Heterojunction Organic Solar Cells Research Articles

Improvement of Stability for Small Molecule Organic Solar Cells by Suppressing the Trap Mediated Recombination.

To understand the degradation mechanism of organic solar cells (OSCs), the charge dynamics of conventional and inverted planar heterojunction OSCs based on boron subthalocyanine chloride (SubPc) and fullerene (C60) with identical buffers during the air exposure were investigated. The results of light intensity dependent open circuit voltage show that the bimolecular recombination is dominated in the fresh devices, regardless of the device structure. The appearance of transient peak in photocurrent after turn-on and the light intensity independent turn-off traces in transient photocurrent suggest that the rapid degradation of conventional device is due to the energy loss originated from the aggravated trap mediated recombination. In contrast, the half-lifetime of inverted device is ∼25 times longer than the conventional one. The improvement of stability is ascribed to the decrease of the trap generation possibility and the suppression of trap mediated recombination in the case of inverted structure, where the penetration of oxygen and water through buffer layer is avoided.

Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells

The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer–polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semicrystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈140 °C) at temperatures of 90, 100, and 110 °C for time periods from 2 min up to 250 min. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1 nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.

Fabrication and characterization of semiconducting single-walled carbon nanotube-based bulk hetero junction organic solar cell using spin coating technique

The fabrication of bulk hetero junction (BHJ) organic solar cell has been developed with the cell structure: Graphene/S-SWCNT-C60 blend/Al. In this work, a glass substrate is used. Initially, a gra...

Carrier injection dynamics in heterojunction solar cells with bipolar molecule

A boron subphthalocyanine chloride (SubPc) is a bipolar molecule and is used in hetero-junction organic solar cells. Here, we investigated the carrier injection dynamics from the donor α-sexithiophene (6T) or acceptor C60 layers to the bipolar SubPc layer by means of the femtosecond time-resolved spectroscopy. We observed gradual increase of the SubPc– (SubPc+) species within ≈300 ps. The increases are interpreted in terms of the exciton diffusion within the 6T (C60) layer and subsequent electron (hole) injection at the interface. In 6T/SubPc heterojunction, the electron injection is observed even at 80 K. The robust electron injection is ascribed to the efficient charge separation within the 6T layer under photo exciation at 400 nm.

Equivalent Circuit Model of Semiconducting Single Walled Carbon Nano Tube based Bulk Hetero Junction Organic Solar Cell in Comparison with Experimental Results

In this paper, the characteristics of organic bulk heterojunction solar cells have been designed and modeled using MATLAB R2010a version. Here, organic solar cell is considered as a diode and an equivalent circuit is developed with series and shunt resistances to obtain the I-V characteristics of the model. Non-linear equations are obtained in by solving the equivalent circuit model. These non-linear equations are solved using Newton-Raphson method. The theoretical results thus obtained are compared with practical results and an observation made is that are theoretical results are close to the results obtained from fabricated structure.

CuSbS2‐Sensitized Inorganic–Organic Heterojunction Solar Cells Fabricated Using a Metal–Thiourea Complex Solution

The device performance of sensitizer-architecture solar cells based on a CuSbS2 light sensitizer is presented. The device consists of F-doped SnO2 substrate/TiO2 blocking layer/mesoporous TiO2 /CuSbS2 /hole-transporting material/Au electrode. The CuSbS2 was deposited by repeated cycles of spin coating of a Cu-Sb-thiourea complex solution and thermal decomposition, followed by annealing in Ar at 500 °C. Poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)) (PCPDTBT) was used as the hole-transporting material. The best-performing cell exhibited a 3.1 % device efficiency, with a short-circuit current density of 21.5 mA cm(-2) , an open-circuit voltage of 304 mV, and a fill factor of 46.8 %.

