and ZnO films on mixed halide perovskite - ACS Publications

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Atomic layer deposition of electron selective SnO and ZnO films on mixed halide perovskite: compatibility and performance Adam Hultqvist, Kerttu Aitola, Kári Sveinbjörnsson, Zahra Saki, Fredrik Larsson, Tobias Torndahl, Erik M. J. Johansson, Gerrit Boschloo, and Marika Edoff ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07627 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Atomic layer deposition of electron selective SnOx and ZnO films on mixed halide perovskite: compatibility and performance Adam Hultqvist†*, Kerttu Aitola‡, Kári Sveinbjörnsson‡, Zahra Saki‡ , Fredrik Larsson†, Tobias Törndahl†, Erik Johansson‡, Gerrit Boschloo‡ and Marika Edoff† †

Solid State Electronics, Department of Engineering Sciences, Uppsala University, 751 21

Uppsala, Sweden ‡

Physical Chemistry, Department of Chemistry – Ångström Laboratory, Uppsala University, 751

20 Uppsala, Sweden ⁑

Department of Physics, Sharif University of Technology, 145 88 Teheran, Iran

*

Corresponding authorCorresponding author e-mail: [email protected]

Keywords: Perovskite solar cell, atomic layer deposition, interfaces, electron selective layers, precursor chemistry

ABSTRACT

ACS Paragon Plus Environment

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The compatibility of atomic layer deposition directly onto FAPBI3:MAPbBr3 perovskite films is investigated by exposing the perovskite films to the full or partial atomic layer deposition processes for the electron selective layer candidates ZnO and SnOx. Exposing the samples to the heat, the vacuum and even the counter reactant of H2O of the atomic layer deposition processes does not appear to alter the perovskite films in terms of crystallinity, but the choice of metal precursor is found to be critical. The Zn precursor Zn(C2H5)2 either by itself or in combination with H2O during the ZnO ALD process is found to enhance the decomposition of the bulk of the perovskite film into PbI2 without even forming ZnO. In contrast the Sn precursor Sn(N(CH3)2)4 does not seem to degrade the bulk of the perovskite film and conformal SnOx films can successfully be grown on top of it using atomic layer deposition. Using this SnOx film as the electron selective layer in inverted perovskite solar cells results in a lower power conversion efficiency of 3.4 % than the 8.4 % for the reference devices using phenyl-C70-butyric acid methyl ester. However, the devices with SnOx show strong hysteresis and can be pushed to an efficiency of 7.8 % after biasing treatments. Still these cells lacks both open circuit voltage and fill factor compared to the references, especially when thicker SnOx films are used. Upon further investigation, a possible cause of these losses could be that the perovskite/SnOx interface is not ideal and more specifically found to be rich in Sn, O and halides, which is probably a result of the nucleation during the SnOx growth and which might introduce barriers or alter the band alignment for the transport of charge carriers.

1. INTRODUCTION Inorganic-organic perovskite solar cells (PSC) have recently showed a tremendous development in terms of performance reaching above 20 %.1-3 While the original device


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configuration is inspired by the solid state version of the dye-sensitized solar cells, it has recently become apparent that there is a lot to gain in terms of performance and stability by finding new stacks and materials that are better suited for the inorganic-organic perovskite absorber.4,5 The restrictions on finding new materials and deposition methods for the PSC is somewhat limited by the perovskite material itself as it is sensitive to heat and water exposure. Furthermore, it is known that the common hole-conducting material Spiro-OMeTAD and the Au counter electrode suffer from long term stability issues especially at elevated temperatures (of over 50 °C). Alternative contact materials such as the different oxides SnO2 and ZnO and NiOx have been therefore suggested. In more detail, when depositing layers directly on top of the perovskite it is imperative to not heat the material to temperatures of over 150 °C or expose it to water containing environments such as water vapor or water based solutions in order to keep it from decomposing.6 Furthermore, the perovskite crystal is quite soft and is thus susceptible to impeding kinetic energy from e.g. a sputtering process. The perovskite surface is also generally somewhat rough, which creates shadowing effects that reduce the film uniformity for direct path depositions as evaporation. Finally, even if the perovskite is kept from decomposing and the desired conformal covering layer is formed with a good quality on top of it, it is crucial that the newly formed interface has a high electronic quality to minimize recombination that could otherwise hamper the device performance. Atomic layer deposition (ALD) is in principle able to fulfill all of the requirements listed above with its potentially low temperature, relatively dry, soft, and conformal growth.7 In previous studies however it has been shown that the perovskite can decompose when exposed to the well-known ALD process for Al2O3 that uses Al(CH3)3 (TMA) and either H2O or O3 as the


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aluminum and oxygen precursors respectively.8,9 In one of those studies on a (CH3NH3)PbI3 (MAPbI3) perovskite a different process was developed to deposit Al2O3 using Al(OCH(CH3)2)3 and CH3COOH as the aluminum and oxygen precursors respectively. This new process did not decompose the MAPbI3 and the main conclusion was that the decomposition was avoided since the process no longer used H2O as the oxygen precursor. In contrast, in two Al2O3 was successfully deposited using TMA and H2O at 100 °C, both on MAPbI3 and on the Cl-modified version of it (CH3NH3)PbI3-δClδ.10,11 One of the studies did however notice that the perovskite surface had been altered during ALD.11 The same study found similar results when investigating other metal oxide ALD processes where the H2O pulse has been complemented with an oxygen plasma to lower the growth temperature.11 In this study we continue to investigate the possibility of depositing metal oxides by ALD directly on perovskite. To improve the chances of successful ALD we have chosen to deposit the ALD layers onto the mixed halide perovskite formamidinium lead iodide:methylammonium lead bromide (CH(NH2)2, CH3NH3)Pb(I,Br)3 (FAPbI3:MAPbBr3), which has previously been shown to be resilient to temperatures above 100 °C and to moderately high humidity levels.12 Our findings support this by showing that water vapor, heat, and vacuum exposure during partial ALD processes do not decompose the bulk of the perovskite film. In contrast, the choice of metal precursor is found to be crucial to avoid decomposition of the perovskite bulk. We have exemplified this by trying to grow two different transparent electron selective layer (ESL) candidates, ZnO and SnOx. These materials were specifically selected as they are interesting ESL candidates for perovskite top cells in tandem with Si or Cu(In,Ga)Se2 (CIGS) bottom cells, as their ALD processes are very similar in terms of pressure and temperature, and since they both use H2O as the counter reactant. The bulk of the perovskite film very clearly decomposes in the


