Passivating Grain Boundaries via Graphene Additive for Efficient Kesterite Solar Cells.

Grain boundaries (GBs)-triggered severe non-radiative recombination is recently recognized as the main culprits for carrier loss in polycrystalline kesterite photovoltaic devices. Accordingly, further optimization of kesterite-based thin film solar cells critically depends on passivating the grain interfaces of polycrystalline Cu2 ZnSn(S,Se)4 (CZTSSe) thin films. Herein, 2D material of graphene is first chosen as a passivator to improve the detrimental GBs. By adding graphene dispersion to the CZTSSe precursor solution, single-layer graphene is successfully introduced into the GBs of CZTSSe absorber. Due to the high carrier mobility and electrical conductivity of graphene, GBs in the CZTSSe films are transforming into electrically benign and do not act as high recombination sites for carrier. Consequently, benefitting from the significant passivation effect of GBs, the use of 0.05 wt% graphene additives increases the efficiency of CZTSSe solar cells from 10.40% to 12.90%, one of the highest for this type of cells. These results demonstrate a new route to further increase kesterite-based solar cell efficiency by additive engineering.

Over 16% Efficient Solution-Processed Cu(In,Ga)Se2 Solar Cells via Incorporation of Copper-Rich Precursor Film.

Solution processing of Cu(In,Ga)Se2 (CIGS) absorber is a highly promising strategy for a cost-effective CIGS photovoltaic device. However, the device performance of solution-processed CIGS solar cells is still hindered by the severe non-radiative recombination resulting from deep defects and poor crystal quality. Here, a simple and effective precursor film engineering strategy is reported, where Cu-rich (CGI >1) CIGS layer is incorporated into the bottom of the CIGS precursor film. It has been discovered that the incorporation of the Cu-rich CIGS layer greatly improves the absorber crystallinity and reduces the trap state density. Accordingly, more efficient charge generation and charge transfer are realized. As a result of systematic processing optimization, the champion solution-processed CIGS device delivers an improved open-circuit voltage of 656 mV, current density of 33.15 mA cm-2 , and fill factor of 73.78%, leading to the high efficiency of 16.05%.

Se-Assisted Performance Enhancement of Cu2ZnSn(S,Se)4 Quantum-Dot Sensitized Solar Cells via a Simple Yet Versatile Synthesis.

The earth-abundant Cu2ZnSnS4 (CZTS) quantum dots (QDs) have emerged as one potential substitute to toxic cadmium or rare indium QDs, but their application in quantum dot-sensitized solar cells (QDSSCs) is still limited by the improper particle size and the rigorous synthesis and ligand exchange conditions. Herein, we developed a one-pot hot injection method by using Tri-n-octylphosphine oxide (TOPO) as the solvent and oleylamine as the capping agent to synthesize Cu2ZnSn(S,Se)4 (CZTSSe) QDs with adjustable size and narrow size distribution. The key feature of this approach is that we can take advantage of the high-temperature nucleation, low-temperature growth, and strong reducibility of NaHB4 to prepare small-sized CZTSSe QDs without using 1-dodecanethiol (DDT) and to extend the light harvesting range through Se incorporation. After Se incorporation, it turns out that the conduction band (CB) level of CZTSSe QDs decreases, implying that the injection driving force of the electron to the CB of TiO2 films becomes weaker and a larger recombination would be induced at the TiO2/QDs/electrolyte interface. Benefiting from the broadened optoelectronic response range, the induced higher Jsc (16.80 vs 14.13 mA/cm2) finally leads to the increase of the conversion efficiency of CZTSSe QDSSC from 3.17% to 3.54% without further modification. Despite the fact that the efficiency is still far behind those of literature reported values through use of other chalcogenide sensitizers, this DDT-free approach solves the main hindrance for the application of CZTSSe QDs in QDSSCs and holds a more convenient way for ligand exchange, light absorption improvement, and particle size control.

p-type Li, Cu-codoped NiOx hole-transporting layer for efficient planar perovskite solar cells.

p-type inorganic hole transport materials of Li, Cu-codoped NiOx films were deposited using a simple solution-based process. The as-prepared films were used as hole selective contacts for lead halide perovskite solar cell. An enhanced power conversion efficiency of 14.53% has been achieved due to the improved electrical conductivity and optical transmittance of the Li, Cu-codoped NiOx electrode interlayer.

Segmented Control of Selenization Environment for High-Quality Cu2ZnSn(S,Se)4 Films Toward Efficient Kesterite Solar Cells.

