Ultrathin Wide-Bandgap a-Si:H/oxide Transparent Photovoltaic Devices with Improved Open-Circuit Voltage via Electron Transport Layer Optimization

Transparent Photovoltaic (TPV) devices open novel and unexplored pathways for the development of alternative photovoltaic technologies with a clear aim towards distributed applications, focusing on on-site generation while reducing visual impact as well as other inconveniences found in conventional PV applications. Implementation of TPV architectures can potentially allow for a wide array of applications, for example, smart windows in façades and other glazing elements such as vehicle windows, or canopies. Other examples can be the use of TPV to ubiquitously power up sensors and future IoT devices with a low-power draw. In order to deploy these applications into everyday life, it is crucial to develop the technology to be cost-effective. To do so, the pursuit of functional devices that rely on cheap, earth-abundant, stable, and non-toxic materials is of paramount importance if lab research is to be translated to industry. Additionally, the use of industrially scalable techniques is also an important requirement.<br/>In this work, we report the development of wide-bandgap inorganic-based TPV devices that are fabricated using an ultrathin a-Si:H as a highly transparent absorber and oxide layers that are acting as carrier selective contacts and transparent electrical contacts. The basic device structure is FTO/AZO/a-Si:H/MoO₃/ITO, where FTO and ITO serve as back and front contacts while AZO is also used as electron transport layer. We will also report results on structures including V₂O₅ as hole transport material and AZO as top contact, comparing them against the reference HTL/front-contact stack, MoO₃/ITO. We will present results showing that the introduction of an intrinsic ZnO nanometric layer in-between the AZO ETL and a-Si:H leads to an overall performance increase, with the most notable improvement coming from an increased V_oc as compared to reference devices, as well as an increase in FF. Preliminary results show that there is an optimal thickness value for the ZnO interfacial film, and beyond this value there is a worsening of the device performance, which has been attributed to an increase in series resistance leading to poor transport in the TPV devices. This increase in PCE directly translates into an increased Light Utilization Efficiency (LUE), as the introduction of the ZnO interfacial film does not reduce Average Visible Transmittance (AVT) or Color Rendering Index (CRI) of the devices.<br/>At the present time, spectrophotometry and J-V measurements performed in dark and under AM1.5G illumination (from the front and back sides) as well as Spectral Response measurements (under no bias as well as under light and electrical bias) have been carried out. As a preliminary result, we present devices with PCE&gt;1% and AVT=40% (LUE&gt;0.4%), overcoming the barrier of PCE ≤0.5% that has been observed in literature for state-of-the-art oxide-based devices. Record device efficiency achieved with these devices in our lab is PCE = 2%. We provide robust statistics showing that the process is reproducible thanks to the use of PE-CVD and ALD techniques, which allow for chemical composition tailoring and high film conformality even at the nanometric scale. These techniques are industrially scalable and could thus allow for a rapid TRL transition towards higher tiers. Ongoing characterization with XPS is also being performed. Ongoing work is exploring further optimization of the ETL via introduction of novel dipolar materials based on organic polymers: (PEI) and exploring different generations of dendrimeric molecules (PAMAM).<br/>The work presented is of high relevance for the development of the field of inorganic thin film Transparent PV technologies, especially those working on extremely thin absorbers. It might also spark interest in thin film technologies in general that rely on advanced ETL/Active layer/HTL architectures.<br/><br/>This work has received funding from the European Union H2020 Framework Programme under Grant Agreement no. 826002 (Tech4Win).