Characterization of degradation in thin-film photovoltaic module performance parameters

This paper characterizes and compares the degradation observed in thin-film module performance. Three commercially available thin-film modules comprising a-Si:H, a-Si:H/a-SiGe:H/a-SiGe:H and CuInSe₂ technologies were used in this study. After an initial indoor assessment the modules were deployed outdoors and periodically taken down for indoor assessment. Results obtained indicate that the a-Si modules degraded by the classical Staebler–Wronski effect. The CuInSe₂ module, though known to have long-term performance stability, also degraded in this study. The CuInSe₂ module showed shunting behaviour before outdoor exposure. This shunting behaviour was enhanced when the module was deployed outdoors under open-circuit conditions. A comparison of the modules’ performances outdoors indicates that the low bandgap CuInSe₂ material performs best at high air mass values. This paper emphasizes the importance of being able to analyze module degradation.     

Thermal stability of slow and fast cure EVA encapsulant material for photovoltaic module manufacturing process

Ethylene vinyl acetate (EVA) is a copolymer encapsulant used as an interlayer in the industrial photovoltaic module encapsulation process. EVA serves to provide the functions of structural support, electrical isolation, physical isolation/protection and thermal conduction for the solar cell circuit. This paper focuses on thermal properties of slow and fast cure EVA encapsulant material. Phase transition and thermal stability parameters of uncured EVA materials have been studied by employing thermo gravimetric analysis, thermal differential analysis and differential scanning calorimetric. EVA fast cure encapsulant material exhibits superior thermal stability in the range of operational cell temperature.     

Pre-deployment evaluation of amorphous silicon photovoltaic modules

In this study, the indoor evaluation of amorphous silicon modules was conducted using extended visual inspection and various electrical characterisation tools. The visual inspection, which included low-magnification optical microscopy, revealed several defects resulting from physical damage and bad scribing. These defects, as well as poor material quality, are likely contributors to the degradation in performance observed during the measurement of current–voltage characteristics under standard conditions, as well as at different temperature and irradiance levels. The observed degradation is carefully analysed in this paper.     

Investigation of delamination in an edge-defined film-fed growth photovoltaic module

In this study, we have carefully analysed the performance degradation in an edge-defined film-fed growth (EFG) module due to delamination and moisture ingress. Our results showed that this degradation is directly related to the delamination as well as the moisture ingress. We also found that during hot, dry periods (December–January), there is a small reversal in degradation, which is due to the regression of the moisture ingress. The presence of moisture in the delaminated regions has led to the deterioration in cell interconnects. This is observed by the increase in series resistance with time.     

Changes in the temperature coefficients of the characteristics of amorphous silicon solar cells subjected to light degradation and recovery

Measuring changes in temperature coefficients is an effective way to estimate the performance of a solar cell. We investigated changes in the temperature coefficients of the I–V characteristics of amorphous silicon (a-Si) solar cells subjected to light degradation and recovery. There is a good correlation between change in the temperature coefficient (Ψ) and the degradation/recovery state of a cell’s conversion efficiency (η). This relationship can be expressed by Ψ_η=−0.0052Δη−0.45. Therefore, the temperature coefficient corresponding to the degradation/recovery state can be estimated.     

Flexible amorphous and microcrystalline silicon tandem solar modules in the temporary superstrate concept

Encapsulated and series-connected amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) based thin film silicon solar modules were developed in the superstrate configuration using an aluminum foil as temporary substrate during processing and a commodity polymer as permanent substrate in the finished module. For the development of μc-Si:H single junction modules, aspects regarding TCO conductivity, TCO reduction, deposition uniformity, substrate temperature stability and surface morphology were addressed. It was established that on sharp TCO morphologies where single junction μc-Si:H solar cells fail, tandem structures consisting of an a-Si:H top cell and a μc-Si:H bottom cell can still show a good performance. Initial aperture area efficiencies of 8.2%, 3.9% and 9.4% were obtained for fully encapsulated amorphous silicon (a-Si:H) single junction, microcrystalline silicon (μc-Si:H) single junction and a-Si:H/μc-Si:H tandem junction modules, respectively.     

Efficient hybrid organic/inorganic photovoltaic cells utilizing n-type pentacene and intrinsic/p-type hydrogenated amorphous silicon

We demonstrate efficient hybrid inorganic/organic p-i-n photovoltaic (PV) devices with a p-type-doped hydrogenated amorphous silicon (a-Si:H), intrinsic a-Si:H, and an organic semiconductor, pentacene. The correlation between the electrical properties of the PV devices and the morphological properties of the pentacene films were investigated using absorption spectroscopy, X-ray diffraction, and scanning electron microscopy. The maximum power conversion efficiency can be increased by one order with respect to the devices using different thicknesses of a pentacene layer from 0.32% at 10 nm to above 3.0% at 30 nm. Photocarriers in PVs are suggested to be mainly generated in the intrinsic a-Si:H layer. The pentacene layer is used as the exciton-blocking and electron-transport layer. Thus, the structural quality of pentacene films plays an important role in PV performance.     

