Organic thin-film solar cell employing a novel electron-donor material

A highly efficient organic thin-film solar cell based on a heterojunction structure employing a novel electron-donor (ED) material, tetraphenyldibenzoperiflanthene (DBP), has been demonstrated for the first time. An organic photovoltaic (OPV) cell with 0.033-cm² active area, comprising DBP as an ED layer, fullerene C₆₀ as an electron-acceptor (EA) layer, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as an exciton-blocking (EB) layer, has exhibited an open-circuit voltage (V_oc) of 0.92 V, a short-circuit current density (J_sc) of 6.3 mA/cm² and a conversion efficiency of 3.6% at 100-mW/cm² simulated AM1.5G sunlight. Meanwhile, those of a conventional cell employing copper phthalocyanine (CuPc) for an ED layer are 0.51 V, 4.3 mA/cm², and 1.4%, respectively. The high V_oc and J_sc of the DBP-based cell is attributed to the DBP's highest occupied molecular orbital (HOMO) level 5.5 eV and the effective light absorption, respectively.     

Highly efficient 1 μm thick CdTe solar cells with textured TCOs

Thickness reduction of CdTe absorption layer down to 1 μm has been achieved by controlling the temperature profile used during the close-spaced sublimation (CSS) growth. Transparent conducting oxides, such as indium tin oxide (ITO) and textured fluorine doped tin oxide (SnO₂:F) films have been investigated as transparent electrodes for such 1-μm-thick CdTe absorption layers to increase the incident light confinement and thus to achieve higher conversion efficiency. The contribution in solar cell performance has been found in the case of textured TCOs with optimum haze ratio (roughness). Conversion efficiencies of 10.6% (V_oc: 0.75 V, J_sc: 22.02 mA/cm², FF: 0.64, area: 1 cm²) and 11.2% (V_oc: 0.78 V, J_sc: 22.6 mA/cm², FF: 0.63) have been achieved for only 0.6-μm-thick CdTe absorption layers with SnO₂:F-TCO of 11% and 3% of haze ratios, respectively.     

TiO2–Au plasmonic nanocomposite for enhanced dye-sensitized solar cell (DSSC) performance

Anatase TiO₂ nanoparticles dressed with gold nanoparticles were synthesized by hydrothermal process by using mixed precursor and controlled conditions. Diffused Reflectance Spectra (DRS) reveal that in addition to the expected TiO₂ interband absorption below 360 nm gold surface plasmon feature occurs near 564 nm. It is shown that the dye sensitized solar cells made using TiO₂–Au plasmonic nanocomposite yield superior performance with conversion efficiency (CE) of ～6% (no light harvesting), current density (J_SC) of ～13.2 mA/cm², open circuit voltage (V_oc) of ～0.74 V and fill factor (FF) 0.61; considerably better than that with only TiO₂ nanoparticles (CE ～ 5%, J_SC ～ 12.6 mA/cm², V_oc ～ 0.70 V, FF ～ 0.56).     

Influence of electrolytes on the photovoltaic performance of organic dye-sensitized nanocrystalline TiO2 solar cells

We have studied the influence of electrolytes on the photovoltaic performance of mercurochrome-sensitized nanocrystalline TiO₂ solar cells using LiI, LiBr, and tetraalkylammonium iodides as the electrolyte. Short-circuit photocurrent density (J_sc) and open-circuit photovoltage (V_oc) depended strongly on the electrolyte. J_sc of 3.42 mA cm⁻² and V_oc of 0.52 V were obtained for the LiI electrolyte and J_sc of 2.10 mA cm⁻² and V_oc of 0.86 V were obtained for the Pr₄NI electrolyte. This difference in photovoltaic performance was due to the change in the conduction band level of the TiO₂ electrode. Large V_oc of 0.99 V was obtained for the LiBr electrolyte due to the large energy gap between the conduction band level of TiO₂ and the Br/Br₂ redox potential. Solar cell performance also depended strongly on organic solvent, suggesting that the physical properties of solvents such as Li ion conductivity and donor number affect photovoltaic performance.     

