Carbon-based perovskite solar cells (C-PSCs) differ from the traditional structures in that the metal electrode is eliminated, replaced by a carbon electrode typically prepared using simple methods, such as doctor blading, screen printing and painting, by a C paste. Depending on the temperature that is required for the annealing of the C paste, in order for the electrode to form, they can be distinguished in two main categories: high-temperature (HT) and low-temperature (LT) C-PSCs. Both structures have the same working principle: upon illumination, excitons are produced by the perovskite absorbing layer, forming free charge carriers (electrons and holes); electrons are drawn to the electron transport layer (ETL) and holes are collected from the hole transport layer (HTL). The C electrode, having an appropriate work function (W_F), similar to Au, and Fermi level above the valence band edge of the perovskite, can draw the carriers to the external circuit. Moreover, carbon has the capability to absorb oxygen and moisture, thus protecting the perovskite and providing highly stable devices. Even though the initial reports of C-PSCs achieved PCEs much lower than the conventional devices, typically below 10%, the intensive research in this field has led to optimized devices with comparable PCEs to the high-performing metal electrode PSCs, with higher stability and at a significantly lower cost (Figure 1).

The representative HT device configuration comprises of multiple oxide layers on top of which the perovskite is deposited and formed after annealing. In particular, a compact hole blocking layer, typically TiO₂ or SnO₂ grown from precursor solutions, is first deposited on FTO glass substrates, to minimize recombination losses and prevent shunting pathways. Next, an Electron Transport Layer (ETL) is deposited, which is most commonly the wide bandgap semiconductor Titanium Dioxide (TiO₂) in a mesoporous structure, followed by an additional mesoporous layer, typically ZrO₂ or AlO₃. This layer acts as a spacer, preventing the direct contact of TiO₂ with the C electrode, which would cause a short circuiting of the device. Finally, the mesoporous C electrode is deposited from a starting C paste, which is then annealed at a high temperature to remove binders and organic substances and increase the conductivity of the resulting electrode. The perovskite precursor solution is then infiltrated through the triple mesoscopic stack and the perovskite film is formed after annealing (Figure 2). This type of devices are also referred to as “monolithic” and additionally to being HTL-free, highly stable and able to be prepared under ambient conditions, they can also be entirely prepared by printing methods, which gives an advantage when moving to up scaling.

A variety of carbon materials have been used in order to prepare high-performing C pastes, which would result in C electrodes of high conductivity, low series resistance and great mechanical stability. Among these, the combination of carbon black (CB) and graphite has been established as a high-performing formulation [18,19,20,21], while carbon nanotubes (CNTs) have also shown promising results [22,23]. In all of the highest-performing HT C-PSCs the perovskite formulations that have been used are Pb-based, with the most common being the AVA_xMA_(1−x)PbI₃, while recently efforts have turned to the incorporation of multiple cation structures, for the enhancement of performance and stability (Table 1). In the typical HT C-PSC structure, no HTL is employed; however, the state of the art PSCs of this structure have achieved PCE of 17%, when a Nickel Oxide (NiO) layer has been employed as the HTL [24], combined with a mixed cation-mixed halide perovskite (Figure 3).

The low-temperature C-PSC configuration, besides the obvious difference of the C paste requiring a low-temperature annealing to form the C electrode, differs from the HT in two major aspects: (a) the C electrode is deposited after the perovskite deposition and crystallization on the appropriate substrate and (b) the mesoporous layers can be omitted and replaced by planar ones. Particularly for the spacer layer, this can be entirely eliminated, since the perovskite layer intervenes between the C electrode and the ETL, thus preventing a short circuit at this interface. This type of devices are also referred to as “paintable” (Figure 4).

