Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Gettering effects by aluminum upon the dark and illuminated I–V characteristics of N–P–P silicon solar cells

Impurity gettering is an essential process step in silicon solar cell technology. A widely used technique to enhance silicon solar cell performance is the deposition of an aluminum layer on the back surface of the cell, followed by a thermal annealing. The aluminum thermal treatment is typically done at temperatures around 600°C for short times (10–30 min). Seeking a new approach of aluminum annealing at the back of silicon solar cells, a systematic study about the effect the above process has on dark and illuminated I–V cell characteristics is reported in this paper. We report results on silicon solar cells where annealing of aluminum was done at two different temperatures (600°C and 800°C), and compare the results for cells with and without aluminum alloying. We have shown that annealing of the aluminum in forming gas at temperatures around 800°C causes improvement of the electrical cell characteristics. We have also made evident that for temperatures below 250 K, the predominant recombination process for our cells is trap-assisted carrier tunneling for both annealing temperatures, but it is less accentuated for cells with annealing of aluminum at 800°C. For temperatures above 250 K, the recombination proceeds through Shockley–Read–Hall trap levels, for cells annealed at both temperatures. Furthermore, it seems from DLTS measurements that there is gettering of iron impurities introduced during the fabrication processes. The transport of impurities from the bulk to the back surface (alloyed with aluminum) reduces the dark current and increases the effective diffusion length as determined from dark I–V characteristics and from spectral response measurements, respectively. All these effects cause a global efficiency improvement for cells where aluminum is annealed at 800°C as compared to conventional cells where the annealing was made at 600°C.     

串并联太阳能电池弧形光伏组件的发电性能研究

将弧形光伏组件安装在建筑和汽车上获取电能,已受到人们越来越多的关注。为获得更高的输出功率,有必要研究由互连太阳能电池组成的、电流不匹配的弧形光伏组件的特性。研究重点关注由串并联太阳能电池组成的弧形光伏组件的发电性能,设计了不同曲率的非平面微型光伏模块,并通过测量获取光伏模块的参数。与平面光伏模块相比,弧形光伏模块的发电量较小。此外,利用二极管模型分析了光伏模块的特性,说明并联比串联功率高的原因。最后研究了实际应用中太阳能电池的互连问题。结果表明,在理想模型下并联能获取更多电能,但大尺寸的光伏模块会产生更大电流,可能会在实际运行中产生额外损耗。因此,在实际应用中设计弧形光伏组件时也应考虑太阳能电池的互连。     

Influence of the layer thickness and hydrogen dilution on electrical properties of large area amorphous silicon p–i–n solar cell

The aim of this work is to present data concerning the optimization of performances of a large area amorphous silicon p–i–n solar cell (30×40 cm²) deposited by plasma enhanced chemical vapour deposition (PECVD) at 27.12 MHz. In this work the solar cell was split into small areas of 0.126 cm², aiming to study the device performance uniformity, where emphasis was put on the role of the n-layer thickness. The solar cells were studied through the spectral response behaviour in the 400–750 nm range as well as by the behaviour of the AC impedance. Solar cells with fill factor of 0.58, open circuit voltage of 0.83 V, short circuit current density of 17.14 mA/cm² and an efficiency of 8% were obtained at growth rates higher than 0.3 nm/s.     

Status of amorphous silicon solar cell technologies in Japan

The present situation of a-Si solar cell technologies in Japan is reviewed and the future prospects toward the industrialization of a-Si solar cell modules for power generation are presented. The conversion efficiency, the stability, and the yield have reached a satisfied level to use a-Si solar cells for electric power generation. The last issue is a production cost, which will be overcome by developing the market and extracting the merits of a-Si solar cells instead of following the production process and applications of conventional solar cell modules.     

