Novel D-A Polymer Donors For Polymer Solar Cells

Polymer solar cells(PSCs)have attracted enormous attention due to their low-cost,ease to processing,light weight,mechanical flexibility,and potential capability for large-area and high-throughput roll-to-roll manufacturing production.At present,further improving the efficiency is still the research focus for PSCs.In this thesis,two species novel D-A copolymer donor materials were applied to PSCs to prepare binary and ternary PSCs,respectively.Finally,we achieved a high power conversion efficiency(PCE)and gave a detailed explanation through a series of characterizations.The main results and innovations of this thesis can be summarized as follows:The first chapter summarized the development history and research status of organic solar cells(OSCs),classified the OSCs based on the device structure,elaborated its working mechanism,and introduced commonly used materials including donor/acceptor and conventional methods to improve the device performance.In the second chapter,the application of novel electron donors in binary polymer solar cells(PSCs)was studied.?-extended vinylidenedithiophenmethyleneoxindole(VTI)unit had been incorporated to polymeric conjugated backbones affording two donor-acceptor copolymers such as P1 and P2.Both VTI-based copolymers exhibited broad absorption profiles in the visible region.Bulk heterojunction solar cell-based P2 and [6,6]-phenyl-C71-butyric acid methyl ester(PC71BM)blend afforded an improved power conversion efficiency(PCE)value of 4.75%,which was much higher than the PCE of 2.12% for the PSCs based on P1:PC71BM.These results indicated that increasing the degree of conjugation in the direction of the vertical backbone can improve the performance of polymers with the same conjugated backbone,and highlighted the great potential of VTI unit as a building blocks for constructing high performance polymer semiconductors for PSCs.In the third chapter,the application of novel electron donor in ternary PSCs was studied.A dialkoxyl-substituted naphthodithieno[3,2-b] thiophene-based copolymer(PV12)was applied as a third component for ternary polymer solar cells(PSCs)based on PCDTBT1-x:PV12x:PC71BM.In Al-only ternary PSCs with x=0.15,a power conversion efficiency(PCE)of 6.73% was achieved due to simultaneous enhancement in the VOC,JSC and FF,which was much higher than the PCE of 5.28% for Al-only binary PSCs based on PCDTBT:PC71BM and 2.93% for Al-only binary PSCs based on PV12:PC71BM.The PCE for ternary PSCs reached 7.69% when IIDTh-NSB as a cathode interlayer was introduced between the ternary active layer and Al.Ultraviolet photoemission spectroscopy measurements demonstrated a cascade-type energy level alignment at the ternary PCDTBT(D1)/PV12(D2)/PC71BM(A)junction.As a result,the voltage limitation was overcome at the D1/D2/A junction.Ultraviolet-visible absorption and external quantum efficiency spectra showed that PV12 had complementary absorption to PCDTBT in the solar spectrum.Atomic force microscopy and transmission electron microscopy images showed that the morphology and phase separation of the active layer were optimized by adding PV12.Photoluminescence spectral investigations suggested that the F?rster resonance energy transfer from PCDTBT to PV12 occurred in the ternary PSCs under illumination.Finally,the PCEs of the ternary PSCs were not as sensitive to the thickness of the active layer as those of the binary PSCs,which was important for the roll-to-roll coating processing of organic photovoltaic modules.