The Role of the Interfaces in Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (HPSCs) have achieved an impressive power conversion efficiency (PCE) of 25.2% in 2019. At this stage, it is of paramount importance to understand in detail the working mechanism of these devices and which physical and chemical processes govern not only their power conversion efficiency but also their long-term stability. The interfaces between the perovskite film and the charge transport layers are among the most important factors in determining both the PCE and stability of HPSCs. Herein, an overview is provided on the recent advances in the fundamental understanding of how these interfaces influence the performance of HPSCs. Firstly, it is discussed how the surface energy of the charge transport layer, the energy level alignment at the interfaces, the charge transport in interfacial layers, defects and mobile ions in the perovskite film, and interfacial layers or at the interfaces affect the charge recombination as well as hysteresis and light soaking phenomenon. Then it is discussed how the interfaces and interfacial materials influence the stability of HPSCs. At the same time, an overview is also provided on the various design strategies for the interfaces and the interfacial materials. At the end, the outlook for the development of highly efficient and stable HPSCs is provided

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

Tin Halide Perovskites:From Fundamental Properties to Solar Cells

Metal halide perovskites have unique optical and electrical properties, which make them an excellent class of materials for a broad spectrum of optoelectronic applications. However, it is with photovoltaic devices that this class of materials has reached the apotheosis of popularity. High power conversion efficiencies are achieved with lead-based compounds, which are toxic to the environment. Tin-based perovskites are the most promising alternative because of their bandgap close to the optimal value for photovoltaic applications, the strong optical absorption, and good charge carrier mobilities. Nevertheless, the low defect tolerance, the fast crystallization, and the oxidative instability of tin halide perovskites currently limit their efficiency. The aim of this review is to give a detailed overview of the crystallographic, photophysical, and optoelectronic properties of tin-based perovskite compounds in their multiple forms from 3D to low-dimensional structures. At the end, recent progress in tin-based perovskite solar cells are reviewed, mainly focusing on the detail of the strategies adopted to improve the device performances. For each subtopic, the current challenges and the outlook are discussed, with the aim to stimulate the community to address the most important issues in a concerted manner

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

Grain-Specific Transitions Determine the Band Edge Luminescence in Dion–Jacobson Type 2D Perovskites

The photophysics of 2D perovskites incorporating 1,4-phenylenedimethanammonium (PDMA) as spacer cations is studied. PDMAPbI4 and PDMASnI4 exhibit absorption and luminescence spectra dominated by excitonic transitions and an emission due to two different states. Low-temperature studies reveal a time-dependent red shift of 12 meV that is correlated with grain-specific luminescence spectra observed in optical micrographs. For the Pb-variant, grains of red-shifted and lower intensity band edge emission simultaneously exhibit a more pronounced luminescence from a broad defect-related band around 2 eV. This suggests the grain-specific emission to be related to local defects. These observations have important consequences for the understanding of luminescence of 2D perovskites, for which peak splitting of the band edge emission is a common, yet not completely resolved observation.</p

Effect of the Device Architecture on the Performance of FA(0.85)MA(0.15)PbBr(0.45)I(2.55) Planar Perovskite Solar Cells

Hybrid perovskite solar cells have attracted an unprecedented research attention due to their skyrocketing record power conversion efficiency (PCE), which now exceeds 23% in less than a decade from the initial PCE of 3.8%. Besides the excellent optoelectronic properties of the perovskite absorbers, the high efficiencies are also dependent on preparation methods and advanced device engineering. In this study, the role of the device architecture (planar n-i-p vs inverted p-i-n structure) and of the charge-selective interlayer on the photophysical properties of the perovskite absorber and device performance are explored. FA(0.85)MA(0.15)PbBr(0.45)I(2.55) (MA = methylammonium, FA = formamidinium) as the perovskite absorber and chloride-capped TiO2 colloidal nanocrystals (TiO2-Cl) and poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as close-to-the-substrate layers in the conventional and inverted structures are employed, respectively. Extremely different device performances are demonstrated by the two structures. The device where the active layer is deposited on TiO2-Cl displays a champion PCE of 19.9%, while the one using PEDOT:PSS gives about 15.1% efficiency. The photophysical and electrical investigations indicate that the TiO2-Cl/perovskite interface has lower number of traps, underlining the importance of interfaces for achieving highly performing perovskite solar cells

