Yuhang LiupersName, Jingbo ZhaopersName, Zhengke LipersName, Cheng MupersName, Wei MapersName, Huawei HupersName, Kui JiangpersName, Haoran LinpersName, Harald AdepersName, He YanpersName (2014date). “Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells..” Nature Communicationstitle·j.
Object of study — thick-filmterm·key polymer:fullereneterm·key solar cells and the aggregation-controlled morphologyterm·key of their blends
Research question — how to obtain high-efficiency polymer solar cellsterm not tied to one fullereneterm·key or thin active layers
Contribution — an aggregationterm·key and morphologyterm·key control approach and polymer design rule yielding multiple >10% thick-filmterm·key cells
phenomenon · claim-type · cross-domain-transfer · mechanism · claim-relation · replication-status · limitation — grounded in the full text| phenomenon | temperature-dependent polymer aggregation controlling blend morphology — “This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes.” | |
| claim-type | mechanism — “This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes.” | |
| method — evidence | — “Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains.” | |
| object of study — evidence | — “Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains.” | |
| research question — evidence | — “For instance, current high-efficiency (>9.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor.” | |
| contribution — evidence | — “To summarize, we report that exquisite control of aggregation results in high-performance thick-film PSCs for three different donor polymers and 10 polymer:fullerene combinations, all of which yielded efficiencies higher than the previous state of the art (9.5%).” | |
| mechanism | warm casting then aggregation on cooling The polymer is cast from warm solution where it is disaggregated and aggregates as the film cools and dries. — “Consequently, devices are always cast from warm solutions (60–80 °C) of PffBT4T-2OD, which then aggregates during the cooling and film-forming process.” | mechanisms[0] |
| mechanism | side-chain branching sets steric hindrance First-position branching gives too much steric hindrance and poor aggregation; second-position branching gives an optimal tradeoff. — “These comparisons indicate that PffBT4T-1ON’s alkyl chains cause too much steric hindrance, which results in poor aggregation and crystallinity.” | mechanisms[1] |
| mechanism | pure domains limit recombination Pure polymer domains minimize bimolecular recombination, which impure phases suffer in thick films. — “In general, PSCs with significantly impure polymer phases exhibit detrimental bimolecular charge recombination when the polymer film is too thick, whereas pure phases can help to minimize recombination 23 .” | mechanisms[2] |
| sense | near-optimum morphology - a blend morphology good enough for high efficiency; the paper argues there is more than one — “The high polymer crystallinity and thus excellent hole transport ability, combined with sufficiently pure polymer domains, are the main reasons why the PSCs exhibit high FFs and efficiency even when the active layer is 300 nm thick.” | senses[0] |
| sense | thick-film PSC - a cell with a ~300 nm active layer, needed for industrial application — “Besides the sensitivity of the choice of fullerenes, another important limitation for PTB7 family polymers is that they cannot perform well when relatively thick active layers (~300 nm) are used in the PSC device.” | senses[1] |
| control | polymer films on Si/ZnO as well as Si substrates — held — “To rule out the effect of substrate properties on the crystallinity of polymer film samples, we also investigated polymer films on Si/ZnO substrates and found that the polymer films have similar scattering profiles ( Supplementary Fig. 8 ) on these two types of substrates (Si/ZnO and Si).” — rules out the substrate, rather than the polymer, setting film crystallinity — “To rule out the effect of substrate properties on the crystallinity of polymer film samples, we also investigated polymer films on Si/ZnO substrates and found that the polymer films have similar scattering profiles ( Supplementary Fig. 8 ) on these two types of substrates (Si/ZnO and Si).” | controls[0] |
| control | independent certification of one of the best cells by an accredited laboratory — held — “One of our best cells was sent to an accredited solar cell calibration laboratory (Newport Corporation) for certification, confirming an efficiency of 10.36±0.22%” — rules out in-house measurement error inflating efficiency — “One of our best cells was sent to an accredited solar cell calibration laboratory (Newport Corporation) for certification, confirming an efficiency of 10.36±0.22%” | controls[1] |
| relation | PSC —same-as→ polymer solar cell — ( ) — “Among these, polymer solar cell (PSC) 1 2 3 4 5 6 technology is an excellent example of low-cost production because PSCs can be produced using extremely high-throughput roll-to-roll printing methods similar to those used to print newspapers 7 .” — “polymer solar cell (PSC)” | concept_edges[0] |
| weight | PSC —same-as→ polymer solar cell — 0.95 — “Among these, polymer solar cell (PSC) 1 2 3 4 5 6 technology is an excellent example of low-cost production because PSCs can be produced using extremely high-throughput roll-to-roll printing methods similar to those used to print newspapers 7 .” | concept_edges[0] |
| relation | SCLC —same-as→ space charge limited current — ( ) — “The reason why PTB7 does not perform well in thick-film PSCs is partially owing to the relatively low hole transport ability (space charge limited current (SCLC) mobility ~6 × 10 −4 cm 2 V −1 s −1 ; ref. 17 ) related to the low crystallinity of the PTB7 polymer.” — “space charge limited current (SCLC)” | concept_edges[1] |
| weight | SCLC —same-as→ space charge limited current — 0.95 — “The reason why PTB7 does not perform well in thick-film PSCs is partially owing to the relatively low hole transport ability (space charge limited current (SCLC) mobility ~6 × 10 −4 cm 2 V −1 s −1 ; ref. 17 ) related to the low crystallinity of the PTB7 polymer.” | concept_edges[1] |
| relation | GIWAXS —same-as→ Grazing incident wide-angle X-ray diffraction — ( ) — “Grazing incident wide-angle X-ray diffraction (GIWAXS) 26 reveals the molecular packing and orientational texture of pure PffBT4T-2OD and PffBT4T-2OD:fullerene blend films.” — “Grazing incident wide-angle X-ray diffraction (GIWAXS)” | concept_edges[2] |
| weight | GIWAXS —same-as→ Grazing incident wide-angle X-ray diffraction — 0.95 — “Grazing incident wide-angle X-ray diffraction (GIWAXS) 26 reveals the molecular packing and orientational texture of pure PffBT4T-2OD and PffBT4T-2OD:fullerene blend films.” | concept_edges[2] |
| relation | R-SoXS —same-as→ resonant soft X-ray scattering — ( ) — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” — “resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c )” | concept_edges[3] |
| weight | R-SoXS —same-as→ resonant soft X-ray scattering — 0.95 — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” | concept_edges[3] |
| relation | AFM —same-as→ atomic force microscopy — ( ) — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” — “atomic force microscopy (AFM; Supplementary Fig. 3 )” | concept_edges[4] |
| weight | AFM —same-as→ atomic force microscopy — 0.95 — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” | concept_edges[4] |
