*3.2. Design of HTL-Free Hybrid Hetero-Homojunction Cell*

In this section, the HTL-free configuration is investigated. A carbon-electrode can be utilized which is considered a good candidate when compared with traditional metal electrodes. The reason is because carbon is air-stable and inert to ion migration [40,57,58]. Figure 12 shows the illuminated *J–V* (Figure 12a) and quantum efficiency (Figure 12b) of the HTL-free structure, which is shown in the inset of the figure. For such an initial HTL-free configuration, the photovoltaic parameters are *Voc* = 0.7513 V, *Jsc* = 19.63 mA/cm2, *FF* = 58.77% and PCE = 8.67%.

**Figure 12.** HTL-free structure characteristics: (**a**) the illuminated *J*–*V*, (**b**) the quantum efficiency.

The optimization of the performance of our proposed HTL-free p-n homojunction absorber lead-free PSC is carried out in the coming subsections. Firstly, the effect of pabsorber doping variation is illustrated. Secondly, the work function of the different back contact materials is studied to find out its influence on the device performance. Then, the effect of the absorber defect density is investigated. Finally, the impact of different ETL materials is examined and the best ETL material, which gives the optimum performance for the designed HTL-free configuration, is determined.
