Bulk heterojunction solar cells based on self-assembling disc-shaped liquid crystalline material

In this article, effect of addition of disc-shaped liquid crystalline material, namely 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene, in poly (3-hexylthiophene): [6, 6]-phenyl-C61-butyric acid methyl ester containing bulk heterojunction (BHJ) solar cells has been investigated. These disc-shaped molecules organise into ordered columnar hexagonal structures through intermolecular π ? π interactions as monitored by polarised light optical microscopy. Current–voltage characteristics of the device prepared with liquid crystal layer exhibited a short-circuit current of 10.5 mA cm^?2 and a fill factor of 35%. The resultant power conversion efficiency (PCE) was 1.54%. The influence of varying the thickness of liquid crystal layer and annealing on these solar cells was also studied. The short circuit current and PCE of 12.9 mA cm^?2 and 2.3% was achieved for these BHJ solar cells containing self-organised discotic liquid crystals in the active layer under one sun condition after annealing.     

Bulk heterojunction solar cells made from carbazole copolymer and fullerene derivative with an inserted layer of discotic material with improved efficiency

Bulk heterojunction photovoltaic cells based on composites of copolymer poly [N-90-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] and the fullerene derivative [6,6]-phenyl C71-butyric acid methyl ester with an inserted layer of discotic liquid crystalline material (2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene) between the interface of active layer and hole transporting layer has been reported. Different hole transporting layers deposited on indium tin oxide substrates such as poly (3,4-ethylenedioxythiophene)-poly (styrenesulphonate) or molybdenum trioxide has been used in these devices. All the devices with inserted discotic liquid crystal layer showed better performance than the reference cells. Power conversion efficiency of 5.14% was achieved for these photovoltaic solar cells containing self-organised discotic liquid crystal layer of 30 nm thickness under one sun condition which is substantial jump as compared to earlier reports. The mobility of holes in the discotic liquid crystal inserted devices was found to be of the order of 10^–6 cm² V^–1 s^–1 due to which high values of current density was achieved. The influence of varying the thickness of liquid crystal layer and annealing on the photovoltaic parameters of these devices was also studied.     

Bulk heterojunction perovskite solar cells incorporated with solution-processed TiOx nanoparticles as the electron acceptors

In the past ten years, perovskite solar cells were rapidly developed, but the intrinsic unbalanced charge carrier diffusion lengths within perovskite materials were not fully addressed by either a planar heterojunction or meso-superstructured perovskite solar cells. In this study, we report bulk heterojunction perovskite solar cells, where perovskite materials CH₃NH₃PbI₃ is blended with solution-processed n-type TiO_x nanoparticles as the photoactive layer. Studies indicate that one-step solution-processed CH₃NH₃PbI₃:TiO_x bulk-heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap-induced charge recombination. Thus, bulk heterojunction perovskite solar cells by CH₃NH₃PbI₃ mixed with 5 wt% of TiO_x, which is processed by one-step method rather than typical two-step method, show a short-circuit current density of 20.93 mA/cm², an open-circuit voltage of 0.90 V, a fill factor of 80% and with a corresponding power conversion efficiency of 14.91%, which is more than 30% enhancement as compared with that of perovskite solar cells with a planar heterojunction device structure. Moreover, bulk heterojunction perovskite solar cells possess enhanced device stability. All these results demonstrate that perovskite solar cells with a bulk heterojunction device structure are one of apparent approaches to boost device performance.     

Achieving Efficient Thick Film All-polymer Solar Cells Using a Green Solvent Additive

Advances in organic photovoltaic technologies have been geared toward industrial high-throughput printing manufacturing, which requires insensitivity of photovoltaic performance regarding to the light-harvesting layer thickness. However, the thickness of light-harvesting layer for all polymer solar cells(all-PSCs) is often limited to about 100 nm due to the dramatically decreased fill factor upon increasing film thickness, which hampers the light harvesting capability to increase the power conversion efficiency, and is unfavorable for fabricating large-area devices. Here we demonstrate that by tuning the bulk heterojunction morphology using a non-halogenated solvent, cyclopentyl methyl ether, in the presence of a green solvent additive of dibenzyl ether, the power conversion efficiency of all-PSCs with photoactive layer thicknesses of over500 nm reached an impressively high value of 9%. The generic applicability of this green solvent additive to boost the power conversion efficiency of thick-film devices is also validated in various bulk heterojunction active layer systems, thus representing a promising approach for the fabrication of all-PSCs toward industrial production, as well as further commercialization.     

