Although the substitution of Cu by Ag to suppress CuZn defects offers several advantages in overcoming the large open-circuit voltage (Voc) deficit for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, an excellent performance has not been achieved to date primarily due to the Fermi level pinning at the CdS/absorber interface and large recombination at the absorber/Mo interface. Herein, we developed a composition grading strategy to achieve a V-shaped Ag-graded structure with a higher Ag content on both the back and front surfaces of the (Cu,Ag)2ZnSn(S,Se)4 (CAZTSSe) layer. The key advantages of this Ag-graded structure are as follows: the higher content towards the CdS/absorber interface can create weak n-type donor defects and retard Fermi level pinning, whereas the lower content at the interlayer maintains the conductivity and light absorption; moreover, the other higher content towards Mo back contact can effectively suppress the recombination and improve the utilization of long-wave incident light. By appropriately adjusting the Ag gradient, we demonstrated a significant increase in Voc, and an unexpected conversion efficiency of 11.2% was achieved. This is the highest efficiency achieved to date for Ag-substituted CZTSSe solar cells, and the result supports a new aspect that synthesis of a composition-graded CAZTSSe absorber has great potential for future research.

•Directly adding In2Se3 into precursor solution to prepare In-doped CZTSSe layer.•Precisely controlled carrier concentration and charge transport process are achieved.•High doping content would induce larger recombination and performance losses.•FF becomes the dominant parameter for cell performance due to the decrease of Rs.The doping of indium into absorber layer has been proven to be beneficial for hybrid buffer structured Cu2ZnSn(S, Se)4 (CZTSSe) solar cells, but it is difficult to precisely control the doping content only relying on In diffusion from In2S3 emitter. Herein, we use the solution method to prepare In-substituted CZTSSe layer by directly doping In2Se3 into metal chalcogenides precursor solution. The doping content can be precisely controlled and the influences of indium doping on cell performance are systematically discussed. Mott-Schottky and resistivity measurements indicate that the carrier concentration and charge transport process are monotonically enhanced with increasing In/(In + Sn) content (0 at.%, 3 at.%, 6 at.% and 12 at.%). The improvements of series resistance also lead to a monotonic increase of fill factor (FF) from 52.1% to 64.8%. Due to the larger recombination at high doping content, the 6 at.% In content device finally contributes the highest power conversion efficiency of 8.47%. It is expected that a precisely controlled indium doping could further improve cell performance for efficient hybrid buffer structured devices.Download high-res image (74KB)Download full-size image

•Synthesize small-sized CZTS QDs by using 1-dodecanethiol as capping ligand.•The strong bonded nature cation-DDT units are first exchanged by Cd-oleate.•A type-II core/shell structure is formed during the ligand exchange procedure.•The improvement of electron transport and recombination processes are achieved.The quaternary Cu2ZnSnS4 (CZTS) QDs had been successfully introduced into quantum dot-sensitized solar cells (QDSC) via hydrolysis approach in our previous work [Green Chem. 2015, vol. 17, p. 4377], but the obtained cell efficiency was still limited by low open-circuit voltage and fill factor. Herein, we use 1-dodecanethiol (DDT) as capping ligand for fairly small-sized CZTS QDs synthesis to improve their intrinsic properties. Since this strong bonded capping ligand can not be replaced by 3-mercaptopropionic acid (MPA) directly, the nature cation (Cu, Zn or Sn)-DDT units of QDs are first exchanged by the preconjugated Cd-oleate via successive ionic layer adsorption and reaction (SILAR) procedure accompanied with the formation of a core/shell structure. The weak bonded oleic acid (OA) can be finally replaced by MPA and the constructed water soluble CZTS/CdSe QDSC achieves an impressive conversion efficiency of 4.70%. The electron transport and recombination dynamic processes are confirmed by intensity-modulated photocurrent spectroscopy (IMPS)/intensity-modulated photovoltage spectroscopy (IMVS) measurements. It is found that the removal of long alkyl chain is conducive to improve the electron transport process and the type-II core/shell structure is beneficial to accelerate electron transport and retard charge recombination. This effective ligand removal strategy is proved to be more convenient for the applying of quaternary QDs in QDSC and would boost a more powerful efficiency in the future work.

