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Trap state engineering has been widely employed to manipulate the dynamics of ion migration in metal halide perovskites (MHPs), a crucial factor associated with the performance and stability of MHP-based devices. However, the specific roles of different trap states remain poorly understood due to their complicated spatial and energetics distributions. Herein, we propose a methodology for independently regulating the distributions of bulk shallow and surface deep trap states in MHPs. By combining in situ photoluminescence spectroscopy with wide-field imaging microscopy, we elucidate the effect of surface trap states on promoting long-range interparticle ion migration. Interestingly, we ascertain with time-resolved photoelectric techniques that the majority of mobile ionic defects involved in ion migration are predominantly contributed by bulk trap states. Our findings suggest that targeted manipulation of bulk shallow and surface deep trap states can be effective in reducing the number of mobile ions and decelerating the rate of ion migration, respectively.

Organic spacers play a crucial role in two-/three-dimensional (2D/3D) hybrid perovskite-based solar cells that integrate the advantages of the stability of 2D perovskites and the high efficiency of 3D perovskites. However, improving the stability of the device without compromising the high photovoltaic efficiency in 2D/3D perovskite solar cells using large organic cations in 2D perovskite is a challenge. Herein, we synthesized 2-thiopheneethylammonium iodide (TEAI) and found that TEAI can perform as a spacer for 2D/3D hybrid perovskites. TEAI with a bulky aromatic spacer in 2D/3D hybrid perovskites can effectively induce crystalline growth and orientation, leading to a longer carrier lifetime, higher carrier mobility, fewer surface, and grain boundary defects. The inclusion of TEAI also improves surface hydrophobicity. The 2D/3D hybrid perovskite device based on TEAI achieved the highest efficiency of 18.75% compared with the 16.83% efficiency of the 3D perovskite device. After aging for 1000 h in an ambient atmosphere, the unpackaged 2D/3D hybrid perovskite solar cells maintained approximately 82% of their initial efficiency, whereas that of the controlled devices decreased to approximately 47% of their original performance. These results suggest that TEAI can be used as an organic spacer for the preparation of high-efficiency and high-stability 2D/3D perovskite solar cells.

Ion migration in perovskite solar cells (PSCs) is a crucial factor governing both photoelectric performance and physicochemical stability of the devices. However, the related fundamental research, especially the quantitative studies of the ion migration dynamics at a device level, has long been limited by the lack of analytical methods. Herein, we report a novel circuit-switched transient photoelectric technique by which the migration dynamics specific to the ions in PSCs is closely inspected. By decoding the comprehensive information on the interaction between ions and charge carriers, we put forward a model of polarization-induced trap states, which can not only precisely account for the photoelectric behavior of PSCs but also be applicable to other photovoltaic systems with different device architectures, such as conventional silicon photovoltaic cells and dye-sensitized solar cells. Furthermore, we demonstrate the bifunctional role of the fullerene derivative in suppressing ion migration and accumulation, which provides an effective protocol of ion dynamics manipulation, besides the empirical strategy of defect passivation, toward high-efficiency and high-stability PSC devices.

High-efficiency single-photon upconversion photoluminescence (SPUC-PL) has been reported in diverse lead halide perovskite materials, bringing them unique applications in optics and optoelectronics. However, because of the lack of reliable protocols for tuning the individual SPUC-PL indexes, their feasibility and flexibility in practice are still highly limited. In this work, the mechanism of the perovskite-based SPUC-PL is systematically studied. We report that the energetic distribution of the intragap electronic states determines the SPUC-PL quantum yield and the anti-Stokes shift, two critical spectroscopic indexes, in a trade-off manner. On the basis of this finding, we propose a viable method of tuning SPUC-PL parameters in terms of the intragap state engineering strategy. Specifically, by controlling the thermal annealing temperature, the SPUC-PL quantum yield and the anti-Stokes shift can be effectively adjusted as desired. This work provides new insights into the deployment of lead halide perovskites as the controllable SPUC-PL media.

Among diverse strategies of improving the power conversion efficiency of perovskite solar cells (PSCs), one of the most effective pathways is to simultaneously promote charge transport and suppress charge recombination. Graphite carbon nitride (CN) is a promising additive for blocking the charge recombination in PSCs by passivating the trap states, which, however, usually leads to terrible charge transport performance as a side effect. Herein, we propose that this trade-off issue can be smoothly solved by simply replacing the intrinsic CN with its Li-doped counterpart (CN-Li). Specifically, introducing the CN-Li into perovskite films results in larger grain size with greater crystallinity, which further benefits the reduction of intrinsic defects and the elongation of the fluorescence lifetime. At the device level, the CN-Li modification not only dramatically passivates the trap states and suppresses the charge recombination but also decreases the series resistance and hence boosts the charge transport. Consequently, the CN-Li-modified PSC reaches a champion power conversion efficiency of 18.86% with a prominent short-circuit current density of 24.16 mA cm–2. This work provides new insights into the efficiency optimization strategy of PSCs by simultaneously promoting charge transport and suppressing charge recombination.

