BACK JUNCTION SOLAR CELL WITH SELECTIVE FRONT SURFACE FIELD
Solar cells and methods for their manufacture are disclosed. An example method may include fabricating an n-type silicon substrate and introducing n-type dopant to one or more first and second regions of the substrate so that the second region is more heavily doped than the first region. The substrate may be subjected to a single high-temperature anneal cycle to form a selective front surface field layer. Oxygen may be introduced during the single anneal cycle to form in situ front and back passivating oxide layers. Fire-through of front and back contacts as well as metallization with contact connections may be performed in a single co-firing operation. The firing of the back contact may form a p + emitter layer at the interface of the substrate and back contacts, thus forming a p-n junction at the interface of the emitter layer and the substrate. Associated solar cells are also provided.
1 . A solar cell of the back junction type having an emitter layer opposite an illuminated surface of the solar cell, the solar cell comprising:
a p-type emitter layer;
an n-type base layer overlying the p-type emitter layer so as to define a p-n junction at the interface of the p-type emitter layer and the n-type base layer; and
an n-type front surface field layer overlying the n-type base layer comprising:
one or more first doped regions, and
one or more second doped regions, wherein the second doped regions are more heavily doped than the first doped regions so as to form a selective front surface field.
2 . The solar cell of claim 1 wherein the solar cell is formed from a monocrystalline Czochralski-grown silicon substrate, and wherein the n-type base layer and one or more first and second doped regions of the n-type front surface field layer are doped with phosphorus.
3 . The solar cell of claim 1 wherein the p-type emitter layer comprises aluminum, and wherein the p-type emitter layer is formed by liquid phase epitaxial regrowth.
4 . The solar cell of claim 1 , further comprising:
a passivating oxide layer overlying the n-type front surface field layer.
5 . The solar cell of claim 4 , further comprising:
an antireflection layer overlying the passivating oxide layer, wherein the antireflection layer comprises an amorphous silicon nitride layer.
6 . The solar cell of claim 5 , further comprising:
one or more screen-printed contacts formed over the antireflection layer, wherein the one or more screen-printed contacts are in electrical communication with the more heavily doped one or more second regions of the n-type front surface field layer through the antireflection layer and the passivating oxide layer.
7 . The solar cell of claim 1 wherein the one or more first doped regions and the one or more second doped regions of the n-type front surface field layer comprise implanted dopant.
8 . The solar cell of claim 3 , further comprising:
a self-aligning screen-printed aluminum contact formed from an aluminum paste, wherein the emitter layer and the aluminum contact are both formed from the aluminum paste, and wherein the emitter layer overlies the aluminum contact.
9 . A method for forming a solar cell of the back junction type, comprising:
fabricating an n-type base layer;
fabricating a p-type emitter layer such that the n-type base layer overlies the p-type emitter layer, wherein fabricating the p-type emitter layer further comprises:
applying a contact layer to one side of the base layer, and
alloying the contact layer with at least a portion of the base layer; and
doping one or more first doped regions and one or more second doped regions so as to form an n-type front surface field layer such that the n-type front surface field layer overlies the n-type base layer, wherein the second doped regions are more heavily doped than the first doped regions so as to form a selective front surface field.
10 . The method of claim 9 , further comprising:
forming a passivating oxide layer over the n-type front surface field layer.
11 . The method of claim 10 wherein the passivating oxide layer and the n-type front surface field layer are formed during a single anneal cycle.
12 . The method of claim 10 , further comprising:
depositing an amorphous silicon nitride layer over the passivating oxide layer thereby forming an antireflection coating.
13 . The method of claim 12 , further comprising:
screen-printing one or more front contacts on the amorphous silicon nitride layer in alignment with the more heavily doped one or more second regions of the n-type front surface field layer.
14 . The method of claim 13 , further comprising:
firing the one or more front contacts thereby electrically connecting the one or more front contacts with the n-type front surface field layer through the amorphous silicon nitride layer and the passivating oxide layer.
15 . The method of claim 14 , wherein fabricating the p-type emitter layer further comprises:
forming a contact on the surface of the p-type emitter layer such that the p-type emitter layer overlies the contact; and
wherein the p-type emitter layer comprises a p-type emitter layer formed by liquid phase epitaxial regrowth, and wherein the contact is electrically connected to the p-type emitter layer.
16 . The method of claim 9 wherein the one or more first and second doped regions are formed by introducing dopant by ion implantation.
17 . The method of claim 16 wherein introducing dopant to the one or more first doped regions comprises uniformly introducing dopant to the one or more first and second doped regions, and wherein introducing dopant to the one or more second doped regions comprises introducing additional dopant through a mask to the one or more second doped regions.
18 . The method of claim 16 wherein introducing dopant to the one or more first and second doped regions occurs during a single ion implantation step.
19 . The method of claim 17 wherein the n-type base layer is doped with phosphorus, and wherein introducing dopant to one or more first and second doped regions comprises introducing phosphorus dopant.
20 . A back junction solar cell comprising:
a first n-type region;
a second n-type region;
a third n-type region, wherein the second and third n-type regions overlie the first n-type region; and
a p-type emitter layer formed on the surface of the first n-type region opposite the second and third n-type regions, wherein the interface of the p-type emitter layer and the first n-type region defines a p-n junction.