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The role of halide substitution in a lead-free 2D hybrid perovskite is investigated on the BZA(2)SnX(4) (BZA = benzylammonium; X = Cl, Br and I) system, by reporting the synthesis and the structural analysis by means of single-crystal X-ray diffraction data of the two novel compositions BZA(2)SnBr(4) and BZA(2)SnCl(4). In addition, the mixed composition BZA(2)Sn(BrxI1-x)(4) (0

Engineering two-/three-dimensional (2D/3D) perovskite solar cells is nowadays a popular strategy for efficient and stable devices. However, the exact function of the 2D/3D interface in controlling the long-term device behavior is still obscure. Here, we reveal a dynamical structural mutation of the 2D/3D interface: the small cations in the 3D cage move towards the 2D layer, which acts as an ion scavenger. If structurally stable, the 2D layer physically blocks the ion movement at the interface boosting the device stability. Otherwise, the 2D layer embeds them, dynamically self-transforming into a quasi-2D structure. The judicious choice of the 2D constituent is decisive in controlling the 2D/3D kinetics and improving the device lifetime, opening a new avenue for perovskite interface design.

Invited for this month's cover are collaborators from University of Pavia, Ecole Polytechnique Federale de Lausanne, University of Messina and Istituto Italiano di Tecnologia. The cover picture shows the crystal structure of a Ruddlesden-Popper quasi-2D perovskite with chemical formula (PEA)2 MA39 Pb40 I121 (with PEA: phenylethylammonium and MA: methylammonium). The subscript 40 indicates the number of PbI6 octahedra separated by a double layer of PEA cations. Such quasi-2D perovskites exhibit efficient photovoltaic performances and higher stability with respect to the pure 3D counterpart (MAPbI3 ). This article is part of the Special Collection on "Perovskite Materials and Devices". Read the full text of the article at 10.1002/cplu.202000777.

Funder: Engineering and Physical Sciences Research Council; Id: http://dx.doi.org/10.13039/501100000266, Funder: Cambridge Trust Scholarship, Funder: Robert Gardiner Scholarship, Funder: Royal Society; Id: http://dx.doi.org/10.13039/501100000288, Halide perovskite materials offer an ideal playground for easily tuning their color and, accordingly, the spectral range of their emitted light. In contrast to common procedures, this work demonstrates that halide substitution in Ruddlesden–Popper perovskites not only progressively modulates the bandgap, but it can also be a powerful tool to control the nanoscale phase segregation—by adjusting the halide ratio and therefore the spatial distribution of recombination centers. As a result, thin films of chloride‐rich perovskite are engineered—which appear transparent to the human eye—with controlled tunable emission in the green. This is due to a rational halide substitution with iodide or bromide leading to a spatial distribution of phases where the minor component is responsible for the tunable emission, as identified by combined hyperspectral photoluminescence imaging and elemental mapping. This work paves the way for the next generation of highly tunable transparent emissive materials, which can be used as light‐emitting pixels in advanced and low‐cost optoelectronics.

2D hybrid perovskites (2DP) are versatile materials, whose electronic and optical properties can be tuned through the nature of the organic cations (even when those are seemingly electronically inert). Here, it is demonstrated that fluorination of the organic ligands yields glassy 2DP materials featuring long‐lived correlated electron–hole pairs. Such states have a marked charge‐transfer character, as revealed by the persistent Stark effect in the form of a second derivative in electroabsorption. Modeling shows that electrostatic effects associated with fluorination, combined with the steric hindrance due to the bulky side groups, drive the formation of spatially dislocated charge pairs with reduced recombination rates. This work enriches and broadens the current knowledge of the photophysics of 2DP, which will hopefully guide synthesis efforts toward novel materials with improved functionalities., An anomalous, persistent, Stark effect from electroabsorption and nanosecond transient absorption measurement related to the presence of long‐lived screened electron–hole pairs in 2D perovskite is presented. A peculiarity in these materials, contrasting with large exciton binding energies, is presented, revealing a complex photophysical response and highlighting the role of cation's chemistry on the material's optical response.

