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7 Sustainability, Photovoltaics, and the CZTS Cell

7 Sustainability, Photovoltaics, and the CZTS Cell

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layer technology and increasing efficiencies, but there are limits to each (efficiency

is literally limited by the Shockley–Queisser limit). The alternative is to (a) use less,

(b) use material science and engineering concepts to utilize these materials more

effectively (e.g., nanotechnology), or (c) find replacements for resource-scarce materials. It is the latter approach that has led to the development of the “earth abundant”

photovoltaics based on Cu2Zn(Sn1−xGex)S,Se (CZTS or CZTSe).

CZTS and derivatives are members of the kesterite crystal family where kesterite refers to a relatively abundant mineral of formula Cu2 Zn 0.75 Fe 2 + 0.25Sn1.3S4 .

In effect, the CZTS/Se cells replace the rare elements of indium and gallium with

their neighbors zinc and tin on the periodic table. Like the chalcogenides covered

in Section 7.3.2.2, the kesterites have the same basic copper/sulfur structure, but the

M(III) ions have been replaced with an equal number of M(II) and M(IV) atoms

(Figure 7.83). The CZTS/Se materials are p-type absorbers in which modification

of the ratio of S to Se allows for tuning the band gap, as can be gleaned from comparing the values for CZTS (1.45 eV) and CZTSe (0.94 eV; Table 7.1). Band gaps

in the range of 1−1.5 eV can be attained (Mitzi et  al. 2011). Recently, CZTS or

CZTSe cells substituted with germanium in varying ratios (replacing tin) have been

investigated to further tune the band gap and address device limitations. A 40%

Ge-doped CZTSe (Cu1.5ZnSn0.5Ge0.4Se4) shows a shift to a slightly larger band gap

(1.15 eV vs. 1.08 eV for pure CZTSe) but with essentially no improvement in PCE

(the Ge-substituted cell gives a PCE of 9.1% compared to the pure CZTSe cell’s

9.07%) (Bag et al. 2012).

Research effort into the use of CZTS/Se materials in solar cells is very active.

A comparison of a variety of CZTSSe cells with a CIGSSe standard (with a PCE

of 13.8%) was carried out to begin to understand the limitations of CZTSSe cell

efficiencies. The devices were fabricated on a molybdenum-coated glass substrate

followed by the absorber material (either CZTSSe or CIGSSe), then CdS/ZnO/ITO

(a)



(b)



Cu

Cu



Zn

Se



Sn

Zn

Zn



Cu

Cu



Se



Se

Sn



Se



Sn

Sn



Se

Zn



Cu

Cu

Se



Cu

Se



Zn

Zn

Se



Sn

Se



Cu

Cu

Se



Cu



Cu



Zn



Se



Se



Se



Se

Zn

Zn



Cu



Cu

Cu

Cu



Cu

Cu



Zn

Se



Se



Sn

Se



Cu

Se

Cu

Cu



Cu



Cu



Zn



Sn



Cu



FIGURE 7.83  The unit cells of (a) kesterite and (b) CZTSSe. (Reprinted with permission

from Fan, F.-J., L. Wu, M. Gong et al. 2013. Composition- and band-gap-tunable synthesis

of Wurtzite-derived Cu2ZnSn(S1–xSex)4 nanocrystals: Theoretical and experimental insights.

ACS Nano 7 (2):1454–1463. Copyright 2013 American Chemical Society.)



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279



with a Ni-Al contact grid and MgF2 antireflective coating. Three major findings were

reported:

• VOC. The CZTS/Se cells showed low open-circuit voltages relative to their

band gap. This was attributed to significant recombination at the buffer/

absorber interface.

• Fill factor. Compared to the CIGSSe standard, the fill factors of the

CZTSSe cells were low, presumably a result of high series resistance. This

may be due to a thick MoSe2 layer at the back contact in the hydrazineprocessed cells.

• External quantum efficiency. The CZTSSe cells showed a poor response

at long wavelengths, possibly because of a very low carrier lifetime (which

would also result in the low VOCs and could be related to the high recombination at the interface) (Mitzi et al. 2011; Todorov et al. 2010).

