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Efficient ultrathin organic solar cells with sustainable #-carotene as electron donor Varun Vohra, Takayuki Uchiyama, Shusei Inaba, and Yoshiko Okada-Shudo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06255 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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ACS Sustainable Chemistry & Engineering

Efficient

ultrathin

organic

solar

cells

with

sustainable -carotene as electron donor Varun Vohra†,* Takayuki Uchiyama†, Shusei Inaba†, Yoshiko Okada-Shudo† † Department of Engineering Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu City, 182-8585 Tokyo, Japan Corresponding Author * Email: [email protected]; Telephone: +81-42-443-5359 (Office)

ACS Paragon Plus Environment

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ABSTRACT. -carotene (bCar) is an abundant natural organic semiconductor that can be extracted from tomatoes or carrots at extremely low costs. Using natural bCar as electron donor combined with a C70 derivative (PC71BM) as electron acceptor in bulk heterojunction active layers, we successfully fabricated efficient inverted organic solar cells (OSCs) processed in air without encapsulation. Unlike conventional OSCs produced with synthetic materials, higher short-circuit current densities are achieved in ultrathin active layers (~30 nm) compared to thicker ones (~ 90 nm). This peculiar behavior can be ascribed to the low hole transport properties of bCar that limit the charge collection efficiency in 90 nm-thick bCar:fullerene OSCs. Our results demonstrate that higher boiling point solvents induce crystalline transformation of bCar in thin active layers resulting in OSCs with fill factors around 35% and average power conversion efficiencies (PCEs) of 0.58%. These devices demonstrate stable operation under constant illumination and are the best performing bCar-based OSCs published to date. They exhibit a 4-fold increase in PCE compared to previously reported bCar:fullerene OSCs, thus opening the path to low-cost yet efficient bCar photovoltaic device fabrication.

KEYWORDS: Carotenoids; Fullerenes; Bulk heterojunction solar cells; Photosynthetic molecule; Natural pigment.


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Introduction Bulk heterojunction (BHJ) organic solar cells (OSCs) employing organic semiconductors and fullerene derivatives as active materials were introduced at the end of the 20th century.1 Through advanced chemistry and materials design, BHJ-OSCs now produce power conversion efficiencies (PCEs) over 14%.2,3 However, the numerous synthetic steps to fabricate efficient synthetic semiconductors increase their impact on the environment (use of hazardous organic solvents) and their cost-performance indices (CPIs). In fact, even simple thiophene-based homopolymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) cost over 600 USD/g according to Sigma-Aldrich. P3HT combined with phenyl-C61-butyric acid methyl ester (PC61BM) or phenyl-C71-butyric acid methyl ester (PC71BM) into BHJ-OSCs can produce PCE around 5% but generally yields PCEs close to 4%.4,5 As active layer solutions are typically prepared with electron donor concentrations of a few mg/ml, we define the CPI of the electron donors as the ratio of their cost per milligram to the PCE of the OSCs that employ these materials. P3HT thus has a CPI of 0.12 USD/(mg.%). Note that this CPI calculation does not take into account the cost of the electron acceptor to emphasize the potential of natural pigments as low-cost electron donors with respect to synthetic materials. For example, carotenoids can be extracted from tomatoes at a low cost using green solvents,6-8 and are consequently of great interest as sustainable electron donors for hybrid solar cells and OSCs fabrication.9-13 -carotene (bCar) have a short polyacetylene-like molecular structure (Figure 1) and can thus be expected to exhibit a p-type semiconducting behavior. These natural photosynthetic pigments have an absorption spectrum extending from 350 nm to 600 nm with a maximum absorption coefficient at 452 nm of 249 l.g-1.cm-1 (over 130,000 M-1.cm-1), which far exceeds that of P3HT with a maximum value around 8,000 M-1.cm-1.14,15 As the S0 to S1 optical transition in bCar is symmetry


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forbidden, light absorption is generally associated with an electron promotion from the S0 ground state to the S2 excited state.

