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We have studied four 2D layered perovskites, including OA2PbI4 (RP phase), ODAPbI4 and BDAPbI4 (DJ phase), (GA)MAPbI4 (ACI phase), where OA is [(C m H2m+1)NH3](m = 8), ODA is [NH3(CH2) m NH3](m = 8), BDA is [NH3(CH2) m NH3](m = 4), and GA is [C(NH2)3]; RP, DJ, and ACI means Ruddlesden–Popper, Dion–Jacobson and alternating cations in the interlayer, respectively. The temperature dependence of absorption and photoluminescence (PL) spectra have been measured. From which the average phonon energy (electron-phonon interaction strength) is analyzed as around 34 (80), 47 (184), 50 (402), and 63 (758) with the unit of meV for OA2PbI4, ODAPbI4, BDAPbI4, and (GA)MAPbI4, respectively. Larger phonon energy indicates the involvement of more phonons in organic spacer layer, with the corresponding stronger electron-phonon interaction. Furthermore, ultrafast transient absorption spectroscopy proves that, when the excitation photon energy is serval hundred meV higher than bandgap, the excitons still are the major photoexcitations in OA2PbI4, but polarons are major one in ODAPbI4, BDAPbI4, and (GA)MAPbI4 films, no matter the excitonic transitions dominate the absorption at their band edges. This work proves the organic spacers can regulate electron–phonon interaction then optoelectronic properties in 2D perovskites profoundly, which have implications toward future rational design for relevant devices.

Coherent optical control of the magnetization in ferromagnetic (FM) mediums using ultrafast nonthermal effect paves a promising avenue to improve the speed and repetition rate of the magnetization manipulation. Whereas previously, only heat-induced or helicity-dependent magnetization dynamics are demonstrated in metallic ferromagnets. Here, the linearly-polarized light control of magnetization is demonstrated in FM Co coupled with ferroelectric (FE) BiFeO

Highly robust, swiftly reversible thermochromic nature of a two-dimensional (2D) perovskite of (PDMA)(CH3NH3)n-1PbnI3n+1, nominally prepared as n = 2 is found, where PDMA = C6H4(CH2NH3)2. A wide band gap variation from 700 to 430 nm is observed between room temperature and >60 °C under ambient conditions, resulting from moisture absorption and desorption. X-ray diffraction and Fourier-transform infrared spectroscopy are performed to analyze the hydrated and dehydrated states. Furthermore, the (PDMA)(CH3NH3)n-1PbnI3n+1 film is demonstrated as an active material for smart windows and thermochromic solar cells, which could lower the inside air temperature in an enclosed space and supply a power conversion efficiency of more than 0.5% at a high ambient temperature, respectively. Overall, we may pave a pathway for exploring the novel phenomena and applications of Dion-Jacobson 2D perovskites.

Flexible pressure sensors (FPSs) are widely demanded for accurate pressure sensing on complex surfaces. FPSs using metal nanoparticles (NPs) as the sensing material have high conductivity and good mechanical properties, but poor gauge factor as a trade-off. In this work, a novel design of the AgNP-based FPS is proposed and testified with the help of the electrohydrodynamics-printing technology, so that the gauge factor can be improved from 8 to 286.8, resulting from the localized micro-zone piezoresistance effect. The working principle of the piezoresistive characteristic is discussed, and two methods to further improve and adjust the gauge factor are proposed and experimentally demonstrated.

Using substrates at room temperature or at 110 °C, two-dimensional (2D) perovskite films and solar cells using mixed butylammonium (BA) and formamidinium (FA) cations, i.e., (BA)2(FA)n−1PbnI3n+1 (n...

Bulk heterojunction (BHJ) organic solar cells prepared by blending a semiconductor donor (D) with an acceptor (A) polymer have achieved significant progress with power conversion efficiencies (PCEs...

