Abstract

　The energy level alignment of the perovskite and hole transporting materials (HTMs) is essential for increasing the open-circuit voltage (Voc) and enhancing the performance of perovskite solar cells (PSCs). In this work, new sequentially fluorinated poly(triarylamine) polymers (PTAA, 1F-PTAA, and 2F-PTAA) with tuned highest occupied molecular orbital (HOMO) energy levels are developed and applied as HTMs into PSCs. The fluorination approach successfully leads to stepwise downshifting of the HOMO levels of PTAA derivatives, resulting in an obvious increase in the Voc and power conversion efficiency (PCE) of the PSCs. In particular, introduction of 1F-PTAA polymer in (FAPbI3)0.85(MAPbBr3)0.15-based mesoporous n-i-p structure PSCs achieves the high stabilized PCE of 21.2% at the maximum power point with improved Voc of 1.14 V. To elucidate the importance of the optimized degree of fluorination of PTAA polymers on the photovoltaic performances, the optical, electrical, photophysical properties, and doping behaviors of the fluorinated PTAA derivatives are investigated.

Abstract

　Perovskite solar cells (PSCs) require both high efficiency and good long-term stability if they are to be commercialized. It is crucial to finely optimize the energy level matching between the perovskites and hole-transporting materials to achieve better performance. Here, we synthesize a fluorene-terminated hole-transporting material with a fine-tuned energy level and a high glass transition temperature to ensure highly efficient and thermally stable PSCs. We use this material to fabricate photovoltaic devices with 23.2% efficiency (under reverse scanning) with a steady-state efficiency of 22.85% for small-area (~0.094 cm2) cells and 21.7% efficiency (under reverse scanning) for large-area (~1 cm2) cells. We also achieve certified efficiencies of 22.6% (small-area cells, ~0.094 cm2) and 20.9% (large-area, ~1 cm2). The resultant device shows better thermal stability than the device with spiro-OMeTAD, maintaining almost 95% of its initial performance for more than 500 h after thermal annealing at 60 °C.

Abstract

　The presence of excess lead iodide in halide perovskites has been key for surpassing 20% photon-to-power conversion efficiency. To achieve even higher power conversion efficiencies, it is important to understand the role of remnant lead iodide in these perovskites. To that end, we explored the mechanism facilitating this effect by identifying the impact of excess lead iodide within the perovskite film on charge diffusion length, using electron-beam-induced current measurements, and on film formation properties, from grazing-incidence wide-angle X-ray scattering and high-resolution transmission electron microscopy. Based on our results, we propose that excess lead iodide in the perovskite precursors can reduce the halide vacancy concentration and lead to formation of azimuthal angle-oriented cubic α-perovskite crystals in-between 0° and 90°. We further identify a higher perovskite carrier concentration inside the nanostructured titanium dioxide layer than in the capping layer. These effects are consistent with enhanced lead iodide-rich perovskite solar cell performance and illustrate the role of lead iodide.

Abstract

　Methylammonium lead iodide perovskites have limited practical applications due to a lack of stability under operating conditions. Environmental stability under heating has not yet been achieved, although recent studies modifying perovskites with long organic cations have reported the progress of stabilisation under humid conditions. In this work, we report the structural evolution of long alkylammonium-modified MAPbI3 and its functions for highly efficient and stable solar cells. As an encapsulating agent, octylammonium (OA) cation produced individual MAPbI3 grains in full armour without the formation of layered structures in contrast to butylammonium (BA) and phenethylammonium (PEA) cations. Our OA-armoured MAPbI3 achieved a stabilised power conversion efficiency of 20.1% without a deterioration of charge transport properties due to highly preferential orientation suppressing non-radiative recombination. The structural features also led to a much improved thermal stability at 85 °C in ambient atmosphere retaining 80% of the initial efficiency after 760 h without any encapsulation, as well as water tolerance. This work addresses widespread concerns associated with the photovoltaic efficiency and stability of MAPbI3 by exploring the inter-relationship between structural features and their functions in surfactant-modified perovskites.

