WO2016146715A1

WO2016146715A1 - Method and apparatus for nanowire film production

Info

Publication number: WO2016146715A1
Application number: PCT/EP2016/055741
Authority: WO
Inventors: Johan BORGSTRÖM; Niklas MÅRTENSSON; Umear NASEEM; Per Viklund; Jaime CASTILLO LEON; Klaus KUNTZE
Original assignee: Sol Voltaics Ab
Priority date: 2015-03-16
Filing date: 2016-03-16
Publication date: 2016-09-22

Abstract

A method of fabrication of an ordered nanowire film comprising the steps of: adding nanowires to a liquid comprising a solvent, wherein each nanowire has a first end and a second opposite end, and a character which is different adjacent to the first end from adjacent to the second end; providing the liquid on a substrate; thinning the liquid into a film; positioning nanowires in the liquid film such that they are affected, based on said character, to be oriented in a predetermined direction with respect to a surface of the film; removing the solvent from the liquid film, such that the nanowires are moved into aligned position closer to each other by capillary force or surface tension; and solidifying the liquid film to a solid film. The invention also includes an apparatus for carrying out the method.

Description

Method and Apparatus for Nanowire Film Production

Technical field

The present invention relates to semiconductor devices and in particular to nanowire semiconductor devices. More specifically, the invention relates to a method and apparatus for nanowire film production.

Background

Langmuir - Blodgett (LB) methods, spin coating, evaporation induced self- assembly and convective assembly methods are used today, primarily to fabricate monolayers on research scale. LB equipment for batch production of thin layers on wafers up to 8' size is used industrially in the semiconductor industry and for surface treatment of optical components

There is relatively little prior art on aligning and fabrication of ordered nanowire films but much prior art on fabrication of films with particles. Example of convective assembly method: Prevo and Velev, 2004 where they use 1,1 μιη polystyrene spheres to produce 10 cm2 smooth areas of ordered films at speeds of about 36 mm/h (Example of spin coating: US patent 4801476, 1989) where they obtain 45 cm2 (3 " wafer) by spinning at 3400 rpm until dry). They produce an ordered array of 0,5 μιη polystyrene latex sphere at relatively low speed (about 0,06 m2/h). Example of E- field alignment of nanowires for NW film fabrication is given in US published application 2013/0203242, incorporated herein by reference in its entirety.

Traditional coating methods such as example spray coating, dip coating, slot-die and other coating methods, for example using a doctor blade, are continuous industrial scale methods that usually produce polymer films or multilayers of particles at relatively high speeds. The particles are here often embedded in a polymer matrix and the thickness of the coated layers can be down to a few micrometers.

The LB and convective assembly are usually slow but can a produce a high quality monolayer over large areas. They usually operate in a batch mode and their productivity is thus low. In contrast, traditional coating methods have a high productivity, operate in a continuous mode but the quality of the films on a micro scale is usually less good than what can be obtained with LB or convective assembly and the production of monolayers is usually not demonstrated.

Furthermore, assembly of rod like particles, for example carbon nano tubes or CdSe nanowires on solid and liquid surfaces generally leads to horizontal organization where the rodlike particles lay down on the surface. Evaporative assembly of rodlike particles also often tends to result in disordered arrangements, where the rodlike particles are arranged randomly (called below an amorphous glass), which makes these arrangements not suitable for fabrication of solar panels. Summary

According to one aspect, a method for fabrication of an ordered nanowire film is provided, comprising the steps of:

adding nanowires to a liquid comprising a solvent, wherein each nanowire has a first end and a second opposite end, and a character which is different adjacent to the first end from adjacent to the second end;

providing the liquid on a substrate;

thinning the liquid into a film;

positioning nanowires in the liquid film such that they are affected, based on said character, to be oriented in a predetermined direction with respect to a surface of the film;

removing the solvent from the liquid film, such that the nanowires are moved into aligned position closer to each other by capillary force or surface tension; and

solidifying the liquid film to a solid film.

According to a second aspect, an apparatus for producing a nanowire solid film comprising a colloidal crystal or oriented liquid crystal nanowire solid film is provided, comprising an injector device connected to a container for holding a liquid comprising nanowires; a substrate carrier device for moving a substrate by the injector device in a saturated vapor atmosphere, such that a liquid injected onto a substrate is thinned to a liquid film; a positioning stage configured to affect nanowires in the liquid film, which nanowires have first and second opposite ends, and a character which is different adjacent to the first end from adjacent to the second end, to become oriented in a predetermined direction with respect to a surface of the film; an evaporation device positioned spaced apart from the injector device, in which the saturated vapor is exhausted, such that a substrate is passed from the injector device into the evaporation device where a solvent of the liquid is vaporized, such that the nanowires are moved into aligned position closer to each other by capillary force or surface tension.

