JinWoo Lee

Characterization of the Electronic Properties of Wide Bandgap CuIn(SeS) 2 Alloys

The electronic properties of sulfur containing CIS chalcopyrite alloys have been characterized using junction capacitance methods. Two devices were examined; one containing CuIn(S,Se) 2 alloy with a 1:2 S:Se ratio and a bandgap near 1.15eV, and the other an endpoint CuInS2 alloy with a bandgap slightly above 1.5eV. Drive-level capacitance profiling measurements indicated hole carrier densities of less than 1 x 10 15 cm -3 and 1.5 x 10 16 cm -3 , respectively. Transient photocapacitance (TPC) sub-bandgap spectroscopic measurements revealed sharp bandtails plus a broad defect band within the bandgap of each alloy. The TPC spectra for the CuInS2 sample revealed a couple of unusual features, including a bandtail signal that reversed sign below 250K. This indicated poorer hole collection than electron collection in the low temperature regime. Comparing these results to TPC spectra obtained previously for Cu(InGa)Se 2 alloys indicate some similarities but also some striking differences.

Role of Bulk Defect States in Limiting CIGS Device Properties

We have used sustained light-soaking in the near-infrared (780 nm wavelength) to modify the properties of the absorber layer in CIGS solar cells, and thus study the relationship between the absorber electronic properties and the device performance. Through this light-soaking treatment we can increase the hole carrier density in the CIGS absorber, as well as the density of the commonly observed 0.3 eV bulk deep acceptor, by up to a factor of 5. The device performance was periodically recorded under the same 780 nm light, and was found to degrade due to a reduction in fill factor and short circuit current. We demonstrate that these changes can be largely attributed to a reduction in the photogenerated carrier collection, due to a decrease in the depletion width. Based on these data, we estimate the minority carrier diffusion length to be about 0.2 mum. At the same time, no change in open circuit voltage was observed, indicating that the recombination rate through the dominant recombination channel remained nearly constant. This means that it cannot be the 0.3 eV bulk defect. Detailed SCAPS modeling was carried out and reinforces this conclusion

Material Properties of a-SiGe:H Solar Cells as a Function of Growth Rate

We have examined a series of a Si,Ge:H alloy devices deposited using both RF and VHF glow discharge in two configurations: SS/n + /i ( a-SiGe:H ) /p + /ITO nip devices and SS/n+/i (a-SiGe:H)/Pd Schottky contact devices, over a range of deposition rates. We employed drive-level capacitance profiling (DLCP), modulated photocurrent (MPC), and transient junction photo-current (TPI) measurement methods to characterize the electronic properties in these materials. The DLCP profiles indicated quite low defect densities (mid 10 15 cm -3 . to low 10 16 cm -3 depending on the Ge alloy fraction) for the low rate RF (∼1A/s) deposited a-SiGe:H materials. In contrast to the RF process, the VHF deposited a-SiGe:H materials did not exhibit nearly as rapid an increase of defect density with the deposition rate, remaining well below 10 17 cm -3 . up to rates as high as 10A/s. Simple examination of the TPI spectra on theses devices allowed us to determine valence band-tail widths.. Modulated photocurrent (MPC) obtained for several of these a-SiGe:H devices allowed us to deduce the conduction band-tail widths. In general, the a-Si,Ge:H materials exhibiting narrower valence band-tail widths and lower defect densities correlated with the best device performance.

Electroabsorption Measurements on Bifacial CIGS Solar Cell Devices

We report the first studies of electroabsorption in Cu(InGa)Se 2 (CIGS) solar cell devices. We utilized a bifacial CIGS device with a Ga/(In+Ga) ratio of 0.8 (bandgap of 1.5 eV) deposited onto semi-transparent (40 nm thick) Mo coated glass as the back contact. By modulating the electric field using a small sinusoidal potential of amplitude δV across the CIGS layer, we were able to detect the modulation ΔT of the transmitted light. This was examined as a function of photon energy, DC bias, temperature, and modulation frequency (100 Hz to 10 kHz) and had a maximum amplitude of ΔT/T ≈ 10 −5 for δV = 0.3 V. Very different characteristics were obtained for near bandgap light (1.3 eV) compared to photon energies considerable smaller (