Morphology-dependent charge recombination through localized states in polymer/polymer blend solar cells

Polymer solar cells (PSCs) based on the PBDTTT-EFT (polymer donor):N2200 (polymer acceptor) blend have been fabricated with the active layer processed with various solvents including o-dichlorobenzene (o-DCB), chlorobenzene (CB), chloroform (CF), and CF:p-xylene (PX) mixed solvent. A highest power conversion efficiency (PCE) of 5.37% is achieved with the active layer processed with CF:PX mixed solvent. The dependence of short-circuit current density (J_SC) on incident light intensity indicates that the charge recombination is to some extent suppressed in the device with the active layer processed with the CF:PX mixed solvent and results in an enhanced J_SC. Morphology studies disclosed that the domains are preferential face-on orientation in the active layers processed with o-DCB, CB and CF, while it shows a combined face-on and edge-on orientation in CF:PX-processed film. The long-lived trap-assisted charge recombination originated from the active layer morphological variance has been focused on and investigated. And the nanosecond transient absorption experiment further demonstrated that the PBDTTT-EFT:N2200 film processed with o-DCB shows obvious long-lived trap-assisted charge recombination, while the trap-assisted charge recombination is effectively suppressed in the PBDTTT-EFT:N2200 film processed with CF:PX mixed solvent. This implies that deep localized traps correlated with the PBDTTT-EFT:N2200 morphology are reduced by using the CF:PX mixed solvent to process the active layer. In addition, the domains with a mixed orientation in the active layer may also enhance the three-dimensional charge transport and hence improve the charge-collection efficiency.     

有机体异质结太阳能电池的数值模拟

引入了一种可以直观展现出聚合物体异质结太阳能电池的伏安特性的数值模型,该模型是由双分子复合以及温度和电场效应下自由载流子产生的机制。这个在聚合物材料中的到很好的体现,该材料中空间电荷效应只起到微弱的作用,从而在模型中形成相对恒定的电场。此外,在短路条件下只有7%的自由载流子由于双分子复合而消失。该模型证明在PPV/PCBM太阳能电池中随着空穴迁移率的增加和减少受体0.5电子伏特将得到5.5%最高转换效率。     

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 5 wt% 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 1 wt% P3HT to the host system. This study reveals a promising way to achieve high efficiency solar cells using a low bandgap polymer.     

Influences of charge of conjugated polymer electrolytes cathode interlayer for bulk-heterojunction polymer solar cells

Two fluorene-based conjugated polymer electrolyte (CPE) poly[(9,9-bis(6′-(N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNBr) and poly[9,9-bis(4′-sulfonatobutyl)fluorene-alt-2,7-(9,9-dioctylfluorene)] sodium salt (PFSO₃Na), bearing amine groups and anionic sulfonate groups on side chains respectively, are synthesized and applied as cathode interlayer in polymer solar cells. Both of the hydrophilic CPEs can well modify the interfacial properties and allow ohomic contact between the activelayer and cathode. The opposite charges exert great influence on the effective work function of cathode and interfacial interaction through the orientation of the interfacial dipole at the active layer/metal electrode interface, subsequently influence the resulting device performance. Compared with the cationic PFNBr, PFSO₃Na with anionic sulfonate groups can dramatically reduce the work function of Al by accumulation of the polar groups at the PFSO₃Na/Al interface to induce more favorable the interfacial dipole. The better energy alignment for electron extraction and transportation at active layer/Al interface is confirmed by a significant enhancement of V_OC. The better wettability and morphology of PFSO₃Na on the active layer and the more effective motion of sodium counterion further modify the barrier to facilitate electron extraction and transportation. Moreover, 14% and 22% performance enhancement can also be achieved respectively, when PFNBr and PFSO₃Na are used as interlayers for low bandgap poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT)-based solar cells.     

Backbone regulation of a bithiazole-based wide bandgap polymer donor by introducing thiophene bridges towards efficient polymer solar cells

Miscibility and morphology of the active layers have significant influence on the photovoltaic performance of polymer solar cells (PSCs). Chemical strategies, especially molecular structure design, have been proven to be crucial for polymer donor materials. In this work, two wide bandgap D-A copolymer donors composed of tripropylsilyl substituted bithienyl-benzodithiophene as donor (D) unit and dialkyl substituted bithiazole as acceptor (A) unit were designed and synthesized. By introducing thiophene π-bridges into the backbone, the miscibility and morphological properties of the materials are effectively tuned, leading to tremendous progress in power conversion efficiency from 0.95% to 10.73% with m-ITIC as the acceptor. The results demonstrate that manipulating molecular distortion can be an effective strategy to regulate molecular self-assembly behavior of the polymer donors and achieve excellent aggregation properties, blend miscibility, and photovoltaic performance of the PSCs.     

