Search

Two-dimensional (2D) metal halide perovskites are promising tunable semiconductors. Previous studies have focused on Pb-based structures, whereas the multilayered Sn- and Ge-based analogues are largely unexplored, even though they potentially exhibit more diverse structural chemistry and properties associated with the more polarizable

Lead-free organic–inorganic halide perovskites have gained much attention as nontoxic alternatives to CH3NH3PbI3 in next-generation solar cells. In this study, we have examined the geometric and electronic properties of methylammonium germanium iodide CH3NH3GeI3 using density functional theory. Identifying a suitable functional to accurately model the germanium halide perovskites is crucial to allow the theoretical investigation for tuning the optoelectronic properties. The performance of various functionals (PBE, PBE+D3, PBEsol, PBEsol+D3, HSE06, and HSE06+D3) has been evaluated for modelling the structure and properties. The calculation of electronic properties was further refined by using the quasiparticle GW method on the optimized geometries, and that has an excellent agreement with the experiment. We report from our GW calculations that the characteristic of the density of states for CH3NH3GeI3 resembles the density of states for CH3NH3PbI3 and the effective masses of the charge carriers of CH3NH3GeI3 are comparable to the effective masses of CH3NH3PbI3 as well as silicon used in commercially available solar cells.

Methyl-ammonium lead, tin, and germanium iodide perovskites are very interesting compounds for photovoltaics because they present a high absorption capacity for solar radiation. The microscopic contributions to the absorption coefficients and efficiencies of these perovskites are analyzed and quantified. To achieve this goal, both absorption coefficients and efficiencies are split as an exact many-species expansion. The absorption coefficients have been obtained from first-principles and have been used later to obtain efficiencies. Furthermore, by using the absorption coefficients instead of the band gap as the criterion for absorption, the efficiencies have also been quantified as a function of cell thickness. The contributions of inorganic cations are larger than those of organic cation atoms for low energies. As a consequence, as the thickness decreases, the contribution to the efficiency of inorganic cations and iodine increases, whereas that of the organic cation decreases. © 2020 American Chemical Society.

In this paper, we have investigated the structural, optoelectronic and elastic properties of AGeI3 (A = K, Rb and Cs) using the density functional theory with generalized gradient approximation (GGA) for potential exchange correlation. The modified Becke–Johnson (mBJ-GGA) potential approximation is also used for calculating the optoelectronic properties of the material. The results show that the band structure of the perovskites AGeI3 (have a semiconductor behavior with direct band gap at R–R direction, the gap energy values calculated with mBJ-GGA, for each compound as following: 0.58, 0.63, and 0.71 eV, respectively. The optical properties, such as real and imaginary parts of the dielectric functions, refractive index, reflectivity, conductivity and absorption coefficient are investigated. As results, these compounds are competent candidates photovoltaic application like light harvester.

The synthesis and properties of the hybrid organic/inorganic germanium perovskite compounds, AGeI3, are reported (A = Cs, organic cation). The systematic study of this reaction system led to the isolation of 6 new hybrid semiconductors. Using CsGeI3 (1) as the prototype compound, we have prepared methylammonium, CH3NH3GeI3 (2), formamidinium, HC(NH2)2GeI3 (3), acetamidinium, CH3C(NH2)2GeI3 (4), guanidinium, C(NH2)3GeI3 (5), trimethylammonium, (CH3)3NHGeI3 (6), and isopropylammonium, (CH3)2C(H)NH3GeI3 (7) analogues. The crystal structures of the compounds are classified based on their dimensionality with 1–4 forming 3D perovskite frameworks and 5–7 1D infinite chains. Compounds 1–7, with the exception of compounds 5 (centrosymmetric) and 7 (nonpolar acentric), crystallize in polar space groups. The 3D compounds have direct band gaps of 1.6 eV (1), 1.9 eV (2), 2.2 eV (3), and 2.5 eV (4), while the 1D compounds have indirect band gaps of 2.7 eV (5), 2.5 eV (6), and 2.8 eV (7). Herein, we report on the second harmonic generation (SHG) properties of the compounds, which display remarkably strong, type I phase-matchable SHG response with high laser-induced damage thresholds (up to ∼3 GW/cm(2)). The second-order nonlinear susceptibility, χS(2), was determined to be 125.3 ± 10.5 pm/V (1), (161.0 ± 14.5) pm/V (2), 143.0 ± 13.5 pm/V (3), and 57.2 ± 5.5 pm/V (4). First-principles density functional theory electronic structure calculations indicate that the large SHG response is attributed to the high density of states in the valence band due to sp-hybridization of the Ge and I orbitals, a consequence of the lone pair activation.