Figure 1. Molecular structure of bCar; and schematic representations of the inverted OSC architecture and the energetic levels of active layer materials. The ground and excited states of bCar correspond to those measured by ultraviolet photoelectron spectroscopy in a previous study.12 The energetic levels of PC61BM and PC71BM were collected from Ossila. The electrons in the S2 state relax non-radiatively (femtosecond scale) to the S1 state which has an energy of -3.3eV. As the lowest unoccupied molecular level (LUMO) of PC61BM and PC71BM can be found at -3.7eV and -3.9eV, respectively, the electrons populating the S1 excited state of bCar can be transferred to the LUMO of these fullerene acceptors to produce photoinduced charge separated states.12 In fact, Wang et al. demonstrated that bCar:PC61BM OSCs can generate PCEs up to 0.15%. As bCar extracted from tomatoes costs 9 USD/g (SigmaAldrich), the CPI of these OSCs has a value of 0.06 USD/(mg.%), corresponding to half of the CPI for the best performing P3HT OSCs. Note that these electron donors can be produced by liquid-liquid extractions from commonly available natural tomatoes and are thus suitable active materials for advanced optoelectronic devices fabricated in low resource developing countries.


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By considerably reducing the amount of hazardous organic solvents required for synthetic electron donor production and because they can be recovered from household wastes, employing bCar in OSCs aligns well with several of the 12 principles of green chemistry (Prevention, Less Hazardous Chemical Syntheses, Use of Renewable Feedstock) and also with the Sustainable Development Goal 7 adopted by the United Nations (Affordable and Clean Energy). To produce functional bCar OSCs with the UV-absorbing PC61BM, Wang et al. used relatively thick active layers to harvest enough sunlight.12 Due to the low hole mobility (H) of bCar, the optimized devices exhibited low fill factors (FF) of 28% and short-circuit current densities (Jsc) of 0.86 mA/cm2. Here, we demonstrate that bCar:PC71BM BHJ active layers can yield higher PCEs than bCar:PC61BM ones in inverted OSC architectures (Figure 1). This active material combination produces thin films with a strong absorption in the 400~500 nm wavelength region where sunlight (AM1.5g) is most abundant. As a result, ultrathin active layers (~ 30 nm) can be employed, in which holes and electrons can percolate more efficiently to their respective electrodes. By analyzing the electrical parameters of stable bCar:PC71BM OSCs prepared in air, we elucidate the uncommon thickness dependence behavior of these photovoltaic devices based on low-cost natural dyes. We find that chlorobenzene (CB) yields higher performances than chloroform (CF) in both OSCs and hole only devices, owing to the crystalline transformation of bCar in the active layers. The optimized ultrathin bCar:PC71BM active layers processed in air produce an average PCE of 0.58%, which corresponds to a 4-fold increase with respect to previous studies on OSCs employing bCar as electron donor.12 The resulting OSCs consequently have a CPI below 0.016 USD/(mg.%), which is almost an order of magnitude lower than that of P3HT-based OSCs.


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Experimental Section

Materials and Device Fabrication

bCar (≥97%) and the solvent employed for this study were acquired from Sigma-Aldrich and used as received without further purification. The fullerene derivatives were purchased from Luminescence Technology Corp. All active layer solutions were prepared with a donor:acceptor ratio of 1:4 in either CF or CB. For OSC fabrication, a 30 nm-thick ZnO layer was formed on cleaned ITO pattern glass substrates (Atsugi Micro) by spin-coating a precursor solution composed of 500 mg ZnOAc·H2O and 101.4 mg 2-aminoethanol in 5 ml of 2-methoxyethanol at 3000 rpm for 30s. The formed thin films were annealed at 200°C in air for 30 min followed by slow cooling back to room temperature. The 90 nm, 60 nm, 30 nm and 20 nm-thick active layers from CF were prepared with a total concentration of 20 mg/ml spin-coated at 3000 and 4000 rpm, and 10 mg/ml spin-coated at 6000 and 8000 rpm, respectively. To produce the 30 nm-thick active layers from CB, a solution with a total concentration of 37.5 mg/ml was spin-coated at 8000 rpm. The devices were finalized by placing the substrates in vacuum (< 10-3 Pa) for at least 2 hours to remove any residual solvent trace and by subsequently evaporating MoO3 (10 nm) and Ag (65 nm) under these low pressure conditions. For hole only devices, the devices were prepared following the exact same preparation method but ZnO was replaced with a 40 nm-thick PEDOT:PSS layer spin-coated at 4000 rpm from a water dispersion (Clevios AI4083) and annealed at 200°C for 10 min. The substrates for absorption and PL measurements were deposited on glass/ZnO substrates using CF and CB. For single component films, concentrations of 10 and 15 mg/ml were employed for CF and CB, respectively.


