Atomic layer deposition of Zn1−xMgxO buffer layers for Cu(In,Ga)Se2 solar cells

Fabrication of Zn_1−xMg_xO films by atomic layer deposition (ALD) has been studied for use as buffer layers in Cu(In,Ga)Se₂ (CIGS)‐based solar cell devices. The Zn_1−xMg_xO films were grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in the temperature range from 105 to 180°C. Single‐phase ZnO‐like films were obtained for x < 0·2, followed by a two phase region of ZnO‐ and MgO‐like structures for higher Mg concentrations. Increasing optical band gaps of up to above 3·8 eV were obtained for Zn_1−xMg_xO with increasing x. It was found that the composition of the Zn_1−xMg_xO films varied as an effect of deposition temperature as well as by increasing the relative amount of magnesium precursor pulses during film growth. Completely Cd‐free CIGS‐based solar cells devices with ALD‐Zn_1−xMg_xO buffer layers were fabricated and showed efficiencies of up to 14·1%, which was higher than that of the CdS references. Copyright © 2006 John Wiley & Sons, Ltd.     

Thin‐film Cu(In,Ga)(Se,S)2‐based solar cell with (Cd,Zn)S buffer layer and Zn1−xMgxO window layer

(Cd,Zn)S buffer layer and Zn_1−x Mg_x O window layer were investigated to replace the traditional CdS buffer layer and ZnO window layer in Cu(In,Ga)(Se,S)₂ (CIGSSe)‐based solar cell. (Cd,Zn)S with band‐gap energy (E _g) of approximately 2.6 eV was prepared by chemical bath deposition, and Zn_1−x Mg_x O films with different [Mg]/([Mg] + [Zn]) ratios, x , were deposited by radio frequency magnetron co‐sputtering of ZnO and MgO. The estimated optical E _g of Zn_1−x Mg_x O films is linearly enhanced from 3.3 eV for pure ZnO (x  = 0) to 4.1 eV for Zn_0.6Mg_0.4O (x  = 0.4). The quality of the Zn_1−x Mg_x O films, implied by Urbach energy, is severely deteriorated when x is above 0.211. Moreover, the temperature‐dependent current density‐voltage characteristics of the CIGSSe solar cells were conducted for the investigation of the heterointerface recombination mechanism. The external quantum efficiency of the CIGSSe solar cell with the (Cd,Zn)S buffer layer/Zn_1−x Mg_x O window layer is improved in the wavelength range of 320–520 nm. Therefore, a gain in short‐circuit current density up to about 5.7% was obtained, which is higher conversion efficiency of up to around 5.4% relative as compared with the solar cell with the traditional CdS buffer layer/ZnO window layer. The peak efficiency of 19.6% was demonstrated in CIGSSe solar cell with (Cd,Zn)S buffer layer and Zn_1−x Mg_x O window layer, where x is optimized at 0.211. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of window layer composition in Cd1−xZnxS/CdTe solar cells

To improve CdS/CdTe cell/module efficiencies, CdS window layer thinning is commonly applied despite the risk of increased pin‐hole defects and shunting. An alternative approach is to widen the band gap of the window layer (2.42 eV for CdS) via alloying, for example, by forming compositions of Cd_1−xZn_xS. In this study, the performance of Cd_1−xZn_xS/CdTe thin‐film solar cells has been studied as a function of x (from x = 0 to 0.9), widening the window layer band gap up to and over 3.4 eV. Optimum Cd_1−xZn_xS compositions were clearly identified to be around x = 0.7, and limitations to the achievable photocurrent and conversion efficiencies have been addressed. Copyright © 2012 John Wiley & Sons, Ltd.     

Temperature‐dependent current‐voltage and lightsoaking measurements on Cu(In,Ga)Se2 solar cells with ALD‐Zn1‐xMgxO buffer layers

