3 solar cell with a 1020-nm-thick CsPbBr3 layer showed a conversion efficiency of 22.5% (VOC = 1.502 V, JSC = 53.7 mA/cm2, and FF = 0.574) under blue LED light with an intensity of 206 mW/cm2. Higher light intensity of the blue LED illumination improved the VOC; however, FF deteriorated due to the effect of series resistance. Our theoretical calculation indicated that an ideal CsPbBr3 solar cell shows a conversion efficiency of about 61% for monochromatic light with a wavelength of 450 nm. Further improvements of device performance, especially VOC and FF by improving the electron and hole transport layer and reducing the series resistance, are important to improve the device performance.

FIG. 6.

Dependence of the device parameters on the illumination intensity of the blue LED. The device parameters of three solar cells with different CsPbBr3 layer thicknesses are summarized.

FIG. 6.

Dependence of the device parameters on the illumination intensity of the blue LED. The device parameters of three solar cells with different CsPbBr3 layer thicknesses are summarized.

Close modal

In conclusion, we investigated the effect of high-temperature (500 °C) PDA on the properties of CsPbBr3 thin films prepared by sequential PbBr2 and CsBr evaporation. We found that the grain size of CsPbBr3 films increased with an increase in the PDA time. A large grain with a size of above 2 μm was observed in the film with PDA at 500 °C for over 60 min. The carrier diffusion length of CsPbBr3 films was also increased by the PDA, and a carrier diffusion length of 0.5 μm was obtained under the best PDA condition. The fabricated CsPbBr3 solar cell using CsPbBr3 with PDA showed a conversion efficiency of 6.62% (VOC = 1.46 V, JSC = 6.57 mA/cm2, and FF = 0.688). The conversion efficiency for monochromatic light with a wavelength of 450 nm was estimated to be 29.1%. These results indicate that the present device does not have sufficient conversion efficiency for OWPT using blue laser or LED as a light source; however, there is plenty of room for improvement of the device performance.

This work was supported by the JSPS Grants-in-Aid for Scientific Research (KAKENHI) (Grant No. 17H03532).

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