Joel Troughton

Molecular encapsulation is a promising strategy to reduce non-radiative losses and enhance stability of organic solar cells, via reduced aggregation in the polymer phase. Encapsulated polymers demonstrated enhanced electroluminescence and better donor–acceptor intermixing.

The efficiency of tin–lead perovskite solar cells (TLPSC) has been consistently increasing. However, their photostability continues to be a persistent challenge. Besides the oxidation of tin (Sn), the presence of methylammonium (MA) has been correlated with poor device photostability, which still remains unclear. In this work, we unravel the influence of the A-site cation choice, MA vs. cesium (Cs), on the photostability and performance of TLPSCs where Cs_0.25−xMA_xFA_0.75Sn_0.5Pb_0.5I₃ (FA: formamidinium; x = 0.0–0.25) is used as a photoactive layer. We observe that the addition of MA (i.e. increasing x) improves the device efficiency while reducing the photostability. This reduction is attributed to light-induced interfacial barriers, surface recombination, and ion diffusion. We found that MA volatilizes from the surface of MA-rich devices during illumination, leading to an irreversible performance loss after light exposure. We further demonstrate that MA content rather than the choice of the hole-transport layer (PEDOT:PSS vs. NiO_x) dominates the device photostability. Under continuous 1 sun illumination, devices without MA and with the use of NiO_x as a transport layer are able to retain 80% of their efficiency for 690 h, while MA-rich cells retain 80% of their efficiency only for the first 3 h of operation. Our work provides insights into the development of photostable TLPSC.

Monolithically stacked tandem solar cells present opportunities to absorb more of the sun's radiation while reducing the degree of energetic loss through thermalization. In these applications, the bandgap of the tandem's constituent subcells must be carefully adjusted so as to avoid competition for photons. Organic photovoltaics based on nonfullerene acceptors (NFAs) have recently exploded in popularity owing to the ease with which their electrical and optical properties can be tuned through chemistry. Here, highly complementary and efficient 2‐terminal tandem solar cells are reported based on a wide bandgap amorphous silicon absorber, and a narrow bandgap NFA bulk‐heterojunction with power conversion efficiencies (PCEs) exceeding 15%. Interface engineering of this tandem device allows for high PCEs across a wide range of light intensities both above and below “1 sun.” Furthermore, the addition of an inorganic silicon subcell enhances the operational stability of the tandem by reducing the light‐stress experienced by the bulk heterojunction, resolving a long‐standing stumbling block in organic photovoltaic research.

A bilayer of Nb-TiO₂ and bathocuproine forms a highly ohmic contact between a wide variety of semiconducting materials and metal electrodes. This enables performance and stability improvements in a range of electronic devices.

Two novel star-shaped triarylamine-based hole transport materials with triphenylamine (STR1), or a partially oxygen-bridged triphenylamine (STR0), as core and para-substituted triphenylamine side arms were synthesized, fully characterized and studied in perovskite solar cells. Their thermal, optical, electrochemical and charge transport properties were examined and compared in the context of their molecular structure. Due to its more planar configuration, STR0 showed a red-shifted absorption in comparison with STR1. STR0 also forms a more stable amorphous glassy state and showed higher glass transition temperature than STR1 and spiro-OMeTAD. These HTMs were tested in perovskite solar cells using a device configuration of FTO/bl-TiO₂/mp-TiO₂/CH₃NH₃PbI₃/HTM/Au showing a power conversion efficiency of 13.3% for STR0 and 11.5% for STR1. The STR0-based devices showed higher fill factor and better reproducibility than spiro-OMeTAD-based cells. Without dopant additives, solar cells based on STR0 exhibited a good photocurrent density of 16.63 mA cm⁻² and the efficiency improved from a starting PCE of 3.9% to 6.6% after two weeks of storage.

Charge accumulation at the electron and hole transport layers generates anomalous electrical behavior in perovskite solar cells (PSCs). Hysteresis in the current–voltage characteristic and recombination at the interfaces are the clearest manifestations of this phenomenon, which compromises device performance and stability. Here, the underlying charge‐carrier dynamics of a variety of PSCs are investigated by analyzing their transient photocurrent response. Towards shorter time scales, PSCs often show increasingly severe hysteretic responses. This phenomenon is correlated with the presence of interfacial accumulated charges that hinders the photogenerated carrier extraction process. However, introducing passivating contacts improves the carrier‐injection properties and the devices become completely hysteresis free. These results underline the importance of contact passivation for PSCs and the need to further develop new passivating interlayers that simultaneously eliminate charge‐carrier recombination and provide selective transport for each carrier type at the PSC's contacts.

Perovskite solar cells based on organolead halides such as CH₃NH₃PbX₃ (X = Cl, Br, and I) have rapidly established themselves as the frontrunners among emerging photovoltaic technologies. However, their commercial application has been hindered to date in part due to their susceptibility to degradation by UV radiation or heat in the presence of moisture. Herein we investigate the relationship between the physical properties of several polymer encapsulants (poly(methylmethacrylate) (PMMA), ethyl cellulose, polycarbonate and poly(4-methyl-1-pentene)) and their ability to function as barrier layers to improve the stability of CH₃NH₃PbI_3−xCl_x films under prolonged thermal degradation at 60 °C, 80 °C and 100 °C. In all cases, polymer-coated CH₃NH₃PbI_3−xCl_x films showed retarded thermal degradation compared to the uncoated films, as indicated by the quantitative decay of the perovskite band edge in the UV/Vis absorption spectrum and the appearance of PbI₂ peaks in the powder X-ray diffraction pattern. However, the extent of this reduction was highly dependent on the physical properties of the polymer encapsulant. Notably, PMMA-coated CH₃NH₃PbI_3−xCl_x films showed no visible signs of degradation to PbI₂ after extended heating at 60 °C. However, concomitant studies by epifluorescence microscopy (FM) revealed deterioration of the CH₃NH₃PbI_3−xCl_x film quality, even in the presence of a polymer-coating, at much shorter heating times (29 h), as evidenced by quenching of the film fluorescence, which was attributed to grain aggregation and the formation of associated non-radiative trap sites. Since grain aggregation occurs on a shorter timescale than chemical degradation to PbI₂, this may be the limiting factor in determining the resistance of organolead halide perovskite films to thermal degradation.