Hot carrier solar cells and the potential of perovskites for breaking the Shockley-Queisser limit

In this review article, we discuss the working mechanism of hot carrier solar cells (HCSCs), their prerequisites from a material point of view and consider power conversion efficiencies that could reasonably be achieved with these devices. We review phonon decay pathways and proposed design rules for hot carrier absorbers established for classical bulk materials, as well as engineering efforts based on nanostructuring. Our main focus, however, lies on the recently emerged class of metal halide perovskites that not only exhibits tremendous potential in standard solar cells, but also offers highly promising hot carrier lifetimes. We discuss possible origins for this encouraging observation and point out pathways for future research towards HCSCs that break the Shockley-Queisser limit.</p

Cooling, Scattering, and Recombination-The Role of the Material Quality for the Physics of Tin Halide Perovskites

Tin-based perovskites have long remained a side topic in current perovskite optoelectronic research. With the recent efficiency improvement in thin film solar cells and the observation of a long hot carrier cooling time in formamidinium tin iodide (FASnI(3)), a thorough understanding of the material's photophysics becomes a pressing matter. Since pronounced background doping can easily obscure the actual material properties, it is of paramount importance to understand how different processing conditions affect the observed behavior. Using photoluminescence spectroscopy, thin films of FASnI(3) fabricated through different protocols are therefore investigated. It is shown that hot carrier relaxation occurs much faster in highly p-doped films due to carrier-carrier scattering. From high quality thin films, the longitudinal optical phonon energy and the electron-phonon coupling constant are extracted, which are fundamental to understanding carrier cooling. Importantly, high quality films allow for the observation of a previously unreported state of microsecond lifetime at lower energy in FASnI(3), that has important consequences for the discussion of long lived emission in the field of metal halide perovskites

Photophysics of pentacene-doped picene thin films

Here were report a study of picene nano-cristalline thin films doped with pentacene molecules. The thin films were grown by supersonic molecular beam deposition with a doping concentration that ranges between less than one molecules of pentacene every 104 picene molecules up to about one molecule of pentacene every 102 of picene. Morphology and opto-electronic properties of the films were studied as a function of the concentration of dopants. The optical response of the picene films, characterized by absorption, steady-state and time-resolved photoluminescence measurements, changes dramatically after the doping with pentacene. An efficient energy transfer from the picene host matrix to the pentacene guest molecules was observed giving rise to an intense photoluminescence coming out from pentacene. This efficient mechanism opens the possibility to exploit applications where the excitonic states of the guest component, pentacene, are of major interest such as MASER. The observed mechanism could also serve as prototypical system for the study of the photophysics of host guest systems based on different phenacenes and acenes.Comment: 15 pages, 6 figure

Addition of Ammonium Thiocyanate Alters the Microstructure and Energetic Landscape of 2D/3D Perovskite Films

Mixtures of low-dimensional and 3D perovskite phases have attracted significant attention due to their improved stability with respect to purely 3D perovskites. One of the strategies to gain control over the complex crystallization of these 2D/3D perovskite films and obtain well-ordered thin films is through the additive engineering of the precursor solution. In this work, the influence of ammonium thiocyanate addition on the microstructural and optical properties of thin films of (PEA)2(MA)n−1PbnI3n+1 is investigated for different n values.</p

Extrinsic nature of the broad photoluminescence in lead iodide-based Ruddlesden-Popper perovskites

Two-dimensional metal halide perovskites of Ruddlesden–Popper type have recently moved into the centre of attention of perovskite research due to their potential for light generation and for stabilisation of their 3D counterparts. It has become widespread in the field to attribute broad luminescence with a large Stokes shift to self-trapped excitons, forming due to strong carrier–phonon interactions in these compounds. Contrarily, by investigating the behaviour of two types of lead-iodide based single crystals, we here highlight the extrinsic origin of their broad band emission. As shown by below-gap excitation, in-gap states in the crystal bulk are responsible for the broad emission. With this insight, we further the understanding of the emission properties of low-dimensional perovskites and question the generality of the attribution of broad band emission in metal halide perovskite and related compounds to self-trapped excitons

Impact of two diammonium cations on the structure and photophysics of layered Sn-based perovskites

