Flexible solar cells achieve 19% PCE gain

Researchers from the Chinese Academy of Sciences (CAS) have tweaked the material of ternary organic solar cells (TOSCs) and achieved efficiencies similar to those of conventional solar cells. Organic photovoltaic solar cells (OSCs) are a type of solar cell that uses organic materials, typically composed of small molecules or polymers, to convert sunlight into electricity, distinct from traditional inorganic solar cells, which use crystalline silicon or other inorganic materials.

One of the advantages of OSCs is their flexibility and light weight. They can be cheaply manufactured in flexible rolls rather than rigid panels — using solution-based processes, like inkjet printing — making them suitable for diverse applications such as sensors, portable chargers or wearable electronics. OSCs can also be designed to be semi-transparent or in various colours, allowing for aesthetic integrations into buildings, windows or other structures.

However, OSCs have lower power conversion efficiencies (PCE) than inorganic solar cells. TOSCs have changed the situation, to a certain extent. Unlike traditional binary organic solar cells, which consist of a donor material and an acceptor material, TOSCs include an additional third component, often referred to as the “guest”. This guest component is introduced to optimise various aspects of the solar cell’s operation, from tweaking the cell’s internal energy flows to improving how the cell converts light into power. The guest component can also broaden the spectrum of light that can be absorbed. By selecting a guest material that absorbs light in a range not covered by the donor or acceptor, the overall sunlight absorption of the cell can be increased.

Given the different functions that the guest component can play, its specific location within the solar cell ‘sandwich’ or matrix can alter performance. “Depending on its placement, the guest component can either transfer energy lightning-fast or help capture more sunlight,” Li Yonghai, co-author of the study, said.

There are three location possibilities: embedded in the donor material, embedded in the acceptor material or dispersed in some way between the interface of donor and acceptor, forming blended, alloy-like structures (aggregations). The researchers from CAS used a guest component called LA1 in the TOSC. LA1 is a small molecule acceptor that the researchers modified with phenylalkyl side chains — a functional group (collections of atoms within molecules that have their own set of properties) that is used in the design of organic materials for use in photovoltaic devices. The LA1 was modified with the phenylalkyl side chain to improve its crystallinity and alignment while maintaining compatibility, in turn enhancing its performance in the TOSC.

The researchers also regulated the distribution of their guest component by experimenting with various conditions that govern interaction with the host components, including host/guest compatibility, surface energy, crystalline kinetics and intermolecular interactions. By doing so, they found alloy-like aggregations within most of the guest molecules, which permeated and dispersed into the host molecules. The crystalline size of these embedded host/guest ‘alloys’ can be fine-tuned for improved electric charge transport and suppressed charge recombination. As a result, the researchers were able to achieve PCE gains of over 15%, and then by combining the guest component with the Y6 family of acceptors as host component, they achieved greater efficiency gains of over 19%. The research findings have been published in Advanced Materials.

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