The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

Conjugated polymer-based organic solar cells

Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

We demonstrate that semiconductor nanorods can be used to fabricate readily processed and efficient hybrid solar cells together with polymers. By controlling nanorod length, we can change the distance on which electrons are transported directly through the thin film device. Tuning the band gap by altering the nanorod radius enabled us to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum. A photovoltaic device consisting of 7-nanometer by 60-nanometer CdSe nanorods and the conjugated polymer poly-3(hexylthiophene) was assembled from solution with an external quantum efficiency of over 54% and a monochromatic power conversion efficiency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers. Under Air Mass (A.M.) 1.5 Global solar conditions, we obtained a power conversion efficiency of 1.7%.

http://science.sciencemag.org/content/sci/295/5564/2425.full.pdf

Hybrid Nanorod-Polymer Solar Cells

Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

http://chialvo.org/Curso/UBACurso/DIA7/Papers/Aono_NatMat_07_Ionic%20Switching%20Memory.pdf

Nanoionics-based resistive switching memories

Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges

Degradation-induced changes in the structural and optical properties of the polyfluorene-based blue emitting films and LEDs are examined using spectroscopic (FTIR, UV−vis, photo- and electroluminescence), analytical (FTIR and ESCA), and scanning probe microscopy techniques. The materials studied are oligomers (DP ∼ 10) of 9,9-di-n-hexylfluorene and its random copolymer with anthracene. In situ FTIR monitoring is used to characterize chemical changes in the active layer of operating LED devices. Two primary mechanisms of degradation are identified. In the first, photooxidation of the polymer matrix leads to the formation of an aromatic ketone, most likely fluorenone at the chain terminating monomer units, which quenches the fluorescence. The second process promotes aggregate formation, which then leads to loss of luminous intensity by exciton transfer and relaxation through excimers.

Electrical and Photoinduced Degradation of Polyfluorene Based Films and Light-Emitting Devices

Light‐emitting diodes made with poly(2‐methoxy‐5(2′‐ethyl)hexoxy‐phenylenevinylene) (MEH‐ PPV) using indium‐tin‐oxide (ITO) as anode and Ca as cathode have been examined as they age during operation in a dry inert atmosphere. Two primary modes of degradation are identified. First, oxidation of the polymer leads to the formation of aromatic aldehyde, i.e., carbonyl which quenches the fluorescence. The concomitant chain scission results in reduced carrier mobility. ITO is identified as a likely source of oxygen. The second process involves the formation of localized electrical shorts which do not necessarily cause immediate complete failure because they can be isolated by self‐induced melting of the surrounding cathode metal. We have not identified the origin of the shorts, but once they are initiated, thermal runaway appears to accelerate their development. The ultimate failure of many MEH‐PPV devices occurs when the regions of damaged cathode start to coalesce.

Degradation and failure of MEH‐PPV light‐emitting diodes

We have studied polyaniline and polyethylenedioxythiophene transparent electrodes for use as hole-injecting anodes in polymer light emitting diodes. The anodes were doped with a variety of polymer and monomer-based acids and cast from either water or organic solvents to determine the effect of the dopant and solvent on the hole-injection properties. We find that the anodes with polymeric dopants have improved device quantum efficiency and brightness relative to those with small molecule dopants, independent of conductivity, solvent, or type of conducting polymer. For the most conducting polymer anodes [σ>2(Ωcm)−1], diodes could be made without an indium tin oxide underlayer. These diodes show substantially slower degradation.

/pdf/polymeric-anodes-for-improved-polymer-light-emitting-diode-1f6kjhs7rq.pdf

Polymeric anodes for improved polymer light-emitting diode performance

We demonstrate that the resistive switching phenomenon observed in organic semiconductor layers containing granular metal particles conforms to a charge storage mechanism described by Simmons and Verderber [Proc. R. Soc. A 391, 77 (1967)]. The space-charge field due to the stored charge inhibits further charge injection from the electrodes. The equilibrium current–voltage curve is N shaped and the low and high resistance states are obtained by applying voltage close to the local maximum and minimum, respectively.

Mechanism for bistability in organic memory elements

ESR measurements have been performed on samples of neutral polypyrrole, polypyrrole-perchlorate, oxygen-doped polypyrrole, and poly($\ensuremath{\beta}$,${\ensuremath{\beta}}^{\ensuremath{'}}$-dimethylpyrrole perchlorate). A narrow Lorentzian line is observed in oxidized, highly conducting films, but no correlation between conductivity and either linewidth or intensity is found. Electrochemically cycled films, which remain highly conducting, show little or no detectable ESR absorption. It is deduced that the ESR signal seen in the as-prepared material does not arise from the current-carrying species, but rather from accidental neutral $\ensuremath{\pi}$-radical defects. The absence of paramagnetism in the metallic state is discussed within the framework of bipolaron formation.

John Campbell Scott

Papers

Electrical and Photoinduced Degradation of Polyfluorene Based Films and Light-Emitting Devices

Degradation and failure of MEH‐PPV light‐emitting diodes

Polymeric anodes for improved polymer light-emitting diode performance

Mechanism for bistability in organic memory elements

Electron-spin-resonance studies of pyrrole polymers: Evidence for bipolarons