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.     

>16% thin‐film epitaxial silicon solar cells on 70‐cm2 area with 30‐µm active layer,porous silicon back reflector,and Cu‐based top‐contact metallization

We demonstrate the use of a copper‐based metallization scheme for the specific application of thin‐film epitaxial silicon wafer equivalent (EpiWE) solar cells with rear chemical vapor deposition emitter and conventional POCl₃ emitter. Thin‐film epitaxial silicon wafer equivalent cells are consisting of high‐quality epitaxial active layer of only 30 µm, beneath which a highly reflective porous silicon multilayer stack is embedded. By combining Cu‐plating metallization and narrow finger lines with an epitaxial cell architecture including the porous silicon reflector, a J_sc exceeding 32 mA/cm² was achieved. We report on reproducible cell efficiencies of >16% on >70‐cm² cells with rear epitaxial chemical vapor deposition emitters and Cu contacts. Copyright © 2011 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.     

Autodiffusion: a novel method for emitter formation in crystalline silicon thin‐film solar cells

The in situ formation of an emitter in monocrystalline silicon thin‐film solar cells by solid‐state diffusion of dopants from the growth substrate during epitaxy is demonstrated. This approach, that we denote autodiffusion, combines the epitaxy and the diffusion into one single process. Layer‐transfer with porous silicon (PSI process) is used to fabricate n‐type silicon thin‐film solar cells. The cells feature a boron emitter on the cell rear side that is formed by autodiffusion. The sheet resistance of this autodiffused emitter is 330 Ω/□. An independently confirmed conversion efficiency of (14·5 ± 0·4)% with a high short circuit current density of (33·3 ± 0·8) mA/cm² is achieved for a 2 × 2 cm² large cell with a thickness of (24 ± 1) µm. Transferred n‐type silicon thin films made from the same run as the cells show effective carrier lifetimes exceeding 13 µs. From these samples a bulk diffusion length L > 111 µm is deduced. Amorphous silicon is used to passivate the rear surface of these samples after the layer‐transfer resulting in a surface recombination velocity lower than 38 cm/s. Copyright © 2006 John Wiley & Sons, Ltd.     

Thin‐film (25.5 μm) solar cells from layer transfer using porous silicon with 32.7 mA/cm2 short‐circuit current density

We fabricate a 25.5‐μm‐thick monocrystalline Si solar cell with a confirmed power conversion efficiency of 15.4% and an area of 3.88 cm² using a layer transfer process with porous Si. The process is free of photolithography and contains no high‐temperature oxidation steps. We investigate three design features that improve the short‐circuit current density to a value of 32.7 mA/cm² under AM1.5 illumination. The detached back reflector contributes 2 mA/cm², a reduced front‐surface reflectance accounts for an additional 2 mA/cm² and a reduced base doping increases the current density by 1 mA/cm². Copyright © 2002 John Wiley & Sons, Ltd.     

CuOx/a‐Si:H heterojunction thin‐film solar cell with an n‐type µc‐Si:H depletion‐assisting layer

We showed that thin n‐type CuO_x films can be deposited by radio‐frequency magnetron reactive sputtering and demonstrated the fabrication of n‐CuO_x/intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) heterojunction solar cells (HSCs) for the first time. A highly n‐doped hydrogenated microcrystalline Si (n‐µc‐Si:H) layer was introduced as a depletion‐assisting layer to further improve the performance of n‐CuO_x/i‐a‐Si:H HSCs. An analysis of the external quantum efficiency and energy‐band diagram showed that the thin depletion‐assisting layer helped establish sufficient depletion and increased the built‐in potential in the n‐CuO_x layer. The fabricated HSC exhibited a high open‐circuit voltage of 0.715 V and an efficiency of 4.79%. Copyright © 2015 John Wiley & Sons, Ltd.     

19%‐efficient and 43 µm‐thick crystalline Si solar cell from layer transfer using porous silicon

We present a both‐sides‐contacted thin‐film crystalline silicon (c‐Si) solar cell with a confirmed AM1.5 efficiency of 19.1% using the porous silicon layer transfer process. The aperture area of the cell is 3.98 cm². This is the highest efficiency ever reported for transferred Si cells. The efficiency improvement over the prior state of the art (16.9%) is achieved by implementing recent developments for Si wafer cells such as surface passivation with aluminum oxide and laser ablation for contacting. The cell has a short‐circuit current density of 37.8 mA cm⁻², an open‐circuit voltage of 650 mV, and a fill factor of 77.6%. Copyright © 2011 John Wiley & Sons, Ltd.     

