What is the Bulk Photovoltaic effect?

Over the past few decades there has been a great interest in the field of photovoltaics for their use in sustainable energy generation. However, as the Shokley-Queisser limit, that dictates the maximum theoretical efficiency of a solar cell, has been met there has been a need to research alternatives. One such phenomena is the Bulk photovoltaic effect something that I hope to give you a brief insight into below.

Unlike the traditional photovoltaic effect, where a p-n junction is used to create electron hole pairs, the bulk photovoltaic effect is seen in materials with broken inversion symmetry, thus displaying non-linear optical properties. A non-linear optical process is one where the response of the medium depends non-linearly on the electric field of the beam. When exposed to high energy photons, the electrons and holes will become spatially separated leading to the generation of a built-in electric field and thus electrical polarisation, producing a d.c shift current. This is complimented with the ballistic current which results from the electrons being able to pass through the material in a direction determined by the symmetry of the crystal without losing energy. Both of these currents will result in the production of a voltage when connected to an external circuit.

The effectiveness of the Bulk Photovoltaic effect in being able to harness a photovoltage that is greater than the material’s band gap stems from the fact that it can use a wide range of wavelengths unlike the traditional photovoltaic effect which only allows the absorption of specific wavelengths. Certain materials are required for this phenomenon to be observed, including transition metal dichalcogenides and certain perovskite oxides with broken inversion symmetry. The aim is to find materials that have an optimal band structure, crystal symmetry, charge transport properties and materials that display minimal scattering to enhance the ballistic current. The problem with many of these materials is the difficulty in making them, especially on a large scale. Methods such as mechanical exfoliation are incredibly slow and offer very low yields, only allowing small flakes of a few microns wide to be made. These flakes can often crack rendering them unusable only to have to start the long process again, something which I have frustratingly experienced many times before.

With the effects of climate change driving the production of sustainable energy, I think it is clear to see the importance of this rather unheard-of phenomenon and I hope I have been successful in giving you a brief insight into the complicated world of physics wherein it lies!

References:
Panoiu N . ‘Chapter 1 – Introduction to nonlinear optics at the nanoscale’ in ‘Fundamentals and Applications of Nonlinear Nanophotonics’, 2024
Chan Y-H . ‘Giant exciton-enhanced shift currents and direct current conduction with subbandgap photo excitations produced by many-electron interactions’ 2021
Zhenbang Dai . ‘Phonon-Assisted Ballistic Current from First-Principles Calculations’ 2021
Zhenbang Dai . ‘Recent progress in the theory of bulk photovoltaic effect’ 2023