We present a monolithic perovskite/silicon tandem cell enabling a high Voc and a stabilized power output of 18%.

Singlet exciton fission is an exciton multiplication process that occurs in certain organic materials, converting the energy of single highly-energetic photons into pairs of triplet excitons. This could be used to boost the conversion efficiency of crystalline silicon solar cells by creating photocurrent from energy that is usually lost to thermalisation. An appealing method of implementing singlet fission with crystalline silicon is to incorporate singlet fission media directly into a crystalline silicon device. To this end, we developed a solar cell that pairs the electron-selective contact of a high-efficiency silicon heterojunction cell with an organic singlet fission material, tetracene, and a PEDOT:PSS hole extraction layer. Tetracene and n-type crystalline silicon meet in a direct organic-inorganic heterojunction. In this concept the tetracene layer selectively absorbs blue-green light, generating triplet pairs that can dissociate or resonantly transfer at the organo-silicon interface, while lower-energy light is transmitted to the silicon absorber. UV photoemission measurements of the organicinorganic interface showed an energy level alignment conducive to selective hole extraction from silicon by the organic layer. This was borne out by current-voltage measurements of devices subsequently produced. In these devices, the silicon substrate remained well-passivated beneath the tetracene thin film. Light absorption in the tetracene layer created a net reduction in current for the solar cell, but optical modelling of the external quantum efficiency spectrum suggested a small photocurrent contribution from the layer. This is a promising first result for the direct heterojunction approach to singlet fission on crystalline silicon.

Tungsten oxide (WO x ) can be incorporated into amorphous/crystalline silicon heterojunction solar cells as hole contact and for interface modification between p-type amorphous silicon and indium tin oxide. This paper aims at understanding the influence of tungsten oxides properties on silicon heterojunction solar cells. Using in-system photoelectron spectroscopy on thermally evaporated WO x layers, it was verified that WO x with a stoichiometry close to WO 3 features a work function close to 6 eV and is therefore suitable as hole contact on silicon. Additionally the oxygen vacancy concentration in WO x was measured using photoelectron spectroscopy. High oxygen vacancy concentrations in WO x lead to a low band bending in the WO x /silicon-junction. Furthermore solar cells were fabricated using the same WO x , and the band bending in these cells is correlated with their fill factors (FF) and open circuit voltages (V OC ).Combining these results, the following picture arises: Positively charged oxygen vacancies raise the Fermi-level in WO x and reduce the band bending at the WO x /silicon-junction. This, in turn, leads to reduced V OC and FF. Thus, when incorporating WO x into silicon solar cells it is important to minimize the oxygen vacancy density in WO x . Therefore deposition methods, enabling adjustment of the WO x stoichiometry are preferable.

We present a three-terminal (3T) tandem approach for the interconnection of a perovskite top cell with an interdigitated back contact (IBC) silicon heterojunction (SHJ) bottom cell. The general viability of our cell design is further verified with drift-diffusion simulations indicating efficient charge carrier transport throughout the whole device and an efficiency potential of ≈27% using readily available absorber and contact materials. Our experimental proof-of-concept device reaches a combined PCE of 17.1% when both subcells are operating at their individual maximum power point. To emulate different operation conditions, the current-voltage characteristics of both cells were obtained by measuring one subcell while the other cell was set to a fixed bias voltage. Only a slight mutual dependence of both subcells was found. As determined by electrical simulations, it likely stems from the resistance of the electron contact on the cell's rear side, which is shared by both subcells. The optimisation of this contact turns out to be a major design criterion for IBC 3T tandems. We demonstrate that our current proof-of-concept cells are limited by this series resistance as well as by optical losses, and we discuss pathways to approach the simulated efficiency potential by an optimised device design.

The impact of post-deposition hydrogen plasma treatment (HPT) on passivation in amorphous/crystalline silicon (a-Si:H/c-Si) interfaces is investigated. Combining low temperature a-Si:H deposition and successive HPT, a high minority carrier lifetime >8 ms is achieved on c-Si 〈100〉, which is otherwise prone to epitaxial growth and thus inferior passivation. It is shown that the passivation improvement stems from diffusion of hydrogen atoms to the heterointerface and subsequent dangling bond passivation. Concomitantly, the a-Si:H hydrogen density increases, leading to band gap widening and void formation, while the film disorder is not increased. Thus, HPT allows for a-Si:H band gap and a-Si:H/c-Si band offset engineering.