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The finale of a trilogy: Comparing terpolymers and ternary blends with structurally similar backbones for use in organic bulk heterojunction solar cells

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Abstract

Building on our previous works that compared the efficacy of terpolymers vs. ternary blends in improving the performance of bulk heterojunction organic solar cells, the final piece of this series of studies focuses on comparing terpolymer and ternary blends constructed with two polymers with structurally similar backbones (monoCNTAZ and FTAZ) yet markedly different open circuit voltage (Voc) values. Terpolymers and ternary blends of five different ratios were studied and the results demonstrate that while the overall performance of both the systems is similar, the ternary blends exhibit higher short circuit current (Jsc) values, while the terpolymers exhibit higher Voc values. Investigation of the charge transfer state using low-energy external quantum efficiency (EQE) indicates that the ternary blends are governed by a parallel-like mechanism, while the terpolymer does not follow this mechanism. The key morphological difference between the systems, as elucidated by resonance soft X-ray scattering (RSoXS), is the slightly smaller size (∼60 nm) of domains in the ternary blends compared to that of the terpolymer (∼80 nm), which may affect exciton harvesting in the terpolymer system and lead to lower Jsc values. In addition, a lower driving force for the formation of charge transfer (CT) state is also likely to contribute to the lower Jsc values in the terpolymer system. All together, the data show that structurally similar (perhaps even miscible) polymers still exhibit key differences in performance when paired in terpolymers vs. ternary blends and allow us to further illuminate the underlying mechanisms of such complex systems.

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... Compared to the widely studied fluorine substitution, the cyano functional group is a stronger electron-withdrawing functional group due to the inductive and resonance effects; thus, adding the cyano substituent to conjugated polymer backbones can decrease the highest occupied molecular orbital (HOMO) energy level of the polymer by a larger degree and shift the absorption of the polymers to a longer wavelength, which benefits the V OC and J SC of the solar cells, respectively. 17,18 Currently, cyano units are widely used in nonfullerene acceptors end groups; however, cyano-substituted polymers with high performance have not been widely reported. 19−23 In our previous study, Li et al. developed a benzotriazole (TAZ)-acceptor-moiety-based polymer with two cyano units on the TAZ unit (CNTAZ, renamed diCNTAZ herein for clarity) 21 and achieved an efficiency of 6.0%. ...
... In order to get a comprehensive understanding of the effect of the cyano substitution on polymers, in this study, we synthesized a series of polymers with varying amounts of cyano groups on the TAZ unit: HTAZ (0), monoCNTAZ 17 (1), and diCNTAZ (2). Along with understanding the optical, electrochemical, and photovoltaic characteristics, we also explored changes in morphology, charge transfer state energy, charge carrier density and lifetime, and nongeminate recombination rate as the number of cyano substituents changes. ...
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... In a recent study, random terpolymers based on fluorinated benzotriazole, cyano benzotriazole and BDT monomers were reported and properties were compared with a ternary blend of two alternating polymers and PC 71 BM (Figure 8; Kelly et al., 2018). The random terpolymers are prepared from CNTAZ and FTAZ in 9:1 P81, 7:3 P82, 1:1 P83, 3:7 P84, and 1:9 P85 feed ratios, respectively. ...
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Mixing different compounds to improve functionality is one of the pillars of the organic electronics field. Here, the degree to which the charge transport properties of the constituent materials are simply additive when materials are mixed is quantified. It is demonstrated that in bulk heterojunction organic solar cells hole mobility in the donor phase depends critically on the choice of the acceptor material, which may alter the energetic disorder of the donor. The same holds for electron mobility and disorder in the acceptor. The associated mobility differences can exceed an order of magnitude compared to pristine materials. Quantifying these effects by a state-filling model for the open-circuit voltage (VOC) of ternary Donor:Acceptor1:Acceptor2 (D:A1:A2) organic solar cells leads to a physically transparent description of the surprising, nearly linear tunability of the VOC with A1:A2 weight ratio. It is predicted that in binary OPV systems, suitably chosen donor and acceptor materials can improve the device power conversion efficiency (PCE) by several percentage points, for example from 11% to 13.5% for a hypothetical state-of-the-art organic solar cell, highlighting the importance of this design rule.
