This invention relates generally to solar cells made of semi¬ conductor material and more particularly to such cells in¬ cluding amorphous semiconductor material.

Amorphous (non-crystal) silicon is a material of great poten¬ tial interest for solar cell applications. This interest arises from the fact that amorphous silicon is a highly efficient absorber of light and is at the same time extremely easy and cheap to prepare. The optical absorption charac- teristics are such that 0.5 μ thic layer of amorphous silicon will absorb as much solar energy as a 250 μm thick layer of crystalline silicon. Hence, the use of amorphous silicon as an optical absorber leads to the use of 500 times less material than that required' for a single crystal cell.

Amorphous silicon is also impressively inexpensive. It can be obtained by RF plasma deposition from a silane-based (SiCl.) gaseous source or by either sputtering or evaporation of silicon from either silicon powder or a charge of polycrystalline material. The RF deposition process is of special interest because it is simple to add dopants during the deposition process. Thus, p-type, n-type and intrinsic layers can be grown during one deposition run by appropriate control of the gases admitted to the plasma deposition chamber.

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However, amorphous silicon cells made by any of the tech¬ niques described above are typically of very low efficienc The reason for this is that the carrier lifetime in the amorphous layer is very short and, hence, the collection o electrons and holes from such a layer is difficult.

It is an object of the present invention to provide a sola cell including amorphous semiconductor material and a meth of making the solar cell.

It is a further object of the present invention to provide a highly efficient, inexpensive solar cell and method of making the solar cell.

These and other objects are achieved by a solar cell which includes n-type and p-type polycrystalline or crystalline layers separated by an amorphous layer of semiconductor material and to a method of making the solar cell.

Figure 1 is a perspective view of a solar cell in accordan with the present invention.

Figure 2 is a partial sectional view showing the accelerat voltage across the amorphous layer disposed between the cr talline or polycrystalline doped layers.

Figures 3A-E show the steps for one method of forming a so cell in accordance with the present invention.

Figures 4A-E show the steps for an alternate method of for ing a solar cell in accordance with the present invention.

Referring to Figure 1, the solar cell comprises a first la 11 spaced from a second layer 12 by a layer 13 of amorphou or πicrocrystalline material. Layers 11 and 12 may, for

example, be crystalline or polycrystalline semiconductor material. One of the layers is p-type and the other n-type. The amorphous layer may be an intrinsic layer. Suitable con¬ tacts 14 may be formed on the upper surface. The contacts may be in the form of a comb which with relatively narrow and long teeth 16 and a wide back 17 whereby to provide for maxi¬ mum penetration of solar energy into the cell, as shown by the rays 18. The layer 12 includes the second contact where¬ by the voltage generated between the p-type and n-type layers by carriers formed by the impinging solar energy can be recovered.

The amorphous layer serves as the optically active region in which the carriers are generated. The absorption properties of the amorphous layer are such that it will absorb substan- tially all of the carrier producing solar energy that strikes the cell.. Referring to Figure 2, the contact potential that normally develops across a p-n or a p-i-n junction is shown between the doped layers 11 and 12. For the purpose of this example, one assumes that the holes that leave the p-type region to form the space charge layer in the p-type material recombine with the electrons in the amorphous layer, namely the electrons coming from the n-type region. Thus, the amorphous layer is sandwiched between p-type and n-type semiconductive layers that are either single crystal or sufficiently large grain polycrystalline material to permit the development of space charge in the normal manner for a p-i-n junction. The two space charge layers shown in Figure 2 produce an electric field in the intrinsic amor- phous layer whose value is E = φ/L.

As an example, for Φ= 0.7 volts and L = 0.5 μm , one obtains E = 14,000 volts/cm, a rather large field. With this field carriers that are generated in the amorphous layer can drift to the crystalline contact layers in a time

T = L μ Φ

where. is the carrier mobility. Taking the value of μ of

10cm 2/Vsec as representative of typical carrier mobi.li•¬ ties in an amorphous silicon film one finds that the trans time across the amorphous layer to be

T -≥. 0 ,7ns

Hence, a substantial fraction of the photogenerated carrie can be collected so long as the carrier lifetime is 1 ns o more. Without this electric field, however, the transit time would be governed by the carrier diffusion process. For the mobility assumed, the carrier diffusion length for 1 ns lifetime would be

1 ss 0.12 μ

Carriers generated in most of the amorphous layers cannot collected into the contact regions without the presence of the electric field. Hence, the electric field offers a technique for most of the carriers generated in the amor¬ phous region to be collected even when the carrier lifetim is so short that diffusion transport cannot possibly provide an efficient means for transporting carriers to th collecting layers.

