CN110224033B - Iron oxide photo-anode system embedded with silicon pn junction and preparation method - Google Patents
Iron oxide photo-anode system embedded with silicon pn junction and preparation method Download PDFInfo
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- 239000010703 silicon Substances 0.000 title claims abstract description 110
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 title claims abstract description 109
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- XOLBLPGZBRYERU-UHFFFAOYSA-N tin dioxide Chemical compound O=[Sn]=O XOLBLPGZBRYERU-UHFFFAOYSA-N 0.000 claims description 3
- 229910001887 tin oxide Inorganic materials 0.000 claims description 3
- 239000002184 metal Substances 0.000 claims description 2
- 238000003764 ultrasonic spray pyrolysis Methods 0.000 claims description 2
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 abstract description 20
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- VCJMYUPGQJHHFU-UHFFFAOYSA-N iron(3+);trinitrate Chemical compound [Fe+3].[O-][N+]([O-])=O.[O-][N+]([O-])=O.[O-][N+]([O-])=O VCJMYUPGQJHHFU-UHFFFAOYSA-N 0.000 description 2
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- CBCDPJRPMPTFKA-UHFFFAOYSA-N [Nb+3].CC[N-]CC.CC[N-]CC.CC[N-]CC.CC(C)(C)[N] Chemical compound [Nb+3].CC[N-]CC.CC[N-]CC.CC[N-]CC.CC(C)(C)[N] CBCDPJRPMPTFKA-UHFFFAOYSA-N 0.000 description 1
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- WHXTVQNIFGXMSB-UHFFFAOYSA-N n-methyl-n-[tris(dimethylamino)stannyl]methanamine Chemical compound CN(C)[Sn](N(C)C)(N(C)C)N(C)C WHXTVQNIFGXMSB-UHFFFAOYSA-N 0.000 description 1
- 229910000484 niobium oxide Inorganic materials 0.000 description 1
- URLJKFSTXLNXLG-UHFFFAOYSA-N niobium(5+);oxygen(2-) Chemical compound [O-2].[O-2].[O-2].[O-2].[O-2].[Nb+5].[Nb+5] URLJKFSTXLNXLG-UHFFFAOYSA-N 0.000 description 1
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Abstract
The invention belongs to the field of photoelectric conversion and new energy, and provides an iron oxide photo-anode system with a silicon pn junction embedded therein and a preparation method thereof, aiming at solving the technical problem that the iron oxide photo-anode in the prior art can not realize complete photolysis of water, wherein the iron oxide photo-anode system comprises an iron oxide absorption layer, a p-type silicon doping layer, an n-type silicon substrate, a back conducting layer and a back waterproof insulating layer; the p-type silicon doping layer and the n-type silicon substrate form a silicon pn junction; the shape of the silicon pn junction is a pyramid array structure; and a transparent conductive tunneling layer is arranged between the p-type silicon doping layer and the ferric oxide absorption layer. The embedded silicon pn junction enables the silicon layer to absorb incident light to generate larger photovoltage, the photovoltage and the iron oxide absorption layer form a series relation, namely the voltage with the magnitude is added to the iron oxide layer, the starting voltage of the iron oxide photoanode is effectively reduced, the conductivity of the iron oxide absorption layer and the collection efficiency of photogenerated carriers are improved, and therefore complete photowater splitting is achieved.
Description
Technical Field
The invention relates to an iron oxide photo-anode system embedded with a silicon pn junction and a preparation method thereof, in particular to an energy band interface regulation and control technology when the photo-anode is used for completely photolyzing water, belonging to the field of photoelectric conversion and new energy.
Background
The photoelectrochemical cell taking a photoelectricity as a core is an effective way which is expected to realize hydrogen production by photolyzing water with the help of solar energy at low cost. The semiconductor material absorbs sunlight to generate photon-generated carriers which participate in the oxidation and reduction reaction of water (generate hydrogen), and the conversion of solar energy into high-energy green fuel is completed.
