Power Sources for Operation of Spacecraft Systems: Photovoltaics (PV) Fundamentals

The photovoltaic (PV) effect occurs when two dissimilar materials form a junction that generates electrical potential when illuminated with photons.
- Electron and Hole Generation: Photon absorption transfers energy to electrons, creating charge carriers (electrons and holes) that separate at the junction, forming a voltage.
- Charge carriers are accelerated by the electric field, producing current if the circuit is closed.
- Power generation is the product of voltage and current.
- Unconverted Power: Unconverted power increases cell temperature and dissipates energy as heat.

Semiconductors: Understanding the roles of p-type and n-type semiconductors is foundational for PV technology.
- A basic p/n junction is integral to the operation of solar cells.
- Efficiency of a solar cell is a critical performance metric.
- Voltage-current (I-V) and power curves illustrate cell performance.

Periodic Table Reference
- Silicon (Si): Group IV
- Boron (B): Group III (used for p-type doping)
- Phosphorus (P): Group V (used for n-type doping)
Types of Doping in Silicon:
- p-Type Silicon (Si:B):
- Doping with boron creates an excess of holes in the structure.
- Boron has three valence electrons and bonds with three neighboring silicon atoms, leaving an unsatisfied bond which creates holes.
- n-Type Silicon (Si:P):
- Doping with phosphorus results in excess electrons.
- Phosphorus has five valence electrons and bonds with four neighboring silicon atoms, leaving one extra unbound electron, which enhances electrical conductivity.

Movement of Carriers:
- In p-type material, holes move while negatively charged acceptor ions remain fixed.
- In n-type material, electrons move while positively charged donor ions remain fixed.
Behavior of Doped Silicone:
- When a boron atom accepts an electron, it becomes negatively charged (B⁻) – a fixed negative ion in the lattice.
- When a phosphorus atom donates an electron, it becomes positively charged, maintaining overall charge neutrality in n-type silicon.

Built-in Electric Field:
- The p-n junction creates a built-in electric field essential for the photovoltaic effect.
- When p-type and n-type materials come into contact, diffusion of electrons and holes occurs due to concentration gradients.
- Electrons from the n-type region diffuse into the p-type region, while holes from the p-type region diffuse into the n-type region.
- This diffusion leads to a depletion region where fixed ion charges remain, establishing an electric field.
Equilibrium and Charge Dynamics:
- Once in equilibrium, the built-in electric field acts on the electrons and holes, pulling them towards opposing charges.
- In the depletion region, positively charged donor ions and negatively charged acceptor ions remain fixed, leading to the establishment of a built-in potential that separates photo-generated electron-hole pairs.

Energy Band Diagram:
- Visualizes the alignment of Fermi levels of p-type and n-type semiconductors at equilibrium.
- The Fermi level represents the energy level at which the probability of finding an electron is 50% at absolute zero temperature.
- In p-type materials, the Fermi level is close to the valence band; in n-type materials, it is closer to the conduction band.
Key Steps in Solar Cell Operation:
1. Generation of electron-hole pairs under illumination.
2. Separation of electrons and holes at the junction due to the built-in electric field.
3. Collection of electrons and holes at the terminals for power generation.

Irradiance and Efficiency:
- Photovoltaic cells are typically tested under air mass zero (AM0) conditions representing outer space.
- Space-qualified efficiency is crucial; n-on-p silicon solar cells outperform p-on-n designs due to enhanced radiation tolerance and minority carrier longevity.
Energy Conversion Efficiency (A0 or PCE):
- Defined as: $\eta = \frac{P_{electrical~power~output}}{solar~power~impinging~on~the~cell}$
- Not all absorbed energy is converted into electricity, leading to efficiencies significantly less than 100%.
- Example: A blue photon (3 eV) generates approximately 0.5 eV of electricity, losing 2.5 eV as heat during the process.
- Additional energy is lost to reflection as well.
Triple-Junction Solar Cells:
- Composed of GaInP₂/GaAs/Ge stacked on germanium substrates, achieving energy conversion efficiencies of ~32% at the beginning of life.
- Each junction captures different segments of the solar spectrum (short wavelengths, visible/near IR, and longer wavelengths).
- Significantly reduces parasitic losses and optimizes current densities.

Conducting Mesh: A thin mesh collects current while allowing light to penetrate
- The spacing of the mesh must balance conductivity and light blockage.
Anti-Reflective Coating: The cell’s front face often has an anti-reflective coating to minimize reflection and maximize light absorption.

I-V and P-V Performance:
- Short-Circuit Current (Isc): Current under short-circuited conditions at full illumination; approximates photocurrent (Is).
- Open-Circuit Voltage (Voc): Maximum photovoltage when no current is drawn.
- The I-V curve is critical for characterizing solar cell performance under various conditions.
Steady-State Equivalent Circuit:
- Represents a solar cell as a constant current source shunted by a diode.
- The series resistance (Rₛ) hinders current flow and is caused by material resistivity.
- Shunt resistance (Rsh) represents leakage currents across the junction.

Factors affecting performance:
- Rsh ideally should equal infinity (no leakage), and Rₛ should be zero (no resistance).
- Both Rsh and Rₛ are temperature-dependent, impacting output and efficiency during high-temperature conditions.

A technique essential for maintaining optimum operation at variable sunlight conditions.
- The MPPT dynamically adjusts voltage and current from the panels to optimize power output, critical for solar charging systems.

Provide important information to design engineers, including:
- Typical electrical parameters like current and voltage metrics.
- Radiation degradation factors, thermal properties, dimensions, and weights.
- Identifies the junction material used.

Understanding PV fundamentals, including the behaviour of p/n junctions, energy band dynamics, operational conditions, and efficiency evaluation, is vital for effective utilization and advancement of photovoltaic technologies, especially in spacecraft systems where reliability and performance under varying conditions are paramount.