2.1 PV Cell Technologies and Manufacturing

PV Cell Technologies and Manufacturing

Photovoltaic (PV) cells represent the foundational energy conversion units of solar electric systems. Designing, installing, and commissioning solar arrays requires a rigorous technical understanding of semiconductor physics, atomic doping, photon absorption mechanisms, and cell architectures. The solar industry has rapidly transitioned from legacy aluminum back surface field (Al-BSF) and standard monocrystalline cells to advanced passivated and heterojunction architectures. Evaluating cell technologies enables solar professionals to optimize system energy yield, minimize thermal losses, mitigate degradation mechanisms, and match module specifications to site-specific environmental conditions.

Semiconductor Physics and the P-N Junction

At the atomic scale, crystalline silicon (c-Si) is a tetravalent semiconductor, meaning each silicon atom shares four valence electrons in a tetrahedral diamond cubic crystal lattice. In an intrinsic (pure) silicon crystal at absolute zero, all valence electrons are locked into covalent bonds, leaving the conduction band empty and rendering pure silicon an electrical insulator.

Bandgap Energy and Photon Absorption

The energy differential between the top of the valence band and the bottom of the conduction band is known as the bandgap energy ($E_g$). For crystalline silicon at room temperature ($300\text{ K}$), the bandgap is approximately $1.12\text{ electron-volts (eV)}$.

When incident sunlight strikes a silicon solar cell, incoming photons interact with the semiconductor material through three primary pathways:

1. Absorption and Electron-Hole Pair Generation ($h\nu \ge E_g$): When an incoming photon possesses energy equal to or greater than the silicon bandgap ($h\nu \ge 1.12\text{ eV}$, corresponding to wavelengths $\lambda \le 1107\text{ nm}$), the photon transfers its energy to a valence electron. This excites the electron across the forbidden bandgap into the conduction band, leaving behind a vacant state called an electron vacancy or hole in the valence band. This event creates a mobile electron-hole pair (EHP).
2. Thermalization Losses ($h\nu > E_g$): When a high-energy photon (such as ultraviolet or blue light) strikes the cell with energy significantly exceeding $1.12\text{ eV}$, the excess kinetic energy ($h\nu - E_g$) cannot be converted into electrical potential. Instead, the excited electron rapidly relaxes to the conduction band edge, dissipating the excess energy as lattice vibrations (heat). Thermalization represents one of the largest fundamental losses in single-junction PV cells.
3. Sub-Bandgap Transmission ($h\nu < E_g$): Photons with energy less than $1.12\text{ eV}$ (wavelengths in the infrared spectrum beyond $1107\text{ nm}$) lack sufficient energy to excite an electron. These photons pass directly through the silicon without being absorbed, generating zero electrical carriers.

Doping: N-Type vs. P-Type Silicon

To establish a functional solar cell, intrinsic silicon is intentionally altered through chemical doping to create regions of differing electrical polarity:

• P-Type Doping (Boron or Gallium): Silicon is doped with a trivalent element from Group III of the periodic table, traditionally Boron ($B$) or increasingly Gallium ($Ga$). Because Group III atoms possess only three valence electrons, their incorporation into the four-coordinate lattice creates an electron deficiency or positive hole. In p-type silicon, holes serve as the majority charge carriers, while electrons are the minority carriers.
• N-Type Doping (Phosphorus): Silicon is doped with a pentavalent element from Group V of the periodic table, typically Phosphorus ($P$). Phosphorus atoms contribute five valence electrons; four participate in covalent bonding with neighboring silicon atoms, leaving the fifth electron loosely bound and free to enter the conduction band at room temperature. In n-type silicon, electrons serve as majority carriers, and holes are minority carriers.

Formation of the Depletion Region and Built-in Field

When p-type and n-type silicon layers are brought into intimate contact, a P-N junction forms:

1. Carrier Diffusion: Free electrons from the n-type side spontaneously diffuse across the interface into the p-type side to recombine with available holes, while holes diffuse from the p-type side into the n-type side.
2. Space-Charge Region (Depletion Zone): As mobile carriers recombine near the interface, they leave behind fixed, uncompensated ionized dopant atoms: positively charged donor ions ($P^+$) on the n-side and negatively charged acceptor ions ($B^-$ or $Ga^-$) on the p-side. This region becomes completely depleted of free, mobile charge carriers.
3. Built-in Electric Field: The uncovered positive and negative ions create an internal electrostatic field oriented from the n-side toward the p-side, establishing a contact potential of approximately $0.6\text{ to }0.7\text{ V}$ in crystalline silicon.

