Novel profiled thin-film polycrystalline silicon solar cells onstainless steel substrates

In order to obtain higher conversion efficiencies while keeping the manufacturing cost low in thin-film PV technologies, a possible low bandgap material is amorphous silicon germanium. Although record efficiencies in excess of 15% have been reported for triple-junction solar cells comprising these alloys, concerns regarding the stability and quality of this material still need to be overcome. Another approach is the introduction of thin-film micro- or polycrystalline silicon with a band gap of 1.1 eV, deposited at a temperature that is low enough to allow cheap, “foreign” carrier materials. Apart from the application of a modified PECVD method utilizing frequencies in the VHP domain, the hot wire CVD (HWCVD) method appears a particularly promising technique for the deposition of high-quality thin-film intrinsic or doped poly-Si. In this contribution, special attention will be paid to the latest developments in the application of hot-wire deposited silicon thin films in solar cells. By implementing a profiled hydrogen-diluted HWCVD growth scheme that produces a thin small-grained seed layer on top of a thin n-layer, we have been able to obtain fast polycrystalline growth of the intrinsic layer of an n-i-p solar cell. An efficiency of 4.41% is obtained and the fill factor is 0.607. The current density is close to 20 mA/cm² for an i-layer that is only 1.22 μm thick. The cell is deposited on plain stainless steel and thus does not comprise a back reflector     

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.     

A Polymer/Carbon‐Nanotube Ink as a Boron‐Dopant/Inorganic‐Passivation Free Carrier Selective Contact for Silicon Solar Cells with over 21% Efficiency

Traditional silicon solar cells extract holes and achieve interface passivation with the use of a boron dopant and dielectric thin films such as silicon oxide or hydrogenated amorphous silicon. Without these two key components, few technologies have realized power conversion efficiencies above 20%. Here, a carbon nanotube ink is spin coated directly onto a silicon wafer to serve simultaneously as a hole extraction layer, but also to passivate interfacial defects. This enables a low‐cost fabrication process that is absent of vacuum equipment and high‐temperatures. Power conversion efficiencies of 21.4% on an device area of 4.8 cm² and 20% on an industrial size (245.71 cm²) wafer are obtained. Additionally, the high quality of this passivated carrier selective contact affords a fill factor of 82%, which is a record for silicon solar cells with dopant‐free contacts. The combination of low‐dimensional materials with an organic passivation is a new strategy to high performance photovoltaics.     

Wafer bonded four‐junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cells with 44.7% efficiency

Triple‐junction solar cells from III–V compound semiconductors have thus far delivered the highest solar‐electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four‐junction solar cell architectures with optimum bandgap combination are found for lattice‐mismatched III–V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs‐based top tandem solar cell structure was bonded to an InP‐based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four‐junction solar cell with a new record efficiency of 44.7% at 297‐times concentration of the AM1.5d (ASTM G173‐03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III–V multi‐junction solar cells having four and in the future even more junctions. Copyright © 2014 John Wiley & Sons, Ltd.     

Silicon heterojunction solar cell: a new buffer Layer concept with low-temperature epitaxial silicon

Amorphous silicon/crystalline silicon heterojunction solar cells, deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique, have been fabricated using different technologies to passivate defects at the heterointerface: without treatment, the insertion of a thin intrinsic amorphous layer or that of a thin intrinsic epitaxial layer. The open circuit voltage of heterojunction solar cells fabricated including an intrinsic amorphous buffer layer is strangely lower than in devices with no buffer layer. The structure of the amorphous buffer layer is investigated by high resolution transmission electron microscope observations. As an alternative to amorphous silicon, the insertion of a fully epitaxial silicon layer, deposited at low temperature with conventional PECVD technique in a hydrogen-silane gas mixture, was tested. Using the amorphous silicon/crystalline silicon (p a-Si/i epi-Si/n c-Si) heterojunction structure in solar cells, a 13.5% efficiency and a 605-mV open circuit voltage were achieved on flat Czochralski silicon substrates. These results demonstrate that epitaxial silicon can be successfully used to passivate interface defects, allowing for an open circuit voltage gain of more than 50 mV compared to cells with no buffer layer. In this paper, the actual structure of the amorphous silicon buffer layer used in heterojunction solar cells is discussed. We make the hypothesis that this buffer layer, commonly considered amorphous, is actually epitaxial.     

