Hysteresis-Less CsPbI2Br Mesoscopic Perovskite Solar Cells with a

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Hysteresis-less CsPbI2Br mesoscopic perovskite solar cells with high open circuit voltage exceeding 1.3 V and 14.86 % of power conversion efficiency Do Hun Kim, Jin Hyuck Heo, and Sang Hyuk Im ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03413 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Applied Materials & Interfaces

Hysteresis-less CsPbI2Br Mesoscopic Perovskite Solar Cells with High Open Circuit Voltage Exceeding 1.3 V and 14.86 % of Power Conversion Efficiency Do Hun Kim,† Jin Hyuck Heo,† and Sang Hyuk Im*,†

†Department

of Chemical and Biological Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul 136-713, Republic of Korea

KEYWORDS. cesium lead halide, perovskite, solar cells, high open circuit voltage,

thermal stability

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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

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ABSTRACT

A High performance and hysteresis-less mesoscopic CsPbI2Br perovskite solar cells (PSCs) are demonstrated by adapting hole transporting materials (HTMs) with

controlled highest occupied molecular orbital (HOMO) values. The used model HTMs

are poly-3-hexylthiophene (P3HT), poly-triarylamine (P-TAA), poly-fluoren-8-triarylamine

(PF8-TAA), and poly-indenofluoren-8-triarylamine (PIF8-TAA) and their HOMO energy

levels position to -4.98, -5.09, -5.45, and -5.52 eV, respectively. By controlling the

HOMO of the HTMs, the average open-circuit voltages of 25 mesoscopic CsPbI2Br PSCs are controllable from 1.11 ± 0.030 V for P3HT HTM-based device to 1.17 ± 0.023,

1.21 ± 0.027, and 1.27 ± 0.028 V for P-TAA, PF8-TAA, and PIF8-TAA HTM-based

device. As a result, the PIF8-TAA HTM-based mesoscopic PSC exhibits the highest


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open-circuit voltage of 1.31 V and power conversion efficiency (PCE) of 14.20 % for

forward scan condition and 14.86 % for reverse scan condition under 1 sun illumination (100 mW/cm2 AM1.5G). In addition, the un-encapsulated mesoscopic CsPbI2Br PSCs exhibited 10-14 % of PCE degradation compared to its initial efficiency in maximum power point tracking under continuous 1 sun light soaking at 85 oC for 1000 h.

1. Introduction


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Solar energy has been recognized as renewable, abundant, safe, and sustainable

energy resource compared with fossil fuel. Hence, wealth of studies has been done to

generate electricity, which is the most convenient and safe energy form in modern life,

from the solar energy. Accordingly commercialized crystalline Si solar cells have

significantly contributed to generate electricity. However, their power generation cost is

still more expensive than that of fossil fuel because they require expensive vapor

process to obtain high purity silicon and high energy consumption to make single

crystalline silicon.

Recently, solution processable next generation solar cells with high efficiency and low

processing cost have been of great interest because they can expedite to reach grid

parity. Among the next generation solar cells such as organic photovoltaics, dye-

sensitized solar cells, quantum dots (QDs) solar cells, metal chalcogenide solar cells,

and metal halide perovskite solar cells, the perovskite solar cells (PSCs) have

considered as a promising candidate for next generation solar cell because they have

excellent properties such as high absorptivity, convenient bandgap tunability, small


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exciton binding energy, and solution processability. Since Kojima et al. reported a liquid junction CH3NH3PbI3 (MAPbI3) and MAPbBr3 PSC,1 the record efficiency of PSC have reached over 23 % for single junction and over 28 % for tandem solar cell.2 Namely, the

researches of PSCs have focused on the single junction solar cells such as rigid solar

cells, flexible solar cells, semi-transparent solar cells including building and vehicle

integrated photovoltaics and the tandem solar cells.

Except the efficiency of PSCs, the stability of metal halide perovskite materials against

heat, air, and humidity is challenging issue to find commercialization. Mixed halide perovskites3-7 such as MAPbI3-xBrx, MAPbI3-xClx, and CH(NH2)2PbI3-xBrx (FAPbI3-xBrx), mixed cation halide perovskites8-10 such as FA1-xMAxPbI3-yBry and FA1-x-yMAxCsyPbI3zBrz,

and all-inorganic metal halide perovskites11-14 such as G' $3( 3, CsPbI3-xBrx,

Cs3Bi2I9, and CsSnI3 have been intensively studied. If we just take the tandem device and stability issue into account, the CsPbI3-xBrx (x = 2) all-inorganic mixed metal halide perovskite is a good candidate because of its proper band gap and good stability.


