Solar cell

Abstract

A solar cell comprising a crystalline silicon substrate, a semiconductor layer arranged on a back surface of the substrate which is configured not to face a radiative source, when the solar cell is in use, and a transparent-conductive region arranged on a surface of the semiconductor layer, wherein the transparent conductive region comprises: a first layer having a first work function; and a second layer having a second work function and being interposed between the first layer and the semiconductor layer; wherein the second work function of the second layer is greater than the first work function of the first layer.

Claims

1. A solar cell comprising: a substrate comprised of crystalline silicon; a first semiconductor layer arranged on a back surface of the substrate; a second semiconductor layer arranged on a front surface of the substrate which is configured to face a radiative source, when the solar cell is in use; a first transparent-conductive region arranged on a surface of the first semiconductor layer, wherein the first transparent conductive region comprises: a first rear layer having a first rear work function; and a second rear layer having a second rear work function and being interposed between the first rear layer and the first semiconductor layer; wherein the second rear work function of the second rear layer is greater than the first rear work function of the first rear layer; and a second transparent-conductive region being arranged on a surface of the second semiconductor layer, wherein the second transparent-conductive region comprises: a first front layer having a first front work function; and a second front layer having a second front work function and being interposed between the first front layer and the second semiconductor layer; and a third front layer being interposed between the second front layer and the second semiconductor layer and arranged directly on the second semiconductor layer; wherein the second front work function of the second front layer is greater than the first front work function of the first front layer; wherein the third front layer is configured with a third front work function which is greater than the second front work function of the second front layer; and wherein the second semiconductor layer comprises amorphous silicon (a-Si).

2. The solar cell according to claim 1, wherein the first semiconductor layer is further configured with a positive conductivity type.

3. The solar cell according to claim 1, wherein the second rear work function of the second rear layer is configured to be less than the work function of the first semiconductor layer.

4. The solar cell according to claim 1, wherein a difference between the work function of the first semiconductor layer and the second rear work function of the second rear layer is less than the difference between the work function of the first semiconductor layer and the first rear work function of the first rear layer.

5. The solar cell according to claim 1, wherein the second rear work function of the second rear layer is configured to be up to 15% greater than the first rear work function of the first rear layer.

6. The solar cell according to claim 1, wherein the first transparent-conductive region comprises a third rear layer being interposed between the second rear layer and the first semiconductor layer, wherein the third rear layer is configured with a third rear work function which is greater than the second rear work function of the second rear layer.

7. The solar cell according to claim 6, wherein the third rear layer is arranged directly on the first semiconductor layer.

8. The solar cell according to claim 7, wherein the third rear work function of the third rear layer is configured to be up to 15% greater than the second rear work function of the second rear layer.

9. The solar cell according to claim 7, wherein the third rear work function of the third rear layer is configured to be less than the work function of the first semiconductor layer.

10. The solar cell according to claim 1, wherein the work function of a layer of the first transparent-conductive region that is furthest from the substrate is greater than 3.5 eV and less than 4.5 eV, and/or the work function of a layer of the first transparent-conductive region that is closest to the substrate is greater than 5.0 eV and less than 6.0 eV.

11. The solar cell according to claim 1, wherein the work function of the first semiconductor layer is greater than 5.0 eV and less than 6.0 eV.

12. The solar cell according to claim 6, wherein the first transparent-conductive region has a thickness of less than 500 nm, and wherein each of the layers of the first transparent-conductive region has a thickness of at least 20 nm and no more than 50 nm.

13. The solar cell according to claim 6, wherein at least one of the layers of the first transparent-conductive region is formed of a metal oxide material, and the first semiconductor layer is comprised of amorphous silicon (a-Si).

14. The solar cell according to claim 1, wherein the second semiconductor layer is configured with a negative conductivity type.

15. The solar cell according to claim 1, wherein the second semiconductor layer defines an accumulator layer.

16. The solar cell according to claim 1, wherein at least one of the layers of the second transparent-conductive region is formed of a metal oxide material.

17. A solar module comprising a plurality of solar cells according to claim 1, wherein the plurality of solar cells are electrically coupled together.

18. The solar cell according to claim 1, wherein the second rear work function of the second rear layer is configured to be up to 10% greater than the first rear work function of the first rear layer.

19. The solar cell according to claim 1, wherein the second rear work function of the second rear layer is configured to be at least 10% and up to 15% greater than the first rear work function of the first rear layer.

