Boosting the efficiency and stability of perovskite solar cells through facile molecular engineering approaches

Solar Energy 199 (2020) 136–142

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Boosting the efficiency and stability of perovskite solar cells through facile molecular engineering approaches

T

Seckin Akin Department of Metallurgical and Materials Engineering, Karamanoglu Mehmetbey University, 70100 Karaman, Turkey

A R T I C LE I N FO

A B S T R A C T

Keywords: Perovskite solar cells SnO2 electron transporting layer Cesium fluoride Zwitterion molecules Stability

Not only the poor interaction but also trap states at interfaces and grain boundaries are suspected to be responsible for carrier losses in perovskite solar cell (PSC) architecture, leading to inferior photovoltaic performance and long-term stability. Here, facile and effective molecular engineering approaches have been reported by employing a CsF-doped SnO2 electron-transporting layer (ETL) and inserting zwitterion molecules as building blocks between perovskite and hole-transporting layer (HTL). The modification of SnO2 by alkali metal fluoride significantly improved the opto-electronic properties, indicating rapid extraction of photogenerated electrons and better light-harvesting. On the other hand, zwitterion interlayer demonstrated a considerable passivation in multiple defect states at grain boundaries of perovskite film. This strategy yielded an open-circuit voltage (VOC) of 1.23 V for triple-cation perovskite composition with the loss in potential of only 0.37 V. As a result, a considerable efficiency of 21.7% was achieved with negligible hysteresis. More importantly, such engineering approaches exhibited an admissible long-term stability under continuous light soaking at the maximum power point (MPP) tracking by retaining 90% of initial efficiency after ~800 h. In short, these initiatives have simultaneously improved the photovoltaic performance and long-term stability of PSCs. This work severely highlights the utility of molecular engineering approaches in perovskite devices and provides the basis for facilitating industrial applications in the near future.

1. Introduction Organic–inorganic metal halide perovskite solar cells (PSCs) are an emerging photovoltaic technology that consist of the perovskite absorber positioned between an electron-transporting layer (ETL) and a hole-transporting layer (HTL) (Kim et al., 2012; Liu et al., 2019; Seo et al., 2018a; Turren-Cruz et al., 2018; Xiang et al., 2019). In such architecture, each component as well as the corresponding interfaces play a of great importance role on the photovoltaic performance (Akin et al., 2019b). However, in addition to efficiency, the instability issue associated from solution deposition methods of various constituents in perovskite devices, which get aggravated in the presence of traps, is still one of the main challenges that PSC community is facing currently (Akin et al., 2019a; Arora et al., 2019; Seo et al., 2018a). Therefore, further improvements in performance and stability merely require significant control of recombination processes associated from trap states which dominate the non-radiative charge recombination (Stolterfoht et al., 2019; Yadav et al., 2018). Such trap states are mainly considered to be related by vacancies and defects located at the surface and grain boundaries of perovskite layer (Sherkar et al., 2017a, 2017b; Tress et al., 2015). Despite the difficulty of

knowing the nature of these defects and differentiating them, their formation could be finely inspected by controlling the growth and morphology of the perovskite and other layers. Owing to its prominent impact on the operational stability and efficiency of PSCs, the reduction of defect states over perovskite layer as well as corresponding interfaces has attracted tremendous attention both from the scientific community and photovoltaic industry (Chen, Q. et al., 2019; Chen, Y.H. et al., 2019). Among the various strategies, interface engineering is one of the most efficient approaches to achieve perovskite layers with high crystallinity, smooth surface, and large grains, which implicitly reduce the grain boundary dependent trap states (Akin et al., 2019a; Liu et al., 2019; Tavakoli et al., 2019). Recently, the insertion of zwitterion molecules, as an alternative to widely used building blocks, onto perovskite layer as an interfacial layer (IL) has gathered a great attention in order to mitigate the non-radiative recombination in perovskite devices, yielding both improved open-circuit voltage (VOC) and operational stability (Choi et al., 2018; Islam et al., 2019; Wang et al., 2017). In addition to its suppression role in ion migration, it can simultaneously passivate defect states of perovskite layer owing to its different functional groups capable of passivating multiple defect types. Very

E-mail address: [email protected]. https://doi.org/10.1016/j.solener.2020.02.025 Received 18 December 2019; Received in revised form 4 February 2020; Accepted 6 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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(b) Intensity (a.u.)

