low-bandgap conjugated polymers enabling solution...

The first observation of the photovoltaic effect in a liq-uid cell was in the 1800s. It took a long time before the development of Schottky barrier devices and pn junc-tion devices produced the forerunners of the advanced photovoltaic technologies that are now commonplace. Over a century lapsed before the modern silicon solar cell was developed in the Bell Labs in 1954 (REF.1).

The main metric used to characterize the perfor-mance of solar cells is their power conversion efficiency (PCE). The maximum theoretical efficiency is defined by the ShockleyQueisser limit2. This thermodynamic balance predicts an optimal bandgap of ~1.4 eV for a single-junction photovoltaic cell under AM1.5G 1 sun illumination with a maximum efficiency of ~31%. In addition to optimizing the bandgap, it was proposed that a good solar cell material must have strong photo-luminescence efficiency. On the basis of this design rule, a remarkably high PCE of 28.8% was demonstrated in a single-junction GaAs solar cell in 2012 (REF.3).

Despite successes in the development of the tech-nology, photovoltaic energy accounts for only a small fraction of our energy mix (~0.2% in the United States)4. The photovoltaic market is growing rapidly: the cumulative installed photovoltaic capacity reached over 300 GW at the end of 2016 (with ~25% installed in 2016 alone), which is capable of supplying 1.8% of the worlds totalelectricity consumption4. Currently, the solar photo voltaic industry is dominated by in or-ganic crystalline silicon photovoltaics, which makes up approximately 90% of the market. One of the main

bottlenecks of silicon-based photovoltaic technology is that its installation is expensive and time-consuming. To overcome this and several other technical and envi-ronmental issues, the scientific community is pursuing next-generation technologies.

Organic photovoltaic (OPV) technology has attracted growing interest in the past decade, owing to features such as synthetic versatility, low-temperature processing, low material utilization, light weight and flexible form factor58. Organic and polymeric semiconductors are carbon-based materials that possess rather different semi-conductor properties from crystalline semi conductors such as Si or GaAs. The organic materials have small dielectric constants, which make them more excitonic than free-carrier semiconductors. One of the milestones in OPV technology was the realization of the importance of the film morphology9 in 2005 and its controlled forma-tion10. Subsequent advances involving new polymers for OPV cells have continued to set new PCE records7,8. In parallel, the invention of infrared-absorbing low-bandgap polymers has enabled device engineers to turn their attention to tandem solar cell architectures, an idea that has been widely practised by the inorganic photo voltaic community. A timeline of important breakthroughs in organic photovoltaics is presented in FIG.1.

In this Review, we first discuss progress in the design of donoracceptor (DA) low-bandgap polymers, which has broadened the boundaries of OPV technology in the past decade. We then discuss solution-processable tandem OPV cells from pure polymerpolymer tandem solar

1Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, California 90095, USA.2Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China.

Correspondence to G.L. and Y.Y. [email protected]; [email protected]

doi:10.1038/natrevmats.2017.43Published online 25 Jul 2017

Low-bandgap conjugated polymers enabling solution-processable tandem solar cellsGang Li1,2, Wei-Hsuan Chang1 and Yang Yang1

Abstract | The technology of polymer-based organic photovoltaic (OPV) cells has made great progress in the past decade, with the power conversion efficiency increasing from just a few percent to around 12%, and the stability increasing from hours to years. One of the important milestones in this progress has been the invention of infrared-absorbing low-bandgap polymers, which allows the OPV cells to form effective tandem structures for harvesting near-infrared energy from the solar spectrum. In this Review, we focus on the progress in low-bandgap conjugated polymers and several tandem OPV cells enabled by these low-bandgap polymers. Specifically, we cover polymer-based tandem solar cells; hybrid tandem solar cells combining polymers with hydrogenated amorphous silicon; and unconventional solar cells. For each of these technologies, we address the challenges and offer our perspectives for future development.

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cells to hybrid tandem cells combining low-bandgap poly-mer with hydrogenated amorphous silicon and emerg-ing unconventional cells incorporating low-bandgap polymers.

Organic semiconductors and solar cellsOrganic and polymeric semiconductors are discrete molecules with weak van der Waals bonding between molecules11. The structure of such materials is created by bonds formed between carbon sp2 orbitals, and bonds formed by parallel overlapping of leftover carbon pz orbitals. For a small molecule such as ethylene or 1,3-butadiene (FIG.2a), the splitting between the bond-ing () and antibonding (*) is large. As the overlap-ping of the carbon pz orbitals increase (for example, through poly merization of ethylene molecules), the bonds further spread out into bands. The topmost band is referred to as the highest occupied molecular orbital (HOMO) and the lowest * band is referred to as the lowest un occupied molecular orbital (LUMO). The energy difference between the HOMO and LUMO deter-mines the bandgap (Eg) of the resulting molecule or poly-mer. Once the repeating unit is large enough, splitting of the orbitals can bring the HOMO and LUMO closer to each other, and the bandgap is thus reduced, becoming equivalent to the energy of ultraviolet or visible light. The intermolecular interactions in organic semiconductors are relatively weak van der Waals and interactions. Therefore, the electrons travel through a hopping mech-anism12, and the charge carrier mobility is several orders lower than that of inorganic crystalline semiconductors. The state of electronhole pairs formed after optical or thermal excitation is also different. Instead of forming free electrons and holes, photoexcitation in organic semicon-ductors creates Frenkel excitons, which cannot dissociate under room temperature. Pioneering work on double- layer thermally evaporated OPVs13,14, observations of

ultrafast electron transfer15 and bulk-heterojunction (BHJ) OPVs16,17 introduced the concept of using electron donor and acceptor materials with energy differences in chemical potentials to release the carriers from strongly bonded excitons. The donoracceptor external quantum efficiency (EQE, number of electrons generated per 100 absorbed photons, which is a function of wavelength or photon energy) of BHJ solar cells depends on the effi-ciencies of the following steps: photon absorption and exciton generation; exciton diffusion; charge separation; and charge collection9. For the electrical currentvoltage characterization of a solar cell, three key parameters need to be extracted7,8: short-circuit current (density) Isc (Jsc) at V = 0; open-circuit voltage (Voc) at I = 0; and fill factor, which is equal to the maximum power output (Imax Vmax) from the IV curve divided by (Isc Voc). Typically, an EQE of over 60% and a fill factor of over 60% is viewed as a benchmark. Exciton dissociation and charge separa-tion happens at the polymeracceptor interface, where electrons at the acceptor LUMO level and holes at the polymer HOMO level form charge-transfer states. The Voc is determined by the charge-transfer state. An impor-tant unresolved question is how much energy can be lost from exciton energy while still maintaining efficient charge separation. This voltage loss can be evaluated by the parameter known as bandgapVoc offset, which is defined as (Woc = (Eg/q) Voc), where q is the elementary charge18. A considerable challenge that persists for OPV cells is reducing the voltage loss, as the Woc is still much larger than that in inorganic solarcells.

