Increase In Voc Research Articles

Influence of conjugated π-linker in D–D–π–A indoline dyes: towards long-term stable and efficient dye-sensitized solar cells with high photovoltage

The judicious choice of conjugated π-linkers is a critical strategy towards the energy-level engineering of donor–π–acceptor (D–π–A) sensitizers. Given that the vinyl bond on π-bridge thiophene segment can deteriorate its intrinsic photo-stability, we systematically study three D–D–π–A indoline dyes C-CA, WBS-1T and WBS-1F with different conjugated π-linkers (vinylthiophene, thiophene and furan) for high efficiency, long-term stable dye-sensitized solar cells (DSSCs). Compared with the vinylthiophene unit in C-CA, the conjugated π-linker of the thiophene or furan group in WBS-1T and WBS-1F can improve the solar cell performance with a great enhancement in the open-circuit photovoltage (VOC). As an overall result of the upshift of the TiO2 conduction band (CB) edge and the slow charge recombination, the VOC values are in the order WBS-1F (779 mV) > WBS-1T (756 mV) > C-CA (670 mV). Moreover, the CB edge shift of TiO2 is the major contribution to the large difference in VOC, accounting for 80% of the enhancement. Both the stepped light-induced transient measurements (SLIT) and the molecular dipole simulation are accounted for well by the observed superior photovoltage upon removal of the vinyl group in the conjugated π-linker. The higher molecular dipole moments can bring forth a more effective charge separation between donor and acceptor units, resulting in a remarkable increase in VOC. Using a liquid electrolyte, WBS-1F shows an impressively high efficiency of 9.49% with a high photovoltage of 779 mV. Its efficiency reaches 8.03% with ionic-liquid electrolyte while it reduces to 7.60% after a 1000 h aging test. Our work has shown that for D–π–A organic dyes, the vinyl unit in the conjugated π-linker is detrimental to the molecular dipole moment, the upshift of TiO2 CB edge, and the suppression of charge recombination, as well as the photo-stability.

Carbon Doped Nano-Crystalline TiO<sub>2</sub> Photo-Active Thin Film for Solid State Photochemical Solar Cells

Carbon doped titanium dioxide (C-TiO2) is considered as a promising photocatalytic material due to its optical absorption extended in the visible region compared to pure TiO2. However, in the field of photovoltaic’s, use of C-doped nano-crystalline titanium dioxide (C-TiO2) electrodes for light absorption has been considered to be unnecessary so far. In this context, we report here on the use of C-TiO2 nano-crystalline electrodes in photochemical solar cells devices (PCSC). Carbon doping has reduced the band gap of TiO2 to 2.41 eV and 2.25 eV with increase in the doping extent for the 9 mM C-TiO2 and 45 mM C-TiO2 respectively. The C-TiO2 electrodes were first used as photo electrodes for solar cells, exhibiting JSC of 1.34651 mA/cm2, VOC 0.683 V, FF 50.23% and η 0.46%. for the 9 mM C-TiO2 and exhibiting JSC of 1.34651 mA/cm2, VOC 0.815 V, FF 54.3% and η 0.59% for the 45 mM C-TiO2. The fabricated solar cell devices have shown an increase in VOC of up to 0.815 V, which is higher than that of 0.7 V for dye sensitized solar cells. The doping of carbon in TiO2 lattice was closely studied by SEM, XRD, RS and UV-Vis spectroscopy.

