Performance Of Bulk Heterojunction Solar Cells Research Articles

Influence of β-linkages on the morphology and performance of DArP P3HT–PC61BM solar cells

Direct arylation polymerization (DArP) has emerged as a greener and more atom-efficient alternative to Stille polymerization. Despite the attractiveness of this method, DArP is known to produce β-linkages in polymers, which have β-protons available for activation. Here, we report the influence of the β-defect content in DArP poly(3-hexylthiophene) (P3HT) on the performance of bulk-heterojunction solar cells and the morphology of pristine polymers and their blends with PC61BM in thin films and compare with Stille P3HT containing 0% β-defects as a reference point. The optical and electrochemical properties as well as the hole mobilities of pristine polymers remain virtually the same when the amount of β-defects is limited to 0.75% or lower, as evidenced by UV–visible absorption spectra, cyclic voltammetry and space-charge-limited current (SCLC) mobility measurements. However, an increase of β-defect concentration to 1.41% significantly affects the oxidation onset, UV–visible absorption profile and hole mobility of P3HT. The key result of this study is that the photovoltaic performance of DArP P3HT with 0% β-defects is remarkably close to that of Stille P3HT, whereas the performance of DArP P3HT with 0–0.75% β-defects does not differ dramatically from that of Stille P3HT and could potentially be improved upon by individual optimization of the processing conditions.

Influence of Cathode Work Functions on the Photovoltaic Properties of MEH-PPV: TiO<sub>2</sub> Bulk Heterojunction Solar Cell

Bulk heterojunction solar cell has received significant attention over the past decade due to low cost power generation and the potential to develop a clean renewable energy source [. We investigated the effect of different type of metal cathodes on the power conversion efficiency of bulk heterojunction solar cell based on a blend of conjugated polymer poly [2-methoxy 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with titanium dioxide (TiO2). In this case of study, Aluminum (Al) and gold (Au) has been chosen as the metal cathode due to the difference of work function and their wide application in hybrid solar cell. We show that the choice of metal cathode plays a role in determining overall device efficiency through their impact on short-circuit current, open circuit voltage and fill factor due to the influence of work function. It is found that the device employing Al metal cathode which has low work function is showing a comparable performance to the Au metal electrode with fill factor of over 20 % and a power conversion efficiency of 3.3x10-3 %. Overall it is demonstrated that the matching between the work function of the cathode and photoactive layer MEH-PPV: TiO2 is the most important factor towards best bulk heterojunction solar cell performance.

Analysis of ITO cleaning protocol on surface properties and polymer: Fullerene bulk heterojunction solar cell performance

In the traditional organic solar cell architecture, indium tin oxide (ITO) serves as the transparent electrode and substrate for solution-cast device layers. Hydrophobic contaminants are known to modify ITO wettability and other properties, making selection of an effective cleaning scheme essential. In this work, we demonstrate that certain solvent-based routines provide essentially equivalent surface properties as coupled solvent+UV-ozone protocols reported in literature. Surface energy analysis further reveals that solvent selection can effectively tune ITO charge transport. Fabricated devices showed comparable performance, indicating that interfacial resistances are not limiting factors in the model OSC system and that UV-ozone exposure could be eliminated to reduce commercialization costs.

Synthesis of 5H-Dithieno[3,2-b:2′,3′-d]pyran as an Electron-Rich Building Block for Donor–Acceptor Type Low-Bandgap Polymers

We describe the detailed synthesis and characterization of an electron-rich building block, dithienopyran (DTP), and its application as a donor unit in low-bandgap conjugated polymers. The electron-donating property of the DTP unit was found to be the strongest among the most frequently used donor units such as benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units. When the DTP unit was polymerized with the strongly electron-deficient difluorobenzothiadiazole (DFBT) unit, a regiorandom polymer (PDTP–DFBT, bandgap = 1.38 eV) was obtained. For comparison with the DTP unit, polymers containing alternating benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units and the DFBT unit were synthesized (PBDT–DFBT and PCPDT–DFBT). We found that the DTP based polymer PDTP–DFBT shows significantly improved solubility and processability compared to the BDT or CPDT based polymers. Consequently, very high molecular weight and soluble PDTP–DFBT can be obtained with less bulky side chains. Interestingly, PDTP–DFBT shows excellent performance in bulk-heterojunction solar cells with power conversion efficiencies reaching 8.0%, which is significantly higher than PBDT–DFBT and PCPDT–DFBT based devices. This study demonstrates that DTP is a promising building block for high-performance solar cell materials.

