of 1/1
hotplate dry 300 o C spin-coat 3000rpm ,30s Mo Mo Mo repeatto obtain desired thickness Mo sulfurization in S vapor 500 o C ,1 hr Film characterizations XRD,SEM ,EDS apply ink Spin coating+ sulfurization Sonicationin and w ash w ith EtO H,dispersed in EtO H D rop-cast+ sulfurization + ED S results Cu 1.6 ZnSnS 4 Im purities (atom % ):N (8% ),Cl(1% ) XR D available + ED S results Cu 1.6 ZnSn 0.8 S 2.8 Im purities (atom % ):N (35% ),C (35% ) XR D available Very poor adhesion Ink 5 Ink D F5 Ink Type 2: inorganicm etal saltsand chalcogen sources dispersed in solvents (Example: CuCl, Zn(NO) 2 , SnCl 2 , thiourea dispersed in non-toxic solvents) Advantages: disadvantages: non-toxicsolvents reaction by-products easy to form ulate m aterial dispersion im purities Development of Cu 2 ZnSnS 4 Inks for Solution Processable Thin Film Solar Cells Feng Jiang and Anthony Muscat [email protected] Department of Chemical and Environmental Engineering University of Arizona, Tucson, AZ 85721 Introduction Comparing to CuInGaS 2 (CIGS), kesterite Cu 2 ZnSnS 4 (CZTS) has been considered as a more ideal absorber material for the next generation low-cost solar cell, due to the lack of rear and expensive indium. With a direct band gap of 1.0-1.5 eV and absorption coefficient larger than 10 4 cm -1 , CZTS based thin film solar cell has reached reported efficiency up to 9.6%. Although vacuum sputtering has been used to deposit CZTS film, it is difficult to control the chemical stoichiometry during the deposition, resulting undesired crystalline phases and impairing the cell efficiency. Solution processable thin film fabrication not only opens the door for precise control of film stoichiometry, but also creates the opportunity for being adapted into large scale, continuous roll-to-roll solar cell manufacturing. This work gives a quick review of three types of major ink formulations for CZTS cells. Then we present our experiment results on one of these formulations, which is based on having inorganic metal salts and chalcogen sources dispersed in common organic solvents. Concentration, pH, and precursor ratio were systematically changed to optimize the ink formulation in order to obtain an adhesive and stoichiometric CZTS film. Review on Inks The requirements of an ink for thin film solar cell fabrication: - contains metal chalcogenides or ready-to-react metal and chalcogen sources - has metal precursors close to the oxidation states of target material - minimizes the impurity contents that is hard to remove - has a good material dispersion (i.e. no severe agglomeration) - uses non-toxic solvents with low boiling points - has an adjustable viscosity that is adaptable to the deposition methods - is easy to formulate and process Why these requirements - to fabricate a dense film with large grain size and small number of voids: lower impurities in the ink; minimize reactions that impede crystallization; accelerate solvent evaporation - to incorporate the ink into mass production line InkType 1: m etalsand m etal chalcogenidesdissolved in solvents (Example: Cu 2 S, Zn, SnSe, S and Se dissolved in N 2 H 4 ) Advantages: disadvantages: no carbon content toxicsolvent good m aterial dispersion no reaction by-product Equipm entsand Prelim inary results Ink Types Major Equipments Preliminary results Ink type 1 glove box dense, low -voids, large grain CZTSfilm , cell efficiency 10.1% 1 Ink type 2 N/A CZTSfilm w ith sm aller grainsand som e voids, cell efficiency4.1% 2 Ink type 3 m illing m achine, vacuum oven dense, low -voids, large grain CZTS and CIGS film s, cell efficiencyexceeds10% 3 Reference: 1. Prog. Photovolt: Res. Appl., 2012, 20, 6-11 2. Adv. Energy Mater., 2011, 1, 732-735 Ink Type 3: nanoparticlesofm etal oxides/chalcogenides dispersed in solvents (Example: CuO, In 2 O 3 , Ga 2 O 3 (for CIGS) and Cu 2 S, Zn, Sn, and S (for CZTS) dispersed in non-toxic solvents) Advantages: disadvantages: non-toxicsolvents m aterial dispersion low reaction by-products form ulate process no orlow carbon content Experiments on Inks Type #2 ink # Cu (conc.) Zn (conc.) Sn (conc.) Thiourea (conc.) Solvent (volume ratio) 1 Cu(N O 3 ) 2 0.8 M Zn(acetate) 2 0.5 M SnCl 2 0.5 M 4 M H 2 O :EG =1:1 2 Cu(N O 3 ) 2 0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.5 M 6 M H 2 O :EG =1:2 3 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.5 M 6 M H 2 O:NH 4 OH:EG=7:4:9 4 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.5 M 6 M H 2 O :EtO H =5:1 5 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH:NH 4 O H =3:16:1 6 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EG:NH 4 O H =3:16:1 7 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH:NH 4 O H =3:36:1 8 Cu(N O 3 ) 2 