A Polyfluoroalkyl-Containing Non-fullerene Acceptor Enables Self-Stratification in Organic Solar Cells

The elaborate control of the vertical phase distribution within an active layer is critical to ensuring the high performance of organic solar cells (OSCs), but is challenging. Herein, a self-stratification active layer is realised by adding a novel polyfluoroalkyl-containing non-fullerene small-molecule acceptor (NFSMA), EH-C₈F₁₇, as the guest into PM6:BTP-eC9 blend. A favourable vertical morphology was obtained with an upper acceptor-enriched thin layer and a lower undisturbed bulk heterojunction layer. Consequently, a power conversion efficiency of 18.03 % was achieved, higher than the efficiency of 17.40 % for the device without EH-C₈F₁₇. Additionally, benefiting from the improved charge transport and collection realised by this self-stratification strategy, the OSC with a thickness of 350 nm had an impressive PCE of 16.89 %. The results of the study indicate that polyfluoroalkyl-containing NFSMA-assisted self-stratification within the active layer is effective for realising an ideal morphology for high-performance OSCs.     

Efficient charge collection with sol–gel derived colloidal ZnO thin film in photovoltaic devices

Polymer bulk heterojunction photovoltaic cell was fabricated by inserting a sol–gel derived ZnO thin film as an electron collecting layer between the fluorine-doped SnO₂ (FTO) and polymer-fullerene blend active layer. We demonstrated that the performance of device depends on sol concentration and the sol–gel process. Ammonia treatment on the ZnO film improved the efficiency of the device due to the effective removal of acetate group on the film. The short circuit current density was further increased by fine-tuning the thickness of ZnO film. The photovoltaic cell with this structure (FTO/ZnO film/polymer-fullerene blend/Au) produced a power conversion efficiency of 2.01% under simulated AM1.5G illumination of 100 mW/cm².     

Dye-sensitised solar cells with iodine-free discotic electrolytes

Discotic mesogenic molecules viz., hexahexylthiotriphenylene (HHTT) and hexahexyloxytriphenylene were applied, for the first time, as iodine-free redox electrolyte in dye-sensitised solar cells (DSSCs). The cell shows open circuit voltage (V_oc) of 0.95 V, short circuit current density (J_sc) of 0.534 mA/cm², fill factor 88.24% and overall power conversion efficiency (η) 0.45% in a typical fluorine doped tin oxide/TiO₂/N719/HHTT/Pt DSSC configuration. Scanning electron microscopy was used to study surface profile of electrolytes while electrochemical impedance spectroscopy was used to understand the electrochemical behaviour of electrolytes. The photovoltaic parameters were measured under standard conditions using Oriel solar simulator class AAA. These first results demonstrate the potential of the discotic molecules as charge transporter and mediator and show promise to be used in iodine-free DSSCs.     

Self-assembled hybrid polymer-TiO2 nanotube array heterojunction solar cells

Films comprised of 4 microm long titanium dioxide nanotube arrays were fabricated by anodizing Ti foils in an ethylene glycol based electrolyte. A carboxylated polythiophene derivative was self-assembled onto the TiO2 nanotube arrays by immersing them in a solution of the polymer. The binding sites of the carboxylate moiety along the polymer chain provide multiple anchoring sites to the substrate, making for a stable rugged film. Backside illuminated liquid junction solar cells based on TiO2 nanotube films sensitized by the self-assembled polymeric layer showed a short-circuit current density of 5.5 mA cm-2, a 0.7 V open circuit potential, and a 0.55 fill factor yielding power conversion efficiencies of 2.1% under AM 1.5 sun. A backside illuminated single heterojunction solid state solar cell using the same self-assembled polymer was demonstrated and yielded a photocurrent density as high as 2.0 mA cm-2. When a double heterojunction was formed by infiltrating a blend of poly(3-hexylthiophene) (P3HT) and C60-methanofullerene into the self-assembled polymer coated nanotube arrays, a photocurrent as high as 6.5 mA cm-2 was obtained under AM 1.5 sun with a corresponding efficiency of 1%. The photocurrent action spectra showed a maximum incident photon-to-electron conversion efficiency (IPCE) of 53% for the liquid junction cells and 25% for the single heterojunction solid state solar cells.     

