Impact of the chemical preparation of the electrical n-contact on the performance of perovskite solar cells

Motivation: Solar energy is an alternative, sustainable energy source for mankind. Finding a convenient way to convert sunlight energy into chemical energy is a key step towards realizing large-scale solar energy utilization like artificial photosynthesis. A previous stage would be the complete study of a material with a high absorption capacity of sunlight and in this context, the perovskite type solar cells are presented. Perovskite solar cells are one of the most promising photovoltaic low-cost technologies due to the fast increase in efficiency from 3% in 2009 to 22% in 2016. In this work it has been studied how the combination of the main dopants in the n-contact of the solar cell, impacts on the optoelectronic properties of the device.Methods: Perovskite solar cell reference devices: a titanium dioxide (TiO2) compact layer was deposited onto FTO-coated glass by spray pyrolysis and performing as electron transporter material. A mesoporous TiO2 layer was deposited by spin coating using a particle paste and then sintered. The perovskite was made using one-step deposition method. A solution of Spiro-OMeTad as Hole Transporter Material was prepared and spun-coated. Finally, an 80 nm layer of gold was thermally evaporated on the top of the cell as cathode under high vacuum. The mesoporous layer was doped with lithium and TiCl4 respectively to study the electronic properties of the n-contact. The characterization of all the devices is carried out under a solar simulator, fluorescence and absorption analysis, electrochemical impedance spectroscopy and intensity modulated photocurrent spectroscopy to know the charge extraction.Results: Under environmental conditions and without a controlled atmosphere, reference cells were built with a 13% efficiency, quite close to the state-of-the devices currently fabricated in top research groups. It has been observed that doping the compact and mesoporous layers respectively with TiCl4, the best configuration from the electronic point of view is with the TiCl4 is deposited on the mesoporous layer. In the test with Lithium, a deleterious effect on all the properties of the cell is observed. Currently, some tests are being completed where Lithium and TiCl4 are combined, as the best configuration according to literature.Conclusions: The chemical and physical treatment of the n-contact in perovskite solar cells is crucial to ensure the best performance of the resulting photovoltaic device

The cluster model: A hierarchically-ordered assemblage of random-packing spheres for modelling microstructure of porous materials

A new structural model based on the premises widely used for describing the structure of random materials, and especially aerogels, is introduced. Aerogels are described as an assemblage of randomly-packed spheres in several hierarchically-ordered levels. A new algorithm has been developed for constructing structural models from these premises using computer simulation. Subsequently, several techniques based both on the Monte Carlo technique and on geometrical considerations for characterizing real systems have been simulated, and textural parameters of the models have been obtained, including specific surface area, specific porous volume and the apparent density of the systems. This characterization process yields a set of parameters used for testing the capacity of the models to reproduce the structure of several real systems, like aerogels. Special attention has been paid to the pore size distribution calculations: the Monte Carlo integration and the triangulation algorithms have been compared.Comisión Interministerial de Ciencia y Tecnología MAT2005-158

Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells

Perovskite solar cells are known to show very long response time scales, on the order of milliseconds to seconds. This generates considerable doubt over the validity of the measured external quantum efficiency (EQE) and consequently the estimation of the short-circuit current density. We observe a variation as high as 10% in the values of the EQE of perovskite solar cells for different optical chopper frequencies between 10 and 500 Hz, indicating a need to establish well-defined protocols of EQE measurement. We also corroborate these values and obtain new insights regarding the working mechanisms of perovskite solar cells from intensity-modulated photocurrent spectroscopy measurements, identifying the evolution of the EQE over a range of frequencies, displaying a singular reduction at very low frequencies. This reduction in EQE is ascribed to additional resistive contributions hindering charge extraction in the perovskite solar cell at short-circuit conditions, which are delayed because of the concomitant large low-frequency capacitance

Efficient Modelling of Ion Structure and Dynamics in Inorganic Metal Halide Perovskites

Metal halide perovskites (MHPs) are nowadays one of the most studied semiconductors due to their exceptional performance as active layers in solar cells. Although MHPs are excellent solid-state semiconductors, they are also ionic compounds, where ion migration plays a decisive role in their formation, their photovoltaic performance and their long-term stability. Given the above-mentioned complexity, molecular dynamics simulations based on classical force fields are especially suited to study MHP properties, such as lattice dynamics and ion migration. In particular, the possibility to model mixed compositions is important since they are the most relevant to optimize the optical band gap and the stability. With this intention, we employ DFT calculations and a genetic algorithm to develop a fully transferable classical force field valid for the benchmark inorganic perovskite compositional set CsPb(Br_xI_(1-x))_3 (x = 0,1/3,2/3,1). The resulting force field reproduces correctly, with a common set of parameters valid for all compositions, the experimental lattice parameter as a function of bromide/iodide ratio, the ion-ion distances and the XRD spectra of the pure and mixed structures. The simulated thermal conductivities and ion migration activation energies of the pure compounds are also in good agreement with experimental trends. Our molecular dynamics simulations make it possible to predict the compositional dependence of the ionic diffusion coefficient on bromide/iodide ratio and vacancy concentration. For vacancy concentrations of around 9 10^21 cm^-3, we obtained ionic diffusion coefficients at ambient temperature of 10^-11 and 10^-13 cm2/s for CsPbBr3 and CsPbI3, respectively. Interestingly, in comparison with the pure compounds, we predict a significantly lower activation energy for vacancy migration and faster diffusion for the mixed perovskites

Vacuum template synthesis of multifunctional nanotubes with tailored nanostructured walls

