The present invention relates to a method for surface modification procedure of semiconducting nanoparticles with polymer or polymer nanowire, ink formulation and electronic devices comprising the semiconducting nanoparticles obtainable by the procedure of the invent ion.

Solar ceils or photovoltaic cells (PV ceils) are electronic devices, which convert light energy directly into electric energy.

Many di ferent types of solar cells are known in the art. For example silicon-based solar cells are obtained from monocrystal silicon wafer as they are used for the manufacture of semiconductors. This manufacture process is rather expensive.

With the aim of reducing production costs organic solar cells have been developed in the recent years.

An organic solar ceil (also called polymer solar cell) is made of organic material that is hydrocarbon compounds and particularly polymers. The advantage of a polymer solar cell compared to the silicon solar ceils are:

lower production costs,

high current efficiency through t in layer-large scale technology, flexibility, transparence and easy handl i ng (physical properties of polymer materials)

- environmental friendly (carbon-based material)

easy adjustment to the light spectrum through selective polymer production,

Colored solar cells for architecture design.

However current organic solar cells show reduced energy conversion efficiency and lifetime compared to solar cells based on inorganic semiconducting material. Therefore, a hybrid organ ic solar cell, which is a combination o both advantages from polymer and inorganic materials, can fulfi l l the requirement to improve energy conversion efficiency and lifetime since both polymer and inorganic materials absorb light in a wider range of solar spectrum than the pure polymer solar cells and the silicon solar cells. T!ie most efficient organic solar cells make use of Donor-Acceptor (D-A) systems that is of the combination of semiconducting materials, which after light absorption allow quasi instantaneous («1 ps) transfer of a charge carrier between donor and acceptor (i.e. combination of polymer and fuiierenes). Absorption of photons with energy higher than the gap between HOMO and LUMO of either D or A triggers production of excitons which split up for a short time due to local electric field on the D-A contact surface. The transport of the charge carrier occurs selectively in the semiconductorafter the split. The charge earner hops within an amorphous or microcrystalline semiconducting structure comprising a whole range of energy barriers in a disordered way. Thereby carriers also meet molecule and phase limits, and recombine so the charge split is lost.

Hybrid solar ceils and printable electronic devices consists of an active layer, which is a nanostructured composite of semiconducting nanoparticles and semiconducting polymer (bulk-he teroj unction devices). Ideally, the nanostructured active layer must be formed by nanometer size domains that ensure the maximum interface area between both materials in order to create the maximum number of excitons possible. On the other hand, it is necessary to provide pathways between semiconducting nanoparticles and semiconducting polymer for good electron and hole transportations within layer, which subsequently leads to a high device performance. This requires that the semiconducting nanoparticles should ideally form a three dimensional net structure surrounded by polymer matrix, where each particle is directly in contact with others.

To enhance performance of hybrid solar cells and printable electronic devices optimal morphology of semiconducting polymer and semiconducting nanoparticles have been investigated:

