Generalized quantum efficiency analysis for non-ideal solar cells: Case of Cu2ZnSnSe4

Abstract

Detailed quantum efficiency (QE) analysis of a nanoparticle-based Cu2ZnSnSe4 (CZTSe) solar cell has been conducted to understand photogenerated carrier collection in the device. Specifically, voltage-dependent analysis has been considered to characterize both diffusion limitations and recombination limitations to carrier collection. Application of a generalized QE model and corresponding experimental and analytical procedures are presented to account for non-ideal device behavior, with specific consideration of photogenerated charge trapping, finite absorber thickness, back-surface recombination, and recombination of photogenerated carriers via interface, space-charge-region limited, and/or band tail limited recombination mechanisms. Analysis of diffusion limited collection results in extraction of the minority carrier diffusion length, mobility, back surface recombination velocity, and absorption coefficient. Additionally, forward bias QE measurements afford analysis of the dominant recombination mechanism for photogenerated carriers. For the analyzed CZTSe device, diffusion limitations are not expected to play a significant role in carrier collection in forward bias. However, voltage-dependent carrier collection, previously identified to contribute to open-circuit voltage limitations, is attributed to high recombination rates via band tail states/potential fluctuations in forward bias. A consideration of the assumptions commonly applied to diffusion length, band gap, and band tail extraction is also discussed.

Identifying the Real Minority Carrier Lifetime in Nonideal Semiconductors: A Case Study of Kesterite Materials

“…The ts-TRPL data have been uniformly scaled in Figure 4b to the measured initial carrier injection Δn t=0 , which allows for determining the free carrier density p 0 ≈ 1.8 × 10 16 cm −3 for CZTSSe. This value is in good agreement with the free carrier density measured by capacitance-voltage profiling on devices measured here (see the Supporting Information) and on equivalently processed CZTSSe devices, [20,43] illustrating the benefit of intensity-dependent TRPL in extracting this parameter for absorbers without the need for device fabrication.…”

“…First, no correlation between the measured PL decay times and device performance over a wide range (1%-12%) of device efficiencies (shown in Figure 2) and open-circuit voltage V OC (see the Supporting Information) can be found from published TRPL data. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] These results are in contrast to that found from TRPL analysis of chalcopyrites and CdTe where a correlation with device efficiency and V OC is clear, as τ n is a measure of recombination losses. [33][34][35][36] However, for kesterites the connection between PL decay time and the assumed τ n is not apparent, as the reported decay times represent arbitrary measurement excitation conditions and data analysis procedures; characteristic decay times from a variety of fitting regions and techniques are reported for measured TRPL data.…”

“…Additionally, TRPL data measured on kesterites are typically reported with poor signal-to-noise ratio, with PL decays generally resolved over 1-3 orders of magnitude of signal decay. [5][6][7][9][10][11][12]14,15,18,[20][21][22][23][24] As various transport mechanisms can occur within this decay range care must be taken in interpreting such signal.…”

“…Data are taken from refs. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The solid lines represent device simulation results bounded by S F = 10 2 -10 6 cm s −1 and S B = 10 2 -10 7 cm s −1 ; the dashed line represents a lower bound on the efficiency for a reduced mobility gap E µ = E PL,Peak .…”

Deposition Technologies of High-Efficiency CIGS Solar Cells: Development of Two-Step and Co-Evaporation Processes

“…The local maxima of |dEQE/dλ| determined by using a quadratic fit were at the wavelengths of 1072 nm, 1073 nm, and 1082 nm for the CIGS films prepared by the single-stage, bi-layer, and three-stage processes, respectively, as illustrated in Figure 7a [33]. The optical energies corresponding to the local maxima yield E g + E U /2 [33,34], where E U is the Urbach energy. Taking the Urbach energy of around 20 meV for the CIGS films [20], the corresponding bandgap energies of the CIGS films prepared by the single-stage, bi-layer, and three-stage processes were 1.15 eV, 1.15 eV, and 1.14 eV, respectively.…”