N. Naghavi

Optical approaches to improve the photocurrent generation in Cu(In,Ga)Se2 solar cells with absorber thicknesses down to 0.5 μm

Improving the optical management is a key issue for ultrathin based solar cells performance. It can be accomplished either by trapping the light in the active layer or by decreasing the parasitic absorptions in the cell. We calculate the absorption of the different layers of Cu(In,Ga)Se2 (CIGSe) based solar cell and propose to increase the absorption in the CIGSe layer by optimizing three parameters. First, by increasing the transmitted light to the cell using a textured surface of ZnO:Al front contact which functions as a broadband antireflection layer. Second, by replacing the CdS/i-ZnO buffer layers with ZnS/ZnMgO buffer layers which have higher energy bandgaps. Third, by replacing the Mo back contact with a higher reflective metal, such as silver or gold. Calculations show that modifying these layers improves the total absorption by 32% in a 0.5 μm thick CIGSe absorber. These predicted improvements of the short circuit current are confirmed experimentally.

Insights on the influence of surface roughness on photovoltaic properties of state of the art copper indium gallium diselenide thin films solar cells

The influence of Cu(In,Ga)Se2 (CIGSe) surface roughness on the photovoltaic parameters of state of the art devices is reported, highlighting the importance of the roughness of the as-grown CIGSe absorbers on solar cell efficiencies. As-grown CIGSe surface is progressively smoothed using a chemical etch, and characterized by SEM, AFM, XPS, μ-Raman spectroscopy, x-ray diffraction (XRD), and reflectivity. The decrease of roughness has no marked influence on crystal structure and surface composition of the absorber. The main effect is that the total reflectivity of the CIGSe surface increases with decreasing roughness. The samples are processed into solar cells and characterized by current-voltage measurements. While the open circuit voltage (Voc) and fill factor remain constant, the short circuit current (Jsc) decreases markedly with decreasing roughness, resulting in a reduction of the solar cell efficiency from 14% down to 11%, which exceeds the expected decrease from increased reflectivity. Quantum effici...

Solution Processing Route to High Efficiency CuIn(S,Se)2 Solar Cells

Inorganic semiconductors have properties that are notoriously difficult to control due to the deleterious impact of crystalline imperfections, and this is especially so in solar cells. In this work, it is demonstrated that materials grown using wet chemistry processes for the preparation of nanocristalline precursors can achieve the same performance as the best state of the art, namely conversion efficiencies above 11% with CuInS2. Interestingly, due to the growth process, the active material inherit a porous morphology that is shown to play a part in the performance and functionality of the active material. The new device morphology leads to a device operation closer to that of nanoscale organic interpenetrated solar cells or dye sensitized solar cells than to those of standard polycrystalline ones.

Using radiative transfer equation to model absorption by thin Cu(In,Ga)Se 2 solar cells with Lambertian back reflector

We investigate the optical absorption in a thin Cu(In,Ga)Se(2) solar cell with a Lambertian white paint beneath a transparent back contact. Although this configuration has been proposed more than 30 years ago, it turns out that rigorous simulation of Maxwells equations demand powerful numerical calculations. This type of approach is time consuming and does not provide a physical insight in the absorption mechanisms. Here, we use the radiative transfer equation to deal with multiple scattering of the diffuse part of the light. The collimated part is treated accounting for wave effects. Our model is in good agreement with optical measurements.

Revisiting the interpretation of biased luminescence: Effects on Cu(In,Ga)Se2 photovoltaic heterostructures

We analyzed the luminescence signal under electrical bias (Lum-V) for several Cu(In,Ga)Se2 solar cells having different absorber growth processes and different buffer layers such as CdS and ZnS. A numerical model is developed taking into account optical and electrical properties of the complete heterostructures. It appears that the absorber-buffer interface has a crucial role in explaining the different behaviors. Our interpretation is based on the quasi Fermi level splitting (QFL) linked to both the applied voltage and the luminescence intensity. Lum-V experiments and its dependence on illumination intensity are discussed and could be used to access transport properties when looking at the depth variation of the QFL and offer a classification of the possible cases.

