“…The linear fits provided the exponents α = 0.99 for the PffBT4T-2OD:PC 70 BM, α = 1.00 for the PffBT4T-2OD:PC 70 BM w/DIO, α = 0.92 for the PffBT4T-2OD:bis-PDI and α = 0.96 for the PffBT4T-2OD:bis-PDI w/DIO systems. A stronger deviation of α from 1 for the bis-PDI-based OSCs indicates that J SC for these devices is limited by bimolecular recombination losses 40 , however such deviation in α could also be assign to space charge limited effects 42,43 or to inefficient charge extraction caused by an improper choice of charge carrier-collecting electrodes 44,45 .Figure 5( a ) Light intensity ( I ) dependence current density ( J SC ) and ( b ) charge carrier lifetime (τ) versus charge carrier density (n) for PffBT4T-2OD:PC 70 BM and PffBT4T-2OD:bis-PDI based OSCs. Scattered points represent experimental data points, while the dashed and solid lines demonstrate the fitting according to the equations in ( a ) and in ( b ), respectively 41,48 .…”
Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.
“…The linear fits provided the exponents α = 0.99 for the PffBT4T-2OD:PC 70 BM, α = 1.00 for the PffBT4T-2OD:PC 70 BM w/DIO, α = 0.92 for the PffBT4T-2OD:bis-PDI and α = 0.96 for the PffBT4T-2OD:bis-PDI w/DIO systems. A stronger deviation of α from 1 for the bis-PDI-based OSCs indicates that J SC for these devices is limited by bimolecular recombination losses 40 , however such deviation in α could also be assign to space charge limited effects 42,43 or to inefficient charge extraction caused by an improper choice of charge carrier-collecting electrodes 44,45 .Figure 5( a ) Light intensity ( I ) dependence current density ( J SC ) and ( b ) charge carrier lifetime (τ) versus charge carrier density (n) for PffBT4T-2OD:PC 70 BM and PffBT4T-2OD:bis-PDI based OSCs. Scattered points represent experimental data points, while the dashed and solid lines demonstrate the fitting according to the equations in ( a ) and in ( b ), respectively 41,48 .…”
Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.
“…However, in the initial stage, PDI derivatives suffer from over-strong propensity of aggregation, leading to the formation of large crystalline domains [26][27][28][29]. Moreover, some PDI derivatives intend to form excimers that perform as traps for excitation energy which severely hinder the efficiency of exciton splitting and the further performance of the OSCs [30][31][32][33].…”
“…The origin of the micrometer L D in rubrene is in long-lived triplet excitons that diffuse through Dexter mechanism 15 . Acceptor materials like perylenediimides (PDIs), whose optoelectronic properties can be controlled by functionalization at peri-, bay-and orthopositions 20,21,22,23 , can also yield long range energy transfer even for singlet excitons 24 . Several strategies have been envisioned to induce longer L D , such as promotion of singletfission to increase the yield of long living triplets 20 .…”
Exciton diffusion is at the heart of most organic optoelectronic devices' operation, and it is currently the most limiting factor to their achieving high efficiency. It is deeply related to molecular organization, as it depends on intermolecular distances and orbital overlap. However, there is no clear guideline for how to improve exciton diffusion with regard to molecular design and structure. Here, we use single-crystal charge-transfer interfaces to probe favorable exciton diffusion. Photoresponse measurements on interfaces between perylenediimides and rubrene show a higher photocurrent yield (+50%) and extended spectral coverage (+100 nm) when there is increased dimensionality of the percolation network and stronger orbital overlap. This is achieved by very short interstack distances in different directional axes, which favors exciton diffusion by a Dexter mechanism. Even if the core of the molecule shows strong deviation from planarity, the similar electrical resistance of the different systems, planar and nonplanar, shows that electronic transport is not compromised. These results highlight the impact of molecular organization in device performance and the necessity of optimizing it to take full advantage of the materials' properties.
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