Environmental Impacts from Photovoltaic Solar Cells Made with Single

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Environmental Impacts from Photovoltaic Solar Cells Made with Single Walled Carbon Nanotubes Ilke Celik, Brooke E. Mason, Adam B Phillips, Michael J. Heben, and Defne S Apul Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06272 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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Environmental Science & Technology

Environmental Impacts from Photovoltaic Solar Cells Made with Single Walled Carbon Nanotubes Ilke Celik*Ϯ1, Brooke E. Mason*Ϯ1, Adam B. Phillips2, Michael J. Heben2, Defne Apul1 School of Solar and Advanced Renewable Energy, Department of Civil Engineering, University of Toledo 2801 W. Bancroft St., Toledo, OH 43606, USA 2 School of Solar and Advanced Renewable Energy, Wright Center for Photovoltaics Innovation and Commercialization, Department of Physics and Astronomy, University of Toledo 2801 W. Bancroft St., Toledo, OH 43606, USA * Corresponding authors’ email: [email protected], phone: (419) 530-8120; email: [email protected], phone: (419) 530-8120. Ϯ Contributed equally to this work 1

ABSTRACT An ex-ante life cycle inventory was developed for single walled carbon nanotube (SWCNT) PV cells, including a laboratory-made 1 % efficient device and an aspirational 28 % efficient four-cell tandem device. The environmental impact of unit energy generation from the mono-Si PV technology was used as a reference point. Compared to monocrystalline Si (mono-Si), the environmental impacts from 1% SWCNT was ~18 times higher due mainly to the short lifetime of three years. However, even with the same short lifetime, the 28% cell had lower environmental impacts than mono-Si. The effects of lifetime and efficiency on the environmental impacts were further examined. This analysis showed that if the SWCNT device efficiency had the same value as the best efficiency of the material under comparison, to match the total normalized impacts of the mono- and poly-Si, CIGS, CdTe, and a-Si devices, the SWCNT devices would need a lifetime of 2.8, 3.5, 5.3, 5.1, and 10.8 years, respectively. It was also found that if the SWCNT PV has an efficiency of 4.5 % or higher, its energy payback time would be lower than other existing and emerging PV technologies. The major impacts of SWCNT PV came from the cell’s materials synthesis.

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1. Introduction The future seems bright for photovoltaic (PV) solar cells. Predictions and policy recommendations suggest a large growth in PV installations to mitigate climate change.1 Currently, approximately 1% of global electricity generated is from PV, but this number is expected to grow to 16% by 2050.2,3 During this growth, some shortcoming of today’s PV technologies will need to be addressed. Currently, 90% of commercial PV modules are made from mono or polycrystalline silicon (mono-Si or poly-Si), with best-recorded module efficiencies of 22.4% and 18.5%, respectively.2,4 Silicon is an earth abundant material, yet processing it into high grade silicon is energy intensive and costly.5 The remaining 10% of the PV cells are thin film technology, primarily cadmium telluride (CdTe) and copper-indium-gallium-diselenide (CIGS), both with best-recorded module efficiencies of 17.5%.2,4 Thin film efficiencies are beginning to rival the efficiencies of silicon,6 and thin-film PVs are lighter, require less materials, and, unlike rigid wafer-based crystalline silicon, can be installed in flexible formats. Yet, the geopolitical supply risk of Te and In is considered to be a barrier for terawatt scale installation of PV.7–10 One emerging technology that may grow rapidly and change the status quo is the use of single walled carbon nanotubes (SWCNT) in thin film solar cells. SWCNTs are hollow tubes of graphene that are already incorporated in diverse commercial products with vast potential to be used in many other future applications11 The electronic and optical properties of SWCNTs are dependent on the diameter and chirality of the individual structures, and these characteristics are specified by the (n,m) index of each species or tube-type.12 The (n,m) index of SWCNTs in a sample can be controlled during synthesis13 and/or isolated through post-synthesis separation.14–18 This flexibility within the materials system provides routes to prepare SWCNT samples with a wide range of optical band gaps that overlap well with the distribution of photon energies in the solar spectrum. Beyond these excellent optoelectronic properties, SWCNTs are relatively inert19 and, if used in PV, would not suffer the instability issues observed in other emerging PV materials such as organometal halide perovskites,20 polymers21 and organics.22 SWCNTs have already been incorporated into several types of PV devices in the laboratory, for example as a replacement for transparent conducting oxides,23 a novel back contact for thin film PV,24 or to assist carrier transport in polymer PV devices.25,26 Recent advancements in electronic type separation of SWCNTs27–31 have allowed semiconducting (s-)SWCNTs to be isolated in sufficient purity to enable use in the sunlight absorbing layer of PV devices. Recently, Shea and Arnold32 reported a device with a PCE of 1% using a single index of s-SWCNT, while Gong et al.33 fabricated a device with a PCE of 3.1% by using mixture of s-SWCNT species. While these PCEs are low compared to commercially available PV technologies, the results represent the first demonstration of SWCNTs working as the active semiconducting layer that is critical to the operation of a PV device. As methods for preparing bulk amounts of specific types of s-SWCNTs improve, PCEs will likely increase as well. In fact, a theoretical PCE of 28% has been estimated for a SWCNT PV constructed with four tandem sub-cells, each using a different (n,m) species to absorb different portions of the solar spectrum.34 With continued research progress, it is reasonable to expect that such high performance limits will be approached. Given the promise of SWCNT PV, it is imperative that its environmental impacts be determined to inform future solar PV planning. Would SWCNT PV reduce the greenhouse gas emissions from