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Sutton et al. reported that the 9.8 % CsPbI2Br normal (n-i-p) type planar-structured PSC comprised of glass/F-doped tin oxide (FTO)/blocking TiO2 (bl-TiO2)/CsPbI2 $JF F' 4%'

8

5

K'; ;I'$

(

@1 1I + +I'

(spiro-OMeTAD)/Ag with

significant current density-voltage (J-V) hysteresis with respect to scan direction.15 Nam

et al. reported 10.7 % n-i-p type CsPbI2Br PSC by optimizing thermal annealing temperature of perovskite film to 280 oC.16 Bai et al. improved the n-i-p type CsPbI2Br PSC efficiency to 13.47 % by doping the CsPbI2Br perovskite film with MnCl2 and introducing CsPbI2Br QDs interlayer between CsPbI2Br and poly-triarylamine (PTAA) for efficient hole extraction.17 Yin et al. reported a 14.78 % record efficiency of n-i-p type

CsPbI2Br PSC by forming dense uniform perovskite film through PbI2(DMSO) and PbBr2(DMSO) precursor.18 Bai et al. reported a 14.81 % of n-i-p type the CsPbI2Br PSC formed on anti-reflection coated glass/FTO substrate. In contrast, the inverted (p-i-n)

type CsPbI2Br PSCs adapting NiOx hole transporting layer (HTL) exhibited lower efficiency (~13.7 %) than the n-i-p type ones.19-21 In addition, the n-p heterojunction type

CsPbI2Br PSCs composed of FTO/electron transporting layer (ETL) such as TiO2 or SnO2/CsPbI2Br/carbon with ~10 % efficiency were also reported.22,23


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So far, most researches of CsPbI2Br PSCs have focused on the formation of uniform CsPbI2Br perovskite film, interface engineering, and ETL for n-i-p type planar structured solar cells, which are more difficult to reduce J-V hysteresis with respect to the scan direction due to the unbalanced electron flux and hole flux.24-29 Here, we fabricated the

n-i-p type hysteresis-less mesoscopic CsPbI2Br PSCs composed of FTO/blTiO2/mesoscopic TiO2 (m-TiO2)/CsPbI2Br/HTL/Au as a basic model device and enhanced the open-circuit voltage of the mesoscopic CsPbI2Br PSCs by adapting hole transporting materials (HTMs) with different highest occupied molecular orbital (HOMO)

values.

2. Experimental section

2.1. Preparation of CsPbI2Br perovskite solution


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For preparing CsPbI2Br perovskite solution, 1 M CsPbI2Br was prepared by mixing CsBr (Sigma-Aldrich) and PbI2 (Sigma-Aldrich) (1:1 M ratio) in dimethyl sulfoxide (DMSO) at 60 oC until completely dissolved.

2.2. Device fabrication

For mesoscopic CsPbI2Br perovskite solar cells device fabrication, we deposited ~50 nm-thick TiO2 blocking layer on a cleaned F-doped tin oxide (FTO, Pilkington, TEC8) by SPD (spray pyrolysis deposition) method with 20 mM of TAA (titanium diisopropoxide

bis(acetylacetate)) (Sigma-Aldrich) solution at 500

oC.

Mesoporous TiO2 (m-TiO2)

electron conductor with ~500 nm-thickness was deposited by spin coating and subsequent calcination at 500 oC for 1h. CsPbI2Br perovskite solution was spin-coated on the m-TiO2/bl-TiO2/FTO substrates at 2500 rpm for 300 s and dried on a hotplate at 100, 150

oC

for 5 min respectively. Poly-triarylamine (PTAA, EM index) hole-

transporting materials (HTMs) were spin-coated on CsPbI2Br/m-TiO2/bl-TiO2/FTO


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substrate at 4000 rpm for 30 s by using HTM/toluene (15 mg/ 1 mL) with 7.5 OL Li-

bis(trifluoromethanesulfonyl) imide (Li-TFSI, Sigma-Aldrich)/Acotonitrile (170 mg/ 1mL)

and 7.5 OL tert-butylpyridin (t-BP, Sigma-Aldrich)/Acetonitrile (1 mL/ 1 mL) additives.

Finally, Au counter electrode was deposited by thermal evaporation.

2.3. Materials characterization

Absorption data of the CsPbI2Br films were measured with a Shimadzu model UV 3600

Plus, TRPL characteristics were measured by a ChronosBH-ISS technology measuring

equipment. Morphological images of the surface and cross-section of devices were

obtained by field emission scanning microscopy (FE-SEM, FEI, Quanta 250 FEG). The

crystallographic structure and crystallinity were analyzed by X-ray diffraction (XRD,

Rigaku, Smartlab). The UPS measurement were performed with a photoelectron

spectrometer (Kratos Inc., AXIS-Ultra DLD). The setup was equipped with helium

discharge lamp with excitation energt of 21.2 eV. Photo electron spectroscopy in air


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(PESA) measurements were recorded with RikenKeiki AC-2 surface analyzer with a 10

nW of UV source. The samples for PESA were prepared on a glass substrate with the

same condition to that used for CsPbI2Br and HTM fabrication in solar cell devices.

