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Physicochemical Interface Engineering of CuI/Cu as an Advanced Potential Hole Transporting Materials/Metal Contact Couples In Hysteresis-free Ultra Low-cost and Large-area Perovskite Solar Cells Pariya Nazari, Fatemeh Ansari, Bahram Abdollahi Nejand, Vahid Ahmadi, Masoud Payandeh, and Masoud Salavati-Niasari J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07061 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Physicochemical Interface Engineering of CuI/Cu as an Advanced Potential Hole Transporting Materials/Metal Contact Couples In Hysteresis-free Ultra Low-cost and Largearea Perovskite Solar Cells

Pariya Nazari†,Ψ, Fatemeh Ansari§,Ψ, Bahram Abdollahi Nejand†,‡,* , Vahid Ahmadi*,†, Masoud Payandeh†, Masoud Salavati-Niasari§

†

School of Electrical and Computer Engineering, Tarbiat Modares University, Tehran-Iran

§ Institute

of Nano Science and Nano Technology, University of Kashan, Kashan, Iran

‡Nanomaterial

Research Group, Academic Center for Education, Culture and Research (ACECR) on TMU, Tehran, Iran, Ψ These authors have a same contribution in this work

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ACS Paragon Plus Environment


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Abstract Low-cost fabrication of perovskite solar cells while fusing large-area deposition potential along with high power conversion efficiency and durability, paves the way for scaling up the perovskite solar cells. In this work, a novel approach in the simultaneous deposition of hole transporting and contact layers is introduced. The interface engineering is conducted in order to grow CuI as a hole transporting material at the interface of Cu contact and perovskite layer through which a great reproducibility and durability are achieved. The prepared devices shows 9.24% and 8.3% power conversion efficiencies for 0.1cm2 and 1 cm2 active areas, respectively.

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1. Introduction

Regarding to the overwhelming urgency to reach green energy along with the necessity of using earth-abundant materials in fabrication of low-cost solar cells, substantial heed has been paid to organometal halide perovskite materials as enthralling photovoltaic materials with an approximate direct optical band gap of 1.5 eV. This potential light absorber has numerous advantages such as broad range of light absorption including the visible to near infrared spectra with high extinction coefficient (1.5×104 cm−1 at 550 nm) and long diffusion length around 1 μm 1-3 . Despite the reported 21.1% efficiency4 5 for common perovskite solar cells, the 14.7% power conversion efficiency was reported for all inorganic based electron and hole transporting materials in perovskite solar cells using carbon counters6. Foremost, material selection and engineering electron transport material (ETM) and hole transport material (HTM) in order to extract electron and hole, respectively, from organometal trihalide perovskite absorber have a tremendous impact on perovskite solar cells efficiency. Although numerous intriguing researches have been conducted on improving the perovskite solar cells performance and resolving some issues such as durability against moisture, other serious problems remain thus far. Case in point, the production cost of these devices is high due to the use of some expensive compounds in normal and inverted architectures. Among these materials are 2,29,7,79-tetrakis(N,N-di-p-methoxy phenylamino)-9,99- spiro bifluorene (SpiroOMeTAD)

7-9,

(P3HT),

Poly[bis(4-phenyl)

poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) (2,4,6-trimethylphenyl)-amine]

10-12,

Poly(3-hexylthiophene)

(PTAA),

phthalocyanine-3,4’,4’’,4’’’-tetra-sulfonated acid tetra sodium salt (TS-CuPc)

and 13,

Copper

and Perylene

(C20H12) 14 as HTM, [6,6]-phenyl- C61-butyric acid methyl ester (PCBM)10, 15-16 as ETM, as well as noble metals (Au, Ag, Pt) as the counter electrodes. Materials such as carbon 17-18, NiO 16, 19-20, CuI 21,

Cu2O22, CuSCN

23,

and GeO2 24-25 were found as potential candidates for replacing expensive

HTMs. Earth-abundant Cu based hole transport materials such as CuI 26, Cu2O27, and CuSCN 28-30 drawn considerable attention in the fabrication of perovskite solar cells because of their immense 3



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hole mobility and low-cost fabrication methods and materials. To date, as a record, the fabricated solar cell using CuSCN as an inorganic HTM showed 15.6% efficiency

