CA2177493A1

CA2177493A1 - Concrete solar cell

Info

Publication number: CA2177493A1
Application number: CA002177493A
Authority: CA
Inventors: John R. Arthur; Robert K. Graupner; Tyrus K. Monson; James A. Van Vechten; Ernest Wolff
Original assignee: John R. Arthur; Robert K. Graupner; Tyrus K. Monson; James A. Van Vechten; Ernest Wolff; State Of Oregon, Acting By And Through The State Board Of Higher Education On Behalf Of Oregon State University
Current assignee: Oregon State University
Priority date: 1993-12-10
Filing date: 1994-12-02
Publication date: 1995-06-15
Also published as: NL9420039A; KR960706694A; US5415700A; WO1995016279A1; US5672214A; TW279272B; AU1298795A; DE4499682T1; JPH09506472A; AU686716B2

Abstract

An inexpensive, robust concrete solar cell (10) comprises a photovoltaic material embedded in and extending beyond the major surfaces (16 and 18) of a matrix layer (14). The matrix layer typically comprises a high strength, cementitious material such as a macrodefect free cement. The photovoltaic material comprises particles (12) of high-resistivity single crystal silicon, typically ball milled from ingot sections unsuitable for slicing into silicon wafers. The ingot sections include unprecipitated dissolved oxygen that is electrically activated by a low temperature annealing process to produce n-type silicon, even in silicon crystals that include a p-type dopant. An aluminum sheet (28) positioned on the backside of the matrix layer, is briefly melted together with the silicon particles to produce a p-type aluminum-doped silicon region (22) that forms a pn junction with the n-type region (24) of the particle. The aluminum sheet also provides the electrical contact to the p-type regions. The front surface of the matrix layer, from which the n-portion of the silicon particle protrudes, is covered with a translucent indium tin oxide conductive layer (30) that provides electrical contacts to the n-portion of the pn junction and digitated electrode (32) for conducting current off the cell. A voltage is generated between the two conductive layers when light incident on the photovoltaic particle through the indium tin oxide conductive layer creates charge carriers.

Description

WO g5/16279 2 1 7 ~ ~ ~ 3 PCTIUS94/13810 .
CONCRETE SOLAR CE~L
Terhn i rA 1 FiPl d This invention relates to an economical, robuElt solar cell.
~ark~ro~ln-l o~ the Invention Solar cells convert light into useful energy, such as electricity or chemical energy. The high cost of solar cells, however, has prevented them from rn"~ret;n~
with convPntinn~l deviceg for gPnPr~t~n~ power. Solar 2 0 cells have typically been limited to low power applications, such as rllr~ tr~r~, or niche applir~tinn~, such as powering sp~rer~A f t ~ buoys, or other remote equipment .
Solar cells are typically constructed by forming 25 a pn junction on a wafer of single crystal, electronic grade sPm1 rnnr~lrtor silicon. The pn junction is typically formed parallel to the major surfaces of the silicon wafer. One side of the pn junction is electrically contacted by a conductor on the back surf ace of the solar 30 cell, while the other side of the pn junction is rnnt~ctPc by a metallic grid on the front surface of the solar cell.
~ight inri~lPnt on the cell creates electron-hole pairs that cause a voltage difference between the conductor on the back surf ace of the cell and the conductive grid on 35 the front surface of the cell ~3ecause such cells require electronic grade semicnnrl-~rtr,r silicon, they are expensive Wo 9S/16279 217~3 to manufacture. Such cells are relatively frayile and typically reouire mn-lnt;n~ in a protective onrlnql~re having a cover of a tr~nqluc-~nt material , i . e., a material that tra~smits a portion o_ the; nr; rlc~nt light .
Another type of solar cell that is constructed from spheres of metallic grade silicon is described in ~evine, et al., "~asic Properties of the Spheral Solara Cell, ~ Prore~d1n~Ts of the Twentv Second IEEE Photovolt~;r ~'rlnferrnr~, Vol. 2, pp. 1045-48 (1991). Spheres of metallic grade silicon somewhat smaller than 1. 0 mm in diameter and ;nrl~ ;n~ a p-type dopant are purified, and an outer shell of each sphere i8 doped with an n- type material to form a pn junction. The spheres are bonded to a flexible All-m; foil, and electrical rnnt~rtq are f ormed between the aluminum and the outer n - type shell .
The spheres are etched to allow f ~nrm-t; nn of an electrical contact to the inner p-type material. Although such cell9 are purportedly cheaper to produce than cells using wafers of electronic grade semicnn~llrtor silicon, the ---nllf~rtl~re of such cells is complex. FurthP t, such cells, like previous cells, are relatively f ragile and must be mounted in a protective module having a tr~nql~lront glass or polymer superstrate.
Another type of solar cell uses silicon crystals f~mh~ rl in a frit glass ;nqlll~tnr, :~uLLuuLlded by clear 11YdLUbL~ c acid. The voltage across the silicon crystals causes an electrorh~m; ~ l reaction that produces gaseous lydLU~jt`ll, liS~uid bromine, and heat. The solar energy is thus stored as chemical energy in the l-ydLuyell and bromine, which can be used in a fuel cell. ~cKee, et al., "Development and Evaluation of the Texas Instruments Solar Energy System, " 16th T~Z PVSc Proce~-l;nr.s, p. 257 (1982~ .

