**4.1 Photovoltaic cells**

The widely studied hybrid perovskite solar cells with high performance are usually made from polycrystalline films; however, the current studies have also focused on the developments and optimization of single crystal perovskite solar cells, owing to their significant advantages. Huang et al. fabricated photovoltaic devices based on MAPbI3 bulk crystals by depositing gold (Au) as anodes and gallium (Ga) as cathodes (**Figure 6a**) [63]. A red-shift of 50 nm of the EQE cutoff to 850 nm showed that MAPbI3 crystals increased the upper limit of short-circuit current density (*JSC*) compared with the polycrystalline solar cells from 27.5 mA/cm2 to 33.0 mA/cm2 . Notably, as compared with the perovskite polycrystalline solar cells, the bulk crystal devices showed much lower efficiency, which was attributed to the fact that photogenerated carriers could not be fully collected in a thick active layer. Much thinner MAPbBr3 monocrystalline films grown on indium tin oxide (ITO)-coated glass were applied into the solar cells, and the devices showed the best cell performance with a fill factor (*FF*) of 0.58, a *JSC* of 7.42 mA/cm2 , an open-circuit voltage (*VOC*) of

**111**

**4.2 Photodetectors**

**Figure 6.**

*[74], Copyright 2019.*

*4.2.1 In visible region*

are explained in **Table 1** briefly.

*Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives*

1.24 V, and a *PCE* of 5.37% (**Figure 6b**) [54]. To enhance the device performance, Huang's group further fabricated crystal solar cells through interface engineering

*6a, schematic of MAPbI3 crystal solar cell.* Science *[63], copyright 2015. 6b, dark and illuminated* J-V *curves of MAPbBr3 crystal solar cells with a device illustration in the inset. Adv. Mater. [54], Copyright 2016. 6c, device structure of single-crystal solar cells.* Nature Commun. *[72], Copyright 2017. 6d, schematic of MAPbI3 crystal solar cells with lateral structure.* Adv. Mater. *[73], Copyright 2016. 6e, cross-sectional SEM image of a MAPbI3 crystal device. 6f, statistical summary of photovoltaic parameters from 12 devices.* ACS Energy Lett*.* 

a *FF* of 74.1%, and a *PCE* of 16.1% [72]. The single crystal solar cell also displayed the better device stability of remaining nearly unchanged after storage in air for 30 days. In addition to the vertical-structured solar cells, Huang's group also fabricated the lateral structure perovskite crystal device (**Figure 6d**) [73], which showed a *VOC* of 0.82 V and the highest *PCE* of 5.36% at 170 K. More recently, a 20-μm MAPbI3 single crystal inverted *p-i-n* solar cell with a *PCE* as high as 21.09% and a *FF* up to 84.3% was fabricated [74], of which the cross-sectional SEM image and photovoltaic performance are shown in **Figure 6e** and **f**. To further realize the optimized performance of perovskite crystal solar cells, more efforts will be performed

Photodetectors which can convert incident light into electrical signals are critical

, a *VOC* of 1.06V,

(**Figure 6c**), of which the best device showed a *JSC* of 20.5mA/cm2

to enhance the sample quality and to design promising device structures.

for various industrial and scientific applications. To evaluate the photodetector performance, several parameters are important, including responsivity (*R*), detectivity (*D*\*), Gain (*G*), and linear dynamic range (*LDR*), which are listed and

Huang's group fabricated perovskite crystal photodetectors that exhibited a high sensitivity capacity, which led to a narrow-band photo-response with a full width at half maximum (FWHM) of less than 20 nm under V = −1 V (**Figure 7a**) [49]. *EQE* spectra of the single crystals showed a narrow peak near the absorption edge, which promised a detection application at a specific wavelength, with a peak *D*\* over 2 × 1010 Jones at 570 nm under V = −4 V (**Figure 7b**). Also, Huang et al. further fabricated vertical structured perovskite crystal photodetectors by using the non-wetting hole transport layer-coating substrates [75]. The noise currents are as low as 1.4 and 1.8 fA/Hz1/2 at an 8-Hz frequency for the devices based on MAPbBr3

*DOI: http://dx.doi.org/10.5772/intechopen.95046*

*Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives DOI: http://dx.doi.org/10.5772/intechopen.95046*