Organic semiconducting emitters integrated with butterfly-mimetic photonic crystals (PhCs) are fascinating for dramatic advantages over light outcoupling efficiency and multifunctional strain sensors, as well as the key step toward electrically pumped lasers. Herein, an unprecedentedly direct mesoscale self-assembly into 1D PhCs is reported through a covalently gridization-driven approach of wide-bandgap conjugated polymers. A simple solvent-casting procedure allows for in situ self-assembly of the state-of-the-art conjugated nanopolymer, poly{[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]grid}-co-{[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]grid} (PODPFG), into well-defined multilayer architectures with an excellent toughness (30-40 J m

Absorption and photoluminescence spectra at various temperatures below 200 K had been used to study the optical properties of poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) fi...

Films of two-dimensional (2D) perovskite (PEA)2(MA)n-1PbnI3n+1 (PEA = phenylethylammonium, MA = methylammonium) series (n = 1, 2, 3) show high stability in contrast to their three-dimensional counterparts. Development of an effective means to fabricate 2D perovskite solar cells which can achieve a better power conversion efficiency (PCE) and have less current density-voltage (J-V) hysteresis, is crucial to boost their applications. We present a one-step solution process to fabricate the 2D perovskite (PEA)2(MA)2Pb3I10 as light absorbers in solar cells by adding methylammonium acetate (MAAc) in a precursor solution. With an appropriate molar ratio of MAAc, the 2D perovskite films show a smoother surface, higher coverage, and better crystallinity, resulting in an optimal PCE of 5.7%, and importantly, no J-V hysteresis.

Highly robust, swiftly reversible thermochromic nature of a two-dimensional (2D) perovskite of (PDMA)(CH

In this work, the ultrafast transient absorption spectroscopy (TAs) was utilized to first investigate the charge transfer from the emerging FAPbI3 (FA = CH(NH2)2) perovskite quantum dots (PQDs) to charge transport layers. Specifically, we compared the TAs in pure FAPbI3 PQDs, PQDs grown with both electron and hole transfer layers (ETL and HTL), and PQDs with only ETL or HTL. The TA signals induced by photoexcited electrons decay much faster in PQDs samples with the ETL (~20 ps) compared to the pure FAPbI3 PQDs (&gt, 1 ns). These results reveal that electrons can effectively transport between coupled PQDs and transfer to the ETL (TiO2) at a time scale of 20 ps, much faster than the bimolecular charge recombination inside the PQDs (&gt, 1 ns), and the electron transfer efficiency is estimated to be close to 100%. In contrast, the temporal evolution of the TA signals in the PQDs with and without HTL exhibit negligible change, and no substantive hole transfer to the HTL (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], PTAA) occurs within 1 ns. The much slower hole transfer implies the further potential of increasing the overall photo-carrier conversion efficiency through enhancing the hole diffusion length and fine-tuning the coupling between the HTL and PQDs.

The temperature dependence of absorption and photoluminescence (PL) spectroscopies were used to study the optical properties of 2D perovskite films, including n = 1 and 3 of (PEA)2(CH3NH3)n−1[PbnI3n+1] (PEA = C6H5(CH2)2NH3). In (PEA)2[Pb1I4] (PEPI), excitons coupling to optical phonons with an average energy of ∼30 meV dominate the photophysics of absorption and PL. (PEA)2(CH3NH3)2[Pb3I10] (shortened as PMPI3), nominally prepared as n = 3, actually was a mixture of multiple layered perovskites with various n. In absorption, a PMPI3 film presents respective n materials’ excitonic features, coupling to phonons with an average energy of ∼30 meV; in analyzing PL peaked singly at ∼1.6 eV and its width as a function of temperature, we found that PMPI3 behaves like PEPI at around 80 K but like 3D perovskite near room temperature, with three times larger electron–phonon interaction strength compared to that in PEPI.