Thin‐film GaAs epitaxial lift‐off solar cells for space applications

In the present work the space compatibility of thin‐film GaAs solar cells is studied. These cells are separated from their GaAs substrate by the epitaxial lift‐off (ELO) technique and mounted behind a CMG cover glass which at the same time serves as a stable carrier for the thin film cells. In the present initial stage of development these cells have an average efficiency of about 15·4% under AM0 illumination due to not yet optimized grid contacts and anti‐reflection coatings. Inspection after irradiation by 1 MeV electrons, thermal vacuum and thermal cycling experiments reveal that degradation of the cells is largely due to delamination and micro‐cracking. Based on these results, glass dehydration and adhesive degassing procedures are implemented in the ELO cell processing. As a consequence, even in this premature phase, newly produced cells show a radiation hardness comparable to or better than that of commercially available GaAs cells on Ge substrates and are virtually unaffected by severe thermal cycling. Copyright © 2005 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.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Multicrystalline silicon solar cells exceeding 20% efficiency

This paper presents the first conversion efficiency above 20% for a multicrystalline silicon solar cell. The application of wet oxidation for rear surface passivation significantly reduces the process temperature and therefore prevents the degradation of minority‐carrier lifetime. The excellent optical properties of the dielectrically passivated rear surface in combination with a plasma textured front surface result in a superior light trapping and allow the use of substrates below 100 μm thickness. A simplified process scheme with laser‐fired rear contacts leads to conversion efficiencies of 20·3% for multicrystalline and 21·2% for monocrystalline silicon solar cells on small device areas (1 cm²). Copyright © 2004 John Wiley & Sons, Ltd.     

Efficiency (>15%) for thin‐film epitaxial silicon solar cells on 70 cm2 area offspec silicon substrate using porous silicon segmented mirrors

We report on the beneficial use of embedded segmented porous silicon broad‐band optical reflectors for thin‐film epitaxial silicon solar cells. These reflectors are formed by gradual increase of the spatial period between the layer segments, allowing for an enhanced absorption of low energy photons in the epitaxial layer. By combining these reflectors with well‐established solar cell processing by photolithography, a conversion efficiency of 15·2% was reached on 73 cm² area, highly doped offspec multicrystalline silicon substrates. The corresponding photogenerated current densities (J_sc) were well above 31 mA/cm² for an active layer of only 20 µm. Copyright © 2010 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.     

Back‐contacted back‐junction n‐type silicon solar cells featuring an insulating thin film for decoupling charge carrier collection and metallization geometry

In this study, back‐contacted back‐junction n‐type silicon solar cells featuring a large emitter coverage (point‐like base contacts), a small emitter coverage (point‐like base and emitter contacts), and interdigitated metal fingers have been fabricated and analyzed. For both solar cell designs, a significant reduction of electrical shading losses caused by an increased recombination in the non‐collecting base area on the rear side was obtained. Because the solar cell designs are characterized by an overlap of the B‐doped emitter and the P‐doped base with metal fingers of the other polarity, insulating thin films with excellent electrical insulation properties are required to prevent shunting in these overlapping regions. Thus, with insulating thin films, the geometry of the minority charge carrier collecting emitter diffusion and the geometry of the interdigitated metal fingers can be decoupled. In this regard, plasma‐enhanced chemical vapor deposited SiO₂ insulating thin films with various thicknesses and deposited at different temperatures have been investigated in more detail by metal‐insulator‐semiconductor structures. Furthermore, the influence of different metal layers on the insulation properties of the films has been analyzed. It has been found that by applying a SiO₂ insulating thin film with a thickness of more than 1000 nm and deposited at 350 °C to solar cells fabricated on 1 Ω cm and 10 Ω cm n‐type float‐zone grown silicon substrates, electrical shading losses could be reduced considerably, resulting in excellent short‐circuit current densities of more than 41 mA/cm² and conversion efficiencies of up to 23.0%. Copyright © 2012 John Wiley & Sons, Ltd.     

High‐efficiency microcrystalline silicon single‐junction solar cells

This short communication highlights our latest results towards high‐efficiency microcrystalline silicon single‐junction solar cells. By combining adequate cell design with high‐quality material, a new world record efficiency was achieved for single‐junction microcrystalline silicon solar cell, with a conversion efficiency of 10.69%, independently confirmed at ISE CalLab PV Cells. Such significant conversion efficiency could be achieved with only 1.8 µm of Si. Copyright © 2013 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.     

