Polymer Solar Cells Research Papers (original) (raw)

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- Polymer Chemistry, Polymer Solar Cells

A series of novel low-bandgap triphenylaminebased conjugated polymers (PCAZCN, PPTZCN, and PDTPCN) consisting of different electron-rich donor main chains (N-alkyl-2,7-carbazole, phenothiazine, and cyclopentadithinopyrol, respectively) as well as cyano-and dicyano-vinyl electronacceptor pendants were synthesized and developed for polymer solar cell applications. The polymers covered broad absorption spectra of 400-800 nm with narrow optical bandgaps ranging 1.66-1.72 eV. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the polymers measured by cyclic voltammetry were found in the range of À5.12 to À5.32 V and À3.45 to À3.55 eV, respectively. Under 100 mW/cm 2 of AM 1.5 white-light illumination, bulk heterojunction photovoltaic devices composing of an active layer of electron-donor polymers (PCAZCN, PPTZCN, and PDTPCN) blended with electron-acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester or [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) in different weight ratios were investigated. The photovoltaic device containing donor PCAZCN and acceptor PC 71 BM in 1:2 weight ratio showed the highest power conversion efficiency of 1.28%, with V oc ¼ 0.81 V, J sc ¼ 4.93 mA/cm 2 , and fill factor ¼ 32.1%. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5812-5823, 2010

Bulk-heterojunction (BHJ) solar cells based on organic small molecules and polymers are the focus of increasing attention by science and commerce. In organic photovoltaic devices, a conjugated polymer layer is used as the donor, while a fullerene-based derivative is used as the acceptor. Poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is one of the most common interfacial materials used for organic BHJ solar cells. However, PEDOT:PSS is acidic and hygroscopic in nature, and it inherits microstructural inhomogeneities that cause not only gradual degradation, but a complete failure of BHJ solar cell devices. There is a growing interest in graphene-based solar cells because graphene-based materials offer ease of solution processability, high optical transparency, and high
power conversion efficiency. Graphene has been actively investigated for use as a transparent conducting electrode, and as a photoactive layer in fabricating solar cell devices. Power conversion efficiency in the range of 10% to 15% for graphene and inorganic semiconductor-based hybrid heterojunction solar cells, and 15.6% for graphene-containing perovskite solar cells has been observed. Organic materials-based solar cells degrade not only from environmental exposure, but also from
photo-oxidation caused by light illumination. In addition to higher power conversion efficiency, stability in graphene-based solar cells is critically important for commercial applications. In this review article, the stability of graphene-based heterojunction solar cells under atmospheric conditions is evaluated. Current studies show that the insertion of a graphene buffer layer into solar cell heterostructures stops
degradation and enhances stability in solar cell devices. Long-term environmental stability of graphenebased heterojunction solar cells for commercial applications is discussed.

Concern about global warming and diminishing fossil fuel reserves have accelerated searches for low cost sources of renewable energy. Organic photovoltaic (OPV) cells are one such source. They have couple of advantages over the conventional semiconductors. Organic solar cells have the potential to be low cost and efficient solar energy convertors, with a promising energy balance. The applications of thermoelectric polymers at low temperatures, especially conducting polymers, have shown various advantages such as easy and low cost of fabrication, light weight, and flexibility. However efficiency remains quite low. Thus efforts have been made to increase the efficiency by varying the fabrication parameters. Poly (3-hexythiophene) (P3HT) and 1-(3-methoxycarbonyl) propyl-1-phenyl [6, 6] C 61 (PCBM) are the most studied polymer blend materials around the world for bulk heterojunction structure of an organic solar cells (OSCs). This research article is a survey on tremendous literature published that exhibit solar cells based on blends of P3HT and PCBM. The basic structure of a P3HT: PCBM heterojunction solar cell and accurate methods for measurement of the power conversion efficiency (PCE) were also discussed. Standard method using Air Mass 1.5 Global (1000Wm -2 , AM1.5G) solar spectrum is advised after finding abnormalities in the PCE reported. It is noticed that optimum thickness and area of every layer in the cell structure is important. A detailed discussion on thermal annealing and solvent annealing approaches to improve device performance is presented. The effects of these two approaches on improving polymer crystallinity, light absorption in the polymer, carrier transport, and blend film nano-morphology, etc. are summarized. Polymer morphology has proven to be extremely important in determining the optoelectronic properties in polymer-based devices. We also investigate the effects of polymer morphology too on the PCE of the cell. Another important parameter affecting the efficiency discussed is the Molecular weight ratio of P3HT and PCBM blend active layer. Future directions and challenges on polymer solar cell development are also discussed Keywords— P3HT: PCBM organic blend layer, Polymer Photovoltaic, Organic solar cells, bulk heterojunctions.

