Strain effects on halide perovskite solar cells

Abstract

This review systematically describes the origins, characterization and implications of strain in perovskite solar cells and proposes novel control strategies.

Physical Mechanism and Chemical Trends in the Thermal Expansion of Inorganic Halide Perovskites

“…A high temperature also accelerates chemical reactions and instabilities of functional layers such as perovskite films and organic hole transport materials. , In addition, thermally induced stress and strain in PSCs resulting from significant thermal expansion mismatch between perovskites and substrates − also constitute an important degradation mechanism, but one that is just beginning to be explored . For example, the residual tensile stresses in perovskite films deposited on glass substrates can exceed 50 MPa, a value high enough to deform copper. , …”

“…12 For example, the residual tensile stresses in perovskite films deposited on glass substrates can exceed 50 MPa, a value high enough to deform copper. 13,14 The basic formula of halide perovskites is ABX 3 , where A is an alkali metal or organic ammonium cation [e.g., Cs + , methylammonium CH 3 NH 3 + (MA), or formamidinium NH 2 CH�NH 2 + (FA)], B is a divalent metal cation (e.g., Pb 2+ , Sn 2+ , or Ge 2+ ), and X is a halogen anion (I − , Br − , or Cl − ). The chemical design space in halide systems is extensive due to the weak ionic bonding, and a succession of derived structures [e.g., double perovskites and Ruddlesden−Popper (RP) perovskites] can be obtained via elemental substitution and altering the chemical stoichiometry.…”

Defect Passivation Results in the Stability of Cesium Lead Halide Perovskite Nanocrystals

“…Colloidal nanocrystals (NCs) of lead halide perovskites (LHPs) exhibit exciting optoelectronic properties for various device applications because of their high photoluminescence quantum yields (PLQYs), high absorption coefficients, size, and composition-tunable optoelectronic properties. − However, these materials are enriched with surface defects due to the presence of a large number of halide vacancies . These defect states act as the trapping centers for charge carriers, resulting in the lowering of PLQY and stability of LHPs. , All of these factors greatly affect the power conversion efficiency (PCE) as well as the lifetime of photovoltaic devices. − Thus, there is a need to engineer the defect chemistry via modifying the surface of LHPs, which can tailor the optoelectronic properties and enhance the stability for better practical applications.…”

Strain Release and Defect Passivation in Formamidinium-Dominated Perovskite via a Novel in-Plane Thermal Gradient Assisted Crystallization Strategy

“…Recently, many reports have demonstrated that the tensile strain causes lattice distortion of the microscopic crystal structure, weakens the bonds, induces the defects, reduces the activation energy for ion migration, and further accelerates the degradation of perovskites. ,, The tensile strain is mainly caused by the heterogeneous crystallization of the polycrystalline perovskite and the thermal expansion mismatch (e.g., 3.70 × 10 –6 K –1 for glass, 9.86 × 10 –5 K –1 for α-FAPbI 3 ) between the perovskites and the substrates during the annealing process, which can hardly be modulated by the postannealing treatment. − Strain engineering has been developed as an efficient approach to enhance the performance and stability of PSCs, because it can affect the band structure of the perovskite, the formation energy of defects, the activation energies for halide ion migration, and the intrinsic stability of the photoactive perovskite phase. − In this context, three approaches have been reported to mitigate the harmful tensile strain in the fabrication process of perovskite films: (i) decreasing the local crystal misorientation by optimizing the nucleation and film growth of the perovskite; (ii) fabricating the perovskite films under a low-temperature procedure or reducing the gap of thermal expansion coefficients between the perovskite and the substrate; , (iii) utilizing additives with various flexible chains to facilitate the release of residual strain at grain boundaries. , Despite their effectiveness in bringing down the tensile strain in perovskite films, these approaches possess limitations to further enhancing the efficiency and stability of PSCs . Moreover, a high annealing temperature (e.g., 150 °C) is vital to promote sufficient conversion from δ phase to α phase of FAPbI 3 perovskite.…”