Record amorphous silicon single‐junction photovoltaic module with 9.1% stabilized conversion efficiency on 1.43 m2

Providing two‐thirds of the total stabilized power of thin‐film tandem MICROMORPH^TM technology, the amorphous junction remains a key element in the quest for higher efficiencies. This paper reports and summarizes a considerable work to achieve a record large‐area amorphous silicon single‐junction photovoltaic module. New hardware has been developed and known process steps have been accurately tuned and combined with new features of cell design. Effort was focused on the deposition of high‐quality and low‐defect a‐Si:H layers that has promoted an improved device stability and resistance against light induced degradation. Efficient light management has been used, and module design has been revised. The word‐record performance reported in this paper for a large‐area (1.43 m²) stabilized module conversion efficiency (total area) was measured and certified by Swiss PV Module Test Center to be 9.1%. Copyright © 2016 John Wiley & Sons, Ltd.     

5% Efficient evaporated solid‐phase crystallised polycrystalline silicon thin‐film solar cells

The first energy conversion efficiencies of over 5% are reported for evaporated solid‐phase crystallised (SPC) polycrystalline silicon thin‐film solar cells. All cells have a size of 2 cm² and are formed on planar glass superstrates. Back surface reflectance is provided by a simple coating with commercial white paint. The best cells have short‐circuit current densities of about 19 mA/cm² and external quantum efficiencies peaking at above 80%. The diffusion length in the base of the solar cells is larger than the base thickness, providing significant room for further efficiency improvements via an increased thickness of the base layer. Additional improvements are expected via the use of textured glass sheets, boosting the light trapping capabilities of the cells. Copyright © 2009 John Wiley & Sons, Ltd.     

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites for highly efficient nanocrystalline silicon thin‐film solar cells

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites (p‐nc‐SiC:H) were prepared using a plasma‐enhanced chemical vapor deposition system with a mixture of CH₄, SiH₄, B₂H₆ and H₂ gases. The influence of hydrogen dilution on the material properties of the p‐nc‐SiC:H films was investigated, and their roles as window layers in hydrogenated nanocrystalline silicon (nc‐Si:H) solar cells were examined. By increasing the R_H (H₂/SiH₄) ratio from 90 to 220, the Si―C bond density in the p‐nc‐SiC:H films increased from 5.20 × 10¹⁹ to 7.07 × 10¹⁹/cm³, resulting in a significant increase of the bandgap from 2.09 to 2.23 eV in comparison with the bandgap of 1.95 eV for p‐nc‐Si:H films. For the films deposited at a high R_H ratio, the Si nanocrystallites with a size of 3–15 nm were formed in the amorphous SiC:H matrix. The Si nanocrystallites played an important role in the enhancement of vertical charge transport in the p‐nc‐SiC:H films, which was verified by conductive atomic force microscopy measurements. When the p‐nc‐SiC:H films deposited at R_H = 220 were applied in the nc‐Si:H solar cells, a high conversion efficiency of 8.26% (V_oc = 0.53 V, J_sc = 23.98 mA/cm² and FF = 0.65) was obtained compared to 6.36% (V_oc = 0.44 V, J_sc = 21.90 mA/cm² and FF = 0.66) of the solar cells with reference p‐nc‐Si:H films. Further enhancement in the cell performance was achieved using p‐nc‐SiC:H bilayers consisting of highly doped upper layers and low‐level doped bottom layers, which led to the increased conversion efficiency of 9.03%. Copyright © 2015 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.     

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.     

