Solar cell with reduced shading and method of producing the same

A solar cell is disclosed having electrical connections only on the rear side to reduce shading and improve efficiency of the cell. The solar cell includes a crystalline silicon substrate exhibiting crystallographic planes on the front and rear sides and a flat, doped emitter layer on at least the front side. The solar cell also includes a plurality of elongated slots aligned parallel to crystallographic planes and extending through the entire thickness of the silicon substrate of the solar cell. A high doping, corresponding to the conductivity type of the emitter is located in the slots. Two contact patterns are located on the rear side of the solar cell. The first is for electrical connection to the bulk material and the second is for electrical connection to the emitter and at least partly overlaps the slots. The slots are crystallographically etched anisotropically from the rear side and taper toward the front side of the cell. Also disclosed is a method for producing a solar cell. In this method, a plurality of slots are etched into, and extending through the entire thickness of, a crystalline silicon substrate in an alkaline, crystal-oriented and masked manner. A flat emitter layer is produced by diffusion of a dopant. Finally a first and second contact pattern are produced on the rear side of the solar cell by imprinting and burning in a conductive path. The second contact pattern is formed to overlap the slots.

Reduced shading can, for example, be achieved in a solar cell in which both 
the n and the p contacts are located on the rear side. In this way the 
front side is not shaded by any contact and is therefore available without 
restriction for the irradiation of light. 
A solar cell without front-side metallization is known, for example, from 
R. A. Sinton, P. J. Verlinden, R. A. Crane, R. M. Swanson, C. Tilford, J. 
Perkins and K. Garrison, "Large-Area 21% Efficient Si Solar Cells", Proc. 
of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, 1993, 
pages 157 to 161. To produce same, varyingly doped areas are generated 
side by side in a plurality of masking steps and are metallized or 
contacted by applying a multilayer metal structure on top thereof. The 
metal structures are applied by thin-film techniques. 
One drawback is that the method needs a plurality of masking steps and is 
complex as a result. All the charge carriers also have to reach the rear 
side of the solar cell by way of diffusion, there being a greater 
probability of charge carrier recombination which in turn reduces the 
solar cell's collection efficiency. 
Another idea for a solar cell without front-side metallization is known 
from the article entitled "Emitter Wrap-Through Solar Cell" by James M. 
Gee et al. in a paper for the 23rd Photovoltaic Specialists Conference 
1993, Louisville, pages 265 to 270. The solar cell described there 
comprises an emitter layer placed close to the front side with a pn 
junction adjacent thereto. Contact holes drilled and metallized by means 
of a laser connect the emitter layer to metallized contacts positioned on 
the rear side. The rear-side contacts are also disposed on the rear side 
interdigital to the "front-side contacts". This solar cell suffers from 
the disadvantage of a high number of contact holes that have to be drilled 
with a laser, this large number requiring about 10,000 contact holes per 
solar cell for a typical solar cell 100 cm.sup.2 in size and a typical gap 
of 1 mm between the contact holes. This reduces the throughput in 
automated production. Furthermore, the contact holes and the associated 
contacts disposed on the rear side have to be adjusted in relation to one 
another. Undesired structural transformations in the silicon may also be 
produced in the contact holes drilled with a laser, thereby making it 
possible to create additional recombination centers for pairs of charge 
carriers which further reduce collection efficiency. The reduced 
mechanical strength may lead to rupture in these solar cells. 
The present invention's object is to design a solar cell without front-side 
contacts which create shade; such a solar cell is simple and inexpensive 
to produce and satisfies other requirements for a high-output solar cell. 
In accordance with the invention, this object is solved by a solar cell 
according to claim 1. Preferred embodiments of the invention and a method 
of producing same can be gathered from further claims. 
