Advances in CIGS thin film solar cells with emphasis on the alkali element post-deposition treatment

In the past tens of years,the power conversion efficiency of Cu(In,Ga)Se2(CIGS)has continuously improved and been one of the fastest growing photovoltaic technologies that can also help us achieve the goal of carbon emissions reduction.Among several key advances,the alkali element post-deposition treatment(AlK PDT)is regarded as the most important finding in the last 10 years,which has led to the improvement of CIGS solar cell efficiency from 20.4%to 23.35%.A profound understanding of the influence of alkali element on the chemical and electrical properties of the CIGS absorber along with the underlying mechanisms is of great importance.In this review,we summarize the strategies of the alkali element doping in CIGS solar cell,the problems to be noted in the PDT process,the effects on the CdS buffer layer,the effects of different alkali elements on the structure and morphology of the CIGS absorber layer,and retrospect the progress in the CIGS solar cell with emphasis on the alkali element post deposition treatment.

Back interface passivation for ultrathin Cu(In,Ga)Se_(2) solar cells with Schottky back contact: A trade-off of electrical effects

Back interface passivation reduces the back recombination of photogenerated electrons, whereas aggravates the blocking of hole transport towards back contact, which complicate the back interface engineering for ultrathin CIGSe solar cells with a Schottky back contact. In this work, theoretical explorations were conducted to study how the two contradictory electrical effects impact cell performance. For ultrathin CIGSe solar cells with a pronounced Schottky potential barrier(E_(h)> 0.2 eV), back interface passivation produces diverse performance evolution trends, which are highly dependent on cell structures and properties. Since a back Ga grading can screen the effect of reduced recombination of photogenerated electrons from back interface passivation, the hole blocking effect predominates and back interface passivation is not desirable. However, when the back Schottky diode merges with the main pn junction due to a reduced absorber thickness,the back potential barrier and the hole blocking effect is much reduced on this occasion. Consequently, cells exhibit the same efficiency evolution trend as ones with an Ohmic contact, where back interface passivation is always advantageous.The discoveries imply the complexity of back interface passivation and provide guidance to manipulate back interface for ultrathin CIGSe solar on TCOs with a pronounced Schottky back contact.

Fabrication of high-quality ZnS buffer and its application in Cd-free CIGS solar cells

This paper provides the fabrication of Cd-free Cu(In,Ga)Se2(CIGS) solar cells on soda-lime glass substrates. A high quality ZnS buffer layer is grown by chemical bath deposition(CBD) process with ZnSO4-NH3-SC(NH2)2 aqueous solution system. The X-ray diffraction(XRD) result shows that the as-deposited ZnS film has cubic(111) and(220) diffraction peaks. Scanning electron microscope(SEM) images indicate that the ZnS film has a dense and compact surface with good crystalline quality. Transmission measurement shows that the optical transmittance is about 90% when the wavelength is beyond 500 nm. The bandgap(Eg) value of the as-deposited ZnS film is estimated to be 3.54 eV. Finally, a competitive efficiency of 11.06% is demonstrated for the Cd-free CIGS solar cells with ZnS buffer layer after light soaking.

In-situ growth of a CdS window layer by vacuum thermal evaporation for CIGS thin film solar cell applications

Highly crystalline and transparent CdS films are grown by utilizing the vacuum thermal evaporation （VTE） method. The structural, surface morphological, and optical properties of the films are studied and compared with those prepared by chemical bath deposition （CBD）. It is found that the films deposited at a high substrate temperature （200 ℃） have a preferential orientation along （002） which is consistent with CBD-grown films. Absorption spectra reveal that the films are highly transparent and the optical band gap values are found to be in a range of 2.44 eV-2.56 eV. Culnl_xGaxSe2 （CIGS） solar cells with in-situ VTE-grown CdS films exhibit higher values of Voc together with smaller values of Jsc than those from CBD. Eventually the conversion efficiency and fill factor become slightly better than those from the CBD method. Our work suggests that the in-situ thermal evaporation method can be a competitive alternative to the CBD method, particularly in the physical- and vacuum-based CIGS technology.

