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Stability Enhancement in Perovskite Solar Cells with Perovskite/Silver-Graphene Composites in Active Layer Tahmineh Mahmoudi, Yousheng Wang, and Yoon-Bong Hahn ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02201 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Energy Letters

Stability Enhancement in Perovskite Solar Cells with Perovskite/Silver-Graphene Composites in Active Layer Tahmineh Mahmoudi, Yousheng Wang, and Yoon-Bong Hahn*

School of Semiconductor and Chemical Engineering, Solar Energy Research Center, Chonbuk National University, 567 Baekjedaero, Deokjin-gu, Jeonju-si 54896, Republic of Korea

* Corresponding Author: [email protected]

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ABSTRACT: For practical use of perovskite solar cells (PSCs) the long-term stability of devices should be insured. The device stability is attributed to the degradation of perovskite molecule due to moisture and ions migration and the thermal and light instability under sun light illumination. To solve the stability issues, we proposed a new approach, i.e. fabrication of PSCs based on perovskite/silver nanoparticlesanchored reduced-graphene oxide (perovskite/Ag-rGO) composites. The Ag-rGO in the composite not only suppresses the ions migration but improves the thermal and light stability. The composite-based solar cells thermally aged at 90 oC for 90 h sustained over 94% of the initial value of power conversion efficiency (PCE), while a pristine PSC showed 39 % decrease in its initial PCE. More importantly, the long-term stability of composite-based PSCs was exceptional, retaining almost 100 % of the initial values of performance parameters over 330 days under ambient conditions (i.e., 25-30 oC, 45-57% humidity).

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ACS Energy Letters

It is known that the degradation and poor stability of perovskite materials has been a bottleneck in the commercial development of high-performance and long-term stability perovskite solar cells (PSCs) which can withstand sustained operation in ambient environment.1-9 There are major reasons for PSCs instability; the instability due to moisture and ions migration, intrinsic instability under heat and/or light soaking condition, instability attributed to thermal aging and photo-induced chemical reactions, etc.10-16 To control the degradation phenomenon and improve the PSCs stability several methods were reported, including incorporation of perovskite/nickel oxide composites,7-9 composition engineering,17-21 synthesis of perovskite materials using mixed cations22-25 and mixed anions,26-28 introducing carbon electrode,29-32 and solvent engineering for better crystal growth.33-36 Son et al. used a series of metal iodides (i.e., LiI, NaI, KI, RbI and CsI) in pure and mixed cation/anion perovskite materials to minimize the current-voltage hysteresis and interfacial trap densities.37 Aeuneh et al. deposited inorganic core/shell Au-SiO2 nanoparticles at interface between bl- and mp-TiO2 layers to modify and decrease the accumulation of produced cations by direct plasmonic process.38 Hu et.al. reported that PSCs showed light-stability with bigger grain size and improved film microstructure by using wide band gap mixed-halide perovskite (MAPbI3–xBrx) and non-wetting hole transporting layer (PTAA).39 Xing et al. studied the effect of light on ion migration in both poly- and single-crystal PSCs and reported that in single-crystalline perovskite the ion migration is too slow to be detected.40 Lin et al. reported that a bromine replacement with iodide not only impedes ion migration in PSCs but also maintains higher light absorption capacity.41 It has been reported that using inorganic hole transporting layers such as MoS2, MoS2 quantum dot/graphene and CuSCN allow PSCs devices to retain device performance and surpassed the stability of spiro-OMeTAD based PSCs.42-44 However, major challenges still exist to prevent perovskite



crystals from degradation and achieve long-lasting PSCs.45-47 Furthermore, little work has been done on the inclusion of MAPbI3-xClx/silver nanoparticles-anchored reduced-graphene oxide (perovskite/Ag-rGO) composites in active layer. Here, to solve the stability issues attributed to the degradation of perovskite materials due to moisture and ions migration and the thermal and photo instability, we proposed a new approach, i.e. fabrication of PSCs based on the perovskite/Ag-rGO composites in active layer. The Ag-rGO in the active layer not only impedes the destructive ions migration and diffusion but also accelerates the charge transport and improves the thermal and photo stability of the cells, resulting in long-term stability of PSCs. Perovskite solar cells (PSCs) with FTO/bl-TiO2 (50 nm)/mp-TiO2 (130 nm)/Al2O3 (100 nm)/(MAPbI3-xClx/Ag-rGO (350 nm))/Spiro-OMeTAD (50 nm)/Au (100 nm) configuration were fabricated, and its schematic and cross-sectional SEM image and corresponding energy band diagram are illustrated in Figure 1 and Figure S2a, respectively. Details of synthesis of p-type Ag-rGO and devices fabrication are available in Experimental Methods in Supporting Information. The structural TEM images and p-type semiconducting characteristics of Ag-rGO are presented in Figure S1. It is worthwhile to note that the functional role of Ag nanoparticles (NPs) on rGO films is to mainly increase the work function of rGO from 4.22 eV to 4.95 eV (see Figure S2a). Moreover, as-synthesized Ag-rGO shows a p-type behavior (Figure S1b) with high carrier mobility of 3x105 cm2/Vs and a greater conductivity of 9x106 S/m48 than pristine graphene (i.e., 1.59x105 S/m),49 indicating that the Ag NPs enhance charge separation and conduction in a photovoltaic device. Generally, thermal aging accelerates the movement of ions and the decomposition and degradation of perovskite crystals. At a thermal aging condition the perovskite material 4 ACS Paragon Plus Environment

