Investigation of Properties Limiting Efficiency in Cu2ZnSnSe4-Based Solar Cells (original) (raw)
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Investigation of Properties Limiting Efficiency in Cu2ZnSnSe4-Based Solar Cells
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.
Zn(O,S) film is a promising low-cost and environment-friendly Cd-free buffer layer for chalcopyrite and kesterite thin film solar cells. However, the devices with Zn(O,S) buffer layer usually suffer from poor interface performance, resulting in a much lower efficiency, especially for kesterite solar cells. Here, the band fluctuation caused by ZnO secondary phase in Zn(O,S) layer is identified as the main reason deteriorating the device performance. By a concentrated ammonium etching and subsequent soft annealing treatment, the detrimental ZnO and Zn(OH) 2 secondary phases are eliminated from the Zn(O,S) layer and the hetero-junction performance is improved significantly. Consequently, the power conversion efficiency of the Zn(O,S)/CZTSe solar cells was improved from 1.17% to a favorable value of 7.2%. Temperature dependent J–V properties reveal a defect level assisted charge carrier transport mechanism across the Zn(O,S)/CZTSe interface. These encouraging results imply that Zn(O,S) buffer layer is a promising substitution for toxic CdS in future manufacturing of high performance thin film solar cells.
Low open circuit voltage (Voc) values have been widely reported in kesterite Cu2ZnSnSe4 (CZTSe)-based thin film solar cells. So far, a complete understanding of the main sources of these low performances is far from clear. In this work, a theoretical model for CZTSe solar cell with record efficiency is presented. Among the different device loss mechanisms, trap-assisted tunneling recombination is introduced as the major hurdle to boost Voc values. Detailed comparison of the simulation results to the measured device parameters shows that our model is able to reproduce the experimental observations. Finally, it is found that a further solar cell efficiency enhancement of up to 19.4% with an open circuit voltage close to 708 mV can be achieved by using more resistive CdS layers which is in contradiction to p-n junction behavior. In this way, a MIS performance is proposed to promote Voc and efficiency values. As a result, this approach could help to solve at least one of the main issues of this technology.
Solar Energy Materials and Solar Cells, 2015
In this paper, we examine photoluminescence spectra (PL) of Cu 2 ZnSnSe 4 /CdS/ZnO solar cells, based on an absorber layer fabricated by selenization of sputtered Cu, Zn, Sn multilayers, via temperaturedependent and illumination power-dependent measurements. We observe anomalous emission behavior: the PL peak initially decreases with increasing temperature (red shift) in the temperature range 10-60 K, followed by a blue shift at higher temperature in the range 60-180 K. A recombination model is proposed that is able to explain both temperature dependent PL as well as power-dependent PL. The important aspect of the proposed model is taking into account the presence of strong potential fluctuations in the absorber layer, which can also contribute to the low V oc values generally observed in Cu 2 ZnSnSe 4 , based solar cells.
Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se)2 solar cell
Journal of Physics and Chemistry of Solids, 2003
Temperature-dependent current-voltage measurements are used to determine the dominant recombination mechanism in thin-film heterojunction solar cells based on Cu(In,Ga)(S,Se) 2 absorbers with chemical bath deposited ZnS buffer layer. The measurements are carried out in the dark and under illumination in the temperature range 200-320 K. The activation energy of the recombination under illumination follows the absorber band gap energy E g ¼ 1:07 eV of bulk Cu(In,Ga)(S,Se) 2 . The thermal dependence of the diode ideality factor is described by classical Shockley -Read -Hall (SRH) recombination via an exponential distribution of trap states in the bulk of the absorber. In the dark, the current flow is dominated by tunnelling enhanced bulk recombination with a tunnelling energy E 00 ¼ 18 meV. Two activation energies higher than E g ; namely 1.21 and 1.40 eV, have been found. These results may be explained by dominant recombination in a region close to the surface of the Cu(In,Ga)(S,Se) 2 absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se) 2 solar cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy E 00 influenced by illumination. q
Microstructural analysis of 9.7% efficient Cu2ZnSnSe4 thin film solar cells
Applied Physics Letters, 2014
This work presents a detailed analysis of the microstructure and the composition of our record Cu 2 ZnSnSe 4 (CZTSe)-CdS-ZnO solar cell with a total area efficiency of 9.7%. The average composition of the CZTSe crystallites is Cu 1.94 Zn 1.12 Sn 0.95 Se 3.99 . Large crystals of ZnSe secondary phase (up to 400 nm diameter) are observed at the voids between the absorber and the back contact, while smaller ZnSe domains are segregated at the grain boundaries and close to the surface of the CZTSe grains. An underlying layer and some particles of Cu x Se are observed at the Mo-MoSe 2 -Cu 2 ZnSnSe 4 interface. The free surface of the voids at the back interface is covered by an amorphous layer containing Cu, S, O, and C, while the presence of Cd, Na, and K is also observed in this region. V C 2014 AIP Publishing LLC. [http://dx.
Quantification of surface ZnSe in Cu2ZnSnSe4-based solar cells by analysis of the spectral response
Solar Energy Materials and Solar Cells, 2014
Absorber layers consisting of Cu 2 ZnSnSe 4 (CZTSe) and surface ZnSe in variable ratios were prepared by selenization of electroplated Cu/Sn/Zn precursors and completed into full devices with up to 5.6% power conversion efficiency. The loss of short circuit current density for samples with increasing ZnSe content is consistent with an overall reduction of spectral response, pointing to a ZnSe current blocking behavior. A feature in the spectral response centered around 3 eV was identified and attributed to light absorption by ZnSe. A model is proposed to account for additional collection of the carriers generated underneath ZnSe capable of diffusing across to the space charge region. The model satisfactorily reproduces the shape of the spectral response and the estimated ZnSe surface coverage is in good qualitative agreement with analysis of the Raman spectral mapping. The model emphasizes the importance of the ZnSe morphology on the spectral response, and its consequences on the solar cell device performance.
Temperature dependent electrical characterization of thin film Cu 2 ZnSnSe 4 solar cells
Journal of Physics D: Applied Physics, 2016
Impedance spectroscopy (IS) and current-voltage characteristics measurements were applied to study properties of a Cu 2 ZnSnSe 4 (CZTSe) thin film solar cell. IS measurements were done in the frequency range 20 Hz to 10 MHz. The measurement temperature was varied from 10 K to 325 K with a step ∆T = 5 K. Temperature dependence of V oc revealed an activation energy of 962 meV, which is in the vicinity of the band gap energy of CZTSe and hence the dominating recombination mechanism in this solar cell is bulk recombination. Different temperature ranges, where electrical properties change, were found. Interface states at grain boundaries with different properties were revealed to play an important role in impedance measurements. These states can be described by introducing a constant phase element in the equivalent circuit.
2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014
In this work we report the Cu doping of chemical bath deposited CdS for the preparation of Cu 2 ZnSnSe 4 /CdS:Cu hetero-junctions. We demonstrate that increasing the Cu concentration in the reaction bath several fundamental properties of CdS are changed (optical, crystalline, morphological), positively impacting in the resulting devices. The improvement of the efficiency from 1.6% to 6.1%, is mainly explained by the large increases on VOC, obtaining the higher voltage value reported to now in the literature for this hetero-junction. We propose the formation of a metal-insulator-semiconductor (MIS) type device to explain our experimental results, which opens the possibility to use the MIS structure to solve, at least in part, the voltage deficit problems of kesterites.