The objective the Theme is to develop new materials and processes that enable high-efficiency organic and perovskite solar cells with enhanced photoactive layer stability. Successful commercialization of organic and perovskite solar-cell modules requires the convergence of appropriate power conversion efficiency, low-cost manufacturing, and required lifetime. While each of these parameters is unique to the specific application, it is generally agreed that a lifetime specification of ten years is minimal for most applications; shorter lifetimes can, however, be acceptable for applications outside traditional power generation markets (for which the guaranteed performance has to be 25 years), such as mobile and wearable devices. Although significant progress has been made in both high-efficiency devices and high throughput processing, attention is now being increasingly focused to improve the lifetime of the device components with respect to the requirements of specific potential products. There are several specific and separate sources of instability in an organic or hybrid solar-cell device. Initial devices often suffered from delamination of the multilayer stack, leading to ingress of ambient water and oxygen, which could then attack the primary oxidizable component of the device, namely the low-workfunction cathode. Two approaches can be taken to solve this issue, namely improved encapsulation, adhesive, and barrier function of the device, as well as shifting to an alternative architecture – the inverted device – where a higher work function cathode can be employed. However, other sources of instability have remained and manifested over time. The three primary intrinsic processes involving the photoactive layer and leading to reduced device lifetime involve: electrochemistry, morphological instability, and photochemistry. The scope of Theme B will cover these critically important aspects.

Monocrystalline Perovskite Film Solar Cells: Model Systems for Studying the Impact of Grain Boundaries and Transport Layers on Stability

This project is dedicated to improving the stability of hybrid organic-inorganic lead trihalide perovskite solar cells. Despite the rapid growth in the efficiency of perovskite solar cells in a brief period of time (currently reaching more than 20%), major issues concerning the stability and degradation of these cells remain a major obstacle on the path to their commercialization. The degradation and operational instability of such cells can be compounded from several device components that are vulnerable to moisture and photodegradation. These components span a broad range of materials classes including: the organic hole transport layer, grain boundaries in the perovskite layer, and UV photocatalytic degradation at the perovskite/electron-conductor interface. To tackle these challenges, we will study new organic hole transporters, perovskite crystal growth, perovskite devices and electron conductor processing, charge transport and recombination dynamics, and perform computational materials modeling.

Impact of External Crosslinkers on the Efficiency & Stability of Small Molecule Solar Cells

The stability of the active-layer morphology is a critical aspect in ensuring lifetime of organic solar cells. On deposition of the active layer, and with the desired morphology achieved, it is essential to "lock" that morphology in order to maintain high power conversion efficiencies and to avoid device degradation over time. In polymer-fullerene bulk-heterojunctions (BHJs), one effective way of addressing morphological stability is to crosslink the BHJ thin film, either via "built-in" crosslinkable moieties directly appended to the polymer/fullerene systems, or by intimately mixing an external crosslinker in the polymer-fullerene blends, which are then solution-cast into BHJ thin films that include the external crosslinker. Here, as we consider the use of small molecule donors and acceptors as alternatives to polymer donors and fullerene acceptors for high-efficiency BHJ solar cells, the effectiveness and outcome of similar crosslinking strategies remains to be shown. We will explore and develop this approach to stabilization of the morphology, while simultaneously ensuring that crosslinking the active layer does not inadvertently cause a significant performance decrease, through device testing and optimizations and photo-physical characterization.

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