Thanks to the very high efficiencies in the sunlight-to-electricity conversion gained in the last ten years, Perovskite Solar Cells (PSCs) have attracted tremendous interest, stating as one of the leading technologies in the next-generation photovoltaic field . Within them, the hole-transporting material (HTM), capable of transporting the holes formed after photo-excitation of the perovskite to the counter-electrode, plays an essential role in improving the performance of the device. A breakthrough has been represented by the introduction of small organic molecules as HTMs capable of forming a very thin self-assembled monolayer (SAM) onto the ITO electrode and extracting the photogenerated hole from the perovskite layer. This work aimed to design, synthesize and characterize spectroscopically and electrochemically a new family of six organic precursors of SAMs based on the tricyclic structure of the dithieno[3,2-b:2',3'-d]pyrrole and bearing a phosphonic acid as the anchoring group onto the ITO electrode. Different aryl substituents in 2- and 4-positions have been introduced to modulate the photo-electrochemical properties of the final SAMs, while a common and straightforward synthetic procedure based on Pd-catalyzed cross-coupling reactions has been optimized for all the compounds. All the new molecules have been successfully tested for the realization of PSC devices and the most promising results have been obtained with QM06 and QM10, outperforming the PCE values of the standard literature reference 2-MeOPACz. Optimization of the PSC construction and full characterization of the devices is ongoing.

Thanks to the very high efficiencies in the sunlight-to-electricity conversion gained in the last ten years, Perovskite Solar Cells (PSCs) attracted tremendous interest, stating as one of the leading technologies in the next-generation photovoltaic field. To increase stability and performance, several alternatives to the original device architecture and the materials composing the PSC layers have been investigated. About the hole-transporting layer (HTL), a breakthrough has been represented by the introduction of small organic molecules capable of forming a very thin self-assembled monolayer (SAM) onto the ITO electrode and extracting the photogenerated hole from the perovskite layer. The main advantages of using SAMs in comparison to traditional hole-transporting materials (HTMs) are their cost-effectiveness and their stability, as well as the pointlessness additives. In this work, we investigated the properties of a new family of six organic precursors of SAMs (MS36-45-47 and QM06-08-10) based on the tricyclic structure of the dithieno[3,2-b:2',3'-d]pyrrole and bearing a phosphonic acid as the anchoring group to the ITO electrode. Different aryl substituents in 2- and 6-positions have been introduced to modulate the photo-electrochemical properties of the final SAMs, while a common and straightforward synthetic procedure has been optimized for all the compounds. A preliminary screening of test PSCs containing the new compounds outlined QM06 and QM10 as the best-performing SAM precursors, outperforming the PCE values of the standard literature reference 2-MeOPACz. Optimization of the PSC construction and full characterization of the devices is ongoing.

Perovskite solar cells (PSCs) are one of the emerging technologies in the photovoltaic industry. Within them, the hole-transporting material (HTM), capable of transporting the holes formed after photo-excitation of the perovskite to the counter-electrode, plays an essential role in improving the performance of the device. This work aimed to design, synthesize and characterize spectroscopically and electrochemically a new class of compounds capable of forming self-assembling monolayers (SAMs) to be used as organic HTMs for inverted-configuration perovskite solar cells (p-i-n PSCs). Six new SAMs with a common structure containing a dithieno[3,2-b:2',3'-d]pyrrolic (DTP) core acting as a heteroaromatic functional group, a propyl chain as a spacer and a phosphonic acid as an anchor group were synthesized and characterized. A common synthetic procedure based on Pd-catalyzed cross-coupling reactions was developed for all the molecules, enabling them to be obtained in a few steps and with good yields. All the DTP-based derivatives possess the appropriate spectroscopic and electrochemical properties to act properly as HTMs within a PSC. All the new molecules have been successfully tested for the realization of p-i-n PSC devices and the most promising results in terms of the efficiency of conversion of sunlight into electric current (18.42%) have been obtained with QM06, slightly lower than the efficiency obtained with the literature reference SAM, MeO-2PACz (20.4 %).

