Cu(In,Ga)Se2太阳电池缓冲层ZnS薄膜性质及应用

在含有ZnSO4,SC(NH2)2,NH4OH的水溶液中采用CBD法沉积ZnS薄膜,XRF和热处理前后的XRD测试表明,ZnS沉积薄膜为立方相结构,薄膜含有非晶态的Zn(OH)2.光学透射谱测试表明,制备的薄膜透过率(λ＞500nm)约为90%,薄膜的禁带宽度约为3.51eV.ZnS薄膜沉积时间对Cu(In,Ga)Se2太阳电池影响显著,当薄膜沉积时间在25～35min时,电池的综合性能最好.对比了不同缓冲层的电池性能,采用CBD-CdS为缓冲层的电池转换效率、填充因子、开路电压稍高于CBD-ZnS为缓冲层的无镉电池,但无镉电池的短路电流密度高于前者,两者转换效率相差2%左右.ZnS可以作为CIGS电池的缓冲层,替代CdS,实现电池的无镉化.     

Cu(In,Ga)Se2太阳电池缓冲层ZnS薄膜性质及应用

在含有ZnSO4,SC(NH2)2,NH4OH的水溶液中采用CBD法沉积ZnS薄膜,XRF和热处理前后的XRD测试表明,ZnS沉积薄膜为立方相结构,薄膜含有非晶态的Zn(OH)2.光学透射谱测试表明,制备的薄膜透过率(λ＞500nm)约为90%,薄膜的禁带宽度约为3.51eV.ZnS薄膜沉积时间对Cu(In,Ga)Se2太阳电池影响显著,当薄膜沉积时间在25～35min时,电池的综合性能最好.对比了不同缓冲层的电池性能,采用CBD-CdS为缓冲层的电池转换效率、填充因子、开路电压稍高于CBD-ZnS为缓冲层的无镉电池,但无镉电池的短路电流密度高于前者,两者转换效率相差2%左右.ZnS可以作为CIGS电池的缓冲层,替代CdS,实现电池的无镉化.     

Surface photovoltage spectroscopy on Cu(In,Ga)(S,Se)2/ZnS‐nanodot/In2S3 systems

Single layers and combined layer systems with Cu(In,Ga)(S,Se)₂, ZnS‐nanodot (nd) and In₂S₃ layers were investigated by surface photovoltage spectroscopy in the Kelvin‐probe arrangement and compared with the open‐circuit voltage (V_OC) of solar cells. The In₂S₃ and ZnS‐nd layers were prepared by the spray ion layer gas reaction (ILGAR) technique from Indium chloride (InCl₃), Indium acetylacetonate (In(acac)₃) and Zinc acetylacetonate, respectively. The surface photovoltage signals of Cu(In,Ga)(S,Se)₂ were larger for the Cu(In,Ga)(S,Se)₂/ZnS‐nd/In₂S₃ than for the Cu(In,Ga)(S,Se)₂/In₂S₃ layer system showing that a ZnS‐nd layer additionally passivated the Cu(In,Ga)(S,Se)₂ surface. ILGAR In₂S₃ deposition from InCl₃ precursor solution led to a modification of surface defects of ZnS‐nd and to generation of defect states below the band gap of Cu(In,Ga)(S,Se)₂, which has not been observed for deposition from Indium acetylacetonate precursor. Defect generation during ILGAR In₂S₃ deposition with InCl₃ precursor resulted in a lower V_OC of Cu(In,Ga)(S,Se)₂/ZnS‐nd/In₂S₃/ZnO : Al solar cells. Copyright © 2012 John Wiley & Sons, Ltd.     

ILGAR In2S3 buffer layers for Cd‐free Cu(In,Ga)(S,Se)2 solar cells with certified efficiencies above 16%

In₂S₃ buffer layers have been prepared using the spray ion layer gas reaction deposition technique for chalcopyrite‐based thin‐film solar cells. These buffers deposited on commercially available Cu(In,Ga)(S,Se)₂ absorbers have resulted in solar cells with certified record efficiencies of 16.1%, clearly higher than the corresponding CdS‐buffered references. The deposition process has been optimized, and the resulting cells have been studied using current–voltage and quantum efficiency analysis and compared with previous record cells, cells with a thermally evaporated In₂S₃ buffer layer and CdS references. Copyright © 2012 John Wiley & Sons, Ltd.     

