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.

We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate that a high build-in potential forms at the hybrid junction leading to strong inversion of the n-Si surface. By extracting work function and valence band edge of the polymer from ultraviolet photoelectron spectroscopy, a band diagram of the hybrid n-Si/PEDOT:PSS heterojunction is presented. The current-voltage characteristics were analyzed using Schottky and abrupt pn-junction models. The magnitude as well as the dependence of dark saturation current on n-Si doping concentration proves that the transport is governed by diffusion of minority charge carriers in the n-Si and not by thermionic emission of majorities over a Schottky barrier. This leads to a comprehensive explanation of the high observed open-circuit voltages of up to 634 mV connected to high conversion efficiency of almost 14%, even for simple planar device structures without antireflection coating or optimized contacts. The presented work clearly shows that PEDOT:PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction.

We show that the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) can successfully be applied as a hole selective front contact in silicon heterojunction (SHJ) solar cells. In combination with a superior electron selective heterojunction back contact based on amorphous silicon (a-Si), mono-crystalline n-type silicon (c-Si) solar cells reach power conversion efficiencies up to 14.8% and high open-circuit voltages exceeding 660 mV. Since in the PEDOT:PSS/c-Si/a-Si solar cell the inferior hybrid junction is determining the electrical device performance we are capable of assessing the recombination velocity (v I) at the PEDOT:PSS/c-Si interface. An estimated v I of ~400 cm/s demonstrates, that while PEDOT:PSS shows an excellent selectivity on n-type c-Si, the passivation quality provided by the formation of a native oxide at the c-Si surface restricts the performance of the hybrid junction. Furthermore, by comparing the measured external quantum efficiency with optical simulations, we quantify the losses due to parasitic absorption of PEDOT:PSS and reflection of the device layer stack. By pointing out ways to better passivate the hybrid interface and to increase the photocurrent we discuss the full potential of PEDOT:PSS as a front contact in SHJ solar cells.

PSS interface continues to further oxidize. Thus, without further surface treatment, an unstable Si suboxide will always be present at the hybrid interface.

To investigate the hole transport across amorphous/crystalline silicon heterojunctions, solar cells with varying band offsets were fabricated using amorphous silicon