WO2012163976A1

WO2012163976A1 - Continuous process for the synthesis of ternary or quaternary semiconducting nanoparticles based on ib, iiia, via elements of the periodic classification

Info

Publication number: WO2012163976A1
Application number: PCT/EP2012/060167
Authority: WO
Inventors: Frank Rauscher; Min FU; Leslaw Mleczko; Werner Hoheisel
Original assignee: Bayer Intellectual Property Gmbh
Priority date: 2011-06-03
Filing date: 2012-05-30
Publication date: 2012-12-06

Abstract

A process for the continuous production of ternary or quaternary semiconducting nanoparticles based on IB, IIIA, VIA elements of the periodic classification, in particular low-toxic AglnS₂; and CuInS₂ in the liquid phase is described. A uniform size of semiconducting nanoparticles is obtained in a size range from 1 nm to 20 nm, showing chalcopyrite or wurzite crystal structure. The size and crystal structure of the semiconducting nanoparticle can be controlled by adjusting reaction parameters in particular concentration, temperature, flow and selected ligands.

Description

C ontinuous process for the synthesis of ternary or quaternary Semiconducting Nanoparticles based on ΙΒ, ΙΙΙΑ, VIA elements of the periodic classification

The present invention is related to a process for continuous production of ternary or quaternary semiconducting nanoparticles based on IB, ill A, VIA elements of the periodic classification, in particular low-toxic AgInS2 and CuInSi in the liquid phase.

With the development of nanotechnology, nano- material science has become an indispensable important field in the current material science development. The progress of nano -material researches is bound to push physics, chemistry, biology and many other disciplines to a new level, and at the same time, also brings new opportunities in technological researches in 21st century. With a growing urgency of the energy issues, solar cells as a renewable, clean energy has attracted worldwide attention. Applying nano- material and technology to the solar cells might greatly increase the conversion efficiency of the current solar cells, lower the production cost of the solar ceils, and promote the development of new types of solar cells. Under such circumstances, the development of nano-material to be used in solar cells is becoming a new challenge.

State of the Art:

Semiconducting nanoparticles are characterized by a quantum confinement of both the electron and hole in all three dimensions, which leads to an increase in the effective band gap of the material and shift both the optical absorption and emission of semiconducting nanoparticles depending on nanoparticle size: larger sized particles show smaller band gap and smaller particles show larger band gap wherein for a larger band gap a. nanoparticle size of 1 to 20 nm is usually preferred. These nanoparticles can be applied in the fields of printable electronics, functional polymer films, bio-diagnose, energy supply (e.g. solar ceils), energy storage, advanced displays applications.

Up to now, a series of binary semiconductor nanocrystals such as CdSe, CdTe, CdS, PbSe, etc., have been investigated for nanocrystal-based solar ceil, biolabeling and functional film applications. However, most of these semiconductors contain toxic heavy metals such as Cd, Pb, Te and so on, which limit their comprehensive application. For this reason, the state of the art turned its attention to alternative non or low-toxic semiconductor compounds, in particular ternary or quaternary semiconductors based on IB, IDA, VIA elements of the periodic classification (I-III-VI semiconductors) have shown promising results for applications in these fields. Especially ternary semiconductors CuInS2, CuInSe2 and A lnS; and in particular CuInS2 and AgInS2 because of their lower toxicity have been receiving considerable attention as thin film absorbing materials in photovoltaic solar ceils. They show direct band gap that is well matched to the solar spectrum, high energy conversion efficiency, high light absorption coefficient (a=10⁵crrr^!), low production cost and lower toxicity, which are favorable properties for use in photovoltaic ceils.

The nanoscaie semiconductors have been synthesized both in gas phase and liquid phase processes. Typically the liquid phase synthesis is easier to realize in large scale production. Meanwhile the liquid process can be either conducted as a batch or a continuous process. Discontinuous processes have been studied most extensively.

Some approaches employed to prepare Ι-Ι ΙΙ-ΥΊ semiconducting nanoparticles have been reported, such as solvothermal method, single precursor decomposition method and hot injection techniques. The published methods are based on batch process.

CN101054198A describes preparation of CuInS2 nanoparticles by solvothermal method in an autoclave. First, copper salt, indium salt, sul fur source and an alkylamine were dispersed in the solvent to prepare the reaction precursor, which was heated for 12-24 hours to produce CuInS2 nanocrystal with 13-17nm in size. However this process is uneconomic and difficult to scale up because of long reaction time. Another limitation is that pressure is used in the system which means high requirement for used equipment. The particle size is also comparably too large to exhibit the quantum confinement necessary for adequate luminescence which limits applications in solar cell or other photoeiectron devices.

Wet-chemical precipitation method has also been used to produce CuInS2 nanoparticles [P.Guha, Materials Letters, 2003, 57:1786-1791 ]. In this process precursors are dissolved in aqueous solution and precipitation process is controlled by adjusting pH value of reaction solution and concentration of reaction substance. The obtained particles are comparably large (around 2~3μιη) with significant agglomeration, which size is far above the desirable quantum confinement and leads to the lost of the luminescence properties, thus limiting applications in solar ceil or other photoeiectron devices.

