Effectiveness of full spectrum light soaking on solar cell degradation analysis

This study examined the effect of UV-spectrum light soaking on solar cell degradation tests. An indoor light soaking test was evaluated over three different spectral ranges: “UV only”, “UV blocked” and “Full spectrum”. a-Si:H and poly-crystalline silicon solar cell technologies were studied by light soaking tests with the same optical filter configurations.The I–V measurement results demonstrated that “UV only” irradiated solar cells exhibited the smallest output power degradation, which was only half of a percent variation compared with the full spectrum light soaking case. Using a filter that excluded the effect of the UV spectral range on light soaking, the “UV blocked” case also exhibited a significant output power degradation of the solar cells. A comparative analysis of the solar cell response, based on the I–V characteristics and the diode ideality factor under these different light soaking spectra, demonstrated the importance of the full spectrum light soaking test in the evaluation of the long-term performance of solar cells.     

Cu2ZnSn(S1–xSex)4 thin film solar cells prepared by water‐based solution process

The kesterite Cu₂ZnSn(S_1–xSe_x)₄ (CZTSSe) thin film solar cell has been developed rapidly due to its excellence in structural and optical properties and its abundance in raw materials. Both vacuum‐based and solution‐based methods have been successfully employed to fabricate CZTSSe thin film solar cells. In this Letter, we report an environmentally friendly, water‐based, solution process for fabrication of high‐efficiency CZTSSe thin film solar cells. High quality CZTSSe thin film is obtained by selenization under high temperature and Se vapor. An efficiency of 6.2% is achieved on CZTSSe thin film solar cell fabricated by such water‐based solution process. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-temperature-processed ZnO thin films as electron transporting layer to achieve stable perovskite solar cells

Morphology and surface property of ZnO thin films as electron transporting layer in perovskite solar cells are crucial for obtaining high-efficient and stable perovskite solar cells. In this work, two different preparation methods of ZnO thin films were carried out and the photovoltaic performances of the subsequent perovskite solar cells were investigated. ZnO thin film prepared by sol–gel method was homogenous but provided high series resistance in solar cells, leading to low short circuit current density. Lower series resistance of solar cell was obtained from homogeneous ZnO thin film from spin-coating of colloidal ZnO nanoparticles (synthesized by hydrolysis–condensation) in a mixture of 1-butanol, chloroform and methanol. The perovskite solar cells using this film achieved the highest power conversion efficiency (PCE) of 4.79% when poly(3-hexylthiophene) was used as a hole transporting layer. In addition, the stability of perovskite solar cells was also examined by measuring the photovoltaic characteristic for six consecutive weeks with the interval of 2 weeks. It was found that using double layers of the sol–gel ZnO and ZnO nanoparticles provided better stability with no degradation of PCE in 10 weeks. Therefore, this work provides a simple method for preparing homogeneous ZnO thin films in order to achieve stable perovskite solar cells, also for controlling their surface properties which help better understand the characteristics of perovskite solar cells.     

Nanoimprint texturing of transparent flexible substrates for improved light management in thin‐film solar cells

We present a nanoimprint based approach to achieve efficient light management for solar cells on low temperature transparent polymer films. These films are particularly low‐priced, though sensitive to temperature, and therefore limiting the range of deposition temperatures of subsequent solar cell layers. By using nanoimprint technology, we successfully replicated optimized light trapping textures of etched high temperature ZnO:Al on a low temperature PET film without deterioration of optical properties of the substrate. The imprint‐textured PET substrates show excellent light scattering properties and lead to significantly improved incoupling and trapping of light in the solar cell, resulting in a current density of 12.9 mA/cm², similar to that on a glass substrate. An overall efficiency of 6.9% was achieved for a flexible thin‐film silicon solar cell on low cost PET substrate. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Plasmonic fractals: ultrabroadband light trapping in thin film solar cells by a Sierpinski nanocarpet

Plasmonic Sierpinski nanocarpet as back structure for a thin film Si solar cell is investigated. We demonstrate that ultra-broadband light trapping can be obtained by placing square metallic nanoridges with Sierpinski pattern on the back contact of the thin film solar cell. The multiple-scale plasmonic fractal structure allows excitation of localized surface plasmons and surface plasmon polaritons in multiple wavelengths leading to obvious absorption enhancements in a wide frequency range. Full wave simulations show that 109 % increase of the short-circuit current density for a 200 nm thick solar cell, is achievable by the proposed fractal back structure. The amount of light absorbed in the active region of this cell is more than that of a flat cell with semiconductor thickness of 1,000 nm.     