- by Vedant Pravin
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- P3HT, Polymer Solar Cells

The fill factor is an important param-eter that determines the power con-version efficiency of an organic photo-voltaic solar cell.There are several fac-tors that significantly influence FF and these factors interact with each other in-tricately.Due to this understanding FF is quite important.The relationship be-tween Fill Factor(FF) and process vari-ables have been demonstrated with series and shunt resistances,and this provided a chance to understand the electrical de-vice behaviour.in the blend layer, series resistance(R s) and shunt resistance(R sh) were varied by controlling the morphol-ogy of the blend layer,the regioregularity of conjugated polymer, and the thickness of the blend layer.At the interface between the cathode including PEDOT:PSS and the blend layer, cathode conductivity was controlled by varying the structure of the cathode or adding an additive.

- by Vedant Pravin
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- P3HT, PCBM, Polymer Solar Cells

Due to serious energy crisis in the world, solar energy has gained much importance due to its availability in abundant quantity. India aims to generate 20 GW Solar power by the year 2020, but by end of 2010 it could have been achieved only 0.5 GW. High capital cost and low efficiency is the main problem for penetration and reach of solar PV systems. Efficiency of a solar panel is nearly 20-25%. So to increase further the overall efficiency of the solar power generation system, a Maximum Power Point Tracking algorithm is necessary. Photovoltaic modules have a single operating point where the output of the voltage and current results in the maximum power output. MPPT is a system to extract maximum obtainable solar power from a PV module. Maximum power point (MPP) varies depending on the angle of sunlight on the surface of the panel and cell temperature. This paper reviews working of MPPT and short description of different MPPT techniques.

A strategy of the fine-tuning of the degree of intrachain charge transfer and aromaticity of polymer backbone was adopted to design and synthesize new polymers applicable in photovoltaics. Three conjugated polymers P1, P2, and P3 were synthesized by alternating the electron-donating dithieno[3,2-b:2 0 3 0 -d]pyrrole (D) and three different electronaccepting (A) segments (P1: N-(2-ethylhexyl)phthalimide; P2: 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole; and P3: thiophene-3-hexyl formate) in the polymer main chain. Among the three polymers, P2 possessed the broadest absorption band ranging from 300 to 760 nm, the lowest bandgap (1.63 eV), and enough low HOMO energy level (À5.27 eV) because of the strong intrachain charge transfer from D to A units and the appropriate extent of quinoid state in the main chain of P2, which was convinced by the theoretical simulation of molecular geometry and front orbits. Photovoltaic study of solar cells based on the blends of P1-P3 and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) demonstrated that P2:PCBM exhibited the best performance: a power conversion efficiency of 1.22% with a high open-circuit voltage (V OC ) of 0.70 V and a large short-circuit current (I SC ) of 5.02 mA/cm 2 were achieved.

- by Dan Deng
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- Materials Engineering, Organic solar cells, Polymer Solar Cells, Energy Levels
- by Frédéric CHANDEZON and +1
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- Materials Engineering, Organic Photovoltaics, Optoelectronics, Polymer Solar Cells

Two new soluble alternating phenylenevinylene copolymers S and L which contained dithienbenzothiadiazole moieties were synthesized by Heck coupling. The repeating unit of L was longer than that of S and contained two additional phenylene rings and two cyano-vinylene bonds. Both copolymers were stable up to about 350 C and afforded char yield of 52-66% at 800 C in N 2 . Their absorption spectra were broad and extended up to about 600 nm with a longer wavelength maximum at 447-502 nm and optical band gap of $ 2.0 eV. These copolymers emitted yellow light in solution with PL max-imum at 551-580 nm and orange-red light in thin film with PL maximum at 588-661 nm. The emission maximum of L was considerably red-shifted relative to S. Photovoltaic cells based on S (or L) as donor and [6,6]phenyl C61-butyric acid methyl ester as acceptor were investigated. V

- by Svetlana van Bavel
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- Engineering, Electron Tomography, Morphology, Organic solar cells

Highly efficient bulk-heterojunction polymer solar cells based on two indacenodithiophene-containing low bandgap, high mobility polymers in both of their conventional and inverted device configurations are demonstrated. The highest power conversion efficiency obtained from the conventional structure device is 6.3%, while the PCE of the inverted device reaches 4.9%. These results are very encouraging for further material and device optimization.

- by Ying-Tse Sun
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- Engineering, Solar Cell, Organic solar cells, Energy Conversion

Main approaches to the design of organic bulk heterojunction photovoltaic structures are generalized and systematized. Novel photovoltaic materials based on fullerenes, organic dyes and related compounds, graphene, conjugated polymers and dendrimers are considered. The emphasis is placed on correlations between the chemical structure and properties of materials. The effect of morphology of the photoactive layer on the photovoltaic properties of devices is analyzed. Main methods of optimization of the photovoltaic properties are outlined.