Polythiophene have been intensively studied for use in polymer/fullerene bulk- heterojunction photovoltaic devices because they absorb light throughout the entire visible spectrum and show high hole mobility, which are the two most important parameters for a successful donor in these devices. Moule et al investigated the influence of polymeric nanoparticles on the morphology of the bulk-heterojunetion active layer of solar cells fabricated with mixtures of polymers and concludes that both the size of the crystalline polymer domain (preformed or formed during postprocessing treatment) and the amount of amorphous matrix that acts as a glue between the crystalline domains are critical for good charge transport in bulk- heterojunction solar cells [J. Phys.CSiem. C. Vol 1 1 2. No. 33, 2008, p. 12583- 12589]. Berson et al. describe use of P o I y ( 3 - h e x y 11 h i op h e n e ) (P3HT) nanofibers mixed with a molecular acceptor in solution to obtain in a simple process a highly efficient active layer for organic solar cells with a demonstrated power conversion efficiency (PCE) of up to 3.6 % without the need for thermal post- treatment to obtain optimal performance [Berson et al, Adv. Funct. Mat. 2007, 17, 1377- 1384]. The synthesis of semiconducting nanoparticles is usually carried out by a soivothermal process, wherein primary ligands, such as trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), oleic acid (OA), tetradecylphosphonic acid (TDPA), tri-n-Butylphosphine (TBP), octylphosphonic acid (OPA) or other high- boiling hydrophobic ligands, are used as protective ligands covering around the surface of nanoparticles to shield them from unintentional agglomeration, subsequently results as a stable dispersion of nanoparticles. However, these primary ligands are large molecule, which affects on t e electrical conductivity o nanoparticles since they act as insulating layer. The use of those semiconducting nanoparticles without an additional step to modify surface of nanoparticles in hybrid solar ceils or other printable electronic applications leads to very low performance due to the insulating layer of primary ligands surrounding nanoparticle surface. It is known that following a defined procedure for the post-synthesis treatment of the semiconducting nanoparticles is advantageous for the ink formulation leading to higher performance printable electronics devices. To improve the conductivity o semiconducting nanoparticle-semiconducting nanoparticle and of semiconducting nanoparticle-semiconducting polymer, the primary ligands coming from the synthesis of semiconducting nanoparticles, should be completely removed from nanoparticle surfaces prior the device fabrication, A post-synthesis procedure comprises the semiconducting nanoparticles being washed with methanol prior to iigand exchange (US2007/0132052A1). The washed semiconducting nanoparticles are then treated to remove the insulating primary ligands being necessary for the synthesis to substitute them with preferably low molecular weight, volatile and/or fairly conductive species without causing agglomerations. TGA-MS analysis conducted on the washed nanoparticles show that methanol washing only achieves partial removal of primary iigand layer B. Nevertheless, although the complete elimination of the ligands in the final layer is a must, the lack of these compounds leads to particle stability problems in the formulation of the ink. Xu et al. describe the preparation of poly(3-hexylthiophene) ( P3 HT) cadmium selenide quantum dot (CdSe QD) hybrid coaxial nanowires by a stepwise self-assembly process in a poor solvent. After preparation CdSe nanocrystals are precipitated into acetone and redispersed by hexane more than three times, then redispersed in cyclohexanone. In order to ensure the P3HT combined with CdSe by replacing ligands, a long stirring time and high temperature are necessary. CdSe QDs are deposited compactly onto the P3HT nanowires by non-covalent interactions between P3HT and CdSe. When illuminated with white light, the hybrid nanowires show enhanced photoconductivity compared with the pristine P3HT nanowires and the blended nanocomposites so Xu et al. conclude that the one-dimensional structure of the coaxial nanowires contributes to the charge transfer significantly. [Xu et al, Macromol. Rapid Commun. 2009, 30, 1419-1423]. Challenges in the design of hybrid buik-heterojunction solar cells are therefore on one hand ensuring a stable/metas table dispersion of semiconductor nanoparticles and polymers and on the other hand enabling a charge transfer from or to the nanoparticles and between them. A further challenge is the final domain size or morphology of the final product is defined by the materials and solvent used in the formulation of the ink as well as by the coating technique used. Ultimately the formation of the domains, takes place during the casting and drying of the ink and this usually leads to a low reproducibility and limited control due to the high number of variabl es involved in the process.

Therefore, the object of the present invention is to provide an ink formulation which can lead to a better control of the final nano-domain size, should on one hand ensure a stable? metastabie dispersion of semiconductor nanoparticles and polymers and on the other hand enable a charge transfer from or to the nanoparticles and between them.

The solutions of the present invention are materials and processes wherein primary ligands from inorganic semiconducting nanoparticles are completely or nearly completely removed and obtained semiconducting nanoparticles are directly decorated with semiconducting polymer and/or semiconducting crystalline polymer nanowires.

