Ternary Bulk Heterojunction Solar Cells Research Articles

Rationalizing the Influence of Tunable Energy Levels on Quantum Efficiency to Design Optimal Non‐Fullerene Acceptor‐Based Ternary Organic Solar Cells

AbstractNon‐fullerene acceptor (NFA)‐based ternary bulk heterojunction solar cells (TSC) are the most efficient organic solar cells (OSCs) today due to their broader absorption and quantum efficiencies (QE) often surpassing those of corresponding binary blends. The impact on QE of the energetics driving charge transfer at the electron donor:electron acceptor (D/A) interfaces is studied in blends of PBDB‐T‐2F donor with several pairs of lower bandgap NFAs. As in binary blends, the ionization energy offset between donor and acceptor (ΔIE) controls the QE and maximizes for ΔIE > 0.5 eV. However, ΔIE is not controlled by the individual NFAs IEs but by their average, weighted for their blending ratio. Using this property, the QE of a PBDB‐T‐2F:IEICO binary blend that has an insufficient ΔIE for charge generation is improved by adding a deep IE third component: IT‐4F. Combining two NFAs enables to optimize the D/A energy alignment and cells’ QE without molecular engineering.

Heat transfer in binary and ternary bulk heterojunction solar cells

Ternary strategy is one of the most commonly used methods to boost the performance of organic solar cells (OSCs) from a binary blend of donor and acceptor. Fullerene derivatives are popular choices for the ternary component as they could benefit the electrical property. However, the ternary component could also affect other physical properties of the bulk-heterojunction (BHJ). Among these properties, heat transfer has rarely been reported, despite its relevance for thermal durability of OSCs. Here, we employ scanning photothermal deflection technique to study thermal diffusion properties of binary PM6:Y7 and ternary PM6:Y7:X BHJs, where X = PC71BM, ICBA, and N2200. It is found that fullerene derivatives deteriorate the thermal diffusivity (D) of blend films and the device thermal durability, despite enhancing the electrical and device performance. In contrast, when an n-type conjugated polymer N2200 is used as the ternary component, both the electrical and thermal properties are enhanced, with improved power conversion efficiency and prolonged device thermal durability. These results offer a perspective on how to choose a favorable third component. Fullerene derivatives are not necessarily the optimal choice for ternary component for BHJ cells because of the inferior thermal properties.

Fluorinated Zinc and Copper Phthalocyanines as Efficient Third Components in Ternary Bulk Heterojunction Solar Cells.

Fluorinated zinc and copper metallophthalocyanines MPcF48 are synthesized and incorporated as third component small molecules in ternary organic solar cells (TOSCs). To enable the high performance of TOSCs, maximizing short-circuit current density (J SC) is crucial. Ternary bulk heterojunction blends, consisting of a polymer donor PTB7-Th, fullerene acceptors PC70BM, and a third component MPcF48, are formulated to fabricate TOSCs with a device architecture of ITO/PFN/active layer/V2O5/Ag. Employing copper as metal atom substitution in the third component of TOSCs enhances J SC as a result of complementary absorption spectra in the near-infrared region. In combination with J SC enhancement, suppressed charge recombination, improved exciton dissociation and charge carrier collection efficiency, and better morphology lead to a slightly improved fill factor (FF), resulting in a 7% enhancement of PCE than those of binary OSCs. In addition to the increased PCE, the photostability of TOSCs has also been improved by the appropriate addition of CuPcF48. Detailed studies imply that metal atom substitution in phthalocyanines is an effective way to improve J SC, FF, and thus the performance and photostability of TOSCs.

Higher Mobility and Carrier Lifetimes in Solution‐Processable Small‐Molecule Ternary Solar Cells with 11% Efficiency

AbstractSolution‐processed small molecule (SM) solar cells have the prospect to outperform their polymer‐fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk‐heterojunction (BHJ) charge carrier mobility (<10‐4 cm2 V‐1 s‐1) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making large‐scale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC71BM can be used to achieve SM‐based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10‐4 cm2 V‐1 s‐1), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC71BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC71BM solar cells demonstrate long‐term shelf stability (dark) for >1000 h, in 55% humidity air environment.

Utilizing Energy Transfer in Binary and Ternary Bulk Heterojunction Organic Solar Cells.

