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Hybrid halide perovskites have great potential for application in optoelectronic devices. However, an understanding of some basic properties, such as charge-carrier transport, remains inconclusive, mainly due to the mixed ionic and electronic nature of these materials. Here, we perform temperature-dependent pulsed-voltage space-charge-limited current measurements to provide a detailed look into the electronic properties of methylammonium lead tribromide (${\mathrm{MA}\mathrm{Pb}\mathrm{Br}}_{3}$) and methylammonium lead triiodide (${\mathrm{MA}\mathrm{Pb}\mathrm{I}}_{3}$) single crystals. We show that the background carrier density in these crystals is orders of magnitude higher than that expected from thermally excited carriers from the valence band. We highlight the complexity of the system via a combination of experiments and drift-diffusion simulations and show that different factors, such as thermal injection from the electrodes, temperature-dependent mobility, and trap and ion density, influence the free-carrier concentration. We experimentally determine effective activation energies for conductivity of (349 \ifmmode\pm\else\textpm\fi{} 10) meV for ${\mathrm{MA}\mathrm{Pb}\mathrm{Br}}_{3}$ and (193 \ifmmode\pm\else\textpm\fi{} 12) meV for ${\mathrm{MA}\mathrm{Pb}\mathrm{I}}_{3}$, which includes the sum of all of these factors. We point out that fitting the dark current density-voltage curve with a drift-diffusion model allows for the extraction of intrinsic parameters, such as mobility and trap and ion density. From simulations, we determine a charge-carrier mobility of 12.9 cm${}^{2}$/Vs, a trap density of 1.52 \ifmmode\times\else\texttimes\fi{} 10${}^{13}$ cm${}^{\ensuremath{-}3}$, and an ion density of 3.19 \ifmmode\times\else\texttimes\fi{} 10${}^{12}$ cm${}^{\ensuremath{-}3}$ for ${\mathrm{MA}\mathrm{Pb}\mathrm{Br}}_{3}$ single crystals. Insights into charge-carrier transport in metal-halide perovskite single crystals will be beneficial for device optimization in various optoelectronic applications.

The behavior of hot carriers in metal-halide perovskites (MHPs) present a valuable foundation for understanding the details of carrier-phonon coupling in the materials as well as the prospective development of highly efficient hot carrier and carrier multiplication solar cells. Whilst the carrier population dynamics during cooling have been intensely studied, the evolution of the hot carrier properties, namely the hot carrier mobility, remain largely unexplored. To address this, we introduce a novel ultrafast visible pump - infrared push - terahertz probe spectroscopy (PPP-THz) to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide. We find a decrease in mobility upon optically depositing energy into the carriers, which is typical of band-transport. Surprisingly, the conductivity recovery dynamics are incommensurate with the intraband relaxation measured by an analogous experiment with an infrared probe (PPP- IR), and exhibit a negligible dependence on the density of hot carriers. These results and the kinetic modelling reveal the importance of highly-localized lattice heating on the mobility of the hot electronic states. This collective polaron-lattice phenomenon may contribute to the unusual photophysics observed in MHPs and should be accounted for in devices that utilize hot carriers., Comment: 28 pages, 4 figures, 77 references

Cadmium and barium doped TiO2 nanoparticles and their respective heterostructured nanomaterials (TiO2/ZnO) were successfully synthesized through simple sol-gel and reflux methods respectively. Optical, structural and morphological analysis of as synthesized material were done by using UV-Vis spectroscopy, X-ray diffraction (XRD), Fourier transform infra-red (FT-IR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-Ray analysis (EDX). The band gap values were calculated from the optical absorption spectra of nanomaterials. The band gap of TiO2 NPs (3.10 eV) was effectively tuned up to 2.82 eV by doping it with different concentrations of cadmium and barium followed by coupling with ZnO nanoparticles. The crystalline nature and phase purity of the materials were confirmed through XRD. Morphological analysis and elemental composition studies were performed with the help of SEM and EDX. Functional groups were detected through FTIR analysis. The nanostructured materials were used in combination with P3HT (as a hole conducting polymer) to fabricate photoactive blend for dye sensitized solar cell. The photovoltaic performances of fabricated device were investigated by using current voltage (I-V) measurements. The results showed that metal doped TiO2 and their respective nano structures (TiO2/ZnO) showed better conversion efficiencies than undoped TiO2. The maximum conversion efficiency (eta) was observed for Pyrocatechol violet sensitized Cd-TiO2/ZnO heterostructured nanomaterial which is 1.84%. The corresponding current density was found to be 13.11 mA/cm(2)., Higher education commission of Pakistan [6169/Federal//NRPU/RD/HEC/16], We are highly thankful to Higher education commission of Pakistan under project No-6169/Federal//NRPU/R&D/HEC/16 for financial support and Quaid-i-Azam University, Islamabad, for providing labo-ratory and other facilities.

We report a material screening study for phase transitions of multication hybrid halide perovskite films. Two hundred sixty-six films processed with flash infrared annealing were optically and structurally characterized. This data was compiled into a database to use as a reliable guide for fundamental studies of halide perovskites. We determine the optimum conditions for the formation of the mixed-cations halide perovskite active phase. We subjected the films to different stress conditions (light, temperature, humidity) resulting in five compositions that were thermodynamically stable. From these, the photoinduced phase instability process of the hybrid perovskite films was explored. These intrinsic stability tests showed that the correct multication combination plays a fundamental role in the crystal growth and thermodynamic stability of the films.

Semiconductors based on bismuth halides have gained attention for a wide range of electronic applications, including photovoltaics, light-emitting diodes, and radiation detectors. Their appeal is due to their low toxicity, high environmental stability under ambient conditions, and easy processability by a wide range of scalable methods. The performance of Bi-based semiconductors is dictated by electron–phonon interactions, which limit carrier mobilities and can also influence optoelectronic performance, for example, by giving rise to a large Stokes shift for photoluminescence, unavoidable energy loss channels, or shallow optical absorption onsets. In this Perspective, we discuss the recent understanding of how polarons and self-trapped excitons/carriers form in Bi-based semiconductors (particularly for the case of Cs2AgBiBr6), their impact on the optoelectronic properties of the materials, and the consequences on device performance. Finally, we discuss the opportunities that control of electron–phonon coupling enables, including stable solid-state white lighting, and the possibilities of exploiting the strong coupling found in bipolarons for quantum technologies.