- by Gennadiy Bulavko
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- Photovoltaics, Organic Semiconductors, Solar Cell, Photonics

A two-stage approach that combines electrodeposition with redox replacement has been used to deposit Pt/Pb nanoparticles on fluorine doped tin oxide (FTO) glass. The deposition takes place by cycling between the two steps in a single electrolyte bath containing both Pb2+ and Pt2+ ions. In the first step, Pb is electrodeposited on FTO glass at a potential E1 and in

- by Benjamin Wilson
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- Engineering, Materials Science, Fuel Cells, Solar Cell

Optical and photovoltaic properties were studied for ternary photovoltaic cells containing a traditional donor-acceptor bulk-heterojunction (BHJ) active layer modified with polymethine dye molecules in a broad range of compositions and wavelengths. An effect of composition induced optical transparency, due to the strong modification of the density of states, was observed for symmetrical compositions with approximately equal amount of components. Based on our spectroscopic ellipsometry and atomic force microscopy (AFM) studies we can suggest that the variation of the refractive index, which is significantly reduced in the visible range for ternary systems, is involved in the physical mechanism of the phenomenon. Despite of an addition of the IR absorbing component (which allows broadening of the absorption band to up to 800 nm) no improvement in the power conversion efficiency (PCE) is observed in comparison to the binary BHJ system (P3HT:PCBM). Nevertheless, we believe that further advance of the efficiency will be possible if the energy levels will be chemically designed to avoid formation of charge traps at the BHJ interface during light excitation. Such fine adjustment of the system should become possible with a proper choice of polymer:dye composition due to a high versatility of the polymethine dyes demonstrated in previous studies.

- by Gennadiy Bulavko
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- Photovoltaics, Renewable Energy, Energy Conservation, Photonics

This paper describes the fabrication of P3HT based bulk heterojunction solar cell using ZnS nanopar-ticles of sphalarite and wurtzite crystal structure and discusses their photovoltaic characteristics. It is found that the device of P3HT : ZnS can give higher Voc and surface modified wurtzite ZnS is able to give higher Voc (~ 1.3 V) as well as higher current density (229.60 A/cm 2) and comparable efficiency (0.032 %). Active layer (P3HT : ZnS) showed a broad optical absorption from 320 nm to 650 nm and it is seen that the absorption intensity increased in the blend than the pristine P3HT.

- by abdulkareem thottoli
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- Polymer Solar Cells

A class of cross-linked polythiophenes with different ratio (2%, 4%, 8%) of vinylene-terthiophenevinylene (VTThV) conjugated bridges (PT-VTThV2, PT-VTThV4, and PT-VTThV8) were synthesized. The cross-linking influenced the absorption spectra of the polymer solutions very little, but it resulted in blueshift of the absorption spectra of the polymer films by ca. 28-39 nm. Cyclic voltammograms display that the p-doping/dedoping and n-doping/dedoping processes of all the cross-linked polythiophenes are reversible and the electrochemical bandgaps increased a little with the increase of the content of the conjugated bridges. The hole mobility values determined from the space-charge-limited current (SCLC) model reached 4.70 × 10 -3 for PT-VTThV2, 2.58 × 10 -3 for PT-VTThV4, and 9.48 × 10 -4 cm 2 /(V s) for PT-VTThV8, respectively. The hole mobility of PT-VTThV2 with 2% VTThV conjugated bridges is about three orders higher than that of the corresponding polymer P1 without the conjugated bridges. The power conversion efficiency of the polymer solar cell based on the blend of PT-VTThV2 and PCBM (1:1, w/w) reached 1.72% under the illumination of AM 1.5, 100 mW/cm 2 , which is two times of that of the device based on P1. The results indicate that the crosslinking with the VTThV conjugated bridges obviously improved charge transportation the photovoltaic properties of the conjugated polymers.

- by Yingping Zou
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- Materials Engineering, Polymer science, Conducting Polymers, UV/Vis spectroscopy

Water-soluble sulfonated monomer based on 3,4-propylenedioxythiophene (ProDOT-sultone) was synthesized and characterized for the first time via the O-alkylation of the corresponding unreactive , -disubstituted hydroxyl group with propane sultone in the presence of a catalytic amount of 1,4-diazabicyclo[2.2.2]octane (DABCO). This new monomer was oxidatively polymerized to produce regioregular water-soluble conjugated anionic polyelectrolyte, which was then used for the fabrication of solid-state electrochromic devices using the layer-by-layer (LbL) deposition method. These solid-state devices were found to exhibit better electrochromic properties in terms of color contrast, switching time, coloration efficiency (CE), surface control electroactivity, and conductivity in thin films compared with the corresponding water-soluble regiorandom 3,4-ethylenedioxythiophene (EDOT) derivative. For the 40 and 80 bilayer solid-state electrochromic devices, the electrochemical contrast was observed to be 31 and 40% at 570 nm with fast solid-state switching times of 100 and 220 ms, respectively, indicating faster movement of the ions in and out of the films. Furthermore, the CE was found to be as high as 250 cm 2 /C for the 80 bilayer device and was independent of the device thickness, indicating the full accessibility of all of the ionic sites, even in thicker films. Four-point probe conductivity of the LbL and in situ conductivity of solution cast films were found to be in the range of 10 -4 and 10 -3 S/cm, respectively.

