Improving the Long‐Term Stability of Doped Spiro‐Type Hole‐Transporting Materials in Planar Perovskite Solar Cells

Abstract

The third generation of photovoltaic technologies has attracted the attention of the scientific community since the groundbreaking report by Miyasaka and coworkers with the introduction of perovskite solar cells (PSCs). [1] In just a few years of development, PSCs have reached exceptional power conversion efficiencies (PCEs) passing from the initial 3.8% in 2009 to the recently certified 25.5%. [2] Organic-inorganic metal halide perovskites with the chemical formula ABX 3 (where A ¼ small organic cation, such as methylammonium (MA) or formamidinium (FA), B ¼ Pb 2þ , and X ¼ Br À or I À ) exhibit outstanding intrinsic properties, [3] including wide light absorption, [4] high charge-carrier mobilities, [5] long charge diffusion length, [6] and solution-processed fabrication, which can be precisely tuned through compositionally engineered modifications. [7][8][9][10] The ambipolar behavior showed by organic-inorganic perovskites enables their use in conventional and inverted architectures, where the perovskite is sandwiched between n-type and p-type charge-transporting layers. [11] Despite the great performances and simpler fabrication of planar devices, the most featured

A Trifluoroethoxyl Functionalized Spiro‐Based Hole‐Transporting Material for Highly Efficient and Stable Perovskite Solar Cells

“…[1][2][3] In principle, an ideal HTM needs to meet the following prerequisites: 1) proper energy level alignment with the perovskite absorber for the efficient extraction of photogenerated holes; 2) high hole mobility for efficient hole transport to the electrode; 3) high solubility and glass transition temperature (T g ) to improve film morphology quality; 4) good interfacial contact between perovskite and HTM for effective charge separation and transfer; and 5) good hydrophobicity and protective effect on the perovskite layer to ensure long-term stability. [3][4][5][6] Although several compounds with newly developed core structures have been shown as promising alternative HTMs, [7][8][9][10][11][12][13][14] the current 2,2 0 ,7,7 0 -tetrakis(N,N-di-4-methoxyphenylamino)-9,9 0 -spirobifluorene (Spiro-OMeTAD) adopting a spiro-corestructure is still the dominant HTM employed for achieving high efficiencies [15][16][17][18][19] because of its desirable properties, such as solution-processable, high T g , and amorphous glassy state. [20,21] However, Spiro-OMeTAD also exhibits unfavorable defects induced by temperature, film quality, and environmental conditions, including limited long-term stability and unsatisfactory hole mobility and conductivity.…”

“…Although several compounds with newly developed core structures have been shown as promising alternative HTMs, [ 7–14 ] the current 2,2′,7,7′‐tetrakis( N , N ‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) adopting a spiro‐corestructure is still the dominant HTM employed for achieving high efficiencies [ 15–19 ] because of its desirable properties, such as solution‐processable, high T g , and amorphous glassy state. [ 20,21 ] However, Spiro‐OMeTAD also exhibits unfavorable defects induced by temperature, film quality, and environmental conditions, including limited long‐term stability and unsatisfactory hole mobility and conductivity.…”

Influence of alkyl chain length on the photovoltaic properties of dithienopyran-based hole-transporting materials for perovskite solar cells

“…The three new HTMs show a similar redox behavior with three reversible oxidations, with the first half-wave potential in the range of 0.57–0.61 V. The HOMO energy levels are estimated at −5.03, −5.01 and −5.05 eV for DTPA-Et , DTPA-Bu and DTPA-Hex , respectively, exhibiting a slightly stronger donor ability in comparison to spiro-OMeTAD measured under similar conditions (−5.16 eV). 22 Therefore, the new HTMs offer a good alignment between their HOMO levels and the valence band edge of the triple cation perovskite [(FAPbI 3 ) 0.87 (MAPbBr 3 ) 0.13 ] 0.92 [CsPbI 3 ] 0.08 situated at −5.70 eV, which points to an efficient injection of holes from the HTM to the perovskite. The energy levels of the different constituents of the PSC are schematically illustrated in Fig.…”

“…16,17 Furthermore, the incorporation of sulfur atoms in the structure of the HTMs has been described as a successful approach for enhancing the interaction with the Pb 2+ cations of the photoactive layer. [18][19][20] One of the examples of sulfur-containing systems is the 5H-dithieno[3,2-b:2 0 , 3 0 -d]pyran (DTP) moiety, which provides an interesting core because: (i) the relationship between asymmetry and photovoltaic properties has been little explored, 21,22 the asymmetric DTP can be used as an object of study to more deeply probe the symmetry-related correlations, and (ii) DTP has a pyran ring, which has been extensively used in dye-sensitized solar cells (DSSCs), 23 organic light-emitting diodes (OLEDs) and non-linear optics. 24 DTP has an oxygen atom that confers electron-rich and high-donating character to the structure with respect to its homologues.…”