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of 1H, 13C, 15N, 14N, 133Cs, 87Rb, 39K, 207Pb, 119Sn, 113Cd, 209Bi, 115In, 19F and 2H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research. Solid-state NMR is useful to study the local structure, dynamics and dopant speciation in metal halide perovskites. This Perspective describes the practical aspects of the method that make it broadly applicable to optoelectronic materials.

Intramolecular singlet fission (iSF) has shown potential to improve the power conversion efficiency in photovoltaic devices by promoting the splitting of a photon-absorbing singlet exciton into two triplet excitons within a single molecule. Among different possibilities, the donor-acceptor modular strategy of copolymers has shown great promise in its ability to undergo iSF under certain conditions. However, the number of iSF donor-acceptor copolymers reported in the literature remains remarkably narrow and clear trends for the molecular design of better candidates have not yet been established. In this work, we identify the trade-off between the main iSF requirements of the donor-acceptor strategy and formulate design rules that allow them to be tuned simultaneously in a fragment-based approach. Based on a library of 2944 donor-acceptor copolymers, we establish simple guidelines to build promising novel materials for iSF. These consist in (1st) selecting an acceptor core with high intrinsic singlet-triplet splitting, (2nd) locating a donor with a larger monomer frontier molecular orbital (FMO) gap than that of the acceptor, and (3rd) tuning the relative energy of donor and/or acceptor FMOs through functionalization to promote photoinduced charge transfer in the resulting polymer. Remarkably, systems containing benzothiadiazole and thiophehe-1,1-dioxide acceptors, which have been shown to undergo iSF, fulfill all criteria simultaneously when paired with appropriate donors. This is due to their particular electronic features, which make them highly promising candidates in the quest for iSF materials.

With the exponential rise in the market value and number of devices part of the Internet of Things (IoT), the demand for indoor photovoltaics (IPV) to power autonomous devices is predicted to rapidly increase. Lead‐free perovskite‐inspired materials (PIMs) have recently attracted significant attention in photovoltaics research, due to the similarity of their electronic structure to high‐performance lead‐halide perovskites, but without the same toxicity limitations. However, the capability of PIMs for indoor light harvesting has not yet been considered. Herein, two exemplar PIMs, BiOI and Cs3Sb2ClxI9‐x are examined. It is shown that while their bandgaps are too wide for single‐junction solar cells, they are close to the optimum for indoor light harvesting. As a result, while BiOI and Cs3Sb2ClxI9‐x devices are only circa 1%‐efficient under 1‐sun illumination, their efficiencies increase to 4–5% under indoor illumination. These efficiencies are within the range of reported values for hydrogenated amorphous silicon, i.e., the industry standard for IPV. It is demonstrated that such performance levels are already sufficient for millimeter‐scale PIM devices to power thin‐film‐transistor circuits. Intensity‐dependent and optical loss analyses show that future improvements in efficiency are possible. Furthermore, calculations of the optically limited efficiency of these and other low‐toxicity PIMs reveal their considerable potential for IPV, thus encouraging future efforts for their exploration for powering IoT devices.

ConspectusA new generation of semiconducting materials based on metal halide perovskites has recently been launched into the scientific spotlight, exhibiting outstanding optoelectronic properties and providing promise for the development of efficient optical devices. As a vivid example, solar cells made from these materials have quickly reached conversion efficiencies exceeding 25%, now on par with well-established technologies, like silicon. Their widespread success is due, in part, to a unique ability to retain high-quality optoelectronic performance while being easily solution-processed into thin films. This feature is what defines them as a brand-new class of optoelectronic materials, with the ability to compete with traditional semiconductors requiring higher processing costs, like the III-Vs or II-IVs. However, the interesting photophysics of metal halide perovskites come with a catch; their soft ionic lattice promotes complex thermal-induced phase transitions and a high capacity for postsynthetic compositional changes, e.g., halide anion exchange. Such dynamic behavior has ultimately made understanding several important structure-property relationships ambiguous and obstructed a clear path toward commercialization due to inherent phase instability.Our aim in this Account is to highlight the fundamental aspects of metal halide perovskites that dictate a stable crystal structure and enable efficient anion exchange, through the lens of thermodynamic preference and phase formation energies. Taking the all-inorganic CsPbI3-xBrx system as a suitable case study, we focus on several ways in which its thermodynamically unstable perovskite structure can be maintained at room temperature and elucidate the restructuring pathways taken during destabilization. In addition, we will discuss the origin and mechanisms of postsynthetic anion exchange in CsPbX3 (X = I, Br, Cl) perovskites, with emphasis made toward direct visualization using in situ optical microspectroscopy and arriving at quantitative results. For several notable features of halide perovskites dealt with in this Account, e.g., strain stabilization, nonperovskite phase restructuring pathway, and lattice anion diffusion, we attempt to rationalize them using state-of-the-art materials modeling techniques.It is within this spirit that we not only modify a broad range of properties existing within metal halide perovskites but also regulate them for enhanced material functionality. For example, controlling partial phase changes and local replacement of halide composition in CsBX3 (B = Pb, Sn and X = I, Br, Cl) nanowires can facilitate the formation of optoelectronic heterojunctions, due to the abrupt change in local crystal structure and the correlated transition in optoelectronic properties. From this combined perspective, metal halide perovskites appear as highly dynamic systems, whereby structural and compositional modifications have a large impact on the underlying phase stability and optoelectronic properties. Thus, we highlight several scientific aspects important to the fundamental understanding of metal halide perovskites, ranging from the underlying mechanism and kinetics through which phase destabilization and anion exchange take place, to tuning the thermodynamic energy landscape using external stimuli. We anticipate that providing a clear perspective for these topics will help deepen our knowledge of the nature of ionic semiconductors and provide the stimulus required to build new research directions toward utilizing halide perovskites within versatile optoelectronic devices.

