Two-Step Photon Upconversion Solar Cells

• Two-Step Photon Upconversion Solar Cells convert sub-bandgap photons into high-energy carriers through sequential absorption and energy transfer.
• They employ advanced architectures, including double-tunnel junctions and quantum dots, to enhance carrier extraction and boost output current and voltage.
• Materials strategies and precise interface engineering mitigate losses, enabling solar cells to potentially exceed traditional Shockley–Queisser efficiency limits.

Two-Step Photon Upconversion Solar Cells (TPU-SCs) are a class of advanced photovoltaic devices designed to surpass conventional single-junction solar cell efficiency limits by utilizing below-bandgap photons through sequential absorption and upconversion processes. These systems integrate mechanisms such as intraband transitions, triplet sensitization, singlet fission, and nanophotonic field enhancements to convert sub-bandgap photons into higher-energy carriers or photons that can then be absorbed by a standard solar cell, boosting both output current and voltage.

1. Fundamental Upconversion Mechanisms in TPU-SCs

TPU-SCs leverage a sequence of two energy absorption events—the so-called two-step process—to convert low-energy (infrared or sub-bandgap) photons into usable high-energy carriers. The archetypal device comprises a wide-gap semiconductor (WGS) and a narrow-gap semiconductor (NGS), often bridged by a quantum dot (QD) layer or intermediate states (Matsuzawa et al., 4 Sep 2025).

• Interband Excitation: Initial absorption occurs in the NGS (e.g., GaAs or InAs QDs), generating electrons in its conduction band.
• Intraband Excitation/Transition: These electrons are driven by internal fields toward the interface with the WGS (e.g., Al₀.₃Ga₀.₇As). Sub-bandgap photons then promote accumulated electrons into the conduction band of the WGS via intraband transitions; this process is further enhanced by QD incorporation as it relaxes optical selection rules, boosting intraband transition rates.
• Photon Upconversion: Alternatives include triplet-triplet annihilation (TTA), diffusion-free intramolecular TTA (Mattiello et al., 2022), singlet fission photon multipliers (Futscher et al., 2018), or thermally mediated upconversion (Boriskina et al., 2013), all of which share the objective of converting two low-energy photons into a single above-bandgap carrier or photon.

2. Device Architectures and Interface Engineering

Optimal TPU-SC architectures depend on careful management of electronic interfaces, carrier confinement, and spatial overlap between states.

• Double-Tunnel-Junction Structure: An n–p–n double-tunnel junction beneath the QD layer significantly improves TPU performance. It confines electrons at the conduction-band-edge discontinuity, increases their lifetime, and accelerates extraction post-intraband absorption, reducing recombination and back-diffusion (Matsuzawa et al., 4 Sep 2025).
• Quantum Well Islands (QWIs) and Quantum Dots: InAs QWIs and QDs are utilized to maximize the spatial energy transfer cross section, enhancing the probability of exciton-exciton interaction before recombination (Tex et al., 2011). The geometric expansion of quantum islands increases local process rates such as excitonic Auger upconversion.
• Hybrid Sensitizer Layers: Implementation of electronic-doped nanocrystals (CdSe:Au) with conjugated ligands (e.g., 9-anthracene acid) allows for ultrafast (∼1–2 ps) hole routing, suppressing parasitic charge transfer and yielding near-100% Dexter-type triplet energy transfer efficiency (Ronchi et al., 2020).

3. Rate Equations and Physical Models

TPU-SC operation is quantitatively described via coupled rate equations governing carrier and exciton populations:

• Effective Decay and Exciton Kinetics:

$$\frac{1}{\tau_e} = \frac{1}{\tau_i} + \frac{1}{\tau_p(n)}$$

where $\tau_e$ is the effective lifetime, $\tau_i$ the intrinsic state decay, and $\tau_p(n)$ the upconversion process constant, which depends on exciton density and hence on spatial cross section (Tex et al., 2011).

• Interface-Driven Population Dynamics:

$$\frac{\partial n_{NGS}}{\partial t} = G_{inter,784} - \frac{n_{NGS}}{\tau_{NGS-to-ht}} + \frac{n_{ht}}{\tau_{ht-to-NGS}}$$

$$\frac{\partial n_{ht}}{\partial t} = \frac{n_{NGS}}{\tau_{NGS-to-ht}} - \frac{n_{ht}}{\tau_{ht-to-NGS}} - \frac{n_{ht}}{\tau_{rec,ht}} + G_{up,784} - G_{up,1319} - \frac{n_{ht}}{\tau_{th}}$$

where $n_{NGS}$, $n_{ht}$ denote electron densities in the NGS and at the heterointerface, and various $G$ and $\tau$ terms specify generation rates and lifetimes under different excitations (Matsuzawa et al., 4 Sep 2025).

