PHOTOVOLTAIC DEVICE AND METHODS OF FORMING THE SAME

Abstract

Methods and devices are described for a photovoltaic device. The photovoltaic device includes a glass substrate, a semiconductor absorber layer formed over the glass substrate, a metal back contact layer formed over the semiconductor absorber layer, and a p-type back contact buffer layer formed from one of MnTe, Cd.sub.1-xMn.sub.xTe, and SnTe, the buffer layer disposed between the semiconductor absorber layer and the metal back contact layer.

Claims

1. A method of manufacturing a photovoltaic device comprising the steps of: depositing a semiconductor absorber layer on a substrate, wherein the semiconductor absorber layer is formed from CdTe; depositing a p-type back contact buffer layer on the semiconductor absorber layer, wherein the p-type back contact buffer layer is formed from MnTe, and wherein the depositing a p-type back contact buffer layer step is an evaporation step whereby the MnTe back contact buffer layer is deposited on the semiconductor absorber layer by impinging evaporated MnTe onto the semiconductor absorber layer, the evaporated MnTe generated by heating a source of MnTe; and depositing a back contact layer on the p-type back contact buffer layer.

2. The method of claim 1, wherein the evaporation step is performed at a temperature of up to about 1200° C.

3. The method of claim 1, wherein the MnTe is doped up to about 10.sup.19 cm.sup.−3.

4. A method of manufacturing a photovoltaic device comprising the steps of: depositing a semiconductor absorber layer on a substrate, wherein the semiconductor absorber layer is formed from CdTe; depositing a p-type back contact buffer layer on the semiconductor absorber layer, wherein the depositing a p-type back contact buffer layer step is a sputtering step whereby the MnTe back contact buffer layer is deposited on the semiconductor absorber layer by sputtering MnTe with a MnTe target onto the semiconductor absorber layer; and depositing a back contact layer on the p-type back contact buffer layer.

5. The method of claim 4, wherein the sputtering step is performed at a temperature of up to about 300° C.

6. The method of claim 4, wherein the MnTe is doped up to about 10.sup.19 cm.sup.−3.

7. A method of manufacturing a photovoltaic device comprising the steps of: depositing a semiconductor absorber layer on a substrate, wherein the semiconductor absorber layer is formed from CdTe; depositing a p-type back contact buffer layer on the semiconductor absorber layer, wherein the depositing a p-type back contact buffer layer step is a chemical reaction step involving: a. depositing MnI.sub.2 solution onto the semiconductor absorber layer by one of application of liquid MnI.sub.2 thereon and evaporating MnI.sub.2 onto thereon; and b. annealing the MnI.sub.2-coated semiconductor absorber layer to form the p-type back contact; and depositing a back contact layer on the p-type back contact buffer layer.

8. The method of claim 7, wherein the annealing step is conducted at a temperature of from about 400° C. to about 650° C. in an oxygen-deficient environment.

9. The method of claim 7, wherein the annealing step is conducted in the presence of a flow of an inert gas to remove annealing byproduct gases.

10. The method of claim 7, wherein the p-type back contact buffer layer is formed from SnTe.

11. The method of claim 10, wherein the depositing a p-type back contact buffer layer step is a sputtering step whereby the SnTe back contact buffer layer is deposited on the semiconductor absorber layer by sputtering SnTe with a SnTe target onto the semiconductor absorber layer.

12. The method of claim 10, wherein the depositing a p-type back contact buffer layer step is a vapor transport deposition step whereby SnTe is deposited on the semiconductor absorber layer.

13. The method of claim 10, wherein the SnTe back contact buffer layer has a thickness from about 10 nm to about 500 nm.

14. The method of claim 7, wherein the p-type back contact buffer layer is formed from Cd.sub.1-xMn.sub.xTe.

15. The method of claim 14, wherein the Cd.sub.1-xMn.sub.xTe comprises Cd.sub.0.5Mn.sub.0.5Te.

Description

DRAWINGS

[0011] The above, as well as other advantages of the present disclosure, will become readily apparent to those skilled in the art from the following detailed description, particularly when considered in the light of the drawings described hereafter.

[0012] FIG. 1 is a schematic of a photovoltaic device as known in the art;

[0013] FIG. 2 is an energy band diagram of the photovoltaic device of FIG. 1;

[0014] FIG. 3 is a schematic of a photovoltaic device according to the invention;

[0015] FIG. 4 is an energy band diagram of one embodiment of the photovoltaic device of FIG. 3 and

[0016] FIG. 5 is an energy band diagram of another embodiment of the photovoltaic device of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should also be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. In respect of the methods disclosed, the order of the steps presented is exemplary in nature, and thus, is not necessary or critical unless recited otherwise.

