Low Bandgap Nonfullerene Acceptor Research Articles

Effect of diazine isomers on photovoltaic properties of simple and efficient wide band gap polymer donor materials

In this paper, we designed and synthesized six new wide band gap donor-acceptor (D-A) copolymers, PZ2-PZ7, with diazine isomers (pyrazine, pyridazine, pyrimidine) as the A unit and alkylthiophene- or alkylthiothienyl-substituted BDT as the D unit. The effect of diazine isomeric A units and side-chain substituents on molecular structure, optoelectronic properties and photovoltaic performances of the resulting D-A copolymers were investigated in details. Six low-cost polymers were easily obtained by minimal reaction steps and cheap raw materials. All the polymers exhibited wide band gaps (WBGs) (>1.9 eV) and deep-lying HOMO energy levels (<−5.48 eV), which is convenient for paring with low band gap nonfullerene acceptors (NFAs). Theoretical simulation calculation indicates that diazine isomeric A unit has great impact on the planarity of polymer backbone. Bulk heterojunction polymer solar cells (PSCs) were fabricated with these polymers as donor materials and representative NFAs (Y6 and ITIC) as acceptor materials. After optimization, pyrazine-based polymers PZ2 and PZ3 exhibited higher PCEs than those of pyridazine- and pyrimidine-based ones (PZ4-PZ7) regardless of the acceptors. Finally, the PZ2:Y6-based PSC achieved the highest power conversion efficiency (PCE) up to 11.21 %, which could be attributed to more balanced carrier mobility, increased charge dissociation probability, reduced charge recombination, and superior active layer morphology. This work suggests that diazine-based WBG polymers, especially for pyrazine, would become as very promising donor candidates for constructing low-cost and efficient NFA-based PSCs.

Investigation of Hole-Transfer Dynamics through Simple EL De-Convolution in Non-Fullerene Organic Solar Cells.

In conventional fullerene-based organic photovoltaics (OPVs), in which the excited electrons from the donor are transferred to the acceptor, the electron charge transfer state (eECT) that electrons pass through has a great influence on the device's performance. In a bulk-heterojunction (BHJ) system based on a low bandgap non-fullerene acceptor (NFA), however, a hole charge transfer state (hECT) from the acceptor to the donor has a greater influence on the device's performance. The accurate determination of hECT is essential for achieving further enhancement in the performance of non-fullerene organic solar cells. However, the discovery of a method to determine the exact hECT remains an open challenge. Here, we suggest a simple method to determine the exact hECT level via deconvolution of the EL spectrum of the BHJ blend (ELB). To generalize, we have applied our ELB deconvolution method to nine different BHJ systems consisting of the combination of three donor polymers (PM6, PBDTTPD-HT, PTB7-Th) and three NFAs (Y6, IDIC, IEICO-4F). Under the conditions that (i) absorption of the donor and acceptor are separated sufficiently, and (ii) the onset part of the external quantum efficiency (EQE) is formed solely by the contribution of the acceptor only, ELB can be deconvoluted into the contribution of the singlet recombination of the acceptor and the radiative recombination via hECT. Through the deconvolution of ELB, we have clearly decided which part of the broad ELB spectrum should be used to apply the Marcus theory. Accurate determination of hECT is expected to be of great help in fine-tuning the energy level of donor polymers and NFAs by understanding the charge transfer mechanism clearly.

P-Type Conjugated Polymers Containing Electron-Deficient Pentacyclic Azepinedione.

Bisthienoazepinedione (BTA) has been reported for constructing high-performing p-type conjugated polymers in organic electronics, but the ring extended version of BTA is not well explored. In this work, we report a new synthesis of a key building block to the ring expanded electron-deficient pentacyclic azepinedione (BTTA). Three copolymers of BTAA with benzodithiophene substituted by different side chains are prepared. These polymers exhibit similar energy levels and optical absorption in solution and solid state, while significant differences are revealed in their film morphologies and behavior in transistor and photovoltaic devices. The best-performing polymers in transistor devices contained alkylthienyl side chains on the BDT unit (pBDT-BTTA-2 and pBDT-BTTA-3) and demonstrated maximum saturation hole mobilities of 0.027 and 0.017 cm2 V-1 s-1. Blends of these polymers with PC71BM exhibited a best photovoltaic efficiency of 6.78% for pBDT-BTTA-3-based devices. Changing to a low band gap non-fullerene acceptor (BTP-eC9) resulted in improved efficiency of up to 13.5%. Our results are among the best device performances for BTA and BTTA-based p-type polymers and highlight the versatile applications of this electron-deficient BTTA unit.

