Enhanced Charge Transfer, Transport and Photovoltaic Efficiency in All-Polymer Organic Solar Cells by Polymer Backbone Fluorinationt

We successfully designed and synthesized two BDT-BT-T （BDT=benzo[1,2-b：4,5-b＇]dithiophene, BT-T=4,7-dithien-2-yl-2,1,3-benzothia- diazole） based polymers as the electron donor for application in all-polymer solar cells （all-PSCs）. By adopting N2200 as the electron acceptor, we system- atically investigated the impact of fluorination on the charge transfer, transport, blend morphology and photovoltaic properties of the relevant alI-PSCs. A best power conversion efficiency （PCE） of 3.4% was obtained for fluorinated PT-BT2F/N2200 （BT2F=difluorobenzo[c][1,2,5]thiadiazole） alI-PSCs in com- parison with that of 2.7% in non-fluorinated PT-BT/N2200 （BT=benzothiad（azole） based device. Herein, all-polymers blends adopting either non-fluori- nated PT-BT or fluorinated PT-BT2F exhibit similar morphology features. In depth optical spectrum measurements demonstrate that molecular fluorina- tion can further enhance charge transfer between donor and acceptor polymer. Moreover, all-polymer blends exhibit improved hole mobilities and more balanced carriers transport when adopting fluorinated donor polymer PT-BT2F. Therefore, although the PCE is relatively low, our findings may become important in understanding how subtle changes in molecular structure impact relevant optoelectronic properties and further improve the performance of all-PSCSs.

Synthesis of ultra-narrow PbTe nanorods with extremely strong quantum confinement

Monodisperse, high-quality, ultra-narrow PbTe nanorods were synthesized for the first time in a one-pot, hot-injection reaction using trans-2-decenoic acid as the agents for lead precursors and tris(diethylamino)phosphine telluride together with free tris(diethylamino)phosphine as the telluride precursors. High monomer reactivity, rapid nucleation and fast growth rate derived from the new precursors led to the anisotropic growth of PbTe nanocrystals at low reaction temperatures(<150℃). In addition, the aspect ratio of PbTe nanorods could be largely adjusted from 4 to 15 by tuning the Pb to Te precursor molar ratio and reaction temperatures. Moreover, the synthesized ultra-narrow PbTe nanorods exhibited extremely strong quantum confinement and presented unique optical properties. We revealed that the diameter and length of PbTe nanorods could significantly affect their optical properties, which potentially offer them new opportunities in the application of optoelectronic and thermoelectric devices and make them desired subjects for multiple exciton generation and other fundamental physics studies.

Controlling molecular weight of naphthalenediimide-based polymer acceptor P(NDI2OD-T2)for high performance all-polymer solar cells

A widely-used naphthalenediimide （NDI） based electron acceptor P（NDI2OD-T2） with different number- average molecular weight （Mn） of 38 （N2200L）, 56 （N2200M）, 102 （N2200H） kDa were successfully prepared. The effect of molecular-weight on the performance of all-polymer solar cells based on Poly（5-（5-（4,8- bis（ 5-decylthiophen-2-yl ）-6-methylbenzo[1,2-b： 4,5-b＇]dithophen-2-yl ）thiophen-2-yl ）-6,7-difluoro-8- （5-methylthiophen-2-yl）-2,S-bis（3-（octyloxy）phenyl）quinoxaline） （P2F-DE）：N2200 was systematically investigated. The results reveal that N2200 with increased M. show enhanced intermolecular interac- tions, resulting in improved light absorption and electron mobility. However, the strong aggregation trend of N2200H can cause unfavorable morphology for exciton dissociation and carrier transport. The blend film using N2200 with moderate M. actually develops more ideal phase segregation for efficient charge separation and transport, leading to balanced electron/hole mobility and less carrier recombi- nation. Consequently, all-polymer solar cells employing P2F-DE as the electron donor and N2200M as the electron acceptor show the highest efficiency of 4.81%, outperforming those using N2200L （3,07~;） and N2200H （S,92%）. Thus, the Mn of the polymer acceptor plays an important role in all-polymer solar ceils, which allows it to be an effective parameter for the adjustment of the device morphology and efficiency.