Ternary Blend Bulk Heterojunction Research Articles

Organic Thin‐film Solar Cells Using Benzotrithiophene Derivatives Bearing Acceptor Units as Non‐Fullerene Acceptors

AbstractNew star‐shaped non‐fullerene acceptors (5Z,5′Z,5′′Z)‐5,5′,5′′‐((benzo[1,2‐b : 3,4‐b′ : 5,6‐b′′]trithiophene‐2,5,8‐triyltris(4‐octylthiophene‐5,2‐diyl))tris(methaneylylidene))tris(3‐octyl‐2‐thioxothiazolidin‐4‐one) (1: BTT‐OT‐ORD) and 2,2′,2′′‐((5Z,5′Z,5′′Z)‐((benzo[1,2‐b : 3,4‐b′ : 5,6‐b′′]trithiophene‐2,5,8‐triyltris(4‐octylthiophene‐5,2‐diyl))tris(methaneylylidene))tris(3‐octyl‐4‐oxothiazolidine‐5,2‐diylidene))trimalononitrile (2: BTT‐OT‐OTZDM) with a benzotrithiophene core, alkyl‐thiophen units, and acceptor units were designed and synthesized. The HOMO‐LUMO levels of 1 and 2 were determined by photoemission spectroscopy and UV‐Vis absorption spectroscopy. Binary blend and ternary blend bulk heterojunction (BHJ) organic solar cells with non‐fullerene acceptors 1 and 2 were fabricated with the inverted device structures of glass/ITO/ZnO/active_layer/MoO3/Ag. Both binary blend BHJ solar cells with 1 and 2 show lower JSC and larger VOC values than P3HT : PCBM solar cells. On the other hand, ternary blend BHJ organic solar cells, including 10 % of 1, exhibited a larger power conversion efficiency than P3HT : PCBM solar cells because the JSC value was largely improved.

Designing ternary blend all-polymer solar cells with an efficiency of over 10% and a fill factor of 78%

In recent years, ternary blend bulk-heterojunction (BHJ) all-polymer solar cells have been gradually developed to better utilize the solar irradiance spectrum. However, power conversion efficiencies remain below 10%, mainly because of the low fill factor. Generally, the fill factor of all-polymer solar cells is limited mainly by the competition between the recombination and extraction of free charges. Here, we design advanced ternary blend all-polymer solar cells with a high fill factor of 78%, thus demonstrating how such recombination thresholds can be overcome. These results can be attributed to the high and balanced bulk charge mobility, reduced recombination, and optimized morphology, as well as the intimate mixing properties of the two donors in the photoactive layer.

A new strategy to engineer polymer bulk heterojunction solar cells with thick active layers via self-assembly of the tertiary columnar phase.

We report that the addition of a non-photoactive tertiary polymer phase in the binary bulk heterojunction (BHJ) polymer solar cell leads to a self-assembled columnar nanostructure, enhancing the charge mobilities and photovoltaic efficiency with surprisingly increased optimal active blend thicknesses over 300 nm, 3-4 times larger than that of the binary counterpart. Using the prototypical poly(3-hexylthiophene) (P3HT):fullerene blend as a model BHJ system, we discover that the inert poly(methyl methacrylate) (PMMA) added in the binary BHJ blend self-assembles into vertical columns, which not only template the phase segregation of electron acceptor fullerenes but also induce the out-of-plane rotation of the edge-on-orientated crystalline P3HT phase. Using complementary interrogation methods including neutron reflectivity, X-ray scattering, atomic force microscopy, transmission electron microscopy, and molecular dynamics simulations, we show that the enhanced charge transport originates from the more randomized molecular stacking of the P3HT phase and the spontaneous segregation of fullerenes at the P3HT/PMMA interface, driven by the high surface tension between the two polymeric components. The results demonstrate a potential method for increasing the thicknesses of high-performance polymer BHJ solar cells with improved photovoltaic efficiency, alleviating the burden of stringently controlling the ultrathin blend thickness during the roll-to-roll-type large-area manufacturing environment.

Influence of Surface Energy on Organic Alloy Formation in Ternary Blend Solar Cells Based on Two Donor Polymers.

