Simulation of losses in thin‐film silicon modules for different configurations and front contacts

A simulation tool for the quantification of electrical losses in thin‐film modules using a one‐ and two‐dimensional electrical PSpice model is presented. Two main sources of electrical losses are examined: monolithic contacts (MC) and front contacts made of a transparent conductive oxide (TCO) layer with or without a metal finger grid. Our study was focussed on amorphous and micromorph silicon modules in substrate or superstrate configuration. Results show that front contact losses (TCO losses and finger losses) prevail. While, under assumption that their subcell performances are the same, performance of amorphous silicon (a‐Si) modules do not depend on the configuration, the superstrate micromorph silicon module has a relatively slight (below 2%) advantage over the substrate counterpart due to lower electrical losses in the MC. Losses of the front contact made of a thick TCO layer or of thin TCO layer and metal finger grid on top were studied for both modules in substrate configuration and optimisation results are presented. Use of thin TCO layer and optimised finger grid and solar cell geometry is competitive and these modules can even outperform the optimised amorphous or micromorph silicon module with thick TCO front contact. In all optimised cases under standard test conditions, total relative losses can be minimised to around 10%. Copyright © 2008 John Wiley & Sons, Ltd.     

A thin‐film silicon solar cell and module

We have developed a new light‐trapping scheme for a thin‐film Si stacked module (Si HYBRID PULS module), where a (a‐Si:H/transparent interlayer/microcrystalline Si) thin‐film was integrated into a large‐area solar cell module. An initial aperture efficiency of 13·1% has been achieved for a 910 × 455 mm Si HYBRID PLUS module, which was independently confirmed by AIST. This is the first report of the independently confirmed efficiency of a large‐area thin‐film Si module with an interlayer. The 19% increase of short‐circuit current of this module was obtained by the introduction of a transparent interlayer that caused internal light‐trapping. A mini‐module was shown to exhibit a stabilized efficiency of 12%. Outdoor performance of a Si HYBRID (a‐Si:H / micro‐crystalline Si stacked) solar cell module has been investigated for over 4 years with two different kinds of module (top and bottom cell limited, respectively). The HYBRID modules limited by the top cell have exhibited a more efficient performance than the modules limited by the bottom cell, in natural sunlight at noon. Copyright © 2005 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.     

An alternative method for spectral response measurements of large‐area thin‐film photovoltaic modules

A new method for the spectral response measurement of large‐area single and multi‐junction thin‐film photovoltaic modules is presented, making use of a chopped monochromatic beam produced from a continuous source with band pass filters and lock‐in technique. The beam is projected onto part of the test module and superimposed over continuous bias light of variable colour. The procedure for the determination of the absolute spectral response is presented, and the influence of the intrinsic non‐uniformity of the monochromatic beam is investigated. The results obtained are compared with those from two other methods of spectral response measurement, providing a validation of the proposed experimental setup. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrically conductive anti‐reflecting nanostructure for chalcogenide thin‐film solar cells

Electrically conducting aluminum (Al)‐doped ZnO nanorods (NRs) film has been introduced as an anti‐reflective (AR) layer for effective light trapping in chalcogenide thin‐film solar cells. Results indicate that the Al‐doping significantly reduced the electrical contact resistance between the Ag top electrode and the AR layer. The Al‐doped ZnO NRs exhibited low average reflectance (4.5%) over the entire visible and near‐infrared range, and changed the nature of electrical contact between the Ag electrode and the AR layer from Schottky to Ohmic. Finally, the CuInS₂ solar cell coated with the Al‐doped ZnO NRs exhibited huge enhancement in photovoltaic efficiency from 9.57% to 11.70% due to the lowering series resistance and the increase in the short‐circuit current density, when compared with that of a solar cell without the AR layer. Copyright © 2014 John Wiley & Sons, Ltd.     

3‐D optical modeling of thin‐film silicon solar cells on diffraction gratings

An effective rigorous 3‐D optical modeling of thin‐film silicon solar cells based on finite element method (FEM) is presented. The simulation of a flat single junction thin‐film silicon solar cell on thick glass (i.e., superstrate configuration) is used to validate a commercial FEM‐based package, the High Frequency Structure Simulator (HFSS). The results are compared with those of the reference software, Advanced Semiconductor Analysis (ASA) program, proving that the HFSS is capable of correctly handling glass as an incident material within very timely, short, and numerically stable calculations. By using the HFSS, we simulated single junction thin‐film silicon solar cells on glass substrates textured with one‐dimensional (1‐D) and two‐dimensional (2‐D) trapezoid‐shaped diffraction gratings. The correctness of the computed results, with respect to an actual device, is discussed, and the impact of different polarizations on spectral response and optical losses is examined. From the simulations carried out, optimal combinations for period and height in both 1‐D and 2‐D grating configurations can be indicated, leading to short‐circuit current percentage increase with respect to a flat cell of, respectively, 25.46% and 32.53%. With very limited computer memory usage and computational time in the order of tens of minutes for a single simulation, we promote the usage of 3‐D FEM as a rigorous and efficient way to simulate thin‐film silicon solar cells. Copyright © 2012 John Wiley & Sons, Ltd.     