Nairn et ai. describes a single-source precursor ([(TOP)2CuIn(SR)4] (TOP ) (octyl)sP; R alky! ) decomposition method by UV light to synthesize CuInS2 NPs and yields organic soluble 2 nm nanoparticles with narrow size distribution [Nano Lett., 2006, 6, 1218-1223]. Castro et ai. [Chem. Mater. 2003, 15, 3142-3147; J. Phys. Chem. B, 2004, 108 (33), 12429-12435] also used molecular single-source precursor (PPSi3)2CuIn(SEt)4 in the presence of hexanethiol in dioctylphthalate to form colloidal CuInS2 at 200 °C. The obtainable average size of the nanocrystais can be increased from 2 to 4 nm by raising the reaction temperature from 200 °C to 250 °C. It is known that CuInS2 can be prepared to n-type or p-type semiconductor. The particles with In-rich are n-type semiconductor, and the particles with Cu-rich are p-type semiconductor. The published papers teach that the ratio of Cu and In will effect the conduction type and quantum efficiency of CuInS2 semiconductors; however the conduction type could not be adjusted using the described decomposition methods.

Further disadvantages of the single-source precursor decomposition methods are that these processes are complex because the single -precursor has to be prepared in advance, and some raw materials are toxic thus limiting scaiing-up and industrialization.

Good mixing and stable temperature control are known to be crucial in the formation process of nanoparticles. A facile method was developed in EP 2263977 wherein low cost raw materials copper salt, indium salt, 1 -dodecanethiol and non-polar solvents were used to prepare CuInS2 nanoparticles. CuInS nanoparticles synthesized are from 2 to 10 nm size with morphologies as sphere, triangle, slice and rods. Defined sizes and morphologies could be synthesized by simply adjusting process parameters. The highest quantum yield was around 10%, which is the best results known in the state of the art for such nanoparticles [EP 2263977; Chem. Mater., 2008, 20, 6434-6443]. Fast heat-up and heat-removal are needed for controlled core generation, growth and quenching. The batch process of EP 2263977 yields to high quality nanoparticles because of the good temperature control and mixing which however can only be achieved in small scale. For the synthesis of larger quantities, several batches would have to be carried out in parallel or sequentially. Fluctuations in the implementation of the various batches would inevitably lead to a loss of quality. Therefore there is a need for a process for continuous synthesis of high quality I-III-VI group ternary or quaternary semiconductors, wherein good mixing and stable temperature control can be reliably achieved when scaiing-up.

DEI 02006055218 describes a continuous process for the synthesis of binary system semiconducting nanoparticles in a microreaction system wherein temperature and volume flows are controlled and a separation of nucleation and growth process is achieved. The system comprises components such as micro heat exchanger and residence reactor, which can optimize the chemical and engineering process parameters and thus allow production of nearly monodisperse and morphologically uniform nanoparticles. Ligands such as trioctylphosphine, trioctylphosphine oxide and oleic acid are used to control the growth of nanoparticles. DEI 02006055218 teaches that the separation between nucleation and growth process allows precise adjustment of ideal individual process parameters leading to optimal control of particle properties compared to batch processes.

Weil in Luan continuously synthesized CdSe nanoparticles in a capillary reactor. Different sizes of CdSe and CdS nanoparticles could be obtained by adjusting the feed ratio of precursors at constant temperature and residence time. Also capillary reaction allowed fast mixing and efficient heat ex change which guarantied precise control of parameters and high reproducibility [Nanoscale Res. Lett., 2008, 3, 134-139].

However the disadvantages of capillary reactor are its limitation to extremely low volume flows and wide residence distribution because of the laminar flow profile usually resulting in a broad particle size distribution. Furthermore, the poor mixing situation in capillary reactor during reaction induces the formation of crystal with much surface defect and side-product, which significantly reduced photoiuminescence efficiency.

Moreover ternary or quaternary I-III-VI semiconducting nanoparticies are complex in comparison to the above mentioned binary systems. For example various cations have different reaction speed under different iigands, therefore nanoparticies with different composition and crystal structure will form depending on the selection of Iigands. Different composition or crystal structure will significantly affect the crystal optical properties and its applications in electronics. Objective of the invention

The objective of the present invention is to provide a continuous process for production of ternary or quaternary semiconducting nanoparticies based on IB, I II A. VIA elements of the periodic classification (I- III- VI semiconducting nanoparticies). The process should provide reliable reproducible quality of nanoparticies, i.e. monodispersion, smaller particle size and narrow particle size distribution and also allow manufacture of these semiconducting nanoparticies in large quantities. Another objective of the present invention is to synthesize ternary or quaternary I-III-VI semiconducting nanoparticies with defined crystal structures. Subject of invention

A new liquid synthesis process via microreaction technology based on simply solvothermal method was developed to overcome the obstacles in traditional batch process such as low reproducibility of particle properties, high production cost and scale -up problems and quality of nanoparticies (such as particles size distribution, emission range, crystal structure and quantum yield).