2.4. Device characterization

The external quantum efficiency (EQE) was measured by a power source (ABET, 150W

Xenon lamp, 13014) with a monochromator (Dongwoo Optron Co., Ltd., MonoRa500i)

and a potentiostat (Ivium, IviumStat). The current density-voltage (J-V) curves were

measured by a solar simulator (Peccell, PEC-L01) with a potentiostat (IVIUM, IviumStat)

at under illumination of 1 Sun (100 mW/cm 2 AM 1.5G) and a calibrated Si-reference

cell certificated by JIS (Japanese Industrial Standards). The J-V curves of all devices

were measured by masking the active area with a metal mask having open hole of 0.096 cm2 .


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3. Results and discussion

Optical and electronic properties of CsPbI2Br perovskite material were analyzed as shown in Figure 1. For this, we prepared a 1 M of CsPbI2Br perovskite solution by dissolving equimolar CsBr (3 mmol) and PbI2 (3 mmol) in 3 ml of dimethyl sulfoxide (DMSO) solvent as shown in Figure 1(a). The CsPbI2Br perovskite solution was then spin-coated on a cleaned glass substrate at 2500 rpm for 300 s and consecutively heattreated it on a hot plate at 100, 150 oC for 5 min. A scanning electron microscopy (SEM)

surface image of as-prepared CsPbI2Br perovskite film in Figure 1(b) indicates that the perovskite film has even surface without pin-holes and relatively rough surface. An

atomic force microscopy (AFM) topology image in Figure S1 indicates the root mean

square (rms) roughness of perovskite film is 20.4 nm. A photograph of as-prepared

CsPbI2Br perovskite film was shown in an inset of Figure 1(c). An X-ray diffraction (XRD) pattern of the perovskite in Figure 1(c) confirms that the CsPbI2Br perovskite has cubic crystalline phase. UV-visible absorption spectrum of the perovskite film in Figure 1(d)

and Tauc plot in a corresponding inset graph indicate that the on-set absorption band


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edge and extrapolated optical bandgap energy of the CsPbI2Br perovskite is ~660 nmwavelength and ~1.92 eV, respectively. To check the electronic energy state, we

conducted an ultraviolet photoelectron spectroscopy (UPS) measurement as shown in

Figure 1(e) and (f). The zoom-up photoemission spectrum confirms that the onset

photoemission and valence band edge is 17.36 eV and 1.84 eV, respectively.

Accordingly, a calculated valence band maximum (VBM) energy (EVBM = Q - Ecut-off + EVB, where Q = 21.22 eV for He I, Ecut-off = 17.36 eV, and

EVB = 1.84 eV) of the

CsPbI2Br is 5.7 eV.

Figure 2(a) is a schematic device structure of mesoscopic CsPbI2Br PSC composed of FTO/bl-TiO2/m-TiO2/CsPbI2Br/HTMs with different HOMO values/Au and corresponding energy band diagram. Upon illumination of light, electron-hole pairs are generated in the

CsPbI2Br perovskite and most electrons (holes) generated in the perovskite are promptly transferred/transported into TiO2 ETL (HTM). Some electrons can be directly transported through the CsPbI2Br perovskite itself infiltrated in m-TiO2 layer and are then transferred/transported into bl-TiO2 ETL.25 The electrons (holes) are then moved


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into FTO (Au) electrode. By connecting the FTO and Au electrode with external circuit,

the electrons generated by solar cell can be continuously flowed through the external

circuit. Figure S2 is the SEM cross-sectional images and top-surface images of FTO/bl-

TiO2/mesoscopic TiO2/CsPbI2Br/HTMs with different HOMO values indicating that the thickness of each layer is ~500 nm for FTO, ~50 nm for bl-TiO2, ~500 nm for m-TiO2, ~200 nm for CsPbI2Br over-layer, and ~40 nm for HTMs, respectively. The SEM topsurface images in Figure S2(e-h) display that all HTMs were fully covered on top of

CsPbI2Br perovskite layer without pin-holes. The different morphology of SEM topsurface image of P3HT HTM sample (Figure S2(e)) and the others (Figure S2(f-h))

might be associated with the fact that the P3HT is a semi-crystalline polymer so it

makes dense film, whereas the other HTMs are amorphous polymers.

Chemical structures of HTMs with different HOMO values were shown in Figure 2(b).

The used HTMs are poly-3-hexylthiophene (P3HT), poly-triarylamine (P-TAA), poly-

fluoren-8-triarylamine (PF8-TAA), and poly-indenofluoren-8-triarylamine (PIF8-TAA).

The photoelectron spectroscopy in air (PESA) spectra in Figure 2(c) indicate that the


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VBM or HOMO of CsPbI2Br, P3HT, P-TAA, PF8-TAA, and PIF8-TAA is -5.70, -4.98, 5.09, -5.45, and -5.52 eV, respectively. The measured VBM of CsPbI2Br by PESA was well matched with the result of UPS so we can think that the measured values are

reasonable.

