MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells

Electrochimica Acta xxx (xxxx) xxx

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MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells Ya-Nan Zhang a, b, *, Bo Li b, Lin Fu b, Qun Li a, Long-Wei Yin b, ** a

College of Chemistry and Chemical Engineering, Taishan University, Tai’an, 271021, PR China Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan, 250061, PR China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 August 2019 Received in revised form 8 November 2019 Accepted 10 November 2019 Available online xxx

We adopt Metal-Organic-Framework (MOF)-derived zinc oxide (ZnO) as electron extraction material for hybrid cationic perovskite solar cells for the first time, breaking the prevailing paradigm of using oxides nanoparticle as electron extraction layer. MOF-derived ZnO with a polyhedral morphology and abundant internal porous structure can increase light harvesting ability and optimize the interfacial contact with perovskite. In contrast to conventional ZnO nanoparticles, the introduction of MOF-derived ZnO will achieve more efficient electron extraction, reduction of trapped state density and lower electron-hole recombination probability, thus significantly increase the fill factor and short-circuit current density of the cells. MOF-derived ZnO based perovskite solar cells exhibit a champion power conversion efficiency of 18.1% coupled with improved fill factor of 0.74 and short-circuit current density of 22.1 mA cm
2. Simultaneously, there is almost no hysteresis effect, and performance attenuation of the device in the ambient atmosphere over time can be suppressed. The performance improvement of perovskite solar cells stems from improved light harvesting efficiency in a wide wavelength range, as well as enhanced carrier extraction efficiency resulted from the increase of interface area between MOF-derived ZnO and perovskites. © 2019 Elsevier Ltd. All rights reserved.

Keywords: MOF-Derived ZnO Perovskite solar cells Electron extraction Light harvesting

1. Introduction Organic-inorganic hybrid perovskite materials are one of the hottest candidates for high efficient and low-cost solar cells [1,2]. Their photoelectric properties including the outstanding optical absorption, the long equilibrium carrier diffusion length and the simple processing methods, which make them have great application prospects in the field of photovoltaics [3]. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been increased rapidly at an overwhelmingly rate, and continuously optimized by film engineering, interface engineering and the like [4,5]. Since ZnO is inexpensive and abundant in nature reserves, it is one of the widely used electron transport materials (ETM) in solar cells. The physical properties and energy band position of ZnO are

* Corresponding author. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan, 250061, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y.-N. Zhang), [email protected] (L.-W. Yin).

similar to TiO2, and even more attractive owing to its high electron mobility and structural variability [6e8]. From the point view of preparation method, the ZnO nano-layer can be deposited via spincoating at low temperature, which will be advantageous for the energy-saving and massive production of photovoltaic devices, and can also be applied in flexible devices [9e11]. Unfortunately, although ZnO has these advantages, the PCE of regular structured PSCs based on pure ZnO was still hovering around 15e16%, which is obviously expressively lower than those devices based on TiO2 or SnO2 [12e14]. In addition, ZnO devices suffer from performance instability, such as the instability caused by chemical residues in the manufacturing process, is a serious problem restricting the further application of such devices [15]. According to the research of Cheng et al. [16], not only the residual hydroxyl and acetate in the growth solution will aggravate the decomposition of perovskite, but also the chemical residues such as hydroxyl on the surface of zinc oxide will cause photochemical reaction, which will further accelerate the decomposition of perovskite [17]. Nonetheless, perovskite deposited on annealed ZnO exhibits better stability, which is due to the decomposition and volatilization of unstable surface hydroxyl groups and residual organic ligands during the high temperature

https://doi.org/10.1016/j.electacta.2019.135280 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280

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annealing. Optimizing the quality of ZnO ETMs may still be the main focus to improve charge collections and reduce the recombination. Kim et al. [18] have shown that high density of states in perovskite conduction bands accumulate free carriers, which leading to a longer extraction time. As a consequence, nanostructures with larger specific surface area than planar structures can observably enhance the carrier extraction efficiency. In addition, nanostructures can also increase the light harvesting in PSCs because they can act as scattering centers. ZnO nanoparticles are traditional and efficient nanomaterial in the field of optoelectronics and can simply form porous and compact films by facile solution method [19,20]. According to the research of Zhang et al. [21], the size of ZnO nanoparticles has a significant effect on the performance of solar cells. With large contact area, the ~40 nm ZnO particles in which perovskite permeates completely gives the best performance. ZnO nanowalls have been used in photovoltaic cells due to their excellent electronic transport performance. Compared with planar ZnO film, ZnO nanowalls provide much larger contact area and more direct electronic path between ZnO and perovskite material, which enhances the electron transportation and collection efficiency at ZnO nanowalls/perovskite interfaces [22]. Considering that the sintered MOF-derived oxides have good crystallinity and regular nanostructure, the pre-annealing of the oxide which eliminates chemical residue is conductive to enhance the stability of the solar cells. The large specific surface area and uniform pore network of the MOF-derived porous oxide facilitates the effective penetration and sufficient filling of the perovskite, which can increase the contact area between the ETM/perovskite [23,24]. Perovskite filling into the internal voids of the electron transport layer (ETL) can bring about two improvements. One is to increase the internal light scattering, that is, increase the effective light harvest and improve the light utilization rate [25e27]. Another is that increased contact area is conducive to the rapid transport of electrons and reducing electronic recombination [28,29].