Currently, hydrogen production by photoelectrochemical cells is difficult to apply and popularize. The key problem is that most photoelectrode materials cannot realize water photolysis (namely, water needs to be decomposed) only by solar energyA certain bias voltage is applied). Although few wide bandgap semiconductor materials (e.g. KTaO)3) Complete photolysis of water can theoretically be achieved (i.e. without the need for an external bias), but these materials absorb only uv light, i.e. the vast majority of solar light cannot be utilized. In addition, these wide bandgap semiconductor materials have poor stability in aqueous solutions. In order to obtain a photoelectrochemical cell system with higher solar energy conversion efficiency, the photoelectrode material has a moderate forbidden bandwidth and good chemical stability in an aqueous solution.
Iron oxide (alpha-Fe)2O3) The material has the characteristics of excellent chemical stability, proper forbidden band width (1.9-2.3 eV, theoretical conversion efficiency can reach 12.9% -16.8%), good environmental compatibility and the like, and is an ideal photo-anode material. However, their minority carrier lifetime is short, resulting in the generation of photogenerated carriers that cannot be efficiently extracted and collected when the iron oxide is thick (several hundred nanometers or more). In addition, the position of the potential of the conduction band is lower than H+/H2And (3) the potential, which causes photo-generated electrons, cannot satisfy photo-reduction water reaction without bias voltage (i.e. the ferric oxide photo-anode system cannot realize complete photo-water splitting). In order to solve the above problems, it is common practice to introduce another light absorbing layer below the iron oxide absorbing layer, and theoretically satisfy the thermodynamic requirement of complete photolysis of water by combining the band positions of the materials of the inner and outer light absorbing layers. Although the photoelectrochemical cell system constructed by the double absorption layers can realize complete water photolysis theoretically, the photoelectrode system is difficult to realize complete water photolysis or has very small photocurrent without bias voltage because the photovoltage generated by the heterojunction of the inner absorption layer and the outer absorption layer is relatively small and the photocurrent of the whole photoelectrode system is relatively small because the carrier recombination on the interface of the inner/outer absorption layers is serious.
Disclosure of Invention
The invention aims to solve the technical problems that the ferric oxide photo-anode in the prior art can not realize complete water photolysis, and the ferric oxide is not matched with the energy bands of the photoelectrode of the double absorption layers constructed by other light absorption layers, so that the carrier recombination is serious and the photo-generated voltage is small. The technical scheme is as follows:
an iron oxide photo-anode system embedded with a silicon pn junction, wherein the photo-anode is of a composite layer structure and is characterized in that: the light-emitting diode comprises an iron oxide absorption layer, a p-type silicon doping layer, an n-type silicon substrate, a back conductive layer and a back waterproof insulating layer in sequence along a light incidence direction; the p-type silicon doping layer and the n-type silicon substrate form a silicon pn junction; the shape of the silicon pn junction is a pyramid array structure; a transparent conductive tunneling layer is arranged between the p-type silicon doping layer and the ferric oxide absorption layer, and the thickness of each transparent conductive tunneling layer is equal.
Preferably, the thickness of the iron oxide layer is 50-150 nm;
preferably the concentration of boron doping in the p-type silicon doped layer ranges from 5.0 × (10)18～1019)cm-3The depth is 0.1 to 0.3 μm.
Preferably, the concentration of phosphorus doped in the n-type silicon substrate is in the range of 5.0 × (10)14～1015)cm-3The thickness of the substrate is 200 to 600 μm.
The thickness of the transparent conductive tunneling layer is preferably 10-50 nm.
Preferably, the pyramid array is in a close-packed shape, and the sizes are randomly distributed in a certain range (namely, the height is 0.5-3 μm, and the length of the bottom edge is 0.7-4 μm).
