5.2 Quantum dot solar cells and photovoltaics

Quantum dot solar cells harness the unique properties of these nanoscale semiconductors to boost energy conversion. By tuning bandgaps and leveraging multiple exciton generation, they can absorb more light and generate more current than traditional cells.

Various quantum dot solar cell designs exist, from sensitized to heterojunction architectures. While efficiency and stability are improving, challenges remain. Quantum dots also show promise in enhancing existing solar technologies through integration and hybrid approaches.

Quantum Dots for Solar Cell Efficiency

Unique Optical and Electronic Properties

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• Quantum dots have unique optical and electronic properties that make them promising for enhancing solar cell efficiency
• These properties include tunable bandgap, high absorption coefficient, and multiple exciton generation
• The tunable bandgap allows for the absorption of a broader range of the solar spectrum (visible to near-infrared), leading to increased photocurrent generation and improved solar cell efficiency
• Quantum dots exhibit a high absorption coefficient, enabling efficient light harvesting even in thin layers, which reduces material consumption and cost

Luminescent Down-Shifting and Multiple Exciton Generation

• Quantum dots can be used as luminescent down-shifting materials, absorbing high-energy photons (UV) and re-emitting them at lower energies (visible), which can be more efficiently absorbed by the solar cell
• This process reduces thermalization losses and improves the overall energy conversion efficiency
• Multiple exciton generation in quantum dots enables the creation of multiple electron-hole pairs from a single high-energy photon, potentially overcoming the Shockley-Queisser limit and boosting solar cell efficiency
• In multiple exciton generation, a single photon with energy greater than twice the bandgap can generate two or more excitons, increasing the photocurrent and efficiency

Quantum Dot Solar Cell Architectures

Quantum Dot-Sensitized Solar Cells (QDSSCs)

• QDSSCs employ quantum dots as sensitizers on a mesoporous wide-bandgap semiconductor scaffold (TiO2), enabling efficient charge separation and transport
• The quantum dots absorb light and inject electrons into the TiO2 scaffold, while the holes are transferred to a redox electrolyte or hole transport material
• QDSSCs offer advantages such as easy fabrication, low cost, and the ability to tune the absorption spectrum by selecting appropriate quantum dot materials

Quantum Dot Heterojunction Solar Cells

• Quantum dot heterojunction solar cells incorporate quantum dots as the absorber layer in a p-n junction or p-i-n structure, offering the potential for high open-circuit voltage and efficient charge extraction
• The quantum dots are sandwiched between electron and hole transport layers, forming a heterojunction that facilitates charge separation and collection
• Examples of quantum dot heterojunction solar cells include PbS quantum dot solar cells and CdTe quantum dot solar cells

Colloidal Quantum Dot Solar Cells

• Colloidal quantum dot solar cells utilize a thin film of quantum dots as the active layer, deposited using solution-based techniques (spin-coating, inkjet printing), which allows for low-cost and large-area fabrication
• The quantum dots are typically passivated with organic ligands to improve stability and charge transport properties
• Colloidal quantum dot solar cells can be fabricated on flexible substrates, enabling the development of lightweight and portable solar devices

Tandem Quantum Dot Solar Cells

• Tandem quantum dot solar cells combine multiple quantum dot layers with different bandgaps to absorb different portions of the solar spectrum, potentially achieving higher efficiencies than single-junction cells
• By stacking quantum dot layers with decreasing bandgaps from top to bottom, a larger portion of the solar spectrum can be efficiently harvested
• Tandem quantum dot solar cells can be designed to minimize thermalization losses and optimize current matching between the different subcells

Performance and Stability of Quantum Dot Solar Cells

Key Performance Parameters

• The performance of quantum dot solar cells is evaluated using key parameters such as power conversion efficiency (PCE), open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF)
• PCE represents the overall energy conversion efficiency of the solar cell, determined by the ratio of output electrical power to input solar power
• Voc is the maximum voltage generated by the solar cell under open-circuit conditions, influenced by the bandgap and recombination processes
• Jsc is the maximum current density generated by the solar cell under short-circuit conditions, determined by the light absorption and charge collection efficiency
• FF is a measure of the "squareness" of the current-voltage (I-V) curve, indicating the quality of the solar cell

