High-efficiency multicrystalline silicon solar cells by liquid phase-epitaxy

Thin-film silicon cells produced on crystalline silicon substrates have the potential to achieve high cell efficiencies at low cost. We have used a modified liquid-phase epitaxy growth process to produce very smooth, high-quality silicon films on multicrystalline silicon substrates. Photoconductivity decay measurements indicate that the minority carrier lifetimes in these layers are at least 10 μs, sufficient to achieve cell efficiencies in excess of 16%. This efficiency potential is confirmed in small area cells, which have displayed efficiencies up to 15.4%. Further improvements up to 17% efficiency are possible in the short term, even without the introduction of any light-trapping schemes into the device structure.     

Modelling of material properties influence on back junction thin polycrystalline silicon solar cells

The influence of polycrystalline silicon properties on the performances of thin back junction solar cells has been investigated by means of a 3-dimensional model taking into account grain size, grain boundary recombination, volumic recombination, and surface recombination. The drastic influence of front surface recombination has been confirmed. The grain size has been shown to be of minor importance provided the grain size is not too small and the grain boundaries are correctly passivated. An optimal base thickness has been determined which is all the smaller that the material is more imperfect.     

The role of rear surface in thin silicon solar cells

The aim of this work is the study of the light-trapping ability of rear surface relief for thin silicon solar cells. With a simple model, the conditions needed to achieve efficient thin cells are discussed first. It is shown how the combined effect of back surface passivation and the confinement of light impacts the cell performance. If the illuminated face is not textured, light-trapping must be accomplished by the rear surface. A rear saw-tooth relief grating is proposed as a good back reflector producing also the tilt of reflected rays. Since technological limitations lead relief features to have sizes near the range of the optical wavelengths, the behavior of such structures is analyzed using a rigorous electromagnetic approach. The influence of the depth-to-period ratio of the grating in the internal reflectivity is analyzed. Finally, it is calculated that the back internal reflectivity is much higher when the exit medium is air than when it is aluminum.     

Can we improve the record efficiency of CdS/CdTe solar cells?

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaic systems. The two key properties of this material are its near ideal band gap for photovoltaic conversion efficiency of 1.45 eV, and its high optical absorption coefficient. Thin film CdTe solar cells are typically hetero-junctions with CdS being the n-type partner, or window layer. Efficiencies as high as 16.5% have been achieved.In this paper we make a physical analysis of the typical CdS/CdTe superstrate solar cell, and we show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell. We show that a current estimate for the maximum efficiency of hetero-junction CdS/CdTe solar cells is around 17.5%, in contrast to old theoretical predictions, which calculate about 30% efficiencies for ideal homo-junction CdTe solar cells. This analysis explains why the record efficiency for this kind of cells has been stable for the last 10 years, going up by less than 1% from 15.8% to only 16.5%.     

The influence of drift fields in thin silicon solar cells

The influence of electric “drift” fields in the base of silicon solar cells on device performance is investigated. The drift fields are the result of a nonuniform dopant density in the base material. Numerical modelling is carried out for a range of representative cell structures and two different models for the dependence of the minority carrier lifetime on the dopant density. The cell design variables, in particular the dopant densities and the thicknesses of the device regions, are optimized with respect to the cell efficiency. Comparison of optimized cells incorporating a drift field with those not having a drift field, shows that a drift field can offer only small efficiency advantages for particular cell structures and recombination parameters, and only if large variations in dopant concentration can be achieved.     

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%.     

Substrates for thin crystalline silicon solar cells

Choice of substrate for thin crystalline silicon solar cells requires a compromise between cost and quality. There are three generic substrate types, namely a transparent substrate (such as glass), an opaque substrate (such as a ceramic or metal) and low-cost multicrystalline silicon. Glass has the advantage of eliminating absorption within the substrate. However, the larger effective diffusion length, the improved surface passivation and the increased process flexibility obtainable with an opaque substrate, particularly low-cost multicrystalline silicon, may considerably outweigh the modest optical benefits of a transparent substrate. In this paper it is shown that the advantage in effective diffusion length that is required of a cell grown on an opaque substrate in order to offset the light-trapping advantages of a glass substrate is about a factor of two.     

Control of LPCVD ZnO growth modes for improved light trapping in thin film silicon solar cells

By controlling the deposition parameters of low pressure chemical vapor deposition (LPCVD) grown ZnO transparent conductive oxide (TCO), we show that it is possible to achieve a switch of crystallographic orientations and to obtain novel surface nanotexture. The new layers are (0 0 0 2) oriented and exhibit light scattering at larger angles than state-of-the-art TCO used in thin film silicon solar cells. This property translates into a strong enhancement of the top amorphous cell current in micromorph tandem devices.     

Thin film solar cells on honeycomb-structured substrates for photovoltaic building blocks

Honeycomb-structured solar cell is proposed for photovoltaic building block applications. Honeycomb-like substrates were prepared either by a conventional semiconductor processing or by a low cost wet-chemical method, and amorphous Si thin film solar cells were fabricated on these substrates. We have demonstrated one of the essential requirements for building block application, which is the low sensitivity of the light incidence angles on the power conversion efficiency; and we have identified the critical processing issues through the experimental study using various thin film deposition methods. This honeycomb-structured solar cell is a promising candidate for the future photovoltaic building block applications enabling the inherent high strength-to-weight ratio and higher efficiency at an oblique light incidence.     