Analysis of high-efficiency silicon solar cells

Performance data on high-efficiency concentrator (>18 percent at 25 suns) silicon solar cells are compared to results from an exact numerical model in which all parameters for the calculation are taken from the existing literature on bulk silicon. The numerical solution of the transport equations includes the effects of Fermi-Dirac statistics, bandgap narrowing, and Auger recombination. Cell performances as a function of sunlight concentration are predicted with reasonable accuracy using this model. Evidence for the existence of bandgap-narrowing effects is found by comparing experimental data to calculated values of spectral quantum efficiencies and open-circuit voltages under a variety of lifetime assumptions. The validity of using superposition with simple diode equations to approximate the behavior of silicon solar cells also is examined.     

27.5-percent silicon concentrator solar cells

Recent advances in silicon solar cells using the backside point-contact configuration have been extended resulting in 27.5-percent efficiencies at 10 W/cm²(100 suns, 24°C), making these the most efficient solar cells reported to date. The one-sun efficiencies under an AM1.5 spectrum normalized to 100 mW/cm²are 22 percent at 24°C based on the design area of the concentrator cell. The improvements reported here are largely due to the incorporation of optical light trapping to enhance the absorption of weakly absorbed near bandgap light. These results approach the projected efficiencies for a mature technology which are 23-24 percent at one sun and 29 percent in the 100-350-sun (10-35 W/ cm²) range.     

New interconnection technology for enhanced module efficiency

In order to enhance solar modular efficiency, an innovative interconnection method for solar cells has been developed. The solar cells are two-dimensionally interconnected to a large-area, shingle-roof patterned solar cell array. Test samples were fabricated using silicon solar cells with conventional cell structures. Packing densities over 96% and module efficiencies of 17.3% and 13.4% (AM 1.5, 100 mW/cm² ) were obtained for single-crystalline and polycrystalline silicon solar cells, respectively     

Eco-friendly fabrication of PBDTTPD:PC71BM solar cells reaching a PCE of 3.8% using water-based nanoparticle dispersions

In this work we report on the eco-friendly processing of PBDTTPD:PC₇₁BM organic solar cells using water-based nanoparticle (NP) dispersions. The polymer:fullerene NPs are prepared using the miniemulsion-solvent evaporation method, despite employing high-boiling solvents. Polymer solar cells are fabricated from these blend NPs and the device characteristics are studied in function of annealing time and temperature. The photoactive layer formation is carefully analyzed using atomic force microscopy (AFM). Annealing for longer times significantly increases the power conversion efficiency (PCE), up to 3.8%, the highest value reported for surfactant based NP solar cells. Our work shows that the low bandgap polymer PBDTTPD has the ability to afford reasonable efficiencies in NP solar cells in combination with PC₇₁BM and paves the way to a truly eco-friendly processing of organic photovoltaics (OPVs).     

High‐efficiency solar cells on phosphorus gettered multicrystalline silicon substrates

Measurements of the dislocation density are compared with locally resolved measurements of carrier lifetime for p‐type multicrystalline silicon. A correlation between dislocation density and carrier recombination was found: high carrier lifetimes (>100 µs) were only measured in areas with low dislocation density (<10⁵ cm⁻²), in areas of high dislocation density (>10⁶ cm⁻²) relatively low lifetimes (<20 µs) were observed. In order to remove mobile impurities from the silicon, a phosphorus diffusion gettering process was applied. An increase of the carrier lifetime by about a factor of three was observed in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test the effectiveness of the gettering in a solar cell manufacturing process, five different multicrystalline silicon materials from four manufacturers were phosphorus gettered. Base resistivity varied between 0·5 and 5 Ω cm for the boron‐ and gallium‐doped p‐type wafers which were used in this study. The high‐efficiency solar cell structure, which has led to the highest conversion efficiencies of multicrystalline silicon solar cells to date, was used to fabricate numerous solar cells with aperture areas of 1 and 4 cm². Efficiencies in the 20% range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm² (20·3%) and 4 cm² (19·8%) cells were achieved on 0·6 and 1·5 Ω cm, respectively. This proves that multicrystalline silicon of very different material specification can yield very high efficiencies if an appropriate cell process is applied. Copyright © 2006 John Wiley & Sons, Ltd.     

