3.2.1. Influence of Doping

In this subsection, the effect of p-doping variation on the HTL-free configuration is performed for two values of n-doping, namely 1 × 1017 and 1 × <sup>10</sup><sup>18</sup> cm−3. Figure 13a shows the influence of such doping variation on the photovoltaic parameters. It is obvious that the optimum p-side doping is a little above 5 × <sup>10</sup><sup>16</sup> cm−<sup>3</sup> (exactly at 6.3 × <sup>10</sup><sup>16</sup> cm<sup>−</sup>3), giving an n-type doping of 1 × <sup>10</sup><sup>17</sup> cm−3. In this case, the optimum PCE is 8.69% which indicates a slight enhancement, 0.02%, compared to the starting PCE (8.67%). The variation of the photovoltaic parameters with the p-doping concentration can be explained from the perception of the recombination rate, as shown in Figure 13b, which illustrates the recombination rate across the device distance at the short-circuit condition. As can be seen, the recombination rate increases considerably when *NA* is 1 × <sup>10</sup><sup>18</sup> cm−3. In addition, for *NA* = 6.3 × <sup>10</sup><sup>16</sup> cm−3, the recombination rate is decreased near the ETL interface and deep inside the absorber, which implies a higher short-circuit current. Although the rate is higher when moving towards the back contact, this high rate is not effective because the generation rate is minimized since that region is relatively far away from the light source.

**Figure 13.** (**a**) Impact of p-layer doping variation on the HTL-free configuration photovoltaic parameters for two values of n-layer doping, and (**b**) recombination rates for three different values of p-layer doping.

#### 3.2.2. Influence of Back Metal Work Function

In this subsection, different metal contacts with distinct work function values are examined. From Figure 14a, it is clear that optimum performance occurs by using the carbon-based contact at 5.4 eV, which also satisfies the flat band condition. For work function values lower than 5.1 eV, the performance is deteriorated. Therefore, it is recommended to use carbon as a back contact metal in the HTL-free architecture. The explanation of the improvement of the open circuit voltage, and hence the efficiency, when increasing the work function could be attributed to the built-in potential enhancement. This can be deduced from the energy band diagram plotted in Figure 14b, in which three different values of work functions are plotted, showing that the highest *Vbi* is obtained when using carbon contact.

**Figure 14.** Impact of different metal contacts with distinct work function values on the HTL-free configuration photovoltaic parameters. (**a**) Photovoltaic parameters, and (**b**) energy band diagram drawn at the short-circuit condition for three different metal electrodes (work function values are also indicated).