The Effect of Ga Content on the Recombination Behavior of Grain Boundaries in Cu(In,Ga)Se2 Solar Cells

ABSTRACTThe highest efficiency CuIn1-xGaxSe2 (CIGS) based solar cells have been produced from films with x∼0.3 which gives a value of Eg around 1.1-1.2eV. Increasing the Ga content of the CIGS absorber provides an increase in Voc, allows tuning of the band gap that can enhance performance under actual operating conditions, and potentially makes it possible to use CIGS films in multi-junction devices. However, champion cells have not yet been produced for values of x significantly greater than 0.3. This work focuses on how increased Ga content in CIGS films affects the recombination behavior of grain boundaries. Cathodoluminescence spectral imaging (CLSI) measurements on fully processed devices allow us to compare device properties with recombination behavior and optical properties of grain boundaries in films with different Ga content. Our data suggests that grain boundaries in high efficiency films with x∼0.3 exhibit a significant red shift in the CL spectra whereas grain boundaries in films with higher Ga content typically show either a small shift or none at all. This shift indicates band bending near the boundaries which could enhance charge separation and subsequent collection of carriers generated near grain boundaries. This is investigated statistically to identify trends in different regions of the films.

Effects of Ga- and Al-Codoped ZnO Buffer Layer on the Performance of Inverted Polymer Solar Cells

A Ga, Al-doped zinc oxide (GAZO) buffer layer was applied to inverted polymer solar cells (PSCs) based on P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C61-butyric acid methyl ester] blend films. The work function of the GAZO layer on indium-tin oxide (ITO) was measured to be 4.45 eV. The insertion of the GAZO layer between the ITO electrode and the P3HT:PCBM blend film in the inverted PSC led to an improved short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor (FF) compared to those of the reference cell without GAZO layer. The Jsc enhancement in the inverted PSC with the GAZO layer was attributed to both the effective electron extraction and the increased crystallinity of P3HT, and the work function difference between Ag and GAZO layer induced the increase in Voc. The improved FF value was due to the lowered series resistance and elevated shunt resistance. Thus, the power conversion efficiency of the device with the GAZO layer was improved by more than 200% relative to the reference cell.

Device Characteristics of CZTSSe Thin‐Film Solar Cells with 12.6% Efficiency

A 12.6% Cu2ZnSnSxSe4–x (CZTSSe) solar cell is presented with detailed device characteristics. Both short-circuit current density (Jsc) and open circuit voltage (Voc) increase in the 12.6% champion, relative to previous devices, due to better bulk CZTSSe quality and improved optical architecture. The reduction in Voc deficit shows opportunities to push CZTSSe solar cells to higher efficiency.

Graphene Quantum Dots from a Facile Sono-Fenton Reaction and Its Hybrid with a Polythiophene Graft Copolymer toward Photovoltaic Application

A new and facile approach for synthesizing graphene quantum dots (GQDs) using sono-Fenton reaction in an aqueous dispersion of graphene oxide (GO) is reported. The transmission electron microscopy (TEM) micrographs of GQDs indicate its average diameter as ∼5.6 ± 1.4 nm having a lattice parameter of 0.24 nm. GQDs are used to fabricate composites (PG) with a water-soluble polymer, polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethylaminoethyl methacrylate)] [PT-g-P(MeO2MA-co-DMAEMA), P]. TEM micrographs indicate that both P and PG possess distinct core-shell morphology and the average particle size of P (0.16 ± 0.08 μm) increases in PG (0.95 ± 0.45 μm). Fourier transform infrared and X-ray photoelectron spectrometry spectra suggest an interaction between -OH and -COOH groups of GQDs and -NMe2 groups of P. A decrease of the intensity ratio of Raman D and G bands (ID/IG) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp(2) domain, and in PG, the π-π* absorption of polythiophene (430 nm) of P disappears. The photoluminescence (PL) peak of GQD shifts from 450 to 580 nm upon a change in excitation from 270 to 540 nm. PL emission of PG at 537 nm is quenched, and it shifts toward lower wavelength (∼430 nm) with increasing aging time for energy transfer from P to GQDs followed by up-converted emission of GQDs. Both P and PG exhibit semiconducting behavior, and PG produces an almost reproducible photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with an indium-titanium oxide/PG/graphite device using the N719 dye exhibit a short-circuit current (Jsc) of 4.36 mA/cm(2), an open-circuit voltage (Voc) of 0.78 V, a fill factor of 0.52, and a power conversion efficiency (PCE, η) of 1.76%. Extending the use of GQDs to fabricate DSSCs with polypyrrole, both Voc and Jsc increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components.