Further aspects of the invention are outlined in the claims.

Brief description of the drawings

Figure 1 is a schematic side cross sectional view of an apparatus for the manufacturing of a nanowire ("NW") film by deposition on a liquid interface.

Figure 2 is a schematic side cross sectional view of an alternative apparatus for the manufacturing of a NW film by deposition on a liquid interface where the atmosphere is incombustible.

Figure 3 is a schematic side cross sectional view of an alternative apparatus for the manufacturing of a NW film by deposition on a liquid interface where the solvent is removed by osmosis/diffusion into the liquid (subphase).

Figure 4 is a schematic perspective view of an exemplary capture device designed for continuous film capture by dip-coating.

Figure 5 is a schematic side cross sectional view of an alternative apparatus for the manufacturing of a NW film on a solid substrate.

Figure 6 is a schematic side cross sectional view of an alternative apparatus for the manufacture of a NW film on a solid substrate in a continuous process.

Figures 7 A and 7B are schematic plots of film thickness and volume percent nanowires versus time, respectively, according to one embodiment of the method of making a nanowire film.

Figure 8 shows schematic steps in a method of making the nanowire film.

Figure 9 shows schematic steps in a method of making a nanowire ink for use in the method of Figure 8.

Figure 10 shows schematic steps in a method of making a nanowire solar cell made by combining the steps of the methods of Figures 8 and 9.

Figure 11 is schematic side cross sectional view of a nanowire solar cell.

Detailed description

Definitions Nanowires are nanoscale structures that have a diameter (for cylindrical nanowires) or width (for non-cylindrical nanowires, such as nanowires having a hexagonal cross sectional shape in a plane perpendicular to its axis) less than 1 micron, such as 2-500 nm such as 100-300 nm. The length, however, may be at least 0.5 microns, such as 0.5 to 3 microns, such as 1 to 2 microns. For nanowires used in embodiments of the invention, the height to width aspect ratio may be more than 5, even more than 10, such as 10-12.

Thin, solid NW film (final product) definition: A monolayer of nanowires with a volume fraction of NWs of above 50 %. The thin, solid NW film has a thickness = 1 NW length + 50%, Area = >2 cm2. The thin solid film contains immobile NWs. The NWs are oriented. Preferably, the NWs are oriented with their axes substantially perpendicular to the underlying surface (e.g., over 50% of the nanowires, such as over 80% of the nanowires have an axis position within 20 degrees, such as within 10 degrees of perpendicular to the underlying surface, such that the majority of the nanowires "stand on end" side by side vertically).

Thin, liquid NW film definition: A film with thickness from 1 mm down to 1 NW length + 50%. The thin liquid film contains mobile NWs.

Bulk dispersion of NWs definition: a dispersion that contains mobile NWs and has a viscosity that can be handled in the equipment. The nanowires are disordered, thus pointing in any direction (isotropic).

Liquid crystal definition: a material having properties between those of conventional liquid and those of solid crystal. Here we consider matter where the nanowires are orientationally ordered but free to flow and move relative to each other. The degree of orientational order can vary, but for the material to be considered a liquid crystal there must be some degree (e.g., at least 50%) of orientational order. A liquid crystal of nanowires is anisotropic, having different properties in different directions.

Oriented liquid crystal definition: a particular type of nanowires of one embodiment is where the different ends of the nanowire contain different materials. For example, it may contain a seed particle in one end or a different type of doping or different doping levels in one of the ends compared to the other end. For example, this includes axial junction semiconductor nanowires having a p-n or p-i-n junction where one end has an opposite conductivity type (e.g., p or n) to that of the other end of the nanowire (e.g., n or p). An oriented liquid crystal is then a state of matter where, for example the end with the seed particle is pointing preferentially in one direction - the orientation of the nanowires is unidirectional. The nanowires are fluid and can move relative to each other.

Colloidal crystal definition: a colloidal crystal is an ordered array of colloid particles, analogous to a standard crystal. The particles in a colloidal crystal have permanent average positions relative to each other that repeat over long distances. The material is not fluid and the nanowires are unable to move relative to each other.

"Frozen liquid crystal" (can also be a "frozen, oriented liquid crystal") definition: the frozen liquid (oriented) crystal is a state of matter where nanowires are immobile relative to each other but still have a preferential orientation (and the material is anisotropic). The immobilization of the nanowires can be done by either removing the solvent (until the dispersion is solidified) or by crosslinking molecules in the dispersion media, or reactive groups grafted onto the nanowires.