Abstract

　As the efficiency of perovskite solar cells (PSCs) reached more than 22%, the large-area fabrication of PSCs became another issue receiving growing attention. For large-area PSCs, more reproducibility is required to precisely control the crystallization behavior of perovskites. A two-step process has been preferred to apply large-area coatings of perovskite because of its better reproducibility, but the process has suffered from slow and incomplete conversion of PbI2 to perovskite. In this paper, we propose a fast, simple, two-step method—mediator extraction treatment (MET)—for the preparation of a high-quality perovskite film. In MET, a pre-deposited PbI2–DMSO complex film is converted into a peculiar PbI2 film with a porous morphology and unusual crystallographic orientation via the removal of DMSO. PbI2 could be completely converted into MAPbI3 by a fast reaction with MAI molecules. We demonstrate that this MET process in MAPbI3-based PSCs can achieve 18.8% of the maximum power conversion efficiency (PCE) using spin-coating, and 18.3% of the maximum PCE using slot-die coating with a uniform distribution in a 10 ×10 cm2 substrate at a laboratory scale. Moreover, over 18% of PCE could be achieved in only 100 s, and with room-temperature processing.

Abstract

　The stability of a perovskite solar cell (PSC) is enhanced significantly by applying a customized thin-film encapsulation (TFE). The TFE is composed of a multilayer stack of organic/inorganic layers deposited by initiated chemical vapor deposition and atomic layer deposition, respectively, whose water vapor transmission rate is on the order of 10−4 g m−2 d−1 at an accelerated condition of 38 °C and 90% relative humidity (RH). The TFE is optimized, taking into consideration various aspects of thermosensitive PSCs. Lowering the process temperature is one of the most effective methods for minimizing the thermal damage to the PSC during the monolithic integration of the TFE onto PSC. The direct deposition of TFE onto a PSC causes less than 0.3% degradation (from 18.5% to 18.2%) in the power conversion efficiency, while the long-term stability is substantially improved; the PSC retains 97% of its original efficiency after a 300 h exposure to an accelerated condition of 50 °C and 50% RH, confirming the enhanced stability of the PSC against moisture. This is the first demonstration of a TFE applied directly onto PSCs in a damage-free manner, which will be a powerful tool for the development of highly stable PSCs with high efficiency.

Abstract

　In Sn-based halide perovskite solar cells (PSCs), the oxidation of Sn2+ to Sn4+ under ambient air leads to unwanted p-type doping in the perovskite film, which is a main reason for increased background carrier density and low efficiency. Here, we find that the introduction of bromide into formamidinium tin iodide (CH(NH2)2SnI3, FASnI3) lattice significantly lowers the carrier density of perovskite absorber, which is thought to be a result of reduction of Sn vacancies. It reduces the leakage current of devices, increases recombination lifetime, and finally improves open-circuit voltage and fill factor of the resulting devices employing mesoporous TiO2 as an electron transport layer. Consequently, a high power conversion efficiency (PCE) of 5.5% is achieved with an average PCE of 5%, and after encapsulation the devices are highly stable over 1000 h under continuous one sun illumination including the ultraviolet region. This study suggests a simple approach for improving stability and efficiency in FASnI3-based PSCs.

Abstract

　Successful commercialization of perovskite solar cells (PSCs) in the near future will require the fabrication of cells with high efficiency and long-term stability. Despite their good processability at low temperatures, the majority of organic conductors employed in the fabrication of high-efficiency PSCs [e.g., 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) and poly(triaryl amine) (PTAA)] have low thermal stability. In order to fabricate PSCs with excellent thermal stability, both the constituent material itself and the interface between the constituents must be thermally stable. In this work, we focused on copper phthalocyanine (CuPC) as a model hole-transporting material (HTM) for thermally stable PSCs since CuPC is known to possess excellent thermal stability and interfacial bonding properties. The CuPC-based PSCs recorded a high power conversion efficiency (PCE) of ∼18% and maintained 97% of their initial efficiency for more than 1000 h of thermal annealing at 85 °C. Moreover, the device was stable under thermal cycling tests (50 cycles, −45 to 85 °C). The high PCE and high thermal stability observed in the CuPC-PSCs were found to arise as a result of the strong interfacial and conformal coating present on the surface of the perovskite facets, located between CuPC and the perovskite layer. These results will provide an important future direction for the development of highly efficient and thermally stable PSCs.

