Sanghyun Paek

Hybrid lead halide perovskite solar cells (PSCs) have emerged as potential competitors to silicon‐based solar cells with an unprecedented increase in power conversion efficiency (PCE), nearing the breakthrough point toward commercialization. However, for hole‐transporting materials, it is generally acknowledged that complex structures often create issues such as increased costs and hazardous substances in the synthetic schemes, when translated from the laboratory to manufacture on a large scale. Here, we present cyclobutane‐based hole‐selective materials synthesized using simple and green‐chemistry inspired protocols in order to reduce costs and adverse environmental impact. A series of novel semiconductors with molecularly engineered side arms were successfully applied in perovskite solar cells. V1366‐based PSCs feature impressive efficiency of 21 %, along with long‐term operational stability under atmospheric environment. Most importantly, we also fabricated perovskite solar modules exhibiting a record efficiency over 19 % with an active area of 30.24 cm².

The photovoltaic stability of nonencapsulated cells containing Cs _x (FAMAGua) _1−x ( x = 0.1; x = 0.2; x = 0.3) stored in the dark under air conditions for 4000 h (a), and devices under continuous light soaking at MPP tracking (b).

A non-reactive material generates beneficial band-bending resulting in higher photovoltaic performance and also stability by suppressing Pb⁰ at the perovskite surface.

A set of novel hole-transporting materials (HTMs) based on π-extension through the carbazole units were designed and synthesized via a facile synthetic procedure. The impact of isomeric structural linking on their optical, thermal, electrophysical, and photovoltaic properties were thoroughly investigated combining experimental and simulation methods. Ionisation energies of HTMs were measured and are suitable with a triple-cation perovskite active layer ensuring efficient hole injection. New materials were successfully applied in perovskite solar cells (PSCs) and yielded a promising efficiency up to almost 18% under standard 100 mW cm-2 global AM1.5G illumination and show better stability tendency outperforming that of spiro-OMeTAD. This work provides guidance for the molecular design strategy of effective hole conducting materials for perovskite photovoltaics and similar electronic devices.

Methylammonium-free perovskite was prepared by using a (PEA)₂PbI₄nanosheet/chlorobenzene suspension as the anti-solvent for enhanced device stability and photovoltaic performances.

Perovskite solar cells have set a new milestone in terms of efficiencies in the thin film photovoltaics category. Long-term stability of perovskite solar cells is of paramount importance but remains a challenging task. The lack of perovskite solar cells stability in real-time operating conditions erodes and impedes commercialization. Further improvements are essential with a view to delivering longer-lasting photovoltaic (PV) performances. An ideal path in this direction will be to identify novel dopants for boosting the conductivity and hole mobility of hole transport materials (HTMs), and by so doing the usage of hygroscopic and deliquescent additive materials can be avoided. Pyridine-based ionic liquids represent a well-known class of ultra-hydrophobic materials, which are suitable for their application in opto-electrical devices. The present work demonstrates the employment of ionic liquids into a dissymmetric fluorene-dithiophene, FDT (2’,7’ -bis(bis(4-methoxyphenyl)amino) spiro[cyclopenta[2,1-b:3,4-b’]dithiophene-4,9’-fluorene]) based HTM to understand the doping mechanisms. N-heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyidinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI) as p-type dopant for FDT was found to increase the conductivity of FDT, to higher geometrical capacitance, to facilitate homogeneous film formation, and to enhance device stability. Our findings open up a broad range of hole-transport materials to control the degradation of the underlying water-sensitive active layer by substituting hygroscopic element.

We reveal that 2D/3D interfaces are dynamical in nature which is detrimental for long term perovskite solar cells stability.

Four spirobisacridine (SBA) hole-transporting materials were synthesized and employed in perovskite solar cells (PSC). The molecules bear electronically inert alkyl chains of different length and bulkiness, attached to in-plane N-atoms of nearly orthogonal spiro-connected acridines. Di-p-methoxyphenylamine (DMPA) substituents tailored to the central SBA-platform define electronic properties of the materials mimicking the structure of the benchmark 2,2’,7,7’-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9’-spirobifluorene (spiro-MeOTAD), while the alkyl pending groups affect molecular packing in thin film and affect long-term performance of PSCs. Devices with SBA based hole transporting layers (HTL) attain effi-ciencies on par with spiro-MeOTAD. More importantly, solar cells with the new HTMs are hysteresis-free and demonstrate good operational stability, despite being doped as spiro-MeOTAD. The best performing MeSBA DMPA retained 88% of the initial efficiency after 1000 h ageing test under a constant illumination. The results clearly demonstrate: SBA-based com-pounds are potent candidates for a design of new HTMs for PSCs with improved longevity.

Surface passivation of the perovskite photo absorber is a key factor to improve the photovoltaic performance. So far robust passivation strategies have not yet been revealed. Here, we demonstrate a successful passivation strategy which controls the Fermi-level of the perovskite surface by improving the surface states. Such Fermi-level control caused band-bending between the surface and bulk of the perovskite, which enhanced the hole-extraction from the absorber bulk to the HTM side. As an added benefit, the inorganic passivation layer improved the device light stability. By depositing a thick protection layer on the complete device, a remarkable waterproofing effect was obtained. As a result, an enhancement of V_OC and the conversion efficiency from 20.5% to 22.1% was achieved. We revealed these passivation mechanisms and used perhydropoly(silazane) (PHPS) derived silica to control the perovskite surface states.

Combining halide perovskites with tailored dimensionality into two/three-dimensional (2D/3D) systems has revealed a powerful strategy to boost the performances of perovskite photovoltaics (PVs). Despite the recent advances, a clear understanding on the intimate link between interface structure and physics is still missing, leading so far to a blind optimization of the 2D/3D PVs. Here, we reveal the impact of 2D/3D crystal alignment in driving the interface charge-recombination dynamics. The 2D crystal growth and orientation is manipulated by specific fluorination of the phenethylammonium (PEA), used here as the organic cation backbone of the 2D component. By means of time-resolved optoelectronic analysis from femto- to microsecond, we demonstrate a static function of the 2D as an electron barrier and homogeneous surface passivant, together with a dynamic role in retarding back charge recombination. Our results reveal a crucial dependence of such beneficial effect with the 2D crystal orientation, leading to an enhanced open circuit voltage (VOC) if 2D lies parallel on the 3D. Such findings provide a deep understanding and delineate precise guidelines for the smart design of multidimensional perovskite interfaces for advanced PV and beyond.