Au

480

SnO2 CsF:SnO2

Sn 3d5/2

Sn 3d3/2

485

490

495

500

(c) Intensity (a.u.)

(a)

(e)

Cs 3d3/2

720 725 730 735 740 745 750

Binding energy (eV)

Binding energy (eV)

(d)

Cs 3d5/2

SnO2 CsF:SnO2

(f)

1 μm

1 μm

1 μm

Fig. 1. (a) The device configuration employed in this work. High-resolution XPS spectra of (b) Sn 3d and (c) Cs 3d peaks in the pristine and CsF:SnO2 films. SEM micrographs of (d) CsF:SnO2 film on FTO, (e) perovskite film on SnO2, and (f) perovskite film on CsF:SnO2.

performance of the device is correlated with an increase in the short circuit current density (JSC). As compared to LiF; CsF possesses lower diffusivity, thereby can provide relatively higher lifetime (Mitul et al., 2018; Yi et al., 2019). Very recently, CsF was adopted to perovskite devices by incorporating CsF into the PbI2 seed layer to obtain perovskite layer via the two-step method (Yi et al., 2019). It is reported that the additive engineering greatly enhanced the perovskite crystallization leading to an extended carrier lifetime with better efficiency. In this study, CsF was doped into SnO2 (CsF:SnO2) ETL for the first time. The incorporation of CsF into ETL not only facilitated the formation of larger perovskite grains, but also in improving the optoelectronic properties of SnO2 ETL, which ultimately results in improved efficiency. The increase in efficiency revealed that Cs atoms occupy interstitial sites, producing electrons, which results in higher electron mobility whereas F atoms occupied in substitutional oxygen sites reduce the oxygen vacancies in the structure and lower the absorption in the visible region, resulting higher JSC. Moreover, in order to control the recombination processes associated with trap states at grain boundaries, a thin layer of D4TBP was first inserted on the perovskite layer in n-i-p architecture. As expected, an excellent efficiency approaching 22% with VOC as high as 1.21 V (1.23 V, the highest) was achieved with decreasing density of defects after passivation. This can be ascribed to enhanced perovskite microstructure with reduced surface traps. More importantly, a considerable improvement has been achieved in long-term operational and shelf stability in the presence of D4TBP interlayer. This study presents a significant potential to open up new opportunities for boosting the efficiency and long-term stability of perovskite devices by molecular engineering approaches.

recently, a new zwitterionic amino acid, namely 4-tert-butyl-D-phenylalanine (D4TBP), with functional passivation groups, including carboxyl, amine, isopropyl, phenethyl and tert-butyl-phenethyl, was employed in inverted type perovskite devices (Yang et al., 2019). Such a simple and generic strategy entirely passivated the ionic and/or neutral defects of the perovskite layer, indicating a reduced VOC deficit. However, there was no study about the stability of perovskite devices in the presence of D4TBP. Another attractive aspect related to molecular engineering approach for fewer defects is to achieve high-quality perovskite absorber films. The quality of perovskite film is strongly dependent on the physical/ morphological properties of ETL in normal architecture. Although TiO2 has been extensively employed as an ETL in regular device architecture, low electron mobility (~10−4 cm2 V−1 s−1), requirement of high temperature sintering, and insufficient band-offset of TiO2 slowly turns the direction of the wind towards SnO2 (Xiong et al., 2018; Zhu et al., 2016) in many successful state‐of‐the‐art PSCs. Despite the remarkable properties of SnO2 including wide bandgap (> 3.6 eV), high electron mobility, anti-reflection property, and deeper conduction band, it is still not the best alternate in ETLs because of the ongoing major issues such as relatively low electrical conductivity and high rates of recombination (Park et al., 2016; Roose et al., 2018). In order to improve the charge transport properties of SnO2 ETLs, various strategies have been developed such as doping, treatment, and interfacial layer. Recently published reports indicated that alkali metal fluorides e.g., cesium fluoride (CsF) and lithium fluoride (LiF), suppress the charge recombination and facilitate charge transport by reducing trap states and passivating oxygen vacancy defects in the metal oxide based ETLs (Chang et al., 2013; Ling et al., 2019; Wei et al., 2013). The doping or treatment of alkali metal fluorides into various optoelectronic devices showed a great reduction in resistivity for all performing systems, resulting in enhanced electron injection. However, the insertion of alkali metal fluorides into solar cells is fairly limited with several reports. Ling et al., first investigated the light-soaking effect of LiF in polymer solar cells employing LiF doped ZnO as an ETL (Ling et al., 2019). As a result, trap state density at the ETL/active layer interface significantly reduced while electron extraction efficiency increased. In another study, a very high electron mobility of ZnO up to 11.2 cm2 V−1 s−1 was achieved by LiF doping (Chang et al., 2013). Such results are indeed supported by the fact that the better

