Dielectric Front Passivation for Cu(In,Ga)Se₂ Solar Cells: Status and Prospect

Abstract

Cu(In,Ga)Se2 (CIGSe) solar cells are among the most efficient thin‐film solar cells on lab scale. However, this thin‐film technology has relatively large upscaling losses for commercial technology. To tackle this, paradigm shifts are proposed that allow for simpler, cost‐effective, and efficient CIGSe solar cells. Front passivation using dielectric layers is one of the options being investigated as this is widely used in Si technology. Research on front passivation for CIGSe is in an early stage and no improvements are made yet. A close comparison with silicon technology is made to understand why it seems to be more difficult for CIGSe solar cells. In general, chemical passivation is less effective, resulting in higher interface defect densities than seen for Si. Also, field‐effect passivation requires positive charges, which have not been implemented yet on the CIGSe front surface. Finally, for Si passivation, often a high‐temperature annealing step is applied, which is not possible for CIGSe. It is proposed to apply a dielectric tunneling layer with positive fixed charges in combination with an electron transport layer to move forward. A list of potential dielectric layers that could be suitable for CIGSe is provided.

Graded Bandgap Ultrathin CIGS Solar Cells

“…First, bandgap grading by changing the Ga content in the absorber is considered a traditional way to improve V oc for a thinner absorber layer [15][16][17][18][19][20][21][22][23][24][25][26]. Many research works have delighted the positive impact of bandgap grading profiles in the generation and collection of minority carriers [26][27][28][29][30][31][32][33][34]. It has been found that double grading allows benefiting from the advantages of both high grading at the front and back side of the cell [30,34].…”

“…Many research works have delighted the positive impact of bandgap grading profiles in the generation and collection of minority carriers [26][27][28][29][30][31][32][33][34]. It has been found that double grading allows benefiting from the advantages of both high grading at the front and back side of the cell [30,34]. However, Ga grading is not a complete solution for passivation and rear recombination [1,8].…”

“…Then, it is very necessary to investigate the effect of the graded bandgap, electron affinity, absorber doping density, and operating temperature on cell performance. The conventional buffer layer of CdS is nearly optimum for CIGS with a smaller bandgap ranging from 1 to 1.35 eV but shows poor alignment for a higher Ga ratio CIGS absorber with a higher bandgap [29][30][31][32][33][34]. The CdS possesses a low bandgap of 2.4 eV, which is not sufficient to cover the maximum shorter wavelength of photons, resulting in absorption losses.…”

“…The CdS possesses a low bandgap of 2.4 eV, which is not sufficient to cover the maximum shorter wavelength of photons, resulting in absorption losses. For high efficiency, it is necessary to replace the CdS buffer layer (BL), which still contains the toxic Cd, with better suited BL to form a good heterojunction [33][34][35][36]. Introducing ZnS as a better n-type BL due to its high-bandgap semiconducting and better lattice match with CIGS [34][35][36][37][38][39][40], it has been found that using dielectric passivation layers at the front could increase light absorption and be sufficient to establish long-term stability [34,35].…”

“…For high efficiency, it is necessary to replace the CdS buffer layer (BL), which still contains the toxic Cd, with better suited BL to form a good heterojunction [33][34][35][36]. Introducing ZnS as a better n-type BL due to its high-bandgap semiconducting and better lattice match with CIGS [34][35][36][37][38][39][40], it has been found that using dielectric passivation layers at the front could increase light absorption and be sufficient to establish long-term stability [34,35]. Up to date, no improvements have been observed for solar cells based on absorber/passivation structures [34][35][36].…”