Flexible crystalline ultra-thin Si solar cells
A novel, low cost method for manufacturing flexible crystalline ultra-thin Si solar cells using previously fabricated inflexible crystalline Si solar cells. A stack of metal layers is coated onto a front side of previously completed inflexible crystalline Si solar cells. The stack serves as a bonding layer as well as an electrically conducting layer between the inflexible solar cell and the carrier substrate. The front side of the coated inflexible Si solar cell is bonded onto the carrier substrate. Back side layers from the starting inflexible solar cell are removed, as is much of the base layer, so that only a thin base layer remains, with the thin base layer and emitter region having a total thickness of between 1 μm and 30 μm and the final cell having a total thickness of about 10 to about 125 μm.
1. A method for making a flexible crystalline ultra-thin silicon (Si) solar cell, comprising:
receiving a pre-existing inflexible crystalline Si solar cell comprising a back contact layer;
a back-surface field layer disposed on an upper surface of the back contact layer; a base layer disposed on an upper surface of the back-surface field layer; an emitter disposed on an upper surface of the base layer, a front side of the emitter having a textured surface; an antireflective coating (ARC) layer disposed on the textured front side of the emitter; and a plurality of first top metal contact grids on an upper surface of the ARC layer, the metal contact grids extending from below a front surface of the emitter to above front surface of the ARC layer;
preparing a carrier substrate by depositing a first adhesion layer on an upper surface of a flexible, conductive foil; depositing a first metal base layer on an upper surface of the adhesion layer; and depositing a first metal interlayer on an upper surface of the first metal base layer;
preparing the pre-existing inflexible crystalline Si solar cell for mounting on the carrier substrate, the preparation including depositing a planarization metal layer on an upper surface of the ARC layer; depositing a second adhesion layer on an upper surface of the planarization metal layer; depositing a second metal base layer on an upper surface of the second adhesion layer; and depositing a second metal interlayer on an upper surface of the second metal base layer, the second adhesion layer, the second metal base layer, and the second metal interlayer being formed from the same material as the corresponding first adhesion layer, first metal base layer, and first metal interlayer in the carrier substrate;
inverting the prepared pre-existing inflexible crystalline Si solar cell to that the second metal interlayer in the pre-existing inflexible crystalline Si solar cell faces the first metal interlayer in the carrier substrate;
bonding the inverted pre-existing inflexible crystalline Si solar cell to the carrier substrate to form a bonded structure situated on the flexible conductive foil, wherein the first metal interlayer is bonded to the second metal interlayer to form a blended bonded metal layer comprising metals from the first and second metal interlayers mixed with metals from the first and second metal base layers, and wherein the first textured ARC and the first metal grid pillars are contained within the bonded structure;
completely removing the back layer and the back surface field layer from the bonded structure;
partially removing the base layer from the bonded structure so that the remaining base layer has a reduced thickness of between about 1 and about 30 μm;
texturing a front surface of the remaining base layer;
depositing a passivating tunneling layer on the textured front surface of the base layer;
depositing carrier-selective contact layers on the passivating tunnel layer;
depositing a transparent conducting oxide (TCO) layer on the carrier-selective layer; and
forming a set of second top metal contact grids on a front surface of the TCO layer, the second top metal contact grids extending from below the front surface of the base layer to above the front surface of the TCO layer;
wherein a total thickness of ultra-thin Si regions in the completed solar cell is between about 1 and about 20 μm.
2. The method according to claim 1 , wherein the flexible conductive foil has a thickness of about 10 to about 125 μm.
3. The method according to claim 1 , wherein the blended bonded region in the flexible crystalline ultra-thin Si solar cell has a thickness of about 0.1 to about 1 μm.
4. The method according to claim 1 , wherein the pre-existing inflexible Si solar cell comprises a (100) oriented monocrystalline Si wafer.
5. The method according to claim 1 , wherein the pre-existing inflexible Si solar cell comprises a multicrystalline Si wafer.
6. The method according to claim 1 , wherein the pre-existing inflexible Si solar cell is bonded to the carrier substrate by means of transient liquid phase (TLP) diffusion bonding.