The organic dye diketopyrrolopyrrole (DPP) was utilized as a core between two PDI units. DPP-based small molecules have been reported to organize in the film with solvent vapor annealing (SVA) treatment, which was a motivation for its use [108]. DPP is a relatively weaker acceptor than PDI, which creates a unique all-acceptor-based, push-pull system with strong panchromatic light absorption. McAfee et al. [84] demonstrated the facile synthesis of PDI-DPP-PDI using direct-heteroarylation (DHA) cross-coupling methods. The optical absorption spectra of the PTB7-Th:PDI-DPP-DPI blend showed strong light absorption from 400 to 800 nm with an increase at 586 nm upon SVA treatment for 2–5 min, which is attributed to the crystallization of the PDI-DPP-PDI molecule. A PCE of 5.6% (Voc = 0.98 V; Jsc = 11.32 mA/cm²; FF = 50.1%) was achieved using chloroform SVA treatment for 2.5 min. The J–V curves and EQE spectra of the OPV devices with and without SVA treatment of the photoactive layer show that the SVA treatment led to a significant increase in photocurrent generation in the visible region.

Following this, McAfee et al. [87] demonstrated a multigram synthesis of PDI-DPP-PDI, and the NFA molecule was blended with four different BDT-based donor polymers (PTB7-Th, PBDB-T, J61, and TTFQx-T1) to identify best donor/acceptor pairings. The best OPV device performance was obtained using the donor polymer PTB7-Th, providing a PCE of 6.2% (V_oc = 0.98 V; J_sc = 14.52 mA/cm²; FF = 44%). The pairing with TTFQx-T1 provided devices with a slightly lower PCE of 5.7% (V_oc = 1.02 V; J_sc = 11.53 mA/cm²; FF = 49%). Pairing with PBDB-T and J61 resulted in device performance drops, demonstrating PCEs of 2.3% (V_oc = 1.0 V; J_sc = 5.77 mA/cm²; FF = 40%) and 3.1% (V_oc = 0.99 V; J_sc = 6.66 mA/cm²; FF = 48%), respectively. Consistent for all blends was the use of a short SVA treatment to crystallize the PDI-DPP-PDI acceptor and boost the OPV performance.

Further developing this type of NFA, McAfee et al. [86] investigated the role of other dye-based cores, including DPP with branched side chains (DPP-B), isoindigo (ISI-B), and thienylisoindigo (TII-B). The compounds were paired with the donor polymer, PTB7-Th, devices were fabricated, and the OPV performance was measured. The devices based on TII-B, DPP-B, and ISI-B showed much lower performances of 0.4% (Voc = 0.93 V; Jsc = 1.09 mA/cm²; FF = 34%), 2.9% (Voc = 0.97 V; Jsc = 6.0 mA/cm²; FF = 50%), and 2.6% (Voc = 1.03 V; Jsc = 6.97 mA/cm²; FF = 37%), respectively, compared to devices based on the parent PDI-DPP-PDI NFA.

Cann et al. explored the optoelectronic properties of N-annulated PDI NFA dimers containing different cores connected to the PDIs with acetylene linkers [109]. They found that by linking the PDI units to the dye cores through acetylene linkers, steric interactions are minimized. This allows for strong electronic communication between the PDIs and the dye cores, leading to intense panchromatic optical absorption. Motivated and with a goal to develop narrow-bandgap PDI-based NFAs, the formazanate (BF₂fz) chromophore was investigated as an electron-deficient core with strong light absorption [110]. A new NFA (PDI-BF₂fz-PDI), comprised of a BF₂fz core acetylene-linked to N-annulated PDI endcaps, was introduced by Koeing et al. [88]. Prior, SVA treatment was proven to induce crystallization of DPP-containing PDIs, leading to significant changes in optical absorption [82]. After the 5 min chloroform SVA treatment, a red shift of 150 nm in the onset of absorption was observed in both neat and blend films. Proof-of-concept OPV devices were fabricated to evaluate the capabilities of the new molecule as an NFA. The PDI-BF₂fz-PDI acceptor was used as a third component in a ternary blend composed of FBT (donor polymer) and PC₆₁BM (fullerene acceptor) with a blend ratio of (1:1:0.5 w/w) and a 10 mg/mL total concentration. OPV devices with active layers treated under chloroform SVA conditions showed enhanced photovoltaic performance compared to as-cast devices. The PCEs doubled due to J_sc enhancement. Surface morphologies (as determined by AFM) showed distinct differences between the as-cast and SVA-treated blend films. The root mean square surface roughness of the films increased from 1.9 nm for the as-cast films to 4.8 nm for the SVA-treated films. The enhanced performance in OPV devices was attributed to the aggregation of PDI-BF₂fz-PDI into phase-separated domains rather than crystallization.

