No abstract

Organic solar cells (OSCs) are promising photovoltaic devices and zinc oxide (ZnO) is a commonly used electron transport layer (ETL) in OSCs. However, the conventional spin-coating ZnO layer is currently limiting its efficiency potential. Herein, it is shown for the first time that atomic layer deposition (ALD), which allows for controlled thin film growth with atomic-scale control, can effectively be used to optimize the ZnO for nonfullerene OSCs. First, density functional theory (DFT) calculations are discussed to show the impact of doping ZnO with zirconium (Zr) on its density of states and detail the synthesis of Zr doped ZnO films by ALD using a supercycle approach. A 2.4% Zr concentration is found to be optimal in terms of optoelectronic properties and sufficiently low defect density. The champion efficiency of 14.7% for a PM6:N3-based nonfullerene OSC with Zr-doped ZnO ETL are obtained, which is %1% absolute higher compared to a device with an undoped ZnO ETL. This improvement is attributed to a lower series resistance, a suppressed surface recombination, and an enhanced current extraction resulting from the Zr-doped ZnO. This work demonstrates the potential of atomic-scale engineering afforded by ALD towards achieving the ultimate efficiency of OSCs.

The industry for producing silicon solar cells and modules has grown remarkably over the past decades, with more than a 100-fold reduction in price over the past 45 years. The main solar cell fabrication technology has shifted over that time and is currently dominated by the passivated emitter and rear cell (PERC). Other technologies are expected to increase in market share, including tunnel-oxide passivated contact (TOPCon) and heterojunction technology (HJT). In this paper, we examine the cost potential for using atomic layer deposition (ALD) to form transition metal oxide (TMO) layers (MoO x , TiO x and aluminium-doped zinc oxide [AZO]) to use as lower cost alternatives of the p-doped, n-doped and indium tin oxide (ITO) layers, respectively, the layers normally used in HJT solar cells. Using a bottom-up cost and uncertainty model with equipment cost data and process experience in the lab, we find that the production cost of these variations will likely be lower per wafer than standard HJT, with the main cost drivers being the cost of the ALD precursors at highvolume production. We then considered what efficiency is required for these sequences to be cost effective in $/W and discuss whether these targets are technically feasible. This work motivates further work in developing these ALD TMO processes to increase their efficiency towards their theoretical limits to take advantage of the processing cost advantage.

The production of graphene films is of importance for the large-scale application of graphene-based materials; however, there is still a lack of an efficient and effective approach to synthesize graphene films directly on dielectric substrates. Here, we report the controlled growth of ultrathin carbon films, which have a similar structure to graphene, directly on silicon substrates in a process of seeded chemical vapor deposition (CVD). Crystalline silicon with a thermally grown 300 nm oxide layer was first treated with 3-trimethoxysilyl-1-propanamine (APS), which was used as an anchor point for the covalent deposition of small graphene flakes, obtained from graphite using the Hummers’ method. Surface coverage of these flakes on the silicon substrate was estimated by scanning electron microscopy (SEM) to be around only 0.01% of the total area. By treating the covalently deposited graphene as seeds for CVD growth, the coverage was increased to >40% when using ethanol as the carbon source. Examination of the carbon thin films with SEM, X-ray photoelectron spectroscopy, and Raman spectroscopy indicated that they consist of domains of coherent, single-layer graphene produced by the coalescence of the expanding graphene islands. This approach potentially lends itself to the production of high-quality graphene films that may be suitable for device fabrication.

Chemical vapor deposition (CVD) has great potential to produce graphene films at large-scale. However, CVD production of graphene films usually requires a catalytic metal substrate, such as copper. Recently we have developed a new method to grow graphene films directly on crystalline silicon wafers with a thermally grown 300 nm oxide layer, using a seeded-CVD growth approach. The use of methane as the feedstock and optimized graphene seeds has led to enhanced film formation, which SEM, X-ray photo-electron and Raman spectroscopies indicate consist of graphene layers formed by the coalescence of expanding “graphene seeds”. The resultant films have regions of single graphene crystallites within them as a result of lateral growth of the seeds. In addition, we have observed that the unilateral conductivity of the graphene films is consistent with the presence of graphene nanoribbons and as such has potential application in device fabrication.