Stabilizing semi-transparent perovskite solar cells with a polymer composite hole transport layer

Semi-transparent perovskite solar cells (ST-PSCs) have broad applications in building integrated photovoltaics. However, the stability of ST-PSCs needs to be improved, especially in n-i-p ST-PSCs since the doped 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (Spiro-OMeTAD) is unstable at elevated temperatures and high humidity. In this work, a π-conjugated polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T) is selected to form a polymer composite hole transport layer (HTL) with Spiro-OMeTAD. The sulfur atom of the thiophene unit and the carbonyl group of the polymer interact with the undercoordinated Pb²⁺ at the perovskite surface, which stabilizes the perovskite/HTL interface and passivates the interfacial defects. The incorporation of the polymer also increases the glass transition temperature and the moisture resistance of Spiro-OMeTAD. As a result, we obtain ST-PSCs with a champion efficiency of 13.71% and an average visible light transmittance of 36.04%. Therefore, a high light utilization efficiency of 4.94% can be obtained. Moreover, the encapsulated device can maintain 84% of the initial efficiency after 751 h under continuous one-sun illumination (at 30% relative humidity) at the open circuit and the unencapsulated device can maintain 80% of the initial efficiency after maximum power tracking for more than 1250 h under continuous one-sun illumination.