1.1.

Principles of Sensitized Triplet–Triplet Annihilation-Based Upconversion

UC of low-energy photons from a noncoherent radiation source, such as the sun, is most frequently a multistep process that involves the sensitized population of metastable optically dark electronic states of a given system.⁵ These can transit to higher emissive states by receiving additional energy through energy transfer (ET) from other sensitizers (ions or molecules) or by mutual annihilation upon biexcitonic collisions. The radiative transition from the reached higher energy level to the ground state leads to the emission of the desired higher energy photon. There are two main reasons why most frequently an ET process is involved: first, it is not necessary that one sort of ion or molecule provides all needed energy levels; second, an excited ion or molecule does not have to absorb more than one photon, which would be an extremely unlikely event requiring high excitation intensity.

Indeed, to realize devices working under standard solar irradiance, traditional UC mechanisms, such as second-harmonic generation or sequential multiphoton absorption by lanthanide ions, are still not suitable because they require coherent and/or high-intensity irradiation to reach high efficiencies.^5,9 Conversely, the sTTA-UC also occurs at excitation intensities as low as a few $\mathrm{mW}{\mathrm{cm}}^{-2}$ and exhibits quantum efficiencies (${\mathrm{QY}}_{\mathrm{uc}}$) at low power that are orders of magnitude higher than those of the aforementioned processes.^10–12 In this multistep UC route, energy absorbed by a sensitizer is transferred to a second moiety, the emitter, by means of a triplet–triplet ET process. Subsequently, two optically dark emitter triplets ($E_{\mathrm{T}}$) undergo triplet–triplet annihilation (TTA) to produce one excited singlet state, from which a photon with higher energy than those absorbed is emitted [Fig. 2(a)]. Since its demonstration in solution with concentrated sunlight and thanks to its surprisingly high efficiency at low powers, materials for sTTA-UC have been intensively studied and developed to achieve higher performances and to cover a larger fraction of the visible (Vis)–near-infrared (NIR) spectrum.^7,12,30–32 By choosing the proper emitter/sensitizer pair, UC quantum efficiencies (${\mathrm{QY}}_{\mathrm{uc}}$) above 30% have been obtained at excitation power densities of 10s of $\mathrm{mW}{\mathrm{cm}}^{-2}$. This remarkable value is not far from the maximum theoretical yield of 50% arising from the energy conservation law.^13,33 Figure 2(b) shows the gradual increment of ${\mathrm{QY}}_{\mathrm{uc}}$ achieved in the last 10 years. Notably, despite a slower initial development, a good performance is now assessed even for solid-state materials. Nowadays, the research on sTTA-UC for solar applications is indeed focused on the design and development of solid systems that match the strict requirements for technological applications. The crucial issue for solid sTTA-UC is to keep the proper mixing of sensitizer and emitter moieties while maintaining the exciton diffusivity that maximizes the ET and the TTA steps as in low-viscosity solutions. If these requirements are met, the excitation intensity threshold $I_{\mathrm{th}}$ required to obtain the maximum ${\mathrm{QY}}_{\mathrm{uc}}$ can be achieved below the solar irradiance, thus allowing for the design and development of UC-assisted devices.³⁴ However, despite several materials based on polymers,^21,35–38 supramolecular gels,^39–41 organic⁴² or polymeric^22,43 glasses, and hybrid nanocrystal/dye films²³ showed interesting performances at low powers, examples of prototype solar devices are still in the embryonal state.

Fig. 2

(a) Energy-level diagram for the sTTA-UC process. A sensitizer with absorptance ${\alpha }_{\mathrm{sens}}$ is excited into a singlet state ($S_S$) that efficiently undergoes intersystem crossing (ISC) into the triplet state ($S_{\mathrm{T}}$). ET then competes with back energy transfer (BET) to $E_{\mathrm{T}}$. These $E_{\mathrm{T}}$ can either spontaneously decay (with a rate constant $k_{\mathrm{SD}}$) or undergo TTA to an excited singlet state of the emitter ($E_{\mathrm{S}}$). Emitter quintet states ($E_{\mathrm{Q}}$) are not energetically accessible. (b) Evolution of sTTA-UC quantum efficiency ${\mathrm{QY}}_{\mathrm{uc}}$ from its demonstration with sunlight in 2006, both in liquid- [Refs. 13 and 1415.16.17.18.19.–20] and solid-state environment [Refs. 2122.23.24.25.26.27.28.–29]. The dashed line marks the theoretical upper limit of 50% in the UC process.

