UV nanoimprint for the replication of etched ZnO:Al textures applied in thin‐film silicon solar cells

In this work, we present a technology for a high precision nanostructure replication process based on ultraviolet nanoimprint lithography for the application in the field of thin‐film photovoltaics. The potential of the technology is demonstrated by the fabrication of microcrystalline silicon thin‐film prototype solar cells. The high accuracy replication of random microstructures made from sputtered and etched ZnO:Al, used to scatter the incident light in thin solar cells, is shown by local topography investigations of the same 7.5 × 7.5 µm² area on the master and the replica. Different types of imprint resists and imprint moulds were investigated to find the optimal, high precision replication technology. Two types of thin‐film silicon solar cells, in p‐i‐n and n‐i‐p configuration, were fabricated to study the potential of the imprint technology for different applications. It is shown that solar cells deposited on an imprinted glass hold similar performances compared with reference solar cells fabricated with a standard process on textured ZnO:Al. Thus, it is demonstrated that the replication of light scattering structures by using an imprint process is an attractive method to decouple the scattering properties from the layer forming the electrical front contact. Because a simple and cheap high throughput process is used, this study additionally proves the relevance for the industrial mass production in the field of photovoltaics. Copyright © 2013 John Wiley & Sons, Ltd.     

Ex‐ante environmental and economic evaluation of polymer photovoltaics

The use of polymer materials for photovoltaic applications is expected to have several advantages over current crystalline silicon technology. In this paper, we perform an environmental and economic assessment of polymer‐based thin film modules with a glass substrate and modules with a flexible substrate and we compare our results with literature data for multicrystalline (mc‐) silicon photovoltaics and other types of PV. The functional unit of this study is ‘25 years of electricity production by PV systems with a power of 1 watt‐peak (W_p)’. Because the lifetime of polymer photovoltaics is at present much lower than of mc‐silicon photovoltaics, we first compared the PV cells per watt‐peak and next determined the minimum required lifetime of polymer PV to arrive at the same environmental impacts as mc‐silicon PV. We found that per watt‐peak of output power, the environmental impacts compared to mc‐silicon are 20–60% lower for polymer PV systems with glass substrate and 80–95% lower for polymer PV with PET as substrate (flexible modules). Also in comparison with thin film CuInSe and thin film silicon, the impacts of polymer modules, per watt‐peak, appeared to be lower. The costs per watt‐peak of polymer PV modules with glass substrate are approximately 20% higher compared to mc‐silicon photovoltaics. However, taking into account uncertainties, this might be an overestimation. For flexible modules, no cost data were available. If the efficiency and lifetime of polymer PV modules increases, both glass‐based and flexible polymer PV could become an environment friendly and cheap alternative to mc‐silicon PV. Copyright © 2009 John Wiley & Sons, Ltd.     

Perspectives of crystalline Si thin film solar cells: a new era of thin monocrystalline Si films?

A large number of competing approaches are currently being investigated around the world to develop crystalline silicon thin film solar cells on foreign substrates. These approaches can be broadly classified according to the crystalline state of the Si films employed: (i) thin film solar cells based on nano‐ or microcrystalline Si‐films; (ii) cells fabricated from large‐grained polycrystalline Si and (iii) recent approaches utilizing the transfer of monocrystalline Si films. The paper discusses prospects and limitations of these approaches and describes device results based on the transfer of quasi‐monocrystalline Si films. Using Si absorber films epitaxially grown on quasi‐monocrystalline Si, we achieve a conversion efficiency of 13˙6% for a 4 cm² sized thin film solar cell on glass. In contrast to the limited performance of polycrystalline Si thin film solar cells imposed by the presence of grain boundaries, transfer approaches are expected to result in thin film solar cell efficiencies in the range of 15 – 18% depending on process maturity and complexity. The transfer of monocrystalline Si films therefore opens a new avenue to an efficient and competitive Si‐based thin film technology. Copyright © 2000 John Wiley & Sons, Ltd     

薄膜太阳电池技术发展趋势浅析

低成本、高效率的薄膜太阳电池是未来光伏产业发展的重要方向之一。主要介绍了目前备受关注的薄膜太阳电池,包括硅基薄膜太阳电池、铜铟镓硒与铜锌锡硫薄膜太阳电池,及砷化镓薄膜太阳电池等,简述了它们的各自特点、研究现状、主要技术路线和产业化发展等情况。最后展望了薄膜太阳电池未来的发展趋势。     