2. Results and discussion The device configuration employed in the present work is FTO/ CsF:SnO2/ Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3/ D4TBP/ spiroOMeTAD/ Au as shown in Fig. 1a, where CsF dopant was poured into precursor solution of SnO2 to obtain high-quality ETLs. In order to further reduce interfacial non-radiative recombination and accordingly enhance the photovoltaic performance, D4TBP molecule-based interlayer (IL) was used to reduce the defect density at grain boundaries and surface of perovskite layer (Fig. S1). In the first section of this work, alkali metal halide material, CsF, 137

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remarkable reproducibility with a narrow distribution between 20.2 and 20.5% for PCE (Fig. 2d). Note that the improved PCE is partially contributed from the increased JSC value which can be speculated from the enhanced lightharvesting capability of perovskite film associated by better optical properties of CsF:SnO2 ETL (Kam et al., 2019). In addition to optical properties, electrical properties of an ETL also affect the charge transfer process. In this manner, the conductivity (σ ) of pristine SnO2 and CsF:SnO2 ETLs was estimated from the slope of linear sweep voltammetry (LSV) curves shown in Fig. S8 by the following equation of σ = d/ AR where A is the active area, d is the thickness, and R is the resistance of ETLs acquired from V = IR . The larger slope of CsF:SnO2 ETL curve implies 3-fold higher electrical conductivity (5.4 × 10−5 S cm−1) as compared to pristine SnO2 ETL (1.9 × 10−5 S cm−1). The enhancement in conductivity can properly reduce non-ideal space charge distribution and improves electron transport, thus leading to an increase in both JSC and FF parameters (Akin, 2019). Steady-state photoluminescence (PL) measurement was performed to evaluate the electron transfer kinetics between perovskite and ETLs. Fig. 2e comparatively shows PL spectra of perovskite films on glass or different ETLs. Arguably, a more strongly quenched PL in the CsF:SnO2/ perovskite stack was observed, indicating more efficient electron extraction from perovskite layer to corresponding ETL (Seo et al., 2018b). This is consistent with the better electrical properties of CsF:SnO2 film. The charge transfer kinetics between perovskite and different ETLs were further investigated by time-resolved PL (TRPL) decays. As shown in Fig. 2f, the decay time τ10 (at which the maximum PL intensity decreases by a factor of 10) is shorter for the SnO2 film doped with CsF (19.7 ns) than that of pristine SnO2 (45.2 ns), pointing a reduced nonradiative recombination in perovskite film, given the improved film quality with CsF:SnO2 ETL (Fig. 1f). These results indicate a better interfacial binding and more rapid charge transfer at the CsF:SnO2/perovskite interface compared to pristine SnO2/perovskite (Shi et al., 2019). Such an ideal interfacial property is helpful to achieve highperformance photovoltaic devices. On the other hand, the improvement in VOC (~30 mV) associated from low trap states in CsF:SnO2 ETL can be ascribed to effectively suppressed non-radiative recombination in the device (Akin et al., 2019a; Liu et al., 2019; Seo et al., 2018a). To probe differences in the trap-state density (Nt ), space charge limited current (SCLC) measurements in electron only devices (FTO/ SnO2 or CsF:SnO2/ Au structure) were performed to evaluate the trap levels (Fig. 3a and b). The corresponding Nt value is determined by the equation of Nt = (2εε0 VTFL)/ eL2 where e is the charge of electron, ε0 is the vacuum permittivity, ε is the relative dielectric constant of ETL, L is the thickness of ETL, and VTFL is the trap-filled limit voltage which is estimated from the kink point in the related curves (0.84 V for pristine SnO2 and 0.34 V for CsF:SnO2). The Nt in CsF:SnO2 is estimated as 1.3 × 1015 cm−3, which is 6-fold lower than that of pristine SnO2 (8.4 × 1015 cm−3). As discussed earlier, CsF substitution can efficiently passivate the electron traps and boost the charge carrier dynamics of the corresponding ETLs, leading to higher electron mobility ( μe ) . This result highlights the effectiveness of using CsF additive for defect passivation and performance enhancement. To further explore the interfacial charge transfer and recombination dynamics, electrochemical impedance spectroscopy (EIS) measurement was performed at different voltages. The Nyquist plots of devices with different ETLs and equivalent circuit are shown in Fig. S9. In general, the semicircle in the high-frequency region is related with the charge transfer resistance (Rct) whereas the semicircle in the low-frequency region represents the recombination resistance (Rrec) (Akin et al., 2018). A lower Rct value (0.59 kΩ to 0.44 kΩ) was achieved for CsF:SnO2 ETL employing PSC while Rrec in device with CsF:SnO2 ETL (8.03 kΩ) is much higher than Rrec in device with pristine SnO2 ETL (5.62 kΩ), indicating an effectively suppressed charge recombination as coinciding with the VOC in J–V curve (Chen, J. et al., 2019; Shi et al.,

