Interfacial Charge Recombination Between e−−TiO2 and the I−/I3− Electrolyte in Ruthenium Heteroleptic Complexes: Dye Molecular Structure−Open Circuit Voltage Relationship

Origin of Intersystem Crossing in Red-Light Absorbing Bodipy Derivatives: Time-Resolved Transient Optical and Electron Paramagnetic Resonance Spectral Studies with Twisted and Planar Compounds

We studied the intersystem crossing (ISC) property of red-light absorbing heavy atom-free dihydronaphtho[b]-fused Bodipy derivatives (with phenyl group attached at the lower rim via ethylene bridge, taking constrained geometry, i.e., BDP-1 and the half-oxidized product BDP-2) and dispiroflourene[b]-fused Bodipy (BDP-3) that have a twisted π-conjugated framework. BDP-1 and BDP-3 show strong and sharp absorption bands (i.e., ε = 2.0 × 105 M-1 cm-1 at 639 nm, fwhm ∼491 cm-1 for BDP-3). BDP-1 is significantly twisted (φ = 21.6°), while upon mono-oxidation, BDP-2 becomes nearly planar on the oxidized side (φ = 3.5°). Interestingly, BDP-2 showed efficient ISC (triplet state quantum yield, ΦT = 40%) due to S1/T2 state energy matching. Long-lived triplet excited state was observed (τT = 212 μs in solution and 2.4 ms in polymer matrix), and ISC takes 4.0 ns. Differently, twisted BDP-1 gives weak ISC only 5%, ISC takes 7.7 ns, and the triplet state is populated only with addition of ethyl iodide. Time-resolved electron paramagnetic resonance spectra of BDP-1 revealed the coexistence of two triplet states, with drastically different zero-field splitting D parameters of -2047 MHz and -1370 MHz, respectively, along with varying sublevel population ratios. We demonstrate that the ISC is not necessarily enhanced by torsion of the π-conjugation framework; instead, S1/Tn state energy matching is more efficient to induce ISC even in compounds that have planar molecular structures.

The importance of anchoring ligands of binuclear sensitizers on electron transfer processes and photovoltaic action in dye-sensitized solar cells

The relatively low photon-to-current conversion efficiency of dye-sensitized solar cells is their major drawback limiting widespread application. Light harvesting, followed by a series of electron transfer processes, is the critical step in photocurrent generation. An in-depth understanding and fine optimization of those processes are crucial to enhance cell performance. In this work, we synthesize two new bi-ruthenium sensitizers with extended anchoring ligands to gain insight into underlying processes determining photovoltaic action mechanisms. The structure of the compounds has been confirmed, and their properties have been thoroughly examined by various techniques such as NMR, IR, elemental analysis UV-Vis, cyclic voltammetry, and electroabsorption. The experimental characterization has been supported and developed via extensive quantum-chemical calculations, giving a broad view of the presented molecules' properties. Finally, the DSSC devices have been assembled utilizing obtained dyes. The photovoltaic and EIS measurements, combined with performed calculations and fundamental dyes characterization, unraveled an intramolecular electron transfer as an initial step of the electron injection process at the dye/semiconductor interface. The overall photovoltaic action mechanism has been discussed. Our study demonstrates the significance of the anchoring group architecture in the molecular design of new sensitizers for DSSC applications.

Charge Recombination Suppressing in Dye-Sensitized Solar Cells by Tuning Dielectric Constant of Triphenylamine Dyes with Altering π-Bridge from Naphthalene to Anthracene Units

Charge recombination reactions (CRRs) are responsible for a major contribution of power conversion efficiency (PCE) loss in dye-sensitized solar cells (DSSCs). This study tracks the impact of the dielectric constant...

