Enhancing Solar Cell Performance with Fullerene Interlayer on ZnO Rod Arrays

1. JC01 – 26 Oct. 2009 “Enhancing hybrid solar cells using a fullerene interlayer”, J.Huang, C.Chou, C.Lin, National Taiwan University, Solar Energy Materials & Solar Cells, In Press. “On the origin of the VOC of polymer-fullerene solar cells”, K.Vandewal, K.Tvingstedt, A.Gadisa, O.Ingasas, J.Manca, Hasselt University, Belgium, Nature Materials, 11 Oct. 2009.

2. Background: ZnO rod array Hybrid Cells • Motivation • Good hole-blocking layer • Ideal morphology • Higher mobility • Problems • Lower surface area • Poor polymer infiltration • Poor wetting • Band alignment notas good? Images from I. Gonzalez-Valls & M. Lira-Cantu, Energy & Environ. Sci., 2 19, 2008.

3. Methods • Uses All solution processing • ZnO spin-coated seed layer from Zn-acetate dihydrate • Arrays grown in Zn-nitrate solution/90°C oven then rinsed in water • Rest is similar to our method using dichlorobenzene • Inverted cell w/higher work function Ag & no PEDOT • Compared w/& w/out interlayer both w/& w/out ZnO rod array (still smooth ZnO layer) • Modifies the interface between BHJ & ZnO rod arrays using PCBM interlayer • Enhances performance • Better infiltration of rod array • Enables active film to order, allowing increased optical absorption Claims ~300nm

4. Field-Emission? SEM, EDX, and XRD Rods after PCBM spin-coating XRD PCBM crystallites! EDX

5. Polymer Infiltration of Rod Array • Cross sectional SEM shows voids w/out the interlayer • How methodical is this? • Is there a better way to measure this?(X-ray tomography?)

6. Energy Diagram & IV- Characteristics • Cell is inverted! • Uses more “air-stable” high-work function electrodes • No PEDOT: says it corrodes ITO • Tested 8 samples for each structure • Better performance (Voc, Jsc, FF, PCE) w/interlayer • PCE 2.35% to 3.2% • Jsc10.1 to 11.7 mA/cm2 attribute to better contact • FF 0.45 to 0.5 (variance?) • Voc 0.51V to 0.55V w/stdevof 1.5mV • ΔHOMO-LUMO for P3HT-ZnO is 0.4eV • Don’t suggest a reason for the increased Voc

7. P3HT Crystallization • UV-vis→ more absorption • “the fullerene interlayer assists the self-organization of P3HT chains during the solidification of the blend films” • AFM on active layer → higher roughness • σRMS~15nm (b) vs ~7nm (c) • References another paper suggesting rougher=more ordered P3HT • How well did theysample the area • How conclusive is this evidence? • Why not do XRD? • Claim interlayer causes this • I see no connection

8. Test w/out ZnO array • Still have smooth ZnO layer w/& w/out interlayer • See no difference in IV or UV-vis • Say nothing about XRD or crystallization • Suggests PCBM “reblends” into the heterojuntion • Doesn’t suggest why the difference beyond… • Never use this un-scientific language: • “…fullerene interlayer coated on the flat ZnO film is very thin and the amount of PCBM molecules is very small.” • Extremely poor explanation

9. My Take • An initial investigation showing better performance • Definitely successful PCBM interlayer that is crystallized (difficult) • PCBM interlayer maybe helps interface wetting & infiltration • So few samples, could be fluke • Don’t buy cause-effect relationship of PCBM interlayer causing better P3HT crystallization. • Part of the better Jsc is due to this & could be caused by other experimental factors not discussed • Can not definitively say interlayer causes better incorporation • Should have done more comparisons w/out rods since that shows no effect. • Perhaps it shows this IS a fluke? • Voc results don’t jive with HOMO/LUMO theory