Abstract
Addition of alkali dopants is essential for achieving high-efficiency conversion efficiency of thin film solar cells based on chalcogenide semiconductors like Cu(In,Ga)Se2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTSSe also called kesterite). Whereas the treatment with potassium allows boosting the performance of CIGS solar cells as compared to the conventional sodium doping, it is debated if similar effects can be expected for kesterite solar cells. Here the influence of potassium is investigated by introducing the dopant during the solution processing of kesterite absorbers. It is confirmed that the presence of potassium leads to an enhanced grain growth and a ten-fold lower potassium concentration is sufficient for obtaining grain size similar to sodium-containing absorbers. Potassium is located predominantly at grain boundaries and it suppresses incorporation of sodium into the absorber layer. The potassium doping increases the apparent carrier concentration to ∼2×1016 cm-3 for a potassium concentration of 0.2 at%. The potassium-doped solar cells yield conversion efficiency close to 10%, on par with only sodium-doped samples. Co-doping with potassium and sodium has not revealed any beneficial synergetic effects and it is concluded that both dopants exhibit similar effects on the kesterite solar cell performance.
Highlights
Kesterite solar cells achieved up to 12.6 % conversion efficiency[1] but still suffer under a pronounced reduction of the open circuit voltage (VOC), often expressed as VOC – deficit, which is defined as VOC – deficit = Eg/q – VOC with Eg the bandgap and q the elementary electric charge
For simplicity the samples are denoted with molarity values throughout the article even though it is worth stressing that actual alkali concentrations inside kesterite absorbers measured by inductively coupled plasma mass spectrometry (ICP-MS) are significantly lower than nominal and do not exceed 0.2 at%
We suggest a similar mechanism for a potassium assisted grain growth improvement as was suggest earlier for sodium:[15] Chemisorption of gaseous Se during the high temperature selenization step leads to the formation of
Summary
Solution processed CZTS absorber layers.[12] For solar cell devices, a reduction in the series resistance was observed upon potassium addition[12] whereas the carrier concentration could be increased for potassium as compared to sodium, rubidium, cesium or undoped samples.[9,13]. In this study we investigate the impact of potassium addition on the kesterite absorber morphology, composition, lattice structure and the photovoltaic parameters of resulting kesterite solar cells. This approach exploits the advantage of solution processing which enables a controlled and homogeneous incorporation of alkali elements in to the precursor layer. In order to explore any synergetic effects between potassium and sodium, controlled amounts of KCl were added on top of a constant amount NaCl, and results are compared to the samples with only KCl addition
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