Kazuyoshi Nakada

Amorphous silicon oxide passivation films for silicon heterojunction solar cells studied by hydrogen evolution

The passivation mechanism of ultrathin (~6 nm) a-SiO:H films was studied by effective lifetime measurements and thermal desorption spectroscopy. An extremely high effective lifetime and a low surface recombination velocity of 6.3 ms and 1.6 cm/s, respectively, were achieved after postannealing treatment. For samples with high lifetime, the low-temperature hydrogen effusion peak related to molecular hydrogen shifted to higher temperatures independently of passivation material, indicating that desorption temperature directly reflects the changes in surface passivation. The same trend was observed for the high-temperature peak related to atomic hydrogen effusion for samples passivated by a-SiO:H. Additionally, when compared with a-Si:H, the high-temperature peak for a-SiO:H shifted to higher temperatures owing to O backbonding. Moreover, it was found that the FWHM of desorption peaks increased owing to the convoluted desorption from different Si3−nOn–Si–H (n = 0, 1, 2, and 3) configurations.

Silicon heterojunction solar cells with high surface passivation quality realized using amorphous silicon oxide films with epitaxial phase

The epitaxial growth of the i-layer of crystalline silicon heterojunction solar cells has been widely accepted as harmful to surface passivation. In our experiments, however, although a very rough epitaxial phase in the intrinsic a-Si1−xOx:H passivation layer was confirmed by transmission electron microscopy and spectroscopic ellipsometry, a high effective lifetime and an implied-VOC of over 720 mV were achieved with lifetime samples. The high passivation quality was confirmed by the obtained open circuit voltages of 728 and 721 mV for n- and p-type solar cells, respectively, with an a-Si1−xOx:H/p-µc-Si1−xOx:H stack rear structure. These results indicate that, contrary to the common knowledge, high surface passivation quality can be achieved even when the epitaxial phase is present.

Interfacial quality improvement of Cu(In,Ga)Se2 thin film solar cells by Cu-depletion layer formation

Se irradiation with time, t Se, was introduced after the second stage of a three-stage process to control the Cu2Se layer during Cu(In,Ga)Se2 (CIGS) deposition. Open circuit voltage and fill factor of CIGS solar cells could be improved by introducing Se irradiation. We concluded that the control of the Cu2Se layer led to the formation of a Cu-depletion CIGS layer (CDL), which improved conversion efficiency owing to suppression of interfacial recombination by a valence band offset formed between CIGS and the CDL. Finally, highest efficiency of 19.8% was achieved with t Se of 5 min. This very simple and new technique is promising for the improvement of photovoltaic performance.

Application of n-type microcrystalline silicon oxide as back reflector of crystalline silicon heterojunction solar cells

We investigated the application of n-type hydrogenated microcrystalline silicon oxide (n-µc-Si1−xOx:H) as an alternative back reflector material for n-type heterojunction solar cells. The effect of the CO2 and PH3 flow rates on the refractive index, oxygen content, conductivity, and crystalline fraction (Xc) of n-µc-Si1−xOx:H films was evaluated. By controlling the film oxygen content, the refractive index could be widely changed, while the absorption coefficient at wavelengths exceeding 800 nm was practically zero. We found that the insertion of an n-µc-Si1−xOx:H layer between the back surface field and the rear electrode improves the reflectance at long wavelengths. The improvement in reflectance resulted in higher internal quantum efficiency in the IR range, suggesting that the application of n-µc-Si1−xOx:H as a back reflector is promising for reducing long-wavelength losses in heterojunction solar cells.

Impact of roll-over-shaped current–voltage characteristics and device properties of Ag(In,Ga)Se2 solar cells

The roll-over shape often observed in the current–voltage curve of Ag(In,Ga)Se2 (AIGS) solar cells degrades the open circuit voltage (V OC) and particularly the fill factor (FF). The origin of the roll-over shape was investigated by experimental measurements and device simulation. By combining AC Hall measurement and the peel-off process, we estimated the AIGS hole concentration to be 2.2 × 1012 cm−3. Theoretical simulation revealed that the roll-over shape is attributed to this low hole concentration. Under an applied forward bias, the band bending near the back contact of the AIGS layer forms an intrinsic semiconductor owing to the injected electrons, leading to the formation of an inverted diode. To solve this issue, the addition of NaF by the postdeposition treatment of the AIGS layer was performed. As a result, the hole concentration of the AIGS layer increased, significantly improving its V OC, FF, and conversion efficiency.

