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Versatile ternary organic solar cells: a critical review
Publication date (Print):
2016
Journal:
Energy Environ. Sci.
Publisher:
Royal Society of Chemistry (RSC)
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Abstract
Ternary organic solar cells enjoy both the enhanced light absorption by incorporating multiple organic materials in tandem solar cells and the simplicity of processing conditions that are used in single bulk heterojunction solar cells.
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Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells
Yuhang Liu, Jingbo Zhao, Zhengke Li … (2014)
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 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 newspapers7. PSCs 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 architecture6 8 9 10 can be easily implemented with PSCs and has proven to improve PSC efficiency by ~40–50% (refs 6, 8). As PSCs are two-component, donor–acceptor material systems, it is generally important to control the morphology 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 PSCs 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 fullerene, PC71BM, to achieve their best performance11 12 13. In general, the morphologies14 15 16 and thus performance of state-of-the-art donor polymers (for example, PTB7 (refs 11, 17) and PBDT-DTNT18) are sensitive to the choice of fullerene and replacing PC71BM with another C60-based or non-PCBM fullerene decreases PSC efficiency to 6-7% (refs 11, 18, 19, 20) The dominant role of PC71BM places serious constraints on PSC material development, because the properties of the polymers must be precisely matched with fixed targets set by PC71BM. As tandem PSCs require two sets of perfectly matching polymer/fullerene 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 fullerenes and more generally to remove material constraints to achieve tandem PSCs with 15–20% efficiency envisioned by Brebac and colleagues9 10 21. The development of polymer:fullerene material systems that are morphologically insensitive to fullerene choice will remove these material constraints, and greatly accelerate material development for single-junction and tandem PSCs10 22. Another important fundamental issue for the PSC field is how to control the morphology of polymer:fullerene blends to achieve the best PSC performance. There is likely more than one near-optimum PSC morphology. The famous PTB7 family donor polymers enabled one type of the near-optimum PSC morphology, as high external quantum efficiencies (EQEs~80%) have been reported for PTB7-based cells11. However, the PTB7-based PSC materials and devices have certain limitations. 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. Thick-film PSCs are important for the industrial application of PSCs, 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-film PSCs is partially owing to the relatively low hole transport ability (space charge limited current (SCLC) mobility ~6 × 10−4 cm2 V−1 s−1; ref. 17) related to the low crystallinity 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-film PSCs14 23 24. The PTB7-based materials systems are characterized by relatively impure polymer domains25, which could be a reason why these polymers do not perform well in thick-film PSCs. Clearly, there is a need for new materials systems that explore a different ‘near-optimum’ PSC morphology in order to achieve thick-film PSCs 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-film PSCs based on three different donor polymers and 10 polymer:fullerene 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 PSC performances in this report are achieved via the formation of an ‘optimum PSC morphology’ that contains highly crystalline, sufficiently pure, yet reasonably small polymer domains. 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. Importantly, this ipso facto near-perfect morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers during casting and is insensitive to the choice of fullerenes. Taking advantage of the robust polymer:fullerene morphology enabled by the three donor polymers, many non-traditional fullerenes are also used. Traditional PCBMs, the most dominant fullerenes in PSCs, are out-performed by several other non-traditional fullerenes, clearly indicating the benefits of exploring different fullerenes and the robust morphology formation. Comparative studies on several structurally similar polymers reveal that the 2-octyldodecyl (2OD) alkyl chains sitting on quaterthiophene is the key structural feature that causes the polymers’ highly temperature-dependent aggregation behaviour that allows for the processing of the polymer solutions at elevated temperature, and, more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process. The branching position and size of the branched alkyl chains are critically important in enabling an optimal aggregation behaviour. With our approach, PSC production is no longer constrained by the use of a single fullerene or by a very thin active layer. Our aggregation and morphology control approach and polymer design rules can be applied to multiple polymer:fullerene materials systems and will allow the PSC community to explore many more polymers and fullerene 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 PSCs. Results PSC 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-2OD (Fig. 1a). PffBT4T-2OD is a material that enables six cases of high-efficiency (9.6–10.8%), high FF (73–77%) and thick-film (250–300 nm) PSCs (Table 1) when combined with traditional PCBM and many non-traditional fullerenes (Fig. 1b). A typical J–V plot of a PffBT4T-2OD:fullerene PSC is shown in Fig. 1c, with EQE spectra shown in the inset. The benefits of thick-film PSCs are obvious. The thick cell exhibits 10–20% higher EQE values, and the effective absorption bandwidth of a thick PSC can be increased as the result of a ~20 nm red-shift of the ‘leading, low energy edge’ of a PSC’s EQE spectrum. Combined, these account for a ~30% increase in short circuit current (J SC). Taking advantage of PffBT4T-2OD’s excellent aggregation properties (as delineated further below), we synthesized more than a dozen known or new fullerene derivatives (Fig. 1b) to find the best acceptor match for PffBT4T-2OD. All of these fullerenes form similar morphologies with PffBT4T-2OD and can produce PSCs with high efficiencies in the range of 8.6–10.8% (Table 1 and Supplementary Table 1). The best efficiency (10.4%) in the C60 family was achieved by PC61PM (Fig. 1b), and the most commonly used C60-based fullerene, PC61BM, is not the best match for PffBT4T-2OD. Polymer crystallinity and hole mobility 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. 