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ZnO process despite of ALD being a surface reaction process. Upon further investigation it even decomposes when exposed to the Zn precursor Zn(C2H5)2 (DEZ) itself. In contrast, a fully covering and uniform SnOx film is grown on top of a seemingly unharmed bulk perovskite during the SnOx process, which uses Sn(N(CH3)2)4 (TDMASn) as the Sn precursor. To evaluate the performance of the SnOx ESL, perovskite solar cells using an inverted structure design were manufactured and compared to reference devices using phenyl-C70-butyric acid methyl ester (PC70BM) electron selective layers. We find that SnOx works as an electron selective layer in PSC if the film is thin and if a gold electron contact is used. If a different contact configuration is used, we find that the solar cell performance is severely hampered by what appears to be an energy barrier, despite of the seemingly intact bulk of the perovskite film below and the theoretically good band alignments. Since the bulk of the perovskite is unharmed we turn our focus, just as the previously mentioned study by Zardetto et al.,11 towards the SnOx/perovskite interface by using X-ray photoemission spectroscopy (XPS) depth profiling and find it to be halide enriched. Thus the SnOx process is not yet fully compatible with the perovskite film, but these initial results are nevertheless promising for ALD on perovskite as they show that it is possible to grow conformal films on the perovskite and that the resulting nonideal reactions are limited to the interface between the perovskite and the SnOx. 2. EXPERIMENTAL METHODS 2.1 Synthesis Two different PSC are used in this study. Figure 1a shows the first stack of soda lime glass (SLG)/SnO:F (FTO)/TiO2/meso porous TiO2 (mp-TiO2)/FAPbI3:MAPbBr3/ALD-oxide which is used for most of the analysis techniques whereas the second stack of SLG/InSnO (ITO)/NiOx/FAPbI3:MAPbBr3/SnOx/electron contact, shown in Figure 1b, is used to evaluate


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SnOx as an ESL in a solar cell. The perovskite recipe is slightly altered to have a lower PbI2 molar concentration (PbI2:MAI ratio 1:1) for the device stack, because it gives better performance in the inverted solar cell configuration. The analysis stack is produced by first cutting a conducting SnO2:F (FTO) glass substrates into 2.5 x 1.5 cm2 pieces. The substrates are washed by sonication in 2% RBS™50 solution (Fluka), deionized water, acetone and ethanol for 15 minutes each. Once dried, a dense layer of TiO2 is deposited onto the substrates by spray-pyrolysis at 500 °C by using an isopropanol solution containing 0.2 M Ti(OCH(CH3)2)4 (Sigma-Aldrich) and 2 M CH3COCH2COCH3 (SigmaAldrich) in isopropanol solution. After cooling to room temperature, a mesoporous TiO2 layer is deposited on the substrates by spin-coating a nanoparticle solution (150 mg of 30 NR-D Dyesol paste in 1 ml ethanol) at 4000 rpm for 30 s followed by annealing at 500 °C for 30 min. The perovskite precursor solution is prepared by dissolving PbI2 (TCI), formamidinium iodide (FAI) (Dyenamo), PbBr2 (Alfa Aesar), and methylammonium bromide (MABr) (Dyenamo) in 4:1 (CH3)2NC(O)H:(CH3)2SO

(DMF:DMSO)

(anhydrous,

Sigma-Aldrich)

using

molar

concentrations of 1.1:1:0.2:0.2 to prepare a (FAPbI3)0.83(MAPbBr3)0.17 mixture. The solutions are heated to 100 °C to dissolve the inorganic salts but otherwise always kept at room temperature. 75 µl of the perovskite solution is dispersed onto the mesoporous TiO2 substrates followed by spin-coating at 1000 rpm for 10 s and 4000 rpm for 30 s. During the second spin-coating step an anti-solvent is injected onto the film after 15 seconds using 125 µl of chlorobenzene. The perovskite films are then annealed at 100°C for 60 minutes on a hotplate. For the device stacks, conducting indium tin oxide (ITO) glass substrates are cut into dimension of 2.5 x 1.5 cm2 and the ITO is then patterned by chemical etching using zinc powder and hydrochloric acid. The substrates are washed by sonication in 2% RBS™50 solution (Fluka),


