## 3 Results and Discussion
### Device Performance
The two HTMs that are investigated in this study have been used extensively in many different solar cell devices ranging from polymer solar cells, to hybrid perovskite solar cells, to quantum dot solar cells [30; 31; 32; 33; 34; 35; 36; 37]. P3 HT has been reported to have its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels at \(-5.1\) and \(-2.9\) eV respectively [34; 38], while spiro-OMeTAD has its HOMO level at \(-5.22\) eV [28]. The P3 HT in this study was undoped, while spiro-OMeTAD was doped with cobalt and lithium (see Supporting Information (SI)) according to previous studies to lower the Fermi level and increase the conductivity [39]. A third hole conductor, poly((2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl)-2,5-thiophenediyl) (TQ1), was also investigated in this study, as it has been shown to take a similar role as P3 HT in organic polymer solar cells [40]. However, using TQ1 did not result in any comparable voltage and thus no efficiency was measured, see Figure S9. As mentioned before, different thicknesses of the CQD layers were investigated. In the case of spiro-OMeTAD, three, four, six, and eight layers of CQDs were investigated, were four layers gave the highest efficiency as seen in Figure S10a-c. While the four layer cell had the highest efficiency, the fill factor is very low, which can be attributed to pin holes in the very thin film. For thicker CQD films (six and eight layers) the solar cells show a higher fill factor, but the photovoltage and photocurrent are lower and the series resistance increases in the devices, which decreases the overall efficiency drastically. Important to note that also the thickness of TiO\({}_{2}\) was investigated with an optimal thickness around 60 nm. P3 HT was investigated at thicknesses of six layers of CQDs, which gave the record efficiency (see Figure 3b). However, at a separate occasion different thickness of the Ag\({}_{2}\)S CQD layer was investigated (6, 9, and 12 layers), were the efficiency was comparably similar for all thicknesses (see Figure S10d). For no HTM, cells with six and nine layers were investigated, were the one with nine layers gave the highest efficiency (Figure S11). The thickness of the CQD layer was therefore found to influence the efficiency of the devices, and depending on the HTM, a different optimal thickness was obtained. This can be due to the limited transport length or diffusion length of the photogenerated charges in the CQD layer, and the ability of the HTMs to extract the holes from the CQD layer. Comparing spiro-OMeTAD and P3 HT based devices with the same CQD layer thickness (six layers of CQDs, see Supporting Information), we observe a higher photocurrent from the P3 HT based device, which again confirms the more efficient hole extraction with P3 HT. If the hole extraction from the CQD layer is efficient a thicker CQD layer may be used, compared to for a less efficient hole extraction layer, however at the same time the charge recombination at the HTM/CQD interface must be considered, which may also differ for different HTMs. Also the maximum thickness of the CQD layer is dependent on the electron extraction from the CQD layer to the TiO\({}_{2}\) layer. As mentioned above, the IPCE spectrum indicates that the electron extraction is not optimal, and the overall rather low currents from the solar cells also suggests that the electron extraction is not optimal. While the CQD dispersion stability is high and it has been possible to store the CQD dispersion for several months without a considerable change in its soluble and optical properties, the CQD films are unstable under strong illumination, resulting in bleaching of the device, especially during measurements. As previous studies have reported Ag\({}_{2}\)S as being highly stable even in working devices under illumination [22], this photo instability can be attributed to the specific conditioning of the nanocrystals during spin-coating them into the film.