Donoracceptor-based polymersThe state-of-the-art p-type polymer semiconductors in OPV are donoracceptor (DA) conjugated polymers, which consist of at least two alternating moieties along their polymer backbone: an electron-rich donor and an electron-deficient acceptor. Compared with the classic

Nature Reviews | Materials

1970s 1986 1992 1995 2005 2006 2007 2009 2010 2011 2012 2014 2015 2016

Invention of conductive polymers and synthetic metals

Triple-junction tandem polymer solar cell (PCE ~12%)

Non-fullerene acceptor ternary polymer solar cell (PCE 12.2%)

Tangs bilayer organic solar cell

Patent of bulk-heterojunction polymer solar cell synthesis of regioregular-P3HT

P3HT and PDTPDFBT tandem polymer solar cell (PCE 10.6%)

PCPDTBT:PCBM solar cell (PCE 5.5%)

Single-junction polymer solar cell (PCE 11.7%) a-Si:PDTPDFBT hybrid tandem cell (PCE 10.5%)

PBDT-TT (or PTB) polymers: certified solar cell (PCE >8%)

P3HT and PCPDTBT tandem polymer solar cell (PCE 6.5%)

45% bulk-heterojunctionP3HT:PCBM solar cells via morphology manipulation

Discovery of polymerfullerene charge transfer

Discovery of fullerene

Invention of PCBM

Inverted polymer solar cell

Single-junction polymer solar cell (PCE 10.1%)

P3HT and PBDTTDPP tandem polymer solar cell (PCE 8.6%)

Reports of polymerfullerene and polymerpolymer bulk-heterojunctionsolar cells

Figure 1 | A brief timeline of discovery and development of organic polymers for solar cells. The advances shown in the boxes are some of the milestones in the development of polymer solar cells, including key conjugated materials discovery, device structure innovation and efficiency breakthroughs. a-Si, amorphous silicon; PCE, power conversion efficiency. Polymers are defined in the text or depicted in FIG.3.

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P3HT (poly(3-hexylthiophene-2,5-diyl)) polymer with only one moiety, the DA polymer design offers clear advantages of flexible tuning of energy bandgap, energy level, transport properties and soon.

In the early 1990s, intramolecular charge-transfer behaviour was observed in a polymer consisting of a thiophene donor moiety and pyridine acceptor moi-ety units19. The absorption and fluorescence spectra of the resulting DA polymer were redshifted, indicating a lower Eg. Soon thereafter, a semiconducting polymer with a very low Eg of less than 0.5 eV was reported20,21. These studies led to the emergence of low-bandgap DA polymers for spectral engineering22,23.

There are two main mechanisms that are responsible for the lower bandgap of DA polymers: delocalization of electrons along the polymer backbone, and hybridi-zation of frontier orbitals. First, the DA structure aids the delocalization of electrons along the conjugated back-bone, which helps to stabilize the quinoid structure over the backbone, leading to a smaller energy gap24. Second,

hybridization of frontier orbitals can further reduce the energy gap25: as shown in FIG.2b, as two moieties become closer to each other by chemical bonding, the HOMOs and LUMOs of the donor and acceptor start to interact with each other. Once the electrons are redistributed, a new set of HOMO and LUMO levels of the bonded compound is produced, which results in a smaller band-gap. This concept also explains the fact that for the DA polymer, the resulting HOMO level is largely affected by the donor, and the LUMO is largely affected by the accep-tor26. Recent studies suggest that these two mechanisms are closely related and could be mutually beneficial27.

Constructing DA polymers starts by forming a CC bond between two functionalized moieties. Transition metal-catalysed cross-coupling reactions (for example, Sonogashira, Heck, SuzukiMiyaura or Stille reactions, or direct arylation couplings) are used to form such bonding. Comprehensive reviews on these polymeriza-tion methodologies and their mechanisms can be found elsewhere26,28,29.

The first design consideration for a DA OPV poly mer is the bandgap, which depends largely on the strength of the electron-pushing (and withdrawing) abilities of the donor (and acceptor). In this Review, we focus on poly-mers that have bandgaps less than 1.6 eV and thus have a near-infrared photoresponse. Representative DA low-bandgap polymer structures are shown in FIG.3.

The HOMO and LUMO energy levels of a polymer are of crucial importance because they are directly linked to the Voc and the charge-separation efficiency of the OPV. The rule of thumb when designing a p-type photo voltaic polymer is to downshift the HOMO level while keeping the LUMO level above that of the n-type mat erial30. Good solubility in common organic solvents is also needed for solution processing, which can be affected by factors such as molecular mass, side-chains, rigidity of the polymer backbone and the strength of the polymers intermolecu-lar interaction. Trial-and-error experiments are typically still needed31.

Benzothiadiazole and difluorobenzothiadiazole units. Benzothiadiazole (BT) and its derivatives have been widely used as electron-deficient units. The first example32, PCPDTBT (polymer 1 in FIG.3; Eg = 1.4 eV), is a small-bandgap polymer made by alternating the electron-rich cyclopenta[2,1-b;3,4-b]dithiophene (CPDT) and the BT units. The PCE of the poly-mer1-based device was improved from 3 to 5.5% through nano scale morphology tuning using solvent additives (that is, diiodooctane)10. Replacement of the CPDT unit of the polymer by a silicon-substituted analogue, dithieno [3,2-b:2,3-d]silole (DTS), to form PSBTBT (poly mer 2, otherwise known as Si-PCPDTBT) has a negligible effect on the bandgap3335.