When incident light generates electron-hole pairs within or near the depletion zone, this built-in electric field sweeps the photogenerated minority electrons across the junction to the n-type side and repels the photogenerated holes toward the p-type side. This spatial separation prevents carrier recombination and drives a continuous photocurrent ($I_{ph}$) through an external electrical load.

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Comparison of Solar Cell Technologies

Modern commercial solar modules are fabricated using diverse cell architectures, each offering distinct trade-offs between conversion efficiency, temperature coefficients, degradation susceptibility, and manufacturing cost.

Monocrystalline Silicon Architectures

Monocrystalline silicon (mono c-Si) wafers are sliced from single, continuous cylindrical ingots grown via the Czochralski (Cz) process. Because the entire wafer exhibits an unbroken crystal lattice with zero grain boundaries, monocrystalline cells demonstrate superior electron mobility and commercial efficiencies.

• P-Type PERC (Passivated Emitter and Rear Cell): For years the industry workhorse, PERC enhances standard silicon cells by applying a dielectric passivation film (typically aluminum oxide $Al_2O_3$ capped with silicon nitride $SiN_x$) across the rear cell surface. Microscopic laser contact openings penetrate this dielectric film to allow local metal contact. The dielectric layer serves two vital functions: it drastically reduces rear-surface carrier recombination velocity, and it reflects unabsorbed light back through the silicon wafer for a secondary absorption opportunity. Commercial PERC module efficiencies average $20.0\%\text{ to }21.5\%$. However, traditional boron-doped PERC cells suffer from light-induced degradation (LID).
• N-Type TOPCon (Tunnel Oxide Passivated Contact): TOPCon represents the modern standard for utility and distributed generation. Built on n-type silicon wafers, TOPCon introduces an ultra-thin silicon oxide tunnel layer ($\approx 1.5\text{ nm}$) combined with a heavily doped polycrystalline silicon ($poly\text{-}Si$) rear surface layer. The tunnel oxide is thin enough to allow majority carriers (electrons) to quantum-tunnel through to the electrical contact while physically blocking minority carriers (holes), virtually eliminating surface recombination. Because n-type silicon contains zero boron, TOPCon is completely immune to boron-oxygen light-induced degradation. TOPCon modules achieve commercial efficiencies of $22.0\%\text{ to }23.5\%$, deliver higher open-circuit voltages ($V_{oc} > 710\text{ mV}$), and exhibit an outstanding temperature coefficient of power ($\approx -0.30\%/^\circ\text{C}$).
• N-Type Heterojunction Technology (HJT / SHJ): HJT merges crystalline and thin-film physics by sandwiching an n-type monocrystalline silicon wafer between ultra-thin layers of intrinsic (undoped) amorphous silicon ($a\text{-}Si:H$) and doped amorphous silicon films, capped by a transparent conductive oxide (TCO) layer. The intrinsic amorphous silicon provides nearly flawless surface passivation, yielding ultra-high open-circuit voltages exceeding $740\text{ mV}$ and commercial module efficiencies of $22.5\%\text{ to }24.0\%$. HJT exhibits an industry-leading temperature coefficient of power ($\approx -0.26\%/^\circ\text{C}$) and near-perfect bifacial symmetry ($90\%\text{ to }95\%$). Its primary constraints are higher manufacturing capital expenditures and a low thermal processing threshold ($<200^\circ\text{C}$), which requires low-temperature soldering alloys.

Polycrystalline Silicon (Poly c-Si)

Polycrystalline (multicrystalline) wafers are cast by melting silicon fragments in ceramic crucibles and allowing them to solidify into blocks containing countless individual crystallites. The boundaries between adjacent crystals create structural defects that trap charge carriers and accelerate recombination. While historically dominant due to lower manufacturing costs, poly-Si modules feature modest efficiencies ($16.5\%\text{ to }18.0\%$) and poor temperature coefficients ($\approx -0.38\%/^\circ\text{C}$ to $-0.40\%/^\circ\text{C}$), making them largely obsolete in modern installations.