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deionized water, acetone, ethanol and 2-propanol for 20 minutes each. Once dried, the substrates are treated for 20 minutes in a UV-Ozone cleaner. A hole selective NiOx layer is prepared by a sol-gel method. The sol-gel mixture is prepared in methanol by adding diethanolamine (SigmaAldrich), to a concentration of 0.3 M, and then dissolving an equimolar amount of nickel (II) acetate (Sigma-Aldrich) while the solution is stirred and heated at 70 °C. The solution is deposited on the ITO substrates by spin-coating at 3000 rpm for 30 s, which is followed by annealing in an oven at 350 °C for 1 h. The perovskite solution is prepared by dissolving PbI2 (TCI), FAI (Dyenamo), PbBr2 (Alfa Aesar), MABr (Dyenamo) in 4:1 DMF:DMSO (anhydrous, Sigma-Aldrich) in molar concentrations of 1:1:0.2:0.2 to prepare a (FAPbI3)0.83(MAPbBr3)0.17 mixture. 75 µl of the perovskite solution is deposited on the ITO/NiOx substrate by spin-coating at 1000 rpm for 10 s and subsequently at 5000 rpm for 20 s. During the second spin-coating step an anti-solvent is injected onto the film after 15 seconds using 140 µl of chlorobenzene (anhydrous, Sigma-Aldrich). The perovskite films are then annealed at 100°C for 30 minutes on a hotplate. Control devices a prepared using the same procedure for the bottom stack of ITO/NiOx/ FAPbI3:MAPbBr3. The stack is then spin-coated with a PC70BM solution (20 mg/ml in chlorobenzene) at 1000 rpm for 30 s to form a thin electron transport layer. Finally, silver-back contacts (150 nm) are thermally evaporated onto the PC70BM layer. Both of the ESL candidates ZnO and SnOx are ALD coated on top of the mixed perovskite. The ZnO ALD uses a homebuilt hot wall reactor, a growth temperature of 125 °C, DEZ as the metal precursor, and H2O as the counter reactant.13 The ZnO ALD cycle sequence is DEZ/N2purge/H2O/N2-purge with pulse times of 0.25/5/0.5/5 s respectively. SnOx ALD is performed in a commercial Microchemistry F120 system using a growth temperature of 95, 120, or 150 °C,


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TDMASn as the metal precursor, and H2O as the counter reactant.14 A cycle sequence of TDMASn/N2-purge/H2O/N2-purge with pulse times of 0.4/2/0.4/2 s respectively is used for the SnOx ALD

Figure 1. (a) Thin film stack for analyzing the effects of full and partial ALD process exposure. (b) Inverted solar cell stack used to evaluate SnOx as an electron selective layer. (c) The three different top contact configurations of the solar cell stack. Three options for electron conductors, shown in Figure 1c, are used for the inverted stack devices in this study. Firstly, an 80 nm Au film with an area of 0.4x0.5 cm2 is thermally evaporated directly onto the SnOx layer which resembles the PCBM/Au structure that has been used in previous studies with good results.15 Secondly, a stack of ZnO:Al/metal grid, where the ZnO:Al is a sputtered TCO and the metal grid is an evaporated stack of Ni/Al/Ni that only covers 2 % of the 0.5 cm2 solar cell surface. This stack is derived from similar top stacks that are used in top illuminated CIGS thin film solar cells.16 Lastly, a ZnO:Al/Au hybrid where the Au layer is deposited as mentioned above onto the ZnO:Al film. 2.2 Analysis To analyze the impact of the ALD on perovskite growth the samples were investigated by the following bulk and surface sensitive analysis techniques: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). θ-2θ XRD is


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measured by a Siemens D5000 system using a parallel plate setup with an X-ray mirror on the tube side and a 0.4° parallel plate collimator on the detector side. A Physical Electronics Quantum 2000 Scanning ESCA microprobe and Al Kα radiation is used to measure the XPS spectra. The spectra are corrected for shifts due to charging by checking the position of the C-C 1s peak that is present in all of the spectra due to sample air exposure. To measure XPS depth profiles an Ar plasma with a sputtering voltage of 500 V over a 1x1 mm2 area is used for 8 min in between each XPS scan. For each scan the elemental peaks are fitted with Gauss-Lorentz distributions and Shirley backgrounds, and the resulting peak areas are integrated. Crosssectional SEM micrographs are acquired with a Zeiss Merlin SEM using a field emission electron gun, an in-lens detector and charge compensation. The current-voltage characteristics of the solar cells with Au contacts are measured from the glass side under simulated sunlight, 1000 W/m2 intensity and AM1.5 G spectral distribution, with a Newport solar simulator (model 91160) equipped with a 300 W xenon arc lamp and a Keithley 2400 source meter. The intensity of the solar simulator is calibrated by a certified reference silicon solar cell (Fraunhofer ISE). A 0.126 cm2 circular shadow mask is used to define the active area of the solar cell and a voltage scan speed of 50 mV/s is used. The solar cells with ZnO:Al and a Ni/Al/Ni grid stack are measured from the glass side with a tungsten halogen lamp calibrated to give the same current density for a Hamamatsu S1337-66BR Si calibration cell as the AM 1.5 spectra does and with a sweep speed of 500 mV/s. In addition JV measurements of the cells with a Ni/Al/Ni grid stack are also done from the topside for comparison. 3. RESULTS AND DISCUSSION 3.1 Analysis


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To better distinguish between the different chemical reactions that take place during an ALD process a sample set with perovskite films from two similar batches are treated to both partial and full ALD processes. The set uses the previously described analysis stack shown in Figure 1a. A detailed summary of the different samples and treatments is found in Table 1 and a photograph of them is shown in Figure 2. Table 1. Analysis samples and the corresponding treatment conditions that they are exposed to during full or partial ALD processes. Sample

Temperature Pressure [°C] [mbar]

Preheating* Chemical exposure

Duration

Reference 1A

RT

1000

No

None

-

Reference 2B

RT

1000

No

None

-

VacuumB

RT

~5*10-6

No

None

1h

HeatedB

120

~3

No

N2

1h

H2OA

120

~5

Yes

H2O + N2

5 min

DEZB

120

~5

Yes

DEZ + N2

30 s

ZnOB

120

~5

Yes

100 cycles ZnO ALD

30 min

TDMASnA

120

~5

Yes

TDMASn + N2

30 s

SnOxA

120

~5

Yes

500 cycles ALD

SnOx 40 min

A,B

Represents two different batches of perovskite. *Preheating is performed at 120 °C for 30 min and with a 3 mbar N2 background pressure.