This short review aims at summarizing the current challenges related to poor Perovskite Solar Cells (PSCs) stability which nowadays puts severe constrains on near future device commercialization. As a game changer in the field of photovoltaics (PVs), PSCs are highly efficient and cheap to fabricate. However, they suffer from poor long-term stability upon exposure to heat, moisture, oxygen and light, and combinations thereof. Poor device stability originates from intrinsic instability issues of the perovskite active layer itself, as well as extrinsic factors due to partial degradation of the layers composing the device stack. Here we briefly review the chemical and physical processes responsible for intrinsic material instability, and we highlight possible solutions to overcome it; we then consider the whole device, discussing properties and interactions of the stacked layers. Finally, particular emphasis is placed on the need of shared standards for stability tests, which should include detailed report on experimental conditions over a statistically significant number of samples, allowing for a direct comparison of results across different groups and fostering a rapid advance of our understanding of degradation mechanisms and of the solutions to overcome them.

International audience; Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)2PbI4 and (Lf)2PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.

Surface passivation of the perovskite photo absorber is a key factor to improve the photovoltaic performance. So far robust passivation strategies have not yet been revealed. Here, we demonstrate a successful passivation strategy which controls the Fermi-level of the perovskite surface by improving the surface states. Such Fermi-level control caused band-bending between the surface and bulk of the perovskite, which enhanced the hole-extraction from the absorber bulk to the HTM side. As an added benefit, the inorganic passivation layer improved the device light stability. By depositing a thick protection layer on the complete device, a remarkable waterproofing effect was obtained. As a result, an enhancement ofVOCand the conversion efficiency from 20.5% to 22.1% was achieved. We revealed these passivation mechanisms and used perhydropoly(silazane) (PHPS) derived silica to control the perovskite surface states.

Hybrid perovskites are currently one of the most active fields of research owing to their enormous potential for photovoltaics. The performance of 3D hybrid organic–inorganic perovskite solar cells has increased at an incredible rate, reaching power conversion efficiencies comparable to those of many established technologies. However, the commercial application of 3D hybrid perovskites is inhibited by their poor stability. Relative to 3D hybrid perovskites, low-dimensional — that is, 2D — hybrid perovskites have demonstrated higher moisture stability, offering new approaches to stabilizing perovskite-based photovoltaic devices. Furthermore, 2D hybrid perovskites have versatile structures, enabling the fine-tuning of their optoelectronic properties through compositional engineering. In this Review, we discuss the state of the art in 2D perovskites, providing an overview of structural and materials engineering aspects and optical and photophysical properties. Moreover, we discuss recent developments along with the main limitations of 3D perovskites and assess the advantages of 2D perovskites over their 3D parent structures in terms of stability. Finally, we review recent achievements in combining 3D and 2D perovskites as an approach to simultaneously boost device efficiency and stability, paving the way for mixed-dimensional perovskite solar cells for commercial applications. Combining low-dimensional and 3D perovskites is a promising approach to achieve stable and efficient solar cells. In this Review, we discuss the structural, optical and photophysical properties of low-dimensional perovskites, compare the stability and efficiency of 2D and 3D perovskite devices, and consider 2D/3D composites as a strategy to increase the stability of perovskite solar cells.

The power conversion efficiency of perovskite solar cells is drastically affected by photocarrier dynamics at the interfaces. Experimental measurements show quenching of the photoluminescence (PL) ...

Two dimensional (2D) organic-inorganic hybrid perovskites have recently attracted enormous attention due to their higher environmental stability with respect to three-dimensional (3D) perovskites and larger structural tunability. The layered structure relaxes constraints on the dimensions of the organic cations that alternate the inorganic sheets, opening up a large choice on the organics, ultimately enabling the creation of tunable layered perovskites. Here, we report on a series of fluorous cations, varying in size and shape, as building blocks for a new family of fluorous 2D lead-iodide perovskites. These display a large tunability in the optical and dielectric properties depending on the structure of the fluorous cations. Importantly, despite the invariant inorganic framework, the 2D perovskite electronic structure is strongly affected by the cation size. The longer the cation, the smaller the 2D perovskite band gap and the exciton binding energy (reducing from 400 meV down to 130 meV). Such variation is induced by the strain in the inorganic sheet, resulting in a more dispersed valence and conduction bands, in turn yielding a smaller band gap. In addition, a smaller effective mass for the 2D perovskite with the longest cation is calculated, for which improved transport properties are anticipated. Importantly, the fluorous moiety confers extreme stability to the 2D perovskite and enhances the hydrophobic character of the perovskite surface, which remains perfectly stable for more than one month in ambient conditions.

