Polarons in perovskite solar cells: effects on photovoltaic performance and stability

Abstract

Organic-inorganic hybrid perovskites manifest unique photophysical properties in terms of long carrier lifetime, low recombination rate and high defect tolerance, enabling them promising candidates in optoelectronic devices. However, such advancing properties are unexpected in the perovskite materials with moderate charge mobility. Recent investigations have revealed that these appealing properties were endowed as the formation of large polarons in perovskite crystals due to the coupling of photogenerated carrier and polarized crystal lattice, which largely affected carrier transport dynamics and structural stability of perovskite solar cells (PSCs). In this review, crystal structure of perovskite lattice and formation mechanism of polaron are firstly elucidated. And then, the modulation of polaron states in PSCs including large polaron stabilization, polaron facilitated charge transport, hot carrier solar cells, and polaron related stability issues such as polaron induced metastable defects, polaronic strain, photostriction are systematically investigated. In the end, prospect in the further understanding and manipulation of polaron related phenomenon towards highly efficient and stable PSCs is suggested.

Accurate and Efficient Computation of the Fundamental Bandgap of the Vacancy-Ordered Double Perovskite Cs2TiBr6

“…Based predominantly on evidence from DFT studies − ,− and optical experiments, Cs 2 TiBr 6 has been identified as a promising thin-film solar cell absorber material. , However, highly efficient hybrid organic–inorganic solar cell absorbers show exciton binding energies ∼15 meV, , enabling rapid thermal dissociation of the exciton (at k B T ∼ 25 meV), and the subsequent formation of Wannier–Mott polarons. It has been proposed that weakly bound polarons reduce recombination rates while retaining moderate charge carrier mobility. − The large difference between the fundamental and optical bandgaps of Cs 2 TiBr 6 at zero-temperature indicates significant exciton binding and electron–phonon interaction energies, apparently precluding Cs 2 TiBr 6 as an efficient solar cell absorber. Alternatively, this work supports applications for Cs 2 TiBr 6 such as white LEDs, broadband photodetectors, or lasers. ,, Accurate spectral properties are important in the design of new ionic semiconductors for diverse applications ranging from solar cells to memristors and pressure induced emission .…”

“…In contrast to conventional semiconductors, MHPs are polar materials where excited charge carriers can distort the ionic lattice creating a potential well, where they are self-trapped as a polaron . There has been increasing evidence that polaronic trapping in a soft, ionic crystalline lattice facilitates the spatial separation of excited charge carriers, resulting in decreased recombination rates combined with moderate carrier mobilities. − In addition, excitons with short lifetimes can facilitate the efficient absorption of solar radiation. Hybrid organic–inorganic MHP thin-film solar cell absorbers have achieved certified power conversion efficiencies (PCE) of up to 26.1% for a single-junction at low-cost, with the possibility of layered absorbers to further increase PCE. ,, However, stability and lead-toxicity impede their large-scale commercialization. − …”

Fabrication strategies for high quality halide perovskite films in solar cells

“…[6][7][8][9][10][11] Benefiting from the hybrid composition, halide perovskites exhibit excellent properties such as adjustable band gap, high carrier mobility, large optical absorption coefficient, small exciton binding energy, and solution processability. [12][13][14][15][16][17] These properties are crucial to achieve high performance and low cost solar cells.…”

Perspective on High-Resolution Characterizations of Polarons in Halide Perovskites

“…Polarons have a profound impact on material properties such as electron−phonon interactions, charge transport, and ion migration and, therefore, affect device functionalities and efficiencies. 6 Halide perovskites are a promising class of optoelectronic material. Both small polarons and large polarons are present in halide perovskites due to their ionic lattice with large polarity, playing an important role in properties such as charge transport and phase segregation.…”

“…Small polarons are trapped on a single ion, while large polarons move in a band with a large effective mass. Polarons have a profound impact on material properties such as electron–phonon interactions, charge transport, and ion migration and, therefore, affect device functionalities and efficiencies …”