Semitransparent organic photovoltaics with wide geographical adaptability as sustainable smart windows – Nature

 

Report on Semitransparent Organic Photovoltaics for Sustainable Smart Windows

Executive Summary

This report details the development and analysis of high-efficiency semitransparent organic photovoltaics (ST-OPVs) designed for building-integrated applications. The research establishes a novel figure of merit for material selection, achieves record-breaking light utilization efficiency (LUE), and demonstrates significant energy-saving potential across diverse geographical climates. These advancements directly support the achievement of several United Nations Sustainable Development Goals (SDGs), including SDG 7 (Affordable and Clean Energy), SDG 11 (Sustainable Cities and Communities), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action). The findings confirm the multifunctionality of ST-OPVs as a key technology for constructing sustainable, energy-saving smart windows.

1. Introduction: Advancing Sustainable Energy through Innovative Photovoltaics

The integration of renewable energy technologies into urban environments is critical for achieving global sustainability targets. Semitransparent organic photovoltaics (ST-OPVs) represent a transformative technology with the potential to turn building facades and windows into power-generating assets, directly contributing to SDG 7 (Affordable and Clean Energy) and SDG 11 (Sustainable Cities and Communities). Unlike traditional solar panels, ST-OPVs can be integrated without compromising architectural aesthetics, offering a dual function of energy generation and smart daylight management. However, widespread adoption is contingent on overcoming challenges related to efficiency, stability, and cost-effectiveness. This study addresses these challenges by systematically optimizing materials and device architecture to enhance performance and demonstrate real-world applicability, thereby promoting SDG 13 (Climate Action) by reducing the carbon footprint of buildings.

2. Material Selection and Optimization for Sustainability

2.1. A Novel Parameter for Responsible Material Screening (FoM_LUE)

To accelerate the development of high-performance ST-OPVs and align with SDG 12 (Responsible Consumption and Production), a dimensionless parameter, Figure of Merit for Light Utilization Efficiency (FoM_LUE), was established. This parameter enables the quantitative screening of photoactive materials by evaluating their potential based on a combination of:

• Average Visual Transmittance (AVT)
• Optical Bandgap
• Potential Current Density

A higher FoM_LUE value indicates a material’s superior potential for creating efficient and transparent solar cells.

2.2. Screening Results

A series of state-of-the-art photoactive materials were evaluated using the FoM_LUE parameter.

1. Polymer Donors: PBOF exhibited the highest FoM_LUE value of 0.084, significantly outperforming other common donors like PM6 and D18-Cl.
2. Acceptors: The non-fullerene acceptor (NFA) eC9 achieved the highest FoM_LUE value of 0.161.
3. Blends: A ternary blend of PBOF:eC9:PC61BM was identified as the most promising combination, with a calculated FoM_LUE of 0.091, indicating its superior potential for high-efficiency ST-OPVs.

3. Device Performance and Efficiency Milestones

3.1. Opaque Device Optimization

To validate the material screening, opaque OPV devices were fabricated. Through comprehensive optimization of the hole transport layer, active layer composition, and additives, a device based on the PBOF:eC9:PC61BM blend achieved a power conversion efficiency (PCE) of 18.60%. This high efficiency was attributed to:

• Suppressed charge recombination.
• Improved exciton dissociation and charge collection efficiencies (96.99% and 88.74%, respectively).
• Higher and more balanced charge mobility.
• Reduced non-radiative recombination energy loss.

3.2. Record-Breaking Semitransparent Device Performance

Leveraging the optimized ternary blend, ST-OPVs were fabricated. By integrating an aperiodic band-pass filter (ABPF) as a transparent top electrode, a new benchmark for ST-OPV performance was set. This achievement is a significant contribution to SDG 7, pushing the boundaries of clean energy generation in transparent applications.

• Power Conversion Efficiency (PCE): 12.19%
• Average Visual Transmittance (AVT): 49.62%
• Light Utilization Efficiency (LUE): 6.05% (the highest reported value for semitransparent solar cells).