Hybrid perovskite solar cells have been capturing an enormous research interest in the energy sector due to their extraordinary performances and ease of fabrication. However, low device lifetime, mainly due to material and device degradation upon water exposure, challenges their near-future commercialization. Here, we synthesized a new fluorous organic cation used as organic spacer to form a low-dimensional perovskite (LDP) with an enhanced water-resistant character. The LDP is integrated with three-dimensional (3D) perovskite absorbers in the form of MA(0.9)FA(0.1)PbI(3) (FA = NH2CH = NH2+, MA = CH3NH3+) and CS(0.1)FA(0.74)MA(0.13)PbI(2.48)Br(0.39). In both cases, a LDP layer self-assembles as a thin capping layer on the top of the 3D bulk, making the perovskite surface hydrophobic. Our easy and robust approach, validated for different perovskite compositions, limits the interface deterioration in perovskite solar cells yielding to >20% power conversion efficient solar cells with improved stability, especially pronounced in the first hours of functioning under environmental conditions. As a consequence, single and multijunction perovskite devices, such as tandem solar cells, can benefit from the use of the waterproof stabilization here demonstrated, a concept which can be further expanded in the perovskite optoelectronic industry beyond photovoltaics.

An understanding of the improvements achieved in the use of cosolvents for methylammonium lead triiodide (MAPbI3):C70 blend films (MAPbI3:C70) processing is presented. A comparative study using aromatic (i.e., o-xylene and o-dichlorobenzene) and aliphatic (i.e., methylcyclohexane and chlorocyclohexane) cosolvents proves the nature of the cosolvent interacting with fullerene to be determining for achieving enhanced devices. UV–vis spectra of the different C70 solutions suggest a major impact of the solute–aromatic solvent interactions on the optoelectronic properties. The effect of aromatic and aliphatic solvents in the electronic structure of C70 crystals, obtained from the different solutions, is indeed demonstrated by electron energy loss spectroscopy. Morphological studies show elimination of pinholes (field emission scanning electron microscopy) and different nanometric features related to fullerene (atomic force microscopy) in MAPbI3:C70 blend films processed using aromatic cosolvents. A severe quenc...

With a power conversion efficiency (PCE) exceeding 22 %, perovskite solar cells (PSCs) have thrilled photovoltaic research. However, the interface behavior is still not understood and is a hot topic of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, which makes perovskite interface physics intriguing. Herein, through femtosecond transient absorption spectroscopy with spectral coverage extending into the crucial IR region, the ultrafast interface-specific processes at standard and newly molecularly engineered perovskite interfaces in state-of-the-art PSCs are interrogated. Ultrafast interfacial charge injection occurs with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the timescale for the first step involved in the complex charge-transfer process. This is also true for 20 % efficient devices measured under real operation, for which the femtosecond injection is followed by a slower picosecond component. These findings provide compelling evidence for the femtosecond interfacial charge-injection step and demonstrate a robust method for the straightforward identification of interfacial non-equilibrium processes on the ultrafast timescale. Fil: Grancini, Giulia. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Viola, Daniele. Politecnico di Milano; Italia Fil: Lee, Yonghui. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Saliba, Michael. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Paek, Sanghyun. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Cho, Kyung Taek. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Orlandi, Simonetta. Consiglio Nazionale delle Ricerche; Italia Fil: Cavazzini, Marco. Consiglio Nazionale delle Ricerche; Italia Fil: Fungo, Fernando Gabriel. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina Fil: Hossain, Mohammad I.. Qatar Environment And Energy Research Institute; Qatar Fil: Belaidi, Abdelhak. Qatar Environment And Energy Research Institute; Qatar Fil: Tabet, Nouar. Qatar Environment And Energy Research Institute; Qatar Fil: Pozzi, Gianluca. Consiglio Nazionale delle Ricerche; Italia Fil: Cerullo, Giulio. Politecnico di Milano; Italia Fil: Nazeeruddin, Mohammad Khaja. Ecole Polytechnique Federale de Lausanne; Suiza

