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A Design Based on Stair-case Band Alignment of Electron Transport Layer for Improving Performance and Stability in Planar Perovskite Solar Cells

Among the n-type metal oxide materials used in the planar perovskite solar cells, zinc oxide (ZnO) is a promising candidate to replace titanium dioxide (TiO2) due to its relatively high electron mobility, high transparency, and versatile nanostructures. Here, we present the application of low temperature solution processed ZnO/Al-doped ZnO (AZO) bilayer thin film as electron transport layers (ETLs) in the inverted perovskite solar cells, which provide a stair-case band profile. Experimental results revealed that the power conversion efficiency (PCE) of perovskite solar cells were significantly increased from 12.25 to 16.07% by employing the AZO thin film as the buffer layer. Meanwhile, the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) were improved to 20.58 mA/cm2, 1.09V, and 71.6%, respectively. The enhancement in performance is attributed to the modified interface in ETL with stair-case band alignment of ZnO/AZO/CH3NH3PbI3, which allows more efficient extraction of photogenerated electrons in the CH3NH3PbI3 active layer. Thus, it is demonstrated that the ZnO/AZO bilayer ETLs would benefit the electron extraction and contribute in enhancing the performance of perovskite solar cells.


Introduction
Organometal halide perovskite solar cells (e.g., CH 3 NH 3 PbX 3 , X = Cl, Br, I) have drawn much attention in current renewable solar research owing to their excellent optical and electronic properties, including strong absorption band that span the visible region, 1 direct band gap (∼ 1.5 eV), 2 long carrier diffusion length (100 -1000 nm), 3 high charge-carrier mobility (∼10 cm 2 V −1 s −1 ), 4 and significantly low-cost fabrication process. 5,6 These superior optoelectronic properties enable an increase in power conversion efficiencies (PCEs) of planar heterojunction (PHJ) perovskite devices from 3.8% in 2009 7 to 22.1% in 2016. 8 Due to the potential of organometal halide perovskites, there have been a great number of studies regarding the carrier dynamics of perovskites, 9 development of new materials 10,11 , optimization of perovskite absorber layer thickness, crystallinity and surface coverage, [12][13][14] and low-temperature processing 15,16 over the past few years. The state-of-art perovskite solar cells are based on two different device architectures: mesoporous 17 and planar heterojunctions. [18][19][20] For the n-type metal oxide materials in the cell, titanium dioxide (TiO 2 ) is commonly employed in the mesoporous scaffold. Although TiO 2 -based perovskite solar cells may pave the way to high PCE devices, several disadvantages of TiO 2 were reported such as low electron mobility and high annealing temperature (above 450 • C) in order to form crys-talline TiO 2 film in anatase phase. 21 Accordingly, among other n-type metal oxide materials used in inverted solar cells, zinc oxide (ZnO) has been extensively studied as a substitute to TiO 2 due to its similar electron affinity, relatively high electron mobility, high transparency, and versatile nanostructures. 22,23 For high performance inverted perovskite solar cell, the selection of the electron collection layer with hole blocking capability and low resistivity pathway for efficient electron extraction is necessary. ZnO have been demonstrated to be an effective electron collection material due to its various nanostructures that can be easily achieved by solution process for more efficient charge extraction and transport. 24,25 Recently, several studies of mesostructured perovskite solar cell based on ZnO nanorod arrays have been used to replace the mesoporous TiO 2 nanostructures in the conventional perovskite solar cells. 26,27 For instance, Mahmood et al. have obtained remarkably PCE of 16.1% in ZnO mesoscopic perovskite solar cells based on synergistically combining mesoscale control with nitrogen doping and surface modification of ZnO nanorod arrays. 28 However, it was found that severe decomposition of CH 3 NH 3 PbI 3 could occur at the ZnO/perovskite interface due to excessive OH − groups and chemical residuals on the ZnO surface. 29 To overcome the unstable decomposition process of CH 3 NH 3 PbI 3 to PbI 2 which tends to occur at the ZnO/perovskite interface, several approaches have been developed to solve decomposition issue: (i) high temperature annealing of ZnO layer, 30 (ii) employing polymer interlayers between ZnO/perovskite to avoid the direct interaction, 31,32 and (iii) the use of aluminum (Al) as a dopant in ZnO to passivate the OH − group. 33,34 Previously, Tseng et al. have achieved a PCE of 17.6%, which was considered to be the highest for ZnO-based perovskite solar cell by using the sputtered Al-doped ZnO (AZO) thin film as electron transport layer (ETL). 34 In the above research, it is worth noting that the use of AZO as ETL could synergistically produce higher efficiency and stability for perovskite solar cells. However, to fabricate better thin film quality of AZO ETLs with high conductivity and crystallinity, preparation by physical vapor deposition (PVD) system with the use of vacuum chamber is required which could lead to an increase in production cost and hinder the scale-up technologies in roll-to-roll fabrication of large area solar cells. Therefore, the development of solution-processed AZO ETLs by sol-gel technique is desired for it presents several advantages, including facile synthesis, solution-based growth, long-term stability, and low temperature processability that are suitable for flexible photovoltaics.
Inspired by these concepts based on effective electron extraction offered by ZnO ETL 35 and great passivation ability of AZO, 34 we adopted a synergistic strategy of using lowtemperature solution-processed ZnO/AZO bilayer as an ETL which can offer better electron extraction and transportation due to the stair-case band alignment in the ZnO/AZO/perovskite interface. In addition, the ZnO/AZO bilayer configuration can improve the interfacial contact to active layer of perovskite and suppress the charge recombination, ultimately resulting in enhancement in PCE, J sc , and V oc of the device. Detailed investigations on the prominent effect of ZnO/AZO bilayer as an ETL for the inverted PHJ perovskite solar cells were presented. The V oc and J sc values of the devices containing ZnO/AZO bilayer were enhanced compared to those of devices containing only ZnO. Moreover, the transport properties of the ZnO and interfacial AZO layer were systematically investigated by using ultraviolet photoelectron spectroscopy (UPS) and absorption measurements. Better charge transfer was found between ZnO/AZO bilayer and perovskite which leads to higher V oc and J sc in the device. A maximum PCE of 16.07% was obtained for inverted planar perovskite solar cells comprising ZnO/AZO bilayer ETL that is 24% higher than those containing only ZnO ETL.

