Interfacial Molecular Doping of Metal Halide Perovskites for Highly Efficient Solar Cells

Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way. Here, a small molecule of 4,4′,4″,4″′-(pyrazine-2,3,5,6-tetrayl) tetrakis (N,N-bis(4-methoxyphenyl) aniline) (PT-TPA) which can effectively p-dope the surface of FA_xMA_1−xPbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite films is reported. The intermolecular charge transfer property of PT-TPA forms a stabilized resonance structure to accept electrons from perovskites. The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times. Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT-TPA. After applying PT-TPA into perovskite solar cells, the doping-induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there. The optimized devices demonstrate an increased photovoltage from 1.12 to 1.17 V and an efficiency of 23.4% from photocurrent scanning with a stabilized efficiency of 22.9%. The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques.     

Molecule‐Doped Nickel Oxide: Verified Charge Transfer and Planar Inverted Mixed Cation Perovskite Solar Cell

Both conductivity and mobility are essential to charge transfer by carrier transport layers (CTLs) in perovskite solar cells (PSCs). The defects derived from generally used ionic doping method lead to the degradation of carrier mobility and parasite recombinations. In this work, a novel molecular doping of NiO_x hole transport layer (HTL) is realized successfully by 2,2′‐(perfluoronaphthalene‐2,6‐diylidene)dimalononitrile (F6TCNNQ). Determined by X‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, the Fermi level (E_F) of NiO_x HTLs is increased from ?4.63 to ?5.07 eV and valence band maximum (VBM)‐E_F declines from 0.58 to 0.29 eV after F6TCNNQ doping. The energy level offset between the VBMs of NiO_x and perovskites declines from 0.18 to 0.04 eV. Combining with first‐principle calculations, electrostatic force microscopy is applied for the first time to verify direct electron transfer from NiO_x to F6TCNNQ. The average power conversion efficiency of CsFAMA mixed cation PSCs is boosted by ≈8% depending on F6TCNNQ‐doped NiOx HTLs. Strikingly, the champion cell conversion efficiency of CsFAMA mixed cations and MAPbI₃‐based devices gets to 20.86% and 19.75%, respectively. Different from passivation effect, the results offer an extremely promising molecular doping method for inorganic CTLs in PSCs. This methodology definitely paves a novel way to modulate the doping in hybrid electronics more than perovskite and organic solar cells.     

Thermoelectric properties of GdBaCo2−x Fe x O5+δ ceramics

The influence of Fe doping on the lattice structure and thermoelectric properties of GdBaCo_2?x Fe _x O_5+δ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ceramics was studied from room temperature to 525 K. The results show that presence of Fe will increase both electrical resistivity and Seebeck coefficients of the samples. Due to the lattice misfit and carrier concentration reduction caused by Fe, the thermal conductivity of the sample decreases. The activation energy of conductivity is larger than that of thermopower calculated in the semiconductive region, which means that the conduction mechanism may be determined as a small polaron hopping model. The optimum Fe doping amount is x = 0.6, which results in a ZT value 0.02 at 373 K.     

FA0.88Cs0.12PbI3−x(PF6)x Interlayer Formed by Ion Exchange Reaction between Perovskite and Hole Transporting Layer for Improving Photovoltaic Performance and Stability

Interface engineering to form an interlayer via ion exchange reaction is reported. A FA_0.88Cs_0.12PbI₃ formamidinium (FA) perovskite layer is first prepared, then FAPF₆ solution with different concentrations is spin‐coated on top of the perovskite film, which leads to a partial substitution of iodide by PF₆^? ion. The second phase with nominal composition of FA_0.88Cs_0.12PbI_3?x(PF₆)_x is grown at the grain boundary, which has island morphology and its size depends on the FAPF₆ solution concentration. The lattice is expanded and bandgap is reduced due to inclusion of larger PF₆^? ions. The power conversion efficiency (PCE) is significantly enhanced from 17.8% to 19.3% as a consequence of improved fill factor and open‐circuit voltage (V_oc). In addition, current–voltage hysteresis is reduced. Post‐treatment with FAPF₆ reduces defect density and enhances carrier lifetime, which is responsible for the improved photovoltaic performance and reduced hysteresis. The unencapsulated device with post‐treated perovskite film demonstrates better stability than the pristine perovskite, where the initial PCE retains over 80% after 528 h exposure under relative humidity of around 50–70% in the dark and 92% after 360 h under one sun illumination.     

