Polycrystalline ZnO: B grown by LPCVD as TCO for thin film silicon solar cells

Conductive zinc oxide (ZnO) grown by low pressure chemical vapor deposition (LPCVD) technique possesses a rough surface that induces an efficient light scattering in thin film silicon (TF Si) solar cells, which makes this TCO an ideal candidate for contacting such devices. IMT-EPFL has developed an in-house LPCVD process for the deposition of nanotextured boron doped ZnO films used as rough TCO for TF Si solar cells. This paper is a general review and synthesis of the study of the electrical, optical and structural properties of the ZnO:B that has been performed at IMT-EPFL.The influence of the free carrier absorption and the grain size on the electrical and optical properties of LPCVD ZnO:B is discussed. Transport mechanisms at grain boundaries are studied. It is seen that high doping of the ZnO grains facilitates the tunnelling of the electrons through potential barriers that are located at the grain boundaries. Therefore, even if these potential barriers increase after an exposition of the film to a humid atmosphere, the heavily doped LPCVD ZnO:B layers show a remarkable stable conductivity. However, the introduction of diborane in the CVD reaction induces also a degradation of the intra-grain mobility and increases over-proportionally the optical absorption of the ZnO:B films. Hence, the necessity to finely tune the doping level of LPCVD ZnO:B films is highlighted. Finally, the next challenges to push further the optimization of LPCVD ZnO:B films for thin film silicon solar cells are discussed, as well as some remarkable record cell results achieved with LPCVD ZnO:B as front electrode.     

Potential of thin-film silicon solar cells by using high haze TCO superstrates

Potential improvements in the performance of tandem amorphous silicon/microcrystalline silicon (a-Si:H/μc-Si:H) solar cells, related to the TCO superstrates with enhanced scattering properties are studied. In particular, optical effects of a high haze double textured (W-textured) SnO₂:F TCO superstrate are analyzed and compared to the properties of the pyramidal type SnO₂:F TCO superstrate. Solar cell with W-textured superstrate exhibits higher long-wavelength external quantum efficiency of the bottom μc-Si:H cell than the one with pyramidal type TCO superstrate. Optical simulations are employed to study the potential improvements of the solar cell performance if ideal haze parameter (H = 1) and/or a broad angular distribution function (Lambertian) of scattered light are applied to textured interfaces in the solar cell structure. Simulations reveal significant improvements in long-wavelength quantum efficiencies if a broad angular distribution function of scattered light is applied. Optical losses in the cells with enhanced scattering properties are analysed and evaluated in terms of short-circuit current losses in the supporting layers and losses due to reflected light.     

Effect of TiO2 nanopatterns on the performance of hydrogenated amorphous silicon thin-film solar cells

We investigate how TiO₂ nanopatterns formed onto ZnO:Al (AZO) films affect the performance of hydrogenated amorphous silicon (a-Si:H) solar cells. Scanning electron microscopy results show that the dome-shaped TiO₂ nanopatterns (300 nm in diameter) having a period of 500 nm are formed onto AZO films and vary from 60 to 180 nm in height. Haze factor increases with an increase in the height of the nanopatterns in the wavelength region below 530 nm. Short circuit current density also increases with an increase in the height of the nanopatterns. As the nanopatterns increases in height, the fill factor of the cells slightly increases, reaches maximum (0.64) at 100 nm, and then decreases. Measurements show that a-Si:H solar cells fabricated with 100 nm-high TiO₂ nanopatterns exhibit the highest conversion efficiency (6.34%) among the solar cells with the nanopatterns and flat AZO sample.     

Modeling of light scattering properties from surface profile in thin-film solar cells by Fourier transform techniques

The optical properties of textured surfaces in the micro and nanometer range are of interest in manifold topics, such as thin-film silicon photovoltaics. Light scattering models, which are based on Fourier transform techniques, are applied to calculate both, the angularly resolved scattering and the haze. Therein, topography, measured by atomic force microscopy, and the refractive index are used as input data. In this study, these models are applied to zinc oxide/air interfaces and to zinc oxide/hydrogenated amorphous silicon interfaces. Results obtained from zinc oxide/air interfaces are compared to the measured scattering properties.     

Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells

High-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) is a widely applicable method of deposition over a large area at a high rate for fabricating silicon thin-film solar cells. This investigation presents the properties of hydrogenated amorphous silicon (a-Si:H) films and the preparation of highly-efficient p-i-n solar cells using an RF (27.1 MHz) excitation frequency. The influence of the power (10-40 W) and pressure (20-50 Pa) used during the deposition of absorber layers in p-i-n solar cells on the properties and mechanism of growth of the a-Si:H thin films and the solar cells is studied. The a-Si:H thin films prepared under various deposition conditions have widely varying deposition rates, optical-electronic properties and microstructures. When the deposition parameters were optimized, amorphous silicon-based thin-film silicon solar cells with efficiency of 7.6% were fabricated by HF-PECVD. These results are very encouraging for the future fabrication of highly-efficient thin-film solar cells by HF-PECVD.     

