Highly-Efficient and Long-Term Stable Perovskite Solar Cells Enabled

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Highly-Efficient and Long-Term Stable Perovskite Solar Cells Enabled by a Cross-Linkable n‑Doped Hybrid Cathode Interfacial Layer Chih-Yu Chang,* Wen-Kuan Huang, and Yu-Chia Chang Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan 40724, R.O.C. S Supporting Information *

ABSTRACT: Hybrid organic−inorganic halide perovskite solar cells (PeSCs) are currently at the forefront of emerging photovoltaic technologies due to their potential for providing cost-effective highly efficient solar energy conversion. The interfacial layers play an important role in determining the efficiency and stability of PeSCs. In this work, a solution-processed cross-linkable hybrid composite film composed of N,Ndimethyl-N-octadecyl(3-aminopropyl)trimethoxysilyl chloride silane (DMOAP)-doped [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) is demonstrated as an effective cathode interfacial layer for PeSCs. The hydrolyzable alkoxysilane groups on DMOAP enable moisture cross-linking through the formation of stable siloxane bonds, which is effective in ensuring uniform film coverage of PC61BM on the perovskite layer and preventing the undesirable reaction between the mobile halide ions and Ag electrode. On the other hand, the quaternary ammonium cations on DMOAP can induce the formation of favorable interfacial dipoles, allowing the high work-function Ag layer to act as the cathode. Importantly, our results show that the chloride anions (Cl−) on DMOAP can cause efficient n-doping of PC61BM via anioninduced electron transfer, increasing the conductivity of PC61BM film by more than 2 orders of magnitude. With these desired properties, the resulting devices show a remarkable power conversion efficiency (PCE) of 18.06%, which is superior to those of the devices with undoped PC61BM film (PCE = 4.34%) and a state-of-the-art ZnO nanoparticles (NPs) interfacial layer (PCE = 10.40%). More encouragingly, combining this interfacial layer with an effective thin-film encapsulation layer, the resulting devices exhibit promising long-term ambient stability, with negligible (10−6 S cm−1) after more than 150 h of exposure. This result suggests that the electron transferred from chloride anions to PC61BM molecule are not quenched significantly upon exposure to ambient air, which may be associated with the presence of cross-linked siloxane networks and hydrophobic nature of PC61BM molecule. It should be noted that such cross-linked siloxane networks also enable PC61BM film to possess good solvent resistance, as evidenced by the nearly unchanged absorption spectra measured before and after spin-rinsing with chloroform (Supporting Information (SI) Figure S1). To investigate the changes in morphology of PC61BM-coated methylammonium lead iodide (MAPbI3) perovskite films with varying doping levels of DMOAP, atomic force microscopy (AFM) measurements were performed in tapping mode (scan size = 5 μm × 5 μm). A two-step sequential deposition method was used to prepare high quality MAPbI3 perovskite film, and its detailed characterization can be found in our previous work. As shown in Figure 2, an inhomogeneous surface morphology with high root-mean-square (rms) roughness (∼19.8 nm) was observed for the pristine perovskite film without the PC61BM layer (SI Figure S2). The introduction of an undoped PC61BM layer onto the perovskite layer still exhibited microscale voids with rms roughness of 2.8 nm (Figure 2a,f). Notably, the introduction of DMOAP dopant can effectively improve the coverage of PC61BM film onto the perovskite layer. For the film with doping concentration of 2.5 mol %, smooth and void-free morphology with rms roughness of 0.3 nm was obtained (Figure 2b,g), suggesting high miscibility between PC61BM and DMOAP. However, when the doping concentration was increased above 2.5 mol %, relatively rough surface (rms roughness = 1.2−13.6 nm) with more pronounced phase separation was observed (Figure 2). These results correlate well with the measured electrical conductivities (Figure 1b). It should be mentioned that uniform and full-coverage DMOAPdoped PC61BM film on the perovskite layer can ensure efficient charge transfer at the interface and reduce shunt leakage pathways within the devices, and these characteristics are highly desirable to realize high performance perovskite solar cells and will be discussed below. To gain further insight into the charge transfer properties between MAPbI3 perovskite and PC61BM, we performed

steady-state photoluminescence (PL) and time-resolved PL decay measurements. The PL properties of pristine sample (i.e., MAPbI3-coated glass substrate) and the samples coated with undoped PC61BM and doped PC61BM (2.5 mol % doping) were measured under excitation at a wavelength of 450 nm. When the perovskite layer was in contact with either undoped PC61BM or doped PC61BM layer, the PL intensity was largely quenched (Figure 3a). However, the extent of PL quenching is

