Yogesh Kumar Dongre^a*, Sanjay Tiwari^b*,

Photonics Research Laboratory (PRL), S.O.S in Electronics and Photonics ,Pt. Ravishankar Shukla University, Raipur, C.G.

*Corresponding author:yogielectro@gmail.com

[Received: 28October 2021; Revised: 12 March 2022; Accepted:13 March 2022]

Abstract. Inverted Bulk heterojunctions (Ag/MoO3/PCDTBT-PC70BM/ZnO/ITO) Organic Solar cells, based on Organic (Polymer) materials is fabricated and characterized in this work. PCDTBT-PC70BM was synthesized by chloroform, chlorobenzene and o-dichlorobenzene (organic solvent). Surface morphology of ZnO and PCDTBT-PC70BM were studied. Bulk heterojunctions of active material are formed by the mixture of PCDTBT donor and PC70BM an acceptor in a random manner. For Sufficient transportation of charge carrier (electron and hole), hole transport (HT) and electron transport (ET) layers was deposited. ZnO is used as an ETM and synthesized by using Sol-Gel technique. MoO3 thin film deposited over the active material, enhances hole transformation because of band gap tuning with Ag and active materials. Absorbance and Photoluminescence spectra of polymer material with different organic solvents were studied and results were discussed in this work. o-dichlorobenzene enhance the absorption of PCDTBT/PC70BM. At 400 nm, 90% of sun light is absorbed, and 70% absorption is figure out in 500- 550nm wavelength. The Photo-luminescence of PCDTBT/PC70BM thin film in different organic solvents is ranging from 650nm to 750nm. At 700nm, 20% is shown for chloroform, 40% for chlorobenzene and highest 80% is achieved by o-dichlorobenzene. J-V value is obtained from a solar simulator which irradiates the sun spectrum 1.5 AM, for all the devices having cell area 0.045 cm2. For concentration (1:1) ratio in different organic solvents like chloroform, chlorobenzene and o-dichlorobenzene, (3.5, 4.2, and 5.8) %, PCE were obtained respectively.

Keywords: BHJ, ETM, HTM, Sol-Gel Techniques, PCE.

Introduction

Solar energy is a one of the best promising alternative energy sources for today’s energy. Solar cell is a device, working on photovoltaic effect convert sun energy into electricity. The electricity generated from solar cells is green, clean and sustainable. Now a day 90% electricity is produced by thermal power plant where fossil fuels including Coal, Oil and Petroleum gases are used. At the time of production of electricity, they burn and produces harmful gases like Carbon dioxide (CO₂), acids of Sulphur (S), Carbon (C) and Nitrogen (N), the byproduct of fossil fuels is directly affected to our Environment and Human Being [Frederica Perera, 2018, ]. Another issues related to fossil fuels are its availability; it is finite to be finished within some year and unbalanced distribution inthe earth [IEA World energy outlook, 2017)]. Requirement of green, clean and sustainable energy researcher look around the solar cell. Solar cells are classified by the active material which converts light into electricity,  we have 1^st to 4^th generation’s solar cells, 1^st generation’s cells are mainly based on Silicon Technology, the highest reorted cell efficiency (single crystal cell) of ~ 28%. With the advancement of Si technology single crystalline silicon (c-Si), multi-crystalline silicon and amorphous silicon (a-Si) were used [Kiran Ranabhat, et al. (2016), Martin A. Green, et al. (2014),]. In Second generation solar cell, hydrogenated amorphous silicon (a-Si:H), cadmium telluride (CdTe) as well as cpper indium diselenide, (CuInSe₂) and its related alloys like Copper Indium Gallium diselenide, CuInxGa₁-xSe₂ (CIGS) were used in place of Si to reduce thecost of Si solar cell, with reportedefficiencies of CIGS and CdTe 20% and 17%, respectively [R. G. Nrel, (2010), Bagher, A, M; et al. (2015)]. Because of the high processing cost of Silicon (Si) the researcher worked for alternatives in Silicon (Si), and the thin film solar technology was invented. Organic/polymer and dye-sensitized solar cells were fabricated in 3^rd generation. Organic photovoltaic solar cell (OPV) technology employs semiconducting polymers as low-cost materials, ease of fabrication and tuneable bandgap a suitable alternative to inorganic semiconductors (silicon, CdTe and CIGS). The efficiencies for dye-sensitized and polymer single cells using chlorinated acceptor are relatively ~ 14% and 16% reported [Sanjay Tiwari et al. (2017), Sanjay Tiwari et al. (2017)]. Combination of Organic and Inorganic materials, represent 4^th generation’s solar cells, which is basically dealing with perovskite materials, perovskite was discovered in 2006 by Miyasaka and his co-workers [Kojima, et al. (2006),  Kojima, et al. (2009)]. This new material has a highest reported efficiency of ~19.3% [Zho, et al. (2014)]. The limiting factor of this technology is un-stability in environment and hazardous because of Pb.