Controllable Formation of Ordered Vacancy Compound for High Efficiency Solution Processed Cu(In,Ga)Se2 Solar Cells

Solution processing of Cu(In,Ga)Se₂ (CIGS) absorber makes it cost-competitive in the photovoltaic market. It is reported that copper-poor ordered vacancy compound (OVC) is crucial for high performance CIGS solar cells. However, in solution process method, controllable formation of OVC is unavailable and limited research has been carried out. In this work, the controllable formation of the OVC phase on the CIGS surface is successful by controlling the selenization temperature and intentional variation of Cu/(In+Ga) stoichiometry in precursors for top layers and bulk layers deposition. The effects of OVC contents on the device performance are investigated. The CIGS thin film with OVC phase exhibits a lower valence band position. Meanwhile, the CIGS devices with optimized OVC content show decreased interface defects density and better carrier collection ability. The above advantages translate into a champion PCE of 16.39% for CIGS device with OVC phase, which is the champion performance among non-hydrazine solution-processed CIGS solar cells. The results demonstrate that the controllable formation of OVC phase approach should make a significant contribution to the efficiency promoting of solution processed CIGS solar cells.     

Electronic structure of the Zn(O,S)/Cu(In,Ga)Se2 thin‐film solar cell interface

The electronic band alignment of the Zn(O,S)/Cu(In,Ga)Se₂ interface in high‐efficiency thin‐film solar cells was derived using X‐ray photoelectron spectroscopy, ultra‐violet photoelectron spectroscopy, and inverse photoemission spectroscopy. Similar to the CdS/Cu(In,Ga)Se₂ system, we find an essentially flat (small‐spike) conduction band alignment (here: a conduction band offset of (0.09 ± 0.20) eV), allowing for largely unimpeded electron transfer and forming a likely basis for the success of high‐efficiency Zn(O,S)‐based chalcopyrite devices. Furthermore, we find evidence for multiple bonding environments of Zn and O in the Zn(O,S) film, including ZnO, ZnS, Zn(OH)₂, and possibly ZnSe. Copyright © 2016 John Wiley & Sons, Ltd.     

Impact of RbF and NaF Postdeposition Treatments on Charge Carrier Transport and Recombination in Ga-Graded Cu(In,Ga)Se2 Solar Cells

Two key strategies for enhancing the efficiency of Cu(In,Ga)Se₂ solar cells are the bandgap gradient across the absorber and the incorporation of alkali atoms. The combined incorporation of Na and Rb into the absorber has brought large efficiency gains compared to Na-containing or alkali-free layers. Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition-graded Cu(In,Ga)Se₂ solar cells. Electron lifetimes are found to be highly dependent on the film composition and morphology, varying from tens of nanoseconds in the energy notch to only ≈100 ps in the Ga-rich region near the Mo-back contact. NaF PDT improves recombination lifetimes by a factor of 2–2.5 in all regions of the absorber, whereas the effectiveness of the RbF PDT is found to decrease for higher Ga-concentrations. Electron mobility measured in the absorber region with large grains is promoted by both alkali PDTs. The data suggest that NaF PDT passivates shallow defect states (Urbach tail) throughout the Cu(In,Ga)Se₂ film (including the interior of large grains), whereas the additional RbF PDT is effective at grain boundary surfaces (predominantly in regions with medium to low Ga-concentrations).     

Defect Engineering in Earth-Abundant Cu2ZnSn(S,Se)4 Photovoltaic Materials via Ga3+-Doping for over 12% Efficient Solar Cells

The efficiency of earth-abundant Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is considerably lower than the Shockley–Queisser limit. One of the main reasons for this is the presence of deleterious cation disordering caused by Sn_Zn antisite and 2Cu_Zn+Sn_Zn defect clusters, resulting in a short minority carrier lifetime and significant band tailing, leading to a large open-circuit voltage deficit, and hence, low efficiency. In this study, Ga-doping is used to increase the CZTSSe solar cell efficiency to as high as 12.3%, one of the highest for this type of cells. First-principles calculations show that the preference of Ga³⁺ occupying Zn and Sn sites has a benign effect on suppressing the formation of the Sn_Zn deep donor defects by upwardly shifting the Fermi level, which is further confirmed by deep-level transient spectroscopy characterization. Besides, the Ga dopants can also form defect-dopant clusters, such as Ga_Zn+Cu_Zn and Ga_Zn+Ga_Sn, which also have positive effects on suppressing the band-tailing states. The defect engineering via Ga³⁺-doping may suppress the band-tailing defect with a decreased Urbach energy, elevate the minority carrier lifetime, and in the end, enhance the V_OC from 473 to 515 mV. These results provide a new route to further increase CZTSSe-based solar cell efficiency by defect engineering.     