Torsion-Induced Nonradiative Relaxation of the Singlet Excited State of meso-Thienyl Bodipy and Charge Separation, Charge Recombination-Induced Intersystem Crossing in Its Compact Electron Donor/Acceptor Dyads

We prepared a series of meso-thienyl boron-dipyrromethene (Bodipy) derivatives to investigate the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The photophysical properties of the compounds were studied by steady-state and femtosecond/nanosecond transient absorption spectroscopy, as well as density functional theory (DFT) computations. Different from the meso-phenyl Bodipy analogues, the meso-thienyl Bodipy are weakly fluorescent. Based on femtosecond transient absorption and DFT computations, we propose that the torsion of the thienyl group and the distortion of the Bodipy core (19.7 ps) in the S₁ state lead to a conical intersection on the potential energy surface as an efficient nonradiative decay channel (408 ps), which is responsible for the observed weak fluorescence as compared to the meso-phenyl analogue. The increased fluorescence quantum yield (from 5.5 to 14.5%) in viscous solvents supports this hypothesis. With the electron donor 4'-hydroxylphenyl moiety attached to the meso-thienyl unit, the fast charge separation (CS, 15.3 ps) and charge recombination (CR, 238 ps) processes outcompete the torsion-induced nonradiative decay and induce fast ISC through the SOCT-ISC mechanism. The triplet quantum yield of the electron donor/acceptor dyad is highly dependent on solvent polarity (Φ_T = 1.9-45%), which supports the SOCT-ISC mechanism, and the triplet-state lifetime is up to 247.3 μs. Using the electron donor-acceptor dyad showing SOCT-ISC as a triplet photosensitizer, efficient triplet-triplet annihilation (TTA) upconversion was observed with a quantum yield of up to 6.0%.

Weakened Triplet-Triplet Annihilation of Diiodo-BODIPY Moieties without Influence on Their Intrinsic Triplet Lifetimes in Diiodo-BODIPY-Functionalized Pillar[5]arenes

The triplet-triplet annihilation (TTA) effect of sensitizers themselves can lead to the additional quenching of lifetimes of triplet states; therefore, how to weaken the TTA effect of sensitizers is an urgent issue to be resolved for their further applications. Besides, it remains a tremendous challenge for constructing supramolecular systems of photosensitizers based on photosensitizer-functionalized pillararenes because there have been very few investigations on them. Thus, 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (DIBDP) and ethoxy pillar[5]arene (EtP5) were utilized to synthesize a DIBDP-functionalized pillar[5]arene (EtP5-DIBDP), a cyano-containing DIBDP (G) used as a guest molecule was also prepared, and they were used to investigate the electron-transfer mechanism between EtP5 and DIBDP moieties and weaken the TTA effect of DIBDP moieties. The theoretical computational results of frontier molecular orbitals and isosurfaces of spin density preliminarily predicted that the cavities of the EtP5 moiety had influence on the fluorescence emission of DIBDP units but not on their triplet states in EtP5-DIBDP. The fluorescence emission intensities in a variety of solvents with different polarities and electrochemical studies revealed that there was electron transfer from EtP5 to the DIBDP units, and the electron-transfer process had influence on the fluorescence emission but not on the triplet states of DIBDP moieties in EtP5-DIBDP, which verified the results of density functional theory calculations. The triplet state lifetimes of EtP5-DIBDP were longer than those of DIBDP and G and the photooxidation abilities of EtP5-DIBDP were better than those of DIBDP and G at a high concentration (1.0 × 10^-5 M) in various solvents; in contrast, the intrinsic triplet state lifetimes and singlet oxygen quantum yields (Φ_Δ) of DIBDP, G, and EtP5-DIBDP were very similar. This was because the steric hindrance of EtP5 moieties could weaken the TTA effect of DIBDP moieties without influencing their intrinsic triplet state lifetimes in EtP5-DIBDP.

POM-based dyes featuring rigidified bithiophene π linkers: potential high-efficiency dyes for dye-sensitized solar cells

A series of POM-based dyes with a triphenylamine electron donor group, cyanoacrylic acid electron acceptor group and different π linkers of thiophene derivatives were systematically investigated to analyze the influence of a rigidified bithiophene with fastening atoms (C and N) on the performance of dye-sensitized solar cells (DSSCs) based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.

