Broadband MoS2 Field‐Effect Phototransistors: Ultrasensitive Visible‐Light Photoresponse and Negative Infrared Photoresponse

Inverse photoresponse is discovered from phototransistors based on molybdenum disulfide (MoS₂). The devices are capable of detecting photons with energy below the bandgap of MoS₂. Under the illumination of near‐infrared (NIR) light at 980 and 1550 nm, negative photoresponses with short response time (50 ms) are observed for the first time. Upon visible‐light illumination, the phototransistors exhibit positive photoresponse with ultrahigh responsivity on the order of 10⁴–10⁵ A W^?1 owing to the photogating effect and charge trapping mechanism. Besides, the phototransistors can detect a weak visible‐light signal with effective optical power as low as 17 picowatts (pW). A thermally induced photoresponse mechanism, the bolometric effect, is proposed as the cause of the negative photocurrent in the NIR regime. The thermal energy of the NIR radiation is transferred to the MoS₂ crystal lattice, inducing lattice heating and resistance increase. This model is experimentally confirmed by low‐temperature electrical measurements. The bolometric coefficient calculated from the measured transport current change with temperature is ?33 nA K^?1. These findings offer a new approach to develop sub‐bandgap photodetectors and other novel optoelectronic devices based on 2D layered materials.     

UV–Vis–NIR Full‐Range Responsive Carbon Dots with Large Multiphoton Absorption Cross Sections and Deep‐Red Fluorescence at Nucleoli and In Vivo

Carbon dots (CDs), with excellent optical property and cytocompatibility, are an ideal class of nanomaterials applied in the field of biomedicine. However, the weak response of CDs in the near‐infrared (NIR) region impedes their practical applications. Here, UV–vis–NIR full‐range responsive fluorine and nitrogen doped CDs (N‐CDs‐F) are designed and synthesized that own a favorable donor‐π‐acceptor (D‐π‐A) configuration and exhibit excellent two‐photon (λ_ex = 1060 nm), three‐photon (λ_ex = 1600 nm), and four‐photon (λ_ex = 2000 nm) excitation upconversion fluorescence. D‐π‐A‐conjugated CDs prepared by solvothermal synthesis under the assistance of ammonia fluoride are reported and are endowed with larger multiphoton absorption (MPA) cross sections (3PA: 9.55 × 10^?80 cm⁶ s² photon^?2, 4PA: 6.32 × 10^?80 cm⁸ s³ photon^?3) than conventional organic compounds. Furthermore, the N‐CDs‐F show bright deep‐red to NIR fluorescence both in vitro and in vivo, and can even stain the nucleoli of tumor cells. A plausible mechanism is proposed on the basis of the strong inter‐dot and intra‐dot hydrogen bonds through N? H···F that can facilitate the expanding of conjugated sp² domains, and thus not only result in lower highest occupied molecular orbital‐lowest unoccupied molecular orbital energy level but also larger MPA cross sections than those of undoped CDs.     

Stable Light‐Emitting Diodes Using Phase‐Pure Ruddlesden–Popper Layered Perovskites

State‐of‐the‐art light‐emitting diodes (LEDs) are made from high‐purity alloys of III–V semiconductors, but high fabrication cost has limited their widespread use for large area solid‐state lighting. Here, efficient and stable LEDs processed from solution with tunable color enabled by using phase‐pure 2D Ruddlesden–Popper (RP) halide perovskites with a formula (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n?1Pb_nI_3n+1 are reported. By using vertically oriented thin films that facilitate efficient charge injection and transport, efficient electroluminescence with a radiance of 35 W Sr^?1 cm^?2 at 744 nm with an ultralow turn‐on voltage of 1 V is obtained. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Phase‐pure 2D perovskites exhibit >14 h of stable operation at peak operating conditions with no droop at current densities of several Amperes cm^?2 in comparison to mixtures of 2D/3D or 3D perovskites, which degrade within minutes.     