Simulated annealing electro-photonic optimization of organic solar cells

Micro-patterned organic solar cells can exhibit enhanced light absorption properties due to a photonic crystal effect [Tumbleston et al., Appl. Phys. Lett. 94, 043305 (2009)]. Here, a three-dimensional model of light absorption and charge carrier transport in micro-patterned materials is presented and applied to the design of organic bulk heterojunction (BHJ) solar cells. Rigorous coupled wave analysis is used to simulate the multiple scattering and absorption of light in a layered solar cell device. The non-linearly coupled steady-state electric field and charge transport equations are solved iteratively by a sequence of linear partial differential equations (PDEs). Each linear PDE is discretized by an exponential upwind finite difference scheme, and the preconditioned conjugate gradient method is applied to the resulting algebraic system. The electro-photonic solver is coupled with the simulated annealing optimization algorithm to investigate the effect of micro-patterning upon performance of BHJ solar cells. Starting from the baseline configuration of a cell formed from flat layers of optimal thickness, the optimization algorithm leads to improvements of up to 15% in energy conversion efficiency by patterning both the front and back electrodes with a periodic ridge shape, with conformally coated layers in-between.

Homogeneous bulk heterojunction networks via surface energy matching at polymer/fullerene interfaces

This work presents homogeneous bulk heterojunction (BHJ) networks between a push-pull π-conjugated polymer donor, poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene)-alt-5,5-(4,7-bis(2,2-bithien-5-yl)-2,2-dimethyl-2H-benzimidazole)) (PININE-BBTMBI) and a fullerene derivative acceptor, [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). By introducing a bi-thiophene unit between the electron-rich and electron-deficient moieties in a repeating unit, the surface energy of PININE-BBTMBI increased to that of PC71BM, enhancing its miscibility in PININE-BBTMBI/PC71BM blends. Direct evidence obtained via atomic force microscopy and the performance of BHJ solar cells indicate that the introduction of the bi-thiophene unit leads to homogeneous BHJ networks and high-performance BHJ solar cells by matching the surface energies at the polymer/fullerene interfaces.

Investigation of the morphology of an MEH-PPV/PCBM active layer and its application to bulk hetero-junction solar cell performance

Bulk hetero-junction solar cells based on blends of poly[2-methoxy, 5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and [6,6]-phenylen C61-butyric acid methyl ester (PCBM) were studied. The morphological of devices were investigated where varying the nitrogen plasma treatment, the casting solvents, and the thermal treatment. An optimized morphology was found to enhance the device performance to a power conversion efficiency of up to 2.5%, are measured using a Keithley 2425 source unit with an A.M 1.5G solar simulator light source (100 W/cm2).

Influence of NiO anode buffer layer prepared by solution on performance of bulk-heterojunction solar cells

We investigate the properties of NiO thin films prepared by sol–gel solution and the photovoltaic performance of bulk-heterojunction (BHJ) solar cells based on NiO thin films as anode buffer layers with various film thickness and amorphous–crystalline phase. Optoelectronic properties and surface morphology of NiO films strongly depend on the thickness and phase of the NiO. The performance of solar cells based on nanocrystaline (nc-) NiO exhibits better photovoltaic characteristics in comparison to those employing an amorphous (a-) NiO. The obtained power conversion efficiency (PCE, η) of a solar cell with a 20-nm-thick nc-NiO is 2.24%, with a short-circuit density (JSC) of 7.77mA/cm2, open-circuit voltage (VOC) of 0.57V, and fill factor (FF) of 0.50.

Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives

The aim of this study is to quantitatively investigate the effect of different fullerene type (PC60BM and PC70BM) on various morphological structures and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) P3HT/PCxBM solar cells. The solar cells are fabricated by spin coating without thermal annealing. The quantitative investigations of three-dimensional self-organized nanostructures are performed by using combined grazing-incidence small- and wide-angle X-ray scattering technique (GISAXS/GIWAXS). Two types of nanostructures are observed due to the phase separation in the BHJ films during the processing. They include (1) intercalated PCxBM molecules around boundary of P3HT crystalline domain and within amorphous domain and (2) aggregated PCxBM clusters in PCxBM domains. The lamellar spacing of P3HT crystalline domains in P3HT/PC70BM is larger than that in P3HT/PC60BM. This result indicates more interfacial areas are generated between PC70BM and P3HT at the molecular scale for more efficient charge separation. On the other hand, the size, volume fraction, partial attachment, and spatial distribution of PC60BM clusters are larger than that of PC70BM clusters, which reveals more efficient electron transport in P3HT/PC60BM. We deduce the correlation between nanostructures and PCE (3.25% and 2.64%, respectively, for P3HT/PC70BM and P3HT/PC60BM). The structure of fullerene intercalated with P3HT rather than the size of fullerene cluster plays a major role in the PCE performance of BHJ solar cell without thermal annealing.

Improved thin film morphology and bulk-heterojunction solar cell performance through systematic tuning of the surface energy of conjugated polymers

A detailed model study has shown that thin film morphology and bulk-heterojunction solar cell performance can be significantly improved by systematic tuning of the surface energy of the conjugated donor polymer through side-chain functionalization. Thiophene-flanked diketopyrrolopyrrole (DPP) moieties with different contents of cyanohexane side chains were incorporated into three low band-gap conjugated copolymers (PIDTDPP1, PIDTDPP2 and PIDTDPP3) consisting of indacenodithiophene (IDT) donors and DPP acceptors. The resulting polymers possessed good solubility in common organic solvents and showed similar energy levels, bandgaps, and hole mobilities. However, the introduction of cyano groups onto the terminal of side-chains significantly changed their surface energy. Topographical images obtained from atomic force microscopy (AFM) proved that a better matched surface energy between polymer and PC71BM had led to enhanced miscibility, which resulted in better BHJ film morphology. Consistent with the surface energy enhancement, the performance of BHJ photovoltaic devices increased from 0.97% for PIDTDPP1, to 2.16% for PIDTDPP2 then to 3.67% for PIDTDPP3. These results clearly reveal that tuning surface energy is an effective way to improve the morphology of the BHJ active layer and efficiency of the photovoltaic device.

Effect of trichlorobenzene additive on the performance and morphology of polyfluorene and fullerene derivative bulk heterojunction solar cells

The influence of additive, namely 1,3,5-trichlorobenzene (TCB), on the morphology and performance of bulk heterojunction (BHJ) organic solar cells was studied based on a 1:2 (w/w) blend of benzothiadiazole/thiophene-based copolymers (PFTBzTT) to [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The active layers were deposited by spin-coating from solutions using chloroform, with different additive concentrations from 0 to 36 mg/ml. The addition of solvent additive into the polymer solution was able to improve the performance of BHJ solar cells. The maximum power conversion efficiency (%PCE) of 0.85% was obtained for a cell with the TCB concentration of 12 mg/ml after annealing at 180°C for 20 min. From atomic force microscopy (AFM) images, the films processed without TCB appear smoother than those with the TCB additive. A large extent of segregation was also observed in the films processed with a high TCB concentration. The optical images of the thin films suggest that the optimum concentrations of the additive cause the polymer self-organisation, resulting in some aggregation of large PCBM crystals. © 2011 Canadian Society for Chemical Engineering

Quinacridone-Based Electron Transport Layers for Enhanced Performance in Bulk-Heterojunction Solar Cells

A water/alcohol-soluble small molecule based on the commercially available pigment quinacridone is employed as an electron transport layer in organic photovoltaics. The quinacridone derivative is utilized in solution-processed bulk-heterojunction solar cells to improve primarily the fill factor of the devices, contributing to an upwards of 19% enhancement in the power conversion efficiency relative to the control devices with no electron transport layer. The facile synthesis of the quinacridone derivative coupled with the ease of device fabrication via solution processing provide a simple, yet effective means of improving the performance of existing organic photovoltaic cells.

