Effects of heteroatom substitution in spiro-bifluorene hole transport materials.
ABSTRACT: Three new spirofluorene-based hole transport materials, Spiro-S, Spiro-N, and Spiro-E, are synthesized by replacing the para-methoxy substituent in 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-MeOTAD) with methylsulfanyl, N,N-dimethylamino and ethyl groups. Their properties as hole transport materials in perovskite solar cells are investigated. The impact of replacing the para-methoxy substituent on bulk properties, such as the photophysical properties, HOMO/LUMO energy level, hole extraction properties and morphologies of perovskite thin films are investigated. Their optoelectronic and charge-transport properties and performance in perovskite solar cells are compared with the current benchmarked and structurally-related hole transport material (HTM) Spiro-MeOTAD. Surprisingly, the methylsulfanyl substituted spirofluorene shows the highest power conversion efficiency of 15.92% among the investigated spirofluorenes, which is an over 38% increase in PCE compared with that of Spiro-MeOTAD under similar device fabrication conditions.
Project description:Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized mostly with expensive spiro-MeOTAD hole-transporting material. PSCs are demonstrated that achieve stabilized efficiencies exceeding 20% with straightforward low-cost molecularly engineered copolymer poly(1-(4-hexylphenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole) (PHPT-py) based on Rutin-silver nanoparticles (AgNPs) as the hole extraction layer. The Rutin-AgNPs additive enables the creation of compact, highly conformal PHPT-py layers that facilitate rapid carrier extraction and collection. The spiro-MeOTAD-based PSCs show comparable efficiency, although their operational stability is poor. This instability originated from potential-induced degradation of the spiro-MeOTAD/Au contact. The addition of conductive Rutin-AgNPs into PHPT-py layer allows PSCs to retain >97% of their initial efficiency up to 60 d without encapsulation under relative humidity. The PHPT-py/ Rutin-AgNPs-based devices surpass the stability of spiro-MeOTAD-based PSCs and potentially reduce the fabrication cost of PSCs.
Project description:Hole-transport materials (HTMs) play an important role as hole scavenger materials in the most efficient perovskite solar cells (PSCs). Here, for the first time, two Ag-based metal organic complexes (HA1 and HA2) are employed as a new class of dopant-free hole-transport material for application in PSCs. These HTMs show excellent conductivity and hole-transport mobility. Consequently, the devices based on these two HTMs exhibit unusually high fill factors of 0.76 for HA1 and 0.78 for HA2, which are significantly higher than that obtained using spiro-OMeTAD (0.69). The cell based on HA1-HTM in its pristine form achieved a high power conversion efficiency of 11.98% under air conditions, which is comparable to the PCE of the cell employing the well-known doped spiro-MeOTAD (12.27%) under the same conditions. More importantly, their facile synthesis and purification without using column chromatography makes these new silver-based HTMs highly promising for future commercial applications of PSCs. These results provide a new way to develop more low-cost and high conductivity metal-complex based HTMs for efficient PSCs.
Project description:We report on solid-state mesoscopic heterojunction solar cells employing nanoparticles (NPs) of methyl ammonium lead iodide (CH(3)NH(3))PbI(3) as light harvesters. The perovskite NPs were produced by reaction of methylammonium iodide with PbI(2) and deposited onto a submicron-thick mesoscopic TiO(2) film, whose pores were infiltrated with the hole-conductor spiro-MeOTAD. Illumination with standard AM-1.5 sunlight generated large photocurrents (J(SC)) exceeding 17 mA/cm(2), an open circuit photovoltage (V(OC)) of 0.888 V and a fill factor (FF) of 0.62 yielding a power conversion efficiency (PCE) of 9.7%, the highest reported to date for such cells. Femto second laser studies combined with photo-induced absorption measurements showed charge separation to proceed via hole injection from the excited (CH(3)NH(3))PbI(3) NPs into the spiro-MeOTAD followed by electron transfer to the mesoscopic TiO(2) film. The use of a solid hole conductor dramatically improved the device stability compared to (CH(3)NH(3))PbI(3) -sensitized liquid junction cells.
Project description:We report the crystal structure and hole-transport mechanism in spiro-OMeTAD [2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene], the dominant hole-transporting material in perovskite and solid-state dye-sensitized solar cells. Despite spiro-OMeTAD's paramount role in such devices, its crystal structure was unknown because of highly disordered solution-processed films; the hole-transport pathways remained ill-defined and the charge carrier mobilities were low, posing a major bottleneck for advancing cell efficiencies. We devised an antisolvent crystallization strategy to grow single crystals of spiro-OMeTAD, which allowed us to experimentally elucidate its molecular packing and transport properties. Electronic structure calculations enabled us to map spiro-OMeTAD's intermolecular charge-hopping pathways. Promisingly, single-crystal mobilities were found to exceed their thin-film counterparts by three orders of magnitude. Our findings underscore mesoscale ordering as a key strategy to achieving breakthroughs in hole-transport material engineering of solar cells.