Novel two-step CdS deposition strategy to improve the performance of Cu_2ZnSn(S,Se)_4 solar cell

Kesterite Cu_2ZnSn(S,Se)_4(CZTSSe)solar cells have drawn worldwide attention for their promising photovoltaics performance and earth-abundant element composition,yet the record efficiency of this type of device is still far lower than its theoretical conversion efficiency.Undesirable band alignment and severe non-radiative recombination at CZTSSe/CdS heterojunction interfaces are the major causes limiting the current/voltage output and overall device performance.Herein,we propose a novel two-step CdS deposition strategy to improve the quality of CZTSSe/CdS heterojunction interface and thereby improve the performance of CZTSSe solar cell.The two-step strategy includes firstly pre-deposits CdS thin layer on CZTSSe absorber layer by chemical bath deposition(CBD),followed with a mild heat treatment to facilitate element inter-diffusion,and secondly deposits an appropriate thickness of CdS layer by CBD to cover the whole surface of pre-deposited CdS and CZTSSe layers.The solar energy conversion efficiency of CZTSSe solar cells with two-step deposited CdS layer approaches to 8.76%(with an active area of about 0.19 cm~2),which shows an encouraging improvement of over 87.98% or 30.16% compared to the devices with traditional CBD-deposited CdS layer without and with the mild annealing process,respectively.The performance enhancement by the two-step CdS deposition is attributed to the formation of more favorable band alignment at CZTSSe/CdS interface as well as the effective decrease in interfacial recombination paths on the basis of material and device characterizations.The two-step CdS deposition strategy is simple but effective,and should have large room to improve the quality of CZTSSe/CdS heterojunction interface and further lift up the conversion efficiency of CZTSSe solar cells.     

A two-step method combining electrodepositing and spin-coating for solar cell processing

Electrochemistry provides a simple and promising method for preparing organic solar cells (OSCs). In this paper, we present a two-step solution-based method to prepare bilayer heterojunction OSCs by electrodepositing polythiophene (PTh) and then spin-coating chloroform solution of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) onto the PTh layer. The influence of film thickness on performance of bilayer solar cells was investigated, and the best performance was achieved when the thickness of PTh and PCBM was 15 nm and 30 nm, respectively. The optimized solar cell showed power conversion efficiency of 0.1% under the illumination of AM 1.5 (100 mW cm⁻²) simulated solar light. This solution-based method offers a new way for processing bilayer OSCs.     

用CdSe/ZnS量子点提高体异质结有机太阳电池的效率

通过掺杂吸收光谱在可见光波段的量子点可提高聚合物对可见光的吸收,因此掺杂CdSe/ZnS核-壳结构量子点(CQDs)能提高聚(3-己基噻吩):[6,6]-苯基-C61-丁酸甲酯(P3HT:PCBM)体异质结太阳电池的能量转换效率.本文研究了CdSe/ZnS量子点在P3HT:PCBM中的不同掺杂比例及其表面配体对太阳电池光伏性能的影响,优化器件ITO(氧化铟锡)/PEDOT:PSS(聚(3,4-乙撑二氧噻吩:聚苯乙烯磺酸)/P3HT:PCBM:(CdSe/ZnS)/Al的能量转换效率达到了3.99%,与相同条件下没有掺杂量子点的参考器件ITO/PEDOT:PSS/P3HT:PCBM/Al相比,其能量转换效率提高了45.1%.     

Unsymmetrical zinc phthalocyanines containing thiophene and amine groups as donor for bulk heterojunction solar cells