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Figure 2. Photograph of FAPbI3:MAPbBr3 perovskite films treated to partial ALD conditions using H2O, DEZ, or TDMASn or to the full ALD processes for either ZnO or SnOx. None of our analysis techniques yields any measurable differences between Reference 2 and the vacuum only or heated only samples and those results have therefore been omitted. The lack of differences is not surprising as we have chosen FAPbI3:MAPbBr3 which as discussed above has shown to be more resilient against degradation conditions such as heat.12 3.1.1 XRD Figure 3 shows the θ-2θ XRD diffractograms performed on the sample set to check for one of the main signs of degradation, the decomposition of the perovskite crystal into crystalline lead halides. Unfortunately, this method is not able to pick up small changes in the amount of crystalline material at e.g. the interface where the ALD reaction takes place. However, significant decomposition of the bulk of the perovskite has previously been found to be triggered and enhanced by elevated temperatures and water exposure.4,17,18 Thus even though only the surface is exposed to water initially, the degradation still reaches the bulk of the film. It is therefore interesting to see if exposing the sample set to ALD induces similar effects that are clearly distinguishable in XRD.It should also be noted that the FAPbI3:MAPbBr3 intrinsically contains crystalline PbI2 from growth12,19 and that the analysis therefore is based on major


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changes to the crystallinity. The diffractogram for the water exposed sample in this study does however not show any visible change in crystallinity compared to the corresponding reference. Similarly, the samples exposed to TDMASn and the 500c SnOx ALD process does not show any distinguishable changes in bulk crystallinity of the perovskite film. In addition as seen in Figure 2, the SnOx exposed sample appears to be a uniform dark purple to the naked eye, for further details regarding the reflectance in the visible range, see Figure S1 in the supplementary information. There are no peaks from the SnOx film present, but the film is expected to be x-ray amorphous at the growth temperature of 120 °C.14 Furthermore, there are no indications of any tin halides. In contrast the samples exposed to DEZn, and the 100c ZnO ALD process show severe reduction of the bulk perovskite peaks and a very distinct increase for the PbI2 peak. Since PbI2 has a yellow color, the XRD results correlates well with the yellowing of the DEZ exposed sample and the completely yellow ZnO exposed sample, seen in Figure 2. Thus it seems that exposing the perovskite to DEZ enhances the decomposition process especially in conjunction with the H2O pulses of the full ZnO ALD process. Furthermore, based on previous experiments, ZnO is expected to be in the hexagonal wurtzite structure for this ALD growth temperature, but this sample shows none of the corresponding peaks.14 Similarly, to the SnOx exposed sample there are no ZnI2 or ZnBr2 peaks. Furthermore, no peaks corresponding to elemental Pb are found. This is in contrast to when the perovskite precursors PbI2 and PbBr2 are exposed to DEZ as shown in Figure S2 of the supporting information. In that case we suspect that an oxidationreduction reaction of the lead-iodides causes the formation of crystalline Pb (see Equation S1).


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Figure 3. XRD diffractograms of perovskite films after being subjected to a partial or a full ALD process, where the different reflections are identified as follows: * perovskite, x PbI2, o TiO2, + FTO, # δ-FAPbI3, and - ZnO.12 A,B indicate which perovskite batch the samples belong to. 3.1.3 XPS To supplement the bulk analysis, XPS spectra are acquired from the top of the stack after the different ALD exposures. The analysis of the Br 3d peak for the samples is omitted since the signal to noise ratio is very low in our setup, preventing us from reliably making a more detailed analysis including peak shifts and peak splits. The analysis of the unexposed reference sample shows that both the I 3d3/2 in Figure 4a and the Pb 4f7/2 in Figure 4b have peak energies of 619.3 and 138.5 eV, that correspond to a Pb-I bond seen for perovskite in previous studies20,21. In contrast, N 1s has a peak energy of 400.5 eV corresponding to an organic matrix, but which is a bit lower compared to previous studies on methylammonium only containing films.20,21 Possibly, the lower N 1s peak energy in this study


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is related to a different chemical environment of N in formamidinium compared to methylammonium. More interestingly after exposing the stack to DEZ, Zn peaks appear. The Zn 2p3/2, shown in Figure 4e, has a binding energy of 1022.6 eV. The presence of Zn is further supported by X-ray fluorescence (XRF) measurements see table S1 in the supporting information. Finding a peak after only a DEZ exposure is somewhat surprising as it took quite a number of ALD ZnO cycles to distinguish Zn peaks in a previous study.11 We also calculate the kinetic energy of the Zn LMM Auger peak to be 987 eV and can from there deduce, using a Wagner plot, that the Zn is most likely bound to a CO3, OH or organic group rather than as ZnO, ZnI2 or ZnBr222. Figure 4d shows that a weak O 1s peak is also found for this sample, which is a bit contradictive as the DEZ exposure itself should not include any O, but possibly the DEZ treated surface more readily binds O to the surface during the subsequent air exposure compared to the untreated reference. This sample also shows broadening of the I 3d3/2peak compared to the reference sample, as shown in Figure 4a. Similarly, as seen in Figure 4b there is also a shift for the Pb 4f7/2 peak towards higher energies. A similar shift and broadening was recently found in studies where the Pb-I bond is believed to change from its perovskite form to pure PbI2 due to degradation. 20,21 At the same time the N 1s peak is reduced as seen in Figure 4c, suggesting that N leaves the sample during the exposure. The findings for the Pb 4f7/2, I 3d3/2 and N 1s peaks are in line with the decomposition process suggested for the bulk material above, where N leaves the sample in form of organic halides as MAI and the bulk decomposes into lead halides as PbI2. Additionally, it is also in line with the XRD results that show an increasing signal intensity for the PbI2 phase for this sample.


