CIGSSe Solar Cells Research Articles

High-Performance Sub-Micron Cigsse Solar Cells Optimized for Sodium Doping by Adjusting Diffusion Barriers

High-Performance Sub-Micron Cigsse Solar Cells Optimized for Sodium Doping by Adjusting Diffusion Barriers

Impact of the Buffer/Absorber Interface on the Metastability of Fill Factor Temperature Coefficients in CIGSSe Solar Cells

AbstractThe metastable behavior of the fill factor temperature coefficient (βFF,rel) in Cu(In,Ga)(S,Se)2 solar cells is investigated for absorbers with a different Ga content in the front surface, and for different buffer materials. The buffer/absorber interface region is suggested to be the location which causes the metastability in βFF,rel. Temperature dependent current–voltage curves, light‐biased external quantum efficiency, and low temperature capacitance–voltage measurements are performed to characterize the electrical properties and metastable defects in the investigated solar cell devices. The presence of Ga in the absorber front surface, together with the buffer layer material plays a significant role in the metastable behavior. Additionally, energy dispersive X‐ray spectroscopy indicates that the InxSy buffer layers allow for elemental interdiffusion from absorbers with a Ga‐richer front surface, whereas a Zn(O,S) buffer shows to be rather resilient to atomic diffusion. The elemental interdiffusion between the buffer and absorber layers is proposed to be the cause of the defects causing the observed βFF,rel metastability.

Temperature coefficient characterization of CIGSSe solar cells with layer modifications

One of the advantages of Cu(In,Ga)(S,Se)2 (CIGSSe) thin film solar cells is the lower temperature sensitivity of the solar cell parameters as compared to conventional silicon-based technologies. To further improve and to tailor CIGSSe temperature behavior for different environments, it is important to gain a detailed understanding of the microscopic mechanisms involved.The goal of the current study is to elucidate the impact of the individual functional layers within the CIGSSe solar cell stack on the temperature dependence of the overall cell performance. For this purpose, temperature-dependent current-voltage measurements were performed on a selected set of CIGSSe cells, containing a systematic variation of functional layers.The experimental results demonstrate that the absorber layer exhibits the largest influence on the temperature dependence of the power efficiency. An analysis of the contributing parameters revealed that the temperature-dependence of the power efficiency is mainly governed by a change in the open-circuit voltage. The temperature coefficient of the open-circuit voltage is analyzed in terms of the cell's physical parameters, including their minimum band gap and the temperature-dependence of the dark saturation current.

Enhancement of efficiency and external quantum efficiency of CIGSSe solar cell by replacement and inserting buffer and Cu2O ER-HTL layer

In this article, the performance of CIGSSe solar cells have been analyzed in two stages. Firstly, a conventional structure MgF2/ZnO:B/CdS/CIGSSe/Mo has been simulated based on experimental data to validate the results. Three toxic-free, buffer layer materials ZnSnO, ZnS and ZnMnO have been used in the conventional structure. Secondly, the recombination loss has been reduced and external quantum efficiency (EQE) has been increased by adding p-type cuprous oxide (p-Cu2O) layer at back surface field (BSF) contact. BSF acts as an electron reflected-hole transport layer (ER-HTL) in a conventional structure and proposed CIGSSe solar cell. The efficiency and EQE of the proposed Type-II solar cells have been increased by 5.94% and 2% respectively by optimizing cell parameters in terms of thickness, energy bandgap and doping concentration. It is shown that the proposed solar cell with ZnMnO as buffer layer achieves a conversion efficiency of 28.86%.

Interfacial modification mechanism by aging effect for high-performance Cd-free and all-dry process Cu(In,Ga)(S,Se)2 solar cells

Modification of the hetero-junction interface in Cd-free and all-dry process Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells by introducing the aging process for the bare CIGSSe films is demonstrated, ultimately achieving the highest conversion efficiency of 22.0%. In this article, two positive effects of the aging process are discussed. First, additional phases, such as Inx(O,S)y and Cu-deficient phases, are naturally formed on the CIGSSe surface. Second, unintentional segregation of Zn-rich particles at the (Zn,Mg)O/CIGSSe interface during the (Zn,Mg)O deposition is suppressed. These effects lead to the reduction of recombination at the hetero-junction interface and dramatic enhancement of performance for the Cd-free and all-dry process CIGSSe solar cells.