The dual localized surface plasmonic effects of gold nanodots and gold nanoparticles enhance the performance of bulk heterojunction polymer solar cells

In this study, we investigated the effects of plasmonic resonances induced by gold nanodots (Au NDs), thermally deposited on the active layer, and octahedral gold nanoparticles (Au NPs), incorporated within the hole transport layer, on the performance of bulk heterojunction polymer solar cells (PSCs) based on poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C₆₁butyric acid methyl ester (PC₆₁BM). Thermal deposition of 5.3-nm Au NDs between the active layer and the cathode in a P3HT:PC₆₁BM device resulted in the power conversion efficiency (PCE) of 4.6%—that is 15% greater than that (4.0%) for the P3HT:PC₆₁BM device without Au NDs. The Au NDs provided near-field enhancement through excitation of the localized surface plasmon resonance (LSPR), thereby enhancing the degree of light absorption.     

Magnetoconductance of polymer–fullerene bulk heterojunction solar cells

The organic magnetoconductance (MC) effects in poly(3-hexylthiophene): [6,6]-phenyl-C61-butyricacid methylester based bulk heterojunction solar cells were studied in dark and under illumination. The correlations between the MC and current character were revealed in this study. Results show that the dark current always exhibits a negative MC whereas a sign change in MC under illumination occurs at the bias around the open circuit voltage V_oc. We suggest that the positive MC in photocurrent is due to the field dependent conversion of singlet electron–hole pairs to triplet states and the negative MC is associated with space charge limited current with traps. Other possible mechanisms about the magnetoconductance effects are also discussed.     

In-situ synthesis of metal nanoparticle-polymer composites and their application as efficient interfacial materials for both polymer and planar heterojunction perovskite solar cells

A new approach for the synthesis of gold nanoparticles (Au NPs) via a simple and fast in-situ generation method using an amine-containing polymer (PN4N) as both stabilizer and reducing agent is reported. The application of the Au NPs-PN4N hybrid material as efficient interfacial layer in different types of solar cells was also explored. The synthesized Au NPs show good uniformity in size and shape and the Au NPs doped PN4N hybrid composites exhibit high stability. Amine-containing polymers are good cathode interfacial materials (CIMs) in polymer solar cells (PSCs) and planar heterojunction perovskite solar cells (PVKSCs). The performance of the PSCs with Au NPs doped PN4N CIMs is largely improved when compares to devices with pristine PN4N CIM due to the enhanced electronic properties of the doped PN4N. Furthermore, by incorporating larger Au NPs into PEDOT:PSS to enhance absorption of the light harvesting layer, power conversion efficiencies (PCEs) of 6.82% and 13.7% are achieved for PSC with PCDTBT/PC₇₁BM as the light harvesting materials and PVKSC with a ∼280 nm-thick CH₃NH₃PbI_3−xCl_x perovskite layer, respectively. These results indicate that Au NPs doped into both PEDOT:PSS and PN4N interlayers exhibited a synergistic effect in performance improvement of PSCs and PVKSCs.     

Efficient ternary blend polymer solar cells with a bipolar diketopyrrolopyrrole small molecule as cascade material

We present a ternary strategy to enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (PSCs) with a bipolar small molecule as cascade material. A bipolar diketopyrrolopyrrole small molecule (F(DPP)₂B₂), as the second electron acceptor, was incorporated into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C₆₁-butyric-acidmethyl-ester (PC₆₁BM) blend to fabricate ternary blend PSCs. The introduction of the bipolar compound F(DPP)₂B₂ can not only broaden the light absorption of the active layer because of its absorption in near infrared region but also play a bridging role between P3HT and PC₆₁BM due to the cascaded energy level structure, thus improving the charge separation and transportation. The optimized ternary PSC with 5 wt% F(DPP)₂B₂ content delivered a high PCE of 3.92% with a short-circuit current density (J_sc) of 9.63 mA cm⁻², an open-circuit voltage (V_oc) of 0.62 V and a fill factor (FF) of 64.90%, showing an 23% improvement of PCE as compared to the binary systems based on P3HT:PC₆₁BM (3.18%) or P3HT:F(DPP)₂B₂ (3.17%). The results indicate that the ternary PSCs with a bipolar compound have the potential to surpass high-performance binary PSCs after carefully device optimization.     

