Design of Solar CellsD.G. Ast • Crystalline Si solar cell • Amorphous Si solar cell The first is an example of a classical “bulk”cell and the second one the prototypical “thin” film solar cell. If I don’t get to it but you can ask me after class
The classical solar cell always has has an n+ top layer (“emitter”) and a much thicker p type layer (“base”). The terms were carried over from the bipolar transistor that preceded the solar cell
The parameters here are 1. Material 2. Doping concentrations (emitter and base) 3. The Generation rate (a function of the sunlight intensity) 4. The area 5. The sheet resistance of the emitter, Rs 6. The external load 7. The operating temperature (Eg decreases with increasing T)
G is the generation rate Lh,Le are the minority carried diffusion length Dh,De are the corresponding diffusion coefficients. Relation to materials properties L = D D = (kT/q) is a function of doping. It’s not very sensitive to impurities. Typical D’s are 50 cm2/s . However the life time can vary from 10-9 sec to milliseconds, depending on how clean the material is.
Lets consider the cell step by step 1. Front grid
1. Design consideration, front grid 1. Large cross section for low ohmic resistance (Rohmic lowers fill factor) 2. Minimum area for minimum shading. 3. Maximum conductivity in the emitter layer That is maximum doping, maximum thickness. Both . interfere with it’s collection efficiency (later) The trade off is well understood and M. Green’s book has the basic design tools.
Parameters : Current density and voltage at maximum FF (Fill Factor), sheet resistance of emitter, sheet resistance of busbar and fingers.
ECE’s learn more about contacts: 1.transfer length and 2.current crowding but at the high doping of the emitter we can neglect it Note that the optimized solution nearly balances ohmic losses in the emitter and the contact (8.6%) vs shading (10.6%)
Engineering work around # 1 Still some shading….
Engineering work-around # 2: Martin Green’s record cell. The grid deflects light into a light trapping structure
3. Engineering work around # 3 (An other M.Green invention !) The metal line is very narrow but very high (aspect ration w/h<<1), theopposite of a conventional cell. Economically feasible because of laser grooving ! In production by BP solar (licensed from M. Green)
Engineering work around # 4: Dick Swanson’s rear contact solar cell
Dick Swanson Martin Green
Moving on ... 2. The anti-reflex coating
2. Design considerations AR coating Si has a refractive index of ~ 4 (wavelength dependent) It is therefore has a reflectivity of ~ 30…35 % Engineering Solution (~ 1932.. Zeiss) An intermediate layer of n ~ 2 that cancels the air-AR and AR/Si reflection
The design rules are well understood. Optimum n of AR is nintermediate = nsilicon
Limitations of single layer AR 1. Zero reflection possible for only one wavelength 2. And normal incidence for 1. Note that n=2.3 looks really good for Si under glass !
Multiple AR coatings covering a wide array of wavelength and incident angles are available (T* by Zeiss, e.g.) and used extensively by the optical industry (camera lenses etc). The technique is too expensive to be used in solar cells at present . Zeiss T* coatings on glass. Note <0.1% reflectivity over visible range. Used in expensive eyeglasses. Source: Zeiss Inc
Ww.appliedfilms.com Note: SiNxmade by PECVD has an adjustable n between 1.7 (N rich) and 2.3 (Si rich), is an excellent diffusion barrier, and a copious source of atomic hydrogen when RTA annealed. More on that later. It therefore has become the AR coating of choice.
Engineering work around # 1 Surface patterning: The “black” space cell, employing a chemically etched surface to reduce reflection Note: Emitter profile must follow surface profile: Requires to diffuse after etch
Engineering work-around # 2 Chemically leach glass to generate a smooth transition between n=1 (air) and n= 1.48 (glass) In 2004 The Fraunhofer Institute for Silicate (ISC) research developed a coating solution enabling the production of enhanced solar light transmittance properties. This solution is used to dip low-iron glass panes to coat them with a thin wet film. After drying, these coated panes are put to a temperature treatment to create a wipe-proof weather-resistance layer composed of porous silicon dioxide. This technique was jointly enhanced with Fraunhofer ISE while FLABEG GmbH & Co. KG applied it to a complete industrial process for manufacturing anti-reflex coated glass for solar applications. Finally, MERCK KGaA used the formula for the coating solution for large-scale technical synthesis Note: Investigated in the 1990’s by Corning, but problems with durability, weathering, and dirt removal proved formidable
Moving on to step 3…. 3. The top layer, I.e. the “emitter”
Design considerations: 1. The emitter must be highly conductive to minimize the metallic grid line shading. That means it must be highly doped, and should be thick. Both factors lower the sheet resistance (Ohms/) the current transport controlling parameter and reduce ohmic losses. 2. High doping reduces minority carrier lifetime. Hence reduces photon collection by top layer. 3. Making the emitter very thin minimizes conversion losses in the “dead” emitter layer but, technically, is not easy to do (Lindmeyer’s famous Violet Cell, see www.jjbenergy.orgfor a history of Dr. Lindmeyer)
First bad news : Unfortunately high doping => low = > Low D => low L = D. Remember L must be > than emitter thickness to collect holes.
