Project financed by UEFISCDI, Executive Agency for Higher Education, Research, Development and Innovation, IDEAS Programme, Exploratory Research Projects (PCE)

Host institution: Politehnica University Timişoara 

Project registration code: PN-II-ID-PCE-2012-4-0398

Contract number: 77 / 02.09.2013

Project title: New fabrication concept of silver nanowire/polyaniline transparent, conductive and flexible electrodes for solar cells

Project summary: A great challenge in the actual research of solar-to-electricity conversion is the construction of flexible solar cells. Although indium tin oxide (ITO) deposited on plastic is traditionally used for organic solar cells and light emitting diodes, solutions are searched to replace the ITO layer and to manufacture cheap transparent conducting electrodes. Silver nanowires (AgNWs) are a promising candidate to replace ITO due to their high electric conductivity and corrosion resistance, but there is still the issue of increased resistance on wire contacts. The aim of the project is to develop transparent, conductive and flexible electrodes for solar cells based on silver nanowire/polyaniline hybrid materials and to offer a new technical solution to decrease the sheet resistance of the silver nanowires embedded in the polymer matrix.

Work plan

Main objectives:

1. Synthesis and characterization of silver nanowires with controlled aspect ratio (2013).

2. Synthesis and characterization of indium and tin nanoparticles (2014)

3. Development and characterization of transparent conductive electrodes on flexible substrates using silver nanowires and assessment of their electrical and optical properties (2014)

4. Synthesis and morphological, structural and compositional characterization of silver nanowires modified with tin and indium nanoparticles (2015)

5. Preparation of electroconductive inks based on modified Ag nanowires (2015)

6. Deposition of a conducting polymer on previously manufactured electrodes and their use in the construction of dye-sensitized solar cells (2016).

Project start date: 02.09.2013

Project completion date: 30.09.2016

Total funding: 994,400 lei

2013:              91,300 lei

2014:              263,970 lei

2015:              300,761 lei

2016:              338,369 lei

Project manager: Assoc. Prof. Andrea Kellenberger

Team members:

Nyari Terezia                        senior researcher                 work contract from 12.09.2013

Banica Radu Nicolae          postdoctoral researcher      work contract from 12.09.2013

Dan Mircea Laurentiu         PhD student                         work contract from 12.09.2013

Locovei Cosmin                   postdoctoral researcher      work contract from 24.09.2013

Baies Radu                           postdoctoral researcher      work contract from 12.09.2013

Bucur Alin                             PhD student                         work contract from 12.09.2013

Ursu Daniel Horatiu           PhD student                         work contract from 03.02.2014

Cseh Liliana                         senior researcher                 work contract from 03.02.2014

Capota Paul Cristian           master student                      work contract from 03.02.2014

Vaszilcsin Nicolae               senior researcher                 work contract from 12.11.2014

Poienar Maria                       postdoctoral researcher

Kumar Sanjay                       postdoctoral researcher

Project status report 2013 -2015

Objective 1: Solvothermal synthesis and morphological, structural and compositional characterization of silver nanowires with controlled aspect ratio (2013).

The aim of this objective was the synthesis of silver nanowires (AgNWs) by the reduction of silver ions in ethylene glycol in the presence of surfactants. If the surfactant used is polyvinylpyrrolidone (PVP), the PVP : Ag molar ratio and Cl^- ion concentration in the system have been shown to be essential for obtaining a reaction product rich in nanowires. To control the aspect ratio (diameter and length) of silver nanowires, the effect of synthesis conditions, such as injection temperature of Ag⁺ ions, rate of temperature increase of the reaction medium, molar ratio Ag : surfactant, temperature and synthesis time have been investigated.

The obtained silver nanostructures were characterized by X-ray diffraction, UV-Vis spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis and transmission electron microscopy.

XRD patterns of the silver nanowires show the presence of cubic-phase Ag grown preferentially along the direction (111) as the major phase, together with tetragonal phase Ag. EDX analysis revealed the presence of silver, small amounts of chlorine due to traces of AgCl (consistent with XRD spectra) and carbon from the PVP adsorbed on the surface of nanowires. TG / DTG-ATD studies have shown the oxidative desorption of PVP in the temperature range 320-480°C. UV-Vis spectra show an absorption maximum located at 378 nm attributed to plasmonic absorption of silver nanowires. An additional peak at 352 nm corresponds to macrocrystalline silver. Absorption maxima located at wavelengths of 385 nm are assigned to overlapped plasmonic absorptions of particles with different shapes, other than nanofibers. SEM micrographs show that regardless of the injection temperature of the precursor, silver nanowires with a very narrow size dispersion are obtained, the average diameter is about 50 nm in both cases. Injecting the precursor at 25°C leads to an average length of 3.5 to 4 μm, resulting in an average form factor (aspect ratio = L / D) of 80. Besides nanowires,  nanoparticles with various shapes, essentially spherical and cubic are obtained, that can be easily removed by centrifugation. The average length of the nanowires obtained by the injection of precursor at 160°C is about 5 μm, resulting in a higher average form factor of 100. Also, the amount of silver nanoparticles having another shapes is lower, as a result we can conclude that the optimum temperature of the precursor injection is 160°C.

