Side chain engineering on D18 polymers yields 18.74％ power conversion efficiency

Donor-acceptor (D-A) conjugated copolymers contain-ing fused-ring acceptor units demonstrate outstanding per-formance in organic solar cells (OSCs)[1-13].We have in-vented highly efficient D-A copolymer donors D18 and D18-CI by using a fused-ring acceptor unit,dithieno[3',2':3,4;2",3":5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT)[1,2].OSCs with D18 or D18-CI gave power conversion efficiencies(PCEs) of 18.56％ and 18.69％,respectively[3,4].Side chain engineering is an effective approach to improve the per-formance of conjugated polymers in optoelectronic devi-ces[14-16].The alkyl side chains not only determine polymers' solubility,but also influence their crystallinity and mobility.In this work,we develop two efficient donors D18-B and D18-CI-B via side chain engineering on D18 polymers (Fig.1(a)).These donors offer PCEs up to 18.74％ (certified 18.2％) in tern-ary OSCs.     

D18,an eximious solar polymer!

Organic solar cell(OSC)is a promising photovoltaic technology with great commercialization potential due to the advantages like solution-processing,roll-to-roll fabrication,lightweight,flexibility,and semitransparency[1?7].Conjugated polymer donors are key materials for OSCs[8].     

18.69％ PCE from organic solar cells

The exquisite design and persistent development of fused-ring-acceptor-unit-based copolymer donors and Y-series nonfullerene acceptors(NFAs)have pushed the power conversion efficiencies(PCEs)for organic solar cells onto the 18％level[1-21].Our group invented copolymer donors D18 and D18-Cl[2,3].D18∶Y6,D18-Cl∶N3 and D18∶N3 solar cells have delivered outstanding PCEs of 18.22％,18.13％and 18.56％,respectively[2-4].Ternary solar cells based on a poly-mer donor,a NFA and a fullerene acceptor show great poten-tial since they combine good light-harvesting capability of NFA and good electron-mobility of fullerene[5].     

Post-sulphuration enhances the performance of a lactone polymer donor

Remarkable progress has been made in conjugated copoly-mer donors due to the development of novel fused-ring ac-ceptor (FRA) building units[1-15].The copolymers based on FRA units have delivered excellent power conversion efficien-cies (PCEs) up to 18.69％ in organic solar cells (OSCs)[16].Fused-ring aromatic lactones are promising FRA units[17-19].     

Monolithic perovskite/silicon tandem solar cells offer an efficiency over 29％

Organic-inorganic hybrid perovskite semiconductors pos-sessing superior optoelectronic properties (e.g.long carrier dif-fusion lengths,high optical absorption coefficient,low ex-citon binding energy,and high defect tolerance) are attract-ing serious attention.The certified power conversion effi-ciency (PCE) for single-junction perovskite solar cells have ex-ceeded 25％[1,2].As a very promising PCE-enhancement strategy,tandem structure made by stacking a perovskite cell on a market-dominant silicon cell can yield much higher PCEs beyond the Shockley-Queisser limit of single-junction devices without adding substantial cost[3].To satisfy current-match-ing in tandem configuration,the top perovskite cell requires an ideal bandgap of ～1.7 eV rather than the ones (～1.5-1.6 eV)typically used for highly efficient single-junction perovskite devices given that the bottom silicon cell holds a bandgap of 1.12 eV[4].Such wide-bandgap perovskites achieved through I/Br alloying usually suffer from photoinduced phase segrega-tion and relatively low radiative efficiency,which inevitably result in large open-circuit voltage (Voc) deficits[5,6].Several strategies like adjusting perovskite composition[7,8],additive engineering[9,10],and upper surface passivation[11,12] have been utilized to stabilize these wide-bandgap perovskites and improve film quality to reduce Voc losses.The reported pe-rovskite/silicon tandem devices suffer from low Voc (＜1.9 V)and PCEs (≤28％)[13].There is still a large room for enhancing PCEs given that the predicted PCE limit is beyond 30％ for this tandem technology[14].     