Despite organic/inorganic lead halide perovskite solar cells becoming one of the most promising next‐generation photovoltaic materials, instability under heat and light soaking remains unsolved. In this work, a highly hydrophobic cation, perfluorobenzylammonium iodide (5FBzAI), is designed and a 2D perovskite with reinforced intermolecular interactions is engineered, providing improved passivation at the interface that reduces charge recombination and enhances cell stability compared with benchmark 2D systems. Motivated by the strong halogen bond interaction, (5FBzAI)2PbI4 used as a capping layer aligns in in‐plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V oc, a twofold improvement compared with its analogous monofluorinated phenylethylammonium iodide (PEAI) recently reported. This endows the system with high power conversion efficiency of 21.65% and extended operational stability after 1100 h of continuous illumination, outlining directions for future work., 2D perovskites are of great importance to increase both the efficiency and stability of perovskite interfaces. Motivated by the stronger halogen bond interaction, (5FBzAI)2PbI4 used as a capping layer in 3D/2D systems self‐organizes with an in‐plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V oc, and remarkable operational stability.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most studied conductive polymers, holding great potential in many applications such as thermoelectric generators, solar cells, and memristors. Great efforts have been invested in trying to improve its mechanical and electrical properties and to elucidate the structure-property relationship. In this work, a systematic and quantitative study of the effect of solvent polarity and solution processing on the film structure and conductivity is presented. By using grazing-incidence wide-angle X-ray scattering (GIWAXS) together with atomic force microscopy (AFM), the importance of the quality of the PEDOT crystal packing is highlighted as a key factor to reach improved electrical conductivity, rather than the overall degree of crystallinity. Moreover, the (re)structuring mechanisms occurring during the film formation and film exposure processes are also studied by in situ GIWAXS. Different intermediate precursor stages and different pathways to reach improved crystallinity are reported depending on the used solvent. The structural results are interpreted looking at the solvent nature and the PSS/solvent affinity. With this contribution, a guidance is hoped to be given not only on how to improve the PEDOT:PSS electrical conductivity, but also on how to tune the film structural or electrical property for different applications.

Perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites suffer from ion migration, which causes overshoot of luminance over time during operation and reduces its operational lifetime. Here, we demonstrate 3D/2D hybrid PeLEDs with extremely reduced luminance overshoot and 21 times longer operational lifetime than 3D PeLEDs. The luminance overshoot ratio of 3D/2D hybrid PeLED is only 7.4% which is greatly lower than that of 3D PeLED (150.4%). The 3D/2D hybrid perovskite is obtained by adding a small amount of neutral benzylamine to methylammonium lead bromide, which induces a proton transfer from methylammonium to benzylamine and enables crystallization of 2D perovskite without destroying the 3D phase. Benzylammonium in the perovskite lattice suppresses formation of deep-trap states and ion migration, thereby enhances both operating stability and luminous efficiency based on its retardation effect in reorientation., Ion migration can induce overshoot of luminance in normal 3D perovskite light-emitting diode devices and results in reduced lifetime. Here Kim et al. show that the ion migration and overshoot can be suppressed in 3D/2D hybrid perovskites, leading to 21 times longer operational lifetime.

Knowledge of the mechanism of formation, orientation, and location of phases inside thin perovskite films is essential to optimize their optoelectronic properties. Among the most promising, low toxicity, lead-free perovskites, the tin-based ones are receiving much attention. Here, an extensive in situ and ex situ structural study is performed on the mechanism of crystallization from solution of 3D formamidinium tin iodide (FASnI(3)), 2D phenylethylammonium tin iodide (PEA(2)SnI(4)), and hybrid PEA(2)FA(n)(-1)Sn(n)I(3)(n)(+1) Ruddlesden-Popper perovskites. Addition of small amounts of low-dimensional component promotes oriented 3D-like crystallite growth in the top part of the film, together with an aligned quasi-2D bottom-rich phase. The sporadic bulk nucleation occurring in the pure 3D system is negligible in the pure 2D and in the hybrid systems with sufficiently high PEA content, where only surface crystallization occurs. Moreover, tin-based perovskites form through a direct conversion of a disordered precursor phase without forming ordered solvated intermediates and thus without the need of thermal annealing steps. The findings are used to explain the device performances over a wide range of composition and shed light onto the mechanism of the formation of one of the most promising Sn-based perovskites, providing opportunities to further improve the performances of these interesting Pb-free materials.

In this progress report, the recent work in the field of light-emitting field-effect transistors (LEFETs) based on colloidal quantum dots (CQDs) as emitters is highlighted. These devices combine the possibility of electrical switching, as known from field-effect transistors, with the possibility of light emission in a single device. The properties of field-effect transistors and the prerequisites of LEFETs are reviewed, before motivating the use of colloidal quantum dots for light emission. Recent reports on these quantum dot light-emitting field-effect transistors (QDLEFETs) include both materials emitting in the near infrared and the visible spectral range-underlining the great potential and breadth of applications for QDLEFETs. The way in which LEFETs can further the understanding of the CQD material properties-their photophysics as well as the carrier transport through films-is discussed. In addition, an overview of technology areas offering the potential for large impact is provided.

