Luminescent solar concentrators (LSCs) are devices that concentrate light using a transparent matrix doped with chromophores. LSCs have the potential to outperform other concentration technologies due to their ability to concentrate diffuse light and reshape the solar spectrum to match the optimum spectral characteristics for photovoltaics. This study compares different LSC technologies, including solar windows, within a simulated real-world environment and outlines the impact of upcoming technologies to determine their commercialization potential. We utilized a year of real-world solar spectrum data from Boulder and Amsterdam and define the power efficiencies from different LSC configurations. We find that silicon PV generally outperforms even the best LSC over a calendar year. We outline the potential opportunities in the application of LSC systems relative to traditional photovoltaics.

For perovskite-on-silicon tandem technology to achieve even higher efficiencies than have been demonstrated in recent work1, it is paramount to identify fundamental electrical bottlenecks that are present in the state-of-the-art devices. One such bottleneck, an overly high diode ideality factor (IF) of the perovskite cell, limits the achievable Fill Factor (FF), and it is driven by non-favorable charge recombination characteristics within the perovskite bulk and interfaces. As a result, there have been recent efforts to quantify perovskite-on-silicon IFs using optical techniques2, and earlier work evaluating different methods to accurately determine perovskite IF in single junction solar cells3. In this work we extend the importance of the IF to being a key variable for the design of tandem perovskite-on-silicon solar cells, due to its strong influence over whether tandem sub-cells should be current-matched or intentional current mismatched. Calculations are presented to quantify these links, and its implications on cell design discussed, in view of achieving record R&D cells, and how this may differ for real world applications. We ultimately conclude that minimizing the perovskite cell IF not only increases the achievable efficiency by >3% absolute, but also reduces the requirement to intentionally design mismatched tandems, thereby allowing for improved performance across locations and seasonally dependent spectra. We then evaluate different electrical methods to rapidly quantify sub-cell IFs so that it can be used as a tool to screen for cell improvements with statistically significant results and we discuss its use in terms of being able to fingerprint device regions that have excessive non-radiative charge carrier recombination. A number of case studies from Oxford PV devices are used throughout the work.

We present a soft-stamping method to selectively print a homogenous layer of CdSeTe/ZnS core-shell quantum dots (QDs) on top of an array of Si nanocylinders with Mie-type resonant modes. Using this new method, we gain accurate control of the quantum dot's angular emission through engineered coupling of the QDs to these resonant modes. Using numerical simulations we show that the emission into or away from the Si substrate can be precisely controlled by the QD position on the nanocylinder. QDs centered on a 400 nm diameter nanocylinder surface show 98% emission directionality into the Si substrate. Alternatively, for homogenous ensembles placed over the nanocylinder top-surface, the upward emission is enhanced 10-fold for 150 nm diameter cylinders. Experimental PL intensity measurements corroborate the simulated trends with cylinder diameter. PL lifetime measurements reflect well the variations of the local density of states at the QD position due to coupling to the resonant cylinders. These results demonstrate that the soft imprint technique provides a unique manner to directly integrate optical emitters with a wide range of nanophotonic geometries, with potential applications in LEDs, luminescent solar concentrators, and up- and down-conversion schemes for improved photovoltaics.

Singlet fission is a carrier multiplication mechanism that could make silicon solar cells much more efficient. The singlet-fission process splits one high-energy spin-singlet exciton into two lower-energy spin-triplet excitons. We calculated the efficiency potential of three technologically relevant singlet-fission silicon solar cell implementations. We assume realistic but optimistic parameters for the singlet-fission material and investigate the effect of singlet energy and entropic gain. If the transfer of triplet excitons occurs via charge transfer, the maximum efficiency is 34.6% at a surprisingly small singlet energy of 1.85 eV. For the Dexter-type triplet energy transfer, the maximum efficiency is 32.9% at a singlet energy of 2.15 eV. For Förster resonance energy transfer (FRET), the triplet excitons are first transferred into a quantum dot, from which they then undergo FRET into silicon. For this transfer mechanism, the maximum efficiency is 28.% at a singlet energy of 2.33 eV. We show that the efficiency gain from singlet fission is larger the more efficient the silicon base cell is, which stands in contrast to tandem perovskite–silicon solar cells.

Singlet fission is one of the most promising routes to overcome the single-junction efficiency limit for solar cells. Singlet fission-enhanced silicon solar cells are the most desirable implementation, but transfer of triplet excitons, the product of singlet fission, into silicon solar cells has proved to be very challenging. Here, we report on an all optical measurement technique for the detection of triplet exciton quenching at semiconductor interfaces, a necessary requirement for triplet exciton or charge transfer. The method relies on the growth of individual, single-crystal islands of the singlet fission material on the silicon surface. The islands have different heights, and we correlate these heights to the quenching efficiency of triplet excitons. The quenching efficiency is measured by spatially resolved delayed fluorescence and compared to a diffusion–quenching model. Using silicon capped with a blocking thermal oxide and aromatic monolayers, we demonstrate that this technique can quickly screen different silicon surface treatments for triplet exciton quenching.

The rational combination of tetracene (Tc) with crystalline silicon (c-Si) could greatly enhance c-Si solar cell efficiencies via singlet fission. The Tc/c-Si energy-level alignment (ELA) is though...