～1.2 V open-circuit voltage from organic solar cells

Organic solar cells (OSCs) have achieved rapid advance due to the continuous development of high-performance key materials.Recently,the power conversion efficiencies (PCEs)of OSCs under 1 Sun condition (AM 1.5 G,100 mW/cm2) are striving toward 19％[1-5].The PCE improvement benefits from the largely enhanced short-circuit current density (Jsc) and fill factor (FF).However,these cells show relatively low open-circuit voltage (Voc) around 0.8-0.9 V.The rise of Internet of Things (loT) industry has promoted the indoor application of solar cells.OSCs can afford higher PCEs under various indoor light as compared to 1 Sun condition[6,7],but they present lower Voc[8].Fabricating tandem devices is an effective strategy to boost the performance of OSCs.Sub-cells with syn-chronously high Voc,Jsc and FF are highly desired in tandem cells,while these sub-cells are still limited[9].Thus,improving Voc without sacrificing Jsc and FF is an urgent mission in OSCs.     

A chlorinated copolymer donor demonstrates a 18.13% power conversion efficiency

The rapid development of low-bandgap(LBG)nonfullerene acceptors and wide-bandgap(WBG)copolymer donors in recent years has boosted the power conversion efficiency(PCE)of organic solar cells(OSCs)to the 18%level[1?21].The commercialization of OSCs is highly expected.However,critical issues like the cost and the stability also determine whether OSCs can enter the market or not[22].     

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

Star nonfullerene acceptors like ITIC[1],IDIC[2],O-IDTBR[3],IT-4F[4],CO,8DFIC[5],Y6[6]etc.continuously emerge and keep pushing the power conversion efficiency(PCE)of organic sol-ar cells forward.These small molecules generally show nar-row bandgaps,excellent visible to NIR light-harvesting capabil-ity,good electron mobility,suitable energy levels and miscibil-ity 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 non-fullerene acceptors also received considerable interests[8].Ow-ing to complementary light absorption,high hole-mobility and deep HOMO levels,wide-bandgap(WBG)conjugated co-polymers are ideal donor partners for the low-bandgap non-fullerene acceptors.     

Investigating the reason for high FF from ternary organic solar cells

Organic solar cells (OSCs) have attracted huge attention because of their unique merits[1-3].In last few years,thanks to the design of new materials and device engineering,the power conversion efficiencies (PCEs) of OSCs have surpassed 18％[4-8].The PM6:Y6 cells are efficient binary cells,offering high PCEs over 16％[9-11].The high performance originates from the efficient free charge generation and the ground state dipole field at the donor-acceptor interface that pro-motes the exciton dissociation[12].To further boost the per-formance of PM6:Y6 cells,ternary architectures were adop-ted.Significant improvements in short-circuit current density(Jsc) and open-circuit voltage (Voc) were realized.However,most of the ternary cells still suffer from low fill factor (FF) (gen-erally ＜78％) (Table S1).The FF is generally determined by the competition between recombination and extraction of charge carriers[13-15].The interfacial electronic structures have nonnegligible impacts on charge transport in OSCs,also influ-encing the FF[16,17].Previously,Bao et al.demonstrated that the energy of positive integer charge transfer (ICT) states(EICT+) of PM6 is equal to the energy of negative ICT states(EICT-) of Y6,leading to no potential step at the PM6:Y6 inter-face and an ideal binary host system[18].To avoid the forma-tion of potential step in the ternary system,the EICT-of the second acceptor should be lower than (or equal to) EICT+ of the donor,thus suppressing the ICT trap-assisted recombina-tion[1].In this work,we carefully incorporate a second accept-or EH-IDTBR into the host PM6:Y6 blend (Fig.1(a)).From the ultraviolet photoelectron spectroscopy (UPS)-derived energy levels (Fig.1(b)),the negative ICT states of EH-IDTBR (EICT-=4.25 eV) is smaller than the positive ICT states of PM6 (EICr+ =4.5 eV),suggesting no ground state charge transfer at the PM6:EH-IDTBR interface,thus avoiding interfacial ICT trap-assisted recombination.     