**Figure 6.**

*6a, schematic of MAPbI3 crystal solar cell.* Science *[63], copyright 2015. 6b, dark and illuminated* J-V *curves of MAPbBr3 crystal solar cells with a device illustration in the inset. Adv. Mater. [54], Copyright 2016. 6c, device structure of single-crystal solar cells.* Nature Commun. *[72], Copyright 2017. 6d, schematic of MAPbI3 crystal solar cells with lateral structure.* Adv. Mater. *[73], Copyright 2016. 6e, cross-sectional SEM image of a MAPbI3 crystal device. 6f, statistical summary of photovoltaic parameters from 12 devices.* ACS Energy Lett*. [74], Copyright 2019.*

1.24 V, and a *PCE* of 5.37% (**Figure 6b**) [54]. To enhance the device performance, Huang's group further fabricated crystal solar cells through interface engineering (**Figure 6c**), of which the best device showed a *JSC* of 20.5mA/cm2 , a *VOC* of 1.06V, a *FF* of 74.1%, and a *PCE* of 16.1% [72]. The single crystal solar cell also displayed the better device stability of remaining nearly unchanged after storage in air for 30 days.

In addition to the vertical-structured solar cells, Huang's group also fabricated the lateral structure perovskite crystal device (**Figure 6d**) [73], which showed a *VOC* of 0.82 V and the highest *PCE* of 5.36% at 170 K. More recently, a 20-μm MAPbI3 single crystal inverted *p-i-n* solar cell with a *PCE* as high as 21.09% and a *FF* up to 84.3% was fabricated [74], of which the cross-sectional SEM image and photovoltaic performance are shown in **Figure 6e** and **f**. To further realize the optimized performance of perovskite crystal solar cells, more efforts will be performed to enhance the sample quality and to design promising device structures.

#### **4.2 Photodetectors**

Photodetectors which can convert incident light into electrical signals are critical for various industrial and scientific applications. To evaluate the photodetector performance, several parameters are important, including responsivity (*R*), detectivity (*D*\*), Gain (*G*), and linear dynamic range (*LDR*), which are listed and are explained in **Table 1** briefly.

#### *4.2.1 In visible region*

Huang's group fabricated perovskite crystal photodetectors that exhibited a high sensitivity capacity, which led to a narrow-band photo-response with a full width at half maximum (FWHM) of less than 20 nm under V = −1 V (**Figure 7a**) [49]. *EQE* spectra of the single crystals showed a narrow peak near the absorption edge, which promised a detection application at a specific wavelength, with a peak *D*\* over 2 × 1010 Jones at 570 nm under V = −4 V (**Figure 7b**). Also, Huang et al. further fabricated vertical structured perovskite crystal photodetectors by using the non-wetting hole transport layer-coating substrates [75]. The noise currents are as low as 1.4 and 1.8 fA/Hz1/2 at an 8-Hz frequency for the devices based on MAPbBr3


#### **Table 1.**

*Parameters for evaluating the perovskite single crystal photodetectors.*

#### **Figure 7.**

*7a, schematic of device structure. 7b,* D*\* spectrum and total noise at* −*4 V.* Nature Photonics *[49], copyright 2015. 7c, illustration of planar-integrated MAPbBr3 photodetector.* Nature Commun. *[42], Copyright 2015. Photograph of* ≈*100 photodetectors on a perovskite crystal wafer (7d) and the* R *values (7e).* Adv. Mater. *[55], Copyright 2016.*

and MAPbI3, respectively. Additionally, the photocurrent responses of both the MAPbBr3 and MAPbI3 devices were linear, and their *LDR*s are up to 256 and 222 dB, respectively. Sun's group introduced a planar-type photodetector on the MAPbI3 crystal (001) facet with a highest *R* value of 953A/W and *EQE* of 2.22 × 105 % at a light power density of 2.12nW/cm<sup>2</sup> [76]. Wei's group used a two-step method to fabricate a self-powered photodetector based on a MAPbBr3 crystal core-shell heterojunction [77]. The device showed a broad photo-response ranging from 350 to 800 nm and a peak *R* up to 11.5 mA/W. Hu's group fabricated photodetectors based on MAPbI3 single crystal nanowires and nanoplates by transferring them to SiO2/ Si slides [78]. The highest On/Off ratio approached 103 under a light illumination of 73.7 mW/cm2 .

