Mechanically grooved, double sided, buried contact silicon solar cells

The groove formation is an important step in the production of buried contact solar cells. This often involves the alignment and scribing of individual wafers in turn, which results in a low production throughput and a possible increase in the cost of production. To increase the production throughput and reduce the cost of scribing, a batch scribing of wafers would be necessary. However, the batch grooving of cells is not easily obtained with the scribing techniques such as laser, photolithography and plasma etching, but can be achieved with the mechanical scriber. The mechanical scriber using rotating blades is restricted to cutting in only one direction while the use of a diamond point scriber in conjunction with a drafting plotter (as used in this work) is capable of scribing 63 (4 inch) wafers in a matrix on the plotter. The adapted plotter was used to form the grooves of the double sided buried contact (DSBC) silicon solar cells. The DSCB structure is based on the single sided buried contact (SSBC) cells [1] with the added advantages of potentially higher conversion efficiency and lower production cost. The use of mechanical scribing rather than laser scribing to form the grooves in the silicon substrate has no detrimental effect on the performance of the device and may have the potential to significantly reduce production costs.     

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.     

Laser patterning of P2 interconnect via in thin-film CIGS PV modules

The use of laser scribed Cu(In,Ga)Se₂ as a high-speed and low-stress alternative to the standard mechanical scribing has been investigated for thin-film PV modules on glass substrates. Laser scribe lines were smooth, narrow and straight compared to the chipped and jagged mechanically scribed lines. Modules fabricated using laser patterning have reached a 15% module efficiency and fill factors as high as 73%. Contact resistances were found to be in the range of for standard scribing and for the laser-formed contacts. In spite of the elevated contact resistance performance is comparable to conventionally processed modules and dry heat stability tests are encouraging.     

整形激光掺杂制备PERC电池选择性发射极研究

研究整形激光掺杂制备选择性发射极(SE)对p型单晶硅钝化发射极局域背接触(PERC)太阳电池的表面金字塔形貌、掺杂分布及电池性能的影响。整形激光具有能量分布均匀、对硅片损伤小等优点。通过改变激光的功率以及激光划线速度在p型单晶硅PERC太阳电池制备了不同掺杂分布的发射极。结合3D激光显微镜、扫描电子显微镜、EDS能谱分析、四探针方阻测试仪、电化学电容法等测试分析方法表征了样品的表面形貌、方阻变化、掺杂浓度曲线和电学性能。本文结合光斑重叠率将不同激光功率和激光划线速度采用公式统一转化为激光能量密度,从而得出制备选择性发射极的最佳激光能量密度。研究结果表明,当激光能量密度为0.97 J/cm2时,电池效率可以稳定提升0.25%以上,体现了整形激光SE技术应用于PERC电池的应用潜力。     

Microcrystalline silicon thin film solar modules on glass

We developed microcrystalline silicon (μc-Si:H) thin film solar modules on textured ZnO-coated glass. The single junction (p–i–n) cell structure was prepared by plasma-enhanced chemical vapour deposition (PECVD) at substrate temperatures below 250 °C. Front ZnO and back contacts were prepared by sputtering. A process for the monolithic series connection of μc-Si:H cells by laser scribing was developed. These microcrystalline p–i–n modules showed aperture area efficiencies up to 8.3% and 7.3% on aperture areas of 64 and 676 cm², respectively. The temperature coefficient of the efficiency was −0.4%/K.     

Preparation and performance of thin film CdTe mini-module

The film deposition process and integrated technology of the CdTe mini-module with high efficiency are key steps to manufacture large-area modules. In this paper, CdS, CdTe and ZnTe:Cu films with a substrate area of 7×10 cm² were deposited by chemical bath deposition, close-spaced sublimation and vacuum co-evaporation, respectively. The uniform films were prepared after their thicknesses, structures and electronic characteristics were studied as the function of deposition parameters. The films of SnO₂:F, CdTe, etc, were scribed by Kr-lamp-pumped Q-switch YAG:Nd laser. The pumped lamp current, Q-switch frequency and scribing rate were optimized. The scribing efficiency of the base frequency light was compared with that of doubled frequency light. The integrated structure design was optimized after simulating. Then the CdTe mini-module of 7.03% efficiency was gained with a total area of 54 cm² and nine integrated elementary cells.     

