Aurelien Sokeng Djoumessi

End‐group fluorination of the well‐known non‐fullerene acceptor (NFA) ITIC extends the absorption spectrum to the near‐infrared (NIR), increasing the solar cells’ photocurrent compared to the non‐fluorinated version. Here, ITIC and its fluorinated variants (asymmetric ITIC‐2F ^* and symmetric ITIC‐4F) were synthesized, and the effect of fluorination on physicochemical, optical, electronic, and photovoltaic properties was systematically investigated. Fluorination increases the acceptors’ electron affinity (EA), consequently reducing the device photovoltage. The NFA's molecular quadrupole moment (QM) increases with the degree of fluorination, which improves exciton dissociation and reduces charge recombination at the donor‐acceptor (D‐A) interface. Simultaneously, the NFA ionization energy (IE) increases, facilitating hole transfer to the donor. Both effects improve the internal quantum efficiency (IQE) of solar cells upon NFA fluorination. Donor fluorination increases the donor's IE, potentially hampering hole transfer. Unfortunately, cyclic voltammetry (CV) recordings of single materials fail to accurately describe the energetic conditions at D‐A heterojunctions. Here, the unique capability of energy‐resolved electrochemical impedance spectroscopy (ER‐EIS) to detect the density of states (DOS) shows that the donor's IE is lower in blends than in the pristine material, thereby enabling hole transfer. In combination with the increased NFAs EA, enhanced charge generation quantum yields are obtained; however, ultimately, a trade‐off exists between photocurrent and photovoltage.

Like other carbon nanoparticles, diamond nanoparticles, also known as nanodiamonds (NDs), tend to aggregate when they are dispersed in solution or when they are cast on a substrate. This is mainly due to the versatility of functional groups present on their surface. Previous studies have reported the use of several techniques, including chemical modification, surface active compound usage, and mechanical milling using tiny zirconia beads, for destroying the ND aggregates. Herein, we focus on the deposition of hydrogen‐terminated NDs (H‐NDs) for use as electron transport layer material in inverted organic solar cells and we investigate different approaches to prevent or to eliminate aggregation during the coating of films of H‐NDs, including the reduction of the ND concentration in the dispersions and the blending of H‐NDs powder with additives or binders such as styrene‐butadiene rubber, carboxymethyl cellulose, a combination of both, fluoride‐based polyvinylidene fluoride, and the conjugated polyelectrolyte poly(9,9‐bis(3′‐(N, N‐dimethyl)‐N‐ethylammonium‐propyl‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene))dibromide (PFN‐Br), in dispersions. The film morphology and more specifically the device's performance and stability were improved in several cases and single nanoparticles of diamonds with small sizes (<20 nm) were observed.

Diamond nanoparticles so‐called nanodiamonds (NDs) have recently experienced raising scientific interest due to interesting optical and electronic properties, nontoxicity, biocompatibility, and large surface area. Another significant feature of NDs is the versatility of the surface chemistry, where various functional groups can be attached. This provides an excellent platform for adjusting NDs properties and functions for many applications including in photovoltaic devices. Herein, high‐pressure high‐temperature (HPHT) NDs are tested as charge extraction material in organic solar cells using various surface chemistries: as‐received (HPHT ND‐ar), oxidized (HPHT ND‐O), and hydrogenated (HPHT ND‐O‐H) NDs. Despite the high work function values (≈5.3 eV) of HPHT ND‐ar and HPHT ND‐O, which make these materials normally suitable for hole extraction, devices made with them failed. In contrast, the work function decreases upon hydrogenation (≈4.5 eV) of the beforehand oxidized NDs, making them interesting for electron extraction. By employing such HPHT ND‐O‐H for electron extraction layers, PBDB‐T:ITIC‐based devices reach 77%, while PM6:Y6‐based devices reach even 85% of the performance when process on standard ZnO electron transport layers. Improvement of the film‐forming qualities of this new electron extraction material is expected to further improve the performance.

Thermal annealing (TA) is one of the most used processing techniques for the fabrication of polymer solar cells. The method is not only used to dry solution‐processed films but also for improving the electrical conductivity and the optical absorption of the materials after coating as films. Its use is questionable in combination with high throughput manufacturing techniques such as roll‐to‐roll large‐scale production since annealing times of several minutes already put up high demands on oven lengths. Furthermore, the commonly used flexible substrates are not compatible with high processing temperatures. Therefore, there is a need for more rapid treatment techniques, which do not negatively affect the plastic substrate. Herein, it is successfully demonstrated that flash lamp annealing (FLA) can be a valid alternative to TA. The FLA technique is applied at various pulse durations and thus energy doses to PEDOT:PSS and SnO₂ films used as charge extraction layers in conventional and inverted solar cells architectures. In combination with PM6:Y6 photoactive layer, the obtained device's performances are comparable or even better than the devices treated with the classical TA on a hotplate. In terms of energy efficiency, the FLA is clearly more efficient than the TA, even at the size of prototypes.

With the latest prominent rise in the development of organic solar cells (OSCs), there is a continuous influx of interested students in the OSC research and development field.¹ Lately, researchers have been fascinated mainly by novel non-fullerene acceptors (NFAs) combined with high-performing mid-bandgap polymers.^2,3 However, poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM) blend is one of the most-studied photoactive layers in OSCs.^{4 – 9} According to Wantz et al.,¹⁰ a remarkable number of papers were published between 2002 and 2010. After a decade, there is still interest in P3HT:PCBM-based devices due to their novel properties and useful aspects compared to NFA-based solar cells.^{11 – 13} Due to the polymers versatile properties, many organic materials have been developed for photovoltaic applications over the last 20 years of applications.^{14 – 17} With the development of novel NFAs that match well with the old P3HT, efficiencies in the range of 10% and beyond are within sight. One of the greatest benefits that P3HT offers over the novel mid-bandgap polymers is that it can be synthesized with a high yield at relatively low costs. Generally speaking, the more complex the synthesis, the higher the associated costs, making it very hard to reach the same economics as with P3HT production.¹⁸ In addition, NFAs that are tailor-made for P3HT nicely complement its limited absorption range and may thus be the perfect partner for P3HT. Moreover, there is still ongoing research for materials that can boost P3HT-based OSCs over the 10% efficiency limit. Despite significant advancements concerning OSC power conversion efficiency (PCE) over the past decade, there is still an ongoing debate over the physical processes restraining the device performance.^19,20

As the device performance and stability of polymer solar cells strongly depend on the interfacial charge extraction layers, the hole transport layer (HTL) properties are crucial. Furthermore, unfavorable interactions with the electrode or the photoactive layer should be screened and prevented. Organic solar cells of conventional architecture by varying the HTL material and layer stack systematically between PEDOT:PSS and a sol–gel‐derived tungsten oxide (WO₃) are investigated. The impact of various HTLs in the solar cells is investigated by optical and electrical characterization. Interestingly, a triple‐layer WO₃/PEDOT:PSS/WO₃ configuration results in the best device performance specifically compared with the use of pristine WO₃ and pristine PEDOT:PSS hole extraction layers. The triple layer also shows an increased reproducibility in the lifetime, which, combined with the improvement in the efficiency, can be the keys for expectable revenue.