In this paper, lightsoaking and temperature‐dependent current‐voltage (JVT) measurements on Cu(In,Ga)Se₂ solar cells with atomic layer deposited Zn_1‐xMg_xO buffer layers are presented. A range of Mg concentrations are used, from pure ZnO (x=0) to 26% Mg (x=0·26). Since this kind of solar cells exhibit strong metastable behaviour, lightsoaking is needed prior to the JVT‐measurements to enable fitting of these to the one‐diode model. The most prominent effect of lightsoaking cells with Mg‐rich buffer layers is an increased fill factor, while the effect on cells with pure ZnO buffer is mainly to increase V_oc·. The activation energy is extracted from JVT‐measurement data by applying three different methods and the ideality factors are fitted to two different models of temperature‐dependence. A buffer layer consisting either of ZnO or Zn_1‐xMg_xO with a minor Mg content gives solar cells dominated by interface recombination, which probably can be related to a negative conduction band offset. A relatively high Mg content in the buffer layer (x=0·21) leads to solar cells dominated by recombination in the space charge region. The recombination is interpreted as being tunnelling‐enhanced. The situation in between these Mg concentrations is less clear. Before lightsoaking, the sample with x=0·12 has the highest efficiency of 15·3%, while after lightsoaking the sample with x=0·21 holds the best efficiency, 16·1%, exceeding the value for the CdS reference. The J_sc values of the Zn_1‐xMg_xO cells surpass that of the reference due to the larger bandgap of Zn_1‐xMg_xO compared to CdS. Copyright © 2009 John Wiley & Sons, Ltd.     

Study of Interfacial Reactions Between Sn(Cu) Solders and Ni-Co Alloy Layers

The interfacial reactions between electroplated Ni-yCo alloy layers and Sn(Cu) solders at 250°C are studied. For pure Co layers, CoSn₃ is the only interfacial compound phase formed at the Sn(Cu)/Co interfaces regardless of the Cu concentration. Also, the addition of Cu to Sn(Cu) solders has no obvious influence on the CoSn₃ compound growth at the Sn(Cu)/Co interfaces. For Ni-63Co layers, (Co,Ni,Cu)Sn₃ is the only interfacial compound phase formed at the Sn(Cu)/Ni-63Co interfaces. Unlike in the pure Co layer cases, the Cu additives in the Sn(Cu) solders clearly suppress the growth rate of the interfacial (Co,Ni,Cu)Sn₃ compound layer. For Ni-20Co layers, the interfacial compound formation at the Sn(Cu)/Ni-20Co interfaces depends on the Cu content in the Sn(Cu) solders and the reflow time. In the case of high Cu content in the Sn(Cu) solders (Sn-0.7Cu and Sn-1.2Cu), an additional needle-like interfacial (Ni _x ,Co _y ,Cu_1−x−y )₃Sn₄ phase forms above the continuous (Ni _x ,Cu _y ,Co_1−x−y )Sn₂ compound layer. The Ni content in the Ni-yCo layer can indeed reduce the interfacial compound formation at the Sn(Cu)/Ni-yCo interfaces. With pure Sn solders, the thickness of the compound layer monotonically decreases with the Ni content in the Ni-yCo layer. As for reactions with the Sn(Cu) solders, as the compound thickness decreases, the Ni content in the Ni-yCo layers increases.     

Electronic structure of Ge1 − x − ySixSny ternary alloys for multijunction solar cells

Ternary group‐IV alloys have a wide potential for applications in infrared devices and optoelectronics. In connection with photovoltaic applications, they are among the most promising materials for inclusion in the next generation of high‐efficiency multijunction solar cells, because they can be lattice matched to substrates as GaAs and Ge, offering the possibility of a range of band gaps complementary to III–V semiconductors. Apart from the full decoupling of lattice and band structures in Ge_{1 − x − y}Si_xSn_y alloys, experimentally confirmed, they allow preparation in a controllable and large range of compositions, thus enabling to tune their band gap. Recently, optical experiments on ternary alloy‐based films, photodetectors measured the direct absorption edges and probed the compositional dependence of the direct gap. The nature of the fundamental gap of Ge_{1 − x − y}Si_xSn_y alloys is still unknown, as neither experimental data on the indirect edges nor electronic structure calculations are available, as yet. Here, we report a first calculation of the electronic structure of Ge_{1 − x − y}Si_xSn_y ternary alloys, employing a combined tight‐binding and virtual crystal approximation method, which proved to be useful to describe group‐IV semiconductor binary alloys. Our results confirm predictions and experimental indications that a 1eV band gap is indeed attainable with these ternary alloys, as required for the fourth layer plan to be added to present‐day record‐efficiency triple‐junction solar cells, to further increase their efficiency, for example, for satellite applications. When lattice matched to Ge, we find that Ge_{1 − x − y}Si_xSn_y ternary alloys have an indirect gap with a compositional dependence reflecting the presence of two competing minima in the conduction band. Copyright © 2013 John Wiley & Sons, Ltd.     

