In Operando, Photovoltaic, and Microscopic Evaluation of Recombination Centers in Halide Perovskite-Based Solar Cells.

ACS applied materials & interfaces (2021)

Authors:

Arava Zohar, Michael Kulbak, Silver H Turren-Cruz, Pabitra K Nayak, Adi Kama, Anders Hagfeldt, Henry J Snaith, Gary Hodes, David Cahen

Abstract:

The origin of the low densities of electrically active defects in Pb halide perovskite (HaP), a crucial factor for their use in photovoltaics, light emission, and radiation detection, remains a matter of discussion, in part because of the difficulty in determining these densities. Here, we present a powerful approach to assess the defect densities, based on electric field mapping in working HaP-based solar cells. The minority carrier diffusion lengths were deduced from the electric field profile, measured by electron beam-induced current (EBIC). The EBIC method was used earlier to get the first direct evidence for the n-i-p junction structure, at the heart of efficient HaP-based PV cells, and later by us and others for further HaP studies. This manuscript includes EBIC results on illuminated cell cross sections (in operando) at several light intensities to compare optoelectronic characteristics of different cells made by different groups in several laboratories. We then apply a simple, effective single-level defect model that allows deriving the densities (Nr) of the defect acting as recombination center. We find Nr ≈ 1 × 1013 cm-3 for mixed A cation lead bromide-based HaP films and ∼1 × 1014 cm-3 for MAPbBr3(Cl). As EBIC photocurrents are similar at the grain bulk and boundaries, we suggest that the defects are at the interfaces with selective contacts rather than in the HaP film. These results are relevant for photovoltaic devices as the EBIC responses distinguish clearly between high- and low-efficiency devices. The most efficient devices have n-i-p structures with a close-to-intrinsic HaP film, and the selective contacts then dictate the electric field strength throughout the HaP absorber.

Universal Current Losses in Perovskite Solar Cells Due to Mobile Ions

ADVANCED ENERGY MATERIALS (2021) ARTN 2101447

Authors:

Jarla Thiesbrummel, Vincent M Le Corre, Francisco Pena-Camargo, Lorena Perdigon-Toro, Felix Lang, Fengjiu Yang, Max Grischek, Emilio Gutierrez-Partida, Jonathan Warby, Michael D Farrar, Suhas Mahesh, Pietro Caprioglio, Steve Albrecht, Dieter Neher, Henry J Snaith, Martin Stolterfoht

Balanced Charge Carrier Transport Mediated by Quantum Dot Film Post-organization for Light-Emitting Diode Applications.

ACS applied materials & interfaces 13:22 (2021) 26170-26179

Authors:

Yuljae Cho, Jongchul Lim, Meng Li, Sangyeon Pak, Zhao-Kui Wang, Ying-Guo Yang, Antonio Abate, Zhe Li, Henry J Snaith, Bo Hou, SeungNam Cha

Abstract:

In light-emitting diodes (LEDs), balanced electron and hole transport is of particular importance to achieve high rates of radiative recombination. Most quantum dot (QD)-based LEDs, however, employ infinitesimal core-shell QDs which inherently have different electron and hole mobilities. As QDs are the core building blocks of QD-LEDs, the inherent mobility difference in the core-shell QDs causes significantly unbalanced charge carrier transport, resulting in detrimental effects on performances of QD-LEDs. Herein, we introduce a post-chemical treatment to reconstruct the QD films through the solvent-mediated self-organization process. The treatment using various poly-alkyl alcohol groups enables QD ensembles to transform from disordered solid dispersion into an ordered superlattice and effectively modulate electron and hole mobilities, which leads to the balanced charge carrier transport. In particular, ethanol-treated QD films exhibit enhanced charge carrier lifetime and reduced hysteresis due to the balanced charge carrier transport, which is attributed to the preferential-facet-oriented QD post-organization. As a result, 63, 78, and 54% enhancements in the external quantum efficiency were observed in red, green, and blue QD-LEDs, respectively. These results are of fundamental importance to understand both solvent-mediated QD film reconstruction and the effect of balanced electron and hole transport in QD-LEDs.

Charge-Carrier Mobility and Localization in Semiconducting Cu2AgBiI6 for Photovoltaic Applications.

ACS energy letters 6:5 (2021) 1729-1739

Authors:

Leonardo RV Buizza, Adam D Wright, Giulia Longo, Harry C Sansom, Chelsea Q Xia, Matthew J Rosseinsky, Michael B Johnston, Henry J Snaith, Laura M Herz

Abstract:

Lead-free silver-bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu2AgBiI6 using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu2AgBiI6 are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm2 V-1 s-1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ∼29 meV and a direct band gap near 2.1 eV, these findings highlight Cu2AgBiI6 as an attractive lead-free material for photovoltaic applications.

Adduct-based p-doping of organic semiconductors.

Nature Materials Nature Research (2021)

Authors:

Nobuya Sakai, Ross Warren, Fengyu Zhang, Simantini Nayak, Junliang Liu, Sameer V Kesava, Henry J Snaith, Moritz Riede, Yen-Hung Lin, Himansu S Biswal, Xin Lin, Chris Grovenor, Tadas Malinauskas, Aniruddha Basu, Thomas D Anthopoulos, Vytautas Getautis, Antoine Kahn, Pabitra K Nayak

Abstract:

Electronic doping of organic semiconductors is essential for their usage in highly efficient optoelectronic devices. Although molecular and metal complex-based dopants have already enabled significant progress of devices based on organic semiconductors, there remains a need for clean, efficient and low-cost dopants if a widespread transition towards larger-area organic electronic devices is to occur. Here we report dimethyl sulfoxide adducts as p-dopants that fulfil these conditions for a range of organic semiconductors. These adduct-based dopants are compatible with both solution and vapour-phase processing. We explore the doping mechanism and use the knowledge we gain to 'decouple' the dopants from the choice of counterion. We demonstrate that asymmetric p-doping is possible using solution processing routes, and demonstrate its use in metal halide perovskite solar cells, organic thin-film transistors and organic light-emitting diodes, which showcases the versatility of this doping approach.