Disordered glass definition: a glass is an amorphous state of matter. For the nanowires of the present embodiment, this state is a state where the nanowires are isotropically oriented or oriented in a state without long range orientational order (for example horizontally disordered).

In one embodiment, the assembly of particles into a thin film is provided in a continuous process. The assembly process is driven by the removal of the solvent and the confinement of the particles into a thin film.

In one embodiment, the particles, such as nanowires, are added as dispersion onto a surface (solid or liquid) in a compartment with a predetermined environment, preferably a constant environment where the solvent is not removed. The liquid dispersion is then spread or continuously moved, into a second (or several)

compartment(s) with an environment where the solvent is removed. The rate of solvent removal along the direction of solvent removal/drying can be controlled by varying for example the composition and velocity of the fluid flows and the temperature profile. Preferably, the process parameters are selected so that a steady state profile is obtained, i.e. the composition of the drying film at each point along the drying direction is constant with time. The produced films can be monolayers or multilayers of particles, depending on the process parameters, and they may be embedded in a matrix material (for example a polymer) or as free particles. The particles may be spherical particles or rod like particles, preferably semiconductor nanowires for solar cells. The examples below depict variations of the method of fabrication of a nanowire film, comprising nanowires, and nanowire devices. The nanowire devices can be complex in structure, comprising multiple radial and axial semiconductor nanowire layers, dielectric passivation layers and be formed from several semiconductors, but preferably, in light interacting devices preferably semiconductor nanowires with p-i-n or pn-junctions, selected from the group of direct band gap semiconductors, exemplified by III-V semiconductor materials, such as Ill-arsenide (e.g., GaAs, GaAsP, etc.), III- phosphide (e.g., InGaP, InP, InAsP, InGaAsP, etc.) and Ill-nitride (e.g., AlGalnN, etc.). The nanowires in the film can be positioned in an ordered fashion, exemplified by either one of the states of a colloidal crystal, liquid crystal or oriented liquid crystal. By introducing the method step of electrical alignment of the direction of the nanowires, as described in US published application 2013/0203242, incorporated herein by reference in its entirety, preferably in proximity in the step of positioning the nanowires in an ordered fashion, films with ordered nanowires, sharing an single alignment direction (i.e., "vertical" nanowires with majority of nanowires with major axis substantially perpendicular to the underlying liquid or solid surface) can be fabricated.

The bulk dispersion of NWs is introduced/deposited to a substrate (liquid or solid) at a controlled rate. The introduction of the liquid bulk dispersion is performed simultaneously across the width of the substrate by the equipment. The dispersion may be mechanically dragged or spontaneously spread, thus transitioning from bulk dispersion into a thin, liquid NW film. Further thinning of the thin, liquid NW film is done by continued dragging and/or spreading and, in addition, removal of the solvent. The removal of the solvent will also drive a NW concentration increase in the thin, liquid NW film.

The equipment allows for matching between the rate of introduction of the bulk dispersion and the rate of removal of the solvent. The further thinning of the thin, liquid NW film leads to further concentration increase. The rate of solvent removal may be controlled such that the thin liquid film remains fluid during a time long enough for the nanowires to order and orient, for example into a liquid crystal or an oriented liquid crystal. If the solvent is removed too fast the formation of a disordered glass structures will prevent the nanowires to reach the desired oriented liquid crystal structure. In this phase the transport of nanowires within the thin liquid film can take place by convective flows induced by the solvent removal. As the film thickness reaches dimensions comparable to the nanowire length, the occurrence of capillary drag/ surface tension forces acting upon the NWs cause the NWs to assemble even faster. The thin, liquid NW film will subsequently transform into a thin, solid NW film (final product) by further removal of solvent or by

curing/crosslinking a polymer that may be part of the bulk dispersion or added at a later point. The temperature profile along the process flow can be adjusted to optimize the quality of the produced film as it will influence for example the viscosity, mobility of particle/nano wires, surface tension and the evaporation rate.

