Molecular Aggregation of Naphthalene Diimide(NDI) Derivatives in Electron Transport Layers of Inverted Perovskite Solar Cells and Their Influence on the Device Performance

Abstract

One of key factorst od esign applicable electron transport layers (ETLs) for perovskite solar cells is the morphology of ETLs since ag ood morphology would help to facilitate the carrier transport at two interfaces (perovskite\ETL and ETL\cathode). However,o ne drawback of most organic ETL smallm olecules is the internal undesired accumulation, which would cause the formation of inappropriate morphology and rough ETL surface. Here, by elaborately designing the side chains of NDI derivatives, the molecular interaction could be modified to achievet he aggregation in different degrees, whichw ould eventually affect the accumulation of molecules and surfaceq ualities of ETLs. By speculating from the comparison between the absorption spectra of solutions and films, the sequence of extent of molecule interaction and aggregation was built among three NDI derivatives, which is furtherc onfirmed by direct evidence of atomic force microscopy (AFM)i mages. Then, carrier exaction abilities are simply studied by steady-state photoluminescence spectroscopy.T he carrier transport process is also discussed based on cyclic voltammetry,t ime-resolved photoluminescence spectroscopy and mobility.N DIF1 are proven to have the appropriate internal aggregation to smooth the contact with cathodea nd low series resistance, and ad evice performance of 15.6 %i sa chieved. With the ability of preventing the thermal diffusion of Ag towards the perovskite surface due to the strongi nteraction between molecules, NDIF2 at high concentration shows the highest fill factor (80 %).

Improving the Fill Factor of Perovskite Solar Cells by Employing an Amine-tethered Diketopyrrolopyrrole-Based Polymer as the Dopant-free Hole Transport Layer

“…The reason could be that the aggregated islands could continue to grow under high concentration and then were integrated with each other to form many relatively smooth and thin P25NH domains, which were further overlapped layer by layer to form the multilayer morphology. 12 Because P25NH films fabricated from a high concentration showed a smooth and lamellar morphology, the related device could possess a high fill factor (83.2%). However, the thicker transport layer would lead to a lower J sc and a lower PCE (17.3%), and these data are summarized in Figure S10 and Table S2.…”

“…The reason could be that under low concentration, weak interactions among P3HT molecules might lead to poor aggregation, making P3HT more difficult to cover the uneven perovskite surface with rough grains. 12 Another evidence of insufficient coverage of P3HT could be offered in the comparison of PL spectra. Insufficient coverage of the perovskite surface by P3HT with a low concentration resulted in the direct exposure of the perovskite surface to the metal anode and caused a short circuit of the whole device, leading to very poor performance of PSCs.…”

“…The first organic–inorganic hybrid lead halide perovskite solar cell (PSC) was reported by Miyasaka et al in 2009 and the subsequent dramatic improvement in power conversion efficiencies (PCEs, exceeding 25.2%) makes scientists believe that PSCs would be the most competitive candidate for next-generation solar cells. − The device architectures of PSCs range from mesoscopic to planar structures with regular (n–i–p) or inverted (p–i–n) layouts. − The regular PSCs usually contain mesoporous titanium oxide (TiO 2 ) as the electron transport layer (ETL) . However, the drawbacks of TiO 2 are obvious, including the photocatalytic effect of TiO 2 -induced deep trap sites on the surface of TiO 2 , the degradation of perovskite films caused by the accumulated charges at the interface of TiO 2 /perovskite owing to the electron extraction performance of TiO 2 , and the high-temperature treatment process. − Other n-type metal oxides (i.e., SnO 2 , ZnO, and WO 3 ) have also been explored as ETLs. − Among them, SnO 2 displays several charming advantages including good mobility and high stability, low-temperature preparation, excellent transparency, and suitable band alignment to perovskite, making it very promising as an electron transport material in PSCs. − Thus, it is logical and reasonable for us to employ SnO 2 as ETLs in the construction of our PSCs.…”

Perinone—New Life of an Old Molecule

“…In 2000, this efficiency was more than doubled by incorporating an exciton-blocking layer, inserted between the photoactive organic layers and the metal cathode in the solar cell [153]. The performance of organic electronic devices are tightly related to the microstructure of the molecular active layer and are affected by factors such as material solubility [154], donor/acceptor ratio [155], thermal/solvent annealing [156], charge separation efficiency [157], aggregations of active molecules [158,159], and use of additives [160]. The molecules with large planar structures tend to form strong aggregation and excessively large domains (up to ≈1 µm) in bulk heterojunction solar cells which is detrimental to the organic solar cells performance.…”

Synthesis and electronic properties of nitrogen-doped π-extended polycyclic aromatic dicarboximides with multiple redox processes

“…In this regard, π-conjugated fused-ring systems, such as polycyclic aromatic dicarboximides [4][5][6][7] (PADI) and their heteroatom-doped analogues are emerging classes of semiconducting materials 8 derived from the well-known family of polycyclic aromatic hydrocarbons [9][10][11][12][13] (PAH). These materials have been successfully applied in organic (opto)electronics [14][15][16][17][18] and more recently, as electrodes in energy storage devices due to their remarkable redox behavior. [19][20][21][22][23] Among them, π-conjugated compounds containing imide groups, such as naphthalene 24 and perylene diimides (NDI and PDI) 1, 2, [25][26][27] are among the most successful n-type organic semiconductors, due to the strong electron-withdrawing character of the imide group, which lowers the LUMO energy levels of the semiconductor, facilitating electron injection and charge stabilization through the π-conjugated systems.…”