Influence of the orientation of methylammonium lead iodide perovskite crystals on solar cell performance

Perovskite solar cells are emerging as serious candidates for thin film photovoltaics with power conversion efficiencies already exceeding 16%. Devices based on a planar heterojunction architecture, where the MAPbI(3) perovskite film is simply sandwiched between two charge selective extraction contacts, can be processed at low temperatures (<150 degrees C), making them particularly attractive for tandem and flexible applications. However, in this configuration, the perovskite crystals formed are more or less randomly oriented on the surface. Our results show that by increasing the conversion step temperature from room temperature to 60 degrees C, the perovskite crystal orientation on the substrate can be controlled. We find that films with a preferential orientation of the long axis of the tetragonal unit cell parallel to the substrate achieve the highest short circuit currents and correspondingly the highest photovoltaic performance

Heterostructures of skutterudites and germanium antimony tellurides – structure analysis and thermoelectric properties of bulk samples

Heterostructures of germanium antimony tellurides with skutterudite-type precipitates are promising thermoelectric materials due to low thermal conductivity and multiple ways of tuning their electronic transport properties. Materials with the nominal composition [CoSb2(GeTe)_(0.5)]_x(GeTe)_(10.5)Sb_2Te_3 (x = 0–2) contain nano- to microscale precipitates of skutterudite-type phases which are homogeneously distributed. Powder X-ray diffraction reveals that phase transitions of the germanium antimony telluride matrix depend on its GeTe content. These are typical for this class of materials; however, the phase transition temperatures are influenced by heterostructuring in a beneficial way, yielding a larger existence range of the intrinsically nanostructured pseudocubic structure of the matrix. Using microfocused synchrotron radiation in combination with crystallite pre-selection by means of electron microscopy, single crystals of the matrix as well as of the precipitates were examined. They show nano-domain twinning of the telluride matrix and a pronounced structure distortion in the precipitates caused by GeTe substitution. Thermoelectric figures of merit of 1.4 ± 0.3 at 450 °C are observed. In certain temperature ranges, heterostructuring involves an improvement of up to 30% compared to the homogeneous material

TAGS-related indium compounds and their thermoelectric properties – the solid solution series (GeTe)_xAgIn_ySb_(1-y)Te_2 (x = 1–12; y = 0.5 and 1)

Various members of the solid solution series (GeTe)_xAgIn_ySb_(1−y)Te_2 can be obtained by quenching high-temperature phases (x = 12 for y = 1 and x > 5 for y = 0.5). In contrast, high-temperature and high-pressure conditions (2.5 GPa, 350 °C) are required for the synthesis of members with In contents >3.6 atom% (such as x 5 adopt the α-GeTe structure type (3 + 3 coordination). Thus, in all samples investigated, 3 or 4 cations are disordered at one Wyckoff position. The quenched high-temperature or high-pressure phases, respectively, are almost homogeneous. Their powder X-ray diffraction patterns suggest pure phases; yet, high-resolution electron microscopy occasionally reveals a very small extent of nanoscopic precipitates as well as dislocations and twinning. (GeTe)_(5.5)AgIn_(0.5)Sb_(0.5)Te_2 shows a maximal ZT value of 0.75 even when (partial) decomposition into the TAGS material (GeTe)_(11)AgSbTe_2 and chalcopyrite-type AgInTe_2 has occurred at 300 °C. (GeTe)_(5.5)AgInTe_2 prepared under high-pressure conditions exhibits a ZT value of 0.6 at 125 °C, i.e. far below the decomposition temperature and thus is an interesting new low-temperature thermoelectric material

Temperature-dependent studies of exciton binding energy and phase-transition suppression in (Cs,FA,MA)Pb(I,Br)3_{3} perovskites

Multiple-cation mixed-halide (Cs,FA,MA)Pb(I,Br)3 perovskites containing cesium, formamidinium (FA), and methylammonium (MA) possess excellent properties for a wide range of optoelectronic applications such as thin-film photovoltaics or lasers. We investigate the role of excitons and the exciton binding energy EB, relevant for the effectiveness of charge separation in solar cells, as well as the temperature-dependent bandgap energy Eg which is used as an indicator for crystal phase transitions. Generalized Elliott fits of absorption spectra offer the possibility to determine both EB and Eg. However, since excitonic effects are non-negligible even at room temperature, a careful and detailed analysis of the spectra is crucial for a correct interpretation. Therefore, an additional evaluation based on a so-called f-sum rule is applied to achieve an improved reliability of the results at higher temperatures. The obtained EB values of 20–24 meV for Cs-containing mixed perovskite compounds are below the ones of 24–32 meV and 36–41 meV for pure methylammonium lead iodide (MAPbI3) and bromide (MAPbBr3), respectively, and, thus, facilitate charge-carrier separation in photovoltaic applications. Furthermore, temperature-dependent (T = 5–300 K) studies of Eg in (Cs,FA,MA)Pb(I,Br)3 indicate a suppressed crystal phase transition by the absence of any phase-transition related signatures such as the well-known jump of about 100 meV in MAPbI3. We verify these results using temperature-dependent electroreflectance spectroscopy, which is a very reliable technique for the direct and non-destructive determination of optical resonances of the absorber layer in complete solar cells. Additionally, we confirm the suppression of the phase transition in Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 by temperature-dependent X-ray diffraction