Arrays of ZnO/AZO (Al-doped ZnO) nanocables: A higher open circuit voltage and remarkable improvement of efficiency for CdS-sensitized solar cells

Photoelectrode of nanocables (NCs) structure of ZnO nanowires (NWs) coated with Al-doped ZnO (AZO) shells was investigated for CdS quantum dots sensitized solar cells (QDSSCs). ZnO NWs serve as the frame for the preparation of AZO shells, in which electron transport more rapidly due to the more higher electron mobility of AZO (n-ZnO) than that of i-ZnO. AZO shells were assembled onto the surface of ZnO NWs via a spin-coating method. Optical band-gap of the ZnO/AZO films varies from 3.19eV for pure ZnO to 3.25eV for AZO (15%) depending on the Al-doping concentration. The PL intensity of AZO/ZnO, Voc, Jsc and η of the cells first increased and then decreased with the increase in the Al-doping (from 0% to 20%) and post-annealed temperature. Remarkably, the value of Voc can achieve above 0.8V after Al-doping. The dark current and absorption spectrum provided direct evidence of the increase in Jsc and Voc, respectively. Moreover, we discussed the effect of Al-doping on optical band-gap of the samples and the transfer of electron.

Rear passivation of commercial multi-crystalline PERC solar cell by PECVD Al2O3

Low-cost high-efficiency solar cells are the key to achieve grid parity with photovoltaic devices. Multi-crystalline silicon (mc-Si) solar cells are low-cost but low-efficiency. High-quality rear passivation is essential for the achievement of this goal. This dielectric stack, in conjunction with a local back surface field, produced mc-Si passivated emitter and rear cell (PERC) which efficiency exceeded 18.6%. The peak efficiency of 18.63% was achieved with Jsc of 37.44mA/cm2 and Voc of 638mV on commercial grade mc-Si wafers. Detailed characterization and modeling revealed that the increase in Voc and Jsc is the result of increased back surface reflectance from 74.8% to 90.7% and reduced back surface recombination velocity from 1800 to 260cm/s. It was observed that the efficiency of mc-Si PERC solar cells decreased in degradation experiment.

Enhancement of VOC without Loss of JSC in Organic Solar Cells by Modification of Donor/Acceptor Interfaces

Organic solar cells (OSCs) are promising low‐cost devices for generating electricity. In addition to fill factor, the short circuit current density (JSC) and the open circuit voltage (VOC) are two key factors that have critical influence on the device performance. The energy levels of the donor and acceptor materials are crucial for achieving a high JSC and VOC. However, the interfacial structures between the organic materials substantially affect the JSC and VOC through the energy of the charge transfer (CT) states and the charge separation and recombination reaction kinetics. Here, it is reported that separating the donor and acceptor layer in bilayer OSCs with a thin insulating layer increases the energy of the CT state by weakening the Coulomb interaction at the interface and this also suppresses photoinduced CT and recombination. Although these effects usually increase VOC and decrease JSC, the trade‐off is avoided by doping the insulating layer with a dye to utilize the energy transfer process. The increase in VOC without the reduction in JSC enhances the conversion efficiency of the OSCs by 30%.

Simultaneous enhancement in both large-area coatability and photovoltaic performance of inverted organic solar cells with co-solvent

We report our observation of simultaneous enhancement in large-area coatability and photovoltaic performance for blade-coated inverted P3HT:PCBM organic solar cells with DCB:hexane co-solvent. The addition of hexane improves greatly the wettability of P3HT:PCBM blend layer on Cs2CO3 treated ITO and leads to excessively higher P3HT surface concentration due to the incongruent evaporation of hexane and DCB. A post-processing light soaking was found to further improve the photovoltaic performance for blade-coated devices prepared with co-solvent by adjusting the P3HT surface concentration ratio for more favorable carrier transport, as evidenced by the disappearance of current suppression at forward bias and significant increase in Voc after light soaking. Since large-area manufacturing is the key to full commercialization of organic solar cells, the use of co-solvent, combined with light soaking, may be crucial for the development of inverted organic solar cells.