Thin-Film Photovoltaic Technologies

Thin-film modules deposit semiconductor absorber layers measuring only a few micrometers in thickness directly onto glass, metal, or polymer substrates:

• Cadmium Telluride (CdTe): CdTe is a direct-bandgap semiconductor ($E_g \approx 1.44\text{ eV}$) that matches the solar spectrum with an exceptionally high optical absorption coefficient. Over $99\%$ of incident sunlight is absorbed within the first 1 to 2 micrometers of material. CdTe modules demonstrate commercial efficiencies of $18.5\%\text{ to }19.5\%$, low temperature coefficients ($\approx -0.32\%/^\circ\text{C}$), superior low-light spectral response, and the lowest embodied carbon footprint of any commercial technology.
• Copper Indium Gallium Diselenide (CIGS): CIGS features a direct, tunable bandgap ranging from $1.0\text{ to }1.7\text{ eV}$. While achieving laboratory efficiencies over $23\%$, commercial modules typically reach $14.5\%\text{ to }17.0\%$. CIGS is valued for flexible, lightweight modules and building-integrated PV (BIPV), though it requires stringent barrier seals to prevent moisture-induced degradation.
• Amorphous Silicon (a-Si): A disordered, non-crystalline silicon alloy characterized by high absorption but low carrier mobility. Amorphous silicon suffers from significant light-induced degradation known as the Staebler-Wronski Effect, stabilizing at commercial efficiencies of only $6.0\%\text{ to }10.0\%$. It is primarily confined to small consumer devices and specialized architectural glazing.

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Modern Module Architectures: Half-Cut Cells and Bifaciality

Beyond basic semiconductor chemistry, mechanical and structural module architectures play a vital role in real-world electrical performance.

Half-Cut (Split-Cell) Architecture

Traditional modules utilized full square or pseudo-square cells interconnected in a continuous series string. Modern modules employ laser cutting to cleave standard wafers (such as M10 $182\text{ mm}$ or G12 $210\text{ mm}$) into two equal halves:

1. Current Halving: Because photogenerated current is directly proportional to cell surface area, halving cell size cuts cell operational current exactly in half ($I_{half} = I_{full} / 2$).
2. Resistive Loss Reduction: Internal resistive power losses across cell busbars, contact fingers, and interconnect ribbons follow Joule's Law: $P_{loss} = I^2 R$ By reducing current by half, internal resistive dissipation drops to one-quarter: $P_{loss,half} = \left(\frac{I}{2}\right)^2 R = \frac{1}{4} I^2 R$ This $75\%$ reduction in internal series resistance losses delivers a net module power gain of $5\text{ to }10\text{ W}$ under identical irradiance.
3. Split Junction Box and Shading Tolerance: Half-cut modules divide the internal circuit into two independent, parallel upper and lower sub-arrays serviced by three decentralized junction boxes containing bypass diodes. When inter-row shading, snow accumulation, or roof parapets cast shade across the lower half of a portrait-oriented module, the top sub-array continues producing $100\%$ of its rated output. In a traditional full-cell module, identical shading would reverse-bias all three diode strings, collapsing total module output to zero.

Bifacial PV Modules and Ground Albedo Gain

Bifacial modules replace traditional opaque backsheets with either tempered glass on both sides (dual-glass / glass-glass) or specialized transparent backsheets, enabling the rear surface of the cell to capture reflected and diffuse irradiance.

• Bifaciality Factor ($\phi$): The ratio of rear-side maximum power to front-side maximum power when tested independently under Standard Test Conditions: $\phi = \frac{P_{mp,rear}}{P_{mp,front}} \times 100\%$ Typical values range from $65\%\text{ to }75\%$ for P-type PERC, $80\%\text{ to }85\%$ for N-type TOPCon, and $90\%\text{ to }95\%$ for Heterojunction (HJT) cells due to their fully symmetrical wafer structure.
• Albedo Gain: Ground surface reflectance—the albedo ($\rho$)—determines the quantity of optical radiation available to the rear cell face. Ground albedo varies dramatically by surface:
  • Asphalt / Dark Shingles: $10\%\text{ to }15\%$
  • Green Grass / Native Soil: $15\%\text{ to }20\%$
  • Concrete / Weathered Gravel: $25\%\text{ to }35\%$
  • Crushed White Rock / Calcite: $50\%\text{ to }60\%$
  • White TPO / PVC Cool Roof Membrane: $60\%\text{ to }80\%$
  • Fresh Snow Cover: $75\%\text{ to }90\%$
• System Design Factors: Rear-side energy generation increases overall module yield by $5\%\text{ to }25\%$. Achieving maximum albedo gain requires optimizing ground clearance height, ground cover ratio (GCR), and single-axis tracker torque tube spacing to avoid rear structural shading.

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Photovoltaic Cell Technology Comparison