Boosting internal quantum efficiency via ultrafast triplet transfer to 2H-MoTe2 film.

Organic systems often allow to create two triplet spin states (triplet excitons) by converting an initially excited singlet spin state (a singlet exciton). An ideally designed organic/inorganic heterostructure could reach the photovoltaic energy harvest over the Shockley-Queisser (S-Q) limit because of the efficient conversion of triplet excitons into charge carriers. Here, we demonstrate the molybdenum ditelluride (MoTe2)/pentacene heterostructure to boost the carrier density via efficient triplet transfer from pentacene to MoTe2 using ultrafast transient absorption spectroscopy. We observe carrier multiplication by nearly four times by doubling carriers in MoTe2 via the inverse Auger process and subsequently doubling carriers via triplet extraction from pentacene. We also verify efficient energy conversion by doubling the photocurrent in the MoTe2/pentacene film. This puts a step forward to enhancing photovoltaic conversion efficiency beyond the S-Q limit in the organic/inorganic heterostructures.

Insights into High-Efficiency Ag-Alloyed CZTSSe Solar Cells Fabricated through Aqueous Spray Deposition.

Kesterite Cu2ZnSnSe4 (CZTSe), Cu2ZnSn(S,Se)4 (CZTSSe), and Cu2ZnSnS4 (CZTS) solar cells show considerably lower open-circuit voltages than their theoretical values. The large open-circuit voltage deficiency (Vocdef) hinders the improvement of the power conversion efficiency (PCE) and the development of the pathway to mass production of kesterite solar cells. The main reason behind the Vocdef is considered to be the low formation energy of Cu/Zn disorders and their highly distributed defect complexes. To diminish the Cu/Zn disorder, we substituted Ag with a relatively large atomic radius into the host CZTSSe as (AgxCu1-x)2ZnSn(S,Se)4 (ACZTSSe) and investigated its beneficial effect in a systematic way. The ACZTSSe absorbers were all fabricated using aqueous spray pyrolysis in ambient air. The device performance was found to increase up to the optimum Ag substitution and decrease after the optimum Ag substitution. Admittance spectroscopy revealed that the optimal substitution of Ag reduced the Cu-/Zn-related defects, that is, charge recombination centers, which further mitigates the band tailing issue and enhances the PCE of the solar cell, and higher Ag substitution induced the generation of deeper defects, which decreases the PCE back. At the optimum Ag content of Ag/(Ag + Cu) = ∼9%, the ACZTSSe solar cell with the highest PCE of 11.83% was obtained, where both the interface recombination and bulk recombination were found to be minimized.

Device Characteristics of Band gap Tailored 10.04% Efficient CZTSSe Solar Cells Sprayed from Water-Based Solution.

A diverse S/(S + Se) ratio of Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) thin-film solar cells is derived by the water-based spray pyrolysis approach. By fine-tuning the S/(S + Se) ratio, base CZTSSe device efficiency has significantly improved from 7.02 to 10.04% by minimizing the Voc deficit up to 616 mV and increasing fill factor (FF) from 56.42 to 62.38%. As the S/(S + Se) ratio was increased from 0 to 0.4, surface compactness was observed to be improved with slightly decreased grain size, which increased shunt resistance and resultantly increased FF. However, when S-alloying was more than S/(S + Se) = 0.4, grain size decreased too much and had a detrimental effect on device performance. To deeply understand the role effect of the S/(S + Se) ratio, detailed spectroscopic analysis is performed with admittance spectroscopy, temperature-dependent current-voltage characteristic (J-V-T), time-resolved photoluminescence, and Raman depth profiling. Experimental results revealed that the different power conversion efficiency limiting factors were developed with various S/(S + Se) ratios. High density of deep defect states generated with the S/(S + Se) ≥ 40% content and larger conduction band offset observed with red kink were formed in the Se/(S + Se) ≥ 80% content. Hence, in order to get the high-efficient CZTSSe solar cell, fine tuning of the S/(S + Se) ratio is necessary.

Flexible Cu2ZnSn(S,Se)4 solar cells with over 10% efficiency and methods of enlarging the cell area.

For kesterite copper zinc tin sulfide/selenide (CZTSSe) solar cells to enter the market, in addition to efficiency improvements, the technological capability to produce flexible and large-area modules with homogeneous properties is necessary. Here, we report a greater than 10% efficiency for a cell area of approximately 0.5 cm2 and a greater than 8% efficiency for a cell area larger than 2 cm2 of certified flexible CZTSSe solar cells. By designing a thin and multi-layered precursor structure, the formation of defects and defect clusters, particularly tin-related donor defects, is controlled, and the open circuit voltage value is enhanced. Using statistical analysis, we verify that the cell-to-cell and within-cell uniformity characteristics are improved. This study reports the highest efficiency so far for flexible CZTSSe solar cells with small and large areas. These results also present methods for improving the efficiency and enlarging the cell area.