Tandem Type Solar Cells Research Articles

Orientation control of two-dimensional perovskite (CH3(CH2)3NH3)2(CH3NH3) n−1PbnI3n+1 (n = 2) thin films by thermal annealing

It has been attempted to preferentially orientate Pb-I layers in two-dimensional (2D) organic-inorganic hybrid perovskite thin films (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 perpendicular to substrates only by thermal annealing after spin coating of a reagent solution for improvements in the energy-conversion-efficiency of solar cells. It is found from X-ray diffraction measurements that the ratio of diffraction intensity from the (202) plane to that from the (060) plane becomes larger in thermally annealed (50 °C–135 °C) samples. This indicates that the Pb-I layer tends to grow perpendicular to the surface of the substrate. In particular, the ratio has reached 8.2, which is larger compared with the ratio of 2.7 for the randomly oriented powder sample, for the sample prepared on SnO2 substrates. Such (202) oriented films seem to contribute to improvements in the energy-conversion-efficiency of tandem-type solar cells utilizing the 2D perovskite thin films as an active layer of the top cell.

Thermodynamic limit of tandem solar cells under different solar spectra and their perovskite top solar cell

Abstract Thermodynamic (detailed balance) limits of single-junction and tandem-type solar cells are calculated under different solar spectra, which are spectra of blackbody (6000 K), air mass 0, air mass 1.5 global (1.5G), and air mass 1.5 direct (1.5D). The solar cells are described by blackbody under room temperature of 298.15 K. It is disclosed that optimized bandgaps of single-junction solar cells with the highest conversion efficiencies of 31.6, 30.6, 33.7 and 33.1% are 1.29, 1.25, 1.34, and 1.34 eV under illuminations of blackbody (6000 K), air mass 0, air mass 1.5G, and air mass 1.5D, respectively. Conversion efficiencies of 43.2, 42.2, 45.5, and 45.1% are further increased for double-junction tandem solar cells with optimized bandgaps of bottom and top cells under spectra of blackbody (6000 K), air mass 0, air mass 1.5G, and air mass 1.5D, respectively. Moreover, detailed balance limits of tandem solar cells with perovskite top cells with bandgap range of 1.60–1.63 eV were realized with high conversion efficiency, when bandgap values of bottom cells are optimized in a range of 0.8–1.01 eV, and at 1.1 eV. It is also disclosed that the optical matching between top cell absorber and bottom cell absorber should be optimized for the high cell performances.

(Keynote) Development History of High Efficiency Silicon Heterojunction Solar Cell- from Discover to Practical Use

Silicon heterojunction (SHJ) solar cells are attracting attention as high-efficiency Si solar cells. The basicSHJ structure is p-type amorphous Si (a-Si)/i-type (non-doped) a-Si/n-type mono-crystalline Si (c-Si). Wehave developed and actively evolved this SHJ solar cells from early 1990s, and introduced the moduleequipped with SHJ solar cells named as well-known “HIT”. The features of HIT are: (1) high efficiency,(2) good temperature characteristics, that is, a small output decrease even in the temperature environmentactually used, (3) easy application to double-sided power generation (bifacial module) using symmetricstructure. Although bifacial module has been receiving much attention in recent years, we launched it asearly as 2000.How was this excellent solar cell born? Originally, it was born in the process of developing thin-filmpolycrystalline Si (poly-Si) solar cells as the bottom cells of tandem-type solar cells with a-Si top cells.As an initial stage of the development, we used p-type a-Si as a window layer and n-type c-Si wafer as aphotovoltaic layer because the characteristics of c-Si was clearer than thin film poly-Si. The insertion of ilayerbetween p-type a-Si and n-type c-Si came from two ideas to improve the interface properties. Onewas a structural approach (control the Si bonding network between c-Si and a-Si with microcrystal bufferlayer), and the other was an attempt to suppress the mutual diffusion of impurities. For the former, it wasconfirmed that it was better to use a-Si instead of microcrystals to passivate the c-Si surface effectively,and for the latter, it was found that better characteristics could be obtained by surface passivation of ilayerwith few defects, instead of the effect of impurity separation. The surface passivation technologyusing amorphous semiconductor film was a new invention different from conventional passivation usinginsulating films such as SiOx and SiNx. With this new structure, a large improvement of about 30 mV inopen circuit voltage (Voc) was confirmed. Next we applied this approach to the back side, and again wefound that i-layer could greatly reduce interface defects. Thus, the basic structure of the symmetrical SHJsolar cell was determined. With this structure (Ag electrode/TCO/(p/i)-a-Si/n-c-Si/(i/n)-a-Si/TCO/Agelectrode), we achieved a conversion efficiency of 20% (cell size of 1 cm x 1 cm) in 1994, and after thatthe efforts for mass production started. To expand to a practical size, we newly developed the electrodeforming technology using low-temperature curing type silver paste, and successfully started the massproduction in 1997.In parallel, our R&D team continued to make various conversion efficiency improvements whichincluded optimizing a-Si deposition conditions, developing new TCO materials and deposition methods,and lowering the resistivity of silver paste. In addition, we succeeded in demonstrating in experiments forthe first time that the open-circuit voltage increased associated with the thinning of wafers that had beenpredicted in simulations. The excellent surface passivation capability of i-layer minimized losses whenthinning wafers and maintained the conversion efficiency, which was a very advantageous feature fromthe viewpoint of cost reduction. At the end of 2012 (announced in 2013), we achieved a cell conversionefficiency of 24.7% with a wafer thickness of 98 μm, demonstrating that both low cost and highefficiency can be obtained. Some of these technologies were introduced into mass production.In 2014, we applied SHJ technology the back-contact type solar cell, and achieved a conversionefficiency of 25.6% with both the high Isc due to elimination of front electrode and the high Voc due toexcellent surface passivation. This was a new world record at that time for non-concentrating siliconbasedsolar cells at the research level, the previous one having stood for 15 years.