Highly transparent singlet fission solar cell with multistacked thin metal contacts for tandem applications

Singlet fission solar cells combined with silicon photovoltaics allow the construction of parallel tandem solar cells, which benefit from better usage of high‐energy photons. A key limiting factor for the performance of such a tandem configuration is the transparency of the singlet fission front cell. Here we show highly transparent singlet fission solar cells with a top contact of thin Ca:Ag blends. The optimized contact leads to 81% average solar cell transmittance in the near‐infrared while maintaining more than half the short‐circuit current density compared with an opaque device. We simulate the performance of the parallel tandem stack and assess the improvements needed to fully realize the potential of singlet fission in this device configuration.     

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.     

双光栅结构薄膜太阳能电池的优化

为了提高单晶硅薄膜太阳能电池短路电流密度和转换效率, 采用在单晶硅薄膜太阳能电池正背面分别集成硅介质光栅和铝金属光栅的方法, 并利用有限时域差分法软件仿真研究了两种光栅的周期、厚度、占空比对单晶硅薄膜太阳能电池短路电流密度和光转换效率的影响。结果表明, 通过优化可得当正背面光栅都处于最优值时(介质光栅占空比F=0.8、介质光栅周期P=0.632μm、介质光栅厚度h_g=0.42μm; 金属光栅占空比F₁=0.9、金属光栅周期P=0.632μm、金属光栅厚度h_m=0.005μm), 短路电流密度可达35.15mA/cm2, 转换效率为43.35%;将最优光栅单晶硅薄膜太阳能电池与传统单晶硅薄膜太阳能电池对比, 无论是光程路径还是吸收效率, 光栅单晶硅薄膜太阳能电池都有显著的提高。这为以后制备高性能薄膜太阳能电池提供了理论指导。     

A New View of Microcrystalline Silicon: The Role of Plasma Processing in Achieving a Dense and Stable Absorber Material for Photovoltaic Applications

To further lower production costs and increase conversion efficiency of thin‐film silicon solar modules, challenges are the deposition of high‐quality microcrystalline silicon (μc‐Si:H) at an increased rate and on textured substrates that guarantee efficient light trapping. A qualitative model that explains how plasma processes act on the properties of μc‐Si:H and on the related solar cell performance is presented, evidencing the growth of two different material phases. The first phase, which gives signature for bulk defect density, can be obtained at high quality over a wide range of plasma process parameters and dominates cell performance on flat substrates. The second phase, which consists of nanoporous 2D regions, typically appears when the material is grown on substrates with inappropriate roughness, and alters or even dominates the electrical performance of the device. The formation of this second material phase is shown to be highly sensitive to deposition conditions and substrate geometry, especially at high deposition rates. This porous material phase is more prone to the incorporation of contaminants present in the plasma during film deposition and is reported to lead to solar cells with instabilities with respect to humidity exposure and post‐deposition oxidation. It is demonstrated how defective zones influence can be mitigated by the choice of suitable plasma processes and silicon sub‐oxide doped layers, for reaching high efficiency stable thin film silicon solar cells.     

SHORT COMMUNICATION: Thin‐film free‐standing monocrystalline si solar cells with heterojunction emitter

We propose a novel approach to thin‐film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS‐HJ). High crystallographic quality mono‐Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free‐standing mono‐Si (FMS) films were processed into solar cells by creating a‐a‐Si/c‐Si heterojunction. In our preliminary experiments a thin‐film FMS‐HJ solar cell with 9.6% efficiency was realized in a 20‐μμm‐thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.     

UV‐nano‐imprint lithography technique for the replication of back reflectors for n‐i‐p thin film silicon solar cells

Texturing of interfaces in thin film silicon solar cells is essential to enhance the produced photocurrent and thus the efficiencies. A UV nano‐imprint‐lithography (UV‐NIL) replication process was developed to prepare substrates with textures that are suitable for the growth of n‐i‐p thin film silicon solar cells. Morphological and optical analyses were performed to assess the quality of the replicas. A comparison of single junction amorphous solar cells on the original structures and on their replicas on glass revealed good light trapping and excellent electrical properties on the replicated structures. A tandem amorphous silicon/amorphous silicon (a‐Si/a‐Si) cell deposited on a replica on plastic exhibits a stabilized efficiency of 8.1% and a high yield of 90% of good cells in laboratory conditions. It demonstrates the possibility to obtain appropriate structure on low cost plastic substrate. Copyright © 2010 John Wiley & Sons, Ltd.     