[0018] FIG. 1 is a schematic representation of a photovoltaic device 10 as known in the art. The photovoltaic device 10 includes a glass substrate 12 on which a thin conductive oxide (TCO) layer 14, formed from a F-doped SnO.sub.2, for example, is deposited. A buffer layer 16, formed from SnO.sub.2, for example, is deposited on the TCO layer 14. The buffer layer 16 may also be formed from a zinc tin oxide, cadmium tin oxide, or other transparent semiconducting oxide or a combination thereof, as desired. The CdS buffer layer is option, and if the layer is present it may be continuous or non-continuous and the layer may cover all or a portion of the device and/or all or a portion of a layer or a substrate underlying the buffer layer. An n-type window layer 18, formed from CdS, for example, is deposited on the buffer layer 16, followed by a p-type semiconductor absorber layer 20, formed from CdTe, for example. The absorber layer 20 may also be formed from CdZnTe, CdSTe, CIGS, amorphous silicon, crystalline silicon, or GaAs, for example, as desired. A metal back contact 22 is deposited or formed on the absorber layer 20. The back contact may be formed from MoN.sub.x/Al, ZnTe:Cu, CdSe, MgTe, HgTe, or ZnTe/Al bilayer other suitable semiconductor/metal multilayers, and the like, for example.

[0019] An exemplary energy band diagram of the photovoltaic device of FIG. 1 is shown in FIG. 2. Band gap energy for the TCO layer 14 is depicted as 24, band gap energy of the buffer layer 16 is depicted as 26, band gap energy of the window layer 18 is depicted as 28, band gap energy of the absorber layer 20 is depicted as 30, and the band gap energy of the back contact layer 22 is depicted as 32. As shown in FIG. 2, the conduction band edge and the valence band edge bend downward by A near the junction of the absorber layer 20 and the back contact layer 22. This is due to the back contact layer 22 having a lower work function than that of the absorber layer 20. The downward bending of the band edges increases an electron diffusion current into the back contact layer 22 and limits a maximum achievable open-circuit voltage V.sub.oc to V.sub.bi−Δ. V.sub.bi (built-in potential) is described as an upper limit to the open-circuit voltage (V.sub.oc) of a photovoltaic device under illumination. Therefore, a decrease in V.sub.bi by an amount, Δ, is a decrease in the upper limit to the maximum achievable V.sub.oc.

[0020] FIG. 3 is a schematic representation of a photovoltaic device 34 according to an embodiment of the invention. The photovoltaic device 34 includes a substrate layer 36, a TCO layer 38, a buffer layer 40, a window layer 42, a semiconductor absorber layer 44, and a back contact layer 46 similar to those described with respect to the layers of the photovoltaic device 10. However, the photovoltaic device 34 includes a back contact buffer layer 48 disposed between the back contact layer 46 and the absorber layer 44. The back contact buffer layer 48 is formed from a p-type material, such as SnTe, MnTe, or Cd.sub.1-xMn.sub.xTe. MnTe and SnTe are particularly suitable as materials for forming the back contact buffer layer 48 due to good lattice structure matches with the CdTe semiconductor absorber layer 44. MnTe and SnTe are also particularly suitable due to having a higher hole concentrations than CdTe to induce an upward band bending in CdTe to reduce electron diffusion into the back contact layer 46, as illustrated in FIGS. 4 and 5 and discussed further herein below. The back contact buffer layer 48 improves band alignment between the back contact layer 46 and the absorber layer 44 which leads to an optimized performance of the photovoltaic device 34.

[0021] Similarly, Cd.sub.1-xMn.sub.x Te is a suitable back contact buffer layer 48 because the presence of

[0022] Mn with CdTe increases a room temperature band gap thereof linearly with a Mn faction x at the rate of about 13 mV/% Mn up to x=about 0.5. That is, a maximum band gap increase is obtainable for Cd.sub.0.5Mn.sub.0.5Te, though x may be between 0 and about 1, as desired. Furthermore, Cd.sub.1-xMn.sub.xTe has a very small mismatch, about 1%, with CdTe. Therefore, an amount of interface states at a junction between CdTe and Cd.sub.1-xMn.sub.xTe is minimized, thereby optimizing performance of the photovoltaic device. A Cd.sub.1-xMn.sub.xTe back contact buffer layer may be prepared using techniques such as metalorganic chemical vapor deposition (MOCVD), sputtering, and molecular beam epitaxy (MBE), for example.

[0023] Favorable results have been obtained using the back contact buffer layer 48 formed from MnTe for at least the following reasons: MnTe has low vapor pressure suitable for vapor transport deposition (VTD) processes; about 100% solubility in CdTe; a band gap of about 3.2 eV; and due to Mn vacancies, the MnTe may be doped up to about 10.sup.19 cm.sup.−3. The MnTe back contact buffer layer 48 may be deposited on the absorber layer 44 using known deposition processes, but positive results have been obtained using a high temperature evaporation process, a sputtering processing