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Active Layer Characterizations

The absorption spectra of the thin films were measured using a JASCO V-670 UV-Vis spectrometer and solid-state photoluminescence (PL) was measured with a JASCO FP-6500 spectrofluorometer. The PL spectra presented in this study were measured by using excitation wavelengths of 450 nm and 580 nm, respectively. The raw data was normalized by the absorption intensity of the films at 450 nm or 580 nm to account for the differences in thickness and/or content of active molecule in each film. AFM images of active layers were collected in contact mode.

Device Characterizations

The photovoltaic characteristics of the devices were measured with a Keithley 2401 sourcemeter using a solar simulator with a power intensity of 100 mW/cm2 (1sun) and a spectral shape corresponding to AM1.5g. Series (Rs) and shunt (Rsh) resistances were calculated using the inverse of the J-V slopes at Voc and Jsc, respectively. The photovoltaic performances correspond to the average values of 12 devices and the standard deviation of their PCEs. For the hole mobility calculations, the J-V characteristics were fitted with the following space-charge limited current model equation (Eq. 1): J=

V2 9 μ ε ε 0 r 8 L3

Eq.1

where 0, r and L correspond to the permittivity of free space, the relative dielectric constant and the active layer thickness, respectively.


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Results and Discussion Unlike regular device architectures with Ca or Al as top cathode material, in inverted devices, the top anode corresponds to a combination of MoO3 and Ag which are both stable in air and remove the necessity for an additional encapsulation process in a nitrogen-filled glovebox.16 Using CF as the solvent, we first compare the photovoltaic performances of inverted bCar:PC61BM devices with previously published results.12 Our inverted devices exhibit similar performances to the regular devices, with average PCEs of 0.17% and 0.15%, respectively (Table 1 and Figure S1). However, as no nitrogen-filled glovebox was employed, the fabrication process is considerably simplified and devices can be produced with low financial and energetical investments. For similar active layer thicknesses (85~90 nm), the bCar:PC71BM OSCs with a stronger absorption in the visible exhibit an average Jsc 44% higher than bCar:PC61BM OSCs. Although the Jsc of OSC is expected to increase with active layer thickness owing to a larger amount of absorbed sunlight, the 30 nm-thick BHJ deposited using CF (BHJCFs) produce a Jsc 2.5 times higher than 90 nm-thick ones with average Jsc values of 3.51 and 1.41 mA/cm2, respectively. We use CF, a low boiling point solvent, to compare various active layer thicknesses while limiting the impact of potential phase separation. This uncommon thickness dependent behavior may be related to the low H of bCar and the resulting low collection efficiency of charges generated in the bulk of thick BHJ-CFs. In fact, the Rs values displayed in Table 1 suggest that charges can percolate to the electrodes more efficiently in thinner films. The Rs measured under light irradiation for 30 nm and 90 nm-thick BHJ-CFs have values of 1.8 and 5.1 Ω.cm2, respectively.


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Table 1. Photovoltaic parameters of OSCs fabricated with bCar as electron donor electron acceptor

thickness (nm)

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω.cm2)

Rsh (Ω.cm2)

PC61BMa

80

0.86

0.63

28.0

0.15

-

-

PC61BMb

85

0.98

0.56

30.3

0.17±0.01

6.9

705

PC71BMb

90

1.41

0.61

32.7

0.28±0.03

5.1

580

PC71BMb

60

2.27

0.52

34.1

0.40±0.02

4.8

329

PC71BMb

30

3.51

0.46

32.9

0.53±0.01

1.8

219

PC71BMb 20 3.47 0.39 30.5 0.41±0.02 a Data collected from previous study using regular architecture OSCs.12

1.6

166

b

Average data collected from 12 inverted OSCs and the standard deviation of their PCE.

On the other hand, the active layer thickness dependence of Rsh indicates that leak current increases with decreasing active layer thickness. According to the equivalent circuit model of OSCs, the open-circuit voltage (Voc) decreases with increasing leak current,17 which is consistent with the lower Voc values measured in thinner active layers as compared to thicker ones. The opposing effects of thickness variation on charge collection and leak current balance each other, resulting in a constant FF value of 33~34% for active layers with a thickness ranging from 30 nm to 90 nm. The increase in Jsc in 30 nm-thick active layers compensates for the Voc reduction to produce an average PCE of 0.53%, a value 1.9 times larger than that of 90 nm-thick active layer OSCs (0.28%).


