## 3 Results and Discussion
### Device Performance  
Photovoltaic performance of the Ag\({}_{2}\)S CQD solar cells with P3 HT and spiro-OMeTAD HTM layers, and without any HTM layer were measured using a solar simulator with AM1.5G 100 mW/cm\({}^{2}\) illumination from a Newport Xe lamp combined with a Keithley 2400 Sourcemeter. The working area of the solar cells was 0.126 cm\({}^{2}\) as defined by a black metallic aperture. The resulting J-V curves for the best solar cells are displayed in Figure 3b and the resulting photovoltaic parameters, such as short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and PCE, and are summarized in Table 1 (see Supplementary Materials for non-rounded data). The best cell with P3 HT as a HTM was constructed with a CQD solid of six CQD layers and the best cell without any HTM was constructed with a CQD solid of nine CQD layers, and the best cell with spiro-OMeTAD was constructed with four CQD layers. Comparing the results, it can be concluded that the HTM has a strong influence on the device characteristics. The CQD solar cells with P3 HT as a HTM show higher photocurrent but slightly lower photovoltage, while the CQD solar cells with spiro-OMeTAD HTM show higher photovoltage and slightly lower photocurrent. The solar cells with no HTM show much lower current, but relatively high voltage. We can therefore conclude that for the devices with a HTM layer a larger photocurrent can be obtained, which indicates that the HTM is important in the extraction of the holes from the CQD layer. At the same time the photovoltage is affected by the HTM, which may be a result of either different charge extraction efficiency or different recombination rate of the photogenerated charges for the devices with different HTMs. The charge extraction and charge recombination at the CQD/HTM interface depends on the energy levels and molecular structure at the interface and can be affected both by the individual properties of the materials, but also on the interaction between the HTM and the CQD layer [28; 30]. The J-V curve for the P3 HT based device shows a slight s-shape close to short-circuit. This has been obtained for several of the tested devices, and indicates that there is a barrier for charge extraction in the devices, and the IPCE results further down, using an ASB-XE-175 Xe light source connected to a Spectral Products monochromator and AB300 series controller, suggests that the TiO\({}_{2}\)/CQD interface may limit the device performance. The fabricated devices (see Figure 3c) show a promising voltage up towards 0.5 V, but have a very low current and fill factor, which limits the efficiency drastically. J-V measurements in the dark (Figure S8) show that the devices are limited by a considerable dark current, which indicates that the recombination at the interfaces in the device is very important. As seen in Figure S8, P3 HT has a lower dark current than spiro-OMeTAD. Further, the IPCE spectrum (see Figure 3d), shows a broad photocurrent response. This is in agreement with the broad absorption spectrum of the Ag\({}_{2}\)S CQDs, and shows the potential for utilization of a large part of the solar spectrum with solar cells based on Ag\({}_{2}\)S CQDs. In the device the light with short wavelength is absorbed close to the TiO\({}_{2}\) layer and light with longer wavelength is absorbed closer to the HTM layer, since the Ag\({}_{2}\)S CQDs have larger light absorption in the short wavelength region (compare Figure 1a, Figure 3a,d). The rather low efficiencies obtained in the devices we therefore relate to the TiO\({}_{2}\)/CQD interface and further enhancement of the device efficiency should in future be focused on this interface. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**HTM** & **VOC (V)** & **JSc (mAcm\({}^{-2}\))** & **FF** & **Eff. (\%)** \\ \hline Spiro-OMeTAD & 0.47 & \(-1.9\) & 0.28 & 0.25 \\ P3 HT & 0.37 & \(-2.5\) & 0.38 & 0.34 \\ - & 0.45 & \(-1.1\) & 0.52 & 0.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Device parameters from the devices shown in Figure 3b.