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Carbon film electrode based square-centimeter scale planar perovskite solar cells exceeding 17% efficiency
Materials Science in Semiconductor Processing 107 (2020) 104809
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Carbon film electrode based square-centimeter scale planar perovskite solar cells exceeding 17% efficiency Hang Su a, b, Junyan Xiao a, b, *, Qianhui Li a, b, Chao Peng a, b, Xinxin Zhang c, Chi Mao a, Qin Yao a, Yujie Lu a, Zhiliang Ku b, Jie Zhong b, Wei Li b, Yong Peng b, Fuzhi Huang b, Yi-bing Cheng b a
School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, PR China State Key Laboratory of Advanced Technologies for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, PR China Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei, PR China b c
A B S T R A C T
Carbon electrode-based perovskite solar cells have shown some advantages in materials cost and long term stability compared to the metal electrode-based devices. Among all the reported carbon electrode techniques, self-adhesive carbon film electrode via press transfer process can achieve high PCE comparable with the Au electrode device in small area perovskite solar cells (~ 0.1 cm2). Herein, we further promote the carbon film electrode to match the practical square-centimeter scale device. Pre-assembled with highly conductive substrates as graphite paper or aluminum foil, the double layered composite carbon electrode can significantly reduce the series resistance, thus retaining the high photoelectric performance of large area device (1 cm2). Especially with the graphite paper/carbon film electrode, a highest PCE of 17.02% was obtained, which achieved 92% of the PCE of 18.56% in small device. The soft and compressible nature can be the superiority of graphite paper as conductive substrate in carbon film based electrode compared with aluminum foil.
1. Introduction Recent years, perovskite solar cells (PSCs) based on metal halide perovskite materials as light absorber have developed rapidly, and their certified power conversion efficiency (PCE) has already reached 25.2% [1–6]. In order to realize the industrialization of perovskite solar cells, the stability performance of the device should be improved. Meanwhile, the cost of the fabrication process and raw materials should be reduced. Top electrode in perovskite solar cell plays an extremely important role in the overall performance and cost. Therefore, conductive carbon ma terials with good chemical stability, suitable Fermi level and low cost were considered as ideal choice for top electrode [7,8]. Graphite based conductive carbon paste [7–15], carbon nanotube [16–18] and gra phene [19,20] have been adopted in PSCs as top electrode with various device configurations. Among all these carbon electrode techniques, self-adhesive carbon film electrode via press transfer process exhibited its unique advantage which can match well with high performance perovskite absorber and organic hole-transporting materials (HTM) in device. The carbon film electrode based PSC can achieve a high PCE of 19.2%, comparable with the standard Au electrode device [21]. However, the carbon electrode is significantly poorer than evapo rated metal electrodes in the aspect of conductivity, which will result in
poorer performance in large area device. Specifically, the sheet resis tance of vapor-deposited metal film (~100 nm thick) can be less than 1 Ω sq-1, while the value of carbon electrodes are always more than 10 Ω sq-1 and even exceeding 100 Ω sq-1 [16,19,21]. Although some effort have been made to enhance the conductivity of carbon top electrode, such as optimizing carbon paste or combining with highly conductive extraction electrode [22–24], square-centimeter sized PSCs with carbon electrode and high PCE (>15%) have been rarely reported [25–27]. Depending on the carbon film electrode, herein, we pre-assemble it with highly conductive substrates as graphite paper and aluminum foil, which have been used to enhance the conductivity of carbon electrode in PSCs [24,28], thus promoting this technique to match the practical square-centimeter scale planar structured PSCs. Due to the significantly reduced series resistance (Rs) in the whole PSCs, the PCE of large area device (1 cm2) with graphite extraction electrode can achieve 17.02%, which is about 92% of the highest PCE of 18.56% in small device (0.1 cm2). The large device with aluminum foil can also exhibit high PCE of 15.41%, and was compared with the former case. These results ob tained in square-centimeter scale devices are obviously better than other ones reported in literature and represent good prospect.
* Corresponding author. School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, PR China. E-mail address: [email protected] (J. Xiao). https://doi.org/10.1016/j.mssp.2019.104809 Received 3 July 2019; Received in revised form 24 October 2019; Accepted 28 October 2019 Available online 2 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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Materials Science in Semiconductor Processing 107 (2020) 104809
2. Results and discussion
during forward scan (FS) and reverse scan (RS) can be achieved since the perfect ohmic contact between carbon film and spiro-OMeTAD, which coincide with our previous research [21]. The steady-state output characteristics of best carbon electrode based device was also tested. As shown in Fig. 3a, under the maximum power point (MPP) of 950 mV, PSCs based on the connecting layer/ graphite electrode gave a steady-state current density of about 19 mA cm-2, yielding a steady-state efficiency of 18.05%, near to the J-V result. The external quantum efficiency (EQE) of this piece of solar cell was also measured to confirm the accuracy of the J-V measurement, as shown in Fig. 3b. The integrated current density was 20.48 mA cm-2, which is in good agreement with the experimentally obtained Jsc. This over 18% PCE is better than our previous result in planar PSCs (17.5%) [21], due to the optimized carbon electrode and SnO2/CsFAMA mixed perovskite system. The preliminary stability tests have been further carried out. As shown in Fig. 3c, PSCs based on different electrodes were stored in ambient atmosphere under open circuit and dark condition without any encapsulation (Temperature: 20 � C; Humidity: 30%). They were measured every two days for 1500 h under one sun illumination. Finally, PSCs based on the connecting layer/graphite electrode can retain 90.5% of its initial efficiency, indicating relatively better stability than Au electrode based PSCs (82.3%). In previous researches, carbon electrode and novel carbon based additives, such as carbon quantum dots, gra phene and graphdiyne, can improve the stability of perovskite devices due to the ions diffusion resistance and hydrophobic property of carbon films [21,36–38]. However, the connecting layer/aluminum foil elec trode based devices show an even worse