Inhibiting Charge Recombination in cis-Ru(NCS)2 Diimine Sensitizers with Aromatic Substituents

A series of cis-[Ru(LL)(dcbH₂)(NCS)₂] compounds, where dcbH₂ = 2,2'-bipyridine-4,4'-dicarboxylic acid and LL = 1,10-phenanthroline (Ru(phen)), 4,7-dipyrrole-1,10-phenanthroline (Ru(pyr)), 4,7-diindole-1,10-phenanthroline (Ru(ind)), or 4,7-dicarbazole-1,10-phenanthroline (Ru(cbz)), was investigated for application as sensitizers in mesoporous TiO₂ dye-sensitized solar cells (DSSCs). A systematic increase in the number of rings of the aromatic substituents at the 4,7-positions of the 1,10-phenanthroline allowed tuning of the molecular size of the sensitizers and the energy stored in the excited state while maintaining the same ground-state Ru^3+/2+ reduction potentials. These small structural changes had a significant influence on the rates and/or efficiencies of electron injection, back-electron transfer, recombination to oxidized mediators, lateral self-exchange electron transfer, and regeneration through iodide oxidation that were reflected in distinct photoelectrochemical performance of full operating DSSCs. The global efficiencies, open-circuit voltages, and short-circuit current densities of the DSSCs consistently followed the trend Ru(pyr) < Ru(ind) < Ru(phen) < Ru(cbz), and the most optimal performance of Ru(cbz) was ascribed to dramatically slower recombination to the oxidized redox mediators. Transient photovoltage and transient absorption experiments both revealed significantly slower recombination as the size of the aromatic substituents increased with Ru(cbz) providing the most promising behavior for application in dye sensitization.

Iodine binding with thiophene and furan based dyes for DSCs

Donor–π-bridge–acceptor dyes with thiophene have been shown to bind I₂ leading to diminished dye-sensitized solar cell performances relative to furan based analogues.

Correlation Between Charge Recombination and Lateral Hole-Hopping Kinetics in a Series of cis-Ru(phen')(dcb)(NCS)2 Dye-Sensitized Solar Cells

Four complexes of the general form cis-Ru(phen')(dcb)(NCS)₂, where dcb is 4,4'-(CO₂H)₂-2,2'-bipyridine and phen' is 1,10-phenanthroline (phen), 4,7-(C₆H₅)₂-phen (Ph₂-phen), 4,7-(CH₃)₂-phen (Me₂-phen), or 3,4,7,8-(CH₃)₄-phen (Me₄-phen), were anchored to mesoporous TiO₂ thin films for applications as sensitizers in dye-sensitized solar cells (DSSCs). The compounds displayed metal based reductions E^o(Ru^III/II) = 1.01 ± 0.05 V vs NHE and were potent reductants competent of excited-state electron transfer to TiO₂ with yields ϕ_inj ≥ 0.75 in acetonitrile electrolytes. Average charge recombination rate constants, k_cr, abstracted from nanosecond transient absorption measurements, and the apparent diffusion coefficients for lateral hole-hopping, abstracted from chronabsorptometry measurements, showed the same sensitizer dependency: Ru(Me₄-phen) > Ru(Ph₂-phen) > Ru(Me₂-phen) ≈ Ru(phen). When used in operational solar cells, Ru(Ph₂-phen) was most optimal with an efficiency of (6.6 ± 0.5)% in ionic liquids under 1 sun illumination. The superior performance of Ru(Ph₂-phen) was traced to a higher injection yield and more efficient regeneration due to an unusually small sensitivity of k_cr to the number of injected electrons.

Aminophenyl/carboxyphenylporphyrins as sensitizers for dye-sensitized solar cells

A series of tetraphenyl meso-substituted porphyrins with either p-amino or p-carboxy substituents has been studied as dyes for standard dye-sensitized solar cells (DSSCs). The porphyrins with greater numbers of amino groups generally show greater efficiency, primarily due to higher photocurrents; open-circuit voltage and fill factors are comparable. The most efficient sensitizer was the trans disubstituted zinc porphyrin, with an overall solar energy conversion efficiency of 5.66%, slightly higher than the triamino zinc porphyrin at 5.18%. The improved efficiency is attributed to the well known push–pull effect in porphyrinsensitized DSSCs with electrondonating groups opposite to the electronwithdrawing anchoring (carboxy) groups. It is suggested that porphyrin derivatives with cis diaminophenyl groups show somewhat diminished efficiency due to difficulties with regeneration of the dye from its oxidized form.

