DE3526908A1 - Semiconductor layer made of a transition metal dichalcogenide, method for the preparation thereof and use of such semiconductor layers for solar cells - Google Patents

Abstract

Semiconductor layers made of a transition metal dichalcogenide, especially having the composition MX2(M = W, Mo; X = S, Se, Te), prepared in a specially adapted process, enable the use of said layers in solar cells. Large-area layers made of two-dimensional crystals 4 having good photoelectric behaviour are provided. The semiconductor layer is generated at the boundary layer 5 of a melt 2 situated within a crystallisation vessel 1 and the gas phase 3 located thereabove of the melt material. The two-dimensional crystals 4 which, with large surface areas 8 and small lateral steps 9, grow to form the semiconductor layer and have the composition MX2 are suitable as photovoltaic and photoelectric large-area semiconducting layers for solar cells. <IMAGE>

Description

Semiconductor layer made of a transition metal dichalcoge-

nid, method for their production and use of such semiconductor layers for solar cells The invention relates to semiconductor layers made of transition metal dichalcogenides, specially adapted processes for their production and the use of such Semiconductor layers for solar cells.

Because of their optical and electronic properties, they come in layered form Group VI transition metal dichalcogenides for efficient solar energy conversion seriously to be considered as promising materials. With indirect transitions at low energy, those of direct transitions with high absorption capacity (a ~ 105 cm 1) at photon energies of around 1.5 eV (WSe2, MoSe2), and minority carrier diffusion lengths of around 2 pm may make it possible to use solar energy effectively converting it into electrical energy using thin-film structures.

Investigations on photoelectrochemical solar cells have shown that, although with these semiconductors, an exceptional stability against photocorrosion was achieved, depending on the samples, large variations in the photocurrent (disc), occur in the photovoltage (of) and in the fill factor. Let these differences based on variations in surface morphology and deviations in the stoichiometry. In particular, it was found that stages in the Surfaces of the layered grid material for the solar lar cell yield are disadvantageous. This also applies to doping fluctuations within a crystal.

An extensive material selection has led to the development of a photoelectrochemical Solar cell with n-WSe2 led, which converts solar energy into electrical energy with a Converts efficiency of 10%. By chemical transport (CVT) from this material Crystals produced must, however, with regard to their surface structure and their Doping profiles are specially selected because this process increases the reproducibility and the uniformity of the crystals is not guaranteed.

In addition, only relatively small crystallites of a maximum of 50 mm by means of chemical transport (CVT).

Considerable efforts have been made to make it two-dimensional growing MX2 materials (M = Mo, W; X = S, Se, Te) as large, uniform to represent photoactive layers. All the methods investigated so far led to high concentrations of steps with defects that reduce the energy yield Extremely reduce recombination processes.

The use of tellurium as a melt for cultivation is already known of crystals which contain elements of groups II to IV. Also are with it Crystals of transition dichalcogenides have already been produced. Apparently it is so far, however, it has not been possible to produce thin, large-area polycrystalline films of MX2 materials from such a melt.

As the light absorption of some transition metal dichalcogenides very high in the wavelength range of interest for solar energy conversion is, these advantages can then come into their own when it is possible to thin smooth To have layers available that efficiently convert solar energy into electrical Can transform energy.

The invention is therefore based on the object of such large-area Layers of two-dimensionally crystallized transition metal dichalcogenide material with good photoelectric behavior.

The semiconductor layers with which this object according to the invention is solved are characterized by the features listed in claim 1.

Transition metal dichalcogenides have proven to be particularly suitable the composition MX2 or Ml, xMxX2 with M = tungsten (W) and / or molybdenum (Mo) and X = sulfur (S), Se (selenium) or tellurium (Te).

The manufacture of these is of essential importance for the invention Products. The changes that the material has to undergo afterwards to have the desired properties are only possible through specially adapted process steps accessible. Claim 3 contains the comprehensive technical according to the invention Teaching on this.

This method thus sees the orientation of two-dimensional growing, Lattice-like crystals of the composition MX2 or M1 ~ XMxX2 with M = W, Mo or M1 x = W; Mx = Mo and X = S, Se, Te with the aim before, large-scale to produce semiconducting layers, which are responsible for the conversion of solar energy suitable in electrical energy. The crystal growth of the semiconductor layer takes place here at the surface of the melt or at the interface between the melt and the gas phase of the melt material, whereby the crystals, their specific Weight is higher than that of the melt, move upwards against gravity.

It can advantageously be used a melt, in particular consists of liquid tellurium (Te). This melt material remains in a wide temperature range liquid and has a low vapor pressure. It also has the property that Compound in particular the composition MX2 easy to solve.

Selenium (Se) is also well suited as a melt material also mixtures of selenium and tellurium as well as lead chloride (PbCl2) and bismuth (Bi).