Efficiency enhancement of flexible Cu(In,Ga)Se2 solar cells deposited on polyimide-coated soda lime glass substrate by alkali treatment

Alkali treatment effects on Cu(In,Ga)Se2 (CIGS) solar cells deposited on polyimide-coated soda lime glass (PI-coated SLG) were investigated. CIGS on PI-coated SLG shows Na diffusion from the substrate, which should be controlled to obtain high efficiencies. Further incorporation of Na was achieved by enhancing diffusion from the substrate or by external incorporation using post-deposition treatment (PDT) methods. Both methods lead to a high efficiency of approximately 15%. Moreover, aside from Na, K was also incorporated by KF-PDT, resulting in efficiency improvement from 12% for an untreated CIGS to more than 18% at the maximum substrate temperature of 450 °C, which is comparable to CIGS deposited at higher temperatures using the same equipment. It was also found that the alkali concentration of CIGS deposited on PI-coated SLG shows almost the same behavior as that of a film deposited on a rigid glass, suggesting that the deposition technique for CIGS on the rigid glass can be applied to flexible substrates.

Conduction band offset engineering in wide-bandgap Ag(In,Ga)Se2 solar cells by hybrid buffer layer

Ag(In,Ga)Se2 (AIGS) is one of the promising candidates for the top cell absorber in the tandem structure. However, the conversion efficiency of AIGS solar cells is still lower than that required for the top cell. In this study, to improve the conversion efficiency of AIGS solar cells, we controlled the conduction band offset (CBO) at the buffer layer/ZnO and buffer layer/AIGS interfaces. The reduction in interface recombination at the CdS buffer layer/AIGS interface was achieved by introducing a ZnS(O,OH) buffer layer instead of a CdS buffer layer, although the fill factor (FF) decreased markedly because the CBO at the ZnS(O,OH)/ZnO interface prevented the electron flow under a forward bias. We found that the introduction of a CdS/ZnS(O,OH) hybrid buffer layer is efficient in controlling the CBO at both the buffer layer/AIGS and buffer layer/ZnO interfaces and improving the solar cell conversion efficiency.

Fabrication and characterization of sputtered Cu2O:N/c-Si heterojunction diode

Nitrogen doped cuprous oxide (Cu2O:N) deposited by reactive sputtering was applied to a crystalline silicon (c-Si) based heterojunction diode. The current density–voltage (J–V) characteristics of the fabricated diode showed rectifying characteristics with a high rectifying ratio of 105 at ±1 V. The capacitance–voltage measurements revealed the existence of a large conduction band offset and a small valence band offset at the Cu2O:N/c-Si interface, which implies that Cu2O:N is a suitable material for a hole selective emitter layer in n-type c-Si based heterojunction solar cells. Detailed analysis of the temperature dependent J–V characteristics showed that the diode current was limited by interface recombination originated from Fermi level pinning at the Cu2O:N/c-Si interface.

Improvement of Cu(In, Ga)Se 2 photovoltaic performance by adding Cu-poor compounds Cu(In, Ga) 3 Se 5 at Cu(In, Ga)Se 2 /CdS interface

We report on the improvement of Cu(In, Ga)Se₂ (CIGS) photovoltaic performance by inserting Cu-poor compounds Cu(In, Ga)₃Se₅ at CIGS/CdS interface. It was experimentally found that formation of the Cu(In, Ga)₃Se₅ layer at the CdS/CIGS interface was quite important to boost the photovoltaic performance. Cd diffusion was promoted by introducing the layer, and conductivity type turned to n-type near the surface. As a result, cell performance was improved owing to suppression of recombination at the interface by stronger band-bending. Additionally, it was shown that the interval time before the growth of the Cu(In, Ga)₃Se₅ layer is important to eliminate the Cu intermixing in CIGS.

Crystal growth mechanism of Cu2ZnSn(S,Se)4 thin films fabricated from nanoparticles

In this paper, we study the reaction mechanisms involved in the transformation from the precursor prepared with nanoparticles to the Cu2ZnSn(S,Se)4 phase during solid-phase sulfurization and selenization. The top-view of the film observed by scanning electron microscopy showed that crystal grains suddenly grow at temperatures above 550 °C. For all samples, impurity phases were not detected by X-ray diffraction and Raman spectroscopy, which indicates that intermediate phases such as binary and ternary compounds are not necessary to produce the Cu2ZnSn(S,Se)4 phase. During sintering, the film was first sulfurized because of the difference between the vapor pressures of S and Se. Subsequently, the S of Cu2ZnSn(S,Se)4 was replaced by Se, causing the generation of a defective layer. For the sample sintered at 600 °C, Cu-rich, Zn-rich, and Sn-rich phases were detected by transmission electron microscopy analysis. Additionally, a diagrammatic Cu2ZnSn(S,Se)4 film growth model was proposed to illustrate the detailed reaction mechanism during precursor sintering.