Tin halide perovskites make up the only lead-free material class endowed with optoelectronic properties comparable to those of lead iodide perovskites. Despite significant progress, the device efficiency and stability of tin halide perovskites are still limited by two potentially related phenomena, i.e., self-p-doping and tin oxidation. Both processes are likely related to defects; thus, understanding tin halide defect chemistry is a key step toward exploitation of this class of materials. We investigate the MASnI3perovskite defect chemistry, as a prototype of the entire materials class, using state-of-the-art density functional theory simulations. We show that the inherently low ionization potential of MASnI3is solely responsible of the high stability of tin vacancy and interstitial iodine defects, which are in turn at the origin of the material p-doping. Tin vacancies create a locally iodine-rich environment that could promote Sn(II) → Sn(IV) oxidation. The higher band edge energies of MASnI3compared to those of MAPbI3lead to the emergence of deep electron traps associated with undercoordinated tin defects (e.g., interstitial tin) and the suppression of deep transitions associated with undercoordinated iodine defects that are typical of MAPbI3. Thus, while lead iodide perovskites are dominated by iodine chemistry, tin chemistry dominates tin iodide perovskite defect chemistry. Mixed tin/lead perovskites exhibit an intermediate behavior and are predicted to be potentially free of deep traps. Compositional alloying with different metals is finally explored as a strategy for mitigating defect formation and self-p-doping in tin iodide perovskites.

Improving the performances of photovoltaic (PV) devices by suppressing nonradiative energy losses through surface defect passivation and enhancing the stability to the level of standard PV represents one critical challenge for perovskite solar cells. Here, reported are the advantages of introducing a tetrapropylammonium (TPA+) cation that combines two key functionalities, namely surface passivation of CH3NH3PbI3 nanocrystals through strong ionic interaction with the surface and bulk passivation via formation of a type I heterostructure that acts as a recombination barrier. As a result, nonencapsulated perovskite devices with only 2 mol% of TPA+ achieve power conversion efficiencies over 18.5% with higher VOC under air mass 1.5G conditions. The devices fabricated retain more than 85% of their initial performances for over 1500 h under ambient conditions (55% RH ± 5%). Furthermore, devices with TPA+ also exhibit excellent operational stability by retaining over 85% of the initial performance after 250 h at maximum power point under 1 sun illumination. The effect of incorporation of TPA+ on the structural and optoelectronic properties is studied by X-ray diffraction, ultraviolet–visible absorption spectroscopy, ultraviolet photon–electron spectroscopy, time-resolved photoluminescence, and scanning electron microscopy imaging. Atomic-level passivation upon addition of TPA+ is elucidated employing 2D solid-state NMR spectroscopy.

Metal halide perovskite semiconductors have sprung to the forefront of research into optoelectronic devices and materials, largely because of their remarkable photovoltaic efficiency records above 25% in single-junction devices and 28% in tandem solar cells, achieved within a decade of research. Despite this rapid progress, ionic conduction within the semiconductor still puzzles the community and can have a significant impact on all metal halide perovskite-based optoelectronic devices because of its influence upon electronic and optoelectronic processes. This phenomenon thus also makes the interpretation of electrical characterization techniques, which probe the fundamental properties of these materials, delicate and complex. For example, space-charge limited current measurements are widely used to probe defect densities and carrier mobilities in perovskites. However, the influence of mobile ions upon these measurements is significant but has yet to be considered. Here we report the effect of mobile ions upon electronic conductivity during space-charge limited current measurements of MAPbBr3 single crystals and show that conventional interpretations deliver erroneous results. We introduce a pulsed-voltage space-charge limited current procedure to achieve reproducible current-voltage characteristics without hysteresis. From this revised pulsed current-voltage sweep, we elucidate a lower bound trap-density value of 2.8 ± 1.8 × 1012 cm-3 in MAPbBr3 single crystals. This work will lead to more accurate characterization of halide perovskite semiconductors and ultimately more effective device optimization.

In this work we present a method to quantitatively measure the optical absorption of single nanowires that can be applied over a wide range of temperatures and with a high enough sensitivity to enable the measurement of below-band-gap absorption (as well as the absorption of single molecules). The method is based on accurately measuring the heat flow coming from a nanowire when it is illuminated by a laser beam. We experimentally verify this method by measuring the absorption of both a zincblende and a wurtzite GaAs, a wurtzite GaP, and a superlattice Zn3P2 nanowire. Furthermore, we find that the Zn3P2 nanowires have the largest absorption of all these materials. We analyze the advantages and disadvantages of the method and study its range of applicability.

Electron transfer theories predict that rates of charge transfer vary with the dielectric properties of the environment where the reaction occurs. An appropriate description of this relation for molecular sensitized semiconductors in electrolytes must account for the restricted geometry of these systems compared to “free” molecules in solution. Here, we explore the extent to which dielectric properties of the surrounding medium can explain the rates of charge transfer processes, measured using transient absorption spectroscopy, involving photo-oxidized thiophene–carbazole-based molecules on oxide semiconductors in inert or redox-active electrolytes. We observe no clear correlation between the activation energy of hole hopping between molecules on oxide surfaces or the recombination rate between photogenerated electrons in the oxide and holes on the adsorbed molecules and the dielectric properties of the surrounding solvent. The activation energy of hole hopping tends to increase with time following initial photogeneration of the holes, which we attribute to energetic disorder in the molecular monolayer. The recombination rate in different solvents scales with the hole hopping rate. It can also be varied by adding inert salts in the electrolyte and by controlling the access of cations in solution to the oxide surface. Finally, we show that fast electron transfer from cobalt complexes to photo-oxidized molecules in solvents with low polarity is verified, but the kinetics are limited by the ionic dissociation. Our study highlights the importance of electronic coupling between the redox-active components and their solvation, besides the reorganization energy and the driving force, in the determination of electron transfer rates at molecular sensitized interfaces in electrolytes.

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

The molar ratio of ZnO-CdO has an important effect on the photo-induced charge activities of the composite/n-Si structures. Thus, the relationship between the molar ratio of ZnO-CdO and the photo-induced charge generation was revealed in detail. The optical characteristics of thin films obtained by different molar ratio of two semiconductors were studied analytically using absorbance, transmittance, and reflectance measurements. The current-voltage (I-V), transient photo-current (I-t), and transient photo-capacitance (C-t) techniques were used to investigate the photoresponse properties of Al/ZnO-CdO/n-Si/Al diodes. The results reveal that the composites with 5/0, 4/1, and 3/2 molar ratio of ZnO-CdO display the photo-induced charge activity, and the results further manifest that the electronic parameters of the diodes depend on the molar ratio of ZnO-CdO.