Influence of the stoichiometry of tin-based 2D/3D perovskite active layers on solar cell performance

2D/3D mixed tin perovskites have the advantages of high crystallinity and preferential orientation compared to pure 3D tin perovskite. However, solar cells based on 2D/3D mixed tin perovskites are still limited by low power conversion efficiency (PCE) when compared to their lead-based counterparts. It is essential to gain deeper insight into the factors that limit the performance of these solar cells in order to further improve them. In this work, we demonstrate that the starting stoichiometry of 2D/3D (PEA(0.08)FA(x)SnI(3)) tin perovskite films influences their crystallization and photophysical properties as well as the solar cell performance. The reference 2D/3D film (x = 0.92, where x refers to the stoichiometry of the precursors) is highly crystalline with the 3D phase preferentially oriented and a small amount of 2D phase located at the bottom of the film. The reference solar cell delivers a PCE of about 8.0%. 2D/3D films with even higher FA concentration (x > 1.0) mainly consist of poorly crystalline and randomly oriented 3D phases, with much higher trap density compared to the reference film. The corresponding solar cells therefore suffer from severe trap-assisted charge recombination, and deliver a poor PCE o

Field-Effect Transistors Based on Formamidinium Tin Triiodide Perovskite

To date, there are no reports of 3D tin perovskite being used as a semiconducting channel in field-effect transistors (FETs). This is probably due to the large amount of trap states and high p-doping typical of this material. Here, the first top-gate bottom-contact FET using formamidinium tin triiodide perovskite films is reported as a semiconducting channel. These FET devices show a hole mobility of up to 0.21 cm(2) V-1 s(-1), an I-ON/OFF ratio of 10(4), and a relatively small threshold voltage (V-TH) of 2.8 V. Besides the device geometry, the key factor explaining this performance is the reduced doping level of the active layer. In fact, by adding a small amount of the 2D material in the 3D tin perovskite, the crystallinity of FASnI(3) is enhanced, and the trap density and hole carrier density are reduced by one order of magnitude. Importantly, these transistors show enhanced parameters after 20 months of storage in a N-2 atmosphere

Impact of the Hole Transport Layer on the Charge Extraction of Ruddlesden-Popper Perovskite Solar Cells

Recent works demonstrate that polyelectrolytes as a hole transport layer (HTL) offers superior performance in Ruddlesden-Popper perovskite solar cells (RPPSCs) compared to poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The factors contributing to such improvement need to be systematically investigated. To achieve this, we have systematically investigated how the two HTLs affect the morphology, crystallinity, and orientation of the Ruddlesden-Popper perovskite (RPP) films as well as the charge extraction of the RPPSCs. PEDOT:PSS as a HTL leads to RPP films of low crystallinity and with a number of large pinholes. These factors lead to poor charge carrier extraction and significant charge recombination in the RPPSCs. Conversely, a PCP-Na HTL gives rise to highly crystalline and pinhole-free RPPSC films. Moreover, a PCP-Na HTL provides a better energy alignment at the perovskite/HTL interface because of its higher work function compared to PEDOT:PSS. Consequently, devices using PCP-Na as HTLs are more efficient in extracting charge carriers

Tuning the Energetic Landscape of Ruddlesden-Popper Perovskite Films for Efficient Solar Cells

Ruddlesden-Popper perovskite films deposited with different methods show very diverse phase segregation and composition. When DMSO is used as solvent, the conventional method based on spin-coating and annealing produces very poor devices, whereas the vacuum-assisted method proposed here allows obtaining devices with efficiency up to 14.14%. The conventional method gives rise to a three-dimensional (3D)-like phase on the top of the film but dominant n = 2 phase with large domains (∼40 μm) at the bottom of the film. These n = 2 domains are oriented with their inorganic slabs parallel to the substrate and inhibit the charge transport in the vertical direction. Consequently, severe monomolecular and bimolecular charge recombination occurs in the solar cells. Conversely, the vacuum-assisted method yields films with a 3D-like phase dominant throughout their entire thickness and only a small amount of n ≤ 2 domains of limited dimensions (∼3 μm) at the bottom, which facilitate charge transport and reduce charge recombination.</p