Advanced Materials Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells --Manuscript Draft-Manuscript Number:
adma.201505140R1
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Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells
Article Type:
Communication
Section/Category: Keywords:
perovskite, passivation, charge collection, Al2O3, PbI2.
Corresponding Author:
Mohammad Khaja Nazeeruddin, Dr. Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology Lausanne, SWITZERLAND
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Please submit a plain text version of your Dear Prof. Gregory, cover letter here. We are submitting our manuscript entitled “Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells” to be considered for publication in If you are submitting a revision of your Advanced Materials as communication. We consider that our work to be of immense manuscript, please do not overwrite your interest to the readers of Advanced Materials and in particular perovskite photovoltaic original cover letter. There is an community for the following reasons. opportunity for you to provide your Upsurge in development of efficient perovskite solar cells commenced with pure responses to the reviewers later; please CH3NH3PbI3 but record-high efficiencies have been realized in devices fabricated do not add them here. using mixed organic-inorganic lead halide perovskites. Several studies have shown compositional engineering or alloying of the organic-inorganic lead halide perovskites to harness the solar energy more efficiently. However, optimization of interface engineering is crucial to enhance the performance of the mixed organic-inorganic lead halide perovskite solar cell efficiency. In this work, we investigated passivation effect in the perovskite solar cells. We found that the use of mesoporous electron transport layer (ETL) is better for charge separation owing to the increased surface area. The Al2O3 passivation layer was found to be beneficial for the mp-TiO2 based cells, but should be used selectively only on the compact-TiO2 surface. Improved FF and Jsc were observed from the surface passivation yielding power conversion efficiency of 18.64% under 1 sun. The data provide information that charge separation and charge transport in the interface can be managed separately in perovskite solar cells. We believe our finding shows that further enhancement in perovskite solar cell efficiency can be achieved from the controlled surface passivations, which would contribute towards the fundamental understanding of surface passivation, charge separation and charge transport at the interface. Thank you very much for your kind consideration. I shall look forward to hear from you. Please see a list of possible referees for our manuscript; 1.Prof. Lars Kloo, Royal Institute of Technology(KTH), Sweden.
[email protected],+46-8790 9343. 2.Dr. Henk Bolink, Instituto de Ciencia Molecular, Universidad de Valencia, C/ Catadratico J. Beltran nr 2 Paterna, Valencia 46980, Spain. Email:
[email protected] 3.Dr. Emilio Palomares, Group Leader & ICREA Research Professor, Catalan Institution for Research and Advanced Studies (ICREA) & Institute of Chemical Research of Catalonia (ICIQ) The Barcelona Institute of Science and Technology ( BIST), Av. Països Catalans 16 – 43007 Tarragona (Spain), Phone +34 977920200 (Ext. 241) – Fax +34 977920223,
[email protected] 4.Prof. Shuzi Hayase Kyushu Institute of Technology, Graduate School of Life Science and Systems Engineering, Hibikino, Wakamatsuku Kitakyushu 808-0196. Email:
[email protected] 5.Prof. Hiroshi Segawa, The University of Tokyo, Japan.
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tokyo.ac.jp, +81-3-5452-5295 We would be grateful if you could avert sending our manuscript to Prof. Nam-Gyu Park, and Prof. Sang Il Seok, owing to the conflicts of interest. Kind regards, Md. K. Nazeeruddin Corresponding Author Secondary Information: Corresponding Author's Institution:
Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology
Corresponding Author's Secondary Institution: First Author:
Mohammad Khaja Nazeeruddin, Dr.
First Author Secondary Information: Order of Authors:
Mohammad Khaja Nazeeruddin, Dr. Yong Hui Lee Jingshan Luo Min-Kyu Son Peng Gao Kyung Taek Cho Jiyoun Seo Michael Graetzel Shaik M Zakeeruddin
Order of Authors Secondary Information: Abstract:
The Al2O3 passivation layer was found to be beneficial for the mp-TiO2 based cells, but should be used selectively, only on the c-TiO2 surface. We also found such a passivation layer can be formed on the perovskite layer during thermal annealing. The PbI2-rich layer was proven to suppress surface recombinations yielding power conversion efficiency 18.64% under 1 sun.
