| The bulk heterojunction (BHJ) structure has been widely applied for the construction of efficient organic solar cells. Typically, the heterojunction is formed as a result of phase segregation of donor and acceptor mixture by solution processing. In this thesis, issues with the structure of solution-processed polymer(donor)/fullerene(acceptor) BHJ solar cells are addressed. Two new approaches, where an electrochemically grown conducting polymer nano-network is used to fabricate the nano-structured polymer/fullerene heterojunction solar cells, are introduced and investigated in an attempt to remove the electrical discontinuity and misconnection introduced by solution processing and improve carrier transport and extraction while maintaining high exciton generation and dissociation rate in polymer-fullerene BHJ solar cells. In the first approach, 3-dimensional (3-D) nano-networked polymer/fullerene BHJ solar cells are fabricated by infiltrating fullerene into openings of the electropolymerized 3-D nano-porous polymer. With this approach, efficient exciton dissociation can be realized, and importantly, excellent continuity of both donor and acceptor phases and proper connection of each phase to the corresponding electrode can be accomplished, therefore allowing effective collection of the free carriers. Power conversion efficiency of 3.0 % is demonstrated. In the second approach, fullerene is infiltrated into the carbon nanotube (CNT) network which has been first electrochemically wrapped with a thin polymer layer, to form the CNT-polymer core-shell nanowire network / fullerene BHJ solar cell. The thin polymer shell can ensure high exciton dissociation rate and efficient free hole transport to the CNT core which serves as an efficient network for extracting holes out of the heterojunction. Thus on top of all the advantages from the first approach, the second approach also addresses the low hole mobility issue with conventional BHJ solar cells. Power conversion efficiency of 3.07 % is demonstrated. Moreover, the nano-structured polymer/fullerene BHJ solar cells are investigated with different polymers and are also suitable for flexible solar cell applications. The results show the feasibility of the new approaches and their potential to offer a new direction to tackle the problems associated with solution-processed polymer/fullerene BHJ solar cells. |
