Bulk and surface investigation of narrow bandgap Cu(In,Ga)Se2 solar cells
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Single-junction solar cell efficiency is limited to 33%, in which the majority of losses are due to thermalization of carriers. Thermalization can be reduced by creating solar cells with two junctions, or tandem solar cells, in which a wide bandgap (Eg) solar cell on top absorbs high energy light and a narrow Eg solar cell on bottom absorbs the remaining light. The two most commercially viable candidates for the bottom cell are Si and Cu(In,Ga)Se2 (CIGS). CIGS is well suited to be the light absorber in a bottom cell of a tandem solar cell due to its high absorption coefficient and tunability of its bandgap to the ideal 1.0 eV. However, historically there has been a large efficiency difference between CIGS solar cells with narrow Eg = 1.0 eV with conversion efficiency (η) = 15.0% compared to standard CIGS solar cells with Eg = 1.2 eV and η = 21.7% in 2015, due to a deficit in the open-circuit voltage (VOC). This dissertation addresses the fundamental differences between narrow and standard Eg CIGS and how to improve the efficiency of narrow Eg CIGS solar cells. ☐ CuInSe2 (CIS) solar cells were alloyed with small amounts of Ga and Ag with the aim of improving bulk properties. Ga alloying leads to increased VOC and decreased short-circuit current (JSC) due to increased Eg. Ag alloyed devices lead to improved long wavelength current collection relevant to bottom cells in tandems but suffer from low VOC due to interface recombination. Ga alloying was also used to form a gradient in Ga/(Ga+In) composition throughout the depth of the absorber, a strategy used in standard CIGS solar cells to improve current collection, while keeping the surface Ga-free. The maximum substrate temperature (Tsub) during deposition was discovered to have significant influence on the Ga/(Ga+In) depth profile and device performance. Absorber layers grown at Tsub = 580°C have high VOC but diffusion of Ga towards the front which reduces JSC and fill factor (FF), whereas a lower Tsub = 500°C leads to an ideal Ga/(Ga+In) profile with higher JSC and FF, but less VOC increase. The VOC increases in graded samples were not understood by changes in Eg or charge carrier concentration, so SCAPS-1D was used to simulate the device performance. The simulations showed that recombination at the back contact reduces CIS performance even at standard film thickness but is prevented with a Ga gradient. It was further revealed that reducing defects at the front region of devices with a gradient is important for obtaining the highest VOC. ☐ The quality of CIGS absorber layers can be improved with in situ alkali-fluoride surface treatments after the absorber deposition. X-ray photoelectron spectroscopy measurements on KF-treated CIGS revealed differences in F products and surface Cu concentration related to the presence of Ga. However, similar device performance and CdS buffer growth are seen in KF-treated CIGS despite these chemical differences. CIS+KF devices demonstrate improved VOC, doping, and tolerance of thin CdS layers, whereas Ag alloyed ACIS+KF devices have low VOC due to additional interface recombination. ☐ Finally, processing modifications to improve the VOC and JSC of CIS+KF led to a certified record CIS solar cell η = 16.0%. Higher efficiency narrow Eg CIGS solar cells were enabled by a Ga gradient reducing the back contact recombination that limits the CIS solar cell efficiency, along with alkali fluoride treatments to improve the p-n junction, and the champion device combined a Ga gradient + NaF treatment to give η = 17.4%. These improvements will allow for CIGS to be used as a bottom cell in a tandem solar cell.
Description
Keywords
Narrow bandgap, Solar cells