Barrierless charge generation at non-fullerene organic heterojunctions comes at a cost

The advent of non-fullerene acceptors has enabled organic solar cells to reach power conversion efficiencies that were previously thought unreachable. However, in a recent Nature Materials article, Karuthedath and colleagues show the electrostatic environment at interface might put limit how much further they can be improved. To maximize solar-to-electric efficiency heterojunctions, we need design rules for electron-donating electron-accepting components allow efficient charge pair generation. Recently, co-workers have shown absorbed-photon-to-current such heterojunction depends primarily on offset ionization energies (ΔIE) two materials becomes compromised when ΔIE<0.5eV.1Karuthedath S. Gorenflot J. Firdaus Y. Chaturvedi N. De Castro C.S.P. Harrison G.T. Khan J.I. Markina A. Balawi A.H. Peña T.A.D. et al.Intrinsic limits low-bandgap acceptor cells.Nat. Mater. 2021; 20: 378-384Crossref PubMed Scopus (106) Google Scholar A finding is interfacial energy transfer surprisingly competes with transfer. findings from this study important implications heterojunctions. Organic semiconductors attracted interest application photovoltaics largely because chemical tuneability, diversity, processability materials. low dielectric constant inherent disorder results strong Coulombic binding photo-induced excitations, heterojunction, consisting an electron donor (typically conjugated polymer) molecule), generally needed drive separation. In commonly accepted picture, exciton, ES1, dissociates into charge-transfer (CT) excited state ECT<ES1, CT then independent charges (denoted by charge-separated ECS) deliver photocurrent. generation (and therefore measured photocurrent) difference hole solid state, but increasing comes expense photovoltage any given optical absorption gap. It thus been long-standing goal establish exactly large needs be. Establishing separation efficiency—interfacial gap relationship has, however, difficult. Although comparison affinity (EA) or (IE) should yield transfer, respectively, measurements separate are unable capture effects microstructure mixed system electrostatics. Further, different techniques (e.g., cyclic voltammetry photoelectron spectroscopy) often disagree affinities particularly hard determine. More conveniently, estimated directly electroluminescence high-precision external quantum (EQE) measurements, following observations ECT obtained way correlates open-circuit voltage (VOC) cell, ES1−ECT taken indicate driving introduction so-called (NFAs), oligomeric light-absorbing small molecules, whose energetic properties easily tuned, brought step-change both device range could explored. Particularly intriguing several photocurrent generated lower offsets than observed, including reports vanishingly offset,2Zhong Causa’ M. Moore G.J. Krauspe P. Xiao B. Günther F. Kublitski Shivhare R. Benduhn BarOr E. al.Sub-picosecond near-zero force polymer:non-fullerene blends bilayers.Nat. Commun. 2020; 11: 833Crossref (74) Scholar,3Perdigón-Toro L. Zhang H. Yuan Hosseini S.M. Wolff C.M. Zuo G. Stolterfoht Zou Gao al.Barrierless Free Charge Generation High-Performance PM6:Y6 Bulk Heterojunction Non-Fullerene Solar Cell.Adv. 32: e1906763Crossref (161) remarkably slow separation.4Qian D. Zheng Z. Yao Tress W. Hopper T.R. Chen Li Liu al.Design minimizing losses high-efficiency 2018; 17: 703-709Crossref (487) improved performance NFA-based compared fullerene-based junctions attributed factors disorder, greater crystallinity, sharp onset, higher constant, hybridization exciton states. Considering retains some energy, it remained enigma high photon-to-electron offset. Perdigon-Toro al. recently addressed question performance, relatively low-offset blend PM6:Y6.3Perdigón-Toro Using temperature dependent showed that, despite offset, was barrier-free. explain this, considered effect molecular structure alignment concluded molecule’s IE EA changed close changing contribution associated Y6 quadrupole moment. underlying theory proposes rough solid-state material varies as moves away interface.5Poelking, C., Andrienko, (2015). Design Rules Donor-Acceptor Heterojunctions: Pathway Splitting Detrapping. https://doi.org/10.1021/jacs.5b02130Google At interface, molecule surrounded experiences interaction, mainly moments surrounding interaction would experience while deep phase. This “bending,” bias potential B, tenths eV size either sign, depending moment presented molecules and, therefore, orientation shape.6Schwarze Schellhammer K.S. Ortstein K. Gaul C. Hinderhofer Perdigón Toro Scholz Roland al.Impact levels heterojunctions.Nat. 2019; 10: 2466Crossref (52) similar also made other systems,7Saladina Simón Marqués Wöpke Göhler Allain Blanchard Cabanetos al.Charge Photogeneration Cells: Influence Excess Energy Electrostatic Interactions.Adv. Funct. 31: 2007479Crossref (14) Scholar,8Dong Nikolis V.C. Talnack Chin Y.C. Londi X. Mannsfeld S.C.B. Spoltore al.Orientation electrostatics drives homojunction 4617Crossref (29) up now result not generalized; lead effects. explored set 23 donor:acceptor combinations. They eliminate light-harvesting measuring spectrally averaged internal (IQE) each function ΔIE. Their remarkable entire family follow behavior, IQE saturates ΔIE ∼0.5 eV. authors B five NFAs studied found all cases same tending increase CT-state apart, illustrated Figure 1, which helps dissociate Conversely, reduces exciton-to-CT-state transition, introduces barrier (i.e., ΔIE<B). conclude minimum 0.5 barrier-less ensure maximum charge-generation efficiency. Interestingly, find no correlation EA, ΔEA, assign ultra-fast Förster resonant (FRET) blends, which, outcompeting funnels through hole-transfer pathway. Two conclusions drawn study. For ΔIE, presence positive dissociation (ES1−ECT) increases (ECT−ECS), making harder form states easier no-bias case. suggests unreliable indicator ECS, includes value will vary case case, instead measured. Second, work origin state. Together, these suggest exhibit Voc IQE, both, limiting maximal effficiency photovoltaic devices. why increases, achieved quadrupole-moment-bearing Y-series NFAs, come increased fill-factor, whereas voltage-related still exceed presents potentially disheartening news competitiveness terms efficiency, there caveats. first sparse data points offsets, does include record-breaking novel polymer donors D18 yielded lowest yet. Possibly materials, optimized morphology, ΔIE=0.5eV. used is, necessity, approximate doesn’t yet consider specific microstructures sign B; particular relative standing lying) domains known greatly affect environment.8Dong Additionally, local affects S1→CT transition rate energies.4Qian Exploiting effects, control microstructure, enable reduction necessary second main observation paper FRET causes rather photoexcited lower-bandgap renders unimportant. Efficient reported between thanks Stokes shift, enabling long-distance transport.9Firdaus Le Corre V.M. Huang Chattopadhyay Nahid M.M. Nugraha M.I. Lin al.Long-range diffusion acceptors.Nat. 5220Crossref (78) role process offers parallels photosynthetic generation, where light harvesting via cascade spatially separated separation, leading ∼100% quasi-morphology independent.10Mohapatra A.A. Tiwari V. Patil ternary cells: Recent insights future directions.Energy Environ. Sci. 14: 302-319Crossref interesting junction architectures, especially quaternary blends. remains seen whether exploited ease limitations discussed above, minimized areas recombination summary, Karuthedath’s highlights importance accurately determining involved heterointerfaces explanation assist raises challenging questions identified circumvented, better electronic innovative designs exploiting long-range FRET.