Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy

Recombination in Organic Bulk Heterojunction Solar Cells: Small Dependence of Interfacial Charge Transfer Kinetics on Fullerene Affinity

We investigate the causes for obtaining higher open-circuit voltage in solar cells that use a fullerene with a smaller electron affinity. Using impedance spectroscopy technique, we show that the change of fullerene LUMO energy has very little influence on the kinetic rate of charge transfer across the interface. In terms of the Marcus theory, large reorganization energy values govern the recombination kinetic rate, which is only slightly dependent on the fullerene LUMO energy, and also depends weakly on the energy location of recombining carriers within the broad density of states. Since the recombination rate is very similar in the different devices, we conclude that the larger open-circuit voltage is due to the larger donor HOMO/acceptor LUMO offset.

How the charge-neutrality level of interface states controls energy level alignment in cathode contacts of organic bulk-heterojunction solar cells

Electronic equilibration at the metal-organic interface, leading to equalization of the Fermi levels, is a key process in organic optoelectronic devices. How the energy levels are set across the interface determines carrier extraction at the contact and also limits the achievable open-circuit voltage under illumination. Here, we report an extensive investigation of the cathode energy equilibration of organic bulk-heterojunction solar cells. We show that the potential to balance the mismatch between the cathode metal and the organic layer Fermi levels is divided into two contributions: spatially extended band bending in the organic bulk and voltage drop at the interface dipole layer caused by a net charge transfer. We scan the operation of the cathode under a varied set of conditions, using metals of different work functions in the range of ∼2 eV, different fullerene acceptors, and several cathode interlayers. The measurements allow us to locate the charge-neutrality level within the interface density of sates and calculate the corresponding dipole layer strength. The dipole layer withstands a large part of the total Fermi level mismatch when the polymer:fullerene blend ratio approaches ∼1:1, producing the practical alignment between the metal Fermi level and the charge-neutrality level. Origin of the interface states is linked with fullerene reduced molecules covering the metal contact. The dipole contribution, and consequently the band bending, is highly sensitive to the nature and amount of fullerene molecules forming the interface density of states. Our analysis provides a detailed picture of the evolution of the potentials in the bulk and the interface of the solar cell when forward voltage is applied or when photogeneration takes place.

Enhanced conversion efficiency and surface hydrophobicity of nano-roughened Teflon-like film coated poly-crystalline Si solar cells

Nano-roughened Teflon-like film coated poly-crystalline Si photovoltaic solar cells (PVSCs) with enhanced surface hydrophobicity and conversion efficiency (η) are characterized and compared with those coated by a Si nanorod array or a standard SiN anti-reflection layer. The Teflon-like film coated PVSC surface reveals a water contact angle increasing from 89.3° to 96.2° as its thickness enlarges from 22 to 640 nm, which is much larger than those of the standard and Si nanorod array coated PVSC surfaces (with angles of 55.6° and 32.8°, respectively). After nano-roughened Teflon-like film passivation, the PVSC shows a comparable η(10.89%) with the standard SiN coated PVSC (η = 11.39%), while the short-circuit current (I(SC)) is slightly reduced by 2% owing to the slightly decreased UV transmittance and unchanged diode performance. In contrast, the Si nanorod array may offer an improved surface anti-reflection with surface reflectance decreasing from 30% to 5% at a cost of optical scattering and randomized deflection, which simultaneously decrease the optical transmittance from 15% to 3% in the visible region without improving hydrophobicity and conversion efficiency. The Si nanorod array covered PVSC with numerous surface dangling bonds induced by 1 min wet-etching, which greatly reduces the open-circuit voltage (V(OC)) by 10-15% and I(SC) by 30% due to the reduced shunt resistance from 3 to 0.24 kΩ. The nano-scale roughened Teflon-like film coated on PVSC has provided better hydrophobicity and conversion efficiency than the Si nanorod array covered PVSC, which exhibits superior water repellant performance and comparable conversion efficiency to be one alternative approach for self-cleaning PVSC applications.

Benzotriazole-bridged sensitizers containing a furan moiety for dye-sensitized solar cells with high open-circuit voltage performance

Two new benzotriazole-bridged sensitizers are designed and synthesized (BTA-I and BTA-II) containing a furan moiety for dye-sensitized solar cells (DSSCs). Two corresponding dyes (BTA-III and BTA-IV) with a thiophene spacer were also synthesized for comparison. All of these dyes performed as sensitizers for DSSCs, and the photovoltaic performance data of these benzotriazole-bridged dyes showed a high open-circuit voltage (V(oc): 804-834 mV). Among the four dyes, DSSCs based on BTA-II, with a furan moiety and branched alkyl chain, showed the highest V(oc) (834 mV), a photocurrent density (J(sc)) of 12.64 mA cm(-2), and a fill factor (FF) of 0.64, corresponding to an overall conversion efficiency (η) of 6.72%. Most importantly, long-term stability of the BTA-I-IV-based DSSCs with ionic-liquid electrolytes under 1000 h light-soaking was demonstrated, and BTA-II exhibited better photovoltaic performance of up to 5.06% power conversion efficiency.