Improving the photoelectric conversion efficiency and long-term stability are two substantial goals for the research work of perovskite photovoltaics, which can be realized through appropriate surface modification methods. In this work, we report a bifunctional modification strategy using (3-chloropropyl)trimethoxysilane with the aim to increase the efficiency and stability of the as-fabricated perovskite solar cells (PSCs) simultaneously. The mechanism of such bifunctional modification is studied with systematic spectroscopic and transient photoelectric characterizations. Specifically, the enhancement of efficiency results from the chlorine-induced passivation of iodine-vacancy trap states, as evidenced by the decrease of trap-state density through the interaction between chlorosilane and Pb2+, which further leads to the prolonged photoluminescence lifetime of the intrinsic perovskite films and suppressed charge recombination of photovoltaic devices. The corresponding champion device possesses a power conversion efficiency of 18.14%, 16% higher than that of the control device. In addition, the modified perovskite films exhibit superior moisture resistance owing to the introduced hydrophobic silane network, and the corresponding devices could preserve ∼85% of the initial efficiency after 2 months’ storage under ambient air conditions. The proposed bifunctional modification strategy and the mechanism studies provide new insights into both efficiency and stability engineering of the PSCs.

The realization of room-temperature preparation of the perovskite solar cells without a thermal annealing step could simplify the fabrication process and reduce the energy consumption. N,N-Dimethylacetamide (DMAc) is a promising candidate to replace N,N-dimethylformamide (DMF) as a perovskite precursor solvent with the advantage of room-temperature processability, while the influencing mechanism of DMAc and DMF solvents on controlling nucleation and crystallization kinetics has received limited attention. In light of this, we investigate the morphological structure and spectral properties of the perovskite films and verify the successful fabrication of high-quality α-phase perovskites from DMAc at room temperature. Further infrared and Raman spectroscopy reveals that the weakened interaction between DMAc and solutes helps perovskites to rapidly nucleate and crystallize from the solvent, promoting complete phase transformation to form a perovskite film. In situ UV–vis spectroscopy demonstrates the solvent effect on the crystallization kinetic process during film drying at room temperature. This work provides valuable insights into the preparation of high-quality perovskite films at room temperature, thereby driving the production of cost-effective perovskite devices.

Cation composition engineering of perovskite polycrystals, such as substituting the typically used methylamine (MA+) with formamidine (FA+) wholly or partially, is a reliable protocol for fabricating high-efficiency perovskite solar cells (PSCs). Hitherto, it is still in debate how and to what extent the FA+substitution influences the ion migration, which restricts the further optimization of PSCs. In this work, ion migration kinetics in MAPbI3-based and FAPbI3-based PSCs was systematically studied by a series of time-resolved photoelectric measurements. We find that by replacing MA+with FA+, owing to the steric effect of the large-sized FA+, the cation migration is significantly suppressed, and the migration rate of halide anions is reduced. However, FA+results in a structural tolerance factor exceeding the ideal range, which leads to a drastic increase in halide defects and eventually aggravates the ion migration in FAPbI3PSCs. These findings provide unique insights into the manipulation of ion migration in PSCs via the strategy of cation composition engineering.

Exciton and charge dynamics of poly(3-hexylthiophene) (P3HT) blended with covalently linked monomeric (MP), dimeric (BP), or trimeric (TP) fullerene derivatives are investigated by using near-infrared time-resolved absorption spectroscopy under low excitation photon fluence. A low blending ratio (10%, w/w) of the fullerene derivative allows us to examine the effect of linked-fullerene number on charge photogeneration. With reference to P3HT:MP, the P3HT:BP blend shows a one-fold higher exciton dissociation rate and significantly alleviated nongeminate charge recombination in spite of a similar P3HT packing order. However, TP with even larger size tends to aggravate the geminate and nongeminate recombination in picosecond-to-nanosecond time scales. The primary exciton and charge dynamics are discussed in correlation with the nanoscopic film morphology. The results suggest that the dumbbelled fullerene derivative BP as an electron-withdrawing material is promising in boosting the free charge photoproduction in polymer solar cells.

