Fundamentals of Solar Cell Design. Rajender Boddula

Figure 1.16 Fused five membered ring - thienothiophene dicyanorhodanine.

Yuvraj Patil et al. designed and synthesized two low band gap small non-fullerene acceptor molecules DPP7 and DPP8 (Figure 1.17) and investigated their solar cell parameters [14]. Diketopyrrolopyrrole and tetracyano-diene fragments acted as acceptor and carbazole moiety represents as donor part leading to D-A-D type architecture. Polymer was employed as donor material in the solar cell fabrications. The complimentary absorption of polymer-donor gives wide absorption for the blend. The configuration of the solar cell fabricated was: ITO/PEDOT:PSS/P: DPP7 or DPP8/PFN/Al. PV parameters obtained for DDP7 was 4.86% efficiency and for DPP8 was 7.19% efficiency. It was informed that the superior performance of DPP8 was due to its structure, which contributed mainly to the morphology of the film as well as to the betterment of the charge mobility.

Figure 1.17 DPP-thiophene-tetracyanobutadiene-carbazole hybrid.

Jia Sun et al. synthesized two ultralow band gap small non-fullerene acceptor molecules, INPIC and INPIC-4F (Figure 1.18) [15]. These are interesting molecules in terms of design that there are contiguously nine rings fused together to form donor part of the molecule and flanked on either side by electron withdrawing groups like dicyano-indenone (INPIC) and dicyano-difluro-indenone, making them as A-D-A–type architecture. INPIC and INPIC-4F exhibited absorptions in 600- to 900-nm region, thereby complementing with the PBDB-T polymer acting as donor, and further the blend of donor-acceptor absorption encompassed 350 to 900 nm. Solar cell devices were fabricated according to the given configuration: ITO/ZnO/active layer/MoO3/Ag and the PV parameters were as follows: INSPIC-4F displayed impressive parameters Voc = 0.85V, Jsc = 21.6 mA, FF = 71.5%, PCE = 13.13%, whereas INSPIC showed only 4.31% efficiency. The difference between the two molecules INSPIC and INSPIC-4F was “fluoro” substitution and the same was reflected in solar cell efficiency improving from 4.31% to 13.13%. INSPIC-4F/PBDB-T blend morphology exhibited well defined texture, high charge mobility, and improved light absorption property, which were cited in favor of excellent efficiency shown by the INSPIC-4F.

Figure 1.18 Fused nine membered ring - fluorodicyanoindenone.

Zhuping Fei et al. synthesized low band gap non-fullerene acceptor small molecule, C8-ITIC (Figure 1.19) [16]. Acceptor molecule has seven contiguously fused rings flanked on either side by indacenodicyano electron withdrawing group and carries four n-octyl alkylchains. Authors employed two donors like (i) PBDB-T polymer and (ii) PFBDB-T polymer. Solar cell devices were fabricated by adopting given configuration: ITO/In₂O₂/ZnO/active layer/MoO₃/Ag. Impressive PV parameters were obtained with conversion efficiency of 13.2% using C8-ITIC and PFBDB-T blend. The energy loss noted in the solar cell device is less than 0.56 eV. Non-fluorinated polymer PBDB-T with C8_ITIC blend recorded lower efficiency. Authors claim that polymer backbone selective fluorination is another important factor to achieve higher conversion efficiencies in organic solar cell devices.

Jianfei Qu et al. synthesized four A-D-A–type non-fullerene small molecule [17] acceptors. Alkyl groups C₂, C₄, C₆, and C₈ were selected and were attached to the rhodanine end group having ring Nitrogen (Figure 1.20). Alkyl groups attached did not have much effect on their absorption properties. But these alkyl groups played a role on the film properties like, crystalinity, molecular packing, manifesting on the PV parameters. PBDB-T polymer was chosen as donor along with one of the acceptors synthesized as a blend material. PV measurements were determined with an inverted device structure: ITO/ZnO/active layer/MoO₃/Ag. PBDB-T polymer donor with C6 small-molecule acceptor blend gave an excellent efficiency of 8.26% PCE. Furthermore, introducing thermal annealing with iodooctane solvent improved the efficiency to 9.29%. Other PV parameters observed were also good: Voc = 0.89V; Jsc = 15.80 mA/cm²; and FF = 58.12%. Investigations reported in this work indicated that effect of alkyl chain length has a pivotal role in tuning the PV parameters, in making suitable films.

Figure 1.19 Fused seven membered ring - dicyanoindenone.

Kaili Wang et al. synthesized calamatic shaped A-D-A–type nonfullerene small-molecule acceptors, CPDT-4Cl and CPDT-4F (Figure 1.21), by varying chloro and fluoro substituents [18]. CPDT-4Cl and CPDT-4F exhibited absorptions extending in to 900-nm region. PBDB-T was employed as a polymer donor and its light absorption has complementarity with the two CPDT-4Cl and CPDT-4F acceptor molecules and the blend absorption of these (PBDB-T and CPDT-4Cl, and PBDB-T and CPDT-4F) cover 400- to 980-nm region. PV parameters were evaluated by adopting conventional device structure like: ITO/PEDOT-PSS/[PBDBT+Acceptor]/Phen-NaDPO/Ag having 9.47% efficiency for CPDT-4Cl and 9.26% efficiency for CPDT-4F, respectively. Other parameter like Jsc was found to be impressive like 21.3 mA/cm² for [CPDT-4Cl + PBDB-T] blend and 20.1 mA/cm² for [CPDT-4F + PBDB-T] blend. Authors express that the non-fullerene–type acceptors with NIR absorption have great scope to improve the organic solar cell efficiency.

Figure 1.20 Fused seven membered ring acceptors with variation in N-alkyl chain length.

Figure 1.21 Calamatic shaped non-fullerene small-molecule acceptors.

Eun Yi Ko et al. synthesized small acceptor molecules [19] containing dicyanovinylene (DCV₂) and tricycanovinylene (TCV₂) groups (Figure 1.22) as strong electron accepting moieties. Material properties were determined for IDT(DCV)₂, IDT(TCV)₂, and IDTT(TCV)₂ (Figure 1.22) and PTB7-Th polymer used as donor to evaluate PV parameters with an inverted cell structure like ITO/Zno/PTB7-Th+Small Molecule/MoO₃/ Ag. Reasonably good efficiency (2.8% to ~4%) was observed for all the prepared small acceptor molecules. Interestingly, these fabricated devices exhibited relatively high Jsc values as 11.02 to 11.98 mA/cm². Thus, fabricated devices were stored in dark without encapsulation for about 1000 h and the device stability was monitored by recording absorption spectrum. The devices were found to be stable to oxygen, moisture, and carbondioxide for over a period of 1,000 hours indicating excellent shelf stability.

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