Shih-Yuan Wei

Room-Temperature Chemical Solution Treatment for Flexible ZnS(O,OH)/Cu(In,Ga)Se2 Solar Cell: Improvements in Interface Properties and Metastability

We demonstrate an effective room-temperature chemical solution treatment, by using thioacetamide (S treatment) or thioacetamide-InCl3 (In-S treatment) solution, on Cu(In,Ga)Se2 (CIGSe) surface to engineer the ZnS(O,OH)/CIGSe interface and junction quality, leading to enhanced efficiency and minimized metastability of flexible solar cells. The control device without treatment reveals a relatively low efficiency of 8.15%, which is significantly improved to 9.74% by In-S treatment, and 10.39% by S treatment. Results of X-ray photoelectron spectroscopy suggest that S is incorporated into CIGSe surface forming CIGSSe by S treatment, whereas a thin In-S layer is formed on CIGSe surface by In-S treatment with reduced amount of S diffusing into CIGSe. PL spectra and TRPL lifetime further reveal that S incorporation into CIGS surface may substitute the OSe and/or directly occupy the vacant anion site (VSe), resulting in the effective passivation of the recombination centers at CIGSe surface. Moreover, reducing the concentrations of VSe may thereby decrease the density of (VCu-VSe) acceptors, which can minimize the metastability of ZnS(O,OH)/CIGSe solar cells. With S treatment, the light soaking (LS) time of ZnS(O,OH)/CIGSe device is reduced approximately to one-half of control one. Our approach can be potentially applied for alternative Cd-free buffer layers to achieve high efficiency and low metastability.

An ammonia-free chemical-bath-deposited ZnS(O,OH) buffer layer for flexible Cu(In,Ga)Se2 solar cell application: an eco-friendly approach to achieving improved stability

A low-cost, environmentally friendly process of chemical-bath-deposited (CBD) ZnS(O,OH) buffer layers for flexible Cu(In,Ga)Se2(CIGS)-based solar cells is reported. Unlike the conventional CdS buffer layers, in which toxic cadmium and ammonia are used, our CBD-ZnS(O,OH) buffer layer is fabricated without the addition of ammonia or other complexing agents. The ammonia-free ZnS(O,OH) buffer layer is robust under light illumination so the metastabilities in the device characteristics (the so-called light-soaking effect), typically observed in CIGS/CBD-ZnS(O,OH) solar cells, are eliminated. To further adjust the composition of the ammonia-free CBD ZnS(O,OH) buffer layer, we employed an oxygen plasma post-treatment so that the oxygen content of the CBD buffer layer could be controlled in a straightforward way and the conduction band offset at the CIGS/ZnS(O,OH) interface could be engineered. Significantly reduced series resistance was observed after optimizing the oxygen content, leading to 10.1% cell efficiency. Our ammonia-free process demonstrates a comparable efficiency to the conventional process but without any light soaking.

Investigation of sodium effects on CIGS thin films deposited by sputtering from a single quaternary

We investigated sodium effects on Cu(In,Ga)Se2 thin films deposited by sputtering from a single quaternary target without any further post-selenization or sulfurization. We conducted current-voltage measurement to gather device output parameters. We then used capacitance spectroscopy including capacitance-voltage measurement, admittance spectroscopy, and drive-level capacitance profile to investigate sodium effects in our system. Photoluminescence measurement was made to analyze radiative recombination between energy levels. In addition, we used electron probe x-ray analyzer to analyze atomic composition of CIGS thin film. With the help of the techniques mentioned above, we came to a conclusion that sodium behaves as a catalyzer for oxygen atoms to neutralize selenium vacancies.

The effect of Ag incorporation on the phase stability, crystallinity and band structure on the (Cu,Ag)2ZnSn(S,Se)4 kesterite solar cells

The effect of Ag incorporation on the kesterite CZTSSe thin film by vacuum process has been investigated aiming for the band engineering of the kesterite solar cells. Phase stability, crystalinity, morphology and optical properties of Ag2ZnSnSe thin film were firstly reported and measured by XRD, SEM and UV-visible spectrometer in this work. We have successfully fabricated the ACZTSe thin films over a wide range of composition and the optical band gap could be controlled by the Ag/(Cu+Ag) ratio in a range from 1.06eV to 1.3 eV. Finally, the cell performance around 4% was demonstrated by the ACZTSSe devices.