Because the research and development are in the early stages, these modest results

are actually very promising. One major hurdle with respect to their further improvement, however, lies in the development of better methods of preparing the absorber

material as a pure, single-phase thin film. The complexity of the kesterite material

leads to challenges in phase segregation and control of the elemental composition

which naturally impacts device performance. Both vacuum and nonvacuum techniques for film preparation have been explored and the nonvacuum technique of wet

chemical deposition using hydrazine appears to be the most successful (although use

of hydrazine presents its own issues in terms of toxicity) (Bag et al. 2012). Naturally,

various other methods are under development, including synthesis of CZTS nanocrystals followed by sintering with selenium vapor (Ford et al. 2011) or electrodeposition (Peter 2011). Ultimately an ink-based approach—where the precursor material

could be rolled onto the substrate—is a goal for large-scale production of these and

many other PV devices (Mitzi et al. 2011).



7.8 CONCLUSIONS

Beyond resource availability, the sustainability of photovoltaics requires careful

assessment of the cradle-to-grave cycle at the end of the PV life cycle: what do we

do with expended PVs? Recycling is a highly desirable option because of the limited

supply of many PV materials. In the case of CdTe solar cells, both cadmium and

tellurium are toxic so that reclaiming these elements from CdTe solar cells is imperative. But recovery is complicated and costly. Land use, too, is a piece of the PV sustainability puzzle: given the generally low efficiency of photovoltaics, a huge amount

of land area is required to generate large amounts of usable solar energy. Obviously,

a candid focus on sustainability in all of its aspects is required as we contemplate the

use of photovoltaics to help meet our growing energy appetite.

Innovations to increase the efficiency of solar PV devices continue at a rapid pace;

this chapter has only introduced a few of the more common configurations. Tandem

cells, intermediate band solar cells, inorganic–organic hybrid cells, organometallic

photovoltaics, dye-sensitized QD devices—every day encouraging new findings are



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reported. The key concerns associated with solar energy persist, however: persistently low efficiency, high cost, and questionable sustainability. In summary,

• Given the current levels of efficiency, the contributions of solar photovoltaics to our huge energy needs are likely to be small.

• The cost of materials and processing is, of course, an important concern. The

switch from crystalline silicon cells to amorphous silicon meant lower manufacturing costs, but also decreased efficiency. And cost is also related to…

• Sustainability. As noted above, limited resources, toxicity of some materials, and concerns about land use cloud the picture for solar PV technology.

Nevertheless, PVs are much more environmentally friendly with respect to

lifecycle air emissions per GWh than fossil fuel-based electricity (Fthenakis

et al. 2008). With the current astonishing growth rate in PV production and

installations, plus the astonishing amount of research effort being devoted

to solar PV technology, it is certain that this area of energy generation will

continue to grow.



OTHER RESOURCES

Books

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Brendel, R. 2003. Thin-Film Crystalline Silicon Solar Cells. Physics & Technology. Weinheim,

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Fonash, S.J. 2010. Solar Cell Device Physics, 2nd ed. Burlington, MA: Academic Press/

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Kitai, A. 2011. Principles of Solar Cells, LEDs and Diodes. West Sussex, UK: John Wiley &

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Nelson, J. 2003. The Physics of Solar Cells. London, UK: Imperial College Press.

Pagliaro, M., G. Palmisano, and R. Ciriminna. 2008. Flexible Solar Cells. Weinheim, FRG:

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Shah, A. 2012. Thin-film silicon solar cells. In Practical Handbook of Photovoltaics.

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Waltham, MA/Oxford, UK: Academic Press.



Online Resources

U.S. Department of Energy/Energy Efficiency and Renewable Energy/Solar: http://www.eere.

energy.gov/topics/solar.html

International Energy Agency/Solar PV: http://www.iea.org/topics/solarpvandcsp/

European Photovoltaic Industry Association: http://www.epia.org/home/



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8



Biomass



8.1 INTRODUCTION

When people think “sustainable energy,” the energy source that often springs to mind

is, naturally, biomass. Humans have been using biomass for energy since the dawn

of our time. Not only is it our personal source of energy (as in food), we have also

used it to cook our food and heat our homes for millennia. As a potential energy

source, biomass is relatively abundant (ranked third, after oil and coal) and has supplied more than 90% of the fuel and energy needs of the United States until the

mid-nineteenth century (Champagne 2008). In several European countries, biomass

makes a considerable contribution to energy supply and consumption and it is still a

significant source of energy in developing countries (Pereira et al. 2012). Among the

various energy solutions presented in this book, biomass-to-energy conversions are

arguably the most sustainable and are considerably cleaner than coal, for example,

in that biomass-derived energy generates far fewer NOx or SOx emissions. Another

advantage of the use of biomass is its amenability to small-scale installations with the

concomitant promise of energy availability and economic development in rural and

developing areas. Biomass led the way in renewable energy consumption in 2011 in

the United States, with 4.4 quadrillion BTU being consumed (hydroelectric power

was a distant second with 3.2 quad) (U.S. Energy Information Administration 2012).