was employed to modify the opto-electronic properties of SnO2 ETL in the absence of IL between perovskite and HTL. The effect of CsF doping on the crystallinity of SnO2 film was analyzed by XRD patterns. As obviously seen in Fig. S2, the obtained peaks confirmed the formation of the SnO2 structure without any impurity phases and the samples remain the current phase irrespective of the doping. The elemental composition of pristine SnO2 and CsF:SnO2 layers atop FTO was examined using XPS analysis; as shown in Fig. 1b and 1c. The occurrence of Cs 3d5/2 and Cs 3d3/2 peaks confirms the existence of Cs+ cation in SnO2 layer. On the other hand, two peaks in the pristine SnO2 film with binding energies at 487.1 eV (Sn 3d5/2) and 495.5 eV (Sn 3d3/2) shifted to 486.9 and 495.3 eV for the CsF:SnO2 layer, respectively. This slight shift to lower binding energies indicates the chemical interaction between SnO2 and CsF. Moreover, low level of F 1 s peak located at 687.4 eV (Fig. S3) confirms the existency of small amount of F¯ halide after thermal treatment. The existency of F¯ halide can facilitate the passivation of trap states associated from oxygen vacancies, indicating an improved charge transport properties (Ling et al., 2019). Besides, a reduction in oxygen vacancies leads to decrease of optical absorption in visible region. As clearly seen in the transmittance curves of ETLs shown in Fig. S4, CsF doping slightly improved the optical transmission of the SnO2 layer on FTO substrate with no significant difference in bandgaps. Such a better transparency facilitates the transmission more of the incident light, which can be absorbed by the perovskite layer, resulting in more efficient light harvesting within the perovskite devices. Beside optical properties, surface morphology of perovskite layer is mightily dependent on the quality of ETL surface (Gong et al., 2018; Salim et al., 2015). The uniform and dense surface of CsF:SnO2 ETL without pinholes (Fig. 1d) demonstrated a considerable effect on the growth mechanism of perovskite layer. As compared to pristine SnO2 ETL seen in Fig. 1e, perovskite film with fewer defects can be obtained via molecular engineering approach in ETLs (Fig. 1f). Moreover, it consists of larger grain domains with an average size of ~480 nm as delineated in Fig. S5. The reduced number of grain boundaries can suppress the nonradiative recombination pathways and ion migration in the device architecture (Sherkar et al., 2017a). In order to evaluate the bulk properties of perovskite film on different ETLs, XRD was conducted and given in Fig. S6. The intensity of main peak at ~14.2° became stronger with a lower full width at half maximum (FWHM) on CsF:SnO2 ETL. It further confirms the improved perovskite film quality and enlarged grains shown in Fig. 1f and S5. To evaluate the effect of CsF doping on the photovoltaic performance of PSCs, SnO2 films employing various CsF concentrations (0 to 5 mol.%) have been tested as ETL in device structure. The highest average efficiency collected from four independent cells was obtained with 2 mol.% concentration value of CsF (Fig. S7). With further increasing of CsF concentration, the excess CsF molecules serve as an insulator agent, resulting in lower Jsc and FF. Fig. 2a shows the J–V characteristics of planar-type PSCs based on SnO2 or CsF:SnO2 ETLs, with the photovoltaic parameters, including JSC, VOC, FF, and PCE summarized in Table 1. The best-performing device based on pristine SnO2 showed an efficiency of 19.26% with Jsc = 22.57 mA cm−2, VOC = 1.13 V, and FF = 0.75. As expected, when CsF:SnO2 was employed as ETL, the JSC, VOC, and FF parameters increased to 23.18 mA cm−2, 1.16 V, and 0.76, respectively, yielding a PCE up to 20.48%. The obtained efficiency was further confirmed by time dependent measurements in order to avoid inflated values from instant J–V scans. Under maximum power point (MPP) tracking, the stabilized power output (SPO) of the CsF:SnO2 ETL employing device reached within several seconds to a stabilized value of ~20.4%, yielding an SPO-to-PCE ratio of ~0.97 (Fig. 2b). The IPCE spectra of PSCs with and without CsF additive are shown in Fig. 2c. The integrated photocurrent against wavelength is in excellent agreement with J–V measurements, with a discrepancy of no more than 2%. As evident from the histogram of efficiency distribution, the devices with CsF:SnO2 ETL showed 138