The Zn(S,O,OH)/ZnMgO buffer in thin‐film Cu(In,Ga)(Se,S)2‐based solar cells part II: Magnetron sputtering of the ZnMgO buffer layer for in‐line co‐evaporated Cu(In,Ga)Se2 solar cells

A ZnS/Zn_1‐xMg_xO buffer combination was developed to replace the CdS/i‐ZnO layers in in‐line co‐evaporated Cu(In,Ga)Se₂(CIGS)‐based solar cells. The ZnS was deposited by the chemical bath deposition (CBD) technique and the Zn_1‐xMg_xO layer by RF magnetron sputtering from ceramic targets. The [Mg]/([Mg] + [Zn]) ratio in the target was varied between x = 0·0 and 0·4. The composition, the crystal structure, and the optical properties of the resulting layers were analyzed. Small laboratory cells and 10 × 10 cm² modules were realized with high reproducibility and enhanced stability. The transmission is improved in the wavelength region between 330 and 550 nm for the ZnS/Zn_1‐xMg_xO layers. Therefore, a large gain in the short‐circuit current density up to 12% was obtained, which resulted in higher conversion efficiencies up to 9% relative as compared to cells with the CdS/i‐ZnO buffer system. Peak efficiencies of 18% with small laboratory cells and 15·2% with 10 × 10 cm² mini‐modules were demonstrated. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of Cross-Interaction between Ni and Cu on Growth Kinetics of Intermetallic Compounds in Ni/Sn/Cu Diffusion Couples during Aging

The solid-state, cross-interaction between the Ni layer on the component side and the Cu pad on the printed circuit board (PCB) side in ball grid array (BGA) solder joints was investigated by employing Ni(15 μm)/Sn(65 μm)/Cu ternary diffusion couples. The ternary diffusion couples were prepared by sequentially electroplating Sn and Ni on a Cu foil and were aged isothermally at 150, 180, and 200°C. The growth of the intermetallic compound (IMC) layer on the Ni side was coupled with that on the Cu side by the mass flux across the Sn layer that was caused by the difference in the Ni content between the (Cu_1−x Ni _x )₆Sn₅ layer on the Ni side and the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side. As the consequence of the coupling, the growth rate of the (Cu_1−x Ni _x )₆ Sn₅ layer on the Ni side was rapidly accelerated by decreasing Sn layer thickness and increasing aging temperature. Owing to the cross-interaction with the top Ni layer, the growth rate of the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side was accelerated at 150°C and 180°C but was retarded at 200°C, while the growth rate of the Cu₃Sn layer was always retarded. The growth kinetic model proposed in an attempt to interpret the experimental results was able to reproduce qualitatively all of the important experimental observations pertaining to the growth of the IMC layers in the Ni/Sn/Cu diffusion couple.     

Gradient Conduction Band Energy Engineering Driven High-Efficiency Solution-Processed Cu2ZnSn(S,Se)4/ZnxCd1–x S Solar Cells

The photovoltaic performance of the environmentally friendly Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is lower than its predecessor Cu(In,Ga)Se₂ solar cells. Severe carrier recombination at the CZTSSe/CdS interface is one major reason that results in a large open-circuit voltage loss. Doping zinc into CdS is a feasible strategy to modifying the CdS buffer layer film, but the present methods are not satisfactory. In this study, novel zinc incorporation strategy is developed to deposit a gradient composition ternary Zn_xCd_1–xS buffer layer for optimizing the heterojunction interface. The application of gradient composition Zn_xCd_1–xS buffer layer constructs a gradient conduction band energy configuration in the CZTSSe/buffer layer interface, which highly reduces the interface recombination. The suppressed interface recombination contributes to the enhanced open circuit voltage and device performance. Consequently, the CZTSSe solar cell based on gradient composition Zn_xCd_1–xS buffer layers achieves champion efficiency of 12.35% with V_OC of 504.81 mV, J_SC of 36.90 mA cm⁻², and FF of 66.28%. It is worth noting that flammable and the toxic hydrazine solvent are replaced by the safe and low-toxic 2-methoxyethanol, making it more promising for the future commercialization of CZTSSe solar cells.     