In the scheme, the embedded silicon pn junction can enable the silicon layer to absorb incident light to generate larger photovoltage (the single n-type or p-type silicon doping layer cannot realize the function), the photovoltage and the iron oxide absorption layer form a series connection relation, namely, the voltage with the magnitude is added to the iron oxide layer, and the starting voltage of the iron oxide photoanode can be effectively reduced. The silicon pn junction is in a pyramid shape, and the geometrical characteristics of the pyramid array shape are pyramids which comprise any pyramid such as a triangular pyramid and a rectangular pyramid; the pyramid array shape can (1) lead the iron oxide layer which is grown on the pyramid array to be micro-nano structured, thereby greatly improving the specific surface area of the iron oxide film and enhancing the light absorption efficiency in unit volume; (2) although the intensity of sunlight penetrating through the iron oxide layer is obviously attenuated, the light absorption capacity of the silicon layer can be enhanced through pyramid shape processing, so that the number of photogenerated carriers in silicon is equivalent to that of photogenerated carriers in the oxide layer (if the numbers of photogenerated carriers corresponding to the two are greatly different, the smaller number of photogenerated carriers determines the output performance of the whole photoanode). The transparent conductive tunneling layer arranged between the p-type silicon doping layer and the iron oxide layer can avoid the problem of energy band mismatching caused by direct contact of p-type silicon and iron oxide and the problem of serious carrier recombination at a silicon/iron oxide interface. In addition, the metal elements in the transparent conductive tunneling layer can also be used as a doping source of the iron oxide layer, so that the conductivity of the iron oxide layer and the collection efficiency of photo-generated carriers can be improved, and the effect of completely photolyzing water can be realized.
On the basis of the technical scheme, the preparation method of the iron oxide photo-anode system with the embedded silicon pn junction is also provided, and comprises the following steps:
a. preparing a silicon pyramid array by using an n-type (100) silicon wafer as a substrate and adopting an alkaline wet etching silicon technology;
b. carrying out boron doping on the silicon pyramid array to obtain a p-type silicon doping layer;
c. growing a transparent conductive tunneling layer by using a silicon pn junction pyramid as a substrate and adopting an Atomic Layer Deposition (ALD) technology;
d. growing an iron oxide absorption layer on the surface of the transparent conductive tunneling layer by an ultrasonic spray pyrolysis method;
e. manufacturing a conductive layer on the back of the n-type silicon substrate, and leading out an external lead;
f. and coating a waterproof insulating layer on the conductive layer.
Further, in step c, the metal element in the transparent conductive tunneling layer can be thermally diffused into the iron oxide absorption layer when step d is performed. So that the grown iron oxide absorption layer has better electrical properties. Preferably, the transparent conductive tunneling layer is niobium-doped tin oxide, since niobium or tin diffuses into the iron oxide absorption layer more easily than other metal elements and can form n-type doping to iron oxide.
Further, in step a, the preferable concentration range of n-type silicon doped with phosphorus is 5.0 × (10)14～1015)cm-3. The silicon pyramid arrays are closely arranged and randomly distributed in a certain range in size (the height is 0.5-3 mu m, and the length of the bottom edge is 0.7-4 mu m). This featureOn one hand, the silicon pn junction pyramid array can be ensured to have good light limiting effect and large specific surface area, and simultaneously, the transparent conductive tunneling layer and the iron oxide absorption layer which grow subsequently can be ensured to be conformally and completely coated on the silicon pn junction pyramid (but the bottom edge is not feasible when the length is too small and the height is too high).
Further, in step b, the preferred concentration range of boron doping is 5.0 × (10)18～1019)cm-3The junction depth is 0.1 to 0.3 μm. The doping concentration and the junction depth in the range can form a pn junction with excellent electrical performance with n-type silicon, and can generate larger photovoltage.
Further, in the step c, the thickness of the transparent conductive layer is 10-50 nm. At the moment, the transparent conducting layer can effectively isolate silicon and ferric oxide to form a tunneling layer, and the collection of photogenerated holes in the ferric oxide and photogenerated electrons in a silicon pn junction is promoted.
