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

Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organicdevices have not yet reached those of their inorganic counterparts (η ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400094498

Organic solar cells: An overview

Review of selected electrode–solution interactions which determine the performance of Li and Li ion batteries

An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems' applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems' applications, and permittivity and loss in ferroelectric films-important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures. (c) 2006 American Institute of Physics.

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Ferroelectric thin films: Review of materials, properties, and applications

Transparent conductors as solar energy materials: A panoramic review

We deposited epitaxial Ba0.4Sr0.6TiO3 (BST) films via laser ablation on MgO and LaAlO3 (LAO) substrates for tunable microwave devices. Postdeposition anneals (∼1100 °C in O2) improved the morphology and overall dielectric properties of films on both substrates, but shifted the temperature of maximum dielectric constant (Tmax) up for BST/LAO and down for BST/MgO. These substrate-dependent Tmax shifts had opposite effects on the room-temperature dielectric properties. Overall, BST films on MgO had the larger maximum dielectric constant (e/e0⩾6000) and tunability (Δe/e⩾65%), but these maxima occurred at 227 K. 30 GHz phase shifters made from similar films had figures of merit (ratio of maximum phase shift to insertion loss) of ∼45°/dB and phase shifts of ∼400° under 500 V (∼13 V/μm) bias, illustrating their utility for many frequency-agile microwave devices.

Large dielectric constant (ε/ε0>6000) Ba0.4Sr0.6TiO3 thin films for high-performance microwave phase shifters

The power conversion efficiency of organic and hybrid solar cells is commonly reduced by a low open-circuit voltage (VOC). In these cases, the VOC is significantly less than the energy of the lowest energy absorbed photon, divided by the elementary charge q. The low photovoltage originates from characteristically large band offsets between the electron donor and acceptor species. Here a simple method is reported to systematically tune the band offset in a π-conjugated polymer–metal oxide hybrid donor–acceptor system in order to maximize the VOC. It is demonstrated that substitution of magnesium into a zinc oxide acceptor (ZnMgO) reduces the band offset and results in a substantial increase in the VOC of poly(3-hexylthiophene) (P3HT)–ZnMgO planar devices. The VOC is seen to increase from 500 mV at x = 0 up to values in excess of 900 mV for x = 0.35. A concomitant increase in overall device efficiency is seen as x is increased from 0 to 0.25, with a maximum power-conversion efficiency of 0.5 % obtained at x = 0.25, beyond which the efficiency decreases because of increased series resistance in the device. This work provides a new tool for understanding the role of the donor–acceptor band offset in hybrid photovoltaics and for maximizing the photovoltage and power-conversion efficiency in such devices.

Band‐Offset Engineering for Enhanced Open‐Circuit Voltage in Polymer–Oxide Hybrid Solar Cells

We report on the effects of titanium doping (0–7at.%) on the optical and electrical properties of In2O3 using combinatorial deposition and analysis techniques. Maximum mobilities are observed at Ti concentrations of 1.5–2.5at.% and are >80cm2∕Vs in sputtered films. The carrier concentration increased with titanium content to a high of 8.0×1020cm−3. Data show that one carrier is generated per added Ti between 1 and 3at.%. Conductivities up to 6260Ω−1cm−1 were observed. These remained very high >5000Ω−1cm−1 across a wide compositional range. The optical transparency is high (>85%) in a wide spectral range from 400nm to at least 1750nm. The work function of titanium-doped indium oxide varies substantially over the studied compositional range.

Titanium-doped indium oxide: A high-mobility transparent conductor

Thin-film small molecule/polymer hybrid bilayer photovoltaic cells have been constructed, exhibiting power conversion efficiencies of 0.71% under 80 mW/cm2 white light illumination. The parameters influencing the photovoltage of these devices are explored by reversing the order of the photoactive layers while maintaining the same electrode configuration. It has been found that the properties of the organic photoactive layers play an important role in determining the direction of current flow and the photovoltage of the device. Comparison is made to analogous pure small molecule bilayer devices, and conclusions about some of the factors influencing device efficiency are drawn. It has been shown that ordering of the band offsets of the two organic materials plays an important role in determining the polarity of the photocurrent and the photovoltage of the device.

D. S. Ginley

Papers

Large dielectric constant (ε/ε0>6000) Ba0.4Sr0.6TiO3 thin films for high-performance microwave phase shifters

Band‐Offset Engineering for Enhanced Open‐Circuit Voltage in Polymer–Oxide Hybrid Solar Cells

Titanium-doped indium oxide: A high-mobility transparent conductor

Polymer—perylene diimide heterojunction solar cells

Combinatorial studies of Zn-Al-O and Zn-Sn-O transparent conducting oxide thin films