Interestingly, the optimal performance of a blend of polymer 2 with [6,6]-phenyl-C71-butyric acid methyl ester (PSBTBT:PC71BM) can be achieved without using solvent additives, and the CSi bond results in better polymer stacking. Replacing Si with Ge (polymer 3) leads to similar bandgap and device performance36, but 3 has a different face-on orientation in the thinfilm.


a

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HOMO

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Eg

b

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*

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Ethylene 1,3-Butadiene Oligomer Polymer

10 n

*

LUMO

LUMO

HOMO

HOMO

D DA A

Figure 2 | Principles of organic semiconductor and donoracceptor polymers. a|Relationship between the bandgap and the number of overlapping pz orbitals. b|Orbital interactions of donor (D) and acceptor (A) units, which results in a smaller bandgap in donoracceptor (DA) polymers. Eg, bandgap; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital. Part b is modified with permission from REF.7, American Chemical Society.

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NS

N

N

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S S

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7: PBDTTDPP, X = S8: PBDTTSeDPP, X = Se

4: PDTPDFBT 5: PffBT4T-2OD

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16: N2200

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Figure 3 | Molecular structures of representative donoracceptor low-bandgap polymers. The representative building blocks for low-bandgap organic photovoltaic polymers include benzothiadiazole (BT), diketopyrrolopyrrole (DPP), thieno[3,4-b]thiophene (TT) , isoindigos, and thieno[3,4-c]pyrrole-4,6-dione (TPD).

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Very recently, an oxygen-inserted donor, dithieno-[3,2-b:2,3-d]pyran (DTP) was introduced as a strong electron donor. When DTP is polymerized with a difluoro benzothiadiazole unit, the polymer obtained, PDTPDFBT (polymer 4), has a smaller Eg (1.38 eV); an OPV based on 4 showed an 8.0% PCE. The main improvement in the PCE compared with OPVs based on 13 comes from its low-lying HOMO and small band-gap, which result in a large Voc and high Jsc. In addition, this polymer presented an EQE of ~60% in the near- infrared region, which is among the highest reported so far37. When combined with a wide-bandgap subcell made of P3HT and indene-C60 bisadduct (P3HT:ICBA), this polymer led to the first tandem polymer solar cell with a PCE above 10%. DFBT was recently polymerized with different thiophene units to form a series of poly-mers with structures similar to the polymer known as PffBT4T-2OD (polymer 5). Through an interesting high-temperature solution-casting processing method38, a PffBT4T-2OD:PC71BM single-junction solar cell showed high PCE and particularly high fill factor of 10.8% and 77%, respectively. Using a similar structure, a high-performing solar cell was processed with a non-halogenated solvent39.

Diketopyrrolopyrrole units. The first DPP-based poly-mer, poly(diketopyrololpyrrole-terthiophene) or PDPP3T (polymer 6)40, has a bandgap of 1.31 eV, which allows absorption up to 930 nm. After optimization of the molec-ular mass, a PCE of ~6% was achieved41. Combining DPP with thienyl-substituted BDT (BDTT) to form PBDTTDPP (polymer 7) resulted in a bandgap of 1.44 eV in 2012, and the Voc was higher (that is, the energy loss was reduced) compared with DPP polymers that had pre-viously been reported. A PCE of ~6.2% was achieved42. Modification of 7 with selenium incorporation on the DPP unit to give PBDTTSeDPP (polymer 8), led to a redshifted absorption spectrum and PCE of 7.2%43. Other notable DPP polymers in recent years include polymers based on thieno[3,4b]thiophene-flanked DPP (DPPTT; for example, DPPTT-T, polymer 9) with an impressive Jsc of 18.6 mAcm2, reported in 2013 (REF.44), and an 8.8% PCE solar cell with Jsc of 23.5 mA cm2 reported in 2015 (REF.45). DPP-based polymers remain a popular OPV poly mer building block46,47, and an interesting morphol-ogy formation mechanism48 and molecular packing49 have been demonstrated in DPP-based polymers.

Thieno[3,4b]thiophene units. Thieno[3,4-b]thiophene (TT) units were introduced in 2009 as a strong accep-tor unit in OPV50,51. TT units were copolymerized with the benzo[1,2-b:4,5-b]dithiophene (BDT) unit52, giving a series of polymers with bandgaps of ~1.6 eV. A mile-stone in OPV technology was achieved with a solar cell based on the copolymer PTB7 (polymer 10)50 (contain-ing BDT and TT units, with diiodooctane as an addi-tive in morphology tuning), which exhibited a PCE of 7.4% without device architecture optimization. This high PCE is believed to be related to the high planarity of the BDT unit, the low-lying HOMO caused by fluo-rine53,54, and an appropriate side-chain length to optimize the polymer molecular mass and solubility55. Through

different chemical modifications (such as PBDT-TT-CF and PTB7-Th (polymer 11)), this family of polymers has revolutionized the field56,57. Substantial improvements on PTB-based polymers were subsequently made through interfacial layer engineering of the OPV device. A conju-gated polyelectrolyte was used as an interfacial layer in an inverted device structure, which improved the PCE from 7.4 to 9.2% when using PTB7 (REF.58) and to 10% when using PTB7-Th (REF.59). A small-molecule zwitterionic non-conjugated electrolyte60 has also been shown to be an effective charge-injection layer for polymer solar cells, enabling a PCE of over 10% using PTB-Th polymer61.

The processing additives required in the examples pro-vided above have high boiling points, which is undesirable in terms of ease and cost of fabrication. In one example of additive-free processing, S was replaced with Se to pro-duce PBDTSe-TT (polymer 12) and a PCE of ~9%62. It is believed that the higher intermolecular interaction caused by stronger SeSe interaction gives a favourable morphol-ogy63. In another example, an alkylthio-substituted PTB polymer, PBDTT-S-TT, showed a decent PCE of 8.4% without use of processing additives64.