Further, in the step d, the thickness of the iron oxide absorption layer is 50-150 nm. If too thin, the light absorption of the iron oxide is too weak, and if too thick, photogenerated carriers in the iron oxide that are too far from the surface cannot be extracted due to their limited diffusion length.
The scheme has the advantages that:
(1) the pyramid-shaped silicon pn junction is used as the inner absorption layer, so that the subsequently grown iron oxide outer absorption layer is ensured to have the pyramid shape, and the whole photo-anode has a good light trapping effect and a large specific surface area.
(2) The silicon layer is constructed as a pn junction, so that when the silicon absorbs incident light transmitted through the iron oxide layer, a large photovoltage is generated. The photovoltage and the ferric oxide absorption layer form a series connection relation, and can effectively promote the separation of photocarrier in the ferric oxide layer and the water oxidation reaction on the surface of the photovoltage.
(3) The transparent conductive through layer is grown between silicon and ferric oxide by adopting the ALD technology, so that the grown transparent conductive through layer can be deposited on the surface of the silicon pyramid in a shape-preserving manner, the thickness can be controlled to be 0.1nm, and the uniformity, the interface passivation effect and the carrier through effect of the transparent conductive through layer are further ensured.
The foregoing is a summary of the present invention, and in order to provide a clear understanding of the technical means of the present invention and to be implemented in accordance with the present specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings.
Drawings
FIG. 1: the structure schematic diagram of the iron oxide photo-anode with the embedded silicon pn junction;
wherein: 1-1 is an n-type silicon substrate, and 1-2 is a p-type silicon doped layer; 1-3 is a transparent conductive tunneling layer, 1-4 is an iron oxide absorption layer, 1-5 is a back conductive layer, and 1-6 is a waterproof insulating layer;
FIG. 2: scanning electron microscope images of pyramid-shaped silicon pn junctions;
FIG. 3: a scanning electron microscope image of the silicon pn junction after a transparent conductive tunneling layer and an iron oxide absorption layer are grown in sequence;
FIG. 4: the current-voltage characteristic of a photo-anode system constructed when the iron oxide layer grows on the FTO substrate and the pyramid array silicon pn junction substrate respectively; wherein: 4-1 is the dark current-voltage characteristic of a photoanode system constructed when an iron oxide layer grows on a pyramid arrayed silicon pn junction substrate; 4-2 is the current-voltage characteristic of a photoanode system constructed when an iron oxide layer grows on a pyramid arrayed silicon pn junction substrate under the illumination of AM1.5G; 4-3 is the dark current-voltage characteristic of the photoanode system constructed when the iron oxide layer grows on the FTO substrate; 4-4 is the current-voltage characteristic of the photoanode system under AM1.5G illumination constructed when the iron oxide layer grows on the FTO substrate. Note that the current-voltage curves for both cases 4-1 and 4-3 almost coincide.
Detailed Description
For a more clear explanation of the invention, reference is made to the following description, taken in conjunction with the accompanying drawings and examples:
example one
An iron oxide photo-anode system embedded with a silicon pn junction, as shown in figure 1: the photoanode is of a composite layer type structure and sequentially comprises an iron oxide absorption layer 1-4, a p-type silicon doping layer 1-2, an n-type silicon substrate 1-1, a back conductive layer 1-5 and a back waterproof insulating layer 1-6 along a light incidence direction; the p-type silicon doping layer and the n-type silicon substrate form a silicon pn junction: (1) the shape of the silicon pn junction is a pyramid array structure; (2) a transparent conductive tunneling layer 1-3 is arranged between the p-type silicon doping layer 1-2 and the ferric oxide absorption layer 1-4, and the thickness of each transparent conductive tunneling layer is equal.