2.1.

Silicon-Based Devices

The Australian research group lead by Schmidt^2,45,46 is a pioneer in the sTTA-UC field not only for the deep study of the fundamental photophysics occurring in the process but also for the development of numerical models and prototype devices that describe and exploit the sTTA-UC. In particular, they focused on the efficiency enhancing of silicon-based SC cells, the most diffuse solar devices commercially available. In 2012, they reported the first integrated a-Si:H/TTA-UC photovoltaic device as a proof-of-principle of a third-generation device, and they demonstrated how to characterize such an upconversion-enhanced solar cell (UC-SC) so that its performance under solar conditions may be evaluated.⁴⁷ As shown in Fig. 4, they realized a multilayer device based on the use of a thin bifacial a-Si:H $p\text{-}i\text{-}n$ cell coupled to a sTTA-UC solution where the tetrakisquinoxalinoporphyrin (${\mathrm{PQ}}_4\mathrm{Pd}$) and its derivative ${\mathrm{PQ}}_4\mathrm{PdNA}$ were used as sensitizers and rubrene as an annihilator/emitter moiety for NIR-to-VIS UC. With a 50-nm-thick UC-SC, they observed an overall relative energy conversion efficiency increase of about 0.1% under 48 suns of noncoherent white excitation.

Fig. 4

(a) An illustration of the integrated a-Si:H $p\text{-}i\text{-}n/\mathrm{UC}$ device. Low-energy (shown in red) photons pass though the $p\text{-}i\text{-}n$ structure and excite dye molecules in the UC unit, which subsequently returns photons of a shorter wavelength (yellow). (b) The molecules utilized for photochemical UC: two Pd porphyrins, ${\mathrm{PQ}}_4\mathrm{Pd}$ and ${\mathrm{PQ}}_4\mathrm{PdNA}$, were employed as sensitizers in combination with the highly efficient TTA emitter rubrene. (c, d) Spectral characterization: comparison of the EQE (black for 50 nm, gray for 100-nm $i$-layer) and transmission curves (dashed) of the respective a-Si:H SC with the absorption cross section (s, orange or red) and emission (green) profile of the UC unit constituents. (c) 50-nm a-Si:H SC and ${\mathrm{PQ}}_4\mathrm{Pd}$, (d) 100-nm a-Si:H SC and ${\mathrm{PQ}}_4\mathrm{PdNA}$. Reproduced from Ref. 47 with permission from The Royal Society of Chemistry.

A following improvement of the device architecture lead to an enhancement of $\sim 0.2\%$ under 19 suns,⁴⁸ a value further enlarged with the inclusion of a back reflector to improve the harvesting of the upconverted light by the silicon layer.⁴⁹

2.2.

Organic and Hybrid Devices

The electronic properties of the most efficient sTTA-UC pairs allow for the exploitation of light in the NIR and Vis spectral ranges. Therefore, it is not difficult to imagine that these systems can be coupled with a SC with an energy bandgap wider than silicon as dye-sensitized solar cells (DSSC). DSSCs are a photovoltaic technology that has generated significant interest across a broad range of research fields since their first report.⁵⁰ Unfortunately, despite the large effort made, the highest reported device efficiencies remain relatively low. However, this is partially compensated for by low projected manufacturing costs and their strong performance in ambient and diffuse lighting conditions.

The large driving forces required for effective charge transfer in DSSCs necessitate the use of absorbers with large absorption thresholds.^51,52 This large energy gap, therefore, results in a significant section of the solar spectrum remaining unused by the cell, which can be recovered using a suitable UC system.⁵³ In 2013, Nattestad et al.⁵⁴ demonstrated that a DSSC cell displays enhanced current under subbandgap illumination at a low light concentration (3 suns) when an SC has been coupled to the a ${\mathrm{PQ}}_4\mathrm{PdNA}/\text{rubrene}$ solution, whose upconverted emitted photons are recaptured by the D149 dye.

