Rough ZnO layers by LP-CVD process and their effect in improving performances of amorphous and microcrystalline silicon solar cells

Doped ZnO layers deposited by low-pressure chemical vapour deposition technique have been studied for their use as transparent contact layers for thin-film silicon solar cells.Surface roughness of these ZnO layers is related to their light-scattering capability; this is shown to be of prime importance to enhance the current generation in thin-film silicon solar cells. Surface roughness has been tuned over a large range of values, by varying thickness and/or doping concentration of the ZnO layers.A method is proposed to optimize the light-scattering capacity of ZnO layers, and the incorporation of these layers as front transparent conductive oxides for p–i–n thin-film microcrystalline silicon solar cells is studied.     

Effect of surface roughness of top cover layer on the efficiency of dye-sensitized solar cell

In this research, the effect of surface roughness of flexible polymeric top cover layer on the efficiency of a dye-sensitized solar cell (DSSC) was investigated. In order to assess the durability of polyethylene terephthalate (PET) and polyethylene terephthalate naphthalate (PEN) the wear properties of these materials were first investigated. It was found that PEN was about 2.3 times more wear resistive than PET. PEN was used for further investigation regarding the effect of surface roughness on the light transmittance and electrical efficiency of DSSC. The surface roughness of PEN was varied by sand blasting to simulate an erosion process. It was found that PEN with a maximum average surface roughness of 1.6 μm resulted in 10% decrease in light transmittance and consequently degraded the cell efficiency of DSSC by 13%. The decrease in electrical efficiency due to increasing surface roughness was found to be proportional to the light transmittance.     

Development of micromorph tandem solar cells on flexible low-cost plastic substrates

We report on the development of fully flexible microcrystalline and micromorph tandem solar cells directly on low-cost substrates like poly-ethylen-terephtalate (PET) and poly-ethylen-naphtalate (PEN). The cells are deposited in nip or nip/nip configuration on the plastic substrate coated with a highly reflecting Ag–ZnO back contact. Light trapping is achieved by combining a periodically textured substrate and a diffusing ZnO front contact. Single-junction microcrystalline cells with a stable efficiency of 8.7% are achieved with an i-layer thickness of 1.2 μm. In tandem devices we obtain an efficiency of 10.9% (initial) with an open circuit voltage of 1.35 V and a fill factor (FF) of 71.5%. These cells are slightly top limited with 11.26 and 11.46 mA/cm² in the amorphous (270 nm thick) and the microcrystalline (1.2 μm thick) sub-cells, respectively. We introduce an intermediate reflector (IR) between the bottom and the top cell because it allows increasing the top cell current without compromising the stability by a thicker absorber. The IRs consist of either an ex-situ ZnO or a low refractive index P-doped silicon–oxygen compound deposited in-situ with a plasma process that is fully compatible with solar cell processing. We demonstrate significant current improvement (up to 8% relative) using both kinds of IRs.     

Study on diffusion barrier layer of silicon-based thin-film solar cells on polyimide substrate

ZnO and Ni films were used as the diffusion barrier layer between Al and n-type μc-Si:H for the hydrogenated amorphous silicon (a-Si:H) solar cells on polyimide (PI) substrate. The electrical, optical and uniformity properties of ZnO or Ni film influence strongly the performance and uniformity of solar cells. The uniformity of the solar cells with ZnO diffusion barrier layer degraded with the increasing thickness of ZnO film. The uniformity of solar cells with Ni diffusion barrier layer was more than 90%, which was generally better than those with ZnO film. A power-to-weight ratio of 200 W/kg was obtained for a-Si:H thin-film solar cell on PI substrate with a size of 14.8 cm².     

TCO and light trapping in silicon thin film solar cells

For thin film silicon solar cells and modules incorporating amorphous (a-Si:H) or microcrystalline (μc-Si:H) silicon as absorber materials, light trapping, i.e. increasing the path length of incoming light, plays a decisive role for device performance. This paper discusses ways to realize efficient light trapping schemes by using textured transparent conductive oxides (TCOs) as light scattering, highly conductive and transparent front contact in silicon p–i–n (superstrate) solar cells. Focus is on the concept of applying aluminum-doped zinc oxide (ZnO:Al) films, which are prepared by magnetron sputtering and subsequently textured by a wet-chemical etching step. The influence of electrical, optical and light scattering properties of the ZnO:Al front contact and the role of the back reflector are studied in experimentally prepared a-Si:H and μc-Si:H solar cells. Furthermore, a model is presented which allows to analyze optical losses in the individual layers of a solar cell structure. The model is applied to develop a roadmap for achieving a stable cell efficiency up to 15% in an amorphous/microcrystalline tandem cell. To realize this, necessary prerequisites are the incorporation of an efficient intermediate reflector between a-Si:H top and μc-Si:H bottom cell, the use of a front TCO with very low absorbance and ideal light scattering properties and a low-loss highly reflective back contact. Finally, the mid-frequency reactive sputtering technique is presented as a promising and potentially cost-effective way to up-scale the ZnO front contact preparation to industrial size substrate areas.     