Black phosphorus quantum dots for efficient photovoltaic application., Black phosphorus quantum dots (BPQDs) are proposed as effective seed-like sites to modulate the nucleation and growth of CsPbI2Br perovskite crystalline thin layers, allowing an enhanced crystallization and remarkable morphological improvement. We reveal that the lone-pair electrons of BPQDs can induce strong binding between molecules of the CsPbI2Br precursor solution and phosphorus atoms stemming from the concomitant reduction in coulombic repulsion. The four-phase transition during the annealing process yields an α-phase CsPbI2Br stabilized by BPQDs. The BPQDS/CsPbI2Br core-shell structure concomitantly reinforces a stable CsPbI2Br crystallite and suppresses the oxidation of BPQDs. Consequently, a power conversion efficiency of 15.47% can be achieved for 0.7 wt % BPQDs embedded in CsPbI2Br film-based devices, with an enhanced cell stability, under ambient conditions. Our finding is a decisive step in the exploration of crystallization and phase stability that can lead to the realization of efficient and stable inorganic perovskite solar cells.

Solubilized fullerene derivatives have revolutionized the development of organic photovoltaic devices, acting as excellent electron acceptors. The addition of solubilizing addends to the fullerene cage results in a large number of isomers, which are generally employed as isomeric mixtures. Moreover, a significant number of these isomers are chiral, which further adds to the isomeric complexity. The opportunities presented by single-isomer, and particularly single-enantiomer, fullerenes in organic electronic materials and devices are poorly understood however. Here, ten pairs of enantiomers are separated from the 19 structural isomers of bis[60]phenyl-C61-butyric acid methyl ester, using them to elucidate important chiroptical relationships and demonstrating their application to a circularly polarized light (CPL)-detecting device. Larger chiroptical responses are found, occurring through the inherent chirality of the fullerene. When used in a single-enantiomer organic field-effect transistor, the potential to discriminate CPL with a fast light response time and with a very high photocurrent dissymmetry factor (gph = 1.27 ± 0.06) is demonstrated. This study thus provides key strategies to design fullerenes with large chiroptical responses for use as chiral components of organic electronic devices. It is anticipated that this data will position chiral fullerenes as an exciting material class for the growing field of chiral electronic technologies.

The performance of perovskite solar cells is under direct control of the perovskite film quality and controlling the crystalinity and orientation of solution-processed perovskite film is a fundamental challenge. In this study, we present a scalable fabrication process for heteroepitaxial growth of mixed-cation hybrid perovskites (FA(1-x-y)MA(x)Cs(y))Pb(I1-xBrx)(3) in ambient atmospheric condition by using a Crystal Engineering (CE) approach. Smooth and mesoporous thin film of pure crystalline intermediate phase of PbX2 center dot 2DMSO is formed by deposition of supersaturated lead/cesium halides solution. Kinetically fast perovskite nucleation is achieved by rapid intercalation of formamidinium iodide (FAI) and methylammonium bromide (MABr) into the intermediate layer trough solvent assisted S(N)1 ligand exchange. Finally, heteroepitaxially perovskite growth is accomplished via Volmer-Weber crystal growth mechanism. All the layers are deposited under atmospheric condition (relative humidity (RH) 50-75%) with high reproducibility for various device and module dimensions. In particular, perovskite solar modules (Pmax similar to 550 mW) are successfully fabricated by blade coating under atmospheric condition. The CE approach remarkably improves the device performance by reaching a power conversion efficiency of 18.4% for small area (0.1 cm(2)), 16.5% on larger area (1 cm(2)) devices, and 12.7% and 11.6% for blade-coated modules with an active area of 17 and 50 cm(2), respectively. Non-encapsulated triple cation solar cells and modules show promising stability under atmospheric shelf life and light soaking conditions.

Formamidinium lead triiodide (FAPbI3) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI3 conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI3 is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI3 into black α-FAPbI3 at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI3 exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI3. This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI3. Accordingly, pure FAPbI3 p–i–n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

Hybrid perovskites are currently one of the most active fields of research owing to their enormous potential for photovoltaics. The performance of 3D hybrid organic–inorganic perovskite solar cells has increased at an incredible rate, reaching power conversion efficiencies comparable to those of many established technologies. However, the commercial application of 3D hybrid perovskites is inhibited by their poor stability. Relative to 3D hybrid perovskites, low-dimensional — that is, 2D — hybrid perovskites have demonstrated higher moisture stability, offering new approaches to stabilizing perovskite-based photovoltaic devices. Furthermore, 2D hybrid perovskites have versatile structures, enabling the fine-tuning of their optoelectronic properties through compositional engineering. In this Review, we discuss the state of the art in 2D perovskites, providing an overview of structural and materials engineering aspects and optical and photophysical properties. Moreover, we discuss recent developments along with the main limitations of 3D perovskites and assess the advantages of 2D perovskites over their 3D parent structures in terms of stability. Finally, we review recent achievements in combining 3D and 2D perovskites as an approach to simultaneously boost device efficiency and stability, paving the way for mixed-dimensional perovskite solar cells for commercial applications. Combining low-dimensional and 3D perovskites is a promising approach to achieve stable and efficient solar cells. In this Review, we discuss the structural, optical and photophysical properties of low-dimensional perovskites, compare the stability and efficiency of 2D and 3D perovskite devices, and consider 2D/3D composites as a strategy to increase the stability of perovskite solar cells.