- by vaibhav sinha
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- Engineering, Materials Engineering, Technology, Advanced Materials

We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, fieldeffect charge mobility, as well as organic photovoltaic characteristics between thiophene-and selenophene-bridged donor-acceptor low-band-gap copolymers. In these copolymers, a significant reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase of the short circuit current density (J SC ) but a slight decrease of the open circuit voltage (V OC ) of their bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a selenophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6bis(dodecyloxy)-4,7-di(selenophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pCzSe) or poly{4,8-bis(2ethylhexyloxy)benzo[1,2-b;4,5-b 0 ]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo [c] [1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9Hcarbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly {4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b 0 ]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Judging from our device data, the potential Se-Se interactions of the selenophene-bridged donor-acceptor copolymers, which is presumably beneficial for the fill factor (FF) of BHJ OPVs, is rather susceptible to the device fabrication conditions. † Electronic supplementary information (ESI) available: The drain current-voltage plots and transistor transfer characteristics of copolymers, details of computational study, 1 H NMR spectra of polymers. See

- by Hung Yang Chen
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- Polymer Solar Cells

The effects of cathode modifi cation by a conjugated polymer interlayer PFPA1 on the performance of reversed organic solar cells (substrate/cathode/active layer/transparent anode) based on different active material systems and different substrate electrodes are systematically investigated. A reduction of the work function irrespective of the substrate cathode used is observed upon the deposition of the PFPA1 interlayer, which is further related to an improved built-in electric fi eld and open-circuit voltage. The amphiphilic character of the PFPA1 interlayer alters the surface energy of the substrate cathode, leading to the formation of a better active layer morphology aiding effi cient exciton dissociation and photocurrent extraction in the modifi ed solar cells. Hence, internal quantum effi ciency is found to be signifi cantly higher than that of their unmodifi ed counterparts, while optically, the modifi ed and unmodifi ed solar cells are identical. Moreover, the deep highest occupied molecular orbital (HOMO) of the PFPA1 interlayer improves the selectivity for all investigated substrate cathodes, thus enhancing the fi ll factor.

- by Wolfgang Tress
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- Organic solar cells, Physical sciences, Interfaces, CHEMICAL SCIENCES

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US$ within the next few years. Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5-6% have been certified at NREL and AIST usually on devices with small active areas. The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10-12%. Several reasons for the power conversion efficiency limitation have been identified. Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor-acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells.

- by Mauro Morana
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- Engineering, Carbon, Organic Semiconductors, Solar Cell

Hybrid quantum dot solar cell (HQDSC) based on solution-processed blends of poly(3-hexylthiophene) (P3HT) with PbS quantum dots (QDs) is a potential candidate toward practical use for its low material cost and simple fabrication process. However, P3HT is highly incompatible with oleic acid (OA)-capped PbS QDs (OA-PbS QDs) due to strong phase separation, giving poor quality in the desired bi-continuous networks morphology and thus leading to inefficient charge collection. Here, for the first time, a block copolymer of P3HT with polystyrene (P3HT-b-PS) was confirmed to improve the miscibility between the polymers and OA-PbS QDs, leading to the formation of a desirable bi-continuous network morphology, as predicted by us via dissipative dynamic simulations previously. The bi-continuous network morphology for charge transport is an ideal morphology in bulk heterojunction solar cells. For the active layer, using the block copolymer P3HT-b-PS as the donor and PbS QDs as the acceptor at the weight ratio of 1 : 20, the power conversion efficiency (PCE) of HQDSC was found to be 4.18%, which is higher than P3HT and PbS QDs (3.66%) having the same weight ratio even though the content of the P3HT component in P3HT-b-PS was 28% less than that of homo-polymer of P3HT. The formation of the desired morphology for electron and hole collections of the device with the block copolymer was confirmed via scanning electron microscopy. Further, the addition of P3HT into the blend of the block copolymer with OA-PbS QDs still retains the desired morphology. Therefore, further improvement of PCE was made by taking the blend of P3HT and P3HT-b-PS at the weight ratio of 0.7 : 0.3 as the donor, thus achieving the PCE of 4.91%, which is better than that of P3HT alone by 1.25% and P3HT-b-PS alone by 0.73%. Thus, this methodology could be applicable for hybrid solar cells with a low bandgap
molecular or polymeric material as the donor.