30

, however, in these

structures organic materials such as PCBM were used as electron transport materials to complete the device. Hence, engineering all inorganic HTMs and ETMs in the fabrication of perovskite solar cells would set the stage for durable, robust, low-cost, and stable perovskite solar cells. CuSCN and CuI mostly show higher current density because of higher conductivity of CuI and CuSCN than small molecule HTM such as spiro-OMeTAD and high-molecular weight HTMs such as PTTA 31. Cuprous iodide is a low-cost Cu-based inorganic hole transport material showing higher hole mobility of 9.3 cm2 V-1 s-1

32,33

in comparison to organic hole transport materials such as spiro-

OMeTAD (5.3×10-5 cm2 V-1 s-1 34). Besides, compared to spiro-OMeTAD, CuI is an inorganic hole transport material which is more robust and stable in the moistures atmosphere. In the most recent works in the robust perovskite solar cells including inverted and normal architectures, CuI showed a strong potential in the fabrication of perovskite solar cells, presenting superb current density and stability21, 23, 26. Different methods were reported in the deposition of CuI layer on the substrate which mostly are conducted by chemical deposition approaches from dissolved CuI in acetonitrile or di-npropyl sulfide-chlorobenzene mixture21,

23, 26, 35,

thermal evaporation36-37, and hydrothermal

methods38. In our previous works, as a new approach, we used iodinazation method in reaching compact and high-quality CuI thin films as a potential inorganic HTM in the fabrication of all inorganic durable perovskite solar cells39-40. In this work, as a novel approach, we fabricate a large-area ultra low-cost and hysteresis-free perovskite solar cell using CuI/Cu as HTM and counter electrode. As Cu has the same work function as ITO, it could potentially collect the extracted holes from CuI layer. In this regard, by direct deposition of Cu onto perovskite layer containing extra methylammonium iodide (MAI) followed by slight annealing, cuprous iodide is derived from Cu counter at the interface of the perovskite and Cu layer by the interaction of excess iodine in MAI and Cu atoms. This uniform and well crystalline CuI 4


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layer demonstrates a strong hole extraction behavior and wide band gap to prevent the charge recombination at the interface of the CuI and perovskite layer even in the case of direct contact with ETM layer. Thereupon, the introduced almost all inorganic based perovskite solar cell in this effort proposes an efficient way to fabricate perovskite solar cell which may hit the future of low-cost and large-area perovskite solar cells market.

2. Experimental Section

2.1 Device fabrication FTO-coated glass substrates were patterned by Zn powder and 2M HCl etching solution. The patterned FTO substrates were first rinsed with detergent and deionized water, then cleaned in an ultrasonic bath with acetone, ethanol, and deionized water for about 15 min. Thereafter it was dried with clean dry air. A hole blocking layer of TiO2 was deposited by spin-coating of an acidic solution of titanium isopropoxide in anhydrous ethanol (2000 rpm for 30s) and then annealed at 500 ℃ for 30 minutes. The substrate was immersed in 40 mM TiCl4 (Sigma-Aldrich) aqueous solution for 30 minutes at 70 ℃ and washed with distilled water. Furthermore, it was annealed at 500 ℃ for 30 min to obtain a uniform compact layer for preventing short-circuits between the FTO and the final gold electrode. The mesoporous TiO2 layer was spin coated (4500 rpm for 30s) by a solution including TiO2 paste diluted in anhydrous ethanol with contributions of 2:7 weighting, respectively. The sample was annealed at 500 ℃ for 30 minutes after drying on a hot plate. The final mesoporous layers were treated in 40 mM TiCl4 again and annealed at 500 ℃ for 30 minutes. Deposition of perovskite layer and CuI/Cu couple described in details as following.

2.2 Synthesis of CH3NH3I The CH3NH3I was synthesized by reacting 24 mL of CH3NH2 and 10 mL of HI in a 250 mL roundbottom flask at 0℃ for 2 h with stirring. The precipitate was collected using a rotary evaporator 5



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through the careful removal of the solvent at 50℃. The as-obtained product was re-dissolved in 100 mL absolute ethanol and precipitated with addition of 300 mL diethyl ether, and this procedure was repeated thrice. The final CH3NH3I was collected and dried at 60℃ in a vacuum oven for 24 h.