Article
Recent studies demonstrated that with proper selection of chemically compatible constituents, the open-circuit voltage (Voc) of ternary-blend solar cells can be tuned across the composition window of the active layer. In this study, we probed the limit of the offset between the lowest unoccupied molecular orbital (LUMO) energy levels of the two acceptors in ternary blends containing one donor and two acceptors. We demonstrate, for the first time, that ternary-blend active layers with two acceptors having energy-level difference between their LUMO levels exceeding 0.4 eV can still result in solar cells exhibiting composition-dependent open-circuit voltage (Voc). Our results suggest strong electronic interactions between the acceptors, with the electron wavefunction delocalized over multiple molecules. These findings have broadened the library of possible candidates for active layers of ternary-blend solar cells with tunable Voc and established guidelines for the design of next-generation of materials for efficient performance of such devices.
Article
Using small molecule donor (SMD) semiconductors in organic photovoltaics (OPVs) has historically afforded lower power conversion efficiencies (PCEs) than their polymeric counterparts. The PCE difference is attributed to shorter conjugated backbones, resulting in reduced intermolecular interactions. Here, a new pair of SMDs is synthesized based on the diketopyrrolopyrrole–benzodithiophene–diketopyrrolopyrrole (BDT-DPP2) skeleton, but having fluorinated and fluorine-free aromatic side-chain substituents. Ternary OPVs having varied ratios of the two SMDs with PC61BM as the acceptor exhibit tunable open-circuit voltages (Vocs) between 0.833 V and 0.944 V, due to a fluorination-induced shift in energy levels and the electronic “alloy” formed from the miscibility of the two SMDs. A 15% increase in PCE is observed at the optimal ternary SMD ratio, with the short-circuit current density (Jsc) significantly increased to 9.18 mA/cm2. The origin of Jsc enhancement is analyzed via charge generation, transport, and diffuse reflectance measurements, and is attributed to increased optical absorption from a maximum in film crystallinity at this SMD ratio, observed by grazing incidence wide-angle X-ray scattering.
Article
In recent years, ternary solar cells have become an important photovoltaic-technology since they combine the merits of both single junction and tandem solar cells with their processing ease, mechanical flexibility, and lightweight. To date, their efficiency has reached over 12%, which paved the way for commercialization of organic solar cells. In this review, we first present general principles of ternary solar cells, followed by a comprehensive review of recent advances in ternary systems including the D1:D2:A system and D:A1:A2 system. In the end, we summarize the fundamentals and provide a prospect on organic ternary solar cells.
Article
Non-fullerene acceptors (NFAs) are becoming a serious contender to fullerene-based electron acceptors in organic photovoltaics, due to their structural versatility and easily tunable optical and electronic properties. However, NFA-based solar cells often have a decreased short-circuit current (Jsc) and fill factor (FF) compared to their fullerene-based counterparts. Here, we investigate the fundamental causes of this decrease in the performance of solar cells using a non-fullerene acceptor (SF-PDI2) paired with two polymer donors, FTAZ and PyCNTAZ, compared with their fullerene-based counterparts. Through a number of experimental techniques and morphological studies, we show that the SF-PDI2-based solar cells suffer from insufficient charge generation, transport, and collection when compared with the PCBM-based solar cells. The SF-PDI2-based solar cells show increased bimolecular recombination, which, together with other recombination loss mechanisms in these cells, causes a significant decrease in their Jsc and FF. Notably, the less pure domains, low electron mobility (on the order of 10⁻⁵ cm² V⁻¹ s⁻¹), and imbalanced mobility (in regard to the hole mobility) further explain the low FF. On the other hand, the higher open-circuit voltage (Voc) in the SF-PDI2 devices is mainly due to the increase in the CT state energy. It is worth mentioning that the PyCNTAZ-based devices show an ultralow charge separation energy (ΔECS), close to 0 eV. Our results demonstrate that further increasing the mobility (both of electrons and holes) in these NFA-based solar cells would be a viable approach to further enhance the efficiency of these new types of solar cells, ideally, without losing the high Voc of such cells.
Article
We present a novel ternary organic solar cell with uncommonly thick active layer (>300 nm), featuring thickness invariant charge carrier recombination and delivering 11% power conversion efficiency (PCE). The ternary blend was used to demonstrate photovoltaic modules of high technological relevance both on glass and flexible substrates, yielding 8.2% and 6.8% PCE, respectively.