Mixed halide perovskites are considered to have great application prospects in the field of optoelectronics due to their bandgap tunability. However, their applications are limited by phase instability under illumination. Here, we have used PL measurement to investigate phase segregation in mixed halide perovskite films with and without post-treatment using methylammonium halide solutions. Our findings reveal the PL evolution is strongly dependent on the morphology of perovskite films: the PL peak from small I-rich clusters is observed at the initial time in the films with unreacted lead halides, accompanied by faster PL evolution with increase of the concentration of methylammonium halide solution; adding excess methylammonium halide solution to the perovskite film leads to the disappearance of that PL peak. We attribute the difference of PL evolution in various films to the different evolution of local electric fields, which are induced by carrier trapping at grain boundaries. Our results indicate the local electric fields play an essential role in the halide segregation process.

Partial circularly polarized photoluminescence (CPL) with a polarization degree (P) of 2% from CH3NH3PbBr3 film at 77 K is observed, excited by a circularly polarized 532 nm laser. However, P decreases to 0.3% when excited using a circularly polarized 447 nm laser, and to zero using a linear polarized laser. The Hanle effect is measured to study spin relaxation in CH3NH3PbBr3 film, yielding a spin lifetime of ∼240 ps at 77 K, which however becomes one order of magnitude smaller when the temperature increases to 110 K. The existence of a CPL that is dependent on the helicity of the excitation laser, and the unexpectedly long spin lifetime with large spin–orbit coupling, suggest the potential of hybrid perovskites for spintronic applications.

A novel and simple microfabrication method for in-line micro-cavity based fiber Mach–Zehnder interferometers (FMZIs) is proposed and demonstrated by nanosecond (ns) pulsed fibers lasers for the first time. Instead of using high-end femtosecond laser for the fabrication process, only conventional ns-pulsed fiber lasers of modest optical powers are required in our approach. Such fabricated fiber MZ interferometer has exhibited a high sensitivity of −20048 nm/RIU (refractive index unit) for the index sensing application. The achieved index sensitivity is in good agreement with the theoretical estimation.

The all-polymer solar cells composed of wide band gap polymer PTP8 as the donor and N2200 as the acceptor exhibit a broad absorbance in the range of 300-900 nm, thanks to the complementary absorption of the near-infrared light by N2200. Although N2200 shows reasonably high electron mobility, the contribution of the photo-generated excitons in N2200 to the power conversion of the PTP8/N2200 solar cell is insignificant. Here, the hole transfer from the N2200 to PTP8 in PTP8/N2200 blend films was investigated by utilizing the ultrafast transient absorption spectroscopy. The spectral fingerprints of the ground state bleaching and the hole polaron-induced absorption of the PTP8 are identified under selective excitation of the N2200 component, which unambiguously indicate the hole transfer from the N2200 to PTP8. The hole transfer is slow (~100 ps), comparable to the geminate exciton recombination rate, consequently limiting the transfer efficiency and carrier generation. The hole-transfer efficiency depends on...

We report a semiconductor saturable absorber mirror $Q$ -switched holmium (Ho3+)-doped ZrF4–BaF2–LaF3–AlF3–NaF fiber laser operating at ~1190 nm in a linear cavity. Stable $Q$ -switched operation was established at a threshold pump power of 92.8 mW with a repetition rate of 26.1 kHz and a pulsewidth of $4~\mu \text{s}$ . The repetition rate increased and the pulsewidth decreased with an increasing pump power. When the pump power was increased to 830 mW, 800-ns $Q$ -switched pulses with a pulse energy of $0.18~\mu \text{J}$ at a repetition rate of 170 kHz were generated.

Organo-lead halide perovskites show excellent optoelectronic properties; however, the unexpected inconsistency in forward–backward I–V characteristics remains a problem for fabricating solar panels. Here we have investigated the reasons behind this “hysteresis” by following the changes in photocurrent and photoluminescence under electric field poling in transverse CH3NH3PbI3-based devices from 300 to 10 K. We found that the hysteresis disappears at cryogenic temperatures, indicating the “freeze-out” of the ionic diffusion contribution. When the same device is cooled under continuous poling, the built-in electric field from ion accumulation brings significant photovoltaic effect even at 10 K. From the change of photoluminescence upon polling, we found a second dipole-related mechanism which enhances radiative recombination upon the alignment of the organic cations. The ionic origin of hysteresis was also verified by applying a magnetic field to affect the ion diffusion. These findings reveal the coexistenc...