| relation | 2OD alkyl chains —causes→ temperature-dependent aggregation behaviour — causes — “The common structural feature of the three donor polymers, the 2OD alkyl chains on quaterthiophene, causes a temperature-dependent aggregation behaviour that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process.” — “the 2OD alkyl chains on quaterthiophene, causes a temperature-dependent aggregation behaviour” | concept_edges[5] |
| weight | 2OD alkyl chains —causes→ temperature-dependent aggregation behaviour — 0.9 — “The common structural feature of the three donor polymers, the 2OD alkyl chains on quaterthiophene, causes a temperature-dependent aggregation behaviour that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process.” | concept_edges[5] |
| effect direction | spin rates —causes→ π – π ordering — reduces — “As shown in Fig. 2d and Supplementary Table 2 , the π – π ordering decreases markedly with increasing spin rates.” — “the π – π ordering decreases markedly with increasing spin rates” | concept_edges[6] |
| relation | spin rates —causes→ π – π ordering — decreases — “As shown in Fig. 2d and Supplementary Table 2 , the π – π ordering decreases markedly with increasing spin rates.” | concept_edges[6] |
| weight | spin rates —causes→ π – π ordering — 0.8 — “As shown in Fig. 2d and Supplementary Table 2 , the π – π ordering decreases markedly with increasing spin rates.” | concept_edges[6] |
| relation | Unnecessarily long alkyl chains —causes→ less pure polymer domains — cause — “Unnecessarily long alkyl chains (for example, 2DT) cause several detrimental effects including weaker laminar stacking, poorer absorption properties and less pure polymer domains.” | concept_edges[7] |
| weight | Unnecessarily long alkyl chains —causes→ less pure polymer domains — 0.8 — “Unnecessarily long alkyl chains (for example, 2DT) cause several detrimental effects including weaker laminar stacking, poorer absorption properties and less pure polymer domains.” | concept_edges[7] |
| relation | PTB7 family polymers —contrasted-with→ 4T-2OD based polymers — While — “While PTB7 family polymers do not exhibit a strong temperature-dependent aggregation property and are often processed from room temperature solutions, the 4T-2OD based polymers are processed from warm solutions to utilize their temperature-dependent aggregation property so that the morphology and extent of molecular ordering can be explicitly controlled during casting.” | concept_edges[8] |
| weight | PTB7 family polymers —contrasted-with→ 4T-2OD based polymers — 0.85 — “While PTB7 family polymers do not exhibit a strong temperature-dependent aggregation property and are often processed from room temperature solutions, the 4T-2OD based polymers are processed from warm solutions to utilize their temperature-dependent aggregation property so that the morphology and extent of molecular ordering can be explicitly controlled during casting.” | concept_edges[8] |
| outcome | control — held — “To rule out the effect of substrate properties on the crystallinity of polymer film samples, we also investigated polymer films on Si/ZnO substrates and found that the polymer films have similar scattering profiles ( Supplementary Fig. 8 ) on these two types of substrates (Si/ZnO and Si).” | controls[0] |
| outcome | control — held — “One of our best cells was sent to an accredited solar cell calibration laboratory (Newport Corporation) for certification, confirming an efficiency of 10.36±0.22%” | controls[1] |
| replication | original | |
| limitation | R-SoXS cannot give absolute composition without absolute flux normalization. — “Owing to a lack of absolute flux normalization, the absolute composition cannot be obtained by only R-SoXS.” | limitations[0] |
| limitation | The polymer dielectric constant is assumed to be ~3 in the mobility fits. — “The dielectric constant ε r is assumed to be ~3, which is a typical value for conjugated polymers.” | limitations[1] |
| limitation | PTB7 family polymers perform poorly with ~300 nm active layers. — “Besides the sensitivity of the choice of fullerenes, another important limitation for PTB7 family polymers is that they cannot perform well when relatively thick active layers (~300 nm) are used in the PSC device.” | limitations[2] |
finding · quantity · variable · definition · key-term — grounded in the full text| finding | Thick-film polymer solar cells reach up to 10.8% efficiency for multiple polymer:fullerene combinations. — “Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains.” | content_tags.findings[0] |
| finding | The blend morphology is set by the donor polymer's temperature-dependent aggregation and is insensitive to the fullerene. — “This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes.” | content_tags.findings[1] |
| finding | PffBT4T-2OD aggregation is insensitive to the presence of fullerenes. — “Importantly, studies for PffBT4T-2OD pure films and PffBT4T-2OD blend films with two different fullerenes yield similar trends ( Supplementary Table 2 and Supplementary Fig. 4 ), demonstrating that the aggregation of PffBT4T-2OD is insensitive to the presence of fullerenes.” | content_tags.findings[2] |
| finding | Fast spin rates and hot substrates give disordered films with lower hole mobility and 3.6% efficiency. — “The corresponding hole-only and PSC devices fabricated using high spin rates and high substrate temperature also exhibit markedly decreased hole mobilities (3.1 × 10 −3 cm 2 V −1 s −1 ; Supplementary Table 3 ) and PSC efficiencies (3.6%; Supplementary Table 4 ).” | content_tags.findings[3] |
| finding | Lower molecular weight batches aggregate more weakly and reach only 7.7% efficiency. — “batches of PffBT4T-2OD exhibit weaker aggregation and thus lower efficiency (7.7%) than the high molecular weight” | content_tags.findings[4] |
| finding | Solution aggregation seen by UV-Vis may screen materials for warm casting. — “Furthermore, the aggregation behaviour as observed by UV-Vis might serve as a useful screening tool to identify materials that yield good devices when cast from warm solutions.” | content_tags.findings[5] |
| quantity | up to 10.8%, FF up to 77% — — “Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains.” | content_tags.quantities[0] |
| quantity | ~40–50% — — “Most importantly, a tandem cell architecture 6 8 9 10 can be easily implemented with PSCs and has proven to improve PSC efficiency by ~40–50% (refs 6 , 8 ).” | content_tags.quantities[1] |
| quantity | ~8.5 nm vs ~2 nm — — “The (010) coherence length (that is, extent of ordering) of PffBT4T-2OD:PC 61 PM blend films was calculated using Scherrer analysis 27 to be ~8.5 nm, which corresponds to ~24 π -stacked copolymers.” | content_tags.quantities[2] |
| quantity | ~30–40 nm — — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” | content_tags.quantities[3] |
| quantity | ~30% — — “Combined, these account for a ~30% increase in short circuit current ( J SC ).” | content_tags.quantities[4] |
| quantity | 3.1 × 10−3 cm2 V−1 s−1; 3.6% — — “The corresponding hole-only and PSC devices fabricated using high spin rates and high substrate temperature also exhibit markedly decreased hole mobilities (3.1 × 10 −3 cm 2 V −1 s −1 ; Supplementary Table 3 ) and PSC efficiencies (3.6%; Supplementary Table 4 ).” | content_tags.quantities[5] |
| quantity | 7.7% — — “batches of PffBT4T-2OD exhibit weaker aggregation and thus lower efficiency (7.7%) than the high molecular weight” | content_tags.quantities[6] |
| quantity | 9.5% — — “To summarize, we report that exquisite control of aggregation results in high-performance thick-film PSCs for three different donor polymers and 10 polymer:fullerene combinations, all of which yielded efficiencies higher than the previous state of the art (9.5%).” | content_tags.quantities[7] |