In 2016, a significant step forward in this research was made by Akindoyo et al.,⁵⁵ who presented a study where, for the first time, the sTTA-UC enhancement of an SC performance is combined with the ability to embed the chromophores in polymer matrices that can be tailored to films and many different shapes and sizes. The synthetic flexibility and longevity of these sTTA-UC films are of the utmost importance for economically practical applications. The films are indeed robust against oxygen quenching and can be readily tailored to existing SC architectures [Fig. 5(a)]. The photovoltaic performance of the UC-assisted DSSC device [Figs. 5(b) and 5(c)] was measured under both high-power coherent laser and low-power incoherent light irradiation.⁵⁶ The resulting photovoltaic performance is summarized in Fig. 5(c). In a DSSC based on a $5\text{-}\mu \mathrm{m}$-thick mesoporous ${\mathrm{TiO}}_2$ layer and a single sTTA-UC film, the short-circuit photocurrent $J_{\mathrm{sc}}$ was enhanced by 13.6% and 20.3% upon illumination with the equivalent of 1 sun and 2 suns, respectively. The addition of a rear reflector to utilize more effectively the upconverted photons almost doubles the $J_{\mathrm{sc}}$ enhancement observed.

Fig. 5

(a) Schematic illustration of sTTA-UC film-assisted absorption of photons with energies below the HOMO−LUMO gap of the chromophore sensitizer D131 employed in a DSSC device. (b) Normalized (yellow) external quantum efficiency spectrum of a D131-sensitized ${\mathrm{TiO}}_2$ photoelectrochemical cell, (red) Q-band absorption of PdTPBP, and (blue) UC-emission spectrum of a sTTA-UC film. (c) sTTA-UC induced the short-circuit current density ($J_{\mathrm{sc}}$) enhancement using an LED source (625 nm) illumination of the DSSC device with $5\text{-}\mu \mathrm{m}$ sensitized ${\mathrm{TiO}}_2$ layer. Error bars represent standard deviation of five measurements. Reprinted with permission from Ref. 56. Copyright 2016 American Chemical Society.

An alternative device architecture for UC-enhanced DSSC was proposed by the group lead by Hanson in 2015, inspired by the concept of intermediate band cells (vide infra, Sec. 2.3).⁵⁷ To facilitate the sTTA-UC and the charge separation of the upconverted excitons, they used an inorganic−organic interface based on self-assembled bilayers of a TTA molecular pair on metal oxide surfaces. This self-assembled motif via metal ion linkages is shown in Fig. 6(a) and provides a simple and modular method for multimolecular assembly on a metal oxide surface.^58,59 This strategy is not diffusion limited and offers unprecedented geometric and spatial control of the donor−acceptor interactions at an interface. The bilayer film depicted is prepared by stepwise soaking of nanocrystalline ${\mathrm{TiO}}_2$ or ${\mathrm{ZrO}}_2$ films in three separate solutions of 4,4′-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid (DPPA), $\mathrm{Zn}{({\mathrm{CH}}_3\mathrm{COO})}_2$, and Pt(II)tetrakis(4-carboxyphenyl)porphyrin (PtTCPP). Anthracene and platinum porphyrin derivatives were selected as the TTA-UC pair because they are known to exhibit efficient UC-emission. Figure 6(b) shows the electronic transitions and relative energetics for DPPA and PtTCPP and the conduction band (CB) energies of ${\mathrm{TiO}}_2$ and ${\mathrm{ZrO}}_2$. The authors demonstrated the generation of photocurrent with subbandgap photons by monitoring the peak photocurrent for ${\mathrm{TiO}}_2$-DPPA-Zn-PtTCPP with respect to 532-nm excitation. The photocurrent density exhibits a change in slope from 1.2 to 0.61 that is reminiscent of the quadratic to linear behavior typical for sTTA-UC [Fig. 6(c)]. This change in slope is in contrast to that for ${\mathrm{TiO}}_2$-PtTCPP, which exhibits a slope of 0.53 throughout the entire excitation intensity range from $\sim 300$ to $7000\mathrm{mW}{\mathrm{cm}}^{-2}$. Under the same conditions, a similar linear dependence ($\text{slope}=0.5$) is observed for the common DSSC dye employed N3, which is known to undergo a one photon-to-one electron photocurrent generation mechanism.⁶⁰ By combining intensity dependence and incident photon-to-electron conversion efficiency measurements, they further demonstrated that these bilayer films effectively generate a photocurrent by two different mechanisms: (1) direct excitation and electron injection from the acceptor molecule and (2) low-energy light absorption by the sensitizer molecule followed by sTTA-UC and electron injection from the upconverted state. Notably, the power conversion efficiency from the upconverted photons is the highest yet reported for an SC integrated with sTTA-UC.⁶¹

