Organic-inorganic hybrid material and method for silicon surface passivation

Abstract

A relevant technological challenge is the low cost and abundant materials development for silicon surface passivation for applications in optoelectronic devices, in particular in solar cells by scalable industrial methods. In the present invention, a new hybrid material comprising PEDOT:PSS and transparent conducting oxide nanostructures is developed and a method is proposed to fabricate the composite material that passivates well the silicon surface to be used by means of a thin composite film of thickness below 200 nm.

Claims

1. Hybrid organic-inorganic material consisting of an organic conductor polymer matrix and transparent conducting oxide nanostructures as filler, where the conductive polymer is PEDOT:PSS and the nanostructures are dispersed in the polymer in a ratio of 0.1-10% wt., wherein the hybrid organic-inorganic material having tin and/or titanium oxide nanoparticles undoped or doped with Cr, Al, or Li with a cationic percentage range of 1 to 40%, and the nanoparticles have sizes between 1-65 nm.

2. Hybrid organic-inorganic material consisting of an organic conductor polymer matrix and transparent conducting oxide nanostructures as filler, where the conductive polymer is PEDOT:PSS and the nanostructures are dispersed in the polymer in a ratio of 0.1-10% wt., the hybrid organic-inorganic material further having tin and/or titanium oxide nanowires undoped or doped with Cr, Al, or Li with a cationic percentage range of 1 to 40%, and the nanowires have sections of up to a 100 nm and lengths of up to 1000 nm.

3. The hybrid organic-inorganic material, as claimed in claim 1, having a mixture with controlled ratios of the tin and/or titanium oxide nanoparticles and/or the tin and/or titanium oxide nanowires.

4. The hybrid organic-inorganic material, as claimed in claim 1, having other filler such as carbon nanotubes, Si nanoparticles or nanowires, Al.sub.2O.sub.3 nanoparticles or nanowires, and SiN.sub.x nanoparticles or nanowires, or a mixture thereof.

5. The hybrid organic-inorganic material, as claimed in claim 1, having Ethylene Glycol as dispersant added to the dispersion of the organic conductor.

6. A method for fabricating a hybrid composite containing an organic conductor as host and semiconducting nanostructures as filler, comprising at least the steps of: providing an aqueous dispersion of a conductive polymer, providing transparent conducting oxide nanostructures with controlled size and doping by techniques such as hydrolysis, modified Pechini method or vapour-solid method, and adding the nanostructures in the ratios lower than 10% wt., at the aqueous dispersion under ultra-sonication, wherein the nanostructures including tin and/or titanium oxide nanostructures undoped or doped with Cr, Al, or Li with a cationic percentage range of 1 to 40%, and the nanostructures have sizes between 1-65 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention is directed to the fabrication of a hybrid organic-inorganic film to be used for passivation of Si surface in Si-based devices, in particular, Si-based solar cells, as well as to developing the method of deposition of the hybrid films. The deposition process here used involves rapidity and low costs as compared with other chemical methods employed so far, such as, e.g., chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD). In the embodiments described below, the combination of inorganic transparent conducting oxide (TCO) nanostructures, like tin oxide and titanium oxide (rutile and/or anatase) nanoparticles and/or nanowires, with the organic p-type semiconductor known as PEDOT:PSS results, by spin coating deposition, into a thin film with thickness below 200 nm, preferably in a range of 90-150 nm, even preferably 100-130 nm. Oxide nanostructures might also be replaced by other convenient nanostructures such as carbon nanotubes, Si nanoparticles or nanowires, Al.sub.2O.sub.3 nanoparticles or nanowires, and SiN.sub.x nanoparticles or nanowires, or a mixture thereof. In comparison to the use of a film made exclusively of PEDOT:PSS, the composite thin film containing nanoparticles improves the passivation properties of the Si surface increasing lifetime of the charge carriers and the conductivity of the layer (as described in embodiment 1), with a slight modification on the light absorption as well.