Growth,characterization and transport properties of Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S mixed crystals

The polycrystalline Pb _x Zn_1−x S semiconductor powder with (0 ≤ x ≤ 0·5) has been prepared by controlled co-precipitation method from an alkaline medium using thiourea as a sulphide ion source. Pellets are made with these powders applying 10 ton/sq.cm. pressure and sintered at 800°C for 2 h in nitrogen atmosphere. X-ray studies of these samples have indicated that the compounds are polycrystalline in nature with mixed hexagonal and cubic structure of ZnS and cubic structure of PbS. Lattice parameters (a and c) of all the compounds are determined from the X-ray data and are found to decrease nonlinearly with increase in Pb concentration (x). It is also observed that the grain size of the crystallites increases in samples with x = 0−0·5. Scanning electron micrographs have shown that both cubic and hexagonal crystallites are present in the mixed crystals. The electrical conductivity in Pb _x Zn_1−x S is found to decrease with increase in composition (x = 0−0·5), whereas it increases at all temperatures in all samples. Mobility of charge carrier concentration is found to increase with increasing temperature. The increase in carrier mobility in Pb _x Zn_1−x S samples may be due to reduced grain boundary potential. In Pb _x Zn_1−x S samples with x = 0−0.3, the sum of the activation energy due to charge carriers and grain boundary potential is equal to the activation energy due to conductivity.     

Effects of annealing on the optical bandgap of amorphous Ga5Se95-xSbx during crystallization

Abstract

The optical bandgap of as-deposited and annealed films of amorphous Ga₅Se_95-xSb_x (a-Ga₅Se_95-xSb_x) (where x = 0, 5, 10 and 15) have been studied as a function of photon energy in the wavelength range 400–900 nm. Thin films were induced by thermal annealing for 1 h at a temperature below their crystallization temperatures. It has been found that the optical bandgap increases with increasing annealing temperature. For as-deposited films, it has been found that the optical bandgap and refractive index decrease while the extinction coefficient k increases on incorporation of antimony into the Ga-Se system. The temperature dependence (312–392 K) of dc conductivity of bulk samples of a-Ga₅Se_95-xSb_x has also been reported. It has been found that the decrease in activation energy may be due to the decrease in optical bandgap on adding antimony to the present system.     

Structural and Optical Properties of Co-Doped ZnO Thin Films

Co-doped zinc oxide thin films (Zn_1–xCo_xO) have been deposited on c-plane sapphire substrates by dual-beam pulsed laser deposition. The films have lattice parameters similar to that of ZnO, and the lattice parameters are closely distributed. The films grew along a preferred direction, following the epitaxial relationship Zn_1–xCo_xO (0001)substrate (0001). Excitonic emission was suppressed at higher Co-dopant concentration in ZnO because of increase in the distortion of host lattice and defects. When more Zn is replaced by Co, more impurity levels are developed within the bandgap, and more defect are generated. Under our experimental conditions, the bandgap of the films tends to increase with increasing dopant concentration.     

Lattice parameters and dielectric properties of (1 ? x)Bi(Mg1/2Ti1/2)O3 · xBiCoO3 perovskite solid solutions

(1 − x)Bi(Mg_1/2Ti_1/2)O₃ · xBiCoO ceramics have been prepared at high pressures (6 GPa) and temperatures (1370–1570 K). Perovskite solid solutions have been obtained in the composition ranges 0 < x < 0.6 and 0.8 < x < 1. The Bi(Mg_1/2Ti_1/2)O₃-based perovskite phase (x < 0.6) has an orthorhombic structure (sp. gr. Pnnm), which persists up to the decomposition temperature of the material. The BiCoO₃-based phase (x > 0.8) has a tetragonal structure (sp. gr. P4mn). In the range 0.6 < x < 0.8, the ceramics consisted of the two perovskite phases. The lattice parameters of the solid solutions with x < 0.6 are linear functions of composition. The dielectric properties of the orthorhombic solid solutions have been studied using impedance spectroscopy. The temperature dependences of dielectric permittivity ɛ′ and dielectric loss tanδ exhibit different behaviors at low and high temperatures. With increasing x, the boundary between these regions shifts from about 450 to 300 K. At high temperatures, both ɛ′ and tanδ rise steeply. The dc conductivity of the solid solutions with x < 0.6 exhibits Arrhenius behavior. The activation energies for charge transport in the ceramics studies are presented.     