Lowering the electron affinity (i.e., raising the LUMO energy level) [31] of PDI is desired to increase the operating V_oc in OPV devices by maximizing the energy gap between the HOMO of the donor and the LUMO of the acceptor. The use of electron-rich thiophene cores (donor) with the PDI dye (acceptor), thus applying the donor–acceptor (D–A) strategy, was carried out to precisely tune energy levels and optical absorption. N-annulated PDI dimers bearing various thiophene linkers (PDI-core-PDI) were reported by a group [89,90,91,92,93]. Hendsbee et al. demonstrated that the use of thiophene or bithiophene as the core destabilized the LUMO energy levels to 3.47 and 3.54 eV and electrochemical bandgaps of 2.22 and 2.04 eV, respectively, relative to the parent N-hexyl PDI dimer (tPDI₂N-Hex; LUMO energy level = 3.76 eV; electrochemical band gap = 2.23 eV) [89]. The PDI NFAs with thiophene cores were paired with the PTB7-Th donor polymer to deliver OPV devices with high V_oc values above 1 V. However, the inability of the NFAs to form crystalline domains when compared to tPDI₂N-Hex resulted in poor device performances. To tackle this design drawback, thiophene core fusion was proposed to enhance the PDI self-assembly and increase the acceptor domain crystallinity. Laventure et al. developed a fused, thiophene-based PDI acceptor (F-NPDI₂), which lead to devices with a higher V_oc (ca. 1.1 V) [90] and improved device PCEs.

Further chemical modification of the thiophene-based dimers was introduced by Welsh et al., using dithienophosphole and borane incorporation [91,92]. P-phenyl dithienophosphole oxide (Th₂PO) was used as a core, providing the NFA PDI-Th₂PO-PDI. Again, OPV devices based on this NFA and the PBDB-T donor polymer had high V_oc values (1.1 V), but only moderate PCE values (ca. 2.3%). In contrast, incorporating boron into the molecular framework, namely, PDI-Th₂B-PDI, led to a twisting of the structure [92]. With the addition of the borane unit, the electrochemical bandgap increased from 2.12 eV for PDI-Th₂PO-PDI to 2.20 eV in PDI-Th₂B-PDI. OPV devices using this NFA, and the medium-gap donor polymer TTFQx-T1(QX1) provided a PCE of 1.8% (Voc = 1 V; Jsc = 5.3 mA/cm²; FF = 33%) for the devices with as-cast active layers. With thermal annealing of the active layers for 10 min at 180 °C, PCEs increased to 2.3% (Voc = 1 V; Jsc = 6.9 mA/cm²; FF = 33%). Use of a 5% diphenyl ether solvent additive (DPE) during photoactive film formation, in addition to thermal annealing for 10 min at 180 °C, led to a PCE of ~3.9% (Voc = 1.0 V; Jsc = 9.3 mA/cm²; FF = 37%), nearly double that of the phosphine oxide analogue. For the PDI-Th₂B-PDI, the overlap of the thiophene π* orbitals and the boron P_z orbital [111] helps to stabilize the LUMO, while for PDI-Th₂PO-PDI, the LUMO is stabilized through the overlap of the bithiophene π* orbital and exocyclic P—R σ* orbitals [112]. Both have a higher electron affinity than the parent bithiophene analogue (PDI-Th₂-PDI), but they have different geometries due to the fused planar structure of Th₂PO and the trigonal structure of Th₂B, providing a comparison of the effect of molecule shape on device characteristics.