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 length (GIWAXS 2D patterns shown in Fig. 2a,b and Supplementary Fig. 1). The (010) coherence length (that is, extent of ordering) of PffBT4T-2OD:PC61PM blend films was calculated using Scherrer analysis27 to be ~8.5 nm, which corresponds to ~24 π-stacked copolymers. In contrast, the (010) coherence length of PTB7:PC61BM, for example, is only ~2 nm (ref. 16). Owing to the high crystallinity and preferential face-on orientation of polymer domains, relatively high SCLC hole mobility of 1.5–3.0 × 10−2 cm2 V−1 s−1 were obtained for various PffBT4T-2OD:fullerene blend films in a hole-only diode device configuration (Supplementary Fig. 2). The importance of mobility for good FF was recently illustrated23. Polymer:fullerene domain size and average domain purity In addition, resonant soft X-ray scattering14 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 polymers16 25. R-SoXS can also reveal the average composition variations, which are indicative of the average purity of the polymer and fullerene regions as well as a possible third phase of polymer-rich domains26 31. An annealing sequence on PffBT4T-2OD:fullerene 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% fullerene averaged over all PffBT4T-2OD domains in the film as measured by X-ray microscopy (Fig. 3c)25 32 33. 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 recombination23. These morphological data show that PffBT4T-2OD can form a polymer:fullerene morphology containing highly crystalline and sufficiently pure yet reasonably small polymer domains. Note that PTB7-type polymers have been the best donor polymer in PSCs for the past few years. By its very nature of high performance in thin films, PTB7 can form a ‘near-optimum’ PSC morphology characterized by relatively low molecular ordering, relatively low hole mobilities and impure polymer domains25. PffBT4T-2OD exhibits high molecular ordering (‘crystallinity’), high hole mobilities and purer polymer domains, which appears to be a different ipso facto ‘near-optimum’ PSC morphology. Although PTB7 enabled great thin-film PSC performance, PffBT4T-2OD offers high performance even in thick-film PSCs owing to the high mobility of the highly ordered and sufficiently pure polymer domains it forms. Morphology control via temperature-dependent aggregation We attribute PffBT4T-2OD’s excellent performance and robust morphology to its significant temperature-dependent aggregation behaviour that can be exploited during device fabrication. The UV-Vis absorption spectra exhibit a marked red-shift when a low concentration PffBT4T-2OD solution in 1,2-dichlorobenzene (DCB) is lowered from 85 to 25 °C (Fig. 1d). At elevated temperature, PffBT4T-2OD 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 aggregation of the polymer chains in solution at that temperature. Note that the absorption spectrum of the 25 °C solution of PffBT4T-2OD is almost identical to that of the optimized PffBT4T-2OD solid film (Fig. 1d), which is observed to be highly crystalline by GIWAXS. Consequently, devices are always cast from warm solutions (60–80 °C) of PffBT4T-2OD, which then aggregates during the cooling and film-forming process. To understand details of PffBT4T-2OD’s aggregation behaviour during the film-forming process, the critical π–π molecular ordering ((010) coherence length and intensity of the (010) peak) is determined with X-ray diffraction (XRD) for a series of PffBT4T-2OD films spun at different rates. As shown in Fig. 2d and Supplementary Table 2, the π–π ordering decreases markedly with increasing spin rates. As PffBT4T-2OD exhibits a strong yet progressively evolving aggregation property, the extent of PffBT4T-2OD’s aggregation depends upon temperature and concentration changes, the film drying time and the kinetics of aggregation. 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 rates provides PffBT4T-2OD sufficient time to aggregate and to form crystalline polymer domains with large coherence lengths. 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. Not surprisingly, high substrate temperatures were found to have a similar effect to fast spin rates. PffBT4T-2OD:fullerene films prepared with fast spin rates/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 PSC devices fabricated using high spin rates and high substrate temperature also exhibit markedly decreased hole mobilities (3.1 × 10−3 cm2 V−1 s−1; Supplementary Table 3) and PSC efficiencies (3.6%; Supplementary Table 4). These morphological, spectroscopic and electric data demonstrate that PffBT4T-2OD’s morphology is mainly controlled by the progress of its aggregation during the film-casting process until the film is dry, which locks-in the length scale of the morphology. PffBT4T-2OD’s strong yet well-controllable aggregation property allows for convenient optimization of processing conditions that led to a near-ideal polymer:fullerene morphology that is insensitive to the choices of fullerene. This approach of controlling the extent of polymer aggregation during a warm solution casting process is different from the common processing protocol of PTB7 family polymers that are typically processed at room temperature. Discussion The key structural feature of PffBT4T-2OD that enables its pronounced, yet gradual temperature-dependent aggregation (Fig. 1d) is the second-position branched alkyl chains (2OD) on a quaterthiophene (4T-2OD). To elucidate this aspect, we contrast PffBT4T-2OD with two structurally very similar polymers. These two polymers have the same backbone but their alkyl chains 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-2OD, 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 crystallinity (GIWAXS pattern shown in Fig. 4c) and thus PSC devices of only ~0.6% efficiency. PffBT4T-3OT exhibits the other extreme, showing excessive aggregation 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 chains cause too much steric hindrance, which results in poor aggregation and crystallinity. PffBT4T-3OT’s alkyl chains provide too little steric hindrance that makes aggregation of PffBT4T-3OT too strong even at 85 °C and