CIGS太阳能电池缓冲层ZnS薄膜的制备与表征

以硫酸锌、(NH4)2S2O3混合溶液为前驱体溶液,加入少量的柠檬酸钠和丙三醇为络合剂和分散剂,采用化学浴沉积法在玻璃衬底上成功制备了表面均匀的ZnS薄膜。研究了沉积时间和退火时间对ZnS薄膜质量的影响,并运用扫描电镜(SEM)、X射线衍射(XRD)、紫外-可见光光度计对薄膜进行分析和表征。结果表明:在沉积时间为90m in,退火温度为200℃时制得的薄膜性能较好,晶体结构为纤锌矿结构。制备的薄膜透过率(λ>400nm)约为80%,薄膜的禁带宽度约为3.75eV。通过添加少量的分散剂丙三醇可以改善ZnS薄膜质量。退火温度为300℃,薄膜表面形貌均匀致密。     

Interconnection between Trait,Structure, and Composition of Grain Boundaries in Cu(In,Ga)Se2 Thin‐Film Solar Cells

Grain boundaries (GBs) are crucial for solar cells incorporating polycrystalline absorbers and particularly for those characterized by small grain sizes (≈2 µm). For example, random GBs in Si solar cells are found to have a detrimental effect on the cell performance being characterized by an increased recombination activity relative to grains. Yet, their role in Cu(In,Ga)Se₂ (CIGS) solar cells still remains controversial. The recent electron‐beam‐induced current (EBIC) study shows that 58% of the GBs in CIGS exhibit enhanced electrical properties considered to be benign (for the device performance). Yet, they coexist with 16% detrimental GBs (reduced electrical properties) and 27% neutral ones (no change in electrical property when compared with the bulk). In the present study, these different GBs are investigated by combining EBIC with electron backscattered diffraction and atom probe tomography techniques on identical GBs. For the first time, a successful correlation is shown (for any device) that interconnects the GB characteristics to its composition. Sufficient statistics demonstrate that the collective fluctuations of all elements at GBs determine its trait. In general, benign (detrimental) GBs are characterized by Cu depletion (enrichment) that favored the formation of donor (acceptor) defects.     

Cu(In,Ga)Se2 superstrate solar cells: prospects and limitations

Superstrate solar cells were prepared by thermal evaporation of Cu(In,Ga)Se2 onto ZnO coated glass substrates. For the first time, photo‐conversion efficiencies above 11% were reached without the necessity of additional light soaking or forward biasing of the solar cell. This was achieved by modifying the deposition process as well as the sodium doping. Limitations of the superstrate device configuration and possible ways to overcome these were investigated by analyzing the hetero‐interface with electron microscopy and X‐ray photoemission spectroscopy measurements, combined with capacitance spectroscopy and device simulations. A device model was derived that explains how on the one hand the GaO_x, which forms at the CIGSe/ZnO interface, reduces the interface recombination. On the other hand how it limits the efficiency by acting as an electron barrier at the hetero‐interface presumably because of a high density of negatively charged acceptor states like Cu_Ga. The addition of sodium enhances the p‐type doping of the absorber but also increases the net doping within the GaO_x. Hence, a trade‐off between these two effects is required. The conversion efficiency was found to decrease over time, which can be explained in our model by field‐induced diffusion of sodium cations out from the GaO_x layer. The proposed device model is able to explain various effects frequently observed upon light soaking and forward biasing of superstrate devices. Copyright © 2014 John Wiley & Sons, Ltd.     