Herein, based on these precedents, we tried to use MOF-derived ZnO (MZnO) with dodecahedron porous structures as an electron extraction material for improving optical harvesting efficiency and electron extraction efficiency of perovskite solar cells. We investigated influence of MOF-derived ZnO on the absorbance of the active layer, interface electron transport and carrier recombination. The PSCs devices with excellent photovoltaic performance and satisfying durability were obtained. Compared to the ZnO nanoparticle layer, the perovskite solar cell with MOF derived ZnO increases short-circuit current density (JSC) by 11% and PCE by 20% mainly due to improved light harvesting efficiency (LHE) and electron collection. As a consequence, PCE is increased from 15.1% to 18.1% by changing the ZnO nanoparticle layer to the MOF derived ZnO one. This opens up a new path for the selection of ETLs in perovskite solar cells.

2. Results and discussion 2.1. Structure and morphology ZIF-8 MOFs with different sizes were synthesized by the coordination reaction of Zn2þ (Fig. S1). By comparing the effect of different sizes of MZnO on the performance of perovskite films and PSCs (Figs. S2eS3), it was determined that the most suitable ETL material was a MOF-derived ZnO having a particle size of about 120 nm (Fig. 1b and blue curve in Figs. S2eS3). The following systematic study will be based on this type of MOF-derived ZnO (abbreviated as MZnO). As shown in Fig. 1, the synthesized MOF particles are uniform and exhibit dodecahedral morphology. After heat treatment at 400  C, the MOF precursor transforms to crystalline ZnO. The MOF-derived ZnO polyhedrons (Fig. 1c) still retain the polyhedron shape of the MOF crystals and shrink slightly due to the decomposition of organic species. At the same time, it can be distinctly displayed that the polyhedron is composed of ZnO nanoparticles, which makes it has a large specific area and a porous structure. The MZnO prepared by high temperature sintering has

Fig. 1. (a) SEM and (b) TEM images of ZIF-8 polyhedrons; (c) TEM images and (d) XRD patterns of MZnO polyhedrons.

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280

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good crystallinity. The XRD pattern (Fig. 1d) shows typical peaks corresponding well with (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes of ZnO (PDF 36e1451). The perovskite (PVSK) film deposited on MZnO is dense and uniform, and the grain size is about 200 nm (Fig. S4). TEM image of MZnO-PVSK powder shows that the morphology of MZnO particles does not change significantly and is still relatively uniform after PVSK infiltration (Fig. 2a). It can be observed that the outer layer of MZnO is coated with PVSK and its thickness is approximately 3e5 nm (Fig. 2b inset). In the high-resolution TEM images, the presence of PVSK grains in the pores between the particles inside the MZnO polyhedron can be observed (Fig. 2c), while the MZnO polyhedral surface has a PVSK cladding (Fig. 2d), which proves that PVSK is adequately penetration between the MZnO particles and the internal pores of MZnO. Such large-area contact between PVSK and MZnO facilitates improved light utilization and photogenerated electrons extraction. Fig. 2e shows a select area electron diffraction pattern of MZnO-PVSK. The diffraction rings match well with the (002), (110), and (103) planes of ZnO and the (111), (002), and (022) planes of PVSK, respectively. The textural properties of the MZnO-PVSK and MZnO polyhedrons are studied by nitrogen adsorption-desorption experiments. Typical coexistence of micropores and mesoporous in MZnO polyhedrons can be found, as shown in the N2 adsorptiondesorption isotherms (Fig. 2f, black). The existence of large volume micropores formed during the pyrolysis of organic ligands is indicated by the approximate vertical rise at the low pressure region (P/P0~0) [30]. The presence of mesopores is suggested by the hysteresis loop at P/P0 of 0.85e0.99 [31]. The specific surface area is measured to be 73.01 m2 g
1, with a mesoporous and microporous volume of 0.66 and 0.03 cm3 g
1, respectively. The textural

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properties show an enormous change after PVSK is permeated into pores of MZnO. The specific area reduces to 10.86 m2 g
1, forecasting that some pores are occupied by PVSK grains, which is accordance with the results displayed by the TEM figures. The vanishing of the vertical rise at P/P0~0 manifests the extinction of micropores, revealing the mesoporous volume and microporous volume decrease to 0.15 and 0.0044 cm3 g
1. An evident conclusion can be drawn from the above discussions that PVSK is successfully penetrated into pores of MZnO polyhedrons matrix, thus forming a coessential MZnO-PVSK heterojunction structure. This structure is favorable for PVSK using light scattering between MZnO particles to enhance the utilization of light, and is also conducive to the electron transport and extraction between PVSK and MZnO, both of which are beneficial to the amelioration of device properties. 2.2. Optoelectronic property In order to testify the effect of MZnO on light utilization, the absorption spectrum of the PVSK active layer and its addition of different ETLs were measured (Fig. 3a). The conventional nanoparticle ZnO (NPZnO) contributes little to the light absorption of the active layer and does not substantially affect the absorption density of PVSK. However, UVevisible absorption spectrum of the MZnOPVSK film shows an increase about 10% than that of PVSK film over nearly the whole visible spectrum, indicating an enhanced light absorption of perovskite layer. The increase in absorbance can be ascribed to the regular polyhedral morphology and poriferous structure of MZnO which can be used as extra scattering centers to enhance the internal light scattering, allowing the active layer to have more efficient light harvesting and improve light utilization efficiency [26e28]. Using the following formula:

Fig. 2. (aed) TEM images and (e) diffraction patterns of MZnO-PVSK powder samples. (f) N2 adsorption-desorption isotherms and pore distribution of MZnO and MZnO-PVSK complexes (inset).