Surface morphology and light scattering properties of plasma etched ZnO:B films grown by LP-MOCVD for silicon thin film solar cells

LP-MOCVD deposited ZnO:B thin films, post-etched by argon plasma processes, were investigated in this study in order to optimise the ZnO:B/p-layer interface when the ZnO:B is used as front electrode of p-i-n a-Si:H solar cells. At varying etching time different surface roughness was obtained and the evolution of the surface morphology was correlated with the texture characteristic and its scattering properties. Atomic force microscopy data were analysed and discussed together with the scattering properties, which are haze parameter and angular resolved scattering (ARS) distribution.The presence of several preferential scattering angles was hypothesized and a deconvolution approach was applied to each angular scattering curve. For each fixed preferential scattering angle θ_i we associated a Gaussian distribution of the scattered light amount related to a well-defined scattering surface. The different preferential scattering angles were correlated to different scattering phenomena, the modifications of the angular scattering curves well agreed with SEM and AFM images.It is well known that a:Si-H solar cells fabricated on MOCVD deposited ZnO:B substrates show poor FF and V_oc values with good J_sc value. We demonstrated that only an effective sharp edge rounding off produced by an appropriately long plasma etching treatment is able to make MOCVD deposited ZnO:B perfectly suitable for high quality a-Si:H based devices.     

Contact resistivity measurements of the buried Si-ZnO:Al interface of polycrystalline silicon thin-film solar cells on ZnO:Al

An experimental method is developed for contact resistivity measurements of a buried interface in polycrystalline silicon (poly-Si) thin-film solar cell devices on aluminum doped zinc oxide (ZnO:Al) layers. The solar cell concept comprises a glass substrate covered with a temperature-stable ZnO:Al film as transparent front contact layer, a poly-Si n⁺/p⁻/p⁺ cell, as well as a metal back contact. Glass/ZnO:Al/poly-Si/metal test stripe structures are fabricated by photolithographic techniques with the ZnO:Al stripes locally bared by laser ablation. The high-temperature treatments during poly-Si fabrication, e.g. a several hours lasting high-temperature step at 600 °C, are found to have no detrimental impact on the ZnO:Al/Si interface contact resistivity. All measured ρ_C values range well below 0.4 Ω cm² corresponding to a relative power loss ΔP below 3% for a solar cell with 500 mV open circuit voltage and 30 mA/cm² short circuit current density. By inclusion of a silicon nitride (SiN_x) diffusion barrier between ZnO:Al and poly-Si the electrical material quality of the poly-Si absorber can be significantly enhanced. Even in this case, the contact resistivity remains below 0.4 Ω cm² if the diffusion barrier has a thickness smaller than 10 nm.     

Instabilities in reactive sputtering of ZnO:Al and reliable texture-etching solution for light trapping in silicon thin film solar cells

Texture etched zinc oxide is often used as transparent front contact for silicon thin film solar cells. Reactive sputtering is a potentially low-cost process. However, process stability, film uniformity, and reproducibility are challenges to be solved. Oscillations of the control signal and subsequent reaction of the plasma emission control with moving substrates from rotatable metallic targets cause fluctuations of aluminum doped zinc oxide (ZnO:Al) properties. Solutions to overcome such variations during the reactive sputtering process are discussed. However, effects on film properties, especially on etching behavior, cannot be totally removed. To achieve good light scattering properties for solar cell application ZnO:Al films are usually etched in dilute hydrochloric acid. An etch process based on hydrofluoric acid has been developed to tune the surface texture for a given ZnO:Al material. One feature of this process is the relaxed requirement on ZnO:Al film properties as the reactively sputtered ZnO:Al films do not necessarily possess optimized film structure for the HCl etch. Solar cells with optimized ZnO:Al front contacts achieved conversion efficiency well above 11%.     

Rapid thermal-plasma annealing of ZnO:Al films for silicon thin-film solar cells

Rapid thermal annealing of sputter-deposited ZnO and Al-doped ZnO (AZO) films with and without an amorphous silicon (a-Si) capping layer was investigated using a radio-frequency (rf) argon thermal plasma jet at atmospheric pressure. The resistivity of bare ZnO films on glass decreased drastically from 10⁶ to 10³ Ω·cm at maximum surface temperatures T_max above 650 °C, whereas the resistivity increased from 10^− 4 to 10^− 3-10^− 2 Ω·cm for bare AZO films. On the other hand, the resistivity of AZO films with a 30-nm-thick a-Si capping layer remained below 10^− 4 Ω·cm, even after TPJ annealing at a T_max of 825 °C. X-ray diffraction and X-ray photoemission electron studies revealed that the film crystallization of both AZO and a-Si layers was promoted without the formation of an intermixing layer. Additionally, the crystallization of phosphorous- and boron-doped a-Si layers at the sample surface was promoted, compared to that of intrinsic a-Si under identical plasma annealing conditions. The role of the a-Si capping layer on sputter-deposited AZO and ZnO films during TPJ annealing is demonstrated. The effects of the mixing of phosphorous and boron impurities in a-Si:H during TPJ annealing of flat and textured AZOs are also discussed.     