(2) The SnO.sub.2 and TiO.sub.2 nanoparticles may be grown by different synthesis routes such as hydrolysis (by using SnCl.sub.2.2H.sub.2O or Ti(OBu).sub.4 and 1-butanol precursors for the fabrication of tin or titanium oxide nanoparticles respectively), or by a modification of the Pechini Method (as the one followed in patent ES201400759). The dimensions of the nanoparticles range from 1 to a maximum of approximately 80 nm (they should not be larger than the thickness of the spin-coated film), preferably 1 to 65 nm, even preferably 5 to 50 nm.

(3) SnO.sub.2 and TiO.sub.2 nanowires, grown by a vapour-solid method which avoids the use of catalyst or external substrates, can be also used as filler in the formation of the composite, as described in embodiment 2. Metallic Sn have been used as precursor for the fabrication of SnO.sub.2 nanowires, while for the growth of TiO.sub.2 nanowires TiN powder has been used as precursor. Temperatures of 800 C. or 900 C. have been used for the fabrication of SnO.sub.2 and TiO.sub.2 nanowires, respectively, which reach length of hundreds of nm and widths of tens of nm. By length of hundreds of nanometer, we mean from about 10 nm to about 1 000 nm, preferably 50 to 950 nm, even preferably 100 to 900 nm. By widths of tens of nm, we mean from 1 nm to about 100 nm, preferably 10 nm to about 100 nm. Either anatase and/or rutile phases can be used for the titanium oxide nanostructures, whereas rutile is obtained for tin oxide nanostructures. Doping elements may include Cr, Al or Li as acceptors for both SnO.sub.2 and TiO.sub.2 nanostructures. The dopant concentrations that have been used are 10, 20 and 30% cat. The % of dopant is less than 50%. Although we have worked with a range of 10-30%, a preferred range is of 1-40%, preferably 1-35%, even preferably 1-30%.

(4) The PEDOT:PSS may be used dispersed in water at 1.3% v/v, it presents a sheet resistance below 100 (/sq) and conductivities up to =1000 S/cm. Some additives can be used for different purposes, as described in embodiment 3. In the case of ethylene glycol (EG), it is reported that it improves the electrical conductivity of PEDOT:PSS by aligning the polymer chains. In our case, EG exhibits another interesting property which consists of avoiding the agglomeration of the nanoparticles during spin coating, because this compound acts as a dispersant. Really interesting is as well the use of isopropanol (IPA) before the spin-coating, because this compound helps the deposition on hydrogenated Si, especially after HF cleaning, and improves homogeneity of the deposited layers by making hydrophilic the silicon surface, as described in embodiment 3.

(5) The composition ratio PEDOT:PSS dispersion to nanostructures may be between 0.25 to 5 wt. %, although dispersion in a broader range (0.1-10 wt. %) are also expected to work. This parameter is crucial for the passivation behaviour of the hybrid composite, as life time charge carrier values vary as a function of the concentration of nanostructures in the composited film (as described in embodiment 1).

(6) Dispersions of the nanostructures in PEDOT:PSS have been made in the desired concentrations and under ultra-sonication. Once the dispersions were ready, spin-coating was carried out at room temperature and without a need of processing under vacuum. The spin-coating recipe followed in this case consists of three different steps: initialization (500 r.p.m during 2s), covering (3000 r.p.m during 30s) and drying (4000 r.p.m during 40s) Immediately after spin-coating, consequent thermal annealing was performed in order to evaporate water from PEDOT:PSS, which is diluted on it. The thermal annealing was performed at 120 C. during 20 min, and preferred temperatures ranges are 100-130 C. and even preferred 110-125 C. Preferred time ranges are 10-30 min, even preferred 15-25 min.

(7) A single composite layer may be deposited on top of the silicon substrate, and in some embodiments a bilayer or multilayers may be deposited too by repeating the process on a previously spin coated layer, as described in embodiment 4.

