perovskite solution and device stability for

PEROVSKITE SOLUTION AND DEVICE STABILITY FOR PHOTOVOLTAIC PURPOSES

A Project Report Submitted on the Successful Completion of a Summer Project by Spandan Anupam 1711136 National Institute of Science Education and Research (NISER)

Submitted to: Prof. Narayan Pradhan IACS, Kolkata

Department of Materials Science INDIAN ASSOCIATION FOR THE CULTIVATION OF SCIENCE June 2018

Approval The Project Report “Perovskite device and solution stability for photovoltaic uses” submitted by SPANDAN ANUPAM, First Year Integrated MSc., NISER, to the Department of Materials Science, IACS Kolkata, has been accepted as satisfactory for the fulfillment of the requirements for the completion of a summer project from 20.05.18 to 10.06.2018

Supervisor’s Signature and Date: ........................................... Professor Narayan Pradhan Date:

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Declaration I, hereby, declare that the work presented in this Project is the outcome of the investigation performed by me under the supervision of Dr Narayan Pradhan, Professor, Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata.

Signature and Date: ........................................... Spandan Anupam Date:

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Acknowledgments The internship opportunity I had under Professor Narayan Pradhan was a great chance for learning and professional development. Therefore, I consider myself as a very lucky individual as I was provided with an opportunity to be a part of it. I am also grateful for having a chance to meet so many wonderful people and professionals who led me though this internship period. Bearing in mind previous I am using this opportunity to express my deepest gratitude and special thanks to the Mr. Suman Bera, who in spite of being extraordinarily busy with his duties, took time out to hear, guide and keep me on the correct path and allowing me to carry out my project at their esteemed organization and extending during the training. I express my deepest thanks to Mr. Anirban Dutta for taking part in useful decision & giving necessary advice and guidance, which were extremely valuable for my study both theoretically and practically. I perceive as this opportunity as a big milestone in my career development. I will strive to use gained skills and knowledge in the best possible way, and I will continue to work on their improvement, in order to attain desired career objectives. Sincerely, Spandan Anupam Place: Date:

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Indian Association for the Cultivation of Science

Solution and device stability of cubic CsPbI3 Project Report

Spandan Anupam1 , Narayan Pradhan2∗ 1 National Institute of Science Education and Research, Bhubaneswar 2 Indian Association for the Cultivation of Science, Kolkata

Abstract:

Recent advancements and growing needs of the people ask for better, more efficient, printable, thin solar cells that will harvest the power of the mightiest energy provider on earth. Perovskites have been, for long, seen as materials which can accomplish these tasks, with a few shortcomings, which need to be catered by us. This project report covers the basic hurdles and some of the methods recently used, to harvest the advantages of using perovskites as cubic CsPbI3 , at room temperature. The same applies for photodetectors, which work on basically the same principle and have a similar architecture.

Keywords: Perovskite | Photodetectors |

A

ll inorganic perovskites have been in the spotlight today, because of their favorable bandgap and a less hygroscopic property compared to organic inorganic, or methyl ammonium based perovskites. Tandem solar

cells have been seen as the future of solar cells, with a possible PCE of more than 40%[1, 2], which is a lot, compared to the near 20% PCE of other cells. But along with these great performance data, we face a lot of problems in the stabilization of these materials. The major problem faced by everybody is that we may stabilize the nanocrystals in the solution and may stabilize a thin film on a glass substrate, but the process we find, for converting them from the first form to the other must be carried out in such a way that neither do we destroy these crystals, nor should they destroy themselves after getting coated.

1.

Introduction

Perovskites are the class of crystals having the form ABX3 , where A, being the largest atoms, occupy the body center, B, being the second largest occupy the edge centres and X sit in cubic positions, for a perfect crystal. ∗

The co-ordination number of A will be 12 and of B, will be 6.

