Origin of enhancement in open-circuit voltage by adding ZnO to nanocrystalline SnO2 in dye-sensitized solar cells

SnO2 + ZnO working electrodes for dye-sensitized solar cells were made by mixing a nanocrystalline SnO2 colloidal dispersion with ZnO or Zn(CH3COO)2. Addition of ZnO or Zn(CH3COO)2 enhanced the open-circuit voltage (V(oc)) of the cells with respect to cells containing only SnO2. Dependence of the electron lifetime in the electrodes on short-circuit photocurrent density (J(sc)) gave evidence against the assumption that the suppression of back electron transfer to the electrolyte is the origin for the V(oc) enhancement by addition of Zn. V(oc) dependence on temperatures indicated a decrease in the combined capacitance of the mixed electrode. The slope of the V(oc) dependence versus the logarithm of J(sc) indicated that the contribution of unpinning of the band to the enhancement of V(oc) could be neglected. From the cyclic voltammograms of the electrodes, the combined capacitance of the mixed electrode was 1 order of magnitude smaller than that of SnO2. The decrease in the combined capacitance in the mixed electrode could be explained by the decrease in the chemical capacitance of SnO2, thus the shift of the conduction band position toward the vacuum level. X-ray photoelectron spectra of Sn 3d(5/2) peaks showed a shift toward lower binding energy with an increasing amount of added Zn. This was attributed to an increase in the surface potential toward the negative direction, which might have resulted from a dipole moment formed by Zn on the surface of SnO2.     

Significant efficiency improvement of the black dye-sensitized solar cell through protonation of TiO2 films

This paper describes the influence of acid pretreatment ofTiO2 mesoporous films prior to dye sensitization on the performance of dye-sensitized solar cells based on [(C4H9)4N]3[Ru(Htcterpy)(NCS)3] (tcterpy = 4,4',4"-tricarboxy- 2,2',2"-terpyridine), the so-called black dye. The HCl pretreatment caused an increase in overall efficiency by 8%, with a major contribution from photocurrent improvement. It is speculated, from the analysis of incident photon-to-electron conversion efficiency, UV-vis absorption spectra, redox properties of the dye and TiO2, and the impedance spectra of the dye-sensitized solar cells, that photocurrent enhancement is attributed to the increases in electron injection and/or charge collection efficiency besides the improvement of light harvesting efficiency upon HCl pretreatment. Open-circuit photovoltage (V(oc)) remained almost unchanged in the case of significant positive shift of flat band potential for TiO2 upon HCl pretreatment. The suppression of electron transfer from conduction band electrons to the I3- ions in the electrolyte upon HCl pretreatment, reflected by the increased resistance at the TiO2/dye/electrolyte interface and reduced dark current, resulted in a V(oc) gain, which compensated the V(oc) loss due to the positive shift of the flat band. Using the HCl pretreatment approach, 10.5% of overall efficiency with the black dye was obtained under illumination of simulated AM 1.5 solar light (100 mW cm(-2)) using an antireflection film on the cell surface.     

Incorporation of functionalized single-wall carbon nanotubes in dye-sensitized TiO2 solar cells

Effects of incorporation of acid-treated single-wall carbon nanotubes (a-SWCNs) in TiO(2) film and of anchorage of dye-linked, a-SWCNs (hereafter dye-SWCNs) to the TiO(2)/electrolyte interface on photocurrent-voltage characteristics of dye-sensitized solar cells were studied. Compared with an unmodified cell, the modified cell with the a-SWCNs in TiO(2) film showed a 25% increase in short-circuit photocurrent (J(sc)). The J(sc) increase is correlated with improved connectivity between the a-SWCNs and the TiO(2) particles and with enhanced light scattering by TiO(2) clusters formed in the presence of the a-SWCNs. In the case of anchoring dye-SWCNs to the TiO(2)/electrolyte interface, the open-circuit voltage (V(oc)) increased by as much as 0.1 V, possibly due to the basicity of the TiO(2) surface from NH groups of ethylenediamine moieties of the anchored dye-SWCNs.     