Life cycle assessment of thin‐film GaAs and GaInP/GaAs solar modules

This paper presents an environmental comparison based on life cycle assessment (LCA) of the production under average European circumstances and use in The Netherlands of modules based on two kinds of III–V solar cells in an early development stage: a thin‐film gallium arsenide (GaAs) cell and a thin‐film gallium‐indium phosphide/gallium arsenide (GaInP/GaAs) tandem cell. A more general comparison of these modules with the common multicrystalline silicon (multi‐Si) module is also included. The evaluation of the both III–V systems is made for a limited industrial production scale of 0·1 MW_p per year, compared to a scale of about 10 MW_p per year for the multi‐Si system. The here considered III–V cells allow for reuse of the GaAs wafers that are required for their production. The LCA indicates that the overall environmental impact of the production of the III–V modules is larger than the impact of the common multi‐Si module production; per category their scores have the same order of magnitude. For the III–V systems the metal‐organic vapour phase epitaxy (MOVPE) process is the main contributor to the primary energy consumption. The energy payback times of the thin‐film GaAs and GaInP/GaAs modules are 5·0 and 4·6 years, respectively. For the multi‐Si module an energy payback time of 4·2 years is found. The results for the III–V modules have an uncertainty up to approximately 40%. The highly comparable results for the III–V systems and the multi‐Si system indicate that from an environmental point of view there is a case for further development of both III–V systems. Copyright © 2006 John Wiley & Sons, Ltd.     

Corrosion effects in thin‐film photovoltaic modules

Electrochemical corrosion effects can occur in thin‐film photovoltaic (PV) modules that are fabricated on tin‐oxide‐coated glass when operating at high voltages and at elevated temperatures in a humid climate. The current study shows that this corrosion is associated with a delamination of the tin oxide layer from the glass, which is caused by sodium accumulation near the interface between the tin oxide and the glass and by the ingression of moisture into the PV module from the edges. This corrosion in thin‐film PV modules can be significantly reduced by altering the growth conditions of the tin oxide or by using zinc oxide as a transparent conductive oxide electrode. Copyright © 2003 John Wiley & Sons, Ltd.     

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

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

Interdigitated back‐contact heterojunction solar cell concept for liquid phase crystallized thin‐film silicon on glass

We present an interdigitated back‐contact silicon heterojunction system designed for liquid‐phase crystallized thin‐film (~10 µm) silicon on glass. The preparation of the interdigitated emitter (a‐Si:H(p)) and absorber (a‐Si:H(n)) contact layers relies on the etch selectivity of doped amorphous silicon layers in alkaline solutions. The etch rates of a‐Si:H(n) and a‐Si:H(p) in 0.6% NaOH were determined and interdigitated back‐contact silicon heterojunction solar cells with two different metallizations, namely Al and ITO/Ag electrodes, were evaluated regarding electrical and optical properties. An additional random pyramid texture on the back side provides short‐circuit current density (j_SC) of up to 30.3 mA/cm² using the ITO/Ag metallization. The maximum efficiency of 10.5% is mainly limited by a low of fill factor of 57%. However, the high j_SC, as well as V_OC values of 633 mV and pseudo‐fill factors of 77%, underline the high potential of this approach. Copyright © 2015 John Wiley & Sons, Ltd.     

Wire bonding as a cell interconnection technique for polycrystalline silicon thin‐film solar cells on glass

The interconnection of solar cells is a critical part of photovoltaic module fabrication. In this paper, a high‐yield, low‐cost method for interconnecting polycrystalline silicon thin‐film solar cells on glass is presented. The method consists of forming adjacent, electrically isolated groves across the cells using laser scribing, and then forming wire bonds over each laser scribe, resulting in series interconnection of the individual solar cells. Wire bonds are also used to connect the first and last solar cell in the string to external (tabbing) leads, forming a mini‐module. A layer of white paint is then applied, which acts as both an encapsulation layer and an additional back surface reflector. Using this method, an 8·3% efficient mini‐module has been fabricated. By exploiting recent developments in wire bonding technology, it appears that this process can be automated and will be capable of forming solar cell interconnections on large‐area modules within relatively short processing times (∼10 min for a 1 m² module). Copyright © 2010 John Wiley & Sons, Ltd.     

Broadband light trapping with disordered photonic structures in thin‐film silicon solar cells

We theoretically investigate light trapping with disordered 1D photonic structures in thin‐film crystalline silicon solar cells. The disorder is modelled in a finite‐size supercell, which allows the use of rigorous coupled‐wave analysis to calculate the optical properties of the devices and the short‐circuit current density J_sc. The role of the Fourier transform of the photonic pattern in the light trapping is investigated, and the optimal correlation between size and position disorder is found. This result is used to optimize the disorder in a more effective way, using a single parameter. We find that a Gaussian disorder always enhances the device performance with respect to the best ordered configuration. To properly quantify this improvement, we calculate the Lambertian limit to the absorption enhancement for 1D photonic structures in crystalline silicon, following the previous work for the 2D case [M.A. Green, Progr. Photovolt: Res. Appl. 2002; 10 (4), pp. 235–241]. We find that disorder optimization can give a relevant contribution to approach this limit. Finally, we propose an optimal disordered 2D configuration and estimate the maximum short‐circuit current that can be achieved, potentially leading to efficiencies that are comparable with the values of other thin‐film solar cell technologies. Copyright © 2013 John Wiley & Sons, Ltd.     