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Figure 2. (a) AFM topographic images and (b) absorption spectra of -carotene:PC71BM active layers deposited from CF or CB. To further improve the performances of OSCs employing bCar as electron donor, we compare 30 nm-thick active layers deposited from CB (boiling point: 131°C) and CF (boiling point: 61.2°C). The AFM images in Figure 2(a) suggest that finer nanoparticle-like assemblies are formed at the surface of BHJ-CBs as compared to BHJ-CFs. Due to the overlapping absorption of the two active materials and the larger ratio of PC71BM employed, no clear evidence of crystalline transformation could be observed from the BHJ-CFs and BHJ-CBs absorption spectra (data not shown). However, major changes in spectral distribution are found in the absorption spectra of bCar films deposited using CF or CB (Figure 2(b)). The stronger contribution of peaks at 480 nm and 520 nm relative to the peak at 445 nm in films deposited from CB can be ascribed to the formation of carotenoid crystallites.18 The space-charge limited current fittings of 30 nm-thick hole only devices in Table 2 and Figure 3(a) indicate an 8.5-fold increase in H in BHJ-CB devices with respect to BHJ-CF ones. This is consistent with the lower Rs values obtained for


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BHJ-CB compared to BHJ-CF in OSCs. The crystalline transformation of bCar using the higher boiling point solvent thus yields an increase in FF due to a more efficient charge collection as well as higher Voc values resulting from a lower trap density (Table 2 and Figure 3(b)).

Figure 3. J-V curves from (a) hole only devices and (b) OSCs employing BHJ-CF and BHJ-CB as active layers. Table 2. Impact of processing solvent on the optoelectronic properties of OSCs employing ultrathin (~30 nm) active layers

Solvent

H 2 (cm /(Vs))

Rs (Ω.cm2)

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

CF

0.8 x 10-4

1.8

3.51

0.46

32.9

0.53±0.01

CB

6.8 x 10-4

1.5

3.27

0.51

34.8

0.58±0.02

Despite the fact that enhanced hole transport in BHJ-CB should positively affect the Jsc of the OSCs, slightly lower Jsc values are obtained for BHJ-CBs compared to BHJ-CFs. The slower film formation kinetics of BHJ-CB compared to BHJ-CF could produce larger phase separated


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bCar-rich and PC71BM-rich domains, which would result in a smaller donor:acceptor interface yielding lower Jsc values.

Figure 4. PL spectra of (a) PC71BM, BHJ-CF and BHJ-CB excited and normalized to their absorption at 580 nm (b) bCar, BHJ-CF and BHJ-CB excited and normalized at 450 nm; (c) schematic representations of the active layer nanomorphologies of BHJ-CF and BHJ-CB. Figure 4(a) displays the PL spectrum of PC71BM thin films excited at 580 nm along with the spectra of the blend films measured under the same conditions. These spectra were normalized by the absorption of the corresponding films at 580 nm, a wavelength where PC71BM absorbs strongly compared to bCar. When comparing the single component and blend films, we can clearly observe a strong quenching of the PC71BM PL independently of the employed solvent, indicating that photons absorbed by the electron acceptor efficiently dissociate into holes and electrons at the donor:acceptor interface in both BHJ-CF and BHJ-CB. On the other hand, when


