Liquid-junction Quantum Dot-sensitized Solar Cells Research Articles

Pb5Sb8Se17-δ nanocrystals: A new solar absorber material with an optimal bandgap and an efficiency near 6 % under 0.05 sun

Ternary semiconductor Pb5Sb8S17 has been shown to be a candidate solar absorber. This work presents a new solar absorber of the same group – Pb5Sb8Se17. Pb5Sb8S17 precursor nanocrystals (NCs) are first synthesized using solution processing. A subsequent S to Se ion exchange transforms the Pb5Sb8S17 precursor into the monoclinic Pb5Sb8Se17-δ phase. The substitution of S by Se reduces the band gap Eg from 1.75 (Pb5Sb8S17) to 1.41 eV (Pb5Sb8Se17-δ), increasing the optical absorption band from 300 to 750 (Pb5Sb8S17) to 300–900 nm (Pb5Sb8Se17-δ). The Eg of 1.4 eV equals to the ideal Eg for the Shockley-Queisser-limit. Liquid-junction quantum dot-sensitized solar cells (QDSSCs) are fabricated from the Pb5Sb8Se17-δ NCs with three types of counter electrode (CE): Pt, Au and NiSe. The cell using NiSe CE yields the best PCE of 2.73% (1 sun). Under reduced sun intensities, the PCE increases to 5.47% (0.1 sun) and further to 5.91% (0.05 sun). The external quantum efficiency (EQE) spectrum yields an EQE-integrated photocurrent Jph of 26.8 mA/cm2. The PCE of Pb5Sb8Se17 is 50% higher than that of the host Pb5Sb8S17 (3.95%). The optimal Eg, a high Jph and a high PCE suggest that Pb5Sb8Se17-δ has potential to be an efficient solar absorber.

Tunable Optical Properties in SnxSb2–yS3: A New Solar Absorber Material with an Efficiency of near 5%

This work investigates the synthesis of a new ternary alloyed metal sulfide semiconductor SnxSb2–yS3 and its application in solar cells. SnxSb2–yS3 nanocrystals were synthesized by incorporating Sn2+ ions into the host binary Sb2S3 semiconductor using a two-step sequential ionic layer adsorption reaction (SILAR) process. The ternary SnxSb2–yS3 semiconductor maintains the monoclinic crystalline structure of the binary Sb2S3 host with a small expansion in lattice constants relative to that of Sb2S3. Energy-dispersive X-ray analysis revealed the nominal chemical composition of the sample with eight SILAR cycles to be Sn0.52Sb1.48S3. The energy gap Eg of SnxSb2–yS3 decreases with increasing Sn content x, resulting in a tunable Eg from 620 to 800 nm (i.e., 2.0–1.5 eV) for x = 0–0.56. Liquid-junction quantum dot-sensitized solar cells were fabricated, for the first time, from the prepared SnxSb2–yS3 nanocrystals using the polyiodide electrolyte. The best cell yielded an efficiency of 2.58% with the photovoltaic parameters of Jsc = 14.04 mA/cm2, Voc = 0.46 V, and FF = 39.9% under 1 sun. The efficiency improved to a respectable value of 4.89% under the reduced light intensity of 0.05%. The external quantum efficiency (EQE) spectrum has a maximal EQE of 71.8% at λ = 500 nm and covers the spectral range of 300–800 nm, which is significantly broader than that (300–620 nm) of the host Sb2S3. The broader optical absorption band increases light harvesting and results in a Jsc ≈ 64% larger than that of the host Sb2S3. The result demonstrates the tunable optical properties of SnxSb2–yS3 by controlling the cationic Sn and Sb compositions, which is a favorable property for a solar absorber.

Microwave-Assisted Hydrothermal Synthesis of CuS Nanoplate Films on Conductive Substrates as Efficient Counter Electrodes for Liquid-Junction Quantum Dot-Sensitized Solar Cells

CuS nanoplate films were synthesized on FTO conducting glass substrates via a facile microwave-assisted hydrothermal (MHT) route and applied directly as a low-cost and high-efficiency counter electrode (CE) for liquid-junction quantum dot-sensitized solar cells (QDSSC). The two-dimensional structure of nanoplate provide high surface area for more active catalytic sites and easy accessibility for polysulfide electrolyte, which leads to the remarkable decrease in charge transfer resistance and the enhancement of catalytic activity of CE. As a result, the liquid-junction QDSSC assembled with the prepared CuS-2 CE achieves maximum power conversion efficiency (PCE) of 4.97%, which is superior than that of CuS fabricated by conventional chemical bath deposition method based cell (PCE 4.06%) and the 100 nm thin sputtered Pt/FTO CE based cell (PCE 3.08%). Furthermore, the prepared CuS exhibits highly chemical stability in polysulfide electrolyte, resulting in reproducible performance. These results show the potential application of MHT synthetic method in preparing metal chalcogenide CE for high-performance liquid-junction QDSSC in the future.

