Simultaneous GI-WAXS diffraction patterns and JV measurement of IBC solar cells during in situ anneal.

The growth-kinetics of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) crystals, on two different length-scales, is shown to be controlled by the thickness of the polymer layer within a PCBM-polymer bilayer. Using a model amorphous polymer we present evidence, from in situ optical microscopy and grazing-incidence X-ray diffraction (GIXD), that an increased growth-rate of nanoscale crystals impedes the growth of micron-sized, needle-like PCBM crystals. A combination of neutron reflectivity and GIXD measurements, also allows us to observe the establishment of a liquid-liquid equilibrium composition-profile between the PCBM layer and a polymer-rich layer, before crystallization occurs. While the interfacial composition-profile is independent of polymer-film-thickness, the growth-rate of nanoscale PCBM crystals is significantly larger for thinner polymer films. A similar thickness-dependent behavior is observed for different molecular weights of entangled polymer. We suggest that the behavior may be related to enhanced local-polymer-chain-mobility in nanocomposite thin-films.

We gated both p-type, and n-type, organic nanowire (NW) films with an aqueous electric double layer (EDL) in thin-film transistor (TFT) architectures. For p-type NWs, we used poly(3-hexylthiophene) (P3HT) NWs grown via two different routes. Both can be gated with water, resulting in TFTs with threshold lower than for conventionally cast P3HT films under the same gating conditions. However, TFT drain currents are lower for NWs than for conventional P3HT films, which agrees with similar observations for ‘dry’ gated TFTs. For n-type NWs, we have grown ‘nanobelts’ of poly(benzimidazobenzophenanthroline) (BBL) by a solvent/non-solvent mixing route with later displacement of the solvent, and dispersion in a non-solvent. Water-gating such films initially failed to give an observable drain current. However, BBL nanobelts can be gated with the aprotic solvent acetonitrile, giving high n-type drain currents, which are further increased by adding salt. Remarkably, after first gating BBL NW films with acetonitrile, they can then be gated by water, giving very high drain currents. This behaviour is transient on a timescale of minutes. We believe this observation is caused by a thin protective acetonitrile film remaining on the nanobelt surface.

Chemiresistors were prepared from the common organic semiconductor, poly(3-hexylthiophene) (P3HT), which is known to respond to atmospheric humidity with an increase of its conductance. Chemiresistors with different P3HT morphologies were compared: a solution-cast film versus an organic nanowire 'mesh.' It was found that both types of chemiresistors show no response for an exposure to less than 50% relative humidity. When humidity exceeds this threshold, the conductance of both chenniresistors increases. The conductance response of the nanowire device is far stronger as compared to that of the solution-cast film. This difference can be justified by the much larger surface area in the nanowire morphology. We conclude that organic nanowires constitute a favourable morphology for organic sensor technology, and recommend their wider employment in organic sensor applications.

Substrate and top electrode effects on P3HT lamellar crystallization, were studied using the full device structure of a bulk-hetorojunction P3HT:PCBM organic solar cell, during thermal annealing. P3HT: PCBM chlorobenzene solutions (50 wt %) were spin coated on quartz, HMDS treated SiO2, SiO2/PEDOT: PSS, ITO/PEDOT: PSS. The structural evolution of these thick (∼100nm) films was probed during a realistic device-processing annealing cycle at 140°C for ∼53 minutes by time-resolved synchrotron Grazing Incidence X-Ray Diffraction (GI-XRD). The 20 nm- thick coated Aluminium layer on top of ITO/PEDOT: PSS/P3HT: PCBM acts as a nucleation site for the P3HT crystalline growth during annealing, increasing the number of the edge-on lamellae, and preferentially increasing the domain size of face on and edge-on lamellae, with minor growth of the randomly oriented lamellae.

A real-time crystallographic analysis of P3HT/PCBM films during thermal annealing is reported, detailing the temporal variation of crystallization, disorder, and orientational spread during the annealing. Five P3HT/PCBM chlorobenzene solutions with different P3HT concentrations (0, 33, 50, 67, 100 wt %) were spin coated on SiO2 substrates. The thick films (similar to 100 nm) were studied during annealing (50 min at 140 degrees C), with in situ synchrotron grazing incidence X-ray diffraction (GI-XRD) and a sampling time < 8 s. For the first time, the evolution of the crystal structure is analyzed taking into account P3HT paracrystallinity. The following were observed: a predominance of edge-on P3HT lamellae in the as-spun and annealed films; changes in concentration-dependent edge-on lamellar orientation spread along the alkyl-stacking direction and paracrystalline disorder after annealing; a permanent lamellar stretching just along the alky-stacking direction after annealing; an increase in the P3HT domain size along only the alkyl-stacking direction for the edge-on lamellae, with dynamics consistent with PCBM acting as a plasticizer for P3HT; and finally, an increase in the PCBM concentration at the sample-air interface. We show that the paracrystalline correction is important to calculate correctly the domain size as deduced from GI-XRD.

Crystallization and phase segregation during thermal annealing lead to the increase of power-conversion efficiency in poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. An understanding of the length and time scale on which crystallization and phase segregation occur is important to improve control of the nanomorphology. Crystallization is monitored by means of grazing incidence X-ray diffraction in real time during thermal annealing. Furthermore, the change in film density is monitored by means of ellipsometry and the evolution of carrier mobilities by means of field effect transistors, both during annealing. From the combination of such measurements with those of device performance as a function of annealing time, it is concluded that the evolution of microstructure involves two important time windows: i) A first one of about 5 minutes duration wherein crystallization of the polymer correlates with a major increase of photocurrent; ii) a second window of about 30 minutes during which the aggregation of PCBM continues, accompanied by an increase in the fill factor.

Grazing incidence X-ray diffraction (GI-XRD) is used to characterize the crystallographic dynamics of low molecular weight (LMW) and high molecular weight (HMW) poly(3-hexylselenophene) (P3HS) films and blend films of P3HS with [6-6-]-phenyl-C(61) -butyric acid methyl ester (PCBM) as a function of 'step-by-step' thermal annealing, from room temperature to 250 °C. The temperature-dependent GIXRD data show how the melting point of P3HS crystallites is decreased by the presence of PCBM. P3HS crystallite domain sizes dramatically increase upon annealing to the P3HS melting temperature. The formation of well-oriented HMW P3HS crystallites with the (100) plane parallel to the substrate (edge-on orientation), when cooled from melt, are observed. We compare the behaviour of P3HS pure and blend films with that of poly(3-hexyl)thiophene (P3HT) pure and PCBM blended films and suggest that the similar temperature dependent behaviour we observe may be a common to polythiophene and related polymers and their blends.

The effect of the addition of 1,8-octanedithiol (ODT) during processing on the microstructure of blend films of 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) and [6,6]-phenyl-C71 butyric acid methyl ester ([70]PCBM) is studied. Grazing incidence X-ray diffraction and absorption spectroscopy show that the crystalline order of PCPDTBT increases when ODT is introduced in the solution phase either to neat polymer systems or to blends with [70]PCBM. The increased crystalline order is accompanied by less dispersive hole transport in the polymer, and leads to a more efficient formation of a percolating fullerene network within the blend. This contributes to an increase in photocurrent generation. However, the bimolecular recombination rate as determined from photovoltage transients increases upon addition of ODT, limiting the power conversion efficiency to values well below those expected from the energy levels of PCPDTBT. We propose some explanations for this increase in bimolecular recombination, based also on variable angle spectroscopic ellipsometry measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011