TiO2 band shift by nitrogen-containing heterocycles in dye-sensitized solar cells: a periodic density functional theory study

A density functional theory (DFT) method (periodic DMol3) with full geometry optimization was used to study the adsorption of nitrogen-containing heterocycles such as pyrazole, imidazole, 1,2,4-triazole, pyridine, pyrimidine, pyrazine, and 4-t-butylpyridine (TBP) on TiO2 anatase (101), (100), and (001) surfaces. All structures displayed a negative shift in the TiO2 Fermi level upon adsorption of N-containing heterocycles. Additionally, the heterocycles were examined as an additive in an I-/I3- redox electrolyte solution of dye-sensitized TiO2 solar cell. The DFT results indicated that the negative shift of TiO2 Fermi level was due to the adsorbate dipole moment component normal to the TiO2 surface plane, and corresponded to the enhanced open-circuit photovoltage (Voc) and the reduced short-circuit photocurrent density (Jsc) in a dye-sensitized solar cell.     

The effect of Al2O3 barrier layers in TiO2/dye/CuSCN photovoltaic cells explored by recombination and DOS characterization using transient photovoltage measurements

Solid-state dye-sensitized solar cells of the type TiO(2)/dye/CuSCN have been made with thin Al(2)O(3) barriers between the TiO(2) and the dye. The Al(2)O(3)-treated cells show improved voltages and fill factors but lower short-circuit currents. Transient photovoltage and photocurrent measurements have been used to find the pseudo-first-order recombination rate constant (k(pfo)) and capacitance as a function of potential. Results show that k(pfo) is dependent on V(oc) with the same form as in TiO(2)/dye/electrolyte cells. The added Al(2)O(3) layer acts as a "tunnel barrier", reducing the k(pfo) and thus increasing V(oc). The decrease in k(pfo) also results in an increased fill factor. Capacitance vs voltage plots show the same curvature (approximately 150 mV/decade) as found in TiO(2)/dye/electrolyte cells. The application of one Al(2)O(3) layer does not cause a significant shift in the shape or position of the capacitance curve, indicating that changes in band offset play a lesser role in the observed V(oc) increase. Cells made with P25 TiO(2) have, on average, 2.5 times slower recombination rate constants (longer lifetimes) than those made with colloidal TiO(2). The cells with P25 also show 2.3 times higher trap density (DOS), which results in little change in the V(oc) between the two types of TiO(2). It is further noted that the recombination current in these cells cannot be calculated from the total charge times the first order rate constant.     

TiO2纳米片/巢状分级结构纳米阵列薄膜的制备及其在染料敏化太阳能电池中的应用

采用水热合成法在氟掺杂二氧化锡(FTO)导电玻璃基底上得到TiO2纳米阵列薄膜,并进一步通过NaOH溶液水热处理制备了由巢状纳米阵列及纳米片覆盖层构成的TiO2纳米阵列分级结构一体化薄膜.采用场发射扫描电镜(FE-SEM),X射线衍射(XRD),紫外-可见(UV-Vis)漫反射光谱和吸收光谱技术对TiO2薄膜的结构和性质进行表征.FE-SEM结果表明:分级结构TiO2薄膜膜厚为1.5μm,薄膜由一层纳米片覆盖层(约0.2μm高)和一层巢状纳米阵列层(约1.3μm高)组成.XRD谱图表明TiO2薄膜为锐钛矿相.UV-Vis光谱显示分级结构TiO2薄膜具有较强的光捕获能力和染料吸附能力.TiO2纳米片/巢状分级结构纳米阵列薄膜作为光阳极,可有效地提高染料敏化太阳能电池的光电转换效率,其短路电流(Jsc)为7.79mA·cm-2,开路电压(Voc)为0.80V,填充因子(FF)为0.40,光电转换效率(η)为2.48%,其光电转换效率较TiO2纳米阵列薄膜提高了近10倍.     

Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br-/Br3- electrolytes

A series of novel metal-free organic dyes TC301-TC310 with relatively high HOMO levels were synthesized and applied in dye-sensitized solar cells (DSCs) based on electrolytes that contain Br(-)/Br(3)(-) and I(-)/I(3)(-). The effects of additive Li(+) ions and the HOMO levels of the dyes have an important influence on properties of the dyes and performance of DSCs. The addition of Li(+) ions in electrolytes can broaden the absorption spectra of the dyes on TiO(2) films and shift both the LUMO levels of the dyes and the conduction band of TiO(2), thus leading to the increase of J(sc) and the decrease of V(oc). Upon using Br(-)/Br(3)(-) instead of I(-)/I(3)(-), a large increase of V(oc) is attributed to the enlarged energy difference between the redox potentials of electrolyte and the Fermi level of TiO(2), as well as the suppressed electron recombination. Incident photon to current efficiency (IPCE) action spectra, electrochemical impedance spectra, and nanosecond laser transient absorption reveal that both the electron collection yields and the dye regeneration yields (Φ(r)) depend on the potential difference (the driving forces) between the oxidized dyes and the Br(-)/Br(3)(-) redox couple. For the dyes for which the HOMO levels are more positive than the redox potential of Br(-)/Br(3)(-) sufficient driving forces lead to the longer effective electron-diffusion lengths and almost the same efficient dye regenerations, whereas for the dyes for which the HOMO levels are similar to the redox potential of Br(-)/Br(3)(-), insufficient driving forces lead to shorter effective electron-diffusion lengths and inefficient dye regenerations.     

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.     

A highly efficient organic sensitizer for dye-sensitized solar cells

We have synthesized a highly efficient organic dye for a dye-sensitized solar cell; the overall solar-to-energy conversion efficiency was 9.1% at AM 1.5 illumination (100 mW cm(-2)): short-circuit current density (J(sc)) = 18.1 mA cm(-2), open circuit photovoltage (V(oc)) = 743 mV and fill factor (ff) = 0.675.     

Oligothiophene-containing coumarin dyes for efficient dye-sensitized solar cells

We have developed oligothiophene-containing coumarin dyes fully functionalized for dye-sensitized nanocrystalline TiO(2) solar cells (DSSCs). DSSCs based on the dyes gave good performance in terms of incident photon-to-current conversion efficiency (IPCE) in the range of 400-800 nm. A solar energy-to-electricity conversion efficiency (eta) of 7.4% was obtained with a DSSC based on 2-cyano-3-[5'-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-[2,2']bithiophenyl-5-yl]acrylic acid (NKX-2677) under simulated AM 1.5G irradiation (100 mW cm(-2)) with a mask: short-circuit current density (J(sc)) = 13.5 mA cm(-2); open-circuit voltage (V(oc)) = 0.71 V; fill factor (FF) = 0.77. Transient absorption spectroscopy measurements indicated that electron injection from NKX-2677 to the conduction band of TiO(2) is very rapid (<100 fs), which is much faster than the emission lifetime of the dye (1.0 ns), giving a highly efficient electron injection yield of near unity.     

染料敏化太阳能电池的二氧化钛膜性能研究

以二氧化钛(TiO₂)纳米粉(P25)为原料,把它研磨成胶状,用涂敷法制得TiO₂纳米多孔膜,并组装成太阳能电池,用100W氙灯作为模拟太阳光,对电池进行光电性能测试.根据电池的短路电流(I_sc)、开路电压(V_oc)和填充因子(ff)等指标来反映电池的性能.研究表明,分散剂乙酰丙酮、OP乳化剂、研磨时间和热处理后的保温时间长短对TiO₂膜的性能均有很大的影响.其结果是,乙酰丙酮0.15mL、OP乳化剂0.10mL、研磨时间1h和保温时间0.5h时,TiO₂膜的光电性能较好,I_sc、V_oc和ff分别为8.85mA、567mV和0.445.并用XRD和比表面及孔隙分析仪对TiO₂膜进行了表征.     

Bulk heterojunction photoelectrochemical cells consisting of oxotitanyl phthalocyanine nanoporous films and I(3)(-)/I(-) redox couple

Photoelectrochemical cells based on oxotitanylphthalocyanine (TiOPc) films and an I(3)(-)/I(-) redox couple have been constructed. The TiOPc films were prepared on an indium-tin oxide coated glass plate (ITO) by the micellar disruption method and characterized by their unique nanoporous structure. A photocurrent action spectrum for input radiation directed through the ITO/TiOPc film, film-thickness dependence, and morphological investigation revealed that the cells consisted of a bulk heterojunction formed between the nanoporous TiOPc films and the liquid I3-/I- electrolyte, resulting in a larger short-circuit current (J(sc)= 2.1 mA/cm(2)), open-circuit voltage (V(oc)= 0.11 V), fill factor (ff= 0.31), and hence a larger energy conversion efficiency (eta= 0.13% for an incident white-light intensity of 53 mW/cm2) than the bilayer structure composed of the vaccum-evaporated TiOPc compact film and the I(3)(-)/I(-) electrolyte (J(sc)= 0.16 mA/cm(2), V(oc)= 0.018 V, ff = 0.27, and eta = (1.5 x 10(-3)%).     