Structure,band gap and energy level modulations for obtaining efficient materials in inverted polymer solar cells

Two new donor–acceptor (D–A) polymers composed of benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and thiadiazolo[3,4-c]pyridine (PyTZ) as acceptor were designed and synthesized. Compared to the polymer based on BDT and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT), the planarity, energy levels, and band-gaps of the new polymers were fine adjusted by incorporating conjugated alkylthienyl side chains to BDT and substitute a stronger acceptor PyTZ for 2,1,3-benzothiadiazole. The new polymers exhibit broad absorption from 300 to 800 nm in both solution and film state. The polymer band gaps and energy levels are close to the optimal values. As a result, power conversion efficiencies (PCEs) of 4.84% and 5.11% were obtained for inverted polymer solar cells based on these new polymers. The PCEs are significantly higher than those of the BDT–DTBT based polymers (2–4%).     

Defect engineering on all-inorganic perovskite solar cells for high efficiency

Perovskite solar cells based on organic-inorganic hybrid perovskite materials have become a research hotspot in photovoltaics field due to their outstanding power conversion efficiency (PCE)[1].Nonetheless, the organic cations are volatile and have rotation freedom, which is not good for photoand thermal-stability of the devices.Fortunately, these issues can be solved by all-inorganic perovskites, which consist of Cs, Pb and I (or Br)[2, 3].Moreover, the all-inorganic perovskites, such as CsPbl3, are excellent candidates as top-cell absorbers in tandem solar cells because of their suitable bandgaps and high stability.All-inorganic perovskites were first used as light absorbers in solar cells in 2015[4, 5].All-inorganic perovskite solar cells experienced rapid development in last few years, and the PCE reaches 20.4％ at the end of 2020[6].Most recently, Meng et al.pushed the PCE to ＞21.0％ (unpublished).Despite these advances, we should recognize that there still remains a big gap between the PCEs for all-inorganic perovskite solar cells and hybrid perovskite solar cells (Fig.1(a))[7, 8].The PCE for hybrid cells has reached 80％ of the theoretical limit, while the PCE for all-inorganic cells is still below 70％ of the theoretical limit[9].A detailed analysis on performance parameters of these cells suggests that the PCE for all-inorganic cells is mainly limited by the open-circuit voltage (Voc) and fill factor (FF) (Fig.1(b)).In solar cells, these two parameters correlate to non-radiative charge loss caused by defects[10].Therefore, defect engineering is the most critical approach for achieving higher PCE for all-inorganic perovskite solar cells.     

Bithiazole-based copolymer with deep HOMO level and noncovalent conformational lock for organic photovoltaics

To match with the state-the-art-of non-fullerene acceptor for polymer solar cells (PSCs), it is urgent to develop high performance wide-bandgap (WBG) copolymer donors with deep highest occupied molecular orbital (HOMO) level and highly planar backbone to further promote the device efficiency. A new WBG copolymer PBDTBTz, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithi ophene))-alt-(5,5’-(-4,4’-dinonyl-2,2’-bithiazole))], which is based on benzodithiophene (BDT) as donor unit and 4,4’-dinonyl-2,2’-bithiazole (BTz) as acceptor unit is prepared for non-fullerene PSCs. Due to the strong electron-withdrawing effect, BTz unit can dramatically lower the HOMO level of PBDTBTz to −5.60 eV. Notably, double noncovalent conformational locks (N⋯S) of BTz are formed in the backbone to reduce the steric hindrance and favor a highly planar geometry for efficient charge transport and molecular packing. As a result, the device based on PBDTBTz as donor and 3,9-bis(2-methylene-((3-(1,1-dic-yanomethylene)-6,7-difluoro)-indanone))-5,5,11, 11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d']-s-indaceno[1,2-b:5,6-b']dithiophen e(IT-4F) as acceptor afforded a high open circuit voltage (V_oc) of 0.92 V, which is the highest value for IT-4F-based PSCs reported so far. Furthermore, the device operated well with a negligible driving force (ΔE_HOMO) of as small as 0.06 eV. These results revealed that combination of electron-withdrawing bithiazole and double noncovalent conformational lock of N⋯S is a promising molecular design concept of polymer donor with deep and planar structure for high performance PSCs.     