Isoindigo units. Another small-bandgap polymer family is based on isoindigos. Because of its internal lactam ring, isoindigo has a very strong electron-withdrawing prop-erty. The first thienyl-derived isoindigo (TII)-based pol-ymer 13 was reported in 2013 (REF.65). Each thiophene substitution resulted in a ~0.2 eV reduction in bandgap. As a result, a 1.4 eV bandgap polymer was produced using one benzene and one thiophene, and a 1.15 eV bandgap polymer was achieved by using fully thiophene-replaced isoindigo (that is, TII). Although these polymers have very good charge-carrier transport properties, the best PCE achieved (using PTBII2T) was a mere 4%. This is possibly because of their low-lying LUMO levels (and thus insufficient LUMOLUMO offset), which affects the charge-separation efficiency in the device. TII was further copolymerized with different donor groups such as BDT, CPDT and fluorene66, and the bandgap was suc-cessfully lowered to around 1.0 eV. However, the solar cell performance is not attractive enough, owing to low current generation.

Other donoracceptor lowbandgap polymers. Organic chemistry provides many opportunities to improve the performance of OPV cells. For example, a strong electron acceptor unit, thieno[3,4-c]pyrrole-4,6-dione (TPD), can be easily synthesized. In 2010, the TPD moiety was copolymerized with BDT units to form a copolymer (PBDTTPD) with a bandgap of 1.81 eV and very deep HOMO level of 5.57 eV (REFS6769). Among reports from several groups, the best PCE so far for this system is 6.8%. Alternatively, the TPD moiety copolymerized with the DTS unit gives the lower-bandgap polymer PDTSTPD, with a bandgap of 1.73 eV and deep HOMO level, and a higher PCE of 7.3%70. The silicon atom in the DTS unit can be replaced with a germanium atom to form the dithienogermole (DTG) unit71: the new polymer PDTGTPD (polymer 14) and interface engineering pushed the PDTGTPD-based device to 8.1% PCE72.

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The benzothiadiazole unit mentioned above is a powerful and interesting moiety, the chemical modi-fication of which can lead to new functional building blocks. In the BT unit, replacing the top sulfur atom with nitrogen leads to the benzotriazole (TAZ) unit, which is more electron-accepting than the BT moiety. When the benzene ring in BT is converted to a pyridine ring (that is, one carbon atom in the ring is replaced by a nitrogen atom), this leads to a more electron-deficient pyridyl-thiadiazole unit. Copolymerizing this PT unit with an electron-donating indacenodithiophene unit leads to a PTIDT copolymer, and a polymer solar cell with 3.4% PCE was reported in 2011, using PC71BM as the acceptor. The electronic properties of the PTIDT polymer can be further improved by controlling the regiochemistry. Formed by precisely arranged pyridyl nitrogen atoms of the PT along the polymer backbone, the regioregular PTIDT polymer PIPT-G (polymer 15) shows good solar cell performance. The inverted OPV efficiency was further improved from 4.6% in a regioran-dom (PIPT-RA) donor system to 6.7% in a regioregular (PIPT-RG) donor system, although the two polymers have identical optical bandgaps of 1.60 eV, clearly show-ing the power of regioregularity73. Copolymerizing with CPDT units in a regioregular manner, PT-based low-bandgap polymers with an optical bandgap of ~1.1 eV were realized74, which also showed excellent mobility in transistors. An even lower-bandgap (0.7 eV) conjugated DA polymer based on thiadiazoloquinox-aline as the acceptor absorbs up to 1,476 nm and shows good mobility in organic thin-film transistors75.

Low-bandgap polymers in particular, polymers containing perylene diimide (PDI) or naphthalene diimide (NDI) have potential as acceptors in high- performance all-polymer OPVs. A notable example is the copolymer of NDI-bithiophene (N2200, polymer 16) with an optical bandgap of ~1.45 eV, first used in an n-type organic thin-film transistor76. With N2200 as the

acceptor, and a medium-bandgap PBDTTAZ co poly-mer (17, Eg = 1.91 eV) as the donor, a PCE of 8.3% was achieved, with an excellent EQE and fill factor (75% and 70%, respectively)77. The parameters of OPV devices based on representative low-bandgap polymers are shown in TABLE1.

Solution-processable tandem solar cellsThere are two main losses under the ShockleyQueisser assumptions. First, low-energy photons beyond the semi conductor band edge are optically lost. Second, photons of higher energy than the bandgap will ther-mally relax, with the excess energy lost through ther-malization. The most effective way to minimize these two fundamental losses is to adopt a multijunction or tandem structure, which consists of several semiconduc-tors with different bandgaps to ensure better coverage of the solar spectrum. In such designs, each semi conductor is responsible for narrow-band absorption, such that thermal loss is reduced.

In traditional multijunction cells, the subcells are interconnected by tunnel junctions, which provide low resistance connections. A tunnel junction has the struc-ture of a heavily doped pn junction (p++n++ junction), which has the opposite direction to that of the two sub-cells, such that it produces a photovoltage in the same direction as those generated by the subcells. Because of the heavy doping, the space-charge region for the tunnel junction is very narrow. Under a small forward bias or any reversed bias, when the current tunnels through the nar-row space-charge region, the tunnelling diode behaves like a resistor. Under sufficiently large forward bias, when the current surpasses a threshold tunnelling cur-rent, thermionic emission dominates the tunnelling diode. The threshold tunnelling current must be larger than the photocurrent of the tandem cell78.

Strictly speaking, the multijunction or tandem cells mentioned above, in which two subcells are connected in

Table 1 | Single-junction cells

Donor:acceptor Eg (eV) Voc (V) Jsc (mA cm2) FF (%) PCE (%) Refs

PCPDTBT:PC71BM 1.46 0.62 16.2 55 5.2 10

PSBTBT (or Si-PCPDTBT):PC71BM 1.5 0.576 14.92 61 5.2 35

PDTPDFBT:PC71BM 1.38 0.7 18 63.4 8 37

PffBT4T-2OD:TC71BM 1.63 0.77 18.8 75 10.8 38

PBDTTSeDPP:PC71BM 1.38 0.69 16.8 62 7.2 43

PTB7:PC71BM 1.6 0.74 14.5 68.97 7.4 50

PTB7:PC71BM 1.6 0.74 17.2 72 9.15 58

PTB7-Th:PC71BM 1.59 0.825 17.43 73.78 10.61 59

PBDTSe-TT:PC71BM 1.59 0.83 15.4 69 8.8 62

PBDTTPD:PC61BM 1.73 0.85 11.5 68 6.8 69

PDTGTPD:PC71BM 1.69 0.865 13 65.7 7.4 72

PBDTTAZ:N2200 1.91 (PBDTTAZ) and 1.45 (N2200)

0.83 14.18 70.24 8.27 77

Parameters of representative low-bandgap polymer-based organic photovoltaic devices. Eg, bandgap; FF, fill factor; Jsc, short-circuit current density; PCE, power conversion efficiency; Voc, open-circuit voltage. Polymers are defined in the text or depicted in FIG. 3.