Syntheses of materials
Methylammonium iodide (MAI) preparation aqueous HI (57 wt% in water), methylamine (CH 3 NH 2 , 40 wt% in aqueous solution), PbI 2 (99.998%), dimethyl sulfoxide (DMSO), and diethyl ether were purchased from Alfa Aesar and used without further purification. CH 3 NH 3 I was synthesized by the method described in literature. 20 Briefly, CH 3 NH 3 I was synthesized by reacting aqueous HI with CH 3 NH 2 at 0 • C for 2 h in three-neck flask under a N 2 atmosphere with constant stirring. A white precipitate (CH 3 NH 3 I) formed during rotary evaporation of the solvent. The precipitated white powder was collected, washed three times with diethyl ether, and then dried under vacuum at 60 • C overnight. This dried CH 3 NH 3 I white powder was eventually stored in a glove box for further fabrication. For the ZnO sol-gel solution which was synthesized according to a modified procedure reported in literatures. 36 Typically, zinc acetate dihydrate, aluminum chloride hexahydrate, 2-methoxethanol, and monoethanolamine (MEA) were used as the starting materials, solvent, and stabilizer, respectively. Zinc acetate dihydrate were first dissolved in a mixture of 2-methoxethanol. The molar ratio of MEA to zinc acetate dihydrate was maintained at 1.0 and the concentration of

Device fabrication and characterization
Substrates of the cells are fluorine-doped tin oxide (FTO) conducting glass (Ruilong; thickness 2.2 mm, sheet resistance 14 Ω/square). Before use, the FTO glass was first washed with mild detergent, rinsed with distilled water several times and subsequently with ethanol in an ultrasonic bath, finally dried under air stream. The ZnO/AZO bilayer thin films were sequentially deposited on the FTO glass substrate via spin coating method. The coating solution was dropped onto the FTO substrate which was rotated at 3000 rpm for 30s, followed by thermal annealing at 200 • C for 30 min on a hot plate to evaporate the solvent and remove organic residuals. Next, the substrates were transferred to a glove box for further deposition of the perovskite active layer through the two-step spin-coating method. PbI 2 (48 wt%)+KCl (1 wt%) additive were dissolved in 1ml dimethyl sulfoxide; CH 3 NH 3 I (4.25 wt%) was dissolved in 1 ml 2-propanol solvent. Both solutions were kept on a hot plate at 70 • C overnight. A hot PbI 2 solution was spin-coated onto ZnO thin film and annealed directly (70 • C, 10 min). The hot CH 3 NH 3 I solution was then spin-coated onto the PbI 2 film; the structure was kept on the hot plate at 100 • C for 5 min to form a crystalline perovskite film.
The hole-transporting spiro-OMeTAD material (5 wt% External quantum efficiency (EQE) spectra were measured under monochromatic illumination (Enlitech, QE-R3011). Devices were encapsulated before they were removed for EQE measurement. The micro-photoluminescence spectra (µ-PL, Horiba Jobin Yvon HR-800) of the ZnO, ZnO/AZO bilayer and perovskite thin films were obtained by using a 325 nm He-Cd CW laser as the excitation source with a 2400 grooves/mm grating in the backscatter-ing geometry. Time-resolved PL measurements were carried out by a time-correlated single photon counting (TCSPC) system and samples were photoexcited by using a 405 nm pulse laser source. All of the measurements were carried out at room temperature (RT). To efficiently extract electrons from CH 3 NH 3 PbI 3 active layer, a 10-nm thick solgel processed AZO layer was grown at the ZnO/perovskite interface. Since AZO has a higher conduction band compared to that of ZnO, with high electron density and faster electron mobility, the ZnO/perovskite recombination would be restrained. The corresponding cross sectional image of FTO/ZnO/AZO/perovskite device was shown in Fig. 1(b). The ZnO/AZO thin layer was deposited on the FTO substrate by spin-coating method. Then, the CH 3 NH 3 PbI 3 or perovskite active layer was deposited onto the ZnO/AZO bilayer using two-step spin-coating method. A uniform distribution of perovskite film was formed on the top of the ZnO/AZO bilayer. The PHJ perovskite devices were finally fabricated using spiro-OMeTAD as a hole transporting layer (HTL), MoO 3 and silver as top electrode. The thicknesses of ZnO and AZO layers were varied to optimize the peformance of solar cells (See supporting information, Figure S1). The best thicknesses of ZnO and AZO layers found are are shown in Fig. 2(a). The VBM energy levels (relative to vacuum level) are calculated by using eq. 1. 37

