Minimalist Design of Efficient, Stable Perovskite Solar Cells - ACS

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Minimalist Design of Efficient, Stable Perovskite Solar Cells Xin Yin, Jifeng Zhai, Tianwei Wang, Wanru Jing, Lixin Song, Jie Xiong, and Frank K. Ko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21692 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Minimalist Design of Efficient, Stable Perovskite Solar Cells Xin Yina, b*, Jifeng Zhaia, b, Tianwei Wangb, Wanru Jingb, Lixin Songa, b*, Jie Xionga, b*, Frank Koc a.

College of Materials and Textiles, b. Key Laboratory of Advanced Textile Materials and

Manufacturing Technology of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, 310018, China c.

Department of Materials Engineering, University of British Columbia, Vancouver, Canada

Corresponding

author Tex: +86 571 86843586; E-mail address: [email protected];

[email protected]; [email protected] Key words: perovskite solar cell; minimalist architecture; FTO modification; NiO; stability. Abstract Feasible production process and excellent device stability are significant prerequisites for the practical application of perovskite solar cells (PSCs). Herein, a systemic strategy is developed to fabricate the stable, minimalist PSCs without conventional electron/hole transport layer. The engineering is carried out by the surface modification of FTO substrate and incorporation of perovskite film with NiO nanoparticles (NPs). Notably, the surface modification can impart an unexpected porous structure to the FTO substrate, thereby fabricating the efficient diffusion and deposition of perovskite. Besides, the incorporated NiO NPs passivate the defects of perovskite film, resulting in the increase of perovskite grain size, decrease of grain boundary density and increase of film thickness. Synergistic improvements in film quality and interfacial contract enhance charge transport/extraction capacity and suppress electron/hole recombination. Consequently, the stabilized efficiency of 14.65% is realized for this modified FTO (M1

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FTO)/MAPbI3-NiO/Ag device, with excellent moisture and thermal stability. Overall, this work provides a viable strategy for accelerating the commercialization of PSCs due to the significant process simplification and cost reduction. Introduction Over the past few years, halide perovskite solar cells (PSCs) have arisen enormous attention due to the rapid development of power conversion efficiency (PCE). Benefiting from the characteristics of considerable light absorption, excellent charge transport ability and suitable bandgaps1-4, the certified efficiency of ˃23% has been achieved for mesoporous PSCs, which can be comparable to commercial silicon solar cells5. Therefore, numerous works are devoted to developing a feasible strategy to accelerate commercialization of PSCs6-9. However, from the perspective of device fabrication, tedious process, such as high-temperature process of inorganic electron transport layer (ETL) or modification of organic hole transport layer (HTL), is not conducive to its practical application10, 11. Some efforts are urgently needed to focus on the design and optimization of device architecture. Interestingly, Edri’s group proposed that the presence of electron/perovskite and hole/perovskite interfaces is not a precondition for device performance12. Thus, eliminating the charge transport layer (ETL/HTL) is the promising approach to simplify fabrication process13-16. For example, Kelly et al. proposed a new type of simplified PSCs by removing ZnO ETL without performance degradation (ITO/MAPbI3/PTAA/Au)

17.

Further, Huang et al. employed FTO

surface modification approach to improve quality of perovskite film and promote charge separation, achieving an ETL-free PSCs with efficiency of 15%18. Notably, the surface modification of FTO has been reported in dye-sensitized solar cell, which is beneficial to improve 2

