Interface band gap narrowing behind open circuit voltage losses in Cu2ZnSnS4 solar cells (original) (raw)
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Journal of Applied Physics, 2014
We investigate point defects in the buffer layers CdS and ZnS that may arise from intermixing with Cu(In,Ga)(S,Se)2 (CIGS) or Cu2ZnSn(S,Se)4 (CZTS) absorber layers in thin-film photovoltaics. Using hybrid functional calculations, we characterize the electrical and optical behavior of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities in the buffer. We find that In and Ga substituted on the cation site act as shallow donors in CdS and tend to enhance the prevailing n-type conductivity at the interface facilitated by Cd incorporation in CIGS, whereas they are deep donors in ZnS and will be less effective dopants. Substitutional In and Ga can favorably form complexes with cation vacancies (A-centers) which may contribute to the “red kink” effect observed in some CIGS-based devices. For CZTS absorbers, we find that Zn and Sn defects substituting on the buffer cation site are electrically inactive in n-type buffers and will not supplement the donor doping at the interface as in CIGS/CdS or ZnS...
Origin of Interface Limitation in Zn(O,S)/CuInS2Based Solar Cells
Copper indium disulfide (CuInS 2) grown under Cu-rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination, resulting in poor solar cell performance. An unfavorable "cliff"-like conduction band alignment at the buffer/CuInS 2 interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in chemical bath-deposited Zn(O,S)/co-evaporated CuInS 2-based devices. Temperature-dependent current− voltage analyses indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bulk band gap, confirming the dominant recombination channel to be present at the Zn(O,S)/CuInS 2 interface. However, photoelectron spectroscopy measurements indicate a small (0.1 eV) "spike"like conduction band offset at the Zn(O,S)/CuInS 2 interface, excluding an unfavorable energy-level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also suggests Fermi-level pinning not to be the main reason, leaving nearinterface defects (as recently observed in Cu-rich CuInSe 2) as the likely reason for the performance-limiting interface recombination.
RECOMBINATION OF ELECTRONS AND DONORS IN SEMICONDUCTORS. Technical Note No. 7
1961
A description is given of a calculation of the recombination of electrons in the conduction band of a semiconductor with ionized donor impurities. The process which we assume consists in the initial capture of the electron in an excited state of the donor center followed by a transition to the ground state. This mechanism is most effective in the case in which all transitions are accompanied with emission or absorption of phonons.
Materials Today Energy, 2019
Interface engineering of CdS/CZTS(Se) is an important aspect of improving the performance of buffer/ absorber heterojunction combination. It has been demonstrated that the crossover phenomenon due to the interface recombination can be drastically eliminated by interface modification. Therefore, in-depth studies across the CdS/CZTS(Se) junction properties, as well as effective optimization processes, are very crucial for achieving high-efficiency CZTSSe solar cells. Here, we present a comprehensive study on the effects of soft-baking (SB) temperature on the junction properties and the corresponding optoelectronic and interface-structural properties. Based on in-depth photoemission studies corroborated with structural and composition analysis, we concluded that interdiffusion and intermixing of CZTSSe and CdS phases occurred on the Cu-poor surface of CZTSSe at elevated SB temperatures, and the interface dipole moments induced by electrostatic potential fluctuation were thus significantly eliminated. In contrast, with low SB temperature, the CdS/CZTSSe heterojunction revealed very sharp interface with very short interdiffusion, forming interface dipole moments and drastically deteriorating device performance. These post thermal treatments also significantly suppress defect energy level of interface measured by admittance spectroscopy from 294 to 109 meV due to CdS/CZTSSe interdiffusion. Meanwhile, the interdiffusion effects on the shift of valence band maximum, conduction band minimum and band offset across the heterojunction of thermally treated CdS/CZTSSe interface are spatially resolved at the atomic scale by measuring the local density of states with cross-sectional scanning tunneling microscopy and spectroscopy. A significant enhancement in the power conversion efficiency from 4.88% to 8.48% is achieved by a facile interface engineering process allowing a sufficient intermixing of CdS/Cd and CZTSSe/Se phases without detrimental recombination centers.
