Researchers propose new method to reduce solar cell energy loss, boosting efficiency to near 43%

Researchers at the Institute of Photonic Sciences (ICFO) in Barcelona have experimentally demonstrated a method to reduce energy loss by controlling the direction of light emitted by solar cells, potentially increasing their power generation capacity. The method, published in the journal Energy & Environmental Science, may offer a pathway to surpass the traditional theoretical efficiency limit of single-junction solar cells. The study focuses on a loss mechanism known as Boltzmann loss, which occurs when a material capable of absorbing light can also emit light.

Why is the direction of emitted light so critical? Solar cells receive sunlight from a relatively narrow direction. The sun occupies only a small patch of sky, meaning sunlight entering a solar cell can only arrive within a narrow cone. However, photons generated inside a solar cell can typically be emitted across a much wider range of directions. According to ICFO researchers, this mismatch between the narrow acceptance cone and the broad emission cone reduces the useful power that can be extracted from the device.

The need to control light emission direction

The researchers sought to reduce this loss by making solar cells more selective in the direction they emit light. The team, led by ICFO and UPC professor Jordi Martorell, included Dr. Francisco Bernal Texca, Chiara Cortese, Dr. Mariia Kramarenko, and Dr. Quan Liu. They used an inverted organic solar cell based on a PM6:Y6 material blend and transformed it into an optical cavity.

This structure suppressed large-angle photon emission, effectively narrowing the cell's emission cone.

The researchers achieved this effect using a silver-based front electrode and a layered structure, with a second silver electrode placed on the back of the cell. The metal front electrode provides high reflectivity across a wide range of emission angles. Combined with the back electrode and its surrounding layers, this formed an optical cavity. The cavity performs two functions simultaneously: suppressing light emission at larger angles while efficiently allowing sunlight from the source to enter. This makes the angular distribution of emitted light more similar to that of incoming sunlight.

Challenging the conventional efficiency limit

The researchers note that this method does not require more complex manufacturing processes than many other strategies explored to push photovoltaic efficiency beyond conventional limits. The study targets the well-known Shockley–Queisser limit, a model describing the theoretical efficiency ceiling of conventional single-junction solar cells under model assumptions. ICFO sets this limit at 33.16%. The researchers say that if the emission cone can be narrowed sufficiently to better match the sun's absorption cone, efficiencies approaching 43% could theoretically be achieved.

This does not mean the researchers have produced a 43%-efficient solar cell. The study demonstrates an optical strategy for reducing the relevant loss mechanism in organic photovoltaic devices. "Our work represents a significant advance in the field of solar energy conversion because it opens a completely unexplored path to surpass the Shockley–Queisser limit," said lead author Dr. Francisco Bernal Texca.

The researchers also emphasize that the principle is not limited to organic photovoltaic technology. Although the experiments were conducted using organic solar cells, Martorell says the concept can in principle be applied to different photovoltaic technologies. If this approach can be translated into more efficient practical devices, controlling the direction of photon emission could become another tool for extracting more power from the same amount of sunlight. According to the researchers, higher photovoltaic efficiency can also reduce the land and materials needed for a given amount of solar power generation, while lowering the cost per watt.

Potential impact of the new method on solar technology

The new method proposed in this study addresses the energy loss problem in solar cells by adjusting the direction of light emission to boost efficiency, showing potential to surpass conventional theory. The Shockley–Queisser limit has long been a challenge for photovoltaic technology, and this research advance may provide new thinking for future solar cell designs. Higher photovoltaic efficiency not only increases energy output but may also reduce the land and materials required, lowering costs — a significant implication for the advancement of renewable energy.