Fluorinated Low‐Dimensional Ruddlesden–Popper Perovskite Solar Cells with over 17% Power Conversion Efficiency and Improved Stability

Low‐dimensional Ruddlesden–Popper perovskites (RPPs) exhibit excellent stability in comparison with 3D perovskites; however, the relatively low power conversion efficiency (PCE) limits their future application. In this work, a new fluorine‐substituted phenylethlammonium (PEA) cation is developed as a spacer to fabricate quasi‐2D (4FPEA)₂(MA)₄Pb₅I₁₆ (n = 5) perovskite solar cells. The champion device exhibits a remarkable PCE of 17.3% with a J_sc of 19.00 mA cm^?2, a V_oc of 1.16 V, and a fill factor (FF) of 79%, which are among the best results for low‐dimensional RPP solar cells (n ≤ 5). The enhanced device performance can be attributed as follows: first, the strong dipole field induced by the 4‐fluoro‐phenethylammonium (4FPEA) organic spacer facilitates charge dissociation. Second, fluorinated RPP crystals preferentially grow along the vertical direction, and form a phase distribution with the increasing n number from bottom to the top surface, resulting in efficient charge transport. Third, 4FPEA‐based RPP films exhibit higher film crystallinity, enlarged grain size, and reduced trap‐state density. Lastly, the unsealed fluorinated RPP devices demonstrate superior humidity and thermal stability. Therefore, the fluorination of the long‐chain organic cations provides a feasible approach for simultaneously improving the efficiency and stability of low‐dimensional RPP solar cells.     

Lasing from Mechanically Exfoliated 2D Homologous Ruddlesden–Popper Perovskite Engineered by Inorganic Layer Thickness

2D Ruddlesden–Popper perovskites (RPPs) have aroused growing attention in light harvesting and emission applications owing to their high environmental stability. Recently, coherent light emission of RPPs was reported, however mostly from inhomologous thin films that involve cascade intercompositional energy transfer. Lasing and fundamental understanding of intrinsic laser dynamics in homologous RPPs free from intercompositional energy transfer is still inadequate. Herein, the lasing and loss mechanisms of homologous 2D (BA)₂(MA)_n?1Pb_nI_3n+1 RPP thin flakes mechanically exfoliated from the bulk crystal are reported. Multicolor lasing is achieved from the large‐n RPPs (n ≥ 3) in the spectral range of 620–680 nm but not from small‐n RPPs (n ≤ 2) even down to 78 K. With decreasing n, the lasing threshold increases significantly and the characteristic temperature decreases as 49, 25, and 20 K for n = 5, 4, and 3, respectively. The n‐engineered lasing behaviors are attributed to the stronger Auger recombination and exciton–phonon interaction as a result of the enhanced quantum confinement in the smaller‐n perovskites. These results not only advance the fundamental understanding of loss mechanisms in both inhomologous and homologous RPP lasers but also provide insights into developing low‐threshold, substrate‐free, and multicolor 2D semiconductor microlasers.     

Designing Efficient Energy Funneling Kinetics in Ruddlesden–Popper Perovskites for High‐Performance Light‐Emitting Diodes

Mixed Ruddlesden–Popper (RP) perovskites are of great interest in light‐emitting diodes (LEDs), due to the efficient energy transfer (funneling) from high‐bandgap (donor) domains to low‐bandgap (acceptor) domains, which leads to enhanced photoluminescence (PL) intensity, long PL lifetime, and high‐efficiency LEDs. However, the influence of reduced effective emitter centers in the active emissive film, as well as the implications of electrical injection into the larger bandgap donor material, have not been addressed in the context of an active device. The electrical and optical signatures of the energy cascading mechanisms are critically assessed and modulated in a model RP perovskite series ((C₈H₁₇NH₃)₂(CH(NH₂)₂)_m?1Pb_mBr_3m+1). Optimized devices demonstrate a current efficiency of 22.9 cd A^?1 and 5% external quantum efficiency, more than five times higher than systems where funneling is absent. The signature of nonideal funneling in RP perovskites is revealed by the appearance of donor electroluminescence from the device, followed by a reduction in the LED performance     

Highly Oriented Thin Films of 2D Ruddlesden‐Popper Hybrid Perovskite toward Superfast Response Photodetectors