(b)

25

20.36%

20 20 PCE (%)

80

20

22.57

1132

0.75

19.26

60

15

23.18

1161

0.76

20.48

40

10

5

2

FF

15

IPCE (%)

VOC (mV)

10

10 5

SnO2

0 0.0

0.3

0.6

0.9

Voltage (V)

10

0 0

1.2

PL intensity (a.u.)

CsF:SnO2

6 4 2

30

40

50

0 400

60

Time (s)

(e)

SnO2 8

20

(f)

Glass SnO2 CsF:SnO2

5

CsF:SnO2

CsF:SnO2 10

SnO2

20

SnO2

CsF:SnO2

Counts

25

18.92% JSC (mA/cm 2)

SPO (%)

15

(c) 100

1

500

600

700

0 800

Wavelength (nm)

PL intensity (a.u.)

(d)

Current density (mA/cm2)

(a)

JSC (mA/cm )

Solar Energy 199 (2020) 136–142

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Glass SnO2 CsF:SnO2 Fitting lines

0 18.0

18.5

19.0

19.5

20.0

20.5

670

21.0

710

750

790

830

0

300

Wavelength (nm)

PCE (%)

600

900

1200

Time (ns)

Fig. 2. (a) J–V curves of the champion cells in reverse scanning at scanning rate of 50 mV/s. (b) The steady-state output of efficiency. (c) The IPCE and integrated JSC spectra of corresponding cells. (d) PCE histogram of devices employing different ETLs. (e) The steady-state PL and (f) TRPL spectra of the perovskite films on different substrates.