The solar cell according to the invention is built up from a 
(110)-orientation crystalline silicon substrate. This material enjoys the 
advantage that it exhibits (111) planes aligned vertical to the (110) 
surface. Anisotropic etching oriented toward the crystal structure makes 
it possible to generate depressions, holes or openings with a high aspect 
ratio and two vertical side walls in the (110) substrate. The solar cell 
according to the invention has a plurality of elongated slots aligned 
parallel to (111) planes and extending through the entire thickness of the 
silicon substrate or breaking through this substrate. The inner surfaces 
of the slots have a high doping corresponding to the conductivity type of 
the flat emitter layer generated at least on the front side. A grid-like 
first contact pattern is located on the rear side of the solar cell for 
electrically connecting the bulk material. Interdigital thereto, a second 
grid-like contact pattern which overlaps with the slots at least in part 
and thus ensures the emitter layer's electrical connection is disposed on 
the rear side. 
The front side of the solar cell according to the invention is unimpaired 
except for the slots and has a high-grade surface which enables good 
passivation and an effective antireflection layer. Because of the good 
anisotropic etchability in (110)-oriented silicon, the slots can be 
generated with high aspect ratios of e.g. 1:600 in the silicon substrate. 
This makes it possible to minimize the size of the slots and hence the 
surface losses. Slots that have been anisotropically etched in (110) 
silicon have side walls which consist of (111) planes. Two of these planes 
are disposed vertical to the substrate surface, whereas the two "narrow 
sides" extend at an angle through the substrate. When etching from the 
rear side of the silicon substrate, the cross section of the slots 
therefore tapers toward the front side, so that as a result, the surface 
losses are further reduced by the slots on the front side. The elongated 
extension of the slots makes it easier to adjust the second contact 
pattern which overlaps the slots on the rear side. 
The silicon substrate is highly doped in the slots. This creates current 
paths of electrically sufficient conductivity which connect the front side 
of the solar cell to the rear side, or to the contact pattern applied 
there. A sufficiently dense pattern of slots and the relatively low 
substrate thickness cause the current paths to remain short for charge 
carriers collected on the front side. In this way, the solar cell's series 
resistance is also low and a high fill factor is made possible. 
In an advantageous embodiment of the invention, a so-called tricrystal 
wafer is used as a substrate, as is known for example from an article by 
G. Martinelli in Solid State Phenomena Vol. 32 to 33, 1993, pp. 21-26. 
Such a wafer comprises three monocrystalline regions that are tilted 
toward one another and which in themselves are each (110)-oriented. The 
boundary areas between the monocrystalline regions extend radially toward 
the middle of the wafer so that the monocrystalline regions form sectors 
of the tricrystal wafer. Two of the three boundary areas are first-order 
twin grain boundaries on (111) planes which are particularly low in 
imperfections. 
A solar cell according to the invention produced from such a tricrystal 
wafer enjoys the advantage that the mechanical stability of the wafer and 
hence of the solar cell is substantially increased compared to a 
monocrystalline substrate. In this way the substrate thickness can be 
reduced to values of 30 to 70 .mu.m without having to take an increased 
risk of rupture into consideration during processing. The tricrystal wafer 
is particularly suitable for the invention because it only has 
(110)-oriented surfaces or makes (110)-oriented silicon substrates 
sufficiently available for the first time. Crystal pulling of 
monocrystalline (110)-oriented rods is much more difficult than that of 
conventional (100) silicon rods, since crystal rearrangements and 
structural loss are produced more quickly, such loss causing the pulling 
process to be stopped too early. Crystal pulling of a tricrystal, on the 
other hand, is 2 to 3 times faster than is the case with (110)-oriented 
silicon rods. A cone is not necessary at the end of the rod. It can 
therefore be performed quasi-continuously and without rearrangement. A 
crucible can be used up to ten times. 
A solar cell having a thinner silicon substrate enjoys other technical 
advantages in addition to the saving in material. Using a thinner 
substrate, the demand placed on a high-output solar cell that the 
diffusion length of the minority charge carriers should be greater than 
the three-fold thickness of the substrate is already satisfied by a 
material of a lower electronic quality. A thinner silicon substrate in a 
solar cell therefore results in lower recombination losses than a thicker 
substrate. 