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series, should be called two-terminal tandem solar cells. It is possible to have a third conductive terminal to sep-arate or connect the two subcells. In this three-terminal tandem configuration, the middle terminal is a shared common terminal, and the two subcells are connected in parallel. In this case, the subcell currents do not need to match; however, the photovoltages of the two cells must be matched and maximized. A four-terminal tandem device consists of two complete two-terminal subcells that are connected either in series or parallel. One should keep in mind that each terminal is a highly conductive layer most often a transparent conductive layer. The versatility of the connection scheme provided by the three-terminal and four-terminal tandem devices comes with drawbacks: for example, additional optical loss asso-ciated with transparent terminals; the extra cost of the terminals; and the process compatibility of the extra ter-minals with the device. This issue of process compatibil-ity could be particularly problematic in organic tandem solar cells because the use of sputtered transparent oxides as extra terminals introduces a vacuum process, which breaks from the continuous solution process and may damage the organic layers. Moreover, for solution-pro-cessed metal-nanowire-based transparent electrodes, dif-ficulties arise, such as finding orthogonal solvents and achieving a uniform coating that can be applied to a large area. In the following sections, we focus on two-terminal tandem OPV devices.

Polymerpolymer tandem solar cellsThe first two-terminal double-junction solar cell is based on GaInP and GaAs as subcells79. P3HT (1.9 eV) has a similar large bandgap to GaInP (1.85 eV), and the pro-cessing is easy and well-controlled, which makes P3HT a promising building block for polymer tandem cells. High-quality low-bandgap polymers, particularly those men-tioned in the previous section, were invented later than P3HT, and have led to great progress in the development of solution-processed tandem OPV devices.

In the early years in OPV development, tandem poly-mer solar cells were limited by the available polymers. So-called homo-tandem solar cells (that is, using iden-tical polymers as active layers) were adopted by several groups. Examples include four-terminal tandem devices using two single-junction subcells made of poly(2-meth-oxy-5-(2-ethylhexyloxy)-p-phenylenevinylene) and PCBM (MEH-PPV:PC61BM) to create a semitranspar-ent OPV80; a two-terminal tandem polymer cell based on two subcells of poly(2-methoxy-5-(3,7-dimethyloc-tyloxy)-1,4-phenylenevinylene) and PCBM (MDMO-PPV:PC61BM)81; two P3HT:PC61BM subcells82,83; and two subcells of poly(N-90-hepta-decanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)) and PCBM (PCDTBT:PC61BM)84.

Conventionalconfiguration tandem polymer solar cells. Conventional solar cell configurations use transparent conductive oxides on substrates as natural positive con-tacts. In 2006, tandem polymer cells were demonstrated that had complementary absorptions similar to those in inorganic tandem cells85.

Although polymers are solution-processable, the interconnecting layer in tandem OPV cells also needs to be as solution-processable as possible to allow low-cost fabrication. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), with a high work function and processing advantages, was a choice for many of the early tandem polymer OPV devices. Titanium oxide (TiOx (REF.86)), zinc oxide (ZnOx)87 and other n-type inorganic metal oxides produced through solgel chemistry are compatible with solution process-ing and are good candidates for the n-type side of the interconnectinglayer.

In 2007, an important breakthrough in polymer tandem solar cells was reported: a solution-processed interconnecting layer with TiOx/PEDOT:PSS88, linking a low-bandgap PCPDTBT cell (1.4 eV bandgap), and a wide-bandgap cell of P3HT and PC71BM. The PCE of the tandem cell (6.5%) was substantially higher than that of the front and back cells (3% and 4.7%, respectively). However, the device still suffered from low EQE of the low-bandgap subcell and a small fillfactor.

Using an improved polymer PSBTBT/Si-PCPDTBT, a 0.5 nm Al/nanocrystalline TiO2/PEDOT:PSS inter-connecting layer structure was developed for a tandem polymer solar cell89. The ultrathin aluminium layer improved both the wettability and electrical contact of the TiO2 film on the bottom P3HT-blend film. In another study, TiO2:Cs was used as an electron-transport layer by incorporating Cs2CO3 into nanocrystallineTiOx. This led to Cs-doping of TiOx, which reduced the work func-tion. Although the subcells had PCEs of 3.77 and 3.94%, the tandem cell reached 5.84%90. The same tandem cell structure, when P3HT:ICBA replaced the P3HT:PCBM subcell91, also enabled the first tandem polymer OPV cell with PCE over 7%.

Zinc oxide nanoparticles were used as the n-type layer together with an interestingly modified PEDOT:PSS dis-persion as the p-type layer in a tandem solar cell92,93. In this configuration, insertion of a thin metal layer and ultraviolet excitation to dope the ZnO layer were found to be important to improve the contact. Other intercon-nection layers using n-metal/p-structures have also been reported, for example, with LiF as the n-type layer, and transition metal oxides such as V2O5, MoO3 and WO3 (REFS94,95) as the p-type layer96.

Invertedconfiguration tandem polymer solar cells. Structurally opposite to conventional architecture, the inverted polymer solar cells use n-type interface lay-er(s) to modify the high intrinsic work function of trans-parent conductive oxide electrodes, such that opposite polarity can be realized. Early demonstrations included the use of transition metal oxides as the p-type interfa-cial layer and Cs2CO3 to act as the n-type layer94,95; and the use of ZnO as a transparent conductive oxide modi-fication layer and a silver electrode for the cathode97. The concept of inverted polymer solar cells was embraced warmly by the community because the structure ena-bles the removal of reactive metals, which substantially boosted the stability of the solar cell. In addition, the new structure is also more manufacturing-friendly.

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The parameters of representative low-bandgap poly-mer-based organic tandem solar cells discussed in this subsection are shown in TABLE2.

