Tandem Solar Cells: Working Principle, Structure, Efficiency & Advantages

 

Description 

Learn about tandem solar cells, their working principle, structure, efficiency, advantages, challenges, and future.

Introduction

The theoretical limits of single solar cell are 30%. The William Shockley and Hans Joachim Queisser made discovery about 50 years ago, called it Shockley – Quisser limit. They theoretically calculated that solar panels with one single layer suffer efficiency limitations as they are unable to absorb full solar light. In the tandem cell, two solar cells stack one on top of the other in which top cells are transparent, convert high energy photons into electricity. The bottom cell absorbs lower energy photons. The solar spectrum splits into two layers dramatically reduces thermalization losses. This will enable theoretically efficiency far better than single layer cells.

Operating Principle

In a tandem cell, the incident sunlight passes through the top sub cell first. High energy photons (blue and green wavelengths) are absorbed by the wide band gap perovskite layer, converting them at higher voltage similarly lower energy photons (red and near-infrared) transmitted through the top cell are captured by the silicon bottom cell. This spectrum splitting dramatically reduces thermalization losses in single junction cells.

Structure

The tandem devices are commonly fabricated in either two terminals or four terminals’ configurations. The sub cells are electrically connected in series and same current flows through all layers in the two terminals tandems. This design offers simple module integration and reduced wiring complexity but current matching between top and bottom cells.

The perovskite silicon tandem solar cells have strong efficiency potential. The perovskite layer absorbs high energy protons, while the silicon cell converts the lower energy photons. The structure of Perovskite tandem solar cell and function of each layer is explained here.

 

Structure of Perovskite Tandem solar cell

 

The function of each layer is given here.

 

Layer	Function
Glass substrate	Provide mechanical support and protects solar cell from environment
Anti reflection coating (ARC)	Minimizes light reflection and increases the amount of sunlight entering the cell
Transparent conductive oxide (TCO)	Conducts electrical current, when sunlight pass through it
Electron transport layer (ETL)	Extracts and transports electrons from the perovskite absorber to the electrode while blocking holes.
Perovskite absorber (Top Cell)	Absorbs high energy (short-wavelength) sunlight and generates electron-hole pairs
Hole transport layer (HTL)	Extracts and transports holes from the perovskite layer while blocking electrons
Recombination / Interconnection Layer	Electrically connects the top and bottom cells and enables carrier recombination between them
N type silicon layer	Selectively collects electrons and reduces recombination losses in the silicon cell.
Silicon absorber (Bottom Cell)	Absorbs lower energy (long wavelength) light that passes through the perovskite layer
P type Silicon Layer	Collects holes generated in the silicon absorber and transports them to the electrode
Rear TCO (if present)	Provides a conductive path and improves current collection.

 

Recombination Junction

The junction is the most critical component unique to tandem cells. It must allow electrons flow from the bottom sub cell to recombine with holes from the top sub cell (completing the internal circuit) with minimal optical absorption and electrical resistance. Transparent conductive oxides (ITO, AZO) or ultrathin metallic layers are commonly used.

Current Matching

In a two terminal series connected tandem, the sub-cell generating the lower photocurrent than total output — a condition known as current matching. The perovskite layer thickness and band gap must be precisely tuned so that both sub cells produce equal photocurrents under the AM1.5G (Air mass 1.5 Global) solar spectrum.

AM1.5G* - Air mass is the path length in which light takes through atmosphere normalized to the shortest possible path length to surface when sun is directly overhead. The 1.5 means sun light travelled through path length 1.5 times longer than direct overhead sunlight at sea level (AM 1.0G)

Salient Feature of the Perovskite Tandem Solar Cells

The following are the salient feature of the Perovskite Tandem Solar Cells.

•        The laboratory efficiency as per LONGI (Green energy Technology Company) achieves 33.9% efficiency in 2024.

•        The theoretical efficiency of two junction solar cell roughly 45% to 47% under concentrated sunlight.

•        It can capture both high energy and low energy solar photons

•        The standard monocrystalline silicon solar panel efficiency around 22 – 24% in commercial models. Its efficiency needs to reach around 33 – 36% to produce 50% more efficiency per unit area. An individual solar cell cannot achieve this due to Shockley – Queisser limit caps them at around 29.4%. The tandem solar cells drastically increase power output.

•        The perovkites degraded when exposed to heat, moisture, UV light during fabrication processes.

•        In a monolithic two terminal tandems, the entire device output is limited by lowest performing sub cell.

•        The mismatch of wavelength lead to unused photons resulting significant leakage current. 

FAQs 

What are tandem solar cells?

Tandem solar cells combine two photovoltaic materials to improve efficiency.

How efficient are tandem solar cells?

Laboratory efficiencies exceed 33%, with theoretical efficiencies of about 45–47% under concentrated sunlight.

Why is perovskite used?

It efficiently absorbs high-energy photons while silicon absorbs lower-energy photons.

What are the challenges?

Stability, moisture, UV degradation, and current matching.

Are tandem solar cells the future?

They are among the most promising next-generation solar technologies.