The working principle that the iron oxide photo-anode system embedded with the silicon pn junction can completely photolyze water is as follows: when sunlight with a wide spectrum is incident on the surface of the photo anode, due to the light trapping effect of the surface pyramid structure, most incident light enters the photo anode, short waves and long waves are absorbed by the iron oxide layer and the silicon layer respectively, and incident light with the wavelength of more than 1100nm is reflected by the back conductive layer of the photo anode and leaves from the upper surface of the photo anode. Incident light absorbed by the silicon layer generates photogenerated carriers, the photogenerated carriers are effectively separated under the promotion of a built-in electric field of a pn junction, and a larger photovoltage (the direction of the electric field is from the bottom layer of the photoanode to the top layer) can be observed under the open circuit state. The photogenerated carriers in the iron oxide layer are effectively separated in the depletion layer near the solid/liquid interface. The photoproduction holes in the ferric oxide layer are extracted to a solid/liquid interface to participate in the oxidation reaction of water; the photoproduction electrons generated in the silicon pn junction are transported to the counter electrode through the back electrode to participate in the reduction reaction of water; the photo-generated electrons in the iron oxide and the photo-generated holes in the silicon pn junction are annihilated through the transparent conductive tunneling layer. The carrier beam current corresponding to the equilibrium state of the oxidation reaction and the reduction reaction of water is represented by the observed photocurrent density.
Example two
A preparation method of an iron oxide photo-anode system with embedded silicon pn junction comprises the following steps:
1) and (3) carrying out standard RCA cleaning by adopting an n-type silicon wafer with the resistivity of 1-5 omega cm.
2) Reacting in a mixed solution of potassium hydroxide and isopropanol at 80 ℃ for 60 minutes, and cleaning the silicon wafer to obtain an n-type silicon pyramid array structure as shown in figure 2.
3) Carrying out boron doping on the n-type silicon pyramid array obtained in the step 2) by adopting a thermal diffusion mode, wherein the doping concentration is 2.0 multiplied by 1019cm-3The junction depth was 200 nm. Protecting the back of n-silicon during thermal diffusion so that boron doping only occurs on the front of the silicon pyramid structure。
4) And putting the prepared silicon pn junction pyramid array into a cavity of an atomic layer deposition system, and alternately growing tin oxide and niobium oxide with different cycle times (such as 50 and 5 times, and repeating for 10 times) by taking tetra (dimethylamino) tin as a tin source and tert-butyliminotris (diethylamino) niobium as a niobium source.
5) The obtained sample was treated at 600 ℃ for 30 minutes in an air atmosphere to obtain a silicon pn junction whose surface is covered with a transparent conductive layer.
6) The sample is put into an ultrasonic spray coating system, and atomized and sprayed for 30 minutes at the injection rate of 0.5mL/min on a substrate at the temperature of 80 ℃ by taking 0.005mol/L ferric nitrate as a precursor liquid.
7) And (3) putting the sample obtained in the step into a tubular annealing furnace, and carrying out heat treatment at 700 ℃ for 2 hours in an air atmosphere. An iron oxide composite structure with embedded silicon pn junction is obtained, as shown in fig. 3.
8) And coating an In-Ga conductive layer on the back surface of the prepared composite structure, and leading out an external lead.
9) And coating 704 silica gel to completely cover the conductive layer to obtain the final photo-anode.
10) And immersing the prepared photo-anode into 1mol/L NaOH aqueous solution, taking a platinum mesh electrode as a counter electrode and an Ag/AgCl electrode as a reference electrode, and connecting the three electrodes by using an electrochemical workstation to construct a three-electrode test system.
