Morphological Investigation of Small Molecule Solution Processed Polymer Solar Cells Based

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

Morphological Investigation of Small Molecule Solution Processed Polymer Solar Cells Based on Spin Coating Technique1 Liyakath Reshma 1, Kannappan Santhakumar 2,a 1 – School of Electronics Engineering, VIT University, Vellore, Tamil Nadu, India 2 – Carbon Dioxide and Green Technologies Centre, VIT University, Vellore, Tamil Nadu, India a – ksanthakumar@vit.ac.in DOI 10.2412/mmse.42.77.422 provided by Seo4U.link

Keywords: polymer solar cell, small molecule, bulk heterojunction, spin coating, power conversion efficiency.

ABSTRACT. Organic solar cells are one of the best candidates to overcome the traditional energy depletion and energy pollution, because they use simple processing techniques to fabricate and they are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost effective devices. Here we explore an efficient solution-processed polymer bulk heterojunction solar cells based on the combination of a small molecular donor ((DTS(PTTh2)2) and an acceptor (PC71BM ) by using chlorobenzene as a solvent in order to obtain the mixing morphology through spin coating. PEDOT: PSS was used as a surface modifier to reduce the work function of the conductors. The molecular aggregations in chlorobenzene solvent were investigated by means of UV–visible spectra and photoluminescence measurements. The surface morphology of the active layers deposited was examined using atomic force microscopy. The current density–voltage (J–V) characteristics of the photovoltaic cells were measured under the illumination by using Oriel 1000W solar simulator and the maximum power conversion efficiency has been reported for this polymer combination. These results indicate that the spin coating technique can be a viable alternative to the highcost and vacuum-deposited ITO for mass production and low cost roll-to-roll based solar cells.

Introduction. The world’s demand for usable energy increases every year, with an expected increase from 479 trillion joule (505 quadrillion Btu) in 2008 to 730 trillion joule (770 quadrillion Btu) in 2035, an increase of 52 %. In order to meet this demand, non-renewable fossil fuels, mostly coal, and renewable sources of useful energy will need to be deployed. As fossil fuels will eventually run out and their use seems to be as the main contributor to the increase of the global greenhouse effect, more research done on the development and deployment of alternative technologies for renewable energy production. Sunlight is an abundant and virtually eternally renewable energy source, with 174 petawatt of power arriving at the earth’s atmosphere and about 89 petawatt is absorbed by land and water. Even using only a fraction of this enormous amount of power may significantly meet the world’s growing demand for power. Solar cells, which rely on the photovoltaic effect, transform sunlight into electricity and in order to successfully utilize solar power, developing well performing and cost-effective photovoltaic devices is paramount. Solar cells can be categorized into two different kinds, inorganic and organic ones. The former having current commercial power efficiency between 15 and 20 %, up to 25 % for more refined silicon cells and top lab-scale efficiencies of more than 40 % reached with lab-scale multi-junction devices consisting of various inorganic semiconductors and the usage of light concentration techniques. However, the performance of organic solar cells (or Organic Photovoltaic’s OPV's) is considerably lower with a commercial efficiency of about 3 to 5 % and a current top efficiency of 1 0 . 6 %. Organic electrondonor/electron-acceptor blends are a key ingredient of “plastic” photovoltaic devices, whose development raises an ever-increasing scientific interest due to their low cost, easy production process and mechanical flexibility. Thus, OPV research has taken a new direction in exploring the uses of 1

