Design Of Novel Organic Dyes For Dye-Sensitized Solar Cells

Dye sensitizers have been recognized as one of the most important parts for dye-sensitized solar cells (DSSC) to get high performance. Designing highly efficient sensitizers with high molar extinction coefficients and broad absorption spectra for DSSC is an urgent task and important issue.In this work, we design and screen a series of organic dyes with donor-π-acceptor (D-π-A) structure by modifying their three parts of the structure, respectively, based on synthesized typical organic dyes, and further use density functional theory (DFT) and time-dependent DFT (TDDFT) approach to investigate the molecular orbital energy levels, absorption spectra, light harvesting efficiency of these newly designed dyes. We design thirty novel D-π-A structures of BUCT1-BUCT30 by using the screened functional group as donor,π bridge and acceptor. The main contents are summarized as following.1. We investigated the energy levels and the absorption spectra of the experimentally synthesized phenothiazine Dyel-Dye4 with DTF/ TDDFT and revealed the discrepancy for photoelectric energy conversion efficiency (PCE) of the dyes with different acceptor position sensitized DSSC devices. We also found that the planar structure of the electron donor and the electron acceptor is greatly beneficial for photoinduced transfer of electron from the electron donor to the electron acceptor, strong light-harvesting ability and broaden absorption spectra. The finding provides reference for further designing organic dyes.2. Based on the experimentally synthesized dye PO(TA-St-CA), we design and screen a series of triphenylamine derivative dyes with D-π-A structure D1-D4, Pi1-Pi 12 and A1-A7 using different electron donors,π bridges and electron acceptors. Results indicate that the donor group in D2,π bridges in Pi10-Pi12 and the acceptor group in A7 are promising functional groups. By using the screened functional groups as donor, π-bridge and acceptor, we design six novel D-π-A structures of BUCT1-BUCT6. The calculated results indicate that BUCT1-BUCT6 dyes show smaller HOMO-LUMO energy gaps, higher molar extinction coefficients and obvious redshifts compared to experimentally synthesized PO dye. In particular, the newly designed BUCT2 dye exhibits not only 215nm redshifts and higher molar extinction coefficient with increment of 32.4% compared to PO dye, but also the extremely broad absorption spectrum covering entire visible range up to the near-IR region of 1100 nm. Therefore, the BUCT2 dye is a very promising candidate for highly effective DSSCs with near-infrared light harvesting up to 1100 nm. And the electron acceptor of dye with two -CN groups and sulfonic acid group would be likely to become a promising acceptor for the design and synthesis of dye.3. Based on the experimentally synthesized dye d01(D5), we design and screen a series of dyes d02-d06 with different electron donors, such as diphenyl ethylene benzene, phenothiazine and perylene etc. And we further design a series of dyes d07-dl2 by modifying the donor of d06 with-O-CH3,-C4H9 and benzene groups. Results indicate that the donor in dl2 is promising functional electron donor. So, we design six novel D-π-A structures of BUCT7-BUCT12 by using the electron donor in d12 as donor,π bridges in Pi10-Pi12 as π-bridge, dicyanocyclic acid and dicyano vinylsulfonic acid as acceptor. The calculated results indicate that BUCT7-BUCT12 dyes show smaller HOMO-LUMO energy gaps, higher molar extinction coefficients and obvious redshifts compared to experimentally synthesized d01 dye. In particular, the newly designed BUCT8 dye exhibits not only 133 nm redshifts and higher molar extinction coefficient with increment of 74.1% compared to d01 dye, but also the extremely broad absorption spectrum covering entire visible range up to the near-IR region of 1000 nm. Interestingly, we find that BUCT8, which almost just the same structure with BUCT7 except the electron acceptor, has not only a redshift of 24 nm but also broader absorption spectra compared to BUCT7. Similar results were observed in the comparisons of BUCT9 and BUCT10, BUCT11 and BUCT12. Therefore, we believe that the dyes with sulfonic acid group as the electron acceptor are superior to the ones with carboxylic acid group.4. Based on the experimentally synthesized dye SH-6, we design 54 kinds of dyes with different monocyclic ring (such as thiophene, furan and pyridine etc.), bicyclic ring (such as EDOT and bithiophene etc.), tricyclic ring (such as s-DTT etc.) and their combinations as π bridges. By investigating the molecular orbital energy levels, absorption spectra, light harvesting efficiency of these dyes, we screened a series of excellent performance π bridges such as the π bridge in Pd2, Pd4, Pel, Pe2 and Pe3. By exploring the combined functional groups as π bridge and dicyano vinylsulfonic acid as acceptor, we design eighteen novel D-π-A structures of BUCT13-BUCT30. The calculated results indicate that BUCT13-BUCT30 dyes show smaller HOMO-LUMO energy gaps, higher molar extinction coefficients and obvious redshifts compared to experimentally synthesized SH-6 dye, and the best dyes are F13 and F18 among the screened dyes F1-F35. In particular, the maximum absorption peaks of eight kinds of dyes are greater than 650 nm among the newly designed dyes. BUCT27, has the maximum molar extinction coefficient, exhibits 234 nm redshifts and higher molar extinction coefficient with increment of about 80% compared to SH-6 dye. BUCT19, the maximum absorption peak is 705 nm, exhibits not only 269 nm redshifts and higher molar extinction coefficient with increment of about 50% compared to SH-6 dye, but also the extremely broad absorption spectrum covering entire visible range up to the near-IR region of 1250 nm.In general, we design a series of D-π-A structure organic dyes with high molar extinction coefficients and broad absorption spectra by DFT /TDDFT. It is expected that this work can provide a new strategy for the design of new dyes for highly effective DSSC, and can also trigger experimental scientists to synthesize the new dyes and further confirm the excellent performance of this type dye-sensitized device.