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DOI: 10.1002/((please add manuscript number)) Article type: Communication
Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells Yong Hui Lee, 1,* Jingshan Luo,2 Min-Kyu Son, 2 Peng Gao,1 Kyung Taek Cho,1 Jiyoun Seo, 2 Shaik M. Zakeeruddin,2 Michael Grätzel,2 and Mohammad Khaja Nazeeruddin1,* Dr. Y. H. Lee, Dr. P. Gao, K. T. Cho, Prof. M. K. Nazeeruddin 1 Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland E-mail:
[email protected],
[email protected] Dr. J. Luo, Dr. M.-K. Son, J. Seo, Dr. S. M. Zakeeruddin, Prof. M. Grätzel 2 Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland Keywords: perovskite, passivation, charge collection, Al2O3, PbI2
The organic-inorganic metal halide based perovskite solar cell is of primary interest due to its superior performance compared to dye-sensitized solar cells (DSCs),[1] and has achieved a power conversion efficiency (PCE) over 20%.[2] Since Kojima’s work,[3] various perovskite materials, deposition methods and charge carrier dynamics of the cells have been investigated to realize high PCEs.[4-15] Perovskite solar cells basically adopt a similar device structure of solid-state DSCs composed of FTO/compact-TiO2 (c-TiO2)/mesoporous-TiO2 (mp-TiO2)/Dye/hole transport layer (HTM)/Au.[14] Although there have been several studies to find alternative structures,[6,8,12] the highest efficiency is still from the mp-TiO2 based device structure.[2] In this structure, it is known that the surface passivation of primary metal oxide electrodes with secondary metal oxides suppresses the interfacial recombination.[16] Several metal oxides such as ZnO, ZrO2, MgO, Al2O3, Nb2O5 and Y2O3 were explored as efficient capping materials in the liquid state or solid-state DSCs.[16,17] However, it might be difficult to apply such concept directly to the perovskite solar cells because of new features of perovskite. Unlike mono-layer dyes, the perovskite layer has a volume which makes the device less dependent on the surface area of the electron transport layer (ETL). Actually, high efficiencies are often obtained in a 1
planar type cell.[18] Secondly, perovskite functions as hole transporter, absorber and electron 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
transporter while dyes form a monolayer and play only as a sensitizer that requires additional electrolytes or HTMs.[6] The difference in junctions and materials indicates that results of the intended treatment can be different although the same materials were similarly used. Therefore, it is necessary to study the influence of such surface treatments in a certain structure of the perovskite solar cells before extending their applications. Here we introduced a thin Al2O3 layer on TiO2 by the atomic layer deposition (ALD) process,[19] and investigated its passivation effect in the perovskite solar cells. In comparison of two different cells with different perovskite layers, we found that thermal heating also generates a passivating region in the perovskite layer which results in increase of open-circuit voltage (Voc). Al2O3 is one of the most commonly used surface passivation materials on TiO2 in DSCs. As an insulator of ca. 8.8 eV bandgap (for bulk, crystalline), it is not suitable for charge transfer from dyes to TiO2 if it is conformally coated on the TiO2 surface.[20] Thus it was believed it can work efficiently only by the tunneling effect when it is extremely thin. Conventionally, sol-gel[21,22] and ALD[23] methods were usually used to get such an extremely thin film, and ALD was particularly favored for the mesoporous structure. ALD is based on self-limiting surface reactions by sequential exposure of the substrate to different gas phase precursors, enabling precise thickness control at the angstrom or monolayer level and deposition on high aspect ratio nanostructures with excellent step coverage.[24] Figure 1 depicts a schematic of the typical ALD processes for the deposition of Al2O3 thin film. During the deposition, two different gas phase molecules of Al(CH3)3 (trimethyl aluminium, TMA) and H2O are alternately exposed to a substrate in a repeated A-pulse-purge-B-pulse-purge sequence.[19] It is known that as-deposited film in a low temperature process has an amorphous property (Eg: ~6.2 eV),[25] which can be converted to crystalline phase after postsintering. During the sintering step, the as-deposited film comes to undergo shrinkage in its volume to be converted to crystalline phase that results in isolated island morphology of the 2
film.[26] We expect this kind of partially pored Al2O3 layer gives a passivation effect to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
