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)

/pdf/effects-of-heavy-alkali-elements-in-cu-in-ga-se2-solar-cells-18s7rj88wq.pdf

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

Three types of CIGS devices with varying treatments of RbF post-deposition treatment (PDT) and sodium are subjected to accelerated stress test (AST) conditions at elevated temperature (65 °C), voltage bias (short- vs open-circuit), and illumination (AM1.5 vs dark). RbF treatment with reduced CdS thickness shows an improvement in Voc and efficiency in this sample series, and also results in high FF and doping ~ 1016cm−3. Cells with reduced sodium show a decrease in Voc and infrared QE results suggest a higher optical minimum bandgap. Heat and light soaking experiments at 50, 65, and 75 °C, open and short-circuit junction bias under AM1.5G illumination suggest stabilization of CIGS solar cells with addition of Na and RbF. SCAPS-1D simulations suggest reduction in thermally ionized defect density in ordered vacancy compound (OVC) and changes in conduction band offset with RbF post-deposition treatment. Further, simulations show an increase in shallow acceptor and donor density after open-circuit and short-circuit AST respectively. Preliminary light soaking results for oxidized CIGS with and without Na are also discussed.

Effects of Alkali and Oxidation Treatments on Efficiency and Stability of CdS/CIGS Solar Cells

Nine types of CIGS cells were studied with permutations of sodium content, buffer type (CdS or Zn(O, S)), and post-deposition treatment (PDT) with KF, RbF, or oxidation. JV, QE, and CV measurements are presented for several cells of each type. KF PDT with CdS buffer resulted in the highest efficiency. Although Zn(O, S) buffer cells with typical sodium content had relatively low efficiency, RbF PDT significantly improved performance. Oxidation alone significantly increased the efficiency of sodium deficient cells with CdS buffer. Numerical modeling was conducted to connect device properties to the observed performance variations.

CIGS Device Models with Variations of Buffer Layer, Alkali Content, and Oxidation

Voltage losses reduce the photovoltaic conversion efficiency of thin‐film solar cells and are a primary efficiency limitation in Cu(In,Ga)Se2. Herein, voltage loss analysis of Cu(In,Ga)Se2 solar cells fabricated at three institutions with variation in process, bandgap, absorber structure, postdeposition treatment (PDT), and efficiency is presented. Nonradiative voltage losses due to Shockley–Read–Hall charge carrier recombination dominate and constitute >75% of the total compared to <25% from radiative voltage losses. The radiative voltage loss results from nonideal absorption and carriers in band tails that stem from local composition‐driven potential fluctuations. It is shown that significant bulk lifetime improvements are achieved for all alkali PDT processed absorbers, chiefly associated with reductions in nonradiative recombination. Primary voltage loss contributions (radiative and nonradiative) change little across fabrication processes, but variation in submechanisms (bulk lifetime, net acceptor concentration, and interface recombination) differentiate nonradiative loss pathways in this series of solar cells.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/solr.202300075

Stefan Paetel

Papers

Effects of Alkali and Oxidation Treatments on Efficiency and Stability of CdS/CIGS Solar Cells

CIGS Device Models with Variations of Buffer Layer, Alkali Content, and Oxidation

Nonradiative Recombination Dominates Voltage Losses in Cu(In,Ga)Se2 Solar Cells Fabricated using Different Methods