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Unlike the DEZ exposure the TDMASn exposure does not shift either of the Pb 4f7/2, I 3d3/2 or N 1s peaks, but slightly reduces the Pb 4f7/2 and I 3d3/2 peaks, as seen in Figures 4a, 4b, and 4c. Just like for the DEZ exposure a weak O 1s signal is found for this sample as seen Figure 4d. Once more, there is no O present during the TDMA Sn exposure, but it is possible that the metalO bonds form as the sample is exposed to air afterwards. Figure 4b shows that there is a weak Sn 3d5/2 peak, which indicates that Sn does get deposited onto the perovskite surface, but not in large amounts even for this prolonged TDMASn pulse. The sample exposed to the full ALD SnOx process shows a distinct O 1s peak at 530.6 eV, which indicates a metal-oxygen bond. This sample also show a strong Sn 3d5/2 peak at 487.0 eV, which corresponds to a Sn-O bond. Both of these results points at this ALD process indeed forming the wanted SnOx film. XRF also verifies the added presence of Sn on the sample as can be seen in table S1 in the supporting information. Furthermore, the Pb, I, and N signals are either gone or significantly reduced. The small remnants that are left of N could be due to an incomplete reaction of the TDMA Sn precursor. Nevertheless, the lack of Pb and I peaks indicates that the SnOx is fully covering the perovskite surface, which is also confirmed by SEM, as seen in figure 5. In addition, SEM shows that the SnOx film is conformal with a thickness of 53±2 nm, which agrees well with the dark purple interference color seen in Figure 2. Performing similar measurements on samples exposed to 200, and 400 cycles of SnOx ALD gives film thicknesses of 23±2 and 44±2 nm respectively. The thicknesses are plotted versus the amount of cycles in figure 6. To model an ALD film on film growth the measurements are fitted with a linear equation. A good fit is established for a growth rate of 0.1 nm/cycle, which is expected from this ALD SnOx process.14 Furthermore, the fit suggests a positive thickness offset at 0


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cycles, which hints at non-ideal reactions taking place during the film nucleation before film on film growth is achieved.


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Figure 4. XPS spectra of the (a) I 3d3/2, (b) Pb 4f7/2, (c) N 1s, (d) O 1s, (e) Zn 2p3/2, and (f) Sn 3d5/2 peaks for the different samples. A,B indicate which perovskite batch the samples belong to.

Figure 5. Cross-sectional SEM image of the FTO/mp-TiO2/Perovskite/SnOx stack. The figure has been partially colored to help identify the different layers.

Figure 6. Measured film thicknesses in SEM as function of the amount of cycles used in SnOx ALD. The dashed line represents a linear fit of the measurements that models an ALD film on film growth. 3.1.4 Analysis remark The XPS and XRD analysis results points at DEZ exposure enhancing the perovskite decomposition into lead halides and organo halides. Furthermore, after the exposure, Zn is found to stick to the surface, but not in a crystalline form or binding to I, O or Br, but rather to an


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organic amorphous matrix. This seems to be further enhanced by introducing water pulses during ZnO ALD. In contrast, TDMASn exposure and even the full ALD SnOx exposure do not induce significant changes to the bulk of the underlying perovskite and in the case of the TDMASn exposure even the perovskite surface remains almost intact showing the stark contrast between using different metal precursors during ALD. 3.2 Device performance Since the analysis results of ALD SnOx on perovskite seemed promising a set of devices using the SnOx layer are fabricated using the inverted solar cell stack shown in Figure 1b and c. The resulting solar cell parameters for an ALD growth temperature of 120 °C is shown in Table 2. For the cells with Au top contacts there is a clear dependence in performance on the amount of ALD SnOx cycles, as seen in Figure 7a. Above 50 cycles the short circuit current density (Jsc) and the fill factor (FF) are vastly reduced, whereas cells with fewer cycles retain a diode like behavior. A peculiar thing occurs when the diode like devices are measured by consecutive reverse JV sweeps, as illustrated in Figure 7b, namely that the open circuit voltage (Voc) and FF increases for each sweep. As can be seen in Tablee S3 in the supporting information this Voc gain is not exclusive for the solar cells with 50 cycles of SnOx, but is also present in the cells with fewer or more SnOx cycles. Thus it is possibly an intrinsic effect of the perovskite/SnOx interface that is formed during ALD. The device with 50 cycles in Figure 7b goes from a power conversion efficiency of 3.4 % (Voc of 460 mV, Jsc of 19.1 mA/cm2 and FF of 38 %) to 7.8 % (Voc of 720 mV, Jsc of 20.8 mA/cm2 and FF of 52 %) after reverse sweeping 6 times. In contrast, if a single forward sweep is performed after the 6 consecutive reverse sweeps the same cell gives a reduced efficiency of 5.1 % (Voc of 550 mV, Jsc of 20.6 mA/cm2 and FF of 45 % as shown in figure 6b). Compared to previous studies where major hysteresis effects on Voc have been seen


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and investigated there is no intentional added step of pretreatment voltage bias before J-V in this study and all of the J-V measurements are performed using the same light intensity using and the same J-V sweep speed.23-29 Thus the origin of this effect is at this point not know, but our preliminary initial guess is that the reverse sweeps themselves acts as a form of pre voltage bias treatment for the next reverse scan despite of these scans ending at 0 V. For the other top contacts ZnO:Al/Au, and ZnO:Al/metal grid, and independently of the amount of SnOx cycles, the solar cell parameter values show very small Jsc and FF. Thus very similar to when an Au contact is used with more than 50 cycles of SnOx ALD. The low FF and Jsc characteristics are persistent even when the SnOx growth temperature is lowered to 85 °C or increased to 150 °C, when the samples with a ZnO:Al/metal grid are illuminated from the top, when the illumination intensity is changed, when the illumination spectra are changed, and for both forward and reverse JV sweeps. In other words, whatever hinders the current in these structures is seemingly not dependent on the incoming light, SnOx bulk properties that change slightly with deposition temperature or by bias induced ion movement.