Effective passivation of silicon surface by AZO films: Application in bifacial solar cells

Aluminum rich zinc oxide (AZO) film was made on silicon substrates by spin coating using a precursor containing Al and Zn in the 20% atomic ratio. The films were annealed in air atmosphere and in hydrogen ambient. The minimum reflectance (Rλ) in air annealed (AZO:Air) samples was ∼2.5% in the vicinity (±50nm) of 750nm, however, the value was marginally lower (∼2%) in hydrogen annealed (AZO:H2) samples. The average Rλ over 300–1200nm range was 18.5% and 14.1% respectively. An additional XRD peak of H2Al10O16 at 2θ=51.5° corresponding to (121) reflection plane was observed in the AZO:H2 film. The AZO films were applied on the front and back surfaces of bifacial crystalline silicon (c-Si) solar cells where an improvement both in short circuit current density (Jsc) and open circuit voltage (Voc) was observed. The values of Jsc and Voc after AZO:Air were increased by 23% and ∼10mV respectively w.r.t without AZO layer cells which on subsequent annealing in H2 raised to 32% and ∼21mV. Jsc (AZO:Air) was used to calculate the expected enhancement in Voc which matched with the measured increase in Voc. The improvement in Jsc is due to reduced reflectance and, hence, the observed increase in Voc (AZO:Air) is attributed to it. On the other hand, increase in Voc after AZO:H2 step was much more and cannot be accounted only by Rλ, and therefore, the additional enhancement in Voc is attributed to back surface passivation that has occurred after annealing of the solar cells in hydrogen ambient. This attribution is substantiated by the internal and external quantum efficiencies (IQE and EQE) after back and front illumination of the solar cells and minority carrier lifetime measurements. The IQE data was used to calculate the diffusion length (Ln) of the base region where the value of Ln did not change after AZO:Air layer (=130μm for Cell #1) but its value substantially increased (=244μm) after the AZO:H2 treatment. This observation further reiterates our claim that the AZO film has surface passivation property.

Dependence of electrical and structural properties on the thickness of n-type μc-Si thin-film silicon solar cells grown by linear facing target sputtering

In this work, the construction of an (Al+Ag)/n+-Si/p-Si/Al thin-film silicon (TF-Si) solar cell is presented. The maximum achievable current density generated by a planar solar cell, with an optimum antireflection coating, for different values of the cell thickness are analysed. Both electrical and optical properties of (Al+Ag)/n+-Si/p-Si/Al TF-Si solar cell have been studied, which assumes the generation of one electron–hole pair per photon and a collection efficiency of unity. A reduction in thickness can lead to an increase in VOC, it can have the opposite effect if surface recombination is not reduced simultaneously. Thin-film solar cell has an absorbed photon flux density about three times higher at each interface, implying that carrier generation at the interfaces of the thinner cell is about three times higher. Finally, for larger grain size, the recombination at the grain boundaries (GBs) of Si will mainly degrade VOC, and not JSC. From the obtained results, it is evident that the larger the grain size, the better the performance of the device. However, the interface recombination has a strong influence on each parameter.

Compression-Induced Open Circuit Voltage Increase in All-Polymer Solar Cells with Lithium Fluoride Nanolayers

We report an open circuit voltage (VOC) increase by physical pressing of all-polymer solar cells with bulk heterojunction (BHJ) films of poly(3-hexylthiophene) and poly(9,9-dioctylfluorene-co-benzothiadiazole). When compression force was applied in the direction normal to the BHJ film plane, VOC was increased for all-polymer solar cells with a lithium fluoride (LiF) nanolayer (∼1 nm thick) between the BHJ and cathode layers. However, VOC was rather decreased for all-polymer solar cells without the LiF layer upon compression. Interestingly, the VOC change (both increase and decrease) was linearly proportional to the intensity of pressing (compression) forces. The reason for the VOC increase in the device with the LiF layer has been assigned to the improved LiF dipole alignment and interfacial contacts during rearrangement of the nanolayer (LiF) nanomorphology upon pressing.

