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31st European Photovoltaic Solar Energy Conference and Exhibition; 62-66, Among the four features unique to hot-carrier solar cells (HC-SCs): (i) carrier thermalization time and (ii) carrier equilibration time in the absorber, (iii) energy-selection width, and (iv) conductance of the energyselective contacts (ESCs), the requisites of (i)-(iii) have been clarified. To find the required values of (iv), we have improved the detailed balance model of HC-SC operation to involve a finite value of the ESC conductance. We have also derived a formulation to calculate the conductance of resonant tunneling diodes (RTDs) consisting of semiconductor quantum dots (QDs) and quantum wells (QWs) based on the rigorous solutions of the effective-mass Hamiltonians, and found that the requisite of (iv) can be, in principle, fulfilled using the QD- and QW-RTDs.

31st European Photovoltaic Solar Energy Conference and Exhibition; 210-215, A series of a-Si/a-Si double tandem solar cells (PSCs) have been fabricated over the Al:ZnO (~210 nm) /Ag nano particles (~50 nm lateral dimension)/ZnO (tc nm) multilayers working as front TCOs with tc varying from ~5 nm to ~50 nm and compared with a reference double tandem solar cell (RSC) deposited on an AZO substrate. Measurement of the external quantum efficiency (EQE) of the front cell in each PSC shows a large enhancement in the wavelength range (L): 550 nm to 725 nm together with a poor EQE at shorter wavelength range (S) < 550 nm in comparison to the front cell EQE of the RSC. Theoretical modeling of the solar cell structure based on finite difference time domain method shows surface plasmon resonance of Ag capped with ZnO occurring around 550 nm. A PL emission band above 530 nm associated with the oxygen related defect in ZnO has been correlated with the enhanced EQE for the range L. The absorption by the Ag nano particles and the recombination at the front TCO/a- Si:H interface degrades the EQE over S range. Existence of an optimum value of ZnO thickness has been proposed to improve the performance of the solar cells.

31st European Photovoltaic Solar Energy Conference and Exhibition; 164-168, Current cell interconnection technology in a PV module is based on soldering three or four solder coated Cu ribbons on top and bottom of the cells, connecting them in series to create a string. An alternative approach being proposed by several equipment manufacturers is based on replacing the ribbons with multiple wires: this should enable at the same time higher module efficiency and significant cost reduction through consistent Ag paste reduction at cell level. On the other hand, a module reliability risk related to the introduction of a new technology has to be considered. Purpose of this work is to make a technical and economical assessment on the viability of a wire soldered interconnection with optimized metallization.

31st European Photovoltaic Solar Energy Conference and Exhibition; 100-103, DSM’s light trapping film (LTF) is applied to bifacial modules to improve the energy yield for vertical, east/west facing applications. The LTF increases the current both at test conditions (STC) and under high albedo with about 4% compared to planar solar glass, due to the better anti-reflection (AR) and light trapping properties. Without LTF, the current drop due to reflection at the air/glass interface increases from 4% to 45% for incident angles between 0 and 85°, With LTF it remains between 1 and 4%, coupling almost all light at all angles into the module. Optical, thermal and electrical modelling together with time resolved meteorological data results in time resolved output power. This shows that for east/west facing vertical modules 60% of the calculated energy output is generated at incidence angle >55°, versus 20% for equator-facing modules. Applying the annual energy yield (AEY) model to bifacial modules in vertical applications, we show that there is >6% gain in AEY by the addition of LTF. In general, as LTF is most effective for light incident at higher angles, systems that have a large contribution from either (or both) diffuse or ground reflected light will benefit the most from addition of LTF.

31st European Photovoltaic Solar Energy Conference and Exhibition; 6-11, Perovskite/crystalline silicon tandem solar cells are one of the most promising approaches to reach conversion efficiencies beyond 30% at reasonable costs. In the present paper, we present our research directions and recent results. This includes fundamental studies on the perovskite material properties such as sub-band gap absorption and complex refractive index, as well as the development of a broadband transparent rear electrode. The electrode development enabled the fabrication of semitransparent perovskite cells with efficiencies up to 14.7%, which then translates to four-terminal tandem measurements with total stabilized efficiency of up to 22.8%. Furthermore, we present first results on monolithic perovskite/silicon tandems with total current of 32.25 mA/cm2 on a flat wafer and finally provide optimization guidelines with optical simulations.

31st European Photovoltaic Solar Energy Conference and Exhibition; 20-24, The inter-annual variability (IAV) of the solar resource is an important uncertainty component in annual energy production (AEP) estimates of photovoltaic (PV) plants. Based on studies in the Mediterranean and Black Sea region, a figure of 4 to 6% is sometimes used by consultants. For the extremely arid climates in the Atacama Desert in the North of Chile such generic values are expected to unnecessarily penalise developers seeking project financing. In the absence of reliable and consistent long-term ground measurements for this region, and based on previous in-house validation work and industry experience, long-term mesoscale satellite-derived datasets from the SolarGIS® database were found to be the best available data source to estimate the IAV of the global horizontal irradiance (GHI). It was found that none of the 14 sample sites considered in this study displayed an IAV exceeding 2%. While this result is subject to limitations, the observed performance of the model relative to measured data suggests that the uncertainty in the IAV value is small compared to the systematic error that otherwise arises from using a 4 to 6% generic figure for sites in the Atacama.

31st European Photovoltaic Solar Energy Conference and Exhibition; 146-148, To prevent PID (potential induced degradation), we have developed coating materials with high water-repellency. The coating solution consists of fluorine-hybrid of inorganic and organic components, and the resultant coated layer has attained environmentally tough properties. Only by wiping photovoltaic (PV) modules with cloth and the coating solution followed by drying, coated modules show that the losses of the normal powers (LNP) are less than 0.7%, after duration tests at 85°C and 85%RH, regardless of loading continuously electrical current. Further the present material has a function of anti-reflection (AR). A single coated layer on glass substrate of a c- Si PV module, gives ~3% of enhancement of the light-trapping performance (LTP) and to improve 2.99% of the power generation efficiency, respectively.

