Minwoo Nam

Broadband-absorbing hybrid solar cells with efficiency greater than 3% based on a bulk heterojunction of PbS quantum dots and a low-bandgap polymer

Currently existing common conjugated polymer:PbS quantum dot (QD)-based hybrid bulk heterojunction (BHJ) solar cells show efficiencies of less than 1% owing to improper bandgap alignment and poor coupling at the hybrid material interfaces. However, herein we report that PbS QD-based hybrid BHJ solar cells provide greatly increased efficiencies of more than 3%, which is attributed to the employment of a new kind of donor polymer poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-alt-4,7(2,1,3-benzothiadiazole) (PSBTBT), the optimization of donor–acceptor (D–A) band-offsets by strategically changing the diameter of PbS QDs in the donor matrix, and the modification of hybrid material interfaces via a chemical treatment of ligands around the QD surface. The optimized hybrid nanocomposite device performs well in good combination between the low-bandgap polymer and near-infrared (NIR)-absorbing PbS QDs, and it enables a broad spectral response from UV to NIR under an energetically favorable type-II heterojunction system, leading to high efficiencies of up to 3.48% under an air mass 1.5G illumination. The efficiency is higher than 3.39%, which corresponds to the efficiency value for the purely organic device fabricated in this study by utilizing [6,6]-phenyl-C71-butyric acid methyl ester (PCBM), which is the most widely studied electron acceptor in BHJ systems. These findings suggest that our hybrid BHJ blends are very promising, not only for use as energy conversion platforms to substitute all-organic or all-inorganic systems, but also for optoelectronic devices that require a broad spectral reaction.

Broadband and ultrahigh optical haze thin films with self-aggregated alumina nanowire bundles for photovoltaic applications

Optical haze is one of the most important factors to be considered in the development of efficient photonic and optoelectronic devices by manipulating the light behavior. In this work, we fabricated a high optical haze film composed of self-aggregated alumina nanowire arrays and applied this novel structure to improve the energy conversion efficiency of organic photovoltaic (OPV) devices. We controlled the optical properties of films, such as total/diffuse transmittance and haze value, by changing the packing density of nanowires during the wet etching process. By optimizing the etching conditions, the nanowire bundle arrays enabled us to obtain an ultrahigh optical haze value up to ∼98% or high transmittance up to ∼96%. By simply attaching the haze film onto the front glass surface of the OPV device, we significantly elongated the optical path length in the active layer, thereby achieving an overall efficiency of 9.01% out of the bare device of 8.17%, resulting in 10.28% enhancement.

Development of Hybrid Photovoltaic Cells by Incorporating CuInS2 Quantum Dots into Organic Photoactive Layers

Hybrid photovoltaic (PV) cells containing CuInS2 quantum dots (CIS QDs) incorporated into organic PV layers were developed through a simple solution process. Four types of such cells were fabricated by blending poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) and CIS QDs in weight ratios of 1:1:0, 1:1:0.2, 1:1:0.5, and 1:1:1. The hybrid PV cell with CIS QDs exhibited increased light absorption and greater quantum efficiency than the purely organic PV cell. In cell performance tests under an air mass (AM) 1.5 solar simulator, the short-circuit current of the hybrid cells with CIS QDs reached up to 10.12 mA/cm2; the corresponding power conversion efficiency was 2.76%, which was 13% higher than that of purely organic PV cells. This indicates that the CIS QDs function as an additional light absorber encouraging device performance improvement.

Broadband energy-harvesting hybrid solar cells employing nanocomposites of polythiophene:ternary PbSSe nanocrystals

Hybrid solar cells (HSCs) incorporating narrow band gap PbSSe nanocrystals (NCs) in a conjugated polymer were developed for the extensive use of solar radiation up to the near infrared (NIR) range. The developed cell, composed of poly(3-hexylthiophene) (P3HT):PbSSe NC composites with 1:1 weight ratio, exhibited a large light-harvesting efficiency over a broad range, in which the external quantum efficiency reached 0.3% at 1100 nm. These promising results were attributed to the well-assembled percolation networks with large interfacial junctions and the efficient use of broad solar spectrum due to the use of narrow band gap ternary NCs.

Long-term efficient organic photovoltaics based on quaternary bulk heterojunctions

A major impediment to the commercialization of organic photovoltaics (OPVs) is attaining long-term morphological stability of the bulk heterojunction (BHJ) layer. To secure the stability while pursuing optimized performance, multi-component BHJ-based OPVs have been strategically explored. Here we demonstrate the use of quaternary BHJs (q-BHJs) composed of two conjugated polymer donors and two fullerene acceptors as a novel platform to produce high-efficiency and long-term durable OPVs. A q-BHJ OPV (q-OPV) with an experimentally optimized composition exhibits an enhanced efficiency and extended operational lifetime than does the binary reference OPV. The q-OPV would retain more than 72% of its initial efficiency (for example, 8.42–6.06%) after a 1-year operation at an elevated temperature of 65 °C. This is superior to those of the state-of-the-art BHJ-based OPVs. We attribute the enhanced stability to the significant suppression of domain growth and phase separation between the components via kinetic trapping effect.