W0 95J16279 217 7 ~ 9 ~ PCTIUS94113810 Summarv of the Invention An object of the present invention is, therefore, to ;nPYrPnQively convert light into a useful energy source.
Another object of this invention is to produce an economical, large surface area solar cell for converting solar rA~l;At;nn into electricity.
A further object of this invention is to produce such a solar cell that i9 sufficiently robust to function with little or no mA1ntPnAnce for PYt~nr1P~ periods in outdoor envi~ ~ Q.
Yet another object of this invention is to reduce the cogt of rl;r~p~ Q~inr~ of scrap silicon produced during the mAnllf-Alr~tllring of silicon wafers.
lS The present invention comprises an ArpArAt~Q
for converting light into useable energy and a method for r-n1lfActllring the -Arr-r-t1~Q. Particleg of a photovoltaic material are ' ~ in and extend beyond the maj or surfaces of a dielectric matrix, such as a high strength 20 cementitious material. I,ight; nr; ~lPnt on the photovoltaic material generates charge carriers that travel to the portion of the photovoltaic particles -YtPn~lin~ beyond the dielectric matrix layer, where a voltage i8 produced.
Applicants refer to the -nmh;nAtinn of photovoltaic 25 particles with a cementitious matrix as a "concrete solar cell. "
In a preferred Pmhorl; t, the photovoltaic particles comprise single crystal silicon. The silicon particles can be provided, for example, by c- n~t;n;
30 high-resistivity by-products of silicon wafer production, such as ingot ends that are unguitable f or waf er production. Silicon particles that are not ~- osed of an n-type material are annealed to electr;cA11y activate dissolved oxygen to convert the silicon to an n- type Wo 95116279 , Pcr/uss4/13810 2177~

material .
The dielectric matrix material is typically a rigid, weather-resistant material, such as a high strength cement, with a macrodefec~ free cement ("MDFC") being the pre~erred material.
An A l Tmi nl~m sheet i9 positioned on one side of the .1; ~l ertriC matrix layer and rnntArt~ the silicon particles that extend beyond the layer. The aluminum and n-type silicon are melted together at the interface using the 577~C eutectic process and rpqol;ri;fied~ leaving a portion of the silicon doped with ~lllm;nl~m atoms to create a p-type region in each n-type particle, thereby forming a pn junction and electrical rnnt~rts between the ~ nllm sheet and the p-type region. The front side o~ the dielectric matrix layer, from which the n-type portion of the silicon particles protrude, is covered with a conductive layer, such as an indium tin oxide (nITOn) layer, that provides electrical contacts to the n-type portion A digitated, metallic grid may be added to reduce the sheet resistance of the front surface. A
protective layer, such as a trAnqlurPnt MDFC layer may be added over the ITO layer and metallic grid. The invention can also be made using other junction types, such as a Schottky barrier junction, instead of a pn junction. The invention can also be used as part of an electrochemical cell .
~arge area, robust photovoltaic panels can be constructed, ~or example, as shingles for plAI - on the roofs or sides of structures or on concrete railroad ties.
Additional objects and advAntages of the present invention will be aFparent from the ~ollowing detailed description o~ a pre~erred Pmhori;mPrt thereof, which proceeds with reference to the ~ L~ ying drawings.