Article
Fluorinating conjugated polymers is a proven strategy for creating high performance materials in polymer solar cells, yet few studies investigated the importance of the fluorination method. We compare the performance of three fluorinated systems: a poly(benzodithieno-dithienyltriazole) (PBnDT-XTAZ) random copolymer where 50% of the acceptor units are difluorinated, PBnDT-mFTAZ where every acceptor unit is monofluorinated, and a 1:1 physical blend of the difluorinated and nonfluorinated polymer. All systems have the same degree of fluorination (50%), yet via different methods (chemically vs. physically, random vs. regular). We show that these three systems have equivalent photovoltaic behavior: ~5.2% efficiency with a short circuit current (Jsc) at ~11 mA cm-2, an open circuit voltage (Voc) at 0.77 V, and a fill factor (FF) of ~60%. Further investigation of these three systems demonstrate that the charge generation, charge extraction, and charge transfer state are essentially identical for the three studied systems. Transmission electron microscopy shows no significant differences in the morphologies. All these data illustrate that not only is it possible to improve performance via regular or random fluorination, but also by physical addition via a ternary blend. Thus, our results demonstrate the versatility of incorporating fluorine in the active layer of polymer solar cells to enhance device performance.
Article
Ternary organic solar cells (OSCs), with a simple structure, can be easily adopted as sub‐cells in a tandem design, thereby further enhancing the power conversion efficiency (PCE). Considering the potential to surpass the theoretical PCE limit in OSCs, we incorporated a benzo[1,2‐b;4,5‐b']dithiophene‐based small molecule into a poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b']dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)):[6,6]‐phenyl‐C71‐butyric acid methyl ester host system. A hitherto unrealized PCE of 12.1% was achieved at the optimized composition of the ternary blend. The ternary blend surprisingly had a face‐on and edge‐on coexistent texture, which is far better than that of the face‐on orientated host film. To the best of our knowledge, this intriguing result refutes for the first time a general paradigm that high‐performance OSCs are unambiguously linked to face‐on structures. Therefore, our study provides a new platform for refining the theoretical underpinning of multiple blending OSCs.
Article
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)ethyl)thiophen-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, 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.
Article
Highly efficient electron extraction is achieved by using photoconductive cathode interlayer in inverted ternary organic solar cells (OSCs) where a near-IR absorbing porphyrin molecule is used as the sensitizer. The OSCs show improved device performance when the ratio of the two donors varies in a large region and the maximum power conversion efficiency up to 11.03% is demonstrated.
Article
This study demonstrates high-performance, ternary-blend polymer solar cells by modifying a binary blend bulk heterojunction (PPDT2FBT:PC71BM) with the addition of a ternary component, PPDT2CNBT. PPDT2CNBT is designed to have complementary absorption and deeper frontier energy levels compared to PPDT2FBT, while being based on the same polymeric backbone. A power conversion efficiency of 9.46% is achieved via improvements in both short-circuit current density (JSC) and open-circuit voltage (VOC). Interestingly, the VOC increases with increasing the PPDT2CNBT content in ternary blends. In-depth studies using ultraviolet photoelectron spectroscopy and transient absorption spectroscopy indicate that the two polymers are not electronically homogeneous and function as discrete light harvesting species. The structural similarity between PPDT2CNBT and PPDT2FBT allows the merits of a ternary system to be fully utilized to enhance both JSC and VOC without detriment to fill-factor via minimized disruption of semi-crystalline morphology of binary PPDT2FBT:PC71BM blend. Further, by careful analysis, charge carrier transport in this ternary blend is clearly verified to follow parallel-like behavior.
Article
A regularly alternating terpolymer and a random terpolymer were synthesized from the constituent units of two donor–acceptor polymers with complementary absorption. They were then compared to a physical blend of these two donor–acceptor polymers in order to investigate the best means of extending the light absorption range in bulk heterojunction (BHJ) solar cells. While all three methods broadened the light absorption, the physical blend provided the best improvement in power conversion efficiency (4.10% vs 3.63% and 2.67% for the random and regular terpolymers, respectively). This is due to the increase in aggregation in the physical blend, as demonstrated in the UV–vis spectra, which likely leads to higher local mobility and less recombination. This study shows that in order to effectively increase the light absorption (and therefore performance) of a polymer:fullerene based BHJ solar cell, a terpolymer must retain a structure which allows sufficient aggregation.