suppress the surface recombination. With the increased number of ALD cycles, the partial pores will be filled up to form a thick and dense Al2O3 layer on the substrate. However, such a dense layer is expected to be unsuitable for the device since it can completely block the charge transfer from the perovskite to TiO2. Thus the first step should be finding the optimal thickness of the Al2O3 layer. In the previous DSC works, the coating of Al2O3 was usually done on the mesoporous metal oxide electrodes including a FTO or FTO/bl-TiO2 substrate.[16,17,21-23] Kay et al. reported enhanced PCEs with coating of thin layers of Al2O3 (0.16 nm).[16] Zhang et al. reported that only 1-cycle coating (ca. 0.19 nm) of Al2O3 layer showed the highest improvement in PCE.[17] These studies clearly show that an extremely thin layer is required for the passivation effect. We varied the number of ALD cycles to get different thickness and coverage of the Al2O3 layer. Cyclic Voltammetry (CV) technique was used to get the current density by dividing the current value with the area of the anode. So in this case, that the current density is inversely proportional to the resistance of the interface.[26] According to the results shown in Figure 2a, it is found that c-TiO2 layer (20 nm-thick) prepared by spray pyrolysis deposition (SPD) and Al2O3 layer (5 nm-thick) by ALD completely block the charge transfer from FTO to the electrolyte, which indicates this film is pin-hole free. On the other hand, films show a certain increase of current as the thickness of Al2O3 decreases, and it reaches the maximum value with a bare FTO film. Figure 2b visually shows the SEM crosssectional image of the Al2O3 coated film on the FTO with 60 ALD cycles. We found ca. 5 nm-think Al2O3 layer is uniformly deposited on the FTO substrate. Difference in the surface morphology between a bare FTO and an Al2O3 coated FTO is compared in Figure S1, Supporting Information. It is still not clear if this conductivity is generated by tunneling or from pin-holes in the film. However, assuming the shrinkage during crystallization in the sintering process, we put the greater possibility on the later. 3
A SEM cross-sectional image of the typical perovskite solar cell adopting FTO/c1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
TiO2/ALD-Al2O3 (A-Al2O3)/mp-TiO2/CH3NH3PbI3 (MAPbI3)/PTAA/Au is shown in Figure 3a. Figure 3b demonstrates possible heterojunctions in this device. It is reported that MAPbI3 could have an ambipolar,[27] or intrinsic,[28] or n/p types[29] characteristics by the difference in composition, deposition method and post-heating condition. We used a stoichiometric composition for MAPbI3, and the device is a n-i-p structure.[28] In our preceding test, it was found that charge separation mainly takes over in the interface of TiO2/perovskite rather than that of MAPbI3/HTM. Thus, as a preliminary, we differently introduced the Al2O3 layers only on I1 surface and both I1 and I2 surfaces, and investigated the difference. The optimal number of cycles was confirmed as 6 (Figure S2, Supporting Information). It was confirmed that such a thin coating did not influence the light absorption as presented in Figure S3, Supporting Information. Firstly, two surfaces of I1 and I2 were passivated with Al2O3 simultaneously, which is expected to prevent a direct contact of HTM with TiO2 besides the surface passivation. However, we found that this is not beneficial for the device performance, but greatly decreases short-circuit current density (Jsc), fill factor (FF) and Voc even with one cycle of the Al2O3 coating. This is in contrary to the previous results studied in the DSCs,[17,21] but could be explained as the different combining mechanism onto TiO2 of dyes and the perovskite. Secondly, we coated Al2O3 on a planar substrate of FTO/c-TiO2 which allows us a simpler comparison. In the J-V measurement, the increase of Voc is shown, but the PCE decreased dramatically compared to the reference cell (with FTO/c-TiO2 substrate) owing to the decreased Jsc and FF. This result is a good agreement with the previous study on the surface passivation in DSCs.[22] We note that the difference of wettability between Al2O3 and TiO2 surfaces could result in the difference of the device performance. However, in comparison of c-TiO2 and c-TiO2/mp-TiO2 films, it is clearly demonstrated that difference of TiO2 surface area strongly influences on the cell efficiency.[30-33] Deficient electron quenching causes strong charge recombination and it would be reflected to PL emission. As known, 4
Al2O3 is an insulator, thus the increased coverage of Al2O3 with increased ALD cycles means 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