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investigating the PL quenching behavior of bCar excited at 450 nm, we could clearly observe that active layers deposited from CB have a lower PL quenching rate than BHJ-CF (Figure 4(b)). Note that when a high boiling point deposition solvent such as o-dichlorobenzene (DCB) is employed, the PL quenching rate decreases furthermore with respect to BHJ-CF (Figure S2). These differences in PL quenching behavior can be understood through the excited-state dynamics of bCar and PC71BM. Compared to the relatively long decay times of PC71BM excitons (up to 500 ps), electrons in the S1 state of bCar relax much faster to the ground state in less than 10 ps.12,19 Consequently, larger phase separated domains will affect the bCar PL quenching more strongly than that of PC71BM. PL quenching rates for BHJ-CF, BHJ-CB and BHJ-DCB have values of 99%, 97% and 90%, respectively (Table S1). The PL behaviors in the three types of active layer thus suggest that formation of bCar nanocrystals induces larger phase separated domains in BHJ-DCB and BHJ-CB compared to BHJ-CF (Figure 4(c)) and that some of the excitons generated within the bCar nanocrystals present in BHJ-CB and BHJ-DCB may undergo relaxation to the S0 ground state before reaching the donor:acceptor interface, thus yielding a lower charge generation efficiency. These findings correlate well with the slightly lower Jsc obtained in the devices employing CB despite the enhanced hole transport properties of BHJ-CB films relative to BHJ-CFs. In fact, larger bCar crystals should be formed in active layers deposited with the slow drying solvent DCB which correlates well with the lower PL quenching ratio of BHJ-DCB compared to BHJ-CF and BHJ-CB, as well as the low average Jsc (2.64 mA/cm2) of BHJ-DCBs, which is 25% lower than the value measured for BHJ-CFs (Table S2). This further confirms that, although the formation of crystalline bCar will have a positive effect on the FF of the OSCs, large crystalline structures will notably reduce the charge generation efficiency and thus, the Jsc of the devices. Nonetheless, the higher values of Voc and FF


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obtained for BHJ-CB-OSCs largely compensate the minor decrease in Jsc with respect to BHJCF ones, thus producing an average PCE of 0.58%, which is approximately 10% higher than that of BHJ-CF-OSCs. Conclusions In summary, we have demonstrated that OSCs based on extremely thin active layers composed of a natural carotenoid, bCar, combined with a fullerene derivative absorbing in the visible, PC71BM, can yield PCEs up to 0.58%. Compared to previous reports on OSC employing bCar as electron donor and PC61BM as electron acceptor, we were able to improve the PCE almost 4-fold by using an electron acceptor that can absorb an abundant amount of sunlight and by enhancing charge collection efficiency (thinner active layer, increased crystallinity) in inverted device architectures that remove the necessity for an additional encapsulation step in a nitrogen-filled glovebox. These devices exhibit a stable operation under constant irradiation for more than 24 hours (data not shown). Although the PCEs of bCar BHJ-OSCs remain relatively low compared to synthetic conjugated materials, our results open the path to extremely low-cost and sustainable fabrication of renewable energy harvesting devices. Supporting Information. J-V curves corresponding to the data presented in Table 1 as well as PL spectra, PL quenching ratios and photovoltaic properties of 30 nm-thick active layers (CF, CB and DCB) can be found in the Supporting Information file. AUTHOR INFORMATION [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The experimental work was supported by the Japan Society for the Promotion of Science through the Grant-in-aid for Young Scientists (B) program (Grant No. 17K14549). The authors are grateful to Prof. Qing Shen for providing access to PL measurement equipment. REFERENCES (1) Yu, G.; Hummelen, J. C.; Wudl, F.; Heeger, A.J. Polymer Photovoltaic Cells – Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789-1791, DOI 10.1126/science.270.5243.1789. (2) Zhang, H.; Yao, H.; Hou, J.; Zhu, J.; Zhang, J.; Li, W.; Yu, R.; Gao, B.; Zhang, S.; Hou, J. Over 14% Efficiency in Organic Solar Cells Enabled by Chlorinated Nonfullerene SmallMolecule Acceptors. Adv. Mater. 2018, 30, 1800613, DOI 10.1002/adma.201800613. (3) Xiao, Z.; Jia, X., Ding, L. Ternary Organic Solar Cells Offer 14% Power Conversion Efficiency. Sci. Bull. 2018, 62, 1562-1564, DOI 10.1016/j.scib.2017.11.003. (4) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617-1622, DOI 10.1002/adfm.200500211. (5) Kim, J.Y.; Lee, K.; Coates, N.E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A.J. Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007, 317, 222-225, DOI 10.1126/science.1141711. (6) Sabio, E.; Lozano, M.; Montero de Espinosa, V.; Mendes, R.L.; Pereira, A.P.; Palavra, A.F.; Coelho, J.A. Lycopene and β-Carotene Extraction from Tomato Processing Waste


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Table of Content (TOC) Graphic:

Synopsis: Efficient photovoltaic devices produced with sustainable natural dyes remove the necessity for advanced chemistry and the resulting hazardous chemical wastes/bi-products.


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Efficient ultrathin organic solar cells with ... - ACS Publications

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