Effect of Additives on the Performance of Non-Fullerene Based Organic Solar Cells in Non-Halogenated

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Effect of Additives on the Performance of Non-Fullerene Based Organic Solar Cells in Non-Halogenated Solvents1 L. Reshma1, V. Sai Saraswathi2, P. Induja3, M. Shivashankar4, K.Santhakumar1,a 1 – School of Electronics Engineering, VIT University, Vellore, Tamil Nadu, India 2 – School of Bio Science & Technology, VIT University, Vellore, Tamil Nadu, India 3 – School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India 4 – Carbon Dioxide and Green Technologies Centre, VIT University, Vellore, Tamil Nadu, India a – jhansiranidvr@gmail.com DOI 10.2412/mmse.69.45.881 provided by Seo4U.link

Keywords: polymer solar cell, non-fullerene acceptor, spray coating, power conversion efficiency, air stability.

ABSTRACT. Achieving highly stable and reliable organic solar cells relies on the advancement of good performance and enthusiastically reasonable hole transporting buffer layers tuned in to the anode and the photoactive materials of the solar cell stack. We explore the photophysics of all polymer solar cells based on the blends of the low band gap polymers poly(3-hexylthiophene) (P3HT) as a donor and poly {[N,N-9-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) as an acceptor blend active layer in 2-methyl anisole with 2% 1,8 diiodooctane (DIO) using air brush spray coating method. Polyethyleneimine ethoxylated (PEIE) is used as a surface modifier and SnO2 was used as an anode to minimize chemical damage of the transparent conducting electrode. The fabricated films were characterized and the solar cell performance was evaluated. An efficiency of 5.6 % was achieved and the devices are highly stable, retaining 75% of its original efficiency after being stored in air even without encapsulation.

Introduction. Solar cells are one of the best candidates to overcome traditional energy depletion and environmental pollution. Especially, organic solar cells (OSCs) represent an exciting class of renewable energy technology; they are lightweight, flexible and have a low production cost with a scalable approach for solar energy conversion [1]. Over the last two decades, the efficiency of these devices has improved significantly, in particular through the development of solution-processed bulk heterojunction (BHJ) OSCs [2,3] based on interpenetrating networks of polymer donors and acceptors that exhibit power conversion efficiencies (PCEs) over 10% mostly with fullerene-based electron acceptors [4]. Very recently, however, highly efficient solution-processable non-fullerene acceptors have been discovered and their performance is more or less comparable to that of conventional fullerene-based acceptors. The low-band-gap polymers of P3HT and P(NDI2OD-T2) were used as an electron-rich donor and as an electron-deficient acceptor respectively. 2-Methyl anisole, a halogen free greener organic solvent was selected as they are the most attractive processing solvents providing enough solubility and favourable morphology to improve the performance of P3HT: P(NDI2OD-T2) solar cell device and their environmental accumulation can also be significantly mitigated. In this paper, we report the effect of processing conditions on the performance of P3HT: P(NDI2OD-T2) based cells, and the nano-scale morphology of active layers using spraycoating technique [5] were 2-methyl anisole was used as the solvent [6,7]. The parameters such as spraying time and substrate-nozzle distance were varied and the coated active layers of P3HT: P(NDI2OD-T2) were investigated.

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© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Experimental Materials used. P3HT (Sigma Aldrich, đ?&#x2018;&#x20AC;đ?&#x2018;&#x160; 87 kg molâ&#x2C6;&#x2019;1 ), regioregularity 98%, polydispersity < 2) and P(NDI2OD-T2) from polyera corporation . P3HT had a molecular weight (MW) of average Mn 54,000 â&#x20AC;&#x201C; 75,000, whereas P(NDI2OD-T2) had a MW of 96.6 kg molâ&#x2C6;&#x2019;1 and poly diversity index of about 4.0. 2-Methyl anisole purchased from sigma Aldrich was used as the solvent. The molecular structures of P3HT, P(NDI2OD-T2) are shown in the Fig. 1(a).

a)

b)

c)

Fig. 1. (a) molecular structure of P3HT and P(NDI2OD-T2), (b) Device Architecture, (c) relative energy level diagram.

Fabrication of OSCď&#x201A;˘s and device characterization. Photovoltaic devices used an ITO/PEIE/P3HT:P(NDI2OD-T2)/Al architecture, with Polyethyleneimine ethoxylated ( PEIE ) (5 nm) coated on indium tin oxide (ITO) glass substrates were used as the transparent substrate and aluminium as the top electrode. Commercially available pre-patterned 12 ď &#x2014;/h sheet resistance ITO substrates were cleaned with detergent, ultrasonicated in acetone and isopropyl alcohol for 15 min, and dried in an oven at 120 °C. UV-ozone treatment was then performed for 15 min and plasma etched prior to coating with a 5 nm layer of PEIE. Each PEIE solution was spin coated on the ITO substrate at 5000 rpm for 40s and then thermally annealed at 110 °C for 10 min. Fig. 1 (b) illustrate the configurations of OSCď&#x201A;˘s in the form of a sandwich structure of the photoactive polymeric layer between an anode electrode of indium tin oxide (ITO) and a metal cathode of aluminium (Al), which has a structure of ITO(180nm)/SnO2 (xnm)/PEIE(5nm)/P3HT:P(NDI2OD-T2) (120nm)/Al(100nm).