| quantity | 10.36±0.22% — — “One of our best cells was sent to an accredited solar cell calibration laboratory (Newport Corporation) for certification, confirming an efficiency of 10.36±0.22%” | content_tags.quantities[8] |
| variable | spin rates — “As shown in Fig. 2d and Supplementary Table 2 , the π – π ordering decreases markedly with increasing spin rates.” | content_tags.variables[0] |
| variable | coherence length — “The (010) coherence length (that is, extent of ordering) of PffBT4T-2OD:PC 61 PM blend films was calculated using Scherrer analysis 27 to be ~8.5 nm, which corresponds to ~24 π -stacked copolymers.” | content_tags.variables[1] |
| definition | coherence length — “The (010) coherence length (that is, extent of ordering) of PffBT4T-2OD:PC 61 PM blend films was calculated using Scherrer analysis 27 to be ~8.5 nm, which corresponds to ~24 π -stacked copolymers.” | content_tags.definitions[0] |
| key-terms | polymer solar cell, fullerene, aggregation, morphology, thick-film, alkyl chains, hole mobility, crystallinity, GIWAXS, R-SoXS, PffBT4T-2OD |
| concept | PSC — polymer solar cell — “Among these, polymer solar cell (PSC) 1 2 3 4 5 6 technology is an excellent example of low-cost production because PSCs can be produced using extremely high-throughput roll-to-roll printing methods similar to those used to print newspapers 7 .” | concepts[0] |
| concept | polymer solar cell — not defined in this paper's own words; a solar cell whose active layer is a donor polymer:fullerene blend — “Among these, polymer solar cell (PSC) 1 2 3 4 5 6 technology is an excellent example of low-cost production because PSCs can be produced using extremely high-throughput roll-to-roll printing methods similar to those used to print newspapers 7 .” | concepts[1] |
| concept | SCLC — space charge limited current — “The reason why PTB7 does not perform well in thick-film PSCs is partially owing to the relatively low hole transport ability (space charge limited current (SCLC) mobility ~6 × 10 −4 cm 2 V −1 s −1 ; ref. 17 ) related to the low crystallinity of the PTB7 polymer.” | concepts[2] |
| concept | space charge limited current — not defined in this paper's own words; the current regime used to extract hole mobility from hole-only diodes — “The reason why PTB7 does not perform well in thick-film PSCs is partially owing to the relatively low hole transport ability (space charge limited current (SCLC) mobility ~6 × 10 −4 cm 2 V −1 s −1 ; ref. 17 ) related to the low crystallinity of the PTB7 polymer.” | concepts[3] |
| concept | GIWAXS — Grazing incident wide-angle X-ray diffraction — “Grazing incident wide-angle X-ray diffraction (GIWAXS) 26 reveals the molecular packing and orientational texture of pure PffBT4T-2OD and PffBT4T-2OD:fullerene blend films.” | concepts[4] |
| concept | Grazing incident wide-angle X-ray diffraction — not defined in this paper's own words; X-ray scattering at grazing incidence that reveals molecular packing in films — “Grazing incident wide-angle X-ray diffraction (GIWAXS) 26 reveals the molecular packing and orientational texture of pure PffBT4T-2OD and PffBT4T-2OD:fullerene blend films.” | concepts[5] |
| concept | R-SoXS — resonant soft X-ray scattering — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” | concepts[6] |
| concept | resonant soft X-ray scattering — not defined in this paper's own words; X-ray scattering that measures domain size and average composition variation — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” | concepts[7] |
| concept | AFM — atomic force microscopy — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” | concepts[8] |
| concept | atomic force microscopy — not defined in this paper's own words; a surface imaging method — “In addition, resonant soft X-ray scattering 14 15 25 28 29 30 (R-SoXS; Fig. 2c ) and atomic force microscopy (AFM; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2OD:fullerene films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 .” | concepts[9] |
| concept | coherence length — extent of ordering — “The (010) coherence length (that is, extent of ordering) of PffBT4T-2OD:PC 61 PM blend films was calculated using Scherrer analysis 27 to be ~8.5 nm, which corresponds to ~24 π -stacked copolymers.” | concepts[10] |
| concept | 2OD alkyl chains — not defined in this paper's own words; second-position branched 2-octyldodecyl side chains on quaterthiophene — “The common structural feature of the three donor polymers, the 2OD alkyl chains on quaterthiophene, causes a temperature-dependent aggregation behaviour that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process.” | concepts[11] |
| concept | temperature-dependent aggregation behaviour — not defined in this paper's own words; the polymer disaggregating in warm solution and aggregating as it cools — “The common structural feature of the three donor polymers, the 2OD alkyl chains on quaterthiophene, causes a temperature-dependent aggregation behaviour that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process.” | concepts[12] |
| concept | spin rates — not defined in this paper's own words; the rotation speed used in spin casting the film — “As shown in Fig. 2d and Supplementary Table 2 , the π – π ordering decreases markedly with increasing spin rates.” | concepts[13] |
| concept | π – π ordering — not defined in this paper's own words; stacking order of the conjugated backbones — “As shown in Fig. 2d and Supplementary Table 2 , the π – π ordering decreases markedly with increasing spin rates.” | concepts[14] |
| concept | Unnecessarily long alkyl chains — not defined in this paper's own words; side chains longer than needed, such as 2DT — “Unnecessarily long alkyl chains (for example, 2DT) cause several detrimental effects including weaker laminar stacking, poorer absorption properties and less pure polymer domains.” | concepts[15] |
| concept | less pure polymer domains — not defined in this paper's own words; polymer regions containing more fullerene — “Unnecessarily long alkyl chains (for example, 2DT) cause several detrimental effects including weaker laminar stacking, poorer absorption properties and less pure polymer domains.” | concepts[16] |
| concept | PTB7 family polymers — not defined in this paper's own words; the previous record donor polymers, based on fluorinated thieno[3,4-b]thiophene — “While PTB7 family polymers do not exhibit a strong temperature-dependent aggregation property and are often processed from room temperature solutions, the 4T-2OD based polymers are processed from warm solutions to utilize their temperature-dependent aggregation property so that the morphology and extent of molecular ordering can be explicitly controlled during casting.” | concepts[17] |
| concept | 4T-2OD based polymers — not defined in this paper's own words; the paper's three donor polymers carrying 2OD chains on quaterthiophene — “While PTB7 family polymers do not exhibit a strong temperature-dependent aggregation property and are often processed from room temperature solutions, the 4T-2OD based polymers are processed from warm solutions to utilize their temperature-dependent aggregation property so that the morphology and extent of molecular ordering can be explicitly controlled during casting.” | concepts[18] |
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Aggregationterm·key and morphologyterm·key control enables multiple cases of high-efficiency polymer solar cellsterm.