Incorporation of diketopyrrolopyrrole dye to improve photovoltaic performance of P3HT:PC71BM based bulk heterojunction polymer solar cells

Ternary blend solar cells have been intensively studied in recent years to harvest more photons over the near-IR region. In this work, the effects of adding a diketopyrrolopyrrole dye (py-DPP) into a conventional P3HT:PC₇₁BM based bulk heterojunction photovoltaic cell are investigated. The near infrared absorption of the blend is enhanced by the doped py-DPP dye, leading to more than 20% increased power conversion efficiency compared to the P3HT:PC₇₁BM binary system. The highest efficiency of 4.05% is achieved for a P3HT:PC₇₁BM blend with 2.4 wt % of py-DPP.     

Non-peripheral octahexylphthalocyanine doping effects in bulk heterojunction polymer solar cells

The improvement of long-wavelength sensitivity in bulk heterojunction organic thin-film solar cells based on poly(3-hexylthiophene) (P3HT) by the addition of the soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) is reported. C6PcH₂ possesses near-infrared absorption and can be mixed with a P3HT:1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM) bulk heterojunction active layer. By doping C6PcH₂, the photosensitivity in the long-wavelength region was improved, and the energy conversion efficiency reached 3.0% at a composition ratio of P3HT:C6PcH₂:PCBM = 10:3:10. We discuss the principle of photoconversion in the bulk heterojunction solar cell based on the P3HT:C6PcH₂:PCBM active layer by taking into consideration the existence of both highly ordered P3HT domains and hexagonal columnar structures of C6PcH₂, and the microphase separation of P3HT and C6PcH₂ in the active layer.     

Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

The fill factor of polymer bulk heterojunction solar cells (PSCs), which is mainly governed by the processes of charge carrier generation, recombination, transport and extraction, and the competition between them in the device, is one of the most important parameters that determine the power conversion efficiency of the device. We show that the fill factor of PSCs based on thieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C₇₁-butyric acid methylester (PC₇₁BM) blend that only have moderate carrier mobilities for hole and electron transport, can be enhanced to 76% by reducing the thickness of the photoactive layer. A drift–diffusion simulation study showed that reduced charge recombination loss is mainly responsible for the improvement of FF, as a result of manipulating spatial distribution of charge carrier in the photoactive layer. Furthermore, the reduction of the active layer thickness also leads to enhanced built-in electric field across the active layer, therefore can facilitate efficient charge carrier transport and extraction. Finally, the dependence of FF on charge carrier mobility and transport balance is also investigated theoretically, revealing that an ultrahigh FF of 80–82% is feasible if the charge mobility is high enough (∼10⁻³–10⁻¹ cm²/V s).     

Synthesis and characterization of D-A-A type regular terpolymers with narrowed band-gap and their application in high performance polymer solar cells

Two novel D-A-A type regular terpolymers of PBDT-DTQ and PIDT-DTQ were designed and synthesized, in which benzodithiophene and indacenodithiophene building blocks were employed as the D unit, and quinoxaline building block was introduced as the A unit. Owing to the strong intra-molecular charge transfer effect in polymer backbone, much broader absorption spectra covering from 320 to 800 nm with the narrowed band-gap were obtained for the developed D-A-A type polymers in contrast to their corresponding D-A type polymers. On the other hand, compared with PBDT-DTQ, PIDT-DTQ exhibited a deeper HOMO energy level and also higher charge carrier mobility. To investigate the photovoltaic properties of PBDT-DTQ and PIDT-DTQ in detail, bulk hetero-junction polymer solar cells with a structure of ITO/PEDOT: PSS/Active Layer/Ca/Al were fabricated. PBDT-DTQ-based solar cells exhibited a moderate PCE value of 3.84%, however, an increased J_sc of 11.42 mA/cm², V_oc of 0.86 V and FF of 65.77% was achieved for PIDT-DTQ-based device, leading to the maximum PCE up to 6.41%. Our results here demonstrated that using the regular terpolymers as electron donor materials could be an efficient way to broaden the absorption of polymers and improve the photovoltaic performance of PSCs.     