31st European Photovoltaic Solar Energy Conference and Exhibition; 48-51, Multi-Quantum Well (MQW) solar cells are devices intended for use as single junction or within multijunction cells, and presenting interestingly large open-circuit voltages. However, due to their particular band structure, complex transport properties are involved. In this contribution, the influence of the host pin material (either GaAs or InGaP) is investigated. For the InGaP case, extremely high open-circuit voltages are observed, larger than what could be expected from a detailed balance argument. We explain this behavior by reduced hole transport properties under forward voltage, so that the MQW contribution to the recombination is reduced. Nevertheless, as seen on the external quantum efficiency, the MQW are active at short circuit, allowing increased current by absorption below the bandgap of the host material.

31st European Photovoltaic Solar Energy Conference and Exhibition; 15-19, The recent technological shift towards photovoltaic (PV) systems operating at voltages as high as 1500 Vdc combined with the trend towards transformerless inverters in order to reduce the system complexity and cost is increasing the risk of Potential-induced-degradation (PID) in contemporary PV modules. It is well known in the PV industry that PID phenomenon is one of the major technical challenges with an incomplete fundamental understanding. To minimize the risk of PID there are several approaches at different levels of the PV cell and module laminate (e.g. cell anti reflective coating, encapsulants, etc.). Yingli is going beyond current standards and is striving for PID resistance for voltages of up to 1500Vdc, without sacrificing power output to deliver a reliable, manufactural, and cost-effective solution to the problem. In this work, we study the potential of resolving PID on the cell-level by modifying the Silicon Nitride (SiNx) anti-reflection coating (ARC) and compare it to a module-level approach using ionomer encapsulants. We identified SiNx coatings that yield PID resistance at 1000 Vdc, albeit at cost of power loss. Ionomers show full suppression of PID at no initial power loss, which was verified by an independent testing lab. In addition, we demonstrated that modules made with ionomer film surpassed standard reliability tests. Results of PID exposure tests at 1500Vdc will be presented.

31st European Photovoltaic Solar Energy Conference and Exhibition; 117-120, It is currently possible to fabricate crystalline silicon solar cells with the absorber thickness ranging from a few hundreds of micrometres (conventional wafer-based cells) to devices as thin as 1 μm. In this work, we use a model single-junction solar cell to calculate the limits of energy conversion efficiency and estimate the optimal absorber thickness. We analyse the roles of bulk and surface recombination mechanisms. Considering the main parameters that impact the solar cell performance, we quantify the requirements for achieving a given efficiency, which helps us to establish a proper design strategy for high efficiency silicon solar cells. Finally, we review an analytic electro-optical model of silicon solar cells, which is a reliable alternative to more complex device simulators.

31st European Photovoltaic Solar Energy Conference and Exhibition; 35-38, Two-dimensional carrier distribution in a phosphorus (P)-implanted emitter is observed by scanning capacitance microscopy (SCM). The SCM image shows a clear contrast at an almost constant depth along the surface of a silicon wafer with a pyramidal texture. Segregation of carriers in convex and concave regions of the pyramidal texture was not observed. A non-monotonic SCM depth profile is discussed based on the applied bias and carrier concentration. Junction depth of top, valley, and side wall of pyramidal texture is evaluated from SCM depth profile. The junction depth of the side wall is nearly equal to that of the valley. Since the vertical junction depth immediately beneath the surface of the texture is determined by diffusion from the side wall, it is longer than those of the side wall and valley. Based on the SCM images, formation processes of p-n junction in a P-implanted emitter are discussed.

31st European Photovoltaic Solar Energy Conference and Exhibition; 252-258, Solar energy development has to be based on unique innovative technologies. Its role in the power industry of the future will be defined by the potentialities in the field of development and practical use of new physical principals, manufacturing techniques, materials and designs to create solar energy converters of high competitive ability. In this work, the most perspective directions for the development of the silicon based solar power have been overviewed and the basic manufacturing technologies for photoelectric converters and semiconductor materials designed at VIESH have been presented that include: chloride-free solar-grade silicon manufacturing technology, matrix solar cell technology providing an efficiency of 25% to 30% for conversion of concentrated solar radiation, thermal and laser radiation, silicon solar module encapsulation technology providing the enhanced power performance and life span of up to 50 years, new solar radiation concentration methods making it possible to achieve uniform solar cell irradiation, for concentration ratios ranging from 50 to 100.

31st European Photovoltaic Solar Energy Conference and Exhibition; 205-209, The influence of Aluminum Oxide (Al1-xOx) films grown on two types of silicon micro wire (Si-MW) substrates by thermal atomic layer deposition (ALD) technique as a passivation layer was investigated, compared to the one grown on conventional flat wafer substrate. When the growth rate of the Al1-xOx film became closed to the ideal growth rate (0.11 nm/cycle), the Al1-xOx film has then shown good-passivation effect and were found to be uniformly deposited. Compared to the samples grown on flat substrate and the Si-MW substrate fabricated using ICP-dry etching technique, the Al1-xOx film deposited on the Si-MW substrate using Deep-RIE technique, blessed us with implied Voc of 0.646 V after annealing. We investigated implied Voc of Al1-xOx films and reflectance properties for Si-MW substrates with various wire-array sizes ((1)D=2 um, P=4 um, (2)D=3 um, P=5 um, (3)D=5 um, P=8 um, (4)D=7 um, P=10 um, (5)D=10 um, P=13 um). We found that the relation between implied Voc and reflectance properties for each of wire array size of Si-MW substrates, is in a trade-off.