E-mail: [email protected]

1

Solution and device stability of cubic CsPbI3

The first perovskite to be discovered was the mineral, CaTiO3 . The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovski (1792 to 1856). It’s crystal structure was first described by Victor Goldschmidt in 1926. The perovskite of our interest here, is CsPbI3 , which exists in two phases, alpha and gamma. Of these two, the gamma phase is more stable and it from alpha to gamma when the temperaFigure 1.

The two phases of CsPbI3 [3]

ture goes below 300 o C. Various methods for stabilization have been used to lock the

phase of the perovksite, in solutions. One of them, which involves making of long chain organic compounds ,which have a NH3+ end. This long compound gets attracted to the highly ionic perovskite, and all that remains now is a huge organic, non ionic tail which gets dissolved in a non polar solvent and forms a colloid.

2.

Why all inorganic?

Organic inorganic perovskites have also shown a great power conversion efficiency and good photodetectivity values. But many of them have shortcomings. All inorganic may also pose many problems to us, but it fits better to our interests. The reasons why the whole attention is towards developing the all inorganic solar cells is[4]: • Their hygroscopic, and volatile nature. • They can’t withstand adverse temperature conditions. CsPbI3 has been seen to be stable from temperatures starting from 300 o C. • They do not posses the band gap needed for tandem solar cells. alpha CsPbI3 has a bandgap if 1.73 eV ideally, compared to the required 1.7 eV for tandem cells. • They show high current-voltage hysteresis[5]. All of these factors lead us to work more towards all inorganic solar cells, and figure out some methods to apply on the surface so as to decrease the rate of degradation of these cells to their orthorhombic gamma forms.

3.

The ideal combination

Cesium lead iodide was specifically chosen because the cubic phase satisfies the coditions needed for a photovoltaic device, with the band gap and position being at the values that are needed for being used as the active material for a PV device. DFT calculations and approximations are close to the experimentally obtained value. This

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Spandan Anupam, Suman Bera, Narayan Pradhan

material has the lowest bandgap of all the other possible permutations, which allows us to choose this over the others, as a higher bandgap won’t be suitable for proper PV uses. Moreover, lead based perovskites have lesser number of trap states[6] than the ones experimented with other materials like Sn, Sb etc. However, doping some percentage of other materials helps coming in more into the bandgap range and also in increasing the stability of the overall structure.

3.1.

Experimental proof

A recent paper by Yang et al[7] shows us what can be experimental limits of using these crystals by using a single crystal nanorod as a device. It is known that the highest order we can feasibly obtain in a macroscopic system will be by using a single crystal system, as the amount of defects and imperfections we obtain in a crystal will be significantly lower than a stacked, imperfect structure we obtain by just spin coating several layers of the material. They report a detectivity of 5.17x1013 Jones, five times the maximum ever obtained by anyone else considering that they didn’t use a single crystal arrangement. This gives us an idea of what we can obtain on making the whole structure a highly ordered arrangement. Currently, the Figure 2. focus is slowly going towards bridging the gap between crystals and making

SEM Image of the single nanorod used by Yang et al

them more conductive.

4.

Challenges

On the way to making these devices feasible, we have to overcome a lot of problems, have to find our way around them. The challenges include some inherent, and some difficulties made by us to overcome stability issues, which now, pose a problem. Three simple problems can be easily listed out as[8]: • Inherent low stability in the α phase under normal conditions • High recombination frequency on using the stabilized colloid as the active material • High toxicity, carcinogenic property of lead based compounds Here, just the first two challenges will be mentioned, and discussed.

4.1.

Low stability of the material:

The low stability of this certain phase of the crystal may be attributed to the Goldschmidt tolerance factor[9] value. The value is defined as: √ t = (rCs +rI )/( 2(rP b +rI ))

3

Solution and device stability of cubic CsPbI3

Now the numberical value we get from this fraction gives us the idea of how stable this structure is, for a cubic structure. For a perfect crystal lattice, we get the value to be 1. For most practical purposes, the value of 0.8 to 1 gives us a moderately stable structure. There may be two ways to stabilize these crystals, one, by increasing the surface energy of the aggregate or, by doping something to make the t.f favourable.