Novel Approaches in the Design of Donor-Acceptor Oligomeric and Polymeric Materials for Photovoltaic Applications: D/A Blend versus Self-assembly of D/A by Covalent or Non-Covalent Interaction

Organic semiconducting materials involving small -conjugated molecules as well as polymers are fast growing members in the family of potential materials for harvesting non-conventional source of energy like the solar radiation.1-8 Organic photovoltaic cells (OPVs) are a revolutionary technology which converts the solar radiations into electrical energy using organic/polymeric semiconducting material (active layer). The technology has become very promising due to its low fabrication cost, lightweight, large area and mechanical flexibility. The major breakthrough in OPV was created by Tang et al. who reported the first hetero junction organic solar cell with power conversion efficiency (PCE) ~ 1 %.1,9Although intensely researched for more than two decades, the efficiency of organic solar cell remains lower compared to efficiency based on inorganic crystalline silicon which has reached up to ~ 25 %. However, promising breakthroughs in terms of better device performance efficiencies, device designs as well as longer outdoor performance of test modules have kept the field ongoing.10-13 The basic OPV device architecture consists of a photo-active layer of organic /polymeric semiconductor sandwiched between indium tin oxide (ITO) electrode (anode) and metal electrode (cathode).3 Tang implemented modifications involving a bilayer heterojunction OPV device comprising of p-type donor and n-type acceptor as active layer, which improved the device performance.1,9 The working principle of such device involves the light absorption by active layer which lead to creation of electron-hole pair i.e. exciton. The next step is the dissociation of excitons; which recombined by emitting photon or non-radioactive recombination if eventual separation to its component electron and hole did not occur within the short exciton diffusion lenght.1,3, 6-8 Once the charges are separated into electrons and hole, they move towards the respective electrode and finally charge collection leads to electric current. The charge extraction is driven by internal electric field in cell caused by different work function of electrodes. It is widely accepted that the exciton diffusion at D/A interface is the most important step that depends upon how the donor and acceptor are arranged in the nanoscopic length scale of about 10-20 nm which is critical to the exciton life time. Generally, the life time of exciton is very short with diffusion length limited to 5-20 nm after which they will recombine and fail to generate free charges.3, 6, 8 The exciton diffusion is considered to be the most important aspect during design of active layer of solar cell. Therefore, morphology of the active layer component (donor-acceptor) plays a vital role in the device efficiency of solar cells.14

Simulation Studies on Semiconducting Single Walled Carbon Nanotube Based Bilayer and Bulk Hetero Junction Organic Solar Cells

The characteristics of the bilayer and bulk hetero junction (BHJ) model organic solar cell have been modeled and simulated using Silvaco Atlas TCAD. The cell structure used is Graphene/S-SWCNT:C60/Al. The simulations are carried out to obtain the I-V characteristics of an OSC for both the models. The parameters obtained from I-V characteristics are open circuit voltage (Voc) and short circuit current (Isc), maximum voltage (Vm) and maximum current (Im). Using those parameters short circuit current density (Jsc), fill factor (FF) and efficiency are calculated. In bilayer model OSC, efficiency decreased (1.6 to 0.27%) by increasing the thickness of active layer. It shows poor agreement between the exciton diffusion and absorption lengths. Therefore, BHJ model OSC is designed to overcome the exciton diffusion length limitations in bilayer model. It is found that, the efficiency increases with an increase in active layer thickness (5% to 10.4%) for BHJ model. The simulations are carried out to study the effect of active layer thickness on those parameters.

Simulation of Organic Solar Cell at Different Active Layer Thickness

The J-V characteristics of organic hetero junction solar cell of different active layer thicknesses have been simulated by AMPS-1D software. A composite of P3HT: PCBM is used as photo active layer material, sandwiched between a transparent ITO electrode and Al back side contact. The simulation results clearly demonstrated that the dark J-V characteristics and electric field across the junction of hetero junction solar cell are affected by the active layer thickness.