ZnO:Al绒面透明导电薄膜的制备及分析

利用中频脉冲磁控溅射方法,采用Al掺杂(质量百分比2%)的Zn(纯度99.99%)金属材料为靶材制备平面透明导电ZnO:Al(ZAO)薄膜。利用湿法腐蚀方法,将平面ZAO薄膜在0.5%的稀盐酸中浸泡一定时间后,形成表面凹凸起伏的绒面结构。研究了平面ZAO薄膜的结构特性以及衬底温度、溅射功率和腐蚀时间对绒面ZAO薄膜表面形貌的影响,并对腐蚀前后薄膜的电阻变化进行了分析。结果表明:高温、低功率条件下制备的绒面ZAO薄膜表面形貌较好,在硅薄膜太阳电池中具有潜在的应用前景。     

Organic photovoltaic devices: influence of the cell configuration on its performences

On the basis of our thin film technology we have proceeded to the study of different cell configurations: two-layers D/A organic solar cells deposited by vacuum evaporation and bulk D/A heterojunction material based on a discontinuous D/A network thin film obtained by spin coating. We have also tested different transparent conductive oxides (TCO: ITO, ZnO). These TCO films have been used as under or upper electrode. The organic materials were zinc-phthalocyanine (Zn-Pc) or poly vinyl(N-carbazole) (PVK) as electron donor and 1,4-diaminoanthraquinone (1,4-DAAQ) as electron acceptor. A PEDOT/PSS thin film was often intercalated between the organic and the TCO in order to improve the current characteristics.Results can be summarized as follows:

• The cells efficiency does not depend strongly on the nature of the TCO used in the present work.

• The performance of the p–n bilayer ZnPc/1,4-DAAQ depends strongly on the surface roughness of the structure. The fill factor (FF) of the current–voltage characteristics varies from 0.2, when the structure is glass/TCO/(PEDOT/PSS)/ZnPc/1,4-DAAQ/Al, to 0.6, when the structure is glass/Al/DAAQ/ZnPc/ PEDOT/PSS/”mechanical contact”/TCO. It is shown that this behaviour is related to the columnar growth properties of the 1,4-DAAQ films.

• Best efficiency has been achieved with bulk D/A heterojunction material based on PVK/1,4-DAAQ blend (efficiency=0.5%). The electrochemical measure of its HOMO (5.7 eV) and LUMO (3.8 eV), by comparison with PVK (HOMO=5.7 eV, LUMO=2.2 eV), shows that these values should be optimized. Effectively, the electron affinity of the donor should be significantly smaller than that of the acceptor, while the ionization potential of the acceptor should be significantly greater than that of the donor, which is not the case presently.

复合绒面透明导电薄膜研究

在常规非晶硅电池绒面SnO2衬底上，采用Zn：Al重量比为5％的金属靶直流反应磁控溅射沉积ZnO，构成复合绒面SnO2／ZnO透明导电膜。控制适当ZnO厚度，既能保持SnO2绒面效果，又可阻挡H离子对SnO2的还原作用，可作为微晶硅电池的前电极。文中对ZnO沉积条件以及复合膜的形貌、电光性能进行了讨论。     

Next generation interconnective laser patterning of CIGS thin film modules

Laser patterning of thin film solar cells has proven technically feasible for all layers but still remains a challenging topic for research and development. We present a method where P2 laser patterning of CIGS thin film solar modules is performed after deposition of the ZnO:Al layer, as opposed to the current state-of-the-art where patterning is performed before the ZnO:Al layer is deposited. This method takes full advantage of the potential of laser processing and works by creating an interconnecting scribe line that “welds” the ZnO:Al and CIGS layers into an electrical contact. In this work we present experimental results of this process on five different sets of CIGS with slightly varying thickness and Cu content. While optimization of the process with respect to the various layers has not been performed, the initial results for CIGS thin film modules show working experimental modules in each set. The best experimental module has a fill factor of 71% and its performance is within the standard deviation of mechanically patterned references. An experimental submodule of 30×30 cm² size with 10% efficiency was manufactured.     

