High Bandgap (1.9 eV) Polymer with Over 8% Efficiency in Bulk Heterojunction Solar Cells

Abstract

(PBDT-FTAZ), [ 9 ] thieno [3,4-c ]pyrrole-4,6-dione (PBDT-TPD) [ 10 ] and 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c :4,5-c ′] dithiophene-4,8-dione (PDBT-T1) [ 11 ] showed PCEs not more than 8% in conventional single junction solar cells. Up to now, there are only few high bandgap polymers with PCE over 8% were reported ( Table 1 ). [ 2e , 12 ] In order to further improve the performances of tandem solar cells, high bandgap (>1.9 eV) conjugated polymers with low-lying highest occupied molecular orbitals (HOMOs) amenable to larger open-circuit voltages ( V oc ), and comparably high PCEs (over 8%) in BHJ devices are required. [ 4 ] Recently, the high bandgap copolymer PBDT-FTAZ which consists of alternating units of benzodithiophene (BDT) and fl uorinated benzotriazole (FTAZ) has attracted considerable interest due to the high PCE of 7% and fi ll factor (FF) of over 0.7 in BHJ devices with thick active layers. [ 9 ] It will be very interesting to further improve the photovoltaic performance of this polymer via fi ne-tuning its chemical structure.Donor polymers based on 2D-conjugated BDT units substituted with alkylthiophene side chains have been shown to exhibit high performance with excellent thermal stability, hole mobility, and spectral coverage. [ 13 ] Li and co-workers [ 14 ] attached conjugated thiophene side chains instead of alkyl substituents on the BDT unit (BDTT) and synthesized the 2D-conjugated copolymer from BDTT and fl uorinated benzotriazole. Nevertheless, the HOMO energy level of this copolymer was relatively high (−5.22 eV), which limited V oc s of the devices to less than 0.8 V and the PCEs recorded were less than 7%.Recently, incorporation of alkylthio side chains on the 4,8-di(thiophen-2-yl)benzo[1,2-b :4,5-b ′]dithiophene (BDTT) unit has been used to effectively down-shift the HOMO energy levels and thereby increase V oc s of the donor polymers. [ 15 ] This is mainly attributed to π-electron accepting capability of divalent sulfur from the p-orbital of the carbon-carbon double bond into its empty 3d-orbitals. [ 16 ] Li and co-workers [ 17 ] reported a BDTTbased low bandgap polymer by introducing branched alkylthio side chains onto the BDTT unit of the well-known photovoltaic polymer, PTB7. This polymer had a low-lying HOMO energy level and highly enhanced hole mobility and photovoltaic properties. Subsequently, Hou and co-workers [ 18 ] also attached linear octylthio side chains on the BDTT unit to improve the short-circuit current ( J sc ) (17.46 mA cm −2 ) of the resulting low bandgap (1.51 eV) polymer PBDT-TS1 while keeping the high V oc (0.8 eV) of the original backbone and achieved a PCE of 9.48%.

Isoindigo‐3,4‐Difluorothiophene Polymer Acceptors Yield “All‐Polymer” Bulk‐Heterojunction Solar Cells with over 7 % Efficiency

“…5.5 eV for PBFTAZ) and approximately 4.0 eV (vs.c a. [11] Theh igher J SC values for the 2BO/2HD devices (13.2 mA cm À2 )c ompared to those of the control PC 71 BM-based devices (11.4 mA cm À2 )result in part from the more efficient spectral absorption of the all-PSCs (PC 71 BMs absorption being limited to the short-wavelength region where the photon flux is only modest). Note that the EA estimate is 0.1 eV lower than that of the fullerene acceptor PCBM (4.1-4.2 eV) and slightly lower than those of PDI/NDI-based polymer acceptors.…”

“…Zuschriften difluorothiophene [2F]T motifs, [10] and show that the appropriately functionalized poly(isoindigo-alt-3,4-difluorothiophene) analogue,namely PIID[2F]T (Figure 1a), can achieve PCEs as high as 7.3 %inB HJ solar cells with PBFTAZ [11] as the polymer donor (model system; Figure 1b). PBFTAZ (E opt % 1.9 eV) and PIID[2F]T (E opt % 1.7 eV) provide spectral complementary across the UV/Vis spectrum (300-750 nm), yielding high short-circuit current densities (J SC )o fc a.…”

“…0.7 V; Table 1). [11] Theh igher J SC values for the 2BO/2HD devices (13.2 mA cm À2 )c ompared to those of the control PC 71 BM-based devices (11.4 mA cm À2 )result in part from the more efficient spectral absorption of the all-PSCs (PC 71 BMs absorption being limited to the short-wavelength region where the photon flux is only modest). Thus,i mportantly, all-PSCs based on 2BO/2HD achieve PCEs higher by approximately 50 %r elative to those of the PC 71 BM-based BHJ solar cells (max.…”

High‐Efficiency Polymer Solar Cells Over 13.9% With a High VOC Beyond 1.0 V by Synergistic Effect of Fluorine and Sulfur

“…The side chains flanking on the two‐dimensional (2D) conjugated unit BDT, such as phenyl substituted BDT (BDTP) and thienyl substituted (BDTT), play an critical role not only in the solubility for polymer but also in their optical and electrical properties . Compared with the two building blocks, the weaker electron‐donating nature of BDTP has relatively lower HOMO energy levels, which has been used to effectively lower the polymer energy levels than thienyl substituted BDTT unit . For example, Li et al reported an efficient ITIC‐based PSC with a wide bandgap polymer donor of J52.…”

8.0% Efficient All‐Polymer Solar Cells with High Photovoltage of 1.1 V and Internal Quantum Efficiency near Unity

“…The synthesis of PBDTS-TPD is inspired by our previous work, where we found that the alkylthio-side groups instead of the alkyl-side groups can improve the solubility and downshift the HOMO level of the resulting polymer. [37] For comparison, the alkyl and alkylthiosubstituted donor polymers were synthesized and evaluated in both all-PSCs and PC 71 BM-based PSCs with conventional and inverted device structures. Compared to the PBDTS-TPD:PC 71 BM solar cell with a V oc of 1.0 V and a PCE of 7.1%, the PBDTS-TPD:PNDI-T all-PSC attains a higher PCE of 8.0% with an enhanced V oc of 1.1 V. The high V oc of this all-PSC benefits from the down-lying HOMO level of the donor and uplying LUMO of the acceptor, which agrees well with our design motif.…”