Second bad news: The rapid decrease in minority carrier life time at high doping is due to Auger recombination. There is nothing we can do… purifying the material will not help. Note: Slope looks like 2, suggesting 1/n2
Auger recombination of an electron hole occurs by transfer excess energy and momentum to an electron in the conduction band or valence band. The more holes or electrons in these bands, the more likely the process. Analysis shows that at high doping it goes with n2.
Estimate for 1019/cm3 n doping Dh ~ 1…2 cm2/s ~ 10-9 sec Lh ~ 10-4…10-5 cm = 0.1 to 1 um Engineering conclusion: To collect any minority carriers from the emitter, we need to make it thinner than 0.5 um => and save the sheet resistancy !
Third. Good news weak absorption may save us !…. To absorb light in a 1 um thick layer you need an of > 106 m-1. Inspection shows that only blue light can do this !
Optimum Design 1. Emitter thickness < 0.5 um or less 2. Highly doped for low s 3. Since contacts are nearby, recombination of minority carriers in the emitter at the contact need to be minimized.
Engineering work around # 1 on contact recombination An ultrathin oxide layer can be tunneled through by majority carriers (electrons) but not by holes. The recombination velocity for most of n+ is low as most of it sees oxide
Engineering workaround # 2 on emitter contact recombination The design concept is to keep most of the metal (required to keep ohmic loss down) away from the n+ layer. Tunneling still exists and contributes.
Basic design considerations • 1. The base must be thick enough to absorb all wavelength present in AM 1.5 spectrum • 2. Minority carriers generated in the base must cross the junction. Thus Lemust be greater than the base thickness • 3. To stop loss of minority carriers we must turn off: • Bulk recombination • Surface recombination • Contact recombination
To catch each photon in Si, it has to be about 300 um thick. For a 100%, catching the most far out IR even 1000 um => 1 mm ! A direct bandgap material, such as GaAs requires only about 1 um ! That is the lure of the thin film cell with direct bandgap materials !
Basic Design Consideration 1. Ln > 300 um 2. Requires L > 300 um > D. 3. As D is pretty much constant at low doping to 30 to 40 cm2/s (see next page ) it follows that > 2.5 10-5sec
Electron minority carrier life time vs doping (close up of previous plot). Design Looks feasible ! As long as we are below 1E17/cc we should be ok (Auger wise)
Basic design consideration 1. There is no electric field in the base as the base doping is uniform. 2. The electron does a random walk 3. The junction is a sink, with boundary condition ce,exces = 0 4. The collection efficiency, hence, falls of with distance x from the p-n junctions as exp(- x/L) If the electron recombines at the contact it is a lost electron !
First consideration: Carrier collection probability Collection probability of a minority carrier as a function of distance from p-n junction. Neglects recombination.
Second : Help is on the way ..most light gets absorbed in top layer The number of collected carriers (neglecting recombination) is the product of generation (drops rapidly with depth because of absorption) and collection probability. Why is the concentration zero at the front and back ? Because of surface recombination
The care and feeding of minority carriers. Suppress recombination ! 1. Recombination in the bulk 2. Recombination at surfaces. Expressed as 1/ = S x cexces (cm/sec x #/cm3 ) Give recombination rate per cm2 of surface 3. Recombination at contacts.
1. Bulk Recombination in the base is dominated by traps: Impurities and crystal defects. But we need cheap material ! The solution is to process the cell in such that the Silicon “improves”
2 Surface recombination All surfaces promote recombination. An analysis shows that for a base a surface recombination velocity < 30 cm/sec is required for surface recombination not to dominate. The only interface that accomplishes that is a high quality Si/SiO2 interface.
3. Recombination at contacts Contacts are highly doped regions. No matter what we do, we will be at the Auger limit. In reality more terrible things can happen (dopant induced misfit dislocations etc….) but we leave it at that.
We now understand ! To tame surface recombination High quality Si. Float zone is best. Low or no doping. Resistivity of > 50 Ohm-c The basic design of the high efficiency cell High quality SiO2
The basic Swanson design (Rear contact cell) Contacts are unavoidable evils. We keep them as small as possible and put them somewhere where they do the least harm. Their optimum size and spacing is an extremely interesting design exercise P+ doped contact N+ doped contact