Light trapping and optical losses in microcrystalline silicon pin solar cells deposited on surface-textured glass/ZnO substrates

Influence of front TCO thickness, surface texture and different back reflectors on short-circuit current density and fill factor of thin film silicon solar cells were investigated. For the front TCO studies, we used ZnO layers of different thickness and applied wet chemical etching in diluted HCl. This approach allowed us to adjust ZnO texture and thickness almost independently. Additionally, we used optical modeling to calculate optical absorption losses in every layer. Results show that texture and thickness reduction of front ZnO increase quantum efficiency over the whole spectral range. The major gain is in the red/IR region. However, the higher sheet resistance of the thin ZnO causes a reduction in fill factor. In the back reflector studies, we compared four different back reflectors: ZnO/Ag, Ag, ZnO/Al and Al. ZnO/Ag yielded the best, Al the worst light trapping properties. Furthermore, the Ag back contact turned out to be superior to ZnO/Al for microcrystalline cells. Finally, the smooth ZnO/Ag back contact showed a higher reflectivity than the rough one. We prepared pin cells with rough and smooth ZnO/Ag interface, leaving the roughness of all other interfaces unchanged.     

Deposition of transparent and conductive Al-doped ZnO thin films for photovoltaic solar cells

The effect of the substrate temperature on the optoelectronic properties of ZnO-based thin films prepared by rf magnetron sputtering has been studied. Three different targets (Zn/Al 98/2 at%, ZnO:Al 98/2 at% and ZnO:Al₂O₃ 98/2 wt%) have been investigated in order to compare resulting samples and try to reduce the substrate temperature down to room temperature. From the ZnO:Al₂O₃ target, transparent conductive zinc oxide has been obtained at 25°C with the average optical transmission in the 400–800 nm wavelength range, T = 80–90% and resistivity, = 3−5 × 10⁻³ Ωcm. In Al:Zn0 layers, the spatial distribution of the electrical properties across the substrate placed parallel to the target has been improved by depositing at high substrate temperatures, above 200°C. Besides, owing to diffusion processes of CuInSe₂ and CdS take place at 200°C, an AI:ZnO/CdS/CuInSe₂ polycrystalline solar cell made with the Al:ZnO deposited at 25°C as the transparent conductive oxide, has shown a more efficient photovoltaic response, η = 6.8%, than the one measured when the aluminium-doped zinc oxide has been prepared at 200°C, η = 1.8%.     

Chemical and electronic interface structure of spray pyrolysis deposited undoped and Al-doped ZnO thin films on a commercial Cz-Si solar cell substrate

We have studied differences in the interface between undoped and Al-doped ZnO thin films deposited on commercial Si solar cell substrates. The undoped ZnO film is significantly thicker than the Al-doped film for the same deposition time. An extended silicate-like interface is present in both samples. Transmission electron microscopy (TEM) and photoelectron spectroscopy (PES) probe the presence of a zinc silicate and several Si oxides in both cases. Although Al doping improves the conductivity of ZnO, we present evidence for Al segregation at the interface during deposition on the Si substrate and suggest the presence of considerable fixed charge near the oxidized Si interface layer. The induced distortion in the valence band, compared to that of undoped ZnO, could be responsible for considerable reduction in the solar cell performance.     

Effect of electron beam irradiation on polydopamine and its application in polymer solar cells

Polydopamine (PDA) films were irradiated by an electron beam linear accelerator at different irradiation doses ranging from 10 to 150 kGy. The irradiated samples were characterized by Fourier transform infrared spectrometry, UV‐Vis, scanning electron microscopy, atomic force microscope, X‐ray diffraction, cyclic voltammetry, electrochemical impedance spectroscopy, and thermogravimetric analysis. Changes in surface morphology, chemical structure, contact angle, frontier orbitals, and bandgap were analyzed. PDA films modified by electron beam irradiation were used in the interface design and control of polymer solar cells. Devices with the structures of ITO/ZnO/PTB7:PC₇₁BM/MoO₃/Al and ITO/PDA‐ZnO/PTB7:PC₇₁BM/MoO₃/Al were fabricated. The solar cells with a 100‐kGy electron beam‐irradiated PDA film‐modified ZnO electron transport layer had a significantly improved short circuit current density, and its efficiency reached a maximum value. The short circuit current density and power conversion efficiency reached 13.70 mA/cm² and 3.82%, respectively. Electron beam irradiation is an effective method to modify PDA, which can be used as an interface modifier in polymer solar cells.     

