Letian Dou

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%.

25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research

Organic photovoltaic (OPV) technology has been developed and improved from a fancy concept with less than 1% power conversion efficiency (PCE) to over 10% PCE, particularly through the efforts in the last decade. The significant progress is the result of multidisciplinary research ranging from chemistry, material science, physics, and engineering. These efforts include the design and synthesis of novel compounds, understanding and controlling the film morphology, elucidating the device mechanisms, developing new device architectures, and improving large-scale manufacture. All of these achievements catalyzed the rapid growth of the OPV technology. This review article takes a retrospective look at the research and development of OPV, and focuses on recent advances of solution-processed materials and devices during the last decade, particular the polymer version of the materials and devices. The work in this field is exciting and OPV technology is a promising candidate for future thin film solar cells.

Solution-processed hybrid perovskite photodetectors with high detectivity

Photodetectors capture optical signals with a wide range of incident photon flux density and convert them to electrical signals instantaneously. They have many important applications including imaging, optical communication, remote control, chemical/biological sensing and so on. Currently, GaN, Si and InGaAs photodetectors are used in commercially available products. Here we demonstrate a novel solution-processed photodetector based on an organic-inorganic hybrid perovskite material. Operating at room temperature, the photodetectors exhibit a large detectivity (the ability to detect weak signals) approaching 10(14) Jones, a linear dynamic range over 100 decibels (dB) and a fast photoresponse with 3-dB bandwidth up to 3 MHz. The performance is significantly better than most of the organic, quantum dot and hybrid photodetectors reported so far; and is comparable, or even better than, the traditional inorganic semiconductor-based photodetectors. Our results indicate that with proper device interface design, perovskite materials are promising candidates for low-cost, high-performance photodetectors.

Controllable self-induced passivation of hybrid lead iodide perovskites toward high performance solar cells.

To improve the performance of the polycrystalline thin film devices, it requires a delicate control of its grain structures. As one of the most promising candidates among current thin film photovoltaic techniques, the organic/inorganic hybrid perovskites generally inherit polycrystalline nature and exhibit compositional/structural dependence in regard to their optoelectronic properties. Here, we demonstrate a controllable passivation technique for perovskite films, which enables their compositional change, and allows substantial enhancement in corresponding device performance. By releasing the organic species during annealing, PbI2 phase is presented in perovskite grain boundaries and at the relevant interfaces. The consequent passivation effects and underlying mechanisms are investigated with complementary characterizations, including scanning electron microscopy (SEM), X-ray diffraction (XRD), time-resolved photoluminescence decay (TRPL), scanning Kelvin probe microscopy (SKPM), and ultraviolet photoemission spectroscopy (UPS). This controllable self-induced passivation technique represents an important step to understand the polycrystalline nature of hybrid perovskite thin films and contributes to the development of perovskite solar cells judiciously.

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.

Atomically thin two-dimensional organic-inorganic hybrid perovskites

Flat perovskite crystals Bulk crystals and thick films of inorganic-organic perovskite materials such as CH3NH3PbI3 have shown promise as active material for solar cells. Dou et al. show that thin films—a single unit cell or a few unit cells thick—of a related composition, (C4H9NH3)2PbBr4, form squares with edges several micrometers long. These materials exhibit strong and tunable blue photoluminescence. Science, this issue p. 1518 Several inorganic-organic perovskite materials grown as atomically thin crystals exhibit strong photoluminescence. Organic-inorganic hybrid perovskites, which have proved to be promising semiconductor materials for photovoltaic applications, have been made into atomically thin two-dimensional (2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick single-crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift as compared to the bulk crystal. The high-quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials.

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.

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