Through judicious molecular engineering, novel dopant-free star-shaped D-pi-A type hole transporting materials coded KR355, KR321, and KR353 were systematically designed, synthesized and characterized. KR321 has been revealed to form a particular face-on organization on perovskite films favoring vertical charge carrier transport and for the first time, we show that this particular molecular stacking feature resulted in a power conversion efficiency over 19% in combination with mixed-perovskite (FAPbI(3))(0.85)(MAPbBr)(0.15). The obtained 19% efficiency using a pristine hole transporting layer without any chemical additives or doping is the highest, establishing that the molecular engineering of a planar donor core, p-spacer and periphery acceptor leads to high mobility, and the design provides useful insight into the synthesis of next-generation HTMs for perovskite solar cells and optoelectronic applications.

A flexible, saddle-like, π-conjugated skeleton composed of four fused thiophene rings forming a cyclooctatetrathiophene (CoTh) with four triphenylamines (CoTh-TTPA) is presented as a hole-transporting material (HTM) for perovskite solar cells. The new HTM shows a bright red color stemming from a direct conjugation between the TPA groups and the central CoTh scaffold. This results in a charge transfer band due to the combination of the weak acceptor moiety, the CoTh unit, and the electron-donating p-methoxytriphenylamine groups. CoTh-TTPA exhibits a suitable highest-occupied molecular orbital (HOMO) level in relation to the valence band edge of the perovskite, which ensures efficient hole extraction at the perovskite/HTM interface. It has been applied as the HTM in combination with a mixed perovskite ([FAPbI3]0.85[MAPbBr3]0.15) and a state-of-the-art triple cation perovskite ([(FAPbI3)0.87(MAPbBr3)0.13]0.92[CsPbI3]0.08) reaching noticeable light-to-energy conversion efficiencies of 16.3 and 15.9%, respectively. These values are slightly lower than those measured for the benchmark spiro-OMeTAD HTM. The HTM properties have been analyzed by means of photoluminescence and conductivity experiments, which demonstrated a better hole extraction and conductivity for spiro-OMeTAD.