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280

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Fig. 3. (a) Absorption-fluorescence spectra and (b) LHE of PVSK films with and without MZnO. (c) TRPL spectra of PVSK with different ZnO as ETL. (d) Dark IeV measurement of the electron-only devices based on different ETL displaying VTFL kink point behavior, inset is EIS spectra of PSCs with different ZnO as ETL.

LHE ¼ ð1
RÞð1
10
A Þ calculate the light harvesting efficiency, where R is reflectance (Fig. S5) and A is absorbance. Fig. 3b exhibits LHE and LHE enhancement spectra, where the perovskite layer with MZnO has slightly higher LHE than that without MZnO in the visible range, specifically the introduction of MZnO increases the LHE by 4e7% over the wavelength range of 400e730 nm. A higher LHE value manifests that more incident photons arrive in the films. The enhancement of LHE is mainly due to the decreased reflectance and the increased scattering efficiency [28,32]. The steady state photoluminescence (PL) is a simple and efficient method for studying the trap filling and electron extraction effect between the EEM and PVSK. For comparison, we collect the PL spectra of the PVSK films deposited on different ZnO/glass substrates and are displayed in Fig. 3a. The PVSK film shows a robust PL peak at 789 nm. Sure enough, the excited ZnO/PVSK films exhibit a reduction in PL intensity, manifesting the fluorescence quenching produced by the effective electron extraction of ZnO EEM. Interestingly, the sample of NPZnO-PVSK quenches the PL by 69%, while the MZnO-PVSK quenches the PL exceed 93%. The remarkable PL quenching obtained in the presence of the MZnO ETL indicates that PVSK sufficiently penetrated MZnO is conducive to efficient electron extraction [33]. Apart from the dropping PL intensity, the PL peaks of the MZnO-PVSK sample shows an obvious blue-shifted PL peak from 789 to 785 nm upon excitation. Huang and his colleagues [34] reported that the presence of surface trap states on the perovskite surface or grain boundaries would cause

band bending near the surface of the film, resulting in a significant bandgap lower than that of the bulk with a red-shifted PL peak. Weakening the trap states can restore of the bandgap and show blue shift of the PL peak. The results identify that MZnO could decrease the trap states of the perovskite film, thus decreasing the electron-hole recombination positions. That is to say, MZnO exhibits more efficient electron extraction and collection than NPZnO. The dynamics of charge transfer and recombination occurring at the MZnO-PVSK interface is further elucidated by transient PL decay (Fig. 3b). The decay time is obtained by fitting the curve with the bi-exponential decay function. The perovskite film shows a comparatively long PL lifetime of 152.89ns in the absence of ETL quencher. In the case of ZnO-PVSK films, the PL lifetimes decrease significantly with values of 54.14 and 13.18 ns for NPZnO and MZnO films, respectively. Commonly, better electron extraction through the accepting layer in perovskite-based devices can generally lead to shorter lifetime and stronger PL quenching [34,35]. The consistency between the TRPL lifetime trend acquired in ETL/PVSK films and the effective charge extraction proved from steady PL measurements further indicates that MZnO can effectively decrease charge recombination in the perovskite film. The photoinduced exciton behavior quenched at the MZnO/PVSK interface clearly confirms that the introduction of MZnO enhances the electron extraction and collection performance, thereby reducing the exciton recombination rate [36]. In order to clarify the charge transfer and recombination rates in essence and further elucidate

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280

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the effect of MZnO on the performance of PSCs, the PSCs based on diverse ETL are fabricated and their Nyquist plots are shown in the inset of Fig. 3d. The main semicircle in the low frequency range is related to recombination resistance (Rrec), which is inversely proportional to the carrier recombination rate [37]. Interestingly, the perovskite solar cell with the MZnO ETL has a higher Rrec than that of the NPZnO based cell, that is, the device based on MZnO ETL has the lower carrier recombination rate. We can argue for sure that compared with NPZnO, MZnO has a larger contact interface with PVSK. MZnO and PVSK infiltrating into its interior form a structure similar to a bulk heterojunction, making the interface structure more compact. Such a structure is advantageous to accelerate the electron extraction efficiency, and also reduces the electron-hole recombination probability [29]. In order to obtain the interface properties and further verify the role of MZnO on the performance of PSCs, the recorded EIS data (ReV curve) are depicted in Fig. S6a. Owing to the identical fabrication condition and parameters for all devices, the difference of recombination resistance (Rrec) could be identified only as the effect of different electron extraction materials. For perovskite solar cell, the recombination resistance depends on the applied potential difference, presenting an approximately exponential reduction with applied potential difference. It is well known that the Rrec value is inversely proportional to the electron recombination rate [37]. It is clear that the MZnO sample shows lower recombination rate than that of the NPZnO one. The charge carrier density of the samples can be analyzed by the Mott-Schottky plot (Fig. S6b). The donor density (Nd) can be calculated by the slope of the MottSchottky plots via the equation [38]