31st European Photovoltaic Solar Energy Conference and Exhibition; 25-28, A knowledge-based approach towards developing a new generation of solar energy conversion devices requires a fast and direct feedback between sophisticated analytics and state-of-the-art processing/test facilities for all relevant material classes. A promising approach is the coupling of synchrotron-based X-ray characterization techniques, providing the unique possibility to map the electronic and chemical structure of thin layers and interface regions – with relevant in-system/in-situ sample preparation or in-operando analysis capabilities in one dedicated laboratory. EMIL, the Energy Materials In-situ Laboratory Berlin, is a unique facility at the BESSY II synchrotron light source. EMIL will be dedicated to the in-system, in-situ, and in-operando X-ray analysis of materials and devices for energy conversion and energy storage technologies including photovoltaic applications and photoelectrochemical processes. EMIL comprises up to five experimental end-stations, three of them can access X-rays in an energy range of 80 eV – 10 keV. For example, one key setup of EMIL combines a suite of advanced spectroscopic characterization tools with industry-relevant deposition facilities in one integrated ultra-high vacuum system. These deposition tools allow the growth of PV devices based on silicon, compound semiconductors, hybrid heterojunctions, and organo-metal halide perovskites on up to 6’’ sized substrates. EMIL will serve as a research platform for national and international collaboration in the field of photovoltaic/photocatalytic energy conversion and beyond. In this paper, we will provide an overview of the analytic and material capabilities at EMIL.

31st European Photovoltaic Solar Energy Conference and Exhibition; 136-138, The need for controlled initial vapor droplet size, and, final nano-particle size correlation models has captured the attention of many material scientist involved in production of semiconductor nano-particles using ultrasonic spray pyrolysis (USP). The exploration of unique sizes and shapes as they relate to various properties has become a great quest for large field applications. USP has been, for the last three decades, one of the major techniques used to produce a wide variety of nano-materials either in thin film and/or in powder form. However the lack of universal droplet correlation models that yield agreement between the theoretically estimated droplet sizes and the experimentally determined droplet sizes has been one of the biggest obstacles in the development of USP systems on a large industrial scale. To meet these demands, this paper exposes the conflicts of the conventional droplet correlation models and finally presents a new droplet correlation nanoparticle correlation model for TiO2 nanoparticles produced for USP.

31st European Photovoltaic Solar Energy Conference and Exhibition; 216-219, The present paper reports results of an incorporation of p+ doped Si nanoparticles (NPs) in (Al2O3)x (B2O3)1-x films deposited on n-type Si. Si NPs with diameter ~20 nm have been dispersed in a sol solution, which was used for a spin coating deposition followed by an additional annealing at 850oC. The (Al2O3)x (B2O3)1-x films were deposited on quartz and n-type Si substrates. The optical study reveals that Si NPs embedded in (Al2O3)x(B2O3)1-x leads to higher absorption in the visible spectral range compared to dielectric layer without nanoparticles. AFM studies show that the incorporated Si NPs into dielectric layer have quite uniform distributions on the film surface. Тhe sheet resistance of (Al2O3)x(B2O3)1-x films with Si NPs, deposited on quartz substrates is about 250 Ohm/sq. Similar films deposited on n-type Si can be considered as a new type of heterojunction structures, which is the subject of study in this work. The diode rectifying behavior of (Al2O3)x(B2O3)1-x p+ Si NPs/n-Si structure has been demonstrated by means of the current- voltage measurements. The rectification ratio (RR) is about 7,4x103 at +/- 2 V and a reverse current of I= 1.7.10-7 A at -2V has measured. The structure shows the photovoltaic effect. It is concluded that (Al2O3)x(B2O3)1-x : p+Si NPs layers can be considered as a promising option for the formation of ultra thin emitters for photodiodes and Si based solar cells.

31st European Photovoltaic Solar Energy Conference and Exhibition; 52-54, Silicon nanowall solar cells are expected to exceed the efficiency limit of bulk Si solar cells owing to quantum confined enlarged bandgap with optical absorption properties. However, one of the concerns of nanowall cells is the presence of surface states to increase the recombination velocity. In this work, photovoltaic (PV) properties of ultra-thin lateral p+-i-n+ Si solar cells with Si layer thickness below 10 nm have been characterized under AM1.5 light illumination, and the impact of thickness on PV characteristics has been examined. With thinner Si layer less than 6 nm, open-circuit voltage (VOC) has shown gradual increment, which is predicted by theoretical calculation of bandgap enlargement. The short-circuit current (ISC) has shown linear dependency on thickness from 3.3 to 7.6 nm, which indicates that generated photocurrent is linearly proportional to volume of Si layer. However, the gradual current reduction with anode voltage application caused the FF degradation. This is mainly caused by series resistance due to missing dopants. Therefore, formation of junctions is a critical issue for ultra-thin Si solar cells.

29th European Photovoltaic Solar Energy Conference and Exhibition; 207-209, In this paper we present ECN’s new high-voltage module concept which is based on MWT backcontact technology, and test its performance under shadow conditions. Controlled realistic shading tests were performed for three shade cases that are typical for PV applications in the built environment (a pole shade, a tree shade and a dormer shade). The relative power output (shaded over normal) of the high-voltage module was compared to that of a conventional three string, 60-cell module. The new high-voltage module significantly outperformed the conventional module: its shade performance was 1.1 to 4.2 times better, and its shade linearity was around 80%.