A series of benzene octyl ether adducted fullerene derivatives (PCBOEs) of higher LUMO levels with reference to PCBM were synthesized and were blended with P3HT to assemble polymer solar cells (PSCs). Upon increasing the adduct order, the PCBOE-based PSCs exhibited higher open-circuit voltage (VOC) but increasingly lower short-circuit current density (JSC) with respect to the P3HT:PCBM device. Morphological analyses of the P3HT:fullerene blends reveal different extents of fullerene aggregation, which are shown to impact intimately on the charge photogeneration dynamics. An increase of adduct order is found to accelerate geminate charge recombination and to induce nanosecond nongeminate charge recombination that is absent in the cases of monoadducted PCBOE and PCBM. The severer charge recombination found for higher order fullerene adducts is ascribed to the deterioration of electron mobility owing to the discontinuity of the fullerene phase. The results suggest that, upon increasing the adduct order, the electron and hole transport in photoactive layers turn from ambipolarly balanced to space-charge limited states, a crucial issue for the application of multiadducted fullerenes as the electron accepting materials of PCSs.

Primary charge photogeneration dynamics in the films of polyfluorene copolymer PFDTBT (poly([2,7-(9,9-bis(3,7-dimethyloctyl)fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)])) blended with fullerene derivatives, with variation of fullerene/polymer lowest unoccupied molecular orbital level offsets (−ΔEL) over 110–320 meV, are investigated by using near-infrared transient absorption spectroscopy under low excitation photon fluence. Time-resolved spectra combined with spectroelectrochemical characterization enable us to verify the signature spectra of the S1exciton of the polymer, the interfacial charge transfer (ICT) state, and the charge-separated (CS) state, with which the species-associated kinetics are derived via decomposition of the time-resolved data matrices. For a neat film the interchain CT state is generated irrespective of the excitation photon energy; however, the CS state forms merely upon the above-gap excitation with a quantum yield of 15%, which subsequently recombines into the S1exciton within 1 ps. For blend films a dual-path scheme of free charge formation is revealed: The S1exciton transforms in a parallel manner into the ICT and CS states with branching ratios ΦICT:ΦCS≈ 3:1 and time constants of 180–800 fs depending on −ΔEL. When −ΔELis relatively large, i.e., 0.29 or 0.32 eV for PFDTBT blended with PC61BOE (4-(pentyl-[6,6]-C61)benzene octyl ether) or PC61BM (phenyl-[6,6]-C61-butyric acid methyl ester), respectively, the ICT state further dissociates into the CS state with a time constant of ∼300 ps and an efficiency exceeding 50%. This channel, however, closes when −ΔEL< 0.20 eV, and the ICT state fully recombines back to the ground state within a few hundred picoseconds. The large ICT-to-CS branching ratio and high dissociation efficiency of the ICT state consolidate the crucial role of this state in photocurrent generation. On the other hand, the process of exciton-to-CS transition is found to obey Marcus’s nonadiabatic electron transfer mechanism with a coupling strength V= 55 ± 7 cm–1and a reorganization energy λ = 0.33 ± 0.02 eV, whereas the dissociation of the ICT state can be accounted for by Braun–Onsager’s model of e––h+dissociation. The dynamic properties of the ICT state and roles of −ΔELin yielding charge species (ICT and CS) revealed in the present work may shed light on the development of new photovoltaic materials.

The primary dynamics of exciton and charge photogeneration in the neat P3HT (poly(3-hexylthiophene)) and the blend P3HT/PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) films were investigated by the use of near-infrared femtosecond transient absorption spectroscopy with varying excitation photon energy and fluence. The effects of film morphology were examined by comparing the solvent vapor annealed (SVA) and the carbon disulfide (CS2) cast films. Spectroelectrochemistry was employed to characterize the cationic polaron P3HT•+to facilitate the assignments of the transient spectra. Time-resolved spectroscopy revealed two different types of polarons, the delocalized (DP, absorbing over 630–830 nm) and the localized (LP, 750–1100 nm) ones inhabiting the crystalline and the disordered P3HT phases, respectively. In addition, the characteristic absorption of the long-sought anionic polaron P3HT•–is proposed to be a broad-band spectrum extending up to ∼1300 nm with a maximum at ∼1080 nm. For SVA neat P3HT films under the low-fluence photoexcitation, ∼1012photons·cm–2·pulse–1, the overall polaron yield (DP + LP) at 1.45 ns was determined to be <7% after the blue-edge excitation at 460 nm, or to be negligible after the red-edge excitation at 620 nm. For the P3HT/PCBM blends under the low-fluence photoexcitation, the prompt and the delayed polaron formation with comparable contribution are found to be independent of the excitation wavelengths; the former is governed by the instantaneous charge separation at the donor–acceptor interfaces, whereas the latter relies on the exciton diffusion in P3HT crystallites with a time constant of 7–10 ps. The effects of film morphology and photon energy on the charge photogeneration dynamics are discussed in terms of the disordered intermediate between the P3HT crystallite and the PCBM aggregate, which is suggested to be in less dimension for SVA films than that for thermally annealed films, and thereby to be advantageous in minimizing the traps or blocks of charge carriers.