The compositional dependence of the open-circuit voltage (Voc) in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of polymers, which may be influenced by a number of attributes, including crystallinity, the random copolymer effect, or surface energy. Four ternary blend systems featuring poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25), poly(3-hexylthiophene-co-(hexyl-3-carboxylate)), herein referred to as poly(3-hexylthiophene-co-3-hexylesterthiophene) (P3HT50-co-3HET50), poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), and an analog of P3HTT-DPP-10% with 40% of 3-hexylthiophene exchanged for 2-(2-methoxyethoxy)ethylthiophen-2-yl (3MEO-T) (featuring an electronically decoupled oligoether side-chain), referred to as P3HTTDPP-MEO40%, are explored in this work. All four polymers are semicrystalline and rich in rr-P3HT content and perform well in binary devices with PC61BM. Except for P3HTTDPP-MEO40%, all polymers exhibit similar surface energies (∼21-22 mN/m). P3HTTDPP-MEO40% exhibits an elevated surface energy of around 26 mN/m. As a result, despite the similar optoelectronic properties and binary solar cell performance of P3HTTDPP-MEO40% compared to P3HTT-DPP-10%, the former exhibits a pinned Voc in two different sets of ternary blend devices. This is a stark contrast to previous rr-P3HT-based systems and demonstrates that surface energy, and its influence on miscibility, plays a critical role in the formation of organic alloys and can supersede the influence of crystallinity, the random copolymer effect, similar backbone structures, and HOMO/LUMO considerations. Therefore, we confirm surface energy compatibility as a figure-of-merit for predicting the compositional dependence of the Voc in ternary blend solar cells and highlight the importance of polymer miscibility in organic alloy formation.

Designing ternary blend bulk heterojunction solar cells with reduced carrier recombination and a fill factor of 77%

In recent years the concept of ternary blend bulk heterojunction (BHJ) solar cells based on organic semiconductors has been widely used to achieve a better match to the solar irradiance spectrum, and power conversion efficiencies beyond 10% have been reported. However, the fill factor of organic solar cells is still limited by the competition between recombination and extraction of free charges. Here, we design advanced material composites leading to a high fill factor of 77% in ternary blends, thus demonstrating how the recombination thresholds can be overcome. Extending beyond the typical sensitization concept, we add a highly ordered polymer that, in addition to enhanced absorption, overcomes limits predicted by classical recombination models. An effective charge transfer from the disordered host system onto the highly ordered sensitizer effectively avoids traps of the host matrix and features an almost ideal recombination behaviour. Carrier recombination in organic solar cells usually limits their optoelectronic performance, in particular their fill factor. Gasparini et al show that adding an ordered polymer to a ternary blend reduces carrier recombination, achieving a fill factor of 77%.

Effect of blend composition on ternary blend organic solar cells using a low band gap polymer

This work investigates the influence of blend composition in ternary blend bulk heterojunction organic solar cells composed of poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The use of the low band gap Si-PCPDTBT vastly improves the spectral response, when compared to a P3HT:PC71BM binary system. The optimum blend composition occurred at a total polymer concentration of 50wt%. At low polymer concentrations, the P3HT phase was amorphous in nature. Increasing the polymer content led to the formation of crystalline polymer domains, as evidenced by XRD measurements. This significantly enhanced the charge carrier transport throughout the active layer. XPS depth profiles indicated that variations in the polymer content also influenced the mixing between the Si-PCPDTBT and the P3HT host matrix. This analysis showed that the 50wt% was conducive to a larger interaction between the two polymers. A comprehensive analysis of the relative contributions of each molecule to the photoluminescence suggested that the polymer concentration not only affects the film microstructure, it also influences the photoluminescence quantum yield of the blend. This is caused by alterations to the recombination mechanisms occurring in the constituent materials, which, in turn, influences photocurrent generation. This result shows that the overall polymer content must be chosen carefully to strike a delicate balance between improved absorption and effective charge generation and collection.

