On the Origin of Tail States and Open Circuit Voltage Losses in Cu(In,Ga)Se₂

Abstract

The detrimental effect of tail states on the radiative and non‐radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage (VOC) in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single‐crystal films. The results demonstrate that sodium and potassium decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of sodium or potassium incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high‐efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC.

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

“…While the exponential expression applied to fit the low energy side of the PL emission spectra is similar to Urbach energy analysis for absorptance (Figure 1b), fitting emission spectra at low temperature directly quantifies defect states that introduce potential fluctuations. [59,60] In addition, consistent with the potential fluctuation model, the blue PL shift with injection is observed. [59] The magnitude of the shift at 4K is 𝛽 = 20.8 meV decade −1 , which is higher than for high-efficiency Cu(In,Ga)Se 2 (𝛽 = 2-17 meV decade −1 ).…”

“…[59] The magnitude of the shift at 4K is 𝛽 = 20.8 meV decade −1 , which is higher than for high-efficiency Cu(In,Ga)Se 2 (𝛽 = 2-17 meV decade −1 ). [60,62] PL analysis for solar cells 5 fabricated by evaporation and VTD at NREL is summarized in Figure S7 (Supporting Information). These samples have higher recombination, and the optical diode factor at low temperature increases to 𝛼 = 1.8±0.1.…”

“…Using the tunneling model, we estimated N ≤ 10 17 cm −3 for Cu(In,Ga)Se 2 solar cells, [77] consistent with lower 𝛾 for Cu(In,Ga)Se 2 . [60] Figure S9 (Supporting Information) shows carrier dynamics in device 5. The low temperature lifetime for the band tail emission (1.18-1.24 eV) is 520 ns, and room temperature lifetime is 8 ns.…”

“…We now consider how low dopant activation impacts potential fluctuations and V rad OC . The electrostatic model developed by Sklovskii and Efros for disordered semiconductors with potential fluctuations 𝛾 [71] was applied to Cu(In,Ga)Se 2 [60] and As-doped CdSeTe [57] 𝛾 = q 2 4𝜋𝜀𝜖 0…”

“…The agreement is perhaps better than expected, because the electrostatic model applies to order-of-magnitude comparisons. [71,57,60] Independent of simplifications of the analytical expression, Moseley et al used a random number generator to distribute donors and acceptors screened by the Coulomb potentials and estimated similar 𝛾 for comparable activation. [57] It appears dopant activation is a key aspect determining electronic properties, supporting the hypothesis that As incorporated differently than As Te contributes to potential fluctuations.…”