A paradigmatic change: linking fullerenes to electron acceptors

The potential of Lu(3)N@C(80) and its analogues as electron acceptors in the areas of photovoltaics and artificial photosynthesis is tremendous. To this date, their electron-donating properties have never been explored, despite the facile oxidations that they reveal when compared to those of C(60). Herein, we report on the synthesis and physicochemical studies of a covalently linked Lu(3)N@C(80)-perylenebisimide (PDI) conjugate, in which PDI acts as the light harvester and the electron acceptor. Most important is the unambiguous evidence--in terms of spectroscopy and kinetics--that corroborates a photoinduced electron transfer evolving from the ground state of Lu(3)N@C(80) to the singlet excited state of PDI. In stark contrast, the photoreactivity of a C(60)-PDI conjugate is exclusively governed by a cascade of energy-transfer processes. Also, the electron-donating property of the Lu(3)N@C(80) moiety was confirmed through constructing and testing a bilayer heterojunction solar cell device with a PDI and Lu(3)N@C(80) derivative as electron acceptor and electron donor, respectively. In particular, a positive photovoltage of 0.46 V and a negative short circuit current density of 0.38 mA are observed with PDI/Ca as anode and ITO/Lu(3)N@C(80) as cathode. Although the devices were not optimized, the sign of the V(OC) and the flow direction of J(SC) clearly underline the unique oxidative role of Lu(3)N@C(80) within electron donor-acceptor conjugates toward the construction of novel optoelectronic devices.     

Structural tuning of low band gap intermolecular push/pull side-chain polymers for organic photovoltaic applications

A series of novel low band gap donor-acceptor (D-A) type organic co-polymers (BT-F-TPA,BT-CZ-TPA and BT-SI-TPA) consisting of electron-deficient acceptor blocks both in main chains (M1) and at the pendant (M2) were polymerized with different electron rich donor (M3-M5) blocks,i.e.,9,9-dihexyl-9H-fluorene,N-alkyl-2,7-carbazole,and 2,6-dithinosilole,respectively,via Suzuki method.These polymers exhibited relatively low band gaps (1.65-1.88 eV) and broad absorption ranges (680-740 nm).Bulk heterojunction (BHJ) solar cells incorporating these polymers as electron donors,blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron-acceptors in different weight ratios were fabricated and tested under 100 mW/cm2 of AM 1.5 with white-light illumination.The photovoltaic device containing donor BT-SI-TPA and acceptor PC71BM in 1:2 weight ratio showed the best power conversion efficiency (PCE) value of 1.88％,with open circuit voltage (Voc) =0.75 V,short circuit current density (Jsc) =7.60 mA/cm2,and fill factor (FF) =33.0％.     

Efficient ternary blend bulk heterojunction solar cells with tunable open-circuit voltage

To explore the potential of ternary blend bulk heterojunction (BHJ) photovoltaics as a general platform for increasing the attainable performance of organic solar cells, a model system based on poly(3-hexylthiophene) (P3HT) as the donor and two soluble fullerene acceptors, phenyl-C(61)-butyric acid methyl ester (PC(61)BM) and indene-C(60) bisadduct (ICBA), was examined. In all of the solar cells, the overall ratio of polymer to fullerene was maintained at 1:1, while the composition of the fullerene component (PC(61)BM:ICBA ratio) was varied. Photovoltaic devices showed high short-circuit current densities (J(sc)) and fill factors (FF) (>0.57) at all fullerene ratios, while the open-circuit voltage (V(oc)) was found to vary from 0.61 to 0.84 V as the fraction of ICBA was increased. These results indicate that the V(oc) in ternary blend BHJ solar cells is not limited to the smallest V(oc) of the corresponding binary blend solar cells but can be varied between the extreme V(oc) values without significant effect on the J(sc) or FF. By extension, this result suggests that ternary blends provide a potentially effective route toward maximizing the attainable J(sc)V(oc) product (which is directly proportional to the solar cell efficiency) in BHJ solar cells and that with judicious selection of donor and acceptor components, solar cells with efficiencies exceeding the theoretical limits for binary blend solar cells could be possible without sacrificing the simplicity of a single active-layer processing step.     