EMBODIMENTS OF THE INVENTION

(8) The present invention is additionally illustrated by means of the following embodiments, which are not intended to be limiting its scope.

Embodiment 1

(9) An hybrid composite thin film, is fabricated by PEDOT:PSS combined with tin oxide and/or titanium oxide doped or undoped nanoparticles in different concentrations. In addition to the rutile SnO.sub.2 nanoparticles, either rutile or anatase TiO.sub.2 nanoparticles can be used, with different properties as a function of the crystalline phase. The SnO.sub.2 and TiO.sub.2 (rutile) nanoparticles used in this embodiment, with sizes ranging from 5 to 50 nm, have been fabricated by an hydrolysis method, using SnCl.sub.2.2H.sub.2O or Ti(OBu).sub.4 and 1-butanol precursors, respectively. The reduced dimensions of the nanoparticles facilitate their dispersion and deposition by the spin-coating process, which results in a layer with good homogeneity. An image of the composited thin film 125 nm thick spin-coated onto a n-Si substrate containing 0.5% wt. SnO.sub.2 nanoparticles is shown in FIG. 1. The spin coating process has been carried out following three different steps described in the section Detailed description of the invention: initialization, covering and drying. This process is followed by a thermal annealing at 120 C. for 20 min Different concentrations of semiconducting oxide nanoparticles (0.25 to 5% wt.) have been used in the formation of the composite, although a broader range of dispersion (0.1-10% wt.) is also expected to work. Measurements of the carrier lifetime have been performed by using a photoluminescence (PL) imaging system in order to study the surface recombination at the Si surface and thus the passivation behaviour of the composite thin film spin coated on n-Si. A LIS-R1 PL imaging setup from BT Imaging with an excitation wavelength of 808 nm and a constant illumination intensity of 4.210.sup.2 W/cm.sup.2 has been used. In this case, a 40 nm thick layer of hydrogenated amorphous silicon (a-Si:H) has been deposited by sputtering on the back side of the Si wafer used as a substrate as a reference passivation layer. The a-Si:H passivated surface has a low surface recombination velocity (SRV) and the Si wafers have a high bulk lifetime of several milliseconds. Hence, the SRV from the front Si surface passivated with pristine PEDOT:PSS could be calculated with a small error. As an example, FIG. 2 shows the carrier lifetime vs. the concentration of nanoparticles when SnO.sub.2 is used as filler in the composite. Best results have been achieved when using SnO.sub.2 (0.5% wt.) and rutile TiO.sub.2 (1% wt) concentrations, which confirms that a control of the composition is crucial for the passivation performance of the hybrid composite thin film. Carrier lifetime values of hundreds of s have been achieved. In addition to the undoped nanoparticles, doped nanoparticles can be also employed. As for example, Cr, Al or Li have been used as a dopant in SnO.sub.2 and TiO.sub.2 (anatase) nanoparticles in a concentration ranging between 10 and 30% cat. The selection of dopants is based on achieving a p-type character for the metal oxide nanoparticles, and therefore the spirit of this embodiment is not limited to the previous selection.

Embodiment 2

(10) SnO.sub.2 and TiO.sub.2 (rutile) nanowires, with hundreds of nm length and tens of nm width, fabricated by a vapor solid process have been also employed as filler in the composite layer. Tin oxide or titanium oxide nanowires have been fabricated by a vapour-solid method, using metallic Sn or TiN precursors and temperatures of 800 C. or 900 C., respectively. The nanowires show dimensions of hundreds of nm length and tens of nm width. The nanowires have been added to the PEDOT:PSS dispersion in 0.25 to 5% wt. concentrations, although concentrations in a broader range (0.1-10% wt.) are also expected to work. Despite the fact that the carrier lifetime values measured for the nanowires-based composites are lower than those for the nanoparticles, their characteristic morphology can improve some other relevance optical properties for the solar-cell performance, such as the absorbance. In this case the homogeneity of the spin-coated films is not as good as for the nanoparticles, due to the dimensions of the elongated structures which can be easily tangled, thus hindering their dispersion. However, layers with good homogeneity can be also spin coated on a n-Si substrate, by paying special attention to the process and including adequate additives to avoid aggregation such as EG or IPA to facilitate spin coating of PEDOT on Si, as indicated in the embodiment number 3. Upon using the other scalable chemical methods, larger carrier lifetime might be expected. Doped nanowires can be also employed, as described for the nanoparticles in embodiment 1.