Results and discussion
where hν = 21.22 eV is the incident photon energy and the high binding energy cut off (E cutof f ) spectra of ZnO and AZO obtained from Fig. 2 where n = 1/2 for indirect transition, n = 2 for direct transition; A is proportional constant, α is the absorption coefficient, h is Planck's constant, ν is frequency of incident photon. Here the E g was determined by using (αhν) 2 = A(hν − E g ) since ZnO and AZO are both direct transition in nature. As shown in Figure S3, where V (z), n(z), N d (z), and /ε(z) denote the electrostatic potential energy, electron carrier density, dopant concentration, and dielectric constant at position z, respectively. In Fig. 3(a), the band profile of ZnO has a weak band bending, since the built-in electric field is partially screened by the free carriers in ZnO, which is slightly n-type. An extra electron trapping potential can be created (as indicated by the dotted line) by the electron-hole attraction after photo-excitation. These trapped electrons in the triangular well can hinder the charge separation process and part of them will recombine with holes in CH 3 NH 3 PbI 3 .
In Fig. 3(b), a thin AZO layer (which has higher free electron density than ZnO due to Al doping) is inserted between ZnO and CH 3 NH 3 PbI 3 . Since the conduction band minimum The surface morphologies of the pure ZnO ETL, the ZnO/AZO bilayer ETL, and the active layer of CH 3 NH 3 PbI 3 , respectively, are shown in Fig. 4. As observed in Fig. 4(a) Fig. S5.
of the device based on ZnO/AZO bilayer ETLs is due to the higher light absorption within the range from 300 to 400 nm [See Fig. 6(c)] and the better charge collection efficiency.
The discrepancy (∼ 5%) between the integrated J sc from EQE and J sc obtained from the J-V curve is generally observed in the perovskite cell due to the difference in measurement methods. In order to investigate the origin of enhanced J sc , PL measurements were carried out to study the capability for the electron transfer from perovskite to ETLs. As shown in  Fig. 6 (d). The fast decay (τ 1 ) process was considered to be the result of the free carriers in the perovskite through transport to ETL or HTL, whereas the slow decay (τ 2 ) process to be the result of radia-tive decay. 33 The τ 1 of ZnO/AZO/perovskite/spiro-OMeTAD sample decreased to 1.52 ns, which was much smaller than that of the ZnO/perovskite/spiro-OMeTAD (2.14 ns). This indicated that charger transfer between perovskite and ZnO/AZO bilayer is faster than that without AZO interlayer. The results of PL quenching and lifetime are consistent with the more efficient charge separation due to the stair-case band alignment of our device structure with ZnO/AZO bilayer ETL. It is thus evident that the electrical properties are strongly influenced by employing AZO as the interlayer.
Electrochemical impedance measurements were carried out for perovskite solar cells con-  Figure S6). The lower ZnO thin film which can be explained by the Al-doping. 33 The higher binding energy of second peak (532.97 eV) corresponds to the OH − or O − 2 ions in the AZO shifts toward higher binding energy and has lower intensity than in ZnO thin film. The above result indicates that AZO thin film is more oxygen deficient. Thereby the OH − group passivation enhances the thermal stability and durability of perovskite solar cells containing ZnO/AZO ETL.

Conclusion
In summary, we have successfully demonstrated how the low temperature solution processed ZnO/AZO bilayer ETL improves the efficiency of PHJ perovskite solar cell. In this study, the PCE obtained can exceed 16%. The introduction of AZO layer not only helps the formation of uniform surface morphology to fill the defects but also lead to a stair-case band alignment in the ZnO/AZO/perovskite heterostruucture, which is shown to yield better electron extraction, thereby significantly enhance the device performance. It is noteworthy that the results obtained for our optimized device surpass the state-of-the-art PCE and V oc of solution processed ZnO-based perovskite solar cells. Moreover, the device prepared with ZnO/AZO bilayer ETLs showed long-term stability; only 9% degradation of its initial PCE after being kept in the dark for over 45 days. This approach pave a new way for the development of low-cost perovskite solar cells.

Supporting Information Available
The following files are available free of charge. J-V curves at different molar concentrations of ZnO and ZnO/AZO ETLs, Tauc plots for optical bandgap extraction, UPS spectra of 3% AZO and 5% AZO films and corresponding energy-level diagram, J-V curves at different concentrations of AZO interlayers, and Histograms of device performance parameters.

Author Information
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