For crystal growth a vessel is required, which is one of the desired Layer size corresponding interface between melt and gas space guaranteed. That The vessel can in particular consist of quartz. Other possible materials for the Crystal growing vessels are aluminum oxide or silicon-aluminum alloys.

More detailed information on the individual steps of a preferred Embodiments of the method according to the invention are set out in claim 7.

The semiconductor material of the composition MX2, which is called polycrystalline Layer with good alignment and a small proportion of steps, i.e. the lowest possible proportion of edge surfaces which are perpendicular to the layer surface is to crystallize, the melt is added, the amount correspondingly the volume of the desired slice is calculated.

The homogenization can be done mechanically or by ultrasound. If the solubility is reduced as a result of lowering the temperature, it crystallizes Semiconductor material from, and the semiconductor layer grows on the surface of the melt.

The crystallization at the interface between the surface of the melt and gas phase of the melt can be influenced and improved by a plate, which is located at the interface, i.e. either held close to the melt or even slidingly touches the surface of the melt. This becomes a Temperature gradient achieved for the crystallization nuclei. In the case of the sliding The plate can come into contact with the microcrystals to be grown which are rolled on under pressure Compounds are coated, causing crystallization to the lower plate surface is improved.

The separation of the grown semiconductor crystal layer from the melt can either be brought about by a temperature gradient, whereby the Melt condenses at a colder point in the crystallization vessel, or E.g. also by applying a vacuum, whereby the melt can flow through if necessary a sintered layer of quartz glass can be filtered off. Furthermore there is also a side Discharge of the melt and subsequent sublimation of the melt or the like.

possible.

The inventive use of such semiconductor layers lies with solar cells. It can be photovoltaic as well as photoelectric Acting on solar cells.

The essential importance of the invention lies in the greatest possible Alignment of the crystal faces for layer growth parallel to the van der Waals surface.

This results in crystal layers with a smooth, low-step surface generated. By avoiding steps on the layer lattice crystal films their effect as recombination centers avoided, so that a recombination of Charge carriers and losses during energy conversion no longer or only in insignificant extent occur, whereby the photocurrent, the photovoltage and the The fill factor of a solar cell operated with this material can be increased.

Further details of the invention are provided below in context explained in more detail with the description of the pictorial representations. Show: 1: an illustration to explain the crystal growth of a semiconductor layer at the interface between a melt and a melt material gas phase; Fig. 2: Representations of several methods for eliminating the melt by a) filtration, b) distillation and c) discharge with subsequent sublimation; Fig. 3: Scanning electron microscope Images of a MoSe2 layer that was produced on the surface of a tellurium melt became; 4: a diagram for the photoelectrochemical behavior of n-MoSe2, which was obtained from a tellurium melt; Fig. 5: the performance characteristics a MoSe2 layer in a regenerative electrochemical solar cell with J- / J2 as a redox system under AM1 exposure; Fig. 6: the photocurrent-voltage characteristic a silver-treated n-MoSe2 electrode, which in the presence of J / J2 from a tellurium melt and Figs. 7 and 8: Illustrations for the explanation of the Accumulation of crystals on the surface of the melt to reach the side beginning growth of a semiconductor layer, in Fig. 8 on the underside of a Plate.

First of all, however, we should focus on the production of semiconductor layers from transition metal dichalcogenides, especially the composition MX2 (M = W, Mo; X = S, Se, Te) or the composition M1 XMXX2 (M1-x = W, Mx = Mo) of an exemplary embodiment will be discussed in more detail, in which tellurium (Te) is used as melt 2 and gas phase 3.

In this exemplary embodiment on a laboratory scale.

powdery transition metal diselenide, double-distilled tellurium (Te) added in a ratio of 1: 100. On every 10 g of this mixture were in addition, 100 mg selenium (Se) were added for better control of the desired reaction. The starting material was then placed in an ampoule made of quartz with a diameter of 20 mm encapsulated and at a temperature of 1050 OC heated. Included formed in the quartz ampoule, which served as a crystallization vessel, at the bottom a tellurium melt 2 and, above it, a tellurium gas phase 3. Afterward the material became with a temperature gradient of a few 1/100 degrees per hour chilled. During the cooling process, transition metal dichalcogenide crystals began to form 4 both in the liquid melt 2 and in the melt / vapor intermediate layer to be deposited. There was an effect which was convection and surface tension includes and crystallization in the tellurium melt / vapor interface 5 facilitated although the density of the semiconductor material is greater than the density the melt 2. The mechanism, interpreted as the Marangoni effect, is shown schematically in FIG shown.

In Fig. 1, a crystal growing vessel 1 is shown, which is one of the desired Semiconductor layer size corresponding interface of melt 2 and gas phase space 3 must guarantee. The crystallization vessel 1 can in particular consist of quartz. It can also be used as aluminum oxide, zirconium oxide or silicon-aluminum alloys other materials can be used for the crystallization vessel 1.