(11) Moreover, mixed SnO.sub.2/TiO.sub.2 nanoparticles and/or nanowires can be used as filler in the composite in order to exploit the properties of both materials in the design of the passivation layer.

Embodiment 3

(12) In this invention some additives can be added to the organic polymer PEDOT:PSS in order to improve its performance. The use of Ethylene glycol (EG) not only involves an improvement of the electrical conductivity of the polymer, due to alignment of the polymer chains, but also enhances the dispersion of the nanoparticles, which is a relevant parameter to be taken into account. Using EG in a range of concentration of 3-4.5 wt. % as a dispersant avoids the agglomeration of nanoparticles (nanowires) during the spin-coating and results in a higher homogeneity of the composite film. An improvement of the homogeneity of the spin-coated layer has been also achieved by using isopropanol (IPA) and/or standard RCA cleaning (W. Kern and D. Puotinen, RCA Rev., 31, 187 (1970)) before the spin coating process. For the IPA cleaning, the Si substrate is placed on the spinner platform and IPA is dropped covering the Si surface for 90 seconds prior to the PEDOT:PSS deposition. Then, the sample is dried by a conventional spinning process. By using IPA or RCA cleaning procedures the deposition of the layer on Si is improved, as shown in FIG. 3, which also results in a better homogeneity of the film, due to the improvement in the hydrophilic character of the surface. The homogeneity of the passivation layer is improved by adding EG and/or IPA, without detriment in the passivation properties which remains unchanged.

Embodiment 4

(13) As a fourth embodiment, the spin coating technique employed in this invention can be subsequently repeated in order to fabricate multilayer structures. Once a high homogeneous layer is deposited, it can be used as a substrate on top of which a new layer could be deposited. Following this procedure layers with a concentration gradient of TCO nanostructures in PEDOT:PSS can be fabricated, as well as multilayers with tuned optical properties, as an example, making use of the different properties achieved by doping and the combination of materials with different band gaps. This embodiment adds functionality, while keeping high homogeneity in the layers and involving low costs. This can widen the performance and applicability of this invention in the field of solar cells, and other optoelectronic devices.

DESCRIPTION OF FIGURES

(14) FIG. 1. Optical image of a composite 125 nm thick layer presenting good homogeneity as deposited by spin-coating on a n-Si substrate (3.53.5 mm). The layer is composed of PEDOT:PSS and SnO.sub.2 nanoparticles in a 0.5% wt. concentration.

(15) FIG. 2. Life time variation of the charge carriers as a function of the SnO.sub.2 nanoparticles concentration at the composite acquired by illumination from the PEDOT:PSS/nanoparticle frontal surface or the a-Si:H back side.

(16) FIG. 3. Optical image of the PEDOT:PSS and SnO.sub.2 nanoparticles dispersion spin-coated on (a) n-Si without previous treatment, and (b) n-Si substrate pre-treated with IPA. Better homogeneity is observed in image (b) due to the hydrophilic character of the Si substrate induced by IPA treatment. (c) Image of drops of PEDOT:PSS and SnO.sub.2 nanoparticles deposited on n-Si substrates, either cleaned (right) or not (left) by RCA as indicated on the image, inducing the former the hydrophilic property of silicon.

(17) The above described is merely examples of the present invention and they do not intend to limit the present invention. Any modifications and changes without departing from the scope of the spirit of the present invention are deemed as within the scope of the present invention. The scope of the present invention is to be interpreted with the scope as defined in the claims.