stability, with only 64.2% of the initial PSC remained, which may cause by unsatisfactory long term mechanical connection property. From the statistical results in Table 1, the average PCE of PSCs based on Au electrode is 18.50%. And the average value of the PSCs based on the connecting layer/graphite electrode is 17.40%, which is close to the former with sound reproducibility. With aluminum foil in place of graphite paper, lower PCE values (15.38% on average) and worse reproducibility. Although the active area is small, bare carbon film based extraction electrode still bring about extra series resistance, which lead to the large Rs over 10 Ω cm2, thus yielding average PCE of 12.16% with poor FF. Apparently, pre-assembling with graphite paper and aluminum foil can benefit the carbon film electrode and PSCs device performance. However, compared with graphite paper based electrode, aluminum foil based electrode exhibit unsatisfactory reproducibility and stability. The average Jsc value of connecting layer/aluminum foil devices (19.72 mA cm-2) is even lower than that of bare connecting layer device (20.63 mA cm-2). So it is important to figure out the differences between these two sorts of electrode. In reality during our experiments, some different phenomena can be observed in different electrodes. After the preparation of the connecting layer/aluminum foil electrode, the carbon film deposited on the aluminum foil is easily loosened or even peeled off. This issue may due to the smooth surface of the aluminum foil and the incompatibility be tween aluminum foil and the resin in carbon paste. The poor stability performance of corresponding PSCs can be at least partially caused by this problem. Moreover, irreversible creases can be hardly avoided in aluminum foil substrate during the preparation, as shown in Fig. 1b. These creases may cause the ununiformity of carbon film coated on aluminum foil, thus resulting in poor reproducibility. Cross-sectional SEM images of various carbon based electrode before and after press transfer were shown in Fig. 4 for detailed investigation. Comparing the samples in Fig. 4a, b, e and f, we can see that the thickness of the connecting layer carbon film decreases by about 15 μm and 17 μm respectively after pressing, which proves that the incom pressibility of aluminum foil and the good compressibility of the con necting layer carbon film. ComparingFig. 4c and d, it can be seen that the thickness of the connecting layer/graphite electrode decreases by
In previous work, we have developed a self-adhesive macroporous carbon film via solvent exchange process [21]. However, when trying to apply this carbon electrode onto larger device, we faced two difficulties. The first problem was maintaining the mechanical integrity of large area self-supporting carbon film within the whole fabrication process. And the second one was the unnegligible high resistance in large area device caused by carbon film. Therefore, combining the self-adhesive carbon with highly conductive substrate is a natural idea. In this research, we name the individual or combined self-adhesive macroporous carbon film as connecting layer. Low cost commercial graphite paper and aluminum foil are selected as extraction layer for the double layer composite electrode, which are both highly conductive (sheet resistance < 1 Ω sq-1) and stable under organic solvent. Fig. 1a and Fig. 1b show the prepared connecting layer/graphite electrode and the connecting layer/aluminum foil electrode, respec tively. Fig. 1c presents the schematic diagram for the preparation pro cess of the double layer composite carbon film electrode. The commercial carbon paste was blade coated directly on graphite paper or aluminum foil to form a wet film, and then transferred together into ethanol for solvent-exchange process. Composite carbon film was ob tained after removed from ethanol and dried in air. Planar structured PSCs with SnO2 as electron selective layer and mixed cation perovskite absorber have been proved to be a well per formed system, and can be fabricated via low temperature process [29–34]. Through pressure transfer method, we prepared connecting layer/aluminum foil electrode, connecting layer/graphite electrode and bare connecting layer carbon film electrode onto the top of FTO/S nO2/Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3/spiro-OMeTAD substrates to complete the small area PSCs (0.1 cm2) [35]. The photovoltaic characteristics (J-V curves) of champion devices with various top elec trodes are shown in Fig. 2. More specifically, champion and statistical photovoltaic parameters are summarized in Table 1. As standard com parison, the highest efficiency of PSCs based on Au electrodes is 19.04%, with short-circuit current (Jsc) of 21.98 mA cm-2, open-circuit voltage (Voc) of 1.110 V and fill factor (FF) of 0.78, respectively. For PSCs based on connecting layer/graphite electrode, a high PCE up to 18.56% has been achieved with Jsc of 21.11 mA cm-2, Voc of 1.117 V and FF of 0.79. The highest efficiency of the connecting layer/aluminum foil electrode based PSCs is slightly lower than the connecting layer/graphite elec trode, which is 17.15%, with Jsc of 20.91 mA cm-2, Voc of 1.090 V and FF of 0.75. Bare connecting layer based electrode exhibits relatively poor performance, of which the best result is PCE 14.93%, Jsc 20.11 mA cm-2, Voc 1.104 V and FF 0.67. In Fig. 2, connecting layer/graphite and con necting layer/aluminum foil based devices exhibit slightly lower Jsc value than standard Au device. Besides, negligible hysteresis behavior
Fig. 1. (a) Photograph of graphite paper and connecting layer/graphite elec trode; (b) Photograph of connecting layer/aluminum foil electrode; (c) Sche matic diagram of the solvent-exchange preparation process of double layer composite carbon film electrode. 2
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Materials Science in Semiconductor Processing 107 (2020) 104809
Fig. 2. J-V curves of PSCs with (a) connecting layer/graphite electrode, (b) connecting layer/aluminum foil electrode, (c) connecting layer carbon film electrode, (d) Au electrode. Table 1 Statistical photovoltaic parameters of different electrodes based 0.1 cm2 PSCs. Electrode type
Voc (V)
Jsc (mA cm-2)
FF
PCE (%)
Rs (Ω cm2)
connecting layer/graphite
champion average
1.117 1.099�0.022
21.11 20.62�0.51
0.79 0.76�0.04
18.56 17.40�1.16
3.58 4.05�0.77
connecting layer/aluminum foil
champion average
1.090 1.090�0.042
20.91 19.72�1.33
0.75 0.70�0.06
17.15 15.38�1.77
4.07 7.90�3.83
connecting layer
champion average
1.086 1.016�0.070
20.16 20.63�0.72
0.65 0.58�0.07
14.29 12.16�2.13
11.39 16.19�4.80
Au
champion average
1.110 1.090�0.015
21.98 21.65�0.41
0.78 0.76�0.03
19.04 18.50�0.50
3.57 3.95�0.41
Fig. 3. (a) The steady-state output current and calculated PCE at 950 mV bias. (b) External quantum efficiency (EQE) spectra and integrated current density. (c) Stability test kept in ambient atmosphere without any encapsulation.
