Abstract
AbstractTwo key strategies for enhancing the efficiency of Cu(In,Ga)Se2 solar cells are the bandgap gradient across the absorber and the incorporation of alkali atoms. The combined incorporation of Na and Rb into the absorber has brought large efficiency gains compared to Na‐containing or alkali‐free layers. Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition‐graded Cu(In,Ga)Se2 solar cells. Electron lifetimes are found to be highly dependent on the film composition and morphology, varying from tens of nanoseconds in the energy notch to only ≈100 ps in the Ga‐rich region near the Mo‐back contact. NaF PDT improves recombination lifetimes by a factor of 2–2.5 in all regions of the absorber, whereas the effectiveness of the RbF PDT is found to decrease for higher Ga‐concentrations. Electron mobility measured in the absorber region with large grains is promoted by both alkali PDTs. The data suggest that NaF PDT passivates shallow defect states (Urbach tail) throughout the Cu(In,Ga)Se2 film (including the interior of large grains), whereas the additional RbF PDT is effective at grain boundary surfaces (predominantly in regions with medium to low Ga‐concentrations).
Highlights
Alkali atom incorporation in Cu(In,Ga)Se2 (CIGSe) absorbers along with an engineered bandgap-gradient profile (obtained of the Na-treated CIGSe absorber.[7,9,11,12] Other studies showed that the increase in VOC and fill factor (FF) can result from the passivation of non-radiative recombination centers at grain boundaries (GBs), evidenced by several atom probe tomography studies showing Na accumulation predominantly at GBs compared toDr Y.-H
We do not seem to observe this proportional increase and would tend to argue that Na has a more significant effect in the bulk/grain interiors (GIs). This is corroborated by a 1.5-fold increase in electron mobility in the NaF treated sample (11.2 cm2V−1s−1 in the No PDT, 16.3 cm2V−1s−1 in the NaF PDT sample) which we calculate from the electron drift time constant in the H-LG region where far fewer GBs are visible compared to the H-SG volume
Utilizing transient absorption spectroscopy (TAS), we have investigated identically Ga-graded CIGSe layers with 3 different post-deposition processing conditions (No PDT, NaF, and NaF+RbF PDT)
Summary
Alkali atom incorporation in Cu(In,Ga)Se2 (CIGSe) absorbers along with an engineered bandgap-gradient profile (obtained of the Na-treated CIGSe absorber.[7,9,11,12] Other studies showed that the increase in VOC and FF can result from the passivation of non-radiative recombination centers at grain boundaries (GBs), evidenced by several atom probe tomography studies showing Na accumulation predominantly at GBs compared toDr Y.-H. Alkali atom incorporation in Cu(In,Ga)Se2 (CIGSe) absorbers along with an engineered bandgap-gradient profile (obtained of the Na-treated CIGSe absorber.[7,9,11,12] Other studies showed that the increase in VOC and FF can result from the passivation of non-radiative recombination centers at grain boundaries (GBs), evidenced by several atom probe tomography studies showing Na accumulation predominantly at GBs compared to. Steier Department of Chemistry Centre for Processable Electronics grain interiors (GIs).[13,14,15,16,17,18,19,20] Transient optical studies correlated increased charge carrier lifetime with optimal Na atom incorporation[21,22,23] Eid et al utilized ultrafast pump-probe reflectance
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