Fig. 6

(a) Chemical structure of DPPA and PtTCPP and a schematic representation of the bilayer film on a metal oxide surface (${\mathrm{MO}}_2$-DPPA-Zn-PtTCPP). (b) Electronic transitions and relative energetics for DPPA and PtTCPP and the CB energies of ${\mathrm{TiO}}_2$ and ${\mathrm{ZrO}}_2$. ($S_n=\text{singlet state}$, $T_n=\text{triplet state}$, ISC = intersystem crossing, TET = triplet energy transfer, TTA = triplet−triplet annihilation). (c) Peak photocurrent density from ${\mathrm{TiO}}_2$-DPPA-Zn-PtTCPP and ${\mathrm{TiO}}_2$-PtTCPP with respect to 532-nm excitation intensity. (${\mathrm{TiO}}_2$ thin film working electrode, platinum counter electrode, 0.3 M TBAClO4 in ${\mathrm{N}}_2$ deaerated MeCN electrolyte at 0-V applied potential). Reproduced from Ref. 58 with permission from The Royal Society of Chemistry.

Fig. 7

(a) Structure of the emitter A, and sensitizers PtP, and PdP molecules with the schematic representation of (b) the dual sensitized bilayer and (c) dual sensitized trilayer films. (d) Absorption spectra for the dual sensitized bilayer and trilayer as well as (e) their constituent single sensitizer films on ${\mathrm{TiO}}_2$. The calculated spectra, from the scaled sum of the single sensitized films, are shown as dashed orange lines. Reprinted with permission from Ref. 62. Copyright 2017 American Chemical Society.

Further developing this strategy, to overcome the intrinsic limitation of porphyrin-like sensitizers, i.e., their narrow absorption bandwidth that limits the system absorptance, Dilbeck et al.⁶² recently introduced a trilayer architecture as a new system for incorporating multiple sensitizers into sTTA-UC film-based DSSC (Fig. 7). By means of interlayer ET from both sensitizers in the bilayer, and in energy cascade for the trilayer, it is possible to efficiently absorb low-energy light and generate acceptor triplet-excited states. A photocurrent enhancement is observed in the multisensitizer films due to cooperative broadband absorption of employed light harvesters. The efficiency of the trilayer device ($1.2\times {10}^{-3}\%$), with high sensitizer density and directional ET that cascades toward the charge separation interface, marks another record among the performance of integrated sTTA-UC-SCs currently reported. Importantly, intensity dependence measurements demonstrate that high triplet densities and peak UC efficiency can be achieved at subsolar irradiance using dual sensitized self-assembled multilayer films. Even though the device efficiency is still low, these results suggest that an integrated sTTA-UC DSSC offers a viable means of enhancing solar energy harvesting efficiencies upon several improvements, including optimizing overlap between the sensitizer(s) and the solar spectrum, increasing injection yield from the singlet-excited state of the acceptor molecule, maximizing the regeneration rate, and reducing the triplet quenching losses.

2.3.

Intermediate Band Solar Cells

Intermediate band solar cells (IBSCs) are a third-generation SC technology where, as the name suggests, the inclusion of an absorbing band in the energy gap of the “main” semiconducting material allows for the increase in the efficiency, potentially exceeding the Shockley–Queisser limit. The new band opens a second absorption channel; therefore, the photoexcitation of carriers into the CB can occur from direct absorption from the valence band (VB) or after consequent absorption of lower energy photons from the VB to the intermediate band (IB) and then from the IB to the CB.