Experimental study of variations of the solar spectrum of relevance to thin film solar cells

The influence of variations in the incident solar spectrum on solar cells is often neglected. This paper investigates the magnitude of this variation and its potential influence on the performance of thin film solar cells in a maritime climate. The investigation centres on the analysis of a large number of measurements carried out in Loughborough, UK, at 10 min intervals over a period of 30 months. The magnitude of the spectral variation is presented both on a daily and a seasonal basis. Of the different thin film materials studied, amorphous silicon is shown to be the most susceptible to changes in the spectral distribution, with the “useful fraction” of the light varying in the range +6% to −9% of the annual average, with the maximum occurring in summer time.     

Highly reflective nanotextured sputtered silver back reflector for flexible high-efficiency n-i-p thin-film silicon solar cells

High reflectivity is essential when a metal is used as back contact and reflector in thin-film silicon solar cells. We show that thermal annealing at 150 °C improves the reflectivity of silver films deposited by sputtering at room temperature on nanotextured substrates. The annealing provokes two interlinked effects: rearrangement of the silver layer with a modification of its morphology and an increase of up to 42% in the grain size of the polycrystalline film for the preferential orientation as measured by X-ray diffraction. The main consequence of these two mechanisms is a large increase in the reflectivity of silver when measured in air. This reflectivity increase is also noticeable in devices: amorphous silicon thin-film solar cells grown on annealed silver films yield higher internal and external quantum efficiencies compared to cells grown on as-deposited silver. The morphology modification smoothes down the substrate, which is revealed by a clear increase of the open-circuit voltage and fill factor of the cells grown on top. An amorphous silicon cell with a 200 nm nominally thick i-layer fabricated on a flexible plastic substrate yielded an initial efficiency close to 10% with 15.9 mA/cm² of short-circuit current using highly reflective annealed textured silver. We also propose, for industrial purpose, the sputtering of thin silver layer (120 nm) under moderate substrate temperature (∼150 °C) to increase the layer reflectivity, which avoids lengthening of the back reflector fabrication.     

Effect of sodium addition on Cu-deficient CuIn1−xGaxS2 thin film solar cells

Chalcopyrites are important contenders among solar-cell materials due to direct band gap and very high-absorption coefficients. Copper-indium-gallium disulfide (CIGS2) is a chalcopyrite material with a near-optimum band gap of 1.5 eV for terrestrial as well as space applications. At FSEC PV Materials Laboratory, record efficiency of 11.99% has been achieved on a 2.7 μm CIGS2 thin film prepared by sulfurization. There are reports of influence of sodium on copper-indium-gallium selenide (CIGS) as well as copper-indium disulfide (CIS2) solar cells. However, this is the first of its kind approach to study the effect of sodium on CIGS2 solar cells and resulting in encouraging efficiencies. Copper-deficient CIGS2 thin films were prepared with and without the addition of sodium fluoride (NaF). Effects of addition of NaF on the microstructure and device electrical properties are presented in this work.     

Optical confinement of the intermediate layer between Si and alumina substrate in thin film Si solar cells

Enhancement of the optical confinement effect by an intermediate layer (IML) between Si and alumina substrate in thin film Si solar cells was studied. The dependence of the optical confinement effect on refractive index of the IML and on thickness of Si was separately investigated by hemispherical reflectance measurement of the following two series of samples. In the first case, SiO_xN_y, SiN_x or TiO₂ was deposited as the IML in the multilayer, Si/IML/alumina. In the second case, Si layers with different thicknesses were formed. The study showed that in certain conditions the IML could enhance the optical absorption of Si layer in thin film Si solar cells.     

CVD of CuGaSe2 for thin film solar cells with various transport agents

Chemical vapor deposition (CVD) in an open tube system was employed to deposit single-phase CuGaSe₂ thin films on plain and Mo-coated glass substrates. The use of HCl and ternary CuGaSe₂ source material resulted in non-stoichiometric volatilization of the source material. The use of binary source materials – Cu₂Se and Ga₂Se₃ – in combination with I₂ and HCl as the respective transport agents yielded single-phase CuGaSe₂ thin films while the source materials were volatilized stoichiometrically. Mo/CuGaSe₂/CdS/ZnO devices were fabricated from these samples exhibiting an open-circuit voltages up to V_oc=853 mV.     

150-mm layer transfer for monocrystalline silicon solar cells

We report on recent improvements concerning the transfer of monocrystalline silicon layers to plastic substrates for flexible solar cell applications. Finite element numerical modeling of the etching current density distribution allows for optimizing our electrochemical etching setup for separation layer formation. By modifying the setup according to the simulation results, we are now able to transfer 25 μm thick monocrystalline silicon sheets with up to 150 mm in diameter.     