Given the availability of biomass, its very low levels of pollutants and the wide variety of conversion options for deriving usable energy from biomass, the contributions

of biomass to our future energy needs are certain to grow.



8.1.1  Carbon Neutrality

One of the most prevalent reasons for the use of biomass as an energy source is its

promise of being “carbon-neutral.” But what do we really mean by carbon neutrality? Recall the carbon cycle from Chapter 1: the biomass, as it grows, removes

carbon from the atmosphere in the form of CO2 and fixes it into carbohydrates and

other carbon-containing material. A tree is a natural carbon sequestration agent.

When we then convert that biomass into energy (as in combustion), we generate

CO2 but conservation of matter dictates that we are neither creating nor destroying

matter: carbon neutral. But is biomass really carbon neutral? Fertilizers and pesticides (which may be carbon-containing) are often used in growing the material.

Machinery, probably powered by fossil fuels, will be used in planting, harvesting,

processing, and transporting the biomass. In terms of life-cycle analysis for sustainability, all these factors must be taken into consideration. That said, if the biomass

can be produced in a carbon-neutral way and the CO2 generated during the energy

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conversion process is sequestered, biomass energy can actually reduce the amount

of carbon in the atmosphere (Milne and Field 2013). However, an in-depth study

of the thermodynamics of energy production from biomass came to the conclusion

that biofuel production on an industrial scale is inherently unsustainable (Patzek

and Pimentel 2005). Biomass is an important part of the overall sustainable energy

solution, but it is not a panacea.



8.1.2 Biomass Considerations

8.1.2.1  Energy Density and Land Use

A major consideration for biomass-derived energy is the amount of energy the biomass actually contains. As we saw in Table 1.5, different materials inherently possess different quantities of energy. Crude oil is an excellent source of energy in that it

is easily transported and energy rich: at 42 MJ/kg, it is one of the most energy-dense

fuels. Methane is even higher, at 55 MJ/kg, and even coal has a respectable energy

density (27–32 MJ/kg). Compared to these fossil fuels, the energy density of biomass

pales: dry carbohydrate biomass has an energy density in the range of 15–20 MJ/

kg (Champagne 2008; da Rosa 2009; Sørensen 2007). This, of course, translates to

needing more biomass to produce the same amount of heat or power, which translates into higher transportation costs, processing costs, and so on.

The relatively low energy density of biomass also means that issues associated with land use must be taken into account. Expansion of land use for biofuel

production can lead to deforestation (particularly in tropical areas), reducing any

potential benefit to using biomass. The best land for agriculture must be used to

grow food for a hungry global population. The “ideal” energy crop should be able

to be grown on marginal land with little use of fertilizer or pesticides and, potentially, under drought conditions (or at least needing minimal water). Furthermore,

energy crops should not be grown at the expense of biodiversity. Given the current

level of energy consumption, it would take almost three times all the land currently

cultivated for agriculture to satisfy our energy needs through biomass conversions

(Barber 2009).

8.1.2.2  Soil and Water

Beyond the enormous area of land needed lies the concern of soil quality. Repeated

removal of biomass (as in harvesting of corn stover for energy purposes rather than

tilling it back into the soil) may impact the long-term soil quality (Johnson 2013).

Water usage, as intimated above, is another major concern: the more biomass-derived

energy sources expand, the greater the stress on water supplies. For example, up to

six gallons of water are required for every one gallon of bioethanol produced (Aden

2007). Biomass may have a moderate carbon footprint, but its water footprint is

huge. It is true that only a small percentage of the biomass produced by photosynthesis is currently being cultivated, harvested, and used—but how much can be used

sustainably? As with any approach to energy generation, the massive demand for

energy demand accentuates the need to be careful in considering the use of biomass

for energy generation.



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