Earth-abundant quaternary chalcogenides are promising candidate materials for thin-film solar cells. Here we have synthesized Cu2NiSnS4 nanocrystals and thin films in a novel zincblende type cubic phase using a facile hot-injection method. The structural, electronic, and optical properties are studied using various experimental techniques, and the results are further corroborated within first-principles density functional theory based calculations. The estimated direct band gap ∼ 1.57 eV and high optical absorption coefficient ∼ 106 cm–1 indicate potential application in a low-cost thin-film solar cell. Further, the alignments for both conduction and valence bands are directly measured through cyclic voltametry. The 1.47 eV electrochemical gap and very small conduction band offset of −0.12 eV measured at the CNTS/CdS heterojunction are encouraging factors for the device. These results enable us to model carrier transport across the heterostructure interface. Finally, we have fabricated a CNTS solar cell device for the first time, with high open circuit voltage and fill factor. The results presented here should attract further studies.

To improve the constraints of kesterite Cu 2 ZnSnS 4 (CZTS) solar cell, such as undesirable band alignment at p–n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu 2 Zn 1– x Cd x SnS 4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental–theoretical approach was employed to characterize and assess the optoelectronic properties of Cu 2 Zn 1– x Cd x SnS 4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu 2 Zn 1– x Cd x SnS 4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu 2 Zn 1– x Cd x SnS 4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu 2 CdSnS 4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p–n junction in the ultrafast time scale and highlight a route to improve device performances.

To boost the power conversion efficiency (PCE) of quantum dot solar cell (QDSC) by long-lived charge separated state, we are introducing CdSe{Au} nanohybrid material (NHM) which acts as a better light harvester than CdSe quantum dot (QD) alone. Steady state absorption studies show broadening of the absorption band of CdSe{Au} NHM up to 800 nm. The steady state and time-resolved luminescence studies reveal ultrafast electron transfer from CdSe QD to Au nanoparticles (NPs), forming a charge separated state. The measured PCE of the CdSe{Au} NHM is 4.39% which is significantly higher than pure CdSe QDs (3.37%). The enhancement of PCE has been explained by femtosecond transient absorption (TA) and electrochemical impedance spectroscopy (EIS). The ultrafast TA studies suggest subpicosecond electron transfer from CdSe QDs to Au NP and slower charge recombination in NHM. Interestingly 3 times higher recombination resistance at the interface of TiO 2 −CdSe{Au} as compared to TiO 2 −CdSe is shown by EIS measurements, which has also explained the enhancement of PCE for the NHM. To the best of our knowledge this is the first report of PCE for any kind of metal−semiconductor NHM.

Cyclic voltammetric and femtosecond transient absorption (TA) measurements on Cu+-doped CdSe nanocrystals (NCs) were utilized to reveal the energetics of the electroactive Cu+ dopant with respect to the band energies of CdSe NC host and the influence of Cu in tuning the carrier dynamics, respectively. Oxidation–reduction peaks due to an electroactive dopant within CdSe NC host have been traced to determine its energy level which was correlated to the dopant emission energy and Stokes shift. The low doping density of Cu does not significantly alter the band structure of CdSe as the shape of the TA spectra remains similar before and after doping. However, Cu+ acts as a hole localizing center decoupling the electronic wave function from the hole leading to slower Auger-assisted electron cooling in doped NCs. As hole localization to Cu+ is the primary step for dopant emission, in the presence of hole quenchers (aminophenols) the dopant emission gets drastically quenched. Interestingly, once hole is captured by Cu+ due to strong affinity for electron, external quenchers (nitrophenols) are unable to capture the electron as confirmed from steady state and time-resolved measurements establishing the role of Cu as an internal sensitizer for the charge carriers.

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Ternary Cu 2 SnS 3 (CTS) is an attractive nontoxic and earth-abundant absorber material with suitable optoelectronic properties for cost-effective photoelectrochemical applications. Herein, we report the synthesis of high-quality CTS nanoparticles (NPs) using a low-cost facile hot injection route, which is a very simple and nontoxic synthesis method. The structural, morphological, optoelectronic, and photoelectrochemical (PEC) properties and heterojunction band alignment of the as-synthesized CTS NPs have been systematically characterized using various state-of-the-art experimental techniques and atomistic first-principles density functional theory (DFT) calculations. The phase-pure CTS NPs confirmed by X-ray diffraction (XRD) and Raman spectroscopy analyses have an optical band gap of 1.1 eV and exhibit a random distribution of uniform spherical particles with size of approximately 15–25 nm as determined from high-resolution transmission electron microscopy (HR-TEM) images. The CTS photocathode exhibits excellent photoelectrochemical properties with PCE of 0.55% (fill factor (FF) = 0.26 and open circuit voltage (Voc) = 0.54 V) and photocurrent density of −3.95 mA/cm 2 under AM 1.5 illumination (100 mW/cm 2 ). Additionally, the PEC activities of CdS and ZnS NPs are investigated as possible photoanodes to create a heterojunction with CTS to enhance the PEC activity. CdS is demonstrated to exhibit a higher current density than ZnS, indicating that it is a better photoanode material to form a heterojunction with CTS. Consistently, we predict a staggered type-II band alignment at the CTS/CdS interface with a small conduction band offset (CBO) of 0.08 eV compared to a straddling type-I band alignment at the CTS/ZnS interface with a CBO of 0.29 eV. The observed small CBO at the type-II band aligned CTS/CdS interface points to efficient charge carrier separation and transport across the interface, which are necessary to achieve enhanced PEC activity. The facile CTS synthesis, PEC measurements, and heterojunction band alignment results provide a promising approach for fabricating next-generation Cu-based light-absorbing materials for efficient photoelectrochemical applications.

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