More stable and less lead for perovskite solar cells – MORELESS

The first strategy consists in the search for lead deficient HPs materials (d-HPs) (A,A’)1+xPb1-xI3-x (A, A’: organic cation); this new type of hybrid perovskites, patented by partners 1 and 3, which contain less lead while keeping a 3D architecture and which are more stable than MAPI, offers increased flexibility of its chemical composition. We propose to focus on this new type of hybrid perovskite by preparing new materials through substitutions on the A, A’, Pb and I sites. We further expect that such lead and iodide deficient materials may favor the substitution of Pb2+ by Bi3+ (3 Pb2+ = 2 Bi3+). Finally, we are also convinced that other kinds of d-HPs materials can be discovered.
The second strategy seeks for lead-free materials based on iodobismuthate (or iodoantimonate) networks. These materials are known to be stable and easily prepared as thin films. While non-perovskite compounds have been mainly used for PSCs applications, we propose to focus on perovskite network based materials, particularly the rare but known 1D anion of trans corner-sharing octahedra and the (deficient) 2D perovskite network Bi2/3I4-, reported once by selecting organic cations able to self-assemble through weak interactions. Bi3+(Sb3+)-containing 3D structures will also be targeted using neutral molecule that will able to template the formation of perovskite networks or by using a monovalent cation A+ which will lead to bismuth(antimony)-rich M3+/A+ perovskite networks.
Improvement of the performances of the first set of HPs will be fostered through an integrative and synergetic approach closely involving characterization of materials up to devices as well as modeling necessary to afford fundamental understanding. Finally, keeping in mind that the stability of new materials is an important issue of MORELESS, solar cell efficiencies as high as 17 % for lead-deficient HPs materials and 7 % for lead-free materials are targeted for this project.

The MA+ cation, present in d-MAPI, has been substituted by formamidinium (FA+), a cation of comparable size, known to lead to compounds that are more stable to moisture. The compounds (FA, HEA)1+xPb1-xI3-x and (FA,TEA)1+xPb1-xI3-x (d-FAPI) (TEA+ = thioethylammonium) were thus obtained in pure phase in the form of crystals, crystallized powders and thin layers for several x values, and characterized by x-ray diffraction, absorption spectroscopy and solid state NMR. Thin films are extremely stable under ambient conditions compared to a-FAPI or (Cs, FA) PbI3 references. DFT calculations carried out on structural models built from the experimental structure, well reproduce the evolution of the (direct) gap observed as a function of the x value. These results involving the 4 project partners were published in ACS Applied Materials and Interfaces in 2019 (DOI :10.1021/acsami.9b00210).
Perovskite solar Cells were prepared using d-MAPI-HEA compounds discovered initially. Their development and optimization led for the d-MAPI-HEA0.10 phase to an average efficiency of 8% with a record cell at 10.28%. More recently, the d-FAPI-TEA0.04 system has been the subject of a similar study and optimization. This work resulted in 7.84% efficiency on average and a record cell of 8.22%.
The chemistry of lead-free materials was also explored, in particular by selecting two types of cations derived from the methylviologen (MV2+). In fact, as the known (MV)BiI5 compound, suitable for PSC, but insoluble in solvents used for spin-coating, derivative cations were prepared. Thus, by substituting the methyl group with the amino group (aminoviologen, AmV2+), the compound (AmV)BiI5, exhibiting a broad band absorption in the visible region, was obtained in the form of a thin layer. Initial PSC tests have shown the photovoltaic effect with, however, low photoconversion yields (<1%).

We plan to prepare and characterize by X-ray diffraction, solid-state NMR, absorption of other d-HPs (A, A'ouA''0.5)1+xPb1-xI3-x with A = MA+ or FA+, and a monocation A'+ of the X-(CH2) 2NH3+ type (like HEA+ (X = OH) or TEA+ (X = SH)) with X = CN (CNEA+), I (IEA+), or a dication A''2+, cystaminium (NH3(CH2)2SS(CH2)2NH3)2+ (cys2+) or (NH3CH2CHOHCH2NH3)2+ (dic2+). The materials should be prepared as thin layers and their stability should be assessed. These various phases will have to be tested in solar cells. Of particular interest is the system with IEA+. In fact, in the form of crystals, 2D phases of RP type (n = 1-5) are formed, while in the form of films d-HP phases seem to form preferentially (study in progress). For comparison, the system based on iodopropylammonium (I(CH2)3NH3+) will also be explored. Finally, DFT calculations will be undertaken on this series, in particular to analyze the effect of the presence of a halogen in the organic barrier.