decrease of TiO2 surface. We can see the gradual change of current through CV data measured with an aqueous electrolyte, which proves the observed data are less influenced by wettability. The trend shown in CV measurement corresponds with the device performance shown in Figure S2. We found AlOx films show a hydrophobic property, but amorphous films are converted to Al2O3 after baking at 500 oC. Now we confirm again that securing the sufficient contact surface of TiO2 is the key to enable the efficient charge separation as mentioned in the previous reports.[30-33] Consequently, use of mp-TiO2 layer is one of the best solutions for the high efficiency MAPbI3 based solar cells. Then, we find quite interesting behavior of the device when we applied the Al2O3 layer only on the c-TiO2 layer with the intact mp-TiO2 surface. The highest PCE was obtained by the increased FF and Voc after the surface passivation. Comparison of photoluminescence (PL) emission spectra in Figure 3d clearly supports the variation of J-V curves displayed in Figure 3c. Distinguished improvement in J-V curve hysteresis was not observed with the Al2O3 passivation (Figure S4, Supporting Information) because hysteresis is known to occur mainly due to the deficient charge transfer rate in the perovskite/TiO2 interface.[30,32,33] Therefore the role of the surface passivation could be confined just to minimize charge recombination between c-TiO2 and mpTiO2 or perovskite. We believe this is the first demonstration that charge separation and collection could be managed independently in the perovskite solar cells. And it should be noted that this is the most successful result with the high efficiency devices while previous works remained in low efficiencies. Formamidinium lead iodide (FAPbI3) is another potential light absorber with narrower bandgap of 1.48 eV.[7,34-36] Recently Jeon et al reported that mixing 15 mol % of MAPbBr3 greatly stabilizes the FAPbI3 in perovskite phase, enabling to achieve the certified 17.91% of PCE.[37] We found the trend shown in MAPbI3 with the Al2O3 passivation is similarly reproduced in FAPbI3-MAPbBr3 cells as shown in Table 1 and Figure S5, Supporting 5
Information. According to the SEM cross-sectional image in Figure 4a, use of the mixed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
perovskite seems quite successful to form a dense layer. A thick and pin-hole free perovskite layer is formed inside and over the mp-TiO2 layer by the assistance of chlorobenzene dropping. This is expected to show superior absorption by the sufficient absorber thickness. Compared with MAPbI3, improved absorbance and quantum efficiency are shown in Figure 4b. For IPCE, slightly lower calculated Jsc are observed for both cells, but they reasonably match with measured values shown in J-V curves (Figure 4c). It is quite interesting that these two perovskites have almost same bandgaps which was resulted by the tuned bandgap of FAPbI3 with MAPbBr3 as verified by absorbance and incident photon to current conversion efficiency (IPCE) spectra in Figure 4b.[9,37,38] Nevertheless, they surely show different Voc values as displayed in Figure 4c. Considering the better absorption and longer carrier diffusion length, the higher current density and longer lifetime (Figure 4d) are expected from FAPbI3-MAPbBr3. However, it should be answered why the FAPbI3-MAPbBr3 shows a higher Voc. On the Voc enhancement, there have been some reports explaining that the increased grain (or crystal) size and thickness of the perovskite layer enhanced Voc. However, it is necessary to clarify, according to SEM images shown in several papers,[39-42] that the enhancement of Voc is dominantly from the enhanced surface coverage and improved charge collection by optimizing the thickness of the perovskite layer, rather than the increased grain size. Moreover, we find those of the measured Voc data usually remain in low values of below 1.0 V, which make it difficult to see if it really enhances Voc with the certain processing. On the morphological aspect of the perovskite layer, solvent dropping method is thought to be an excellent way to get conformal and pin-hole free films. (Figure 3b, Figure 4a) Both perovskite layers show very nice coverage, which prove the observed difference of Voc is less associated with the surface coverage. By comparing SEM morphologies shown in the recent papers,[2,3942]
we exclude the thickness effect out of our consideration as it is rather related to the
absorption. It is also because the recombination by the increased series resistance, affecting 6
the Voc value, is proportional to the thickness of the practical layer.[30] Instead, we carefully 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
