A team led by Kunzmann reported carrier multiplication of nearly 130% by harnessing singlet fission to inject electrons from pentacene-based molecules into a dye-sensitized solar cell. Published in Angewandte Chemie in 2018, the study demonstrated that a single absorbed photon could generate roughly 1.3 electrons, a result that challenges conventional assumptions about the upper limits of photovoltaic efficiency. The finding sits within a broader line of research stretching back over a decade, where multiple groups have pushed external quantum efficiency past the 100% barrier using organic materials paired with silicon.
How One Photon Produces More Than One Electron
Standard solar cells operate under a simple constraint: one photon in, one electron out. High-energy photons that carry more energy than the cell’s bandgap waste the surplus as heat. Singlet fission sidesteps that loss. In this process, a single high-energy photon excites a molecule into a singlet state, which then rapidly splits into two lower-energy triplet states. Each triplet can, in principle, inject a separate electron into the circuit. A singlet fission quantum yield of 100% means one excited carrier reaches the electrode per absorbed photon; a yield of 200% means two triplets successfully transferred their energy.
The Kunzmann study used a novel pentacene dimer, labeled P2, and compared it against a pentacene monomer, P1. Both molecules underwent singlet fission, generating triplet excited states through what the authors described as a rapid, spin-allowed process. Efficient electron injection from those triplet states was confirmed for both P1 and P2, but the dimer architecture proved more effective. The abstract reports carrier multiplication of nearly 130%, meaning that for every 100 photons absorbed, roughly 130 electrons were injected into the titanium dioxide electrode of the n-type dye-sensitized cell. This quantitative result appears both in the journal article and in the corresponding PubMed entry, underscoring that the effect was measured at the device level rather than inferred solely from spectroscopy.
In practical terms, the pentacene dimer acts as a spectral converter. It absorbs high-energy photons that silicon or titanium dioxide would otherwise handle inefficiently, splits the excitation into two triplets, and then transfers those excitations as separate electrons into the semiconductor. Because the singlet fission step is ultrafast, it can outcompete nonradiative decay channels that typically dissipate excess energy as heat. The challenge is ensuring that both triplets survive long enough, migrate to the interface, and inject their charges without recombining. Kunzmann’s device architecture, by carefully matching energy levels between the organic layer and the oxide electrode, demonstrated that this delicate sequence can be made to work reproducibly.
Earlier Breakthroughs That Set the Stage
The 130% result did not emerge in isolation. Several years earlier, a team publishing in Science demonstrated that singlet fission could yield greater than 100% external quantum efficiency in a working photovoltaic device, reporting a peak EQE of roughly 109% at 670 nm in pentacene-based organic photovoltaic cells. That experiment was the first clean proof that the two-for-one photon trick could actually deliver extra current, not just extra excitons, in a real device. By directly measuring the photocurrent as a function of wavelength and comparing it with the number of incident photons, the researchers showed that the current exceeded what would be possible if each photon produced only one collected electron.
A separate effort indexed by the U.S. Department of Energy pushed the number higher by using slow-light techniques to boost absorption in thin pentacene films, achieving an external quantum efficiency of 126%. In that work, the team engineered photonic structures that lengthened the optical path within the absorbing layer, effectively giving each photon more opportunities to interact with the pentacene without increasing material thickness. The resulting device, documented in an OSTI report, demonstrated that optical design can be just as important as molecular selection in exploiting singlet fission.
Another group demonstrated a silicon–singlet fission tandem architecture that also exceeded the 100% benchmark, using an organic layer to harvest high-energy photons and pass their energy into a crystalline silicon cell beneath. By stacking a singlet fission material on top of silicon, the device could convert blue and green light into two lower-energy excitations while allowing red and near-infrared photons to pass through to the silicon junction. This tandem configuration, described in an energy letters study, proved that the concept could work with the dominant commercial solar material rather than being confined to niche organic cells.