about 41 μm, this major change of thickness is mainly due to the compressibility of graphite paper. In addition, it can be seen that the connecting layer carbon film has good interface contact with the graphite paper before and after hot pressing, which cannot be distin guished from the cross-sectional view. This combination between con necting layer carbon film and graphite paper cannot be separated by a thin blade without damage. So it can be inferred that the two layers are embedded to each other to some extent. By contrast, as given in Fig. 4g, sometimes visible gap between the connecting layer carbon film and the aluminum foil can be observed. The charge transfer may be greatly
hindered due to the poor interface contact between the connecting layer carbon film and the aluminum foil. Further more, the liner surface roughness of three carbon electrodes before and after pressing were measured with a step profiler, as shown in Fig. 5. Before the pressing, the three carbon electrodes have extremely rough surfaces due to relative crude processing technique and the porous nature of the connecting layer carbon film. Within 2.5 mm length, the undulation range can even exceed 20 μm, far greater than the total thickness of SnO2/perovskite/spiro-OMeTAD functional multilayer (less than 1 μm) in device. From the changes after pressing, we can see 3
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Materials Science in Semiconductor Processing 107 (2020) 104809
Fig. 4. The cross-sectional SEM images of the bare connecting layer carbon film (a) before and (b) after press. The cross-sectional SEM images of the connecting layer/graphite (c) before and (d) after press. The cross-sectional SEM images of the connecting layer/aluminum foil before (e) and (f) after press. (g) The interface contact between the connecting layer and the aluminum foil.
Fig. 5. (a) The roughness of the connecting layer/graphite before and after press. (b) The roughness of the connecting layer/aluminum foil before and after press. (c) The roughness of the connecting layer before and after press.
that the liner surface roughness of the connecting layer carbon film is significantly reduced after pressing, indicating that the porous structure in the carbon film become dense. Compared with bare connecting layer
sample, connecting layer/graphite exhibits flatter surface since graphite paper provides extra compressibility, which can contribute to the better contact with underlayer in device. However, as shown in Fig. 5b, the
Fig. 6. J-V curves of 1 cm2 PSCs with (a) connecting layer/graphite electrode, (b) connecting layer/aluminum foil electrode, (c) connecting layer carbon film electrode. (d) Photographs of 1 cm2 devices based on different electrodes. 4
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surface of connecting layer/aluminum foil sample is very smooth but not flat enough. The surface undulation still exceeds 10 μm, which can be due to the plasticity and elasticity of aluminum foil. In the preparation process of the connecting layer/aluminum foil electrode, the planeness of aluminum foil can be easily changed due to plastic deformation. But it cannot be well controlled because of the elastic nature. So in the final device, the gap between the aluminum foil and the HTL may be unful filled with carbon film, which not only hinders the transport of charges but also leaves plenty voids for ambient air. Eventually, poorer repro ducibility and stability in connecting layer/aluminum foil electrode based devices will thus happen. To examine the practicability of the composite electrode, devices with 1 cm2 active area were prepared, as shown in Fig. 6d. Through the J-V curves in Fig. 6 and photovoltaic parameters in Table 2, similar re sults with the former small devices can be found. The average PCE of 1 cm2 PSCs based on connecting layer/graphite electrode can be 14.79%, and the best value is 17.02%, with Jsc of 20.94 mA cm-2, Voc of 1.130 V, FF of 0.72 and Rs of 4.79 Ω cm2. And for the connecting layer/ aluminum foil electrode based one, the highest is 15.41% with Jsc of 20.56 mA cm-2, Voc of 1.117 V, FF of 0.67 and Rs of 10.70 Ω cm2. These two best results maintain 92% and 90% the PCEs of the values in small area devices, and should be among the absolute high level of carbon electrode based PSCs (see Table S1 in Supplementary Information). What’s more, we tried the connecting layer/graphite electrode in a 5 cm � 5 cm perovskite series module, and achieved a PCE of 7.49% without much optimization on the module technique (see Fig. S1 in Supplementary Information). Considering the conductivity of graphite paper, the increased Rs should be attributed to the larger FTO area. And in the case of aluminum foil based device, FTO and further deteriorating electrical contact can be both the reasons. The poor carrier transporting even results in neglectable hysteresis behavior in Fig. 6b. Under negative control condition, the best large PSCs based on bare carbon film elec trode has only 6.06% PCE, 14.36 mA cm-2 Jsc and 0.38 FF. The great Rs value up to 22.59 Ω cm2 will affect not only FF but even photocurrent density significantly in this high efficiency perovskite system, thus leading to PCE less than 50% the value in small device. Finally, to emphasize the cost advantage of our composite carbon electrode, we give a brief calculation as follow. Since the device per formances are similar with our carbon based electrode and standard Au electrode, we can compare the materials cost for the two sorts of devices with a same total active area. In our experiment, 120 g carbon paste (commercially available, about $27 per kilogram) is needed for 1 m2 carbon film, and the cost of graphite paper is about $4 per square-meter. Thus the total materials cost of the composite carbon electrode should be $7.2 per square-meter. While in the case of Au electrode, assuming the utilization rate of gold to be 100%, the materials cost can be easily calculated as about $100 per square-meter, which is 14 times higher than carbon electrode. Moreover, the blade coating and hot-press equipment for our composite carbon electrode have obviously lower cost than the vacuum evaporation equipment for Au electrode.