High-crystalline-quality absorbers with fewer defects are crucial for further improvement of open-circuit voltage (VOC) and efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. However, the preparation of high-quality CZTSSe absorbers remains challenging due to the uncontrollability of the selenization reaction and the complexity of the required selenization environment for film growth. Herein, a novel segmented control strategy for the selenization environment, specifically targeting the evaporation area of Se, to regulate the selenization reactions and improve the absorber quality is proposed. The large evaporation area of Se in the initial stage of the selenization provides a great evaporation and diffusion flux for Se, which facilitates rapid phase transition reactions and enables the attainment of a single-layer thin film. The reduced evaporation area of Se in the later stage creates a soft-selenization environment for grain growth, effectively suppressing the loss of Sn and promoting element homogenization. Consequently, the mitigation of Sn-related deep-level defects on the surface and in the bulk induced by element imbalance is simultaneously achieved. This leads to a significant improvement in nonradiative recombination suppression and carrier collection enhancement, thereby enhancing the VOC. As a result, the CZTSSe device delivers an impressive efficiency of 13.77% with a low VOC deficit.

Synergistic Crystallization Modulation and Defects Passivation in Kesterite via Anion-Coordinate Precursor Engineering for Efficient Solar Cells.

It has been validated that enhancing crystallinity and passivating the deep-level defect are critical for improving the device performance of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Coordination chemistry interactions within the Cu-Zn-Sn-S precursor solution play a crucial role in the management of structural defects and the crystallization kinetics of CZTSSe thin films. Therefore, regulating the coordination environment of anion and cation in the precursor solution to control the formation process of precursor films is a major challenge at present. Herein, a synergetic crystallization modulation and defect passivation method is developed using P2S5 as an additive in the CZTS precursor solution to optimize the coordination structure and improve the crystallization process. The alignment of theoretical assessments with experimental observations confirms the ability of the P2S5 molecule to coordinate with the metal cation sites of CZTS precursor films, especially more liable to the Zn2+, effectively passivating the Zn-related defects, thereby significantly reducing the defect density in CZTSSe absorbers. As a result, the device with a power conversion efficiency of 14.36% has been achieved. This work provides an unprecedented strategy for fabricating high-quality thin films by anion-coordinate regulation and a novel route for realizing efficient CZTSSe solar cells.

Progress and prospectives of solution-processed kesterite absorbers for photovoltaic applications.

Solar cells based on emerging kesterite Cu2ZnSn(S,Se)4 (CZTSSe) materials have reached certified power conversion efficiency (PCE) as high as 13.6%, showing great potential in the next generation of photovoltaic technologies because of their earth-abundant, tunable direct bandgap, high optical absorption coefficient, environment-friendly, and low-cost properties. The predecessor of CZTSSe is Cu(In,Ga) Se2 (CIGS), and the highest PCE of CIGS fabricated by the vacuum method is 23.35%. However, the recorded PCE of CZTSSe devices are fabricated by a low-cost solution method. The characteristics of the solvent play a key role in determining the crystallization kinetics, crystal growth quality, and optoelectronic properties of the CZTSSe thin films in the solution method. It is still challenging to improve the efficiency of CZTSSe solar cells for future commercialization and applications. This review describes the current status of CZTSSe solar cell absorbers fabricated by protic solvents with NH (hydrazine), protic solvents with SH (amine-thiol), aprotic solvents (DMSO and DMF), ethylene glycol methyl ether-based precursor solution method (EGME), and thioglycolic acid (TGA)-ammonia solution (NH3H2O) deposition methods. Furthermore, the performances of vacuum-deposited devices and solution-based processed devices are compared. Finally, the challenges and outlooks of CZTSSe solar cells are discussed for further performance improvement.

2D Ti3C2-MXene Serving as Intermediate Layer between Absorber and Back Contact for Efficient CZTSSe Solar Cells.

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has been considered as the most promising absorber material for inorganic thin-film solar cells. Among the three main interfaces in CZTSSe-based solar cells, the CZTSSe/Mo back interface plays an essential role in hole extraction as well as device performance. During the selenization process, the reaction between CZTSSe and Mo is one of the main reasons that lead to a large open circuit voltage (VOC) deficit, low short circuit current (Jsc), and fill factor. In this study, 2D Ti3C2-MXene was introduced as an intermediate layer to optimize the interface between the CZTSSe absorber layer and Mo back contact. Benefiting from the 2D Ti3C2-MXene intermediate layer, the reaction between CZTSSe and Mo was effectually suppressed, thus, significantly reducing the thickness of the detrimental Mo(S,Se)2 layer as well as interface recombination at the CZTSSe/Mo back interface. As a result, the power conversion efficiency of the champion device fabricated with the 2D Ti3C2-MXene intermediate layer was improved from 10.89 to 13.14% (active-area efficiency). This study demonstrates the potential use of the 2D Ti3C2-MXene intermediate layer for efficient CZTSSe solar cells and promotes a deeper understanding of the back interface in CZTSSe solar cells.

