Zhiqiang Zhou

Growth of Cu2ZnSnSe4 Film under Controllable Se Vapor Composition and Impact of Low Cu Content on Solar Cell Efficiency

It is a challenge to fabricate high quality Cu2ZnSnSe4 (CZTSe) film with low Cu content (Cu/(Zn + Sn) < 0.8). In this work, the growth mechanisms of CZTSe films under different Se vapor composition are investigated by DC-sputtering and a postselenization approach. The composition of Se vapor has important influence on the compactability of the films and the diffusion of elements in the CZTSe films. By adjusting the composition of Se vapor during the selenization process, an optimized two step selenization process is proposed and highly crystallized CZTSe film with low Cu content (Cu/(Zn + Sn) = 0.75) is obtained. Further study of the effect of Cu content on the morphology and photovoltaic performance of the corresponding CZTSe solar cells has shown that the roughness of the CZTSe absorber film increases when Cu content decreases. As a consequence, the reflection loss of CZTSe solar cells reduces dramatically and the short circuit current density of the cells improve from 34.7 mA/cm(2) for Cu/(Zn + Sn) = 0.88 to 38.5 mA/cm(2) for Cu/(Zn + Sn) = 0.75. In addition, the CZTSe solar cells with low Cu content show longer minority carrier lifetime and higher open circuit voltage than the high Cu content devices. A champion performance CZTSe solar cell with 10.4% efficiency is fabricated with Cu/(Zn + Sn) = 0.75 in the CZTSe film without antireflection coating.

CZTSe solar cells prepared by electrodeposition of Cu/Sn/Zn stack layer followed by selenization at low Se pressure.

Cu2ZnSnSe4 (CZTSe) thin films are prepared by the electrodeposition of stack copper/tin/zinc (Cu/Sn/Zn) precursors, followed by selenization with a tin source at a substrate temperature of 530°C. Three selenization processes were performed herein to study the effects of the source of tin on the quality of CZTSe thin films that are formed at low Se pressure. Much elemental Sn is lost from CZTSe thin films during selenization without a source of tin. The loss of Sn from CZTSe thin films in selenization was suppressed herein using a tin source at 400°C (A2) or 530°C (A3). A copper-poor and zinc-rich CZTSe absorber layer with Cu/Sn, Zn/Sn, Cu/(Zn + Sn), and Zn/(Cu + Zn + Sn) with metallic element ratios of 1.86, 1.24, 0.83, and 0.3, respectively, was obtained in a selenization with a tin source at 530°C. The crystallized CZTSe thin film exhibited an increasingly (112)-preferred orientation at higher tin selenide (SnSex) partial pressure. The lack of any obvious Mo-Se phase-related diffraction peaks in the X-ray diffraction (XRD) diffraction patterns may have arisen from the low Se pressure in the selenization processes. The scanning electron microscope (SEM) images reveal a compact surface morphology and a moderate grain size. CZTSe solar cells with an efficiency of 4.81% were produced by the low-cost fabrication process that is elucidated herein.

The effects of sodium on the growth of Cu(In,Ga)Se2 thin films using low-temperature three-stage process on polyimide substrate

In this work, Cu(In,Ga)Se2 (CIGS) absorbers were deposited on polyimide substrates using the low-temperature three-stage process, and different sodium incorporation methods were applied to investigate the effects of Na on the CIGS growth. It was found that Na affected the CIGS film growth significantly during the second stage of the process, and resulted in a higher Ga content near the back contact. The CIGS thin films with different Na-incorporation methods exhibited similar electrical parameters. A modelling investigation was also implemented by using the wxAMPS software package to quantitatively analyse the effects of different Ga gradient to the device characteristics. Finally, the device performance showed little difference between the post-deposition treatment of NaF and the co-evaporation of NaF during the third stage.

Controllable Growth of Ga Film Electrodeposited from Aqueous Solution and Cu(In,Ga)Se2 Solar Cells

Electrodepositon of Ga film is very challenging due to the high standard reduction potential (-0.53 V vs SHE for Ga3+). In this study, Ga film with compact structure was successfully deposited on the Mo/Cu/In substrate by the pulse current electrodeposition (PCE) method using GaCl3 aqueous solution. A high deposition rate of Ga3+ and H+ can be achieved by applying a large overpotential induced by high pulse current. In the meanwhile, the concentration polarization induced by cation depletion can be minimized by changing the pulse frequency and duty cycle. Uniform and smooth Ga film was fabricated at high deposition rate with pulse current density 125 mA/cm2, pulse frequency 5 Hz, and duty cycle 0.25. Ga film was then selenized together with electrodeposited Cu and In films to make a CIGSe absorber film for solar cells. The solar cell based on the Ga film presents conversion efficiency of 11.04%, fill factor of 63.40%, and Voc of 505 mV, which is much better than those based on the inhomogeneous and rough Ga film prepared by the DCE method, indicating the pulse current electrodeposition process is promising for the fabrication of CIGSe solar cell.

Organocatalyzed enantioselective [3 + 3] annulation for the direct synthesis of conformationally constrained cyclic tryptophan derivatives

An enantioselective formal [3 + 3] annulation of 1-methylindoline-2-thiones and 4-arylmethylideneoxazolin-5(4H)-ones has been developed by the use of an L-tert-leucine-derived bifunctional tertiary amine-squaramide catalyst, which furnished a series of optically active conformationally strained β-branched cyclic tryptophan derivatives in acceptable yields with good to excellent diastereo- and enantioselectivities.