Abstract Although the field of polymer solar cellterm has seen much progress in device performance in the past few years, several limitations are holding back its further development. For instance, current high-efficiency (>9.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullereneterm·key acceptor. Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-filmterm·key polymer solar cellsterm for multiple polymer:fullereneterm·key combinations via the formation of a near-ideal polymer:fullereneterm·key morphologyterm·key that contains highly crystalline yet reasonably small polymer domains. This morphologyterm·key is controlled by the temperature-dependent aggregation behaviourterm·dom of the donor polymers and is insensitive to the choice of fullerenesterm·key. The uncovered aggregationterm·key and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullereneterm·key materials, potentially leading to significantly improved performance and increased design flexibility.
Conventional inorganic solar cells can achieve high efficiencies but are produced through complicated, costly processes. The desirability of lower costs is driving the development of several third-generation solar technologies. Among these, polymer solar cellterm (PSCterm) 1 2 3 4 5 6 technology is an excellent example of low-cost production because PSCsterm can be produced using extremely high-throughput roll-to-roll printing methods similar to those used to print newspapers 7 . PSCsterm also offer several other advantages: vacuum processing and high-temperature sintering are not needed, and no toxic materials are used in the end product. Most importantly, a tandem cell architecture 6 8 9 10 can be easily implemented with PSCsterm and has proven to improve PSCterm efficiency by ~40–50% (refs 6 , 8 ). As PSCsterm are two-component, donor–acceptor material systems, it is generally important to control the morphologyterm·key of the donor:acceptor blends and to find an optimal materials combination with excellent optical and electronic properties. In the last few years, record-efficiency PSCsterm were achieved with only three donor polymers (which all belong to a specific polymer family based on fluorinated thieno[3,4- b ]thiophene, for example, PTB7) that are, furthermore, constrained to be used with a specific fullereneterm·key, PC 71 BM, to achieve their best performance 11 12 13 . In general, the morphologies 14 15 16 and thus performance of state-of-the-art donor polymers (for example, PTB7 (refs 11 , 17 ) and PBDT-DTNT 18 ) are sensitive to the choice of fullereneterm·key and replacing PC 71 BM with another C 60 -based or non-PCBM fullereneterm·key decreases PSCterm efficiency to 6-7% (refs 11 , 18 , 19 , 20 ) The dominant role of PC 71 BM places serious constraints on PSCterm material development, because the properties of the polymers must be precisely matched with fixed targets set by PC 71 BM. As tandem PSCsterm require two sets of perfectly matching polymer/fullereneterm·key materials, the constraint on their development is compounded. It has thus been pointed out that it is crucial to have the flexibility of being able to use different fullerenesterm·key and more generally to remove material constraints to achieve tandem PSCsterm with 15–20% efficiency envisioned by Brebac and colleagues 9 10 21 . The development of polymer:fullereneterm·key material systems that are morphologically insensitive to fullereneterm·key choice will remove these material constraints, and greatly accelerate material development for single-junction and tandem PSCsterm 10 22 .
Another important fundamental issue for the PSCterm field is how to control the morphologyterm·key of polymer:fullereneterm·key blends to achieve the best PSCterm performance. There is likely more than one near-optimum PSCterm morphologyterm·key. The famous PTB7 family donor polymers enabled one type of the near-optimum PSCterm morphologyterm·key, as high external quantum efficiencies (EQEs~80%) have been reported for PTB7-based cells 11 . However, the PTB7-based PSCterm materials and devices have certain limitations. Besides the sensitivity of the choice of fullerenesterm·key, another important limitation for PTB7 family polymersterm is that they cannot perform well when relatively thick active layers (~300 nm) are used in the PSCterm device. Thick-filmterm·key PSCsterm are important for the industrial application of PSCsterm, and thick films should also further increase the absorption strength of the solar cell and thus cell efficiency. The reason why PTB7 does not perform well in thick-filmterm·key PSCsterm is partially owing to the relatively low hole transport ability (space charge limited currentterm·mth (SCLCterm·mth) mobility ~6 × 10 −4 cm 2 V −1 s −1 ; ref. 17 ) related to the low crystallinityterm·key of the PTB7 polymer. There has been also evidence that high purity of the polymer domain may be an important factor to achieve efficient thick-filmterm·key PSCsterm 14 23 24 . The PTB7-based materials systems are characterized by relatively impure polymer domains 25 , which could be a reason why these polymers do not perform well in thick-filmterm·key PSCsterm. Clearly, there is a need for new materials systems that explore a different ‘near-optimum’ PSCterm morphologyterm·key in order to achieve thick-filmterm·key PSCsterm that have comparable or higher efficiencies than state-of-the-art PTB7 materials systems.
In the following, we report the achievement of high-performance (efficiencies up to 10.8% and fill factors (FFs) up to 77%) thick-filmterm·key PSCsterm based on three different donor polymers and 10 polymer:fullereneterm·key combinations, all of which exhibit efficiencies higher than the previous state of the art. In contrast to state-of-the-art PTB7-based materials systems, the high PSCterm performances in this report are achieved via the formation of an ‘optimum PSCterm morphologyterm·key’ that contains highly crystalline, sufficiently pure, yet reasonably small polymer domains. The high polymer crystallinityterm·key and thus excellent hole transport ability, combined with sufficiently pure polymer domains, are the main reasons why the PSCsterm exhibit high FFs and efficiency even when the active layer is 300 nm thick. Importantly, this ipso facto near-perfect morphologyterm·key is controlled by the temperature-dependent aggregation behaviourterm·dom of the donor polymers during casting and is insensitive to the choice of fullerenesterm·key. Taking advantage of the robust polymer:fullereneterm·key morphologyterm·key enabled by the three donor polymers, many non-traditional fullerenesterm·key are also used. Traditional PCBMs, the most dominant fullerenesterm·key in PSCsterm, are out-performed by several other non-traditional fullerenesterm·key, clearly indicating the benefits of exploring different fullerenesterm·key and the robust morphologyterm·key formation. Comparative studies on several structurally similar polymers reveal that the 2-octyldodecyl (2OD) alkyl chainsterm·key sitting on quaterthiophene is the key structural feature that causes the polymers’ highly temperature-dependent aggregation behaviourterm·dom that allows for the processing of the polymer solutions at elevated temperature, and, more importantly, controlled aggregationterm·key and strong crystallization of the polymer during the film cooling and drying process. The branching position and size of the branched alkyl chainsterm·key are critically important in enabling an optimal aggregationterm·key behaviour. With our approach, PSCterm production is no longer constrained by the use of a single fullereneterm·key or by a very thin active layer. Our aggregationterm·key and morphologyterm·key control approach and polymer design rules can be applied to multiple polymer:fullereneterm·key materials systems and will allow the PSCterm community to explore many more polymers and fullereneterm·key materials and to optimize their combinations (energy offsets, bandgap and so on) under a well-controlled morphological landscape, which would greatly accelerate the materials and process development towards improved PSCsterm.