The fabrication of IBSCs has always been limited by strict constraints of the intermediate absorption band, namely (i) a generally low-absorption cross section, (ii) the need to use spectrally selective absorbers, i.e., to avoid the excitation of carriers to the IB via a higher energy photon, and (iii) the rapid nonradiative relaxation from the IB to the VB. Organic IBSCs based on sTTA-UC allow overcoming all the above constraints by exploiting the unique electronic structures of the employed components. In detail, the extremely high molar extinction coefficient of metalated porphyrins allows for high absorption of light, while the narrow absorption peak selects only the wavelengths that will be used to activate the TTA process. Simultaneously, the $E_{\mathrm{T}}$ that are populated via TTA are metastable dark states with an extremely low oscillator strength that makes them very long-lived and avoids fast nonradiative relaxation. For all these reasons, IBSCs based on sTTA-UC are a new, exciting class of devices potentially able to harvest subbandgap photons.

The concept of IBSCs was presented in 1997 by Luque and Martí,⁶³ while the idea of fabricating an organic IBSC based on sTTA-UC first came by Ekins-Daukes and Schmidt⁶⁴ in 2008. They demonstrated that the most efficient electronic structure for an sTTA-UC IBSC features a symmetrical band structure arrangement, with the IB lying at around midgap. The power conversion efficiency has been calculated to be up to 40.6% for 1.9-eV excitation light under 1-sun illumination, exceeding the Shockley–Queisser limit. However, no significant IBSC has been fabricated until very recently.² The first TTA-UC-based IBSC was demonstrated by Simpson et al.⁵⁹ in 2015 by staining a screen-printed commercial ${\mathrm{TiO}}_2$ paste with an upconverting solution composed of Pt(II)-tetrabenzotetraphenylporphyrin (PtTBTPP, the sensitizer) and 4,4′-(anthracene-9,10-diylbis(ethyne-2,1-diyl))dibenzoic acid (BDCA, the emitter). The energy-level schematic of the prototype devices is shown in Fig. 8. This IBSC is shown to generate photocurrent when illuminated with incoherent light whose energy is well below the bandgap of ${\mathrm{TiO}}_2$ ($560\mathrm{nm}<\lambda <640\mathrm{nm}$), while a control device with no BDCA showed no photocurrent under the same conditions.

Fig. 8

Energy-level diagram of an IBSC, with excitation of BDCA (left structure, blue states) occurring either by direct excitation with a single high-energy photon ($\mathrm{h}{\upsilon }_1$) or by two low-energy photon ($\mathrm{h}{\upsilon }_2$) absorptions, commencing with photoexcitation of PtTBTPBP (right structure, red states). Left inset demonstrates energetically favorable electron injection into the ${\mathrm{TiO}}_2$ CB from BDCA ($S_1$) but not BDCA ($T_1$), while the right inset shows the absorption spectra of PtTBTPP and BDCA (measured in DMF). Reproduced from Ref. 59 with permission from The Royal Society of Chemistry.

A different approach to sTTA-UC-based IBSCs has been recently given by Lin et al.⁶⁵ They fabricated an all-solid-state organic cell with the active layer composed of a blend of PtTPTBP and $\alpha$-sexithiophene ($\alpha$-6T) as the sensitizer and emitter, respectively. The acceptor layer for the IBSC was diindenoperylene and was compared with C60, which has a charge transfer state with a much lower energy than the triplet states of $\alpha$-6T. Therefore, the cell with C60 has a direct funneling of charges from $\alpha$-6T triplets to C60, bypassing TTA and serving as a model, control system. With their IBSC, the authors were able to reach a 12% increase in photocurrent with respect to the control system under subbandgap illumination intensity lower than $0.1\mathrm{mW}{\mathrm{cm}}^{-2}$. The examples reported above are interesting and promising steps forward to the realization of efficient IBSC based on sTTA-UC due to its photophysical properties that overcome the major limitations of this SC technology.