The influence of amino group on PCDTBT-based and P3HT-based polymer solar cells: Hole trapping processes

Abstract

Polymer solar cells (PSCs) based on aliphatic-amino-functionalized materials presented low performance with negligibly small efficiency, the prime mechanism of which is found to be hole trapping induced by the amine end groups. We propose that such hole trapping behavior depends on the relative energetic position of the hole transport states and the trapping states. Herein, we comparatively study the photovoltaic properties of PSCs based on amino-functionalized fullerene derivative blended with poly [N-9′-heptadecanyl-2, 7-carbazole-alt-5, 5-(4′, 7′-di-2-thienyl-2′, 1′, 3′-benzothiadiazole)] (PCDTBT) or poly (3-hexylthiophene) (P3HT). The former polymer has a lower-positioning highest occupied molecular orbital (HOMO) level, whereas the latter has a comparable HOMO level relative to the ionization state of tertiary aliphatic amine in energy. Our investigation confirms our proposition, revealing an ultrafast trapping process in PCDTBT:amino-group-functionalized fullerene derivative film, which seriously crippled hole transport, consequently results in very poor device performance. In contrast, trapping process is almost negligible in P3HT systems.

Water/Ethanol Soluble p-Type Conjugated Polymers for the Use in Organic Photovoltaics

“…Upon applying a thin layer of TPD (3 nm), the V OC was marginally improved to 0.34 V, leading to an efficiency of 0.13%. Although the devices based on TQ1A showed photovoltaic response, the PCE is much lower compared to that of TQ1:PC 61 BM devices (Kroon et al, 2012), which could be due to the charge traps formed by the amino groups, indicated from the electrochemistry results (Cai et al, 2015).…”

“…It has been previously reported that amino groups are potential hole traps in the BHJ devices containing aminefunctionalized active materials, due to that the highest occupied molecular orbital (HOMO) level of p-type polymer lies below the oxidation state of amine (Cai et al, 2015). Hence, electrochemistry was also performed on triethylamine to compare its oxidation process with amine-functionalized TQ1A.…”

“…Nevertheless, the onset oxidation potential of pyridine was measured to be ∼1.0 V, which is far above the value for the TQ1P4 backbone of 0.32 V (Figure 3B), revealing the low-lying ionization state of pyridine groups compared to the pyridinefunctionalized polymer. Hence, the introduced side-chain with pyridine pendant group potentially affects hole transfer in the pyridine-functionalized polymer to a much lower degree compared to tertiary amine functionalities (Cai et al, 2015). The irreversibility of the CV is also a concern for the use of the materials in OPV devices, however, some high performing donor polymers used in OPVs also exhibit irreversible CVs (Xu X. et al, 2016;Zhang et al, 2017;Li et al, 2018).…”

“…Materials with amino and pyridine functionalities have been previously reported as interface layers to reduce the electrode work function (Duan et al, 2013;Wu et al, 2016;Sharma et al, 2017), which is related to the presence of nitrogens with free electron pair in these materials. Since tertiary amines and pyridine groups are well known for reducing the work function of the underlying metal oxide electrodes (van Reenen et al, 2014;Cai et al, 2015;George et al, 2016), studying these donor polymers in devices with an inverted structure (ITO/ZnO/BHJ/MoO 3 /Ag) would be ideal as the work function of the underlying cathode layer would be further altered in an energetically favorable direction. However, it is equally important to also understand the interaction of the amine/pyridine groups of the donor polymer with the top anode interface, due to its potential to influence the energetics at the BHJ-MoO 3 interface.…”

“…In another approach, water/alcohol soluble conjugated polymers (WS) have been investigated as active layer materials for water/alcohol-based solar inks. Through the side-chain engineering (Huang et al, 2010;Mei and Bao, 2013;He et al, 2014;Hu et al, 2015;Park et al, 2015;Wu et al, 2016) i.e., replacing the hydrophobic aliphatic substituent with hydrophilic polar side groups, for example, ionic moieties [e.g., sulfonate groups (Mwaura et al, 2005), quaternary ammonium salt (Hu et al, 2015)] and non-ionic functional groups such as tertiary amine (Duan et al, 2013;Li et al, 2013;Lv et al, 2014;Ma et al, 2014;Cai et al, 2015) and oligoethylene glycol (OEG) (Søndergaard et al, 2011;Nguyen et al, 2017;Kim et al, 2018;Lee et al, 2018). However, the ionic moieties have been reported to act as charge carrier traps/recombination site (Yang et al, 2007;Duan et al, 2013), and the WSCPs designed based on this approach have only been successfully applied as interface layer materials for OPVs (Huang et al, 2010;Wu et al, 2016;Xu B. et al, 2016).…”