Panchromatic sensitization of nanocrystalline TiO2 with cis-Bis(4-carboxy-2-[2'-(4'-carboxypyridyl)]quinoline)bis(thiocyanato-N)ruthenium(II)

We compared the spectral (IR and Raman), electrochemical, and photoelectrochemical properties of nanocrystalline TiO(2) sensitized with the newly synthesized complex [NBu(4)](2)[cis-Ru(Hdcpq)(2)(NCS)(2)] (1; [NBu(4)](+) = tetrabutylammonium cation; H(2)dcpq = 4-carboxy-2-[2'-(4'-carboxypyridyl)]quinoline) with those of TiO(2) sensitized with [NBu(4)](2)[cis-Ru(Hdcbpy)(2)(NCS)(2)] (2; H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and [NBu(4)](2)[cis-Ru(Hdcbiq)(2)(NCS)(2)] (3; H(2)dcbiq = 4,4'-dicarboxy-2,2'-biquinoline). Complex 1 achieved efficient sensitization of nanocrystalline TiO(2) films over a wide visible and near-IR region, generating a large short-circuit photocurrent. The absorbed photon-to-current conversion efficiency decreased in the order 2 > 1 > 3 with the decrease in the free energy change (-Delta G(inj)) of the electron injection from the ruthenium complex to TiO(2). The open-circuit photovoltages (V(oc)'s) of dye-sensitized solar cells decreased in the order 2 > 1 > 3 with the increase in the dark current resulting from reverse electron transfer from TiO(2) to I(3)(-). The sensitizer-dependent V(oc) value can be interpreted as a result of reverse electron transfer through the sensitizing dye molecules.     

Effect of solvent and additives on the open-circuit voltage of ZnO-based dye-sensitized solar cells: a combined theoretical and experimental study

We have investigated the role of electrolyte composition, in terms of solvent and additive, on the open-circuit voltage (V(oc)) of ZnO-based dye-sensitized solar cells (DSSCs) using a combined experimental and theoretical approach. Calculations based on density functional theory (DFT) have been performed in order to describe the geometries and adsorption energies of various adsorbed solvents (nitromethane, acetonitrile and dimethylformamide) and p-tert-butylpyridine (TBP) (modeled by methylpyridine) on the ZnO (100) surface using a periodic approach. The densities of states (DOS) have been calculated and the energy position of the conduction band edge (CBE) has been evaluated for the different molecules adsorbed. The effect of the electrolyte composition on the standard redox potential of the iodide/triiodide redox couple has been experimentally determined. These two data values (CBE and standard redox potential) allowed us to determine the dependence of V(oc) on the electrolyte composition. The variations determined using this method were in good agreement with the measured V(oc) for cells made of electrodeposited ZnO films sensitized using D149 (indoline) dye. As in the case of TiO(2)-based cells, a correlation of V(oc) with the donor number of the adsorbed species was found. The present study clearly points out that both the CBE energy and the redox potential variation are important for explaining the experimentally observed changes in the V(oc) of DSSCs.     

Illumination intensity dependence of the photovoltage in nanostructured TiO2 dye-sensitized solar cells