A photovoltaic cell from p-type boron-doped amorphous carbon film

Boron-doped amorphous carbon (a-C(B)) films were prepared on n-type silicon using pulsed laser deposition technique of a graphite target. The a-C(B) films have been proved to be p-type by the formation of a heterojunction between the a-C(B) film and n-Si. The device of a-C(B)/n-Si structure yielded an open-circuit voltage (V_oc) of 0.27 V and a short-circuit current density (J_sc) of 2.2 mA/cm² under illumination (AM1.5 100 mW/cm²). According to calculation, the energy conversion efficiency and fill factor were found to be about 0.3% and 0.53, respectively.     

Ribbon Si solar cells with efficiencies over 18% by hydrogenation of defects

We have fabricated 4 cm² solar cells on String Ribbon Si wafers and edge-defined film-fed grown (EFG) Si wafers with using a combination of laboratory and industrial processes. The highest efficiency on String Ribbon Si wafer is 17.8% with an open circuit voltage (V_oc) of 620 mV, a short circuit current density (J_sc) of 36.8 mA/cm² and a fill factor (FF) of 0.78. The maximum efficiency on EFG Si is 18.2% with a V_oc of 620 mV, a J_sc of 37.5 mA/cm² and a FF of 0.78. These are the most efficient ribbon Si devices made to date, demonstrating the high quality of the processed Si ribbon and its potential for industrial cells. Co-firing of SiN_x and Al by rapid thermal processing was used to boost the minority carrier lifetime of bulk Si from 3–5 μs to 70–100 μs. Photolithography-defined front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The effects of firing temperature and time were studied to understand the trade-off between hydrogen retention and Al-doped back surface field (Al-BSF) formation. Excellent bulk defect hydrogenation and high-quality thick Al-BSF formation was achieved in a very short time (1 s) at firing temperatures of 740–750 °C. It was found that the bulk lifetime decreases at annealing temperatures above 750 °C or annealing time above 1 s due to dissociation of hydrogenated defects.     

CIS thin film solar cells with evaporated InSe buffer layers

This paper describes the investigations of CIS-based solar cells with a new In_xSe_y (IS) buffer layer. Studies were concentrated on determining the deposition conditions to get In_xSe_y thin films with adequate properties to be used in substitution of the CdS buffer layer, usually employed in the fabrication of this type of devices. Before the solar cell fabrication, the buffer layers grown by evaporation of the In₂Se₃ compound were characterized through transmittance and X-ray diffraction measurements. It was found that good results can be obtained using indium selenide film as the buffer layer, grown in the In₂Se₃ phase.Solar cells with structure Mo/CIS/In₂Se₃/ZnO were fabricated. The ZnO layer was deposited by reactive evaporation and the absorber CIS layer was grown on Mo by a two-stage process. The preliminary results obtained with this type of solar cells are J_sc=30.8 mA/cm², V_oc=0.445 V, FF≈0.6 and η=8.3% with an irradiance of 100 mW/cm². Solar cells fabricated using a CdS buffer layer deposited by CBD on CIS substrate, prepared under the same conditions used in the fabrication of Mo/CIS/In₂Se₃/ZnO cells, gave the following results: V_oc=0.43 V, J_sc=34 mA/cm², FF≈0.63 and η=9.2%.     

Organic ultraviolet photovoltaic diodes based on copper phthalocyanine as an electron acceptor

Organic ultraviolet (UV) light-sensitive photovoltaic (PV) diodes, based on 4, 4′, 4″-tris-(2-methylphenyl phenylamino) triphenylamine (m-MTDATA) as an electron donor and copper phthalocyanine (CuPc) as acceptor, have been fabricated. The PV diode exhibits high open-circuit voltage (V_OC) of 1.05 V under illumination of 365 nm UV light with 1.7 mW/cm², although the CuPc was generally used as electron donor in other PV diodes. And the short-circuit current (I_SC) of 54.6 μA/cm², fill factor (FF) of 0.304 and power conversion efficiency (η_e) of 1.03% are respectively achieved. This diode can accurately detect the UV radiation according to photo-generated voltage signal.     