2177493 ~^T'~ 9~ 3~1 Brief Description of the Drawin~rq Fig. l is a plan view of a solar cell of thepresent invention.
Figs. 2 and l are sectional views taken along 5 respective lines 2--2 and 3--3 of Fig. l.
Fig. 4 is an enlarged, fragmentary view of the area l~h~1le~ ~'4~ of Fig. 2.
Detailed DescriPtion of Preferred ~ ts Figs. 1-4 show a photovoltaic cell 10 that lo represents a preferred PmhO~; ~ of a solar cell of the present invention. Photovoltaic cell lo comprises photovoltaic particles 12: ' -'1e' in a dielectric matrix layer 14 having first and second major surfaces 16 and 18_ Photovoltaic particles 12 and matrix layer 14 together 15 form a concrete layer 20.
Each photovoltaic parti~cle 12 includes a portion 22 of a p-type material and a portion 24 of an n-type material that together form a pn ~unction 26 at their interfaGe and extend beyond major surfaces 16 and 18, 20 respectively, to electrically contact a conductive layer 28 and a tr~nql~ nt conductive layer 30, respectively. A digitated electrode 32 positioned on translucent cn~n~l~r~ive layer 30 reduces the effective electrical sheet resistance of translucent rnrl'~llf't; ve 2s layer 30. Photovoltaic particles 12 and the various layers shown in the figures are t:~yy~:Lc.ted for clarity.
IJight 34 incident on photovoltaic particle 12 and having enersy greater than the band gap energy of the photovoltaic material comprising particle 12 passes 30 through translucent conductive layer 30 and creates in photovoltaic particle 12 charge carriers, i . e., a c~n~ i on band electron and a valence band hole . An electric field within the pn junction, known in the art as the "built-in field, n causes the electrons to move toward ~DIC1-16~159.1 2279~ 0001 21 '~ 7 4 ~ ~ P~ s941l38lo translucent conductive layer 30 and causes the holes to move toward conductive layer 28, thereby producing a voltage between conductive layers 28 and 30 that can be used to do work such as driving an electrical load or an 5 electrochemical cell.
Matrix layer 14 is pref erably , ~ od of a high flexural strength, i.e., greater than 10 MPa, cement, such as a macrodefect free cement ( "M3FC" ) . An M~FC is a cement that i5 exceptionally strong because, unlike 10 ordinary cement, it cnntA i nq essentially no large voids .
For example, an M~FC may contain less than 2 percent voids ~y volume, with esgentially no voids larger than 15 /~m.
Such large voids rnncirlPrAhl y weaken normal cement . The flexural strength of M~FC is, therefore, two orders of 15 magnitude greater than that of normal cement, and its f racture energy is f ive orders of magnitude greater than that of ordinary cement. MDFC can be formed from many cementitious materials by carefully controlling the grain size, using a high shear mixer that deflsr~ Ateq the 20 grains, lubricating the particles with a water soluble organic polymer, and casting or r:ql Pn~pring the cement at irr~lPrAtP pressures of between apprnYiir-tply 5 MPa and 50 MPa. The proportion of water in the M~FC composition is typically less than 25 percent, and preferably less than 25 12 percent, by weight, although the _mount of water should not be so low that a plastic dough- like ghApPAhl e cementitious composition cannot be formed.
A preferred M~FC, as described in ~Microstructural and Microrh~mi rAl t~hAr~rtprization of a 30 Calcium Aluminate-Polymer Composite (MDF Cement), "
Popoola, et al., 74 J. Am. Cer_mic Soc., pp. 1928-33 ( 199 1 ), i nrl lltipq calcium A 1 llml nA te cement, poly (vinyl alcohol/acetate), glycerine plasticizer, and distilled water. The poly(vinyl alcohol/acetate) may be 79.3 mol96 .1 i' WO 9~/16279 21 7 7 4 9 ~ PCTIUS9J/13810 `