makes it difficult to process. PffBT4T-2OD’s second-position branched alkyl chains offer an optimal tradeoff that allows for controllable aggregation of PffBT4T-2OD during the film-forming process. More discussions on the impact of alkyl chain branching positions are provided in Supplementary Note 1. Several structurally similar donor polymers containing quaterthiophene substituted with second-position branched alkyl chains were reported in the literature34 35 36 37, including a recent report in which a polymer with longer alkyl chains, FBT-Th4(1,4) (named as PffBT4T-2DT for simplicity in this paper, structure shown in Fig. 4a), achieved 7.64% efficiency34. The difference of PffBT4T-2OD and PffBT4T-2DT devices can be understood from the following results. R-SoXS (Fig. 4e) and prior AFM studies34 show that the average domain size of PffBT4T-2DT:fullerene films is too large (~100 nm). The R-SoXS data furthermore show that the average purity of the polymer/polymer-rich domains in an PffBT4T-2DT:fullerene film is ~87% of that in PffBT4T-2OD:fullerene film. Lower purity of average polymer/polymer-rich domains can result in significant recombination for thick-film PSC devices and thus lower performance14 23 24. Regarding molecular ordering, the two-dimensional GIWAXS mapping of PffBT4T-2DT:PC71BM films34 shows that PffBT4T-2DT:fullerene films exhibit weak laminar packing peaks, which are significantly weaker than those of PffBT4T-2OD:fullerene 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-2OD’s, as more fullerene is expelled by the crystalline polymer domains of PffBT4T-2OD. Lastly, the absorption coefficient of PffBT4T-2DT is lower than that of PffBT4T-2OD owing to longer alkyl chains that does not contribute to light absorption (Supplementary Fig. 5). These studies show that the branching position and the size of the alkyl chains are critically important in obtaining the optimal aggregation properties of PffBT4T-2OD. Insufficient aggregation (for example, PffBT4T-1ON) and unnecessarily long alkyl chains (for example, PffBT4T-2DT) resulted in low crystallinity and/or impure polymer domains. Excessive aggregation (for example, PffBT4T-3OT) makes the processing and aggregation difficult to control. Similar to recent observations32 36 38, the molecular weight of PffBT4T-2OD has a significant impact on its aggregation property and performance. Lower molecular weight (M n=16.6 kDa, M w=29.5 kDa) batches of PffBT4T-2OD exhibit weaker aggregation and thus lower efficiency (7.7%) than the high molecular weight (M n=47.5 kDa, M w=93.7 kDa) PffBT4T-2OD batches (Supplementary Table 5 and Supplementary Fig. 6). The impacts of polymer molecular weight on PSC 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 chains. Both PBTff4T-2OD and PNT4T-2OD exhibit significant temperature-dependent aggregation behaviour that leads to processing and morphology control and thus efficiency (including >10% for thick-film PSCs; Table 1, Supplementary Table 1 and Supplementary Fig. 7) comparable to those achieved by PffBT4T-2OD-based PSCs. R-SoXS and AFM studies confirmed that the polymer domain size of these two new polymers are similar to that of PffBT4T-2OD (30–40 nm). XRD characterization of PBTff4T-2OD:fullerene and PNT4T-2OD:fullerene films also showed strong (010) π–π stacking peaks that are similar to those observed for PffBT4T-2OD. Note that PNT4T-2OD also significantly outperforms its analogue polymer with 2-decyltetradecyl (2DT) alkyl chains35, providing another example that supports the critical importance of the size of the alkyl chains. 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 chains 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 chains nor were warm-casting methods used that optimally harnessed aggregation. They thus failed to reveal the connections between chemical structure, polymer aggregation during warm processing, morphology formation, polymer crystallinity and consequently PSC performance. Our study uncovered a new approach of aggregation and morphology control enabled by a structural feature (2OD alkyl chain) that is seemingly simple and commonly known, yet has surprisingly profound impact on PSC performances. The wide ranging applicability of our morphology control approach is supported by the three polymers and over 10 polymer:fullerene combinations that all yielded similar blend morphology and high-efficiency thick-film PSCs. 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. 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 polymers12 13 17. The PTB7 family polymers 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 chains sitting on the first and fourth thiophenes. The difference in the chemical structures caused different aggregation properties, based on which different processing protocols are used. 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. 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%). 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. This results in a near-ideal polymer:fullerene morphology (containing highly crystalline, preferentially orientated, yet small polymer domains) that is controlled by polymer aggregation during casting and thus insensitive to the choice of fullerenes. The branching position and size of the branched alkyl chains are critically important in enabling a well-controllable aggregation behaviour. Unnecessarily long alkyl chains (for example, 2DT) cause several detrimental effects including weaker laminar stacking, poorer absorption properties and less pure polymer domains. Our structural design rationales and aggregation and morphology control approach offer a new route to achieve high-performance thick-film PSCs 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 PC71BM), 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 PSC community to explore many more polymers and fullerene 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 PSCs. 