PerspectiveApplied Physics

Powering up perovskite photoresponse

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Science  24 Mar 2017:
Vol. 355, Issue 6331, pp. 1260-1261
DOI: 10.1126/science.aam7154

The most notable scientific milestone in photovoltaics in the past several years is the emergence of solar cells based on hybrid organic-inorganic perovskite materials. While conventional silicon and thin-film solar cells have seen steady improvements in their power-conversion efficiencies (PCEs) spanning several decades, hybrid perovskite solar cells have already reached a certified 22.1% PCE (1), matching conventional solar cell technologies in only a few years since their first device architecture was tested. Setting the stage for a disruptive technology in the field of photovoltaics is the seemingly winning combination of properties of hybrid perovskite materials: high absorption coefficient and a tunable energy band gap in wavelengths ideal for solar cells; long diffusion lengths and lifetimes for photogenerated charge carriers, which easily dissociate into efficiently collected electrons and holes; Earth-abundant elemental composition; and their compatibility with low-cost and low-temperature fabrication methods (25). On page 1288 of this issue, Blancon et al. (6) report on the observation of an enhanced photoresponse for layered perovskite materials. The results add, literally, a new dimension to the further development of high-performance perovskite solar cells.

2D perovskites and layer-edge states

The breakup of 3D metal-halide perovskites into networks of reduced dimensionality with new electronic states known as layer-edge states opens the possibility for wide-ranging applications in solar cells.

GRAPHIC: V. ALTOUNIAN/SCIENCE

There are crucial factors debilitating hybrid perovskite materials in solar cells. A variety of causes contribute to their instability, including moisture and light-induced degradation; defective surface structure; and ion migration under operating conditions (7, 8). In essence, hybrid perovskites are ionic salts, readily soluble in polar solvents, with the unit cell formula AMX3 (X is a halogen, M is a metal that can coordinate to six halides, and A is a small organic or inorganic cation). The perovskite structure is a three-dimensional (3D) network of corner-sharing [MX6] octahedra, balanced with an equal number of positively charged cations residing within the small cavity spaces left behind by the metal halide network (see the figure). The geometric restriction imposed by the size of the cavity limits the choice of A-site species to a small set of cations such as methylammonium (MA), formamidinium (FA), or Cs; and the M-site to Pb, Sn, or Ge. Of the latter group of elements, Pb-based perovskites are the most stable. These geometric constraints for 3D perovskites have made it difficult to find materials within this family of compounds that are both stable and possess the optoelectronic characteristics demanded by solar cell applications.

Substituting the small A-site cation, either partially or completely, with organic cations larger than the perovskite network cavities leads to the breakup of this metal halide network into sheets, rods, or clusters, spatially separated by sublattices of the large cations. Such compounds are often referred to as reduced-dimensionality or low-dimensional (2D for sheets, 1D for rods, and 0D for isolated clusters) perovskite derivatives (912). Owing to the large number of possible cations compared to the 3D counterpart, the potential set of such compounds is practically enumerable. Moreover, the flexibility offered by the unrestricted size of the organic cation allows for freedom to tailor the properties of the materials (through the appropriate use of functional organic molecules) such that they are made intrinsically more robust against the degradation factors affecting 3D perovskites. Furthermore, the relaxed geometric requirements in low-dimensional perovskites offer additional paths for designing perovskites with more environmentally benign elements than Pb, but without compromising performance.

Despite these advantages, the large exciton binding energy and quantum confinement induced by the low dimensionality and layered nature of these compounds implies that they are inherently excitonic materials, in which the photogenerated electron-hole pairs are strongly bound and thus tend to recombine before they can separate into free charges and be collected. The prevailing wisdom is that these reduced-dimensionality perovskites are potentially useful for light-emitting applications, but are inefficient for solar cells and photodetectors, for which efficient electron-hole separation is a prerequisite. The results of Blancon et al. might change this widely accepted view.

Blancon et al. reinvestigated a type of reduced-dimensionality perovskite known as Ruddlesden-Popper-type 2D perovskites. In these 2D structures, the [PbI6] octahedra are arranged in equal-thickness sheets periodically separated by large n-butylammonium (BA) cations (see the figure). The thickness of the sheet can be varied from a one-octahedra-thick layer (n = 1) up to five-octahedra-thick layers (n = 5) by controlling the ratio of small MA to large BA cations. While studying single crystals (n ≤ 2) of these materials with time-resolved photoluminescence, the authors noted that excitons generated in the bulk of the crystals recombined quickly (see the figure). Yet, surprisingly, for crystals with n > 2, excitons that reached the edge of the crystal within a picosecond time scale encountered new electronic states, called layer-edge states (LESs). In these states, the separated charge carriers survive for a long time before recombining to reemit a photon. Consequently, for n > 2 2D perovskites, fabricating solar cells from polycrystalline films of crystal grain sizes smaller than the diffusion length of the excitons (10 to 100 nm) guarantees that a maximal number of excitons will reach LESs, and thus can be separated and collected efficiently. Indeed, the study observed very low PCEs for cells based on n = 1 and 2 (no LESs), and impressively high efficiencies as soon as n exceeded 2 (LESs present).

The discovery of LESs that protect charges from fast recombination in 2D perovskites is in stark contrast to 3D perovskite surfaces, which tend to be defective and riddled with undesirable trap states that prevent charge carriers from being collected. Additionally, from a fundamental physics and materials property standpoint, the precisely controlled quantum confinement of these 2D perovskites with LESs makes them ideal systems for studying the physics of highly confined free-carrier states.a situation that is difficult to replicate in other materials systems. The finding of Blancon et al. opens the possibility for engineering similar LESs in other members of the low-dimensional perovskite family; as such, a large group of materials that photovoltaic researchers mostly overlooked in the past now become prime competitors in optoelectronics, offering an expanded toolbox to remediate many of the impediments plaguing 3D perovskites.

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