High concentration photovoltaic systems applying III–V cells

High concentration systems make use of the direct solar beam and therefore are suitable for application in regions with high annual direct irradiation values. III–V PV cells with a nominal efficiency of up to 39% are readily available in today's market, with further efficiency improvements expected in the years ahead. The relatively high cost of III–V cells limits their terrestrial use to applications under high concentration, usually above 400 suns. In this way the relatively high cell cost is compensated through the low amount for cells needed per kW nominal system output.This paper presents a state of the art of high concentration photovoltaics using III–V cells. This PV field accounts already for more than 20 developed systems, which are commercially available or shortly before market introduction.     

Large area multicrystalline silicon solar cells in high volume production environment—history, status, new processes, technology transfer issues

Cast multicrystalline silicon (mc-Si) solar cell technology, accounted for nearly 41% of all the PV modules manufactured worldwide in 2000. Since 1995 the use of cast mc-Si as a substrate has increased every year and that increase is expected to accelerate in the coming years as the PV market grows further. This impressive growth has been enabled by several factors—wafer suppliers, improvements in casting technology, sawing technology and cell process technology. In this paper the enabling factors will be discussed. The new processes used to enhance the efficiency of the cast multicrystalline silicon solar cells and the criteria for technology transfer will also be discussed.     

Super high-efficiency multi-junction and concentrator solar cells

III–V compound multi-junction (MJ) (tandem) solar cells have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications.We have proposed AlInP–InGaP double hetero (DH) structure top cell, wide-band gap InGaP DH structure tunnel junction for sub cell interconnection, and lattice-matched InGaAs middle cell. In 2004, we have successfully fabricated world-record efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cells with an efficiency of 37.4% at 200-suns AM1.5 as a result of widening top cell band gap, current matching of sub cells, precise lattice matching of sub cell materials, proposal of InGaP–Ge heteroface bottom cell, and introduction of DH-structure tunnel junction. In addition, we have realized high-efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cell modules (with area of 7000 cm²) with an out-door efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd-generation solar cells in addition to 1st-generation crystalline Si solar cells and 2nd-generation thin-film solar cells. We are now challenging to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.     

Present status and future of crystalline silicon solar cells in Japan

Crystalline silicon solar cells show promise for further improvement of cell efficiency and cost reduction by developing process technologies for large-area, thin and high-efficiency cells and manufacturing technologies for cells and modules with high yield and high productivity.In this paper, Japanese activities on crystalline Si wafers and solar cells are presented. Based on our research results from crystalline Si materials and solar cells, key issues for further development of crystalline Si materials and solar cells will be discussed together with recent progress in the field. According to the Japanese PV2030 road map, by the year 2030 we will have to realize efficiencies of 22% for module and 25% for cell technologies into industrial mass production, to reduce the wafer thickness to 50–100 μm, and to reduce electricity cost from 50 Japanese Yen/kWh to 7 Yen/kWh in order to increase the market size by another 100–1000 times.     

Degradation of transparent metal–insulator–semiconductor solar cells due to heating effects

All the output parameters of the metal–insulator–semiconductor solar cells are degraded after heating. Also the dark current and the non-ideality factor are increased with heating. A reduction in the built-in potential has been detected. The capacitance–voltage–frequency measurements indicate the presence of interface states. These states are heavily occupied by electrons. Heating will increase the density of these states and consequently reduce the barrier height and the overall cell efficiency.     

A novel choice for the photovoltaic–thermoelectric hybrid system: the perovskite solar cell

Most of the recent studies about the photovoltaic cell‐thermoelectric generator (PV‐TEG) hybrid system pay their attention to silicon PV cells. This paper is to estimate the feasibility and features of the integrated system consisting of the emerging perovskite solar cells and thermoelectric modules. The results in this paper show that the temperature coefficient of the perovskite solar cell is lower than 2‰. Because of such a lower temperature coefficient, the efficiency of the perovskite solar cell‐TEG hybrid system can amount to 18.6%, while the efficiency of the single perovskite solar cell is 17.8%. Therefore, the perovskite solar cell is a reasonable choice for the PV‐TEG hybrid system. By altering the thermal concentration, the volume of the TEG material can be decreased, and the cost of the hybrid system can be remarkably reduced. To study the influence of the thermal concentration on the performance of the hybrid system, a three‐dimensional numerical model of the hybrid system is developed in this paper. When the thermal concentration ration is lower than 100, the temperature drop is lower than 3 K, and the decline in the conversion efficiency caused by the thermal concentration can be neglected for the proposed PV‐TEG hybrid system. Copyright © 2016 John Wiley & Sons, Ltd.     

