The development of quantum dot solar cells (QDSCs) has followed a trajectory marked by incremental improvements in efficiency, driven by advances in materials synthesis, device engineering, and processing techniques. The unique properties of quantum dots, such as tunable bandgaps, multiple exciton generation, and solution processability, have positioned them as a promising alternative to conventional and emerging photovoltaic technologies. This overview traces the key milestones in QDSC efficiency and contextualizes progress relative to other thin-film and third-generation solar cells.
Early research in QDSCs began in the 1990s, with initial demonstrations of photocurrent generation from quantum dot films. However, efficiencies were below 1% due to poor charge transport and high recombination losses. The first significant breakthrough came in 2005, when researchers achieved 2.7% efficiency using PbS quantum dots with a Schottky junction architecture. This demonstrated the feasibility of colloidal quantum dots as light absorbers and spurred further exploration of materials and device designs.
By 2010, the introduction of heterojunction architectures, combining PbS quantum dots with metal oxide electron transport layers such as TiO2, pushed efficiencies to 5%. This period also saw the development of ligand exchange strategies, where long insulating ligands on quantum dot surfaces were replaced with shorter conductive molecules like mercaptopropionic acid. These exchanges improved charge transport and reduced recombination, addressing one of the major bottlenecks in early QDSCs.
The period from 2011 to 2015 marked rapid progress, with efficiencies climbing from 6% to nearly 10%. Key innovations included the optimization of quantum dot surface chemistry, the introduction of hybrid passivation techniques, and the use of graded bandgap structures to enhance light absorption and charge collection. In 2015, a milestone was reached with a certified efficiency of 10.6% for a PbS QDSC, achieved through a combination of improved hole transport layers and reduced interfacial defects.
Between 2016 and 2020, research focused on advanced device architectures such as tandem cells and the integration of quantum dots with other materials. In 2017, a two-terminal tandem QDSC achieved 12% efficiency by combining PbS quantum dots with perovskite layers, leveraging the complementary absorption profiles of the two materials. This period also saw the exploration of alternative quantum dot compositions, including CdTe and CuInSe2, though PbS remained the dominant material due to its tunable bandgap and strong absorption.
Recent years have seen incremental but steady progress, with record efficiencies reaching 16.6% in 2022 for a single-junction PbS QDSC. This was enabled by advances in defect passivation, interface engineering, and the development of novel charge transport materials. The introduction of zwitterionic ligands, which simultaneously passivate surface traps and improve charge mobility, has been particularly impactful. Additionally, the use of scalable deposition techniques such as spray coating and roll-to-roll processing has improved the commercial viability of QDSCs.
Comparisons with other emerging photovoltaic technologies reveal both strengths and challenges for QDSCs. Perovskite solar cells, for example, have seen faster efficiency gains, reaching over 25% in laboratory settings due to their exceptional optoelectronic properties and rapid materials optimization. However, QDSCs offer advantages in stability and tunability, with degradation rates often lower than those of perovskites under operational conditions. Organic photovoltaics, another solution-processable technology, have plateaued around 18% efficiency but face challenges in environmental stability and large-scale reproducibility. Quantum dot solar cells thus occupy a niche where their unique properties—such as infrared absorption and potential for low-cost manufacturing—complement rather than compete directly with these alternatives.
The evolution of QDSC efficiency has been closely tied to advances in materials chemistry and device physics. Early devices suffered from high trap densities and poor charge extraction, but systematic improvements in surface passivation and interface engineering have mitigated these issues. The development of new ligand chemistries, such as halide and hybrid organic-inorganic ligands, has been critical in reducing non-radiative recombination. Similarly, the optimization of device architectures, including the transition from Schottky junctions to p-i-n and n-i-p structures, has enhanced charge collection and open-circuit voltage.
Processing techniques have also matured, moving from lab-scale spin coating to more scalable methods like blade coating and inkjet printing. These advances have improved the reproducibility and uniformity of quantum dot films, enabling larger-area devices without significant efficiency losses. The integration of QDSCs with silicon and perovskite tandems has further expanded their potential, though challenges in current matching and stability remain.
In summary, the historical progress of quantum dot solar cells reflects a concerted effort to address fundamental materials and device challenges. From initial efficiencies below 1% to recent achievements exceeding 16%, the field has leveraged innovations in quantum dot synthesis, surface chemistry, and device architecture. While QDSCs have not yet matched the efficiency peaks of perovskites or the maturity of silicon, their unique attributes—including bandgap tunability, solution processability, and stability—continue to drive research and development. The trajectory suggests a technology that is steadily overcoming its early limitations, with potential for niche applications where its specific advantages are paramount. Comparisons with other emerging photovoltaics highlight a diverse landscape where different technologies may ultimately find complementary roles in the global energy mix.