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 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). The polymer crystallinity 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 mobility were measured using the SCLC method by using a device architecture of ITO/V2O5/PffBT4T-2OD (300 nm)/V2O5/Al by taking current–voltage curves and fitting the results to a space charge limited form, where the SCLC 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 mobility, 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. GIWAXS characterization GIWAXS 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 scattering R-SoXS transmission measurements were performed at beamline 11.0.1.2 at the ALS30. Samples for R-SoXS 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 Si3N4 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-SoXS. In order to get a sense of how pure the domains are, we annealed the PffBT4T-2OD/fullerene 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. AFM characterization AFM 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-2OD was estimated from the ratio of the PL intensity of a PffBT4T-2OD:fullerene film sample to that of the PffBT4T-2OD 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 N2 glovebox at 800 r.p.m. to obtain thicknesses of ~300 nm. (The spin casting of high-performance PSC 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 rate samples are described in Supplementary Note 5). The polymer/fullerene 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 MoO3 or V2O5 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 mm2, 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, cm2, FF=72.0±1.5 (Supplementary Fig. 11). Author contributions Y.L. synthesized PffBT4T-2OD; J.Z. designed PNT4T-2OD, synthesized 5,10-Dibromonaphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole and carried out AFM measurements; Z.L. synthesized PBTff4T-2OD; J.Z., H.L., Y.L., H.H. and Z.L. synthesized fullerenes; C.M., H.H., K.J. and H.Y. fabricated and tested PSC devices; W.M. carried out GIWAXS and R-SoXS measurements and analysis; K.J. carried out XRD analysis; H.L. synthesized PNT4T-2OD; H.A. supervised GIWAXS and R-SoXS 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: Liu, Y. et al. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells. Nat. Commun. 5:5293 doi: 10.1038/ncomms6293 (2014). Supplementary Material Supplementary Information Supplementary Figures 1-11, Supplementary Tables 1-6, Supplementary Notes 1-5, Supplementary Methods and Supplementary References
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A polymer tandem solar cell with 10.6% power conversion efficiency
Jingbi You, Letian Dou, Ken Yoshimura … (2013)
Organic photovoltaics (OPV) is an emerging photovoltaics (PV) technology with promising properties such as low cost, flexibility, light weight, transparency and large-area manufacturing compatibility1 2 3 4 5 6. Polymer solar cells (PSCs) based on conjugated polymers as electron donor materials blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor material have been the leading candidates in OPV in the past several years and achieved ~8% power conversion efficiency (PCE) using a single bulk heterojunction device structure7 8 9 10. To use solar radiation more effectively, a useful approach is to stack multiple photoactive layers with complementary absorption spectra in series to make a tandem PSC11 12 13 14 15 16 17 18 19. Typically, for a double-junction cell, such a tandem structure consists of a front cell with a high-bandgap material, an interconnecting layer, and a rear cell with a low-bandgap (LBG) material11 12 13 14 15 16 17 18 19. Compared with a single-junction device using low-bandgap materials, the multijunction/tandem structure can reduce thermalization loss of photonic energy during the photon-to-electron conversion process, and maximize the open circuit voltage (V OC). The high bandgap material in the front cell, which is responsible for the absorption of high-energy photons provide higher V OC than the low bandgap material. Therefore, by adopting polymers with matched absorption spectra, a tandem solar cell can effectively utilize the photonic energy and optimize open-circuit voltage, which leads to high PCE. In serial connected tandem solar cell, it is very obvious that subcell current balancing is critical for achieving high efficiency18. Inorganic multi-junction/tandem solar cells have gone a long way and the latest cell efficiency of 43.5% have been certified20, which shows the great potential of tandem solar cell. For polymer tandem solar cells, Hadipour et al. 13 demonstrated a polymer tandem solar cell consisting of two subcells with two different materials with about 0.57% efficiency in 2006, which is higher than each of the subcell’s efficiencies. In 2007, Kim et al. used a new interconnecting layer structure to bridge two higher performance single junction polymer PV cells to realize a tandem structure with 6.5% PCE14. More recently, Janssen et al. 18 has studied the effect of current matching on the tandem device performance, which provides more insight to achieve high-performance tandem PSC. However, the polymer tandem solar cells’ performance has been limited to around 7% efficiency in the last 4 years mainly due to the lack of high-performance low-bandgap polymers15 16 17 18 with high V OC and high external quantum efficiency (EQE) at long wavelengths. Recently, we designed a new low bandgap polymer PBDTT-DPP with improved quantum efficiency (EQE~;50%) at long wavelength, and successfully achieved an inverted tandem PSC17 with PCE of certified 8.6% (19 21). These progresses have translated the tandem polymer PV from a concept to a real OPV technology breakthrough, and paved a solid ground for achieving higher efficiencies in the future. With the progress in inter-connecting layer development, the main technical obstacle to break 10% barrier with a tandem PSC is the low bandgap polymer. The high bandgap cell in an inorganic multi-junction solar cell typically has a band gap of ~1.9 eV. In the PSC field, several polymers with ~1.9 eV such as poly-(3-hexylthiophene) (P3HT)2 and poly N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)22 have shown excellent performance, with short circuit current density (J SC) over 10 mA cm−2, and high EQE of ~70% from 400−600 nm. With 70% fill factor (FF) and over 1.5 V open circuit voltage in a tandem cell, a J SC of 10 mA cm−2 will lead to over 10% PCE. To achieve 10 mA cm−2 J SC in a low bandgap polymer cell between 600 nm and longer wavelength, an EQE close to 90% are required for polymer that absorbs up to 800 nm (~1.55 eV), or 60% for one that absorbs to 900 nm (~1.38 eV)23. So far, with a low bandgap cell of ~1.4 eV, the PSC shows only 5–6% PCE19 24 25 26 27. Particularly, most of the low bandgap polymers show low quantum efficiency ( 60% from 710 to 820 nm and the spectral response extends to 900 nm. This leads to a PCE of 7.9%. The polymer