A glass thinning and texturing method for light incoupling in thin‐film polycrystalline silicon solar cells application

For polycrystalline silicon (poly‐Si) thin‐film solar cells on ～3 mm borosilicate glass, glass thinning reduces the glass absorption and light leaking to neighbouring cells; the glass texturing of the sun‐facing side suppresses reflection. In this Letter, a labour‐free wet etching method is developed to texture and thin the glass at the same time in contrast to conventionally separated labour‐intensive glass thinning and texturing processes. For 2 cm² size poly‐Si thin‐film solar cells on glass superstrate, this wet etching successfully thins down the glass from 3 mm to 0.5 mm to check the ultimate benefit of the process and introduces a large micron texture on the sun‐facing glass surface. The process enhances J_sc by 6.3% on average, with the optimal J_sc enhancement of 8%, better than the value of 4.6% found in the literature. This process also reduces the loss in external quantum efficiency (EQE loss), which is due to light leaking to neighbouring cells, dramatically. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Identification of intra‐grain and grain boundary defects in polycrystalline Si thin films by electron paramagnetic resonance

We investigate the characteristics of intra‐grain and grain boundary defects in polycrystalline Si films, by employing quantitative electron paramagnetic resonance measurements on liquid phase crystallized layers with an average grain size of 200 µm and tailored solid phase crystallized Si layers with similar intra‐grain morphology but systematically varied grain sizes between 0.25 µm and 1 µm. The defect characteristics are found to be composed of two distinctive g ‐values of g = 2.0055 and 2.0032, which are attributed to grain boundary defects and intra‐grain defects, respectively. Additional hydrogenation leads to a reduction of the overall defect concentration, while a rapid thermal annealing process primarily heals intra‐grain defects.

     

Study of detached back reflector designs for thin‐film silicon solar cells

We present a precise and flexible method to investigate the impact of diverse detached reflector designs on the optical response of p–i–n thin‐film silicon solar cells. In this study, the term detached reflectors refers to back reflectors that are separated from the silicon layers by an intermediate rear dielectric of several micrometers. Based on the utilization of a highly conductive n‐doped layer and a local electrical contact scheme, the method allows the use of non‐conductive rear dielectrics such as air or transparent liquids. With this approach, diverse combinations of back reflector and rear dielectric can be placed behind the same solar cell, providing a direct evaluation of their impact on the device performance. We demonstrate the positive effect of a rear dielectric of low refractive index on the light trapping and compare the performance of solar cells with an air/Ag and a standard ZnO/Ag back reflector design. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unlinking absorption and haze in thin film silicon solar cells front electrodes

We study the respective influence of haze and free carrier absorption (FCA) of transparent front electrodes on the photogenerated current of micromorph thin film silicon solar cells. To decouple the haze and FCA we develop bi‐layer front electrodes: a flat indium tin oxide layer assures conduction and allows us to tune FCA while the haze is adjusted by varying the thickness of a highly transparent rough ZnO layer. We show how a minimum amount of FCA leads only to a few percents absorption for a single light path but to a strong reduction of the cell current in the infrared part of the spectrum. Conversely, a current enhancement is shown with increasing front electrode haze up to a saturation of the current gain. This saturation correlates remarkably well with the haze of the front electrode calculated in silicon. This allows us to clarify the requirements for the front electrodes of micromorph cells. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication of polycrystalline silicon films by Al‐induced crystallization of silicon‐rich oxide films

Polycrystalline silicon (poly‐Si) films were fabricated by aluminum (Al)‐induced crystallization of Si‐rich oxide (SiO_x) films. The fabrication was achieved by thermal annealing of SiO_x /Al bilayers below the eutectic temperature of the Al–Si alloy. The poly‐Si film resulting from SiO_1.45 exhibited good crystallinity with highly preferential (111) orientation, as deduced from Raman scattering, X‐ray diffraction, and transmission electron microscopy measurements. The poly‐Si film is probably formed by the Al‐induced layer exchange mechanism, which is mediated by Al oxide.     

