H. Zogg

Efficiency enhancement of Cu(In,Ga)Se2 solar cells due to post-deposition Na incorporation

Cu(In,Ga)Se2 (CIGS) absorber layers for thin-film solar cells were grown without sodium. Na was diffused into some of the absorbers after growth, which led to strongly improved device performance compared with Na-free cells. Efficiencies of 13.3% and 14.4% were achieved at substrate temperatures as low as 400 and 450 °C, respectively. With the post-deposition treatment, the effects of Na on CIGS growth are excluded, and most of the Na is expected to reside at grain boundaries. The dominating cause for Na-induced device improvements might be passivation of grain boundaries.

Effects of NaF coevaporation on structural properties of Cu(In,Ga)Se2 thin films

Abstract Cu(In,Ga)Se 2 (CIGS) layers have been deposited using the three-stage process on Mo coated soda-lime glass substrates with an alkali diffusion barrier. Sodium has been incorporated in such layers by NaF coevaporation, by deposition of NaF precursor layers and by diffusion of Na from a glass substrate without barrier. Scanning electron microscope pictures showed a reduction in grain size of the CIGS films when Na was available during growth. This effect depends on the Na concentration of the finished film, but not on the incorporation method. Furthermore, the [Ga]/[In] concentration ratio is decreased in Na containing CIGS layers at depths at around one-quarter of the absorber thickness. X-ray diffraction patterns indicate a preferred orientation change only in the case of NaF precursors, which is attributed to CIGS growth on a modified surface and/or to the high Na availability during the initial stages of film growth. Solar cells with an efficiency exceeding 15% have been processed from such absorber layers containing Na.

Development of efficient and stable back contacts on CdTe/CdS solar cells

To make CdTe/CdS solar cells highly efficient, a Cu containing back contact (BC) is generally used. These cells degrade due to Cu diffusion to the front contact which causes shunting; this is shown with secondary ion mass spectroscopy (SIMS) depth profiling. To get a stable but still highly efficient cell, different BC materials and etching treatments were investigated. Chemical etching creates a back surface field (BSF) due to a p+-doped Te-rich CdTe surface. To overcome the naturally existing Schottky barrier between p-CdTe and any metal, a thin buffer layer was evaporated prior to the metallization. Amongst the many investigated BC materials, the most suitable are Sb or Sb2Te3 as a buffer and Mo for metallization. These cells showed high stability under accelerated tests corresponding to 70 years.

Na incorporation into Cu(In,Ga)Se2 for high-efficiency flexible solar cells on polymer foils

Incorporation of a small amount of sodium into Cu(In,Ga)Se2 (CIGS) absorbers for thin-film solar cells is well known to enhance conversion efficiencies. Usually, Na is added in a way such that it is present during CIGS growth and therewith influences the growth kinetics. We have used post-deposition Na in-diffusion into as-grown, Na-free absorbers and observed typical efficiency improvements. This suggests that in general the main positive Na effect originates from changes in the electronic absorber properties rather than from modification of CIGS growth kinetics. At low substrate temperatures, Na impedes CIGS phase formation. This may explain why absorbers grown at substrate temperatures below 450°C in the presence of Na yield inferior cells compared with post-deposition-treated CIGS. We have developed post-deposition Na incorporation for the processing of flexible CIGS solar cells on polyimide substrates. A conversion efficiency of 14.1% under AM1.5 standard test conditions was independently measured. T...