Photovoltaic technology is an alternative resource for renewable and sustainable energy and low costs organic photovoltaic devices such as bulk-heterojunction (BHJ) solar cells, which are selective candidates for the effective conversion of solar energy into electricity. Asymmetric phthalocyanines containing electron acceptor and donor groups create high photovoltaic conversion efficiency in dye sensitized solar cells. In this study, a new unsymmetrical zinc phthalocyanine was designed and synthesized including thiophene and amine groups at peripherally positions for BHJ solar cell. The structure of the targeted compound (4) was characterized comprehensively by FT-IR, UV–Vis, ¹H-NMR, and MALDI-TOF MS spectroscopies. The potential of this compound in bulk heterojunction (BHJ) photovoltaic devices as donor was also researched as function of blend ratio (blend ratio was varied from 0.5 to 4). For this purpose, a series of BHJ devices with the structure of fluorine doped indium tin oxide (FTO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/ ZnPc:[6,6]- phenyl-C61- butyric acid methyl ester (PCBM) blend/Al with identical thickness of ZnPc:PCBM layer were fabricated and characterized. Photo current measurements in 4 revealed that the observed photo current maximum is consistent with UV-vis spectra of the compound of 4. Preliminary studies showed that the blend ratio has a critical effect on the BHJ device performance parameters. Photovoltaic conversion efficiency of 6.14% was achieved with 4 based BHJ device.     

The use of sublimable chlorotricarbonyl bis(phenylimino)acenaphthene rhenium(I) complexes as photosensitizers in bulk-heterojunction photovoltaic devices

A series of sublimable substituted chlorotricarbonyl bis(phenylimino)acenaphthene rhenium(I) complexes was synthesized and used in the fabrication of photovoltaic devices. The hole and electron carrier mobilities of these complexes are in the order of 10⁻³ to 10⁻⁴ cm² V⁻¹ s⁻¹. Heterojunction devices with CuPc/complex/C₆₀ (CuPc = copper phthalocyanine) as the active layer and bulk heterojunction devices with complex:C₆₀ as the active layer were fabricated. The rhenium complexes function as photosensitizer in the devices, and exhibit optical absorption in the region between 500 and 550 nm within which other components in the device do not absorb. Other devices with hole transport materials, exciton blocking materials, and different active layer thickness were also fabricated. Variation of substitution groups in the ligand did not show significant difference in device performance. The best power conversion efficiency of the devices was measured to be 1.29% under illumination of AM1.5 simulated solar light.     

An eco‐friendly water‐soluble fluorene‐based polyelectrolyte as interfacial layer for efficient inverted polymer solar cells

A conjugated polyelectrolyte poly(9,9‐bis(3′‐[(N,N‐dimethyl)‐N‐ethylammonium]‐propyl)‐2,7‐fluorene dibromide) (PFBr) with the feature of environmental friendliness and cheapness was successfully used in polymer solar cells (PSCs) as the cathode interfacial layer (CIL). And we successfully demonstrate that the PFBr can build interfacial dipoles at the CIL/cathode interfaces, leading to reduce cathode work functions and improve open‐circuit voltages, which decrease interfacial energy loss at the cathode. It not only improves the electron transfer efficiency but also inhibits the charge carrier recombination at the contact interface. Impedance spectra revealed that the optimal device with the smallest charge transport time constant of 2.83 microseconds was achieved under the concentration of 2 mg mL⁻¹ of PFBr, which suggests efficient electron transport on the interface between the organic active layer and the indium tin oxide cathode. Moreover, as a consequence, the power conversion efficiency of the PSCs increases to 3.83% (with PFBr as CIL) from 1.89% (without any CIL), based on the poly(3‐hexylthiophene) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester bulk heterojunction active layer. Therefore, our observation can demonstrate PFBr is a prospective candidate as CIL for constructing low‐cost, large‐area, and flexible PSCs.     

Soluble narrow‐band‐gap copolymers containing novel cyclopentadithiophene units for organic photovoltaic cell applications

Five novel conjugated copolymers ( P1 – P5 ) containing coplanar cyclopentadithiophene (CPDT) units (incorporated with arylcyanovinyl and keto groups in different molar ratios) were synthesized and developed for the applications of polymer solar cells (PSCs). Polymers P1 – P5 covered broad absorption ranges from UV to near infrared (400–900 nm) with narrow optical band gaps of 1.38–1.70 eV, which are compatible with the maximum solar photon reflux. Partially reversible p‐ and n‐doping processes of P1 – P5 in electrochemical experiments were observed, and the proper molecular design for highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of P1 – P5 induced the highest photovoltaic open‐circuit voltage in the PSC devices, compared with those previously reported CPDT‐based narrow‐band‐gap polymers. Powder X‐ray diffraction (XRD) analyses suggested that these copolymers formed self‐assembled π‐π stacking and pseudobilayered structures. Under 100 mW/cm² of AM 1.5 white‐light illumination, bulk heterojunction PSC devices containing an active layer of electron donor polymers P1 – P5 mixed with electron acceptor [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) in the weight ratio of 1:4 were investigated. The PSC device containing P1 gave the best preliminary result with an open‐circuit voltage of 0.84 V, a short‐circuit current of 2.36 mA/cm², and a fill factor of 0.38, offering an overall power conversion efficiency (PCE) of 0.77% as well as a maximal quantum efficiency of 23% from the external quantum efficiency (EQE) measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2073–2092, 2009     