Structural and optical properties of textured ZnO:Al films on glass substrates prepared by in-line rf magnetron sputtering

Textured ZnO:Al films with excellent light scattering properties as a front electrode of silicon thin film solar cells were prepared on glass substrates by an in-line rf magnetron sputtering, followed by a wet-etching process to modify the surface morphologies of the films. Deposition parameters and wet etching conditions of the films were controlled precisely to obtain the optimized surface features. All as-deposited films show a strong preferred orientation in the [0 0 1] direction under our experimental conditions. The microstructure of the films was significantly affected by working pressure and film compactness was reduced with increasing working pressure, while the effect of a substrate temperature on the microstructure is less pronounced. A low resistivity of 4.25×10⁻⁴ Ω cm and high optical transmittance of above 80% in a visible range were obtained in the films deposited at 1.5 mTorr and 100 °C. After wet etching process, the surface morphologies of the films were changed dramatically depending on the microstructure and film compactness of the initial films. By controlling the surface feature, the haze factor and angular resolved distribution of the textured ZnO:Al films were improved remarkably when compared with commercial SnO₂:F films. The textured ZnO:Al and SnO₂:F films were applied as substrates for a silicon thin film solar cells with tandem structure of a-Si:H/μc-Si:H. Compared with the solar cells with the SnO₂:F films, a significant enhancement in the short-circuit current density of the μc-Si:H bottom cell was achieved, which is due to the improved light scattering on the highly textured ZnO:Al film surfaces in the long wavelength above 600 nm.     

Effects of substrate temperature on the properties of facing-target sputtered Al-doped ZnO films

ZnO:Al films (Al 2.5 wt%) were deposited using a DC facing targets magnetron sputtering via two ZnO targets mixed with Al₂O₃. The structural, electrical and optical properties of the deposited films were strongly influenced by substrate temperature. Films with better texture, higher transmission, lower resistivity and larger carrier concentration were obtained for the samples fabricated at higher substrate temperature. The optimal condition for deposition of ZnO:Al film with the lowest resistivity of 3.18×10⁻⁴ Ω cm, the highest carrier concentration of 4.58×10²⁰ cm⁻³, and a transmission toward 85% in the visible range was obtained at 200 °C. This film proposes a promising future for the application of the practical window and contact layers for solar cells.     

Surface texturing of sputtered ZnO:Al/Ag back reflectors for flexible silicon thin-film solar cells

Al-doped zinc oxide/silver (ZnO:Al/Ag) back reflectors for silicon thin-film solar cells with an n-i-p configuration were prepared on flexible stainless steel substrates by dc magnetron sputtering. The surface morphologies of the back reflectors were modified by changing the deposition temperature of the Ag films to improve the light-scattering properties on the back reflectors, resulting in the enhancement of the light-trapping effect in the solar cells. By elevating the deposition temperature from room temperature to 500 °C, the surface roughness of the Ag films increased from 6.62 to 46.64 nm. The films at 500 °C had coarse surface features with irregular grain size distributions between 200 and 900 nm, whereas the films produced at low temperatures below 100 °C had smooth surfaces consisting of small grains between 100 and 200 nm. Even after the 100-nm thick ZnO:Al films were deposited on the modified Ag surfaces, the surface microstructure of the ZnO:Al/Ag bilayers was similar to that of the Ag films. The surface roughness of bilayers increased from 7.12 to 39.30 nm with coarsening the Ag surfaces. Haze factor (a ratio of diffuse reflectance to total reflectance) of Ag films was enhanced remarkably from 59% to 74% in a wide wavelength range from 350 to 1100 nm with increasing the surface roughness of the Ag films from 6.62 nm to 46.64 nm. Enhancement in the haze factor was due to the increase of diffuse reflectance on the Ag films, because the total reflectance did not change much with increasing surface roughness of the Ag films. This increasing roughness indicated that the light scattering from the rough surface of the back reflectors improved. The enhanced light scattering from the back reflectors influenced the performance of the solar cells mainly in terms of the short-circuit current density (J_sc). Compared to the back reflectors with smooth surface features, leading to a J_sc value of 9.94 mA/cm², the back reflectors with large surface roughness improved the J_sc value of the solar cells to 13.36 mA/cm² without detrimental changes in the fill factor (FF) and open circuit voltage (V_oc); they eventually increased the conversion efficiency of the solar cells from 5.59% to 7.60%.     

