Role of the Dopants on the Morphological and Transport Properties of Spiro-MeOTAD Hole Transport Layer

The use of a solid hole transport layer (HTL) was transformational for the recent perovskite solar cell (PSC) revolution in solar energy technology. Often high efficiency PSC devices employ heavily doped hole transport materials such as spiro-MeOTAD. Independent of HTL chemistry, lithium-bis-trifluoromethanesulfonyl-imide (LiTFSI) and tert-butylpyridine (TBP) are commonly used as additives in HTL formulations for PSCs. LiTFSI and TBP were originally optimized for dye-sensitized solar cells, where their roles have been extensively studied. However, in the case of PSCs, the function of TBP is not clearly understood. In this study, properties of the HTL composite deposited on flat silicon substrates were systematically measured at several length scales, e.g., macroscopically (profilometry, 4-point probe conductivity, and thermogravimetry-differential thermal analysis), microscopically, and at the nanoscale to investigate film morphology, conductivity, and dopant distribution. Microscopic distributions of spiro-MeOTAD, LiTFSI, and TBP were determined using 2D Fourier transform infrared (FTIR) microscopy and electrostatic atomic force microscopy (EFM). Our findings reveal that the main role of TBP is to prevent phase segregation of LiTFSI and spiro-MeOTAD, resulting in a homogeneous hole transport layer. These properties are critical for charge transport in the HTL bulk film as well as at the perovskite/HTL and HTL/electrode interfaces and for efficient solar cell performance.