Sae Byeok Jo

Stable Low-Bandgap Pb–Sn Binary Perovskites for Tandem Solar Cells

A low-bandgap (1.33 eV) Sn-based MA0.5 FA0.5 Pb0.75 Sn0.25 I3 perovskite is developed via combined compositional, process, and interfacial engineering. It can deliver a high power conversion efficiency (PCE) of 14.19%. Finally, a four-terminal all-perovskite tandem solar cell is demonstrated by combining this low-bandgap cell with a semitransparent MAPbI3 cell to achieve a high efficiency of 19.08%.

Rational Design of Dipolar Chromophore as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells

In this paper, an electron donor-acceptor (D-A) substituted dipolar chromophore (BTPA-TCNE) is developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PVSCs). BTPA-TCNE is synthesized via a simple reaction between a triphenylamine-based Michlers base and tetracyanoethylene. This chromophore possesses a zwitterionic resonance structure in the ground state, as evidenced by X-ray crystallography and transient absorption spectroscopies. Moreover, BTPA-TCNE shows an antiparallel molecular packing (i.e., centrosymmetric dimers) in its crystalline state, which cancels out its overall molecular dipole moment to facilitate charge transport. As a result, BTPA-TCNE can be employed as an effective dopant-free HTM to realize an efficient (PCE ≈ 17.0%) PVSC in the conventional n-i-p configuration, outperforming the control device with doped spiro-OMeTAD HTM.

Highly Efficient Perovskite–Perovskite Tandem Solar Cells Reaching 80% of the Theoretical Limit in Photovoltage

Organic-inorganic hybrid perovskite multijunction solar cells have immense potential to realize power conversion efficiencies (PCEs) beyond the Shockley-Queisser limit of single-junction solar cells; however, they are limited by large nonideal photovoltage loss (V oc,loss ) in small- and large-bandgap subcells. Here, an integrated approach is utilized to improve the V oc of subcells with optimized bandgaps and fabricate perovskite-perovskite tandem solar cells with small V oc,loss . A fullerene variant, Indene-C60 bis-adduct, is used to achieve optimized interfacial contact in a small-bandgap (≈1.2 eV) subcell, which facilitates higher quasi-Fermi level splitting, reduces nonradiative recombination, alleviates hysteresis instabilities, and improves V oc to 0.84 V. Compositional engineering of large-bandgap (≈1.8 eV) perovskite is employed to realize a subcell with a transparent top electrode and photostabilized V oc of 1.22 V. The resultant monolithic perovskite-perovskite tandem solar cell shows a high V oc of 1.98 V (approaching 80% of the theoretical limit) and a stabilized PCE of 18.5%. The significantly minimized nonideal V oc,loss is better than state-of-the-art silicon-perovskite tandem solar cells, which highlights the prospects of using perovskite-perovskite tandems for solar-energy generation. It also unlocks opportunities for solar water splitting using hybrid perovskites with solar-to-hydrogen efficiencies beyond 15%.

Highly crystalline Zn2SnO4 nanoparticles as efficient electron-transporting layers toward stable inverted and flexible conventional perovskite solar cells

In this work, we have successfully utilized a facile hydrothermal method to prepare crystalline Zn2SnO4 nanoparticles (ZSO NPs) and applied them as an efficient electron-transporting layer (ETL) via a simple room-temperature solution process for perovskite solar cells (PVSCs). The superior semiconducting properties of this ZSO-based ETL enable an efficient (PCE: 17.7%) inverted p–i–n PVSC to be fabricated with respectable ambient stability. It can retain over 90% of its original PCE after being stored under ambient conditions for 14 days under 30 ± 5% relative humidity. Moreover, it also facilitates the fabrication of efficient conventional n–i–p PVSCs with a PCE of ∼14.5% and ∼11.4% achieved on a glass/ITO rigid substrate and PEG/ITO flexible substrate, respectively.

