# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 1 Introduction  
Colloidal semiconductor quantum dots (QDs) are nanometer-sized crystals made and suspended in solution. Because they can be processed in solution, have facilely tunable properties, and demonstrate unique photophysics, they are intriguing for optoelectronic devices, imaging, and fundamental science. The size and size distribution of the QD ensemble matter because they directly control the energy levels of the material (i.e., the highest occupied molecular orbital to the lowest unoccupied molecular orbital (HOMO-LUMO) gap or band gap). This observation can be explained from two points of view. The first viewpoint is a bottom-up description where the combination of molecular orbitals (energy states) with each additional atom bonded to the QD narrow the HOMO-LUMO gap until there is a negligible change, resulting in the \"bulk\" material. In the second top-down viewpoint, spatially confining the exciton (a bound electron-hole pair) wavefunction by making a crystal smaller thereby increases the energy required to excite the QD (e.g., the bandgap), analogous to the particle-in-a-box description [1; 2].  
There are several ways to synthesize colloidal QDs, which are reviewed elsewhere [3; 4; 5; 6; 7; 8; 9]. The main method, hot injection, follows a reaction pathway as described by La Mer's model of nucleation and growth [10]. After injection, the precursor concentration in solution increases and exceeds the nucleation threshold (i.e., the solution is supersaturated); at this point, a burst of nucleation occurs and continues along with QD growth until the precursor concentration drops below the nucleation threshold and enters the growth regime. During growth, size focusing can occur if the QD growth rate is size-dependent, i.e., if smaller QDs grow faster than larger ones then the size distribution will narrow. During this time however, the high surface energy of small QDs drives their dissolutionand precipitation onto larger QDs leading to an increase in the size distribution--A process called Ostwald's ripening [5; 11]. Influencing these processes by reaction temperature, precursor reactivity, and/or reaction kinetics allows for control over the size and size-distribution [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Once all the reactants are used and the concentration approaches zero, the growth regime ends. Typically, around half of the QD's atoms reside at the surface [24; 25]. With such a high ratio of atoms exposed at the surface versus in the core, it is not surprising that surface chemistry and surface environment dominate many properties of QDs--not just by size alone [26].  
With these two factors in mind (the size and the surface), this Review sheds light upon photoexcitations in colloidal semiconductor QD films and devices, from absorption to extraction (and the time between). We start with a brief overview of the processes that occur at each time scale ranging from a few femtoseconds to microseconds before giving an in-depth discussion of the progress of Pb-based QD solar cells (QDSCs). For in-depth discussions on each subtopic, we refer the interested readers to more specific reviews [27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41].