The dependence of electron mobility on strain, channel direction, and substrate orientation is theoretically studied for the germanium n-channel metal-oxide-semiconductor field-effect transistors. For the unstrained channel, (111) substrate can provide the highest mobility among the three orientations, mainly due to its largest quantization mass and smallest conductivity mass in L valley. The tensile strain parallel to the [(1) over bar 10] channel direction on (111) substrate gives 4.1 times mobility of Si at 1 MV/cm, and the mobility enhancement starts to saturate for the strain larger than 0.5%. The compressive strain of similar to 1.5% transverse to [(1) over bar 10] on (111) substrate yields 2.9 times mobility enhancement at 1 MV/cm. (c) 2007 American Institute of Physics

With Al2O3 passivation on the surface of Cu(In,Ga)Se2, the integrated photoluminescence intensity can achieve two orders of magnitude enhancement due to the reduction of surface recombination velocity. The photoluminescence intensity increases with increasing Al2O3 thickness from 5 nm to 50 nm. The capacitance-voltage measurement indicates negative fixed charges in the film. Based on the first principles calculations, the deposition of Al2O3 can only reduce about 35% of interface defect density as compared to the unpassivated Cu(In,Ga)Se2. Therefore, the passivation effect is mainly caused by field effect where the surface carrier concentration is reduced by Coulomb repulsion.

junction quality, thus increasing open circuit voltage (V oc ) and fill factor (FF). It has also been observed that KF-PDT allows a steeper Ga gradient [8] and a thinner CdS buffer layer [6,8] to be employed without having a negative impact on device performance; therefore, the short circuit current (J sc ) can be improved after optimizing Ga gradient and CdS thickness.Though the improvement in device parameters has been generally observed, the underlying mechanism of KF-PDT is still unclear. One possible reason for the improved junction quality is the formation of a Cu-depleted surface layer, which may enlarge the surface bandgap [9] and promote the in-diffusion of Cd [6] during buffer layer deposition. However, the formation of a Cu-depleted surface does not seem to be always observed. For example, Mansfield et al. have claimed that no Cudepleted surface formed on the selenized CIGS after KF-PDT. [10] Another interesting observation is the reduction of Na content in CIGS after KF-PDT, nevertheless the consequences of which have not yet been discussed in literature. Further investigation is still required to fully understand the effects of KF-PDT.The research into CIGS deposited by three-stage coevaporation or two-step processes (metal deposition and postselenization) continues to advance in the pursuit of mass production and low-cost processing. In addition, various fabrication processes of CIGS thin films have been demonstrated and continue to be developed, for example, fabrication from electrodeposited [11] or nanoparticle-based [12] precursor layers. In these processes, however, selenization is indispensable. The need for a postselenization process inevitably increases production costs. Some researchers have attempted to eliminate this unfavorable and time-consuming process. For example, in a hydrazine-based process, device-quality CIGS absorbers were obtained by annealing the spin-coated precursors in an inert atmosphere, yielding cell efficiency of over 15%. [13] One of the key factors of eliminating selenization in the hydrazine-based process is that extra Se can be added into the precursor ink so extra Se in the atmosphere is not required during annealing. [14] However, the main disadvantage of the hydrazine-based process is its toxicity and instability.Directly sputtering from a single CIGS target is another promising process that could produce efficient CIGS absorbers Direct sputtering of a single quaternary Cu(In,Ga)Se 2 (CIGS) target without postselenization is a promising approach to fabricating CIGS absorbers. However, the device efficiency of the quaternary-sputtered CIGS is limited to 10%-11% due to the low and uncontrollable Se supply during the quaternary sputtering process. Here, an enhanced efficiency of 14.1% is reported by directly sputtering from a CIGS target without extra Se supply followed by sequential postdeposition treatments (PDT) of NaF and KF. The effects of different post-treatments of alkali metals on quaternary-sputtered CIGS thin films are discussed in detail. A Cu-depleted su...

Kesterite with a high Ag content processed at low temperature without CuZnantisite defects using aqueous spray pyrolysis reaches 10% efficiency.

Among different process routes for Cu(In,Ga)Se 2 (CIGS) solar cells, sufficient Se supply is commonly required to obtain high-quality CIGS films. However, supplying extra Se increases the cost and the complexity. In this work, we demonstrate that extra Na incorporation can substantially increase efficiency of Se-deficient CIGS solar cells, fabricated by sputtering from a quaternary CIGS target without extra Se supply, from 1.5% to 11.0%. The Se-deficient CIGS device without extra NaF reveals a roll-over I-V curve at room temperature as well as significantly reduced J sc and fill factor at low temperatures. The electrical characteristics of Se-deficient CIGS films are well explained and modeled by the low p-type doping due to high density of compensating donors and the presence of deep defects possibly originating from the anti-bonding levels of Se vacancies. The significant improvement after extra Na incorporation is attributable to the Na-induced passivation of Se vacancies and the increased p-type doping. Our result suggests that extra Na addition can effectively compensate the Se deficiency in CIGS films, which provides a valuable tuning knob for compositional tolerance of absorbers, especially for the Se-deficient CIGS films. We believe that our findings can shine light on the development of novel CIGS processes, distinct from previous ones fabricated in Se-rich atmosphere.

No abstract

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.

A low-cost, environmentally friendly process of chemical-bath-deposited (CBD) ZnS(O,OH) buffer layers for flexible Cu(In,Ga)Se 2 (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. † Electronic supplementary information (ESI) available. See