Strain relaxation and phase regulation in quasi-2D perovskites for efficient solar cells

Branched iso-butylammonium (iso-BA(+)) tends to form a perfectly vertical crystal orientation and thus is a good choice for spacer cations. Currently, a planar p-i-n structure with PEDOT:PSS as a hole transport layer (HTL) is commonly used for iso-BA based devices. PTAA is a better candidate for hole transport compared to PEDOT:PSS, but iso-BA based perovskite solar cells (PSCs) with a PTAA HTL are rarely reported. In this work, we find that iso-BA based perovskite films prepared on a PTAA HTL have significant tensile strain, which results in the destruction of the PTAA layer after spin-coating the PCBM solution in chlorobenzene (CB) on top of the PTAA/(iso-BA)(2)MA(n-1)Pb(n)I(3n+1) (n = 4) film arising from the CB penetrating through the perovskite layer. To address this issue, we introduce 4-fluoro-phenethylammonium (F-PEA(+)) as a second spacer cation (SSC), which effectively releases the tensile strain in the perovskite film, thereby preventing the CB penetration. Moreover, the introduction of the F-PEA SSC promotes the formation of large-n phases in the perovskite film, achieving uniform phase distribution. The photo-generated excitons in small-n phases can be efficiently transferred to the adjacent large-n phases, and then the free carriers are transported in an interpenetrating charge transport network constructed by large-n phases, which greatly improves the exciton dissociation and carrier transport in the PSCs. By introducing the F-PEA SSC, we obtain a remarkable PCE of 17.17%, the highest efficiency so far for iso-BA based PSCs. Due to the release of tensile strain and optimization of phase distribution, the stability of the devices is also dramatically improved after introducing the F-PEA SSC.