Читать книгу Fundamentals of Solar Cell Design - Rajender Boddula - Страница 23

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 reported in non-fullerene all small-molecule solar cells.

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 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 sides carrying diketopyrrolopyrrole with thiophene moiety. Acceptor and donor exhibited complementarity in their light absorption spectra. The donor-acceptor blend absorption starts at ~350 nm and ends at ~780 nm. Inverted solar cells were fabricated with configuration of: ITO/Zno/Blend of DFDT(DPP)₂ + IDIC or DFDT(DPP)₂ + IDC-4F/MoO₃/Ag, to generate PV parameters. DFDT(DPP)₂ + IDIC-4F Blend displayed PCE of 9.43% with Voc = 0.86 V; Jsc = 16.83 mA/cm²; FF = 65%. Authors inform that donor acceptor interaction in the blend leads to good crystallinity as well as improved morphology, and these are also the factors responsible for the improvement of efficiency.

Figure 1.38 Tetrathiophene linked with DPP.

Huan Li et al. synthesized [36] a new A-D-A–type small donor NDTSR molecule (Figure 1.39). NDTSR has naphthalene fused with four thiophenes is the central core, and further, it is attached to three thiophene units on both sides and with rhodanine as terminal group at both ends. IDIC and ITIC (Figure 1.39) were taken as small-molecule acceptors. Solution phase light absorption for donor and acceptors indicate the complementarity in their light absorption. NDTSR with acceptor blend exhibited light absorption covering ~350- to 780-nm region. Conventional and simple configuration was adopted for fabricating the solar cell as: ITO/ PEDOT-PSS/Blend NDTSR + IDIC or NDTSR + ITIC/Ca/Al and the PV parameters were determined. The NDSTR + IDIC blend provided poor efficiency of 1.71%, whereas NDTSR + ITIC blend gave very good result by showing 8.05% efficiency. The big difference observed in using IDIC and ITIC was attributed to the donor-acceptor molecular interactions leading to the formulation of ordered film, which could facilitate the charge mobility/migration effectively.

Figure 1.39 Fused NDTSR with rhodanine end group.

Sachin Badgujar et al. prepared [37] two small molecules, one as donor—BDT3TR, and another as acceptor—O-IDTBR (Figure 1.40), for the purpose of studying their all small-molecule solar cell efficiency. Donor and acceptor were selected because of their complementarity in their light absorption spectrum. Blend of donor-BDT3TR and acceptor–OIDTBR absorption occurred in ~350- to ~650-nm region. Solar cell device structure adopted was: ITO/PEDOT-PSS/Blend of BDT3TR + O-IDTBR/ ZnO/CPE/Al for measuring the PV parameters. An efficiency 6.96 % was recorded with other parameters as, Voc = 1.06 V; Jsc = 12.10 mA/cm²; FF = 55%. Authors mentioned that complimentary light absorption by donor and acceptor and high lying HOMO level of O-IDTBR could be the reasons behind the higher efficiency observed in these investigations. Further, they advocated that this was the first all small-molecule BHJOSCs.

Figure 1.40 Linear BDTT linked trithiophene with rhodanine end group.

Jisu Hong et al. reported [38] the synthesis of three small-molecule acceptors (Figure 1.41). These were two naphthalenediimides linked to thiophene - NDICN-T: i) NDICN-T linked to bithiophene—NDICN-BT and ii) NDICN-T linked to (E)-1,2-di(thiophene-2-yl)ethane—NDICNTVT. The small-molecule donor employed was DTS-F. UV-visible light absorption and photoluminescence spectra were recorded for three acceptors and one donor and were found to have considerable overlapping. PV properties were obtained by fabricating conventional or simple solar cell architecture like: ITO/PEDOT-PSS/Blend of DTS-F with acceptor molecule/LiF/Al. The blend DTS-F with NDICN-TVT gave satisfactory efficiency of 3.01. Furthermore, to probe the solar cell fabrications and efficiency, authors evaluated film morphology, femtosecond transient absorption on films, and charge transport dynamics. These investigations inform that NDI (Naphthalene Diimide) derivatives can be probed as lead molecules in further studies.

Figure 1.41 NDIs linked with spacer.