Over the past three years, a particularly exciting and active area of research within the field of organic photovoltaics has been the use of non-fullerene acceptors (NFAs). Compared with fullerene acceptors, NFAs possess significant advantages including tunability of bandgaps, energy levels, planarity and crystallinity. To date, NFA solar cells have not only achieved impressive power conversion efficiencies of ~13–14%, but have also shown excellent stability compared with traditional fullerene acceptor solar cells. This Review highlights recent progress on single-junction and tandem NFA solar cells and research directions to achieve even higher efficiencies of 15–20% using NFA-based organic photovoltaics are also proposed.
Solar energy plays a key role in solving serious environmental problems and the terawatt energy challenge the world is facing today. The pursuit of clean, renewable, cost-effective and high-performance photovoltaic technologies has attracted tremendous efforts in both academia and industry. In particular, solution-processed organic photovoltaics (OPVs) devices have attracted considerable attention in the last two decades because they possess several advantages such as easy and low-cost fabrication, flexibility and light weight, and the potential of optical transparency, to name a few1,2,3,4,5.
At the early stages of OPV development (1950s–1980s), active layers typically consisted of single-component organic materials, which yielded very low power conversion efficiencies (PCEs)6. In the 1980s, bilayer OPVs were first reported by Tang7 who introduced duo organic component systems (electron donor (D) and electron acceptor (A)) . The significantly enhanced photocurrents and efficiencies that resulted were later understood to be due to the D/A heterojunction that provided a driving force to overcome the exciton binding energy in organic semiconductors. To markedly enlarge the size of D/A interfaces in the active layer, which can boost the proportion of exciton dissociation, the bulk heterojunction (BHJ) concept (Fig. 1a) was reported in the 1990s in polymer–fullerene8 and polymer–polymer9 systems. There are two photocurrent generation channels in BHJ OPVs—the excitons can be generated both from donors (‘channel 1’) and acceptors (‘channel 2’). The working mechanism in BHJ OPVs10,11,12 is shown in Fig. 1b: (1) absorption of photons and creation of excitons; (2) diffusion of excitons to D/A interfaces; (3) dissociation of excitons to free charge carriers (holes and electrons) at D/A interfaces; (4) transportation of the charge carriers to electrodes; and (5) extraction of the charge carriers at electrodes.
In BHJ OPVs, the limiting absorption efficiency in the solar spectrum is a function of the bandgap, while the optimum bandgap for a heterojunction is a function of the difference between the optical and electrical gap. Many systems developed in the early age of BHJ OPVs, such as P3HT:PCBM (poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester), had a sub-optimally large bandgap13. Over the past 10 years, numerous studies have focused on the following aspects: design and synthesis of high-mobility and strongly absorbing polymer or small-molecule donor materials14,15,16,17,18,19 and on new acceptor materials with strong absorption and high lowest unoccupied molecular orbital (LUMO) energy levels20,21; fabrication of ternary blend OPVs based on donor, acceptor and multi-functional third components (to control the microstructure or to assist with light harvesting, charge separation or transportation)22,23,24,25; control of the morphology of the active layers via thermal annealing, solvent annealing and solvent additives26,27,28; utilization of new transporting materials to modify the surface between the active layer and the electrodes29,30; development of the inverted device geometry to optimize the vertical separation of the active layer and to enhance device stability31,32; and the use of tandem structures to cover the whole solar spectrum (complementary absorption of sub-cells) and to increase photon utilization, in particular, reducing the thermal energy loss33,34 (Fig. 1c). So far, BHJ OPVs based on polymer donors and fullerene derivative acceptors have shown the best performance with a National Renewable Energy Laboratory (NREL)-certified PCE of 11.5%35.
However, there are still some challenges that remain for state-of-the-art polymer:PCBM solar cells. First, the widely used fullerene derivative acceptor PCBM exhibits weak absorption in the visible and near-infrared (NIR) regions, and thus it can only provide a weak ‘channel 2’ for charge generation36. The exceptionally weak absorption in the first excited state of C60-based fullerenes results from the high symmetry of these molecules37. Second, PCBM has restricted opportunities for variation of its bandgap because there are only a limited number of different chemical structures that can be easily modified while still keeping the C60:C70 core more or less intact. Third, spherically structured PCBM easily crystallizes and aggregates, which reduces the long-term stability of devices38,39. Finally, representative polymer:PCBM solar cells yield large open circuit voltage (VOC) losses (Eg/q – VOC), where Eg is the optical bandgap of the blend layer and q is the elementary charge, of 0.8–1.3 V, which are much higher than those of solar cells based on gallium arsenide (GaAs), silicon or perovskite (0.3–0.5 V)40,41. This large loss relates to the non-sharp absorption edge of the BHJ, as well as non-radiative losses that occur in most OPV systems.
Over the past three years, exciting and rapid progress on non-fullerene acceptors (NFAs) has provided effective means to overcome these challenges42,43,44,45,46,47,48,49. Compared with fullerene acceptors, NFAs possess some distinct advantages: tunable bandgaps that can broaden the absorption in the NIR region—the high oscillator strength can induce the absorption to be as strong as possible, which is close to the band edge50; tunable energy levels that can adjust the energy-level alignments of OPVs to achieve suitable energy offsets and high VOC; tunable planarity and crystallinity that can control the morphology of the active layer and improve device stability.
