The performance of bulk-heterojunction (BHJ) solar cells is strongly correlated with the nanoscale structure of the active layer. Various processing techniques have been explored to improve the nanoscale morphology of the BHJ layer, e.g., by varying the casting solvent, thermal annealing, solvent annealing, and solvent additives. This paper highlights the role of residual solvent in the "dried" BHJ layer, and the effect of residual solvents on PCBM diffusion and ultimately the stability of the morphology. We show that solvent is retained within the BHJ fi lm despite prolonged heat treatment, leading to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM agglomerates. The addition of a small volume fraction of nitrobenzene to the casting solution inhibits the diffusion of PCBM in the dry fi lm, resulting in smaller PCBM agglomerates, and improves the fi ll factor of the BHJ device to 0.61 without further tempering. The addition of nitrobenzene also increases the P3HT crystalline content, while increasing the onset temperature for melting of P3HT side chains and backbone. The melting temperature for PCBM is also higher with the nitrobenzene additive present.

Broadband femtosecond transient absorption spectroscopy is used to explore the mechanisms underlying excited-state and ground-state exciton relaxation in poly(3-hexylthiophene) (P3HT) solution. We focus on the picosecond spectral shifts in the ground and excited states of P3HT, using pumpÀprobe (PP) and pumpÀdumpÀ probe (PDP) techniques to investigate exciton relaxation mechanisms. Excited-state PP signals resolved a dynamic stimulated emission Stokes shift and ground-state reorganization; PDP signals resolved a blue-shifting nonequilibrium ground-state bleach. Initial structural reorganization is shown to be faster in the excited state. Ground-state reorganization is shown to be dependent on dump time, with later times resulting in relatively more population undergoing slow (∼20 ps) reorganization. These observations are discussed in the context of structural relaxation involving small-scale (<1 ps) and largescale (>1 ps) planarization of thiophene groups following photoexcitation. Excited-state and ground-state dynamics are contrasted in terms of electronic structure defining the torsional potential energy surfaces. It is shown that the primary excitonic relaxation mechanism is excited-state self-trapping via torsional relaxation rather than exciton energy transfer.

For the last decade, researchers have attempted to construct photovoltaic (PV) devices using a mixture of inorganic nanoparticles and conjugated polymers. The goal is to construct layers that use the best properties of each material e.g., flexibility from the polymer and high charge mobility from the nanoparticles or blue absorbance from the polymer complementing red absorbance from the nanoparticles. This critical review discusses the main obstacles to efficient hybrid organic/inorganic PV device design in terms of contributions to the external and internal quantum efficiencies. We discuss in particular the role that ligands on the nanoparticles play for mutual solubility and electronic processes at the nanoscale. After a decade of work to control the separation distance between unlike domains and the connectivity between like domains at the nanoscale, hybrid PV device layers are gaining in efficiency, but the goal of using the best properties of two mixed materials is still elusive. IntroductionThere is a serious concern that the global temperature will increase by 1 to 6 C and the CO 2 concentration will exceed the range from 570 to 970 parts per million during the 21 st century. 1 These numbers create tremendous awareness towards the increasing need for renewable energy resources. Approximately 80% of the world energy supply still comes from fossil fuels. 2 Unfortunately, photovoltaic (PV) energy only contributes 0.04% of the total energy production, 3 although the earth receives enough energy in an hour to fulfil the yearly demand. The main reason for low penetration of PV into the energy market is the high cost of providing power from PV. We review here hybrid organic/inorganic PV devices that, due to ease of fabrication, have the chance to greatly reduce the cost of PV energy.The current PV industry is dominated by silicon-based photovoltaics. There are other types of PVs such as single and multi-junction thin films that have also captured a significant part of the PV market because of their high power conversion

The composition of polymer-fullerene blends is a critical parameter for achieving high effi ciencies in bulk-heterojunction (BHJ) organic photovoltaics. Achieving the "right" materials distribution is crucial for device optimization as it greatly infl uences charge-carrier mobility. The effect of the vertical concentration profi le of materials in spin-coated BHJs on device properties has stirred particularly vigorous debate. Despite available literature on this subject, the results are often contradictory and inconsistent, likely due to differences in sample preparation and experimental considerations. To reconcile published results, the infl uence of heating, surface energy, and solvent additives on vertical segregation and doping in polymer-fullerene BHJ organic photovoltaics are studied using neutron refl ectometry and near edge X-ray absorption fi ne structure spectroscopy. It is shown that surface energies and solvent additives greatly impact heat-induced vertical segregation. Interface charging due to Fermi level mismatch increases (6,6)-phenyl-C 61 -butyric acid methyl ester (PCBM)-enrichment at the BHJ/cathode interface. Currentvoltage measurements show that self-assembly of interfaces affects the opencircuit voltage, resulting in clear changes to the power conversion effi ciency.

Surface photovoltage spectroscopy (SPS) was used to probe photon induced charge separation in thin films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) as a function of excitation energy. Both positive and negative photovoltage signals were observed under sub-band-gap (<2.0 eV) and super-band-gap (>2.0 eV) excitation of the polymer. The dependence of the spectra on substrate work function, thermal annealing, film thickness, and illumination intensity was investigated, allowing the identification of interface, charge transfer (CT), and band-gap states in the amorphous and crystalline regions of the polymer films. The ability to probe these states in polymer films will aid the development and optimization of organic electronic devices such as photovoltaics (OPVs), light-emitting diodes (OLEDs), and field effect transistors (OFETs). The direction and size of the observed photovoltage features can be explained using the depleted semiconductor model.