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ACS Energy Letters

decomposes to HI, MA and PbI2, and the HI and MA diffuse to leave the perovskite for their smaller radius and lower energy barrier. Thus, to stimulate ions migration and diffusion the device samples were placed on a hot plate at 90 oC in dark under argon atmosphere for 90 h.

Figure 1. (a) Schematic of perovskite/Ag-rGO composite based solar cell structure, (b) cross-sectional SEM image of the cell, and (c) illustration of blocking of iodide ions and water molecules by graphene sheet in the active layer.

Thermal stability of the perovskite/Ag-rGO composite films was examined with UV-vis analysis as a function of Ag-rGO content with varying thermal aging time (see Figure S3 a-f). Compared to the pristine perovskite (i.e., MAPbI3-xClx) film (a), the composite films show enhancement in light absorption capability with incorporation of Ag-rGO (b-e) and yields the best light harvesting at 6 wt% Ag-rGO (d). Normalized absorbance plot clearly exhibits that the composite with an optimal loading of 6 wt% Ag-rGO has remarkable thermal stability, i.e. retaining 97 % of absorption intensity over 90 h (f). This result strongly suggests that the presence of graphene sheets could interdict the delocalized ions movement in perovskite layer and thus suppress defects generation as well as degradation, which is attributed to the fact that among the ions and molecules in perovskite material iodide has the smallest diameter of 0.412 nm, but it cannot penetrate through graphene sheet having lattice constant of 0.246 nm (Figure 1c). The 5 ACS Paragon Plus Environment


performance of the perovskite/Ag-rGO composite cells was examined as a function of the content of Ag-rGO in the composite (see Figure S2b and Table S1). Compared to the pristine device (PCE = 11.74%), the composite-based cells showed a best performance with 37% increase in PCE (i.e., 16.1 %) with optimal content of 6 wt% Ag-rGO. Such improvement is attributed to high carrier mobility of p-type Ag-rGO, which enhances charge (i.e., hole) transfer

Figure 2. XPS spectra of iodide on gold electrode in (a) perovskite and (b) perovskite/AgrGO based solar cells after thermal aging for 0, 24 and 48 h.

at the interfaces of Ag-rGO/perovskite phase. The charge carrier mobility of the perovskite/AgrGO composite film via hall measurements was 50 cm2/Vs, which is 4.2 times larger than that of pristine perovskite film (12 cm2/Vs), confirming the faster carrier transport in the composite film. Unless mentioned, the optimal content of 6 wt% Ag-rGO in the composite was used for devices fabrication. It is worthwhile to note that compared to the pristine and perovskite-rGO composite based cells, the perovskite-rGO based cell showed a worst photovoltaic performance (PCE = 8.386 %), justifying the role of Ag NPs in the perovskite/Ag-rGO composite. J-V characteristics and photovoltaic parameters with and without Ag NPs are included in Figure S2b and Table S1. 6 ACS Paragon Plus Environment

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ACS Energy Letters

The XPS analysis of I 3d was taken from gold electrode for both pristine perovskite and perovskite/Ag-rGO composite based cells after thermally aging at 90 oC because iodide ions migrate towards the upper electrode layer. Figure 2 shows that the iodide concentration on Au electrode in pristine perovskite devices significantly increases with thermal aging time and peaks shifted to lower binding energy (a), indicating migration of more iodide ions to the upper electrode. By contrast, for the perovskite/Ag-rGO composite based solar cells, the intensity of iodide peaks remains nearly unchanged, indicating that the ions migration is much suppressed in the photoactive layer in the course of thermal heating (b). As the iodide ions are blocked by the rGO sheets, they need to go around the graphene sheets during the thermal aging to leave the perovskite layer, which prolongs the migration path because a longer diffusion path reduces the diffusion flux and hinders the ions to leave perovskite layer.50,51 To observe ions distribution across the cell structure, the FIB-EDX (focused ion beam with energy dispersive X-ray) analysis was performed on as-fabricated and thermally-aged devices (Figure S4d-e). The top view SEM images of perovskite layer with and without Ag-rGO are provided in Figure S4a-b. Compared to pristine perovskite (a), surface morphology of perovskite/Ag-rGO composite layer exhibit pinhole-free, highly crystallized dense-grained with micronmeter-size grains (b). The EDX elemental mapping of pristine perovskite-based devices indicates that iodide ions migrate to upper layer during the thermal aging. By contrast, the iodide ions migration and diffusion is significantly suppressed in the perovskite/Ag-rGO composite based devices. This result confirms that the presence of rGO hinders ions migration and effectively blocks the diffusion paths. Moreover, the FT-IR analysis of Ag-rGO, perovskite, and perovskite/Ag-rGO composite films exhibit that the peaks of N-H, C-H and C-N in perovskite shift towards a lower wave number in perovskite/Ag-rGO composite (Figure S5). This suggests that chemical bonding 7 ACS Paragon Plus Environment


