There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Solution-processed bulk-heterojunction solar cells have gained serious attention during the last few years and are becoming established as one of the future photovoltaic technologies for low-cost power production. This article reviews the highlights of the last few years, and summarizes today's state-of-the-art performance. An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic technology towards the magic 10% regime. A cost model supplements the technical discussions, with practical aspects any photovoltaic technology needs to fulfil, and answers to the question as to whether low module costs can compensate lower lifetimes and performances.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.200801283

Polymer‐Fullerene Bulk‐Heterojunction 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

In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, physical flexibility and large area coverage1. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the infrared for telecommunications (1,300–1,600 nm), thermal imaging (1,500 nm and beyond), biological imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (>1,900 nm), and solar cells (800–2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond ∼800 nm (refs 2,3). Sensitizing conjugated polymers with infrared-active nanocrystal quantum dots provides a spectrally tunable means of accessing the infrared while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the infrared. We achieve, in a solution-processed device and with sensitivity far beyond 800 nm, harvesting of infrared-photogenerated carriers and the demonstration of an infrared photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the infrared spectrum.

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Solution-processed PbS quantum dot infrared photodetectors and photovoltaics

In this Review, we summarize recent work on modeling of organic/metal and organic/organic interfaces. Some of the models discussed have a semiempirical approach, that is, experimentally derived values are used in combination with theory, and others rely completely of calculations. The models are categorized according to the types of interfaces they apply to, and the strength of the interaction at the interface has been used as the main factor. We explain the basics of the models, their use, and give examples on how the models correlate with experimental results. We stress that given the complexity of organic/metal and organic/organic interface formation, it is crucial to know the exact way in which the interface was formed before choosing the model that is applicable, as none of the models presented covers the whole range of interface interaction strengths (weak physisorption to strong chemisorption).

Energy‐Level Alignment at Organic/Metal and Organic/Organic Interfaces

X-ray photoelectron spectroscopy: Towards reliable binding energy referencing

The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

Reliable determination of chemical state in x-ray photoelectron spectroscopy based on sample-work-function referencing to adventitious carbon: Resolving the myth of apparent constant binding energy of the C 1s peak

The C 1s signal from ubiquitous carbon contamination on samples forming during air exposure, so called adventitious carbon (AdC) layers, is the most common binding energy (BE) reference in X-ray photoelectron spectroscopy studies. We demonstrate here, by using a series of transition-metal nitride films with different AdC coverage, that the BE of the C 1s peak EBF varies by as much as 1.44 eV. This is a factor of 10 more than the typical resolvable difference between two chemical states of the same element, which makes BE referencing against the C 1s peak highly unreliable. Surprisingly, we find that C 1s shifts correlate to changes in sample work function ϕSA , such that the sum EBF+ϕSA is constant at 289.50±0.15 eV, irrespective of materials system and air exposure time, indicating vacuum level alignment. This discovery allows for significantly better accuracy of chemical state determination than offered by the conventional methods. Our findings are not specific to nitrides and likely apply to all systems in which charge transfer at the AdC/substrate interface is negligible.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cphc.201700126

C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy.

X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern materials science for the determination of chemical bonding as evidenced by more than 10 000 XPS papers published annually. A literature survey reveals that in the vast majority of cases an incorrect referencing of the binding energy scale is used, neglecting warnings that have been formulated from the early days of the technique. Consequences for the data reliability are disastrous and decades of XPS work require revisiting. The purpose of this Viewpoint is to highlight the existing problems, review the criticism and suggest ways forward.

/pdf/compromising-science-by-ignorant-instrument-calibration-need-3hi3mhspad.pdf

Grzegorz Greczynski

Papers

X-ray photoelectron spectroscopy: Towards reliable binding energy referencing

The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

Reliable determination of chemical state in x-ray photoelectron spectroscopy based on sample-work-function referencing to adventitious carbon: Resolving the myth of apparent constant binding energy of the C 1s peak

C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy.

Compromising Science by Ignorant Instrument Calibration-Need to Revisit Half a Century of Published XPS Data.