electrodes exhibit improved performance in perovskite solar cells. Especially for the carbon film electrode composited with graphite paper, high PCEs of 18.56% and 17.02% were achieved in 0.1 cm2 and 1 cm2 area devices respectively, which are the best results in carbon electrode based planar PSCs. Within pressure transfer process, graphite paper substrate exhibits soft and compressible nature, which can guarantee desired smooth and flat surface for ideal interface contact. In contrast, aluminum foil exhibits uncontrollable plasticity and elasticity, thus affect the PCE, stability and reproducibility of the corresponding de vices. Therefore, our results may offer a solution for practical large area carbon based PSCs. 4. Experimental section Materials: The SnCl2‧2H2O was obtained from Aladdin. For mamidinium iodide (FAI) and methylammonium bromine (MABr) were purchased from Lumtec, Taiwan. Lead iodide (PbI2), lead bromine (PbBr2) and organic HTM Spiro-OMeTAD were purchased from Xi’an Polymer Light Technology Corp. CsI and KI were purchased from SigmaAldrich. Carbon paste was purchased from LinchWin Printing Material Co., Ltd. All the chemicals were used directly as received. Carbon Film Based Electrodes Preparation: Bare carbon film was pre pared according to the literature [21]. And the composite electrodes were prepared with even easier process as follow. Firstly, the commer cial conductive-carbon paste was coated on conductive substrates (graphite paper and aluminum foil) by simple doctor-blading. Then, the substrates with wet carbon film were immersed in ethanol to exchange solvent at room temperature for 10 min. Finally, the substrates with ethanol stained carbon film was taken out and dried to form carbon based double-layer composite electrodes. Device Fabrication: The FTO substrates were first patterned by femtosecond laser, then cleaned through detergent, pure water and ethanol under ultrasonic for 20 min, respectively. The substrates were then dried with dry-air gas flow and then treated by UV-ozone for 15 min before use. Then compact SnO2 film was grown by chemical bath deposition on FTO substrates directly as electron selective layer. The precursor solution of CsFAMA mixed perovskite was prepared by dis solving 1.4 M mixture of metal lead salts composed of 0.85 portion PbI2 (548.6 mg) and 0.15 portion PbBr2 (77.07 mg), and 1.3 M organic cation composed of 0.85 portion FAI (190.12 mg) and 0.15 portion MABr (21.84 mg) in 0.8 ml mixture solvent of DMF/DMSO (4:1, by volume), with the addition of 34 μl CsI solution (pre-dissolved as a 2 M stock so lution in DMSO). This precursor solution was dropped onto SnO2 layer and spin-coated at a rate of 6000 rpm for 30 s. Then a certain amount of green anti-solvent ethyl acetate was poured onto the spinning substrate at the last 5 s. Right after this, the film was annealed at 120 � C for 45 min to achieved the desired Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 perov skite layer. Then Spiro-OMeTAD solution doped with Li-TFSI, FK209 and 4-tert-butylpyridine was spin-coated on the perovskite films at 3000 rpm for 30 s to form hole-transporting layer. Finally, for Au de vices, 80 nm of gold was evaporated on the top of Spiro-OMeTAD layer as the top electrode; for carbon electrode based devices, the film elec trodes were directly pressed onto Spiro-OMeTAD layer under a pressure of 0.8 MPa at 50 � C for 20 s via a pneumatic hot-press. Materials and Devices Characterizations: The microscopic morphology
3. Conclusion Pre-assembled with highly conductive graphite paper and aluminum foil to form double layer composite structures, the carbon film based Table 2 Statistical photovoltaic parameters of different electrode based 1 cm2 PSCs. Voc (V)
Jsc (mA cm-2)
FF
PCE (%)
Rs (Ω cm2)
connecting layer/graphite
champion average
1.130 1.106�0.025
20.94 19.58�1.18
0.72 0.66�0.06
17.02 14.79�2.17
4.79 9.73�4.51
connecting layer/aluminum foil
champion average
1.117 1.121�0.063
20.56 18.17�2.14
0.67 0.60�0.08
15.41 13.20�2.21
10.70 16.09�5.74
connecting layer
champion average
1.105 1.032�0.073
14.36 15.45�2.27
0.38 0.33�0.05
6.06 4.89�1.17
22.59 44.50�22.50
5
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were investigated by a field emission scanning electron microscopy (FESEM, Zeiss Ultra Plus). The photocurrent density-voltage (J-V) curves were carried out by using a solar simulator (Oriel 94023A, 300 W) with a source meter (Keithley2400) under 100 mW cm 2 illumination (AM 1.5G) with a scan rate of 10 mV s-1. An external quantum efficiency (EQE) measurement system (QEX10, PV Measurements, Inc.) was used to measure the EQE of the devices across a wavelength range of 300–850 nm.