The relative energy level diagram is illustrated in the Fig. 1 (c). Polymer blends were spin-coated from P3HT:P (NDI2OD-T2) solutions of varying concentration of 1:1; 1:2, 1:3, and 1:4 using 2methyl anisole solvent. In each case, the initial solution was prepared in a glovebox with the measured P (NDI2OD-T2) and P3HT polymers allowed to dissolve in a hot solution for at least an hour. By introducing the nitrogen gas with the pressure of 8.5 x 10-4 Pa into the spray apparatus, the solution of P3HT: P (NDI2OD-T2) were spray cast onto the PEIE film to form the active layer with the thickness ranging from 120 â&#x20AC;&#x201C; 125 nm. The blend films were prepared with different time periods from 10 â&#x20AC;&#x201C; 40 s and the substrate-nozzle distances were varied from 10 to 30 cm. Once the active layer has been deposited, the MoO3 hole transporting layer (HTL) and 100 nm aluminium top electrode were deposited via thermal evaporation for a final thickness of ~5 nm and ~100nm, respectively.

Completed devices were annealed at 130° C for 10 minutes inside a glove box and then encapsulated with epoxy resin and soda-lime cap. SnO2 buffer layers with different thicknesses of 5â&#x20AC;&#x201C;15 nm were deposited onto ITO transparent anodes by RF magnetron sputtering. All absorption measurements were performed using a Cary 5000 UVâ&#x20AC;&#x201C;Visâ&#x20AC;&#x201C;NIR double-beam spectrophotometer in the two-beam transmission mode. Absorption spectra of P3HT, P(NDI2OD-T2), and P3HT:P(NDI2OD-T2) films were taken near the centre of solar cells lacking the top electrode. The surface morphology of the blend layers was examined by atomic force microscopy (AFM) using a Seiko Instruments SPA400SPI4000 operating under ambient condition. The current density (J)-voltage (V) characteristics were measured using a Keithley 2420 m in dark and under illumination of a sun 2000 solar simulator (Abet)

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

with 100 mw/cm2 AM 1.5 G spectrum. All measurements were performed under ambient atmosphere at room temperature in open air.

Results and Discussion. The absorption spectra of spray coated P3HT, P(NDI2OD-T2) and active layers of P3HT: P(NDI2OD-T2) thin films from 1:1 to 1:4 weight ratios in 2-methyl anisole with 2% 1,8 diiodooctane (DIO) are shown in Fig. 2 (a). For the absorption spectra of P3HT film fabricated from 2-methyl anisole, wavelength of the absorption peak (max) is at 526 nm. The pure P (NDI2ODT2) film has a broad near-IR absorption band extending from 550 nm to 850 nm and a -* absorption feature at 390 nm. The extinction coefficient (abs) were calculated by using Beer-Lambert Law from the absorption spectra of both films and solution of P3HT: P(NDI2OD-T2) For the films, the values of abs were 5320, 2710, 1860, 1564 and 1610 for P3HT: P(NDI2OD-T2) concentration of 1:0, 1:3, 1:2, 1:1, 1:4, respectively. However, for the solutions, the values of abs were 40, 44, 42, 45 and 47 for similar concentration of P3HT: P(NDI2OD-T2). The extinction coefficients of the solution were lower than that of the films by about two orders of magnitude. Hence, the inter-chain interaction among the P3HT chains results into more delocalized conjugated  electrons, the lowering of the bandgap between -* transition [8], [9].

To evaluate the role of the blend morphology, the topography of the thin films was investigated by atomic force microscopy (AFM). The tendency of the polymer–polymer blends to phase separate is generally described to low entropy of mixing and is governed by a spinodal decomposition of the blend. The properties of the spray solution not only affect the thickness optimization by a proper choice of nozzle-substrate distance, but also play an important role on morphology of the films. The topography and surface roughness of the films were investigated by atomic force microscopy (AFM).