Efficiency enhancement of Cu(In,Ga)Se2 thin‐film solar cells by a post‐deposition treatment with potassium fluoride

AbstractAlkali‐free Cu(In,Ga)Se2(CIGS) absorbers grown on Mo‐coated alumina (Al2O3) substrates were doped with potassium (K) after CIGS growth by a potassium fluoride (KF) post‐deposition treatment (PDT). The addition of K to the absorber leads to a strong increase in cell efficiency from 10.0% for the K‐free cell to 14.2% for the K‐doped cell, mainly driven by an increase in the open‐circuit voltage Voc and the fill factor FF, and to an increase in the net charge carrier density. Hence K doping by KF‐PDT is comparable to doping with Na.magnified imageJ –V characteristics of a CIGS solar cell with KF‐PDT (red solid line) and of a K‐free reference cell without treatment (black dashed line). The solar cell treated with K shows a strong increase in Voc and FF compared to the solar cell without K.(© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrochemical approach to enhance the open-circuit voltage (Voc) of dye-sensitized solar cells (DSSCs)

The net enhancement of open-circuit voltage (Voc) of dye-sensitized solar cells (DSSCs) was achieved with the novel electrolyte composed of interhalogen based binary-redox couples without regard to the choice of sensitizers. The interhalogen ion, I2Br−, was formed in the conventional iodine-based electrolyte by both chemically and electrochemically and it was found to produce an extra redox pair, (I−, Br−)/I2Br−, with new energy state at a more positive potential than that of I−/I3−. The Fermi level of the electrolyte shifted positively by the weighted-average of the two redox systems. It induced the increase of Voc up to 30, 60, and 50mV for cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (N719), cis-dicyano-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) (Ruthenizer 505 or Ru505), and 2′,4′,5′,7′-tetrabromofluorescein (eosin Y) dyes, respectively, without affecting the short circuit current much. It corresponded to the enhancement of the overall power conversion efficiency (η) up to 8, 14, and 13%, respectively. Moreover, the degree of enhancement of Voc was controllable by varying [I2Br−] since the higher the contribution of [I2Br−] the more positive shift in the energy level of the binary redox couples.

Enhanced open-circuit voltage in organic photovoltaic cells with partially chlorinated zinc phthalocyanine

The open-circuit voltage (V OC) in organic photovoltaic cells has been shown to depend on a number of parameters including the energy levels of the active materials, active layer structure, illumination intensity, and operating temperature. Here we report, a significant increase in V OC from 0.43 to 0.63 V in zinc phthalocyanine (ZnPc)/C60 planar heterojunction photovoltaic cells operated at room temperature under 1 sun AM1.5G solar illumination, when a home-synthesized and purified ZnPc source materials was used instead of a commercially obtained (and home purified) ZnPc source. While the two ZnPc source materials have nearly identical UV–Vis and IR absorption properties, the home-synthesized ZnPc contains chlorinated derivatives and has half of the electrical defect density (on the order of 1016 cm−3) as compared to the commercial ZnPc. The improved V OC in devices with the home-synthesized ZnPc is contributed from both a lower dark current and a higher magnitude of photocurrent. Additional experiments revealed that the different device characteristics are mostly associated with processes occurring at the ZnPc/C60 interface, which we attribute to nongeminate recombination of charges built-up on either side of the interface.

