Optical confinement in high-efficiency a-Si solar cells with textured surfaces

The experimental spectral response and reflectance of high-efficiency a-Si solar cells are systematically investigated by using an optical simulation based on realistic optical properties of the transparent conducting oxide (TCO), a-Si, and metal electrode, in order to improve the spectral response. It is shown that a practically important optical loss results from absorption by the TCO, which is enhanced by the optical confinement effect. This suggests that improvement in the spectral response is possible by suppressing the optical confinement in the TCO.     

Low-temperature a-Si:H/ZnO/Al back contacts for high-efficiency silicon solar cells

The paper analyses the electronic transport of high-efficiency silicon solar cells with high-quality back contacts that use a sequence of amorphous (a-Si) and microcrystalline (μc-Si) silicon layers prepared at a maximum temperature of 220 °C. Our best solar cells having diffused emitters with random texture and full-area a-Si/μc-Si contacts have an independently confirmed efficiency of 21.0%. An alternative concept uses a simplified a-Si layer sequence combined with Al-point contacts and yields a confirmed efficiency of 19.3%. Analysis of the internal quantum efficiency (IQE) shows that both types of back contacts lead to effective diffusion lengths L_eff exceeding the wafer thickness considerably. Fill factor limitations for the full area contacts result from non-ideal diode behavior, possibly due to the injection dependence of the interface recombination velocity.     

Fabrication technology for large-area a-Si solar cells

The fabrication process technology for large-area a-Si photovoltaic (PV) modules and their performance are reviewed. Our present technology enables us to provide 10% efficient large-area submodules with a stabilized efficiency of 8.5%. To study the practicability of the a-Si solar panels, we carried out an outdoor test for our a-Si modules. The results show that the a-Si solar PV modules generate power very efficiently in outdoor systems. The advantage of the a-Si modules under outdoor uses is presented and discussed.     

Cost effective process for high-efficiency solar cells

A new method for patterning the rear passivation layers of high-efficiency solar cells with a mechanical scriber has been developed and successfully adapted to fabricate high-efficiency passivated emitter and rear cell (PERC). Three types of the rear contact patterns: dot patterns with a photolithography process, line and dashed line patterns with a mechanical scriber process have been processed in order to optimize the rear contact structure. An efficiency of 19.42% has been achieved on the mechanical-scribed (MS)-PERC solar cell on 0.5 Ω cm p-type FZ-Si wafer and is comparable to that of conventional PERC solar cells fabricated by using photolithography process. The mechanical scriber process shows great potential for commercial applications by achieving high efficiency above 20% and by significantly reducing the fabrication costs without an expensive photolithography process. Low-cost Ni/Cu metal contact has been formed by using a low-cost electroless and electroplating. Nickel silicide formation at the interface enhances stability and reduces the contact resistance resulting in an energy conversion efficiency of 20.2% on 0.5 Ω cm FZ wafer.     

New concepts for high-efficiency silicon solar cells

This paper gives an overview about recent activities in the industrial application of high-efficiency monocrystalline silicon solar cells. It also presents the latest results achieved at Fraunhofer ISE, especially a new patented process for the formation of back-contact points on dielectrically passivated cells called laser-fired contacts and its application to thin wafers.     

非晶硅太阳电池的原理

非晶硅太阳电池是２０世纪７０年代中期发展起来的一种新型薄膜太阳电池，与其他太阳电池相比，非晶硅电池具有以下突出特点：（１）制作工艺简单，在制备非晶硅薄膜的同时就能制作pin结构。（２）可连续、大面积、自动化批量生产。（３）非晶硅太阳电池的衬底材料可以是玻璃、不锈钢等，因而成本小。（４）可以设计成各种形式，利用集成型结构，可获得更高的输出电压和光电转换效率。（５）薄膜材料是用硅烷（SiH４）等的辉光放电分解得到的，原材料价格低。１非晶硅太阳电池的结构、原理及制备方法非晶硅太阳电池是以玻璃、不锈钢及特种…     

Heteroepitaxial technologies on Si for high-efficiency solar cells

The improvements of the AlGaAs crystal quality grown on Si substrate and the AlGaAs/Si tandem solar cell have been studied with varying the growth conditions. The crystal quality of the AlGaAs layer was evaluated by photoluminescence, deep level transient spectroscopy, time-resolved photoluminescence and double crystal X-ray diffraction while varying the thermal cycle annealing temperature. The optimum thermal cycle annealing temperature and the buffer layer thickness for the growth of high efficiency AlGaAs/Si tandem solar cells have been presented. The active-area conversion efficiency of 21.2% and 21.4% (AMO and 1 sun at 27°C) has been demonstrated with two-terminal and four-terminal configuration, respectively, by the perfect photocurrent matching between the top cell and the bottom cell.     