2.3 Perovskite deposition method The perovskite layer was prepared by two-step spin-coating method9 in which 600 mg PbI2 solution (1.3 M) was dissolved in 1 ml DMF under stirring at 70 ℃ for overnight.The hot retained PbI2 solution at 110℃ was spin coated for 20 seconds at 6000 rpm. After spinning, the film was dried at 40 ℃ for 3 min and then at 100 ℃ for 5 min. After cooling the layers to room temperature, 150 μl of CH3NH3I (50 mg/mL) in 2-propanol were spin coated on the PbI2 coated substrates. The perovskite layers were annealed at 130℃ for 20 min.

2.4 Deposition of CuI thin film The final one-step deposition of CuI/Cu conducted by easy thermal evaporation of 120 nm Cu on the perovskite layer containing an excess amount of MAI on the top of the surface through a 0.1 cm2 and 1 cm2 shadow masks (DTT, Nanostructure coater, Iran). To reach a PSK/Cu, no thermal annealing was performed while to reach a PSK/CuI/Cu structure, the thermal evaporated Cu on the PSK/MAI was annealed at 100℃ for 10 minutes in nitrogen atmosphere.

2.5 Prepared device by Spiro-OMeTAD/Au Right after cooling down the annealed perovskite layers to room temperature, the spiro-OMeTADbased hole transporting layer (72 mg spiro-OMeTAD, 17.5 μl lithium bis(trifluoromethane sulfonyl)imide (Li-TFSI) solution (520 mg Li-TFSI in 1 ml acetonitrile) and 26.6 mg 4-tertbutylpyridine all dissolved in 1 ml chlorobenzene) were deposited by spin-coating at 2000 rpm for 30 sec. By keeping the deposited spiro-OMeTAD in a desiccator for 12 hours, the thin 100 nm Au 6


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contact was deposited on the spiro-OMeTAD by thin stainless steel shadow mask to create a 0.1 cm2 active area.

2.6 Thin film and device characterization To study the microstructure field emission scanning electron microscopy (FE-SEM, S4160 Hitachi Japan) was used. The phase structure and crystal size of films were also investigated by X-ray diffraction (XRD, Philips Expert- MPD). XRD was performed in a θ-2θ mode using Cu-Kα with a wavelength of 1.5439 ˚A radiation. All the XRD experiments were performed at grazing incident angle of 2°. The photocurrent-voltage (I–V) characteristics of solar cells were measured under one sun (AM1.5G, 100mW/cm2) illumination with a solar simulator (Sharif solar simulator). Steadystate PL measurements were acquired using a Varian Cary Eclipse (USA) fluorescence spectrometer.

3. Results and Discussion

Charge generation, dissociation, and transportation rule the performance of the prepared perovskite solar cells. In this regard, engineering the energy band diagram and physical properties of materials would govern the perovskite solar cells efficiency and performance. Figure 1 shows the energy level diagrams of FTO/TiO2/CH3NH3PbI3 /CuI/Cu devices incorporating Cu layer as a counter electrode and Cu atoms provider as for the growth of CuI layer at the interface of Cu and CH3NH3PbI3. The device configuration shows a 40 nm compact TiO2, 300 nm mesostructured TiO2/Perovskite layer, 200 nm upper perovskite layer, and 120 nm Cu electrode (Figure 1.a) which a thin CuI layer is grown at the interface of Cu and perovskite layer (Figure 1.b) to complete the device. As it is shown in band diagrams, the generated electron can easily transfer to TiO2 layer because of the high electron affinity of TiO2 at the interface of TiO2 and perovskite layer while the generated holes can be collected by Cu layer (Figure 1. a). Therefore, as schematically shown, the generated electrons would recombine at the interface of Cu and perovskite layers. On the other side, by growing 7