Spectral response of a-Si:H p–i–n solar cells

The spectral response of a typical thin-film a-Si:H p–i–n solar cell has been investigated using the simulation RAUPV2. The peak in the external quantum efficiency has been observed to shift towards the violet part of the spectrum on decreasing the cell thickness. Moreover, the height of the peak increases as cell thickness is decreased. This is correlated with an enhancement in cell performance for thinner cells, due to a general increase in the drift field within the cell. The external quantum efficiency of a cell with an optimal concentration of phosphorous in the intrinsic layer has also been investigated. The external quantum efficiency for this cell is similar to that of the thinner cell, and is associated with the enhancement of the drift field near the p/i interface that is brought about by the phosphorous doping of the intrinsic layer. However, the integrated recombination for the thinner cell and the phosphorous-profiled cell differ significantly at long wavelengths, despite their similarity at shorter wavelengths. This effect is due to the weakening of the drift field near the n/i interface in the phosphorous-profiled cell.     

Novel materials for high-efficiency III–V multi-junction solar cells

As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm². In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells.     

A simple method for quantifying spectral impacts on multi-junction solar cells

A method to quantify spectral effects on the electric parameters of multi-junction solar cells is presented. The method is based on measuring the short circuit current of at least two monitor cells. Ideally these monitor cells have the same spectral responses as the subcells in the investigated multi-junction solar cell. In contrast to the subcells, the current of the individual monitor cells can be measured separately. This allows conclusions to be drawn about the spectral impact on the current mismatch of the multi-junction solar cell. A spectrometric evaluation method is then applied.The method has been tested experimentally with three concentrator modules using III-V triple-junction solar cells. These modules were measured outdoors for several months under variable solar spectral conditions. In parallel, the IV curves of the modules and the current of two component cells were measured. A spectral parameter Z was derived from the monitor cell current signals, which was correlated to the short circuit current and the fill factor of the modules. A linear correlation was found between Z and the normalized short circuit current of the concentrator modules. Translation equations were derived from the linear correlation. These enable the calculation of a module’s short circuit current under any spectral conditions. In particular, the short circuit currents of the modules were derived for direct normal irradiance of 850 W/m² and spectral conditions corresponding to the AM1.5d low AOD spectrum. This is an important step towards comparing the performance of modules which show strong spectral sensitivity. Future rating methods can benefit from the presented simple method for quantifying spectral impacts on multi-junction solar cells. Furthermore, the method can be of interest for tuning the spectrum of pulsed solar simulators.     

I–V curve simulation by multi-module simulator using I–V magnifier circuit

This paper presents an I–V curve simulation of PV array/modules using multi I–V magnifier circuits. The circuit magnifies an I–V output of a pn photo-sensor, which is regarded as a small solar cell, by analog technology. About 30 W I–V curve simulator circuits were made and their characteristics were evaluated. LED light irradiated into the photo-sensor works like irradiation of sun light on real PV modules. It has been confirmed that each voltage gain and current gain of the circuit is independently adjustable and the circuit magnifies an I–V output of the photo-sensor successfully. FF of the circuit can be modified by shunt register connected to the photo-sensor in parallel. The circuit showed enough response ability to apply the maximum power point tracker evaluation of PV inverters. Temperature dependence of module output can be simulated by temperature control of the photo-sensor. The result of output characteristics of series connection of the I–V magnifier circuits suggests that the simulator system composed of multi I–V magnifier circuit could simulate partial shading effect of PV array output.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

Reduction of optical losses in colored solar cells with multilayer antireflection coatings