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Declaration of competing interest The authors declare no conflict of interest. Acknowledgements The authors acknowledge the financial support by The National Key Research and Development Program of China (2018YFB1500104), The Technological Innovation Key Project of Hubei Province (2018AAA048), The Fundamental Research Funds for the Central Uni versities (WUT: 2019IVA004). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104809. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visiblelight sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] H. Kim, C. Lee, J. Im, K. Lee, T. Moehl, A. Marchioro, S. Moon, R. HumphryBaker, J. Yum, J. Moser, M. Gratzel, N. Park, Lead iodide perovskite sensitized all solidstate submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Rep. 2 (2012) 591. [3] J. Burschka, N. Pellet, S. Moon, R. Humphry-Baker, P. Gao, M. Nazeeruddin, M. Gr€ atzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316–319. [4] N. Jeon, J. Noh, Y. Kim, W. Yang, S. Ryu, S. Seok, Solvent engineering for highperformance inorganic-organic hybrid perovskite solar cells, Nat. Mater. 13 (2014) 897–903. [5] N. Jeon, J. Noh, W. Yang, Y. Kim, S. Ryu, J. Seo, S. Seok, Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476–480. [6] N.R.E.L, Best research-cell efficiencies (accessed October, 2019), https://www.nre l.gov/pv/cell-efficiency.html. [7] Y. Cai, L. Liang, P. Gao, Promise of commercialization: carbon materials for lowcost perovskite solar cells, Chin, Phys. B 27 (2018), 018805. [8] Z. Ku, Y. Rong, M. Xu, T. Liu, H. Han, Full printable processed mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells with carbon counter electrode, Sci. Rep. 3 (2013) 3132. [9] F. Zhang, X. Yang, M. Cheng, W. Wang, L. Sun, Boosting the efficiency and the stability of low cost perovskite solar cells by using CuPc nanorods as hole transport material and carbon as counter electrode, Nano Energy 20 (2016) 108–116. [10] H. Zhou, Y. Shi, Q. Dong, H. Zhang, Y. Xing, K. Wang, Y. Du, T. Ma, Holeconductor-free, metal-electrode-free TiO2/CH3NH3PbI3 heterojunction solar cells based on a low-temperature carbon electrode, J. Phys. Chem. Lett. 5 (2014) 3241–3246. [11] H. Chen, Z. Wei, H. He, X. Zheng, K. Wong, S. Yang, Solvent engineering boosts the efficiency of paintable carbon-based perovskite solar cells to beyond 14%, Adv. Energy Mater. 6 (2016), 1502087. [12] Q. Chu, B. Ding, J. Peng, H. Shen, X. Li, Y. Liu, C. Li, C. Li, G. Yang, T. White, K. Catchpole, Highly stable carbon-based perovskite solar cell with a record efficiency of over 18% via hole transport engineering, J. Mater. Sci. Technol. 35 (2019) 987–993. [13] H. Ye, Z. Liu, X. Liu, B. Sun, X. Tan, Y. Tu, T. Shi, Z. Tang, G. Liao, 17.78% efficient low-temperature carbon-based planar perovskite solar cells using Zn-doped SnO2 electron transport layer, Appl. Surf. Sci. 478 (2019) 417–425. [14] J. Duan, Y. Zhao, B. He, Q. Tang, High-purity inorganic perovskite films for solar cells with 9.72% efficiency, Angew. Chem. Int. Ed. 57 (2018) 3787–3791.
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Carbon film electrode based square-centimeter scale planar perovskite solar cells exceeding 17% efficiency Hang Su a, b, Junyan Xiao a, b, *, Qianhui Li a, b, Chao Peng a, b, Xinxin Zhang c, Chi Mao a, Qin Yao a, Yujie Lu a, Zhiliang Ku b, Jie Zhong b, Wei Li b, Yong Peng b, Fuzhi Huang b, Yi-bing Cheng b a
School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, PR China State Key Laboratory of Advanced Technologies for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, PR China Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei, PR China b c
A B S T R A C T
Carbon electrode-based perovskite solar cells have shown some advantages in materials cost and long term stability compared to the metal electrode-based devices. Among all the reported carbon electrode techniques, self-adhesive carbon film electrode via press transfer process can achieve high PCE comparable with the Au electrode device in small area perovskite solar cells (~ 0.1 cm2). Herein, we further promote the carbon film electrode to match the practical square-centimeter scale device. Pre-assembled with highly conductive substrates as graphite paper or aluminum foil, the double layered composite carbon electrode can significantly reduce the series resistance, thus retaining the high photoelectric performance of large area device (1 cm2). Especially with the graphite paper/carbon film electrode, a highest PCE of 17.02% was obtained, which achieved 92% of the PCE of 18.56% in small device. The soft and compressible nature can be the superiority of graphite paper as conductive substrate in carbon film based electrode compared with aluminum foil.
1. Introduction Recent years, perovskite solar cells (PSCs) based on metal halide perovskite materials as light absorber have developed rapidly, and their certified power conversion efficiency (PCE) has already reached 25.2% [1–6]. In order to realize the industrialization of perovskite solar cells, the stability performance of the device should be improved. Meanwhile, the cost of the fabrication process and raw materials should be reduced. Top electrode in perovskite solar cell plays an extremely important role in the overall performance and cost. Therefore, conductive carbon ma terials with good chemical stability, suitable Fermi level and low cost were considered as ideal choice for top electrode [7,8]. Graphite based conductive carbon paste [7–15], carbon nanotube [16–18] and gra phene [19,20] have been adopted in PSCs as top electrode with various device configurations. Among all these carbon electrode techniques, self-adhesive carbon film electrode via press transfer process exhibited its unique advantage which can match well with high performance perovskite absorber and organic hole-transporting materials (HTM) in device. The carbon film electrode based PSC can achieve a high PCE of 19.2%, comparable with the standard Au electrode device [21]. However, the carbon electrode is significantly poorer than evapo rated metal electrodes in the aspect of conductivity, which will result in
poorer performance in large area device. Specifically, the sheet resis tance of vapor-deposited metal film (~100 nm thick) can be less than 1 Ω sq-1, while the value of carbon electrodes are always more than 10 Ω sq-1 and even exceeding 100 Ω sq-1 [16,19,21]. Although some effort have been made to enhance the conductivity of carbon top electrode, such as optimizing carbon paste or combining with highly conductive extraction electrode [22–24], square-centimeter sized PSCs with carbon electrode and high PCE (>15%) have been rarely reported [25–27]. Depending on the carbon film electrode, herein, we pre-assemble it with highly conductive substrates as graphite paper and aluminum foil, which have been used to enhance the conductivity of carbon electrode in PSCs [24,28], thus promoting this technique to match the practical square-centimeter scale planar structured PSCs. Due to the significantly reduced series resistance (Rs) in the whole PSCs, the PCE of large area device (1 cm2) with graphite extraction electrode can achieve 17.02%, which is about 92% of the highest PCE of 18.56% in small device (0.1 cm2). The large device with aluminum foil can also exhibit high PCE of 15.41%, and was compared with the former case. These results ob tained in square-centimeter scale devices are obviously better than other ones reported in literature and represent good prospect.
