Skip to main content

Development of thin film Solar cells based on WIde band Gap kesterite absorbers

Periodic Reporting for period 2 - SWInG (Development of thin film Solar cells based on WIde band Gap kesterite absorbers)

Reporting period: 2016-12-01 to 2018-05-31

The aim of this project is to develop wide band gap thin film solar cells based on kesterite absorbers for future application in high efficiency and low cost tandem PV devices. The SWInG working group focuses both on the development of the processes for the synthesis of such solar cells based on the Cu2ZnXY4 (with X = Sn, Si, Ge and Y = S, Se) compounds, and on the understanding of the physical and electrical properties of the high band gap absorber in order to reach high conversion efficiency. The key research challenges are: developing up-scalable processes for the synthesis of the absorbers; defining the specifications for high quality wide band gap absorbers as well as suitable back contact and buffer/window layers; and assessing the potential of this technology for PV applications. The wide band gap thin film solar cells developed in this project are expected to reach state-of-the-art efficiencies. Publication of specifications for the synthesis of a high quality Cu2ZnXY4 absorber as well as suitable back and front contacts are expected. The lead users will be PV modules manufacturers and companies that design and produce the machines for the synthesis of such devices. The results will be disseminated and communicated to the European PV industries and the scientific community. The intensive exchange of researchers between the partners during the project will also lead to an enhanced European collaboration in the research field of thin film solar cells.
The main technical results achieved:
WP 2 - Two different reliable preparation routes for Cu2ZnGe(S,Se)4 absorbers were developed which resulted in functional solar cells that achieved up to 8.4 % power conversion efficiency, which is the world record value for this material.
WP 3 - The combination of etching (HCl-route), passivation (chemical and annealing), and buffer layer deposition optimization led to the following best efficiencies ever reported for these particular absorber materials: (a) 8.4 % as already mentioned above, (b) kesterite solar cell with Zn(O,S) buffer layer (7.5 %), (c) best reported fraction of theoretically achievable VOC for any kesterite solar cell (68 %, while record Cu2ZnGe(S,Se) solar cells reach 55 to 60 %).
WP 4 - ITO has been shown to be a good back contact candidate, but requires a protective top layer (i.e. Al2O3, IZO, or TiO2) in order to remain transparent after exposure to the H2Se atmosphere needed for the absorber deposition process. An ITO back contact with a 30 nm IZO or TiO2 protective layer has an average absorption of 8 and 18 %, respectively, in the near infra-red regime.
WP 5 – For large-area prototypes compositional and thickness lateral uniformity is shown at the precursor level. CZGSe absorbers combined with interface chemical treatments and top layer deposition as described above gives the best device efficiency of 6.4 %, while the average efficiency value on 31 cm2 is 5.8 %.
Their exploitation:
Midsummer developed new equipment for manufacturing (i.e. the implementation of radiofrequency (RF) sputtering into the equipment at Midsummer, which now constitutes as a basis for the development of a commercial RF sputtering module to be added to the company product portfolio). Imec, ZSW, TNO, and CNRS focused on know-how creation (for patents, licenses, new funding ...) where ZSW focused on the new absorber, CNRS on the front interface and buffer, and TNO on the rear interface and transparent rear contact. And finally, HZB and UG created new knowledge via optimizing their advanced characterization/simulation capabilities (e.g. the development of a novel IPES spectrometer to the stage of a prototype).
SWInG moved wide band gap kesterite technology from TRL 2 to TRL 3-4. TRL 3 was achieved in WP2, where two different reliable preparation routes for Cu2ZnGe(S,Se)4 absorbers were developed. It was also achieved in WP3, where the combination of etching (HCl-route), passivation (chemical and annealing), and buffer layer deposition optimization led to the best performing solar cells. And it was achieved in WP4, where transparent back contacts were employed in wide-band gap CZGeSe kesterite devices intended to be used as top cell in a four terminal photovoltaic tandem cell. TRL 4 was done in WP5, where compositional and thickness lateral uniformity is shown at the precursor level, and CZGSe absorbers combined with interface chemical treatments and top layer deposition gave an average efficiency value of 5.8 % on 31 cm2.
SWInG provided better scientific understanding and guidance enabling the players concerned to frame strategic choices concerning future energy technologies and to integrate them in the future energy system. Hard x-ray photoelectron spectroscopy (HAXPES) and soft x-ray emission spectroscopy (XES) were employed for chemical and electronic structure analysis of the layers developed and their interfaces. Additionally, important parameters of all layers developed have been measured by advanced electrical and material characterization. All this data enabled the development of a SCAPS model for the solar cell structure developed in this project. This model has been applied for the outlook of this technology. This project is also a scientific material research project for the TF-PV application. It brings more understanding of the basic issues in improving the efficiency of wide band gap solar cells, and it also assisted improving the efficiency of standard band gap kesterite solar cells (see e.g. the results obtained at other institutes as IREC, where a pinch of Ge is now used to developed standard band gap high-efficiency kesterite solar cells with high reproducibility).
New approaches to existing technologies with potential for significant improvements in the overall performance have been developed. Record solar cell efficiencies have been obtained: (i) Cu2ZnGe(S,Se)4 absorbers that resulted in functional solar cells that achieved up to 8.4 % power conversion efficiency, which is the world record value for this material, (ii) similar kesterite solar cells with Zn(O,S) buffer layer of 7.5 % efficiency, (iii) best reported fraction of theoretically achievable VOC for any kesterite solar cell, i.e. 68 %, while record Cu2ZnGe(S,Se)4 solar cells reach 55 to 60 % only, and (iv) solar cells with a CZGeSe absorber material and an ITO back contact with IZO or TiO2 protective layer, where the efficiency was 85 to 90 % of reference cells with Mo back contact (with those with a TiO2 layer yielding highest VOC).
SWInG developed new, out-of-the-box or advanced innovative ideas that provide new impetus to technology pathways, to new solutions, and to new contributions to the energy challenge in Europe or worldwide. This project is a decisive first step to significantly improve the performance of the current PV technologies for a reasonable increase of the fabrication costs and to sustain the development of the TF-PV market in Europe. During the duration of the project tandem approaches became an important field of research, where many PV candidates are still battling to become the preferred top or bottom solar cell choice. A scientific fight that is not yet finalized and will continue in future EU calls. Additionally, his project also enabled new idea’s where very wide band-gap materials will be involved in the development of transparent PV windows, i.e. H2020 project Tech4Win.