裁剪型有机光伏小分子及其衍生物的研究与应用#

有机太阳能电池光电转化效率已经突破13%,这主要归因于活性层材料的不断丰富与改进。其中,以聚合物为母体的裁剪型分子,其相比于聚合物具有明确的分子量,共轭长度可调,高消光系数,优良的结晶性等优势。本文简要介绍裁剪型分子在二元本体异质结体系,三元体系,非富勒烯体系中的应用及我们组的相关研究工作,总结了其特点并对其应用前景做了展望。     

Role of balanced charge carrier transport in low band gap polymer:Fullerene bulk heterojunction solar cells

Lowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylenevinylene] and 6,6‐phenyl C₆₁‐butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Electro-deposition under a modulated electrical field as an enhanced method for the preparation of an efficient photoanode of dye-sensitized solar cells

This paper describes an application of a new electro-deposition method in a modulated electrical field in order to have an efficient semiconductor coating on a conductive substrate. The prepared film was used as a photoanode of dye-sensitized solar cells (DSSCs). Electro-deposition of nanoparticles usually was performed by applying a DC electrical field in a suspension. In the DC field, a homogeneous layer could not be performed because of unwanted electrochemical reactions that might occur on the substrate surface. Modulated electrical fields based on pulsed AC and a sweeping voltage profile were used. The photovoltaic performance of the assembled solar cells showed a significant difference between the films produced in different electrical field conditions. Under the illumination of simulated AM 1.5 sunlight (100 mW cm^?2) with an aperture black mask, the energy conversion efficiency of 2.45% (V _OC = 768 mV, J _SC = 4.74 mA cm^?2, FF = 67%) was obtained with a thin layer of TiO₂ nanoparticles deposited in a pulsed waveform voltage. A crack-free and uniform porous layer produced in this condition showed an enhancement of about seven times over the photoanode prepared using conventional DC electrical field with the same voltage amplitude. The solar cell efficiency was increased to 4.22% (V _OC = 735 mV, J _SC = 7.92 mA cm^?2, FF = 72%) by just increasing the TiO₂ film thickness and using a blocking layer beneath the semiconductor layer. Moreover, a higher electron recombination lifetime presented better electron transport and collection efficiency of the film deposited in the pulsed electrical field conditions.     

Enhanced Performance of Organic Photovoltaic Cells Fabricated with a Methyl Thiophene‐3‐Carboxylate‐Containing Alternating Conjugated Copolymer

A new donor‐acceptor copolymer, containing benzodithiophene (BDT) and methyl thiophene‐3‐carboxylate (3MT) units, is designed and synthesized for polymer solar cells (PSCs). The 3MT unit is used as an electron acceptor unit in this copolymer to provide a lower highest occupied molecular orbital (HOMO) level for obtaining polymer solar cells with a higher open‐circuit voltage (V_OC). The resulting bulk heterojunction PSC made of the copolymer and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) exhibits a power conversion efficiency (PCE) up to 4.52%, a short circuit current (J_SC) of 10.5 mA·cm‐², and a V_OC of 0.86 V.     

Synthesis and characterization of a narrow‐bandgap polymer containing alternating cyclopentadithiophene and diketo‐pyrrolo‐pyrrole units for solar cell applications

We have synthesized a narrow‐bandgap conjugated polymer ( PCTDPP ) containing alternating cyclopentadithiophene (CT) and diketo‐pyrrolo‐pyrrole (DPP) units by Suzuki coupling. This PCTDPP exhibits a low band gap of 1.31 eV and a broad absorption band from 350 to 1000 nm, which allows it to absorb more available photons from sunlight. A bulk heterojunction polymer solar cell incorporating PCTDPP and C₇₀ at a blend ratio of 1:3 exhibited a high short‐circuit current of 10.87 mA/cm² and a power conversion efficiency of 2.27%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1669–1675, 2010