Materials and Methods

Materials and Device Structure:

Bulk Heterojunction (BHJ) concept with Inverted Architecture: Low work function metals (easily transport generated charge carrier) like calcium (Ca), barium (Ba) or aluminum (Al) were applied as cathodes or in between cathodes and active material [Kai Wang, et al. (2016)]. Whenever exposure of oxygen or water from the environment is subjected to electrode caused an almost immediate oxidation of these electrodes, resulting in a fast degradation of the power conversion efficiency of the device. By introducing different architecture so-called inverted design,this fast electrode degradation could be overcome [Shaheen, S. E; et al. (2001), Green, M. A. et al. (2011)]. The low work function metals have been replaced by transparent oxides like zinc oxide or titanium dioxide. In inverted device design all layers can be deposited from solution and no vacuum process is required [Green, M. A. et al. (2011), Hou, J. et al. (2008)]. In bulk heterojunction concept we introduce a mixture of donor usually conjugated polymers, oligomers or conjugated pigments, and for acceptor fullerene derivatives were used in a specific molecular ratio (Weight) and deposited to form a blend over the surface of the ETL. A nanometers domain sizes blend film weredeposited because of nano meter size effects blend allowing for excitons with short lifetimes to reach an interface and dissociate due to the large donor-acceptor interfacial area [Waldauf, C. et al. (2006)]. The molecular arrangement (Hydrocarbon chain) of PCDTBT donor material is shown in figure 1(a).

Figure 1.(a) Molecular arrangement (Hydrocarbon chain) of PCDTBT donor material

The process of photon absorption and exciton generation takes place on a junction, the donor materials is responsible for that. Once the charge carriers (electron and holes) are generated, it is easily transported to their respective layer like (ETL and HTL) by acceptor blend. The molecular arrangement (Hydrocarbon chain) of PC₇₀BM (Fullerene derivatives) acceptor material is shown in figure 1(b).

Because of blend morphology of the active layer, it is easy to harvest all the incident photon and transport the charge into electrode [Krebs, F.C; (2008), Zhou, Y; et al. (2012)]. Here we are combine this Bulk heterojunctions concepts with novel inverted architecture to increases the stability and enhancement of PCE of organic solar cell. Different device geometries of organic solar cell are described in figure 2.

Figure 3.(a) Non-inverted Architecture                                Figure 3.(b) Inverted Architecture      

 BHJ device is shown in figure 3(a & b) both having non-inverted and inverted architecture. The devices were fabricated, consists of layers of different materials, such as for bottom electrode a transparent oxide of indium tin (ITO), an electron transport layer (ETL), ZnO were deposited. As the active layer the donor polymer/acceptor molecule blend (mixture of PCDTBT-PC₇₀BM), for hole transport layer MoO₃ and Ag for top electrode were used.

Band gap of materials used in device:

ITO (Indium Tin Oxide) is used as a bottom electrode of the device. The work function of the ITO is -4.7, which is less as compare to ETL material work function because of work function suitability electron can beeasily move from ETL to electrodes. In our work inorganic buffer layers of ZnO, work function -4.71 to -5.1, betweenthe active layers and the electrode have been deposited using spin coater, which act as electron-transport layer or hole blocking buffer layers. PCDTBT (conjugated polymer) and PC₇₀BM (Fullerene derivatives) are used as an active material, which absorb light and generate charge carriers [White, M. S; et al. (2006)].The work function of PCDTBT and PC₇₀BM are (-3.6 to -5.5), (-4.3 to -8.0) respectively. To enhance the hole transportation to the top electrode here a very high work function material (-6.7 to -9.71), MoO₃ is deposited. The role of the MoO₃ in this device is, its block the transportation of electron from active layer to top electrode and it’s provided a better surface and good adhesion for Ag. Because of the high electrical conductivity Ag (-4.3) is used as a top electrode of the device. Energy band gap of different materials are shown in figure 4.

 Figure 4. Energy band gaps of different materials used in device.

 In the fabrication of BHJ organic solar cells, varieties of materials were used. In below table 3(a) we have given the details of materials and chemicals. In this work ZnO is synthesized by Sol-Gel techniques and active material is synthesized using different organic solvent with different concentration ratio.

 Table 1. Shows the Material/Chemicals used for fabrication of BHJ organic solar cell.