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electricity generation more than current commercial PVs? Would other types of environmental impacts be lower than today’s technologies? Could SWCNT PV be manufactured with lower energy inputs and, therefore, lower cost? In this paper, these questions are addressed using the life cycle assessment (LCA) methodology. While LCAs have previously been conducted for SWCNT manufacturing35–41 and products that utilize them,42–45 this work is the first to analyze the use of SWCNT for electricity production within a PV device. 2. Methodology 2.1 Goal and Scope An ‘ex-ante’ LCA model was created to estimate the potential environmental impacts of a SWCNT PV solar cell. Shea and Arnold’s 1% efficient laboratory scale device was modeled to represent today’s technology. To bracket the range of performance expected, an aspirational 28% efficient device was also modeled. The extraction of raw materials and the production of the modules were included in the model. For the use phase impacts, potential toxicity resulting from damaged cells were investigated. Balance of system (BOS) was not taken into account, because they were expected to be similar for the compared systems.46 Transport, and disposal phase were not considered, for lack of data. Results were interpreted i) by comparing the environmental impacts from SWCNT PV to those from today’s commercial PV solar cells, ii) by analyzing the importance of materials versus manufacturing impacts from SWCNT PV and further inspecting the impacts from materials, iii) evaluating the toxicity from possible release of SWCNTs and silver to the environment during the use phase, and iv) by analyzing the primary energy demand (PED) and the energy payback time (EPBT) of SWCNT PV. EPBT is one of the most commonly reported sustainability indicators for PV systems.47 EPBT provides a useful comparison to other technologies since its value is independent of the device lifetime, which is highly uncertain for emerging PVs. The functional unit of the study was 1 kWh of electricity generated within the lifetime of the PV cell. The inventory data were developed for an area of 1 m2. To estimate the results per functional unit, Equation 1 was used with various assumptions for PCE (1%32 and 28%34 SWCNT), lifetime (SWCNT- 3 years and commercial PV- 30 years), active area (90%), performance ratio (75%), and irradiation (1700 kWh/m2-yr).48 In calculating the environmental impacts of the commercial PV technologies, the best-recorded module efficiencies were used (22.4% mono-Si, 18.5% polySi, 12.2% amorphous-Si, 17.5% CdTe, and 17.5% CIGS)4. As a conservative estimate, the lifetime of SWCNT PV was assumed to be three years which is comparable to other organic PV cells.49,50 A sensitivity analysis was also performed to understand the implications of lifetime and PCE assumptions. Amorphous-Si (a-Si) was included for a more comprehensive analysis that can be interpreted in the context of earlier studies but a-Si has lost importance in the commercial market due to competition from CdTe and CIGS. 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝑘𝑘𝑘𝑘ℎ = where,

𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝑚𝑚2

𝐼𝐼 × 𝜂𝜂 × 𝑃𝑃𝑃𝑃 𝑥𝑥 𝐿𝐿𝐿𝐿

(Eqn 1)

Impactm2 = impact per 1 m2module area; 3



125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170

I = insolation constant (kWh/m2-yr); ŋ = module PCE (%); PR = performance ratio of the module (%); Lt = lifetime of the PV technology (yr). 2.2 Structure and Processing of SWCNT PV The device structure for the 1% cell was taken from Shea and Arnold32 which provides sufficient detail to allow a life cycle inventory to be developed (Figure 1a). The top layer is the transparent anode consisting of ITO coated glass. Prior to deposition of the other layers, the ITO is cleaned with ethanol and water.32 A solution of a single species of SWCNTs, acting as the electron donor layer (absorber layer), is deposited onto the ITO via doctor-blade casting. Doctor blade casting draws a metal blade across the substrate above the substrate surface resulting in a uniform thickness of solution. This process is repeated several times until the desired layer thickness is achieved. The SWCNTs were synthesized via the CoMoCAT method.51,52 CoMoCAT produces a small distribution of SWCNTs including the (7,5) tube-type needed for the PV cell, which can be separated out using a polymer,27 aqueous two phase,16 or two column separation process.17 After SWCNT deposition, the sample is heated to 110 °C for one hour in a nitrogen atmosphere to completely remove the solvent. Then C60 fullerenes are synthesized, purified and separated53 before being evaporated onto the sample. The C60 acts as the electron acceptor layer, which has a higher electron affinity and ionization potential than the electron donor layer. (A thermal evaporator vaporizes the material inside a vacuum, allowing the material to condense onto the substrate). The last two layers are also evaporated onto the sample. Bathocuproine (BCP) is the hole transport layer, which prohibits exciton quenching by the cathode surface.54 Silver is used as the cathode. The basic structure of the 28% aspirational cell was taken from Tune and Shapter’s theoretical model. This device consists of four sub-cells, each with a SWCNT absorber layer consisting of SWCNTs with a single specific tube type.34 In absence of other detailed structural information, the 28% cell was modeled using the same concepts used in the current state-of-the-art devices that produce efficiencies in the 1 to 3% range.32,33 The 1% and 3% device structures are similar to one another and also bear similarity to other organic PV devices with a transparent conducting anode, electron donor layer, electron acceptor layer, hole transport layer, and metal cathode (Figure 1). However, the 1% device consists of a single type of s-SWCNT, while the 3.1% device consists of mixture of s-SWCNTs species. The sub-cell structures were modeled similar to the 1% device because a rational extension of this structure lends itself to the construction of the theoretical 28% device. For simplicity, the electron donor, electron acceptor and hole transport layers of the 1% device structure were repeated to form each sub-cell. The complete device consisted of four subcells with tunnel contact layers between each sub-cell and only one anode and one cathode. For the tunnel layers a 10 nm SWCNT layer deposited via doctor blade casting, as previous results55 indicate SWCNTs can be an effective tunnel layer material. Several assumptions based upon this device structure are inherent in our analysis. First is that the materials specifically used to fabricate the 1% device will be used for the hypothetical 28% tandem device. While this may ultimately not be the case, the values generated here are believed to be representative since (1) the function of each layer will remain the same and be required in the final device and (2) the overall impact from these materials relative to other materials in the device is