Trans-polyacetylene [$t\text{\ensuremath{-}}{(\mathrm{CH})}_{x}$] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in $t\text{\ensuremath{-}}(\mathrm{CH}{)}_{x}$; this prediction, however, has led to sharp debate. To resolve this saga, we use subpicosecond transient photomodulation spectroscopy in the mid-IR spectral range (0.1--1.5 eV) in neat $t\text{\ensuremath{-}}{(\mathrm{CH})}_{x}$ thin films. We show that odd-parity singlet excitons are the primary photoexcitations in $t\text{\ensuremath{-}}{(\mathrm{CH})}_{x}$, similar to many other nondegenerate $\ensuremath{\pi}$-conjugated polymers. The exciton transitions are characterized by two photoinduced absorption (PA) bands at 0.38 and 0.6 eV, and an associated photoluminescence band at $\ensuremath{\sim}1.5\text{ }\text{ }\mathrm{eV}$ having similar polarization memory. The primary excitons undergo internal conversion within $\ensuremath{\sim}100\text{ }\text{ }\mathrm{fs}$ to an even-parity (dark) singlet exciton with a PA band at $\ensuremath{\sim}1.4\text{ }\text{ }\mathrm{eV}$. We also find ultrafast photogeneration of charge polarons when pumping deep into the polymer continuum band, which are characterized by two other PA bands in the mid-IR and associated photoinduced IR vibrational modes.

Rapid emergence of solar cells based on mixed organic-inorganic halide perovskite have led to high power conversion efficiencies of over 20% in recent years. Looking for a simple and convenient way to fine-control of the perovskite film morphology is becoming one of the main issues. In this work, we explore the effect of adding NH 4 Cl on crystallization process and optical properties of perovskite. With adding NH 4 Cl, the perovskite films prepared by one-step method present better morphology than films without adding NH 4 Cl, namely, smoother surface and better coverage which result in uniform and much more stable photoluminescence intensities as well as longer lifetime of photoexcitations. More importantly, the photovoltaic cells fabricated with the addition of the NH 4 Cl have far better performance than the cells without additives. Therefore, one-step fabrication method can also control the morphology of perovskite films finely for both optical application and solar cells with adding proper additive.

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT) with below-gap excitation. Compared to above-gap excitation, the polarons exhibited thermally activated and multi-trap limited bimolecular recombination too, but with narrower distribution profile in polaron states in millisecond time regime. However, magnetophoto-induced absorption (MPA) of polarons presented much weaker magnetic field effect in below-gap excitation (∼0.5 mT) than in above-gap excitation (order of ∼10 mT), although the underline mechanism is same as spin mixing of polaron pairs by the hyperfine interaction at small field region.

The photoluminescence from exciplex states at lower energy than the bandgap of CH3NH3PbI3 (MAPbI3) was observed in both MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM bilayer films. At the same time, using continuous wave photoinduced absorption (PIA) spectroscopy, we observed the PIA band at 0.14 eV, which was due to the positive or negative polarons in perovskite for MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM film, respectively. The binding energies of both kinds of polarons are about ∼40 meV, with the radius of phonon cloud of about 5.9 nm. Our finding may supply direct spectroscopic evidence for the existence of exciplex states in heterojunction of hybrid halide perovskite and charge transport materials as well as consequent large polarons in perovskite.

The rapid improvements in perovskite solar cells have led to high power conversion efficiencies of over 20% in recent years. Despite this remarkable performance, the mechanism of thermal instability for those films is still one of the problems that we have to face. In this work, we systematically measure the thermal stability of CH 3 NH 3 PbI 3−x Cl x films on substrates of ITO, PEDOT:PSS/ITO, TiO 2 /ITO, at various temperatures. Comparing their counterparts on Glass, PEDOT:PSS/Glass, TiO 2 /Glass, we found that the films on substrates with ITO are always less thermal stable, resulting from charge transfer between perovskite and ITO substrates, in particular at higher annealing temperatures. Therefore, it was believed that more thermal degradation can occur in CH 3 NH 3 PbI 3−x Cl x film with less neutral electrical environment.

