Water/Alcohol Soluble Thickness-Insensitive Hyperbranched Perylene Diimide Electron Transport Layer Improving the Efficiency of Organic Solar Cells

The electron transport layer (ETL) is very crucial for enhancing the device performance of polymer solar cells (PSCs). Meanwhile, thickness-insensitive and environment-friendly water/alcohol soluble processing are two essential requirements for large-scale roll-to-roll commercial application. Based on this, we designed and synthesized two new n-type ETLs with tetraethylene pentamine or butyl sulfonate sodium substituted tetraethylene pentamine as the branched side chains and high electron affinities perylene diimide (PDI) as the central core, named as PDIPN and PDIPNSO3Na. Encouragingly, both PDIPN and PDIPNSO3Na can effectively reduce the interfacial barrier and improve the interfacial contact. In addition, both of them can exhibit strong n-type self-doping effects, especially the PDIPN with higher density of negative charge. Consequently, compared to bare ITO, the PCE of the devices with ITO/PDIPN and ITO/PDIPNSO3Na ETLs has increased to 3–4 times. Our research results indicate that n-type self-doping PDI-based ETL PDIPN and PDIPNSO3Na could be promising candidates for ETL in environment-friendly water/alcohol soluble processing large-scale PSCs.

isopropanol. After the ITO substrates were dried by nitrogen purging and treated the surface with UV ozone for 15 min, the PDIPN and PDIPNSO3Na (1.0 mg mL −1 ) dissolved in methanol solutions were spin-coated on the ITO at 3000 r/min for 1 min.
The active layer P3HT/PC61BM (1:0.8 w/w), a blend solution of 10 mg of P3HT and 8 mg of P61CBM dissolved in 0.5 mL o-dichlorobenzene solution with a concentration of 20 mg mL −1 and was prepared and by spin-casted at 800 rpm for 30 s. And then the film was dried in the glove box for 2 hours, subsequently annealed at 150°C for 10 min in a nitrogen-atmosphere glove box. Finally, the anode buffer layer MoO3 (7 nm) and Ag (90 nm) electrode was sequentially deposited by thermal evaporation. The effective area of each cell was 0.04 cm 2 . The current−voltage (J−V) curve was measured by a Keithley 2400 Source Meter under simulated solar light (100 mW/cm 2 , AM 1.5 G, Abet Solar Simulator Sun 2000).

Synthesis
The detailed synthetic routes of the target compounds are displayed in Scheme S1.
The PDIPN was obtained by one-step finished reaction between 3,4,9,10-perylenetetracarboxylic diimides and tetraethylene pentamine. Then, PDIPNSO3Na was prepared by the substitution reaction between PDIPN and 1,4-butanesultone. The chemical structures of the PDIPN and PDIPNSO3Na are confirmed by 1 H nuclear magnetic resonance spectra ( 1 H NMR) ( Figure S1) and UV-Vis absorption (Figure 1). Due to the hydrophilic branched chains encasing the hydrophobic perylene diimides nucleus, the nuclear magnetic resonance signal peaks of the benzene ring of PDIPN and PDIPNSO3Na can't be detected. However, from the UV-Vis spectra, we can clearly observe strong absorption band at about 400-600 nm, which are attributed to the absorption of perylene diimides nucleus.

Synthesis of PDIPNSO3Na.
PDIPN (0.73 g, 1.0 mmol) and dried THF (60 mL) were added to 250 mL round bottom flask under the nitrogen atmosphere, the reaction mixture was cooled to 0 C in an ice bath, then NaH 1.5 g (60%) was slowly added to the solution and stired at 0 C for 0.5 h, the solution was gradually warmed to room temperature and stirred for 6-8h. Then, 1,4-butanesultone (13.6 g, 0.1mol) was added to the reaction mixture, and the reaction solution was reacted at 70  C overnight. After cooling to room temperature, the reaction solution was poured into plenty of acetone solution and filtered, the obtained filter residue was washed with acetone and dried under vacuum yield purple red solid (71%).     Figure S3. Atomic force microscopy (AFM) tapping mode height images (above) and phase images (below) of the surface of (a, c) ITO/PDIPN and (b, d) ITO/PDIPNSO3Na. (a)