4


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small (vide infra). Also, note that, in general, the ex-ante nature of the study is expected to overestimate the impacts because most of these materials are currently made in small batches and scaling to manufacturing levels would introduce new efficiencies into the process. Another assumption inherent in this analysis is that the separation of the SWCNTs into the needed specific tube types will have little environmental effects. Several widely varying methods have been used to separate SWCNTs into (n,m) species such as polymer27 (used for the 1% device) or DNA14 wrapping, ultracentrifugation15 (used for the 3.1% device), spontaneous aqueous two phase separation,16 and column chromatography.17 The number of different ways in which SWCNT separations may now be done suggests that a simple, industrial scale, low energy intensity process can be developed. For example, column chromatography, in which a solution of surfactant stabilized SWCNTs are passed through a column containing hydrogel beads by gravitational force could be such a method.30,31 The interactions between the ionic moieties on the gel beads selectively interact with the SWCNTs,18,56 resulting in separation of the SWCNTs. This method has already been used to generate SWCNT solutions with a narrow range of (n, m) species. Because this method relies on gravity to pull the solution through the column, it would be expected to have very little environmental impact. Indeed, the impacts due to separation were found to be small based on a quick estimation (Tables S.5 and S.6 show that the separation process may lead to ~2-3% increase in environmental impacts of SWCNT PV cell). In the end, it is also possible that separation may not even be necessary considering, there are efforts to allow direct synthesis of specific SWCNT species,13 making separation unnecessary. Consequently, these aspects have not been included in this calculation.

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TCA- ITO Coated Glass (0.2 cm) EDL- (6,4) SWCNT (100 nm)

EAL- C60 Fullerenes (86 nm)

TCA- ITO Coated Glass (0.2 cm tune) Patterning

HTL- Bathocuproine (100 nm)

Substrate cleaning

TBL- SWCNT (10 nm)

EDL- (7,5) SWCNT (100 nm)

Doctor blading


Heated


BL - Bathocuproine (100 nm)


1 cell

TBL- SWCNT (10 nm)

Thermo-evaporation





Thermoevaporation

1 cell

TBL- SWCNT (10 nm)


MC- Silver (100 nm)


Thermo-evaporation

194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211

1 cell

HTL- Bathocuproine (100 nm) MC - Silver (100 nm)

(a)

(b)

Figure 1. (a) The relative thicknesses, pre and post treatments, and deposition methods of the five cell layers: transparent conducting anode (TCA), electron donor layer (EDL), electron acceptor layer (EAL), hole transport layer (HTL), and metal cathode (MC). Three steps are investigated for the C60 and SWCNT layers: synthesis, separation and purification. The data for these two layers consist of the aggregate of these steps, except for the separation step for SWCNT. (b) The 28% SWCNT cell is created by stacking four of the 1% cells,less the TCA and MC layers on top of each other to form a tandem PV structure. The individual sub-cells are connected by adding a tunnel barrier layer (TBL) of SWCNTs between each repeated cells. The 1% and 28% cells are 2000.4 μm and 2001.3 μm thick, respectively. The largest component for both cells is the solar glass, which is 2000.0 μm (0.2 cm) thick.

2.3 Life Cycle Inventory The life cycle inventory data, including material inputs, electricity inputs, and waste outputs (Table 1), were collected from literature and compiled using GaBi 6.0 software and the EcoInvent v2.1 database. Electricity inputs include synthesis of the materials as well as pre-treatment, deposition,

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1 cell

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212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233


and post-treatment of the materials onto the cell. Chemicals unavailable in the database were modeled using the procedure by Geisler et al.57 To produce poly(vinyl phenyl ketone) and ophenylenediamine for the BCP layer an average chemical reaction energy requirement (5.58E-05 MJ/kg) was used.58 The different layers were assumed to be deposited by low-temperature solution based methods. Deposition efficiencies were estimated based on our laboratory experience: 50% for ITO patterning, 90% for doctor blade casting, 90% for C60 thermal evaporation, and 20% for BCP and silver thermal evaporation. Inventory for screen printing was used in lieu of doctor blading since similar devices was used for the depositions. For modeling the commercial PV technologies, the environmental impacts were extracted from EcoInvent database. This data has been reported in an aggregated format which includes lamination. In our model, the SWNCT PV cell does not include lamination. However, this should not create a major error in comparing SWNCT PV to commercial PV as 90% of the commercial PV laminate phase’s impacts come from cell production (see Table S.4). Union for the Coordination of Transmission of Electricity (UCTE) European electricity mix was used in all our models. Table 1. Aggregated inventory for manufacturing a 1 m2 SWCNT cells with 1% 32 and 28%.34 efficiencies. The first column describes whether the line item is a material input (M), electricity input (E), or a waste output (W). The primary energy demand (PED) is also included for each input. PED is about three times larger than the electricity input for each input. In the comments the literature source is cited.

