Partial replacement of Pb in CsPbI3 perovskite solar cells with Ca enhances power conversion efficiency to 13.5% under reverse scan (stabilised at 13.3%), without sacrificing stability.

Singlet fission is an exciton multiplication process in organic molecules in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley−Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extraction of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorganic PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley−Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a solution-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and timeresolved measurements combined with analytical modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs >95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the solution phase (135 ± 5%) before quantitative harvesting of the triplet excitons (95 ± 5%) via a low concentration of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is critical to engineer the surface of the QD with a triplet transfer ligand and that bimolecular decay of triplets is potentially a major loss pathway which can be controlled via tuning the concentration of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the solution-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between organic semiconductors and QDs, one that can provide clear design rules for new materials.

Photon upconversion (PUC) via triplet-triplet annihilation (TTA) from near-infrared (NIR) to blue photons could have important applications especially to bio-imaging and drug delivery accompanied by photochemical reaction. The fundamental challenges in achieving this has been the large anti-Stokes shift combined with the need to efficiently sensitize within the biological transparency window (700-900 nm). This calls for materials combinations with minimal energy losses during sensitization and minimal energy requirements to drive efficient TTA. Here, we demonstrate efficient PUC converting from NIR energy to blue photons using the commercially available material 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-Ac) as the annihilator. With a conventional triplet sensitizing system, TIPS-Ac performed TTA efficiency of 77 ± 3 % despite a relatively small driving force, compared to conventional TTA material converting from NIR to blue, for the TTA of less than 0.32 eV. Combined with Pt(II) meso-Tetraphenyl Tetrabenzoporphine (PtTPBP), which is a heavy atom triplet sensitizer that directly generates triplets upon NIR photon excitation, the resulting system allowed for an anti-Stokes shift of 1.03 eV. Our results highlight the use of direct triplet generation via NIR excitation as a useful path to achieving large anti-Stokes shift and also show that high TTA efficiencies can be achieved even in the absence of large driving energies for the TTA process.

Hybrid inorganic–organic materials such as quantum dots (QDs) coupled with organic semiconductors have a wide range of optoelectronic applications, taking advantage of the respective materials’ strengths. A key area of investigation in such systems is the transfer of triplet exciton states to and from QDs, which has potential applications in the luminescent harvesting of triplet excitons generated by singlet fission, in photocatalysis and photochemical upconversion. While the transfer of energy from QDs to the triplet state of organic semiconductors has been intensely studied in recent years, the mechanism and materials parameters controlling the reverse process, triplet transfer to QDs, have not been well investigated. Here, through a combination of steady-state and time-resolved optical spectroscopy we study the mechanism and energetic dependence of triplet energy transfer from an organic ligand (TIPS-tetracene carboxylic acid) to PbS QDs. Over an energetic range spanning from exothermic (−0.3 eV) to endothermic (+0.1 eV) triplet energy transfer we find that the triplet energy transfer to the QD occurs through a single step process with a clear energy dependence that is consistent with an electron exchange mechanism as described by Marcus–Hush theory. In contrast, the reverse process, energy transfer from the QD to the triplet state of the ligand, does not show any energy dependence in the studied energy range; interestingly, a delayed formation of the triplet state occurs relative to the quantum dots’ decay. Based on the energetic dependence of triplet energy transfer we also suggest design criteria for future materials systems where triplet excitons from organic semiconductors are harvested via QDs, for instance in light emitting structures or the harvesting of triplet excitons generated via singlet fission.