Optoelectronic and material properties of nanocrystal-based CZTSe absorbers with Ag-alloying

Abstract

In this work, the benefits of Ag-alloying in kesterite solar cells are explored in terms of tunable band gap, improved grain growth, improved minority carrier lifetime, reduced defect formation, and reduced potential fluctuations for (Ag,Cu) 2 ZnSnSe 4 (ACZTSe) absorbers relative to Cu 2 ZnSnSe 4 (CZTSe). The enhanced optoelectronic properties are shown to scale here with the degree of Ag-alloying in ACZTSe. The impacts of these effects on device performance are discussed, with improvement in average device performance/open-circuit voltage reported for ACZTSe (5%-Ag) absorbers relative to CZTSe absorbers with similar band gap. These initial results are promising for the Ag-alloyed ACZTSe material system as V OC limitations are the primary cause of poor device performance in kesterite solar cells, and cation substitution presents a unique method to tune the defect properties of kesterite absorbers. Herein, nanoparticle synthesis and large-grain ACZTSe absorber formation is described followed by material and optoelectronic characterization. Additionally, RTP processing is presented to achieve fully selenized large-grain chalcogenide absorbers from sulfide nanocrystal inks.In addition to modification of the defect properties, Ag-alloying may also be beneficial for band gap tuning/grading of the absorber for improved performance. For CZTSSe, the absorber band gap (E G ) is determined mainly by Cu d orbital and S/Se p orbital anti-bonding (valance band maximum -VBM) and Sn s orbital and S/Se sp

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

“…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.…”

“…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 .…”

Suppressed Deep Traps and Bandgap Fluctuations in Cu2CdSnS4 Solar Cells with ≈8% Efficiency

“…Specifically, we calculate the Urbach energy from the inverse slope of the linear portion in the sub-bandgap region of the plot between ln(−ln(1 − EQE)) and E − E g . [45] We extract an Urbach energy of ≈40-55 meV for all of the samples. Although the E U values are slightly overestimated due to inefficient charge-carrier collection at long wavelengths (see Section 2.4.3), the values measured here are consistent with those extracted from photothermal deflection spectroscopy measurements by Huang et al, where an E U of 53 meV was measured for both Cu 2 ZnSnS 4 and Cu 2 CdSnS 4 .…”

Efficient (Cu1−xAgx)2ZnSn(S,Se)4 solar cells on flexible Mo foils

“…This observation suggests that there is a turning point in the curve of E g vs. Ag concentration, which explains the controversial observation of previous works, i.e., it is reported in ref. 17,18 and 20 that the band gap increases with the ratio of Ag/(Ag + Cu), but it is found in ref. 19 that the band gap decreases with the ratio of Ag/(Ag + Cu).…”