Enabling white color tunability in complex 3D-printed composites by using lead-free self-trapped exciton 2D perovskite/carbon quantum dot inks.

The generation of stable white light emission using lead-free perovskites remains a huge challenge in the development of future display and lighting technologies, due to fast material deterioration and the decrease of the color quality. In this work, we report a combination of diverse types of 2D A2SnX4 (A = bulky cation, X = Br, I) perovskites exhibiting self-trapped exciton (STE) emission and blue luminescent carbon quantum dots (CQDs), with the purpose of generating A2SnX4/CQD inks with a broadband emission in the visible region and a tunable white light color. By varying the concentration of the 2D perovskite, the white emission of the mixtures is modulated to cool, neutral, and warm tonalities, with a PL quantum yield up to 45%. From the combinations, the PEA2SnI4/CQD-based ink shows the longest stability, due to suitable surface ligand passivation provided by the capping ligands covering the CQDs, compensating the defect sites in the perovskite. Then, by incorporating the PEA2SnI4/CQDs inks into an acrylate polymer matrix, the quenching of the PL component from the perovskite was restrained, being stable for >400 h under ambient conditions and at a relative humidity of ∼50%, and allowing the preparation of complex 3D-printed composites with stable white emission tonalities. This contribution offers an application of STE-based Sn-perovskites to facilitate the future fabrication of lead-free white-light optoelectronic devices.

Correction: Influence of the carbazole moiety in self-assembling molecules as selective contacts in perovskite solar cells: interfacial charge transfer kinetics and solar-to-energy efficiency effects.

[This corrects the article DOI: 10.1039/D3NA00811H.].

Photo-Cross-Linked Fullerene-Based Hole Transport Material for Moisture-Resistant Regular Fullerene Sandwich Perovskite Solar Cells.

Despite the high efficiencies currently achieved with perovskite solar cells (PSCs), the need to develop stable devices, particularly in humid conditions, still remains. This study presents the synthesis of a novel photo-cross-linkable fullerene-based hole transport material named FT12. For the first time, the photo-cross-linking process is applied to PSCs, resulting in the preparation of photo-cross-linked FT12 (PCL FT12). Regular PSCs based on C60-sandwich architectures were fabricated using FT12 and PCL FT12 as dopant-free hole transport layers (HTLs) and compared to the reference spiro-OMeTAD. The photovoltaic results demonstrate that both FT12 and PCL FT12 significantly outperform pristine spiro-OMeTAD regarding device performance and stability. The comparison between devices based on FT12 and PCL FT12 demonstrates that the photo-cross-linking process enhances device efficiency. This improvement is primarily attributed to enhanced charge extraction, partial oxidation of the HTL, increased hole mobility, and improved layer morphology. PCL FT12-based devices exhibit improved stability compared to FT12 devices, primarily due to the superior moisture resistance achieved through photo-cross-linking.

CPE-Na-Based Hole Transport Layers for Improving the Stability in Nonfullerene Organic Solar Cells: A Comprehensive Study.

Organic photovoltaic (OPV) cells have experienced significant development in the last decades after the introduction of nonfullerene acceptor molecules with top power conversion efficiencies reported over 19% and considerable versatility, for example, with application in transparent/semitransparent and flexible photovoltaics. Yet, the optimization of the operational stability continues to be a challenge. This study presents a comprehensive investigation of the use of a conjugated polyelectrolyte polymer (CPE-Na) as a hole layer (HTL) to improve the performance and longevity of OPV cells. Two different fabrication approaches were adopted: integrating CPE-Na with PEDOT:PSS to create a composite HTL and using CPE-Na as a stand-alone bilayer deposited beneath PEDOT:PSS on the ITO substrate. These configurations were compared against a reference device employing PEDOT:PSS alone, as the HTL increased efficiency and fill factor. The instruments with CPE-Na also demonstrated increased stability in the dark and under simulated operational conditions. Device-based PEDOT:PSS as an HTL reached T80 after 2500 h while involving CPE-Na in the device kept at T90 in the same period, evidenced by a reduced degradation rate. Furthermore, the impedance spectroscopy and photoinduced transient methods suggest optimized charge transfer and reduced charge carrier recombination. These findings collectively highlight the potential of CPE-Na as a HTL optimizer material for nonfluorine OPV cells.

