Recently developed organic-inorganic hybrid perovskite solar cells combine low-cost fabrication and high power conversion efficiency. Advances in perovskite film optimization have led to an outstanding power conversion efficiency of more than 20%. Looking forward, shifting the focus toward new device architectures holds great potential to induce the next leap in device performance. Here, we demonstrate a perovskite/perovskite heterojunction solar cell. We developed a facile solution-based cation infiltration process to deposit layered perovskite (LPK) structures onto methylammonium lead iodide (MAPI) films. Grazing-incidence wide-angle X-ray scattering experiments were performed to gain insights into the crystallite orientation and the formation process of the perovskite bilayer. Our results show that the self-assembly of the LPK layer on top of an intact MAPI layer is accompanied by a reorganization of the perovskite interface. This leads to an enhancement of the open-circuit voltage and power conversion efficiency due to reduced recombination losses, as well as improved moisture stability in the resulting photovoltaic devices.

2D perovskites is one of the proposed strategies to enhance the moisture resistance, since the larger organic cations can act as a natural barrier. Nevertheless, 2D perovskites hinder the charge transport in certain directions, reducing the solar cell power conversion efficiency. A nanostructured mixed-dimensionality approach is presented to overcome the charge transport limitation, obtaining power conversion efficiencies over 9%.

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CHNHPbI are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10 cms is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

following statement: These materials can be processed from solution and yet exhibit optoelectronic materials properties described in classic solid-state physics textbooks. This unprecedented combination has led to photovoltaic devices that can be processed at room temperature and achieve performance levels similar to industry giant polycrystalline silicon, from a starting point of 3.8% in 2009. [1][2][3][4] The first light-emitting electrochemical cells [5] and diodes [6][7][8] have also been introduced recently, leading to tunable light emission spectra and internal quantum efficiencies exceeding 15%.The road towards these achievements has been marked by a constant improvement of perovskite deposition techniques fueled by our increasing understanding of the crystallization processes. The choice of deposition technique, either from solution or vapor, and the composition and order in which precursor species are applied has a great influence on the crystallization kinetics. Here, one can distinguish one-step methods where the perovskite is formed from a precursor mixture dissolved in a common solution, and two-step methods where the inorganic compound is deposited first and transformed to the perovskite phase later by addition of the organic constituents. [9][10][11] Furthermore, many parameters during processing can have an effect on the crystallization mechanism and consequently on film morphology, such as the choice of solvents, concentrations and processing additives that are often not incorporated into the final product. [11][12][13] We discuss this aspect in Section 2, where we focus our attention on the most commonly employed solution-based techniques. Additionally, as for any new technology trying to displace an incumbent technology, higher efficiencies, lower costs, reproducibility, stability, and sustainability are paramount. In particular, reproducibility has been a major challenge in perovskite solar cells from the beginning: small variations in morphology, like crystal size, film roughness, and pinholes can have detrimental effects on photovoltaic performance. [14] On the other hand, current-voltage measurements often showed a hysteresis that is rarely seen in other photovoltaic technologies. [15] These topics are highlighted in Sections 3 and 4. Finally, in Section 5 we discuss the framework for market introduction, such as cost reduction by choosing low-cost charge transport layers, or how the lifetime of perovskite absorbers can be extended by the choice of materials composition.Hybrid metal halide perovskites have become one of the hottest topics in optoelectronic materials research in recent years. Not only have they surpassed everyone's expectations and achieved similar performance as tried and true polycrystalline silicon photovoltaic devices, but they are also finding applications in a variety of different fields, including lighting. The main advantages of hybrid metal halide perovskites are simple processability, compatible with large-scale solution processing such as roll-to-roll printing, a...

Recently, hybrid perovskites have gathered much interest as alternative materials for the fabrication of highly efficient and cost-competitive solar cells; however, many questions regarding perovskite crystal formation and deposition methods remain. Here we have applied a two-step protocol where a crystalline PbI2 precursor film is converted to MAPbI3-xClx perovskite upon immersion in a mixed solution of methylammonium iodide and methylammonium chloride. We have investigated both films with grazing incidence small-angle X-ray scattering to probe the inner film morphology. Our results demonstrate a strong link between lateral crystal sizes in the films before and after conversion, which we attribute to laterally confined crystal growth. Additionally, we observe an accumulation of smaller grains within the bulk in contrast with the surface. Thus, our results help to elucidate the crystallization process of perovskite films deposited via a two-step technique that is crucial for controlled film formation, improved reproducibility, and high photovoltaic performance.

Grazing‐incidence X‐ray scattering (GIXS) methods have proven to be a valuable asset for investigating the morphology of thin films at different length scales. Consequently, GIXS has been applied to the fast‐progressing field of organometal halide perovskites. This exciting class of materials has propelled research in the areas of cheap and sustainable photovoltaics, light emitting devices, and optoelectronics in general. Especially, perovskite solar cells (PSC) have seen a remarkable rise in power conversion efficiencies, crossing the 20% mark after only five years of research. This research news outlines GIXS studies focusing on the most challenging research topics in the perovskite field today: Current–voltage hysteresis, device reproducibility, and long‐term stability of PSC are inherently linked to perovskite film morphology. On the other hand, film formation depends on the choice of precursors and processing parameters; understanding their interdependence opens possibilities to tailor film morphologies. Owing to their tunability and moisture resistance, 2D perovskites have recently attracted attention. Examples of GIXS studies with different measurement and data analysis techniques are presented, highlighting especially in‐situ investigations on the many kinetic processes involved. Thus, an overview on the toolbox of GIXS techniques is linked to the specific needs of research into organometal halide perovskite optoelectronics.

Wide band gap perovskites such as methylammonium lead bromide are interesting materials for photovoltaic applications because of their potentially high open-circuit voltage. However, the fabrication of high-quality planar films has not been investigated in detail for this material. We report a new synthesis approach for the fabrication of bromide-based perovskite planar films based on the control of the deposition environment. We achieve dense layers with large and perfectly oriented crystallites 5−10 μm in size. Our results show that large crystal sizes can be achieved only for smooth indiumdoped tin oxide substrates, whereas lateral perovskite crystal growth is limited for the rougher fluorine-doped tin oxide substrates. We additionally correlate photocurrent and perovskite crystal properties in photovoltaic devices and find that this parameter is maximized for ordered systems, with internal quantum efficiencies approaching unity. Hence, our work not only gives a new pathway to tune morphology and crystal orientation but also demonstrates its importance for planar perovskite solar cells.

Efficient perovskite solar cells can be produced by a wide variety of different methods. Previous results show that controlling the film morphology is critical to enhance the efficiency of the prepared devices. Here, grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) is used to study the morphology and especially the orientation distribution of CH3NH3PbI3 (MAPI) perovskite films prepared by five typical methods. We find that the investigated one‐step methods produce non‐oriented films, while the two‐step deposition methods produce varying degrees of orientation depending on the method chosen to convert the highly oriented PbI2 precursor, providing direct evidence for different perovskite conversion mechanisms. In particular, we show that the morphology and crystal orientation of MAPI films is tunable by varying the spin‐coating temperature and by adding chloride to the conversion solution. By relating the precursor morphology to the resulting MAPI film morphology, we link the observed preferential orientations to specific conversion mechanisms, thereby establishing GIWAXS as an important tool toward a rational development of new synthesis methods.