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Charge‐Selective Contact Materials for Perovskite Solar Cells (PSCs)
Abstract
This chapter attempts to give a systematic introduction about the choices of both electron‐ and hole‐selective contact materials for efficient perovskite solar cells (PSCs). A huge number of hole‐selective electron‐blocking materials (HTMs) have been synthesized for PSCs, which can be generally divided into organic and inorganic HTMs. In typical PSCs, the electron‐selective contact material (ETM) is responsible for selective extraction of electrons at the anode contact, adopted from dye‐sensitized solar cells (DSSCs) and blocking holes from recombination with injected electrons. Similar to its counterpart HTM, the ETM can be divided into inorganic ETM, organic ETM, and composite ETM depending on the nature of the material. It is desirable that selective contact materials have larger bandgaps than that of perovskite films to avoid the excitons from recombining at the electrode. Also, it is crucial to design and synthesize selective contact materials with high conductivity to reduce the series resistance of the PSCs.
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Dimethoxydiphenylamine-substituted dispiro-oxepine derivative 2,2',7,7'-tetrakis-(N,N'-di-4-methoxyphenylamine)dispiro-[fluorene-9,4'-dithieno[3,2-c:2',3'-e]oxepine-6',9''-fluorene] (DDOF) has been
designed and synthesized using a facile synthetic route. The novel hole transporting material (HTM) was fully characterized and tested in perovskite solar cells exhibiting a remarkable power conversion efficiency of 19.4%. More importantly, compared with spiro-OMe-TAD-based devices, DDOF shows significantly improved stability. The comparatively comprehensive solid structure study is attempted to disclose the common features of good performance HTMs. These achievements clearly demonstrated that the highly hindered DDOF can be an effective HTM for the fabrication of efficient perovskite solar cells and further enlightened the rule of new HTM's design.
Interfacial engineering of the meso-TiO2 surface through a modified sequential deposition procedure involving a novel PbI2-HMPA complex pretreatment is conducted as a reproducible method for preparing MAPbI3-based perovskite solar cells providing the highest efficiencies yet reported with polymer HTM layer. Grazing incidence X-ray diffraction depth profiling confirms the formation of a perovskite film with PbI2-rich region close to the electron transport layer (ETL) due to the strong interaction of HMPA with PbI2, which successfully retarded the dissolution of PbI2 phase when depositing the perovskite layer on top. These results are further confirmed by energy-dispersive X-ray spectroscopy performed in a scanning transmis-sion electron microscope, which reveals that the I/Pb ratio in samples treated with the complex is indeed reduced in the vicin-ity of the ETL contact when compared to samples without the treatment. The engineered interface leads to an average pow-er conversion efficiency of 19.2% (reverse scan, standard deviation s.d. < 0.2) over 30 cells (best cell at 19.5% with high FF of 0.80).
Amino donor groups-substituted zinc(II) phthalocyanines (ZnPcs) BI25, BL07 and BL08 were obtained via cyclotetramerization of suitable phthalonitriles that were synthesized by Pd catalyzed amination reaction between 4-iodophthalonitrile and a selected secondary amine. The BI25, BL07 and BL08 ZnPcs were characterized using spectroscopic and electrochemical methods, and used as hole transporting layer in perovskite solar cells. The open circuit voltage (VOC) of perovskite solar cell devices reached close to 1 V, and the short-circuit current density (JSC) values evaluated from J-V curves were 16.2, 8.42, and 16.9 mA/cm2, respectively, demonstrating influence of the HOMO levels of ZnPcs. A power conversion efficiency of 11.75% was obtained in the case of BI25, which is the highest value ever reported using ZnPcs as HTMs in perovskite solar cells with traditional device geometry.
Donor-π-Bridge-Donor type oligomers (D-π-D) have been studied intensively as active materials for organic optoelectronic devices. In this study, we introduce three new D-π-D type organic semiconductors incorporating thiophene or thienothiophene with two electron-rich TPA units, which can be synthesized easily from commercially available materials. A thorough comparison of their optoelectronic and structural properties were conducted revealing the strong influence of longitudinal π-bridge conjugation extension on both the solid structure of organic semiconductive material theirselves and photo-voltaic performance when applied as hole transporting material (HTM) in perovskite solar cells. Single-crystallography and Time-resolved photoluminescence (TRPL) studies indicate that these coplanar donor-π-donor type HTMs can be promising alternatives to state-of-the-art spiro-OMeTAD, due to the multiple intermolecular short contacts as charge transporting channels and efficient charge extraction property from perovskite layer. The optimized devices with PEH-9 exhibited an impressive PCE of 16.9 % under standard global AM 1.5 illumination with minimized hysteretic behaviour, which is comparable to that of the devices using spiro-OMeTAD under similar conditions. Ambient stability after 400 h revealed that 93% of the energy conversion efficiency was retained for PEH-9, indicating that the devices had good long-term stability.