29th European Photovoltaic Solar Energy Conference and Exhibition; 310-312, In our previous work, we have designed and fabricated a series of multiple quantum wells (MQWs) solar cell which were different from conventional MQWs with the ultra-thin barrier ( < 4 nm) aiming at very efficient carrier extraction via carrier tunneling mechanism in MQWs region. Preliminary results indicated that utilization of such a thin barrier design, MQWs solar cell can improve carrier collection efficiency exhibiting as an enhanced current density with the minimized open circuits voltage loss. So far, investigations have been only focused on transport mechanism of carriers generated in MQWs region. Photon generated carriers from bulk material in a p-i-n MQWs cell has never been discussed in details on the transport mechanism through periodical well and barrier part, though it has very much significance as well. In this report, influence from barrier in MQWs region has been discussed on carriers generated in the bulk in term s of barrier thickness and potential barriers’ height.

29th European Photovoltaic Solar Energy Conference and Exhibition; 221-224, We report a comprehensive comparison of structural and electrical investigations of high quality and high indium content InGaN epilayers in forms of bulk and multiple quantum wells (MQWs) to be used as active regions in solar cells. Solar cell designs have active regions that include 200 nm In0.26Ga0.74N and InxGa1-xN/GaN (x~ 0.10, x~0.20 and x~0.30) MQWs repeated for 25 and 35 cycles which were grown on sapphire (0001) by metal-organic chemical vapor deposition (MOCVD). The surface characterization of the epilayers was investigated by means of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The detailed characterization of crystalline quality and the morphological evolution of high In content InGaN MQWs were studied by High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) in addition to compositional data provided by Energy dispersive X-ray spectroscopy (EDX) and High resolution X-ray diffraction (HRXRD). It was found that changing the type of high In content InGaN active layer from bulk to very thin QW epilayers enhances greatly the crystalline quality by decreasing the defect density and the phase separation within the InGaN epilayers to a large extent. For solar cells based on InxGa1-xN/GaN MQWs (x~0.10, x~0.20 and x~0.30), decreasing both the growth thickness of InGaN QWs and the number of wells improved the structural and surface quality. The designs with fully strained 25 cycles InxGa1-xN/GaN MQWs (varied from x~0.10 to 0.30) epilayers shows very low defect density and excellent structural quality showing that they are highly promising for future applications such as solar cells. Besides, very high quality crystalline structure of the solar cell having 25 cycles of In0.1Ga0.9N (3 nm)/GaN (10 nm) MQWs, which was fabricated into a solar cell, showed a photovoltaic response. In addition, owing to excellent mechanical robustness, high resistance to energy radiation, alignment of energy bands favorable with silicon, strong optical absorption, and good thermal stability, InGaN based solar cells can be explored to be integrated with standard silicon PV technology. The innovative hybrid integration of InGaN solar cells with silicon solar cells and their incorporation in very high efficiency modules of reduced area would lead to a major break-through in the photovoltaic domain.

29th European Photovoltaic Solar Energy Conference and Exhibition; 53-57, We propose to use metal nanogrids to enhance light absorption in ultrathin GaAs solar cells with 120 nm-thick and 220 nm-thick absorbers. Thanks to the transfer of the cells on a silver back mirror leading to Fabry Perot resonances in the semiconductor cavity, the short circuit current is increased by 40%. With an additional antireflection coating and a nanostructured back mirror, we obtain Jsc=17.4 mA/cm2 and 21.2 mA/cm2 for 120 nm and 220 nm-thick absorbers, respectively. It represents a two-fold enhancement of the Jsc compared to the non-transferred solar cells. We also show that is possible to realize really absorptive semiconductor devices within a few tens of nanometers. As a proof of concept, a super-absorber device consisting of a front nanogrid deposited on a 25 nm-thick GaAs layer absorbs more than 80% of the light over the 450-830 wavelength range.

29th European Photovoltaic Solar Energy Conference and Exhibition; 198-201, Photovoltaics are widely used method to tap solar energy, but the disadvantage in this method is that besides being costlier it occupies much space. So it is proposed to design three dimensional structures. In this research 3 dimensional natural structures are designed and compared with a conventional two dimensional module structure. Firstly the number of cells is made constant then the base area is made constant. These designs were experimentally studied with prototypes. It was to find out the power output and plot day profile of all these designs on a certain sunny day in the university premises. The purpose was to analyse them and to find out a better 3 Dimensional Photovoltaic system if 3D systems are more productive than the conventional modules. Firstly the location of our experimentation is studied, then the types of cells are compared and one of it is selected and purchased and the purchased cell’s characteristics are found out. The circuit diagram for the selected structures is designed and fabricated. The structure selected for this research is Tree structure and Ellipsoid which is to be compared with a two dimensional module prototype. Then the obtained results are tabulated and graphs are drawn and the best structure out of them is selected. It is seen that the Tree structure outperforms the other models. For the constant base area study it was found out that the tree structure can produce twice the amount of power as 2D module. With possibilities to increase the number of cells and size of the solar tree than the fabricated prototype, it can be said that the area required for a certain solar power system can be reduced to more than half by using the solar tree structure.

29th European Photovoltaic Solar Energy Conference and Exhibition; 247-249, For tandem solar cells, increasing the generated photocurrent due to absorption of sunlight and minimizing recombination of generated minority carriers is crucial for improving the solar cell performance which is possible by using lateral composition modulated (LCM) structure. LCM GaInP has a smaller band gap compared to bulk GaInP, and using them in the top cell of multi-junction solar cell is effective for increasing the light absorption and the generated photocurrent. In this work we investigate the effect of embedding LCM GaInP on the solar cell performance. Simulation results reveal that for LCM GaInP layer thickness up to 40 nm, the efficiency of the solar cell can be increased. This is due to the increase in open circuit voltage owing to the increased light absorption by both In-rich and Ga-rich GaInP layers of LCM structure. With further increase in LCM GaInP layer thickness, the increase in efficiency is due to the increase in short circuit current density which is attributed to the enhanced light absorption by the In-rich GaInP. These results show the potential of LCM GaInP structure as the top cell material for realizing high-efficiency multi-junction tandem solar cells.