 Experimental

Device Fabrication and Material Synthetization

Patterning, Cutting and Cleaning of ITO Coated Glass

ITO Coated Glass (L 50mm x W 50mm x T 1.1mm) having resistivity of 10 Ω/sq. from TechInstro is used as a substrate, ITO Coated Glass were patterned by a mixture of Zinc powder from Sigma-Aldrich and DI water. Here Kepton Tape was used for selective masking of ITO. Tape were fixe within line without any air gap, and also the Zinc powder solution was applied in unmasked region of substrate. Due to Chemical reaction ITO were etched from unmasked region and proceed for cutting and cleaning. The cutting of patterned substrate were done byDiamond based cutter system with measurement unit, here 16 samples each having dimensions (L 12 mm x W 8 mm x T 1.1 mm) are made from single substrate, so for device fabrication, we have 16 patterned substrate. The patterned Substrate was cleaned by using DI Water, Soap solution, Acetone and finally IPA followed one by one each for 10 Minutes in Ultra-sonicator, this cleaning process makes chemically stable and contamination free epitaxial surfaces for the subsequent fabrication process. After solution based cleaning, substrate were proceeded for Plasma Ashing, where Oxygen plasma (~ 10 minutes) is used to remove organic materials impurity and making the surface more pure. Table 3(b) shows the processing of substrate using pictures of each process.

 Synthesization and deposition of ZnO as an ETL

Theory

Here an electron transport layer (ETL) is deposited in between ITO and active materials, the purpose of an electron transport layer (or of a hole blocking layer) between the active layer and the cathode is to reduce the recombination of the free charge carriers (electrons and holes) and transport electron easily to cathode. Zinc oxide (ZnO) is used as an ETL materials because of work function suitability with both active materials and cathode interlayer and also better surface compatibility with both the active layer and the cathode, this causes ZnO leading to less surface defects and more efficient electron transport towards the cathode to more efficient electron extraction [Radzimska, A. K; et al. (2014)].  Apart from this the conductive nature, transparent, and ease fabrication using spin coater as thin films (nm regime) and generally insoluble in common organic solvents used for polymer active layer is made ZnO has suitable candidate for ETL, here ZnO were synthesized using sol-gel method.

 Sol-Gel synthetization of ZnO (Zinc Oxide) and deposition using LAF spin coater

ETL (Electron Transport Layer) of ZnO (Zinc Oxide) in paste form has synthesized using Sol-Gel Techniques. Zinc Acetate (Dehydrate) Zn(CH₃COO)_2. 2H₂O (ZAD, 0.5 M) (98%), Ethanolamine NH₂CH₂CH₂OH (MEA, 0.5 M) (98%) and 2 Methoxy ethanol anhydrous C₃H₈O₂ (2-ME) (99.8%), all the materials were supplied from Sigma-Aldrich. Materials were mixed according to their molecular weight. In this chemical reactions, a base material Ethanolamine (MEA) act as a stabilizer, and initiate the growth of ZnO, by increasing the PH value of chemical mixture[Radzimska, A. K; et al. (2014)]. Solvent 2-ME was taken in a glass bottle, and then Zinc Acetate precursor solutions were mixed followed by Ethanolamine in appropriate amount and after steering of 1 hour we found a transparent ZnO solution. Further prepared ZnO solution was applied to cleaned ITO coated glass for thin film coating, here laminar air flow (LAF) Programmable Spin coater was used for 60 seconds at 2000 rpm. After deposition of ZnO (~ 15 nm, thin), samples were proceeding for annealing at 200^0C for 10 minutes at hot plate. Process of preparation and deposition of ZnO is figure out in a table 3(c).

Organic active material (PCDTBT/PC₇₀BM) preparation with organic solvent and deposition

 PCDTBT conjugated polymers and PC₇₀BM fullerene derivatives are used as an active layer material. The precursor mixture solutions of the PCDTBT and PC₇₀BM were prepared with organic solvent (3 wt%) chloroform, dichlorobenzene and o-dichlorobenzene with a ratio of 1:1:1 respectively, Organic chemicals were supplied from Sigma-Aldrich. The weight measurement has done in Weight measurement machine and for liquid we have used a syringe, all the materials were mixed according to their molecular weight and kept for 12 hours steering for proper mixing at 60⁰Ctemperature.After that it has spin coated on the ZnO coated substrate at 1000 rpm for 1 minute and annealed at 130°C for 10 minutes, the thickness of deposit PCDTBT/PC₇₀BM is around 120 nm, all the procedures were done in Glove Box. Synthetization of PCDTBT/PC₇₀BM with their appropriate organic solvent (o-dichlorobenzene, dichlorobenzene and Chloroform) and deposition over the surface of ZnO.