The open-circuit voltage (V(oc)) dependence on the illumination intensity (phi0) under steady-state conditions in both bare and coated (blocked) nanostructured TiO2 dye-sensitized solar cells (DSSCs) is analyzed. This analysis is based on a recently reported model [Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 8774] which describes the rate of interfacial electron transfer from the conduction band of TiO2 to acceptor electrolyte levels (recombination). The model involves two possible mechanisms: (1) direct, isoenergetic electron injection from the conduction band and (2) a two-step process involving inelastic electron trapping by band-gap surface states and subsequent isoenergetic transfer of trapped electrons to electrolyte levels. By considering the variation of V(oc) over a wide range of illumination intensities (10(10) < phi0 < 10(16) cm(-2) s(-1)), three major regions with different values of dV(oc)/d phi0 can be distinguished and interpreted. At the lower illumination intensities, recombination mainly involves localized band-gap, deep traps at about 0.6 eV below the conduction band edge; at intermediate photon fluxes, recombination is apparently controlled by a tail of shallow traps, while, for high enough phi0 values, conduction band states control the recombination process. The high phi0 region is characterized by a slope of dV(oc)/d log phi0 congruent with 60 mV, which indicates a recombination of first order in the free electron concentration. The study, which was extended to different solar cells, shows that the energy of the deep traps seems to be an intrinsic property of the nanostructured TiO2 material, while their concentration and also the density ([symbol: see text]t approximately 10(18)-10(19) cm(-3)) and distribution of shallow traps, which strongly affects the shape of the V(oc) vs phi0 curves, change from sample to sample and are quite sensitive to the electrode preparation. The influence of the back-reaction of electrons from the fluorine-doped tin oxide (FTO) conducting glass substrate with electrolyte tri-iodide ions on the V(oc) vs phi0 dependence characteristic of the DSSC is analyzed. It is concluded that this back-reaction route can be neglected, even at low light intensities, when its rate (exchange current density, j0), which can vary over 4 orders of magnitude depending on the type of FTO used, is low enough (j0 < or = 10(-8)A cm(-2)). The comparison of V(oc) vs phi0 measurements corresponding to different DSSCs with and without blocking of the FTO-electrolyte contact supports this conclusion.     

Sputtered Nb2O5 as an effective blocking layer at conducting glass and TiO2 interfaces in ionic liquid-based dye-sensitized solar cells

The thin Nb(2)O(5) layer works as a remarkable blocking layer when deposited by the rf magnetron sputtering method between fluorine-doped tin oxide and a mesoporous TiO(2) layer, improving open-circuit photovoltage (V(oc)) and fill factor (FF) with power conversion efficiency over 5.5% at 1 sun irradiation of the dye-sensitized TiO(2) solar cells using ionic liquid electrolytes.     

Constructing organic D-A-π-A-featured sensitizers with a quinoxaline unit for high-efficiency solar cells: the effect of an auxiliary acceptor on the absorption and the energy level alignment

Four organic D-A-π-A-featured sensitizers (TQ1, TQ2, IQ1, and IQ2) have been studied for high-efficiency dye-sensitized solar cells (DSSCs). We employed an indoline or a triphenylamine unit as the donor, cyanoacetic acid as the acceptor/anchor, and a thiophene moiety as the conjugation bridge. Additionally, an electron-withdrawing quinoxaline unit was incorporated between the donor and the π-conjugation unit. These sensitizers show an additional absorption band covering the broad visible range in solution. The contribution from the incorporated quinoxaline was investigated theoretically by using DFT and time-dependent DFT. The incorporated low-band-gap quinoxaline unit as an auxiliary acceptor has several merits, such as decreasing the band gap, optimizing the energy levels, and realizing a facile structural modification on several positions in the quinoxaline unit. As demonstrated, the observed additional absorption band is favorable to the photon-to-electron conversion because it corresponds to the efficient electron transitions to the LUMO orbital. Electrochemical impedance spectroscopy (EIS) Bode plots reveal that the replacement of a methoxy group with an octyloxy group can increase the injection electron lifetime by a factor of 2.4. IQ2 and TQ2 can perform well without any co-adsorbent, successfully suppress the charge recombination from TiO(2) conduction band to I(3)(-) in the electrolyte, and enhance the electron lifetime, resulting in a decreased dark current and enhanced open circuit voltage (V(oc)) values. By using a liquid electrolyte, DSSCs based on dye IQ2 exhibited a broad incident photon-to-current conversion efficiency (IPCE) action spectrum and high efficiency (η=8.50?%) with a short circuit current density (J(sc)) of 15.65?mA?cm(-2), a V(oc) value of 776?mV, a fill factor (FF) of 0.70 under AM 1.5 illumination (100?mW?cm(-2)). Moreover, the overall efficiency remained at 97% of the initial value after 1000?h of visible-light soaking.     