Figures 1 to 6 illustrate embodiments of apparatus 100, 200, 300, 400, 500, 600 for forming the nanowire film. The embodiments of apparatus 100, 200, 300, 400 of Figures 1-4 provide, such as with a pump 102, nanowire ink 104 (comprising nanowires 108 dispersed in a solvent 107) onto a liquid surface 106 in a tank 109 in a coating chamber 110. The liquid 105 may be water or another liquid. The ink 104 may be provided in a first part 111 of an apparatus 100, 200, 300, 400 that contains a saturated vapor atmosphere 112 (such as humidified air, humidified inert gas (e.g., nitrogen or noble gas), air or inert gas saturated with organic material, such as ethyl acetate, etc.) which prevents or reduces nanowire ink solvent 107 evaporation. The ink 104 spreads on the liquid surface 106, forming a liquid nanowire film 116. The saturated atmosphere 112 is removed in a second part 114 of the apparatus 100, 200, 300, 400 with an air flow exhaust 130, to allow the solvent 107 of the ink 104 to evaporate from the liquid surface 106. The solidified nanowire film 116a is then removed from a third part 118 of the apparatus 100, 200, 300, 400 using a conveyor web or another removal device 119.

Figure 2 illustrates another embodiment of an apparatus 200 for the manufacture of a solid nanowire thin film 116a. The embodiment illustrated in Figure 2 is similar to the embodiment illustrated in Figure 1. However, in the embodiment illustrated in Figure 2, the apparatus 200 includes a fourth part 115 located between the second part 114 and the third part 118. The fourth part 115 of the apparatus 200 includes a chamber 117 in which an inert gas 121, such as nitrogen or argon is supplied. The supply of the inert gas 121 may be controlled by a mass flow controller. The evaporation rate of the solvent 107 from the liquid nanowire thin film 116 may be controlled by the flow of inert gas 121 supplied to the chamber 117 in the fourth part 115 of the apparatus 200. Figure 3 illustrates another embodiment of an apparatus 300 for the manufacture of a solid nanowire thin film 116a. The embodiment illustrated in Figure 3 is also similar to the embodiment illustrated in Figure 1. However, in this embodiment, the second part 114a of the apparatus 300 does not exhaust the solvent via an exhaust system 130. Rather, the second part 114a of the apparatus 300 is configured to use a liquid extraction technique at 301, such as osmosis or diffusion, to remove the solvent 107 from the liquid thin nanowire film 116 to form the solid thin nanowire film 116a. Liquid may also be supplied at 302.

The apparatus of Figures 5-6 provides the nanowire ink onto a solid surface 120 in a chamber 110. The ink 104 may be provided in a first part 111 of an apparatus 500, 600 that contains a saturated vapor atmosphere 112 (such as humidified air, humidified inert gas (e.g., nitrogen or noble gas), air or inert gas saturated with organic material, such as ethyl acetate, etc.) which prevents or reduces nanowire ink solvent 107 evaporation. The nanowires 108 are then moved to a second part 114 of the apparatus 500, 600 (e.g., by motorized substrate translation or by movable web substrate) where the saturated atmosphere 112 is removed (e.g., exhausted) to allow the solvent 107 of the ink 104 to evaporate from the solid surface 120. The saturated atmosphere 112 may be replaced by dry air 112a at a third part 118 of the apparatus 500, 600 where the solid nanowire film 116a is removed from the apparatus 500, 600.

In various embodiments, a process step for orienting nanowires is carried out. In one embodiment, schematically illustrated in Fig. 5, this is accomplished by means of applying an electric field over the nanowire ink. This may be obtained by means of an electric field device 501, comprising a first electrode 502 and a second electrode 503. The first electrode 502 may be provided underneath the substrate, preferably as close to the substrate as possible. Where the substrate is conductive, the lower electrode 502 may be arranged to contact the substrate 120. The second electrode 503 is preferably provided over the substrate, close to the nanowire film 116. In one embodiment, the second, upper, electrode 503 is placed in very close relation to the first electrode 502, such that it will touch the surface of the nanowire film 116 when passing through the electric field device 501. In an alternative embodiment, the electrode spacing is selected such that the upper surface of the applied nanowire film 116 passes under the upper electrode 503 without contacting it. The spacing between the first 502 and second 503 electrodes may be in the range of 10μιη-1 mm or higher, e.g. in the range of 100- 500μηι. The electrodes may be connected to a voltage supply unit 504, which may be configured to provide a voltage to generate an electric field between the electrodes 502 and 503. Where nanowires have a composition that entails different electrical characteristics along the elongate extension, the nanowires will turn to align with the electrical field. The composition involving varying electrical characteristics may e.g. comprise a metal particle at one end, such as a gold particle, or e.g. an axial p-n junction. Further details of various embodiments configured to apply an electric field are provided with reference to Fig. 8 below.