1 C2

¼

2ðVbi
VÞ , A2 qεε0 Nd

where A is elec-

trode area, q is electronic charge, ε is relative dielectric constant of the sample, ε0 is permittivity of the vacuum, Nd is donor density, Vbi is built-in potential, and V is applied potential difference. The carrier concentration of MZnO cell is estimated to be 1.29  1015 cm
3, higher than that of NPZnO sample (7.19  1014 cm
3). In addition, the built-in potential can be estimated using the intercept of the linear regime with the x-axis of Mott-Schottky curves to about 1.14 V [39], suggesting that the photo-generated carriers can be separated efficiently by the presence of highly built-in field. The relatively higher built-in potential of the MZnO sample can not only suppress the back transfer of electrons from the ETL to the perovskite layer, but also benefit the charge collection and transfer of the photo-generated carriers [40]. We have fabricated electron-only devices to test the electron trap-state density in the perovskite absorber via space charge limited current (SCLC) analysis. The IeV curves of the devices based on NPZnO and MZnO are displayed in Fig. 3d. At low applied potential difference, the ohmic response of electronic-only devices is represented by a linear relationship (labeled in red). When the bias exceeds the inflection point, the current increases rapidly and nonlinearly (labeled in orange), indicating that the trap-states are fully filled [41]. The trap-state density can be calculated by the trapfilled limit voltage (VTFL) employing equation [42]: VTFL ¼

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generated holes. By decreasing these electron traps, the trapmediated electron-hole recombination is suppressed, thus effectively extracting charge carriers from PVSK into MZnO [43]. 2.3. Photovoltaic performance For the sake of improve the photoelectric properties of PSCs, the J-V properties of PSCs based on different ZnO ETL are conducted. The J-V curves with forward and reverse scan direction for the champion devices of the MZnO cells and corresponding NPZnO cells are depicted in Fig. 4a. The solar cell based on NPZnO ETL yield a JSC of 19.9 mAcm
2, a VOC of 1.12 V and a FF of 0.68, producing a PCE of 15.1%. From the point of view of photovoltaic, PSC based on MZnO manifests the optimized performance with a PCE of 18.1%, quantifying the highest JSC of 22.1 mA cm
2, a FF of 0.74 and a VOC of 1.11 V. The JSC and FF show an increasing tendency after introducing MZnO, which may be due to the optimizing of light absorption, interface contact, carrier transfer and recombination behavior [44]. As described above, the enhanced light scattering of the MZnO polyhedrons improves the light absorption characteristics of the solar cell, and the enhancement in current density is inseparably linked to enhanced light harvest and light utilization efficiency. For another, it is suggested that the enlarged interface areas between MZnO and PVSK is beneficial to extract more free electrons from perovskite into ZnO EEM, thus significantly enhance electron extraction efficiency [45]. Meanwhile, sufficient percolation of the perovskite inside the 3D nanostructures MZnO layer would offer more directly electron channel, thus allowing more electrons generated nearby the perovskite/HTM interface to arrive ETL layer [28]. In general, the JSC and FF enhancement of perovskite solar cells is due to the decrease of trap state density, enhanced PL quenching and increased composite resistance proved by the above tests, which can cause the suppression of carrier recombination. The efficiency of MZnO solar cells is close to or even higher than that of PSCs prepared by ZnO ETL reported at present [46,47]. In addition, the J-V curves of the optimized presenting cells show that the

qNt L2 2εε0 ,

where q is the elementary charge of the electron, Nt is the trap-state density, L is the perovskite film thickness, ε is the relative dielectric constant of perovskite, and ε0 is the vacuum permittivity. The VTFL of the perovskite films deposited on NPZnO and MZnO substrates are 0.15 and 0.35 V, respectively. Consequently, the electron Nt of NPZnO device is 4.45  1014 cm
3, whereas the electron Nt of MZnO device is decreased to 1.91  1014 cm
3, demonstrating that the film character is greatly optimized. The PL spectra (Fig. 3a) exhibit that the peak of MZnO-PVSK blue-shifted from 789 nm to 785 nm, verifying that the trap-states in the perovskite absorber are effectively passivated. What is more, the existence of trapped electrons will enlarge the opportunity of recombination with the photo-

Fig. 4. (a) Champion cell performance and hysteresis, (b) Cell efficiency histograms and (c) EQE curves for PSCs based on different ETLs; (d) Stability of MZnO-PVSK film and PSCs.

