The depletion of oil reserves, spiraling fuel costs, uncertain energy supplies, and belated recognition of fossil-fuel-induced climate change have sparked an urgent demand for sustainable energy. Of all the sustainable sources, solar stands alone as the only one that can be exploited throughout the world, irrespective of climate or geography, without detriment to the environment, and with sufficient theoreticalcapacity to meet the world's entire energy needs single-handedly.

The goal for all renewables is to drive presubsidy production costs below 3 /kWhr, a value that would allow them to compete with coal and nuclear power. This is feasible for wind, hydroelectric, and geothermal energy (which can already produce electricity at less than 5 /kWhr) but is a distant prospect for solar energy (which is 15–30 /kWhr).

The main technology for solar electricity is Si photovoltaics (PV), in which Si solar cells directly convert sunlight to electricity via the photovoltaic effect. Commercial panels have high power efficiencies of up to 20%, and guaranteed lifetimes of 25 years. They provide a reliable, space-efficient means of generating electricity with minimal maintenance. However, their costs are high and may rise further as growing solar production places a strain on existing manufacturing facilities. Until now, the PV industry has cut costs by extensive use of depreciated processing plants and Si off-casts from the semiconductor industry. This is unsustainable and a capacity crunch is looming that will only be overcome through huge capital investment in new dedicated facilities.

This investment is starting to occur. For example, Hemlock Semiconductor Corporation is building a $1 billion facility for polysilicon – the feedstock of most modern PV cells – with a planned annual output of 17?000 metric tonnes, equating to ∼2 GW of solar cell production each year. Given such high upfront capital costs, the roll-out of Si PV is bound to proceed in a slow, cautious manner with capacity increasing incrementally. The US government predicts PV capacity will scrape 10 billion kWhrs in 2030, barely 0.2% of projected energy usage and way below the levels of other renewables.

To accelerate uptake, the PV industry must side-step the need for high capital expenditure by tapping into unused capacity in existing facilities. The requisite ‘slack’ does not exist in conventional semiconductor foundries, which already operate at close to capacity. The only industry that could readily accommodate the high-area demands of large-scale PV right now is printing, where even a fairly small press can operate at rates of 100 m2/min.

Successful exploitation of this capacity requires compatible PV technologies that can be processed in existing printing plants without significant reconfiguration of equipment or processes. There is a limited range of printable PV materials, of which solution-processable organic semiconductors are arguably the most appealing since they share many of the chemical and rheological properties of conventional inks and pigments. Organic semiconductors are already used in some mobile phone displays and are under development for larger displays too, where their low materials costs, ease-of-processing, simple device architectures, and minimal component counts promise significant savings. Their use in PV is less well developed and the highest reported efficiencies and lifetimes (6%, 1 year) still lag far behind Si cells. Major innovations in nanoscale science and engineering will be needed if printable organic photovoltaics (OPV) is to mature into a viable technology. Critical challenges include: new printable PV materials, electrodes, and device structures for improved manufacturability, lifetime, and efficiency; new techniques for controlling the nanoscale morphologies of the deposited layers; and effective packaging techniques to protect against damaging exposure to O2 and H2O.

Solving these challenges will require significant investment in scientific innovation. This is important for technology companies in high-wage, western economies as it offers considerable opportunities for intellectual property acquisition. Crucially for such companies, the adaptation of OPV to existing printing facilities allows a ‘fabless’ business model to be pursued, where proprietary technologies are developed in-house and manufacturing is outsourced to existing third-party manufacturers. The ability to piggyback on prebuilt facilities has many advantages: it eliminates upfront capital expenditure, reduces time to market, speeds up profitability, drives cost competition among manufacturers, lowers financial risk, and enables local manufacturing.

Christoph Brabec, chief technology officer of Konarka, points out, “The importance of OPV as a solar technology lies as much in its compatibility with fabless manufacturing as it does in the technical merits of the materials themselves.” Yes, they’re cheap and easily handled, but without a business framework that can manage the huge manufacturing demands, this counts for little. OPV is still an early stage, high-risk technology, but its successful adaptation to print manufacturing would be genuinely revolutionary, enabling solar electricity to be deployed on a scale and at a rate that is unimaginable with Si.

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DOI: 10.1016/S1369-7021(07)70260-2