Results PSCterm device performance Among the three donor polymers, we developed that achieved power conversion efficiency>10%, we first focus on poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)- alt -(3,3′′′-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)], PffBT4T-2ODterm·key ( Fig. 1a ). PffBT4T-2ODterm·key is a material that enables six cases of high-efficiency (9.6–10.8%), high FF (73–77%) and thick-filmterm·key (250–300 nm) PSCsterm ( Table 1 ) when combined with traditional PCBM and many non-traditional fullerenesterm·key ( Fig. 1b ). A typical J – V plot of a PffBT4T-2ODterm·key:fullereneterm·key PSCterm is shown in Fig. 1c , with EQE spectra shown in the inset. The benefits of thick-filmterm·key PSCsterm are obvious. The thick cell exhibits 10–20% higher EQE values, and the effective absorption bandwidth of a thick PSCterm can be increased as the result of a ~20 nm red-shift of the ‘leading, low energy edge’ of a PSCterm’s EQE spectrum. Combined, these account for a ~30% increase in short circuit current ( J SC ). Taking advantage of PffBT4T-2ODterm·key’s excellent aggregationterm·key properties (as delineated further below), we synthesized more than a dozen known or new fullereneterm·key derivatives ( Fig. 1b ) to find the best acceptor match for PffBT4T-2ODterm·key. All of these fullerenesterm·key form similar morphologies with PffBT4T-2ODterm·key and can produce PSCsterm with high efficiencies in the range of 8.6–10.8% ( Table 1 and Supplementary Table 1 ). The best efficiency (10.4%) in the C 60 family was achieved by PC 61 PM ( Fig. 1b ), and the most commonly used C 60 -based fullereneterm·key, PC 61 BM, is not the best match for PffBT4T-2ODterm·key.
Polymer crystallinityterm·key and hole mobilityterm·key Grazing incident wide-angle X-ray diffractionterm·mth (GIWAXSterm·mth) 26 reveals the molecular packing and orientational texture of pure PffBT4T-2ODterm·key and PffBT4T-2ODterm·key:fullereneterm·key blend films. Both exhibit a high degree of molecular order, as evidenced by strong lamellar (100), (200) and even (300) reflection peaks and, more importantly, a large (010) coherence lengthterm·dom (GIWAXSterm·mth 2D patterns shown in Fig. 2a,b and Supplementary Fig. 1 ). The (010) coherence lengthterm·dom (that is, extent of ordering) of PffBT4T-2ODterm·key:PC 61 PM blend films was calculated using Scherrer analysis 27 to be ~8.5 nm, which corresponds to ~24 π -stacked copolymers. In contrast, the (010) coherence lengthterm·dom of PTB7:PC 61 BM, for example, is only ~2 nm (ref. 16 ). Owing to the high crystallinityterm·key and preferential face-on orientation of polymer domains, relatively high SCLCterm·mth hole mobilityterm·key of 1.5–3.0 × 10 −2 cm 2 V −1 s −1 were obtained for various PffBT4T-2ODterm·key:fullereneterm·key blend films in a hole-only diode device configuration ( Supplementary Fig. 2 ). The importance of mobility for good FF was recently illustrated 23 .
Polymer:fullereneterm·key domain size and average domain purity In addition, resonant soft X-ray scatteringterm·mth 14 15 25 28 29 30 (R-SoXSterm·mth; Fig. 2c ) and atomic force microscopyterm·mth (AFMterm·mth; Supplementary Fig. 3 ) analysis reveals that the various PffBT4T-2ODterm·key:fullereneterm·key films all exhibit multi-length scale morphologies with reasonably small median domain sizes of ~30–40 nm, which is similar to previous cases of high-performance polymers 16 25 . R-SoXSterm·mth can also reveal the average composition variations, which are indicative of the average purity of the polymer and fullereneterm·key regions as well as a possible third phase of polymer-rich domains 26 31 . An annealing sequence on PffBT4T-2ODterm·key:fullereneterm·key blends revealed that the non-annealed devices presented here exhibited almost 90% average purity compared with the asymptotic limit ( Fig. 3a,b ), which corresponds to an unusually low residual concentration of 3.2% fullereneterm·key averaged over all PffBT4T-2ODterm·key domains in the film as measured by X-ray microscopy ( Fig. 3c ) 25 32 33 . In general, PSCsterm with significantly impure polymer phases exhibit detrimental bimolecular charge recombination when the polymer film is too thick, whereas pure phases can help to minimize recombination 23 . These morphological data show that PffBT4T-2ODterm·key can form a polymer:fullereneterm·key morphologyterm·key containing highly crystalline and sufficiently pure yet reasonably small polymer domains. Note that PTB7-type polymers have been the best donor polymer in PSCsterm for the past few years. By its very nature of high performance in thin films, PTB7 can form a ‘near-optimum’ PSCterm morphologyterm·key characterized by relatively low molecular ordering, relatively low hole mobilities and impure polymer domains 25 . PffBT4T-2ODterm·key exhibits high molecular ordering (‘crystallinityterm·key’), high hole mobilities and purer polymer domains, which appears to be a different ipso facto ‘near-optimum’ PSCterm morphologyterm·key. Although PTB7 enabled great thin-film PSCterm performance, PffBT4T-2ODterm·key offers high performance even in thick-filmterm·key PSCsterm owing to the high mobility of the highly ordered and sufficiently pure polymer domains it forms.
Morphologyterm·key control via temperature-dependent aggregationterm·key We attribute PffBT4T-2ODterm·key’s excellent performance and robust morphologyterm·key to its significant temperature-dependent aggregation behaviourterm·dom that can be exploited during device fabrication. The UV-Vis absorption spectra exhibit a marked red-shift when a low concentration PffBT4T-2ODterm·key solution in 1,2-dichlorobenzene (DCB) is lowered from 85 to 25 °C ( Fig. 1d ). At elevated temperature, PffBT4T-2ODterm·key is well dissolved and disaggregated. At progressively lower temperatures, a strong 0-0 transition peak at ~700 nm emerges with significant strength at 25 °C, indicating strong aggregationterm·key of the polymer chains in solution at that temperature. Note that the absorption spectrum of the 25 °C solution of PffBT4T-2ODterm·key is almost identical to that of the optimized PffBT4T-2ODterm·key solid film ( Fig. 1d ), which is observed to be highly crystalline by GIWAXSterm·mth. Consequently, devices are always cast from warm solutions (60–80 °C) of PffBT4T-2ODterm·key, which then aggregates during the cooling and film-forming process.