Here, we first review the recent progress on single-junction NFA solar cells in terms of the different bandgaps of NFAs. In particular, the controllable film morphology, improved stability, small energy offsets and small VOC losses of single-junction NFA solar cells will be addressed. In addition to enhancing single-junction solar cells, NFA solar cells are also compatible with the organic tandem solar cell architecture, and are highly promising with regard to enabling further performance boosts using the tandem structure. Second, we briefly discuss the tandem concept and recent progress on tandem OPVs. Next, we discuss the limited reports of tandem NFA solar cells. Finally, we forecast the hot research area of tandem NFA solar cells, and propose some promising research directions and highlight key issues that need to be overcome to achieve even higher solar cell efficiencies of 15–20% in the future. The realization of this goal will provide a great opportunity to motivate future industrial manufacture of OPVs.
Single-junction NFA solar cells
In the history of the development of NFAs, many novel and important NFAs have been synthesized such as those based on perylene diimide (PDI)36,51,52, naphthalene diimide (NDI)53,54,55, benzothiadiazole (BT)56,57 and more9,58,59. These NFAs, including the linear-shaped/star-shaped small molecules and polymers, showed great potential for controlling bandgaps, energy levels, planarity and crystallinity by careful molecular design. Although the efficiencies of solar cells based on these NFAs were lower than those of solar cells based on fullerenes at that time, these pioneering works that attempted to find alternatives to fullerenes significantly boosted the development of NFAs.
In this section, due to the rapid developments in this field, we will review only those works reported in the past five years. First, we will focus on single-junction NFA solar cells based on NFAs with different bandgaps. Then, the controllable film morphology, improved stability, small energy offsets and small VOC losses of single-junction NFA solar cells will be discussed. The molecular structures of NFAs discussed in this Review are shown in Fig. 2 and are presented in terms of their different bandgaps, and the molecular structures of the donors are shown in Supplementary Figs. 1 and 2.
Based on wide-bandgap NFAs (>1.9 eV)
P3HT:PC61BM is a classical and widely used D:A combination in OPVs41. Wide-bandgap NFAs were first used in P3HT-based OPVs to replace PC61BM. For example, Zhao et al.60 synthesized a PDI dimer-based NFA (SF-PDI2) with a bandgap of 2.0 eV. This NFA is a PDI dimer with one spirobifluorene (SF) as the bridge, resulting in a twisted structure that can avoid excessive strong π–π interactions. The PCE of solar cells based on P3HT:SF-PDI2 was only 2.4% due to the substantial absorption overlap between the wide-bandgap P3HT (1.9 eV) and wide-bandgap NFA (>1.9 eV). To reduce the absorption overlap between the donor and the acceptor and to broaden the absorption range of the active layer in the NIR region, medium-bandgap polymers were employed as the donor in a blend with wide-bandgap NFAs. For example, in 2013, NFA solar cells consisting of the medium-bandgap donor PBDTTT-C-T and the wide-bandgap NFA bis-PDI-T-EG were fabricated61. After morphology optimization by the addition of the solvent additive 1,8-diiodooctane, the degree of phase separation was increased and a PCE of 4% was achieved—the best result up to that point. In 2015, NFA solar cells based on the medium-bandgap donor PTB7-Th and the wide-bandgap NFA TPE-PDI4 were reported62. The solar cells based on PTB7-Th:TPE-PDI4 exhibited NIR absorption (~770 nm) and a relatively good PCE of 5.5%. Recently, higher PCEs of approximately 9% were achieved in NFA solar cells also by combining wide-bandgap acceptors and medium-bandgap donors63,64,65. For example, a high PCE of 9.5% was achieved for a NFA solar cell based on SF-PDI2, which initially showed poor performance. After changing the donor from P3HT (1.9 eV) to P3TEA (1.7 eV), the absorption overlap between the donor and the acceptor was reduced and the absorption region of the blend layer was broadened, leading to a boosted short circuit current density JSC from 5.9 mA cm–2 (ref. 60) to 13.3 mA cm–2 (ref. 64). Nuckolls et al.63,66 synthesized a new class of highly efficient acceptors consisting of helical conjugated PDI oligomers with a wide bandgap. The PDI acceptor used (hPDI4) possesses a nonplanar molecular structure due to the steric congestion in the cove areas defined by the junction point between the PDIs. After blending with the donor PTB7-Th, these NFA solar cells based on hPDI4 exhibited a high PCE of 8.4%.
