Fundamentals of Solar Cell Design. Rajender Boddula
Читать онлайн книгу.
Huo et al. in their research paper [30] discussed the synthesis of two small-molecule donors (DRBDT-TVT and DRBDT-STVT) (Figure 1.33). These two small molecules have BDT central core linked on either side with three thiophene units carrying rhodanine terminal groups, attached to central core DBT with two thiophenes linked with E geometry double bond and differ in carrying S-alkyl group. IDIC small molecule was used as acceptor in these all small-molecule BHJOSC studies to generate PV parameters and to understand the role of structure in improving the efficiency of solar cell. Blend of IDIC with DRBDT-TVT or DRBDT-STVT displayed light absorption in ~350- to 780-nm region. Conventional device with architecture-ITO/PEDOT:PSS/DRBDT-TVT or DRBDT-STVT + IDIC/PDIN/Al was adopted to measure the solar cell parameters. Both the molecules exhibited decent efficiency like PCE of 6.63% for DRBDT-TVT and 6.51% for DRBDT-STVT. These results were compared with PC71BM acceptor based solar cell parameters to evaluate the advantages of all small-molecule BHJOSC.
Figure 1.33 BDT linearly linked trithiophene derivatives.
Yunchuang Wang et al. reported [31] the synthesis of three nonfullerene small acceptor molecules, IDIC8-Me, IDIC8-H, IDIC8-F (Figure 1.34), and a donor small molecule—DRCN5T. Indoceneodithiophene unit is the core with vinyledene dicyano moiety as end group in these three acceptor molecules. The three acceptor small molecules, IDIC8-Me, IDIC8-H, and IDIC8-F, differ in their group substitution leading to small changes in their HOMO-LUMO energy levels and also matched with donor small-molecule DRCN5T energy levels. Light absorption of blends prepared falls in to the region of ~350 to 750 nm. A conventional and simple solar cell architecture was adopted as: ITO/PEDOT:PSS/DRCN5T + IDIC8-Me or IDIC8-H or IDIC8-F/PDINO/Al, and the PV parameters were determined. PC Efficiency of 6.31% for IDIC8-Me, 8.00% for IDIC-H and 8.42% for IDIC8-F were observed. The fluorine substitution effected the change in the HOMO-LUMO energy levels compared to the methyl derivative synthesized and this could be rationale behind the 8.42% efficiency observed in these studies. Authors point out that linearly linked thiophene moiety with suitable end groups will improve the efficiency.
Figure 1.34 Linearly linked pentathiophene with vinyldicyanoindenones.
Haijun Bin et al. described [32] the synthesis of two small-molecule donors (H11 and H12: Figure 1.35) with core structure BDTT flanked by thiophene-fluorobenzotriazole which was attached both sides with thiophene-vinylenecyanoester group as electron withdrawing group. IDIC was used as small-molecule acceptor. Light absorption spectrum of these donors and acceptors found to display complementarity and covered a wide range of absorption. Solar cells were fabricated with a conventional device structure of ITO/PEDOT-PSS/Blend of IDIC + H11 or H12/ PDINO/Al with and without annealing at 120°C. PCE observed for H11 is 9.73% and for H12 5.51%, under these fabrication conditions. Low lying HOMO energy level, higher charge mobility, and more orderly nature of film formation were the reasons informed for the higher efficiency found for the H11. Authors advocate that BDTT linked with benzotriazole moiety is new scaffold with decent efficiency of 9.73% and has the potential to improve the PCE involving further design of small molecules.
Figure 1.35 BDTT core linked benzotriazole derivatives.
Xiafei Cheng et al. synthesized [33] A-D-A–type conjugated four small donor molecules (Figure 1.36) differing in substitution: DRTT-T (alkylthiophene substituent); DRTT-R (ethylhexyl substituent); DRTT-OR (alkoxythiophene substituent); and DRTT (no substituent). Rhodanine group was attached to BDTT, terminally on either side with the central core thienothiophene moiety. Density Functional Theory informed that DRTT-OR and DRTT mould in to almost planar conformation, while DRTT-T and DRTT-R moulded in to twisted conformation due to the introduction of bulky substituents on TT units. These molecules were found to be soluble green solvents. F-2Cl was selected as small acceptor molecule to blend with the above four donor small molecules. Active blend prepared from F-2Cl and donor molecule provided light absorption covering ~350- to 780-nm region. Simple or conventional device architecture adopted for these molecules as ITO/PEDOT-PSS/Blend of F-2Cl + Donor/PDINDO/ Al, without annealing and with annealing at ~120°C to measure the photo voltaic parameters. DRTT-T exhibited decent efficiency like 9.37%, Voc = 0.95V; Jsc = 15.72 mA/cm2; FF = 62.8%, whereas DRTT-R displayed 10.45% efficiency, Voc = 1.00 V; Jsc = 16.82 mA/cm2; FF = 62.6% using THF solvent. Chloroform as solvent also afforded good results for the same compounds. The other two planar small-molecule donors, DRTT-OR and DRTT, showed satisfactory efficiencies. Twisted nature of the molecules, charge mobility, and film morphology factors are influential in displaying higher efficiencies.
Xinxin Li et al. explained in their paper [34] the synthesis of A-D-A– type small donor molecule (P2TBR; Figure 1.37) for the purpose of fabricating all small-molecule BHJOSCs using IDIC as a small acceptor molecule. P2TBR was a non-fused p-dialkoxybenzene at center core with linearly attached thiophene and then BDTT and rhodanine terminal at both sides of center core. P2TBR and IDIC showed complementarity of absorption in solution phase absorption studies. P2TBR and IDIC blend film, after SV annealing, displayed light absorption in the range ~350 to 750 nm. It was informed that solvent vapor annealing improved intermolecular interactions between donor and acceptor molecules. All small-molecule BHJOSC were fabricated using P2TBR donor and IDIC as acceptor blend, adopting a simple and conventional architecture like: ITO/PEDOT-PSS/P2TBR + IDIC ble3nd/ZnO/Al to determine PV parameters. Excellent efficiency of 11.5% was observed for the device fabricated (as given above) along with Voc = 0.94; Jsc = 17.5 mA/cm2; and FF = 70.5. Authors claim that non-fused, linearly linked, smallmolecule donor with p-dialkoxybenzene core structure has potential to further achieve higher efficiencies.
Figure 1.36 Thienothiophene with BDTT Core linked with rhodanine.
Figure 1.37 Dialkoxybenzene linked BDTT with rhodanine end group.
Zuojia Li et al. synthesized [35] two small acceptor molecules differing in their fluorine substitution (Figure 1.38), to understand the fluorine effect on the PV parameters of all small-molecule BHJOSCs. IDIC has five rings fused continuously with vinylilenedicyano indacene (IDIC) or vinylilenedicyano tetrafluoro indocene (IDIC-4F) end groups at both ends. DFDT(DPP)2 was selected as donor which contains four thiophene units linked linearly with 2-Fluorines attached to each thiophene in the middle of the molecule, both