Manufacturing method for transparent electric windows using dye-sensitized TiO2 solar cells

The transparent electric windows based on dye-sensitized nanocrystalline TiO₂ solar cells have been prepared. The solar cell consists of dye-sensitized TiO₂ electrode with a TiO₂ layer of an about 8 μm thickness and of a 80×80 mm² active area, Pt counter electrode and redox electrolyte. The solar cell shows a transmittance of approximately 60% in the visible range and an open-circuit voltage (V_oc) of 0.64 V and a short-circuit photocurrent (J_sc) of 250 mA. A moderately transparent electric window composed of nine unit solar cells in series generates V_oc of 5.7 V and J_sc of 220 mA at one sun light intensity.     

Photovoltaic effect in single-layer organic solar cell devices fabricated with two new imidazolin-5-one molecules

Organic solar cells were fabricated with two new imidazolin-5-one molecules as active layers. The use of imidazolin-5-ones, derivatives of a biomolecule chromophore, for photovoltaic applications is particularly attractive due to its biodegradable nature and tunable properties. Single-layer devices with two analogues of imidazolin-5-ones were prepared and characterized. Devices fabricated with one of the molecules as the active layer showed a maximum J_sc of 0.52 μA cm⁻² and V_oc of 0.68 V at an incident power of 20.32 mW cm⁻², while the other set of devices showed a maximum J_sc of 0.63 μA cm⁻² and V_oc of 0.57 V at the same incident power.     

Photovoltaic characteristics of phosphorus-doped amorphous carbon films grown by r.f. plasma-enhanced CVD

The phosphorus-doped amorphous carbon (n-C:P) films were grown by r.f. power-assisted plasma-enhanced chemical vapor deposition at room temperature using solid phosphorus target. The influence of phosphorus doping on material properties of n-C:P based on the results of simultaneous characterization are reported. Moreover, the solar cell properties such as series resistance, short circuit current density (J_sc), open circuit current voltage (V_oc), fill factor (FF) and conversion efficiency (η) along with the spectral response are reported for the fabricated carbon based n-C:P/p-Si heterojunction solar cell were measured by standard measurement technique. The cells performances have been given in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination condition (100 mW/cm², 25 °C). The maximum of V_oc and J_sc for the cells are observed to be approximately 236 V and 7.34 mA/cm², respectively for the n-C:P/p-Si cell grown at lower r.f. power of 100 W. The highest η and FF were found to be approximately 0.84% and 49%, respectively. We have observed the rectifying nature of the heterojunction structures is due to the nature of n-C:P films.     

Preparation and properties of CdS/CdTe thin film solar cell produced by periodic pulse electrodeposition technique

CdS/CdTe solar cells have been prepared by periodic pulse electrodepositionmethod. 10.8% efficient cell was made with open circuit voltage (V_oc)≈753mV, short-circuit current (J_sc)≈23.6 mA/cm² and fill factor (FF)≈0.61. Current-voltage-temperature measurments showed the variation of ideality factor (A) from 1.88 at 344 K to 4.49 at 202 K whereas voltage factor (α) was almost constant above 276 K. The junction transport is possibly dominated by a tunneling mechanism. Capacitance measurements gave the value of diffusion potential as 1.2 eV, ionized charged density as 5.9 × 10¹⁵ cm⁻³ and number of interface states (N_IS) as 2.8 × 10¹¹ cm⁻² eV⁻¹ at zero volt bias. Measurements of open circuit voltage (V_oc) with temperature gave the value of barrier height as 1.42 eV.     

Improved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer

A comparative study of the cell performance of CIGS thin-film solar cells fabricated using ZnO:Al and ZnO:B window layers has been carried out. ZnO:B films were deposited by RF magnetron sputtering using an undoped ZnO target in a B₂H₆–Ar gas mixture. The short-circuit current (J_sc) was found to improve upon the replacement of the ZnO:Al layer with ZnO:B layers. This improvement in J_sc is attributed to an increase in quantum efficiency due to the higher optical transmission of the ZnO:B layer in the near-infrared region. The best cell fabricated with a MgF₂/ZnO:B/i-ZnO/CdS/CIGS/Mo structure yielded an active area efficiency of 18.0% with V_oc=0.645 V, J_sc=36.8 mA/cm², FF=0.76, and an active area of 0.2 cm² under AM 1.5 illumination.     