**113**

*Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives*

the fabricated photodetector possessed a high *G* (above 104

tals [79], which displayed the *R* as high as 1.6 × 107

density of a 385-nm laser (**Figure 8d**) [85].

wavelength as long as 1064 nm (**Figure 8f**).

with a sensitivity of 80 μC/Gyaircm2

high potential for practical applications.

*4.2.3 In near-infrared (NIR) region*

808-nm laser (∼10 mW/cm2

*4.2.4 In X-ray region*

*4.2.2 In ultraviolet (UV) region*

Although perovskite crystal photodetectors have shown better performance, macroscopic crystals cannot be grown on a planar substrate, restricting their potential for device integration. To overcome this shortcoming, Bakr et al. grew large-area planar-integrated crystal films onto the ITO-patterned substrates (**Figure 7c**) [42], and

fabricated a photodetector based on a thin perovskite crystal wafer by the space-limited crystallization method, which has about 100 pairs of interdigitated Au wire electrodes (**Figure 7d**) [55], and the *R* increased linearly as the radiance intensity decreased (**Figure 7e**). Moreover, Su's group sputtered the thin Au electrodes on a large-area MAPbBr3 thin crystal to fabricate a narrowband photodetector [56]. Furthermore, Ma's group reported the superior-performance photodetectors based on MAPbBr3 thin crys-

UV detection is a key technology in the fields of flame detection [80], remote security monitoring [81], environmental monitoring [82], and so forth. Researchers have endeavored to develop UV photodetectors based on perovskite crystals considering their excellent UV absorption properties. Visible-blind UV photodetectors based on MAPbCl3 crystals a suitable bandgap of about 3.11 eV were fabricated (**Figure 8a**) [60], and the device showed the dark current as low as 4.15 × 10−7 A at 15 V and a drastically high stability (**Figure 8b**). Planar-integrated MAPbCl3 crystal UV photodetectors on ITO-deposited glass substrate were reported by Sargent et al. (**Figure 8c**) [83], which showed decreased *R* and *G* values as increased power

NIR photodetectors have widespread uses in telecommunications [86], as well as thermal and biological imaging [87–90]. Meredith's group demonstrated the perovskite crystal that overcame the large bandgap and presented photodetectors with performance metrics appropriate for NIR detection by using the trap-related linear sub-gap absorption (**Figure 8e**) [84]. A strong NIR photo-response was achieved in photodiodes based on MAPbI3 crystals illuminated by a continuous

In addition to the common light detections from UV to IR, perovskite crystals have been employed for the detection of X-rays, which have important applications in medical diagnostics, clinical treatment, and the non-destructive testing of products [53]. Huang et al. fabricated a sensitive MAPbBr3 crystal X-ray detector with the structure of Au/MAPbBr3/crystal/C60/BCP/Ag or Au (**Figure 9a**) [53]. Through reducing the bulk defects and passivating surface traps, the devices showed a detection efficiency of 16.4% at a near zero bias under irradiation with continuum X-ray energy up to 50 keV. The lowest detectable X-ray dose rate was 0.5 μGyair/s

achieved in *α*-Se-based X-ray detectors (**Figure 9b**). An X-ray detector based on *p-i-n* diode array made of a thick MAPbBr3 single crystal was introduced by Chen's

group [94], which displayed the highest sensitivity of 23.6μC/mGyaircm2

). The photodiodes could also respond to a laser with a

, which is four times higher than the sensitivity

, indicating

Hz) relative to other perovskite devices. Furthermore, Liu's group

) and a high gain-bandwidth

.

A/W and the highest *G* up to 5 × 107

*DOI: http://dx.doi.org/10.5772/intechopen.95046*

product (above 108

#### *Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives DOI: http://dx.doi.org/10.5772/intechopen.95046*

Although perovskite crystal photodetectors have shown better performance, macroscopic crystals cannot be grown on a planar substrate, restricting their potential for device integration. To overcome this shortcoming, Bakr et al. grew large-area planar-integrated crystal films onto the ITO-patterned substrates (**Figure 7c**) [42], and the fabricated photodetector possessed a high *G* (above 104 ) and a high gain-bandwidth product (above 108 Hz) relative to other perovskite devices. Furthermore, Liu's group fabricated a photodetector based on a thin perovskite crystal wafer by the space-limited crystallization method, which has about 100 pairs of interdigitated Au wire electrodes (**Figure 7d**) [55], and the *R* increased linearly as the radiance intensity decreased (**Figure 7e**). Moreover, Su's group sputtered the thin Au electrodes on a large-area MAPbBr3 thin crystal to fabricate a narrowband photodetector [56]. Furthermore, Ma's group reported the superior-performance photodetectors based on MAPbBr3 thin crystals [79], which displayed the *R* as high as 1.6 × 107 A/W and the highest *G* up to 5 × 107 .

