Development of back‐junction back‐contact silicon solar cells based on industrial processes

We have presented simplified industrial processes to fabricate high performance back‐junction back‐contact (BJBC) silicon solar cells. Good optical surface structures (solar averaged reflectance 2.5%) and high implied open‐circuit voltage (0.695 V) have been realized in the BJBC cell precursors through wet chemical processing, co‐diffusion, P ion implantation and annealing oxidation, as well as laser patterning and plasma enhanced chemical vapour deposition passivation processes. We have achieved a certified high efficiency of close to 22% on BJBC silicon solar cells with the size of 4.04 cm² by using screen printing and co‐firing technologies. The manufacturing process flow further successfully yields efficiency of around 21% BJBC silicon solar cells with enlarged sizes of 6 × 6 cm². The present work has demonstrated that the commercialization of low‐cost and high‐efficiency BJBC solar cells is possible because we have used processes compatible with existing production lines. Copyright © 2017 John Wiley & Sons, Ltd.     

Progression of metalorganic chemical vapour‐deposited CdTe thin‐film PV devices towards modules

This paper reports important developments achieved with CdTe thin‐film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high‐transparency window layer (Cd_0.3Zn_0.7S) and a device‐activation anneal. These increased the fill factor and open‐circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd_0.3Zn_0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm² in order to assess its impact on the device series resistance and fill factor. Finally, micro‐module devices utilising series‐connected 2‐cm² sub‐cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module‐to‐cell efficiency ratio of 0.9 was demonstrated for a six‐cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of concentrator modules with dome‐shaped Fresnel lenses and triple‐junction concentrator cells

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm² 400 × module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm² area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm² module with 20 solar cells connected in series was 28·9% in house. The temperature‐corrected efficiency of the 550 × module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side‐by‐side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III‐V solar cell system are discussed and a 20‐year lifetime under concentrated flux exposure proven. The fail‐safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis of industrial c‐Si solar cell's front metallization by advanced numerical simulation

The influence of the front metallization on the performance of industrially fabricated c‐Si solar cells is quantified by means of a combination of optical, semiconductor, and circuit modeling. Special attention is given to the front metallization of conventionally processed emitters. First, we verify our model by reproducing the measured I–V curves of industrially fabricated cells. Based on this, the potential changes in performance are predicted by variations in finger spacing, finger resistivity, busbar numbers, and contact resistivity. We also assess the benefits derived from the higher aspect ratios of the fingers: if the fingers are 120 (or 60 µm) wide, increasing the finger height from 15 to 25 µm increases their efficiency by 0.06% (or 0.7%) absolute if a conventional emitter is used. This implies that, if the fingers cannot be made as narrow as 60 µm in industrial mass production, the economic gains from higher aspect ratios may be rather limited, because there is a trade‐off between improved cell efficiency and the increasing cost of silver. Simulations show that, for the light‐induced plating approach, the seed layer must be made as narrow as 50 µm in order to achieve an efficiency gain of 1% absolute with a plating of 10 µm silver. Copyright © 2011 John Wiley & Sons, Ltd.     

Metal fingers on grain boundaries in multicrystalline silicon solar cells

We have developed a method of applying a net‐like finger grid to the front of multicrystalline (mc) silicon solar cells, which lies mainly on the grain boundaries (grain‐boundary‐oriented‐finger GBOF grid). This net has no busbars. It is drawn by a plotter, using screen printing paste dispensed through a fine tube. The power output of cells contacted in this manner has been tested in a statistical study of pairs and triplets of cells of size 100 × 100 mm² (Bayer) and 103 × 103 mm² (Eurosolare). In the pairs study, pairs of neighbouring wafers of the original ingot were processed into solar cells. One wafer received a GBOF grid, the other got the same grid rotated by 90°, and so had little coverage of grain boundaries. In the study of triplets the third wafer of each triplet was equipped with a standard H‐pattern of the same shading as the GBOF grid. In the pairs study, we find that under approximately standard conditions there is an 89% chance that the GBOF grid increases power output over cells with an identical, but 90° rotated, grid, the most probable increase being 2.6%. The triplets study shows that there is an 87% chance that the GBOF grid increases power output over cells with the standard H‐pattern, the most probable increase being 2.5%. Copyright © 2002 John Wiley & Sons, Ltd.     