Rapid phase segregation of P3HT:PCBM composites by thermal annealing for high-performance bulk-heterojunction solar cells

The performances of bulk-heterojunction (BHJ) solar cells are investigated for time-dependent thermal annealing with different morphology evolution scales, having special consideration for the diffusion and aggregation of fullerene derivative molecules based on blends of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Meaningfully, rapid formation of dot-like and needle-like crystalline PCBM structures of a few micrometers up to 60 μm in size is obtained with thermal annealing treatment from 2 to 15 min, which dynamically reflects a fast process of PCBM molecule and cluster aggregation. Upon ultrasonic-assisted processing and annealing treatment, the scale of P3HT crystals is drastically increased in view of X-ray diffraction (XRD) patterns, leading to a high hole mobility. And, the P3HT domains can be gradually converted into larger P3HT crystals approved by the decreased full width at half-maximum in the XRD patterns. Corresponding current–voltage curves are measured in quantity and we propose a model to explain the effect of the crystalline degree of P3HT domains and aggregation of PCBM molecules and clusters on the phase segregation, expressing a viewpoint towards high performance of BHJ solar cells.

Improvement of CdSe/P 3HT bulk hetero-junction solar cell performance due to ligand exchange from TOPO to pyridine

Cadmium selenide (CdSe) nanoparticles of 2–7 nm diameter were synthesized by a hot injection method and their TOPO ligands exchanged with pyridine by a liquid–liquid extraction process. The effects of the ligand exchange on the material properties of the nanoparticles and device performance of the bulk hetero-junction (BHJ) solar cell were systematically investigated. The wavelength shift to a shorter value observed in the PL spectra of the pyridine-capped CdSe indicates that the size of nanoparticles decreased due to the exchange of the surface ligands of the CdSe from TOPO to pyridine. Improvements in the power conversion efficiency of BHJ solar cells made with pyridine-capped CdSe are mainly attributed to the enhanced current collection, J sc =0.43 mA/cm 2 (TOPO) to 1.4 mA/cm 2 (pyridine).

A New Class of Semiconducting Polymers for Bulk Heterojunction Solar Cells with Exceptionally High Performance

Solar cells based on the polymer-fullerene bulk heterojunction (BHJ) concept are an attractive class of low-cost solar energy harvesting devices. Because the power conversion efficiency (PCE) of these solar cells is still significantly lower than that of their inorganic counterparts, however, materials design and device engineering efforts are directed toward improving their output. A variety of factors limit the performance of BHJ solar cells, but the properties of the materials in the active layer are the primary determinant of their overall efficiency. The ideal polymer in a BHJ structure should exhibit the following set of physical properties: a broad absorption with high coefficient in the solar spectrum to efficiently harvest solar energy, a bicontinuous network with domain width within twice that of the exciton diffusion length, and high donor-acceptor interfacial area to favor exciton dissociation and efficient transport of separated charges to the respective electrodes. To facilitate exciton dissociation, the lowest unoccupied molecular orbital (LUMO) energy level of the donor must have a proper match with that of the acceptor to provide enough driving force for charge separation. The polymer should have a low-lying highest occupied molecular orbital (HOMO) energy level to provide a large open circuit voltage (V(oc)). All of these desired properties must be synergistically integrated to maximize solar cell performance. However, it is difficult to design a polymer to fulfill all these requirements. In this Account, we summarize our recent progress in developing a new class of semiconducting polymers, which represents the first polymeric system to generate solar PCE greater than 7%. The polymer system is composed of thieno[3,4-b]thiophene and benzodithiophene alternating units. These polymers have low bandgaps and exhibit efficient absorption throughout the region of greatest photon flux in the solar spectrum (around 700 nm). The stabilization of the quinoidal structure from thieno[3,4-b]thiophene is believed to be primarily responsible for these properties. Additionally, the rigid backbone enables the polymer to form an assembly with high hole mobility. Proper side chains on the polymer backbone ensure good solubility and miscibility with fullerene acceptors. The flexibility in structural tuning on the polymer backbone provides the polymers with relatively low-lying HOMO energy levels and enhanced V(oc), short-circuit current density (J(sc)), and fill factor (FF) and, thus, enhanced PCE. All of these features indicate that the polymer system exhibits a host of properties that are indeed synergistically combined, leading to the enhancement in solar cell output. Our preliminary results demonstrate why these polymers are excellent materials for solar energy conversion and represent prime candidates for further improvements through research and development.