Formation of vertical concentration gradients in poly(3-hexylthiophene-2,5-diyl): Phenyl-C61-butyric acid methyl ester-graded bilayer solar cells

In the present work, we demonstrate that graded bilayer solar cells provide a very interesting alternative to the bulk heterojunction active layers commonly used in organic photovoltaic cells. One of the main advantages of this type of active layers is the possibility to optimize independently both donor and acceptor layers. Using various process methods, we obtain active layers that demonstrate a donor–acceptor vertical concentration gradient. These devices exhibit not only a high fill factor but also a remarkable increase in open-circuit voltage (Voc). In order to understand the influence of the film morphology over the device parameters, we provide a complete study using energy-dispersive x-ray spectroscopy elemental mapping of the device cross sections, showing evidence that ideal donor–acceptor concentration gradient are required to obtain high fill factors. Furthermore, we use a simple equivalent electrical model to extrapolate device parameters such as reverse saturation current for a clearer understanding of the origin of the Voc increase.

Electrophoretically self-assembled mixed metal oxide-TiO2 nano-composite film structures for photoelectrochemical energy conversion: Probing of charge recombination and electron transport resistances

Nanotitania/metal oxide composite films are prepared via the coupling of low DC voltage Electrophoretic Deposition (EPD) of P25 TiO2 nanoparticles with simultaneous electrolytic deposition of hydrous metal oxides, namely ZnO, MgO and Al2O3. The nanocomposite films are built into dye-sensitized solar cell photoanodes and their interfacial charge recombination and electronic resistances are investigated. The nano hydrous oxides were found to co-deposit (1–3 wt%) uniformly within the TiO2 film (forming island-like nanodeposits) significantly increasing film adhesion. Analysis via Electrochemical Impedance Spectroscopy and Open Circuit Voltage Decay techniques found, among the three composite films, the TiO2–Al2O3 electrode to exhibit the highest charge recombination resistance at the TiO2/electrolyte interface (Rrec) and as consequence an increase in Voc. However, its conversion efficiency (4.14%) was the lowest because it suffered from very high electron transport resistance (Rt) in the TiO2 network. By comparison, the TiO2–MgO film resulted in 5.40% efficiency and the TiO2–ZnO film in 5.85% efficiency-both exhibiting significantly lower Rt resistance. The obtained results point to the need for simultaneous optimization of the nanocomposite TiO2/metal oxide film structure in terms of high interfacial charge recombination resistance and low overall electron transport resistance.

Performance Enhancement of Dye-Sensitized Solar Cells Using an Ester-Functionalized Imidazolium Iodide as the Solid State Electrolyte

Linking an ester group to the imidazolium ring has been demonstrated to improve solar cell performance in terms of short-circuit photocurrent (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) in particular, when the imidazolium iodide mixed with iodine and LiI is used as a solid state electrolyte of dye-sensitized solar cells. Herein, the effect of ester group on solar cell performance has been investigated by means of intensity modulated photocurrent/photovoltage and electrochemical impedance spectroscopy. From the alkyl- to ester-functionalized imidazolium iodide, the increase in Jsc is attributed to the increased charge collection efficiency due to the enhanced conductivity, the increase in Voc is caused by the upward shift of conduction band edge of TiO2, which compensates for the voltage loss arising from the higher charge recombination rate, and the remarkable increase in FF is attributed to the decreased series resistance along with the increased Voc and decreased diode quality factor.

Post-annealing Effects on Characteristics of Crystalline Germanium Solar Cells with the Double Heterostructure

We fabricated crystalline germanium (c-Ge) heterojunction solar cells with a double heterostructure (DH) which consists of hydrogenated amorphous silicon (a-Si:H) layers. In the a-Si:H/c-Ge DH solar cell, the improvement of the conversion efficiency is obtained compared to the c-Ge solar cell having single heterostructure. We investigated effects of post-annealing on characteristics of Ge heterojunction solar cells. The open-circuit voltage (VOC) of Ge heterojunction solar cells increases with increasing post-annealing temperatures above 150 °C. The increase of VOC by post-annealing could be mainly explained by the improvement of passivation quality of a-Si:H for the c-Ge surface. Our results suggest that the a-Si:H double heterostructure is beneficial not only for c-Si-based heterojunction solar cells but also for c-Ge-based heterojunction solar cells.