Liudmila Larina

Electronic structure study of lightly Nb-doped TiO2 electrode for dye-sensitized solar cells

To improve the conversion efficiency of dye-sensitized solar cells (DSSCs) it is necessary to understand the electronic structure of the TiO2–dye–electrolyte interface in detail. A sturdy junction at the interface can be provided by modifying the electronic structure of the TiO2 electrode with Nb doping. The Nb-doped TiO2 was prepared by a sol–gel method followed by a hydrothermal treatment; the Nb content was varied from 0.5 to 3.0 mol%. The X-ray photoelectron spectroscopy showed that the Fermi level of TiO2 electrode shifted away from the conduction band minimum (CBM) when the Nb content is low (≤1.5 mol%) and shifted toward the CBM when the Nb content is high (≥2.5 mol%). The shift of Fermi level with low Nb doping was due to the passivation of the oxygen vacancies at the TiO2 nanoparticle surface. Intraband states were formed when dopant content was 1.5 and 2.5 mol%. We have found that the photovoltaic parameters of DSSCs based on doped TiO2 sensitized with a cis-[Ru(dcbpyH)2(NCS)2](NBu4)2, N719 dye, are closely related to the electronic structure of the Nb-doped TiO2 electrode. The changes of short circuit current and open circuit voltage of DSSCs were explained in relation to the electronic structure of the TiO2 electrode. The best efficiency of 8.0% was demonstrated by DSSCs with 2.5 mol% Nb-doped TiO2.

Alignment of energy levels at the ZnS/Cu(In,Ga)Se2 interface

Further understanding of the electronic structure at the ZnS/Cu(In,Ga)Se2 interface is necessary to enhance the electron injection across the interface in Cu(In,Ga)Se2 solar cells. The valence band structure and shallow core levels were investigated by ultraviolet photoelectron spectroscopy depth profile analysis with He II line excitation. ZnS film was grown by a chemical bath deposition on a Cu(In,Ga)Se2 absorber deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The discontinuity of 2.0 eV in the valence band edge at the ZnS/Cu(In0.7Ga0.3)Se2 interface was directly determined. This type of valence band offset yields a spike conduction band alignment of 0.25 eV. The positions of the VBM and the Zn 3d core-level emission of the buffer underwent the substantial shifts of 0.36 eV and 0.64 eV to a lower binding energy levels during the etching process. The shifts are ascribed to the contribution of the band bending in the ZnS buffer layer and its graded chemical composition. This study is the first to determine the small conduction band offset at the interface formed by the chemical bath deposited ZnS layer and the Cu(In0.7Ga0.3)Se2 absorber. Our results also provide information toward the design optimization of the optoelectronic properties of the ZnS/Cu(In0.7Ga0.3)Se2 interface. To enhance the electron injection from Cu(In0.7Ga0.3)Se2 absorber to ZnS layer further lowering of the energy barrier is required. For this purpose, the bandgap of ZnS should be reduced by controlling the crystal structure and composition or its Fermi level should be upward shifted by appropriate doping.

Design of energy band alignment at the Zn1−xMgxO/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells

The electronic band structure at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface was investigated for its potential application in Cd-free Cu(In,Ga)Se(2) thin film solar cells. Zn(1-x)Mg(x)O thin films with various Mg contents were grown by atomic layer deposition on Cu(In(0.7)Ga(0.3))Se(2) absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn(0.9)Mg(0.1)O and Zn(0.8)Mg(0.2)O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn(1-x)Mg(x)O (x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn(1-x)Mg(x)O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn(1-x)Mg(x)O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn(1-x)Mg(x)O films results in the flat (0 eV) and cliff (-0.23 eV) conduction band alignments at the Zn(0.8)Mg(0.2)O/Cu(In(0.7)Ga(0.3))Se(2) and Zn(0.9)Mg(0.1)O/Cu(In(0.7)Ga(0.3))Se(2) interfaces, respectively. The experimental results suggest that the bandgap energy of Zn(1-x)Mg(x)O films is the main factor that determines the conduction band offset at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface. Based on these results, we conclude that a Zn(1-x)Mg(x)O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn(1-x)Mg(x)O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn(1-x)Mg(x)O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.