A solar cell with a tricrystalline silicon substrate is sufficiently stable 
even when there is a plurality of slots breaking through the substrate. It 
is nevertheless advantageous for the slots extending parallel to (111) 
planes to be offset against one another so that several slots which might 
assist a rupture of the substrate parallel to the crystal planes as a 
result of the predetermined "perforation" are not arranged in succession 
into one and the same (111) plane. 
A first and a second contact pattern on the rear side of the solar cell are 
preferably applied as thick-film contacts and particularly as conductive 
pastes to be sinter-fused. The first and second contact patterns form an 
interdigital structure in which finger-like contacts are alternately 
arranged, the finger-like contacts of the first and second contact 
patterns engaging with one another like the teeth of a zip fastener. Each 
contact pattern comprises at least one bus structure which connects all 
the finger-like contacts together. One of the bus structures is preferably 
arranged circumferentially close to the edge of the solar cell's rear 
side. The surface-area proportions of the first and second contact 
patterns are preferably approximately equal because identical charge 
quantities have to be transported for both charge carrier types, thus 
minimizing the series resistance. 
The method of producing the solar cell according to the invention will now 
be explained in more detail on the basis of exemplary embodiments and the 
associated ten figures. The figures belong solely to the exemplary 
embodiments and should not be regarded as restrictive.

The starting point for the process according to the invention is an e.g. 
p-doped (110)-orientation silicon wafer 1. The slots or a pattern of slots 
are produced in the first step. For this purpose, an oxide or nitride 
layer 2 is first applied all over the entire surface area of the front 
side VS and the rear side RS. Rectangular openings 3 that correspond to 
the slot pattern are then photolithographically defined and freely etched 
in this oxide or nitride layer 2. FIG. 1 illustrates this procedural step 
on the basis of a diagrammatic cross section through a silicon substrate; 
this cross section is not true to scale. 
In accordance with the pattern of openings 3 defined in the masking layer 
2, slots 4 are now produced in the substrate 1 by means of 
crystal-oriented alkaline etching. FIG. 2 illustrates this state after 
removal of the masking layer 2. 
FIG. 3: a flat, n.sup.+ -doped emitter layer 5, e.g. at a depth of 0.3 to 2 
.mu.m, is produced on all the surfaces of the silicon substrate 1, 
including the slots, as a result of the phosphorus doping that takes place 
all over. 
FIG. 4: a passivation layer 6, e.g. an oxide or nitride layer which is 
usually 70 nm thick, is applied all over each surface in the next step. 
FIG. 5: the electrical contacts are applied to the rear side in a 
thick-film technique in the next step. As regards the first contact 
pattern 7, finger-like contacts, in addition to the slots 4, are for 
example applied to the rear side RS in order to contact the bulk material, 
i.e. to contact the inner p-doped substrate region. This may be brought 
about for example by imprinting a conductive screen-printing paste that 
contains silver or aluminum particles and which can be sintered. The paste 
contains either aluminum or another dopant that generates p doping, such 
as boron. A second contact pattern 8 is applied over the slots 4 at least 
in part, e.g. by imprinting a silver-containing conductive paste. The 
first and second contact patterns 7 and 8 are grid-shaped and each 
comprise at least one bus structure and finger-like contacts emanating 
therefrom. The two contact patterns are arranged on the rear side of the 
substrate such that the finger-like contacts interdigitally engage with 
one another and are spatially separate from one another. FIG. 5 shows the 
configuration after this procedural step. 
FIG. 6: in the next step, the contacts are burned in and sintered, the 
passivation layer 6 beneath the contact patterns 7 and 8 being alloyed in 
an electrically conducting manner. The dopant contained in the paste for 
the first contact pattern 7 generates a p.sup.+ doping 9 which 
overcompensates the emitter layer 5 and produces the ohmic contact with 
the internally positioned p-doped region of the substrate 1. The material 
of the second contact pattern 8 produces a conductive connection to the 
n.sup.+ -doped region 5, the emitter layer. 