29th European Photovoltaic Solar Energy Conference and Exhibition; 298-302, For the purpose of the application to the top cell in all-silicon tandem solar cells, we have fabricated silicon nanowire arrays using metal assisted chemical etching with silica nanoparticles (MACES) method. In this method, the diameter of SiNWs can be controlled to around 30 nm. However, to control the bandgap of SiNWs using the quantum size effect, the diameter has to be less than 5 nm. In this study, to reduce the diameter, we tried ozone thermal oxidation of SiNWs. Ozone enables us oxidation at lower temperature than oxygen. SiNW arrays fabricated by MACES were oxidized under 24.9wt% ozone at 700 °C for 3 hours. From an energy dispersive x-ray spectroscopy (EDS) mapping, it was confirmed that the oxygen concentration was increased in the whole of SiNW arrays after the oxidation. Cross-sectional transmission electron microscopy (TEM) image showed that the silicon-part-diameter of a SiNW was reduced to about 5 nm around the top of the SiNW. Therefore, ozone oxidation process is promising method to reduce the diameter of SiNWs at lower temperature.

29th European Photovoltaic Solar Energy Conference and Exhibition; 25-30, The grain-boundary light-beam-induced-current (GB-LBIC) technique is a versatile method for simultaneous determination of grain and grain boundary (GB) properties in polycrystalline semiconductors and semiinsulators without the need of charge collection by p-n juctions or Schottky contacts. Here, the GB itself acts as charge collector and generates a characteristic peak-and-trough (PAT) current profile. We investigate by 2D optoelectronic numerical simulations the nature of this PAT signal on different models for the GB. We find that generally the charge at the grain-boundary that causes a built-in-field around the GB is responsible for the PAT signal. However, a PAT can also be generated for structures with a high resistivity GB or a high recombination GB, without built-in fields. Moreover, these findings will demonstrate that the GB-LBIC method allows the determination - in materials with grain boundaries - of the minority carrier diffusion length of the grain bulk, without being influenced by GB recombination.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1-4, In this paper, we give a historic overview over the development of upconverter solar cell devices and show how performance has increased in the past few years. We present measurements of upconverter solar cell devices showing a world-record increase in short-circuit current density due to upconversion of 0.5% relative for a bifacial crystalline silicon solar cell. Finally, we discuss concepts to further increase upconversion performance: photonic structures that combine local irradiance enhancement and resonant emission amplification and spectral concentration to enlarge the utilized spectral range

29th European Photovoltaic Solar Energy Conference and Exhibition; 120-122, Two types of the solar simulators were compared in several types of experiments involving crystalline silicon, amorphous silicon solar cells and test structures of transparent oxides and buffer layers used in thin film solar cells. Photo response of solar cells illuminated by commercially available Xenon arc based simulator was used as a benchmark for comparison. Compact light emitting diode (LED) based solar simulator with six LED groups of different colors was the second illumination source in our study. AAA class requirements are met by both solar simulators, but substantial performance differences appear when testing various devices. These differences are attributed to the differences of quantum efficiency spectra of measured devices, surface reflectivity variations for mini-modules and an increase of photo response non-uniformity once the photocurrent contribution is reduced from several diode groups. Numerical calculations for multilayer structures were used for further comparison studies and were employed to clarify the possible origin of the observed phenomena.

29th European Photovoltaic Solar Energy Conference and Exhibition; 307-309, We present electrical and optical characterization of InGaN/GaN nanowires on silicon with a goal to develop an intermediate band solar cell based on this novel material system. We demonstrate current-voltage characteristics illuminated with up to 16 suns AM 1.5G illumination, and the improved photoluminescence properties of InGaN/GaN quantum dots inside GaN nanowireswith an AlGaN shell. A sub-bandgap contribution to the quantum efficiency is observed, demonstrating the use of InGaN/GaN quantum dots as a possible route towards efficiency beyond the Shockley-Queisser limit.

29th European Photovoltaic Solar Energy Conference and Exhibition; 263-265, It is proposed to utilize liquid phase epitaxy for GaAs and GaP structures with metal quantum dots by the pulse cooling and heating of the solution-melt. Metals such as ytterbium and calcium are evaluated as potential candidate for a material that experienced metal-semiconductor transition due to stress caused by the difference in thermal expansion between substrate and metal.

29th European Photovoltaic Solar Energy Conference and Exhibition; 277-279, In this work n-ZnO/p-CuO heterojunctions were fabricated by respectively depositing ZnO by spin coating and CuO by RF-sputtering on ITO covered glass substrates. The structural and optical properties of the p-CuO and n-ZnO films were investigated by X-ray diffraction and UV-VIS spectroscopy. The electrical junction properties were characterized by currentvoltage (I-V) characteristics. Additionally, the both capacitance and conductance versus applied DC voltage (C-V,, G-V,…) measurements were realized at room temperature

29th European Photovoltaic Solar Energy Conference and Exhibition; 359-361, Intermediate bandgap solar cells (IBSCs) are predicted to take the power conversion efficiencies beyond 60% and quantum dots inserted in the intrinsic region of IBSCs, will play an essential role in the realizing this future technology. Inspite of suffering from drawback of formation of 2-D wetting layer during the growth, Stranski- Krastanov (SK) growth mode has been popular over years. Recently, sub-monolayer (SML) growth was suggested as an alternative. Dots formed by this new method are expected to provide better confinement as compared to conventional one. In this paper, we present study of optimization of number of stacks followed by post growth annealing using photoluminescence measurements.