Designing III–V multijunction solar cells on silicon

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells featuring a Si subcell also serving as substrate. Subject to this Si bandgap constraint, we design optimum cell structures that we show depart from the unconstrained ideal. In order to progress to manufacturable designs, the use of III–V materials is considered, using a novel growth method capable of yielding low defect density III–V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current–voltage characteristics using the same parameters. The method yields a single‐parameter fit in terms of the Shockley–Read–Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few per cent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III–V and Si materials. Copyright © 2014 John Wiley & Sons, Ltd.     

微晶硅薄膜微结构对稳定性的影响

实验研究了不同晶化率的微晶硅（μc-Si）薄膜的光衰退现象,提出了制备高稳定性硅薄膜太阳电池材料选取方案。研究结果表明,μc-Si中的非晶硅（a-Si）组分是导致光衰退的主要原因,晶化率越高,材料越稳定;过渡区靠近μc-Si材料区域的μc-Si材料,由于具有更好的稳定性和光敏特性,适于制备μc-Si太阳能电池;过渡区附...     

A comparative human health,ecotoxicity, and product environmental assessment on the production of organic and silicon solar cells

A life cycle assessment case study involving organic photovoltaic technology using phenyl‐C₆₁‐butyric acid methyl ester and poly(3‐hexylthiophene) is presented. Although solar technology converts freely available solar radiation into more useful forms of energy, potential environmental impacts can occur during the life cycle of the product. A cradle‐to‐gate life cycle assessment is completed, comparing organic solar cells with traditional silicon‐based cells across 18 multiple criteria. The functional unit is defined as the production of 1 watt‐peak of electricity produced. The inventory is based on prospective organic solar cell technology and two traditional silicon technologies. The results demonstrate that from a life cycle perspective, organic solar cells can outperform conventional silicon solar cells with impacts reduced by 93%. The energy payback time for the default organic photovoltaic cell was 0.21 years (75 days) compared with multicrystalline silicon and amorphous silicon's 2.7 and 2.2 years, respectively. The minimum required lifetime of the organic cells, so that their impacts were no worse than amorphous silicon's over 25 years, was measured between 1.2 and 8.9 years. Results of the sensitivity analysis demonstrate that consideration of manufacturing routes (e.g., fullerene or solar cell production) can be targeted using life cycle assessment for further improvements in the environmental, human health, and ecotoxicity profile of organic solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

晶体硅太阳电池扩散气氛场均匀性研究

为降低晶体硅太阳电池的制造成本,从扩散气氛场角度提出实验方法,优化扩散工艺均匀性。该研究方法可改善太阳电池电性能并对产业化生产起指导作用。     

Passivation Properties and Formation Mechanism of Amorphous Halide Perovskite Thin Films

Lead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high-quality crystals with tunable bandgaps in the visible and near-infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin-film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells.     

Monolithic Perovskite/Silicon Tandems with >28% Efficiency: Role of Silicon-Surface Texture on Perovskite Properties

Textured silicon wafers used in silicon solar cell manufacturing offer superior light trapping, which is a critical enabler for high-performance photovoltaics. A similar optical benefit can be obtained in monolithic perovskite/silicon tandem solar cells, enhancing the current output of the silicon bottom cell. Yet, such complex silicon surfaces may affect the structural and optoelectronic properties of the overlying perovskite films. Here, through extensive characterization based on optical and microstructural spectroscopy, it is found that the main effect of such substrate morphology lies in an altering of the photoluminescence response of the perovskite, which is associated with thickness variations of the perovskite, rather than lattice strain or compositional changes. With this understanding, the design of high-performance perovskite/silicon tandems is rationalized, yielding certified power conversion efficiencies of >28%.     

