A Combined Theoretical and Experimental Study

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Mechanistic Insights from Functional Group Exchange Surface Passivation: A Combined Theoretical and Experimental Study Wei Zhang, Azar Sadollahkhani, Yuanyuan Li, Valentina Leandri, James M. Gardner, and Lars Kloo ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00050 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Energy Materials

Mechanistic Insights from Functional Group Exchange Surface Passivation: A Combined Theoretical and Experimental Study Wei Zhang, a Azar Sadollahkhani, a ,† Yuanyuan Li, b Valentina Leandri, a James M. Gardner, a Lars Kloo a* a)

Department of Chemistry, Applied Physical Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden.

b)

Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden.

KEYWORDS

Perovskite solar cells, polyhedral oligomeric silsesquioxane (POSS), passivation, DFT calculation, stability.

ABSTRACT

ACS Paragon Plus Environment

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Four different functional groups including amino (-NH2), phosphine (-PH2), hydroxyl (-OH) and thiol (-SH) were combined with POSS (polyhedral oligomeric silsesquioxane) molecules in order to investigate how functional groups affect the surface passivation of POSS systems. Results from density-functional theory (DFT) calculations, functional group amino (-NH2) with adsorption energy 86 (56) kJ mol-1 is consistently better than that of thiol (-SH) with adsorption energy 68 (43) kJ mol-1 for different passivation mechanisms. Theoretical studies on the analogous POSSOH and POSS-PH2 systems show similar adsorption energies. Two of the systems were also investigated experimentally, aminopropyl isobutyl POSS (POSS-NH2) and mercaptopropyl isobutyl POSS (POSS-SH) were applied as passivation materials for MAPbI3 (MA = methylammonium) perovskite and (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 (FA = formamidinium) perovskite films. The same conclusion was drawn based on the results from contact angle studies, X-ray diffraction (XRD), as well as the stability of solar cells in ambient atmosphere, indicating the vital importance of choice of functional groups for passivation of the perovskite materials.

INTRODUCTION

Due to simple fabrication methods and rapidly increasing power conversion efficiencies (PCEs), perovskite solar cells have attracted much attention in recent years.1-4 Methylammonium lead(II) iodide (MAPbI3), one type of commonly used perovskite materials, was first structurally characterized in 1978.5 However, it was not applied into solar cells as a light absorber until 2009 by Miyasaka et al.6 showing an efficiency of around 3.8%. From then on, thousands of studies have focused on the optimization of photovoltaic devices employing perovskite materials, and certified efficiencies have steadily increased to 23.7% thus challenging both monocrystalline Si and GaAs as thin film solar cell materials.7 Although the typical efficiencies of perovskite-based


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solar cells are close to or even supercede the ones of silicon solar cells, perovskite solar cells are still far from commercialization. The biggest problem is the lack of stability of the perovskite material. It is known that MAPbI3 shows very limited tolerance against humidity, temperature and UV light. Much effort has been made in order to overcome the instability.8 One of the employed strategies is composition engineering. Grätzel et al.9 introduced formamidinium (FA) into MAPbI3 for the first time, and the resulting perovskite (MA)x(FA)1-xPbI3 opened a new route to stable perovskite materials. Furthermore, by incorporation of bromide anions, Jeon et al.10 found that (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 shows a better phase stability and device power conversion efficiency than other systems. Recently, by adding the inorganic cations cesium11 or rubidium,12 Saliba et al. found that the resulting triple-cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. As a result, a more reproducible device performance with efficiencies over 21% could be achieved. Furthermore, the effects of adding a polymer directly into the MAPbI3 perovskite precursor solution was studied by Liu et al.13 A polymer denoted J71 was used as additive in the precursor solution and resulted in perovskite films with less pin holes and large grain size. They found that using this method, they could obtain devices showing efficiencies over 19%, and with improved stability versus moisture. Another strategy to increase the stability of perovskite-based devices is to encapsulate the device to avoid any contact with water in the ambient atmosphere. Lee et al.14 used poly(methyl methacrylate) (PMMA) and reduced graphene oxide (rGO) to coat the MAPbI3 devices and found that the devices exhibit a negligible change in air, at a temperature of 35 oC, and a humidity of 40% for a test period of 1000 h. Idígoras et al.15 used adamantine to encapsulate the MAPbI3 devices. The photovoltaic performance was not affected within the first 60 s after immersing the encapsulated cells into liquid water. Apart from the above described methods,