Boosting Solar Cell Performance via Centrally Localized Ag in Solution-Processed Cu(In,Ga)(S,Se)2 Thin Film Solar Cells

Fabrication of Cu(In,Ga)(S,Se)2 (CIGSSe) absorber films from environmentally friendly solutions under ambient air conditions for use in solar cells has shown promise for the low-cost mass production of CIGSSe solar cells. However, the limited power conversion efficiency (PCE) of these solar cells compared with their vacuum-processed counterparts has been a critical setback to their practical applications. This study aims to fabricate solution-processed CIGSSe solar cells with high PCEs by incorporation of Ag into the precursor layer of the CIGSSe absorber films. The results showed that Ag doping promoted grain growth by accelerating Se uptake irrespective of location within the CIGSSe film. Nevertheless, uniform Ag doping formed crevices that lowered the PCE of the cells, while centrally localizing the doped Ag prevented the formation of crevices, resulting in high PCEs up to 15.3%. Our results demonstrate that carefully doping Ag into a selected area of the precursor layer of CIGSSe films can realize solution-processed chalcopyrite solar cells with high PCE.

Transparent Electrode and Buffer Layer Combination for Reducing Carrier Recombination and Optical Loss Realizing over a 22%-Efficient Cd-Free Alkaline-Treated Cu(In,Ga)(S,Se)2 Solar Cell by the All-Dry Process.

The structures of K or Cs alkaline-treated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are developed, and their carrier recombination rates are scrutinized. It is determined that short-circuit current density (JSC) is enhanced (decreased optical loss), when ZnS(O,OH), (Cd,Zn)S, and Zn0.8Mg0.2O buffers with a large band gap energy (Eg) are applied as a replacement of CdS buffer. The JSC is further increased, reducing the optical loss more, when Zn0.9Mg0.1O:B is used as the transparent conductive oxide (TCO) with a larger Eg and lower free carrier absorption than those of ZnO:Al. Furthermore, all carrier recombination rates throughout the devices with K or Cs treatment, especially at the buffer/absorber interface and in the quasi neutral region, are reduced, thereby reducing open-circuit voltage deficit (VOC,def), well consistent with the simulated ones. The carrier recombination rate at the buffer/absorber interface is further decreased, when the CdS and (Cd,Zn)S buffers, deposited by chemical bath deposition, are applied, leading to the greater reduction of the VOC,def and the high conversion efficiency (η) of about 21%. Under the trade-off between VOC,def and optical loss, the highest η of 22.6% is attained with the lowest power loss (or the highest VOC × JSC) in the Cs-treated Cd-free CIGSSe solar cell with an optimized structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:B, fabricated by the all-dry process, where the Zn0.8Mg0.2O buffer is prepared by the sputtering method. This occurs because the JSC is the highest attributable to the larger Eg of Zn0.8Mg0.2O buffer than those of the CdS and (Cd,Zn)S.

Effect of Alkali Treatment on Photovoltaic Performances of Cu(In,Ga)(S,Se)2 Solar Cells and Their Absorber Quality Analyzed by Urbach Energy and Carrier Recombination Rates

Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are fabricated by all-dry process, where CIGSSe absorbers with and without alkali (K and Cs) treatments are used. Urbach energy (EU) is estimated in long-wavelength edge of external quantum efficiency. It is revealed that reduced EU through K and Cs treatments yields decreased open-circuit voltage deficit. Carrier recombination rates across devices are reduced through K and Cs treatments, corresponding to increased carrier lifetimes and decreased EU. Conversion efficiency of Cd-free CIGSSe solar cells with a structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:Al is enhanced from about 17.8% (none alkali) to 18.5% (with K) and to 20.3% (with Cs).

Sulfur-Alloying Effects on Cu(In,Ga)(S,Se)2 Solar Cell Fabricated Using Aqueous Spray Pyrolysis.

We fabricated Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells using aqueous spray based deposition, which is inexpensive and covers a large area. To apply the sprayed film to a photoabsorber of a solar cell, post-sulfo-selenization was carried out. Through the sulfo-selenization process, we were able to fabricate various S-alloyed CIGSSe films from S/(S + Se) = 0 (S-0.0) to S/(S + Se) = 0.4 (S-0.4). CIGSSe solar cells were made with the S-alloyed CIGSSe absorbers. Power conversion efficiency of CIGSSe solar cell was found to be increased with S-alloying up to S-0.3, and the best efficiency of 10.89% was obtained with the S-0.3 CIGSSe absorber. Comparison study of S-alloyed CIGSSe solar cells showed that enhanced efficiency in S-0.3 solar cell is due to the increased open-circuit voltage and an improved fill factor, which is induced by S-alloying. In addition, admittance spectroscopy revealed that the defect density of the deep level was developed in the S-alloyed S-0.3 CIGSSe absorber. However, the defect density was observed to be rather reduced. Details of characterization and analysis results are discussed in this paper.