grains with less grain boundaries have been obtained, indicating the penetration of D4TBP molecules into the perovskite film along the grain boundaries (Fig. 4e). To assess the passivation effect of IL molecule on the triple-cation perovskite composition, PL measurement was performed (Fig. S11). The obtained emission intensity is ~2.1 times higher than that of pristine perovskite, indicating that the functional groups in IL effectively passivate the surface defects. Additionally, one can observe a negligible blue shift in the PL peak which supports the interaction between perovskite and D4TBP. After insertion of passivation layer, the best-performing PSC delivered a JSC of 23.01 mA cm−2, a VOC of 1.21 V, a FF of 0.78, yielding a considerable PCE of 21.7% (> 21.5% stabilized PCE over 200 s) (Fig. 4a). Similar trend in all photovoltaic parameters was obtained in the average values of FTO/ CsF:SnO2/ perovskite/ D4TBP/ spiro-OMeTAD/ Au (50 devices) as compared to FTO/ SnO2/ perovskite/ spiro-OMeTAD/ Au (20 devices) and FTO/ CsF:SnO2/ perovskite/ spiro-OMeTAD/ Au (20 devices) configurations (Fig. S12). It is also worth mentioning that a VOC between 1.19 and 1.23 V is consistently achievable. This strategy yielded the highest VOC of 1.23 V at a bandgap of 1.60 eV with the loss in potential of only 0.37 V (Eg/q – VOC), which is even less than that of crystal silicon solar cells (0.38 V). Such an improvement in VOC is consistent with the PL results. The VOC of best-performing device was measured as a function of time under illumination, confirming that the VOC stabilizes above 1.20 V (Fig. 4b). On the other hand, despite the slightly lower JSC upon interlayer, IPCE spectra in Fig. 4c shows an effective light harvesting across the entire visible spectral region with a compatible integrated current density. Such a negligible decrease in JSC in the presence of interlayer can be

2019). Fig. 3c and 3d show the Rct and Rrec as a function of applied voltages. A similar trend has been obtained in all voltage steps where the CsF:SnO2 ETL based-device exhibits a lower Rct and higher Rrec as compared to those for pristine SnO2 ETL based-devices. This further confirms the improved electron extraction and reduced interface recombination by the introduction of CsF:SnO2 as an ETL. In the second section of this work, interface of perovskite/HTL has been designed to passivate the charge defects (defect-healing) associated from the ionic nature of perovskite materials. In this manner, a thin interface layer (IL) of zwitterionic amino acid, namely 4-tert-butylD-phenylalanine (D4TBP), was employed on perovskite layer to retard the trap-assisted non-radiative charge carrier recombination at grain boundaries. As shown in the cross-section SEM image in Fig. 4f, the performing planar type devices were prepared in a device configuration of FTO/ CsF:SnO2/ perovskite/ D4TBP/ spiro-OMeTAD/ Au. D4TBP IL acting as a crosslinker between the neighboring perovskite grains has both charged functional passivation groups and simultaneously passivate neutral and charged ionic defects on perovskite film (Zheng et al., 2017, 2018). Particularly, phenyl groups in D4TBP structure exhibited a great potential to mitigate the point defects at grain boundaries associated with neutral I2 by demonstrating formation of charge transfer complexes between the conjugated ring and I2 whereas carboxyl and amine groups can heal charged defects via electrostatic interactions (Fig. S10). Such passivation layer was formed on perovskite surface by spin-coating of D4TBP precursor solution to avoid the possible risks induced by perovskite morphology changes in case of mixing the additives in perovskite precursor. As obviously seen in SEM image of D4TBP-treated perovskite surface, more compact and dense perovskite

Table 1 The best and average (20 devices) photovoltaic parameters of devices employing pristine SnO2 and CsF:SnO2 as ETL. ETLs

SnO2 CsF:SnO2

JSC (mA·cm−2)

VOC (V)

FF

PCE (%)

Best

Average

Best

Average

Best

Average

Best

Average

1.13 1.16

1.13 ± 0.01 1.16 ± 0.01

22.6 23.2

22.4 ± 0.2 23.2 ± 0.1

0.75 0.76

0.74 ± 0.01 0.76 ± 0.01

19.3 20.5

19.0 ± 0.3 20.4 ± 0.1

139

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0

10

Current (A)

-1

10

(b)

SnO2

0

CsF:SnO2

10

VTFL= 0.34 V

VTFL= 0.84 V 15

nt= 8.39 x 10 cm

Current (A)

(a)