Average photovoltaic properties of 25 n-i-p type mesoscopic CsPbI2Br PSCs with different HTMs were shown in Figure 3(a-d). Average open-circuit voltage (Voc) of 25 devices with different HTMs shown in Figure 3(a) indicates that the average Voc is 1.11 ± 0.030, 1.17 ± 0.023, 1.21 ± 0.027, and 1.27 ± 0.028 V for P3HT, P-TAA, PF8-TAA,

and PIF8-TAA device, respectively. Averages short-circuit current density (Jsc) of P3HT, P-TAA, PF8-TAA, and PIF8-TAA device were 12.78 ± 0.407, 14.29 ± 0.394, 14.05 ± 0.503, and 13.78 ± 0.501 mA/cm2, respectively as shown in Figure 3(b). Averages fill

factor (FF) of P3HT, P-TAA, PF8-TAA, and PIF8-TAA device were 76.77 ± 3.08, 79.58

± 2.73, 78.35 ± 1.93, and 76.69 ± 1.88 %, respectively a shown in Figure 3(c).

Accordingly, averages power conversion efficiency (PCE) of P3HT, P-TAA, PF8-TAA,

and PIF8-TAA device were 10.92 ± 0.969, 13.34 ± 0.914, 13.33 ± 0.868, and 13.43 ±


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0.920 %, respectively as shown in Figure 3(d). All photovoltaic parameters of the n-i-p

type mesoscopic CsPbBr2I PSCs were summarized in Table 1.

The current density-voltage (J-V) curves of champion devices with different HTMs were shown in Figure 4(a) and Figure S3. The Voc of P3HT, P-TAA, PF8-TAA, and PIF8-TAA device was 1.14, 1.21, 1.24, and 1.31 V, respectively, and its corresponding PCE was 12.40, 14.50, 14.73, and 14.86 %, respectively. Similar to the results of 25 average devices, the Voc of mesoscopic CsPbI2Br PSC was gradually increased as the HOMO of HTM increases from -4.98 eV for P3HT to -5.09, -5.45, and -5.52 eV for P-TAA, PF8-TAA, and PIF8-TAA, respectively, because the Voc depends on the difference between electron Fermi level and hole Fermi level of the device. The difference between bandgap (Eg) and Voc of the mesoscopic CsPbI2Br devices was 0.78, 0.71, 0.68, and 0.61 eV for P3HT, P-TAA, PF8-TAA, and PIF8-TAA device, respectively. Namely, we could reduce the Voc loss of the mesoscopic CsPbI2Br PSC simply by adjusting HOMO of HTM. In general, the Voc of mesoscopic PSC is lower than that of planar type PSC because the potential is lost by injection of electrons from perovskite into m-TiO2.25 Therefore, it should be noted that the achieved Voc of mesoscopic CsPbI2Br PSC is significantly high. Figure S4 confirms that the planar type CsPbI2Br PSC (Voc = 1.39 V) composed of FTO/bl-TiO2/CsPbI2Br/PIF8-TAA/Au has higher Voc than the mesoscopic type one (Voc = 1.31 V). The SEM cross-sectional image of planar type PSC was shown in Figure S5. The planar type PIF8-TAA HTM-based device exhibited 1.39 V Voc, 13.79


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mA/cm2 Jsc, 72.33 % FF, and 13.86 % PCE for forward scan condition and 1.39 V Voc, 13.96 mA/cm2 Jsc, 75.61 % FF, and 14.67 % PCE for reverse scan condition as shown in Figure S4.

Unlike planar type CsPbI2Br PSCs, the mesoscopic PSCs do not have significant J-V hysteresis with respect to the scan direction as shown in Figure 4 and S4. The J-V

hysteresis of PSCs can be reduced by balancing electron flux and hole flux, which is function of number of charge carriers per unit interface area and unit time.24 So the

bottleneck of electron flow is generally occurred at TiO2 bulk and/or TiO2/perovskite interface because the electron mobility and conductivity of bl-TiO2 and m-TiO2 is much smaller than the electron mobility and conductivity of perovskite and the hole mobility

and conductivity of perovskite and HTM. Hence, in PSCs with TiO2 ETL, the mesoscopic PSCs have smaller J-V hysteresis than the planar type ones due to the

higher m-TiO2/perovskite interface area. To reduce J-V hysteresis of planar type PSCs, the TiO2 ETL needs to be modified or replaced to new ETM with higher electron mobility and conductivity such as SnO2 and ZnO. However, both SnO2 and ZnO have relatively weak chemical stability against perovskite so exhibit poorer long-term stability than the


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TiO2 ETL-based PSCs although they have smaller J-V hysteresis than the TiO2 ETLbased ones. For instance, the ZnO ETL has weak chemical stability in acidic and basic condition so more stable Zn2SnO4 has been developed.30 The SnO2 ETL-based PSCs also have smaller J-V hysteresis due to better charge injection from perovskite into the SnO2 than the TiO2 ETL-based ones and exhibit good light-soaking stability,31-33 but they have potential to be damaged by reaction with HI once the perovskite is decomposed at damp heat (85 oC/85 % relative humidity) condition.

