Xiao Zhang, Sai Ma, Jingbi You, Yang Bai, and Qi Chen,†

1Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China

2Experimental Center of Advanced Materials, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China

3Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

4Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100081, China

Abstract: Interfacial engineering has made an outstanding contribution to the development of high-efficiency perovskite solar cells (PSCs). Here, we introduce an effective interface passivation strategy via methoxysilane molecules with different terminal groups. The power conversion efficiency (PCE) has increased from 20.97% to 21.97% after introducing a 3-isocyanatopropyltrimethoxy silane (IPTMS) molecule with carbonyl group, while a trimethoxy[3-(phenylamino)propyl] silane (PAPMS) molecule containing aniline group deteriorates the photovoltaic performance as a consequence of decreased open circuit voltage. The improved performance after IPTMS treatment is ascribed to the suppression of non-radiative recombination and enhancement of carrier transportation. In addition, the devices with carbonyl group modification exhibit outstanding thermal stability, which maintain 90% of its initial PCE after 1500 h exposure. This work provides a guideline for the design of passivation molecules aiming to deliver the efficiency and thermal stability simultaneously.

Key words: perovskite solar cells; terminal groups; interfacial engineering; thermal stability

1. Introduction

Organic-inorganic halide perovskite solar cells (PSCs)have attracted substantial attention due to their unique properties, such as high absorption coefficient[1,2], long carrier diffusion length[3-5]and low-cost fabrication[6]. The power conversion efficiency (PCE) of PSCs quickly increased from 3.8% to 25.5% over the past decade, which reached that of single-crystal silicon solar cells[7-12]. Although perovskite materials have more benign defects compared to other semiconductors[13],the defects located at the grain boundary or interface are still harmful to the device efficiency because of trap-assisted nonradiative recombination[10,14-17], and device stability[18,19], in particular.

Various strategies have been developed to minimize the non-radiative recombination losses, such as composition engineering[20,21], assisted by bulk[22]or surface post-treatment[23].Among them, surface passivation has made a significant contribution to the enhancement of device efficiency and stability over the past decade[10,11,24-28]. Since then, many passivation molecules have been reported to reduce surface traps by neutralizing the surface charge or uncoordinated sites. For example, Lewis base molecules were generally used to passivate surface under-coordinated Pb ions by donating electrons or sharing electron pairs[29]. In addition, Lewis acid molecules can accept electrons from the Lewis base defects on perovskite films, such as phenyl-C61-butyric acid methyl ester(PCBM)[30,31]. In order to maximize the impact of surface molecules modification on device performance, zwitterion have been applied to passivate both positively and negatively charge defects[32-34]. The above successful attempts inspire us to explore more effective molecules and explain the mechanism of improved performance.

In this study, we introduce methoxysilane with different terminal groups, including sulfhydryl, carbonyl and aniline groups, as a surface passivation layer aiming to enhance the device efficiency and thermal stability. It is found that the terminal group of methoxysilane is critical for enhanced PCEs.Among the molecules in investigation, the devices based on IPTMS with a carbonyl group shows the best PCE and a stabilized PCE of nearly 22%. Additionally, the devices with carbonyl group modification exhibit outstanding thermal stability, which maintain 90% of its initial PCE after 1500 h exposure.

2. Experiment section

2.1. Materials

The SnO2colloid precursor (tin (IV) oxide, 15 wt% in H2O colloidal dispersion), (3-mercaptopropyl)trimethoxysilane(MPTMS) and trimethoxy[3-(phenylamino)propyl] silane were purchased from Alfa Aesar.N,N-dimethylformamide (DMF,99.9%), dimethyl sulfoxide (DMSO, 99.5%), isopropanol (IPA,99.99%), chlorobenzene (CB, 99.9%), lead iodine (PbI2) and cesium iodine were obtained from Sigma Aldrich. Lithium bistrifluorosulfonyl imide (Li-TFSI) was purchased from Aldrich. 4-terbutyl pyridine (tBP) was purchased from Innochem. FAI was purchased from Greatcell Solar. MACl and poly[bis(4-phenyl)(2,4,6-trimethylphenyl) amine (PTAA) were purchased from Xi’an Polymer Light Technology in China. 2,2’,7,7’-tetrakis (N,N-di-4-methoxyphenylamine)-9,9’-spirobifluorene (Spiro-OMe-TAD) was purchased from Lumtec. 3-Isocyanatopropyltrimethoxy silane was purchased from Aladdin.