Ternary blend bulk heterojunction polymer solar cells based on double donors and single acceptor with ultra wideband absorption

Ternary blend bulk heterojunction polymer solar cells based on double donors and single acceptor with ultra wideband absorption

Ternary Blend Bulk-Heterojunction Solar Cells Based on Active Layers of PTB7, PC71BM, and PC61BM

Bulk-heterojunction solar cells were fabricated using ternary blend dichlorobenzene solutions of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) with different weight ratios between PC61BM and PC71BM. In all the solar cells, the overall weight ratio of polymer to fullerene was maintained at 1:1.5, while the composition of the fullerene component (PC61BM:PC71BM) was varied. The measurement results of the solar cell performance showed that the open-circuit voltage notably increased for PC61BM weight fractions between 10% and 90%, while it decreased at 100%. The short-circuit current showed the most significant increase in the PC61BM weight fraction range between 50% and 60%. A power conversion efficiency of 3.4% was achieved when the PC61BM weight fraction was between 50% and 60%. These results may suggest that the transport of the photoexcited electrons between the cathode and the PC61BM/PC71BM nanodomains was enhanced.

Ternary Blend Bulk-Heterojunction Solar Cells Based on Active Layers of PTB7, PC71BM, and PC61BM

Organic bulk-heterojunction (BHJ) solar cells are highly efficient solar cells with additional advantages such as low-cost production, large area, and mechanical flexibility, as they can be fabricated by solution processing without using high vacuum equipment [1]. On the other hand, an extremely high PCE of 7.5% has recently been reported for BHJ solar cells by Murray et al. [2]. The cells were fabricated using a p-type semiconductor, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7), an n-type semiconductor, [6,6]-phenyl-C71-butyric-acid-methyl-ester (PC71BM), and a lithium fluoride (LiF) inserted Al cathode on an indium tin oxide (ITO) coated glass substrate. These promising results inspired many recent studies on BHJ solar cells based on PTB7:PC71BM, and various techniques were proposed to further improve the performance of the BHJ solar cells. Ternary or quaternary blend solar cells have also been reported to exhibit interesting performances. Lin et al. [3] have demonstrated a significant improvement in PCE by adding the PTB7 polymer to a poly[[4,8-bis[(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-bnodithiophene-2,6-diyl]] (PCD TBT):PC71BM host system in order to form a ternary-blend BHJ solar cell; the addition of 5 wt% of PTB7 resulted in the greatest PCE improvement. Such studies on ternary-blend solar cells showed performance improvements as a result of adding minute amounts of additives to the host active layers. Ohori et al. recently reported significant improvements in the performance of BHJ solar cells based on ternary-blend organic semiconductor materials with smaller amounts of P3HT additives into the PTB7:PC61BM active layers [4]. They suggested that the improvement of their solar cells was due to enhanced carrier transport paths. Xu et al. reported a significant increase in the open-circuit voltage (Voc) of quaternary-blend BHJ solar cells based on P3HT as a donor, and PC61BM, indene-C60 bisadduct (ICBA) and silicon phthalocyanine bis(trihexylsilyl oxide) as acceptors [5]. Cheng et al. fabricated ternary blend BHJ solar cells based on PTB7:ICBA:PC71BM, and found that the solar cells showed the maximum PCE at the ICBA weight fraction of 15% of the total weight of ICBA and PC71BM [6]. They suggested that the PCE improvement was due to the cascade electron transfer through the lowest unoccupied molecular orbital (LUMO). Notably, ternary blend solar cells based on PTB7, PC61BM, and PC71BM may have similar LUMO cascade structures, but they have not yet been reported. In this paper, we report on BHJ solar cells based on ternary blend solutions of PTB7, PC71BM, and PC61BM. Bulk-heterojunction solar cells were fabricated using ternary blend dichlorobenzene solutions of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) with different weight ratios between PC61BM and PC71BM. In all the solar cells, the overall weight ratio of polymer to fullerene was maintained at 1:1.5, while the composition of the fullerene component (PC61BM:PC71BM) was varied. The ultraviolet-visible absorption spectra of these ternary blend films showed that the photon absorptions at wavelengths between 300 and 800 nm continuously decreased with the increase of the PC61BM weight fraction in the PC61BM and PC71BM total weight. The measurement results of the solar cell performance showed that the open-circuit voltage notably increased for PC61BM weight fractions between 10% and 90%, while it decreased at 100%. The short-circuit current showed the most significant increase in the PC61BM weight fraction range between 50% and 60%. A power conversion efficiency of 3.4% was achieved when the PC61BM weight fraction was between 50% and 60%. These results may suggest that the transport of the photoexcited electrons between the cathode and the PC61BM/PC71BM nanodomains was enhanced. [1] C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, Adv. Funct. Mater., 11 (2001) 15. [2] I. P. Murray, S. J. Lou, L. J. Cote, S. Loser, C. J. Kadleck, T. Xu, J. M. Szarko, B. S. Rolczynski, J. E. Johns, J. Huang, L. Yu, L. X. Chen, T. J. Marks, and M. C. Hersam, J. Phys. Chem. Lett., 2 (2011) 3006. [3] R. Lin, M. Wright, B. P. Veettil, and A. Uddin, Synth. Met., 1927 (2014) 113. [4] Y. Ohori, S. Fujii, H. Kataura, and Y. Nishioka, Jpn. J. Appl. Phys., 45 (2015) 04DK09. [5] H. Xu, H. Ohkita, H. Benten, and S. Itoh, Jpn. J. Appl. Phys., 53 (2014), 01AB10. [6] P. Cheng, Y. Li, and X. Zhan, Energy Environ. Sci., 7 (2014) 2005.