The amount of that added to the tellurium melt 2.

Transition metal dichalcogenide is desired according to the volume To calculate semiconductor layer.

A homogenization of the mixture from melt 2 and in between 600 OC and 1000 OC dissolved semiconductor material can be done mechanically or by ultrasound take place. After the growth process has ended, it crystallizes in the subsequent process Reduction of the solubility of the melt as a result of a lower temperature adjustment the semiconductor layer on the melt. Good results are achieved if 1050 0C is selected as the starting temperature for the temperature decrease.

The crystallization at the interface 5 between the tellurium melt 2 and tellurium gas phase 3 can be influenced and improved by a plate 10 (see. Fig. 8) which is held close to the tellurium melt and in the extreme case the surface of the melt 2 slidingly touches. The plate 10 causes a temperature gradient for the formation of crystal nuclei. With sliding contact with the melt 2 can the plate 10 with microcrystals of the compounds to be grown, which under Pressure rolled to be coated, causing crystallization to lower Plate area is improved.

After completion of the growth process, the crystal layer 4 separated from the melt 2. This can be done in three different ways, for example, as shown in Fig. 2 take place.

According to FIG. 2a, a vacuum can be applied to the crystallization vessel 1 and the melt 2 through a sintered layer 7 in the direction of the arrows 6 the vessel 1 can be filtered off. According to FIG. 2 b, a temperature gradient can be used the melt 2 condenses at a colder point in the crystallization vessel 1 where T1 <T2. According to FIG. 2 c, the lateral drainage Melt 2 and subsequent sublimation a separation of the crystal layer 4 from brought about the melt.

MoSe2, MoTe2 and WSe2 were prepared in experiments.

The crystallite dimensions reached 60 mm2 with a thickness of approximately 0.3 mm. The transition metal diselenides obtained in this way had a stoichiometry MSe2xTex (M = Mo, W) with X <0.1. The tellurium content of the crystals could be reduced by heating the samples in a selenium atmosphere to 650 ° C will. The MoSe2 material examined still showed detectable amounts of tellurium and some transition metal impurities.

All the prepared semiconductor layers were photoactive.

MoSe2 and MoTe2 had n-type conductivity, and WSe2 showed p-type conductivity. The semiconductors showed 2 photocurrent densities of a few mA / cm2 under simulated AM1 conditions. MoSe2 was chosen for the study because of its high photopotential.

The morphological structure of a semiconductor layer obtained is in 3 is shown in scanning electron microscope images of a MoSe2 layer, which was produced on the surface of a tellurium melt 2, the starting temperature for cooling was 1050 ° C. These recordings show that the crystals 4 are parallel have grown together to form their layer surfaces and have a firm lateral connection exhibit hexagonal crystallites along their edges. The crystals are similar Mica has grown together in large leaves or large layers of material. The crystallites are partly on top of each other, from which the formation of small steps results. The detail X in Fig. 1 shows schematically the large-scale growth surfaces 8 of the crystals 4 perpendicular to the arrow c and the small lateral steps 9 parallel to arrow c.

The photoelectric properties of these semiconductor layers are shown in FIG. This shows the photoelectrochemical behavior of n-MoSe2, which was obtained from a tellurium melt 2. The photocurrents were measured without adding redox systems to the electrolyte.

Fig. 5 shows the performance characteristics of a MoSe2 layer in a power generating, regenerative electrochemical solar cell with J / J2 as Redox system.

The exposure is standardized to AM1 ratios, which are the solar ones Intensity at the earth's surface, with perpendicular incidence of light to 0 m above sea level (NN).

Through chemical and electrochemical treatment of the layer surface, which the effect already reduced by the technical teaching according to the invention on the step surfaces further reduced, the photoelectric properties the layer surface can be further improved. An example of the properties of a Fig. 6 shows such a surface-treated layer as a photocurrent-voltage characteristic an n-MoSe2 electrode treated with silver ions (Ag +), which is made from a tellurium melt 2 was drawn. This proves that the production of semiconductor layers two-dimensional growing MX2 compounds with useful photoelectric properties for solar energy conversion in photoelectrochemical and photovotaic solar cells leads. The technical realization of large-area, polycrystalline thin-film layers e.g. MoSe2 and WSe2 is possible for the construction of such solar cells.

Fig. 7 shows the surface growth of crystals 4 to achieve lateral growth, which can be attributed to the Marangoni effect is. The surface tension a is a critical parameter for the formation of Crystallization nuclei and not only on the concentration of the melt 2 but also dependent on their temperature. The temperature gradient AT Ax in the plane of the The surface of the melt 2 results in a force F x parallel to the surface, whose origin is the difference in surface tension a. There is one Convection, which the crystallization nuclei from the mass of the melt 2 to the Surface transported at points of the higher temperature T1 of the vessel 1 and a subsequent transport of the slowly growing crystals 4 along the boundary layer between melt 2 and gas phase 3 causes. The larger crystallites in the places with a higher surface tension a do not follow the convection of the melt 2, since the increasing surface tension a counteracts the sinking of the crystallites.