0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH:NH 4 O H =3:16:1 9 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:IPA:N H 4 O H =3:16:1 10 Cu(N O 3 ) 2 0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH:NH 4 O H =3:16:1 11 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH:HCl=3.8:16:0.2 12 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH:NH 4 O H =3.5:16:0.5 13 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH=1:4 14 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M H 2 O:EtOH:NH 4 OH=1:1:8 15 CuCl0.8 M Zn(N O 3 ) 2 0.5 M SnCl 2 0.4 M 6 M EtOH:NH 4 O H =4:1 Ethylene glycol (EG) yielded CuS impurity phase, identified by the diffraction peak at 15 o in the XRD spectra. Ink 5 and 8 yielded CZTS crystalline film. However, ink 8 has a short storage time and large aggregation formed after days of storage. Ink 5 is stable and suitable with deposition process, such as spin coating. Again, EG in Ink 6 yielded CuS phase. Inks without XRD were not stable and thus not suitable for making films. Problem: film made by ink 5 is very porous, containing small particles. Tried to solve this issue by collecting particles and re-crystallizing them to form a new film. Outlook: ink #2 uses thiourea as S source, which brings C content into the film. To avoid this issue, ink type #3 could be a good candidate. Experiment on ink type #3 is undergoing. Film Fabrication Process Using Formulated Inks

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  • Development of Cu2ZnSnS4 Inks for Solution Processable Thin Film Solar CellsFeng Jiang and Anthony [email protected] of Chemical and Environmental EngineeringUniversity of Arizona, Tucson, AZ 85721IntroductionComparing to CuInGaS2 (CIGS), kesterite Cu2ZnSnS4 (CZTS) has been considered as a more ideal absorber material for the next generation low-cost solar cell, due to the lack of rear and expensive indium. With a direct band gap of 1.0-1.5 eV and absorption coefficient larger than 104 cm-1, CZTS based thin film solar cell has reached reported efficiency up to 9.6%. Although vacuum sputtering has been used to deposit CZTS film, it is difficult to control the chemical stoichiometry during the deposition, resulting undesired crystalline phases and impairing the cell efficiency. Solution processable thin film fabrication not only opens the door for precise control of film stoichiometry, but also creates the opportunity for being adapted into large scale, continuous roll-to-roll solar cell manufacturing. This work gives a quick review of three types of major ink formulations for CZTS cells. Then we present our experiment results on one of these formulations, which is based on having inorganic metal salts and chalcogen sources dispersed in common organic solvents. Concentration, pH, and precursor ratio were systematically changed to optimize the ink formulation in order to obtain an adhesive and stoichiometric CZTS film. Review on InksThe requirements of an ink for thin film solar cell fabrication: - contains metal chalcogenides or ready-to-react metal and chalcogen sources - has metal precursors close to the oxidation states of target material - minimizes the impurity contents that is hard to remove - has a good material dispersion (i.e. no severe agglomeration) - uses non-toxic solvents with low boiling points - has an adjustable viscosity that is adaptable to the deposition methods - is easy to formulate and processWhy these requirements - to fabricate a dense film with large grain size and small number of voids: lower impurities in the ink; minimize reactions that impede crystallization; accelerate solvent evaporation - to incorporate the ink into mass production line without extra costs and safety issuesReference:1. Prog. Photovolt: Res. Appl., 2012, 20, 6-11 2. Adv. Energy Mater., 2011, 1, 732-7353. Thin Solid Films, 2003, 431-432, 53-57 Experiments on Inks Type #2Ethylene glycol (EG) yielded CuS impurity phase, identified by the diffraction peak at 15o in the XRD spectra.Ink 5 and 8 yielded CZTS crystalline film. However, ink 8 has a short storage time and large aggregation formed after days of storage. Ink 5 is stable and suitable with deposition process, such as spin coating. Again, EG in Ink 6 yielded CuS phase. Inks without XRD were not stable and thus not suitable for making films.Problem: film made by ink 5 is very porous, containing small particles. Tried to solve this issue by collecting particles and re-crystallizing them to form a new film.Outlook: ink #2 uses thiourea as S source, which brings C content into the film. To avoid this issue, ink type #3 could be a good candidate. Experiment on ink type #3 is undergoing.Film Fabrication Process Using Formulated Inks