A critical appraisal of the factors affecting energy production from amorphous silicon photovoltaic arrays in a maritime climate

Contradictory reports exist in the literature regarding the energy production from amorphous silicon photovoltaic arrays. The majority claims high-energy output compared to crystalline silicon arrays of the same power rating (i.e. high kW h/kW_p), but some reports point to less favourable comparisons. The reasons for these conflicting reports are investigated using long-term measurements of the I–V characteristics of a number of amorphous silicon devices, in conjunction with in situ measurements of the solar spectrum and other relevant environmental parameters. It is shown that the variation in the performance of devices produced by different manufacturers is so significant that one cannot speak of the performance of amorphous silicon devices in general; one has to investigate each type of amorphous silicon panel separately.

The causes of differences in energy production are investigated in detail. The major factor impacting on the seasonal performance in the UK is identified to be variations in the solar spectrum. Single junction devices exhibit some seasonal thermal annealing but multi-junctions do not show this effect at a significant level.

Scope for further improvement is identified, largely in the photon absorption. The response to different spectra can be modified to some extent, which would bridge the gap between the best and the worst performers in the field. It is also shown that in the case of multi-junction devices an optimised current matching might bring a 5% increase in energy production for this location. Differences in the magnitude of the fill factor have been identified to be the second most significant cause for performance variation between the different samples in the test, suggesting additional scope for improvement.     

Determination of thermal properties of crosslinked EVA encapsulant material in outdoor exposure by TSC and DSC methods

Apart from chemical structure, thermal properties of polymers depend mainly on polymer morphology, which is affected by the processing conditions. Outdoor exposure of the materials leads to changes in polymer morphology. The main objective of this experimental investigation was to better understand the changes due to thermal transitions and the molecular organizations of the crosslinked EVA encapsulant material after aging in outdoor exposure. The thermal properties of unaged and aged EVA encapsulant material were measured by thermal analysis methods as TSC and DSC. For the aged EVA samples, the distinctive feature of these results is that there are two different endothermic processes due to the recrystallization phenomenon. Furthermore, the difference of the magnitude of peak current by TSC technique suggests increased crosslinking exposure occurring selectively in the high temperature phase as a result of outdoor exposure.     

Optical modeling of finely textured amorphous silicon solar cells

It has long been known that the use of finely textured transparent conducting oxide layers substantially improves the performance of thin film amorphous silicon (a-Si:H) solar cells. Major efforts to understand the nature of this effect and to fully capture its potential have been made by researchers using advanced modeling techniques. In this work, modeling the oblique angle optical performance and use of an effective medium approximation to simulate microrough interfaces suggests that effective interface grading makes a significant contribution to optical enhancement.     

Practical application of building integrated photovoltaic (BIPV) system using transparent amorphous silicon thin-film PV module

An analysis has been carried out on the first practical application in Korea of the design and installation of building integrated photovoltaic (BIPV) modules on the windows covering the front side of a building by using transparent thin-film amorphous silicon solar cells. This analysis was performed through long-term monitoring of performance for 2 years. Electrical energy generation per unit power output was estimated through the 2 year monitoring of an actual BIPV system, which were 48.4 kWh/kWp/month and 580.5 kWh/kWp/year, respectively, while the measured energy generation data in this study were almost half of that reported from the existing data which were derived by general amorphous thin-film solar cell application. The reason is that the azimuth of the tested BIPV system in this study was inclined to 50° in the southwest and moreover, the self-shade caused by the projected building mass resulted in the further reduction of energy generation efficiency. From simulating influencing factors such as azimuth and shading, the measured energy generation efficiency in the tested condition can be improved up to 47% by changing the building location in terms of azimuth and shading, thus allowing better solar radiation for the PV module. Thus, from the real application of the BIPV system, the installation of a PV module associated with azimuth and shading can be said to be the essentially influencing factors on PV performance, and both factors can be useful design parameters in order to optimize a PV system for an architectural BIPV application.     

Non-destructive degradation analysis of encapsulants in PV modules by Raman Spectroscopy

Raman Probe Spectroscopy as a powerful tool for analyzing the degradation behavior of encapsulants in c-Si based PV modules is reported. A non-destructive and quick testing method is needed in order to follow material changes during the aging of PV modules. Two types of c-Si PV modules with ethylene vinyl acetate (EVA) encapsulation have been aged indoors under damp-heat conditions (85% r.h./85 °C) and under combined UV/moisture conditions, respectively. The aged modules as well as a non-aged reference module for each type were analyzed by Raman Spectroscopy. The degradation of the encapsulant was observed, resulting in an increasing fluorescence background, as well as changing intensities of EVA Raman peaks. A lateral non-uniformity of the fluorescence intensity of and the EVA CH stretching vibration intensity ratios, depending on the position above the cell as well as from the aging condition, could be observed and it might be an indicator for the diffusion of water in the encapsulant.     