Efficiency enhancement of P3HT:PCBM polymer solar cells using oligomers DH4T as the third component

We assembled a ternary blend bulk heterojunction polymer solar cell (PSCs) containing P3HT (donor) and PC61BM (acceptor) incorporated with a dihexyl-quaterthiophene (DH4T) small molecule oligomer as a third component. By optimizing the contents of DH4T, we increased the power conversion efficiency of ternary P3HT:DH4T:PC61BM PSCs to 4.17% from 3.44% of binary P3HT:PC61BM PSCs under AM 1.5 G of 100 mW/cm2 intensity. The major improvement is from the increase of the short circuit current and fill factor that is due to the increased light absorption at short wavelength, the balanced charge carrier transportation and the enhanced hole evacuation by a DH4T-enriched layer at the anode interface. In this work, we demonstrate that the efficiency of the PSCs can be enhanced by using low-bandgap conjugated polymer and its oligomer as donors and fullerene derivatives as acceptors.

Narrow band gap conjugated polymer for improving the photovoltaic performance of P3HT:PCBM ternary blend bulk heterojunction solar cells

A new low band gap conjugated polymer PBDO-T-TDP is synthesized and used as a sensitizer in P3HT:PCBM based BHJ solar cells and its effect on their photovoltaic properties is investigated by blending them at various weight ratios.

Performance improvement of low bandgap polymer bulk heterojunction solar cells by incorporating P3HT

This work demonstrates a significant improvement of device performance by incorporating the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) into a low bandgap polymer poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host system, to form a ternary blend bulk heterojunction solar cell. The P3HT concentration was varied from 1 to 5wt% in the host system. P3HT functions as a morphology control agent in this ternary system. A small weight percentage of P3HT can enhance the light absorption, polymer phase separation, exciton separation and charge carrier mobilities. These results are supported by UV–vis spectroscopy, X-ray diffraction, photoluminescence analysis and other characterisation methods. The highest average power conversion efficiency improvement of 10% was achieved by adding 1wt% P3HT to the host system. This study reveals a promising way to achieve high efficiency solar cells using a low bandgap polymer.

Ternary Bulk Heterojunction Photovoltaic Cells Composed of Small Molecule Donor Additive as Cascade Material

To explore the potential of ternary blend bulk heterojunction (BHJ) solar cells as a general platform for improving the performance of organic photovoltaics, we studied a ternary BHJ system based on poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), and DTDCTB. The optimized ternary structure containing a weight ratio of 20% DTDCTB as the cascade material demonstrates a ∼25% improvement of the power conversion efficiency (PCE) as compared to the binary P3HT/PC61BM solar cells. A systematic spectroscopic study is carried out to elucidate the underlying mechanism of charge transfer in the ternary system. Wavelength-dependent external quantum efficiency measurement confirms the contribution of DTDCTB to the enhanced photocurrent. Photoinduced absorption spectroscopy and transient photovoltage measurement reveal unambiguously that charges generated in DTDCTB are efficiently transferred to and subsequently transported in P3HT and PC61BM. The results also suggest that despite th...

