Round‐robin measurement intercomparison of c‐Si PV modules among Asian testing laboratories

Because of recent advances in the production and installation of photovoltaic (PV) systems, the international conformity of PV module performance measurement has become increasingly important. The increase in PV production sites is particularly significant in the Asian region. The present paper summarizes and discusses the results of a round‐robin intercomparison of crystalline silicon modules among national laboratories and certified testing laboratories in the Asian region conducted from 2009 to 2011. Most of the values of P_max measured at the different laboratories were within a ±2% range, although some P_max results showed differences of up to about 3%. This result is comparable to that obtained in the recent intercomparison among international laboratories. Possible sources of difference in the measured values of I_sc, V_oc, FF, and P_max are discussed, for further improvement of international conformity in PV measurement technologies. Copyright © 2012 John Wiley & Sons, Ltd.     

Life‐cycle assessment of photovoltaic modules: Comparison of mc‐Si,InGaP and InGaP/mc‐Si solar modules

A higher conversion efficiency of photovoltaic modules does not automatically imply a lower environmental impact, when the life‐cycle of modules is taken into account. An environmental comparison is carried out between the production and use phase, except maintenance, of an indium–gallium–phosphide (InGaP) on multicrystalline silicon (mc‐Si) tandem module, a thin‐film InGaP cell module and a mc‐Si module. The evaluation of the InGaP systems was made for a very limited industrial production scale. Assuming a fourfold reuse of the GaAs substrates in the production of the thin‐film InGaP (half) modules, the environmental impacts of the tandem module and of the thin‐film InGaP module are estimated to be respectively 50 and 80% higher than the environmental impact of the mc‐Si module. The energy payback times of the tandem module, the thin‐film InGaP module and the mc‐Si module are estimated to be respectively 5.3, 6.3 and 3.5 years. There are several ways to improve the life‐cycle environmental performance of thin‐film InGaP cells, including improved materials efficiency in production and reuse of the GaAs wafer and higher energy efficiency of the metalorganic chemical vapour deposition process. Copyright © 2003 John Wiley & Sons, Ltd.     

Analysis of degradation mechanisms of crystalline silicon PV modules after 12 years of operation in Southern Europe

The long‐term reliability of photovoltaic modules is crucial to ensure the technical and economic viability of PV as a successful energy source. The analysis of degradation mechanisms of PV modules is key to ensure current lifetimes exceeding 25 years. This paper presents the results of the investigations carried out on the degradation mechanisms of a crystalline silicon PV installation of 2 kWp after 12 years of exposure in Málaga, Spain. The analysis was conducted by visual inspection, infrared thermography and electrical performance evaluation. By visual inspection, the most relevant defects in the modules were identified and ranked according to their frequency. The electrical performance was assessed by comparing the characteristic parameters of the individual modules, obtained by outdoor measurements at the start and end of the exposure period. The correlation of the visual defects and the shifts in the electrical parameters was analysed. The results presented show that glass weathering, delamination at the cell‐EVA interface and oxidation of the antireflective coating and the cell metallization grid were the most frequently occurring defects found. The total peak power loss, including the initial light induced degradation, was 11.5%, which corresponded almost totally to a loss in short‐circuit current. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparative life‐cycle energy payback analysis of multi‐junction a‐SiGe and nanocrystalline/a‐Si modules

Despite the publicity of nanotechnologies in high tech industries including the photovoltaic sector, their life‐cycle energy use and related environmental impacts are understood only to a limited degree as their production is mostly immature. We investigated the life‐cycle energy implications of amorphous silicon (a‐Si) PV designs using a nanocrystalline silicon (nc‐Si) bottom layer in the context of a comparative, prospective life‐cycle analysis framework. Three R&D options using nc‐Si bottom layer were evaluated and compared to the current triple‐junction a‐Si design, i.e., a‐Si/a‐SiGe/a‐SiGe. The life‐cycle energy demand to deposit nc‐Si was estimated from parametric analyses of film thickness, deposition rate, precursor gas usage, and power for generating gas plasma. We found that extended deposition time and increased gas usages associated to the relatively high thickness of nc‐Si lead to a larger primary energy demand for the nc‐Si bottom layer designs, than the current triple‐junction a‐Si. Assuming an 8% conversion efficiency, the energy payback time of those R&D designs will be 0.7–0.9 years, close to that of currently commercial triple‐junction a‐Si design, 0.8 years. Future scenario analyses show that if nc‐Si film is deposited at a higher rate (i.e., 2–3 nm/s), and at the same time the conversion efficiency reaches 10%, the energy‐payback time could drop by 30%. Copyright © 2010 John Wiley & Sons, Ltd.     