also enables over 10 mA cm−2 J SC in tandem solar cells. As a result, certified 10.6% PCE is achieved under standard reporting conditions (25 °C, 1,000 Wm−2, IEC 60904-3 global). Results Polymers design and characterization The chemical structure of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT are shown in Fig. 1a. The synthesis procedures can be found in the Methods and Supplementary Fig. S1. All polymers showed molecular weight of around 20 kDa and poly dispersity index of around 2.5. The ultraviolet-visible absorption spectra of these polymers in solid state are shown in Fig. 1b. As shown in the figure 1b, the main absorption range of PCPDT-BT covers from 500 to 850 nm; the absorption onset is located at around 850 nm, indicating an optical bandgap of 1.48 eV. Interestingly, by adding two F atoms onto the BT unit, the absorption spectrum of the new polymer PCPDT-DFBT shows a blue shift of around 30 nm (bandgap of around 1.51 eV), whereas the addition of F atoms did not affect the absorption spectrum of other reported polymer systems9 29 30 31 32 33. To further lower the bandgap to match a P3HT-based wide bandgap cell in a tandem structure, a strong electron-donating oxygen atom is introduced into the CPDT unit to form the DTP unit. By co-polymerizing with the strong electron-withdrawing DFBT unit, the polymer PDTP-DFBT shows significantly lower bandgap. The absorption spectrum of PDTP-DFBT is ~80 nm red-shifted (Fig. 1b) compared with PCPDT-DFBT and the bandgap is calculated to be 1.38 eV. The HOMO and lowest unoccupied molecular orbital level (LUMO) energy levels of the three polymers were determined by cyclic voltammetry (CV), and the results are shown in Fig. 1c. The HOMO and LUMO levels of PCPDT-BT are located at −5.18 and −3.56 eV, respectively. After adding two F atoms, PCPDT-DFBT shows a much deeper HOMO level (−5.34 eV) whereas the LUMO level (−3.52 eV) is almost unchanged. The deeper HOMO level of PCPDT-DFBT is desired to enhance the V OC, the similar LUMO level satisfies the exciton dissociation requirement. After adding the O atom, PDTP-DFBT shows slightly higher HOMO level (−5.26 eV) and lower LUMO (−3.61 eV), agrees with the lower optical bandgap. All the measured CV results are in accordance with density functional theory (DFT) calculations (Supplementary Fig. S2). It should be noted that the LUMO difference between polymer (~−3.6 eV) and PCBM (~−4.0 eV)19 is about 0.4 eV, there are sufficient driving force to dissociate the exciton at the bulk heterojunction interface3. Single junction solar cell devices Single junction photovoltaic cells based on the three polymers (and PC71BM as acceptor) were fabricated in an inverted device structure34 35 36 37 38. The PV performances are shown in Fig. 2a. For PCPDT-BT, the device showed a V OC of 0.62 eV and a J SC of 10 mA cm−2, but the FF was only 50%. For PCPDT-DFBT, the V OC of the device was increased significantly to 0.85 V, as adding the F atoms lowered the HOMO level of the polymer (Fig. 1c). Both J SC and FF increased slightly compared with PCPDT-DFBT device. Upon the insertion of the electron-donating oxygen atom, the PDTP-DFBT-based devices showed significantly enhanced J SC of 17–18 mA cm−2, which is among the highest reported J SC in PSCs so far. At the same time it gave a relatively high V OC close to 0.7 V, indicating the energy level tuning was successful to balance photocurrent generation and photovoltage. The performance of devices based on these three polymers is summarized in Table 1. The EQE of these devices are shown in Fig. 2b. The EQE of PCPDT-BT and PCPDT-DFBT-based devices are around 40% peak (~35% average) and 50% peak (~45% average), respectively. The PDTP-DFBT-based devices showed much higher peak EQE over 60% (average ~55%), and the photoresponse extends to 900 nm. Plasmon mapping based on energy-filtered transmission electron microscopy (EFTEM) was used to investigate the morphology of active layer39 40 41 42. The electron energy loss spectra of PDTP-DFBT and PDTP-DFBT:PC71BM are shown in Supplementary Fig. S3. It can be seen that the polymer and blend system has a peak around 22.5 and 24.2 eV, respectively, which are consistent with the previous reports39 40 41 42, where the plasmon peak of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) was found to be around 26 eV (39 40 41 42). Therefore, EFTEM enables us to use 20 and 30 eV energy loss to distinguish polymer and fullerene-rich domain, respectively, and provide more detailed morphology information than traditional TEM technique. The energy loss images of these three polymers blended with PC71BM at energy loss of 20 and 30 eV are shown in Fig. 3. The energy loss images at 20 and 30 eV for PCPDT-BT: PC71BM and PCPDT-DFBT: PC71BM active layers (using chlorobenzene (CB) with 3% (vol.) 1, 8-diiodooctane (DIO)) are shown in Fig. 3b, respectively. Nanoscale fibril features were clearly seen from on both polymer blend films, indicating nice phase separation of polymer and fullerene for PCPDT-BT and PCPDT-DFBT system. On the other hand, it can be found that the PC71BM-rich domain (the dark region in 20 eV energy loss image, and bright region in 30 eV energy loss image) are as large as ~200 nm, which could be not good enough for charge separation and transport. The PDTP-DFBT: PC71BM film shows finely phase separation with the feature size of about 20–30 nm, and it is achieved using pure dichlorobenzene (DCB) solvent without the assistance of solvent additive (Fig. 3h). This type of morphology is expected to improve charge separation and transport10 22. For comparison, the zero loss images, formed with only elastically scattered electrons39, have also been collected. Figure 3a,d and g are the zero loss images of PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-DFBT:PC71BM films, respectively. The zero loss images show the similar trend as plasmon energy loss mapping, with PBDT-DFBT:PC71BM owns a finer structure without large aggregation domain. Transport property is critically important for photovoltaic devices. Space-charge-limited current (SCLC) method was used to derive the hole mobilities of these three polymers. Hole-only devices were fabricated and the SCLC results are shown in Supplementary Fig. S3. On the basis of equation 2 (see Methods) the mobilities are estimated to be 5.1 × 10−4, 4.8 × 10−4 and 3.2 × 10−3 cm2 V−1s−1 for PCPDT-BT, PCPDT-DFBT and PDTP-DFBT, respectively. The carrier mobilities