AGeI3 (A: Cs+ (I), MeNH3+ (II), HC(NH2)2+ (III), CH3C(NH2)2+ (IV), C(NH2)3+ (V), Me3NH+ (VI), iPrNH3+ (VII)) polar hybrid inorganic/organic semiconductors are prepared from a solution of GeI4 in aqueous HI and H3PO2 by precipitation and crystallization through the addition of stoichiometric amounts of AI (stirring at 120 °C until half of the solution is evaporated, 25 °C, 24 h, 80-90% yield).

Computational screening based on density-functional-theory calculations reveals Ge as a candidate element for replacing Pb in halide perovskite compounds suitable for light harvesting. Experimentally, three AGeI3 (A = Cs, CH3NH3 or HC(NH2)2) halide perovskite materials have been synthesized. These compounds are stable up to 150 °C, and have bandgaps correlated with the A-site cation size. CsGeI3-based solar cells display higher photocurrents, of about 6 mA cm−2, but are limited by poor film forming abilities and oxidising tendencies. The present results demonstrate the utility of combining computational screening and experimental efforts to develop lead-free halide perovskite compounds for photovoltaic applications. NRF (Natl Research Foundation, S’pore) Accepted version

The syntheses of high purity GeI 2 and GeI 4 from the elements are described. The vapour pressure over solid germanium (II) iodide has been measured by the static method with a membrane-gauge manometer from T =550 K to T =650 K and the molar enthalpy and entropy of sublimation calculated by second-law and third-law methods. Enthalpy differences for solid and liquid GeI 2 were measured in the temperature interval (330 to 770) K by drop calorimetry. The temperature, enthalpy and entropy of melting of germanium (II) iodide were determined. The values of the standard molar thermodynamic properties for solid and liquid GeI 2 are presented from T =298.15 K to T =781 K. We have used the maximum likelihood method in the treatment of the data obtained together with previous reported values. On the basis of this work we recommend a set of standard enthalpies of formation and absolute entropies for the Ge–I compounds.

The first monomeric mono- and binuclear clathrochelates with trifluoromethylgermanium-containing capping groups have been prepared by template condensation of cyclohexanedion-1,2-dioxime, diacetylazineoxime and diacetylmonoxime hydrazone with IGe(CF3)3 on an Fe2+ ion. The trigonal-antiprismatic geometry at the Fe2+ coordination polyhedron for the complexes obtained has been established from 57Fe Mossbauer and UV spectra.

In this study, solar cell capacitance simulator (SCAPS) was utilized to investigate a tandem solar device with Methyl-ammonium germanium iodide (MAGeI3), an organic perovskite, as top cell active layer, and crystalline silicon (c-Si) as the bottom cell. Validation studies were done against established single-junction device structures including MAPbI3 and c-Si. Our simulation-based results showcase a robust congruence with experimental data, with obtained power conversion efficiency (PCE) values of 20.19%, and 23.01% for MAPbI3 and c-Si based single-junction devices, respectively. For the thesis of this work, both four-terminal (4-T) and two terminal (2-T) perovskite-on-silicon tandem architectures were investigated. Our numerical simulation for the monolithic stacked 2-T tandem structure of MAGeI3-on-c-Si produced a PCE of 28.71 %, while a PCE of 32.2 % was obtained for the standard MAPbI3-on-c-Si. For the current matching condition in the 2-T tandem devices, the optimum thickness of MAGeI3 and MAPbI3 were found to be 626 nm and 223 nm respectively. For the mechanically stacked 4-T configurations, MAGeI3-on-c-Si and MAPbI3-on-c-Si produced PCE values of 29.85 % and 33.67%, respectively, with optimum top cell absorber thickness values of 1.7 µm and 1.4 µm, respectively. Each device architecture was optimised by carrying out extensive studies including modulations in absorber thickness, bulk defect density, interfacial defect density, and back contact metal work function. Our in-depth analyses highlight a remarkable potential for MAGeI3 to be utilized as the top cell of a perovskite-on-silicon solar device, boasting high efficiency and intrinsic non-toxicity.