A multifunctional fullerene interlayer in colloidal quantum dot-based hybrid solar cells

Chemically modifying the surfaces of colloidal quantum dots (QDs) offers an effective approach to improve their photovoltaic performances. Ligand exchange processes, however, tend to cause nanoscale cracks throughout the QD-based films, which increases the leakage current and magnitude of recombination losses. Here, we have developed a multifunctional [6,6]-phenyl C61 butyric acid methyl ester (PC60BM) cathode interlayer for use in polymer–QD hybrid bulk heterojunction (BHJ) solar cells. The PC60BM layer deposited onto the hybrid BHJ film via solution uniformly covered the nanocracks produced by QD surface ligand exchange with thiol and facilitated electron transport to the cathode. The PC60BM layer also improved photon harvesting at short wavelengths and formed an efficient vertical donor–acceptor/acceptor′ (D–A/A′) junction in the interfacial areas between the hybrid blend and PC60BM. The efficient vertical junction increased the probability of ultrafast exciton dissociation, provided pathways for effective transport and extraction of photo-generated electrons, and blocked holes to reduce recombination losses. These combined advantages significantly enhanced the overall efficiency of the hybrid solar cells over the current state-of-the-art efficiency.

Solution-processed inverted solar cells using an inorganic bulk heterojunction of iron pyrite nanocrystals and cadmium selenide quantum dots with a polymeric hole-transport medium

Inverted solar cells were developed using a donor–acceptor (D–A) bulk heterojunction (BHJ) of iron pyrite nanocrystals (FeS2 NCs) and cadmium selenide quantum dots (CdSe QDs). The all-inorganic BHJ nanocomposites showed broad-range absorption and effective dynamics of charge dissociation and transfer through bi-continuous D–A interactions under a type II band-offset system. The developed solar cell, which contained an optimized ratio of FeS2–CdSe in the blend and a polymeric hole-transport layer between the photoactive layer and the anode electrode, showed a significantly enhanced photovoltaic efficiency. The long-term ambient stability of the inverted devices was better than that of conventional devices fabricated in this study, but was relatively lower than the air stability of other NC solar cells. Our findings suggest the potential of FeS2 NCs as an environmentally benign, earth-abundant material for solution-processed photovoltaic applications. The merits of an FeS2–CdSe D–A BHJ system with an inverted device architecture and its long-term stability are addressed.

Development of a Polymer Coating-Based Microlens Array on Isotropically Wet-Etched Quartz Substrates for Maskless Lithography Application

A polymer coating-based microlens array (MLA) was developed on an isotropically wet-etched quartz substrate for maskless photolithography application. The developed MLA showed excellent light focusing ability and uniformity, and a dense fill factor. The obtained focal length ranged from 32.2 to 45.4 µm depending on the curvature of quartz and the thickness of an ultraviolet (UV) adhesive. A small spot size of 1.55 µm and an uniformly focused beam intensity were obtained at the focal plane, confirming that the fabricated MLA has excellent uniformity and good focal ability. The fabricated MLA was applied to UV lithography. Beams were well focused onto a photoresist when UV passed through the MLA. Depending on the variable distance from the MLA, beam size on the photoresist was controlled. Variable micropatterns were realized on the photoresist. Even at high a temperature, the interface between quartz and the UV adhesive was thermally stable and lens performance characteristics remained unchanged.

Spectrum conversion technology via upconversion and downshifting for photovoltaics

We report the energy transition of light by lanthanide materials for upconversion or downshifting. This transition is enhanced by photon management with nanostructures and offers the possibility of harnessing the entire solar spectrum.

Solar cell packaged by a microlens array and its long-term optical efficiency enhancement

A technique of mounting a microlens array (MLA) on a solar cell as an encapsulation layer is presented. The uniform cylinder-shaped MLA was fabricated through simple and cost-effective micromachining processes. The efficiency of the triple-junction InGaP/GaAs/Ge solar cell was considerably enhanced by replacing a bare glass cover with the developed MLA as a surface protection layer. This is attributed to efficient conveyance of the refractive light into bare photoactive regions of the solar cell to avoid the gridlines. Under the optimal mounting condition with an optimal height of optical spacer, the MLA effect was maximized resulting in a 16.8% increase in power conversion efficiency (PCE) than that of the control device. The efficiency of the MLA-packaged solar cell remained for a long time without degradation. The MLA can therefore replace a conventional glass (or film) as a means of encapsulation layer to enhance photovoltaic performances of GaAs-based solar cells.