Terminal Groups of Nonfullerene Acceptors: Design and Application

Materials design plays a critical role in improving the power conversion efficiency (PCEs) of organic solar cells (OSCs). Recent advances in nonfullerene acceptors (NFAs) have achieved great success and made a large contribution to the rapid increase in PCEs. The current state-of-the-art OSCs have PCEs of >19%, demonstrating their great potential for use in practical applications. All high-efficiency NFAs adopt an A–D–A or A–DA′D–A (A = acceptor and D = donor) structure. Modulating the electron push–pull effect using an alternating D–A structure has proven to be an effective molecular design method. Specially, for NFAs, the design and application of terminal groups are of great significance. In this Perspective, a brief introduction is given to the development of terminal groups and representative materials. We highlight the critical role of the molecular electrostatic potential in evaluating the electron-withdrawing strength. The applications of terminal groups in designing wide-, middle-, and low-bandgap NFAs are discussed. Then, an outline of the other functions of terminal groups in affecting the intermolecular packing, blend morphology, and energy loss is presented. Furthermore, insights into the key issues that should be considered when developing new terminal groups, including efficiency, cost, and stability, are discussed.

Incorporating a weak acceptor unit into PTB7-Th to tune the open circuit voltage for non-fullerene polymer solar cells

PTB7-Th is one of the most effective polymer donors which have drawn huge attention in organic solar cells. However, the highest occupied molecular orbital (HOMO) level of PTB7-Th is relatively high, which limit open circuit voltage (Voc) of the cell devices. In this work, to develop the new members of PTB7-Th family materials with higher Vocs, two new random terpolymers PTB7-Th(0.9) and PTB7-Th(0.8) have been synthesized, in which contains 10% or 20% benzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxylate (VBDTC, a weak acceptor unit) moiety in PTB7-Th. It was found that the incorporating of VBDTC unit has great influence on optical and electronic properties of the related polymers. Comparison with PTB7-Th, the PTB7-Th(0.9) and PTB7-Th(0.8) exhibit blue shift in the UV−visible spectra, which forming a more complementary absorption with the low band gap non-fullerene acceptors, like ITIC and Y6. Increasing the percentage of VBDTC moiety, it can sufficiently deepen the low-lying HOMO level of the related polymers, which would expect to an enhancement in Voc. Non-fullerene solar cells blended with ITIC, PTB7-Th(0.9) exhibited a Voc of 0.839 V, with power conversion efficiency (PCE) of 4.86%. Interestingly, PTB7-Th(0.8) showed superior photovoltaic performance with a Voc of 0.862 V, and PCE of 5.24%. And when blended with Y6, the PCE of PTB7-Th(0.8) based device was further improved to 6.13%, due to increasing the short-circuit current (Jsc). The comprehensive study reveals that it is an effective strategy to tune the absorption spectra and deepens the HOMO energy level of the PTB7-Th related polymers by incorporation of a weak acceptor unit, thus, achieving higher Voc as well as photovoltaic performance.

Broadband Organic Ternary Bulk Heterojunctions Photodetector Based on Non-Fullerene Acceptor with Enhanced Flat-Spectrum Response Range from 200 to 1100 nm.

