Increased performance of inverted organic photovoltaic cells using a cationically functionalized fullerene interfacial layer
Introduction
Electrode modification using interfacial layers (IFLs) to enhance the performance of inverted organic photovoltaic cells (i-OPVs) has attracted increased research interest in recent years [1], [2], [3]. Modification of electrodes using IFLs has been shown to have a significant impact on the magnitude of the cell׳s open-circuit voltage (Voc) [2], [3], [4]. This enhancement of Voc can result from the modification of the electrode work function to increase the built-in potential, as well as enhanced alignment of the electrode work function with the active layer quasi Fermi level for electrons or holes. IFL modification of electrodes can also provide charge selectivity by introducing an energy barrier to a specific sign of charge. This charge selectivity reduces interfacial recombination at the collecting electrode resulting in an increase in Voc [5]. To maximize performance of devices, it is also important to ensure a low series resistance (Rs) to maximize charge extraction and enhance the fill factor (FF) [6].
Numerous IFLs have been investigated to modify the transparent electron-collecting electrode, typically indium tin oxide (ITO), in i-OPVs. These include n-type wide bandgap semiconductors such as TiO2 [7] and ZnO [8], graphene [9], fullerene films [10], [11] and self-assembled monolayers (SAMs) [12], [13], conjugated polymers [1], and conjugated polyelectrolytes (CPEs) [14]. The use of ion-containing IFL materials, such as CPEs, are particularly attractive as they posses orthogonal solubility to most active layer bulk heterojunction (BHJ) blends which facilitates solution processability. Furthermore, CPEs containing cationic functional groups have been shown to effectively reduce the ITO work function, which defines the ITO polarity and promotes Fermi level alignment with the acceptor phase in the inverted structure [15], [16], [17].
Fullerene based IFLs have recently gained interest as efficient electron collecting electrode interfacial layers (n-IFLs) in inverted and conventional BHJ solar cells. Fullerene derivatives are advantageous as n-IFLs due to their energy-level alignment with the PCBM acceptor phase, resulting in efficient charge transfer with minimal voltage loss. In addition, the low-lying HOMO level of fullerenes enhances carrier selectivity by inhibiting hole transfer from the donor polymer. Cationically functionalized fullerene n-IFLs have been demonstrated by Jen et al. in conventional OPVs [18]; however, due to poor solvent resistance, fullerene n-IFLs in the inverted geometry have often required chemical cross-linking or the use of fullerene SAMs which are chemically bound to the substrate [19], [20]. The relatively low conductivity of fullerene n-IFLs has also often required the use of an underlying n-type IFL such as TiO2 to reduce Rs [2], [12]. To overcome this shortcoming, chemical n-doping of fullerene IFLs by co-evaporation with Cs2Co3 or incorporation of n-dopants, such as decamethylcobaltacene or alkali carbonates have been reported [10], [20], [21].
Herein we demonstrate the use of the highly conductive, alcohol-soluble, cationically functionalized fullerene derivative N,N,N-trimethyl-5-(N-methyl-3,4-[60]fulleropyrrolidin-2-yl)pentan-1-aminium bromide (NMFP-Br) as an efficient electron collecting electrode interfacial layer in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) inverted organic photovoltaic cells. The incorporation of NMFP-Br as an n-IFL results in a substantial increase of the Voc from 0.41 V to 0.60 V and the overall power conversion efficiency (PCE) from 2.1% to 3.5% under AM 1.5G illumination. The orthogonal solubility of NMFP-Br allows for sequential solution processing without the need of chemical cross-linking. The unusually high conductivity of this material results in a significant decrease in the series resistance, relative to control devices, even when the n-IFL thickness is increased fivefold.
Section snippets
Synthesis of N,N,N-trimethyl-5-(N-methyl-3,4-[60]fulleropyrrolidin-2-yl)pentan-1-aminium bromide (NMFP-Br)
Sarcosine (0.3 g, 3.4 mmol) and 6-bromo-hexanal (240 mg, l.34 mmol) were added to a solution of C60 (654 mg, 0.91 mmol) in deoxygenated toluene (600 mL). The solution was heated to a reflux for 2 h turning from purple to a dark brown at which point it was cooled to room temperature, concentrated to ~100 mL and then purified by silica gel column chromatography using toluene as eluent. Eluting first was unreacted purple starting material, followed by the mono-adduct (300 mg) followed by multiple addition
Results and discussion
The average J–V curves of inverted devices with and without NMFP-Br n-IFLs are shown in Fig. 2 and properties are tabulated in Table 1. The J–V curves initially display an S-shaped kink which is often observed in inverted devices incorporating metal oxide or CPE interlayers [17], [30]. In the case of inverted cells incorporating a metal oxide interlayer, this effect has been attributed to an electron extraction barrier at the metal oxide/active layer interface [30], however, the reason for the
Conclusion
In conclusion, we have demonstrated a significant power conversion efficiency increase of an inverted P3HT:PCBM bulk heterojunction photovoltaic cell though the use of a cationically functionalized fullerene derivative NMFP-Br interfacial layer, relative to devices with no interfacial layer. The PCE increased an average of 60%, from 2.0% to 3.2%, with a maximum PCE value of 3.5% observed. An increase in device Voc of up to 200 mV is observed upon incorporation of NMFP-Br n-IFLs, resulting from a
Acknowledgments
This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-07ER15907. This project made use of equipment in the SUNRISE Photovoltaic Laboratory supported by the Oregon Built Environment and Sustainable Technologies (BEST) Signature Research Center. The authors would like to acknowledge Samantha Young and Clay Easterly for their contributions to this work. M.C.L. and C.D.W. also
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