Fig. 1. (Color online) (a) Chemical structure of PAPMS, MPTMS and IPTMS passivation layer used in this work. (b) Schematic illustration of the formation process of perovskite films based on a sequential deposition. (c) XPS spectra of Pb 4f for the perovskite films with and without siloxane treatment. (d) XRD pattern of perovskite films with and without siloxane treatment.

2.2. Film and device fabrication

The ITO substrate was sequentially cleaned with detergent deionized water, acetone and ethanol. Before use, the substrates were treated with ultraviolet ozone for 45 min.The SnO2colloidal dispersion (2.67%, diluted by deionized water) was spin-coated on the substrate at 4000 rpm for 30 s,and then annealed at 150 °C for 30 min. After that, a 1.5 M PbI2mixed with 7% CsI in DMF:DMSO (9 : 1) solvent was spincoated onto SnO2at 2300 r.p.m for 30 s and dried at 70 °C for 1 min. After cooling the PbI2film to room temperature, a mixed organic cation solution (FAI 0.52 M; MACl 0.13 M) was spin-coated onto PbI2at 2700 rpm for 30 s and then annealed at 150 °C for 10 min in air. 0.05 mL PAPMS, 0.5 mL MPTMS and 0.05 mL IPTMS solution dissolved in 1 mL CB solution, respectively, and further spin-coated onto the perovskite surface at 4000 rpm for 30 s, and then annealed at 150 °C for 10 min. A spiro-OMeTAD solution was spin-coated onto the perovskite film at a spin rate of 3000 rpm for 30 s.Spiro-OMeTAD solution was prepared with the following compositions: 75 mg spiro-OMeTAD, 35μL Li-TFSI (260 mg/mL in acetonitrile) and 30μL 4-ter-butyl pyridine in 1 mL CB. For the thermal and humidity stability test, 0.5 mL PTAA solution(10 mg/mL in CB) with additives of 10μL Li-TFSI (260 mg/mL in acetonitrile) and 10μL 4-ter-butyl pyridine mixed with 0.5 mL spiro-OMeTAD solution, and spin-coated at 1500 rpm for 30 s to replace spiro-OMeTAD as a hole transport layer. Finally, a 100 nm gold electrode was deposited by thermal evaporation.

2.3. Film and device characterization

X-ray diffraction (XRD) patterns were recorded on a PANalytical X-ray diffractometer. Current-voltage characteristics were recorded by using a Keithley 2400 source-measure unit.The photocurrent was measured under AM1.5G illumination at 100 mW/cm2under a Newport Thermal Oriel 91192 1000 W solar simulator. Light intensity was calibrated with a National Institute of Metrology (China) calibrated KG5-filtered Si reference cell. The effective area of each cell was 0.102 cm2defined by masks for all the photovoltaic devices discussed in this work. External quantum efficiencies (EQE) were performed on an Enli Technology (Taiwan) EQE measurement system. A calibrated silicon diode with a known spectral response was used as a reference. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an XPS AXIS Ultra DLD (Kratos Analytical). The transient photocurrent(TPC) decay measurements were obtained on Molex 180081-4320 simulating one sun working condition. The electrochemical impedance spectroscopy (EIS) measurements were determined by the electrochemical workstation (Germany,Zahner Company), employing light emitting diodes driven by Export (Germany, Zahner Company). The photoluminescent(PL) mapping spectra were performed on an Enli Technology measurement system.

Fig. 2. (Color online) Photovoltaic performance distribution of PSCs with different termination groups of methoxysilane (a) PCE, (b) open-circuit voltage (Voc), (c) short-circuit current density (Jsc), (d) fill factor (FF) for control, PAPMS, MPTMS and IPTMS devices. (e) Current density-voltage(J-V) curves of the control device and self-cross-linking devices with PAPMS, MPTMS, and IPTMS. (f) EQE spectra for the control and IPTMS devices. (g, h) Steady-state power output for the best-performing of (g) control and (h) IPTMS device.

3. Results and discussion

Chemical structures of methoxysilane employed in this work are shown in Fig. 1(a). Previous research has proved that sulfhydryl, carbonyl and aniline groups have a strong interaction with perovskites and can effectively passivate defects.Inspired by these results, we selected three silanes with different terminal functional groups to study the effect of surface functionalization on device performance and long-term stability. The perovskite polycrystalline films are prepared using a two-step spin-coated method and the schematic is depicted in Fig. 1(b). The methoxysilane molecular is dissolved in chlorobenzene solution, and spin-coated on the surface of perovskite films to form the passivation layer.

