A subject that caught my attention during the 39th edition of the Photovoltaic Specialist Conference (PVSC) was a poster from Toshiba Corporation [1] about the study of a homojunction CIGS (Copper Indium Gallium Selenium) solar cell. CIGS solar cells are gaining more and more interest in the photovoltaic community as a thin film solar cell due to the material’s high absorptivity, low cost and relatively high power conversion efficiency. 

Standard CIGS solar cell consists of a p-type CIGS base, n-type CdS emitter and a ZnO transparent conductive oxide. This heterojunction between CIGS and CdS results in a conduction band offset. The heterojunction structure is used due to the fact that it is hard to get high enough levels of n-type doping in CIGS. P-type doping in CIGS is usually done intrinsically through Cu vacancies, which act as acceptors. To achieve n-type doping, a donor material would need to be introduced into CIGS. 

In their poster, the Toshiba corporation group reported achieving n-type CIGS with CdS doping up to a level of 1×1016 cm-2. They were able to achieve a high enough doping level to use a CIGS layer as the emitter in a homojunction CIGS solar cell. From this point, they have grown a homojunction CIGS solar cell leading to a power conversion efficiency of 17.2% (See J-V characteristics). This relatively high efficiency device shows the feasibility of a CIGS homojunction. Another talk about n-type CIGS was given by Professor Angus Rockett [2]. This talk was about nitrogen doped CIGS. He was mentioning the possibility in the future to achieve high level of doping in CIGS in order to eventually obtain CIGS tunnel junctions. From a modelling perspective, these two ideas could lead into very interesting designs of a CIGS tandem cell consisting of homojunction subcells. One of the main advantages of CIGS is the fact that it has a tuneable bandgap ranging from 1.0eV to 1.7eV which covers a good portion of the solar spectrum. The change in the bandgap can be achieved by varying the molar fraction x, corresponding to the gallium to indium ratio in CuIn1-xGaxSe2.  A high bandgap CIGS subcell on top of a low bandgap CIGS subcell connected together in series with a CIGS tunnel junction could lead into high power conversion efficiency while potentially reducing the cost compared to III-V based multi-junction solar cell. This type of technology is certainly not achievable in the short term, but it could definitely be an interesting exercise to model this type of device in order to have an idea of what level of efficiency we could potentially achieve from it.

Fred Bouchard

 

-Frédéric Bouchard

Undergraduate student, Year 4

Sunlab, University of Ottawa

References:

[1] N. Nakagawa, et al., “Feasibility study of homojunction CIGS solar cells”, 39th IEEE PVSC, 2013.

[2] A. Rockett, et al., “Nitrogen doped chalcopyrites as contacts to CdTe photovoltaics”, 39th IEEE PVSC, 2013.

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