Optimizing annealing steps for crystalline silicon solar cells with screen printed front side metallization and an oxide‐passivated rear surface with local contacts

Silicon solar cells that feature screen printed front contacts and a passivated rear surface with local contacts allow higher efficiencies compared to present industrial solar cells that exhibit a full area rear side metallization. If thermal oxidation is used for the rear surface passivation, the final annealing step in the processing sequence is crucial. On the one hand, this post‐metallization annealing (PMA) step is required for decreasing the surface recombination velocity (SRV) at the aluminum‐coated oxide‐passivated rear surface. On the other hand, PMA can negatively affect the screen printed front side metallization leading to a lower fill factor. This work separately analyzes the impact of PMA on both, the screen printed front metallization and the oxide‐passivated rear surface. Measuring dark and illuminated IV‐curves of standard industrial aluminum back surface field (Al‐BSF) silicon solar cells reveals the impact of PMA on the front metallization, while measuring the effective minority carrier lifetime of symmetric lifetime samples provides information about the rear side SRV. One‐dimensional simulations are used for predicting the cell performance according to the contributions from both, the front metallization and the rear oxide‐passivation for different PMA temperatures and durations. The simulation also includes recombination at the local rear contacts. An optimized PMA process is presented according to the simulations and is experimentally verified. The optimized process is applied to silicon solar cells with a screen printed front side metallization and an oxide‐passivated rear surface. Efficiencies up to 18.1% are achieved on 148.8 cm² Czochralski (Cz) silicon wafers. Copyright © 2009 John Wiley & Sons, Ltd.     

Lead‐free on‐laminate laser soldering: a new module assembling concept

We demonstrate lead‐free laser soldering of standard industrial solar cells. The laser‐soldered contacts stay stable for more than 240 accelerated ageing cycles by humidity–freeze test and withstand peel forces in excess of 10 N/cm. Laser soldering is demonstrated while the cells are lying on the ethylene vinyl acetate (EVA) foil. This permits to connect the solar cells to the lead‐free tinned copper ribbons directly on the lamination materials. We also demonstrate soldering on the bottom side by lasering through the glass and the non‐polymerized EVA. With the aid of a pick and place robot it thus becomes possible to avoid all string handling. On‐laminate laser soldering (OLLS) technique, which permits a high level of automation and process control, induces little thermal and mechanical stress, reduces the handling and should thus be of particular advantage for assembling modules with very thin cells at a high yield. Copyright © 2007 John Wiley & Sons, Ltd.     

Laser structuring for back junction silicon solar cells

We demonstrate mask‐free fabrication of a 22·0%‐efficient crystalline Si solar cell by applying laser ablation of Si and by laser ablation of protective coatings. The bulk absorber material is a p ‐type float zone silicon wafer and the designated cell area is 4 cm². While the processing time of our laboratory‐type of laser system is far too slow for industrial processing, we estimate on the basis of our experiments that laser processing of 12·5 × 12·5 cm²‐sized solar cells in just a few seconds is feasible with commercially available equipment. Copyright © 2006 John Wiley & Sons, Ltd.     

Bifacial concentrator Ag‐free crystalline n‐type Si solar cell

We report results obtained using an innovative approach for the fabrication of bifacial low‐concentrator thin Ag‐free n‐type Cz‐Si (Czochralski silicon) solar cells based on an indium tin oxide/(p⁺nn⁺)Cz‐Si/indium fluorine oxide structure. The (p⁺nn⁺)Cz‐Si structure was produced by boron and phosphorus diffusion from B‐ and P‐containing glasses deposited on the opposite sides of n‐type Cz‐Si wafers, followed by an etch‐back step. Transparent conducting oxide (TCO) films, acting as antireflection electrodes, were deposited by ultrasonic spray pyrolysis on both sides. A copper wire contact pattern was attached by low‐temperature (160°C) lamination simultaneously to the front and rear transparent conducting oxide layers as well as to the interconnecting ribbons located outside the structure. The shadowing from the contacts was ~4%. The resulting solar cells, 25 × 25 mm² in dimensions, showed front/rear efficiencies of 17.6–17.9%/16.7–17.0%, respectively, at one to three suns (bifaciality of ~95%). Even at one‐sun front illumination and 20–50% one‐sun rear illumination, such a cell will generate energy approaching that produced by a monofacial solar cell of 21–26% efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

Achievement of 17.9% efficiency in 30 × 30 cm2 Cu(In,Ga)(Se,S)2 solar cell sub‐module by sulfurization after selenization with Cd‐free buffer