CuI and CuSCN as Hole Transport Materials for Perovskite Solar Cells Vinod E. Madhavan1*, Ahmer Ali Bozdar Baloch1, Afsal Manekkathodi1, Dhanasekaran Thirunvukkaarasu1, I. Zimmermann2, C. Roldán- Carmona2, G. Grancini2, M. Buffiere1, Mohammad Khaja Nazeeruddin2, A. Belaidi1 and Nouar Tabet1 1 Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar Foundation, P. O. Box 5825, Doha, Qatar 2 Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland *vmadhavan@qf.org.qa. ABSTRACT: Perovskite based solar cell is an important area of research for solar energy harvesting and for green energy revolution. The materials used for a thin film solar cell are considerably less with respect to the prevailing silicon based solar cells. There is an interest in copper based inorganic materials, especially copper iodide (CuI) and copper thiocyanate (CuSCN) as hole transport materials (HTMs) in perovskite solar cells (PSCs) for efficient, inexpensive and stable energy harvesting. Copper based films exhibit higher conductivity and wider-band-gap. The existing organic HTMs are expensive and have low hole mobility. The reported high power conversion efficiency (PCE) of CuI and CuSCN based perovskite solar cells are 6% and 20.3% respectively [Christians et al. J. Am. Chem. Soc., 2014, N Arora et al. Science 2017]. The perovskite solar cells based on inorganic CuI and CuSCN can be more stable and cost-effective with respect to spiro-OMeTAD HTM. We present our work on mixed perovskite in the form of (FAPbI3)0.85(MAPbBr3)0.15 in combination with CuI and CuSCN HTMs that lead to efficiencies 5.16% and 15.43% respectively in an n-i-p architecture under full sun illumination. The CuI based device displayed a short-circuit current density (Jsc) of 8.98 mA/cm2, Voc of 898.57 mV and fill factor (FF) of 0.64. Under similar conditions, the device with CuSCN, showed a significant increase in the Jsc (8.95 mA/cm2 to 20.86 mA/cm2) and Voc (from 898.57 mV to 1060 mV) (Figure1). A reference cell with standard spiro-OMeTAD HTM showed a PCE of 19.65% (with Voc of 1180 mV and Jsc of 22.70 mA/cm2). The high Jsc when using CuSCN with respect to CuI is mainly due to the effective charge extraction between the perovskite and CuSCN. However there is a strong quenching in the photoluminescence measurements in both CuI and CuSCN cases, which suggests that excellent hole injection is taking place from the perovskite active layer into the CuI and CuSCN hole transport materials (Figure 2). It is worth noting that CuI based perovskite devices showed a PCE of 6% with excellent photocurrent stability and 2 orders higher electrical conductivity that lead to higher fill factors [Christians et al. J. Am. Chem. Soc., 2014]. Impedance spectroscopy measurements revealed high recombination rate in CuI devices [Huangfu et al, Applied Surface Science 2015]. This could explain the low Voc and Jsc despite the quenching of the photoluminescence spectrum in the presence of CuI and CuSCN. Further studies are in progress in-order to find out the origin of the recombination and how to remediate them. Various device structure models (p-i-n and n-i-p geometry) with copper based HTMs are simulated with SCAPS software to find out the efficient structures with minimum losses. The results shows that the CuSCN based cells are more promising and can be used to prepare high efficiency perovskite solar cells. References J.A. Christians, R.CM. Fung, P.V. Kamat, J. Am. Chem. Soc. 2014, 136, 2, 758–764 pp. N. Arora, M. I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S.M. Zakeeruddin, M. Grätzel, Science 2017,10.1126/science.aam5655. M. Huangfu, Y. Shen, G. Zhu, K. Xu and M. Cao*,F. Gu and L. Wang, Appl. Surf. Sci. 2015, 357-B, 2234–2240pp.

Moisture, in the form of ambient humidity, has a significant impact on methylammonium lead halide perovskite films. In particular, due to the hygroscopic nature of the methylammonium component, moisture plays a significant role during film formation. This issue has so far not been well understood and neither has the impact of moisture on the physical properties of resultant films. Herein, we carry out a comprehensive and well-controlled study of the effect of moisture exposure on methylammonium lead halide perovskite film formation and properties. We find that films formed in higher humidity atmospheres have a less continuous morphology but significantly improved photoluminescence, and that film formation is faster. In photovoltaic devices, we find that exposure to moisture, either in the precursor solution or in the atmosphere during formation, results in significantly improved open-circuit voltages and hence overall device performance. We then find that by post-treating dry films with moisture exposure, we can enhance photovoltaic performance and photoluminescence in a similar way. The enhanced photoluminescence and open-circuit voltage imply that the material quality is improved in films that have been exposed to moisture. We determine that this improvement stems from a reduction in trap density in the films, which we postulate to be due to the partial solvation of the methylammonium component and "self-healing" of the perovskite lattice. This work highlights the importance of controlled moisture exposure when fabricating high-performance perovskite devices and provides guidelines for the optimum environment for fabrication. Moreover, we note that often an unintentional water exposure is likely responsible for the high performance of solar cells produced in some laboratories, whereas careful synthesis and fabrication in a dry environment will lead to lower-performing devices.

The performance of hybrid solar cells is strongly affected by the device morphology. In this work, we demonstrate a poly(3-hexylthiophene-2,5-diyl)/TiO2 hybrid solar cell where the TiO2 photoanode comprises an array of tree-like hyperbranched quasi-1D nanostructures self-assembled from the gas phase. This advanced architecture enables us to increase the power conversion efficiency to over 1%, doubling the efficiency with respect to state of the art devices employing standard mesoporous titania photoanodes. This improvement is attributed to several peculiar features of this array of nanostructures: high interfacial area; increased optical density thanks to the enhanced light scattering; and enhanced crystallization of poly(3-hexylthiophene-2,5-diyl) inside the quasi-1D nanostructure.