Two-Step Cooling Strategy for Synergistic Control of CuZn and SnZn Defects Enabling 12.87% Efficiency (Ag,Cu)2ZnSn(S,Se)4 Solar Cells

Abundant intrinsic defects and defect clusters in Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells lead to severe nonradiative recombination and limited photoelectric performance. Therefore, developing effective method to suppress the detrimental defects is the key to achieve high-efficiency solar cell. Herein, a convenient two-step cooling strategy in selenization process is reported to suppress the Cu_Zn and Sn_Zn defects and defect clusters synergistically. The results show that rapid cooling during section from selenization temperature to turning temperature can inhibit the volatilization of Sn and restrain the corresponding Sn-related defects, while slow cooling during the subsequent temperature section can reduce the degree of Cu-Zn disorder. Benefitting from the synergistic effect of two-step cooling, a significantly lowered concentration of Sn_Zn and Cu_Zn defect and their defect clusters [2Cu_Zn+Sn_Zn] in absorber is observed, meanwhile, a reduced band tailing effect and promoted carrier collection efficiency of the photovoltaic device is obtained. Finally, a device with improved open-circuit voltage (V_oc) of 505.5 mV and efficiency of 12.87% is achieved. This study demonstrates the impact of cooling process on defects controlling for the first time and provides a simple and effective new strategy for intrinsic defect control, which may be universal in other inorganic thin film solar cells.     

Solution‐based processing of Cu(In,Ga)Se2 absorber layers for 11% efficiency solar cells via a metallic intermediate

In this work, a low cost solution‐based method for the deposition of uniform Cu‐In‐Ga layers compatible with roll‐to‐roll processing is described. As ink system we use metal carboxylates dissolved in a mixture of a nitrogen containing base and an alcohol. This solution can be coated homogeneously under inert atmosphere using a doctor blade technique. With this method and appropriate precursor concentrations, crack‐free metal layers with dry‐film thicknesses of more than 700 nm can be deposited in one fast step. For the controlled film formation during the drying of the solvents a flow channel has been used to improve the evaporative mass transport and the convective gas flows of any unwanted organic species. Due to the absence of organic binders with high molecular weight, this step allows the formation of virtually pure metal layers. Elementary analyses of the dried thin films reveal less than 5 wt% of carbon residues at 200°C. In situ X‐ray diffraction data of the drying step show the formation of Cu‐In‐Ga alloys. The subsequent processing of Cu(In,Ga)Se₂ chalcopyrites with evaporated elemental selenium takes place in a separate tube oven under inert atmosphere. Photoelectric measurements of cells with CdS buffer and ZnO window layer reveal a short‐circuit current of 29 mA/cm², an open‐circuit voltage of 533 mV, and a fill factor of 0.69 under standard conditions. Thus efficiencies of up to 11% on 0.5 cm² area without antireflective coating have been achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of high growth rates on evaporated Cu(In,Ga)Se2 layers and solar cells

Thin film solar cells based on polycrystalline Cu(In,Ga)Se₂ were prepared by elemental co‐evaporation using modified three‐stage processes on soda lime glass substrates at a low substrate temperature of 450°C intended for application on polyimide foils. The growth rates in the different stages of the growth process were varied, and it was observed that the final composition profile and structural quality of the film are mainly determined by the growth rate in the third stage. Application of high growth rates in the second stage was found to have no significant impact on layer morphology and gallium grading profile, which was confirmed by scanning electron microscopy, secondary ion mass spectroscopy, and x‐ray diffraction measurements. On the other hand, scanning electron microscopy cross sections revealed that high growth rates in the third stage lead to a fine‐grained structure toward the surface as well as smaller grains toward the back contact. Secondary ion mass spectroscopy and x‐ray diffraction measurements of such layers revealed a pronounced gallium grading profile, while Raman spectroscopy showed strong occurrence of group III‐rich phases in the near‐surface region. The final device performance was found to deteriorate by about 10% relative to the baseline process efficiency when growth rates of up to 500 nm min⁻¹ were applied in the second stage or 600 nm min⁻¹ in the third stage. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of the Cu(In,Ga)Se2 thickness and Ga grading on solar cell performance

Thin‐film solar cells with Cu(In,Ga)Se₂ (CIGS) absorber layers ranging from 1.8 to 0.15 μm in thickness were fabricated by co‐evaporation, with both homogeneous and Ga/(Ga + In) graded composition. The absorption of the CIGS layers was determined and compared with corresponding QE measurements in order to obtain the optical related losses. The material characterization included XRD as well as cross‐sectional SEM analysis. Devices with CIGS layers of all thicknesses were fabricated, and down to 0.8–1 μm they showed a maintained high performance (η ∼ 15%). When the CIGS layer was further reduced in thickness the loss in performance increased. The main loss was observed for the short‐circuit current, although the loss was not only due to a reduced absorbance. The open‐circuit voltage was essentially not affected by the reduction of the CIGS thickness, while the fill factor showed a slight decrease. The fill factor loss was eliminated by introducing a Ga/(Ga+In) graded CIGS, which also resulted in an increased open‐circuit voltage of 20–30 mV for all CIGS thicknesses. Device results of 16.1% efficiency at 1.8 μm CIGS thickness, 15.0% at 1.0 μm and 12.1% at 0.6 μm (total area without anti‐reflective coating) were achieved. Copyright © 2002 John Wiley & Sons, Ltd.     