Layered metal-halide perovskites have shown great promise for applications in optoelectronic devices, where a large number of suitable organic cations give the opportunity to tune their structural and optical properties. However, especially for Sn-based perovskites, a detailed understanding of the impact of the cation on the crystalline structure is still missing. By employing two cations, 2,2′-oxybis(ethylammonium) (OBE) and 2,2′-(ethylenedioxy)bis(ethylammonium) (EDBE), we obtain a planar 〈100〉 and a corrugated 〈110〉-oriented perovskite, respectively, where the hydrogen bonding between the EDBE cations stabilises the corrugated structure. OBESnI4 exhibits a relatively narrow band gap and photoluminescence bands compared to EDBESnI4. In-depth analysis shows that the markedly different optical properties of the two compounds have an extrinsic origin. Interestingly, thin films of OBESnI4 can be obtained both in black and red colours. This effect is attributed to a second crystalline phase that can be obtained by processing the thin films at 100 °C. Our work highlights that the design of the crystal structure as obtained by ligand chemistry can be used to obtain the desired optical properties, whereas thin film engineering can result in multiple crystalline phases unique to Sn-based perovskites.</p

Synthesis, Optical and Electrochemical Properties of High-Quality Cross-Conjugated Aromatic Polyketones

This paper describes the synthesis and characterization of three new aromatic polyketones with repeating units based on 2,2′-(2,5-dihexyl-1,4-phenylene) dithiophene (PTK), 2,2′-(9,9-dihexyl-9H-fluorene-2,7-diyl)dithiophene (PFTK), and 4,7-bis(3-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (PBTK). These polymers were obtained with a one-pot Suzuki-Miyaura cross-coupling-promoted homopolymerization to afford high-quality, defect-free polymers. Experimental and theoretical studies were applied to investigate their optical and electrical properties. The cross-conjugated nature of aromatic polyketones imparts excellent thermal stability. Exposure to acid converts the cross-conjugation to linear-conjugation, enabling the dynamic tuning of optoelectronic properties

Influence of morphology on photoluminescence properties of methylammonium lead tribromide films

The morphology of hybrid perovskite thin films plays a crucial role for their photophysical properties. However, the underlying mechanisms are still unclear. To gain further insight into this phenomenon, methylammonium lead tribromide films of different morphology were investigated using photoluminescence spectroscopy. Photostability measurements demonstrate three mechanisms: (A) reversible degradation of the photoluminescence, depending positively on the grain-boundary density, which is presumably caused by photo-induced bromide vacancies, (B) enhancement of the photoluminescence intensity in the presence of oxygen and moisture and (C) destruction of the perovskite after several minutes of ultraviolet illumination with excitation power above 100 W/cm(2). Both the intensity and the lifetime of the photoluminescence were significantly smaller in films with micrometer-sized crystallites compared to granular films. This is ascribed to crystals being partially isolated in the former, causing smaller diffusion lengths, whereas the carriers in the granular films can diffuse from grain to grain resulting in higher photoluminescence lifetime and intensity

Elucidating the Structure and Photophysics of Layered Perovskites through Cation Fluorination

Optoelectronic devices based on layered perovskites containing fluorinated cations display a well-documented improved stability and enhanced performance over non-fluorinated cations. The effect of fluorination on the crystal structure and photophysics, however, has received limited attention up until now. Here, 3-fluorophenethylammonium lead iodide ((3-FPEA)(2)PbI4) single crystals are investigated and their properties to the non-fluorinated ((PEA)(2)PbI4) variant are compared. The bulkier 3-FPEA cation increases the distortion of the inorganic layers, resulting in a blue-shifted absorbance and photoluminescence. Temperature-dependent photoluminescence spectroscopy reveals an intricate exciton substructure in both cases. The fluorinated variant shows hot-exciton resonances separated by 12 to 15 meV, values that are much smaller than the 40 to 46 meV found for (PEA)(2)PbI4. In addition, high-resolution spectra show that the emission at lower energies consists of a substructure, previously thought to be a single line. With the analysis on the resolved photoluminescence, a vibronic progression is excluded as the origin of the emission at lower energies. Instead, part of the excitonic substructure is proposed to originate from bound excitons. This work furthers the understanding of the photophysics of layered perovskites that has been heavily debated lately