Chun-Chao Chen

A polymer tandem solar cell with 10.6% power conversion efficiency

An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high-performance low-bandgap polymers was the major limiting factor for achieving high-performance tandem solar cell. Here we report the development of a high-performance low bandgap polymer (bandgap <1.4 eV), poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothia diazole)] with a bandgap of 1.38 eV, high mobility, deep highest occupied molecular orbital. As a result, a single-junction device shows high external quantum efficiency of >60% and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency under standard reporting conditions (25 °C, 1,000 Wm−2, IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.

An Efficient Triple‐Junction Polymer Solar Cell Having a Power Conversion Efficiency Exceeding 11%

Tandem solar cells have the potential to improve photon conversion efficiencies (PCEs) beyond the limits of single-junction devices. In this study, a triple-junction tandem design is demonstrated by employing three distinct organic donor materials having bandgap energies ranging from 1.4 to 1.9 eV. Through optical modeling, balanced photon absorption rates are achieved and, thereby, the photo-currents are matched among the three subcells. Accordingly, an efficient triple-junction tandem organic solar cell can exhibit a record-high PCE of 11.5%.

Solution-processed small-molecule solar cells: breaking the 10% power conversion efficiency.

A two-dimensional conjugated small molecule (SMPV1) was designed and synthesized for high performance solution-processed organic solar cells. This study explores the photovoltaic properties of this molecule as a donor, with a fullerene derivative as an acceptor, using solution processing in single junction and double junction tandem solar cells. The single junction solar cells based on SMPV1 exhibited a certified power conversion efficiency of 8.02% under AM 1.5 G irradiation (100 mW cm−2). A homo-tandem solar cell based on SMPV1 was constructed with a novel interlayer (or tunnel junction) consisting of bilayer conjugated polyelectrolyte, demonstrating an unprecedented PCE of 10.1%. These results strongly suggest solution-processed small molecular materials are excellent candidates for organic solar cells.

Systematic Investigation of Benzodithiophene- and Diketopyrrolopyrrole-Based Low-Bandgap Polymers Designed for Single Junction and Tandem Polymer Solar Cells

The tandem solar cell architecture is an effective way to harvest a broader part of the solar spectrum and make better use of the photonic energy than the single junction cell. Here, we present the design, synthesis, and characterization of a series of new low bandgap polymers specifically for tandem polymer solar cells. These polymers have a backbone based on the benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) units. Alkylthienyl and alkylphenyl moieties were incorporated onto the BDT unit to form BDTT and BDTP units, respectively; a furan moiety was incorporated onto the DPP unit in place of thiophene to form the FDPP unit. Low bandgap polymers (bandgap = 1.4-1.5 eV) were prepared using BDTT, BDTP, FDPP, and DPP units via Stille-coupling polymerization. These structural modifications lead to polymers with different optical, electrochemical, and electronic properties. Single junction solar cells were fabricated, and the polymer:PC(71)BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In the single-layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 3-6%. When the polymers were applied in tandem solar cells, PCEs over 8% were reached, demonstrating their great potential for high efficiency tandem polymer solar cells.

10.2% Power Conversion Efficiency Polymer Tandem Solar Cells Consisting of Two Identical Sub‐Cells

Polymer tandem solar cells with 10.2% power conversion efficiency are demonstrated via stacking two PDTP-DFBT:PC₇₁ BM bulk heterojunctions, connected by MoO₃/PEDOT:PSS/ZnO as an interconnecting layer. The tandem solar cells increase the power conversion efficiency of the PDTP-DFBT:PC₇₁ BM system from 8.1% to 10.2%, successfully demonstrating polymer tandem solar cells with identical sub-cells of double-digit efficiency.

A Selenium‐Substituted Low‐Bandgap Polymer with Versatile Photovoltaic Applications

IO N Organic photovoltaic (OPV) devices provide an opportunity to utilize the solar energy effi ciently while maintaining low cost. [ 1 ] To harvest a greater part of the solar spectrum, lowering the energy bandgap of the active material is a major task for materials scientists. The design and synthesis of low-bandgap (LBG) conjugated polymers for use as electron donor materials for bulk heterojuction (BHJ) polymer solar cell (PSC) applications have attracted remarkable attention during the last decade. [ 2 ] The reasons for pursuing LBG polymers include: 1) The Shockley-Quiesser equation indicates a bandgap of around 1.4 eV is ideal for a single junction solar cell device. [ 3 ]

Visibly transparent polymer solar cells produced by solution processing.

