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In bulk heterojunction organic photovoltaics, electron-donating and electron-accepting materials form a distributed network of heterointerfaces in the photoactive layer, where critical photo-physical processes occur. However, little is known about the structural properties of these interfaces due to their complex three-dimensional arrangement and the lack of techniques to measure local order. Here, we report that molecular orientation relative to donor/acceptor heterojunctions is an important parameter in realizing high-performance fullerene-based, bulk heterojunction solar cells. Using resonant soft X-ray scattering, we characterize the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on (−1) orientations relative to these heterointerfaces. By manipulating the degree of molecular orientation through the choice of molecular chemistry and the characteristics of the processing solvent, we are able to show the importance of this structural parameter on the performance of bulk heterojunction organic photovoltaic devices featuring the electron-donating polymers PNDT-DTBT, PBnDT-DTBT or PBnDT-TAZ. [ABSTRACT FROM AUTHOR]

Abstract: A bicontinuous, percolating bulk heterojunction morphology is integral to organic polymer solar cells. Understanding the factors affecting the miscibility of photovoltaic polymers with a fullerene electron acceptor molecule is a key to controlling the morphology. Starting from discreet pure phases – a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bilayer film – the evolution of the P3HT–PCBM interface was studied with particular attention to the role of residual solvent in P3HT on PCBM interdiffusion. This investigation shows that in the bilayer geometry PCBM can rapidly diffuse into amorphous P3HT, but phase separation is maintained if the P3HT layer is cast from a very volatile solvent or if it is annealed prior to casting the PCBM overlayer to complete the bilayer geometry. [Copyright &y& Elsevier]

We report organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer, a topography that exhibits a 3-fold enhancement of the absorption in specific regions of the solar spectrum in part through multiple excitation resonances. The photonic crystal geometry is fabricated using a materials-agnostic process called PRINT wherein highly ordered arrays of nanoscale features are readily made in a single processing step over wide areas (∼4 cm2) that is scalable. We show efficiency improvements of ∼70% that result not only from greater absorption, but also from electrical enhancements. The methodology is generally applicable to organic solar cells and the experimental findings reported in our manuscript corroborate theoretical expectations. [ABSTRACT FROM AUTHOR]

The minority carrier transport length is a critical parameter limiting the performance of inexpensive Cu2O-ZnO photovoltaic devices. In this letter, this length is estimated to be ∼430 nm for electrochemically deposited Cu2O by linking the cell's carrier generation profile with back and front incident photon-to-electron conversion efficiency measurements to a one-dimensional transport model. This critical length explains the losses typically presented by these devices and appears to correlate well with the microcrystalline film structure. The consequences of the magnitude of the length on device design with the aim of improving solar cell performance are described. [ABSTRACT FROM AUTHOR]