* Corresponding author. School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, PR China. E-mail address: [email protected] (J. Xiao). https://doi.org/10.1016/j.mssp.2019.104809 Received 3 July 2019; Received in revised form 24 October 2019; Accepted 28 October 2019 Available online 2 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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2. Results and discussion
during forward scan (FS) and reverse scan (RS) can be achieved since the perfect ohmic contact between carbon film and spiro-OMeTAD, which coincide with our previous research [21]. The steady-state output characteristics of best carbon electrode based device was also tested. As shown in Fig. 3a, under the maximum power point (MPP) of 950 mV, PSCs based on the connecting layer/ graphite electrode gave a steady-state current density of about 19 mA cm-2, yielding a steady-state efficiency of 18.05%, near to the J-V result. The external quantum efficiency (EQE) of this piece of solar cell was also measured to confirm the accuracy of the J-V measurement, as shown in Fig. 3b. The integrated current density was 20.48 mA cm-2, which is in good agreement with the experimentally obtained Jsc. This over 18% PCE is better than our previous result in planar PSCs (17.5%) [21], due to the optimized carbon electrode and SnO2/CsFAMA mixed perovskite system. The preliminary stability tests have been further carried out. As shown in Fig. 3c, PSCs based on different electrodes were stored in ambient atmosphere under open circuit and dark condition without any encapsulation (Temperature: 20 � C; Humidity: 30%). They were measured every two days for 1500 h under one sun illumination. Finally, PSCs based on the connecting layer/graphite electrode can retain 90.5% of its initial efficiency, indicating relatively better stability than Au electrode based PSCs (82.3%). In previous researches, carbon electrode and novel carbon based additives, such as carbon quantum dots, gra phene and graphdiyne, can improve the stability of perovskite devices due to the ions diffusion resistance and hydrophobic property of carbon films [21,36–38]. However, the connecting layer/aluminum foil elec trode based devices show an even worse stability, with only 64.2% of the initial PSC remained, which may cause by unsatisfactory long term mechanical connection property. From the statistical results in Table 1, the average PCE of PSCs based on Au electrode is 18.50%. And the average value of the PSCs based on the connecting layer/graphite electrode is 17.40%, which is close to the former with sound reproducibility. With aluminum foil in place of graphite paper, lower PCE values (15.38% on average) and worse reproducibility. Although the active area is small, bare carbon film based extraction electrode still bring about extra series resistance, which lead to the large Rs over 10 Ω cm2, thus yielding average PCE of 12.16% with poor FF. Apparently, pre-assembling with graphite paper and aluminum foil can benefit the carbon film electrode and PSCs device performance. However, compared with graphite paper based electrode, aluminum foil based electrode exhibit unsatisfactory reproducibility and stability. The average Jsc value of connecting layer/aluminum foil devices (19.72 mA cm-2) is even lower than that of bare connecting layer device (20.63 mA cm-2). So it is important to figure out the differences between these two sorts of electrode. In reality during our experiments, some different phenomena can be observed in different electrodes. After the preparation of the connecting layer/aluminum foil electrode, the carbon film deposited on the aluminum foil is easily loosened or even peeled off. This issue may due to the smooth surface of the aluminum foil and the incompatibility be tween aluminum foil and the resin in carbon paste. The poor stability performance of corresponding PSCs can be at least partially caused by this problem. Moreover, irreversible creases can be hardly avoided in aluminum foil substrate during the preparation, as shown in Fig. 1b. These creases may cause the ununiformity of carbon film coated on aluminum foil, thus resulting in poor reproducibility. Cross-sectional SEM images of various carbon based electrode before and after press transfer were shown in Fig. 4 for detailed investigation. Comparing the samples in Fig. 4a, b, e and f, we can see that the thickness of the connecting layer carbon film decreases by about 15 μm and 17 μm respectively after pressing, which proves that the incom pressibility of aluminum foil and the good compressibility of the con necting layer carbon film. ComparingFig. 4c and d, it can be seen that the thickness of the connecting layer/graphite electrode decreases by
In previous work, we have developed a self-adhesive macroporous carbon film via solvent exchange process [21]. However, when trying to apply this carbon electrode onto larger device, we faced two difficulties. The first problem was maintaining the mechanical integrity of large area self-supporting carbon film within the whole fabrication process. And the second one was the unnegligible high resistance in large area device caused by carbon film. Therefore, combining the self-adhesive carbon with highly conductive substrate is a natural idea. In this research, we name the individual or combined self-adhesive macroporous carbon film as connecting layer. Low cost commercial graphite paper and aluminum foil are selected as extraction layer for the double layer composite electrode, which are both highly conductive (sheet resistance < 1 Ω sq-1) and stable under organic solvent. Fig. 1a and Fig. 1b show the prepared connecting layer/graphite electrode and the connecting layer/aluminum foil electrode, respec tively. Fig. 1c presents the schematic diagram for the preparation pro cess of the double layer composite carbon film electrode. The commercial carbon paste was blade coated directly on graphite paper or aluminum foil to form a wet film, and then transferred together into ethanol for solvent-exchange process. Composite carbon film was ob tained after removed from ethanol and dried in air. Planar structured PSCs with SnO2 as electron selective layer and mixed cation perovskite absorber have been proved to be a well per formed system, and can be fabricated via low temperature process [29–34]. Through pressure transfer method, we prepared connecting layer/aluminum foil electrode, connecting layer/graphite electrode and bare connecting layer carbon film electrode onto the top of FTO/S nO2/Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3/spiro-OMeTAD substrates to complete the small area PSCs (0.1 cm2) [35]. The photovoltaic characteristics (J-V curves) of champion devices with various top elec trodes are shown in Fig. 2. More specifically, champion and statistical photovoltaic parameters are summarized in Table 1. As standard com parison, the highest efficiency of PSCs based on Au electrodes is 19.04%, with short-circuit current (Jsc) of 21.98 mA cm-2, open-circuit voltage (Voc) of 1.110 V and fill factor (FF) of 0.78, respectively. For PSCs based on connecting layer/graphite electrode, a high PCE up to 18.56% has been achieved with Jsc of 21.11 mA cm-2, Voc of 1.117 V and FF of 0.79. The highest efficiency of the connecting layer/aluminum foil electrode based PSCs is slightly lower than the connecting layer/graphite elec trode, which is 17.15%, with Jsc of 20.91 mA cm-2, Voc of 1.090 V and FF of 0.75. Bare connecting layer based electrode exhibits relatively poor performance, of which the best result is PCE 14.93%, Jsc 20.11 mA cm-2, Voc 1.104 V and FF 0.67. In Fig. 2, connecting layer/graphite and con necting layer/aluminum foil based devices exhibit slightly lower Jsc value than standard Au device. Besides, negligible hysteresis behavior