• Dynamic Balance—Local vs. Non-Local Processes:

Non-local (photon-recycling, TS-TPA) and local (Auger) processes compete with respective time constants that scale with exciton density and spatial overlap. The ratio $x = \eta_{non}/\eta_{loc}$ (internal quantum efficiencies of each channel) determines which pathway dominates under specific illumination conditions (Tex et al., 2011).

4. Experimental Validation and Efficiency Enhancement

TPU-SC designs are validated by multiwavelength excitation experiments, photoluminescence studies, and quantum efficiency measurements:

• Two-Color Excitation: Using simultaneous IR and visible illumination, devices with double-tunnel-junctions show superlinear photocurrent enhancements, indicating improved intraband absorption and electron extraction over conventional designs (Matsuzawa et al., 4 Sep 2025).
• Photonic and Metasurface Enhancement: Double-layer metasurface architectures (stacked silicon nanodisc arrays and photonic crystal slabs) create split Rayleigh–Wood anomalies, resulting in resonant near-field amplification and efficient light trapping. Upconversion photoluminescence enhancement factors of ∼2.7 compared to planar substrates are realized, even with only ∼1% of upconversion material directly exposed to the near-field (Manley et al., 2020).
• Gold Nanostructures and Field Concentration: Topologically optimized gold structures atop Er³⁺-doped films achieve field concentration factors exceeding 30, with upconversion enhancement near 913× at moderate excitation intensities (Christiansen et al., 2019), demonstrating the role of electromagnetic design in under-solar-flux conditions.
• Singlet Fission Multiplier Layer: Films combining organic singlet fission materials and quantum dots efficiently split a singlet exciton into two triplets, then transfer energy for bandgap-matched emission, leading to photoluminescence and operational stability improvements over tandem architectures (Futscher et al., 2018).

5. Materials Strategies and Loss Mitigation

Selection and structuring of materials are pivotal to suppressing recombination, quenching, and parasitic losses.

• Quantum Dot Engineering: Maximizing Urbach tail states within QD devices facilitates both 1PA and 2PA contributions to carrier generation, with measured coefficients (e.g., $B_{2PA} \approx 5.7\,\text{cm/GW}$) highlighting the role of engineered tailing states in upconversion (Li et al., 2015).
• Triplet Energy Alignment and Quenching: Proper energetics (ΔE) between sensitizer and emitter triplet levels are mandatory to localize long-lived excitons on the emitter, maximizing TTA yield. Excessive sensitizer concentration can, however, accelerate triplet quenching, reducing efficiency; optimal device loading is strictly below typical solubility limits for high performance (Jefferies et al., 2019).
• Barrier Layers and Interface Passivation: The use of 2D perovskite spacers (e.g., PEA₂PbI₄) between 3D perovskite sensitisers and organic emitters (rubrene/DBP) effectively mitigates singlet exciton back-transfer via FRET, allowing enhanced upconversion at lower excitation power densities (Sloane et al., 9 May 2025).

6. Impact, Limitations, and Future Directions

TPU-SCs—by integrating double-tunnel-junction structures, hybrid sensitizers, nanophotonic field enhancers, and interface-engineered layers—offer a principled route to exceeding the Shockley–Queisser efficiency limit. Quantum dot and nanophotonic integration relax selection rules and provide tunable spatial overlap, while materials strategies targeting efficient triplet formation and transfer further boost conversion rates.

Limitations persist in achieving sufficient upconversion efficiency under one-sun conditions, controlling exciton migration in the solid state, and optimizing device architectures for maximal spatial cross section and minimum recombination. The required balance between barrier thickness for back-transfer suppression and charge transfer efficiency presents a continuing challenge for practical device realization.

Continued research in designing tailored nanostructures, advanced interface engineering, and systematic theoretical-experimental optimization is suggested to approach the predicted limiting efficiencies (e.g., up to 73% under ideal TPV conditions (Boriskina et al., 2013)) and fully realize the spectrum-harvesting potential of TPU-SCs. Advances in computational modeling (such as open-source device design simulators) are expected to accelerate this optimization and deployment trajectory.