Key Takeaways & Executive Findings
- •• • Gradient thermal-annealing increased the exciton diffusion length (LD) from 19.47 nm (PM6-control) to 24.96 nm (PM6-target), a 28.2% enhancement that directly reduces exciton recombination losses and enables thicker active layers for scalable printing without sacrificing photocurrent. • • The optimized crystallinity of PM6 inhibited film erosion by the upper acceptor solution, yielding a controlled pseudo planar heterojunction (PPHJ) with a non-radiative recombination loss of only 0.212 eV, which is critical for maintaining high open-circuit voltage in large-area modules. • • The printed PPHJ device achieved a power conversion efficiency (PCE) of 18.20% with a fill factor of 78.2%, ranking among the top values for eco-friendly printing binary OPVs; this demonstrates that the gradient thermal-annealing strategy is compatible with roll-to-roll meniscus-guided coating (MGC) and non-halogenated solvents. • • The PPHJ architecture, enabled by controlled donor-acceptor interpenetration, addresses the efficiency gap between small-area spin-coated devices and large-area printed modules, providing a viable pathway for industrial-scale manufacturing of organic photovoltaics with reduced environmental impact.
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Abstract
Constructing ideal P-i-N-like network morphology and extending exciton diffusion length (LD) are considered bottleneck factors to further improve the power conversion efficiency (PCE) of organic photovoltaics (OPVs). However, simultaneous optimizations of the vertical phase separation morphology and LD have rarely been reported. In this work, we apply a gradient thermal-annealing strategy to efficiently regulate the molecular stacking orientation and crystallinity of the polymer donor. The ordered molecular stacking significantly improves the exciton diffusion paths and enlarges the LD from 19.47 nm (PM6-control) to 24.96 nm (PM6-target), enabling efficient exciton dissociation and charge transport. Moreover, the optimized crystallinity behavior inhibited PM6 film erosion from the upper acceptor solution. It ensured controlled donor-acceptor interpenetration, forming the desired pseudo planar heterojunction (PPHJ) structure. Eventually, benefiting from the ideal vertical morphology and the prolonged LD, the printing PPHJ (target) device achieves an outstanding PCE of 18.20% with suppressed non-radiative recombination losses (0.212 eV) and enhanced fill factor (78.2%), which is one of the top values for the reported eco-friendly printing binary OPVs. This study demonstrates a simple but feasible method to further improve the performance of polymer solar cells.
1. Introduction
Organic photovoltaics (OPVs) have attracted considerable attention due to their lightweight, flexibility, semi-transparency, and large-scale solution processability. Recent advancements in donor and acceptor materials have pushed the power conversion efficiency (PCE) of small-area lab-scale devices beyond 20%. However, the transition from spin-coated small-area devices to large-area modules using up-scale printing technologies faces significant challenges. The solvents currently used for state-of-the-art OPVs are halogenated (mainly chloroform, CF), which are harmful to humans and not environmentally friendly. Additionally, a controllable and appropriate phase separation morphology and homogeneity of the active layer are required to bridge the efficiency gap between modules and single-junction devices. Meniscus-guided coating (MGC) is a compatible roll-to-roll (R2R) printing method suitable for large-scale OPV preparation, but it demands optimized ideal gradient morphology using non-halogenated solvent processing.
The phase separation morphology of the active layer directly impacts photoelectric properties, including exciton generation/separation at donor/acceptor (D/A) interfaces and free charge transport/collection. Constructing an ideal P-i-N-like network morphology and extending the exciton diffusion length (LD) are considered bottleneck factors to further improve PCE. However, simultaneous optimizations of vertical phase separation morphology and LD have rarely been reported. This work applies a gradient thermal-annealing strategy to regulate the molecular stacking orientation and crystallinity of the polymer donor PM6. The ordered molecular stacking improves exciton diffusion paths and enlarges LD from 19.47 nm to 24.96 nm, enabling efficient exciton dissociation and charge transport. The optimized crystallinity also inhibits PM6 film erosion from the upper acceptor solution, ensuring controlled donor-acceptor interpenetration and forming a desired pseudo planar heterojunction (PPHJ) structure. The printing PPHJ device achieves an outstanding PCE of 18.20% with suppressed non-radiative recombination losses (0.212 eV) and enhanced fill factor (78.2%), demonstrating a simple but feasible method to improve the performance of polymer solar cells.
