Organic Photovoltaics

Developing the science and technology of organic bulk heterojunction (BHJ) photovoltaic devices can lead to a new breed of cheap, lightweight, and flexible power sources. While tremendous progress in their power conversion efficiency has been demonstrated ( ηp ˜5% is reported for a single BHJ layer), obtaining an efficiency of ~10% (comparable to amorphous silicon) has been elusive.  Yet it should be attainable if organic materials with the right absorbance can be found. As shown in the figure below, we have fabricated P3HT-C60 devices with a fill-factor that is comparable to that of a single crystalline silicon solar cell. The difference in their power conversion efficiencies (and short circuit currents) is due to the much narrower optical absorption in the organic photovoltaic device, which can be widened by the addition of other polymer(s) that are absorbing in the needed spectral range.


Glove box with integrated  spincoater and evaporator  for fabricating organic solar cells and photodetectors in an inert environment.

Figure 1. Characteristics of a P3HT:PCBM-C60 BHJ organic PV device (blue) and a single-crystalline silicon solar cell (red).  Fill factors are nearly identical.  The biggest difference is in their short-circuit current which is due to the much wider spectral range of the optical absorption in silicon, as indicated by their quantum efficiencies shown in the inset.


“Organic Photovoltaic Devices Based on a Block Copolymer / Fullerene Blend,” R. P. Barber, Jr., R. D. Gomez, W. N. Herman, D. B. Romero, Organic Electronics 7, 508(2006).

“High fill-factor organic bulk heterojunction photovoltaic devices using a highly conducting hole-doped polymer transport layer,” M. Ballarotto, W.N. Herman, and D. B. Romero, in press in Organic Thin Films for Photonics Applications,  W.N. Herman, S. Flom, and S. Foulger, eds. (ACS Symposium Series 1039, 2010), Chap 14.

“Low band gap small molecules for near-infrared photovoltaic applications,” M. Ballarotto, W.N. Hermana, D.B. Romero, J. Photon. Energy 1, 011102 (Jan 04, 2011); doi:10.1117/1.3528044.

“Low bandgap small molecules for near-infrared photovoltaic applications,”  M.Ballarotto, W.N Herman, D.B. Romero, Proc. SPIE 7777-38(2010).

 “Understanding the current-voltage characteristics of bulk heterojunction photovoltaic devices”, M.Ballarotto, W.N Herman, D.B. Romero, Proc. SPIE, 7777-46(2010).

“Effect of gold nanopillar arrays on the absorption spectrum of a bulk heterojunction organic solar cell,” S.-J. Tsai, M. Ballarotto, D. B. Romero, W. N. Herman, H.-C. Kan, and R. J. Phaneuf, Optics Express 18, A528(2010).