The process of the in vention al lows the synthesis of ternary or quaternary semiconducting nanoparticies based on IB, II I A, VIA elements of the periodic classification (I-III-VI semiconducting nanoparticies). In particular IB element is selected from copper and silver, IDA element is selected from gallium and indium, and VI is selected from sulfur and seienide. I - 111 - V^" I semiconducting nanoparticies in the present invention are therefore CuInS2, AglnS:, CuInSe2, AgInSe2, CuGaS2, CuGaSe2, AgGaS2, AgGaSe2, CuJnGaS, CuAglnS, CuInGaSe, CuAglnSe, AgEiGaS, AglnGaSe, CuAgGaS, CuAgGaSe. Preferably, l-l ll-Vl semiconducting nanoparticies in present invention are CUE1S2, AgInS2, CuInSe2, AgInSe₂, CuGaS¾ CuInGaS, CuAglnS, AglnGaS.

Most preferably, i-III-VI semiconducting nanoparticies in present invention are CuInS2, AgInS2 because of lower toxicity.

The formation of nanoparticies comprises three consecutive steps: mixing of precursors, nucleation and growth. For the formation of uniform nanoparticies with a narrow size distribution the clean separation of these processes and their independent control is an indispensable prerequisite. Nucleation of nanoparticies initiates from a supersaturated solution at a nucleation temperature, and then growth of nanoparticies follows and is conducted at a growth temperature. The temperature of nucleation of nanoparticies is normally lower than that of growth.

The stable reaction conditions allow the production of high quality nanoparticies with lower surface crystal defect, monodispersion and narrow size distribution.

First object of the present invention is a method for continuous preparation of ternary or quaternary semiconducting nanoparticies based on IB, I IIA, VIA elements of the periodic classification (I-III-VI semiconducting nanoparticies) comprising the following steps:

a) Cation starting material, anion starting material, and at least one iigand are mixed in at least one solvent to form a synthesis mixture, then

b) the synthesis mixture is brought to temperature for nucleation, and then

c) the synthesis mixture is brought to temperature for particle growth.

In a further step d) the synthesis mixture is cooled down, the cooling temperature being lower then the temperature of nucleation and particle growth.

In one embodiment of the present invention, when cation starting material and anion starting material don't generate nucleation below nucleation temperature, synthesis mixture containing cation starting material, anion starting material, at least one ligand and at least one solvent is prepared. Typical synthesis procedure is: cation starting material, anion starting material, and at least one ligand able to coordinate with the cations are mixed in at least one solvent and degassed in N? protection. Optionally the mixture may be have to be heated or stirred to form clear or turbid flowable solution with a viscosity below 50 mPas preferably below 10 mPas.

Cation starting material in the sense of the present invention is a cation source, soluble in the synthesis solution by mixing or after heat treatment, normally metal salts selected from a group comprising copper salt, indium salt, silver salt and gallium salt. Suitable copper salt is copper (I) acetate, copper (II) acetate, copper stearate, copper chloride, copper indine, copper nitrate, copper (I) sulfate, (PPli3)2CuIn(SEt)4 and (PPti3)2CuIn(SePh)4 or any mixture thereof. Suitable Indium salt is indium acetate, indium chloride, indium stearate, indium nitrate, indium sulfate, (PPii3)2CuIn(SEt)4 and (PPli3)2CuIn(SePh)4 or any mixture thereof. Suitable silver salt are silver nitrate, silver sulfate, silver acetate, or any mixture thereof. Suitable gallium salt is gallium chloride, gallium acetate, gallium sulfate, gallium stearate or any mixture thereof.

Anion starting material in the sense of the present invention is an anion source soluble in the synthesis solution by mixing or after heat treatment, which is selected from the following group:

- Se starting material can be selected from the group comprising selenium, bis (trim ethylsilyl) selenide, or any mixture thereof.

- S starting material can be selected from the group comprising aikanethiois having one or more sulfhydryl functional groups, carbon disulfide, sulfur or any mixture thereof. Aikanethiois having one or more sulfhydryl functional group are preferably octylthiol, isooctylthiol, dodecylthioi, hexadecanethioi, octadecanethiol, 1,8-dioctyl thiols, 1,6-dioctyl thiols. Solvent is preferably a non or low polar solvent with high boiling point. The solvent should be stable and degrade as little as possible at reaction temperature. Preferably the boiling point of solvent is above 200 °C, more preferably above 240 °C. Solvent can be selected from the group comprising 1 -octadecene, diphenyl ether, dioctyl ether, diheptyl ether, octadecane, olefin, Diphyl THT (hydrogenated terphenyl), Diphyl DT (isomeric ditolyl ether), or any mixture thereof.

Ligand can be selected from the group comprising oleylamine, oleic acid, trioctyiphosphine, trioctyiphosphine oxide, myristic acid, aikanethiois having one or more than one sulfhydryl functional groups, or any mixture thereof. Aikanethiois having one or more sulfhydryl functional group are preferably octylthiol, isooctylthiol, dodecylthioi, hexadecanethioi, octadecanethiol, 1 ,8-dioctyl thiols, 1,6-dioctyl thiols. It. is known that use of oleylamine induces wurzite or zincbland-type CuInS2 nanoparticies; Hexadecylamine induces wurzite-type CuInS2 nanoparticies; aikane thiol (such as dodecanethiol) induces chaicopyrite-type CuInS2 nanoparticies; trioctylphosphite and triphenyiphosphite induce wurzite-type CuInS2 nanoparticies [Material Transactions, 2008, 49, 435-438; Chem. Mater. 2009, 21, 2607-2613].

