A pinboard by
Long Ye

Postdoctoral Research Scholar, North Carolina State University


Polymer solar cells

An organic solar cell or plastic solar cell is a type of photovoltaic that uses organic electronics, a branch of electronics that deals with conductive organic polymers or small organic molecules, for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect. My research interest is Developing high-efficiency (printed) organic solar cells, particularly fullerene-free polymer:small molecule and all-polymer solar cells.


An Easily Accessible Cathode Buffer Layer for Achieving Multiple High Performance Polymer Photovoltaic Cells

Abstract: Here we report a successful efficiency improvement strategy in both conventional and inverted polymer solar cells (PSCs) based on multiple polymer blends, using a feasible and commercially available cathode buffer layer (CBL), namely barium hydroxide [Ba(OH)2], to modify the photoactive blend and cathode contacts. High performance PSCs with an identical Ba(OH)2 buffer layer were fabricated based on the multiple light-harvesting PBDT-TS1:PC71BM, PffBT4T-2OD:PC71BM, and PBDT-TS1:N2200 blends. The conventional PSC with Ba(OH)2 as the CBL showed a higher power conversion efficiency (PCE) of 9.65% based on the PBDT-TS1:PC71BM system under the illumination of 100 mW/cm2. For the inverted cells based on the PffBT4T-2OD:PC71BM system, the PCE can be improved from 4.26% (without CBL) to 9.02% after inserting the Ba(OH)2 buffer layer. More importantly, the Ba(OH)2 buffer layer presents similar positive effects in the conventional and inverted all-polymer devices based on a new combination, i.e., the PBDT-TS1:N2200 system. The dramatic enhancement in device performance resulted from the suitable work function of Ba(OH)2, extremely high transmittance, and excellent film-forming capability. Therefore, inserting Ba(OH)2 as the CBL is a simple, low-cost, and widely applicable method to simultaneously improve the conventional and inverted photovoltaic device performance.

Pub.: 13 Nov '15, Pinned: 27 Jun '17

Perovskite-polymer hybrid solar cells with near-infrared external quantum efficiency over 40%

Abstract: In the past several years, conjugated polymers and organometal halide perovskites have become regarded as promising light-absorbing materials for next-generation photovoltaic devices and have attracted a great deal of interest. As the main part of this contribution, we describe the enhancement of near-infrared (NIR) photoresponse of well-known CH3NH3PbI3−xClx-based solar cells by the integration of bulk heterojunction (BHJ) small band gap polymer:fullerene absorbers. Particularly, the integration of a commercially available polymer PDPP3T and PCBM-based BHJ boosts the peak external quantum efficiency (EQE) by up to 46% in the NIR region (800−1000 nm), which is outside of the photoresponsive region (300−800 nm) of conventional perovskite solar cells. This substantial improvement in the EQE over the NIR region offers an additional current density of ∼5 mA cm−2 for the control perovskite solar cell, and a high power conversion efficiency (PCE) of over 12% was obtained in the perovskite/BHJ-based solar cells. In addition, the insertion of the BHJ absorber consisting of a small band gap polymer PDTP-DFBT and PCBM also results in nearly 40% EQE for the perovskite/BHJ solar cell. The results also reveal that controlling over the polymer/PCBM weight ratio for a BHJ absorber is the key to achieving the optimal efficiency for this type of perovskite-polymer hybrid solar cell.

Pub.: 15 Dec '15, Pinned: 27 Jun '17

Green‐Solvent‐Processed All‐Polymer Solar Cells Containing a Perylene Diimide‐Based Acceptor with an Efficiency over 6.5%

Abstract: To realize high power conversion efficiencies (PCEs) in green‐solvent‐processed all‐polymer solar cells (All‐PSCs), a long alkyl chain modified perylene diimide (PDI)‐based polymer acceptor PPDIODT with superior solubility in nonhalogenated solvents is synthesized. A properly matched PBDT‐TS1 is selected as the polymer donor due to the red‐shifted light absorption and low‐lying energy level in order to achieve the complementary absorption spectrum and matched energy level between polymer donor and polymer acceptor. By utilizing anisole as the processing solvent, an optimal efficiency of 5.43% is realized in PBDT‐TS1/PPDIODT‐based All‐PSC with conventional configuration, which is comparable with that of All‐PSCs processed by the widely used binary solvent. Due to the utilization of an inverted device configuration, the PCE is further increased to over 6.5% efficiency. Notably, the best‐performing PCE of 6.58% is the highest value for All‐PSCs employing PDI‐based polymer acceptors and green‐solvent‐processed All‐PSCs. The excellent photovoltaic performance is mainly attributed to a favorable vertical phase distribution, a higher exciton dissociation efficiency (Pdiss) in the blend film, and a higher electrode carrier collection efficiency. Overall, the combination of rational molecular designing, material selection, and device engineering will motivate the efficiency breakthrough in green‐solvent‐processed All‐PSCs.