- by Pavel Komarov
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- Organic solar cells, Solar Energy, Polymer Solar Cells

Inverted bulk heterojunction solar cells were fabricated using poly(3-hexylthiophene) (P3HT) blended with two different fullerene derivatives namely phenyl-C61-butyric acid methyl ester (PC 60 BM) and indene-C 60 bis-adduct (IC 60 BA). The effects of annealing temperatures on the morphology, optical and structural properties were studied and correlated to differences in photovoltaic device performance. It was observed that annealing temperature significantly improved the performance of P3HT:IC 60 BA solar cells while P3HT:PC 60 BM cells showed relatively less improvement. The performance improvement is attributed to the extent of fullerene mixing with polymer domains. Energy filtered transmission electron microscopy (EFTEM) and x-ray diffraction (XRD) results showed that ICBA mixes with disordered P3HT much more readily than PC 60 BM which leads to lower short circuit current density and fill factor for P3HT:IC 60 BA cells annealed below 120 1C. Annealing above 120 1C improves the crystallinity of P3HT in case of P3HT:IC 60 BA whereas in P3HT:PC 60 BM films, annealing above 80 1C leads to negligible change in crystallinity. Crystallization of P3HT also leads to higher domain purity as seen EFTEM. Further it is seen that cells processed with additive nitrobenzene (NB) showed enhanced short circuit current density and power conversion efficiency regardless of the fullerene derivative used. Addition of NB led to nanoscale phase separation between purer polymer and fullerene domains. Kelvin probe force microscopy (KPFM) images showed that enhanced domain purity in 1 3 5 7

- by Devendra Khatiwada
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- Polymer Solar Cells

In this work, the role of fullerene loading on the nanomorphology and photovoltaic performance of alternating copolymer... more

In this work, the role of fullerene loading on the nanomorphology and photovoltaic performance of alternating copolymer poly{2-octyldodecyloxy-benzo[1,2-b;3,4-b]dithiophene-alt-5,6-bis(dodecyloxy)-4,7bis(thiophen-2-yl)-benzo[c][1,2,5]-thiadiazole}(PBDT-ABT-1) blend films were investigated. Morphology of blend films with different PCBM mixing ratios and solvent additive were studied using atomic force microscopy (AFM) and energy filtered transmission electron microscopy (EFTEM). From the AFM and EFTEM images it was observed that 1:1,1:2,1:3 weight ratio of PBDT-ABT-1:PCBM intermixed domain showed increased domain size with increasing PCBM weight ratio due to amorphous nature of polymer which tends to intermix with the polymer. X-ray diffraction measurements on the pristine polymer and blend films casted without additive showed amorphous nature of PBDT-ABT-1. EFTEM donor acceptor composite images showed intermixed domain and was conducted to see the bulk morphology i.e domain composition in the granular morphology . Furthermore, EFTEM composite image for additive cast film gave clear insight that addition of solvent additive di-iodooctane leads to more purer domain. Photo CELIV measurement indicated fast charge extraction in case of the optimized nanomorphology sample with the weight ratio of 1:2 corresponding to highest bimolecular recombination life time and mobility.

- by Abu Mitul, Ph.D. and +1
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- Morphology, Organic solar cells, EFTEM, AFM

Organic dyes can be used as a polar dipole component to create strong local electrical field and promote exciton dissociation in polymer bulk heterojunction (BHJ) solar cells. Some polymethine dyes are characterized with strong IR absorption and therefore can significantly improve light energy harvesting characteristics in such devices due to extension of the absorption spectrum of the photoconducting layer to the IR region. In a view of these opportunities, photovoltaic properties of BHJ systems constituted with poly-3-hexylthiophene (P3HT) as an electron donor, 6,6-phenyl-C61-butyric acid methyl ester (PCBM) fullerene-type electron acceptor, and cation polymethine or merocyanine dyes were studied depending on the dye concentration and under different light excitation conditions. A new physical effect of photoinduced current hysteresis was observed in dye containing BHJ solar cells, when the dye concentration was in the range of 16–60%. The photocurrent hysteresis is detected only when simultaneously polythiophene and dye molecules are excited with light (white light excitation), while no current hysteresis can be seen in the dark or under selective excitation of the polymer component (490 nm) or dye component (740 nm) of the BHJ system. In addition, significant reduction of the photocurrent collected from the dye-containing devices under illumination with white light in comparison to a selective excitation of the polymer component only is observed.

- by Gennadiy Bulavko
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- Photovoltaics, Organic Semiconductors, Solar Cell, Photonics
- by Priti Tiwana
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- Polymer Solar Cells
- by Raja Siram
- •
- Engineering, Technology, Derivatives, The

Recently, head-to-tail regioregular poly(3-hexylthiophene) (P3HT) has been widely used as an active material in fabricating polymer optoelectronic devices. This study employs UV-vis absorption, photoluminescence (PL), X-ray diffraction (XRD) and the space-charge limited current (SCLC) model to elucidate the effect of light illumination on the optic and optoelectronic properties of P3HT. The degraded performance of P3HT, such as low absorbance, PL emission, and charge-carrier mobility is caused mostly by a reduction in the degree of conjugated structure and the lower crystallinity. r

- by Yi-Ming Chang
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- Engineering, Solar Cell, Physical sciences, Degradation
- by Meng-Hung Tsai
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- Engineering, Nanoparticles, Physical sciences, CHEMICAL SCIENCES