Complementing these device demonstrations, another experiment used engineered photonic crystals to create slow-light modes in pentacene films, reporting an EQE of 126% at specific wavelengths. By carefully tuning the interaction between light and the absorbing medium, the researchers were able to intensify the electric field within the organic layer without sacrificing transparency or introducing large reflection losses. Together, these studies established a toolkit (molecular design, optical engineering, and tandem integration) that later work like Kunzmann’s could draw upon.
Why 130% Is Not the Same as 130% Efficient Panels
Numbers like 109%, 126%, or 130% can be easy to misinterpret. They refer specifically to the ratio of injected electrons to absorbed photons at particular wavelengths, not to overall power conversion efficiency or to the fraction of total sunlight converted into usable electricity. A solar cell can extract more than one electron per photon from high-energy light while still losing energy at other wavelengths, through recombination of charges before they reach the contacts, or via resistive losses in the wiring and electrodes. When all these factors are accounted for, the net power output of today’s singlet fission devices remains modest.
The theoretical benchmark for a conventional single-junction cell is the Shockley–Queisser limit, which caps silicon devices at around one-third of the incident solar power under standard conditions. Singlet fission offers a route around this limit by more efficiently using high-energy photons that would otherwise be wasted as heat. An analysis from the National Renewable Energy Laboratory estimated that a simple multilayered solar cell based on singlet fission could increase photovoltaic power conversion efficiency by more than 43% above the Shockley–Queisser limit. The NREL modeling, presented in a technical report, assumes idealized materials and perfect interfaces, but it highlights the scale of the opportunity if the underlying physics can be fully harnessed in manufacturable devices.
Even if practical efficiencies fall short of those theoretical projections, incremental gains can have outsized impacts at scale. A few percentage points of additional efficiency can translate into smaller panel areas for the same power output, reduced material costs, and lower balance-of-system expenses. In that context, the ability to generate 1.3 or more electrons per absorbed high-energy photon is not a curiosity; it is a potential lever for reshaping the economics of solar power.
Recent Results Signal Continued Progress
Research published years after the Kunzmann study has continued to push charge-generation numbers upward. A preprint indexed by NASA’s Astrophysics Data System describes an exciton fission–enhanced silicon solar cell combining zinc phthalocyanine with tetracene, reporting a peak charge generation efficiency of (138 ± 6)% per photon absorbed in the tetracene layer. In that architecture, tetracene serves as the singlet fission material, splitting high-energy excitations into two triplets, while zinc phthalocyanine facilitates energy transfer and spectral matching to the underlying silicon. The authors present their findings in an arXiv manuscript, emphasizing that the reported multiplication factor is measured at the level of charge collection rather than just exciton formation.
The jump from 109% in 2013 to 126%, then 130%, and now potentially 138% traces a clear trajectory. Each step has involved a different material system or optical strategy, from neat pentacene films to pentacene dimers to tetracene–zinc phthalocyanine blends coupled with silicon. The trend suggests that singlet fission is a flexible platform rather than a one-off trick tied to a single molecule. By adjusting molecular packing, energy-level alignment, and light-management structures, researchers have repeatedly found ways to convert more of the sun’s high-energy photons into usable charge.
Significant hurdles remain before singlet fission moves from the lab to commercial rooftops. Organic layers must withstand years of ultraviolet exposure and thermal cycling without degrading, interfaces need to be engineered to minimize recombination while remaining manufacturable at scale, and the added complexity of tandem or multilayer designs must be justified by clear cost and performance benefits. Nonetheless, the steady march of reported carrier multiplication, from just over unity to well above 1.3 electrons per photon in optimized structures, indicates that the underlying physics is robust. As materials scientists and device engineers refine these concepts, singlet fission–enabled solar cells could evolve from scientific curiosities into practical tools for pushing photovoltaic efficiency beyond long-accepted limits.
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*This article was researched with the help of AI, with human editors creating the final content.