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a thin CuI layer at the interface of the Cu and perovskite layers, the generated holes would preferably extract by wide band gap CuI layer through which CuI will act as a potential electron blocking layer. Fabrication of the devices begins by deposition of compact and mesostructure of TiO2 followed by two-step deposition of perovskite in which 1.25 M PbI2 solution in DMF is spin-coated on the mTiO2 layer and subsequently 50 mg/mL MAI in 2-propanol is spin-coated on the PbI2 layer followed by annealing at 130℃ for 20 minutes (Figure 2). Finally a thin 120 nm Cu layer is thermal evaporated on the perovskite layer to complete the devices. The interface engineering of the devices is conducted by annealing the prepared devices on the hot plate at 100℃ for 10 minutes. As a result, a thin CuI layer is grown at the interface of Cu and perovskite layer. Accordingly, no more front contact deposition is needed and the upper residual Cu layer would be an appropriate contact to collect the extracted carriers from CuI layer. The comprehensive experimental details are explained in the experimental section. Copper shows a high reactivity to halide atoms especially iodine while exposing to their gas, liquid, or solid state. Cu could extract the iodine atoms from unstable iodide composition with different reaction kinetic. At room temperature, due to high bonding energy of nitrogen and iodide in MAI compound, Cu could not extract the iodide to reach lower energy state by forming the CuI structure (Figure 2), while at the higher temperature of 100℃, the kinetic energy would provide the activation energy needed for reaction of Cu atoms with iodide from methylammonium iodide molecules (Formula 1) through which methyl ammine and hydrogen would be released (Figure 3.b and Equeation 1). ln(K)= -Ea/R(1/T)+lnA

(1)

Where A is a constant related to the geometry needed, K is the rate constant, R is the gas constant (8.314 J/mol-K), and T is the temperature in Kelvin. Cu+CH3NH3I→ CuI+CH3NH2+1/2H2↑

(1)

8


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As shown in the Figure 3.a, by deposition of Cu on the perovskite layer which contains residual MAI on the perovskite surface, due to partial interaction of hot Cu atoms with MAI, a slight CuI thin film forms in the interface of Cu and perovskite layers (Figure 3.a, b, and C). By thermal annealing of the devices on the hot plate at diferent temperatures of 90, 100, 110, 120, and 130℃, CuI peaks strengthen while the peaks related to Cu and MAI decreases drastically due to ineraction of Cu and MAI in the interface at different annealing temperatures. Interestingly, no trace of methylamine peaks was observed in the XRD peaks due to very small amount of methylamine in the chemical reaction of CH3NH3I and Cu to CuI, CH3NH2, and H2 (Figure 3.a and Equeation 1). On the other side, by increasing the annealing temperature, the PbI2 phase is observed at 110, 120, and 130℃ which describes decomposition of CH3NH3PbI3 structure to PbI2 and CH3NH3I. The produded CH3NH3I molecules from decomposition of the perovskite structure provides further iodine resource for further CuI formation in the interface of Cu and perovskite layer. Besides, regarding to annealing the devices at air atmospher, an slight growth of CuO layer from Cu contact is observed which could detoriorate the device performanc (Figure 3.a). The cross-section images of the prepared devices at different annealing temperatures show a well surface coverage of the CuI/Cu on the perovskite upper layer (Figure 4.a-f). As shown in the Figure 3.a and Figure 4.a-f, by increasing the annealing temperature, Cu contact produces CuI layer in the interface of the perovskite and Cu layer. At higher annealing temperatures of 120 and 130℃, decomposition of perovskite to PbI2 and CH3NH3I is distiguashible (Figure 3.a and Figure 4.e and f). Figure 4.g and Figure 4.i show the surface morphoogies of perovskite and perovskite/CuI layers. To study the grwown CuI in the interface of Cu and perovskite layer, the Cu contact layer is partially peeled off from the surface of the perovskite while breaking the devices (Figure 4.h). As shw in the Figure 4.i, the grown CuI layer on the perovskite layer produces a uniform layer of CuI on the perovskite layer. Accordingly, the coherent charge transfer from perovskite to Cu contact through the CuI layer could be conducted. 9