FIG. 7: in the next step, the first and second contact patterns 7 and 8 can 
be used as a self-adjusting mask to optionally separate the pn junction 
between the first and second contact patterns, e.g. by plasma etching, 
depressions 13 being produced between the first and second contact 
patterns. If the p+ doping 9, which simultaneously represents a back 
surface field (BSF), prevents a conductive connection between the contact 
pattern 7 and the emitter layer, plasma etching is not, however, 
necessary. 
FIG. 8: in one version of the process (following on from the procedural 
step according to FIG. 4), the passivation layer 6 and emitter layer 5 are 
removed in a lift-off technique, e.g. by brief plasma etching, in a region 
14 which is provided to receive the first contact pattern. This region is 
therefore somewhat larger in dimension than the first contact pattern. 
FIG. 9: the first and second contact patterns 7, 8 are then applied e.g. by 
means of imprinting and are optionally burned in. The first contact 
pattern may in turn contain a doping suitable for generation of a BSF. 
Following on from the state illustrated in FIG. 4, however, it is also 
possible to first apply the second contact pattern 8 and to use it as a 
mask for the lift-off technique in order to remove the passivation layer 6 
and emitter layer 5, whereby recesses that correspond to the regions 14 
and reach right into the bulk material are produced. The first contact 
pattern 7 is then applied in these recesses. In this version, it is 
advantageous for the second contact pattern 8 to be generated with a 
larger surface area than the first contact pattern 7 in order to keep a 
maximum emitter surface after the lift-off process. 
In any case, the first and second contact patterns 7 and 8 are applied such 
that the two do not overlap and are electrically separate from one 
another. 
FIG. 10 depicts in a perspective illustration the rear side of the silicon 
substrate 1 with one of the slots 4. This slot comprises two opposite 
vertical walls 11 which correspond to (111) planes in the substrate. The 
narrow sides of the slots 4, on the other hand, are delimited by crystal 
faces 12 extending at an angle thereto and which also represent (111) 
planes. When defining the slot pattern in the masking layer 2 at the start 
of the process, it is borne in mind that the longitudinal axis of the 
slots is disposed parallel to the vertical (111) planes. Length 1 and 
width b of the slots (on the rear side) are chosen such that an opening 
which breaks through the substrate 1 is just produced during 
crystal-oriented etching. The slot width b is set to 5 to 50 .mu.m and 
varies for example from 15 to 20 .mu.m. The slot length l depends on the 
thickness of the silicon substrate 1. The length l is preferably chosen 
such that the virtual intersection of the faces 12 which delimit the 
narrow sides of the slot is disposed just above the front side VS of the 
silicon substrate 1. This produces a slot which is examined from the front 
side VS of the substrate 1 and whose "length" corresponds to b and whose 
"width" is minimized parallel to the slot length l. 
FIG. 11 shows a tricrystal wafer which is preferably used as a substrate 
for the solar cell according to the invention. This wafer comprises three 
monocrystalline regions M1, M2 and M3 which are all (110) oriented, but 
tilted toward one another. 
In the figure, the tricrystal wafer is disposed such that a first-order 
twin grain boundary KG12 having (111) planes as crystal faces that delimit 
the grain is produced between the monocrystalline regions M1 and M2. The 
grain boundary KG13 between M1 and M3 is also a first-order twin grain 
boundary with delimiting (111) crystal planes. An optimally grown 
tricrystal having the two first-order twin grain boundaries has ideal 
internal angles between the different monocrystalline regions which amount 
to exactly 109.47.degree. for W1 and exactly 125.26.degree. for W2 and W3. 
Internal angles which deviate therefrom also result in a stable tricrystal 
wafer which can be obtained by sawing it out of corresponding tricrystal 
rods, whereby reliable handling of the corresponding wafer is guaranteed 
down to wafer thicknesses of 30 .mu.m without any increased risk of 
rupture. Wafer thicknesses that are preferred for a solar cell range for 
example from 60 to 150 .mu.m. 