Modified co-evaporation process for fabrication of 4 cm × 4 cm large area flexible CIGS thin film solar cells on polyimide substrate

A modified three-stage co-evaporation process has been studied in our work for the fabrication of large area Cu(In,Ga)Se2 (CIGS) thin-film solar cells on flexible polyimide substrates. According to our results, the open circuit voltage and fill factor are improved significantly by using the modified process. In order to quantitatively analyze the effects of the modified band-gap gradient in CIGS thin film, a modeling investigation is carried out by employing a simulation program, wxAMPS. The simulation indicates that the device improvement is not only attributed to the refined Ga gradient profile, but also to the better crystalline quality. The elemental evaporation rates and the content of Cu are found to be key factors for the large-area solar cell preparation. Finally, 16 cm2 CIGS thin-film solar cells on PI substrates are fabricated, and the highest cell efficiency has achieved 7%.

Modified Back Contact Interface of CZTSe Thin Film Solar Cells: Elimination of Double Layer Distribution in Absorber Layer

Abstract Double layer distribution exists in Cu2SnZnSe4 (CZTSe) thin films prepared by selenizing the metallic precursors, which will degrade the back contact of Mo substrate to absorber layer and thus suppressing the performance of solar cell. In this work, the double‐layer distribution of CZTSe film is eliminated entirely and the formation of MoSe2 interfacial layer is inhibited successfully. CZTSe film is prepared by selenizing the precursor deposited by electrodeposition method under Se and SnSex mixed atmosphere. It is found that the insufficient reaction between ZnSe and Cu‐Sn‐Se phases in the bottom of the film is the reason why the double layer distribution of CZTSe film is formed. By increasing Sn content in the metallic precursor, thus making up the loss of Sn because of the decomposition of CZTSe and facilitate the diffusion of liquid Cu2Se, the double layer distribution is eliminated entirely. The crystallization of the formed thin film is dense and the grains go through the entire film without voids. And there is no obvious MoSe2 layer formed between CZTSe and Mo. As a consequence, the series resistance of the solar cell reduces significantly to 0.14 Ω cm2 and a CZTSe solar cell with efficiency of 7.2% is fabricated.

Effect of Sn Content in a CuSnZn Metal Precursor on Formation of MoSe2 Film during Selenization in Se+SnSe Vapor

The preparation of Cu2ZnSnSe4 (CZTSe) thin films by the selenization of an electrodeposited copper–tin–zinc (CuSnZn) precursor with various Sn contents in low-pressure Se+SnSex vapor was studied. Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) measurements revealed that the Sn content of the precursor that is used in selenization in a low-pressure Se+SnSex vapor atmosphere only slightly affects the elemental composition of the formed CZTSe films. However, the Sn content of the precursor significantly affects the grain size and surface morphology of CZTSe films. A metal precursor with a very Sn-poor composition produces CZTSe films with large grains and a rough surface, while a metal precursor with a very Sn-rich composition procures CZTSe films with small grains and a compact surface. X-ray diffraction (XRD) and SEM revealed that the metal precursor with a Sn-rich composition can grow a thicker MoSe2 thin film at CZTSe/Mo interface than one with a Sn-poor composition, possibly because excess Sn in the precursor may catalyze the formation of MoSe2 thin film. A CZTSe solar cell with an efficiency of 7.94%was realized by using an electrodeposited metal precursor with a Sn/Cu ratio of 0.5 in selenization in a low-pressure Se+SnSex vapor.

The influence of thermal cracking selenium source temperature on CIGS absorber and device performance in co-evaporation processes

Se source cracking is reported with positive influence on CIGS absorber and device. We apply thermal cracking selenium source to co-evaporation processes, and investigate the influence of cracking Se source temperature on the CIGS absorber and device performance. The absorber films are investigated by XRF, SEM, XRD (GIXRD), SIMS, CV and Current-Voltage characterization. As the cracking temperature increase, the film topography tend to exhibit copper poor morphology, namely layered structure. The film preferred orientation was not significantly influenced by cracking temperature. For the device fabricated, we find that high thermal crack temperature decreases the open circuit voltage and fill factor. Further analysis of the IV data, we conclude that major recombination occurs in the space charge region. Though no iron material is applied in thermal cracking unit, the Fe intensity of high cracking temperature sample is still higher than those of low cracking temperature ones. Also, the cracking Se source facilitate the diffusion of Ga and In, which is similar to elevating substrate temperature. CV measurement unveiled that deep level defect concentration is higher in high cracking temperature samples. Further investigation and experiment including IVT, AS measurement and removal of material containing iron in Se source is carrying out to determine whether these defects are introduced only by impurities or by selenium activity induced atomic vacancy or substitution defects.

Novel high-efficiency c-Si compound heterojunction solar cells: HCT (Heterojunction with Compound Thin-layer)

A new concept of developing high-efficiency c-Si heterojunction solar cells is proposed in this work. By replacing a-Si:H thin films in HIT solar cells with appropriate compound semiconductors, we propose novel heterojunction structures which allow higher efficiency than that of HIT solar cells, and this novel type of solar cells is denominated HCT (Heterojunction with Compound Thin-layer). The compound heterojunction materials are preliminarily selected from binary semiconductors whose lattice constants, energy bands and coefficients of thermal expansion match the ones of c-Si, and their feasibilities are investigated by using a solar cell numerical modeling tool, wxAMPS. The modeling investigation indicates that HCT possess better energy band structures at hetero-interfaces than HIT. Finally, this paper concludes the compound selection standards, and suggests several novel research topics in the real device implementation of high-efficiency HCT solar cells.