© 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

different materials [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% [4] mostly with fullerene-based electron acceptors. Here, we report a solution-processed small-molecule donor: 5,5’ - bisf (4-(7-hexylthiophen – 2 - yl) thiophen – 2 – yl ) - [1, 2, 5] thiadiazolo [3, 4-c] pyridineg - 3, 3’- di - 2-ethylhexylsilylene - 2,2’ bithiophene, DTS (PTTh2) 2. This small molecule donor exhibits excellent solubility in organic solvents, strong optical absorption, especially from 600 to 800 nm, and a field-effect hole mobility of ~0.1 cm2 V-1 s-1. The design of DTS (PTTh2) 2 incorporates the [1,2,5] thiadiazolo [3, 4c]pyridine(PT) unit as a strategic building block. The PT heterocycle has a high electron affinity. .Furthermore, the asymmetric nature of PT allows for mono functionalization, resulting in facile and high yielding molecular synthesis. In this we investigated the morphologies of active layers deposited by using a spin coating process instead of spray coating techniques [5] for better morphology and we studied the effects of the interfacial contact on the device performance and device physics [6] in small molecule BHJ solar cells based on (DTS (PTTh2) 2) as the electron donor and [6, 6] -phenyl C71butyric acid methyl ester (PC71BM) as the acceptor. PEDOT: PSS was utilized as the Hole Transport Layer [7], [8] because it has an appropriate work function and sufficiently high electrical conductivity, to be a suitable candidate for a hole transporting material. Chlorobenzene was used as the solvent, as they are the most attractive processing solvents providing enough solubility and favourable morphology to improve the performance of the solar cell device and their environmental accumulation can also be significantly mitigated and ITO was used as the electrode. In this paper, we report the effect of processing conditions on the performance of DTS(PTTh2)2: PC71BM based cells, and the nano-scale morphology of active layers using spin coating technique were Chlorobenzene (CB) was used as the solvent.

Experimental.

Materials used. DTS(PTTh2)2 and PC71BM were purchased from 1-material. These chemicals act as an electron donor (D) and acceptor (A) materials, respectively. Chlorobenzene (CB) was purged with nitrogen to remove residual oxygen prior to use. PEDOT:PSS was purchased from HC Strak (Newtown, MA Bayer AG) and passed through a 0.20 mm filter before spin-coating.

Fabrication of OSCs. Pre-patterned indium tin oxide (ITO)-coated glass with a sheet resistance of 12 Ω/square 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. A film of PEDOT: PSS was spin cast (3000 rpm for 30 s) on top of the ITO substrates and was dried for 15 min at 140°C. The active layer solutions were prepared by mixing the polymer and PC71BM in different blend ratios of 1:1; 1:1.5, 1:2 and 1:3 using CB solution mixed with Dichlorobenzene (DCB) and spin coated on the top of buffer layer with a speed of 800 rpm for 60 s. Then the samples were transferred into a vacuum chamber to deposit Al (100 nm) on top of the active layers under a pressure of 2.0×10-6 Torr.

The active surface area of the device was 0.12 cm2.The design of the solar cell device was 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 aluminum (Al). The relative energy level diagram and device construction of ITO/PEDOT:PSS/ DTS(PTTh2)2:PC71BM/Al were illustrated in Fig. 1. The thickness of all the active layers thicknesses was well controlled in the range of 120 -125 nm as measured by Dektak II profilometer. The active surface area of the device was 12mm2.

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

Fig. 1. Relative energy level diagram and Device Architecture.

Device characterization. All absorption measurements were performed using a Cary 5000 UV–Vis– NIR double-beam spectrophotometer in the two-beam transmission mode. Absorption spectra of DTS(PTTh2)2 , PC71BM, and DTS(PTTh2)2:PC71BM films were taken near the center of solar cells lacking the top electrode. PL spectra were measured for DTS(PTTh2)2, PC71BM, and DTS(PTTh2)2:PC71BM films, which were spin coated onto quartz substrates, using a calibrated fluorescence spectrophotometer (FP-6500,JASCO). The surface morphology of the blend layers was examined by atomic force microscopy (AFM) using a Seiko Instruments SPA400-SPI4000 operating under ambient condition. All AFM images were taken in the dynamic force mode at an optimal force.

