All-perovskite–based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite–based tandem devices has been limited by the lack of high-efficiency, low–band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs). We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low–band gap (~1.25 electron volt) perovskite films. The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers. These improved properties enable our demonstration of >20% efficient low–band gap PSCs. When combined with wider–band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite–based polycrystalline thin-film tandem solar cells.

molecule based OPVs these gains arise primarily from enhanced short-circuit photocurrent ( J sc ), due to the enhanced dispersion and molecular organization of the donor and acceptor phases, and through the use of new donor polymers with higher ionization potentials (IP) and lower band-gaps than the original poly(thiophene) donors, simultaneously extending the spectral response of the OPV and increasing the open-circuit voltage ( V oc ). [3][4][5] Increasing IP in the donor poly mer increases the frontier orbital energy differences which is known to control the V OC followingwhere E HOMO and E LUMO are the transport energy levels for active layer materials minus what has been an observed offset of 0.3 V [ 5 ] which is related to the polaron pair binding energy. [ 6 ] It has been shown that increasing the frontier orbital energy difference also lowers the probability for dark charge transfer at the D/A inter face, decreasing the reverse saturation current density ( J sat ) which helps V oc to reach its maximum obtainable value. [7][8][9] Full dispersion of the donor and acceptor materials in these devices also increases the probability for charge recombination at the contact electrodes since both donor and acceptor

Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky 2D cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic 2D spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluoro-phenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained power conversion efficiency >13% for (F-PEA)2MA4Pb5I16 based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.

We report the charge carrier recombination rate and spin coherence lifetimes in single crystals of two-dimensional (2D) Ruddlesden−Popper perovskites PEA 2 PbI 4 •(MAPbI 3 ) n−1 (PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4). Layer thickness-dependent charge carrier recombination rates are observed, with the fastest rates for n = 1 because of the large exciton binding energy, and the slowest rates are observed for n = 2. Room-temperature spin coherence times also show a nonmonotonic layer thickness dependence with an increasing spin coherence lifetime with increasing layer thickness from n = 1 to n = 4, followed by a decrease in lifetime from n = 4 to ∞. The longest coherence lifetime of ∼7 ps is observed in the n = 4 sample. Our results are consistent with two contributions: Rashba splitting increases the spin coherence lifetime going from the n = ∞ to the layered systems, while phonon scattering, which increases for smaller layers, decreases the spin coherence lifetime. The interplay between these two factors contributes to the layer thickness dependence.

We have utilized a commercially available metal-organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoO x ) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal-organic precursor by 115 C in air. Acetonitrile solutions spin-cast in a N 2 atmosphere and annealed in air yield continuous thin films of MoO x . Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoO x and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoO x thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C 61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoO x HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo 5+ gap states in the MoO x HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoO x HCL.