Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells

摘要： Incorporating fluorine (–F) substituents along the main-chains of polymer donors and acceptors is an effective strategy toward efficient bulk-heterojunction (BHJ) solar cells. Specifically, F-substituted polymers often exhibit planar conformations, leading to favorable packing, and electronic coupling. However, the effects of fluorine substituents on the charge generation and recombination characteristics that determine the overall efficiency of BHJ active layers remain critically important issues to examine. In this report, two PBDT[2X]T polymer analogs –poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-thiophene] [PBDT[2H]T] and its F-substituted counterpart poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluoro-thiophene] [PBDT[2F]T]—are studied to systematically examine how –F substituents impact the blend morphology, charge generation, carrier recombination and extraction in BHJ solar cells. Considering the large efficiency differences between PBDT[2H]T- and PBDT[2F]T-based BHJ devices, significant emphasis is given to characterizing the out-of-plane morphology of the blend films as vertical phase-separation characteristics are known to have dramatic effects on charge transport and carrier extraction in polymer-fullerene BHJ solar cells. Herein, we use electron energy loss spectroscopy (EELS) in tandem with charge transport characterization to examine PBDT[2X]T-fullerene blend films. Our analyses show that PBDT[2H]T and PBDT[2F]T possess very different charge generation, recombination and extraction characteristics, resulting from distinct aggregation, and phase-distribution within the BHJ blend films.

Corrigendum: Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells

Comparison of ZnO buffer layers prepared by spin coating or RF magnetron sputtering for application in inverted organic solar cells

摘要： We compared the electrical, optical, structural, and morphological properties of radio-frequency (RF) magnetron-sputtered ZnO and solution-processed ZnO nanoparticle (NP) buffer layers on ITO cathodes for use in inverted polymer solar cells (IPSCs). Continuous sputtering resulted in integration of the ZnO buffer layer in the ITO cathodes, which were then used as transparent cathodes for IPSCs. Although the electrical, optical, and morphological properties as well as work function of RF-sputtered ZnO film were similar to those of solution-processed ZnO NP film, the power conversion efficiency (PCE) of IPSCs with an RF-sputtered ZnO buffer layer was much lower than that of IPSCs with a solution-processed ZnO NP buffer layer due to vertical phase segregation of the organic active layer. However, intentional bias sweeping of IPSCs with an RF-sputtered ZnO buffer layer improved performance due to diffusion of PC70BM through the PV-D4610 donor layer and formation of a suitable heterojunction structure. Based on transmission electron microscope examination and dark current-voltage curves, we suggest a possible mechanism to explain the difference in behavior of RF-sputtered ZnO and solution-processed ZnO NP buffer layers in IPSCs.