The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.

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Polymer photovoltaic cells : enhanced efficiencies via a network of internal donor-acceptor heterojunctions

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

Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

The path to ubiquitous and low-cost organic electronic appliances on plastic

ELECTROLUMINESCENT devices have been developed recently that are based on new materials such as porous silicon1 and semiconducting polymers2,3. By taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals4–6, and of electroluminescent polymers7, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV)8–10 with electrons injected into a multilayer film of cadmium selenide nanocrystals. Close matching of the emitting layer of nanocrystals with the work function of the metal contact leads to an operating voltage11 of only 4V. At low voltages emission from the CdSe layer occurs. Because of the quantum size effect19–24 the colour of this emission can be varied from red to yellow by changing the nanocrystal size. At higher voltages green emission from the polymer layer predominates. Thus this device has a degree of voltage tunability of colour.

Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer

In this paper it is demonstrated that the characteristics of light‐emitting diodes based upon MEH‐PPV [more fully known as poly(2‐methoxy,5‐(2’‐ethyl‐hexoxy)‐1,4‐phenylene‐ vinylene)] are determined by tunneling of both the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating voltage of the device as well as its efficiency. A model is developed that clearly explains the device characteristics of a wide range of diodes based upon MEH‐PPV. The turn‐on voltage for an ideal device is shown to be equal to the band gap, i.e., 2.1 eV for MEH‐PPV, and is slightly lower at 1.8 eV for an indium‐tin oxide/MEH‐PPV/Ca device. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I–V characteristics, while the larger barrier determines the device efficiency. In indium‐tin‐oxide/MEH‐PPV/Ca devices, the barrier to hole injection is ...

Carrier tunneling and device characteristics in polymer light‐emitting diodes

This paper demonstrates that the characteristics of light- emitting diodes (LEDs) based upon MEH-PPV, (more fully known as poly(2-methoxy,5-(2'-ethyl-heroxy)-1,4-phenylene-vinylene)), are determined by tunneling of both the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating-voltage of the device as well as its efficiency. A Model is developed which clearly explains the device characteristics of a wide range of diodes based upon MEH-PPV. The turn-on voltage for an ideal device is shown to be equal to the band-gap, i.e. 2.1eV for MEH-PPV, and is slightly lower at 1.8eV for an indium-tin oxide/MEH-PPV/Ca device. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I-V characteristics while the larger barrier determines the device efficiency. In ITO/MEH-PPV/Ca devices, the barrier to hole injection is 0.2eV and the barrier to electron injection is only 0.1eV. We show that reducing the hold barrier to 0.1eV using a polyaniline anode halves the operating voltage and increases device efficiency.

Ian D. Parker

Papers

Carrier tunneling and device characteristics in polymer light‐emitting diodes

Carrier tunneling and device characteristics in polymer light-emitting diodes