### *4.2.2 In ultraviolet (UV) region*

UV detection is a key technology in the fields of flame detection [80], remote security monitoring [81], environmental monitoring [82], and so forth. Researchers have endeavored to develop UV photodetectors based on perovskite crystals considering their excellent UV absorption properties. Visible-blind UV photodetectors based on MAPbCl3 crystals a suitable bandgap of about 3.11 eV were fabricated (**Figure 8a**) [60], and the device showed the dark current as low as 4.15 × 10−7 A at 15 V and a drastically high stability (**Figure 8b**). Planar-integrated MAPbCl3 crystal UV photodetectors on ITO-deposited glass substrate were reported by Sargent et al. (**Figure 8c**) [83], which showed decreased *R* and *G* values as increased power density of a 385-nm laser (**Figure 8d**) [85].

#### *4.2.3 In near-infrared (NIR) region*

NIR photodetectors have widespread uses in telecommunications [86], as well as thermal and biological imaging [87–90]. Meredith's group demonstrated the perovskite crystal that overcame the large bandgap and presented photodetectors with performance metrics appropriate for NIR detection by using the trap-related linear sub-gap absorption (**Figure 8e**) [84]. A strong NIR photo-response was achieved in photodiodes based on MAPbI3 crystals illuminated by a continuous 808-nm laser (∼10 mW/cm2 ). The photodiodes could also respond to a laser with a wavelength as long as 1064 nm (**Figure 8f**).

#### *4.2.4 In X-ray region*

In addition to the common light detections from UV to IR, perovskite crystals have been employed for the detection of X-rays, which have important applications in medical diagnostics, clinical treatment, and the non-destructive testing of products [53]. Huang et al. fabricated a sensitive MAPbBr3 crystal X-ray detector with the structure of Au/MAPbBr3/crystal/C60/BCP/Ag or Au (**Figure 9a**) [53]. Through reducing the bulk defects and passivating surface traps, the devices showed a detection efficiency of 16.4% at a near zero bias under irradiation with continuum X-ray energy up to 50 keV. The lowest detectable X-ray dose rate was 0.5 μGyair/s with a sensitivity of 80 μC/Gyaircm2 , which is four times higher than the sensitivity achieved in *α*-Se-based X-ray detectors (**Figure 9b**). An X-ray detector based on *p-i-n* diode array made of a thick MAPbBr3 single crystal was introduced by Chen's group [94], which displayed the highest sensitivity of 23.6μC/mGyaircm2 , indicating high potential for practical applications.

#### **Figure 8.**

*8a, device architecture of MAPbCl3 crystal photodetector. 8b,* I-V *curves of the photodetector under UV light (*λ *= 365 nm) and in the dark.* J. Phys. Chem. Lett. *[60], Copyright 2015. 8c, schematic of planar-integrated MAPbCl3 UV-detectors. 8d,* R *and* G *values vs. incident light power.* Adv. Mater*. [83], Copyright 2016. 8e, sub-gap electron trap state absorptions. 8f,* R *values of MAPbI3 photo-resistors under the illumination above the gap (visible, 600 nm) and below the gap (NIR, 900 nm).* Laser Photonics Rev. *[84], copyright 2016.*

#### **Figure 9.**

*9a, structure of MAPbBr3 crystal X-ray detector. 9b, X-ray-generated photocurrent at various dose rates.*  Nature Photonics *[53], copyright 2016. 9c, attenuation coefficient and penetration depth of MAPbI3 and CdTe. 9d, photocurrent and a fit with Hecht model generated by Cu* Kα *X-ray radiation (8 keV) in a MAPbI3 crystal.* Nature Photonics *[91], copyright 2016. 9e, pictures of guard ring electrode side, anode side and side view of a MAPbBr2.94Cl0.06 crystal detector.* **9f***, 137Cs energy spectrum obtained by crystal, CZT and NaI (Tl) detectors.* Nature Mater. *[92], Copyright 2017. 9 g, schematic of a Schottky-type MAPbI3 detector with asymmetrical electrode and the energy level diagram. Energy-resolved spectrum by Schottky-type MAPbI3 detector (9 h) under 241Am 59.5 keV* γ*-ray under* −*50 V and (9i) under 57Co 122 keV* γ*-ray under* −*70 V.* ACS Photonics *[93], copyright 2018.*