In an early successful inverted tandem solar cell, P3HT:PCBM and PSBTBT:PCBM subcells were con-nected using a thin Al connecting layer inserted between p-type MoO3 and n-type ZnO, to achieve a 5.1% PCE tandem cell98. The electrical conductivity, processability and robustness of the interconnection layer was improved by introducing higher-conductivity PEDOT:PSS (such as PH500 or PH1000) with greater thickness, which was applied to various low-bandgap polymers. The PBDTTDPP polymer (7, Eg = 1.44 eV) complemented well the wide-bandgap P3HT, enabling an inverted tan-dem polymer solar cell (with PBDTTDPP:PCBM and P3HT:ICBA subcells) with a 8.62% record PCE42 as certified by the National Renewable Energy Laboratory (NREL). The PCE of the tandem cell was improved to 9.5% by using Se-containing PBDTTSeDPP (8, Eg = 1.38 eV), which provided greater spectral comple-mentarity with P3HT43. Furthermore, the low-bandgap polymer PDTPDFBT (4, Eg = 1.38 eV) showed ~8.0% PCE in single-junction configuration37. The higher quantum efficiency of this polymer at long wavelengths (650900 nm) benefited the inverted tandem architecture when used with a P3HT cell, which covers the wave-length range of ~350650 nm. This spectrum matching leads to a high tandem cell photocurrent and a tandem polymer solar cell with a NREL-certified PCE of 10.6%99.

Coating a large area uniformly with a very thin layer of ~1020 nm is a big challenge for OPV fabrication. For a configuration based on aluminium-doped ZnO, it was shown that the thickness of the low-conductivity PEDOT:PSS connecting layer could range from about 50 to above 60 nm, which is the optical interference max-imum in tandem cells. This tandem polymer solar cell configuration seems to be a promising structure that is friendly to large-scale production100.

Triple-junction tandem polymer solar cells93,101,102 with low-bandgap polymers have also been reported, with PCEs close to 12%. The all-solution-processed triple junc-tion is another step forward in the manufacturability of

tandem cells. There is also renewed interest in homo-tan-dem polymer solar cells using new-generation low-band-gap polymers, such as PDTPDFBT (polymer 4), which exhibits a homo-tandem cell PCE of 10.2%103. In FIG.4, a comparison is provided of the configurations of conven-tional double-junction88, inverted double-junction99, and triple-junction101 polymer tandem cells, together with the absorption spectra of representative wide-bandgap, medium-bandgap and low-bandgap polymers101.

The inverted tandem OPV structure has also been expanded into the field of OPVs based on non-fullerene acceptors. Using N2200 polymer as the acceptor and a BDT-based donor polymer P2F-DO, an inverted homo-tandem polymer cell was developed with a PCE of 6.7% (compared with 4.7% for the single-junction device)104. In another example, hetero-tandem solar cells were prepared using n-type organic small- molecule acceptors and polymer donors with different bandgaps in an inverted architecture105. After optimi-zation of the MoO3/Ag/PFN connecting layer, a PCE of 8.5% was achieved (compared with 6.1 and 6.3% for P3HT:SF(DPPB)4 and PTB7-Th:IEIC subcells, respec-tively (PFN, poly[(9,9-bis(3-(N,N-dimethylamino)pro-pyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)]; DPPB, 1,4-bis(diphenylphosphino)butane; IEIC, a non-fullerene electron acceptor).

Polymerinorganic hybrid tandem cellsThe simple, coating-compatible processing of polymer solar cells, which can be performed under ambient con-ditions, makes them valuable candidates for integration with other solar cell technologies to form hybrid tandem solar cells. Hydrogenated amorphous silicon (a-Si:H) cells have a long history and proven industrial track record, with desirable high Voc of ~0.92 V and a high fill factor. The bandgap of a-Si:H is ~1.7 eV (that is, it absorbs until ~750 nm), with major absorption below 650 nm wavelength. This is comparable to the wide-bandgap subcell of GaInP, or P3HT, and thus may be suitable for tandem cell. However, the efficiency of single-junction a-Si:H solar cells is typically

The first OPV device combining a low-bandgap poly-mer with a-Si:H was reported in 2011 (REF.108). However, despite the PCE of the a-Si:H cell being 4.9%, the hybrid tandem cell yielded a PCE of just 1.84%. The main limi-tation might have been the quality of the PCPDTBT poly-mer, because the PCE of the single-junction cell was 2.2%, compared with the PCE of 5.5% originally reported10. The connecting layer used might also have been responsible for the poor efficiency. In another example109, a PCE of 5.7% was achieved for a P3HT:PCBM and a-Si:H tandem cell with an indium tin oxide (ITO) and PEDOT:PSS connectinglayer.

By incorporating more efficient, lower-bandgap and spectrally complementary polymer subcells (for exam-ple, Si-PCPDTBT:PC71BM with a AZO/PEDOT:PSS (where AZO is aluminium-doped zinc oxide) connect-ing layer110, or PDPP3T:PCBM with an ITO/PEDOT:PSS connecting layer)111, hybrid tandem solar cells with PCEs of ~7.5% were realized. In these works, the polymer sub-cells suffered from low near-infrared EQE (

to achieve low cost, because the a-Si:H process is itself relatively mature and low cost; this allows more tolerance of non-solution steps, such as sputtering ITO on top of a-Si before the solution processsteps.

Unconventional applicationsLow-bandgap polymers also contribute to unconven-tional solar cells. The parameters of representative low-bandgap polymer/inorganic hybrid and unconven-tional solar cell devices are shown in TABLE3. One exam-ple is multicomponent BHJ cells, particularly those with two donors with one acceptor (that is, a ternary OPV). The availability of many high-performance donor mat-erials with varied properties (such as bandgap, energy level and molecular orientation) is the main driving force enabling the multicomponent BHJ concept115. Small molecules116, dyes117, polymers118 or quantum dots119 can function as the additional components; these sys-tems have been reviewed elsewhere120,121. The additional components may improve charge and energy transfer122 and may work independently (that is, parallel) or cou-ple electronically121,123. Early efforts on multicomponent BHJ OPV have proved the concept but the efficiency remains low. Recently, however, a ternary dual-polymer donor solar cell with a PCE of 10% was demonstrated124. Theoretically, the ternary system is limited by the ShockleyQueisser limit of the single-junction solar cell. However, the possibility of using a wide range of differ-ent materials offers opportunities to achieve higher PCE. We are still advancing our understanding of the complex nature of OPV systems, and the research community has a long way to go. Conflicting proposed design rules and mechanisms for successful ternary polymer solar cell systems have been reported, such as whether the molec-ular orientation of the multiple donor molecules has a substantial role in determining the success of ternary OPVs. We have provided evidence that this is the case in a model system, and provided guidelines by which an efficient OPV with more than three components can

be achieved115. However, cases have been reported that do not follow these guidelines123. We therefore suggest that researchers consider hidden parameters, such as the role and degree of crystallinity, the donoracceptor interactions and the effect of phase separation in multiple component BHJ systems.

