Читать книгу Fundamentals of Solar Cell Design - Rajender Boddula - Страница 22

Research on BHJ organic solar cells resulting from fullerene as acceptor is dealt at length for the past several years. During the course of investigations by many researchers, it was found that fullerene has limitations in using it as acceptor in solar cell devices. The shortcomings or limitations with properties of fullerene are like negligible light absorption in NIR and visible region, shape of the molecule, solubility, controlling the film texture or film morphology, stability of fullerene, and very limited tunability of energy levels of fullerene. These limitations lead the researchers to explore novel materials to initiate work on non-fullerene derived acceptors for BHJ organic solar cells to replace fullerene and take the system to improve efficiency of organic solar cells. The following part describes BHJOSCs involving non-fullerene acceptors with polymer donors.

Dongxue Liu et al. reported two low band gap acceptor materials where in, linearly fused 6 five membered rings having four “Sulfur” atoms in the ring structure acted as donor central core moiety and the remaining part as acceptor at both sides (Figure 1.4) [1]. Thiophene and selenophene moieties are embedded as π-spacers to improve conjugation and also for intra-molecular charge transfer (ICT) leading to red shift in the absorption spectrum. But this thiophene or selenophene insertion did not affect the HOMO-LUMO values of the small acceptor molecules, 4TO-T-4F and 4TO-Se-4F. Two end groups with electron withdrawing nature, 2-(5,6-difluoro-3-oxo-2-2,3-dihydro-1H-inden-1-ylidene)malononitrile, helped in enhancing ICT. The optical band gaps estimated for 4TO-T-4F and 4TO-Se-4F were 1.3 and 1.27 eV, respectively. Optical, thermal, and electrochemical properties determined were found to be suitable as non-fullerene acceptors. Polymer PTB7-Th was used as donor, and the other two, 4TO-T-4F and 4TO-Se-4F, acted as non-fullerene smallmolecule acceptors in organic solar cell fabrications and the configuration of the cell was: IndiumTinOxide/ZnO/[Donor-PTB7-Th:Acceptor]blend/ MoO3/Ag. Solar cell performance was determined as 8.7% (PCE) for 4TO-T-4F with loss of energy of 0.55 eV and 7.4% (PCE) for 4TO-Se-4F with loss of energy of 0.57 eV.

Figure 1.4 Vinylidine dicyano–difluoroindanone compounds.

Benzothiadiazole central ring fused with pyrrole and bithiophene rings on both sides (seven rings contiguously fused together) and carrying electron withdrawing end groups 2-(5,6-difluoro-3-oxo-2-2,3-dihydro-1H-inden-1-ylidene)malononitrileand2-(5,6-dichloro-3-oxo-2-2,3-dihydro-1H-inden-1-ylidene)malononitrile was the main core designed and developed (Figure 1.5) by Yong Cui et al. for organic solar cells as low band gap organic materials [2]. Various techniques like UV-Vis-NIR absorption, fluorescence, thermal, electrochemical, AFM, TEM, and GIWAXS characterizations (morphology of films) as well as computational, X-ray diffraction (molecular packing), charge carrier mobility, TPV measurements, and highly sensitive EQE and EQEEL measurements were applied to generate the data. OPV fabrications were conducted using PBDB-TF as donor and BTP-4F and BTP-4Cl as non-fullerene acceptors. OPV cells were fabricated with an inverted configuration of Indium Tin Oxide/ZnO nano-particles/[donor polymer PBDB-TF:BTP-4X] blend/MoO3/Al, where PBDB-TF polymer was selected as electron donor material. Photo-conversion efficiency determined from solar cell fabrications was 15.6% for BTP-4F and 16.5% for BTP-4Cl with 0.834 and 0.867 V, respectively. The high PCE was attributed to the chlorine effect by reducing the non-radiative energy (~0.26 eV) loss. Further, it is expected that fine tuning the low band gap material properties has a great potential to achieve higher conversion efficiencies.

Figure 1.5 Fused benzo-thiadiazole–dicyanoindenone compounds.

