Deposition of intrinsic hydrogenated amorphous silicon for thin‐film solar cells – a comparative study for layers grown statically by RF‐PECVD and dynamically by VHF‐PECVD

Hydrogenated amorphous silicon (a‐Si:H) is conventionally deposited using static plasma‐enhanced chemical vapor deposition (PECVD) processes. In this work, a very high frequency (VHF) dynamic deposition technique is presented, on the basis of linear plasma sources. This configuration deploys a simple reactor design and enables continuous deposition processes, leading to a high throughput. Hence, this technique may facilitate the use of flexible substrates. As a result, the production costs of thin‐film silicon solar cells could be reduced significantly. We found a suitable regime for the homogeneous deposition of a‐Si:H layers for growth rates from 0.35–1.1 nm/s. The single layer properties as well as the performance of corresponding a‐Si:H solar cells are investigated and compared with a state‐of‐the‐art radio frequency (RF) PECVD regime. By analyzing the Fourier transform infrared spectroscopy spectra of single layers, we found an increasing hydrogen concentration with deposition rate for both techniques, which is in agreement with earlier findings. At a given growth rate, the hydrogen concentration was at the same level for intrinsic layers deposited by RF‐PECVD and VHF‐PECVD. The initial efficiency of the corresponding p–i–n solar cells ranged from 9.6% at a deposition rate of 0.2 nm/s (RF regime) to 8.9% at 1.1 nm/s (VHF regime). After degradation, the solar cell efficiency stabilized between 7.8% and 5.9%, respectively. The solar cells incorporating intrinsic layers grown dynamically using the linear plasma sources and very high frequencies showed a higher stabilized efficiency and lower degradation loss than solar cells with intrinsic layers grown statically by RF‐PECVD at the same deposition rate. Copyright © 2012 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.     

TCOs for nip thin film silicon solar cells

Substrate configuration allows for the deposition of thin film silicon (Si) solar cells on non‐transparent substrates such as plastic sheets or metallic foils. In this work, we develop processes compatible with low T_g plastics. The amorphous Si (a‐Si:H) and microcrystalline Si (µc‐Si:H) films are deposited by plasma enhanced chemical vapour deposition, at very high excitation frequencies (VHF‐PECVD). We investigate the optical behaviour of single and triple junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of low pressure CVD (LP‐CVD) ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 µm thick LP‐CVD ZnO. We observe that, for a‐Si:H, the cell performance depends critically on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a‐Si:H, a thick LP‐CVD ZnO front contact provides more light trapping on the 2D periodic substrate. Then, we investigate the influence of the thick and thin TCOs in conjunction with thick absorbers (µc‐Si:H). Because of the different nature of the optical systems (thick against thin absorber layer), the antireflection effect of ITO becomes more effective and the structure with the flat TCO provides as much light trapping as the rough LP‐CVD ZnO. Finally, the conformality of the layers is investigated and guidelines are given to understand the effectiveness of the light trapping in devices deposited on periodic gratings. Copyright © 2008 John Wiley & Sons, Ltd.     

Efficient and Flexible Thin Film Amorphous Silicon Solar Cells on Nanotextured Polymer Substrate Using Sol–gel Based Nanoimprinting Method

The mechanical flexibility of substrates and controllable nanostructures are two major considerations in designing high‐performance, flexible thin‐film solar cells. In this work, we proposed an approach to realize highly ordered metal oxide nanopatterns on polyimide (PI) substrate based on the sol‐gel chemistry and soft thermal nanoimprinting lithography. Thin‐film amorphous silicon (a‐Si:H) solar cells were subsequently constructed on the patterned PI flexible substrates. The periodic nanopatterns delivered broadband‐enhanced light absorption and quantum efficiency, as well as the eventual power conversion efficiency (PCE). The nanotextures also benefit for the device yield and mechanical flexibility, which experienced little efficiency drop even after 100,000 bending cycles. In addition, flexible, transparent nanocone films, obtained by a template process, were attached onto the patterned PI solar cells, serving as top anti‐reflection layers. The PCE performance with these dual‐interfacial patterns rose up to 8.17%, that is, it improved by 48.5% over the planar device. Although the work was conducted on a‐Si:H material, our proposed scheme can be extended to a variety of active materials for different optoelectronic applications.     