Another thiophene building block, thieno[3,4-c]pyrrole-4,6-dione (TPD), was used as a core to create the NFA PDI-TPD-PDI [93]. TPD is a commonly used building block for constructing organic photoactive materials with an electron-deficient aromatic system, a site for side-chain engineering, and activated CH bonds for facile functionalization using DHA methods [113]. The PDI-TPD-PDI NFA molecule can be considered a hybrid structure of the previously designed NFAs: PDI-DPP-PDI [86] and tPDI₂N-Hex [81]. The use of TPD was to provide enough steric bulk between the two PDI units to twist the structure and prevent significant aggregation, mimicking the effect in tPDI₂N-Hex while being electron-deficient, maintaining a low-lying LUMO energy level, and having a linear octyl chain (such as within PDI-DPP-PDI) to enable the molecule to self-organize within the film. The introduction of the TPD core did not have large impact on the ionization potential and electron affinity values (IP = 5.9 eV, EA = 3.7 eV, and E_elec = 2.2 eV) relative to tPDI₂N-Hex with no core. A PCE of ~3.28% (Voc = 1.05 V; Jsc = 7.40 mA/cm²; FF = 42%) was realized for OPV devices using the donor polymer PDBD-T with PDI-TPD-PDI. The use of the DPE solvent additive during BHJ film formation, which induced phase separation, was key.

Indancenodithiophene (IDT) is a rigid, rod-like organic conjugated building block that has been used to create one of the most successful NFAs to date, namely, ITIC [56,114]. Payne et al. used IDT as a core for four PDI units (synthesized via a 4-fold DHA coupling method), providing the tetrameric NFA IDT–NPDI₄, which exhibited with a pseudo-butterfly structure [94]. The ease of functionalization of the thiophene-based core provides a simple synthetic pathway towards tetrameric PDIs.

To test the validity of IDT-NPDI₄ as an NFA, OPV devices were fabricated by blending it with the PTB7-Th donor polymer. Various amounts of a chloronaphthalene (CN) additive of 3%, 5%, and 7% in chlorobenzene, used during BHJ film formation, achieved device PCEs of 3.03% (Voc = 1.03 V; Jsc = 8.53 mA/cm²; FF = 35%), 3.41% (Voc = 1.02 V; Jsc = 9.62 mA/cm²; FF = 35%), and 2.9% (Voc = 1.00 V; Jsc = 8.44 mA/cm²; FF = 34%), respectively. High V_oc values >1 were obtained with modest J_sc values (8–10 mA/cm²) but poor FFs. The contribution of the IDT-NPDI₄ NFA to light harvesting was confirmed by EQE measurements.

A follow-up work by Koenig et al., which used a less rigid π-conjugated cyclooctatetrathiophene (Th₄) core, introduced the tetrameric NFA Th₄PDI₄ [95]. The new NFA design was proposed to provide a different molecular shape when compared with IDT-NPDI₄; this might be the reason behind the poor FFs obtained. Testing the viability of Th₄PDI₄ as an NFA, OPV devices were fabricated by pairing it with a PTB7-Th donor polymer. A device PCE of 2.2% (Voc = 1.01 V; Jsc = 5.1 mA/cm²; FF = 42%) was obtained for devices with as-cast active layers. The addition of various amounts of CN additive (1%, 3%, and 5% CN) during the BHJ film formation enhanced the device PCEs to 3.61% (Voc = 0.98 V; Jsc = 9.5 mA/cm²; FF = 39%), 4.52% (Voc = 0.99 V; Jsc = 10.6 mA/cm²; FF = 43%), and 4.16% (Voc = 1.00 V; Jsc = 9.3 mA/cm²; FF = 45%), respectively. EQE measurements of devices with and without the 3% CN additive showed a significant enhancement of the photocurrent generation when additives were used for processing the BHJs. While the IDT-NPDI₄ and Th₄PDI₄ tetrameric NFAs only provided modest PCEs, facile synthesis using DHA methods opens the possibility for a larger number of tetrameric structures to be designed and synthesized for probing structure–function relationships.