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surface passivation represents another effective way to increase the device stability.16 Yang et al.17 demonstrated that hydrophobic tertiary and quaternary alkyl ammonium cations can successfully be assembled on top of the MAPbI3 perovskite surface as water-resisting layers via a facile surface functionalization technique. Such layers can protect the perovskite films under high relative humidity (90±5)% over 30 days. More importantly, devices based on such films can retain the photovoltaic capacity of bulk perovskites, with power conversion efficiencies over 15%. Zhao et al.18 utilized molecules containing benzene rings to passivate FAPbI3 perovskite films and discovered that among aniline, benzylamine, and phenethylamine, only benzylamine increases the solar cell efficiency and stabilizes the perovskite films for more than four months in moisture-rich air. Polyhedral oligomeric silsesquioxanes (POSS) are organic-inorganic hybrid compounds with an inorganic cubic core and peripheral organic groups and with a typical size of around 1.5 nm.19 Functionalization of the outer organic groups offers the possibility to change the chemical properties of the POSS molecules. Since it was first synthesized in 1946,20 POSS molecules have been widely used in many areas, including polymers,21 catalysis,22 and solar cells.23 Nguyen et al.24 modified the POSS molecules to become hydrophobic and used them to coat paperboards to efficiently screen them from water. As a result, the water resistance and the water vapor barrier properties of the paperboard were significantly improved. In 2016, A POSS with an amino group was successfully applied as a capping ligand for the perovskite material (CH3NH3)PbBr3 (MAPbBr3). Zhang et al.25 found that the POSS molecules containing an amino group could easily passivate the surface of MAPbBr3 nanocrystals, thus controlling the crystal size and increasing the perovskite material stability in LED devices. The perovskite material CsPbX3 (X = Br or I) capped by POSS containing a thiol group was also studied by Rogach et al,26 and a high water resistance


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and prevention of anion exchange could be achieved. Recently, Chen et al27 and Zhou et al28 also investigated POSS-NH2 in perovskite solar cells, and extremely low density of trap-states in the perovskite were observed. These studies highlight POSS molecules as attractive passivating agents for perovskite materials. Hence, in this work, aminopropyl isobutyl POSS (POSS-NH2), phosphoruspropyl isobutyl POSS (POSS-PH2), hydroxypropyl isobutyl POSS (POSS-OH) and mercaptopropyl isobutyl POSS (POSS-SH) molecules were initially included in a calculational study in order to gain insights into how the functional groups affect the surface passivation. Furthermore, based on the results from the calculations, POSS-NH2 and POSS-SH were selected as passivation materials for experimental studies on MAPbI3 and (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 perovskite solar cells regarding both power conversion efficiencies (PCEs) and long-term stabilities. One main advantage of the POSS materials is that they are inexpensive (Table S1) and, due to their large sizes, POSS molecules are expected to work better as moisture barrier.17 The molecular structures of the four materials investigated in this study are shown in Figure 1.

O

Si O O Si

O

O

Si O

Si

O

O

Si O O

Si

Si

POSS-NH2

O Si

O

NH2

O Si Si O O O O Si O Si O Si

O

O

Si O O

Si

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Si O

PH2

O

Si O O Si

O

O

Si O

Si

O

O

Si O O

Si

Si

O

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Si O

Si

O

O

Si

POSS-OH

O

Si O

O

O

Si O

Si O O

Si O

Si

SH

O Si

O

POSS-SH

Figure 1. Molecular structures of the neutral POSS-NH2, POSS-PH2, POSS-OH and POSS-SH.