Probing the Highly Efficient Electron Transfer Dynamics between Zinc Protoporphyrin IX and Sodium Titanate Nanosheets

Sodium titanate nanosheets (NaTiO2 NS) have been prepared by a new method and completely characterized by TEM, SEM, XRD, EDX, and XPS techniques. The sensitization of nanosheets is carried out with Zn protoporphyrin IX (ZnPPIX). The emission intensity of ZnPPIX is quenched by NaTiO2 NS, and the dominant process for this quenching has been attributed to the process of photoinduced electron injection from excited ZnPPIX to the nanosheets. Time resolved fluorescence measurement was used to elucidate the process of electron injection from the singlet state of ZnPPIX to the conduction band of NaTiO2 NS. Electron injection from the dye to the semiconductor is very fast (ket ≈ 10(11) s(-1)), much faster than previously reported rates. The large two-dimensional surface offered by the NaTiO2 NS for interaction with the dye and the favorable driving force for electron injection from ZnPPIX to NaTiO2 NS (ΔGinj = -0.66 V) are the two important factors responsible for such efficient electron injection. Thus, NaTiO2 NS can serve as an effective alternative to the use of TiO2 nanoparticles in dye sensitized solar cells (DSSCs).

Influence of Ancillary Ligands in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have motivated many researchers to develop various sensitizers with tailored properties involving anchoring and ancillary ligands. Ancillary ligands carry favorable light-harvesting abilities and are therefore crucial in determining the overall power conversion efficiencies. The use of ancillary ligands having aliphatic chains and/or π-extended aromatic units decreases charge recombination and permits the collection of a large fraction of sunlight. This review aims to provide insight into the relationship between ancillary ligand structure and DSSC properties, which can further guide the function-oriented design and synthesis of different sensitizers for DSSCs. This review outlines how the new and rapidly expanding class of chelating ancillary ligands bearing 2,2'-bipyridyl, 1,10-phenanthroline, carbene, dipyridylamine, pyridyl-benzimidazole, pyridyl-azolate, and other aromatic ligands provides a conduit for potentially enhancing the performance and stability of DSSCs. Finally, these classes of Ru polypyridyl complexes have gained increasing interest for feasible large-scale commercialization of DSSCs due to their more favorable light-harvesting abilities and long-term thermal and chemical stabilities compared with other conventional sensitizers. Therefore, the main idea is to inspire readers to explore new avenues in the design of new sensitizers for DSSCs based on different ancillary ligands.

Enhanced photovoltaic performance of dye-sensitized solar cells using a new photoelectrode material: upconversion YbF3-Ho/TiO2 nanoheterostructures

New up-conversion YbF3-Ho/TiO2 (UC/TiO2) nanoheterostructures are designed and explored as an efficient photoelectrode material to yield dye-sensitized solar cells (DSSCs) with enhanced performance. In this study, we analyze the photogenerated charge transfer properties of the UC/TiO2 nanoheterostructures via surface photovoltage (SPV) and transient photovoltage (TPV) techniques, and the interfacial dynamics of charge transfer and recombination processes in DSSCs using electrochemical impedance spectroscopy (EIS) and open circuit photovoltage decay (OCVD) techniques. It is found that these UC/TiO2 nanoheterostructures combine the upconversion function of YbF3-Ho and the semiconductive merits from TiO2. More importantly, the hetero-junction interface in the UC/TiO2 nanoheterostructures not only induces direct electron-injection from YbF3-Ho to TiO2 by utilizing near-infrared light, but also further improves the existing merits of TiO2 through facilitating the interfacial photoinduced charge separation, suppressing the photoinduced charge recombination and prolonging the lifetimes of excited electrons, which can give further improvement of the photovoltaic performances. When integrating the UC/TiO2 nanoheterostructures into DSSCs, an overall energy conversion efficiency of 8.0% is achieved. There is a 23% enhancement in the overall conversion efficiency and a 19% improvement in the photocurrent, compared to the pristine devices.

Design, synthesis and DSSC performance of o-fluorine substituted phenylene spacer sensitizers: effect of TiO2 thickness variation

The influence of o-fluorine substituted phenyl π-spacer dyes with varying donors and different TiO₂ film thickness of 9 and 12 μm show improved V_oc, J_sc and PCE measured under one sun.

Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Accelerated inner charge transfer in porphyrins promotes a broad light-harvesting ability up to 840 nm and a conversion efficiency of 9.2%.

