Alternative Organic Spacers for More Efficient Perovskite Solar Cells Containing Ruddlesden–Popper Phases

The
halide perovskite Ruddlesden–Popper (RP) phases are
a homologous layered subclass of solution-processable semiconductors
that have aroused great attention, especially for developing long-term
solar photovoltaics. They are defined as (A′)2(A)n−1PbnX3n+1 (A′ = spacer cation, A = cage
cation, and X = halide anion). The orientation control of low-temperature
self-assembled thin films is a fundamental issue associated with the
ability to control the charge carrier transport perpendicular to the
substrate. Here we report new chemical derivatives designed from a
molecular perspective using a novel spacer cation 3-phenyl-2-propen­ammonium
(PPA) with conjugated backbone as a low-temperature strategy to assemble
more efficient solar cells. First, we solved and refined the crystal
structures of single crystals with the general formula (PPA)2(FA0.5MA0.5)n−1PbnI3n+1 (n = 2 and 3, space group C2) using X-ray
diffraction and then used the mixed halide (PPA)2(Cs0.05(FA0.88­MA0.12)0.95)n−1­Pbn(I0.88Br0.12)3n+1 analogues to achieve more efficient devices. While forming
the RP phases, multiple hydrogen bonds between PPA and inorganic octahedra
reinforce the layered structure. For films we observe that as the
targeted layer thickness index increases from n =
2 to n = 4, a less horizontal preferred orientation
of the inorganic layers is progressively realized along with an increased
presence of high-n or 3D phases, with an improved
flow of free charge carriers and vertical to substrate conductivity.
Accordingly, we achieve an efficiency of 14.76% for planar p–i–n
solar cells using PPA-RP perovskites, which retain 93.8 ± 0.25%
efficiency with encapsulation after 600 h at 85 °C and 85% humidity
(ISOS-D-3).