A broadband multilayer antireflection coating for thin film CdSeTe/CdTe solar cells

Thin film cadmium telluride (CdTe) photovoltaics (PV) is the most successful second-generation PV technology, with a current installed capacity of over 30 GWp, predominantly at utility scale. Recent improvements in the buffer layer of the device, switching from cadmium sulphide (CdS) to transparent magnesium-doped zinc oxide (MZO), tin oxide (SnO2), or zinc oxide (ZnO), and the addition of selenium to the absorber layer, have expanded the wavelength range over which CdTe devices operate, from 400–850 nm to 350–900 nm. These changes have resulted in higher efficiency devices. As a result, an optimized antireflection (AR) coating design is required to improve the efficiency further. A six-layer AR coating of SiO2 and ZrO2, building on a previous four-layer design for CdTe devices, has been designed, modeled, and fabricated on 3.8-mm thick fluorine-doped tin oxide coated TEC™15 substrates, reducing reflection by 3.38% absolute. Electrical measurements of a CdSeTe/CdTe device before and after addition of the AR coating show an increase in short-circuit current density (Jsc) of almost 1 mAcm−2, a relative increase of 3.45%, and a 0.6% increase in the conversion efficiency of the device, from 16.93% to 17.53%, which is a relative increase of 3.54%. Unlike conventional single layer AR coatings this multilayer coating is stable even under the high processing temperatures required in module manufacturing, so could be supplied by glass manufacturers. This newly optimized broadband AR coating on will enable significantly higher conversion efficiencies and help push CdSeTe/CdTe module efficiencies higher.</p

Measurement of band alignment between ZnO based front emitters and CdCl₂ treated CdSeTe/CdTe absorbers

Thin film CdSeTe/CdTe solar cells have achieved > 22% record efficiency and generated solar electricity at a cost as low as 3 US cents per kW.hr in large scale utilities. In this work, the use of ZnO based n-type emitters is considered with the aim of improving the thin film CdSeTe/CdTe photovoltaic device efficiency still further. The measured conduction band offsets (CBOs) between ZnO and CdSeTe are determined to be in the “cliff” conformation. These CBOs will be optimized to achieve a “spike” conformation by incorporating suitable dopants to achieve high device efficiency. This work identifies new pathways to highly efficient ZnO based n-type emitters for arsenic doped CdTe solar cells. In particular, we have identified Sn, Ce and Cs as suitable dopants to generate the preferred ‘spike’ in the band alignment between the emitter layer and the CdSeTe absorber. Suitably doped emitter layers will be used in device research to reduce the deficit in open circuit voltage.</p

Nonplanar spray-coated perovskite solar cells

Spray coating is an industrially mature technique used to deposit thin films that combines high throughput with the ability to coat nonplanar surfaces. Here, we explore the use of ultrasonic spray coating to fabricate perovskite solar cells (PSCs) over rigid, nonplanar surfaces without problems caused by solution dewetting and subsequent "run-off". Encouragingly, we find that PSCs can be spray-coated using our processes onto glass substrates held at angles of inclination up to 45° away from the horizontal, with such devices having comparable power conversion efficiencies (up to 18.3%) to those spray-cast onto horizontal substrates. Having established that our process can be used to create PSCs on surfaces that are not horizontal, we fabricate devices over a convex glass substrate, with devices having a maximum power conversion efficiency of 12.5%. To our best knowledge, this study represents the first demonstration of a rigid, curved perovskite solar cell. The integration of perovskite photovoltaics onto curved surfaces will likely find direct applications in the aerospace and automotive sectors

Direct integration of perovskite solar cells with carbon fiber substrates

Integrating photovoltaic devices onto the surface of carbon-fiber-reinforced polymer substrates should create materials with high mechanical strength that are also able to generate electrical power. Such devices are anticipated to find ready applications as structural, energy-harvesting systems in both the automotive and aeronautical sectors. Here, the fabrication of triple-cation perovskite n–i–p solar cells onto the surface of planarized carbon-fiber-reinforced polymer substrates is demonstrated, with devices utilizing a transparent top ITO contact. These devices also contain a “wrinkled” SiO2 interlayer placed between the device and substrate that alleviates thermally induced cracking of the bottom ITO layer. Devices are found to have a maximum stabilized power conversion efficiency of 14.5% and a specific power (power per weight) of 21.4 W g−1 (without encapsulation), making them highly suitable for mobile power applications.</p