hydrolyzed with a 1, 700 unit degree of polymerization and a medium particle size of 12 ~m. Photovoltaic particles 12 may be added to the cement dough before r~ n~Pring into a layer preferably between So ~,m and 100 ~ in ~hl rkn~
rl~ron~; nrj upon the average dimensions of the photovoltaic particles 12 . Thicker layers, such as layers of 400 ~ m in thickness, are sturdier and easier to produce but produce a less eficient photovoltaic cell 10. Other high strength cements, such as a portland- or po77nlAntc-type cement, can also be used to embed photovoltaic particles 12 in the construction of photovoltaic cell 10.
Adding photovoltaic particles 12 to the cement bef ore it is processed in the high shear mixer that deflor~ t~ the cement grains produces a cement having good mechanical strength but may damage the photovoltaic particles 12. Adding photovoltaic particles 12 to the cement in a low force mixer after it hag been ~1~flor~ tP~l ig less likely to damage photovoltaic particles 12 but may introduce voids, which produces a weaker cement, and adversely affects the setting time. Alternatively, photovoltaic particles 12 can be distributed onto ron~ rt;ve layer 28 and then an uncured ~DFC layer lg can be calendered onto photovoltaic particles 12 and conductive layer 28.
Calendering i9 performed preferably using hard rubber rollers that compress the cement paste and leave the ends of photovoltaic particles 12 P~ nrli n~ slightly beyond the cement paste. f'~l on~l~ring may also be performed using pliable sheets of plastic or rubber. The calendering scrubs the top surface of photovoltaic particles 12 free of both oxide and cement and drives photovoltaic particle~ 12 into rnn~ r-t~n~ layer 28 with sufficient force to break the ~nq~ inr; oxide layers on the photovoltaic particles 12 and conductive layer 28, Wo 9~/16279 PCT/US94113810 2177~93 which may comprise, f or example, an aluminum ~oil . In some cases, however, it may be desirable to improve the electrical connection between particles 12 and cnn~t~lct;ve layer 28 by performing an additional ~leAn;nq step to 5 remove an insulating layer of cement f rom the ends of photovoltaic particles 12.
An efficient photovoltaic cell 10 has a large proportion of the volume of co~crete layer 20 comprised of photovoltaic particles 12. Too large a volume proportion 10 of photovoltaic particles 12 would, however, reduce the mechanical stre~gth of photovoltaic cell 10 and increase the probability of l~nltPC;rlhlf. electrical contacts between differe~t photovoltaic particles 12. A surface area of 35 percent photovoltaic particles 12 has been ~tt~tn~ while 15 r~tnt:~inin~ a 8uffiL i~ntly gtro~g photovoltaic cell 10.
~ he rh~r~ct~ristics of ~IDFC make it very suitable f or use in photovoltaic cell 10 . D~FC is translucent, electrically ;nq~ t;n~, and can be cast i~to sheets as thin as 20 ~m. It boD.ds with silico~
20 photovoltaic particles and with ~ll-m;nl-m rrnrtllctive layers. It is tough, strong"~q~ntt~lly nol.~uLuus~ and water resistant and can withgtand a wide range o~
enviL, ~1 t: t~res. The relative tt;~olec~ric constant of calcium Al..m;n~m-based ~DFC is typically between 7 and 9, which ig less than the 11. 8 relative dielectric constant of silico~. This dif f ere~ce in dielectric constants results in light being rf~fr~rt~t from the ~FC i~to the silico~ and light f rom withi~ the silicoL bei~y r~f r~cta~i back into the silicon . The ~)FC

3 0 thus acts as an antiref lectior. coating to enhance the efficiency of photovoltaic cell 10. ûther types of ~mFC
can have lower dielectric constants, and thus would perf orm this function even better .
A photovoltaic cell 10 composed of ~5)FC _as ~ WO 95/16279 2 1 7 7 ~ 9 ~ PCr~S94113810 sufficient structural strength and is sufficiently robust that it can be used on the sides or roofg of ~ tn~q without a protective structure and cover glass. However, an optional translucent protective layer 42 of thin MDFC
5 can be applied as a protective, antireflective coating over digitated electrodes 32 and trAnql~c~nt conductive layer 30 to provide further enviLl tAl protection for photovoltaic cell lO. An additional protective layer ~not shown~ can also be applied over cnn~11c~;ve layer 28.
Photovoltaic particles 12 are high-resistivity (greater than 25 mn-cm) n-type semirnn~ rtors when they are hPr~ d into MDFC layer 14. The particles can be made from high-resistivity semirnnrl~lct~r silicon doped with n-type electron donor impurities, such as pho,ilhoLL1S, arsenic, or antimony. The particles can also be made from semirnn-l-1ctor silicon that is undoped or doped with p-type, electron acceptor impurities by electrically activating dissolved oxygen in the silicon to change the silicon to n- type . The oxygen is electrically activated by annealing, typically between 425C and 475C, to move oxygen atoms from interstitial positions to lattice sites where they can donate valence band electrons. In a typical r7OrhrAl qk; -grown silicon crystal, annealing can activate apprn~ tP1 y 3 x lO16 atoms per cm' of oxygen, which is a sufficient rnnr~ntrAt;nn to change high-resistivity p- type silicon into high- resistivity n- type silicon .
Photovoltaic particles 12 can be f ormed by comminuting scrap sections of ingots grown f or producing silicon wafers to be used in the m~nl1fA~t-1re of ;ntegrAte~
circuits and from silicon r~mA;n;ng in the growing crucible af ter an ingot is grown. Ingot sections such as the seed and tail ends that are unsuitable for slicing into waf ers are suitable f or use in photovoltaic cell lO