Despite tremendous progress in efficiency and stability, perovskite solar cells are still facing the challenge of upscaling. Here we present unique advantages of reactive polyiodide melts for solvent- and adduct-free reactionary fabrication of perovskite films exhibiting excellent quality over large areas. Our method employs a nanoscale layer of metallic Pb coated with stoichiometric amounts of CH3NH3I (MAI) or mixed CsI/MAI/NH2CHNH2I (FAI), subsequently exposed to iodine vapour. The instantly formed MAI3(L) or Cs(MA,FA)I3(L) polyiodide liquid converts the Pb layer into a pure perovskite film without byproducts or unreacted components at nearly room temperature. We demonstrate highly uniform and relatively large area MAPbI3 perovskite films, such as 100 cm2 on glass/fluorine-doped tin oxide (FTO) and 600 cm2 on flexible polyethylene terephthalate (PET)/indium tin oxide (ITO) substrates. As a proof-of-concept, we demonstrate solar cells with reverse scan power conversion efficiencies of 16.12% (planar MAPbI3), 17.18% (mesoscopic MAPbI3) and 16.89% (planar Cs0.05MA0.2FA0.75PbI3) in the standard FTO/c(m)-TiO2/perovskite/spiro-OMeTAD/Au architecture. Reactive polyiodide melt-assisted conversion of metallic lead nanolayers into hybrid lead halide films opens a new branch of scalable technologies for perovskite photovoltaics.

Poly(2-alkyl-2-oxazoline)s (PAOx) are regaining interest for biomedical applications. However, their full potential is hampered by the inability to synthesise uniform high-molar mass PAOx. In this work, we proposed alternative intrinsic chain transfer mechanisms based on 2-oxazoline and oxazolinium chain-end tautomerisation and derived improved polymerization conditions to suppress chain transfer, allowing the synthesis of highly defined poly(2-ethyl-2-oxazoline) s up to ca. 50 kDa (dispersity (D)

Hybrid halide perovskite solar cells were first demonstrated in 2009 with cell efficiency quickly soaring from below 10% to more than 23% in a few years. Halide perovskites have the desirable processing simplicity but are very fragile when exposed to water and heat. This fragility represents a great challenge for the achievement of their full practical potential in photovoltaic technologies. To address this problem, here we review the recent development of the mixed-dimensional perovskites, whereby the trade-off between power conversion efficiency and stability of the material can be finely tuned using organic amine cations with different sizes and functionalities., Organic−inorganic metal halide perovskite solar cells possess high efficiency and low processing cost but suffer poor stability. Here Gao et al. review the recent progress on the 2D–3D mixed perovskites and suggest that greatly improved stability can be achieved without compromising the efficiency.

In this contribution, the two key parameters, the polarity and side chain length have been changed to study their effects on n-type organic thermoelectrics of a series of fullerene derivatives. Fullerene derivatives bearing either an alkyl side chain or ethylene glycol (EG) side chains of different lengths are used as the host molecules for molecular doping. It is found that the polar EG side chains can enable better miscibility with the polar dopant than the alkyl side chain, which leads to more than 5-fold enhancement of doping efficiency. Beyond the doping efficiency, another crucial parameter of molecular doping, the molecular order, is readily acquired by simultaneous control of the polarity and the length of the side chain. A polar side chain with an appropriate chain length can contribute to increasing Seebeck coefficients of doped fullerene derivatives more effectively than an alkyl side chain, likely due to the resultant good miscibility and high molecular order. As a result, an optimized power factor of 23.1 μW m-1 K-2 is achieved in the fullerene derivative with a tetraethylene glycol side chain. This represents one of the best n-type organic thermoelectrics. Additionally, EG side chains can improve the air stability of n-doped fullerene derivatives films as compared to an alkyl side chain. Our work sheds light on the design of side-chains in efficient n-type small molecules thermoelectric materials and contributes to the understanding of their thermoelectric properties.

Attaining control over the surface traps in halide perovskites is critical for the tunability of ultimate device characteristics. Here, we present a study on the modulation of photophysical properties, surface traps, and recombination in MAPbI3 single crystals (MSCs) with methylamine (MA) vapor surface treatment. Transient photoluminescence spectroscopy in conjunction with density functional theory calculations reveals that nonradiative recombination related to Pb2+ becomes mitigated after MA vaporing while radiative recombination via bimolecular path tends to increase, which originates from the passivation of Pb ions with the Lewis base nitrogen in MA. In contrast to the broad photoresponse in the pristine MSC photodiodes, application of MA surface treatments leads to a spectral narrowing effect (SNE) in MSCs with the response peak width

The structural phase behavior of high-quality single crystals of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) was revisited by combining Raman scattering and photoluminescence (PL) measurements under high hydrostatic pressure up to ca. 10 GPa. The single crystals were specially grown with the final thickness needed for pressure experiments, retaining their high quality due to a less invasive preparation procedure, which avoids sample thinning. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4, 2.7, and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in the line width of the phonon modes of the inorganic cage, similar to the changes observed at the tetragonal-to-orthorhombic phase transition occurring at around 160 K but ambient pressure. This behavior is interpreted as evidence for the locking of the organic cations in the cage voids above 2.7 GPa due to the reduced volume and symmetry of the unit cell. At the third phase transition, reported here for the first time, the PL is greatly affected, whereas the Raman spectrum experiences only subtle changes related to a splitting of some of the peaks. This behavior may indicate a change mostly in the electronic structure with little effect on the crystal structure. Strikingly, the sharp Raman features observed at high pressures do not support amorphization of MAPbI3 with onset at 3 GPa, as claimed by most of the high-pressure (X-ray) literature. We interpret this apparent discrepancy in terms of the degree of disorder introduced at different length scales in the perovskite lattice by the pressure-induced freeze-out of the methylammonium cation motion., The Spanish Ministerio de Economí a, Industria y Competitividad, is gratefully acknowledged for its support through Grant No. SEV-2015-0496 in the framework of the Spanish Severo Ochoa Centre of Excellence program and through Grants MAT2015-70850-P (HIBRI2) and CSD2010-00044 (Consolider NANOTHERM). A.F.-L. acknowledges a FPI fellowship from the Spanish Ministerio co-financed by the European Social Fund. B.C. and O.J.W. thank the EPSRC for Ph.D. studentship funding via the CSCT CDT (EP/ G03768X/1, EP/L016354/1). Financial support is also acknowledged from the European Research Council through project ERC CoG648901.

