Trap-State Passivation by Nonvolatile Small Molecules with

May 24, 2019 - Trap-state passivation has been validated to efficiently improve the performance of perovskite solar cells. Small volatile molecules an...
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Article Cite This: J. Phys. Chem. C 2019, 123, 14223−14228

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Trap-State Passivation by Nonvolatile Small Molecules with Carboxylic Acid Groups for Efficient Planar Perovskite Solar Cells Lin Guan, Nan Jiao, and Yiping Guo* State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Material Building D, 800 Dongchuan Road, Minhang District, Shanghai 200240, China

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ABSTRACT: Trap-state passivation has been validated to efficiently improve the performance of perovskite solar cells. Small volatile molecules and polymers both have the ability to reduce the trap states in perovskite. Herein, we demonstrate the feasibility of passivation by nonvolatile small molecules with carboxylic acid groups such as benzoic acid, p-phthalic acid, and trimesic acid. They can obviously increase the fill factor of the photocurrent density−voltage curves. Furthermore, the relationship between the molecular structure and passivation effect is proposed by fixing the concentration of carboxylic acid groups. Trimesic-acid-doped perovskite solar cells significantly increase the power conversion efficiency from 12.52 ± 0.67 to 14.51 ± 0.81 (champion efficiency, 15.81%) under standard AM 1.5G illumination. Our work expands the chemical additives in perovskite and further demonstrates how the additive molecular structure influences the performance of solar cells.

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passivation as effective additives in the perovskite layer.17 Additionally, benzoic acid (BA) has been widely used as a “template” to form self-assembled monolayers (SAMs) to modify the charge transfer layers.18−20 Considering its carboxylic acid groups, BA and its derivatives may also be potential to passivate defects in perovskite layers. On the other hand, PSCs by using mesoporous TiO2 as the electron transfer layer (ETL) can achieve a PCE of nearly over 20% without hysteresis.21,22 However, the stability of perovskite can be negatively affected by compact TiO2 under ultraviolet illumination23,24 due to light-induced desorption of surface-adsorbed oxygen.25 Research studies show that SnO2based PSCs exhibit better stability.26,27 Besides, the SnO2 layer can be sintered at a lower temperature than mesoporous TiO2 (>450 °C). Therefore, SnO2 or SnOx is widely considered to replace TiO2 to improve the stability27−29 of planar PSCs. In this work, we obtained effective defect passivation results by doping BA, p-phthalic acid (PTA), and trimesic acid (TA) in perovskite films using SnOx as the ETL. First, we optimized the doping concentration of carboxylic acid groups (c(−OOH)) by adding different amounts of BA. Second, BA, PTA, and TA were doped separately at the same c(−OOH) to discuss whether the concentration of molecules and relative position of −OOH can affect the passivation. Interestingly, we find that BA-doped samples statistically exhibited lower VOC than that of reference samples (ref for short), while doping PTA or TA successfully enhanced the VOC. By doping 0.067%

INTRODUCTION Owing to suitable band gaps, facile film deposition methods, and excellent optical properties, hybrid organometal halide perovskites have been extensively investigated for optoelectronic applications, especially perovskite solar cells (PSCs). During recent years, a certificated power conversion efficiency (PCE) of 24.2% has been realized after continuously rapid improvement in the quality of perovskite film and structural optimization.1−5 It is well known that grain boundaries and undercoordinated ions substantially exist in polycrystalline perovskite films, like other semiconductor (e.g., Si) films.6−8 Therefore, defect states at grain boundaries in perovskite films would unavoidably lead to higher nonradiative recombination rates and energy loss. The difference of electrical potentials and ion migration rates between grain boundaries and bulks has been investigated by Yun et al.9 and Shao et al.10 Such a difference would exert harmful microcosmic effects on electrical properties, which finally result in macroscopically worse performance of PSCs, such as lower open circuit voltage (VOC), fill factor (FF), longterm stability, etc. Thus, defect passivation of perovskite films is a wise strategy for boosting PCE, especially in planar PSCs. Small volatile molecules, such as chloroform and pyridine, have been applied to passivate defect states effectively by coordinating with Pb atoms.11−15 Zuo et al.16 demonstrated the passivation effect of long-chain polymers with different functional groups (amino, Lewis base pyridine, and carboxylic acid groups). These polymers can be stably localized at grain boundaries and strongly coordinated with perovskite. However, it is hard to form high quality films for polymer-doped perovskite via conversional antisolvent methods. On the other hand, nonvolatile small molecules are rarely reported for © 2019 American Chemical Society

Received: March 20, 2019 Revised: May 24, 2019 Published: May 24, 2019 14223

DOI: 10.1021/acs.jpcc.9b02621 J. Phys. Chem. C 2019, 123, 14223−14228

Article

The Journal of Physical Chemistry C

Figure 1. (a) J−V curves of ref, 0.2% BA, 0.1% PTA, and 0.067% TA. (b) J−V hysteresis behavior and (c) steady-state JSC and output power of ref and 0.067% TA.