Lead-halide perovskites (PVKs) based solar cells have seen remarkable consideration due to improved efficiency, ease of fabrication, versatility and flexibility. However, the existence of lead, a toxic material, raises several concerns about the environment and the health of living beings and hinders their future commercialization. Therefore, a considerable surge in searching for an alternate lead-free PVK has started in the past few years. Several lead-free PVK solar cells have been proposed; nonetheless, achievable conversion efficiency from these devices is not up to the mark due to some inherent losses. Therefore, a comprehensive theoretical analysis is needed to understand the root of these losses for uplifting efficiency. Thus, the results of a specific modelling technique for all-inorganic lead-free PVK-based solar cells, namely cesium tin germanium halide (CsSnGeI3), to achieve the highest feasible efficiencies. The current simulation uses an electron transport material (ETM) of TiO2 and a hole transport material (HTM) of Spiro-OMeTAD to sandwich PVK layers of CsSnGeI3 (Eg=1.5 eV) for the PSCs. The device is subjected to further analysis and optimization of active layer thickness, defect density, operating temperature, defect density, capacitance-voltage (C-V) and impedance analysis to investigate the different performance parameters. The optimized conversion efficiency of 28.4% has been achieved with CsSnGeI3-based PSC. Results reported in this study may pave the way to developing lead-free PVK solar cells through higher conversion efficiencies.

There is an emerging demand for nanodiamonds with controlled structures or shapes for applications in biomedical imaging and sensing. Synthetic conditions proceeding at less harsh temperatures and pressures are considered crucial to control nanodiamond formation process. We report agermaniumiodide (GeI4) mediated synthesis of nanodiamonds from diamondoid molecules and alkane hydrocarbon under moderate high-pressure high-temperature (m-HPHT) conditions. For the first time, GeI4is used to generate nanodiamonds at 3.5GPa and 500°C, which is considerably lower than the conditions reported for common HPHT methods for nanodiamond synthesis. The strategy reported herein allows synthesizing nanodiamonds based on well-defined molecular precursors, which paves the way to a bottom-up diamond synthesis designed at a molecular level.

Germanium halide perovskites are an attractive alternative to lead perovskites because of their well-suited optical properties for photovoltaic applications. However, the power conversion efficiencies of solar cells based on germanium perovskites remained below 0.2% so far, and also, the device stability is an issue. Herein, we show that modifying the chemical composition of the germanium perovskite, i.e., introducing bromide ions into the methylammonium germanium iodide perovskite, leads to a significant improvement of the solar cell performance along with a slight enhancement of the stability of the germanium perovskite. With substitution of 10% of the iodide with bromide, power conversion efficiencies up to 0.57% were obtained in MAGeI2.7Br0.3 based solar cells with a planar p–i–n architecture using PEDOT:PSS as hole and PC70BM as electron transport layer.

Recently, two-dimensional organic–inorganic perovskites have attracted increasing attention due to their unique photophysical properties and high stability. Here we report a lead-free, two-dimensional perovskite, (PEA)2GeI4 (PEA = C6H5(CH2)2NH3+). Structural characterization demonstrated that this 2D perovskite structure is formed with inorganic germanium iodide planes separated by organic PEAI layers. (PEA)2GeI4 has a direct band gap of 2.12 eV, in agreement with 2.17 eV obtained by density functional theory (DFT) calculations, implying that it is suitable for a tandem solar cell. (PEA)2GeI4 luminesces at room-temperature with a moderate lifetime, exhibiting good potential for photovoltaic applications. In addition, 2D (PEA)2GeI4 is more stable than 3D CH3NH3GeI3 in air, owing to the presence of a hydrophobic organic long chain. This work provides a direction for the development of 2D Ge-based perovskites with potential for photovoltaic applications.

Lead iodide perovskite has long been considered to be promising for photovoltaic applications due to the high power conversion efficiency. However, the toxicity and instability of lead iodide perovskite limit its widespread use. Recently, germanium iodide perovskite CH3NH3GeI3 has attracted much attention due to the lead-free and high distorted structure. In this study, we exhibited a detailed theoretical investigation to predict total ferroelectric polarization of about 13.8 μC cm−2 and disentangle the contributions of MA cation (MA = CH3NH3), the center charge of MA cation and Ge2+ ion relative to the inorganic framework. Based on the electronic structure, optical absorption as well as carrier mobility of the system are further calculated, which exhibit strong anisotropy along the same direction. Further regulating atomic structure to strengthen or weaken the polarization, variable optical absorption spectra and hence tunable efficiencies are obtained. By analyzing the electronic structure with applied increased polarization, we conclude that polarization driven optical absorption can be attributed to an increased p–p orbital transition from I to Ge. In the case of CH3NH3GeI3, we discuss the relationship between ferroelectricity and photovoltaic performance, reveal the potential of utilizing polar material for higher solar conversion, and speculate that it is a general rule to raise the efficiency by enhancing the polarization.

