Effects of Oxide Contact Layer on the Preparation and Properties of CH₃NH₃PbI₃ for Perovskite Solar Cell Application

In perovskite solar cells, oxide electron transport layers (ETL) and their interface with the organometal trihalides are key to achieve efficient and stable devices. In the present work we investigate ZnO and TiO₂ ETLs and their influence on the preparation of CH₃NH₃PbI₃ film by two different techniques. In the “one-step” technique, a solution is used for the deposition of a precursor layer which is dripped and subsequently annealed. In the “two-step” sequential technique, a PbI₂ precursor layer is converted into perovskite. We show that, on ZnO, the annealing treatment of the “one-step” deposited layer is optimum for a duration time of only 2 min. This duration is much less critical for the TiO₂ underlayer. Long annealing times produce the degradation of the pigment and formation of PbI₂. It is also shown that the “one-step” technique gives better results for the sensitization of smooth oxide underlayers whereas the “two-step” one must be utilized for rough or structured underlayer sensitization. The best solar cell performances were achieved by combining a low-overvoltage electrodeposited ZnO layer, a planar architecture, and a perovskite layer prepared by a “one-step” deposition-dripping route. A maximum overall conversion efficiency of 15% was measured for the ZnO-based perovskite solar cell. Cell impedance spectra have been measured over a large applied voltage range. Their analysis, using an ad-hoc equivalent circuit, shows that charge recombinations are reduced for the “one-step” perovskite and that a better interface with the oxide is produced in that case

Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells

The partial replacement of methylammonium by formamidinium and cesium in organolead trihalide materials is of great importance to improve the performance and stability of photovoltaic solar cells. However, the effect of multiple cations on the cell functioning and their electrical characteristics remains to be clarified. By using the impedance spectroscopy technique, we have investigated the electrical response to a small ac perturbation applied to solar cells implementing hybrid perovskites with various compositions, polarized over a large potential range. The solar cell preparation protocols have been optimized to reach power conversion efficiencies higher than 17%. The impedance response has been investigated both under light and in the dark to discriminate the light sensitive parameters. The spectra have been carefully analyzed using an <i>ad hoc</i> equivalent circuit, and the data have been discussed in the light of the existing literature. The spectra showed no intermediate frequency inductive loop due to the absence of multistep charge transfer involving surface states. A large inductive loop is found to be the signature of poorly functioning solar cells. Except for the high frequency capacitance, which is the bulk response of perovskite, the other parameters are influenced by interface and contact phenomena, ionic conductivity and charge accumulations. The scaling of the low frequency capacitance with the hysteresis amplitude is clearly stated by our comprehensive study. Moreover, no diffusion impedance due to the diffusion of ionic species is observed. However, ion mobility results in a strong effect on recombinations and has a strong influence on the low frequency impedance response of the system

Theoretical Procedure for Optimizing Dye-Sensitized Solar Cells: From Electronic Structure to Photovoltaic Efficiency

International audienc

Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response

The implementation of efficient hole blocking layers (BLs) is of vital importance to achieve high efficiency in solar cells using organolead trihalide materials as the solar light absorber. BLs permit electronic charge separation and avoid the recombination of charges by blocking the hole transfer to the anode. In this study, BLs have been prepared by aerosol spray pyrolysis and spin-coating using various solution compositions. The morphological, optical, microstructural and crystalline properties as well as the phase composition of these layers have been characterized by scanning electron microscopy, Raman spectroscopy, and optical measurements. Their blocking ability has been evaluated by cyclic voltammetry. A strong relationship has been found between these properties and the solar cell <i>J</i>–<i>V</i> characteristics and performances. Overall, we figured out that the sprayed BLs were thin, highly compact, covering, and conformal. No cracks or pinholes were found because the precursor underwent degradation and condensation directly at high temperature. They were made of the pure anatase phase and were perfectly blocking. They produced high-efficiency perovskite solar cells. The BL has also an influence on the impedance spectroscopy response of the cells over the whole frequency range from several hundreds of Hz to some tens of mHz. We have found that the inductive loop if present for all the cells was more pronounced and observed over a larger potential range in the case of the most poorly working devices with inaccurate BLs. An equivalent electrical circuit is proposed to fit the full spectra. The various electrical parameters of the circuit have been compared for the various BLs and thoroughly analyzed. Influences of the BL on the internal resistance, recombinations, interfacial traps, and the inductive response are detailed

Impact of Organic Hole Transporting Material and Doping on the Electrical Response of Perovskite Solar Cells