SEM micrographs of the synthesized silver nanowires obtained by injecting the precursor at 25°C (a - c) and 160°C (d - f). TEM images of silver nanowires and nanoparticles with different shapes (g) and the bipyramidal pentagonal shape nucleation center of a nanowire (h).

Objective 2: Development and characterization of transparent conductive electrodes on flexible substrates using silver nanowires and assessment of their electrical and optical properties (2014)

Transparent and conductive electrodes have been obtained by depositing thin films of AgNWs on transparent polyethylene terephthalate (PET) sheets by doctor blade method. An intermediate thin layer of polymethylmethacrylate (PMMA) was applied to increase the adhesion of silver nanowires on PET support. In the next step, successive layers of AgNWs were deposited from a suspension of AgNWs in ethanol, also by means of doctor blade method with intermediate drying in an oven after each layer for 15 minutes at 50°C and cooling at room temperature another 15 to 20 minutes. By this procedure, samples with 2, 4 and respectively 6 layers of AgNWs have been prepared.

 Images of transparent and conductive films based on silver nanowires obtained by depositing 2 layers (a), 4 layers (b) and 6 layers (c) of AgNWs on PET/PMMA.

The obtained silver nanostructures were characterized by X-ray diffraction (X`Pert PRO MPD PANalytical diffractometer, CuKα radiation, λ=1.54184 Å, Bragg-Bretano geometry), UV-Vis spectroscopy (Lambda 950 Perkin-Elmer spectrometer with integrating sphere) and scanning electron microscopy (FEI Inspect S). Sheet resistance of the transparent electrodes obtained from AgNWs deposited on PMMA/PET was determined by four point probe measurement using a Jandel RM3000 Test unit.

SEM micrographs of AgNWs films on PMMA/PET support for 2 (a), 4 (b) and 6 (c) successive layers of AgNWs. UV-Vis diffuse transmittance spectra of PMMA/PET (a) and of AgNWs transparent films with 2 (b), 4 (c) and 6 (d) successive layers.

SEM images reveal that the deposition of two layers of AgNW on the substrate leads to the formation of isolated “islands” and “peninsulas” of silver nanowires unconnected or joined only through few nanowires. Increasing the number of deposited layers, these "islands" connect together electrically, leading to the formation of electrically conductive layers. The samples obtained by the deposition of 6 successive layers reveal an agglomerated, three-dimensional network structure of nanowires, composed of a large quantity of uniformly distributed nanowires. Diffuse transmittance spectra indicates good transparency in the visible, which decreases with increasing number of layers, reaching 75% for 2 layers, 65% for four layers and 58% for six layers of deposited AgNWs.

Electrical measurements indicate good conductivities for samples with 4 and 6 layers of AgNWs. Sheet resistance can be further increased by 23 % by heating the film at 150°C for 10 minutes and 30 % by heating the film at 150°C for 40 minutes, due to improved electric contacts between the nanowires.

Objective 3: Synthesis and morphological, structural and compositional characterization of indium and tin nanoparticles (2014)

The synthesis of indium nanoparticles involves reduction of dissolved In (III) ions to In (0). The reduction is followed by nucleation, leading to the formation of metal nanoparticles. Since it is envisaged to obtain a narrow particle size distribution, the nucleation step must be separated from the growth step. A strong and rapid supersaturation of the solution in indium leads to a high density of nucleation sites and reduced particle size. Increasing the nucleation sites density can be done mainly by two ways: (i) use an excess of one of the reactants; (ii) increasing the reaction rate by increasing temperature. Sodium borohydride was used as reducing agent and the reaction was conducted at high temperatures (100°C). Since In (0) nanoparticles are prone to oxidation, the synthesis was carried out under inert atmosphere (argon). To prevent agglomeration of nanoparticles a surfactant (trisodium citrate) was used. In (0) nanoparticles covered with trisodium citrate were synthesized according to the procedure described in the literature, leading to nanoparticles with a diameter in the range 10-20 nm. The dimension of nanoparticles was established by TEM and the size distribution by dynamic light scattering with a Nanosizer ZS; the morphology has studied by SEM. Moreover, the optical properties of nanoparticles were quantified by plasmon resonance absorption via UV-Vis.