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The photovoltaic characteristics of the prepared devices are analyzed by AM1.5 illumination of one sun by sun-simulator at 25±2 ℃ and 28% moisture. Figure 5.a demonstrates the J-V analysis of the fabricated device before (PSK/Cu) and after (PSK/CuI/Cu) interface engineering using a 0.1 cm2 masked active area. Completing the device by thermal deposition of Cu on the perovskite layer as HTM-free perovskite solar cells does not show a significant efficiency in comparison to outstanding enhanced devices in presence of CuI as HTM layer (Figure 5.a and Table 1). The HTM-free device employing Cu as a top contact, which has a 4.7eV work function, would not provide an appropriate energy state in extracting the generated holes. On the other side, due to the close work function of Cu to the conduction band of perovskite layer (Figure 1.a), the high recombination rate of carriers would be inevitable. Thermal annealing of deposited Cu thin film on the PSK/MAI produces PSK/CuI/Cu junction in which a very strong electron blocking and hole-extracting layer forms at the interface of the Cu and perovskite layers. This architecture introduces a great energy band diagram through which the photovoltaic parameters enhances. At annealing temperature of 100℃, the open circuit voltage and fill factor of devices dramatically boosted from 0.48V and 32% to 0.85V and 47% by interface engineering technique, which may be attributed to the decreasing the immense recombination rate of the carriers at the interface of the perovskite and Cu layer. Simultaneously, the short circuit density of the devices surges from 14.02 to 22.99 mA which can be described by a lower rate of charge recombination in increasing the charge extraction by appropriate energy states of the electron (TiO2) and hole (CuI) transporting materials. A negligible hysteresis was observed in our devices in two scanning directions of open circuit voltage to short circuit density (FB-SC) and short circuit density to open circuit voltage (SC-FB) with 0.1 cm2 active area (Figure 5.b). The low hysteresis could be described by low surface traps and fast charge extraction by CuI thin film at the interface of Cu and perovskite layer in transporting the extracted holes to Cu contact. Furthermore, the CuI layer driven from Cu contact proposes a great

10


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junction between CuI and Cu at the interface of CuI and Cu, which may help to decline the hysteresis in this kind of devices by rapid charge collection and low density of traps. To study the annealing effect on the photovoltaic properties of devices, the prepared devices are annealed at different annealing temperatures of 90, 100, 110, 120, and 130℃. As shown in the Figure 5.c, by increasing the annealing temperature from 90 to 100℃, the photovoltaic parameters enhance due to an appropriate formation of CuI thickness in the interaface of Cu and perovskite layers. By rising the annealing temperature up to 130℃, despite incresaing the thickness of the CuI layer, the photovoltaic properties of the devices detoriorate which can be ascribed to perovskite layers decomposition to PbI2 and MAI (Figure 3.a and Figure 5.c). Reproducibility of prepared devices introduces the scale-up potential of the process in the fabrication of perovskite solar cells. In this regard, engineering the fabrication process in shortening the required steps of the process, substituting the expensive materials with the cheaper ones, and employing a deposition route to reach large-area active layers, provides the primitive and pivotal requirements for proposing the potential process as scalable perovskite solar cells. As shown in Figure 6.a, fabrication of our device shows a great reproducibility as a result of simultaneous deposition of CuI/Cu by solid interface engineering technique. The total statistical analysis of 10 devices is shown in Figure 5.b. Hence, the present technique would pave the way in the deposition of large-area scalable perovskite solar cells. Extracting the generated carriers in the p-i-n solar cells by n-type electron and p-type hole transporting materials plays the main role in decreasing the charge recombination in the perovskite absorber layers. The PL analysis is conducted to study the charge extraction potential in the present device employing TiO2 as ETM and CuI as a HTM layer compared to conventional spiro-OMeTAD HTM. As shown in the Figure 6.c, CuI shows a great potential in almost complete extracting the holes from perovskite layer through which the PL is completely quenched by contacting the CuI and perovskite layers. Therefore, in this case, CuI shows considerable potential to extract holes more 11



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effective than spiro-OMeTAD, which has been widely used in most efficient perovskite solar cells. Thereupon, the present work proposes an easy approach to reach efficient and durable low-cost perovskite solar cells. On the other side, TiO2 potentially extract the generated electrons from perovskite in transporting the carriers to the FTO contact. As shown in the Figure 6.d, the perovskite solar cells which is prepared by inorganic HTM of CuI and inorganic ETM of TiO2, presents the high photo-stability which may ascribe to the higher stability of electron and hole transporting materials against light and temperature rather than organic one. Evaluating the process to fabricate large-area devices is the first step in scaling up the solar cells, which was reported in some highlighted works 41. In this regard, we deposited a Cu/CuI double layer with a 1 cm2 active area in which a notable PCE of 8.3% was obtained (Figure 7.a). This large-area potential deposition of final HTM/Contact layers could be used in different architectures of perovskite solar cells in examining the capability of reported work in the scaling up vision and approach. As widely reported, fabrication of inorganic base perovskite solar cells using inorganic electron and hole transport materials in normal and inverted architectures proposes high durability and stability against moisture and mechanical damages

20, 39, 42.