Interfacial passivation is a crucial technique for improving the performance of perovskite solar cells (PSCs) by suppressing nonradiative recombination. Incorporating electron-rich functional groups into organic semiconductors can combine the advantages of Lewis bases and organic semiconductors to achieve defect passivation of perovskite films and interfacial charge transport improvement simultaneously. However, interlayers generated by organic semi- conductors are often destroyed during the deposition of the hole transport layer (HTL) in n-i-p PSCs. This prevents the accurate evaluation of interfacial pas- sivation effects. Herein, a pyromellitic derivative, 2,6-bis(4-(bis(4-methoxyphenyl) amino)phenyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2 H,6 H)-tetraone (Pyr-TPA), containing four carbonyl groups that can passivate defects and enhance hole transport while simultaneously acting as a stable interlayer at the perovskite/HTL interface due to its ideal solubility profile is introduced. As a result, Pyr-TPA as an interlayer can minimize nonradiative recombination loss, resulting in a power conversion efficiency of up to 24.16%. Additionally, the interfacial Pyr-TPA passivation layer also exhibits strong resistance to moisture and ion migration, leading to enhanced long-term ambient stability of PSCs based on this material. Findings provide valuable insights into developing efficient and stable PSCs with simple and effective organic semiconductor interfacial passivation materials.

High-dimensionality Ruddlesden-Popper (RP) perovskites, with general formula R(2)A(n-1)B(n)X(3n+1) and high n values (n >= 5), are regarded as viable materials for photovoltaics because they feature higher stability if compared to the 3D perovskite, i.e., ABX(3), still maintaining good charge absorption and transport properties. When integrated into the actual solar cells, however, scattered, sometimes contradictory results are reported among different deposition procedures and different cations, especially for higher n resulting in not uniform morphology and mixed composition. Herein, high-dimensionality RP perovskites with n = 1, 4, 10, 20, and 40 values are systematically investigated considering the interplay between the formation of 2D domains, their distribution along the active layer, the active layer thickness, and the solar cells' performance. Given the complexity of the investigated system, combined advanced structural/morphological analyses are performed to explain solar cells' performance, finding that the 2D phase segregates at the interface with the top electrode, acting as a barrier for charge extraction, overall decreasing the short-circuit current (J(sc)). Reducing the relative amount of bulky alkylammonium cation with respect to the methylammonium, the 2D perovskite overlayer is intentionally decreased leading to a recovery of the J(sc) values, corroborating the hypothesis.

During the last decade, perovskite solar technologies underwent an impressive development, with power conversion efficiencies reaching 25.5% for single junction devices, and 29.8% for Silicon-Perovskite tandem configurations. Even though research mainly focused on improving the efficiency of perovskite photovoltaics (PV), stability and scalability remain fundamental aspects for a mature PV technology. For n-i-p structure perovskite solar cells, using poly-triaryl(amine) (PTAA) as hole transport layer (HTL) allowed to achieve marked improvements in device stability compared to other common hole conductors. For p-i-n structure, PTAA is also routinely used as dopant-free HTL, but problems in perovskite films growth, as well as its limited resistance to stress and imperfect batch-to-batch reproducibility, hamper its use for device upscaling. Following previous computational investigations, in this work we report the synthesis of two small-molecule organic HTLs (BPT-1,2), aiming to solve the above-mentioned issues and allow upscale to module level. By using BPT-1 and methylammonium-free perovskite max. PCEs of 17.26% and 15.42% on small area (0.09 cm2) and mini-module size (2.25 cm2), respectively, were obtained, with a better reproducibility than with PTAA. Moreover, BPT-1 was demonstrated to yield more stable devices compared to PTAA under ISOS-D1, T1 and L1 accelerated life test protocols, reaching maximum T90 values >1000 hours on all tests.

The power conversion efficiency of the formamidinium tin iodide (FASI) solar cells constantly increases, with the current record power conversion efficiency approaching 15%. The literature reports a broad anomaly distribution of the photoluminescence (PL) peak position. The PL anomaly is particularly relevant to photovoltaic applications since it directly links the material's bandgap and subgap defects energy, which are crucial to extracting its full photovoltaic potential. Herein, the PL of FASI polycrystalline thin film and powder is studied. It is found that a distribution of PL peak positions in line with the distribution available in the literature systematically. The distribution in PL is linked to the octahedral tilting and Sn off-centering within the perovskite lattice, influenced by the procedure used to prepare the material. Our finding paves the way toward controlling the energy distribution of tin perovskite and thus preparing highly efficient tin halide perovskite solar cells.

