Moving Beyond the 80-Year-Old Solar Cell Equation

Physicists from Swansea University and Åbo Akademi University have made a significant breakthrough in solar cell technology by developing a new analytical model that allows for a better understanding and improved performance of thin-film photovoltaic (PV) devices.

For nearly eight decades, the so-called Shockley diode equation has explained how electricity flows through solar cells; the electricity that powers your home or charges your battery bank. But a new study challenges that traditional understanding for a specific class of next-generation solar cells: thin-film solar cells

These thin-film solar cells, made of flexible and cheap materials, are limited in performance due to factors that existing analytical models cannot fully explain.

A new study published in PRX Energysheds light on how these solar cells achieve optimal efficiency. It reveals the critical balance between harvesting electrical energy generated by light and minimizing losses due to recombination, in which electrical charges cancel each other out.

“Our findings provide key insights into the mechanisms that drive and limit charge accumulation and ultimately power conversion efficiency in low-mobility photovoltaic devices,” said lead author Dr Oskar Sandberg from Åbo Akademi University in Finland.

Previous analytical models for these solar cells had a blind spot: “injected carriers”—charges entering the device from the contacts. These carriers significantly affect recombination and limit efficiency.

“Traditional models simply did not give the full picture, especially in the case of thin-film cells with low-mobility semiconductors,” explained lead researcher, Assistant Professor Ardalan Armin from Swansea University.

“Our new study addresses this gap by introducing a new diode equation specifically tailored to account for these crucial injected carriers and their recombination with photogenerated carriers.”

“Recombination between injected and photogenerated charges is not a major problem in traditional solar cells such as silicon solar cells, which are hundreds of times thicker than next-generation thin-film solar cells such as organic solar cells,” Dr. Sandberg added.

Associate Professor Armin said: “One of the most brilliant theoretical physicists of all time, Wolfgang Pauli once said: ‘God created mass; the surface was the devil’s work.’ Since thin-film solar cells have much larger interfacial areas per mass than traditional silicon; it is no wonder that they are more drastically affected by the ‘devil’s work’ – that is, the recombination of valuable photogenerated charges with charges injected near the interface.”

This new model offers a new framework for designing more efficient thin-film solar cells and photodetectors, optimizing existing devices, and analyzing material properties. It can also help train machines used to optimize devices, representing a significant step forward in the development of next-generation thin-film solar cells.