Both flat-spectrum responsivity and high external quantum efficiency (EQE) of bulk heterojunction organic photodetectors (BHJ OPDs) are greatly in demand and still challenging to realize from the ultraviolet (UV) to near-infrared (NIR) regions. In this article, conjugated polymer donor poly(3-hexylthiophene) (P3HT) and PTB7-Th are blended with a low band gap nonfullerene acceptor (NFA) IEICO-4F to form a ternary BHJ active layer, thereby forming a BHJ OPD with a broadband responsivity spectrum from UV to visible light to NIR region (200-1100 nm). Under 6 V voltage and in the range from 280 to 810 nm, the ternary BHJ OPD shows a relatively flat responsivity spectrum, and the highest responsivity is 1.348 A/W, which is 1.34 times that of the binary BHJ OPD. Specifically, the ternary BHJ OPD achieved the highest EQE at 285 nm and as high as 449.31%. In addition, the ternary OPD detectivity (D*) is 2.65 times that of the binary BHJ OPD. Therefore, ternary BHJ as an active layer provides an effective method to develop BHJ OPDs with an expanded response range, higher responsivity, improved EQE, and broadband spectrum with flat spectral response from the UV to NIR region.

Donor End-Capped Alkyl Chain Length Dependent Non-Radiative Energy Loss in All-Small-Molecule Organic Solar Cells.

A critical bottleneck for further efficiency breakthroughs in organic solar cells (OSCs) is to minimize the non-radiative energy loss (eΔVnr ) while maximizing the charge generation. With the development of highly emissive low-bandgap non-fullerene acceptors, the design of high-performance donors becomes critical to enable the blend with the electroluminescence quantum efficiency to approach or surpass the pristine acceptor. Herein, by shortening the end-capped alkyl chains of the small-molecular donors from hexyl (MPhS-C6) to ethyl (MPhS-C2), the material obtained aggregation that wasinsensitive to thermal annealing (TA) along withcondensed packing simultaneously. The former leads to small phase separation and suppressed upshifts of the highest occupied molecular orbital energy level during TA, and the latter facilitates its efficient charge-transport at aggregation-less packing. Hence, the ΔVnr decreases from 0.242 to 0.182V, from MPhS-C6 to MPhS-C2 based OSCs. An excellent PCE of 17.11% is obtained by 1,8-diiodoctane addition due to almost unchanged high Jsc (26.6mA cm-2 ) and Voc (0.888V) with improved fill factor, which is the record efficiency with the smallest energy loss (0.497eV) and ΔVnr (0.192V) in all-small-molecule OSCs. These results emphasize the potential material design direction of obtaining concurrent TA-insensitive aggregation and condensed packing to maximize the device performances with a super low ΔVnr .

Semitransparent organic solar cells with light utilization efficiency of 4% using fused-cyclopentadithiophene based near-infrared polymer donor

Semi-transparent organic solar cells (ST-OSCs) have attracted significant attention for various applications such as building-integrated photovoltaics and smart window technologies. Although various low-band gap non-fullerene acceptors have been developed, near-infrared (NIR) absorbing polymeric donors have been rarely reported for highly efficient ST-OSCs. In this study, we synthesize cyclopentadithiophene (CPDT) core-based NIR-absorbing polymer donors, PL1 and PL2. These polymer donors show strong NIR absorption capabilities with high transmittance in visible wavelength region and deep-lying HOMO levels for cascade energy level alignment with low-bandgap non-fullerene acceptors. The opaque device with PL2 and PCBM exhibits higher power conversion efficiency of 11.62% than that of the device with PL1 (PCE: 7.54%) due to lower charge-carrier recombination loss and balanced charge-carrier mobilities. Most significantly, incorporation of PL2 into ST-OSCs results in a PCE of 9.91 %, average visible transmittance of 40.4 % and light utilization efficiency (LUE) of 4.00 %. This work achieves the one of the highest LUE in ST-OSCs based on new NIR-absorbing polymer donors.

Alkyl Chain Engineering of Low Bandgap Non-Fullerene Acceptors for High-Performance Organic Solar Cells: Branched vs. Linear Alkyl Side Chains.