29th European Photovoltaic Solar Energy Conference and Exhibition; 325-327, This paper calculates the effects of interface thermal conductance of energy selective contacts on the efficiency of hot carrier solar cells (HCSC). It proposes that hot lattice could enable HCSCs not to depend on the long thermal lifetime and other ideal conditions and hence open a new approach to HCSC. This is due to the fact that when lattice temperature approaching carrier temperature, thermalization loss drops. Calculation shows thermalized heat is enough to heat thin absorbers to above 800K with low interface thermal conductance less than 100W/m2/k. In this scenario, efficiency above 40% of a single cell is predicted by using more practical parameters.

29th European Photovoltaic Solar Energy Conference and Exhibition; 243-246, Polydimethylsiloxane (PDMS) nano-structured layers were investigated as possible candidates for inexpensive antireflection coatings to be used on materials having high refractive index, such as semiconductors. A mature manufacturing process, developed for moulding a single layer of micro-hemispheres on a PDMS film, was used to create a single layer of nano-hemispheres that can act like a nano-structured coating. The nano-hemispheres on the PDMS film were created by using a self-assembly process and have a size smaller than 300nm. The performance of the nano-structured coating was measured by comparing the reflection of a Silicon substrate with and without the presence of the nano-coating. The measurements were performed at different incidence angles and over a wide range of wavelengths, ranging from 400nm to 900nm. The results show a reduction in the spectral reflectance of the Silicon surface, even at high incidence angles, once the PDMS nano-coating is applied. With respect to the uncoated silicon it was obtained a reduction in the reflected components of more than 30% in the spectral range 400nm ÷ 900nm.

29th European Photovoltaic Solar Energy Conference and Exhibition; 76-79, We propose an experimental method for sensitive detection of carrier interaction and filtering in hot carrier PV structures. The method was successfully tested on PbSe/InP heterostructure. Development of the method required extension of Richardson’s thermionic emission theory towards the case of small barrier heights. Remarkably, the exact solution including Fermi-Dirac statistics was obtained in a simple closed form. It includes the Richardson’s solution as a specific case. We applied linear thermal conductivity model for estimation of the carrier temperature vs. heating power dependence. The new generalized theory allows quantitative interpretation of the experimental data, however, the temperature of the hot electron gas turned out to be very large.

29th European Photovoltaic Solar Energy Conference and Exhibition; 161-164, One way to increase module efficiency is to use deep structured ribbons as cell connectors in photovoltaic modules. The conventional soldered, flat ribbon connector is replaced with a glued, deep structured ribbon. When light strikes a cell it is reflected at the ribbons and, in a conventional cell, lost. The structured ribbons and the glass/air interface redirect some of this light back to the cell. In this paper we compare the yield of conventional photovoltaic modules with soldered flat and glued flat connector ribbons to glued deep structured ribbons, both under laboratory conditions and in field. We found that the new design increases short circuit current by 2 % and yield by 1.5 %. To analyse this effect in detail, simulations are conducted that confirm the experimental findings.

29th European Photovoltaic Solar Energy Conference and Exhibition; 280-283, Thermophotovoltaics has recently garnered significant interest in the field of renewable energy. This is due to their versatility and promise to achieve efficiencies well beyond the Shockeley-Queisser Limit that dictates the performance of photovoltaics [1,2]. Thermophotovoltaics generate electricity from heat converted into infrared photons. In this study, we focus our attention to Ga1-xTlxP, a tertiary compound with the potential of having a lower bandgap with increasing Thallium (Tl) content. TlP belongs to a group of materials known as semi-metals and possesses an optical bandgap below zero [3]: This implies that the material bandgap can theoretically change from 2.26 eV (x=0) to -0.27 eV (x=1). Presented in this study are the implications such a band structure change could have on the associated properties such as the optical bandgap and lattice constant of GaTlP with varying Tl content. The versatility in Tl content allows for a number of candidate substrates including GaAs, Ge, and InP.

29th European Photovoltaic Solar Energy Conference and Exhibition; 147-151, Glass-to-Glass (GG) encapsulation scheme and low temperature cell metallization-interconnection technologies known as Smart Wire Connection Technology (SWCT) are promising alternatives to standard design for highly efficient, long lasting photovoltaic modules. The focus of this work was to study the compatibility of different types of encapsulants currently available on the market with these new technologies in terms lamination process and module durability. The feasibility of photovoltaic modules based on GG and SWCT for high-efficiency solar cells is proven using novel laminating lines with which a process cycle time of 4min can be achieved. The results obtained show that thermoplastic polyolefin elastomer (TPO) encapsulants have superior performances compared to other encapsulant types with regards to both the lamination process conditions and the module reliability.

29th European Photovoltaic Solar Energy Conference and Exhibition; 46-49, Due to a unique structure, a silicon nanowire matrix provides natural light trapping and anti-reflection effects suitable for photovoltaic applications. Thus depositing thin film silicon directly on top of them and forming radial junctions is a promising approach for a new generation of efficient solar cells. Apart from the light trapping, their major advantage is decoupling of the light absorption from the carrier collection direction. This allows the use of very thin active layer, providing more efficient collection in materials with low minority carrier diffusion lengths and also minimizes the light-induced degradation. It has been found that the density of radial junctions is a very important parameter for controlling the device performances. By optimizing the nanowire density, we have demonstrated 8.1 % efficiency for PIN hydrogenated amorphous silicon (a-Si:H) solar cells deposited directly on the TCO coated glass with only 100 nm of active layer thickness. We have adopted two different approaches to improve the short-circuit current density and open-circuit voltage. Firstly, replacing n-type a-Si:H by more transparent amorphous silicon oxide has enhanced the blue response of the solar cell. Secondly, we have shown that a p-type a-Si:H coating on the crystalline silicon nanowire, before depositing the intrinsic layer, is very effective in reducing the nanowire surface recombination and consequently increasing the open-circuit voltage.