Reflection at transparent V-grooved surface

Reflection at transparent V-grooved surface was studied on the basis of geometrical analysis and ray tracing using Fresnel's formulas. For normally incident light, the reflectivity at the V-grooved surface of tilt angles between 0° and 30° and between 45° and 54° are approximated by the reflectivities and the squared reflectivities, respectively, at a smooth surface. For obliquely incident light, the V-grooved surface is effective to reduce the reflectivity at large angle of incidence by reducing the actual angle of striking the surface. In addition, the V-grooved surface of large tilt angle reduces the reflectivity at a small angle of incidence due to multiple reflections between adjacent opposite surfaces of the groove. The daily average reflectivity monotonically decreases with increasing tilt angle from 30° to its upper limit (59.1° for n=1.5) in the analysis. When normally incident light enters the layer and a part of it reflects at the rear surface, the escape from the upper surface can be prevented by serial total internal reflections at the upper surface of medium tilt angle (30.9°–41.0° for n=1.5).     

Recovery of minority carrier lifetime in low-cost multicrystalline silicon

Lifetime of minority carriers has been widely identified to be the key material parameter determining the conversion efficiency of pn-junction silicon solar cells. Impurities and defects in the silicon crystal lattice reduce the charge carrier lifetime and thus limit the performance of the solar cells. Removal of impurities by silicon material purification is often contradictory with low cost production of photovoltaic devices. In this paper, we present experimental results of an efficient gettering technique which can be applied to low cost processing of multicrystalline silicon solar cells without any additional process steps or compromises with optimal device design parameters. This technique is based on well-known phosphorous gettering. We have discovered that if the silicon wafers are kept in the furnace after the emitter diffusion at the 700°C, significant improvement in the lifetime will take place. At this temperature the properties of the pn-junction remain unaffected meanwhile many lifetime killers are still mobile. The time needed for this temperature program can be easily modified in order to respond to the material quality variations in substrates originating from different parts of multicrystalline ingot. Better control of lifetime can lead to higher degree of starting material utilization.     

Production of biohydrogen from hydrolyzed bagasse with thermally preheated sludge

Production of biohydrogen from dark fermentation is an interesting alternative to producing renewable fuels because of its low cost and various usable substrates. Cellulosic content in plentiful bagasse residue is an economically feasible feedstock for biohydrogen production. A statistical experimental design was applied to identify the optimal condition for biohydrogen production from enzymatically hydrolyzed bagasse with 60-min preheated seed sludge. The bagasse substrate was first heated at 100 °C for 2 h and was then hydrolyzed with cellulase. Culture of the pretreated bagasse at 55 °C provided a higher H₂ production performance than that obtained from cultures at 45 °C, 65 °C, 35 °C and 25 °C. On the other hand, the culture at pH 5 resulted in higher H₂ production than the cultures at pH 6, pH 4 and pH 7. The optimal culture condition for the hydrogen production rate was around 56.5 °C and pH 5.2, which was identified using response surface methodology. Moreover, the pretreatment of bagasse under alkaline conditions gave a thirteen-fold increase in H₂ production yield when compared with that from preheatment under neutral condition.     

Influence of laser scribing in the electrical properties of a-Si:H thin film photovoltaic modules

The third (P3) laser patterning step of thin film photovoltaic devices is studied experimentally using a diode pumped solid state laser with 532 nm wavelength and a delay generator. The effect on the electrical characteristics of the devices due to the patterning process is investigated by performing scribes on single, thin-film solar cells. As it is shown, in this type of experiments the inertia in the motion systems or in the scanner controlling the direction of the laser beam plays a critical role in the results. By controlling externally the output of the laser beam it is possible to overcome the inertia and investigate the real effect of the P3 laser scribing on the device electrical characteristics. When the laser scribing conditions are optimized and the inertia in the system is taken care of, the P3 process has very little effect on the device electrical characteristics. Translated to modules this means that by optimizing the P3 process, the decrease in the efficiency found when up-scaling from single cells to modules can be minimized (as far as the P3 process is concerned) to that coming from the removed area.     