Efficiency Achievements and Stability Challenges

• The PCE of quantum dot solar cells has been steadily increasing over the years, with the highest reported values exceeding 16% for single-junction cells and 25% for tandem cells
• These efficiencies are approaching those of established photovoltaic technologies (silicon, CIGS), demonstrating the potential of quantum dot solar cells
• The stability of quantum dot solar cells is crucial for their practical application and is affected by factors such as air and moisture sensitivity, thermal stability, and photo-stability
• Quantum dots are often sensitive to oxygen and moisture, leading to degradation of their optical and electronic properties over time
• Thermal stability is important for maintaining the performance of quantum dot solar cells under elevated temperatures, as encountered in real-world operating conditions
• Photo-stability refers to the ability of quantum dots to withstand prolonged exposure to light without significant degradation, which is essential for long-term device reliability

Strategies for Improving Stability

• Encapsulation techniques, such as the use of barrier layers (Al2O3, SiO2) and protective coatings (polymers), can significantly improve the stability of quantum dot solar cells under ambient conditions
• These encapsulation layers prevent the ingress of oxygen and moisture, protecting the quantum dots from degradation
• The development of air-stable and environmentally friendly quantum dot materials, such as lead-free perovskite quantum dots (CsPbBr3, FAPbI3), is an active area of research to address the stability and toxicity concerns
• Surface passivation of quantum dots with organic ligands or inorganic shells can also enhance their stability by reducing surface defects and preventing oxidation

Quantum Dots vs Other Photovoltaic Technologies

Integration with Silicon Solar Cells

• Quantum dots can be integrated with various established photovoltaic technologies to enhance their performance and functionality
• The incorporation of quantum dots into silicon solar cells, either as a down-shifting layer or as a luminescent solar concentrator, can improve the efficiency of silicon-based devices
• Down-shifting quantum dots can convert high-energy photons into lower-energy photons that are more effectively absorbed by silicon, reducing thermalization losses
• Luminescent solar concentrators based on quantum dots can concentrate and guide sunlight onto a smaller area of silicon solar cells, increasing the power output per unit area

Hybrid Organic-Inorganic Solar Cells

• Quantum dots can be combined with organic semiconductors to create hybrid organic-inorganic solar cells, leveraging the advantages of both materials, such as high absorption and charge transport properties
• Organic semiconductors offer high absorption coefficients and flexibility, while quantum dots provide tunable bandgaps and efficient charge generation
• Hybrid solar cells can be fabricated using solution-based processes, enabling low-cost and large-area production
• Examples of hybrid organic-inorganic solar cells include polymer-quantum dot solar cells and small molecule-quantum dot solar cells

Integration with Perovskite Solar Cells

• The integration of quantum dots with perovskite solar cells, either as a separate layer or as a compositional component, can enhance light harvesting and charge carrier dynamics
• Quantum dots can be used as a surface passivation layer on perovskite films, reducing defects and improving the stability of perovskite solar cells
• Incorporating quantum dots into the perovskite absorber layer can broaden the absorption spectrum and enhance charge separation and transport
• Quantum dot-perovskite tandem solar cells can achieve high efficiencies by combining the complementary absorption ranges of both materials

Tandem with Emerging Photovoltaic Technologies

• Quantum dots can be used in tandem with other emerging photovoltaic technologies, such as dye-sensitized solar cells or organic solar cells, to create multi-junction devices with complementary absorption ranges
• In dye-sensitized solar cells, quantum dots can be used as co-sensitizers alongside organic dyes, extending the absorption spectrum and improving the photocurrent generation
• Organic solar cells can be combined with quantum dots in a tandem configuration, where the quantum dots absorb the near-infrared region of the spectrum, complementing the absorption of organic materials
• These tandem devices can overcome the limitations of individual technologies and achieve higher efficiencies through the efficient utilization of the solar spectrum