[0024] For example, to form the device 34 having the MnTe back contact buffer layer 48 using high temperature evaporation or sputtering processes, the window layer 42 and absorber layer 44 are deposited using VTD processes on a TEC10 glass substrate 36. The window layer 42 and the absorber layer 44 are then treated with CdCl.sub.2, as known in the art. The surface of the CdCl.sub.2-treated absorber layer 44 is then cleaned with a dilute HCl solution. When using an evaporation process, a MnTe source is then heated to evaporate the MnTe. The evaporated MnTe is then impinged upon the absorber layer 44 to deposit the MnTe back contact buffer layer 48 thereon. Alternatively when using a sputtering process, MnTe may be sputtered onto the absorber layer 44 with a MnTe target with a temperature of the substrate layer 36 from about room temperature to about 300° C. The target thickness of the back contact buffer layer 48 is from about 10 nm to about 500 nm. Once the MnTe back contact buffer layer 48 is deposited, processing of the device 34 continues through to packaging.

[0025] FIG. 4 shows a band energy band diagram of the photovoltaic device 34 of FIG. 3 where the back contact buffer layer 48 is formed from MnTe. Band gap energy for the TCO layer 38 is depicted as 48, band gap energy of the buffer layer 40 is depicted as 50, band gap energy of the window layer 42 is depicted as 52, band gap energy of the absorber layer 44 is depicted as 54, the band gap energy of the MnTe back contact buffer layer 48 is depicted 56, and the band gap energy of the back contact layer 46 is depicted as 58. As shown in FIG. 4, the higher work function of the MnTe back contact buffer layer 48 causes an upward CdTe band bending A when the MnTe back contact buffer layer 48 is deposited on the CdTe absorber layer 44. Because MnTe has a higher conduction band edge than CdTe, i.e. a conduction band offset of about 1.7 eV, the MnTe back contact buffer layer 48 performs as an electron reflector, thereby substantially minimizing, if not eliminating, diffusion of electrons into the back contact layer 46. Due to the upward band bending A, a higher limit on achievable V.sub.oc is increased to V.sub.bi+Δ, thereby improving the performance of the photovoltaic device 34.

[0026] Using the back contact buffer layer 48 formed from SnTe may have favorable results for the following reasons: SnTe has a vapor pressure of about 0.03 atm at 1000° C., only slightly higher than that of CdS; a work function of about 5.1 eV; a band gap of from about 0.2 eV to about 0.3 eV; a melting point at about 795° C.; and due to Sn vacancies, the SnTe may be intrinsically doped up to about 1.5×10.sup.21 cm.sup.−3 at room temperature. The SnTe may be deposited on the absorber layer 44 using known deposition processes, but favorable results may be obtained using a VTD process and a sputtering process.

[0027] To form the device 34 having the SnTe back contact buffer layer using the VTD process, the window layer 42 and absorber layer 44 are deposited using VTD processes on the TEC10 glass substrate 36. The SnTe is deposited on the absorber layer 44 using a VTD process with the same or similar conditions as the VTD process to deposit the CdS since SnTe has a similar vapor pressure thereto. The target thickness of the SnTe back contact buffer layer 48 is from about 10 nm to about 500 nm. Prior to deposition of the SnTe back contact buffer layer 48, the window layer 42 and the absorber layer 44 are then treated with CdCl.sub.2, and the surface of the CdCl.sub.2-treated absorber layer may then be cleaned with a dilute HCl solution.

[0028] To form the device 34 having the SnTe back contact buffer layer 48 using the sputtering process, the window layer 42 and absorber layer 44 are deposited using VTD processes on the TEC10 glass substrate 36. The window layer 42 and the absorber layer 44 are then treated with CdCl.sub.2. The surface of the CdCl.sub.2-treated absorber layer is then cleaned with a dilute HCl solution. SnTe is sputtered onto the absorber layer 44 with a SnTe target at a temperature of from about room temperature to about 300° C. The target thickness of the back contact buffer layer 48 is from about 10 nm to about 500 nm. Once the SnTe back contact buffer layer 48 is deposited, processing of the device 34 continues through to packaging.

[0029] FIG. 5 shows an energy band diagram of the photovoltaic device 34 of FIG. 3 where the back contact buffer layer 48 is formed from SnTe. Band gap energy for the TCO layer 38 is depicted as 60, band gap energy of the buffer layer 40 is depicted as 62, band gap energy of the window layer 42 is depicted as 64, band gap energy of the absorber layer 44 is depicted as 66, the band gap energy of the MnTe back contact buffer layer 48 is depicted 68, and the band gap energy of the back contact layer 46 is depicted as 70. As shown in FIG. 4, the higher work function of the SnTe back contact buffer layer 48 causes an upward CdTe band bending A when the SnTe back contact buffer layer 48 is deposited on the CdTe absorber layer 44. Due to the upward band bending A, a higher limit on achievable V.sub.oc is increased to V.sub.bi+Δ. Furthermore, the SnTe back contact buffer layer 48 performs as an electron reflector, thereby substantially minimizing, if not eliminating, diffusion of electrons into the back contact layer 46.

[0030] While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the disclosure, which is further described in the following appended claims.