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280

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hysteresis phenomenon can be almost neglected. The histograms of two sets of 30 devices prepared in an identical fabrication and measurement condition are plotted in Fig. 4b, based on NPZnO and MZnO as ETL of the PSCs, respectively (The box charts of parameters are shown in Fig. S2). The excellent normal distribution of MZnO devices demonstrates that it has better reproducible and reliable. More than 80% of the MZnO cells exhibit an overall efficiency exceeding of 17%, proving the conceptual validity of the MOF derived ZnO-perovskite heterojunction design. Fig. 4c exhibits the EQE spectra of devices based on these two ETLs. The photo-response range reaches its maximum in the visible region, which is consistent with the absorption spectrum (Fig. 3a). MZnO devices have stronger optical response than NPZnO devices, and more than 80% of the optical response platforms are found in a wide range of 360e650 nm, which is in agreement with the higher current density in J-V curves. In contrast to the difference between absorber curves in the absorption spectrum, the MZnO device in the EQE spectrum has stronger spectral response. This validated that on the one hand, MZnO as photosensitizer enhances light absorption. On the other hand, MZnO can decrease the carrier recombination, which promotes electron transport and further enhances EQE[27,28]. The MZnO-perovskite observed in Fig. 3 exhibits enhanced PL quenching, reduced TRPL lifetime and decreased trap density, which is in accordance with the fact that the heterojunction can reduce the defect density sites at the ETL/ perovskite interface, in the meanwhile increase the transfer rate of

electron across this interface. Therefore, compared with the control devices, in the MZnO-PVSK devices the charge extraction to the external circuit competes much more advantageous with recombination losses. The stability of prepared MZnO-PVSK film in the relative humidity of around 40% is investigated. Fig. 4d exhibits the relevant UVevis spectra data recorded at different storage time in the environment. We demonstrate that the MZnO-PVSK films show remarkable stability against humid stress. Similarly, it is worth noting that more than 95% of absorbance intensity can be retained after 45 d. Apart from this, for the sake of preliminarily measure the stability of PSCs based on MZnO, unpackaged fabricated cells were stored in 40% RH air ambient and determined their J-V performance periodically. Taken together, the residual PCE of the aged device based on MZnO in 30 days is shown to be 77% of its original value, reflecting its extremely remarkable stability. As shown in Fig. 5, the MOF derived ZnO polyhedron has a dodecahedral morphology and a large number of pore structures. Large particles of MZnO can act as light scattering centers, increasing the light harvesting efficiency of the active species in the PSCs. In the next place, the porous MZnO can offerti a large contact area with the perovskite material, perovskite can adequately penetrate between the polyhedron and into the pores of MZnO. This complete and sufficient contact provides a more direct path for the transport of photo-generated electrons, thereby enhancing the charge extraction and transport efficiency at the MZnO/perovskite interface.

Fig. 5. The Schematic diagrams of formation of MOF-Derived ZnO Polyhedra and their use as ETLs to enhance light harvesting and electrons extraction.

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280

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3. Conclusions In conclusion, we pioneer an updated tactic to enhance electron transport and extraction in PSCs via using MOF-derived ZnO as ETL. It can be ascertained that the introduction of MZnO leads to quenched PL intensity, reduced electron lifetime, enlarged charge recombination resistance and decreased density of trap states, indicating efficient electron extraction and suppressed electronholes recombination rates, which boosts electron transport and increase JSC and FF. It has been found that MZnO with a special morphology and massive internal pores can induce higher lightabsorbing densities and improve the optical utilization efficiency of the perovskite. The increased interface area between MZnO and PVSK promotes carrier extraction efficiency. This MOF-derived ZnO is employed to optimize the interface in PSCs and significantly enhance device property, yielding VOC of 1.11 V, JSC of 22.1 mA cm
2, FF of 0.74, and a PCE of 18.1% close to the most advanced ZnO devices. This device has virtually negligible hysteresis and has remarkable stability in a certain humidity environment. We believe that this work can provide diversified choice and innovative designs with desirable performance for perovskite solar cell ETLs. 4. Methods Materials FTO (F:SnO2, 8U/sq) substrate, spiro-MeOTAD, Formamidine iodine (FAI) and Ag are purchased from OPV Tech Co., Ltd. Other chemical reagents are purchased from Aladdin Biochemical Technology Co., Ltd. and used without any further purification. Synthesis of MOF-derived ZnO Dodecahedrons 1 g of zinc nitrate hexahydrate (Zn(NO3)2 $6H2O) and 1e2 g of dimethyl imidazole were dissolved in 50 ml of methanol, respectively, and then the dimethyl imidazole solution was poured into the zinc nitrate solution with stirring. The mixture was stirred for 0.5 h and the solution was allowed to stand for 24 h. The supernatant was then drained off, the white precipitate was centrifuged, washed three times with methanol, and dried overnight at 60  C to obtain the MOF product (ZIF-8). The product was sintered in air at 400  C for 3 h to obtain MOF-derived ZnO (abbreviated as MZnO). The MOFs prepared using dimethyl imidazole amounts of 1 g, 1.5 g, and 2 g were named MOF1, MOF2 and MOF3, respectively. The zinc oxides obtained after sintering these three MOFs are named MZnO1, MZnO2, MZnO3, respectively. Perovskite Solar Cell Preparation The structure composition of the PSCs device using MOF-derived ZnO as the ETL is: FTO/cTiO2/ MZnO/perovskite/HTM/Ag. The compact TiO2 blocking layer (cTiO2 for short) was deposited on the patterned glasses which were ultrasonically cleaned and oxygen plasma treatment by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol (2000 rpm, 30s), and annealed at 500  C for 30 min. The appropriate amount of MZnO powder was ultrasonically dispersed in ethanol, and then spin-coated the MZnO suspension solution on cTiO2/FTO substrate. After the solvent was volatilized, the perovskite film was one-step spin coated with FA0.83Cs0.17PbI2.5Br0.5 precursor solution on 70  C preheated substrate at 3000 rpm for 30s, followed by annealing at 70  C for 2 min and 130  C for 5 min. The perovskite precursor solution was prepared by dissolving stoichiometric FAI, CsBr, PbBr2 and PbI2 in DMSO-DMF solution (dimethylsulfoxide and N,N-dimethylformamide,v:v ¼ 1:4) and then filtered by syringe filter (0.45 mm, JinTeng). Subsequently, the hole transport layer of spiro-MeOTAD solution (68 mM spiroMeOTAD, 55 mM tert-butylpyridine (TBP) and 9 mM lithium bis (trifluoromethanesulfonyl) imide (Li-TFSI) in 1 mL chlorobenzene) was deposited by spin-coating at 4000 rpm for 30 s. Finally, the samples were placed in the dark overnight and Ag counter

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electrodes were thermal evaporated to complete the fabrication of PSCs. The control group when preparing the PSCs device used the purchased nanoparticle zinc oxide (abbreviated as NPZnO) as the electron transport layer. Characterization X-ray diffraction (XRD) spectra were measured by Philips Rigaku D/Max-kA diffractometer. The surface/ cross-section morphologies and microstructures were characterized using field-emission scanning electron microscopy (FESEM, Hitachi SU-70) and high-resolution transmission electron microscopy (TEM, Tecnai 20U-Twin). The surface area and porosity distributions were performed by Brunauer-Emmett-Teller (BET) method and Barrett Joyner Halenda (BJH) method. The timeresolved photoluminescence (TRPL) and steady-state photoluminescence (SSPL) tests were carried out using all functional fluorescence spectrometer (Edinburgh, FLS920) with an excitation wavelength of 450 nm. The ultravioletevisible absorption spectra were obtained on a TU-1901 spectrophotometer. Photocurrentpotential (J-V) performances were analyzed by a solar simulator (Newport, Class 3 A, 94023 A) and data collected with a Keithley 2440 Source meter with Scan velocity ¼ 100 mV/s under AM1.5G illumination (100 mW cm
2). The PSCs measured using a metal aperture to define the active area about 0.09 cm2. Electron-only devices (FTO/cTiO2/MZnO/perovskite/Ag) were fabricated to calculate the electron trap-state density of the devices. The dark J-V characteristics of the electron-only devices were measured by a Keithley 2400 source. Electrochemical impedance spectroscopy (EIS) was performed under 1 sun light with a frequency from 1 MHz to 0.1 Hz, amplitude of 5 mV (Princeton Parstate2273 A) and fitted with the software Zview. The External quantum efficiency (EQE) was conducted by a monochromator (Newport Cornerstone 260) equipped with a power source (Newport 300 W xenon lamp, 66920) and a multimeter (Keithley 2400). Acknowledgements This work was supported by the National Nature Science Foundation of China (No.: 51702228, 51872171), Shandong Province Natural Science Foundation (ZR2017BEM014, ZR201801290005), Tai’an Science and Technology Development Plan (2018GX0075), the Talent Introduction Project of Taishan University (Y-012018017) and Shandong Province Higher Educational Science and Technology Program (J17KA023). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135280. References [1] T. Zhou, M. Wang, Z. Zang, L. Fang, Stable dynamics performance and high efficiency of ABX3 -type super-alkali perovskites first obtained by introducing H5O2 cation, Adv. Energy Mater. (2019) 1900664. [2] T. Miyasaka, Perovskite photovoltaics: rare functions of organo lead halide in solar cells and optoelectronic devices, Chem. Lett. 44 (2015) 720e729. [3] H.J. Snaith, Perovskites: the emergence of a new era for low-cost, high-efficiency solar cells, J. Phys. Chem. Lett. 4 (2013) 3623e3630. [4] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643e647. [5] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S.I. Seok, Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells, Nat. Mater. 13 (2014) 897e903. [6] K. Mahmood, B.S. Swain, A. Amassian, Double-layered ZnO nanostructures for efficient perovskite solar cells, Nanoscale 6 (2014) 14674e14678. [7] Q. Zhang, C.S. Dandeneau, X. Zhou, G. Cao, ZnO Nanostructures for dyesensitized solar cells, Adv. Mater. 21 (2009) 4087e4108. [8] Z.L. Wang, Zinc oxide nanostructures: growth, properties and applications, J. Phys. Condens. Matter 16 (2004) R829eR858.