Based on medium-bandgap NFAs (1.9–1.5 eV)
The combination of medium-bandgap donors and wide-bandgap NFAs works well (PCEs of approximately 9%), similarly to the combination of medium-bandgap donors and the wide-bandgap fullerene acceptors67. It is interesting that the bandgaps of donors and acceptors can be changed without changing the absorption region of the blend layer. However, this field progressed slowly due to the difficulty in tuning the bandgap of PCBM. Considering that one of the advantages of NFAs is the ease of tuning the bandgap, a medium-bandgap (1.6 eV) NFA ITIC was synthesized in 2015 (ref. 68). ITIC is an A–D–A push–pull small-molecule acceptor with an indacenodithiophene (IDT)-type structure69,70,71 as the core and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN)72 as the end groups. Previously, IDT-type structures have been widely used in many donor polymers that exhibit good hole mobilities and broad and strong absorption due to the rigid and coplanar structure. Meanwhile, the INCN unit exhibits strong electron-withdrawing ability, which can lead to deep LUMO energy levels (–3.8 to –3.9 eV). ITIC can work well with many donors. For example, a NFA solar cell based on ITIC and a wide-bandgap donor J71 has been reported with a high PCE of 11.4%73. Because ITIC is a model NFA, a number of works focusing on structural modifications of ITIC have been published recently—these structure modifications affect the energy levels and absorption regions of NFAs as well as the film morphology of the corresponding active layers. Zhan et al.74 modified the side chains of ITIC from phenyl alkyl chains to thienyl alkyl chains (ITIC-Th)74, and Li et al.75 changed the position of the alkyl chains on phenyl (meta-alkyl-phenyl substitution, m-ITIC)75. Compared with ITIC, ITIC-Th and m-ITIC exhibited similar bandgaps but with different molecular packing behaviours. After blending with the wide-bandgap donors PTFB-O and J61, respectively, high PCEs of 11–12%75,76 could be achieved for these NFA solar cells. Hou et al. employed thienyl to replace the phenyl in the end groups of ITIC77. Compared with ITIC, the modified NFA (ITCC) showed an up-shifted LUMO energy level (0.14 eV) due to the more electron-donating nature of thiophene compared with benzene. As a result, NFA solar cells based on PBDB-T:ITCC exhibited higher VOC (1.01 V versus 0.93 V) and PCEs (11.4% versus 10.6%) than those based on PBDB-T:ITIC. In addition to modifying side chains and end groups, Zhan et al.78 modified the core of ITIC by reducing the number of fused rings from seven (ITIC) to five (IDIC). After blending with the wide-bandgap donor PTFBDT-BZS, the PCE of this NFA solar cell exceeded 11%. Also based on the IDT unit, another medium-bandgap (1.63 eV) NFA (O-IDTBR) has been reported79. This NFA exhibited a planar molecular backbone with strong intermolecular packing. A PCE of 6.3% could be achieved for NFA solar cells with the donor P3HT, which could be further increased to 7.7% by the addition of the second NFA IDFBR. The limited vitrification of the crystalline O-IDTBR phase by IDFBR for IDFBR fractions of up to 30% led to the preservation of the three-phase microstructure, which was favourable for photocurrent generation, electron transport and reduced bimolecular recombination80. In addition to small molecules, another important medium-bandgap acceptor is a PDI-based polymer. In 2016, Yan et al. increased the PCE of NFA solar cells based on a PDI polymer to approximately 8% by molecular engineering (PDI-V)81.
Based on narrow-bandgap NFAs (<1.5 eV)
Solar cells based on medium-bandgap NFAs displayed PCEs of 11–12%. To further increase photon utilization, which can improve the PCE limit of OPVs, researchers have broadened the absorption range of the blend layer by further reducing the bandgap of the acceptor. For example, a NFA (IHIC) based on six fused rings was reported82. IHIC has a strong electron-donating group end-capped with a strong electron-withdrawing group and, as a result, it exhibited broad and strong absorption (bandgap of 1.38 eV) with a maximum extinction coefficient of 1.6 × 105 M–1 cm–1. The PCE of NFA solar cells based on PTB7-Th:IHIC approached 10%. More importantly, this active layer exhibited strong NIR absorption (600–900 nm) but weak visible absorption, and therefore could sufficiently utilize the NIR light to achieve high efficiency, while simultaneously maintaining high visible transparency. Hou et al.83 employed alkoxyl chains to replace the alkyl chains on π-bridged thiophenes in another IDT-based model NFA (IEIC84), achieving a reduced bandgap from 1.57 eV (IEIC) to 1.34 eV (IEICO) due to the introduction of electron-donating groups. A PCE of 8.4% could be achieved for NFA solar cells with PBDTTT-E-T as the donor and IEICO as the acceptor. After employing fluorine (F) atoms on the end groups of IEICO (IEICO-4F), the bandgap could be further reduced to 1.24 eV, resulting from the enhanced intramolecular charge transfer effect. A very high JSC of 25 mA cm–2 with high PCE of approximately 11% could be obtained for NFA solar cells with mixed donors (PTB7-Th and J52). As shown in Fig. 3a, the IEICO-4F-based device showed very high external quantum efficiency (EQE) values in the NIR region from 600–900 nm, with the edge of the photoelectron response reaching near 1,000 nm (ref. 85). Based on this strategy, a high PCE of 13.1% (Fig. 3b, the inset is the distribution of PCE) was reported in another study by the same group, which employed PBDB-T-SF:IT-4F (a NFA derived from ITIC by the addition of F atoms to its end groups)86. In addition to small molecules, the NDI-based conjugated polymer N2200 is another important narrow-bandgap (1.48 eV) NFA. After blending with the donor J51, a PCE of 8.3% with fill factor (FF) of 70.2% was achieved for the resulting NFA solar cells87.