-3

X

-2

10

-3

-1

10

15

X -2

10

-3

10

10

Ohmic

Ohmic

SCLC

-2

-1

10

10 -3 10

0

10

10

-2

(d)

SnO2

-2

-2

Rct (ohm cm )

1200

Rrec (ohm cm )

CsF:SnO2

800

400

0.2

0.4

0

10

10

Voltage (V)

1600

0.0

-1

10

Voltage (V)

0

SCLC

-4

-4

10 -3 10

(c)

-3

nt= 1.27 x 10 cm

0.6

0.8

4

5x10

SnO2 CsF:SnO2

4

4x10

4

3x10

4

2x10

4

1x10

0

1.0

0.0

0.2

Voltage (V)

0.4

0.6

0.8

1.0

Voltage (V)

Fig. 3. SCLC curves of the different ETLs based on (a) SnO2 (b) CsF:SnO2. (c) Rct and (d) Rrec plots as a function of applied voltage, extracted from the Nyquist plots.

(b)

25 20

5

15 50

0

Current density (mA/cm2 )

80

20

60

15

40

10

20

5

> 1.20 V

1.00

100 150 200

Time (s) 0.3

0.6

0.9

0.50

1.2

0

Voltage (V)

(d)

25

0.75 10

0 0.0

>21.5%

100

IPCE (%)

20

VOC (V)

15 10

(c)

1.50

1.25

SPO (%)

Current density (mA/cm2 )

(a)

JSC (mA/cm2)

grain boundaries (Akin et al., 2019a; Liu et al., 2019; Vidal et al., 2019). To evaluate the effect of trap-assisted recombination on the devices, light intensity-dependent Voc curves were employed and shown in Fig. 5a. The logarithmic light intensity-dependent Voc curve shows slopes of 1.58 kT/q for the control device and 1.21 kT/q for the IL employing device. The smaller slope of the IL employing device indicates a lower trap-assisted recombination as compared to control

ascribed to the blue-shifting effect of D4TBP material as shown in Fig. S11 and possible interfacial states. Fortunately, the insignificant loss in JSC parameter is not more than compensated by the increased FF and VOC parameters. More importantly, D4TBP-treated perovskite employing devices exhibited negligible hysteresis compared to bare perovskite employing devices (Fig. 4d and S13). This clearly states that D4TBP-based passivation layer can block the ion migration along the

20

40

60

80

0 400

100

600

700

0 800

Wavelength (nm)

Time (s)

24

500

(e)

(f)

20

Au Spiro-OMeTAD IL

16 12 8

V OC Scan JSC direction (mA/cm2) (mV)

FF

Perovskite

PCE (%)

4 Forward

0 0.0

23.06

0.3

1198

0.6

0.78 21.5

0.9

1 μm

1.2

CsF:SnO2 Glass/FTO

500 nm

Voltage (V) Fig. 4. Photovoltaic and morphological characteristics of the PSCs based on bare and passivated perovskite films. (a) J–V curve of the best-performing device (reverse scanning at a scan rate of 50 mV/s). The inset shows steady-state output of efficiency. (b) Stabilized VOC as a function of time. (c) IPCE spectrum as a function of the wavelength and the corresponding integrated JSC values. (d) Hysteresis curve of the best-performing device. (e) Top-view SEM micrograph of D4TBP-treated perovskite film. (f) Cross-sectional SEM micrograph of a complete device. 140

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(a)

(b) 1018 tDOS (cm-3eV -1)

1.2

VOC (V)

n = 1.21 kT/q 1.1

n = 1.58 kT/q

1017

without IL with IL

without IL with IL

1.0 10

1016 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55

100

Ea (eV)

Light intensity (mW/cm2)

Fig. 5. (a) Light intensity-dependent Voc for PSCs with and without IL. (b) Trap density of states (tDOS) extracted from thermal admittance spectroscopy (TAS) measurement of PSCs with and without IL.

