Molecular design of non-fullerene acceptors is of vital importance for high-efficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach, as it may introduce an undesirable steric hindrance that reduces charge transport in non-fullerene acceptors. Here we show the design and synthesis of a highly efficient non-fullerene acceptor family by substituting the beta position of the thiophene unit on a Y6-based dithienothiophen[3,2-b]-pyrrolobenzothiadiazole core with branched alkyl chains. It was found that such a modification to a different alkyl chain length could completely change the molecular packing behaviour of non-fullerene acceptors, leading to improved structural order and charge transport in thin films. An unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of the branched alkyl chain topology in tuning the molecular packing and blend morphology, which leads to improved organic photovoltaic performance. Molecular design of acceptor and donor molecules has enabled major progress in organic photovoltaics. Li et al. show that branched alkyl chains in non-fullerene acceptors allow favourable morphology in the active layer, enabling a certified device efficiency of 17.9%.

Non-fullerene acceptors with branched side chains and improved molecular packing to exceed 18% efficiency in organic solar cells

Improving power conversion efficiency (PCE) is important for broadening the applications of organic photovoltaic (OPV) cells. Here, a maximum PCE of 19.0% (certified value of 18.7%) is achieved in single-junction OPV cells by combining material design with a ternary blending strategy. An active layer comprising a new wide-bandgap polymer donor named PBQx-TF and a new low-bandgap non-fullerene acceptor (NFA) named eC9-2Cl is rationally designed. With optimized light utilization, the resulting binary cell exhibits a good PCE of 17.7%. An NFA F-BTA3 is then added to the active layer as a third component to simultaneously improve the photovoltaic parameters. The improved light unitization, cascaded energy level alignment, and enhanced intermolecular packing result in open-circuit voltage of 0.879 V, short-circuit current density of 26.7 mA cm-2 , and fill factor of 0.809. This study demonstrates that further improvement of PCEs of high-performance OPV cells requires fine tuning of the electronic structures and morphologies of the active layers.

Single-Junction Organic Photovoltaic Cell with 19% Efficiency

Solar cell efficiency tables (version 57)

In organic photovoltaics, morphological control of donor and acceptor domains on the nanoscale is the key for enabling efficient exciton diffusion and dissociation, carrier transport and suppression of recombination losses. To realize this, here, we demonstrated a double-fibril network based on a ternary donor-acceptor morphology with multi-length scales constructed by combining ancillary conjugated polymer crystallizers and a non-fullerene acceptor filament assembly. Using this approach, we achieved an average power conversion efficiency of 19.3% (certified 19.2%). The success lies in the good match between the photoelectric parameters and the morphological characteristic lengths, which utilizes the excitons and free charges efficiently. This strategy leads to an enhanced exciton diffusion length and a reduced recombination rate, hence minimizing photon-to-electron losses in the ternary devices as compared to their binary counterparts. The double-fibril network morphology strategy minimizes losses and maximizes the power output, offering the possibility of 20% power conversion efficiencies in single-junction organic photovoltaics. 

Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology

With the recent emergence of a new class of high-performance nonfullerene acceptors (NFAs), organic solar cells (OSCs) have entered a new phase of research featuring high power conversion efficienc...

New Phase for Organic Solar Cell Research: Emergence of Y-Series Electron Acceptors and Their Perspectives