To understand details of PffBT4T-2ODterm·key’s aggregationterm·key behaviour during the film-forming process, the critical π – π molecular ordering ((010) coherence lengthterm·dom and intensity of the (010) peak) is determined with X-ray diffraction (XRD) for a series of PffBT4T-2ODterm·key films spun at different rates. As shown in Fig. 2d and Supplementary Table 2 , the π – π orderingterm·dom decreases markedly with increasing spin ratesterm·mth. As PffBT4T-2ODterm·key exhibits a strong yet progressively evolving aggregationterm·key property, the extent of PffBT4T-2ODterm·key’s aggregationterm·key depends upon temperature and concentration changes, the film drying time and the kinetics of aggregationterm·key. During a slow spin process (for example, 700 r.p.m.), it takes a relatively long time for the solution and film to dry, during which the temperature of the substrate and the wet film also decreases significantly. When using an ultra-fast rate (for example, 5,000 r.p.m.), however, the solvent evaporates more quickly, which results in kinetically quenched, poorly ordered films, whereas slow spin ratesterm·mth provides PffBT4T-2ODterm·key sufficient time to aggregate and to form crystalline polymer domains with large coherence lengthsterm·dom. Importantly, studies for PffBT4T-2ODterm·key pure films and PffBT4T-2ODterm·key blend films with two different fullerenesterm·key yield similar trends ( Supplementary Table 2 and Supplementary Fig. 4 ), demonstrating that the aggregationterm·key of PffBT4T-2ODterm·key is insensitive to the presence of fullerenesterm·key. Not surprisingly, high substrate temperatures were found to have a similar effect to fast spin ratesterm·mth. PffBT4T-2ODterm·key:fullereneterm·key films prepared with fast spin ratesterm·mth/high substrate temperatures show a decrease in the 0-0 transition peaks and a pronounced shift in the 0-0 transition energy in their UV-Vis absorption spectra, indicative of significant disorder ( Fig. 2e ). The corresponding hole-only and PSCterm devices fabricated using high spin ratesterm·mth and high substrate temperature also exhibit markedly decreased hole mobilities (3.1 × 10 −3 cm 2 V −1 s −1 ; Supplementary Table 3 ) and PSCterm efficiencies (3.6%; Supplementary Table 4 ). These morphological, spectroscopic and electric data demonstrate that PffBT4T-2ODterm·key’s morphologyterm·key is mainly controlled by the progress of its aggregationterm·key during the film-casting process until the film is dry, which locks-in the length scale of the morphologyterm·key. PffBT4T-2ODterm·key’s strong yet well-controllable aggregationterm·key property allows for convenient optimization of processing conditions that led to a near-ideal polymer:fullereneterm·key morphologyterm·key that is insensitive to the choices of fullereneterm·key. This approach of controlling the extent of polymer aggregationterm·key during a warm solution casting process is different from the common processing protocol of PTB7 family polymersterm that are typically processed at room temperature.
Discussion The key structural feature of PffBT4T-2ODterm·key that enables its pronounced, yet gradual temperature-dependent aggregationterm·key ( Fig. 1d ) is the second-position branched alkyl chainsterm·key (2OD) on a quaterthiophene (4T-2OD). To elucidate this aspect, we contrast PffBT4T-2ODterm·key with two structurally very similar polymers. These two polymers have the same backbone but their alkyl chainsterm·key are branched at the first or third side-chain carbon atom (for ease of comparison, these two polymers are named as PffBT4T-1ON and PffBT4T-3OT; Fig. 4a ). In contrast to PffBT4T-2ODterm·key, PffBT4T-1ON is disaggregated at 85 °C, and more importantly, also disaggregated at 25 °C ( Fig. 4b ). As a result, PffBT4T-1ON cannot aggregate easily during the film-forming process, leading to films with poor crystallinityterm·key (GIWAXSterm·mth pattern shown in Fig. 4c ) and thus PSCterm devices of only ~0.6% efficiency. PffBT4T-3OT exhibits the other extreme, showing excessive aggregationterm·key at both 25 and 85 °C. During our attempt to process PffBT4T-3OT, the PffBT4T-3OT solution quickly becomes a gel ( Fig. 4d ) even before the start of spin casting. These comparisons indicate that PffBT4T-1ON’s alkyl chainsterm·key cause too much steric hindrance, which results in poor aggregationterm·key and crystallinityterm·key. PffBT4T-3OT’s alkyl chainsterm·key provide too little steric hindrance that makes aggregationterm·key of PffBT4T-3OT too strong even at 85 °C and makes it difficult to process. PffBT4T-2ODterm·key’s second-position branched alkyl chainsterm·key offer an optimal tradeoff that allows for controllable aggregationterm·key of PffBT4T-2ODterm·key during the film-forming process. More discussions on the impact of alkyl chainterm·key branching positions are provided in Supplementary Note 1 .
Several structurally similar donor polymers containing quaterthiophene substituted with second-position branched alkyl chainsterm·key were reported in the literature 34 35 36 37 , including a recent report in which a polymer with longer alkyl chainsterm·key, FBT-Th 4 (1,4) (named as PffBT4T-2DT for simplicity in this paper, structure shown in Fig. 4a ), achieved 7.64% efficiency 34 . The difference of PffBT4T-2ODterm·key and PffBT4T-2DT devices can be understood from the following results. R-SoXSterm·mth ( Fig. 4e ) and prior AFMterm·mth studies 34 show that the average domain size of PffBT4T-2DT:fullereneterm·key films is too large (~100 nm). The R-SoXSterm·mth data furthermore show that the average purity of the polymer/polymer-rich domains in an PffBT4T-2DT:fullereneterm·key film is ~87% of that in PffBT4T-2ODterm·key:fullereneterm·key film. Lower purity of average polymer/polymer-rich domains can result in significant recombination for thick-filmterm·key PSCterm devices and thus lower performance 14 23 24 . Regarding molecular ordering, the two-dimensional GIWAXSterm·mth mapping of PffBT4T-2DT:PC 71 BM films 34 shows that PffBT4T-2DT:fullereneterm·key films exhibit weak laminar packing peaks, which are significantly weaker than those of PffBT4T-2ODterm·key:fullereneterm·key films. The smaller degree of laminar packing of PffBT4T-2DT is consistent with the lower average purity of PffBT4T-2DT polymer domains compared with that of PffBT4T-2ODterm·key’s, as more fullereneterm·key is expelled by the crystalline polymer domains of PffBT4T-2ODterm·key. Lastly, the absorption coefficient of PffBT4T-2DT is lower than that of PffBT4T-2ODterm·key owing to longer alkyl chainsterm·key that does not contribute to light absorption ( Supplementary Fig. 5 ). These studies show that the branching position and the size of the alkyl chainsterm·key are critically important in obtaining the optimal aggregationterm·key properties of PffBT4T-2ODterm·key. Insufficient aggregationterm·key (for example, PffBT4T-1ON) and unnecessarily long alkyl chainsterm·dom (for example, PffBT4T-2DT) resulted in low crystallinityterm·key and/or impure polymer domains. Excessive aggregationterm·key (for example, PffBT4T-3OT) makes the processing and aggregationterm·key difficult to control. Similar to recent observations 32 36 38 , the molecular weight of PffBT4T-2ODterm·key has a significant impact on its aggregationterm·key property and performance. Lower molecular weight ( M n =16.6 kDa, M w =29.5 kDa) batches of PffBT4T-2ODterm·key exhibit weaker aggregationterm·key and thus lower efficiency (7.7%) than the high molecular weight ( M n =47.5 kDa, M w =93.7 kDa) PffBT4T-2ODterm·key batches ( Supplementary Table 5 and Supplementary Fig. 6 ). The impacts of polymer molecular weight on PSCterm performances are discussed in details in the Supplementary Note 2 .