The characteristic of Cu2ZnSnS4 thin film solar cells prepared by sputtering CuSn and CuZn alloy targets

Recent study shows that the main reason for limiting CZTS device performance lies in the low open circuit voltage, and crucial factor that could affect the V_oc is secondary phases like ZnS existing in absorber layer and its interfaces. In this work, the Cu₂ZnSnS₄ thin film solar cells were prepared by sputtering CuSn and CuZn alloy targets. Through tuning the Zn/Sn ratios of the CZTS thin films, the crystal structure, morphology, chemical composition and phase purity of CZTS thin films were characterized by X-Ray Diffraction (XRD), scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) and Raman spectroscopy. The statistics data show that the CZTS solar cell with a ratio of Zn/Sn = 1.2 have the best power convention efficiency of 5.07%. After HCl etching process, the CZTS thin film solar cell with the highest efficiency 5.41% was obtained, which demonstrated that CZTS film solar cells with high efficiency could be developed by sputtering CuSn and CuZn alloy targets.     

Buried emitter solar cell structures: Decoupling of metallisation geometry and carrier collection geometry of back contacted solar cells

We present a novel solar cell structure, the “buried emitter solar cell”. This concept is designed for decoupling the metallisation geometry from the geometry of the carrier collecting p–n junction in back‐contacted (and in particular back‐junction) solar cells without requiring electrical insulation by dielectric layers. The most prominent features of this device structure are a carrier collecting emitter that covers close to 100% of the total cell area and an effective electrical insulation between emitter and base metallisation via a p⁺–n⁺ junction. The experimental results presented in this paper report a 19.5% efficient “buried emitter solar cell”, where 50% of the solar cell's rear side exhibit a p⁺–n⁺ junction. This preparation technique implies covering a boron‐doped p‐type emitter with an n‐type surface layer that can be efficiently surface‐passivated by thermal oxidation. All structuring of this cell has been performed by laser processing without any photo‐lithography. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced photovoltaic performance of solution-processed Sb2Se3 thin film solar cells by optimizing device structure

Thin-film solar cells have attracted worldwide attention due to their high efficiency and low cost. Antimony selenide (Sb₂Se₃) is a promising light absorption material candidate for thin-film solar cells due to its suitable band gap, abundance, low toxicity, and high chemical stability. Herein, we fabricate an Sb₂Se₃ thin film solar cell using a simple hydrazine solution process. By controlling the thickness of the photoactive layer and inserting a poly(3-hexylthiophene) hole-transporting layer, an Sb₂Se₃ solar cell with a power conversion efficiency of 2.45% was achieved.     

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)     

Enhancement of silicon solar cell performances due to light trapping by colloidal metal nanoparticles

Photovoltaics is the most promising technology for the future of green energy production. To fully realize the potential use of photovoltaic technology, low manufacturing cost and high working photoconversion efficiency must be obtained. Light trapping by metal nanoparticles is an attractive strategy in thin film as well as in bulk silicon solar cells aimed to confine light within the active layer to promote the photon absorption and therefore achieving higher efficiency. In this paper, we tested the deposition of silver and gold nanoparticles on bulk silicon solar cells by colloidal technique in order to enhance their photovoltaic conversion efficiency by means of Plasmonic Light Scattering by metal nanoparticles. The feasible Plasmonic Light Scattering related enhancement was examined using spectral response and I–V measurements. Relative increases of the total delivered power under simulated solar irradiation were observed for cells both with and without antireflection coating using silver and gold nanoparticles.     

Optimized emitter contacting on multicrystalline silicon thin film solar cells

We present an optimized contacting scheme for multicrystalline silicon thin film solar cells on glass based on epitaxially crystallized emitters with a thin Al₂O₃ layer and a silver back reflector. In a first step a 6.5 µm thick amorphous silicon absorber layer is crystallized by a diode laser. In a second step a thin silicon emitter layer is epitaxially crystallized by an excimer laser. The emitter is covered by an Al₂O₃ layer with a thickness ranging from 1.0 nm to 2.5 nm, which passivates the surface and acts as a tunnel barrier. On top of the Al₂O₃ layer a 90–100 nm thick silver back reflector is deposited. The Al₂O₃ layer was found to have an optimal thickness of 1.5 nm resulting in solar cells with back reflector that achieve a maximum open‐circuit voltage of 567 mV, a short‐circuit current density of 27.9 mA/cm², and an efficiency of 10.9%. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

UV-illuminated chemical bath deposition of CdS buffer layer for CIGSSe solar cells

In this paper, we compare the performance of Cu(In,Ga)(S,Se)₂ (CIGSSe) thin film solar cells with a CdS buffer layer grown by chemical bath deposition (CBD) with UV irradiation of 365 nm or 254 nm at an output power of 8 W. The effects of UV light irradiation on the CBD-CdS thin film deposition mechanism were investigated through chemical and electro-optical studies. UV light irradiation during the solution process promotes the hydrolysis of thiourea, thereby inhibiting the formation of the intermediate products being developed on the reaction pathways and decreasing the solution pH. Therefore, the efficiency of the CdS/CIGSSe solar cells was improved because of the increased elemental ratio of S/(S + O) in the CdS thin film. This very simple and effective approach can be used to control the S/O ratio of the CdS thin film fabricated by the CBD process without artificially controlling the process temperature, solution pH or concentration.     

