The application of perovskite photovoltaics is hampered by issues related to the operational stability upon exposure to external stimuli, such as voltage bias and light. The dynamic control of the properties of perovskite materials in response to light could ensure the durability of perovskite solar cells, which is especially critical at the interface with charge-extraction layers. We have applied a functionalized photochromic material based on spiro-indoline naphthoxazine at the interface with hole-transport layers in the corresponding perovskite solar cells with the aim of stabilizing them in response to voltage bias and light. We demonstrate photoinduced transformation by a combination of techniques, including transient absorption spectroscopy and Kelvin probe force microscopy. As a result, the application of the photochromic derivative offers improvements in photovoltaic performance and operational stability, highlighting the potential of dynamic photochromic strategies in perovskite photovoltaics.

AbstractThe stability of hybrid organic–inorganic halide perovskite semiconductors remains a significant obstacle to their application in photovoltaics. To this end, the use of low‐dimensional (LD) perovskites, which incorporate hydrophobic organic moieties, provides an effective strategy to improve their stability, yet often at the expense of their performance. To address this limitation, supramolecular engineering of noncovalent interactions between organic and inorganic components has shown potential by relying on hydrogen bonding and conventional van der Waals interactions. Here, the capacity to access novel LD perovskite structures that uniquely assemble through unorthodox S‐mediated interactions is explored by incorporating benzothiadiazole‐based moieties. The formation of S‐mediated LD structures is demonstrated, including one‐dimensional (1D) and layered two‐dimensional (2D) perovskite phases assembled via chalcogen bonding and S–π interactions. This involved a combination of techniques, such as single crystal and thin film X‐ray diffraction, as well as solid‐state NMR spectroscopy, complemented by molecular dynamics simulations, density functional theory calculations, and optoelectronic characterization, revealing superior conductivities of S‐mediated LD perovskites. The resulting materials are applied in n‐i‐p and p‐i‐n perovskite solar cells, demonstrating enhancements in performance and operational stability that reveal a versatile supramolecular strategy in photovoltaics.

The synthesis and control of properties of p-type ZnO is crucial for a variety of optoelectronic and spintronic applications; however, it remains challenging due to the control of intrinsic midgap (defect) states. In this study, we demonstrate a synthetic route to yield colloidal ZnO quantum dots (QD) via an enhanced sol-gel process that effectively eliminates the residual intermediate reaction molecules, which would otherwise weaken the excitonic emission. This process supports the creation of ZnO with p-type properties or compensation of inherited n-type defects, primarily due to zinc vacancies under oxygen-rich conditions. The in-depth analysis of carrier recombination in the midgap across several time scales reveals microsecond carrier lifetimes at room temperature which are expected to occur via zinc vacancy defects, supporting the promoted p-type character of the synthesized ZnO QDs.

Incorporating organic semiconductor building blocks as spacer cations into layered hybrid perovskites provides an opportunity to develop new materials with novel optoelectronic properties, including nanoheterojunctions that afford spatial separation of electron and hole transport. However, identifying organics with suitable structure and electronic energy levels to selectively absorb visible light has been a challenge in the field. In this work, we introduce a new lead-halide-based Ruddlesden-Popper perovskite structure based on a visible-light-absorbing naphthalene-iminoimide cation (NDI-DAE). Thin films of (NDI-DAE)(2)PbI4 show a quenched photoluminescence and transient absorption dynamics consistent with the formation of a charge transfer state or free charge carriers when either the inorganic or organic layer is photoexcited, suggesting the formation of a type II nanoheterostructure. Time-resolved microwave conductivity analysis supports free charge generation with sum mobilities up to 4 x 10(-4) cm(2) V-1 s(-1). Mixed halide (NDI-DAE)(2)Pb(I-x Br1-x )(4 )films show modified inorganic layer band gaps and a photoluminescent reversed type I nanoheterostructure with high bromide content (e.g., for x = 0). At x = 0.5, transient absorption and microwave conductivity measurements provide strong evidence that selective visible-light absorbance by the NDI-DAE cation generates separated free carriers via hole transfer to the inorganic layer (leaving photogenerated electrons in the organic layer), which represents an important step toward enhancing light harvesting and affording the spatial separation of charge carrier transport in stable layered perovskite-based devices.

