High efficiency CdTe solar cell with treated graphene

Abstract

A solar cell includes a doped CdTe layer; a graphene layer over the CdTe layer; and metal contacts over the graphene layer. Advantageously, the metal contacts are composed of Pt or MoO.sub.3-x. The doped CdTe layer can be composed of a p-doped CdTe layer, and the solar cell comprises an n-doped CdS layer beneath the CdTe layer. The solar cell can include a conducting oxide layer beneath the CdS layer. The conducting oxide layer can be composed of SnO.sub.2:F. The solar cell can include a glass layer beneath the conducting oxide layer. The graphene contacting the MoO contacts particularly alleviates the issue of the inability of prior CdTe solar cells to collect holes and could increase efficiency by about 5%.

Claims

1. A solar cell, comprising: a doped CdTe layer; a graphene layer over the CdTe layer; and metal contacts over the graphene layer.

2. The solar cell according to claim 1, wherein the metal contacts are composed of Pt.

3. The solar cell according to claim 1, wherein the metal contacts are composed of MoO.sub.3-x.

4. The solar cell according to claim 1, wherein the doped CdTe layer comprises a p-doped CdTe layer, and the solar cell comprises an n-doped CdS layer beneath the p-doped CdTe layer.

5. The solar cell according to claim 4, comprising a conducting oxide layer beneath the CdS layer.

6. The solar cell according to claim 5, wherein the conducting oxide layer is composed of SnO.sub.2:F.

7. The solar cell according to claim 5, comprising a glass layer beneath the conducting oxide layer.

8. A solar cell, comprising: a first doped Group II-VI layer; a graphene layer over the first doped Group II-VI layer; and metal contacts over the graphene layer, wherein the metal contacts are composed of Pt or MoO.sub.3-x.

9. The solar cell according to claim 8, wherein the metal contacts are composed of Pt.

10. The solar cell according to claim 8, wherein the metal contacts are composed of MoO.sub.3-x.

11. The solar cell according to claim 8, wherein the first Group II-VI layer comprises a p-doped CdTe layer, and the solar cell comprises an n-doped CdS layer beneath the p-doped CdTe layer.

12. The solar cell according to claim 11, comprising a conducting oxide layer beneath the CdS layer.

13. The solar cell according to claim 11, wherein the conducting oxide layer is composed of SnO.sub.2:F.

14. The solar cell according to claim 13, comprising a glass layer beneath the conducting oxide layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic cross-sectional view of an exemplary embodiment solar cell of the invention.

DETAILED DESCRIPTION

[0014] While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.

[0015] This application claims the benefit of U.S. Provisional Application Ser. No. 63/047,702, filed Jul. 2, 2020, which is herein incorporated by reference in its entirety.

[0016] FIG. 1 illustrates a solar cell 10, formed of a glass layer 14, a conducting oxide layer layer 18 such as a fluorine-doped, tin oxide (SnO.sub.2:F) layer, a doped Group II-VI material such as an n-doped cadmium sulfide (n-CdS) layer 20, a doped Group II-VI material such as a p-doped cadmium telluride (p-CdTe) layer 24, a graphene layer 30, a platinum (Pt) or molybdenum oxide (MoO.sub.3-x) layer 34 and contacts 38, 42. The Pt and MoO.sub.3 have very low work function and they dope graphene deeply and hence graphene Fermi level matches that of CdTe, enabling easy extraction of holes from the solar cell. The contacts can be composed of Pt or MoO.sub.3-x material. Light “L” impinges on the glass side of the solar cell 10.

[0017] Solar cells using a CdTe layer, and methods of fabricating such solar cells are disclosed in U.S. Pat. Nos. 10,340,405; 9,837,563; and 8,912,428, all herein incorporated by reference to the extent that the references are not contradictory to the present disclosure.

[0018] The work function of intrinsic graphene is ≈4.5 eV and is known to vary as much as ±1.2 eV with electrical or contact doping. With an appropriate contact metal, such as Pt (with work function of 5.9 eV) or MoO.sub.3-x (with work function of 6.9 eV) the graphene work function can be matched or lowered to that of p-doped CdTe. Graphene is inert and attaches to the surface only by van der Waals interaction, thus avoiding complicated compound formation. Graphene has high mobility and with the heavy doping by the contact metal, its sheet resistance can be reduced to 30-50Ω (ohms). Low sheet resistance and matched work functions remove the barrier at the interface and thus improve efficiency. The basic design is shown in FIG. 1 and the predicted performance under various improvements are shown in Table 1. By comparing row 1 and row 3, we note that reducing the work function to zero or negative, as promised by the MoOx-doped graphene, the solar cell efficiency can increase by over 5.5%.

[0019] The work function p-CdTe is −5.9 eV. MoO.sub.3-x-covered graphene has a work function of −6 eV or more. The lower or matched work function enables efficient collection of holes and thus results in higher efficiency.

TABLE-US-00001 TABLE 1 n-CdTe: n.sub.D n-CdTe: n.sub.T p-CdTe: n.sub.A n-CdTe: n.sub.T (ϕ.sub.M − ϕ.sub.CdTe) Efficiency 1 × 10.sup.18 2 × 10.sup.14 1 × 10.sup.15 2 × 10.sup.14 0.6 16.61 1 × 10.sup.18 2 × 10.sup.12 1 × 10.sup.15 2 × 10.sup.12 0.6 17.74 1 × 10.sup.18 2 × 10.sup.14 1 × 10.sup.15 2 × 10.sup.14 0.0 or −ve 22.20 1 × 10.sup.18 2 × 10.sup.12 1 × 10.sup.15 2 × 10.sup.12 0.0 or −ve 29.80 1 × 10.sup.18 2 × 10.sup.12 1 × 10.sup.14 2 × 10.sup.12 0.0 or −ve 30.14

[0020] Table 1 indicates the calculated efficiency under various defect density and work function conditions. Note that matched work function (column 5) increases the efficiency by over 5.5% (compare 6.sup.th column of rows 1 and 3)

[0021] From the foregoing, it will be observed that numerous variations and modifications may be incorporated without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred.