Device Characteristics of CZTSSe Thin‐Film Solar Cells with 12.6% Efficiency

Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined and new entries since July 2014 are reviewed. URI: http://onlinelibrary.wiley.com/doi/10.1002/pip.2573/pdf [1] Authors: GREEN Martin A. EMERY Keith HISHIKAWA Y. WARTA W. DUNLOP Ewan Publication Year: 2015 Type: Articles in Journals

Solar Cell Efficiency Tables (Version 45)

We report on the use and effect of the alkali elements rubidium and caesium in the place of sodium and potassium in the alkali post deposition treatment (PDT) as applied to Cu(In,Ga)Se2 (CIGS) solar cell absorbers. In order to study the effects of the different alkali elements, we have produced a large number of CIGS solar cells with high efficiencies resulting in a good experimental resolution to detect even small differences in performance. We examine the electrical device parameters of these fully functional devices and observe a positive trend in the I –V parameters when moving from devices without PDT to KF-, RbF-, and eventually to CsF-PDT. A diode analysis reveals an improved diode quality for cells treat-ed with heavier alkalis. Furthermore, secondary ion mass spectrometry (SIMS) measurements reveal a competitive mechanism induced within the class of alkali elements in the CIGS absorber induced by the alkali post deposition treatment. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

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Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%

Solar Cell Efficiency Tables (Version 39)

Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20%. This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.

Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells

Behavior of Na and RbF‐Treated CdS/Cu(In,Ga)Se 2 Solar Cells with Stress Testing under Heat, Light, and Junction Bias

We present four-terminal perovskite-Si and perovskite-CIGS tandem solar modules that are fully scalable to commercial dimensions. Starting from small cells of 0.13 cm_2, we scale up the tandem devices by a factor of 30 to modules of 4 cm_2. By using a low-loss module design and optimized light coupling, we demonstrate perovskite-Si and perovskite-CIGS tandem solar modules which outperform the stand-alone devices on the large area, representing a key milestone for perovskite-based tandem photovoltaics. Moreover, the tandem solar module architecture is compatible with industrially scalable fabrication techniques, thus opening the route towards production of high-performance large-area perovskite-based tandem solar modules.

Efficient, large-area scalable Perovskite-Si and Perovskite-CIGS tandem solar modules

Rubidium fluoride (RbF) postdeposition treatment (PDT) has been shown to improve the performance of Cu(In <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${}_\mathrm{x}$</tex-math></inline-formula> Ga <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_\mathrm{1-x}$</tex-math></inline-formula> )Se <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> (CIGS) photovoltaic devices. In this study, temperature-dependent current voltage (JVT) and time-resolved photoluminescence (TRPL) experiments were combined with modeling using the solar cell capacitance simulator (SCAPS) computer code to investigate the effect of the RbF PDT. Two devices, one as-deposited and one with RbF PDT, were deposited by a three stage coevaporation process. JVT measurements suggest the dominant recombination mechanism may be tunneling-enhanced recombination via bandtail states, but that defect states in the bandgap can also be important. RbF PDT is shown to decrease the characteristic energy of the bandtails. TRPL data show an increase in the minority carrier lifetime after RbF PDT, leading to an improved open-circuit voltage. SCAPS modeling indicates that the dominant recombination mechanism is dependent on the specific defect makeup of a device, suggesting that small changes in processing conditions can impact device behavior. This explains the observation that, for some devices, defect states in the gap dominate while others, as is the case here, appear to be dominated by bandtails. 

Evaluating Recombination Mechanisms in RbF Treated Cu(In${}_\mathrm{x}$Ga$_\mathrm{1-x}$)Se$_{2}$ Solar Cells

Partial substitution of Cu by Ag in Cu(In,Ga)Se2 (CIGS) solar cells is advantageous as it allows lower temperature growth while maintaining high performance. To understand the role of Ag on device performance, we present a comprehensive analysis of (Ag,Cu)(In,Ga)Se2 (ACIGS) samples with an [Ag]/([Ag]+[Cu]) (AAC) ratio varying from 7% to 22%. The analysis involves a set of material and device characterization techniques as well as numerical simulations. Multiple electrical and material properties show a systematic dependence on the increased Ag content. These include a carrier-density decrease, a grain-size increase, and a flattened [Ga]/([Ga] + [In]) (GGI) profile leading to a higher minimum band gap energy and a reduced back grading. Although the best performing device (PCE = 18.0%) in this set has an AAC = 7%, cells with higher Ag contents have an advantage of a smoother absorber surface which is attractive for tandem applications, despite their slightly inferior conversion efficiencies (PCE = 16.4% for 22% Ag).

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Impact of Ag content on device properties of Cu(In,Ga)Se2 solar cells

Despite the efficiencies above 20% achieved with (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells, further efficiency improvements are necessary. One possibility is the implementation of a postdeposition treatment (PDT) process. The aim of this study is therefore to investigate the effect of rubidium-fluoride (RbF)-PDT on the performance and physical properties of the ACIGS absorber. For this purpose, the RbF source temperature of the PDT process of ACIGS films with Ag/(Ag+Cu) (AAC) ratios of 5% from a multistage co-evaporation inline process was systematically varied. It was shown that the efficiency of the devices is reduced by the PDT process, unlike observed for Cu(In,Ga)Se2 (CIGS) absorber, and is strongly influenced by the amount of rubidium. The behavior can be attributed to a strong reduction of the doping, which results from a changed doping mechanism. Furthermore, evidence for the formation of an additional layer was found. In addition, deep level transient spectroscopy (DLTS) measurements were performed on the samples showing a strong signal at low temperatures. This minority trap signal is strongly influenced by the amount of Rb and shows a systematically changing energetic position towards the middle of the band gap and an increasing density. Based on pulse variation measurements, the associated defect could be identified as an extended defect, indicating a location of the defect at grain boundaries. 

Stefan Paetel

Papers

Behavior of Na and RbF‐Treated CdS/Cu(In,Ga)Se 2 Solar Cells with Stress Testing under Heat, Light, and Junction Bias

Efficient, large-area scalable Perovskite-Si and Perovskite-CIGS tandem solar modules

Evaluating Recombination Mechanisms in RbF Treated Cu(In${}_\mathrm{x}$Ga$_\mathrm{1-x}$)Se$_{2}$ Solar Cells

Impact of Ag content on device properties of Cu(In,Ga)Se2 solar cells

How small changes make a difference: Influence of low silver contents on the effect of RbF‐PDT in CIGS solar cells