conclude such difference in Voc is strongly associated with the surface passivation by PbI2. By comparing X-ray diffraction (XRD) data of the MAPbI3 and FAPbI3-MAPbBr3 as shown in Figure 5a, we confirm our presumption is quite acceptable. Normally, perovskite layers require a post-annealing at a mild temperature to remove residual solvents and complete the conversion from the precursors. In case of MAPbI3, we dried the perovskite film for 10 min at 100 °C. For FAPbI3-MAPbBr3, however, we found it requires longer postannealing time to reach high efficiencies. According to XRD spectra, FAPbI3-MAPbBr3 layer shows a high crystallinity perovskite phase even in a short time drying of 2 min while we find a poor device performance with this film. At the longer annealing, we find that the peak intensities of PbI2 and δ-FAPbI3 negelegible at the short annealing time become distinctly visible. These minor peaks might be from the imbalance of the precursor composition and difficulty in stabilizing yellow phase FAPbI3 to black one.[36,37] We also find those products do not make a compound with perovskite, but exist as separated phases. The detailed explanation requires further study, but the improved PCE, mainly by the increased Voc, is achieved at the prolonged annealing. Considering the annealing process, FTO glass faces a hot plate, and the loss of weak organic cations and anions would happen predominantly on the top surface I3 shown in Figure 3b. On this issue, Supasai et al. reported that obvious mass transfer (should be a sublimation of MAI) on the top of the perovskite layer was observed during mild thermal annealing.[29] Accordingly, the precipitate of PbI2 should happen mainly on the surface, which does not block the charge transfer completely just as shown in our previous work.[42] Chen et al. reported an in-depth study on the passivation effect of PbI2 on the perovskite films prepared by vapor assisted solution process (VASP).[43] In their work, conduction band minimum (CBM) and valence band maximum (VBM) of PbI2 measured by UPS are 3.45 and 5.75 eV, while MAPbI3 normally known to have 3.93 and 5.43 eV, respectively. This is expected to form energy barriers to prevent excitons for the surface 7
defects and traps states.[43] They also proposed that upward CB bending of perovskite by the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
interfacial PbI2 is beneficial to reduce carrier recombination. Wang et al. also found that increased amount of mixed PbI2 changes Fermi level of the MAPbI3 film upward.[44] Considering FAPbI3-MAPbI3 has a same bandgap edge position with MAPbI3,[38] our result is a good agreement with above reports. We attribute the role of PbI2 is quite similar with that of Al2O3 layer on TiO2 surface that enhances FF and Voc. In this case, the locally formed PbI2 layer plays a role to reduce charge recombination as Al2O3 does. Consequently, the film is likely to have a graded composition caused by loss of thermally weak cations and anions (mainly MABr) on the surface which might change the charge mobilites of the perovskite layer. The visualized intensity of the δ-FAPbI3 peak after longer heating is evidence of partial decomposition of the perovskite layer. We believe this kind of film engineering related to defect chemisty is more suitable for thicker perovskite layers like MAPbIxCl3-x and FAPbI3MAPbBr3 because longer heating causes larger loss of the absorber layer. Based on our understanding, we fabricated perovskite solar cells comprised of FTO/AAl2O3/mp-TiO2/FAPbI3-MAPbBr3/PTAA/Au. (Figure 6) It is notable that this device has a similar structure of bulk heterojunction organic photo voltaics (BHJOPVs). By assigning the role of charge separatioin to mp-TiO2, and passivating the FTO surface with Al2O3, we fabricated cells demonstrating 16.88% of PCE under one sunlight illumination. In summary, we investigated passivation effect in the perovskite solar cells. As light absorbers, MAPbI3 and FAPbI3-MAPbBr3 were prepared on c-TiO2 and mp-TiO2 substrates for the systemic comparison. In comparison of c-TiO2 and mp-TiO2, we found use of mesoporous TiO2 is better for charge separation owing to the increased surface area. The Al2O3 passivation layer was found to be beneficial for the mp-TiO2 based cells, but should be used selectively only on the c-TiO2 surface. Improved FF and Jsc were observed from the surface passivation. The result provides us information that charge separation and charge transport in the interface can be managed separately in perovskite solar cells. We also found 8
such passivation layer can be formed on the peorvksite layer itself during thermal annealing. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
The PbI2-rich perovskite layer by the loss of organic cations and anions was proven to act as the passivation layer suppressing surface recombinations, thus enhance and Voc. We believe our finding shows that further enhancement in perovskite solar cell efficiency can be achieved from the contorlled surface passivations.