Enhanced intrinsic stability of the bulk heterojunction active layer blend of polymer solar cells by varying the polymer side chain pattern

Organic photovoltaics (OPVs) have acquired huge attention over the past years as potential renewable energy sources, adding attractive features such as aesthetics, semi-transparency, flexibility, large area printability, improved low-light performance, and cost-effectiveness to the well-known Si-based photovoltaics. Steady improvements in OPV power conversion efficiencies are continuously reported, notably for bulk heterojunction solar cells based on conjugated polymer:fullerene blends. However, apart from efficiency and cost, the stability of organic solar cell devices is of particular concern. Among the different factors contributing to OPV instability, gradual loss of the optimum phase-separated nanomorphology of the photoactive layer blend is a critical parameter. In this paper, we present the results of ‘shelf-life’ accelerated lifetime tests performed for devices containing a range of functionalized poly(3-alkylthiophene) (P3AT) donor polymers upon prolonged thermal stress. By the incorporation of functional moieties on the side chains of P3HT-based copolymers, a remarkable improvement of the intrinsic stability of the active layer blend morphology is accomplished, even for fairly low built-in ratios (5–15%) and without crosslinking to covalently anchor the polymer and/or fullerene molecules. Moreover, these alterations do not influence the initial power conversion efficiencies to a large extent. As such, the presented approach can be regarded as an attractive paradigm for OPV active layer stability.     

Efficiency improvement of polymer solar cells by iodine doping

A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C₆₀ butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I₂) doping concentrations. The short circuit current density (J_sc) was increased to 8.7 mA/cm² from 4 mA/cm², meanwhile the open circuit voltage (V_oc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage.     

Conjugated dendritic oligothiophenes for solution-processed bulk heterojunction solar cells

This mini-review summarizes the recent achievements of developing conjugated dendritic oligothiophenes (DOT) for use in solution-processed bulk heterojunction (BHJ) solar cells. These DOTs are structurally defined molecules with relatively high molecular weight. Therefore, this novel class of thiophene based material possesses not only some advantages of oligomers,such as defined and monodispersed molecular structure,high chemical purity, but also some characteristics of polymers, for example, good solution-processability.In addition, the step-by-step approach of its synthesis allows precise fimctionalization of dendritic backbones with desired moieties, which is helpful to finely tune the optical and electronic properties of materials. Power conversion efficiencies (PCE) of BHJ solar cells were achieved up to 2.5% when functionalized thiophene dendrimers were used as electron donor and electron acceptor was a fullerene derivative. These results indicated that dendritic oligothiophenes are a novel class of the materials of electron donor for solution-processed organic solar cells.     

Wide range thickness effect of hole-collecting buffer layers for polymer:fullerene solar cells

Here we report the wide range thickness effect of hole-collecting buffer layers (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)) on the performance of polymer:fullerene solar cells with blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The thickness of the PEDOT:PSS layers was controlled from 3 nm to 625 nm, followed by characterizations such as optical transmittance, electrical resistances in the in-plane and out-of-plane directions, work functions, contact angles, and device performances. Results showed that the optical transmittance was gradually decreased with the PEDOT:PSS thickness but a maximum value was measured for other properties in the thickness range of 10–30 nm. The device performance was noticeably improved with only 3 nm-thick PEDOT:PSS layer, while it was almost similar in the thickness range of 30–225 nm in the presence of gradual decrease in the surface roughness. The similar device performance between 30 nm and 225 nm has been assigned to the compensation effect between the reduced electrical resistance (increased conductivity) and the decreased optical transmittance as the thickness of the PEDOT:PSS layer increased.     

Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer

TiO₂ sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO₂ nanoparticles (TiO₂ NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H₂O in the process of peptization both the crystallinity and particle size of TiO₂ NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO₂ NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO₂ NPs/Al showed the short-circuit current (J_sc) of 12.83 mA/cm² and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO₂ buffer layer. The stability measurement showed that PSC devices with a TiO₂ NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H₂O diffusion into the active layers. The observations indicate that TiO₂ NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.     

Synthesis and characterization of the conjugated polymers tethered with dipolar side chains containing a benzothiadiazole entity for bulk heterojunction solar cells

New conjugated copolymers (P1?P3) containing dipolar side chains connected to the main chain via triphenylamine donors have been synthesized and characterized. The side chains of these polymers have an electron deficient benzothiadiazole moiety in the spacer, but with different acceptors at the end. By changing the acceptor moieties of the side chain, the absorption spectra and HOMO/LUMO gaps of the polymers can be fine-tuned, ranging from 1.86 to 1.59 eV. Solution processed bulk heterojunction (BHJ) solar cells using these polymers as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor were fabricated and measured under 100 mW cm^?2 of AM 1.5 illumination. The cell based on the blend of P1/PCBM (1:1, w/w) exhibited the highest power conversion efficiency of 1.78%, with open circuit voltage (V_oc) = 0.79 V, short circuit current (J_sc) = 6.63 mA cm^?2 and fill factor (FF) = 0.34, respectively.