US chemists have shown that trace impurities - below the sensitivity of standard characterisation techniques - can halve the efficiency of bulk heterojunction (BHJ) solar cells. Their finding means that initially promising materials for device applications may have been written off prematurely owing to their low efficiencies.
Unlike traditional inorganic solar cells, polymer cells do not immediately create charge carriers but instead create electron-hole pairs called excitons over a donor-acceptor interface, which migrate to the electrodes. In BHJ solar cells, the donor-acceptor blend is mixed, creating this interface throughout the cell and transporting the charge carriers to the electrodes through an interpenetrating network. Although this network shows a significant improvement in efficiency over traditional cells, it is also highly sensitive to impurities, which can affect the carrier mobility and act as traps, increasing the chance of charge recombination before the exciton reaches an electrode.
A team led by Guillermo Bazan and Alan Heeger from the University of California, Santa Barbara, noticed that organic semiconductors' effectiveness could vary from batch to batch. As the power conversion efficiencies of BHJ solar cells depend on the average molecular weight of the polymer, the team started by examining the definition of the structures and found that trace impurities in the donor of below 1% could halve device performance.
'We had no expectation of such a strong effect,' Bazan tells Chemistry World. 'Looking back, it makes sense based on the intense purity requirements of silicon solar cells, but it took us a long time to actually detect the impurities.' The standard characterisation techniques showed no difference between samples and it wasn't until the team examined mass spectroscopy results that they found the minute impurity: a methyl terminated by-product.
Igor Perepichka, who studies organic p-functional systems at Bangor University, UK, comments that it was surprising that a by-product with 'very similar energy levels and spectral characteristics has so drastic an effect on the performance'. Jianguo Mei from DuPont, China, raised concerns that the degree of purification might affect the commercialisation of the technology, pointing out that extra purification could negatively impact the cost effectiveness of such products.
Bazan sees it in a more positive light, highlighting the number of BHJ solar cells with unexpectedly low efficiencies and looks forward to seeing if this information could lead to BHJ solar cells outperforming their polymer counterparts in the future.
- W L Leong et al, Chem. Sci., 2012, 10.1039/c2sc20157g