The two-donor/one-acceptor ternary OPV concept was recently extended to one-donor/two-acceptor ter-nary OPV systems, which was particularly boosted by remarkable progress in non-fullerene acceptors. Recently, a ternary polymer cell was reported with a record 12.2% PCE that was based on one wide-bandgap polymer (PBDB-T) and two acceptors (non-fullerene IT-M and one fullerene derivative, bis[70]PCBM)125. In another recent example, two non-fullerene acceptors were used to form a ternary cell with well-known PTB7-Th, to achieve over 11% PCE126.

In organo-metal halide perovskite solar cell research, it was found that BHJ PBDTTSeDPP (polymer8):PCBM can function well on top of the perovskite layer127 (FIG.5c). In a double-layer device, the near-infrared contribution of the polymer BHJ was clearly beyond the EQE spectrum of the perovskite. More interestingly, the overall Voc of the device (0.94 V) was dominated by the perovskite, which has a significantly higher Voc than the BHJ cell (0.66 V). Therefore, the structure can enable appreciably smaller Voc loss of long-wavelength photons. A similar phenom-enon was observed up to a 970 nm cut-off response using PDPP3T (polymer 6)128. Later, an impressive Voc of 1.07 V was reported for a ternary BHJ system with polymer:P-C71BM:N2200, which used another DPP-based polymer with bandgap ~1.38 eV (REF.129). Future research should focus on elucidating the mechanisms of this integrated perovskiteOPV device, particularly the Voc loss, which is even smaller than for the perovskite solar cell itself. The efficiency of these integrated devices, however, has not yet reached that of the perovskite solar cell. Further optimizing the performance of the integrated device to test the limitation is also an area of interest.


a a-Si:H/polymer double-junction solar cell

b a-Si:H/polymer triple-junction solar cell

c Perovskite/polymer solar cell

Ca/metal electrode

Low-bandgap polymer BHJ cell

p-type ICL:PEDOT:PSS or MoO3

TCO:ITO or AZO

a-Si:H pin cell Thick a-Si:H pin cell

Thin a-Si:H pin cellGlass/AZO

Ca/metal electrode


PEDOT:PSS

ITO

Glass/AZO

MoO3/metal electrode


Perovskite cell

TiO2

Substrate/ITO

Figure 5 | Variety of hybrid tandem solar cells containing low-bandgap polymers. a|Typical structure of planar hybrid tandem (double-junction) device containing a-Si:H/low-bandgap polymer. b|Device structure of hybrid tandem triple-junction solar cell containing a-Si:H/a-Si:H/low-bandgap polymer. c|Device structure of integrated perovskite/bulk-heterojunction polymer photovoltaic cell. a-Si:H, hydrogenated amorphous silicon; AZO, aluminium-doped zinc oxide; BHJ, bulk heterojunction; ICL, interconnecting layer; ITO, indium tin oxide; TCO, transparent conductive oxide. Polymers are defined in the text or depicted in FIG.3.

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Summary and perspectivesLow-bandgap polymers, including single-junction, pure and hybrid polymer tandem, and unconventional structures, have been extremely important in the pro-gress of OPV technology. However, substantial effort is still required to make the technology economically feasible. Most notably, in terms of device performance, OPV devices are at a disadvantage compared with other photovoltaic technologies.

Photovoltage loss is the most significant loss in OPV cells, which is quantified in terms of the offset, Woc, between the bandgap and the Voc. Some state-of-the-art low-bandgap polymers (for example, PBDTTDPP and PDTPDFBT) can reach an offset of ~0.7 V. Although much smaller than that of classical polymers (such as P3HT (Woc >1.0 V), PCDTBT (Woc ~0.91 V) and even PBDT-TT (Woc ~0.85 V)), the open-current voltage loss is still too high compared with that of c-Si and orga-no-lead halide perovskite solar cells (~0.40.5 V) and GaAs (~0.3 V). Non-fullerene acceptors have further reduced the open-current voltage loss, with several recent systems showing values of ~0.6 V, including near-infrared OPVs using low-bandgap acceptors127,128,130.

The energy-level difference between the bandgap and charge-transfer state can be very small or often negligible, and yet still give effective charge separation in a OPV system, with a PCE of 9.5% and an offset of 0.61 V (REF.131). Theoretically, it has been shown that two-junction OPV cells with subcell bandgaps of 1.4 and 1.9 eV could reach a PCE of 25% with an offset of 0.6 V in both cells, a tandem cell fill factor of 80%, and EQE of 90%132. Even with 80% EQE, which is almost reached in state-of-the-art single-junction OPVs, a 2223% PCE tandem OPV is theoretically possible, and if realized, would close the gap between OPV and other photovol-taic technologies considerably.

The synthesis possibilities of acceptors with tunable HOMOLUMO levels provide opportunities to push the limits of photovoltage in OPV cells. Reducing the photo-voltage to 0.5 V or even 0.4 V may soon be achievable. Many low-bandgap polymers were originally deemed not to be promising for OPV cells owing to the low Voc

when used with fullerene acceptors. New, tunable accep-tors could bring new life to these material and repre-sent a gold mine of opportunities. Hybrid tandem cells also provide promising new research dimensions. The perovskite/BHJ OPV cell gives a remarkably low offset of 0.33 V. However, the mechanism remains to be eluci-dated, and it is not clear how much further this can be optimized. A low-bandgap polymer/a-Si OPV hybrid tandem cell can feasibly attain a high fill factor of close to 80%, which is very encouraging and should further stimulate research efforts in hybrid tandem solar cells, as well as pure tandem OPV research.

