Lithium-doping inverts the nanoscale electric field at the grain boundaries in Cu₂ZnSn(S,Se)₄ and increases photovoltaic efficiency

Abstract

Passive grain boundaries (GBs) are essential for polycrystalline solar cells to reach high efficiency. However, the GBs in Cu2ZnSn(S,Se)4 have less favorable defect chemistry compared to CuInGaSe2. Here, using scanning probe microscopy we show that lithium doping of Cu2ZnSn(S,Se)4 changes the polarity of the electric field at the GB such that minority carrier electrons are repelled from the GB. Solar cells with lithium-doping show improved performance and yield a new efficiency record of 11.8% for hydrazine-free solution-processed Cu2ZnSn(S,Se)4. We propose that lithium competes for copper vacancies (forming benign isoelectronic LiCu defects) decreasing the concentration of ZnCu donors and competes for zinc vacancies (forming a LiZn acceptor that is likely shallower than CuZn). Both phenomena may explain the order of magnitude increase in conductivity. Further, the effects of lithium doping reported here establish that extrinsic species are able to alter the nanoscale electric fields near the GBs in Cu2ZnSn(S,Se)4. This will be essential for this low-cost Earth abundant element semiconductor to achieve efficiencies that compete with CuInGaSe2 and CdTe.

Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?

“…As of today, several teams employing this approach have reached effi ciencies higher than 10%. [ 13,14,16 ] Here again, carbon residues from the solvent have been reported to remain in some instances, and appeared at the bottom of the CZTSSe absorber layer. Interestingly and contrarily to other semi-conductors, CZTSSe appears quite tolerant to carbon impurities.…”

“…A record 11.8% device was achieved by this approach, employing a molecular solution-based CZTSSe absorber. [ 13 ] (5 of 21) 1502276…”

Efficiency Enhancement of Cu2ZnSn(S,Se)4 Solar Cells via Alkali Metals Doping

“…The formation energy of Li Zn (0.15 eV) is lower than Li atoms occupying Cu vacancy (0.23 eV) and the Li Zn defect could effectively increase hole density. [ 15,16 ] Therefore, a proper amount of Li ions could effectively augment the carrier concentration. As radii of Na and K ions are larger than Cu and Zn ions, they both need more energy to overcome the barrier of replacing constituents of CZTSSe.…”

The importance of back contact modification in Cu2ZnSnSe4 solar cells: The role of a thin MoO2 layer

“…repelling minority carriers (electrons) and attracting holes would lead to a better device performance [54]. It is important to note that this would be the typical behavior that could be expected for a polycrystalline semiconductor, since GBs contain numerous defects that enhance recombination and lower devices performance.…”