AFM topographic images for P3HT: P (NDI2OD-T2) films with different blend ratios: 1:1(a); 1:2(b); 1:3(c); 1:4(d) are shown in the Fig. 2 (b). At lower P(NDI2OD-T2) loadings with blend ratio of 1:1, 1:2, the blends showed uneven and larger number of granular aggregations with a size distribution between 50-100 nm, which were uniformly dispersed in the P3HT matrix. On further increasing the acceptor material concentration to 1:3 ratio, the blend showed such high miscibility that the homogeneous films were obtained with smoother surfaces and this improved the device efficiency to a greater extent. For the ratio of 1:4, the surfaces of the blend films become increasingly uneven and large P(NDI2OD-T2) aggregations were observed, this results in lower absorption when compared to other blend ratio. The SnO2 film is composed of nano-sized particles with root mean square (RMS) roughness of ~3 nm. The PEIE film itself is very smooth with RMS roughness of 0.369 nm, which was determined independently by spray coating a relatively thick film on ITO (180 nm). However, for the ultra-thin layer of PEIE used in our device fabrication, the roughness of the PEIE film is predominantly influenced by the under layer. Hence, the RMS roughness of the PEIE coated SnO 2 film is similar to that of SnO2 (3.112 nm). The current density-voltage (J–V) characteristics of photovoltaic cells with various interfacial layers under AM 1.5G irradiation at 100 mW cm -2 were examined. The effect of the D/A weight ratio along with the influence of the solvent on the photovoltaic behaviour of the PSC is summarized in Table 1. The observed open circuit voltage is consistent with the HOMOD LUMOA difference expected from the energy level of P3HT and P (NDI2OD-T2). Indeed, according to the typical energy loss in P3HT-based cells (ca. 0.35 V), the maximum predictable open circuit voltage is about 0.69V, and it showed a short-circuit current density of about 12.4 mA cm-2 and a fill factor of 54.93%. In this respect, the P3HT/P (NDI2OD-T2) interface has been shown to be highly efficient for charge transfer and free carrier generation.

Specifically, for 20 cm 30 s when the substrate-to-nozzle distance was increased to 20 cm the layers with uniform thickness were observed and it showed maximum efficiency of about 5.6% for 1:3 blend ratio with Voc of 0.68, Jsc of 12.9 mA/cm2. And FF of 63.84 % and reduced at the ratio of 1:1, 1:2 and 1:4 for P3HT: P (NDI2OD-T2) blend because of its less uniformity and thickness and similarly for SnO2 concentration. When the thickness is around 20cm, it showed maximum efficiency of about 5.6% with Voc of 0.68, Jsc of 12.9mA/cm2, and FF of 63.84 % and reduced at the other thickness level as shown in the Table 2. It can be explained by the fact that, with decreasing P(NDI2OD-T2) loading, a large number of P(NDI2OD-T2) clusters with size above 100 nm dispersed in the film not only MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

reduce the P3HT: P (NDI2OD-T2) interface but also act as charge traps resulting in a strong deterioration of photovoltaic performance.

Fig. 2. (a) Absorption spectra for P3HT: P(NDI2OD-T2) blends, (b) AFM topographic images for P3HT: P(NDI2OD-T2) blend with different blend ratios: (a) 1:1, (b) 1:2 (c)1:3, (d)1:4 Table 1. Photovoltaic Performance of the BHJ Polymer Solar Cells Composed of ITO/PEIE/P3HT: P (ND12OD-T2)/MoO3/Al fabricated with different weight ratios under AM 1.5G illumination of 100 mWcm-2 Voc, (V)

Jsc, (mA cm-2)

FF, (%)

PCEavg, (%)

weight ratio

Active layer (nm)

1:1

130

0.68  0.1

9.2  0.3

43.16  0.2

2.7  0.1

1:2

127

0.67  0.3

10.8 0.2

48.37 0.3

3.5  0.1

1:3

123

0.69  0.2

12.4 0.3

54.93  0.2

4.7  0.2

1:4

125

0.67  0.1

10.1 0.2

44.33 0.3

3.0  0.3

P3HT:P(ND12OD-T2),

Summary. We have explored the photovoltaic properties of the P(NDI2ODT2) in the blend with the P3HT using 2-methyl anisole solvent with 2% 1,8 diiodooctane (DIO) by varying spray time in ambient atmosphere. High fill factor in all-polymer solar cells have been demonstrated for the first time with values of nearly 64%, suggesting a highly balanced mobility into the polymer-blend thin films. Thus, using high mobility electron transporting polymers such as P (NDI2OD-T2) enables FF values comparable with those reported for fullerene-based devices. Spectral and morphological investigation of P3HT: P (NDI2OD-T2) blends reveals that these low band gap polymers exhibited uniform surface morphology and thickness for 1:3 blend ratio with 5% SnO2 concentration and attained a maximum power conversion efficiency of about 5.6%. Thus the morphological properties and the device efficiency achieved indicates that the P3HT:P (NDI2OD-T2) system is a promising all-polymer system for further device optimization and for practical use of non-fullerene OSCs. However, several limiting factors still hinder to reach high efficiencies as for instance the photoactive blend morphology, thus further optimizations are necessary. The use of additive molecules may eventually lead to a better morphology and to an overall improvement of the device performance. In addition, the electronic structure of the blend could play an important role on the ultimate efficiency.

Adjusting the D and A, HOMO and LUMO levels by combining P(NDI2OD-T2) with new highperforming p-type polymers would allow us to minimize the energy loss due to the LUMO offset.

Acknowledgement. This study was supported by DST, New Delhi under Young Scientist Scheme (Grant No. YSS/2015/001104), CSIR New Delhi under Extramural Research (Grant No. 01(2865)/16/EMR-II) and VIT University under RGEMS Fund.

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