Size distribution of In nanoparticles. TEM and HR-TEM images of In nanoparticles.

A narrow size distribution is obtained, with a diameter of nanoparticles ranging from 10 to 30 nm, and a mean diameter of 18.7 nm. These results are confirmed by TEM images which show monocrystalline In nanoparticles with 10 nm diameter, some of them agglomerated in clusters with about 40 nm diameter. TEM images of Sn nanoparticles show large clusters of agglomerated nanoparticles.

(a)                                     (b)                                   (c)                                (d)

STEM (a), TEM (b), HR-TEM (c, d) images of Sn nanoparticles.

Objective 4: Synthesis and morphological, structural and compositional characterization of silver nanowires modified with tin and indium nanoparticles (2015)

The synthesis of modified silver nanowires (mAgNWs) by deposition of In and Sn nanoparticles on them aimed at solving an important aspect of optimizing the mAgNWs-based transparent electrodes, namely to increase the transmittance/resistivity ratio (T/R). The T/R ration can be increased in three ways: (i) Increasing the transmittance by reducing the density of nanowires; (ii) Lowering the resistivity by decreasing the number of contacts between wires; (iii) Lowering the resistivity by welding of wires with the decrease of contact resistance.

The results of the previous stages indicated that reduction of the density of nanowires leads to the improvement of transmittance but also to the rise of surface resistance due to the formation of clusters of silver nanowires shaped as "islands" which are electrically unbound or weakly bound to each other. To reduce the number of these "islands" composed of silver nanowires, a first measure taken is to increase the aspect ratio of the nanowires by increasing their average length. Thus, we synthesized silver nanowires with high aspect ratios. By XRD and SEM characterization it was observed that the use of PVP with a high degree of polymerization (Mw 1,300,000) leads to nanowires with aspect ratios higher than 100 in the case of using AgCl for the heterogeneous nucleation process. Reducing the degree of polymerization of PVP leads to samples with considerable larger size distribution, characterized by a mixture of nanowires having several tens of microns in length and diameters varying between a few dozen and a few hundred nanometers.

In order to deposit In and Sn nanoparticles on silver nanowires, the method chosen was the precipitation of In and Sn nanoparticles in the presence of silver nanowires which may act as nucleation centers in a heterogeneous nucleation process.

XRD profiles corresponding to modified silver nanowires shows that the only crystalline phase present is cubic metallic silver. Therefore, X-ray diffraction is not suitable to identify the decoration of silver nanowires with In and Sn nanoparticles, primarily due to the extremely low In(Sn)/Ag mass ratio.

 TEM images indicate that the major part of indium that is decorating the silver nanowires consists of quasi-spherical particles having dimensions of 30-50 nm and 10 nm, depending on the synthesis temperature. HR-TEM images indicate that In metal nanoparticles are covered with a layer of amorphous citrate. Between the monocrystalline core and the organic layer, the crystal lattice is dominated by structural defects, which implies a superficial oxidation of In nanoparticles. EDX maps unequivocally demonstrate that In and Sn nanoparticles are present on the surface of silver nanowires, and that during the reduction of In³⁺ ions it has not occurred any alloying process implying diffusion of In atoms in the silver nanowires. But this process takes place by the heat treatment of nanowires decorated with metal nanoparticles.

Objective 5: Preparation of electroconductive inks based on modified Ag nanowires (2015)

For the preparation of inks and determination of the most appropriate dispersing media, the ethanol suspension of nanowires was subjected to vacuum evaporation at ambient temperature. The resulted solid mass was redispersed in different solvents (water, absolute ethanol, dichloroethane and n-hexane) obtaining different inks. The dielectric constants at 25°C of these solvents are 80.1, 24.5, 8.93 and 1.88, respectively. Polyethylene terephthalate (PET) and aluminum metal foil were used as substrates for the evaluation of the most suitable dispersion media. Another substrate (PETP) was manufactured by treating PET in oxygen plasma for 10 minutes. As working gas a mixture of 75% (vol.) Ar and 25% (vol.) O₂ was used. The PET and Al substrates were washed before use with 98% ethanol. The PETP substrate was washed with ethanol only before the treatment in oxygen plasma.