In other various embodiments, orientation of the nanowires in the film 116 may be carried out by providing chemically functionalized nanowires. In such embodiments, orientation is obtained through a combination of surface-functionalization of the wires and choice of liquids in a two-liquid. As an example, one or both ends of a nanowire may be configured such that out of the opposite ends of the nanowire, one is more hydrophobic and one is more hydrophilic, compared to each other. The nanowires may thus e.g. be provided in an ink 104 which comprises two different liquids, of different density. When injected through the slit 124 of an ink injector, the denser liquid may sink to the bottom towards the substrate, leaving the less dense liquid on top. The orientation of the nanowires may be obtained by the opposite ends of the nanowires being attracted to the different liquids. In an alternative embodiment, now with reference to Figs 1-3, the ink 104 may include one type of liquid containing functionalized nanowires, whereas the liquid 105 on which the ink is provided acts as a complementary liquid, such that the functionalized nanowires orient themselves in the junction between the liquids. Various examples for preparing functionalized nanowires have been presented by the instant applicant Sol Voltaics in PCT/IB2015/053094. As exemplified therein, in one embodiment a "functionalizing compound" may be applied to one and of the nanowire, or different compounds may be applied to opposite ends of a nanowire. Such a compound may comprise a surface affixing functional group (called "anchor") and a second functional group 119A that gives the compound its specific properties. Table I of that application lists different kinds of anchor groups as well as different kinds of second functional groups and backbones. Some non-limiting exemplary combinations of anchors and second functional groups as well as backbones are shown in the examples column. Exemplary solvents are also listed. Functionalizing compounds suitable for nanowire assembly and capture are also described in U.S. Provisional Application No. 61/623,137, filed on April 12, 2012, hereby incorporated by reference in its entirety. Furthermore, a functionalizing polymer can be used instead of functionalizing compounds.

It should be noted that a process step for orienting nanowires, e.g. as described above, may be applied to any of the embodiments described herein with respect to Figs 1-11.

Solid substrate 120 non-limiting example: In one embodiment, the ink (bulk NW dispersion) 104 is introduced to the solid substrate 120 through a thin slit 124 (0.05-1 mm wide), spreading the ink 104 into a thin line across the width of the substrate 120. The substrate 120 is dragged perpendicular to the width of the substrate 120 while continuously introducing the ink 104 (bulk dispersion). The equipment may contain several compartments 111, 114, 118 with controlled vapor saturation and/or laminar gas flow which allows for the control of position and rate of solvent 107 removal.

Evaporation and dragging of the substrate 120 will thin the liquid film 116 and increase the NW 108 concentration in the liquid film 116. At a place >2 mm from the point of bulk dispersion introduction, e.g. slit 124, referred to as "steady state drying front" 126, the thin, liquid NW film 116 will transition into a solid (or semi-solid) thin NW film 116a. At a place between the point of introduction of the bulk dispersion, e.g. slit 124, and the steady state drying front 126, the NWs 108 are moved into ordered positions by means of a capillary force or surface tension, which is realized by the thinning of the liquid NW film 116. The initial liquid film thickness, the thinning of the liquid film 116, the concentration increase (liquid removal) rate and the relative position of the steady state drying front 126 to the point of bulk dispersion introduction, e.g. slit 124, are affected and controlled by the slit- substrate distance (typically 0.1- lmm), the substrate dragging speed (typically 0.01-10 mm/s), the rate of bulk dispersion introduction (typically 0.001 to 0.2 ml/min) and the gas flow rate (typically 5-50 1/min). Other ranges may be used depending on the apparatus 500, 600 and process conditions. In the embodiment illustrate in Figure 6, the substrate 120 is a movable web wound on two spindles 120a, 120b.

Liquid surface non-limiting example: In another embodiment, a few drops of ethylacetate is first added to a liquid interface 106 (with polymer subphase of 0.5% PVA) in a rectangular container of glass (a small kuvette) 109. N2 gas is saturated by bubbling through a gas-washing bottle filled with ethyl acetate. The flow of N2 gas is controlled by a mass flow controller (MFC) to be 20 ml/min and injected into the compartment 110 where the nanowire ink 104 is to be injected. The exhaust of air is done by connecting to the ventilation system 130 and it is verified by smoke that the flow profile in the container 110 is of the desired type to realize an equipment of the type depicted in Figure 1. A dispersion of nanowires in Ethylacetate is then deposited on the liquid surface 106 leading to the formation of a steady-state drying front 126 on the liquid 105.