Plasmonic Local Electric Field-Enhanced Interface toward High-Efficiency Cu2ZnSn(S,Se)4 Thin-Film Solar Cells.

The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have shown a continuous rise in power conversion efficiencies in the past years. However, the encountered interfacial problems with respect to charge recombination and extraction losses at the CdS/CZTSSe heterojunction still hinder their further development. In this work, an additional plasmonic local electric field is imposed into the CdS/CZTSSe interface through the electrostatic assembly of a two-dimensional (2D) ordered Au@SiO2 NP array onto an aminosilane-modified surface absorber. The interfacial electric properties are tuned by controlling the coverage particle distance, and the finite-difference time domain (FDTD) simulation demonstrates that the strong near-field enhancement mainly occurs near the p-n junction interface. It is shown that the imposed local electric field leads to interfacial electrostatic potential (Velec) augmentation and improves the charge extraction and recombination processes. These electric benefits enable remarkable improvements in open-circuit voltage (Voc) and short-circuit current (Jsc), leading to the cell efficiency being increased from 10.19 to 11.50%. This work highlights the dramatic role of the plasmonic local electric field and the use of the 2D Au@SiO2 NP array to modify a surface absorber instead of the extensively used ion passivation, providing a new strategy for p-n junction engineering in kesterite photovoltaics.

Interface Engineering for High-Efficiency Solution-Processed Cu(In,Ga)(S,Se)2 Solar Cells via a Novel Indium-Doped CdS Strategy.

Indium doping of cadmium sulfide (CdS) by chemical bath deposition (CBD) can be an efficient strategy to boost the CIGSSe efficiency. However, limited by the extremely low solubility of In2S3, it is difficult to increase the In doping contents and inhibit the band energy-level regulation for CdS through the traditional CBD process. In this work, we perform a novel CBD method to prepare an indium-doped CdS (In:CdS) buffer, in which the indium source is sequentially slowly added in the growing aqueous solution. In this process, the In ion concentration involved in the real-time deposition is significantly reduced. Thus, compact and uniform In:CdS with higher indium doping content is obtained. Indium doping can elevate the CdS conduction band edge and construct a more favorable spike band alignment with a CIGSSe absorber. Moreover, it introduces efficient carrier transport and reduced interface defect density. As a result, improved CIGSSe heterojunction quality is realized by utilizing In:CdS. Also, the solution-processed CIGSSe device with In:CdS as a buffer yields a high efficiency of 16.4%, with a high VOC of 670 mV and an FF of 75.3%.

Suppressing interface recombination in CZTSSe solar cells by simple selenization with synchronous interface gradient doping.

The main bottleneck in the development of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is their very low VOC due to severe carrier recombination. Specifically, due to the poor defect environment and unfavorable band structure, carrier recombination at the front interface is considered to be one of the most serious issues. Thus, to reduce the interface recombination and VOC deficit, we propose a convenient and effective strategy for Cd gradient doping near the front interface during selenization. The formed Cd gradient significantly reduced the CuZn defects and related [2CuZn + SnZn] defect clusters near the CZTSSe-CdS heterojunction, thus significantly suppressing the interface recombination near the heterojunction. Benefitting from the formed Cd gradient, a champion device with 12.14% PCE was achieved with the VOC significantly improved from 432 mV to 486 mV. The proposed element gradient doping strategy can offer a new idea for selenization and element gradient doping in other photoelectric devices.

Local Cu Component Engineering to Achieve Continuous Carrier Transport for Enhanced Kesterite Solar Cells.