Researchers have extensively studied the effect of side-chain engineering N-annulated PDI dimers on OPV device performance [81,83,96,99,103,105] through the functionalization of the pyrrolic N-atoms. Nazari eta al. developed three PDI dimers using benzyl-based moieties at the pyrrolic N-atoms. Benzyl, 4-fluorobenzyl, and pentafluorobenzyl were used as side chains to form the compounds tPDI₂N-benzyl (1), tPDI₂N-benzylF (2) and, tPDI₂N-benzylF5 (3) [98]. The photovoltaic characteristics of (1), (2), and (3) were evaluated by blending them with PTB7-Th or QX1 for BHJ layer formation. OPV devices with PTB7-Th: NFA and QX1: NFA BHJ blends demonstrated a similar trend in terms of photovoltaic characteristics. The PTB7-Th:1 blend provided the best device performance, demonstrating a PCE of 5.7% (Voc = 0.96 V; Jsc = 11.7 mA/cm²; FF = 51%) compared to PCEs of 4.9% (Voc = 0.90 V; Jsc = 11.0 mA/cm²; FF = 49%) and 4.2% (Voc = 0.89 V; Jsc = 10.2 mA/cm²; FF = 47%) for devices based on the PTB7-Th:2 and PTB7-Th:3 BHJ blends, respectively. Switching to the QX1 polymer donor resulted in higher device PCEs yet similar metrics, regardless of the acceptor used: QX1:1 (PCE= 5.8%; Voc = 0.99 V; Jsc = 14.1 mA/cm²; FF = 42%), QX1:2 (PCE= 5.8%; Voc = 0.96 V; Jsc = 15.1 mA/cm²; FF = 41%), and QX1:3 (PCE= 5.5%; Voc = 0.96 V; Jsc = 14.4 mA/cm²; FF = 40%). It is important to note that the introduction of F-atoms into the benzyl side chains stabilizes the frontier molecular orbital energy levels, which directly correlate to a drop in V_oc values for the OPV devices.

The N-alkyl chain modification of twisted PDI dimers and its effect on OPV device performance were first reported by Hendsbee et al. [81,99]. First, ethyl-(7b) and hexyl-functionalized (7 or tPDI₂N-Hex) N-annulated PDI dimers provided device PCEs of 5.5% and 5.1%, respectively, when paired with a PTB7-Th donor polymer. Using the P3TEA donor polymer with PDI NFA 7b led to the higher device PCE of 7.5% (Voc = 1.13 V; Jsc = 11 mA/cm²; FF = 61%). The promising photovoltaic performance coupled with the simple, scalable, and reproducible synthesis encouraged researchers to further optimize these PDI derivatives and use green solvent processing methods [99]. A series of eight different PDI dimers functionalized with alkyl chains with lengths ranging from linear methyl (C1) to octyl (C8) and the branched alkyl chain 2-ethylhexyl (C8b) were synthesized and characterized, and their use in OPV devices was evaluated. Inverted-type OPV devices were fabricated with BHJ blends comprising PTB7-Th and each PDI NFA, all processed from the green solvent 2Me-THF. It was found that regardless of alkyl chain length, comparable photovoltaic performances were achieved. Ethyl- and hexyl-functionalized PDI dimers gave good device PCEs of 4.8% (Voc = 0.95 V; Jsc = 11.4 mA/cm²; FF = 43%) and 5.5% (Voc = 0.96 V; Jsc = 12.1 mA/cm²; FF = 45%). The branched 2-ethylhexyl side-chain functionalized PDI dimer (C8b, tPDI₂N-EH) provided the best device performance with a PCE of 6.5% (Voc = 0.96 V; Jsc = 15.4 mA/cm²; FF = 43%).

As mentioned prior, the insertion of acetylene units as spacers in tPDI₂N-Hex was proposed as a strategy to control the steric hindrance between the PDI subunits [97]. Adding one acetylene spacer led to partially twisted dimer when compared to tPDI₂N-Hex, which is highly twisted. Adding two acetylene spacers rendered the PDI subunits coplanar. However, acetylene spacers were found to reduce the material’s solubility in organic solvents and thus induced strong aggregation in both solution and thin films. The latter led to a dramatic deterioration in OPV device performance (PCE = 1.3%, Voc = 0.95 V; Jsc = 4.4 mA/cm²; FF = 32%) compared to the devices using the parent tPDI₂N-Hex (PCE = 5.2%, Voc = 0.96 V; Jsc = 11.5 mA/cm²; FF = 47%). Thus, in-line with the current literature on PDI-based NFAs, molecular-based NFAs using PDI subunits perform better in OPV devices when the structure has non-planar (i.e., twisted) arrangements of the PDIs.