wo 9~/l6279 PCT/US94/138~/) 2177~3 of the present invention. Photovoltaic particles 12 are pref erably milled to an octahedral shape havi~g an average particle size of 50 Im to 100 ~m using a ball mill _nd sieves. An average grai~ size of 50 ym would produce a more ef f icient photovoltaic cell 10, but such a cell would be more difficult to produce than a photovoltaic cell 10 having a larger grain slze. The use of ball mills and sieves for producing particles of uniform size is well known in the powder metallurgy art . Apprn~ t~l y 65 percent of the silicon produced in the United States is of a high- resistivity type suitable f or such use . High resistivity p-type scrap silico~ is usable but must be u~ u.uLed before the dissolved oxygen is precipitated, 80 the oxygen is available to be electrically activated by Annp~l ;n~ to convert the p-type gcrap to _n n-type material as described above. The present inventio~ thus provides a bPn~f;r;Al use for scrap material that i8 a currently a costly waste ~ POSA 1 problem f or the silicûn i~dustry .
Milli~g photovoltaic particles 12 from larger crystals of semirnn~rtnr silico~ can cause crystal defects in photovoltaic particles 12. Such defects cau8e high surface ,~ ' ~nAt;nn velocities and low minority carrier l;fPt; 9 that reduce the Pff;~;Pnry of photovoltaic cell 10.
The amount of crystal structure damage can be reduced by addi~g a lubricant during the milling process to reduce the e~ergy of 'nlltirTm Suitable lubricants include thoge typically u8ed in the 8lici~g opPrAt; nn of silicon wafer ma~ufacturing. Other methods also believed to be useable for reducing the crystal structure damage include AnnpAl; n~ the photovoltaic particles 12 prior to casting them into MDFC layer 14 and etching photovoltaic particles 12 to enhance stable crystal facets and 217 7 4 9 ~ PCTIUS94/13810 passivate dislocations.
Another technique that may be useful f or improving minority carrier lifetimes and surface rec~ ` in~t;on velocity includes growing an oxide layer on 5 photovoltaic particles 12 and then heating them to apprn~ tPl y 1, 000C in the presence of lime to convert a portion of the oxide to calciated silica, thereby passivating dislocations. This method may also enhance the mechanical properties of cell 10 by increasing the 10 adhesive between photovoltaic cell 10 and the MDFC
material. The surface rPc~m~in~t;nn velocity and minority carrier lifetime may also be; ~ Jve:d by forming an n-type layer on photovoltaic particles 12 using chemical vapor deposition or organo-metallic chemical vapor deposition.
15 The surface of the photovoltaic particles 12 may also be passivated by reacting with the cement, with the degree of passivation being rll~t~rm; n~d by the type of cement used.
Photovoltaic particles 12 can also be f ormed from electronic grade polycr,Ystalline silicon or from 20 metallurgical grade gilicon as described by Jules D.
Levine, et al., in "Basic Properties of the Spheral Solar~
Cell, n Proce~l;n--s of the TwentY Sernn-l T~R~ PhotovoltA;~
Onnference. Vol. 2, pp. 1045-48 (1991) and U.S. Pat. No.
5, 069, 740 for ~Production of Semiconductor Grade Silicon 25 Spheres from Metallurgical Grade Silicon Particles. n Such spheres have diameters on the order of a millimeter and require, therefore, a CULLe~ 1;n~1Y thicker MDFC
layer 14. Other photovoltaic materials, such as silicon carbide and gallium rhnqFh;~l~, can be used in photovoltaic 30 particles 12. The starting material for creating photovoltaic particles 12 can be varied ~ rl~nri; n~ on the price and availability of the various raw materials.
P- type portions 22 of photovoltaic particles 12 are preferably formed simult~n~ollqly with the formation of o 95116279 t~ " , PCTruS94/13810 217~ 3 electrical connections between photovoltaic particles 12 and conductive layer 28. Conductive layer 28 typically comprises an Alllminllm foil. The Alllm;mlm foil and the silicon o~ photovoltaic particles 12 are briefly melted 5 together at their interface by using, for example, a rapid thermal annealer or by applying a high voltage between the Al--min.lm sheet and a ti~-coated calenderi~g roller on the opposite side of the ~DFC layer 14 . The PUtPrt; r reaction at 577C results in a rnnrPntr~t;nn of apprn~;~-tPly 10 3 x lol8 Alllmlnllm atoms per cm' in the rP~ol;~l;f;P~l silicon. Because Al mlm is an electron acceptor, the Alll--;nllm doped silicon is a p-type semirnn~ tor. The interface between All~m;nllm-doped p-type region 22 and the L~ ~ n; n~ n- type region 24 of photovoltaic particles 12 15 results in a pn junction 26 that provides an; nt~rnA