Solution Processable Fluorenyl Hexa‐peri‐hexabenzocoronenes in Organic Field‐Effect Transistors and Solar Cells

AbstractThe organization of organic semiconductor molecules in the active layer of organic electronic devices has important consequences to overall device performance. This is due to the fact that molecular organization directly affects charge carrier mobility of the material. Organic field‐effect transistor (OFET) performance is driven by high charge carrier mobility while bulk heterojunction (BHJ) solar cells require balanced hole and electron transport. By investigating the properties and device performance of three structural variations of the fluorenyl hexa‐peri‐hexabenzocoronene (FHBC) material, the importance of molecular organization to device performance was highlighted. It is clear from 1H NMR and 2D wide‐angle X‐ray scattering (2D WAXS) experiments that the sterically demanding 9,9‐dioctylfluorene groups are preventing π–π intermolecular contact in the hexakis‐substituted FHBC 4. For bis‐substituted FHBC compounds 5 and 6, π–π intermolecular contact was observed in solution and hexagonal columnar ordering was observed in solid state. Furthermore, in atomic force microscopy (AFM) experiments, nanoscale phase separation was observed in thin films of FHBC and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) blends. The differences in molecular and bulk structural features were found to correlate with OFET and BHJ solar cell performance. Poor OFET and BHJ solar cells devices were obtained for FHBC compound 4 while compounds 5 and 6 gave excellent devices. In particular, the field‐effect mobility of FHBC 6, deposited by spin‐casting, reached 2.8 × 10−3 cm2 V−1 s and a power conversion efficiency of 1.5% was recorded for the BHJ solar cell containing FHBC 6 and PC61BM.

Combined effects of MoO3 interlayer and PC70BM on polymer photovoltaic device performance

Abstract The performance of bulk-heterojunction solar cells fabricated from a donor–acceptor structured polymer designed to exhibit the color green (PGREEN) as the electron-donor, along with [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM) as the electron-acceptor, are demonstrated with power conversion efficiency (PCE) up to 3.2% under air mass 1.5G, 100 mW/cm2. The effects of anode interface layers (PEDOT:PSS, MoO3) and different acceptors (PC60BM, bis-PC60BM, PC70BM) on the cell performance were investigated. We found that the combined effect of PC70BM and a MoO3 anode interlayer exhibits a 69% enhancement in PCE compared to cells with PEDOT:PSS and PC60BM. This enhancement in PCE is attributed to combined effects of improved charge transport and strong light absorption by PC70BM, and efficient hole extraction by the MoO3 interlayer.

Thickness dependence and solution-degradation effect in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester based solar cells

The performance of bulk-heterojunction solar cells made with poly(3-hexylthiophene) (P3HT) as the donor and phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor depends strongly on various factors in fabrication processes. This work studies the effect of the thickness of the P3HT:PCBM layer produced in the spin coating process on photovoltaic performances. Thickness dependent optical absorption in P3HT:PCBM layer is numerically modelled and the results are compared with experimental data. In addition, it is analyzed how degradation of air-exposed P3HT:PCBM blends depends on the storage time in nitrogen atmosphere.

Improving performance of organic solar cells using amorphous tungsten oxides as an interfacial buffer layer on transparent anodes

Thermally evaporated tungsten oxide (WO 3) films are investigated as a buffer layer on anodes to improve the performance of bulk-heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C 61 butyric acid methyl ester (PCBM-60). Characterization of the WO 3 film under study shows that it is amorphous with the conductivity on the order of 10 −6 S/cm and can effectively planarize an originally rough ITO surface down to the RMS roughness of 0.88 nm. Insertion of 5–30 nm-thick WO 3 layers resulted in power conversion efficiency and fill factor as high as 3.1% and 0.70, respectively, showing an improvement with a good batch-to-batch consistency over conventional cells. The observed improvement is related mainly to the relatively large shunt resistance of WO 3-based cells under illumination that is linked to a low carrier recombination resulting in high fill factor and open-circuit voltage.

Photovoltaic characterization of hybrid solar cells using surface modifiedTiO2 nanoparticles and poly(3-hexyl)thiophene

We report on the photovoltaic performance of bulk heterojunction solarcells using novel nanoparticles of 6-palmitate ascorbic acid surface modifiedTiO2 as an electron acceptor embedded into the donor poly(3-hexyl)thiophene (P3HT)matrix. Devices were fabricated by using P3HT with varying amounts of redTiO2 nanoparticles(1:1,1:2,1:3 w–w ratio). The devices were characterized by measuring current–voltage characteristics undersimulated AM 1.5 conditions. Incident photon to current efficiency (IPCE) was spectrallyresolved. The nanoscale morphology of such organic/inorganic hybrid blends was alsoinvestigated using atomic force microscopy (AFM).