Polymer acceptors for all-polymer solar cells

Bulk-heterojunction polymer solar cells(PSCs)as a clean and renewable energy resource have attracted great atten-tion from both academia and industry[1-20].Recently non-fullerene PSCs based on polymer donors(PDs)and small mo-lecule acceptors(SMAs)have achieved remarkable success with the power conversion efficiencies(PCEs)over 18％[21-26].Among various PSCs,all-polymer solar cells(all-PSCs)consist of PDs and polymer acceptors(PAs),showing unique merits including superior stability and mechanical robustness.How-ever,the development of all-PSCs lag behind SMAs-based PSCs due to the scarcity of high-performance PAs[6].     

Benzo[1,2-b:4,5-b′]dithiophene and benzotriazole based small molecule for solution-processed organic solar cells

A novel deep HOMO A₁-π-A₂-D-A₂-π-A₁ type molecule (D(CATBTzT)BDT), which terminal electron-withdrawing octyl cyanoacetate group is connected to a benzo[1,2-b:4,5-b′]dithiophene (BDT) core through another electron-accepting benzotriazole block, has been synthesized, characterized, and employed as electron donor material for small molecule organic solar cells (SM-OSCs). By simple solution spin-coating fabrication process, D(CATBTzT)BDT/PC₆₁BM based OSCs exhibit a power conversion efficiency (PCE) of 3.61% with a high open-circuit voltage of 0.93 V. The D(CATBTzT)BDT based solar cells device also can show high FF of 72% with PCEs of 2.31% which is one of the best FF results for solution-processed SM-OSCs.     

Voc deficit in kesterite solar cells

As containing earth-abundant elements,kesterite (Cu2Zn-Sn(S,Se)4,CZTSSe) semiconductors have great potential to be low-cost and environmental-friendly inorganic absorbers.However,the record power conversion efficiency (PCE) for CZTSSe solar cells is only 12.6％[1,2],much lower than that for Cu(In,Ga)Se2 (CIGS) solar cells (23.35％)[3].The key issue for kes-terite solar cells is the large open-circuit voltage deficit(Voc,def,the gap between Voc and Shockley-Queisser limit VocsQ) or small Voc gain (Voc/VocSQ).The Voc/VocSQ is higher than 85％ for high-performance CIGS solar cells but only 61％for current world-record CZTSSe device[2].Many factors may cause the Voc loss of kesterite:(1) the narrow phase stability makes it difficult to achieve highly uniform absorber composi-tion,which can result in bandgap fluctuation and secondary phases;(2) the similar ionic size of Cu and Zn leads to high con-centration of Cu-Zn antisite defects (Cu-Zn disorder),which may cause electrostatic potential fluctuation and band tail-ing;(3) the multi-element composition and the variable valence of Sn lead to complicated defect property,causing seri-ous recombination in absorber bulk and interfaces[4-8].Identi-fy the most critical one and its origin is crucial for further im-proving device efficiency.     

Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells

To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HD-DPP-A-PY as a donor and [6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₀BM) as an acceptor exhibited PCEs as high as 3.15%.     

Enhanced Performance of Organic Solar Cells with Increased End Group Dipole Moment in Indacenodithieno[3,2‐b]thiophene‐Based Molecules

Four new molecular donors are reported using a D1‐A‐D2‐A‐D1 structure, where D1 is an oligiothiophene, A is a benzothiadiazole, and D2 is indacenodithieno[3,2‐b]thiophene. The resulting materials provide efficiencies as high as 6.5% in organic solar cells, without the use of solvent additives or thermal/solvent annealing. A strong correlation between the end group (D1‐A) dipole moment and the fill factor (FF), mobility, and loss in the open‐circuit voltage (V_OC) is observed. Indacenodithieno[3,2‐b]thiophene‐fluorobenzothiadiazole‐terthiophene (IDTT‐FBT‐3T) possesses the largest end group dipole moment, and in turn, has the highest mobility, FF, and power conversion efficiency in devices. It also has a similarly high V_OC (0.95 V) to the other materials (0.93–0.99 V), despite possessing a much higher highest occupied molecular orbital (HOMO) energy level.     