Inverted perovskite solar cells (IPSCs) have different advantages for future commercialization compared to the regular PSC, expecially due to their long stability. In the PSC architecture, Hole-Transporting Materials (HTMs) play a critical role to capture holes formed after the light absorption in the perovskite layer and transport them to the cathode. Spiro-OMeTAD, is the best performing organic molecule to date, however rationally designed HTMs might be crucial to improve performances of (IPSCs). We decided to synthetize two new HTMs which were based on a double phenothiazine scaffold, having triphenylamine (TPA) groups as substituents, and with a different central core: a phenyl (HTM1) and dibenzofuran group (HTM2) (figure 1). The synthesis took advantage of Pd-catatyzed Suzuki and Buchwald-Hartwig coupling reactions to assemble different molecular fragments. Optimization of the reaction conditions gave the desired molecules in good yields. The corresponding devices showed optimal results regarding both efficiency and long stability, proving HTM1 and HTM2 to be excellent candidates as hole carrier in inverted PCS.

Nanocluster aggregation sources based on magnetron-sputtering represent precise and versatile means to deposit a controlled quantity of metal nanoparticles at selected interfaces. In this work, we exploit this methodology to produce Ag/MgO nanoparticles (NPs) and deposit them on a glass/FTO/TiO2 substrate, which constitutes the mesoscopic front electrode of a monolithic perovskite-based solar cell (PSC). Herein, the Ag NP growth through magnetron sputtering and gas aggregation, subsequently covered with MgO ultrathin layers, is fully characterized in terms of structural and morphological properties while thermal stability and endurance against air-induced oxidation are demonstrated in accordance with PSC manufacturing processes. Finally, once the NP coverage is optimized, the Ag/MgO engineered PSCs demonstrate an overall increase of 5% in terms of device power conversion efficiencies (up to 17.8%).

Perovskite solar cells (PSCs) are one of the emerging technologies in the field of photovoltaics. Hole transporting materials (HTMs) play essential roles in improving the device performances; their main task is to capture the hole formed after light absorption in the perovskite layer and transport it to the cathode. The goal of this study was to synthesize new, cheaper and more stable organic HTMs than Spiro-OMeTAD, an organic molecule showing the best performances in PSC devices to date. Organometallic reactions were used to form new carbon-carbon and carbon-nitrogen bonds to assemble different molecular fragments in simple way; in particular, we used palladium catalyzed Suzuki-Miyaura and Buchwald-Hartwig cross-couplings to prepare two new HTMs (HTM1 and HTM2, see figure 1). These compounds were then used to study how small structural modifications may affect the performance of the resulting PSCs.

MXenes are a recent family of 2D materials with very interesting electronic properties for device applications. One very appealing feature is the wide range of work functions shown by these materials, depending on their composition and surface terminations, that can be exploited to adjust band alignments between different material layers. In this work, based on density functional theory calculations, how mixed terminations of F, OH, and/or O affect the work function of TiC MXene is analyzed in detail, covering the whole phase-space of mixtures. The TiC/CHNHPbI (MAPbI) perovskite coupled system for solar cell applications is also analyzed. A strong nonlinear behavior is found when varying the relative concentrations of OH, O, and F terminations, with the strongest effect of the OH groups in lowering the work function, already at a relative amount of 25%. A surprising minimum work function is found for relative OH:O fraction of 75:25, explained in terms of the nonlinear electronic response in screening the surface dipoles.

The operation of halide perovskite optoelectronic devices, including solar cells and LEDs, is strongly influenced by the mobility of ions comprising the crystal structure. This peculiarity is particularly true when considering the long-term stability of devices. A detailed understanding of the ion migration-driven degradation pathways is critical to design effective stabilization strategies. Nonetheless, despite substantial research in this first decade of perovskite photovoltaics, the long-term effects of ion migration remain elusive due to the complex chemistry of lead halide perovskites. By linking materials chemistry to device optoelectronics, this study highlights that electrical bias-induced perovskite amorphization and phase segregation is a crucial degradation mechanism in planar mixed halide perovskite solar cells. Depending on the biasing potential and the injected charge, halide segregation occurs, forming crystalline iodide-rich domains, which govern light emission and participate in light absorption and photocurrent generation. Additionally, the loss of crystallinity limits charge collection efficiency and eventually degrades the device performance.