We have achieved 17.9% efficiency in a 30 × 30 cm² Cu(In,Ga)(Se,S)₂ solar cell sub‐module prepared by selenization and sulfurization processes with a Cd‐free buffer. The development of an absorber layer, transparent conducting oxide window layer, and module design was the key focus. This permitted 1.8% higher efficiency than our last experimental result. The quantity and the injection time of the sodium were controlled, resulting in higher open circuit voltage (V_oc) and short circuit current (J_sc). In order to increase J_sc, we changed the thickness of the window layer. Boron‐doped zinc oxide was optimized for higher transmittance without reducing the fill factor. The uniformity of each layer was improved, and patterns were optimized for each module. Therefore, V_oc, J_sc, and FF could be theoretically improved on the reported results of, respectively, 20 mV, 2 mA/cm², and 1.4%. The module's efficiency was measured at the Korea Test Laboratory to compare with the data obtained in‐house. Various analyses were performed, including secondary ion mass spectroscopy, photoluminescence, quantum efficiency, solar simulator, and UV–vis spectrometry, to measure the cell's depth profile, carrier lifetime, external quantum efficiency, module efficiency, and transmittance, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

New interdigital design for large area dye solar modules using a lead‐free glass frit sealing

A new interdigital design for large area dye solar modules is developed for an area of 30×30 cm². This design requires fewer holes in the glass substrate for electrolyte filling, than the conventional strip design. A complete manufacturing process of this module—ranging from screen printed layers to semi‐automated colouring and electrolyte filling—in a laboratory‐scale baseline is illustrated. As primary sealing method, a durable glass frit sealing is used. It is shown, that the lead (Pb) content present in many glass frit powders contaminates the catalytic platinum electrode during the sintering process, resulting in a lowering of the fill factor. A screen printable lead‐free glass frit paste is developed, which solves this problem. Long term stability tests are presented on 2·5 cm² dye solar cells, which have been completely sealed with glass frit. In consecutively performed accelerated ageing tests under 85°C in the dark (about 1400 h) and continuous illumination with visible light (1 sun, about 1700 h), a 2·5 cm² dye solar cell with an electrolyte based on propylmethylimidazolium iodide showed an overall degradation of less than 5% in conversion efficiency. In a subsequently performed thermal cycling test (−40°C to +85°C, 50 cycles) a 2·5 cm² dye solar cell with the same electrolyte composition also showed only a slight degradation of less than 5% in conversion efficiency. Copyright © 2006 John Wiley & Sons, Ltd.     

Reflector materials for two‐dimensional low‐concentrating photovoltaic systems: the effect of specular versus diffuse reflectance on the module efficiency

Photovoltaic modules in two‐dimensional low‐concentrating systems with specular parabolic reflectors often experience high local irradiance that causes high local currents and cell temperatures. This generally results in power losses. The use of low‐angle scattering reflectors gives a smoother irradiance distribution, which results in a higher fill factor. In order to study how the choice of reflector material influences system performance, two different reflector materials (anodised aluminium and lacquered rolled aluminium laminated on a plastic substrate) were compared. The total and diffuse reflectance spectra of the reflector materials were measured, the integrated hemispherical and specular solar reflectance values calculated, and the angular distributions of scattered light investigated. Two geometrically identical 3× concentrating photovoltaic systems with semi‐parabolic over edge reflectors of the different materials were tested outdoors. While the anodised aluminium reflector, which had higher hemispherical and specular solar reflectance, resulted in a higher short‐circuit current, the low‐angle scattering lacquered foil gave a higher fill factor, due to a smoother image of the sun on the module surface, and an equally high calculated annual electricity production. Given its low price, the latter reflector should thus be more cost‐effective in low‐concentrating photovoltaic systems. Copyright © 2005 John Wiley & Sons, Ltd.     

DC‐sputtered ZnO:Al as transparent conductive oxide for silicon heterojunction solar cells with µc‐Si:H emitter

Silicon heterojunction (SHJ) solar cells are highly interesting, because of their high efficiency and low cost fabrication. So far, the most applied transparent conductive oxide (TCO) is indium tin oxide (ITO). The replacement of ITO with cheaper, more abundant and environmental friendly material with texturing capability is a promising way to reduce the production cost of the future SHJ solar cells. Here, we report on the fabrication of the SHJ solar cells with direct current‐sputtered aluminum‐doped zinc oxide (ZnO:Al) as an alternative TCO. Furthermore, we address several important differences between ITO and the ZnO:Al layers including a high Schottky barrier at the emitter/ZnO:Al interface and a high intrinsic resistivity of the ZnO:Al layers. To overcome the high Schottky barrier, we suggest employing micro‐crystalline silicon (µc‐Si:H) emitter, which also improves temperature threshold and passivation of the solar cell precursor. In addition, we report on the extensive studies of the effect of the ZnO:Al deposition parameters including layer thickness, oxygen flow, power density and temperature on the electrical properties of the fabricated SHJ solar cells. Finally, the results of our study indicate that the ZnO:Al deposition parameters significantly affect the electrical properties of the obtained solar cell. By understanding and fine‐tuning all these parameters, a high conversion efficiency of 19.2% on flat wafer (small area (5 × 5 mm²) and without any front metal grid) is achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