Pub.: 16 Dec '15, Pinned: 27 Jun '17

Quantitative Morphology–Performance Correlations in Organic Solar Cells: Insights from Soft X-Ray Scattering

Abstract: Organic/polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. One of the key factors contributing to the rapid and continuous progress of organic photovoltaics (OPVs) is the control and optimization of photoactive-layer morphology. The impact of morphology on photovoltaic parameters has been widely observed. However, the highly complex and multilength-scale morphology often formed in efficient OPV devices consisting of compositionally similar components impose obstacles to conventional morphological characterizations. In contrast, due to the high compositional and orientational sensitivity, resonant soft X-ray scattering (R-SoXS), and related techniques lead to tremendous progress of characterization and comprehension regarding the complex mesoscale morphology in OPVs. R-SoXS is capable of quantifying the domain characteristics, and polarized soft X-ray scattering (P-SoXS) provides quantitative information on orientational ordering. These morphological parameters strongly correlate the fill factor (FF), open-circuit voltage (Voc), as well as short-circuit current (Jsc) in a wider range of OPV devices, including recent record-efficiency polymer:fullerene solar cells and 12%-efficiency fullerene-free OPVs. This progress report will delineate the soft X-ray scattering methodology and its future challenges to characterize and understand functional organic materials and provide a non-exhaustive overview of R-SoXS characterization and its implication to date.

Pub.: 04 May '17, Pinned: 27 Jun '17

9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor.

Abstract: In the last two years, polymer solar cells (PSCs) developed quickly with n-type organic semiconductor (n-OSs) as acceptor. In contrast, the research progress of nonfullerene organic solar cells (OSCs) with organic small molecule as donor and the n-OS as acceptor lags behind. Here, we synthesized a D-A structured medium bandgap organic small molecule H11 with bithienyl-benzodithiophene (BDT) as central donor unit and fluorobenzotriazole as acceptor unit, and achieved a power conversion efficiency (PCE) of 9.73% for the all organic small molecules OSCs with H11 as donor and a low bandgap n-OS IDIC as acceptor. A control molecule H12 without thiophene conjugated side chains on the BDT unit was also synthesized for investigating the effect of the thiophene conjugated side chains on the photovoltaic performance of the p-type organic semiconductors (p-OSs). Compared with H12, the 2D-conjugated H11 with thiophene conjugated side chains shows intense absorption, low-lying HOMO energy level, higher hole mobility and ordered bimodal crystallite packing in the blend films. Moreover, a larger interaction parameters (χ) was observed in the H11 blends calculated from Hansen solubility parameters and differential scanning calorimetry measurements. These special features combined with the complementary absorption of H11 donor and IDIC acceptor resulted in the best PCE of 9.73% for nonfullerene all small molecule OSCs up to date. Our results indicate that fluorobenzotriazole based 2D conjugated p-OSs are promising medium bandgap donors in the nonfullerene OSCs.

Pub.: 23 Mar '17, Pinned: 27 Jun '17

Comparing non-fullerene acceptors with fullerene in polymer solar cells: a case study with FTAZ and PyCNTAZ

Abstract: Non-fullerene acceptors (NFAs) are becoming a serious contender to fullerene-based electron acceptors in organic photovoltaics, due to their structural versatility and easily tunable optical and electronic properties. However, NFA-based solar cells often have a decreased short-circuit current (Jsc) and fill factor (FF) compared to their fullerene-based counterparts. Here, we investigate the fundamental causes of this decrease in the performance of solar cells using a non-fullerene acceptor (SF-PDI2) paired with two polymer donors, FTAZ and PyCNTAZ, compared with their fullerene-based counterparts. Through a number of experimental techniques and morphological studies, we show that the SF-PDI2-based solar cells suffer from insufficient charge generation, transport, and collection when compared with the PCBM-based solar cells. The SF-PDI2-based solar cells show increased bimolecular recombination, which, together with other recombination loss mechanisms in these cells, causes a significant decrease in their Jsc and FF. Notably, the less pure domains, low electron mobility (on the order of 10−5 cm2 V−1 s−1), and imbalanced mobility (in regard to the hole mobility) further explain the low FF. On the other hand, the higher open-circuit voltage (Voc) in the SF-PDI2 devices is mainly due to the increase in the CT state energy. It is worth mentioning that the PyCNTAZ-based devices show an ultralow charge separation energy (ΔECS), close to 0 eV. Our results demonstrate that further increasing the mobility (both of electrons and holes) in these NFA-based solar cells would be a viable approach to further enhance the efficiency of these new types of solar cells, ideally, without losing the high Voc of such cells.

