Wei-Hao Ho

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.

Effects of Se flux on the properties of the quaternary-sputtered CIGS thin film solar cells

Sputtering from a single CIGS compound target is a promising process to fabricate CIGS absorbers for mass production. However, only few reports demonstrate efficient CIGS cells without post-selenization. The need for the additional post-selenization step may be possibly due to the low Se supply during deposition, which originates from the low Se content in the target. In this study, we proposed a method to supply extra Se, sputtering from a Se target. With this, the Se supply during the quaternary sputtering process is adjusted nominally from 1 to 1.24. The effects of Se supply during the quaternary sputtering process on the film and device properties were studied. Efficiency of 11% was obtained with medium Se flux.

The role of Ag in aqueous solution processed (Ag,Cu)2ZnSn(S,Se)4 kesterite solar cells: antisite defect elimination and importance of Na passivation

CuZn antisite defects have been regarded as the efficiency limiting factor of kesterite solar cells because they cause band tailing and loss of open circuit voltage (VOC). To suppress CuZn antisite defects, Ag-alloyed (Ag,Cu)2ZnSn(S,Se)4 (ACZTSSe) kesterite has been proposed. Although efficiency enhancement is realized by a limited amount of Ag addition (Ag/(Ag + Cu) ≤ 20%), the antisite defects may not be completely eliminated by the small amount of Ag. Further enhancement by using a high Ag content (Ag/(Ag + Cu) ≥ 20%) has not been achieved yet. In this work, the effects of Ag on the phase stability, crystal structure, defect properties and device performance are investigated. Experimental results demonstrate that the optimized formation temperature of kesterite is reduced with increasing Ag content. We demonstrate that Ag2ZnSn(S,Se)4 is ordered kesterite and CuZn antisite defects are effectively eliminated as 35% of Cu is replaced by Ag in ACZTSSe. To further increase the efficiency of the high Ag-content kesterite, 1 at% Na addition to the precursor is critical to passivate the interface as well as grain boundaries and increase the carrier concentration. An efficiency of 10% for ACZTSSe solar cells is reported with Ag/(Ag + Cu) = 35% at a processing temperature as low as 470 °C through an environment-friendly chemical spray pyrolysis process using aqueous solution. This study demonstrates the feasibility of controlling antisite defects and band-gap engineering by a larger amount of Ag substitution in the kesterite system to solve the VOC deficit problem.

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%.

The comparison of different chemical reaction routes for CBD-ZnS applied on Cu(In, Ga)Se2 solar cells

The most high efficiency cadmium-free CIGS solar cell is zinc sulfide fabricated by chemical bath deposition. However, on account of the presence of Zn(OH)2 phase during chemical reaction, it leads to poor cell performance attributed in low fill factor and shows distorted I-V curve. The cell performance can be enhanced by continuous illumination, which has been found by many groups called light-soaking effect. In this paper, we compared two different chemical processes with and without ammonia hydroxide etching. The well-known ammonia etching effect for removing Zn(OH)2 phase has also been observed in this work. By comparing the different chemical processes with ammonia etching, we noticed the difference of compositions due to chemical reaction routes. Based on the research for chemical routes, we further to optimize CBD-ZnS process deposited on CIGS substrates. Owing to the decrease of Zn(OH)2 concentration by optimized CBD-ZnS, the cell efficiency can be improved in Voc and FF. Moreover, the time for lightsoaking treatment can be shortened to less than 20 minutes.

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.