Colloidal quantum dots have found many applications and patterning them on micro- and nanoscale would open a new dimension of tunability for the creation of smaller scale (flexible) electronics or nanophotonic structures. Here we present a simple, general, one-step top-down patterning technique for colloidal quantum dots by means of direct optical or electron beam lithography. We find that both photons and electrons can induce a solubility switch of both PbS and CdSe quantum dot films. The solubility switch can be ascribed to cross-linking of the organic ligands, which we observe from exposure with deep-UV photons (5.5 eV) to extreme-UV photons (91.9 eV), and low-energy (3-70 eV) as well as highly energetic electrons (50 keV). The required doses for patterning are relatively low and feature sizes can be as small as tens of nanometers. The luminescence properties as well as carrier lifetimes remain similar after patterning.

In this work, we investigate a tetracene/Si singlet-triplet down-conversion solar cell geometry in which we control the directional emission of quantum dots (QDs). In this system, photons in the visible range (450-550 nm) excite high-energy singlet-excitons in tetracene that rapidly convert into two triplet-excitons at about half the energy. The triplet-exciton energy is transferred to the QDs that subsequently emit at 1000-1100 nm. A significant loss channel is the QD emission that is directed upwards, so anisotropic downward emission into the underlying Si cell is essential. Here, we demonstrate directional light emission of CdSe/ZnS core-shell quantum dots (QDs) coupled to Si Mie resonators fabricated on a Si solar cell. By varying the shape and size of the Mie resonator, interference of the electric and magnetic multipolar modes supported by the resonator is controlled. Placing the QDs in the near-field of the resonator enables efficient coupling of the QD transition dipole with these multipolar modes. Using numerical FDTD calculations we show that the QD emission is efficiently directed into the solar cell. We fabricate nanostructures on a Si substrate using electron-beam lithography and reactive-ion etching. Using soft-stamp imprinting we selectively print CdSe/ZnS QDs (peak emission 800 nm) on top of the nanostructures. We then map the QD emission anisotropy for different Mie resonator sizes, using photoluminescence spectroscopy. Photoluminescence lifetimes show a systematic increase from 7 ns to 17 ns, for Si nanocylinder diameter from 200 to 425 nm (cylinder height 125 nm), consistent with the varying nanostructure resonances as found in FDTD simulations. The anisotropic downward emission demonstrated in this work of QDs coupled to Si nanostructures can enhance the efficiency of a tetracene/Si down-conversion system. Moreover, this work can impact a broad range of other applications in which directional emission is relevant, in solid-state lighting, integrated optics, and photovoltaics.

Efficient light management is essential to achieve high-efficiency solar cell geometries: ensuring the absorption of sunlight in the active region is a fundamental principle of a photovoltaic device. By controlling the spectrum and angular distribution of light in the device, the absorption in the active region can be enhanced. We investigate two solar cell geometries in which we control the directional emission of quantum dots (QDs): a tetracene\Si solar cell based on singlet-triplet down-conversion and a luminescent solar concentrator (LSC) device.

Singlet fission in tetracene generates two triplet excitons per absorbed photon. If these triplet excitons can be effectively transferred into silicon (Si), then additional photocurrent can be generated from photons above the bandgap of Si. This could alleviate the thermalization loss and increase the efficiency of conventional Si solar cells. Here, we show that a change in the polymorphism of tetracene deposited on Si due to air exposure facilitates triplet transfer from tetracene into Si. Magnetic field-dependent photocurrent measurements confirm that triplet excitons contribute to the photocurrent. The decay of tetracene delayed photoluminescence was used to determine a transfer efficiency of ∼36% into Si. Our study suggests that control over the morphology of tetracene during the deposition will be of great importance to boost the triplet transfer yield further.

One way for solar cell efficiencies to overcome the Shockley-Queisser limit is downconversion of high-energy photons using singlet fission (SF) in polyacenes like tetracene (Tc). SF enables generation of multiple excitons from the high-energy photons which can be harvested in combination with Si. In this work we investigate the use of lead sulfide quantum dots (PbS QDs) with a band gap close to Si as an interlayer that allows Foerster Resonant Energy Transfer (FRET) from Tc to Si, a process that would be spin-forbidden without the intermediate QD step. We investigate how the conventional FRET model, most commonly applied to the description of molecular interactions, can be modified to describe the geometry of QDs between Tc and Si and how the distance between QD and Si, and the QD bandgap affects the FRET efficiency. By extending the acceptor dipole in the FRET model to a 2D plane, and to the bulk, we see a relaxation of the distance dependence of transfer. Our results indicate that FRET efficiencies from PbS QDs to Si well above 50 % are be possible at very short, but possibly realistic distances of around 1 nm, even for quantum dots with relatively low photoluminescence quantum yield.

Unusually long charge carrier lifetime in methylammonium lead halide perovskites is a result of the Rashba-split indirect bandgap. At high pressure the bandgap becomes purely direct, with shorter carrier lifetime and higher radiative efficiency.

Methylammonium lead iodide (MAPI) is thought to be a direct band gap semiconductor. The nature of this direct bandgap stands in contrast to the long charge carrier lifetimes of up to 15μs that this material exhibits. I present experimental evidence for the presence of an indirect transition 60meV below the direct transition. Charge carriers in (MAPI) decay via an indirect transition, requiring a phonon for the decay. The necessary phonon contribution renders the decay inefficient, thus slow.