Silicon cantilevers (Tip radius: *10 nm; SI-DF20; Seiko Instruments Inc.), with a spring constant of 14 N/m and a resonance frequency of 136 kHz, were used to record AFM images. 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) with 100 mw/cm2 AM 1.5 G spectrum. The EQE measurement was performed using a Jobin-Yvon Triax spectrometer, a Jobin-Yvon xenon light source, a Merlin lock-in amplifier, a calibrated Si UV detector, and an SR570 low noise current amplifier. The intensity of the solar simulator was calibrated by standard Si photovoltaic cell. All measurements were performed under ambient atmosphere at room temperature in open air.

3. Results and discussion.The absorption spectra of the spin coated DTS(PTTh2)2, PC71BM and active layers of DTS(PTTh2)2 : PC71BM thin films from 1:1 to 1:4 weight ratios in chlorobenzene were analyzed. The polymer shows strong absorption from 450 to750 nm. However the absorption from 300 to 400 nm is relatively weak. To compensate the absorption of DTS(PTTh2)2 , PC71BM, which has strong absorption in the visible range, is used as the acceptor. The absorption spectra of blend film prepared in CB: DCB exhibited a stronger absorption extended from 300 to 800 nm than that of the film prepared in CB solution. This indicates that DCB added into CB modify the absorption of blend layers and can harvest solar photons more effectively than the film prepared from CB under the same conditions. Two broad absorption peaks at around 624 and 682 nm are attributed to the *transition [9] of the DTS(PTTh2)2 polymer whereas the broad absorption coverage in the range from 320 to 500 nm is due to absorption of PC71BM. To obtain a deeper insight in the relation between the morphology and performance of polymer/fullerene bulk heterojunction solar cells, devices have been characterized via AFM. Altering the blend ratio is one of the common methods of controlling the morphology of the active layer during device fabrication, and further influencing the device performance. The AFM topographic images of the DTS(PTTh2)2: PC71BM are shown in Fig. 2. At lower DTS(PTTh2)2 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 DTS(PTTh2)2 matrix. On further increasing the acceptor material concentration to MMSE Journal. Open Access www.mmse.xyz

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

1:3 and 1:4 ratio, the blend showed such high miscibility that the homogeneous films were obtained with smoother surfaces and for more deeper insights, the devices were characterized via SEM, the blend ratio of 1:4 showed uniform surface morphology of the blended region as shown in Fig.3. 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 observed open circuit voltage is consistent with the HOMOD LUMOA difference expected from the energy level of DTS(PTTh2)2 and PC71BM. Indeed, according to the typical energy loss in DTS(PTTh2)2-based cells (ca. 0.35 V), the maximum predictable open circuit voltage is about 0.80V, and it showed a short-circuit current density of about 12.5 mA cm-2 and a fill factor of 45.20% with a power conversion efficiency of about 4.56%.. In this respect, the DTS(PTTh2)2/PC71BM interface has been shown to be highly efficient for charge transfer and free carrier generation.

Fig. 2. AFM topographic images for DTS(PTTh2)2:PC71BM blend with different blend ratios: (a) 1:1; (b) 1:2 (c) 1:3; (d) 1:4.

a)

b)

Fig. 3. SEM images for DTS(PTTh2) 2:PC71 BM blend a)1:3, b) 1:4.

Summary. BHJ solar cells were fabricated using DTS(PTTh2)2:PC71BM layers prepared in CB with Dichlorobenzene solvents in an ambient atmosphere. The results clearly indicate that the device performance is strongly influenced by the solvent additive from which the active layer was prepared.

This result shows that a strong relationship between the device performance and the active layer morphology that allows efficient exciton dissociation and charge transport pathways in an optimized polymer and fullerene interface. The choice of the main solvents is equally important in producing such structure. The addition of co-solvents leads to smoother films, less heterogeneous surface features, particularly in the distribution of polymer and fullerene phases, and much improved PCE values in the resulting solar cells. The efficiency of bulk heterojunction solar cells is rapidly MMSE Journal. Open Access www.mmse.xyz

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

increasing in the current years, so an accurate optimization of each layer and interface composing the device is strictly required in order to avoid loss mechanism and to achieve higher and higher performances. We anticipate that the device performances could be further improved by varying other parameters such as solvents and substrate temperatures.

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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