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the photovoltaic performance19. Besides, a widely recognized fact is that halide perovskite could serve as the excellent electron/hole transport material20, 21. Therefore, some attention has been focused on the development of HTL-free PSCs22. Ye et al. fabricated HTL-free PSCs with FTO/compact TiO2/perovskite/Au structure, while the efficiency was improved to 10.04%23. And Zhao et al. incorporated the hole-conductor material (carbon nanotubes, CNTs) into perovskite to improve the quality of film. In this, the CNTs can act as rapid charge transport pathway, resulting in the PCE of 11.6%24. Yet despite these promising results, systematic considerations for further simplifying device architecture and reducing commercial burden of process are still insufficient. In this work, we demonstrate the minimalist PSCs with FTO/MAPbI3/Ag structure, which can greatly reduce process cycles and costs. However, the poor quality of perovskite film and FTO/perovskite interface contract lead to the undesirable PCE (only 5.71%) and device stability. Therefore, further modifications for fabricating the minimalist PSCs are provided from the standpoint of improving efficiency and stability. Here, the engineering is carried out in two steps. Firstly, the commercial pristine FTO (P-FTO) feature an unexpected surface characteristic of porous structure via simple modification. This porous structure is favorable for the deposition of perovskite film and improves the interfacial contract. To further passivate the defects of perovskite film and enhance stability, NiO nanoparticles (NPs) are employed as the additive to induce better crystallization of perovskite and reduce electron/hole recombination. As expected, the promising PCE of 14.65% is achieved with excellent moisture and thermal stability. More importantly, significantly simplified process provides a viable strategy for commercialization of PSCs, compared with conventional n-i-p mesoporous devices. Results and discussion 3

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From the perspective of comprehensively simplifying device fabrication, we first fabricate the minimalist PSCs with FTO/MAPbI3/Ag structure, by eliminating the conventional ETL and HTL. The MAPbI3 film is directly deposited on the P-FTO substrate via typical one-step method. Fig. 1a shows the current-voltage (J-V) curves of P-FTO/MAPbI3/Ag, and the photovoltage parameters are summarized in Table 1. Obviously, this PSCs exhibits a poor efficiency of 5.71% with severe hysteresis. Besides, the investigation of stability in Fig. 1b demonstrates a fact that device performance degraded rapidly after 100 h. It is mainly ascribed to the poor quality of MAPbI3 film. As displayed in Fig. S1 (supporting information), the thickness of perovskite film is only ~300 nm. Some interfacial defects would exist due to the poor quality of MAPbI3 film, which is not conducive to the effective charge separation and device stability25.

Fig. 1 The J-V curves of P-FTO/MAPbI3/Ag PSCs in different scan directions (a). And the evaluation of device stability under ambient conditions (b)

To optimize the interfacial contract and charge extraction, the P-FTO substrate was modified via the simple method, as shown in Fig. 2a. Briefly, under the electrochemical treatment with reaction of SnO2 + 4H+ = Sn4+ + 2H2O, the substrate gradually features a surface characteristic of porous structure. The variation of surface morphology is demonstrated in Fig. 2b. The P-FTO substrate exhibits a flat surface consisting of numerous sharp SnO2 crystals. In comparison, the 4

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surface of modified FTO (M-FTO) possesses the distinct pore structure. And the porousness becomes more obvious as the etching time increases (Fig. S2). Here, the optimal porous morphology of the M-FTO is achieved by etching for 20 min. Fig. 2c demonstrates that this MFTO (20 min) is not significantly altered in resistance and thickness compared to P-FTO substrate. Notably, the electrical conductivity would decrease with further etching (Fig. S3). Besides, this M-FTO possesses the smaller water contact angle of 45.8° than that of P-FTO (67.2°), indicating the better surface wettability (Fig. S4). This improved wettability could promote the diffusion of the perovskite precursor on the substrate, facilitating the acquisition of high-quality perovskite films in the subsequent device preparation process26, 27. Fig. 2d shows the corresponding XRD patterns of M-FTO. The diffraction peaks at 27.4°, 34.6°, 38.8°, 52.3°, 62.5°, and 66.3°, corresponding to the (110), (101), (200), (211), (310), and (301) planes, can be well matched for that of P-FTO17,

18.

This indicates that there would be little difference in the structural

characteristics between these substrates. The transmission spectra (Fig. 2e) reveal that the transmittance of M-FTO is slightly lower than that of the P-FTO. It can be ascribed to the light scattering effect of porous structure.

