Influence of polymer structure on electronic properties and performance as donor materials in bulk heterojunction solar cells

2015-03-12Essential to the achievement of higher efficiencies in polymer:fullerene bulk heterojunction solar cells is the development of a polymer donor material that simultaneously meets the requirements of a broad absorption profile, suitable energy levels and a favorable morphology with fullerene derivatives. In an effort to pursue the ideal polymer, copolymerizing electron‐rich and electron‐poor monomers in a perfectly alternating manner (alternating donor‐acceptor (D-A) copolymers) has produced state‐of‐the‐art polymer samples through the “monomer approach”. As an alternative strategy, polymer properties can be modulated by the strategic combination of monomers through the “polymer approach”. Following this concept, in particular, a number of random copolymers have been reported in literature, revealing their distinct advantages in the ease of synthesis, as well as in the convenience and broad scope of property tuning ability. ❧ This thesis has been inspired by the previous works on electroactive polymers for organic photovoltaics that are synthesized through the “polymer approach”, especially those with random conjugated backbones. Focus in this thesis is on the synthesis of electroactive polymers following the “polymer approach” and their structure‐property relationships. Chapter 1 reviews the reported examples of random conjugated polymers in the context of organic solar cell applications, with an emphasis on their synthetic strategies, structure‐property relationships and photovoltaic device performance. Several specific strategies to produce random copolymers are recognized and discussed according to comonomer structures and reaction features. In particular, three popular strategies reviewed in Chapter 1, namely the random regioregular polythiophenes, random D‐A copolymers and semi‐random D‐A copolymers, will be reflected in the polymer structures reported in Chapter 2, 3 and 4. In addition, a brief summary is included in Chapter 1 on the advantages of random conjugated copolymers, especially in comparison to alternating D‐A copolymers. ❧ Chapter 2 describes the investigation of two sets of copolymers containing varying percentages of strongly electron‐donating 3-hexyloxythiophene units (3HOT). The polymers synthesized here include both random regioregular polythiophenes and semi‐random diketopyrrolopyrrole (DPP) ‐containing copolymers. The influence of 3HOT content on UV−Vis absorption, HOMO energy levels, polymer crystallinity and polymer:fullerene solar cell performance, especially the open‐circuit voltage (Voc), is discussed in detail. Importantly, this study demonstrates that significant changes in polymer electronic properties can be induced with only small percentage of 3HOT comonomers in random and semi‐random conjugated polymers. ❧ Chapter 3 continues the investigation on the effect of 3HOT units on polymer properties, but with another parent polymer based on alternating 2,7-carbazole and dithienyl-2,1,3-benzothiadiazole. A 2,1,3-benzothiadiazole‐based comonomer containing 3HOT units was used to replace varied amounts of dithienyl-2,1,3-benzothiadiazole and incorporated into polymer backbones following the random D‐A copolymer strategy. The relationships between polymer composition and properties are described. ❧ Chapter 4 describes the preparation of a series of random DPP‐functionalized regioregular poly(3-hexylthiophene)s (rr-P3HTs) and their application as donor materials in fullerene‐based solar cells. Functionalized rr-P3HTs with varied amounts of azide side groups were synthesized, and post‐polymerization “click” reactions were used to attach DPP derivatives onto the rr-P3HT backbones. Structure‐property‐performance relationships of these polymers are analyzed, particularly in terms of UV‐Vis absorption, HOMO energy levels, polymer crystallinity and solar cell Voc. Due to the separated conjugation between P3HT backbones and DPP side chains, these polymers are considered as two donor components that are physically constrained in one phase. Comparison is made between the binary blend solar cells based on these DPP‐functionalized rr-P3HTs and the ternary blend devices based on separate P3HT and DPP derivative donors. Dramatic differences in their device performance shed light on the working principles of ternary blend solar cells. ❧ Chapter 5 describes the use of post‐polymerization “click” reactions in preparing non‐conjugated polymers with electroactive pendent groups. The synthetic strategy toward functionalization of stereoregular polymers with electroactive pendents is explored. This work demonstrates another platform of the “polymer approach” in achieving distinct structural features of electroactive polymers