A universal ligand for lead coordination and tailored crystal growth in perovskite solar cells

Abstract

Chemical environment and precursor-coordinating molecular interactions within a perovskite precursor solution can lead to important implications in structural defects and crystallization kinetics of a perovskite film. Thus, the opto-electronic quality of such films can be boosted by carefully fine-tuning the coordination chemistry of perovskite precursors via controllable introduction of additives, capable of forming intermediate complexes. In this work, we employed a new type of ligand, namely 1-phenylguanidine (PGua), which coordinates strongly with the PbI₂ complexes in the perovskite precursor, forming new intermediate species. These strong interactions effectively retard the perovskite crystallization process and form homogeneous films with enlarged grain sizes and reduced density of defects. In combination with an interfacial treatment, the resulted champion devices exhibit a 24.6% efficiency with outstanding operational stability. Unprecedently, PGua can be applied in various PSCs with different perovskite compositions and even in both configurations: n-i-p and p-i-n, highlighting the universality of this ligand.

Fig. 1. The interaction of PbI₂ and PGua (a) chemical structure of PGua and GuaX; (b) binding energy of DMSO and PGua with PbI₂ precursor complexes. Replacement of DMSO with PGua in a high-valent iodoplumbate [PbI₄]²⁻ results in the release of iodide ions. The following color code is used for the atomic representations: purple, I; cyan, Pb; blue, N; red, O; yellow, S; gray, C; pinkish white, H. Distances between ligands and iodide and the Pb center are given in units of Å, highlighted in the color of the respective species. (c) ¹H NMR spectra of PGua solution and the mixed solution of PGua and PbI₂, which were dissolved in DMSO-d₆; (d) photos of different DMSO solutions, the concentration in all cases is 0.1 M; (e) XPS core level signals of Pb 4f of PbI₂ film and of PbI₂(PGua) complex film; (f) XRD patterns of PGua and PbI₂(PGua) complex films. The molar ratio of PGua and PbI₂ is 1 : 1 in the above experiments (e) and (f).

Fig. 2. Opto-electronic properties of perovskite films. Spatially resolved photoluminescence (PL) measurements of (a) reference perovskite film and (b) perovskite films with PGua, scale bar – 6 μm. (c) Absolute PL measurement of a reference and PGua-treated perovskite films on glass with respective values for PLQY and QFLS. (d) Generation-dependent PLQY measurements fitted according to a two-trap level SRH model. (e) TRPL measurements of reference and PGua-treated perovskite films at ∼1 sun equivalent intensity and calculated differential lifetime based on the multi-exponential fits (white lines).

Fig. 3. DFT calculations on the passivation mechanism. Optimized geometry structures of the perovskite slabs passivate with one PGua molecule for (a) the surface with one PbI₂ vacancies (V_PbI2), see Fig. S10 (ESI†) for top view, and (b) for the surface with iodine Frenkel defects (V⁺_I/I⁻_i). Main interacting atomic distances and location of the defects are also highlighted with dashed circles. Following color code is used for the atomic representations: purple, I; cyan, Pb; blue, N; red, O; yellow, S; gray, C; pinkish white, H.

Fig. 4. PSCs photovoltaic performance. (a) JV-curves of reference and PGua treated perovskite films obtained from forward voltage sweep (FS) and reverse voltage sweep (RS). The inset shows the incident photon to electron conversion efficiency (IPCE) with an integrated J_SC. (b) Pseudo JV-curves of reference and PGua treated perovskite films on glass reconstructed from the intensity-dependent QFLS measurements (inset), showing potential PCE of the corresponding films. (c) V_oc, FF and PCE loss analysis based on the data extracted from the light intensity-dependent measurements (complementary data can be found in Note S4, ESI†). (d) JV- and pseudo JV-curves and characteristics of the manufactured PSC with PGua and interfacial passivation with CEAI.

Fig. 5. The stability test of PSCs. (a) Long-term stability of unencapsulated devices from different conditions under MPPT in N₂ atmosphere at room temperature; and (b) thermal stability of unencapsulated devices from different conditions at 85 °C in the N₂ atmosphere, five devices for each condition.