#### *4.2.5 In gamma-ray (γ-ray) region*

Similar to X-ray detectors, the *γ*-ray detectors are also widely used in many fields, owing to the non-invasive detections. However, *γ*-ray detectors need to work in a weak radiation field pulse mode and perform event-by-event detections to sort out the intensity vs. the energy of the radiation quanta. Large and balanced *μ* and *τ* are needed for high-energy detection. Huang et al. reported high-quality MAPbI3 crystals that were applied to *γ*-ray detection with a 4% efficiency when operating in the *γ*-voltaic mode [63]. Kovalenko et al. demonstrated MAPbI3 crystals for *γ*-ray detection (**Figure 9c**), and a 59.6 keV 241Am energy spectrum was acquired [91]. A fit of bias dependence of photocurrent with Hecht model indicated a high *μτ* product of ∼10−2 cm2 /V (**Figure 9d**) [95, 96].

**115**

**Figure 10.**

cm2

*Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives*

crystal process to fabricate a low-cost *γ*-ray detector [92]. MAPbBr2.94Cl0.06 crystals with a larger *μτ* product were equipped with a guard ring electrode to mitigate their leakage current (**Figure 9e**). The 137Cs energy spectrum obtained by such crystals with a full-energy peak resolution of 6.50% is compared with the spectrum obtained by CZT and NaI(Tl) detectors (**Figure 9f**). A high-performance MAPbI3 crystal *γ*-ray spectrometer was designed by Kanatzidis et al. [93], and the asymmetrical electrodes (Schottky-type) were applied to prohibit the hole injection from the anode or to reduce the leakage current (**Figure 9g**). The best energy resolution of the device for 241Am 59.5 keV *γ*-rays was ∼12%; while the best energy resolution

With the exceptional PL efficiency and high color purity, perovskite crystals can also perform as high-performance LEDs [97]. Most of the existing perovskite LEDs employ a polycrystalline film with sizes of nanometers to micrometers, and coherent light emission is a challenge [98]. In Yu's work, the LEDs with the structure of ITO/MAPbBr3 micro-platelet/Au cathode had the turn-on voltage of about 1.8 V

The excellent properties, including a small trap density, long lifetime and electron–hole diffusion length, and large carrier mobility, also make perovskite crystals suitable for laser devices with low lasing thresholds and high qualities. Xiong's group grew typical MAPbI3 triangular nano-platelets and optically pumped them by a femtosecond-pulsed laser (**Figure 10b**) [100], and the peaks centered at *λ* = 776.7, 779.2, 781.9, 784.3, and 786.8 nm appeared over the spontaneous emission band with a FWHM of ∼1.2 nm (**Figure 10c**), when the pump fluence was increased to 40.6 μJ/

. Zhu et al. demonstrated room-temperature lasing via using MAPbI3 crystal nanowire, which had a broad tenability covering the NIR to visible region [101].

*10a, light emission intensity vs. time of a perovskite LED at* −*193°C. inset: A microscopic image at t = 12 h.* ACS Nano *[99], copyright 2018. 10b, schematic for optical setup of a CH3NH3PbI3 nanoplatelet. 10c, evolution from spontaneous emission to lasing in a typical CH3NH3PbI3 nanoplatelet. Inset left: Optical image of a nanoplate and plot of integrated* P*out. Inset right: PL decay curve below (pink) and above (dark green) the threshold.*  Nano Lett. *[100], Copyright 2014. 10d, nanowire emission spectra. Inset: Integrated emission intensity and FWHM vs.* P*.* Nature Mater*. [101], Copyright 2015. 10e, integrated PL intensity as a function of excitation density.* Adv. Mater. *[102], Copyright 2015. 10f, emission spectra of perovskite microplates excited by different* 

*pump densities. Inset: Integrated PL intensity vs. pump density.* Adv. Mater*. [103], Copyright 2016.*

dopant compensation of MAPbBr3 single

(**Figure 10a**) [99].