Effects of Damkhöler number of evaporation on the morphology of active layer and the performance of organic heterojunction solar cells fabricated by electrospray method

The electrospray (ES) process has emerged as a scalable and efficient fabrication method for organic photovoltaic cells (OPVs). However, the ES process could often involve numerous parameters, which impose a major challenge on uncovering the interplay among process, morphology of active layers, and device performance. This work attempts to reduce the parameter space and capture the essence of the ES process using the Damkhöler (Da) number of evaporation, which is the ratio of the droplet residence time over evaporation time. We first derived an explicit equation for Da that links nine different parameters affecting the process. Experimental results indicate that Da number shows strong effect on morphology and crystallinity of the active layer composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Most remarkably, the power conversion efficiency exhibits monotonically dependence on Da values spanning more than one order of magnitude, e.g., from 0.13 to 1.52. This finding suggests that Da number analysis can reduce the large parameter space in electrospray deposition, thus provides a simple way to control and predict the morphology of active layer and the performance of the solar cells.

A 12%-efficient upgraded metallurgical grade silicon-organic heterojunction solar cell achieved by a self-purifying process.

Low-quality silicon such as upgraded metallurgical-grade (UMG) silicon promises to reduce the material requirements for high-performance cost-effective photovoltaics. So far, however, UMG silicon currently exhibits the short diffusion length and serious charge recombination associated with high impurity levels, which hinders the performance of solar cells. Here, we used a metal-assisted chemical etching (MACE) method to partially upgrade the UMG silicon surface. The silicon was etched into a nanostructured one by the MACE process, associated with removing impurities on the surface. Meanwhile, nanostructured forms of UMG silicon can benefit improved light harvesting with thin substrates, which can relax the requirement of material purity for high photovoltaic performance. In order to suppress the large surface recombination due to increased surface area of nanostructured UMG silicon, a post chemical treatment was used to decrease the surface area. A solution-processed conjugated polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was deposited on UMG silicon at low temperature (<150 °C) to form a heterojunction to avoid any impurity diffusion in the silicon substrate. By optimizing the thickness of silicon and suppressing the charge recombination at the interface between thin UMG silicon/PEDOT:PSS, we are able to achieve 12.0%-efficient organic-inorganic hybrid solar cells, which are higher than analogous UMG silicon devices. We show that the modified UMG silicon surface can increase the minority carrier lifetime because of reduced impurity and surface area. Our results suggest a design rule for an efficient silicon solar cell with low-quality silicon absorbers.

Improvement of organic solar cell efficiency by solution-processed doping of pentacene in PEDOT:PSS layer

In the current research, organic solar cells (OSCs) with various concentrations of pentacene in Poly(ethylenedioxythiopene):Poly(styrenesulfonate) (PEDOT:PSS) interface layer were investigated for better hole extraction. The ITO/Pentacene + PEDOT:PSS/P3HT:PCBM/Al-fabricated solar cell fabricated via brush coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The ITO/Pentacene + PEDOT:PSS/P3HT:PCBM/Al solar cells deliver a VOC ~350 mV and 2.57% efficiency. It is observed that the optimized concentration of pentacene doping in PEDOT:PSS layer, along with an active layer of P3HT and PC60BM, doubles the efficiency of the device, when compared with pristine PEDOT:PSS layer. The degradation studies of the fabricated bulk heterojunction OSCs reveal that the degrading abilities of ITO/Pentacene + PEDOT:PSS/P3HT:PCBM/Al solar cells are 60% more better than those of ITO/PEDOT:PSS/P3HT:PCBM/Al devices. Thus, this work will ultimately contribute toward fully solution processed painted device, which will provide low-cost manufacturing and improved stability of pentacene-based organic photovoltaics.

PEDOT:PSS-free organic solar cells fabricated on buffer and anode integrated Ta-doped In2O3 films

We developed poly(3,4-ethylene dioxylene thiophene):poly(styrene sulfonic acid) (PEDOT:PSS)-free organic solar cells (OSCs) using buffer and anode-integrated Ta-doped In2O3 (ITaO) films. By co-sputtering of Ta2O5 and In2O3 targets, we obtained buffer and anode-integrated ITaO electrodes with a sheet resistance of 17.11Ω/square, a transmittance of 93.45%, and a work function of 4.9eV, all of which are comparable to conventional ITO anodes. Since ITaO electrodes have similar electrical and optical transmittance to those of ITO films, the conventional bulk heterojunction OSC with ITaO anode showed a power conversion efficiency (PCE) of 3.348% similar to the OSCs (3.541%) with an ITO anode. In addition, OSCs fabricated on an ITaO electrode successfully operated without an acidic PEDOT:PSS buffer layer and showed a PCE of 2.634%, which was much higher than the comparable OSC with an ITO anode. Therefore, co-sputtered ITaO films simultaneously acting as a buffer and an anode layer can be considered promising transparent electrodes for cost-efficient and reliable OSCs because they can eliminate the use of an acidic PEDOT:PSS buffer layer.