electric f ield that drives the photo- induced charge carriers to rnn~lllr~tnr~ 28 and 30.
The depth of pn junction 26 i8 controlled by controlling the energy applied to melt the silicon-20 ~l mlm interface. Alternatively, the depth of the pnjunction 26 can be controlled by limiting the quantity of Alllmin~lm available, for example, by vacuum depositing a thin layer of Altlm;nllm onto ~mPC layer 14 before melting the Alllm; -silicon interface. Arter the pn junction is 25 f ormed f rom the thin A l llm; layer, an additional rnn~ tnr, such as an Al mlm foil, is applied onto major sur~ace 16 to increase the cross - sectional area and reduce the sheet resistance of rnntlllrtive layer 28. The Al 'nllm foil can be bonded to the ~lPrns;te~ Alllm~nllm by heating 30 both layers above the 577C Alllm;nllm-silicon ~--te~t;c temperature. ~ preferred conductive layer 28 has a th; rknPsl~ of approxi~mately 100-150 ~n. P-type region 22 extends into photovoltaic particle 12 a distance 44, preferably equal to the lesser of apprn~;m-tply 10 ~m or Wo 95116279 3 PCTIllS94113~10 half of the rl; ~ r of photovoltaic particle 12 .
Another method of controlling the depth of pn junction 26 entails depositing a layer of aluminum apprnY1m~tPl y 2 ~Lm in th1 rkn~cs onto a conductive 5 substrate, such as a steel backing sheet, that has a melting temperature sign;f;riznrly higher than the 577~C
--m,n11m-silicon eutectic t , tl1re. Upon heating to 577C, the aluminum at the silicon interface melts to form the pn junctions and electrical rnntilCtS. The steel lO remains solid, ~s~nt;Ally soldered to the silicon by the aluminum. Because of the detrimental ef f ect of heavy metal atoms, such as iron, on minority carrier lietimes in silicon, t~ t~res during formi t1nn of the pn j11nct1nnc should remain low to preclude significant 15 diffusion of atoms from the metallic backing sheet into the silicon.
If photovoltaic particles 12 are not mixed into the matrix material before caleAdering, they can be deposited directly onto cnn~ rt;ve layer 28. The matrix 20 material can then be deposited onto the photovoltaic particles 12 and cnn~i~~rt1ve layer 28 to form concrete layer 20. For example, silicon particles can be distributed on an i~l m1m foil, and then a IIIDPC can be rAlPnrl~red onto the Al11m;n~1m foil uging a cnn~ t;ve.
2S e.g., tin-coated, roller having a sufficiently high voltage applied between the tin roller and the i~l vm;
foil to weld the photovoltaic particles 12 to the A1 foil. During the weld, the p-type region would be formed by the eutectic process and the electrical contact between 30 the A1--m1n--m and the silicon would be esti~hl;ch~d. Any tin deposited by the roller onto the ~FC or the silicon would simply become a part of an indium tin oxide conductive layer î O .
Alternatively, the photovolta c particles 12 can 217~3 14 be deposited onto conductive layer 2~ and pn jllnrt; ~nq 26 could be formed, ~or example, by rapid thermal AnnP;~l ;n~, The matrix ~tar; il ig then deposited o~to rnn~;~lrt;ve layer 28, for example, by calendering with hard rubber 5 rollers a~d cured to form matrix layer 14. ~hether matrix layer 14 is deposited before or after pn jllnrtic~nq 26 are formed depends in part upon the ability of the matrix material to withstand without dam~age the short period at 577-C required to form pn jllnrt;nnq 26.
To reduce deteriorAtinn of the matrix material during junction form-tinn, the t ~ tllre required for junction fr,rr-t;nn can be reduced by adding one or more additional .'l l t.q at the photovoltaic particle-rnnrlllrt;ve layer interface. For example, a~ additional 15 alloying m -tpr; Al, such as tin, gallium, or zinc, can be added to Al 'nllm at the interface between cnn~illrtive layer 28 and photovoltaic particles 12. Such three-c, systems can have a melting t~ ,-r~tl~re lower than the silicon Al 'nllm binary system. The alloying 20 material can be added, for example, by ev~rrJr~t;n~ it onto major surface 16 or by depositing it onto or incorpor~t;
it into a metallic foil used to form conductive layer 28.