MEH-PPV和PCBM共混体系光电池中电荷传输及性能与光强的关系

以MEH-PPV{poly[2-methoxy-5-(2'-ethylhexoxy)]-1,4-phenylene vinylene}作为电子给体材料, PCBM[1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61]作为电子受体材料, 制成了共混体系的高性能太阳电池. 光电池在100 mW/cm2强度光照下, 其开路电压Voc为0.8 V, 短路电流密度Jsc为5.06 mA/cm2, 填充因子FF为48.1%, 能量转换效率η为1.93%. UV-Vis及PL图表明, MEH-PPV与PCBM之间没有发生化学变化, 但有明显的荧光猝灭, 说明光生激子能有效地快速分离, 并在各自的传输网络中传递. 分析了光照及暗导I-V曲线的物理意义, 探讨了MEH-PPV与PCBM之间的电荷传输, 研究了在不同强度的光照下器件性能的变化. 随着光强的增加, 器件的短路电流密度线性增大, 开路电压也略有升高, 并联电阻和填充因子下降, 串联电阻变化不明显. 分析了其物理机理, 并进行了合理的解释.     

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.     

Charge recombination in organic photovoltaic devices with high open-circuit voltages

A detailed charge recombination mechanism is presented for organic photovoltaic devices with a high open-circuit voltage. In a binary blend comprised of polyfluorene copolymers, the performance-limiting process is found to be the efficient recombination of tightly bound charge pairs into neutral triplet excitons. We arrive at this conclusion using optical transient absorption (TA) spectroscopy with visible and IR probes and over seven decades of time resolution. By resolving the polarization of the TA signal, we track the movement of polaronic states generated at the heterojunction not only in time but also in space. It is found that the photogenerated charge pairs are remarkably immobile at the heterojunction during their lifetime. The charge pairs are shown to be subject to efficient intersystem crossing and terminally recombine into F8BT triplet excitons within approximately 40 ns. Long-range charge separation competes rather unfavorably with intersystem crossing--75% of all charge pairs decay into triplet excitons. Triplet exciton states are thermodynamically accessible in polymer solar cells with high open circuit voltage, and we therefore suggest this loss mechanism to be general. We discuss guidelines for the design of the next generation of organic photovoltaic materials where separating the metastable interfacial charge pairs within approximately 40 ns is paramount.     

Large open-circuit voltage polymer solar cells by poly(3-hexylthiophene) with multi-adducts fullerenes

Polymer solar cells (PSCs) made by poly(3-hexylthiophene) (P3HT) with multi-adducts fullerenes, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), PC61BM-bisadduct (bisPC61BM) and PC61BM-trisadduct (trisPC61BM), were reported. Electrochemistry studies indicated that PC61BM, bisPC61BM and trisPC61BM had step-up distributional lowest unoccupied molecular orbital (LUMO) energy. PSCs made by P3HT with above PC61BMs show a trend of enlarged open-circuit voltages, which is in good agreement with the energy difference between the LUMO of PC61BMs and the HOMO of P3HT. On the contrary, reduced short-circuit currents (Jsc) were observed. The investigation of photo responsibility, dynamics analysis based on photo-induced absorption of composite films, P3HT:PC61BMs and n-channel thin film field-effect transistors of PC61BMs suggested that the short polaron lifetimes and low carrier mobilities were response for reduced Jsc. All these results demonstrated that it was important to develop an electron acceptor which has both high carrier mobility, and good compatibility with the electron donor conjugated polymer for approaching high performance PSCs.     