The following defined process can be divided into the following steps, removal of primary ligands from semiconducting nanoparticles by washing and cleaning, synthesis of crystalline polymer nanowire, optionally in situ, and stabilization of nanoparticles after the primary ligand removal by redispersing them with a semiconducting polymer solution or semiconducting crystalline polymer nanowires in a proper solvent. The process of the invention provided a stable ink formulations; the use of the crystalline polymer nanowires in the formulation lead to a better control of the final nano-domain size, since the polymer nanodomains are formed prior to the casting and drying of the ink. The inks formulated can be subsequently used for hybrid solar cell fabrication or other printable electronics applications.

Nanoparticles of the present invention are inorganic semiconducting nanoparticles in form of quantum dots (QD), nanorods (NR), tripods, tetrapods or other multipeds, which comprise at least one of PbS, InAs, InP, PbSe, CdS, CdSe, CdTe, In _x Ga _1-x As, (Cd-Hg)Te, ZnSe(PbS), ZnS(CdSe), ZnSe(CdS), PbO(PbS), CuInS ₂ , CuInSe ₂ and PbS0 ₄ (PbS).

For the removal of primary ligands semiconducting nanoparticles are washed with a suitable organic solvent or solvent-mixture. Suitable washing agents are alcohols like methanol, ethanol, propanol, or higher linear (C4 to C8 containing alcohol) or branched alcohols (such as mono-methylated alcohols, polyisoprenoid alcohols, isobutyl alcohol, sec-butyl alcohol, tert-butyl alcohol, as well as m ixtures thereof. Methanol, ethanol and propanol as well as mixtures thereof are preferred.

100 mg of nanoparticles are usually dispersed in 1 mi of dispersion solvent and washing agent is added to a ratio of dispersion solvent to washing agent of 1 :5 to 1 : 100, preferably 1 :5 to 1 :50, most preferably 1 :5 to 1 :20. Semiconducting nanoparticles precipitate and the precipitate is usually separated using centrifugation method.

Preferably the washing step is repeated several times. A redispersable dry powder can be obtained.

After washing, the re -precipitation of semiconducting nanoparticles by methanol shows clear supernatant after a short time of centrifugation, which facilitates time consumption as well as the removal of rest primary ligands. Semiconducting nanoparticles are usually isolated by centrifugation or other method known in the art. Then they are optionally dried under nitrogen flow under an inert atmosphere to prevent oxidation or other undesired reactions for analysis. A redispersable dry powder is obtained after washing and drying.

As already mentioned TGA-MS analysis conducted on the washed nanoparticles show that methanol washing according to U S2007/0132052 A 1 only achieves partial removal of pri mary ligand layer.

This is why washed semiconducting nanoparticles are then treated with cleaning molecules, which can remove as much as possible of the primary iigands (e.g. . trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), oleic acid (OA), tetradecyiphosphonic acid (TDPA), tri-n-Butyiphosphine (TBP), octylphosphonic acid (OPA) or other high-boiling hydrophobic iigands) from the surface of the semiconducting particles after separation from the synthesis broth.

The selection of cleaning molecules should base on the following criteria

a. An affinity should be at least in the same range as the affinity of the primary iigands from the synthesis to nanopart icles. Most preferably the affinity should be even higher than the affinity of the primaiy Iigands from the synthesis to nanoparticles (e.g. a!kanethio! )

b. be volatile, i.e. a low boiling point (< 150 °C) and high vapour pressure to open up the possibility for easy evaporation during the annealing step after formation of the active layer in case that these molecules are still on nanoparticle surfaces.

c. do not form any covalent bond with the semiconducting nanoparticles

Usually cleaning molecules with functional groups containi ng nitrogen, sulfur, carbon or phosphor are used for this procedure. Especially preferred are thiols and particularly short- chain alkane thiol molecule (C2 to C6 containing thiol) such as Ethane thiol, Ethanedithiol.