~n alloying material can be chosen for its effects as a dopant, as well as its ability to lower junction f~rr~~tinn 25 t~ rAtl~re~ Binary 9ygtems other than the silicon ~1llm;n--m 9y9tem could algo be uged to form the iunction_.
For example, antimony can be used to form np ~l~nrt;rnq in p-type silicon particles.
If ,m, trix layer 14 is formed from a rPmPntit;o~lq 30 material other tha~ a MDFC, it will typically have pores that can fill with a conductive ~-tPr;Al during prorPqqing and cause short circuits between conductive layers 28 and 3C. Such short circuits can be prevented by filling the pores using an electrodepositio~ proces~ and then 217 7 ~ 9 ~ PCTIUS94/13810 1~
nl~;rl;7;n~ or ;~nml;7;ng the exposed top surface of the deposited m~t~r;i~l to form an inq~ tin~ layer. Short circuits through voids in concrete layer 20 caused by imperfect deposition of the matrix material can be 5 similarly prevented. Shorted photovoltaic particles 12, i . e ., those in which both p- type region 22 and n-type region 24 contact the same conductive layer 28 or 30, can be isolated by an ~nnri;7~t;nn process, similar to that described in U.S. Pat. No. 5,192,400 to Parker et al. for 10 "Method of Tqol~tinrJ Shorted Silicon Spheres." The resistivity between cnn~lllrt;ve layers 28 and 30 across concrete layer 20 is preferably greater than 250 n/cml to ensure a suf f iciently small leakage current .
After the pn junotions are formed, tr:~nql Uc~nt 15 conductive layer 30 is formed, for example, by depositing a layer of indium tin oxide, preferably apprn~rim~t~ly 5 ~an in thickness, onto major surface 18. ~etallic digitated electrode 32, deposited on translucent conductive layer 30 by known terhn; ~q, exhibits lower resistivity than the 20 indium tin oxide of tr~nql~lc~nt rnn~l~lrtive layer 30 and, therefore, reduces the electrical resistance between photovoltaic particles 12 and an electrical load (not shown~ driven by photovoltaic cell 10 by effectively reducing the sheet registance of translucent cnn~lllct;ve 25 layer 30. The area covered by digitated electrodes 32 is sufficiently small so that the increase in efficiency caused by the decreased electrical registivity i9 greater than the decrease in ef f iciency caused by blocking some of the inri~ nt light 34. Optionally, protective M~FC
30 layer 42 can be applied over digitated electrode 32 and translucent conductive layer 30, and a second protective MDFC layer can be applied over conductive layer 28.
Although photovoltaic cell 10 could be produced as wide strips with arbitrary lengths, such a WO 9~/16279 PCT/US94/13810 ~177~3 i conf iguration would regult in a low voltage, large current device. It would be preferable to configure multiple panels o~ photovoltaic cell l0 in a series to increase the voltage output. One method of series comlecting 5 photovoltaic cell l0 would be to ~qqPmhle them as roofing shingles, with cnn~lllrtive layer 28 of each course electrically connected to the nnn~ t;ve layer 30 of the subsequent course, so that the voltage dif f erence between each course and the f irst course increases with each l0 sllhcp~lpnt course.
Photovoltaic cell l0 has s~ff;riPnt structural strength and is stlff;riPntly robust that it can be used on the outside of structures, 8uch as concrete hll;lfl;n~q, concrete railroad ties, and roof s, with no cover ~lass or 15 other support structures. The rigidity of a typical photovoltaic cell l0 Pnh~nnps its usefulness as a bll; 1 tl;
material when compared to prior art, such as ~
foil-matrix cells. Photovoltaic cell l0 is, therefore, ;nPYrPnqive to install a~d requires little or no 20 r-;ntpn~nne Care must be taken, however, when photovoltaic cell l0 is installed onto structures of convPntion~l concrete that the water content of the convPnt; nn~ 1 concrete does not corrode the aluminum of photovoltaic cell l0 . This can be ~rcn-~rl; ChP-l by placing 25 a layer of MDFC between clllm;mlm and the ConvPntinn~l concrete .
It will be ob~ious that many changes may be made to the above-described details of the invention without departing from the underlying pr;n~;rlPq thereof. For 3C example, pn junction 26 can be formed by methods other than those described. A photovoltaic cell can also be constructed using other junctio~ types, such as a Schottky barrier junction, a heterojunction, or a metal-in~ tor semiconductor junction, in place of a pn junction. A