Although all mesoscopic CsPbI2Br PSCs irrespective of kind of HTM have same device structure and thickness, the P3HT HTM-based PSCs exhibited smaller Jsc than the others as shown in Figure 3(b) and Figure 4(a). To elucidate the reason, the external

quantum efficiency (EQE) spectra of all devices were measured as shown in Figure 4(b).

The P3HT HTM-based PSC only exhibited additional EQE lost at 450-660 nm-

wavelength regions in the EQE spectra. In the n-i-p type mesoscopic CsPbI2Br PSC, the incident light passes through FTO/bl-TiO2/m-TiO2 layer and is then absorbed by CsPbI2Br layer. The light not fully absorbed by CsPbI2Br passes through the HTM layer, which absorbs the light, and is then reflected by Au metal electrode. The reflected light

is re-absorbed by the HTM and is then fully re-absorbed by the CsPbI2Br layer. The


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absorptivity of CsPbI2Br perovskite significantly depends on the wavelength as shown in Figure 1(d). It has strong absorption below ~450 nm-wavelength region and its

absorptivity is exponentially reduced to the on-set band edge (~660 nm-wavelength).

This implies that the light below ~450 nm-wavelength region might be fully absorbed by

the CsPbI2Br perovskite layer, but the light at 450-660 nm-wavelength regions is not fully absorbed by the perovskite layer and is then absorbed by the HTM. Because the

absorptivity of CsPbI2Br is gradually reduced at 450-660 nm-wavelength regions, more light can be absorbed by the HTM as the wavelength increases. As shown in Figure 4(c),

unlike the P3HT HTM, the other HTMs have only absorption below ~450 nm-wavelength

region, so they did not have EQE lost at whole wavelength, but the P3HT HTM has

strong absorption at visible region, so it had EQE lost at 450-660 nm-wavelength

regions. The EQE is a product of light harvesting efficiency, charge separation efficiency,

and charge collection efficiency. The EQE values below ~450 nm-wavelength region

were similar regardless of kind of HTM, the charge separation efficiency and charge

collection efficiency of the mesoscopic PSCs seem to be similar. Therefore, the

difference of EQE spectrum of P3HT-based PSC is attributed to the absorption by P3HT


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HTM. The calculated Jsc values of all devices from the integration of the EQE spectra were 12.90, 14.45, 14.41, and 14.39 mA/cm2, respectively, and the calculated Jsc values are well matched with the measured Jsc in the J-V curves in Figure 4(a).

To understand the device operation kinetics, we measured transient photoluminescence

(TRPL) decay spectra of all PSCs as shown in Figure 4(d). The bi-exponent fitting

parameters of the spectra were summarized in Table S1. The average PL life-time of

CsPbI2Br

perovskite,

m-TiO2/CsPbI2Br,

CsPbI2Br/P3HT,

CsPbI2Br/P-TAA,

CsPbI2Br/PF8-TAA, and CsPbI2Br/PIF8-TAA was 11.27, 0.95, 2.22, 2.38, 2.38, and 2.46 ns, respectively. This clearly confirms that the generated electrons and holes can

be transferred to m-TiO2 ETM and HTMs, but the charge transfer at CsPbI2Br/m-TiO2 interface is more effective than the charge transfer at CsPbI2Br/HTM interface. Irrespective of kind of HTM, the PL life-times were similar. So the charge transfer

efficiencies of the CsPbI2Br PSCs are similar as expected from EQE spectra. It should be noted that the charge transfer rates were similar although the driving force of hole


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extraction from CsPbI2Br to HTM is reduced in the order of P3HT T P-TAA T PF8-TAA T PIF8-TAA.

Finally, we checked long-term stability of the n-i-p type mesoscopic CsPbI2Br PSCs with different HTMs as shown in Figure 5. The maximum power point (MPP) of un-

encapsulated each device was recorded with continuous light soaking time under 1 sun irradiation (100 mW/cm2 AM1.5G) and 85 oC condition for 1000 h as shown in Figure

5(a). The J-V curves of all devices at initial (t = 0 h) and final stage (t = 1000 h) were

shown in Figure 5(b-e). The corresponding photovoltaic parameters of all devices at

initial and final stage were summarized in Table 2. The P3HT, P-TAA, PF8-TAA, PIF8-

TAA device exhibited 10.1, 14.4, 14.9, and 14.2 % of PCE degradation compared to its

initial efficiency, respectively. Except the P3HT HTM-based PSC, the other HTM-based

PSCs exhibited similar PCE degradation of 14-15 %, whereas the P3HT HTM-based

PSC had PCE degradation of 10.1 %. The better long-term stability of the P3HT HTM-

based PSC might be attributed to the better barrier properties of semi-crystalline P3HT HTM against moisture and oxygen than the amorphous TAA-based HTMs.34