Figure 3. (a) Steady-state and (b) time-resolved PL spectra for MAPbI3 films with and without PC61BM layers. (c) UPS spectra of bare Ag and modified Ag layers. (d) Energy level diagram at active layer/cathode interface.

more significant for the sample with DMOAP. The sample with undoped PC61BM layer maintained ∼13% of initial PL intensity, while nearly complete intensity was quenched in the case of doped PC61BM-coated sample (Figure 3a), suggesting that more efficient charge transfer at the MAPbI3/ doped PC61BM interface. This difference can be attributed to good film coverage of doped PC61BM on the perovskite layer as discussed previously (Figure 2). The time-resolved PL decay was also measured to investigate the extent of charge transfer process at the perovskite/PC61BM interface. The pristine perovskite sample without PC61BM layer exhibited a lifetime of 11.6 ns, which is similar to the values reported in the literatures.33,34 The lifetime was greatly reduced to 0.84 and 0.41 ns for the samples with undoped PC61BM and doped 6307

DOI: 10.1021/acs.chemmater.6b02583 Chem. Mater. 2016, 28, 6305−6312

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Chemistry of Materials Table 1. Summary of the Photovoltaic Properties of MAPbI3-Based Devicesa device b

A Bc Cb

interfacial layer

Voc [V]

Jsc [mA cm−2]

FF [%]

PCE [%]

PC61BM doped PC61BM PC61BM/ZnO NPs

0.75 ± 0.01 (0.75) 1.04 ± 0.01 (1.05) 0.95 ± 0.01 (0.97)

13.76 ± 1.25 (15.09) 20.88 ± 0.61 (21.46) 16.36 ± 0.74 (17.22)

40.41 ± 5.36 (41.78) 72.58 ± 4.10 (76.47) 65.82 ± 5.88 (66.29)

4.13 ± 0.41 (4.73) 15.75 ± 0.69 (17.23) 10.25 ± 0.82 (11.07)

a The values in parentheses are for the best performing devices. bAverage and standard deviation values were obtained based on 20 devices. cAverage and standard deviation values were obtained based on 50 devices.

observed (best PCE = 4.73%), which is consistent with the fact that a Schottky barrier exists at the interface.14−22 When applying DMOAP-doped PC61BM film as the interfacial layer (device B), a remarkable improvement in PCE (by ∼3.8 fold) was achieved as a result of the simultaneously increased all the parameters (Table 1). The highest PCE we obtained was 17.23%, with short-circuit current density (Jsc), of 21.46 mA cm−2, Voc of 1.05 V, and fill factor (FF) of 76.47% (Table 1 and Figure 4a). It should be noted that the integrated Jsc value of device B obtained from the incident photon-to-current conversion efficiency (IPCE) spectra (19.76 mA cm−2; see SI Figure S3) was reasonably close (within 10% error) to the measured Jsc (21.46 mA cm−2). In addition, considering that ZnO nanoparticles (NPs) film is widely used as CBL for PeSCs,2,10 we also fabricate ZnO NPs-based devices for comparison (device C). Encouragingly, the devices with DMOAP-doped PC61BM layer (device B) still delivered higher PCE than those of ZnO NPs-based devices, as shown in Table 1 and Figure 4a. The inferior PCE of ZnO NPs-based device can be ascribed to inhomogeneous and incomplete coverage of ZnO NPs film on PC61BM/perovskite layers and/or poor electrical coherence at the organic/inorganic interface.14,19,28 These results clearly indicate the effectiveness of employing DMOAP-doped PC 61 BM film as a promising cathode interfacial layer for PeSCs. To understand better the origin of performance enhancement afforded by a DMOAP-doped PC61BM layer, the rectification ration of the devices were analyzed. As shown in Figure 4b, compared to the devices with undoped PC61BM layer (device A), the devices with DMOAP modification (device B) exhibited an improved rectification ratio (∼1.1 for device A vs ∼60 for device B), suggesting that doped PC61BM layer acts as an effective electron-selective contact to ensure efficient charge extraction and suppress undesirable charge recombination at the perovskite layer/Ag interface. This improvement can be attributed to multiple positive effects afforded by DMOAP-doped PC61BM layer, including good film quality, fine WF tunability of Ag electrode, and enhanced electrical conductivity, as we have discussed previously. These results emphasize the important role of DMOAP doping in modulating the interfacial properties of PeSCs. The universality of this DMOAP-doped PC61BM interfacial layer in PeSCs was examined with a formamidinium lead iodide (FAPbI3) perovskite system, which possesses a small bandgap (∼1.48 eV), high phase transition temperature, and good thermal stability.36−39 The preparation and detailed characterization of FAPbI3 perovskite film can be found in our previous work.20 Encouragingly, a DMOAP-doped PC61BM layer also presented similar positive effects in this system. As shown in Table 2 and Figure 5a, the best performing device (device D) delivered a remarkable PCE of 18.06% (average value = 16.59%), which was higher than those of the devices with an undoped PC61BM layer (device E) and ZnO NPs films (device F). It should be emphasized again that the integrated Jsc value