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Fig. 2 Schematic diagram of etching process of FTO (a). Surface morphologies of P-FTO and M-FTO (b). The variation of resistance and thickness for the P-FTO and M-FTO (c). The corresponding XRD patterns (d) and the UV-vis transmission spectra (e).

Impacts of P-FTO and M-FTO substrate on quality of MAPbI3 film are evaluated by FESEM in Fig. 3a. As observed, the MAPbI3 film on M-FTO exhibits a uniform and dense surface morphology, whereas the film on P-FTO has obvious defects. This indicates the M-FTO effectively promote the diffusion and deposition of perovskite film due to its porous structure. Fig. 3b demonstrates the enhanced crystallinity of perovskite film on M-FTO. And the UV-vis absorption spectra (Fig. 3c) indicate that better absorbance is also achieved for M-FTO, which is beneficial to the light-harvesting capacity of film28. Further, Fig. 3d displays the representative JV curves of M-FTO/MAPbI3/Ag PSCs. An increase in short-circuit current density (Jsc), opencircuit voltage (Voc) and fill factor (FF) can be observed for M-FTO based device. And the efficiency is boosted to 11.15%. This enhancement should be primarily attributed to the optimized 6

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interfacial contract through improved quality of MAPbI3 film. To directly obtain the information of interfacial charge transfer and recombination process, the electrochemical impedance spectra (EIS) were characterized, as presented in Fig. S5. Compared with P-FTO based devices, the MFTO based devices possess smaller series resistance (Rs) and larger recombination resistance (Rrec)29, 30. It indicates that the M-FTO substrate could promote the interface charge extraction and reduce recombination, resulting in the better device performance. Notably, further etching would degrade device performance because of the reduced conductivity of FTO substrate (Fig. S3 and S6, Table S1). Besides, the device stability is shown in Fig. 3e. Different from the rapid decomposition of P-FTO based device within 100 h, the M-FTO based device remain 36% of initial PCE after 360 h under ambient air conditions because of the improved film quality. Nevertheless, the problems of J-V hysteresis and device stability are still serious.

Fig. 3 Top-view FESEM images of MAPbI3 film on P-FTO and M-FTO (a). The XRD patterns (b) and UV-vis M-FTO/MAPbI3/Ag based PSCs.

To address above issues, NiO NPs were incorporated into the MAPbI3 film to further improve the device performance. Fig. S7a-c demonstrate that the average size distribution of NiO NPs is 7

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below 30 nm. The XRD pattern verifies the formation of pure phase of NiO NPs28, 31. Such this NiO additive could inhibit nucleation and slow down the crystallization of MAPbI3, leading to an increase in grain size and film thickness32,

33,

as illustrated in Fig. 4a. And the fact that the

crystallization process of MAPbI3-NiO film is slower than that of MAPbI3 film can be proved by bottom photographs. Fig. 4b and Fig. S5d also indicate that the MAPbI3-NiO film (0.12 mg/mL) exhibits the larger perovskite size than that of the reference MAPbI3 film. And the XRD results suggest that the MAPbI3-NiO film present an excellent crystallinity of perovskite phase at 14.3°, 26.7°, 28.5° and 32.1° than the others (Fig. S8). To our best knowledge, the larger grain size and better crystallinity of perovskite contribute to the reduction of grain boundaries and improvement of light-harvesting ability34. For the UV-vis absorption spectra in Fig. S9, it is evidence that the introduction of NiO NPs enhances the light absorption, thereby improving the light-harvesting ability of perovskite film. To gain deep insight into the role of NiO NPs in photovoltaic performance, comparisons of the M-FTO/MAPbI3-NiO/Ag PSCs and M-FTO/MAPbI3/SpiroOMeTAD/Ag reference PSCs are demonstrated. Fig. S10 shows the corresponding cross-sectional SEM images of these devices respectively. Consistent with our predictions, the incorporation of NiO NPs induces an increase in overall perovskite grain size than that of the pristine film, whereas the film thickness increases from ~390 nm to ~582 nm. The corresponding J-V curves of these devices are presented in Fig. 4c, d. In this, the optimal concertation of NiO NPs is 0.12 mg/mL. Apparently, the device of MAPbI3-NiO delivers the considerable PCE of 14.65% with Jsc of 19.41 mA/cm2, Voc of 1008 mV, and FF of 74.87%, slightly better than that of the reference device (14.18%, 19.10 mA/cm2, 998 mV, and 74.390%). Notably, the device performance would decrease once the NiO concentration is further increased over 0.12 mg/mL, which is mainly because of the 8