Optimization of CdTe thin‐film solar cell efficiency using a sputtered,oxygenated CdS window layer

A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high‐efficiency devices on tin oxide‐coated soda‐lime‐glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short‐circuit current. Meanwhile, fill factor is maintained, and open‐circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%‐efficient cell on 3.2‐mm soda‐lime glass. This window material was shown to be compatible with SnO₂‐based transparent conducting oxide and high resistance transparent coated substrates using in‐line compatible processes. Copyright © 2015 John Wiley & Sons, Ltd     

World‐record Cu(In,Ga)Se2‐based thin‐film sub‐module with 17.4% efficiency

We report a new certified world‐record efficiency for thin‐film Cu(In,Ga)Se₂‐based photovoltaic sub‐modules of 17.4% (aperture area). The record efficiency of the 16 cm², monolithically integrated, sub‐module has been independently confirmed by Fraunhofer ISE. The record device is the result of extensive co‐optimization of all processing steps. During the optimization process, strong focus has been put on the scalability of processes to cost‐effective mass production, as reflected, for example, in Cu(In,Ga)Se₂ deposition time and substrate temperature. Device manufacturing as well as results of electrical and material characterization is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel series connection concept for thin film solar modules

The paper introduces a new type of series connection that considerably increases the active area and thus the efficiency of a thin film module by a superior arrangement of the patterning grooves. The new concept is mainly based on pointwise contacts instead of continuous, stripe‐like contacts between adjacent cell stripes combined with a modified arrangement of the isolation grooves. We describe the functionality of the innovative series connection and present a method to calculate the optimal cell stripe geometry for cells based on the new series connection concept. Finally, the applicability of the new concept for a laser‐patterned thin film silicon solar module is demonstrated. The new series connection leads to a relative efficiency increase of approximately 3% compared with the standard series connection for thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.     

Improved performance in ZnO/CdS/CuGaSe2 thin‐film solar cells

We report the growth and characterization of improved efficiency wide‐bandgap ZnO/CdS/CuGaSe₂ thin‐film solar cells. The CuGaSe₂ absorber thickness was intentionally decreased to better match depletion widths indicated by drive‐level capacitance profiling data. A total‐area efficiency of 9·5% was achieved with a fill factor of 70·8% and a V_oc of 910 mV. Published in 2003 by John Wiley & Sons, Ltd.     

Through‐the‐glass spectroscopic ellipsometry for analysis of CdTe thin‐film solar cells in the superstrate configuration

Polycrystalline CdS/CdTe thin‐film solar cells in the superstrate configuration have been studied by spectroscopic ellipsometry (SE) using glass side illumination. In this measurement method, the first reflection from the ambient/glass interface is rejected, whereas the second reflection from the glass/film‐stack interface is collected; higher order reflections are also rejected. The SE analysis incorporates parameterized dielectric functions ε for solar cell component materials obtained by in situ and variable‐angle SE. In the SE analysis of the complete cells, a step‐wise procedure ranks the fitting parameters, including thicknesses and those defining the spectra in ε, according to their ability to reduce the root‐mean‐square deviation between the simulated and measured SE spectra. The best fit thicknesses from this analysis are found to be consistent with electron microscopy. Based on the SE results, the solar cell quantum efficiency (QE) can be simulated without any free parameters, and comparisons with measured QE enable optical model refinements as well as identification of optical and electronic losses. These capabilities have wide applications in photovoltaic module mapping and in‐line monitoring. Copyright © 2016 John Wiley & Sons, Ltd.     

Silicon thin‐film solar cells with rectangular‐shaped grating couplers

As an alternative to randomly textured transparent conductive oxides as front contact for thin‐film silicon solar cells the application of transparent grating couplers was studied. The grating couplers were prepared by sputtering of aluminium‐doped zinc oxide (ZnO) on glass substrate, a photolithography and a lift‐off process and were used as periodically textured substrates. The period size and groove depth of these transparent gratings were tuned independently from each other and varied between 1 and 4 μm and 100–600 nm. The optical properties of rectangular‐shaped gratings and the opto‐electronic behaviour of amorphous and microcrystalline silicon solar cells with integrated grating couplers as a function of the grating parameters (period size P and groove depth h_g) are presented. The optical properties of the gratings are discussed with respect to randomly textured substrates and the achieved solar cell results are compared with the opto‐electronic properties of solar cells deposited on untextured (flat) and randomly textured substrates. Copyright © 2005 John Wiley & Sons, Ltd.