Thin-film tandem solar cells, comprising of a perovskite top junction and a radiation hard

Organic–inorganic perovskite solar cells have experienced a remarkable development. In a short period of time power conversion efficiencies have jumped to values of more than 22%. However, the stability of these devices is an important subject. The stability of CH3NH3PbI3 perovskite films is investigated using visible and ultraviolet light in oxygen atmosphere and in vacuum. Illumination in O2 atmosphere results in a swift degradation. Oxygen acts as a catalyst decomposing methylammonium ions (CH3NH3+) into CH3NH2 and hydrogen. In vacuum, another degradation mechanism is observed. Prolonged illumination of the samples with photons from blue and UV light‐emitting diodes also results in dissociation of the methylammonium ion into CH3NH2 and hydrogen. In both cases the resulting molecules are highly mobile at room temperature and diffuse out of the samples. The light‐induced dissociation of CH3NH3+ is accompanied by the generation of localized defects in the band gap of the perovskite. Furthermore, the experimental data clearly show that the molecular orbitals of CH3NH3+ are not in resonance with the energy bands of the perovskite lattice.

Perovskite solar cells with transparent contacts may be used to compensate for thermalization losses of silicon solar cells in tandem devices. This offers a way to outreach stagnating efficiencies. However, perovskite top cells in tandem structures require contact layers with high electrical conductivity and optimal transparency. We address this challenge by implementing large-area graphene grown by chemical vapor deposition as a highly transparent electrode in perovskite solar cells, leading to identical charge collection efficiencies. Electrical performance of solar cells with a graphene-based contact reached those of solar cells with standard gold contacts. The optical transmission by far exceeds that of reference devices and amounts to 64.3% below the perovskite band gap. Finally, we demonstrate a four-terminal tandem device combining a high band gap graphene-contacted perovskite top solar cell (E g = 1.6 eV) with an amorphous/crystalline silicon bottom solar cell (E g = 1.12 eV).

Organic-inorganic perovskites are well suited for optoelectronic applications. In particular, perovskite single and perovskite tandem solar cells with silicon are close to their market entry. Despite their swift rise in efficiency to more than 21%, solar cell lifetimes are way below the needed 25 years. In fact, comparison of the time when the device performance has degraded to 80% of its initial value (T lifetime) of numerous solar cells throughout the literature reveals a strongly reduced stability under illumination. Herein, the various detrimental effects are discussed. Most notably, moisture- and heat-related degradation can be mitigated easily by now. Recently, however, several photoinduced degradation mechanisms have been observed. Under illumination, mixed perovskites tend to phase segregate, while, further, oxygen catalyzes deprotonation of the organic cations. Additionally, during illumination photogenerated charge can be trapped in the NH antibonding orbitals causing dissociation of the organic cation. On the other hand, organic-inorganic perovskites exhibit a high radiation hardness that is superior to crystalline silicon. Here, the proposed degradation mechanisms reported in the literature are thoroughly reviewed and the microscopic mechanisms and their implications for solar cells are discussed.

Although highly energetic proton irradiation forms localized trap states in triple cation perovskites, solar cells possess exceptional radiation hardness.