20. The solar cell according to claim 6, wherein the third rear work function of the third rear layer is up to 10% greater than the second rear work function of the second rear layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described by way of example only, with reference to the Figures, in which:

(2) FIG. 1 is a schematic illustrating the layers of a solar cell;

(3) FIG. 2 is a close-up view of a front transparent-conductive region of the solar cell of FIG. 1;

(4) FIG. 3 is a close-up view of a back transparent-conductive region of the solar cell of FIG. 1; and

(5) FIG. 4 is a flow chart illustrating a method of forming the solar cell of FIG. 1.

DETAILED DESCRIPTION

(6) Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art.

(7) FIG. 1 schematically illustrates a solar cell 10 comprising, among other layers, a semiconductor substrate 12 comprising a first (i.e. front) surface 14, upon which light from a radiative source (e.g. the sun) is incident during normal use, and a second (i.e. back) surface 16 that is opposite the front surface 14. That is, the front surface 14 may be configured in use to face the sun, whereas the back surface 16 may be configured in use to face away from the sun.

(8) The substrate 12 divides the solar cell 10 into a front portion 18 that is forward (i.e. in front of) of the substrate 12, and a back portion 20 that is rearward of the substrate 12. Light incident on the solar cell 10 passes through the front portion 18, the substrate 12 and then the back portion 20.

(9) Each of the front and back portions 18, 20 comprises a plurality of layers which are arranged to define separate layered structures. The front portion 18 (also referred to herein as a front layered structure 18) is arranged opposite the front surface 14 of the substrate 12 and the back portion 20 (also referred to herein as a back layered structure 20) is arranged opposite the back surface 16 of the substrate 12. The constituent layers of the front and back layered structures 18, 20 are sequentially deposited (or e.g. diffused or implanted) onto the respective front and back surfaces 14, 16 of the substrate 12.

(10) Each of the layers of the front and back portions 18, 20 are configured with a width, a length and a depth. The width and length of each layer is measured in perpendicular directions that are aligned with the front and back surfaces 14, 16 of the substrate 12. For each layer, its width and length is substantially greater than its depth, which is measured in a direction that is perpendicular to the front and back surfaces 14, 16 of the substrate 12.

(11) The solar cell 10 is a back emitter solar cell (and, in particular, a back emitter heterojunction solar cell 10). As such the solar cell 10 is provided with an emitter 50 and an accumulator 52 arranged either side of the substrate 12. Accordingly, the emitter 50 forms part of the back portion 20 and the accumulator 52 forms part of the front portion 18.

(12) According to the illustrated embodiment, the substrate 12 is an n-type monocrystalline silicon wafer which forms a p-n junction with the p-type emitter layer 50. The accumulator layer 52 is configured to have n-type such that it can extract electrons from the substrate 12. The emitter and accumulator layers 50, 52 are each formed of a doped amorphous silicon (a-Si) material, which is doped with corresponding elements in order to achieve the prescribed conductivity type, as would be understood by the skilled person.

(13) The front portion 18 comprises a front passivation layer 28 which is interposed between the front surface 14 of the substrate 12 and the accumulator 52. A back passivation layer 30, of the back portion 20, is interposed between the emitter 50 and the back surface 16 of the substrate 12. Each of the passivation layers 28, 30 are formed of intrinsic amorphous silicon material, as would be understood by the skilled person.

(14) The emitter and accumulator layers 50, 52 each have a depth of 12 nm and the passivation layers 28, 30 each have a depth of 3 nm (as measured in the vertical direction shown in FIG. 1).

(15) The solar cell 10 is further provided with a transparent-conductive (TC) region 46, (also referred to herein as a front TC region 46), which is arranged at a front surface 54 of the accumulator 52. A further TC region 48 (also referred to herein as a back TC region 48) is arranged at a back surface 44 of the emitter 50.

(16) The TC regions 46, 48 are each textured to provide an anti-reflective surface of the solar cell 10, as shown in FIGS. 1 to 3. A front electrode 40 is provided at a front textured surface 56 of the front TC region 46 and a back electrode 42 is provided at a back textured surface 58 of the back TC region 48. The front and back electrodes 40, 42 are formed of silver.

(17) The front and back TCO regions 46, 48 each have a thickness of less than 100 nm (as measured in the vertical direction shown in FIG. 1), and they are each formed of indium tin oxide (ITO). However, the composition of each of the TC regions 46, 48 is varied across its depth, as will be described in more detail below.

(18) The front and back TC regions 46, 48 will now be described in more detail with reference to FIGS. 2 and 3, respectively.