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4. Conclusion

We successfully fabricated the high performance and hysteresis-less mesoscopic

CsPbI2Br PSCs composed of FTO/bl-TiO2/m-TiO2/CsPbI2Br/HTM/Au by adapting HTMs with controlled HOMO values. The measured VBM of CsPbI2Br perovskite and HOMO values of P3HT, P-TAA, PF8-TAA, and PIF8-TAA HTM was -5.70, -4.98, -5.09, -5.45,

and -5.52 eV, respectively. Accordingly, the average Voc values of the 25 mesoscopic CsPbI2Br PSCs were gradually increased from 1.11 ± 0.030 V for P3HT HTM-based device to 1.17 ± 0.023, 1.21 ± 0.027, and 1.27 ± 0.028 V for P-TAA, PF8-TAA, and

PIF8-TAA HTM-based device and their corresponding PCEs were 10.92 ± 0.969, 13.34

± 0.914, 13.33 ± 0.868, and 13.43 ± 0.920 %, respectively. Due to the highest Voc of the PIF8-TAA HTM-based PSC, it exhibited the highest PCE of 14.20 % for forward scan condition and 14.86 % for reverse scan condition under 1 sun illumination (100 mW/cm2

AM1.5G). The insignificant J-V hysteresis of the mesoscopic CsPbI2Br PSCs with respect to the scan direction might be attributed to the more balanced electron and hole


21


Page 22 of 43

flux than the planar type CsPbI2Br PSCs. From the TRPL decay spectra, we found that the charge carriers generated in the CsPbI2Br perovskite layer can be transferred to both m-TiO2 and HTM because the average PL life-time of CsPbI2Br perovskite, mTiO2/CsPbI2Br,

CsPbI2Br/P3HT,

CsPbI2Br/P-TAA,

CsPbI2Br/PF8-TAA,

and

CsPbI2Br/PIF8-TAA was 11.27, 0.95, 2.22, 2.38, 2.38, and 2.46 ns, respectively. Due to better stability of CsPbI2Br all inorganic perovskite material than the organic/inorganic hybrid perovskites, the un-encapsulated mesoscopic CsPbI2Br PSCs exhibited 10-14 % of PCE degradation compared to its initial efficiency under continuous 1 sun light soaking at 85 oC for 1000 h.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website

at DOI:


22

Page 23 of 43 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


Fitting parameters of TRPL decay spectra, AFM topology image of perovskite film, SEM

cross-sectional images of perovskite solar cells with different hole transporting maerials,

J-V curves of champion devices, J-V curves of planar type perovskite solar cell, and

SEM cross-sectional image of planar type device.

AUTHOR INFORMATION

Corresponding authors:

*E-mail: [email protected] (S. H. Im)

Author Contributions

D.H.K and J.H.H. contributed equally to this study. D.H.K. prepared perovksite materials

and characterized their properties. J.H.H fabricated devices and analized thier

properties. S.H.Im designed concept and revised a manuscript.


23


Page 24 of 43

Acknowledgements.

This study was supported by the National Research Foundation of Korea (NRF) under

the Ministry of Science, ICT & Future Planning (Basic Science Research Program (No.

2014R1A5A1009799), the Technology Development Program to Solve Climate Change

(No. 2015M1A2A2055631) and the Global Frontier R&D Program of the Center for

Multiscale Energy Systems (No. 2018M3A6A7055631)) and the Ministry of Trade,

Industry & Energy, Republic of Korea (New & Renewable Energy Core Technology

Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP)

(No. 20183010013820, No. 20163010012470)).

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List of Tables and Figure captions

Table 1. Photovoltaic parameters of the n-i-p type mesoscopic CsPbBr2I PSCs with different HTMs at 1 sun condition (100 mW/cm2 AM1.5G). Table 2. Photovoltaic parameters of the mesoscopic CsPbI2Br PSCs with different HTMs at initial (t = 0 h) and final (t = 1000 h) stage.

Figure 1. (a) Photograph of all inorganic CsPbI2Br perovskite solution, (b) scanning electron microscopy (SEM) surface image of as-prepared CsPbI2Br perovskite film, (c) X-ray diffraction (XRD) pattern of CsPbI2Br perovskite film: inset = photograph of CsPbI2Br perovskite film, (d) UV-visible absorption spectrum of the CsPbI2Br perovskite film: inset = Tauc plot, and (e, f) ultra violet photoelectron spectroscopy (UPS) spectra of CsPbI2Br perovskite film: (e) the magnified section of the photoemission on-set and (f) the valance band edge. Figure 2. (a) Schematic device structure of n-i-p type mesoscopic CsPbI2Br perovskite solar cell (PSC) and corresponding its band energy diagram, (b) chemical structure of hole transporting materials (HTMs) of P3HT, P-TAA, PF8-TAA, and PIF8-TAA, and (c) photoelectron spectroscopy in air (PESA) spectra of CsPbI2Br, P3HT, P-TAA, PF8-TAA, and PIF8-TAA. Figure 3. Average photovoltaic properties of 25 mesoscopic CsPbI2Br PSCs with different HTMs of P3HT, P-TAA, PF8-TAA, and PIF8-TAA: (a) open-circuit voltage (Voc), (b) short-circuit current density (Jsc), (c) fill factor (FF), and (d) power conversion efficiency (PCE).


