Source and effects of sodium in solution-processed kesterite solar cells

Abzieher, Tobias; Schnabel, Thomas; Hetterich, M.; Powalla, Michael; Ahlswede, Erik

doi:10.1002/pssa.201532619

Cited by 27 publications

(31 citation statements)

References 41 publications

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“…Since the early 1990's, it is known that Na leads to significant improvements on CIGS solar cells [33]. For kesterite solar cells, the effects of Na were thoroughly investigated [34][35][36][37] and it has been shown that Na diffusion from SLG has similar positive effects on absorber morphology and device properties in kesterite devices [37,38]. Na treatment increases grain size and improves the overall absorber layer morphology, increases doping concentration and was shown to yield improved opto-electronic properties of the absorber layer.…”

Section: Sodium (Na)mentioning

confidence: 99%

Doping and alloying of kesterites

et al. 2019

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Attempts to improve the efficiency of kesterite solar cells by changing the intrinsic stoichiometry have not helped to boost the device efficiency beyond the current record of 12.6%. In this light, the addition of extrinsic elements to the Cu 2 ZnSn(S,Se) 4 matrix in various quantities has emerged as a popular topic aiming to ameliorate electronic properties of the solar cell absorbers. This article reviews extrinsic doping and alloying concepts for kesterite absorbers with the focus on those that do not alter the parent zinc-blende derived kesterite structure. The latest state-of-the-art of possible extrinsic elements is presented in the order of groups of the periodic table. The highest reported solar cell efficiencies for each extrinsic dopant are tabulated at the end. Several dopants like alkali elements and substitutional alloying with Ag, Cd or Ge have been shown to improve the device performance of kesterite solar cells as compared to the nominally undoped references, although it is often difficult to differentiate between pure electronic effects and other possible influences such as changes in the crystallization path, deviations in matrix composition and presence of alkali dopants coming from the substrates. The review is concluded with a suggestion to intensify efforts for identifying intrinsic defects that negatively affect electronic properties of the kesterite absorbers, and, if identified, to test extrinsic strategies that may compensate these defects. Characterization techniques must be developed and widely used to reliably access semiconductor absorber metrics such as the quasi-Fermi level splitting, defect concentration and their energetic position, and carrier lifetime in order to assist in search for effective doping/alloying strategies.

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Section: Sodium (Na)mentioning

confidence: 99%

Doping and alloying of kesterites

et al. 2019

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“…So far the most research attention has been paid to sodium, resulting in many thorough investigations which revealed grain size enhancement, passivation of grain boundaries, and an increase in net hole concentration as the major beneficial effects of sodium treatments. [3][4][5][6] Also lithium addition has shown to improve device performance by boosting the electronic quality of the CZTSSe absorber material and grain boundaries. [7] First studies on the effect of potassium addition confirmed advantageous effects on kesterite absorber growth and optoelectronic properties similar to Na.…”

mentioning

confidence: 99%

“…[3][4][5][6] Also lithium addition has shown to improve device performance by boosting the electronic quality of the CZTSSe absorber material and grain boundaries. [7] First studies on the effect of potassium addition confirmed advantageous effects on kesterite absorber growth and optoelectronic properties similar to Na.…”

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confidence: 99%

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

Haass

Andrés

Figi

et al. 2017

Advanced Energy Materials

107

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kesterite material with its optimal bandgap and absorption coefficient. [2] Alkali treatment of kesterite solar cells is one of the measures to reduce the high V OC -deficit and most of today's >10% efficiency kesterite devices utilize the beneficial effects of alkali elements on absorber layer morphology and optoelectronic properties. So far the most research attention has been paid to sodium, resulting in many thorough investigations which revealed grain size enhancement, passivation of grain boundaries, and an increase in net hole concentration as the major beneficial effects of sodium treatments. [3][4][5][6] Also lithium addition has shown to improve device performance by boosting the electronic quality of the CZTSSe absorber material and grain boundaries. [7] First studies on the effect of potassium addition confirmed advantageous effects on kesterite absorber growth and optoelectronic properties similar to Na. [8,9] Several studies comparing different alkali elements and their effect on solar cell properties and device performance have recently been published. [10][11][12][13] Table 1 compares the effects on device performance by extracting a ranking of the various alkali elements in each publication in the order of their capability to improve device performance. It is apparent from these rankings that no consistent experimental results have been obtained, which triggers two questions: (i) Why do the published results differ so much? (ii) Which alkali element possesses the highest potential for efficiency improvements?This paper aims to unveil the discrepancy between the recently published results comparing the effects of alkali treatments on device performance. Our hypothesis is that each alkali element requires a different absorber composition to achieve the highest PV performance and we therefore prepared a comprehensive set of samples with different alkali elements and alkali concentrations as well as various metal ratios. All samples were thoroughly characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), inductively coupled plasma-mass spectrometry (ICP-MS), as well as current-voltage (J-V), capacitance-voltage (C-V), time-resolved photoluminescence (TRPL), and external quantum efficiency (EQE) measurements.The methodology used for absorber synthesis is based on the solution process described elsewhere, [14] allowing for accurate alkali incorporation by simply adding alkali chlorides to the solution. [15] Figure 1a illustrates the matrix of sample compositions prepared with five different alkali elements: lithium (Li), Sodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the r...

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“…However, the Na and O concentrations correlate for all samples over all temperatures. The concentrations are increased at the air/surface and the CZTS/Mo interfaces for all samples, indicating segregation of Na and O at these interfaces . The increased SIMS intensity may also be caused by a change in sputter yield and ionization efficiency at these interfaces, making quantitative assessment challenging .…”

Section: Resultsmentioning

confidence: 98%

Dynamic Impurity Redistributions in Kesterite Absorbers

Grini

Aboulfadl

Ross

et al. 2020

Physica Status Solidi (b)

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Cu2ZnSn(S,Se)4 is a promising nontoxic earth‐abundant solar cell absorber. To optimize the thin films for solar cell device performance, postdeposition treatments at temperatures below the crystallization temperature are normally performed, which alter the surface and bulk properties. The polycrystalline thin films contain relatively high concentrations of impurities, such as sodium, oxygen and hydrogen. During the treatments, these impurities migrate and likely agglomerate at lattice defects or interfaces. Herein, impurity redistribution after air annealing for temperatures up to 200 °C and short heavy water treatments are studied. In addition, nonuniformities of the sodium distribution on a nanometer and micrometer scale are characterized by atom probe tomography and secondary ion mass spectrometry, respectively. Sodium and oxygen correlate to a greater extent after heat treatments, supporting strong binding between the two impurities. Redistributions of these impurities occur even at room temperature over longer time periods. Heavy water treatments confirm out‐diffusion of sodium with more incorporation of oxygen and hydrogen. It is observed that the increased hydrogen content does not originate from the heavy water. The existence of an “ice‐like” layer on top of the Cu2ZnSnS4 layer is proposed.

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Source and effects of sodium in solution-processed kesterite solar cells

Cited by 27 publications

References 41 publications

Doping and alloying of kesterites

Doping and alloying of kesterites

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

Dynamic Impurity Redistributions in Kesterite Absorbers

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