Ohmic-like contact formation at the rear interface between Cu(In,Ga)Se2 and ZnO:Al in a lift-off Cu(In,Ga)Se2 solar cell

A lift-off Cu(In,Ga)Se2 (CIGS) solar cell, which could serve as the top solar cell in a tandem-type solar cell, was fabricated with a superstrate-type structure consisting of soda-lime glass (SLG)/epoxy/Al/n-type ZnO:Al (AZO)/ZnO/CdS/p-type CIGS/n-type AZO (back electrode)/Al. This study proposes a method of realizing the ohmic-like contact between p-type CIGS and n-type AZO in a lift-off CIGS solar cell to improve the conversion efficiency (η). The sputtering conditions under which the n-type AZO layer was deposited, specifically the sample position and radio frequency (RF) power density, were optimized to form ohmic-like contact through trap-assisted tunneling recombination. The sputtering damage near the CIGS surface that induced the trap-assisted tunneling recombination was evaluated by examining the photoluminescence peak intensity. Ultimately, ohmic-like characteristics were attained for the p-type CIGS/n-type AZO interface under optimal AZO deposition conditions, with the sample positioned 0cm from the AZO target center and an RF power density of 2.4W/cm2. The proposed method was successfully applied to the fabrication of a lift-off CIGS solar cell. Consequently, an η value of 9.2% was obtained for the lift-off CIGS solar cell, which is 70% that of a substrate-type CIGS solar cell.

広帯域増感色素と有機金属ハライドペロブスカイトを用いた高効率有機系タンデム太陽電池

Tandem-type solar cell is a multiple-layered solar cell to decrease thermal- and optical-energy losses. However, typical organic photovoltaics has been difficult to apply tandem photovoltaics because of the lack of organic photovoltaic cells with high open-circuit voltage (VOC) and/or near-infrared (NIR) photoelectric conversion. In this study, we constructed tandem-type organic photovoltaics using high VOC organometal halide perovskite solar cell and NIR dye-sensitized solar cell. Consequently, over 16% power conversion efficiency which is highest record among tandem-type organic photovoltaics has been accomplished.

Effect of p-Type Semiconductor Electrode on Photovoltaic Properties in n/p Tandem-Type Dye-Sensitized Solar Cell

To improve the light-to-electric energy conversion efficiency of a dye-sensitized solar cell, a p-type semiconductor electrode was introduced to the cell so that n/p tandem-type solar cell was composed. In the tandem cell, the TiO2 adsorbed with N719 dye molecules and the p-type semiconductor were used an anode and a cathode, respectively, and acetonitrile solution containing iodide was done as an electrolyte. The cathode was prepared on a surface of platinum layer formed on a FTO glass plate by plasma spattering CuxBiyOz as the p-type semiconductor. Short-circuit current density and conversion efficiency of the tandem cell were larger than those in a cathode without the p-type semiconductor layer. The p-type semiconductor layer will be able to absorb low energy photons passed through the anode. After ZnO particles were coated on a surface of the p-type semiconductor layer, open-circuit voltage was increased. Such coating of the particles will be effective to depress a back-transfer at the interface between the p-type semiconductor layer and the electrolyte and band-bending of the p-type semiconductor.

Transparent Conducting Titanium-Doped Zinc Oxide Films Prepared by PLD in Magnetic Field

In this study, a remarkable feature of the method that fabricates Titanium-Doped Zinc Oxide (TZO) films is used to deposit films in a magnetic field caused by the rare earth magnet, NdFeB arranged between the target and substrate. The magnetic field is applied perpendicular to the plume. As a result, the resistivity of the films fabricated in the magnetic field decreased from 1.26×10−3 to 6.99×10−4 Ω•cm. Furthermore, the average transmittance in the visible wavelength region (400-700 nm) increased from 77.0% to 81.3%, and that in the wavelength region applicable to the tandem type solar cells (400-1500 nm) increased from 84.2% to 85.5% for the films fabricated in the magnetic field. Furthermore, an average transmittance in the wavelength region corresponding to the whole solar spectra (400-2500 nm) was 71.4%.