The thin liquid NW film 116 can be transformed into a thin solid NW film 116a by removing all solvent 107 (if the dispersion 104 only contains solvent(s) 107 and NWs 108) or by crosslinking. The crosslinking can be realized by incorporating a crosslinking agent into the dispersion, for example a UV curable crosslinker such as TMPTMA (Trimethylolpropanetrimethacrylate) or HDDMA

(Hexanedioldimethacrylate) and a suitable photoinitiator such as for example BIP (Bisphonyl containing compound) and then exposing the composition to UV radiation. An alternative way to crosslink the film is to introduce two reactants in different phases and let them meet at the interface. This can for example be done if the dispersion contains an isocyanate compound such as for example HMDI

(Hexamethyledediisocyanate) and the subphase contains a polyol such as for example ethyleneglycol or polyethyleneglycol or pentaerytritol. This procedure will produce a cross linked polyurethane film at the interface.

Figures 7A and 7B illustrate the property of the film and the degree of order on respective plots of film thickness and volume percent nanowires versus time, based on the above described method.

As shown in Figure 7A, starting at point Al, the solvent is removed (e.g., by evaporation or osmotic effect (dissolution/diffusion into substrate/subphase) to increase the nanowire concentration and decrease the thickness of the nanowire liquid film.

Starting at point A2, convective flows move the nanowires as the solvent is removed.

Starting at point A3, capillary drag/surface tension effect move the nanowires when the film thickness is close to the nanowire size (e.g., nanowire length +/- 50%). For example, the nanowire concentration increase and/or crosslinking of a polymer matrix around the nanowires is used to drag the nanowires into position and to retain the nanowires in an ordered fashion (e.g., aligned with the axis substantially perpendicular to the underlying surface). Starting at point A4, the final drying of the residual nanowire ink solvent finalizes the fixing of the nanowires in the ordered fashion.

Figure 7B illustrates the degrees of ordering/orientation of nanowires in their structural arrangements. At point Bl, the nanowires are disordered. At point B2, the nanowires are in a liquid crystal arrangement. At point B3, the nanowires are in an oriented liquid crystal arrangement. At point B4, the nanowires are in a colloidal crystal/frozen liquid crystal arrangement.

Figure 8 illustrates the steps in the above described method. During the deposition step, the nanowire ink 104 (e.g., semiconductor nanowires 108 in a solvent 107, such as an organic solvent, e.g., toluene, ethylacetate or ethanol with optional polymer components, such as PMMA or PVA) is deposited on a surface 106 or 120. As described above, the liquid ink 104 (e.g., the nanowire ink solvent) is thinned into a film 116 by providing the ink 104 to a solid or liquid surface 106, 120 in a saturated atmosphere 112 which prevents or reduces evaporation of the solvent 107, allowing the ink 104 to spread out.

During the orientation and vertical alignment step, the nanowires 108 are oriented substantially vertically using the convective flows, the electric field (for nanowires containing a junction) and/or capillary drag/surface tension. During the convective self assembly and aggregation step polymer matrix cross linking and/or the capillary drag/surface tension care used to close pack and/or link the nanowires in place in the colloidal crystal arrangement.

Preferably, the electrical field application occurs before positioning the nanowires 108 in the ordered fashion (e.g., while the nanowires 108 can still move in the solvent 107 prior to forming the frozen liquid crystal or colloidal crystal solid film 116a). As described above, the solvent 107 from the liquid film 116 is removed by moving the liquid film 116 into a non-saturated atmosphere 112a (e.g., dry air) to evaporate the solvent 107 to form the solid colloidal crystal film 116a. If the substrate is a solid substrate 120, it may be a conductive electrode, and the electric field may be applied between the solid substrate 120 and a counter electrode 220 from a voltage or current source 222.

The bottom two images show nanowire 108 orientation in a liquid polymer 107 using an electric field, where the polymer may be cross linked by UV irradiation or heat and high density alignment of the nanowires using an electric field (e.g., 5x1010 nano wires per ml of ink 104).

Figure 9 shows the steps in preparation of the nano wire ink 104. The nanowires 108 may be formed by the aerotaxy™ method described in US Application serial number 14/403,427 and its PCT parent application publication number WO

2013/176619 Al, both incorporated herein by reference in their entirety. Other nanowire fabrication methods may also be used. The nanowires 108 are then dispersed in a liquid 107 (e.g., an organic solvent such as ethanol, ethylacetate or toluene) and then optionally encapsulated in a shell 128 in the liquid 107. For example, the method described in PCT publication number WO 2013/154490 A2 may be used to disperse the nanowires 108. The nanowires 108 may be encapsulated in an inorganic (e.g., silicon oxide) or organic (e.g., polymer) shell 128 if desired. The bottom two images in Figure 9 show photographs of dispersed and encapsulated nanowires 108 which form the ink 104.

