A π-extended triphenylamine based dopant-free hole-transporting material for perovskite solar cells via heteroatom substitution

Asymmetric Triphenylethylene-Based Hole Transporting Materials for Highly Efficient Perovskite Solar Cells

Molecular hole-transporting materials (HTMs) having triphenylethylene central core were designed, synthesized, and employed in perovskite solar cell (PSC) devices. The synthesized HTM derivatives were obtained in a two- or three-step synthetic procedure, and their characteristics were analyzed by various thermoanalytical, optical, photophysical, and photovoltaic techniques. The most efficient PSC device recorded a 23.43% power conversion efficiency. Furthermore, the longevity of the device employing V1509 HTM surpassed that of PSC with state-of-art spiro-OMeTAD as the reference HTM.

D-π-A-Type Pyrazolo[1,5-a]pyrimidine-Based Hole-Transporting Materials for Perovskite Solar Cells: Effect of the Functionalization Position

Donor−acceptor (D−A) small molecules are regarded as promising hole-transporting materials for perovskite solar cells (PSCs) due to their tunable optoelectronic properties. This paper reports the design, synthesis and characterization of three novel isomeric D-π-A small molecules PY1, PY2 and PY3. The chemical structures of the molecules consist of a pyrazolo[1,5-a]pyrimidine acceptor core functionalized with one 3,6-bis(4,4′-dimethoxydiphenylamino)carbazole (3,6-CzDMPA) donor moiety via a phenyl π-spacer at the 3, 5 and 7 positions, respectively. The isolated compounds possess suitable energy levels, sufficient thermal stability (Td > 400 °C), molecular glass behavior with Tg values in the range of 127−136 °C slightly higher than that of the reference material Spiro-OMeTAD (126 °C) and acceptable hydrophobicity. Undoped PY1 demonstrates the highest hole mobility (3 × 10−6 cm2 V−1 s−1) compared to PY2 and PY3 (1.3 × 10−6 cm2 V−1 s−1). The whole isomers were incorporated as doped HTMs in planar n-i-p PSCs based on double cation perovskite FA0.85Cs0.15Pb(I0.85Br0.15)3. The non-optimized device fabricated using PY1 exhibited a power conversion efficiency (PCE) of 12.41%, similar to that obtained using the reference, Spiro-OMeTAD, which demonstrated a maximum PCE of 12.58% under the same conditions. The PY2 and PY3 materials demonstrated slightly lower performance in device configuration, with relatively moderate PCEs of 10.21% and 10.82%, respectively, and slight hysteresis behavior (−0.01 and 0.02). The preliminary stability testing of PSCs is also described. The PY1-based device exhibited better stability than the device using Spiro-OMeTAD, which could be related to its slightly superior hydrophobic character preventing water diffusion into the perovskite layer.

An optimal molecule-matching co-sensitization system for the improvement of photovoltaic performances of DSSCs

Three biphenyl co-sensitizers (4OBA, 8OBA and 12OBA) with different terminal oxyalkyl chains were synthesized and co-sensitized respectively with the main dye (NP-1) in co-sensitized solar cells (co-DSSCs). The effects of the terminal oxyalkyl chains on the photophysical, electrochemical and photovoltaic properties of the co-DSSCs were systematically investigated. The optimal molecular matching relationship between the co-sensitizers and the main dye was obtained through density functional theory (DFT) calculations. Consequently, 4OBA has the most appropriate three-dimensional (3D) molecular structure, which could not only fill the gap between the large-size dyes but also plays a partial shielding role, inhibiting dye aggregation and electron recombination, therefore yielding the highest power conversion efficiency (PCE) for the co-DSSCs with NP-1@4OBA. This study suggests that adjusting the terminal oxyalkyl chains of the co-sensitizers can be used to enhance the intramolecular charge transfer efficiency and inhibit electron recombination, ultimately improving the photovoltaic performances of the co-DSSCs.

Rational design of small molecule hole-transporting materials with a linear π-bridge for highly efficient perovskite solar cells

Developing highly efficient small molecule hole-transporting materials (HTMs) to improve the performance of devices is one of the hot topics in the progress of perovskite solar cells (PSCs). In this work, a series of molecules are designed by utilizing benzocyclobutadiene (C1), bicyclo[6.2.0]decapentaene (C2), naphthalene (C3), biphenylene (C4), fluorene (C5), and azulene (C6) as the π-cores, and p-methoxydiphenylamine (R1), p-methoxytriphenylamine (R2) and p-methoxydiphenylamine-substituted carbazole (R3) as the peripheral groups. For isolated molecules, frontier molecule orbitals, absorption and emission spectra, Stokes shift, stability, solubility, and hole mobility are assessed by density functional theory calculations along with the Marcus theory of electron transfer. The molecules adsorbed on the surface of CH₃NH₃PbI₃ are used to simulate the interfacial properties between HTMs and perovskites. Our results indicate that varying the central bridge and the terminal groups has a remarkable influence on the properties. The designed R2-Cn (n = 3, 4, 5) have deeper HOMO levels, stronger absorption in the UV region, and larger Stokes shift than Spiro-OMeTAD. They also possess good solubility, stability, and hole mobility. The proper alignment of interfacial energy levels ensures the transfer of holes from CH₃NH₃PbI₃ to the studied molecules and blocks the backflow of electrons simultaneously. Significant charge redistributions around the interfacial region benefit the separation and transfer photogenerated electron-hole pairs. The results of the present study can be further employed in the process of synthesizing new HTMs with promising features.