Chen Weijie

In article number 1700264, Sven Hüttner, Pablo Docampo, and co-workers give an overview of hybrid metal halide perovskites for solar cell applications. The focus is on the series of challenges that have been overcome and those still remaining to be solved. In particular, the authors lay out their understanding of the perovskite crystallization process and how this knowledge can be harnessed to enable better performing devices; how to overcome reproducibility and hysteresis issues; and the long-term prospects of the technology in terms of stability and sustainability. Image by Criss Hohmann (Nanosystems Initiative Munich).

In hybrid photovoltaics an organic and an inorganic semiconductor are combined in the active layer to have the advantages of both material classes in a single device. In article number 1700248, Peter Müller-Buschbaum and co-workers review research related to hybrid solar cells which combine conjugated polymers with inorganic materials such as titanium dioxide, zinc oxide, silicon, germanium and quantum dots. Hybrid solar cells based on crystalline Si are discussed for comparison. Particular emphasis is put on different routes to tailor nanostructures of the organic or inorganic component. Cover Image by Christoph Hohmann, Nanosystems Initiative Munich (NIM).

Rapid improvement in photoconversion efficiency (PCE) of solution processable organometallic hybrid halide based perovskite solar cells (PSCs) have taken the photovoltaic (PV) community with a surprise and has extended their application in other electronic devices such as light emitting diodes, photo detectors and batteries. Together with efforts to push the PCE of PSCs to record values >22% – now at par with that of crystalline silicon solar cells – origin of their PV action and underlying physical processes are also deeply investigated worldwide in diverse device configurations. A typical PSC consists of a perovskite film sandwiched between an electron and a hole selective contact thereby creating ESC/perovskite and perovskite/HSC interfaces, respectively. The selective contacts and their interfaces determine properties of perovskite layer and also control the performance, origin of PV action, open circuit voltage, device stability, and hysteresis in PSCs. Herein, we define ideal charge selective contacts, and provide an overview on how the choice of interfacing materials impacts charge accumulation, transport, transfer/recombination, band-alignment, and electrical stability in PSCs. We then discuss device related considerations such as morphology of the selective contacts (planar or mesoporous), energetics and electrical properties (insulating and conducting), and its chemical properties (organic vs inorganic). Finally, the outlook highlights key challenges and future directions for a commercially viable perovskite based PV technology.

The past few years marked a new era of organometallic halide hybrid perovskite efficient solar cell technology. To capitalize the potential of this new class of materials in solar cells, in particular, and in any electronic devices in general, an understanding of interfacial physical processes is crucial. Herein, a comprehensive analysis of the role of interfaces in determining the PV performance and long term operational stability of this PV technology is provided.

Recently, due to the possibility of thinning down to the atomic thickness to achieve exotic properties, layered materials have attracted extensive research attention. In particular, PbI₂, a kind of layered material, and its perovskite derivatives, CH₃NH₃PbI₃ (i.e., MAPbI₃), have demonstrated impressive photoresponsivities for efficient photodetection. Herein, the synthesis of large-scale, high-density, and freestanding PbI₂ nanosheets is demonstrated by manipulating the microenvironment during physical vapor deposition. In contrast to conventional two-dimensional (2D) growth along the substrate surface, the essence here is the effective nucleation of microplanes with different angles relative to the in-plane direction of underlying rough-surfaced substrates. When configured into photodetectors, the fabricated device exhibits a photoresponsivity of 410 mA W⁻¹, a detectivity of 3.1 × 10¹¹ Jones, and a fast response with the rise and decay time constants of 86 and 150 ms, respectively, under a wavelength of 405 nm. These PbI₂ nanosheets can also be completely converted into MAPbI₃ materials via chemical vapor deposition with an improved photoresponsivity up to 40 A W⁻¹. All these performance parameters are comparable to those of state-of-the-art layered-material-based photodetectors, revealing the technological potency of these freestanding nanosheets for next-generation high-performance optoelectronics.

High-density, crystalline, and freestanding PbI₂ and MAPbI₃ nanosheets are synthesized on a large-scale through the nucleation of microplanes on rough-surfaced substrates by manipulating the microenvironment during physical vapor deposition. When configured into photodetectors, they exhibit efficient photodetection with excellent performance in responsivity, detectivity, etc.

Efficient solar–thermal energy conversion is essential for the harvesting and transformation of abundant solar energy, leading to the exploration and design of efficient solar–thermal materials. Carbon-based materials, especially graphene, have the advantages of broadband absorption and excellent photothermal properties, and hold promise for solar–thermal energy conversion. However, to date, graphene-based solar–thermal materials with superior omnidirectional light harvesting performances remain elusive. Herein, hierarchical graphene foam (h-G foam) with continuous porosity grown via plasma-enhanced chemical vapor deposition is reported, showing dramatic enhancement of broadband and omnidirectional absorption of sunlight, which thereby can enable a considerable elevation of temperature. Used as a heating material, the external solar–thermal energy conversion efficiency of the h-G foam impressively reaches up to ≈93.4%, and the solar–vapor conversion efficiency exceeds 90% for seawater desalination with high endurance.

A hierarchical graphene foam (h-G foam) with continuous porosity is designed and grown by plasma-enhanced chemical vapor deposition. This foam shows dramatic enhancement of broadband and omnidirectional absorption of sunlight, with an external solar–thermal energy conversion efficiency of ≈93.4%. The solar–vapor conversion efficiency exceeds 90% for seawater desalination.

Perovskite solar cells (PSCs) use perovskites with an APbX₃ structure, where A is a monovalent cation and X is a halide such as Cl, Br, and/or I. Currently, the cations for high-efficiency PSCs are Rb, Cs, methylammonium (MA), and/or formamidinium (FA). Molecules larger than FA, such as ethylammonium (EA), guanidinium (GA), and imidazolium (IA), are usually incompatible with photoactive “black”-phase perovskites. Here, novel molecular descriptors for larger molecular cations are introduced using a “globularity factor”, i.e., the discrepancy of the molecular shape and an ideal sphere. These cationic radii differ significantly from previous reports, showing that especially ethylammonium (EA) is only slightly larger than FA. This makes EA a suitable candidate for multication 3D perovskites that have potential for unexpected and beneficial properties (suppressing halide segregation, stability). This approach is tested experimentally showing that surprisingly large quantities of EA get incorporated, in contrast to most previous reports where only small quantities of larger molecular cations can be tolerated as “additives”. MA/EA perovskites are characterized experimentally with a band gap ranging from 1.59 to 2.78 eV, demonstrating some of the most blue-shifted PSCs reported to date. Furthermore, one of the compositions, MA_0.5EA_0.5PbBr₃, shows an open circuit voltage of 1.58 V, which is the highest to date with a conventional PSC architecture.

Tolerance factors based on novel molecular descriptors are introduced and subsequently implemented experimentally in multication methylammonium/ethylammonium (EA) perovskite solar cells. It is shown that surprisingly large quantities of EA can be incorporated into the perovskite structure, which results in one of the highest reported open-circuit voltages for perovskite solar cells.