A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'″-di(2-octyldodecyl)-2,2';5',2″;5″,2'″-quaterthiophen-5,5'″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C butyric acid methyl ester (PC BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC BM solar cells show significant efficiency loss under simulated solar irradiation ("burn in" degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC BM devices.

Here, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance.

Increasing the open circuit voltage (Voc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction and their loss mechanisms inside a device upon illumination. Herein we report the important origin of Voc losses in methylammonium lead iodide perovskite (MAPI) based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the PEDOT:PSS hole transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in Voc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50-nm-thick device compared to 230-nm-thick one). In contrast, accumulated positive charges at the TiO2/MAPI interface modify interfacial energy band bending, which leads to an increase in Voc for the TiO2/MAPI conventional devices (70 mV increase in 50-nm-thick device compared to 230-nm-thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.

Nanoscale morphology is critical to determining the device efficiency of bulk heterojunction organic solar cells, and the ideal structure is often described as a threephase network with one well-mixed phase for efficient charge separation and two purer phases for efficient charge transport. In order to understand such nanoscale morphology, we have performed detailed spectroscopic investigations and identified the three-phase morphology evolution in one of the classic blend systems, P3HT:PCBM. The impact of different phases on polymer molecular (chain conformational) order, blend thermal and optical properties were monitored in situ using resonant Raman, absorption and photoluminescence spectroscopy techniques. Semi-crystalline P3HT was found to accommodate up to ~25% PCBM (by weight) in its amorphous phase, with very little impact on either polymer molecular order or aggregation. Higher concentrations of PCBM resulted in a greater proportion of amorphous mixed phase and reduced polymer molecular order and aggregation. On the other hand, the formation of crystalline purer phases via phase separation was evident during in situ thermal annealing, revealing a consistent glass transition temperature (Tg) of ~50 °C in blends with up to 50 %wt PCBM. This indicates similar local chemical compositions in the amorphous mixed phase present in blends despite different overall blend ratios. A much higher Tg (80-100 °C) was observed for blends with >50 %wt PCBM, indicating a stronger impact of PCBM on P3HT molecular order and thermal properties, requiring a higher annealing temperature to ensure formation of the preferred 3-phase morphology.

Printable organic sensors fabricated from solution-processed π-conjugated polymers (π-CPs) are promising candidates to detect volatile organic compounds (VOCs) due to the intriguing physical, chemical and electronic properties of π-CPs. These...