Experiments conducted within the present invention have shown that oieic acid induces wurzite-type CuInS2 nanoparticies, oieyiamine induce both chalcopyrite and wurzite-type CuInS2 nanoparticies depending on reaction time and temperature, and myristic acid induce wurzite-type CuInS2 nanoparticies.

For better photo!uminescence quantum yield in the production of CuInS2 aikanethioi as iigand is preferred. For the preparation of l-ll l-VI semiconducting nanoparticies the molar ratio of different cations was found to have a strong effect on the quality and properties of produced material.

For ternary nanoparticies, moiar ratio of two cations is (1~5):(5~1), preferred (1~2):(2~1). For quaternary nanoparticies, molar ratio of any two cations is (1~5):(5~1) and preferred (1~2):(2~1), and the moiar ratio of the third cation to any one of above two cations is lower than 5 and preferred lower than 2.

A total amount of metal (i.e. copper, indium, silver and gallium) salts is in a concentration from 0.001 M to 1 M, preferably from 0.005 M to 0.5 M, and more preferably from 0.008 to 0.1 M in solvent. The molar ratio of iigand to the total amount of metal salts is above 1, preferably from 1 to 80, and more preferably from 2 to 50.

For the moiar ratio of anion starting material to metal salts, normally higher production yield is obtained when anion starting material is in excess (more than stoichiometric amount). Preferably the moiar ratio of the total amount of anion starting material to the total amount of metal salts is from 0.5 to 50 preferably from 1 to 25, and more preferably from 1 to 10.

Typically the temperature for precursor solution preparation ranges from 25°C to 220°C, preferably from 40°C to 200°C, more preferably from 50°C to 180°C. In another embodiment of the present invention, cation-containing precursor solution and anion- containing precursor solution are prepared separately. Synthesis mixture is then prepared by mixing cation-containing precursor solution and anion-containing precursor solution. In cation-containing precursor solution, a total amount of metal (i.e. copper, indium, gallium and silver) salts is usually in a concentration from 0.001 M to 2 M, preferably from 0.005 M to 0.1 M, and more preferably from 0.008 to 0.2 M in solvent. The molar ratio of ligand to the total amount of metal salts is above 1 , preferably from 1 to 80, and more preferably from 2 to 50. Typically the temperature for cation-containing precursor solution preparation ranges from 25°C to 220°C, preferably from 40°C to 200°C, more preferably from 50°C to 180°C.

Anion-containing precursor solution comprises anion starting material and at least one solvent. Typical procedure for anion-containing precursor solution comprises mixing anion starting material with a solvent in flask and degassing to form an optical clear solution by simply stirring, heating or supersonicating. The previously mentioned selected ligand may be added if the anion source is not readily soluble in the solvents used.

Solvent for anion-containing precursor solution can be selected from from the group comprising 1 - octadecene, diphenyl ether, dioctyl ether, diheptyl ether, octadecane, olefin, Diphy! THT (hydrogenated terphenyl), Diphyl DT (isomeric ditolyl ether), oleylamine, oleic acid, trioctylphosphine, trioctylphosphine oxide, myristic acid, alkanethiols having one or more than one sulfhydryl functional groups, or any mixture thereof. Alkanethiols having one or more sulfhydryl functional group are preferably octylthiol, isooctylthiol, dodecyithioi, hexadecanethiol, octadecanethiol, 1 ,8 -dioctyl thiols, 1 ,6 -dioctyl thiols.

For the molar ratio of anion to cation, normally higher production yield is obtained when anion starting material is in excess (more than stoichiometric amount). Preferably molar ratio of anion starting material to the total amount of metal salts is from 1 to 50, and more preferably from 1 to 25. Mixing of cation-containing precursor solution and anion-containing precursor solution can be achieved by traditional agitation or in a micro mixer.

Typically the continuous process of the present invention is conducted in a microreaction system. Microreaction system usually refers to micro structured operation units with inner dimensions of from 10 nm to 1 mm, comprising elements such as micromixer, micro heat exchanger micro residence reactor, sensor (such as temperature sensor, pressure sensor, pH sensor), actuator ( such as pressure controller, etc.) etc.

Used micromixer is typically based on the multilamination principle wherein the streams of fluids to be mixed are separately fanned out in a large number of thin lamellae. The lamellae of the two fluids are alternately arranged so that an interdigital configuration is generated. Due to diffusion and secondary flows, the molecules of the fluids mix rapidly and efficiently. During the mixing of precursor solutions, a micromixer keeps them in good mixing condition. Compared to a conventional heat exchanger, used micro heat exchanger shows higher specific surface area, which increases the heat transfer efficiency therefore the synthesis mixture can be heated or cooled within several seconds, which permits separation of nucieation and growth progresses of iianoparticies by simply controlling the temperatures. Typically the heating/cooling procedure is conducted with a heating/cooling rate of > 1 K s, preferably with a heating/cooling rate of > 10 K s and most preferably with a heating/cooling rate of > 100 K/s.