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lower FF and Voc (details in Fig. S11, Table S2). Besides, the hysteresis effect of devices is quantified in Fig. S12. As seen, the lower hysteresis can be observed for M-FTO/MAPbI3-NiO/Ag PSCs. We ascribe these improvements to the superior quality of MAPbI3-NiO film. Larger perovskite grain, fewer grain boundaries and thicker film thickness can synergistically promote the better charge transport and extraction capacity of device to suppress hysteresis35. To determine our hypothesis, the impedance spectra were employed to investigate the internal resistance and charge transfer kinetics of these devices in Fig. 4e. Based on the information provided in Fig. S5 and Fig. 4e, significant improved Rrec indicates that the incorporated NiO NPs can effectively reduce charge recombination and resistive loss, thereby achieving better performance23, 36. Fig. 4f displays external IPCE spectra response of the reference and MAPbI3-NiO based PSCs. A slight enhancement is observed for MAPbI3-NiO based PSCs in absorption wavelength range, in agreement with the UV-vis spectra. This result further indicates the a more efficient electrons injection is achieved for devices with NiO NPs modification. And the integrated Jsc is approximately consistent with the corresponding J-V curves (Fig. S13). Besides, the stabilized PCE of 14.61% and photocurrent of 17.43 mA/cm2 is achieved with M-FTO/MAPbI3-NiO/Ag device, synergistically suggesting the effect of NiO NPs on improving hysteresis (Fig. 4g). Meanwhile, the distributions of device performance parameters among 70 cells in Fig. 4h confirm the considerable reproducibility of device.

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Fig. 4 Schematic diagram of role of NiO NPs and the corresponding photographs of perovskite film during the crystallization process in 100 °C (a). Top-view FESEM images of MAPbI3-NiO film (b). J-V curves of MFTO/MAPbI3-NiO/Ag and M-FTO/MAPbI3/Spiro-OMeTAD/Ag devices in different scan directions (c, d). The corresponding impedance spectra measured under 1 sun illumination at Voc (e), IPCE spectra (f), steady-state power outputs (g) and histogram of PCEs measured on more than 70 cells (h). Table 1 Photovoltaic parameters of the devices: P-FTO/MAPbI3/Ag, M-FTO/MAPbI3/Ag, MFTO/MAPbI3/Spiro-OMeTAD/Ag and M-FTO/MAPbI3-NiO/Ag. Devices P-FTO/MAPbI3/Ag M-FTO/MAPbI3/Ag M-FTO/MAPbI3/SpiroOMeTAD/Ag M-FTO/MAPbI3-NiO/Ag

Scan direction

Jsc (mA/cm2)

Voc (mV)

FF (%)

PCE (%)