Challenges in the design and synthesis of self-assembling molecules as selective contacts in perovskite solar cells.

Self-assembling molecules (SAMs), as selective contacts, play an important role in perovskite solar cells (PSCs), determining the performance and stability of these photovoltaic devices. These materials offer many advantages over other traditional materials used as hole-selective contacts, as they can be easily deposited on a large area of metal oxides, can modify the work function of these substrates, and reduce optical and electric losses with low material consumption. However, the most interesting thing about SAMs is that by modifying the chemical structure of the small molecules used, the energy levels, molecular dipoles, and surface properties of this assembled monolayer can be modulated to fine-tune the desired interactions between the substrate and the active layer. Due to the important role of organic chemistry in the field of photovoltaics, in this review, we will cover the current challenges for the design and synthesis of SAMs PSCs. Discussing, the structural features that define a SAM, (ii) disclosing how commercial molecules inspired the synthesis of new SAMs; and (iii) detailing the pros- and cons- of the reported synthetic protocols that have been employed for the synthesis of molecules for SAMs, helping synthetic chemists to develop novel structures and promoting the fast industrialization of PSCs.

One-Step Solution Deposition of Tin-Perovskite onto a Self-Assembled Monolayer with a DMSO-Free Solvent System.

We show for the first time DMSO-free tin-based perovskite solar cells with a self-assembled hole selective contact (MeO-2PACz). Our method provides reproducible and hysteresis-free devices with MeO-2PACz, having the best device PCE of 5.8 % with a VOC of 638 mV.

Monodentate versus Bidentate Anchoring Groups in Self-Assembling Molecules (SAMs) for Robust p-i-n Perovskite Solar Cells.

Current improvement in perovskite solar cells (PSCs) has been achieved by interface engineering and fine-tuning of charge-selective contacts. In this work, we report three novel molecules that can form self-assembled layers (SAMs) as an alternative to the most commonly used p-type contact material, PTAA. Two of these molecules have bidentate anchoring groups (MC-54 and MC-55), while the last one is monodentate (MC-45). Besides the PTAA comparison, we also compared those two types of molecules and their effect on the solar cell's performance. Devices fabricated with MC-54 and MC-55 showed a remarkable field factor (about 80%) and a better current density, leading to higher efficient solar cells in comparison to MC-45 and PTAA. Moreover, mono- and bidentate present higher stability and reproducibility in comparison to PTAA.

Influence of the carbazole moiety in self-assembling molecules as selective contacts in perovskite solar cells: interfacial charge transfer kinetics and solar-to-energy efficiency effects.

The use of self-assembled molecules (SAMs) as hole transport materials (HTMs) in p-i-n perovskite solar cells (iPSCs) has triggered widespread research due to their relatively easy synthetic methods, suitable energy level alignment with the perovskite material and the suppression of chemical defects. Herein, three new SAMs have been designed and synthesised based on a carbazole core moiety and modified functional groups through an efficient synthetic protocol. The SAMs have been used to understand the SAM/perovskite interface interactions and establish the relationship between the SAM molecular structure and the resulting performance of the perovskite-based devices. The best devices show efficiencies ranging from 18.9% to 17.5% under standard illumination conditions, which are very close to that of our benchmark EADR03, which has been recently commercialised. Our work aims to provide knowledge on the structure of the molecules versus device function relationship.

Novel Spiro-Core Dopant-Free Hole Transporting Material for Planar Inverted Perovskite Solar Cells.