Optimization of Mo/Cu(In,Ga)Se2/CdS/ZnO Hetero-Junction Solar Cell Performance by Numerical Simulation with SCAPS-1D

The paper presents a one-dimensional simulation study of chalcopyrite Cu(In,Ga)Se2(CIGS)solar cells,where the effects of the variation of CIGS,CdS,and ZnO layers are presented.Additionlly the influence of the variation of doping and the defects density of shallow uniform donors and acceptors types are also presented.The analyse of the simulation results shows that recombination inside the space charge region(SCR)decrease more our CIGS solar cell model performance.We also found that the electrical parameters increase with increasing CIGS absorber doping density exception of JSC values that reach their maximum at 1016cm-3 and decrease due to recombination of charge carriers in the p-n junction particularly the recombination inside the SCR.We also stressed the fact that the effects of shallow uniforme donor density is very low on the performance of our CIGS solar cell model is important because it will allow to control the width of space charge region from shallow uniform acceptors defect density that has a strong influence on the different electrical parameters.Yet,good optimization of performance of the CIGS-based solar cell necessarily passes though a good control of the space charge region width and will constitute a boosting perspective for the preparation of our next paper.We contact that the results obtained of the numerical simulation with SCAPS-1D show a good agreement comparatively of the literature results.The simulation of our CIGS solar cell presents best performances if the values of the absorber layer thickness is in the range of 0.02 to 0.03μm,the buffer layer thickness is in the range of 0.02 to 0.06μm and the defects density of shallow uniform acceptors types is in the range of 1015 to 1017cm-3.

Evaluation of electrical and optical characteristics of ZnO/CdS/CIS thin film solar cell

In this study, device modeling and simulation are conducted to explain the effects of each layer thickness and temperature on the performance of ZnO/CdS/CIS thin film solar cells. Also, the thicknesses of the CIS and CdS absorber layers are considered in this work theoretically and experimentally. The calculations of solar cell performances are based on the solutions of the well-known three coupling equations： the continuity equation for holes and electrons and the Poisson equation. Our simulated results show that the efficiency increases by reducing the CdS thickness. Increasing the CIS thickness can increase the efficiency but it needs more materials. The efficiency is more than 19% for a CIS layer with a thickness of 2 μm. CIS nanoparticles are prepared via the polyol route and purified through centrifugation and precipitation processes.Then nanoparticles are dispersed to obtain stable inks that could be directly used for thin-film deposition via spin coating.We also obtain x-ray diffraction（XRD） peak intensities and absorption spectra for CIS experimentally. Finally, absorption spectra for the CdS window layer in several deposition times are investigated experimentally.

退火处理对磁控溅射制备CIGSe吸收层的影响

在Se气氛中对磁控溅射CIGSe靶材制备的CIGSe薄膜进行退火处理。采用SEM、XRD、Raman、XRF、Hall等方法观察和分析了退火的主要工艺参数对薄膜表面形貌、组织结构、成分及电学性能的影响,并制备了CIGSe太阳电池。结果表明,采用磁控溅射CIGSe靶材+Se气氛中退火处理的方法,可制备得到成分均匀、电学性能优良、单一黄铜矿相的CIGSe薄膜;退火温度和退火时间是影响退火后薄膜质量的主要因素。退火温度低于350℃时,退火效果不明显。退火温度在400℃,退火时间达120min时,薄膜完成再结晶过程,并制得单一黄铜矿相的CIGSe薄膜;退火过程存在Cu-Se二次相的析出和消融,同时具有为薄膜补Se的作用。该文采用该方法制备出的CIGSe太阳电池最高转换效率为5.44%。

磁控溅射法制备CIGSe吸收层的工艺与性能研究

采用中频交流磁控溅射方法直接溅射CIGSe靶材,制备得到用于太阳电池吸收层的CIGSe薄膜。采用SEM、XRD、Raman、XRF、Hall等方法观察和分析主要工艺参数基底温度和溅射气压对薄膜表面形貌、组织结构、成分、电阻率以及载流子浓度的影响。结果表明,基底温度在250℃以上时,薄膜的黄铜矿相特征明显;当基底温度在250～300℃时,薄膜的电阻率和载流子浓度在理想范围之内。提高溅射气压,薄膜的沉积速率降低,当溅射气压达到0.7～0.9Pa时,溅射速率趋于稳定;溅射气压增大,薄膜的结晶性增强,溅射气压达到0.7Pa时,薄膜的XRD特征峰较明显;提高溅射气压,薄膜的电阻率降低,载流子浓度升高,当溅射气压达到0.7Pa时,薄膜电阻率和载流子浓度在理想范围之内。基底温度和溅射气压对薄膜成分无影响,薄膜成分主要由靶材成分决定。