A widely used component of high-efficiency perovskite solar cells (PSCs) is the molecular hole-transport material (HTM) spiro-OMeTAD. This organic solid needs to be p-doped to acquire sufficient hole conductivity. However, the conventional doping method using LiTFSI in the air is slow, sensitive to the environment, and may lead to the deterioration of the PSCs by unintended oxidation or dopant migration. It is thus highly desirable to develop fast doping approaches that avoid exposing the PSC to ambient air and easy-to-move dopant ions. We report here that light absorption by spiro-OMeTAD itself triggers redox photochemistry that has so far been ignored. Strikingly, we found that Y(III) or La(III)-tBP complexes catalyze the symmetry-breaking charge separation of photo-excited spiro-OMeTAD, resulting in the efficient p-doping of the HTM. Using this photo-redox process, we realize PSCs with superior stability over cells using conventional doping that show no degradation under continuous illumination over 1,000 h.

Organic materials can tune the optical properties in layered (2D) hybrid perovskites, although their impact on photophysics is often overlooked. Here, we use transient absorption spectroscopy to probe the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) 2D perovskite phases. We show the formation of charge transfer excitons in DJ phases, resulting in a photoinduced Stark effect which is shown to be dependent on the spacer size. By using electroabsorption spectroscopy, we quantify the strength of the photoinduced electric field, while temperature-dependent measurements demonstrate new features in the transient spectra of RP phases at low temperatures resulting from the quantum-confined Stark effect. This study reveals the impact of spacer size and perovskite phase configuration on charge transfer excitons in 2D perovskites of interest to their advanced material design.

An improved optical design for nanosecond diffuse reflectance (DR) spectroscopy is presented. The in-situ analysis of the electron back-reaction and dye regeneration processes in efficient opaque dye-sensitized solar cell devices (DSCs) was scrutinized for the first time using nanosecond DR spectroscopy. The efficient DSC device is based on an opaque TiO2 double-layer film comprising 400 nm light-scattering particles and 20 nm optically transparent particles. Transmission-based laser techniques are not suitable for studying these or other devices by using the opaque morphologies of TiO2 films. However, time-resolved DR flash photolysis enables the exploration of photophysical processes in a broad variety of opaque or highly light-absorbing and light-scattering materials. We experimentally verified the three important components of DR-based spectroscopy: optical configuration, sample condition, and theoretical quantitative optical models. The large optical angle for diffusive light enables efficient light collection and measurement at a relatively low power. We tested the steady-state and time-resolved concentration dependence of the Kubelka-Munk theory for the quantitative analysis of time-resolved results and observed that the dynamics of electron back-reactions are strongly affected by the morphological parameters of the TiO2 films. With a lifetime of 50 mu s, the kinetics of electron back-recombination in the device's photoanode, which is manufactured with 400 nm TiO2 particles and 20 nm TiO2 particles, are 2 orders of magnitude faster than what has been reported to date for 20 nm particles (1 ms). In contrast to electron back-recombination, the dye regeneration process is not influenced by the TiO2 film morphology.

An electron-accepting spacer di-cation based on naphthalene diimide is incorporated into layered and quasi-layered perovskite structure, and the effect of the resulting type II heterostructure on the optoelectronic structure is investigated.

Metal halide perovskites (MHPs) have shown an incredible rise in efficiency, reaching as high as 25.7%, which now competes with traditional photovoltaic technologies. Herein, we excluded CsX and RbX, the most commonly used cations to stabilize FAPbI3, from the bulk of perovskite thin films and applied them on the surface, as passivation agents. Extensive device optimization led to a power conversion efficiency (PCE) of 24.1% with a high fill factor (FF) of 82.2% upon passivation with CsI. We investigated in-depth the effect of CsI passivation on structural and optoelectronic properties using X-ray diffraction (XRD), angle resolved X-ray photoelectron spectroscopy (ARXPS), Kelvin Probe Force (KPFM) microscopy, time-resolved photoluminescence (TRPL), photoluminescence quantum yield (PLQY) and electroabsorption spectroscopy (TREAS). Furthermore, passivated devices exhibit enhanced operational stability, with optimized passivation with CsI leading to a retention of ~90% of initial PCE under 1 Sun illumination with maximum power point tracking for 600 h.