Hole-transporting materials (HTMs) have demonstrated their crucial role in promoting charge extraction, interface recombination, and device stability in perovskite solar cells (PSCs). Herein, we present the synthesis of a novel dopant-free spiro-type fluorine core-based HTM with four ethoxytriisopropylsilane groups (Syl-SC) for inverted planar perovskite solar cells (iPSCs). The thickness of the Syl-SC influences the performance of iPSCs. The best-performing iPSC is achieved with a 0.8 mg/mL Syl-SC solution (ca. 15 nm thick) and exhibits a power conversion efficiency (PCE) of 15.77%, with Jsc = 20.00 mA/cm2, Voc = 1.006 V, and FF = 80.10%. As compared to devices based on PEDOT:PSS, the iPSCs based on Syl-SC exhibit a higher Voc, leading to a higher PCE. Additionally, it has been found that Syl-SC can more effectively suppress charge interfacial recombination in comparison to PEDOT:PSS, which results in an improvement in fill factor. Therefore, Syl-SC, a facilely processed and efficient hole-transporting material, presents a promising cost-effective alternative for inverted perovskite solar cells.

Electro- and Photoinduced Interfacial Charge Transfers in Nanocrystalline Mesoporous TiO2 and TiO2/Iron Porphyrin Sensitized Films under CO2 Reduction Catalysis.

Electro- and photochemical CO2 reduction (CO2R) is the quintessence of modern-day sustainable research. We report our studies on the electro- and photoinduced interfacial charge transfer occurring in a nanocrystalline mesoporous TiO2 film and two TiO2/iron porphyrin hybrid films (meso-aryl- and ß-pyrrole-substituted porphyrins, respectively) under CO2R conditions. We used transient absorption spectroscopy (TAS) to demonstrate that, under 355 nm laser excitation and an applied voltage bias (0 to -0.8 V vs Ag/AgCl), the TiO2 film exhibited a diminution in the transient absorption (at -0.5 V by 35%), as well as a reduction of the lifetime of the photogenerated electrons (at -0.5 V by 50%) when the experiments were conducted under a CO2 atmosphere changing from inert N2. The TiO2/iron porphyrin films showed faster charge recombination kinetics, featuring 100-fold faster transient signal decays than that of the TiO2 film. The electro-, photo-, and photoelectrochemical CO2R performance of the TiO2 and TiO2/iron porphyrin films are evaluated within the bias range of -0.5 to -1.8 V vs Ag/AgCl. The bare TiO2 film produced CO and CH4 as well as H2, depending on the applied voltage bias. In contrast, the TiO2/iron porphyrin films showed the exclusive formation of CO (100% selectivity) under identical conditions. During the CO2R, a gain in the overpotential values is obtained under light irradiation conditions. This finding was indicative of a direct transfer of the photogenerated electrons from the film to absorbed CO2 molecules and an observed decrease in the decay of the TAS signals. In the TiO2/iron porphyrin films, we identified the interfacial charge recombination processes between the oxidized iron porphyrin and the electrons of the TiO2 conduction band. These competitive processes are considered to be responsible for the diminution of direct charge transfer between the film and the adsorbed CO2 molecules, explaining the moderate performances of the hybrid films for the CO2R.

Self-Assembled Molecules for Hole-Selective Electrodes in Highly Stable and Efficient Inverted Perovskite Solar Cells with Ultralow Energy Loss.

Good selective contacts are necessary for solar cells that are efficient and have long-term stability. Since 1998, with the advent of solid-state dye sensitized solar cells (DSSC), Spiro-OMeTAD has become the reference hole-transporting material. Yet, for efficient solar cells Spiro-OMeTAD must be partially oxidized with chemical dopants, which compromises the long-term stability of the solar cell. Alternatively, semiconductor polymers such as PTAA have been also studied, matching or improving the solar cell characteristics. However, PTAA-based devices lack long-term stability. Moreover, both Spiro-OMeTAD and PTAA are expensive materials to synthesize. Hence, approaches toward increasing the solar cell stability without compromising the device efficiency and decreasing the manufacturing cost are very desirable. In this work we have modified Spiro-OMeTAD, by an easy-to-use methodology, by introducing a carboxylic acid anchoring group (Spiro-Acid), thereby allowing the formation of self-assembled monolayers (SAMs) of the hole-transporting material in dopant-free p-i-n hybrid perovskite solar cells (iPSCs). The resulting device showed a champion efficiency of 18.15% with ultralow energy loss, which is the highest efficiency among Spiro-OMeTAD-based iPSCs, and a remarkable fill factor of over 82%, as well as excellent long-term illumination stability. Charge transfer and charge carrier dynamics are studied by using advanced transient techniques to understand the interfacial kinetics. Our results demonstrate that the Spiro-OMeTAD-based SAMs have a great potential in producing low-cost iPSC devices, due to lower material usage, good long-term stability, and high performance.