Figure 10 illustrates the method steps of Figures 8 and 9 in combination.

Specifically, the nanowires 108 fabricated by aerotaxy™ are modified (e.g., dispersed and optionally encapsulated in shell 128) in a liquid 107 to form the ink 104. The ink 104 is then deposited on a liquid or solid surface 106, 120, and the nanowires 108 are aggregated and aligned using the above described evaporation and optional electric field alignment method to form the solid nanowire thin film 116a. The solid nanowire thin film 116a is then removed from the surface 106, 120 and provided into a solar cell. If desired, the liquid carrier 107 may contain a monomer or polymer which is cross linked to form a matrix 129 around the aligned nanowires 108 to allow the ease of removing the nanowire film 116b from the surface 106, 120.

Thus, as described above, any equipment may be used that forms a thin liquid film

116 from a bulk dispersion 104 and further transforms the liquid thin film 116 into a thin solid film 116a, 116b.

Preferably, during the above described process, the nanowire concentration increase profile and the rate of concentration increase are controlled along with controlling composition of particle (i.e., nanowire) dispersion and the injector, e.g. pump 102 or slit 124, to form the solid thin film 116a. Likewise, the viscosity, thickness and concentration of the thin liquid NW film 116 may be controlled during the process. The components (particles, solvents, co-solvents and additives) of the process and the container materials for the coating chamber 110 may be selected to achieve the desired particle-particle interactions during the drying process to form the solid thin film 116a.

If desired, an external field that can order the nanowires 108 in different ways (for example preferential orientation) may be applied during the process. Optionally, chemical heterogeneity of particles may be used to control the organization of the nanowires 108 in the dried film 116a (order, orientation, or pattern formation). The solid substrate 120 may be treated with a surface treatment used for capture to control the organization of nanowires 108 in the drying film 116. If desired, the fixing or curing the nanowires arrangement may optionally be added at some point along the drying direction by for example, UV-curing or adding a crosslinking agent (introduced as a vapor or a liquid into the drying film 116).

The dried film 116a, 116b may be a free standing film or a nanowire 108 in a polymer matrix film 129 which is removed from the surface of the liquid or solid 106, 120 and placed into a solar cell as an assembly. In an embodiment, the substrate 120 with the captured assembly 112 of nanowires 108 can be placed into a solar cell 501 if the nanowires 108 have a pn junction 508C, as shown in Figure 11. For example, as schematically illustrated in Figure 11, the substrate 120 contains semiconductor (e.g., GaAs, InP, etc.) nanowires 108 positioned substantially perpendicular (e.g., with the longest axis 80 to 100 degrees, such as 90 degrees) to upper capture surface of the substrate. The nanowires 108 in this embodiment have an axial pn junction 508C located between a lower first conductivity type (e.g., n or p type) segment 508A and an upper second conductivity type (e.g., p or n type) segment 508B of the opposite conductivity type. In the solar cell 501, electrodes provide electrical contact to the nanowires 108. For example, the solar cell 501 may contain an upper electrode (e.g., transparent electrode) 510 in electrical contact with the upper segment 508B of the nanowires and an electrically conductive or semiconductor substrate 520 may provide an electrical contact to the lower segment 508A of the nanowires. An insulating or encapsulating material 512 may be located between the nanowires 108. Alternatively, the nanowires may contain a radial rather than an axial pn junction, in which case segment 508B is formed as a shell surrounding a nanowire core 508A such that the pn junction extends substantially perpendicular to the substrate capture surface. While the nanowire thin film 116a is preferably used in a solar cell, it may be used in other devices, such as surface layers of electronic devices (e.g., coatings of multi-junction solar cells, screens, batteries, fuel cells, etc.), medical devices (e.g., coating of surfaces for implants), surface treatments of 3D printed objects, etc.

