| Development of renewable clean energy is necessary to achieve sustainable development for human society. Solar energy is one of the most abundant renewable energy. However, solar energy should be stored for a large-scale application due to its low energy density and intermittent irradiation. A photoelectrochemical (PEC) water splitting cell is a promising technology to store solar energy as clean hydrogen energy in one step. A low-cost p/n tandem cell (maximum theory efficiency-27%) is considered to be a desirable solar water splitting cell, in which an n-type semiconductor photoanode is connected directly with a p-type semiconductor photocathode. Recently, the performance of Fe2O3 and Ta3N5 photoanodes have been greatly improved. Therefore, a suitable photocathode should be explored to match these photoanodes to achieve a high efficiency p/n tandem cell. Among numerous photocathode materials, CuIn1-xGaxS2 and Cu2ZnSnS4 (CZTS) have received considerable attention due to their optimal band gaps (about 1.5 eV) and photoelectric parameters.However, impurity phases and small grain sizes usually are considered to limit conversion efficiencies of both quaternary compound semiconductors (CuIn1-xGaxS2 and Cu2ZnSnS4). In this study, the performance of CuIn0.7Ga0.3S2 and CZTS photocathodes have been improved by different methods. The highlights are listed as follows:(1) The PEC performance of CuIn0.7Ga0.3S2 photocathodes was improved by selective etching of a CuAu metastable phase. The segregation phases of CuxS and metastable CuAu ordering phase were selectively removed by a electrochemical etching treatment. The photocurrent of a CuIn0.7Ga0.3S2 photocathode was significantly enhanced by selective electrochemical etching of a CuAu ordering phase, but not increased after etching of CuxS. High interface transfer efficiency played a key role for higher performance in the etched sample after eliminating a CuAu ordering phase. After CdS and Pt coating, the Pt/CdS/CuIn0.7Ga0.3S2 photocathode exhibited a high solar photocurrent density of 6.0 mA cm-2 at 0 VRHE under AM 1.5G simulated sunlight (100 mW cm-2) irradiation, which was comparable with the highest recorded photocurrent on a CuIn1_xGaxS2 microcrystal photocathode with the same surface modification.(2) The ZnS impuiry phase in CZTS was eliminated by decreasing the precursor ratio of Zn/Sn and a higher PEC performance was achieved on the pure phase of CZTS photocathode. A pure CZTS was obtained by decreasing Zn/Sn precursor ratios to 0.6 and the formation of ZnS impurity phase was inhibited. The formation mechanism of CZTS and ZnS impurities were also investigated. Crystalline CZTS and Cu2SnS3, as well as amorphous ZnS formed after sulfur-oleylamine was injected into the mixture solution. Amorphous ZnS reacted with Cu2SnS3 to form CZTS during sulfur annealing at high temperature. However, if the Zn/Sn ratio was too high, the excess ZnS could not be eliminated even after a longer reaction time. The ZnS impurity phase increased the resistance of CZTS film. The photocurrent of CZTS photocathode was improved by eliminating the ZnS impurity phase.(3) The grain size of CZTS was increased by aging precursor solution and a higher PEC performance was obtained on the large-grained CZTS photocathode. Reducing grain boundaries’ recombination could improve performance of the samples. We obtained a vertical-penetration CZTS thin film with micron-scale grain sizes by aging precursor solution. Moreover, the mechanism of aging promoting CZTS grain growth was also investigated in details. The aging process led to Sn segregation from the precursor solution. When the sample was calcined in air, SnS2 intermediate product formed, which evaporated and played as a fluxing agent to promote CZTS grain growth during sulfuration at high temperature. The large-grained CZTS films photocathode exhibited much higher photocurrent than those of the samples without aging. |
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