Key Takeaways & Executive Findings
- •• • Sequential crystallization manipulation enabled a 20.82% PCE in organic solar cells with high tolerance of active layer thickness, as reported in Nat Mater 2025, 24: 444–453, directly addressing the thickness-PCE trade-off that limits scalable manufacturing. • • The active layer thickness in OSCs is typically ~100 nm; increasing thickness to reduce pinholes often causes PCE drop, but controlled crystallization sequence mitigates this, allowing thicker films without sacrificing performance. • • Exciton diffusion length is on the order of 10 nm, necessitating BHJ interpenetrating networks with domain sizes comparable to this length to ensure efficient exciton dissociation; sequential crystallization optimizes domain purity and alignment. • • The third-generation OSCs utilize materials with bandgaps of 1–4 eV and high absorption coefficients, but short exciton lifetimes and diffusion lengths require precise morphological control to achieve PCEs competitive with silicon (>25%) and thin-film (10–15%) technologies.
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Abstract
The evolution of photovoltaic technology has progressed from first-generation silicon solar cells, which achieved 4% power conversion efficiency (PCE) in 1954 and now exceed 25% PCE, to second-generation thin-film cells (CIGS, CdTe, a-Si) with 10–15% PCE, and third-generation organic solar cells (OSCs) that offer tunable properties, low cost, and flexibility. OSCs rely on bulk heterojunctions (BHJs) formed by casting mixed donor-acceptor solutions, where morphology and crystallinity critically govern exciton separation and charge transport. The diffusion length of excitons (~10 nm) necessitates interpenetrating percolated networks with orderly aligned conjugated polymers to maximize intramolecular conduction and suppress recombination. However, increasing active layer thickness to reduce pinholes often decreases PCE. This study investigates sequential crystallization during BHJ formation, a process typically assisted by annealing and controlled by solvent selection and evaporation. By manipulating crystallization sequence, the authors aim to optimize the nanoscale morphology, achieving a balance between thickness tolerance and efficient charge extraction. The findings, contextualized with recent breakthroughs such as the 20.82% efficiency reported by Chen et al. (Nat Mater, 2025), underscore the importance of crystallization kinetics in overcoming the trade-off between active layer thickness and device performance, providing a pathway for scalable, high-efficiency organic photovoltaics.
1. Introduction
The commercial dominance of silicon solar cells, with efficiencies exceeding 25% and decades of manufacturing maturity, has been challenged by the promise of low-cost, flexible organic photovoltaics (OPVs). However, OSCs have stalled at the threshold of commercialization due to a fundamental trade-off: increasing the active layer thickness to reduce pinhole defects and improve manufacturing yield inevitably degrades power conversion efficiency (PCE). This degradation stems from the short exciton diffusion length (~10 nm) in organic semiconductors, which mandates a bulk heterojunction (BHJ) morphology with nanoscale interpenetrating donor-acceptor networks. Conventional BHJ formation via concurrent phase separation and crystallization often yields suboptimal domain sizes and purity, leading to charge recombination and poor thickness tolerance.
This study addresses the bottleneck by investigating sequential crystallization during BHJ formation, a process where the donor and acceptor phases crystallize in a controlled order rather than simultaneously. By tailoring solvent evaporation kinetics and annealing protocols, the authors demonstrate that sequential crystallization can decouple the morphological evolution, enabling thicker active layers (>100 nm) without compromising exciton separation or charge transport. The approach leverages recent insights from Chen et al. (Nat Mater, 2025), who achieved 20.82% PCE with high thickness tolerance through crystallization sequence manipulation. The findings provide a mechanistic framework for optimizing BHJ morphology, potentially accelerating the transition of OSCs from laboratory curiosities to industrially viable, scalable photovoltaic modules.
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JIA Xiaoxiao, ZHANG Qifeng, CAO Guozhong (2025). Sequential Crystallization during the Formation of Bulk Heterojunction in Organic Solar Cells. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3274-3
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Frequently Asked Questions
What is the primary failure mechanism that limits the power conversion efficiency of organic solar cells when the active layer thickness is increased?
The primary failure mechanism is the short exciton diffusion length, on the order of 10 nm, which restricts the distance excitons can travel before recombining. In thicker active layers, typically beyond 100 nm, the probability of excitons reaching the donor-acceptor interface for dissociation decreases, leading to recombination losses and a drop in PCE. Additionally, increased thickness can exacerbate charge transport limitations due to low carrier mobility and morphological defects, further reducing fill factor and short-circuit current.
How does sequential crystallization improve the trade-off between active layer thickness and PCE compared to conventional concurrent crystallization?
Sequential crystallization decouples the crystallization of donor and acceptor phases, allowing for the formation of purer domains and better control over the interpenetrating network. This results in optimized domain sizes that are commensurate with the exciton diffusion length (~10 nm), enhancing exciton dissociation efficiency. Consequently, thicker active layers can be employed without sacrificing exciton harvesting, as demonstrated by a 20.82% PCE with high thickness tolerance (Nat Mater, 2025, 24: 444–453). This contrasts with concurrent crystallization, which often yields amorphous or mixed phases that impede charge transport.
What are the scalability bottlenecks for implementing sequential crystallization in industrial roll-to-roll manufacturing of organic solar cells?
Scalability bottlenecks include precise control of solvent evaporation kinetics and annealing profiles over large areas, as sequential crystallization is highly sensitive to processing conditions. Roll-to-roll coating demands uniform temperature and solvent gradients, which are challenging to maintain at high speeds. Additionally, the technique may require compatible donor-acceptor pairs with distinct crystallization temperatures, limiting material selection. Cost parity with silicon (>25% PCE) remains elusive, as OSCs currently achieve ~20.82% PCE in lab-scale devices, and scaling up often introduces defects that reduce efficiency by 10–20% relative.
What are the operational stability and degradation rates of OSCs fabricated with sequential crystallization under real-world stress conditions (e.g., humidity, temperature, illumination)?
Operational stability remains a critical challenge; OSCs typically exhibit short lifespans compared to silicon. Under accelerated aging (85°C/85% relative humidity, 1 sun illumination), unencapsulated OSCs can degrade by 20–50% in PCE within 1000 hours. Sequential crystallization may improve morphological stability by reducing mixed phases, but degradation mechanisms such as photo-oxidation of the active layer and electrode interdiffusion persist. Encapsulation and UV filters are necessary, but add cost. Long-term stability data for sequentially crystallized BHJs are limited, though the 20.82% PCE device (Nat Mater, 2025) likely employed encapsulation, with degradation rates not specified in the abstract.
How does the cost of organic solar cells with sequential crystallization compare to silicon and thin-film technologies on a $/W basis?
OSCs promise low-cost manufacturing due to solution processing and abundant materials, with potential module costs below $0.50/W compared to silicon at ~$0.30/W and thin-film (CdTe, CIGS) at ~$0.40/W. However, sequential crystallization adds complexity to the coating process, potentially increasing capital expenditure and reducing yield. Current lab-scale OSCs with 20.82% PCE are not cost-competitive due to small area and manual processing. For cost parity, roll-to-roll production with >15% module efficiency and >10-year lifetime is required, which remains unproven. The third-generation OSCs utilize cheaper materials than CIGS or CdTe, but encapsulation and barrier films add significant cost.
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