Abstract

　The formation of a dense and uniform thin layer on the substrates is crucial for the fabrication of high-performance perovskite solar cells (PSCs) containing formamidinium with multiple cations and mixed halide anions. The concentration of defect states, which reduce a cell’s performance by decreasing the open-circuit voltage and short-circuit current density, needs to be as low as possible. We show that the introduction of additional iodide ions into the organic cation solution, which are used to form the perovskite layers through an intramolecular exchanging process, decreases the concentration of deep-level defects. The defect-engineered thin perovskite layers enable the fabrication of PSCs with a certified power conversion efficiency of 22.1% in small cells and 19.7% in 1-square-centimeter cells.

Abstract

　Perovskite solar cells (PSCs) exceeding a power conversion efficiency (PCE) of 20% have mainly been demonstrated by using mesoporous titanium dioxide (mp-TiO2) as an electron-transporting layer. However, TiO2 can reduce the stability of PSCs under illumination (including ultraviolet light). Lanthanum (La)–doped BaSnO3 (LBSO) perovskite would be an ideal replacement given its electron mobility and electronic structure, but LBSO cannot be synthesized as well-dispersible fine particles or crystallized below 500°C. We report a superoxide colloidal solution route for preparing a LBSO electrode under very mild conditions (below 300°C). The PSCs fabricated with LBSO and methylammonium lead iodide (MAPbI3) show a steady-state power conversion efficiency of 21.2%, versus 19.7% for a mp-TiO2 device. The LBSO-based PSCs could retain 93% of their initial performance after 1000 hours of full-Sun illumination.

Abstract

　Photoresponsive change in the basal spacing of cationic azo dye-layered silicate intercalation compounds was investigated using X-ray diffraction under controlled humidity conditions to investigate the mechanism of the photoresponse triggered by trans–cis photoisomerization. The molecular design of azo dyes was conducted and two cationic azo dyes (phenylazobenzene and phenylazonaphthalene) were used to obtain intercalation compounds with different packing and hydration. Whereas the trans-phenylazobenzene-layered silicate hardly adsorbed water vapor, the trans-phenylazonaphthalene intercalation compound strongly interacted with water. Both azo dyes photoisomerized in the interlayer space reversibly. Under the relative humidity of 95%, the basal spacing of the phenylazobenzene–silicate increased upon UV irradiation, confirming that the change in the basal spacing was caused by the photoinduced hydration. Under the relative humidity of 6.8%, the basal spacing of the phenylazonaphthalene–silicate decreased upon UV irradiation, suggesting that the packing of the phenylazonaphthalene in the interlayer space was changed to be compacted by the photochemical conversion to the cis-form.

Abstract

　We have designed and synthesized fluorinated indolo[3,2-b]indole (IDID) derivatives as crystalline hole-transporting materials (HTM) for perovskite solar cells. The fluorinated IDID backbone enables a tight molecular arrangement stacked by strong π–π interactions, leading to a higher hole mobility than that of the current HTM standard, p,p-spiro-OMeTAD, with a spherical shape and amorphous morphology. Moreover, the photoluminescence quenching in a perovskite/HTM film is more effective at the interface of the perovskite with IDIDF as compared to that of p,p-spiro-OMeTAD. As a consequence, the device fabricated using IDIDF shows superior photovoltaic properties compared to that using p,p-spiro-OMeTAD, exhibiting an optimal performance of 19%. Thus, this remarkable result demonstrates IDID core-based materials as a new class of HTMs for highly efficient perovskite solar cells.