CIGSeS薄膜太阳电池的H2S硫化研究

采用H2S硫化的方法制备铜铟镓硒硫(Cu(In1-xGax)(Se1-ySy)2,CIGSeS)吸收层,研究硫化温度和时间对CIGSeS吸收层和电池性能的影响。结果表明,550~580℃硫化处理时,电池的开路电压得到有效提升,但硫化温度为600℃时,薄膜表面出现局部开裂现象,电池短路电流和效率严重下降。硫化处理可促进铜铟镓硒(Cu(In1-xGax)Se2,CIGSe)吸收层内Ga元素向表面扩散,Ga和S的共同作用可有效提高吸收层表面带隙和太阳电池的开路电压。最终,采用580℃硫化30 min条件下制备的太阳电池开路电压比硫化处理前提高约80 mV,填充因子提高约5%,最高效率为13.69%,有效面积效率为14.62%。而采用580℃硫化10 min后再降温硫化20 min的变温硫化工艺不仅可减少热预算,而且可制备出同580℃恒温硫化30 min时转换效率相当的太阳电池。

Designing novel thin film polycrystalline solar cells for high efficiency:sandwich CIGS and heterojunction perovskite

Heterojunction and sandwich architectures are two new-type structures with great potential for solar cells.Specifically,the heterojunction structure possesses the advantages of efficient charge separation but suffers from band offset and large interface recombination;the sandwich configuration is favorable for transferring carriers but requires complex fabrication process.Here,we have designed two thin-film polycrystalline solar cells with novel structures：sandwich CIGS and heterojunction perovskite,referring to the advantages of the architectures of sandwich perovskite（standard）and heterojunction CIGS（standard）solar cells,respectively.A reliable simulation software wxAMPS is used to investigate their inherent characteristics with variation of the thickness and doping density of absorber layer.The results reveal that sandwich CIGS solar cell is able to exhibit an optimized efficiency of 20.7%,which is much higher than the standard heterojunction CIGS structure（18.48%）.The heterojunction perovskite solar cell can be more efficient employing thick and doped perovskite films（16.9%）than these typically utilizing thin and weak-doping/intrinsic perovskite films（9.6%）.This concept of structure modulation proves to be useful and can be applicable for other solar cells.

Study on the performance of titanium film as a diffusion barrier layer for CIGS solar-cell application on stainless-steel substrates

In this paper,pure titanium(Ti)thin films deposited by radio frequency sputtering were used as a diffusion barrier layer in a flexible copper indium gallium selenium(CIGS)solar cell on a stainless-steel foil and characterized by X-ray diffraction,scanning electron microscopy and second ion mass spectroscopy measurement methods.The influences of the magnetron sputtering pressure on the surface morphology and preferred crystal orientation of Ti films are discussed.It was found that the Ti film showed a(001)preferred orientation and smooth surface topography at lower deposition pressure,while(002)preferred orientation and relatively rough surface topography at higher deposition pressure.In addition,Ti films made with different process pressures were deposited as the barriers and the second ion mass spectroscopy results indicated that a Ti film with the thickness of 200 nm was able to effectively block Fe and Cr diffusion from the stainless-steel foil into the CIGS absorber across the molybdenum back contact.The Ti barrier significantly improved the conversion efficiency of the CIGS solar cell.