Superstrate-type CuInSe2-based thin film solar cells by a low-temperature process using sodium compounds

Superstrate-type solar cells with a Au/CuInSe₂(CIS)/In_xSe_y,/ZnO : Al/glass structure were investigated. The CIS films were deposited by coevaporation method with intentionally incorporated Na₂S at a substrate temperature of 350°C. Even at relatively low substrate temperatures, sodium compounds enhanced the (1 1 2) preferred orientation of the chalcopyrite structure, and also improved the cell performance. The In_xSe_y buffer layers disappeared after CIS deposition by interdiffusion. Preliminary cells yielded an efficiency of 7.5% with V_oc, = 430 mV, J_sc = 29.4 mA/cm² and FF = 0.60. The light soaking and forward bias effects were observed for these cells.     

Enhanced photovoltaic performance of inverted organic solar cells with In-doped ZnO as an electron extraction layer

In the present work, a systematic study has been carried out to understand the effect of In doping on the various properties of the ZnO nanocrystalline thin films. In-doped ZnO nanocrystalline thin films with different indium concentrations (1.98%, 4.03%, 6.74%, 8.62% and 10.48% In) have been synthesized by sol–gel method. The grain size and surface roughness of the In-doped ZnO thin films are observed to be smaller than those of the ZnO thin films. 6.74% In-doped ZnO films with a low resistivity of 1.95 × 10⁻³ Ω cm and a high mobility of 2.19 cm² V⁻¹ S⁻¹ have been prepared under optimal deposition conditions. Inverted organic solar cells containing In-doped ZnO as an electron extraction layer with the structure indium tin oxide (ITO)/In-doped ZnO/poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT): [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM)/MoO₃/Al have been fabricated. The inverted organic solar cell with 6.74% In-doped ZnO exhibited a power conversion efficiency of 5.58%, which is the best efficiency reported so far for these type of solar cells. The device performance has been optimized by varying the indium doping concentration. The results clearly demonstrate that significant improvement in power conversion efficiency can be obtained by incorporating In into the ZnO films.     

High-Ts amorphous top cells for increased top cell currents in micromorph tandem cells

In the present paper, the authors discuss the application of amorphous p–i–n solar cells containing i-layers which are deposited at high substrate temperatures as top cells in amorphous silicon/microcrystalline silicon tandem (“micromorph”) solar cells. Increasing the substrate temperature for the deposition of intrinsic a-Si : H results in a reduced optical gap. The optical absorption is enhanced and thereby the current generation. A high-current generation within a relatively thin amorphous top cell is very interesting in the context of micromorph tandem cells, where the amorphous top cell should contribute a current of at least 13 mA/cm² for a total cell current density of 26 mA/cm². A detailed study of the intrinsic material deposited by VHF-GD at 70 MHz at substrate temperatures between 220°C and 360°C is presented, including samples deposited from hydrogen-diluted silane plasmas. The stability of the films against light soaking is investigated employing the μ₀τ₀ parameter, which has been shown to be directly correlated to the cell performance. The paper discusses in detail the technological problems arising from the insertion of i-layers deposited at high substrate temperatures into solar cells. These problems are specially pronounced in the case of cells in the p–i–n (superstrate) structure. The authors demonstrate that an appropriate interface layer at the p/i-interface can largely reduce the detrimental effects of i-layer deposition at high temperatures. Finally, the application of such optimized high-temperature amorphous cells as top cells in micromorph tandem cells is discussed. Current densities of 13 mA/cm² in the top cell are available with a top cell i-layer thickness of only 250 nm.     

RF magnetron sputtered ZnO:Al thin films on glass substrates: A study of damp heat stability on their optical and electrical properties

ZnO:Al thin films were deposited on glass substrates by RF magnetron sputtering from a powder compacted ceramic target. Structural, electrical and optical properties of the films with different thickness were characterized. The damp heat stability of ZnO:Al thin film was investigated for its application in thin-film solar cells. After the 1000 h damp heat treatment in harsh conditions of 85% relative humidity at 85 °C for all samples, a degradation of electrical properties was observed, while the transmissions of the films were almost unchanged. Thick films with a relative large grain size could form compact structure to resist the corrosion by oxygen and water molecules.