RESULTS AND DISCUSSION

In order to understand the mechanisms behind surface passivation, we considered three different hypotheses illustrated in Figure 2 and DFT calculations were used for modelling. The first


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hypothesis (neutral) is based on hydrogen bonding between POSS-NH2 (or the other neutral, functionalized POSS molecules) and iodide ions in the perovskite film. The second hypothesis (basic) involves a chemical reaction on the surface of MAPbI3 and the formation of direct Pb-N (or P, O, S) interaction by the release of hydrogen iodide (HI). The structural models used in the DFT study only involved the interaction between the resulting anionic POSS molecules and the perovskite film slab. The third hypothesis (acid) is based on formed POSS-NH3+ (or the other protonated, cationic POSS cations) to replace an MA+ cation in the surface layer of the perovskite slab.

Figure 2. Three hypothetical molecular models of surface passivation. In order to compare these molecular hypotheses, perovskite slab models were constructed based on a fully cubic model of the methylammonium lead iodide structure (Pb-I distance of 4.142 Å) with some modifications. The perovskite model chosen follows the one extracted from a combined scanning tunneling microscopy (STM) and theoretical study by Zhong and co-workers,29 based on the (001) surface and with the top Pb-I layer removed, leaving a reactive surface of iodides interspaced by the organic cations (Figure 3, Figure S1). In addition, the organic cations were replaced by Cs+ cations in order to avoid orientational/symmetrical effects. The resulting composition of the CsPbI3 slab used was Cs27Pb48I120, thus with a formal total charge of +3. In addition, test calculations were also performed by replacing the Cs+ cations with Li+ in order to


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detect any spurious effects from the polarizable character of the cesium ion. In studies of adsorption of the POSS- NH3+/-PH3+/-OH2+/-SH2+ cations, a central Cs+ cation was removed.

Figure 3. The geometrically optimized structures of (a) POSS-NH2 and (b) POSS-SH: cyan = Si, red = O, grey = C, black = H, blue = N, and yellow = S. As a result of modeling hypothesis 1, adsorption of the neutral POSS-NH2/-PH2/-OH/-SH molecules to the CsPbI3 model surface is rather strong, 56, 42, 16 and 43 kJ mol-1, respectively, but the adsorption mode deviates somewhat from presumptions. It was expected that the main form of interaction would be via hydrogen bonding, albeit less pronounced from the 3rd-period functional groups, from the functionalized arm of the POSS molecules to iodide ions at the top of the perovskite model. However, focusing on the POSS molecules also studied experimentally it can be noted that the coordination in addition involves electrostatic interaction between the partially negatively charged N- and S- atom, respectively, and a central Cs+ ion in the perovskite slab. The S-Cs/N-Cs distances in the optimized structures are 3.31 and 3.82 Å, to be compared with the close H-I hydrogen interactions of 3.34 and 3.35 Å in the two cases. It is also notable that the POSS-NH2/-SH molecule exposes a large surface of the hydrocarbon arms of the cage for secondary interactions of agostic C-H…surface type contributing to the overall adsorption energy. The model structures for POSS-NH2 and POSS-SH are shown in Figure 4, while the rest are shown


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in Figure S2. Replacing the Cs+ ions for Li+ to investigate if the soft character of Cs+ significantly affects this qualitative picture shows that the coordination mode prevails.

Figure 4. Coordination model of a perovskite slab from a geometrically optimized structure of (a) POSS-NH2 and (b) POSS-SH. Regarding the perovskite slab, gold = Cs+ or Li+, pink = I, and blue = Pb. Hypothesis 2 was suggested in a previous study,30 in which a halide can be lost during the annealing process, resulting in coordinatively non-saturated Pb atoms both on the crystal surface and at the grain boundaries. These non-saturated Pb atoms may then act as electronic trap states.3133

However, it is difficult to determine how many halides that may be lost during annealing process

and this makes an identification of a suitable model for calculations difficult. Besides, it is clear from the reaction conditions that extremely basic conditions will be required to accomplish deprotonation to generate such coordination, where the negative –NH-/-PH-/-O-/-S- donor groups could work as replacement ligands for Pb2+ or Cs+ in the perovskite slab. At face value this appears chemically dubious. In addition, it is clear that extraction of a proton from the neutral POSS molecules in the presence of the perovskite slab represents something more complex than just a deprotonation. It is beyond the scope of this work to fully explore the bonding consequences, but electronic states are significantly affected. This hypothesis is, in conclusion, judged less likely.


