Search

US-12622089-B2 - Solar cell

US12622089B2US 12622089 B2US12622089 B2US 12622089B2US-12622089-B2

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.

Inventors

  • Kenta Nakayashiki
  • Shu Yunn CHONG
  • Ngeah Theng CHUA

Assignees

  • REC SOLAR PTE. LTD.

Dates

Publication Date
20260505
Application Date
20211220
Priority Date
20201230

Claims (20)

  1. 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. 2 . The solar cell according to claim 1 , wherein the first semiconductor layer is further configured with a positive conductivity type.
  3. 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. 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. 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. 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. 7 . The solar cell according to claim 6 , wherein the third rear layer is arranged directly on the first semiconductor layer.
  8. 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. 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. 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. 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. 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. 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. 14 . The solar cell according to claim 1 , wherein the second semiconductor layer is configured with a negative conductivity type.
  15. 15 . The solar cell according to claim 1 , wherein the second semiconductor layer defines an accumulator layer.
  16. 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. 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. 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. 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. 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

CROSS REFERENCE TO RELATED APPLICATIONS This application is a U.S. National Phase of International Application No. PCT/EP2021/086830 filed Dec. 20, 2021, which claims priority to Great Britain Application No. GB 2020730.4 filed Dec. 30, 2020, both of which are incorporated herein by reference in their entirety. FIELD OF THE DISCLOSURE The present disclosure relates to solar cells and methods for forming the same. BACKGROUND Solar modules for providing electrical energy from sunlight comprise an array of solar/photovoltaic cells, each comprising a multi-layer semiconductor structure arranged between one or more front and back electrodes. The substrate typically forms a p-n junction with an emitter layer which is arranged on a surface of the substrate (i.e. one of the substrate and emitter layers being an n-type material and the other being a p-type material). The p-n junction facilitates the generation of an electric current in response to light incident on the solar cell. A surface field layer (e.g. front or back surface field layer), is arranged on an opposite surface of the substrate to the emitter layer. The surface field layer is doped (i.e. with the opposite charge type to the emitter) and is configured to extract charge carriers from the substrate. The emitter and surface field layers are typically formed of amorphous silicon (a-Si) whereas the substrate is formed of crystalline silicon (c-Si) so as to convey a heterojunction technology type (HJT) solar cell. With such HJT solar cells, a transparent conducting oxide layer (TCO) is interposed between the surface field layer and one of the electrodes, and a further TCO layer is interposed between the emitter layer and the other of the electrodes. The TCO layers are arranged to extract charge carriers from the active layers (e.g. the surface field and emitter layers) of the solar cell and transport them to the respective electrodes. To maximise the efficiency of solar cell, it is important to maximise the optoelectronic properties of the TCO layers. However, since transparent materials are generally insulators, and conductive materials tend to have metallic properties, there is a fundamental trade-off between the optical and electrical properties of such materials. Accordingly, there is still a need to increase the optical properties of the TCO layers of such solar cells, whilst also improving their charge carrier transport properties. SUMMARY According to a first aspect there is provided a solar cell comprising a substrate (e.g. a crystalline silicon substrate), a semiconductor layer arranged on a back surface of the substrate which is configured not to face a radiative source (or is configured to face away from a radiative source), when the solar cell is in use, and a transparent-conductive region being arranged on a surface of the semiconductor layer. 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. The second work function of the second layer is greater than the first work function of the first layer. In an embodiment, the semiconductor layer is interposed between the substrate and the transparent-conductive region. During the operation of a known solar cell, photo-generated carriers are collected by the TCO and transported to an electrode. Such a TCO layer may be configured with a low work function which increases its conductivity so as to increase the transportation of photo-generated carriers to the electrode. However, the low work function of the TCO layer can lead to an increase in the contact resistance with a semiconductor layer (e.g. amorphous silicon, a-Si) of the solar cell, upon which the TCO layer is arranged. The increased contact resistance results from the formation of a potential barrier (e.g. a parasitic Schottky barrier) at the interface between the TCO and the semiconductor layer. This potential barrier generates a diffusion potential which hinders the collection of photo-generated carriers by the TCO layer, and thereby decreases the efficiency of the solar cell. The transparent-conductive region of the present invention is provided with a second layer which is arranged between the first layer and semiconductor layer. The second layer is configured with a work function which is greater than the work function of the first layer. As such, the second work function of the second layer is more suitably matched to the valence band, or conduction band, of the semiconductor layer so as to reduce the parasitic potential barrier. As such, the transparent-conductive region is able to extract more photo-generated carriers from the semiconductor layer, which thereby increases the fill factor (FF) and efficiency of the solar cell. Furthermore, the first layer of the transparent-conductive region is configured with a smaller work function which results in lower transparency (compared with