Figure 2. Statistical results of (a) VOC, (b) JSC, (c) FF, and (d) PCE with different additives.

TA, the PCE can be increased to 14.51 ± 0.81 (champion efficiency, 15.81%) versus ref (12.52 ± 0.67).

mixture of DMF, DMSO, and GBL (4:3:3 v/v) for 1.0 M perovskite precursor solution under constant stirring at 50 °C. BA, TMA, and TA (0.036 M) were dissolved, respectively, in DMSO for further doping in MAPbI3 solution. The perovskite layer was deposited on an SnOx film at 1000 rpm for 15 s and 5000 rpm for 25 s. Then, 200 μL of CB was added dropwise at about 14 s before the end of the second step. Spiro-OMeTAD solution was prepared by dissolving 72.3 mg of SpiroOMeTAD, 28.8 μL of t-BP, and 17.5 μL of Li-TFSI solution (520 mg/mL in acetonitrile) in 1 mL of CB. It was spin-coated on the perovskite layer at 4000 rpm for 30 s. Finally, an 80 nm Ag counter electrode was thermally evaporated on the SpiroOMeTAD layer to complete the fabrication of devices. Characterization. X-ray diffraction (XRD) patterns were measured by a Rigaku D/MAX 2400 diffractometer (Cu Kα). UV−vis spectra were measured by an Agilent UV−vis spectrophotometer. Morphologies of perovskite films and cross-section of PSCs were characterized by a Hitachi S4800 field-emission scanning electron microscope (SEM). A Keithley 2400 source meter was applied to record the J−V curves of PSCs under AM 1.5G illumination. External quantum efficiency (EQE) spectra and integrated current density were

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EXPERIMENTAL SECTION Materials. PbI2 (99.9%), 4-tert-butylpyridine (t-BP), and Spiro-OMeTAD were purchased from Yingkou Optimal Choice Trade CO., Ltd. (China). N,N-Dimethylformamide (DMF, anhydrous, 99.8%), gamma-butyrolactone (GBL, anhydrous, 99.9%), and dimethylsulfoxide (DMSO, anhydrous) were purchased from Aladdin Reagent. Chlorobenzene (CB, anhydrous, 99.8%) and Li bis(trifluoromethanesulfonyl)imide (Li-TFSI) were purchased from Sigma-Aldrich. SnCl2· 2H2O (SP) came from MACKLIN reagent. MAI came from Shanghai MaterWin New Materials Co, Ltd. (China). Solar Cell Fabrication. Patterned fluorine-doped tin oxide (FTO) glass was soaked in piranha solution for 10 min and ultrasonically cleaned with deionized water and ethanol for 30 min, respectively. SnCl2·2H2O (0.08 M) was dissolved in ethanol and then the solution was stirred under an oxygen atmosphere for about 1 day. The precursor was spin-coated on FTO substrates at 3000 rpm for 30 s, followed by annealing at 190 °C for 1 h. PbI2 and MAI (1:1 n/n) were dissolved in a 14224

DOI: 10.1021/acs.jpcc.9b02621 J. Phys. Chem. C 2019, 123, 14223−14228

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The Journal of Physical Chemistry C Table 1. Device Parameters of ref, 0.2% BA, 0.1% PTA, and 0.067% TA ref 0.2% BA 0.1% PTA 0.067% TA

JSC (mA/cm2)

VOC (V)

sample 1.018 1.015 1.054 1.074

(0.996 (0.977 (1.015 (1.029

± ± ± ±

0.016) 0.020) 0.028) 0.027)

20.56 21.38 21.32 21.54

(20.05 (20.66 (20.54 (20.77

± ± ± ±

FF

0.45) 0.67) 0.47) 0.51)

0.656 0.723 0.695 0.705

(0.625 (0.676 (0.658 (0.679

PCE (%) ± ± ± ±

0.023) 0.020) 0.033) 0.024)