ABSTRACT We present a dual length morphological model for the active layer of bulk-heterojunction, polymer-based solar cells using results from neutron and X-ray scattering techniques. Two critical characteristic lengths are found in the mixtures composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A characteristic length at 15 nm is the local characteristic of the P3HT crystals and PCBM agglomerations, which is independent of the bulk composition upon relaxation by thermal annealing. Conversely, a larger bicontinuous structure described by Teubner–Strey model with phase distances between 23 and 35 nm forms only after thermal annealing, which is highly correlated to the bulk compositions. These results suggest phase separation between the polymer and fullerene can only be partially manipulated by simple processing techniques such as coating conditions and annealing, and a more rigorous design of the morphology should be implemented in the future. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

- by Wenluan Zhang
- •
- Engineering, Small angle X-ray and neutron scattering, Morphology, Nanocomposites

We investigated optical, electrical and mechanical properties of indium tin oxide (ITO) on flexible polyethylene terephthalate (PET) substrate, considering bulk-heterojunction (BHJ) polymer solar cells applications. Encapsulation of flexible solar cells with the architecture PET/ITO/PEDOT:PSS/P3HT:PCBM (or P3HT:PCBM:AZ-NDI-4)/Al was done by direct brush-painting with nail enamel. Active cell layer blends of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) with regioregular or regiorandom poly(3-hexylthiophene-2,5-diyl) (P3HT) were applied. Additionally for this role the mixture of regioregular P3HT:PCBM with naphthalene diimide–imine with four thiophene rings AZ-NDI-4 was tested. Obtained photovoltaic (PV) and optical (UV–vis) results of the flexible polymer solar cells were compared with the same architecture of devices on the glass/ITO substrate.

- by Maciej Sibiński and +2
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- Materials Engineering, PET, Polymer Solar Cells, ITO

Recognition of top download/ most popular paper for December, 2015 (in SPIE digital library)

The present manuscript reports the design, synthesis and characterization of three starshaped polymers consisting of three different arylimides such as perylene (PR)-, naphthalene (NT)-and benzene (BZ) tetracarboxylicdiimide as core and polyfluorene (PF) as arms. Chemical structure of star-shaped polymers was aimed at broadening as much as possible their absorption profile. Arylimide cored star polymers (PF-BZ, PF-NT and PF-PR) were prepared through palladium catalyzed Suzuki polycondensation to tune the band gap of the polymers. The prepared polymers were characterized by elemental analysis, NMR, GPC, UV-Vis, photoluminescence and cyclic voltammetry studies. Electrochemical and optical responses of three polymers revealed the lowering of band gap from linear PF to starshaped polymers. TCSPC study confirmed the partial energy transfer from PF arms to arylimide cores. The unexpected keto defect in linear PF was also reduced by preparation of star polymer with large arylimide cores. TGA exhibited the enhancement of thermal stability of star polymer than linear PF. By using star polyfluorenes as the donor and [6,6]-phenyl C61butyric acid methyl ester (PCBM) as the acceptor, bulk heterojunction (BHJ) solar cells of the structure ITO/PEDOT:PSS/star polymer: PCBM/Al were fabricated and studied with a solar simulator under AM1.5G (100 mW/cm 2) irradiation intensity. Those cells showed the open circuit voltage (V oc) $0.52-0.55 V, the short circuit current density (J sc

- by SUDIP MALIK
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- Materials Engineering, Chemical Engineering, Physics, Materials Science

Inverted polymer solar cells (iPSCs) have shown a great potential in organic photovoltaics due to its enhanced stability and large-scale roll-to-roll processing. Metallated conjugated polymers also show promise as a donor material in the PSCs. In this paper, the role of solvents on the performance of inverted structure metallated PSCs was understood. Chlorobenzene (CB) and toluene were used as solvents to obtain the optimum morphology for improved device efficiency. To understand the charge transport properties and charge carrier lifetime in metallated polymer iPSCs, atomic force microscopy and transient photocurrent measurements were performed. The results showed that the CB as a solvent can improve the morphology of the active layer and enhance the performance of metallated iPSCs.

- by LAL MOHAMMAD
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- Organic Chemistry, Polymer Chemistry, Polymer science, Morphology

In this paper, we demonstrate that low-temperatureprocessed
aluminum-doped zinc oxide (AZO)/ethoxylated
polyethylenimine (PEIE) electron transport layer (ETL) significantly
improves the performance of poly[(4,4-bis(2- ethylhexyl)
dithieno[3,2-b:2,3-d]silole)-2,6-diyl-alt-(2,1,3 benzothiadiazole)-
4,7-diyl] (PSBTBT)-based inverted organic solar cells. Different
metal oxides, including zinc oxide (ZnO), AZO, ZnO/PEIE, and
AZO/PEIE, were used as ETL. The optical and morphological
properties of ZnO, AZO, PEIE, and their combination layers
were investigated in order to find the favorable ETLs for inverted
structure PSBTBT:PC70BM solar cells. Transient photocurrent,
photoinduced charge extraction by linearly increasing voltage, and
atomic force microscopy were performed to understand the effects
of charge transport, recombination, and morphological changes
on device performance. Among the various layers, including ZnO,
AZO, ZnO/PEIE, and AZO/PEIE, the low-temperature-processed
AZO/PEIE combination ETL was the best performing interface
layer that achieved the highest device fill factor and energy conversion efficiency.