Development of photovoltaic modules integrated with a metal curtain wall

New type photovoltaic (PV) modules integrated with a metal curtain wall have been developed. In this research, experimental walls (Yoshino et al., Proc. 1st World Conf. on Photovoltaic Energy Conversion, Hawaii, 1994, pp. 969–972) were made in 1994 for evaluating the energy output performance of the PV modules. Basic data on output characteristics and temperature were obtained accordingly under different PV submodule installation conditions. Based on these fundamental data, the demonstration wall was designed and constructed. From the results of examinations, PV submodule installation conditions, the annual energy output, and the temperature rise depression effect are described in this paper. Major contents of the module specifications, the construction method, and results of construction material performance tests are also described.     

Analysis of the temperature history of amorphous silicon photovoltaic module outdoors

To analyze the effects of the module temperature and irradiance on photovoltaic (PV) modules outdoors, contour graphs were made for the performance ratio (PR) of single-crystalline Si (sc-Si) and amorphous Si (a-Si) PV modules. The result shows that the PR of the sc-Si PV module decreases with increase in module temperature. In contrast, the PR of the a-Si PV module shows unique temperature dependence influenced by temperature history. Dependence of environmental factors of the PR of the a-Si PV module was analyzed in almost the same temperature seasons, i.e., Spring and Fall. The PR in Fall was higher than that in Spring. The result shows that the recovered performance in summer may continue for Fall. The contour graph of remainder of PR in Fall and Spring was made. The environmental condition that the PR in Fall was higher was clarified.     

Electrode distance dependence of photo-induced degradation in hydrogenated amorphous silicon

Electrode distance between cathode and anode is one of the important parameters for fabricating hydrogenated amorphous silicon (a-Si:H) using plasma-enhanced chemical-vapor-deposition system with parallel plate electrodes. In this work, we have investigated the relationship between electrode distance and stability of a resulting a-Si:H. The stability is improved with decreasing electrode distance. At shorter electrode distance, formation of higher silane-related-reactive species is suppressed by heating effect of gas molecules near the cathode due to a proximity to the heated anode. Using cathode heating method, the stability of a-Si:H is improved even at long electrode distances.     

Preparation of very stable and low hydrogen content amorphous silicon films by hydrogen-radical CVD method

High-quality a-Si : H films have been prepared by a hydrogen-radical CVD method. Films with high photosensitivity and high stability against light illumination could be prepared from both Si₂H₆ and SiH₄. It was found that hydrogen content is considerably low compared with films prepared by other deposition methods. These results suggest that hydrogen radicals contribute to the improvement of film properties.     

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.     

Effects of spectral variation on the device performance of copper indium diselenide and multi-crystalline silicon photovoltaic modules

We present results of an experimental investigation of the effects of the daily spectral variation on the device performance of copper indium diselenide and multi-crystalline silicon photovoltaic modules. Such investigations are of importance in characterization of photovoltaic devices. The investigation centres on the analysis of outdoor solar spectral measurements carried out at 10 min intervals on clear-sky days. We have shown that the shift in the solar spectrum towards infrared has a negative impact on the device performance of both modules. The spectral bands in the visible region contribute more to the short circuit current than the bands in the infrared region while the ultraviolet region contributes least. The quantitative effects of the spectral variation on the performance of the two photovoltaic modules are reflected on their respective device performance parameters. The decrease in the visible and the increase in infrared of the late afternoon spectra in each case account for the decreased current collection and hence power and efficiency of both modules.     

Computer simulation of the effect of phosphorous doping of the i-layer in a thin-film a-Si:H p–i–n solar cell

The simulation RAUPV2 has been used to model a thin-film a-Si:H p–i–n solar cell, fabricated at the Rand Afrikaans University. For a physically acceptable set of input parameters, the simulated J–V curve agrees very well with the empirical J–V curve, under AM1.5 g illumination. The effect of boron- and phosphorous doping of the i-layer (B- and P-profiling) was studied. It was found that boron doping of the i-layer greatly reduced cell performance. On the other hand, there seemed to be an optimal phosphorous concentration in the i-layer, P_opt, for which cell performance, measured in terms of maximum power output, was a maximum. It was observed that as the P concentration in the i-layer was increased towards P_opt, the recombination rate in the front of the i-layer decreased, whilst that in the back part of the i-layer increased. The short-circuit current was seen to decrease under P-profiling. It was seen that as a consequence of P-profiling, the drift field in the back part of the i-layer was relatively insensitive to the effect of an applied voltage, for applied voltages up to about 0.55 V.