Reverse

14.11

832

48.64

5.71

Forward

13.68

813

38.21

4.25

Reverse

18.65

920

64.98

11.15

Forward

18.19

899

56.50

9.24

Reverse

19.10

998

74.39

14.18

Forward

19.06

981

68.14

12.74

Reverse

19.41

1008

74.87

14.65

Forward

19.05

1002

73.03

13.94

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Further, to evaluate effects of the incorporation of NiO NPs on stability, we test the moisture stability of perovskite film. And the relative humidity is regulated at 85 ± 10% at room temperature. Fig. 5a presents the absorbance (at 600 nm) of perovskite film as a function of exposure time (detail absorption spectra are provided in Fig. S14). Significant decrease in absorbance is observed for MAPbI3 film, while only slight changes is for MAPbI3-NiO film. The enhanced stability could be ascribed to the reduced grain boundary density caused by NiO NPs. Besides, the corresponding photos of perovskite film with different exposure times are displayed in Fig. 5b. Apparently, the MAPbI3 film almost completely decomposed into PbI2 after 24 h, whereas no significant changes were observed in MAPbI3-NiO film. And this phase transformation can be confirmed by corresponding XRD patterns (Fig. S15). Besides, the mechanism of perovskite film stability is demonstrated in Fig. 5c. The reference MAPbI3 film possesses many grain boundaries, which can be confirmed by SEM in Fig. 3b. And it has been proposed that the grain boundary might be the ingression pathways for moisture37, 38. Hence, the MAPbI3 grains would easily degrade into PbI2. For the MAPbI3-NiO film, the incorporation of NiO NPs can effectively increase grain size and passivate grain boundary, thus protecting perovskite film from moisture. Besides, the chemical bonding formed between NiO additives and MAPbI3 is favor for stabilizing the perovskite film39, 40.

As displayed in Fig. S16, the peaks of Ni-N and Ni-O in the 400-500 cm-1 region demonstrate

that the chemical interaction exists between NiO and MAPbI3. This interaction synergistically improves the long-term stability of the perovskite.

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Fig. 5 The photos of films exposed to ~85 ± 10% RH at room temperature for 24 h (a). The corresponding absorption evaluation (600 nm) of the MAPbI3 and MAPbI3-NiO films (b). Schematics diagram of improved structural stability of MAPbI3-NiO (c).

Finally, we further evaluate the moisture stability of M-FTO/MAPbI3-NiO/Ag devices under ambient and 50-70% RH conditions. As illustrated in Fig. 6a, the 87% the initial performance is maintained for MAPbI3-NiO PSCs after 480 h under ambient conditions, while the reference devices showed 55% PCE loss. When stored in 50-70% RH environment (Fig. 6b), the MAPbI3NiO PSCs degrades about 48% of initial PCE after 300 h. Conversely, the reference devices exhibit a significant degradation in performance, leaving only 16% of initial PCE after 60 h. Besides, these devices were also aged at 65 ± 5 °C condition to investigate thermal stability, as displayed in Fig. 6c. Obviously, the MAPbI3-NiO PSCs still retained 64% of initial value after 300 h, whereas the 12

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reference devices degraded to 40% of initial PCE after 200 h. These results indicate that the incorporation of NiO NPs plays an important role in improving both efficiency and device stability.

Fig. 6 The Long-term stability of M-FTO/MAPbI3-NiO/Ag based devices under various conditions: ambient conditions (a), 50-70% RH (b) and 65 ± 5 °C (c).

Conclusions In conclusion, we developed a systemic strategy to fabricate the minimalist PSCs with desirable efficiency and excellent stability. The engineering is carried out by the surface modification of FTO substrate and incorporation of perovskite film with NiO NPs to boost the charge transport/extraction. Consequently, the stabilized PCE of 14.65% is achieved for the MFTO/MAPbI3-NiO/Ag PSCs, better than that of M-FTO/MAPbI3/Spiro-OMeTAD/Ag (14.19%). This is mainly attributed to the construction of porous structure and incorporation of NiO NPs, which passivates the FTO/perovskite interface and improves the quality of perovskite film respectively. Hence, the improved charge transport and suppressed electron/hole recombination is achieved. Notably, the devices exhibit the excellent moisture and thermal stability. Finally, this work provides a feasible approach for assisting in commercialization of PSCs arising from its simplified process, reduced cost and long-term stability. Experimental Section Experimental details are provided in Supporting information. Conflicts of interest 13