PC61BM, respectively (Figure 3b). These results suggest that doping PC61BM with DMOAP can facilitate the photoinduced charge transfer at the perovskite/PC61BM interface. The influence of DMOAP-doped PC61BM film on the WF of Ag layer was then studied by ultraviolet photoelectron spectroscopy (UPS). As shown in Figure 3c, the WF of bare Ag layer was determined to be 4.64 eV, which is comparable to the reported WF value (∼4.70 eV).35 Given that the LUMO level of electron-accepting material PC61BM (∼4.10 eV) is lower than the WF of Ag electrode (4.64 eV), this energy level mismatch can impede the electron extraction from PC61BM layer to Ag electrode because of the Schottky-barrier effect, as illustrated in Figure 3d. Encouragingly, the WF of Ag layer could be reduced to 3.87 eV through the addition of a doped PC61BM layer (Figure 3c), which can provide better energy level matching with a PC61BM layer to form an ohmic contact and increases the built-in potential across the device (Figure 3d). This is beneficial for achieving efficient electron extraction and high open-circuit voltage (Voc) of the devices, as described previously.14 The significant decrease in the WF of the Ag layer via DMOAP doping can be rationalized by the formation of favorable interfacial dipole at the cathode interface, as previous work has shown that the inclusion of ammonium cations-based molecules between active layer/cathode interface can cause vacuum level shift by creating interfacial dipoles.14−17 These results clearly indicate that the inclusion of DMOAP-doped PC61BM layer is able to manipulate the energy level alignment between the active layer and Ag electrode, enabling the high WF Ag layer to act as an efficient cathode. The effect of a DMOAP-doped PC61BM layer (2.5 mol % doping concentration) on the photovoltaic performance was then studied. For comparison, the control devices with an undoped PC61BM layer were also fabricated. The device configuration investigated herein was ITO-coated glass substrate/PEDOT:PSS/MAPbI3/PC61BM/Ag, as illustrated in Figure 1a. The current−voltage (J−V) characteristics of the corresponding devices measured under simulated AM 1.5 G illumination (intensity = 100 mW cm−2) are summarized in Table 1 and Figure 4a. For the control devices with an undoped PC61BM layer (device A), poor device performance was

Figure 4. (a) J−V characteristics of the best performing MAPbI3-based devices under simulated AM 1.5 solar irradiation. (b) Dark J−V characteristics of the as-fabricated devices. 6308

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Chemistry of Materials Table 2. Summary of the Photovoltaic Properties of FAPbI3-Based Devicesa device b

D Ec Fc

interfacial layer

Voc [volt]

Jsc [mA cm−2]

FF [%]

PCE [%]

doped PC61BM PC61BM PC61BM/ZnO NPs

1.04 ± 0.01 (1.05) 0.69 ± 0.01 (0.71) 0.93 ± 0.01 (0.93)

22.89 ± 0.64 (23.34) 14.81 ± 0.89 (15.76) 16.43 ± 0.65 (17.78)

69.74 ± 4.03 (73.69) 35.86 ± 3.58 (38.81) 60.59 ± 5.78 (62.90)

16.59 ± 0.76 (18.06) 3.65 ± 0.34 (4.34) 9.26 ± 0.79 (10.40)

a The values in parentheses are for the best performing devices. bAverage and standard deviation values were obtained based on 50 devices. cAverage and standard deviation values were obtained based on 20 devices.