Tackling Energy Loss for High‐Efficiency Organic Solar Cells with Integrated Multiple Strategies

Limited by the various inherent energy losses from multiple channels, organic solar cells show inferior device performance compared to traditional inorganic photovoltaic techniques, such as silicon and CuInGaSe. To alleviate these fundamental limitations, an integrated multiple strategy is implemented including molecular design, interfacial engineering, optical manipulation, and tandem device construction into one cell. Considering the close correlation among these loss channels, a sophisticated quantification of energy-loss reduction is tracked along with each strategy in a perspective to reach rational overall optimum. A novel nonfullerene acceptor, 6TBA, is synthesized to resolve the thermalization and VOC loss, and another small bandgap nonfullerene acceptor, 4TIC, is used in the back sub-cell to alleviate transmission loss. Tandem architecture design significantly reduces the light absorption loss, and compensates carrier dynamics and thermalization loss. Interfacial engineering further reduces energy loss from carrier dynamics in the tandem architecture. As a result of this concerted effort, a very high power conversion efficiency (13.20%) is obtained. A detailed quantitative analysis on the energy losses confirms that the improved device performance stems from these multiple strategies. The results provide a rational way to explore the ultimate device performance through molecular design and device engineering.

Highly Efficient Porphyrin-Based OPV/Perovskite Hybrid Solar Cells with Extended Photoresponse and High Fill Factor

Employing a layer of bulk-heterojunction (BHJ) organic semiconductors on top of perovskite to further extend its photoresponse is considered as a simple and promising way to enhance the efficiency of perovskite-based solar cells, instead of using tandem devices or near infrared (NIR)-absorbing Sn-containing perovskites. However, the progress made from this approach is quite limited because very few such hybrid solar cells can simultaneously show high short-circuit current (JSC ) and fill factor (FF). To find an appropriate NIR-absorbing BHJ is essential for highly efficient, organic, photovoltaics (OPV)/perovskite hybrid solar cells. The materials involved in the BHJ layer not only need to have broad photoresponse to increase JSC , but also possess suitable energy levels and high mobility to afford high VOC and FF. In this work, a new porphyrin is synthesized and blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to function as an efficient BHJ for OPV/perovskite hybrid solar cells. The extended photoresponse, well-matched energy levels, and high hole mobility from optimized BHJ morphology afford a very high power conversion efficiency (PCE) (19.02%) with high Voc , JSC , and FF achieved simultaneously. This is the highest value reported so far for such hybrid devices, which demonstrates the feasibility of further improving the efficiency of perovskite devices.

A regioregular conjugated polymer for high performance thick-film organic solar cells without processing additive

To address the challenges of reliability and facile processability of a benchmark polymer PTB7-Th, one of the most prevailing donor materials utilized in organic solar cells, we have systematically investigated the correlations among chemical structure, processing, morphology and device performance. Our study reveals that the regioregularity of PTB7-Th plays a crucial role in polymer properties as well as the fabrication process of devices. The regioregular polymer entirely consisting of favourable repeat units is capable of realizing high power conversion efficiency (>10%) in organic solar cells without using any solvent additive and tedious processing treatments. More importantly, the device efficiency based on this regioregular polymer is insensitive over a broad range of film thickness (from 100 nm to >400 nm). This will be very advantageous for manufacturing highly efficient and stable polymer solar cells by high throughput fabrication processes.