Controllable film morphology
Controlling the morphology in BHJ organic solar cells is a complex issue. Inorganic solar cells have high mobility and thus high crystallinity. Optimizing charge mobility is one approach for improving OPVs, but it is not the only method. It has long been recognized that the carrier mobility balance88 might be more important than absolute carrier mobility in BHJ OPVs. As NFA molecules can be designed to be highly crystalline or highly amorphous, going beyond the once effective PCBM acceptor molecules with spherical symmetry is clearly a seminal step to unlock the molecular design potential. The electron mobility of NFAs can be tuned to match the hole mobility of the donor. This balanced charge carrier mobility in NFA solar cells favours a high FF. In the early stages of development, solar cells based on the monomeric PDI acceptor exhibited very low PCE (0.1%) due to the highly planar configuration of PDI, which could induce strong intermolecular π–π stacking, leading to large crystalline domains and severe phase separation. The atomic force microscopy height image of PBDTTT-C-T:monomeric PDI is shown in Fig. 3c (ref. 61). To overcome this morphology problem, linear-shaped PDI dimer acceptors and three-dimensional PDI acceptors were synthesized and employed in solar cells. For example, Yu et al.89 developed a three-dimensional acceptor named TPB based on a BDT-Th core with four PDI end-groups. The BDT-Th unit had a coplanar π-conjugated backbone, which was conjugated through at least three directions with each terminal. Density functional theory (DFT) calculations showed that four PDIs in the TPB molecule formed a cross-like molecular geometry, while they were still partially conjugated with the BDT-Th core. This molecule exhibited a twisted three-dimensional structure as well as good conjugation of the whole molecule. As a result, the JSC and PCE of the PTB7-Th:TPB-based device could reach up to 18 mA cm–2 and 8.5%, respectively. Similar to PDI-based NFAs, IDT-based NFAs also exhibited tunable planarity and crystallinity through modifications of the IDT core (the number of fused rings), side chains or end groups, which could be used to control the film morphology. For example, the NFA m-ITIC was synthesized through side-chain isomerism engineering of the alkyl-phenyl substituents of ITIC (ref. 75). 2D grazing incident wide-angle X-ray scattering (GIWAXS) patterns of the pure ITIC film and m-ITIC film are shown in Fig. 3d. For ITIC, both the lamellar (100) reflection and π−π stacking (010) reflection were competitively observed along the out-of-plane direction, confirming that face-on and edge-on crystallites coexisted. In contrast, m-ITIC showed better defined scattering peaks and stronger intensities with a predominant face-on crystalline orientation, as shown by the clear and strong (100) diffraction in the in-plane direction and π−π stacking (010) diffraction in the out-of-plane direction. In addition, the enhanced crystallinity and preference for face-on orientation of m-ITIC compared with that of ITIC could increase the electron mobility. The enhanced electron mobility of m-ITIC, which was a result of the change in morphology, led to a more balanced charge carrier mobility in the active layer (the ratios of hole/electron mobilities in J61:m-ITIC and J61:ITIC were 1.18 and 1.97, respectively). As a result, minor modification of the NFA structure could significantly change the film morphology and enhance the PCE of NFA solar cells from 10.6% (ITIC-based) to 11.8% (m-ITIC-based). Overall, while the enhanced planarity and crystallinity of NFAs improve charge transport and photon absorption, they may also negatively affect charge separation. However, at the same time, reduced planarity and crystallinity of NFAs can also enhance the material solubility and charge separation but may induce severe charge recombination. Hence, as a result of tunable planarity and crystallinity, NFAs with appropriate planarity and crystallinity can be selected to blend with specific donors to maximize the solar cell performance.
For both academic and industrial purposes, obtaining high PCEs and good stability is the goal for OPVs90. Despite high PCEs of 12–13% being achieved, insufficient stability is the limiting factor for future industrial production. To this end, Chen et al.91 reported a spirobifluorene core-based NFA (SF(DPPB)4). Thanks to the non-spherical and nonplanar structure of SF(DPPB)4, which was amorphous, devices based on P3HT:SF(DPPB)4 exhibited excellent thermal stability at 150 oC for up to 3 hours (Fig. 3e). McCulloch et al.92 synthesized another non-spherical and nonplanar NFA (FBR) that reduced the tendency of crystallization and helped to prevent the formation of large crystalline domains in the BHJ blend composition over extended lifetimes92. It was found that the film morphology of P3HT:FBR remained nearly unchanged before and after heating. Although these amorphous NFAs exhibited a lower tendency to aggregate, they can still exhibit sufficient phase separation with donors by suitable molecular design, which can minimize the trade-off between high efficiency and good thermal stability. In addition to thermal stability, NFA solar cells have also exhibited enhanced ambient stability compared with solar cells based on PCBM79,80,81. For example, McCulloch et al.80 tested the ambient stability of NFA (O-IDTBR and IDFBR) solar cells and some benchmark PCBM-based solar cells. Devices were stored at room temperature in ambient conditions under both dark and light (1 sun) conditions. After 1,200 hours in air and under dark conditions, the NFA solar cells retained 80% of their PCE, while all of the PCBM-based solar cells were no longer operational after 800 hours. In addition, the NFA solar cells exhibited better air photo-stability, retaining 85% of their initial performance after 90 hours. In contrast, the PCE of PCBM-based devices dropped to 20% of their initial value. The enhanced ambient stability of NFA-based devices compared with PCBM-based devices was attributed to the solvent-additive-free process used in their fabrication. During the life cycle of OPVs, mechanical stress always exists. Unfortunately, the major PCBM-based solar cells are unstable as a result of this mechanical stress93. Kim et al.94 fabricated solar cells based on the polymer donor PBDTTTPD and the polymer acceptor P(NDI2HD-T). As shown in Fig. 3f, the PBDTTTPD:P(NDI2HD-T) film showed much higher toughness relative to the PBDTTTPD:PC61BM film due to its lower tensile moduli (0.43 GPa versus 1.76 GPa) and much higher elongation (7.16% versus 0.12%)94. The NFA-based film showed much better mechanical stability than the PCBM-based film because the polymer acceptor was intrinsically more ductile than the fullerene and could better entangle with the other polymer chains with strengthened interfaces.