From flat to nanostructured photovoltaics: balance between thickness of the absorber and charge screening in sensitized solar cells

Nanoporous metal oxide electrodes provide a high internal area for dye anchoring in dye-sensitized solar cells, but the thickness required to extinguish the solar photons also enhances recombination at the TiO(2)/electrolyte interface. The high extinction coefficient of inorganic semiconductor absorber should allow the reduction of the film thickness, improving the photovoltage. Here we study all-solid semiconductor sensitized solar cells, in the promising TiO(2)/Sb(2)S(3)/P3HT configuration. Flat and nanostructured cells have been prepared and analyzed, developing a cell performance model, based on impedance spectroscopy results, that allows us to determine the impact of the reduction of metal oxide film thickness on the operation of the solar cell. Decreasing the effective surface area toward the limit of flat samples produces a reduction in the recombination rate, increasing the open circuit potential, V(oc), while providing a significant photocurrent. However, charge compensation problems as a consequence of inefficient charge screening in flat cells increase the hole transport resistance, lowering severely the cell fill factor. The use of novel structures balancing recombination and hole transport will enhance solid sensitized cell performance.

Enhanced performance and air stability of 3.2% hybrid solar cells: how the functional polymer and CdTe nanostructure boost the solar cell efficiency

A record high PCE of up to 3.2% demonstrates that the efficiency of hybrid solar cells (HSCs) can be boosted by utilizing a unique mono-aniline end group of PSBTBT-NH(2) as a strong anchor to attach to CdTe nanocrystal surfaces and by simultaneously exploiting benzene-1,3-dithiol solvent-vapor annealing to improve the charge separation at the donor/acceptor interface, which leads to efficient charge transportation in the HSCs.

Double-layer coating of SrCO3/TiO2 on nanoporous TiO2 for efficient dye-sensitized solar cells

Surface modification plays a crucial role in improving the efficiency of dye-sensitized solar cells (DSSCs), but the reported surface treatments are in general superior to the untreated TiO(2) but inferior to the typical TiCl(4)-treated TiO(2) in terms of solar cell performance. This work demonstrates a two-step treatment of the nanoporous titania surface with strontium acetate [Sr(OAc)(2)] and TiCl(4) in order, each step followed by sintering. An electronically insulating layer of SrCO(3) is formed on the TiO(2) surface via the Sr(OAc)(2) treatment and then a fresh TiO(2) layer is deposited on top of the SrCO(3) layer via the TiCl(4) treatment, corresponding to a double layer of Sr(OAc)(2)/TiO(2) coated on the TiO(2) surface. As compared to the typical TiCl(4)-treated DSSC, the Sr(OAc)(2)-TiCl(4) treated DSSC improves short-circuit photocurrent (J(sc)) by 17%, open-circuit photovoltage (V(oc)) by 2%, and power conversion efficiency by 20%. These results indicate that the Sr(OAc)(2)-TiCl(4) treatment is better than the often used TiCl(4) treatment for fabrication of efficient DSSCs. Charge density at open circuit and controlled intensity modulated photocurrent/photovoltage spectroscopy reveal that the two electrodes show almost same conduction band level but different electron diffusion coefficient and charge recombination rate constant. Owing to the blocking effect of the SrCO(3) layer on electron recombination with I(3)(-) ions, the charge recombination rate constant of the Sr(OAc)(2)-TiCl(4) treated DSSC is half that of the TiCl(4)-treated DSSC, accounting well for the difference of their V(oc). The improved J(sc) is also attributed to the middle SrCO(3) layer, which increases dye adsorption and may improve charge separation efficiency due to the blocking effect of SrCO(3) on charge recombination.

Novel Zn-Sn-O nanocactus with excellent transport properties as photoanode material for high performance dye-sensitized solar cells

A novel chemically stable Zn-Sn-O nanocactus structure has been synthesized for the first time using a hydrothermal method. The Zn-Sn-O nanocactus structure comprises a Zn poor-Zn(2)SnO(4) plate and Zn-doped SnO(2) nanothorns growing on the plate, both of which have high electron mobilities. The nanocactus is used as the photoanode of dye-sensitized solar cells (DSSCs). The overall power conversion efficiency (PCE) for the Zn-Sn-O nanocactus film reaches 2.21%, which is twice the previous reported efficiency of pure SnO(2). Electrochemical impedance spectroscopy (EIS) measurements show that the Zn-Sn-O nanocactus film has a good effective diffusion length and high intrinsic electron mobility. After TiCl(4) treatment of the Zn-Sn-O nanocactus film, the current density increases nearly three times and the PCE increases to 6.62%, which compares favourably with the P25 DSSCs (6.97%) and is much higher than that of the SnO(2) (1.04%) or Zn(2)SnO(4) (3.7%)-based DSSCs.