Study of transport properties of Ge50− x Sb y Te100+ x − y thin film alloy

The transport properties of Ge_50−xSb_yTe_100+x−y Alloys where 0 ≤ x ≤ 15 and 0 ≤ y ≤ 30 in the thin films state were studied. The temperature dependence of the d.c. electrical conductivity measurements for all prepared composition with different thickness, shows that, all samples behaves as a semiconductor up to a specific temperature at which an abrupt transition appears. That transition is due to the change of the semiconductor from non-degenerate to degenerate state. The temperature dependence of a.c. electrical conductivity measurements shows that, the a.c. conductivity for all films are frequency independent in the tested frequency range from 0.12 to 10 kHz. Thermoelectric power was found to have positive sign indicating that these alloys are p-type semiconductors. The value of temperature coefficient γ was found to be in the order of 10⁻⁴ (eV/K). The calculation of the free-charge carrier concentration and charge mobility shows that, the abrupt transition appears in thermoelectric power and conductivity are due to an abrupt increase in the mobility.     

Composition-Dependent Passivation Efficiency at the CdS/CuIn1-xGaxSe2 Interface

The bandgap of CuIn_1-xGa_xSe₂ (CIGS) chalcopyrite semiconductors can be tuned between ≈1.0 and ≈1.7 eV for Ga contents ranging between x = 0 and x = 1. While an optimum bandgap of 1.34 eV is desirable for achieving maximum solar energy conversion in solar cells, state-of-the-art CIGS-based devices experience a drop in efficiency for Ga contents x > 0.3 (i.e., for bandgaps >1.2 eV), an aspect that is limiting the full potential of these devices. The mechanism underlying the limited performance as a function of CIGS composition has remained elusive: both surface and bulk recombination effects are proposed. Here, the disentanglement between surface and bulk effects in CIGS absorbers as a function of Ga content is achieved by comparing photogenerated charge carrier dynamics in air/CIGS and surface-passivated ZnO/CdS/CIGS samples. While surface passivation prevents surface recombination of charge carriers for low Ga content (x < 0.3; up to 1.2 eV bandgap), surface recombination dominates for higher-bandgap materials. The results thus demonstrate that surface, rather than bulk effects, is responsible for the drop in efficiency for Ga contents larger than x ≈ 0.3.     

Improving the physical properties of polycrystalline-deficient Pr0.8Sr0.2−x□xMnO3 (0 ≤ x ≤ 0.2) compounds for electronic devices

Deficient strontium effects upon structural and spectroscopic properties of Pr_0.8Sr_0.2MnO₃ have been investigated. Pr_0.8Sr_0.2?x□_xMnO₃ (0?≤?x?≤?0.2) powder samples have been elaborated by the conventional ceramic method. All the samples crystallized in the orthorhombic perovskite system with Pnma (for x?=?0 and 0.05) and Pbnm (for x?=?0.1 and 0.2) space groups. The impedance spectroscopy measurements, the AC conductivity was investigated in the frequency range 40 to 5?×?10⁶ Hz and in the temperature range 80–500 K. DC measurements show that the studied compounds exhibit a semiconductor behavior in the temperature range studied. We have demonstrated that the conduction mechanism is governed by the hopping process at high temperatures as observed for doped perovskite materials. The AC conductivity results were well described by the Jonscher power law. The evolution of the frequency exponent s was determined in order to investigate the conduction mechanism in deficient structures. A discrepancy between the results obtained with the parent sample and deficient structures was observed. Analysis of the complex impedance shows that our structures obey Cole–Cole model. Nyquist plots were obtained using the Maxwell–Wagner equivalent circuit model. The evolution of the real and imaginary parts ε′ and ε″ of the permittivity was studied as a function of frequency, temperature, and strontium deficiency. Both ε′ and ε″ exhibit a strong dependence on these parameters. A step-like decrease in the dielectric constant ε′ was observed which may have originated from the space charge polarization and blocking electrode effect. The dielectric loss ε″ decreased with increasing frequency but increased with temperature for all studied samples. This result was correlated with the conduction mechanism which is governed by grain boundaries at low frequencies and grains at high frequencies. In addition, it affected the capacitance values in each frequency interval.