Small energy offset (~0.1 eV)
In fullerene-based solar cells, it has been suggested that a LUMO level offset of 0.3 eV is sufficient for efficient charge separation95. However, this has recently been called into question as a result of the development of NFA solar cells. For example, NFA solar cells based on P3TEA:SF-PDI2 with a small LUMO level offset of 0.05 eV and high PCE of 9.5% have been reported64. The electron transfer (characteristic decay half-life of 3 ps) was an order of magnitude faster than the exciton lifetime (30 ps) of the donor polymer P3TEA. Furthermore, a significant population (>23%) of excitons dissociated into charges within 100 fs (Fig. 3g). In addition, the photoluminescence (PL) quenching efficiency of the blend film was about 87%. These data demonstrated that fast and efficient charge separation could be realized with a small energy offset. Also, NFA solar cells with a small LUMO level offset of 0.1 eV have been fabricated96. The PL intensity of the PTB7-Th:IDT-2BR:PDI-2DTT ternary blend decreased by 96% and 94% relative to that of the pure donor PTB7-Th and the pure acceptor IDT-2BR, respectively. The strong PL quenching indicated that very efficient charge transfer happens from donor to acceptor or from acceptor to donor, rather than the charges being consumed by luminescence (radiative recombination), which suggested that efficient charge separation could be realized even with a small energy offset. In addition, the excited-state lifetime of the PTB7-Th:IDT-2BR:PDI-2DTT ternary blend film was only 0.44 ns. The short excited-state lifetime indicated a high proportion of photoinduced charges being transferred. As a result, a high PCE of 10.1% was achieved for NFA solar cells with small energy offset. Another possible contribution factor to the small threshold for charge separation might be the low disorder in these NFA OPV systems. It’s worth noting that the LUMO offset may be an unreliable metric for the interfacial offset within a device, due to variations in the degree of crystallization, and electrostatic and other effects.
Small V OC loss (0.5–0.6 V)
Compared with the small VOC loss of the highly efficient GaAs, silicon or perovskite solar cells, the VOC loss of organic solar cells is much larger, which has been viewed as a large unavoidable loss mechanism due to the Frenkel exciton. The VOC loss in solar cells consists of three parts97: (1) ΔV1 (Eg/q – VOC, SQ) is due to the radiative recombination originating from the absorption above the bandgap—this loss is unavoidable for any type of solar cell (typically between 0.25 and 0.30 V; SQ is the Shockley–Queisser limit); (2) ΔV2 (ΔVOC, abs) is due to the additional radiative recombination originating from the absorption below the bandgap—this loss is negligible for GaAs, silicon or perovskite solar cells but usually very high for PCBM-based solar cells (up to 0.7 V); and (3) ΔV3 (ΔVOC, nr) is due to the non-radiative recombination—this loss in organic solar cells (approximately 0.4 V) is usually higher than that in GaAs, silicon or perovskite solar cells (approximately 0.2 V). The large VOC loss (large ΔV2 and ΔV3) seriously limits further efficiency enhancement of OPVs. Fortunately, the new NFA solar cells show unexpected potential in reducing the VOC loss. Yan et al.64 fabricated NFA solar cells based on P3TEA:SF-PDI2 and achieved a small VOC loss of 0.61 V with high PCE of 9.5%. As shown in Fig. 3h (Fourier-transform photocurrent spectroscopy (FTPS)-EQE), the absorption onset of the D:A blend almost overlaps with that of the pure P3TEA, indicating that no sub-gap charge transfer (CT) state absorption is observed, which minimized the energy difference between the singlet exciton on the donor and/or acceptor and the CT states. As a result, ΔV2 of this NFA solar cell was as small as 0.07 V, which is much smaller than that of the benchmark organic solar cells based on P3HT:PCBM (0.67 V) and PTB7:PCBM (0.20 V). It is worth noting that a negligible ΔV2 was also recently observed in some PCBM-based solar cells97,98,99. This reduced radiative recombination is the result of the small energy offset (0.05 eV) that is sufficient for efficient charge separation. Not only ΔV2 but ΔV3 of this P3TEA:SF-PDI2 NFA solar cell (0.26 V) was smaller than that of the solar cells based on P3HT:PCBM (0.38 V) and PTB7:PCBM (0.39 V), which was attributed to the relatively high electroluminescence quantum efficiency (EQEEL) value of 5 × 10–5. Compared with PDI-based NFA solar cells, the IDT-based NFA solar cells also exhibit great potential in reducing VOC loss. Very high PCEs of 10–12% with small VOC losses of 0.50–0.60 V have been reported for IDT-based NFA solar cells80,85,100,101,102. For example, McCulloch et al.100 reported NFA solar cells based on PffBT4T-2DT:IDTBR that achieved a high PCE of 10% with small ΔV3 of 0.27 V (suppressed non-radiative recombination) and total VOC loss of 0.55 V. Furthermore, Hou et al. fabricated NFA solar cells based on PTB7-Th:IEICO-4F (ref. 85) and PB3T:IT-M (ref. 102). After device optimization, small VOC losses (0.51 V and 0.60 V) and very high PCEs (10.9% and 11.9%) were achieved for IEICO-4F-based and IT-M-based NFA solar cells, respectively. It is worth mentioning that the small VOC loss may decrease the driving force for energy transfer between donor and acceptor, which will decrease JSC. However, the broadened absorption of NFAs can utilize more photons that can contribute to JSC and therefore minimize the trade-off between VOC and JSC.
Tandem NFA solar cells
In this section, we briefly review the tandem concept and recent progress on tandem OPVs. Subsequently, we discuss the limited reports of tandem NFA solar cells, including tandem NFA solar cells with complementary absorption and homo-tandem NFA solar cells. The photovoltaic properties of these tandem NFA solar cells are listed in Table 1.
The tandem solar cell concept
Tandem solar cells were invented to overcome the limitations of single-junction solar cells, and comprise two or more sub-cells that absorb light in complementary wavelength ranges stacked together103 (Fig. 4a). In this way, the photon utilization efficiency can be significantly improved due to the better solar spectral matching. In addition, a major loss mechanism for single-junction solar cells is the photovoltage loss, which is induced by thermalization of hot carriers when the energy of photons is greater than the bandgap. The tandem structure is a proven way to overcome the Shockley–Queisser limit104 in all types of solar cell, by reducing thermal loss. As a result, inorganic tandem solar cells with efficiency up to 38.8% (under 1 sun AM 1.5G conditions) have been achieved105.