In the present work, a compact and uniform

deposition of CuI/Cu could protect the perovskite layer against water molecules diffusion through the Cu and CuI layer. The durability of the fabricated cells were traced by keeping them at 25℃ ambient atmosphere with 28 ± 2 percent moisture, compared with the similar structure fabricated with spiro-OMeTAD and gold as a standard sample. As shown in Figure 7.b, the fabricated perovskite cell with CuI/Cu as a HTM/Contact couple retains its stability up to 30 days without getting the advantages of encapsulation, while the prepared devices with spiro-OMeTAD/Au as HTM/Contact couple losses its PCE in less than 24 hours. The PCE improvement at first day of working could be ascribed to further interface engineering at room temperature, surface defects decrement, and releasing the residual strain at the interface of perovskite20, CuI and Cu layers. Accordingly, the 12


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inorganic CuI/Cu couple along with acting as a successful and strong HTM and contact layers, respectively, would provide a great surface protection against the diffusion of moisture into the perovskite layer. Hence, this would be a big step in the easy fabrication of durable and inexpensive perovskite solar cells, which may pave the way for large-area fabrication of perovskite solar cells. The comparative analyses of FE-SEM and J-V are conducted to study the morphology and photovoltaic parameters of devices prepared with two structures of FTO/TiO2/PSK/CuI/Cu and FTO/TiO2/PSK/Spiro-OMeTAD/Au. In this regard, the HTM/metal contacts are different while the bottom structure of FTO/TiO2/PSK are kept same. As shown in the Figure 8.a, same as spiroOMeTAD/Au layers, the CuI/Cu layers show good uniformity covering the perovskite layer in which the great charge extraction conducts. The lower Voc and fill factor in the prepared devices using CuI/Cu layers compare to spiro-OMeTAD/Au layers, may come from possible defects in the interfaces of CuI and perovskite layers which facilitate the interfacial charge recombination. On the other side, strong charge extraction properties of CuI from perovskite layer (Figure 6.c), and its high hole mobility than spiro-OMeTAD presents higher current density in the FTO/TiO2/PSK/CuI/Cu devices.

4. Conclusion

In this work, a physicochemical interface engineering was introduced in reaching CuI/Cu double layer as a potential HTM/Metal contact couple in the fabrication of low-cost, large-area, hysteresisfree, reproducible, and durable perovskite solar cells. In this architecture, the Cu layer directly deposited on the two-step perovskite layer containing an excess amount of methylammonium iodide as an iodine provider in producing the CuI at the interface of Cu metal contact and perovskite layers. The prepared device shows the hysteresis-free photovoltaic properties with maximum power conversion efficiency of 9.24% and long-term durability proposing an environment and market friendly perovskite solar cells. 13



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Acknowledgements The authors acknowledge financial support from Tarbiat Modares University and the equipment services from NOPL laboratory. The authors also acknowledge the Iran Nanotechnology Initiative Council (INIC) and Iran Science Elites Federation (ISEF) for the partial support of this project.