In this work, we report the synthesis and photovoltaic properties of IEBICO-4F, IEHICO-4F, IOICO-4F, and IDICO-4F non-fullerene acceptors (NFAs) bearing different types of alkyl chains (2-ehtylhexyl (EH), 2-ethylbutyl (EB), n-octyl (O), and n-decyl (D), respectively). These NFAs are based on the central indacenodithiophene (IDT) donor core and the same terminal group of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F), albeit with different side chains appended to the thiophene bridge unit. Although the side chains induced negligible differences between the NFAs in terms of optical band gaps and molecular energy levels, they did lead to changes in their melting points and crystallinity. The NFAs with branched alkyl chains exhibited weaker intermolecular interactions and crystallinity than those with linear alkyl chains. Organic solar cells (OSCs) were fabricated by blending these NFAs with the p-type polymer PTB7-Th. The NFAs with appended branched alkyl chains (IEHICO-4F and IEBICO-4F) possessed superior photovoltaic properties than those with appended linear alkyl chains (IOICO-4F and IDICO-4F). This result can be ascribed mainly to the thin-film morphology. Furthermore, the NFA-based blend films with appended branched alkyl chains exhibited the optimal degree of aggregation and miscibility, whereas the NFA-based blend films with appended linear alkyl chains exhibited higher levels of self-aggregation and lower miscibility between the NFA molecule and the PTB7-Th polymer. We demonstrate that changing the alkyl chain on the π-bridging unit in fused-ring-based NFAs is an effective strategy for improving their photovoltaic performance in bulk heterojunction-type OSCs.

Charge transport and recombination in wide-bandgap Y6 derivatives-based organic solar cells

The power conversion efficiency of nonfullerene-based organic solar cells (OSCs) has recently exceeded 18%, thanks to the constant effort to identify the key properties governing the OSCs performance and development of better photovoltaic materials. With its superior properties, low-bandgap Y6 and its derivatives have emerged as one of the most popular nonfullerene acceptors (NFAs) for OSCs. In most cases, these low bandgap NFAs were based mainly on the most widely used and successful end-group 1,1-dicyanomethylene-3-indanone (IC). On the other hand, wide-bandgap Y6 derivatives are still scarce. Attempts to increase the NFA’s bandgap by incorporating electron-rich end-groups often end up with NFAs with poor performance. In this work, we compare two wide-bandgap Y6 derivatives with different end-groups, and their distinct device performance is correlated with their charge transport and recombination properties. Electronic measurements on solar cell devices and device physics results are presented to discuss charge transport and recombination within the device.

Morphology manipulation for highly miscible photovoltaic blend of carboxylate-substituted polythiophene:Y6

Polythiophenes and its derivatives (PTs) have attracted extensive research interest in the field of organic solar cells (OSCs) owing to their benefits of low synthesis cost and high scalability of synthesis. However, the efficiency of PT-based photovoltaic devices is far lower than that of the popular push-pull conjugated polymer-based devices. To elevate the photovoltaic performance, pairing PTs with low-bandgap acceptors is highly desirable. Nevertheless, when pairing the prevailing low-bandgap non-fullerene acceptor Y6 with a representative PT derivative named PDCBT-Cl, the OSC delivered an extremely low efficiency of ∼0.1%. The crucial reason for the abysmal performance is the poor morphology with insufficient phase separation induced by the high miscibility between PDCBT-Cl and Y6. How to tune the morphology and promote phase separation is challenging and important for the highly miscible blend system to achieve performance breakthrough. Given that PDCBT-Cl and Y6 are crystalline, in the present study, we promote the formation of phase separation for PDCBT-Cl:Y6 blend by control molecular crystallization, using thermal annealing (TA) at various temperatures. We observed that, phase-separated morphology appeared when TA temperature was higher than the aggregation transition temperature of PDCBT-Cl. Upon TA at 160 °C, ordered molecular packing and appropriate phase separation were obtained for PDCBT-Cl:Y6 blend film, and the device efficiency increased dramatically to ∼10%, which was ∼90 times higher than that of the as-cast counterpart. This study demonstrates that promoting crystallization is an effective means to induce phase separation for the highly miscible blend system with crystallinity.

Vacuum-deposited organic solar cells utilizing a low-bandgap non-fullerene acceptor

A new low-bandgap D–A type non-fullerene acceptor is designed and synthesized, which is successfully applied in full-vacuum-deposited organic solar cells and realizes a PCE of 0.86%.