2D hybrid perovskites have shown great promise in the photodetection field, due to their intriguing attributes stemming from unique structural architectures. However, the great majority of detectors based on this 2D system possess a relatively low response speed (≈ms), making it extremely urgent to develop new candidates for superfast photodetection. Here, a new organic–inorganic hybrid perovskite, (PA)₂(FA)Pb₂I₇ (EFA, where PA is n‐pentylaminium and FA is formamidine), which features the 2D Ruddlesden–Popper type perovskite framework that is composed of the corner‐sharing PbI₆ octahedra is reported. Significantly, photodetectors fabricated on highly oriented thin films, which exhibit a perfect orientation parallel to 2D inorganic perovskite layers, exhibit a superfast response time up to ≈2.54 ns. To the best of the knowledge, this figure‐of‐merit catches up with that of the top‐ranking commercial materials, and sets a new record for 2D hybrid perovskite photodetectors. Moreover, extremely high photodetectivity (≈1.73 × 10¹⁴ Jones, under an incident power intensity of ≈46 µW cm^?2), considerable switching ratios (>10³), and low dark current (≈10 pA) are also achieved in the detector, indicating its great potential for high‐efficiency photodetection. These results shed light on the possibilities to explore new 2D candidates for assembling future high‐performance optoelectronic devices.     

2D Ruddlesden–Popper Perovskites Microring Laser Array

3D organic–inorganic hybrid perovskites have featured high gain coefficients through the electron–hole plasma stimulated emission mechanism, while their 2D counterparts of Ruddlesden–Popper perovskites (RPPs) exhibit strongly bound electron–hole pairs (excitons) at room temperature. High‐performance solar cells and light‐emitting diodes (LEDs) are reported based on 2D RPPs, whereas light‐amplification devices remain largely unexplored. Here, it is demonstrated that ultrafast energy transfer along cascade quantum well (QW) structures in 2D RPPs concentrates photogenerated carriers on the lowest‐bandgap QW state, at which population inversion can be readily established enabling room‐temperature amplified spontaneous emission and lasing. Gain coefficients measured for 2D RPP thin‐films (≈100 nm in thickness) are found about at least four times larger than those for their 3D counterparts. High‐density large‐area microring arrays of 2D RPPs are fabricated as whispering‐gallery‐mode lasers, which exhibit high quality factor (Q ≈ 2600), identical optical modes, and similarly low lasing thresholds, allowing them to be ignited simultaneously as a laser array. The findings reveal that 2D RPPs are excellent solution‐processed gain materials potentially for achieving electrically driven lasers and ideally for on‐chip integration of nanophotonics.     

Mixed Lead–Tin Halide Perovskites for Efficient and Wavelength‐Tunable Near‐Infrared Light‐Emitting Diodes

Near‐infrared (NIR) light‐emitting diodes (LEDs), with emission wavelengths between 800 and 950 nm, are useful for various applications, e.g., night‐vision devices, optical communication, and medical treatments. Yet, devices using thin film materials like organic semiconductors and lead based colloidal quantum dots face certain fundamental challenges that limit the improvement of external quantum efficiency (EQE), making the search of alternative NIR emitters important for the community. In this work, efficient NIR LEDs with tunable emission from 850 to 950 nm, using lead–tin (Pb‐Sn) halide perovskite as emitters are demonstrated. The best performing device exhibits an EQE of 5.0% with a peak emission wavelength of 917 nm, a turn‐on voltage of 1.65 V, and a radiance of 2.7 W Sr^?1 m^?2 when driven at 4.5 V. The emission spectra of mixed Pb‐Sn perovskites are tuned either by changing the Pb:Sn ratio or by incorporating bromide, and notably exhibit no phase separation during device operation. The work demonstrates that mixed Pb‐Sn perovskites are promising next generation NIR emitters.     

Near‐Infrared Photodetectors Based on MoTe2/Graphene Heterostructure with High Responsivity and Flexibility

2D transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their impressively high performance in optoelectronic devices. However, efficient infrared (IR) photodetection has been significantly hampered because the absorption wavelength range of most TMDCs lies in the visible spectrum. In this regard, semiconducting 2D MoTe₂ can be an alternative choice owing to its smaller band gap ≈1 eV from bulk to monolayer and high carrier mobility. Here, a MoTe₂/graphene heterostructure photodetector is demonstrated for efficient near‐infrared (NIR) light detection. The devices achieve a high responsivity of ≈970.82 A W^?1 (at 1064 nm) and broadband photodetection (visible‐1064 nm). Because of the effective photogating effect induced by electrons trapped in the localized states of MoTe₂, the devices demonstrate an extremely high photoconductive gain of 4.69 × 10⁸ and detectivity of 1.55 × 10¹¹ cm Hz^1/2 W^?1. Moreover, flexible devices based on the MoTe₂/graphene heterostructure on flexible substrate also retains a good photodetection ability after thousands of times bending test (1.2% tensile strain), with a high responsivity of ≈60 A W^?1 at 1064 nm at V _DS = 1 V, which provides a promising platform for highly efficient, flexible, and low cost broadband NIR photodetectors.     