FIG. 11 shows an exemplary embodiment for the configuration of first and 
second contact patterns on the rear side of a tricrystal wafer. In 
accordance with the orientation illustrated in FIG. 9, the two lower 
shanks of the "star" formed by the grain boundaries form first-order twin 
grain boundaries. The slots in the tricrystal wafer are preferably 
disposed such that their length l is aligned parallel to one of the 
first-order twin grain boundaries. The slots are preferably aligned 
parallel to that first-order twin grain boundary which is closest to the 
slot. Corresponding to the configuration of the tricrystal wafer depicted 
in FIG. 9, the slot pattern is aligned parallel to the grain boundary KG13 
in a first wafer half on the left of the imaginary axis A, but aligned 
parallel to the grain boundary KG12 in the wafer half on the right of the 
axis A. The slots are preferably offset against one another so that slots 
arranged side by side in a row do not end up in one and the same (111) 
plane. They are preferably offset against one another by more than a whole 
slot width. 
A second contact pattern 8 suitable therefor and overlapping all the slots 
is illustrated for example in FIG. 12. The first contact pattern 7 has a 
bus structure which is arranged circumferentially close to the substrate 
edge. Contact fingers emanating therefrom point at an angle to the 
substrate's central axis. The second contact pattern 8, on the other hand, 
has a central bus structure which is for example disposed parallel to the 
axis A shown in FIG. 9. The finger-like contacts emanating therefrom are 
disposed interdigital to the first contact structure 7 without touching 
same. The geometrical alignment of the first contact pattern 7 [should 
read: 7] is chosen in the exemplary embodiment such that the contact 
fingers are aligned parallel to the length l of the slots and they 
therefore overlap in terms of length. The first contact pattern 7 does not 
overlap any of the slots. But it is also possible to change around the 
assignment of the contact patterns to the p and n-doped areas of the solar 
cell so that for example the contact pattern with the circumferential bus 
structure overlaps the slots and therefore contacts the n-doped areas, 
whereas the contact pattern with the central bus structure serves to 
contact the p-doped bulk material. 
The width of the finger-like contacts for the first and second contact 
patterns is set for example to about 300 .mu.m. Such a contact pattern can 
be created reliably and reproducibly using conventional screen printing 
techniques. Much wider or narrower finger-like contacts are also possible, 
however. Corresponding to the gap between the slots, the finger-like 
contacts of a contact structure are spaced about 3 mm apart from one 
another. 
One or more antireflection layers of a suitable thickness can then also be 
applied to the passivation layer 6, e.g. further oxide, nitride or 
titanium oxide layers. 
A solar cell according to the invention produced in this manner meets all 
the prerequisites necessary for achieving collection efficiency of more 
than 20%. The demand that the diffusion length be greater for the minority 
charge carriers than the three-fold thickness of the silicon substrate is 
satisfied by the solar cell according to the invention with inexpensive CZ 
silicon in which the diffusion length L exceeds the substrate thickness d 
by 1.5 times (where d=60 .mu.m, L.gtoreq.120 .mu.m). High surface quality, 
expressed by a low surface recombination velocity S, can be simply and 
reliably achieved by means of passivation layers on both the front and 
rear sides. High surface quality of S&lt;1000 cm/s can be set over the 
emitter by means of oxide passivation. As regards the rear-side quality, a 
surface recombination velocity of S&lt;100 cm/s is required, which can be 
achieved in the solar cell according to the invention even without further 
measures. Requisite shading losses of less than 4 percent are also 
exceeded by means of the solar cell according to the invention, since it 
exhibits virtually no shading. Low requisite reflection values of &lt;4 
percent are obtained by using standard antireflection layers. A high fill 
factor of at least 80 percent is also achieved by the invention. 
Another advantage of solar cells with contacts applied only to the rear 
side is that it is easier to mechanically connect different solar cells to 
form a module, because no more lead-ins on the front side are necessary in 
order to solder corresponding connections on. This simplifies the 
connecting process and increases procedural reliability. The solar cells 
according to the invention are therefore fully automated and can be 
produced on an industrial scale.