Micro residence reactor is preferred to significantly improve heat transfer and for a faster and more efficient heating of synthesis mixture. Most preferred micro residence reactor is a microstructured residence reactor with heat exchanging and static mixing features, which allows sustained cross-mixing during the reaction and reduce the formation of side-products. Preferred micro residence reactor comprises a tube or capillary reactor, has a heat transfer surface area to volume ratio (A/V ratio) of at least 1000 1/m and also comprises static mixing features to enhance cross-mixing of the synthesis mixture and narrow the residence time distribution within the tube or capillary reactor.

In the micro residence reactor the synthesis mixture can be heated or cooled by heating media to achieve the desired temperature. Microwave can also be used to quickly heat the synthesis mixture.

When using a microreaction system, precise control of reaction conditions (e.g. increasing rate, residence time, temperature) contributes to high reproducibility of particles properties. Preferably the microreaction system for the continous preparation of ternary or quaternary l-l ll-VI semiconducting nanoparticles comprises at least one micro heat exchanger and at least one micro residence reactor, wherein step b) is conducted in the micro heat exchanger and step c) is conducted in the micro residence reactor. In other words detailed synthesis in micro reaction system comprises the following steps: synthesis mixture is pumped into at least one micro heat exchanger and heated to expected temperature for nucieation, and then the synthesis mixture is brought to expected temperature in at least one micro residence reactor for particle growth. It. is preferred that the synthesis mixture is cooled down, the cooling temperature being lower than the temperature of nucleation and the growth temperature. More preferably, the synthesis mixture is cooled in a further micro heat exchanger. it is preferred that the reaction process is monitored by online analytical measurement using online analytical measurement device.

Precursor soiutions or synthesis mixture can be stored in heatable containers. The solutions or synthesis mixture are normally at room temperature. When precursor solutions or synthesis mixture is solid or sticky at room temperature, it has to be heated to keep the solution as one phase liquid.

Temperature of precursor solutions or synthesis mixture in heatable containers is normally from room temperature to 180 °C, preferably from 25 °C to 100 °C in all cases below temperature for nucleation. The micro heat exchangers for heating or cooling usefully have heat transfer surface area to volume ratio (AN ratio) > 20,000 1/min, preferably > 25,000 1/m and in particular > 30,000 i/min and allows setting nucleation temperature > 200 °C, preferably from 220 °C to 300 °C and in particular from 230 °C to 290 °C. A counter flow micro heat exchanger is preferred. The residence time in micro heat exchangers for heating or cooling is usefully set out from 90 s to 0.18 s, preferably from 72 s to 0.36 s, and particu iariy from 36 s to 0.9 s at a flow rate of the synthesis mixture of 0.2 ml/min up to lOOmi/min, preferably from 0.25ml/min up to 50mi/min, and especially from 0.5ml/min to 20ml/min. Micro residence reactor (4) usefully has heat transfer surface area to volume ratio (A/V ratio) > 800 1/m, preferably > 1000 1/m and in particular > 1200 1/m. The set temperature is usually > 200 °C, preferably from 220 °C to 300 °C, and more preferably from 230 °C to 270 °C. Normally the set temperature in micro residence reactor for particle growth is less than or equal to that in micro heat exchanger for nucleation.

The micro residence reactor ensures the good radial mixing, high efficient heat transfer and narrow residence distribution. The average residence time in micro residence reactor is usefully from 18 s to 150 min, preferably from 36 s to 120 min, and especially from 90 s to 60 min at a flow rate of the synthesis mixture from 100 ml / min to 0.2 ml / min, preferably from 50 mi / min to 0.25 mi / min and more preferably from 20 ml / min to 0.5 ml / min. More than one micro residence reactors and/or micro heat exchangers can be connected sequentially for increasing the production capacity.

It is preferred that the micro residence reactor comprises static mixing internals and heat exchanging features. Such a micro residence reactor is significantly narrower than that a tube or capillary and allows a narrow residence time distribution of synthesis mixture so nanoparticles with a narrow size distribution can be obtained.

In a further step of the process, anti-solvent is mixed with the cooled synthesis mixture for temporary precipitation of semiconducting nanoparticles. It is preferred, that the mixing procedure with is operated continuously in a micro mixer. The mixing time in micro mixer (9) is usually < 10 s, preferably < 5 s, more preferably < 0.5 s.

In a further st ep of the process, semiconducting nanoparticles are separated and washed.

In a separation step, semiconducting nanoparticles are separated from the cooled synthesis mixture. Standard procedures such as ultrafiltration, membrane filtration, dialysis, centrifugation and evaporation can be used. Anti-soivent, targeted for launch of the aggregation and precipitation of the nanoparticles, can be used to separate semiconducting nanoparticles from the cooled synthesis mixture. Anti-solvent are typicaily selected from the group comprising acetone, methanol, ethanol, iso-propanol, propanol, or any mixture thereof. In a washing step, the separated semiconducting nanoparticles are typically redispersed in an appropriate dispersion solvent and anti-solvent is added and mixed, and then semiconducting nanoparticles are separated by using the above standard procedures (ultrafiltration, membrane filtration, dialysis, centrifugation and evaporation). The semiconducting nanoparticles are preferably washed 3 to 4 times through repeating the above washing procedure.