A novel method of fabrication of gadolinia doped bismuth oxide nanoceramic via the sol-gel technique is reported. Their thermal, structural and morphological properties are described by measurements of Differential Thermal Analysis/Thermal Gravimetry, X-ray Powder Diffraction and Scanning Electron Microscopy. The samples have stable high ion conductive face centered cubic δ-phase nanocrystalline structure. The electrical measurements of the nanoceramic powders were carried out in the temperature range of (689-1091 K) using 4-point probe technique. There is a transition between two distinct regions at 720 °C, which can be attributed to the order-disorder transition. This observation is supported by the differential thermal analysis measurements. The experimental results show that the value of conductivity increases with increasing temperature over linear parts characterized by two different activation energies. The conductivity data over whole measured temperature range were fitted the Arrhenius plot and it shows two linear regions with different slopes which correspond to low-temperature range (689-975 K) and high-temperature range (999- ersus q/kT plot as 1.25 eV and 2.81 eV for low-temperature range and high-temperature range, respectively.

- by Prof.Dr. İbrahim USLU
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- Polymer Engineering, Polymer Chemistry, Biomaterials, Composites

Ternary bulk-heterojunction polymer solar cells have been fabricated using a polymer blend (PBDB-T:PBDTTPD) as a donor in combination with a small molecule ITIC as a non-fullerene acceptor. In addition to the similarity in their energy levels, both PBDB-T and PBDTTPD polymers demonstrated rather similar crystalline or lamellar packing structure to support an alloy-type blend model, which well-explained the progressively monotonic change in photovoltaic VOC against the compositions. In addition, photoinduced electron-transfer (PET) between the PBDB-T/PBDTTPD polymers forming an intermediate charge separation state between the donors. A synergistic improvement in exciton generation rate, enhancement in charge carrier transport with good balance, and charge separation rate due to PET all contribute to the achieve a photovoltaic device with high efficiency

- by Zhiqun He
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- Polymer Solar Cells
- by Hyungtak Seo
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- Engineering, Advanced Materials, Physical sciences, CHEMICAL SCIENCES

A series of poly{(3-hexylthiophene)-co-[3-(6-hydroxyhexyl)thiophene]}:titania (P3HT-OH:TiO 2 ) hybrids were synthesized via the in situ polycondensation of titanium (IV) n-butoxide in the presence of P3HT-OH. Introducing a hydroxyl moiety onto the side-chain of poly(3hexylthiophene) (P3HT) significantly promotes the polymer-titania interaction, resulting in the formation of homogeneous hybrid colloids. The UV-vis spectra of P3HT-OH:TiO 2 films demonstrate that TiO 2 markedly affects the stacking structure and the chain conformation of P3HT-OH. The maximum absorption wavelength of these hybrid materials can be tailor-made by merely varying the weight percentage of TiO 2 . Moreover, P3HT-OH:TiO 2 can be further utilized as an efficient compatibilizer in preparing photoactive P3HT:P3HT-OH:TiO 2 films with excellent miscibility. The photovoltaic cell based on such a hybrid exhibited a 2.4-fold higher value of power-conversion efficiency compared to the cell based on P3HT:TiO 2 .

- by Yi-Ming Chang
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- Engineering, Photovoltaic Cell, CHEMICAL SCIENCES, Hybrid Materials

and sharing with colleagues.

- by Raul Salazar
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- Engineering, Organic Photovoltaics, Physical sciences, CHEMICAL SCIENCES

Optical coherence tomography (OCT) is a noninvasive technique providing cross-sectional images of a tooth structure. This review describes the use of OCT for detecting dental caries, tooth fractures, and interfacial gaps in intraoral... more

- by Piotr Targowski
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- Engineering, Solar Cell, Optical coherence tomography, Solar Energy Materials

Two alternating conjugated copolymers TTP and TTT were synthesized by Heck coupling of 2,3-bis-(5-bromothiophene-2-yl)acrylonitrile with 1,4-dihexyloxy-2,5-divinylbenzene and 3-hexyl-2,5divinylthiophene, respectively. The absorption spectra of the copolymers in THF solution showed three maxima around 270, 370 and 460 nm with optical band gaps of 2.30-2.34 eV. The electrochemically estimated band gaps of copolymers were 2.04-2.10 eV. The thin film absorption spectra were broad and extended about from 250 to 600 nm with a long wavelength maximum near 470 nm. The copolymers emitted yellow-orange light with maximum at 528-551 nm in THF solution and 567-610 nm in thin film. TTP showed the most red shifted emission maximum between the copolymers. The performance of the photovoltaic cells which were fabricated using blends of the copolymers with 6,6-phenyl C61-butyric acid methyl ester (PCBM) (1:1 and 1:4, w/w) as well as pure copolymers was investigated.