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280

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Y.-N. Zhang et al. / Electrochimica Acta xxx (xxxx) xxx

[9] H. Liu, Z. Huang, S. Wei, L. Zheng, L. Xiao, Q. Gong, Nano-structured electron transporting materials for perovskite solar cells, Nanoscale 8 (2016) 6209e6221. [10] P. Zhang, J. Wu, T. Zhang, Y. Wang, D. Liu, H. Chen, L. Ji, C. Liu, W. Ahmad, Z.D. Chen, S. Li, Perovskite solar cells with ZnO electron-transporting materials, Adv. Mater. 30 (2018) 1703737. [11] U. Ryu, S. Jee, J.S. Park, I.K. Han, J.H. Lee, M. Park, K.M. Choi, Nanocrystalline titanium metal-organic frameworks for highly efficient and flexible perovskite solar cells, ACS Nano 12 (2018) 4968e4975. [12] D.Y. Liu, T.L. Kelly, Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques, Nat. Photonics 8 (2014) 133e138. [13] J. Song, W. Hu, X.-F. Wang, G. Chen, W. Tian, T. Miyasaka, HC(NH2)2PbI3 as a thermally stable absorber for efficient ZnO-based perovskite solar cells, J. Mater. Chem. 4 (2016) 8435. [14] R.T. Ginting, E.-S. Jung, M.-K. Jeon, W.-Y. Jin, M. Song, J.-W. Kang, Low-temperature operation of perovskite solar cells: with efficiency improvement and hysteresis-less, Nano Energy 27 (2016) 569e576. [15] J. Yang, B.D. Siempelkamp, E. Mosconi, F.D. Angelis, T.L. Kelly, Origin of the thermal instability in CH3NH3PbI3 thin films deposited on ZnO, Chem. Mater. 27 (2015) 4229e4236. [16] Y. Cheng, Q.D. Yang, J. Xiao, Q. Xue, H.-W. Li, Z. Guan, H.L. Yip, S.W. Tsang, Decomposition of organometal halide perovskite films on zinc oxide nanoparticles, ACS Appl. Mater. Interfaces 7 (2015) 19986e19993. [17] P. Zhang, J. Wu, Y. Wang, H. Sarvari, D. Liu, Z.D. Chen, S. Li, Enhanced efficiency and environmental stability of planar perovskite solar cells by suppressing photocatalytic decomposition, J. Mater. Chem. 5 (2017) 17368e17378. [18] H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. Fabregat-Santiago, E.J. JuarezPerez, N.-G. Park, J. Bisquert, Mechanism of carrier accumulation in perovskite thin-absorber solar cells, Nat. Commun. 4 (2013) 2242. [19] Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Inverted polymer solar cells integrated with a low-temperature-annealed sol-gel-derived ZnO Film as an electron transport layer, Adv. Mater. 23 (2011) 1679e1683. [20] M. Ahmad, E. Ahmed, Y. Zhang, N. Khalid, J. Xu, M. Ullah, Z. Hong, Preparation of highly efficient Al-doped ZnO photocatalyst by combustion synthesis, Curr. Appl. Phys. 13 (2013) 697e704. [21] R. Zhang, C. Fei, B. Li, H. Fu, J. Tian, G. Cao, Continuous size tuning of monodispersed ZnO nanoparticles and its size effect on the performance of perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 9785e9794. [22] J.-F. Tang, Z.-L. Tseng, L.-C. Chen, S.-Y. Chu, ZnO nanowalls grown at lowtemperature for electron collection in high-efficiency perovskite solar cells, Sol. Energy Mater. Sol. Cells 154 (2016) 18e22. [23] U. Ryu, S. Jee, J. Park, I.K. Han, J.H. Lee, M. Park, K.M. Choi, Nanocrystalline titanium metal-organic frameworks for highly efficient and flexible perovskite solar cells, ACS Nano 12 (2018) 4968e4975. [24] T.-H. Chang, C.-W. Kung, H.-W. Chen, T.-Y. Huang, S.-Y. Kao, H.-C. Lu, M.H. Lee, K.M. Boopathi, C.-W. Chu, K.-C. Ho, Planar heterojunction perovskite solar cells incorporating metal-organic framework nanocrystals, Adv. Mater. 27 (2015) 7229e7235. [25] S.D. Sung, D.P. Ojha, J.S. You, J. Lee, J. Kim, W.I. Lee, 50 nm sized spherical TiO2 nanocrystals for highly efficient mesoscopic perovskite solar cells, Nanoscale 7 (2015) 8898e8906. [26] B.C. Moon, J.H. Park, D.K. Lee, N. Tsvetkov, I. Ock, K.M. Choi, J.K. Kang, Broadband light absorption and efficient charge separation using a light scattering layer with mixed cavities for high-performance perovskite photovoltaic cells with stability, Small 13 (2017) 1700418. [27] W.-Q. Wu, F. Huang, D. Chen, Y.-B. Cheng, R.A. Caruso, Thin Films of Dendritic anatase titania nanowires enable effective hole-blocking and efficient lightharvesting for high-performance mesoscopic perovskite solar cells, Adv. Funct. Mater. 25 (2015) 3264e3272. [28] S. Jang, J. Yoon, K. Ha, M. Ki, D. Kim, S.M. Kim, S.M. Kang, S.J. Park, H.S. Jung, M. Choi, Facile fabrication of three-dimensional TiO2 structures for highly