Acceleration and mitigation of carrier‐induced degradation in p‐type multi‐crystalline silicon

Recently, a new carrier‐induced defect has been reported in multi‐crystalline silicon (mc‐Si), and has been shown to be particularly detrimental to the performance of passivated emitter and rear contact (PERC) cells. Under normal conditions, this defect can take years to fully form. This Letter reports on the accelerated formation and subsequent passivation of this carrier‐induced defect through the use of high illumination intensity and elevated temperatures resulting in passivation within minutes. The process was tested on industrial mc‐Si PERC solar cells, where degradation after a 100 hour stability test was suppressed to only 0.1% absolute compared to 2.1% for non‐treated cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Light‐induced degradation of PECVD aluminium oxide passivated silicon solar cells

Light‐induced degradation (LID) has been identified to be a critical issue for solar cells processed on boron‐doped silicon substrates. Typically, Czochralski‐grown silicon (Cz‐Si) has been reported to suffer from stronger LID than block‐cast multicrystalline silicon (mc‐Si) due to higher oxygen concentrations. This work investigates LID under conditions practically relevant under module operation on different cell types. It is shown that aluminium oxide (AlOx) passivated mc‐Si solar cells degrade more than a reference aluminium back surface field mc‐Si cell and, remarkably, an AlOx passivated Cz‐Si solar cell. The defect which is activated by illumination is shown to be doubtful a sole bulk effect while the AlOx passivation might play a certain role. This work may contribute to a re‐evaluation of the suitability of boron‐doped Cz‐ and mc‐Si for solar cells with very high efficiencies. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Multi‐crystalline silicon solar cells with metal‐assisted nano‐texturing using HNO3 as hole injection agent

In this study, metal‐assisted etching (MAE) with nitric acid (HNO₃) as a hole injecting agent has been employed to texture multi‐crystalline silicon wafers. It was previously proven that addition of HNO₃ enabled control of surface texturing so as to form nano‐cone shaped structures rather than nanowires. The process parameters optimized for optically efficient texturing have been applied to multi‐crystalline wafers. Fabrication of p‐type Al:BSF cells have been carried out on textured samples with thermal SiO₂/PECVD‐SiN_x stack passivation and screen printed metallization. Firing process has been optimized in order to obtain the best contact formation. Finally, j_sc enhancement of 0.9 mA/cm² and 0.6% absolute increase in the efficiency have been achieved. This proves that the optimized MAE texture process can be successfully used in multi‐crystalline wafer texturing with standard passivation methods.

J –V curves and SEM images of the nano and iso‐textured samples. j_sc enhancement of 0.9 mA/cm² together with 0.6% absolute efficiency gain was observed on nano‐textured samples.     

非晶硅太阳电池宽光谱陷光结构的优化设计

陷光是改善薄膜太阳电池光吸收进而提高其效率的关键技术之一. 以非晶硅（α-Si）薄膜太阳电池为例,设计了一种新的复合陷光结构：在Ag背电极与硅薄膜之间制备一维Ag纳米光栅,并通过保形生长在电池前表面沉积织构的减反膜. 采用有限元数值模拟方法,研究了该复合陷光结构对电池光吸收的影响,并对Ag纳米光栅的结构参数进行了优化. 模拟结果表明：该复合陷光结构可在宽光谱范围内较大地提高太阳电池的光吸收;当Ag纳米光栅的周期P为600 nm,高度H为90 nm,宽度W为180 nm时,在AM1.5光谱垂直入射条件下α-Si薄膜电池在300–800 nm波长范围内总的光吸收较无陷光结构的参考电池提高达103%,其中在650–750 nm长波范围内的光子吸收率提高达300%以上. 结合电场强度分布,对电池在各个波段光吸收提高的物理机制进行了分析. 另外,该复合陷光结构的引入,还较大地改善了非晶硅电池对太阳光入射角度的敏感性. 关键词： 非晶硅太阳电池 陷光 银纳米光栅 数值模拟     

锥形二维光子晶体太阳电池数值模拟

采用时域有限差分方法,模拟研究在本征吸收层引入锥形二维光子晶体(2D PC)后,其结构参数变化对单结微晶硅电池各膜层吸收的影响规律.研究表明,2D PC的纵横比(高度与周期之比)对电池本征吸收具有决定性影响.周期小于1μm时,本征吸收随着纵横比的增大先上升后下降,纵横比为1时达到最大值;周期大于1μm时,本征吸收达到最大值的纵横比小于1,且周期越大,实现本征吸收最大化的纵横比越小.当周期为0.5μm,纵横比为1时,锥形2D PC电池的本征吸收达到峰值,短路电流密度为27.8 mA/cm2;与平面结构相比,短路电流密度提升5.8 mA/cm~2,相对增加27%.该研究突破了以往认为绒面陷光效果主要取决于绒面形貌横向特征尺寸的观点,对实验获取最佳的周期或随机绒面陷光结构具有指导意义.     

