Efficiency enhancement in organic solar cells with ferroelectric polymers

Abstract

Polymeric organic photovoltaic (OPV) cells with polymer-fullerene bulk heterojunction are promising candidates for future low-cost, high-performance energy sources, owing to their low material and processing costs and mechanical flexibility. 1,2 High efficiencies of 6-8% have been realized in polymeric OPVs by reducing both the optical bandgap and the highest occupied molecular orbital of the semiconducting polymers, and by optimizing the morphology of polymerfullerene blend film with thermal annealing and solvent annealing. [3][4][5][6][7][8][9] Although further optimization of the bandgap and highest occupied molecular orbital level of the semiconducting polymer is possible, 3 the path to increasing OPV efficiency to 15% must include recovery of significant energy loss even in the relatively high efficiency devices demonstrated so far. [10][11][12] There are five main causes of reduced efficiency in OPV devices: energy level misalignment, insufficient light trapping and absorption, low exciton diffusion lengths, and non-radiative recombination of charges or chargetransfer excitons (CTEs), which consist of electrons at the acceptor and holes at the donor bound by Coulomb attraction, and low carrier mobilities. 11,13 In many of the most efficient polymer-fullerene OPV devices, 50% or more of the energy loss is caused by the recombination of CTEs (Reference 11). For example, if we look at the most intensively studied material system, regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend, the photocurrent at the maximum power output point is only 70% of what could be extracted by externally applying a large reverse bias voltage to the device. 14 The purpose of the present work is to show how to achieve greater efficiency using a large internal electrical field provided by the permanent electrical polarization of a ferroelectric (FE) polymer layer. To understand how this innovation works, we need to understand how internal electric fields affect charge extraction efficiency. Figure 1a, CTEs form right after the photoinduced electron transfer. The next step is to separate them and enable them to contribute to the photocurrent. CTEs can be treated as a precursors of free carriers, and their bandgap sets a maximum value for the open-circuit voltage (V oc ; References 10, 15-23). The CTEs can be lost by non-radiative recombination when the dissociation driving forces (temperature and electric field) are small. The non-radiative recombination of CTEs, of course, reduces the photocurrent. In addition, the non-radiative recombination of CTEs reduces the free charge concentration, and then reduces the quasi-Fermi-energy (or chemical-potential) difference between electrons and holes, resulting in a lower V oc , reducing the output power even further. A strong reduction of the non-radiative recombination is required to reduce the fundamental efficiency loss. 11,12,24 Using the detailed balance theory, which was used to calculate the theoretical efficiency limit of a p-n junction sol...

Storage of an electric field for photocurrent generation in ferroelectric-functionalized organic devices

“…Considering that the SLs with free carriers inside can act as electrodes for poling, only the dimension of such SLs determines the V c ( ¼ E c d f ). The f in dc devices with small dimensional ferroelectric components, such as the OPV devices in previous reports 14,15 , may be sufficient for poling. As shown in Supplementary Note 1 and Supplementary Fig.…”

“…However, it is difficult to determine the contribution from the oriented E s , since random distributed local fields and field amplification due to its linear dielectric property may also help for photocurrent generation. Considering an even smaller dimension of the ferroelectric components and a larger f with two injecting contacts (the resultant bipolar injection can induce plenty of free carriers for poling) of the OPV devices in the previous reports 14,15 , the ferroelectric polarization may have already occurred, and the resultant oriented E s may play a partial role for the performance enhancements in their devices. Since the large amount of free charges due to the dc photocurrent, which may induce good compensation for polarization, there should not be a large E s across the whole devices, as proposed in Yuan et al 15 In summary, the double-wave method and photocurrent transients have been employed to probe net fields in organic materials from poled P(VDF-TrFE) films.…”

“…14), which is much larger than that induced by a conventional electrode potential difference (typically 1-10 MV m À 1 ). A recent attempt 15 has been made to utilize the 'strong' net electric field from poled ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) monolayers at electrode interfaces to improve exciton dissociation in OPV devices, despite the fact that the introduction of an organic semiconductor with a low permittivity must induce a large depolarization field (E d ) [16][17][18] if there are not enough charges injected for compensation/screening during poling. It was argued that the resultant enhancement originates from chemical reactions at the electrode interfaces that can modify the work function of the electrodes 19 .…”

Piezo‐Phototronic Effect Enhanced Flexible Solar Cells Based on n‐ZnO/p‐SnS Core–Shell Nanowire Array

“…Thus, NWs‐based PV will have great application in flexible power source compared to bulk materials. However, a large source of loss in this type of device is nonradiative recombination of the photogenerated carriers 8. Therefore, new strategies are necessary to reduce this recombination and significantly increase device efficiency.…”

Origin of the efficiency enhancement in ferroelectric functionalized organic solar cells

“…A prime example of the latter is tandem solar cells. 4 In a recent article, Yuan et al 5 reported on the efficiency enhancement in organic solar cells by incorporating a ferroelectric polymer interlayer between the electrodes and the photoactive layer. The enhanced efficiency was explained by the presence of an electric field due to the polarization of the ferroelectric layer, leading to enhanced exciton dissociation and, therefore, increased power conversion efficiency from 1%-2% to 4%-5%.…”