     

Synthesis,structure and electrical studies of praseodymium doped barium zirconium titanate

Praseodymium (Pr) doped barium zirconium titanate with nominal composition (Ba_1−xPr_x)(Zr_0.52Ti_0.48)O₃ (x = 0.1 and 0.2) were synthesized using solid state reaction method. X-ray analysis conform the formation of cubic phase Pr-doped barium zirconium titanate along with minor pyrochloric phase. The increase in grain size after primary investigation reveals the influence of Pr ions on the domain structure and its microstructure. In order to correlate the effect of the chemical composition with the conduction mechanism, different AC electrical parameters have been addressed. The frequency dependant tangent loss of the sample was less for both the ceramics. The temperature dependence results show that the dielectric parameters and resistivity increases as Pr-content in the ceramic increases; this is attributed to the grain size and dipole dynamics. Complex impedance (Z*) plots show frequency dependent behavior as the response for the grain resistance mechanisms. This mechanism has been represented by an equivalent circuit. The temperature dependence of the electrical conductivity and Seebeck coefficient showed n-type non-degenerated semiconductor in the measured temperature range. The temperature dependent conductivity measurement suggests a novel negative temperature coefficient of resistance behavior of the samples. Furthermore, the frequency dependent conductivity plot shows increasing behavior.     

High-pressure Bi(Mg1 ? xZnx)1/2Ti1/2O3 perovskite solid solutions

We report the preparation of Bi(Mg_{1 − x} Zn _x )_1/2Ti_1/2O₃ ceramics at a pressure of 6 GPa and temperatures from 1370 to 1570 K. In the composition ranges 0 < x < 0.5 and 0.8 < x < 1, the ceramics consist of perovskite solid solutions. The solid solutions with x < 0.5 have an orthorhombic structure (sp. gr. Pnnm), and those with x > 0.7 have a tetragonal structure (sp. gr. P4mn). On heating at atmospheric pressure, the solid solutions with 0.1 < x < 0.5 undergo an irreversible orthorhombic-to-rhombohedral (sp. gr. R3c) phase transition, accompanied by heat absorption and an increase in specific volume. The transition temperature decreases with increasing x. The reduced perovskite cell parameter of the solid solutions increases linearly with x. The Curie temperature of the rhombohedral solid solutions exceeds the decomposition temperature of the perovskite phase.     

Design for Highly Piezoelectric and Visible/Near‐Infrared Photoresponsive Perovskite Oxides

Defect‐engineered perovskite oxides that exhibit ferroelectric and photovoltaic properties are promising multifunctional materials. Though introducing gap states by transition metal doping on the perovskite B‐site can obtain low bandgap (i.e., 1.1–3.8 eV), the electrically leaky perovskite oxides generally lose piezoelectricity mainly due to oxygen vacancies. Therefore, the development of highly piezoelectric ferroelectric semiconductor remains challenging. Here, inspired by point‐defect‐mediated large piezoelectricity in ferroelectrics especially at the morphotropic phase boundary (MPB) region, an efficient strategy is proposed by judiciously introducing the gap states at the MPB where defect‐induced local polar heterogeneities are thermodynamically coupled with the host polarization to simultaneously achieve high piezoelectricity and low bandgap. A concrete example, Ni²⁺‐mediated (1–x)Na_0.5Bi_0.5TiO₃‐xBa(Ti_0.5Ni_0.5)O_3–δ (x = 0.02–0.08) composition is presented, which can show excellent piezoelectricity and unprecedented visible/near‐infrared light absorption with a lowest ever bandgap ≈0.9 eV at room temperature. In particular, the MPB composition x = 0.05 shows the best ferroelectricity/piezoelectricity (d₃₃ = 151 pC N^–1, Pr = 31.2 μC cm^–2) and a largely enhanced photocurrent density approximately two orders of magnitude higher compared with classic ferroelectric (Pb,La)(Zr,Ti)O₃. This research provides a new paradigm for designing highly piezoelectric and visible/near‐infrared photoresponsive perovskite oxides for solar energy conversion, near‐infrared detection, and other multifunctional applications.     

Thermal Transport Properties of Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Mixed Crystals Measured by Means of the Photopyroelectric Method

The concentration dependence of the thermal conductivity and thermal diffusivity were determined for Cd_1-x Mg _x Se mixed crystals in the temperature range between 20 ^◦C and 40 ^◦C. To determine the thermal transport properties, the photopyroelectric setup in the back detection configuration was constructed. In the concentration range 0< x <0.36, both thermal conductivity and thermal diffusivity were found to decrease with increasing magnesium concentration as well as with increasing temperature. The observed concentration dependence is discussed in the framework of the Adachi model.     