IEEE Journal of Photovoltaics, 2015
We have investigated different nonidealities in Cu 2 ZnSnSe 4-CdS-ZnO solar cells with 9.7% conversion efficiency, in order to determine what is limiting the efficiency of these devices. Several nonidealities could be observed. A barrier of about 300 meV is present for electron flow at the absorber-buffer heterojunction leading to a strong crossover behavior between dark and illuminated current-voltage curves. In addition, a barrier of about 130 meV is present at the Mo-absorber contact, which could be reduced to 15 meV by inclusion of a TiN interlayer. Admittance spectroscopy results on the devices with the TiN backside contact show a defect level with an activation energy of 170 meV. Using all parameters extracted by the different characterization methods for simulations of the two-diode model including injection and recombination currents, we come to the conclusion that our devices are limited by the large recombination current in the depletion region. Potential fluctuations are present in the devices as well, but they do not seem to have a special degrading effect on the devices, besides a probable reduction in minority carrier lifetime through enhanced recombination through the band tail defects.
ACS Applied Energy Materials, 2018
The optimisation of the interface between back contact and absorber is one of the main challenges to improve the electrical behaviour and further enhance the efficiencies of Cu 2 ZnSn(S,Se) 4 (CZTS(e)) solar cell devices. In this work, Mo/Si x N y thin films with various film thicknesses were introduced as an interfacial layer to explore its influence on opto-electronic properties of the pure sulphide CZTS thin film solar cells. The Si x N y was deposited through plasma enhanced chemical vapour deposition (PECVD). The film thickness and stress of the Mo/Si x N y films were controlled to improve the adhesion of the CZTS layer and reduce the chances of cracking the deposited films. Energy dispersive X-Ray spectroscopy (EDS) mapping measurements performed directly on the cross-section of Mo/Si x N y /CZTS/Mo films indicate that the Si x N y intermediate layer can effectively inhibit the formation of a highly resistive MoS 2 layer and decomposition of CZTS at the CZTS/Molybdenum (Mo) interface region. A reduced efficiency was obtained with a Si x N y modified back contact compared with the devices without this layer. This could be due to the increased recombination and poor hole extraction stemming from the very low valance band maximum of Si x N y obtained from ultraviolet photoelectron spectroscopy (UPS) measurements. Temperature dependent current density-voltage (T-JV) and temperature dependent transient photovoltage (T-TPV) measurements were used to uncover insights into the internal recombination dynamics of the charge carriers.
ACS Applied Energy Materials, 2020
To improve the constraints of kesterite Cu 2 ZnSnS 4 (CZTS) solar cell, such as undesirable band alignment at p−n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu 2 Zn 1−x Cd x SnS 4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental−theoretical approach was employed to characterize and assess the optoelectronic properties of Cu 2 Zn 1−x Cd x SnS 4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu 2 Zn 1−x Cd x SnS 4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu 2 CdSnS 4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p−n junction in the ultrafast time scale and highlight a route to improve device performances.
Recombination Losses in Solar Cells Based on n-ZnS(n-CdS) / p-CdTe Heterojunctions
The recombination losses in ancillary and absorber layers of solar cells based on n-ZnS / p-CdTe and n-CdS / p-CdTe heterojunctions with ITO and ZnO current-collecting frontal contacts were calculated. The effect of recombination losses in solar cells with structure ITO(ZnO) / CdS(ZnS) / CdTe on the short-circuit current (Jsc) and the efficiency (η) of photovoltaic devices at different window layer thickness CdS (ZnS) (50-300 nm) and at invariable of current-collecting layer thickness (200 nm) were investigated. The influence of recombination velocity (S 10 7-10 9 cm/s) on the main features of solar cells was researched. It was established that solar cells with structure ZnO/ZnS/CdTe at the concentration of uncompensated acceptors in absorber layer (Na – Nd) 10 15-10 17 cm – 3 and at window layer thickness 50 nm at recombination velocity S 10 7 cm/s have the highest efficiency values (15.9-16.1 %).