Nanostructures in Te/Sb/Ge/Ag (TAGS) Thermoelectric Materials Induced by Phase Transitions Associated with Vacancy Ordering

Te/Sb/Ge/Ag (TAGS) materials with rather high concentrations of cation vacancies exhibit improved thermoelectric properties as compared to corresponding conventional TAGS (with constant Ag/Sb ratio of 1) due to a significant reduction of the lattice thermal conductivity. There are different vacancy ordering possibilities depending on the vacancy concentration and the history of heat treatment of the samples. In contrast to the average α-GeTe-type structure of TAGS materials with cation vacancy concentrations <3%, quenched compounds like Ge_(0.53)Ag_(0.13)Sb_(0.27)□_(0.07)Te_1 and Ge_(0.61)Ag_(0.11)Sb_(0.22)□_(0.06)Te_1 exhibit “parquet-like” multidomain nanostructures with finite intersecting vacancy layers. These are perpendicular to the pseudocubic 111 directions but not equidistantly spaced, comparable to the nanostructures of compounds (GeTe)_nSb_2Te_3. Upon heating, the nanostructures transform into long-periodically ordered trigonal phases with parallel van der Waals gaps. These phases are slightly affected by stacking disorder but distinctly different from the α-GeTe-type structure reported for conventional TAGS materials. Deviations from this structure type are evident only from HRTEM images along certain directions or very weak intensities in diffraction patterns. At temperatures above 400 °C, a rock-salt-type high-temperature phase with statistically disordered cation vacancies is formed. Upon cooling, the long-periodically trigonal phases are reformed at the same temperature. Quenched nanostructured Ge_(0.53)Ag_(0.13)Sb_(0.27)□_(0.07)Te_1 and Ge_(0.61)Ag_(0.11)Sb_(0.22)□_(0.06)Te_1 exhibit ZT values as high as 1.3 and 0.8, respectively, at 160 °C, which is far below the phase transition temperatures. After heat treatment, i.e., without pronounced nanostructure and when only reversible phase transitions occur, the ZT values of Ge_(0.53)Ag_(0.13)Sb_(0.27)□_(0.07)Te_1 and Ge_(0.61)Ag_(0.11)Sb_(0.22)□_(0.06)Te_1 with extended van der Waals gaps amount to 1.6 at 360 °C and 1.4 at 410 °C, respectively, which is at the top end of the range of high-performance TAGS materials

Blue-Green Color Tunable Solution Processable Organolead Chloride-Bromide Mixed Halide Perovskites for Optoelectronic Applications.

Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425-570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1-x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride-bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.We acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme (Cambridge) for the Physics of Sustainability. Support from the Deutsche Forschungsgemeinschaft (NIM Excellence Cluster) is gratefully acknowledged. A.S. acknowledges the funding and support from the Indo-UK APEX project. F.D. acknowledges funding and support from a Herchel Smith fellowship. M.D.V. acknowledges funding and support from the ERC-StG 337739-HIENA. A.S. thanks Dr. D. Di for the insightful discussions. P. D. gratefully acknowledges support from the European Union in the form of a Marie Curie Intra-European fellowship.This is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/acs.nanolett.5b0236

A general approach for hysteresis-free, operationally stable metal halide perovskite field-effect transistors.

Despite sustained research, application of lead halide perovskites in field-effect transistors (FETs) has substantial concerns in terms of operational instabilities and hysteresis effects which are linked to its ionic nature. Here, we investigate the mechanism behind these instabilities and demonstrate an effective route to suppress them to realize high-performance perovskite FETs with low hysteresis, high threshold voltage stability (ΔVt 1 cm2/V·s at room temperature. We show that multiple cation incorporation using strain-relieving cations like Cs and cations such as Rb, which act as passivation/crystallization modifying agents, is an effective strategy for reducing vacancy concentration and ion migration in perovskite FETs. Furthermore, we demonstrate that treatment of perovskite films with positive azeotrope solvents that act as Lewis bases (acids) enables a further reduction in defect density and substantial improvement in performance and stability of n-type (p-type) perovskite devices

Influence of the orientation of methylammonium lead iodide perovskite crystals on solar cell performance

Perovskite solar cells are emerging as serious candidates for thin film photovoltaics with power conversion efficiencies already exceeding 16%. Devices based on a planar heterojunction architecture, where the MAPbI3 perovskite film is simply sandwiched between two charge selective extraction contacts, can be processed at low temperatures (&lt;150 °C), making them particularly attractive for tandem and flexible applications. However, in this configuration, the perovskite crystals formed are more or less randomly oriented on the surface. Our results show that by increasing the conversion step temperature from room temperature to 60 °C, the perovskite crystal orientation on the substrate can be controlled. We find that films with a preferential orientation of the long axis of the tetragonal unit cell parallel to the substrate achieve the highest short circuit currents and correspondingly the highest photovoltaic performance