Aluminum‐doped Zn1 − xMgxO as transparent conductive oxide of Cu(In,Ga)(S,Se)2‐based solar cell for minimizing surface carrier recombination

A 19.5%‐efficient Cu(In,Ga)(S,Se)₂ (CIGSSe)‐based solar cell is obtained by replacing traditional CdS/ZnO buffer layers with Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O buffer layers for increasing short‐circuit current density because band‐gap energies of Cd_0.75Zn_0.25S and Zn_0.79Mg_0.21O are wider than those of CdS and ZnO, respectively. This yields the increase in external quantum efficiency in a short wavelength range of approximately 320 to 550 nm. Moreover, difference of conduction band minimum (E _C) between Zn_1 − x Mg_x O:Al (transparent conductive oxide, TCO) layer and CIGSSe absorber is optimized by varying [Mg]/([Mg] + [Zn]), x . It is revealed that Zn_1 − x Mg_x O:Al films with [Mg]/([Mg] + [Zn]) in a range of 0.10 to 0.12, enhancing E _g from 3.72 to 3.76 eV, are appropriate as TCO because of their enhanced mobility and decreased carrier density. Addition of 12% Mg into ZnO:Al to form Zn_0.88Mg_0.12O:Al as TCO layer effectively decreases surface carrier recombination and improves photovoltaic parameters, especially open‐circuit voltage and fill factor. This is the first experimental proof of the concept for optimizing E _C difference between TCO and absorber to minimize surface carrier recombination. Ultimately, conversion efficiency (η ) of CIGSSe solar cell with alternative Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O/Zn_0.88Mg_0.12O:Al (TCO) layers is enhanced to 20.6%, owing to control of total E _C alignment, which is higher η up to 12.6% relative as compared with the solar cell with traditional CdS/ZnO/ZnO:Al layers.     

Growth kinetics,properties, performance,and stability of atomic layer deposition Zn–Sn–O buffer layers for Cu(In,Ga)Se2 solar cells

A new atomic layer deposition process was developed for deposition of Zn–Sn–O buffer layers for Cu(In,Ga)Se₂ solar cells with tetrakis(dimethylamino) tin, Sn(N(CH₃)₂)₄, diethyl zinc, Zn(C₂H₅)₂, and water, H₂O. The new process gives good control of thickness and [Sn]/([Sn] + [Zn]) content of the films. The Zn–Sn–O films are amorphous as found by grazing incidence X‐ray diffraction, have a high resistivity, show a lower density compared with ZnO and SnO_x, and have a transmittance loss that is smeared out over a wide wavelength interval. Good solar cell performance was achieved for a [Sn]/([Sn] + [Zn]) content determined to be 0.15–0.21 by Rutherford backscattering. The champion solar cell with a Zn–Sn–O buffer layer had an efficiency of 15.3% (V_oc = 653 mV, J_sc(QE) = 31.8 mA/cm², and FF = 73.8%) compared with 15.1% (V_oc = 663 mV, J_sc(QE) = 30.1 mA/cm², and FF = 75.8%) of the best reference solar cell with a CdS buffer layer. There is a strong light‐soaking effect that saturates after a few minutes for solar cells with Zn–Sn–O buffer layers after storage in the dark. Stability was tested by 1000 h of dry heat storage in darkness at 85 °C, where Zn–Sn–O buffer layers with a thickness of 76 nm retained their initial value after a few minutes of light soaking. Copyright © 2011 John Wiley & Sons, Ltd.     

Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

Tandem solar cell structures require a high‐performance wide band gap absorber as top cell. A possible candidate is CuGaSe₂, with a fundamental band gap of 1.7 eV. However, a significant open‐circuit voltage deficit is often reported for wide band gap chalcopyrite solar cells like CuGaSe₂. In this paper, we show that the open‐circuit voltage can be drastically improved in wide band gap p‐Cu(In,Ga)Se₂ and p‐CuGaSe₂ devices by improving the conduction band alignment to the n‐type buffer layer. This is accomplished by using Zn_1−x Sn_x O_y , grown by atomic layer deposition, as a buffer layer. In this case, the conduction band level can be adapted to an almost perfect fit to the wide band gap Cu(In,Ga)Se₂ and CuGaSe₂ materials. With an improved buffer band alignment for CuGaSe₂ absorbers, evaporated in a 3‐stage type process, we show devices exhibiting open‐circuit voltages up to 1017 mV, and efficiencies up to 11.9%. This is to the best of our knowledge the highest reported open‐circuit voltage and efficiency for a CuGaSe₂ device. Temperature‐dependent current‐voltage measurements show that the high open‐circuit voltage is explained by reduced interface recombination, which makes it possible to separate the influence of absorber quality from interface recombination in future studies.     