The semiconducting nanoparticles after this step are named cleaned nanoparticles. The amount of primary Iigands from the synthesis after the cleaning treatment was shown to be surprisingly low (less than 3 wt% on semiconducting nanopart icle surfaces). T!ie cleaned nanoparticies usually precipitate due to the higher density compared to the continuous phase in which they are dispersed. Therefore, after the washing procedure, the nanoparticies are usually precipitated and separated as sediment. First object of the present invention is therefore a method for the post-synthesis treatment of semiconducting inorganic nanoparticies with the following step:

removal of primary ligands from semiconducting nanoparticies by washing with an organic solvent or solvent mixture and cleaning with cleaning molecules cleaning molecules wherein cleaning molecules show an affinity in the same range as the affinity of the primaiy ligands from the synthesis to nanoparticies, are volatile and do not form any covaient bond with the semiconducting nanoparticies leading to cleaned semiconducting nanoparticies.

Further object of the present invention are therefore cleaned semiconducting inorganic nanoparticies obtainable by first washing with an organic solvent or solvent-mixture then treatment with cleaning molecules, wherein cleaning molecules show an affinity in the same range as the affinity of the primary ligands from the synthesis to nanoparticies, are volatile and do not form any covaient bond with the semiconducting nanoparticies..

Obtained nanoparticies can be redispersed in a semiconducting polymer solution in an organic solvent or in a mixture of semiconducting polymer nanowires in a specific organic solvent. Semiconducting nanoparticies decorated with semiconducting polymer or semiconducting polymer nanowires as substitution ligands is obtained.

So in a preferred embodiment of the method of the present invention cleaned semiconducting inorganic nanoparticies are stabilized with a semiconducting polymer solution or semiconducting crystalline polymer nanowires, optionally prepared in situ, dispersed in a proper solvent, wherein the semiconducting polymer solution or the crystalline polymer nanowires as semiconducting polymer substitution ligands contain moieties susceptible to interact with the surface of the semiconducting nanoparticies.

The in situ preparation of the crystalline polymer nanowires used in the method invention is also part of the present invention. A further object of the present invention is therefore a semiconducting nanoparticle decorated with semiconducting polymer and / or semiconducting crystalline polymer nanowires as substitution ligands, wherein the semiconducting polymer solution or crystalline polymer nanowires contains moieties susceptible to interact with the surface of the semiconducting nanoparticles obtainable according to the method of the present invention.

Suitable semiconducting polymer substitution ligands must contain moieties susceptible to interact with the surface of the nanoparticles, so the nanoparticle dispersion is stabilized and at the same time allow direct contact between the two semiconducting components of the hybrid systems that is semiconducting nanoparticles and semiconducting polymers, allowing a direct electron transport between them.

Suitable semiconducting polymers substitution ligands are poiythiophene, poiypyrroi or polyani l ine. which present functional moieties capable to interact with the surface of the (thioi-)cleaned semiconducting nanoparticles. Usually the semiconducting polymers substitution ligands are solved in a solvent capable of dissolving them later referred to as substitution solvent. In the polymer solution the polymer chains of the substitution ligands attract the semiconducting cleaned nanoparticles and act as a stabilizer, wrapping up the cleaned semiconducting nanoparticles. Also semiconducting crystalline polymer nanowires dispersed in the above mentioned polymer solution or in a suitable substitution solvent were found to be able to stabilize cleaned semiconducting nanoparticles.

In the decoration/substitution step cleaned semiconducting nanoparticles (usually in a solid form) and a polymer (nanowire) stock solution are usually prepared. It is preferred that the cleaned semiconducting nanoparticles are stirred (typically over 12 hours at room temperature, between 20 - 28 °C) and precipitated. Decoration/substitution is achieved in mixing the polymer or the polymer nanowire stock solution into the cleaned semiconducting nanoparticles with a suitable weight ratio, depending on semiconducting nanoparticles and semiconducting polymer, usually 75:25 to 95:5, preferably 80:20 to 90: 10. The addition of the polymer nanowires can be carried out by physical mixture followed by mild agitation or the crystalline polymer nanowires can be directly synthesized in situ in the presence of the treated semiconducting nanoparticle sediment. All the steps are prepared under inert atmosphere inside the glove -box.