Single-junction and tandem OPV cells are currently both at the same PCE level (~12%). This is a similar sit-uation to around 5years ago, when tandem OPV cells reached a PCE level of 89%, slightly higher than that of single-junction OPV cells at the time. Tandem OPV cells are much more complicated than single-junction cells, but have greater potential to reach higher performance. With respect to key device parameters, the Voc is generally retained in tandem configurations (that is, the low offset in single-junction cells is easily translated to the tandem structure). However, a particular challenge lies in estab-lishing how the high EQE (~80%) and fill factor (80%) of single-junction OPV cells can be realized in tandem structures (which are now at ~60% near-infrared EQE and ~65% fill factor). Interface engineering innovation59,98 and plasmonic130 approaches may hold the key. In parallel, exploring high-performance semitransparent OPV cells to realize high efficiency in four-terminal tandem struc-ture is still a meaningful way of mitigating risks of the technical difficulty in two terminal monolithic tandem cell, although this is associated with significant addi-tional cost. Other challenges that exist in tandem OPV research include overcoming the issues of solar cell yield in scale-up and device/module reliability.

In summary, OPV technology faces both great chal-lenges and opportunities. The ultimate answer to photo-voltaic technology may be the marriage of more than one existing photovoltaic technology, particularly the print-able ones133. Progress is encouraging, and we anticipate breakthroughs in the next fewyears.

Table 3 | Hybrid tandem and unconventional cells

Configuration Inorganic cell Organic cell Interconnecting layer Voc (V) Jsc (mA/cm2) FF (%) PCE (%) Refs

Hybrid a-Si:H pin PDTPDFBT:PC71BM ITO/PEDOT:PSS 1.544 9.8 69.2 10.5 112

Hybrid and triplet

a-Si:H pin 60 nm; a-Si:H pin 400 nm

Si-PCPDTBT:PC71BM ITO/PEDOT:PSS 2.26 6.83 75.8 11.7 113

Ternary PTB7:PBDTTSeDPP:PC71BM 0.69 18.7 67.4 8.7 115Ternary PTB7:PID2:PC71BM 0.72 16.8 68.7 8.22 122Ternary, NFA PBDB-T:IT-M:bis[70]PCBM 0.952 17.39 73.7 12.2 125Perovskite, BHJ CH3NH3PbI3 DOR3TTBDT:PC71BM 0.99 21.2 67.9 14.3 127Perovskite, BHJ CH3NH3PbI3 PBDTTSeDPP:PC BM 0.94 20.6 62 12 127Parameters of representative hybrid solar cells based on low-bandgap polymer/inorganic cells and unconventional solar cell devices. a-Si:H, hydrogenated amorphous silicon; BHJ, bulk heterojunction; FF, fill factor; ITO, indium tin oxide; Jsc, short-circuit current density; NFA, non-fullerene acceptor; PCE, power conversion efficiency; Voc, open-circuit voltage; IT-M, methyl-modified non-fullerene acceptor; PBDB-T, poly(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b] dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c]dithiophene-4,8-dione)); PID2, poly-3-oxothieno[3,4-d]isothiazole-1,1-dioxide/benzodithiophene. Other polymers are defined in the text or depicted in FIG.3.

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AcknowledgementsThis work is supported by the US Office of Naval Research (ONR, Grant No. N00014-14-1-0648, programme director: P. Armistead), Air Force Office of Scientific Research (AFOSR, Grant No. FA238615-14108, programme director: C. Lee) and National Science Foundation (CHE 1230598, programme director: L.S. Sapochak; DMR1335645, programme director: C.Ying). This work was also supported by the funding for Project of Strategic Importance provided by The Hong Kong Polytechnic University (project code: 1-ZE29). The authors thank I. Wang for help in editing and proofreading this article before submission.

Competing interests statementThe authors declare no competing interests.

Publishers noteSpringer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

How to cite this articleLi, G., Chang, W.-H. & Yang, Y. Low-bandgap conjugated poly-mers enabling solution-processable tandem solar cells. Nat. Rev. Mater. 2, 17043 (2017).

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_Hlk485827673_Hlk485827694_Hlk485886427_Hlk485827848_Hlk485827684_Hlk485907672_Hlk485828970_Hlk485829035_Hlk485895163_Hlk485829390_Hlk485885956_Hlk485885658_Hlk485831584_Hlk485886361_Hlk485886543_Hlk485887702_Hlk485887873_Hlk485888933_Hlk485889253_Hlk485889297_Hlk485889868_Hlk485890843Abstract | The technology of polymer-based organic photovoltaic (OPV) cells has made great progress in the past decade, with the power conversion efficiency increasing from just a few percent to around 12%, and the stability increasing from hours to yearFigure 1 | A brief timeline of discovery and development of organic polymers for solar cells. The advances shown in the boxes are some of the milestones in the development of polymer solar cells, including key conjugated materials discovery, device structOrganic semiconductors and solar cellsFigure 2 | Principles of organic semiconductor and donoracceptor polymers. a|Relationship between the bandgap and the number of overlapping pz orbitals. b|Orbital interactions of donor (D) and acceptor (A) units, which results in a smaller bandgap inFigure 3 | Molecular structures of representative donoracceptor low-bandgap polymers. The representative building blocks for low-bandgap organic photovoltaic polymers include benzothiadiazole (BT), diketopyrrolopyrrole (DPP), thieno[3,4b]thiophene (TT) Table 1 | Single-junction cellsSolution-processable tandem solar cellsTable 2 | Tandem solar cellsFigure 4 | Various tandem polymer solar cells. a|Conventional tandem (double-junction) polymer solar cell structure. b|Inverted tandem (double-junction) polymer solar cell structure. c|Structure of an inverted triple-junction tandem polymer solar ceFigure 5 | Variety of hybrid tandem solar cells containing low-bandgap polymers. a|Typical structure of planar hybrid tandem (double-junction) device containing aSi:H/low-bandgap polymer. b|Device structure of hybrid tandem triple-junction solar cellTable 3 | Hybrid tandem and unconventional cellsSummary and perspectives

low-bandgap conjugated polymers enabling solution...

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