29th European Photovoltaic Solar Energy Conference and Exhibition; 43-45, Conversion of solar radiation to electricity with Photon Enhanced Thermionic Emission (PETE) uses both photonic and thermal excitation of charge carriers in a semiconductor, and theoretically may achieve conversion efficiency exceeding 50%. This work investigates the effect of the front grid of electrical contacts on the performance of a PETE converter with a Si based cathode, using a 2-D detailed simulation. The impact of shading by the contacts, charge transport at the contact surface, and dependence on operating temperature are analyzed. The results show that the preferred device design is very different depending on the temperature, due to the different contributions of photonic vs. thermal excitation.

29th European Photovoltaic Solar Energy Conference and Exhibition; 202-206, Irradiance differences on PV cell level can cause the total output power of a PV panel to drop disproportionally. If no action is taken, a single cell can limit the output current of the entire panel. Traditionally this is solved by placing a bypass diode in parallel to a substring of cells. However, when such a diode is active, it creates a significant voltage drop, and this dissipates power that we would rather see at the output. The SmartPV project, an IWT-SBO project funded by the Flemish Government (Flanders, Belgium), aims at increasing the power yield of PV panels under partial shading conditions. With each of the partners (imec, Ghent University, University of Leuven, VITO) working in different areas, we at Ghent University developed a smart bypass. It takes over the function of a bypass diode, while minimizing the power loss. It is a low-voltage-drop switch that is activated when a failing cell is detected. The NIMBUS chip is a fully integrated smart bypass. The NIMBUS chip is designed for a load current of 5A, while limiting the voltage drop to 100mV. When larger load currents need to be provided, several NIMBUS chips can be placed in parallel. An integrated synchronization circuit ensures that all parallel NIMBUS chips work as one. A prototype of the NIMBUS was created, and the first measurements are presented. Also, the first steps in creating a full module with NIMBUS devices are taken.

29th European Photovoltaic Solar Energy Conference and Exhibition; 356-358, AgInTe2 was introduced as an alternative for bottom cells for high efficient tandem solar cells. Thin films were fabricated by RF sputtering deposition on glass substrates under ambient Ar for duration of 30 min using the AgInTe2 source targets. Sputtering gas pressure and sputtering power were optimized first and determined as 0.5 Pa and 100 W, respectively. After the deposition, annealing was carried out in ambient N2 for 30 min at temperatures from 100oC to 350oC. The characterizations of thin films were performed by UV-Vis absorption spectra, XRD and Hall-effect measurements and fabrication of solar cells. The AgInTe2 sputtered layer found to be so promising for a bottom cell with significantly low carrier concentration as low as 6.9 × 1011 /cm3 and high mobility 1.2 × 104/cm2(Vs)-1 at 200oC deposition and annealing temperature. AgInTe2 solar cells were fabricated as AgInTe2/In2S3/TiO2/FTO structure with Au electrode. As a primary cell results conversion efficiency of 1.13% was achieved with Voc of 232 mV, Jsc of 13.6 mA/cm2 with 35.6 FF. Additionally, doping of AgInTe2 by metals including Al, Ni, Cu and Zn was also also carried out and analyzed after sputtering. High quality p-type AIT obtained by Zn doping with carrier density of 2.14×1013 /cm3 and mobility of 3.87×104 /cm2(Vs)-1 where best n-type AIT could be achieved by Ni doping with carrier density of -6.51×1011 /cm3 and mobility of 8.53×104 /cm2(Vs)-1.

29th European Photovoltaic Solar Energy Conference and Exhibition; 84-87, Undoped amorphous hydrogenated silicon carbide (a-SiC:H) thin films were deposited on n-type crystalline Si (c-Si) substrates by PECVD method from a CH4/SiH4/H2 gas mixture. Influence of the gas flow ratio of CH4 and SiH4 has been studied in order to identify the electrical behavior of the prepared thin films. The currentvoltage (I-V), capacitance-voltage (C-V) and complex impedance characteristics of the a-SiC:H/c-Si structures were investigated. Electrical transport mechanisms of prepared samples were determined from the measured temperature dependence of I-V curves. Influence of flow ratio of CH4 and SiH4 on built-in voltage was evaluated from the capacitance-voltage characteristics. Relaxation time and equivalent circuit of prepared samples were determined from impedance spectroscopy measurements.

29th European Photovoltaic Solar Energy Conference and Exhibition; 158-160, Novel module technologies have previously proven that cost of crystalline silicon solar cell processing can be significantly decreased if front busbars and rear soldering pads are omitted. Most of the available technologies are proprietary therefore the aim of this work is to search for directly accessible interconnection technologies such as conductive film (CF) to replace the soldering process and allow for interconnection of busbar-free solar cells. The paper investigated if cells without front busbars and rear pads can be modified to allow for a stable interconnection during extended damp heat (DH) and thermo-cycling (TC) test. For this standard 60 cells modules comprising cells with and without busbars and rear pads were interconnected by means of CF. The results showed that due to the poor adhesion of CF to the screen printed Al layer no possibility exist the module to pass TC200, therefore AlAg or equal is required. Electrical results for cells without busbars but with pads showed superior cell to module (CTM) losses and promising climatic testing data for CF if compared to soldered references but with further optimization work required on the metallization grid. For standard solar cells all obtained data make the CF interconnection a viable and robust technology to replace the standard soldering coming along with better CTM results.

29th European Photovoltaic Solar Energy Conference and Exhibition; 127-130, Hydrogen (H), introduced into crystalline (c-) or amorphous (a-) silicon (Si), is important for modifying Si properties, required for solar cell (SC) photovoltaics. To establish correlation of microscopic H bonding and distribution with the macroscopic properties of Si materials, and to theoretically interpret experimental spectra (from, e.g., non-destructive infrared or optical techniques) we carried out finite temperature ab-initio molecular dynamics (AIMD) for both c-Si and a-Si:H. Starting with H implanted into c-Si and a-Si:H, we simulated the H dynamics, stability, and dissociation product evolution at various temperatures. For a-Si:H, realistic numerical samples are prepared by melting and subsequent annealing at different rates. The vibrational frequencies of hydrogen complexes such as BCH, the H2* and H2** dimers are in good agreement with experiments available. The complexes observed in AIMD are associated with characteristic features in the electronic and optical spectra of the host Si material.