PIN型和NIP型硅薄膜太阳电池中绒面陷光结构和陷光性能研究

对于硅薄膜太阳电池来说, 无论是PIN型还是NIP型太阳电池, 采用绒面陷光结构来提高入射光的有效利用率是提高太阳电池效率的重要方法之一.本文采用标度相干理论对PIN和NIP型电池的绒面结构的陷光性能进行了数值模拟. 结果表明: PIN电池中前电极和NIP电池中背电极衬底粗糙度分别为160和40 nm时可获得理想的陷光效果; 在不同粗糙度背电极衬底上制备a-SiGe:H电池发现, 使用40和61.5 nm 背电极可获得相当的短路电流密度, 理论分析和实验得到了一致的结果. 关键词： 陷光结构 光散射能力 标量相干理论 硅基薄膜太阳电池     

Absorption enhancement of ultrathin crystalline silicon solar cells with frequency upconversion nanosphere arrays

A design of ultrathin crystalline silicon solar cells patterned with α-NaEr_(0.2)Y_(0.8)F_4 upconversion nanosphere(NSs) arrays on the surface was proposed. The light trapping performance ofα-NaEr_(0.2)Y_(0.8)F_4 NSs with different ratios of sphere diameter to sphere pitch was systematically studied by COMSOL Multiphysics. The influence of different NS diameters and ratio to the average optical absorption of ultrathin crystalline silicon solar cell was calculated, as well as the short circuit current densities. The results show that the average optical absorption of solar cells with 2.33 μm silicon covered by α-NaEr_(0.2)Y_(0.8)F_4 NSs of 100 nm in diameter and 5.2 in ratio has improved by 8.5% compared to planar silicon solar cells with the same thickness of silicon. The light trapping performance of different thicknesses of silicon solar cells with the optimized configuration of NSs was also discussed. The results indicate that our structure enhances the light absorption. The presented model will be the basis for further simulations concerning frequency upconversion of α-NaEr_(0.2)Y_(0.8)F_4 materials.     

Photonic nanostructures for advanced light trapping in silicon solar cells: the impact of etching on the material electronic quality

Dry plasma etching, commonly used by the Photonics community as the etching technique for the fabrication of photonic nanostructures, could be a source of device performance limitations when used in the frame of silicon photovoltaics. So far, the lack of silicon solar cells with state‐of‐the‐art efficiencies utilizing nanophotonic concepts shows how challenging their integration is, owing to the trade‐off between optical and electrical properties. In this study we show that dry plasma etching results in the degradation of the silicon material quality due to (i) a high density of dangling bonds and (ii) the presence of sub‐surface defects, resulting in high surface recombination velocities and low minority carrier lifetimes. On the contrary, wet chemical anisotropic etching used as an alternative, leads to the formation of inverted nanopyramids that result in low surface recombination velocity and low density of dangling bonds. The proposed inverted nanopyramids could enable high efficiency photonic assisted solar cells by offering the potential to achieve higher short‐circuit current without degrading the open circuit voltage. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Light‐induced degradation in indium‐doped silicon

Light‐induced degradation of charge carrier lifetime was observed in indium‐doped silicon. After defect formation, an annealing step at 200 °C for 10 min deactivates the defect and the initial charge carrier lifetime is fully recovered. The observed time range of the defect kinetics is similar to the well known defect kinetics of the light‐induced degradation in boron‐doped samples. Differences between defect formation in boron‐ and indium‐doped silicon are detected and discussed. A new model based on an acceptor self‐interstitial A_Si–Si_i defect is proposed and established with experimental findings and existing ab‐initio simulations.

     

Light‐induced degradation and regeneration of multicrystalline silicon Al‐BSF and PERC solar cells

Light‐induced degradation (LID) is a well‐known problem faced by p‐type Czochralski (Cz) monocrystalline silicon (mono‐Si) wafer solar cells. In mono‐Si material, the physical mechanism has been traced to the formation of recombination active boron‐oxygen (B–O) complexes, which can be permanently deactivated through a regeneration process. In recent years, LID has also been identified to be a significant problem for multicrystalline silicon (multi‐Si) wafer solar cells, but the exact physical mechanism is still unknown. In this work, we study the effect of LID in two different solar cell structures, aluminium back‐surface‐field (Al‐BSF) and aluminium local back‐surface‐field (Al‐LBSF or PERC (passivated emitter and rear cell)) multi‐Si solar cells. The large‐area (156 mm × 156 mm) multi‐Si solar cells are light soaked under constant 1‐sun illumination at elevated temperatures of 90 °C. Our study shows that, in general, PERC multi‐Si solar cells degrade faster and to a greater extent than Al‐BSF multi‐Si solar cells. The total degradation and regeneration can occur within ～320 hours for PERC cells and within ～200 hours for Al‐BSF cells, which is much faster than the timescales previously reported for PERC cells. An important finding of this work is that Al‐BSF solar cells can also achieve almost complete regeneration, which has not been reported before. The maximum degradation in Al‐BSF cells is shown to reduce from 2% (relative) to an average of 1.5% (relative) with heavier phosphorus diffusion.     

