LECTURE 6

Tandem Solar Cells: An Overview

Introduction to Tandem Solar Cells

• Tandem solar cells consist of two solar cells stacked together, enhancing overall efficiency by utilizing both top and bottom layers.

• Various types of solar cells have been discussed previously, including silicon, thin film (CDS, CDT, CIGS), dye-sensitized, quantum dot-sensitized, and perovskite solar cells.

Efficiency Challenges in Single Solar Cell Configurations

• The theoretical maximum efficiency for single junction solar cells is around 33%, as indicated by the Shockley-Queisser limit; however, this has not been achieved practically.

• While some semiconductor solar cells like gallium arsenide can reach efficiencies close to 30% in lab settings, they fall short in large-scale applications compared to silicon-based solutions.

Limitations of Current Solar Technologies

• Many existing solar cell technologies do not meet high-efficiency criteria necessary for effective power generation for domestic and industrial use.

• A graphical representation shows that most current solar cell efficiencies are below the theoretical limits established by the Shockley-Queisser model.

Understanding Losses in Solar Cell Efficiency

• Key losses affecting efficiency include thermalization loss (energy lost when electrons emit phonons instead of being absorbed) and transmission loss (loss of photons with energy below the band gap).

• Only a small fraction of incident photons contribute to electricity generation due to these losses; thus addressing these limitations is crucial for improving efficiency.

Potential Solutions: Variable Band Gaps and Tandem Configurations

• One proposed solution involves using materials with variable band gaps; however, fabricating such materials remains challenging.

• The tandem configuration allows two different band gap solar cells to work simultaneously under the same radiation conditions. This multi-junction approach aims to maximize photon absorption across a broader spectrum.

Understanding Multi-Junction Solar Cells

Energy Absorption in Solar Cells

• Higher energy and smaller band gap solar cells can absorb light of lower energy, allowing for more efficient photon utilization.

• A perovskite solar cell with a larger band gap absorbs higher energy blue photons while transmitting lower energy red photons to a smaller band gap solar cell.

• In single junction configurations, 50% of photons are lost as only one type (either red or blue) can be absorbed at a time.

Solar Spectrum Analysis

• The solar spectrum shows that the green band accounts for approximately 53% of irradiance, while red photons contribute around 34%.

• Pink photons have about 6% irradiance and correspond to lower energy levels, relevant for silicon solar cells.

Band Gap and Absorption Characteristics

• Two distinct bands are identified: the red band for lower energy photon absorption and the blue band for higher energy photon absorption.

• This dual-band approach allows multi-junction configurations to absorb both red and blue photons, enhancing overall efficiency.

Multi-Junction Configuration Design

• A multi-junction configuration consists of two solar cells: one with a large band gap (EG large) absorbing blue photons and another with a small band gap (EG small) absorbing red photons.

Tandem Configuration Types

• There are two types of tandem configurations:

• Two-Terminal (2T): Both solar cells are stacked together with integrated connections.

• Disconnected: Each cell has separate electrical connections, reducing potential defects from lattice mismatch.

Advantages and Challenges of Tandem Configurations

• The advantage of the 2T configuration is integration which maximizes space but may introduce defects due to material mismatches.

• Disconnected configurations allow independent operation but require careful management of electrical connections to maintain efficiency.

Solar Cell Configurations and Quantum Efficiency

Overview of Solar Cell Types

• The discussion begins with the comparison between large EG solar cells and small silicon solar cells, emphasizing that both types operate on similar principles.

• Introduction of the 4T (four-terminal) configuration for solar cells, contrasting it with the previously mentioned 2T (two-terminal) configuration.

Mechanism of Light Absorption

• A schematic representation is provided for perovskite and silicon solar cells, highlighting their respective layers including electron transport and hole transport layers.

• The mechanism involves higher energy photons being absorbed by the perovskite layer while lower energy photons pass through to be absorbed by the silicon layer.

Quantum Efficiency Analysis

• A plot illustrates quantum efficiency against wavelength, showing that perovskite solar cells achieve about 85% efficiency in lower wavelength regimes (higher energy).

• As wavelengths increase, quantum efficiency decreases for both cell types; perovskite does not absorb red photons effectively, leading to absorption by silicon instead.

Open Circuit Voltage Values

• Discussion on open circuit voltage (VOC), noting various configurations yield different VOC values. For example:

• Densitized solar cell: VOC ~0.8V

• Perovskite-silicon combination: VOC ~1.74V

Multi-Junction Solar Cells and Their Drawbacks

• Multi-junction configurations can significantly enhance open circuit voltage compared to single junction setups, thus improving overall efficiency.

• However, tandem solar cells face challenges such as reflection losses due to multiple material layers affecting light absorption.

Sources of Losses in Tandem Solar Cells

• Reflection loss is identified as a primary source of power loss in tandem configurations due to several integrated material layers which can reflect incoming light rather than absorbing it effectively.

Understanding Reflection and Parasitic Losses in Solar Cells

Refractive Index Mismatch

• The concept of refractive index mismatch is introduced, highlighting its role in causing reflection loss when light transitions between different layers, such as air and solar cell materials.

Minimizing Reflection Loss

• Anti-reflection coatings are discussed as a method to reduce reflection loss. Additionally, creating microstructural changes or surface roughness on silicon solar cells can help trap light but may lead to structural mismatches with subsequent layers.

Types of Losses in Tandem Solar Cells

• Reflection loss is identified as a significant issue for tandem configurations of solar cells. Parasitic losses are also mentioned, which occur due to absorption by non-active layers within the solar cell structure.

Understanding Parasitic Losses

• Parasitic losses arise from photons being absorbed by various layers instead of the primary absorbing materials (perovskite and silicon). This leads to inefficiencies since the goal is for these materials to absorb most photons.

Reducing Charge Carrier Density

• Lowering charge carrier density can minimize parasitic absorption since interactions between photons and free charge carriers contribute to these losses. However, this approach risks introducing electrical losses due to reduced current flow through the material.

Optimizing Efficiency in Solar Cell Technologies

Balancing Different Types of Losses

• The discussion emphasizes the need for careful material selection and configuration to optimize efficiency while minimizing reflection, parasitic, and electrical losses in tandem or multi-junction solar cells.

Historical Context of Solar Cell Efficiency

• A summary of historical advancements in solar cell technology is presented, showing progress from 1940–1950 onwards. Current efficiencies for crystalline silicon are around 25%.

Future Potential of Perovskite Solar Cells

• Perovskite solar cells show promise with potential efficiencies reaching about 30%, although stability remains a significant concern that needs addressing.

Multi-Junction Solar Cells: High Efficiency Prospects

• Multi-junction solar cells have shown remarkable efficiency improvements, potentially reaching up to 50%. Research continues at laboratory scales with hopes for practical applications on rooftops if integration challenges can be overcome.