4.2.

High recombination frequency:

Another problem we have been facing in making of any of these devices is the high recombination frequencies of these materials. This can be increased by providing a constant potential difference across the material, or by decreasing the distance between the electrodes. The constant potential difference is created by the difference in the ionization energies of the ITO and the metal contacts. The other can be obtained by introducing nanorods[10] into the active layer, which basically decreases the distance between the contacts. Many different patterns have been tried out and IDT has been seen to be useful. Stabilization at any stage may be controlled by replacing the iodide ion by bromide or chloride, but the bandgap in any of those cases go to over 2 eV, which cannot be accepted for device formation for detecting a radiation in the solar spectrum, as the energy of all of these lie in the 1.1 to 1.7 eV range.

5.

Solution stabilization

Stabilization of the perovksite crystals has to be done at two phases, one while in the solution, that is, when the crystals are formed, and one when the device is fabricated. The solution stability can be brought out by many means as listed:

5.1.

Alloying for a better tolerance factor: From the formula, we may notice that the tolerance factor of the pure perovskite is less than 0.8, that is, the Cs atom is too small for the central atom. For the same cause, groups have been trying to work for either substituting the Cs atom with formadimium ion (FA) or substituting the Pb atom with smaller atoms[9, 11, 12, 13, 14], to push the tolerance factor in range, practically alloying the material to increase stability. Successful LEDs, solar cells have been fabricated with PCE values of around 14%,

Figure 3.

Tolerance factor comparision, by Li et al

which promote the making of these kinds of cells. But along with all the advantages we may see, alloying with such materials can hamper with other

properties of the cell. For example, the volatile, hygroscopic nature of the organic materials are brought along with the stability obtained, high temperature stability is lost. The figure above, shows schematically how the t.f

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Spandan Anupam, Suman Bera, Narayan Pradhan

is tuned using FA and Cs.

5.2.

Long chain hydrogen bonding:

Perovskite crystals in bulk always promotes degradation in the phase from black to yellow. So, long ions like olyelammonuim (OAm) are used for keeping the nanocrystals apart and suspending them in a non polar solvent, as a colloid. The basic idea behind all the passivation and stabilization processes is the same, increase the surface energy to such a level that the phase stabilization energy cannot cross the Figure 4.

barrier. The ammonium ions are used here, to have

NMR image of the aggregate by Ravi et al

a hydrogen bond with the halide ions, and crystals are kept separately[15]. The NMR, DFT results in Ravi et al’s paper shows that the bonding formed between the OAm+ and the halide ion is basically a hydrogen bond, where the flat peak in the NMR graph confirms us a weak interaction.

5.3. In

a

Long ion co-ordination: new

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al[16], the

it

has

exact

been reason

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that

stability

the

reaction

in

the

tem-

solution.

They carried out the usual protocol, with some changes that changed the whole reaction. Instead of carrying out the reaction at 120 -160 o C, they carried out the reaction at almost 100 o C higher temperature, that is, at 260 o

C. The stabilization by OAm and HI, forming the OAm+ was done at that

temperature, and it was noticed that the ice quenching was not required, this time. On further NMR studies of the sample, they found out that the flattened peak, as reported by Ravi et al, was no longer found and the place was occupied by multiple sharp peaks, suggesting a strong co-ordinate bond Figure 5.

NMR image of the aggregate by Dutta et al

with the perovskite. The solution could then be converted into a powdered form, and stored for days. Samples prepared by other processes would get degraded back to their gamma phase, but these samples didn’t, for a longer

period of time suggesting a strong attachment with the material. The problem with this method will be though, to extract the perovskite, without the long chain ligands for device preparation, as these are highly stable under these conditions.

5

Solution and device stability of cubic CsPbI3

5.4.

Bidentate ligand passivation:

Pan et al[17], in their endeavor, used a different ligand to make the structure stable.