Although the traditional Cu-poor architecture addresses many limitations for Cu2ZnSn(S,Se)4 solar cells, its further development still encounters a bottleneck in terms of efficiency, primarily arising from the inferior charge transport within the quasineutral region and enlarged recombination at back contact. On the contrary, the electrical benign kesterite compound with higher Cu content may compensate for these shortages, but it will degrade device performance more pronouncedly at front contact because of the Fermi level pinning and more electric shunts. Based on the electric disparities on their independent side, in this work, we propose a new status of Cu component by exploring a large grain/fine grain/large grain trilayer architecture with higher Cu content near back contact and lower Cu content near front contact. The benefits of this bottom Cu-higher strategy are that it imposes a concentration gradient to drive carrier diffusion toward front contact and decreases the valence band edge offset in the rear of the device to aid in hole extraction. Also, it maintains the Cu-poor architecture at the near surface to facilitate hole quasi-Fermi level splitting. In return, the local Cu component engineering-mediated electric advances contribute to the highest efficiency of 12.54% for kesterite solar cells using amine-thiol solution systems so far.

Enhancing Grain Growth for Efficient Solution-Processed (Cu,Ag)2ZnSn(S,Se)4 Solar Cells Based on Acetate Precursor.

Material crystallinity is the overriding factor in the determination of the photoelectric properties of absorber materials and the overall performance of the photovoltaic device. Nevertheless, in the Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic device, the bilayer or trilayer structure for the absorber has been broadly observed, which is generally harmful to the cell performance because the probability of photogenerated carrier recombination at grain boundaries significantly increased. Herein, our experiment reveals that the application of anions to a new family of (Cu,Ag)2ZnSn(S,Se)4 (CAZTSSe) materials leads to an increase in grain size and crystallinity. It is inspiring that using acetate starting materials in the precursor solution, a uniform, compact, and pinhole-free CAZTS precursor film was obtained, and the smoothness of the films surpassed that of films fabricated from the oxide route. More importantly, the crystallization of the CAZTSSe film has been considerably enhanced after selenization, and large grains going through the entire absorber layer was successfully obtained. Additionally, it is observed that the Voc accompanied by excellent crystallinity improved significantly due to the pronouncedly reduced carrier recombination loss at grain boundaries. As a consequence, the power conversion efficiency (PCE) of the CAZTSSe photovoltaic device is successfully increased from 10.35% (oxide route) to 11.32% (acetate route). Importantly, our work attests to the feasibility of tuning the crystallization of the CZTSSe film by simple chemistry.

High Efficiency CIGS Solar Cells by Bulk Defect Passivation through Ag Substituting Strategy.

Cu(In,Ga)Se2 (CIGS) is considered a promising photovoltaics material due to its excellent properties and high efficiency. However, the complicated deep defects (such as InCu or GaCu) in the CIGS layer hamper the development of polycrystalline CIGS solar cells. Numerous efforts have been employed to passivate these defects which distributed in the grain boundary and the CIGS/CdS interface. In this work, we implemented an effective Ag substituting approach to passivate bulk defects in CIGS absorber. The composition and phase characterizations revealed that Ag was successfully incorporated in the CIGS lattice. The substituting of Ag could boost the crystallization without obviously changing the band gap. The C-V and EIS results demonstrated that the device showed enlarged Wd and beneficial carrier transport dynamics after Ag incorporation. The DLTS result revealed that the deep InCu defect density was dramatically decreased after Ag substituting for Cu. A champion Ag-substituted CIGS device exhibited a remarkable efficiency of 15.82%, with improved VOC of 630 mV, JSC of 34.44 mA/cm2, and FF of 72.90%. Comparing with the efficiency of an unsubstituted CIGS device (12.18%), a Ag-substituted CIGS device exhibited 30% enhancement.

Plasmon-mediated nonradiative energy transfer from a conjugated polymer to a plane of graphene-nanodot-supported silver nanoparticles: an insight into characteristic distance.

Hybrid nanostructures comprising conjugated polymers (CPs) and plasmonic metals show excellent performance in light harvesting. However, the energy transfer mechanism of the CP film to nearby metal nanoparticles, especially knowledge of the characteristic distance, is still unclear. Here, quenching of the emission of a CP film in proximity to a monolayer of graphene-nanodot-supported silver nanoparticles (GND-Ag NPs) is investigated. Uniform Ag NPs with D = 3.2 nm were grown on GNDs in situ under mild light irradiation, and a series of bilayer structures of GND-Ag NPs/CPs were constructed by spin-coating blue, green and red light-emitting poly(9,9-dioctylfluorene) (PFO), poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV),respectively, on top of the GND-Ag NP plane. The spacer distance was controlled by the layers of assembled polyelectrolytes. Both steady and transient photoluminescence (PL) spectra showed emission quenching of the bilayer structures, providing the maximum efficiency of 99% for the F8BT films. The surface density of GND-Ag NPs and the spacer distance-dependent PL quenching data were analyzed within the plasmonic resonant energy transfer model, and the extracted characteristic distances are 6 nm, 3 nm and 10 nm for the PFO, F8BT and MEH-PPV systems, respectively. Current-sensing atomic force microscopy shows that the GND-Ag NPs/F8BT film exhibits enhanced electrical conductivity. These results are believed to be important for the development of plasmonic enhanced polymer photovoltaics and photocatalysis.