Layer-by-layer deposition films of copper phthalocyanine derivative; their photoelectrochemical properties and application to solution-processed thin-film organic solar cells

Ultrathin films serving as a light-harvesting and hole-transporting material were fabricated by layer-by-layer deposition of a water-soluble phthalocyanine derivative, copper(II) phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid tetrasodium salt (CuPcTS), and poly(diallyldimethylammonium chloride). The blue-shift of absorption peak and the absorption dichroism of the Q band indicated that CuPcTS molecules in the layer-by-layer films form cofacial dimers or oligomers and that their molecular planes take a three- or two-dimensional orientation in a direction parallel to the substrate depending on a drying process of the film during the deposition. The diffusion constant of hole carriers among CuPcTS molecules in the film was evaluated to be 6.5 × 10^− 11 cm² s^− 1 in an acetonitrile solution from potential step chronoamperometry measurement. Solution-processed thin-film organic solar cells with a triple-layered structure were developed by combining a hole-transporting layer made of poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate), a light-harvesting layer of CuPcTS, and an electron-transporting layer of fullerene, in this sequence. Photovoltaic properties of the cells strongly depended on the thickness of CuPcTS films and can be maximized by controlling the thickness at ca. 10 nm.     

Influence of working pressure on ZnO:Al films from tube targets for silicon thin film solar cells

Mid-frequency magnetron sputtering of aluminum doped zinc oxide films (ZnO:Al) from tube ceramic targets has been investigated for silicon based thin film solar cell applications. The influence of working pressure on structural, electrical, and optical properties of sputtered ZnO:Al films was studied. ZnO:Al thin films with a minimum resistivity of 3.4 × 10⁻⁴ Ω cm, high mobility of 50 cm²/Vs, and high optical transmission close to 90% in visible spectrum region were achieved. The surface texture of ZnO:Al films after a chemical etching step was investigated. A gradual increase in feature sizes (diameter and depth) was observed with increasing sputter pressure. Silicon based thin film solar cells were prepared using the etched ZnO:Al films as front contacts. Energy conversion efficiencies of up to 10.2% were obtained for amorphous/microcrystalline silicon tandem solar cells.     

Controlled synthesis of ZnO branched nanorod arrays by hierarchical solution growth and application in dye-sensitized solar cells

We demonstrate the controlled synthesis of ZnO branched nanorod arrays on fluorine-doped SnO₂-coated glass substrates by the hierarchical solution growth method. In the secondary growth, the concentration of Zn(NO₃)₂/hexamethylenetetramine plays an important role in controlling the morphology of the branched nanorod arrays, besides that of diaminopropane used as a structure-directing agent to induce the growth of branches. The population density and morphology of the branched nanorod arrays depend on those of the nanorod arrays obtained from the primary growth, which can be modulated though the concentration of Zn(NO₃)₂/hexamethylenetetramine in the primary growth solution. The dye-sensitized ZnO branched nanorod arrays exhibit much stronger optical absorption as compared with its corresponding primary nanorod arrays, suggesting that the addition of the branches improves light harvesting. The dye-sensitized solar cell based on the optimized ZnO branched nanorod array reaches a conversion efficiency of 1.66% under the light radiation of 1000 W/m². The branched nanorod arrays can also be applied in other application fields of ZnO.     

The potential of textured front ZnO and flat TCO/metal back contact to improve optical absorption in thin Cu(In,Ga)Se2 solar cells

The role of additionally textured front transparent conductive oxide − TCO (ZnO:Al) and flat TCO/metal contact on optical improvements in thin Cu(In,Ga)Se₂ (CIGS) solar cells are investigated by means of numerical simulations. A de-coupled analysis of two effects related to additional texturing of front surface of ZnO:Al TCO − (i) enhancement of light scattering and (ii) decreased total reflectance (antireflective effect) − reveals that the improvements in quantum efficiency, QE, and short-circuit current, J_SC, of the solar cell originate from an antireflective effect only. In order to improve optical properties of the back contact the introduction of a TCO layer (undoped ZnO) between CIGS and back metal contact is investigated from the optical point of view. In addition to ZnO/Mo, a highly reflective ZnO/Ag contact (ZnO is also assumed to work as a protection layer for Ag) is also included in simulations. Results show significant increase in reflectance related to introduced ZnO in front of Mo. Drastically increased reflectance is obtained if ZnO/Mo is substituted with ZnO/Ag. The improvements in QE and J_SC of a thin CIGS solar cell, related to ZnO/metal contacts are presented.     

