in dramatically improved environmental and structural stability. [2,3] However, the exciton binding energy of layered phases in 2D perovskites could be as high as 300 meV although this value could be reduced with the increased thickness of inorganic layer, resulting in relatively inferior photovoltaic performance compared to 3D perovskite solar cells (PSCs). [13] Thus, it is urgent to develop more efficient organic spacers as well as device engineering to obtain both highly efficient and stable 2D PSCs for future industry application.Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) structures are two main archetypes of layered 2D perovskites to date. [14][15][16][17][18][19] The general formula of a 2D RP and DJ perovskite is (L′) 2 A n−1 M n X 3n+1 and (L″)A n−1 M n X 3n+1 , where L′ and A are monovalent cations, L″ is a divalent organic cation, M is a divalent metal cation, X is a halide ion, and n is the number of corner-sharing [MX 6 ] 4− octahedral slabs. [5] Typically, a van der Waals gap exist in organic spacers between adjacent inorganic [MI 6 ] 4octahedron slabs in 2D RP perovskites. [1] 2D RP perovskites have been shown enhanced stability, while the weak interlayer interactions in 2D RP perovskites with obvious van der Waals gaps cannot sufficiently stabilize their layered structure. In contrast, charge transport and stability could be enhanced in 2D DJ perovskites, where the neighboring inorganic slabs were bridged by diammonium spacers, resulting in shortened interslab distance and eliminated van der Waals gap. [1] So far, the aliphatic diammonium spacers, such as 1,4-butanediamine (BDA) and 1,3-propanediamine (PDA), have been studied in 2D DJ perovskite devices with promising photovoltaic performance. [20][21][22] However, the electrically insulating aliphatic diammonium spacers could block efficient charge transport due to their low dielectric constant. [23] In comparison with the aliphatic spacers, the aromatic spacers typically exhibit a larger dielectric constant and better conductivity due to the delocalized π-electrons along the molecular backbone, which could weaken the dielectric confinement effect by reducing dielectric mismatch between corner-sharing inorganic slabs and adjacent organic spacer layer. [23][24][25] Recently, several aromatic diammonium cations such as pyridinium-based aromatic spacers (3AMP, 4AMP), phenyl-based aromatic spacer (PDMA) and thiophene-based aromatic spacer (ThDMA) have been developed as organic spacers in 2D DJ PSCs to enhance the device performance. [23,[26][27][28][29][30] These single aromatic ringbased bulky spacers used so far still cannot directly contribute 2D Dion-Jacobson (DJ) perovskites have become an emerging photovoltaic material with excellent structure and environmental stability due to their lacking van der Waals gaps relative to 2D Ruddlesden-Popper perovskites. Here, a fused-thiophene-based spacer, namely TTDMAI, is successfully developed for 2D DJ perovskite solar cells. It is found that the DJ perovskite using TTDMA spacer with extended π-conjuga...
The passivation of the intrinsic surface defects of perovskites by organic functional materials has a great potential to retard charge recombination and enhance charge extraction. However, unsatisfactory device performance and a lack of in-depth understanding of the defect passivation mechanism make rational molecule design for efficient solar cells a great challenge. Herein, two solution-processable two-dimensional (2D) conjugated polymers, namely, 2DP-F and 2DP-O, have been synthesized for perovskite solar cells (PSCs). It is found that these materials could passivate surface defects, transport and extract hole carriers, hamper moisture invasion, and impede diffusion of Li+ cations into the perovskite film. As a result, champion efficiencies of 23.31% and 24.08% were achieved for 2DP-F- and 2DP-O-based devices, respectively, coupled with dramatically improved stability. These results indicate that our proposed 2D polymers could be promising multifunctional materials for further boosting the efficiency and improving the stability of PSCs.
The dominant hole transport material (HTM) in state-of-the-art perovskite solar cells (PSCs) is Spiro-OMeTAD, which needs to be doped using hydrophilic dopants to improve its hole mobility and conductivity, resulting in inferior device stability. Here, we propose an effective molecular design strategy to construct dopant-free polymer HTMs by selecting four structurally related polymers and investigating their structureproperty relationship. It is found that the donor and acceptor units with longitudinal conjugate extension, such as BDT-T and BDD, could not only enhance the planarity of the conjugated polymer backbone and tune the energy levels but also promote the face-on orientation, resulting in superior charge extraction and transport. The optimized device utilizing dopant-free polymer HTM shows a high open-circuit voltage of 1.19 V and a champion efficiency of 24.04 % with greatly improved operational stability, making it among the best performance PSCs based on dopant-free HTMs.
2D Dion–Jacobson (DJ) perovskites have become emerging photovoltaic materials owing to their intrinsic structure stability. However, as insulating aliphatic cations are widely used as spacers, the interactions between the spacers and inorganic layers in DJ perovskites have rarely been studied. Here, an organic semiconductor spacer with two covalently connected thiophene rings, namely bithiophene dimethylammonium (BThDMA), is successfully developed for 2D DJ perovskite solar cells (PSCs). An important finding is that there are strong orbital interactions between the conjugated organic spacer and adjacent inorganic layers, whereas no such interactions exist in DJ perovskite using an aliphatic octane‐1,8‐diaminium (ODA) spacer with similar length. The BThDMA spacer with multiple conjugated aromatic rings can also induce crystal growth with large grain size and preferred vertical orientation, resulting in reduced trap density and improved charge‐carrier mobility. As a result, the optimized device based on (BThDMA)MAn−1PbnI3n+1 (nominal n = 5) shows an excellent PCE of 18.1% with negligible hysteresis, which is a record efficiency for 2D DJ PSCs using a spacer with two or more covalently linked aromatic rings. These findings provide a novel and important insight on achieving efficient and stable 2D DJ perovskite solar cells by developing organic semiconductor spacers.
Integrated perovskite/organic solar cells (IPOSCs) have shown great potential in broadening the light absorption range and improving the photovoltaic performance. However, the severe interface charge recombination and unmatched energy levels between perovskite and organic photoactive layers hinder their performance improvement. Here, an efficient interface passivation strategy for IPOSCs based on a layered Ruddlesden-Popper (RP) perovskite and high photovoltaic performance is successfully demonstrated. It is found that an ultrathin conjugated polymer (PM6) layer could passivate the surface defects of perovskite film, tuning the energy level and suppress the nonradiative recombination loss, leading to efficient interface contact between RP perovskite and organic photoactive layers, boosting the open-circuit voltage from 1.06 to 1.12 V and the efficiency from 17.23% to 19.15%. Importantly, the optimized device shows extended photocurrent response to 930 nm with a peak intensity close to 50% from 800 to 931 nm. The results indicate that interface passivation using a functionalized polymer could be an efficient strategy to improve the photovoltaic performance of integrated devices.
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