Doped Spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluoren) at present is the most commonly used hole transport material (HTM) in n-i-p type perovskite solar cells, enabling high efficiencies around 22%.^1,2 However, the required dopants were shown to induce non-radiative recombination of charge carriers and foster degradation of the solar cell.³ Oxidized states in doped Spiro-OMeTAD further cause substantial absorption losses in monolithic n-i-p type perovskite/silicon tandem solar cells, where incident light enters the device through the HTM.⁴

Here, in a novel approach, highly conductive and inexpensive water-free PEDOT (poly (3, 4-ethylene dioxythiophene) is used to replace the dopants in Spiro-OMeTAD. The resulting Spiro-OMeTAD/PEDOT mixed films (SpiDOT) achieve higher lateral conductivities than layers of doped Spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of the PEDOT, suppressing non-radiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages (V_OC) up to 1.19 V in single-junction solar cells. Increasing the concentration of dopant-free Spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improve the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped Spiro-OMeTAD as HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest non-radiative recombination loss in the V_OC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as HTM.

Due to the lack of dopants, SpiDOT films also show no absorption from oxidized states of Spiro-OMeTAD. Further, the minimum required layer thickness for SpiDOT is found to be only a third of the one for Spiro-OMeTAD (50 vs. 150 nm). Together with the high V_OC values obtained in single-junction devices, this renders SpiDOT as a promising HTM for monolithic n-i-p type perovskite/silicon tandem solar cells. By combining experimentally determined layer thickness constraints with optical simulations, a semi-empirical efficiency assessment for the different tandem structures is performed. As a result, replacing doped Spiro-OMeTAD with SpiDOT as HTM is projected to decrease parasitic absorption losses and enhance the achievable efficiency. Furthermore, the simulations highlight the importance of Fresnel reflection losses in n-i-p type perovskite/silicon tandem solar cells and we present several strategies on how to minimize their impact. Supported by initial experiments, we predict that employing a light management foil atop the tandem device allows to further raise the tandem device efficiency to above 27%.

The herein introduced combination of dopant-free Spiro-OMeTAD and a highly conductive PEDOT, therefore, represents a promising novel hole contact, both in single-junction and tandem solar cells.