In this Review, the extreme care that must be taken when predicting the optical properties of conjugated polymers via the oligomer approach, and when comparing theoretical and experimental data, is illustrated. In the first part, conceptual strategies for the correct determination of optical transitions from experimental spectra and relevant extrapolation procedures at the polymer limit are introduced. The impact of conformational, substitution, solvent, and solid‐state effects on the optical properties is discussed in light of experimental data reported for molecular backbones based on phenylene, phenylenevinylene, and thiophene repeat units. A comparison is then made between experimental results and those provided by standard quantum‐chemical methods, to assess their reliability.

Oligophenylenevinylenes (OPV) with a series of distance-controlling, electron-donating, and/or electron-withdrawing substituents are deposited from vapor phase and solution as ultrathin films or nanoparticles with diameters of 20−200 nm. In some cases the systems are doped at levels of 10-5−10-3 with energy accepting OPV's of longer chainlengths. Absorption and fluorescence spectra, steady-state and time-resolved anisotropies, and radiative and nonradiative deactivation rates of these systems are investigated and compared to the corresponding properties in dilute solutions. Fluorescence yields of the parent oligomers and their alkyl or oxyalkyl derivatives are high in solution with an “infinite chain” limit of ΦF ≈ 0.5 and an upper radiative rate constant limit of k r ∞ = (1 ± 0.3) × 109 s-1. Yields and k r decrease strongly in films and nanoparticles because of H-aggregate formation. However, doping with fluorescent acceptors can increase the yields up to ΦF → 0.7. Introduction of electron-withdrawing -CN and -SO2CF3 substituents reduces ΦF in low viscous dilute solutions almost to zero. High viscosities and condensation to solid phases will raise the yields up to ΦF → 0.6 because of suppression of nonradiative torsional deactivation and formation of J-aggregates with high k r.

The present Perspective critically re-examines the photophysics of paradistyrylbenzene (DSB) as a prototype of herringbone-arranged H-aggregates to resolve the apparent contradiction of the frequently reported "aggregation-induced emission quenching" in H-aggregates on one side and highly emissive DSB crystals on the other and discusses the signatures and fate of excitons in single-and polycrystalline samples, including size and polarization effects.

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US$ within the next few years. [1] Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5-6% have been certified at NREL and AIST usually on devices with small active areas.[2]The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10-12%.[3] Several reasons for the power conversion efficiency limitation have been identified.[1] Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor-acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. [4][5][6] The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells.

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly­(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A–D–A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2′-((2Z,2′Z)-((12,13-bis­(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo­[3,4-e]-thieno­[2″,3″:4′,5′]­thieno-[2′,3′:4,5]­pyrrolo-[3,2-g]­thieno-[2′,3′:4,5]­thieno-[3,2-b]­indole-2,10-diyl)­bis­(methanylylidene))­bis­(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))­dimalononitrile) with an A–DA′ D–A configuration has further propelled the PCEs to go beyond 15% due to smaller E loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure–property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

The spectroscopic response of a poly(3‐hexylthiophene)/[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT/PCBM)‐based bulk heterojunction solar cell is extended into the near infrared region (NIR) of the spectrum by adding the low bandgap polymer poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b´]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] [PCPDTBT] to the blend. The dominant mechanism behind the enhanced photosensitivity of the ternary blend is found to be a two‐step process: first, an ultrafast and efficient photoinduced charge transfer generates positive charges on P3HT and PCPDTBT and a negative charge on PCBM. In a second step, the positive charge on PCPDTBT is transferred to P3HT. Thus, P3HT serves two purposes. On the one hand it is involved in the generation of charge carriers by the photoinduced electron transfer to PCBM, and, on the other hand, it forms the charge transport matrix for the positive carriers transferred from PCPDTBT. Other mechanisms, such as energy transfer or photoinduced charge transfer directly between the two polymers, are found to be absent or negligible.

The influence of environmental factors on the degradation process of P3HT film has been investigated quantitatively. The decay kinetics of the polymer absorption during variation of intensity and spectral distribution of the incident light, oxygen concentration, humidity level as well as temperature are monitored using infrared and UV/vis absorption spectroscopy. Additionally, the oxygen diffusion into the polymer film has been investigated using fluorescence spectroscopy under the same experimental conditions. The degradation process is light initiated with a strong increase of the effectiveness toward the ultraviolet region of the spectrum. The observed photo oxidation is not oxygen diffusion limited although an activation energy of 26 kJmol -1 was observed for both degradation and oxygen diffusion. The observed kinetics, especially its dependence on wavelength of the incident light, point to a radical-based degradation process in the solid state rather than a singlet oxygen-based mechanism as it is observed in the liquid phase. Furthermore the presence of humidity strongly affects the degradation process although water itself does not decompose the polymer. Changing the structure of the polymer from regioregular to regiorandom significantly accelerates the degradation, probably due to the higher triplet yield of the regiorandom polymer.

The intermolecular arrangement in the solid state and the consequences on the optical and photophysical properties are studied on different derivatives of oligophenylenevinylenes by UV/VIS absorption and angular-resolved polarized fluorescence spectroscopy. Unsubstituted distyrylbenzene (DSB) organizes in a herringbone manner, with the long axes of the molecules oriented in parallel, but the short axes almost perpendicular to each other. Fluorinated distyrylbenzene (F(12)DSB) as well as the DSB:F(12)DSB cocrystals prefer cofacial pi-stacking in the solid state. For all structures, the consequence of the parallel alignment of the transition moments is a strongly blueshifted H-type absorption spectrum and a low radiative rate constant k(F). Significant differences are observed for the emission spectra: the perpendicular arrangement of the short axes in DSB crystals leads to only very weak intermolecular vibronic coupling. Hence the emission spectrum is well structured, very similar to the one in solution. For F(12)DSB and DSB:F(12)DSB, the cofacial arrangement of the adjacent molecules enables strong intermolecular vibronic coupling of adjacent molecules. Thus, an unstructured and strongly redshifted excimerlike emission spectrum is observed. The differences in the electronic nature of the excited states are highlighted by quantum-chemical calculations, revealing the contribution of interchain excitations to the electronic transitions.