Fullerenes and derivatives as electron transport materials in perovskite solar cells

In this study, two fullerenes (C60, C70) and their methano-substitutions (PC61BM, PC71BM), as electron transport materials (ETMs) in perovskite solar cells (Pero-SCs), were systematically studied. As being used as ETMs, methanofullerenes, though with lower electron mobility compared to the counterpart pristine fullerenes, lead to higher power conversion efficiencies (PCEs) of Pero-SCs. The difference is likely caused by the fill-out vacancies and smoother morphology of the interfaces between ETM and perovskite layers, as they were prepared by different methods. In addition, compared to C60 and PC61BM, C70 and PC71BM showed priority in terms of short-circuit current density, which should be attributed to fast free charge extraction abilities.     

Tetrathienoanthracene-based copolymers for efficient solar cells

A series of semiconducting copolymers (PTAT-x) containing extended π-conjugated tetrathienoanthracene units have been synthesized. It was shown that the extended conjugation system enhanced the π-π stacking in the polymer/PC(61)BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC(61)BM.     

Low-bandgap poly(thiophene-phenylene-thiophene) derivatives with broaden absorption spectra for use in high-performance bulk-heterojunction polymer solar cells

Two low-bandgap (LGB) conjugated polymers ( P1 and P2) based on thiophene-phenylene-thiophene (TPT) with adequate energy levels have been designed and synthesized for application in bulk-heterojunction polymer solar cells (PSCs). The absorption spectral, electrochemical, field effect hole mobility and photovoltaic properties of LGB TPT derivatives are investigated and compared with poly(3-hexylthiophene) (P3HT). Photophysical studies reveal bandgaps of 1.76 eV for P1 and 1.70 eV for P2, which could effectively harvest broader solar spectrum. In addition, the thin film absorption coefficients of P1 and P2 are 1.6 x 10 (5) cm (-1) (lambda approximately 520 nm) and 1.4 x 10 (5) cm (-1) (lambda approximately 590 nm), respectively. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open circuit voltage while blending them with fullerene derivatives as electron acceptors. Furthermore, both materials show sufficient hole mobility (3.4 x 10 (-3) cm (2)/Vs for P2) allowing efficient charge extraction and a good fill-factor for PSC application. High-performance power conversion efficiency (PCE) of 4.4% is obtained under simulated solar light AM 1.5 G (100 mW/cm (2)) from PSC device with an active layer containing 25 wt% P2 and 75 wt% [6,6]-phenyl-C71-butyric acid methyl ester (PC 71BM), which is superior to that of the analogous P3HT cell (3.9%) under the same experimental condition.     

以C_(70)衍生物为电子受体的高效聚合物固体薄膜太阳能电池

以PC[70]BM(phenyl C71-butyric acid methyl ester)取代PC[60]BM(phenyl C61-butyric acid methyl ester)作为电子受体材料,以MEH-PPV(poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene])为电子给体材料,制成了本体异质结(bulk heterojunction,BHJ)聚合物太阳能电池.MEH-PPV/PC[70]BM器件在AM1.5G(80 mW/cm2)模拟太阳光的光照条件下得到了3.42%的能量转换效率,短路电流值达到了6.07 mA/cm2,开路电压0.85 V,填充因子为53%.通过紫外可见吸收光谱和外量子效率的研究,发现PC[70]BM作为电子受体,对扩大光谱的吸收范围和增加活性层的吸收系数有明显的作用.同时比较了不同溶剂对该体系器件性能的影响.通过原子力显微镜(AFM)、光暗导I-V曲线等研究,分析了1,2-二氯苯有利于给体相和受体相的微相分离和载流子的传输的原因.     

Dihydronaphthyl-based [60]fullerene bisadducts for efficient and stable polymer solar cells

Dihydronaphthyl-based [60]fullerene bisadduct derivative, NC(60)BA, was synthesized at mild temperature in high yield. NC(60)BA not only possesses a LUMO energy level 0.16 eV higher than PC(61)BM but also has amorphous nature that can overcome thermal-driven crystallization. The fabricated P3HT:NC(60)BA-based polymer solar cells exhibit superior photovoltaic performance and thermal stability compared to PC(61)BM-based devices under the same conditions.     