Sputtered Inx(O,S)y Buffer Layers for Cu(In,Ga)Se2 Thin-Film Solar Cells: Engineering of Band Alignment and Interface Properties

We propose a simple approach to engineering the sputtered Inx(O,S)y/Cu(In,Ga)Se2 heterojunction, in terms of band alignment and interface properties. The band alignment was tailored by tuning the base pressure of the sputtering deposition to incorporate oxygen into deposited In2S3 layers (termed as Inx(O,S)y). The interface properties were improved by optimizing the air-annealing temperature on Inx(O,S)y/Cu(In,Ga)Se2 stacked layers. Increasing the base pressure raises the O/(S + O) ratio contained in deposited Inx(O,S)y films and thus widens the band gaps. This could effectively tailor the conduction band offset (ΔEC) at the Inx(O,S)y/Cu(In,Ga)Se2 interface from a cliff (-0.25 eV) to a nearly flat band (0.07 eV) alignment. On the other hand, the extra air annealing at 235 °C did not significantly change the band alignment but did ameliorate the interface properties by reducing the Cu content at the Cu(In,Ga)Se2 surface and diminish the interface defect density induced by sputtering damages. The former might enhance the type of inversion and increase the hole barrier at the interface, preventing the detrimental recombination behavior. The latter could effectively strengthen the junction quality. Consequently, our approach substantially enhances the cell efficiency from 2.30% to 11.04%.

Influence of NaF addition on Cu2ZnSn(S,Se)4 thin film solar cells

The Cu2ZnSn(S,Se)4 (CZTSSe) absorbers were fabricated by two step process which consists of deposited stacking precursor followed by selenization. We investigated the influence of adding sodium into CZTSSe absorbers via sputtering NaF layer during the fabrication of precursor, or evaporating NaF layer on top of CZTSSe absorbers followed by vacuum annealing. We discovered that the NaF layer at the bottom of precursor can change the morphological properties of CZTSSe absorbers; on the contrary, the NaF layer on the top of precursor show no significant change. The increasing thickness of NaF layer revealed positive dependence on crystallinity of CZTSSe absorbers and increased the carrier concentration. However, we found device performance to be worse than those without NaF addition.

Fabrication and characterization of Cu2ZnSn(S, Se)4 with aluminum doping by spray pyrolysis followed by selenization

Spray pyrolysis is a simple way to fabricate CZTSSe and has potential for low-cost industrial manufacturing. Doping of elements can be easily conducted using spray pyrolysis by addition of dopant salts into solution. Aluminum doping in CZTSSe might be an alternative way to adjust bandgap for application to tandem cell or bandgap grading. In this paper, we demonstrate the Al-doing of CZTSSe by spray pyrolysis. Aluminum is introduced by adding AlCl3·6H2O into the precursor solution of CZTSSe. With aluminum in CZTSSe, the crystal structure remained kesterite, but c/a ratio is slightly changed. However, small grain layer was observed when aluminum was incorporated in this film, but this layer might be reduced by optimizing selenization condition.

Fabrication and characterization of Cu 2 ZnSn(S, Se) 4 solar cells by spray-deposited precursor stacks

Non-toxic, earth abundant absorber material CZTSSe thin film was successfully fabricated by spray pyrolysis, which is a low-cost, convenient and non-vacuum process. In this work, CZTSSe thin film is made from spray-deposited precursor stacks with subsequent sulfurization and selenization. The effect of the selenization time on the CZTSSe growth was investigated. X-ray diffraction, Raman spectrum and scanning electron microscope are used to characterize the precursors and CZTSSe thin films. The device is characterized by current-voltage measurements. By this process, the power conversion efficiency of the fabricated device reaches 3.1%.

Selenium treatment on the polycrystalline CuIn 1−x Ga x Se 2 thin films sputtered from a quaternary target

In this work, selenium treatment at 250-350°C on the polycrystalline CuIn1-xGaxSe2 (CIGS) thin films sputtered from a quaternary target has been investigated in order to passivate anionic defects which induce the current-blocking behavior and lowering open circuit voltage. The CIGS thin films were selenized in a closed-space graphite container. The result of selenization was characterized by Raman spectroscopy, EQE and the current-voltage-temperature measurement. After selenization at 350°C, the current-blocking behavior is inhibited and Voc increases from 310mV to 640mV. Until now, the efficiency near 9% can be obtained by an optimized selenization process.

Effect of injection rate on the growth of Cu 2 ZnSnS 4 nanoparticles synthesized by hot injection method

In this study, we have obtained uniform and high quality Cu2ZnSnS4 (CZTS) nanocrystals which were synthesized by using the hot injection method. The effect of injection rate on growth mechanism was investigated. The injection rates were varied from 2.5 to 20mL/h. The CZTS nanocrystals were characterized by TEM, XRD, and Raman. The Raman spectrum revealed good crystallinity, and TEM images showed uniform dispersion. As the injection rates increased, the CZTS particles size become larger and less uniform, which was consistent with LaMer model. The composition of CZTS nanocrystals was almost close to stoichiometry, and we obtained the CZTS nanoparticles with good crystallinity for the case injected at 10mL/h.