Organic–inorganic lead halide perovskites have shown photovoltaic performances above 20% in a range of solar cell architectures while offering simple and low-cost processability. Despite the multiple ionic compositions that have been reported so far, the presence of organic constituents is an essential element in all of the high-efficiency formulations, with the methylammonium and formamidinium cations being the sole efficient options available to date. In this study, we demonstrate improved material stability after the incorporation of a large organic cation, guanidinium, into the MAPbI3 crystal structure, which delivers average power conversion efficiencies over 19%, and stabilized performance for 1,000 h under continuous light illumination, a fundamental step within the perovskite field.

Perovskite solar cells (PSCs) have experienced an outstanding advance in power conversion efficiency (PCE) by optimizing the perovskite layer morphology, composition, interfaces, and charge collection efficiency. To enhance PCE, here we developed a new method i.e., engineering a compositional gradient thinly at the rear interface between the perovskite and the hole transporting materials. We demonstrate that charge collection is improved and charge recombination is reduced by formation of an engineered passivating layer, which leads to a striking enhancement in open-circuit voltage (VOC). The passivation effect induced by constructing an additional FAPbBr3−xIx layer on top of the primary (FAPbI3)0.85(MAPbBr3)0.15 film was proven to function as an electron blocking layer within the perovskite film, resulting in a final PCE of 21.3%. Our results shed light on the importance of the interfacial engineering on the rear surface of perovskite layers and describe an innovative approach that will further boost the PSC efficiency.

Complex compositional engineering of mixed halides/mixed cations perovskites has recently fostered a rapid progress in perovskite solar cell technology. Here we demonstrate that when 10% of formamidinium (FA+) is simply added into methylammonium lead iodide (MAPbI3) a highly crystalline and compositionally uniform perovskite is formed, self-organizing into a stable “quasi-cubic” phase at room temperature. We reached power conversion efficiency of over 20.2%, the highest value reported to date for FAxMA1−xPbI3 perovskite.

Easily accessible tetra-​5-​hexylthiophene-​, tetra-​5-​hexyl-​2,​2'-​bisthiophene-​substituted zinc phthalocyanines (ZnPcs) and tetra-​tert-​Bu ZnPc are employed as hole-​transporting materials in mixed-​ion perovskite [HC(NH2)​2]​0.85(CH3NH3)​0.15Pb(I0.85Br0.15)​3 solar cells, reaching the highest power conversion efficiency (PCE) so far for phthalocyanines. Results confirm that the photovoltaic performance is strongly influenced by both, the individual optoelectronic properties of ZnPcs and the aggregation of these tetrapyrrolic semiconductors in the solid thin film. The optimized devices exhibit PCE of 15.5​% when using tetra-​5-​hexyl-​2,​2'-​bisthiophene substituted ZnPcs, 13.3​% for tetra-​tert-​Bu ZnPc, and a record 17.5​% for tetra-​5-​hexylthiophene-​based analog under std. global 100 mW cm-​2 AM 1.5G illumination. These results boost up the potential of soln.-​processed ZnPc derivs. as stable and economic hole-​transport materials for large-​scale applications, opening new frontiers toward a realistic, efficient, and inexpensive energy prodn.

Despite the impressive photovoltaic performances with power conversion efficiency beyond 22%, perovskite solar cells are poorly stable under operation, failing by far the market requirements. Various technological approaches have been proposed to overcome the instability problem, which, while delivering appreciable incremental improvements, are still far from a market-proof solution. Here we show one-year stable perovskite devices by engineering an ultra-stable 2D/3D (HOOC(CH2)4NH3)2PbI4/CH3NH3PbI3 perovskite junction. The 2D/3D forms an exceptional gradually-organized multi-dimensional interface that yields up to 12.9% efficiency in a carbon-based architecture, and 14.6% in standard mesoporous solar cells. To demonstrate the up-scale potential of our technology, we fabricate 10 × 10 cm2 solar modules by a fully printable industrial-scale process, delivering 11.2% efficiency stable for >10,000 h with zero loss in performances measured under controlled standard conditions. This innovative stable and low-cost architecture will enable the timely commercialization of perovskite solar cells., Up-scaling represents a key challenge for photovoltaics based on metal halide perovskites. Using a composite of 2D and 3D perovskites in combination with a printable carbon black/graphite counter electrode; Grancini et al., report 11.2% efficient modules stable over 10,000 hours.