Nonlinear optical properties (nonlinear refraction index and two-photon absorption coefficient) of chalcoiodide glasses of the GexS90 − xI10 system obtained by fusing germanium, germanium iodide and sulfur in an evacuated quartz ampoule were studied as a function of their macrocomposition. The dependence of nonlinear properties proved to be nonmonotonic. For stoichiometric ratio of the number of Ge and S atoms, the nonlinear refraction index reaches its minimum value and the two-photon absorption coefficient is maximal.

Compared with organic-inorganic perovskites, all-inorganic cesium-based perovskites without volatile organic compounds have gained extensive interests because of the high thermal stability. However, they have a problem on phase transition from cubic phase (active for photo-electric conversion) to orthorhombic phase (inactive for photo-electric conversion) at room temperature, which has hindered further progress. Herein, novel inorganic CsPb1-x Gex I2 Br perovskites were prepared in humid ambient atmosphere without a glovebox. The phase stability of the all-inorganic perovskite was effectively enhanced after germanium addition. In addition, the highest power conversion efficiency of 10.8 % with high open-circuit voltage (VOC ) of 1.27 V in a planar solar cell based on CsPb0.8 Ge0.2 I2 Br perovskite was achieved. Furthermore, the highest VOC up to 1.34 V was obtained by CsPb0.7 Ge0.3 I2 Br perovskite, which is a remarkable record in the field of all-inorganic perovskite solar cells. More importantly, all the photovoltaic parameters of CsPb0.8 Ge0.2 I2 Br perovskite solar cells showed nearly no decay after 7 h measurement in 50-60 % relative humidity without encapsulation.

01 August Compared with organic‐inorganic perovskites, all‐inorganic cesium‐based perovskites without volatile organic compounds have gained extensive interests because of the high thermal stability. However, they have a problem on phase transition from cubic phase (active for photo‐electric conversion) to orthorhombic phase (inactive for photo‐electric conversion) at room temperature, which has hindered further progress. Herein, novel inorganic CsPb1−xGexI2Br perovskites were prepared in humid ambient atmosphere without a glovebox. The phase stability of the all‐inorganic perovskite was effectively enhanced after germanium addition. In addition, the highest power conversion efficiency of 10.8 % with high open‐circuit voltage (VOC) of 1.27 V in a planar solar cell based on CsPb0.8Ge0.2I2Br perovskite was achieved. Furthermore, the highest VOC up to 1.34 V was obtained by CsPb0.7Ge0.3I2Br perovskite, which is a remarkable record in the field of all‐inorganic perovskite solar cells. More importantly, all the photovoltaic parameters of CsPb0.8Ge0.2I2Br perovskite solar cells showed nearly no decay after 7 h measurement in 50–60 % relative humidity without encapsulation.

Owing to high power conversion efficiency and low-cost, methyl-ammonium lead-based halide (viz. CH3NH3PbI3) Perovskites have been emerging as the innovative candidate in the development of optoelectronic devices. However, the toxic lead in these materials is a major hurdle in its commercialization. Thus, there is an urgent need to replace lead with an appropriate element. Ethyl-ammonium based lead-free hybrid halide perovskites may be an alternative photovoltaic (PV) absorber material with appropriate band gap, high stability and non-toxic properties. Herein, we have investigated structural, electronic, optical, thermoelectric and thermodynamic properties of ethyl-ammonium germanium iodide (CH3CH2NH3GeI3 or EAGeI3) by full-potential augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code within the density functional theory (DFT). In this paper, we have found that EAGeI3 has direct band gap of 1.3 eV and high absorption coefficient greater than 104 cm−1 indicating its suitability as PV absorber material. We have also calculated thermoelectric coefficients as a function of carrier concentration, chemical potential and temperature. The thermodynamic calculations have been done within the quasi-harmonic approximation. As EAGeI3 has been studied first time for PV applications, the present study may open a new vista for more exhaustive experimental and theoretical investigations in search of non-toxic and eco-friendly PV materials.