Contribution of dark current density to the photodetecting properties of thieno[3,4-b]pyrazine-based low bandgap polymers

Recently, near infrared (NIR) organic photodetectors (OPDs) have been extensively studied. Bulk heterojunction NIR OPDs composed of a high-bandgap polymer donor (PD) and a low-bandgap non-fullerene acceptor (NFA) showed the best performance, whereas the low-bandgap PD-based OPDs were relatively unsuccessful due to the high level of dark current density (Jd) under a negative bias. In this study, we synthesized three low-bandgap PDs based on a thieno[3,4-b]pyrazine (TP) moiety and developed red-NIR OPDs by blending them with a low-bandgap NFA. We found that the PD having a shallow HOMO energy level generated the largest ground-state electron transfer at negative bias, which overestimated the responsivity (R) and detectivity (D*) in OPDs. Notably, under weak light irradiation of 0.1 mW/cm2 at -2V, the contribution of Jd on Jph reached 99.6%. Thus, we modified the existing R and D* equations to better understand photodetecting properties at low light intensity, and these modified equations gave more realistic R and D* values in OPDs. On the other hand, a low-bandgap PD showing low Jd in OPDs was highly beneficial to detect a low light signal because the Jd negligibly contributed to Jph in OPDs. The low Jd values of OPDs at negative bias resulted in a high on/off signal ratio and constant R and D* values at different light intensities.

Single‐Junction Organic Photovoltaic Cell with 19% Efficiency

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 0.879V, short-circuit current density of 26.7mA 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.

Low-bandgap nonfullerene acceptor based on thieno[3,2-b]indole core for highly efficient binary and ternary organic solar cells

A low-bandgap nonfullerene acceptor (NFA) TIT-2FIC based on thieno[3,2-b]indole-thiophenes core has been developed. Compared with the analogue NFAs DTC(4Ph)-4FIC and IT-4F, TIT-2FIC exhibited remarkably red-shifted absorption, and up-shifted HOMO energy level. In addition, TIT-2FIC showed interesting universal miscibility with the donors nonfluorinated PBDB-T and fluorinated PM6, therefore the corresponding organic solar cells achieved promising power conversion efficiencies (PCEs) of 11.80% and 13.00%, respectively, which are higher compared to the counterpart IT-4F based cells. Furthermore, the ternary PM6:TIT-2FIC:Y6 cell pronounced a high PCE of 17.22%, being significantly improved from that of 16.04% for the binary PM6:Y6 cell. Similar improvement in PCEs from 13.41% to 14.46% was also observed in the ternary PM6:TIT-2FIC:IT-4F cell with TIT-2FIC as the third component. These results indicated that TIT-2FIC is universally applicable as an acceptor with nonfluorinated or fluorinated polymer donor materials in both binary and ternary cells.

Semitransparent organic solar cells based on all-low-bandgap donor and acceptor materials and their performance potential

Recent advances in organic solar cells (OSCs) based on large-bandgap donors and low-bandgap non-fullerene acceptors (NFAs) have increased the power conversion efficiency (PCE) of OSCs to ~18%. However, these state-of-the-art OSCs have strong absorption in the visible region, limiting their application in semitransparent organic solar cells (STOSCs). In this study, an all-low-bandgap system based on a low-bandgap polymer donor (PM2) and a low-bandgap NFA (Y6-BO), was introduced as the light-harvesting layer for STOCSs with absorption mainly localized in the near-infrared (NIR) spectrum from 600 to 900 nm. The corresponding opaque OSCs exhibited the highest PCE among reported all-low-bandgap OSC systems, and the corresponding STOSCs showed higher visible light transmittances (VLTs) and light utilization efficiencies (LUEs) than the reference devices based on state-of-the-art PM6:Y6-BO OSC system with broad range absorption from visible to NIR. Optical simulations predicted that the PM2:Y6-BO-based STOSCs have a greater potential to realize higher VLTs and PCEs and better PCE retention (PCEsemitransparent/PCEopaque) than those from the PM6:Y6-BO-based STOSCs. Guided by these simulations, PM2-based STOSCs with VLTs exceeding 40% and PCEs of ~6% were achieved. Further recombination analysis suggested that the PM2-based devices experienced more severe charge recombination and energy losses, indicating there is further room for PCE improvement by designing new all-low-bandgap systems. Overall, this work shows the great potential of all-low-bandgap systems in realizing STOSCs with high PCEs and VLTs, which is promising for the commercialization of OSCs as power-generating window applications.