High open-circuit voltage of the solution-processed organic solar cells based on benzothiadiazole–triphenylamine small molecules incorporating π-linkage

Two novel small molecular photovoltaic (PV) materials, BDPTBT and BDATBT were designed and synthesized, consisting of 5,6-bis-(octyloxy)benzo[c][1,2,5]thiadiazole (DOBT) as electron-withdrawing core (A), and triphenylamine (TPA) as electron-donating side group (D). Moreover, the benzene and ethynylbenzene as π-linkage were introduced to form donor–π-acceptor–π-donor (D–π-A–π-D) typed molecular structures, respectively. To fully investigate the linkage effect of a series of small molecules, two reference compounds BDCTBT and BDETBT were also studied systematically, consisting of 2-phenylacrylonitrile and styrene as π-linkage, respectively. As a result, the π-linkage units, benzene, styrene, ethynylbenzene and 2-phenylacrylonitrile played an important role in modifying molecular structure and improving PV performance. Bulk heterojunction (BHJ) solar cells based on BDPTBT/PC₆₁BM and BDATBT/PC₆₁BM yielded the power conversion efficiencies (PCEs) of 2.99% and 2.03%, respectively. Notably, BDATBT based device showed a high open-circuit voltage (Voc) of 1.03 V. Compared to the results we have reported previously, the reference devices based on BDCTBT/PC₆₁BM and BDETBT/PC₆₁BM with the optimized weight ratio showed dramatically enhanced PCEs of 4.84% and 3.40%, respectively, and BDCTBT based device showed a high Voc of 1.08 V. To our knowledge, the Voc of 1.08 V is the highest voltage reported to date for devices prepared from solution-processed small-molecule-donor materials, and the PCE of 4.84% is the highest efficiency reported so far for D–A–D-typed benzothiadiazole (BT)–TPA based solution-processed small molecules PV devices.     

Triisopropylsilyl-Substituted Benzo[1,2-b:4,5-c′]dithiophene-4,8-dione-Containing Copolymers with More Than 17% Efficiency in Organic Solar Cells

Considering the special functions of fused-ring aromatic building blocks and Si-atom in high-performance donor–acceptor-conjugated materials at the same time, herein the synthesis of a novel fused-ring tricyclic heterocycle, triisopropylsilyl-substituted benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (iBDD-Si), an isomer of well-known benzo[1,2-c:4,5-c′]dithiophene-4,8-dione is presented. The iBDD-Si-based copolymer series (PM6, PM6-5Si, PM6-10Si, and PM6-15Si) is synthesized via Stille polymerization, revealing fine-tuned optical and electrochemical properties, and molecular packing with varying iBDD-Si contents in the backbone. Organic solar cells are fabricated by pairing the copolymer donors with nonfullerene acceptor N3 and characterized. High power conversion efficiency of more than 17% is achieved using the PM6-5Si-based solar cell, which is attributed to the balanced charge transport, enhanced charge generation/dissociation kinetics, and minimized total energy and recombination losses. It is demonstrated that iBDD-Si is a promising backbone toolbox for various high-performance conjugated materials.     

All-polymer solar cells

Organic solar cells(OSCs)show a promising commercializa-tion prospect with their power conversion efficiencies(PCEs)exceeding 18％[1-6].Among various types of OSCs,all-poly-mer solar cells(all-PSCs)with a physical blend of p-and n-type polymer as the active layer to harvest solar irradiation at-tract growing attention due to their unique advantages like ex-cellent morphological stability,and mechanical durability[7].Re-cently,great progresses have been achieved in this field includ-ing the development of high-performance polymer accept-ors and the advances in morphology regulation[8-13].Particu-larly,a PCE of 17.20％has been realized very recently by all-PSCs via properly aligned energy levels and optimal active-lay-er morphology[8].This achievement has significantly reduced the PCE gap between all-PSCs and small molecular acceptor-based OSCs,indicating the bright future of all-PSCs.There-fore,a highlight on these important progresses is timely and will effectively drive the development of all-PSCs.