On voltage, photovoltage, and photocurrent in bulk heterojunction organic solar cells

Abstract

The interpretation of voltage, photovoltage, and photocurrent in polymer/fullerene bulk heterojunction (BHJ) solar cells is discussed in terms of fundamental device models and results of capacitance spectroscopy. First we establish the relationship between the applied voltage (which is the difference of Fermi levels) and the variation of electrostatic potential that governs the drift field. We then show the most common distribution of carriers and Fermi levels in the blend layer, supported on experimental results of impedance spectroscopy of P3HT:PCBM solar cells. We arrive at the conclusion that charge separation and charge transportation has very little to do with a built-in electric field between metal contacts, while kinetics plays a major role in photocurrent production. Finally, we discuss the key factors relevant to understand device properties and power conversion efficiencies of the BHJ solar cells: recombination, charge generation, and the current–potential curve, based on the suggested model that emphasizes mobile electrons and holes (that we term quasifree carriers) contributing to the respective Fermi levels.