That shown in FIG. 8 already mentioned above Crystallization vessel 1 has a plate 10 above the melt 2, on which The crystals 4 emerging from the melt 2 grow underneath the surface. For the emergence a corresponding temperature gradient between melt 2 and plate 10, which is surrounded by the gas phase 3 of the melt 2, for example by cooling and thermal insulation of the plate 10 (not shown) can be provided.

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Claims (17)

A semiconductor layer made from a transition metal dichalcogenide, not shown by - a surface structure in which the crystals are aligned two-dimensionally symmetrically, parallel to the layer surface, and in which the proportion of the crystal edge surfaces lying perpendicular to the layer surface is low to negligibly small, 2 - a continuous layer area of more than 50 mm2 - a layer thickness of down to approx. 0.1 mm and - a homogeneous doping in semiconductor material. 2. Semiconductor layer according to claim 1, g e k e n n z e i c h n e t by a transition metal dichalcogenide of the composition MX2 or M1 xMxX2 with M = W, Mo or Mix = W, M = Mo and X = S, Se, Te. 3. A method for producing a semiconductor layer according to claim 1 or 2, that is, that the crystallites (4) are attached the surface of a melt (2) in a crystallization vessel (1) are deposited and grow together to form the semiconductor layer, the surface the melt (2) is covered by the melt material in the gaseous phase (3). 4. The method according to claim 3, d a d u r c h g e k e n n z e i c h n e t that the melt (2) consists of tellurium (Te), selenium (Se) or a mixture of Tellurium (Te) and selenium (Se). 5. The method according to claim 3, d a d u r c h g e k e n n z e i c h n e t that the melt (2) consists of lead chloride (PbCl2). 6. The method according to claim 3, d a d u r c h g e k e n n z e i c h n e t that the melt (2) consists of bismuth (Bi). 7. The method according to any one of claims 3 to 6, d a d u r c h g e k It is noted that 0 - the melt (2) is heated up to approx. 1050 C, - The transition metal dichalcogenide in the amount corresponding to the volume of the to be produced Semiconductor layer corresponds to the melt (2) is added and dissolved in this, - A homogenization of the liquid phase from melt (2) and dissolved semiconductor material is carried out between 1000 ° C and 600 ° C, - the solubility of the melt (2) reduced by lowering the temperature and thereby the deposition of the semiconductor material crystals and their growing together to form the desired semiconductor layer at the interface (5) is brought about between the melt (2) and the gas phase (3) of the melt material, and finally - the semiconductor layer is separated from the melt (2). 8. The method according to claim 7, d a d u r c h g e k e n n z e i c h n e t that as a result of convection and surface tension inclusive influences the crystals forming in the melt (2) migrate to the boundary layer (5) be subjected. 9. The method according to claim 7 or 8, d a d u r c h g e k e n n z e i c h n e t that the crystallization of the semiconductor material and the formation the semiconductor layer at the interface (5) between the melt (2) and the gas phase (3) influenced by a plate (10) arranged in the region of the interface (5) will. 10. The method according to claim 9, d a d u r c h g e k e n n z e 1 c h n e t that the plate (10) is held just above the melt (2). 11. The method according to claim 9, d a d u r c h g e k e n n z e i c h n e t that the plate (10) slidingly touches the surface of the melt (2). 12. The method according to claim 11, d a d u r c h g e k e n n z e i c h n e t that the plate (10) slidingly contacting the surface of the melt (2) with microcrystals of the semiconductor crystal layer to be grown, rolled on under pressure (4) is coated. 13. The method according to any one of claims 7 to 12, d a d u r c h g e it is not indicated that the separation of the crystalline semiconductor layer (4) of the melt (2) by condensation of the melt (2) at a colder point of the crystallization vessel (1) takes place by means of a temperature gradient. 14. The method according to any one of claims 7 to 12, d a d u r c h g e it is not indicated that the separation of the crystalline semiconductor layer (4) from the melt (2) by applying a vacuum (6) to the crystallization vessel (1) and the melt (2) is filtered off. 15. The method according to claim 14, d a d u r c h g e k e n n z e i c hn e t that the abbot filter by means of a sintered layer (7) made of quartz glass he follows. 16. The method according to any one of claims 7 to 12, d a d u r c h g e it is not indicated that the separation of the crystalline semiconductor layer (4) from the melt (2) by laterally draining the melt (2) and by subsequent Sublimation occurs. 17. Use of a semiconductor layer made from a transition metal dichalcogenide according to one of the preceding claims for solar cells.