Fig. 1. (a) Photograph of graphite paper and connecting layer/graphite elec trode; (b) Photograph of connecting layer/aluminum foil electrode; (c) Sche matic diagram of the solvent-exchange preparation process of double layer composite carbon film electrode. 2
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Fig. 2. J-V curves of PSCs with (a) connecting layer/graphite electrode, (b) connecting layer/aluminum foil electrode, (c) connecting layer carbon film electrode, (d) Au electrode. Table 1 Statistical photovoltaic parameters of different electrodes based 0.1 cm2 PSCs. Electrode type
Voc (V)
Jsc (mA cm-2)
FF
PCE (%)
Rs (Ω cm2)
connecting layer/graphite
champion average
1.117 1.099�0.022
21.11 20.62�0.51
0.79 0.76�0.04
18.56 17.40�1.16
3.58 4.05�0.77
connecting layer/aluminum foil
champion average
1.090 1.090�0.042
20.91 19.72�1.33
0.75 0.70�0.06
17.15 15.38�1.77
4.07 7.90�3.83
connecting layer
champion average
1.086 1.016�0.070
20.16 20.63�0.72
0.65 0.58�0.07
14.29 12.16�2.13
11.39 16.19�4.80
Au
champion average
1.110 1.090�0.015
21.98 21.65�0.41
0.78 0.76�0.03
19.04 18.50�0.50
3.57 3.95�0.41
Fig. 3. (a) The steady-state output current and calculated PCE at 950 mV bias. (b) External quantum efficiency (EQE) spectra and integrated current density. (c) Stability test kept in ambient atmosphere without any encapsulation.
about 41 μm, this major change of thickness is mainly due to the compressibility of graphite paper. In addition, it can be seen that the connecting layer carbon film has good interface contact with the graphite paper before and after hot pressing, which cannot be distin guished from the cross-sectional view. This combination between con necting layer carbon film and graphite paper cannot be separated by a thin blade without damage. So it can be inferred that the two layers are embedded to each other to some extent. By contrast, as given in Fig. 4g, sometimes visible gap between the connecting layer carbon film and the aluminum foil can be observed. The charge transfer may be greatly
hindered due to the poor interface contact between the connecting layer carbon film and the aluminum foil. Further more, the liner surface roughness of three carbon electrodes before and after pressing were measured with a step profiler, as shown in Fig. 5. Before the pressing, the three carbon electrodes have extremely rough surfaces due to relative crude processing technique and the porous nature of the connecting layer carbon film. Within 2.5 mm length, the undulation range can even exceed 20 μm, far greater than the total thickness of SnO2/perovskite/spiro-OMeTAD functional multilayer (less than 1 μm) in device. From the changes after pressing, we can see 3
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Fig. 4. The cross-sectional SEM images of the bare connecting layer carbon film (a) before and (b) after press. The cross-sectional SEM images of the connecting layer/graphite (c) before and (d) after press. The cross-sectional SEM images of the connecting layer/aluminum foil before (e) and (f) after press. (g) The interface contact between the connecting layer and the aluminum foil.
Fig. 5. (a) The roughness of the connecting layer/graphite before and after press. (b) The roughness of the connecting layer/aluminum foil before and after press. (c) The roughness of the connecting layer before and after press.
that the liner surface roughness of the connecting layer carbon film is significantly reduced after pressing, indicating that the porous structure in the carbon film become dense. Compared with bare connecting layer
sample, connecting layer/graphite exhibits flatter surface since graphite paper provides extra compressibility, which can contribute to the better contact with underlayer in device. However, as shown in Fig. 5b, the
Fig. 6. J-V curves of 1 cm2 PSCs with (a) connecting layer/graphite electrode, (b) connecting layer/aluminum foil electrode, (c) connecting layer carbon film electrode. (d) Photographs of 1 cm2 devices based on different electrodes. 4
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surface of connecting layer/aluminum foil sample is very smooth but not flat enough. The surface undulation still exceeds 10 μm, which can be due to the plasticity and elasticity of aluminum foil. In the preparation process of the connecting layer/aluminum foil electrode, the planeness of aluminum foil can be easily changed due to plastic deformation. But it cannot be well controlled because of the elastic nature. So in the final device, the gap between the aluminum foil and the HTL may be unful filled with carbon film, which not only hinders the transport of charges but also leaves plenty voids for ambient air. Eventually, poorer repro ducibility and stability in connecting layer/aluminum foil electrode based devices will thus happen. To examine the practicability of the composite electrode, devices with 1 cm2 active area were prepared, as shown in Fig. 6d. Through the J-V curves in Fig. 6 and photovoltaic parameters in Table 2, similar re sults with the former small devices can be found. The average PCE of 1 cm2 PSCs based on connecting layer/graphite electrode can be 14.79%, and the best value is 17.02%, with Jsc of 20.94 mA cm-2, Voc of 1.130 V, FF of 0.72 and Rs of 4.79 Ω cm2. And for the connecting layer/ aluminum foil electrode based one, the highest is 15.41% with Jsc of 20.56 mA cm-2, Voc of 1.117 V, FF of 0.67 and Rs of 10.70 Ω cm2. These two best results maintain 92% and 90% the PCEs of the values in small area devices, and should be among the absolute high level of carbon electrode based PSCs (see Table S1 in Supplementary Information). What’s more, we tried the connecting layer/graphite electrode in a 5 cm � 5 cm perovskite series module, and achieved a PCE of 7.49% without much optimization on the module technique (see Fig. S1 in Supplementary Information). Considering the conductivity of graphite paper, the increased Rs should be attributed to the larger FTO area. And in the case of aluminum foil based device, FTO and further deteriorating electrical contact can be both the reasons. The poor carrier transporting even results in neglectable hysteresis behavior in Fig. 6b. Under negative control condition, the best large PSCs based on bare carbon film elec trode has only 6.06% PCE, 14.36 mA cm-2 Jsc and 0.38 FF. The great Rs value up to 22.59 Ω cm2 will affect not only FF but even photocurrent density significantly in this high efficiency perovskite system, thus leading to PCE less than 50% the value in small device. Finally, to emphasize the cost advantage of our composite carbon electrode, we give a brief calculation as follow. Since the device per formances are similar with our carbon based electrode and standard Au electrode, we can compare the materials cost for the two sorts of devices with a same total active area. In our experiment, 120 g carbon paste (commercially available, about $27 per kilogram) is needed for 1 m2 carbon film, and the cost of graphite paper is about $4 per square-meter. Thus the total materials cost of the composite carbon electrode should be $7.2 per square-meter. While in the case of Au electrode, assuming the utilization rate of gold to be 100%, the materials cost can be easily calculated as about $100 per square-meter, which is 14 times higher than carbon electrode. Moreover, the blade coating and hot-press equipment for our composite carbon electrode have obviously lower cost than the vacuum evaporation equipment for Au electrode.