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.     

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.     

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.     

Deposition of highly efficient microcrystalline silicon solar cells under conditions of low H2 dilution: the role of the transient depletion induced incubation layer

This paper addresses the plasma deposition of highly efficient microcrystalline silicon (μc‐Si:H) p‐i‐n solar cells under conditions of high SiH₄ utilization and low H₂ dilution. It was established that the transient depletion of the initially present SiH₄ source gas induces the formation of an amorphous incubation layer that prevents successful crystallite nucleation in the i‐layer and leads to poor solar cell performance. The effect of this transient depletion induced incubation layer on solar cells was made visible through dedicated solar cell deposition series and selected area electron diffraction measurements. Applying a gas flow procedure at plasma ignition it was succeeded to prepare state‐of‐the‐art μc‐Si:H material and solar cells under low hydrogen dilution conditions, highlighted by μc‐Si:H solar cells of up to 9·5% efficiency prepared using an undiluted source gas flow consisting solely of SiH₄. Copyright © 2007 John Wiley & Sons, Ltd.     

Structural analyses of seeded thin film microcrystalline silicon solar cell

This contribution investigates the effect of seeding the growth of thin film microcrystalline silicon (µc‐Si : H) deposited by radio frequency plasma‐enhanced chemical vapor deposition on the material properties of µc‐Si : H film and the device performance of p‐i‐n and n‐i‐p µc‐Si : H solar cells. By means of Raman measurement, x‐ray diffraction (XRD) and transmission electron microscopy (TEM), we investigate the structure of seeded µc‐Si : H. In particular, the effect of seed layers on the crystallinity development is investigated. Measurements of the depth profile of the crystalline mass fraction using Raman spectroscopy show that seed layers lead to a more rapid and uniform crystallinity development in growth direction. The amorphous incubation layer is suppressed and crystallization begins directly from onset of film growth without evolving through the intermediate growth phases. From TEM analyses, we observe that crystal sizes are not affected by seed layers. Horizontal cracks are however observed to dominate the early growth of µc‐Si : H in p‐i‐n solar cell and this is reduced upon seeding. For the n‐i‐p cells, these cracks are not affected by seeding. XRD results also indicate that the use of seed layers does not affect the crystal sizes but affects the direction of preferential orientation. Solar cell external parameters show that seeding of p‐i‐n solar cells leads mainly to increase in short‐circuit current density, J_sc with a slight drop in open‐circuit voltage, V_oc. For the n‐i‐p cells, a reverse effect is observed. In this case, the V_oc increases and the J_sc decreases. Copyright © 2012 John Wiley & Sons, Ltd.     

Unraveling bulk defects in high‐quality c‐Si material via TIDLS

The current trend in silicon photovoltaics moving towards high‐quality thin mono‐crystalline silicon substrates sets a new challenge for the understanding of recombination mechanisms limiting the final performance of solar cells. Temperature‐ and injection‐dependent lifetime spectroscopy (TIDLS) has been shown to be a promising method for studying of high‐quality material with lifetime above 10 ms where the concentration of electrically active defects is well below the sensitivity of the most well‐known characterization techniques. In particular, when coupled with the Shockley–Read–Hall lifetime recombination model, TIDLS is capable of providing the most important defects' parameters including their energy level and concentration. In this contribution, we show that for a high‐quality silicon material, a thorough evaluation of the surface recombination velocity (SRV) temperature‐ and injection dependence is crucial for an accurate identification of the defects contained in the bulk. A new methodology for the analysis of TIDLS data, called defect parameters contour mapping, is introduced for the first time. By applying it to high‐quality n‐type float zone c‐Si samples passivated by a‐Si:H(i) or an a‐Si:H(i)/a‐Si:H(n) stack, we are able to assert the presence of defects in high lifetime materials in a range of concentration unachievable by any other characterization technique thus far. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental and simulated analysis of front versus all‐back‐contact silicon heterojunction solar cells: effect of interface and doped a‐Si:H layer defects