Experimental Section Deposition of Al2O3. The AlOx layer was deposited by atomic layer deposition (ALD) system (Savannah 100, Cambridge Nanotech). High purity N2 served as the carrier gas with a flow rate of 5 sccm. The deposition was carried out at 120 °C in pulse mode using Trimethylaluminum (TMA) and H2O as the Al and O precursors, respectively. The pulse times for both TMA and water are 0.015 sec. For the crystallization of AlOx, films are annealed at 500 °C for 30 min. Film and Device fabrication. Chemically etched FTO glass (Nippon Sheet Glass) was cleaned with detergent solution, acetone and ethanol. To form a 20 nm-thick TiO2 blocking layer, diluted titanium diisopropoxide bis(acetylacetonate) (TAA) solution (Sigma-Aldrich) in ethanol was sprayed at 450 °C. Mesoporous-TiO2 layers were made by spin-coating a commercially available TiO2 paste (Dyesol 30NRD). Substrates are baked at 500 °C for 30 min before the deposition of the perovskite layer. MAPbI3 precursor solution was prepared by mixing 1.2 M of PbI2 and MAI in DMSO. FAPbI3-MAPbBr3 precursor was prepared by mixing 1.2 M PbI2, 1.1 M FAI, 0.2 M PbBr2, 0.2 M MABr in a mixed solvent of DMF : DMSO = 4 : 1 (volume ratio). Perovskite solutions are successively spin-coated on the ETL substrates at 1,000 rpm for 10 sec and 6,000 rpm for 30 sec, respectively. 100 μL of chlorobenzene[45] was dropped in 20 sec at 6,000 rpm. MAPbI3 and FAPbI3-MAPbBr3 films are annealed at 100 °C for 10 min and 30 min, respectively. The HTM solution was prepared by dissolving 10 mg of PTAA (Emindex) with additives in 1 mL of toluene. As additives, 7.5 9
μL of Li-bis(trifluoromethanesulphonyl) imide from the stock solution (170 mg in 1 mL of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
acetonitrile), and 4 μL of 4-tert-butylpyridine (TBP) were added. The HTM layer was formed by spin-coating the solution at 3,000 rpm for 20 sec, and followed by the deposition of the 60 nm-thick Au electrode by a thermal evaporation. All the preparative work to deposit perovskite and PTAA were done inside the drybox to minimize the influence of moisture.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The authors acknowledge funding from the FONDS NATIONAL SUISSE DE LA RECHERCHE SCIENTIFIQUE, NRP 70, grant N°: 407040_154056 / 1, and the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement n° 604032 of the MESO project, (FP7/2007-2013) ENERGY.2012.10.2.1; NANOMATCELL, grant agreement no. 308997. J. Luo would like to thank EPFL Fellowship co-funded by Marie Curie from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 291771. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) [1] B. O’Regan, M. Grätzel, Nature 1991, 353, 737. [2] W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Science 2015, 348, 1234. [3] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050. [4] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Grätzel, N.-G. Park, Sci. Rep. 2012, 2, 591. 10
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Figure 1. The schematic of the atomic layer deposition (ALD) of AlOx on a metal oxide (MO) substrate. An amorphous AlOx is converted to crystalline Al2O3 with post annealing which reduces the volume of the product layer.
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Figure 2. Cyclic voltammograms and a SEM cross-sectional image. a) CV with a bare FTO and FTOs coated with Al2O3 and TiO2. The measurement was carried out with a Pt wire as a reference electrode, scan rate of 20 mV s-1. The electrolyte solution was 0.5 mM K4Fe(CN)6 + 0.5 mM K3Fe(CN)6 in aqueous 0.5 M KCl, pH 2.5. b) A 5 nm-thick Al2O3 layer (60 ALD cycles) conformally deposited on the FTO substrate. The boundary of the layers is indicated with arrows.