An earth-abundant and relatively less toxic, quatemary Cu2ZnSnSe4 (CZTSe) quantum dots (QDs) were prepared by hot injection method at low temperature to use as a sensitizer for QDSC. The formation of tetragonal phase and stoichiometry were confirmed by X-ray diffraction (XRD), Raman spectroscopy and energy dispersive X-ray (EDX) analysis, respectively. The UV-Vis-NIR and photoluminescence spectroscopy was used to determine the bandgap (1.66 eV) and narrow emission (1050-1130 nm) range. Moreover, transmission electron microscopy (TEM) was used to find out the average size of CZTSe QDs and it was found to be similar to( )5 nm. It can highly adsorb on the porous TiO2 nanofibers (NFs) and enhance the absorbance due to its smaller size. The photoconversion efficiency was investigated using the prepared CZTSe QDs sensitized porous TiO2 NFs based QDSC and its photoconversion efficiency (PCE) was found to be 3.61% which is higher than that of the conventional TiO2 NFs based QDSC (eta approximate to 2.84%).

A method for accurately measuring the relative dielectric constant (er) of thin films of soft, organic materials is described. The effects of the bombardment of these materials with hot Al atoms, the most commonly used top electrode, are mitigated by using electrodes fabricated from eutectic gallium-indium (EGaIn). The geometry of the electrode is defined by injection into microchannels to form stable structures that are nondamaging and that conform to the topology of the organic thin film. The er of a series of references and new organic materials, polymers, and fullerene derivatives was derived from impedance spectroscopy measurements for both Al and EGaIn electrodes showing the specific limitations of Al with soft, organic materials and overcoming them with EGaIn to determine their dielectric properties and provide realistic values of er.

One major advantage of organic electronics is their superior processability relative to traditional silicon-based materials. However, most high-performing polymer semiconductors exhibit poor solubility and require toxic chlorinated solvents coupled with inefficient coating methods such as spin-coating for device fabrication. Therefore, developing polymer semiconductors that are processable in environmentally benign solvents and compatible with effective printing techniques while maintaining good charge transport properties is crucial for the industrialization of low-cost and lightweight plastic electronics. In this study, alkyl flexible linkers with branched tertiary carbon atoms are inserted to a high-mobility diketopyrrolopyrrole-based polymer backbone to suppress polymer aggregation in solution, decrease crystallinity, and increase free volume. The polymer readily dissolves in industrial solvents and shows a 70-fold increase in solubility compared to its fully conjugated counterpart. Furthermore, due t...

In this work we study the photoinduced signatures of polarons in conjugated polymers and the impact of charge carrier delocalisation on their spectra. The variation of film crystallinity for two prototypical systems - blends of the homopolymer P3HT or the donor-acceptor polymer PCPDTBT with PCBM - allows probing changes of the polaron absorption in the mid infrared spectral region. Increased polaron delocalisation entails a shift of the electronic transition to lower energy in both cases. Also, infrared active vibrations soften due to a higher polymer chain order. Our findings help in providing a more complete understanding of polaron properties in conjugated materials and bring the application of the polaron absorption spectrum as an indicator for the environment on a more thoroughly studied foundation.

Colloidal quantum dots are a class of solution-processed semiconductors with good prospects for photovoltaic and optoelectronic applications. Removal of the surfactant, so-called ligand exchange, is a crucial step in making the solid films conductive, but performing it in solid state introduces surface defects and cracks in the films. Hence, the formation of thick, device-grade films have only been possible through layer-by-layer processing, limiting the technological interest for quantum dot solids. Solution-phase ligand exchange before the deposition allows for the direct deposition of thick, homogeneous films suitable for device applications. In this work, fabrication of field-effect transistors in a single step is reported using blade-coating, an upscalable, industrially relevant technique. Most importantly, a postdeposition washing step results in device properties comparable to the best layer-by-layer processed devices, opening the way for large-scale fabrication and further interest from the research community. ISSN:1944-8244 ISSN:1944-8252

The effect of additional Al X Ga 1−X As barrier layer on luminescence and structural behaviors of four cycle InAs/In 0.15 Ga 0.85 As/GaAs based quantum dots (i.e., standard submonolayer (SML) QDs) has been investigated here. Blue-shift in peak emission wavelength, along with narrow full width at half maxima (FWHM) is observed, as evidenced by the photoluminescence measurement results. InAs SML QDs in an In 0.15 Ga 0.85 As/GaAs well with a confinement enhancing Al 0.2 Ga 0.8 As barrier exhibits the lowest FWHM of 10.12 meV with an activation energy of 110 meV. Cross-sectional transmission electron microscopy confirms improvement in QD size distribution and the presence of small QDs of high crystalline quality. Symmetric rocking curves along the [004] Bragg angle affirm that incorporation of the additional barrier improvs the crystalline quality of corresponding heterostructures and yields sharp interfaces with adequate amount of QDs. In addition, the ex situ annealing study exhibits the enhancement in thermal stability of optical properties through integration of this symmetric AlGaAs barrier.

We use temperature-dependent contact resistivity (ρc) measurements to systematically assess the dominant electron transport mechanism in a large set of poly-Si passivating contacts, fabricated by varying (i) the annealing temperature (Tann), (ii) the oxide thickness (tox), (iii) the oxidation method, and (iv) the surface morphology of the Si substrate. The results show that for silicon oxide thicknesses of 1.3–1.5 nm, the dominant transport mechanism changes from tunneling to drift-diffusion via pinholes in the SiOx layer for increasing Tann. This transition occurs for Tann in the range of 850°C-950 °C for a 1.5 nm thick thermal oxide, and 700°C-750 °C for a 1.3 nm thick wet-chemical oxide, which suggests that pinholes appear in wet-chemical oxides after exposure to lower thermal budgets compared to thermal oxides. For SiOx with tox = 2 nm, grown either thermally or by plasma-enhanced atomic layer deposition, carrier transport is pinhole-dominant for Tann = 1050 °C, whereas no electric current through the SiOx layer could be detected for lower Tann. Remarkably, the dominant transport mechanism is not affected by the substrate surface morphology, although lower values of ρc were measured on textured wafers compared to planar surfaces. Lifetime measurements suggest that the best carrier selectivity can be achieved by choosing Tann right above the transition range, but not too high, in order to induce pinhole dominant transport while preserving a good passivation quality.

