Intermediate Band Quantum Dot Solar Cells

Intermediate band quantum dot solar cells (IB QD SCs) represent an advanced photovoltaic architecture designed to exceed the Shockley–Queisser limit by harvesting sub-bandgap photons. In these devices, a series of quantum dots embedded within the intrinsic region of a p–i–n junction introduces discrete energy states—an intermediate band—between the valence and conduction bands. Photons with energy below the host semiconductor bandgap can be absorbed in a two-step process: first promoting electrons from the valence band into the intermediate band, and then into the conduction band, thereby extending the spectral response into the infrared. Quantum dots, typically of InAs, GaSb or type-II heterostructures, offer strong carrier confinement and tunable energy levels via size, composition or strain engineering. Key challenges include maintaining half-filled intermediate band occupancy, minimising non-radiative recombination, controlling carrier lifetimes and managing strain in multi-stacked dot arrays. Recent advances in epitaxial growth, nanostructure design and concentrator integration have driven reported efficiencies closer to practical thresholds. Implementations under concentrated illumination and at room temperature highlight the promise of IB QD SCs for high-efficiency, broadband energy conversion.

Research from Nature Portfolio

Foundational experimental work demonstrated the first InAs/GaAs quantum dot intermediate band solar cell concentrator module, achieving over 15 % conversion under industry-standard testing. Detailed carrier dynamics studies confirmed that sequential two-step absorption via the intermediate band dominates photocurrent generation at high concentration, validating theoretical simulations of intermediate band transport. Another seminal study established a two-step photon up-conversion solar cell employing InAs quantum dots at a hetero-interface of AlGaAs/GaAs. Below-bandgap photons excite carriers into dot states, which are then further excited by a second sub-bandgap photon to contribute to the photocurrent. This up-conversion approach yielded a two-orders-of-magnitude increase in additional photocurrent and enhanced photovoltage, illustrating the benefits of engineered quantum dot arrays for spectral extension and efficiency gains.

Intermediate Band Quantum Dot Solar Cells publication trend

The graph below shows the total number of articles in intermediate band quantum dot solar cells across all publications each year (not limited to Nature Index journals).

Technical terms

Intermediate band: A set of discrete energy levels within the semiconductor bandgap enabling sequential sub-bandgap photon absorption.

Quantum dot: A nanoscale semiconductor crystal that confines carriers in all three spatial dimensions, producing discrete energy states.

Two-step photon absorption: A process in which an electron absorbs two photons of lower energy in succession to transition from the valence to the conduction band via an intermediate state.

External quantum efficiency (EQE): The ratio of collected charge carriers to incident photons as a function of wavelength, indicating spectral response.

References

1. Intermediate-band dynamics of quantum dots solar cell in concentrator photovoltaic modules. Scientific Reports (2014).
2. Two-step photon up-conversion solar cells. Nature Communications (2017).
3. Performance optimization of In(Ga)As quantum dot intermediate band solar cells. Discover Nano (2023).
4. Room Temperature Operation of a Quantum Ratchet Intermediate Band Solar Cell. Solar RRL (2023).