Influence of Polymer Compatibility on the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells

The evolution of the open-circuit voltage (Voc) with composition in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of the polymers. Ternary blends based on poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) with phenyl-C61-butyric acid methyl ester (PC61BM) acceptor were investigated. The Voc is pinned to the lower value of the P3HTT-DPP-10%:PC61BM binary blend even up to 95% PCDTBT in the polymer fraction. This is in stark contrast to the previously investigated system based on P3HTT-DPP-10%, poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25), and PC61BM, where the Voc varied regularly across the full composition range, as explained by an organic alloy model, implying strong physical and electronic interaction between the polymers. Photocurrent spectral response (PSR) and external quantum efficiency (EQE) measurements indicate that the present system does not exhibit the hallmarks of alloy formation. Measured values of the surface energies of the polymers support miscibility of P3HTT-DPP-10% with P3HT75-co-EHT25 but not with PCDTBT. Surface energy is proposed as a figure of merit for predicting alloy formation and compositional dependence of the Voc in ternary blend solar cells and miscibility between polymers is proposed as a necessary attribute for polymer pairs that will display alloy behavior.

Ternary bulk heterojunction solar cells: addition of soluble NIR dyes for photocurrent generation beyond 800 nm.

The incorporation of a tert-butyl-functionalized silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) dye molecule as a third component in a ternary blend bulk heterojunction (BHJ) organic solar cell containing P3HT (donor) and PC60BM (acceptor) results in increased NIR absorption. This absorption yields an increase of up to 40% in the short-circuit current and up to 19% in the power conversion efficiency (PCE) in photovoltaic devices. Two-dimensional grazing incidence wide-angle X-ray scattering (2-D GIWAXS) experiments show that compared to the unfunctionalized dye the tert-butyl functionalization enables an increase in the volume fraction of the dye molecule that can be incorporated before the device performance decreases. Quantum efficiency and absorption spectra also indicate that, at dye concentrations above about 8 wt %, there is an approximately 30 nm red shift in the main silicon naphthalocyanine absorption peak, allowing further dye addition to contribute to added photocurrent. This peak shift is not observed in blends with unfunctionalized dye molecules, however. This simple approach of using ternary blends may be generally applicable for use in other unoptimized BHJ systems towards increasing PCEs beyond current levels. Furthermore, this may offer a new approach towards OPVs that absorb NIR photons without having to design, synthesize, and purify complicated donor-acceptor polymers.

Enhancement of ternary blend organic solar cell efficiency using PTB7 as a sensitizer

We demonstrate a significant improvement of power conversion efficiency (PCE) by the addition of the polymer Poly[[4.8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thio-phenediyl]] (PTB7) to a Poly[2,1,3-benzothiadiazole-4,7-diyl [4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]] (PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host system, to form a ternary blend bulk heterojunction solar cell. This is the first reported investigation using PTB7 in a ternary system. The PTB7 concentration was varied from 1 to 9wt% in the host system. In this ternary blend system, PTB7 works as a sensitizer and greatly enhances the optical performance. We demonstrate that positive charge from PCPDTBT is transferred to PTB7, which acts as an efficient charge transport matrix. The addition of 5wt% PTB7 caused the highest improvement of average PCE, 28%, with reference to the binary PCPDTBT:PC71BM cell. The improved device performance is related to both the sensitized response over a large wavelength region and improved charge carrier transport.