(19) The front TC region 46 comprises first, second and third front layers 22, 24, 26, which all have different compositions. The third front layer 26 is interposed between the accumulator 52 and the second front layer 24, and the second front layer 24 is interposed between the third front layer 26 and the first front layer 22, as shown in FIG. 2.

(20) Each of the first, second and third front layers 22, 24, 26 is configured with a different work function. In particular, the first front layer 22 is configured with a first work function which is smaller than a second work function of the second front layer 24, which is smaller than a third work function of the third front layer 26.

(21) The work function of each layer of the TC regions 46, 48 refers to the difference in energy between the Fermi level of the constituent material of that layer and the energy of free space outside of the material. The work function of a particular layer is described as being greater than that of another layer when its work function energy value (as measured in electron volts, eV) is greater than the work function energy value of the layer against which it is being compared. Furthermore, since the work function of a material is measured on a negative scale, the term greater refers to a work function value which is more negative than the comparative value.

(22) The first work function of the first front layer 22 is approximately 4.0 eV, the second work function of the second front layer 24 is approximately 4.1 eV, and the third work function of the third front layer 26 is approximately 4.2 eV. The work function of the accumulator layer 52 is approximately 4.2 eV and the work function of the front electrode 40 is approximately 4.0 eV.

(23) The front TC region 46 is formed of a stack of transparent-conductive layers having a stepwise increase in work function when moving towards the active layers of the solar cell 10 (the vertically downward direction as shown in FIG. 2). In this way, the third front layer 26 is configured with the highest (e.g. greatest) work function (i.e., of the three layers) such that it has lower conductivity than the first and second layers 22, 24. However, this means the third front layer 26 provides an advantageously transparent window through to the accumulator layer 52 arranged immediately beneath the front TC region 46.

(24) Furthermore, the first front layer 22 is configured with the lowest (e.g. smallest) work function, such that it has greater conductivity than the second and third layers 24, 26. This means the first front layer 22 provides a good electrical contact with front electrode 40. The lower work function also decreases the transparency of the front TC region 46 at the uppermost surface of the solar cell 10. To accommodate for the reduction in transparency, the thickness of first front layer 22 is made as thin as possible in order to increase the number of incident photons that pass through to the photo-active layers.

(25) Finally, the second front layer 24 is configured with an intermediate work function which is chosen so as to provide a balance between the conductivity and transparency of the first and third layers, which the second layer is interposed between. As such, the second front layer 24 provides a bridge in electro-optical properties between the first and third front layers 22, 26.

(26) Similar to the front TC region 46, the back TC region 48 also includes a stack of three back layers 32, 34, 36 as shown in FIG. 3. As with the front layers, each of the first, second and third back layers 32, 34, 36 are formed of indium tin oxide. However, in contrast to the front layers, the three back layers 32, 34, 36 are formed of different material compositions such that their work functions exhibit a stepwise increase when moving towards the active layers of the solar cell 10 (the vertically upward direction as shown in FIG. 3).

(27) In particular, the first back layer 32 has a first work function which is smaller than that of the second and third layers 34, 36. The second back layer 34 has a second work function which is smaller than that of the third layer 36, but greater than that of the first back layer 32. The third back layer 32 has a third work function which is greater than that of both the first and second back layers 32, 34.

(28) The first work function of the first back layer 32 is approximately 4.0 eV, the second work function of the second back layer 34 is approximately 4.75 eV, and the third work function of the third back layer 36 is approximately 5.5 eV. The work function of the emitter layer 50 is approximately 5.5 eV and the work function of the back electrode 42 is approximately 4.0 eV.

(29) According to the illustrated embodiment, the work function of the third layer 36 is more suitably matched to the valence band of the emitter 50, which reduces the possibility of a parasitic potential barrier being formed between the TC region 48 and the emitter 50.

(30) In addition, the first back layer 32 is configured with a relatively low work function such that it has a lower transparency, which increases the reflectance of the TC region 48 at the rearmost surface of the solar cell 10. As a result, more unabsorbed photons may be reflected back towards the photo-active layers of the solar cell 10 by the first back layer 32 of the back TC region 48 when in use.

(31) It will be appreciated that the first back layer 32 is arranged adjacent to the back electrode 42 of the solar cell 10, and since the first back layer 32 has a relatively low work function it also exhibits increased electrical conductivity (compared with the second and third back layers 34, 36). The relatively high conductivity of the first back layer 32 causes an increase in the transfer of photo-generated charge carriers (i.e. holes) into the back electrode 42. Accordingly, the back TC region 48 is able to extract more photo-generated carriers from the emitter 50, which thereby increases the fill factor (FF) of the solar cell 10.