The device also demonstrated excellent operational stability, retaining 80% of its initial efficiency for over 800 hours, a key factor for the long-term viability required by SDG 11 and SDG 12.

4. Multifunctionality for Sustainable Buildings

4.1. Aesthetic and Functional Transparency

For successful integration into buildings, ST-OPVs must offer high-quality visual transparency. The developed devices exhibit excellent aesthetic properties, ensuring they enhance rather than detract from building design, a key consideration for SDG 11.

• Color Rendering Index (CRI): The ABPF-based device achieved a CRI of 90.5, ensuring that colors viewed through the window appear natural.
• Color Appearance: The devices present a neutral, reddish hue, with CIE coordinates close to standard daylight.

4.2. Thermal Insulation and Energy Management

ST-OPVs contribute significantly to building energy efficiency and SDG 13 (Climate Action) by managing solar heat gain. The ABPF-based device demonstrated superior thermal insulation performance.

• Total Solar Energy Rejection (T-SER): 77.49%
• Near-Infrared Energy Rejection (NIR-SER): 88.43%

In a simulated test, the ST-OPV window kept an object 13.6°C cooler than an unshaded space after 10 minutes of illumination, highlighting its effectiveness in reducing cooling loads.

5. Geographical Adaptability and Real-World Impact

5.1. Nationwide Energy-Saving Simulation

To assess the real-world impact on building energy consumption, a transient model was used to simulate the performance of the ST-OPV as a double-glazed window across 371 cities in China, covering five distinct climatic zones. This analysis provides a robust framework for deploying this technology to maximize its contribution to SDG 7 and SDG 13.

5.2. Key Simulation Findings

• Power Generation: The ST-OPV window can generate between 0.35 to 0.77 GJ m^-2 of clean electricity annually.
• Load Reduction: The technology provides a net positive annual load reduction (reduced cooling load minus increased heating load) in over 90% of the cities analyzed.
• Optimal Climate Zone: The “hot summer/warm winter” zone was identified as the most suitable for deployment, achieving a total annual energy benefit (power generation + load reduction) of 1.43 GJ m^-2.
• Influencing Factors: Annual power output was most sensitive to beam solar radiation, while load reduction was primarily influenced by ambient temperature and altitude.

6. Conclusion: A Viable Pathway to Sustainable Urban Development

This research successfully demonstrates a comprehensive approach to developing and validating multifunctional ST-OPVs for sustainable smart windows. By establishing a material screening methodology aligned with SDG 12, a record-breaking LUE of 6.05% was achieved, advancing the goals of SDG 7. The devices offer excellent thermal insulation, operational stability, and aesthetic quality, making them ideal for integration into sustainable urban infrastructure as envisioned by SDG 11. Finally, the extensive geographical analysis confirms their wide adaptability and significant energy-saving potential, providing a tangible solution to reduce building energy consumption and support SDG 13. These results highlight that high-efficiency ST-OPVs are a promising and practical technology for global sustainable development.

1. Which SDGs are addressed or connected to the issues highlighted in the article?

The article on Semitransparent Organic Photovoltaics (ST-OPVs) addresses several Sustainable Development Goals (SDGs) through its focus on renewable energy, energy efficiency, and sustainable infrastructure.

• SDG 7: Affordable and Clean Energy

  The core of the article is the development and improvement of organic photovoltaics, a form of solar energy technology. It directly contributes to increasing the share of renewable energy by exploring ways to make solar cells more efficient and applicable in new contexts, such as “smart windows.” The text emphasizes “constructing sustainable energy-saving smart windows” and revolutionizing the “renewable energy sector.”

• SDG 9: Industry, Innovation, and Infrastructure

  The research represents a significant scientific innovation in material science and clean technology. The article details the creation of a new parameter (FoM_LUE) for evaluating materials, the synthesis of new polymers (PBOF), and the engineering of advanced device structures (e.g., using an aperiodic band-pass filter). This work aims to “upgrade infrastructure” by integrating energy-generating technology directly into buildings (“building-integrated photovoltaics” or BIPV), making them more sustainable and resource-efficient.