H2 plasma treatment at the p/i interface of a hydrogenated amorphous Si absorption layer for high‐performance Si thin film solar cells

Plasma treatment (PT) of the buffer layer for highly H₂‐diluted hydrogenated amorphous silicon (a‐Si:H) absorption layers is proposed as a technique to improve efficiency and mitigate light‐induced degradation (LID) in a‐Si:H thin film solar modules. The method was verified for a‐Si:H single‐junction and a‐Si:H/microcrystalline silicon (µc‐Si:H) tandem modules with a size of 200 × 200 mm² (aperture area of 382.5 cm²) under long‐term light exposure. H₂ PT at the p/i interface was found to eliminate non‐radiative recombination centers in the buffer layer, and plasma‐enhanced chemical vapor deposition at low radio‐frequency power was found to suppress the generation of defects during the growth of a‐Si:H absorption layers on the treated buffer layers. With optimized H₂ PT of the a‐Si:H single‐junction module, the stabilized short circuit current and fill factor increased, and the stabilized open circuit voltage moves beyond its initial value. The results demonstrate 7.7% stabilized efficiency and 10.5% LID for the a‐Si:H single‐junction module and 10.82% stabilized efficiency and 7.76% LID for the a‐Si:H/µc‐Si:H tandem module. Thus, the growth of an a‐Si:H absorption layer on a H₂ PT buffer layer can be considered as a practical method for producing high‐performance Si thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.     

All‐screen‐printed back‐contact back‐junction silicon solar cells with aluminum‐alloyed emitter and demonstration of interconnection of point‐shaped metalized contacts

In the following, high‐efficiency back‐contact back‐junction silicon solar cells with aluminum‐alloyed emitter are described. First, the theoretical background for the cell concept is explained. To that purpose, the bulk lifetime and the front surface field characteristics are considered. Three different process sequences for the phosphorus‐diffused profiles on the front and back surfaces are depicted: One exhibits a shallow field, and two sequences have deeper, driven‐in profiles. For realizing high efficiencies, such cell structures must meet several prerequisites, such as firing‐stable front and rear passivations, and functional small screen‐printed Al structures. Furthermore, it must be possible to create contacts on the Si surfaces using the driven‐in P‐profiles. With such a structure, cell efficiencies of 20.0% are reached. An analysis of the series resistance and area‐weighted recombination is performed. The results are compared with the measured cell parameters. Two‐dimensional simulations show the efficiency potential when decreasing the width of the backside field and when a cell structure, which would inhibit a passivated aluminum‐alloyed p⁺‐emitter, is created. Also, an advanced concept is demonstrated where a point array of both polarities on the cell backside is interconnected externally on module level. To that purpose, the cell is soldered to a printed wiring circuit board by using a reflow soldering process. Copyright © 2013 John Wiley & Sons, Ltd.     

Demonstration of the monolithic interconnection on CIS solar cells by picosecond laser structuring on 30 by 30 cm2 modules

In this paper, we present the selective structuring of all three patterns (P1, P2 and P3) of a monolithic interconnection of CIS (Cu(In,Ga)(S,Se)₂) thin film solar cells by picosecond laser pulses at a wavelength of 1064 nm. We show results for single pulse ablation threshold values and line scribing of molybdenum films on glass (P1), CIS on molybdenum (P2) and zinc oxide on CIS (P3). The purposes of these processes are the p‐type isolation (P1), cell interconnect (P2) and n‐type isolation (P3), which are required for complete cell architecture. The half micron thick molybdenum back electrode can be structured with a process speed of more than 15 m/s at about 15 W average power without detectable residues and damage by direct induced laser ablation from the back side (P1). The CIS layer can be structured selectively down to the molybdenum at process speeds up to 1 m/s at about 15 W average power, due to the precision of direct laser ablation in the ultrashort pulse regime (P2). The ZnO front electrode layer is separated by clean trenches with straight side walls at process speeds of up to 15 m/s at about 10 W average power, as a result of indirect induced laser ablation (P3). A validation of functionality of all processes is demonstrated on CIS solar cell modules (30 × 30 cm²). By replacing one state‐of‐the‐art process by a picosecond laser process at a time, solar efficiencies could be increased for P1 and P2 and stayed on a similar level for P3. After an optimization of the patterning processes in the R&D pilot line of AVANCIS, we achieved a new record efficiency for an all‐laser‐patterned CIS solar module: 14.7% as best value for the aperture area efficiency of a 30 × 30 cm² sized CIS module was reached. Copyright © 2014 John Wiley & Sons, Ltd.     