in the polymer, however, could be affected by morphology, field, recombination or carrier densities effect in polymer:fullerene bulk heterojunction active layers under operating conditions. To get reliable charge carrier mobility of the blending system, photo-induced charge carrier extraction in a linearly increasing voltages (Photo-CELIV) measurements have been conducted in bulk heterojunction solar cells based on the three systems43 44. The maximum voltage is 2 V in our experiment, with active layer thickness ~100 nm, the maximum electric field is thus ~2 × 105 V cm−1. And the charge carriers were extracted after 3 μs fixed delay time. The Photo-CELIV results are shown in Fig. 4, where the t max (the time when the extraction current reaches its maximum value) for PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM are 7.1, 2.8 and 1.1 μs, respectively. The charge mobility can then be calculated to be 1.2 × 10−5, 7.4 × 10−5 and 6.7 × 10−4 cm2 V−1s−1 for PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM, respectively (equation 3, see Methods). The qualitative charge carrier mobility data in the blend systems thus show the same trend, but are lower than those in pure polymer cases, which is consistent with the larger disorder in bulk heterojunction film. The PDTP-BT:PC71BM owns a higher charge carrier mobility than the other two polymer systems, which could contribute to the higher PV performance. For more details of SCLC and Photo-CELIV, please see Methods. Tandem devices based on P3HT:ICBA and PDTP-DFBT:PCBM From the analysis of spectral coverage and the single junction solar cell result, PDTP-DFBT shows high potential for application in tandem solar cells as a rear cell the tandem structure is shown in Fig. 6a. In a tandem structure, a high bandgap polymer, P3HT, combined with Indene-C60Bisadduct (ICBA) fullerene are selected as front cell active materials, and the low bandgap polymer PDTP-DFBT with PC61BM or PC71BM are selected as the rear cell active materials. Sol-gel processed ZnO is used as the electron transport layer, and PEDOT:PSS and MoO3 are used as the hole transport layer for the front and rear cells, respectively. The inverted tandem structure was chosen because of its advantages of a simple, robust device fabrication process and better stability8 34 35 36 37. Figure 5a shows the ultraviolet-visible absorption spectra of P3HT and PDTP-DFBT in the solid state, and the solar radiation spectrum. It can be seen that these two polymers complementarily cover the solar spectrum from 350 to 900 nm with very little spectral overlap, which provides a favourable system combination for optimizing tandem cell performance through independent tuning of two subcells. IC60BA has been shown to be a successful acceptor for the high bandgap polymer P3HT45 46 used in both single junction and tandem PSCs19. Two types of widely used fullerenes (PC61BM and PC71BM) with different absorption coefficients were examined47 here to blend with the low bandgap polymer. The absorptions of polymers blended with acceptors are shown in Fig. 5b. When the low bandgap polymer PDTP-DFBT is blended with PC61BM, the absorption is enhanced slightly in the region of 300–400 nm due to the absorption of PC61BM, and the two subcells show little overlap. Whereas the PDTP-DFBT:PC71BM blend showed strong absorption in the entire visible region, and the overlap of two subcells is more significant. These different overlaps will affect the current match of the tandem solar cell, and thus the efficiency. Figure 5c shows the current versus voltage (J–V) characteristics of single-junction devices under AM1.5G illumination from a calibrated solar simulator with irradiation intensity of 100 mW cm−2. All the single junction cells (P3HT:IC60BA, PDTP-DFBT:PC61BM, and PDTP-DFBT:PC71BM) show excellent performance. Specifically, the P3HT:IC60BA cell shows 6.1% PCE, and the PDTP-DFBT-based single junction cells show 7.1% and 7.9% PCE when blended with PC61BM and PC71BM, respectively. The difference comes from the J SC, which is mainly due to the different absorption coefficients of PC61BM and PC71BM in the region of 300–600 nm. The detailed parameters of the single cell are summarized in Table 2. The EQE of P3HT:IC60BA, PDTP-DFBT:PC61BM, PDTP-DFBT:PC71BM are shown in the Fig. 5d. The wide bandgap polymer cell (P3HT:IC60BA) showed high quantum efficiency from 300–700 nm, with maximum EQE of 70% at about 520 nm. In both of the PDTP-DFBT-based single cells, the maximum EQE is 62% at ~800 nm, and EQE is over 60% from 710 to 820 nm. Accurate tandem cell measurement is a quite complicated procedure and extra care was taken to get reliable data48 49 50. EQE results were first measured in University of California, Los Angeles (UCLA)48, and the tandem devices were then measured using the One-Sun Multi-Source Simulator (recently established at National Renewable Energy Laboratory, (NREL)) based on UCLA EQE data19. The spectral mismatches associated with the re-measured EQEs and the simulator spectra were then recalculated. The spectral mismatches were found to be 1 cm2 ( http://www.heliatek.com/), both clearly show the importance of tandem structure in organic solar cells. Methods Materials P3HT was purchased from Rieke Metals. PC61BM and PC71BM were purchased from Nano-C. The synthesis procedure of CPDT, DTP and DFBT units can be found in Zhu et al. 51 and Yoshimura and Ohya52. The polymers were synthesized as follow (see Supplementary Fig. S1 for the chemical reaction equations): PCPDT-DFBT: CPDT (0.337 g, 0.463 mmol) and DFBT (0.148 g, 0.449 mmol) were dissolved into 10 ml toluene in a flask protected by argon. The solution was flushed by argon for 10 min, then 8 mg of Pd2(dba)3 and 16 mg of P(o-tol)3 was added into the flask. The solution was flushed by argon again for 20 min. The oil bath was heated to 110 °C gradually, and the reaction mixture was stirred for 10 h at 110 °C under argon atmosphere. Then, the mixture was cooled down to room temperature and the polymer was precipitated by addition of 100 ml methanol and the precipitated solid was collected and purified by Soxhlet extraction (acetone for 24 h and then hexane for 24 h). The solid was then dissolved in 30 ml toluene and purified by chromatography on silica gel. Then the solution was concentrated to 10 ml and precipitated in methanol. The title polymer was obtained as dark purple solid, yield ~50%. The polymer can be readily dissolved into CB, DCB and so on. PCPDT-BT and PDTP-DFBT were synthesized using the same procedure. Electrochemical CV The electrochemical CV was conducted with Pt disk, Pt plate and Ag/AgCl electrode as working electrode, counter electrode and reference electrode, respectively, in a 0.1 mol l−1 tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution1. The polymer films for electrochemical measurements were coated from a polymer chloroform solution of concentration ~5 mg ml−1. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured under the same conditions, and it is located at 0.42 V to the Ag/AgCl electrode. It is assumed that the redox potential of Fc/Fc+ has an absolute energy level of −4.80 eV to vacuum. The energy levels of the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) were then calculated according to the following equations: where E ox is the onset oxidation potential versus Ag/AgCl and E red is the onset reduction potential versus Ag/AgCl. Simulated results for the molecular orbital DFT calculations were executed to determine the electronic structures of the two polymers. Two repeating units of the polymers were used as the model compound in the simulation, and methyl groups were used instead of the long side chain. DFT calculations were done using HSE06/6-31G(d). The calculated HOMO/LUMO and bandgap for CPDT-BT, CPDT-DFBT and DTP-DFBT were −4.632/−2.714 and 1.918 eV, −4.827/−2.865 and 1.962 eV, −4.824/−2.914 and 1.910 eV, respectively. These results indicate that by adding two F atoms on the BT unit, CPDT-DFBT shows lower HOMO/LUMO levels and a larger bandgap than CPDT-BT due to the strong electron-withdrawing property of F atom; after further adding an O atom, DTP-DFBT shows lower HOMO/LUMO levels and a smaller bandgap than CPDT-BT. The simulation results are in accordance with experimental results. Morphology characterization EFTEM was used to characterize the morphology of the polymer:fullerene system. Thin films were analysed in a Titan Krios TEM equipped with an energy filter. By electron energy loss spectroscopy, plasmon energies of 20 and 30 eV were used to distinguish the polymer and PCBM, respectively. A slit width of 8 eV was used during the measurement. Mobilities measurement SCLC model measurement: Hole mobility was measured using the SCLC, using a diode configuration of ITO/PEDOT:PSS/polymer:/Au by taking current–voltage current in the range of 0–8 V and fitting the results to a space–charge-limited form, where the SCLC 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 mobility, V is the voltage drop across the device and L is the polymer thickness. The dielectric constant ε r is assumed to be 3, which is a typical value for conjugated polymers. The thickness of the polymer films is measured by using a Dektek profilometer. The thickness of the polymer is ~100, 90 and 100 nm for PCPDT-BT, PCPDT-DFBT and PDTP-DFBT, respectively. The mobilities of these three polymers are 5.1 × 10−4, 4.8 × 10−4 and 3.2 × 10−3 cm2 V−1 s−1. Photo-induced charge carrier extraction in a linearly increasing voltages (Photo-CELIV) measurement: To further measure the mobility of the polymer:fullerene blend system, photo-induced charge carrier extraction in a linearly increasing voltages (Photo-CELIV) measurement was used to determine the charge carrier mobility in bulk heterojunction solar cells44. The device structure is ITO/ZnO/Polymer:PC71BM/MoO3/Ag. A 590-nm dye (Rhodamine Chloride 590) laser pumped by a nitrogen laser (LSI VSL-337ND-S) was used as the excitation source, the pulse energy and pulse width being about 3 μJ cm−2 and 4 ns, respectively. The current of the photodiode was first amplified by using a current amplifier (Femto DHPCA-100), then a preamplifier (SR SSR445A) and finally recorded using a digital oscilloscope (Tektronix DPO 4104). The t max values for PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM are measured to be 7.1, 2.8 and 1.1 μs, respectively. From the active thicknesses of 100, 90 and 100 nm, respectively, the mobilities of PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM are calculated to be 1.2 × 10−5, 7.4 × 10−5 and 6.7 × 10−4 cm2 V−1 s−1, respectively, based on the following equation44. where μ is the mobility, d is the thickness of the active layer, t max is the time when the extraction current is reached, A is the voltage rise speed A=dU/dt. In this measurement, the applied maximum voltage is 2 V, and U offset is chosen to be near the corresponding open-circuit voltage, the time t for voltage increase from U offset to maximum voltage is 30 μs, and j(0) is the capacitive displacement current44. Single-junction devices based on three different polymers The precleaned ITO substrates were first treated with ultraviolet-ozone for 15 min. ZnO nanoparticles were spin-coated on the ITO substrates. The synthesis process of ZnO nanoparticles can be found in You et al.,53 Sun and Sirringhaus54 and Beek et al. 55 Then the active layer was spin-coated on the ZnO surface; PCPDT-BT:PCBM (1:2), PCPDT-DFBT (1:2) and PDTP-DFBT:PCBM (1:2) were dissolved in CB or DCB with a concentration ranging from 5 to 10 mg ml−1 and with stirring at about 80 °C for at least 2 h before spin coating the active layer (heating is helpful to improve the solubility); for the PCPDT-BT and PCPDT-DFBT-based device, to improve the active layer morphology, 3% by volume DIO was added. After spin coating the active layer, the samples were transferred into the evaporation chamber for fabricating the MoO3/Ag electrode. Tandem devices fabrication The precleaned ITO substrates were first treated with ultraviolet-ozone for 15 min. The P3HT:IC60BA (1:1 weight ratio) in DCB solutions with various solid content were spin-casted at 800 r.p.m. for 30 s on top of a ~30 nm layer of ZnO53 54 55. The P3HT:ICBA film is formed through a slow growth process and the thickness is between 180 and 240 nm2. PEDOT:PSS layer was applied on the first subcell active layer and annealed at 150 °C for 10 min. Then a thin layer of ZnO film was spin-casted, followed again by thermal annealing at 150 °C for 10 min. The second subcell active layer of PDTP-DFBT:PCBM (PC61BM or PC71BM) (1:2) from an DCB solution was then spin-casted at 1,500 r.p.m. on top of the ZnO layer, the thickness of PDTP-DFBT is about 80–120 nm. To control the thickness to tune the short circuit current of front and rear cell, we varied the concentration of P3HT (18–22 mg ml−1) and PDTP-DFBT (5–7 mg ml−1) in