Record efficiencies for dry processed cadmium free CIGS solar cells with indium sulfide buffer layers prepared by atomic layer deposition (ALD)

The competitive market environment for the power industry has resulted in new optimization problems that need to be solved such as bidding, pricing, risk management, and market clearing. These problems are complicated by the fact that power systems are nonlinear and some aspects of the problem are ill-formulated. In this paper, we report our formulation of a market clearing optimization problem that maximized the social welfare. The market optimization is solved for cases where the market participants have private information about their generation costs. The proposed formulation incorporates the nonlinear nature of the MW power flows on the transmission grid. The IEEE 30-bus system is modified and used for testing of the proposed techniques.

Plasmonic enhancement of up-conversion in ultrathin layers

Application of up-conversion in photovoltaic is limited by the up-conversion efficiency of materials. We propose to use a realistic plasmonic structure proposed in the literature to exceed this limitation. Erbium doped yttrium fluoride thin layer has been elaborated by ALD at 250°C to be used in such a structure as active up-converter material. Up-conversion properties of samples have been characterized using a confocal microscope. Samples, having a thickness below 100 nm, deposited onto a gold mirror, exhibit an up-conversion visible with naked eyes. The measurement of the enhancement factor of structures associated with the plasmonic resonnator has been performed by luminescence cartography. From these data, total enhancement factor up to 35 has been achieved by the use of the plasmonic structures, as compared to a layer deposited on a bare glass substrate.

Comparative study of patterned TiO 2 and Al 2 O 3 layers as passivated back-contact for ultra-thin Cu(In, Ga)Se 2 solar cells

In this work, a low cost passivated back-contact for ultra-thin Cu(In,Ga)Se2-based (CIGS) solar cells to improve the carrier collection is developed. The current loss due to rear-interface recombination was first estimated with an accurate opto-electrical model. We compared the use of a sol-gel TiO2 and an ALD-Al2O3 layer for the back-contact passivation. 400–420 nm CIGS cells were fabricated on the oxide/Mo substrate with point-contacts patterned by nanoimprint lithography. The use of a patterned-Mo/Al2O3 back-contact leads to an increase of the cell performance compared to the standard Mo back-contact. The passivation effect is discussed and is characterized by photoluminescence.

Electrodeposition of ZnO-doped films as window layer for Cd-free CIGS-based solar cells

The Cu(In,Ga)Se2 (CIGS) thin film solar cell technology has made a steady progress within the last decade reaching efficiency up to 22.3% on laboratory scale, thus overpassing the highest efficiency for polycrystalline silicon solar cells. High efficiency CIGS modules employ a so-called buffer layer of cadmium sulfide CdS deposited by Chemical Bath Deposition (CBD), which presence and Cd-containing waste present some environmental concerns. A second potential bottleneck for CIGS technology is its window layer made of i-ZnO/ZnO:Al, which is deposited by sputtering requiring expensive vacuum equipment. A non-vacuum deposition of transparent conductive oxide (TCO) relying on simpler equipment with lower investment costs will be more economically attractive, and could increase competitiveness of CIGS-based modules with the mainstream silicon-based technologies. In the frame of Novazolar project, we have developed a low-cost aqueous solution photo assisted electrodeposition process of the ZnO-based window layer for high efficiency CIGS-based solar cells. The window layer deposition have been first optimized on classical CdS buffer layer leading to cells with efficiencies similar to those measured with the sputtered references on the same absorber (15%). The the optimized ZnO doped layer has been adapted to cadmium free devices where the CdS is replaced by chemical bath deposited zinc oxysulfide Zn(S,O) buffer layer. The effect of different growth parameters has been studied on CBD-Zn(S,O)-plated co-evaporated Cu(In,Ga)Se2 substrates provided by the Zentrum für Sonnenenergie-und Wasserstoff-Forschung (ZSW). This optimization of the electrodeposition of ZnO:Cl on CIGS/Zn(S,O) stacks led to record efficiency of 14%, while the reference cell with a sputtered (Zn,Mg)O/ZnO:Al window layer has an efficiency of 15.2%.

Zinc Sulfide Based Chemically Deposited Buffer Layers for Electrodeposited CIS Solar Cells

This paper presents a thermodynamic study of Chemical Bath Deposition (CBD) of zinc sulfide based films in aqueous solution. The aim is a better understanding of the effect of temperature and pH on the deposition mechanism and film composition. The formation of solid phases has been predicted by means of the precipitation conditions of ZnO, Zn(OH)2 and ZnS as a function of temperature between 298 and 363 K. Films have been deposited, without ammonia, on the basis of calculated diagrams and first results on solar cells based on electrodeposited CuIn(S,Se)2 layers have been demonstrated