Both recombination and band-edge shift are important factors for the open-circuit voltage (Voc) improvement of metal chalcogenide hole-transport material (HTM) based perovskite solar cells, but it is still not clear that which aspect plays the dominant role in such devices. Herein, we addressed this aspect through employing the band-tunable metal chalcogenide Cu2ZnSnS4 (CZTS) QDs as HTM into perovskite solar cells. By replacing sulfur with selenium atom, the band gap of HTM was tuned from 1.64 eV to 1.14 eV and their influences on cell performances were further discussed. Though the Cu2ZnSnSe4 (CZTSe) device with higher hole transport ability could improve the fill factor (FF), its Voc was still remarkably lower than that of the CZTS device. Electrochemical impedance spectroscopy (EIS) measurements indicated that the Voc loss (45 mV) induced by recombination here was far less than the Voc differences between the two devices (140 mV). After analyzing the band level alignment at TiO2/CH3NH3PbI3/HTM heterojunction, we proposed that the Voc enhancement of CZTS device was mainly ascribed to the more downward valence band-edge shift of HTM. This further approves that developing a wide band gap material without hindering charge injection is more pressing rather than depressing recombination process for future metal chalcogenide HTM researches.

The band-tunable quaternary alloys have more excellent photoelectric properties and stability than their binary or ternary components, but their application as sensitizers in quantum dot-sensitized solar cells (QDSSCs) has seldom been reported. The key feature of this problem is that the fairly small-sized quaternary quantum dots (QDs) require the use of 1-dodecanethiol (DDT) to suppress the growth of QDs, which cannot be displaced by a bifunctional molecular linker. Herein, we developed a novel synthesis and functionalization strategy for presynthesized Cu2ZnSnS4 (CZTS) QDs by utilizing mercapto-acetic acid octyl ester as the capping ligand. Unlike the commonly used ligand exchange approach, the long alkyl chains are removed via a hydrolysis procedure at pH > 7. Benefitting from the broad absorption spectral range, good loading ability and the improvement of the electron transport process after ligand hydrolysis, the constructed “green” CZTS QDSSCs finally achieved an impressive conversion efficiency of 3.29% with a high short-circuit current of 17.48 mA cm−2 without further modification. This efficiency is the first reported value for CZTS QDSSCs so far, which is comparable to most efficiencies for single species sensitizers of around 3%, and demonstrates that it is possible to obtain comparable or even better photovoltaic performance than the toxic cadmium or rare indium QDs.

Cu2ZnSnSxSe4–x (CZTSSe) counter electrodes (CEs) in dye-sensitized solar cells (DSSCs) are commonly developed with porous structures, but their high surface area could also retard electron transport processes owing to the abundant grain boundaries. Herein, we employed a convenient solution method and a rapid heating process to prepare well crystalline CZTSSe CEs in DSSCs. The influence of crystallization of CZTSSe film on DSSCs performances was discussed in depth. The thermogravimetric analysis, phase morphology, conductivity, and electrochemical characteristics of CZTSSe films were performed. It is found that the rapid heating process is beneficial to the formation of well crystalline film with large grains. As the porosity and grain boundaries in the bulk film are dramatically reduced with the enhanced crystallization, the charge transport process is gradually improved. Using cyclic voltammogram and electrochemical impedance spectroscopy measurements, we propose that the accelerating charge transport is of great importance to the photovoltaic performances of DSSCs due to their superior electrocatalytic activities. As the highest cell efficiency was achieved, well crystalline CZTSSe is an efficient CE catalytic material.Keywords: counter electrode; crystallization; Cu2ZnSnSxSe4−x; dye-sensitized solar cells