Engineering Design of P-N Junction Solar Cells & Physics

Engineering Design of P-N Junction Solar Cells

Materials, Physics, Architecture & Electrical Characteristics of Photovoltaic Devices

Semiconductor Engineering | Photovoltaic Systems

Silicon

GaAs

InAs

CdAs

Material Constants & Band Gap Analysis

Si

Silicon

1.12 eV

Most common PV material, dominating the global solar manufacturing industry.

GaAs

GaAs

1.42 eV

High efficiency III-V direct bandgap semiconductor, ideal for space and multi-junctions.

InAs

InAs

0.36 eV

Narrow band gap yielding strong IR absorption, useful for specialized sensors.

CdTe

CdTe (CdAs)

1.50 eV

Leading thin-film candidate offering low cost and excellent optical absorption.

Photon must have energy E_photon ≥ Eg to generate electron-hole pair

Device Architecture — Cross-Section Design

INCIDENT PHOTONS

GLASS WINDOW

Glass Window Layer

Anti-reflective protective layer that optimizes photon transmission.

Nickel Front Contact

Positive terminal (+) laid on the surface edges to minimize shading.

N-TYPE SILICON

N-type Silicon Layer

Ultra-thin, highly doped region favoring rapid electron collection.

Barrier Field

P-N junction depletion region generating internal electric field.

P-TYPE SILICON BASE

P-type Substrate

Thick base layer where the majority of photon absorption occurs.

METAL BACK CONTACT

Back Negative Contact

Negative terminal (−) providing uniform structural grounding.

Thin layers minimize minority carrier recombination before reaching junction.

Device Physics | Cross-Section Analysis

Physical Layout

Current Flow

Dimensional Engineering — Why Thin Layers Matter

The Diffusion Length Problem

1. Thin Region

2. Thick Region

Minority carriers generated far from junction recombine before contributing to current

P and N regions must be kept < diffusion length L = √(D·τ)

Recombination = loss of photogenerated current

Thin layers → higher collection efficiency → higher Isc

L = √(D·τ)

where D = diffusion coefficient, τ = carrier lifetime

Photovoltaic Mechanism

Photon Absorption & Carrier Generation

Photon Incidence

E_photon ≥ E_g = 1.12 eV (Si)

Bond Rupture

Valence electron freed

Pair Generation

Free carriers created

Carrier Separation

E-field sweeps carriers

Energy Band Model

Semiconductor Physics • Quantum Level Process

Sub-bandgap photons (E < E_g) do not generate pairs — absorbed as heat

Junction Dynamics — Depletion Region & Barrier Field

Barrier field sweeps minority carriers across junction

Electrons → N-side, Holes → P-side

Prevents majority carrier flow (barrier)

Built-in Potential

Space Charge Region

Energy Bands

Output Characteristics —

Voc and Isc Under Illumination

Open-Circuit Voltage (Voc)

0.6 V

Voltage across cell terminals when no external load is connected (I = 0)

Voc = (kT/q) · ln(Iph/I0 + 1)

Determined by junction quality and recombination rate

Short-Circuit Current (Isc)

40 mA/cm²

Current when terminals are short-circuited (V = 0)

Directly proportional to incident photon flux and cell area

Isc ∝ irradiance G

AM1.5G, 1000 W/m², 25°C

η = Pmax / Pin = (Voc × Isc × FF) / Pin

V-I Curve Analysis — Maximum Power at the Knee

Voltage V (volts)

Current I (mA/cm²)

P_max = V_mp × I_mp

Maximum Power Point (MPP)

Fill Factor (FF)

FF = P_max / (V_oc × I_sc)

Dark Curve (No Illumination)

Operating at knee =

Maximum power extraction

Photovoltaic Efficiency Analysis

Applications & Engineering Constraints

Key Applications

Satellite Power Systems

High Eg materials (GaAs) preferred for space — radiation hardness, high efficiency at lower temperatures, no atmospheric absorption losses

Terrestrial Solar Farms

Silicon dominates due to low cost, mature fabrication, Eg = 1.12 eV well-matched to AM1.5 solar spectrum

Portable & Consumer Electronics

Thin-film CdTe/InAs used for flexible, low-cost applications

Engineering Constraints & Trade-offs

GaAs:

2× higher efficiency than Si but 100× higher cost

InAs (Eg=0.36eV):

captures IR but high dark current → low Voc

Silicon:

best cost-to-efficiency ratio for terrestrial use

Recombination losses limit practical efficiency (Shockley-Queisser limit ~33%)

Encapsulation & thermal management critical for longevity

Shockley-Queisser Theoretical Limit: ~33% for single-junction cells