High-Performance, Tandem-Type Amorphous Silicon Solar Cell

AbstractMicrocrystalline silicon oxide (μc-SiO) deposited from the gas mixture of silane and carbondioxide using VHF plasma was reported by authors to be the promising material for thin-film silicon solar cell fabricated on glass substrate, comparing with the conventional amorphous silicon oxide or microcrystalline silicon p-layer. High-performance, tandem-type solar cell with amorphous and microcrystalline cells (double cells) has been achieved by ap-plying this μc-SiO to the p-layer of both cells.Further work has been extended to a-Si tandem type solar cell with triple cell configu-ration using amorphous silicon oxide (a-SiO), amorphous silicon germanium (a-SiGe) and microcrystalline Si (μc-Si) as the top, middle and bottom cells, respectively. Up to now, cell efficiency of 15.7 % has been achieved using this novel μc-SiO.

Thirty years trajectory of amorphous and nanocrystalline silicon materials and their optoelectronic devices

A review is given on research trajectory of hydrogenated amorphous and nanocrystalline silicon (a-Si:H and nc-Si) materials with their device applications ongoing since the period of 1970. A brief explanation on the motivation to start amorphous semiconductors research is given to produce a new kind of synthetic semiconductor having continuous energy gap controllability with valency electron controllability. Due to the result of some basic research on the film quality improvement of a-Si:H and nc-Si, some innovative devices had been developed since middle of 1980s in R&D phase such as a-SiC/a-Si heterojunction solar cells, a-Si/a-SiGe and also a-Si/nc-Si tandem type solar cells. Finally, the state of the art on the industrialization of the new devices is introduced and discussed.

Improved Stability of Hydrogenated Amorphous Silicon Solar Cells Fabricated by Triode-Plasma CVD

AbstractWe have employed a triode-type plasma CVD system to fabricate highly stabilized hydrogenated amorphous silicon (a-Si:H) solar cells. The p-i-n type solar cells were fabricated on a textured SnO2/glass substrate (ASAHI VU type). By applying a triode system, the Si-H2 bond density in the film decreased to about one third (from 1.7 at.% for conventional parallel-plate-electrode to 0.6 at.% for a triode configuration), and correspondingly the degradation ratio decreased from 13 % to 10 %. We have achieved the degradation ratio of 5 % by optimizing the player deposition conditions. In case of a triode system, there were minor effects of higher hydrogen dilution in the stabilized efficiency. We have experimented the effects of the substrate temperature for a higher stabilized efficiency. Further improvement in solar efficiency has been made by applying antireflection layers to air/glass and TCO/p interfaces. As a result, we have achieved the stabilized efficiency of 9.22 % (Jsc = 15.9 mA/cm2, Voc = 0.863 V, FF = 0.672) with a degradation ratio of 7.8 %. We have also employed the triode-deposited a-Si:H solar cell to a tandem type solar cell with a structure of a-Si:H/hydrogenated microcrystalline silicon (μc-Si:H). We have achieved the stabilized efficiency of 10.9 % (Jsc = 12.0 mA/cm2, Voc = 1.31 V, FF = 0.691) with a degradation ratio of 7.3 %.

40 Years Trajectory of Amorphous Semiconductor Research

ABSTRACTA review is given on a research trajectory of amorphous and microcrystalline semiconductors and their device applications proceeded since 1970. A brief explanation on the motivation to start amorphous semiconductor research is given to produce a new kind of synthetic semiconductor having continuous energy gap controllability with valency electron controllability through our experience of modulation spectroscopy in semiconductors.The first material we have challenged is Si-As-Te chalcogenide semiconductor which has a very wide vitreous region in Gibb's Triangle. A series of systematic experiments has been carried out in the terrestrial environment since 1971, and also within the TT-500A rocket experiment in 1980, and the Spacelab. J experiments FMPT (First Material Processing Test) project in 1992. The second material is hydrogenated amorphous silicon (a-Si:H) and its alloys started in 1976 just after the Garmisch Partenkirchen ICALS-6. With some basic research on the a-Si:H film deposition technology and film quality improvement, our continuous effort to improve the efficiency bore the tandem type solar cells in 1979, and also new products of a-SiC:H and a-SiGe:H in the early period of 1980s are described. These innovative device structures and materials have bloomed in the middle of 1980s in R & D phase such as a-SiC/a-Si heterojunction solar cells, a-Si/a-SiGe and also a-Si/poly-Si tandem type solar cells, and industrialized in recent few years. New kind of trials on full-color thin film light emitting devices has also been recently initiated with wide range of band gap controllability of a-SiC:H.The third material is microcrystalline silicon (µc-Si) and their alloys which gathers a tremendous R & D effort as a promised candidate for the bottom cell of the a-Si/µc-Si tandem solar cells aimed for the all-round plasma CVD process for the next age thin film photovoltaic devices. In the final part of presentation, a brief discussion will be given on a technological evolution from “bulk crystalline age” to “multilayered thin film age” in the semiconductor optoelectronics toward 21 century.