The tPDI₂N-Hex NFA was introduced as a third component in PBDB-T:PC₆₁BM BHJ blends to fabricate ternary OPVs. All the active layers were processed in air from chlorobenzene, using 3% DIO as solvent additive. It was observed that 10 wt.% tPDI₂N-Hex was enough to enhance the device PCE from 8.1% for the binary blend (PBDB-T: PC₆₁BM) to 8.8% for the ternary blend (PBDB-T:PC₆₁BM:tPDI₂N-Hex). The device performance enhancement was mainly due to an increase in the V_oc from 0.80 V for the binary blend to 0.87 V for the ternary blend. tPDI₂N-Hex has optical absorption from 400 to 600 nm, complementing the optical absorption of the PBDB-T:PC₆₁BM blend.

Owing to the high V_oc (1.0 V) values and the strong optical absorption in the visible range (400–650 nm), OPV devices using the tPDI₂N-EH NFA were evaluated for indoor light harvesting applications [105]. BHJ blends were processed from o-xylene and made use of the FBT donor polymer. Inverted-type OPV devices provided a PCE of 6.5% (Voc = 1.03 V; Jsc = 11.9 mA/cm²; FF = 53%) under 1 sun illumination with spin-coated BHJs. With slot-die coating the BHJs, the inverted-type OPV devices provided a PCE of 4.6% (Voc = 1.02 V; Jsc = 9.5 mA/cm²; FF = 48%). The drop in PCE observed in the transition from spin coating to slot-die coating the BHJs was attributed to the slow drying time associated with the slot-die coating, which allowed for unfavorable phase separation. Under low-light illumination (cold and warm white LED source; 600 lux), OPV devices with slot-die-coated BHJs had a PCE of 9.2% (V_oc ≈ 0.82 V; J_sc ≈ 40.3 µA/cm²; FF ≈ 51%) and an output power of P_out ≈16.6 μW/cm². Compared to OPV devices using other acceptors (PC₆₀BM, ITIC-M, and ITIC-F) and a reference Si diode, the devices based upon tPDI₂N-EH had the highest V_OC values and PCEs second to only the fullerene-based systems. These results demonstrated how N-annulated PDI-based NFAs have utility for indoor photovoltaic applications.

A follow-up work by Farahat et al. introduced OPVs based on the FBT:PC₆₁BM: tPDI₂N-EH (PDI) ternary blend for indoor light harvesting [104]. the PDI acceptor was added to the FBT:PC₆₁BM blend with content at 10–50% of the total concentration. Optimized OPV devices with spin-coated BHJs (ternary blend ratio of 1:1.2:0.3) achieved a PCE of 7.9% (Voc = 86 V; Jsc = 13.4 mA/cm²; FF = 68%) compared to a PCE of 7% (Voc = 76 V; Jsc = 12.5 mA/cm²; FF = 73%) for devices using a binary BHJ blend (i.e., only FBT:PC₆₁BM). Using PEIE/ZnO as the electron transport layer (ETL) instead of ZnO provided similar device performance but with more consistency and reproducibility (PCE = 7.4% for binary and PCE = 7.9% for ternary). Under the optimized processing conditions, OPV devices with slot-die-coated binary blend BHJs had a drop in performance, demonstrating a PCE of 6.8% (Voc = 78 V; Jsc = 12.3 mA/cm²; FF = 71%) when compared to their spin-coated counterparts. On the other hand, OPV devices with slot-die-coated ternary blend BHJs retained performance (PCE= 7.7%; Voc = 86 V; Jsc = 13 mA/cm²; FF = 69%) when compared to their spin-coated counterparts.

The optical absorption spectra of the FBT:PC₆₁BM and FBT:PC₆₁BM:PDI blended films exhibited a strong visible light absorption which directly overlaps with the emission of indoor lights. Under low-light LED illumination [ca. 2000 lux (568 μW/cm² respectively)], OPV devices with slot-die-coated binary or ternary BHJs achieved PCEs of 12% (Voc = 0.63 V; Jsc = 154 μA/cm²; FF = 70%) and 14% (Voc = 72 V; Jsc = 153 μA/cm²; FF = 72%), respectively. Devices with spin-coated BHJs under the same conditions achieved PCEs of 13.1% (Voc = 0.63 V; Jsc = 162 μA/cm²; FF = 73%) and 15.5% (Voc = 72 V; Jsc = 167 μA/cm²; FF = 73%), respectively.

