Researchers at German research institute Forschungszentrum J�lich have investigated which features the ideal recyclable solar cell should have and have concluded that recyclability conflicts with traditional criteria such as efficiency, stability and cost.
The basic idea behind this work was to explore how the basic design of a solar cell would alter if recycling is considered, the research's lead author, Ian Marius Peters, told pv magazine. When looking for a good material to make a solar cell, the defining parameters are absorptivity, charge carrier lifetime, and mobility. As we progress towards mass-produced modules, other parameters enter that are relevant to generating electricity at low cost. The field is further expanded when including sustainability. I introduced a set of parameters to describe the chemical adhesion within and between layers for this purpose, as well as a parameter to describe the inherent entropy.
Finding the ideal recyclable solar cell now becomes an optimization problem for all these parameters, he went on to say. The challenge is that some of them have opposing requirements. For example, I would like to have a very stable module, but, at the same time, Id like to have one in which components are easily separable. The paper identifies several more conflicting requirements. The art of designing the ideal recyclable solar cell then becomes finding good compromises for these conflicting requirements.
In the study The ideal recyclable solar cell, published in nature reviews chemistry, Peters and his colleagues explained that the recyclability of a solar cell is rooted in the chemistry and physics, with three key parameters defining the bonding of its constituent layers: intralayer bonding strength, which refers to the internal cohesion of the materials used; interlayer bonding, which defines the cohesion between the cell layers; and the bonding contrast between adjacent layers, which is key for selective layer-by-layer separation.
Another important factor is what the scientists described as locked-in entropy, which defines the degree of structural and compositional mixing created by the fabrication processes. While not thermodynamically rigorous, it reflects how processes such as diffusion or alloying entangle materials, increasing the energy and complexity of separation at the end of life of the photovoltaics, they further explained.
The research group stressed that all of these parameters are somehow interdependent and that inherent trade-offs between them were inevitable, with the complication of factoring in those elements affecting efficient recycling.
One example is the trade-off between bonding contrast and carrier lifetime. Interfaces with strong bonding contrast such as between soft organic transport layers and crystalline perovskite absorbers can facilitate layer-specific separation, which benefits recyclability, they highlighted. However, this contrast often reflects mechanical, chemical or electronic mismatches at the interface, which can compromise passivation quality, introduce trap states or create energy-level offsets. These effects increase recombination and reduce the carrier lifetime.
The researchers concluded that targeting high solar cell efficiencies could lead to broad structural conflicts with recyclability, as the most efficient PV devices have strongly integrated architectures, including textured surfaces, passivating contact stacks, buried junctions and multilayer coatings.
Such architectures introduce dense networks of functional interfaces and bonded layers, complicating mechanical access and layer-specific separation, the research team emphasized. Even where material intermixing is avoided, the strong physical and chemical integration of components impairs disassembly. In this context, recyclability is not only limited by entropy, but also by the nature of bonding and structural entanglement.
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