Organic-inorganic halide perovskites, such as CH3NH3PbX3 (X = I(-), Br(-), Cl(-)), are attracting growing interest to prepare low-cost solar cells that are capable of converting sunlight to electricity at the highest efficiencies. Despite negligible effort on enhancing materials' purity or passivation of surfaces, high efficiencies have already been achieved. Here, we show that trap states at the perovskite surface generate charge accumulation and consequent recombination losses in working solar cells. We identify that undercoordinated iodine ions within the perovskite structure are responsible and make use of supramolecular halogen bond complexation to successfully passivate these sites. Following this strategy, we demonstrate solar cells with maximum power conversion efficiency of 15.7% and stable power output over 15% under constant 0.81 V forward bias in simulated full sunlight. The surface passivation introduces an important direction for future progress in perovskite solar cells.

Device optimization and ultrafast absorption spectroscopic investigations on the role of semiconducting polymer as hole-transport layer in solid-state dye sensitized solar cells suggest their dual role of dye-regeneration and light-antenna assisting in improved photoconversionefficiencies.© 2012 OSA.

In this work we systematically investigated the role of reduced graphene oxide (rGO) in hybrid perovskite solar cells (PSCs). By mixing rGO within the mesoporous TiO2 (m-TiO2) matrix, highly efficient solar cells with power conversion efficiency values up to 19.54 % were realized. In addition, the boosted beneficial role of rGO with and without Li-treated m-TiO2 is highlighted, improving transport and injection of photoexcited electrons. This combined system may pave the way for further development and optimization of electron transport and collection in high efficiency PSCs.

We report a combined experimental and computational investigation to elucidate the effect of PbI2 on the electronic properties of the TiO2/methylammonium lead-iodide (MAPbI3) perovskite interface. We propose that excess PbI2 from the precursor solution or from a TiO2 pre-treatment may be predominantly located at the interface with the semiconducting oxide, enhancing the interfacial electronic coupling and favorably modifying the electronic energy levels landscape at the interface. This is in turn found to facilitate electron transfer from the MAPbI3 perovskite to the TiO2 electron transport layer as revealed by a fast quenching in the photoluminescence dynamics, confirming the beneficial role of PbI2 at the interface.

A new Zn(II) phthalocyanine (Pc) based low bandgap HTM is introduced for perovskite solar cells. Steady state and time-resolved photoluminescence (PL) measurements indicated an evenly matched hole extraction efficiency between sym-HTPcH and spiro-OMeTAD. On account of the low film quality and resulting high recombination, Zn(II) Pc normally cannot work as an effective HTM. We adopted insulating Al2O3 for the infiltration of sym-HTPcH to form a hybrid interfacial buffer layer, affording perovskite solar cells (PSCs) with an average PCE value of up to 12.3%, which is a significant improvement with respect to the control cell without the meso-Al2O3 layer (4.21%) and is the highest value ever reported for Zn(II) phthalocyanine based devices under AM1.5G standard conditions. A hysteresis test revealed that our device structure with the new HTM exhibited a balanced charge extraction behaviour.

A tremendous advancement in the performances of bulk heterojunction organic solar cells has been motivating a comprehensive investigation of many fundamental aspects regarding the structure–function relationship of these devices. The blend morphology has a crucial role in determining the device operation, affecting both charge separation and transport properties. Despite extensive spectroscopic investigations have been carried out to understand the fundamental photophysical processes, the charge separation mechanisms are still debated. Here we use ultrafast pump–probe spectroscopy to monitor directly photoexcited states dynamics in a promising polymer blend for photovoltaics, based on a low band gap polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), integrated with a fullerene derivative, [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We find that exciton quenching leads directly to two charge populations: charge-transfer sta...