electrodes exhibit improved performance in perovskite solar cells. Especially for the carbon film electrode composited with graphite paper, high PCEs of 18.56% and 17.02% were achieved in 0.1 cm2 and 1 cm2 area devices respectively, which are the best results in carbon electrode based planar PSCs. Within pressure transfer process, graphite paper substrate exhibits soft and compressible nature, which can guarantee desired smooth and flat surface for ideal interface contact. In contrast, aluminum foil exhibits uncontrollable plasticity and elasticity, thus affect the PCE, stability and reproducibility of the corresponding de vices. Therefore, our results may offer a solution for practical large area carbon based PSCs. 4. Experimental section Materials: The SnCl2‧2H2O was obtained from Aladdin. For mamidinium iodide (FAI) and methylammonium bromine (MABr) were purchased from Lumtec, Taiwan. Lead iodide (PbI2), lead bromine (PbBr2) and organic HTM Spiro-OMeTAD were purchased from Xi’an Polymer Light Technology Corp. CsI and KI were purchased from SigmaAldrich. Carbon paste was purchased from LinchWin Printing Material Co., Ltd. All the chemicals were used directly as received. Carbon Film Based Electrodes Preparation: Bare carbon film was pre pared according to the literature [21]. And the composite electrodes were prepared with even easier process as follow. Firstly, the commer cial conductive-carbon paste was coated on conductive substrates (graphite paper and aluminum foil) by simple doctor-blading. Then, the substrates with wet carbon film were immersed in ethanol to exchange solvent at room temperature for 10 min. Finally, the substrates with ethanol stained carbon film was taken out and dried to form carbon based double-layer composite electrodes. Device Fabrication: The FTO substrates were first patterned by femtosecond laser, then cleaned through detergent, pure water and ethanol under ultrasonic for 20 min, respectively. The substrates were then dried with dry-air gas flow and then treated by UV-ozone for 15 min before use. Then compact SnO2 film was grown by chemical bath deposition on FTO substrates directly as electron selective layer. The precursor solution of CsFAMA mixed perovskite was prepared by dis solving 1.4 M mixture of metal lead salts composed of 0.85 portion PbI2 (548.6 mg) and 0.15 portion PbBr2 (77.07 mg), and 1.3 M organic cation composed of 0.85 portion FAI (190.12 mg) and 0.15 portion MABr (21.84 mg) in 0.8 ml mixture solvent of DMF/DMSO (4:1, by volume), with the addition of 34 μl CsI solution (pre-dissolved as a 2 M stock so lution in DMSO). This precursor solution was dropped onto SnO2 layer and spin-coated at a rate of 6000 rpm for 30 s. Then a certain amount of green anti-solvent ethyl acetate was poured onto the spinning substrate at the last 5 s. Right after this, the film was annealed at 120 � C for 45 min to achieved the desired Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 perov skite layer. Then Spiro-OMeTAD solution doped with Li-TFSI, FK209 and 4-tert-butylpyridine was spin-coated on the perovskite films at 3000 rpm for 30 s to form hole-transporting layer. Finally, for Au de vices, 80 nm of gold was evaporated on the top of Spiro-OMeTAD layer as the top electrode; for carbon electrode based devices, the film elec trodes were directly pressed onto Spiro-OMeTAD layer under a pressure of 0.8 MPa at 50 � C for 20 s via a pneumatic hot-press. Materials and Devices Characterizations: The microscopic morphology
3. Conclusion Pre-assembled with highly conductive graphite paper and aluminum foil to form double layer composite structures, the carbon film based Table 2 Statistical photovoltaic parameters of different electrode based 1 cm2 PSCs. Voc (V)
Jsc (mA cm-2)
FF
PCE (%)
Rs (Ω cm2)
connecting layer/graphite
champion average
1.130 1.106�0.025
20.94 19.58�1.18
0.72 0.66�0.06
17.02 14.79�2.17
4.79 9.73�4.51
connecting layer/aluminum foil
champion average
1.117 1.121�0.063
20.56 18.17�2.14
0.67 0.60�0.08
15.41 13.20�2.21
10.70 16.09�5.74
connecting layer
champion average
1.105 1.032�0.073
14.36 15.45�2.27
0.38 0.33�0.05
6.06 4.89�1.17
22.59 44.50�22.50
5
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were investigated by a field emission scanning electron microscopy (FESEM, Zeiss Ultra Plus). The photocurrent density-voltage (J-V) curves were carried out by using a solar simulator (Oriel 94023A, 300 W) with a source meter (Keithley2400) under 100 mW cm 2 illumination (AM 1.5G) with a scan rate of 10 mV s-1. An external quantum efficiency (EQE) measurement system (QEX10, PV Measurements, Inc.) was used to measure the EQE of the devices across a wavelength range of 300–850 nm.
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