Highly Improved Sb2S3 Sensitized‐Inorganic–Organic Heterojunction Solar Cells and Quantification of Traps by Deep‐Level Transient Spectroscopy

The light‐harvesting Sb2S3 surface on mesoporous‐TiO2 in inorganic–organic heterojunction solar cells is sulfurized with thioacetamide (TA). The photovoltaic performances are compared before and after TA treatment, and the state of the Sb2S3 is investigated by X‐ray diffraction, X‐ray photoelectron spectroscopy, and deep‐level transient spectroscopy (DLTS). Although there are no differences in crystallinity and composition, the TA‐treated solar cells exhibit significantly enhanced performance compared to pristine Sb2S3‐sensitized solar cells. From DLTS analysis, the performance enhancement is mainly attributed to the extinction of trap sites, which are present at a density of (2–5) × 1014 cm−3 in Sb2S3, by TA treatment. Through such a simple treatment, the cell records an overall power conversion efficiency (PCE) of 7.5% through a metal mask under simulated illumination (AM 1.5G, 100 mW cm–2) with a very high open circuit voltage of 711.0 mV. This PCE is, thus far, the highest reported for fully solid‐state chalcogenide‐sensitized solar cells.

Effect of crystallinity in small molecular weight organic heterojunction solar cells

Planar-heterojunction (PHJ; pn) solar cells were fabricated using a thermally convertible well soluble benzoporphycene (BPc) precursor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The conversion temperature was controlled in order to investigate the influence of the crystallinity of the organic semiconducting layer in the PHJ solar cell on the cell performance. The crystallinity of BPc increased with annealing temperature, but the surface roughness did not change. The cell with BPc converted at 250 °C had a power conversion efficiency that was 2.5 times better (1.49%) than that of the cell converted at 150 °C, and 1.5 times better than the cell with a vacuum-deposited BPc layer. The mobility of BPc, measured from the space charge limited current characteristics, supports a clear correlation between crystallinity and cell performance. Thus, the importance of increasing the crystallinity and mobility in organic solar cells was experimentally demonstrated.

Solution processed thin films of Nb-doped TiO2 nanoparticles as hole blocking layer for organic solar cells

In this study, Nb-doped TiO2 nanoparticles were synthesized by sol–gel method with controlled doping ratios up to 6mol%. Catalyzed by p-toluene sulfonic acid, alkoxide precursors of Ti and Ni were stabilized by acetylacetone ligand, and converted to the organically modified nanoparticles which were well dispersible in alcohols. Transmission electron microscopy and X-ray diffraction measurements revealed that TiO2 nanoparticle possesses anatase morphologies, and the doping ratios were confirmed by X-ray photoelectron spectroscopy. Uniform thin films of Nb-doped TiO2 were obtained by spin coating dilute alcohol dispersions of Nb-doped nanoparticles. Ellipsometric characterizations were carried out for the as-coated film and the sintered film as well to characterize the indices of refraction, and atomic force microscopy showed subnanometer-scale roughness of Nb-doped films. Owing to the improved roughness via doping, Nb-doped TiO2 thin films exhibited enhanced solar cell efficiencies when used as hole-blocking layer of organic hetero-junction solar cell.