Similar to

Dutta et al, they also found out that peaks come in instead of a blunt hill. Their explanation for the greater stability of the compound is, that the carboxyl oxygen co-ordinates with the lead atoms, increasing the surface energy of the aggregate. On integrating the NMR, it has been found out that both of the oxygen atoms are involved. The ratios is the core level stabilized nanocrystals show that the ratio of OAm to IDA after passivation goes to around 3.3:1, which may mean that we need substantially

Figure 6.

(a)IDA based passivation (b)OAm based passivation by Pan et al

lesser amount of the passivating agent to make the NCs stable. Moreover, the oxygen atoms binding to two different oxygen atoms have much more binding energy than the NH3+ structure, with the energy for IDA being 1.4 eV and for the other being 1.14 eV. The IDA could even do the work at fewer structural distortions, making the conversion to the yellow form less probable.

6.

Device stabilization and improvement

The basic logic behind all of the device stabilization techniques is to increase the surface energy to such a level that the energy required to overcome that level will be more than that, required to break the lattice itself.

6.1.

HI and IPA:

Luo et al[18] discovered that addition of excess HI, then consecutive dipping of the coated substrate in IPA and annealing has made the black phase more stable, with no visible changes for more than 72 hours. The concept behind this technique is a bit different than other surface passivation approaches. It has been observed that the said process facilitates the formation of Cs4 PbI6 , and in turn, turning them into CsPbI3 . The reactions involved are the following: The addition of HI supresses the solubility of PbI2 and forms PbI2 .DMF, which suggests the path of the reactions to be: 4CsI + PbI2 −→ Cs4 PbI6 After addition of IPA, the phase transition will take place as: Cs4 PbI6 −→ CsP bI 3 + 3CsI 3CsI + 3PbI2 −→ 3CsP bI 3

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Spandan Anupam, Suman Bera, Narayan Pradhan

Cs4 PbI6 + 3PbI2 −→ 4CsP bI 3 As reported in the paper, this method of preparation allows for the greater stability of the same, black phase, maybe because of a higher purity of the alpha form, and the lack of proper nucleation points for the gamma phase to start building up. Even preparation of a continuous, big domain may also have huge side effects, as then one nucleation point of the gamma form may destroy the whole sample. But at the same time, small domains can have ions stuck in the crevices, causing lesser current to flow.[19]

6.2.

Poly-vinylpyrrolidone(PVP): Li et al[20] found out that surface passivation carried out by the use of this polymer PVP can lead us to form the cubic form of the required material, owing to a greater surface tension effect on the perovskite crystals. It has been seen that the Cs+ ion has been stabilized by the electron clouds generated by the N-C=O structure. The floating charge over the perovskite increases the surface tension and locks the aggregate in it’s phase. IPA washing of the PVP induced coated substrates show traces of PVP, which suggest the monomers to be separate nucleation sites for the nanocrystals, proving the polymer to be an important part providing the

Figure 7.

Perovskite bound to the eletron clouds created

stability. On comparing with a control of a without PVP perovskite with a layer containing PVP induced perovskite, the tweaked layer shows a un-

degraded layer for around 80 days, which is a commendable feat for alpha cesium lead iodide photovoltaics. This method was used for a solar cell, and the PCE for the device peaked at 10.74%

6.3.

NaYF4 :Yb,Er quantum dots:

This method, by Zhang et al[21] was not actually used to increase the stability of the photovoltaic device, but exploited a highly helpful property of NaYF4 , to make the devices better. The photon upconversion property shown by these materials make the devices more useful at higher wavelength radiations. This property may be explained by the fact that these lanthanide based materials take one or more photons and return back a single photon with much more energy than the ones which it had taken in. Consequently, the whole process narrows down to absorbing light of a higher wavelength and releasing light of a lower wavelength, to which the device can respond. On checking, they found out that this arrangement can detect wavelengths of upto 900 nm and upconvert it upto around 700 nm, which makes that light detectable by the device. This method has helped reap the benifits of other wavelengths of light, and can be used for detecting wavelengths which we weren’t able to, with pristine photodetectors. For any PV device, this may turn out to be useful and may increase the usability, by tuning the detectable spectra.