Broadband Extrinsic Self‐Trapped Exciton Emission in Sn‐Doped 2D Lead‐Halide Perovskites

As emerging efficient emitters, metal‐halide perovskites offer the intriguing potential to the low‐cost light emitting devices. However, semiconductors generally suffer from severe luminescence quenching due to insufficient confinement of excitons (bound electron–hole pairs). Here, Sn‐triggered extrinsic self‐trapping of excitons in bulk 2D perovskite crystal, PEA₂PbI₄ (PEA = phenylethylammonium), is reported, where exciton self‐trapping never occurs in its pure state. By creating local potential wells, isoelectronic Sn dopants initiate the localization of excitons, which would further induce the large lattice deformation around the impurities to accommodate the self‐trapped excitons. With such self‐trapped states, the Sn‐doped perovskites generate broadband red‐to‐near‐infrared (NIR) emission at room temperature due to strong exciton–phonon coupling, with a remarkable quantum yield increase from 0.7% to 6.0% (8.6 folds), reaching 42.3% under a 100 mW cm^?2 excitation by extrapolation. The quantum yield enhancement stems from substantial higher thermal quench activation energy of self‐trapped excitons than that of free excitons (120 vs 35 meV). It is further revealed that the fast exciton diffusion involves in the initial energy transfer step by transient absorption spectroscopy. This dopant‐induced extrinsic exciton self‐trapping approach paves the way for extending the spectral range of perovskite emitters, and may find emerging application in efficient supercontinuum sources.     

Doping and Switchable Photovoltaic Effect in Lead‐Free Perovskites Enabled by Metal Cation Transmutation

Creating defect tolerant lead‐free halide perovskites is the major challenge for development of high‐performance photovoltaics with nontoxic absorbers. Few compounds of Sn, Sb, or Bi possess ns² electronic configuration similar to lead, but their poor photovoltaic performances inspire us to evaluate other factors influencing defect tolerance properties. The effect of heavy metal cation (Bi) transmutation and ionic migration on the defects and carrier properties in a 2D layered perovskite (NH₄)₃(Sb_(1?x)Bi_x)₂I₉ system is investigated. It is shown, for the first time, the possibility of engineering the carriers in halide perovskites via metal cation transmutation to successfully form intrinsic p‐ and n‐type materials. It is also shown that this material possesses a direct–indirect bandgap enabling high absorption coefficient, extended carrier lifetimes >100 ns, and low trap densities similar to lead halide perovskites. This study also demonstrates the possibility of electrical poling to induce switchable photovoltaic effect without additional electron and hole transport layers.     

A Novel Hybrid‐Layered Organic Phototransistor Enables Efficient Intermolecular Charge Transfer and Carrier Transport for Ultrasensitive Photodetection

The interfacial charge effect is crucial for high‐sensitivity organic phototransistors (OPTs), but conventional layered and hybrid OPTs have a trade‐off in balancing the separation, transport, and recombination of photogenerated charges, consequently impacting the device performance. Herein, a novel hybrid‐layered phototransistor (HL‐OPT) is reported with significantly improved photodetection performance, which takes advantages of both the charge‐trapping effect (CTE) and efficient carrier transport. The HL‐OPT consisting of 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) as conduction channel, C8‐BTBT:[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) bulk heterojunction as photoactive layer, and sandwiched MoO₃ interlayer as a charge‐transport interlayer exhibits outstanding photodetection characteristics such as a photosensitivity (I_light/I_dark) of 2.9 × 10⁶, photoresponsivity (R) of 8.6 × 10³ A W^?1, detectivity (D*) of 3.4 × 10¹⁴ Jones, and external quantum efficiency of 3 × 10⁶% under weak light illumination of 32 µW cm^?2. The mechanism and strategy described here provide new insights into the design and optimization of high‐performance OPTs spanning the ultraviolet and near infrared (NIR) range as well as fundamental issues pertaining to the electronic and photonic properties of the devices.     