Low‐temperature formation of local Al contacts to a‐Si:H‐passivated Si wafers

We have passivated boron‐doped, low‐resistivity crystalline silicon wafers on both sides by a layer of intrinsic, amorphous silicon (a‐Si:H). Local aluminum contacts were subsequently evaporated through a shadow mask. Annealing at 210°C in air dissolved the a‐Si:H underneath the Al layer and reduces the contact resistivity from above 1 Ω cm² to 14·9 m Ω cm². The average surface recombination velocity is 124 cm/s for the annealed samples with 6% metallization fraction. In contrast to the metallized regions, no structural change is observed in the non‐metallized regions of the annealed a‐Si:H film, which has a recombination velocity of 48 cm/s before and after annealing. Copyright © 2004 John Wiley & Sons, Ltd.     

Dynamic Pixel Models for a‐Si TFT‐LCD and Their Implementation in SPICE

A dynamic analysis of an amorphous silicon (a‐Si) thin film transistor liquid crystal display (TFT‐LCD) pixel is presented using new a‐Si TFT and liquid crystal (LC) capacitance models for a Simulation Program with Integrated Circuit Emphasis (SPICE) simulator. This dynamic analysis will be useful when predicting the performance of LCDs. The a‐Si TFT model is developed to accurately estimate a‐Si TFT characteristics of a bias‐dependent gate to source and gate to drain capacitance. Moreover, the LC capacitance model is developed using a simplified diode circuit model. It is possible to accurately predict TFT‐LCD characteristics such as flicker phenomena when implementing the proposed simulation model.     

A practical method for the energy rating of c‐Si photovoltaic modules based on standard tests

The performance of a photovoltaic module at Standard Test Conditions (STC) is valuable for comparing the peak performance of different module types. It does not, however, give enough information to accurately predict how much energy a module will deliver when subjected to real operating conditions. There are several proposals for an energy rating for PV modules which attempt to account for the varying operating conditions that one encounters in the field. In this paper, we present an approach with the emphasis on simplicity and practicality that incorporates existing standard measurements to determine the energy output as a function of global in‐plane irradiance and ambient temperature. The method is applied to crystalline Si modules and tested with outdoor measurements, and a good accuracy of prediction of energy production is observed. Finally, a proposal is made for a simple Energy Rating labeling of PV modules. Copyright © 2005 John Wiley & Sons, Ltd.     

Preparation and measurement of highly efficient a‐Si:H single junction solar cells and the advantages of μc‐SiOx:H n‐layers

Reducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc‐SiO_x:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc‐SiO_x:H n‐layers to a‐Si:H single junction solar cells. We report on the comparison between amorphous silicon (a‐Si:H) single junction solar cells with either μc‐SiO_x:H n‐layers or non‐alloyed silicon n‐layers. The origin of the improved performance of a‐Si:H single junction solar cells with the μc‐SiO_x:H n‐layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc‐SiO_x:H material. The solar cell parameters of a‐Si:H solar cells with both types of n‐layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a‐Si:H single junction solar cell with a μc‐SiO_x:H n‐layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.     

Polycrystalline silicon PV modules performance and degradation over 20 years

The presented paper reports the results of the experimental work performed at the European Solar Test Installation, using an array of 70 polycrystalline silicon photovoltaic (PV) modules by the same manufacturer. After almost 20 years of continuous outdoor exposure, the modules were subjected to a comprehensive indoor test plan; in particular, electrical performance measurements were performed, together with a detailed visual analysis. It was also possible to perform a comparison between final and initial data (in particular IV characteristics): module average performance decay is 4.42% for the whole period. Degradation mechanisms, together with their effect on module lifetime, were also analyzed. Results of such a measurement exercise clearly show how PV device reliability over decades can guarantee safe investments, for the benefit of all PV users and stakeholders. The authors are currently installing the modules for further 20 years of outdoor exposure. Copyright © 2012 John Wiley & Sons, Ltd.     

A comparative study on cost and life‐cycle analysis for 100 MW very large‐scale PV (VLS‐PV) systems in deserts using m‐Si,a‐Si,CdTe, and CIS modules

This paper is a study of comparisons between five types of 100 MW Very Large‐Scale Photovoltaic Power Generation (VLS‐PV) Systems, from economic and environmental viewpoints. The authors designed VLS‐PV systems using typical PV modules of multi‐crystalline silicon (12·8% efficiency), high efficiency multi‐crystalline silicon (15·8%), amorphous silicon (6·9%), cadmium tellurium (9·0%), and copper indium selenium (11·0%), and evaluated them by Life‐Cycle Analysis (LCA). Cost, energy requirement, and CO₂ emissions were calculated. In addition, the authors evaluated generation cost, energy payback time (EPT), and CO₂ emission rates. As a result, it was found that the EPT is 1·5–2·5 years and the CO₂ emission rate is 9–16 g‐C/kWh. The generation cost was 11–12 US Cent/kWh on using 2 USD/W PV modules, and 19–20 US Cent/kWh on using 4 USD/W PV module price. Copyright © 2007 John Wiley & Sons, Ltd.     