Figure 7. (a) JV sweeps of inverted SLG/ITO/NiOx/Perovskite/SnOx/Au solar cell stacks as a function of the number of SnOx ALD cycles. (b) The result of 6 consecutive reverse JV sweeps for the solar cell using 50 ALD cycles of SnOx, shown in (a), and the follow up forward JV


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sweep after the 6 reverse sweeps. Reference devices using SLG/ITO/NiOx/Perovskite/PCBM/Ag stacks are included in (a) and (b) for comparison. Table 2. Influence of the SnOx thickness and top contact stack on the average performance of SLG/ITO/NiOx/Perovskite/SnOx/top contact solar cells Cycles Top contact

Illumination

Voc [mV]

Jsc [mA/cm2]

FF [%]

η [%]

10

Au

Bottom

503 ± 37

17.8 ± 1.0

39 ± 5

3.6 ± 0.8

20

Au

Bottom

413 ± 61

16.8 ± 1.5

38 ± 5

2.7 ± 0.8

30

Au

Bottom

476 ± 40

18.6 ± 2.0

39 ± 2

3.5 ± 0.8

50

Au

Bottom

486 ± 39

19.0 ± 1.6

43 ± 3

4.0 ± 0.6

100

Au

Bottom

417 ± 52

5.4 ± 1.5

9±1

0.2 ± 0.1

200

Au

Bottom

298 ± 160

1.2 ± 0.5

14 ± 5

0.0 ± 0.0

50

ZnO:Al/Au*

Bottom

495 ± 0

0.9 ± 0.0

26 ± 0

0.1 ± 0.0

50

ZnO:Al/metal grid

Bottom

679 ± 59

4.8 ± 1.6

16 ± 3

0.5 ± 0.2

50

ZnO:Al/metal grid

Top

840 ± 93

3.0 ± 1.0

16 ± 3

0.4 ± 0.1

Ref.

PCBM/Ag

Bottom

899 ± 13

16.6 ± 1.5

58 ± 8

8.6 ± 1.4

* only a single cell survived the processing. To get better solar cell performance it is imperative to figure out what so detrimentally limits Jsc. Based on the JV analysis it seems like the flow of carriers is severely limited until a large enough voltage is applied. This hints at an energy barrier being introduced that would require the carriers to obtain enough energy to traverse it by e.g. an electric field. However, the theoretical valence and conduction band positions of the perovskite, SnOx, Au and ZnO:Al, shown in Figure 8a, do not suggest that there should be any major energy barriers in the conduction band for the photo excited electrons, if the bulk materials are intact.30-33 Possibly the 0.1 eV barrier in the conduction band could slightly inhibit the current, but the SnOx/ZnO:Al stack has previously


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showed decent performance without a severe blocking behavior for CIGS solar cells.14 Ideally there are thus no problems, but possibly the underlying layers becomes altered as a result of either the SnOx or the ZnO:Al deposition. To check the integrity of the underlying films, XRD is performed on the device structure at various stages and the resulting diffractograms are shown in Figure 8b. From these we deduce that the SnOx process leaves the bulk crystallinity of the underlying layers mostly unchanged and that the subsequent ZnO:Al process does not further induce any bulk crystallinity changes to the underlying films. For the samples that have been exposed to the SnOx process there is however a new peak at an angle of about 12.8°. Upon further investigation using grazing incidence (GI) – XRD, see Figure S3 in supporting information, this peak seems to originate from further down in the perovskite film away from the top surface. As it is a single peak it is not trivial to deduce its crystalline origins, but judging by its location and the present chemical environment it is not unreasonable that it is related to partial changes of either the PbI2 or the perovskite crystal lattice during the SnOx ALD deposition. The SnOx film itself is amorphous, so XPS depth profiling is performed as a complimentary method to check for changes to this material. The XPS depth profile of a 200 cycle SnOx film grown onto the perovskite of the device stack shows in Figure 8c that the SnOx bulk is mostly intact. As the measurement approaches the SnOx/perovskite interface however, there are still quite strong Sn, and O signals, but more interestingly also an early increase of the halide signals, especially when compared to the Pb signal. Thus the Sn, O, and halide rich interface seems to form during the nucleation stage of the ALD process, before pure SnOx starts growing. Based on this, thinner SnOx films using fewer ALD cycles should also form this interface, provided there are enough cycles to form it. A similar XPS depth profile is also achieved at twice the sputtering voltage (1000 V), hinting at a


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negligible influence of preferential sputtering on the trend. We also note that just like in previous studies of perovskite degradation due to heat and light, the Ar plasma sputtering in this study splits the Pb 4f7/2 peak up in both a Pb-I and a Pb0 contribution. 20,21,34-36 To encompass for this, both of these Pb peaks are fitted, integrated and finally added together for the depth profile analysis. If the Sn, O and halide rich interface seen in XPS introduces an energy barrier in the conduction band it could to some extent explain the trends seen in JV for the solar cells. Since the enrichment most likely is created during ALD nucleation, as discussed above, it should introduce a barrier in all of the cells using SnOx, which is indeed what most cells hints at. However, for the cells with very few cycles of ALD SnOx it is possible that the interfacial layer is thin enough for charge carriers to tunnel through or not fully covering the surface enabling a direct connection between the perovskite and the Au contact. The more curious thing is the discrepancy between using an Au or a ZnO:Al contact, where the Au contact works well for thin SnOx films but the ZnO:Al does not. Possibly, the energy difference of about 0.8 eV between the work function of the Au and the electron affinity of the degenerately doped ZnO:Al is enough to significantly alter the built in electric fields of the device and thus change ion migration, electron transport or the tunneling probability through a potential interface barrier. 23,24, ,37,38 In comparison, the reference devices using a PCBM/Ag stack show a much higher Voc and FF, but a lower Jsc. The lower Jsc could be explained by a lower light reflection for the PCBM/Ag stack compared to the SnOx/Au stack. A reduced reflection lower the chances of unabsorbed light being reflected back into the perovskite for another pass and could thus be the cause of the reduced Jsc. The increased Voc is probably the result of a better FAPbI3:MAPbBr3/PCBM