Charge transfer and subsequent separation into free carriers are key processes that govern the efficiencies of third generation solar cell technologies based on donor-acceptor heterojunctions. As these processes typically occur on picosecond to femtosecond timescales, it is necessary to employ ultrafast spectroscopic techniques to further our understanding of these processes in order to provide vital information that can aide in furthering material design. Within the framework of the National Centre of Competence in Research “Molecular Ultrafast Science and Technology” (NCCR MUST), we have developed and utilized a suite of different ultrafast spectroscopic techniques to study charge generation, separation and recombination in a variety of small molecule based organic solar cells and lead halide perovskites. Here, we provide an overview of the main techniques used in our laboratory and the recent results obtained using these spectroscopic techniques.

Layered hybrid organic-inorganic perovskite (LHOIP) materialsconstructed with low-band-gap chromophore-based organic spacer cations are anemerging class of materials that promise unique tunability of their optoelectronicproperties. However, the large size of such chromophore-based spacer cationschallenges their incorporation into a layered perovskite structure and requires furtherinsight into the layered perovskite phase formation mechanism. Herein, we report thepreparation and incorporation of asymmetric naphthalenediimide (NDI) spacerchromophore cations with different amine-bearing alkyl linker chains into thinfilmsof LHOIPs. Usingin situUV-vis spectroscopic kinetic studies of the quantum wellformation, we show that shorter linker chain lengths require higher annealingtemperatures to form the LHOIP structure. Avrami analysis of the layered perovskiteformation shows a larger Avrami coefficient (n= 3.64) for short linker chain-bearing cations compared to that for longer alkyl chain-bearing cations (n= 2.43), suggesting an evolution from three-dimensional to quasi-two-dimensional crystal growth with increasinglinker chain length. Additionally, transient absorption spectroscopy and broad-bandfluorescent upconversion spectroscopy indicatefast photoinduced charge transfer from the inorganic layer to the electron-accepting NDI-spacer cation.

Lead halide perovskites exhibit great potential for light-emitting devices. Enhanced photoluminescence (PL) is obtained in perovskite materials of reduced dimensionalities due to the large exciton binding energy. However, as the nanocrystal size is reduced, the surface-to-volume ratio increases, leading to an abundance of surface defects. Here, a fast PL decay, 3-10 ps, is observed in quasi-1D CsPbBr3 perovskite nanoplatelets using broadband fluorescence upconversion spectroscopy. This decay is attributed to reversible trapping of band-edge excitons into dark states that lie close to the band edge. A simplified model is proposed to further confirm the presence of shallow traps and to fit the data obtained by ultrafast spectroscopy for multiple samples. Finally, the presence of deep trap states in aged nanoplatelets is revealed, likely arising from desorption of the organic capping ligands from the surface. Exciton trapping into these states is slower, 20-30 ps, but leads to a decrease in the photoluminescence quantum yield. These results may not only explain the extended luminescence lifetimes that have been reported for perovskite nanocrystals but also demonstrate the potential of combining ultrafast transient absorption and fluorescence up-conversion to obtain a full description of the spectroscopic properties of the material.

Dataset underpinning the published article: A. Burgos-Caminal, E. Socie, M. E. F. Bouduban and J-E. Moser.Exciton and Carrier Dynamics in Two-Dimensional Perovskites.J. Phys. Chem. Lett. 2020, 11, 7692-7701. DOI: 10.1021/acs.jpclett.0c02425.

The defects present in metal halide perovskite are deleterious to both the performance and stability of photovoltaic devices. Consequently, there is an intense focus on developing defect mitigation strategies. Herein we report a facile strategy that employs methylammonium triiodide (MAI(3)) as an additive to the perovskite precursor solution. We examine the effect of MAI(3) on the structural and optoelectronic properties by X-ray diffraction, density functional theory calculations, molecular dynamics simulations, solid-state nuclear magnetic resonance, steady-state, time-resolved photoluminescence (TRPL), and time-resolved terahertz spectroscopy (TRTS). Specifically, TRPL and TRTS show that MAI(3) suppresses nonradiative recombination and increases the charge carrier mobility. As a result, the champion device shows a power conversion efficiency (PCE) of 23.46% with a high fill factor of >80%. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining similar to 90% of its initial PCE under 1 sun illumination with maximum power point tracking for 350 h.