Aging Effect of a Cu(In,Ga)(S,Se)2 Absorber on the Photovoltaic Performance of Its Cd‐Free Solar Cell Fabricated by an All‐Dry Process: Its Carrier Recombination Analysis

AbstractCd‐free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are fabricated by an all‐dry process (a Cd‐free and all‐dry process CIGSSe solar cell) with aged CIGSSe thin film absorbers. The aged CIGSSe thin films are kept in a desiccator cabinet under partial pressure of oxygen of ≈200 Pa for aging time up to 10 months. It is reported for the first time that aged CIGSSe thin film with increased aging time results in significant enhancement of photovoltaic performance of Cd‐free and all‐dry process CIGSSe solar cells, regardless of the alkali treatment. Based on carrier recombination analysis, carrier recombination rates at the interface and in the depletion region of the Cd‐free and all‐dry process CIGSSe solar cells are reduced owing to avoidance of sputtering damage on CIGSSe absorber surface, which is consistent with the strong electron beam‐induced current signal near CIGSSe surface after the increased aging time. It is implied that the interface and near‐surface qualities are clearly improved through the increased aging time, which is attributable to the self‐forming of Inx(O,S)y near CIGSSe surface, which acts as a buffer layer. Ultimately, the 22.0%‐efficient Cd‐free CIGSSe solar cell fabricated by all‐dry process is achieved with the aged Cs‐treated CIGSSe absorber with the aging time of 10 months.

22%‐efficient Cd‐free Cu(In,Ga)(S,Se)2 solar cell by all‐dry process using Zn0.8Mg0.2O and Zn0.9Mg0.1O:B as buffer and transparent conductive oxide layers

AbstractCd‐free Cu(In,Ga)(S,Se)2 (CIGSSe)‐based solar cell fabricated by all‐dry process is developed to eliminate optical loss in CdS buffer and be readily applied into in‐line process. (Zn,Mg)O:B (BZMO) films can be deposited with the increased band‐gap energy (Eg) by metal organic chemical vapor deposition, and their optical and electrical properties are compared with those of (Zn,Mg)O:Al (AZMO) films prepared by sputtering method. It is shown that the AZMO and BZMO are suitable as transparent conductive oxide (TCO) layer to avoid the optical loss at short wavelength of the solar cell. The free‐carrier absorption of the BZMO films is additionally lower than that of the AZMO, attributable to the lower carrier concentration. The low resistivity of the BZMO films with Mg content up to 0.1 in a range of 3 × 10−2 to 6 × 10−2 Ω.cm is observed. The CIGSSe solar cells are then fabricated with the Cs‐treated CIGSSe layers as the absorbers. Consequently, the CIGSSe solar cell with the conventional CdS buffer layer possesses the conversion efficiency (η) of 21.7%. In addition, Cd‐free CIGSSe solar with a structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:B has the increased short‐circuit current density to 39.8 mA/cm2, thereby enhancing the η to 22.0%. This is because there is the reduced free‐carrier absorption and increased Eg (3.57 eV) of Zn0.9Mg0.1O:B layer, which is proved for the high‐level technology. The recombination rates of the solar cells are moreover discussed.

Structures of Cu(In,Ga)(S,Se)2 solar cells for minimizing open‐circuit voltage deficit: Investigation of carrier recombination rates

AbstractCu(In,Ga)(S,Se)2 (CIGSSe) solar cell structures are designed to reduce open‐circuit voltage deficit (VOC,def) for the enhancement of power conversion efficiency (η) through the optimizations of conduction band offsets not only between the buffer and absorber (ΔEC‐BA) but also between the transparent conductive oxide and absorber (ΔEC‐TA). Voltage‐independent carrier recombination rates at buffer/absorber interface (Ri0), in space‐charge region (Rd0), and in quasi‐neutral region (Rb0) of the CIGSSe solar cells with different structures are separately extracted. Consequently, the development of device structures with similar CIGSSe quality leads to the primary reduction of Ri0, while Rd0 and Rb0 are not varied very much, thus minimizing the VOC,def. It is disclosed that the solar cell structure of CIGSSe/Cd0.75Zn0.25S/Zn0.8Mg0.2O/Zn0.9Mg0.1O:Al together with K‐treated CIGSSe absorber is appropriate with the ΔEC‐BA of 0.31 eV and ΔEC‐TA of 0.37 eV, thereby significantly decreasing Ri0 and VOC,def to approximately 1.1 × 104 cm−2 s−1 and 0.374 V, respectively, and increasing the η to 21.1%.

