Organic solar cells (OSCs) are composed of one or more layers of order 100 nm thickness sandwiched between metallic and transparent electrodes. As such, they are low finesse, multilayer optical cavities where the optical field distribution is governed by the complex refractive indices and thicknesses of all layers in the “solar cell stack”. Optical interference and parasitic absorbance in nonactive layers can have a dramatic effect on the shape of the measured external quantum efficiency (EQE), the parameter often used to optimize device structure and derive critical insight regarding charge generation and extraction. In this communication, we study a model high efficiency OSC system (PCDTBT/PC70BM) as a function of active layer thickness, blend composition and processing. The spectral shapes of the measured EQEs show strong thickness and blend ratio dependence. However, when correctly determined, the internal quantum efficiencies (IQEs) are spectrally flat. The differences in EQE spectral shape predominantly originate from optical interference and parasitic absorptions rather than charge generation or transport phenomena. We also demonstrate similar results for a second model system (PCPDTBT/PC60BM) in which an energy-dependent “IQE-like” response has recently been used to justify the existence of hot excitons. Once again, we show the origin of these spectral phenomena to be optical, not electronic. These cases highlight the fact that thin film organic solar cells (even single junction) must be properly considered as low finesse electro-optical cavities, a point that is not universally appreciated.

A comprehensive study was made of the synthesis of a spectrum of poly(dialkylstannane)s by catalytic dehydropolymerization of dialkylstannanes (dialkyltin dihydrides, R 2 SnH 2 , prepared by reduction of R 2 SnCl 2 ), with R ) ethyl, propyl, butyl, pentyl, hexyl, octyl, and dodecyl. The polymerization reactions were followed by 1 H and 119 Sn NMR spectroscopy, IR spectroscopy (disappearance of the Sn-H vibration), and quantitative measurement of H 2 which evolved during the reaction. Among the numerous metal complexes employed as catalyst, [RhCl(PPh 3 ) 3 ] was found to be particularly suited for the preparation of these inorganic polymers. The reaction parameters temperature, solvent, R 2 SnH 2 concentration, and [RhCl(PPh 3 ) 3 ]/R 2 SnH 2 ratio were varied, with the most prominent influence on the monomer conversion being the temperature. The numberaverage molar masses of the polystannanes were in the range of 1 × 10 4 to 1 × 10 5 g/mol, depending on the reaction conditions. For the generic case of the polymerization of Bu 2 SnH 2 with [RhCl(PPh 3 ) 3 ] as catalyst, it was demonstrated that poly(dibutylstannane) did not form by a random polycondensation, but by growth at a rhodium complex, whereby only a minor part of [RhCl(PPh 3 ) 3 ] was converted to catalytically active species by reaction with tin hydrides. The polymers featured phase transitions into liquid-crystalline states, on occasion at remarkably low temperatures. A particularly high phase transition temperature was observed for poly(dipropylstannane), which also was characterized by a high density, indicative of a particularly favorable packing of the propyl groups.

molecular bulk heterojunction (BHJ), or be arranged in alternating thin layers to create a planar (or linear) heterojunction. The most effi cient OSCs also contain electron/hole blocking and transport layers to facilitate Ohmic extraction at the relevant electrodes. Laboratory-scale power conversion effi ciencies (PCEs) in OSCs now exceed 10% in both solution processed and evaporated junctions, with predictions of >13% within the next 2 years. [3][4][5] However, these PCEs have yet to be translated to the module-scale, with best efforts being serial or parallel connected narrow-strip "minimodules" from IMEC at ≈5-6% (16 cm 2 ), Heliatek 7.7% (140 cm 2 ) and Toshiba 9% (25 cm 2 ). [6][7][8] The narrow-strip architecture is a consequence of the relatively high sheet resistances ( R sh = 10-15 Ω/square) of currently available transparent conducting electrodes: predominantly indium tin oxide (ITO) or fl uorine doped tin oxide (SnO:F). Jin et al. recently showed that the TCE sheet resistance is a dominant scaling parameter controlling cell fi ll factor (FF) for carrier collection path lengths greater than ≈1.0-1.5 cm ( R sh ≈ 10 Ω/square). [ 9 ] Devices with collection path lengths >1 cm in any dimension suffer dramatic loss in FF and short circuit current density which have fi rst order effects upon cell effi ciency. This is clearly a major issue limiting the creation of high effi ciency OSC modules with large active areas (so called monolithic architectures). Connected narrow-strip geometries impose manufacturing complexity and substantial additional cost, plus lead to loss of active area (versus substrate usage) due to the need for extensive interconnection.In this regard, attempts to improve TCE performance have focused on three key strategies: i) The use of very thin, semitransparent metal layers sandwiched between transparent extraction and refractive index matching layers. [10][11][12][13][14][15] The idea behind these insulator/metal/insulator (IMI) stack electrodes originates from low emissivity coatings for windows. [ 16,17 ] By changing the thickness of the individual layers the electrical and optical properties of the stack can be adjusted, allowing for tuning of the sheet resistance and the optical transmission of the electrode. Unfortunately, it is not yet possible to achieve a high conductivity (sheet resistance <5 Ω/square) and a broadband optical transmission from 400-1000 nm for effi cient The high power conversion effi ciencies (PCEs) of laboratory-scale polymerbased organic solar cells are yet to translate to large area modules because of a number of factors including the relatively large sheet resistance of available transparent conducting electrodes (TCEs), and the high defect densities associated with thin organic semiconductor junctions. The TCE problem limits device architectures to narrow connected strips (<1 cm) causing serious fabrication diffi culties and extra costs. Thin junctions are required because of poor charge transport (imbalanced mobilities) in the constituent organic semiconduct...

We present a series of small-molecular trisazobenzene chromophores, including, for instance, 1,3,5-tris{[4-[4-[(4-cyanophenyl)azo]phenoxy]butyryl]amino}benzene that feature a remarkably stable light-induced orientation in initially amorphous thin-film architectures. It is demonstrated that for optimal performance it is critical to design chemical structures that allow formation of both an amorphous and a liquid-crystalline phase. In the present approach, the liquid-crystalline feature was introduced by inserting decoupling spacers between a trisfunctionalized benzene core and the three azobenzene moieties, as well as adding polar end groups to the latter. To compensate for the deleterious reduction of the glass transition temperature associated with the spacers in the compounds, polar units were incorporated between the benzene core and the side groups. Intriguingly, the molecular glasses that feature a latent liquid-crystalline phase display a remarkable "postdevelopment", i.e., an increase of the amplitude of refractive index modulation in holographic experiments after writing of optical gratings is arrested, exceeding 20% for the previously mentioned derivative. Thus, these nonpolymeric, azobenzene-containing compounds presented in this work appear to be attractive candidates for fabrication of stable holographic volume gratings.