Unit

Mass

Mass

1%

28%

PED 1% (MJ)

PED 28% (MJ)

SWCNT material inputs51

EDL - SWCNT

m2

1

1

M

Carbon Monoxide

kg

5.83E-04

2.51E-03

3.44E-02

1.48E-01

M

Molybdenum

kg

3.19E-06

1.37E-05

4.64E-04

1.99E-03

M

Cobalt

kg

3.19E-06

1.37E-05

3.93E-04

1.69E-03

M

Oxygen

kg

1.51E-06

6.50E-06

1.28E-05

5.51E-05

M

Sodium Hydroxide

kg

8.20E-05

3.53E-04

1.07E-03

4.58E-03

M

Deionized Water

kg

4.84E-05

2.08E-04

1.51E-06

6.52E-06

E

Electricity

MJ

1.24E+01

5.32E+01

1.15E+01

4.63E+01

W

Carbon Dioxide

kg

4.58E-04

1.97E-03

-

-

W

Carbon Monoxide

kg

5.87E-05

2.52E-04

-

-

W

Hydrogen

kg

4.20E-06

1.81E-05

-

-

W

Deionized Water

kg

4.29E-05

1.84E-04

-

-

W

Hazardous Waste

kg

4.74E-05

2.04E-04

-

-

EAL- C60

m2

1

1

M

Oxygen

kg

9.46E-03

3.78E-02

8.03E-02

3.21E-01

M

Toulene

kg

1.18E-02

4.71E-02

6.93E-01

2.77E+00

7

Comments

SWCNT CoMoCAT production,35,59 doctor blading,49 annealing60

C60 material inputs53



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M

O-xylene

kg

2.06E-03

8.26E-03

1.26E-01

5.05E-01

E

Electricity

MJ

1.21E+00

4.84E+00

3.27E+00

1.40E+01

W

Carbon Soot Hydrocarbon Emissions Hazardous Waste for Landfilling

kg

1.05E-03

4.21E-03

-

-

kg

1.67E-02

6.68E-02

-

-

kg

8.22E-13

3.29E-12

-

-

HTL- BCP

m2

1

1

M

Benzoic Acid

kg

3.97E-21

1.59E-20

7.77E-19

3.11E-18

M

O-Nitroaniline

kg

3.30E-11

1.32E-10

4.07E-09

1.63E-08

M

Sodium Hydroxide

kg

2.33E-11

9.33E-11

5.13E-10

2.06E-09

M

Ethanol

kg

7.69E-11

3.08E-10

3.42E-09

1.37E-08

M

Zinc

kg

6.22E-11

2.49E-10

0.00E+00

0.00E+00

M

Methanol

Kg

1.14E-10

4.55E-10

3.89E-09

1.54E-08

M

Sodium Dithionite

Kg

1.20E-12

4.78E-12

8.12E-11

3.24E-10

M

Hydrochloric Acid

Kg

3.31E-10

1.32E-09

5.86E-09

2.23E-08

M

Nitrogen

Kg

6.31E-05

2.52E-04

5.68E-04

2.27E-03

M

O-Phenylenediamine

Kg

2.39E-08

9.56E-08

-

-

M

Kg

2.67E-07

1.07E-06

1.33E-05

5.34E-05

Kg

6.55E-11

2.62E-10

-

-

M

Acetic Acid Poly (Vinyl Phenyl Ketone) Ammonia

Kg

1.02E-10

4.07E-10

3.94E-09

1.57E-08

M

Acetone

Kg

2.06E-02

8.26E-02

1.28E+00

5.14E+00

E

Electricity

MJ

4.01E+01

1.60E+02

3.72E+00

1.48E+01

W

Hazardous Waste for Landfilling

Kg

4.68E-13

1.87E-12

-

-

W W

M

C60 production,53 thermoevaporation61

BCP material inputs62 Poly (vinyl phenyl ketone) material input For Ophenylenediamine material input63 O-phenylenediamine material input63 O-phenylenediamine material input63 O-phenylenediamine material input63 O-phenylenediamine material input63 O-phenylenediamine material input63