Low-cost carbon-conductive films were screen-printed on a Plexiglas(R)substrate, and then, after a standard annealing procedure, subjected to femtosecond (fs) laser treatments at different values of total accumulated laser fluence phi(A). Four-point probe measurements showed that, if phi(A)> 0.3 kJ/cm(2), the sheet resistance of laser-treated films can be reduced down to about 15 omega/sq, which is a value more than 20% lower than that measured on as-annealed untreated films. Furthermore, as pointed out by a comprehensive Raman spectroscopy analysis, it was found that sheet resistance decreases linearly with phi(A), due to a progressively higher degree of crystallinity and stacking order of the graphitic phase. Results therefore highlight that fs-laser treatment can be profitably used as an additional process for improving the performance of printable carbon electrodes, which have been recently proposed as a valid alternative to metal electrodes for stable and up-scalable perovskite solar cells.

The [1]benzothieno[3,2-b][1]benzothiophene (BTBT) planar system was used to functionalize the phthalocyanine ring aiming at synthesizing novel electron-rich ?-conjugated macrocycles. The resulting ZnPc-BTBT and ZnPc-(BTBT)4 derivatives are the first two examples of a phthalocyanine subclass having potential use as solution-processable p-type organic semiconductors. In particular, the combination of experimental characterizations and theoretical calculations suggests compatible energy level alignments with mixed halide hybrid perovskite-based devices. Furthermore, ZnPc-(BTBT)4 features a high aggregation tendency, a useful tool to design compact molecular films. When tested as hole transport materials in perovskite solar cells under 100 mAcm-2 standard AM 1.5G solar illumination, ZnPc-(BTBT)4 gave power conversion efficiencies as high as 14.13%, irrespective of the doping process generally required to achieve high photovoltaic performances. This work is a first step toward a new phthalocyanine core engineerization to obtain robust, yet more efficient and cost-effective materials for organic electronics and optoelectronics.

Perovskite solar cells have set a new milestone in terms of efficiencies in the thin film photovoltaics category. Long-term stability of perovskite solar cells is of paramount importance but remains a challenging task. The lack of perovskite solar cells stability in real-time operating conditions erodes and impedes commercialization. Further improvements are essential with a view to delivering longer-lasting photovoltaic (PV) performances. An ideal path in this direction will be to identify novel dopants for boosting the conductivity and hole mobility of hole transport materials (HTMs), and by so doing, the usage of hygroscopic and deliquescent additive materials can be avoided. The present work demonstrates the employment of ionic liquids into a dissymmetric fluorene-dithiophene, FDT (2',7'-bis(bis(4-methoxyphenyl)amino) spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene]) based HTM to understand the doping mechanisms. N-Heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyidinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI) as p-type dopant for FDT was found to increase the conductivity of FDT, to higher geometrical capacitance, to facilitate homogeneous film formation, and to enhance device stability. Our findings open up a broad range of hole-transport materials to control the degradation of the underlying water-sensitive active layer by substituting a hygroscopic element.

The presence of various types of chemical interactions in metal-halide perovskite semiconductors gives them a characteristic "soft" fluctuating structure, prone to a wide set of defects. Understanding of the nature of defects and their photochemistry is summarized, which leverages the cooperative action of density functional theory investigations and accurate experimental design. This knowledge is used to describe how defect activity determines the macroscopic properties of the material and related devices. Finally, a discussion of the open questions provides a path towards achieving an educated prediction of device operation, necessary to engineer reliable devices.