33


Page 34 of 43

Figure 4. (a) Current density-voltage (J-V) curves of mesoscopic CsPbI2Br PSCs with different HTMs, (b) corresponding external quantum efficiency (EQE) spectra and calculated Jsc values from integration of EQE spectra, (c) UV-visible absorption spectra of HTMs, and (d) transient photoluminescence (TRPL) decay spectra of CsPbI2Br perovskite, m-TiO2/CsPbI2Br, CsPbI2Br/P3HT, CsPbI2Br/P-TAA, CsPbI2Br/PF8-TAA, and CsPbI2Br/PIF8-TA film. Figure 5. Long-term stabilities of un-encapsulated mesoscopic CsPbI2Br PSCs with different HTMs: (a) maximum power point (MPP) tracking of un-encapsulated PSCs with continuous 1 sun light soaking time at 85 oC for 1000 h and (b-e) corresponding J-V curves of initial (t = 0 h) and final stage (t = 1000 h): (b) P3HT, (c) P-TAA, (d) PF8-TAA, and (e) PIF8-TAA HTM-based PSC.

Table 1. Photovoltaic parameters of the n-i-p type mesoscopic CsPbBr2I PSCs with different HTMs at 1 sun condition (100 mW/cm2 AM1.5G).

Device

Scan direction 25 devices

P3HT

Champion device

25 devices

Forward

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

1.11 ± 0.030 12.78 ± 0.407 76.77 ± 3.08 10.92 ± 0.969

Forward

1.14

13.23

80.00

12.07

Reverse

1.14

13.25

82.06

12.40

Forward

1.17 ± 0.023 14.29 ± 0.394 79.58 ± 2.73 13.34 ± 0.914

P-TAA Champion

Forward

1.21

14.70

80.12

14.13


34

Page 35 of 43 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


PF8-TAA

device

Reverse

25 devices

Forward

Champion device

25 devices PIF8-TAA

Champion device

1.21

14.72

82.11

14.50

1.21 ± 0.027 14.05 ± 0.503 78.35 ± 1.93 13.33 ± 0.868

Forward

1.24

14.53

78.79

14.20

Reverse

1.24

14.56

81.60

14.73

Forward

1.27 ± 0.028 13.78 ± 0.501 76.69 ± 1.88 13.43 ± 0.920

Forward

1.31

14.52

75.64

14.28

Reverse

1.31

14.55

78.58

14.86

Table 2. Photovoltaic parameters of the mesoscopic CsPbI2Br PSCs with different HTMs at initial (t = 0 h) and final (t = 1000 h) stage.

Device

State

Voc (V)

Jsc (mA/cm2)

FF(%)

Initial

1.14

13.24

81.92

PCE (%) Degradation (%) 12.36

P3HT

10.1 Final

1.12

12.85

77.20

11.11

Initial

1.21

14.71

81.92

14.46

P-TAA

14.4 Final

1.17

13.91

76.08

12.38


35


Initial

1.24

14.55

81.22

Page 36 of 43

14.65

PF8-TAA

14.9 Final

1.19

13.83

75.75

12.47

Initial

1.31

14.54

78.20

14.78

PIF8-TAA

14.2 Final

1.23

13.84

74.49

12.68


36

Page 37 of 43

(b)

(a)

1 µm

(d) 1.0

0.6

1 cm

0.4 0.2

)

2

0.8

h

0.8

(

Absorbance (a.u.)

Normalized intensity (a.u.)

(c) 1.0

0.6 1.80

1.84

1.88

1.92

1.96

2.00

Energy (eV)

0.4 0.2

0.0 10

20

30

40

50

0.0 300

400

2 (degree)

500

600

700

800

0

-1

Wavelength (nm)

(e)

(f) Intensity (a.u.)

Intensity (a.u.)

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


1.84

17.36 19

18

17

16

15

4

3

Binding energy (eV)

2

1

Binding energy (eV)

Figure 1. (a) Photograph of all inorganic CsPbI2Br perovskite solution, (b) scanning electron microscopy (SEM) surface image of as-prepared CsPbI2Br perovskite film, (c) X-ray diffraction (XRD) pattern of CsPbI2Br perovskite film: inset = photograph of CsPbI2Br perovskite film, (d) UV-visible absorption spectrum of the CsPbI2Br perovskite


37


film: inset = Tauc plot, and (e, f) ultra violet photoelectron spectroscopy (UPS) spectra of CsPbI2Br perovskite film: (e) the magnified section of the photoemission on-set and (f) the valance band edge.