In a particular embodiment of the present invention the semiconducting polymers of the hybrid system is also used as a semiconducting polymers substitution ligands. In this case there is no principle restriction in the type of the substitution solvent as long as it is volatile with a boi ling point < 1 50 °C (high vapour pressure). Preferred solvents are e.g. toluene, xylene, methyien chloride, chloroform, chlorobenzene, monochlorobenzene, di- chlorobenzene, tri-chlorobenzene, cyclohexanone. The dispersion should form a stable/ metastable, homogeneous ink. A stable ink with agglomerates of a maximal size of < 100 nm, preferred 20 nm, especially preferred without agglomerates, is obtained directly at the end of the decoration/substitution step and can be further use as an ink in photovoltaic application or other printable electronic applications. Experiments on Nanorods ( N R) have shown that using thiol cleaning and directly redispersing in P3HT, N Rs surprisi ngly tend to arrange itself parallel to each other (see Fig. 3 and 4). Such an orientation has not been observed after methanol washing or after substitution of the primaiy ligands with pyridine. It is to be expected that tetrapods may also prearranged. In this case the perpendicular arm of the tetrapod would bring better contact between electrodes and arranged morphology is expected to improved rjmax.

In a further embodiment o the present invention semiconducting crystalline polymer nanowires are used as substitution ligands. The synthesis o crystal l ine polymer nanowire consists in a solution /precipitation procedure (Berson et al, Adv. Funct. Mater. 17(2007)1377- 1384). The semiconducting polymer is first dissolve in a bad polymer solvent applyin temperature and later on is cooled down slowly with a controlled cooling rate. During this cooling process, small polymer crystal nuclei are formed followed by the growing of those nuclei to reach fi nal ly rod like cristaiiine polymer nanowires. The final nanowires size is dependant of the cooling procedures applied. Typically nanowires with 5- 10 nm wide and hundreds of nm long (between 100 and 500 nm) are obtained. The final product is obtained in the form of an opaque polymer dispersion that remains stable. The solvent used for this synthesis must be a bad polymer solvent able to dissolve the polymer when high or moderate temperature is applied while must act as an antisolvent at the ink; application temperature. Solvents like cyciohexanone, xylene, pyridine are reported to be suitable for the polymer nanowire synthesis. In case the crystalline polymer nanowires are prepared in situ in the presence of the treated semiconducting nanoparticle sediment the solvent used must also be suitable for stable dispersion of the cleaned semiconducting nanoparticles.

The decoration/substitution step is usually conducted as previously mentioned. Depending on used ratio and / or crystallization conditions semiconducting polymer or crystalline polymer nanowires are substitution ligand.

In a preferred embodiment the present invention the semiconducting polymer of the hybrid system is also used as a semiconducting polymer (nanowire) substitution ligands. A stable ink with agglomerates of a maximal size of < 100 nm, preferred 20 nm, especially preferred without agglomerates, is obtained directly at the end of the decoration/substitution step and can be further use as an ink in photovoltaic application or other printable electronic applications. In a further embodiment of the present invention the semiconducting nanoparticles decorated according to the present invention and a further semiconducting polymer material may be dispersed in an ink solvent to form a stable/metastable, homogeneous ink. In this case the ink solvent is compatible with the decorated semiconducting nanoparticles and with the second semiconducting material so that a stable or metastable ink is achieved.

The further semiconducting material in hybrid system may in the context of the present invention be a semiconducting polymer, preferably capable of crystallization for example conductive polymers such as poly(acetylene)s, polyanil ines, poly(pyrrol)s, polyindole, polypyrene. polycarbazole, polyazulene, polyazepin, polyfluorenes, polynapthalenes. preferably polyi p-phenylene vinylene), most preferably conductive polymers with a high affinity to nanoparticles such as polythiophenes, poly(3-alkylthiophenes), polyi p- phenylene sulfide). By mean of high affinity, conductive polymers should contain some components, preferable to coordinate physically or chemically with semiconducting nanoparticle surfaces, such as containing sulfur for using with Cd-based semiconducting nanoparticle. In this latter case both semiconducting material (polymer) and semiconducting crystalline polymer nanowires may act as substitution ligands in the ink according to the present invention. According to the process of the present invention a stable ink comprising the semiconducting nanoparticies decorated with semiconducting polymer and / or semiconducting crystalline polymer nanowires is formed.