Large‐area epitaxial silicon solar cells based on industrial screen‐printing processes

Thin‐film epitaxial silicon solar cells are an attractive future alternative for bulk silicon solar cells incorporating many of the process advantages of the latter, but on a potentially cheap substrate. Several challenges have to be tackled before this potential can be successfully exploited on a large scale. This paper describes the points of interest and how IMEC aims to solve them. It presents a new step forward towards our final objective: the development of an industrial cell process based on screen‐printing for > 15% efficient epitaxial silicon solar cells on a low‐cost substrate. Included in the discussion are the substrates onto which the epitaxial deposition is done and how work is progressing in several research institutes and universities on the topic of a high‐throughput epitaxial reactor. The industrial screen‐printing process sequence developed at IMEC for these epitaxial silicon solar cells is presented, with emphasis on plasma texturing and improvement of the quality of the epitaxial layer. Efficiencies between 12 and 13% are presented for large‐area (98 cm²) epitaxial layers on highly doped UMG‐Si, off‐spec and reclaim material. Finally, the need for an internal reflection scheme is explained. A realistically achievable internal reflection at the epi/substrate interface of 70% will result in a calculated increase of 3 mA/cm² in short‐circuit current. An interfacial stack of porous silicon layers (Bragg reflectors) is chosen as a promising candidate and the challenges facing its incorporation between the epitaxial layer and the substrate are presented. Experimental work on this topic is reported and concentrates on the extraction of the internal reflection at the epi/substrate interface from reflectance measurements. Initial results show an internal reflectance between 30 and 60% with a four‐layer porous silicon stack. Resistance measurements for majority carrier flow through these porous silicon stacks are also included and show that no resistance increase is measurable for stacks up to four layers. Copyright © 2005 John Wiley & Sons, Ltd.     

Highly transparent modulated surface textured front electrodes for high‐efficiency multijunction thin‐film silicon solar cells

To further increase the efficiency of multijunction thin‐film silicon (TF‐Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high‐quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light‐trapping structures in multijunction TF‐Si solar cells. The MST substrates comprise a micro‐textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub‐micron nano‐textured ZnO layer grown by low‐pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in‐coupling and light trapping for the top amorphous silicon (a‐Si:H) solar cell while minimizing the parasitic absorption losses. The crater‐shaped micro‐textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high‐quality nanocrystalline silicon (nc‐Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high‐quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single‐textured state‐of‐the‐art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a‐Si:H/nc‐Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state‐of‐the‐art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high‐efficiency multijunction solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Experimental and simulation study of thin film silicon solar cells with intermediate reflector

Optical and electrical simulations were carried out for thin film silicon solar tandem cells with intermediate reflector layer (IRL) between top and bottom cell and compared with experimental external quantum efficiency and current voltage characteristics results. Reference data were collected from a series of tandem cells with different thicknesses of the top cell absorber layer (160–240 nm), the bottom cell absorber layer (1750–2100 nm), and the transparent conductive oxides based IRL (10–80 nm). It turned out that for capturing correctly the influence of the IRL on the light management as a function of the IRL thickness, the conventional semicoherent approach is not sufficient. Whereas the optical properties of a very thin IRL are governed by interference effects that are best calculated using a fully coherent model, increasingly thicker IRL show a more and more incoherent behavior. By taking into account, the interface morphology and angular light distribution within the cell stack an algorithm for the effective IRL reflectivity was proposed that explains the experimental findings very well. The consecutive electrical simulations were carried out with the device simulator ASA. The dependence of short circuit current density j_sc and fill factor FF on the thickness d_IRL of the IRL is in qualitative agreement between simulation and experiment showing coincident extrema in j_sc(d_IRL) and FF(d_IRL) at the current matching point. Copyright © 2013 John Wiley & Sons, Ltd.     

Electron‐beam crystallized large grained silicon solar cell on glass substrate

Thin film hetero‐emitter solar cells with large‐grained poly‐silicon absorbers of around 10 µm thickness have been prepared on glass. The basis of the cell concept is electron‐beam‐crystallization of an amorphous or nanocrystalline silicon layer deposited onto a SiC:B layer. The SiC:B layer covers a commercially well available glass substrate, serving as diffusion barrier, contact layer and dopand source. For silicon absorber deposition a low pressure chemical vapour deposition was used. The successively applied e‐beam crystallization process creates poly‐silicon layers with grain sizes up to 1 × 10 mm² with low defect densities. The high electronic quality of the absorber is reflected in open circuit voltages as high as 545 mV, which are realized making use of the well‐developed a‐Si:H hetero‐emitter technology. Copyright © 2011 John Wiley & Sons, Ltd.