13.57 14.74 14.57 15.81

(12.52 (13.65 (13.57 (14.51

± ± ± ±

0.67) 0.70) 0.73) 0.81)

Figure 3. Schematic diagram of (a) the device architecture and additives (e.g., TA) doped in perovskite and (b) the interface between perovskite and SnOx with BA, PTA, and TA.

voltage of 0.7734 V exhibits a higher output PCE around 15.5% than ref at 0.7133 V (a PCE of about 12.9%). A steadystate output at longer time is demonstrated in Figure S2. The decay was adjusted to 1 s to prolong the test time. Interestingly, a more obvious fluctuation is found at longer decay time. The unencapsulated sample of 0.067% TA shows a bit better stability than ref. For demonstrating the different effect of defect passivation, the statistical results of VOC, JSC, FF, and PCE are listed in Figure 2a−d and Table 1. Remarkably, the VOC values of 0.1% PTA-doped PSCs (0.1% PTA for short) and 0.067% TA are statistically higher than that of ref, whereas that of 0.2% BAdoped PSCs (0.2% BA for short) seems to be lower as shown in Figure 2a. Zuo et al.16 have reported a higher VOC after defect passivation by poly(acrylic acid) (PAA). Moreover, in accordance with previous reports,19,20 no significant increase in VOC was found after the ETL or hole transfer layer was decorated by a BA SAM. The planar perovskite film was deposited on SnOx as shown in Figure 3a. Thus, the loss of VOC in 0.2% BA can be attributed to the weak contract between BA and perovskite (SnOx) when the −OOH is coordinated with SnOx (perovskite). In contrast to BA, PTA and TA can coordinate with both SnOx and perovskite as illustrated in Figure 3b. Their good contact at the interface, as well as coordination with Pb, makes 0.1% PTA and 0.067% TA show higher VOC than ref. In Figure 2b,c, JSC and especially FF of all doped samples are statistically higher than those of ref. The increase of JSC and FF results from the reduction of the defects and carrier recombination at grain boundaries by additive coordination. Therefore, an obvious promotion of PCE is obtained as shown in Figure 2d. Additionally, the higher FF but lower VOC of 0.2% BA results in a similar PCE in average of 0.2% BA and 0.1% PTA. Compared with 0.1% PTA, the lower concentration of TA seems to be beneficial for the further increase of VOC and FF, which finally leads to the better performance of 0.067% TA. Thus, it can be concluded that the better contact at the interface and lower concentration of the matrix (functional groups with no effect of passivation) can decrease the defects more effectively.

measured by an SM-250 Hyper Monolight system. Perovskite films covered by PMMA were used to characterize the steadystate photoluminescence (PL) spectra and time-resolved PL (TRPL) decay by steady-state and time-resolved fluorescence spectrofluorometers. Electrochemical impedance spectroscopy (EIS) was measured in the dark by impedance/gain-phase analyzer (Solartron SI 1260) from 100 kHz to 10 Hz at different bias voltages. The Fourier transform infrared (FTIR) spectrum was measured by PerkinElmer Spectrum 100. The perovskite films were prepared on NaCl substrates.

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RESULTS AND DISCUSSION PSCs were fabricated with the structure of FTO/SnOx/ perovskite/Spiro-OMeTAD/Ag. Three additives were doped in perovskite (e.g., TA). A suitable c(−OOH) was first investigated by doping BA with different contents (0, 0.08, 0.2, and 0.5% vs MAPbI3). Figure S1 presents the photocurrent density−voltage (J−V) curves of different devices. It is found that the PSC based on 0.2% BA shows the best performance of 14.74% (13.57% of ref), which confirms the passivation effect of −OOH. However, when the content of BA is over 0.5%, the PCE of devices begins to gradually attenuate. Therefore, c(−OOH) was controlled at 0.2% to demonstrate how molecular structure affects the passivation. Then, BA, PTA, and TA were chosen to compare the passivation effect for further study. According to the different amount of carboxylic acid groups on BA, PTA, and TA, their doping concentration was set as 0.2, 0.1, and 0.067%, respectively. The J−V curves of champion devices of ref or doping samples are presented in Figure 1a. All doped samples exhibit better PCE than ref. It is worth noting that the 0.067% TA-doped PSC (0.067% TA for short) shows obviously better performance with a VOC of 1.07 V, a JSC of 20.9 mA/cm2, an FF of 0.704, and a PCE of 15.81%, in comparison with the ref (a VOC of 1.01 V, a JSC of 20.5 mA/ cm2, a FF of 0.656, and a PCE of 13.57%). Both ref and 0.0067% TA have negligible hysteresis as shown in Figure 1b, which reveals a relatively judicious evaluation of PCE. Furthermore, in Figure 1c, steady-state JSC and output power are applied to simulate the real working condition of PSCs. The decay of voltage was 0.17 s. The 0.067% TA at a bias 14225

