Fundamentals of Solar Cell Design. Rajender Boddula

Figure 1.22 Dicyano and tricyano vinylene–based non-fullerene small-molecule acceptors.

Yamin Zhang et al. synthesized [20] non-fullerene acceptor smallmolecule F-2Cl by chlorination of parent molecule (Figure 1.23). PBDB-T polymer donor was difluorinated to make PM6 with changed HOMO and LUMO values. F-2Cl has absorption covering the range of 500 to 800 nm and PM6 has absorption covering the region of 400 to 680 nm with complementarity covering wide absorption range. The conventional solar device structure using F-2Cl as acceptor and PM6 as donor with a solar cell film thickness of 103 nm provided very good efficiency of 12.59% PCE with Voc of 0.94 V; Jsc of 17.96 mA/cm²; and FF of 77%. The total solar cell film thickness was changed to 600 nm by improving the active blend layer to get solar cell parameters like, Voc of 0.879 V; Jsc of 19.61 mA/cm²; FF of 58%; and solar cell efficiency of 10.05%. Authors mentioned that there are some changes leading to decrease the solar cell parameters, but the efficiency of 10.05% is remarkable considering the active layer blend thickness of 600 nm. Authors explained that the morphology of the thick film (600 nm thickness) played key role in the observed efficiency.

Figure 1.23 Dichloro-dicyano-indocinyl based small-molecule acceptors.

Yanbo Wang et al. synthesized [21] seven rings fused contiguously with either sides carrying halo-dicyanoindacenyl group (Figure 1.24) compounds F-H, F-F, F-Cl, and F-Br as non-fullerene small-molecule acceptors. PBDB-T polymer was used as donor in these investigations. A change in energy levels (HOMO and LUMO; Figure 1.24) of three compounds (F-F, F-Cl, and F-Br) carrying halogen was evident upon substituting hydrogen with halogen. All the molecules exhibited strong absorption in the region 550 to 700 nm. Solar cell device structure adopted was ITO/POEDOT-PSS/PBDB-T + Acceptor/PDINDO/Al. The trend of efficiency, 9.59% for F-H, 10.85 for F-F, 11.47 for F-Cl, and 12.05 for F-Br, indicated that halogen substitution improved efficiency of fabricated solar cell. Tuning the light absorption, crystallinity of film and mobilities of non-fullerene acceptors may be contributing factors for improving the performance of fabricated solar cells. Reported investigations inform that design of halogenation strategy on non-fullerene small-molecule acceptors has a role to play in future research.

Andrew Wadsworth et al. [22] synthesized two non-fullerene A-D-A– type small-molecule acceptors with contiguously fused rings (Donor) with attached BTD andrhodanine (O-IDTBR) or dicyanovinylelene (O-IDTBCN)groups (Figure 1.25) on both sides, to understand the role of end groups in tuning the organic solar cell photo voltaic parameters. Deeper lying energy levels are observed for O-IDTBCN carrying strong electron withdrawing dicyano vinylelene group. PTB7-Th low band gap polymer was employed as donor in solar cell fabrications. Overlay of absorption spectra of O-IDTBR, O-IDTBCN and PTB7-Th indicates that a coverage of 400- to 850-nm region. PTB7-Th blended with O-IDTBR or O-IDTBCN were used for the fabrication of inverted OBHJS Cells with an architecture: ITO/ZnO/PTB7-Th+O-IDTBR or O-IDTBCN/MoO₃/Ag. PCE recorded were found to be 9.5% for O-IDTBR and 10.5% for O-IDTBCN, furthermore other photo voltaic parameters were also improved.

Figure 1.24 Halo-dicyanoindacenyl derivatives.

Figure 1.25 Contiguously fused five rings having attached BTD and rhodanine or dicyanovinylelene groups.

      Improved charge separation and collection in [PTB7-Th + O-IDTBCN] blend was explained based on the average charge carrier mobility and lifetime data generated.

Jianfei Qu et al. designed [23] A-D-A–type non-fullerene small-molecule acceptors, ITIC-2Br-γ and ITIC-2Br-m (Figure 1.26), attached with bromine on either side of dicyano-indocinyl group. ITIC-2Br-γ has bromine attached at specific position on the dicyano-indocene whereas the position of bromine attached in ITIC-2Br-m is not specified. ITIC-2Br-γ displayed higher absorption property compared to other bromo compound ITIC-2Br-m. Solid state crystal structure of ITIC-2Br-γ revealed that it induced stronger π-π interactions due to “O” -- “S” and “Br” – “S” proximity. PBDB-T-2F polymer was used as donor with ITIC-2Br-γ or ITIC-2Br-m for making blend material to determine photo voltaic parameters by fabricating cell with inverted configuration: ITO/ZnO/ PBDB-T-2F:acceptor/MoO3/Ag. PBDB-T-2F donor polymer blend with ITIC-2Br-γ acceptor provided very good conversion efficiency like 12.05%. The other combination PBDB-T-2F with ITIC-2Br-m showed lesser conversion like 10.88%. Authors advocate that position of bromine attachment changes the molecular moment influencing film morphology leading to better conversion numbers, indeed which is a “supra-molecular chemistry” concept.

Figure 1.26 Fused seven membered ring with bromovinyldicyanoindenones.

1.7 Recent Research Advances in All Small-Molecule Acceptor and Donor Types

Polymers acting as electron donor materials are used in solar cell fabrications. Indeed, the above section is completely on the same subject. In fact, these polymer donor materials are doing fine for BHJOSCs and the solar cell efficiency reached over 17%. Compared to polymers, smallmolecule donors and acceptors have some definitive advantages or unique merits like: clarity in chemical structure, defined molecular weight, easy purification of small molecule, excellent batch to batch repeatability, easy to synthesize them, low cost preparation, tunable optical, thermal and electrochemical properties, and solubility character to an extent to control the film morphology, as well to establish role of chemical structure in solar cell device performance. These merits made many researchers to work on the functioning of all small-molecule BHJ organic solar cells. Present scenario reveals that all small-molecule BHJOSCs performed very well to show over 14% efficiency. The present part describes the achievements and progress associated with the all small-molecule BHJOSCs.

Liyan Yang et al. reported a combination of non-fullerene wide band gap donor (DRTB-T) and non-fullerene low band gap acceptor (IC-C6IDT-IC) as shown in Figure 1.27, leading to a PCE of 9.08% [24]. Wide band gap donor DRTB-T was designed and synthesized, and material properties were evaluated. Absorption spectrum of donor DRTB-T and acceptor IC-C61DT-IC indicated their complementarity with a coverage of ~90% of the solar spectrum in the region of 300 to 900 nm. Solar cell device fabricated structure was: ITO/MoO3/DRTB-T:IC-C6IDT-IC/Al. Solar cell structure was optimized in terms of it thickness, film morphology, donor/acceptor ratio, and other aspects to get better efficiency. Indeed, it was reported 9.08% PCE with Voc of 0.98V, Jsc of 14.25 mA, and FF of 65%. Authors claim that the efficiency produced is the highest ever