Abstract
Despite the ubiquity and importance of organic hole-transport materials in photovoltaic devices, their intrinsic low conductivity remains a drawback. Thus, chemical doping is an indispensable solution to this drawback and is essentially always required. The most widely used p-type dopant, FK209, is a cobalt coordination complex. By reducing Co(III) to Co(II), Spiro-OMeTAD becomes partially oxidized, and the film conductivity is initially increased. In order to further increase the conductivity, the hygroscopic co-dopant LiTFSI is typically needed. However, lithium salts are normally quite hygroscopic, and thus, water absorption has been suggested as a significant reason for perovskite degradation and therefore limited device stability. In this work, we report a LiTFSI-free doping process by applying organic salts in relatively high amounts. The film conductivity and morphology have been studied at different doping amounts. The resulting solar cell devices show comparable power conversion efficiencies to those based on conventional LiTFSI-doped Spiro-OMeTAD but show considerably better long-term device stability in an ambient atmosphere.
Highlights
Organic hole-transport materials (HTMs) are widely used in solid-state, dye-sensitized solar cells[1,2] and perovskite solar cells (PSCs).[3,4] due to low intrinsic conductivities, the addition of dopants is typically needed
The structures of the synthesized organic salts are based on the molecular building blocks of existing HTMs, which means that the dopant itself can act as the HTM
(MeO-TPD)TFSI can be considered as a substructure of Spiro(TFSI)[2], where one of the fluorene derivatives has been eliminated from the structure. (TBD)TFSI was considered as a substructure of (MeO-TPD)TFSI, in which the phenyl parts have been removed and replaced with methyl groups
Summary
Organic hole-transport materials (HTMs) are widely used in solid-state, dye-sensitized solar cells (ssDSSCs)[1,2] and perovskite solar cells (PSCs).[3,4] due to low intrinsic conductivities, the addition of dopants is typically needed. The complex may protect the perovskite layer from the detrimental effects of the commonly used additives lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (TBP) and thereby reduce the concentration of recombination sites.[10] Copper salts represent another family of inexpensive and widely studied dopants. The reason was attributed to the hygroscopic properties of the lithium salts, while the halogen atoms in the TCNQ molecules were postulated to increase the hydrophobicity of the holetransport layer (HTL) Another investigated material is 1,1,2,2tetrachloroethane (TeCA).[16] This compound was considered as a low-cost, chlorinated organic solvent, which can be used both as a cosolvent and an effective additive to Spiro-. The stability of radical cations from the Spiro-OMeTAD substructures demonstrates a simple but efficient route to the design of organic salts for the purpose of LiTFSI-free and stable perovskite solar cells
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