Fabrication of Cu(In,Ga)Se₂ solar cells with a single graded band profile

Abstract

It was found that Cu(In,Ga)Se 2 (CIGS) solar cells with single graded (SG) band profile and average bandgap E g(avg) of 1.15 eV has higher efficiency than double graded (DG) band profile with the same E g(avg) , despite no intentional bandgapwidening at a CdS/CIGS interface in SG-CIGS. To explain this contradiction, we focused on the band structure at the CdS/CIGS interface and proposed that the contradiction can be explained by the formation of an ordered vacancy chalcopyrite compound (OVC) layer on the surface of CIGS. Device simulation using wxAMPS revealed that if an OVC layer exists at the interface, conversion efficiency of SG-CIGS solar cells overcomes that of DG-CIGS solar cells due to suppression of surface recombination at the interface, leading to open-circuit voltage (V OC ) improvement. We applied the SG structure to CIGS solar cells with high Ga contents. As a result, the conversion efficiency of 15.1% for the SG-CIGS solar cell with E g(avg) of 1.4 eV was achieved, although in contrast, that of the DG-CIGS solar cells was 12.9%. This efficiency is comparable to the highest efficiency in CIGS solar cells with high Ga contents. Therefore, SG band profile is promising structure and it is expected to achieve high efficiency predicted essentially in CIGS solar cells with high Ga contents. Although CIGS solar cells with the highest conversion efficiency have been expected due to good matching the sunlight spectrum in the case of E g ¼ 1.4 eV, practically, the conversion efficiency was deteriorated with increasing Ga content [5,6] because of several reasons [7][8][9].The conduction band profile of typical CIGS solar cells fabricated by the three stage method has double graded (DG) structure which is useful for the improvement of carrier collection efficiency by an internal electric field, and the improvement of open circuit voltage (V OC ) by bandgapwidening in a depletion region. On the other hand, single graded (SG) structure has been reported lower V OC than DG structure due to no bandgap-widening in a depletion region [10]. However, in this study from experimental results we found that SG structure has higher efficiency than DG structure. To explain this contradiction, we focused on the band structure at the CdS/CIGS interface. Moreover, from the experiment and the calculation, we found the policy to improve the efficiency of high-Ga CIGS solar cells.

Effect of Cu‐deficient layer formation in Cu(In,Ga)Se2 solar‐cell performance

“…Notably, V OC achieved the value for ideal interface without DL (0.703 V) even for carrier concentration lower than | N A, CIGS | when SL had Δ E V ≥ 0.15 eV. As already experimentally and theoretically discussed in our previous works, the carrier recombination at the CdS/CIGS interface can be suppressed when Δ E V acts as a hole barrier, reducing hole concentration near the interface. Interestingly, Δ E V especially affected V OC , while FF hardly changed.…”

Efficiency Enhancement of CIGS Solar Cells via Recombination Passivation

“…It is well known that capacitance–voltage ( C–V ) profiling provides more direct evidence for the electrical properties of CIGS solar cells. For instance, the width of the built-in electric field ( V D ) can be obtained from the C – V measurement according to the following equation , where L is the width of the depletion region, V D is the internal electric field intensity, ε is the semiconductor relative permittivity, and N A is the carrier concentration. The V D for the CIGS solar cell is about 0.60 V obtained from the intersection of the 1/ C 2 – V curve with the horizontal axis (Figure d).…”

Benefits of Low Electron-Affinity Material as the N-Type Layer for Cu(In,Ga)S2 Solar Cell

“…Compared to its narrow-band gap counterpart, research into the wide band gap CIG(SSe) 2 has yet to achieve favourable outcomes. As an example, band gap grading design, through Ga-content control, have had a significant impact [11,12], and numerous theoretical studies in band grading designs are still underway [13][14][15]. The additional step of absorber layer's post-deposition treatment (PDT) using alkali [16,17] have also been known to have a positive impact on the CIGSe 2 solar cell's performance.…”