Lars Stolt

Zn(O,S) buffer layers by atomic layer deposition in Cu(In,Ga)Se2 based thin film solar cells: Band alignment and sulfur gradient

Thin film solar cells with the structure sodalimeglass∕Mo∕Cu(In,Ga)Se2∕Zn(O,S)∕ZnO∕ZnO:Al are studied for varying thickness and sulfur content of the Zn(O,S) buffer layer. These Zn(O,S) layers were deposited by atomic layer deposition (ALD) at 120°C. Devices with no or small concentrations of sulfur in the buffer layer show low open-circuit voltages. This is explained by the cliff, or negative conduction-band offset (CBO), of −0.2eV measured by photoelectron spectroscopy (PES) and optical methods for the Cu(In,Ga)Se2 (CIGS)∕ZnO interface. Devices with ZnS buffer layers exhibit very low photocurrent. This is expected from the large positive CBO (spike) of 1.2eV measured for the CIGS∕ZnS interface. For devices with Zn(O,S) buffer layers, two different deposition recipes were found to yield devices with efficiencies equal to or above reference devices in which standard CdS buffer layers were used; ultrathin Zn(O,S) layers with S∕Zn ratios of 0.8–0.9, and Zn(O,S) layers of around 30nm with average S∕Zn ratios...

A novel cadmium free buffer layer for Cu(In,Ga)Se2 based solar cells

Abstract Solar cells based on Cu(In,Ga)Se 2 were prepared replacing the “standard buffer layer” CdS with a In x (OH,S) y thin film. The film is deposited in a chemical bath (CBD) process using an aqueous solution containing InCl 3 and thioacetamide. X-ray photoemission spectroscopy measurements were performed in order to characterize the growth kinetics and the chemical composition. The influence of different concentrations of InCl 3 and thioacetamide in the solution on the electrical properties of the solar cells was studied by measuring the j-V characteristics and the spectral quantum efficiencies. Capacitance-voltage ( C-V ) measurements indicate that the high V ∞ values of devices with the novel buffer layer are correlated with narrower space charge widths and higher effective carrier concentrations in the absorber materials. The achieved conversion efficiency of 15.7% (active area) using the cadmium free In x (OH,S) y buffer demonstrates the potential of this process as an alternative to the standard chemical bath deposition of CdS.

ZnO/CdS/Cu(In,Ga)Se/sub 2/ thin film solar cells with improved performance

This paper reports results from experiments concerning the growth of CuInSe/sub 2/ films on different substrate materials, uncoated, and coated with molybdenum. Specifically the effect on the structure, i.e. preferred orientation, of the polycrystalline films is investigated. It is found that soda-lime float glass results in the most oriented films and also that the highest solar cell conversion efficiency is obtained with devices made from such films. In another set of experiments the effect of various deposition conditions for the ZnO window layer is studied. It is found that optimum performance is not strongly dependent on the deposition process. The highly doped part of the window, ZnO:Al, has been replaced with ITO on some devices and a comparison is made. Finally, ZnO/CdS/CuInSe/sub 2/ and ZnO/CdS/Cu(In,Ga)Se/sub 2/ thin film devices exhibiting active area conversion efficiencies of 15.4% and 16.9%, respectively, are demonstrated.<<ETX>>

Growth of Cu(In,Ga)Se2 thin films by coevaporation using alkaline precursors

Abstract CIGS layers were grown using NaF and LiF precursor layers with different thicknesses deposited on top of the Mo back contact layer prior to the CIGS deposition. The substrates were soda lime glass with and without Na diffusion barriers. Two types of barriers were used, silica and alumina. The CIGS layers were analysed structurally with SEM, TEM and XRD, and solar cell devices were prepared. We found improvements in grain structure, film texture and surface flatness when NaF precursor layers were used as compared to Na-free and Na-poor samples. Also electrically we found that the solar cell efficiency increased with modest additions of Na. Changes in film morphology saturated at an NaF thickness of 30 A and the solar cell efficiency saturated with an NaF precursor thickness of about 100 A. NaF precursor layers were also used at lower deposition temperatures and we observed improved solar cell efficiencies using the NaF precursor layer, even at substrate temperatures as low as 310°C.