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There are no conflicts to declare Acknowledgements The financial support of this work was provided by Zhejiang Provincial Natural Science Foundation of China (LQ19E030020, LZ16E020002); Foundation of Zhejiang Educational Committee (18010048-F); Science Foundation (17012144-Y), and Program of Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education (2017QN04) of Zhejiang Sci-Tech University. References (1) Bi, D.; Yi, C.; Luo, J.; Décoppet, J.; Zhang, F.; Zakeeruddin, S.; Li, X.; Hagfeldt, A.; Grätzel. M. Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%. Nat. energy 2016, 1, 16142. (2) Zhao, Y.; Zhu, K. Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications. Chem. Soc. Rev. 2016, 45, 655-689. (3) Dubey, A.; Adhikari, N.; Wu, F.; Chen, K.; Yang, S.; Qiao, Q. A strategic review on processing routes towards highly efficient perovskite solar cells. J. Mater. Chem. A 2018, 6, 2406. (4) Luo, D.; Yang, W.; Wang, Z.; Sadhanala, A.; Snaith, H.; Zhu, R. Enhanced photovoltage for inverted planar heterojunction perovskite solar cells. Science 2018, 360, 1442-1446 (5) Jeon, N.; Na, H.; Jung, E.; Lee, J.; Seo, J. A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells. Nat. energy 2018, 3, 682-689. (6) Seo, S.; Jeong, S.; Bae, C.; Park, N.; Shin, H. Perovskite Solar Cells with Inorganic Electronand Hole-Transport Layers Exhibiting Long-Term (≈500 h) Stability at 85 °C under Continuous 1 Sun Illumination in Ambient Air. Adv. Mater. 2018, 30, 1801010. 14

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(7) Correa-Baena, J.; Saliba, M.; Buonassisi, T.; Grätzel, M.; Abate, A.; Tress, W.; Hagfeldt, A. Promises and challenges of perovskite solar cells. Science 2017, 358, 739-744. (8) Rossi, F.; Baker, J. Beynon, D.; Hooper, K.; Williams, D.; Wei, Z.; Yasin, A.; Charbonneau, C.; Watson, T. All Printable Perovskite Solar Modules with 198 cm2 Active Area and Over 6% Efficiency. Adv. Mater. Technol. 2018, 1800156. (9) Kim, Y.; Park, E.; Yang, T.; Noh, J.; Shin, T.; Jeon, N.; Seo, J. Fast two-step deposition of perovskite via mediator extraction treatment for large-area, high-performance perovskite solar cells. J. Mater. Chem. A 2018, 6, 12447-12454. (10) Arora, N.; Dar, M.; Hinderhofer, A.; Schreiber, F.; Grätzel, M. Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%. Science 2017, 358, 768-771. (11) Kim, B.; Lee, S.; Jung, H. Recent Progressive Efforts in Perovskite Solar Cells Toward Commercialization. J. Mater. Chem. A 2018, 6, 12215. (12) Edri, E.; Kirmayer, S.; Mukhopadhyay, S.; Gartsman, K.; Hodes, G.; Cahen, D. Elucidating the charge carrier separation and working mechanism of CH3NH3PbI3−xClx perovskite solar cells. Nat. Commun. 2014, 3, 3461. (13) Jiang, X.; Xiong, Y.; Mei, A.; Rong, Y.; Han, H. Efficient Compact-Layer-Free, HoleConductor-Free, Fully Printable Mesoscopic Perovskite Solar Cell. J. Phys. Chem. Lett. 2016, 7, 4142-4146. (14) Zhu, Q.; Bao, X.; Yu, J.; Zhu, D.; Qiu, M.; Yang, R.; Dong, L. Compact Layer Free Perovskite Solar Cells with a High-Mobility Hole Transporting Layer. ACS Appl. Mater. Interfaces 2016, 8, 2652-2657. 15

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