- by Minas Stylianakis
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- Materials Engineering, Condensed Matter Physics, Solar Cell, Photoluminescence
- by Shengwei Shi and +1
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- Polymer Solar Cells

In this paper, the role of fullerene loading on the nanomorphology and photovoltaic performance of alternating copolymer poly{2-octyldodecyloxy-benzo[1,2-b;3,4-b] dithiophene-alt-5,6-bis(dodecyloxy)-4,7bis(thiophen-2-yl)-benzo[c] [1,2,5]-thiadiazole} (PBDT-ABT-1) blend films was investigated. The morphology of blend films with different Phenyl C-60butyric acid methyl ester (PCBM) mixing ratios and solvent additives was studied using atomic force microscopy (AFM) and energy-filtered transmission electron microscopy (EFTEM). AFM and EFTEM images showed difference in the intermixing of polymer with fullerene between 1:1, 1:2, and 1:3 weight ratios. Polymer/PCBM intermixed domain size increases with higher PCBM weight ratios. X-ray diffraction measurements on the pristine polymer and blend films cast without additives did not show any peaks, suggesting an amorphous nature of PBDT-ABT-1. EFTEM images from the donor/acceptor composite showed intermixed polymer-PCBM domains separated by the polymer boundary. Furthermore, EFTEM images for di-iodooctane (DIO) additive cast film revealed purer polymer domain. Photo-charge extraction by linearly increasing voltage measurement exhibited that charge extraction is highest in the nanomorphology sample with a weight ratio of 1:2, corresponding to the lowest bimolecular recombination and the highest charge carrier mobility.

The use of hydrated vanadium(V)oxide as a replacement of the commonly employed hole transporting material PEDOT:PSS was explored in this work. Polymer solar cells were prepared by spin coating on glass. Polymer solar cells and modules comprising 16 serially connected cells were prepared using full roll-to-roll (R2R) processing of all layers. The devices were prepared on flexible polyethyleneterphthalate (PET) and had the structure PET/ITO/ZnO/P3HT:PCBM/V 2 O 5 · (H 2 O) n /Ag. The ITO and silver electrodes were processed and patterned by use of screen printing. The zinc oxide, P3HT:PCBM and vanadium(V)oxide layers were processed by slot-die coating. The hydrated vanadium(V)oxide layer was slot-die coated using an isopropanol solution of vanadyl-triisopropoxide (VTIP). Coating experiments were carried out to establish the critical thickness of the hydrated vanadium(V)oxide layer by varying the concentration of the VTIP precursor over two orders of magnitude. Hydrated vanadium(V)oxide layers were characterized by profilometry, scanning electron microscopy, energy dispersive X-ray spectroscopy, and grazing incidence wide angle X-ray scattering. The power conversion efficiency (PCE) for completed modules was up to 0.18%, in contrast to single cells where OPEN ACCESS Materials 2011, 4 170 efficiencies of 0.4% were achieved. Stability tests under indoor and outdoor conditions were accomplished over three weeks on a solar tracker.

- by Nieves Espinosa
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- Engineering, Materials, CHEMICAL SCIENCES, Polymer Solar Cells

- by Chin-Ti Chen
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- Engineering, Materials Chemistry, CHEMICAL SCIENCES, Polymer Solar Cells

In solution-processed tandem polymer solar cells, one of the most challenging parts is the optimization of interconnecting layers (ICLs) between subcells. In this study, ICLs were optimized for double- and triple-junction polymer solar cells. We investigated the robustness of PEDOT:PSS/AZO/PEIE ICL for tandem polymer solar cells. Solvent testing for ICL robustness showed a uniform coverage of the PEDOT:PSS layer when prepared without adding DMSO and IPA in PEDOT:PSS and with no use of additional filters. The performance of tandem polymer solar cells was investigated on different temperatures of interfacial layer. The PEDOT:PSS annealing temperature can be used from 120 °C to 130 °C, while aluminum-doped zinc oxide (AZO) temperature can be used from 120 °C to 150 °C. Lowering the annealing temperature in ICLs might help to fabricate triple-junction solar cells because they require more layers, and some low-bandgap polymers cannot sustain high temperature. However, reduction in annealing temperature in PEDOT:PSS, AZO, and PEIE led to lower FF and J _sc . A double-junction device V _oc of 1.05 V and a triple-junction V _oc of 1.4 V indicated that the ICLs worked effectively.

- by Abu Mitul, Ph.D.
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- Organic Chemistry, Polymer Chemistry, Polymer science, Morphology