Dispersion solvent in present invention can be selected from toluene, chloroform, hexane, cyciohexane or any mixture thereof. In a further step cleaned semiconducting nanoparticies can be dried in vacuum below 1 10 °C or redispersed in above mentioned appropriate dispersion solvent for analysis.

A uniform size of semiconducting nanoparticies, synthesized by the process in present invention, is obtained. The size range of the nanoparticies is specifically from 1 nm to 20 nm, preferred 2 nm to 10 nm.

The synthesized ternary or quaternary I-III-VI semiconducting nanoparticies are of chalcopyrite and wurzite crystal structure depending on selected ligands and process parameters. The process allows the uniform composition of semiconducting nanoparticies and reduces surface crystal defect which offers a higher photo lumine sc enc e quantum yield. in particular CuInS2 nanoparticies with high photoluminescence quantum yield of 13.9 % without any other post treatment were synthesized.

A further object of the present invention is therefore a semiconducting nanoparticle obtainable by the preparation process of the present invention.

The size and crystal structure of the semiconducting nanoparticl e can be controlled by adjusting reaction parameters in particular concentration, reaction condition (such as flow and temperature) and selected ligands.

The process of the invention can be easily implemented from the laboratory to production scale. A scale- up can be realized by just simply increasing the channel number, number of operation units, or parallel synthesis lines.

Optical and crystal properties of obtained semiconducting nanoparticies were characterized by following method:

1) UV-VIS and Photoluminescence Spectra measurement: UV-VIS absorption spectrum of the sample is measured by UV-VIS spectrophotometer (Specord 40, Analytik Jena). The sample is diluted by toluene until the absorption value of sample is lower than 0.1 at 366nm. The photoluminescence spectrum of the sample is measured by spectrofluorimeter (Fluorolog 3-22, HORIBA Jobin Yvon) at excitation wavelength of 366nm.

2) Photoluminescence quantum yield: Quantum yield of sample is determined by comparing a standard dye with a. known quantum yield. In present invention, ATT0635 is used as standard dye.

Quantum yield of ATT0635 in water is 25% and its emission peak is 659nm. The sample is diluted in toluene following the measurement rules of UV-V IS and photoluminescence spectra. The quantum yield of sample is determined according to the following equation,

wherein,

YQ refers quantum yield of sample

Ys refers quantum yield of standard dye

FQ FS refer integral intensity under photoluminescence spectrum of sample and standard dye, respectively.

AQ -fef As refer absorption at excitation wavelength of sample and standard dye, respectively. DQ^■¾ IX refer the refractive index of the corresponding solvent of sample and standard dye.

3) X-ray diffraction ( XRD )

The crystal structure of samples was measured by X-ray diffraction devices (diffractometer D 5000, Siemens). The sample is dispersed on silicon wafer, dried and measured.

4) Transmission Electron Microscopy ( TEM ) : TEM picture of sample is obtained with a microscope (CM20, Philips). The sample is drop-cast from a diluted dispersion in toluene onto a thin holey carbon film by a copper grid.

The semiconducting nanoparticies of the present invention can be used for the manufacture of electronic devices and in particular for printed eiectronic devices such as solar ceils.

Further objects of the present invention are therefore an ink as well as an eiectronic device comprising the semi-conducting nanoparticies of the present invention. Figures:

Brief description of the figures:

Figure 1 : Diagram of the continuous synthesis process of semiconducting nanoparticies according to the present invention.

Reference number:

1 - h eatable container 1

2 - heatable container 2

3 - micro mixer 4 - micro heat exchanger (for heating)

5 - micro residence reactor

6 - micro heat exchanger (for cooling)

7 - online analytical measurement device

8 - anti-solvent

9 - micro mixer

10 - separation and washing of semiconducting nanoparticies

1 1 - redispersion or drying Figure 2 - TEM pictures of CuinS^ nanoparticies prepared in Example 2

Figure 3 - TEM pictures of CuInS2 nanoparticies prepared in Example 4

Figure 4 - TEM pictures of CuInS2 nanoparticies prepared in Example 5

Figure 5 - TEM pictures of CuInS2 nanoparticies prepared in Example 6

Figure 6 - X RD spectrum of CuInS2 nanoparticies prepared in Example 1

Figure 7 - XRD spectrum of CuInS2 nanoparticies prepared in Example 4

Figure 8 - UV-VIS absorption and photoluminescence spectra of CuInS2 nanoparticies prepared in

Example 1 , Example 2 and Example 3.

Figure 9: Diagram of the continuous synthesis process of semiconducting nanoparticies according to the present invention.

Reference number:

1 - h eatable container 1

2 - micro heat exchanger (for heating)

3 - micro residence reactor

4 - micro heat exchanger (for cooling)

The following examples illustrate the present invention without restricting it thereto.

Example 1 : CuInS nanoparticies synthesis

1) Copper (I) acetate 0.195 g (1.592 mmol), indium acetate 0.466 g (1.595 mmol), 1 -dodecanethiol 4 ml and i -octadecene 160 ml were added into 250 mi three bottle-neck flask in N2 protection. The synthesis mixture was degassed for 30 minutes in N2 (10 L/h) and then heated to 180°C until powders were completely dissolved. The synthesis mixture was cooled down to 100°C.