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junction

formation

leading

to

less

interface

recombination

compared

to

the

FAPbI3:MAPbBr3/SnOx junction.

Figure 8. (a) The work function of Au and the conduction and valence band positions for the FAPbI3:MAPbBr3 perovskite, SnOx, and ZnO:Al.30-32 (b) XRD diffractograms of the inverted SLG/ITO/NiOx/Perovskite/SnOx/ZnO:Al solar cell stack at different manufacturing steps. (c) XPS

depth

profile

of

the

SnOx/perovskite

interface

in

the

partially

completed

ITO/NiOx/Perovskite/SnOx solar cell stack. The Br signal is multiplied by 100 for comparison. While being out of the scope of this study, it would be interesting to further investigate if the metal contact work function or the electron affinity of the TCO plays a role or not for these devices. However, the larger challenge still remains. To form good junctions without unwanted chemical reactions, inhibiting the current or introducing interface recombination, to at least be on


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par with the junction quality of perovskite/PCBM. For that, further knowledge regarding the perovskite and ALD reaction chemistry is required. Perhaps a start for such an investigation would be to try out different perovskite materials, ALD precursors and pretreatments of the perovskite prior to ALD deposition. Maybe even inserting a thin layer of a protective material, such as PCBM, in between the perovskite and the SnOx could be an interesting approach, as shown by the recent publications regarding perovskite/Si tandem solar cells.25,39 4. CONCLUSIONS To summarize, the compatibility of ZnO and SnOx ALD processes with FAPbI3:MAPbBr3 are tested with the goal of finding transparent electron selective layers for inverted perovskite solar cells. While no noticeable changes to the perovskite bulk crystallinity is found for most of the ALD processing conditions, such as exposure to temperatures of around and above 100 °C, H2O vapor pulsing and high vacuum conditions, it seems that the choice of metal precursor is critical for the perovskite integrity. Diethyl-Zn, the Zn precursor, is by itself found to rapidly cause the decomposition of perovskite bulk into Pb-halides and organo-halides, while at the same time reacting with the organic component of the perovskite. In contrast, the Sn precursor tetrakisdimethylamino-Sn does not affect the perovskite bulk or its surface. Using this precursor in the SnOx ALD process does not affect the perovskite bulk either and results in depositing conformal SnOx films on top of the perovskite. SnOx as the electron selective layer in the inverted solar cell stack works best for thin layers, up to 50 ALD cycles, and with an Au contact, but the solar cells’ performance still falls short of that of the reference solar cells’ using PCBM/Ag instead as the ESL/electron contact, mainly because of a lower Voc. The cells with SnOx show quite pronounced hysteresis effects e.g. the 50 ALD SnOx cycle cells initially shows an efficiency of 3.4 %, but it increases to 7.8 % after


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consecutive JV sweeps, mainly due to an increasing Voc. Using thicker films or a transparent ZnO:Al contact instead of Au vastly reduces current transport through the device, in a manner that suggests there to be an energy barrier for the excited electrons. Upon further investigation, the perovskite/SnOx interface is found to be rich in Sn, O, and halides. This is possibly a result of non-ideal chemical surface reactions taking place during the ALD nucleation and possibly the cause of the poor current transport.

ASSOCIATED CONTENT Supporting information The supporting information is available free of charge on the ACS publications website at DOI: Figure of measured total reflection of the SnOx/perovskite stack. Analysis using XRD and XRF of the perovskite precursors PbI2 and PbBr2 after exposing them to full or partial ALD conditions. GI-XRD diffractograms of the partial inverted solar cell stack of SLG/ITO/NiOx/perovskite/SnOx.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected]. Tel: 018-471 7926 Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS


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The authors thank the financial support from the Marcus and Amalia Wallenberg Foundation, SOLAR-ERA.NET, STandUP for Energy, the Swedish Energy Agency, and the Swedish Foundation for Strategic Research. We would also like to acknowledge the X-ray Laboratory and the Ångström Microstrucure Laboratory of Uppsala University and their staff. REFERENCES (1)