Efficiency enhancement of polymer solar cells by incorporating a self-assembled layer of silver nanodisks

We report the efficiency enhancement of polymer solar cells by incorporating a silver nanodisks' self-assembled layer, which was grown on the indium tin oxide (ITO) surface by the electrostatic interaction between the silver particles and modified ITO. Polymer solar cells with a structure of ITO (with silver nanodisks)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevious P VP AI 4083)/poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC₆₁BM)/LiF/Al exhibited an open circuit voltage (V_OC) of 0.61±0.01 V, short-circuit current density (J_SC) of 9.24±0.09 mA/cm², a fill factor (FF) of 0.60±0.01, and power conversion efficiency (PCE) of 3.46±0.07% under one sun of simulated air mass 1.5 global (AM1.5G) irradiation (100 mW/cm²). The PCE was increased from 2.72±0.08% of the devices without silver nanodisks to 3.46±0.07%, mainly from the improved photocurrent density as a result of the excited localized surface plasmon resonance (LSPR) induced by the silver nanodisks.     

Dye-sensitized solar cell based on Rose Bengal dye and nanocrystalline TiO2

Dye-sensitized solar cell is fabricated using Rose Bengal dye (RB) for sensitization of nanocrystalline TiO₂ and that imparts extension in spectral response towards visible region by modifying the semiconductor surface. Further, the photoresponse of the cell was evaluated by analyzing its J–V and impedance characteristics under illumination with metal halide light source of 400 W with an incident light of 73 mW/cm². Various photovoltaic parameters like J_sc, V_oc, FF were evaluated and found to be 3.22 mA, 890 mV, 0.53, respectively, resulting conversion efficiency (η) of 2.09%. Impedance analysis of the cell was carried out to investigate the internal resistance of the cell by recording Cole–Cole plots in between real and imaginary impedance in dark and with illumination under variable biasing, i.e. from 0 to 3 V.     

Characterization of CuInSe2/CdS thin-film solar cells prepared using CBD

CuInSe₂/CdS thin-film heterojunction solar cells were fabricated entirely by chemical bath deposition technique. The illuminated J−V characteristics of the devices prepared with different thicknesses of CdS and CuInSe₂ were studied. The typical solar cell parameters obtained for the best cell are: V_oc = 365 mV, J_sc = 12 mA/cm², FF = 61%, and η = 3.1% under an illumination of 85 mW/cm² on a cell of active area 0.1 cm². The J−V and C−V characteristics under dark condition and the spectral response were also studied for the best cell. The diode quality factor obtained is 1.7.     

Pomegranate leaves and mulberry fruit as natural sensitizers for dye-sensitized solar cells

This study employs chlorophyll extract from pomegranate leaf and anthocyanin extract from mulberry fruit as the natural dyes for a dye-sensitized solar cell (DSSC). A self-developed nanofluid synthesis system is employed to prepare TiO₂ nanofluid with an average particle size of 25 nm. Electrophoresis deposition was performed to deposit TiO₂ nanoparticles on the indium tin oxide (ITO) conductive glass, forming a TiO₂ thin film with the thickness of 11 μm. Furthermore, this TiO₂ thin film was sintered at 450 °C to enhance the thin film compactness. Sputtering was used to prepare counter electrode by depositing Pt thin film on FTO glass at a thickness of 20 nm. The electrodes, electrolyte (), and dyes were assembled into a cell module and illuminated by a light source simulating AM 1.5 with a light strength of 100 mW/cm² to measure the photoelectric conversion efficiency of the prepared DSSCs. According to experimental results, the conversion efficiency of the DSSCs prepared by chlorophyll dyes from pomegranate leaf extract is 0.597%, with open-circuit voltage (V_OC) of 0.56 V, short-circuit current density (J_SC) of 2.05 mA/cm², and fill factor (FF) of 0.52. The conversion efficiency of the DSSCs prepared by anthocyanin dyes from mulberry extract is 0.548%, with V_OC of 0.555 V and J_SC of 1.89 mA/cm² and FF of 0.53. The conversion efficiency is 0.722% for chlorophyll and anthocyanin as the dye mixture, with V_OC of 0.53 V, J_SC of 2.8 mA/cm², and FF of 0.49.     