Bin Kan et al. described the synthesis of small-molecule donor NCBDT-4Cl (ADA type) with ~1.40 eV optical band gap and used it for solar cell fabrication along with polymer (non-fullerene) acceptor PBDB-T-SF (Figure 1.6) [3]. The NCBDT-4Cl (ADA type) donor and acceptor PBDB-T-SF blend provided 300- to 900-nm range absorption with very good molar absorption co-efficient. Solution processed solar cells were constructed with the given cell configuration: ITO/polymer PEDOT-PSS/polymer PBDB-T-SF:NCBDT-4Cl/PDINO/Al—where in PDINO acted as electron transport layer. Fabricated device as casted (without any treatment) produced efficiency of PC 13.1%. The same device after annealing and solvent addition, gave a result of PCE 14.1% along with Voc of 0.85V, Jsc of 22.35 mA cm⁻² and accompanied by loss of energy of 0.55 eV. Authors attributed the excellent display of 14.1% PC efficiency to the blend morphology as well as to the excellent charge transport property due to attached four chlorine atoms in NCBDT-4Cl (ADA type) donor at appropriate places.

Figure 1.6 Fused seven rings with dicyanoindenone end groups.

Contiguously fused five rings as central core moiety linked with thiophene spacer either side and further attached with electron withdrawing indenone group (IEIC and IEICO) small molecules were synthesized by Huifeng Yao et al. (Figure 1.7) as low band gap acceptor materials [4]. Light absorption for IEIC and IEICO extended in to NIR (near infrared) region 600 to 900 nm and the optical band gap determined to be 1.5 eV for IEIC and 1.34 eV for IEICO. The optical band gap difference between that of IEIC and IEICO was attributed to the “alkoxy group” present in IEICO. Device configuration developed after optimization was: ITO/PEDOT: PSS/PBDTTT-E-T:IEICO or IEIC/PFN-Br/Al. Polymer PEDOT:PSS and PFN-Br were used as anode and cathode interface layers. Morphology of the casted film was studied by AFM and TEM to get further insight of the device fabricated. The measured PV parameters were PCE for IEICO is 8.4 %; Voc = 0.82 V; and Jsc = 17.7 mA. Furthermore, the PCE was improved to 10.7% via tandem device fabrication. The loss energy was estimated to be 0.5 eV. PV parameters for IEIC were found to be inferior compared to IEICO. Authors claimed that the design leading to low band gap energy level of 1.34 eV, by introducing alkoxy group at suitable place of the small molecule helped to give higher efficiency.

Figure 1.7 Fused ring linked with thiophene-dicyanoindenone.

Huifeng Yao et al. synthesized linearly fused five rings core flanked by thiophene attached electron withdrawing end groups having efficient ICT, leading to an ultralow band gap acceptor material, IEICO-4F (Figure 1.8) [5]. The IEICO-4F optical band gap was found to be 1.24 eV, charge mobility calculated was 1.14 × 10⁻⁴ cm² V⁻¹ s⁻¹ and the absorption of thin solid film extended in to NIR region with λ_max of ~900 nm. PBDTTT-EFT (PTB-7-Th) and J52 (Figure 1.8) were employed as polymer donors for device fabrication based on their favorable absorption properties and also with suitable energy levels. Fabricated device parameters were: Voc = 0.739 V; Jsc = 22.8 mA; FF = 59.4%; and PCE = 10.0% using polymer donor PBDTTT-EFT (PTB-7-Th), J52 polymer donor exhibited a little smaller values compared to PBDTTT-EFT. It was advocated that ultralow band gap materials definitely have a role to play to improve the solar cell efficiencies.

Figure 1.8 Fused ring coupled with fluorodicyanoindenone.

Huifeng Yao et al. came up with novel small molecules, ITCC and ITIC as non-fullerene acceptor materials for organic solar cells to determine PC efficiency (Figure 1.9) [6]. Linearly fused seven rings acted as donor, which was flanked on both sides with acceptors (thienyl fused indanone end groups) and it was designated as ADA type architecture. Thin films were subjected to GIWAXS investigations to understand whether there existed π-π stacking to some extent and facilitated charge mobility in a better way than ITIC (which was reported earlier). Further, it indicated the role of thieno indanone and the superior design. Device architecture formulated was: ITO/PEDOT-PSS/ITCC or ITIC + PBDB-T/PFN-Br/Al. Solar cell PV parameters determined were: Voc = 1.01 V; Jsc = 15.9 mA; FF = 71%; PCE = 11.4% for ITCC and for ITIC: Voc = 0.93; Jsc = 17.0 mA; FF = 67%; PCE = 10.6%. Authors claim that investigations have high relevance, because of 11.4% PCE noted in non-fullerene solar cell device. The results are of promising nature and suggest that one can improve the PCE to higher levels with these non-fullerene–based solar cells.

Figure 1.9 Fused seven membered ring - thienodicyanoindenones.