Photovoltaic lead-chalcogenide on silicon infrared sensor arrays

MBE growth and infrared device fabrication with epitaxial IV-VI layers on Si substrates are reviewed. Epitaxy on Si substrates is achieved using a stacked BaF[sub 2]/CaF[sub 2] or CaF[sub 2] buffer layer. With buffers containing no BaF[sub 2], standard photolithographic delineation with wet-etching techniques can be used. Photovoltaic IV-VI sensors with cutoff wavelengths ranging from 3 to 14 [mu]m are fabricated in PbS, PbSe[sub 1[minus]x]S[sub x], PbEu[sub 1[minus]x]Se[sub x], PbTe, or Pb[sub 1[minus]x]Sn[sub x]Se layers on Si(111) substrates. They offer the possibility for low-cost infrared focal plane arrays with sensitivities similar to Hg[sub 1[minus]x]Cd[sub x]Te, but with much less demanding material processing steps. A 13-mm-long linear array with 10.5-[mu]m cutoff wavelength has inhomogeneities in cutoff below 0.1 [mu]m. Some arrays were on prefabricated active Si substrates containing the whole readout circuits.First thermal images using these chips are demonstrated. The induced mechanical strain resulting from the different thermal expansion of IV-VIs and Si relaxes down to cryogenic temperatures even after many temperature cycles because of dislocation glide in the main [100] glide planes.

Influence of CdS growth process on structural and photovoltaic properties of CdTe/CdS solar cells

The morphology of CdS layers grown by chemical bath deposition (CBD) and high vacuum evaporation (HVE) have been investigated. The grains of CBD-CdS are more compact and smooth than those of HVE-CdS. The annealing and CdCl 2 treatment cause grain growth, which is stronger for the CdCl 2 treated samples. The grain-size of the as-deposited CdTe on CBD-CdS is about 5 times larger than of those grown on HVE-CdS. The structural and electrical properties of CdTe/CdS solar cells are strongly dependent on the CdS. The grain size of CdCl 2 treated CdTe layers are similar, irrespective of the transparent conducting oxide substrate and CdS deposition method. The e

Growth of high quality epitaxial PbSe onto Si using a (Ca,Ba)F2 buffer layer

ciency of solar cells on thin CBD-CdS is low (about 5.6%) because of pin-holes and a large intermixing of CdTe-CdS. The cells on HVE-CdS yield a higher current density despite thicker HVE-CdS as compared to CBD-CdS. The e

12.8% Efficiency Cu(In,Ga)Se2 solar cell on a flexible polymer sheet

ciency of solar cells on HVE-CdS is 12.3%. ( 2001 Elsevier Science B.V. All rights reserved.

Infrared sensor arrays with 3-12 mu m cutoff wavelengths in heteroepitaxial narrow-gap semiconductors on silicon substrates

Epitaxial layers of PbSe have been grown onto Si(111) by vacuum deposition techniques using a (Ca,Ba)F2 buffer film. For lattice match, the buffer layer is graded with CaF2 at the Si interface and BaF2 at the PbSe interface. Like bulk BaF2, this buffer layer forms a suitable support for the growth of high quality PbSe. The layers grown exhibit smooth surfaces and mobilities as high as those of bulk PbSe, i.e., ∼3×104 cm2 V−1 s−1 at 77 K and >1.2×105 cm2 V−1 s−1 below 20 K. These results are very promising for the fabrication of heteroepitaxial monolithic integrated circuits, with, for example, photovoltaic (Pb,Sn) (S,Se,Te) narrow gap semiconductor infrared detectors on silicon.

Recrystallization in CdTe/CdS

A lift-off process has been developed to obtain Cu(In,Ga)Se 2 solar cells on flexible polymer sheets. The absorber layer is grown by a co-evaporation method on a polyimide layer, which is spin coated on a NaCl covered glass substrate. The NaCl intermediate layer can provide Na to the Cu(In,Ga)Se 2 layer during deposition. After the complete processing of the cells, the NaCl buffer layer is dissolved to separate the glass substrate from the ZnO/CdS/Cu(In,Ga)Se 2 /Mo/polyimide stack. A record conversion efficiency of 12.8% (total area) under AM1.5 illumination was independently measured at FhG/ISE, Freiburg, Germany. Such high efficiency solar cells on light weight and flexible substrates are needed for novel terrestrial and space applications.