Improvement of characteristics of new-type solar cells, having a “transparent conductor/thin Si02 layer with ultrafine metal particles as conductive channels/n-Si” junction

New-type solar cells, having a structure “transparent conductor/thin Si0₂ layer with ultrafine metal islands as conductive channels/n-Si” have been prepared by forming a very thin (< 1.0 nm) silicon oxide (Si0₂) layer as well as platinum (Pt) islands (5–50 nm in size) embedded in it on a single crystal n-type silicon (n-Si) wafer, followed by the deposition of an indium tin oxide (ITO) film (200 nm thick) by the electron-beam evaporation method. The open-circuit photovoltages (V_oc) of the solar cells of the above structure were relatively low, 0.25–0.47 V, but they increased very much to 0.50–0.59 V if a thin (3–10 nm) layer of an organic compound such as copper phthalocyanine (CuPc) was pre-deposited on the Pt-island modified n-Si wafer before the ITO deposition. The reason for the beneficial effect of the pre-deposition of the thin CuPc layer was investigated in detail, and it has been found that certain crystal defects are formed in n-Si near the n-Si/Si0₂ interface during the ITO deposition in the absence of the CuPc layer. The formation of such defects is prevented in the presence of the CuPc layer, which leads to a decrease in surface carrier recombination and hence to the increase in V_oc.     

A new route for fabricating CdO/c-Si heterojunction solar cells

CdO/c-Si solar cells have been made by depositing CdO thin films on p-type monocrystalline silicon substrate by means of the rapid thermal oxidation (RTO) technique using a halogen lamp at 350 °C/45 s in static air. Results on structural, optical, and electrical properties of grown CdO films are reported. The electrical and photovoltaic properties of CdO/Si solar cells are examined. Under AM1 illumination condition, the cell shows an open circuit voltage (V_OC) of 500 mV, a short circuit current density (J_SC) of 27.5 mA/cm², a fill factor (FF) of 60%, and a conversion efficiency (η) of 8.84% without using frontal grid contacts and/or post-deposition annealing. Furthermore, the stability of solar cells characteristics is tested.     

Enhancing the performance of dye-sensitized solar cells by incorporating nanosilicate platelets in gel electrolyte

Two kinds of gel-type dye-sensitized solar cells (DSSCs), composed of two types of electrolytes, were constructed and the respective cell performance was evaluated in this study. One electrolyte, TEOS-Triton X-100 gel, was based on a hybrid organic/inorganic gel electrolyte made by the sol–gel method and the other was based on poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) copolymer. TEOS-Triton X-100 gel was based on the reticulate structure of silica, formed by hydrolysis, and condensation of tetraethoxysilane (TEOS), while its organic subphase was a mixture of surfactant (Triton X-100) and ionic liquid electrolytes. Both DSSC gel-type electrolytes were composed of iodine, 1-propy-3-methyl-imidazolium iodide, and 3-methoxypropionitrile to create the redox couple of I₃⁻/I⁻. Based on the results obtained from the I–V characteristics, it was found that the optimal iodine concentrations for the TEOS-Triton X-100 gel electrolyte and PVDF-HFP gel electrolyte are 0.05 M and 0.1 M, respectively. Although the increase in the iodine concentration could enhance the short-circuit current density (J_SC), a further increase in the iodine concentration would reduce the J_SC due to increased dark current. Therefore, the concentration of I₂ is a significant factor in determining the performance of DSSCs.In order to enhance cell performance, the addition of nanosilicate platelets (NSPs) in the above-mentioned gel electrolytes was investigated. By incorporating NSP-Triton X-100 into the electrolytes, the J_SC of the cells increased due to the decrease of diffusion resistance, while the open circuit voltage (V_OC) remained almost the same. As the loading of the NSP-Triton X-100 in the TEOS-Triton X-100 gel electrolyte increased to 0.5 wt%, the J_SC and the conversion efficiency increased from 8.5 to 12 mA/cm² and from 3.6% to 4.7%, respectively. However, the J_SC decreased as the loading of NSP-Triton X-100 exceeded 0.5 wt%. At higher NSP-Triton X-100 loading, NSPs acted as a barrier interface between the electrolyte and the dye molecules, hindering electron transfer, hence, reducing the cell's photocurrent density. The same behavior was also observed in the PVDF-HFP gel electrolyte DSSC system.