Life‐cycle greenhouse gas effects of introducing nano‐crystalline materials in thin‐film silicon solar cells

Solar PV is widely considered as a “green” technology. This paper, however, investigates the environmental impact of the production of solar modules made from thin‐film silicon. We focus on novel applications of nano‐crystalline Silicon materials (nc‐Si) into current amorphous Silicon (a‐Si) devices. Two nc‐Si specific details concerning the environmental performance can be identified, when we want to compare to a‐Si modules. First, in how far the extra (and thicker) silicon layer (s) affects upstream material requirements and energy use. Second, in how far depositing an extra silicon layer may increase emissions of greenhouse gases as additional emissions of Fluor gases (F‐gases) are associated to this step. The much larger global warming potential of F‐gases (17 200–22 800 times that of CO₂) may lead to higher environmental burdens. To date, no study has yet analyzed the effect of F‐gas usage on the environmental profile of thin‐film silicon solar modules. We performed a life‐cycle assessment (LCA) to investigate the current environmental usefulness of pursuing this novel micromorph concept. The switch to the new micromorph technology will result in a 60–85% increase in greenhouse gas emissions (per generated kWh solar electricity) in case of NF₃ based clean processing, and 15–100% when SF₆ is used. We conclude that F‐gas usage has a substantial environmental impact on both module types, in particular the micromorph one. Also, micromorph module efficiencies need to be improved from the current 8–9% (stabilized efficiency) toward 12–16% (stab. eff.) in order to compensate for the increased environmental impacts. Copyright © 2010 John Wiley & Sons, Ltd.     

Screen‐printed epitaxial silicon thin‐film solar cells with 13·8% efficiency

Amidst the different silicon thin‐film systems, the epitaxial thin‐film solar cell represents an approach with interesting potential. Consisting of a thin active c‐Si layer grown epitaxially on top of a low‐quality c‐Si substrate, it can be implemented into solar cell production lines without major changes in the current industrial process sequences. Within this work, ∼30‐μm‐thick epitaxial layers on non‐textured and highly doped monocrystalline Czochralski (Cz) and multicrystalline (mc) Si substrates have been prepared by CVD. Confirmed efficiencies of 13·8% on Cz and 12·3% on mc‐Si substrates have been achieved by applying an industrial process scheme based on tube and in‐line phosphorus diffusion, as well as screen‐printed front and back contacts fired through a SiN_x anti‐reflection coating. An extensive solar cell characterisation, including infrared lock‐in thermography and spectral response measurements is presented. Copyright © 2003 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.     

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.     

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.     

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.     

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.     

Photocurrent enhancement in thin‐film silicon solar cells by combination of anti‐reflective sub‐wavelength structures and light‐trapping textures

Dielectric films with anti‐reflective sub‐wavelength structures are applied to thin‐film silicon solar cells to improve the light incoupling at the front surface. It is verified that modification of the refractive index of the incident medium using dielectric films with sub‐wavelength structures is beneficial to reduce the average reflectivity of Si solar cells with an anti‐reflective coating based on optical interference. It is also shown that the sub‐wavelength structure must be combined with a proper light‐trapping texture to enhance the absorption within thin‐film silicon solar cells. The effectiveness of dielectric films with sub‐wavelength structures is demonstrated by an increase of the short‐circuit current density of a microcrystalline silicon cell from 29.1 to 30.4 mA/cm² in a designated area of 1 cm². The optical interplay between the dielectric films and the light‐trapping textures is also discussed. Copyright © 2015 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.     

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.     

Polycrystalline silicon thin‐film solar cells on glass by aluminium‐induced crystallisation and subsequent ion‐assisted deposition (ALICIA)

A research project is under way at The University of New South Wales aiming at the realisation of a novel type of polycrystalline silicon thin‐film solar cell on glass. The idea is to first create a thin large‐grained polycrystalline seed layer on glass by aluminium‐induced crystallisation of amorphous silicon and then to epitaxially thicken the seed layer with ion‐assisted deposition. By mid‐2003 this ALICIA project had achieved laboratory cells with voltages of up to 163 mV, as reported elsewhere. In the present paper we give an overview of recent progress (improved Si epitaxy process, improved control of base doping profile due to the use of phosphorus dopants instead of gallium, hydrogen passivation) that has improved the voltages of ALICIA solar cells to 270 mV. Furthermore, the strategy for further voltage improvements is presented. At the present point in time only the voltages of ALICIA cells are known, but obviously solar cells also require current for good efficiency. Hence much improvement in both voltage and current is still needed. Copyright © 2004 John Wiley & Sons, Ltd.     