The carbon pastes that are being used typically consist of carbon black, graphite, polymer, additive and a non-polar solvent. In this type of C-PSC, it is crucial that the components of the paste are compatible with the underlying perovskite layer, in order to avoid its decomposition. It is noteworthy that this architecture benefits the use of free-standing electrodes as well as electrodes that are simply press transferred on the perovskite layer, giving the opportunity for high efficiency devices with an ease of fabrication and an extremely low cost [44,45,46,47]. Additionally, in this structure the perovskite is grown directly on the ETL, avoiding the infiltration through multiple mesoporous layers. This gives a much higher flexibility and a variety of choices for the perovskite structures to be used. This has led to devices with PCEs comparable to the ones of Au-based devices, owing to the potential of multiple cation, multiple halide perovskites for high-performance and high-stability devices, as summarized in Table 2. The current state-of-the-art LT C-PSC has achieved PCE of 19.2%, in devices with self-standing C films used as counter electrode, mixed cation perovskite structure and spiro-OMeTAD as the HTM [44] (Figure 5).

As the interest in C-PSCs is rising, so are the PCE values that are achieved and C-PSCs have now been established as the most promising structure for large-area applications and potential commercialization of perovskite solar modules, owing to their high stability, capability to be processed entirely under ambient conditions, the ability to be HTL-free and, most importantly, their significantly lower cost compared to PSCs with metal electrodes. Hence, it is expected that more progress will be presented in the near years to come. One of the most challenging tasks will be to implement environmentally friendly perovskite structures in C-PSCs, in order to overcome the additional barrier of PSCs towards their commercialization, which are the toxicity issues caused by the Pb content of the perovskite.

Hybrid organic-inorganic lead halide perovskites are compounds with the general chemical formula ABX₃ and octahedral structure. A is the component that generally forms the unit cell and is an organic monocation, typically CH₃NH₃⁺ (methylammonium-MA), HC(NH₂)₂⁺ (formamidinium-FA) or the inorganic cesium (Cs⁺). Component B is a, smaller than A, divalent metal cation, most commonly Pb²⁺ or Sn²⁺, which is located in the center of the cube unit cell, and X is an anion, typically a halogen such as Cl⁻, Br⁻ or I⁻ (Figure 6).

Organic-inorganic hybrid perovskites possess optoelectronic properties that make them suitable for applications as photoactive materials in solar cells. These features include high absorptivity, high charge carrier mobility, long exciton diffusion length, tunable bandgap and low exciton binding energy. The CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ perovskites were first studied and presented as sensitizers in Dye Sensitized Solar Cells (DSSCs) which demonstrated moderate PCE of 3.81% [66]. After that, device engineering, materials composition and chemical modifications have enabled PCEs to rise rapidly and reach 25.5% in 2021 [67], 12 years after their first appearance in the solar cell community. The highest-performing devices, exhibiting PCEs that exceed 20%, employ MA and/or FA as the A cation and Pb as the B cation. Even though the performance of the perovskite based SCs is now comparable in terms of efficiency and stability with Si SCs, at the same time with a much lower cost, there is a major issue towards the commercialization and real-life application of this technology that needs to be tackled, and this is the toxicity and environmental harmfulness of Pb. Especially in consumer electrical and electronic equipment, the use of hazardous materials is prohibited and considering that Pb has been identified as one of the ten hazardous chemicals listed by ROHS in order to avoid its exposure to environment and people, it is mandatory that its use is limited [68,69] (Figure 7). Thus, Pb-based perovskites are unlikely to see a broad industrialization in order to compete with other photovoltaic technologies, unless there is turn to lead-free perovskite materials that would provide stable devices with acceptable PCEs, without endangering the human health and the environment [70].

Towards that direction, the most promising candidates that have been investigated as substitutes of Pb are Tin (Sn), Germanium (Ge), Bismuth (Bi), Antimony (Sb) and Copper (Cu).

