Amidination of ligands for chemical and field-effect passivation stabilizes perovskite solar cells.

Surface passivation has driven the rapid increase in the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, state-of-the-art surface passivation techniques rely on ammonium ligands that suffer deprotonation under light and thermal stress. We developed a library of amidinium ligands, of interest for their resonance effect-enhanced N-H bonds that may resist deprotonation, to increase the thermal stability of passivation layers on perovskite surfaces.

Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal-halide perovskites.

The technique of alloying FA with Cs is often used to promote structural stabilization of the desirable α-FAPbI phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in CsFAPbI perovskite thin-films and solar-cell devices.

Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection.

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500-600 nm-thick nonfullerene blends.

Efficient and Stable Perovskite Solar Cells with a High Open-Circuit Voltage Over 1.2 V Achieved by a Dual-Side Passivation Layer.

Suppressing nonradiative recombination at the interface between the organometal halide perovskite (PVK) and the charge-transport layer (CTL) is crucial for improving the efficiency and stability of PVK-based solar cells (PSCs). Here, a new bathocuproine (BCP)-based nonconjugated polyelectrolyte (poly-BCP) is synthesized and this is introduced as a "dual-side passivation layer" between the tin oxide (SnO ) CTL and the PVK absorber. Poly-BCP significantly suppresses both bulk and interfacial nonradiative recombination by passivating oxygen-vacancy defects from the SnO side and simultaneously scavenges ionic defects from the other (PVK) side.

Static Disorder in Lead Halide Perovskites.

In crystalline and amorphous semiconductors, the temperature-dependent Urbach energy can be determined from the inverse slope of the logarithm of the absorption spectrum and reflects the static and dynamic energetic disorder. Using recent advances in the sensitivity of photocurrent spectroscopy methods, we elucidate the temperature-dependent Urbach energy in lead halide perovskites containing different numbers of cation components. We find Urbach energies at room temperature to be 13.

Probing Charge Generation Efficiency in Thin-Film Solar Cells by Integral-Mode Transient Charge Extraction.

The photogeneration of free charges in light-harvesting devices is a multistep process, which can be challenging to probe due to the complexity of contributing energetic states and the competitive character of different driving mechanisms. In this contribution, we advance a technique, integral-mode transient charge extraction (ITCE), to probe these processes in thin-film solar cells. ITCE combines capacitance measurements with the integral-mode time-of-flight method in the low intensity regime of sandwich-type thin-film devices and allows for the sensitive determination of photogenerated charge-carrier densities.

Direct observation of trap-assisted recombination in organic photovoltaic devices.

Trap-assisted recombination caused by localised sub-gap states is one of the most important first-order loss mechanism limiting the power-conversion efficiency of all solar cells. The presence and relevance of trap-assisted recombination in organic photovoltaic devices is still a matter of some considerable ambiguity and debate, hindering the field as it seeks to deliver ever higher efficiencies and ultimately a viable new solar photovoltaic technology. In this work, we show that trap-assisted recombination loss of photocurrent is universally present under operational conditions in a wide variety of organic solar cell materials including the new non-fullerene electron acceptor systems currently breaking all efficiency records.

Charge-generating mid-gap trap states define the thermodynamic limit of organic photovoltaic devices.

Detailed balance is a cornerstone of our understanding of artificial light-harvesting systems. For next generation organic solar cells, this involves intermolecular charge-transfer (CT) states whose energies set the maximum open circuit voltage V. We have directly observed sub-gap states significantly lower in energy than the CT states in the external quantum efficiency spectra of a significant number of organic semiconductor blends.

Charge Carrier Transport and Generation via Trap-Mediated Optical Release in Organic Semiconductor Devices.

The impact of intermixed donor-acceptor domains in organic bulk heterojunction (BHJ) solar cells, using low-donor-content devices as model systems, is clarified. At low donor contents, the devices are found to exhibit anomalously high open-circuit voltages independent of the donor-acceptor energetics. These observations can be consistently explained by a theoretical model based on optical release of trapped holes, assuming the donors behave as trap sites in the gap of the acceptor.