Solution-processed bulk heterojunction (BHJ) based on electron-donor (D) polymer and acceptor (A) fullerene is a promising technology for organic photovoltaics. Geminate charge recombination is regarded as one of the main loss mechanisms limiting device performances. This stems from the dynamics of the initial charge transfer state (CTS), which depend on the blend morphology, the molecular conformation, and the energetics of the D:A interface. Here we study the photophysics of a crystalline phase-separated blend of regioregular poly(3-hexylthiophene) (P3HT) with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with a coarsened morphology, by mapping the transient absorption signal with submicrometer space and subpicosecond time resolution. At the P3HT:PCBM interface, we detect a long-lived photoinduced dynamic that we assign to a peculiar coherent CTS forming in ∼10 ps, not affected by geminate recombination and characterized by a different polarization with respect to the one in the usual polydispersed ...

Quantum dots (QDs) represent one of the most promising materials for third-generation solar cells due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine-tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next-generation photovoltaic (PV) devices.

We have developed a room-temperature solution processing approach to integrate colloidal anatase titanium dioxide nanorods (TiO2 NRs) and lead sulfide quantum dots (PbS QDs) into a heterostructured p-n junction device. To this aim we have exploited a post-deposition treatment to remove surface-adsorbed ligands by means of UV-light-irradiation of TiO2 NRs and a dilute acid treatment of PbS QDs. Here we report a systematic study on the optimization of the post-deposition treatments and device fabrication. Our approach is fully compatible with plastic device technology and is potentially useful for the integration of crystalline TiO2 as active component into disparate solar cell architectures and organic optoelectronic devices. © 2013 Elsevier B.V. All rights reserved.

Here we identify structural inhomogeneity on a micrometer scale across the surface of a CH3NH3PbI3 perovskite single crystal. At the crystal edge a local distortion of the crystal lattice is responsible for a widening of the optical bandgap and faster photo-carrier recombination. These effects are inherently present at the edge of the crystal, and further enhanced upon water intercalation, as a preliminary step in the hydration of the perovskite material.

A room-temperature low-cost TiS2 p-type contact material is applied for the first time as the hole-transporting material (HTM) in perovskite solar cells, with power conversion efficiencies surpassing 13.5%. Synthesized by a simple two-step hot-injection method, it presents a much lower price per m2 than octakis(4-methoxyphenyl)-9,9′-spirospirobi[9H-fluorene]-2,2′,7,7′-tetramine (spiro-OMeTAD), 30 times lower in price ($0.046 for TiS2 and $1.36 for spiro-OMeTAD at 13.54% efficiency), standing out over most of the reported HTM alternatives.

The interpenetrating morphology formed by the electron donor and acceptor materials is critical for the performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaic (PV) cells. In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c]1, 2, 5 thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative. Grazing incidence X-ray scattering measurements reveal the lower-ordered nature of the BHJ system as well as an intermixing morphology with intercalation of fullerene molecules between the PFBT-T20TT lamella. Steady-state and transient photo-induced absorption spectroscopy reveal ultrafast charge transfer (CT) at the PFBT-T20TT/fullerene interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner. Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain. Published version

Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50 meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.

In this work, we report on similar to 4% power conversion efficiency (PCE) depleted bulk heterojunction (DBH) solar cells based on a high-quality electrode with a three-dimensional nanoscale architecture purposely designed so as to maximize light absorption and charge collection. The newly conceived architecture comprises a mesoporous electron-collecting film made of networked anisotropic metal-oxide nanostructures, which accommodates visible-to-infrared light harvesting quantum dots within the recessed regions of its volume. The three-dimensional electrodes were self-assembled by spin-coating a solution of colloidal branched anatase TiO2 NCs (BNC), followed by photocatalytic removal of the native organic capping from their surface by a mild UV-light treatment and filling with small PbS NCs via infiltration. The PCE similar to 4% of our TiO2 BNC/PbS QD DBH solar cell features an enhancement of 84% over the performance obtained for a planar device fabricated under the same conditions. Overall, the DBH device fabrication procedure is entirely carried out under mild processing conditions at room temperature, thus holding promise for low-cost and large-scale manufacturing.