Extremely low surface recombination in 1 Ω cm n‐type monocrystalline silicon

A key requirement in the recent development of highly efficient silicon solar cells is the outstanding passivation of their surfaces. In this work, plasma enhanced chemical vapour deposition of a triple layer dielectric consisting of amorphous silicon, silicon oxide and silicon nitride, charged extrinsically using corona, has been used to demonstrate extremely low surface recombination. Assuming Richter's parametrisation for bulk lifetime, an effective surface recombination velocity S_eff = 0.1 cm/s at Δn = 10¹⁵ cm^–3 has been obtained for planar, float zone, n ‐type, 1 Ω cm silicon. This equates to a saturation current density J_0s = 0.3 fA/cm², and a 1‐sun implied open‐circuit voltage of 738 mV. These surface recombination parameters are among the lowest reported for 1 Ω cm c‐Si. A combination of impedance spectroscopy and corona‐lifetime measurements shows that the outstanding chemical passivation is due to the small hole capture cross section for states at the interface between the Si and a‐Si layer which are hydrogenated during nitride deposition. (© 2016 The Authors. Phys. Status Solidi RRL published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light trapping for a‐Si:H/µc‐Si:H tandem solar cells using direct pulsed laser interference texturing

We present a‐Si:H/µc‐Si:H tandem solar cells on laser textured ZnO:Al front contact layers. Direct pulsed laser interference patterning (DLIP) was used for writing arrays of one‐dimensional micro gratings of submicron period into ZnO:Al films. The laser texture provides good light trapping which is indicated by an increase in short‐circuit current density of 20% of the bottom cell limited device compared to cells on planar ZnO:Al. The open‐circuit voltage of the cells on laser textured ZnO:Al is almost the same as for cells on planar substrates, indicating excellent growth conditions for amorphous and microcrystalline silicon on the U‐shaped grating grooves. DLIP is a simple, single step and industrially applicable method for large area periodic texturing of ZnO:Al thin films. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Comparison of batch and in‐line PECVD of a‐Si:H passivation layers for silicon heterojunction solar cells

We present PECVD deposition of i‐a‐Si:H in an in‐line configuration for the fabrication of silicon heterojunction solar cells. For industry, in‐line processing has the potential to increase production throughput and yield. We compared batch and in‐line fabrication of i‐a‐Si:H passivation samples with identical plasma conditions and observed that the a‐Si:H material properties do not significantly differ. In batch‐type production the substrate is in the plasma zone at the moment of ignition, whereas for in‐line deposition the substrate is introduced into the plasma zone when steady plasma conditions have been reached. Our preliminary results show that there are depositions conditions that result both for in‐line and batch‐type deposition in good i‐a‐Si:H passivation layers. Therefore both methods can equally well be considered for the production of silicon heterojunction solar cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

In‐situ series connection of an amorphous silicon photovoltaic tandem module

Local shading during thin film deposition monolithically interconnects thin film solar cells into photovoltaic modules. This in‐situ series connection method is, for the first time, applied to an amorphous silicon tandem cell structure. Sequential maskingand mask‐shifting forms the electrical series connection in‐situ, i.e. during the sputtering and plasma depositions of the contact and semiconductor layers of the thin film solar cell structure. The resulting photovoltaic module consists of five amorphous silicon tandem cells with a total module area A_{\rm m} =12.5\mathop {\;{\rm cm}}\nolimits^2. The module exhibits a fill factor {\rm FF}_{\rm m} > 72% and a total area efficiency \eta _{\rm m} =6.2%. Thermographic imaging proves successful patterning by local shading and attributes low shunt resistances of some component cells to single pinhole formation. The dark‐shunt resistance of each cell of the module amounts to R_{{\rm sc}{\rm,dark}{\rm,cell}} > 13\;{\rm k\Omega }\;{\rm cm}^{2} and for the complete module to R_{{\rm sc}{\rm,dark}{\rm,module}} =454\;{\rm k\Omega }\;{\rm cm}^{2}. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)