the solution. The optimized thickness of the front and rear cell are about 220 and 100 nm, respectively. The inverted tandem PSC devices fabrication were completed by thermal evaporation of MoO3/Ag as the anode under vacuum at a base pressure of 2 × 10–6 Torr. Tandem device characterization The device areas were measured in the NREL facility first using optical microscope, and the results range from 0.0998 to 0.104 cm2. EQEs were measured at UCLA and NREL, respectively. A 550- and 700-nm light bias was selected to excite the front and rear cells to measure the EQE of the rear and front cell, respectively. I–V measurements for the tandem cells were performed on the One-Sun Multi-Source Simulator of NREL first by using UCLA EQE data for spectral mismatch correction. The spectral mismatches associated with the re-measured EQEs and the simulator spectra were recalculated. In each case, the spectral mismatches were <0.2% different from those derived from the EQEs. Measurements in NREL were done at 1,000, 500 and 250 W m−2 with and without the mask (~0.04 cm2) on a representative device. The JSC with the mask was about 2% lower than without the mask. Some of this difference is likely due to the extended source of the One-Sun Multi-Source Simulator light (as opposed to a point-like source) and the thickness of the mask relative to the size or the opening. For the different light intensity response measurement, neutral density filters were used to tune the light intensity, and a Si-diode was used to calibrate the light intensity. In examining the NREL certified J-V data, we observed some fluctuation in the J-V curve. We would like to explain the phenomenon as the following: 1) We believe part of the reason for the noise in the data is that there are multiple light sources (we have 9 separated light sources to control the spectrum) each with different intensity versus time characteristics. 2) Smooth I-V curves do not mean they are more accurate then IV curves with fluctuations. For example a halogen light source will give very smooth curves but the spectrum is a terrible match to the reference solar spectrum. 3) More importantly than the noise in the I-V curve was the fact that the spectrum was adjusted so that each cell was within 1% of its target photocurrent under the reference spectrum. 4) Furthermore, from the expansion data in Figure 7 (NREL data, see Figure S7), we can see the fluctuations are at the 1% level or less within the requirements of a Class A simulator. Author contributions J.Y. carried out the fabrication, characterization of inverted single and tandem PSC devices, data collection and analysis. L.D. designed PCPDT-DFBT and performed materials synthesis and characterization of PCPDT-BT and PCPDT-DFBT. K.Y., T.K. and K.O. designed the chemical structure and performed materials synthesis of PDTP-DFBT. T.M. and K.E. performed the certification at NREL. C.C. modified the interlayer, C.C. and J.G. carried out TEM measurements. J.Y., L.D., G.L. and Y.Y. prepared the manuscript. All authors discussed the results and commented on the manuscript. Y.Y. and G.L. conceptualized, planned and supervised the project. Additional information How to cite this article: You, J. et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 4:1446 doi: 10.1038/ncomms2411 (2013). Supplementary Material Supplementary Information Supplementary Figures S1-S7
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Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption.
Yongfang Li (2012)
Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs. In this Account, I discuss the basic requirements and scientific issues in the molecular design of high efficiency photovoltaic molecules. I also summarize recent progress in electronic energy level engineering and absorption spectral broadening of the donor and acceptor photovoltaic materials by my research group and others. For high-efficiency conjugated polymer donors, key requirements are a narrower energy bandgap (E(g)) and broad absorption, relatively lower-lying HOMO (the highest occupied molecular orbital) level, and higher hole mobility. There are three strategies to meet these requirements: D-A copolymerization for narrower E(g) and lower-lying HOMO, substitution with electron-withdrawing groups for lower-lying HOMO, and two-dimensional conjugation for broad absorption and higher hole mobility. Moreover, better main chain planarity and less side chain steric hindrance could strengthen π-π stacking and increase hole mobility. Furthermore, the molecular weight of the polymers also influences their photovoltaic performance. To produce high efficiency photovoltaic polymers, researchers should attempt to increase molecular weight while maintaining solubility. High-efficiency D-A copolymers have been obtained by using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) donor unit and benzothiadiazole (BT), thienopyrrole-dione (TPD), or thiazolothiazole (TTz) acceptor units. The BDT unit with two thienyl conjugated side chains is a highly promising unit in constructing high-efficiency copolymer donor materials. The electron-withdrawing groups of ester, ketone, fluorine, or sulfonyl can effectively tune the HOMO energy levels downward. To improve the performance of fullerene derivative acceptors, researchers will need to strengthen absorption in the visible spectrum, upshift the LUMO (the lowest unoccupied molecular orbital) energy level, and increase the electron mobility. [6,6]-Phenyl-C(71)-butyric acid methyl ester (PC(70)BM) is superior to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) because C(70) absorbs visible light more efficiently. Indene-C(60) bisadduct (ICBA) and Indene-C(70) bisadduct (IC(70)BA) show 0.17 and 0.19 eV higher LUMO energy levels, respectively, than PCBM, due to the electron-rich character of indene and the effect of bisadduct. ICBA and IC(70)BA are excellent acceptors for the P3HT-based PSCs.
Author and article information
Journal
Journal ID (publisher-id): EESNBY
Title:
Energy Environ. Sci.
Abbreviated Title:
Energy Environ. Sci.
Publisher:
Royal Society of Chemistry (RSC)
ISSN
(Print):
1754-5692
ISSN
(Electronic):
1754-5706
Publication date Created: 2016
Publication date
(Print):
2016
Volume: 9
Issue: 2
Pages: 281-322
Article
DOI: 10.1039/C5EE02641E
SO-VID: c06c5c3c-aac6-4865-bc39-56708bab2ebd
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2016
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