The feasibility of using geothermal energy in hydrogen production

A feasibility study exploring the use of geothermal energy in hydrogen production is presented. It is possible to use a thermal energy to supply heat for high temperature electrolysis and thereby substitute a part of the relatively expensive electricity needed. A newly developed HOT ELLY high temperature steam electrolysis process operates at 800 – 1000°C. Geothermal fluid is used to heat fresh water up to 200°C steam. The steam is further heated to 900°C by utilising heat produced within the electrolyser. The electrical power of this process is reduced from 4.6 kWh per normalised cubic meter of hydrogen (kWh/Nm³ H₂) for conventional process to 3.2 kWh/Nm³ H₂ for the HOT ELLY process implying electrical energy reduction of 29.5%. The geothermal energy needed in the process is 0.5 kWh/Nm³ H₂. Price of geothermal energy is approximately 8–10% of electrical energy and therefore a substantial reduction of production cost of hydrogen can be achieved this way. It will be shown that using HOT ELLY process with geothermal steam at 200°C reduces the production cost by approximately 19%.     

Thin film poly-Si formation for solar cells by Flux method and Cat-CVD method

Two methods were examined for the formation of poly-Si films. One is flux method and the other is Cat-CVD method. Flux method was used for forming poly-Si seed films on glass substrates covered with rear electrode. Poly-Si films of a few μm grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 600°C. To make the seed films function as BSF layer for solar cell, boron doping was applied and carrier concentration of 2×10¹⁹/cm³ was obtained which is suitable for highly efficient solar cells. Cat (catalytic)-CVD method was examined for forming poly-Si photo-active layers on the seed films. The films showed deposition gas pressure-dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented films by Cat-CVD method when deposited on (1 1 1) oriented seed films prepared by Flux method. The electrical properties of the film are insufficient at present, may be due to high defect density and the film structure which allows impurity contaminations of oxygen and carbon after film deposition. Although the film quality needs to be improved, poly-Si films whose crystal fraction is more than 85% were obtained at deposition rate of up to around 40 Å/s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin film silicon solar cells.     

Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

An empirical diagnostic method for the evaluation of solar cell grade CuInS₂ absorbers has been developed. The method involves the measurement of the contact angle between water and the CuInS₂ absorber before fabrication of a solar cell. The contact angle is expected to depend upon local inhomogeneity, chemical composition and surface morphology of the CuInS₂ absorber. The variation of these factors on the surface is supported with scanning electron micrographs, chemical analyses, laser scanning photocurrent mapping of various CuInS₂ absorbers and measurements of the solar cell performance. The contact angle has been found to be different at different places on the CuInS₂ surface. Empirically, it was found that for high conversion efficiency solar cells (>8–10.5%), the contact angle on CuInS₂ absorbers ranges between 53° and 63°. For low conversion efficiency solar cells (<6%), it is between 48° and 50°. Therefore, it is seen that contact angle measurements on CuInS₂ absorbers can be used to assess the quality of CuInS₂ absorbers prior to solar cell fabrication.     

Capital cost and economic viability of thermosyphonic solar water heaters manufactured from alternate materials in India

Many companies in India manufacture solar water heaters but these are not becoming popular in the domestic sector because of their high cost. The Ministry of Non-Conventional Energy Sources (MNES), New Delhi is recommending flat-plate collectors with copper (Cu) risers, headers and plate. Therefore, their cost is high. Long term studies have been carried out at the Central Arid Zone Research Institute, Jodhpur, to reduce the cost by replacing copper tubes with galvanised steel (G.S.) tube and copper plate with aluminium (Al) plate. The aluminium plate is wrapped over the G.S. tube by a special wire wound technique so that good contact of plate with risers and headers has been maintained. In this paper performance and testing of solar water heaters having G.S.–Al fin, Cu–Al fin and Cu–Cu fin in flat-plate collectors have been compared. It has been found that performance of all the three heaters is almost similar. The heater can provide 100 litres of hot water at an average temperature 62.0°C at 4 pm that can be retained to 50.4°C when average tap water temperature was 23.9°C. The efficiency of the heater is 51.9%. The cost of the heater with G.S.–Al collector is only Rs. 8,000.00 while it is Rs. 10,250.00 for solar water heaters with Cu–Cu collectors. The payback period of a solar water heater with G.S.–Al collector has been worked out by considering 10% compound annual interest, 5% maintenance cost, 5%, inflation in fuel prices and maintenance cost. The payback period varies between 2.92 years to 4.53 years depending upon which fuel it replaces. The payback periods are in increasing order with respect to fuels: electricity, firewood, LPG, charcoal, and kerosene.     