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Figure captions

Figure 1. Schematic of energy level diagrams for FTO/TiO2/CH3NH3PbI3 /Cu (a) and interface engineered FTO/TiO2/CH3NH3PbI3 /CuI/Cu (b). Figure 2. Fabrication schematic of all inorganic perovskite solar cells and interface engineering of Cu and CH3NH3PbI3 layers. Figure 3. XRD pattern of perovskite/Cu double layer with and without annealing for 10 minutes at RT, 90, 100, 110, 120, and 130℃ (a), the magnified XRD pattern (b and c), and the CuI formation interaction schematic at the interface of CH3NH3PbI3 and Cu layers(d). Figure 4. The cross-section (a-f, and h) and top (g and i) FE-SEM images of the devices with FTO/TiO2/PSK/CuI/Cu structure annealed at RT, 90, 100, 110, 120, and 130℃ (a-f), top image of perovskite layer (g), peeled-off Cu layer from perovskite/CuI layer (h), and top image of CuI morphology on perovskite layer (i). Figure 5. J-V curves of the prepared device before (PSK/Cu) and after (PSK/CuI/Cu) interface engineering (a) for the scan direction of forward bias to short circuit (FB-SC) and short circuit to forward bias (SC-FB) (b), and photovoltaic parametrs in different annealing temperatures (c). Figure 6. J-V curves of the five prepared devices (a), statistics data of the prepared devices (b), PL of single perovskite layer, PSK/TiO2, PSK/CuI/Cu, and PSK/Spiro-OMeTAD double layers (c), and photo-stablity of devices in one sun illumination (d). Figure 7. The J-V curve of the prepared large-area device with 1 cm2 (a) and durability of devices with FTO/TiO2/PSK/CuI/Cu, and FTO/TiO2/PSK/Spiro-OMeTAD/Au structure by 30 days (b). Figure 8. The comparative FE-SEM images (a and b) and J-V ananlysis (c) of prepared devices with FTO/TiO2/PSK/CuI/Cu and FTO/TiO2/PSK/Spiro-OMeTAD/Au (The photovoltaic parameters are listed in the table). 17



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Figure 1. Schematic of energy level diagrams for FTO/TiO2/CH3NH3PbI3 /Cu (a) and interface engineered FTO/TiO2/CH3NH3PbI3 /CuI/Cu (b).

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Figure 2. Fabrication schematic of all inorganic perovskite solar cells and interface engineering of Cu and CH3NH3PbI3 layers.

Figure 3. XRD pattern of perovskite/Cu double layer with and without annealing for 10 minutes at RT, 90, 100, 110, 120, and 130℃ (a), the magnified XRD pattern (b and c), and the CuI formation interaction schematic at the interface of CH3NH3PbI3 and Cu layers(d).

19



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Figure 4. The cross-section (a-f, and h) and top (g and i) FE-SEM images of the devices with FTO/TiO2/PSK/CuI/Cu structures annealed at RT, 90, 100, 110, 120, and 130℃ (a-f), top image of perovskite layer (g), peeled-off Cu layer from perovskite/CuI layer (h), and top image of CuI morphology on perovskite layer (i).

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Figure 5. J-V curves of the prepared device before (PSK/Cu) and after (PSK/CuI/Cu) interface engineering (a) for the scan direction of forward bias to short circuit (FB-SC) and short circuit to forward bias (SC-FB) (b), and photovoltaic parametrs in different annealing temperatures (c). 21



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Figure 6. J-V curves of the five prepared devices (a), statistics data of the prepared devices (b), PL of single perovskite layer, PSK/TiO2, PSK/CuI/Cu, and PSK/Spiro-OMeTAD double layers (c), and photo-stablity of devices in one sun illumination (d).

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Figure 7. The J-V curve of the prepared large-area device with 1 cm2 (a) and durability of devices with FTO/TiO2/PSK/CuI/Cu, and FTO/TiO2/PSK/Spiro-OMeTAD/Au structure by 30 days (b).

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Figure 8. The comparative FE-SEM images (a and b) and J-V ananlysis (c) of prepared devices

with

FTO/TiO2/PSK/CuI/Cu

and

FTO/TiO2/PSK/Spiro-OMeTAD/Au

photovoltaic parameters are listed in the table).

24


(The

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Table 1. Performance parameters of prepared perovskite solar cells before and after interface engineering Device

Scan Direction

Voc (V)

Jsc (mA/cm2)

FF(%)

PCE (%)

FTO/TiO2/PSK/Cu

FB-SC

0.48

14.02

32

2.19

FTO/TiO2/PSK/CuI/Cu

FB-SC

0.85

22.99

47

9.24

FTO/TiO2/PSK/CuI/Cu

SC-FB

0.83

22.95

46.5

8.86

TOC Graphic

25


Cu as Advanced

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