Project description:We fabricated perovskite solar cells using a triple-layer of n-type doped, intrinsic, and p-type doped 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) (n-i-p) as hole transport layer (HTL) by vacuum evaporation. The doping concentration for n-type doped spiro-OMeTAD was optimized to adjust the highest occupied molecular orbital of spiro-OMeTAD to match the valence band maximum of perovskite for efficient hole extraction while maintaining a high open circuit voltage. Time-dependent solar cell performance measurements revealed significantly improved air stability for perovskite solar cells with the n-i-p structured spiro-OMeTAD HTL showing sustained efficiencies even after 840 h of air exposure.
Project description:The simpler the design, the better and more effective it is. Two novel conjugated triarylamine derivatives in donor-?-donor structure employing biphenyl core and pyrene core as ?-bridges, which are termed as Bp-OMe and Py-OMe, have been synthesized and characterized and then applied to perovskite solar cells (PSCs) as hole-transport materials (HTMs) successfully. Using 2,2',7,7'-tetrakis(N,N-di-p-methoxy-phenylamine)-9,9'-spirobiuorene (spiro-OMeTAD) as a relative reference, Py-OMe-based PSCs showed the best power conversion efficiency (PCE) of 19.28% under AM 1.5 G illumination at 100 mW cm-2, which is comparable to that of PSCs based on spiro-OMeTAD with a best PCE of 18.57% with doping. Although Bp-OMe-based PSCs performed with relatively poor PCEs (best PCE of 15.06%) than those of Py-OMe-based PSCs, attributing to the poor planarity and hole mobility, taking the cost into consideration, Bp-OMe and Py-OMe are much more likely to be promising efficient HTMs for PSCs.
Project description:To date, most of the prevailing organic hole-transporting materials (HTMs) used in perovskite solar cells (PVSCs), such as spiro-OMeTAD and PTAA, generally require a sophisticated doping process to ensure their reasonable hole-transporting properties. Unfortunately, the employed dopants/additives and the associated oxidation reactions have been shown to deteriorate the long-term device stability seriously. The exploitation of efficient and stable dopant-free HTMs is thus strongly desired for PVSCs. However, effective molecular design strategies for dopant-free HTMs are still lacking. Thus far, only a few of them yielded comparable performance to their doped counterparts, while their synthetic costs are still high. In this work, a new class of cost-effective small molecule dopant-free HTMs have been developed using readily available fluoranthene as the structural framework. The structure-property correlation of the fluoranthene-based HTMs was carefully investigated by tuning their structural geometry (linear vs. branched), connection between electron-donating and electron-withdrawing moieties (single bond vs. ethylene), and the substitution position of the methoxy side-groups (para- vs. meta-). As a result, the optimized molecule, FBA3, was demonstrated to serve as an efficient dopant-free HTM in a conventional PVSC to deliver an impressive power conversion efficiency of 19.27%, representing one of the best cost-effective dopant-free organic HTMs reported thus far.
Project description: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 ?-? 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.
Project description:Spiro-OMeTAD with symmetric spiro-bifluorene unit has dominated the investigation of hole-transporting material (HTM) for efficient perovskite solar cells (PSCs) despite of its low intrinsic hole conductivity and instability. In this study, we designed and synthesized three asymmetric spiro-phenylpyrazole/fluorene base HTMs, namely: WY-1, WY-2 and WY-3. They exhibit excellent electrochemical properties and hole conductivities. Moreover, the PSC based on WY-1 exhibits the highest power conversion efficiency (PCE) of 14.2%, which is comparable to the control device employing spiro-OMeTAD as HTM (14.8%). These results pave the way to further optimization of both molecular design and device performance of the spiro-based HTMs.
Project description:This data in brief includes forward and reverse scanned current density-voltage (<i>J</i>-<i>V</i>) characteristics of perovskite solar cells with PDPP3T and spiro-OMeTAD as HTL, stability testing conditions of perovskite solar cell shelf life in air for both PDPP3T and spiro-OMeTAD as HTL as per the description in Ref. , and individual <i>J</i>-<i>V</i> performance parameters acquired with increasing time exposed in ambient air are shown for both type of devices using PDPP3T and spiro-OMeTAD as HTL. The data collected in this study compares the device stability with time for both PDPP3T and spiro-OMeTAD based perovskite solar cells and is directly related to our research article "solution processed pristine PDPP3T polymer as hole transport layer for efficient perovskite solar cells with slower degradation" .