DOI: 10.1021/acs.jpcc.9b02621 J. Phys. Chem. C 2019, 123, 14223−14228

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The Journal of Physical Chemistry C

Figure 4. (a) EQE spectra and the integrated current density of ref and 0.067% TA. (b) PL spectra and (c) TRPL decay spectra of perovslite and 0.067% TA-doped perovskite.

the PL spectra of the perovskite layer of ref and 0.067% TA. The doping of TA greatly increases the intensity of PL spectra at about 770 nm, which reveals a lower nonradiative decay rate of 0.067% TA-doped perovskite than that of ref. This result strongly proves that the coordination between Pb and −COOH decreases the defects and recombination of charge carriers at the grain boundary. Therefore, the better performance of 0.067% TA discussed above is in accord with the higher PL spectra. Additionally, TRPL decay spectra in Figure 4c were operated to characterize the quenching of free carriers. The curves are fitted by the double-exponential function. It is well acknowledged that the fast decay lifetime (τ1) and the slow decay lifetime (τ2) account for the radiative bimolecular decay and trap-assisted recombination, respectively. τ1 of the TAdoped perovskite layer is 4.2 ns, which is almost the same as that of ref (3.8 ns). τ2 values of perovskite layers in 0.067% TA and ref are 32.7 and 14.5 ns, respectively. A longer lifetime exhibits a lower recombination rate of free carriers excited by light. Thus, by doping TA, defects in perovskite have been effectively decreased. The less recombination proved by PL spectra and TRPL decay spectra facilitates the carrier transportation in perovskite and finally decrease the energy loss and enhance the performance in PSCs. EIS was widely carried out to study the kinetic processes in PSCs. A bias voltage from 0.7 to 0.9 V and an amplitude sinusoidal voltage of 10 mV were applied on samples to measure the Nyquist plots in the dark. The parameters, such as charge recombination resistance (Rrec) and capacitance, can also be utilized to investigate the defects in perovskite films. Figure 5a shows the Nyquist plots of ref and 0.067% TA at a bias voltage of 0.75 V. The typical semicircle, similarly demonstrated in some articles,16,31 is related to the Rrec at the perovskite films as well as the interfaces of perovskite and charge transfer layers. The series resistance (Rs) of the solar cells is relatively much smaller than Rrec. Thus, Rs cannot be observed clearly in the plots. The larger curve of 0.067% TA is associated with higher Rrec than that of ref. Moreover, the Rrec of ref and 0.067% TA is presented and linear fitted as a function of bias voltage (0.7−0.9 V) in Figure 5b. The Rrec and its difference of the two samples has negative correlation with bias voltage. The results indicate that defect passivation by doping TA limits the charge recombination effectively, which facilitates the charge injection to SnOx and Spiro-OMeTAD for increasing VOC and JSC. As illustrated in Figure 5c, the capacitance (C) is linear fitted under the logarithmic coordinate. C can reflex the relative amount of charge carriers stored at the traps. According to previous research studies,15,32