Yunlong Ma et al. synthesized two novel ladder type low band gap small molecules as non-fullerene acceptors for solar cell fabrication (Figure 1.10) [7]. Both the molecules, DTNIC6 and DTNIC8, were of ladder type because of linearly fused six rings and the two molecules differ in their alkyl groups attached. Both the molecules have strong absorption in 500- to 720-nm region. Film morphology was analyzed using hole and electron mobilities, AFM, TEM, and GIWACS information. PBDB-T polymer as donar and synthesized small molecules as acceptors were used in fabricating solar cell with device architecture as: ITO/TiO2:TOPD/PBDB-T:DTNIC8 or DTNIC6/MoO3/Ag. PV parameters generated were: for DTNIC6: Voc = 0.96 V; Jsc = 7.71 mA; FF = 45.6%; PCE = 3.39%; for DTNIC8: Voc = 0.96 V; Jsc = 12.92 mA; FF = 72.84%; PCE = 9.03%. DTNIC8 (which carried ethylhexyl alkyl chain) exhibited a high photo conversion efficiency compared to DTNIC6 which differed in alkyl chain structure from DTNIC8. The alkyl chain did not influence energy levels and light absorption properties, but exerted sizable effect on the solar cell efficiency. The alkyl chain group effect was believed to have control over film morphology.

Yuze Lin et al. employed (Figure 1.11) medium band gap polymer donor—FTAZ (BG = 2.41 eV) with a non-fullerene low band gap acceptor—IDIC (BG = 1.6 eV) to make solar cells and to understand photo current efficiency [8]. FTAZ and IDIC have complementary absorption to cover 450- to 800-nm region and relatively have high electron and hole mobilities and well-matched energy levels. Single junction solar cell fabrication structure was: ITO/ZnO/FTAZ:IDIC/MoOx/Ag. Solar cell parameters observed were: Voc = 0.840 V; Jsc = 20.8 mA; FF = 71.8%; and PCE = 12.5%, Diiodooctane was used to tune the film morphology in these fabrications. The 12.5% PCE determined for non-fullerene solar cell was very high compared to FTAZ-PCBM blend, which showed only ~6%. Femtosecond transient absorption studies on the casted films indicated the formation of radical cation and radical anion (charged species) and their mobilities. Authors inferred that FTAZ-PCBM combination film provided only ~40% generation of charged species compared to non-fullerene FTAZ-IDIC film combination. Authors claim that non-fullerene blends have superiority over fullerene blends in achieving higher PCE values.

Figure 1.10 Fused six membered ring with dicyanoindenones.

Jie Zhang et al. prepared a small low band gap acceptor molecule-IFTIC (Figure 1.12) for evaluating its solar cell efficiency [9]. IFTIC carried fused bifluorene attached on both sides with thiophene as electron donating central core, with either side holding indenone moiety as electron acceptor. IFTIC showed absorption covering 450 to 700 nm and had suitable energy levels like 5.42 eV HOMO and 3.85 eV LUMO. PTB7-Th polymer was used as donor in these investigations for luminescence quenching (with IFTIC) and as donor material. AFM and TEM techniques were used for monitoring morphology of the casted film. Fabricated device parameters were: Voc = 0.92 V; Jsc = 12.71 mA; FF = 54%; PCE = 6.33%. Authors claim that non-fullerene materials with relatively simple device structure and simple method of preparation of materials make this work attractive.

Figure 1.11 Fused five membered ring with dicyanoindenone.

Oh Kyu Kwon et al. developed a non-fullerene base material PV device which exhibited a good percent of photo current (Figure 1.13). PPDT2FBT, a very well ordered polymer, was used as donor and NIDCS-HO, a small acceptor molecule for building a solar cell device [10]. Interestingly, absorption spectra for polymer donor and small-molecule acceptor displayed complementary absorption, by covering 350- to 700-nm region. Non-fullerene–based conventional single solar cell device adopted structure was given as: ITO/PEDOT:PSS/active layer/Ca/Al. Blend film morphology was investigated systematically using charge mobility data, AFM, TEM, GIWACS, annealing temperature and other techniques. Device PV parameters determined were: Voc = 1.03 V; Jsc = 11.88 mA; FF = 63%; PCE = 7.64%. Investigations indicated complementarity in absorption and suitably placed energy levels along with good film morphology can contribute to the solar cell efficiency.

Figure 1.12 Bifluerene-dicyanoindenone.

