Defect passivation through electrostatic interaction for high performance flexible perovskite solar cells

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Defect passivation through electrostatic interaction for high performance flexible perovskite solar cells Deyu Xin , Shujie Tie , Xiaojia Zheng , Jianguo Zhu , Wen-Hua Zhang PII: DOI: Reference:

S2095-4956(19)30905-2 https://doi.org/10.1016/j.jechem.2019.11.015 JECHEM 1011

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Journal of Energy Chemistry

Received date: Revised date: Accepted date:

9 October 2019 8 November 2019 17 November 2019

Please cite this article as: Deyu Xin , Shujie Tie , Xiaojia Zheng , Jianguo Zhu , Wen-Hua Zhang , Defect passivation through electrostatic interaction for high performance flexible perovskite solar cells, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.11.015

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Communication

Defect passivation through electrostatic interaction for high performance flexible perovskite solar cells Deyu Xin a,b, Shujie Tie a,b, Xiaojia Zheng b,*, Jianguo Zhu a,*, Wen-Hua Zhang b,* a

Department of Materials Science, Sichuan University, Chengdu 610064, Sichuan,

China b

Sichuan Research Center of New Materials, Institute of Chemical Materials, China

Academy of Engineering Physics, Chengdu 610200, Sichuan, China *

Corresponding authors.

E-mail addresses: [email protected] (X. Zheng), [email protected] (J. Zhu), [email protected] (W.-H. Zhang). Declaration of interests ☒The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

ABSTRACT The light weight, good bending resistance and low production cost make flexible perovskite solar cells (PSCs) good candidates in wearable electronics, port-able charger, remote power, and flying objects. High power conversion efficiency (PCE) plays a crucial role on obtaining the high mass specific power of flexible devices. However, the performance for flexible PSCs is still having a large room to be improved. Here, we added the 2-amino-5-cyanopyridine (ACP) molecule with a polar electron density distribution in the perovskite precursor solution to improve the performance of flexible PSCs. The cyano groups with electron-withdrawing ability are expected to passivate positively charged point defects, while amines with electron donating ability are expected to passivate negatively charged point defects in

perovskite films. Thanks to the effectively passivation of defects at the grain boundary and surface of perovskite films, the PCE of flexible PSCs is obviously increased from 16.9% to 18.0%. These results provide a universal approach to improve performance of flexible PSCs by healing the defects in perovskite films through electrostatic interactions.

Keywords: Perovskite solar cell; Flexible; Electrostatic interaction; Defect passivation; Recombination

Organometal halide perovskites (OHPs) have good solubility due to their ionic nature, which enables low-temperature solution processes for perovskite solar cells (PSCs) manufacturing, including screen printing, blade coating, slot-die coating etc [1–4]. These manufacturing methods are well compatible with roll-to-roll (R2R) large-scale manufacturing processes to fabricate lightweight flexible PSCs, which can significantly reduce the manufacturing cost and show strong potential for application in wearable electronics, port-able charger, flying objects etc. [5–7]. Despite the power conversion efficiency (PCE) for PSCs on rigid glass substrate has reached 25.2% at present, the highest PCE for flexible PSCs is only 19.1%, which is still need to be improved for a large extent [8–11]. Defects in light absorb layer always deteriorate the performance of solar cells [12–14]. The defects in perovskites are primarily charged duo to their ionic nature. For example, the desorption of methylammonium cation (MA+) under moisture or heating leaves negatively charged PbI3− antisite defects [15]. While the

undercoordinated Pb ions can form positively charged Pb2+ defects [16]. These charged defects always limit both of the PCE and stability of PSCs. Passivating electronic trap states at surface and GBs has already proved to be a necessary to suppress the charge recombination process and improve the PCE of PSCs [17–20]. For example, the positively charged undercoordinated Pb ions can be passivated by bonding with Lewis base molecules which can donate electrons or share their electron pairs [21,22]. In addition, the charged sites can also be neutralized by other charged molecules or ions [23,24]. Although lots of work has been done on defects passivation, the effectively passivation of electronic defects at GBs in perovskite films still remains a great challenge due to the complexity of defects, and most of the used materials can only passivate one or two kinds of trap states [16,17,25]. Meanwhile, most of the passivation approaches are focused on the treatment of the top surface, which cannot effectively passivate the traps existing at inner GBs of perovskites [26,27]. Moreover, there are limited researches on defect engineering in flexible PSCs at present. In this study, 2-amino-5-cyanopyridine (ACP) molecules were added in the perovskite films to heal the defects at GBs and surface through electrostatic interaction. The electron-withdrawing cyano groups and electron donating amines can bind or coordinate with the positively and negatively charged defects, respectively. Thanks to the effectively healing of defects at the grain boundary and surface of perovskite films, the performance of flexible PSCs is obviously increased from 16.9% to 18.0%. These results open a new avenue for improving performance of flexible

PSCs by passivating the charged defects in perovskites via electrostatic interactions. Fig. 1(a) presents the chemical structure and calculated electrostatic potential profile (ESP) of ACP molecule. It is notable that the strong electron-donating amines unit and electron-withdrawing ability cyano groups make ACP an electronic polarity molecule. The significant electronic polarity in ACP endows its good ability to passivate charged defects in perovskites by a better binding or coordination features [18]. The ACP molecule was directly mixed in Cs0.06FA0.79MA0.15PbI2.55Br0.45 (CsFAMA) perovskite to ensure a good distribution at both GBs and surface of the polycrystalline films. Fourier-transform infrared spectroscopy (FTIR) spectroscopy was used to identify the present of ACP in perovskite films (Fig. 1b) [28]. The new peak around 2220 cm−1, which was introduced by cyano group, demonstrates the present of ACP in perovskites.