A three-step vacuum procedure for the fabrication of vertical TiO2 and ZnO nanotubes with three dimensional walls is presented. The method combines physical vapor deposition of small-molecules, plasma enhanced chemical vapor deposition of inorganic functional thin films and layers and a postannealing process in vacuum in order to remove the organic template. As a result, an ample variety of inorganic nanotubes are made with tunable length, hole dimensions and shapes and tailored wall composition, microstructure, porosity and structure. The fabrication of multishell nanotubes combining different semiconducting oxides and metal nanoparticles is as well explored. This method provides a feasible and reproducible route for the fabrication of high density arrays of vertically alligned nanotubes on processable substrates. The emptying mechanism and microstructure of the nanotubes have been elucidated through SEM, STEM, HAADF-STEM tomography and energy dispersive X-ray spectroscopy. In this article, as a proof of concept, it is presented the straightforward integration of ZnO nanotubes as photoanode in a photovoltaic cell and as a photonic oxygen gas sensorPeer reviewe

Impedance analysis of perovskite solar cells: a case study

Metal halide perovskites are mixed electronic-ionic semiconductors with an extraordinary rich optoelectronic behavior and the capability to function very efficiently as active layers in solar cells, with a record efficiency surpassing 23% nowadays. In this work, we carry out an impedance spectroscopy analysis of two perovskite solar cells with quite distinct optical and electrical characteristics, i.e. MAPbI3 and CsPbBr3-based devices. The main aim of the analysis is to establish how, regardless the inherent complexity of the impedance spectrum due to ionic effects, information like ideality factors, recombination losses and the collection efficiency can be qualitative and quantitatively assessed from impedance experiments at operating conditions

Plasma assisted deposition of single and multistacked TiO2 hierarchical nanotubes photoanodes

We present herein an evolved methodology for the growth of nanocrystalline hierarchical nanotubes combining physical vapor deposition of organic nanowires (ONWs) and plasma enhanced chemical vacuum deposition of anatase TiO2 layers. The ONWs act as vacuum removable 1D and 3D templates, with the whole process occurring at temperatures ranging from RT to 250 °C. As a result, a high density of hierarchical nanotubes with tunable diameter, length and tailored wall microstructures are formed on a variety of processable substrates as metal and metal oxide films or nanoparticles including transparent conductive oxides. The reiteration of the process leads to the development of an unprecedented 3D nanoarchitecture formed by stacking the layers of hierarchical TiO2 nanotubes. As a proof of concept, we present the superior performance of the 3D nanoarchitecture as a photoanode within an excitonic solar cell with efficiencies as high as 4.69% for a nominal thickness of the anatase layer below 2.75 ¿m. Mechanical stability and straightforward implementation in devices are demonstrated at the same time. The process is extendable to other functional oxides fabricated by plasma-assisted methods with readily available applications in energy harvesting and storage, catalysis and nanosensingJunta de Andalucia(FQM 1851 and FQM-2310)España Mineco 201560E055 MAT2016-79866-R MAT2013-40852-R MAT2013-4MAT2013-47192-C3-3-R2900-

The Role of Surface Recombination on the Performance of Perovskite Solar Cells:Effect of Morphology and Crystalline Phase of TiO₂ Contact

Herein, the preparation of 1D TiO2 nanocolumnar films grown by plasma-enhanced chemical vapor deposition is reported as the electron selective layer (ESL) for perovskite solar devices. The impact of the ESL architecture (1D and 3D morphologies) and the nanocrystalline phase (anatase and amorphous) is analyzed. For anatase structures, similar power conversion efficiencies are achieved using an ESL either the 1D nanocolumns or the classical 3D nanoparticle film. However, lower power conversion efficiencies and different optoelectronic properties are found for perovskite devices based on amorphous 1D films. The use of amorphous TiO2 as electron selective contact produces a bump in the reverse scan of the current–voltage curve as well as an additional electronic signal, detected by impedance spectroscopy measurements. The dependence of this additional signal on the optical excitation wavelength used in the IS experiments suggests that it stems from an interfacial process. Calculations using a drift-diffusion model which explicitly considers the selective contacts reproduces qualitatively the main features observed experimentally. These results demonstrate that for a solar cell in which the contact is working properly the open-circuit photovoltage is mainly determined by bulk recombination, whereas the introduction of a “bad contact” shifts the balance to surface recombination.</p

One-reactor plasma assisted fabrication of ZnO@TiO2 multishell nanotubes: assessing the impact of a full coverage on the photovoltaic performance

This paper addresses the fabrication of vertically aligned ZnO@TiO2 multishell nanotubes by a combined full vacuum-plasma approach at mild temperatures. The growth is carried out within the premises of a one-reactor approach, i.e. minimizing the number of vacuum chambers and sample transferences. In this way, the interface between ZnO and TiO2 is fully preserved from humidity thus increasing ZnO durability and stability. These nanostructures are studied by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy in STEM (EDX-STEM). High density one-dimensional arrays of these nanotubes formed on FTO substrates are applied as photoanode in a dye-sensitized solar cell (DSC). The evolution of the dye adsorption capacity and solar cells parameters are explored as a function of the crystallinity and thickness of the TiO2 shell. The results show the critical effect of a full coverage by TiO2 of ZnO core to explain the mixed results found in the literature.Junta de Andalucia FQM 1851 FQM-2310España Mineco Agencia Estatal de Investigación MAT2016–79866-R MAT2013–42900-P MAT2013– 47192-C3-3-RMINECO-CSIC 201560E055Marie Skłodowska-Curie Actions H2020-MSCA-IF-2014PlasmaPerovSol grant 66148