Influence of boron doping on the properties of amorphous and microcrystalline SiC films prepared using ECR-CVD

Hydrogenated silicon carbide films (SiC:H) were deposited using the electron cyclotron resonance plasma chemical vapor deposition (ECR-CVD) method from a mixture of methane, silane, and hydrogen, with diborane as the doping gas. The effect of changes in the percentage of the diborane to reactant gas mixture on the deposition rate, optical bandgap, and photoconductivity were investigated. There is evidence from Raman scattering analysis to show that films deposited at a low microwave power of 150 W were all amorphous and the bandgap decreases as the diborane level is increased whereas films deposited at a high microwave power of 800 W at low diborane levels are highly photoconductive and contain microcrystalline silicon inclusions. These films become amorphous as the diborane level is increased, while the optical bandgap remains relatively unaffected throughout the entire range of diborane levels investigated. The effect of the microwave power was also investigated. The conductivity increases rapidly to a maximum, followed by rapid reduction at high microwave powers. Raman scattering analysis showed evidence of the formation and increase of microcrystalline silicon inclusions and diamond-like components in the films, the former of which could account for the rapid increase and the latter the subsequent decrease in the conductivity.     

The Characteristics of a Hydrogenated Amorphous Silicon Semitransparent Solar Cell When Applying n/i Buffer Layers

In this work, buffer layers with various conditions are inserted at an n/i interface in hydrogenated amorphous silicon semitransparent solar cells. It is observed that the performance of a solar cell strongly depends on the arrangement and thickness of the buffer layer. When arranging buffer layers with various bandgaps in ascending order from the intrinsic layer to the n layer, a relatively high open circuit voltage and short circuit current are observed. In addition, the fill factors are improved, owing to an enhanced shunt resistance under every instance of the introduced n/i buffer layers. Among the various conditions during the arrangement of the buffer layers, a reverse V shape of the energy bandgap is found to be the most effective for high efficiency, which also exhibits intermediate transmittance among all samples. This is an inspiring result, enabling an independent control of the conversion efficiency and transmittance.     

非晶硅顶电池中的n型掺杂层对非晶硅/微晶硅叠层太阳电池性能的影响

采用超高频等离子增强化学气相沉积(VHF-PECVD)技术,逐次高速沉积非品硅顶电池及微晶硅底电池,形成pin/pin型非晶硅/微晶硅叠层电池.通常顶电池的n层与底电池的P层均采用微晶硅材料来形成隧穿复合结,然而该叠层电池的光谱响应测试结果表明,顶电池存在着明显的漏电现象.针对该问题作者提出,在顶电池的微品硅n层中引入非晶硅n保护层的方法.实验结果表明,非晶硅n层的引入有效地改善了顶电池漏电的现象;在非晶硅n层的厚度为6nm时,顶电池的漏电现象消失,叠层电池的开路电压由原来的1.27提高到1.33V,填允因子由60%提高剑63%.     

Efficient silicon heterojunction solar cells based on p‐ and n‐type substrates processed at temperatures < 220°C

We present the optimization and characterization of heterojunction solar cells consisting of an amorphous silicon emitter, a single crystalline absorber and an amorphous silicon rear side which causes the formation of a back surface field (a‐Si:H/c‐Si/a‐Si:H). The solar cells were processed at temperatures <220°C. An optimum of the gas phase doping concentration of the a‐Si:H layers was found. For high gas phase doping concentrations, recombination via defects located at or nearby the interface leads to a decrease in solar cell efficiency. We achieved efficiencies >17% on p‐type c‐Si absorbers and >17·5% on n‐type absorbers. In contrast to the approach of Sanyo, no additional intrinsic a‐Si:H layers between the substrate and the doped a‐Si:H layers were inserted. Copyright © 2006 John Wiley & Sons, Ltd.