Progress on tandem OPVs
The application of the tandem concept in OPVs began with small molecules, through thermal evaporation-based fabrication. In 2002, an ultrathin evaporated metal was employed as the interconnecting layer (ICL) between two sub-cells, and could lead to the addition of the photovoltages of the two sub-cells106. Based on this technology, tandem devices with up to five junctions were successfully fabricated. Compared with solar cells fabricated by vacuum evaporation of small molecules, solution-processable polymer tandem solar cells exhibit some advantages in terms of low cost and low energy consumption34,107. The first polymer tandem solar cell consisting of two sub-cells (covering complementary absorption regions of the solar spectrum) was demonstrated in 2006 by Boer and Janssen et al.108, and the PCE reported was as low as 0.6%. In 2007, Heeger et al.33 reported titanium dioxide (TiO2):poly(3,4-ethylenedioxylenethiophene)-polystylene sulfonic acid (PEDOT:PSS) as the ICL, and the PCE of the polymer tandem solar cells achieved was 6.5%. In 2012, Yang et al.109 employed the inverted structure and PEDOT:PSS/zinc oxide (ZnO) ICL in polymer tandem solar cells, achieving a PCE of 8.6%. In 2013, the same group110 fabricated tandem solar cells based on P3HT:ICBA (front cell) and PDTP-DFBT:PCBM (rear cell) (Fig. 4b), and demonstrated a PCE of 10.6%, which was the first report of a OPV with over 10% efficiency certified by the NREL. It is worth noting that because of their more complex structure, tandem OPVs are more difficult to manufacture and reproduce than single-junction OPVs. There has been encouraging technological progress in this regard such as the works in which large-area and flexible tandem OPVs are shown to have been successfully fabricated by a roll-to-roll process in air111,112.
Over the past three years, there have been three important advancements in tandem solar cells. The first is the development of the ICL. For example, Jen and Chen et al.113 employed an ICL (molybdenum trioxide (MoO3)/silver (Ag)/PFN) in tandem solar cells based on PTB7:PCBM (front cell) and PIDT-PhanQ:PCBM (rear cell)113. The utilization of the semi-transparent Ag layer in the ICL is an effective way to balance current densities between two sub-cells. As a result, a high PCE of ~11% could be achieved. In addition, an ICL based on PF3N-2TNDI/Ag/PEDOT:PSS has been reported114. Due to the presence of the polar amine groups for effective work function modification and the relatively high mobility originating from the NDI-based polymer backbone, PF3N-2TNDI could form ohmic contact with both sub-cells in the tandem solar cells. Therefore, a high PCE of 11.35% could be achieved in solar cells based on PThBDTP:PC71BM (front cell) and DPPEZnP-TEH:PC61BM (rear cell). More importantly, the polymer-based ICL could be applied in flexible tandem OPVs to achieve a PCE over 10%. Very recently, the ZnO/PEDOT:PSS ICL was employed to fabricate a very high efficiency (12.7%) tandem solar cell based on DR3TSBDT:PC71BM (front cell) and DPPEZnP-TBO:PC61BM (rear cell)115.
The second advancement is the development of homo-tandem solar cells. The optimal film thickness of active layers is normally around 100 nm, because of the relatively low carrier mobility of organic semiconductors. As a result, the thin layers limit the overall absorption of the active layer to between 60–80%116. To overcome this limitation, the homo-tandem structure consisting of two sub-cells with the same donor/acceptor was developed. In 2013, homo-tandem solar cells based on PDTP-DFBT:PC71BM were presented116. The light absorption of the active layer in the visible region was pushed from 70% to 90%. As a result, the PCE of OPVs was enhanced from 8.1% (single junction) to 10.2% (homo tandem). Also based on this idea, in 2015, homo-tandem solar cells based on PTB7-Th:PC71BM were reported117. Thanks to the enhancement of light harvesting, a high PCE of 11.3% was achieved for the homo-tandem solar cells, which exhibited a 25% enhancement in efficiency compared with the single-junction solar cells.
The third advancement is the development of triple-junction OPVs. The concept of triple-junction OPVs is to further increase the VOC compared with double-junction OPVs, which is advantageous for solar-energy-driven water splitting. In 2013, a triple-junction OPV with a high PCE of 9.6% and a high VOC of 2.09 V was demonstrated118. In 2014, triple-junction OPVs based on three sub-cells with different bandgaps (1.9, 1.58 and 1.4 eV) were fabricated119 (Fig. 4c). After employing optical simulation and the transfer matrix formalism modelling method, the optimized thicknesses of all sub-cells and ICLs could be achieved to guarantee good current matching. As a result, a high PCE of 11.55% could be achieved for the triple-junction OPVs, with a high VOC of 2.28 V. Also based on the idea of triple-junction OPVs, Jiang et al.120 further improved the PCE of a tandem OPV to 11.83%, with a very high VOC of 2.24 V.