Article
Organic bulk heterojunction solar cells based on ternary blends of two donor absorbers and one acceptor are investigated by experiments and modeling. The commonly observed continuous tunability of the open circuit voltage VOC with the donor1 : donor2 ratio can quantitatively be explained as quasi- Fermi level splitting due to photocreated charges filling a joint density of states that is broadened by Gaussian disorder. On this basis, a predictive model for the power conversion efficiency that accounts for the composition-dependent absorption and the shape of the current–voltage characteristic curve is developed. When all other parameters, most notably the fill factor, are constant, we find that for stateof- the-art absorbers, having a broad and strong absorption spectrum, ternary blends offer no advantage over binary ones. For absorbers with a more narrow absorption spectrum ternary blends of donors with complementary absorption spectra, offer modest improvements over binary ones. In contrast, when, upon blending, transport and/or recombination kinetics are improved, leading to an increased fill factor, ternaries may offer significant advantages over binaries.
Article
Triazole based structural units have been widely used to construct conjugated polymers for optoelectronic applications; yet the design and synthesis of such units have been limited to just a few known examples. We report a general yet versatile synthetic approach toward a diverse set of triazole based conjugated molecules bearing various electron accepting abilities. The structural differences of as-synthesized three new triazole acceptors have a significant impact on the optoelectronic properties of conjugated polymers incorporating these triazoles. Bulk heterojunction solar cells based on one of these new polymers, PyCNTAZ, feature a high open circuit voltage of ∼1 V and a notable efficiency of 8.4% with an active layer thickness around 300 nm.
Article
Ternary-blend bulk-heterojunction solar cells have provided a unique opportunity for tuning the open-circuit voltage (Voc) as the “effective” highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels shift with active-layer composition. Grazing-incidence X-ray diffraction (GIXD) measurements performed on such ternary-blend thin films reveal evidence that the two polymer donors interact intimately; their ionization potentials are thus reflections of the blend compositions. In ternary-blend thin films in which the two polymer donors do not interact physically, the polymer donors each retain their molecular electronic character; solar cells constructed with these ternary blends thus exhibit Vocs that are pinned to the energy level difference between the highest of the two lying HOMO and the LUMO of the electron acceptor. These observations are consistent with the organic alloy model proposed earlier. Quantification of the square of the square-root differences of the surface energies of the components provides a proxy for the Flory–Huggins interaction parameter for polymer donor pairs in these ternary-blend systems. Of the three ternary-blend systems examined herein, this quantity has to be below 0.094 in order for ternary-blend solar cells to exhibit tunable Voc.
Article
The use ternary organic components is currently being pursued to enhance the power conversion efficiency of bulk heterojunction solar cells by expanding the spectral range of light absorption. Here, we report a ternary blend polymer solar cell containing two donor polymers, poly-3-oxothieno[3,4-d]isothiazole-1,1-dioxide/benzodithiophene (PID2), polythieno[3,4-b]-thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as an acceptor. The resulting ternary solar cell delivered a power conversion efficiency of 822% with a short-circuit current density J(sc) of 16.8 mA cm(-2), an open-circuit voltage V-oc of 0.72 V and a fill factor of 68.7%. In addition to extended light absorption, we show that J(sc) is improved through improved charge separation and transport and decreased charge recombination, resulting from the cascade energy levels and optimized device morphology of the ternary system. This work indicates that ternary blend solar cells have the potential to surpass high-performance binary polymer solar cells after further device engineering and optimization.
Article
Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.
Article
Ternary organic photovoltaic donor:acceptor blend active materials composed of three distinct species possess remarkable advantages over neat semiconductors and binary donor:acceptor blends. A blended semiconductor is a foreign concept for inorganic semiconductors, whereas electronically disparate organic semiconductors can be mixed while mutually enhancing the properties of each. This feature allows ternary semiconductors to realize many of the advantages of tandem solar cells in a single layer.
Article
Device performance is recognized to be generally sensitive to morphology in bulk heterojunction solar cells. Through the use of quantitative morphological measurements, it is demonstrated that devices based on benzodithiophene and fluorinated benzotriazole moieties constitute an exception to this design rule and exhibit a range of morphologies that yield similar high performance. In particular, the fill factor (FF) remains above 65% even with factor of two changes in domain size and factor of two changes in relative domain purity. Devices with active layer thicknesses of 250 nm are employed, which are capable of increasing optical absorption to produce high photocurrent. The general insensitivity to both morphology and thickness is likely related to the measured low equilibrium miscibility of fullerene in the polymer of 3-4%. The materials and processes investigated therefore provide insights into functional material design that yield increased processing latitude and may be more amenable to roll-to-roll processing.