18% Efficiency organic solar cells

Fused-ring bislactone building blocks for polymer donors

S⋯O F⋯H Star nonfullerene acceptors like ITIC[1], IDIC[2], O-IDTBR[3], IT-4F[4], COi8DFIC, Y6[6] etc. continuously emerge and keep pushing the power conversion efficiency (PCE) of organic solar cells forward. These small molecules generally show narrow bandgaps, excellent visible to NIR light-harvesting capability, good electron mobility, suitable energy levels and miscibility with the donor materials. PCEs up to 18.56% have been achieved for the state-of-the-art nonfullerene organic solar cells[7]. On the other hand, donor materials matching nonfullerene acceptors also received considerable interests[8]. Owing to complementary light absorption, high hole-mobility and deep HOMO levels, wide-bandgap (WBG) conjugated copolymers are ideal donor partners for the low-bandgap nonfullerene acceptors. Hou et al. developed a WBG copolymer donor PM6 based on a benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) unit[9]. PM6 has been widely applied in nonfullerene solar cells, delivering high PCEs up to 17.8%[10]. Li et al. reported a simple-structured WBG copolymer PTQ10 based on a quinoxaline unit[11]. Solar cells based on PTQ10 and Y6 gave a PCE of 16.53%[12]. Huang et al. developed a WBG copolymer P2F-EHp by using an imide-functionalized benzotrizole (TzBI) unit[13]. P2F-EHp:Y6 solar cells gave a 16.02% PCE. Guo et al. synthesized a random copolymer S1 with a fluorine and ester group functionalized thiophene (FE-T) unit[14]. Owing to the strong electron-withdrawing property of FE-T, S1 has a deep HOMO level and delivered a high open-circuit voltage (Voc) of 0.88 V and a PCE of 16.42%. Ding et al. developed a 2.16 eV ultra-WBG copolymer W1 by using a fluorinated 1,2-dialkoxybenzene (FAB) unit[15]. The FAB unit offers unique and double-side conformation locking in the copolymer backbone, and renders W1 enhanced packing and good hole-transporting capability[16]. W1:Y6 solar cells gave a PCE of 16.23%. Ding et al. also developed several high-performance WBG copolymer donors based on fusedring acceptor units. The WBG copolymer L1 based on a fusedring lactone unit 5H-dithieno[3,2-b:2',3'-d]pyran-5-one (DTP) delivered a 14.36% PCE[17]. A fused-ring thiolactone copolymer D16 based on the 5H-dithieno[3,2-b:2',3'-d]thiopyran-5one (DTTP) unit gave a higher PCE of 16.72%[18]. By using a dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT) unit, which has a larger molecular plane than DTP and DTTP, Ding et al. developed a more efficient WBG copolymer donor D18[19]. D18:Y6 solar cells gave a PCE of 18.22% (certified 17.6%). This is the first time for the PCE of OSCs surpassing 18%. Thick-film D18:Y6:PC61BM ternary cells delivered 16% PCEs with an active layer thickness over 300 nm[20]. Ding et al. further reported a chlorinated analogue of D18, the D18-Cl[21]. Blending D18-Cl with a nonfullerene acceptor N3 yielded a PCE of 18.13% (certified 17.6%). Very recently, Ding et al. pushed the PCE to 18.56% (certified 17.9%) by blending D18 with N3, setting a new PCE record[7]. These works demonstrated the advantages of fused-ring acceptor units in constructing WBG copolymer donors. The strong electron-withdrawing capability and extended molecular planes of these acceptor units gift copolymers deep HOMO levels, enhanced packing and high hole mobility, thus leading to improved Voc, short-circuit current density (Jsc) and fill factor (FF) in solar cells. In this work, we report copolymers P1 and P2 based on a dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]oxadiazole (DTBO) unit (Fig. 1(a)). Compared with DTBT, DTBO has fewer synthetic steps and is more cost-effective[22]. DFT calculations indicate that DTBO renders the copolymer a deeper HOMO level, thus yielding higher Voc in solar cells. Solar cells with P1 and P2 as the donors and Y6 as the acceptor afforded high Voc up to 0.91 V and decent PCEs up to 15.64%. We built the polymer models for D18, P1 and P2 (Fig. 1(b)). Each model has two repeating units, and the alkyl chains were replaced by methyl groups for saving the calculation time. All structures were optimized at the B3LYP/631G(d) level. The DFT-predicted frontier molecular orbitals and energy levels for D18, P1 and P2 are shown in Fig. 1(b). From D18 to P1, DTBT being replaced by DTBO, simultaneous decrease in HOMO and LUMO energy levels was observed. The HOMO and LUMO levels for P1 are –5.05 and –2.69 eV, respectively, which are ~0.1 eV lower than that of D18. A higher Voc was expected for P1-based solar cells since Voc is proportional to the energy difference between donor HOMO and acceptor LUMO[23]. Compared with P1, P2 shows higher HOMO and LUMO levels of –4.94 and –2.61 eV, respectively, due to the removal of electron-withdrawing fluorine atoms. For P1 and P2, the variation trends in DFT-predicted HOMO and LUMO levels are consistent with those from cyclic voltammetry (CV) measurements (vide infra).

Dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]oxadiazole-based polymer donors with deep HOMO levels

A wide-bandgap copolymer donor with a 5-methyl-4H-dithieno[3,2-e:2',3'-g]isoindole-4,6(5H)-dione unit

Two wide-bandgap copolymer donors, W4 and W5, based on mono-fluorine- and mono-alkoxyl-substituted benzene units were developed. W4 and W5 were synthesized from cheap and commercially available starting materials. Organic solar cells based on W4 and W5 gave decent power conversion efficiencies of up to 15.25%.

Jingui Xu

Papers

18% Efficiency organic solar cells

Fused-ring bislactone building blocks for polymer donors

Dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]oxadiazole-based polymer donors with deep HOMO levels

A wide-bandgap copolymer donor with a 5-methyl-4H-dithieno[3,2-e:2',3'-g]isoindole-4,6(5H)-dione unit

Efficient wide-bandgap copolymer donors with reduced synthesis cost