Thermodynamic Limitations of Perovskite Solar Cells in Tropical Climates

Perovskite solar cells (PSCs), particularly carbon-based PSCs (c-PSCs), have emerged as promising candidates for building-integrated photovoltaics (BIPV) due to their high power conversion efficiency (PCE) and potential for semi-transparency. However, their performance in tropical climates, characterized by high temperatures and humidity, is constrained by thermodynamic limitations. This article explores the challenges faced by c-PSCs in tropical environments, focusing on temperature-dependent stability, efficiency, and optical properties, and their implications for BIPV applications.

Background: PSCs in BIPV

PSCs offer a cost-effective alternative to silicon solar cells, achieving PCEs up to 11% in c-PSCs with improved transparency for BIPV glazing [9]. Their high light absorption and tunable bandgap make them ideal for semi-transparent windows, balancing electricity generation with daylight transmission. In tropical climates, where buildings face intense solar radiation and temperatures often exceed 35°C, maintaining PSC stability and performance is critical. Research indicates that temperature significantly affects perovskite stability, with crystal phase changes leading to degradation at extreme conditions [22, 23, 24].

Thermodynamic Challenges in Tropical Climates

Tropical environments pose unique challenges for PSCs due to high temperatures (25–75°C), humidity, and solar intensity. These factors impact:

• Stability: Perovskite materials, such as CH3NH3PbI3 (MAPbI3), undergo phase transitions at elevated temperatures, degrading performance.
• Power Conversion Efficiency (PCE): High temperatures reduce open-circuit voltage (Voc) and short-circuit current density (Jsc), lowering PCE.
• Optical Properties: Temperature affects transmittance, impacting visual and thermal comfort in BIPV applications.
• Thermochromic Behavior: PSCs exhibit reversible transparency changes with temperature, influencing solar heat gain and glare control.

Temperature-Dependent Performance

Studies on c-PSCs, fabricated with TiO2, ZrO2, and carbon electrodes, reveal significant performance variations from 5 to 75°C [9]. Key findings include:

• PCE Trends: PCE peaks at 11% at 25°C but drops significantly above 55°C due to structural changes in MAPbI3, reducing to 4.6% at 75°C.
• Transmittance (AVT): Average visible transmittance (AVT) increases at higher temperatures (29.8% at 75°C), indicating a trade-off with PCE.
• Reversibility: Cooling from 75°C to 5°C restores efficiency, suggesting thermochromic behavior, but prolonged exposure to extreme temperatures risks permanent degradation.

Optical and Thermal Comfort

For BIPV, c-PSCs must balance electricity generation with visual and thermal comfort, measured by:

• Colour Rendering Index (CRI): CRI values above 80 are required for glazing. c-PSCs maintain acceptable CRI across 5–45°C but degrade at higher temperatures due to phase changes.
• Correlated Colour Temperature (CCT): Ideal CCT ranges from 3000–7500 K. c-PSCs exhibit stable CCT at moderate temperatures but shift towards reddish-white tones above 55°C, affecting indoor lighting quality.
• Solar Heat Gain Coefficient (SHGC): SHGC increases with temperature due to higher AVT, allowing more solar energy indoors, which is undesirable in tropical climates.
• Glare Subjective Rating (SR): Evaluated across Köppen climate zones, c-PSCs show effective glare control at 15–45°C but reduced performance at higher temperatures due to increased transmittance.

Temperature and Efficiency Coefficients

The temperature coefficient of transmittance (TCT) and efficiency coefficient of transmittance (ECT) quantify temperature effects:

• TCT: Positive from 5–25°C (indicating increasing transparency) and negative from 25–75°C, reflecting phase transitions.
• ECT: Shows a see-saw relationship between PCE and AVT. Higher PCE correlates with lower AVT, while extreme temperatures increase AVT but reduce PCE.

These coefficients highlight the challenge of optimizing c-PSCs for tropical BIPV, where high temperatures favor transparency over efficiency.

Implications for Tropical Climates

Tropical climates, with temperatures often exceeding 35°C, exacerbate PSC limitations:

• Degradation Risk: Above 55°C, MAPbI3 undergoes morphological changes, reducing PCE and stability. Encapsulation and structural engineering (e.g., WO3 nanoparticle integration) mitigate but don’t eliminate this issue [20, 21].
• Thermal Load: Increased AVT at high temperatures raises SHGC, increasing cooling demands in buildings, counteracting BIPV energy savings.
• Visual Comfort: Reduced CRI and shifting CCT at high temperatures compromise indoor lighting quality, critical for occupant well-being.
• Glare Control: Higher transmittance at elevated temperatures increases glare, particularly in tropical regions with intense sunlight.

Mitigation Strategies

To enhance c-PSC performance in tropical climates:

• Advanced Encapsulation: Develop robust encapsulation to protect against temperature and humidity-induced degradation.
• Material Engineering: Incorporate stable perovskite compositions (e.g., mixed-cation perovskites) to minimize phase transitions.
• Thermochromic Optimization: Leverage thermochromic properties to balance transparency and heat gain, potentially using adaptive coatings.
• Cooling Systems: Integrate passive cooling (e.g., heat sinks) or active cooling to maintain optimal operating temperatures.
• Climate-Specific Design: Tailor c-PSC glazing for tropical conditions, prioritizing low SHGC and high CRI at 35–75°C.

Conclusion

Perovskite solar cells hold immense potential for BIPV in tropical climates due to their efficiency and semi-transparency. However, thermodynamic limitations, particularly temperature-induced phase transitions, significantly reduce PCE and alter optical properties above 55°C. While thermochromic behavior offers reversible performance, prolonged exposure to tropical temperatures risks degradation. By addressing stability through encapsulation, material engineering, and climate-specific designs, c-PSCs can become viable for tropical BIPV, enhancing energy efficiency and occupant comfort in buildings.