C‐Si solar cell modules

In order to meet the rapidly growing demand for solar power photovoltaic systems which is based on public consciousness of global environmental issues, SHARP has increased the production of solar cells and modules over 10‐fold in the last 5 years. Silicon‐based technologies are expected to be dominant in the coming decade. In the course of an increase of the annual production scale to 1000 MW, the efficiency of modules will be improved and the thickness of wafers will be decreased and all this will lead to a drastic price reduction of PV systems. Copyright © 2005 John Wiley & Sons, Ltd.     

High potential of thin (<1 µm) a‐Si: H/µc‐Si:H tandem solar cells

Silicon based thin tandem solar cells were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a 30 × 30 cm² reactor. The layer thicknesses of the amorphous top cells and the microcrystalline bottom cells were significantly reduced compared to standard tandem cells that are optimized for high efficiency (typically with a total absorber layer thickness from 1.5 to 3 µm). The individual absorber layer thicknesses of the top and bottom cells were chosen so that the generated current densities are similar to each other. With such thin cells, having a total absorber layer thickness varying from 0.5 to 1.5 µm, initial efficiencies of 8.6–10.7% were achieved. The effects of thickness variations of both absorber layers on the device properties have been separately investigated. With the help of quantum efficiency (QE) measurements, we could demonstrate that by reducing the bottom cell thickness the top cell current density increased which is addressed to back‐reflected light. Due to a very thin a‐Si:H top cell, the thin tandem cells show a much lower degradation rate under continuous illumination at open circuit conditions compared to standard tandem and a‐Si:H single junction cells. We demonstrate that thin tandem cells of around 550 nm show better stabilized efficiencies than a‐Si:H and µc‐Si:H single junction cells of comparable thickness. The results show the high potential of thin a‐Si/µc‐Si tandem cells for cost‐effective photovoltaics. Copyright © 2010 John Wiley & Sons, Ltd.     

The effects of spectral evaluation of c‐Si modules

Outdoor spectral measurements in sub‐Sahara, South Africa in particular have not been documented probably due to lack of data or lack of proper methodologies for quantifying the spectral effects on photovoltaic performance parameters. Crystalline‐Si modules are widely used for system designs in most cases based on the data provided from indoor measurements or from maritime northern hemispheric conditions. As a result of this, PV systems fail to deliver their intended maximum power output. In this study, a methodology for quantifying outdoor spectral effects of c‐Si modules commonly found in the African continent is presented. The results of three crystalline‐Si modules indicate that these modules are affected as the spectrum shifts during seasons although these devices are perceived (without outdoor data) that their performance is not influenced by the seasonal changes in outdoor spectrum. Copyright © 2010 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.     

Current transport studies of amorphous n/p junctions and its application in a‐Si:H/HIT‐type tandem cells

This paper presents an understanding of the fundamental carrier transport mechanism in hydrogenated amorphous silicon (a‐Si:H)‐based n/p junctions. These n/p junctions are, then, used as tunneling and recombination junctions (TRJ) in tandem solar cells, which were constructed by stacking the a‐Si:H‐based solar cell on the heterojunction with intrinsic thin layer (HIT) cell. First, the effect of activation energy (E_a) and Urbach parameter (E_u) of n‐type hydrogenated amorphous silicon (a‐Si:H(n)) on current transport in an a‐Si:H‐based n/p TRJ has been investigated. The photoluminescence spectra and temperature‐dependent current–voltage characteristics in dark condition indicates that the tunneling is the dominant carrier transport mechanism in our a‐Si:H‐based n/p‐type TRJ. The fabrication of a tandem cell structure consists of an a‐Si:H‐based top cell and an HIT‐type bottom cell with the a‐Si:H‐based n/p junction developed as a TRJ in between. The development of a‐Si:H‐based n/p junction as a TRJ leads to an improved a‐Si:H/HIT‐type tandem cell with a better open circuit voltage (V_oc), fill factor (FF), and efficiency. The improvements in the cell performance was attributed to the wider band‐tail states in the a‐Si:H(n) layer that helps to an enhanced tunneling and recombination process in the TRJ. The best photovoltage parameters of the tandem cell were found to be V_oc = 1430 mV, short circuit current density = 10.51 mA/cm², FF = 0.65, and efficiency = 9.75%. Copyright © 2015 John Wiley & Sons, Ltd.     