Rapid thermal technologies for high-efficiency silicon solar cells

This paper shows that rapidly formed emitters in less than 6 min in the hot zone of a conveyor belt furnace or in 3 min in an rapid thermal processing (RTP) system, in conjunction with a screen-printed (SP) RTP Al-BSF and passivating oxide formed simultaneously in 2 min can produce very simple high-efficiency n⁺-p-p⁺ cells with no surface texturing, point contacts, or selective emitter. It is shown for the first time that an 80 Ω/□ emitter and SP Al-back surface field (BSF) formed in a high throughput belt furnace produced 19% FZ cells and greater than 17% CZ cells with photolithography (PL) contacts. Using PL contacts, we also achieved 19% efficient cells on FZ, >18% on MCZ, and 17% boron-doped CZ by emitter and SP Al-BSF formation in <10 min in a single wafer RTP system. Finally, manufacturable cells with 45 Ω/□ emitter and SP Al-BSF and Ag contacts formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. Compared to the PL cells, the SP cell gave 2% lower efficiency along with a decrease in J_sc and fill factor. This loss in performance is attributed to a combination of the poor blue response, higher series resistance and higher contact shading in the SP devices     

Photoluminescence analysis of InGaP top cells for high-efficiency multi-junction solar cells

A way of evaluate the minority-carrier lifetime by using photoluminescence (PL) measurement is proposed which includes self-absorption. The room-temperature PL intensity is analyzed theoretically for bulk crystals and a device with n⁺-p junction configuration, based on a one-dimensional model. Photoluminescence analysis of In_0.5Ga_0.5P solar cells grown on GaAs and Si substrates by MOCVD (metal organic vapor deposition) have been carried out and compared with the properties of the In_0.5Ga_0.5P solar cells. By improving minority-carrier lifetime, high-efficiency In_0.5Ga_0.5P cells on GaAs substrates with an efficiency of 18.5% have been made.     

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.     

Development status of high-efficiency HIT solar cells

This paper describes the development status of high-efficiency heterojunction with intrinsic thin-layer (HIT) solar cells at SANYO Electric. Presently, the conversion efficiency of our standard HIT solar cell has reached a level of 23.0% for a practical size of (100.4 cm²) substrate. On the other hand, we have developed special technologies for effectively using thinner substrates for HIT solar cells. Surprisingly, we have achieved a quite high open circuit voltage (V_oc) of 743 mV, and a high conversion efficiency of 22.8% using only a 98-μm-thick substrate. A 98-μm-thick cell also exhibits a good temperature coefficient, and allows the thickness of the substrate to be reduced by more than 50% while maintaining its efficiency. These results suggest that the HIT solar cell has the potential to further improve cost-performance.     

Advanced fabrication technologies of integrated-type structure for a-Si solar cells

This paper proposes a new advanced fabrication technology for a low-cost integrated-type a-Si solar cell. Integrated-type cells provide many advantages and have been industrialized with a laser patterning method. However, a higher throughput and more efficient patterning method was required for applying a-Si solar cells to a power generating system. Plasma CVM (Chemical Vaporization Machining) was first applied to advanced patterning because of its advantages of high speed and selectivity. In this method, a plasma generated under high pressure localizes near the wire electrode and concentrates reactive radicals. As a result, we achieved an etching rate of more than 1 μm/s and selective patterning of a 200 μm-wide a-Si layer in 1 s multiline patterning was also developed for large-area modules.     

a-Si alloy three-stacked solar cells with high stabilized-efficiency

a-Si alloy three-stacked solar cells have been studied to improve the stabilized efficiency of a-Si: H based solar cells. Based on the analysis by the individual characterization method of the component cells in stacked type cells, the a-Si :H middle cell was replaced with an a-SiGe :H cell. Furthermore, the optical confinement technology was improved to obtain a high-output current with thin i-layer thickness in the a-SiGe :H bottom cell. By this device design, the initial conversion efficiency was improved up to 12.4% and more than a 10% stabilized efficiency was obtained in a-SiC :H/a-SiGe :H/a-SiGe :H three-stacked cells. These cell characteristics were confirmed by measurements at the JQA Organization (the former JMI Institute).     