Growth of Ultrathin Zn Compound Buffer Layer by a Chemical Bath Deposition for Cu ( In , Ga ) Se2 Solar Cells

A Zn compound buffer layer for Cu(In,Ga)Se 2 (CIGS) solar cells was grown from an alkaline aqueous solution using chemical bath deposition (CBD). To improve the film quality and exclude the cracks in the film, processing parameters such as reagent concentration, deposition time, and temperature profile were varied. Under the optimized CBD process, a uniform and crack-free film was grown on a CIGS substrate with thicknesses ranging from 10 to 60 nm. The controllable thickness of the film was as low as 10 nm. X-ray diffraction and Auger analysis showed that the Zn compound film was in an amorphous state with the ZnS x (OH) y O z composition. A 26% increase in the optical transmittance in the spectral range of 380―600 nm, as compared to a standard CdS buffer layer, was achieved. Finally, by optimization of the CBD process, we formed buffer layers, which enabled the transmission of the short wavelength of the solar spectrum for CIGS absorption.

Growth and Characterization of an In-based Buffer Layer by CBD for Cu ( In , Ga ) Se2 Solar Cells

An In-based buffer layer was deposited by using the chemical bath deposition (CBD) technique from the acetic aqueous solution containing InCl 3 and CH 3 CSNH 2 and its structure and optical properties have been characterized. In 2 S 3 and InOOH phases were found in the buffer layer from the combined results of X-ray photoelectron spectra and X-ray diffraction patterns. The growth of InOOH through the chemical bath deposition has not been previously reported. The compositional ratio of In 2 S 3 and InOOH in the film was ∼3. The Auger In MNN peak and the direct band gap of the InOOH phase are 407.1 and 3.5 eV, respectively. A uniform 30 nm thick In x (OOH, S) y film with a tightly connected grain structure was grown by the chemical bath deposition process. The optical transmittance of the In x (OOH, S) y buffer layer was higher than that of CdS buffer layer, due to an indirect band gap of In 2 S 3 , suggesting that this new film is a good candidate for the buffer layer of Cu(In, Ga)Se 2 solar cells.

Charge Transfer across a ZnO ∕ Electrolyte Interface Induced by Sub-Bandgap Illumination: Role of the Surface States

The role of interfacial bandgap states in sub-bandgap photoinduced electron transfer across a ZnO/electrolyte junction has been analyzed using time-resolved photocurrent measurements in the millisecond regime. The crystallographic structure and morphology of ZnO samples were characterized using X-ray diffraction and scanning electron microscopy measurements. A kinetic model for charge-carrier transport at the ZnO/electrolyte interface based on the intermediacy of the surface states was developed, and the rate equations were analytically solved. A theoretical simulation of the intensity-dependent photocurrent transients was also conducted. Based on an analysis of the experimental data and theoretical predictions, the density of the surface states was determined to be 3.1 X 10 13 cm -2 and the capture cross section was 1.5 X 10 -16 cm 2 . The obtained experimental results are consistent with the developed kinetic model based on a surface-state mediated charge-transfer mechanism.

Growth of a void-free Cu2SnS3 thin film using a Cu/SnS2 precursor through an intermediate-temperature pre-annealing and sulfurization process