Ternary blend bulk heterojunction photovoltaic cells with an ambipolar small molecule as the cascade material

Download options Please wait... Article information DOI https://doi.org/10.1039/C3RA46105J Article type Paper Submitted 29 Aug 2013 Accepted 30 Oct 2013 First published 31 Oct 2013 Download Citation RSC Adv., 2014,4, 1087-1092 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Ternary blend bulk heterojunction photovoltaic cells with an ambipolar small molecule as the cascade material L. Ye, H. Xia, Y. Xiao, J. Xu and Q. Miao, RSC Adv., 2014, 4, 1087 DOI: 10.1039/C3RA46105J To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Lei Ye Hai Xia Yubin Xiao Jianbin Xu Qian Miao

Origin of the Tunable Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Organic Solar Cells

Ternary blend bulk heterojunction organic solar cells comprising either a polythiophene donor and two fullerene acceptors or two polythiophene donors and a fullerene acceptor are shown to have unique electronic properties. Measurements of the photocurrent spectral response and the open-circuit voltage show that the HOMO and LUMO levels change continuously with composition in the respective two-component acceptor or donor pair, consistent with the formation of an organic alloy. However, optical absorption of the exciton states retains the individual molecular properties of the two materials across the blend composition. This difference is attributed to the highly localized molecular nature of the exciton and the more delocalized intermolecular nature of electrons and holes that reflect the average composition of the alloy. As established here, the combination of molecular excitations that can harvest a wide range of photon energies and electronic alloy states that can adjust the open-circuit voltage provides the underlying basis of ternary blends as a platform for highly efficient next-generation organic solar cells.

Compositional Dependence of the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells Based on Two Donor Polymers

Ternary blend bulk heterojunction (BHJ) solar cells containing as donor polymers two P3HT analogues, high-band-gap poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT(75)-co-EHT(25)) and low-band-gap poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), with phenyl-C(61)-butyric acid methyl ester (PC(61)BM) as an acceptor were studied. When the ratio of the three components was varied, the open-circuit voltage (V(oc)) increased as the amount of P3HT(75)-co-EHT(25) increased. The dependence of V(oc) on the polymer composition for the ternary blend regime was linear when the overall polymer:fullerene ratio was optimized for each polymer:polymer ratio. Also, the short-circuit current densities (J(sc)) for the ternary blends were bettter than those of the binary blends because of complementary polymer absorption, as verified using external quantum efficiency measurements. High fill factors (FF) (>0.59) were achieved in all cases and are attributed to high charge-carrier mobilities in the ternary blends. As a result of the intermediate V(oc), increased J(sc) and high FF, the ternary blend BHJ solar cells showed power conversion efficiencies of up to 5.51%, exceeding those of the corresponding binary blends (3.16 and 5.07%). Importantly, this work shows that upon optimization of the overall polymer:fullerene ratio at each polymer:polymer ratio, high FF, regular variations in V(oc), and enhanced J(sc) are possible throughout the ternary blend composition regime. This adds to the growing evidence that the use of ternary blends is a general and effective strategy for producing efficient organic photovoltaics manufactured in a single active-layer processing step.

A study of the factors influencing the performance of ternary MEH-PPV:porphyrin:PCBM heterojunction devices: Electronic effects in porphyrinoid ternary blend bulk heterojunction photovoltaic devices

A series of porphyrins and N-confused porphyrins have been prepared in which the peripheral groups on the porphyrins are kept constant and the porphyrinoids differ only in their electronic nature. The materials have been blended into the active layer of MEH-PPV:PCBM bulk heterojunction solar cells and the performance of the cells is reported and discussed. All of the added porphyrinoids contribute to the photocurrent of the resultant solar cells and result in a broadening of the spectral response of the cells in accordance with the absorption spectra of the porphyrinoid. The efficiency of these devices is shown to correlate strongly with the ionisation potential (IP), and thus highest occupied molecular orbital (HOMO) level, of the added porphyrinoid. We argue that the relative energy of the HOMO levels of the porphyrinoids and the hole transporting polymer in these devices, coupled with the poor charge mobilities of N-confused porphyrins combine to generate porphyrinoid-based hole traps in these devices. This increases recombination within these devices, lowering both the devices' charge densities and open circuit voltages and resulting in reduced cell efficiencies. We show that varying the porphyrinoid added, by metallation or N-alkylation of the N-confused porphyrin, allows us to systematically change the IP of the species and directly affect the power conversion efficiency of the resultant device. The implications of this work for optimising the performance of ternary blend bulk heterojunction solar cells are discussed.