Jakapan Chantana

Sputtered (Zn,Mg)O buffer layer for band offset control in Cu2ZnSn(S,Se)4 solar cells

We fabricated Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with (Zn,Mg)O buffer layers as an alternative to the CdS buffer layer for the improvement of cell performance, where the (Zn,Mg)O layers are deposited by sputtering. However, the solar cell efficiency decreased with the (Zn,Mg)O layer as compared with the CdS layer. Photoluminescence measurements indicated that the damage near the surface of the CZTSSe absorber was induced by the sputtering. To suppress the damage, a 10-nm-thick CdS layer was deposited on the absorber before sputtering. As a result, the efficiency achieved with the (Zn,Mg)O layer was the same as that with the CdS layer. To further improve the efficiency of the cell with the (Zn,Mg)O layer, it is necessary to eliminate sputtering damage. In addition, the conduction band offset of the (Zn,Mg)O/CZTSSe interface is controllable by varying the Mg content. Therefore, the (Zn,Mg)O buffer layer can be suitable against a large-band-gap CZTSSe absorber.

Determination of open-circuit voltage in Cu(In,Ga)Se2 solar cell by averaged Ga/(In + Ga) near its absorber surface

The Cu(In,Ga)Se2 (CIGS) solar cells are fabricated, where CIGS absorbers with various Ga/III, Ga/(In + Ga), profiles are prepared by the so-called “multi-layer precursor method” using multi-layer co-evaporation of material sources. The dependence of cell parameters upon several averaged Ga/III ratios, calculated in different depth ranges of the Ga/III profiles from the absorber surfaces, is performed. It is revealed that open-circuit voltage (VOC) is mainly dependent on the averaged Ga/III, calculated near the surface area of the CIGS absorber (or in space charge region; SCR). The SCR width of ∼0.32 μm can be predicted, corresponding to the calculated value. Finally, the averaged Ga/III near the absorber surface is proposed as an indicator of VOC before solar cell fabrication, as long as carrier concentration is almost constant.

Optimum bandgap profile analysis of Cu(In,Ga)Se2 solar cells with various defect densities by SCAPS

The bandgap of a Cu(In,Ga)Se2 (CIGS) absorbing layer is varied from 1.0 to 1.7 eV by changing the composition ratio of gallium (Ga), realizing an optimum design for solar cell absorbers. In this study, the effects of a graded bandgap profile on the cell performance of a CIGS solar cell are investigated using a device simulator. Moreover, optimum bandgap profiles with various defect densities are simulated. In the case of low defect densities, when the lowest bandgap, Egmin, is inside the space-charge region (SCR), the double-graded structure is effective for achieving high efficiency. However, when Egmin is outside the SCR, the negative gradient from Egmin to the CIGS surface acts as a barrier that impedes the collection of photogenerated electrons, thereby increasing the recombination rate and decreasing cell efficiency. In the case of high defect densities, to decrease the recombination current and improve the efficiency, a more positive gradient from the back contact to the surface is needed.

Relationship between open-circuit voltage in Cu(In,Ga)Se2 solar cell and peak position of (220/204) preferred orientation near its absorber surface

Cu(In,Ga)Se2 (CIGS) absorbers with various Ga/III, Ga/(In+Ga), profiles are prepared by the so-called “multi-layer precursor method” using multi-layer co-evaporation of material sources. It is revealed that open-circuit voltage (VOC) of CIGS solar cell is primarily dependent on averaged Ga/III near the surface of its absorber. This averaged Ga/III is well predicted by peak position of (220/204) preferred orientation of CIGS film near its surface investigated by glancing-incidence X-ray diffraction with 0.1° incident angle. Finally, the peak position of (220/204) preferred orientation is proposed as a measure of VOC before solar cell fabrication.

Influence of conduction band minimum difference between transparent conductive oxide and absorber on photovoltaic performance of thin-film solar cell

Difference of conduction band minimum (EC) between transparent conductive oxide (TCO) and absorber, named ΔEC-TA, in thin-film solar cell is investigated for high cell performance using device simulator. According to the simulation, the optimized ΔEC-TA value is different, depending on the carrier density in buffer layer, ND-B. With ΔEC-TA above 0.6 eV for both ND-Bs of 1.0 × 1013 and 1.0 × 1018 cm−3, the spike is formed at the TCO/buffer interface, thus decreasing cell performances, especially short-circuit current density owing to impeding photo-generated carriers to TCO. On the other hand, with ΔEC-TAs below −0.2 and −0.4 eV for ND-Bs of 1.0 × 1013 and 1.0 × 1018 cm−3, the solar cells demonstrate double diode characteristics, thereby decreasing cell efficiency. Eventually, the optimized ΔEC-TA values for high cell performance are proposed to be in the ranges from −0.2 to 0.6 eV and from −0.4 to 0.6 eV for ND-Bs of 1.0 × 1013 and 1.0 × 1018 cm−3, respectively.