Dithienogermole as a fused electron donor in bulk heterojunction solar cells

We report the synthesis and bulk heterojunction photovoltaic performance of the first dithienogermole (DTG)-containing conjugated polymer. Stille polycondensation of a distannyl-DTG derivative with 1,3-dibromo-N-octyl-thienopyrrolodione (TPD) results in an alternating copolymer which displays light absorption extending to 735 nm, and a higher HOMO level than the analogous copolymer containing the commonly utilized dithienosilole (DTS) heterocycle. When polyDTG-TPD:PC(70)BM blends are utilized in inverted bulk heterojunction solar cells, the cells display average power conversion efficiencies of 7.3%, compared to 6.6% for the DTS-containing cells prepared in parallel under identical conditions. The performance enhancement is a result of a higher short-circuit current and fill factor in the DTG-containing cells, which comes at the cost of a slightly lower open circuit voltage than for the DTS-based cells.     

Conjugated polymers based on dicarboxylic imide‐substituted isothianaphthene and their applications in solar cells

Four new polymers containing a benzo[c]thiophene‐N‐dodecyl‐4,5‐dicarboxylic imide (DIITN) unit including the homopolymer and three donor–acceptor copolymers were designed, synthesized, and characterized. For these copolymers, DIITN unit with low bandgap was selected as an electron acceptor, whereas 5,5′‐(2,7‐bisthiophen‐2‐yl)‐9‐(2‐decyltetradecyl)‐9H‐carbazole), 5,5′‐(3,3′‐di‐n‐octylsilylene‐2,2′‐bithiophene), and 5,5′‐(2,7‐bisthiophen‐2‐yl‐9,9‐bisoctyl‐9H‐fluoren‐7‐yl) were chosen as the electron donor units to tune the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of the copolymers for better light harvesting. These polymers exhibit extended absorption in the visible and near‐infrared range and are soluble in common organic solvents. The relative low lying HOMO of these polymers promises good air stability and high open‐circuit voltage (V_oc) for photovoltaic application. Bulk heterojunction solar cells were fabricated by blending the copolymers with [6,6]‐phenyl‐C61‐butyric acid methyl ester or [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM). The best power conversion efficiency of 1.6% was achieved under simulated sunlight AM 1.5G (100 mW/cm²) from solar cells containing 20 wt % of the fluorene copolymer poly[5,5′‐(2,7‐bisthiophen‐2‐yl‐9,9‐bisoctyl‐9H‐fluoren‐7‐yl)‐alt‐2,9‐(benzo[c]thiophene‐N‐dodecyl‐4,5‐dicarboxylic imide)] and 80 wt % of PC71BM with a high open‐circuit voltage (V_oc) of 0.84 V. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Charge Carrier Dynamics in PDPP-F/PCBM Heterojunction Solar Cells

The morphology dependent of migration and recombination kinetics of charge carriers in the polymer solar cell based on poly {2,7'-9,9-dioctylfluorene-alt-5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione}(PDPP-F) was investigated with photo-induced charge carrier extraction by linearly increasing voltage technique. The recombination coefficient of charge carriers decreased and the mobility of charge carriers increased, after the mass ratio of[6,6]-phenyl-C61-butyric acid methyl ester(PCBM) increased from 3:2 to 2:1. Meanwhile, both of them were sensitive to the applied electric field and could be together responsible for the improvement of voltage performance of polymer solar cell.     