Visibly transparent photovoltaic devices can open photovoltaic applications in many areas, such as building-integrated photovoltaics or integrated photovoltaic chargers for portable electronics. We demonstrate high-performance, visibly transparent polymer solar cells fabricated via solution processing. The photoactive layer of these visibly transparent polymer solar cells harvests solar energy from the near-infrared region while being less sensitive to visible photons. The top transparent electrode employs a highly transparent silver nanowire-metal oxide composite conducting film, which is coated through mild solution processes. With this combination, we have achieved 4% power-conversion efficiency for solution-processed and visibly transparent polymer solar cells. The optimized devices have a maximum transparency of 66% at 550 nm.

Fused Silver Nanowires with Metal Oxide Nanoparticles and Organic Polymers for Highly Transparent Conductors

Silver nanowire (AgNW) networks are promising candidates to replace indium-tin-oxide (ITO) as transparent conductors. However, complicated treatments are often required to fuse crossed AgNWs to achieve low resistance and good substrate adhesion. In this work, we demonstrate a simple and effective solution method to achieve highly conductive AgNW composite films with excellent optical transparency and mechanical properties. These properties are achieved via sequentially applying TiO(2) sol-gel and PEDOT:PSS solution to treat the AgNW film. TiO(2) solution volume shrinkage and the capillary force induced by solvent evaporation result in tighter contact between crossed AgNWs and improved film conductivity. The PEDOT:PSS coating acts as a protecting layer to achieve strong adhesion. Organic photovoltaic devices based on the AgNW-TiO(2)-PEDOT:PSS transparent conductor have shown comparable performance to those based on commercial ITO substrates.

Metal Oxide Nanoparticles as an Electron‐Transport Layer in High‐Performance and Stable Inverted Polymer Solar Cells

Polymer solar cells have many advantages, including transparency, aesthetically pleasing, fl exibility, and light weight. They are particularly compatible with high throughput and low-cost fabrication processes, which make them a promising photovoltaic technology. [ 1–5 ] These properties enable a wide range of potential applications, even for outer space. [ 6 , 7 ] In the last few years, many high-performance polymers with high solar-cell effi ciency have been reported. [ 8–16 ] Among those, benzodithiophene (BDT) and thionothiophene (TT)-based polymers were the fi rst polymer family to break the 7% and 8% effi ciency barriers. [ 8–12 ] Poly{2,6 ′ -4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b] dithiophenealt -5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl) pyrrolo[3,4-c]pyrrole-1,4-dione} (PBDTT-DPP) with a lower bandgap ( ≈ 1.4 eV) showed superior performance in long wavelength regions, which enabled signifi cant progress in tandem solar cells with effi ciency close to 9%. [ 14 ] For historical reasons, these high-effi ciency low-bandgap polymers were mostly evaluated based on standard structures, typically with poly(3,4-ethy lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a hole-transport layer (HTL) and a low-work-function metal such as Ca as the electron-transport layer (ETL). Inverted polymer solar cells have been developed and continue to grow particularly due to their potential for superior device stability and manufacturing compatibility. [ 17–24 ] In the inverted architecture with the classical poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) active layer, several successful n -type buffer layers such as cesium carbonate (Cs 2 CO 3 ), [ 17 ] titanium oxide (TiO 2 ), [ 22 ] Cs-doped TiO 2 , [ 25 ] zinc oxide (ZnO), [ 18 ] and a combination of ZnO and self-assembled monolayers [ 19 ] have been shown to be able to alter the carrier selectivity of the indium tin oxide (ITO) electrode and convert it to a cathode contact. On the anode side, the most widely used are transition metal oxides

Synthesis of 5H-Dithieno[3,2-b:2′,3′-d]pyran as an Electron-Rich Building Block for Donor–Acceptor Type Low-Bandgap Polymers

We describe the detailed synthesis and characterization of an electron-rich building block, dithienopyran (DTP), and its application as a donor unit in lowbandgap conjugated polymers. The electron-donating property of the DTP unit was found to be the strongest among the most frequently used donor units such as benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units. When the DTP unit was polymerized with the strongly electron-deficient difluorobenzothiadiazole (DFBT) unit, a regiorandom polymer (PDTP−DFBT, bandgap = 1.38 eV) was obtained. For comparison with the DTP unit, polymers containing alternating benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units and the DFBT unit were synthesized (PBDT−DFBT and PCPDT−DFBT). We found that the DTP based polymer PDTP−DFBT shows significantly improved solubility and processability compared to the BDT or CPDT based polymers. Consequently, very high molecular weight and soluble PDTP−DFBT can be obtained with less bulky side chains. Interestingly, PDTP−DFBT shows excellent performance in bulk-heterojunction solar cells with power conversion efficiencies reaching 8.0%, which is significantly higher than PBDT−DFBT and PCPDT−DFBT based devices. This study demonstrates that DTP is a promising building block for high-performance solar cell materials.