Haiwei Chen

A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells

Minimizing losses at interfaces Among the issues facing the practical use of hybrid organohalide lead perovskite solar cells is the loss of charge carriers at interfaces. Hou et al. show that tantalum-doped tungsten oxide forms almost ohmic contacts with inexpensive conjugated polymer multilayers to create a hole-transporting material with a small interface barrier. This approach eliminates the use of ionic dopants that compromise device stability. Solar cells made with these contacts achieved maximum efficiencies of 21.2% and operated stably for more than 1000 hours. Science, this issue p. 1192 Tantalum-doped tungsten oxide forms nearly ohmic contacts with conjugated polymers to create efficient hole transporters. A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials without compromising efficiency, stability, or scalability of perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WOx)/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. In a simple device with regular planar architecture and a self-assembled monolayer, Ta-WOx–doped interface–based perovskite solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light stability. By eliminating additional ionic dopants, these findings open up the entire class of organics as scalable hole-transporting materials for perovskite solar cells.

Abnormal strong burn-in degradation of highly efficient polymer solar cells caused by spinodal donor-acceptor demixing

The performance of organic solar cells is determined by the delicate, meticulously optimized bulk-heterojunction microstructure, which consists of finely mixed and relatively separated donor/acceptor regions. Here we demonstrate an abnormal strong burn-in degradation in highly efficient polymer solar cells caused by spinodal demixing of the donor and acceptor phases, which dramatically reduces charge generation and can be attributed to the inherently low miscibility of both materials. Even though the microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation in the solid state, even at room temperature and in the dark. A theoretical calculation of the molecular parameters and construction of the spinodal phase diagrams highlight molecular incompatibilities between the donor and acceptor as a dominant mechanism for burn-in degradation, which is to date the major short-time loss reducing the performance and stability of organic solar cells.

Extending the environmental lifetime of unpackaged perovskite solar cells through interfacial design

Solution-processed oxo-functionalized graphene (oxo-G1) is employed to substitute hydrophilic PEDOT:PSS as an anode interfacial layer for perovskite solar cells. The resulting devices exhibit a reasonably high power conversion efficiency (PCE) of 15.2% in the planar inverted architecture with oxo-G1 as a hole transporting material (HTM), and most importantly, deploy the full open-circuit voltage (Voc) of up to 1.1 V. Moreover, oxo-G1 effectively slows down the ingress of water vapor into the device stack resulting in significantly enhanced environmental stability of unpackaged cells under illumination with 80% of the initial PCE being reached after 500 h. Without encapsulation, ∼60% of the initial PCE is retained after ∼1000 h of light soaking under 0.5 sun and ambient conditions maintaining the temperature beneath 30 °C. Moreover, the unsealed perovskite device retains 92% of its initial PCE after about 1900 h under ambient conditions and in the dark. Our results underpin that controlling water diffusion into perovskite cells through advanced interface engineering is a crucial step towards prolonged environmental stability.

Photoinduced degradation of methylammonium lead triiodide perovskite semiconductors

Photoinduced degradation is a critical obstacle for the real application of novel semiconductors for photovoltaic applications. In this paper, the photoinduced degradation of CH3NH3PbI3 in a vacuum and air (relative humidity 40%) is analyzed by ex situ and advanced in situ technologies. Without light illumination, CH3NH3PbI3 films slowly degrade under vacuum and air within 24 hours. However, we find that CH3NH3PbI3 converts to metallic lead (Pb0) when exposed to vacuum and light illumination. Further, a series of lead salts (e.g. PbO, Pb(OH)2 and PbCO3) are formed when CH3NH3PbI3 is degraded under environmental conditions, i.e. under the combination of light, oxygen and moisture. Photoinduced degradation is found to be determined by the environmental atmosphere as CH3NH3PbI3 films remain very stable under nitrogen conditions. The results from vacuum conditions underpin that the high volatility of the organic component (CH3NH3I) is in conflict with reaching excellent intrinsic stability due to its role in creating ion vacancies. The degradation in air suggests that both oxygen and water contribute to the fast photodecomposition of CH3NH3PbI3 into lead salts rather than water alone. Given these basic yet fundamental understandings, the design of hydrophobic capping layers becomes one prerequisite towards long-term stable perovskite-based devices.

Perovskite solar cells fabricated using dicarboxylic fullerene derivatives

Perovskite solar cells were first fabricated in dye sensitized solar cells. But also, perovskite hybrid solar cells were demonstrated to be among the most promising candidates within the emerging photovoltaic materials with their high power conversion efficiencies and low-cost fabrication. In this work, we design and synthesize a novel benzoic acid fullerene bis adduct material (BAFB) for use in perovskite hybrid organic–inorganic solar cells. The obtained maximum efficiency is reported to be 9.63% using a novel benzoic acid fullerene bis adduct (BAFB) for perovskite heterojunction solar cells.