通过导纳谱表征铜铟镓硒电池中的缺陷

本文通过导纳谱技术表征铜铟镓硒(CIGSe)太阳电池吸收层中缺陷的能量分布,研究了CIGSe太阳电池退火后效率提高的机理.研究发现退火后CIGSe电池的暗电流减小了大约1个数量级,电池的理想因子也从退火前的2.16减小到退火后的1.85.在反向偏压下,退火前CIGSe太阳电池的电容高于退火后的.通过对电池的C-V特性进行1/C^(2)-V线性拟合获得退火前CIGSe电池吸收层中的自由载流子浓度高于退火后,此外还获得了CIGSe电池退火前后的内建电压分别为0.52 V和0.64 V.通过导纳谱的测试发现退火后吸收层中缺陷的激活能降低,但是缺陷浓度几乎不变.缺陷激活能的降低意味着铜铟镓硒太阳能电池中缺陷的SRH(Shockley-read-hall)复合概率降低,因此退火后太阳能电池的开路电压和并联电阻的增大提高了电池的性能.

Piezo-phototronic and pyro-phototronic effects to enhance Cu（In, Ga）Se2 thin film solar cells

Cu（In, Ga）Se2 （CIGS）-based materials have gained remarkable attention for thin-film photovoltaic applications due to their high absorption coefficient, tunable bandgap, compositional tolerance, outstanding stabilities, and high efficiency. A small increase in the efficiency of CIGS solar cells has huge economic impact and practical importance. As such, we fabricated a flexible CIGS solar cell on a mica substrate and demonstrated the enhanced device performance through the piezo- and pyro-phototronic effects based on a ZnO thin film. The device showed enhanced energy conversion efficiency from 13.48% to 14.23% by decreasing the temperature from 31 to 2℃ at a rate of - 0.6℃·s^-1 via the pyro-phototronic effect, and further enhanced from 14.23% to 14.37% via the piezo-phototronic effect by further applying a static compressive strain. A pyro-electric nanogenerator effect was also found to promote the performance of the CIGS solar cell at the beginning of the cooling process. The manipulated energy band of the CIGS/CdS/ZnO heterojunction under the influence of the inner pyroelectric and piezoelectric potentials is believed to contribute to these phenomena. Applying the piezo- and pyro-phototronic effects simultaneously offers a new opportunity for enhancing the output performance of commercial thin film solar cells.

溅射参数对CuInGa预制膜成分和结构的影响

作为CIGSe太阳能电池重要组成部分的CIGSe吸收层可以采用预制膜+硒化两步法制备。文中采用磁控溅射方法制备了成分均匀、具有一定成分比例的CuIn、CuGa、CuInGa预制膜。X射线荧光分析(XRF)结果显示随着溅射电流的增加,CuIn预制膜的Cu/In原子比减小,CuGa预制膜的Cu/Ga原子比保持不变;X射线衍射分析(XRD)结果表明CuIn、CuInGa预制膜主要由Cu2In、CuIn、In相组成,Ga元素以固溶的形式存在于CuInGa预制膜中。对于CuIn预制膜,随着溅射电流的增加,薄膜中的Cu2In相逐渐向CuIn转变。

Effects of silver-doping on properties of Cu(In,Ga)Se_(2) films prepared by Cu In Ga precursors

The AgCuInGa alloy precursors with different Ag concentrations are fabricated by sputtering an Ag target and a CuInGa target.The precursors are selenized in the H_(2)Se-containing atmosphere to prepare(Ag,Cu)(In,Ga)Se_(2)(ACIGS)absorbers.The beneficial effects of Ag doping are demonstrated and their mechanism is explained.It is found that Ag doping significantly improves the films crystallinity.This is believed to be due to the lower melting point of chalcopyrite phase obtained by the Ag doping.This leads to a higher migration ability of the atoms that in turn promotes grain boundary migration and improves the film crystallinity.The Ga enrichment at the interface between the absorber and the back electrode is also alleviated during the selenization annealing.It is found that Ag doping within a specific range can passivate the band tail and improve the quality of the films.Therefore,carrier recombination is reduced and carrier transport is improved.The negative effects of excessive Ag are also demonstrated and their origin is revealed.Because the atomic size of Ag is different from that of Cu,for the Ag/(Ag+Cu)ratio(AAC)≥0.030,lattice distortion is aggravated,and significant micro-strain appears.The atomic radius of Ag is close to those of In and Ga,so that the continued increase in AAC will give rise to the Ag;or Ag;defects.Both the structural and compositional defects degrade the quality of the absorbers and the device performance.An excellent absorber can be obtained at AAC of 0.015.

