Formation Mechanism of Freestanding CH3NH3PbI3 Functional Crystals: In Situ Transformation vs Dissolution–Crystallization

317

205

hole transport materials (HTMs), and metal contact electrode. Discouragingly, the conventional organic HTMs (e.g., spiro-OMeTAD) are usually expensive and unstable. The noble metal electrode is also costly and requires vacuum thermal evaporation device. These problems will hinder the commercialization of PSCs in the future. Fortunately, it was found that HTM-free PSCs could also operate efficiently since the unique ambipolar property of the perovskite allows it to serve not only as a light harvester but also as a hole conductor. [12][13][14][15][16][17] In particular, carbon-based HTM-free PSCs show much promise to solve the above-mentioned problems because carbon materials are earth abundant, low-cost, and environmentally stable. [ 11,[18][19][20][21][22][23][24][25][26] To date, the most effi cient (PCE ≈ 13%) carbon-based PSCs were reported by Han and colleagues. [ 18,19,21,27 ] In these devices, a TiO 2 scaffold layer, a porous ZrO 2 insulating layer, and a porous carbon electrode were sequentially deposited, followed by infi ltration of a perovskite precursor solution. In our previous works, we have simplifi ed the architecture and the fabrication process of the carbon-based PSCs by eliminating the ZrO 2 insulating layer and depositing the perovskite layer by the two-step method, [ 20,[28][29][30] demonstrating the PCE at ≈11%. [ 20,29,30 ] Very recently, a paintable carbon-based PSC was reported by several groups, [31][32][33][34][35] which was fabricated by fi rst depositing the perovskite layer and then printing the carbon electrode with a pre-prepared carbon paste, as illustrated in Figure 1 and Figure S1 (Supporting Information). The whole process is simple, at low temperature and low cost, apparently compatible with roll-to-roll production. However, the PCE of the paintable carbon-based PSCs is relatively low, viz., ≤10%. [31][32][33][34][35][36] Plausibly, the most important limiting factor for achieving high-effi ciency of the paintable carbon-based PSCs lies in the poor contact at perovskite/carbon interface because of the postdeposition process of carbon electrode, which seriously decreases fi ll factor (FF). [31][32][33][34] In order to tackle such a contact problem, it is necessary to produce an even perovskite layer to afford intimate contact at the perovskite/carbon interface. Onestep method has been successfully applied to prepare an even perovskite layer on planer PSCs. [ 8,[37][38][39] However, this method is unsuitable for depositing a good pore-fi lling perovskite layer in a relatively thick mesoscopic TiO 2 scaffold required for the Carbon-based hole transport material (HTM)-free perovskite solar cells (PSCs) have shown much promise for practical applications because of their high stability and low cost. However, the effi ciencies of this kind of PSCs are still relatively low, especially for the simplest paintable carbon-based PSCs, in comparison with the organic HTM-based PSCs. This can be imputed to the perovskite deposition methods that are not very suitable for this kind of devices. A...

“…It is commonly believed that the following reactions tend to occur when dipping PbI 2 in an MAI solution [ 42,50,51 ] PbI s CH NH sol I sol CH NH PbI s …”

Section: Resultsmentioning

confidence: 99%

Solvent Engineering Boosts the Efficiency of Paintable Carbon‐Based Perovskite Solar Cells to Beyond 14%

Chen

Wei

et al. 2016

317

205

“…These two processes compete during crystallization, while in situ conversion is favored in environments with a low MAI concentration. [54] Shortly after, this concept was transferred to thin films as well. [13,37,55] Schlipf et al used GISAXS to gain quantitative information on crystal-size distribution in perovskite thin films fabricated by a sequential deposition method.…”

Section: Two-step (Sequential Deposition)mentioning

confidence: 99%

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

Petrus

Schlipf

Cheng

et al. 2017

303

198

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...

“…[46,47] While the first process is constrained mainly to the bulk of the film where concentration of MAI is low due to being limited by diffusion and the structure from PbI 2 is mostly preserved, the latter process occurs closer to the film surface where the high iodide concentration leads to partial dissolution of the PbI 2 layer and subsequently forms crystals in (002) orientation. This process is reminiscent of what happens in the spin-coating/interdiffusion method where the MAI concentration is higher.…”

Section: Research Newsmentioning

confidence: 99%

Structure of Organometal Halide Perovskite Films as Determined with Grazing‐Incidence X‐Ray Scattering Methods

Schlipf

Müller‐Buschbaum

2017

120

114

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.