Role of Terminal Group Position in Triphenylamine-Based Self-Assembled Hole-Selective Molecules in Perovskite Solar Cells.

The application of self-assembled molecules (SAMs) as a charge selective layer in perovskite solar cells has gained tremendous attention. As a result, highly efficient and stable devices have been released with stand-alone SAMs binding ITO substrates. However, further structural understanding of the effect of SAM in perovskite solar cells (PSCs) is required. Herein, three triphenylamine-based molecules with differently positioned methoxy substituents have been synthesized that can self-assemble onto the metal oxide layers that selectively extract holes. They have been effectively employed in p-i-n PSCs with a power conversion efficiency of up to 20%. We found that the perovskite deposited onto SAMs made by para- and ortho-substituted hole selective contacts provides large grain thin film formation increasing the power conversion efficiencies. Density functional theory predicts that para- and ortho-substituted position SAMs might form a well-ordered structure by improving the SAM's arrangement and in consequence enhancing its stability on the metal oxide surface. We believe this result will be a benchmark for the design of further SAMs.

High Solar-to-Hydrogen Conversion Efficiency at pH 7 Based on a PV-EC Cell with an Oligomeric Molecular Anode.

In the urgent quest for green energy vectors, the generation of hydrogen by water splitting with sunlight occupies a preeminent standpoint. The highest solar-to-hydrogen (STH) efficiencies have been achieved with photovoltaic-electrochemical (PV-EC) systems. However, most PV-EC water-splitting devices are required to work at extreme conditions, such as in concentrated solutions of HClO4 or KOH or under highly concentrated solar illumination. In this work, a molecular catalyst-based anode is incorporated for the first time in a PV-EC configuration, achieving an impressive 21.2% STH efficiency at neutral pH. Moreover, as opposed to metal oxide-based anodes, the molecular catalyst-based anode allows us to work with extremely small catalyst loadings (<16 nmol/cm2) due to a well-defined metallic center, which is responsible for the fast catalysis of the reaction in the anodic compartment. This work paves the way for integrating molecular materials in efficient PV-EC water-splitting systems.

Self-assembled Zn phthalocyanine as a robust p-type selective contact in perovskite solar cells.

The use of self-assembled monolayers (SAMs) as selective charge extracting layers in perovskite solar cells is a great approach to replace the commonly used charge selective contacts, as they can easily modify the interface to enhance the final solar cell performance. Here, we report a novel synthetic approach of the commonly known zinc phtalocyanine (ZnPc) molecule TT1, widely employed in dye-sensitized solar cells and previously used in perovskite solar cells. TT1 is used as a p-type selective contact, and it demonstrates its ability to form SAM on top of the indium tin oxide (ITO) transparent electrode, obtaining higher efficiencies compared to Pedot:PSS based perovskite solar cells. The differences observed, with an enhanced open-circuit voltage and overall efficiency in TT1 devices are correlated with differences in energetics rather than recombination kinetics.

Benzothiadiazole Aryl-amine Based Materials as Efficient Hole Carriers in Perovskite Solar Cells.