Development and application of see-through a-Si solar cells

A new type of translucent amorphous silicon (a-Si) solar cell, called the see-through a-Si solar cell, is developed. It has multiple microscopic holes within its effective area to transmit light and it generates electric power. A series of technical data on the fabrication processing with various patterning and photovoltaic performance are presented. Some examples of application systems such as car sunroof and home interior are introduced and discussed on a wide variety of new areas of PV applications. The see-through a-Si solar cell was mounted on a car sunroof to drive the car's ventilating system or to charge its battery. The ventilating system reduced the interior temperature of the car from 61 to 47°C during daytime parking.     

Phase engineering of a-Si:H solar cells for optimized performance

Until recently, the advances in hydrogenated amorphous silicon (a-Si:H) solar cell performance and stability have been achieved materials prepared with hydrogen dilution following primarily empirical approaches. This paper discusses the recently obtained insights into the growth, microstructure and nature of these materials. Such protocrystalline Si:H materials are more ordered than the a-Si:H obtained without dilution and evolve with thickness from an amorphous phase into first a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase. The development of deposition phase diagrams, characterize their microstructural evolution during growth which can be used to guide the fabrication of solar cell structures in a controlled way. Examples are presented and discussed of their application in solar cell fabrication to obtain a fundamental understanding of the properties of the phase transitions as well as the systematic optimization of cell performance.     

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.     

Optimization of front surface texturing processes for high-efficiency silicon solar cells

This paper presents the results of an experimental study regarding the increase in the efficiency of the silicon solar cells by texturing the front surface. Designing, patterning and surface etching processes led to refined structures with very low losses of the incident optical radiation. Photolithography has been used to generate patterns (disc hole) through the silicon dioxide layer grown at the beginning on silicon wafers. The holes (4 μm in diameter) have been uniformly distributed on the entire surface (2×2) cm² and the distance between the hole centres was determined to be 20 μm. Semispherical walls have been defined in holes by isotropic etching up to join together of the wells.     

Strained and strain-balanced quantum well devices for high-efficiency tandem solar cells

The state of GaAs/InGaAs quantum well solar cell research is reviewed. The effect of strain upon the GaAs/InGaAs cells is discussed and the limits to a strained GaAs/InGaAs cell established. The strain-balance approach is suggested as a means of overcoming the limits inherent to the strained approach and the principle is demonstrated in two differing device configurations. The strain-balance devices show enhanced efficiencies over their strained counterparts and in one case, comparable efficiency to a good GaAs control cell. The application of these cells to tandem structures is discussed, indicating the potential for a substantial efficiency enhancement.     

Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells

Texturing of silicon (Si) wafer surface is a key to enhance light absorption and improve the solar cell performance. While alkaline texturing of single-crystalline Si (sc-Si) wafers was well established, no chemical solution has been successfully developed for multi-crystalline Si (mc-Si) wafers. Reactive-ion-etch (RIE) is a promising technique for effective texturing of both sc-Si and mc-Si wafers, regardless of crystallographic characteristics, and more suitable for thin wafers. However, due to the use of plasma source generated by high power, the wafer surface gets a physical damage during the processing, which requires an additional subsequent damage-removal wet processing. In this work, we developed a damage-free RIE texturing for mc-Si solar cells. An improved self-masking RIE texturing process, developed in this study, produced ∼0.7% absolute efficiency gain on 156×156 mm² mc-Si cells, where the gas ratio and the plasma power density were keys to mitigate the plasma-induced-damage during the RIE processing while maintaining decent surface reflectance. In the self-masking RIE texturing, a mixture of SF₆/Cl₂/O₂ gases was found to significantly affect the surface morphology uniformity and reflectance, where an optimal etch depth was found to be 200-400 nm. We achieved J_sc gain of ∼1.3 mA/cm² while maintaining decent FFs of ∼0.78 without a V_oc loss after optimization of firing conditions.     

Modeling of a-Si/poly-Si and a-Si/poly-Si/poly-Si stacked solar cells

A computer model for the poly-Si thin film-related solar cells is established, with which the solar cells with the structure of single junction poly-Si cell, a-Si/poly-Si tandem cell and a-Si/poly-Si/poly-Si triple cell are simulated. The results indicate that the practical structure for poly-Si-related solar cell is a-Si/poly-Si/poly-Si triple cell with the best matched thickness of 0.23/0.95/3 μm and with optical confinement structure, which has the highest simulated efficiency of 22.74%.