(b)

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3500

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Fig. 1. (a) Structure and electrostatic potential profile of ACP molecule; (b) FTIR spectra of the perovskite films. SEM image for (c) pristine CsFAMA and (d) CsFAMA-ACP films. In general, the introducing of additives will change the morphology of perovskite films. The change of morphology will affect the performance of the device in a large extent [29,30]. Herein, we record the morphology of pristine CsFAMA and CsFAMA with ACP additive (denoted as CsFAMA-ACP). The SEM images shown in Fig. 1(c, d) reveal that the grain size was almost no change after incorporating ACP, thus the influence of morphology change on electronic property of perovskites should be limited. Fig. S1 shows the X-ray diffraction (XRD) patterns of these two samples. Neither new peaks nor peak shifts were found after the addition of ACP, indicating that the addition of ACP doesn’t altered the crystal structure.

Pristine ACP

Normalized absorbance

Absorbance (a.u.)

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Fig. 2. (a) UV-vis absorption spectrum of the perovskite films; (b) normalized absorbance with the linear fit (green line) of the Urbach tail (absorbance was normalized at the band edge 1.7 eV); (c) steady-state PL spectra and (d) time-resolved PL decay curves of perovskite films deposited on glass. Fig. 2(a) shows the UV–vis absorption spectra of perovskite films, and an obvious change in the absorption edge was observed. Charged defects at GBs and surface in perovskites was reported to introduce localized states at or near the conduction band energy level to change band tail width. The Urbach energy (EU) is used to evaluate the band tail width in semiconductor, which can be calculated by Eq. (1), A(E) ∝ e (E/EU) where A is the absorbance, and E is the excitation energy in electronvolts [31].

(1)

Fourier transform photocurrent spectroscopy (FTPS) have been used as a very sensitive measure of the spectral response, and EU with a value of tens meV were demonstrated in high quality perovskite films around room temperature [31,32]. Here, we used UV-vis results to approximately estimate EU in our perovskite layers. EU was estimated to be 20.39 and 11.49 meV for the pristine CsFAMA and CsFAMA-ACP films, respectively (Fig. 2b). The reducing in EU after ACP addition indicates a significant decrease in defects by passivating the charged defects in perovskites. To further characterize the influence of ACP on recombination process in perovskite films, we measured the steady state photoluminescence (PL) and time-resolved photoluminescence (TRPL) of pristine CsFAMA and CsFAMA-ACP on glass substrates. Fig. 2(c) displayed an obvious increase in PL intensity of CsFAMA-ACP, suggesting a reduction of the non-radiative recombination in CsFAMA-ACP film. The longer PL lifetime in CsFAMA-ACP (2397 ns) than CsFAMA (2113 ns) further indicates lower recombination rates in CsFAMA-ACP (Fig. 2d). These results verify that ACP can effectively passivate defects and suppress charge recombination in perovskite films.

Current density (mA cm-2)

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ACP

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Voc (mV)

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Voltage (V)

Fig. 3. (a) Photograph and (b) J-V curves for the flexible devices. To evaluate the effect of ACP on the performance of PSCs, we fabricated planar-structured flexible PSCs (Fig. 3a). SnO2 was prepared on PET/ITO substrate as electron transport layer. Compact perovskite layer was deposited by green anti-solvent assisted crystallization (ASAC) method with methoxybenzene (PhOMe) anti-solvent [33]. Current density-voltage (J-V) curves of PSCs based on pristine CsFAMA and CsFAMA-ACP were recorded under AM 1.5 G, 1 sun illumination (Figs. 3b and S2). The CsFAMA device possessed an open circuit voltage (Voc) of 1105 mV, a short circuit current density (Jsc) of 22.55 mA cm–2, a fill factor (FF) of 68.02%, and a PCE of 16.95% under reverse scan. In contrast, the performance for CsFAMA-ACP device had a Voc of 1120 mV, Jsc of 22.30 mA cm–2, and FF of 72.07%, producing a PCE of 18.00%. The performance improvement was mainly coming from the increases of FF

and Voc. These results demonstrate one can obtain high quality perovskite films with excellent optoelectronic properties by healing the defects in perovskite films through electrostatic interactions. 100 25

EQE (%)

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-Z'' (

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Integrated Jsc (mA cm-2)