*DOI: http://dx.doi.org/10.5772/intechopen.95046*

Huang's group further reported a Cl−

achieved for 57Co 122 keV was 6.8% (**Figure 9h** and **i**).

and could last for at least 54 h with a luminance of ∼5000 cd/m2

**4.3 Light-emitting diodes (LEDs) and lasers**

*Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives DOI: http://dx.doi.org/10.5772/intechopen.95046*

Huang's group further reported a Cl− dopant compensation of MAPbBr3 single crystal process to fabricate a low-cost *γ*-ray detector [92]. MAPbBr2.94Cl0.06 crystals with a larger *μτ* product were equipped with a guard ring electrode to mitigate their leakage current (**Figure 9e**). The 137Cs energy spectrum obtained by such crystals with a full-energy peak resolution of 6.50% is compared with the spectrum obtained by CZT and NaI(Tl) detectors (**Figure 9f**). A high-performance MAPbI3 crystal *γ*-ray spectrometer was designed by Kanatzidis et al. [93], and the asymmetrical electrodes (Schottky-type) were applied to prohibit the hole injection from the anode or to reduce the leakage current (**Figure 9g**). The best energy resolution of the device for 241Am 59.5 keV *γ*-rays was ∼12%; while the best energy resolution achieved for 57Co 122 keV was 6.8% (**Figure 9h** and **i**).

#### **4.3 Light-emitting diodes (LEDs) and lasers**

With the exceptional PL efficiency and high color purity, perovskite crystals can also perform as high-performance LEDs [97]. Most of the existing perovskite LEDs employ a polycrystalline film with sizes of nanometers to micrometers, and coherent light emission is a challenge [98]. In Yu's work, the LEDs with the structure of ITO/MAPbBr3 micro-platelet/Au cathode had the turn-on voltage of about 1.8 V and could last for at least 54 h with a luminance of ∼5000 cd/m2 (**Figure 10a**) [99].

The excellent properties, including a small trap density, long lifetime and electron–hole diffusion length, and large carrier mobility, also make perovskite crystals suitable for laser devices with low lasing thresholds and high qualities. Xiong's group grew typical MAPbI3 triangular nano-platelets and optically pumped them by a femtosecond-pulsed laser (**Figure 10b**) [100], and the peaks centered at *λ* = 776.7, 779.2, 781.9, 784.3, and 786.8 nm appeared over the spontaneous emission band with a FWHM of ∼1.2 nm (**Figure 10c**), when the pump fluence was increased to 40.6 μJ/ cm2 . Zhu et al. demonstrated room-temperature lasing via using MAPbI3 crystal nanowire, which had a broad tenability covering the NIR to visible region [101].

#### **Figure 10.**

*10a, light emission intensity vs. time of a perovskite LED at* −*193°C. inset: A microscopic image at t = 12 h.* ACS Nano *[99], copyright 2018. 10b, schematic for optical setup of a CH3NH3PbI3 nanoplatelet. 10c, evolution from spontaneous emission to lasing in a typical CH3NH3PbI3 nanoplatelet. Inset left: Optical image of a nanoplate and plot of integrated* P*out. Inset right: PL decay curve below (pink) and above (dark green) the threshold.*  Nano Lett. *[100], Copyright 2014. 10d, nanowire emission spectra. Inset: Integrated emission intensity and FWHM vs.* P*.* Nature Mater*. [101], Copyright 2015. 10e, integrated PL intensity as a function of excitation density.* Adv. Mater. *[102], Copyright 2015. 10f, emission spectra of perovskite microplates excited by different pump densities. Inset: Integrated PL intensity vs. pump density.* Adv. Mater*. [103], Copyright 2016.*

From **Figure 10d**, a sharp peak appeared at 787 nm in the representative emission spectra and grew rapidly with increasing the pump laser fluence (*P*) with the lasing threshold *P*Th of ∼595 nJ/cm2 . Additionally, MAPbBr3 crystal square micro-disks were synthesized into a 557-nm single-mode laser based on a built-in whispering gallery mode micro-resonator by Fu's group [102], from which a *P*Th = 3.6 μJ/cm2 was observed, and a sublinear regime was observed below the threshold (**Figure 10e**). Uniform-sized MAPbBr3 microplates were also created by Jiang et al. by using "liquid knife" and were made into lasers [103]. A 400-nm pulsed laser beam was used as a pump source to excite microplates, and a spontaneous emission peak centered at 550 nm with a FWHM of 20 nm was observed (**Figure 10f**).
