Abstract
In the search for highly transparent and non-toxic alternative front layers replacing state-of-the-art CdS in Cu(In,Ga)Se2 thin-film solar cells, alternatives rarely exceed reference devices in terms of efficiency. Full-area ultra-thin aluminium oxide tunnelling layers do not require any contact patterning and thus overcome the main drawback of insulating passivation layers. Even a few monolayers of aluminium oxide can be deposited in a controlled manner by atomic layer deposition, they show excellent interface passivation properties, low absorption, and suitable current transport characteristics on test devices. Depositing a ZnO-based transparent front contact, however, results in extremely poor solar cell performance. The issue is not necessarily a low quality of the alternative front layer, but rather the intricate relation between front layer processing and electronic bulk properties in the absorber layer. We identify three challenges critical for the development of novel front passivation approaches: (i) both Cd and Zn impurities beneficially reduce the high native net dopant concentration in the space charge region, (ii) sputter deposition of ZnO damages the passivation layer resulting in increased interface recombination, (iii) thermal treatments of devices with ZnO layer result in substantial Zn diffusion, which can penetrate the full absorber thickness already at moderate temperatures.
Highlights
Thin-film photovoltaic (PV) devices based on the ternary chalcopyrite Cu(In,Ga)Se2 (CIGS)[1,2,3] are among the most efficient thin-film solar cells[4], having demonstrated efficiencies of 20.8%5 on flexible and 23.35%6,7 on rigid substrates (22.3%8 for pure selenides containing no sulphur)
The physical thickness of the Al2O3 layer on crystalline silicon (c-Si) is 21–22 nm, determined by spectral ellipsometry[35], and we expect a similar thickness on CIGS
We find that 2 Atomic layer deposition (ALD) cycles of AlOx as front passivation result in highly shunted devices, in agreement with the low effective resistance shown in Fig. 4, presumably because direct contact between Al-doped ZnO-based transparent front contact (ZnO) (AZO) and CIGS at pinholes in the passivation layers results in poor junction quality
Summary
Thin-film photovoltaic (PV) devices based on the ternary chalcopyrite Cu(In,Ga)Se2 (CIGS)[1,2,3] are among the most efficient thin-film solar cells[4], having demonstrated efficiencies of 20.8%5 on flexible and 23.35%6,7 on rigid substrates (22.3%8 for pure selenides containing no sulphur). A typical state-of-the-art CIGS thin-film solar cell is sketched in Fig. 1 and is made from a soda-lime glass (SLG) growth substrate, Mo back contact, p-type polycrystalline CIGS absorber layer deposited by vacuum co-deposition, and an n-type front buffer/window stack. This buffer/window stack forms the n-doped side of the p/n-heterojunction, passivates electronic defects at the front interface, and serves as transparent conductive front contact. The most widely used buffer/window stack consists of a CdS buffer layer deposited by chemical bath deposition, and a ZnO-based double window layer with nominally intrinsic i-ZnO and highly conductive Al-doped ZnO (AZO). Alternative Cd-free buffer layer materials, which are still conductive but have a higher optical bandgap than CdS, and recently insulating high-bandgap dielectric passivation layers are two potential approaches to overcome the limitations imposed by CdS
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