Thermal Conductivity of Pb1 – x Mn x Te Single Crystals

The thermal conductivity of Pb_{1 – x} Mn _x Te single crystals is measured in the range 80–300 K. The electronic and lattice components of thermal conductivity in the crystals are evaluated for a parabolic conduction band, arbitrary degeneracy, and elastic scattering of charge carriers. The total thermal conductivity and lattice component are found to decrease with increasing temperature. Increasing the Mn content of the solid solutions also reduces the total and lattice thermal conductivities, while annealing increases them. The density of point defects in the crystals is evaluated. The heat conduction in Pb_{1 – x} Mn _x Te is shown to be dominated by phonons.     

DC conduction behaviour of niobium doped barium stannate

A few compositions with x 0.100 in the system BaSn_1–xNb_xO₃ have been synthesized by solid-state ceramic method. All these are found to be single phase having cubic structure similar to BaSnO₃. Electrical resistivity decreases with increasing niobium concentration up to x = 0.010 and thereafter it increases rapidly for the compositions with x = 0.050 and 0.100. This has been explained in terms of the change in charge compensation behaviour beyond x = 0.010. Measurement of Seebeck coefficient as a function of temperature shows that electrons are the majority charge carriers in compositions with x 0.010 while compositions with x > 0.010, exhibit p-type conductivity showing that holes are the majority charge carriers.     

Fluctuation Conductivity of RE1−2x Ca x M x Ba2Cu3O7−δ (RE=Nd,Y and M=Pr,Th) Superconductors

Polycrystalline samples of RE_1?2x Ca _x M _x Ba₂Cu₃O_7?δ with RE=Nd, Y and M=Pr, Th (with 0.0≤x≤0.10), superconductors were prepared by the standard solid-state method. Resistivity was measured as a function of temperature and doping concentration x. Excess conductivity was analyzed using the modified Lawrence-Doniach (LD) expressions. The fluctuation regions, crossover temperatures, coherence lengths, and effective layer thickness were obtained and the values were compared for both samples. For both samples, it was found that with increasing doping, the crossover temperatures were reduced, while the coherence length decreased. The upper critical field and critical current density were increased with increasing doping concentration due to the introduction of disorder and the enhancement of flux pinning by charge neutral doping. Furthermore, the coherence lengths of the Nd-based samples are larger than that for the Y-based samples by a factor 2. It was found that the value of critical current density in Nd(CaPr)-123 is higher than Y(CaTh)-123, from which it is suggested that CaPr doping is more effective than CaTh doping.     

Dielectric studies of tin based chalcogenide glasses

The dc conductivity and dielectric parameters of glassy system of a-Se₇₀Te_30−x Sn _x (x = 0, 2, 4, 6, 8 and 10) glasses have been investigated. Frequency and temperature dependence of dielectric constants (ε′) and dielectric loss (ε′′) are studied in the frequency range 120–100 kHz and temperature range 300–390 K. Dielectric dispersion is observed when Tin (Sn) is incorporated to a-Se–Te system in the entire temperature range. These results explain that the dc conduction loss is dominated in the present system. From dc conductivity studies it is observed that the dc conductivity and activation energy increases with increasing tin concentration in the present system.     

Efficient and Stable Inverted Perovskite Solar Cells Incorporating Secondary Amines

Large‐bandgap perovskites offer a route to improve the efficiency of energy capture in photovoltaics when employed in the front cell of perovskite–silicon tandems. Implementing perovskites as the front cell requires an inverted (p–i–n) architecture; this architecture is particularly effective at harnessing high‐energy photons and is compatible with ionic‐dopant‐free transport layers. Here, a power conversion efficiency of 21.6% is reported, the highest among inverted perovskite solar cells (PSCs). Only by introducing a secondary amine into the perovskite structure to form MA_1?xDMA_xPbI₃ (MA is methylamine and DMA is dimethylamine) are defect density and carrier recombination suppressed to enable record performance. It is also found that the controlled inclusion of DMA increases the hydrophobicity and stability of films in ambient operating conditions: encapsulated devices maintain over 80% of their efficiency following 800 h of operation at the maximum power point, 30 times longer than reported in the best prior inverted PSCs. The unencapsulated devices show record operational stability in ambient air among PSCs.