Formation of Ni(Ge1−xSnx) layers with solid-phase reaction in Ni/Ge1−xSnx/Ge systems

We have demonstrated the formation of Ni(Ge_1−ySn_y) layers on Ge_1−xSn_x layers by using solid-phase reaction for samples with Sn contents ranging from 2.0% to 6.5%. We have also investigated solid-phase reaction products in Ni/Ge_1−xSn_x/Ge samples after annealing and the crystalline properties of nickel-tin-germanide layer/Ge_1−xSn_x contact structures. After annealing at temperatures ranging from 350 to 550 °C, the formation of polycrystalline Ni(Ge_1−ySn_y) layers has been observed on epitaxial Ge_1−xSn_x layers with Sn contents ranging from 2.0% to 6.5%. We also observed anisotropic crystal deformation of NiGe with the incorporation of Sn atoms into substitutional sites in NiGe. In the case of the Ni/Ge_1−xSn_x/Ge sample with a Sn content of 3.6%, the formation of an epitaxial Ni₂(Ge_1−zSn_z) layer on the Ge_1−xSn_x layer was found. The formation of β-Sn crystallites was observed after annealing at above 450 °C in samples with a high Sn content of 6.5%. This β-Sn formation is due to the precipitation of Sn atoms. In all samples annealed at 350 °C, the morphology of Ni-Ge-Sn layers is smooth and uniform. However, the surface roughness and interface roughness increase for an annealing temperature of 550 °C. In particular, in the sample with a Sn content of 6.5%, the temperature at which agglomeration noticeably occurs is as low as 450 °C.     

Role of the CdS buffer layer as an active optical element in Cu(In,Ga)Se2 thin‐film solar cells

ZnO/CdS/Cu(In,Ga)Se₂ (CIGS) thin‐film heterojunction solar cells with CdS buffer layers of thicknesses between 0 and 85 nm are characterized by current–voltage, quantum efficiency, and optical reflection measurements. We investigate the correlation between the short‐circuit current density and the CdS layer thickness, focusing on the counteracting effects of light absorption and reduced optical reflection induced by the CdS layer. Both effects almost compensate each other for CdS layer thicknesses between 0 and 40 nm. Thus, an optimization of the short‐circuit current density is not achieved by omitting the CdS layer, but rather by replacing the CdS buffer with an alternative buffer material with higher bandgap energy and optical constants similar to those of CdS. Copyright © 2002 John Wiley & Sons, Ltd.     

A comparative study of defect states in evaporated and selenized CIGS(S) solar cells

Current‐voltage, admittance spectroscopy, and drive‐level capacitance profiling measurements were taken on Cu(In_1−xGa_x)(Se_1−yS_y)₂ solar cell devices. The devices were made using two different types of absorbers. One set of absorbers was deposited via physical vapor deposition, while the other set of absorbers was made by selenization of metal precursors. Additionally, each type of absorber was completed with one of two different types of buffer treatments: a CdS layer or a cadmium partial electrolyte surface modification. The devices with the evaporated absorbers had larger values of V_OC, higher carrier densities, lower densities of trapping defects, and likely shallower gap states. Results were qualitatively similar for the CdS and partial electrolyte buffers. Copyright © 2005 John Wiley & Sons, Ltd.     

Formation of Intermetallic Compounds Between Liquid Sn and Various CuNi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Metallizations

Interfacial reactions between liquid Sn and various Cu-Ni alloy metallizations as well as the subsequent phase transformations during the cooling were investigated with an emphasis on the microstructures of the reaction zones. It was found that the extent of the microstructurally complex reaction layer (during reflow at 240°C) does not depend linearly on the Ni content of the alloy metallization. On the contrary, when Cu is alloyed with Ni, the rate of thickness change of the total reaction layer first increases and reaches a maximum at a composition of about 10 at.% Ni. The reaction layer is composed of a relatively uniform continuous (Cu,Ni)₆Sn₅ reaction layer (a uniphase layer) next to the NiCu metallizations and is followed by the two-phase solidification structures between the single-phase layer and Sn matrix. The thickness of the two-phase layer, where the intermetallic tubes and fibers have grown from the continuous interfacial (Cu,Ni)₆Sn₅ layer, varies with the Ni-to-Cu ratio of the alloy metallization. In order to explain the formation mechanism of the reaction layers and their observed kinetics, the phase equilibria in the Sn-rich side of the SnCuNi system at 240°C were evaluated thermodynamically utilizing the available data, and the results of the Sn/Cu _x Ni_1−x diffusion couple experiments. With the help of the assessed data, one can also evaluate the minimum Cu content of Sn-(Ag)-Cu solder, at which (Ni,Cu)₃Sn₄ transforms into (Cu,Ni)₆Sn₅, as a function of temperature and the composition of the liquid solders.     