Au HTM

(a) h

TiO2

-3.78

(c)

-4.0

CsPbI2Br

e

CsPbI2Br TiO2 -5.70

P3HT -4.98 eV PTAA -5.09 eV PF8 -5.45 eV PIF8 -5.52 eV

FTO C6H13

(b)

CsPbI2Br

*

*

C8H17

S

P3HT

C8H17

N

PF8-TAA C8H17

P3HT

Emission yield (cps1/2)

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

Page 38 of 43

P-TAA

PF8-TAA

C8H17

*

PIF8-TAA *

N C8H17

PTAA

C8H17

PIF8-TAA

N

4.0

4.5

5.0

5.5

6.0

6.5

Photon energy (eV)


38

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Figure 2. (a) Schematic device structure of n-i-p type mesoscopic CsPbI2Br perovskite solar cell (PSC) and corresponding its band energy diagram, (b) chemical structure of hole transporting materials (HTMs) of P3HT, P-TAA, PF8-TAA, and PIF8-TAA, and (c) photoelectron spectroscopy in air (PESA) spectra of CsPbI2Br, P3HT, P-TAA, PF8-TAA, and PIF8-TAA.


39


16

(a)

(b)

1.35

15

1.30

Voc (V)

2

J sc (mA/cm )

1.25 1.20 1.15 1.10

14

13

12 1.05 11

1.00

P3HT

P-TAA

PF8-TAA

PFI8-TAA

P3HT

P-TAA

PF8-TAA

PIF8-TAA

P3HT

P-TAA

PF8-TAA

PIF8-TAA

16

88

(d)

(c) 84

14

PCE (%)

80

FF (%)

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

Page 40 of 43

76 72

12

10

68 8

P3HT

P-TAA

PF8-TAA

PIF8-TAA

Figure 3. Average photovoltaic properties of 25 mesoscopic CsPbI2Br PSCs with different HTMs of P3HT, P-TAA, PF8-TAA, and PIF8-TAA: (a) open-circuit voltage (Voc), (b) short-circuit current density (Jsc), (c) fill factor (FF), and (d) power conversion efficiency (PCE).


40

20

80

16

P3HT P3HT-dark P-TAA P-TAA-dark PF8-TAA PF8-TAA-dark PIF8-TAA PIF8-TAA-dark

10

5

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

P3HT P-TAA PF8-TAA PIF8-TAA

60

40

8

20

4

0 300

1.6

0 400

500

600

700

Wavelength (nm)

Voltage (V) 0

(c)1.0

(d) 10 Normalized PL intensity (a.u.)

P3HT P-TAA PF8-TAA PIF8-TAA

0.8 0.6 0.4 0.2 0.0 300

12

2

15

-5 0.0

100

(b)

EQE (%)

Current density (mA/cm2)

(a)20

Absorbance (a.u.)

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


Integraded Jsc (mA/cm )

Page 41 of 43

CsPbI2Br mp-TiO2/CsPbI2Br CsPbI2Br/P3HT CsPbI2Br/P-TAA CsPbI2Br/PF8-TAA

-1

10

CsPbI2Br/PIF8-TAA

-2

10

-3

10

400

500

600

700

800

0

20

40

60

80

Time (ns)

Wavelength (nm)

Figure 4. (a) Current density-voltage (J-V) curves of mesoscopic CsPbI2Br PSCs with different HTMs, (b) corresponding external quantum efficiency (EQE) spectra and calculated Jsc values from integration of EQE spectra, (c) UV-visible absorption spectra of HTMs, and (d) transient photoluminescence (TRPL) decay spectra of CsPbI2Br perovskite, m-TiO2/CsPbI2Br, CsPbI2Br/P3HT, CsPbI2Br/P-TAA, CsPbI2Br/PF8-TAA, and CsPbI2Br/PIF8-TA film.


41



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Figure 5. Long-term stabilities of un-encapsulated mesoscopic CsPbI2Br PSCs with different HTMs: (a) maximum power point (MPP) tracking of un-encapsulated PSCs with continuous 1 sun light soaking time at 85 oC for 1000 h and (b-e) corresponding J-V curves of initial (t = 0 h) and final stage (t = 1000 h): (b) P3HT, (c) P-TAA, (d) PF8-TAA, and (e) PIF8-TAA HTM-based PSC.

TOC

C6H13 *

*

C8H17

S

C8H17

N

PF8-TAA : 14.5 %

P3HT: 12.4 % C8H17

C8H17

*

*

N C8H17

PTAA : 14.73 %

C8H17

PIF8-TAA : 14.86 %

N

Current density (mA/cm2)

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


15

10

Mesoscopic CsPbI2Br device P3HT P-TAA PF8-TAA PIF8-TAA

5

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Voltage (V)


43

Hysteresis-Less CsPbI2Br Mesoscopic Perovskite Solar Cells with a

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