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)     

The influence of sodium on the point defect characteristics in off stoichiometric CuInSe2

The device performance of polycrystalline chalcogenide thin film solar cells is strongly influenced by different kinds of defects within the material. The presence of sodium or other alkali metals like potassium during the deposition process is well known to influence the electronic properties of the solar cell and thus to improve the efficiency of the final device. Structural analysis of neutron powder diffraction data collected at low temperatures and subsequent profile analysis by the LeBail and Rietveld method demonstrates the impact of sodium on the point defect characteristics in off stoichiometric CuInSe₂. The analyzed materials are powder and thin film solar absorber material with addition of NaF and free of sodium. It is illustrated, the so called “sodium effect” cannot be reduced to one single origin. A range of effects, the reduction of In_Cu donors with a followed increase of V_Cu acceptors is possible. The main effect is an increased ordered character of the chalcopyrite crystal structure at off stoichiometric composition, when containing sodium.     

Amorphous silicon solar cells on nano‐imprinted commodity paper without sacrificing efficiency

Paper is a cheap substrate which is in principle compatible with the process temperature applied in the plasma enhanced chemical vapour deposition (PECVD) and hot wire CVD (HWCVD) of thin film silicon solar cells. The main drawback of paper for this application is the porosity due to its fibre like structure. The feature size (micrometre scale) is larger than the thickness of the applied photovoltaic layers. To overcome this problem, UV curable lacquer was used to planarize the surface. Plain 80 grams printer paper was taken as a substrate and the lacquer smoothens the rough surface of the paper such that a designed nanostructure can be imprinted for light scattering. In this manner single junction amorphous silicon solar cells with a HWCVD deposited intrinsic layer were processed on paper, without any concessions to the process temperature of 200 °C. The cell performance is comparable to that of reference cells grown on stainless steel, proving that solar cells can be deposited on paper substrates without sacrificing performance. PV on paper could be applied as ”disposable” power source for gadgets, electronic labelling, remote sensing systems, etc. (Internet of Things). (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

单室沉积p-i-n型微晶硅薄膜太阳电池性能优化的研究

采用原位的氢等离子体处理技术和微晶覆盖技术来降低单室沉积p-i-n型微晶硅薄膜太阳电池中的硼污染问题.通过对不同处理技术所制备电池的电流密度-电压和量子效率测试结果的比较发现,一定的氢处理时间和合适的覆盖层技术都可以在一定程度上提高电池的性能,但每种方法的影响程度各异、文中对此异同进行了分析.通过对电池陷光结构和氢等离子体处理时间的优化,在单室中获得了效率为6.39%的单结微晶硅太阳电池.     

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     

PbS colloidal quantum dot/ZnO‐based bulk‐heterojunction solar cells with high stability under continuous light soaking

PbS colloidal quantum dot (CQD)‐based depleted bulk‐heterojunction solar cells were constructed, using the 1.2 μm thick nanowire array infiltrated with PbS QDs bearing Br ligands. The long‐term stability tests were performed on the solar cells without encapsulation in air under continuous light soaking using a Xe lamp with an AM1.5G filter (100 mW cm^?2). Time course of solar cell performances during the tests showed two time periods with distinct behavior, that is, the initial transient time period and the relatively stable region following it. The power conversion efficiency was found to keep approximately 90% of the initial value at the end of the 3000 h light soaking test. The stability tests suggest that the PbS surface modification or passivation reactions play an important role in achieving such a high stability, and demonstrate that PbS CQD/ZnO nanowire array‐based depleted bulk‐heterojunction solar cells are highly stable. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Minimizing optical losses in bulk heterojunction polymer solar cells

The efficiency that a solar cell can reach is ultimately limited by the number of photons absorbed in its active layer. Bulk heterojunction polymer solar cells are fabricated from a stack of thin film layers, each of which is thinner than a single wavelength from an incident photon within its absorption band. One consequence of this thin film layer stack is a strong optical interference between the various layers that can change the quantity of light dissipated in the active layer by 50%. Here we use optical modeling to quantitatively calculate the dissipation in each of the various layers as functions of wavelength and layer thickness. Using this information the loss free short circuit current density can be calculated (J_scmax). Optimization of J_scmax leads to direct improvements in the efficiency of the solar cell through improved light dissipation in the active layer. The optical properties for a P3HT:PCBM active layer and a model Lorentzian low band gap spectrum are optimized and ideal fabrication conditions are reported for these materials. PACS 72.40.+w; 72.80.Le