29th European Photovoltaic Solar Energy Conference and Exhibition; 229-231, The effects of intrinsic light trapping or sunlight concentration in nanowire array solar cells comprising of optimally engineered radial p-n junction GaAs/AlxGa1-xAs core-shell nanowires is maximized for optimal solar cell efficiency by optimizing the pitch length of the nanowire array and angles of incidence of the sun-rays. Results reveal that the unique property of core-shell nanowires in decoupling light absorption and carrier collection, along with the exceptionally low optical reflection and strong light trapping, enable radial p-n junction core-shell nanowire array to achieve solar cell efficiencies as high as more than 35% (i.e., exceeding the Shockley-Queisser limit) when illuminated by the one-sun AM1.5G solar spectrum. Such high solar cell efficiencies are the result of the intrinsic sunlight concentration effect in nanowire array solar cell.

29th European Photovoltaic Solar Energy Conference and Exhibition; 255-258, Photonic crystals using alternate layers of high and low refractive index material behave like a Bragg reflector which helps in light confinement and entrapment in thin film solar cells. Conventional metal reflectors have the disadvantage of severe intrinsic losses in the longer wavelength due to the surface plasmon modes produced at the metal dielectric interface. In this work we have fabricated Silicon nitride/ Silicon oxynitride stack layer photonic crystals using radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) at low temperature (200ºC). The compositional analysis of the bond concentration of the material has been investigated using Fourier Transform Infrared Spectroscopy (FTIR) and is used to explain the change in the refractive index of the material. We have obtained a range of thin films with refractive index varying between 3.29 for silicon rich Silicon Nitride (SixNy) to Silicon Oxynitride (SiOxNy) films with 1.53 depending on the deposition conditions. Selective broad band reflectors based on SixNy/ SiOxNy photonic crystals can be designed and fabricated by choosing the right refractive index pair and thickness using these materials. A set of such photonic crystal selective broad band reflectors are fabricated and their performance is demonstrated by applying them on silicon thin film solar cells.

29th European Photovoltaic Solar Energy Conference and Exhibition; 69-72, To counter the mismatch in solar panels due to partial shading reconfiguring the cell layouts can prove to be very costly. We instead propose that optimized static cell layouts maximize the energy yield of photovoltaic panels and could be a suitable solution for panels prone to partial shading especially in building integrated PV. We generate 52 layouts under chosen constraints and study them under two common shade scenarios, namely tree and chimney. These layouts include series and parallel connections of the cell-strings and vary in the number and the placement of bypass diodes in the panel. We also compare the performance of the different layouts under uniform irradiance conditions and show that they perform equally well.

29th European Photovoltaic Solar Energy Conference and Exhibition; 210-213, In this work, we use a rigorous electro-optical model to study silicon solar cells with random textures. First, we calculate the efficiency limits of silicon solar cells by considering a p-n junction and solving the driftdiffusion equations as a function of the cell thickness, assuming the Lambertian light trapping. Second, we aim to design realistic solar cells to get as close as possible to the efficiency limits. We use an efficient numerical model, which allows us to study a wide range of the absorber thickness and material parameters. We theoretically demonstrate how to simultaneously increase short-circuit current and improve open-circuit voltage by, respectively, texturing the front surface and decreasing the absorber thickness.

29th European Photovoltaic Solar Energy Conference and Exhibition; 273-276, We have fabricated BaSi2 films by a simple vacuum evaporation method for solar cell applications. Structural analysis shows that dense single-phase BaSi2 films can be grown on silicon and glass substrates at 500 and 600 ˚C, while the film grown at 400 ˚C is oxidized upon exposure to the air. The film growth mechanism is discussed and high substrate temperatures are concluded to be needed for the removal of excess Ba atoms. Electrical characterization reveals that the evaporated BaSi2 films have n-type conductivity. By analyzing the optical transmission and reflection, high absorption coefficients comparable to the epitaxial film are extracted. The indirect absorption edge is suitable for solar cells (1.14 eV). The microwave-detected photoconductivity decay measurement shows that long carrier lifetime for effective usage of photoexcited carriers can be realized by vacuum evaporation.

29th European Photovoltaic Solar Energy Conference and Exhibition; 165-170, The key for increasing the production yield in state of the art crystalline module production lines is the optimization of the stringing and soldering process. Besides the cell to module (CTM) losses the choice of the appropriate solder ribbon defines the mechanical damage during and after the soldering process in the crystalline silicone solar cell. Soft ribbons with very low yield strength offer a possibility to limit the mechanical stress to a low level during the soldering process leading to less cracks, metallization damages and hot spot risk on cell level. Nevertheless the yield strength has to be addressed in alignment with ribbon thickness and width to achieve lowest possible damages. The aim of this work is to investigate the impact of the ribbons mechanical specification as yield strength and dimension on crystal damage, peel strength and metallization failure after soldering. In order to investigate the impact of additional stress and its effect on electrical performance thermo-cycling (TC) was performed. It was found that with larger yield strength the crystal damage strongly increases coming along with a strong reduction in peel strength and Pmpp. Furthermore the ribbon thickness and width could be identified as key parameters for reducing Pmpp losses during TC testing. Finite element simulations were performed to analyse the internal stresses evolution and gain further insight in mechanism leading to cell damage. These results are aligned with the experimental findings. With the results on hand the module maker can optimize the production process in terms of CTM loss and stress reduction.