Silica nanospheres as back surface reflectors for crystalline silicon thin‐film solar cells

In this paper, fabrication of a non‐continuous silicon dioxide layer from a silica nanosphere solution followed by the deposition of an aluminium film is shown to be a low‐cost, low‐thermal‐budget method of forming a high‐quality back surface reflector (BSR) on crystalline silicon (c‐Si) thin‐film solar cells. The silica nanosphere layer has randomly spaced openings which can be used for metal‐silicon contact areas. Using glass/SiN/p⁺nn⁺ c‐Si thin‐film solar cells on glass as test vehicle, the internal quantum efficiency (IQE) at long wavelengths (>900 nm) is experimentally demonstrated to more than double by the implementation of this BSR, compared to the baseline case of a full‐area Al film as BSR. The improved optical performance of the silica nanosphere/aluminium BSR is due to reduced parasitic absorption in the Al film. Copyright © 2007 John Wiley & Sons, Ltd.     

CdTe solar cell in a novel configuration

Polycrystalline thin‐film CdTe/CdS solar cells have been developed in a configuration in which a transparent conducting layer of indium tin oxide (ITO) has been used for the first time as a back electrical contact on p‐CdTe. Solar cells of 7·9% efficiency were developed on SnO_x:F‐coated glass substrates with a low‐temperature (<450°C) high‐vacuum evaporation method. After the CdCl₂ annealing treatment of the CdTe/CdS stack, a bromine methanol solution was used for etching the CdTe surface prior to the ITO deposition. The unique features of this solar cell with both front and back contacts being transparent and conducting are that the cell can be illuminated from either or both sides simultaneously like a ‘bi‐facial’ cell, and it can be used in tandem solar cells. The solar cells with transparent conducting oxide back contact show long‐term stable performance under accelerated test conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Light trapping in Silicon‐Film™ solar cells with rear pigmented dielectric reflectors

This paper presents the novel method of using pigmented dielectric reflectors to provide light trapping in thin‐film silicon solar cells. This type of reflecting material offers many potential advantages over specular metallic reflectors, including low cost, compatibility with high temperatures common to solar cell processing, and high, broadband and diffuse reflectance. As this is the first time this concept is described, the basic theory of the optical behavior of pigmented materials is presented by connecting the basic material properties of pigment and medium to their light‐trapping benefit in thin silicon solar cells. Several general principles leading to maximum light‐trapping benefit are identified, and experimental evidence is presented corroborating these general principles. Light trapping is demonstrated in thin silicon solar cells with pigmented dielectric reflectors by measurement and analysis of external quantum efficiency curves. Copyright © 1999 John Wiley & Sons, Ltd.     

Improving the performance of Sb2Se3 thin film solar cells over 4% by controlled addition of oxygen during film deposition

Sb₂Se₃ has attracted great research interest very recently as a promising absorber material for thin film photovoltaics due to its suitable bandgap, high absorption coefficient, and non‐toxic, low cost, and earth abundant nature. In this work, a significant efficiency improvement to 4.8% of superstrate cadmium sulfide (CdS)/Sb₂Se₃ solar cells is obtained by the controlled addition of oxygen during thermal evaporation of Sb₂Se₃ films. Systematic materials and device physics characterization reveal that oxygen addition during Sb₂Se₃ film evaporation significantly improves the CdS/Sb₂Se₃ heterojunction quality through effective passivation of interfacial defect states, resulting in a substantial enhancement in device circuit voltage and short circuit current density. The 4.8% device is the highest efficiency thus far reported for Sb₂Se₃ thin film solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Use of seed layers for structure,optical, and electronic transport measurements on microcrystalline silicon on glass