7

Solution and device stability of cubic CsPbI3

6.4.

CsPbI3 nanowire based structure:

Waleed et al[22] devised a novel method for making devices more stable than devices with domain based structures. Similar to the solution stability models, the group produced nanowires on the surface of the device, in such a way that the material doesn’t form clusters, and degrade to yellow perovskite. Like in the solution, the colloid is prepared to stop the nanocrystals from clustering by the use of long chain organic compounds with a ionic end, in this case, a sieve like structure is used for supporting different strands of nanowires and making them stand up straight. The ”sieve” used here, is an alumina anodic membrane (AAM), prepared by chemical vapour deposition in an argon environment. These nanowire arrays show commendable stability in ambient air, even stay unharmed in an organic solvent like IPA for around 30 days.The device formed with these arrays will behave as a lot of small, nanowire based cells and will be based on the single crystals of perovskite. But here, the nanocrystals themselves cannot take the process used by the

Figure 8.

methods used while solution processing of the device, so, the AAM is left untouched in the structure. Moreover, it is transparent and has a

Schematic diagram of the structure of the cell used by Waleed et al, showing individual crystal based photovoltaic devices

high resistivity, which makes the cells to work as small, independent cells, which make the work easier for us. Solution processed, chemically prepared perovskites may offer an easier way to make them, but this methods keeps them stable for a longer period of time in the device form. The photodetectivity was found out to be 1.5x108 Jones under a light of intensity 1.5 mW/cm2 , which is commendable considering other devices stabilized through various methods, just because this creates cells with individual crystals and is mostly ordered.

7.

Conclusion

As a conclusion, this report summarized the various stabilization methods that have been used till date, and have been hugely successful in doing so. There will still be improvement in the means and methods, and this area of research will continue to grow. The interest will slowly shift towards lead free, stable perovskites and printable, non toxic solar cells will become an everyday reality. All the challenges mentioned about here, are seriously taken into care, and various groups have been trying hard to make fulfill this dream.

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[18] Paifeng Luo et al. “Solvent Engineering for Ambient-Air-Processed, Phase-Stable CsPbI3 in Perovskite Solar Cells”. In: The Journal of Physical Chemistry Letters 7.18 (2016). PMID: 27569604, pp. 3603–3608. doi: 10.1021/acs.jpclett.6b01576. eprint: https://doi.org/10.1021/acs.jpclett.6b01576. url: https://doi.org/10.1021/acs.jpclett.6b01576. [19] Giles E. Eperon et al. “Inorganic cesium lead iodide perovskite solar cells”. In: J. Mater. Chem. A 3 (39 2015), pp. 19688–19695. doi: 10.1039/C5TA06398A. url: http://dx.doi.org/10.1039/C5TA06398A. [20] Bo Li et al. “Surface passivation engineering strategy to fully-inorganic cubic CsPbI3 perovskites for highperformance solar cells”. In: Nature Communications 9.1 (2018), p. 1076. issn: 2041-1723. doi: 10.1038/ s41467-018-03169-0. url: https://doi.org/10.1038/s41467-018-03169-0. [21] Xisheng Zhang et al. “Stable ultra-fast broad-bandwidth photodetectors based on alpha-CsPbI3 perovskite and NaYF4 :Yb,Er quantum dots”. In: Nanoscale 9 (19 2017), pp. 6278–6285. doi: 10.1039/C7NR02010D. url: http://dx.doi.org/10.1039/C7NR02010D. [22] Aashir Waleed et al. “All Inorganic Cesium Lead Iodide Perovskite Nanowires with Stabilized Cubic Phase at Room Temperature and Nanowire Array-Based Photodetectors”. In: Nano Letters 17.8 (2017). PMID: 28735542, pp. 4951–4957. doi: 10.1021/acs.nanolett.7b02101. eprint: https://doi.org/10.1021/acs. nanolett.7b02101. url: https://doi.org/10.1021/acs.nanolett.7b02101.

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