Characteristics of Zn1−xMgxO:B and its application as transparent conductive oxide layer in Cu(In,Ga)(S,Se)2 solar cells with and without CdS buffer layer

Zn1−xMgxO:B (BZMO) films were deposited by metal organic chemical vapor deposition (MOCVD). Their optical and electrical properties were investigated for suitability as transparent conductive oxide (TCO) of the Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells. It is disclosed that with the increase in the [Mg]/([Mg] + [Zn]) of the BZMO up to 0.12 the optical band-gap energy (Eg) is increased to 3.52 eV, and the resistivity is reduced to about 3.1 × 10−3 Ω·cm owing to the increase in the Hall mobiltity. Moreover, the severe free-carrier absorption in the BZMO film is not observed. The BZMO is consequently appropriate as the TCO layer of the CIGSSe solar cells. The K-treated CIGSSe layers were applied as the absorbers of the solar cells. As a result, the 21.1%-efficient CIGSSe solar cell with a structure of Zn0.88Mg0.12O:B/CdS/CIGSSe/Mo/glass is obtained. In addition, the 20.2%-efficient Cd-free CIGSSe solar with a structure of Zn0.88Mg0.12O:B/Zn0.80Mg0.20O/CIGSSe/Mo/glass, fabricated by all-dry process, is realized with high short-circuit current density (JSC) of 39.7 mA/cm2. The high JSC is attributable to no optical loss in CdS buffer layer, no severe free carrier absorption, and the increased Eg of the BZMO layer.

Impact of Buffer Layer Process and Na on Shunt Paths of Monolithic Series-connected CIGSSe Thin Film Solar Cells

The illuminated current-voltage characteristics of Cu(In,Ga)(S,Se)2 (CIGSSe) thin film solar cells fabricated using two different buffer layer processes: chemical bath deposition (CBD) and atomic layer deposition (ALD) were investigated. The CIGSSe solar cell with the ALD buffer showed comparable conversion efficiency to the CIGSSe solar cell with CBD buffer but lower shunt resistance even though it showed lower point shunt defect density as measured in electroluminescence. The shunt paths were investigated in detail by capturing the high-resolution dark lock-in thermography images, resolving the shunt resistance contributions of the scribing patterns (P1, P3), and depth profiling of the constituent elements. It was found that the concentration of Na from the soda-lime glass substrate played a key role in controlling the shunt paths. In the ALD process, Na segregated at the surface of CIGSSe and contributed to the increase in the shunt current through P1 and P3, resulting in a reduction in the fill factor of the CIGSSe solar cells.

Improving Ga distribution and efficiency of flexible Cu(In,Ga)(S,Se) solar cell using CuGa:Na target route

In this work, we proposed a novel CuGa-NaF (CuGa:Na) target route to improve Ga distribution and device efficiency of flexible Cu(In,Ga)(S,Se) (CIGSSe) solar cells. Analysis involved characterizing the basic material properties of two absorber layers fabricated using sputtering targets of CuGa:Na or Mo: Na. We assessed the uniformity of Ga and Na distribution in the absorber layers. We also examined the effects of various sputtering parameters and sought to optimize the Na content. Finally, we compared the conversion efficiency of the solar cells fabricated using the two Na addition routes (targets of CuGa:Na or Mo:Na target) with the following structure: metal foil/Cr/Mo/CIGSSe/CdS/i-ZnO/ITO/Al. Experimental results show that the CuGa:Na target improved the Ga distribution on the surface of the CIGSSe absorber compared to the Mo:Na target. When applied at a CuGa:Na/In thickness ratio of 300 nm/300 nm, the conversion efficiency of the two devices was as follows: CIGSSe solar cell with Mo: Na-derived absorber layer (7.5%) and CIGSSe solar cell with CuGa: Na-derived absorber layer (9.81%).

A novel two-stage hybrid processing technique towards industrial manufacturing of the Cu(In,Ga)(S,Se)2 solar cell with materially efficient fabrication

The hybrid two-stage processing technique achieved the highly efficient CIGSSe solar cell with high material-utilization and a simple process for commercialization.