(32) Each of the first, second and third front layers 22, 24, 26 and the first, second and third back layers 32, 34, 36 have a depth of approximately 30 nm (as measured in the vertical direction shown in FIGS. 2 and 3). As described above, each of the layers 22, 24, 26, 32, 34, 36 is formed of indium tin oxide. The work function of these indium tin oxide materials is configured during the fabrication of the corresponding layer by adjusting the oxygen flow rate, as is explained in more detail below.

(33) FIG. 4 depicts a method 100 of forming a solar cell, such as those described above. The method comprises a first step 102 of providing a crystalline silicon wafer to define the substrate 12 of the solar cell 10.

(34) In a second method step 104, the method comprises depositing the front and back passivation layers 28, 30 onto the front and back surfaces 14, 16 of the substrate 12, respectively.

(35) A third method step 106 comprises depositing the accumulator 52 and emitter 50 onto the front and back passivation layers 28, 30, respectively. Accordingly, the accumulator and emitter 52, 50 define front and back semiconductor layers, respectively.

(36) The second and third method steps 104, 106 involve arranging (or forming) semiconductor layers on the front and rear surfaces 14, 16 of the silicon wafer substrate 12. This may comprise depositing, diffusing, doping and/or implantation steps. The layers referred to are those forming at least part of the front and rear portions 18, 20 of the solar cell 10 described above (e.g. emitter, accumulator and passivation layers etc.). Each of these steps involves depositing a corresponding semiconductor material using a vapour deposition process (e.g. PECVD). In general, the parameters of the vapour deposition process are configured to determine the composition (e.g. structural and/or chemical) and also the dopant concentration of each layer.

(37) In the fourth method step 108, the method comprises depositing the front and back third layers 26, 36 onto the accumulator and emitter 52, 50, respectively. In a fifth step 110, the method comprises depositing the front and back second layers 24, 34 onto the respective front and back third layers 26, 36. In a sixth step 112, the method comprises depositing the front and back first layers 22, 32 onto the respective front and back second layers 24, 34.

(38) Each of the fourth, fifth and sixth method steps 108, 110, 112, involves depositing front and back TCO layers onto the front and back surfaces of the solar cell 10. Each of these steps involves depositing a corresponding transparent conductive oxide material using a DC magnetron sputtering process. In general, the parameters of the sputtering process are configured to determine the composition (e.g. structural and/or chemical) and also the electrical and optical properties of each layer. For example, the work function of the constituent material of each of the front and back layers of the TC regions 46, 48 is determined by adjusting the parameters of the sputtering process. In particular, each of the layers of the front and back TC regions 46, 48 is deposited using a different oxygen gas flow-rate.

(39) The method of depositing the first front layer 22 involves using a first oxygen flow rate to obtain the first work function. The method of depositing the second front layer 24 includes using a second oxygen flow rate to obtain the second work function. The method of depositing the third front layer 26 comprises a third oxygen flow rate to configure the third work function. The first oxygen flow rate of the first front layer 22 is greater than the second oxygen flow rate used to form the second front layer 24. The second oxygen flow rate of the second front layer 24 is greater than the third oxygen flow rate used to form the third front layer 26.

(40) The method of depositing the first back layer 32 involves using a first oxygen flow rate in order to obtain the first work function. The method of depositing the second back layer 34 includes using a second oxygen flow rate to obtain the second work function. The method of depositing the third back layer 36 comprises a third oxygen flow rate to configure the third work function. The first oxygen flow rate of the first back layer 32 is smaller than the second oxygen flow rate used to form the second back layer 34. The second oxygen flow rate of the second back layer 34 is smaller than the third oxygen flow rate used to form the third back layer 36.

(41) According to an exemplary arrangement of the invention, the front and back TC regions 46, 48 may be deposited separately. For example, the method steps 108, 110, 112 may be performed sequentially on the front side of the solar cell 10, before then performing the corresponding steps 108, 110, 112 on the back side of the solar cell 10. Alternatively, the back layers 32, 34, 36 may be deposited first, before then depositing the front layers 22, 24, 26.

(42) Finally, a seventh method step 114 comprises arranging front and back electrodes 40, 42 on the outermost surfaces of the front and back portions 18, 20 of the solar cell 10.

(43) It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.