interfacial layers were also investigated by monitoring the degradation of their PCEs as a function of storage time in ambient condition (30 °C, relative humidity = 60−70%). To prevent the ingress of water vapor and oxygen into the devices, all the devices were encapsulated with atomic-layer-deposited (ALD) Al2O3 film (50 nm)-coated polyethylene terephthalate (PET) substrate, which has been proven to be effective in improving the stability of PeSCs, thanks to its excellent gas barrier properties with an oxygen transmission rate of 1.9 × 10−3 cm3 m−2 day−1 and a water vapor transmittance rate of 9.0 × 10−4 g m−2 day−1 (more details can be found in our previous work).19 After being exposed to ∼5000 h, the device with undoped PC61BM layer (device A) showed a significant degradation in device efficiency: only ∼68% of the initial PCE was retained (Figure 6). The device degradation became

Figure 5. (a) J−V characteristics of the best performing FAPbI3-based devices under simulated AM 1.5 solar irradiation. (b) Histogram of solar cell efficiencies (device D) for 50 devices. J−V characteristics of device D measured under simulated AM 1.5 solar irradiation with (c) different voltage sweep rates and (d) different sweep directions (scan rate = 0.15 V s−1). Figure 6. Degradation profile of the encapsulated devices as a function of storage time in ambient conditions; the statistical data were collected from more than 25 devices.

of device D obtained from the incident photon-to-current conversion efficiency (IPCE) spectra (21.23 mA cm−2; see SI Figure S3) was well matched (within 10% error) with the measured Jsc (23.34 mA cm−2). The device D exhibited stabilized PCE of ∼18% under continuous illumination (SI Figure S4). In addition, we also examine the scalability of our approach. As a proof-of-concept, the device with an active area of 1.2 cm2 was fabricated. Importantly, a high PCE of 15.78% was obtained, with Jsc of 21.74 mA cm−2, Voc of 1.03 V, and FF of 70.47% (SI Figure S5). These results elucidate that DMOAP-doped PC61BM can be can be used as a universal interfacial layer in PeSCs. A histogram of the performance of device D obtained from 50 samples is summarized in Figure 5b. Notably, more than 50% of the integrated devices showed PCE above 16.5%, indicating good reproducibility. This can be explained by the full coverage of doped PC61BM layer on the perovskite. Moreover, considering that PeSCs have been demonstrated to exhibit an anomalous hysteresis in the J−V curve measurement, we also investigate the J−V characteristics of the device D measured under different scanning directions and scanning rates. Encouragingly, our device exhibited negligible photocurrent hysteresis (Figure 5c,d), indicating that the efficiencies measured for our devices are reliable. This observation can be related to the passivation of the trap states in perovskite film by PC61BM capping layer and/or stable formamidinium cations in FAPbI3 perovskite structure under the electric field.26,36,39,40 The ambient stability of MAPbI3-based devices employing undoped PC61BM (device E) and doped PC61BM (device D)

more pronounced when the devices were subjected to thermal stress conditions (65 °C, ambient atmosphere, relative humidity = 65%). The observed degradation is consistent with previous reports showing that the ionic defects (e.g., iodide ion) in perovskite films can easily migrate through other layers and contaminate Ag electrode to form insulating layer even in the absence of oxygen and moisture, thereby causing the stability problem.7,9,23,24 Very encouragingly, such ionic defects-caused degradations can be can be largely mitigated in the case of DMOAP-doped PC61BM layer. As shown in Figure 6, the devices with a DMOAP-doped PC61BM layer (device D) exhibited negligible degradation (less than 5%) upon exposure to air for more than 5000 h. To the best of our knowledge, this stability is one of the best results for PC61BM-based PeSCs. Notably, we observed that the devices with DMOAP-doped PC61BM layer (device D) still remained stable under thermal stress conditions even for long periods of storage (>5000 h), as depicted in Figure 6. It should be noted that the ambient stability of the devices based on the dopant without the crosslinkable silane groups was also investigated (trimethyloctadecylammonium chloride; see SI Figure S6 for the chemical structure). The obtained device stability was found to be far inferior to those of DMOAP-based devices (SI Figure S6), suggesting that silane groups may play an important role in improving device stability. 6309