• SDG 11: Sustainable Cities and Communities

  The primary application discussed for ST-OPVs is in urban environments, specifically as “smart windows” for buildings. The technology aims to make cities more sustainable by reducing their energy consumption. The article quantifies this impact, noting the windows provide “enhanced thermal insulation” and can lead to an “annual total energy-saving of 1.43 GJ m^-2.” Furthermore, the geographical analysis of performance across different climate zones in China directly addresses the need for climate-resilient and resource-efficient urban planning.

• SDG 12: Responsible Consumption and Production

  The article touches upon sustainable production by highlighting that ST-OPVs are valued for their “low-cost production, and environmental friendliness.” By developing technologies that are less resource-intensive and more environmentally benign than traditional alternatives, this research supports the shift towards more sustainable patterns of production.

2. What specific targets under those SDGs can be identified based on the article’s content?

Based on the article’s focus, several specific SDG targets can be identified:

1. Target 7.2: By 2030, increase substantially the share of renewable energy in the global energy mix.

   The entire study is dedicated to advancing a renewable energy technology. By developing more efficient and versatile solar cells (ST-OPVs), the research aims to expand the applications of solar power, particularly in urban settings through BIPV, thereby contributing to increasing the overall share of renewable energy. The article’s goal is to “revolutionize the renewable energy sector.”

2. Target 7.3: By 2030, double the global rate of improvement in energy efficiency.

   The article strongly emphasizes the energy-saving aspects of ST-OPV windows. They are presented not just as power generators but as multifunctional components that improve building energy efficiency. This is supported by findings on “enhanced thermal insulation,” management of “solar heat gain,” and a calculated “annual total energy-saving of 1.43 GJ m^-2,” which directly aligns with improving energy efficiency.

3. Target 9.4: By 2030, upgrade infrastructure and retrofit industries to make them sustainable, with increased resource-use efficiency and greater adoption of clean and environmentally-sound technologies…

   The concept of “building-integrated photovoltaics (BIPV)” is a direct application of this target. It involves retrofitting or designing buildings (infrastructure) with clean, energy-generating technology. The article’s focus on creating “sustainable energy-saving smart windows” is a clear example of developing technology to make infrastructure more sustainable.

4. Target 9.5: Enhance scientific research, upgrade the technological capabilities of industrial sectors…encouraging innovation…

   This research is a textbook example of Target 9.5 in action. The paper documents a comprehensive scientific investigation, from developing a new theoretical parameter (“FoM_LUE“) to material synthesis, device fabrication, and performance simulation. It aims to push the technological frontier of photovoltaics, reporting a “record-breaking LUE of 5.35%” and later a “highest value” of 6.05%.

5. Target 11.6: By 2030, reduce the adverse per capita environmental impact of cities…

   By reducing the energy required for heating, cooling, and lighting in urban buildings, the ST-OPV technology helps lower the overall environmental footprint of cities. The article’s simulation of “reduced space cooling load” and its reference to a study where “Photovoltaic windows cut energy use and CO2 emissions by 40%” directly connect the technology to mitigating the environmental impact of urban areas.

6. Target 11.b: By 2020, substantially increase the number of cities and human settlements adopting and implementing integrated policies and plans towards…resource efficiency, mitigation and adaptation to climate change…

   The geographical analysis across five different climate zones in China is a key part of the study. By identifying that the “hot summer/warm winter zone is the most suitable for ST-OPV glazing windows,” the research provides crucial data for implementing this technology in a way that is adapted to local climatic conditions, promoting resource efficiency and climate adaptation in urban planning.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

Yes, the article is rich with quantitative indicators that can be used to measure progress towards the identified targets.

