Bulk heterojunction cells polymer:fullerene blends

It is the purpose of this chapter to introduce photoinduced charge transfer phenomena in bulk heterojunction composites, i.e., blends of conjugated polymers and fullerenes. Phenomena found in other organic solar cells such as pristine fullerene cells [11,12], dye sensitised liquid electrolyte [13] or solid state polymer electrolyte cells [14], pure dye cells [15,16] or small molecule cells [17], mostly based on heterojunctions between phthalocyanines and perylenes [18] or other bilayer systems will not be discussed here, but in the corresponding chapters of this book. 

As a synthetic strategy, simple and versatile reactive blending will continue to play a pivotal role in the development of newer materials. For example, the blending technique is being used to produce bulk heterojunction polymer solar cells (polymer/fullerene) and to develop electrically conductive polymer blends using electrically conductive fillers and additives (Huang and Kipouras 2012). 

Vanderzande et al. reported the facile synthesis to 5,6-disubstituted-l,3-dithienylbenzo[c]thiophenes 3.10 via Pd°-catalyzed coupling reaction of 5,6-dichloroterthiophenes 3.9 with an alkyl Grignard reagent (Scheme 1.30) [309, 321]. Chemical polymerization of the 5,6-modified monomers with FcCIb yielded polymers with bandgaps of 1.4-1.8 eV, which are similar to that of poly(dithienylbenzo[c]thiophene) P3.3 [309]. Application of these polymers as donors and fullerene PCBM as acceptor in bulk heterojunction solar cells (BHJSC) was also investigated and reported. An overall power conversion efficiency of 0.3 % and an internal power conversion efficiency of 24% were obtained for PMMA-poly-P3.9c-PCBM (1 2 6) blended devices [321]. 

This approach mimics in some way dye sensitized solar cells because porphyrin/fuUerene clusters serve as sensitizers for buffer tin oxide (Sn02). At the same time, mixed fuUerene/polymer nanostructures resemble bulk heterojunction solar cells since donor (porphyrin) and acceptor (fullerene) molecules are blended together in the active layer. 

Conventional organic bulk heterojunction solar cells do not have hole-blocking electron transport layers (HBETL) at the cathode/active layer interface. Metals with reasonably low-work function (e.g., calcium, aluminium, magnesium, or barium) are typically evaporated directly onto fullerene/polymer blend forming electronic contacts with both donor and acceptor materials under optimal conditions. This situation leads to significant recombination of positive and negative charge carriers at the active layer/cathode interface. 

Usually, the photoactive layer of bulk heterojunction solar cells is a physical blend [1-3] of a rr-conjugated polymer [12] and a fullerene derivative [13-17]. This bulk heterojunction structure inspired the design and preparation of donor... 

---