Ultrafast carotenoid-to-chlorophyll (Car-to-Chl) singlet excitation energy transfer in the cytochrome b6f (Cyt b6f) complex from Bryopsis corticulans is investigated by the use of femtosecond time-resolved absorption spectroscopy. For all-trans-α-carotene free in n-hexane, the lifetimes of the two low-lying singlet excited states, S1(2Ag−) and S2(1Bu+), are determined to be 14.3±0.4ps and 230±10fs, respectively. For the Cyt b6f complex, to which 9-cis-α-carotene is bound, the lifetime of the S1(2Ag−) state remains unchanged, whereas that of the S2(1Bu+) state is significantly reduced. In addition, a decay-to-rise correlation between the excited-state dynamics of α-carotene and Chl a is clearly observed. This spectroscopic evidence proves that the S2(1Bu+) state is able to transfer electronic excitations to the Qx state of Chl a, whereas the S1(2Ag−) state remains inactive. The time constant and the partial efficiency of the energy transfer are determined to be 240±40fs and (49±4)%, respectively, which supports the overall efficiency of 24% determined with steady-state fluorescence spectroscopy. A scheme of the α-carotene-to-Chl a singlet energy transfer is proposed based on the excited-state dynamics of the pigments.

The C‐40 xanthophylls zeaxanthin and astaxanthin were confirmed to form radical cations, Car+, in the electron‐accepting solvent chloroform by direct excitation using subpicosecond time‐resolved absorption spectroscopy in combination with spectroelectrochemical determination of the near‐infrared absorption of Car+. For the singlets, the S2(1Bu+) state and most likely the SX(3Ag−) state directly eject electrons to chloroform leading to the rapid formation of Car+on a timescale of ∼100 fs; the lowest‐lying S1(2Ag−) state, however, remains inactive. Standard reduction potential for Car+was determined by cyclic voltametry to have the value 0.63 V for zeaxanthin and 0.75 V for astaxanthin from which excited state potentials were calculated, which confirmed the reactivity toward radical cation formation. On the other hand, Car+formation from the lowest triplet excited state T1populated through anthracene sensitization is mediated by a precursor suggested to be a solute‐solvent complex detected with broad near‐infrared absorption to the shorter wavelength side of the characteristic Car+absorption. However, ground state carotenoids are able to react with a secondary solvent radical to yield Car+, a process occurring within 16 μs for zeaxanthin and within 21 p for astaxanthin. Among the two xanthophylls together with lycopene and β‐carotene, all having 11 conjugated double bonds, zeaxanthin ranks with the highest reactivity in forming Car+from either the S2(1Bu+) or the ground state. The effects of substituent groups on the reactivity are discussed.

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We have studied, by means of sub-microsecond time-resolved absorption spectroscopy, the triplet-excited state dynamics of carotenoids (Cars) in the intermediate-light adapted LH2 complex (ML–LH2) from Rhodopseudomonas palustriscontaining Cars with different numbers of conjugated double bonds. Following pulsed photo-excitation at 590 nm at room temperature, rapid spectral equilibration was observed either as a red shift of the isosbestic wavelength on a time scale of 0.6–1.0 μs, or as a fast decay in the shorter-wavelength side of the Tn←T1absorption of Cars with a time constant of 0.5–0.8 μs. Two major spectral components assignable to Cars with 11 and 12 conjugated double bonds were identified. The equilibration was not observed in the ML–LH2 at 77 K, or in the LH2 complex from Rhodobacter sphaeroidesG1C containing a single type of Car. The unique spectral equilibration was ascribed to temperature-dependent triplet excitation transfer among different Car compositions. The results suggest that Cars of 11 and 12 conjugated bonds, both in close proximity of BChls, may coexist in an α,β-subunit of the ML–LH2 complex.

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Due to the wavelength-dependent penetration depth of the incident light, charge carriers in perovskite solar cells (PSCs) generated by short-wavelength lights mostly populate near the surface of the perovskite active layer. Thus, these photogenerated charge carriers have to run across the entire longitudinal section of the perovskite to form current, which inevitably causes carrier loss in terms of charge recombination and reduces the photovoltaic performance. Herein, we report that the incidence depth issue of the short-wavelength light can be feasibly overcome by incorporating silica nanoparticles (SiO2NPs) into the perovskite active layer to increase the light scattering effect. As evidenced by the systematic spectroscopic and transient photoelectric studies, SiO2NPs not only diminish the attenuation of the short-wavelength light along the incidence direction within the volume of the perovskite active layer but also promote charge transport and suppress charge recombination. Consequently, the as-modified device exhibits a prominent short-circuit current density promotion with a power conversion efficiency of 17.83%, 17% higher than that of the reference device. More importantly, it is experimentally validated that the SiO2NPs treatment method is viable for sustaining the photocurrent of ultrathin PSC devices, which is of dramatic academic significance in developing semitransparent and flexible modules.