High‐Quality Ruddlesden–Popper Perovskite Films Based on In Situ Formed Organic Spacer Cations

Ruddlesden–Popper perovskites (RPPs), consisting of alternating organic spacer layers and inorganic layers, have emerged as a promising alternative to 3D perovskites for both photovoltaic and light‐emitting applications. The organic spacer layers provide a wide range of new possibilities to tune the properties and even provide new functionalities for RPPs. However, the preparation of state‐of‐the‐art RPPs requires organic ammonium halides as the starting materials, which need to be ex situ synthesized. A novel approach to prepare high‐quality RPP films through in situ formation of organic spacer cations from amines is presented. Compared with control devices fabricated from organic ammonium halides, this new approach results in similar (and even better) device performance for both solar cells and light‐emitting diodes. High‐quality RPP films are fabricated based on different types of amines, demonstrating the universality of the approach. This approach not only represents a new pathway to fabricate efficient devices based on RPPs, but also provides an effective method to screen new organic spacers with further improved performance.     

Large‐Scale Synthesis of Freestanding Layer‐Structured PbI2 and MAPbI3 Nanosheets for High‐Performance Photodetection

Recently, due to the possibility of thinning down to the atomic thickness to achieve exotic properties, layered materials have attracted extensive research attention. In particular, PbI₂, a kind of layered material, and its perovskite derivatives, CH₃NH₃PbI₃ (i.e., MAPbI₃), have demonstrated impressive photoresponsivities for efficient photodetection. Herein, the synthesis of large‐scale, high‐density, and freestanding PbI₂ nanosheets is demonstrated by manipulating the microenvironment during physical vapor deposition. In contrast to conventional two‐dimensional (2D) growth along the substrate surface, the essence here is the effective nucleation of microplanes with different angles relative to the in‐plane direction of underlying rough‐surfaced substrates. When configured into photodetectors, the fabricated device exhibits a photoresponsivity of 410 mA W^?1, a detectivity of 3.1 × 10¹¹ Jones, and a fast response with the rise and decay time constants of 86 and 150 ms, respectively, under a wavelength of 405 nm. These PbI₂ nanosheets can also be completely converted into MAPbI₃ materials via chemical vapor deposition with an improved photoresponsivity up to 40 A W^?1. All these performance parameters are comparable to those of state‐of‐the‐art layered‐material‐based photodetectors, revealing the technological potency of these freestanding nanosheets for next‐generation high‐performance optoelectronics.     

Van der Waals Epitaxial Growth of Atomic Layered HfS2 Crystals for Ultrasensitive Near‐Infrared Phototransistors

As a member of the group IVB transition metal dichalcogenides (TMDs) family, hafnium disulfide (HfS₂) is recently predicted to exhibit higher carrier mobility and higher tunneling current density than group VIB (Mo and W) TMDs. However, the synthesis of high‐quality HfS₂ crystals, sparsely reported, has greatly hindered the development of this new field. Here, a facile strategy for controlled synthesis of high‐quality atomic layered HfS₂ crystals by van der Waals epitaxy is reported. Density functional theory calculations are applied to elucidate the systematic epitaxial growth process of the S‐edge and Hf‐edge. Impressively, the HfS₂ back‐gate field‐effect transistors display a competitive mobility of 7.6 cm² V^?1 s^?1 and an ultrahigh on/off ratio exceeding 10⁸. Meanwhile, ultrasensitive near‐infrared phototransistors based on the HfS₂ crystals (indirect bandgap ≈1.45 eV) exhibit an ultrahigh responsivity exceeding 3.08 × 10⁵ A W^?1, which is 10⁹‐fold higher than 9 × 10^?5 A W^?1 obtained from the multilayer MoS₂ in near‐infrared photodetection. Moreover, an ultrahigh photogain exceeding 4.72 × 10⁵ and an ultrahigh detectivity exceeding 4.01 × 10¹² Jones, superior to the vast majority of the reported 2D‐materials‐based phototransistors, imply a great promise in TMD‐based 2D electronic and optoelectronic applications.     