As it was observed from SEM images, the use of water as solvent leads to the agglomeration of nanowires at the water-air interface in case of aluminum and untreated PET supports. But, the silver nanowires deposited on PETP support by using water as a dispersion medium are more uniformly distributed due to the much higher wettability of the plasma-treated polymer layers. Dichloromethane, similarly to water, formed microdroplets on the surface of the aluminum. In the case of using PET and PETP as supports, the formation of droplets on the substrate surface during drying was not observed so that islands of quasi-agglomerated nanowires are deposited. Due to the high difference in polarity between PVP and hexane, the forces between the PVP molecules adsorbed onto nanowires are much higher than those existing between the molecules of hexane and PVP, so that the wires are strongly agglomerated prior to settling regardless of the nature of the substrate used.

From the cases studied, the most appropriate method for silver nanowires layer deposition is by using of inks with ethanol as a dispersion medium and oxygen plasma treated PET substrates. However, even in this case, it can be observed a fairly high density of nanowires which are electrically unconnected to the mass of nanowires.

Scientific report 2013

Scientific report 2014

Scientific report 2015

Results – Publications

ISI Publications

1. R. Banica, D. Ursu, C. Sarvas, S. F. Rus, S. Novaconi, A. Kellenberger, A.V. Racu, T. Nyari, Electrical properties optimization of silver nanowires supported on polyethylene terephthalate, Particulate Science & Technology

http://www.tandfonline.com/doi/full/10.1080/02726351.2015.1066473.

2. D. Ursu, M. Miclau, R. Banica, N. Vaszilcsin, Photovoltaic performance of Al and Mg doping CuCrO₂ nanoparticles for p-type dye-sensitized solar cells application, Journal of Nanoscience and Nanotechnology­ - accepted

Papers presented at international conferences:

1. R. Banica, R. Baies, R. Bucur, C. Locovei, A. Kellenberger, T. Nyari, Study of liquid phase synthesis of silver nanowires for solar cell applications, 3rd European Energy Conference – E2C 2013, October 27-30, 2013 – Budapest, Hungary.

http://www.e2c2013.mke.org.hu/images/downloads/Final_Programme.pdf

2. R. Banica, R. Baies, D. Ursu. M. Poienar, T. Nyari, Silver nanowires synthesis in the PVP-silver-chloride system, ECO IMPULS 2013, November 7-8, Timisoara, Romania.

http://www.eco-impuls.ro/conference-eco-impuls-2013/full-program/

3. R. Banica, C. Sarvas, S.F. Rus, S. Novaconi, A. Kellenberger, T. Nyari, Optimization of the electrical and mechanical properties of transparent electrodes based on silver nanowires supporrted on polyethylene terephtalate, International Symposium on Flexible Organic Electronics ISFOE 14, 7-10 July, 2014 – Thessaloniki, Greece.

http://www.nanotexnology.com/2014/images/stories/programs2014/ISFOE14_program.pdf

4. R. Banica, C. Sarvas, S.F. Rus, D. Ursu, S. Novaconi, A. Kellenberger, T. Nyari, Manufacture of ultrathin transparent electrodes based on silver nanowires with application to three-dimensional solar cells, International Symposium on Flexible Organic Electronics ISFOE 14, 7-10 July, 2014 – Thessaloniki, Greece.

http://www.nanotexnology.com/2014/images/stories/programs2014/ISFOE14_program.pdf

5. L. Cseh , C. Locovei, O. Marinica, A. Kellenberger, T. Nyari, R. Banica, Synthesis and characterization of indium nanoparticles  as precursor for solar cells , New trends and strategies in the chemistry of advanced materials with relevance in biological systems, technique and environmental protection. New trends and strategies in the chemistry of advanced materials, 5-6 June, 2014 – Timisoara, Romania

http://acad-icht.tm.edu.ro/manifestari/newtrends2014/NewTrends_Program-2014.pdf

6. R. Banica. C. Mosoarca, P.A.Linul, T. Nyari, N. Vaszilcsin, Indium decorated silver nanowires (Link 5), 21^st International Symposium on Analytical and Environmental Problems, 28 September, 2015, Szeged – Ungaria.

http://www2.sci.u-szeged.hu/isaep/index_htm_files/Proceedings%20_of_ISAEP_2015.pdf

7. D. Ursu, R. Banica, N. Vaszilcsin, Photovoltaic performance of (Al, Mg)-doped CuCrO₂ for p-type dye-sensitized solar cells application (Link 6),
Applied Nanotechnology and Nanoscience International Conference, 5 - 7 November, 2015, Paris – France.

http://www.premc.org/doc/ANNIC_2015_Book_Of_Absracts.pdf

Diploma works:

1. Claudia Sarvas – Deposition of thin films based on silver nanowires on transparent substrates, with applications in solar cells, 2014, University Politehnica Timisoara.

2. Arhip Liliana – Silver nanowires to improve the efficiency of solar panels, 2014, University Politehnica Timisoara.