High‐rate deposition of microcrystalline silicon in a large‐area PECVD reactor and integration in tandem solar cells

We study the high‐rate deposition of microcrystalline silicon in a large‐area plasma‐enhanced chemical‐vapor‐deposition (PECVD) reactor operated at 40.68 MHz, in the little‐explored process conditions of high‐pressure and high‐silane concentration and depletion. Due to the long gas residence time in this process, the silane gas is efficiently depleted using moderate feed‐in power density, thus facilitating up‐scaling of the process to large surfaces. As observed in more traditional deposition processes, the deposition rate and performance of device‐quality material are limited by the inter‐electrode gap of the reactor. We significantly increase the cell performances by reducing this gap. X‐ray diffractometry (XRD) and secondary ion mass spectroscopy (SIMS) are used to characterize the microcrystalline material deposited in the modified reactor at a rate of 1 nm/s. Comparison with a microcrystalline process at a low deposition rate demonstrates that the crystallographic orientation of the absorbing layer of the cell and the concentrations of contaminants are strongly correlated and dependent on the process. We use microcrystalline cells with absorber layer grown at a rate of 1 nm/s integrated as bottom cells in amorphous‐microcrystalline (micromorph) tandem solar cells using the superstrate configuration. We report an initial efficiency of 10.8% (9.6% stabilized) for a tandem cell with 1.2 cm² surface. Copyright © 2010 John Wiley & Sons, Ltd.     

8% Efficient thin‐film polycrystalline‐silicon solar cells based on aluminum‐ induced crystallization and thermal CVD

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from polycrystalline‐silicon (pc‐Si) thin films on inexpensive substrates. We recently showed promising solar cell results using pc‐Si layers obtained by aluminum‐induced crystallization (AIC) of amorphous silicon in combination with thermal chemical vapor deposition (CVD). To obtain highly efficient pc‐Si solar cells, however, the material quality has to be optimized and cell processes different from those applied for standard bulk‐Si solar cells have to be developed. In this work, we present the different process steps that we recently developed to enhance the efficiency of pc‐Si solar cells on alumina substrates made by AIC in combination with thermal CVD. Our present pc‐Si solar cell process yields cells in substrate configuration with efficiencies so far of up to 8·0%. Spin‐on oxides are used to smoothen the alumina substrate surface to enhance the electronic quality of the absorber layers. The cells have heterojunction emitters consisting of thin a‐Si layers that yield much higher V_oc values than classical diffused emitters. Base and emitter contacts are on top of the cell in interdigitated finger patterns, leading to fill factors above 70%. The front surface of the cells is plasma textured to increase the current density. Our present pc‐Si solar cell efficiency of 8% together with the fast progression that we have made over the last few years indicate the large potential of pc‐Si solar cells based on the AIC seed layer approach. Copyright © 2007 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.     

Optimal design of periodic surface texture for thin‐film a‐Si:H solar cells

Optical analysis of hydrogenated amorphous silicon (a‐Si:H) solar cells with a periodic texture applied to the interfaces was carried out by two‐dimensional optical simulator. The optical simulator solves the electromagnetic wave equations by means of finite element method using triangular elements for the discretization of space. The periodic texture with rectangular‐like shape acts as a diffraction grating which scatters light into selective angles and thus gives a potential for significant prolongation of optical paths in thin absorber layers of the cells. Optimization of the geometrical parameters (period, height and duty‐cycle) of the periodic texture was carried out in order to obtain the highest photocurrent from a‐Si:H solar cells. The a‐Si:H solar cell with the optimal periodic texture parameters (period of 300 nm, height of 300 nm and duty cycle of 50%) and the absorber layer thickness of 300 nm generates up to 35% more photocurrent in comparison to the cell with flat interfaces. The optical analysis demonstrates that the optimal periodic texture in the a‐Si:H solar cell results in the best trade‐off between the antireflection effect at front interfaces, light scattering efficiency and the absorption losses at realistic metal back contact. Copyright © 2010 John Wiley & Sons, Ltd.