INVESTIGATION AND CHARACTERIZATION OF ORGANIC SOLAR CELLS BASED ON LOW BAND GAP POLYMERS FOR OPTIMAL DEVICE EFFICIENCY

Abstract

The invention of solar cells remains the breakthrough towards clean energy alternatives since its discovery. Organic solar cells may be the replacement to inorganic ones, because of their excellent properties and solution processability leading to low financial and ecological cost, ultra-light weight, good efficiencies and improved stability. Conventional organic solar cells have been fabricated from a blend of active layers of a conjugated material (donor material) and a fullerene derivative (acceptor material) sandwiched between the hole transport layer (HTL) on an indium tin oxide (ITO) positive electrode and the electron transport layer (ETL) on a low work-function metal negative electrode. The HTL in a photovoltaic device plays a pivotal role in device performance. The aim of the thesis is to investigate the low band gap polymeric solar cells based on inexpensive and solution-processable HTL and optimization of device efficiency. The thesis is mainly directed towards: 1. Fabrication of organic solar cells based on low band gap polymer as core problem of the thesis using solution-processable CuSCN as hole transporting layer. 2. Development of low band gap polymeric solar cells using solution-processable CuI as hole transport material. 3. Optimization of composition ratio of P3HT:PC61BM in organic solar cells for optimal device efficiency. The present thesis deals with the fabrication of efficient organic solar cells using different buffer layers as HTL/ETL. Focus was on using inexpensive and solution ix

processable Cu(I) salt as HTL in organic solar cells. The optical property, physical property and morphology of the deposited HTLs were studied using UV-Vis-NIR spectroscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and atomic force microscope (AFM) characterizations. The current density voltage (J-V) characteristics and PCEs were measured with a computer controlled Keithley 2400 source meter. The present thesis consists of six chapters, which are briefly described below. Chapter 1 This chapter of the thesis is devoted towards the extensive literature survey on past and present research work done on organic solar cells including working mechanism, various geometry, buffer layers, both hole and electron transport layers (HTL and ETL) which are used in organic solar cells, are discussed keeping focus on various HTL. It summarizes the general review on various methods for the enhancement in the performance (both stability and efficiency) of organic solar cells by using different HTL, ETL and conducting polymers. Chapter 2 This chapter details the various characterization techniques performed to characterize the parameters of organic solar cells e.g. morphology, I-V measurement etc. The detailed process for the fabrication of solution processed optoelectronic devices such as solar cell is discussed. Equipments require for fabrication process like Glove Box, thermal evaporation systems and spin coating unit with characterization techniques including UV-vis-NIR, XRD, SEM, TEM and AFM etc. are discussed in brief. Chapter 3 describes the utilization of copper (I) thiocyanate (CuSCN) as an efficient and solution processable hole transport layer (HTL) in bulk heterojunction solar cells. The work has been discussed in two subsequent sections. x

Chapter 3A In this section three different combinations of the most studied active layers of P3HT:PC61BM, PCDTBT:PC71BM and PTB7:PC71BM were used for photovoltaic device fabrication with the simplest device structure of ITO/CuSCN/active layer/Al. The use of CuSCN as an HTL has improved light absorption within the active layer and thereby leads to up to 5.94% and 4.60% power conversion efficiencies (PCEs) for these active layers respectively. These results are slightly better when compared to the devices fabricated using thermal deposition of MoO3 and solution processed deposition of PEDOT:PSS as an HTL under similar conditions. Chapter 3B During past few years, significant research on solution processable deposition of copper(I)thiocyanate (CuSCN) as an efficient hole transporting layer (HTL) for excitonic solar cells have been successfully reported. Surprisingly, till now only two solvents diisopropyl sulfide and diethyl sulfide are known which have been used for CuSCN film deposition as a HTL for device fabrication. It is also noticeable that both the solvents are an irritant solvent having very foul smell. In this section we have used eco-friendly and inexpensive solvent dimethyl sulfoxide (DMSO) for solution processed thin film deposition of CuSCN for organic solar cells. The photovoltaic devices were fabricated using two different donor polymers PCDTBT and PTB7 blended with PC71BM as an acceptor material with device structure of ITO/CuSCN/active layer/Al. The power conversion efficiency (PCE) based on CuSCN using DMSO as a deposition solvent have been achieved up to 4.20% and 3.64% respectively, with relative higher fill factor (FF) as compared to previously reported values in literature. In parallel with the above work, investigations were also directed towards development of alternative and universal solvents for copper (I) thiocyanate for xi

fabrication of low band gap polymeric solar cells to boost the utilization of CuSCN. In this connection, we used five different alternative solvents compactable with CuSCN for fabrication of organic solar cells: N,N-dimethylformamide, dioxane, acetonitrile, ethylene glycol, propylene carbonate. Chapter 4 In this chapter, we have shown the performance of solution processable copper iodide (CuI) as an alternative hole transporting layer (HTL) for polymeric solar cells. Optical spectra of the CuI thin film reveal highly transparent and practically no absorption in the range vis-NIR region (450-1110 nm). X-ray diffraction (XRD) patterns of CuI exhibits a p-type semiconductor as well as crystalline nature. The power conversion efficiencies (PCEs) based on CuI as an HTL have been achieved to up to 3.04% and 4.48% for PCDTBT and PTB7 based donor materials blended with PC71BM as an acceptor material respectively with a configuration based on ITO/CuI(40 nm)/active layer (60 nm)/Al (120 nm). Furthermore, we use a wide range of solvents for solution-processed deposition of copper iodide (CuI) thin films as hole transport layer for efficient polymeric solar cells in general. Three different solvents (dimethyl sulfoxide (DMSO), N,N dimethylformamide (DMF) and diisopropyl sulfide) are used for solution-processable HTL for low band gap solar cells. To examine the feasibility of these deposited solvents for HTL, we used two different combination of active layers based on low band gap polymers (PCDTBT: PC71BM and PTB7:PC71BM) for fabrication of solar cells with a device configuration based on ITO/CuI(40 nm)/active layer (60 nm)/Al (120 nm). In Chapter 5 the effects of different composition ratios of P3HT:PC61BM in active layer on photovoltaic parameters were systematically studied in ambient conditions. The P3HT:PC61BM composition ratios range from 1.0:0.4 to 1.0:1.2 in xii

active layer shows relatively good PCE and further decrease or increase of P3HT:PC61BM ratio the resulted devices show very poor PCE. The devices with various composition ratios clearly demonstrated that 1.0:0.8 weight ratio of P3HT:PC61BM has achieved highest power conversion efficiency. Chapter 6 This chapter presents the major conclusions derived from the present work and the scope of the future study in this field has been suggested