2) The synthesis mixture was put into a container at 100 °C and then pumped into microreaction system (Ehrfeld Mikrotechnik Bayer Technology Services GmbH) through FfPLC pump at flow rate 2 ml/min.

3) The synthesis mixture was heated in the first micro heat exchanger (counter flow micro heat exchanger, V « 0,3 mi, A « 0,0076 m², Ehrfeld Mikrotechnik Bayer Technology Services GmbH) for nucleation and obtained nuclei were allowed to grow in the microstructured residence reactor with heat exchanging and static mixing features (Sandwichreactor, V « 30 ml, A « 0,03 m² Ehrfeld Mikrotechnik Bayer Technology Services GmbH), and then the synthesis mixture was quenched in second micro heat exchanger (counter flow heat exchanger, V « 0,3 ml, A « 0,0076 m², Ehrfeld Mikrotechnik Bayer Technology Services GmbH) by cooling down. The set temperatures in first micro heat exchanger and micro residence reactor were 260°C and 260°C respectively; the set temperature in the second micro heat exchanger was 25°C. The residence time was 15 minutes. 4) The cooled reacted solution was collected from the microreaction system and two equivalent volume of acetone was added to form suspension. The suspension was then centrifuged at 9000 rpm for 10 minutes and the supernatant was removed.

5) The separated precipitate was redispersed in toluene, and acetone (same volume as step 4)) was added and mixed to form suspension. The suspension was then centrifuged at 9000 rpm for 10 minutes and the supernatant was removed.

6) The step 5) was repeated for twice. Then the obtained CuInS2 nanoparticies were redispersed in toluene and stored under N; atmosphere.

UV-VIS absorption and photoiumiiiescence spectra of obtained CuInS2 nanoparticies are shown in Figure 8. The emission peak of CuInS2 nanoparticies is at 614 nm. XRD pattern (Figure 6) shows that they are of chalcopyrite crystal structure. A quantum yield of 12% was measured.

Example 2; CuInS2 nanoparticies synthesis

The experiment procedure were the same as the one described in example 1 , but the flow rate in example 2 was decreased to 1 mi / min therefore the residence time was prolonged to 30 minutes accordingly. UV-VIS absorption and photoluminescence spectra of obtained CuInS2 nanoparticles are shown in Figure 8, The emission peak of CuInS2 nanoparticles is at 624 nm. TEM picture (Figure 2) shows that the particle size is around 3.5 nm. A quantum yield of 13.9% was measured. Example 3; CuInS? nanoparticles synthesis

The experiment recipe and procedure were the same as the one described in example I, but the set temperatures in micro heat exchanger and microstructured residence reactor with heat exchanging and static mixing features were 270°C and 250 °C respectively. UV-VIS absorption and photoluminescence spectra of obtained CuInS2 nanoparticles are shown in Figure 8. The emission peak of CuInS₂ nanoparticles is at 682 nm.

Example 4; CuInS2 nanoparticles synthesis

1) Copper (I) acetate 0.732 g (5.971 mmol), indium acetate 1.746 g (5.98 mmol), oleic acid 26.9g and 1 - octadecene 150 ml were added into 250 ml three bottle-neck flask in N2 protection. The mixture was degassed for 30 minutes in N2 (lOL/h) and then heated to 180°C until powders were completely dissolved. The above cation precursor solution was cooled down to 50°C.

2) 1-dodecanethiol 12.67g and Dipliyl TU T 160.4g was mixed to form clear anion precursor solution.

3) The cation and anion precursor solutions were mixed at 50°C and put into a container, then mixture was heated to 100°C and pumped into microreaction system (Ehrfeld Mikrotechnik Bayer Technology Services GmbH) through HPLC pump at a flow rate of 3 ml/min.

4) The precursor solution was heated in the first micro heat exchanger (counter flow heat exchanger, V « 0,3 ml, A « 0,0076 m², Ehrfeld Mikrotechnik Bayer Technology Services GmbH) for nucleation and obtained nuclei were allowed to grow in the micro residence reactor with heat exchanging and static mixing features (Sandwichreactor, V « 30 ml, A « 0,03 m² Ehrfeld Mikrotechnik Bayer Technology Services GmbH). Then the solution was quenched in second micro heat exchanger (counter flow heat exchanger, V « 0,3 ml, A « 0,0076 m², Ehrfeld Mikrotechnik Bayer Technology Services GmbH) by cooling down. The set temperatures in first micro heat exchanger and micro residence reactor were 260°C and 240°C respectively; the set temperature in second micro heat exchanger was 25°C. The residence time is 10 minutes.

5) The cooled synthesis mixture was collected from the microreaction system and two equivalent volume of acetone was added to form suspension. The suspension was then centrifuged at 9000 rpm for 10 minutes and the supernatant was removed.

6) The separated precipitate was redispersed in toluene and acetone (same volume as step 5)) was added and mixed to form suspension. The suspension was then centrifuged at 9000 rpm for 10 minutes and the supernatant was removed. 7) The above procedure 6) was repeated for 3 times. Then the obtained CuInS2 nanoparticles were redispersed in toluene and stored under N · atmosphere.