Ternary CuBiS2 nanoparticles as a sensitizer for quantum dot solar cells

This work investigates the synthesis and application in solar cells of a novel solar absorber material CuBiS2. Ternary copper chalcogenide CuBiS2 nanoparticles were grown on a mesoporous TiO2 electrode by the chemical bath deposition (CBD) method. The synthesized CuBiS2 nanoparticles, size 5–10nm, have an energy gap Eg of 2.1eV. Liquid-junction quantum dot-sensitized solar cells were fabricated from the CuBiS2-sensitized electrode using a polysulfide electrolyte. Three types of counter electrodes (CEs) – Pt, Au and Cu2S – were tested. The photovoltaic performance depends on the CBD reaction time and the CE. The best cell, obtained with the Cu2S CE, exhibited the photovoltaic performance of a short-circuit current density Jsc of 6.87mA/cm2, an open-circuit voltage Voc of 0.25V, a fill factor FF of 36% and a power conversion efficiency η of 0.62%. The present work demonstrates the feasibility of CuBiS2 as a solar energy material.

Pb5Sb8S17 Liquid-Junction Quantum Dot-Sensitized Solar Cells: Improved Performance by Modifying the Particle Size of the TiO2 Electrode

This work investigates the effect of the particle size of the mesoporous TiO2 electrode (mp-TiO2) on the photovoltaic performance of ternary semiconductor Pb5Sb8S17 quantum dot-sensitized solar cells (QDSCs). Due to the large particle sizes of ternary QDs (∼15 nm), the structure of a traditional TiO2 electrode needs modification for the proper operation of a ternary QDSC. Liquid-junction Pb5Sb8S17 QDSCs were fabricated from Pb5Sb8S17 QDs synthesized by the successive ionic layer adsorption reaction process. The mp-TiO2 electrodes were prepared using two TiO2 particle sizes of 20 and 30 nm. The performance of the cells with 30-nm mp-TiO2 electrodes is 60% higher than that of the cells with 20-nm TiO2 electrodes. The best 30-nm cell exhibited a short-circuit current density of 14.21 mA/cm2, an open-circuit voltage of 0.49 V, a fill factor of 44.9% and a power conversion efficiency (PCE) of 3.14% under 1 sun. At the reduced light intensity of 0.1 sun, the PCE increased to 3.95%, which is a 58% increase than the best previous result of 2.5%. The result reveals a general rule: the TiO2 electrode of a ternary QDSC should be constructed using large TiO2 particles (30 nm).

Earth-abundant Cu2SnSe3 thin film counter electrode for high-efficiency quantum dot-sensitized solar cells

We present herein a new facile solution-phase route to the growth of high crystalline quality Cu2SnSe3 (CTSe) thin film on a conductive substrate and demonstrate for the first time its promising application as an efficient counter electrode (CE) in liquid-junction quantum dot-sensitized solar cells (QDSCs). Dissolving Cu2Se and SnSe powders in the thiol-amine mixture forms a homogeneous CTSe molecular precursor solution. High-quality crystalline CTSe thin film electrode can be readily deposited from the above solution via a low-temperature heating step. Powder X-ray diffraction experiment combined with Raman spectroscopy revealed that the resulting CTSe has the monoclinic crystal structure. Electrochemical measurements, impedance spectroscopy, Tafel polarization, and cyclic voltammetry verified that CTSe possesses fascinating electrocatalytic activity for S2−/Sn2− redox couple in aqueous polysulfide electrolyte solution, and is superior to the traditional electrocatalyst, Pt, both in catalytic activity and in electrochemical stability under prolonged potential cycling, rationalizing the improved solar cell device performance, i.e., QDSCs employing CTSe CE showed more than a two-fold improvement in power conversion efficiency relative to that of Pt-based cells. The excellent catalytic performance along with the ease of solution processing of these earth-abundant CTSe materials make them a distinctive choice among the various CEs studied.

Earth-Abundant Cobalt Pyrite (CoS2) Thin Film on Glass as a Robust, High-Performance Counter Electrode for Quantum Dot-Sensitized Solar Cells.

We report a cobalt pyrite (cobalt disulfide, CoS2) thin film on glass as a robust, high-performance, low-cost, earth-abundant counter electrode for liquid-junction quantum dot-sensitized solar cells (QDSSCs) that employ the aqueous sulfide/polysulfide (S(2-)/Sn(2-)) redox electrolyte as the hole-transporting medium. The metallic CoS2 thin film electrode is prepared via thermal sulfidation of a cobalt film deposited on glass and has been characterized by powder X-ray diffraction and electron microscopy. Using the CoS2 counter electrode, CdS/CdSe-sensitized QDSSCs display improved short-circuit photocurrent density and fill factor, achieving solar light-to-electricity conversion efficiencies as high as 4.16%, with an average efficiency improvement of 54 (±14)% over equivalent devices assembled with a traditional platinum counter electrode. Electrochemical measurements verify that CoS2 shows high electrocatalytic activity toward polysulfide reduction, rationalizing the improved QDSSC performance. CoS2 is also less susceptible to poisoning by the sulfide/polysulfide electrolyte, a problem that plagues platinum electrodes in this application; furthermore, CoS2 exhibits excellent stability in sulfide/polysulfide electrolyte, resulting in highly reproducible performance.