Efficiency Enhancement of Cu(In,Ga)Se_2 Solar Cells by Applying SiO_2-PEG/PVP Antire flection Coatings

An effective approach was presented to enhance photoelectric conversion efficiency of Cu(In,Ga)Se2 (CIGS) solar cells by using modified SiO2 antireflection coatings(ARCs) to harvest more incident sunlight.Polyethylene glycol(PEG) and polyvinyl pyrrolidone(PVP) used as additives were introduced into silica sols to prepare SiO2-PEG and SiO2-PVP coatings in the sol-gel dip-coating process,respectively. The different effects of PEG and PVP additives on SiO2 coatings were analyzed and the antireflection performance of SiO2-PEG and SiO2-PVP coatings was investigated. The transmittance over 97% ranging from 450 nm to 700 nm with a maximum transmittance over 99.40% at about 550 nm was achieved for both SiO2-PEG2000A and SiO2-PVP0.5 coatings. The relative efficiencies of CIGS solar cells coated with SiO2-PEG2000A and SiO2-PVP0.5 ARCs were increased by 7.27% and 8.33%,respectively. The modified SiO2 ARCs possessed the advantages of the low manufacturing cost,good adhesion,superior antireflective performance and the feasible method for large area fabrication.

Effect of concentration of cadmium sulfate solution on structural,optical and electric properties of Cd_(1-x)Zn_(x)S thin films

Cd_(1-x)Zn_(x)S thin films were deposited by chemical bath deposition(CBD)on the glass substrate to study the influence of cadmium sulfate concentration on the structural characteristics of the thin film.The SEM results show that the thin film surfaces under the cadmium sulfate concentration of 0.005 M exhibit better compactness and uniformity.The distribution diagrams of thin film elements illustrate the film growth rate changes on the trend of the increase,decrease,and increase with the increase of cadmium sulfate concentration.XRD studies exhibit the crystal structure of the film is the hexagonal phase,and there are obvious diffraction peaks and better crystallinity when the concentration is 0.005 M.Spectrophotometer test results demonstrate that the relationship between zinc content x and optical band gap value E_(g) can be expressed by the equation E_(g)(x)=0.59x^(2)+0.69x+2.43.Increasing the zinc content can increase the optical band gap,and the absorbance of the thin film can be improved by decreasing the cadmium sulfate concentration,however,all of them have good transmittance.At a concentration of 0.005 M,the thin film has good absorbance in the 300-800 nm range,80%transmittance,and band gap value of 3.24 eV,which is suitable for use as a buffer layer for solar cells.

背界面纳米光子结构提高透明导电氧化物基超薄Cu(In, Ga)Se_(2)太阳能电池电学性能的理论探究

透明导电氧化物(TCO)基超薄Cu(In, Ga)Se_(2)(CIGSe)太阳能电池具有建筑光伏一体化的潜力,然而由于背肖特基结的存在,其增大背复合速率S_b在提高空穴传输的同时也增加了光生电子背复合,从而抑制了其性能的提高。本文使用1D-SCAPS软件对背界面纳米光子结构(NPs)如何提高电池的性能进行理论探究,结果表明,背界面NPs的引入产生了复杂的电学效应。一方面,NPs本身不吸收光能,从而降低了背界面附近的有效光吸收体积,导致背界面光生载流子浓度降低,光生电子的背复合显著降低;另一方面,NPs的引入增加了吸收层厚度,导致空间电荷区(SCR)远离背界面,降低了其对光生电子的收集效率,增加了背复合。在高背复合速率(S_(b)=1.0×10^(7)cm·s^(-1))下,光生载流子浓度降低产生的背复合降低大于SCR移动产生的背复合增加,因此总体的背复合降低。与此同时,背复合的降低还缓解了高S_b时的光生电子损耗,从而解除了随S_b增大而增加的背复合对电池性能的抑制。这些发现为设计和优化TCO基超薄CIGSe太阳能电池提供了参考。