It is also possible to use an amorphous layer of one mater germanium, and a collecting layer of another material, sil con. In addition, dopant can be added to the amorphous la while it is being grown to produce a graded ^" impurity profi in the amorphous layer. This procedure provides an extend drift field in the amorphous layer according to U. S. Pate No. 4,001,864 which aids the transport of carriers along the amorphous layers in the manner therein described.

Figure 3 illustrates the steps in one process for forming solar cell in accordance with the present invention.

Starting with a graphite or other suitable substrate 21, an amorphous layer 22 is grown on the substrate by plasma deposition, Figure 3A. The amorphous silicon may be doped. with arsenic to form n-type material. Thereafter, a continuous wave laser or electron beam can be scanned over the layer 22 to recrystallize the layer into large polycry¬ stalline silicon illustrated schematically by the crystals 23, Figure 3B. For example, the laser beam may be generated by a Spectra Physics Model 171 argon ion laser with power output of 11 watts focused onto a 40 micrometer spot scanned over the amorphous silicon layer at a speed of 10 centimeters per second. Next, the recrystallized n-type layer is used as a base on which an amorphous layer 24 of intrinsic silicon can be grown. The amorphous layer may be 0.50 μ thick. The layer may also be formed by plasma deposition as schematically illustrated in. Figure 3C. Plasma deposition is continued with boron doped silicon to deposit a final 0.25 μm layer 26, Figure 3D. Thereafter, a continuous wave laser operating at a power level of approxi- mately 5 watts with other conditions as described above melts only the top 0.25 μm layer of the deposited amorphous film. This recrystallizes the layer 26 that contains the n-type dopant to form crystals 27, Figure 3E. In this way, n-type and p-type layers are formed with high quality large grain polycrystalline material with a layer of intrinsic amorphous silicon separating the polycrystalline layers. Ohmic contacts can be made, for example, by forming comb¬ like contacts on the upper surface by masking .and evapora¬ tion or sputtering and by forming a lower contact layer by evaporation or sputtering.

An alternative method for fabricating a solar cell according to the present invention is described with reference to Fig¬ ure 4. Here a thin layer 31 of arsenic-doped amorphous sili¬ con is deposited on a steel or graphite substrate 32, Figure

4A. The material is then recrystallized by scanning with laser beam as described above to produce large grain n-typ crystallites 33, Figure 4B. ^' The recrysallization may also be carried out with an electron beam or other suitable mea which produce crystals which extend between the two surface

Next, a heavily doped p-type layer (p++) 34 is made directl in the n-type material by implanting boron, Figure 4C, and using a CW laser or electron beam to anneal the implantatio damage under conditions that produce substantially no diff sion of the implanted boron. Figure 4D. By an appropriate choice of implantation conditions, the (p++ -n) junction c be formed at approximately 0.1 μm beneath the surface.

Next, fluorine or other light ions are implanted at a dose and energy that will produce a buried, amorphous layer 35, Figure 4E. For example, implantation of flourine .at a dose of 5 x lO^/CM at an energy of 100 keV followed by a second flourine implantation at a -dose of 5 x lO^/C ^ at an energy of 200 keV will succeed in amorphizing the poly¬ crystalline silicon. The amorphous layer will start approx mately 0.1 μm and continue to a depth of approximately 0.4 In this way we obtain p+ and n+ polycrystalline layers sepa rated by an amorphous region as required in the present in¬ vention. Furthermore, by implanting flourine we introduce species which tends to attach to silicon to reduce dangling bonds in the implantation-amorphized layer, thus improving the carrier lifetime in the amorphous layer.

As a third alternative, it would be possible to start with n-doped metallurgical grade crystalline or polycrystalline silicon substrate (wafers) obtained by casting, film growth (EFG) or conventional Czrochralski techniques. The steps • outlined in Figure 4C-E can then be performed to produce the (p++ -n) junction and the buried amorphous layer.

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Thus, it is seen that there has been provided a solar cell which includes a center region which is highly absorbent to the solar energy to thereby generate carriers which are quickly removed by the voltage between the n-type and p-type layers. Preferably the amorphous layers include a gradient whereby to further enhance the collection of the generated carriers.