Enhanced Performance in Perovskite Organic Lead Iodide Heterojunction Solar Cells with Metal-Insulator-Semiconductor Back Contact

Metal-insulator-semiconductor back contact has been employed for a perovskite organic lead iodide heterojunction solar cell, in which an ultrathin Al2O3 film as an insulating layer was deposited onto the CH3NH3PbI3 by atomic layer deposition technology. The light-to-electricity conversion efficiency of the devices is significantly enhanced from 3.30% to 5.07%. Further the impedance spectrum reveals that this insulating layer sustains part of the positive bias applied in the absorber region close to the back contact and decreases the carrier transport barrier, thus promoting transportation of carriers.

Defect-Induced Loss Mechanisms in Polymer–Inorganic Planar Heterojunction Solar Cells

The effect of ZnO defects on photoexcited charge carrier recombination and forward induced charge transfer was studied in organic-inorganic bilayer organic heterojunction solar cells. Decreased bimolecular recombination via passivation of ZnO nanopariticle defects resulted in longer carrier lifetime as determined by transient photovoltage (TPV) measurements. It was also found by time-resolved photoluminescence (TRPL) measurements that defect passivation decreased the fluorescence lifetime which indicated higher exciton dissociation efficiency. Through passivation of the ZnO nanoparticles defects, the two loss mechanisms were reduced and the power conversion efficiency (PCE) is significantly enhanced.

Electronic structure effect of polythiophene derivatives and nano N-Zn-Ag/TiO2 on performance of organic thin film solar cell

Nanocrystal N-Zn-Ag/TiO2 powders were prepared with N-Zn/TiO2 by photo deposition method. A series of pure polymers P3HT[poly(3-hexylthiophene)], P3OT[poly(3-octylthiophene)], P3DT[poly(3-decylthiophene)] and P3DDT[poly(3-dodecylthiophene)], was synthesized, which were used to synthesize p-n type semiconductor materials P3HT/N-Zn-Ag-TiO2, P3OT/N-Zn-Ag-TiO2, P3DT/N-Zn-Ag-TiO2 and P3DDT/N-Zn-Ag-TiO2 by in situ chemical method. X-Ray diffraction(XRD) and infrared(IR) spectroscopy showed the structure of the polymers and complexes. Ultraviolet-visible(UV-Vis) spectra and cyclic voltammograms(CV) showed the optical and electronic performance of the polymers and complexes. Two new single and double organic thin film heterojunction solar cells were prepared with the above mentioned synthesized powders as raw materials. Current-voltage(I–V) measurements indicate that the conversion efficiency of the single organic thin film heterojunction solar cell is higher than that of the double organic thin film heterojunction solar cells. Single organic thin film heterojunction solar cells based on P3DT/N-Zn-Ag-TiO2 can get a photoelectric conversion efficiency of 0.0408%. The performance of electronic transform between electron donor and acceptor on organic thin film solar cells was researched.

Improved performance of ZnO nanostructured bulk heterojunction organic solar cells with nanowire-density modified by yttrium chloride introduction into solution

Yttrium doped zinc oxide (ZnO) nanowires prepared on seedless indium tin oxide (ITO) substrates were used for nanostructured bulk heterojunction organic solar cells (OSCs). Without patterning by a mask, the optimized areal density of ZnO nanowires decreased the reflection of incident light impinging upon the substrate by more than 50%, resulting in better incident-photon to current efficiencies (IPCEs) than for solar cells without ZnO nanostructures. OSC decorated with yttrium doped ZnO nanostructures showed an increase in current density from 8.9mA/cm2 to 9.9mA/cm2 resulting in 15% better device performance than a planar bulk heterojunction OSC. It was found that the antireflective effect due to ZnO nanowires was maximized with an areal density of 50 nanowires per 100μm2. The areal density was optimized by incorporation of 0.02mM yttrium chloride into the growth solution during nanostructure formation. In addition, OSC that incorporated the optimized areal density and length of ZnO nanostructures showed a 25% improved power conversion efficiency of 2.5% compared to OSC prepared on an ITO substrate without the ZnO nanostructures. Our results indicate that growth of density-controlled ZnO nanowire arrays on seedless ITO substrates using yttrium dopant is simple, low cost, and easily controllable method that enables low temperature fabrication, without patterning, of antireflective structures for significantly enhanced efficiency bulk heterojunction OSCs.