Following the rationale described above, we synthesized two other polymers (poly[(2,1,3-benzothiadiazol-4,7-diyl)- alt -(4′,3′′-difluoro-3,3′′′-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)] (PBTff4T-2OD) and poly[(naphtho[1,2- c :5,6- c ′]bis[1,2,5]thiadiazol-5,10-diyl)- alt -(3,3′′′-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)] (PNT4T-2OD); Fig. 1a ) with significantly different polymer backbones but with similar arrangements of 2OD alkyl chainsterm·dom. Both PBTff4T-2OD and PNT4T-2OD exhibit significant temperature-dependent aggregation behaviourterm·dom that leads to processing and morphologyterm·key control and thus efficiency (including >10% for thick-filmterm·key PSCsterm; Table 1 , Supplementary Table 1 and Supplementary Fig. 7 ) comparable to those achieved by PffBT4T-2OD-based PSCsterm. R-SoXSterm·mth and AFMterm·mth studies confirmed that the polymer domain size of these two new polymers are similar to that of PffBT4T-2ODterm·key (30–40 nm). XRD characterization of PBTff4T-2OD:fullereneterm·key and PNT4T-2OD:fullereneterm·key films also showed strong (010) π – π stacking peaks that are similar to those observed for PffBT4T-2ODterm·key. Note that PNT4T-2OD also significantly outperforms its analogue polymer with 2-decyltetradecyl (2DT) alkyl chainsterm·key 35 , providing another example that supports the critical importance of the size of the alkyl chainsterm·key. The synthesis, characterization and device performance of PBTff4T-2OD and PNT4T-2OD are described in detail in the Supplementary Information ( Supplementary Fig. 7 , Table 1 and Supplementary Table 1 ).
Although second-position branched alkyl chainsterm·key are a well-known structural motif and have been previously used on quaterthiophene-based polymers, previous work did not utilize a polymer with the most suitable alkyl chainsterm·key nor were warm-casting methods used that optimally harnessed aggregationterm·key. They thus failed to reveal the connections between chemical structure, polymer aggregationterm·key during warm processing, morphologyterm·key formation, polymer crystallinityterm·key and consequently PSCterm performance. Our study uncovered a new approach of aggregationterm·key and morphologyterm·key control enabled by a structural feature (2OD alkyl chainterm·dom) that is seemingly simple and commonly known, yet has surprisingly profound impact on PSCterm performances. The wide ranging applicability of our morphologyterm·key control approach is supported by the three polymers and over 10 polymer:fullereneterm·key combinations that all yielded similar blend morphologyterm·key and high-efficiency thick-filmterm·key PSCsterm. Furthermore, the aggregationterm·key behaviour as observed by UV-Vis might serve as a useful screening tool to identify materials that yield good devices when cast from warm solutions.
Note that the chemical structures of the three donor polymers presented in the paper are distinctively different from previous state-of-the-art PTB7 family of polymers 12 13 17 . The PTB7 family polymersterm consist of an electron deficient fluorinated thieno[3,4- b ]thiophene unit and a benzodithiophene unit with alkoxy, alkylthienyl or alkylthiothienyl substitution groups. The three polymers in this paper consist of an electron deficient unit (either difluorobenzothiadiazole or benzothiadiazole or naphthobisthiadiazole) combined with a quaterthiophene unit with two 2OD alkyl chainsterm·dom sitting on the first and fourth thiophenes. The difference in the chemical structures caused different aggregationterm·key properties, based on which different processing protocols are used. While PTB7 family polymersterm do not exhibit a strong temperature-dependent aggregationterm·key property and are often processed from room temperature solutions, the 4T-2OD based polymersterm are processed from warm solutions to utilize their temperature-dependent aggregationterm·key property so that the morphologyterm·key and extent of molecular ordering can be explicitly controlled during casting.
To summarize, we report that exquisite control of aggregationterm·key results in high-performance thick-filmterm·key PSCsterm for three different donor polymers and 10 polymer:fullereneterm·key combinations, all of which yielded efficiencies higher than the previous state of the art (9.5%). The common structural feature of the three donor polymers, the 2OD alkyl chainsterm·dom on quaterthiophene, causes a temperature-dependent aggregation behaviourterm·dom that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregationterm·key and strong crystallization of the polymer during the film cooling and drying process. This results in a near-ideal polymer:fullereneterm·key morphologyterm·key (containing highly crystalline, preferentially orientated, yet small polymer domains) that is controlled by polymer aggregationterm·key during casting and thus insensitive to the choice of fullerenesterm·key. The branching position and size of the branched alkyl chainsterm·key are critically important in enabling a well-controllable aggregationterm·key behaviour. Unnecessarily long alkyl chainsterm·dom (for example, 2DT) cause several detrimental effects including weaker laminar stacking, poorer absorption properties and less pure polymer domainsterm·dom. Our structural design rationales and aggregationterm·key and morphologyterm·key control approach offer a new route to achieve high-performance thick-filmterm·key PSCsterm that cannot be obtained from previous state-of-the-art material systems. Given that the field and record performance in the last few years has been mostly dominated by a single system (PTB7 family with PC 71 BM), the 10 material systems and three polymers based on a single and simple design feature presented here point to a plethora of possible materials combinations that should further improve the performance. Our approach will allow the PSCterm community to explore many more polymers and fullereneterm·key materials and to optimize their combinations (energy offsets, bandgap and so on) under a well-controlled morphological landscape that would greatly accelerate the materials and process development towards improved PSCsterm.
Methods X-ray diffraction XRD data were obtained from a PANanalytical XRD instrument (model name: Empyrean) using the parallel beam mode that is recommended by the instrument manufacturer to characterize thin-film samples. All XRD samples were spin cast on Si substrates to avoid strong scattering background of glass substrates. To rule out the effect of substrate properties on the crystallinityterm·key of polymer film samples, we also investigated polymer films on Si/ZnO substrates and found that the polymer films have similar scattering profiles ( Supplementary Fig. 8 ) on these two types of substrates (Si/ZnO and Si). The polymer crystallinityterm·key is thus rather insensitive to the surface properties of the substrates. More details of XRD characterizations are provided in Supplementary Note 3 .