A high efficiency ruthenium(ii) tris-heteroleptic dye containing 4,7-dicarbazole-1,10-phenanthroline for nanocrystalline solar cells

cis-[Ru(cbz₂-phen)(dcbH₂)(NCS)₂] sensitized cells exhibited higher performance than those by N3. The efficiency is discussed in terms of its thermodynamic and structure.

Distance dependent charge separation and recombination in semiconductor/molecular catalyst systems for water splitting

The photoinduced reduction of three Co electrocatalysts immobilised on TiO2 is 10(4) times faster than the reverse charge recombination. Both processes show an exponential dependence on the distance between the semiconductor and the catalytic core.

Ruthenium sensitizers with a hexylthiophene-modified terpyridine ligand for dye-sensitized solar cells: synthesis, photo- and electrochemical properties, and adsorption behavior to the TiO2 surface

Two novel ruthenium sensitizers with a hexylthiophene-modified terpyridine ligand (TUS-35 and TUS-36) were synthesized to improve the molar absorptivity of the previously reported ruthenium sensitizer (TBA)[Ru{4'-(3,4-dicarboxyphenyl)-4,4″-dicarboxyterpyridine}(NCS)3], TBA = tetrabutylammonium (TUS-21). A relatively strong absorption appeared at ∼380 nm, and the molar absorption coefficient at the metal-to-ligand charge transfer (MLCT) band decreased in TUS-35 by introducing a 2-hexylthiophene unit to the 5-position of the terpyridine-derived ligand. For comparison, a relatively strong absorption was observed at ∼350 nm without decreasing the molar absorption coefficient at the MLCT band in TUS-36 by introducing a 2-hexylthiophene unit to the 4-position of the terpyridine-derived ligand. On the other hand, the energy levels of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of these two sensitizers were found to be almost equal to those of TUS-21. The adsorption behavior of TUS-35 and TUS-36 was similar to that of (TBA)[Ru{4'-(3,4-dicarboxyphenyl)terpyridine}(NCS)3] (TUS-20), which binds to the TiO2 surface by using the 3,4-dicarboxyphenly unit, rather than that of TUS-21, which adsorbs to the TiO2 photoelectrode using one of the carboxyl groups at the terminal pyridines of the terpyridine-derived ligand. Therefore, TUS-35 and TUS-36 are considered to bind to the TiO2 surface by using the 3,4-dicarboxyphenly unit just like TUS-20. The dye-sensitized solar cells (DSCs) with TUS-35 and TUS-36 showed a relatively lower conversion efficiency (6.4% and 5.7%, respectively) compared to the DSC with TUS-21 (10.2%). Open-circuit photovoltage decay and electrochemical impedance spectroscopy measurements revealed that the promoted charge recombination and/or charge transfer of the injected electrons in the TiO2 photoelectrode is a main reason for the inferior performances of TUS-35 and TUS-36.

Probing electron transfer dynamics of pyranine with reduced graphene oxide

A stable reduced graphene oxide (rGO) was prepared and characterized by X-ray diffraction (XRD) and laser Raman spectroscopy. Steady state and time-resolved fluorescence quenching studies have been carried out to elucidate the process of electron transfer from excited pyranine (POH) into the rGO dispersion. POH adsorbed strongly on rGO dispersion with an apparent association constant of 33.4 (mg ml)(-1), and its fluorescence emission was quenched with an apparent association constant of 33.7 (mg ml)(-1). Picosecond lifetime measurements gave the rate constant for the electron transfer process from the excited singlet state of POH into the rGO dispersion as 8.8 × 10(9) s(-1). Laser flash photolysis studies demonstrated the formation of radicals for the evidence of electron transfer between POH and rGO.

Towards the development of functionalized polypyridine ligands for Ru(II) complexes as photosensitizers in dye-sensitized solar cells (DSSCs)

A number of novel ruthenium(II) polypyridine complexes have been designed and synthesized for use as photosensitizers in dye-sensitized solar cells (DSSCs) due to their rich photophysical properties such as intense absorption, long-lived lifetimes, high emission quantum yields and unique redox characteristics. Many of these complexes exhibit photophysical behavior that can be readily controlled through a careful choice of ligands and/or substituents. With this perspective, we review the design and general synthetic methods of some polypyridine ligands based on bipyridine, phenanthroline, terpyridine and quaterpyridine with/without anchoring groups with a view to correlate functionality of ligand structures with the observed photophysical, electroredox and power conversion efficiency of some examples of Ru(II) polypyridyl complexes that have been reported and particularly used in the DSSCs applications. The main interest, however, is focused on showing the development of new polypyridine ligand materials containing long-range electron transfer motifs such as the alkenyl, alkynyl and polyaromatic donor functionalities.