control device without IL showed a dramatical degradation (~60%) in efficiency after ~800 h in accordance with the increased density of the trap states under operational conditions. In comparison, D4TBP-treated PSCs exhibited much better stability, which retained more than 90% of its initial efficiency in the same aging period. Such improved operational stability is presumably related to suppressed non-radiative recombination within the device and can be ascribed to the synergetic passivation effect associated from functional groups, including carboxyl, amine, and phenethyl. More importantly, the D4TBP-treated device self-recovered to an efficiency over 95% of initial efficiency after resting in the dark for several hours at open circuit. Such a reversible loss phenomenon can be attributed to the ion and defect migration within the perovskite layer and accumulation at the interfaces between perovskite and charge transporting layers (Domanski et al., 2017; Kim et al., 2019; Tan et al., 2017). These results show that IL has a crucial role in not only blocking the migration of ions (Li and/or Au) under operational conditions but also introducing a barrier against to penetration of external factors such as moisture and oxygen.

device (Chen et al., 2018; Zhang et al., 2019). This result was further corroborated by trap density measurement under illumination at shortcircuit condition (Fig. 5b). By interrogating the frequency dependent capacitance via thermal admittance spectroscopy (TAS) measurements, the energetic profile of trap density of state (tDOS) can be estimated. The D4TBP employing device showed lower trap density over the whole trap depth region than that of control device. This trend indicates the reduced defect-states not only in the deeper trap region (0.35 to 0.55 eV), which is attributed to defects at the film surface but also in the shallower trap region (0.20 to 0.35 eV) referring the traps at grain boundaries (Shao et al., 2014; Wu et al., 2019). These results corroborate the notion that D4TBP can effectively passivate the defects associated from undercoordinated ions at the perovskite grain surface and reduce trap density. In addition to effects of defect states at the perovskite surface and grain boundaries on the photovoltaic performance, such defect sites have fairly high activity and diffusivity and are more susceptible to attack by moisture and/or oxygen. This limits the stability of perovskite devices in ambient conditions. We therefore monitored the shelf-stability of corresponding PSCs stored in the dark (without encapsulation, temperature of ~25 °C, and relative humidity of 50 ± 10%). The periodically recorded J–V characteristics in Fig. 6a demonstrate that control devices without IL maintained only ~65% of their initial efficiency (~20.2% to ~13.8% in average of 4 devices) after 15 days whereas passivated PSCs retained over 95% of their initial efficiency (~21.4% to ~20.4% in average of 4 devices), proving excellent stability against to moisture and oxygen. Besides shelf-stability, operational stability of corresponding devices under continuous light soaking (100 mW cm−2) at the maximum power point (without encapsulation, under nitrogen atmosphere, at 25 °C, with a white LED) was also evaluated (Fig. 6b). Apparently, the

(a)

24

Unsealed devices in ambient conditions @ 50 ± 10% humidity level

(b)

3. Conclusions In the present work, molecular engineering approaches have been successfully employed to simultaneously improve photovoltaic performance and long-term stability of perovskite devices by the employment of CsF:SnO2 ETL and D4TBP interlayer. CsF incorporation significantly improved the opto-electronic properties of SnO2 ETL whereas the introduction of D4TBP interlayer showed an admissible passivation in defects states at CsFAMA-based perovskite grain boundaries. The bestperforming cell reached to an efficiency of 21.7% (> 21.5% stabilized efficiency over 200 s) with a JSC of 23.01 mA cm−2, a VOC of 1.21 V, a FF of 0.78 in planar architecture. Such a high performance is also Unsealed devices under 1-sun illumination @MPP tracking

1.0

16 12

0.4 CsF:SnO2 /CsFAMA

0.2

th

15 day

Fresh

8

0.6

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PCE (Norm.)

PCE (%)

0.8 20

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0.0 Bare

IL

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IL

0

200

400

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800

Time (h) Fig. 6. Stability test of perovskite devices under different conditions. (a) Shelf-stability curves of the devices under moisture (blue and red curves represent bare and IL-treated devices, respectively, in fresh form whereas cyan and orange curves represent the aged devices). (b) The operational stability of PSCs under continuous illumination at MPP tracking. 141

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crowned with an admissible operational stability at MPP tracking by retaining more than 90% of its initial efficiency after 800 h. These findings highlight the crucial role of the molecular engineering in different layers of perovskite devices for highly efficient and long-term stable devices.

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