Tandem NFA solar cells with complementary absorption
Considering the advantages of NFAs compared with fullerene acceptors, the combination of NFAs and the tandem concept shows great potential for very high performance OPVs: (1) NFAs have easily tunable bandgaps that can broaden the absorption in the NIR region for the rear cell or narrow the absorption to the blue region for the front cell; (2) small VOC loss of NFA sub-cells guarantees small VOC loss of tandem NFA solar cells; (3) the improved thermal stability of the NFA sub-cell is potentially advantageous for maintaining the performance of the front cell, which is usually challenging as a result of the unavoidable thermal treatments of the ICL. Research into this promising area started only recently in 2016. Tandem NFA solar cells based on P3HT:SF(DPPB)4 as the front cell (bandgap of 1.80 eV and VOC loss of 0.70 V) and PTB7-Th:IEIC as the rear cell (bandgap of 1.55 eV and VOC loss of 0.60 V) have been fabricated121. After optimizing the Ag layer thickness in the MoO3/Ag/PFN ICL and the active layer thicknesses of each NFA sub-cell, a good PCE of 8.48% with high VOC of 1.97 V was achieved for the tandem NFA solar cell. To further enhance the photon utilization efficiency of tandem NFA solar cells, Hou et al.83 reduced the bandgap of the rear cell. In their work, the bandgaps of the front cell (PTB7-Th:IEIC) and the rear cell (PTB7-Th:IEICO) were 1.55 eV and 1.34 eV, respectively. Both NFA sub-cells exhibited low VOC losses (0.65 V for the front cell and 0.52 V for the rear cell). As a result, a high PCE of 10.7% was recorded for this tandem NFA solar cell. In 2017, the same group122 changed the recipe of their previously reported tandem NFA solar cells by employing another front cell (PBDD4T-2F:PC71BM) with a larger bandgap of 1.80 eV replacing PTB7-Th:IEIC. Thanks to the better current matching, tandem NFA solar cells based on PBDD4T-2F:PC71BM and PTB7-Th:IEICO exhibited higher JSC (11.5 mA cm–2 versus 10.3 mA cm–2) and PCE (12.8% versus 10.7%) compared with that of tandem NFA solar cells based on PTB7-Th:IEIC and PTB7-Th:IEICO. Very recently, the same group123 synthesized a new NFA (ITCC-M) with a relatively high LUMO energy level (–3.67 eV). Single-junction solar cells based on PBDB-T:ITCC-M exhibited VOC as high as 1.0 V. By employing a sodium-doped polymer (PCP-Na) as the ICL, tandem NFA solar cells based on PBDB-T:ITCC-M as the front cell (bandgap of 1.70 eV and VOC loss of 0.70 V) and PTB7-Th:IEICO as the rear cell (bandgap of 1.34 eV and VOC loss of 0.52 V) were fabricated (Fig. 4d), achieving a very high PCE of 13.8% with enhanced VOC of 1.80 V (Fig. 4d, the inset is the distribution of the PCE). This very high PCE of NFA-based tandem devices has overtaken the efficiencies of both single-junction and tandem devices based on PCBM.
Homo-tandem NFA solar cells
Similarly to the fullerene-based solar cells, the NFA solar cells also suffer from the relatively low mobility of organic semiconductors, which restricts the thickness of the active layer. Thus, the homo-tandem structure is suitable to improve the photon absorption in NFA solar cells. In 2016, NFA solar cells based on P2F-DO:N2200 were fabricated124. As a result of employing the homo-tandem structure, the photon absorption and the EQE were enhanced. The EQE at 500 nm and 750 nm were 50% and 35%, respectively, which was much higher than that of the single-junction device (30% at 500 nm and 20% at 750 nm). As a result, a PCE of 6.7% was achieved for the homo-tandem solar cells, which is a 43% enhancement compared with the PCE of single-junction solar cells. More recently, highly efficient homo-tandem NFA solar cells based on P3TEA:SF-PDI2 as sub-cells and an all solution-processed PEDOT:PSS/ZnO layer as the ICL were demonstrated125. As shown in Fig. 4e, the EQE at 500 nm of the homo-tandem device was 67%, which is much higher than that of the single-junction device (53%) due to the enhanced light harvesting. The PCE of the homo-tandem NFA solar cells was as high as 10.8%. More importantly, a very high VOC over 2.1 V was achieved because of the small VOC loss of the sub-cells (0.61 V). This high voltage reveals great potential for the use of such tandem NFA solar cells in solar-energy-driven water-splitting applications.
Summary and perspective
Compared with organic solar cells based on fullerene acceptors, NFA solar cells possess some advantages. First, the tunable bandgaps of NFAs (typically 1.2–2.2 eV) can broaden the absorption range of the blend layers in the NIR region or narrow the absorption range of the blend layers to the blue region. Due to this feature, high-performance single-junction NFA solar cells can be fabricated with donors and NFAs with complementary absorption. Also, high-performance tandem NFA solar cells can be fabricated with front and rear cells with suitable bandgaps. Second, NFA solar cells with small energy offsets (~0.1 eV) show efficient charge separation. Third, NFA solar cells possess small VOC losses (0.5–0.6 V) with reduced radiative recombination (originating from absorption below the bandgap) and non-radiative recombination. The small VOC loss of NFA sub-cells guarantees a small VOC loss of tandem NFA solar cells. Last, the tunable planarity and crystallinity of NFAs can control the morphology of the active layer and improve device stability. Specifically, the improved thermal stability of the NFA sub-cell is potentially advantageous for maintaining the performance of the front cell during thermal treatments of the ICL. Based on these advantages, very high PCEs of approximately 13%86 and 14%123 have been achieved for single-junction and tandem NFA solar cells, respectively.
