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See detailAbsorber composition: A critical parameter for the effectiveness of heat treatments in chalcopyrite solar cells
Sood, Mohit UL; Elanzeery, Hossam UL; Adeleye, Damilola UL et al

in Progress in Photovoltaics (2020)

Post-device heat treatment (HT) in chalcopyrite [Cu(In,Ga)(S,Se)2] solar cells is known to improve the performance of the devices. However, this HT is only beneficial for devices made with absorbers grown ... [more ▼]

Post-device heat treatment (HT) in chalcopyrite [Cu(In,Ga)(S,Se)2] solar cells is known to improve the performance of the devices. However, this HT is only beneficial for devices made with absorbers grown under Cu-poor conditions but not under Cu excess.. We present a systematic study to understand the effects of HT on CuInSe2 and CuInS2 solar cells. The study is performed for CuInSe2 solar cells grown under Cu-rich and Cu-poor chemical potential prepared with both CdS and Zn(O,S) buffer layers. In addition, we also study Cu-rich CuInS2 solar cells prepared with the suitable Zn(O,S) buffer layer. For Cu-poor selenide device low-temperature HT leads to passivation of bulk, whereas in Cu-rich devices no such passivation was observed. The Cu-rich devices are hampered by a large shunt. The HT decreases shunt resistance in Cu-rich selenides, whereas it increases shunt resistance in Cu-rich sulfides.. The origin of these changes in device performance was investigated with capacitance-voltage measurement which shows the considerable decrease in carrier concentration with HT in Cu-poor CuInSe2, and temperature dependent current-voltage measurements show the presence of barrier for minority carriers. Together with numerical simulations, these findings support a highly-doped interfacial p+ layer device model in Cu-rich selenide absorbers and explain the discrepancy between Cu-poor and Curich device performance. Our findings provide insights into how the same treatment can have a completely different effect on the device depending on the composition of the absorber. [less ▲]

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See detailOPTICAL DEFECT SPECTROSCOPY IN CUINS2 THIN FILMS AND SOLAR CELLS
Lomuscio, Alberto UL

Doctoral thesis (2020)

Pure-sulphide Cu(In,Ga)S2 solar cells have reached certified power conversion efficiency as high as 15.5 %. While this record performance has been achieved by growing the semiconducting absorber at very ... [more ▼]

Pure-sulphide Cu(In,Ga)S2 solar cells have reached certified power conversion efficiency as high as 15.5 %. While this record performance has been achieved by growing the semiconducting absorber at very high temperature with a copper deficient composition, all other previous records were based on chalcopyrite films deposited under Cu excess. Still, this world record is far from the theoretical power conversion achievable in single junction solar cell for this semiconductor (about 30 %), which has a tunable band gap between 1.5 and 2.4 eV. This thesis aims to gain insight into the optoelectronic properties of this semiconductor, particularly CuInS2, looking at their variation as a function of the deposition temperature and of the absorber composition. The investigations are carried out mainly by photoluminescence (PL) spectroscopy, which allows to measure the quasi Fermi level splitting (QFLS), that is an upper limit of the maximum open circuit voltage (VOC) an absorber is capable of. PL spectroscopy is used to get insights onto the electronic defects as well, both the shallow ones, which contribute to the doping, and the deep ones, which enhance non-radiative recombination. By increasing the Cu content in the as-grown compositions, the morphology and microstructure of the thin films improve, as they show larger grains and less structural defects than films deposited with Cu deficiency. The composition affects the QFLS as well, which is significantly higher for sample deposited under Cu excess, in contrast to the observations in selenide chalcopyrite. The increment of the process temperature leads to an improvement of the QFLS too, although absorbers grown in Cu deficiency are less influenced, likely because of a lower sodium content in the high-temperature glass used as substrate. The QFLS increase correlates with the lowering of a deep defect related band, which manifests itself with a peak maximum at around 0.8 eV in room temperature PL spectra. In literature, the low efficiencies exhibited by Cu(In,Ga)S2–based solar cells are often attributed to interface problems at the p-n junction, i.e. at the absorber-buffer layer interface. In this work, the comparison of the QFLS and VOC of pure sulphides CIGS with those measured on selenides clearly points out that the lower efficiencies exhibited by the former are caused also by the intrinsic lower optoelectronic quality of Cu(In,Ga)S2 films. To shed light on the electronic structure, high quality CuInS2 films are deeply investigated by means of low temperature PL. Four shallow defects are detected: one shallow donor at about 30 meV from the conduction band and three shallow acceptors at about 105, 145 and 170 meV from the valence band. The first of these acceptors dominates the band edge luminescence of sample grown with composition close to the stoichiometry, whereas the second deeper acceptor is characteristic of absorbers deposited in Cu rich regime. The deepest of these acceptors seems to be present over a wide range of compositions, although its luminescence is observable only for slight Cu-poor samples with sodium incorporation during the deposition. The quality of the examined films allows the observations of phonon coupling of these shallow defects for the first time in this semiconductor. All these observations on shallow defects and their phonon coupling behaviour allowed to revise the defect model for this semiconductor. The findings of this thesis reveal the strong similarity of the shallow defects structure with selenium based compounds. On the other hand, the presence of deep defects in CuInS2 strongly limits the optoelectronic quality of the bulk material, causing the gap in power conversion efficiencies compared to low-band gap Cu(In,Ga)Se2 solar cells, which show efficiencies above 23%. [less ▲]

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See detailPhonon coupling and shallow defects in CuInS2
Lomuscio, Alberto UL; Sood, Mohit UL; Melchiorre, Michele UL et al

in Physical Review. B (2020), 101(8), 085119-

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See detailOn the chemistry of grain boundaries in CuInS2 film
Schwarz, Torsten; Lomuscio, Alberto UL; Siebentritt, Susanne UL et al

in Nano Energy (2020), 76

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See detailQuasi-Fermi-Level Splitting of Cu-Poor and Cu-Rich CuInS2 Absorber Layers
Lomuscio, Alberto UL; Rödel, Tobias UL; Schwarz, Torsten et al

in Physical Review Applied (2019), 11

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See detailInfluence of stoichiometry and temperature on quasi Fermi level splitting of sulfide CIS absorber layers
Lomuscio, Alberto UL; Melchiorre, Michele UL; Siebentritt, Susanne UL

in 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC (2018, November 29)

CuInS-based solar cells suffer from a low open circuit voltage. Absorbers grown under both Cu-excess and Cudeficiency have been used to fabricate record efficiency photovoltaic cells. In this work, we ... [more ▼]

CuInS-based solar cells suffer from a low open circuit voltage. Absorbers grown under both Cu-excess and Cudeficiency have been used to fabricate record efficiency photovoltaic cells. In this work, we present the influence of stoichiometry on the quality of absorbers by means of calibrated room temperature photoluminescence and quasi Fermi level splitting evaluation (qFLs). Deep defects-related photoluminescence decreases using higher Cu/In ratio, leading to a corresponding improvement in qFLs, with values above 900 meV for high copper rich absorbers. © 2018 IEEE. [less ▲]

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