Category Thin Film Solar Cells Fabrication, Characterization and Applications

RISKS AND PERSPECTIVE

The analysis of major thin films tends to underestimate technical risks (despite Table 11.11) and subsequent comments. Risks are pervasive in thin film development, and major setbacks have already occurred. Perhaps the most universal cause is a lack of science base. Because thin films are almost always different from mainstream electronics materials (as opposed to x-Si, which shares much with the mainstream), thin film development is not much supported by scientific understanding outside of PV. Problems that might otherwise be trivial are magnified. Serious problems such as the Staebler-Wronski Effect in a-Si, multielement stoichiometry and uniformity in CIS, and defects and their interactions in CdTe and its contacts are even harder to overcome...

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OTHER ASPECTS OF THE ‘TERAWATT CHALLENGE’

So far, the potential to achieve very low cost has been emphasized. That is critical to making PV cost effective enough to be used economically. But there are other factors to the ‘TW Challenge.’ The main one is materials availability. To have 10-20 TW of PV energy installed by mid century, we need about 50-100 TW peak, due to the 15-25 % capacity factor of PV. To accomplish this by 2065, for example, would require very high initial growth, resulting in about 4000 GWp of annual PV production in 2065. This means that about 15 TW (not peak) of PV would be installed and producing electricity in 2065 (given an assumed average lifespan of 30 years). Figure 11.6 shows a growth rate for PV to reach 75 TWp installed in 2065.

The materials requirements to meet this challenge have been studied i...

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Results

For the commonalities (BOM) among all thin film modules, a set of distinct substrates were chosen: glass, stainless steel, and polyimide. Only those substrates and encapsulation schemes that are already in common use were chosen (e. g., glass/EVA, Tefzel/EVA), and this could be viewed as a limitation of the study (since they are all rather expensive). However, since encapsulation is crucial for reliability, it seemed the proper choice. To first order, any cost breakthrough in encapsulation is likely to benefit all thin films (and x-Si), and could be treated independently. (If individual technologies have uniquely lower cost options for encapsulation, they should try to develop and implement them as fast as possible to gain a competitive advantage over those given here...

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Approach

A spreadsheet was developed for the estimated component costs of thin film modules of various types. This was done in two parts – so-called commonalities or BOM of most thin film modules; and the unique aspects of each design, being mostly the semiconductors that convert sunlight to electricity (nonBOM). An initial production level of 25 MWp/yr was assumed, which is like the capacity of existing facilities or those that are planned by new start-ups. Then similar estimates were developed for future, larger scale production levels, in which both economies of scale and technical progress were estimated.

In most cases, thin film manufacturing bifurcates into: (i) approaches that take advantage of high quality processes that cost more but produce the best films or (ii) less expensive processes ...

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A BOTTOM UP ANALYSIS OF THIN FILM MODULE COSTS

Almost all thin film PV devices have a great deal in common. They attempt to minimize material costs by using ultrathin semiconductors to convert sunlight to electricity; they attempt to reach adequate sunlight-to-electricity conversion efficiencies; and they require excellent outdoor reliability. In this sense, thin films are a direct response to the high materials costs of wafer silicon PV modules.

However, thin film modules share most functional aspects with wafer silicon modules. That is, they require top and bottom protection from the outdoor environment, so that they can last outdoors about thirty years. They need top and bottom contacts, bus bars, and a connection to an external circuit to carry away current...

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LOW COST AND THE IDEA OF THIN FILMS

The idea of thin films is simple: use mostly low cost material (glass, metal, plastic) and very little high cost semiconductor. A micron or so of semiconductor is about 2-6 g/m2; even ultraexpensive material (say, $1000/kg) only costs pennies per watt at this level. This idea has been around as long as PV, but the difficulty has been developing semiconductors that would work well enough (have high enough conversion efficiency) and then finding ways to actually make them cheaply at high yield. And thin films have their own peculiar stability issues, both intrinsically and at the module level, which have also added to the challenge. However, throughout, the idea of thin films has maintained its power, so that no single failure or long, costly delay has destroyed thin film development...

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‘THE ONLY BIG NUMBER OUT THERE – 125 000 TW’ (QUOTE, NATE LEWIS, 2004)

The world uses about 10 terawatt (TW) of energy (the US, about 3 TW) and by 2050 is projected to need about 30 TW. Thus the world will need about 20 TW of nonCO2 energy to stabilize CO2 in the atmosphere by mid century. For details about non CO2 energy needs for meeting climate change, see Hoffert et al., 1998. Hoffert (NYU), Rick Smalley (Rice Nobel Laureate), and Nate Lewis (CalTech) call this the ‘Terawatt (TW) Challenge,’ and whether thin film PV can meet the challenge is the subject of this study. Shockingly, it turns out that among the nonCO2 options, it is possible that solar is the only one that can (discussed below).

The primary barrier to TWscale use of PV is cost, and that will be the main focus of this study...

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The Terawatt Challenge for Thin Film Photovoltaics

Ken Zweibel

NREL, Golden, CO, USA

11.1 PROLOGUE

It is critical to understand what this report purports to do and what it cannot do. It cannot analyze either company or technology specific information about thin film manufacturing. It cannot give any current actual prices, because they depend on volume and varying specifications. Thin film photovoltaic (PV) manufacturing is changing quickly, and most crucial details are confidential.

So what can this report do? It can assemble in one place a set of technology options, process choices, and device designs and attempt to give a rough estimate of their status and potential. There are long lists of these attributes (e. g., Tables 11.1 and 11.4) that seem to indicate actual costs. But this is not the case...

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