Researchers at the Institute of Photonic Sciences (ICFO) in Spain have developed an optical configuration to reduce Boltzmann losses in organic solar cells and improve their power conversion efficiency.
Boltzmann losses arise from the mismatch between the narrow angular range at which sunlight reaches a solar cell and the much wider range over which the cell emits photons. This difference increases the entropy associated with photon emission, reducing the maximum voltage and efficiency the device can achieve.
The study Inhibited photon emission to overcome Boltzmann losses in an organic solar cell with a cavity configuration, recently published in Energy & Environmental Science, describes an optical cavity designed to restrict photon emission to a narrower angular range, bringing it closer to that of incoming sunlight.
Solar cells lose some of the energy they absorb through several mechanisms, including thermal dissipation and the inability to convert low-energy photons into electricity. Additional losses occur through radiative emission from the photovoltaic material itself. Although this process is inherent to photovoltaic energy conversion, controlling the direction of emitted light can help minimize the associated losses.
The research team investigated this approach using an inverted organic solar cell based on a PM6 blend. They incorporated the device into an optical cavity designed to suppress photon emission at high angles while maintaining efficient absorption of incoming sunlight.
The structure features a front silver electrode that strongly reflects light emitted at wide angles. Together with a rear silver electrode and intermediate layers, it forms an optical cavity that combines high reflectivity over a broad angular range with high transmission in the direction of incident solar radiation.
According to the researchers, this approach could provide a route to exceeding the Shockley-Queisser efficiency limit for single-junction solar cells. The conventional theoretical limit is 33.16%, based on the intrinsic loss mechanisms considered in the Shockley-Queisser model. By narrowing the emission cone to more closely match the angular range of incoming sunlight, efficiencies could, in principle, approach 43%.
The researchers said the optical cavity can be integrated into solar cells using a relatively simple structure without significantly increasing manufacturing complexity. Although demonstrated in an organic photovoltaic device, the concept could potentially be extended to other solar cell technologies.
The findings highlight an alternative strategy for improving photovoltaic efficiency by controlling photon emission rather than focusing exclusively on losses associated with light absorption and energy conversion. Such improvements could increase electricity generation per unit area, potentially reducing the amount of material and land required for a given installed capacity.
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