We have used photoluminescence (PL) and photomodulation (PM) spectroscopy to investigate the reversible spectral changes of PL in CH3NH3PbI3−xBrx films, where x is 1.7. In an as-prepared film, the peak of PL spectra shifts from ∼640 nm near bandedge to ∼750 nm after excitation by a continuous wave (CW) or a pulsed laser with high repetition rate, but keeps at 640 nm excited by same pulsed laser with the repetition rate smaller than 500 Hz. The PM spectroscopy also shows the formation of sub bandgap states after illumination which is responsible for the red shift of PL. The light induced modification of optical properties is reversible after keeping the film out of illumination for several hours at room temperature. We analyze the photoinduced modification to be two-steps processes: the temporary sub bandgap states were first photogenerated in perovskite film, if those states interacting with more coming photons within their lifetimes, light induced metastable states responsible for red-shift of PL will be formed. This instability reduces the electronic bandgap and generates more traps which will degrade the performance of the related photovoltaic devices.

We compare the ultrafast dynamics of the primary photoexcitations in various π-conjugated organic semiconductors, semiconducting single-walled carbon nanotubes (S-NTs) and organometal trihalide perovskites including CH3NH3PbI3 (MAPbI3) and CH3NH3PbI1.1Br1.9 (MAPbI1.1Br1.9), using broadband pump–probe photomodulation spectroscopy in the spectral range of 0.2–2.7 eV with 300 fs time resolution. The primary photoexcitations in single polymer chains and isolated S-NTs have been found to be quasi-one-dimensional (q-1D) excitons, with characteristic photoinduced absorption (PA) band due to intra-band transitions. This conclusion is in agreement with the large exciton binding energy, Eb in polymers and S-NTs (where Eb > 200 meV), illustrating the universal optical characteristic features of q-1D excitons. In three dimensional (3D) semiconductors of organometal trihalide perovskites such as MAPbI3 we found that with above-gap excitation both photo-carriers and excitons are photogenerated; but only photocarriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in MAPbI1.1Br1.9. The contrast between MAPbI3 and MAPbI1.1Br1.9 is ascribed to the difference of Eb, which is ∼20 meV and ∼110 meV, respectively. Our work shows that Eb is one of the crucial parameters that determine the photophysics characteristic of semiconductors, that result in universal occurrence of an exciton PA band, regardless if the compound is q-1D carbon based π-conjugated semiconductors, NTs, or 3D crystalline perovskite semiconductors. At the same time, our work also shows that the broadband ultrafast photomodulation spectroscopy is a powerful tool in analyzing the photophysics of semiconductors, and emphasizes the need for a broad probe spectral range in order to decipher the primary photoexcitation species.

In diluted solid solution using poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and polymethyl methacrylate (PMMA) or polystyrene (PS), both aggregated and extended conformations could be formed according to the weight ratio. Aggregated conformation in as-cast MEH-PPV/PMMA film presented a J-aggregate-like photoluminescence (PL) emission. After annealing at 160 &deg, C, its PL showed characteristics of both J- and H-aggregates at the same time, however, extended conformation showed an oligomer-like emission, which was not sensitive to either measurement temperature or annealing temperature. Thus, the conformation transition between aggregated and extended is unlikely to happen in MEH-PPV/PMMA blends during thermal annealing. On the contrary, in MEH-PPV/PS blends, extended conformation dominated in as-cast film with oligomer-like emissions, after annealing at 160 &deg, C, both J- and H- aggregate-like PL emissions were observed, indicating the conformation transitioned from extended to aggregated. Therefore, our work may suggest a new method to manipulate photophysical properties of conjugated polymers by combining appropriate host matrix and thermal annealing processes.