ZnO nanotubes supported molecularly imprinted polymers arrays as sensing materials for electrochemical detection of dopamine.

In this study, ZnO nanotubes (ZNTs) were prepared onto fluorine-doped tin oxide (FTO) glass and used as supports for MIPs arrays fabrication. Due to the imprinted cavities are always located at both inner and outer surface of ZNTs, these ZNTs supported MIPs arrays have good accessibility towards template and can be used as sensing materials for chemical sensors with high sensitivity, excellent selectivity and fast response. Using K3[Fe(CN)6] as electron probe, the fabricated electrochemical sensor shows two linear dynamic ranges (0.02-5µM and 10-800µM) towards dopamine. This proposed electrochemical sensor has been applied for dopamine determination with satisfied recoveries and precision. More complex human urine samples also confirmed that the proposed method has good accuracy for dopamine determination in real biological samples. These results suggest potential applicability of the proposed method and sensor in important molecule analysis.

Elemental Precursor Solution Processed (Cu1-xAgx)2ZnSn(S,Se)4 Photovoltaic Devices with over 10% Efficiency.

The partial substitution of Cu+ with Ag+ into the host lattice of Cu2ZnSn(S,Se)4 thin films can reduce the open-circuit voltage deficit (Voc,deficit) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this paper, elemental Cu, Ag, Zn, Sn, S, and Se powders were dissolved in solvent mixture of 1,2-ethanedithiol (edtH2) and 1,2-ethylenediamine (en) and used for the formation of (Cu1-xAgx)2ZnSn(S,Se)4 (CAZTSSe) thin films with different Ag/(Ag + Cu) ratios. The key feature of this approach is that the impurity atoms can be absolutely excluded. Further results indicate that the variations of grain size, band gap, and depletion width of the CAZTSSe layer are generally determined by Ag substitution content. Benefiting from the Voc enhancement (∼50 mV), the power conversion efficiency is successfully increased from 7.39% (x = 0) to 10.36% (x = 3%), which is the highest efficiency of Ag substituted devices so far.

Implication of Blocking Layer Functioning with the Effect of Temperature in Dye-Sensitized Solar Cells.

The properties of thin titanium dioxide blocking layers onto TCO in dye-sensitized solar cells (DSCs) have been widely reported as their intensity dependence of illumination intensity. Herein, a further investigation about their functioning with the effect of temperature is developed. The electron recombination process, photovoltage response on illumination intensity and photocurrent-voltage properties for DSCs with/without blocking layer at different temperatures are detected. It is found that the electron recombination via TCO becomes increasingly pronounced with increasing temperature and the effect of blocking layer is extremely temperature dependent. The band bending of the compact layer is more effectively to block electron losses at high temperatures, preventing large falloff of photovoltage. Hence, a resistive layer at the surface of TCO keeps comparable cell performances without falloff over a wide temperature range, while the device without blocking layer shows large decrease by over 10% at high temperature for contrast.

Solution-Processed Cu2ZnSn(S,Se) 4 Thin-Film Solar Cells Using Elemental Cu, Zn, Sn, S, and Se Powders as Source.

Solution-processed approach for the deposition of Cu2ZnSn (S,Se)4 (CZTSSe) absorbing layer offers a route for fabricating thin film solar cell that is appealing because of simplified and low-cost manufacturing, large-area coverage, and better compatibility with flexible substrates. In this work, we present a simple solution-based approach for simultaneously dissolving the low-cost elemental Cu, Zn, Sn, S, and Se powder, forming a homogeneous CZTSSe precursor solution in a short time. Dense and compact kesterite CZTSSe thin film with high crystallinity and uniform composition was obtained by selenizing the low-temperature annealed spin-coated precursor film. Standard CZTSSe thin film solar cell based on the selenized CZTSSe thin film was fabricated and an efficiency of 6.4 % was achieved.

Phase-dependent photocatalytic H2 evolution of copper zinc tin sulfide under visible light.

CZTS exhibited apparently phase-dependent photocatalytic H2 evolution under visible light. Possible factors for the phase-dependent photocatalytic activity of CZTS were discussed in detail.