TEM picture of obtained CuInS2 nanoparticles is shown in figure 3 and shows that the size is around 5.6 nm. XRD pattern (Figure 7) shows that CuInS2 nanoparticles have wurzite crystal structure.

Example 5 : CuInS? nanoparticles synthesis

Sample 5 was achieved when the flow rate was increased to 6ml/min and all other recipes and parameters were same as example 4.

TEM picture of obtained CuInS2 nanoparticles (Figure 4) is shown that the particle size is around 5.4 nm. Example 6: CuInS? nanoparticles synthesis

1) Copper (I) acetate 0.732 g (5.971 mmol), indium acetate 1.746 g (5.98 mmol), oleic acid 26.9 g and

1 -octadecene 150 ml were added into 250 ml three bottle-neck flask in N2 protection. The mixture was degassed for 30 minutes in N2 (l OL/h) and then heated to 180°C until powders were completely dissolved to form cation-containing precursor solution. The above cation precursor solution was cooled down to 50°C.

2) 1-dodecanethiol 12.67g is acted as anion -containing precursor solution.

3) The cation-containing and anion-containing precursor solutions were mixed at 50°C by agitation and put into a heatable container. Then the synthesis mixture was pumped into the microreaction system (Ehrfeld Mikrotechnik Bayer Technology Services GmbH) through 1 1 PLC pump at a flow rate of 6 ml/min.

4) The synthesis mixture was heated to respected temperature in the first micro heat exchanger (counter flow heat exchanger, V « 0,3 ml, A « 0,0076 m², Ehrfeld Mikrotechnik Bayer Technology Services

GmbH) for nucleation and obtained nuclei were allowed to grow in the first micro residence reactor with heat exchanging and static mixing features (Sandwichreactor, V * 30 mi, A « 0,03 m² Ehrfeld Mikrotechn ik Bayer Technology Services GmbH). Then the solution was quenched in second micro heat exchanger (counter flow micro heat exchanger, V « 0,3 ml, A « 0,0076 m², Ehrfeld Mikrotechnik Bayer Technology Services GmbH) by cooling down. The set temperatures in first micro heat exchanger and micro residence reactor were 260°C and 240°C respectively; the set temperature in the second micro heat exchanger was 25°C. The residence time is 5 minutes.

5) The cooled synthesis mixture from microreaction system was collected and two equivalent volume of acetone was added to form suspension. The suspension was then centrifuged at 9000 rpm for 10 minutes and the supernatant was removed. 6) The precipitate was redispersed in toluene, and acetone (same volume as procedure 5)) was added and mixed to form suspension. The suspension was then centrifuged at 9000 rpm for 10 minutes and the supernatant was removed.

7) The above step 6) was repeated for 3 times. Then the obtained CuInS2 nanoparticles were redispersed in toluene and stored under N2 atmosphere.

TEM picture of obtained CuInS2 nanopartieles( Figure 5) is shown that the particle size is around 5 nm.

Claims

Claims:

1) Method for the continuous preparation of ternary or quaternary semiconducting nanoparticles based on IB, M IA, VIA elements of the periodic classification comprising the following steps:

a) Cation starting material, anion starting material, and at least one ligand are mixed in at least one solvent to form a synthesis mixture, then

b) the synthesis mixture is brought to temperature for nucleation, and then

c) the synthesis mixture is brought to temperature for particle growth. 2) Method according to claim 1 wherein in a step d) the synthesis mixture is cooled down, the cooling temperature being lower than the temperature of nucleation and the growth temperature.

3) Method according to one of the claim 1 or 2 wherein for ternary nanoparticles, molar ratio of two cations is (1~5):(5~1) and for quaternary nanoparticles, molar ratio of any two cations is (1~5):(5~1), and the molar ratio of the third cation to any one of above two cations is lower than 5.

4) Method according to one of the claim 1 to 3 wherein cation starting material is one or more metal salt and the synthesis mixture comprises a total amount of metal salt in a concentration from 0.001 M to 1 M.

5) Method according to one of the claim 1 to 4 wherein the ligand is selected from the group comprising oieyiamine, oleic acid, trioctyiphosphine, trioctyiphosphine oxide, myristic acid, alkanethiols having one or more than one sulfhydryl functional groups, as well as mixture thereof. 6) Method according to one of the claim 1 to 5 conducted in a microreaction system comprising at least one micro heat exchanger and at l east one micro residence reactor wherein in step b) is conducted in the micro heat exchanger and step c) is conducted in the micro residence reactor.

7) Method according to claim 6 wherein the micro residence reactor comprises a tube or capillary reactor, has a heat transfer surface area to volume ratio of at least 1000 1 / m and comprises static mixing features.

8) Method according to claim 7 wherein the synthesis mixture has a viscosity below 50 mPas. 9) Ternary or quaternary semiconducting nanoparticle based on IB, 11 B, I I IA, VIA elements of the periodic classification obtainable by the liquid phase preparation process of the present invention. 10) Nanoparticie according to claim 9 are of chalcopyrite or wurzite crystal structures.

11) ink comprising the semi-conducting nanoparticie according to one of the claims 9 or 10.

12) Electronic device comprising the semi-conducting nanoparticie according to one of the claims 9 or 10.