Introduction

Building-Integrated Photovoltaics (BIPV) has emerged as a critical pathway for urban sustainable energy transition. Among various technologies, semi-transparent organic solar cells (ST-OSCs) stand out as ideal candidates for self-powered windows due to their tunable bandgap and intrinsic semi-transparency. However, conventional ST-OSCs face a major bottleneck: to balance transparency and efficiency, the active layer must remain extremely thin (below 80 nm), which creates severe challenges for large-scale industrial manufacturing. Tiny thickness fluctuations can cause dramatic performance drops, and the cell-to-module (CTM) efficiency retention for large-area modules typically stays below 56%.

A recent breakthrough published in Nature Communications by teams from the National Center for Nanoscience and Technology (NCNST) and collaborators addresses this long-standing issue. By combining a donor dilution strategy with slot-die coating under halogen-free solvent conditions, the researchers successfully fabricated ST-OSCs with remarkable thickness tolerance. Even with an ultra-thick 301 nm active layer, the devices maintained high light utilization efficiency (LUE), and 100 cm² modules achieved a CTM ratio of approximately 85%.

Performance Leap in Light Utilization Efficiency

In BIPV applications, semi-transparent cells have long faced a fundamental trade-off: increasing active layer thickness improves photon absorption and power conversion efficiency (PCE), but significantly reduces average visible transmittance (AVT). The industry evaluates ST-OSCs using Light Utilization Efficiency (LUE = PCE × AVT) as the key metric.

This study introduces a donor dilution strategy with a D:A ratio of 1:3, leveraging the fibrous network structure of acceptor materials under specific processing conditions. This approach allows substantial increases in active layer thickness while maintaining high transparency.

The observed data is striking. When the active layer thickness increased from 119 nm to 301 nm, the PM6:Qx-p-4Cl based cells maintained an LUE of 3.02%, demonstrating exceptional thickness robustness. This solves a critical pain point in large-area processing where thin film control has been notoriously difficult.

Figure 1 shows the chemical structures and absorption spectra of PM6:Qx-p-4Cl system, performance trends across different D:A ratios for opaque and semi-transparent devices, and demonstrates how the donor-diluted system outperforms conventional systems in transmittance retention and LUE advantage across varying thicknesses.

Mechanism Behind Thickness Tolerance

Why does the donor dilution approach solve the thickness sensitivity problem? The research team conducted thorough investigations through morphology studies and ultrafast spectroscopy.

Regarding morphological features, slot-die coating under specific conditions promotes ideal aggregation of acceptor molecules, forming continuous interpenetrating fibril-like networks. This structure ensures smooth charge transport even when donor content is extremely low.

For exciton dynamics, experimental measurements revealed that the Qx-p-4Cl acceptor possesses a remarkably long exciton diffusion length of approximately 22.34 nm. This ensures that excitons can effectively reach interfaces and dissociate even in thick, diluted systems.

Charge generation analysis via transient absorption (TA) spectroscopy confirmed that the system maintains efficient and stable charge generation across different thicknesses and ratios.

Figure 2 presents GIWAXS and AFM characterization revealing the fibril network structure, alongside transient absorption spectra and kinetic curves demonstrating robust charge generation and transport in the donor-diluted system.

Film Formation Dynamics: Slot-Die vs Spin Coating

The research further uncovered the physical essence of why slot-die coating outperforms traditional spin coating processes.

Unlike spin coating where films undergo burst-like aggregation in supersaturated states, slot-die coating on heated substrates induces orderly acceptor aggregation already in the liquid phase. This fundamentally changes the morphology evolution pathway.

Viscosity control plays a crucial role as well. Donor dilution reduces solution viscosity, accelerating solvent evaporation and extending the crystallization time after film thinning. This suppresses excessive acceptor aggregation at large thicknesses.

This unique film formation dynamic ensures that during large-area coating, film quality remains less sensitive to process parameter fluctuations, a critical factor for industrial production consistency.

Figure 3 shows in-situ UV-Vis absorption spectroscopy monitoring the acceptor aggregation process, along with comparative schematics of film formation mechanisms under spin coating versus slot-die coating, highlighting the critical regulatory role of heated substrates on morphology evolution.

Industrial Prospects and BIPV Applications

Leveraging the processing advantages from high thickness tolerance, the research team successfully translated the technology into practical applications.

On 100 cm² modules, they achieved 10.40% PCE and 3.32% LUE with a CTM ratio reaching 85%, setting a new benchmark for large semi-transparent modules.

For BIPV function demonstration, the team built a self-powered house model with a 600 cm² power-generating window. Experiments proved the system could drive LCD displays and charge lithium batteries.

The energy-saving benefits are equally impressive. Because the active layer blocks 88.28% of near-infrared radiation, the cell windows reduced indoor temperatures by approximately 9.2°C compared to ordinary glass windows, significantly cutting building energy consumption.

Stability testing showed that after 1000 hours of outdoor exposure, devices retained over 82% of their initial efficiency, demonstrating excellent commercialization potential.

Figure 4 displays the 100 cm² module structure and CTM efficiency statistics, along with BIPV application demonstrations including self-powered electronic device operation, energy storage, and the significant thermal insulation cooling effect curves.

Conclusion and Outlook

This research provides critical support for organic photovoltaics in green building and energy internet applications through several key contributions.

First, it lowers manufacturing barriers by breaking the dependence of ST-OSCs on ultra-thin films. High thickness tolerance translates directly into higher production yields and lower costs.

Second, it enables multi-dimensional carbon reduction. ST-OSC windows contribute green electricity through photovoltaic generation while simultaneously reducing building air conditioning passive energy consumption through excellent thermal insulation.

Third, the technology demonstrates broad applicability. The donor dilution strategy combined with halogen-free solvent processing aligns with green manufacturing trends, clearing obstacles for organic photovoltaics to move toward industrial-scale production lines.

As the world advances toward carbon neutrality, this smart energy solution integrating power generation, energy saving, and aesthetic appeal is transforming every building into a micro green power plant.

Original article: https://www.nature.com/articles/s41467-026-69537-3

Ooitech Perspective

Ooitech believes: donor dilution combined with slot-die coating breaks the thickness tolerance bottleneck of semi-transparent organic solar cells, paving a realistic path for BIPV industrialization and large-area commercial deployment.