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Figure 3. MAPbI3 solar cell with the Al2O3 passivation layer. a) A SEM cross-sectional image of the perovskite solar cell comprising FTO/c-TiO2/A-Al2O3/mpTiO2/MAPbI3/PTAA/Au. b) A schematic depicting heterojunctions in the mp-TiO2 based perovskite solar cells. c) J-V curves. d) Photoluminescence emission spectra of the MAPbI3 films (without PTAA) with different substrates. Reduced recombination is observed with mpTiO2 and A-Al2O3/mp-TiO2 substrates.
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Figure 4. Comparison of MAPbI3 and FAPbI3-MAPbBr3 solar cells. a) A SEM crosssectional image of the complete solar cell consisting of FTO/c-TiO2/A-Al2O3/mp-TiO2/ FAPbI3-MAPbBr3/PTAA/Au. b) IPCE and absorbance spectra of two different perovskite films. c) J-V curves under one sunlight and in the dark. d) PL decays of the perovskite films. (without PTAA)
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Figure 5. Formation of PbI2 on the perovskite film. a) XRD patterns of MAPbI3 and FAPbI3MAPbBr3. b) An energy bandgap diagram of the FAPbI3-MAPbBr3 solar cells. The bandgap of PbI2 generated by decomposition of perovskite during thermal annealing is depicted as a dash lined red box.
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Figure 6. Bulk heterojunction perovskite solar cells made of FTO/A-Al2O3/mp-TiO2/FAPbI3MAPbBr3/PTAA/Au.
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Table 1. Summarized PCEs of MAPbI3 and FAPbI3-MAPbBr3 solar cells with differently structured electron transport layers. Averaged values are obtained from 40 cells of 5 different batches. Devices are measured with a solar simulator under one sunlight intensity and with backward scanning. The scanning setting time is fixed as 200 ms for 10 mV. Jsc Perovskite
Voc
ETL
PCE FF
[mAcm-2]
[V]
c-TiO2
17.80 ± 1.97
0.70 ± 0.03
0.50 ± 0.02
6.23 ± 1.35
c-TiO2/mp-TiO2
20.63 ± 0.72
1.00 ± 0.01
0.74 ± 0.01
15.27 ± 0.87
c-TiO2/A-Al2O3
4.91 ± 0.21
0.78 ± 0.03
0.43 ± 0.04
1.64 ± 0.31
c-TiO2/A-Al2O3/mp-TiO2
21.44 ± 0.32
1.02 ± 0.01
0.77 ± 0.02
17.05 ± 0.63
c-TiO2
10.01 ± 2.58
1.01 ± 0.04
0.60 ± 0.05
6.01 ± 2.65
FAPbI3-
c-TiO2/mp-TiO2
21.76 ± 0.21
1.06 ± 0.01
0.69 ± 0.01
15.91 ± 0.54
MAPbBr3
c-TiO2/A-Al2O3
8.57 ± 1.86
1.02 ± 0.03
0.50 ± 0.07
4.37 ± 1.89
c-TiO2/A-Al2O3/mp-TiO2
22.42 ± 0.30
1.08 ± 0.01
0.74 ± 0.01
17.92 ± 0.72
[%]
MAPbI3
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Surface passivation effect in the perovskite solar cells is investigated. The Al2O3 passivation layer was found to be beneficial for the mp-TiO2 based cells, but should be used selectively, only on the c-TiO2 surface. We also found such a passivation layer can be formed on the perovskite layer during thermal annealing. The PbI2-rich layer was proven to suppress surface recombinations yielding power conversion efficiency 18.64% under 1 sun.
perovskite, passivation, charge collection, Al2O3, PbI2 Yong Hui Lee, 1,* Jingshan Luo,2 Min-Kyu Son, 2 Peng Gao,1 Kyung Taek Cho,1 Jiyoun Seo, 2 Shaik M. Zakeeruddin,2 Michael Grätzel,2 and Mohammad Khaja Nazeeruddin1,* Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells
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