The current-voltage characteristics were tested under dark room or am1.5g (standard solar simulator) illumination, respectively. In addition, a comparative experiment was introduced, i.e. an iron oxide layer was grown on an FTO substrate (without introducing a silicon pn junction and a transparent conductive tunneling layer) under the same process conditions and then processed into a complete photoanode system. As shown in fig. 4, 4-1 is the dark current-voltage characteristic of the photoanode system constructed when the iron oxide layer grows on the pyramid-arrayed silicon pn junction substrate; 4-2 is the current-voltage characteristic of a photoanode system constructed when an iron oxide layer grows on a pyramid arrayed silicon pn junction substrate under the illumination of AM1.5G; 4-3 is the dark current-voltage characteristic of the photoanode system constructed when the iron oxide layer grows on the FTO substrate; 4-4 is the growth of iron oxide layer on FTThe current-voltage characteristic of the photo-anode system constructed in O substrate under AM1.5G illumination. It can be seen that both samples have significant photoresponse. In the dark state, the current and voltage curves for the two samples almost coincide, with a current of almost 0 in the given voltage range. Under AM1.5G illumination, a significant photocurrent was observed only when the comparative sample was biased (i.e., the potential difference between the photoanode and the counter electrode) to greater than 0.15V. The target sample proposed by the scheme of the invention has a remarkable photocurrent density at a bias voltage of-0.3V, and the photocurrent is obviously increased with the increase of the bias voltage. At zero bias, the photocurrent of the target sample reached 0.4mA/cm2Whereas the photocurrent of the comparative sample was negligible. These data demonstrate that significant complete photolysis of water can be achieved with the iron oxide photoanode system of the present invention, whereas an iron oxide photoanode system grown directly on an FTO substrate cannot.
Claims (4)
1. An iron oxide photo-anode system embedded with a silicon pn junction, wherein the photo-anode is of a composite layer structure and is characterized in that: the light-emitting diode comprises an iron oxide absorption layer, a p-type silicon doping layer, an n-type silicon substrate, a back conductive layer and a back waterproof insulating layer in sequence along a light incidence direction; the p-type silicon doping layer and the n-type silicon substrate form a silicon pn junction; the shape of the silicon pn junction is a pyramid array structure; a transparent conductive tunneling layer is arranged between the p-type silicon doping layer and the ferric oxide absorption layer, and the thickness of each part of the transparent conductive tunneling layer is equal; wherein: the thickness of the iron oxide layer is 50-150 nm; the concentration range of boron doping in the p-type silicon doped layer was 5.0 × (10)18～1019)cm-3The depth is 0.1-0.3 μm, and the concentration range of phosphorus doped in the n-type silicon substrate is 5.0 × (10)14～1015)cm-3The thickness of the substrate is 200-600 μm, and the thickness of the transparent conductive tunneling layer is 10-50 nm.
2. The silicon pn junction embedded iron oxide photoanode system of claim 1, wherein: the pyramid array is in a close-packed shape and is distributed randomly.
3. The silicon pn junction embedded iron oxide photoanode system of claim 2, wherein: the transparent conductive tunneling layer is niobium-doped tin oxide.
4. A preparation method of an iron oxide photo-anode system embedded with a silicon pn junction is characterized by comprising the following steps: the method comprises the following steps:
a. preparing a silicon pyramid array on an n-type silicon substrate by adopting an alkaline wet etching silicon technology;
b. carrying out boron doping on the silicon pyramid array to obtain a p-type silicon doping layer; the p-type silicon doping layer and the n-type silicon substrate form a silicon pn junction;
c. growing a transparent conductive tunneling layer by using a silicon pyramid array as a substrate and adopting an atomic layer deposition technology;
d. growing an iron oxide absorption layer on the surface of the transparent conductive tunneling layer by an ultrasonic spray pyrolysis method;
e. manufacturing a conductive layer on the back of the n-type silicon substrate, and leading out an external lead;
f. coating a waterproof insulating layer on the conductive layer;
in the step a, the concentration range of phosphorus doped on the n-type silicon substrate is 5.0 x (10)14～1015)cm-3；
In the step b, the concentration range of boron doping is 5.0 × (10)18～1019)cm-3The junction depth is 0.1-0.3 μm;
in the step c, the metal element in the transparent conductive tunneling layer can be thermally diffused into the iron oxide absorption layer when the step d is carried out; the thickness of the transparent conductive tunneling layer is 10-50 nm;
in the step d, the thickness of the ferric oxide absorption layer is 50-150 nm.
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