Improved Carrier Lifetimes of CdSe Thin Film via Te Doping for Photovoltaic Application

Abstract

Cadmium selenide (CdSe) solar cells have proven to be a remarkable potential top cell for a silicon-based tandem application. However, the defects and short carrier lifetimes of CdSe thin films greatly limit the solar cell performance. In this work, a Te-doped strategy is proposed to passivate the Se vacancy defects and increase the carrier lifetime of the CdSe thin film. The theoretical calculation helps to reveal the mechanism of nonradiative recombination of the CdSe thin film in depth. After Te-doping, the calculated capture coefficient of CdSe can be reduced from 4.61 × 10 −8 cm 3 s −1 to 2.32 × 10 −9 cm 3 s −1 . Meanwhile, the carrier lifetime of CdSe thin film is increased nearly 3fold from 0.53 to 1.43 ns. Finally, the efficiency of the Cd(Se,Te) solar cell is improved to 4.11%, about a relative 36.5% improvement compared to the pure CdSe solar cell. Both theoretical calculations and experiments prove that Te can effectively passivate bulk defects and improve the carrier lifetime of CdSe thin films, deserving further exploration to improve solar cell performance.

Why Increased CdSeTe Charge Carrier Lifetimes and Radiative Efficiencies did not Result in Voltage Boost for CdTe Solar Cells

“…In undoped CdSeTe, defects with E a = 0.14–0.22 eV reduce mobility to <0.1 cm 2 V −1 s −1 . Similar to other Se‐containing semiconductors (e.g., CdSe [ 52,53 ] and Cu(In,Ga)Se 2 [ 50 ] ), and as suggested by theoretical analysis, [ 49 ] these defects are likely related to V Se . Low mobility, low doping, and high back contact barrier all contribute to poor device performance.…”

“…The capture cross‐section and activation energy of V Se 0/+ transition was dependent on the defect geometry assumed in calculation, because V Se in Cd(Se,Te) is composed of not only Se p orbitals, but also Te p orbitals. [ 49 ] The PL broadening at low temperature ( w 0 = 75±2 meV in Figure 2b) can be related to distributions of such defect configurations, and calculated V Se defect energy distribution is similar, 80 meV. [ 49 ] According to the first principles, configuration coordinate diagram for (V Se + + h) and V Se ° charge states has a barrier dependent on Se composition.…”

“…[ 49 ] The PL broadening at low temperature ( w 0 = 75±2 meV in Figure 2b) can be related to distributions of such defect configurations, and calculated V Se defect energy distribution is similar, 80 meV. [ 49 ] According to the first principles, configuration coordinate diagram for (V Se + + h) and V Se ° charge states has a barrier dependent on Se composition. [ 49 ] Such barriers might be related to high radiative efficiencies and charge carrier trapping, explaining >10 µs lifetimes (next section) and PLQY >1%, which are unusual in polycrystalline absorbers.…”

“…considered electronic defect states in CdSe, including Cd(Se,Te) where Te composition was lower than in heterostructures studied here. [ 49 ] They find ≈ 10 16 cm −3 Se vacancies, V Se , and consider three V Se charge states (0/+/2+). The hole capture by V Se 0 (to become V Se + ) is identified as a recombination rate‐limiting step.…”

“…[ 49 ] According to the first principles, configuration coordinate diagram for (V Se + + h) and V Se ° charge states has a barrier dependent on Se composition. [ 49 ] Such barriers might be related to high radiative efficiencies and charge carrier trapping, explaining >10 µs lifetimes (next section) and PLQY >1%, which are unusual in polycrystalline absorbers.…”