Haass, Stefan G.
- Doctoral Thesis
Rights / licenseCreative Commons Attribution-NonCommercial 4.0 International
Worldwide increasing energy needs and negative effects of fossil and nuclear power demand environmental friendly and cost-effective alternative energy sources like solar power. Silicon based solar cells dominate the photovoltaic (PV) market thanks to their high efficiency and matured technology. However, alternative absorber materials are desired that offer lower production cost and employ cheap and readily available constituents with little negative environmental impact. Cu(In,Ga)Se2 (CIGS) and CdTe are chalcogen-based absorber materials with a high absorption coefficient rendering them suitable for the application in thin film solar cells. These thin film technologies offer the advantage of lower material consumption, shorter energy payback time and the possibility of flexible substrates. However, the scarcity, high costs or toxicity of In, Te, Ga and Cd used in these absorber layers demands for alternative materials. Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells present a suitable alternative due to the non-toxicity, abundance of the material’s constituents and similarly high absorption coefficient. Kesterite solar cells exhibit efficiencies of up to 12.6%, demonstrating the high potential of the kesterite material. The relatively low open circuit voltage ((Voc), which is often described quantitatively as Voc - deficit (Voc - deficit=Eg -Voc), still remains the major problem inhibiting further efficiency improvement. The major reason for the Voc - deficit is still under debate, especially whether the main recombination paths are located in the bulk, grain boundaries or interfaces of the device. A major source for recombination are secondary phases such as binary and ternary copper-, zinc- and tin-selenides and -sulphides, which can exist in the absorber or at its interfaces. The secondary phases can impede the carrier transport and lead to increased recombination rates. Prevention of the formation of secondary phases is challenging due to a narrow homogeneity region and an incongruent melting of the kesterite phase. Losses in Voc can also be caused by recombination at the absorber interfaces due to a high density of interface states as well as a non-optimal band alignment. The lack of shallow defect states in kesterite is expected to lower mobility, lifetime and therefore decrease the Voc. Bandgap fluctuations stemming from structural or compositional inhomogeneities, or potential fluctuations due to a high concentration of charged defects can also lead to a higher Voc - deficit. In this thesis two separate approaches are investigated in order to overcome the aforementioned performance limitations. Both approaches are based on a solution process using dimethyl sulfoxide (DMSO), thiourea and metal-chlorides. The absorber fabrication is comprised of two steps, the precursor synthesis and the annealing in a chalcogen atmosphere. The first approach is aiming to improve the annealing in chalcogen atmosphere, thereby leading to the formation of homogeneous absorber layers, free of detrimental secondary phases and with a large-grained morphology. A detailed study conducted in this thesis on different annealing setups eventually results in a 3-stage annealing process under a controlled selenium atmosphere in a SiOx coated graphite box. The improved annealing environment leads to an improved morphology of the absorber layer and the Voc - deficit can be reduced to 0.57 V, which appears to be one of the lowest values reported for kesterite solar cells. The second approach focuses on the understanding of alkali treatments on kesterite solar cells. Sodium treatment has shown significant improvements of bulk properties by enhancing grain growth, increasing doping concentration and passivating grain boundaries. Other alkali elements have also been reported but their impact on electronic properties and device performance is more ambiguous. So far, alkali post deposition treatment of the kesterite absorber has not been thoroughly investigated yet. In the closely related absorber material CIGS, potassium-fluoride post deposition treatment (KF-PDT) of the absorber surface yielded significant improvements in Voc and fill factor leading to record efficiencies above 20% for this material. In this thesis the KF-PDT and potassium bulk treatment on kesterite absorber layers are investigated. It is shown that KF-PDT improves the open circuit voltage of kesterite solar cells. However, a severe blocking of the short circuit current and reduction of fill factor reduces the device efficiencies. Furthermore, quantum efficiency measurements indicate that KF-PDT alters the properties of the buffer layer. In contrast to KF-PDT, potassium bulk treatment leads to an enhanced grain growth and to overall improved photovoltaic parameters resulting in device efficiencies close to 10 %. The impact of potassium as a fluxing agent is more efficient compared to sodium. Additionally, potassium suppresses incorporation of sodium from the soda lime glass. There are no observable synergetic effects between potassium and sodium but both alkali elements equally improve kesterite solar cell performance. Finally, a comprehensive study to unveil the discrepancy between published results is presented comparing the effects of alkali treatments on device performance. The hypothesis is that each alkali element requires a different absorber composition to achieve the highest photovoltaic performance and therefore an extensive set of samples with different alkali elements and alkali concentrations as well as various metal ratios is studied. The investigation reveals a complex dependency of metal ratios, alkali elements and alkali concentrations on the device performance in high-efficiency kesterite solar cells. From Li to Cs the nominal Sn concentration ( Sn/Cu+Zn+Sn) required for best device properties is reduced and the alkali concentration resulting in highest device efficiencies is lower by an order of magnitude for the heavy alkali elements (Rb, Cs) compared to the lighter ones (Li, Na, K). The PV parameters correlate with changes in morphology with best devices exhibiting large grains throughout the whole absorber layer and a low density of grain boundaries. A ranking of best device performances employing alkali treatment resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells. A champion device with 11.5% efficiency is presented using a “high” Li concentration in conjunction with an optimized Sn content. In conclusion, improvements in the annealing environment and alkali treatment of kesterite absorber layers cleared the way for more than 11% conversion efficiency. Further investigations on Li alloying and interface treatments are promising paths to reach anticipated efficiency levels in the future Show more
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ContributorsExaminer: Tiwari, Ayodhya N.
Examiner: Kovalenko, Maksym
Examiner: Björkman-Platzer, Charlotte
Examiner: Romanyuk, Yaroslav E.
Organisational unit02010 - Dep. Physik / Dep. of Physics
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