formed between the Ag-rGO and perovskite atoms is favor for fast charge transfer and stabilizing the perovskite materials. Figure 3 exhibits the IPCE spectra (a), J-V characteristics with forward and reverse direction scanning of the composite based cells (b), and the normalized PCEs of pristine and composite based cells after thermal aging at 90 oC (c) and under continuous light illumination (d). The

Figure 3. (a) IPCE spectra, (b) J-V hysteresis curves with forward and reverse direction scanning, (c) normalized PCE of the pristine perovskite and composite based cells as a function of thermal-aging time, and (d) normalized PCE vs. light illumination. Cell structure is FTO/bl-TiO2 (50 nm)/mp-TiO2 (130 nm)/Al2O3 (100 nm)/MAPbI3-xClx:Ag-rGO (350 nm)/Spiro-OMTAD (50 nm)/Au (100 nm).


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ACS Energy Letters

composite based cells show more light harvesting in the range of 300-780 nm than the pristine perovskite cell, resulting in the integrated photocurrent density Jsc = 22.8 mA/cm2, which is well matched with the measured Jsc (i.e., 23.5 mA/cm2). Furthermore, compared to the severe hysteresis from the pristine perovskite cells (inset, Figure 3 b) the hysteresis is almost removed from the perovskite/ag-rGO composite based devices (b), which is mainly due to the suppression of ions migration and diffusion by graphene sheets in the active layer. To examine the thermal stability, we examined the J-V characteristics of the cells aged at 90 oC as a function of thermal aging time (c). The PCE of pristine perovskite devices showed 39 % decrease in its initial value. By contrast, the perovskite/Ag-rGO composite cells showed strong thermal stability with retaining over 94 % of its initial PCE value after 24 h up to 90 h (also see Figure S6, Tables S2 and S3). These results are in good agreement with the steady state photoluminescence (PL) spectra (Figure S8). For the pristine perovskite film PL spectra showed a sharp emission peak at 770 nm and the peak intensity increased with thermal-aging time, while the perovskite/Ag-rGO composite films showed distinct PL quenching with little increment of peak intensity, which is a clear evidence of efficient charge separation and conduction. Such result indicates that graphene sheets play an important role in not only suppressing the ions migration but also improving the thermal and photo stability. In order to elucidate the influence of Ag-rGO on the crystallinity of perovskite, samples of pristine perovskite and perovskite/Ag-rGO composite films before and after thermal aging at 90 oC for 90 h were subject to XRD analysis. After thermal aging of the pristine perovskite film, a PbI2 peak centered at 12.7° was observed (Figure S7a), representing the formation of PbI2 in the course of thermal aging. By contrast, the perovskite/Ag-rGO composite showed no peak of PbI2 after thermal aging (Figure S7b), indicating that graphene sheets effectively suppress the iodide ions migration, thus not to form PbI2. 9 ACS Paragon Plus Environment


To further explore the effect of thermal aging on the internal electrical characteristics of both the pristine and composite devices, impedance spectra (EIS) measurements were carried out under 1 sun illumination with and without thermal aging at 90 oC for 90 h (see Figure S9b and Table S4). Compared to the pristine perovskite solar cells, the composite based devices show a lower charge transfer resistance (R1) and a higher interfacial recombination resistance (R2) regardless of the thermal aging, indicating fast charge separation and conduction with strong suppression of recombination, attributed to the presence of Ag-rGO, which shows a good agreement with IPCE and PL measurements. This is mainly due to the role of Ag-rGO, i.e. not only impeding the destructive ions migration and diffusion in perovskite phase but also accelerating the charge separation and conduction in the active layer. Additionally, the reproducibility of the perovskite/Ag-rGO based cells is evaluated with 40 devices and results are shown in Figure S10. The distribution of PCE vs. number of cells shows that most of the fabricated devices are stable with excellent reproducibility (PCE = 15-16 %), which is higher than that of pristine perovskite cells (i.e., PCE ~ 10 %). It was reported that iodine reacts with Ag, leading to silver iodide formation and causing a degradation of PSCs.52 To observe the possibility of AgI formation, we performed XPS analysis on the films of Ag-rGO powder, assynthesized and thermally-annealed perovskite/Ag-rGO composite (Figure S12). The Ag-rGO samples showed high binding energy shoulders for the Ag 3d5/2 peak at 369.02 eV and for the Ag 3d3/2 peak at 375.1 eV. The as-synthesized perovskite/Ag-rGO composite films showed no changes in the Ag 3d peaks position, but the thermally-annealed composite film exhibited the peaks of Ag 3d5/2 and Ag 3d slightly shifted (±0.1 eV) to lower binding energy of 368.9 eV and 375.0 eV, respectively, which is probably due to partial oxidation of Ag0 to Ag+. Nevertheless, relatively small atomic ratio of silver to iodide in the composite (i.e., Ag/I = 0.006) and the role