Article
Polymer:fullerene solar cells are demonstrated with power conversion efficiencies over 7% with blends of PBDTTPD and PC61BM. These devices achieve open-circuit voltages (Voc) of 0.945 V and internal quantum efficiencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. Voc’s above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fill factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage-dependent, steady state and time-resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is inefficient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of –0.14 ± 0.06 eV is required for efficient hole transfer. These results suggest that the driving force for hole transfer may be too small for efficient current generation in polymer:fullerene solar cells with Voc values above 1.0 V and that non-fullerene acceptor materials with large optical gaps (>1.7 eV) may be required to achieve both near unity internal quantum efficiencies and values of Voc exceeding 1.0 V.
Article
Ternary solar cells enjoy both an increased light absorption width, and an easy fabrication process associated with their simple structures. Significant progress has been made for such solar cells with demonstrated efficiencies over 7%; however, their fundamental working principles are still under investigation. This Perspective is intended to offer our insights on the three major governing mechanisms in these intriguing ternary solar cells: charge transfer, energy transfer, and parallel-linkage. Through careful analysis of exemplary cases, we summarize the advantages and limitations of these three major mechanisms and suggest future research directions. For example, incorporating additional singlet fission or upconversion materials into the energy transfer dominant ternary solar cells has the potential to break the theoretical efficiency limit in single junction organic solar cells. Clearly, a feedback loop between fundamental understanding and materials selection is in urgent need to accelerate the efficiency improvement of these ternary solar cells.
Article
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.
Article
Recently, researchers have paid a great deal of attention to the research and development of organic solar cells, leading to a breakthrough of over 10% power conversion efficiency. Though impressive, further development is required to ensure a bright industrial future for organic photovoltaics. Relatively narrow spectral overlap of organic polymer absorption bands within the solar spectrum is one of the major limitations of organic solar cells. Among different strategies that are in progress to tackle this restriction, the novel concept of ternary organic solar cells is a promising candidate to extend the absorption spectra of large bandgap polymers to the near IR region and to enhance light harvesting in single bulk-heterojunction solar cells. In this contribution, we review the recent developments in organic ternary solar cell research based on various types of sensitizers. In addition, the aspects of miscibility, morphology complexity, charge transfer dynamics as well as carrier transport in ternary organic composites are addressed.
Article
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 provide the underlying basis of ternary blends as a platform for highly efficient next generation organic solar cells.
Article
Here we demonstrate a conceptually new approach, the parallel-like bulk heterojunction (PBHJ), which maintains the simple device configuration and low-cost processing of single-junction BHJ cells while inheriting the major benefit of incorporating multiple polymers in tandem cells. In this PBHJ, free charge carriers travel through their corresponding donor-polymer-linked channels and fullerene-enriched domain to the electrodes, equivalent to a parallel-like connection. The short-circuit current (J(sc)) of the PBHJ solar cell is nearly identical to the sum of those of the individual "subcells", while the open-circuit voltage (V(oc)) is between those of the "subcells". Preliminary optimization of the PBHJ devices gives improvements of up to 40% in J(sc) and 30% in overall efficiency (η) in comparison with single-junction BHJ devices.
Article
Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.
Article
The development of predictive models relating materials' structures to photovoltaic device performance is crucial for the optimization of organic solar cells. Several factors can favor dissociation of interfacial charge-transfer states, including a large overall free energy loss driving charge separation, large domain sizes, and strong macroscopic electric fields. However, in all these cases, modulating these parameters to enhance charge photogeneration may have a negative impact upon other aspects of device performance. Similarly, increasing the domain size to favor CT-state dissociation can reduce the efficiency of exciton diffusion to the interface, while increasing the internal electric field by reducing the photoactive layer thickness would reduce light absorption. Following Onsager theory, charge photogeneration should be favored by reducing the Coulomb binding energy of the CT states. The introduction of insulator layers at the charge separation interface and the use of redox relays both enhance the performance of dye-sensitized solar cells.