A detailed structural analysis is provided by which the benefits of thin undoped ‘seed layers’ for the preparation of microcrystalline silicon on glass for material characterization are demonstrated. Raman spectroscopy and photothermal deflection spectroscopy (PDS) results reveal that ‘seed layers’ are not only effective for the growth of structurally homogenous films and for an extension of the range of deposition parameters in which highly crystalline material is grown, but also allow for preparing material on glass with properties very close to that of functional layers in thin film solar cells. Films which have successfully been tailored in this way are characterized with respect to electrical conductivity and optical absorption. Regarding conductivity, hydrogenated microcrystalline silicon material grown on a ‘seed layer’ exhibits a structure‐dependent behaviour which is very similar to that observed for material grown on bare glass. Regarding optical absorption spectra, residual interference fringes, which indicate structural non‐uniformities, can be successfully removed by means of ‘seed layers’. As a result, more information is obtainable from PDS, and the data gained in this way are in good agreement with Raman spectroscopy results. Copyright © 2011 John Wiley & Sons, Ltd.     

Thin Film Silicon Photovoltaic Cells on Paper for Flexible Indoor Applications

The present development of non‐wafer‐based photovoltaics (PV) allows supporting thin film solar cells on a wide variety of low‐cost recyclable and flexible substrates such as paper, thereby extending PV to a broad range of consumer‐oriented disposable applications where autonomous energy harvesting is a bottleneck issue. However, their fibrous structure makes it challenging to fabricate good‐performing inorganic PV devices on such substrates. The advances presented here demonstrate the viability of fabricating thin film silicon PV cells on paper coated with a hydrophilic mesoporous layer. Such layer can not only withstand the cells production temperature (150 °C), but also provide adequate paper sealing and surface finishing for the cell's layers deposition. The substances released from the paper substrate are continuously monitored during the cell deposition by mass spectrometry, which allows adapting the procedures to mitigate any contamination from the substrate. In this way, a proof‐of‐concept solar cell with 3.4% cell efficiency (41% fill factor, 0.82 V open‐circuit voltage and 10.2 mA cm^?2 short‐circuit current density) is attained, opening the door to the use of paper as a reliable substrate to fabricate inorganic PV cells for a plethora of indoor applications with tremendous impact in multi‐sectorial fields such as food, pharmacy and security.     

Broadband absorption enhancement in ultra‐thin crystalline Si solar cells by incorporating metallic and dielectric nanostructures in the back reflector

We propose a back reflecting scheme in order to enhance the maximum achievable current in one micron thick crystalline silicon solar cells. We perform 3D numerical investigations of the scattering properties of metallic nanostructures located at the back side and optimize them for enhancing absorption in the silicon layer. We validate our numerical results experimentally and also compare the absorption enhancement in the solar cell structure, both with quasi‐periodic and random metallic nanostructures. We have looked at the interplay between the metallic nanostructures and an integrated back reflector. We show that the combination of metallic nanoparticles and a metallic reflector results in significant parasitic absorption. We compared this to another implementation based on titanium dioxide nanoparticles, which act as a Lambertian reflector of light. Our simulation and experimental results show that this proposed configuration results in reduced absorption losses and in broadband enhancement of absorption for ultra‐thin solar cells, paving the way to an optimal back reflector for thin film photovoltaics. Copyright © 2014 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.     

All Solution‐Processed Chalcogenide Solar Cells – from Single Functional Layers Towards a 13.8% Efficient CIGS Device

Solution processing of inorganic thin films has become an important thrust in material research community because it offers low‐cost and high‐throughput deposition of various functional coatings and devices. Especially inorganic thin film solar cells – macroelectronic devices that rely on consecutive deposition of layers on large‐area rigid and flexible substrates – could benefit from solution approaches in order to realize their low‐cost nature. This article critically reviews existing deposition approaches of functional layers for chalcogenide solar cells with an extension to other thin film technologies. Only true solutions of readily available metal salts in appropriate solvents are considered without the need of pre‐fabricated nanoparticles. By combining three promising approaches, an air‐stable Cu(In,Ga)Se₂ thin film solar cell with efficiency of 13.8% is demonstrated where all constituent layers (except the metal back contact) are processed from solutions. Notably, water is employed as the solvent in all steps, highlighting the potential for safe manufacturing with high utilization rates.     

Investigation on Silicon Thin Film Solar Cells

The preparation,current status and trends are investigated for silicon thin film solar cells.The advantages and disadvantages of amorphous silicon thin film,polycrystalline silicon thin film and mono-crystalline silicon thin film cells are compared.The future development trends are pointed out.It is found that polycrystalline silicon thin film solar cells will be more promising for application with great potential.