ELECTRODE, PHOTOVOLTAIC DEVICE, AND METHOD OF MAKING
In one aspect of the present invention, a transparent electrode, is presented. The transparent electrode includes a substrate and a transparent layer disposed on the substrate. The transparent layer includes (a) a first region including cadmium tin oxide; (b) a second region including tin and oxygen; and (c) a transition region including cadmium, tin, and oxygen interposed between the first region and the second region, wherein an atomic ratio of cadmium to tin in the transition region varies across a thickness of the transition region. The second region further has an electrical resistivity greater than an electrical resistivity of the first region. A photovoltaic device, a photovoltaic module, a method of making is also presented.
1 . A transparent electrode, comprising:
a substrate; and
a transparent layer disposed on the substrate, wherein the transparent layer comprises:
(a) a first region comprising cadmium tin oxide,
(b) a second region comprising tin and oxygen, and
(c) a transition region comprising cadmium, tin, and oxygen interposed between the first region and the second region, wherein an atomic ratio of cadmium to tin in the transition region varies across a thickness of the transition region,
wherein the second region has an electrical resistivity greater than an electrical resistivity of the first region.
2 . The transparent electrode of claim 1 , wherein an atomic ratio of cadmium to tin in the first region is in a range from about 1.2:1 to about 3:1.
3 . The transparent electrode of claim 1 , wherein an atomic ratio of cadmium to tin in the first region is in a range from about 1.5:1 to about 2:1.
4 . The transparent electrode of claim 1 , wherein an atomic ratio of cadmium to tin in the first region is substantially constant across a thickness of the first region.
5 . The transparent electrode of claim 1 , wherein the first region has a thickness in a range from about 100 nm to about 400 nm.
6 . The transparent electrode of claim 1 , wherein the cadmium tin oxide has a substantially single-phase spinel crystal structure.
7 . The transparent electrode of claim 1 , wherein the second region has a thickness in a range from about 20 nm to about 200 nm.
8 . The transparent electrode of claim 1 , wherein the second region further comprises cadmium and an atomic concentration of cadmium is less than about 5%.
9 . The transparent electrode of claim 1 , wherein the second region further comprises cadmium and an atomic concentration of cadmium is less than about 0.5%.
10 . The transparent electrode of claim 1 , wherein the second region is substantially free of cadmium.
11 . The transparent electrode of claim 1 , wherein the transition region has a thickness in a range from about 40 nm to about 100 nm.
12 . The transparent electrode of claim 1 , wherein the atomic ratio of cadmium to tin in the transition region decreases from the first region to the second region.
13 . The transparent electrode of claim 1 , wherein the first region has an electrical resistivity less than about 2×10 −4 Ohms-cm.
14 . The transparent electrode of claim 1 , wherein the second region has an electrical resistivity greater than about 10 −2 Ohms-cm.
15 . The transparent electrode of claim 1 , wherein the transparent electrode has an average optical transmission greater than about 80%.
16 . A photovoltaic device, comprising:
a substrate;
a transparent layer disposed on the substrate;
a first semiconductor layer disposed on the transparent layer;
a second semiconductor layer disposed on the first semiconductor layer; and
a back contact layer disposed on the second semiconductor layer; wherein the transparent layer comprises:
(a) a first region comprising cadmium tin oxide,
(b) a second region comprising tin and oxygen, and
(c) a transition region comprising cadmium, tin, and oxygen interposed between the first region and the second region, wherein an atomic ratio of cadmium to tin in the transition region varies across a thickness of the transition region,
wherein the second region has an electrical resistivity greater than an electrical resistivity of the first region.
17 . The photovoltaic device of claim 16 , wherein the first semiconductor layer comprises cadmium sulfide.
18 . The photovoltaic device of claim 16 , wherein the second semiconductor layer comprises cadmium telluride.
19 . The photovoltaic device of claim 16 , wherein the first semiconductor layer has a thickness in a range from about 30 nm to about 150 nm.
20 . The photovoltaic device of claim 16 , wherein the second semiconductor layer has a thickness in a range from about 1500 nm to about 4000 nm.
21 . The photovoltaic device of claim 16 , wherein the photovoltaic device further comprises a buffer layer interposed between the transparent layer and the first semiconductor layer.
22 . The photovoltaic device of claim 21 , wherein the buffer layer comprises an oxide selected from the group consisting of tin oxide, indium oxide, zinc oxide, and combinations thereof
23 . A photovoltaic module comprising a plurality of photovoltaic devices as defined in claim 16 .
24 . A method, comprising:
disposing a substantially amorphous cadmium tin oxide layer on a substrate; and
thermally processing the substantially amorphous cadmium tin oxide layer to form a transparent layer, wherein thermally processing comprises heating the amorphous cadmium tin oxide layer at a treatment temperature, under vacuum conditions, and for a time duration sufficient to allow formation of:
(a) a first region comprising cadmium tin oxide,
(b) a second region comprising tin and oxygen, and
(c) a transition region comprising cadmium, tin, and oxygen interposed between the first region and the second region, wherein an atomic ratio of cadmium to tin in the transition region varies across a thickness of the transition region,
wherein the second region has an electrical resistivity greater than an electrical resistivity of the first region.
25 . The method of claim 24 , wherein thermally processing comprises non-stoichiometric sublimation of cadmium from the substantially amorphous cadmium tin oxide layer.
26 . The method of claim 24 , wherein thermally processing comprises heating the substantially amorphous cadmium tin oxide layer at the treatment temperature in a range from about 600° C. to about 695° C.
27 . The method of claim 24 , wherein thermally processing is conducted in the presence of argon gas at a pressure less than about 500 Torr.
28 . The method of claim 24 , wherein thermal processing is conducted in the presence of argon gas at a pressure equal to or less than about 10 −3 Torr.
29 . The method of claim 24 , wherein thermally processing comprises heating the substantially amorphous cadmium tin oxide layer for a time duration in a range from about 1 minute to about 70 minutes.
30 . The method of claim 24 , wherein thermally processing comprises forming a first region comprising cadmium tin oxide having a substantially single-phase spinel crystal structure.
31 . The method of claim 24 , wherein disposing a substantially amorphous cadmium tin oxide layer comprises sputtering, chemical vapor depositing, spin coating, or dip coating.
32 . The method of claim 24 , further comprising:
disposing a first semiconductor layer on the transparent layer after the step of thermal processing;
disposing a second semiconductor layer on the first semiconductor layer; and
disposing a back contact layer on the second semiconductor layer to form a photovoltaic device.
33 . The method of claim 24 , further comprising disposing a buffer layer on the transparent layer, wherein the buffer layer is disposed adjacent to the second region.