It is worth noting that almost all reported high-efficiency (>11%) single-junction NFA solar cells have so far been based on ITIC-type acceptors. These NFAs consist of electron-donating fused-ring units (IDT type and derivatives) as the core and electron-withdrawing units (INCN and derivatives) as the end groups. The breakthrough of these ITIC-type acceptors can be attributed to several factors. First, to the significantly improved understanding of how to reduce the optical gap and to control the highest occupied molecular orbital (HOMO) and LUMO energies in push–pull polymers and small molecules used as donors in solar cells14,15,126, information that was also applicable to the design of acceptors. Second, thanks to the rigid and coplanar structure of the fused rings (IDT type and derivatives) and strong intramolecular charge transfer effect on electron-donating units and electron-withdrawing units, strong and broad absorption can be achieved in NFAs. Third, the combination of electron-donating fused-ring units (IDT type and derivatives) and electron-withdrawing units (INCN and derivatives) can realize suitable LUMO energy levels (–3.8 to –3.9 eV) in NFAs, which can provide efficient charge separation driving force and achieve high VOC at the same time. Fourth, the molecular weights of these ITIC-type acceptors are normally higher than 1,200, which means that they exhibit good film-forming properties like polymers. The last factor is the development of high-performance wide-bandgap donors, which exhibit complementary absorption with the medium- or narrow-bandgap ITIC-type acceptors.
We believe that the combination of NFAs and the tandem concept will become the next big thing in the field of OPVs, which has great potential for realizing the next generation of high-performance OPV devices. Although still in the early stages of research, the prospect of tandem NFA solar cells is very promising and attractive. To forecast this hot research area and guide the design rules for future high-performance tandem NFA solar cells, we depict 2D mapping graphs of the future efficiency of tandem NFA solar cells based on the calculation formulas in the Supplementary Information. The calculation adopted a similar methodology and the assumptions of Dennler et al.127. As shown in Fig. 5a,b, very high PCEs of 17% (with assumed VOC loss of 0.60 V, average EQE of 70% and FF of 70%) and 20% (with assumed VOC loss of 0.55 V, average EQE of 75% and FF of 75%) can be achieved with front and rear cells of suitable bandgaps. For the homo-tandem structure (Fig. 5c,d), very high PCEs of 16% (with assumed VOC loss of 0.60 V and FF of 70%) and 18% (with assumed VOC loss of 0.55 V and FF of 75%) can be achieved with sub-cells with suitable bandgaps and high total EQE values of 80–85%. Based on our forecast, there are four important factors for achieving high-performance tandem NFA solar cells: matched bandgaps of sub-cells, small VOC loss of sub-cells, a high EQE value for the tandem device and a high FF for the tandem device.
Some research areas and key issues of NFA solar cells may deserve further attention in the future. (1) Synthesis of new NFA materials with strong absorption in the NIR region and high electron mobility for single-junction OPVs or rear cells in tandem OPVs. The most critical issue for this point is to broaden the absorption up to 1,000 nm, as much as possible close to the band edge. (2) Synthesis of new NFA materials with high LUMO energy levels, strong absorption in the blue region and high electron mobility for the front cell in tandem OPVs. The most critical issue for this point is to realize small VOC loss in the donor:acceptor blend with a wide bandgap. (3) Setting up the relationships between the molecular structures of NFAs and small VOC loss. The key is to link the observed small VOC loss (radiative or non-radiative recombination) with the chemical/physical properties (polarity, aromaticity, geometry and so on) of the donor or acceptor units in NFAs. (4) Realizing further small VOC loss (approximately 0.5 V) in single-junction OPVs or sub-cells in tandem OPVs via device processing or the addition of functional third components. For example, using an NIR donor/acceptor as the third component to broaden the absorption of the active layer (decrease Eg) without decreasing VOC is a promising strategy to reduce VOC loss. (5) Further investigating the device physics of NFA solar cells, including the origin of small VOC loss and charge dynamics. Specifically, setting up appropriate theoretical models for the unusual but efficient charge transfer phenomenon between polymer donors and NFAs will be essential. (6) Enhancing the FF in tandem NFA solar cells by designing new ICLs. For example, ICLs can be designed with some specific functional groups that can form chemical bonds with NFAs. This design can potentially optimize the vertical distribution of the active layer, which will benefit charge extraction and hence enhance the FF. (7) Employment of triple junctions in tandem NFA solar cells to achieve further high PCE and VOC. The most critical issue for this point is to arrange the bandgaps of three sub-cells with the best current matching. (8) Fabrication of hybrid tandem devices128 based on NFA sub-cells and perovskite/silicon/copper indium gallium selenide (CIGS) sub-cells. (9) Adopting techniques used for inorganic solar cells (for example, anti-reflecting layers and concentrators) to further enhance the efficiency of single-junction and tandem NFA solar cells. So far, NFA solar cells have already exhibited high PCEs of approximately 13% and 14% for single-junction and tandem structures, respectively. Further enhancement in the PCEs of NFA solar cells (to approximately 15–20% when NFA solar cells are characterized in laboratories) will provide a great opportunity to realize future industrial manufacture.
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Y.Y. acknowledges the Air Force Office of Scientific Research (AFOSR) (FA2386-15-1-4108), Office of Naval Research (ONR) (N00014-14-1-0648), National Science Foundation (NSF) (ECCS-1509955) and UC-Solar Program (MRPI 328368) for financial support. X.Z. acknowledges the National Science Foundation China (NSFC) (51761165023, 21734001) for financial support. G.L. acknowledges the Project of Strategic Importance provided by The Hong Kong Polytechnic University (1-ZE29) for financial support. All authors acknowledge N. De Marco for help with English language editing.
The authors declare no competing interests.
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Cheng, P., Li, G., Zhan, X. et al. Next-generation organic photovoltaics based on non-fullerene acceptors. Nature Photon 12, 131–142 (2018). https://doi.org/10.1038/s41566-018-0104-9
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