In this work, we demonstrated, for the first time to the best of our knowledge, high-order vortex beams at 604 nm that were directly emitted from a Pr:YLF crystal laser. By combining a blue-diode-pumped v-type cavity for controllable Gouy phase introduction and off-axis crystal rotation for high-order HG modes generation, topological charged 604-nm vortex lasers were attained up to the third order. Furthermore, the vortex beams were found to be lasing at 604 nm that was often deemed less prone to lase compared with 607-nm in the same energy manifold. By a detailed spectroscopic study and rate equation analysis, we proposed and confirmed that the 604-nm transition of Pr:YLF crystal could be favorable in the vortex beam generation by a cavity-loss-induced gain switching process, in which the net gain at 604-nm could surpass that of 607-nm as the cavity loss was amassed beyond a threshold value.

The temperature dependence of absorption and photoluminescence (PL) spectroscopies were used to study the optical properties of 2D perovskite films, including n = 1 and 3 of (PEA)

The organic-inorganic hybrid perovskites show excellent optical and electrical properties for photovoltaic and a myriad of other optoelectronics applications. Using high-field magneto-optical measurements up to 17.5 T at cryogenic temperatures, we have studied the spin-dependent optical transitions in the prototype $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, which are manifested in the field-induced circularly polarized photoluminescence emission. The energy splitting between left and right circularly polarized emission bands is measured to be \ensuremath{\sim}1.5 meV at 17.5 T, from which we obtained an exciton effective $g$ factor of \ensuremath{\sim}1.32. Also from the photoluminescence diamagnetic shift we estimate the exciton binding energy to be \ensuremath{\sim}17 meV at low temperature. Surprisingly, the corresponding field-induced circular polarization is ``anomalous'' in that the photoluminescence emission of the higher split energy band is stronger than that of the lower split band. This ``reversed'' intensity ratio originates from the combination of long electron spin relaxation time and hole negative $g$ factor in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, which are in agreement with a model based on the k\ifmmode\cdot\else\textperiodcentered\fi{}p effective-mass approximation.

In this paper, the generalized binomial states (GBSs) are theoretical presented with second-harmonic generation (SHG) processes by utilizing the Schwinger–Bose operation realization. The GBSs are theoretically demonstrate by a combination of two fundamental-frequency light fields, which are incident upon a second-order nonlinear crystal under the type-II SHG configuration. One incident field is in the number state $$\left| M \right\rangle$$ , and the other is in coherent state of sufficiently strong intensity to be treated as a classical field. It is found that GBSs will eventually convert to the coherent states when the incident number-state field possess sufficiently large number of photons. The presented study provides an alternative and theoretically intriguing scheme to achieve the important GBSs.

Ultrafast charge transfer dynamics in hybrid blend films of a low band-gap polymer poly(2,6-(N-(1-octylnonyl)dithieno[3,2-b:20,30-d]pyrrole)-alt-4,7-(2,1,3-benzothiadiazole)) (PDBT) and PbS quantum dots (QDs) are studied by using ultrafast transient transmission spectroscopy. It is observed that the transient bleaching signal arising from excitons of the PDBT displays a much faster recovery, within the time delay of ≈5 ps, in hybrid films than in the neat PDBT film. In contrast, the bleaching signal resulting from the electron filling of the QDs in hybrid films shows an extra rising component during ≈1–5 ps, which is absent in the pristine QDs. These results indicate the ultrafast electron transfer from the lowest unoccupied molecular orbital energy level of the PDBT to the conduction band of the QDs in the time scale of several ps after laser excitation. A transient absorption signal within 1 ps in the hybrid films is also found, indicating the emergence of charge transfer states (CTs). The CTs formed at the interface of the hybrid blend may facilitate the charge separation and transfer. It is estimated that over 80% of the photoexcited electrons in the PDBT may be transferred into the QDs. The transfer efficiencies show a positive correlation with the power conversion efficiencies of the corresponding hybrid solar cells.