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.

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.

Advances in the Synthesis of Small Molecules as Hole Transport Materials for Lead Halide Perovskite Solar Cells.

Over hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known N2, N2, N2', N2', N7, N7, N7', octakis-(4-methoxyphenyl)-9,9-spirobi-[9,9'-spirobi[9 H-fluorene]-2,2',7,7'-tetramine (spiro-OMeTAD) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly. Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the spiro-OMeTAD molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the "spiro" chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using spiro-OMeTAD. In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure-device function relationship, we describe those examples that always have used spiro-OMeTAD as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it). The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods. Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells.

Supramolecular Coordination of Pb2+ Defects in Hybrid Lead Halide Perovskite Films Using Truxene Derivatives as Lewis Base Interlayers.

Truxene derivatives, due to their molecular structure and properties, are good candidates for the passivation of defects when deposited onto hybrid lead halide perovskite thin films. Moreover, their semiconductor characteristics can be tailored through the modification of their chemical structure, which allows-upon light irradiation- the interfacial charge transfer between the perovskite film and the truxene molecules. In this work, we analysed the use of the molecules as surface passivation agents and their use in complete functional solar cells. We observed that these molecules reduce the non-radiative carrier recombination dynamics in the perovskite thin film through the supramolecular complex formation between the Truxene molecule and the Pb2+ defects at the perovskite surface. Interestingly, this supramolecular complexation neither affect the carrier recombination kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent vs voltage curves of the solar cells under 1 sun illumination.

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'.

Cyclopentadithiophene organic core in small molecule organic solar cells: morphological control of carrier recombination.

Two new planar and symmetrical A-D-A (electron acceptor-electron donor-electron acceptor) small molecules based on a commercial cyclopentadithiophene derivative have been synthesized for solution processed small molecule organic solar cells. The aim was to synthesise the molecules to be energetically identical (similar HOMO-LUMO energy levels) in order to assign the differences observed to changes in the film morphology or to differences in the interfacial recombination kinetics or both. Devices were electrically characterized under one sun simulated (1.5 AM G) conditions by determining current-voltage curves, light harvesting efficiencies and external quantum efficiencies. Moreover, time-resolved photo-induced techniques such as photo-induced charge extraction and photo-induced transient photo-voltage were also performed. The results demonstrate that, despite having the same core, i.e. cyclopentadithiophene, the use of one hexyl chain instead of two in the organic molecule leads to a greater control of the molecular ordering using solvent vapour annealing techniques and also to better solar cell efficiency.

Encapsulation of MEH-PPV:PCBM Hybrids in the Cores of Block Copolymer Micellar Assemblies: Photoinduced Electron Transfer in a Nanoscale Donor-Acceptor System.

The objective of this work is to demonstrate that conjugated polymer:fullerene hybrid nanoparticles encapsulated in the hydrophobic cores of triblock copolymer micelles may successfully act as spatially confined donor-acceptor systems capable of facilitating photoinduced charge carrier separation. To this end, aqueous dispersions of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) nanoparticles were first prepared by solubilization of the polymer in the cores of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblock copolymer, Pluronic F-127 micelles. A number of significant optical spectroscopic changes were observed on transfer of the conjugated polymer from a nonaqueous solvent to the aqueous micellar environment. These were primarily attributed to increased interchain interactions due to conjugated polymer chain collapse during encapsulation in the micellar cores. When prepared in buffer solution, the micelles exhibited good long-term collodial stability. When MEH-PPV micelles were blended by the addition of controlled amounts of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the observed correspondence of photoluminescence emission quenching, quantum yield decreases, and emission lifetime shortening with increasing PCBM concentration indicated efficient photoinduced donor-to-acceptor charge transfer between MEH-PPV and the fullerenes in the cores of the micelles, an assignment that was confirmed by transient absorption spectroscopic monitoring of carrier photogeneration and recombination.

Molecularly Engineered Ru(II) Sensitizers Compatible with Cobalt(II/III) Redox Mediators for Dye-Sensitized Solar Cells.

Thiocyanate-free isoquinazolylpyrazolate Ru(II) complexes were synthesized and applied as sensitizers in dye-sensitized solar cells (DSCs). Unlike most other successful Ru sensitizers, Co-based electrolytes were used, and resulting record efficiency of 9.53% was obtained under simulated sunlight with an intensity of 100 mW cm(-2). Specifically, dye 51-57dht.1 and an electrolyte based on Co(phen)3 led to measurement of a JSC of 13.89 mA cm(-2), VOC of 900 mV, and FF of 0.762 to yield 9.53% efficiency. The improved device performances were achieved by the inclusion of 2-hexylthiophene units onto the isoquinoline subunits, in addition to lengthening the perfluoroalkyl chain on the pyrazolate chelating group, which worked to increase light absorption and decrease recombination effects when using the Co-based electrolyte. As this study shows, Ru(II) sensitizers bearing sterically demanding ligands can allow successful utilization of important Co electrolytes and high performance.

Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J-V Hysteresis.

Methylammonium lead iodide (MAPI) cells of the design FTO/sTiO2/mpTiO2/MAPI/Spiro-OMeTAD/Au, where FTO is fluorine-doped tin oxide, sTiO2 indicates solid-TiO2, and mpTiO2 is mesoporous TiO2, are studied using transient photovoltage (TPV), differential capacitance, charge extraction, current interrupt, and chronophotoamperometry. We show that in mpTiO2/MAPI cells there are two kinds of extractable charge stored under operation: a capacitive electronic charge (∼0.2 µC/cm(2)) and another, larger charge (40 µC/cm(2)), possibly related to mobile ions. Transient photovoltage decays are strongly double exponential with two time constants that differ by a factor of ∼5, independent of bias light intensity. The fast decay (∼1 µs at 1 sun) is assigned to the predominant charge recombination pathway in the cell. We examine and reject the possibility that the fast decay is due to ferroelectric relaxation or to the bulk photovoltaic effect. Like many MAPI solar cells, the studied cells show significant J-V hysteresis. Capacitance vs open circuit voltage (V(oc)) data indicate that the hysteresis involves a change in internal potential gradients, likely a shift in band offset at the TiO2/MAPI interface. The TPV results show that the V(oc) hysteresis is not due to a change in recombination rate constant. Calculation of recombination flux at V(oc) suggests that the hysteresis is also not due to an increase in charge separation efficiency and that charge generation is not a function of applied bias. We also show that the J-V hysteresis is not a light driven effect but is caused by exposure to electrical bias, light or dark.

Efficient and limiting reactions in aqueous light-induced hydrogen evolution systems using molecular catalysts and quantum dots.

Hydrogen produced from water and solar energy holds much promise for decreasing the fossil fuel dependence. It has recently been proven that the use of quantum dots as light harvesters in combination with catalysts is a valuable strategy to obtain photogenerated hydrogen. However, the light to hydrogen conversion efficiency of these systems is reported to be lower than 40%. The low conversion efficiency is mainly due to losses occurring at the different interfacial charge-transfer reactions taking place in the multicomponent system during illumination. In this work we have analyzed all the involved reactions in the hydrogen evolution catalysis of a model system composed of CdTe quantum dots, a molecular cobalt catalyst and vitamin C as sacrificial electron donor. The results demonstrate that the electron transfer from the quantum dots to the catalyst occurs fast enough and efficiently (nanosecond time scale), while the back electron transfer and catalysis are much slower (millisecond and microsecond time scales). Further improvements of the photodriven proton reduction should focus on the catalytic rate enhancement, which should be at least in the hundreds of nanoseconds time scale.

Light-driven organocatalysis using inexpensive, nontoxic Bi2O3 as the photocatalyst.

The development of enantioselective catalytic processes that make use of sunlight as the energy source and nontoxic, affordable materials as catalysts represents one of the new and rapidly evolving areas in chemical research. The direct asymmetric α-alkylation of aldehydes with α-bromocarbonyl compounds can be successfully achieved by combining bismuth-based materials as low-band-gap photocatalysts with the second-generation MacMillan imidazolidinone as the chiral catalyst and simulated sunlight as a low-cost and clean energy source. This reaction also proceeded with high efficiency when the reaction vial was exposed to the morning sunlight on a clear September day in Tarragona, Spain.

Highly efficient dye-sensitized solar cells based on panchromatic ruthenium sensitizers with quinolinylbipyridine anchors.

Panchromatic Ru(II) sensitizers TF-30-TF-33 bearing a new class of 6-quinolin-8-yl-2,2'-bipyridine anchor were synthesized and tested under AM1.5 G simulated solar irradiation. Their increased π conjugation relative to that of the traditional 2,2':6',2''-terpyridine-based anchor led to a remarkable improvement in absorptivity across the whole UV-Vis-NIR spectral regime. Furthermore, the introduction of a bulky tert-butyl substituent on the quinolinyl fragment not only led to an increase in the JSC  value owing to the suppression of dye aggregation, but remarkably also resulted in no loss in VOC in comparison with the reference sensitizer containing a tricarboxyterpyridine anchor. The champion sensitizer in DSC devices was found to be TF-32 with a performance of JSC =19.2 mA cm(-2) , VOC =740 mV, FF=0.72, and η=10.19 %. This 6-quinolin-8-yl-2,2'-bipyridine anchor thus serves as a prototype for the next generation of Ru(II) sensitizers with any tridentate ancillary.

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.

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.

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.

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.].

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.

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.

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.

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.