Synthesis, Characterization And Application Of Donor Materials For Organic Solar Cells

Organic solar cells(OSCs) have been a hot research topic in academia and industry due to the feasibilities in the fabrication of flexible, light-weight and low-cost solar-energy-conversion devices. A rapid development has been achieved for bulkheterojunction(BHJ) solar cells based on polymer/fullerene derivatives as donor/ acceptor pairs, and the power conversion efficiencies(PCE) of single-junction OSCs and tandem BHJ OSCs have reached over 10% and 11%, respectively. So far the conjugated donor-acceptor(D-A) polymers have been the main-stream donor materials for OSCs. The design and synthesis of donor materials with proper frontier orbital energy levels to match those of acceptor materials, generally, fullerene derivatives(PC71BM or PC61BM) is important to approach the high performance organic solar cells.The first part of our work is the design, synthesis and characterization of two D-A type donor polymers P1 and P2 based on 5,6-difluoro-benzo[1,2,5]thiadiazole. Thiophene and selenophene have been introduced into P1 and P2 as donor units, respectively. OSCs with thiophene-based polymer P1 as the donor material demonstrated a PCE up to 5.48% without DIO. Although selenophene-based polymer P2 had the similar frontier orbital energy levels with those of P1, P2: PC71 BM blend film showed a poor performance compared with P1: PC71 BM blend. After the addition of DIO, the both P1: PC71 BM and P2: PC71 BM blend films demonstrated high PCEs above 6.6%. The great enhancement in P2: PC71 BM blend film could be attributed to the optimized morphology and enhanced hole and electron mobilities of the active layer as revealed by atomic force microscope(AFM) and space-charge-limited current(SCLC) measurements.The second part of our work is the design, synthesis and characterization of three D-A polymers P3, P4 and P5 based on 3-fluoro-thiophene[3,4-b]thiophene-2-carboxylic acid ester. These three polymers have the same conjugated backbone, but side chains are different. While keeping side chain the same length, the number of fluorine atoms attached to side chain decreases gradually in polymer P3,P4 and P5. The three polymers have been applied as the donor materials in polymer solar cells. P3: PC71 BM or P4: PC71 BM blend films showed a poor device performance. P5-based OSCs had a PCE up to 6.05% with Jsc of 11.98 mA/cm2, Voc of 0.85 V, and FF of 59.4%. AFM measurements revealed the nanoscale phase separation and continuous network structure in P5: PC71 BM blend film, which was ascribed to the high PCE.