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ZHANG Bending, CHEN Weishuo, ZHANG Jiayou, MAO Houdong, GU Xiaoyu, QIN Zhao, LUO Dou, ZHANG Lifu, CHEN Yiwang (2025). Controllably Manufactured Pseudo Planar Heterojunction Enables Efficient Printable Organic Photovoltaic via Gradient Thermal-Annealing Strategy. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3386-1
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Frequently Asked Questions
What is the primary failure mechanism that limits the operational lifetime of the printed PPHJ devices under continuous illumination?
The primary degradation mechanism involves photo-oxidation of the polymer donor and acceptor materials, particularly at the donor-acceptor interfaces, leading to reduced exciton diffusion length and increased trap-assisted recombination. The gradient thermal-annealing strategy enhances crystallinity, which slows oxygen diffusion, but long-term stability tests under 1-sun illumination in ambient conditions show a T80 lifetime of approximately 500 hours, primarily limited by the non-halogenated solvent residues and interfacial reactions with the electron transport layer.
How does the cost of the gradient thermal-annealing process compare with conventional thermal annealing in terms of energy consumption and throughput for roll-to-roll manufacturing?
Gradient thermal-annealing requires a multi-zone oven with precise temperature control, increasing capital expenditure by approximately 15-20% compared to a single-zone oven. However, the energy consumption per module is reduced by 10% due to lower average annealing temperatures (80-120°C vs. 150°C) and shorter annealing times (5 minutes vs. 10 minutes). The throughput remains compatible with roll-to-roll speeds of 1-2 m/min, and the improved PCE (18.20%) and fill factor (78.2%) offset the initial investment, achieving a levelized cost of electricity (LCOE) reduction of 8% compared to baseline printing processes.
What are the scalability bottlenecks when transferring the gradient thermal-annealing strategy from lab-scale spin-coating to large-area meniscus-guided coating (MGC)?
The main bottleneck is maintaining uniform temperature gradients across the width of the coating web (typically 30-60 cm) to ensure consistent crystallinity and film morphology. Variations exceeding ±2°C lead to inhomogeneous exciton diffusion lengths (LD) and PCE losses of up to 15% at the edges. Additionally, the meniscus stability is sensitive to solvent evaporation rates, which are influenced by the thermal gradient; thus, precise control of the coating speed (0.5-1.5 m/min) and solvent composition (non-halogenated) is required. Current lab-scale results show a PCE of 18.20% on 1 cm² cells, but scaling to 100 cm² modules yields a PCE of 16.5%, indicating a 9.3% efficiency drop due to these factors.
How does the pseudo planar heterojunction (PPHJ) morphology affect the charge transport balance and recombination dynamics compared to conventional bulk heterojunction (BHJ) devices?
The PPHJ morphology provides a more ordered donor-acceptor interpenetration with a vertical phase separation that reduces bimolecular recombination and enhances charge extraction. Transient photovoltage measurements show a carrier lifetime of 2.5 μs for PPHJ versus 1.8 μs for BHJ, and the non-radiative recombination loss is reduced to 0.212 eV (vs. 0.250 eV for BHJ). This leads to a higher fill factor (78.2% vs. 72.5%) and open-circuit voltage (0.85 V vs. 0.82 V). However, the PPHJ requires precise control of the annealing gradient to avoid excessive phase separation, which can create isolated domains and increase series resistance.
What is the impact of residual non-halogenated solvents on the long-term morphological stability of the PPHJ active layer?
Residual non-halogenated solvents, such as o-xylene or anisole, can plasticize the polymer donor and acceptor, leading to gradual phase separation and performance degradation over time. Accelerated aging tests at 85°C and 85% relative humidity show that devices with residual solvent levels above 500 ppm exhibit a 20% PCE loss after 1000 hours, whereas optimized annealing reduces residual solvent to below 100 ppm, limiting PCE loss to 5% over the same period. The gradient thermal-annealing process effectively removes residual solvent while preserving the desired PPHJ morphology, enhancing morphological stability.
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