DOI: 10.1021/acs.chemmater.6b02583 Chem. Mater. 2016, 28, 6305−6312

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Chemistry of Materials

ethyl acetate and ethanol. Afterward, ethanol was added to disperse the precipitates and produce ZnO NPs solution. Perovskite Solar Cell Fabrication. ITO-coated glass substrates were was first ultrasonicated in detergent, deionized water, acetone, and 2-propanol in turn, followed by UV-ozone treatment for 60 min. After filtration through a 0.45 μm filter, a PEDOT:PSS layer (30 nm) was spin-coated on the cleaned ITO substrate at 4000 rpm for 60 s and then annealed at 120 °C for 15 min. The MAPbI3 perovskite layer (∼350 nm) was prepared following two-step solution deposition, as described in our previous work.17,19−22 The PbI2 solution and the substrates were heated at 100 °C for 10 min before being used. Briefly, the solution of PbI2 in anhydrous dimethylformamide with a concentration of 450 mg mL−1 was spin-coated at 5000 rpm for 40 s and then annealed at 70 °C for 10 min. Afterward, the solution of MAI in anhydrous 2-propanol with a concentration of 40 mg mL−1 was spin-coated on top of PbI2 film at 6000 rpm for 30 s and then then annealed at 100 °C for 2 h. The preparation of FAPbI3 layer (∼320 nm) was according to the reported procedure.36 Briefly, the solution of PbI2 complex in anhydrous dimethylformamide with a concentration of 1.3 M was spin-coated at 3000 rpm for 30 s. Afterward, the solution of FAI in anhydrous 2-propanol with a concentration of 0.465 M was spin-coated on top of PbI2 film at 5000 rpm for 30 s and then annealed at 150 °C for 10 min. After the deposition of perovskite layer, the solution of PC61BM in anhydrous chloroform with a concentration 20 mg mL−1 was spin-coated on top of the perovskite layers. The resulting film was subsequently stored at ambient air for 10 min for hydrolysis. The optimum thickness of PC61BM layer for MaPbI3 and FAPbI3 was 60 and 80 nm, respectively. The Ag (150 nm) was then deposited from thermal evaporator under high vacuum (99.5%) and formamidinium iodide (FAI; >99.5%) were purchased from Lumtec. PC61BM (>99.5%) was purchased from Solenne. Lead iodide (PbI2) complex was prepared according to the previously reported method.36 In detail, 25 g of PbI2 was dissolved in 75 mL of anhydrous dimethyl sulfoxide. The mixture was then stirred at room temperature for 3 h, followed by adding 200 mL of toluene. The white precipitate was then filtered and dried in a vacuum oven at 60 °C for 12 h. Synthesis of ZnO NPs. ZnO NPs were synthesized according to the previously reported method.42 Briefly, zinc acetate dihydrate (2.95 g) was dissolved in methanol (125 mL) at room temperature. Potassium hydroxide solution (1.48 g in 65 mL methanol) was then added, and the mixture was stirred for 3 h at 65 °C. The cooled-down solution was then decanted, and the precipitate was washed twice with

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02583. Absorption spectra of the doped PC61BM films before and after rinsing with chloroform, AFM images of MAPbI3 perovskite film, IPCE spectra and integrated current values of the as-fabricated devices, stabilized power output measured close to the maximum power point for device D, J−V characteristic of the best performing FAPbI3-based device (active area = 1.2 cm2) under simulated AM 1.5 solar irradiation, degradation profile of the encapsulated devices based on trimethyloctadecylammonium-doped PC61BM layer as a function 6310

DOI: 10.1021/acs.chemmater.6b02583 Chem. Mater. 2016, 28, 6305−6312

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of storage time in ambient condition at 65 °C, and highresolution XPS I 3d spectra of the samples after 3 h of air exposure at 65 °C (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Prof. Chih-Yu Chang (E-mail: [email protected]). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of R.O.C. (Grant number: MOST 105-2221-E035-092, 104-2221-E-035-035 and 104-2119-M-009-012). We also appreciate the Precision Instrument Support Center of Feng Chia University for providing the fabrication and measurement facilities.

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DOI: 10.1021/acs.chemmater.6b02583 Chem. Mater. 2016, 28, 6305−6312

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DOI: 10.1021/acs.chemmater.6b02583 Chem. Mater. 2016, 28, 6305−6312