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In hypothesis 3, replacement of the Cs+ ion in the perovskite model for the protonated cations POSS-NH3+/-PH3+/-OH2+/-SH2+ gives more promising results, as can be seen in Figure 5 and Figure S3, where the coordinating arms of the POSS-NH3+/-SH2+ cations in a well-behaved and straight-up fashion takes the role of cation substitute. It is notable that the outer part of the coordinating arms very well model a CH3NH3+/CH3SH2+ cation, indicating that also CH3SH2+ cations could make a suitable candidate for the organic-inorganic perovskite materials. The adsorption energies are higher than for the neutral molecules in this model, 86, 62, 86 and 68 kJ mol-1, respectively.

Figure 5. Coordination model of a perovskite slab from a geometrically optimized structure of (a) POSS-NH3+ and (b) POSS-SH2+. Regarding the perovskite slab, gold = Cs+ or Li+, pink = I, and blue = Pb. From the results of calculations on the three hypothetical systems we can make a few general conclusions. Firstly, adsorption at this level of theory is significant and overall corresponds to a weak covalent bond, although secondary interactions also play a role. Secondly, solvent effects are of less direct interest modelling binding effects after solvent evaporation (spin-coating). Otherwise, they would have been of significance being at least in the order of magnitude as the adsorption energies. Thirdly, the adsorption energies are similar for the hypotheses 1 and 3, and


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although all the POSS systems show a consistent preference for hypothesis 3, the differences are not large enough to rule out that both models of adsorption can take place in parallel. Lastly, the POSS-NH2 systems consistently adsorb stronger to the perovskite surface than the POSS-SH system. This result will be further investigated by experiments. Contact angle investigations reveal the MAPbI3 surface properties before and after passivation, as shown in Figure 6, together with the scanning electron microscope (SEM) images. The top part of the picture shows the contact angle, while the bottom part shows the SEM picture of the same film. This allows direct comparison of hydrophobicity and film morphology.


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Figure 6. Contact angle image and SEM top view of the films. (a) MAPbI3; (b) MAPbI3 coated with POSS-NH2; (c) MAPbI3 coated with POSS-SH; (d) The coated POSS-NH2 after washing; (e) The coated POSS-SH after washing. Figure 6(a) shows the contact angle of a pristine MAPbI3 film, which is around 47o, indicating a hydrophilic property of the perovskite surface.34 The corresponding SEM image shows the typical polycrystalline morphology of MAPbI3 with a crystallite size around 200-500 nm.35


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Thereafter, we coated the surface of MAPbI3 with POSS-NH2 and POSS-SH, respectively. The resulting contact angles and SEM images of the films after coating are shown in Figure 6(b) and Figure 6(c). It is clear that both films with POSS-NH2 and with POSS-SH show higher hydrophobicity as indicated by the high contact angles. The contact angle of the POSS-NH2 film is around 107o, while that for POSS-SH is 109o. Not surprisingly, the contact angles are quite similar for both materials, where the molecules contain the same seven isobutyl groups protruding from the POSS cage. The corresponding SEM images show that the whole MAPbI3 perovskite surface is efficiently coved by POSS-NH2 or POSS-SH also showing slight aggregation on the surface. This is also verified by the EDS mapping of the presence of Si on the surface, see Figure S4. It is notable to point out that both POSS-NH2 and POSS-SH molecules as surfactants show a strong tendency to aggregate.36 However, this phenomenon is only observed after the whole perovskite surface has been covered, indicating a stronger interaction between –NH2/-SH and the perovskite surface in comparison with self-aggregation. The passivation effect is further verified by the results from photoluminescence (PL) recorded before and after passivation.18 As shown in Figure S5, PL intensities increase and blue-shift after passivation for both systems. The films were subsequently washed by chlorobenzene, which is the solvent used for the deposition of the holetransport materials when fabricating solar cells after passivation. The SEM images of the films after washing treatment are shown in Figure 6(d) and Figure 6(e). The SEM images display no obvious difference in the films morphology as compared to the pristine MAPbI3 film in Figure 6(a). However, the corresponding contact angles show a significant difference to that of the pristine MAPbI3 film. The film after POSS-NH2 deposition and subsequent washing treatment, Figure 6(d), displays a contact angle of 76o considerably higher than that of the pristine film (47o, Figure 6(b)). In analogy, the corresponding POSS-SH sample shows a contact angle of 67o. It is