In the foregoing, a method of fabrication of an ordered nanowire film has been outlined, with respect to different embodiments. The method may comprise the steps of adding nanowires to a liquid comprising a solvent, wherein each nanowire has a first end and a second opposite end, and a character which is different adjacent to the first end from adjacent to the second end. The liquid may be provided on a substrate, which may be e.g. a thin solid substrate as in Figs 5-10, or a liquid as in Figs 1-4. The liquid may then be thinned into a film, e.g. by moving the substrate away from the point of liquid injection. The nanowires may then be positioned in the liquid film such that they are affected, based on said character, to be oriented in a predetermined direction with respect to a surface of the film. The film may comprise semiconductor nanowires containing a pn or p-i-n junction, such as in Fig. 11, wherein the nanowires have the character of a p-doped region closer to one end and an n-doped region closer to the opposite end. In another embodiment the semiconductor nanowires may have the character of a metal particle attached to one of said ends. In any of these non-limiting examples, the nanowires may be affected to align under the influence of an electric field. More specifically, due to the different character of the nanowire adjacent to the opposing ends, the nanowires may be oriented with the same respective ends aligned in the same substantial direction, under influence of an electric field. In other

embodiments, the nanowires may have the character of a functionalized surface, e.g. by surface treatment at only one end, or different surface treatment at the opposing ends, e.g. as exemplified in the referenced Sol application. As an example, one end may be more hydrophobic than the other. The step of positioning the nanowires may then comprise aligning a direction of the nanowires in a predetermined direction in an interface between said solvent and a second liquid. The second liquid could be provided in the liquid containing nanowires, injected onto the substrate. An alternative example is where a liquid substrate, e.g. as in Figs 1-3, acts as the additional liquid. The method further includes the step of removing the solvent from the liquid film, such that the nanowires are moved into aligned position closer to each other by capillary force or surface tension. This may be accomplished by evaporation of a solvent of the provided liquid that contains nanowires. In a subsequent step, the liquid film may be solidified to a solid film.

Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.

Claims

1. A method of fabrication of an ordered nanowire film comprising the steps of: adding nanowires to a liquid comprising a solvent, wherein each nanowire has a first end and a second opposite end, and a character which is different adjacent to the first end from adjacent to the second end;

providing the liquid on a substrate;

thinning the liquid into a film;

solidifying the liquid film to a solid film.

2. The method of claim 1, wherein solvent is removed through evaporation or osmosis.

3. The method of claim 1, wherein the capillary force or surface tension is realized through the thinning of the liquid by removal of the solvent.

4. The method of claim 1, wherein the step of positioning the nanowires comprises aligning a direction of the nanowires by an electric field.

5. The method of claim 4, wherein the film comprises semiconductor nanowires containing a pn or p-i-n junction, wherein the nanowires have the character of a p-doped region closer to one end and an n-doped region closer to the opposite end.

6. The method of claim 4, wherein the film comprises semiconductor nanowires having the character of a metal particle attached to one of said ends.

7. The method of claim 1, wherein the nanowires have the character of a functionalized surface, wherein the step of positioning the nanowires comprises aligning a direction of the nanowires in a predetermined direction in an interface between said solvent and a second liquid.

8. The method of claim 7, wherein the second liquid forms said substrate.

9. The method of claim 7, wherein the liquid comprises the solvent and the second liquid.

10. The method of claim 1, wherein the ordered fashion comprises one of the states of a colloidal crystal, liquid crystal or oriented liquid crystal.

11. The method of claim 1, wherein thinning the liquid into a film comprises providing the liquid to a solid or liquid surface in a saturated atmosphere which prevents or reduces evaporation of the solvent.

12. The method of claim 9, wherein removing the solvent from the liquid film comprises moving the liquid film in a non-saturated atmosphere to evaporate the solvent and form a colloidal crystal or oriented liquid crystal nanowire solid film.

13. The method of claim 11, wherein the solid film is removed from the solid liquid surface as a free standing film or a nanowire in a polymer matrix film and placed into a solar cell.

14. The of claim 1, comprising the step of passing the substrate under an injector for supplying the liquid as a film, and through an evaporation stage for removal of the solvent.

15. An apparatus for producing a nanowire solid film comprising a colloidal crystal or oriented liquid crystal nanowire solid film, comprising

an injector device connected to a container for holding a liquid comprising nanowires; a substrate carrier device for moving a substrate by the injector device in a saturated vapor atmosphere, such that a liquid injected onto a substrate is thinned to a liquid film;

a positioning stage configured to affect nanowires in the liquid film, which nanowires have first and second opposite ends, and a character which is different adjacent to the first end from adjacent to the second end, to become oriented in a predetermined direction with respect to a surface of the film;

an evaporation device positioned spaced apart from the injector device, in which the saturated vapor is exhausted, such that a substrate is passed from the injector device into the evaporation device where a solvent of the liquid is vaporized, such that the nanowires are moved into aligned position closer to each other by capillary force or surface tension.

16. The apparatus of claim 15, further comprising a curing stage after the evaporation stage.

17. The apparatus of claim 15, wherein a substrate driver is configured to continuously move a substrate from the injector, through the positioning stage, and into the evaporation device.