To develop photosensitizers with high open-circuit photovoltage ( V oc ) is a crucial strategy to enhance the power conversion efficiency (PCE) of co-sensitized solar cells. Here, we show a judiciously tailored organic photosensitizer, coded MS5, featuring the bulky donor N -(2’,4’-bis(dodecyloxy)-[1,1’-biphenyl]-4-yl)-2’,4’-bis(dodecyloxy)- N -phenyl-[1,1’-biphenyl]-4-amine and the electron acceptor 4-(benzo[ c ][1,2,5]thiadiazol-4-yl)benzoic acid. Employing MS5 with a copper (II/I) electrolyte enables a dye-sensitized solar cell (DSC) to achieve a strikingly high V oc of 1.24 V, with the V oc deficit as low as 130 mV and an ideality factor of merely 1.08. The co-sensitization of MS5 with the wider spectral-response dye XY1b produces a highly efficient and stable DSC with the PCE of 13.5% under standard AM1.5 G, 100 mW cm −2 solar radiation. Remarkably, the co-sensitized solar cell (active area of 2.8 cm 2 ) presents a record PCE of 34.5% under ambient light, rendering it very attractive as an ambient light harvesting energy source for low power electronics.

Layered hybrid perovskites based on Dion-Jacobson phases are of interest to various optoelectronic applications. However, the understanding of their structure-property relationships remains limited. Here, we present a systematic study of Dion-Jacobson perovskites based on (S)PbX4 (n = 1) compositions incorporating phenylene-derived aromatic spacers (S) with different anchoring alkylammonium groups and halides (X = I, Br). We focus our study on 1,4-phenylenediammonium (PDA), 1,4-phenylenedimethylammonium (PDMA), and 1,4-phenylenediethylammonium (PDEA) spacers. Systems based on PDA did not form a well-defined layered structure, showing the formation of a 1D structure instead, whereas the extension of the alkyl chains to PDMA and PDEA rendered them compatible with the formation of a layered structure, as shown by X-ray diffraction and solid-state NMR spectroscopy. In addition, the control of the spacer length affects optical properties and environmental stability, which is enhanced for longer alkyl chains and bromide compositions. This provides insights into their design for optoelectronic applications.

Strongly confined, fully inorganic cesium lead halide perovskite nanocrystals are of great promise for light-emitting devices in the blue spectral range owing to their high photoluminescence quantum yields. A combination of broadband fluorescence up-conversion and transient absorption spectroscopies was used to study early exciton dynamics in quasi-1D CsPbBr3 nanoplatelets (3 x 4 x 23 nm3, NPls). This allowed to reveal emitting band-edge states in the NPls that form instantaneously upon photoexcitation and then relax within the first picosecond to lower energy confined hole states (CHSs). The influence of the pump excitation intensity on the latter process was further scrutinized. The band-edge population lifetime was found to increase with the rise of the photon fluence due to CHS filling. When the concentration of NPls in solution becomes very high, nanoparticles overlap, resulting in a decrease in their external absorption cross-section and an increase in the emission Stokes shift due to photon reabsorption.

Low-dimensional hybrid perovskites have emerged as promising materials for optoelectronic applications. Although these materials have already demonstrated enhanced stability as compared to their three-dimensional perovskite analogues, their functionality has been limited by the insulating character of the organic moieties that primarily play a structure-directing role. This is particularly the case for the layered (2D) perovskite materials based on formamidinium lead iodide (FAPbI(3)) that remain scarce. We demonstrate a low-dimensional hybrid perovskite material based on a SPbI4 composition incorporating an electroactive naphthalenediimide (NDI) moiety as an organic spacer (S) between the perovskite slabs and evidence the propensity of the spacer to stabilize the alpha-FAPbI(3) perovskite phase in hybrid low-dimensional SFA(n-1)PbnI(3n+1) perovskite compositions. This has been investigated by means of solid-state nuclear magnetic resonance spectroscopy in conjunction with molecular dynamics simulations and density functional theory calculations. Theoretical calculations suggest an electronic contribution of the organic spacer to the resulting optoelectronic properties, which was confirmed by transient absorption spectroscopy. We have further analyzed these materials by time-resolved microwave conductivity measurements, revealing challenges for their application in photovoltaics.