Front silicon heterojunction and interdigitated all‐back‐contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c‐Si) and intrinsic amorphous silicon [(i)a‐Si:H] layer, and the defects in doped a‐Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c‐Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC‐SHJ devices was simulated to determine the influence of defects at the (i)a‐Si:H/c‐Si interface and in the doped a‐Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC‐SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC‐SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 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.     

Single‐Crystal Germanium Growth on Amorphous Silicon

Growing single‐crystal semiconductors directly on an amorphous substrate without epitaxy or wafer bonding has long been a significant fundamental challenge in materials science. Such technology is especially important for semiconductor devices that require cost‐effective, high‐throughput fabrication, including thin‐film solar cells and transistors on glass substrates as well as large‐scale active photonic circuits on Si using back‐end‐of‐line CMOS technology. This work demonstrates a CMOS‐compatible method of fabricating high‐quality germanium single crystals on amorphous silicon at low temperatures of <450 °C. Grain orientation selection by geometric confinement of polycrystalline germanium films selectively grown on amorphous silicon by chemical vapor deposition is presented, where the confinement selects the fast‐growing grains for extended growth and eventually leads to single crystalline material. Germanium crystals grown using this method exhibit (110) texture and twin‐mediated growth. A model of confined growth is developed to predict the optimal confining channel dimensions for consistent, single‐crystal growth. Germanium films grown from one‐dimensional confinement exhibit a 200% grain size increase at 1 μm film thickness compared to unconfined films, while 2D confinement growth achieved single crystal Ge. The area of single crystalline Ge on amorphous layers is only limited by the growth time. Significant enhancement in room temperature photoluminescence and reduction in residual carrier density have been achieved using confined growth, demonstrating excellent optoelectronic properties. This growth method is readily extensible to any materials system capable of selective non‐epitaxial deposition, thus allowing for the fabrication of devices from high‐quality single crystal material when only an amorphous substrate is available.     

高压高功率VHF-PECVD的微晶硅薄膜高速沉积

采用高压高功率(hphP)甚高频等离子体强强化学气相沉积(VHF-PECVD)法对微晶硅(μc-Si:H)进行高速沉积,在最优沉积条件参数下对hphP和低压低功率(lplP)两组样品沉积速率、光电导、暗电导及光敏性等性能参数进行测试,得到了1.58 nm/s的较高沉积速率、光电性能优秀和更适合薄膜太阳能电池的μc-Si...     

Change in I–V characteristics of thin‐film photovoltaic (PV) modules induced by light soaking and thermal annealing effects

The performance of photovoltaic (PV) modules is generally rated under standard test conditions (STC). However, the performance of thin‐film photovoltaic modules is not unique even under STC, because of the “metastability”. The effects of the light soaking and thermal annealing shall be incorporated into an appropriate energy rating standard. In this study, the change in I–V characteristics of thin‐film PV modules caused by the metastability was examined by repeated indoor measurements in addition to round‐robin outdoor measurements. The investigated thin‐film modules were copper indium gallium (di)selenide (CIGS), a‐Si : H, and a‐Si : H/µc‐Si : H (tandem) modules. The increase in the performance of the CIGS module between the initial and final indoor measurements was approximately 8%. Because of light‐induced degradation, the indoor performance of the a‐Si : H and a‐Si : H/µc‐Si : H modules decreased by approximately 35% and 20%, respectively. The performance was improved by about 4–6% under high temperature conditions after the initial degradation. The results suggest that the performance of thin‐film silicon modules can seasonally vary by approximately 4–6% only due to thermal annealing and light soaking effects. The effect of solar spectrum enhanced the outdoor performance of the a‐Si : H module by about 10% under low air mass conditions, although that of the a‐Si : H/µc‐Si : H modules showed a little increase. The currents of these a‐Si : H/µc‐Si : H modules may be limited by the bottom cells. Therefore, it is required to optimize the effect of solar spectrum in addition to the effects of light soaking and thermal annealing, in order to achieve the best performance for thin‐film silicon tandem modules. Copyright © 2013 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.     