Four novel alternating copolymers bearing 5-fluoro-6-((2-octyldodecyl)oxy)benzo[c][1,2,5]thiadiazole as a strong acceptor unit and 9,9-dioctylfluorene as a strong donor unit with bridging units namely, thienothiophene, selenophene, 3-hexylthiophene, and thiophene were designed and synthesized. The polymers were characterized via 1H NMR spectroscopy, and weight average molecular weights were reported via gel permeation chromatography (GPC). For synthesized novel polymers, the bulk heterojunction solar cells were constructed. Besides, the effects of bridging units on electronic, optical, photovoltaic, and morphological properties were investigated. Among the polymers, the thienothiophene containing polymer P1 exhibited the highest PCE as 4.25% under the illumination of AM 1.5 G with 100 mW/cm2.

Control of the stability, transport, and confinement of charge carriers (electrons and holes) at interfaces is a key requirement to realize robust halide perovskite devices. The PbS–CsPbBr3 interface is atomically matched with low lattice strain, opening the potential for epitaxial growth. We assess the atomic nature of the interface using first-principles density functional theory calculations to identify (1) the thermodynamically stable (100) surface termination of the halide perovskite; (2) the most favorable (100)|(100) contact geometry; (3) the strong interfacial chemical bonding between PbS and CsPbBr3; (4) the type I (straddling) band alignment that enables electron and hole confinement in the lead sulfide layer. The combination of metal halide perovskites and IV–VI semiconductors represents an important platform for probing interfacial chemical processes and realizing new functionality.

ZnS/p-Si heterojunction diode has been successfully fabricated by depositing the ZnS thin films on p-type Si substrates using thermionic vacuum arc technique (TVA). The structural analysis was performed with X-ray diffraction (XRD) and Atomic force microscopy (AFM). The results revealed that ZnS thin film demonstrates nano-crystalline behavior with very smooth and homogeneous surface properties. The type was determined as n-type and the carrier concentration was found approximately 3.1 +/- 10(17) cm(-3) of the ZnS thin film by means of Hall Effect measurement. The dark current-voltage (I-V) and the capacitance- voltage (C-V) measurements with different frequencies were performed to determine the characteristics of the ZnS/p-Si heterojunction diode at room temperature. I-V results show that the diode has a good rectifying characteristic with excellent rectification ratio. The electrical parameters of the diode have been obtained by using current transport mechanism. It was found that the barrier height calculated from dark I-V measurements is in good agreement with the value obtained from C-V measurements at a frequency of 1.5 MHz. The series resistance and the built in potential of the fabricated diode were calculated as 3.6 k Omega and 0.7 V using Cheung and Cheung's equation and C-V measurement, respectively. The low cost and effective film production method were utilized to fabrication of heterojunction diode and to investigate characteristics.

For the design of efficient singlet fission chromophores, knowledge of the factors that govern the singlet fission rate is important. This rate is approximately proportional to the electronic coupling between the lowest (diabatic) spin singlet state that is populated following photoexcitation state and a so-called (TT)-T-1 state. The latter state is characterised by two triplets, each localised on one of two neighbouring molecules, which are coupled into a singlet. Here, we show the applicability of a nonorthogonal configuration interaction approach for the calculation of this electronic coupling. The advantages of this rigorous approach are that (1) the coupling can be calculated directly, (2) it includes important correlation and orbital relaxation effects, and (3) it has a clear chemical interpretation in terms of molecular states. This approach is applied to calculate the electronic coupling for a biradicaloid molecule, viz, the bis(inner salt) of 2,5-dihydroxy-1,4-dimethyl-pyrazinium. The biradicaloid molecule is, based on the energetic criteria, a promising candidate for singlet fission. We show that the electronic coupling between the molecules is also sufficiently large for singlet fission, rendering molecules based on this chemical moiety interesting singlet fission chromophores. (C) 2017 The Author(s). Published by Elsevier B.V.

High-quality radio frequency sputtered ZnO were grown on Si substrates at 400 degrees C at various partial gas pressures (Ar/Ar + O2). Subsequently, to remove as-grown defects, high temperature annealing from 700 to 900 degrees C on as-grown samples in constant oxygen flow for 10 s was performed. X-ray diffraction study confirmed the formation of highly crystalline films with a dominant peak at (002). The sample grown in 50% Ar and 50% O-2 ambient exhibited the lowest linewidth (2 theta = similar to 0.2728 degrees) and highest stoichiometry. Grain size of the as grown samples decreased with increase in the partial pressure of oxygen till a certain ratio (1:1), and photoluminescence (PL) improved with increase in annealing temperature. Low-temperature (18 K) PL measurements showed a near-band-edge emission peak at 3.37 eV, and the highest peak intensity (more than six orders compared to others with narrow linewidth of similar to 0.01272 eV) was exhibited by the sample annealed at 900 degrees C and was six orders higher than that of the as-grown sample. All as-grown samples exhibited dominant visible-range peaks due to emission from defect states.

International audience; A BODIPY thiophene polymer modified by extending conjugation of the BODIPY chromophore is reported. This modification induces tunability of energy levels and therefore absorption wavelengths in order to target lower energies.

Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic inethylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX(3) (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, hut can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClxBr1-x)(3) and MAPb(BrxI1-x)(3) alloys; which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.