A team further demonstrated the compound tPDI₂N-EH as a versatile NFA by pairing it with different donor polymers, processing BHJs under a variety of conditions, and evaluating devices with a range of illumination intensities as detailed below. Achieving high V_oc values ≥ 1 V are desired for OPV devices; thus, quinoxaline-based donor polymers with deep HOMO energy levels are widely used to achieve such metrics. For a group, pairing such polymers with tPDI₂N-EH allowed for the realization of high V_oc OPVs in which the BHJ was processed via slot-die-coating from halogen-free solvents [85,100,101]. Laventure et al. screened a series of BDT-QX donor polymers (QX1, QX2, QX3, QX4 with tPDI₂N-EH and compared BHJ formation using spin coating and slot-die coating from non-halogenated solvents [100]. Spin-coated OPV devices with QX1, QX2, QX3, and QX4, blended with tPDI₂N-EH provided PCEs of 5.7% (Voc = 1.03 V; Jsc = 9.4 mA/cm²; FF = 59%), 4.8% (Voc = 1.03 V; Jsc = 8.5 mA/cm²; FF = 54%), 4.5% (Voc = 1.06 V; Jsc = 7.6 mA/cm²; FF = 55%), and 5.7% (Voc = 1.04 V; Jsc = 9.4 mA/cm²; FF = 59%), respectively. Slot-die-coated devices based on the QX3 polymer resulted in a PCE of 4.1% (Voc = 1.06 V; Jsc = 7.0 mA/cm²; FF = 56%). For all OPV devices, the V_OC was above 1 V. The diphenyl ether (DPE) solvent additive was found to be critical for increasing the device performance while maintaining a fully halogen-solvent-free processing.

Follow-up work by Tontori et al. investigated the effect of solvent additives (DIO and DPE) on the blend morphology and aggregation behavior of BHJ blends based on QX3 and tPDI₂N-EH [101]. Processing the BHJ with a DIO solvent additive induced excessive crystallization of tPDI₂N-EH within the polymer matrix, leading to poor OPV performance, whereas processing the BHJ with DPE had a more suitable effect and, in turn, led to a boost in the OPV performance. OPV devices based on the QX3:tPDI₂N-EH (1:1) BHJ with different solution concentrations (10, 15, and 20 mg/mL) were slot-die-coated from toluene with and without the DPE additive and compared to devices with spin-coated BHJs. OPV devices with BHJs processed with the DPE additive had a PCE of 7.3% (Voc = 1.09 V; Jsc = 12.4 mA/cm²; FF = 54%) compared to a PCE of 6.6% (Voc = 1.09 V; Jsc = 11.7 mA/cm²; FF = 51%) for those processed without the additive. A transition to slot-die-coated BHJs resulted in devices with a PCE of 6.5% (Voc = 1.08 V; Jsc = 10.5 mA/cm²; FF = 58%) compared to a PCE of 6% (Voc = 1.06 V; Jsc = 13.0 mA/cm²; FF = 43%) for devices processed without the additive. Little change was observed in the in device PCEs when transitioning from spin- to slot-die-coating methods (using halogen-free solvents in air) and, in all cases, the Voc values were maintained above 1 V.

Further, the QX3 donor polymer was replaced with a simplified version, namely, PTQ10 polymer, which has a larger band gap (E_gap, ca 2.6 eV) and led to even higher Voc values. OPV devices with a PTQ10:tPDI₂N-EH BHJ processed using slot-die coating from halogen-free solvents had a PCE of 5% (Voc = 1.2 V; Jsc = 8.0 mA/cm²; FF = 52%) under 1 sun illumination (AM1.5G; 100 mW/cm²). Simialr OPV devices with spin-coated BHJs had a slightly higher PCE of 5.5% (Voc = 1.21 V; Jsc = 8.2 mA/cm²; FF = 55%), again demonstrating that the tPDI₂N-EH NFA can be easily transitioned to large-area devices. OPV devices were tested under low-light illumination conditions (ca. 400, 1000, and 2000 lux, both with 2700 K and 6500 K LED light, corresponding to P_in values between 113 and 610 μW/cm²) and provided maximum PCEs of 10.1% (Voc = 1.07 V; Jsc = 105 µA/cm²; FF = 51%) and 10.4% (Voc = 1.08 V; Jsc = 118 µA/cm²; FF = 50%) with 2700 K and 6500 K, respectively. These V_oc values are among the highest reported for indoor OPV devices [43]. To further improve the device PCEs, two strategies were examined: i) ZnO surface modification with PDICA (a carboxylic acid modified PDI), following a previous work reported by Abd-Ellah et al. [102], and ii) using the ternary approach by employing PDI-EDOT-PDI [115] as a third component in PTQ10:tPDI₂N-EH. Using PDICA for ZnO modification enhanced PCEs under both 1 sun (PCE = 6.1%; Voc = 1.24 V; Jsc = 8.9 mA/cm²; FF = 55%) and warm ca. 2000 lux LED illumination (PCE= 11.7%, Voc = 1.07 V; Jsc = 139 µA/cm²; FF = 48%). With the ternary approach, a V_oc of 1.24 V under one sun illumination and 1.11 V under ca. 2000 lux LED illumination were achieved. Although the ternary approach resulted in reducing both the device J_sc and FF, leading to an overall lower PCE under 1 sun, a PCE of ~10% under ca. 2000 lux LED illumination was achieved.