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ACS Energy Letters

of graphene in suppressing iodide ions migration suggest that degradation of PSCs due to silver iodide formation is negligible.

Furthermore, we examined the light-stability property of the composite based devices under continuous and periodic (i.e., 10 s light-on + 2 s light-off) 1 sun light illumination. The device performance parameters with continuous and periodic light illumination are summarized in Tables S5 and S6. The composite-based devices sustained over 93 % of its initial PCE value after 24 min up to 1 h under continuous illumination, but the pristine perovskite devices showed 39 % decrease in its initial PCE due to decomposition of perovskite molecules (Figure 3d). Interestingly, we observed 8.7% loss in PCE under continuous 1 sun illumination (Figure S11a), but surprisingly it showed a rapid recovery under periodic light illumination (i.e., recovered and/or self-healed when the devices were kept in the dark for few seconds between measurements), presenting 6.6% increase in PCE value (Figure S11b). Unlikely, the PCE of pristine perovskite device showed 32.6 % decrease in its initial value under periodic light illumination (Figure S11c). Recently, it has been reported that for PSCs it takes tens of minutes to hours or days to be fully self-healed or recovered, depending on reversible or nonreversible losses which originates from degradations in perovskite layer.52-54 However, the perovskite/Ag-rGO composite devices showed fast recovery, attributed to quick relaxation of the light-activated degradation/or trap states in the dark (see Figure S11b). It is also worthwhile to note that the perovskite/rGO-based PSCs, despite their low PCE, show a similar sustainability of stability under periodic light illumination conditions (see Figure S11d). To examine the long-term stability of the perovskite/Ag-rGO composite based cells, the devices without encapsulation have been exposed to ambient room conditions (25-30 oC, 45-55 % relative humidity) for 330 days, and the results in terms of normalized performance



parameters are shown in Figure 4. Compared to the pristine cells, the perovskite/Ag-rGO composite devices showed a remarkable long-term stability with retaining almost 100 % of the initial values of photovoltaic parameters over 330 days (also see Table S7), which also means a remarkable air-stability. Such results lead to a conclusion that the graphene sheet provides an effective path for rapid transport of photo-induced charges to conduction layers as well as it suppresses the ions migration and thus improves the thermal and light stability of the compositebased PSCs as well the as air-stability. The presence of Ag-rGO also decreases the possibility of perovskite degradation by side-reactions with ambient species (i.e., H2O and O2) and thus helps to avoid perovskite decomposition and enhance interfacial charge transfer.

Figure 4. Long-term stability of pristine perovskite (red circles) and perovskite/Ag-rGO (black circles) based solar cells for 330 days under ambient room condition (25–30 ℃, 45– 55 % humidity): normalized (a) Voc, (b) Jsc, (c) FF and (d) PCE. 12 ACS Paragon Plus Environment

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ACS Energy Letters

In conclusion, long-term stability of perovskite solar cells achieved with incorporation of perovskite/Ag-rGO composite. Functional role of Ag nanoparticles on rGO sheets is to mainly increase the work function of rGO from 4.22 eV to 4.95 eV. The Ag-rGO shows a p-type behavior with high carrier mobility of 3x105 cm2/Vs and a greater conductivity of 9x106 S/m. The Ag-rGO in the active layer not only acts as ions diffusion barrier and efficient pathway of carriers, but also improves the thermal and light stability of the cells. The composite-based cells thermally-aged at 90 oC for 90 h sustained over 94 % of the initial PCE, while the pristine perovskite cells showed 39 % decrease in its initial PCE. Overall the perovskite/Ag-rGO composite cells showed remarkable long-term stability, retaining almost 100 % of the initial values of performance parameters over 330 days under ambient condition, which is critical for practical use of perovskite solar cells.

■ ASSOCIATED

CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergy-lett.8bxxxxx. Additional figures and tables and detailed description of the experimental methods (pdf)

■ AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.



■ ACKNOWLEDGMENT This work was supported by National Leading Research Laboratory program (NRF-20110028899) through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning.

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