The 4,4’-dimethoxydiphenylamine-substituted 9,9'-bifluorenylidene (KR216) hole transporting material has been synthesized using a straightforward two-step procedure from commercially available and inexpensive starting reagents, mimicking the synthetically challenging 9,9'-spirobifluorene moiety of the well-studied spiro-OMeTAD. A power conversion efficiency of 17.8% has been reached employing a novel HTM in a perovskite solar cells.
New zinc porphyrins Y2 and Y2A2 have been utilized in perovskite solar cells (PSCs) specifically as hole-transporting materials (HTMs) rather than photosensitizers. The combination of MAPbI3 as photosensitizer and porphyrins as HTMs is potential alternative to well-known MAPbI3/Spiro-OMeTAD hybrids owing to high performance and versatility toward molecular engineering of porphyrin families. A high efficiency of 16.60% is achieved by n-butyl tethered Y2 HTM (VOC = 0.99 V; JSC = 22.82 mA cm-2) which is comparable to that of Spiro-OMeTAD of 18.03% (VOC = 1.06 V; JSC = 22.79 mA cm-2). Both materials possess similar HOMO level and same order of magnitude of hole-mobility at 10-4 cm2 V-1 s-1. The slightly poorer performance of 10.55% (VOC = 1.01 V; JSC = 17.80 mA cm-2) is obtained for n-dodecyl tethered Y2A2 HTM. This is believed to stem from more surface pinholes while deposited on perovskite leading to an order of magnitude slower mobility.
The past several years have witnessed the rapid emergence of a class of solar cells based on mixed organic–inorganic halide perovskites. Today’s state-of-the-art perovskite solar cells (PSCs) employ various methods to enhance nucleation and improve the smoothness of the perovskite films formed via solution processing. However, the lack of precise control over the crystallization process creates a risk of forming unwanted defects, for example, pinholes and grain boundaries. Here, we introduce an approach to prepare perovskite films of high electronic quality by using poly(methyl methacrylate) (PMMA) as a template to control nucleation and crystal growth. We obtain shiny smooth perovskite films of excellent electronic quality, as manifested by a remarkably long photoluminescence lifetime. We realize stable PSCs with excellent reproducibility showing a power conversion efficiency (PCE) of up to 21.6% and a certified PCE of 21.02% under standard AM 1.5G reporting conditions.
Chlorophylls (Chls) are abundant, naturally occurring pigments that play key roles in light-harvesting and electron/energy transfer in natural photosynthetic apparatus. To demonstrate the idea that Chls are suitable hole transporters, we employed two Chl derivatives, Chl-1 and Chl-2, which self-assembled readily into π-stacking aggregates through a simple spincasting process, in perovskite solar cells (PSCs). The Chl aggregate films exhibit an ultra-smooth film surface, high hole mobility, appropriate energy levels, and efficient hole injection efficiencies that are all key characteristics for efficient hole transporters in PSCs. CH3 NH3 PbI3-x Clx -based PSCs with these Chls as hole transporters were fabricated and compared with P3HT as a standard hole transporter. PSCs based on Chl-1 and Chl-2 without the use of typical additives, such as 4-tert-butylpyridine and lithium bis(trifluoromethanesulfinyl)imide, gave power conversion efficiencies of 11.44 and 8.06 %, respectively. This research provides a unique way to incorporate low-cost and environmentally friendly natural photosynthetic materials in the development of highly efficient photovoltaic devices.
A new Zn(ii) phthalocyanine (Pc) based low bandgap HTM is introduced for perovskite solar cells. Steady state and time-resolved photoluminescence (PL) measurements indicated an evenly matched hole extraction efficiency between sym-HTPcH and spiro-OMeTAD. On account of the low film quality and resulting high recombination, Zn(ii) Pc normally cannot work as an effective HTM. We adopted insulating Al2O3 for the infiltration of sym-HTPcH to form a hybrid interfacial buffer layer, affording perovskite solar cells (PSCs) with an average PCE value of up to 12.3%, which is a significant improvement with respect to the control cell without the meso-Al2O3 layer (4.21%) and is the highest value ever reported for Zn(ii) phthalocyanine based devices under AM1.5G standard conditions. A hysteresis test revealed that our device structure with the new HTM exhibited a balanced charge extraction behaviour.
The 4,4′-dimethoxydiphenylamine-substituted 9,9′-bifluorenylidene (KR216) hole transporting material has been synthesized using a straightforward two-step procedure from commercially available and inexpensive starting reagents, mimicking the synthetically challenging 9,9′-spirobifluorene moiety of the well-studied spiro-OMeTAD. A power conversion efficiency of 17.8 % has been reached employing a novel HTM in a perovskite solar cells.