Molecular modification of spiro[fluorene-9,9′-xanthene]-based dopant-free hole transporting materials for perovskite solar cells

The molecular engineering of organic hole-transporting materials (HTMs) plays an important role in enhancing the performance and stability of perovskite solar cells (PSCs) as well as reducing their fabrication cost. Here, two low-cost spiro-OMeTAD analogues, namely SP-Naph and SP-SMe, featuring a spiro[fluorene-9,9-xanthene] (SFX) central core and asymmetric subunits are designed and synthesized. Specifically, the SFX core in the SP-Naph molecule is substituted with dimethoxyphenylnaphthylamine subunits to enhance conductivity and charge transport properties by expansion of the π-conjugated structure. On the other hand, in the molecular structure of SP-SMe, the methoxy groups (–OMe) from diphenylamine units were partially replaced with the methylsulfanyl groups (–SMe) to increase interaction with the perovskite surface through the “Lewis soft” S atoms. By combining various experimental and simulation methods, thestructure–property relationship of the newly synthesized HTMs was thoroughly investigated. The suitable HOMO energy level with the perovskite layer together with superior photoelectric properties and enhanced thermostability and humidity resistivity are obtained for the SP-SMe HTM. As a result, the planar n–i–p PSC with the dopant-free SP-SMe HTM yields a maximum power conversion efficiency (PCE) of 21.95%, which outperforms that with SP-Naph (20.51%) and doped spiro-OMeTAD (19.23%). Importantly, the device with SP-SMe also reveals enhanced operational stability under continuous 1 sun illumination and thermal stability at 65 °C. These findings provide valuable insight for the rational design of dopant-free organic HTMs based on the SFX core, which would promote the development of highly efficient and stable devices

D-pi-A-pi-D Structured Diketopyrrolopyrrole-Based Electron Donors for Solution-Processed Organic Solar Cells

Solution-processable D-pi-A-pi-D structured two organic small molecules bearing thienyl diketopyrrolopyrrole (TDPP) and furanyl diketopyrrolopyrrole (FDPP) as central acceptor units and cyano on the pi-bridge and phenothiazine as the terminal donor units, coded as TDPP-PTCN and FDPP-PTCN, are designed and synthesized. The CH arylation and Suzuki coupling protocols have been adopted for synthesizing the molecules. Solution-processed organic solar cells (OSCs) were constructed with these molecules as the donors and phenyl-C-71-butyric acid methyl ester as the acceptor yielding power conversion efficiencies (PCE) of 4.0% for FDPP-PTCN and 5.2% for TDPP-PTCN, which is the highest PCE reported so far from the small molecular DPP-phenothiazine-based architecture for solution-based OSCs. The effect of heteroatom substitution on thermal stability and optoelectronic and photovoltaic performances is also systematically investigated herein. This work demonstrates that replacement of oxygen with sulfur in these kinds of small molecules remarkably improves the photovoltaic performance of OSCs

D−π–A−π–D Structured Diketopyrrolopyrrole-Based Electron Donors for Solution-Processed Organic Solar Cells

Solution-processable D−π–A−π–D structured two organic small molecules bearing thienyl diketopyrrolopyrrole (TDPP) and furanyl diketopyrrolopyrrole (FDPP) as central acceptor units and cyano on the π-bridge and phenothiazine as the terminal donor units, coded as TDPP-PTCN and FDPP-PTCN, are designed and synthesized. The C–H arylation and Suzuki coupling protocols have been adopted for synthesizing the molecules. Solution-processed organic solar cells (OSCs) were constructed with these molecules as the donors and phenyl-C71-butyric acid methyl ester as the acceptor yielding power conversion efficiencies (PCE) of 4.0% for FDPP-PTCN and 5.2% for TDPP-PTCN, which is the highest PCE reported so far from the small molecular DPP–phenothiazine-based architecture for solution-based OSCs. The effect of heteroatom substitution on thermal stability and optoelectronic and photovoltaic performances is also systematically investigated herein. This work demonstrates that replacement of oxygen with sulfur in these kinds of small molecules remarkably improves the photovoltaic performance of OSCs

Molecular Engineering of Photosensitizers for Solid‐State Dye‐Sensitized Solar Cells: Recent Developments and Perspectives

Abstract Dye‐sensitized solar cells (DSSCs) are a feasible alternative to traditional silicon‐based solar cells because of their low cost, eco‐friendliness, flexibility, and acceptable device efficiency. In recent years, solid‐state DSSCs (ss‐DSSCs) have garnered much interest as they can overcome the leakage and evaporation issues of liquid electrolyte systems. However, the poor morphology of solid electrolytes and their interface with photoanodes can minimize the device performance. The photosensitizer/dye is a critical component of ss‐DSSCs and plays a vital role in the device‘s overall performance. In this review, we summarize recent developments and performance of photosensitizers, including mono‐ and co‐sensitization of ruthenium, porphyrin, and metal‐free organic dyes under 1 sun and ambient/artificial light conditions. We also discuss the various requirements that efficient photosensitizers should satisfy and provide an overview of their historical development over the years

Impact of rotamer diversity on the self-assembly of nearly isostructural molecular semiconductors

Conformational diversity due to different orientations of structural subunits has a complex impact on morphological disorder of organic semiconductors. Here, we isolate the impact of a specific structural change: replacing bithiophene (biTh) units with thieno[3,2-b]thiophene (TT). We compare four molecules with an alternating donor-acceptor structure (D'-A-D-A-D') composed of a central, electron-rich dithienosilole (DTS) unit flanked by pyridyl-[2,1,3]thiadiazole (PT) or fluorinated benzo[c][1,2,5]thiadiazole (FBT) and end-capped with bithiophene biTh or TT groups. We find that using TT instead of biTh results in an increased degree of order within films cast directly from solution by influencing the self-assembly tendencies of the different molecules. Unlike switching the acceptor subunit, such as FBT for PT, the TT for biTh structural change has little impact on the electronic structure of these molecular semiconductors. Instead, these morphological effects can be understood within the context of the predicted conformational diversity. TT units limit the number of rotational conformations (rotamers) available within this molecular architecture; low rotamer dispersity facilitates self-assembly into ordered domains. As a practical illustration of this greater drive toward self-assembly, we use the TT-containing molecules as donors in bulk heterojunction solar cells with PC70BM. Devices with TT-containing molecules show improved photovoltaic performance compared to their previously characterized biTh analogs (d-DTS(PTTh2)(2) and p-DTS(FBTTh2)(2)) in both as-cast and optimized conditions, with efficiencies up to 6.4% and 8.8% for PT-TT and FBT-TT, respectively. The TT subunit and, more broadly, the strategy of limiting conformational diversity can be readily applied toward the design of solution-processable organic semiconductors with increased as-cast order