Back surface band gap gradings in Cu(In,Ga)Se2 solar cells

Abstract We fabricate and analyse Cu(In,Ga)Se 2 -based solar cells which have a graded band gap by an increased Ga content towards the Mo back contact. The open circuit voltage and the short circuit current strongly improve with the band gap grading. Electronic device analysis reveals that the open circuit voltage increase is directly related to the increased band gap energy at the back surface. We interpret the obtained results to a large extent as reduced back contact recombination by the introduction of an increased band gap close to the back contact.

Diffusion of indium and gallium in Cu(In,Ga)Se2 thin film solar cells

The diffusion of indium and gallium in polycrystalline thin film Cu(In,Ga)Se2 layers has been investigated. Bilayer structures of CuInSe2 on top of CuGaSe2 and vice versa have been fabricated in both a Cu-rich and Cu-poor process (in relation to the ideal stoichiometry). In each process molybdenum coated soda-lime glass with and without a sodium barrier was used. These bilayers were analyzed with secondary ion mass spectrometry, X-ray diffraction, scanning electron microscope and transmission electron microscope equipped with energy dispersive X-ray spectroscopy. It was found that the grain boundary diffusion was not significantly higher than the diffusion inside the grains, also for Cu-rich layers. The diffusion is suggested to mainly proceed via vacant metal sites in the lattice structure. In sodium free films a higher diffusion into the bottom layers, compared to films with sodium, was seen in all cases. This observation was explained with a larger number of vacancies, that facilitates indium and gallium diffusion, in the sodium free films. The difference in diffusion between indium in CGS layers and gallium in CIS layers, in both Cu-rich and Cu-poor processes, was small for layers with sodium. q 2003 Elsevier Ltd. All rights reserved.

Baseline Cu(In, Ga)Se2 device production : Control and statistical significance

Small- and-large area Cu(In,Ga)Se 2 - based solar cells, as well as 20 cm 2 mini-modules are produced using a baseline approach that privileges process simplicity and statistical significance. High-quality devices are controllably obtained, as well as a 14.7% world record mini-module. Both grided and conventional mini-modules are produced and compared. A few processes from our research areas are presented as candidates for baseline integration. Among these, the examples of fast CIGS and thin CIGS are shown. For the latter, Ga-grading is involved and fill factors above 81% have been measured.

Cu(In,Ga)Se2-based thin-film photovoltaic modules optimized for long-term performance

Abstract In this contribution we give an overview of the mechanisms behind degradation of Cu(In,Ga)Se 2 -based modules. Based on the results from a detailed analysis of power losses in modules, prior to and after extended damp heat exposure, we discuss to what extent modules can be designed to achieve enhanced long-term performance. For conventional modules, we show that the stability can be improved by optimizing the interconnect and the front contact. Furthermore, we argue that gridded modules are better from a long-term performance point of view. A novel interconnect structure, specifically designed for long-term durability, is briefly discussed.

Interface study of CuInSe2/ZnO and Cu(In, Ga)Se2/ZnO devices using ALD ZnO buffer layers

Abstract Solar cells based on CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS), with a ZnO buffer layer deposited by atomic layer deposition (ALD), are compared to their reference cells; (CIS or CIGS)/CdS/ZnO. While the CIS/ZnO devices show only slightly lower efficiencies compared to their reference cells, the difference between the CIGS/ZnO devices and their reference cells is larger. In the latter case, the main difference is the lower open circuit voltage of approximately 200 mV of the direct ZnO devices. The valence band offset between CIS and ZnO is determined by ultraviolet photoelectron spectroscopy to −2.2 eV which gives a conduction band offset, ΔEc, of +0.1±0.2 eV. This slightly positive offset is in contrast to our previous result for the CIGS/ZnO interface of ΔEc=−0.2±0.2 eV, and is a possible explanation for the much lower voltage loss observed for the CIS/ZnO devices. Zn diffusion into the different absorbers is investigated by energy dispersive X-ray spectroscopy on transmission electron microscope cross-sections prepared from direct ZnO devices. These cross-sections also show very good coverage of the absorber surface by the (ALD)ZnO layer.

Impurities in Chemical Bath Deposited CdS Films for Cu ( In , Ga ) Se2 Solar Cells and Their Stability

The highest efficiencies for Cu(In,Ga)Se-2-based thin dim solar cells have been achieved with CdS films prepared by a solution growth method known as the chemical bath deposition (CBD) technique. The impurity content in such cadmium sulfide films has bee