Solar modules are becoming an everyday presence in several countries. So far, the installation of such modules has been performed without esthetic concerns, typical locations being rooftops or solar power plants. Building-integrated photovoltaic (BIPV) systems represent an interesting, alternative approach for increasing the available area for electricity production and potentially for further reducing the cost of solar electricity. In BIPV, the visual impression of a solar module becomes important, including its color. The color of a solar module is determined by the color of the cells in the module, which is given by the antireflection coating (ARC). The ARC is a thin film structure that significantly increases the amount of current produced by and, hence, the efficiency of a solar cell. The deposition of silicon nitride single layer ARCs with a dark blue color is the most common process in the industry today and plasma enhanced chemical vapor deposition (PECVD) is mostly used for this purpose. However, access to efficient, but differently colored solar cells are important for the further development of BIPV. In this paper, the impact of varying the color of an ARC upon the optical characteristics and efficiency of a solar cell is investigated. The overall transmittance and reflectance of a set of differently colored single layer ARCs are compared with multilayered silicon nitride ARCs, all made using PECVD. These are again compared with porous silicon ARCs fabricated using an electrochemical process allowing for the rapid and simple manufacture of ARC structures with many tens of layers. In addition to a comparison of the optical characteristics of such solar cells, the effect of using colored ARCs on solar cell efficiency is quantified using the solar cell modeling tool PC1D. This work shows that the use of multilayer ARC structures can allow solar cells with a range of different colors throughout the visual spectrum to retain very high efficiencies.     

Hybrid solar cells based on tetrapod nanocrystals: The effects of compositions and type II heterojunction on hybrid solar cell performance

We synthesized oleic acid capped tetrapod nanocrystals of CdSe, CdTe and type II heterostructured CdTe/CdSe to investigate the effects of nanocrystal compositions and type II heterojunction on the photovoltaic properties of hybrid solar cells. The hybrid solar cell based on the blend of CdSe tetrapod nanocrystals and P3HT with a weight ratio of 6:1 showed the maximum power conversion efficiency of 1.03% under AM 1.5 G condition, and the maximum incident photon to current conversion efficiency of the solar cell was 43% at 415 nm. Although CdTe and CdTe/CdSe tetrapod nanocrystals showed relatively poor performance, the power conversion efficiency and the short circuit current density of the hybrid solar cell based on type II heterostructured CdTe/CdSe tetrapod nanocrystals was 4.4 and 3.9 times higher than that of the solar cell based on CdTe tetrapod nanocrystals, respectively. These results can be explained by the effects of nanocrystal compositions and type II heterojunction on the photovoltaic properties of hybrid solar cells.     

Quantum-efficiency measurements on carbon–hydrogen-alloy-based solar cells

Solar cell devices have been fabricated with amorphous, hydrogenated carbon (a-C : H) as the primary semiconducting material. These devices clearly demonstrate photovoltaic behavior as determined by their I–V curves. To identify photon energies that contribute to the photogenerated current, quantum efficiency measurements have been performed. The fabricated solar cells exhibit a maximum quantum efficiency response in the ultraviolet region. Using the measured quantum efficiency curves, the short-circuit currents for a global AM1.5 spectrum at an irradiance of 1000 W/m² have been calculated. These values are comparable to the actually measured short-circuit currents.     

Silicon solar cells with antireflection diamond-like carbon and silicon carbide films

Optical properties of diamond-like carbon and silicon carbide (SiC) films in dependence on deposition conditions were investigated. It was established that the films having refractive index from 1.6 to 2.3 may be obtained. The film optical bandgap and hardness may be changed from 1.5 to 4 eV and from 1 to 20 GPa, correspondingly. The films were deposited onto the front side of silicon solar cells (SCs). It has been shown that deposition of single- or two-layer diamond-like carbon antireflection (AR) coatings enables the SCs efficiency to be improved 1.35–1.5 times. The improvement is connected with decreasing of reflection losses and passivation of recombination active centers. SiC AR coatings improve the solar cell efficiency up to 1.3 times.