Two new hole selective materials (HSMs) based on dangling methylsulfanyl groups connected to the C-9 position of the fluorene core are synthesized and applied in perovskite solar cells. Being structurally similar to a half of Spiro-OMeTAD molecule, these HSMs (referred as FS and DFS) share similar redox potentials but are endowed with slightly higher hole mobility, due to the planarity and large extension of their structure. Competitive power conversion efficiency (up to 18.6%) is achieved by using the new HSMs in suitable perovskite solar cells. Time-resolved photoluminescence decay measurements and electrochemical impedance spectroscopy show more efficient charge extraction at the HSM/perovskite interface with respect to Spiro-OMeTAD, which is reflected in higher photocurrents exhibited by DFS/FS-integrated perovskite solar cells. Density functional theory simulations reveal that the interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro-OMeTAD. Importantly, the low-cost synthesis of FS makes it significantly attractive for the future commercialization of perovskite solar cells.

Perovskite solar cells have recently revolutionized the field of emerging photovoltaic technologies. They have shown an impressive evolution in the last ten years, jumping from an initial 3.8%[1] to a 24.2%[2] certified efficiency but still have drawbacks to overcome, some of which are related to the use of the expensive Spiro-OMeTAD as hole transport material (HTM) and to the perovskite film morphology, whose frequent inhomogeneity results in low-resistance shunting paths and loss of light absorption in the solar cells, seriously undermining their photovoltaic performances. Phthalocyanines are macrocyclic aromatic compounds that can address both the issues: they possess excellent p-type semiconducting properties that make them appealing materials as hole transporters, having already scored efficiencies up to 17.5%[3] and, recently, above 20%[4] in perovskite-based devices. At the same time, their hydrophobic aromatic core and their chemical and thermal stability make them potentially effective as active layer sealants, passivating its surface defects and increasing the overall stability of the final devices. In this contribution, we will discuss the synthetic approach to electron-rich metallophthalocyanines, including their cost analysis, and some results of their implementation in perovskite-based solar cells. References: [1]Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc. 2009, 131(17), 6050-6051 [2] https://www.nrel.gov/pv/cell-efficiency.html [3]Cho, K. T.; Trukhina, O.; Roldán-Carmona, C.; Ince, M.; Gratia, P.; Grancini, G.; Gao, P.; Marszalek, T.; Pisula, W.; Reddy, P. Y.; Torres, T.; Nazeeruddin, M. K. Adv. Energy Mater. 2017, 1601733. [4]Duong, T. et al., ACS Energy Lett., 2018, 3 (10), 2441-2448

A power conversion efficiency (PCE) as high as 19.7% is achieved using a novel, low-cost, dopant-free hole transport material (HTM) in mixed-ion solution-processed perovskite solar cells (PSCs). Following a rational molecular design strategy, arylamine-substituted copper(II) phthalocyanine (CuPc) derivatives are selected as HTMs, reaching the highest PCE ever reported for PSCs employing dopant-free HTMs. The intrinsic thermal and chemical properties of dopant-free CuPcs result in PSCs with a long-term stability outperforming that of the benchmark doped 2,2 ',7,7 '-Tetrakis-(N,N-di-p-methoxyphenylamine)-9,9 '-Spirobifluorene (Spiro-OMeTAD)-based devices. The combination of molecular modeling, synthesis, and full experimental characterization sheds light on the nanostructure and molecular aggregation of arylamine-substituted CuPc compounds, providing a link between molecular structure and device properties. These results reveal the potential of engineering CuPc derivatives as dopant-free HTMs to fabricate cost-effective and highly efficient PSCs with long-term stability, and pave the way to their commercial-scale manufacturing. More generally, this case demonstrates how an integrated approach based on rational design and computational modeling can guide and anticipate the synthesis of new classes of materials to achieve specific functions in complex device structures.

This work shows an investigation on the reasoning behind the dependence of the perovskite solar cells photovoltaic efficiencies on the relative position of the undoped Spiro-OMeTAD hole transport material respect to the perovskite in the device. We adopt the impedance spectroscopy to enlighten on the modification of the carrier transport mechanisms across the Spiro-OMeTAD/perovskite interface constituting the active part where the main device processes occur. We investigate two interface structures referred to as the direct (or regular, n-i-p) and the inverted (p-i-n) configuration. This work is also intended to further stress on the possible adoption of alternative device structures working with undoped hole transport materials.