(a) 80

0

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Wavelength (nm) 101

1500

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Z' ( 1.14

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T/q 7K

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Current (A)

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3K

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Pristine ACP

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Light intensity (mW cm-2)

Fig. 4. (a) EQE spectra and (b) electrochemical impedance spectroscopy of PSCs; (c) dark current-voltage curves of the electron-only devices for the pristine CsFAMA and CsFAMA-ACP device; (d) light intensity dependence of Voc for the PSCs. Fig. 4(a) shows the external quantum efficiency (EQE) spectra of pristine CsFAMA and CsFAMA-ACP devices. The similar integrated Jsc of both devices is consistent well with the J-V curves shown in Fig. 3(b). To obtain charge transport and transfer kinetics in PSCs, electrical impedance spectroscopy (EIS) was recorded under light condition without bias. Fig. 4(b) displays the Nyquist plots and fitting curves for both devices. Two arcs are obtained in the Nyquist plots, where the first arc at high frequency can be mainly attributed to the charge transport features in bulk materials,

and the arc at low frequency range explains the charge recombination process at interface and GBs. The equivalent circuit diagram used to fit the Nyquist plots is provided in Fig. S3. Both devices showed similar ohmic resistance (Rs) due to the similar device structure. After the addition of ACP, the charge transport resistance (Rtr) decreased from 316 to 237 Ω, which means the PSCs made of CsFAMA-ACP show better charge extraction properties. The recombination resistance (Rrec) value increased from 3484 to 9229 Ω after ACP incorporation, indicating a decrease in the charge recombination rates. The better charge extraction properties and suppressed charge recombination features govern the improvement of FF and Voc. Fig. 4(c) presents the space-charge limited current (SCLC) for electron-only devices (SnO2/perovskite/PCBM). The trap densities (nt) were calculated from the trap-filling region using the following Eq. (2),

𝑛𝑡 =

2𝑉𝑇𝐹𝐿 𝜀𝜀0 𝑒𝐿2

(2)

where VTFL is the trap-filled limit voltage, ε is the relative dielectric constant, ε0 is the vacuum permittivity, e is the elementary charge, and L is the thickness of the perovskite layers [34,35]. The VTFL of the CsFAMA and CsFAMA-ACP films are 0.24 and 0.14 V, respectively. Therefore, the nt was calculated to be 3.1×1015 and 1.8×1015 cm−3 for CsFAMA and CsFAMA-ACP, respectively. The dramatically decrease in trap density in CsFAMA-ACP suggests once again that ACP can effectively decrease charge recombination rates in perovskite films and thus improve the performance of the PSCs. J-V curves measured under light illumination ranging from 12 to 100 mW cm–2

were used to further understand the recombination mechanism in these devices. Voc as a function of the logarithm of the light intensity is shown in Fig. 4(d). It was found that Voc increased monotonically with light intensity for both solar cells, while the slope for CsFAMA-ACP solar cell is smaller than that of pristine CsFAMA device, which indicated a lower diode ideality factor (n) value for CsFAMA-ACP PSCs. It has been reported that a lower n value means less recombination in the space-charge region, revealing suppressed charge recombination in CsFAMA-ACP films.[36, 37] This results consist well with the EIS and SCLC results. Fig. S4 presents the power law dependence of the Jsc with light intensity (J ∝ Iα). A solar cell with no space-charge effect will have an α value close to 1 whereas a solar cell with space-charge-limited current due to carrier imbalance will have a power law relationship with α = 0.75 [38,39]. Both the CsFAMA-pristine and CsFAMA-ACP devices show a linear relation of photocurrent with light intensity with a slope approaches 1 on a double-logarithmic scale, indicating that charge-collection efficiency is independent of light intensity. This implies sufficient electron and hole mobility and less charge-transport barrier existing in the solar cells, which is in agreement with the good photo-response characteristics in EQE. Finally, we evaluate the reliability of our flexible PSCs. Bending fatigue property of the devices under a bending radius of 0.9 cm is shown in Fig. S5. The PCE can retain 89% after 450 bending cycles. Despite obvious reduction on the PCE of the device was found under bending, it is notable that the PCE of the PSC can be recovered to its initial value after a stress relaxation overnight. This PCE

self-recovered phenomenon has been also observed in our previous work [40]. This result demonstrates a good durability of our flexible PSCs. In summary, we report a defect passivating strategy through electrostatic interactions to passivate the defects in flexible PSCs. ACP molecule with polar distributed electron density can effectively passivate the charged electronic defects in perovskite films. The improved charge extraction properties and suppressed charge recombination in perovskites with ACP incorporation obviously boosted the PCE of flexible PSCs from 16.9% to 18.0%. These results provide a universal method to enhance the performance of flexible PSCs by healing the defects in perovskite films.

Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (No. NSFC21773218).

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Current density (mA cm-2)

Graphical Abstract

25

PCE = 18%

20 15 10 5 0 0.0

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Voltage (V)

Flexible perovskite solar cell with power conversion efficiency of 18% is obtained by healing the defects in perovskite films through electrostatic interactions.