Synthesis and Application of Ferroelectric P(VDF‐TrFE) Nanoparticles in Organic Photovoltaic Devices for High Efficiency

Abstract

The power conversion effi ciency (PCE) of organic photovoltaic devices has entered the over 10% era. [ 1,2 ] Interlayers between the organic active layer and the electrodes play important roles in achieving high effi ciencies. Many types of interlayers have been reported, such as metal oxides, [3][4][5] polymer electrolytes, [ 6,7 ] and low-work-function metals. [ 8 ] Generally, these interlayers have the function of bridging the Fermi energy of the electrode and the energy levels of the organic semiconductor and increasing the built-in electric fi eld in the devices. [ 6 ] Recently, we reported a new category of air stable material, ferroelectric polymer poly(vinylidenefl uoride-trifl uoroethylene) (P(VDF-TrFE)), as a universal interlayer, which has tunable and highdensity, aligned dipoles controlled by the external applied bias. [ 9 ] This ferroelectric interlayer can effectively increase or decrease the work function of any electrode to serve as both anode and cathode due to the switchable dipole direction. A strong electric fi eld can be induced in the semiconducting polymers by the unscreened polarization charges of the P(VDF-TrFE), which has been demonstrated to improve the PCE of several types of organic photovoltaic cells (OPVs) to unprecedented values, superior to many other methods. In our previous studies, it was also shown that the polarization of P(VDF-TrFE) on the organic semiconductor fi lms is stable [ 10,11 ] and the surface charges are not screened by photogenerated carriers. [ 10 ] A crystalline β -phase P(VDF-TrFE) fi lm is needed to ensure its ferroelectricity. [ 10 ] Langmuir-Blodgett (LB) deposition is an excellent method for forming highly crystalline P(VDF-TrFE) fi lm as thin as 1 nm. [ 9 ] However, P(VDF-TrFE) LB fi lms generally need a thermal annealing process after the deposition to convert them into the ferroelectric phase at above the Curie temperature. In our previous studies, the P(VDF-TrFE 70:30) LB fi lms were annealed at 135 °C for half an hour to completely convert them to the ferroelectric phase. [ 9,11 ] However, this high temperature thermal annealing process is not compatible with the fabrication process of many high-performance, low bandgap polymer-based OPV devices, such as thieno3,4-b-thiophene/ benzodithiophene (PTBs) [ 6,12,13 ] and poly N -9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole) (PCDTBT). [14][15][16] These polymers are generally heated at 70 °C or vacuumed to remove solvent in the spun fi lms. [ 14 ] A high-temperature annealing at 135 °C will signifi cantly reduce the PCEs of these polymer-based OPVs due to over annealing, causing overly large 6,6-phenyl-C 61 -butyric acid methyl ester (PCBM) or polymer nanodomains. [ 17 ] In addition, it is diffi cult to control the morphology of P(VDF-TrFE) LB fi lms after thermal annealing. [ 18 ] Appropriate P(VDF-TrFE) size of the nanoislands and spacing between them is critically important so that the photogenerated charges can be fully extracted rather than blocked by the large, t...

Arising applications of ferroelectric materials in photovoltaic devices

“…For the FE-OPV device with P(VDF-TrFE) at the organic/electrode interface, the generated electric eld is expected to be switched parallel or antiparallel with the built-in eld, resulting in better and worse device performance. 40,130,136 This was clearly demonstrated by Yuan et al as shown in Fig. 8a.…”

“…Ferroelectric polymers, such as P(VDF-TrFE), can be deposited by various methods including spin coating, 130 electrospinning, 131 nano-imprinting, 40,132,133 and Langmuir-Blodgett (LB) deposition. 70,75 The spin coating from a low boiling point solvent usually leads to an amorphous lm, which can be thermally annealed to increase its ferroelectric phase at a temperature higher than its Curie point.…”

“…One issue in applying this method to the recently developed low bandgap polymer for efficiency enhancement is that the LB deposition used to fabricate monolayers of P(VDF-TrFE) lms is not compatible with the process of these polymers due to the high temperature thermal annealing around 130 C needed to convert the P(VDF-TrFE) LB lm into the ferroelectric phase. For example, many state-of-the-art low bandgap polymers, such as or/and PCBM domains.…”

Ferroelectricity and molecular dynamics of poly(vinylidenefluoride-trifluoroethylene) nanoparticles

“…Also, the resulting polymer nanoparticles are composed of the polymer without modifications in its molecular weight and chemical structure [21]. Although the nanoparticle preparation methods have been applied to a great variety of polymers, studies on ferroelectric polymer nanoparticles are relatively scarce [22]. The modification of some physical properties like the glass transition or the melting and crystallization behavior of polymer nanoparticles in comparison with the bulk behavior has been investigated for some polymers.…”

Fabrication of diisopropylammonium bromide aligned microcrystals with in-plane uniaxial polarization

“…Another class of useful ferroelectrics is polymers [19], most notably those based on vinylidene fluoride (VDF) [20,21]. Copolymers of VDF with trifluoroethylene (TrFE), P(VDF-TrFE), for example, have become the ferroelectric polymer of choice for applications, due in part to the variety of ways they can be prepared in bulk using inexpensive solution-processing methods [22] and can be formed into high-performance thin films and nanostructures [21,[23][24][25][26][27]. The ferroelectric polymers, however, have not proven particularly amenable to synthetic modification for optimizing ferroelectric properties in the way the ferroelectric oxides have [28].…”