Energy transfer has been identified as an important process in ternary organic solar cells. Here, we develop kinetic Monte Carlo (KMC) models to assess the impact of energy transfer in ternary and binary bulk heterojunction systems. We used fluorescence and absorption spectroscopy to determine the energy disorder and Förster radii for poly(3-hexylthiophene-2,5-diyl), [6,6]-phenyl-C61-butyric acid methyl ester, 4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DIBSq), and poly(2,5-thiophene-alt-4,9-bis(2-hexyldecyl)-4,9-dihydrodithieno[3,2-c:3',2'-h][1,5]naphthyridine-5,10-dione). Heterogeneous energy transfer is found to be crucial in the exciton dissociation process of both binary and ternary organic semiconductor systems. Circumstances favoring energy transfer across interfaces allow relaxation of the electronic energy level requirements, meaning that a cascade structure is not required for efficient ternary organic solar cells. We explain how energy transfer can be exploited to eliminate additional energy losses in ternary bulk heterojunction solar cells, thus increasing their open-circuit voltage without loss in short-circuit current. In particular, we show that it is important that the DIBSq is located at the electron donor-acceptor interface; otherwise charge carriers will be trapped in the DIBSq domain or excitons in the DIBSq domains will not be able to dissociate efficiently at an interface. KMC modeling shows that only small amounts of DIBSq (<5% by weight) are needed to achieve substantial performance improvements due to long-range energy transfer.

Efficient ternary bulk heterojunction solar cells with PCDTBT as hole-cascade material

Ternary bulk hetero-junction (BHJ) architecture for organic solar cells has been developed to enhance the power conversion efficiency (PCE) by expanding the light absorption range and smoothing the energy level at the BHJ interface. In this work, we report on a ternary polymer blend solar cell with two donor materials, polythieno[3,4-b]-thiophene/benzodithiophene (PTB7), poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), and using C71-butyric acid methyl ester (PC71BM) as acceptor material. The resultant device shows an optimized PCE of 7.81% (control sample, 7.06%), with an open circuit voltage (Voc) of 0.76V, a short circuit current (Jsc) of 15.4mA/cm2 and a fill factor (FF) of 66.7%. The improved device performance is mainly attributed to the enhanced charge separation, improved hole mobility, and better film morphology. And we find that the third component PCDTBT can reduce charge recombination in the ternary blend system as well. The results bring new insight into the future development of high efficiency ternary organic solar cells.

Functionalized Graphene as an Electron‐Cascade Acceptor for Air‐Processed Organic Ternary Solar Cells

Functionalized graphene nanoflakes (GNFs) are used as an electron‐cascade acceptor material in air‐processed organic ternary bulk heterojunction solar cells. The functionalization is realized via the attachment of the ethylenedinitrobenzoyl (EDNB) molecule to the GNFs. Simulation and experimental results show that such nanoscale modification greatly influences the density of states near the Fermi level. Consequently, the GNF‐EDNB blend presents favorable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels to function as a bridge structure between the poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and the [6,6]‐phenyl‐C71‐butyric‐acid‐methyl‐ester (PC71BM). The improved exciton dissociation and charge transport are associated with the better energy level alignment of the ternary blend and the high electrical conductivity of the GNFs, which act as additional electron transport channels within the photoactive layer. The resulting PCDTBT/GNF‐EDNB/PC71BM ternary organic solar cells, fabricated entirely under ambient conditions, exhibit an average power conversion efficiency enhancement of ≈18% as compared with the binary blend PCDTBT/PC71BM.

Efficient ternary bulk heterojunction solar cells based on small molecules only

Efficient ternary BHJ solar cells fabricated using small molecules—namely BDT6T, ICBA, and PC71BM—achieving a PCE of 6.36%.

Charge transfer state energy in ternary bulk-heterojunction polymer–fullerene solar cells

In ternary bulk heterojunction solar cells based on a semiconducting biphenyl-dithienyldiketopyrrolopyrrole copolymer donor and two different fullerene acceptors that distinctly differ in electron affinity, the open-circuit voltage is found to depend in a slightly sublinear fashion on the relative ratio of the two fullerenes in the blend. Similar effects have previously been observed and have been attributed to the formation of an alloyed fullerene phase possessing electronic levels that are the weighted average of the two components. By analyzing the contribution of the charge transfer (CT)-state absorption to the external quantum efficiency of the ternary blend solar cells as a function of composition, we find no evidence for a CT state formed between the polymer and an alloyed fullerene phase. Rather, the results are consistent with the presence of two distinct CT states, one for each polymer–fullerene combination. The two-state CT model does not, however, explain the sublinear behavior of the open-circuit voltage as a function of the blend composition.

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.

The improved performance in the ternary bulk heterojunction solar cells

The ternary blend films have been fabricated via adding 4,4′-N,N′-dicarbazole-biphenyl (CBP, a hole transport material widely used in organic light emitting diodes) into the poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT: PCBM). Despite the wide bandgap (3.1 eV) of the CBP, the solar cell utilizing the optimized P3HT:PCBM:CBP blend film showed an increase of 16% in power conversion efficiency and 25% in short-circuit current than the compared standard P3HT:PCBM blend film. This is attributed to the fact that the addition of the CBP could enhance the aggregation of the P3HT chains and thereby reduce the hole-electron recombination at the interface of P3HT and PCBM. We provide a simple, effective way to improve the performance of P3HT based bulk heterojunction solar cells.