Pub.: 02 Feb '17, Pinned: 27 Jun '17

High Performance Organic Solar Cells Processed by Blade Coating in Air from a Benign Food Additive Solution

Abstract: Solution processable conjugated organic materials have gained tremendous interest motivated by their potential of low cost, lightweight and especially easy manufacturing of large-area and flexible electronics. Toxic halogen-containing solvents have been widely used in the processing of organic electronics, particularly organic photovoltaics (OPVs). To transition this technology to more commercially attractive manufacturing approaches, removing these halogenated solvents remains one of the key challenges. Our morphological (hard/soft X-ray scattering) and calorimetric characterizations reveal that using o-methylanisole, a certified food additive, as processing solvent can achieve similar crystalline properties and domain spacing/purity with that achieved by widely used binary halogenated solvents (chlorobenzene and 1,8-diiodooctane), thus yielding comparable photovoltaic performance in spin-casted films. To move a step forward, we further present the potential of o-methylanisole as processing solvent in the blade-coating of several cases of OPVs in air. Remarkably, this single nonhazardous solvent yields ∼8.4% and ∼5.2% efficiency in OPVs by respectively blade-coating PBDT-TSR:PC71BM and all-polymeric PBDT-TS1:PPDIODT in ambient air, which are among the highest values for the respective kind of device. We postulate this simple nonhazardous solvent approach will also be applicable in the large area roll-to-roll coating and industrial scale printing of high-efficiency OPVs in air.

Pub.: 22 Sep '16, Pinned: 27 Jun '17

Control of Mesoscale Morphology and Photovoltaic Performance in Diketopyrrolopyrrole-Based Small Band Gap Terpolymers

Abstract: Morphology control is one of the key strategies in optimizing the performance of organic photovoltaic materials, particularly for diketopyrrolopyrrole (DPP)-based donor polymers. The design of DPP-based polymers that provide high power conversion efficiency (PCE) presents a significant challenge that requires optimization of both energetics and morphology. Herein, a series of high performance, small band gap DPP-based terpolymers are designed via two-step side chain engineering, namely introducing alternating short and long alkyls for reducing the domain spacing and inserting alkylthio for modulating the energy levels. The new DPP-based terpolymers are compared to delineate how the side chain impacts the mesoscale morphology. By employing the alkylthio-substituted terpolymer PBDPP-TS, the new polymer solar cell (PSC) device realizes a good balance of a high Voc of 0.77 V and a high Jsc over 15 mA cm−2, and thus realizes desirable PCE in excess of 8% and 9.5% in single junction and tandem PSC devices, respectively. The study indicates better control of domain purity will greatly improve performance of single junction DPP-based PSCs toward 10% efficiency. More significantly, the utility of this stepwise side chain engineering can be readily expanded to other classes of well-defined copolymers and triggers efficiency breakthroughs in novel terpolymers for photovoltaic and related electronic applications.

Pub.: 26 Oct '16, Pinned: 27 Jun '17

High-Efficiency Nonfullerene Organic Solar Cells: Critical Factors that Affect Complex Multi-Length Scale Morphology and Device Performance

Abstract: Organic solar cells (OSCs) made of donor/acceptor bulk-heterojunction active layers have been of widespread interest in converting sunlight to electricity. Characterizing of the complex morphology at multiple length scales of polymer:nonfullerene small molecular acceptor (SMA) systems remains largely unexplored. Through detailed characterizations (hard/soft X-ray scattering) of the record-efficiency polymer:SMA system with a close analog, quantitative morphological parameters are related to the device performance parameters and fundamental morphology–performance relationships that explain why additive use and thermal annealing are needed for optimized performance are established. A linear correlation between the average purity variations at small length scale (≈10 nm) and photovoltaic device characteristics across all processing protocols is observed in ≈12%-efficiency polymer:SMA systems. In addition, molecular interactions as reflected by the estimated Flory–Huggins interaction parameters are used to provide context of the room temperature morphology results. Comparison with results from annealed devices suggests that the two SMA systems compared show upper and lower critical solution temperature behavior, respectively. The in-depth understanding of the complex multilength scale nonfullerene OSC morphology may guide the device optimization and new materials development and indicates that thermodynamic properties of materials systems should be studied in more detail to aid in designing optimized protocols efficiently.

Pub.: 12 Dec '16, Pinned: 27 Jun '17