Novel conjugated copolymers based on dithiafulvalene moiety for bulk heterojunction solar cells

New conjugated copolymers, P1‐P3 , based on dithiafulvalene‐fused entity and different conjugated segments have been synthesized. Incorporation of electron‐deficient conjugated segments into the conjugated copolymers results in red shifting the absorption band and lowering the hole mobility. Bulk heterojunction solar cells using on these polymers as the donor and [6,6]‐phenyl‐C61 ‐butyric acid methyl ester (PC₆₁BM) as the acceptor were fabricated by solution process. The cells based on the blend of P1‐P3 /PC₆₁BM (1:1, w/w) have power conversion efficiencies (PCEs) ranging from 0.53 to 0.93%. Among these, the cell of P1 /PC₆₁BM exhibited the highest open‐circuit voltage at 0.85 V, and the cell of P3/PC₆₁BM exhibited the best PCE at 0.93% with the short‐circuit current (J_SC) of 4.88 mA/cm². © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells

A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC(70)BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC(70)BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.     

The phase behavior of a polymer‐fullerene bulk heterojunction system that contains bimolecular crystals

Polymer:fullerene blends have been widely studied as an inexpensive alternative to traditional silicon solar cells. Some polymer:fullerene blends, such as blends of poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene (pBTTT) with phenyl‐c71‐butyric acid methyl ester (PC₇₁BM), form bimolecular crystals due to fullerene intercalation between the polymer side chains. Here we present the determination of the eutectic pBTTT:PC₇₁BM phase diagram using differential scanning calorimetry (DSC) and two‐dimensional grazing incidence X‐ray scattering (2D GIXS) with in‐situ thermal annealing. The phase diagram explains why the most efficient pBTTT:PC₇₁BM solar cells have 75–80 wt % PC₇₁BM since these blends lie in the center of the only room‐temperature phase region containing both electron‐conducting (PC₇₁BM) and hole‐conducting (bimolecular crystal) phases. We show that intercalation can be suppressed in 50:50 pBTTT:PC₇₁BM blends by using rapid thermal annealing to heat the blends above the eutectic temperature, which forces PC₇₁BM out of the bimolecular crystal, followed by quick cooling to kinetically trap the pure PC₇₁BM phase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Regioregular copolymers of 3-alkoxythiophene and their photovoltaic application

Low band gap conjugated polymers with proper energy levels for charge transfer are required to achieve high-efficiency polymer solar cells. We report the synthesis and characterization of two new regioregular copolymers that are based on 3-alkoxythiophene monomers: poly(3-octylthiophene-2,5-diyl-co-3-decyloxythiophene-2,5-diyl) (POT-co-DOT) and poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5',5' '-diyl} (PF-co-DTB). Compared to the alkyl substituents, the alkoxy side chains on the thiophene units can effectively lower the band gap of copolymers and enhance the charge transfer to electron acceptors such as (6,6)-phenyl C(61)-butyric acid methyl ester (PCBM). The chemical structure and regioregularity of the copolymers were confirmed by NMR. Both copolymers are readily soluble in organic solvents and form high-quality thin films. Electrochemical and photophysical studies reveal band gaps of 1.64 eV for POT-co-DOT and 1.78 eV for PF-co-DTB. Bulk heterojunction photovoltaic devices were fabricated using blends of these copolymers with PCBM as the active layer, ITO-glass as the anode, and aluminum as the cathode. Power conversion efficiency of 1.6% was obtained under simulated solar light AM 1.5 G (100 mW/cm(2)) from a solar cell with an active layer containing 20 wt % PF-co-DTB and 80 wt % PCBM. Regioregular poly(3-decyloxythiophene-2,5-diyl) (P3DOT) was also studied for comparison purposes.     

Development of naphtho[1,2-b:5,6-b']dithiophene based novel small molecules for efficient bulk-heterojunction organic solar cells

Two new small molecules with a rigid planar naphtho[1,2-b:5,6-b']dithiophene (NDT) unit were designed and synthesized. Solution processed bulk-hetereojunction organic solar cells based on blends of the small molecules and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) exhibited promising photovoltaic device performance with a maximum power conversion efficiency up to 2.20% under the illumination of AM 1.5G, 100 mW cm(-2).