• Power Conversion Efficiency (PCE): This is a standard indicator for solar cell performance. The article reports achieving a high PCE of “18.60%” for opaque devices and “12.19%” for the best-performing ST-OPV, measuring progress in renewable energy generation (Target 7.2).
• Light Utilization Efficiency (LUE): This is a key figure of merit introduced in the paper, defined as the product of PCE and Average Visual Transmittance (AVT). It specifically measures the dual-function performance of transparent solar cells. The article reports a record LUE of “6.05%,” serving as a precise indicator for innovation in this field (Target 9.5).
• Annual Power Output: The simulations provide a practical measure of energy generation, showing that the ST-OPV window can generate “0.35 to 0.77 GJ m^-2” of electricity per year. This directly quantifies the contribution to the renewable energy supply (Target 7.2).
• Annual Energy Savings / Load Reduction: This indicator measures the technology’s impact on energy efficiency. The article reports a potential “annual total energy-saving of 1.43 GJ m^-2” and an “annual load reduction” for heating and cooling. This is a direct measure of progress towards Target 7.3 and Target 11.6.
• Thermal Insulation Performance (T-SER and NIR-SER): The Total Solar Energy Rejection (T-SER) and Near-Infrared Solar Energy Rejection (NIR-SER) are specific metrics used to quantify the thermal insulation capability of the windows. The article reports values up to “T-SER of 77.49%” and “NIR-SER of 88.43%,” which are direct indicators of energy efficiency improvements (Target 7.3).
• Operational Stability (T₈₀ Lifetime): This measures the durability and long-term viability of the technology. The reported “T₈₀ lifetime exceeding 800 h” is an indicator of the technology’s readiness for practical application in sustainable infrastructure (Target 9.4).
• Color Rendering Index (CRI): While an aesthetic measure, the CRI is a critical indicator for the practical adoption of this technology in buildings. A high CRI (the article reports values “exceeding 90”) ensures the technology can be integrated without negatively impacting human environments, which is crucial for its widespread adoption in sustainable cities (Target 11.6).

4. Table of SDGs, Targets, and Indicators

SDGs	Targets	Indicators Identified in the Article
SDG 7: Affordable and Clean Energy	7.2: Increase substantially the share of renewable energy in the global energy mix.	Power Conversion Efficiency (PCE): up to 18.60% (opaque) and 12.19% (semitransparent). Light Utilization Efficiency (LUE): a record value of 6.05%. Annual Power Output: 0.35 to 0.77 GJ m-2.
SDG 7: Affordable and Clean Energy	7.3: Double the global rate of improvement in energy efficiency.	Annual Total Energy Saving: 1.43 GJ m-2. Thermal Insulation (T-SER): up to 77.49%. Reduced Building Space Cooling/Heating Load.
SDG 9: Industry, Innovation, and Infrastructure	9.4: Upgrade infrastructure and retrofit industries to make them sustainable…with greater adoption of clean and environmentally-sound technologies.	Integration into buildings as “smart windows” (BIPV). Operational Stability (T80 lifetime): exceeding 800 hours. Mention of “environmental friendliness” and “low-cost production.”
SDG 9: Industry, Innovation, and Infrastructure	9.5: Enhance scientific research, upgrade the technological capabilities…encouraging innovation.	Development of a new evaluation parameter (FoMLUE). Achievement of record-breaking Light Utilization Efficiency (LUE) of 6.05%. Comprehensive device engineering and material screening.
SDG 11: Sustainable Cities and Communities	11.6: Reduce the adverse per capita environmental impact of cities.	Annual energy savings in buildings (1.43 GJ m-2). Implied reduction in CO2 emissions from buildings. High Color Rendering Index (CRI > 90) for aesthetic integration into urban architecture.
SDG 11: Sustainable Cities and Communities	11.b: Increase cities adopting integrated policies and plans towards…resource efficiency, mitigation and adaptation to climate change.	Geographical analysis of ST-OPV performance across 5 climate zones in China. Identification of the most suitable climate zone (“hot summer/warm winter zone”) for optimal performance.

Source: nature.com