First-principles and experimental studies of the IR emissivity of Sn-doped ZnO

The dielectric function and IR emissivity of Zn_1?xSn_xO (x=0, 0.0625) were investigated using a first-principles ultra-soft pseudo potential approach based on density functional theory. Pure ZnO and ZnO doped with 6.25 at.% Sn were synthesized by the solid-state reaction method. The crystal structure, morphology, composition, and IR emissivity in the range 3–14 μm were characterized by various techniques including X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and spectroradiometry. The theoretical and experimental results imply that the IR emissivity of ZnO can be reduced by Sn doping.     

The Hakoniwa method,an approach to predict material properties based on statistical thermodynamics and ab initio calculations

An approach based on statistical thermodynamics and ab initio calculations to predict properties of materials composed of different types of atoms is presented. The key point of what the authors called the “Hakoniwa” method, is to take into account all possible structural supercells constructed by the fixed number of atoms of each species according to the composition of the target material. The conservation of the total number of atoms enables calculating the average value of a material property for a given temperature by applying statistical thermodynamics to the material property values obtained for each of the possible supercells. The application of the Hakoniwa method is illustrated by calculating the average energy gain by mixing Sn and Si atoms in a Ge matrix, as function of the Ge_1−x−ySn_xSi_y composition. The relative stability of each composition is compared allowing predicting the impact of Si doping on the stability of Ge_1−x−ySn_xSi_y films epitaxially grown on a Ge substrate. In addition, the average bandgap is calculated for a given Ge_1−x−ySn_xSi_y composition as a function of the temperature.     

Potential gain in photocurrent generation for Cu(In,Ga)Se2 solar cells by using In2O3 as a transparent conductive oxide layer

This study highlights the potential of atomic layer deposited In₂O₃ as a highly transparent and conductive oxide (TCO) layer in Cu(In,Ga)Se₂ (CIGSe) solar cells. It is shown that the efficiency of solar cells which use Zn‐Sn‐O (ZTO) as an alternative buffer layer can be increased by employing In₂O₃ as a TCO because of a reduction of the parasitic absorption in the window layer structure, resulting in 1.7 mA/cm² gain in short circuit current density (J_sc). In contrast, a degradation of device properties is observed if the In₂O₃ TCO is combined with the conventional CdS buffer layer. The estimated improvement for large‐scale modules is discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Highly Stable Colloidal “Giant” Quantum Dots Sensitized Solar Cells

Colloidal quantum dots (QDs) are widely studied due to their promising optoelectronic properties. This study explores the application of specially designed and synthesized “giant” core/shell CdSe/(CdS)_x QDs with variable CdS shell thickness, while keeping the core size at 1.65 nm, as a highly efficient and stable light harvester for QD sensitized solar cells (QDSCs). The comparative study demonstrates that the photovoltaic performance of QDSCs can be significantly enhanced by optimizing the CdS shell thickness. The highest photoconversion efficiency (PCE) of 3.01% is obtained at optimum CdS shell thickness ≈1.96 nm. To further improve the PCE and fully highlight the effect of core/shell QDs interface engineering, a CdSe_x S_1?x interfacial alloyed layer is introduced between CdSe core and CdS shell. The resulting alloyed CdSe/(CdSe_x S_1?x )₅/(CdS)₁ core/shell QD‐based QDSCs yield a maximum PCE of 6.86%, thanks to favorable stepwise electronic band alignment and improved electron transfer rate with the incorporation of CdSe_x S_1?x interfacial layer with respect to CdSe/(CdS)₆ core/shell. In addition, QDSCs based on “giant” core/CdS‐shell or alloyed core/shell QDs exhibit excellent long‐term stability with respect to bare CdSe‐based QDSCs. The giant core/shell QDs interface engineering methodology offers a new path to improve PCE and the long‐term stability of liquid junction QDSCs.