Realizing a new class of hybrid organic–inorganic multifunctional perovskite

Modification of CH3NH3PbI3 and related hybrid organic–inorganic semiconductors has become an increasingly important effort because of the need to control fundamental material properties. Herein, we closely study material growth to identify the most significant controlling variables determining morphological evolution in a new class of hybrid perovskite alloy. Specifically, drop-casting based perovskite analysis shows that CH3NH3Pb(Mn)yI3, CH3NH3Pb(Fe)yI3, CH3NH3Pb(Co)yI3, and CH3NH3Pb(Ni)yI3 constitute a unique class of hybrid organic–inorganic perovskite in which growth route most strongly determines morphology. Mn, Fe, Co, and Ni consistently modify CH3NH3PbI3 growth, enabling direct perovskite nucleation to compete with growth through solvent induced intermediate states. We show unambiguously that solvent-perovskite co-crystal formation is responsible for the rod-like thin-film morphology that a great deal of work optimizing perovskite growth in planar heterojunction solar cells endeavors to circumvent. In addition to providing insight into the role of growth route in morphological evolution, we also identity the impact of CH3NH3I stoichiometry and the impact of magnetic properties on growth as secondary variables that significantly affect optoelectronic properties. Leveraging this understanding to minimize the impact of morphological phenomena on performance, we closely analyze the compositional impact of these transition metals on optoelectronic quality using CH3NH3Pb(Fe)yI3 as a model system showing that transition metal inclusion of this type leads to trap-assisted recombination within the perovskite bulk that both sharply limits Jsc and causes significant hysteresis. By comparing device performance of Mn, Fe, Co, and Ni based systems, we show that Mn relieves this sharp limitation on Jsc and almost completely eliminates hysteresis. CH3NH3Pb(Mn)yI3 thus allows the implementation of direct perovskite nucleation while minimizing the deleterious impact of transition metal inclusion. PL analysis shows that this material is also more emissive than CH3NH3PbI3, making it ideal for light production as well. Methodology and insights developed herein outline a generalizable approach for navigating complexity of perovskite compositional modification.

Overcoming the Photovoltage Plateau in Large Bandgap Perovskite Photovoltaics

Development of large bandgap (1.80-1.85 eV Eg) perovskite is crucial for perovskite-perovskite tandem solar cells. However, the performance of 1.80-1.85 eV Eg perovskite solar cells (PVKSCs) are significantly lagging their counterparts in the 1.60-1.75 eV Eg range. This is because the photovoltage ( Voc) does not proportionally increase with Eg due to lower optoelectronic quality of conventional (MA,FA,Cs)Pb(I,Br)3 and results in a photovoltage plateau ( Voc limited to 80% of the theoretical limit for ∼1.8 eV Eg). Here, we incorporate phenylethylammonium (PEA) in a mixed-halide perovskite composition to solve the inherent material-level challenges in 1.80-1.85 eV Eg perovskites. The amount of PEA incorporation governs the topography and optoelectronic properties of resultant films. Detailed structural and spectroscopic characterization reveal the characteristic trends in crystalline size, orientation, and charge carrier recombination dynamics and rationalize the origin of improved material quality with higher luminescence. With careful interface optimization, the improved material characteristics were translated to devices and Voc values of 1.30-1.35 V were achieved, which correspond to 85-87% of the theoretical limit. Using an optimal amount of PEA incorporation to balance the increase in Voc and the decrease in charge collection, a highest power conversion efficiency of 12.2% was realized. Our results clearly overcome the photovoltage plateau in the 1.80-1.85 eV Eg range and represent the highest Voc achieved for mixed-halide PVKSCs. This study provides widely translatable insights, an important breakthrough, and a promising platform for next-generation perovskite tandems.

Long-lived, Non-Geminate, Radiative Recombination of Photogenerated Charges in a Polymer:Small-Molecule Acceptor Photovoltaic Blend

Minimization of open-circuit-voltage ( VOC) loss is required to transcend the efficiency limitations on the performance of organic photovoltaics (OPV). We study charge recombination in an OPV blend comprising a polymer donor with a small molecule nonfullerene acceptor that exhibits both high photovoltaic internal quantum efficiency and relatively high external electroluminescence quantum efficiency. Notably, this donor/acceptor blend, consisting of the donor polymer commonly referred to as PCE10 with a pseudoplanar small molecule acceptor (referred to as FIDTT-2PDI) exhibits relatively bright delayed photoluminescence on the microsecond time scale beyond that observed in the neat material. We study the photoluminescence decay kinetics of the blend in detail and conclude that this long-lived photoluminescence arises from radiative nongeminate recombination of charge carriers, which we propose occurs via a donor/acceptor CT state located close in energy to the singlet state of the polymer donor. Additionally, crystallographic and spectroscopic studies point toward low subgap disorder, which could be beneficial for low radiative and nonradiative losses. These results provide an important demonstration of photoluminescence due to nongeminate charge recombination in an efficient OPV blend, a key step in identifying new OPV materials and materials-screening criteria if OPV is to approach the theoretical limits to efficiency.