Deposition of Hole Transport (HT) material and Top Electrode Silver (Ag)

 MoO₃ thin film (approximately 12 nm) were deposited over the active material using Thermal Evaporator at deposition rate 0.1Å/Sec, here Steel mask is used for selective evaporation. High transmittance in the visible region, excellent ambient stability and very high work function (5.3, 5.68, and 6.86 eV) tuning with PCDTBT/PC₇₀BM we were used MoO₃ for HTL material.After deposition of HTL, we have deposited Silver (Ag) electrode (100 nm thin) using the same mask at 0.5 Å/sec deposition rate.

 Figure 7. (a) Pictures of Steel mask for one (1) sample containing eight (8) devices and (b) Deposited thin film of Ag over MoO₃, over the PCDTBT and PC₇₀BM.

Legging and Encapsulation of the device

One of the major challenges associated with the micro devices is there electrical interconnects, in this work one sample containing eight devices, and contacts are associated in bottom and top only. Aluminium (Al) legs were used to connect both the top (Ag) and bottom (ITO) contact. Proper matching of Al legs to ITO and Ag is required otherwise devices are short or no connection. Once the legging done epoxy (Glue) is used to stick the device with transparent glass for encapsulation of devices.

Optical characterization

 In this work Zeta 3D Microscope was used to study the deposited surface of ZnO and Active materials, because for fabrication and better performance of the devices, we require smooth and uniform deposited surface. Here ZnO surface is same for the entire different PCDTBT/PC₇₀BM layer. From the microscopic study ZnO is deposited very uniformly over the ITO, but the variation is observed in PCDTBT/PC₇₀BM layer, due to variation in concentration.

 PCDTBT/PC₇₀BM Absorption Curve with Different Organic Solvents

The absorbance of PCDTBT/PC₇₀BM thin film deposited in different organic solvents are shown in below figure 10 (a), it shows o-dichlorobenzene enhance the absorption of PCDTBT/PC₇₀BM. At 400 nm, 90% of sun light is absorbed, and 70% absorption is figure out in 500- 550nm wavelength. For chloroform and chlorobenzene absorption wavelength are same but the absorption coefficient is low respectively (80, 70) at 400nm and (60, 50) at 500- 550nm.

PCDTBT/PC₇₀BM Photoluminescence curve with different organic solvents

 The luminescences of PCDTBT/PC₇₀BM thin film in different organic solvents are ranging from 650nm to 750nm. At 700nm, 20% is shown for chloroform, 40% for chlorobenzene and highest 80% is achieved by o-dichlorobenzene.

ElectricalCharacterization

J-V Characterization of Devices

 J-V value is obtained from a solar simulator which irradiates the sun spectrum 1.5 AM, for all the devices having cell area 0.045 cm². Both the value of Voc and Jsc tends to increase the fill factor of the devices which leads to increase the overall power conversion efficiency of the devices. For concentration (1:1) ratio in different organic solvents like chloroform, chlorobenzene and o-dichlorobenzene, (3.5, 4.2, and 5.8) %, PCE were obtained respectively. The device performance data is calculated from J-V curve obtained by Solar Simulator. The optimal thickness of ZnO is 15 nm and MoO₃ 12 nm. This deviceshows a maximum PCE 5.8%, with a short circuit current Jsc 8.9mA cm^-2, an open-circuit voltage Voc = 0.86 V, and a fill factor FF = 0.57.

Internal quantum efficiency (IQE) and external quantum efficiency (EQE) of PCDTBT/PC₇₀BM

 The Internal and External quantum efficiency of PCDTBT/PC₇₀BM polymer solar cell is calculated from the solar simulator data (J-V curve), absorption and photoluminescence spectra of PCDTBT/PC₇₀BM material and shown in figure 12 (a&b).

Results and Conclusion

 Here, we have successfully fabricated and characterized an inverted PCDTBT/PC₇₀BM organic solar cell device in order to understand the effects of different organic solvents. Results suggest that the power conversion efficiency is better with o-dichlorobenzene (around 5.8%) as compared to chlorobenzene and chloroform. Here HTL and ETL materials (MoO₃ and ZnO) were used to enhance the charge transportation of the device. Incident photon absorption (photon trapping) is increased by using Inverted architecture. Apart from this cleaning and softly handling of the process is also required for efficiency enhancement.

 Acknowledgment

 The author would like to thank NCPRE-PUMP (INUP), IIT Bombay for their support (Materials and Instruments) and research platform provided to me. The Fabrication work is done in Nano-Electronics lab IIT Bombay and PRL, Raipur. I am very much, thank full to my supervisor Prof. Sanjay Tiwari to provide me an opportunity to work at IIT Bombay and Photonics Research Laboratory, Raipur, and also their moral and technical support for completion of work.

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