Parallel‐connected monolithic dye‐sensitised solar modules

Light‐soaking and high‐temperature storage testing of monolithic dye‐sensitised solar modules with total area module efficiencies above 5% have been performed. Our experiences from the development of a four‐layer monolithic dye‐sensitised solar test cell for comparative testing of material components for dye‐sensitised solar cells have directed our module development to a novel device design consisting of parallel‐connection of individual monolithic cells. The results from the accelerated testing of the modules (total area of 17.0 cm²) with four parallel‐connected cells (active area of 3.38 cm²/cell) are equivalent to those obtained for the monolithic single test cells when using identical device components. The successful transfer from cell to module stability is an important milestone in our ambition to develop a low‐cost Photovoltaic (PV) technology. Moreover, our results indicate that intensified research and development to define the procedures for relevant accelerated testing of dye‐sensitised solar modules is urgently required. Copyright © 2010 John Wiley & Sons, Ltd.     

FLATCON® CPV module with 36.7% efficiency equipped with four‐junction solar cells

In this paper we present the application of high efficiency four‐junction solar cells using SOITEC bonding technology under a Fresnel lens optic and in a FLATCON®‐type CPV module. We demonstrate very high performance. The measurement of a sub‐module, consisting of a four‐junction solar cell adjusted under a single Fresnel lens, showed an efficiency of 38.9%. An 829.6 cm² sized FLATCON®‐type CPV module yielded in an efficiency of 35.0% and 36.7% at CSOC and CSTC, respectively. Thus, both, the sub‐module and the CPV module showed record values, which prove the usefulness of high efficiency four‐junction solar cells in CPV applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Innovation and fabrication of 5.5 generation image‐patterned translucent photovoltaic module by using laser scribing technology

The building‐integrated photovoltaic (PV) technology is one of the most promising applications for amorphous silicon (a‐Si) thin film solar cells. It is necessary to develop more various building‐integrated PV modules, which will provide architects and industries more options for the PV installation to their buildings or construction bodies. In this paper, a new type of a‐Si PV module, called image‐patterned translucent a‐Si PV module, is developed. Any required image can be displayed on the module by using laser processes. In the present result, a 5.5 generation (1100 × 1400 mm) image‐patterned translucent PV module with 10% transmittance exhibits the stabilized maximum power output (P_max) of 92.5 W, which can be further improved by optimizing the laser parameters. The remarkable features of our module such as the image displaying, natural light transmission, and heat reduction create entirely new applications including windows and logos and provide an option that adds personal style and unique design to the building interiors. Copyright © 2011 John Wiley & Sons, Ltd.     

Recent progress of high efficiency Si thin‐film solar cells in large area

Si thin‐film solar cells are suitable to the sunbelt region due to a low temperature coefficient and to building integrated photovoltaics owing to flexible size, easily controllable transmittance, and an aesthetic design. Nevertheless, the application is limited until now due to their low conversion efficiency. We have developed a triple junction cell (a‐Si:H/a‐SiGe:H/µc‐Si:H) providing efficient light utilization. For the high efficiency, we have focused on the smoothing of high haze TCO, a low absorption window layer, a low refractive index interlayer, uniformity control of the thickness and crystalline volume fraction in the microcrystalline silicon layer, and a low absorption back reflector. Through these activities, we have achieved a world record of 13.4% stabilized efficiency in the small size cell (1 cm²) and 10.5% stabilized efficiency in the large area module (1.1 × 1.3 m²), certificated by the National Renewable Energy Laboratory and Advanced Industrial Science and Technology, respectively. This result was presented in solar cell efficiency tables (Version 41). At this moment, we have increased a stabilized efficiency of 11.2% (Output power 160 W) in the large area module. We will report on the advanced materials in detail for high efficiency. Copyright © 2014 John Wiley & Sons, Ltd.