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(31) Correa Baena, J. P.; Steier, L.; Tress, W.; Saliba, M.; Neutzner, S.; Matsui, T.; Giordano, F.; Jacobsson, T. J.; Srimath Kandada, A. R.; Zakeeruddin, S. M.; Petrozza, A.; Abate, A.; Nazeeruddin, M. K.; Grätzel, M.; Hagfeldt, A.Highly Efficient Planar Perovskite Solar Cells through Band Alignment Engineering. Energy Environ. Sci. 2015, 8 (10), 2928–2934. (32) Sachtler, W. M. H.; Dorgelo, G. J. H.; Holscher, A. A. The Work Function of Gold. Surf. Sci. 1966, 5 (2), 221–229. (33) Ryu, Y. R.; Kim, W. J.; White, H. W. Fabrication of Homostructural ZnO p–n Junctions. J. Cryst. Growth 2000, 219 (4), 419–422. (34) Li, Y.; Xu, X.; Wang, C.; Ecker, B.; Yang, J.; Huang, J.; Gao, Y. Light-Induced Degradation of CH3NH3PbI3 Hybrid Perovskite Thin Film. J. Phys. Chem. C 2017, 121 (7), 3904–3910. (35) Niu, G.; Li, W.; Li, J.; Liang, X.; Wang, L. Enhancement of Thermal Stability for Perovskite Solar Cells through Cesium Doping. RSC Adv. 2017, 7 (28), 17473–17479. (36) Zu, F.-S.; Amsalem, P.; Salzmann, I.; Wang, R.-B.; Ralaiarisoa, M.; Kowarik, S.; Duhm, S.; Koch, N. Impact of White Light Illumination on the Electronic and Chemical Structures of Mixed Halide and Single Crystal Perovskites. Adv. Opt. Mater. 2017, 5 (9), 1700139. (37) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5 (20), 1500963. (38) Murata, M.; Chantana, J.; Ashida, N.; Hironiwa, D.; Minemoto, T. Influence of Conduction Band Minimum Difference between Transparent Conductive Oxide and Absorber on Photovoltaic Performance of Thin-Film Solar Cell. Jpn. J. Appl. Phys. 2015, 54 (3), 032301.


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(39) Bush, K. A.; Palmstrom, A. F.; Yu, Z. J.; Boccard, M.; Cheacharoen, R.; Mailoa, J. P.; McMeekin, D. P.; Hoye, R. L. Z.; Bailie, C. D.; Leijtens, T.; Marius Peters, I.; Minichetti, M. C.; Rolston, N.; Prasanna, R.; Sofia, S.; Harwood, D.; Ma, W.; Moghadam, F.; Snaith, H. J.; Buonassi, T.; Holman, Z. C.; Bent, S. F.; McGehee, M. D.23.6%-Efficient Monolithic Perovskite/silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2017, 2, 17009.


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Table of content graphic


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(a) Thin film stack for analyzing the effects of full and partial ALD process exposure. (b) Inverted solar cell stack used to evaluate SnOx as an electron selective layer. (c) The three different top contact configurations of the solar cell stack. 80x44mm (300 x 300 DPI)


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Photograph of FAPbI3:MAPbBr3 perovskite films treated to partial ALD conditions using H2O, DEZ, or TDMASn or to the full ALD processes for either ZnO or SnOx. 80x51mm (300 x 300 DPI)



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XRD diffractograms of perovskite films after being subjected to a partial or a full ALD process, where the different reflections are identified as follows: * perovskite, x PbI2, o TiO2, + FTO, # δ-FAPbI3, and - ZnO.12 A,B indicate which perovskite batch the samples belong to. 160x99mm (300 x 300 DPI)


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XPS spectra of the (a) I 3d3/2, (b) Pb 4f7/2, (c) N 1s, (d) O 1s, (e) Zn 2p3/2, and (f) Sn 3d5/2 peaks for the different samples. A,B indicate which perovskite batch the samples belong to. 80x61mm (300 x 300 DPI)



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Cross-sectional SEM image of the FTO/mp-TiO2/Perovskite/SnOx stack. The figure has been partially colored to help identify the different layers. 80x59mm (300 x 300 DPI)



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Measured film thicknesses in SEM as function of the amount of cycles used in SnOx ALD. The dashed line represents a linear fit of the measurements that models an ALD film on film growth. 80x59mm (300 x 300 DPI)


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(a) JV sweeps of inverted SLG/ITO/NiOx/Perovskite/SnOx/Au solar cell stacks as a function of the number of SnOx ALD cycles. (b) The result of 6 consecutive reverse JV sweeps for the solar cell using 50 ALD cycles of SnOx, shown in (a), and the follow up forward JV sweep after the 6 reverse sweeps. Reference devices using SLG/ITO/NiOx/Perovskite/PCBM/Ag stacks are included in (a) and (b) for comparison. 80x53mm (300 x 300 DPI)



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(a) JV sweeps of inverted SLG/ITO/NiOx/Perovskite/SnOx/Au solar cell stacks as a function of the number of SnOx ALD cycles. (b) The result of 6 consecutive reverse JV sweeps for the solar cell using 50 ALD cycles of SnOx, shown in (a), and the follow up forward JV sweep after the 6 reverse sweeps. Reference devices using SLG/ITO/NiOx/Perovskite/PCBM/Ag stacks are included in (a) and (b) for comparison. 80x53mm (300 x 300 DPI)


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(a) The work function of Au and the conduction and valence band positions for the FAPbI3:MAPbBr3 perovskite, SnOx, and ZnO:Al.25-27 (b) XRD diffractograms of the inverted SLG/ITO/NiOx/Perovskite/SnOx/ZnO:Al solar cell stack at different manufacturing steps. (c) XPS depth profile of the SnOx/perovskite interface in the partially completed ITO/NiOx/Perovskite/SnOx solar cell stack. The Br signal is multiplied by 100 for comparison. 80x55mm (300 x 300 DPI)



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TOC graphics (color) 65x44mm (300 x 300 DPI)


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TOC graphics (black and white) 24x16mm (600 x 600 DPI)



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Total reflectance for an untreated perovskite and for a perovskite coated with approximately 50 nm SnOx. 80x53mm (300 x 300 DPI)


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XRD diffractograms of PbI2 and PbBr2 perovskite precursor films after being subjected to a partial or a full ZnO ALD process. 160x99mm (300 x 300 DPI)



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XRD diffractograms of ITO/NiOx/Perovskite/SnOx for a θ-2θ scan and for GI – XRD scans with angles of incidence of 1.0 °, 0.3 °, and 0.1 ° respectively. 160x102mm (300 x 300 DPI)


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and ZnO films on mixed halide perovskite - ACS Publications

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