Novel ruthenium sensitizers with a dianionic tridentate ligand for dye-sensitized solar cells: the relationship between the solar cell performances and the electron-withdrawing ability of substituents on the ligand

Five novel ruthenium sensitizers (TUS sensitizers) with a dianionic tridentate ligand (pyridine-2,6-dicarboxyamidato and its derivatives) have been synthesized for application to dye-sensitized solar cells (DSCs). These TUS sensitizers have much larger molar absorption coefficients in the wavelength range below 600 nm compared with those of Black dye which is a structural analog and a highly efficient ruthenium sensitizer. The energy levels of HOMOs and LUMOs of TUS sensitizers shifted to the positive direction with increasing the electron-withdrawing ability of the substituents on the dianionic tridentate ligand. The energy levels of HOMO and LUMO showed linear correlation with respect to the Hammett constant of the substituents. The DSCs with TUS sensitizers showed much lower performances than that of Black dye. Both inferior adsorptivity on the TiO2 surface and unfavorable energy levels of HOMOs and LUMOs for the effective electron transfer reactions in the DSCs are considered to be the main reasons for the much lower performances of TUS sensitizers. The conversion efficiency of the DSC with a TUS sensitizer increased with increasing the electron-withdrawing ability of the substituents on the dianionic tridentate ligand. The observed linear relationship between the conversion efficiency and the driving force of the reduction process of the oxidized form of dyes by I(-) suggests that the dye regeneration process is a rate-determining step in the DSCs with TUS sensitizers.

A cyclopenta[1,2-b:5,4-b′]dithiophene–porphyrin conjugate for mesoscopic solar cells: a D–π–D–A approach

In this article, cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) was introduced as a spacer between the porphyrin chromophore and cyanoacetic acid to obtain a porphyrin dye (coded as LW9).

Ruthenium sensitizers having an ortho-dicarboxyl group as an anchoring unit for dye-sensitized solar cells: synthesis, photo- and electrochemical properties, and adsorption behavior to the TiO2 surface

A novel ruthenium sensitizer with a 3′,4′-dicarboxyterpyridine (TUS-28) has been synthesized as an improved model sensitizer of the dye-sensitized solar cells.

PEI@Mg2SiO4: an efficient carbon dioxide and nitrophenol compounds adsorbing material

PEI@Mg₂SiO₄: an efficient carbon dioxide and nitrophenol compounds adsorbing material.

Photo-induced charge recombination kinetics in MAPbI3−xClx perovskite-like solar cells using low band-gap polymers as hole conductors

We measured the charge recombination kinetics using transient photovoltage (TPV) and laser transient absorption spectroscopy (L-TAS) in MAPbI_3−xCl_x perovskite solar cells using low band gap polymers as hole transport materials (HTMs).

The importance of the TiO2/quantum dots interface in the recombination processes of quantum dot sensitized solar cells

Quantum dot sensitized solar cells (QDSSCs) present a promising technology for next generation photovoltaic cells, having exhibited a considerable leap in performance over the last few years. However, recombination processes occurring in parallel at the TiO(2)-QDs-electrolyte triple junction constitute one of the major limitations for further improvement of QDSSCs. Reaching higher conversion efficiencies necessitates gaining a better understanding of the mechanisms of charge recombination in these kinds of cells; this will essentially lead to the development of new solutions for inhibiting the described losses. In this study we have systematically examined the contribution of each interface formed at the triple junction to the recombination of the solar cell. We show that the recombination of electrons at the TiO(2)/QDs interface is as important as the recombination from TiO(2) and QDs to the electrolyte. By applying conformal MgO coating both above and below the QD surface, recombination rates were significantly reduced, and an improvement of more than 20% in cell efficiency was recorded.