A Solid-State Organic Intermediate Band Solar Cell Based on Principles of Triplet-Triplet Annihilation

Background

Organic photovoltaics (OPVs) represent a promising class of solar energy conversion devices due to their mechanical flexibility, lightweight nature, and potential for low-cost, large-area manufacturing. However, a significant limitation of conventional OPVs is their inability to efficiently harvest photons with energies below the optical bandgap of the organic semiconductor materials. This spectral mismatch leads to substantial energy losses, as a considerable fraction of the solar spectrum—particularly in the near-infrared region—remains unutilized. Overcoming this limitation is critical for improving the overall power conversion efficiency of OPVs and making them more competitive with established inorganic photovoltaic technologies.

Current approaches to address sub-bandgap photon harvesting in OPVs have primarily relied on the incorporation of intermediate-band materials or upconversion processes that often require liquid electrolytes or complex device architectures. Liquid-based upconversion systems, while effective in some respects, introduce significant challenges related to device stability, scalability, and integration with solid-state electronics. Additionally, many solid-state upconversion strategies suffer from inefficient energy transfer, limited exciton diffusion, or increased recombination losses, all of which hinder practical implementation. These issues underscore the need for robust, efficient, and fully solid-state solutions that can seamlessly integrate with existing OPV architectures while overcoming the inherent limitations of current methods.

Technology

Solid-state organic intermediate-band photovoltaic devices are designed to enhance solar energy conversion by integrating an organic electron donor, an organic electron acceptor, and a triplet sensitizer within a solid-state architecture. The triplet sensitizer absorbs sub-bandgap photons—light that would otherwise pass through traditional solar cells without generating electricity—and undergoes intersystem crossing to create a triplet exciton. This exciton is transferred to the donor, where two such triplet excitons can combine through triplet-triplet annihilation upconversion (TTA-UC), generating a higher-energy singlet exciton. The singlet exciton is then able to transfer its charge to the acceptor, resulting in increased photocurrent. The device architecture can include a composite layer with the sensitizer dispersed in either the donor or acceptor host, and a spacer layer (5–70 nm thick) to optimize exciton management and minimize losses due to back-transfer, all while avoiding the complications of liquid electrolytes.

This technology is differentiated by its ability to harvest sub-bandgap photons through TTA-UC, thereby increasing the overall photocurrent without sacrificing photovoltage—a significant limitation in conventional photovoltaic devices. By employing a solid-state design, it overcomes the fabrication and durability challenges associated with liquid electrolytes, making it more suitable for practical applications. The inclusion of a spacer layer strategically enhances the efficiency of exciton collection and upconversion, while minimizing recombination losses. Experimental validation, including transient absorption spectroscopy and current-voltage measurements, demonstrates that these devices achieve superior performance, particularly at wavelengths corresponding to the sensitizer’s absorption, and that their output can be further boosted by background illumination. This approach represents a significant advance in organic photovoltaics, enabling more efficient solar energy harvesting from a broader spectrum of sunlight.

Benefits

• Enhances photocurrent by harvesting sub-bandgap photons through triplet-triplet annihilation upconversion (TTA-UC).
• Maintains photovoltage while increasing overall device efficiency.
• Avoids fabrication and durability issues associated with liquid electrolytes by using a solid-state architecture.
• Incorporates a spacer layer to reduce back-transfer losses and improve exciton collection efficiency.
• Enables efficient energy transfer and charge generation via optimized organic donor, acceptor, and triplet sensitizer materials.
• Allows for improved device performance through tuning of spacer layer thickness and background light illumination.
• Supports stable and scalable photovoltaic device fabrication using organic materials and transparent conductive electrodes.

Applications

• High-efficiency solar panels
• Indoor photovoltaic energy harvesting
• Wearable solar-powered devices
• Low-light solar charging systems

Additional info

Solid-state organic intermediate-band photovoltaic devices employ a triplet sensitizer to absorb sub-bandgap photons. Energy transfers to a donor, where triplet-triplet annihilation upconversion generates higher-energy singlets. These singlets transfer charge to an acceptor, producing photocurrent. This liquid-electrolyte-free architecture incorporates a spacer layer to optimize exciton collection and reduce back-transfer of upconverted singlets, enhancing efficiency.