Ternary earth-abundant Cu2SnS3 (CTS) absorbers were synthesized from a Cu/SnS2 stacked precursor by direct annealing in a S atmosphere and by pre-annealing at lower temperature followed by sulfurization. The existing S within the chosen Cu/SnS2 precursor allows avoiding the interface voids commonly generated from a metal precursor. We found that direct annealing of the S-containing precursor at 570 °C in a S atmosphere also generated voids mostly in the middle of the film because a CuS layer is formed on the precursor surface resulting in the discharge of excess S from the SnS2 layer. To eliminate the voids in the CTS film, we developed a two-step annealing process that consists of pre-annealing at 400 °C in N2 and sulfurization at 570 °C in a S atmosphere. The developed process yields a void-free CTS film with a smooth surface and tightly-connected grains. The phase evolution in the CTS films was analyzed by X-ray and Raman spectroscopy, and reaction pathways to form a dense Cu2SnS3 film from the Cu/SnS2 precursor are revealed. Our study demonstrated that appropriate design of annealing could grow a large-grain and dense CTS absorber required for a cost-effective thin film solar cell. Photoluminescence analysis confirmed that the CTS film grown by the two-step annealing process exhibited fewer deep-level defects compared to the film grown by direct annealing in a S atmosphere. The conversion efficiency of the solar cell based on the developed absorber is higher than that of a device using a CTS absorber synthesized by direct sulfurization. However, low values of the open-circuit voltage and fill factor indicate that fine control of the CTS composition is necessary to improve the device performance.

Improvement of electron transport in DSSCs by using Nb-doped TiO 2 electrodes

The performance of dye-sensitized solar cells (DSSCs) is closely related to efficiency of the electron transport within TiO2-dye-electrolyte system. Electron transport can be improved by modification of the electronic structure of TiO2 electrode by doping with niobium (Nb+5). For this purpose, the DSSCs based on undoped and Nb-doped TiO2 layers were fabricated and their PV parameters and electrical properties were studied. The Nb-doped TiO2 was prepared by a sol-gel method followed by a hydrothermal treatment with the Nb content in the range of 0.7 to 3.5 mol%. The transport properties of the TiO2-dye-electrolyte junction were investigated using the electrical impedance spectroscopy. The electron lifetimes, estimated from Bode plots, were found to increase from 8 ms for DSSCs based on undoped TiO2 up to 26 ms for the cells based on TiO2 doped with 2.7 mol% of Nb. We have shown correlation between the parameters of the charge carrier transport and the main DSSC photovoltaic characteristics. The increase in the value of electron lifetime was shown to enhance the value of short circuit current (Jsc). When the Nb doping level was lower than 1.7 mol%, the electrical resistance at the TiO2-electrolyte interface increased leading to rise of the value of the open circuit voltage (Voc). Doping with the 1.7 mol% of Nb led to increase of both Jsc and Voc, and significantly improved the device efficiency.

Wet Pretreatment-Induced Modification of Cu(In,Ga)Se2/Cd-Free ZnTiO Buffer Interface

We report a novel Cd-free ZnTiO buffer layer deposited by atomic layer deposition for Cu(In,Ga)Se2 (CIGS) solar cells. Wet pretreatments of the CIGS absorbers with NH4OH, H2O, and/or aqueous solution of Cd2+ ions were explored to improve the quality of the CIGS/ZnTiO interface, and their effects on the chemical state of the absorber and the final performance of Cd-free CIGS devices were investigated. X-ray photoelectron spectroscopy (XPS) analysis revealed that the aqueous solution etched away sodium compounds accumulated on the CIGS surface, which was found to be detrimental for solar cell operation. Wet treatment with NH4OH solution led to a reduced photocurrent, which was attributed to the thinning (or removal) of an ordered vacancy compound (OVC) layer on the CIGS surface as evidenced by an increased Cu XPS peak intensity after the NH4OH treatment. However, the addition of Cd2+ ions to the NH4OH aqueous solution suppressed the etching of the OVC by NH4OH, explaining why such a negative effect of NH4OH is not present in the conventional chemical bath deposition of CdS. The band alignment at the CIGS/ZnTiO interface was quantified using XPS depth profile measurements. A small cliff-like conduction band offset of -0.11 eV was identified at the interface, which indicates room for further improvement of efficiency of the CIGS/ZnTiO solar cells once the band alignment is altered to a slight spike by inserting a passivation layer with a higher conduction band edge than ZnTiO. Combination of the small cliff conduction band offset at the interface, removal of the Na compound via water, and surface doping by Cd ions allowed the application of ZnTiO buffer to CIGS treated with Cd solutions, exhibiting an efficiency of 80% compared to that of a reference CIGS solar cell treated with the CdS.