Low‐Noise and Large‐Linear‐Dynamic‐Range Photodetectors Based on Hybrid‐Perovskite Thin‐Single‐Crystals

Organic–inorganic halide perovskites are promising photodetector materials due to their strong absorption, large carrier mobility, and easily tunable bandgap. Up to now, perovskite photodetectors are mainly based on polycrystalline thin films, which have some undesired properties such as large defective grain boundaries hindering the further improvement of the detector performance. Here, perovskite thin‐single‐crystal (TSC) photodetectors are fabricated with a vertical p–i–n structure. Due to the absence of grain‐boundaries, the trap densities of TSCs are 10–100 folds lower than that of polycrystalline thin films. The photodetectors based on CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ TSCs show low noise of 1–2 fA Hz^?1/2, yielding a high specific detectivity of 1.5 × 10¹³ cm Hz^1/2 W^?1. The absence of grain boundaries reduces charge recombination and enables a linear response under strong light, superior to polycrystalline photodetectors. The CH₃NH₃PbBr₃ photodetectors show a linear response to green light from 0.35 pW cm^?2 to 2.1 W cm^?2, corresponding to a linear dynamic range of 256 dB.     

N‐Type 2D Organic Single Crystals for High‐Performance Organic Field‐Effect Transistors and Near‐Infrared Phototransistors

Organic field‐effect transistors and near‐infrared (NIR) organic phototransistors (OPTs) have attracted world's attention in many fields in the past decades. In general, the sensitivity, distinguishing the signal from noise, is the key parameter to evaluate the performance of NIR OPTs, which is decided by responsivity and dark current. 2D single crystal films of organic semiconductors (2DCOS) are promising functional materials due to their long‐range order in spite of only few molecular layers. Herein, for the first time, air‐stable 2DCOS of n‐type organic semiconductors (a furan‐thiophene quinoidal compound, TFT‐CN) with strong absorbance around 830 nm, by the facile drop‐casting method on the surface of water are successfully prepared. Almost millimeter‐sized TFT‐CN 2DCOS are obtained and their thickness is below 5 nm. A competitive field‐effect electron mobility (1.36 cm² V^?1 s^?1) and high on/off ratio (up to 10⁸) are obtained in air. Impressively, the ultrasensitive NIR phototransistors operating at the off‐state exhibit a very low dark current of ≈0.3 pA and an ultrahigh detectivity (D*) exceeding 6 × 10¹⁴ Jones because the devices can operate in full depletion at the off‐state, superior to the majority of the reported organic‐based NIR phototransistors.     

FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

Mixed‐halide wide‐bandgap perovskites are key components for the development of high‐efficiency tandem structured devices. However, mixed‐halide perovskites usually suffer from phase‐impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA⁺)‐based mixed‐halide perovskites, MAPb(I_0.6Br_0.4)₃, the halide composition of the spin‐coated perovskite films is preferentially dominated by the bromide ions (Br^?). Additional thermal energy is required to initiate the insertion of iodide ions (I^?) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA⁺) in the precursor solution, it can effectively facilitate the I^? coordination in the perovskite framework during the spin‐coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high‐crystallinity perovskite film with high Br^? content. As a result, high‐quality MA_0.9FA_0.1Pb(I_0.6Br_0.4)₃ perovskite film with a bandgap (E_g) of 1.81 eV is achieved, along with an encouraging power‐conversion‐efficiency of 17.1% and open‐circuit voltage (V_oc) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.     

Quantifying Polaron Formation and Charge Carrier Cooling in Lead‐Iodide Perovskites

Notwithstanding the success of lead‐halide perovskites in emerging solar energy conversion technologies, many of the fundamental photophysical phenomena in this material remain debated. Here, the initial steps following photogeneration of free charge carriers in lead‐iodide perovskites are studied, and timescales of charge carrier cooling and polaron formation, as a function of temperature and charge carrier excess energy, are quantified. It is found, using terahertz time‐domain spectroscopy (THz‐TDS), that the observed femtosecond rise in the photoconductivity can be described very well using a simple model of sequential charge carrier cooling and polaron formation. For excitation above the bandgap, the carrier cooling time depends on the charge carrier excess energy and lattice temperature, with cooling rates varying between 1 and 6 meV fs^?1, depending on the cation. While carrier cooling depends on the cation, polaron formation occurs within ≈400 fs in CH₃NH₃PbI₃ (MAPbI₃), CH(NH₂)₂PbI₃ (FAPbI₃), and CsPbI₃. Its formation time is independent of temperature between 160 and 295 K. The very similar polaron formation dynamics observed for the three perovskites points to the critical role of the inorganic lattice, rather than the cations, for polaron formation.