Solution process of photovoltaic absorber materials

1. Solution process of photovoltaic absorber materials

2. Photovoltaic materials – most of these are chalcogenides Bandgap size: Metal oxide - too wide Metal sulfide Metal selenide Metal Telluride 〉 〉 〉 : Metals : Chalcogens

3. Solution process of inorganic materials Oxides thin film can be synthesized chemically conveniently in oxygen and water containing system Sol-gel process is commonly used for oxides Film formation by hydrolysis and condensation

4. Sol-gel process for chalcogenide materials Example : formation of GeS2 Oxide contamination from incomplete thiolysis of alkoxide precursors Usage of inconvenient and harzardous H2S Inert atmosphere is required

5. Solubility of chalcogenides – some of them are soluble in low-molecular weight amines Solutes: As2S3, As2Se3, As2Te3, Sb2S3, GeSe Solvent: Ethylene diamine, Diethylamine, Triethylamine, n-propylamine

6. Dissolution mechanism – Amine salt cluster formation Nitrogen lone pair donor electrons are transffered to an arsenic acceptor Proton transfer and formation of amine salt Schemetic: As2S3 + n-propylamine

7. Extension of solvent selection – subsequent dissolution of ammonium metal chalcogenide Arsenic Chalcogenide or germanium selenide materials ↓ Dissolution in amine solvent ↓ Removal of amine by solvent drying ↓ Dissolution in oraganic solvents : ex) Dimethyl sulfoxide, Tetrahydrofuran, etc.

8. Soluble ammonium chalcogenide salts Solutes: Ammonium tetrathiomolibate (NH4)2MoS4 Ammonium thiotungstate (NH4)2WS4 Solvents: Ethylene diamine, 1,2-diaminopropane, etc. - - Preparation : [NH4]2[MoO4] + 4 H2S → [NH4]2[MoS4] + 4 H2O

9. Hydrazine method – more kinds of chalcogenides are soluble Hydrazine Solvent can dissove both main group metal and transition metal chalcogenides : Ammonium precursor : Amine solvent : Hydrazine

10. Dissolution mechanism – hydrazinium salt formation Additional chalcogens are employed to improve solubility of metal chalcogenides in hydrazine Redox reaction to form highly soluble hydrazinium salt Hydrazine is strong reducing agent and convert neutral chalcogenide species into corresponding anions ex) Case of tin(IV) chalcogenides 5N2H4 + 2X + 2SnX2 →N2 (gas) + 4N2H5++ Sn2X64- (X = S, Se) Dissolution of hydrazinium salt Hydrazine highly basic and have high value of dielectric constant (51.7 at 25℃) Well-dissociate salt into ion pairs and dissolve ionic species

11. Dimensional reduction – 0D metal framework ex) case of germanium selenide Isolated edge-sharing Ge2Se64- anions which is charge balanced by N2H5+ cations Hydrazinium cations form hydrogen-bonded chains and weaving among Ge2Se64-anions Noticeable interactions between the selenium atoms in one layer and the nitrogen atoms

12. Dimensional reduction – 1D metal framework ex) (N2H4)2ZnTe A nitrogen atom from the hydrazine molecule completes the tetrahedral coordination of each Zn atom (N2H4)2ZnTe has two conformations of α-and β-(N2H4)2ZnTe Thermal decomposition of (N2H4)2ZnTe yields first the hexagonal ZnTe phase (wurtzite), at temperatures as low as 200 ◦C Followed by the thermodynamically more stable zinc blende form (cubic) at temperatures above 350 ◦C ※ (N2H4)2ZnTe is not salt, hydrazine and ZnTe are bonded covalentlye

13. Dimensional reduction – 2D metal framework ex) N4H9Cu7S4 Anionic slab - trilayer structure: distorted hexagonal Cu4S44-top and bottom layers and the central layer comprised entirely of Cu atoms overall thickness of about 4.15Å The remaining hydrazinium cations fill the channels between the anionic chains along the [001] direction Cation spacer – separate anionic slabs

14. These crystals are isolated in single crystal form by gentle evaporation of solvent Amorphous structures Not all metal chalcogenides that dissolve in hydrazine form crystalline materials upon evaporation of the solvent ex) In2Se3, CuInSe2-xSx, Cu(In1-xGax)Se2, and KSb5S8 Neither crystalline nor amorphous hydrazinium salt can form pure crystalline chalcogenides Removal of hydrazine can be performed at low temperature (~150℃) But crystallization of chalcogenide should be performed at least modest temperature (>300℃)