Figure S3 shows the XRD patterns of perovskite layers with three additives (BA, PTA, and TA). They all exhibit the pure perovskite phase without PbI2 and similar full width at half maxima around 0.16 of (110) peaks. No obvious phase change can be seen in the XRD patterns. The cross-sectional SEM images in Figure S4 exhibit the PSCs fabricated on FTO glasses. The SnOx layer spin-coated on FTO seems negligible under the perovskite layer. For identifying the existence of an SnOx layer, top-view images of bare FTO and the SnOx layer are shown in Figure S5. The thickness of perovskite doped or not is similar at around 250 nm, which is independent of the additives. Taking the UV−vis spectrum in Figure S6 and SEM images in Figure S7 into consideration, our doping in perovskite has almost no significant influence on the crystallinity, morphology, or thickness of perovskite films. BA, PTA, and TA doped in perovskite cannot affect the crystallization process of perovskite. Their main distribution at the grain boundary finally results in the weak relation between addition of BA, PTA, or TA and morphologies of perovskite. Therefore, the performance promoted by doping BA, PTA, or TA can be attributed to grain boundary passivation because of the coordination between Pb and −COOH. Further characterization revealing the difference after doping was carried out to certify the effect of reducing the trap states. The FTIR spectrum is demonstrated in Figure S8 to testify the existence of TA in perovskite films. To increase the intensity of TA, we use 1% TA-doped perovskite to show the characteristic peaks of TA. Compared with ref, the transmittance of the peak near 1250 nm is lower than that of the peak around 1100 nm and the width of the peak near 1250 nm in 1% TA-doped perovskite seems larger than that in ref, which is in accordance with the doping of TA. Figure 4a shows the EQE spectra of ref and 0.067% TA. Both curves of ref and 0.067% TA exhibit a wide photoresponse region from 300 to 800 nm. The rapid decrease of EQE at 650 nm is possibly related to the thinner perovskite layer by comparing with other research studies.30 The higher EQE of 0.067% TA than ref means that more electrons generated by light in a wide range are transferred to the electrodes. Integrated JSC calculated by EQE is 19.73 and 18.57 mA/cm2 of 0.067% TA and ref, respectively. This result is a bit lower than the statistical average JSC in Figure 2b. The higher integrated JSC and EQE of 0.067% TA can respond to the lower density of traps, in which the electrons are easily recombined to result in a lower current density. Therefore, the doping of TA prevents the recombination of electrons to realize trap passivation. For a better insight of charge properties in perovskite before and after being doped by TA, Figure 4b shows a comparison of 14226

DOI: 10.1021/acs.jpcc.9b02621 J. Phys. Chem. C 2019, 123, 14223−14228

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The Journal of Physical Chemistry C

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J−V curves of ref, 0.08% BA, 0.2% BA, and 0.5% BA, XRD patterns, UV−vis spectra, and SEM images of perovskite films with or without additives, crosssectional SEM images of ref, 0.2% BA, 0.1% PTA, and 0.067% TA, SEM images of FTO and SnOx deposited on FTO, and normalized steady-state output power of ref and 0.067% TA (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yiping Guo: 0000-0003-0251-450X Notes

The authors declare no competing financial interest.

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Figure 5. Electronic impedance measurements of ref and 0.067% TA. (a) Nyquist plots measured at a bias of 0.75 V in the dark. (b) Rrec and (c) C at different bias voltages. (d) DOS versus electron energy level.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos 11874257 and 11474199). Instrumental Analysis Center of Shanghai Jiao Tong University and National Engineering Research Center for Nanotechnology are sincerely acknowledged for assisting with relevant analyses.

the density of trap states (DOS) are proportional to C. Therefore, the fitting curves of DOS versus energy level in Figure 5d can be calculated from the linear relationship in Figure 5c. The lower CPE and DOS of 0.067% TA at all bias voltages show less defects after TA coordinates with Pb. This result is in agreement with the TRPL decay spectra in analyzing the kinetic properties after doping TA.

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CONCLUSIONS In summary, the doping of BA, PTA, and TA, especially TA, as demonstrated above is efficient to increase the PCE of PSCs. The PCE has been increased from 12.52 ± 0.67 to 14.51 ± 0.81 (champion efficiency, 15.81%) by doping 0.067% TA. The additives do not obviously influence the crystallinity or morphology of the perovskite layers. Therefore, the better performance is attributed to the trap passivation of the coordination between the carboxylic acid groups and Pb. Furthermore, by controlling the concentration of −OOH, the different passivation effect of BA, PTA, and TA can be explained by their concentration and the contract at the interface between SnOx and perovskite. The various VOC values of 0.2% BA and 0.1% PTA seem relative to the change of the interface. The further improvement of 0.067% TA compared to 0.1% PTA should be attributed to the molecular structure. Lower concentration of the matrix (benzene ring in this article) can result in a better passivation effect. Moreover, as depicted in Figure S1, lower coordinated −OOH (0.08% BA) may lead to worse performance than 0.2% BA. Thus, the low concentration of TA and PTA does not greatly decrease the concentration of coordinated −OOH from the discussion above, which partially proved the homogeneous distribution of additives in perovskite layers. The decreased density of trap states in 0.067% TA is successfully evidenced by the investigation of EQE, PL, and TRPL spectra and electronic impedance measurements.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b02621. 14227

DOI: 10.1021/acs.jpcc.9b02621 J. Phys. Chem. C 2019, 123, 14223−14228

Article

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DOI: 10.1021/acs.jpcc.9b02621 J. Phys. Chem. C 2019, 123, 14223−14228