Sunsun Li et al. synthesized a series of novel methoxyl-modified dithieno[2,3-d:2ʹ,3ʹ-dʹ]-s-indaceno[1,2-b:5,6-bʹ]dithiophene-based (ITIC based) low band gap small-molecule acceptors, IT-OM-1, IT-OM-2, IT-OM-3, and IT-OM-4 (Figure 1.14), with A-D-A architecture, for the purpose of developing non-fullerene–based organic solar cells [11]. Position of “methoxy” substitutent on terminal group was systematically varied to understand the positional effect of substitution on optical, electrochemical, charge mobility, and more importantly molecular packing of these isomers. Donor molecule used in these investigations was PBDB-T polymer. PBDB-T donor blended with IT-OM acceptor film morphology was thoroughly investigated using AFM, GIWACS, TEM and charge mobility techniques. Devices were fabricated by adopting conventional cell configuration like: ITO/ZnO/active layer/MoO3/Al to evaluate PV parameters and in particular PCE. Additive 1,8-diodooctane was employed in these fabrications and also annealed at 150°C to make the film. The donoracceptor blend exhibited excellent UV-Visible absorption covering 350- to 800-nm region. Among the four isomers synthesized, IT-OM-2 demonstrated excellent PV parameters like: Voc = 0.93 V; Jsc = 17.53 mA; FF = 73%; PC efficiency of 11.9%. Furthermore, the PCE of >10% maintained when the thickness of the solar cell increased to 250 nm for IT-OM-2 blend system. Authors claim that it is possible to modulate intrinsic molecular properties and also bulk film morphology to achieve excellent PCE in solar cells by designing molecules for fullerene free solar cells.

Figure 1.13 Dialkoxybenzene core–based molecule.

Figure 1.14 Fused seven membered ring - methoxydicyanoindenones.

Huanran Feng et al. described the synthesis of a new non-fullerene A-D-A–type low band gap acceptor small-molecule—FDNCTF (Figure 1.15) [12]. Linearly fused seven rings, three five-membered, two thiophene, and two benzene rings, acting as donor and with electron withdrawing end group (NINCN) flanked on either side of the molecule (FDNCTF) was designed. Properties of FDNCTF were (i) UV-visible absorption reaching near infrared with high molar absorption coefficient (~3 × 10⁵ L mol⁻¹), (ii) exhibited higher charge mobility, and (iii) more ordered arrangement of molecules in the film state, besides other properties. PBDB-T polymer having wide band gap acting as donor was employed for fabricating solar cells with FDNCTF. The solar cell devices were fabricated and the photo voltaic parameters were determined by adopting convention device architecture like ITO/PEDOT:PSS/PBDB-T:FDNCTF/PDINO/Al (PDINO is a cathode interlayer). FDNCTF as a small-molecule low band gap acceptor displayed impressive efficiency of 11.2%. PCE along with other parameters like Voc = 0.93V, Jsc = 16.5 mA, and FF = 72.7%. For comparison purpose, they fabricated solar cell device with FDICTF as small-molecule low band gap acceptor with PBDB-T polymer as donor to understand structural aspects of small molecule on the solar cell efficiency. Interestingly, FDICTF with PBDB-T exhibited 10.06% showcasing the better design of FDNCTF in performance. FDNCTF and PBDB-T blend transient absorption studies indicated that charge mobility was very good as correlated with film morphology. These investigations highlighted that the molecular design with larger and conjugated electron withdrawing end groups improved absorption, molecular packing in the film state which, in turn, improved the solar cell efficiency.

Figure 1.15 Fused seven membered ring - dicyanoindenones.

Feng Liu et al. synthesized low band gap small molecule-ATT1as a non-fullerene acceptor, where dicyano-rhodanine group was attached on both sides of ATT1 (Figure 1.16) [13]. PTB7-Th polymer was selected as a donor in these studies. Solar cell device fabrication adopted a conventional procedure like ITO/PEDOT:PSS/PTB7-Th:ATT-1/PFN/Al. PV parameters determined were: Voc = 0.88V, Jsc = 16.18 mA, FF = 68%, and PCE = 9.78% at a film thickness of 100 nm, the efficiency was found to be increased when the film thickness increased to 130 nm—Voc = 0.87V, Jsc = 16.48 mA, FF = 70%, and PCE = 10.07%. But the efficiency decreased to 9.59% at a film thickness of 160 nm. Central core structure of the molecule ATT1 was more planar, had a high molar absorption coefficient, good charge transporting quality and facilitated film forming with good morphology. Authors believe that this design has capabilities to take forward.

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.

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.