A further object of the present invention is therefore a composition and in particular an ink comprising the inorganic semiconducting nanoparticies decorated with semiconducting polymer and / or semiconducting crystalline polymer nanowires obtainable according to the above mentioned method.

The method of the invention provides a stable ink formulation with agglomerates of a maximal size of < 100 nm, preferred 20 nm, especially preferred without agglomerates. Interestingly the use o the crystalline polymer nanowires in the formulation leads to a better control of the final nanodomain size, since the polymer nanodomains are formed in advance, prior to the casting and drying of the ink. Increasing polymer crystallinity is reported to be the way for increasing electron mobility across the polymer chains and therefore, the photovoltaic performance. The addition of polymer antisolvents to the ink as well as thermal annealing after casting and drying of the ink are usually carried out to improve the polymer crystallinity. Experiments show that use of cleaned semiconducting nanoparticies decorated directly with polymer crystalline nanowires according to the present invention is a valuable alternative way to achieve crystalline polymer nanodomains directly in contact with the nanoparticle surface without needing any additive or thermal annealing. Lower inner resistances and higher efficiencies or better performance of the printable electronics devices can be achieved. In particular in case a further semiconducting polymer material is used further additives may be applied if necessary for performance increase of the printable electronic devices.

Such additives are well known in the state of the art. Suitable additives are nitrobenzene, octanethiois or dihaloalcanes. Some additives may also be used in the ink to trigger further detachment of residual long- chain primary ligands from the nanoparticle surface.

Also combinations of these additives may be suitable.

In case semiconducting polymer alone or in combination with semiconducting crystalline polymer nanowires as substitution ligands a stable ready-for-use ink formulation is readily provided in particular when semiconducting crystalline polymer nanowires are generated in situ.

The inks formulated according to the present invention can be subsequently used for hybrid solar cell fabrication or other printable electronics applications.

They further ensure high performance of those printable electronic devices such as e.g. hybrid solar cells, which contain semiconducting nanoparticles according to the present invention in essential parts.

Also objects of the present invention are a material and an electronic device comprising the inorganic semiconducting nanoparticles of the present invention and in particular a solar cell.

In a further step of the method of the invention the ink is printed on the surface of a printable electronic device using printing methods well known in the state of the art such as ink-jet printing, screen printing, roll-to-roll printing and dried.

It is an further advantage of the present invention that the substitution ligands do not have to be removed in the solar cell active layer or in the printable electronics devices by annealing and/or vacuum steps to further improve the conductivity between the nanoparticles as well as to ensure an efficient charge separation and/or transport between nanoparticles and polymer since the substitution ligand is semiconducting polymer or semiconducting polymer nanowire. Examples of the method and nanoparticles according to the present invention are given without limiting the scope of the present invention to used materials and specific conditions. Figures:

Fig 1 Layer structure of the semiconducting nanoparticles after synthesis according to the state of the art.

Fig. 2 Thermo Gravimetric Analysis - Mass Spectrometry (TGA-MS) measurements after synthesis according to US2007/01 2052 A 1 , washing with MeOH according to US2007/0132052A 1 and cleaning according to the present invention.

Fig. 3 Morphology of the active layer by transmission electron microscopy (TEM) Fig. 4 Cross section analysis by transmission electron microscopy (TEM)

Fig. 5 TEM of cleaned CdSe QDs attached onto P3HT nanowire as prepared in example 4 and analog to morphology observed by Xu et al, Macromol. Rapid. Commun. 16, 30, 1419- 1423 (2009)).

Examples:

Example 1 - Thermo Gravimetric Analysis - Mass Spectrometry (TGA-MS) measurements of CdSe nanoparticle of quantum dot shape (CdSe-QD) with primary ligand triocty!phosphine (=TOP) and oleic acid (= OA).