Material input63

Chemical reactions,57 stirring,60 thermoevaporation61

ITO material and electricity inputs61

m2

1

M

TCA– ITO coated glass Tin

Kg

2.30E-03

7.08E-01

M

Indium

Kg

2.00E-04

4.91E-01

M

Titanium Dioxide

Kg

1.00E-04

8.18E-03

M

Argon

Kg

1.24E-01

8.19E-01

M

Oxygen

Kg

1.00E-04

8.47E-04

M

Solar Glass

Kg

1.54E+00

2.11E+01

M

Ethanol

Kg

2.58E-02

1.15E+00

8


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M

Deionized Water

Kg

3.27E-02

6.06E-04

E

Electricity

MJ

8.27E+01

2.53E+02

W

ITO

Kg

6.41E-01

-

MC - Silver

m2

1

M

Silver

Kg

1.05E-03

1.48E+00

E

Electricity

MJ

1.17E+00 1.77E+00 1.91E+00 kg kg

3.65E+00 3.03E+02 3.66E+02 MJ MJ

Total

234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269

Silver material,32 electricity inputs61

2.4 Life Cycle Impact Assessment The life cycle impact assessment was conducted using the TRACI 2.1 model with ten impact categories: global warming potential including biogenic carbon (kg CO2-equiv.), acidification (kg SO2-equiv.), ecotoxicity (CTUe), eutrophication (kg N-equiv.), ozone depletion (kg CFC 11equiv.), human toxicity - cancer (CTUh), human toxicity – non-cancer (CTUh), resources - fossil fuels (MJ surplus energy), smog (kgO3-equiv.), and human health particulate air (kg PM2.5equiv.).64 In addition, EPBT was calculated as outlined by Bhandari et al.65 EPBT defines the length of time the PV device must be in operation before it ‘pays back’ the PED that was required to produce the PV device. A smaller EPBT is desirable. A benchmark is needed for interpreting results from multiple impact categories with different units that cannot be directly summed or compared among each other. Mono-Si is used as the benchmark as it has the highest environmental impact among commercial technologies and is the second largest PV technology (36 % of global market) after poly-Si (55 % of global market).66 A similar internal normalization approach was used in Celik et al.48 The process involves dividing the results from all models by the impact from mono-Si PV. This normalization was done within each impact category. The normalized scores were then averaged and interpreted as the overall environmental performance of a technology in comparison to mono-Si PV.

3. Results 3.1 Life Cycle Environmental Impacts Table 2 shows the comparison of the normalized environmental impacts of current PV technologies to the 1% and 28% SWCNT cells on a kWh-1 basis. The environmental impact of unit energy generation from the mono-Si PV technology was used as a reference point in Table 2. The 1% SWCNT device is the least environmentally preferable option among all PV technologies that were evaluated (red column in Table 2). Poly-Si and mono-Si impacts are quite similar to one another (yellow shaded as neutral in Table 2); CdTe and CIGS are about 30% lower and a-Si is about 50% lower. Compared to mono-Si, the environmental impacts from 1% SWCNT are 4 to 24 times higher and on average 18 times higher. The primary reason for these very high impacts is the low PCE and to a certain extent the short lifetime assumed for this device. Nevertheless, the 1% SWCNT device is still better than coal; its environmental impacts are half that of a coal power plant (Data shown in Supporting Information Table S1c).

9



270 271 272 273 274 275 276 277 278

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Table 2. Comparison of the normalized environmental impacts of current PV technologies to the 1% and 28% SWCNT cells on a kWh-1 basis. The current PV technologies were assumed to have a 30-year lifetime and the bestrecorded module efficiencies.4 The results are normalized to the impacts from mono-Si with a number less than 1 indicating a smaller comparative impact. The actual values of mono-Si impacts per kWh-1are shown in the last column of the table. The color coding is used to help with interpretation. The numeric values that map to the color code indicate a half standard deviation (0.15) of the normalized impacts from CNT (28%), a-Si, CdTe, CIGS and poly-Si. The data from CNT (1%) was not included as these values are much higher than the rest of the sample population. Here, environmentally preferable defines as less environmental impacts.