Tin is considered to be the most suitable candidate for Pb substitution, because of its similar valence electronic configuration and ionic radius (Sn²⁺ = 115 pm and Pb²⁺ = 119 pm) [72]. Sn-based perovskites have the general formula of ASnX₃, A being MA⁺, Fa⁺ or Cs⁺ cation, and X a halogen anion. They have optical bandgap of 1.2–1.6 eV which is appropriate for light absorbing applications and are characterized by large carrier mobilities and low exciton binding energies [73]. The highest PCEs obtained with Sn-based perovskites is 13.24% in an ITO/PEDOT:PSS/perovskite/C60/BCP/Ag device configuration, with the (FA_1−xEA_x)_0.98EDA_0.01SnI₃ perovskite and 12.96% in a c-TiO₂/absorber/spiro-OMeTAD/Au, with CsSnI₃ perovskite quantum rods [74,75] (Figure 8).

Germanium is another element with valence electronic configuration similar to Pb₂⁺, that qualifies as a potential substitute for low-toxicity PSCs. The ionic radius is 73 pm, which is smaller than Pb and Sn and it possesses analogous electronegativity with Sn; however, it gets more easily oxidized, which makes its handling demanding [70]. The most promising candidates for Ge-based PSCs are ammonium halides (AGeX₃), such as methylammonium germanium triiodide (MAGeI₃), and cesium germanium triiodide (CsGeI₃), owing to their suitable optical bandgap of 1.9 and 1.6 eV, respectively, combined with excellent hole and electron conducting behavior and better stability in ambient conditions than MAPbI₃ [78,79]. The PCEs obtained so far with Ge-based perovskites in PSCs still lay below 1%, in particular 0.57% with MAGeI_2.7Br_0.3 [80]; however, a PCS of 7.11% has been reported in a mixed Sn-Ge perovskite, CsSn_0.5Ge_0.5I₃, and the FTO/PCBM/perovskite/spiro-OMeTAD/Au device structure [81] (Figure 9).

Bismuth materials (Bi), with the structure of A₃Bi₂X₉ have also drawn significant attention as non-toxic substitutes for Pb, because of their similar electronic configuration, very close ionic radius of 103 pm (Pb = 119 pm) and a good value of electronegativity. They typically consist of A cations of MA⁺, Cs⁺, Rb⁺, K⁺, guanidinium, cyclohexylammonium or imidazolium to form a 0D perovskite structure [82]. The highest PCE recorder so far is of the order of 3.59%, in a Cs₃Bi₂I₉–Ag₃Bi₂I₉ bulk heterojunction (BHJ) perovskite solar cell (Figure 10).

Following Bi, antimony (Sb) based perovskites have been evaluated as photoactive materials, since they also exhibit appropriate optoelectronic properties to qualify as efficient absorbers for PSCs. They have the general structure of A₃Sb₂X₉, A being an organic or inorganic cation and X a halogen, while the cation and anion species combination determines the structure and dimensions. The antimony cation has a similar valence electronic configuration with Pb, even though it appears in a trivalent state; therefore, it might exhibit long carrier lifetimes. When incorporated in PSCs, Sb-based perovskites have so far achieved low PCEs, the highest being 3.34%, in a FTO/TiO₂/perovskite/spiro-OMeTAD/Au device configuration, using the MA₃Sb₂I_9−xCl_x perovskite. The PSCs retain 90% of the initial PCE after being stored under ambient conditions for over 1400 h and additionally, semitransparent Sb-based PSCs have been demonstrated with PCEs from 2.62 to 3.06% indicating the great potential for building integrated applications [84] (Figure 11).

Finally, copper (Cu), has also emerged as promising for photovoltaic applications. Because of the small ionic radius (73 pm), layered 2D perovskite structures can be formed, with general formula (RNH₃)₂CuX₄ where RNH₃⁺ is an aliphatic or aromatic cation and X is a halogen. The highest PCE obtained so far is 2.41%, in a FTO/TiO₂/perovskite /Spiro-OMeTAD/Au device configuration and (CH₃NH₃)₂CuCl₄ perovskite [85] (Figure 12).