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Figure 1.27 Trimeric BDT linked rhodanine.

Ruimin Zhou et al. reported the synthesis of DTBDT based three small molecules [25] for the purpose of using them for all small BHJOSCs (ASM-OSC), where ZR1 acts as donor and IDIC-4Cl Y6 acts as an acceptor (Figure 1.28). Overlay of absorption spectra of donor and acceptors reveal that it encompasses 400- to 950-nm region, providing a big light absorption window. ITO/PEDOT-PSS/ZR1 + IDIC-4Cl or Y6 blend/ Al was the device structure adopted for measuring the PV parameters. ZR1 + Y6 combination blend exhibited excellent PCE of 14.34%, and further, it was certified PCE of 14.1%. It was interesting to note that the fabricated ASM-OSC device with [ZR1 + Y6] showed a high Jsc of 24.34 mA/ cm². TEM and RSoXS data for [ZR1 + Y6] blend film forms a hierarchical morphology, which facilitates charge separate ratio and charge transport and hence the device exhibits over 14.34% efficiency. Authors found that the energy loss also got minimized in these devices due to better charge transport mechanism. EQEL measurements developed for these devices found to be at higher side and this higher EQEL number also indicates loss of energy may be of 0.24 eV. Present system provided a high PCE of 14.34%. All small-molecule BHJOSC further become highly functional with careful design to understand the morphology of the film and relations between small-molecule donor-acceptor interactions in forming the films.

Haiyan Chen et al. developed [26] two liquid crystalline small-molecule donors BTR and BTR-Cl. BTR-Cl was prepared by chlorination (Figure 1.29) of core structure involving benzodithiophene attached with terthiophene and rhodanine end group. The chlorine was attached with thiophene moiety linked to central benzene ring. These new materials carrying alkyl chains acquires higher order liquid crystallinity and thereby providing a favorable film morphology leading to higher photo conversion efficiency of the given ASM-BHJOSC. BTR and BTR-Cl act as donor and Y6 as acceptor for fabrication of devices. The red shifted absorption in the film state, compared to the solution phase, is indicative of intermolecular interactions in the film state. Conventional device structure adopted was: ITO/ PEDOT-PSS/BTR or BTR-Cl + Y6 blend/Phen-NaDPO/Ag. The recorded efficiencies were 10.67% with Jsc 22.25 mA/cm² for BTR and 13.6% with Jsc 24.17 mA/cm² for BTR-Cl. The 13.6% efficiency was certified value also. The red shift in the absorption of film state, liquid crystalline property of the small molecules, charge mobilities coupled with GIWASX information indicates the formation of a very good thin film morphology, which leads to higher efficiency.

Figure 1.28 Dicycanoindacenyl and rhodanine end group small-molecule donor and acceptors.

Haijun Bin et al. [27] developed two small-molecule (H21 and H22) donors based on benzodithiophene and alkylsilyl-thiophenyl moieties (Figure 1.30) for fabricating all small-molecule BHJOSC to evaluate PV parameters. IDIC is another small-molecule employed as acceptor in these investigations. H21 or H22 with IDIC as blend the absorption spectrum covers ~380- to780-nm region. Fabrication of solar cell was conducted to determine PV properties using a conventional configuration like: ITO/ PEDOT-PSS/H21 or H22 + IDIC blend/PDINO/Al and the blend annealed at 130 _oC. H21 showed PCE of 7.62% and H22 exhibited higher efficiency at 10.29%. Silyl group and electron withdrawing end group of H21 and H22 played a role in enhancing the all small-molecule BHJOSC’s efficiency. Hole and electron charge transporting properties evaluated indicate that H22 has higher values compared to H21 and hence a higher %PCE was displayed by H22. Present investigations indicate that all small-molecule– based BHJOSCs have several advantages compared to the fullerene-based BHJOSCs.

Figure 1.29 Benzodithiophene based (BTR and BTR-Cl) small-molecule donors.

Figure 1.30 Small-molecule donors from benzodithiophene and alkylsilyl-thienyl–based conjugated side chains.

Beibei Oiu et al. [28] developed two benzodithiophene based small molecules (SM1 and SM2) as donors (Figure 1.31). SM1 has cyanoester as electron withdrawing end group and SM2 carries only ester as electron withdrawing end group. SM1 -- IDIC and SM2 -- IDIC blends exhibited light absorption covering ~350- to 780-nm region. All small-molecule BHJOSCs were fabricated by adopting a simple conventional device structure like: ITO/PEDOT:PSS/SM1 or SM2 + IDIC/PDINO/Al with thermal annealing at 115°C and the PV parameters were determined. SM1 as a donor molecule displayed higher power conversion efficiency (PCE) of 10.11% with Voc 0.905V, Jsc of 15.18 mA/cm², and a FF of 73.55%, whereas the SM2 small molecule showed only 5.32% of PCE. The big difference in SM1 and SM2 molecules efficiencies was attributed to the cyano-ester electron withdrawing end group present in SM1. Morphology of the film was deduced using Photo-induced Force Microscopy (PiFM), a new technique. The charge moblities deduced indicate that SM1 has higher charge mobility compared to SM2.

Figure 1.31 Benzodithiophene based small-molecule donors with electron withdrawing end groups-ester and cyanoester.

Huan Li et al. reported A-D-A–type [29] small molecule of benzodithiophene-based donor type (NDTSR) and used (Figure 1.32) two small-molecule acceptors (IDIC and ITIC) to fabricate All small-molecule BHJOSCs to evaluate PV properties and in particular the efficiencies. NDTSR absorption is complimented by both IDIC and ITIC leading to the blend with an absorption encompassing ~350 to 780 nm. Solar cell was fabricated by adopting cell architecture as ITO/PEDOT-PSS/Active layer/Ca/Al. PV parameters were determined for NDTSR-ITIC blend, with PCE of 1.77% and NDTSR-IDIC blend, with PCE of 8.05%. Charge mobility was found to be higher for NDTSR-IDIC blend compared to NDTSR-ITIC blend. The variation in the PCE values between the two systems was attributed to the difference in the film morphology. Authors state that small-molecule donors or acceptors requires in improving the efficiency of solar cells.

Figure 1.32 Trithieno BDT with rhodenone.

Yong

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