PV Technology Efficiency Lifetime (yr)

poly- monoCNT CNT a-Si CdTe CIGS Si Si 1.0% 28.0% 12.2% 17.5% 17.5% 18.5% 22.4% 3 3 30 30 30 30 30 23.89

0.99

0.79

1.09

0.61

0.93

1.00

Ecotoxicity

20.37

0.83

0.50

1.29

0.94

0.99

1.00

Eutrophication Global Warming Potential Human toxicity, cancer Human toxicity, noncanc. Human Health Particulate Air

26.15

1.11

0.39

0.62

1.19

0.79

1.00

18.66

0.79

0.50

0.56

0.79

0.95

1.00

19.74

0.84

0.65

0.65

0.85

0.91

1.00

15.33

0.62

0.42

1.05

0.76

1.06

1.00

22.07

0.92

0.65

0.75

0.63

0.95

1.00

Smog Air

18.39

0.76

0.46

0.76

0.62

1.02

1.00

Ozone Depletion Air 4.62 Resources, Fossil Fuels 11.46 Average Normalized Impact 18.07

0.19

0.07

0.15

0.19

1.16

1.00

0.50

0.34

0.41

0.42

1.07

1.00

0.76

0.48

0.73

0.70

0.98

1.00

Primary Energy Demand 18.36 0.79 279 most environmentally preferable more environmentally preferable Neutral less environmentally preferable least environmentally preferable 280

0.43

0.45

0.66

0.95

1.00

Impact Categories

Acidification

1.30

10


mono-Si total impacts 8.67E- kg SO205 Equiv./kWh 1.65E01 CTUe/kWh 1.09E04 kg N-Equiv./kWh 2.13E- kg CO202 Equiv./kWh 1.57E09 CTUh/kWh 1.05E08 CTUh/kWh 1.34E- kg PM2.505 Equiv./kWh 1.13E- kg O303 Equiv./kWh 5.62E- kg CFC 1109 Equiv./kWh 2.79E- MJ surplus 02 energy/kWh 4.32E01 MJ/kWh

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Compared to the 1% SWCNT device, the environmental impacts of the 28% SWCNT device are dramatically reduced (23 to 25 times). Even with a short three-year lifetime, the 28% SWCNT device performs better than mono-Si in all impact categories except eutrophication. The second and third highest impacts (acidification and human health particulate air) are very close to the impacts from mono-Si. The overall environmental performances of the 28% SWCNT, CdTe, and CIGS devices are similar (~70 % of mono-Si). Yet, unlike, CdTe and CIGS, the 28% SWCNT device does not have any impact categories that are in the ‘less environmentally preferable’ range. The global warming impacts of the 28% SWCNT and CIGS devices were exactly the same and 21% lower than that of mono-Si. The CdTe device has a lower global warming impact that is 44% lower than that of mono-Si. The lowest impact category in the 28% SWCNT device is ozone depletion (19% of mono-Si). Only a-Si is ‘more environmentally preferable’ for all impact categories. It is very promising that the 28% SWCNT device’s global warming and other environmental impacts are close to or lower than most of the commercial PV technologies despite the short threeyear lifetime assumption. Oxygen, light, moisture, and high temperatures that degrade other organic PV67,68 will likely not cause instability in SWNCT devices. Therefore, the three-year lifetime used in the analysis is likely an underestimation. Furthermore, it may even be possible to create cells that will surpass the theoretical efficiency (28%) that was assumed. For example, Tune and Shapter34 suggested that changes in the thickness of the SWCNT layers could increase the PCE to 41.9% The impact of lifetime and PCE were further explored by independently varying these two parameters (Figure 2). The thin yellow bond that extends from a lifetime of 15 years to a PCE of 15 shows the cases where the average environmental impacts of SWCNT PV equal that of monoSi. This analysis shows that that the environmental impacts of the SWCNT PV will be lower than those of mono-Si except when the product of the lifetime and efficiency of the SWCNT PV is low (

Environmental Impacts from Photovoltaic Solar Cells Made with Single

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