Effect of iodine addition on solid-state electrolyte LiI/3-hydroxypropionitrile (1:4) for dye-sensitized solar cells

It was observed that the ionic conductivity of the solid-state electrolyte LiI/3-hydroxypropionitrile (HPN) = 1:4 (molar ratio) decreased dramatically with increasing iodine (I(2)) concentration, which differs from the conduction behavior of the Grotthuss transport mechanism observed in liquid or gel electrolytes. The short-circuit photocurrent density (J(sc)) of the dye-sensitized solar cell (DSSC) based on this electrolyte system increases with increasing I(2) concentration until LiI/I(2) is 1:0.05 (molar ratio). Beyond this limitation, the J(sc) decreases. At low I(2) concentrations (I(2)/LiI < or = 0.05), the J(sc) is mainly affected by the diffusion of I(3)(-). An increase of the I(2) concentration leads to the enhancement of the diffusion of I(3)(-) and an increase of the J(sc). At high I(2) concentrations (I(2)/LiI > 0.05), the factors, including the increased light absorption by the I(3)(-), the increased recombination of electrons at the photoanode with I(3)(-), and the reduced ionic conductivity of the electrolyte, lead to a decrease of J(sc). At the same time, the open-circuit voltage (V(oc)) of the DSSC decreases monotonically with the ratio of I(2)/LiI due to increased dark current in the DSSC. The increased absorption of visible light by the electrolyte, the enhanced dark current, and the reduced ionic conductivity of the electrolyte contribute to the performance variation of the corresponding solid-state DSSC with increasing I(2) concentration.     

Formation of efficient dye-sensitized solar cells by introducing an interfacial layer of long-range ordered mesoporous TiO2 thin film

Long-range ordered cubic mesoporous TiO 2 films with 300 nm thickness were fabricated on fluorine-doped tin oxide (FTO) substrate by evaporation-induced self-assembly (EISA) process using F127 as a structure-directing agent. The prepared mesoporous TiO 2 film (Meso-TiO 2) was applied as an interfacial layer between the nanocrystalline TiO 2 film (NC-TiO 2) and the FTO electrode in the dye-sensitized solar cell (DSSC). The introduction of Meso-TiO 2 increased J sc from 12.3 to 14.5 mA/cm (2), and V oc by 55 mV, whereas there was no appreciable change in the fill factor (FF). As a result, the photovoltaic conversion efficiency ( eta) was improved by 30.0% from 5.77% to 7.48%. Notably, introduction of Meso-TiO 2 increased the transmittance of visible light through the FTO glass by 23% as a result of its excellent antireflective role. Thus the increased transmittance was a key factor in enhancing the photovoltaic conversion efficiency. In addition, the presence of interfacial Meso-TiO 2 provided excellent adhesion between the FTO and main TiO 2 layer, and suppressed the back-transport reaction by blocking direct contact between the electrolyte and FTO electrode.     

The effect of an atomically deposited layer of alumina on NiO in P-type dye-sensitized solar cells

We present a systematic investigation of the fundamental effects of an atomically deposited alumina (AlO(x)H(y)) onto the NiO films in p-type dye-sensitized solar cells (p-DSCs). With P1 as the sensitizing dye and 0.1 M I(2) and 1.0 M LiI in 3-methoxypropionitrile as the electrolyte, one atomic layer deposition (ALD) cycle of alumina was used to achieve a 74% increase in the overall conversion efficiency of a NiO-based DSC. The open circuit voltage of the cells increased from 0.11 to 0.15 V, and the short circuit current density increased from 0.83 to 0.95 mA/cm(2). Adsorption isotherm studies were performed to show that the amount of dye adsorbed on the NiO-alumina film is slightly lower than the amount adsorbed on the nontreated NiO film. The increased J(sc) was therefore assigned to the increased efficiency of carrier collection at the semiconductor-FTO interface. Our study of the photocurrent onset potentials of NiO and NiO-alumina films with the chopped light measurement technique showed no definitive difference in the onset potential values. However, the DSCs based on NiO-alumina showed a higher recombination resistance value from the electrochemical impedance studies and a higher diode ideality factor from the V(oc) versus ln(light intensity) plots as compared to the DSCs based on untreated NiO. It has thus been established that the increase in V(oc) upon alumina treatment arises due to a higher resistance for electron-hole recombination across NiO surface locally.