空芯光纤硅太阳能电池制备及光捕获研究

本文提出一种空芯光纤结构硅基太阳能电池,并探讨其制备方法和光捕获性能.依据平面电池受光原理和空芯波导的限光机制提出了空芯光纤硅基太阳能电池结构,采用卷曲柔性平面非晶硅薄膜电池制备出圆筒形空芯光纤硅电池.通过对比研究入射光量一定的条件下平面电池和空芯光纤电池的光生电流和电压值,评估空芯光纤电池的光捕获效果.通过测量不同光入射角度和光照强度下空芯光纤电池的光生电流和电压值,揭示光入射角度和光照强度对空芯光纤电池光捕获性能的影响关系.研究表明,空芯光纤硅基电池能将入射光线限制在波导结构内反复吸收和反射,从而在光捕获性能方面较平面电池有所提升(~19.8%).光线入射角度对空芯光纤电池的光捕获性能有较大影响,在30°~50°入射时可以获得较大的光生电压和电流值.在0~100 000 lux的光照强度范围内,光生电压先随光照度增加而增大,而后逐渐趋于恒定值.通过卷曲柔性平面硅电池获得光捕获效率较高的空芯光纤硅电池是可行的,采用结构简单、光线单次入射吸收较低的单节薄膜电池制备空芯光纤电池有望获得更好的光捕获效率提升效果.     

Enhanced efficiency of tri-layered dye solar cells with hydrothermally synthesized titania nanotubes as light scattering outer layer

In the present study dye sensitized solar cells (DSSCs) have been fabricated with a tri-layer photo anode consisting of hydrothermally prepared titania nano tubes (TNT) having a diameter of 9-10 nm and length of several micrometers as outer layer, P25 TiO₂ powder as transparent light absorbing middle layer and a compact TiO₂ inner layer to improve the adhesion of different layers on a transparent conducting oxide coated substrate. In comparison to cells fabricated using TNTs or P25 alone, the tri-layer DSSCs exhibit an enhanced efficiency of 7.15% with a current density of 17.12 mA cm^− 2 under AM 1.5 illumination. The enhancement is attributed to the light scattering generated by TNTs aggregates, reduction in electron transport resistance at the TiO₂/dye/electrolyte interface and an improvement in electron life-time.     

Role of majority and minority carrier barriers silicon/organic hybrid heterojunction solar cells

A hybrid approach to solar cells is demonstrated in which a silicon p-n junction, used in conventional silicon-based photovoltaics, is replaced by a room-temperature fabricated silicon/organic heterojunction. The unique advantage of silicon/organic heterojunction is that it exploits the cost advantage of organic semiconductors and the performance advantages of silicon to enable potentially low-cost, efficient solar cells.     

Large area ZnO:Al films with tailored light scattering properties for photovoltaic applications

Aluminum-doped zinc oxide films on glass are promising substrates for use in thin film solar cells based on amorphous and amorphous/microcrystalline silicon absorber material. The films can be produced by magnetron sputtering on large scale at relative low cost. Especially reactive sputtering of metallic Zn/Al compound targets is a cheap way to produce films at high deposition rate. One drawback of amorphous silicon is the low absorption in the near infrared spectral range. Wet chemical etching has been used to produce a rough TCO surface that enables light trapping in the absorber. The etching behaviour of ZnO:Al films can be tuned by changing oxygen partial pressure during deposition. The etching behaviour is compared to ZnO structure and discussed regarding the performance of solar cells deposited onto the etched films.     

Cube-like mixed-phases TiO2 mesocrystalline hollow boxes from in situ topotactic transformation for highly efficient dye-sensitized solar cells

The cube-like TiO₂ mesocrystalline hollow boxes (TiO₂-MHBs) are fabricated by a topotactic transformation method. TiO₂-MHBs consist of one-dimensional nanorod-like rutile TiO₂ combined with anatase TiO₂. The ordering of TiO₂-MHBs is in favor of efficient electron transport. TiO₂-MHBs with mesoporous structure show excellent light scattering performance. Dye-sensitized solar cell with TiO₂-MHBs light scattering overlayer exhibits an 18.3% increment of cell efficiency (9.51%) compared with the TiO₂ nanoparticles film cell (8.04%). The improved photovoltaic performance is attributed to its unique crystallographic property and highly porous structure, which can enhance light scattering capability, quicken electron transport and electrolyte diffusion, and reduce charge recombination.