Solar-to-electricity conversion efficiency, power conversion efficiency (PCE), and stability are two important aspects of perovskite solar cells (PSCs). However, both aspects are difficult to simultaneously enhance. In the recent two years, two-dimensional (2D)/three-dimensional (3D) stacking structure, designed by covering the 3D perovskite with a thin 2D perovskite capping layer, was reported to be a promising method to achieve both a higher PCE and improved stability simultaneously. However, when reducing the surface defects of 3D perovskite, the thin 2D capping layer itself may probably introduce additional interfacial defects in a 2D/3D stacking structure, which is thought to be able to trigger trap-assisted nonradiative recombination or ion migration. Thus, efforts should be paid to reduce the interfacial defects of 2D hybrid perovskite when serving as a modification layer in a 2D/3D stacking structure PSCs. Here, we demonstrate that bromine (Br) doping of the 2D perovskite capping layer is an efficient strategy to passivate interfacial defects robustly, by which the photoluminescence lifetime is enhanced notably, whereas the interfacial charge recombination is suppressed a lot. As a result, the PCE is enhanced from 18.01% (3D perovskite) to 20.07% (Br-doped 2D/3D perovskite) along with improved moisture stability.
Defects locating within grain boundaries or on the film
surface,
especially organic cation vacancies and iodine vacancies, make the
fabrication of perovskite solar cells (PSCs) with superior performance
a challenge. Organic ammonium iodide is a promising candidate and
has been frequently used to passivate these defects by forming two-dimensional
(2D) perovskite. In this work, it is found that the chain length of
organic ammonium iodide is a crucial factor on the defect passivation
effect. Compared to butylammonium iodide, the hexylammonium iodide
(HAI)-derived 2D perovskite is more efficient in decreasing interfacial
defects, resulting in a notably enhanced photoluminescence lifetime
and a more suppressed interfacial charge recombination process. As
a consequence, the ultimate power conversion efficiency (PCE) has
reached 20.62% (3D + HAI) as compared to 18.83% (3D). Moreover, the
long-term durability of the corresponding PSCs against humidity and
heat is simultaneously improved. This work once again demonstrates
that the 2D/3D structure is promising for further improving the PCE
and stability of PSCs.
For highly interested organolead perovskite based solar cells, the exciton and free carriers are the photoproducts in the working layers. In this study, we revealed their two forms of relations depending on heat-annealing condition. In non-annealed films and single crystal, they are in density-dependent dynamical balance (co-existing). For the sufficiently heat-annealed films, they present a significant emissive exciton-carrier collision (ECC). The two relations indicate the emergence of a subgrain morphology within the tetragonal phase of crystal grain, induced by heat annealing process. Such subgrain structure could be assigned to a ferroelastic twinning structure recently found inside the crystal grain of the films. Since the heat annealing is a general procedure in preparing perovskite working layers, we propose that the ECC and subgrain morphology widely exist in real devices. We suggest that the subgrain structure provides another level of morphological basis for in depth understanding high performance of organolead perovskite working layers.
(MA)Pb(SCN)I, a new pseudohalogen-based 2D perovskite material, was reported as a very stable and promising photo-absorber in PSCs previously. However, the later researchers found that MAPb(SCN)I was not as stable as claimed. Thus, it is very critical to clarify the controversy and reveal the degradation mechanism of MAPb(SCN)I. On the other hand, a large number of studies have indicated that adding a small amount of SCN improves surface topography and crystallinity. However, whether SCN ions can be incorporated into a 3D perovskite film remains debatable. In this work, the thermal degradation pathway of (MA)Pb(SCN)I is revealed by thermal gravimetric and differential thermal analysis coupled with quadrupole mass spectrometry and density functional theory calculations. The decomposition of (MA)Pb(SCN)I has been proved experimentally to be more complex than that of MAPbI, involving four stages and multi-reactions from room temperature to above 500 °C. By combining the experimental results and theoretical calculations, it is found that 2D (MA)Pb(SCN)I actually is unstable when serving as photo-absorber in PSCs. Moreover, the role of SCN in improving the crystallinity of 3D perovskite has also been discussed in detail.
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