Prof Henry Snaith FRS

Abstract:

State-of-the-art perovskite-based solar cells employ expensive, organic hole transporting materials (HTMs) such as Spiro-OMeTAD that, in turn, limits the commercialization of this promising technology. Herein an HTM (EDOT-Amide-TPA) is reported in which a functional amide-based backbone is introduced, which allows this material to be synthesized in a simple condensation reaction with an estimated cost of <$5 g−1. When employed in perovskite solar cells, EDOT-Amide-TPA demonstrates stabilized power conversion efficiencies up to 20.0% and reproducibly outperforms Spiro-OMeTAD in direct comparisons. Time resolved microwave conductivity measurements indicate that the observed improvement originates from a faster hole injection rate from the perovskite to EDOT-Amide-TPA. Additionally, the devices exhibit an improved lifetime, which is assigned to the coordination of the amide bond to the Li-additive, offering a novel strategy to hamper the migration of additives. It is shown that, despite the lack of a conjugated backbone, the amide-based HTM can outperform state-of-the-art HTMs at a fraction of the cost, thereby providing a novel set of design strategies to develop new, low-cost HTMs.

Abstract:

Perovskite-based photovoltaics are an emerging solar technology with lab scale device efficiencies of over 22 %, and significant steps are being made toward their commercialization. Conventionally high efficiency perovskite solar cells are formed from high boiling point, polar aprotic solvent solutions. Methylammonium lead iodide (CH3NH3PbI3) films can be made from a range of solvents and blends; however, the role the solvent system plays in determining the properties of the resulting perovskite films is poorly understood. Acetonitrile (ACN), in the presence of methylamine (MA), is a viable nontoxic solvent for fabrication of CH3NH3PbI3 photovoltaic devices with efficiencies >18 %. Herein we examine films prepared from ACN/MA and dimethylformamide (DMF) and scrutinize their physical and electronic properties using spectroscopy, scanning probe imaging, and ion scattering. Significant differences are observed in the chemistry and electronic structure of CH3NH3PbI3 films made with each solvent, ACN/MA produces films with superior properties resulting in more efficient photovoltaic devices. Here we present a holistic and complete understanding of a high performance perovskite material from an electronic, physical, and structural perspective and establish a robust toolkit with which to understand and optimize photovoltaic perovskites.

Abstract:

Intrinsic and extrinsic optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI3) thin films, whose background hole doping density was varied through SnF2 addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 1020 cm-3, a strong Burstein-Moss effect increases absorption onset energies by ~300meV beyond the band gap energy of undoped FASnI3 (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (1020 cm-3), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 1019 cm-3 and below, the charge-carrier mobility increases with decreasing temperature according to ~T-1.2, suggesting it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2´1018 cm^-3, charge-carrier mobilities reach a value of 67 cm2V-1s-1at room temperature and 470 cm2V-1s-1 at 50 K. Intra-excitonic transitions observed in the THz-frequency photoconductivity spectra at 5K reveal an exciton binding energy of only 3.1 meV for FASnI3, in agreement with the low bandgap energy exhibited by this perovskite.