Semiconducting nanoparticles CdSe-QD with primary ligand trioctyiphosphine (=TOP) and oleic acid (= OA) were prepared according to the method of US2007/01 2052 A 1 . The TGA-MS measurements are performed with dried semiconducting nanoparticles under an inert atmosphere to prevent oxidation or other undesired reactions during the analysis. The amount of the dried sample should be at least 50 mg.

50 mg of dried sample were loaded onto a high-precision balance pan placed in a small electrically heated oven with thermocouple accurately measuring the internal oven temperature.

An inert gas preferably Ar or He and most preferably He with a flow rate of 80 ml/minute was flown into the oven for preferably at least two hours to purge away traces of oxygen and water out of the oven.

Then the temperature of the oven and of the balance pan was gradually increased usually with a gradual temperature increase of 5 K. minute under the inert atmosphere and mass spectroscopy measurements were conducted using a time-of-flight mass spectrometer of type RFT 10 provided by company Kaesdorf.

The range of measurement temperature covered the whole range of decomposition of all substances contained in the sample that is from 300 K to 900 K.

As the sample vaporized, the mass spectrometer detected the elemental components of the organic ligand shell of the dried semiconducting nanoparticles as well as molecular fragments thereof and measured their concentrations.

The composition of the organic ligand shell was subsequently calculated qualitatively and quantitatively. Results of conducted analysis are shown in Fig. 2.

Weight percentage distribution is shown in Fig, 2, column 1.

Example 2 - Preparation of CdSe-QD with primary ligand TOP and OA using MeOH according to the method of US2007/0132052A1 and solar cell preparation.

A - Washing of CdSe-QD with primary ligand TOP and OA using MeOH according to the method of US2007/0132052A1. Above mentioned CdSe-QD with primary ligand TOP and OA were prepared and washed with MeOH according to the method of US2007/01 2052A 1 .

Weight percentage distribution obtained after TGA-MS Analysis according to the method of example 1 is shown in Fig. 2, column 2.

B - Preparation of a solar cell using an ink comprising semiconductingnanoparticles of example 2 - A

100 mg of the CdSe-QD with ligand TOP and OA of example 2- A were dispersed in 1 ml, of the ink solvent (chlorobenzene) as the first stock solution. The 1 1 mg of a second semiconducting material (poly(3-hexylthiophene) (P3HT) were dispersed in 1 m L of the ink solvent (chlorobenzene) as the second stock solution. The two stock solutions were mixed with a weight ratio as 90: 10 (CdSe-QD:P3HT) to form a stable, homogeneous ink.

The ink was spin-coated and dried on a substrate made of indium-tin-oxide coated glass with a layer of Poly(3,4-ethy!enedio.\ythiophene) poly(styrenesulfonate) (PEDOT:PSS) to obtain an active layer. Obtained active layer was annealed by heating at a temperature of

120 °C for 5 minutes.

The back electrode was evaporated in a vacuum thermal deposition chamber. 4 nm of calcium and 150 nm of silver layers were deposited on top of the active layer. After the electrode evaporation, the solar cells were annealed by heating at a temperature of 120 °C for 5 minutes. A max imal power conversion efficiency (rjmax, "eta" max), that is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar ceil is connected to an electrical circuit, was measured for obtained solar cell using a solar simulator under the standardized condition of AMI .5 was measured using a 150W Oriel Solar simulator from Newport. AM refers to air mass and a number refers to air mass coefficient, wherein air mass coefficient characterizes the solar spectrum after the solar radiation has traveled through the earth atmosphere. The AMI .5 is universal used for characterization of solar cell power conversion efficiency. A maximal power conversion efficiency rjmax of 0. 1 5 % was measured for the solar cel ls of example 2.

Example 3 - Washing of CdSe-nanorods (NR) with primary !igands TOP, TOPO and TDPA using MeOH and cleaning with Ethanethiol, subsequently decorating directly with semiconducting polymer poly(3-hexylthiophene) (P3HT) according to the method of the present invention and solar cell preparation.