Seasonal power fluctuations of amorphous silicon thin‐film solar modules: distinguishing between different contributions

Several works report on seasonal fluctuations of power production of amorphous silicon (a‐Si). These oscillations are due to two overlapping phenomena (i) spectral and (ii) the Staebler–Wronski effects. It is hence difficult to assess—for a given location and climatic conditions—which one has the largest impact. By means of a straightforward approach based on two sets of single‐junction a‐Si photovoltaic modules (stored indoors/exposed outdoors) and on two different I–V measurement set‐ups (indoor and outdoor), we were able to separate the different contributions to this phenomenon. For the test‐site of Lugano, seasonal oscillations account for performance variations of a‐Si of ~10% (±5% around an annual average value with a minimum around the mid of January and a maximum around mid‐July). The time‐phase of the overall effect lies in between that of the two distinguished phenomena. (i) Spectral variations seem to have the highest impact on the outdoor performance of a‐Si with an amplitude corresponding to 10.5% (± ~5.2%). Moreover, the influence of spectral variations on the outdoor performance of a‐Si (and for comparison of c‐Si) was modeled, and the experimental data were found to be in excellent agreement with the theoretical simulation; (ii) the Staebler–Wronski effect has a slightly lower influence with an amplitude of ~8% (±4% with a minimum at the middle of February and a maximum around mid‐August). Because of the position (46°N) and average climatic conditions (southern Alpine climate) of Lugano, these observations are possibly representative of a large part of continental Europe. Copyright © 2012 John Wiley & Sons, Ltd.     

The results of performance measurements of field‐aged crystalline silicon photovoltaic modules

This paper presents the results of electrical performance measurements of 204 crystalline silicon‐wafer based photovoltaic modules following long‐term continuous outdoor exposure. The modules comprise a set of 53 module types originating from 20 different producers, all of which were originally characterized at the European Solar Test Installation (ESTI), over the period 1982–1986. The modules represent diverse generations of PV technologies, different encapsulation and substrate materials. The modules electrical performance was determined according to the standards IEC 60891 and the IEC 60904 series, electrical insulation tests were performed according to the recent IEC 61215 edition 2. Many manufacturers currently give a double power warranty for their products, typically 90% of the initial maximum power after 10 years and 80% of the original maximum power after 25 years. Applying the same criteria (taking into account modules electrical performance only and assuming 2·5% measurement uncertainty of a testing lab) only 17·6% of modules failed (35 modules out of 204 tested). Remarkably even if we consider the initial warranty period i.e. 10% of P_max after 10 years, more than 65·7% of modules exposed for 20 years exceed this criteria. The definition of life time is a difficult task as there does not yet appear to be a fixed catastrophic failure point in module ageing but more of a gradual degradation. Therefore, if a system continues to produce energy which satisfies the user need it has not yet reached its end of life. If we consider this level arbitrarily to be the 80% of initial power then all indications from the measurements and observations made in this paper are that the useful lifetime of solar modules is not limited to the commonly assumed 20 year. Copyright © 2008 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.     

High‐rate deposition of a‐Si:H thin layers for high‐performance silicon heterojunction solar cells

In this paper, we describe a technique for high‐quality interface passivation of n‐type crystalline silicon wafers through the growth of hydrogenated amorphous Si (a‐Si:H) thin layers using conventional plasma‐enhanced chemical vapor deposition. We investigated the onset of crystallization of the a‐Si:H layers at various deposition rates and its effect on the surface passivation properties. Epitaxial growth occurred, even at a low substrate temperature of 90 °C, when the deposition rate was as low as 0·5 Å/s; amorphous growth occurred at temperatures up to 150 °C at a higher deposition rate of 4·2 Å/s. After optimizing the intrinsic a‐Si:H layer deposition conditions and then subjecting the sample to post‐annealing treatment, we achieved a very low surface recombination velocity (7·6 cm/s) for a double‐sided intrinsic a‐Si:H coating on an n‐type crystalline silicon wafer. Under the optimized conditions, we achieved an untextured heterojunction cell efficiency of 16·7%, with a high open‐circuit voltage (694 mV) on an n‐type float‐zone Si substrate. On a textured wafer, the cell efficiency was further enhanced to 19·6%. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of initial power stabilization over crystalline‐Si photovoltaic modules maximum power

Measurements that suppliers offer in specification sheets are not always close to the actual power measured in independent laboratories such as CIEMAT. Independent measurements tend to be lower than those printed on the label sometimes even lower than the allowed tolerance indicated by the manufacturer on the same label. Furthermore, a potentially significant power reduction has been reported when Standard EN50380 (which requires photovoltaic (PV) modules to be exposed to more than 20 kWh/m² of sunlight prior to taking the measurements that appear on the label) is followed. This is the initial power stabilization and this work studies the power stabilization that tends to appear in crystalline PV modules. Crystalline PV modules usually decrease in power around 1%, but decreases >4% have also been reported. These power losses are only detected after the mentioned power stabilization. Copyright © 2010 John Wiley & Sons, Ltd.