Quantum efficiency and admittance spectroscopy on Cu(In,Ga)Se2 solar cells

We investigate the electronic transport properties of Cu(In,Ga)Se₂ solar cells by means of quantum efficiency and temperature dependent admittance spectroscopy. A simple evaluation scheme of quantum efficiency data is introduced which accounts for recombinatoric losses in the Us buffer layer and in the Cu(In,Ga)Se₂ absorber. By admittance spectroscopy, we find that the controlled incorporation of Na into the absorber material leads to a shallow acceptor state at about 75 meV above the valence band.     

Wet-etch texturing of ZnO:Ga back layer on superstrate-type microcrystalline silicon solar cells

Surface wet etching is applied to the ZnO:Ga (GZO) back contact in μc-Si thin film solar cells. GZO transparency increases with increasing deposition substrate temperature. Texturing enhances reflective scattering, with etching around 5-6 s producing the best scattering, whereas etching around 5 s produces the best fabricated solar cells. Etching beyond these times produces suboptimal performance related to excessive erosion of the GZO. The best μc-Si solar cell achieves FF=68%, V_OC=471 mV and J_SC=21.48 mA/cm² (η=6.88%). Improvement is attributed to enhanced texture-induced scattering of light reflected back into the solar cell, increasing the efficiency of our lab-made single μc-Si solar cells from 6.54% to 6.88%. Improved external quantum efficiency is seen primarily in the longer wavelengths, i.e. 600-1100 nm. However, variation of the fabrication conditions offers opportunity for significant tuning of the optical absorption spectrum.     

High-efficiency flexible CdTe solar cells on polymer substrates

Development of flexible and lightweight solar cells is interesting for terrestrial and space applications that require a very high specific power (kW/kg) and flexibility for curved shaping or rolling. Flexible CdTe/CdS solar cells of 11% efficiency in superstrate and 7.3% efficiency in substrate configurations have been developed with a “lift-off” approach. However, roll-to-roll manufacturing is desired in future.Therefore, flexible superstrate solar cells were directly grown on commercially available 10 μm thin polyimide (Upilex™) foils. A process for the deposition of ITO (front contact) has been developed to have a stable front contact on the Upilex™ foil. Post-deposition annealing treatments of the ITO/polyimide stacks bring a significant stability to the front contact, having almost the same sheet resistance at the beginning and at the end of the cell fabrication process. Solar cells with AM1.5 efficiency of 11.4% on Upilex™ foils (highest efficiency recorded for flexible CdTe cell) have been developed. A comparison of the cells prepared on different polyimides is presented.     

Thin film silicon solar cells for space applications: Study of proton irradiation and thermal annealing effects on the characteristics of solar cells and individual layers

The paper reports on the effects of a proton irradiation campaign on a series of thin-film silicon solar cells (single- and double-junction). The effect of subsequent thermal annealing on solar cells degraded by proton irradiation is investigated. A low-temperature annealing behaviour can be observed (at temperatures around 100 to 160°C) for microcrystalline silicon solar cells. To further explore this effect, a second proton irradiation campaign has been carried out, but this time on microcrystalline silicon layers. The effect of proton irradiation and subsequent thermal annealing on the optical and electronic properties of microcrystalline silicon is, thus, thoroughly investigated.     

Synchrotron-based spectroscopy for the characterization of surfaces and interfaces in chalcopyrite thin-film solar cells

The experimental CISSY setup was constructed for the surface and interface analysis of chalcopyrite Cu(In_1−xGa_x)(S_ySe_1−y)₂ “CIGSSe” thin-film solar cells operable as laboratory surface analysis system using commercial X-ray and UV sources or as end station at the BESSY synchrotron facility. It houses an X-ray spectrometer for X-ray emission (XES) and an electron analyzer for photoemission (PES) spectroscopy measurements. The techniques deliver information about the chemical and electronic sample structure on a complementary depth scale. With probing depths up to half a micrometer, XES provides information of the near-surface sample bulk. PES in contrast only probes the first monolayers of a sample and hence, is very surface sensitive. The special feature of the CISSY setup is the unique combination of these spectroscopies with in-system sputter and wet-chemical preparation capabilities. Furthermore, a programmable sample manipulator enables laterally resolved measurements or measurements while the sample is constantly moved, reducing damage of radiation sensitive material, as e.g. organics. This arrangement allows for the characterization of real-world sample surfaces and interfaces prepared under controlled conditions such as vacuum or inert gas. The main focus of the CISSY experiments is a better understanding of the solar cell functioning mechanisms by combining the knowledge of the chemical and electronic structures gained by spectroscopic investigation of involved components and the physical device properties.The paper will give an overview of important results from eight years of CISSY research. In that period of time CISSY has tackled three main research areas: (i) the CIGSSe absorber layer; (ii) the buffer/absorber interface; and (iii) laterally resolved and time-resolved measurements, particularly of chalcopyrite thin-film solar cell components and structures. This review will focus on some examples out of these topics including results which could only be obtained by the combination of in-system preparation and analysis capabilities as realized by the CISSY setup.