Study of High Efficiency CZTSSe Solar Cells by Aqueous Spray Deposition Method = 수용액 스프레이 증착법에 의한 고효율 CZTSSe 태양전지 연구
저자
발행사항
인천 : 인천대학교 대학원, 2022
학위논문사항
학위논문(박사)-- 인천대학교 대학원 : 물리학과 2022. 8
발행연도
2022
작성언어
영어
주제어
발행국(도시)
인천
형태사항
117 ; 26 cm
일반주기명
지도교수: JunHo Kim
UCI식별코드
I804:23006-200000629969
소장기관
ABSTRACT
Globally increasing energy demand combined with the limited energy sources (fossil, oil, nuclear power) strongly requires that humankind must seek alternative energy sources which is more environment friendly and much cost effective. One of the most suitable energy sources for this requirement is solar energy. Yet, the photovoltaic (PV) devices which harvest electrical energy from solar source are still in their development period. At current state, Silicon (Si) based solar cells are dominating the worldwide PV market owing to their mature technology and considerably high power conversion efficiency (PCE). However, due to several disadvantages of Si solar cell, it is necessary to obtain alternative solar energy materials which yield high PCE, low production cost and less environmental impact.
Chalcogen based Kesterite Cu2ZnSnSe4 (CZTSe), Cu2ZnSn(S,Se)4 (CZTSSe), Cu2ZnSnS4 (CZTS) absorber materials are highly considered as one of the most promising alternative due to its abundance of constituent elements, suitable opto-electronic properties and low production cost. However until now, the champion CZTSSe devices show PCE ~13% which is nearly the half PCE amount of the best Si solar cell. The origin behind such limited PCE in kesterite solar cells are due to the low open circuit voltage (VOC), which has often described as VOC - deficit (VOCdef = Eg - VOC). In more details, the Shockley-Read-Hall (SRH) recombination in bulk and interface surface, non-optimal band alignment, high density of defects and potential fluctuations lead to a higher VOCdef.
In this thesis work we used eco-friendly aqueous spray deposition method for the CZTSSe thin films and further investegated several approaches to overcome the VOCdef limitation in CTZSSe solar cells. The first approach is aimed to study the anion sulfur (S) and selenium (Se) alloying effect in CTZSSe solar cell and further probe its detrimental role in opto-electronic and defect properties. In this regard, we varied the S/(S+Se) ratio from 0 to 1 with the variation step of 0.2. As the results, when S/(S+Se) ratio was increased from 0 to 0.4, the surface compactness, grain size and shunt resistances were observed to be improved and better fill factor has achieved. However, with higher S-alloying more than S/(S+Se)=0.4, the grain size was too much decreased and detrimental effects were observed on overall device performance. We also found that different PCE limiting factors were developed with various S/(S+Se) ratio. High density of deep defect states were generated with S/(S+Se) ≥ 40% content and larger conduction band offset were formed in the Se/(S+Se) ≥ 80% content. By optimizing the S/(S+Se) ratio, PCE of CZTSSe solar cells significantly improved from 7.02% to 10.04% and Vocdef were minimized up to 616 mV.
The second approach aimed to substitute relatively large atomic radius of Ag into host CZTSSe as (AgxCu1-x)2ZnSn(S,Se)4 (ACZTSSe). We varied the Ag/(Ag+Cu) ratio from 0 to 0.15 and further checked the Ag substitution effects with various spectroscopic methods. As the experimental results, we found that PCE of ACZTSSe devices were increase until the Ag/(Ag+Cu)=0.09 ratio and further decrease back with higher Ag substitution conditions. Moreover, admittance spectroscopy analysis revealed that optimal substitution of Ag/(Ag+Cu)=0.09 reduce Cu/Zn related defects, charge recombination centers effectively. However on the other hand, higher Ag substitution conditions were induced generation of deeper defects which decreases PCE back. Finally with the optimum Ag content, PCE of ACZTSSe solar cell much improved to 11.83% and significantly reduced VOCdef of 560 mV has obtained.
Among above approaches, the Ag alloying into Cu site showed the most reduced Vocdef. However, Ag is not a cheap material and its usage with lower concentration is desirable for the earth abundant CZTSSe solar cells. Therefore in our final third approach, we aimed to study structure engineering to reduce the Ag contents without deteriorating its beneficial effects. In this regard, we proposed insertion of Ag-doped thin layers in front (F), back (B) and front/back (FB) side of the CZTSSe solar cells. As the result, F-passivated structure ensured a higher Voc and enhanced charge carrier extractions whereas B-passivated structure enabled reduction of back contact barrier and improved Jsc. Moreover, the combination of both F- and B-passivated structure, the dual FB structure engineering exhibited the lowest SRH recombination, reduced band tailing and minimized electrostatic fluctuations. Owing to all these benefits, with dual FB-passivated ACZTSSe structure, the highest PCE of 12.43% has obtained with minimum Ag contents of 2-3%. Moreover, the obtained Vocdef was in considerably reduced range of 580 meV.
Summary
This thesis presents study about the limitation factors in CZTSSe based thin film solar cells. CZTSSe absorbers were prepared with eco-friendly aqueous solution (DIW) based spray deposition approach in ambient air condition. Several approaches of cation and anion alloying were applied for deep study of CZTSSe photo absorbers and various types of spectroscopic characterizations were performed for insightful analysis.
First, we systematically studied the significant role of S-alloying in CZTSSe solar cell and revealed its main effect on the device efficiency. We found that different PCE limiting factors were developed depending on the S/(S+Se) ratio. Excessive S alloying (S/(S+Se) ≥ 40%) in the CZTSSe absorber induces deeper defects, limiting the current density drastically by formation of non-radiative charge recombination centers, whereas lower S-alloyed (Se/(S+Se) ≥ 80%) or no S-alloyed CZTSSe solar cell exhibits larger spike type CBO between the absorber and buffer, which is confirmed in the red-kinked J-V curve.
Second, we deeply characterized the Ag-alloyed CZTSSe solar cells. The beneficial effects of optical, electrical and defect properties with Ag-alloying further investigated deeply. The specific amount of Ag alloying (Ag/(Cu+Ag) = ~9 %) is desirable to suppress the electro-static fluctuation and CuZn antisite defects, comparing with devices with lower Ag alloying. We also found that higher Ag alloying ACZTSSe devices has limitations due to the formation of AgZn or ZnSn related deep defects and ZnSe segregation at the interface and boundary region. Thus, fine tuning of Ag/(Ag+Cu) ratio is necessary to suppress both interface and bulk recombination centers in minimum range.
Third, we proposed Ag-passivation structures of front (F), back (B), and dual front/back (FB) passivated absorbers to obtain higher PCE of ACZTSSe solar cells with a minimum Ag content. The F-passivation structure ensured a higher Voc, reduced interface and bulk recombination, whereas the B-passivation structure enabled reduction of back contact barrier and improved collection of photo-generated carriers. The dual FB-passivation structure exhibited the lowest SRH recombination, the longest minority carrier lifetime, reduced band tailings and minimized electrostatic fluctuations. In general, our experimental study reveals that optimum alloying conditions for both anion and cation elements are crucial to minimize limitation factors in CZTSSe absorber films. By using the optimized CZTSSe absorber film, we achieved remarkable PCE of 12.43% (Current world record PCE is 13%) and it further confirms that simple aqueous spray deposition method in ambient air has much competing potential as comparing with vacuum based expensive methods. We believe that our results has significant impact as solar cell community tries to develop processing way with more eco-friendly, less material waste and easy to employ for up-scale productions.
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