Predicting calcite deposition in krafla boreholes

The only part of the Krafla geothermal system where calcite deposits have been observed in boreholes is the Leirbotnar upper production zone (including well KJ-9 that taps both zones), which is liquid dominated at temperatures of 190–210°C. Flow from this part is needed to sustain the low pressure step of the generating turbine. Comparison of wellhead and deep water calcium concentrations has been found the most useful method of predicting the extent of deposition. The deposits generally appear to form above the calculated depth of flashing, and geometric changes in the well may affect the location of their formation. Flow monitoring is the best way of predicting time of blocking.     

Industrial use of the high-temperature geothermal field at Theistareykir, Iceland

The Theistareykir high-temperature geothermal field is located in the region Thingeyjarsýsla about 32 km from the coast and its principal town, Húsavík in northern Iceland. The paper shows that geothermal steam from the Theistareykir field can be piped to an industrial area close to Húsavík having a temperature of 170°C – 180°C at a cost of approximately US$ 4 per tonne while a steam cost of US$ 10 a tonne is quite common in steam consuming industries.The production of alumina is used as an illustration of the possible industrial use of geothermal steam. It is probable that the requirements for aluminium smelter capacity in Iceland may exceed 500,000 tonnes per annum within a decade. One million tonnes of alumina would be needed to achieve this annual aluminium production rate.     

Low cost double-sided buried-contact silicon solar cells

The cost of a solar cell, using the buried-contact technology, can be cut down if the many high temperature processing sequences are reduced to only one. The use of liquid or solid sources at elevated temperatures has been the practice for both emitter and groove diffusions. The use of spin-on dopants would enhance the realization of one high temperature processing. The characterization of groove diffusion with spin-on dopants as carried out in this work has shown some promising results. Also, the use of a laser beam for grooving, as well as driving the dopants into the grooves has resulted in large area cells with energy conversion efficiency in excess of 13% on CZ, p-type, 0.7 Ω cm.     

Development of low cost production technologies for polycrystalline silicon solar cells

To develop a technology of forming grooves for low cost cell production, a multi-blade wheel grinding method was investigated. The process time of groove formation on the surface of 10 × 10 cm² polycrystalline silicon substrate was reduced to 30 s by a newly developed high-speed groove formation machine. Simultaneous formation of junction and anti-reflection coating by atmospheric pressure chemical vapor deposition (APCVD) technique was also investigated. For electrodes formation process, single firing method for both side electrodes made possible to simplify the firing process and to speed up from a conventional speed of 400 mm/min to 5000 mm/min.     

Multi-pressure absorption cycles in solar refrigeration:: A technical and economical study

A technical and economical study of regenerative absorption chillers with multi-pressure cycle has been undertaken as solar operated refrigeration systems. Referred to as advanced absorption chillers they represent one of the new technology options that are under development. Advanced absorption cooling technology offers the possibility of chillers with thermal COPs of 1.5 or greater at driving temperatures of 140°C, which reduces the collector area and the heat rejection requirements compared to current absorption cooling technology. Two different absorption systems have been considered. The first is an advanced, double-effect regenerative absorption cooling system, driven at 140°C, whose efficiency is about 55% of the Carnot efficiency. The second is an ideal, single-effect regenerative absorption system that achieves 70% of the Carnot efficiency driven at 140°C or 200°C. To evaluate the solar performance of a thermally driven chiller requires a separate analysis of the solar availability for a given location compared to the required monthly average solar input. In this analysis different systems, including the vapour compression chillers, have been compared in terms of the thermal and electrical energy input. An effective electrical COP may be computed assuming that the ratio of electrical energy cost to thermal energy cost is four, which is typical of today’s fossil fuel costs. The effective electrical COPs of different technical options can then be compared. Those systems with higher electrical COPs will have lower energy costs. If solar is to be competitive, then the cost of delivered solar thermal energy should be less than the cost of delivered fossil thermal energy.     

Comparative study of conventional vs. one-step-interconnected (OSI) monolithic CdTe modules

We report on the fabrication of 5 × 7.5 cm² CdTe photovoltaic module devices using two alternative routes. One which uses the conventional approach where laser scribing and active layer deposition steps are inter-mixed, and the other via the one-step-interconnection (OSI) process. OSI combines three laser processes with two inkjet processes, depositing insulating and conductive materials. This allows the series interconnection to occur after the deposition of all active layers reducing fabrication time, capital equipment cost and interconnect dead zone. Its suitability for the manufacture of CdTe mini-modules has previously been demonstrated but no direct comparison was made against the conventional process. The structural properties and performance of conventional vs. OSI processed CdTe modules are presented and show comparable performance using both approaches with the OSI showing considerable process simplification.