Chapter 6. Incorporation of Ferroelectric Additives Enhances Built-in Electric
6.1 Introduction
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Chapter 6. Incorporation of Ferroelectric Additives Enhances Built-in Electric
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separated by the internal electric field of the p–n junction and thus, collected by the respective electrodes.2 The strengthening of the internal (built-in) electric field by doping with ferroelectric materials is one of the innovative approach to quickly separate the charges prior to recombination for achieving efficient solar cells.
In particular, the discovery of ferroelectric phenomenon in organic polymers in early 1970s has electrified astounding research interests in these soft materials with appealing physical properties, giving rise to a plethora of applications ranging from transistors and sensors to to several other information technology devices,3 which soon captured rapid attention from the worldwide research communities leading to the discovery of a number of ferroelectric polymers beyond the PVDF homopolymer, such as PVDF-based grafted copolymers, terpolymers.4 Since then, as boosted by the ever-increasing demand for state-of-the-art energy technologies, recent attempts have been made to utilize the built-in electric field provided by the electric polarization of a ferroelectric polymer to improve the PCE in OSCs.5, 1d, 1e For example, Huang et al. incorporated a thin interfacial layer of poled ferroelectric poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) at electrode interface to generate an extra electric field in the active layer, demonstrating an increase in PCE of OSCs after polling the PVDF-TrFE layer by applying a large voltage pulse on the electrode.1d More recently, by mixing a small amount of PVDF-TrFE polymer into the active layer, Chaudhary et al. enhanced the charge collection efficiency and achieved a very high internal quantum efficiency of 100%, consequently leading to improved device performance.1e Nonetheless, the former method has limitation in applying to the many state-of-the-art low-bandgap polymers recently developed since the LangmuirBlodgett (LB) deposition used to fabricate the PVDF-TrFE monolayers is incompatible with their process due to the high-temperature annealing (over 130 °C) needed to convert the high-quality PVDF-TrFE LB film into the ferroelectric phase. For such polymers, the high-temperature annealing directly after casting can give rise to the micrometer-sized phase segregation, dramatically reducing donor- acceptor interfacial area and device performance. In the case of the latter approach, despite the improved PCE as a result of the a local build-in electric field within the unpoled PVDF-TrFE mixed active layer, the positively poled OSC did not show additional enhancement and even the negative poling adversely degraded the device performance. Thereby, the role of PVDF-TrFE and the working mechanisms behind ferroelectric induced electric fields in OSCs are still not completely understood. In addition, both the research works mostly focused on a traditional poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) system referred to as the ‘fruit fly’ platform of OSCs. As a consequence, despite the synergistic effects of electric fields induced by the electric polarization on photovoltaic properties, the achieved maxima PCEs were below 5%.
The large electronegativity difference between the carbon and fluorine atoms give rise to a highly polar C- F bond with a large dipole moment of >6.0 × 10−30 C m leading to the strong electric polarization of PVDF
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crystals generated from the regularly packing of polymer chains such that dipole internal moments are not cancelled out.5b, 5d, 6 The PVDF-based polymers are a semi-crystalline polymer with polymorphs referred to as , , , -phases (directly related to the ferroelectric properties), which are sensitive to external stimuli such as temperature, stress, electric field, strain, or chemical substances.5c, 6-7 Recently, it was reported that their crystal structures and phase transitions can be easy to tune via grafting one polymer onto PVDF-based polymers. Therefore, several studies have been devoted to increase the β crystal phase in PVDF either through copolymerization of vinylidenefluoride (VDF) with trifluoroethylene (TrFE), bromotrifluoroethylene (BTFE), chlorotrifluoroethylene (CTFE) etc. or grafting with low polarizable polymer side chains such as polystyrene (PS), poly(tert-butyl acrylate) (PtBA).5b, 5c, 7b, 8 Like other organic polymers, these materials have several benefits like light weight, ease of processing, and flexibility over their inorganic ferroelectric counterparts offering several opportunities in various electronic devices including organic solar cells, field effect transistors, triboelectric nanogenerator, capacitors, piezo-, and pyroelectric applications.1d, 5b, 8e, 9 Apart from the materials design, innovative device engineering and effective morphology control also play very important roles in increasing the power conversion efficiency (PCE) of OSCs. Currently OSCs have shown very promising PCE beyond 12% through fully optimizing the given binary/ternary system.10 In this study, to advance and forward our understanding of the ferroelectric photovoltaic mechanism, we systematically examine the effects of a family of PVDF-based ferroelectric polymers (PVDF, poly(vinylidene fluoride)-graft-poly(tert-butyl acrylate) (PVDF-g-PBA), PVDF-TrFE, and poly(vinylidene fluoride-trifluoroethylene)-graft-poly(tert-butyl acrylate) (PVDF-TrFE- g-PBA)) as an additive on photovoltaic property of an archetype of high-performance poly(4,8-bis(5-(2- ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-
fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)) (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) system. On the basis of high-performance OSCs based on o-xylene (OXY)/N- methylpyrrolidone (NMP) combination previously reported, the OXY/NMP pair was used as the processing solvent for making OSCs in this study. Note that the solubility of the ferroelectric polymers in NMP is satisfactory but poor in OXY. Thereby, OXY/NMP pair drives the recrystallization of the ferroelectric additives into the active layer matrix, not only yielding built-in electric fields but also a large enhancement of the PCE up to 11.02%. The ferroelectric polarization induced by the simple recrystallization tool requires no additional fabrication step, and is readily applicable to various organic optoelectronic devices. Till now external electric fields are applied (called poling) to polarize the PVDF based ferroelectric material prior to device operation, such that remnant polarizations are left behind after removing the applied field.1d, 9a, 9e, 11
However, these four ferroelectric insulating polymer additives having different dielectric constant display permanent remnant polarization in the host matrix without poling providing a unique approach to strengthen the built-in field.
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