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Perylene diimide (PDI)-based organic photovoltaic devices can potentially deliver high power conversion efficiency values provided the photon energy absorbed is utilized efficiently in charge transfer (CT) reactions instead of being consumed in nonradiative energy transfer (ET) steps. Hitherto, it remains unclear whether ET or CT primarily drives the photoluminescence (PL) quenching of the PDI excimer state in PDI-based blend films. Here, we affirm the key role of the thermally assisted PDI excimer diffusion and subsequent CT reaction in the process of PDI excimer PL deactivation. For our study we perform PL quenching experiments in the model PDI-based composite made of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2-6-diyl] (PBDTTT-CT) polymeric donor mixed with the N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) acceptor. Despite the strong spectral overlap between the PDI excimer PL emission and UV-vis absorption of PBDTTT-CT, two main observations indicate that no significant ET component operates in the overall PL quenching: the PL intensity of the PDI excimer (i) increases with decreasing temperature and (ii) remains unaffected even in the presence of 10 wt % content of the PBDTTT-CT quencher. Temperature-dependent wide-angle X-ray scattering experiments further indicate that nonradiative resonance ET is highly improbable due to the large size of PDI domains. The dominance of the CT over the ET process is verified by the high performance of devices with an optimum composition of 30:70 PBDTTT-CT:PDI. By adding 0.4 vol % of 1,8-diiodooctane we verify the plasticization of the polymer side chains that balances the charge transport properties of the PBDTTT-CT:PDI composite and results in additional improvement in the device efficiency. The temperature-dependent spectral width of the PDI excimer PL band suggests the presence of energetic disorder in the PDI excimer excited state manifold.

The N2-adsorption isotherms of porous solids are received in the form V = f(P/Po). If such data are depicted in the form V(1 − P/Po) = f(P/Po) or V(1 − P/Po)/(P/Po) = f(V(1 − P/Po)), they show an identical inversion point (I-point, Inversion or Inflection point), which corresponds exactly to the monolayer volume Vm. As a result the value of specific surface area S is determined with unique precision via the trivial relationship S (m2 g−1) = 4.356Vm (cm3 g−1) without any use of the classic BET treatment. The derivates dV/d[(P/Po)] vs. (P/Po) and d(V[1 − (P/Po)])/d[(P/Po)] vs. (P/Po) exhibit maxima exactly at the same pressure (P/Po), while the second order derivative d2V/d[(P/Po)]2 vs. (P/Po) exhibits a minimum exactly at the same pressure (P/Po) with the I-point. As a consequence the specific surface area of solids can be determined just from experimental raw (P/Po), V data without any use, neither of linear BET treatment nor the Inversion or Inflection type graphs. For some MCM-41 type mesoporous materials, the I-plots show not one but two Inversion points/I-points. The first inversion/inflection point at lower (P/Po) values, “ILP-point”, corresponds to the completion of the adsorption procedure on the bare surface and the formation of monolayer and is useful for the determination of specific surface area S. The second inversion/inflection point at higher (P/Po) values, “IHP-point”, corresponds to the completion of the pore filling via condensation and may be useful for the determination of mesopore volume.

The performance of organic photovoltaic devices (OPV) with nanostructured polymer:perylene diimide (PDI) photoactive layers approaches the levels of the corresponding polymer:fullerene systems. Nevertheless, a coherent understanding of the difficulty for PDI-based OPV devices to deliver high power conversion efficiencies remains elusive. Here we perform a comparative study of a set of four different polymer:PDI OPV model systems. The different device performances observed are attributed to differences in the nanostructural motif of these composites, as determined by wide-angle X-ray scattering (WAXS) measurements. Long-range structural order in the PDI domain dictates (i) the stabilization energy and (ii) the concentration of the PDI excimers in the composites. The quenching of the PDI excimer photoluminescence (PL) is found to be insensitive to the former, but it depends on the latter. High PL quenching occurs for the low concentration of PDI excimers that are formed in PDI columns with a length comparable to the PDI excimer diffusion length. The stabilization of the PDI excimer state increases as the long-range order in the PDI domains improves. The structural order of the PDI domains primarily affects charge transport. Electron mobility reduces as the size of the PDI domain increases, suggesting that well-ordered PDI domains suffer from poor electronic connectivity. WAXS further reveals the presence of additional intermolecular PDI interactions, other than the direct face-to-face intermolecular coupling, that introduce a substantial energetic disorder in the polymer:PDI composites. Conventional device architectures with hole-collecting ITO/PEDOT:PSS bottom electrodes are compared with inverted device architectures bearing bottom electron-collecting electrodes of ITO/ZnO. In all cases the ZnO-functionalized devices surpass the performance of the conventional device analogues. X-ray photoelectron spectroscopy explains that in PEDOT:PSS-functionalized devices, the PDI component preferentially segregates closer to the hydrophilic PEDOT:PSS electrode, thus impeding the efficient charge extraction and limiting device photocurrent.

In this work we provide evidence of microporosity in the nanostructured mesoporous aluminosilicates MSU-S/W(BEA) synthesized from zeolite BEA seeds, based on the so called I-point plots (refs. 18–20) of the BET equation. Seventeen solids were characterized for their porous characteristics as follows: (i) one mesoporous aluminosilicate 2%Al-MSU-S/W(BEA), (ii) two typical mesoporous MCM-41 materials, one pure silica and one aluminosilicate 2%Al-MCM-41, (iii) three typical microporous zeolites, i.e., zeolites Y, ZSM-5 and beta, (iv) two typical amorphous silica–alumina (ASA) samples, (v) five mechanical mixtures of mesoporous Al-MCM-41 with zeolite beta at different addition percentages, and (vi) four amorphous or partially crystalline ZSM-5 based aluminosilicates. The P,V data from the nitrogen-adsorption isotherms of those porous solids were plotted according to the I-point modification of the BET equation in the form V[1 − (P/P0)]/(P/P0) = CVm − (C − 1)V[1 − (P/P0)]. Plots of V[1 − (P/P0)]/(P/P0) vs. V[1 − (P/P0)] provided lines which have the shape of an inclined V, i.e. >, and variable slopes corresponding to (C − 1) from which the values of the C parameter were determined for the whole range of partial pressures 0 < P/P0 < 1. The inversion point of the above plots, called the I-point, projected onto the V[1 − (P/P0)] axis corresponds exactly to the volume of the monolayer Vm = VI[1 − (PI/P0)], from which the value of the specific surface area SI (m2 g−1) was determined. The shape of the I-plots depended strongly on the type of porous solids, i.e. microporous/zeolitic and partially crystalline zeolitic materials with combined micro- and meso/macroporosity; ordered mesoporous MCM-type materials; ASA materials with random mesoporosity; and mechanical mixtures of zeolite beta with MCM-41. The I-plot for the MSU-S/W(BEA) solid exhibited the combined characteristics of the plots of ordered mesoporous MCM-41 samples and of partially crystalline zeolitic materials, thus proving the presence of microporous domains within the homogeneous mesoporous structure of the MSU-S material. The I-plot result supports the proposed presence of protozeolitic subunits in the framework walls, of MSU-S, even though XRD and electron diffraction TEM studies have shown the material to be free of any zeolitic crystalline phase. The surface areas estimated by the I-method were similar to those provided by the BET model for the zeolitic and the ASA samples, while in the case of the ordered MCM-41 and MSU-S/W(BEA) mesostructures, two surface area values can be estimated based on different interpretations of the I-method, one similar to the value from the BET model and one supported by the modified CPSM model (refs. 31–34).