1 00 mg of the above mentioned CdSe-NR with primary I i gauds trioctylphosphine (TOP), trioctylphosphine oxide (TOPO) and tetradecylphosphonic acid (TDPA) were precipitated by methanol according to the method of US2007/01 2052 A 1 . The semiconducting nanoparticles washed CdSe-N R were separated from the supernatant by centrifugation and dried with nitrogen gas. The dried washed CdSe-NR were redispersed in 1 ml Ethanethiol with a concentration of 100 mg of dried washed CdSe-NR and stirred at room temperature for 12 - 24 hours. Subsequently 10 ml methanol were added into this dispersion to precipitate CdSe-NR and to remove further TOP, TOPO, TDPA and ethanethiol (ratio of dispersion solvent to washing agent 1 : 10). The precipitate was separated from supernatant by centrifugation. The cleaned CdSe-NR were dried with nitrogen gas and redispersed directly in the P3HT stock solution in monochlorobenzene. The mixture has a weight ratio as 90: 10 (CdSe-NR:P3HT) to form a stable and homogeneous ink.

Weight percentage distribution obtained after TGA-MS Analysis according to the method of example 1 is shown in Fig. 2, column 3. The ink was spin-coated and dried on a substrate made of indium-tin-oxide coated glass with a layer of (PEDOT:PSS) to obtain an active layer. Obtained active layer was annealed by heating at a temperature of 120 °C for 5 minutes. The back electrode was evaporated in a vacuum thermal deposition chamber. 4 nm of calcium and 150 nm of silver layers were deposited on top of the active layer. After the electrode evaporation, the solar cells were annealed by heating at a temperature of 120 °C for 5 minutes. The morphology of the active layer and the cross section analysis by transmission electron microscopy (TEM) are illustrated in Fig. 3 and Fig. 4 respectively. It is shown that the direct decoration of cleaned CdSe NRs with P3HT results in a self-organizing structure of CdSe NRs (parallel arrangement between CdSe NRs) in the morphology (analog to the morphology observed by Mouie et al. in J. Phys.Chem. C, Vol 1 12, No. 33, 2008, p. 12583-12589).

TEM of the morphology of an active layer obtained with semiconducting NRs after methanol washing or after substitution of the primary ligands with pyridine did not show any particular orientation.

A maximal power conversion efficiency n _m ax of 0.12 % was measured for the solar cells of example 3 using the method described in example 2-B. The rjmax is in this case very low because NRs arrange themselves also parallel to the substrate.

Example 4 - Washing of CdSe-quantu m dot (QD) with primary ligands TOP and OA using MeOH and cleaning with Ethanethiol, subsequently decorating directly with semiconducting polymer poly(3-hexylthiophene) (P3HT) nanowire according to the method of the present invention

100 mg of the above mentioned CdSe-QD with primary ligands trioctylphosphine (TOP) and oleic acid (OA) were precipitated by methanol according to the method of US2007/0132052A1. The semiconducting nanoparticles washed CdSe-QD were separated from the supernatant by centrifugation and dried with nitrogen gas. The dried washed CdSe-QD were redispersed in 1 mi Ethanethiol with a concentration of 100 mg of dried washed CdSe-QD and stirred at room temperature for 12 - 24 hours. Subsequently 10 ml methanol were added into this dispersion to precipitate CdSe-QD and to remove further TOP, OA and ethanethiol (ratio of dispersion solvent to washing agent 1 : 10). The precipitate was separated from supernatant by centrifugation. The cleaned CdSe-QD were dried with nitrogen gas and redispersed directly in the poly(3-hexylthiophene) (P3HT) nanowire stock solution in cyclohexane prepared according to the method of Xu et ai, Macromol. Rapid. Commun. 16. 30, 1419- 1423 (2009).

The morphology of the active layer and the cross section analysis by transmission electron microscopy (TEM) are illustrated in Fig 5. Fig. 5 shows that cleaned CdSe QDs attach onto P3HT nanowi re analo to morphology observed by Xu et al, Macromol. Rapid. Commun. 16, 30, 1419- 1423 (2009)). However since the concentration of QD in the stock solution is much higher than the number of nanowire some of QDs attached on the nanowire and the rest of the QDs is to be seen in the background.