1) Enhanced Stability and Band Gap Tuning of a-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration. A.Leblanc, N. Mercier,* Magali Allain, Jens Dittmer, Thierry Pauporte´, V. Fernandez, F. Boucher, M. Kepenekian, C. Katan* ; ACS Appl. Mater. Interfaces 2019, 11,20743-20751
2) C. Zheng, O. Rubel, M. Kepenekian, X. Rocquefelte, C. Katan, Electronic properties of Pb-I deficient lead halide perovskites, J. Chem. Phys. 151, 234704 (2019)
3) 2. C. Quarti, C. Katan, J. Even, Physical properties of bulk, defective, 2D and 0D metal halide perovskite semiconductors from a symmetry perspective, J. Phys. Mater. 3, 042001 (2020)

Since a few years, the perovskite solar cells (PSCs) have emerged as a new technology for next-generation photovoltaics. These materials, as exemplified by the archetypal methyl-ammonium lead tri-iodide (CH3NH3)PbI3 (MAPI), have several key advantages: PSCs can be prepared using solution processing at temperature not exceeding 150°C, and their power conversion efficiencies (PCE) reach over 22%. However, PSCs have two main drawbacks: they contain the toxic lead element, and they exhibit chemical instabilities to moisture, oxygen, light, etc.
In this context, the central goal of the MORELESS project is to develop new materials belonging to the family of halide perovskites, suitable for light absorption in PV devices and offering improved stability (“MORE stable” than MAPI) while alleviating the most troubling issue of toxicity (“LESS lead” than MAPI). MORELESS will implement two different strategies. The first strategy consists in the search for lead deficient HPs materials (d-HPs). This new type of hybrid perovskites, (A,A’)1+xPb1-xI3-x (A, A’, organic monocations), discovered recently by the PI, contains less lead while keeping a 3D architecture, is more stable than MAPI and offers increased flexibility of its chemical composition. We propose to focus on this new type of hybrid perovskite by preparing new materials through substitutions on the A, A’, Pb and I sites. MORELESS also aims at discovering new kinds of d-HPs materials. The second strategy seeks for lead-free materials based on iodobismuthate or iodoantimonate networks. These materials are known to be stable and easily prepared as thin films. While non-perovskite compounds have been mainly used for PSCs applications, we propose to focus on 1D and 2D perovskite networks (corner-sharing octahedra) based materials. The next targets will be stabilization of 3D perovskite NMI3 (M= Bi3+/Sb3+), using neutral molecule N, consistently with recent predictions, as well as monovalent cation such as Ag+ in order to stabilize bismuth(antimony)-rich M3+/Ag+ perovskite networks. Once interesting materials will be obtained and characterized (X-ray, NMR, and others), thin films will be prepared to afford well-crystallized, fully covering, efficient light absorbing and adherent thin films. These layers will be characterized by XRD, SEM, EDX, AFM and XPS. Then the PSCs will be prepared and the cell performances determined for the various new perovskites (e.g., J-V curve measurements and impedance spectroscopy). For the best materials, other full studies will be carried out, particularly the aging of the layers will be followed by several techniques. First principles calculations and modeling will be performed both to support the interpretation of available experimental findings, including NMR data, and provide guidance to determine the choice of the next synthetic targets. Available structural data will allow investigation of electronic and optical properties in relation with experimental outcomes and DFT methods will provide complementary insight for foreseen atomic substitutions.
MORELESS is a collaborative project between partners having experience in the field of HPs and a strong expertise in complementary fields that are essential for successful outcome. This multidisciplinary project includes chemistry of materials (task 1), preparation and characterizations of PSCs (task 2) and modeling (task 3). At Moltech-Anjou (Angers, partner 1), the design, and the preparation of materials as well as X-ray characterizations will be assumed by N. Mercier, the coordinator of the project. At the IMMM institute (Le Mans, partner 2), the solid state NMR characterization of materials will be performed by J. Dittmer. In the MPOE-IRCP group of T. Pauporté (Chimie ParisTech, partner 3), material shaping, notably the electrical and optical characterizations and solar cell measurements and aging issue will be carried out. At ISCR (Partner 4) C. Katan will coordinate the theoretical work.