Transmission electron microscopy and electron diffraction are used to study the changes in morphology of composite films of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) in bulk heterojunction solar cells. Thermal annealing produces and stabilizes a nanoscale interpenetrating network with crystalline order for both components. P3HT forms long, thin conducting nanowires in a rather homogeneous, nanocrystalline PCBM film. Both the improved crystalline nature of films and increased but controlled demixing between the two constitutes therein after annealing explains the considerable increase of the power conversion efficiency observed in these devices.

Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO−PPV) as electron donor and crystalline ZnO nanoparticles (nc-ZnO) as electron acceptor have been studied. Composite nc-ZnO:MDMO−PPV films were cast from a common solvent mixture. Time-resolved pump−probe spectroscopy revealed that a photoinduced electron transfer from MDMO−PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived (milliseconds) charge-separated state. The photovoltaic effect in devices, made by sandwiching the active nc-ZnO:MDMO−PPV layer between charge-selective electrodes, has been studied as a function of the ZnO concentration and the thickness of the layer. We also investigated changing the degree and type of mixing of the two components through the use of a surfactant for ZnO and by altering the size and shape of the nc-ZnO particles. Optimized devices have an estimated AM1.5 performance of 1.6% with incident photon to current conversion efficiencies up to 50%. Photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy have been used to gain insight in the morphology of these blends.

The performance of bulk‐heterojunction solar cells based on a phase‐separated mixture of donor and acceptor materials is known to be critically dependent on the morphology of the active layer. Here we use a combination of techniques to resolve the morphology of spin cast films of poly(p‐phenylene vinylene)/methanofullerene blends in three dimensions on a nanometer scale and relate the results to the performance of the corresponding solar cells. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and depth profiling using dynamic time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) clearly show that for the two materials used in this study, 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐methanofullerene (PCBM) and poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV), phase separation is not observed up to 50 wt.‐% PCBM. Nanoscale phase separation throughout the film sets in for concentrations of more than 67 wt.‐% PCBM, to give domains of rather pure PCBM in a homogenous matrix of 50:50 wt.‐% MDMO‐PPV/PCBM. Electrical characterization, under illumination and in the dark, of the corresponding photovoltaic devices revealed a strong increase of power conversion efficiency when the phase‐separated network develops, with a sharp increase of the photocurrent and fill factor between 50 and 67 wt.‐% PCBM. As the phase separation sets in, enhanced electron transport and a reduction of bimolecular charge recombination provide the conditions for improved performance. The results are interpreted in terms of a model that proposes a hierarchical build up of two cooperative interpenetrating networks at different length scales.

Polymer solar cells have the potential to become a major electrical power generating tool in the 21st century. R&D endeavors are focusing on continuous roll-to-roll printing of polymeric or organic compounds from solutionslike newspaperssto produce flexible and lightweight devices at low cost. It is recognized, though, that besides the functional properties of the compounds the organization of structures on the nanometer levels forced and controlled mainly by the processing conditions appliedsdetermines the performance of state-of-theart polymer solar cells. In such devices the photoactive layer is composed of at least two functional materials that form nanoscale interpenetrating phases with specific functionalities, a so-called bulk heterojunction. In this perspective article, our current knowledge on the main factors determining the morphology formation and evolution is introduced, and gaps of our understanding on nanoscale structure-property relations in the field of highperformance polymer solar cells are addressed. Finally, promising routes toward formation of tailored morphologies are presented.

The morphology of composite thin films consisting of a conjugated polymer (poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene], MDMO-PPV) and methanofullerene ([6,6]-phenyl C61 butyric acid methyl ester, PCBM), which are used as the active layer in polymer photovoltaic devices, has been extensively studied using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). Composite MDMO-PPV:PCBM films have been prepared with PCBM concentrations varying from 20 to 90 wt %. PCBM-rich clusters are clearly observed in TEM bright-field mode when the PCBM concentration is increased to ca. 75 wt % in the composite film. The SAED analysis shows that these clusters consist of many PCBM nanocrystals with random crystallographic orientations. Furthermore, we show that these nanocrystals are also present in the homogeneous matrix at PCBM concentrations below 75 wt %. Annealing of the blend films has been performed at temperatures between 60 and 130 °C for different times. In all cases, but especially when the annealing temperature is above the glass transition temperature of MDMO-PPV (∼80 °C), PCBM molecules show high diffusion mobility, resulting in accelerated phase segregation and in the formation of large PCBM single crystals in the film. The observed phase segregation, even at temperatures as low as 60 °C, indicates that the thermal stability of MDMO-PPV:PCBM films will likely limit the long-term performance of solar cells based on these materials.

The influence of various thermal treatment steps on the morphology and the photoconductive properties of a non‐contacted, 50 nm thick blend (50:50 wt.‐%) of [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) and poly(3‐hexyl thiophene) (P3HT) spin‐coated from chloroform has been studied using transmission electron microscopy (TEM) and the electrodeless time‐resolved microwave conductivity technique. After annealing the film for 5 min at 80 °C, TEM images show the formation of crystalline fibrils of P3HT due to a more ordered packing of the polymer chains. The thermal treatment results in a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056 cm2 V–1 s –1 for the non‐annealed sample to 0.044 cm2 V–1 s –1 for the sample annealed at 80 °C. In contrast, the temporal shape of the photoconductivity, with typical decay half‐times, τ1/2, of 1 μs for the lowest excitation intensities, is unaffected by the temperature treatment. Further annealing of the sample at 130 °C results in the formation of three different substructures within the heterojunction: a PCBM:P3HT blend with PCBM‐rich clusters, a region depleted of PCBM, and large PCBM single crystals. Only a minor increase in the amplitude, but a tenfold rise of the decay time of the photoconductivity, is observed. This is explained by the formation of PCBM‐rich clusters and large PCBM single crystals, resulting in an increased diffusional escape probability for mobile charge carriers and hence reduced recombination.

allowed to proceed for 3 h. The product was moved to a separation funnel and an excess of ethanol was added to remove the surfactants and other reagents.Characterization: FESEM images and EDX data were taken with a JEOL JSM-6700 F microscope with an EDX facility. The TEM images were observed with a JEOL EM-2000 EX II microscope. For the TEM analysis, the samples were deposited on a carbon mesh foil supported on a copper grid. The emission spectrum from the silica nanotubes was obtained with a Shimadzu RF-5301 PC spectrofluorophotometer at room temperature. The excitation wavelength for the emission spectrum was 350 nm, which was the maximum absorption wavelength of silica nanotubes dispersed in ethanol.

The morphological evolution of thin composite films based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) upon annealing has been studied by means of transmission electron microscopy (TEM), scanning near-field optical microscopy (SNOM), and confocal Raman microscopy. This system currently is the most promising candidate for high-performance polymer solar cells. TEM bright field and SNOM topography measurements show that segregation and large-scale crystallization of PCBM take place upon thermal annealing. Additional SNOM optical absorbance measurements using laser irradiation sources with two different wavelengths in the visible light range and Raman microscopy are able to detect the component distribution within the thin composite film, which in detail demonstrates high diffusive mobility of PCBM upon annealing.