Four hole transport materials (HTMs) based on a benzothiadiazole (BT) central core have been synthesized and successfully employed in triple-cation mixed-halide perovskite solar cells (PSCs), reaching 18.05% solar to energy conversion efficiency. The synthesis of these HTMs follows the push-and-pull approach to modulate the HOMO energy level by combining the BT group as an electron acceptor and diphenyl- and triphenyl-amines as electron donors. Here we show that despite adjusting the HOMO energy level to that of the perovskite is a believed requisite to achieve efficient interfacial hole transfer, additional factors must be taken into account to design novel and efficient HTMs, such as a high hole mobility, solubility in organic solvents, and thermal stability.

Minimization of Carrier Losses for Efficient Perovskite Solar Cells through Structural Modification of Triphenylamine Derivatives.

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and successfully employed in triple-cation mixed-halide PSCs, reaching efficiencies of 19.4 %. The efficiencies, comparable to those obtained using spiro-OMeTAD, point them out as promising candidates for easily attainable and cost-effective alternatives for PSCs, given their facile synthesis from commercially available materials. Interestingly, although all these HTMs show similar chemical and physical properties, they provide different carrier recombination kinetics. Our results demonstrate that is feasible through the molecular design of the HTM to minimize carrier losses and, thus, increase the solar cell efficiencies.

Improved Carrier Collection and Hot Electron Extraction Across Perovskite, C60, and TiO2 Interfaces.

The use of C60 as an interfacial layer between TiO2 and methylammonium lead iodide perovskite is probed to reduce the current-voltage hysteresis in perovskite solar cells (PSCs) and, in turn, to impact the interfacial carrier injection and recombination processes that limit solar cell efficiencies. Detailed kinetic analyses across different time scales, that is, from the femtoseconds to the seconds, reveal that the charge carrier lifetimes as well as the charge injection and charge recombination dynamics depend largely on the presence or absence of C60. In addition, we corroborate that C60 is applicable in hot carrier PSCs as it is capable of extracting hot carriers generated throughout the early time scales following photoexcitation.

Hot electron injection into semiconducting polymers in polymer based-perovskite solar cells and their fate.

Metal halide perovskites are known to possess upon photoexcitation long-lived hot carriers. By using femtosecond laser transient absorption spectroscopy, we probed in the current work interfacial charge transfer, that is, hot electrons and holes in methylammonium lead iodide perovskite. The focus was, on the one hand, on titanium dioxide as an electron transporting material and, on the other hand, on several organic semiconducting materials as hole transporting materials in perovskite solar cells. An unexpected carrier loss pathway for hot electrons was found in the form of injection into the low lying LUMOs of several organic semiconducting materials. Of great importance is the fact that the final photocurrents of perovskite solar cells scale with the suppression of this newly discovered loss pathway.

Interfacial recombination kinetics in aged perovskite solar cells measured using transient photovoltage techniques.

The reduction of interfacial charge recombination kinetics in perovskite solar cells is key to increase device photovoltaic efficiencies. Thus, it is necessary to fully understand which are the major carrier losses and, thereafter, how they can be minimized. Transient Photovoltage (TPV) has been widely used to study carrier recombination in solar cells under operando conditions. Interestingly, a novel negative transient deflection appears in perovskite solar cells when carrying out TPV measurements and it has been related to the ionic accumulation at the perovskite interfaces, which is a process that requires great attention to fully understand how perovskite solar cells work. Herein, we moved one step further and continuously monitored the evolution of the negative trace with the aging of the perovskite solar cell. Importantly, we demonstrated that the negative signal changes with aging of the solar cells and such a change can be directly related to the enhancement of the open circuit voltage and fill factor of the devices and thus the solar cell efficiency. We postulate that this increase in efficiency is due to better/faster ion redistribution within the perovskite material.

Energetic disorder in perovskite/polymer solar cells and its relationship with the interfacial carrier losses.

Previous reports have observed a direct relationship between the polymer poly(3-hexylthiophene) molecular weight (MW) and the perovskite solar cell (PSC) efficiency. Herein, we analyse how the differences in MW and the differences in energetic disorder influence the interfacial carrier losses in the PSCs under operation conditions and explain the observed differences. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.