H2 plasma treatment at the p/i interface of a hydrogenated amorphous Si absorption layer for high‐performance Si thin film solar cells

Plasma treatment (PT) of the buffer layer for highly H₂‐diluted hydrogenated amorphous silicon (a‐Si:H) absorption layers is proposed as a technique to improve efficiency and mitigate light‐induced degradation (LID) in a‐Si:H thin film solar modules. The method was verified for a‐Si:H single‐junction and a‐Si:H/microcrystalline silicon (µc‐Si:H) tandem modules with a size of 200 × 200 mm² (aperture area of 382.5 cm²) under long‐term light exposure. H₂ PT at the p/i interface was found to eliminate non‐radiative recombination centers in the buffer layer, and plasma‐enhanced chemical vapor deposition at low radio‐frequency power was found to suppress the generation of defects during the growth of a‐Si:H absorption layers on the treated buffer layers. With optimized H₂ PT of the a‐Si:H single‐junction module, the stabilized short circuit current and fill factor increased, and the stabilized open circuit voltage moves beyond its initial value. The results demonstrate 7.7% stabilized efficiency and 10.5% LID for the a‐Si:H single‐junction module and 10.82% stabilized efficiency and 7.76% LID for the a‐Si:H/µc‐Si:H tandem module. Thus, the growth of an a‐Si:H absorption layer on a H₂ PT buffer layer can be considered as a practical method for producing high‐performance Si thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.     

本征微晶硅薄膜的结构演变及其对太阳电池光伏性能的影响

采用高压射频等离子体增强化学气相沉积(RF-PECVD)方法制备本征硅薄膜和n-i-P结构太阳电池,研究了氢稀释率对本征硅薄膜的电学特性和结构特性的影响.采用光发射谱(OES)和喇曼(Raman)散射光谱研究了处于过渡区的本征硅薄膜的纵向结构演变过程.结果表明:光发射谱和喇曼散射光谱可以作为研究硅薄膜的纵向结构演变有效手段.随着氢稀释率的增加,硅薄膜从非晶相向微晶相过渡时,其纵向结构的改变会严重影响硅薄膜太阳电池的光伏性能.     

Low-Temperature Materials and Thin Film Transistors for Flexible Electronics

This paper addresses the low-temperature deposition processes and electronic properties of silicon based thin film semiconductors and dielectrics to enable the fabrication of mechanically flexible electronic devices on plastic substrates. Device quality amorphous hydrogenated silicon (a-Si:H), nanocrystalline silicon (nc-Si), and amorphous silicon nitride (a-SiN/sub x/) films and thin film transistors (TFTs) were made using existing industrial plasma deposition equipment at the process temperatures as low as 75/spl deg/C and 120/spl deg/C. The a-Si:H TFTs fabricated at 120/spl deg/C demonstrate performance similar to their high-temperature counterparts, including the field effect mobility (/spl mu//sub FE/) of 0.8 cm/sup 2/V/sup -1/s/sup -1/, the threshold voltage (V/sub T/) of 4.5 V, and the subthreshold slope of 0.5 V/dec, and can be used in active matrix (AM) displays including organic light emitting diode (OLED) displays. The a-Si:H TFTs fabricated at 75/spl deg/C exhibit /spl mu//sub FE/ of 0.6 cm/sup 2/V/sup -1/s/sup -1/, and V/sub T/ of 4 V. It is shown that further improvement in TFT performance can be achieved by using n/sup +/ nc-Si contact layers and plasma treatments of the interface between the gate dielectric and the channel layer. The results demonstrate that with appropriate process optimization, the large area thin film Si technology suits well the fabrication of electronic devices on low-cost plastic substrates.     

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.