X-ray photoelectron spectroscopy (XPS) measurement is conducted to verify the interaction between the siloxane and uncoordinated Pb atom. As shown in Fig. 1(c), the peaks at 142.5 and 137.6 eV in the control sample are correspond to Pb2+4f7/2and Pb2+4f5/2, respectively, which are not shifted with PAPMS treatment but shifted into a lower binding energy after depositing the IPTMS and MPTMS layer. The shift to a lower binding energy is attributed to electron donation from sulfhydryl and carbonyl groups. We further investigate the crystalline structure of perovskite films with and without siloxane post-treatment by X-ray diffraction (XRD) measurement. As expected, all samples exhibited the similar characteristic diffraction peak positions and intensity, indicating that the crystallinity of perovskite films has not been changed through siloxane surface treatment.

The effects of surface methoxysilane treatment on the photovoltaic performance are estimated with device architecture of ITO/SnO2/FACsPbI3/Spiro-OMeTAD/Au (ITO: indium tin oxide; Spiro-OMeTAD: 2,2′,7,7′-tetrakis (N,N-dipmethoxyphenylamine) - 9,9′-spirobifluorene). We first investigated the dependence of device performance on PAPMS, MPTMS and IPTMS concentration, and the results are plotted in Fig. S1-S3. After optimizing the treatment condition, the device modified with IPTMS exhibited a significantly increased PCE, while PAPMS treatment have a negative impact on device performance (Figs. 2(a)-2(d)). It can be found that the increased performance for IPTMS treated devices is attributed to the improvement of open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF). Additionally, after introducing MPTMS, the devices exhibit the improvedVocbut decreased FF, suggesting that passivated with -SH can minimize surface trap density successfully. These results confirm that the terminal group plays a critical role in the final performance of PSCs.

Fig. 3. (Color online) (a) PL emission intensity mapping of control, PAPMS, MPTMS and IPTMS treated films. (b) EIS spectra of devices based on the pristine and IPTMS treatment. (c) Normalized TPC decay curves for control- and IPTMS-treated devices. (d) Dark I-V curves of devices with the structure of ITO/perovskite/Au for the control and IPTMS treated film.

We further prepared control and target devices with antirefractive coating on the ITO surface, and theJ-Vcurve of champion devices modified with different methoxysilane are plotted in Fig. 2(e). The correspond photovoltaic parameters are listed in Table 1. The best performance of control device exhibits aVocof 1.088 V,Jscof 24.84 mA/cm2and FF of 76.82%, yielding a PCE of 20.78%. In contrast, the device passivated with IPTMS displays a higher efficiency with aVocof 1.12 V,Jscof 25.10 mA/cm2and FF of 78.44%, as a consequence of a promoted PCE up to 21.97%. The integrated current density of control- and IPTMS-treated devices from external quantum efficiency (EQE) spectra are 23.90 and 23.97 mA/cm2, respectively. These results are in good agreement with previous results inJ-Vcurves. We further test the steady-state output power of devices at their maximum power point (Figs. 2(f) and 2(g)). The devices yield stabilized PCE of 20.97% for the control and 21.87% for the IPTMS device.

To elucidate the difference in performance of PSCs treated with different methoxy silane, the photoluminescent(PL) mapping spectra with 450 nm excitation is conducted as shown in Fig. 3(a). The PL intensity of control film presented an inhomogeneous distribution. However, the film with PAPMS treatment showed a wider distribution with lower luminescence intensity than control, corresponding to the damage effects of its on-device performance. In addition, we find that the intensity of films become stronger and more uniform after MPTMS and IPTMS treatment. The enhanced PL intensity indicates that surface non-radiative recombination is suppressed by IPTMS passivation.

We further in-depth study the mechanisms for the performance enhancement-induced IPTMS surface treatment.The electrochemical impedance spectroscopy (EIS) is performed to investigate the interfacial charge transfer and transport kinetics. As shown in Fig. 3(b), the semicircle at high frequency is ascribed to the carrier recombination at the interface, while the arc at low frequency represents carrier transport in the bulk[35]. The corresponding transport resistance(Rtr) and recombination resistance (Rrec) are fitted with equivalent circuit and the results are summarized in Table S1. Compared with the control device, the device modified with IPTMS exhibits a lowerRtrand higherRrec,indicating that a faster carrier transport and lower carrier recombination are obtained for the IPTMS-treated devices.