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important to point out that, although the difference in contact angles after passivation by POSSNH2 and POSS-SH is minor, the time to obtain the stabilizing effect is significantly different, as shown in Figure 7. Films passivated by POSS-NH2 show a strong repulsion towards the water droplet applied already in 40 s, while films passivated by POSS-SH only already after 6 s. The difference in stabilization time indicates a stronger ability against humidity of films passivated by POSS-NH2. Since Both POSS molecules have the same isobutyl groups, the different ability in repelling water molecules after washing with chlorobenzene could mainly be attributed to the stronger attached POSS-NH2 molecules on the surface of MAPbI3, which essentially originates from a stronger binding energy. These results indicate that POSS-NH2 adsorbs more strongly to the surface of MAPbI3, alternatively covers the surface more effectively, than POSS-SH.

120

Contact angle / degrees

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


100 80 60 40 20 0

MAPbI3 MAPbI3 coated with POSS-NH2 MAPbI3 coated with POSS-SH MAPbI3 coated with POSS-NH2, washed MAPbI3 coated with POSS-SH, washed 10

20

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Time / s

Figure 7. Contact angle of MAPbI3 based films. Time zero corresponds to the time when the water droplet was placed onto the film. The same contact angle experiments were performed employing substrates with the mixed (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 perovskite, giving similar results. As shown in Figure S6(a), the contact angle of pristine (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 perovskite is around 60o, indicating a


13


stronger intrinsic water-resistant property of this mixture than of MAPbI3. After coating with POSS-NH2, the film shows an increased hydrophobicity with contact angle around 99o. The film was then washed by chlorobenzene. After washing, the contact angle decreased to 78o (Figure S6(d)) within 50 s (Figure S7). The same behavior was observed for POSS-SH-treated films, as shown in Figure S6(c) and Figure S6(e). In this case, the contact angle decreased from 101o to 73o within 25s after washing (Figure S7). The increased time towards stabilization as compared to MAPbI3 could be linked to the stronger hydrogen bonding ability of the surface of (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15, which in turn is induced by the higher electronegativity of the Br atoms. X-ray diffraction (XRD) was employed to further analyze the properties of the passivated films. All films were stored in a sealed glass container, in which water was placed to provide a high relative humidity (90 ± 5)%. Subsequently, the container was stored in the dark to avoid any influence from light. Every two hours, the samples were investigated and immediately placed back into the container. The typical time of XRD experiment was 15 min. The change in the XRD patterns of the MAPbI3 films, as well as the passivated ones, are shown in Figure 8.

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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

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(c)

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MAPbI3 MAPbI3/POSS-NH2 MAPbI3/POSS-SH

1,0 12 hour

8 hour 6 hour 4 hour 2 hour

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Normalized ratios

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Intensity

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


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Figure 8. X-ray diffractograms of MAPbI3 films within 12 hours under a relative humidity at (90 ±5)%. (a) Without passivation; (b) Passivated by POSS-NH2; (c) Passivated by POSS-SH; (d) Normalized intensity ratios between the (110)-peak of MAPbI3 and (001)-peak of PbI2 versus time. The X-ray diffractogram of a pristine MAPbI3 film is shown in Figure 8(a). The peak at 2θ = 13.03o is related to the crystal plane with the assignment (001) in PbI2, while the peak at 2θ = 14.49o corresponds to the crystal plane with the assignment (110) in MAPbI3. Following the changes in relative intensity of these two peaks provides clear insights about the degradation of MAPbI3. As showed in Figure 8(a), the (001) peak of PbI2 increases significantly with time, indicating that part of MAPbI3 decomposes and PbI2 forms in the MAPbI3 bulk. However, when looking at the passivated films in Figure 8(b) and Figure 8(c), the increase in intensity of the PbI2 (001) peak is effectively retarded. Considering that the position of the film undergoing X-ray measurement may be slightly different from diffractogram to diffractogram, causing a slight difference in the intensity of the peaks, the relative comparison between two peaks is more reliable. Thus, the ratio between the maximum intensities (counts) of (110) peak of MAPbI3 and the (001) peak of PbI2 at different times was calculated according to the equation below:

𝑅𝑎𝑡𝑖𝑜 =

(110) 𝑝𝑒𝑎𝑘 𝑜𝑓 𝑀𝐴𝑃𝑏𝐼3 (001) 𝑝𝑒𝑎𝑘 𝑜𝑓 𝑃𝑏𝐼2 ACS Paragon Plus Environment

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Normalization resulted in the graph shown in Figure 8(d). The results are in good agreement with contact angle conclusions, where the passivated films show better humidity-resistant properties than the pristine MAPbI3 film. The latter degrades quickly in the first two hours, after which only about 40% of the original peak ratio is retained. However, the degradation was significantly suppressed by passivating the perovskite films with either POSS-NH2 or POSS-SH, indicating that surface passivation is an effective way of increasing the stability of MAPbI3 films towards moisture-induced degradation. A further analysis of the XRD patterns shows that POSSNH2 better than POSS-SH protects the perovskite substrates. After storing the samples for 12 hours under a relative humidity of (90±5)%, the original ratio of the peaks from the film passivated by POSS-NH2 is 52%, higher than that for the film passivated by POSS-SH, 39%. Both are clearly than that of the pristine MAPbI3 film at only 13%. The difference in improving the film stability is consistent with the results from the theoretical calculations. The influence on the photovoltaic performance is another important aspect to consider when passivation materials are applied in the solar cells. Hence, we determined the J-V characteristics of standard, non-passivated, MAPbI3-based devices and compared them with those from devices based on passivated MAPbI3 films. The results of champion devices are shown in Figure 9(a). A negligible decrease in the current density (Jsc) and open-circuit voltage (Voc) were observed upon passivation of the MAPbI3 perovskite with either POSS-NH2 or POSS-SH. The fill factor (FF) of these devices exposed to passivation was observed to be essentially constants, from 0.76 to 0.77. The difference here is probably within experimental errors. Overall, the MAPbI3-only device shows an efficiency of 16.25%, while the devices passivated by POSS-NH2 and POSS-SH offer efficiencies around 16.01% and 15.89%, respectively. No significant influence on the hysteresis


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was observed, as shown in Figure S8 and Table S2. On the other hand, the influence of the passivation

step

on

the

photovoltaic

performance

of

the

devices

based

on

(FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 is also negligible; the slight change in power conversion efficiency (PCE) is within experimental error, especially for POSS-SH, indicating that passivation by the POSS molecules has very limited influence on the photovoltaic performance. The photovoltaic parameters are summarized in Table 1.

(b) 25

(a)

Current density / mA cm-2

20


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


15

10

MAPbI3 MAPbI3+POSS-NH2 MAPbI3+POSS-SH

5

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15

10

Mixed Perovskite Mixed Perovskite+POSS-NH2 Mixed Perovskite+POSS-SH

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Voltage / V

Figure 9. J-V curves of photovoltaic champion devices based on non-passivated (red squares) and passivated (blue circles for POSS-NH2 and yellow triangles for POSS-SH) (a) MAPbI3; (b) (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15. Table 1. Detailed photovoltaic parameters of passivated and non-passivated MAPbI3 and (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 champion devices. Current density

Voltage (V)

FF

η ( %)

-2

(mA cm )

MAPbI3 / Mixed Perovskite

20.3 / 21.7

1.05 / 1.11

0.76 / 0.76

16.25 / 18.26

MAPbI3 / Mixed perovskite+ POSS-NH2

20.1 / 22.5

1.04 / 1.11

0.77 / 0.77

16.01 / 19.17


17


MAPbI3 /Mixed perovskite+ POSS-SH

19.9 / 21.8

1.04 / 1.11

0.77 / 0.76

15.89 / 18.38

In order to better understand the influence of the passivation of the MAPbI3 film, the statistical distribution of the photovoltaic parameters including efficiencies are showed in Figure 10.