Spray‐ILGAR indium sulfide buffers for Cu(In,Ga)(S,Se)2 solar cells

Thin‐film indium sulfide buffer layers have been prepared using the Spray‐ILGAR technique for use in chalcopyrite solar cells. Buffers deposited on commercially grown Cu(In,Ga)(S,Se)₂ absorbers have produced cells reaching a certified efficiency of 14·7% and average efficiencies matching the reference solar cells prepared with a conventional cadmium sulfide buffer layer. The process parameters have been optimized and the resulting cells have been studied using current–voltage and temperature–illumination‐dependent current–voltage analysis as well as quantum efficiency measurements. Copyright © 2005 John Wiley & Sons, Ltd.     

Molecular Beam Epitaxy Deposition of In Situ O-Doped CdS Films for Highly Efficient Sb2(S,Se)3 Solar Cells

Antimony selenosulfide (Sb₂(S,Se)₃) is considered as a promising light-harvesting material and has been widely used in solar cells. For high-efficiency Sb₂(S,Se)₃ solar cells, the most commonly used electron-transporting layer of cadmium sulfide (CdS) is generally prepared by chemical bath deposition (CBD) approach. However, the hazardous waste liquid from the chemical bath and the sensitivity of the deposition process to the environment are challenges to practical applications. Herein, a molecular beam epitaxy deposition is reported to prepare CdS films, overcoming the drawbacks of CBD process. Furthermore, through introducing oxygen during the deposition of CdS, the sulfur vacancy defects generated in the vacuum deposition process are suppressed. The performance of Sb₂(S,Se)₃ solar cells is accordingly improved significantly. This improvement is attributed to the following aspects: i) the improved optical transmittance of CdS films. ii) The enhanced [hk1] orientation of Sb₂(S,Se)₃ absorber layer. iii) The improved heterojunction quality and suppressed carrier recombination. As a result, a power conversion efficiency of 8.59% for Sb₂(S,Se)₃ solar cells is achieved. This study provides a novel strategy for preparing electron-transporting layers for efficient chalcogenide thin-film solar cells and sheds new light on large-area solar cell applications.     

World‐record Cu(In,Ga)Se2‐based thin‐film sub‐module with 17.4% efficiency

We report a new certified world‐record efficiency for thin‐film Cu(In,Ga)Se₂‐based photovoltaic sub‐modules of 17.4% (aperture area). The record efficiency of the 16 cm², monolithically integrated, sub‐module has been independently confirmed by Fraunhofer ISE. The record device is the result of extensive co‐optimization of all processing steps. During the optimization process, strong focus has been put on the scalability of processes to cost‐effective mass production, as reflected, for example, in Cu(In,Ga)Se₂ deposition time and substrate temperature. Device manufacturing as well as results of electrical and material characterization is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact of oxygen concentration during the deposition of window layers on lowering the metastability effects in Cu(In,Ga)Se2/CBD Zn(S,O) based solar cell

The purpose of the present paper is to focus on the impact of oxygen gas partial pressure during the sputtering of i‐ZnO and ZnMgO on the transient behavior of Cu(In,Ga)Se₂ (CIGSe) based solar cells parameters when a CBD‐Zn(S,O) buffer layer is used. Based on electrical characterization of cells, it is observed that the effect of light soaking is different on J–V characteristics depending on whether oxygen is or is not present during the first deposition time of the i‐ZnO or ZnMgO layers. In fact, when cells are prepared with standard i‐ZnO, the efficiencies are very low and a pronounced transient behavior is observed. However, when the first 10 nm of i‐ZnO or ZnMgO is formed by sputtered layer without adding oxygen during the process, depending on the thickness of the buffer layer, the transient effects strongly decreases. It is then possible to get stable cells reaching efficiencies quite similar to the CdS reference cells, especially with ZnMgO, without any post‐treatments. Copyright © 2015 John Wiley & Sons, Ltd.