Three-Layer Organic Solar Cell

Abstract Three-layer organic cells composed of perylene pigment/phthalocyanine/quinacridone were examined, and the photocurrent was founded to be greatly affected by the thickness of phthalocyanine layer. Furthermore, the tandem type solar cells were fabricated by combining of several units of the three-layer cell to clarify the relationship between the number of accumulated unit cells and cell characteristics.

The effect of interposing thin oxide layers on the photovoltaic properties of a-Si:H solar cells II between the middle n and p layers of a tandem-type cell

The properties of the p/n junction of hydrogenated amorphous silicon (a-Si) were studied by use of some kinds of a-Si cells. It was concluded that the photoelectromotive force generated at the p/n junction reduces the photovoltage of the tandum-type solar cells, and the rate of the electron-hole recombination lowers their fill factor. However, by interposing a 2-nm-thick TiO2 layer between p and n layers of the tandem-type solar cells, the open-circuit photovoltage was raised from 1.50 to 1.64 V, the fill factor from 0.705 to 0.775, and the energy conversion efficiency was improved ∼10%. Among the metal oxides examined i.e., V2O5, TiO, TiOx, TiO2, NiO, WO3, ITO, Fe2O3, and SiO2, the TiOx (x=1.7) layer showed the best result. In addition, it was observed that the thickness of the p and n layers, which was necessary to form the p/n junction, could be reduced by interposing a metal-oxide layer.

A-SiGe:H Alloy and its Application to Tandem-Type Solar Cell

ABSTRACTAmorphous SiGe:H alloy is the key material in achieving high conversion efficiency with tandem-type amorphous silicon alloy solar cells. Status and issues for this key material are discussed, and efforts made to irprove it are reviewed to obtain directions for higher quality a-SiGe:H alloys. An application of the improved alloy to tandem-type solar cell to achieve 9.6% efficiency for the cell size of 100 cm2 is reported.

Tandem type amorphous solar cells

An efficiency of 8.6 % by the three-stacked tandem type amorphous solar cell which consists of a-Si:H and a-SiGe:H with a size of 10 by 10 cm was obtained. This tandem type solar cell showed stable characteristics for the prolonged light exposure.

Amorphous Solar Cells Using a-Si: H and a-SiGe: H Films

The electric properties of optimized a-SiGe: H films deposited by an R.F. glow discharge in (SiH4+GeH4) and application of these films to the tandem type solar cells were investigated. The dark-conductivity of a-SiGe: H films increase with increasing Ge content and follows quite well to the Meyer-Neldel rule with measured activation energies. Fermi level which was deduced from the activation energy shifts toward mid-band gap under the optimized growth condition. The ratio σph/µn which is related to the lifetime of photogenerated carriers was found nearly constant for various Ge contents in the a-SiGe: H films. Inverted P-I-N type amorphous solar cell with an intrinsic a-SiGe: H film shows higher spectral response in the red region than other kind of a-Si: H aolar cells. The highest conversion efficiency of 8.5% has been obtained by the three junctions stacked cell composed of two inverted a-Si: H P-I-N cells and one inverted P-I-N cell with an intrinsic a-SiGe: H film.

Broadening of Spectral Response Width by a-Si/a-SiGe p-i-n type Solar Cells

We report the MIS type a-Si:H solar cells, plasma analysis using quadrupole mass analyzer and tandem type solar cell which improved spectral response in the long wavelength region. MIS type a-Si:H solar cells inserting TiOx layer as an insulator show the conversion efficiency of 4.4%. The improvement of the collection efficiency in the short wavelength region were accomplished by decrease in interface state density with addition of TiOx film. Plasma polymerized species, (SiHm+)2, increased with the rf power up to 40–50 W and decreased as a result of the decomposition to (SiHm+) species, but in the case of GeH4 reaction, (GeHm+)2 species decreased monotonously with rf power. Suitable condition of a-SiGe:H films deposition are related to the behavior of the polymerization and decomposition of SiH4 and GeH4 gases, and high quality films are obtained at relatively low rf power condition before (GeHm+)2 decomposition. Tandem type amorphous solar cell shows 3.2% coversion efficiency and the collection efficiency in the long wavelength region are clearly improved.