WO 95/16Z7g PCTIUS94113810 21774~

Schottky barrier junction can be formed, for example, at the interface between the silicon o~ a photovoltaic particle 12 and the indium tin oxide of a translucent conductive layer 3 0 . An electrochemical or galvanic cell 5 can also be constructed using the principles of the present invention. In such a cell, corlcrete layer 20 ig immersed in a fluid that electrochemically stores energy from ' nt~ nt light 34 . The scope of the present invention should, therefore, be rl~tPrmln~i only by the lO following claimg.

Claims

1. A concrete solar cell, comprising:
a cementitious layer having first and second major surfaces;
a photovoltaic material embedded in the cementitious layer and extending beyond the first and second major surfaces;
a first electrically conductive material juxtaposed to the first major surface and electrically contacting the photovoltaic material; and a second electrically conductive material that transmits a portion of incident light, the second electrically conductive material being juxtaposed to the second major surface and electrically contacting the photovoltaic material.

2. The concrete solar cell of claim 1 in which the cementitious layer is of a calcium aluminate, Portland cement, or pozzolanic type.

3. The concrete solar cell of claim 1 in which the cementitious layer comprises a cementitious material having a flexural strength of greater than 10 MPa.

4. The concrete solar cell of claim 3 in which the cementitious layer comprises a macrodefect free cement material.

5. The concrete solar cell of claim 3 in which photovoltaic material includes silicon, silicon carbide, or gallium phosphide.

6. The concrete solar cell of claim 3 in which the photovoltaic material includes a pn junction.

7. The concrete solar cell of claim 3 in which the photovoltaic material includes a Schottky barrier junction.

8. The concrete solar cell of claim 1 in which some atoms from the first conductive material are dissolved in the photovoltaic material, thereby altering its electrical characteristics.

9. The concrete solar cell of claim 1 in which the photovoltaic material includes p-type and n-type semiconductor silicon forming a pn junction, the first conductive material includes aluminum, and the p-type material includes aluminum-doped silicon.

10. The concrete solar cell of claim 1 in which the first and second conductive material comprise a fluid for electrochemically storing energy.

11. A method of manufacturing a concrete solar cell, comprising:
providing particles, of a photovoltaic material?
embedding the particles of photovoltaic material in a cementitious layer so that the particles extend beyond major surfaces of the cement layer; and providing electrical contacts to the photovoltaic material.

12. The method of claim 11 in which embedding the particles in a cementitious layer includes embedding the particles in a layer of macrodefect free cement.

13. The method of claim 11 in which embedding the particles in a macrodefect free cement layer includes calendering the cement layer.

14. The method of claim 11 in which providing electrical contacts to the photovoltaic material includes:
contacting the material photovoltaic material with an alloying material to reduce the melting temperature of the photovoltaic material; and melting the photovoltaic material with the alloying material to change the electrical characteristics of the photovoltaic material.

15. The method of claim 14 in which the photovoltaic material includes silicon and the alloying material includes aluminum.

16. The method of claim 11 in which:
providing particles of a photovoltaic material includes providing particles of semiconductor silicon;
providing electrical contacts to the photovoltaic material includes forming an electrical contact with an aluminum conductive layer and forming a pn junction in the photovoltaic material by an aluminum-silicon eutectic process.

17. The method of claim 12 in which providing particles of a photovoltaic material includes comminuting photovoltaic materials to produce particles slightly larger than the thickness of the macrodefect free cement-layer.

18. The method of claim 17 in which providing particles of a photovoltaic material includes increasing the minority carrier lifetime of said photovoltaic material.

19. The method of claim 18 in which increasing the minority carrier lifetime includes providing a lubricant to reduce dislocations while comminuting the photovoltaic material.

20. The method of claim 18 in which increasing the minority carrier lifetime includes annealing the particles.

21. The method of claim 18 in which increasing the minority carrier lifetime includes etching the particles.

22. The method of claim 18 in which increasing the minority carrier lifetime includes forming a layer on the particles.

23. A method of forming a concrete solar cell, comprising:
providing a high-resistivity p-type photovoltaic material;
activating an electrically inactive material in the photovoltaic material to convert the p-type photovoltaic material to an n-type photovoltaic material;
juxtaposing a layer of a second material to the surface of the photovoltaic material to form an interface between the photovoltaic material and the second material;
and forming a pn junction within the photovoltaic material by melting and resolidifying the photovoltaic material and the second material at their interface, whereby atoms from the second material dope the photovoltaic material.

24. The method of claim 23 in which the photovoltaic material includes n-type silicon, the second material includes aluminum, and forming a pn junction includes a 577°C aluminum-silicon eutectic process.