The hole transport material (HTM) layer is a key component of the perovskite solar cells (PSCs) that must be optimized to reach high efficiency. The development of new HTMs alternative to Spiro-OMeTAD and the understanding of the role of doping agents on these layers are important research axes in the field. It requires the use of appropriate characterization tools enabling us to discriminate the bulk and interface effects. In the present paper, we fully analyze the effect of HTM doping and of the material on the impedance response of PSCs. The approach has been implemented on two different molecular HTMs, Spiro-OMeTAD and a new molecular carbazole HTM, called B186, and with various doping levels. We show that limitations by poor doping are characterized by an extra high frequency impedance loop for which capacitance and resistance analysis gives the dielectric constant and conductivity of the material, respectively. However, the low-frequency part of the spectra provides important information on the charge accumulation/outflow and on the recombination levels. More generally, the presented approach is of high practical interest for the development of new organic HTMs and for the optimization of the layer doping

Ab Initio Optimization of Dye-Sensitized Solar Cells: From Individual Components to Interfaces.

International audienc

Enhanced Stability and Bangap Tuning of α-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration

Computational investigations were conducted thanks to HPC resources provided by [TGCC/CINES/IDRIS] under the allocation 2018-A0010907682 made by GENCIInternational audienceWe report room-temperature synthesis of lead- and iodide-deficient α-[HC(NH)]PbI perovskites (abbreviated d-α-FAPI, FA = formamidinium), with the general formula (A',FA)[PbI] (with A' = hydroxyethylammonium (HEA) or thioethylammonium (TEA) cations, 0.04 ≤ x ≤ 0.15). These materials retain a 3D character of their perovskite network despite incorporation of large HEA or TEA cations, demonstrating that the Goldschmidt tolerance factor can be bypassed. We found that thin films of (TEA,FA)[PbI] ( x = 0.04 and 0.13) show exceptional α-phase stability under ambient conditions, 1 order of magnitude higher than α-FAPI and α-(Cs,FA)PbI thin films. d-α-FAPI phases are shown to maintain a direct band gap, which increases monotonously for x ranging from 0 up to 0.20, with characteristics of a p-type semiconductor for low concentrations of vacancies ( x ≤ 0.13) and n-type for larger ones. They offer alternatives to reach the methylammonium- and bromine-free stable α-FAPI-type phase and open new avenues in the field of perovskite solar cells, up to band gap tuning desirable for tandem solar cells

Modeling Dye-Sensitized Solar Cells: From Theory to Experiment

Density functional theory (DFT) and time-dependent DFT are useful computational approaches frequently used in the dye-sensitized solar cell (DSSC) community in order to analyze experimental results and to clarify the elementary processes involved in the working principles of these devices. Indeed, despite these significant contributions, these methods can provide insights that go well beyond a purely descriptive aim, especially when suitable computational approaches and methodologies for interpreting and validating the computational outcomes are developed. In the present contribution, the possibility of using recently developed computational approaches to design and interpret the macroscopic behavior of DSSCs is exemplified by the study of the performances of three new TiO₂-based DSSCs making use of organic dyes, all belonging to the expanded pyridinium family

Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode

Doping ZnO nanowires (NWs) by group IB elements is an important challenge for integrating nanostructures into functional devices with better and tuned performances. The growth of Ag-doped ZnO NWs by electrodeposition at 90 °C using a chloride bath and molecular oxygen precursor is reported. Ag acts as an electrocatalyst for the deposition and influences the nucleation and growth of the structures. The silver atomic concentration in the wires is controlled by the additive concentration in the deposition bath and a content up to 3.7 atomic % is reported. XRD analysis shows that the integration of silver enlarges the lattice parameters of ZnO. The optical measurements also show that the direct optical bandgap of ZnO is reduced by silver doping. The bandgap shift and lattice expansion are explained by first principle calculations using the density functional theory (DFT) on the silver impurity integration as an interstitial (Ag_i) and as a substitute of zinc atom (Ag_Zn) in the crystal lattice. They notably indicate that Ag_Zn doping forms an impurity band because of Ag 4d and O 2p orbital interactions, shifting the Fermi level toward the valence band. At least, Ag-doped ZnO vertically aligned nanowire arrays have been epitaxially grown on GaN(001) substrate. The heterostructure has been inserted in a light emitting device. UV-blue light emission has been achieved with a low emission threshold of 5 V and a tunable red-shifted emission spectrum related to the bandgap reduction induced by silver doping of the ZnO emitter material