(a)


(b) 22,0

1,04

1,02

1,00


21,5


Voltage / V

1,06

21,0 20,5 20,0 19,5 19,0 18,5 18,0

0,98

17,5 MAPbI3

(c)

MAPbI3+POSS-NH2 MAPbI3+POSS-SH


0,85

MAPbI3

(d) 18



17

0,80

16

Efficiency / %

Fill factor

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

Page 18 of 30

0,75

0,70

0,65

15 14 13 12

0,60

11 0,55 MAPbI3


MAPbI3


Figure 10. Statistical distribution of photovoltaic parameters based on MAPbI3 without and with passivation. (a) Voc; (b) Jsc; (c) FF; (d) Efficiencies. As shown in Figure 10, the most obvious change can be noted in a reduction in current density for both POSS materials. The big POSS molecules may block some of the charge carriers at the


18

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interface,37 although the influence is small. The statistical distribution of photovoltaic parameters of devices based on (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 are shown in Figure S9. Electrochemical impedance spectroscopy (EIS) is a powerful tool for the understanding of carrier transport properties in solar cells. Therefore, the influence of the passivation on the perovskite solar cells based both on MAPbI3 and (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 was studied. As shown in Figure 11(a), the observed increase in transport resistance (Rtr) in the devices that include the POSS molecules could explain the slightly lower current densities observed in the MAPbI3-based perovskite solar cells. The impedance results obtained for the mixed perovskite devices in Figure 11(b) are less trivial to interpret. However, the decrease in series resistance (Rs) may reflect the better photovoltaic performance observed. Furthermore, the increase in recombination resistance (Rrec) after passivation by POSS-NH2 may indicate a suppressed recombination loss at the perovskite-HTM interface possibly explaining the increase in photovoltage, as shown in Figure S9(a). The decrease in Rrec after passivation by POSS-SH may in analogy highlight the reason for the lower photovoltaic performance shown in Figure S9(d).

(a) Rs

40

Rtr CPE1

(b) 200


Rrec CPE2

Mixed Perovskite Mixed Perovskite + POSS-NH2 Mixed Perovskite + POSS-SH

180 160 140

-Z'' (ohms)

30

-Z'' (Ohms)

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


20

Rs

Rtr CPE1

120

Rrec CPE2

100 80 60

10

40 20

0

0 0

10

20

30

40

50

60

70

0

50

Z' (Ohms)

100

150

200

250

300

350

Z' (ohms)

Figure 11. Electrochemical impedance spectra (EIS) of devices exposed to passivation and not. (a) MAPbI3; (b) (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15.


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Despite the slight decrease in efficiency observed for the MAPbI3-based solar cells based on passivated films, the analysis of their stability under ambient environment gave impressive results. In this work, 10 devices of each type were stored in the dark, at room temperature, and in ambient atmosphere for 3 months. The relative humidity variations during this period were recorded and are shown in Figure S9(e) and Figure S10. We will firstly discuss the stability of devices based on non-passivated and passivated MAPbI3 films (Figure 12). Both the devices based on nonpassivated and passivated films retain stable Voc’s during 90 days with only a ±3% variation. On the other hand, the Jsc of the devices which have not been treated with the passivating materials significantly drops during 90 days retaining only 84% of the initial current densities. With the previously described results we believe that the reason for that degradation is moisture-induced. Consequently, non-passivated perovskite-based devices show a higher tendency to decompose when exposed to moisture. The degradation induced by water produces PbI2, which is a worse material for light absorption and charge transport,38 therefore causing the noted decrease in Jsc and FF (Figure 12(c)). In contrast to the loss of photocurrent density observed for the devices based on the non-passivated films, the solar cells based on perovskites passivated by POSS-NH2 and POSS-SH retain 100% and 94%, respectively, of their initial Jsc after 90 days. Following a similar trend, the fill factor of devices based on films passivated by POSS-NH2 and POSS-SH are very similar, around respectively arriving at 89% and 88% of their original performance after 90 days. For the corresponding devices based on films not passivated only 72% of the initial FF retained. The agglomerated effects on the power conversion efficiency (PCE) for devices without passivated films, which only retain 61% of their initial PCE. However, these devices based on films passivated by POSS-NH2 retain 89% of the initial efficiency after 90 days, and those passivated by POSS-SH


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85%. The better protection offered by POSS-NH2 agrees well with the previously indicated stronger perovskite film interaction as compared to POSS-SH.