Then, the transient photocurrent (TPC) measurement was conducted to examine the charge transport property of PSCs(Fig. 3(c)). The charge transport constants (τt) slightly decrease from 218.1 to 115.3μs after methoxysilane treatment,which is ascribed to the promotion of carrier extraction from the perovskite to hole transport layer. These results in agreement with the increasement ofJscafter siloxane treatment.What is more, the space-charge limited current (SCLC) method was further employed to understand the effect of IPTMS passivation on perovskite defect density. The darkJ-Vcurves of devices with structure of ITO/perovskite/Au are plotted in Fig. 3(c). There are three distinct regions can be identified in theJ-Vcurve: an ohmic region with an ordern= 1; a child region withn= 2 and a trap-filled limited (TFL) region withn>3. The trap-filled limit voltage (VFTL) is always used to estimate the trap density according to the equation of:

whereeis elementary charge,ntis the trap density,Lis the thickness of perovskite film,ε0is the vacuum dielectric constant, andεis the relative dielectric constant of perovskite(46.9 for FAPbI3). TheVTFLof the control and target device are 0.44 and 0.61, respectively. As expected, the trap density is reduced from 3.75 × 1015cm-3for the control device to 2.70 ×1015cm-3for the target device, which benefits theVocand FF.As mentioned above, we have shown that IPTMS treatment can effectively promote the carrier transportation and reduce the trap density of PSCs, with the better photovoltaic parameters compared with control devices.

Fig. 4. (Color online) Long-term thermal stability of unsealed devices based on pristine, MPTMS and IPTMS treatment (a) PCE, (b) Voc, (c) Jsc and(d) FF.

To examine whether the IPTMS treatment can prevent perovskite degradation, the long-term thermal stability test for devices with and without IPTMS passivation is also conducted at 85 °C in nitrogen. Thermally stable mixed transport layer is adopted as the HTM to avoid the impact of transport layer degradation on the device stability tracking. The initial PCE of control, MPTMS-treated and IPTMS-treated are 19.14%,19.32% and 19.81%, respectively. The evolution of photovoltaic performance parameters for control, MPTMS-treated and IPTMS-treated device aged under thermal condition are plotted in Fig. 4. Surprisingly, the IPTMS-treated device presents excellent thermal stability and the remaining 90.4% of its initial PCE after 1500 h storage, whereas the control device is degraded more than 40% exposed under the same condition.Additionally, the MPTMS-treated device also exhibits better long-term thermal stability than the control device, which maintained near 80% of its initial PCE after 1500 h exposure.These results indicate that different termination groups in crosslink methoxysilane also play an important role in device long-term lifetime.

We believe the improved of thermal stability by IPTMS is likely stemmed from the improved interface between perovskite/HTL. According to the evolution of performance parameters of MPTMS- and IPTMS-treated devices aging at 85 °C in nitrogen (Fig. 4), the MPTMS-treated device shows an obvious drop in FF andVoc, but the IPTMS-treated device only presents a slight decline in FF andVoc. Generally, FF andVocare closely related to the interface characteristics. It indicates that the IPTMS-treated device can maintain better interface contact between the perovskite and hole transport layer than the MPTMS-treated device during a continuous heating process.

What is more, we also tested the humidity stability of devices at 60% RH to examine the protective effect of IPTMS molecule on phase transition. However, the performance of unencapsulated devices degraded rapidly (Fig. S4). The PCE of untreated device decreased from 20.63% to 15.02%, which only retained 73% of its initial performance after 24 h ageing. Although the IPTMS-treated devices have better humidity stability, their PCE has also dropped by nearly 10% at the same storage condition. We guessed that the decreased PCE was ascribed to the destroyed of hole transport materials and the phase transition of FACs-based perovskites. Therefore, we should encapsulate the device or select the more stable HTM materials to further improve the humidity stability.

4. Conclusion

In summary, we introduced an effective passivation layer using a silane coupling agent and systematically investigated the effects of terminal groups on photovoltaic performance. Among the investigated methoxysilane, the IPTMS with carbonyl group exhibited the best performance and longterm thermal stability. The PL mapping results show that the passivation layer can significantly reduce the non-radiative recombination loss. Through the EIS and TPC measurements,we found that the carrier transportation has been improved after IPTMS treatment. As a result, the device with IPTMS presented a PCE of 21.97%, higher than 20.97% for the control device. In addition, the excellent thermal stability also has been observed after siloxane surface passivation, retaining nearly 90% of its initial PCE after 1500 h storage.

Acknowledgements

The authors acknowledge funding support from National Natural Science Foundation of China (21975028,22011540377), National Science Foundation for Young Scientists No. 21805010, Beijing Municipal Science and Technology Project No. Z181100005118002, and Beijing Municipal Natural Science Foundation (JQ19008).

Appendix A. Supplementary materials

Supplementary materials to this article can be found online at https://doi.org/1674-4926/42/11/112201.