## 1 Introduction  
Solar cell technology for energy conversion has been on the rise in the past decades, mainly due to the challenges of global climate change and, consequentially, the need to decrease the usage of fossil fuels. A lot of effort has been put into inquiries of photovoltaic devices, where the aim is to convert solar energy into electrical current. The common long term goal of this research is to find a combination of materials in a solar cell device which has low cost, short energy payback time, abundant materials, that is stable and highly efficient. One of these solar cell systems is the third generation quantum dot solar cells (QDSC). In 2002, some of the device structures seen today for QDSCs were proposed [1]. Since then, different types of QDSCs have been investigated including p-i-n junction solar cells [2; 3; 4; 5; 6; 7; 8; 9; 10; 11], quantum dot sensitized solar cells (QDSCs) [12; 13; 14; 15; 16], and QD-polymer solar cells [17]. State of the art PbS QDSCs achieves most of goals stated above with efficiencies currently around 11.3% [5]. However, there is one last criterion which gets increasingly more important and that is environmental sustainability and more specifically toxicology. The industrial usage of lead in solar cells will increase the demand in handling, manufacturing, and waste handling in production of these devices. Thus, to avoid these risks, choosing a material with low-toxicity and that is more environmentally friendly becomes important. One of these potential materials is Ag\({}_{2}\)S.  
Ag\({}_{2}\)S is a mid-range band gap semiconductor with a band gap of about 1 eV and it has a rather large light absorption coefficient making it a promising candidate in photovoltaic applications. It hasbeen shown that the band gap can be increased to at least 1.1 eV (1120 nm) by decreasing the size of the nanocrystals to about 3 nm, due to relatively weak quantum confinement effects [18; 19]. A few studies have been made on Ag\({}_{2}\)S as an inorganic light-absorber. The stability of Ag\({}_{2}\)S quantum dots produced by successive ionic layer adsorption and reaction (SILAR) as well as their ability to inject electrons into an n-type semi-conductor, such as TiO\({}_{2}\), have been investigated [20]. Above 1% solar to electricity power conversion efficiency (PCE) using a device structure similar to the dye-sensitized solar cells has been achieved [21; 22; 23; 24]. However, there have been no reports on solar cells based on solid CQD layers with Ag\({}_{2}\)S, similar to the solid CQD layers used in the most efficient PbS CQD solar cells discussed above. In this report, we therefore investigate solar cell device based on Ag\({}_{2}\)S CQD solids, and specifically investigate the influence of the hole transport material (HTM) in the device. There are a few methods to produce the Ag\({}_{2}\)S quantum dots. A hot-injection method can be deployed as it is known to give a large amount of quantum dots with a low polydispersity and high yield. This can be done in polar and non-polar solvents, providing quantum dots with a non-polar ligand. This method is common for many other materials such as CdS and PbS [25; 26]. However, this method is difficult to implement at an industrial level as the injection of sulphur precursor is usually performed at a timescale of around a second, which becomes increasingly more difficult at larger volumes. The heat-up synthesis can therefore be advantageous due to its simplicity, and the potential to more easily upscale the synthesis and that the properties (e.g., size and optical properties) from batch to batch are very similar [27]. In this study, the heat-up approach was applied to synthesize Ag\({}_{2}\)S CQDs for the application in solar cells with a p-i-n heterojunction architecture. The solar cells in this work are prepared using a solution-based approach under ambient condition. A thin layer of n-type TiO\({}_{2}\) is used as an electron transport layer.