efficient perovskite solar cells, Nano Energy 22 (2016) 499e506. [29] I.S. Yang, J.S. You, S.D. Sung, C.W. Chung, J. Kim, W.I. Leen, Novel spherical TiO2 aggregates with diameter of 100 nm for efficient mesoscopic perovskite solar cells, Nano Energy 20 (2016) 272e282. [30] Z. Li, C. Li, X. Ge, J. Ma, Z. Zhang, Q. Li, C. Wang, L. Yin, Reduced graphene oxide wrapped MOFs-derived cobalt-doped porous carbon polyhedrons as sulfur immobilizers as cathodes for high performance lithium sulfur batteries, Nano Energy 23 (2016) 15e26. [31] Q. Li, L. Yin, Z. Li, X. Wang, Y. Qi, J. Ma, Copper doped hollow structured manganese oxide mesocrystals with controlled phase structure and morphology as anode materials for lithium ion battery with improved electrochemical performance, ACS Appl. Mater. Interfaces 5 (2013) 10975e10984. [32] S.M. Kang, S. Jang, J. Lee, J. Yoon, D. Yoo, J. Lee, M. Choi, N. Park, Moth-eye TiO2 layer for improving light harvesting efficiency in perovskite solar cells, Small (2016) 201600428. [33] J.B. You, L. Meng, T.B. Song, T.F. Guo, Y. Yang, W.H. Chang, Z.R. Hong, H.J. Chen, H.P. Zhou, Q. Chen, Y.S. Liu, N.D. Marco, Y. Yang, Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers, Nat. Nanotechnol. 11 (2016) 75e81. [34] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells, Nat. Commun. 5 (2014) 5784. [35] P.W. Liang, C.C. Chueh, S.T. Williams, A.K.Y. Jen, Roles of fullerene-based interlayers in enhancing the performance of organometal perovskite thin-film solar cells, Adv. Energy Mater. 5 (2015) 1402321. [36] J. Cao, Y.M. Liu, X.J. Jing, J. Yin, J. Li, B. Xu, Y.Z. Tan, N.F. Zheng, Well-defined thiolated nanographene as hole-transporting material for efficient and stable perovskite solar cells, J. Am. Chem. Soc. 137 (2015) 10914e10917. [37] V. Gonzalez-Pedro, E.J. Juarez-Perez, W.-S. Arsyad, E.M. Barea, F. FabregatSantiago, I. Mora-Sero, J. Bisquert, General working principles of CH3NH3PbX3 perovskite solar cells, Nano Lett. 14 (2014) 888e893. [38] W.A. Laban, L. Etgar, Depleted hole conductor-free lead halide iodide heterojunction solar cells, Energy Environ. Sci. 6 (2013) 3249e3253. [39] P. Yadav, M.I. Dar, N. Arora, E.A. Alharbi, F. Giordano, S.M. Zakeeruddin, €tzel, The role of rubidium in multiple-cation- based high-efficiency M. Gra perovskite solar cells, Adv. Mater. 29 (2017) 1701077. [40] G. Yang, C. Wang, H. Lei, X. Zheng, P. Qin, L. Xiong, X. Zhao, Y. Yan, G. Fang, Interface engineering in planar perovskite solar cells: energy level alignment, perovskite morphology control and high performance achievement, J. Mater. Chem. A. 5 (2017) 1658e1666. [41] T. Niu, J. Lu, R. Munir, J. Li, D. Barrit, X. Zhang, H. Hu, Z. Yang, A. Amassian, K. Zhao, S. Liu, Stable high-performance perovskite solar cells via grain boundary passivation, Adv. Mater. (2018) 1706576. [42] R.H. Bube, Trap density determination by space-charge-limited currents, J. Appl. Phys. 33 (1962) 1733. [43] D. Yang, X. Zhou, R. Yang, Z. Yang, W. Yu, X. Wang, C. Li, S. Liu, R.P.H. Chang, Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells, Energy Environ. Sci. 9 (2016) 3071e3078. [44] G.E. Eperon, V.M. Burlakov, P. Docampo, A. Goriely, H.J. Snaith, Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells, Adv. Funct. Mater. 24 (2014) 151e157. [45] H.S. Kim, J.W. Lee, N. Yantara, P.P. Boix, S.A. Kulkarni, S. Mhaisalkar, M. Gratzel, N.G. Park, High efficiency solid-state sensitized solar cell-based on submicrometer rutile TiO2 Nanorod and CH3NH3PbI3 perovskite sensitizer, Nano Lett. 13 (2013) 2412e2417. [46] J. Song, L. Liu, X. Wang, G. Chen, W. Tian, T. Miyasaka, Highly efficient and stable low-temperature processed ZnO solar cells with triple cation perovskite absorber, J. Mater. Chem. 5 (2017) 13439e13447. [47] Q. An, P. Fassl, Y.J. Hofstetter, D. Becker-Koch, A. Bausch, P.E. Hopkinson, Y. Vaynzof, High performance planar perovskite solar cells by ZnO electron transport layer engineering, Nano Energy 39 (2017) 400e408.

Please cite this article as: Y.-N. Zhang et al., MOF-derived ZnO as electron transport layer for improving light harvesting and electron extraction efficiency in perovskite solar cells, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135280