Doping turns purified silicon into p-type by adding boron, which leaves mobile holes, or into n-type by adding phosphorus, which supplies mobile electrons, and the junction between the two is what makes a solar cell. It only works if the silicon is cleaner than the dopant amount: 99.99% silicon still carries 100 parts per million of impurity, 100 times the carrier’s 1 part per million of dopant, so controlled doping is unrealistic at the purity a village can reach.
Part of Solar Panel Construction
What doping does in a solar cell, the dopant sources and recipes in the corpus, and why the purity of the starting silicon limits it.
Boron Makes Holes And Phosphorus Makes Electrons
Pure silicon conducts poorly. Adding Group III atoms such as boron gives each atom a missing electron, a mobile hole: p-type (P-Type Doping). Adding Group V atoms such as phosphorus gives each atom a spare electron: n-type (N-Type Doping). Where p-type and n-type touch, a junction forms, and light falling near it drives current (P-N Junction).
The carrier’s recipe has two stages. First, melt purified silicon, add a measured trace of boron from borax, stir and cast an ingot, so the bulk is p-type. Second, put wafers in a sealed chamber at 850–900°C, introduce phosphorus vapour from red phosphorus heated separately or from phosphoric acid vapour, hold 15–30 minutes, and cool slowly; phosphorus diffuses into the top 0.2–0.5 µm to make the n-type layer. Junction Formation gives other sources: boron from BBr₃ at 950–1100°C, and P-Type Doping 900–1000°C, neither of which is a village chemical.
The Purity Arithmetic Undermines The Recipe
The carrier says doping levels are roughly 1 atom per million silicon atoms, while Doping Techniques gives 1 part in 10 million to 1 in 1,000. Silicon with 99.99% purity (the carrier’s target) holds 0.01% impurity, which is 100 ppm. Compared with a dopant at 1 ppm, the impurity is 100 times larger; against the low end of 0.1 ppm, it is 1,000 times. Whether this matters depends on which impurities they are: those that are electrically inactive do little harm, but the 99.99% figure does not say which they are. Metallurgical silicon at 98–99% holds 10,000–20,000 ppm.
What You Need
- Purified silicon — Silicon Purification
- A boron source — borax, per the carrier, not purified for this use
- A phosphorus source — bone ash or phosphate rock (phosphorus from bone is covered in Phosphorus Sources as a fertilizer nutrient, not as a doping source)
- A sealed furnace holding 850–900°C, and a way to introduce a vapour
- Measuring tools — a four-point probe or resistance meter (Silicon Purification describes the four-point probe)
Method: What The Corpus Supports
Step 1: Establish the starting purity by measuring resistivity. Pure silicon is about 240,000 Ω·cm; if yours reads 1–100 Ω·cm it is heavily contaminated or doped by background impurities (Silicon Purification).
Step 2: Dope the bulk by adding a trace of boron to a melt and casting the ingot, as in the carrier.
Step 3: Diffuse the n-type layer at 850–900°C for 15–30 minutes and cool slowly.
Step 4: Test the type. The corpus describes a hot-point probe test but the polarity of its reading depends on how the meter is connected, so calibrate on a known n-type and a known p-type sample (for example a salvaged cell or diode) rather than relying on a written sign.
Step 5: Test the junction as a diode: it should conduct one way only (carrier), and a forward-to-reverse resistance ratio above 100:1 is usable (Doping Techniques).
Worked Example: Dopant Parts Per Million In Atoms Per Cubic Centimetre
Silicon has about 5.0 × 10²² atoms per cm³ (general knowledge, not from the other articles). So 1 ppm is 5.0 × 10²² × 10⁻⁶ = 5.0 × 10¹⁶ atoms per cm³. The N-Type Doping and P-Type Doping articles quote resistivities for 10¹⁴ per cm³ (46 Ω·cm n-type), 10¹⁵ per cm³ (4.6 Ω·cm n-type and about 13 Ω·cm p-type). 10¹⁵ per cm³ is 10¹⁵ ÷ 5.0 × 10²² = 2 × 10⁻⁸, or 0.02 ppm, which is 20 parts per billion. Controlled doping at the level those articles work with therefore needs impurity control about 5,000 times tighter than 99.99% purity gives (100 ppm against 0.02 ppm).
Common Mistakes
- Doping before purifying — background impurity sets the type, not your dopant
- Taking the carrier’s 1 ppm as a measured recipe — the sources give a wide range (0.1 ppm to 1,000 ppm)
- Heating red phosphorus in a closed vessel — see the safety note
- Trusting a hot-probe sign from memory — calibrate on known samples
Safety
Phosphorus vapour and borax dusts, a hot furnace and a sealed chamber
Heated phosphorus gives off toxic fumes and ignites in air, and phosphine from phosphorus compounds is highly toxic (general knowledge, not from the other articles). A sealed chamber at 850–900°C that is heated without a vent can burst. Work outdoors or under strong extraction, keep the chamber vented to a safe place, wear eye protection and gloves, and keep ignition and water sources away from phosphorus. The corpus gives no dosing or handling quantities for phosphorus, so treat this step as one to avoid.
What’s Next
- Learn next: Wafer Cutting — slicing the doped ingot
- Combine with: Junction Formation — the junction step in detail