102

100

Normalized current density / %

(b) 104

Normalized voltage / %

(a) 104

100 98 96 94


92 90 0

10

20

30

40

96 92 88 84


80 76

50

60

70

80

90

0

10

20

30

Time / days

40

50

60

70

80

90

60

70

80

90

Time / days

(c) 105

(d) 104

100

Normalized efficiency / %

96 95

Normalized FF / %

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


90 85 80 75


70 65

88 80 72 64


56

60 0

10

20

30

40

50

60

70

80

90

0

10

Time / days

20

30

40

50

Time / days

Figure 12. Normalized photovoltaic parameters of MAPbI3-based devices involving nonpassivated and passivated films. (a) Voc; (b) Jsc; (c) FF; (d) Efficiencies. Each point in the graphs has been calculated from an average of 10 devices. When looking at the stability performance of (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15-based solar cells without and with POSS-NH2 and POSS-SH passivated films, no significant improvement was observed within 3 months (Figure 13). This is in good coherence with the moisture resistance of


21


the pristine mixed material noted above. It has also previously been verified that incorporation of formamidinium (FA) and bromide (Br) ions into MAPbI3 significantly enhances the phase stability of the perovskite structure.10 Therefore, it is reasonable to expect the passivation performed by POSS-NH2 and POSS-SH to play a minor role in this system, especially at low relative humidity (Figure S9(e)).

102

100

Normalized current density / %

(b) 104

Normalized voltage / %

(a) 104

100 98 96 94


92 90 0

10

20

30

40

50

60

70

96 92 88 84 80


76 80

90

100

0

10

20

30

Time / days

40

50

60

70

80

90

100

80

90

100

Time / days

(c) 105

(d) 100

100

Normalized efficiency / %

95

Normalized FF / %

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

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90 85 80 75 70

Mixed Perovskite Mixed Perovskite+POSS-NH2 Mixed Perovskite+POSS-SH

65 60 0

10

20

30

40

50

60

90

80

70


60

50 70

80

90

100

0

10

Time / days

20

30

40

50

60

70

Time / days

Figure 13. Normalized photovoltaic parameters of (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15-based devices involving non-passivated and passivated films. (a) Voc; (b) Jsc; (c) FF; (d) Efficiencies. Each point in the graphs has been calculated from an average of 10 devices.

CONCLUSIONS


22

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In summary, four different functional groups including amino (-NH2), phosphine (-PH2), hydroxyl (-OH) and thiol (-SH) were combined with POSS and subjected to DFT modelling to estimate their adsorption energy with respect to different perovskite surface models. Functional group amino (-NH2) shows the strongest adsorption energy with 86 (56) kJ mol-1 and was compared with thiol group (-SH) in experiments. The results emphasize the importance of choosing suitable functional groups for surface passivation. Also, from mechanical point of view, the theoretical results indicate that the ions POSS-NH3+/POSS-SH2+ could replace MA+ in the surface of MAPbI3, resulting in a passivated hydrophobic surface. However, hydrogen bonding as one of the mechanisms cannot be ruled out according to the adsorption energy and contact angle measurements. The present work offers new insights in surface passivation, and may guide the way for the design of future passivation materials.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge. Experimental method and characterization, DFT calculation method, commercial price of POSS, EDS mapping, photoluminescence, contact angles, SEM, photovoltaic parameters of solar cells and recorded relative humidity. (PDF) AUTHOR INFORMATION Corresponding Author * Lars Kloo, E-mail: [email protected] Present Addresses


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†Solibro Research AB. SE-75651 Uppsala, Sweden. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the Swedish Energy Agency and the Swedish Research Council. The author Wei Zhang thanks the China Scholarship Council (CSC) for financial support.

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TABLE OF CONTENTS


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A Combined Theoretical and Experimental Study

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