Influence of potassium treatment on electronic properties of Cu(In1−xGax)(Se1−ySy)2 solar cells studied by steady state photo-capacitance and admittance spectroscopy

To clarify the mechanism of efficiency enhancement effect by potassium (K) treatment for Cu(In1−xGax)(Se1−ySy)2 (CIGSSe) solar cells, we investigated the physical properties of deep and shallow defect levels of CIGSSe solar cells without or with potassium fluoride treatment via using steady state photo-capacitance and admittance spectroscopy. The results show that the defect density of deep levels (such as 0.8 and 0.7 eV above valance band) are similar, suggesting that the K treatment does not drastically change the deep defect properties. For the shallow levels, the activation energy of interface-related shallow defect decreased from 183 to 120 meV upon K treatment, suggesting the modification of interface. The modified interface may be caused by the diffusion of potassium to grain boundaries and the formation of Cu-depleted layer in the near surface region of CIGSSe, resulting in the prolonged minority carrier lifetime. A new level with activation energy of 250 meV was found after K treatment, and it contributed to the increase of carrier density. These features well explained the enhanced device performance of CIGSSe solar cells by K treatment in terms of defects.

Investigation of correlation between open-circuit voltage deficit and carrier recombination rates in Cu(In,Ga)(S,Se)2-based thin-film solar cells

The temperature-illumination-dependent open-circuit voltage (VOC) method is utilized to separately and quantitatively estimate carrier recombination rates at the buffer/absorber interface, in the space-charge region (SCR), and in the quasi-neutral region (QNR) of Cu(In,Ga)(S,Se)2 (CIGSSe)-based thin-film solar cells with various device structures. The correlation between open-circuit voltage deficits (VOC,def) among the carrier recombination rates of the CIGSSe solar cells with a conversion efficiency (η) above 17% is examined. It is revealed that VOC,def is decreased to 0.373 V with the reduced carrier recombination rate at the buffer/absorber interface through the development of device structures. To further decrease VOC,def (for the improved η), the carrier recombination rates in SCR and QNR are essential to be reduced by the further improvement of CIGSSe quality. Consequently, understanding the quantitative carrier recombination rates across the device, estimated from the temperature-illumination-dependent VOC method, is practical to know which part of the solar cell needs to be developed for high η above 20%.

20% Efficient Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/Cu(In,Ga)(S,Se)2 Solar Cell Prepared by All-Dry Process through a Combination of Heat-Light-Soaking and Light-Soaking Processes.

Development of Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe)-based thin-film solar cells fabricated by an all-dry process is intriguing to minimize optical loss at a wavelength shorter than 520 nm owing to absorption of the CdS buffer layer and to be easily integrated into an in-line process for cost reduction. Cd-free CIGSSe solar cells are therefore prepared by the all-dry process with a structure of Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CIGSSe/Mo/glass. It is demonstrated that Zn0.8Mg0.2O and Zn0.9Mg0.1O:Al are appropriate as buffer and transparent conductive oxide layers with large optical band gap energy values of 3.75 and 3.80 eV, respectively. The conversion efficiency (η) of the Cd-free CIGSSe solar cell without K-treatment is consequently increased to 18.1%. To further increase the η, the Cd-free CIGSSe solar cell with K-treatment is next fabricated and followed by posttreatment called the heat-light-soaking (HLS) + light-soaking (LS) process, including HLS at 110 °C followed by LS under AM 1.5G illumination. It is disclosed that the HLS + LS process gives rise to not only the enhancement of carrier density but also the decrease in the carrier recombination rate at the buffer/absorber interface. Ultimately, the η of the Cd-free CIGSSe solar cell with K-treatment prepared by the all-dry process is enhanced to the level of 20.0%.

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

AbstractA 19.5%‐efficient Cu(In,Ga)(S,Se)2 (CIGSSe)‐based solar cell is obtained by replacing traditional CdS/ZnO buffer layers with Cd0.75Zn0.25S/Zn0.79Mg0.21O buffer layers for increasing short‐circuit current density because band‐gap energies of Cd0.75Zn0.25S and Zn0.79Mg0.21O 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 (EC) between Zn1 − xMgxO:Al (transparent conductive oxide, TCO) layer and CIGSSe absorber is optimized by varying [Mg]/([Mg] + [Zn]), x. It is revealed that Zn1 − xMgxO:Al films with [Mg]/([Mg] + [Zn]) in a range of 0.10 to 0.12, enhancing Eg 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 Zn0.88Mg0.12O: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 EC difference between TCO and absorber to minimize surface carrier recombination. Ultimately, conversion efficiency (η) of CIGSSe solar cell with alternative Cd0.75Zn0.25S/Zn0.79Mg0.21O/Zn0.88Mg0.12O:Al (TCO) layers is enhanced to 20.6%, owing to control of total EC alignment, which is higher η up to 12.6% relative as compared with the solar cell with traditional CdS/ZnO/ZnO:Al layers.