15. Chalcogenide deposition process via hydrazine method Dissolution of chalcogenide, excess chalcogen, and hydrazine Deposition of solution (hydrazinium salt/hydrazine) Drying (hydrazinium salt – can be crystalline or amorphous) Removal of hydrazinium cation by thermal annealing (chalcogenide) Hydrazine is a weakly coordinating ligand and dissociates at a relatively low temperature Reaction (3) represents one of themajor thermal decomposition routes of hydrazine

16. Two forms of hydrazine Anhydrous hydrazine (N2H4) Very reactive – well reduce and solvate many chalcogenides But it means it is toxic, explosive and have to be stored in inert atmosphere to avoid Oxidation as well as CO2 and H2O absorption Hydrazine hydrate (N2H4·H2O) More stability and is easier to manipulate than anhydrous hydrazine But it means it is less reactive and not possible to solvate chalcogenides

17. Hydrazine hydrate is azeotrope of hydrazine and water Azeotrope is a mixture of two or more liquids in such a ratio that is composition cannot be changed by simple distillation N2H4:H2O (68.5:31.5 by weight) BP of N2H4: 113.5℃ BP of mixture : 120.3 ℃ (Negative azeotrope)

18. Azeotrope Solvent combinations that form azeotropes are always nonideal - they deviate from Raoult's law Positive azeotrope Negative azeotrope Because there is maximum/minimum vapor pressure, azeotropes have maximum/minimum boiling point

19. Distllation of azeotrope Distillation of composition A – Vapor composition B Distillate have composition of C & Residue have composition closer to azeotrope No amount of distillation, however, can make either the distillate or the residue arrive on the opposite side of the azeotrope from the original mixture

20. To achieve dissolution of chalcogenide with hydrazine hydrate Formation of hydrazinium salt or hydrazine complex Mainly important to dissolve chalcogenides Dissolution of hydrazinium salt or hydrazine complex Dissolution is not limited to anhydrous hydrazine ex) (N2H5+)2In2Se42- in DMSO/Ethylene diamine Process for synthesis of hydrazinium salt with hydrazine hydrate is required

21. Process with hydrazine hydrate - ZnS 0.0025 mol ZnCl2and 0.0025 mol S or Se autoclaves were filled with hydrazine hydrate up to 50% of the total volume, sealed and maintained at 80℃for 10 days The X-ray powder diffraction (XRD) of them showed no characteristic peaks of Zn, S or Se, ZnO, ZnS or ZnSe

22. Process with hydrazine hydrate - ZnS The IR spectrum of ZnS precursor exhibits vibration bands corresponding to hydrazine molecules, suggesting that N2H4has been intercalated into these precursors ZnS molecular precursors showed that they were fibers with width of about 50–200 nm and length of 3–4 μm

23. Process with hydrazine hydrate - Mn2SnS4(N2H4)2 Elemental Sn (1mmol, 0.120 g), Mn(2 mmol, 0.110 g), and S (4.5 mmol, 0.145 g), hydrazine monohydrate (2 mL, 98%), and distilled water (2 mL) The autoclave was sealed and placed in oven operated at 150℃ for 4 days Basically, prepared Mn2SnS4(N2H4)2 is 1D-reduced

24. Process with hydrazine hydrate - Mn2SnS4(N2H4)2 Two types (A and B) of chains are stacked in the fashion of ...ABAB... along the b axis A chain comprises Mn2S6N4dimers, which consist of MnS4N2octahedra sharing a S...S edge B chain consists of MnS4N2octahedra sharing a corner (S) and further bridged through one H2N-NH2ligand

25. Hydrothermal process with hydrazine hydrate Process over boiling temperature is possible and increases reactivity High pressure increases reactivity Materials react with vapor

26. Future work: Solution process of binary Cu2Se and In2Se3 thin films Cu2Se : Cu2Se + Se + Ethylene diamine In2Se3 : 1. Hydrothermal synthesis of (N2H5+)2In2Se42- 2. Dissolution in DMSO/Ethylene diamine