Asymmetric (or unsymmetrical) linear conjugated molecules have emerged as useful NFAs for high-performance OPV devices [116]. For example, asymmetric PDIs out-performed symmetric PDIs when used as NFAs in OPV devices (PCE = 6.6%; Voc = 0.80 V; Jsc = 14.8 mA/cm²; FF = 56% vs. PCE = 4.5%; V_oc = 0.78 V; Jsc = 14.4 mA/cm²; FF = 40%) [117]. A group reported on three different asymmetric PDIs and tested then as NFAs in OPV devices [82,106,107]. Payne et al. introduced the first asymmetric molecule (PDI-DPP-IQ) based on an NPDI, diketopyrrolopyrrole (DPP) and indoloquinoxaline (IQ) building units connected via C–C bonds in a linear fashion [82]. OPV devices based on PDI-DPP-IQ as an NFA, paired with P3HT as the donor polymer, showed a significant performance enhancement after chloroform SVA treatment, demonstrating a PCE of 0.8% (Voc = 0.75 V; Jsc = 2.5 mA/cm²; FF = 42%) compared to a PCE of 0.1% (Voc = 0.76 V; Jsc = 0.6 mA/cm²; FF = 26%) prior to SVA treatment. The device performance enhancement was mainly attributed to a J_sc enhancement. The SVA treatment crystallized the PDI-DPP-IQ, leading to a red-shifting of the absorption spectrum. The enhancement in FF values after SVA treatment can be explained by the emergence of preferred structural rearrangement, forming well-defined nanomorphologies that enhanced charge extraction [82]. Despite the low PCE values, the new asymmetric chemical design is a guide for the development of future organic materials for organic electronics, especially in terms of bandgap engineering.

Next, the IQ dye was replaced with the N-(alkyl)benzothioxanthene-3,4-dicarboximide (BTXI) dye, delivering the molecule PDI-DPP-BTXI. This molecule exhibited ambipolar charge transport characteristics [106]. When paired with a PTB7-Th donor polymer, the OPV device performance was a modest 0.6% (Voc = 1.0 V; Jsc = 2.1 mA/cm²; FF = 27%). However, upon SVA treatment of the BHJ, the PCE doubled to 1.1% (Voc = 0.95 V; Jsc = 3.8 mA/cm²; FF = 31%), as shown in. Next, the BTXI dye was replaced with a Se-annulated PDI dye (SePDI), producing the molecule NPDI-DPP-SePDI [107]. SVA treatment of the BHJ lead to a slight redshift accompanied by the emergence of a new high energy peak at 568 nm in the optical absorption spectrum. OPV devices with a BHJ comprising a PTB7-Th:NPDI-DPP-SePDI blend showed a slightly enhanced performance PCE of 0.8% (Voc = 0.76 V; Jsc = 2.7 mA/cm²; FF = 38%) with the BHJ SVA treatment compared to a PCE of 0.5% (Voc = 0.76 V; Jsc = 2.0 mA/cm²; FF = 37%) without the BHJ SVA treatment. While the PCEs were low overall, the asymmetric-type PDIs offer unique optical profiles, energy cascades, and films susceptible to crystallization upon SVA treatment; therefore, further work is warranted to create structure–property–function relationships and develop higher-performing photoactive materials.