Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2001-11-14
2003-06-10
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S261000, C136S290000, C257S064000, C257S075000, C257S461000, C438S097000, C438S089000, C438S488000, C438S491000, C023S29500G, C023S300000, C117S924000, C164S122200, C423S348000
Reexamination Certificate
active
06576831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to directionally solidified, multicrystalline silicon having a low proportion of electrically active grain borders, its manufacturing and utilisation, as well as to solar cells comprising said silicon and a method of manufacturing said cells.
2. Background Art
Typically, silicon wafers to be further processed to become solar cells are made of high-purity, positively doped (mostly boron-doped) silicon which after being melt open is converted by directional crystallisation, e.g. by using the SOPLIN technique (solidification by planar interface) to large-sized blocks or bars having a typical weight from 150 to 250 kg. After cooling down, the block material is cut to columns having various cross-section surfaces using belt saws. Then, said columns are cut to disks having a thickness of approximately 200 to 400 &mgr;m using inner hole saws or multiple wire saws. The silicon wafers produced in such manner can then be further processed to become solar cells.
Typically, the surfaces of the silicon disks are at first slightly etched in a bath containing an alkaline medium such as soda lye. Then, phosphorus is diffused into one face of the wafer which changes the conductivity of this layer from positively conductive to negatively conductive. At the interface, an electrical field (the p
transition) originates which separates the light-generated charged particles from one another. Contacts are printed onto the front and back sides to allow carrying-off the electron hole pairs which have been produced by light incidence. Finally, an anti-reflection layer is applied onto the front side which significantly reduces loss due to reflection. The manufacturing sequence may be altered such that the anti-reflection layer is applied prior to printing the contacts which are then burnt through the anti-reflection layer in order to obtain a low transition resistance to the silicon.
The initial material used must meet very high requirements regarding its chemical purity because metallic and non-metallic impurities as well as inclusions of electrically active particles such as silicon carbide (SiC) affect the specific electric resistance, the conductivity type of the silicon as well as the lifetime of the charged particles and/or the free path lengths of the minority charged particles thus having an adverse effect on the efficiency, i.e. the performance of the solar cell. Apart from the impurities caused by foreign atoms, the efficiency of multicrystalline solar cells is affected by extended crystal defects such as grain borders and offsets. In addition to reducing impurities, present research efforts focussing on increasing the efficiency of solar cells on the basis of multicrystalline silicon aim at achieving a reduction of the electric activity of extended crystal defects such as grain borders.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide multicrystalline silicon having low electric activity at the grain borders which in particular is suitable for being used in highly efficient solar cells, and to provide said solar cells.
The subject of this invention is directionally solidified multicrystalline silicon characterised in that electrically active grain borders occurring in less than 3% of the material volume.
The electric grain border activity is defined by measuring the resistance topography. The specific resistance of the resistance topography is determined using the 4-tip technique (H. -F. Hadamovsky, “Werkstoffe der Halbleitertechnik” [Working Material of Semiconductor Technology], p. 183-185, VEB Leipzig 1985.).
To this end, a measuring head comprising four needles spaced apart at 0.63 mm is placed on a silicon sample (e.g. a wafer, a column portion, or another shaped part made of silicon having at least one flat surface) using a pressure of 800 N. A constant current of approx. 300 &mgr;A flows through each two needles. The potential difference generated by the electric field is measured at the other two needles. The specific resistance of the silicon sample is determined according to Ohm's law. Taking into account the geometry of the needles, the specific resistance &rgr; can be calculated using
ρ
=
Δ
U
1
2
π
ln
2
D
equation
(
1
)
(
1
)
where &Dgr;U denotes the potential difference measured, I denotes the constant current, and D denotes the sample thickness wherein D is typically less than 5 mm. The resolution of the resistance measurement is approximately 1 m&OHgr;cm.
If the resistance is measured across a grain border, it no longer copes with Ohm's law. Instead, at least one of the resistance values measured in x or y direction (&rgr;
x
and &rgr;
y
, respectively) is significantly larger than the resistance of the volume material due to the potential barrier occurring at the grain border. The term “active grain border” means that at least one of the two values &rgr;
x
and &rgr;
y
exceeds by more than three times the median value of the specific resistance of the volume material. Such a grain border not only represents a high electric resistance in one direction but also a reactive centre in which electrons and holes recombine while releasing power.
The percentage of electrically active grain borders is determined via resistance topographies applied on silicon disks having a size of 10×10 cm
2
, a basic dotation of
1-5×10
16
cm
−3
and a thickness of 300-350 &mgr;m. The aforementioned pressure force of the tips of 800 N and a measuring current of 300-350 &mgr;A are used. The step width of the individual measurement is 600-700 &mgr;m so that approximately 150×150=22,500 measurements are performed on the entire disk surface.
In the directionally solidified multicrystalline silicon according to the invention, only less than 3% of all measuring points are considered as electrically active grain borders corresponding to the above definition while in the case of prior art materials for multicrystalline solar cells typically 5-15% of the measuring points are associated with electrically active grain borders. The small amount of electrically active grain borders in the silicon according to this invention is not accomplished by a larger grain size (i.e. a smaller total number of grain borders) but by significantly reducing the electric activity of the individual grain borders while retaining the same grain size (the typical grain diameter is 0.5-2 cm).
Most preferably, the specific resistance of the silicon according to this invention ranges between 20 and 10,000 m&OHgr;cm, and even more preferably between 100 and 2,000 m&OHgr;cm.
Surprisingly, it was found that directionally solidified multicrystalline silicon having a negative dotation (hereinafter referred to as N type) has significantly less grain border activity than multicrystalline silicon of the P type, i.e. multicrystalline silicon having a positive dotation. The resistance topographies of both types are shown in FIG.
1
. Active grain borders are found in the P type silicon (dark colour) while no such grain borders whatsoever are found in the N type silicon.
Hence, the directionally solidified multicrystalline silicon according to this invention is preferably N doped.
The N dotation may be accomplished, for example, using phosphorus, arsenic, stibium, lithium and/or bismuth. To obtain a specific resistance of 1,000 m&OHgr;cm, a dope amount of approximately 0.1 ppma corresponding to 5×10
15
atoms/cm
−3
is required.
Another subject of the present invention is a method for manufacturing the silicon according to this invention in which method mono- or multicrystalline silicon is molten open and directionally solidified.
To this end, preferably doped mono- or multicrystalline silicon is molten open with 10
15
-10
17
atoms/cm
3
of P, As, Sb, Li and/or Bi. The dope comprises at least one of the aforementioned elements. The melting is directionally crystal
Hässler Christian
Koch Wolfgang
Stollwerck Gunther
Woditsch Peter
Connolly Bove & Lodge & Hutz LLP
Deutsche Solar GmbH
Diamond Alan
LandOfFree
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