Cyclic voltammetry Cyclic voltammetry was performed in an electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile, both working and counter electrodes were platinum electrode. Ag/AgCl electrode was used as the reference electrode; the Fc/Fc + redox couple was used as an external standard ( Supplementary Fig. 9 and Supplementary Table 6 ).
UV-Vis absorption UV-Vis absorption spectra were acquired on a Gary 50 UV-Vis spectrometer. All film samples were spin cast on ITO/ZnO substrates.
Hole-only device The hole mobilityterm·key were measured using the SCLCterm·mth method by using a device architecture of ITO/V 2 O 5 /PffBT4T-2ODterm·key (300 nm)/V 2 O 5 /Al by taking current–voltage curves and fitting the results to a space charge limited form, where the SCLCterm·mth is described by: Where ε 0 is the permittivity of free space, ε r is the dielectric constant of the polymer, μ is the hole mobilityterm·key, V is the voltage drop across the device and L is the thickness of the polymer. The dielectric constant ε r is assumed to be ~3, which is a typical value for conjugated polymers.
GIWAXSterm·mth characterization GIWAXSterm·mth measurements were performed at beamline 7.3.3 at the Advanced Light Source (ALS) 39 . Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10 keV X-ray beam was incident at a grazing angle of 0.11°–0.15°, which maximized the scattering intensity from the samples. The scattered X-rays were detected using a Dectris Pilatus 1 M photon counting detector.
Resonant soft X-ray scatteringterm·mth R-SoXSterm·mth transmission measurements were performed at beamline 11.0.1.2 at the ALS 30 . Samples for R-SoXSterm·mth measurements were prepared on a PSS modified Si substrate under the same conditions as those used for device fabrication, and then transferred by floating in water to a 1.5 × 1.5 mm, 100-nm thick Si 3 N 4 membrane supported by a 5 × 5 mm, 200 μm thick Si frame (Norcada Inc.). Two dimensional scattering patterns were collected on an in-vacuum CCD camera (Princeton Instrument PI-MTE). The beam size at the sample is ~100 μm by 200 μm. The composition variation (or relative domain purity) over the length scales probed can be extracted by integrating scattering profiles to yield the total scattering intensity. The purer the average domains are, the higher the total scattering intensity. Owing to a lack of absolute flux normalization, the absolute composition cannot be obtained by only R-SoXSterm·mth. In order to get a sense of how pure the domains are, we annealed the PffBT4T-2ODterm·key/fullereneterm·key blend at 130 °C for different length of time, 0, 10, 20, 40 and 120 min. The unannealed sample exhibits very pure domains, that is, almost 90% of the saturated value.
AFMterm·mth characterization AFMterm·mth measurements were performed by using a Scanning Probe Microscope-Dimension 3100 in tapping mode. All film samples were spin casted on ITO/ZnO substrates.
Photoluminescence quenching measurements Photoluminescence spectra were measured on samples on ITO/ZnO substrates upon excitation of a 671-nm laser beam. The PL quenching efficiency of PffBT4T-2ODterm·key was estimated from the ratio of the PL intensity of a PffBT4T-2ODterm·key:fullereneterm·key film sample to that of the PffBT4T-2ODterm·key control sample. ( Supplementary Fig. 10 ) Solar cell fabrication and testing Pre-patterned ITO-coated glass with a sheet resistance of ~15 Ω per square was used as the substrate. It was cleaned by sequential sonications in soap DI water, DI water, acetone and isopropanol for 15 min at each step. After ultraviolet/ozone treatment for 60 min, a ZnO electron transport layer was prepared by spin coating at 5,000 r.p.m. from a ZnO precursor solution (diethyl zinc). Active layer solutions (D/A ratio 1:1.2) were prepared in CB/DCB (1:1 volume ratio) with or without 3% of DIO (polymer concentration: 9 mg ml −1 ). To completely dissolve the polymer, the active layer solution should be stirred on a hot plate at 110 °C for at least 3 h. Before spin coating, both the polymer solution and ITO substrate are preheated on a hot plate at ~110 °C. Active layers were spin coated from the warm polymer solution on the preheated substrate in a N 2 glovebox at 800 r.p.m. to obtain thicknesses of ~300 nm. (The spin casting of high-performance PSCterm films is described in Supplementary Note 4 in details. The processing of PNT4T-2OD also requires the use of a metal chuck as described in Supplementary Note 4 . High-temperature and high spin rateterm·mth samples are described in Supplementary Note 5 ). The polymer/fullereneterm·key films were then annealed at 80 °C for 5 min before being transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of 3 × 10 −6 Torr, a thin layer (20 nm) of MoO 3 or V 2 O 5 was deposited as the anode interlayer, followed by deposition of 100 nm of Al as the top electrode. All cells were encapsulated using epoxy inside the glovebox. Device J – V characteristics was measured under AM1.5G (100 mW cm −2 ) using a Newport solar simulator. The light intensity was calibrated using a standard Si diode (with KG5 filter, purchased from PV Measurement) to bring spectral mismatch to unity. J – V characteristics were recorded using a Keithley 236 source meter unit. Typical cells have devices area of ~5.9 mm 2 , which is defined by a metal mask with an aperture aligned with the device area. EQEs were characterized using a Newport EQE system equipped with a standard Si diode. Monochromatic light was generated from a Newport 300 W lamp source. One of our best cells was sent to an accredited solar cell calibration laboratory (Newport Corporation) for certification, confirming an efficiency of 10.36±0.22%, with V OC =0.7743±0.0077 V, I SC =0.00079±0.00001 A, area=0.0425±0.0001, cm 2 , FF=72.0±1.5 ( Supplementary Fig. 11 ).
Author contributions Y.L. synthesized PffBT4T-2ODterm·key; J.Z. designed PNT4T-2OD, synthesized 5,10-Dibromonaphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole and carried out AFMterm·mth measurements; Z.L. synthesized PBTff4T-2OD; J.Z., H.L., Y.L., H.H. and Z.L. synthesized fullerenesterm·key; C.M., H.H., K.J. and H.Y. fabricated and tested PSCterm devices; W.M. carried out GIWAXSterm·mth and R-SoXSterm·mth measurements and analysis; K.J. carried out XRD analysis; H.L. synthesized PNT4T-2OD; H.A. supervised GIWAXSterm·mth and R-SoXSterm·mth work, and helped design experimental protocols; H.Y. conceived and directed the project; H.A. and H.Y. wrote the paper with input from all authors who reviewed the final paper.
Additional information How to cite this article: LiupersName, Y. et al. Aggregationterm·key and morphologyterm·key control enables multiple cases of high-efficiency polymer solar cellsterm. Nat. Commun. 5:5293 doi: 10.1038/ncomms6293 (2014).
Supplementary Material Supplementary Figures 1-11, Supplementary Tables 1-6, Supplementary Notes 1-5, Supplementary Methods and Supplementary References