Multi layer precursor method for Cu(In,Ga)Se2 solar cells fabricated on flexible substrates

Cu(In,Ga)Se2 (CIGS) solar cells are deemed one of the highest conversion efficiency thin-film solar cells, and the utilization of flexible (stainless steel; SUS) substrates offers several advantages in terms of lowering manufacturing costs. In this work, CIGS films on SUS substrates are deposited by the so-called “multi layer precursor method” using multi layer co-evaporation of material sources. The examination concentrates on the influence of growth temperature and Ga/III, Ga/(In + Ga), profiles of CIGS layers on the cell performance. According to the results, CIGS crystallization is primarily manipulated by growth temperature. Ultimately, the deposition of CIGS on a flexible SUS substrate can be readily controlled to obtain a suitable double Ga/III-grading profile with large grains of CIGS for a high cell efficiency of 16.05%.

Evaluation of junction quality of buffer-free Zn(O,S):Al/Cu(In,Ga)Se2 thin-film solar cells

Cu(In,Ga)Se2 (CIGS) solar cells without buffer layers, namely, buffer-free solar cells, and those with conventional CdS buffer layers were fabricated. Zn(O,S):Al (AZOS) was used as a transparent conductive oxide layer. The AZOS/CIGS interface in the buffer-free solar cells and the AZOS/CdS/CIGS interface in the CdS-buffered solar cells were investigated by bright-field scanning transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and fast Fourier transform. It was found that the open-circuit voltage and fill factor of the buffer-free solar cells are lower than those of the CdS-buffered solar cells, which is attributed to the nonepitaxial growth of AZOS on the CIGS layer, forming defects and deteriorating the junction quality of the buffer-free solar cells.

Highly Efficient 17.6% Tin–Lead Mixed Perovskite Solar Cells Realized through Spike Structure

Frequently observed high Voc loss in tin-lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the Voc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se2 solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved Voc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements.

20% Efficient Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/Cu(In,Ga)(S,Se)2 Solar Cell Prepared by All-Dry Process through a Combination of Heat-Light-Soaking and Light-Soaking Processes

Development of Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe)-based thin-film solar cells fabricated by an all-dry process is intriguing to minimize optical loss at a wavelength shorter than 520 nm owing to absorption of the CdS buffer layer and to be easily integrated into an in-line process for cost reduction. Cd-free CIGSSe solar cells are therefore prepared by the all-dry process with a structure of Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CIGSSe/Mo/glass. It is demonstrated that Zn0.8Mg0.2O and Zn0.9Mg0.1O:Al are appropriate as buffer and transparent conductive oxide layers with large optical band gap energy values of 3.75 and 3.80 eV, respectively. The conversion efficiency (η) of the Cd-free CIGSSe solar cell without K-treatment is consequently increased to 18.1%. To further increase the η, the Cd-free CIGSSe solar cell with K-treatment is next fabricated and followed by posttreatment called the heat-light-soaking (HLS) + light-soaking (LS) process, including HLS at 110 °C followed by LS under AM 1.5G illumination. It is disclosed that the HLS + LS process gives rise to not only the enhancement of carrier density but also the decrease in the carrier recombination rate at the buffer/absorber interface. Ultimately, the η of the Cd-free CIGSSe solar cell with K-treatment prepared by the all-dry process is enhanced to the level of 20.0%.

Impact of Precursor Compositions on the Structural and Photovoltaic Properties of Spray-Deposited Cu2ZnSnS4 Thin Films

Pure sulfide Cu2 ZnSnS4 thin films were fabricated on Mo-coated glass substrates by facile spray deposition of aqueous precursor solutions containing Cu(NO3 )2 , Zn(NO3 )2 , Sn(CH3 SO3 )2 , and thiourea followed by annealing at 600 °C. When a precursor solution containing a stoichiometric composition of Cu, Zn, and Sn was used, the resulting Cu2 ZnSnS4 thin film contained a Cu2-x S impurity phase owing to the evaporation of Sn components during the annealing process. The Cu2-x S impurity in the Cu2 ZnSnS4 thin film was removed by reducing the concentration of Cu in the precursor solution. This resulted in an improvement of the structural features (i.e., grain sizes and compactness) as well as the electric properties such as acceptor densities, the nature of the acceptor defects, and carrier lifetimes. A solar cell based on the Cu2 ZnSnS4 film with an empirically optimal composition showed conversion efficiency of 8.1 %. The value achieved was one of the best efficiencies of Cu2 ZnSnS4 -based cells derived from a non-vacuum process.