Chen Weijie

Polymer photovoltaics are a promising alternative energy for visible-spectrum applications because the absorption spectra of organic semiconductors, including polymer and small-molecule types, are not continuous as in inorganic semiconductors. As a result, the design of organic materials is able to pass visible light and absorb non-visible light such as the infrared. According to the energy distribution of the solar spectrum, more than a half of solar light is distributed within the infrared region. The theoretical efficiency of polymer photovoltaics with only infrared absorption therefore can be as high as a device with only visible absorption. Accordingly, we present a perspective that concisely reviews the developments in transparent polymer photovoltaics and their potential applications in order to engender new ideas on achieving superior transparency, power conversion efficiency, and more practical utilities.

Polymer photovoltaics have become a promising alternative energy due to lightweight properties, environmental friendliness, and solution processability. Transparent organic photovoltaics in particular have been recently receiving more attention in the photovoltaics field due to their unique potential in future applications beyond mere harvesting of solar energy. This perspective is further enhanced by the recent developments of high-performance polymer photovoltaics (including infrared absorbing materials and devices) that could achieve superior visible transparency and power conversion efficiency. To realize high-performance transparent organic photovoltaics, the main strategy is to shift the active-layer absorption spectrum to the infrared region by designing new low-bandgap donors and non-fullerene acceptors, and to reduce the invalid absorption by developing new transparent electrode materials. The potential applications are diverse, especially for spectrum-sensitive ones such as the greenhouse in agriculture. Finally, some key research areas of transparent organic photovoltaics that may deserve further attention are discussed.

Transparent photovoltaic (TPV) technologies offer an exciting approach to produce smart windows on buildings, vehicles, mobile electronics, and greenhouses. In this work, we develop a new approach to TPVs based on halide perovskites where the band gap is precisely tuned with a range of compositions to selectively harvest only UV photons with tailored band gaps between 410 and 440 nm. We develop a processing method to overcome the challenges that exist to control the morphology of these less soluble compositions. Full optimization of these perovskite cells could quickly yield theoretical TPVs with PCEs up to 7% with >99% visible transparency. Such devices would rival state-of-the-art TPVs that selectively harvest near-infrared light while also providing a route to higher-efficiency multi-junction TPVs. Moreover, these initial demonstrations are sufficient to power smart windows and many emerging applications today while providing the high level of aesthetics needed for actual adoption.

Halide perovskite materials have emerged as a potential Si replacement with excellent photovoltaic properties. There has been growing interest in applying halide perovskites to semitransparent and spatially segmented transparent photovoltaics (TPVs) to enable a greater range of deployment routes. However, the continuous-band absorption of these semiconductors prevents near-infrared selective harvesting typically targeted for TPVs with the highest efficiency and transparency needed to meet the aesthetic demands of many potential applications. In this work, we demonstrate TPVs based on perovskite semiconductors where the band gap is sensitively tuned with a range of compositions to selectively harvest only ultraviolet photons with band gaps between 410 and 440 nm. This approach offers theoretical efficiencies up to 7% with >99% visible transparency when precisely targeting band gaps around 435 nm. Practical optimization of these perovskite cells could quickly yield TPVs with power conversion efficiencies rivaling state-of-the-art near-infrared harvesting TPVs today while also providing a route to higher-efficiency multi-junction TPVs.

Commercial heat-control window films applied to the interior or exterior of glass windows to reduce the amount of UV, visible, and infrared light from sunlight are already widely used to improve energy efficiency of buildings, while semitransparent organic photovoltaics (ST-OPVs) have not yet been commercialized for power-generating window applications.

Here we have demonstrated for the first time that power-generation and heat-insulation functions can indeed be integrated together in specially designed ST-OPVs, which not only provide good power-generation properties but also show heat rejection comparable with that of commercial window films. A value-added ST-OPV with high PCE and AVT in addition to an excellent infrared radiation rejection rate have been demonstrated, which paves the way for the new application of OPV technology for both power generation and power saving.

Semitransparent organic photovoltaics (ST-OPVs) have attracted extensive attention due to their potential for integration into the windows of buildings. Herein, we propose a dual-functional ST-OPV device that is not only highly efficient but also very effective for heat insulation. By introducing non-fullerene acceptor with enhanced near-infrared absorption and distributed Bragg reflectors for selectively enhancing the transmittance in visible wavelengths while keeping high reflectance for near-infrared light, the ST-OPVs generate over 6% power conversion efficiency with high visible light transmission of over 25% and outstanding infrared radiation rejection rate of over 80%. Our results show that with proper design of ST-OPVs, they can be used not only for generating power from sunlight but also for solar shading and heat insulation, which opens up a new application of OPVs for both energy harvesting and saving.

The societal commitment toward carbon neutral energy has made photovoltaics the main contender for cost-competitive energy sources. Currently the photovoltaics market is being dominated by silicon technology, which has reached grid parity. With the increasing appetite for renewable energy, thin-film photovoltaic technology will gradually increase its market share. Solution processed perovskite solar cells have made stunning progress in a short time frame, though they also suffer from intrinsic issues. Device long-term instability, triggered by thermal and moisture-induced degradation, remains a challenging task. Here we discover the use of a hydrophobic ionic liquid that can act as a ubiquitous dopant for hole transport materials such as Spiro-OMeTAD and can rival the use of state-of-the-art dopant. This generic route can be applied to other organic semiconductors to increase the charge carrier's density and control the degradation of the underlying water-sensitive perovskite layer.

The use of hydrophobic dopant is paramount for the success of organic semiconductors. Here we demonstrate the use of an N-heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI), to induce dual functionality: as p-type dopant and as additive for state-of-the-art Spiro-OMeTAD hole-transporting material (HTM). This ionic liquid doped HTM was then implemented for perovskite solar cell fabrication and delivered competitive results. The dopant has the ability to substitute state-of-the-art ultra-hygroscopic lithium salt (LiTFSI) and problematic 4-tert-butylpyridine (t-BP) additive. BMPyTFSI was found to increase the conductivity of Spiro-OMeTAD and its use as dopant in HTM reduces charge recombination, improves the film formation by reducing the pinholes on the HTM surface, and allows fabrication of efficient devices. Competitive air stability of solar cells in comparison with their state-of-the-art dopant was found, and these findings open up a broad range of organic semiconductors for hydrophobic ionic liquid-based doping.