An Effective Strategy to Design a Large Bandgap Conjugated Polymer by Tuning the Molecular Backbone Curvature

With the significant progress of low bandgap non-fullerene acceptors, the development of wide bandgap (WBG) donors possessing ideal complementary absorption is of crucial importance to further enhance the photovoltaic performance of organic solar cells. An ideal strategy to design WBG donors is to down-shift the highest occupied molecular orbital (HOMO) and up-shift the lowest unoccupied molecular orbital (LUMO). A properly low-lying HOMO of the donor is favorable to obtaining a high open-circuit voltage, and a properly high-lying LUMO of the donor is conductive to efficient exciton dissociation. This work provides a new strategy to enlarge the bandgap of a polymer with simultaneously decreased HOMO and increased LUMO by increasing the polymer backbone curvature. The polymer PIDT-fDTBT with a large molecular backbone curvature shows a decreased HOMO of -5.38eV and a prominently increased LUMO of -3.35eV relative to the linear polymer PIDT-DTBT (EHOMO = -5.30eV, ELUMO = -3.55eV). The optical bandgap of PIDT-fDTBT is obviously broadened from 1.75 to 2.03eV. This work demonstrates that increasing the polymer backbone curvature can effectively broaden the bandgap by simultaneously decreasing HOMO and increasing LUMO, which may guide the design of WBG conjugated materials.

Carrier losses in non-geminate charge-transferred states of nonfullerene acceptor-based organic solar cells

To understand the current limitations of nonfullerene-based organic solar cells (OSCs), the early-time dynamics of the carrier generation in the high performance bulk heterojunction (BHJ) blend of a semiconducting polymer, PBDB-T, and the low bandgap nonfullerene acceptor, ITIC-m, are investigated. After photoexcitation, photo-induced excitons are separated through the ultrafast (~200 fs) electron transfer process from PBDB-T to ITIC-m and through the fast (3–6 ps) hole transfer process from ITIC-m to PBDB-T. However, a part of the separated charges recombines in the non-geminate (long-range) charge-transferred (CT) states. The yield of mobile carriers is correspondingly decreased by recombination in the CT states. In our measurements, the carrier recombination loss in the CT state is decreased by optimizing the BHJ morphology, especially for showing better electron mobility using a processing additive (1,8-diiodooctane) during the fabrication of the composite film, as evidenced by the decreased CT band intensity at ~30 ps and the increased polaron band intensity, which eventually improve power conversion efficiencies (PCEs).

Highly Efficient Nonfullerene Acceptor with Sulfonyl-Based Ending Groups.

Ending groups play a vital role in regulating the band gap and energy level of low-band gap nonfullerene acceptors (NFAs). In this work, a novel NFA, BTP-IS, is synthesized by adopting sulfonyl-based ending groups. Compared to the ketone counterpart BTP-IC, BTP-IS exhibits a red-shift in absorption spectra with lower lowest unoccupied molecular orbital level. More importantly, the BTP-IS-based organic solar cells with PM6 as donor present a high power conversion efficiency (PCE) of 12.79%, which is much higher than that of the BTP-IC device (PCE of 7.54%). The efficient charge transfer between the polymer donor and NFA acceptor, the balance charge transport, and the fine photoactive morphology bear on the effective exciton dissociation and charge collection in the BTP-IS device, which induces high short circuit current (JSC) and fill factor (FF) values. This research has shed light on designing novel NFAs from the perspective of ending groups.

Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells.

In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor-acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor-NFA interface caused by the acceptors' quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.